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IETF RFC 5740
NACK-Oriented Reliable Multicast (NORM) Transport Protocol
Last modified on Tuesday, November 17th, 2009
 Permanent link to RFC 5740
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Network Working Group B. Adamson
Request for Comments: 5740 Naval Research Laboratory
Obsoletes: 3940 C. Bormann
Category: Standards Track Universitaet Bremen TZI
M. Handley
University College London
J. Macker
Naval Research Laboratory
November 2009
NACK-Oriented Reliable Multicast (NORM) Transport Protocol
 Abstract
This document describes the messages and procedures of the Negative-
ACKnowledgment (NACK) Oriented Reliable Multicast (NORM) protocol.
This protocol can provide end-to-end reliable transport of bulk data
objects or streams over generic IP multicast routing and forwarding
services. NORM uses a selective, negative acknowledgment mechanism
for transport reliability and offers additional protocol mechanisms
to allow for operation with minimal a priori coordination among
senders and receivers. A congestion control scheme is specified to
allow the NORM protocol to fairly share available network bandwidth
with other transport protocols such as Transmission Control Protocol
(TCP). It is capable of operating with both reciprocal multicast
routing among senders and receivers and with asymmetric connectivity
(possibly a unicast return path) between the senders and receivers.
The protocol offers a number of features to allow different types of
applications or possibly other higher-level transport protocols to
utilize its service in different ways. The protocol leverages the
use of FEC-based (forward error correction) repair and other IETF
Reliable Multicast Transport (RMT) building blocks in its design.
This document obsoletes RFC 3940.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
Adamson, et al. Standards Track PAGE 1 
RFC 5740 NORM Protocol November 2009
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Contributions published or made publicly available before November
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Without obtaining an adequate license from the person(s) controlling
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Table of Contents
1. Introduction and Applicability . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.2. NORM Data Delivery Service Model . . . . . . . . . . . . . 5
1.3. NORM Scalability . . . . . . . . . . . . . . . . . . . . . 7
1.4. Environmental Requirements and Considerations . . . . . . 8
2. Architecture Definition . . . . . . . . . . . . . . . . . . . 8
2.1. Protocol Operation Overview . . . . . . . . . . . . . . . 10
2.2. Protocol Building Blocks . . . . . . . . . . . . . . . . . 12
2.3. Design Trade-Offs . . . . . . . . . . . . . . . . . . . . 12
3. Conformance Statement . . . . . . . . . . . . . . . . . . . . 13
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. NORM Common Message Header and Extensions . . . . . . . . 15
4.2. Sender Messages . . . . . . . . . . . . . . . . . . . . . 18
4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . . . . 18
4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . . . . 28
4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . . . . 29
4.3. Receiver Messages . . . . . . . . . . . . . . . . . . . . 47
4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . . . . 47
4.3.2. NORM_ACK Message . . . . . . . . . . . . . . . . . . . 53
4.4. General Purpose Messages . . . . . . . . . . . . . . . . . 55
4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . . . . 55
5. Detailed Protocol Operation . . . . . . . . . . . . . . . . . 55
5.1. Sender Initialization and Transmission . . . . . . . . . . 57
5.1.1. Object Segmentation Algorithm . . . . . . . . . . . . 58
Adamson, et al. Standards Track PAGE 2
RFC 5740 NORM Protocol November 2009
5.2. Receiver Initialization and Reception . . . . . . . . . . 59
5.3. Receiver NACK Procedure . . . . . . . . . . . . . . . . . 59
5.4. Sender NACK Processing and Response . . . . . . . . . . . 62
5.4.1. Sender Repair State Aggregation . . . . . . . . . . . 62
5.4.2. Sender FEC Repair Transmission Strategy . . . . . . . 63
5.4.3. Sender NORM_CMD(SQUELCH) Generation . . . . . . . . . 64
5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation . . . . . . . . 65
5.5. Additional Protocol Mechanisms . . . . . . . . . . . . . . 65
5.5.1. Group Round-Trip Time (GRTT) Collection . . . . . . . 65
5.5.2. NORM Congestion Control Operation . . . . . . . . . . 67
5.5.3. NORM Positive Acknowledgment Procedure . . . . . . . . 75
5.5.4. Group Size Estimate . . . . . . . . . . . . . . . . . 77
6. Configurable Elements . . . . . . . . . . . . . . . . . . . . 77
7. Security Considerations . . . . . . . . . . . . . . . . . . . 80
7.1. Baseline Secure NORM Operation . . . . . . . . . . . . . . 82
7.1.1. IPsec Approach . . . . . . . . . . . . . . . . . . . . 83
7.1.2. IPsec Requirements . . . . . . . . . . . . . . . . . . 85
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 86
8.1. Explicit IANA Assignment Guidelines . . . . . . . . . . . 87
8.1.1. NORM Header Extension Types . . . . . . . . . . . . . 87
8.1.2. NORM Stream Control Codes . . . . . . . . . . . . . . 88
8.1.3. NORM_CMD Message Sub-Types . . . . . . . . . . . . . . 88
9. Suggested Use . . . . . . . . . . . . . . . . . . . . . . . . 89
10. Changes from RFC 3940 . . . . . . . . . . . . . . . . . . . . 90
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 91
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 91
12.1. Normative References . . . . . . . . . . . . . . . . . . . 91
12.2. Informative References . . . . . . . . . . . . . . . . . . 92
Adamson, et al. Standards Track PAGE 3
RFC 5740 NORM Protocol November 2009
1. Introduction and Applicability
The Negative-ACKnowledgment (NACK) Oriented Reliable Multicast (NORM)
protocol can provide reliable transport of data from one or more
senders to a group of receivers over an IP multicast network. The
primary design goals of NORM are to provide efficient, scalable, and
robust bulk data (e.g., computer files, transmission of persistent
data) transfer across possibly heterogeneous IP networks and
topologies. The NORM protocol design provides support for
distributed multicast session participation with minimal coordination
among senders and receivers. NORM allows senders and receivers to
dynamically join and leave multicast sessions at will with minimal
overhead for control information and timing synchronization among
participants. To accommodate this capability, NORM protocol message
headers contain some common information allowing receivers to easily
synchronize to senders throughout the lifetime of a reliable
multicast session. NORM is self-adapting to a wide range of dynamic
network conditions with little or no pre-configuration. The protocol
is tolerant of inaccurate timing estimations or lossy conditions that
can occur in many networks including mobile and wireless. The
protocol can also converge and maintain efficient operation even in
situations of heavy packet loss and large queuing or transmission
delays. This document obsoletes the Experimental RFC 3940
specification.
This document is a product of the IETF RMT working group and follows
the guidelines provided in the Author Guidelines for Reliable
Multicast Transport (RMT) Building Blocks and Protocol Instantiation
documents [RFC 3269].
Statement of Intent
This memo contains the definitions necessary to fully specify a
Reliable Multicast Transport protocol in accordance with the criteria
of IETF Criteria for Evaluating Reliable Multicast Transport and
Application Protocols [RFC 2357]. The NORM specification described in
this document was previously published in the Experimental Category
[RFC 3940]. It was the stated intent of the RMT working group to re-
submit this specifications as an IETF Proposed Standard in due
course. This Proposed Standard specification is thus based on RFC
3940 and has been updated according to accumulated experience and
growing protocol maturity since the publication of RFC 3940. Said
experience applies both to this specification itself and to
congestion control strategies related to the use of this
specification. The differences between RFC 3940 and this document
are listed in Section 10.
Adamson, et al. Standards Track PAGE 4
RFC 5740 NORM Protocol November 2009
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
1.2. NORM Data Delivery Service Model
A NORM protocol instance (NormSession) is defined within the context
of participants communicating connectionless (e.g., Internet Protocol
(IP) or User Datagram Protocol (UDP)) packets over a network using
pre-determined addresses and host port numbers. Generally, the
participants exchange packets using an IP multicast group address,
but unicast transport MAY also be established or applied as an
adjunct to multicast delivery. In the case of multicast, the
participating NormNodes will communicate using a common IP multicast
group address and port number chosen via means outside the context of
the given NormSession. Other existing IETF data format and protocol
standards MAY be applied to describe and convey the necessary a
priori information for a specific NormSession (e.g., Session
Description Protocol (SDP) [RFC 4566], Session Announcement Protocol
(SAP) [RFC 2974], etc.).
The NORM protocol design is principally driven by the assumption of a
single sender transmitting bulk data content to a group of receivers.
However, the protocol MAY operate with multiple senders within the
context of a single NormSession. In initial implementations of this
protocol, it is anticipated that multiple senders will transmit
independently of one another and receivers will maintain state as
necessary for each sender. In future versions of NORM, it is
possible some aspects of protocol operation (e.g., round-trip time
collection) will provide for alternate modes allowing more efficient
performance for applications requiring multiple senders.
NORM provides for three types of bulk data content objects
(NormObjects) to be reliably transported. These types include:
1. static computer memory data content (NORM_OBJECT_DATA type),
2. computer storage files (NORM_OBJECT_FILE type), and
3. non-finite streams of continuous data content (NORM_OBJECT_STREAM
type).
The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is
simply to provide a hint to receivers in NormSessions serving
multiple types of content as to what type of storage to allocate for
received content (i.e., memory or file storage). Other than that
Adamson, et al. Standards Track PAGE 5
RFC 5740 NORM Protocol November 2009
distinction, the two are identical, providing for reliable transport
of finite (but potentially very large) units of content. These
static data and file services are anticipated to be useful for
multicast-based cache applications with the ability to reliably
provide transmission of large quantities of static data. Other types
of static data/file delivery services might make use of these
transport object types, too. The use of the NORM_OBJECT_STREAM type
is at the application's discretion and could be used to carry static
data or file content also. The NORM reliable stream service opens up
additional possibilities such as serialized reliable messaging or
other unbounded, perhaps dynamically produced content. The
NORM_OBJECT_STREAM provides for reliable transport analogous to that
of the Transmission Control Protocol (TCP), although NORM receivers
will be able to begin receiving stream content at any point in time.
The applicability of this feature will depend upon the application.
The NORM protocol also allows for a small amount of out-of-band data
(sent as NORM_INFO messages) to be attached to the data content
objects transmitted by the sender. This readily available out-of-
band data allows multicast receivers to quickly and efficiently
determine the nature of the corresponding data, file, or stream bulk
content being transmitted. This allows application-level control of
the receiver node's participation in the current transport activity.
This also allows the protocol to be flexible with minimal pre-
coordination among senders and receivers. The NORM_INFO content is
atomic in that its size MUST fit into the payload portion of a single
NORM message.
NORM does NOT provide for global or application-level identification
of data content within its message headers. Note the NORM_INFO out-
of-band data mechanism can be leveraged by the application for this
purpose if desired, or identification can alternatively be embedded
within the data content. NORM does identify transmitted content
(NormObjects) with transport identifiers that are applicable only
while the sender is transmitting and/or repairing the given object.
These transport data content identifiers (NormTransportIds) are
assigned in a monotonically increasing fashion by each NORM sender
during the course of a NormSession. Participants, including senders,
in NORM protocol sessions are also identified with unique identifiers
(NormNodeIds). Each sender maintains its NormTransportId assignments
independently and thus individual NormObjects can be uniquely
identified during transport by concatenation of the session-unique
sender identifier (NormNodeId) and the assigned NormTransportId. The
NormTransportIds are assigned from a large, but fixed, numeric space
in increasing order and will be reassigned during long-lived
sessions. The NORM protocol provides mechanisms so the sender
application can terminate transmission of data content and inform the
group of this in an efficient manner. Other similar protocol control
Adamson, et al. Standards Track PAGE 6
RFC 5740 NORM Protocol November 2009
mechanisms (e.g., session termination, receiver synchronization,
etc.) are specified so reliable multicast application variants can
realize different, complete bulk transfer communication models to
meet their goals.
To summarize, the NORM protocol provides reliable transport of
different types of data content (including potentially mixed types).
The senders enqueue and transmit bulk content in the form of static
data or files and/or non-finite, ongoing stream types. NORM senders
provide for repair transmission of data and/or FEC content in
response to NACK messages received from the receiver group.
Mechanisms for out-of-band information and other transport control
mechanisms are specified for use by applications to form complete
reliable multicast solutions for different purposes.
1.3. NORM Scalability
Group communication scalability requirements lead to adaptation of
NACK-based protocol schemes when feedback for reliability is needed
[RmComparison]. NORM is a protocol centered around the use of
selective NACKs to request repairs of missing data. NORM provides
for the use of packet-level forward error correction (FEC) techniques
for efficient multicast repair and OPTIONAL proactive transmission
robustness [RFC 3453]. FEC-based repair can be used to greatly reduce
the quantity of reliable multicast repair requests and repair
transmissions [MdpToolkit] in a NACK-oriented protocol. The
principal factor in NORM scalability is the volume of feedback
traffic generated by the receiver set to facilitate reliability and
congestion control. NORM uses probabilistic suppression of redundant
feedback based on exponentially distributed random backoff timers.
The performance of this type of suppression relative to other
techniques is described in [McastFeedback]. NORM dynamically
measures the group's round-trip timing status to set its suppression
and other protocol timers. This allows NORM to scale well while
maintaining reliable data delivery transport with low latency
relative to the network topology over which it is operating.
Feedback messages can be either multicast to the group at large or
sent via unicast routing to the sender. In the case of unicast
feedback, the sender relays the feedback state to the group to
facilitate feedback suppression. In typical Internet environments,
the NORM protocol will readily scale to group sizes on the order of
tens of thousands of receivers. A study of the quantity of feedback
for this type of protocol is described in [NormFeedback]. NORM is
able to operate with a smaller amount of feedback than a single TCP
connection, even with relatively large numbers of receivers. Thus,
depending upon the network topology, it is possible for NORM to scale
to larger group sizes. With respect to computer resource usage, the
Adamson, et al. Standards Track PAGE 7
RFC 5740 NORM Protocol November 2009
NORM protocol does not need state to be kept on all receivers in the
group. NORM senders maintain state only for receivers providing
explicit congestion control feedback. However, NORM receivers need
to maintain state for each active sender. This can constrain the
number of simultaneous senders in some uses of NORM.
1.4. Environmental Requirements and Considerations
All of the environmental requirements and considerations that apply
to the "Multicast Negative-Acknowledgment (NACK) Building Blocks"
[RFC 5401], "Forward Error Correction (FEC) Building Block" [RFC 5052],
and "TCP-Friendly Multicast Congestion Control (TFMCC) Protocol
Specification" [RFC 4654] also apply to the NORM protocol.
The NORM protocol SHALL be capable of operating in an end-to-end
fashion with no assistance from intermediate systems beyond basic IP
multicast group management, routing, and forwarding services. While
the techniques utilized in NORM are principally applicable to flat,
end-to-end IP multicast topologies, they could also be applied in the
sub-levels of hierarchical (e.g., tree-based) multicast distribution
if so desired. NORM can make use of reciprocal (among senders and
receivers) multicast communication under the Any-Source Multicast
(ASM) model defined in "Host Extensions for IP Multicasting"
[RFC 1112], but it SHALL also be capable of scalable operation in
asymmetric topologies such as Source-Specific Multicast (SSM)
[RFC 4607] where only unicast routing service is available from the
receivers to the sender(s).
NORM is compatible with IPv4 and IPv6. Additionally, NORM can be
used with networks employing Network Address Translation (NAT)
provided that the NAT device supports IP multicast and/or can cache
UDP traffic source port numbers for remapping feedback traffic from
receivers to the sender(s).
2. Architecture Definition
A NormSession is comprised of participants (NormNodes) acting as
senders and/or receivers. NORM senders transmit data content in the
form of NormObjects to the session destination address, and the NORM
receivers attempt to reliably receive the transmitted content using
negative acknowledgments to request repair. Each NormNode within a
NormSession is assumed to have a preselected unique 32-bit identifier
(NormNodeId). NormNodes MUST have uniquely assigned identifiers
within a single NormSession to distinguish between multiple possible
senders and to distinguish feedback information from different
receivers. There are two reserved NormNodeId values. A value of
0x00000000 is considered an invalid NormNodeId (NORM_NODE_NONE), and
a value of 0xffffffff is a "wild card" NormNodeId (NORM_NODE_ANY).
Adamson, et al. Standards Track PAGE 8
RFC 5740 NORM Protocol November 2009
While the protocol does not preclude multiple sender nodes
concurrently transmitting within the context of a single NORM session
(i.e., many-to-many operation), any type of interactive coordination
among NORM senders is assumed to be controlled by the application- or
higher-protocol layer. There are some OPTIONAL mechanisms specified
in this document that can be leveraged for such application-layer
coordination.
As previously noted, NORM allows for reliable transmission of three
different basic types of data content. The first type is
NORM_OBJECT_DATA, which is used for static, persistent blocks of data
content maintained in the sender's application memory storage. The
second type is NORM_OBJECT_FILE, which corresponds to data stored in
the sender's non-volatile file system. The NORM_OBJECT_DATA and
NORM_OBJECT_FILE types both represent NormObjects of finite but
potentially very large size. The third type of data content is
NORM_OBJECT_STREAM, which corresponds to an ongoing transmission of
undefined length. This is analogous to the reliable stream service
provided by TCP for unicast data transport. The format of the stream
content is application-defined and can be "byte" or "message"
oriented. The NORM protocol provides for "flushing" of the stream to
expedite delivery or possibly enforce application message boundaries.
NORM protocol implementations MAY offer either (or both) in-order
delivery of the stream data to the receive application or out-of-
order (more immediate) delivery of received segments of the stream to
the receiver application. In either case, NORM sender and receiver
implementations provide buffering to facilitate repair of the stream
as it is transported.
All NormObjects are logically segmented into FEC coding blocks and
symbols for transmission by the sender. In NORM, a FEC encoding
symbol directly corresponds to the payload of NORM_DATA messages or
"segment". Note that when systematic FEC codes are used, the payload
of NORM_DATA messages sent for the first portion of a FEC encoding
block are source symbols (actual segments of original user data),
while the remaining symbols for the block consist of parity symbols
generated by FEC encoding. These parity symbols are generally sent
in response to repair requests, but some number MAY be sent
proactively at the end of each encoding block to increase the
robustness of transmission. When non-systematic FEC codes are used,
all symbols sent consist of FEC encoding parity content. In this
case, the receiver needs to receive a sufficient number of symbols to
reconstruct (via FEC decoding) the original user data for the given
block.
Transmitted NormObjects are temporarily, yet uniquely, identified
within the NormSession context using the given sender's NormNodeId,
NormInstanceId, and a temporary NormTransportId. Depending upon the
Adamson, et al. Standards Track PAGE 9
RFC 5740 NORM Protocol November 2009
implementation, individual NORM senders can manage their
NormInstanceIds independently, or a common NormInstanceId could be
agreed upon for all participating nodes within a session, if needed,
as a session identifier. NORM NormTransportId data content
identifiers are sender-assigned and applicable and valid only during
a NormObject's actual transport (i.e., for as long as the sender is
transmitting and providing repair of the indicated NormObject). For
a long-lived session, the NormTransportId field can wrap and
previously used identifiers will be re-used. Note that globally
unique identification of transported data content is not provided by
NORM and, if necessary, is expected to be managed by the NORM
application. The individual segments or symbols of the NormObject
are further identified with FEC payload identifiers that include
coding block and symbol identifiers. These are discussed in detail
later in this document.
2.1. Protocol Operation Overview
A NORM sender primarily generates messages of type NORM_DATA. These
messages carry original data segments or FEC symbols and repair
segments/symbols for the bulk data/file or stream NormObjects being
transferred. By default, redundant FEC symbols are sent only in
response to receiver repair requests (NACKs) and thus normally little
or no additional transmission overhead is imposed due to FEC
encoding. However, the NORM implementation MAY be configured to
proactively transmit some amount of redundant FEC symbols along with
the original content to potentially enhance performance (e.g.,
improved delay) at the cost of additional transmission overhead.
This configuration is sensible for certain network conditions and can
allow for robust, asymmetric multicast (e.g., unidirectional routing,
satellite, cable) [FecHybrid] with reduced receiver feedback, or, in
some cases, no feedback.
A sender message of type NORM_INFO is also defined and is used to
carry OPTIONAL out-of-band context information for a given transport
object. A single NORM_INFO message can be associated with a
NormObject. Because of its atomic nature, missing NORM_INFO messages
can be NACKed and repaired with a slightly lower delay process than
NORM's general FEC-encoded data content. The NORM_INFO message can
serve special purposes for some bulk transfer, reliable multicast
applications where receivers join the group mid-stream and need to
ascertain contextual information on the current content being
transmitted. The NACK process for NORM_INFO will be described later.
When the NORM_INFO message type is used, its transmission SHOULD
precede transmission of any NORM_DATA message for the associated
NormObject.
The sender also generates messages of type NORM_CMD to assist in
Adamson, et al. Standards Track PAGE 10
RFC 5740 NORM Protocol November 2009
certain protocol operations such as congestion control, end-of-
transmission flushing, group round-trip time (GRTT) estimation,
receiver synchronization, and OPTIONAL positive acknowledgment
requests or application-defined commands. The transmission of
NORM_CMD messages from the sender is accomplished by one of three
different procedures: single, best-effort unreliable transmission of
the command; repeated redundant transmissions of the command; and
positively acknowledged commands. The transmission technique used
for a given command depends upon the function of the command.
Several core commands are defined for basic protocol operation.
Additionally, implementations MAY wish to consider providing the
OPTIONAL application-defined commands that can take advantage of the
transmission methodologies available for commands. This allows for
application-level session management mechanisms that can make use of
information available to the underlying NORM protocol engine (e.g.,
round-trip timing, transmission rate, etc.). A notable distinction
between NORM_DATA message and some NORM_CMD message transmissions is
that typically a receiver will need to allocate resources to manage
reliable reception when NORM_DATA messages are received. However,
some NORM_CMD messages are completely atomic and no specific
reliability (buffering) state needs to be kept. Thus, for session
management or other purposes, it is possible that even participants
acting principally as data receivers MAY transmit NORM_CMD messages.
However, it is RECOMMENDED that this is not done within the context
of the NORM multicast session unless congestion control is addressed.
For example, many receiver nodes transmitting NORM_CMD messages
simultaneously can cause congestion for the destination(s).
All sender transmissions are subject to rate control governed by a
peak transmission rate set for each participant by the application.
This can be used to limit the quantity of multicast data transmitted
by the group. When NORM's congestion control algorithm is enabled,
the rate for senders is automatically adjusted. In some networks, it
is desirable to establish minimum and maximum bounds for the rate
adjustment depending upon the application even when dynamic
congestion control is enabled. However, in the case of the general
Internet, congestion control policy SHALL be observed that is
compatible with coexistent TCP flows.
NORM receivers generate messages of type NORM_NACK or NORM_ACK in
response to transmissions of data and commands from a sender. The
NORM_NACK messages are generated to request repair of detected data
transmission losses. Receivers generally detect losses by tracking
the sequence of transmission from a sender. Sequencing information
is embedded in the transmitted data packets and end-of-transmission
commands from the sender. NORM_ACK messages are generated in
response to certain commands transmitted by the sender. In the
general (and most scalable) protocol mode, NORM_ACK messages are sent
Adamson, et al. Standards Track PAGE 11
RFC 5740 NORM Protocol November 2009
only in response to congestion control commands from the sender. The
feedback volume of these congestion control NORM_ACK messages is
controlled using the same timer-based probabilistic suppression
techniques as for NORM_NACK messages to avoid feedback implosion. In
order to meet potential application requirements for positive
acknowledgment from receivers, other NORM_ACK messages are defined
and are available for use.
2.2. Protocol Building Blocks
The operation of the NORM protocol is based primarily upon the
concepts presented in the Multicast NACK Building Block [RFC 5401]
document. This includes the basic NORM architecture and the data
transmission, repair, and feedback strategies discussed in that
document. The reliable multicast building block approach, as
described in "Reliable Multicast Transport Building Blocks for One-
to-Many Bulk-Data Transfer" [RFC 3048], is applied in creating the
full NORM protocol instantiation. NORM also makes use of the parity-
based encoding techniques for repair messaging and added transmission
robustness as described in "The Use of Forward Error Correction (FEC)
in Reliable Multicast" [RFC 3453]. NORM uses the FEC Payload ID as
specified by the FEC Building Block document [RFC 5052].
Additionally, for congestion control, this document fully specifies a
baseline congestion control mechanism (NORM-CC) based on the TCP-
Friendly Multicast Congestion Control (TFMCC) scheme [TfmccPaper],
[RFC 4654].
2.3. Design Trade-Offs
While the various features of NORM provide some measure of general
purpose utility, it is important to emphasize the understanding that
"no one size fits all" in the reliable multicast transport arena.
There are numerous engineering trade-offs involved in reliable
multicast transport design and this necessitates an increased
awareness of application and network architecture considerations.
Performance requirements affecting design can include: group size,
heterogeneity (e.g., capacity and/or delay), asymmetric delivery,
data ordering, delivery delay, group dynamics, mobility, congestion
control, and transport across low-capacity connections. NORM
contains various parameters to accommodate many of these differing
requirements. The NORM protocol and its mechanisms MAY be applied in
multicast applications outside of bulk data transfer, but there is an
assumed model of bulk transfer transport service that drives the
trade-offs that determine the scalability and performance described
in this document.
The ability of NORM to provide reliable data delivery is also
governed by any buffer constraints of the sender and receiver
Adamson, et al. Standards Track PAGE 12
RFC 5740 NORM Protocol November 2009
applications. NORM protocol implementations SHOULD operate with the
greatest efficiency and robustness possible within application-
defined buffer constraints. Buffer requirements for reliability, as
always, are a function of the delay-bandwidth product of the network
topology. NORM performs best when allowed more buffering resources
than typical point-to-point transport protocols. This is because
NORM feedback suppression is based upon randomly delayed
transmissions from the receiver set, rather than immediately
transmitted feedback. There are definitive trade-offs between buffer
utilization, group size scalability, and efficiency of performance.
Large buffer sizes allow the NORM protocol to perform most
efficiently in large delay-bandwidth topologies and allow for longer
feedback suppression backoff timeouts. This yields improved group
size scalability. NORM can operate with reduced buffering but at a
cost of decreased efficiency (lower relative goodput) and reduced
group size scalability.
3. Conformance Statement
This RMT Protocol Instantiation document, in conjunction with the
"Multicast Negative-Acknowledgment (NACK) Building Blocks" [RFC 5401]
and "Forward Error Correction (FEC) Building Block" [RFC 5052]
Building Blocks, completely specifies a working reliable multicast
transport protocol that conforms to the requirements described in RFC
2357.
This document specifies the following message types and mechanisms
that are REQUIRED in complying NORM protocol implementations:
+----------------------+--------------------------------------------+
| Message Type | Purpose |
+----------------------+--------------------------------------------+
| NORM_DATA | Sender message for application data |
| | transmission. Implementations MUST |
| | support at least one of the |
| | NORM_OBJECT_DATA, NORM_OBJECT_FILE, or |
| | NORM_OBJECT_STREAM delivery services. The |
| | use of the NORM FEC Object Transmission |
| | Information header extension is OPTIONAL |
| | with NORM_DATA messages. |
| NORM_CMD(FLUSH) | Sender command to excite receivers for |
| | repair requests in lieu of ongoing |
| | NORM_DATA transmissions. Note the use of |
| | the NORM_CMD(FLUSH) for positive |
| | acknowledgment of data receipt is |
| | OPTIONAL. |
Adamson, et al. Standards Track PAGE 13
RFC 5740 NORM Protocol November 2009
| NORM_CMD(SQUELCH) | Sender command to advertise its current |
| | valid repair window in response to invalid |
| | requests for repair. |
| NORM_CMD(REPAIR_ADV) | Sender command to advertise current repair |
| | (and congestion control state) to group |
| | when unicast feedback messages are |
| | detected. Used to control/suppress |
| | excessive receiver feedback in asymmetric |
| | multicast topologies. |
| NORM_CMD(CC) | Sender command used in collection of |
| | round-trip timing and congestion control |
| | status from group (this is OPTIONAL if |
| | alternative congestion control mechanism |
| | and round-trip timing collection is used). |
| NORM_NACK | Receiver message used to request repair of |
| | missing transmitted content. |
| NORM_ACK | Receiver message used to proactively |
| | provide feedback for congestion control |
| | purposes. Also used with the OPTIONAL |
| | NORM Positive Acknowledgment Process. |
+----------------------+--------------------------------------------+
This document also describes the following message types and
associated mechanisms that are OPTIONAL for complying NORM protocol
implementations:
+-----------------------+-------------------------------------------+
| Message Type | Purpose |
+-----------------------+-------------------------------------------+
| NORM_INFO | Sender message for providing ancillary |
| | context information associated with NORM |
| | transport objects. The use of the NORM |
| | FEC Object Transmission Information |
| | header extension is OPTIONAL with |
| | NORM_INFO messages. |
| NORM_CMD(EOT) | Sender command to indicate it has reached |
| | end-of-transmission and will no longer |
| | respond to repair requests. |
| NORM_CMD(ACK_REQ) | Sender command to support |
| | application-defined, positively |
| | acknowledged commands sent outside of the |
| | context of the bulk data content being |
| | transmitted. The NORM Positive |
| | Acknowledgment Procedure associated with |
| | this message type is OPTIONAL. |
Adamson, et al. Standards Track PAGE 14
RFC 5740 NORM Protocol November 2009
| NORM_CMD(APPLICATION) | Sender command containing |
| | application-defined commands sent outside |
| | of the context of the bulk data content |
| | being transmitted. |
| NORM_REPORT | Optional message type reserved for |
| | experimental implementations of the NORM |
| | protocol. |
+-----------------------+-------------------------------------------+
4. Message Formats
There are two primary classes of NORM messages (see Section 2.1):
sender messages and receiver messages. NORM_CMD, NORM_INFO, and
NORM_DATA message types are generated by senders of data content, and
NORM_NACK and NORM_ACK messages generated by receivers within a
NormSession. Sender messages SHALL be governed by congestion control
for Internet use. For session management or other purposes,
receivers can also employ NORM_CMD message transmissions. The
principal rationale for distinguishing sender and receiver messages
is that receivers will typically need to allocate resources to
support reliable reception from sender(s) and NORM sender messages
are subject to congestion control. NORM receivers MAY employ the
NORM_CMD message type for application-defined purposes, but it is
RECOMMENDED that congestion control and feedback implosion issues be
addressed. Additionally, an auxiliary message type of NORM_REPORT is
also provided for experimental purposes. This section describes the
message formats used by the NORM protocol. These messages and their
fields are referenced in the detailed functional description of the
NORM protocol given in Section 5. Individual NORM messages are
compatible with the Maximum Transmission Unit (MTU) limitations of
encapsulating Internet protocols including IPv4, IPv6, and UDP. The
current NORM protocol specification assumes UDP encapsulation and
leverages the transport features of UDP. The NORM messages are
independent of network addresses and can be used in IPv4 and IPv6
networks.
4.1. NORM Common Message Header and Extensions
There are some common message fields contained in all NORM message
types. Additionally, a header extension mechanism is defined to
expand the functionality of the NORM protocol without revision to
this document. All NORM protocol messages begin with a common header
with information fields as follows:
Adamson, et al. Standards Track PAGE 15
RFC 5740 NORM Protocol November 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type | hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: NORM Common Message Header Format
The "version" field is a 4-bit value indicating the protocol version
number. NORM implementations SHOULD ignore received messages with
version numbers different from their own. This number is intended to
indicate and distinguish upgrades of the protocol that are non-
interoperable. The NORM version number for this specification is 1.
The message "type" field is a 4-bit value indicating the NORM
protocol message type. These types are defined as follows:
+------------------+------------------+
| Message | Value |
+------------------+------------------+
| NORM_INFO | 1 |
| NORM_DATA | 2 |
| NORM_CMD | 3 |
| NORM_NACK | 4 |
| NORM_ACK | 5 |
| NORM_REPORT | 6 |
+------------------+------------------+
The 8-bit "hdr_len" field indicates the number of 32-bit words that
comprise the given message's header portion. This is used to
facilitate the addition of header extensions. The presence of header
extensions is implied when the "hdr_len" value is greater than the
base value for the given message "type".
The "sequence" field is a 16-bit value that is set by the message
originator. The "sequence" field serves two separate purposes,
depending upon the message type:
1. NORM senders MUST set the "sequence" field of sender messages
(NORM_INFO, NORM_DATA, and NORM_CMD) so that receivers can
monitor the "sequence" value to maintain an estimate of packet
loss that can be used for congestion control purposes (see
Section 5.5.2 for a detailed description of NORM Congestion
Control operation). A monotonically increasing sequence number
space MUST be maintained to mark NORM sender messages in this
way. Note that this "sequence" number is explicitly NOT used in
Adamson, et al. Standards Track PAGE 16
RFC 5740 NORM Protocol November 2009
NORM as part of its reliability procedures. The NORM object and
FEC payload identifiers are used to detect missing content for
reliable transfer purposes.
2. NORM receivers SHOULD set the "sequence" field to support
protection from message replay attacks of NORM_NACK or NORM_NACK
messages. Note that, depending upon configuration, NORM feedback
messages are sent to the session multicast address or the unicast
address(es) of the active NORM sender(s). Thus, a separate,
monotonically increasing sequence number space MUST be maintained
for each destination address to which the NORM receiver is
transmitting feedback messages.
Note that these two separate purposes necessitate the maintenance of
separate sequence spaces to support the functions described here.
And, in the case of NORM receivers, additional sequence spaces are
needed when feedback messages are sent to the sender unicast
address(es) instead of the session address.
The "source_id" field is a 32-bit value that uniquely identifies the
node that sent the message within the context of a single
NormSession. This value is termed the NORM node identifier
(NormNodeId) and unique NormNodeIds MUST be assigned within a single
NormSession. In some cases, use of the host IPv4 address or a hash
of an address can suffice, but alternative methodologies for
assignment and potential collision resolution of node identifiers
within a multicast session SHOULD be considered. For example, the
techniques for managing the 32-bit "synchronization source" (SSRC)
identifiers defined in the Real-Time Protocol (RTP) specification
[RFC 3550] are applicable for use with NORM node identifiers when an
ASM traffic model is observed. In most deployments of the NORM
protocol to date, the NormNodeId assignments are administratively
configured, and this form of NormNodeId assignment is RECOMMENDED for
most purposes. NORM sender NormNodeId values MUST be unique within
an ASM session so that NORM receiver feedback can be properly
demultiplexed by senders, and NORM receiver NormNodeId values MUST
also be unique for congestion control operation or when the OPTIONAL
positive acknowledgment mechanism is used.
NORM Header Extensions
When header extensions are applied, they follow the message type's
base header and precede any payload portion. There are two formats
for header extensions, both of which begin with an 8-bit "het"
(header extension type) field. One format is provided for variable-
length extensions with "het" values in the range from 0 through 127.
The other format is for fixed-length (one 32-bit word) extensions
with "het" values in the range from 128 through 255.
Adamson, et al. Standards Track PAGE 17
RFC 5740 NORM Protocol November 2009
For variable-length extensions, the value of the "hel" (header
extension length) field is the length of the entire header extension,
expressed in multiples of 32-bit words. The "hel" field MUST be
present for variable-length extensions ("het" between 0 and 127) and
MUST NOT be present for fixed-length extensions ("het" between 128
and 255).
The formats of the variable-length and fixed-length header extensions
are given, respectively, here:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het <=127 | hel | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Header Extension Content |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: NORM Variable-Length Header Extension Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het >=128 | reserved | Header Extension Content |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: NORM Fixed-Length (32-bit) Header Extension Format
The "Header Extension Content" portion of the header extension is
defined for each extension type. Some header extensions are defined
within this document for NORM baseline FEC and congestion control
operations.
4.2. Sender Messages
NORM sender messages include the NORM_DATA type, the NORM_INFO type,
and the NORM_CMD type. NORM_DATA and NORM_INFO messages contain
application data content while NORM_CMD messages are used for various
protocol control functions.
4.2.1. NORM_DATA Message
The NORM_DATA message is generally the predominant type transmitted
by NORM senders. These messages are used to encapsulate segmented
data content for objects of type NORM_OBJECT_DATA, NORM_OBJECT_FILE,
and NORM_OBJECT_STREAM. NORM_DATA messages contain original or FEC-
encoded application data content.
Adamson, et al. Standards Track PAGE 18
RFC 5740 NORM Protocol November 2009
The format of NORM_DATA messages is comprised of three logical
portions: 1) a fixed-format NORM_DATA header portion, 2) a FEC
Payload ID portion with a format dependent upon the FEC encoding
used, and 3) a payload portion containing source or encoded
application data content. Note for objects of type
NORM_OBJECT_STREAM, the payload portion contains additional fields
used to appropriately recover stream content. NORM implementations
MAY also extend the NORM_DATA header to include a FEC Object
Transmission Information (EXT_FTI) header extension. This allows
NORM receivers to automatically allocate resources and properly
perform FEC decoding without the need for pre-configuration or out-
of-band information.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=2| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header_extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_len* | payload_msg_start* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_offset* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data* |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: NORM_DATA Message Format
*IMPORTANT NOTE: The "payload_len", "payload_msg_start" and
"payload_offset" fields are present only for objects of type
NORM_OBJECT_STREAM. These fields, as with the entire payload, are
subject to any FEC encoding used. Thus, when systematic FEC codes
are used, these values can be directly interpreted only for packets
containing source symbols while packets containing FEC parity content
need decoding before these fields can be interpreted.
The "version", "type", "hdr_len", "sequence", and "source_id" fields
Adamson, et al. Standards Track PAGE 19
RFC 5740 NORM Protocol November 2009
form the NORM common message header as described in Section 4.1. The
value of the NORM_DATA "type" field is 2. The NORM_DATA base
"hdr_len" value is 4 (i.e., four 32-bit words) plus the size of the
"fec_payload_id" field. The "fec_payload_id" field size depends upon
the FEC encoding type referenced by the "fec_id" field. For example,
when small block, systematic codes are used, a "fec_id" value of 129
is indicated, and the size of the "fec_payload_id" is two 32-bit
words. In this case the NORM_DATA base "hdr_len" value is 6. The
cumulative size of any header extensions applied is added into the
"hdr_len" field.
The "instance_id" field contains a value generated by the sender to
uniquely identify its current instance of participation in the
NormSession. This allows receivers to detect when senders have
perhaps left and rejoined a session in progress. When a sender
(identified by its "source_id") is detected to have a new
"instance_id", the NORM receivers SHOULD drop their previous state on
the sender and begin reception anew, or at least treat this
"instance" as a new, separate sender.
The "grtt" field contains a non-linear quantized representation of
the sender's current estimate of group round-trip time (GRTT_sender)
(this is also referred to as R_max in [TfmccPaper]). This value is
used to control timing of the NACK repair process and other aspects
of protocol operation as described in this document. Normally, the
advertised "grtt" value will correspond to what the sender has
measured based on feedback from the group, but, at low transmission
rates, the advertised "grtt" SHALL be set to MAX(grttMeasured,
NormSegmentSize/senderRate) where the NormSegmentSize is the sender's
segment size in bytes and the senderRate is the sender's current
transmission rate in bytes per second. The algorithm for encoding
and decoding this field is described in the Multicast NACK Building
Block [RFC 5401] document.
The "backoff" field value is used by receivers to determine the
maximum backoff timer value used in the timer-based NORM NACK
feedback suppression. This 4-bit field supports values from 0-15
that are multiplied by GRTT_sender to determine the maximum backoff
timeout. The "backoff" field informs the receivers of the sender's
backoff factor parameter (K_sender). Recommended values and their
uses are described in the NORM receiver NACK procedure description in
Section 5.3.
The "gsize" field contains a representation of the sender's current
estimate of group size (GSIZE_sender). This 4-bit field can roughly
represent values from ten to 500 million where the most significant
bit value of 0 or 1 represents a mantissa of 1 or 5, respectively,
and the three least significant bits incremented by one represent a
Adamson, et al. Standards Track PAGE 20
RFC 5740 NORM Protocol November 2009
base-10 exponent (order of magnitude). For example, a field value of
"0x0" represents 1.0e+01 (10), a value of "0x8" represents 5.0e+01
(50), a value of "0x1" represents 1.0e+02 (100), and a value of "0xf"
represents 5.0e+08. For NORM feedback suppression purposes, the
group size does not need to be represented with a high degree of
precision. The group size MAY even be estimated somewhat
conservatively (i.e., overestimated) to maintain low levels of
feedback traffic. A default group size estimate of 10,000 ("gsize" =
0x3) is RECOMMENDED for general purpose reliable multicast
applications using the NORM protocol.
The "flags" field contains a number of different binary flags
providing information and hints for the receiver to appropriately
handle the identified object. Defined flags in this field include:
+----------------------+-------+------------------------------------+
| Flag | Value | Purpose |
+----------------------+-------+------------------------------------+
| NORM_FLAG_REPAIR | 0x01 | Indicates message is a repair |
| | | transmission |
| NORM_FLAG_EXPLICIT | 0x02 | Indicates a repair segment |
| | | intended to meet a specific |
| | | receiver erasure, as compared to |
| | | parity segments provided by the |
| | | sender for general purpose (with |
| | | respect to a FEC coding block) |
| | | erasure filling. |
| NORM_FLAG_INFO | 0x04 | Indicates availability of |
| | | NORM_INFO for object. |
| NORM_FLAG_UNRELIABLE | 0x08 | Indicates that repair |
| | | transmissions for the specified |
| | | object will be unavailable |
| | | (one-shot, best-effort |
| | | transmission). |
| NORM_FLAG_FILE | 0x10 | Indicates object is file-based |
| | | data (hint to use disk storage for |
| | | reception). |
| NORM_FLAG_STREAM | 0x20 | Indicates object is of type |
| | | NORM_OBJECT_STREAM. |
+----------------------+-------+------------------------------------+
NORM_FLAG_REPAIR is set when the associated message is a repair
transmission. This information can be used by receivers to help
observe a join policy where it is desired that newly joining
receivers only begin participating in the NACK process upon receipt
of new (non-repair) data content. NORM_FLAG_EXPLICIT is used to mark
repair messages sent when the data sender has exhausted its ability
to provide "fresh" (not previously transmitted) parity segments as
Adamson, et al. Standards Track PAGE 21
RFC 5740 NORM Protocol November 2009
repair. This flag could possibly be used by intermediate systems
implementing functionality to control sub-casting of repair content
to different legs of a reliable multicast topology with disparate
repair needs. NORM_FLAG_INFO is set only when OPTIONAL NORM_INFO
content is actually available for the associated object. Thus,
receivers will NACK for retransmission of NORM_INFO only when it is
available for a given object. NORM_FLAG_UNRELIABLE is set when the
sender wishes to transmit an object with only "best effort" delivery
and will not supply repair transmissions for the object. NORM
receivers SHOULD NOT execute repair requests for objects marked with
the NORM_FLAG_UNRELIABLE flag. There are cases where receivers can
inadvertently request repair of such objects when all segments (or
info content) for those objects are not received (i.e., a gap in the
"object_transport_id" sequence is noted). In this case, the sender
SHALL invoke the NORM_CMD(SQUELCH) process as described in
Section 4.2.3.
NORM_FLAG_FILE can be set as a hint from the sender that the
associated object SHOULD be stored in non-volatile storage.
NORM_FLAG_STREAM is set when the identified object is of type
NORM_OBJECT_STREAM. The presence of NORM_FLAG_STREAM overrides that
of NORM_FLAG_FILE with respect to interpretation of object size and
the format of NORM_DATA messages.
The "fec_id" field corresponds to the FEC Encoding Identifier
described in the FEC Building Block document [RFC 5052]. The "fec_id"
value implies the format of the "fec_payload_id" field and, coupled
with FEC Object Transmission Information, the procedures to decode
FEC-encoded content. Small block, systematic codes ("fec_id" = 129)
are expected to be used for most NORM purposes and systematic FEC
codes are RECOMMENDED for the most efficient performance of
NORM_OBJECT_STREAM transport.
The "object_transport_id" field is a monotonically and incrementally
increasing value assigned by the sender to NormObjects being
transmitted. Transmissions and repair requests related to that
object use the same "object_transport_id" value. For sessions of
very long or indefinite duration, the "object_transport_id" field
will wrap and be repeated, but it is presumed that the 16-bit field
size provides a sufficient sequence space to avoid object confusion
amongst receivers and sources (i.e., receivers SHOULD re-synchronize
with a server when receiving object sequence identifiers sufficiently
out-of-range with the current state kept for a given source). During
the course of its transmission within a NORM session, an object is
uniquely identified by the concatenation of the sender "source_id"
and the given "object_transport_id". Note that NORM_INFO messages
associated with the identified object carry the same
"object_transport_id" value.
Adamson, et al. Standards Track PAGE 22
RFC 5740 NORM Protocol November 2009
The "fec_payload_id" identifies the attached NORM_DATA "payload"
content. The size and format of the "fec_payload_id" field depends
upon the FEC type indicated by the "fec_id" field. These formats are
given in the descriptions of specific FEC schemes such as those
described in the FEC Basic Schemes [RFC 5445] specification or in
other FEC Schemes. As an example, the format of the "fec_payload_id"
format for Small Block, Systematic codes ("fec_id" = 129) from the
FEC Basic Schemes [RFC 5445] specification is given here:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_len | encoding_symbol_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Example: FEC Payload Id Format for 'fec_id' = 129
In this example, FEC payload identifier, the "source_block_number",
"source_block_len", and "encoding_symbol_id" fields correspond to the
"Source Block Number", "Source Block Length", and "Encoding Symbol
ID" fields of the FEC Payload ID format for Small Block Systematic
FEC Schemes identified by a "fec_id" value of 129 as specified by the
FEC Basic Schemes [RFC 5445] specification. The "source_block_number"
identifies the coding block's relative position with a NormObject.
Note that, for NormObjects of type NORM_OBJECT_STREAM, the
"source_block_number" will wrap for very long-lived sessions. The
"source_block_len" indicates the number of user data segments in the
identified coding block. Given the "source_block_len" information of
how many symbols of application data are contained in the block, the
receiver can determine whether the attached segment is data or parity
content and treat it appropriately. Applications MAY dynamically
"shorten" code blocks when the pending information content is not
predictable (e.g., real-time message streams). In that case, the
"source_block_len" value given for an "encoding_symbol_id" that
contains FEC parity content SHALL take precedence over the
"source_block_len" value provided for any packets containing source
symbols. Also, the "source_block_len" value given for an ordinally
higher "encoding_symbol_id" SHALL take precedence over the
"source_block_len" given for prior encoding symbols. The reason for
this is that the sender will only know the maximum source block
length at the time it is transmitting source symbols, but then
subsequently "shorten" the code and then provide that last source
symbol and/or encoding symbols with FEC parity content. The
"encoding_symbol_id" identifies which specific symbol (segment)
within the coding block the attached payload conveys. Depending upon
the value of the "encoding_symbol_id" and the associated
"source_block_len" parameters for the block, the symbol (segment)
Adamson, et al. Standards Track PAGE 23
RFC 5740 NORM Protocol November 2009
referenced will be a user data or a FEC parity segment. For
systematic codes, encoding symbols numbered less than the
source_block_len contain original application data while segments
greater than or equal to source_block_len contain parity symbols
calculated for the block. The concatenation of object_transport_id::
fec_payload_id can be viewed as a unique transport protocol data unit
identifier for the attached segment with respect to the NORM sender's
instance within a session.
Additional FEC Object Transmission Information (FTI) (as described in
the FEC Building Block [RFC 5052]) document is needed to properly
receive and decode NORM transport objects. This information MAY be
provided as out-of-band session information. In some cases, it will
be useful for the sender to include this information "in-band" to
facilitate receiver operation with minimal pre-configuration. For
this purpose, the NORM FEC Object Transmission Information Header
Extension (EXT_FTI) is defined. This header extension MAY be applied
to NORM_DATA and NORM_INFO messages to provide this necessary
information. The format of the EXT_FTI consists of two parts, a
general part that contains the size of the associated transport
object and a portion that depends upon the FEC scheme being used.
The "fec_id" field in NORM_DATA and NORM_INFO messages identifies the
FEC scheme. The format of the EXT_FTI general part is given here.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 64 | hel = 4 | object_size (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_size (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC scheme-specific content ... |
Figure 6: EXT_FTI Header Extension General Portion Format
The header extension type "het" field value for the EXT_FTI header
extension is 64. The header extension length "hel" value depends
upon the format of the FTI for encoding type identified by the
"fec_id" field.
The 48-bit "object_size" field indicates the total length of the
object (in bytes) for the static object types of NORM_OBJECT_FILE and
NORM_OBJECT_DATA. This information is used by receivers to determine
storage requirements and/or allocate storage for the received object.
Receivers with insufficient storage capability might wish to forego
reliable reception (i.e., not NACK for) of the indicated object. In
the case of objects of type NORM_OBJECT_STREAM, the "object_size"
field is used by the sender to advertise the size of its stream
Adamson, et al. Standards Track PAGE 24
RFC 5740 NORM Protocol November 2009
buffer to the receiver group. In turn, the receivers SHOULD use this
information to allocate a stream buffer for reception of
corresponding size.
As noted, the format of the extension depends upon the FEC code in
use, but in general, it contains any necessary details on the code in
use (e.g., FEC Instance ID, etc.). As an example, the format of the
EXT_FTI for small block systematic codes ("fec_id" = 129) is given
here:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 64 | hel = 4 | object_size (msb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| object_size (lsb) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_instance_id | segment_size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_max_block_len | fec_num_parity |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Example: EXT_FTI Header Extension Format for 'fec_id' = 129
In this example (for "fec_id" = 129), the "hel" field value is 4.
The size of the EXT_FTI header extension will possibly be different
for other FEC schemes.
The 48-bit "object_size" serves the purpose described previously.
The "fec_instance_id" corresponds to the "FEC Instance ID" described
in the FEC Building Block [RFC 5052] document. In this case, the
"fec_instance_id" is a value corresponding to the particular type of
Small Block Systematic Code being used (e.g., Reed-Solomon GF(2^8),
Reed-Solomon GF(2^16), etc). The standardized assignment of FEC
Instance ID values is described in RFC 5052.
The "segment_size" field indicates the sender's current setting for
maximum message payload content (in bytes). This allows receivers to
allocate appropriate buffering resources and to determine other
information in order to properly process received data messaging.
Typically, FEC parity symbol segments will be of this size.
The "fec_max_block_len" indicates the current maximum number of user
data segments per FEC coding block to be used by the sender during
the session. This allows receivers to allocate appropriate buffer
space for buffering blocks transmitted by the sender.
The "fec_num_parity" corresponds to the "maximum number of encoding
Adamson, et al. Standards Track PAGE 25
RFC 5740 NORM Protocol November 2009
symbols that can be generated for any source block" as described in
FEC Object Transmission Information for Small Block Systematic Codes
as described in the FEC Building Block [RFC 5052] document. For
example, Reed-Solomon codes can be arbitrarily shortened to create
different code variations for a given block length. In the case of
Reed-Solomon (GF(2^8) and GF(2^16)) codes, this value indicates the
maximum number of parity segments available from the sender for the
coding blocks. This field MAY be interpreted differently for other
systematic codes as they are defined.
The payload portion of NORM_DATA messages includes source data or
FEC-encoded application content. The content of this payload depends
upon the FEC scheme being employed, and support for streaming using
the NORM_OBJECT_STREAM type, when applicable, necessitates some
additional content in the payload.
The "payload_len", "payload_msg_start", and "payload_offset" fields
are present only for transport objects of type NORM_OBJECT_STREAM.
These REQUIRED fields allow senders to arbitrarily vary the size of
NORM_DATA payload segments for streams. This allows applications to
flush transmitted streams as needed to meet unique streaming
requirements. For objects of types NORM_OBJECT_FILE and
NORM_OBJECT_DATA, these fields are unnecessary since the receiver can
calculate the payload length and offset information from the
"fec_payload_id" using the REQUIRED block partitioning algorithm
described in the FEC Building Block [RFC 5052] document. When
systematic FEC codes (e.g., "fec_id" = 129) are used, the
"payload_len", "payload_msg_start", and "payload_offset" fields
contain actual payload_data length, message start index (or stream
control code), and byte offset values for the associated application
stream data segment (the remainder of the "payload_data" field
content) for those NORM_DATA messages containing source data symbols.
In NORM_DATA messages that contain FEC parity content, these fields
do not contain values that can be directly interpreted, but instead
are values computed from FEC encoding the "payload_len",
"payload_msg_start", and "payload_offset" fields for the source data
segments of the corresponding coding block. The actual
"payload_msg_start", "payload_len" and, "payload_offset" values of
missing data content can be determined upon decoding a FEC coding
block. Note that these fields do NOT contribute to the value of the
NORM_DATA "hdr_len" field. These fields are present only when the
"flags" portion of the NORM_DATA message indicate the transport
object is of type NORM_OBJECT_STREAM.
The "payload_len" value, when non-zero, indicates the length (in
bytes) of the source content contained in the associated
"payload_data" field. However, when the "payload_len" value is equal
to ZERO, this indicates that the "payload_msg_start" field be
Adamson, et al. Standards Track PAGE 26
RFC 5740 NORM Protocol November 2009
alternatively interpreted as a "stream_control_code". The only
"stream_control_code" value defined is NORM_STREAM_END = 0. The
NORM_STREAM_END code indicates that the sender is terminating the
transmission of stream content at the corresponding position in the
stream and the receiver MUST NOT expect content (or request repair
for any content) following that position in the stream. Additional
specifications MAY extend the functionality of the NORM stream
transport mode by defining additional stream control codes. These
control codes are delivered to the recipient application reliably,
in-order with respect to the streamed application data content.
The "payload_msg_start" field serves one of two exclusive purposes.
When the "payload_len" value is non-zero, the "payload_msg_start"
field, when also set to a non-zero value, indicates that the
associated "payload_data" content contains an application-defined
message boundary (start-of-message). When such a message boundary is
indicated, the first byte of an application-defined message, with
respect to the "payload_data" field, will be found at an offset of
"payload_msg_start - 1" bytes. Thus, if a NORM_DATA payload for a
NORM_OBJECT_STREAM contains the start of an application message at
the first byte of the "payload_data" field, the value of the
"payload_msg_start" field will be '1'. NORM implementations SHOULD
provide sender stream applications with a capability to mark message
boundaries in this manner. Similarly, the NORM receiver
implementation SHOULD enable the application to recover such message
boundary information. This enables NORM receivers to "synchronize"
reliable reception of transmitted message stream content in a
meaningful way (i.e., meaningful to the application) at any time,
whether joining a session already in progress, or departing the
session and returning. Note that if the value of the
"payload_msg_start" field is ZERO, no message boundary is present.
The "payload_msg_start" value will always be less than or equal to
the "payload_len" value except for the special case of "payload_len =
0", which indicates the "payload_msg_start" field be instead
interpreted as a "stream_control_code"
The "payload_offset" field indicates the relative byte position (from
the sender stream transmission start) of the source content contained
in the "payload_data" field. Note that for long-lived streams, the
"payload_offset" field will wrap.
The "payload_data" field contains the original application source or
parity content for the symbol identified by the "fec_payload_id".
The length of this field SHALL be limited to a maximum of the
sender's NormSegmentSize bytes as given in the FTI for the object.
Note the length of this field for messages containing parity content
will always be of length NormSegmentSize. When encoding data
segments of varying sizes, the FEC encoder SHALL assume ZERO value
Adamson, et al. Standards Track PAGE 27
RFC 5740 NORM Protocol November 2009
padding for data segments with a length less than the
NormSegmentSize. It is RECOMMENDED that a sender's NormSegmentSize
generally be constant for the duration of a given sender's term of
participation in the session, but can possibly vary on a per-object
basis. The NormSegmentSize SHOULD be configurable by the sender
application prior to session participation as needed for network
topology MTU considerations. For IPv6, MTU discovery MAY be possibly
leveraged at session startup to perform this configuration. The
"payload_data" content MAY be delivered directly to the application
for source symbols (when systematic FEC encoding is used) or upon
decoding of the FEC block. For NORM_OBJECT_FILE and
NORM_OBJECT_STREAM objects, the data segment length and offset can be
calculated using the block partitioning algorithm described in the
FEC Building Block [RFC 5052] document. For NORM_OBJECT_STREAM
objects, the length and offset is obtained from the segment's
corresponding embedded "payload_len" and "payload_offset" fields.
4.2.2. NORM_INFO Message
The NORM_INFO message is used to convey OPTIONAL, application-
defined, out-of-band context information for transmitted NormObjects.
An example NORM_INFO use for bulk file transfer is to place MIME type
information for the associated file, data, or stream object into the
NORM_INFO payload. Receivers could then use the NORM_INFO content to
make a decision as to whether to participate in reliable reception of
the associated object. Each NormObject can have an independent unit
of NORM_INFO with which it is associated. NORM_DATA messages contain
a flag to indicate the availability of NORM_INFO for a given
NormObject. NORM receivers will NACK for retransmission of NORM_INFO
when they have not received it for a given NormObject. The size of
the NORM_INFO content is limited to that of a single NormSegmentSize
for the given sender. This atomic nature allows the NORM_INFO to be
rapidly and efficiently repaired within the NORM reliable
transmission process.
When NORM_INFO content is available for a NormObject, the
NORM_FLAG_INFO flag SHALL be set in NORM_DATA messages for the
corresponding "object_transport_id" and the NORM_INFO message SHALL
be transmitted as the first message for the NormObject.
Adamson, et al. Standards Track PAGE 28
RFC 5740 NORM Protocol November 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=1| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header_extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| payload_data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: NORM_INFO Message Format
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM common message header as described in Section 4.1. The
value of the "hdr_len" field when no header extensions are present is
4.
The "instance_id", "grtt", "backoff", "gsize", "flags", "fec_id", and
"object_transport_id" fields carry the same information and serve the
same purpose as NORM_DATA messages. These values allow the receiver
to prepare appropriate buffering, etc., for further transmissions
from the sender when NORM_INFO is the first message received.
As with NORM_DATA messages, the NORM FTI Header Extension (EXT_FTI)
MAY be optionally applied to NORM_INFO messages. To conserve
protocol overhead, NORM implementations MAY apply the EXT_FTI when
used to NORM_INFO messages only and not to NORM_DATA messages.
The NORM_INFO "payload_data" field contains sender application-
defined content that can be used by receiver applications for various
purposes as described above.
4.2.3. NORM_CMD Messages
NORM_CMD messages are transmitted by senders to perform a number of
different protocol functions. This includes functions such as round-
trip timing collection, congestion control functions, synchronization
of sender/receiver repair "windows", and notification of sender
status. A core set of NORM_CMD messages is enumerated.
Additionally, a range of command types remain available for potential
Adamson, et al. Standards Track PAGE 29
RFC 5740 NORM Protocol November 2009
application-specific use. Some NORM_CMD types can have dynamic
content attached. Any attached content will be limited to the
maximum length of the sender NormSegmentSize to retain the atomic
nature of the commands. All NORM_CMD messages begin with a common
set of fields, after the usual NORM message common header. The
standard NORM_CMD fields are:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-type | |
+-+-+-+-+-+-+-+-+ NORM_CMD Content +
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: NORM_CMD Standard Fields
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM common message header as described in Section 4.1. The
value of the "hdr_len" field for NORM_CMD messages without header
extensions present depends upon the "sub-type" field.
The "instance_id", "grtt", "backoff", and "gsize" fields provide the
same information and serve the same purpose as NORM_DATA and
NORM_INFO messages. The "sub-type" field indicates the type of
command to follow. The remainder of the NORM_CMD message is
dependent upon the command sub-type. NORM command sub-types include:
+-----------------------+----------+--------------------------------+
| Command | Sub-type | Purpose |
+-----------------------+----------+--------------------------------+
| NORM_CMD(FLUSH) | 1 | Used to indicate sender |
| | | temporary end-of-transmission. |
| | | (Assists in robustly |
| | | initiating outstanding repair |
| | | requests from receivers). May |
| | | also be optionally used to |
| | | collect positive |
| | | acknowledgment of reliable |
| | | reception from a subset of |
| | | receivers. |
| NORM_CMD(EOT) | 2 | Used to indicate sender |
| | | permanent end-of-transmission. |
Adamson, et al. Standards Track PAGE 30
RFC 5740 NORM Protocol November 2009
| NORM_CMD(SQUELCH) | 3 | Used to advertise sender's |
| | | current repair window in |
| | | response to out-of-range NACKs |
| | | from receivers. |
| NORM_CMD(CC) | 4 | Used for GRTT measurement and |
| | | collection of congestion |
| | | control feedback. |
| NORM_CMD(REPAIR_ADV) | 5 | Used to advertise sender's |
| | | aggregated repair/feedback |
| | | state for suppression of |
| | | unicast feedback from |
| | | receivers. |
| NORM_CMD(ACK_REQ) | 6 | Used to request |
| | | application-defined positive |
| | | acknowledgment from a list of |
| | | receivers (OPTIONAL). |
| NORM_CMD(APPLICATION) | 7 | Used for application-defined |
| | | purposes that need to |
| | | temporarily preempt or |
| | | supplement data transmission |
| | | (OPTIONAL). |
+-----------------------+----------+--------------------------------+
4.2.3.1. NORM_CMD(FLUSH) Message
The NORM_CMD(FLUSH) command is sent when the sender reaches the end
of all data content and pending repairs it has queued for
transmission. This can indicate either a temporary or permanent end-
of-data transmission, but that the sender is still willing to respond
to repair requests. This command is repeated once per 2*GRTT_sender
to excite the receiver set for any outstanding repair requests up to
and including the transmission point indicated within the
NORM_CMD(FLUSH) message. The number of repeats is equal to
NORM_ROBUST_FACTOR unless a list of receivers from which explicit
positive acknowledgment is expected ("acking_node_list") is given.
In that case, the "acking_node_list" is updated as acknowledgments
are received and the NORM_CMD(FLUSH) is repeated according to the
mechanism described in Section 5.5.3. The greater the
NORM_ROBUST_FACTOR, the greater the probability that all applicable
receivers will be excited for acknowledgment or repair requests
(NACKs) AND that the corresponding NACKs are delivered to the sender.
A default value of NORM_ROBUST_FACTOR equal to 20 is RECOMMENDED. If
a NORM_NACK message interrupts the flush process, the sender SHALL
re-initiate the flush process after any resulting repair
transmissions are completed.
Note that receivers also employ a timeout mechanism to self-initiate
NACKing (if there are outstanding repair needs) when no messages of
Adamson, et al. Standards Track PAGE 31
RFC 5740 NORM Protocol November 2009
any type are received from a sender. This inactivity timeout is
related to the NORM_CMD(FLUSH) and NORM_ROBUST_FACTOR and is
specified in Section 5.3. Receivers SHALL self-initiate the NACK
repair process when the inactivity timeout has expired for a specific
sender and the receiver has pending repairs needed from that sender.
With a sufficiently large NORM_ROBUST_FACTOR value, data content is
delivered with a high assurance of reliability. The penalty of a
large NORM_ROBUST_FACTOR value is the potential transmission of
excess NORM_CMD(FLUSH) messages and a longer inactivity timeout for
receivers to self-initiate a terminal NACK process.
For finite-sized transport objects such as NORM_OBJECT_DATA and
NORM_OBJECT_FILE, the flush process (if there are no further pending
objects) occurs at the end of these objects. Thus, FEC repair
information is always available for repairs in response to repair
requests elicited by the flush command. However, for
NORM_OBJECT_STREAM, the flush can occur at any time, including in the
middle of a FEC coding block if systematic FEC codes are employed.
In this case, the sender will not yet be able to provide FEC parity
content for the concurrent coding block and will be limited to
explicitly repairing the stream with source data content for that
block. Applications that anticipate frequent flushing of stream
content SHOULD be judicious in the selection of the FEC coding block
size (i.e., do not use a very large coding block size if frequent
flushing occurs). For example, a reliable multicast application
transmitting an ongoing series of intermittent, relatively small
messages will need to trade-off using the NORM_OBJECT_DATA paradigm
versus the NORM_OBJECT_STREAM paradigm with an appropriate FEC coding
block size. This is analogous to application trade-offs for other
transport protocols such as the selection of different TCP modes of
operation such as "no delay", etc.
Adamson, et al. Standards Track PAGE 32
RFC 5740 NORM Protocol November 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-type = 1 | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: NORM_CMD(FLUSH) Message Format
The "version", "type", "hdr_len", "sequence", and "source_id" fields
form the NORM common message header as described in Section 4.1. In
addition to the NORM common message header and standard NORM_CMD
fields, the NORM_CMD(FLUSH) message contains fields to identify the
current status and logical transmit position of the sender.
The "fec_id" field indicates the FEC type used for the flushing
"object_transport_id" and implies the size and format of the
"fec_payload_id" field. Note the "hdr_len" value for the
NORM_CMD(FLUSH) message is 4 plus the size of the "fec_payload_id"
field when no header extensions are present.
The "object_transport_id" and "fec_payload_id" fields indicate the
sender's current logical "transmit position". These fields are
interpreted in the same manner as in the NORM_DATA message type.
Upon receipt of the NORM_CMD(FLUSH), receivers are expected to check
their completion state THROUGH (including) this transmission
position. If receivers have outstanding repair needs in this range,
they SHALL initiate the NORM NACK Repair Process as described in
Section 5.3. If receivers have no outstanding repair needs, no
response to the NORM_CMD(FLUSH) is generated.
For NORM_OBJECT_STREAM objects using systematic FEC codes, receivers
MUST request "explicit-only" repair of the identified
"source_block_number" if the given "encoding_symbol_id" is less than
the "source_block_len". This condition indicates the sender has not
yet completed encoding the corresponding FEC block and parity content
is not yet available. An "explicit-only" repair request consists of
Adamson, et al. Standards Track PAGE 33
RFC 5740 NORM Protocol November 2009
NACK content for the applicable "source_block_number" that does not
include any requests for parity-based repair. This allows NORM
sender applications to "flush" an ongoing stream of transmission when
needed, even if in the middle of a FEC block. Once the sender
resumes stream transmission and passes the end of the pending coding
block, subsequent NACKs from receivers SHALL request parity-based
repair as usual. Note that the use of a systematic FEC code is
assumed here. Note that a sender has the option of arbitrarily
shortening a given code block when such an application "flush"
occurs. In this case, the receiver will request explicit repair, but
the sender MAY provide FEC-based repair (parity segments) in
response. These parity segments MUST contain the corrected
"source_block_len" for the shortened block and that
"source_block_len" associated with segments containing parity content
SHALL override the previously advertised "source_block_len".
Similarly, the "source_block_len" associated with the highest ordinal
"encoding_symbol_id" SHALL take precedence over prior symbols when a
difference (e.g., due to code shortening at the sender) occurs.
Normal receiver NACK initiation and construction is discussed in
detail in Section 5.3.
The OPTIONAL "acking_node_list" field contains a list of NormNodeIds
for receivers from which the sender is requesting explicit positive
acknowledgment of reception up through the transmission point
identified by the "object_transport_id" and "fec_payload_id" fields.
The length of the list can be inferred from the length of the
received NORM_CMD(FLUSH) message. When the "acking_node_list" is
present, the lightweight positive acknowledgment process described in
Section 5.5.3 SHALL be observed.
4.2.3.2. NORM_CMD(EOT) Message
The NORM_CMD(EOT) command is sent when the sender reaches permanent
end-of-transmission with respect to the NormSession and will not
respond to further repair requests. This allows receivers to
gracefully reach closure of operation with this sender (without
requiring any timeout) and free any resources that are no longer
needed. The NORM_CMD(EOT) command SHOULD be sent with the same
robust mechanism as used for NORM_CMD(FLUSH) commands to provide a
high assurance of reception by the receiver set.
Adamson, et al. Standards Track PAGE 34
RFC 5740 NORM Protocol November 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-type = 2 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: NORM_CMD(EOT) Message Format
The value of the "hdr_len" field for NORM_CMD(EOT) messages without
header extensions present is 4. The "reserved" field is reserved for
future use and MUST be set to an all ZERO value. Receivers MUST
ignore the "reserved" field.
4.2.3.3. NORM_CMD(SQUELCH) Message
The NORM_CMD(SQUELCH) command is transmitted in response to outdated
or invalid NORM_NACK content received by the sender. Invalid
NORM_NACK content consists of repair requests for NormObjects for
which the sender is unable or unwilling to provide repair. This
includes repair requests for outdated objects, aborted objects, or
those objects that the sender previously transmitted marked with the
NORM_FLAG_UNRELIABLE flag. This command indicates to receivers what
content is available for repair, thus serving as a description of the
sender's current "repair window". Receivers SHALL NOT generate
repair requests for content identified as invalid by a
NORM_CMD(SQUELCH).
The NORM_CMD(SQUELCH) command is sent once per 2*GRTT_sender at the
most. The NORM_CMD(SQUELCH) advertises the current "repair window"
of the sender by identifying the earliest (lowest) transmission point
for which it will provide repair, along with an encoded list of
objects from that point forward that are no longer valid for repair.
This mechanism allows the sender application to cancel or abort
transmission and/or repair of specific previously enqueued objects.
The list also contains the identifiers for any objects within the
repair window that were sent with the NORM_FLAG_UNRELIABLE flag set.
In normal conditions, the NORM_CMD(SQUELCH) will be needed
infrequently, and generally only to provide a reference repair window
for receivers who have fallen "out-of-sync" with the sender due to
extremely poor network conditions.
The starting point of the invalid NormObject list begins with the
Adamson, et al. Standards Track PAGE 35
RFC 5740 NORM Protocol November 2009
lowest invalid NormTransportId greater than the current "repair
window" start from the invalid NACK(s) that prompted the generation
of the squelch. The length of the list is limited by the sender's
NormSegmentSize. This allows the receivers to learn the status of
the sender's applicable object repair window with minimal
transmission of NORM_CMD(SQUELCH) commands. The format of the
NORM_CMD(SQUELCH) message is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-type = 3 | fec_id | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| invalid_object_list |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: NORM_CMD(SQUELCH) Message Format
In addition to the NORM common message header and standard NORM_CMD
fields, the NORM_CMD(SQUELCH) message contains fields to identify the
earliest logical transmit position of the sender's current repair
window and an "invalid_object_list" beginning with the index of the
logically earliest invalid repair request from the offending NACK
message that initiated the NORM_CMD(SQUELCH) transmission. The value
of the "hdr_len" field when no extensions are present is 4 plus the
size of the "fec_payload_id" field that is dependent upon the FEC
scheme identified by the "fec_id" field.
The "object_transport_id" and "fec_payload_id" fields are
concatenated to indicate the beginning of the sender's current repair
window (i.e., the logically earliest point in its transmission
history for which the sender can provide repair). The "fec_id" field
implies the size and format of the "fec_payload_id" field. This
serves as an advertisement of a "synchronization" point for receivers
to request repair. Note, that while an "encoding_symbol_id" MAY be
included in the "fec_payload_id" field, the sender's repair window
SHOULD be aligned on FEC coding block boundaries and thus the
"encoding_symbol_id" SHOULD be zero.
Adamson, et al. Standards Track PAGE 36
RFC 5740 NORM Protocol November 2009
The "invalid_object_list" is a list of 16-bit NormTransportIds that,
although they are within the range of the sender's current repair
window, are no longer available for repair from the sender. For
example, a sender application MAY dequeue an out-of-date object even
though it is still within the repair window. The total size of the
"invalid_object_list" content can be determined from the packet's
payload length and is limited to a maximum of the NormSegmentSize of
the sender. Thus, for very large repair windows, it is possible that
a single NORM_CMD(SQUELCH) message cannot include the entire set of
invalid objects in the repair window. In this case, the sender SHALL
ensure that the list begins with a NormTransportId that is greater
than or equal to the lowest ordinal invalid NormTransportId from the
NACK message(s) that prompted the NORM_CMD(SQUELCH) generation. The
NormTransportId in the "invalid_object_list" MUST be ordinally
greater than the "object_transport_id" marking the beginning of the
sender's repair window. This ensures convergence of the squelch
process, even if multiple invalid NACK/squelch iterations are
required. This explicit description of invalid content within the
sender's current window allows the sender application (most notably
for discrete object transport) to arbitrarily invalidate (i.e.,
dequeue) portions of enqueued content (e.g., certain objects) for
which it no longer wishes to provide reliable transport.
4.2.3.4. NORM_CMD(CC) Message
The NORM_CMD(CC) message contains fields to enable sender-to-group
GRTT measurement and to excite the group for congestion control
feedback. A baseline NORM congestion control scheme (NORM-CC), based
on the TCP-Friendly Multicast Congestion Control (TFMCC) scheme of
RFC 4654 is fully specified in Section 5.5.2 of this document. The
NORM_CMD(CC) message is usually transmitted as part of NORM-CC
operation. A NORM header extension is defined below to be used with
the NORM_CMD(CC) message to support NORM-CC operation. Different
header extensions MAY be defined for the NORM_CMD(CC) (and/or other
NORM messages as needed) to support alternative congestion control
schemes in the future. If NORM is operated in a network where
resources are explicitly dedicated to the NORM session and therefore
congestion control operation is disabled, the NORM_CMD(CC) message is
then used solely for GRTT measurement and MAY be sent less frequently
than with congestion control operation.
Adamson, et al. Standards Track PAGE 37
RFC 5740 NORM Protocol November 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-type = 4 | reserved | cc_sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send_time_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_list (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: NORM_CMD(CC) Message Format
The NORM common message header and standard NORM_CMD fields serve
their usual purposes. The value of the "hdr_len" field when no
header extensions are present is 6.
The "reserved" field is for potential future use and MUST be set to
ZERO in this version of the NORM protocol and its baseline NORM-CC
congestion control scheme. It is possible for alternative congestion
control schemes to use the NORM_CMD(CC) message defined here and
leverage the "reserved" field for scheme-specific purposes.
The "cc_sequence" field is a sequence number applied by the sender.
For NORM-CC operation, it is used to provide functionality equivalent
to the "feedback round number" (fb_nr) described in RFC 4654. The
most recently received "cc_sequence" value is recorded by receivers
and can be fed back to the sender in congestion control feedback
generated by the receivers for that sender. The "cc_sequence" number
can also be used in NORM implementations to assess how recently a
receiver has received NORM_CMD(CC) probes from the sender. This can
be useful instrumentation for complex or experimental multicast
routing environments.
The "send_time" field is a timestamp indicating the time that the
NORM_CMD(CC) message was transmitted. This consists of a 64-bit
field containing 32-bits with the time in seconds ("send_time_sec")
Adamson, et al. Standards Track PAGE 38
RFC 5740 NORM Protocol November 2009
and 32-bits with the time in microseconds ("send_time_usec") since
some reference time the source maintains (usually 00:00:00, 1 January
1970). The byte ordering of the fields is "Big Endian" network
order. Receivers use this timestamp adjusted by the amount of delay
from the time they received the NORM_CMD(CC) message to the time of
their response as the "grtt_response" portion of NORM_ACK and
NORM_NACK messages generated. This allows the sender to evaluate
round-trip times to different receivers for congestion control and
other (e.g., GRTT determination) purposes.
To facilitate the baseline NORM-CC scheme described in Section 5.5.2,
a NORM-CC Rate header extension (EXT_RATE) is defined to inform the
group of the sender's current transmission rate. This is used along
with the loss detection "sequence" field of all NORM sender messages
and the NORM_CMD(CC) GRTT collection process to support NORM-CC
congestion control operation. The format of this header extension is
as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 128 | reserved | send_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "send_rate" field indicates the sender's current transmission
rate in bytes per second. The 16-bit "send_rate" field consists of
12 bits of mantissa in the most significant portion and 4 bits of
base 10 integer exponent (E) information in the least significant
portion. The 12-bit mantissa portion of the field is scaled such
that a base 10 mantissa (M) floating point value of 0.0 corresponds
to 0 and a value of 10.0 corresponds to 4096 in the upper 12 bits of
the 16-bit "send_rate" field. Thus:
send_rate = (((int)(M * 4096.0 / 10.0 + 0.5)) << 4) | E;
For example, to represent a transmission rate of 256 kbit/s (3.2e+04
bytes per second), the lower 4 bits of the 16-bit field contain a
value of 0x04 to represent the exponent (E) while the upper 12 bits
contain a value of 0x51f (M) as determined from the equation given
above:
send_rate = (((int)((3.2 * 4096.0 / 10.0) + 0.5)) << 4) | 4;
= (0x51f << 4) | 0x4
= 0x51f4
To decode the "send_rate" field, the following equation can be used:
value = (send_rate >> 4) * (10/4096) * power(10, (send_rate & x000f))
Note the maximum transmission rate that can be represented by this
Adamson, et al. Standards Track PAGE 39
RFC 5740 NORM Protocol November 2009
scheme is approximately 9.99e+15 bytes per second.
When this extension is present, a "cc_node_list" might be attached as
the payload of the NORM_CMD(CC) message. The presence of this header
extension also implies that NORM receivers MUST respond according to
the procedures described in Section 5.5.2.
The "cc_node_list" consists of a list of NormNodeIds and their
associated congestion control status. This includes the current
limiting receiver (CLR) node, any potential limiting receiver (PLR)
nodes that have been identified, and some number of receivers for
which congestion control status is being provided, most notably
including the receivers' current RTT measurement. The maximum length
of the "cc_node_list" provides for at least the CLR and one other
receiver, but can be increased for more timely feedback to the group.
The list length can be inferred from the length of the NORM_CMD(CC)
message.
Each item in the "cc_node_list" is in the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_node_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "cc_node_id" is the NormNodeId of the receiver the item
represents.
The "cc_flags" field contains flags indicating the congestion control
status of the indicated receiver. The following flags are defined:
+--------------------+-------+--------------------------------------+
| Flag | Value | Purpose |
+--------------------+-------+--------------------------------------+
| NORM_FLAG_CC_CLR | 0x01 | Receiver is the current limiting |
| | | receiver (CLR). |
| NORM_FLAG_CC_PLR | 0x02 | Receiver is a potential limiting |
| | | receiver (PLR). |
| NORM_FLAG_CC_RTT | 0x04 | Receiver has measured RTT with |
| | | respect to sender. |
Adamson, et al. Standards Track PAGE 40
RFC 5740 NORM Protocol November 2009
| NORM_FLAG_CC_START | 0x08 | Sender/receiver is in "slow start" |
| | | phase of congestion control |
| | | operation (i.e., the receiver has |
| | | not yet detected any packet loss and |
| | | the "cc_rate" field is the |
| | | receiver's actual measured receive |
| | | rate). |
| NORM_FLAG_CC_LEAVE | 0x10 | Receiver is imminently leaving the |
| | | session and its feedback SHOULD not |
| | | be considered in congestion control |
| | | operation. |
+--------------------+-------+--------------------------------------+
The "cc_rtt" contains a quantized representation of the RTT as
measured by the sender with respect to the indicated receiver. This
field is valid only if the NORM_FLAG_CC_RTT flag is set in the
"cc_flags" field. This one-byte field is a quantized representation
of the RTT using the algorithm described in the Multicast NACK
Building Block [RFC 5401] document.
The "cc_rate" field contains a representation of the receiver's
current calculated (during steady-state congestion control operation)
or twice its measured (during the slow start phase) congestion
control rate. This field is encoded and decoded using the same
technique as described for the NORM_CMD(CC) "send_rate" field.
4.2.3.5. NORM_CMD(REPAIR_ADV) Message
The NORM_CMD(REPAIR_ADV) message is used by the sender to "advertise"
its aggregated repair state from NORM_NACK messages accumulated
during a repair cycle and/or congestion control feedback received.
This message is sent only when the sender has received NORM_NACK
and/or NORM_ACK(CC) (when congestion control is enabled) messages via
unicast transmission instead of multicast. By relaying this
information to the receiver set, suppression of feedback can be
achieved even when receivers are unicasting that feedback instead of
multicasting it among the group [NormFeedback].
Adamson, et al. Standards Track PAGE 41
RFC 5740 NORM Protocol November 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-type = 5 | flags | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| repair_adv_payload |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: NORM_CMD(REPAIR_ADV) Message Format
The "instance_id", "grtt", "backoff", "gsize", and "sub-type" fields
serve the same purpose as in other NORM_CMD messages. The value of
the "hdr_len" field when no extensions are present is 4.
The "flags" field provides information on the NORM_CMD(REPAIR_ADV)
content. There is currently one NORM_CMD(REPAIR_ADV) flag defined:
NORM_REPAIR_ADV_FLAG_LIMIT = 0x01
This flag is set by the sender when it is unable to fit its full
current repair state into a single NormSegmentSize. If this flag is
set, receivers SHALL limit their NACK response to generating NACK
content only up through the maximum ordinal transmission position
(objectTransportId::fecPayloadId) included in the
"repair_adv_content".
When congestion control operation is enabled, a header extension
SHOULD be applied to the NORM_CMD(REPAIR_ADV) representing the most
limiting (in terms of congestion control feedback suppression)
congestion control response. This allows the NORM_CMD(REPAIR_ADV)
message to suppress receiver congestion control responses as well as
NACK feedback messages. The field is defined as a header extension
so that alternative congestion control schemes can be used for NORM
without revision to this document. A NORM-CC Feedback Header
Extension (EXT_CC) is defined to encapsulate congestion control
feedback within NORM_NACK, NORM_ACK, and NORM_CMD(REPAIR_ADV)
messages. If another congestion control technique (e.g., Pragmatic
General Multicast Congestion Control (PGMCC) [PgmccPaper]) is used
Adamson, et al. Standards Track PAGE 42
RFC 5740 NORM Protocol November 2009
within a NORM implementation, an additional header extension MAY need
to be defined to encapsulate any required feedback content. The
NORM-CC Feedback Header Extension format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| het = 3 | hel = 3 | cc_sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_flags | cc_rtt | cc_loss |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cc_rate | cc_reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "cc_sequence" field contains the current greatest "cc_sequence"
value receivers have received in NORM_CMD(CC) messages from the
sender. This information assists the sender in congestion control
operation by providing an indicator of how current ("fresh") the
receiver's round-trip measurement reference time is and whether the
receiver has been successfully receiving recent congestion control
probes. For example, if it is apparent the receiver has not been
receiving recent congestion control probes (and thus possibly other
messages from the sender), the sender SHOULD choose to take
congestion avoidance measures. For NORM_CMD(REPAIR_ADV) messages,
the sender SHALL set the "cc_sequence" field value to the value set
in the last NORM_CMD(CC) message sent.
The "cc_flags" field contains bits representing the receiver's state
with respect to congestion control operation. The possible values
for the "cc_flags" field are those specified for the NORM_CMD(CC)
message node list item flags. These fields are used by receivers in
controlling (suppressing as necessary) their congestion control
feedback. For NORM_CMD(REPAIR_ADV) messages, the NORM_FLAG_CC_RTT
SHALL be set only when all feedback messages received by the sender
have the flag set. Similarly, the NORM_FLAG_CC_CLR or
NORM_FLAG_CC_PLR SHALL be set only when no feedback has been received
from non-CLR or non-PLR receivers. And the NORM_FLAG_CC_LEAVE SHALL
be set only when all feedback messages the sender has received have
this flag set. These heuristics for setting the flags in
NORM_CMD(REPAIR_ADV) ensure the most effective suppression of
receivers providing unicast feedback messages.
The "cc_rtt" field SHALL be set to a default maximum value, and the
NORM_FLAG_CC_RTT flag SHALL be cleared when no receiver has yet
received RTT measurement information. When a receiver has received
RTT measurement information, it SHALL set the "cc_rtt" value
accordingly and set the NORM_FLAG_CC_RTT flag in the "cc_flags"
field. For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the
"cc_rtt" field value to the largest non-CLR/non-PLR RTT it has
Adamson, et al. Standards Track PAGE 43
RFC 5740 NORM Protocol November 2009
measured from receivers for the current feedback round.
The "cc_loss" field represents the receiver's current packet loss
fraction estimate for the indicated source. The loss fraction is a
value from 0.0 to 1.0 corresponding to a range of zero to 100 percent
packet loss. The 16-bit "cc_loss" value is calculated by the
following formula:
"cc_loss" = floor(decimal_loss_fraction * 65535.0)
For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
field value to the largest non-CLR/non-PLR loss estimate it has
received from receivers for the current feedback round.
The "cc_rate" field represents the receiver's current local
congestion control rate. During "slow start", when the receiver has
detected no loss, this value is set to twice the actual rate it has
measured from the corresponding sender and the NORM_FLAG_CC_START is
set in the "cc_flags" field. Otherwise, the receiver calculates a
congestion control rate based on its loss measurement and RTT
measurement information (even if default) for the "cc_rate" field.
For NORM_CMD(REPAIR_ADV) messages, the sender SHALL set the "cc_loss"
field value to the lowest non-CLR/non-PLR "cc_rate" report it has
received from receivers for the current feedback round.
The "cc_reserved" field is reserved for future NORM protocol use.
Currently, senders SHALL set this field to ZERO, and receivers SHALL
ignore the content of this field.
The "repair_adv_payload" is in exactly the same form as the
"nack_content" of NORM_NACK messages and can be processed by
receivers for suppression purposes in the same manner, with the
exception of the condition when the NORM_REPAIR_ADV_FLAG_LIMIT is
set.
4.2.3.6. NORM_CMD(ACK_REQ) Message
The NORM_CMD(ACK_REQ) message is used by the sender to request
acknowledgment from a specified list of receivers. This message is
used in providing a lightweight positive acknowledgment mechanism
that is OPTIONAL for use by the reliable multicast application. A
range of acknowledgment request types is provided for use at the
application's discretion. Provision for application-defined,
positively acknowledged commands allows the application to
automatically take advantage of transmission and round-trip timing
information available to the NORM protocol. The details of the NORM
Positive Acknowledgment Process including transmission of the
NORM_CMD(ACK_REQ) messages and the receiver response (NORM_ACK) are
Adamson, et al. Standards Track PAGE 44
RFC 5740 NORM Protocol November 2009
described in Section 5.5.3. The format of the NORM_CMD(ACK_REQ)
message is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-type = 6 | reserved | ack_type | ack_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| acking_node_list |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: NORM_CMD(ACK_REQ) Message Format
The NORM common message header and standard NORM_CMD fields serve
their usual purposes. The value of the "hdr_len" field for
NORM_CMD(ACK_REQ) messages with no header extension present is 4.
The "ack_type" field indicates the type of acknowledgment being
requested and thus implies rules for how the receiver will treat this
request. The following "ack_type" values are defined and are also
used in NORM_ACK messages described later:
+-----------------------+------------+------------------------------+
| ACK Type | Value | Purpose |
+-----------------------+------------+------------------------------+
| NORM_ACK(CC) | 1 | Used to identify NORM_ACK |
| | | messages sent in response to |
| | | NORM_CMD(CC) messages. |
| NORM_ACK(FLUSH) | 2 | Used to identify NORM_ACK |
| | | messages sent in response to |
| | | NORM_CMD(FLUSH) messages. |
| NORM_ACK(RESERVED) | 3-15 | Reserved for possible future |
| | | NORM protocol use. |
| NORM_ACK(APPLICATION) | 16-255 | Used at application's |
| | | discretion. |
+-----------------------+------------+------------------------------+
The NORM_ACK(CC) value is provided for use only in NORM_ACKs
generated in response to the NORM_CMD(CC) messages used in congestion
control operation. Similarly, the NORM_ACK(FLUSH) is provided for
use only in NORM_ACKs generated in response to applicable
NORM_CMD(FLUSH) messages. NORM_CMD(ACK_REQ) messages with "ack_type"
Adamson, et al. Standards Track PAGE 45
RFC 5740 NORM Protocol November 2009
of NORM_ACK(CC) or NORM_ACK(FLUSH) SHALL NOT be generated by the
sender.
The NORM_ACK(RESERVED) range of "ack_type" values is provided for
possible future NORM protocol use.
The NORM_ACK(APPLICATION) range of "ack_type" values is provided so
that NORM applications can implement application-defined, positively
acknowledged commands that are able to leverage internal transmission
and round-trip timing information available to the NORM protocol
implementation.
The "ack_id" provides a sequenced identifier for the given
NORM_CMD(ACK_REQ) message. This "ack_id" is returned in NORM_ACK
messages generated by the receivers so that the sender can associate
the response with its corresponding request.
The "reserved" field is reserved for possible future protocol use and
SHALL be set to ZERO by senders and ignored by receivers.
The "acking_node_list" field contains the NormNodeIds of the current
NORM receivers that are desired to provide positive acknowledgment
(NORM_ACK) to this request. The packet payload length implies the
length of the "acking_node_list", and its length is limited to the
sender NormSegmentSize. The individual NormNodeId items are listed
in network (Big Endian) byte order. If a receiver's NormNodeId is
included in the "acking_node_list", it SHALL schedule transmission of
a NORM_ACK message as described in Section 5.5.3.
4.2.3.7. NORM_CMD(APPLICATION) Message
This command allows the NORM application to robustly transmit
application-defined commands. The command message preempts any
ongoing data transmission and is repeated up to NORM_ROBUST_FACTOR
times at a rate of once per 2*GRTT_sender. This rate of repetition
allows the application to observe any response (if that is the
application's purpose for the command) before it is repeated.
Possible responses can include initiation of data transmission, other
NORM_CMD(APPLICATION) messages, or even application-defined,
positively acknowledged commands from other NormSession participants.
The transmission of these commands will preempt data transmission
when they are scheduled and can be multiplexed with ongoing data
transmission. This type of robustly transmitted command allows NORM
applications to define a complete set of session control mechanisms
with less state than the transfer of FEC-encoded reliable content
needs while taking advantage of NORM transmission and round-trip
timing information.
Adamson, et al. Standards Track PAGE 46
RFC 5740 NORM Protocol November 2009
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=3| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | grtt |backoff| gsize |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-type = 7 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application-Defined Content |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16: NORM_CMD(APPLICATION) Message Format
The NORM common message header and NORM_CMD fields are interpreted as
previously described. The value of the NORM_CMD(APPLICATION)
"hdr_len" field when no header extensions are present is 4.
The "Application-Defined Content" area contains information in a
format at the discretion of the application. The size of this
payload SHALL be limited to a maximum of the sender's NormSegmentSize
setting. Upon reception, the NORM protocol implementation SHALL
deliver the content to the receiver application. Note that any
detection of duplicate reception of a NORM_CMD(APPLICATION) message
is the responsibility of the application.
4.3. Receiver Messages
The NORM message types generated by participating receivers consist
of the NORM_NACK and NORM_ACK message types. NORM_NACK messages are
sent to request repair of missing data content from sender
transmission, and NORM_ACK messages are generated in response to
certain sender commands including NORM_CMD(CC) and NORM_CMD(ACK_REQ).
4.3.1. NORM_NACK Message
The principal purpose of NORM_NACK messages is for receivers to
request repair of sender content via selective, negative
acknowledgment upon detection of incomplete data. NORM_NACK messages
will be transmitted according to the rules of NORM_NACK generation
and suppression described in Section 5.3. NORM_NACK messages also
contain additional fields to provide feedback to the sender(s) for
purposes of round-trip timing collection and congestion control.
The payload of NORM_NACK messages contains one or more repair
Adamson, et al. Standards Track PAGE 47
RFC 5740 NORM Protocol November 2009
requests for different objects or portions of those objects. The
NORM_NACK message format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=4| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| nack_payload |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 17: NORM_NACK Message Format
The NORM common message header fields serve their usual purposes.
The value of the "hdr_len" field for NORM_NACK messages without
header extensions present is 6.
The "server_id" field identifies the NORM sender to which the
NORM_NACK message is destined.
The "instance_id" field contains the current session identifier given
by the sender identified by the "server_id" field in its sender
messages. The sender SHOULD ignore feedback messages containing an
invalid "instance_id" value.
The "grtt_response" fields contain an adjusted version of the
timestamp from the most recently received NORM_CMD(CC) message for
the indicated NORM sender. The format of the "grtt_response" is the
same as the "send_time" field of the NORM_CMD(CC). The
"grtt_response" value is relative to the "send_time" the source
provided with a corresponding NORM_CMD(CC) command. The receiver
adjusts the source's NORM_CMD(CC) "send_time" timestamp by adding the
time delta from when the receiver received the NORM_CMD(CC) to when
the NORM_NACK is transmitted in response to calculate the value in
the "grtt_response" field. This is the "receive_to_response_delta"
Adamson, et al. Standards Track PAGE 48
RFC 5740 NORM Protocol November 2009
value used in the following formula:
grtt_response = NORM_CMD(CC) send_time + receive_to_response_delta
The receiver SHALL set the "grtt_response" to a ZERO value, to
indicate it has not yet received a NORM_CMD(CC) message from the
indicated sender, and the sender MUST ignore the "grtt_response" in
this message.
For NORM-CC operation, the NORM-CC Feedback Header Extension, as
described in the NORM_CMD(REPAIR_ADV} message description, is added
to NORM_NACK messages to provide feedback on the receiver's current
state with respect to congestion control operation. Alternative
header extensions for congestion control feedback MAY be defined for
alternative congestion control schemes for NORM use in the future.
The "reserved" field is for potential future NORM use and SHALL be
set to ZERO for this version of the protocol.
The "nack_payload" of the NORM_NACK message specifies the repair
needs of the receiver with respect to the NORM sender indicated by
the "server_id" field. The receiver constructs repair requests based
on the NORM_DATA and/or NORM_INFO segments it needs from the sender
to complete reliable reception up to the sender's transmission
position at the moment the receiver initiates the NACK procedure as
described in Section 5.3. A single NORM Repair Request consists of a
list of items, ranges, and/or FEC coding block erasure counts for
needed NORM_DATA and/or NORM_INFO content. Multiple repair requests
can be concatenated within the "nack_payload" field of a NORM_NACK
message. A single NORM Repair Request can possibly include multiple
"items", "ranges", or "erasure_counts". In turn, the "nack_payload"
field MAY contain multiple repair requests. A single NORM Repair
Request has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form | flags | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| repair_request_items |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 18: NORM Repair Request Format
The "form" field indicates the type of repair request items given in
the "repair_request_items" list. Possible values for the "form"
field include:
Adamson, et al. Standards Track PAGE 49
RFC 5740 NORM Protocol November 2009
+--------------------+-------+
| Form | Value |
+--------------------+-------+
| NORM_NACK_ITEMS | 1 |
| NORM_NACK_RANGES | 2 |
| NORM_NACK_ERASURES | 3 |
+--------------------+-------+
A "form" value of NORM_NACK_ITEMS indicates each repair request item
in the "repair_request_items" list is to be treated as an individual
request. A value of NORM_NACK_RANGES indicates the
"repair_request_items" list consists of pairs of repair request items
corresponding to the inclusive ranges of repair needs. The
NORM_NACK_ERASURES "form" indicates the repair request items are to
be treated individually and the "encoding_symbol_id" portion of the
"fec_payload_id" field of the repair request item (see below) is to
be interpreted as an erasure count for the FEC coding block
identified by the repair request item's "source_block_number".
The "flags" field is currently used to indicate the level of data
content for which the repair request items apply (i.e., an individual
segment, entire FEC coding block, or entire transport object).
Possible flag values include:
+-------------------+--------+--------------------------------------+
| Flag | Value | Purpose |
+-------------------+--------+--------------------------------------+
| NORM_NACK_SEGMENT | 0x01 | Indicates the listed segment(s) or |
| | | range of segments needed as repair. |
| NORM_NACK_BLOCK | 0x02 | Indicates the listed block(s) or |
| | | range of blocks in entirety that are |
| | | needed as repair. |
| NORM_NACK_INFO | 0x04 | Indicates NORM_INFO is needed as |
| | | repair for the listed object(s). |
| NORM_NACK_OBJECT | 0x08 | Indicates the listed object(s) or |
| | | range of objects in entirety are |
| | | needed as repair. |
+-------------------+--------+--------------------------------------+
When the NORM_NACK_SEGMENT flag is set, the "object_transport_id" and
"fec_payload_id" fields are used to determine which sets or ranges of
individual NORM_DATA segments are needed to repair content at the
receiver. When the NORM_NACK_BLOCK flag is set, this indicates the
receiver is completely missing the indicated coding block(s), and
that transmissions sufficient to repair the indicated block(s) in
their entirety are needed. When the NORM_NACK_INFO flag is set, this
indicates the receiver is missing the NORM_INFO segment for the
indicated "object_transport_id". Note the NORM_NACK_INFO can be set
Adamson, et al. Standards Track PAGE 50
RFC 5740 NORM Protocol November 2009
in combination with the NORM_NACK_BLOCK or NORM_NACK_SEGMENT flags,
or can be set alone. When the NORM_NACK_OBJECT flag is set, this
indicates the receiver is missing the entire NormTransportObject
referenced by the "object_transport_id". This also implicitly
requests any available NORM_INFO for the NormObject, if applicable.
The "fec_payload_id" field is ignored when the flag NORM_NACK_OBJECT
is set.
The "length" field value is the length in bytes of the
"repair_request_items" field.
The "repair_request_items" field consists of a list of individual or
range pairs of transport data unit identifiers in the following
format.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id | reserved | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 19: NORM Repair Request Item Format
The "fec_id" indicates the FEC type and can be used to determine the
format of the "fec_payload_id" field. The "reserved" field is kept
for possible future use and SHALL be set to a ZERO value and ignored
by NORM nodes processing NACK content.
The "object_transport_id" corresponds to the NormObject for which
repair is being requested, and the "fec_payload_id" identifies the
specific FEC coding block and/or segment being requested. When the
NORM_NACK_OBJECT flag is set, the value of the "fec_payload_id" field
is ignored. When the NORM_NACK_BLOCK flag is set, only the FEC code
block identifier portion of the "fec_payload_id" is to be
interpreted.
The format of the "fec_payload_id" field depends upon the "fec_id"
field value.
When the receiver's repair needs dictate that different forms (mixed
ranges and/or individual items) or types (mixed specific segments
and/or blocks or objects in entirety) are needed to complete reliable
transmission, multiple NORM Repair Requests with different "form" and
or "flags" values can be concatenated within a single NORM_NACK
message. Additionally, NORM receivers SHALL construct NORM_NACK
messages with their repair requests in ordinal order with respect to
Adamson, et al. Standards Track PAGE 51
RFC 5740 NORM Protocol November 2009
"object_transport_id" and "fec_payload_id" values. The
"nack_payload" size SHALL NOT exceed the NormSegmentSize for the
sender to which the NORM_NACK is destined.
NORM_NACK Content Examples:
In these examples, a small block, systematic FEC code ("fec_id" =
129) is assumed with a user data block length of 32 segments. In
Example 1, a list of individual NORM_NACK_ITEMS repair requests is
given. In Example 2, a list of NORM_NACK_RANGES requests AND a
single NORM_NACK_ITEMS request are concatenated to illustrate the
possible content of a NORM_NACK message. Note that FEC coding block
erasure counts could also be provided in each case. However, the
erasure counts are not really necessary since the sender can easily
determine the erasure count while processing the NACK content.
However, the erasure count option can be useful for operation with
other FEC codes or for intermediate system purposes.
Example 1: NORM_NACK "nack_payload" for: Object 12, Coding Block 3,
Segments 2, 5, and 8
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x01 | length = 36 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Adamson, et al. Standards Track PAGE 52
RFC 5740 NORM Protocol November 2009
Example 2: NORM_NACK "nack_payload" for: Object 18, Coding Block 6,
Segments 5, 6, 7, 8, 9, 10; and Object 19 NORM_INFO and Coding Block
1, Segment 3
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 2 | flags = 0x01 | length = 24 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| form = 1 | flags = 0x05 | length = 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id = 129 | reserved | object_transport_id = 19 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_number = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_block_length = 32 | encoding_symbol_id = 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.3.2. NORM_ACK Message
The NORM_ACK message is intended to be used primarily as part of NORM
congestion control operation and round-trip timing measurement. The
acknowledgment type NORM_ACK(CC) is provided for this purpose as
described in the NORM_CMD(ACK_REQ) message description. The
generation of NORM_ACK(CC) messages for round-trip timing estimation
and congestion control operation is described in Section 5.5.1 and
Section 5.5.2, respectively. However, some multicast applications
can benefit from some limited form of positive acknowledgment for
certain functions. A simple, scalable positive acknowledgment scheme
is defined in Section 5.5.3, which can be leveraged by protocol
implementations when appropriate. The NORM_CMD(FLUSH) can also be
used for OPTIONAL collection of positive acknowledgment of reliable
reception to a certain "watermark" transmission point from specific
receivers using this mechanism. The NORM_ACK type NORM_ACK(FLUSH) is
provided for this purpose and the format of the "nack_payload" for
this acknowledgment type is given below. Beyond that, a range of
application-defined "ack_type" values is provided for use at the NORM
Adamson, et al. Standards Track PAGE 53
RFC 5740 NORM Protocol November 2009
application's discretion. Implementations making use of application-
defined positive acknowledgments MAY also make use of the
"nack_payload" as needed, observing the constraint that the
"nack_payload" field size be limited to a maximum of the
NormSegmentSize for the sender to which the NORM_ACK is destined.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version| type=5| hdr_len | sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| server_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| instance_id | ack_type | ack_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_sec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| grtt_response_usec |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| header extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ack_payload (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 20: NORM_ACK Message Format
The NORM common message header fields serve their usual purposes.
The value of the "hdr_len" field when no header extensions are
present is 6.
The "server_id", "instance_id", and "grtt_response" fields serve the
same purpose as the corresponding fields in NORM_NACK messages.
Header extensions can be applied to support congestion control
feedback or other functions in the same manner.
The "ack_type" field indicates the nature of the NORM_ACK message.
This directly corresponds to the "ack_type" field of the
NORM_CMD(ACK_REQ) message to which this acknowledgment applies.
The "ack_id" field serves as a sequence number so the sender can
verify a received NORM_ACK message actually applies to a current
acknowledgment request. The "ack_id" field is not used in the case
of the NORM_ACK(CC) and NORM_ACK(FLUSH) acknowledgment types.
The "ack_payload" format is a function of the "ack_type". The
Adamson, et al. Standards Track PAGE 54
RFC 5740 NORM Protocol November 2009
NORM_ACK(CC) message has no attached content. Only the NORM_ACK
header applies. In the case of NORM_ACK(FLUSH), a specific
"ack_payload" format is defined:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_id | reserved | object_transport_id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| fec_payload_id |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The "object_transport_id" and "fec_payload_id" are used by the
receiver to acknowledge applicable NORM_CMD(FLUSH) messages
transmitted by the sender identified by the "server_id" field.
The "ack_payload" of NORM_ACK messages for application-defined
"ack_type" values is specific to the application but is limited in
size to a maximum of the NormSegmentSize of the sender referenced by
the "server_id".
4.4. General Purpose Messages
Some additional message formats are defined for general purpose in
NORM multicast sessions whether the participant is acting as a sender
and/or receiver within the group.
4.4.1. NORM_REPORT Message
This is an OPTIONAL message generated by NORM participants. This
message can be used for periodic performance reports from receivers
in experimental NORM implementations. The format of this message is
currently undefined. Experimental NORM implementations MAY define
NORM_REPORT formats as needed for test purposes. These report
messages SHOULD be disabled for interoperability testing between
different compliant NORM implementations.
5. Detailed Protocol Operation
This section describes the detailed interactions of senders and
receivers participating in a NORM session. A simple synopsis of the
protocol operation is given here:
1. The sender periodically transmits NORM_CMD(CC) messages as needed
to initialize and collect round-trip timing and congestion
control feedback from the receiver set.
Adamson, et al. Standards Track PAGE 55
RFC 5740 NORM Protocol November 2009
2. The sender transmits an ordinal set of NormObjects segmented in
the form of NORM_DATA messages labeled with NormTransportIds and
logically identified with FEC encoding block numbers and symbol
identifiers. When applicable, NORM_INFO messages MAY optionally
precede the transmission of data content for NORM transport
objects.
3. As receivers detect missing content from the sender, they
initiate repair requests with NORM_NACK messages. The receivers
track the sender's most recent objectTransportId::fecPayloadId
transmit position and NACK only for content that is ordinally
prior to that current transmit position. The receivers schedule
random backoff timeouts before generating NORM_NACK messages and
wait an appropriate amount of time before repeating the NORM_NACK
if their repair request is not satisfied.
4. The sender aggregates repair requests from the receivers and
logically "rewinds" its transmit position to send appropriate
repair messages. The sender sends repairs for the earliest
ordinal transmit position first and maintains this ordinal repair
transmission sequence. FEC parity content not previously
transmitted for the applicable FEC coding block is used for
repair transmissions to the greatest extent possible. If the
sender exhausts its available FEC parity content on multiple
repair cycles for the same coding block, it resorts to an
explicit repair strategy (possibly using parity content) to
complete repairs. (The use of explicit repair is an exception in
general protocol operation, but the possibility does exist for
extreme conditions). The sender immediately assumes transmission
of new content once it has sent pending repairs.
5. The sender transmits NORM_CMD(FLUSH) messages when it reaches the
end of enqueued transmit content and pending repairs. Receivers
respond to the NORM_CMD(FLUSH) messages with NORM_NACK
transmissions (following the same suppression backoff timeout
strategy as for data) if they need further repair.
6. The sender transmissions are subject to rate control limits
determined by congestion control mechanisms. In the baseline
NORM-CC operation, each sender in a NormSession maintains its own
independent congestion control state. Receivers provide
congestion control feedback in NORM_NACK and NORM_ACK messages.
NORM_ACK feedback for congestion control purposes is governed
using a suppression mechanism similar to that for NORM_NACK
messages.
While this overall concept is relatively simple, there are details to
each of these aspects that need to be addressed for successful,
Adamson, et al. Standards Track PAGE 56
RFC 5740 NORM Protocol November 2009
efficient, robust, and scalable NORM protocol operation.
5.1. Sender Initialization and Transmission
Upon startup, the NORM sender immediately begins sending NORM_CMD(CC)
messages to collect round-trip timing and other information from the
potential group. If NORM-CC congestion control operation is enabled,
the NORM-CC Rate header extension MUST be included in these messages.
Congestion control operation SHALL be observed at all times when not
operating using dedicated resources, like in the general Internet.
Even if congestion control operation is disabled at the sender, it
can be desirable to use the NORM_CMD(CC) messaging to collect
feedback from the group using the baseline NORM-CC feedback
mechanisms. This proactive feedback collection can be used to
establish a GRTT estimate prior to data transmission and potential
NACK operation.
In some cases, applications might need the sender to also proceed
with data transmission immediately. In other cases, the sender might
wish to defer data transmission until it has received some feedback
or request from the receiver set indicating receivers are indeed
present. Note, in some applications (e.g., web push), this
indication MAY come out-of-band with respect to the multicast session
via other means. As noted, the periodic transmission of NORM_CMD(CC)
messages MAY precede actual data transmission in order to have an
initial GRTT estimate.
With inclusion of the OPTIONAL NORM FEC Object Transmission
Information Header Extension (EXT_FTI), the NORM protocol sender
message headers can contain all information necessary to prepare
receivers for subsequent reliable reception. This includes FEC
coding parameters, the sender NormSegmentSize, and other information.
If this header extension is not used, it is presumed receivers have
received the FEC Object Transmission Information via other means.
Additionally, applications MAY leverage the use of NORM_INFO messages
associated with the session data objects in the session to provide
application-specific context information for the session and data
being transmitted. These mechanisms allow for operation with minimal
pre-coordination among the senders and receivers.
The NORM sender begins segmenting application-enqueued data into
NORM_DATA segments and transmitting it to the group. For objects of
type NORM_OBJECT_DATA and NORM_OBJECT_FILE, the segmentation
algorithm described in FEC Building Block [RFC 5052] is RECOMMENDED.
For objects of type NORM_OBJECT_STREAM, segmentation will typically
be into uniform FEC coding block sizes, with individual segment sizes
controlled by the application. In most cases, the application and
NORM implementation SHOULD strive to produce full-sized
Adamson, et al. Standards Track PAGE 57
RFC 5740 NORM Protocol November 2009
(NormSegmentSize) segments when possible. The rate of transmission
is controlled via congestion control mechanisms or is a fixed rate if
desired for closed network operations. The receivers participating
in the multicast group provide feedback to the sender as needed.
When the sender reaches the end of data it has enqueued for
transmission or any pending repairs, it transmits a series of
NORM_CMD(FLUSH) messages at a rate of one per 2*GRTT_sender. Similar
to the end of each transmitted FEC coding block during transmission,
receivers SHALL respond to these NORM_CMD(FLUSH) messages with
additional repair requests as needed. A protocol parameter
NORM_ROBUST_FACTOR determines the number of flush messages sent. If
receivers request repair, the repair is provided, and flushing occurs
again at the end of repair transmission. The sender MAY attach an
OPTIONAL "acking_node_list" to NORM_CMD(FLUSH) containing the
NormNodeIds for receivers from which it expects explicit positive
acknowledgment of reception. The NORM_CMD(FLUSH) message MAY be also
used for this OPTIONAL purpose any time prior to the end of data
enqueued for transmission with the NORM_CMD(FLUSH) messages
multiplexed with ongoing data transmissions. The OPTIONAL NORM
positive acknowledgment procedure is described in Section 5.5.3.
5.1.1. Object Segmentation Algorithm
NORM senders and receivers MUST use a common algorithm for logically
segmenting transport data into FEC encoding blocks and symbols so
appropriate NACKs can be constructed to request repair of missing
data. NORM FEC coding blocks are comprised of multi-byte symbols
(segments) transmitted in the payload of NORM_DATA messages. Each
NORM_DATA message will contain one or more source or encoding symbols
identified by the "fec_payload_id" field, and the NormSegmentSize
sender parameter defines the maximum size (in bytes) of the
"payload_data" field containing the content (a "segment"). The FEC
encoding type and associated parameters govern the source block size
(number of source symbols per coding block, etc.). NORM senders and
receivers use these FEC parameters, along with the NormSegmentSize
and transport object size to compute the source block structure for
transport objects. These parameters are provided in the FEC Object
Transmission Information for each object. The block partitioning
algorithm described in the FEC Building Block [RFC 5052] document is
RECOMMENDED for use in computing a source block structure such that
all source blocks are as close to being equal length as possible.
This helps avoid the performance disadvantages of "short" FEC blocks.
Note that this algorithm applies only to the statically sized
NORM_OBJECT_DATA and NORM_OBJECT_FILE transport object types where
the object size is fixed and predetermined. For NORM_OBJECT_STREAM
objects, the object is segmented according to the maximum source
block length given in the FEC Transmission Information, unless the
FEC Payload ID indicates an alternative size for a given block.
Adamson, et al. Standards Track PAGE 58
RFC 5740 NORM Protocol November 2009
5.2. Receiver Initialization and Reception
For typical operation, NORM receivers will join a specified multicast
group and listen on a specific port number for sender transmissions.
As the NORM receiver receives NORM_DATA messages, it will establish
buffering state and provide content to its application as appropriate
for the given data type. The NORM protocol allows receivers to join
and leave the group at will, although some applications might need
receivers to be members of the group prior to start of data
transmission. Thus, different NORM applications MAY use different
policies to constrain the impact of new receivers joining the group
in the middle of a session. For example, a useful implementation
policy is for new receivers joining the group to limit or avoid
repair requests for transport objects already in progress. The NORM
sender implementation MAY impose additional constraints to limit the
ability of receivers to disrupt reliable multicast performance by
joining, leaving, and rejoining the group often. Different receiver
"join policies" might be appropriate for different applications
and/or scenarios. For general purpose operation, a default policy
where receivers are allowed to request repair only for coding blocks
with a NormTransportId and FEC coding block number greater than or
equal to the first non-repair NORM_DATA or NORM_INFO message received
upon joining the group is RECOMMENDED. For objects of type
NORM_OBJECT_STREAM, it is RECOMMENDED the join policy constrain
receivers to begin reliable reception at the current FEC coding block
for which non-repair content is received.
In some deployments, different multicast receivers might have
differing quality of network connectivity. Some receivers may suffer
significantly poorer performance with very limited goodput due to low
connection rate or substantial packet loss. Similar to the "join
policies" described above, a NORM sender implementation MAY choose to
enforce different "service policies" to perhaps exclude exceptionally
poorly performing (or otherwise badly behaving) receivers from the
group. The sender implementation could choose to ignore NACKs from
such receivers and/or force advancement of its logical "repair
window" (i.e., enforcing a minimal level of service) and use the
NORM_CMD(SQUELCH) message to advise those poor performers of its
advance. Note in some cases, the application may need to support the
"weakest member" regardless of the time needed to achieve reliable
delivery. When implemented, the protocol instantiation SHOULD expose
controls to the set of "join" and/or "service" policies available to
support the needs of different applications.
5.3. Receiver NACK Procedure
When the receiver detects it is missing data from a sender's NORM
transmissions, it initiates its NACKing procedure. The NACKing
Adamson, et al. Standards Track PAGE 59
RFC 5740 NORM Protocol November 2009
procedure SHALL be initiated only at FEC coding block boundaries,
NormObject boundaries, upon receipt of a NORM_CMD(FLUSH) message, or
upon an "inactivity" timeout when NORM_DATA or NORM_INFO
transmissions are no longer received from a previously active sender.
The RECOMMENDED value of such an inactivity timeout is:
T_inactivity = NORM_ROBUST_FACTOR * 2 * GRTT_sender
where the GRTT_sender value corresponds to the GRTT estimate
advertised in the "grtt" field of NORM sender messages. A minimum
T_inactivity value of 1 second is RECOMMENDED. The NORM receiver
SHOULD reset this inactivity timer and repeat NACK initiation upon
timeout for up to NORM_ROBUST_FACTOR times or more depending upon the
application's need for persistence by its receivers. It is also
important receivers rescale the T_inactivity timeout as the sender's
advertised GRTT changes.
The NACKing procedure begins with a random backoff timeout. The
duration of the backoff timeout is chosen using the "RandomBackoff"
algorithm described in the Multicast NACK Building Block [RFC 5401]
document using (K_sender*GRTT_sender) for the maxTime parameter and
the sender advertised group size (GSIZE_sender) as the groupSize
parameter. NORM senders provide values for GRTT_sender, K_sender and
GSIZE_sender via the "grtt", "backoff", and "gsize" fields of
transmitted messages. The GRTT_sender value is determined by the
sender based on feedback it has received from the group while the
K_sender and GSIZE_sender values can be determined by application
requirements and expectations or ancillary information. The backoff
factor K_sender MUST be greater than one to provide for effective
feedback suppression. A value of K_sender = 4 is RECOMMENDED for the
Any Source Multicast (ASM) model, while a value of K_sender = 6 is
RECOMMENDED for Single Source Multicast (SSM) operation.
Thus:
T_backoff = RandomBackoff(K_sender*GRTT_sender, GSIZE_sender)
To avoid the possibility of NACK implosion in the case of sender or
network failure during SSM operation, the receiver SHALL
automatically suppress its NACK and immediately enter the "holdoff"
period described below when T_backoff is greater than (K_sender-
1)*GRTT_sender. Otherwise, the backoff period is entered and the
receiver MUST accumulate external pending repair state from NORM_NACK
messages and NORM_CMD(REPAIR_ADV) messages received. At the end of
the backoff time, the receiver SHALL generate a NORM_NACK message
only if the following conditions are met:
Adamson, et al. Standards Track PAGE 60
RFC 5740 NORM Protocol November 2009
1. The sender's current transmit position (in terms of
objectTransportId::fecPayloadId) exceeds the earliest repair
position of the receiver.
2. The repair state accumulated from NORM_NACK and
NORM_CMD(REPAIR_ADV) messages does not equal or supersede the
receiver's repair needs up to the sender transmission position at
the time the NACK procedure (backoff timeout) was initiated.
If these conditions are met, the receiver immediately generates a
NORM_NACK message when the backoff timeout expires. Otherwise, the
receiver's NACK is considered to be "suppressed" and the message is
not sent. At this time, the receiver begins a "holdoff" period
during which it constrains itself to not re-initiate the NACKing
process. The purpose of this timeout is to allow the sender worst-
case time to respond to the repair needs before the receiver requests
repair again. The value of this "holdoff" timeout (T_rcvrHoldoff) as
described in [RFC 5401] is:
T_rcvrHoldoff =(K_sender+2)*GRTT_sender
The NORM_NACK message contains repair request content beginning with
the lowest ordinal repair position of the receiver up through the
coding block prior to the most recently heard ordinal transmission
position for the sender. If the size of the NORM_NACK content
exceeds the sender's NormSegmentSize, the NACK content is truncated
so the receiver only generates a single NORM_NACK message per NACK
cycle for a given sender. In summary, a single NACK message is
generated containing the receiver's lowest ordinal repair needs.
For each partially received FEC coding block requiring repair, the
receiver SHALL, on its FIRST repair attempt for the block, request
the parity portion of the FEC coding block beginning with the lowest
ordinal parity "encoding_symbol_id" (i.e., "encoding_symbol_id" =
"source_block_len") and request the number of FEC symbols
corresponding to its data segment erasure count for the block. On
subsequent repair cycles for the same coding block, the receiver
SHALL request only those repair symbols from the first set it has not
yet received up to the remaining erasure count for that applicable
coding block. Note the sender might have transmitted other
different, additional parity segments for other receivers that could
also be used to satisfy the local receiver's erasure-filling needs.
In the case where the erasure count for a partially received FEC
coding block exceeds the maximum number of parity symbols available
from the sender for the block (as indicated by the NORM_DATA
"fec_num_parity" field), the receiver SHALL request all available
parity segments plus the ordinally highest missing data segments
needed to satisfy its total erasure needs for the block. The goal of
this strategy is for the overall receiver set to request a lowest
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RFC 5740 NORM Protocol November 2009
common denominator set of repair symbols for a given FEC coding
block. This allows the sender to construct the most efficient repair
transmission segment set and enables effective NACK suppression among
the receivers even with uncorrelated packet loss. This approach also
does not demand synchronization among the receiver set in their
repair requests for the sender.
For FEC coding blocks or NormObjects missed in their entirety, the
NORM receiver constructs repair requests with NORM_NACK_BLOCK or
NORM_NACK_OBJECT flags set as appropriate. The request for
retransmission of NORM_INFO is accomplished by setting the
NORM_NACK_INFO flag in a corresponding repair request.
5.4. Sender NACK Processing and Response
The principal goal of the sender is to make forward progress in the
transmission of data its application has enqueued. However, the
sender will need to occasionally "rewind" its logical transmission
point to satisfy the repair needs of receivers who have NACKed.
Aggregation of multiple NACKs is used to determine an optimal repair
strategy when a NACK event occurs. Since receivers initiate the NACK
process on coding block or object boundaries, there is some loose
degree of synchronization of the repair process even when receivers
experience uncorrelated data loss.
5.4.1. Sender Repair State Aggregation
When a sender is in its normal state of transmitting new data and
receives a NACK, it begins a procedure to accumulate NACK repair
state from NORM_NACK messages before beginning repair transmissions.
Note that this period of aggregating repair state does NOT interfere
with its ongoing transmission of new data.
As described in [RFC 5401], the period of time during which the sender
aggregates NORM_NACK messages is equal to:
T_sndrAggregate = (K_sender + 1) * GRTT_sender
where K_sender is the backoff scaling value advertised to the
receivers, and GRTT_sender is the sender's current estimate of the
group's greatest round-trip time. Note, for NORM unicast sessions,
the T_sndrAggregate time can be set to ZERO since there is only one
receiver. Similarly, the K_sender value SHOULD be set to ZERO for
NORM unicast sessions to minimize repair latency.
When this period ends, the sender "rewinds" by incorporating the
accumulated repair state into its pending transmission state and
begins transmitting repair messages. After pending repair
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RFC 5740 NORM Protocol November 2009
transmissions are completed, the sender continues with new
transmissions of any enqueued data. Also, at this point in time, the
sender begins a "holdoff" timeout during which time the sender
constrains itself from initiating a new repair aggregation cycle,
even if NORM_NACK messages arrive. As described in [RFC 5401], the
value of this sender "holdoff" period is:
T_sndrHoldoff = (1 * GRTT_sender)
If additional NORM_NACK messages are received during this sender
"holdoff" period, the sender will immediately incorporate these late-
arriving messages into its pending transmission state if, and only
if, the NACK content is ordinally greater than the sender's current
transmission position. This "holdoff" time allows worst-case time
for the sender to propagate its current transmission sequence
position to the group, thus avoiding redundant repair transmissions.
After the holdoff timeout expires, a new NACK accumulation period can
be started (upon arrival of a NACK) in concert with the pending
repair and new data transmission. Recall receivers are not to
initiate the NACK repair process until the sender's logical
transmission position exceeds the lowest ordinal position of their
repair needs. With the new NACK aggregation period, the sender
repeats the same process of incorporating accumulated repair state
into its transmission plan and subsequently "rewinding" to transmit
the lowest ordinal repair data when the aggregation period expires.
Again, this is conducted in concert with ongoing new data and/or
pending repair transmissions.
5.4.2. Sender FEC Repair Transmission Strategy
The NORM sender SHOULD leverage transmission of FEC parity content
for repair to the greatest extent possible. Recall that receivers
use a strategy to request a lowest common denominator of explicit
repair (including parity content) in the formation of their NORM_NACK
messages. Before falling back to explicitly satisfying different
receivers' repair needs, the sender can make use of the general
erasure-filling capability of FEC-generated parity segments. The
sender can determine the maximum erasure-filling needs for individual
FEC coding blocks from the NORM_NACK messages received during the
repair aggregation period. Then, if the sender has a sufficient
number (less than or equal to the maximum erasure count) of
previously unsent parity segments available for the applicable coding
blocks, the sender can transmit these in lieu of the specific packets
the receiver set has requested. The sender SHOULD NOT resort to
explicit transmission of the receiver set's repair needs until after
exhausting its supply of "fresh" (unsent) parity segments for a given
coding block. In general, if a sufficiently powerful FEC code is
used, the need for explicit repair will be an exception, and the
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RFC 5740 NORM Protocol November 2009
fulfillment of reliable multicast can be accomplished quite
efficiently. However, the ability to resort to explicit repair
allows the protocol to be continue to operate under even very extreme
circumstances.
NORM_DATA messages sent as repair transmissions SHALL be flagged with
the NORM_FLAG_REPAIR flag. This allows receivers to obey any
policies limiting new receivers from joining the reliable
transmission when only repair transmissions have been received.
Additionally, the sender SHOULD flag NORM_DATA transmissions sent as
explicit repair with the NORM_FLAG_EXPLICIT flag.
Although NORM end system receivers do not make use of the
NORM_FLAG_EXPLICIT flag, this message transmission status could be
leveraged by intermediate systems wishing to "assist" NORM protocol
performance. If such systems are properly positioned with respect to
reciprocal reverse-path multicast routing, they need to sub-cast only
a sufficient count of non-explicit parity repairs to satisfy a
multicast routing sub-tree's erasure-filling needs for a given FEC
coding block. When the sender has resorted to explicit repair, then
the intermediate systems SHOULD sub-cast all of the explicit repair
packets to those portions of the routing tree still requiring repair
for a given coding block. Note the intermediate systems will need to
conduct repair state accumulation for sub-routes in a manner similar
to the sender's repair state accumulation in order to have sufficient
information to perform the sub-casting. Additionally, the
intermediate systems could perform NORM_NACK suppression/aggregation
as it conducts this repair state accumulation for NORM repair cycles.
The details of this type of operation are beyond the scope of this
document, but this information is provided for possible future
consideration.
5.4.3. Sender NORM_CMD(SQUELCH) Generation
If the sender receives a NORM_NACK message for repair of data it is
no longer supporting, the sender generates a NORM_CMD(SQUELCH)
message to advertise its repair window and squelch any receivers from
additional NACKing of invalid data. The transmission rate of
NORM_CMD(SQUELCH) messages is limited to once per 2*GRTT_sender. The
"invalid_object_list" (if applicable) of the NORM_CMD(SQUELCH)
message SHALL begin with the lowest "object_transport_id" from the
invalid NORM_NACK messages received since the last NORM_CMD(SQUELCH)
transmission. The list includes as many lower ordinal invalid
"object_transport_ids" that can fit for the NORM_CMD(SQUELCH) payload
size to less than or equal to the sender's NormSegmentSize parameter.
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5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation
When a NORM sender receives NORM_NACK messages from receivers via
unicast transmission, it uses NORM_CMD(REPAIR_ADV) messages to
advertise its accumulated repair state to the receiver set since the
receiver set is not directly sharing their repair needs via multicast
communication. A NORM sender implementation MAY use a separate port
number from the NormSession port number as the source port for its
transmissions. Thus, NORM receivers can direct any unicast feedback
messages to this separate sender port number, distinct from the NORM
session (or destination) port number. Then, the NORM sender
implementation can discriminate unicast feedback messages from
multicast feedback messages when there is a mix of multicast and
unicast feedback receivers. The NORM_CMD(REPAIR_ADV) message is
multicast to the receiver set by the sender. The payload portion of
this message has content in the same format as the NORM_NACK receiver
message payload. Receivers are then able to perform feedback
suppression in the same manner as with NORM_NACK messages directly
received from other receivers. Note that the sender does not merely
retransmit NACK content it receives, but instead transmits a
representation of its aggregated repair state. The transmission of
NORM_CMD(REPAIR_ADV) messages is subject to the sender transmit rate
limit and NormSegmentSize limitation. When the NORM_CMD(REPAIR_ADV)
message is of maximum size (as indicated by the flag
NORM_REPAIR_ADV_FLAG_LIMIT), receivers SHALL consider the maximum
ordinal transmission position value embedded in the message as the
senders current transmission position and implicitly suppress
requests for ordinally higher repair. For congestion control
operation, the sender will also need to provide any information
needed so dynamic congestion control feedback can be suppressed among
receivers. This document specifies the NORM-CC Feedback Header
Extension that is applied for baseline NORM-CC operation. If other
congestion control mechanisms are used within a NORM implementation,
other header extensions MAY be defined. Whatever content format is
used for this purpose SHOULD ensure that maximum possible suppression
state is conveyed to the receiver set.
5.5. Additional Protocol Mechanisms
In addition to the principal function of data content transmission
and repair, there are some other protocol mechanisms to help NORM to
adapt to network conditions and play fairly with other coexistent
protocols.
5.5.1. Group Round-Trip Time (GRTT) Collection
For NORM receivers to appropriately scale backoff timeouts and the
senders to use proper corresponding timeouts, the participants need
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RFC 5740 NORM Protocol November 2009
to use a common timeout basis. Each NORM sender monitors the round-
trip time of active receivers and determines the greatest group
round-trip time. The sender advertises this GRTT estimate in every
message it transmits so receivers have this value available for
scaling their timers. To measure the current GRTT, the sender
periodically sends NORM_CMD(CC) messages containing a locally
generated timestamp. Receivers are expected to record this timestamp
along with the time the NORM_CMD(CC) message is received. Then, when
the receivers generate feedback messages to the sender, an adjusted
version of the sender timestamp is embedded in the feedback message
(NORM_NACK or NORM_ACK). The adjustment adds the amount of time the
receiver held the timestamp before generating its response. Upon
receipt of this adjusted timestamp, the sender is able to calculate
the round-trip time to that receiver.
The round-trip time for each receiver is fed into an algorithm that
assigns weights and smoothes the values for a conservative estimate
of the GRTT. The algorithm and methodology are described in the
Multicast NACK Building Block [RFC 5401] document in the section
entitled "One-to-Many Sender GRTT Measurement". A conservative
estimate helps guarantee feedback suppression at a small cost in
overall protocol repair delay. The sender's current estimate of GRTT
is advertised in the "grtt" field found in all NORM sender messages.
The advertised GRTT is also limited to a minimum of the nominal
inter-packet transmission time given the sender's current
transmission rate and system clock granularity. The reason for this
additional limit is to keep the receiver somewhat event-driven by
making sure the sender has had adequate time to generate any response
to repair requests from receivers given transmit rate limitations due
to congestion control or configuration.
When the NORM-CC Rate header extension is present in NORM_CMD(CC)
messages, the receivers respond to NORM_CMD(CC) messages as described
in Section 5.5.2, "NORM Congestion Control Operation". The
NORM_CMD(CC) messages are periodically generated by the sender as
described for congestion control operation. This provides for
proactive, but controlled, feedback from the group in the form of
NORM_ACK messages. This provides for GRTT feedback even if no
NORM_NACK messages are being sent. If operating without congestion
control in a closed network, the NORM_CMD(CC) messages MAY be sent
periodically without the NORM-CC Rate header extension. In this
case, receivers will only provide GRTT measurement feedback when
NORM_NACK messages are generated since no NORM_ACK messages are
generated. In this case, the NORM_CMD(CC) messages MAY be sent less
frequently, perhaps as little as once per minute, to conserve network
capacity. Note the NORM-CC Rate header extension MAY also be used to
proactively solicit RTT feedback from the receiver group per
congestion control operation even when the sender is not conducting
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RFC 5740 NORM Protocol November 2009
congestion control rate adjustment. NORM operation without
congestion control SHOULD be considered only in closed networks.
5.5.2. NORM Congestion Control Operation
This section describes baseline congestion control operation for the
NORM protocol (NORM-CC). The supporting NORM message formats and
approach described here are an adaptation of the equation-based TCP-
Friendly Multicast Congestion Control (TFMCC) approach [RFC 4654].
This congestion control scheme is REQUIRED for operation within the
general Internet unless the NORM implementation is adapted to use
another IETF-sanctioned reliable multicast congestion control
mechanism. With this TFMCC-based approach, the transmissions of NORM
senders are controlled in a rate-based manner as opposed to window-
based congestion control algorithms as in TCP. However, it is
possible the NORM protocol message set MAY alternatively be used to
support a window-based multicast congestion control scheme such as
PGMCC. The details of such an alternative MAY be described
separately or in a future revision of this document. In either case
(rate-based TFMCC or window-based PGMCC), successful control of
sender transmission depends upon collection of sender-to-receiver
packet loss estimates and RTTs to identify the congestion control
bottleneck path(s) within the multicast topology and adjust the
sender rate accordingly. The receiver with loss and RTT estimates
corresponding to the lowest resulting calculated transmission rate is
identified as the "current limiting receiver" (CLR). In the case of
a tie (where candidate CLRs are within 10% of the same calculated
rate), the receiver with the largest RTT value SHOULD be designated
as the CLR.
As described in [TcpModel], a steady-state sender transmission rate,
to be "friendly" with competing TCP flows, can be calculated as:
S
Rsender = ----------------------------------------------------------
T_rtt*(sqrt((2/3)*p) + 12*sqrt((3/8)*p) * p * (1 + 32*(p^2)))
where
S = nominal transmitted packet size. (In NORM, the "nominal" packet
size can be determined by the sender as an exponentially weighted
moving average (EWMA) of transmitted packet sizes to account for
variable message sizes).
T_rtt = RTT estimate of the current "current limiting receiver"
(CLR).
p = loss event fraction of the CLR.
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RFC 5740 NORM Protocol November 2009
To support congestion control feedback collection and operation, the
NORM sender periodically transmits NORM_CMD(CC) command messages.
NORM_CMD(CC) messages are multiplexed with NORM data and repair
transmissions and serve several purposes, they:
1. Stimulate explicit feedback from the general receiver set to
collect congestion control information.
2. Communicate state to the receiver set on the sender's current
congestion control status including details of the CLR.
3. Initiate rapid (immediate) feedback from the CLR in order to
closely track the dynamics of congestion control for the current
worst path in the group multicast topology.
The format of the NORM_CMD(CC) message is described in Section 4.2.3
of this document. The NORM_CMD(CC) message contains information to
allow measurement of RTTs, to inform the group of the congestion
control CLR, and to provide feedback of individual RTT measurements
to the receivers in the group. The NORM_CMD(CC) also provides for
exciting feedback from OPTIONAL "potential limiting receiver" (PLR)
nodes that might be determined administratively or possibly
algorithmically based upon congestion control feedback. PLR nodes
are receivers that have been identified to have potential for
(perhaps soon) becoming the CLR and thus immediate, up-to-date
feedback is beneficial for congestion control performance. The PLR
list MAY be populated with a small number of receivers the sender
identifies as approaching the CLR loss and delay conditions based on
feedback from the group.
5.5.2.1. NORM_CMD(CC) Transmission
The NORM_CMD(CC) message is transmitted periodically by the sender
along with its normal data transmission. Note the repeated
transmission of NORM_CMD(CC) messages MAY be initiated some time
before transmission of user data content at session startup. This
can be done to collect some estimation of the current state of the
multicast topology with respect to group and individual RTT and
congestion control state.
A NORM_CMD(CC) message is immediately transmitted at sender startup.
The interval of subsequent NORM_CMD(CC) message transmission is
determined as follows:
1. By default, the interval is set according to the current sender
GRTT estimate. A startup initial value of GRTT_sender = 0.5
seconds is RECOMMENDED when no feedback has yet been received
from the group.
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RFC 5740 NORM Protocol November 2009
2. Until a CLR has been identified (based on previous receiver
feedback) or when no data transmission is pending, the
NORM_CMD(CC) interval is doubled up from its current interval to
a maximum of once per 30 seconds. This results in a low duty
cycle for NORM_CMD(CC) probing when no CLR is identified or there
is no pending data to transmit.
3. When a CLR has been identified (based on receiver feedback) and
data transmission is pending, the probing interval is set to the
RTT between the sender and the CLR (RTT_clr).
4. Additionally, when the data transmission rate is low with respect
to the RTT_clr interval used for probing, the implementation
SHOULD ensure no more than one NORM_CMD(CC) message is sent per
NORM_DATA message when there is data pending transmission. This
ensures the transmission of this control message is not done to
the exclusion of user data transmission.
The NORM_CMD(CC) "cc_sequence" field is incremented with each
transmission of a NORM_CMD(CC) command. The greatest "cc_sequence"
recently received by receivers is included in their feedback to the
sender. This allows the sender to determine the age of feedback to
assist in congestion avoidance.
The NORM-CC Rate Header Extension is applied to the NORM_CMD(CC)
message and the sender advertises its current transmission rate in
the "send_rate" field. The rate information is used by receivers to
initialize loss estimation during congestion control startup or
restart.
The "cc_node_list" contains a list of entries identifying receivers
and their current congestion control state (status "flags", "rtt",
and "loss" estimates). The list will be empty if the sender has not
yet received any feedback from the group. If the sender has received
feedback, the list will minimally contain an entry identifying the
CLR. A NORM_FLAG_CC_CLR flag value is provided for the "cc_flags"
field to identify the CLR entry. It is RECOMMENDED the CLR entry be
the first in the list for implementation efficiency. Additional
entries in the list are used to provide sender-measured individual
RTT estimates to receivers in the group. The number of additional
entries in this list is dependent upon the percentage of control
traffic the sender application is willing to send with respect to
user data message transmissions. More entries in the list will allow
the sender to be more responsive to congestion control dynamics. The
length of the list can be dynamically determined according to the
current transmission rate and scheduling of NORM_CMD(CC) messages.
The maximum length of the list corresponds to the sender's
NormSegmentSize parameter for the session. The inclusion of
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RFC 5740 NORM Protocol November 2009
additional entries in the list based on receiver feedback is
prioritized with the following rules:
1. Receivers that have not yet been provided an RTT measurement get
first priority. Of these, those with the greatest loss fraction
receive precedence for list inclusion.
2. Secondly, receivers that have previously been provided an RTT
measurement are included with receivers yielding the lowest
calculated congestion rate getting precedence.
There are "cc_flag" values in addition to NORM_FLAG_CC_CLR used for
other congestion control functions. The NORM_FLAG_CC_PLR flag value
is used to mark additional receivers from which the sender would like
to have immediate, non-suppressed feedback. These can be receivers
the sender algorithmically identified as potential future CLRs or
have been pre-configured as potential congestion control points in
the network. The NORM_FLAG_CC_RTT indicates the validity of the
"cc_rtt" field for the associated receiver node. Normally, this flag
will be set since the receivers in the list will typically be
receivers from which the sender has received feedback. However, in
the case the NORM sender has been pre-configured with a set of PLR
nodes, feedback from those receivers might not have yet been
collected and thus the "cc_rtt" field does not contain a valid value
when this flag is not set. Similarly, a value of ZERO for the
"cc_rate" field here MUST be treated as an invalid value and be
ignored for the purposes of feedback suppression, etc.
5.5.2.2. NORM_CMD(CC) Feedback Response
Receivers explicitly respond to NORM_CMD(CC) messages in the form of
a NORM_ACK(RTT) message. The goal of the congestion control feedback
is to determine the receivers with the lowest congestion control
rates. Receivers marked as CLR or PLR nodes in the NORM_CMD(CC)
"cc_node_list" immediately provide feedback in the form of a NORM_ACK
to this message. When a NORM_CMD(CC) is received, non-CLR or non-PLR
nodes initiate random feedback backoff timeouts similar to those used
when the receiver initiates a repair cycle (see Section 5.3) in
response to detection of data loss. The backoff timeout for the
congestion control response is generated as follows:
T_backoff = RandomBackoff(K_backoff * GRTT_sender, GSIZE_sender)
The RandomBackoff() algorithm provides a truncated exponentially
distributed random number and is described in the Multicast NACK
Building Block [RFC 5401] document. The same backoff factor,
K_backoff = K_sender, as used with NORM_NACK suppression is generally
RECOMMENDED. However, in cases where the application purposefully
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RFC 5740 NORM Protocol November 2009
specifies a very small K_sender backoff factor to minimize the NACK
repair process latency (trading off group size scalability), it is
RECOMMENDED a larger backoff factor for congestion control feedback
be maintained, since there can be a larger volume of congestion
control feedback than NACKs in many cases and some congestion control
feedback latency might be tolerable where reliable delivery latency
is not. As previously noted, a backoff factor value of K_sender = 4
is generally RECOMMENDED for ASM operation and K_sender = 6 for SSM
operation. A receiver SHALL cancel the backoff timeout and thus its
pending transmission of a NORM_ACK(RTT) message under the following
conditions:
1. The receiver generates another feedback message (NORM_NACK or
other NORM_ACK) before the congestion control feedback timeout
expires (these messages will convey the current congestion
control feedback information).
2. A NORM_CMD(CC) or other receiver feedback with an ordinally
greater "cc_sequence" field value is received before the
congestion control feedback timeout expires (this is similar to
the TFMCC feedback round number).
3. When the T_backoff is greater than 1*GRTT_sender. This prevents
NACK implosion in the event of sender or network failure.
4. "Suppressing" congestion control feedback is heard from another
receiver (in a NORM_ACK or NORM_NACK) or via a
NORM_CMD(REPAIR_ADV) message from the sender. The local
receiver's feedback is "suppressed" if the rate of the competing
feedback (Rfb) is sufficiently close to or less than the local
receiver's calculated rate (Rcalc). The local receiver's
feedback is canceled when Rcalc > (0.9 * Rfb). Also, note
receivers that have not yet received an RTT measurement from the
sender are suppressed only by other receivers that have not yet
measured RTT. Additionally, receivers whose RTT estimate has
aged considerably (i.e., they haven't been included in the
NORM_CMD(CC) "cc_node_list" in a long time) might wish to compete
as a receiver with no prior RTT measurement after some long-term
expiration period.
When the backoff timer expires, the receiver SHALL generate a
NORM_ACK(RTT) message to provide feedback to the sender and group.
This message MAY be multicast to the group for most effective
suppression in ASM topologies or unicast to the sender depending upon
how the NORM protocol is deployed and configured.
Whenever any feedback is generated (including this NORM_ACK(RTT)
message), receivers include an adjusted version of the sender
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RFC 5740 NORM Protocol November 2009
timestamp from the most recently received NORM_CMD(CC) message and
its "cc_sequence" value in the corresponding NORM_ACK or NORM_NACK
message fields. For NORM-CC operation, any generated feedback
message SHALL also contain the NORM-CC Feedback header extension.
The receiver provides its current "cc_rate" estimate, "cc_loss"
estimate, "cc_rtt" if known, and any applicable "cc_flags" via this
header extension.
During slow start (when the receiver has not yet detected loss from
the sender), the receiver uses a value equal to two times its
measured rate from the sender in the "cc_rate" field. For steady-
state congestion control operation, the receiver "cc_rate" value is
from the equation-based value using its current loss event estimate
and sender<->receiver RTT information. (The GRTT_sender is used when
the receiver has not yet measured its individual RTT.)
The "cc_loss" field value reflects the receiver's current loss event
estimate with respect to the sender in question.
When the receiver has a valid individual RTT measurement, it SHALL
include this value in the "cc_rtt" field. The NORM_FLAG_CC_RTT MUST
be set when the "cc_rtt" field is valid.
After a congestion control feedback message is generated or when the
feedback is suppressed, a non-CLR receiver begins a "holdoff" timeout
period during which it will restrain itself from providing congestion
control feedback, even if NORM_CMD(CC) messages are received from the
sender (unless the receive becomes marked as a CLR or PLR node). The
value of this holdoff timeout (T_ccHoldoff) period is:
T_ccHoldoff = (K_sender * GRTT_sender)
Thus, non-CLR receivers are constrained to providing explicit
congestion control feedback once per K_sender*GRTT_sender intervals.
However, as the session progresses, different receivers will be
responding to different NORM_CMD(CC) messages and there will be
relatively continuous feedback of congestion control information
while the sender is active.
5.5.2.3. Congestion Control Rate Adjustment
During steady-state operation, the sender will directly adjust its
transmission rate to the rate indicated by the feedback from its
currently selected CLR. As noted in [TfmccPaper], the estimation of
parameters (loss and RTT) for the CLR will generally constrain the
rate changes possible within acceptable bounds. For rate increases,
the sender SHALL observe a maximum rate of increase of one packet per
RTT at all times during steady-state operation.
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RFC 5740 NORM Protocol November 2009
The sender processes congestion control feedback from the receivers
and selects the CLR based on the lowest rate receiver. Receiver
rates are determined either directly from the slow start "cc_rate"
provided by the receiver in the NORM-CC Feedback header extension or
by performing the equation-based calculation using individual RTT and
loss estimates ("cc_loss") as feedback is received.
The sender can calculate a current RTT for a receiver (RTT_rcvrNew)
using the "grtt_response" timestamp included in feedback messages.
When the "cc_rtt" value in a response is not valid, the sender simply
uses this RTT_rcvrNew value as the receiver's current RTT (RTT_rcvr).
For non-CLR and non-PLR receivers, the sender SHOULD use the "cc_rtt"
provided in the NORM-CC Feedback header extension as the receiver's
previous RTT measurement (RTT_rcvrPrev) averaged with the current
measurement ("RTT_rcvrNew") as the receiver's RTT value:
RTT_rcvr = 0.5 * RTT_rcvrPrev + 0.5 * RTT_rcvrNew
For CLR receivers where feedback is received more regularly, the
sender SHOULD maintain a more smoothed RTT estimate upon new feedback
from the CLR where:
RTT_clr = 0.9 * RTT_clr + 0.1 * RTT_clrNew
RTT_clrNew is the new RTT calculated from the timestamp in the
feedback message received from the CLR. The RTT_clr is initialized
to RTT_clrNew on the first feedback message received. Note that the
same procedure is observed by the sender for PLR receivers, and if a
PLR is "promoted" to CLR status, the smoothed estimate can be
continued.
There are some additional periods besides steady-state operation to
be considered in NORM-CC operation. These periods are:
1. during session startup,
2. when no feedback is received from the CLR, and
3. when the sender has a break in data transmission.
During session startup, the congestion control operation SHALL
observe a "slow-start" procedure to quickly approach its fair
bandwidth share. An initial sender startup rate is assumed where:
Rinit = MIN(NormSegmentSize/GRTT_sender, NormSegmentSize) bytes/sec
The rate is increased only when feedback is received from the
receiver set. The "slow start" phase proceeds until any receiver
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RFC 5740 NORM Protocol November 2009
provides feedback indicating loss has occurred. Rate increase during
slow start is applied as:
Rnew = Rrecv_min
where Rrecv_min is the minimum reported receiver rate in the
"cc_rate" field of congestion control feedback messages received from
the group. Note during slow start, receivers use two times their
measured rate from the sender in the "cc_rate" field of their
feedback. Rate increase adjustment is limited to once per GRTT
during slow start.
If the CLR or any receiver intends to leave the group, it will set
the NORM_FLAG_CC_LEAVE in its congestion control feedback message as
an indication the sender SHOULD NOT select it as the CLR. When the
CLR changes to a lower rate receiver, the sender SHOULD immediately
adjust to the new lower rate. The sender is limited to increasing
its rate at one additional packet per RTT towards any new, higher CLR
rate.
The sender SHOULD also track the age of the feedback it has received
from the CLR by comparing its current "cc_sequence" value
(Seq_sender) to the last "cc_sequence" value received from the CLR
(Seq_clr). As the age of the CLR feedback increases with no new
feedback, the sender SHALL begin reducing its rate once per RTT_clr
as a congestion avoidance measure. The following algorithm is used
to determine the decrease in sender rate (Rsender bytes/sec) as the
CLR feedback, unexpectedly, excessively ages:
Age = Seq_sender - Seq_clr;
if (Age > 4) Rsender = Rsender * 0.5;
This rate reduction is limited to the lower bound on NORM
transmission rates. After NORM_ROBUST_FACTOR consecutive
NORM_CMD(CC) rounds without any feedback from the CLR, the sender
SHOULD assume the CLR has left the group and pick the receiver with
the next lowest rate as the new CLR. Note this assumes the sender
does not have explicit knowledge the CLR intentionally left the
group. If no receiver feedback is received, the sender MAY wish to
withhold further transmissions of NORM_DATA segments and maintain
NORM_CMD(CC) transmissions only until feedback is detected. After
such a CLR timeout, the sender will be transmitting with a minimal
rate and SHOULD return to slow start as described here for a break in
data transmission.
When the sender has a break in its data transmission, it can continue
to probe the group with NORM_CMD(CC) messages to maintain RTT
collection from the group. This will enable the sender to quickly
determine an appropriate CLR upon data transmission restart.
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RFC 5740 NORM Protocol November 2009
However, the sender SHOULD exponentially reduce its target rate to be
used for transmission restart as time since the break elapses. The
target rate SHOULD be recalculated once per RTT_clr as:
Rsender = Rsender * 0.5;
If the minimum NORM rate is reached, the sender SHOULD set the
NORM_FLAG_START flag in its NORM_CMD(CC) messages upon restart and
the group SHOULD observe slow-start congestion control procedures
until any receiver experiences a new loss event.
5.5.3. NORM Positive Acknowledgment Procedure
NORM provides options for the source application to request positive
acknowledgment (ACK) of NORM_CMD(FLUSH) and NORM_CMD(ACK_REQ)
messages from members of the group. There are some specific
acknowledgment requests defined for the NORM protocol and a range of
acknowledgment request types left to be defined by the application.
One predefined acknowledgment type is the NORM_ACK(FLUSH) type. This
acknowledgment is used to determine if receivers have achieved
completion of reliable reception up through a specific logical
transmission point with respect to the sender's sequence of
transmission. The NORM_ACK(FLUSH) acknowledgment MAY be used to
assist in application flow control when the sender has information on
a portion of the receiver set. Another predefined acknowledgment
type is NORM_ACK(CC) used to explicitly provide congestion control
feedback in response to NORM_CMD(CC) messages transmitted by the
sender for NORM-CC operation. Note the NORM_ACK(CC) response does
NOT follow the positive acknowledgment procedure described here. The
NORM_CMD(ACK_REQ) and NORM_ACK messages contain an "ack_type" field
to identify the type of acknowledgment requested and provided. A
range of "ack_type" values is provided for application-defined use.
While the application is responsible for initiating the
acknowledgment request and interprets application-defined "ack_type"
values, the acknowledgment procedure SHOULD be conducted within the
protocol implementation to take advantage of timing and transmission
scheduling information available to the NORM transport.
The NORM Positive Acknowledgment Procedure uses polling by the sender
to query the receiver group for response. Note this polling
procedure is not intended to scale to very large receiver groups, but
could be used in a large group setting to query a critical subset of
the group. Either the NORM_CMD(ACK_REQ), or when applicable, the
NORM_CMD(FLUSH) message is used for polling and contains a list of
NormNodeIds of the receivers expected to respond to the command. The
list of receivers providing acknowledgment is determined by the
source application with a priori knowledge of participating nodes or
via some other application-level mechanism.
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RFC 5740 NORM Protocol November 2009
The ACK process is initiated by the sender generating NORM_CMD(FLUSH)
or NORM_CMD(ACK_REQ) messages in periodic rounds. For
NORM_ACK(FLUSH) requests, the NORM_CMD(FLUSH) contains a
"object_transport_id" and "fec_payload_id" denoting the watermark
transmission point for which acknowledgment is requested. This
watermark transmission point is echoed in the corresponding fields of
the NORM_ACK(FLUSH) message sent by the receiver in response.
NORM_CMD(ACK_REQ) messages contain an "ack_id" field that is
similarly echoed in response so the sender can match the response to
the appropriate request.
In response to the NORM_CMD(ACK_REQ), the listed receivers randomly,
with a uniform distribution, transmit NORM_ACK messages over a time
window of (1*GRTT_sender). These NORM_ACK messages are typically
unicast to the sender. (Note NORM_ACK(CC) messages SHALL be
multicast or unicast in the same manner as NORM_NACK messages.)
The ACK process is self-limiting and avoids ACK implosion because:
1. Only a single NORM_CMD(ACK_REQ) message is generated once per
(2*GRTT_sender), and
2. The size of the "acking_node_list" of NormNodeIds from which
acknowledgment is requested is limited to a maximum of the sender
NormSegmentSize setting per round of the positive acknowledgment
process.
Because the size of the included list is limited to the sender's
NormSegmentSize setting, multiple NORM_CMD(ACK_REQ) rounds will
sometimes be necessary to achieve responses from all receivers
specified. The content of the attached NormNodeId list will be
dynamically updated as this process progresses and NORM_ACK responses
are received from the specified receiver set. As the sender receives
valid responses (i.e., matching watermark point or "ack_id") from
receivers, it SHALL eliminate those receivers from the subsequent
NORM_CMD(ACK_REQ) message "acking_node_list" and add in any pending
receiver NormNodeIds while keeping within the NormSegmentSize
limitation of the list size. Each receiver is queried a maximum
number of times (NORM_ROBUST_FACTOR, by default). Receivers not
responding within this number of repeated requests are removed from
the payload list to make room for other potential receivers pending
acknowledgment. The transmission of the NORM_CMD(ACK_REQ) is
repeated until no further responses are needed or until the repeat
threshold is exceeded for all pending receivers. The transmission of
NORM_CMD(ACK_REQ) or NORM_CMD(FLUSH) messages to conduct the positive
acknowledgment process is multiplexed with ongoing sender data
transmissions. However, the NORM_CMD(FLUSH) positive acknowledgment
process MAY be interrupted in response to negative acknowledgment
Adamson, et al. Standards Track PAGE 76
RFC 5740 NORM Protocol November 2009
repair requests (NACKs) received from receivers during the
acknowledgment period. The NORM_CMD(FLUSH) positive acknowledgment
process is restarted for receivers pending acknowledgment once any
the repairs have been transmitted.
In the case of NORM_CMD(FLUSH) commands with an attached
"acking_node_list", receivers will not ACK until they have received
complete transmission of all data up to and including the given
watermark transmission point. All receivers SHALL interpret the
watermark point provided in the request NACK for repairs if needed as
for NORM_CMD(FLUSH) commands with no attached "acking_node_list".
5.5.4. Group Size Estimate
NORM sender messages contain a "gsize" field that is a representation
of the group size and that is used in scaling random backoff timer
ranges. The use of the group size estimate within the NORM protocol
does not demand a precise estimation and works reasonably well if the
estimate is within an order of magnitude of the actual group size.
By default, the NORM sender group size estimate MAY be
administratively configured. Also, given the expected scalability of
the NORM protocol for general use, a default value of 10,000 is
RECOMMENDED for use as the group size estimate. It is also possible
the group size MAY be algorithmically approximated from the volume of
congestion control feedback messages based on the exponentially
weighted random backoff. However, the specification of such an
algorithm is currently beyond the scope of this document.
6. Configurable Elements
The NORM protocol supports a modest number of configurable parameters
that control operation. Most of these need only be set at NORM
sender(s) and the configuration information is communicated to the
receiver set in NORM header and/or header extension fields. A
notable exception to this is the NORM_ROBUST_FACTOR that is presumed
to be a common value preset among senders and receivers for a given
NORM session. The following table summarizes these configurable
elements:
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RFC 5740 NORM Protocol November 2009
+--------------------+----------------------------------------------+
| Configurable | Purpose |
| Element | |
+--------------------+----------------------------------------------+
| Sender initial | Sender's initial estimate of greatest group |
| GRTT Estimate | round-trip time. Affects timing of feedback |
| (GRTT_sender) | suppression and sender command transmissions |
| | at sender startup. |
| Backoff Factor | Sender's scaling factor used for timer-based |
| (K_sender) | feedback suppression. |
| Group Size | Sender's rough estimate of receiver group |
| Estimate | size used in generation of random feedback |
| (GSIZE_sender) | backoff timeout. |
| NORM_ROBUST_FACTOR | Integer factor determining how persistently |
| | (i.e., robust) senders transmit repeated |
| | control messages and receivers self-initiate |
| | timeout-based NACKing in the absence of |
| | sender activity. |
| FEC Type | Sender FEC encoding type. |
| ("fec_id") | |
| Sender segment | Maximum size (in bytes) of the payload |
| size | portion of NORM_DATA and other messages. |
| (NormSegmentSize) | |
| NormNodeId | Unique identifiers pre-assigned to all NORM |
| | session participants. |
+--------------------+----------------------------------------------+
The sender-controlled GRTT estimate (referred to as GRTT_sender in
this document) is used to set and scale various timers associated
with NORM protocol operation. During steady-state operation, the
sender probes the receiver set, adapts to the group round-trip timing
state, and advertises its estimate to the receiver set in the "grtt"
field of relevant NORM protocol messages. However, an initial value
must be assumed at sender startup. A large initial estimate is
conservative and safer with regard to preventing feedback implosion
and starting up congestion control operation, but requires the sender
and receivers to allocate more buffering resources for a given
transmission rate (i.e., larger effective delay*bandwidth product) to
maintain efficient operation. A default initial value of GRTT_sender
= 0.5 seconds is RECOMMENDED.
The sender-controlled Backoff Factor (referred to a K_sender in this
document) is used to scale protocol timers and contributes to the
generation of the random backoff timeout value that facilitates
timer-based feedback suppression. The sender advertises its
configured Backoff Factor to the receiver set in the "backoff" field
of applicable NORM messages and thus no receiver configuration is
necessary. For ASM operation, a default value of K_sender = 4 is
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RFC 5740 NORM Protocol November 2009
RECOMMENDED; for SSM operation, a default value of K_sender = 6 is
RECOMMENDED.
The sender estimate of session Group Size (referred to as
GSIZE_sender in this document) also plays a role in the random
selection of feedback suppression timeout values. The sender
advertises its configured Group Size estimate to the receiver set in
the "gsize" field of applicable NORM messages; thus, no receiver
configuration is necessary. Only a rough estimate (i.e., "order-of-
magnitude") is needed for effective feedback suppression and a
default value of GSIZE_sender = 10,000 is RECOMMENDED as a
conservative estimate for most uses.
The NORM_ROBUST_FACTOR is an integer parameter that determines how
persistently NORM senders transmit control messages (NORM_CMD
messages) such as end-of-transmission flushing, OPTIONAL positive
acknowledgment requests, etc. Additionally, the receivers use their
knowledge of NORM_ROBUST_FACTOR to determine when to consider a NORM
sender inactive and MAY use the factor in determining how
persistently to self-initiate repeated NACK repair requests upon such
timeouts. This parameter is NOT communicated in NORM protocol
message headers and is presumed to be preset to a consistent value
among sender and receivers for a given NORM session. A default value
of NORM_ROBUST_FACTOR = 20 is RECOMMENDED.
Another NORM sender configuration element is the FEC type used to
encode NORM_DATA message content. The FEC type is communicated from
the sender to the receiver set in the "fec_id" field of relevant NORM
message headers. The "fec_id" value corresponds to an IANA-assigned
value identifying the FEC encoding type as described in the FEC
Building Block [RFC 5052] document. Typically, a sender SHOULD use a
consistent FEC encoding for its participation in a session to
simplify receiver state allocation and maintenance, but its
implementations MAY vary the FEC encoding type on a per-object basis
if necessary.
The sender NormSegmentSize setting determines the maximum size of the
payload portion of NORM_DATA and other messages that the sender
transmits. Additionally, the payload size of feedback messages from
receivers to a given sender is limited to that sender's
NormSegmentSize. The NormSegmentSize SHOULD be configured to be
compatible with expected network MTU limitations, given the added
overhead of NORM, UDP, and IP protocol message headers.
Additionally, MTU Discovery MAY be employed by the sender to
determine an appropriate NormSegmentSize. The NormSegmentSize for a
given sender can be determined by receivers from the FEC Object
Transmission Information (FTI) provided either in applied EXT_FTI
header extensions or pre-configured session information.
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RFC 5740 NORM Protocol November 2009
Although it is not technically a configurable element, the receivers
MUST have FEC Object Transmission Information for transmitted
NormObjects to properly buffer, decode, and reassemble the original
content. For loosely organized NORM protocol sessions, the sender
MAY apply the EXT_FTI Header Extension to NORM_DATA and NORM_INFO (if
applicable) messages so that receivers can get this information
without prior coordination. An implementation MAY also apply the
EXT_FTI only to NORM_INFO messages for reduced overhead. Finally,
applications MAY also provide the FTI out-of-band prior to sender
transmission.
Each participant in a NORM protocol session MUST be configured with a
unique NormNodeId value. The NormNodeId value is used by receivers
to identify the sender to which their NACK or other feedback messages
are addressed, and senders use the NormNodeId to differentiate
receivers for purposes of congestion control and OPTIONAL positive
acknowledgment collection. Assignment of unique NormNodeId values
can be done via a priori coordination and/or use of a deconfliction
mechanism external to the NORM protocol itself. The values of
NORM_NODE_NONE = 0x00000000 and NORM_NODE_ANY = 0xffffffff are
reserved and MUST NOT be assigned to NORM participants.
7. Security Considerations
The same security considerations that apply to the Multicast NACK
[RFC 5401], TFMCC [RFC 4654], and FEC [RFC 5052] Building Blocks also
apply to the NORM protocol. In addition to the vulnerabilities to
which any IP and IP multicast protocol implementation is subject,
malicious hosts might engage in excessive NACKing in an attempt to
prevent the NORM sender(s) from making forward progress in reliable
transmission. Receiver "join" and "service" policy enforcement as
described in Section 5.2 can be applied if such activity is detected.
The use of cryptographic peer authentication, integrity checks,
and/or confidentiality mechanisms can be used to provide a more
effective degree of protection from objectionable transmissions from
unauthorized hosts. But in some cases, even with authentication and
integrity checks, the NACK-based feedback of NORM can be exploited by
replay attacks forcing the NORM sender to unnecessarily transmit
repair information. This MAY be addressed in part with network-layer
IP security implementations that guard against this potential
security exploitation or alternatively with a security mechanism
using the EXT_AUTH header extension for similar purposes. Such
security mechanisms SHOULD be deployed and used when available. Use
of security mechanisms will impose additional "a priori"
configuration upon the NORM deployment depending upon the techniques
used.
The NORM protocol is compatible with the use of IP security (IPsec)
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RFC 5740 NORM Protocol November 2009
[RFC 4301], and the IPsec Encapsulating Security Payload (ESP)
protocol or Authentication Header (AH) extension can be used to
secure IP packets transmitted by NORM participants. A baseline
approach to secure NORM operation using IPsec is described below.
Compliant implementations of this specification are REQUIRED to be
compatible with IPsec usage as described in Section 7.1. IPsec can
be used to provide peer authentication, integrity protection, and/or
encryption of packets containing NORM messages.
Additionally, the EXT_AUTH header extension (HET = 1) is reserved for
use by security mechanisms to provide alternatives to IPsec for the
security of NORM messages. The format of this header extension and
its processing is outside the scope of this document and is to be
communicated out-of-band as part of the session description. It is
possible an EXT_AUTH implementation MAY also provide for encryption
of NORM message payloads as well as peer authentication and integrity
protection. The use of this approach as compared to IPsec can allow
for header compression techniques to be applied jointly to IP and
NORM protocol headers. In cases where security analysis deems
encryption of NORM protocol header content to be beneficial or
necessary, the aforementioned use of IPsec ESP might be more
appropriate. Additionally, the EXT_AUTH header extension can be
utilized when NORM is implemented in a network with Network Address
Translation (NAT) systems that are incompatible with use of the IPsec
AH extension. If EXT_AUTH is present, whatever packet authentication
or integrity checks that can be performed immediately upon reception
of the packet MUST be performed before accepting the packet and
performing any congestion-control-related action on it. Some packet
authentication schemes impose a delay of several seconds between when
a packet is received and when the packet can be fully authenticated.
Any appropriate congestion control related action MUST NOT be
postponed by any such packet security mechanism (i.e., security
mechanisms MUST NOT result in poor congestion control behavior).
Consideration MUST also be given to the potential for replay-attacks
that would transplant authenticated packets from one NORM session to
another to disrupt service. To avoid this potential, unique keys
SHOULD be assigned on a per-session basis or NORM sender nodes SHOULD
be configured to use unique "instance_id" identifiers managed as part
of the security association for the sessions.
Note NORM implementations can use the "sequence" field from the NORM
common message header to detect replay attacks. This can be
accomplished if the NORM sender maintains state on actively NACKing
receivers. A cache of such receiver state can be used to provide
protection against NACK replay attacks. NORM receivers MUST also
maintain similar state for protection against possible replay of
other receiver messages in ASM operation as well. For example, a
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RFC 5740 NORM Protocol November 2009
receiver could be suppressed from providing NACK or congestion
control feedback by replay of certain receiver messages. For these
reasons, authentication of NORM messages (e.g., via IPsec) SHOULD be
applied for protection against similar attacks that use fabricated
messages. Also, encryption of messages to provide confidentiality of
application data and protect privacy of users MAY also be applied
using IPsec or similar mechanisms.
When applicable security measures are used, automated key management
mechanisms such as those described in the Group Domain of
Interpretation (GDOI) [RFC 3547], Multimedia Internet KEYing (MIKEY)
[RFC 3830], or Group Secure Association Key Management Protocol
(GSAKMP) [RFC 4535] specifications SHOULD be applied.
While NORM does leverage FEC-based repair for scalability, this alone
does not guarantee integrity of received data. Application-level
integrity-checking of received data content is highly RECOMMENDED.
This recommendation also applies when the IPsec security approach
described below is used for added assurance in data content integrity
given the shared use of IPsec Security Association information among
the group.
7.1. Baseline Secure NORM Operation
This section describes a baseline mode of secure NORM protocol
operation based on application of the IPsec security protocol. This
approach is documented here to provide a baseline interoperable
secure mode of operation. This particular approach represents one
possible trade-off in the level of assurance that can be achieved and
the scalability of multicast group-size given current IPsec
mechanisms and the state required to support them. For example, this
baseline approach specifies the use of a Security Association that is
shared among the receiver set for feedback messages to the sender.
This model requires that the receiver membership receiving the
session keys is trusted and only provides protection from attacks
that are external to the NORM group membership. More stateful and
complex IPsec approaches and key management schemes may be applied
for higher levels of assurance, but those are beyond the scope of
this transport protocol specification. Additional approaches to NORM
security, including other forms of IPsec application, MAY be
specified in the future. For example, the use of the EXT_AUTH header
extension could enable NORM-specific authentication or security
encapsulation headers similar to those of IPsec to be specified and
inserted into the NORM protocol message headers. This would allow
header compression techniques to be applied to IP and NORM protocol
headers when needed in a similar fashion to RTP [RFC 3550] and as
preserved in the specification for Secure Real Time Protocol (SRTP)
[RFC 3711].
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RFC 5740 NORM Protocol November 2009
The baseline approach described is applicable to NORM operation
configured for SSM (or SSM-like) operation where there is a single
sender and the receivers are providing unicast feedback. This form
of NORM operation allows for IPsec to be used with a manageable
number of security associations (SA).
7.1.1. IPsec Approach
For NORM one-to-many SSM operation with unicast feedback from
receivers, each node SHALL be configured with two transport mode
IPsec security associations and corresponding Security Policy
Database (SPD) entries. One entry will be used for sender-to-group
multicast packet authentication and optionally encryption while the
other entry will be used to provide security for the unicast feedback
messaging from the receiver(s) to the sender. Note that this single
SA for NORM receiver feedback messages is shared to protect traffic
from possibly multiple receivers to the single sender.
For each NormSession, the NORM sender SHALL use an IPsec SA
configured for ESP protocol [RFC 4303] operation with the option for
data origin authentication enabled. It is also RECOMMENDED this
IPsec ESP SA be also configured to provide confidentiality protection
for IP packets containing NORM protocol messages. This is suggested
to make the realization of complex replay attacks much more
difficult. The encryption key for this SA SHALL be preplaced at the
sender and receiver(s) prior to NORM protocol operation. Use of
automated key management is RECOMMENDED as a rekey SHALL be REQUIRED
prior to expiration of the sequence space for the SA. This is
necessary so receivers can use the built-in IPsec replay attack
protection possible for an IPsec SA with a single source (the NORM
sender). Thus, the receivers SHALL enable replay attack protection
for this SA used to secure NORM sender traffic. An IPsec SPD entry
MUST be configured to process outbound packets to the session
(destination) address and UDP port number of the applicable
(NormSession).
The NORM receiver(s) MUST be configured with the SA and SPD entry to
properly process the IPsec-secured packets from the sender. The NORM
receiver(s) SHALL also use a common, second IPsec SA (common Security
Parameter Index (SPI) and encryption key) configured for ESP
operation with the option for data origination authentication
enabled. Similar to the NORM sender, is RECOMMENDED this IPsec ESP
SA be also configured to provide confidentiality protection for IP
packets containing NORM protocol messages. The receivers MUST have
an IPsec SPD entry configured to process outbound NORM/UDP packets
directed to the NORM sender source address and port number using this
second SA. To support NORM unicast feedback, the sender's
transmission port number SHOULD be selected to be distinct from the
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RFC 5740 NORM Protocol November 2009
multicast session port number to allow discrimination between unicast
and multicast feedback messages when access to the IP destination
address is not possible (e.g., a user-space NORM implementation).
For processing of packets from receivers, the NORM sender SHALL be
configured with this common, second SA (and the corresponding SPD
entry needed) in order to properly process messages from the
receiver.
Multiple receivers using a common IPsec SA for traffic directed to
the NORM sender (i.e., many-to-one) typically prevents the use of
built-in IPsec replay attack protection by the NORM sender with
current IPsec implementations. Thus the built-in IPsec replay attack
protection for this second SA at the sender MUST be disabled unless
the particular IPsec implementation manages its replay protection on
a per-source basis (which is not typical of existing IPsec
implementations). So, to support a fully secure mode of operation,
the NORM sender implementation MUST provide replay attack protection
based upon the "sequence" field of NORM protocol messages from
receivers. This can be accomplished with a high assurance of
security, even with the limited size (16-bits) of this field,
because:
1. NORM receiver NACK and non-CLR ACK feedback messages are sparse.
2. The more frequent NORM_ACK feedback from CLR or PLR nodes is only
a small set of receivers for which the sender needs to keep more
persistent replay attack state.
3. NORM_NACK feedback messages preceding the sender's current repair
window do not significantly impact protocol operation (generation
of NORM_CMD(SQUELCH) is limited) and could be in fact ignored.
This means the sender can prune any replay attack state that
precedes the current repair window.
4. NORM_ACK messages correspond to either a specific sender
"ack_id", the sender "cc_sequence" for ACKs sent in response to
NORM_CMD(CC), or the sender's current repair window in the case
of ACKs sent in response to NORM_CMD(FLUSH). Thus, the sender
can prune any replay attack state for receivers that precede the
current applicable sequence or repair window space.
The use of ESP confidentiality for secure NORM protocol operation
makes it more difficult for adversaries to conduct any form of replay
attacks. Additionally, a NORM sender implementation with access to
the full ESP protocol header could also use the ESP sequence
information to make replay attack protection even more robust by
maintaining the per-source ESP sequence state that existing IPsec
implementations typically do not provide. The design of this
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RFC 5740 NORM Protocol November 2009
baseline security approach for NORM intentionally places any more
complex processing state or processing (e.g., replay attack
protection given multiple receivers) at the NORM sender since NORM
receiver implementations might often need to be less complex.
This baseline approach can be used for NORM protocol sessions with
multiple senders if the SA pairs described are established for each
sender. For small-sized groups, it is even possible many-to-many
(ASM) IPsec configuration could be achieved where each participant
uses a unique SA (with a unique SPI). In this case, the sender(s)
would maintain an SA for each other participant rather than a single,
shared SA for receiver feedback messages. This does not scale to
larger group sizes given the complex set of SA and SPD entries each
participant would need to maintain.
It is anticipated in early deployments of this baseline approach to
NORM security that key management will be conducted out-of-band with
respect to NORM protocol operation. In the case of one-to-many NORM
operation, it is possible receivers will retrieve keying information
from a central server as needed or otherwise conduct group key
updates with a similar centralized approach. Alternatively, it is
possible with some key management schemes for rekey messages to be
transmitted to the group as a message or transport object within the
NORM reliable transfer session. Similarly, for group-wise
communication sessions, it is possible for potential group
participants to request keying and/or rekeying as part of NORM
communications. Additional specification is necessary to define an
in-band key management scheme for NORM sessions perhaps using the
mechanisms of the automated group key management specifications cited
in this document. Additional specification outside of the scope of
this document would be needed to provide an interoperable approach
for key management in-band of a NORM reliable transport session.
7.1.2. IPsec Requirements
In order to implement this secure mode of NORM protocol operation,
the following IPsec capabilities are REQUIRED.
7.1.2.1. Selectors
The implementation MUST be able to use the source address,
destination address, protocol (UDP), and UDP port numbers as
selectors in the SPD.
7.1.2.2. Mode
IPsec in transport mode MUST be supported. The use of IPsec
[RFC 4301] processing for secure NORM traffic MUST be configured such
Adamson, et al. Standards Track PAGE 85
RFC 5740 NORM Protocol November 2009
that unauthenticated packets are not received by the NORM protocol
implementation.
7.1.2.3. Key Management
An automated key management scheme for group key distribution and
rekeying such as GDOI [RFC 3547], GSAKMP [RFC 4535], or MIKEY [RFC 3830]
is RECOMMENDED for use. Note it is possible for key update messages
(e.g., the GDOI GROUPKEY-PUSH message) to be included as part of the
NORM application reliable data transmission if appropriate interfaces
are available between the NORM application and the key management
daemon. Relatively short-lived NORM sessions MAY be able to use
Manual Keying with a single, preplaced key, particularly if Extended
Sequence Numbering (ESN) [RFC 4303] is available in the IPsec
implementation used. When manual keys are used, it is important that
cryptographic algorithms suitable for manual key use are selected.
7.1.2.4. Security Policy
Receivers MUST accept protocol messages only from the designated,
authorized sender(s). Appropriate key management will provide
authentication, integrity and/or encryption keys only to receivers
authorized to participate in a designated session. The approach
outlined here allows receiver sets to be controlled on a per-sender
basis.
7.1.2.5. Authentication and Encryption
Large NORM group sizes will necessitate some form of key management
that does rely upon shared secrets. The GDOI and GSAKMP protocols
mentioned here allow for certificate-based authentication. It is
RECOMMENDED these certificates use IP addresses for authentication.
7.1.2.6. Availability
The IPsec requirements profile outlined here is commonly available on
many potential NORM hosts. Configuration and operation of IPsec
typically requires privileged user authorization. Automated key
management implementations are typically configured with the
privileges necessary to affect system IPsec configuration.
8. IANA Considerations
Values of NORM Header Extension Types, Stream Control Codes, and
NORM_CMD message sub-types are subject to IANA registration. They
are in the registry named "Reliable Multicast Transport (RMT) NORM
Protocol Parameters" available from http://www.iana.org.
Adamson, et al. Standards Track PAGE 86
RFC 5740 NORM Protocol November 2009
Note the reliable multicast building block components used by this
specification also have their respective IANA considerations, and
those documents SHOULD be consulted accordingly. In particular, the
FEC Building Block used by NORM does REQUIRE IANA registration of the
FEC codecs used. The registration instructions for FEC codecs are
provided in RFC 5052. It is possible additional extensions of the
NORM protocol might be specified in the future (e.g., additional NORM
message types) and additional registries be established at that time
with appropriate IETF standards action.
8.1. Explicit IANA Assignment Guidelines
This document introduces three registries for the NORM Header
Extension Types, Stream Control Codes, and NORM_CMD Message sub-
types. This section describes explicit IANA assignment guidelines
for each of these.
8.1.1. NORM Header Extension Types
This document defines a registry for NORM Header Extensions named
"NORM Header Extension Types".
The NORM Header Extension Type field is an 8-bit value. The values
of this field identify extended header content allowing the protocol
functionality to be expanded to include additional features and
operating modes. The values that can be assigned within the "NORM
Header Extensions" registry are numeric indexes in the range {0,
255}, boundaries included. Values in the range {0,127} indicate
variable-length extended header fields while values in the range
{128,255} indicate extensions of a fixed 4-byte length. This
specification registers the following NORM Header Extension Types:
+-------+----------+--------------------+
| Value | Name | Reference |
+-------+----------+--------------------+
| 1 | EXT_AUTH | This specification |
| 3 | EXT_CC | This specification |
| 64 | EXT_FTI | This specification |
| 128 | EXT_RATE | This specification |
+-------+----------+--------------------+
Requests for assignment of additional NORM Header Extension Type
values are granted on a "Specification Required" basis as defined by
IANA Guidelines [RFC 5226]. Any such header extension specifications
MUST include a description of protocol actions to be taken when the
extension type is encountered by a protocol implementation not
supporting that specific option. For example, it is often possible
for protocol implementations to ignore unknown header extensions.
Adamson, et al. Standards Track PAGE 87
RFC 5740 NORM Protocol November 2009
8.1.2. NORM Stream Control Codes
This document defines a registry for NORM Stream Control Codes named
"NORM Stream Control Codes".
NORM Stream Control Codes are 16-bit values that can be inserted
within a NORM_OBJECT_STREAM delivery object to convey sequenced, out-
of-band (with respect to the stream data) control signaling
applicable to the referenced stream object. These control codes are
to be delivered to the application or protocol implementation with
reliable delivery, in-order with respect to the their inserted
position within the stream. This specification registers the
following NORM Stream Control Code:
+-------+-----------------+--------------------+
| Value | Name | Reference |
+-------+-----------------+--------------------+
| 0 | NORM_STREAM_END | This specification |
+-------+-----------------+--------------------+
Additional NORM Stream Control Code value assignment requests are
granted on a "Specification Required" basis as defined by IANA
Guidelines [RFC 5226]. The full 16-bit space outside of the value
assigned in this specification are available for future assignment.
In addition to describing the control code's expected interpretation,
such specifications MUST include a description of protocol actions to
be taken when the control code is encountered by a protocol
implementation not supporting that specific option.
8.1.3. NORM_CMD Message Sub-Types
This document defines a registry for NORM_CMD message sub-types named
"NORM Command Message Sub-types".
The NORM_CMD message "sub-type" field is an 8-bit value with valid
values in the range of 1-255. Note the value 0 is reserved to
indicate an invalid NORM_CMD message sub-type. The current
specification defines a number of NORM_CMD message sub-types senders
can use to signal the receivers in various aspects of NORM protocol
operation. This specification registers the following NORM_CMD
Message Sub-types:
Adamson, et al. Standards Track PAGE 88
RFC 5740 NORM Protocol November 2009
+-------+-----------------------+--------------------+
| Value | Name | Reference |
+-------+-----------------------+--------------------+
| 0 | reserved | This specification |
| 1 | NORM_CMD(FLUSH) | This specification |
| 2 | NORM_CMD(EOT) | This specification |
| 3 | NORM_CMD(SQUELCH) | This specification |
| 4 | NORM_CMD(CC) | This specification |
| 5 | NORM_CMD(REPAIR_ADV) | This specification |
| 6 | NORM_CMD(ACK_REQ) | This specification |
| 7 | NORM_CMD(APPLICATION) | This specification |
+-------+-----------------------+--------------------+
Future specifications extending NORM MAY define additional NORM_CMD
messages to enhance protocol functionality. NORM_CMD message sub-
type value assignment requests are granted on a "Specification
Required" basis as defined by IANA Guidelines [RFC 5226]. In addition
to describing the command sub-type's expected interpretation,
specifications MUST include a description of protocol actions to be
taken when the command is encountered by a protocol implementation
not supporting that specific option.
This specification already defines an "application-defined" NORM_CMD
message sub-type for use at the discretion of individual applications
using NORM for transport. These "application-defined" commands are
suitable for many application-specific purposes and do not involve
standards action. In any case, such additional messages SHALL be
subject to the same congestion control constraints as the existing
NORM sender message set.
9. Suggested Use
The present NORM protocol is seen as a useful tool for the reliable
data transfer over generic IP multicast services. It is not the
intention of the authors to suggest it is suitable for supporting all
envisioned multicast reliability requirements. NORM provides a
simple and flexible framework for multicast applications with a
degree of concern for network traffic implosion and protocol overhead
efficiency. NORM-like protocols have been successfully demonstrated
within the MBone for bulk data dissemination applications, including
weather satellite compressed imagery updates servicing a large group
of receivers and a generic web content reliable "push" application.
In addition, this framework approach has some design features making
it attractive for bulk transfer in asymmetric and wireless
internetwork applications. NORM is capable of successfully operating
independent of network structure and in environments with high packet
loss, delay, and out-of-order delivery. Hybrid proactive/reactive
Adamson, et al. Standards Track PAGE 89
RFC 5740 NORM Protocol November 2009
FEC-based repairing improve protocol performance in some multicast
scenarios. A sender-only repair approach often makes additional
engineering sense in asymmetric networks. NORM's unicast feedback
capability is suitable for use in asymmetric networks or in networks
where only unidirectional multicast routing/delivery service exists.
Asymmetric architectures supporting multicast delivery are likely to
make up an important portion of the future Internet structure (e.g.,
direct broadcast satellite (DBS) or cable and public-switched
telephone network (PSTN) hybrids, etc.) and efficient, reliable bulk
data transfer will be an important capability for servicing large
groups of subscribed receivers.
10. Changes from RFC 3940
This section lists the changes between the Experimental version of
this specification, RFC 3940, and this version:
1. Removal of the NORM_FLAG_MSG_START for NORM_OBJECT_STREAM,
replacing it with the "payload_msg_start" field in the FEC-
encoded preamble of the NORM_OBJECT_STREAM NORM_DATA payload.
2. Definition of IANA registry for header extension and other
assignments.
3. Removal of file blocking scheme description now specified in the
FEC Building Block document [RFC 5052].
4. Removal of restriction of NORM receiver feedback message rate to
local NORM sender rate (this caused congestion control failures
in high speed operation. The extremely low feedback rate of the
NORM protocol as compared to TCP avoids any resultant impact to
the network as shown in [Mdpcc].)
5. Correction of errors in some message format descriptions.
6. Correction of inconsistency in specification of the inactivity
timeout.
7. Addition of IPsec secure mode description with IPsec
requirements.
8. Addition of the EXT_AUTH header extension definition.
9. Clarification of interpretation of "Source Block Length" when FEC
codes are arbitrarily shortened by the sender.
Adamson, et al. Standards Track PAGE 90
RFC 5740 NORM Protocol November 2009
11. Acknowledgments
(and these are not Negative)
The authors would like to thank Rick Jones, Vincent Roca, Rod Walsh,
Toni Paila, Michael Luby, and Joerg Widmer for their valuable input
and comments on this document. The authors would also like to thank
the RMT working group chairs, Roger Kermode and Lorenzo Vicisano, for
their support in development of this specification, and Sally Floyd
for her early input into this document.
12. References
12.1. Normative References
[RFC 1112] Deering, S., "Host extensions for IP multicasting",
STD 5, RFC 1112, August 1989.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC 4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC 4607] Holbrook, H. and B. Cain, "Source-Specific Multicast
for IP", RFC 4607, August 2006.
[RFC 4654] Widmer, J. and M. Handley, "TCP-Friendly Multicast
Congestion Control (TFMCC): Protocol Specification",
RFC 4654, August 2006.
[RFC 5052] Watson, M., Luby, M., and L. Vicisano, "Forward
Error Correction (FEC) Building Block", RFC 5052,
August 2007.
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 5226, May 2008.
[RFC 5401] Adamson, B., Bormann, C., Handley, M., and J.
Macker, "Multicast Negative-Acknowledgment (NACK)
Building Blocks", RFC 5401, November 2008.
Adamson, et al. Standards Track PAGE 91
RFC 5740 NORM Protocol November 2009
12.2. Informative References
[FecHybrid] Gossink, D. and J. Macker, "Reliable Multicast and
Integrated Parity Retransmission with Channel
Estimation", IEEE GLOBECOMM, 1998.
[McastFeedback] Nonnenmacher, J. and E. Biersack, "Optimal Multicast
Feedback", IEEE INFOCOM, p. 964, March/April 1998.
[MdpToolkit] Macker, J. and B. Adamson, "The Multicast
Dissemination Protocol (MDP) Toolkit", Proc.
IEEE MILCOM, October 1999.
[Mdpcc] Adamson, B. and J. Macker, "A TCP-Friendly, Rate-
based Mechanism for NACK-Oriented Reliable Multicast
Congestion Control", Proc. IEEE GLOBECOMM,
November 2001.
[NormFeedback] Adamson, B. and J. Macker, "Quantitative Prediction
of NACK-Oriented Reliable Multicast (NORM)
Feedback", IEEE MILCOM, October 2002.
[PgmccPaper] Rizzo, L., "pgmcc: A TCP-Friendly Single-Rate
Multicast Congestion Control Scheme", ACM SIGCOMM,
August 2000.
[RFC 2357] Mankin, A., Romanov, A., Bradner, S., and V. Paxson,
"IETF Criteria for Evaluating Reliable Multicast
Transport and Application Protocols", RFC 2357,
June 1998.
[RFC 2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[RFC 3048] Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
Floyd, S., and M. Luby, "Reliable Multicast
Transport Building Blocks for One-to-Many Bulk-Data
Transfer", RFC 3048, January 2001.
[RFC 3269] Kermode, R. and L. Vicisano, "Author Guidelines for
Reliable Multicast Transport (RMT) Building Blocks
and Protocol Instantiation documents", RFC 3269,
April 2002.
[RFC 3453] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L.,
Handley, M., and J. Crowcroft, "The Use of Forward
Error Correction (FEC) in Reliable Multicast",
RFC 3453, December 2002.
Adamson, et al. Standards Track PAGE 92
RFC 5740 NORM Protocol November 2009
[RFC 3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney,
"The Group Domain of Interpretation", RFC 3547,
July 2003.
[RFC 3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC 3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E.,
and K. Norrman, "The Secure Real-time Transport
Protocol (SRTP)", RFC 3711, March 2004.
[RFC 3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,
and K. Norrman, "MIKEY: Multimedia Internet KEYing",
RFC 3830, August 2004.
[RFC 3940] Adamson, B., Bormann, C., Handley, M., and J.
Macker, "Negative-acknowledgment (NACK)-Oriented
Reliable Multicast (NORM) Protocol", RFC 3940,
November 2004.
[RFC 4535] Harney, H., Meth, U., Colegrove, A., and G. Gross,
"GSAKMP: Group Secure Association Key Management
Protocol", RFC 4535, June 2006.
[RFC 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566, July 2006.
[RFC 5445] Watson, M., "Basic Forward Error Correction (FEC)
Schemes", RFC 5445, March 2009.
[RmComparison] Pingali, S., Towsley, D., and J. Kurose, "A
Comparison of Sender-Initiated and Receiver-
Initiated Reliable Multicast Protocols", Proc.
INFOCOMM, San Francisco CA, October 1993.
[TcpModel] Padhye, J., Firoiu, V., Towsley, D., and J. Kurose,
"Modeling TCP Throughput: A Simple Model and its
Empirical Validation", ACM SIGCOMM, 1998.
[TfmccPaper] Widmer, J. and M. Handley, "Extending Equation-Based
Congestion Control to Multicast Applications",
ACM SIGCOMM, August 2001.
Adamson, et al. Standards Track PAGE 93
RFC 5740 NORM Protocol November 2009
Authors' Addresses
Brian Adamson
Naval Research Laboratory
Washington, DC 20375
USA
EMail: adamson@itd.nrl.navy.mil
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
D-28334 Bremen
Germany
EMail: cabo@tzi.org
Mark Handley
University College London
Gower Street
London WC1E 6BT
UK
EMail: M.Handley@cs.ucl.ac.uk
Joe Macker
Naval Research Laboratory
Washington, DC 20375
USA
EMail: macker@itd.nrl.navy.mil
Adamson, et al. Standards Track PAGE 94
NACK-Oriented Reliable Multicast (NORM) Transport Protocol
RFC TOTAL SIZE: 268831 bytes
PUBLICATION DATE: Tuesday, November 17th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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