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IETF RFC 5651
Layered Coding Transport (LCT) Building Block
Last modified on Monday, October 19th, 2009
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Network Working Group M. Luby
Request for Comments: 5651 M. Watson
Obsoletes: 3451 L. Vicisano
Category: Standards Track Qualcomm, Inc.
October 2009
Layered Coding Transport (LCT) Building Block
 Abstract
The Layered Coding Transport (LCT) Building Block provides transport
level support for reliable content delivery and stream delivery
protocols. LCT is specifically designed to support protocols using
IP multicast, but it also provides support to protocols that use
unicast. LCT is compatible with congestion control that provides
multiple rate delivery to receivers and is also compatible with
coding techniques that provide reliable delivery of content. This
document obsoletes RFC 3451.
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.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Luby, et al. Standards Track PAGE 1 
RFC 5651 LCT Building Block October 2009
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................3
2. Rationale .......................................................3
3. Functionality ...................................................4
4. Applicability ...................................................7
4.1. Environmental Requirements and Considerations ..............9
4.2. Delivery Service Models ...................................10
4.3. Congestion Control ........................................13
5. Packet Header Fields ...........................................13
5.1. LCT Header Format .........................................13
5.2. Header-Extension Fields ...................................18
5.2.1. General ............................................18
5.2.2. EXT_TIME Header Extension ..........................20
6. Operations .....................................................23
6.1. Sender Operation ..........................................23
6.2. Receiver Operation ........................................25
7. Requirements from Other Building Blocks ........................26
8. Security Considerations ........................................28
8.1. Session and Object Multiplexing and Termination ...........28
8.2. Time Synchronization ......................................29
8.3. Data Transport ............................................29
9. IANA Considerations ............................................29
9.1. Namespace Declaration for LCT Header Extension Types ......29
9.2. LCT Header Extension Type Registration ....................30
10. Acknowledgments ...............................................30
11. Changes from RFC 3451 .........................................31
12. References ....................................................31
12.1. Normative References .....................................31
12.2. Informative References ...................................32
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RFC 5651 LCT Building Block October 2009
1. Introduction
Layered Coding Transport (LCT) provides transport level support for
reliable content delivery and stream delivery protocols. Layered
Coding Transport is specifically designed to support protocols using
IP multicast, but it also provides support to protocols that use
unicast. Layered Coding Transport is compatible with congestion
control that provides multiple rate delivery to receivers and is also
compatible with coding techniques that provide reliable delivery of
content.
This document describes a building block as defined in [RFC 3048].
This document is a product of the IETF RMT WG and follows the general
guidelines provided in [RFC 3269].
[RFC 3451], which was published in the "Experimental" category and
which is obsoleted by this document, contained a previous version of
the protocol.
This Proposed Standard specification is thus based on and backwards
compatible with the protocol defined in [RFC 3451] updated according
to accumulated experience and growing protocol maturity since its
original publication. Said experience applies both to this
specification itself and to congestion control strategies related to
the use of this specification.
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].
2. Rationale
LCT provides transport level support for massively scalable protocols
using the IP multicast network service. The support that LCT
provides is common to a variety of very important applications,
including reliable content delivery and streaming applications.
An LCT session comprises multiple channels originating at a single
sender that are used for some period of time to carry packets
pertaining to the transmission of one or more objects that can be of
interest to receivers. The logic behind defining a session as
originating from a single sender is that this is the right
granularity to regulate packet traffic via congestion control. One
rationale for using multiple channels within the same session is that
there are massively scalable congestion control protocols that use
multiple channels per session. These congestion control protocols
are considered to be layered because a receiver joins and leaves
channels in a layered order during its participation in the session.
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RFC 5651 LCT Building Block October 2009
The use of layered channels is also useful for streaming
applications.
There are coding techniques that provide massively scalable
reliability and asynchronous delivery that are compatible with both
layered congestion control and with LCT. When all are combined, the
result is a massively scalable reliable asynchronous content delivery
protocol that is network friendly. LCT also provides functionality
that can be used for other applications as well, e.g., layered
streaming applications.
LCT avoids providing functionality that is not massively scalable.
For example, LCT does not provide any mechanisms for sending
information from receivers to senders, although this does not rule
out protocols that both use LCT and do require sending information
from receivers to senders.
LCT includes general support for congestion control that must be
used. It does not, however, specify which congestion control should
be used. The rationale for this is that congestion control must be
provided by any protocol that is network friendly, and yet the
different applications that can use LCT will not have the same
requirements for congestion control. For example, a content delivery
protocol may strive to use all available bandwidth between receivers
and the sender. It must, therefore, drastically back off its rate
when there is competing traffic. On the other hand, a streaming
delivery protocol may strive to maintain a constant rate instead of
trying to use all available bandwidth, and it may not back off its
rate as fast when there is competing traffic.
Beyond support for congestion control, LCT provides a number of
fields and supports functionality commonly required by many
protocols. For example, LCT provides a Transmission Session ID that
can be used to identify to which session each received packet
belongs. This is important because a receiver may be joined to many
sessions concurrently, and thus it is very useful to be able to
demultiplex packets as they arrive according to the session to which
they belong. As another example, there are optional fields within
the LCT packet header for identifying the object about which
information is carried in the packet payload.
3. Functionality
An LCT session consists of a set of logically grouped LCT channels
associated with a single sender carrying packets with LCT headers for
one or more objects. An LCT channel is defined by the combination of
a sender and an address associated with the channel by the sender. A
Luby, et al. Standards Track PAGE 4
RFC 5651 LCT Building Block October 2009
receiver joins a channel to start receiving the data packets sent to
the channel by the sender, and a receiver leaves a channel to stop
receiving data packets from the channel.
LCT is meant to be combined with other building blocks so that the
resulting overall protocol is massively scalable. Scalability refers
to the behavior of the protocol in relation to the number of
receivers and network paths, their heterogeneity, and the ability to
accommodate dynamically variable sets of receivers. Scalability
limitations can come from memory or processing requirements, or from
the amount of feedback control and redundant data packet traffic
generated by the protocol. In turn, such limitations may be a
consequence of the features that a complete reliable content delivery
or stream delivery protocol is expected to provide.
The LCT header provides a number of fields that are useful for
conveying in-band session information to receivers. One of the
required fields is the Transmission Session ID (TSI), which allows
the receiver of a session to uniquely identify received packets as
part of the session. Another required field is the Congestion
Control Information (CCI), which allows the receiver to perform the
required congestion control on the packets received within the
session. Other LCT fields provide optional but often very useful
additional information for the session. For example, the Transport
Object Identifier (TOI) identifies for which object the packet
contains data and flags are included for indicating the close of the
session and the close of sending packets for an object. Header
extensions can carry additional fields that, for example, can be used
for packet authentication or to convey various kinds of timing
information: the Sender Current Time (SCT) conveys the time when the
packet was sent from the sender to the receiver, the Expected
Residual Time (ERT) conveys the amount of time the session or
transmission object will be continued for, and Session Last Change
(SLC) conveys the time when objects have been added, modified, or
removed from the session.
LCT provides support for congestion control. Congestion control MUST
be used that conforms to [RFC 2357] between receivers and the sender
for each LCT session. Congestion control refers to the ability to
adapt throughput to the available bandwidth on the path from the
sender to a receiver, and to share bandwidth fairly with competing
flows such as TCP. Thus, the total flow of packets flowing to each
receiver participating in an LCT session MUST NOT compete unfairly
with existing flow-adaptive protocols such as TCP.
A multiple rate or a single rate congestion control protocol can be
used with LCT. For multiple rate protocols, a session typically
consists of more than one channel, and the sender sends packets to
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RFC 5651 LCT Building Block October 2009
the channels in the session at rates that do not depend on the
receivers. Each receiver adjusts its reception rate during its
participation in the session by joining and leaving channels
dynamically depending on the available bandwidth to the sender
independent of all other receivers. Thus, for multiple rate
protocols, the reception rate of each receiver may vary dynamically
independent of the other receivers.
For single rate protocols, a session typically consists of one
channel and the sender sends packets to the channel at variable rates
over time depending on feedback from receivers. Each receiver
remains joined to the channel during its participation in the
session. Thus, for single rate protocols, the reception rate of each
receiver may vary dynamically but in coordination with all receivers.
Generally, a multiple rate protocol is preferable to a single rate
protocol in a heterogeneous receiver environment, since generally it
more easily achieves scalability to many receivers and provides
higher throughput to each individual receiver. Use of the multiple
rate congestion control scheme defined in [RFC 3738] is RECOMMENDED.
Alternative multiple rate congestion control protocols are described
in [VIC1998] and [BYE2000]. A possible single rate congestion
control protocol is described in [RIZ2000].
Layered coding refers to the ability to produce a coded stream of
packets that can be partitioned into an ordered set of layers. The
coding is meant to provide some form of reliability, and the layering
is meant to allow the receiver experience (in terms of quality of
playout, or overall transfer speed) to vary in a predictable way
depending on how many consecutive layers of packets the receiver is
receiving.
The concept of layered coding was first introduced with reference to
audio and video streams. For example, the information associated
with a TV broadcast could be partitioned into three layers,
corresponding to black and white, color, and HDTV quality. Receivers
can experience different quality without the need for the sender to
replicate information in the different layers.
The concept of layered coding can be naturally extended to reliable
content delivery protocols when Forward Error Correction (FEC)
techniques are used for coding the data stream. Descriptions of this
can be found in [RIZ1997a], [RIZ1997b], [GEM2000], [VIC1998], and
[BYE1998]. By using FEC, the data stream is transformed in such a
way that reconstruction of a data object does not depend on the
reception of specific data packets, but only on the number of
different packets received. As a result, by increasing the number of
layers from which a receiver is receiving, the receiver can reduce
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RFC 5651 LCT Building Block October 2009
the transfer time accordingly. Using FEC to provide reliability can
increase scalability dramatically in comparison to other methods for
providing reliability. More details on the use of FEC for reliable
content delivery can be found in [RFC 3453].
Reliable protocols aim at giving guarantees on the reliable delivery
of data from the sender to the intended recipients. Guarantees vary
from simple packet data integrity to reliable delivery of a precise
copy of an object to all intended recipients. Several reliable
content delivery protocols have been built on top of IP multicast
using methods other than FEC, but scalability was not the primary
design goal for many of them.
Two of the key difficulties in scaling reliable content delivery
using IP multicast are dealing with the amount of data that flows
from receivers back to the sender and the associated response
(generally data retransmissions) from the sender. Protocols that
avoid any such feedback, and minimize the amount of retransmissions,
can be massively scalable. LCT can be used in conjunction with FEC
codes or a layered codec to achieve reliability with little or no
feedback.
Protocol instantiations (PIs) MAY be built by combining the LCT
framework with other components. A complete protocol instantiation
that uses LCT MUST include a congestion control protocol that is
compatible with LCT and that conforms to [RFC 2357]. A complete
protocol instantiation that uses LCT MAY include a scalable
reliability protocol that is compatible with LCT, it MAY include a
session control protocol that is compatible with LCT, and it MAY
include other protocols such as security protocols.
4. Applicability
An LCT session comprises a logically related set of one or more LCT
channels originating at a single sender. The channels are used for
some period of time to carry packets containing LCT headers, and
these headers pertain to the transmission of one or more objects that
can be of interest to receivers.
LCT is most applicable for delivery of objects or streams in a
session of substantial length, i.e., objects or streams that range in
aggregate length from hundreds of kilobytes to many gigabytes, and
where the duration of the session is on the order of tens of seconds
or more.
As an example, an LCT session could be used to deliver a TV program
using three LCT channels. Receiving packets from the first LCT
channel could allow black and white reception. Receiving the first
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RFC 5651 LCT Building Block October 2009
two LCT channels could also permit color reception. Receiving all
three channels could allow HDTV quality reception. Objects in this
example could correspond to individual TV programs being transmitted.
As another example, a reliable LCT session could be used to reliably
deliver hourly updated weather maps (objects) using ten LCT channels
at different rates, using FEC coding. A receiver may join and
concurrently receive packets from subsets of these channels, until it
has enough packets in total to recover the object, then leave the
session (or remain connected listening for session description
information only) until it is time to receive the next object. In
this case, the quality metric is the time required to receive each
object.
Before joining a session, the receivers must obtain enough of the
session description to start the session. This includes the relevant
session parameters needed by a receiver to participate in the
session, including all information relevant to congestion control.
The session description is determined by the sender, and is typically
communicated to the receivers out-of-band. In some cases, as
described later, parts of the session description that are not
required to initiate a session MAY be included in the LCT header or
communicated to a receiver out-of-band after the receiver has joined
the session.
An encoder MAY be used to generate the data that is placed in the
packet payload in order to provide reliability. A suitable decoder
is used to reproduce the original information from the packet
payload. There MAY be a reliability header that follows the LCT
header if such an encoder and decoder is used. The reliability
header helps to describe the encoding data carried in the payload of
the packet. The format of the reliability header depends on the
coding used, and this is negotiated out-of-band. As an example, one
of the FEC headers described in [RFC 5052] could be used.
For LCT, when multiple rate congestion control is used, congestion
control is achieved by sending packets associated with a given
session to several LCT channels. Individual receivers dynamically
join one or more of these channels, according to the network
congestion as seen by the receiver. LCT headers include an opaque
field that MUST be used to convey congestion control information to
the receivers. The actual congestion control scheme to use with LCT
is negotiated out-of-band. Some examples of congestion control
protocols that may be suitable for content delivery are described in
[VIC1998], [BYE2000], and [RFC 3738]. Other congestion controls may
be suitable when LCT is used for a streaming application.
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This document does not specify and restrict the type of exchanges
between LCT (or any protocol instantiation built on top of LCT) and
an upper application. Some upper APIs may use an object-oriented
approach, where the only possible unit of data exchanged between LCT
(or any protocol instantiation built on top of LCT) and an
application, either at a source or at a receiver, is an object.
Other APIs may enable a sending or receiving application to exchange
a subset of an object with LCT (or any PI built on top of LCT), or
may even follow a streaming model. These considerations are outside
the scope of this document.
4.1. Environmental Requirements and Considerations
LCT is intended for congestion controlled delivery of objects and
streams (both reliable content delivery and streaming of multimedia
information).
LCT can be used with both multicast and unicast delivery. LCT
requires connectivity between a sender and receivers, but it does not
require connectivity from receivers to a sender. LCT inherently
works with all types of networks, including LANs, WANs, Intranets,
the Internet, asymmetric networks, wireless networks, and satellite
networks. Thus, the inherent raw scalability of LCT is unlimited.
However, when other specific applications are built on top of LCT,
then these applications, by their very nature, may limit scalability.
For example, if an application requires receivers to retrieve out-of-
band information in order to join a session, or an application allows
receivers to send requests back to the sender to report reception
statistics, then the scalability of the application is limited by the
ability to send, receive, and process this additional data.
LCT requires receivers to be able to uniquely identify and
demultiplex packets associated with an LCT session. In particular,
there MUST be a Transport Session Identifier (TSI) associated with
each LCT session. The TSI is scoped by the IP address of the sender,
and the IP address of the sender together with the TSI MUST uniquely
identify the session. If the underlying transport is UDP, as
described in [RFC 768], then the 16-bit UDP source port number MAY
serve as the TSI for the session. The TSI value MUST be the same in
all places it occurs within a packet. If there is no underlying TSI
provided by the network, transport, or any other layer, then the TSI
MUST be included in the LCT header.
LCT is presumed to be used with an underlying network or transport
service that is a "best effort" service that does not guarantee
packet reception or packet reception order, and that does not have
any support for flow or congestion control. For example, the Any-
Source Multicast (ASM) model of IP multicast as defined in [RFC 1112]
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RFC 5651 LCT Building Block October 2009
is such a "best effort" network service. While the basic service
provided by [RFC 1112] is largely scalable, providing congestion
control or reliability should be done carefully to avoid severe
scalability limitations, especially in the presence of heterogeneous
sets of receivers.
There are currently two models of multicast delivery, the Any-Source
Multicast (ASM) model as defined in [RFC 1112] and the Source-Specific
Multicast (SSM) model as defined in [RFC 4607]. LCT works with both
multicast models, but in a slightly different way with somewhat
different environmental concerns. When using ASM, a sender S sends
packets to a multicast group G, and the LCT channel address consists
of the pair (S,G), where S is the IP address of the sender and G is a
multicast group address. When using SSM, a sender S sends packets to
an SSM channel (S,G), and the LCT channel address coincides with the
SSM channel address.
A sender can locally allocate unique SSM channel addresses, and this
makes allocation of LCT channel addresses easy with SSM. To allocate
LCT channel addresses using ASM, the sender must uniquely chose the
ASM multicast group address across the scope of the group, and this
makes allocation of LCT channel addresses more difficult with ASM.
LCT channels and SSM channels coincide, and thus the receiver will
only receive packets sent to the requested LCT channel. With ASM,
the receiver joins an LCT channel by joining a multicast group G, and
all packets sent to G, regardless of the sender, may be received by
the receiver. Thus, SSM has compelling security advantages over ASM
for prevention of denial-of-service (DoS) attacks. In either case,
receivers SHOULD use packet authentication mechanisms to mitigate
such attacks (see Sections 6.2 and 7).
Some networks are not amenable to some congestion control protocols
that could be used with LCT. In particular, for a satellite or
wireless network, there may be no mechanism for receivers to
effectively reduce their reception rate since there may be a fixed
transmission rate allocated to the session.
LCT is compatible with both IPv4 and IPv6 as no part of the packet is
IP version specific.
4.2. Delivery Service Models
LCT can support several different delivery service models. Two
examples are briefly described here.
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RFC 5651 LCT Building Block October 2009
Push service model
One way a push service model can be used for reliable content
delivery is to deliver a series of objects. For example, a
receiver could join the session and dynamically adapt the number
of LCT channels the receiver is joined to until enough packets
have been received to reconstruct an object. After reconstructing
the object, the receiver may stay in the session and wait for the
transmission of the next object.
The push model is particularly attractive in satellite networks
and wireless networks. In these cases, a session may consist of
one fixed rate LCT channel.
A push service model can be used, for example, for reliable
delivery of a large object such as a 100 GB file. The sender
could send a Session Description announcement to a control channel
and receivers could monitor this channel and join a session
whenever a Session Description of interest arrives. Upon receipt
of the Session Description, each receiver could join the session
to receive packets until enough packets have arrived to
reconstruct the object, at which point the receiver could report
back to the sender that its reception was completed successfully.
The sender could decide to continue sending packets for the object
to the session until all receivers have reported successful
reconstruction or until some other condition has been satisfied.
There are several features Asynchronous Layered Coding (ALC)
provides to support the push model. For example, the sender can
optionally include an Expected Residual Time (ERT) in the packet
header extension that indicates the expected remaining time of
packet transmission for either the single object carried in the
session or for the object identified by the Transmission Object
Identifier (TOI) if there are multiple objects carried in the
session. This can be used by receivers to determine if there is
enough time remaining in the session to successfully receive
enough additional packets to recover the object. If, for example,
there is not enough time, then the push application may have
receivers report back to the sender to extend the transmission of
packets for the object for enough time to allow the receivers to
obtain enough packets to reconstruct the object. The sender could
then include an ERT based on the extended object transmission time
in each subsequent packet header for the object. As other
examples, the LCT header optionally can contain a Close Session
flag that indicates when the sender is about to stop sending
packets to the session and a Close Object flag that indicates when
the sender is about to stop sending packets to the session for the
object identified by the Transmission Object ID. However, these
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flags are not a completely reliable mechanism and thus the Close
Session flag should only be used as a hint of when the session is
about to close, and the Close Object flag should only be used as a
hint of when transmission of packets for the object is about to
end.
On-demand content delivery model
For an on-demand content delivery service model, senders typically
transmit for some given time period selected to be long enough to
allow all the intended receivers to join the session and recover
the object. For example, a popular software update might be
transmitted using LCT for several days, even though a receiver may
be able to complete the download in one hour total of connection
time, perhaps spread over several intervals of time. In this
case, the receivers join the session at any point in time when it
is active. Receivers leave the session when they have received
enough packets to recover the object. The receivers, for example,
obtain a Session Description by contacting a web server.
In this case, the receivers join the session, and dynamically
adapt the number of LCT channels to which they subscribe according
to the available bandwidth. Receivers then drop from the session
when they have received enough packets to recover the object.
As an example, assume that an object is 50 MB. The sender could
send 1 KB packets to the first LCT channel at 50 packets per
second, so that receivers using just this LCT channel could
complete reception of the object in 1,000 seconds in absence of
loss, and would be able to complete reception even in presence of
some substantial amount of losses with the use of coding for
reliability. Furthermore, the sender could use a number of LCT
channels such that the aggregate rate of 1 KB packets to all LCT
channels is 1,000 packets per second, so that a receiver could be
able to complete reception of the object in as little 50 seconds
(assuming no loss and that the congestion control mechanism
immediately converges to the use of all LCT channels).
Other service models
There are many other delivery service models for which LCT can be
used that are not covered above. As examples, a live streaming or
an on-demand archival content streaming service model. A
description of the many potential applications, the appropriate
delivery service model, and the additional mechanisms to support
such functionalities when combined with LCT is beyond the scope of
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RFC 5651 LCT Building Block October 2009
this document. This document only attempts to describe the
minimal common scalable elements to these diverse applications
using LCT as the delivery transport.
4.3. Congestion Control
The specific congestion control protocol to be used for LCT sessions
depends on the type of content to be delivered. While the general
behavior of the congestion control protocol is to reduce the
throughput in presence of congestion and gradually increase it in the
absence of congestion, the actual dynamic behavior (e.g., response to
single losses) can vary.
It is RECOMMENDED that the congestion control mechanism specified in
[RFC 3738] be used. Some alternative possible congestion control
protocols for reliable content delivery using LCT are described in
[VIC1998] and [BYE2000]. Different delivery service models might
require different congestion control protocols.
5. Packet Header Fields
Packets sent to an LCT session MUST include an "LCT header". The LCT
header format is described below.
Other building blocks MAY describe some of the same fields as
described for the LCT header. It is RECOMMENDED that protocol
instantiations using multiple building blocks include shared fields
at most once in each packet. Thus, for example, if another building
block is used with LCT that includes the optional Expected Residual
Time field, then the Expected Residual Time field SHOULD be carried
in each packet at most once.
The position of the LCT header within a packet MUST be specified by
any protocol instantiation that uses LCT.
5.1. LCT Header Format
The LCT header is of variable size, which is specified by a length
field in the third byte of the header. In the LCT header, all
integer fields are carried in "big-endian" or "network order" format,
that is, most significant byte (octet) first. Bits designated as
"padding" or "reserved" (r) MUST by set to 0 by senders and ignored
by receivers in this version of the specification. Unless otherwise
noted, numeric constants in this specification are in decimal form
(base 10).
The format of the default LCT header is depicted in Figure 1.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V | C |PSI|S| O |H|Res|A|B| HDR_LEN | Codepoint (CP)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Congestion Control Information (CCI, length = 32*(C+1) bits) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport Session Identifier (TSI, length = 32*S+16*H bits) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transport Object Identifier (TOI, length = 32*O+16*H bits) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Header Extensions (if applicable) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Default LCT Header Format
The function and length of each field in the default LCT header is
the following.
LCT version number (V): 4 bits
Indicates the LCT version number. The LCT version number for this
specification is 1.
Congestion control flag (C): 2 bits
C=0 indicates the Congestion Control Information (CCI) field is 32
bits in length. C=1 indicates the CCI field is 64 bits in length.
C=2 indicates the CCI field is 96 bits in length. C=3 indicates
the CCI field is 128 bits in length.
Protocol-Specific Indication (PSI): 2 bits
The usage of these bits, if any, is specific to each protocol
instantiation that uses the LCT building block. If no protocol-
instantiation-specific usage of these bits is defined, then a
sender MUST set them to zero and a receiver MUST ignore these
bits.
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Transport Session Identifier flag (S): 1 bit
This is the number of full 32-bit words in the TSI field. The TSI
field is 32*S + 16*H bits in length, i.e., the length is either 0
bits, 16 bits, 32 bits, or 48 bits.
Transport Object Identifier flag (O): 2 bits
This is the number of full 32-bit words in the TOI field. The TOI
field is 32*O + 16*H bits in length, i.e., the length is either 0
bits, 16 bits, 32 bits, 48 bits, 64 bits, 80 bits, 96 bits, or 112
bits.
Half-word flag (H): 1 bit
The TSI and the TOI fields are both multiples of 32 bits plus 16*H
bits in length. This allows the TSI and TOI field lengths to be
multiples of a half-word (16 bits), while ensuring that the
aggregate length of the TSI and TOI fields is a multiple of 32
bits.
Reserved (Res): 2 bits
These bits are reserved. In this version of the specification,
they MUST be set to zero by senders and MUST be ignored by
receivers.
Close Session flag (A): 1 bit
Normally, A is set to 0. The sender MAY set A to 1 when
termination of transmission of packets for the session is
imminent. A MAY be set to 1 in just the last packet transmitted
for the session, or A MAY be set to 1 in the last few seconds of
packets transmitted for the session. Once the sender sets A to 1
in one packet, the sender SHOULD set A to 1 in all subsequent
packets until termination of transmission of packets for the
session. A received packet with A set to 1 indicates to a
receiver that the sender will immediately stop sending packets for
the session. When a receiver receives a packet with A set to 1,
the receiver SHOULD assume that no more packets will be sent to
the session.
Close Object flag (B): 1 bit
Normally, B is set to 0. The sender MAY set B to 1 when
termination of transmission of packets for an object is imminent.
If the TOI field is in use and B is set to 1, then termination of
transmission for the object identified by the TOI field is
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imminent. If the TOI field is not in use and B is set to 1, then
termination of transmission for the one object in the session
identified by out-of-band information is imminent. B MAY be set
to 1 in just the last packet transmitted for the object, or B MAY
be set to 1 in the last few seconds that packets are transmitted
for the object. Once the sender sets B to 1 in one packet for a
particular object, the sender SHOULD set B to 1 in all subsequent
packets for the object until termination of transmission of
packets for the object. A received packet with B set to 1
indicates to a receiver that the sender will immediately stop
sending packets for the object. When a receiver receives a packet
with B set to 1, then it SHOULD assume that no more packets will
be sent for the object to the session.
LCT header length (HDR_LEN): 8 bits
Total length of the LCT header in units of 32-bit words. The
length of the LCT header MUST be a multiple of 32 bits. This
field can be used to directly access the portion of the packet
beyond the LCT header, i.e., to the first other header if it
exists, or to the packet payload if it exists and there is no
other header, or to the end of the packet if there are no other
headers or packet payload.
Codepoint (CP): 8 bits
An opaque identifier that is passed to the packet payload decoder
to convey information on the codec being used for the packet
payload. The mapping between the codepoint and the actual codec
is defined on a per session basis and communicated out-of-band as
part of the session description information. The use of the CP
field is similar to the Payload Type (PT) field in RTP headers as
described in [RFC 3550].
Congestion Control Information (CCI): 32, 64, 96, or 128 bits
Used to carry congestion control information. For example, the
congestion control information could include layer numbers,
logical channel numbers, and sequence numbers. This field is
opaque for the purpose of this specification.
This field MUST be 32 bits if C=0.
This field MUST be 64 bits if C=1.
This field MUST be 96 bits if C=2.
Luby, et al. Standards Track PAGE 16
RFC 5651 LCT Building Block October 2009
This field MUST be 128 bits if C=3.
Transport Session Identifier (TSI): 0, 16, 32, or 48 bits
The TSI uniquely identifies a session among all sessions from a
particular sender. The TSI is scoped by the IP address of the
sender, and thus the IP address of the sender and the TSI together
uniquely identify the session. Although a TSI in conjunction with
the IP address of the sender always uniquely identifies a session,
whether or not the TSI is included in the LCT header depends on
what is used as the TSI value. If the underlying transport is
UDP, then the 16-bit UDP source port number MAY serve as the TSI
for the session. If the TSI value appears multiple times in a
packet, then all occurrences MUST be the same value. If there is
no underlying TSI provided by the network, transport or any other
layer, then the TSI MUST be included in the LCT header.
The TSI MUST be unique among all sessions served by the sender
during the period when the session is active, and for a large
period of time preceding and following when the session is active.
A primary purpose of the TSI is to prevent receivers from
inadvertently accepting packets from a sender that belong to
sessions other than the sessions to which receivers are
subscribed. For example, suppose a session is deactivated and
then another session is activated by a sender and the two sessions
use an overlapping set of channels. A receiver that connects and
remains connected to the first session during this sender activity
could possibly accept packets from the second session as belonging
to the first session if the TSI for the two sessions were
identical. The mapping of TSI field values to sessions is outside
the scope of this document and is to be done out-of-band.
The length of the TSI field is 32*S + 16*H bits. Note that the
aggregate lengths of the TSI field plus the TOI field is a
multiple of 32 bits.
Transport Object Identifier (TOI): 0, 16, 32, 48, 64, 80, 96, or 112
bits.
This field indicates to which object within the session this
packet pertains. For example, a sender might send a number of
files in the same session, using TOI=0 for the first file, TOI=1
for the second one, etc. As another example, the TOI may be a
unique global identifier of the object that is being transmitted
from several senders concurrently, and the TOI value may be the
output of a hash function applied to the object. The mapping of
TOI field values to objects is outside the scope of this document
and is to be done out-of-band. The TOI field MUST be used in all
Luby, et al. Standards Track PAGE 17
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packets if more than one object is to be transmitted in a session,
i.e., the TOI field is either present in all the packets of a
session or is never present.
The length of the TOI field is 32*O + 16*H bits. Note that the
aggregate length of the TSI field plus the TOI field is a multiple
of 32 bits.
5.2. Header-Extension Fields
5.2.1. General
Header Extensions are used in LCT to accommodate optional header
fields that are not always used or have variable size. Examples of
the use of Header Extensions include:
o Extended-size versions of already existing header fields.
o Sender and receiver authentication information.
o Transmission of timing information.
The presence of Header Extensions can be inferred by the LCT header
length (HDR_LEN). If HDR_LEN is larger than the length of the
standard header, then the remaining header space is taken by Header
Extension fields.
If present, Header Extensions MUST be processed to ensure that they
are recognized before performing any congestion control procedure or
otherwise accepting a packet. The default action for unrecognized
Header Extensions is to ignore them. This allows the future
introduction of backward-compatible enhancements to LCT without
changing the LCT version number. Non-backward-compatible Header
Extensions CANNOT be introduced without changing the LCT version
number.
There are two formats for Header Extension fields, as depicted in
Figure 2. The first format is used for variable-length extensions,
with Header Extension Type (HET) values between 0 and 127. The
second format is used for fixed-length (one 32-bit word) extensions,
using HET values from 127 to 255.
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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 (HEC) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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) | Header Extension Content (HEC) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of Additional Headers
The explanation of each sub-field is the following:
Header Extension Type (HET): 8 bits
The type of the Header Extension. This document defines a number
of possible types. Additional types may be defined in future
versions of this specification. HET values from 0 to 127 are used
for variable-length Header Extensions. HET values from 128 to 255
are used for fixed-length 32-bit Header Extensions.
Header Extension Length (HEL): 8 bits
The length of the whole Header Extension field, expressed in
multiples of 32-bit words. This field MUST be present for
variable-length extensions (HETs between 0 and 127) and MUST NOT
be present for fixed-length extensions (HETs between 128 and 255).
Header Extension Content (HEC): variable length
The content of the Header Extension. The format of this sub-
field depends on the Header Extension Type. For fixed-length
Header Extensions, the HEC is 24 bits. For variable-length Header
Extensions, the HEC field has variable size, as specified by the
HEL field. Note that the length of each Header Extension field
MUST be a multiple of 32 bits. Also note that the total size of
the LCT header, including all Header Extensions and all optional
header fields, cannot exceed 255 32-bit words.
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The following LCT Header Extensions are defined by this
specification:
EXT_NOP, HET=0 No-Operation extension. The information present in
this extension field MUST be ignored by receivers.
EXT_AUTH, HET=1 Packet authentication extension. Information used to
authenticate the sender of the packet. 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 RECOMMENDED that senders provide some form of packet
authentication. If EXT_AUTH is present, whatever
packet authentication checks that can be performed
immediately upon reception of the packet SHOULD 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
is fully authenticated. Any congestion control
related action that is appropriate SHOULD NOT be
postponed by any such full packet authentication.
EXT_TIME, HET=2 Time Extension. This extension is used to carry
several types of timing information. It includes
general purpose timing information, namely the Sender
Current Time (SCT), Expected Residual Time (ERT), and
Sender Last Change (SLC) time extensions described in
the present document. It can also be used for timing
information with narrower applicability (e.g.,
defined for a single protocol instantiation); in this
case, it will be described in a separate document.
All senders and receivers implementing LCT MUST support the EXT_NOP
Header Extension and MUST recognize EXT_AUTH and EXT_TIME, but are
not required to be able to parse their content.
5.2.2. EXT_TIME Header Extension
This section defines the timing Header Extensions with general
applicability. The time values carried in this Header Extension are
related to the server's wall clock. The server MUST maintain
consistent relative time during a session (i.e., insignificant clock
drift). For some applications, system or even global synchronization
of server wall clock may be desirable, such as using the Network Time
Luby, et al. Standards Track PAGE 20
RFC 5651 LCT Building Block October 2009
Protocol (NTP) [RFC 1305] to ensure actual time relative to 00:00
hours GMT, January 1st 1900. Such session-external synchronization
is outside the scope of this document.
The EXT_TIME Header Extension uses the format depicted in Figure 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET = 2 | HEL >= 1 | Use (bit field) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| first time value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (other time values (optional) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: EXT_TIME Header Extension Format
The "Use" bit field indicates the semantic of the following 32-bit
time value(s).
It is divided into two parts:
o 8 bits are reserved for general purpose timing information. This
information is applicable to any protocol that makes use of LCT.
o 8 bits are reserved for PI-specific timing information. This
information is out of the scope of this document.
The format of the "Use" bit field is depicted in Figure 4.
2 3
6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|SCT|SCT|ERT|SLC| reserved | PI-specific |
|Hi |Low| | | by LCT | use |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 4: "Use" Bit Field Format
Several "time value" fields MAY be present in a given EXT_TIME Header
Extension, as specified in the "Use-field". When several "time
value" fields are present, they MUST appear in the order specified by
the associated flag position in the "Use-field": first SCT-High (if
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present), then SCT-Low (if present), then ERT (if present), then SLC
(if present). Receivers SHOULD ignore additional fields within the
EXT_TIME Header Extension that they do not support.
The fields for the general purpose EXT_TIME timing information are:
Sender Current Time (SCT): SCT-High flag, SCT-Low flag, corresponding
time value (one or two 32-bit words).
This timing information represents the current time at the sender
at the time this packet was transmitted.
When the SCT-High flag is set, the associated 32-bit time value
provides an unsigned integer representing the time in seconds of
the sender's wall clock. In the particular case where NTP is
used, these 32 bits provide an unsigned integer representing the
time in seconds relative to 00:00 hours GMT, January 1st 1900,
(i.e., the most significant 32 bits of a full 64-bit NTP time
value). In that case, handling of wraparound of the 32-bit time
is outside the scope of NTP and LCT.
When the SCT-Low flag is set, the associated 32-bit time value
provides an unsigned integer representing a multiple of 1/2^^32 of
a second, in order to allow sub-second precision. When the SCT-
Low flag is set, the SCT-High flag MUST be set, too. In the
particular case where NTP is used, these 32 bits provide the 32
least significant bits of a 64-bit NTP timestamp.
Expected Residual Time (ERT): ERT flag, corresponding 32-bit time
value.
This timing information represents the sender expected residual
transmission time for the transmission of the current object. If
the packet containing the ERT timing information also contains the
TOI field, then ERT refers to the object corresponding to the TOI
field; otherwise, it refers to the only object in the session.
When the ERT flag is set, it is expressed as a number of seconds.
The 32 bits provide an unsigned integer representing this number
of seconds.
Session Last Changed (SLC): SLC flag, corresponding 32-bit time
value.
The Session Last Changed time value is the server wall clock time,
in seconds, at which the last change to session data occurred.
That is, it expresses the time at which the last (most recent)
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Transport Object addition, modification, or removal was made for
the delivery session. In the case of modifications and additions,
it indicates that new data will be transported that was not
transported prior to this time. In the case of removals, SLC
indicates that some prior data will no longer be transported.
When the SLC flag is set, the associated 32-bit time value
provides an unsigned integer representing a time in seconds. In
the particular case where NTP is used, these 32 bits provide an
unsigned integer representing the time in seconds relative to
00:00 hours GMT, January 1st 1900, (i.e., the most significant 32
bits of a full 64-bit NTP time value). In that case, handling of
wraparound of the 32-bit time is outside the scope of NTP and LCT.
In some cases, it may be appropriate that a packet containing an
EXT_TIME Header Extension with SLC information also contain an
SCT-High information.
Reserved by LCT for future use (4 bits):
In this version of the specification, these bits MUST be set to
zero by senders and MUST be ignored by receivers.
PI-specific use (8 bits):
These bits are out of the scope of this document. The bits that
are not specified by the PI built on top of LCT SHOULD be set to
zero.
The total EXT_TIME length is carried in the HEL, since this Header
Extension is of variable length. It also enables clients to skip
this Header Extension altogether if not supported (but recognized).
6. Operations
6.1. Sender Operation
Before joining an LCT session, a receiver MUST obtain a session
description. The session description MUST include:
o The sender IP address;
o The number of LCT channels;
o The addresses and port numbers used for each LCT channel;
o The Transport Session ID (TSI) to be used for the session;
Luby, et al. Standards Track PAGE 23
RFC 5651 LCT Building Block October 2009
o Enough information to determine the congestion control protocol
being used;
o Enough information to determine the packet authentication scheme
being used (if one is being used).
The session description could also include, but is not limited to:
o The data rates used for each LCT channel;
o The length of the packet payload;
o The mapping of TOI value(s) to objects for the session;
o Any information that is relevant to each object being transported,
such as when it will be available within the session, for how
long, and the length of the object;
Protocol instantiations using LCT MAY place additional requirements
on what must be included in the session description. For example, a
protocol instantiation might require that the data rates for each
channel, or the mapping of TOI value(s) to objects for the session,
or other information related to other headers that might be required
be included in the session description.
The session description could be in a form such as SDP as defined in
[RFC 4566], or another format appropriate to a particular application.
It might be carried in a session announcement protocol such as SAP as
defined in [RFC 2974], obtained using a proprietary session control
protocol, located on a Web page with scheduling information, or
conveyed via email or other out-of-band methods. Discussion of
session description format, and distribution of session descriptions
is beyond the scope of this document.
Within an LCT session, a sender using LCT transmits a sequence of
packets, each in the format defined above. Packets are sent from a
sender using one or more LCT channels, which together constitute a
session. Transmission rates may be different in different channels
and may vary over time. The specification of the other building
block headers and the packet payload used by a complete protocol
instantiation using LCT is beyond the scope of this document. This
document does not specify the order in which packets are transmitted,
nor the organization of a session into multiple channels. Although
these issues affect the efficiency of the protocol, they do not
affect the correctness nor the inter-operability of LCT between
senders and receivers.
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Several objects can be carried within the same LCT session. In this
case, each object MUST be identified by a unique TOI. Objects MAY be
transmitted sequentially, or they MAY be transmitted concurrently.
It is good practice to only send objects concurrently in the same
session if the receivers that participate in that portion of the
session have interest in receiving all the objects. The reason for
this is that it wastes bandwidth and networking resources to have
receivers receive data for objects in which they have no interest.
Typically, the sender(s) continues to send packets in a session until
the transmission is considered complete. The transmission may be
considered complete when some time has expired, a certain number of
packets have been sent, or some out-of-band signal (possibly from a
higher level protocol) has indicated completion by a sufficient
number of receivers.
For the reasons mentioned above, this document does not pose any
restriction on packet sizes. However, network efficiency
considerations recommend that the sender uses an as large as possible
packet payload size, but in such a way that packets do not exceed the
network's maximum transmission unit size (MTU), or when fragmentation
coupled with packet loss might introduce severe inefficiency in the
transmission.
It is recommended that all packets have the same or very similar
sizes, as this can have a severe impact on the effectiveness of
congestion control schemes such as the ones described in [VIC1998],
[BYE2000], and [RFC 3738]. A sender of packets using LCT MUST
implement the sender-side part of one of the congestion control
schemes that is in accordance with [RFC 2357] using the Congestion
Control Information field provided in the LCT header, and the
corresponding receiver congestion control scheme is to be
communicated out-of-band and MUST be implemented by any receivers
participating in the session.
6.2. Receiver Operation
Receivers can operate differently depending on the delivery service
model. For example, for an on-demand service model, receivers may
join a session, obtain the necessary packets to reproduce the object,
and then leave the session. As another example, for a streaming
service model, a receiver may be continuously joined to a set of LCT
channels to download all objects in a session.
To be able to participate in a session, a receiver MUST obtain the
relevant session description information as listed in Section 6.1.
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If packet authentication information is present in an LCT header, it
SHOULD be used as specified in Section 5.2. To be able to be a
receiver in a session, the receiver MUST be able to process the LCT
header. The receiver MUST be able to discard, forward, store, or
process the other headers and the packet payload. If a receiver is
not able to process an LCT header, it MUST drop from the session.
To be able to participate in a session, a receiver MUST implement the
congestion control protocol specified in the session description
using the Congestion Control Information field provided in the LCT
header. If a receiver is not able to implement the congestion
control protocol used in the session, it MUST NOT join the session.
When the session is transmitted on multiple LCT channels, receivers
MUST initially join channels according to the specified startup
behavior of the congestion control protocol. For a multiple rate
congestion control protocol that uses multiple channels, this
typically means that a receiver will initially join only a minimal
set of LCT channels, possibly a single one, that in aggregate are
carrying packets at a low rate. This rule has the purpose of
preventing receivers from starting at high data rates.
Several objects can be carried either sequentially or concurrently
within the same LCT session. In this case, each object is identified
by a unique TOI. Note that even if a server stops sending packets
for an old object before starting to transmit packets for a new
object, both the network and the underlying protocol layers can cause
some reordering of packets, especially when sent over different LCT
channels, and thus receivers SHOULD NOT assume that the reception of
a packet for a new object means that there are no more packets in
transit for the previous one, at least for some amount of time.
A receiver MAY be concurrently joined to multiple LCT sessions from
one or more senders. The receiver MUST perform congestion control on
each such LCT session. If the congestion control protocol allows the
receiver some flexibility in terms of its actions within a session,
then the receiver MAY make choices to optimize the packet flow
performance across the multiple LCT sessions, as long as the receiver
still adheres to the congestion control rules for each LCT session
individually.
7. Requirements from Other Building Blocks
As described in [RFC 3048], LCT is a building block that is intended
to be used, in conjunction with other building blocks, to help
specify a protocol instantiation. A congestion control building
block that uses the Congestion Control information field within the
Luby, et al. Standards Track PAGE 26
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LCT header MUST be used by any protocol instantiation that uses LCT;
other building blocks MAY also be used, such as a reliability
building block.
The congestion control MUST be applied to the LCT session as an
entity, i.e., over the aggregate of the traffic carried by all of the
LCT channels associated with the LCT session. The Congestion Control
Information field in the LCT header is an opaque field that is
reserved to carry information related to congestion control. There
MAY also be congestion control Header Extension fields that carry
additional information related to congestion control.
The particular layered encoder and congestion control protocols used
with LCT have an impact on the performance and applicability of LCT.
For example, some layered encoders used for video and audio streams
can produce a very limited number of layers, thus providing a very
coarse control in the reception rate of packets by receivers in a
session. When LCT is used for reliable data transfer, some FEC
codecs are inherently limited in the size of the object they can
encode, and for objects larger than this size the reception overhead
on the receivers can grow substantially.
A more in-depth description of the use of FEC in Reliable Multicast
Transport (RMT) protocols is given in [RFC 3453]. Some of the FEC
codecs that MAY be used in conjunction with LCT for reliable content
delivery are specified in [RFC 5052]. The Codepoint field in the LCT
header is an opaque field that can be used to carry information
related to the encoding of the packet payload.
LCT also requires receivers to obtain a session description, as
described in Section 6.1. The session description could be in a form
such as SDP as defined in [RFC 4566], or another format appropriate to
a particular application and may be distributed with SAP as defined
in [RFC 2974], using HTTP, or in other ways. It is RECOMMENDED that
an authentication protocol be used to deliver the session description
to receivers to ensure the correct session description arrives.
It is RECOMMENDED that LCT implementors use some packet
authentication scheme to protect the protocol from attacks. An
example of a possibly suitable scheme is described in [Perrig2001].
Some protocol instantiations that use LCT MAY use building blocks
that require the generation of feedback from the receivers to the
sender. However, the mechanism for doing this is outside the scope
of LCT.
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8. Security Considerations
LCT is a building block as defined in [RFC 3048] and as such does not
define a complete protocol. Protocol instantiations that use the LCT
building block MUST address the potential vulnerabilities described
in the following sections. For an example, see [ALC-PI].
Protocol instantiations could address the vulnerabilities described
below by taking measures to prevent receivers from accepting
incorrect packets, for example, by using a source authentication and
content integrity mechanism. See also Sections 6.2 and 7 for
discussion of packet authentication requirements.
Note that for correct operation, LCT assumes availability of session
description information (see Sections 4 and 7). Incorrect or
maliciously modified session description information may result in
receivers being unable to correctly receive the session content, or
that receivers inadvertently try to receive at a much higher rate
than they are capable of, thereby disrupting traffic in portions of
the network. Protocol instantiations MUST address this potential
vulnerability, for example, by providing source authentication and
integrity mechanisms for the session description. Additionally,
these mechanisms MUST allow the receivers to securely verify the
correspondence between session description and LCT data packets.
The following sections consider further each of the services provided
by LCT.
8.1. Session and Object Multiplexing and Termination
The Transport Session Identifier and the Transport Object Identifier
in the LCT header provide for multiplexing of sessions and objects.
Modification of these fields by an attacker could have the effect of
depriving a session or object of data and potentially directing
incorrect data to another session or object, in both cases effecting
a denial-of-service attack.
Additionally, injection of forged packets with fake TSI or TOI values
may cause receivers to allocate resources for additional sessions or
objects, again potentially effecting a DoS attack.
The Close Object and Close Session bits in the LCT header provide for
signaling of the end of a session or object. Modification of these
fields by an attacker could cause receivers to incorrectly behave as
if the session or object had ended, resulting in a denial-of-service
attack, or conversely to continue to unnecessarily utilize resources
after the session or object has ended (although resource utilization
in this case is largely an implementation issue).
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RFC 5651 LCT Building Block October 2009
As a result of the above vulnerabilities, these fields MUST be
protected by protocol instantiation security mechanisms (for example,
source authentication and data integrity mechanisms).
8.2. Time Synchronization
The SCT and ERT mechanisms provide rudimentary time synchronization
features which can both be subject to attacks. Indeed an attacker
can easily de-synchronize clients, sending erroneous SCT information,
or mount a DoS attack by informing all clients that the session
(respectively, a particular object) is about to be closed.
As a result of the above vulnerabilities, these fields MUST be
protected by protocol instantiation security mechanisms (for example,
source authentication and data integrity mechanisms).
8.3. Data Transport
The LCT protocol provides for transport of information for other
building blocks, specifically the PSI field for the protocol
instantiation, the Congestion Control field for the Congestion
Control building block, the Codepoint field for the FEC building
block, the EXT-AUTH Header Extension (used by the protocol
instantiation) and the packet payload itself.
Modification of any of these fields by an attacker may result in a
denial-of-service attack. In particular, modification of the
Codepoint or packet payload may prevent successful reconstruction or
cause inaccurate reconstruction of large portions of an object by
receivers. Modification of the Congestion Control field may cause
receivers to attempt to receive at an incorrect rate, potentially
worsening or causing a congestion situation and thereby effecting a
DoS attack.
As a result of the above vulnerabilities, these fields MUST be
protected by protocol instantiation security mechanisms (for example,
source authentication and data integrity mechanisms).
9. IANA Considerations
9.1. Namespace Declaration for LCT Header Extension Types
This document defines a new namespace for "LCT Header Extension
Types". Values in this namespace are integers between 0 and 255
(inclusive).
Luby, et al. Standards Track PAGE 29
RFC 5651 LCT Building Block October 2009
Values in the range 0 to 63 (inclusive) are reserved for use for
variable-length LCT Header Extensions and assignments shall be made
through "IETF Review" as defined in [RFC 5226].
Values in the range 64 to 127 (inclusive) are reserved for variable-
length LCT Header Extensions and assignments shall be made on the
"Specification Required" basis as defined in [RFC 5226].
Values in the range 128 to 191 (inclusive) are reserved for use for
fixed-length LCT Header Extensions and assignments shall be made
through "IETF Review" as defined in [RFC 5226].
Values in the range 192 to 255 (inclusive) are reserved for fixed-
length LCT Header Extensions and assignments shall be made on the
"Specification Required" basis as defined in [RFC 5226].
Initial values for the LCT Header Extension Type registry are defined
in Section 9.2.
Note that the previous Experimental version of this specification
reserved values in the ranges [64, 127] and [192, 255] for PI-
specific LCT Header Extensions. In the interest of simplification
and since there were no overlapping allocations of these LCT Header
Extension Type values by PIs, this document specifies a single flat
space for LCT Header Extension Types.
9.2. LCT Header Extension Type Registration
This document registers three values in the LCT Header Extension Type
namespace as follows:
+-------+----------+--------------------+
| Value | Name | Reference |
+-------+----------+--------------------+
| 0 | EXT_NOP | This specification |
| | | |
| 1 | EXT_AUTH | This specification |
| | | |
| 2 | EXT_TIME | This specification |
+-------+----------+--------------------+
10. Acknowledgments
This specification is substantially based on RFC 3451 [RFC 3451] and
thus credit for the authorship of this document is primarily due to
the authors of RFC 3451: Mike Luby, Jim Gemmel, Lorenzo Vicisano,
Luigi Rizzo, Mark Handley, and Jon Crowcroft. Bruce Lueckenhoff,
Luby, et al. Standards Track PAGE 30
RFC 5651 LCT Building Block October 2009
Hayder Radha, and Justin Chapweske also contributed to RFC 3451.
Additional thanks are due to Vincent Roca, Rod Walsh, and Toni Paila
for contributions to this update to Proposed Standard.
11. Changes from RFC 3451
This section summarizes the changes that were made from the
Experimental version of this specification published as RFC 3451
[RFC 3451]:
o Removed the 'Statement of Intent' from the introduction. (The
statement of intent was meant to clarify the "Experimental" status
of RFC 3451.)
o Inclusion of material from ALC that is applicable in the more
general LCT context.
o Creation of an IANA registry for LCT Header Extensions.
o Allocation of the 2 'reserved' bits in the LCT header as
"Protocol-Specific Indication" - usage to be defined by protocol
instantiations.
o Removal of the Sender Current Time and Expected Residual Time LCT
header fields.
o Inclusion of a new Header Extension, EXT_TIME, to replace the SCT
and ERT and provide for future extension of timing capabilities.
12. References
12.1. Normative References
[RFC 768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[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 5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error
Correction (FEC) Building Block", RFC 5052,
August 2007.
Luby, et al. Standards Track PAGE 31
RFC 5651 LCT Building Block October 2009
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 5226, May 2008.
12.2. Informative References
[ALC-PI] Luby, M., Watson, M., and L. Vicisano, "Asynchronous
Layered Coding (ALC) Protocol Instantiation", Work
in Progress, September 2009.
[BYE1998] Byers, J., Luby, M., Mitzenmacher, M., and A. Rege,
"Fountain Approach to Reliable Distribution of Bulk
Data", Proceedings ACM SIGCOMM'98, Vancouver, Canada,
September 1998.
[BYE2000] Byers, J., Frumin, M., Horn, G., Luby, M.,
Mitzenmacher, M., Rotter, A., and W. Shaver, "FLID-DL:
Congestion Control for Layered Multicast", Proceedings
of Second International Workshop on Networked Group
Communications (NGC 2000), Palo Alto, CA,
November 2000.
[GEM2000] Gemmell, J., Schooler, E., and J. Gray, "Fcast
Multicast File Distribution", IEEE Network, Vol. 14,
No. 1, pp. 58-68, January 2000.
[Perrig2001] Perrig, A., Canetti, R., Song, D., and J. Tyger,
"Efficient and Secure Source Authentication for
Multicast", Network and Distributed System Security
Symposium, NDSS 2001, pp. 35-46, February 2001.
[RFC 1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992.
[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.
Luby, et al. Standards Track PAGE 32
RFC 5651 LCT Building Block October 2009
[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 3451] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L.,
Handley, M., and J. Crowcroft, "Layered Coding
Transport (LCT) Building Block", RFC 3451,
December 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.
[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 3738] Luby, M. and V. Goyal, "Wave and Equation Based Rate
Control (WEBRC) Building Block", RFC 3738, April 2004.
[RFC 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566, July 2006.
[RFC 4607] Holbrook, H. and B. Cain, "Source-Specific Multicast
for IP", RFC 4607, August 2006.
[RIZ1997a] Rizzo, L., "Effective Erasure Codes for Reliable
Computer Communication Protocols", ACM SIGCOMM Computer
Communication Review, Vol.27, No.2, pp.24-36,
April 1997.
[RIZ1997b] Rizzo, L. and L. Vicisano, "Reliable Multicast Data
Distribution protocol based on software FEC
techniques", Proceedings of the Fourth IEEE Workshop on
the Architecture and Implementation of High Performance
Communication Systems, HPCS'97, Chalkidiki Greece,
June 1997.
[RIZ2000] Rizzo, L., "PGMCC: A TCP-friendly single-rate multicast
congestion control scheme", Proceedings of SIGCOMM
2000, Stockholm Sweden, August 2000.
[VIC1998] Vicisano, L., Rizzo, L., and J. Crowcroft, "TCP-like
Congestion Control for Layered Multicast Data
Transfer", IEEE Infocom'98, San Francisco, CA,
March 1998.
Luby, et al. Standards Track PAGE 33
RFC 5651 LCT Building Block October 2009
Authors' Addresses
Michael Luby
Qualcomm, Inc.
3165 Kifer Rd.
Santa Clara, CA 95051
US
EMail: luby@qualcomm.com
Mark Watson
Qualcomm, Inc.
3165 Kifer Rd.
Santa Clara, CA 95051
US
EMail: watson@qualcomm.com
Lorenzo Vicisano
Qualcomm, Inc.
3165 Kifer Rd.
Santa Clara, CA 95051
US
EMail: vicisano@qualcomm.com
Luby, et al. Standards Track PAGE 34
Layered Coding Transport (LCT) Building Block
RFC TOTAL SIZE: 85751 bytes
PUBLICATION DATE: Monday, October 19th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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