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IETF RFC 5775
Asynchronous Layered Coding (ALC) Protocol Instantiation
Last modified on Tuesday, April 6th, 2010
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Internet Engineering Task Force (IETF) M. Luby
Request for Comments: 5775 M. Watson
Obsoletes: 3450 L. Vicisano
Category: Standards Track Qualcomm, Inc.
ISSN: 2070-1721 April 2010
Asynchronous Layered Coding (ALC) Protocol Instantiation
 Abstract
This document describes the Asynchronous Layered Coding (ALC)
protocol, a massively scalable reliable content delivery protocol.
Asynchronous Layered Coding combines the Layered Coding Transport
(LCT) building block, a multiple rate congestion control building
block and the Forward Error Correction (FEC) building block to
provide congestion controlled reliable asynchronous delivery of
content to an unlimited number of concurrent receivers from a single
sender. This document obsoletes RFC 3450.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 5775.
Copyright Notice
Copyright (c) 2010 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 Simplified BSD License.
Luby, et al. Standards Track PAGE 1 
RFC 5775 ALC Protocol Instantiation April 2010
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.
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
1.1. Delivery Service Models . . . . . . . . . . . . . . . . . 4
1.2. Scalability . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Environmental Requirements and Considerations . . . . . . 5
2. Architecture Definition . . . . . . . . . . . . . . . . . . . 5
2.1. LCT Building Block . . . . . . . . . . . . . . . . . . . . 7
2.2. Multiple Rate Congestion Control Building Block . . . . . 9
2.3. FEC Building Block . . . . . . . . . . . . . . . . . . . . 10
2.4. Session Description . . . . . . . . . . . . . . . . . . . 11
2.5. Packet Authentication Building Block . . . . . . . . . . . 12
3. Conformance Statement . . . . . . . . . . . . . . . . . . . . 12
4. Functionality Definition . . . . . . . . . . . . . . . . . . . 13
4.1. Packet Format Used by ALC . . . . . . . . . . . . . . . . 13
4.2. LCT Header Extension Fields . . . . . . . . . . . . . . . 14
4.3. Sender Operation . . . . . . . . . . . . . . . . . . . . . 15
4.4. Receiver Operation . . . . . . . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 16
5.1. Baseline Secure ALC Operation . . . . . . . . . . . . . . 18
5.1.1. IPsec Approach . . . . . . . . . . . . . . . . . . . . 18
5.1.2. IPsec Requirements . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
8. Changes from RFC 3450 . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . . 23
Luby, et al. Standards Track PAGE 2
RFC 5775 ALC Protocol Instantiation April 2010
1. Introduction
This document describes a massively scalable reliable content
delivery protocol, Asynchronous Layered Coding (ALC), for multiple
rate congestion controlled reliable content delivery. The protocol
is specifically designed to provide massive scalability using IP
multicast as the underlying network service. Massive scalability in
this context means the number of concurrent receivers for an object
is potentially in the millions, the aggregate size of objects to be
delivered in a session ranges from hundreds of kilobytes to hundreds
of gigabytes, each receiver can initiate reception of an object
asynchronously, the reception rate of each receiver in the session is
the maximum fair bandwidth available between that receiver and the
sender, and all of this can be supported using a single sender.
Because ALC is focused on reliable content delivery, the goal is to
deliver objects as quickly as possible to each receiver while at the
same time remaining network friendly to competing traffic. Thus, the
congestion control used in conjunction with ALC should strive to
maximize use of available bandwidth between receivers and the sender
while at the same time backing off aggressively in the face of
competing traffic.
The sender side of ALC consists of generating packets based on
objects to be delivered within the session and sending the
appropriately formatted packets at the appropriate rates to the
channels associated with the session. The receiver side of ALC
consists of joining appropriate channels associated with the session,
performing congestion control by adjusting the set of joined channels
associated with the session in response to detected congestion, and
using the packets to reliably reconstruct objects. All information
flow in an ALC session is in the form of data packets sent by a
single sender to channels that receivers join to receive data.
ALC does specify the Session Description needed by receivers before
they join a session, but the mechanisms by which receivers obtain
this required information is outside the scope of ALC. An
application that uses ALC may require that receivers report
statistics on their reception experience back to the sender, but the
mechanisms by which receivers report back statistics is outside the
scope of ALC. In general, ALC is designed to be a minimal protocol
instantiation that provides reliable content delivery without
unnecessary limitations to the scalability of the basic protocol.
This document is a product of the IETF RMT WG and follows the general
guidelines provided in [RFC 3269].
Luby, et al. Standards Track PAGE 3
RFC 5775 ALC Protocol Instantiation April 2010
A previous version of this document was published in the
"Experimental" category as [RFC 3450] and is obsoleted by this
document.
This Proposed Standard specification is thus based on and backwards
compatible with the protocol defined in [RFC 3450] 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 BCP 14, [RFC 2119].
1.1. Delivery Service Models
ALC can support several different reliable content delivery service
models as described in [RFC 5651].
1.2. Scalability
Massive scalability is a primary design goal for ALC. IP multicast
is inherently massively scalable, but the best effort service that it
provides does not provide session management functionality,
congestion control, or reliability. ALC provides all of this on top
of IP multicast without sacrificing any of the inherent scalability
of IP multicast. ALC has the following properties:
o To each receiver, it appears as if there is a dedicated session
from the sender to the receiver, where the reception rate adjusts
to congestion along the path from sender to receiver.
o To the sender, there is no difference in load or outgoing rate if
one receiver or a million (or any number of) receivers are joined
to the session, independent of when the receivers join and leave.
o No feedback packets are required from receivers to the sender.
o Almost all packets in the session that pass through a bottleneck
link are utilized by downstream receivers, and the session shares
the link with competing flows fairly in proportion to their
utility.
Thus, ALC provides a massively scalable content delivery transport
that is network friendly.
Luby, et al. Standards Track PAGE 4
RFC 5775 ALC Protocol Instantiation April 2010
ALC intentionally omits any application-specific features that could
potentially limit its scalability. By doing so, ALC provides a
minimal protocol that is massively scalable. Applications may be
built on top of ALC to provide additional features that may limit the
scalability of the application. Such applications are outside the
scope of this document.
1.3. Environmental Requirements and Considerations
All of the environmental requirements and considerations that apply
to the LCT building block [RFC 5651], the FEC building block
[RFC 5052], the multiple rate congestion control building block, and
to any additional building blocks that ALC uses also apply to ALC.
The IP multicast model defined in [RFC 1112] is commonly known as Any-
Source Multicast (ASM), in contrast to Source-Specific Multicast
(SSM) which is defined in [RFC 3569]. One issue that is specific to
ALC with respect to ASM is the way the multiple rate congestion
control building block interacts with ASM. The congestion control
building block may use the measured difference in time between when a
join to a channel is sent and when the first packet from the channel
arrives in determining the receiver reception rate. The congestion
control building block may also use packet sequence numbers per
channel to measure losses, and this is also used to determine the
receiver reception rate. These features raise two concerns with
respect to ASM: The time difference between when the join to a
channel is sent and when the first packet arrives can be significant
due to the use of Rendezvous Points (RPs) and the Multicast Source
Discovery Protocol (MSDP) [RFC 3618] protocol, and packets can be lost
in the switch over from the (*,G) join to the RP and the (S,G) join
directly to the sender. Both of these issues could potentially
substantially degrade the reception rate of receivers. To ameliorate
these concerns, it is recommended during deployment to ensure that
the RP be as close to the sender as possible. SSM does not share
these same concerns. For a fuller consideration of these issues,
consult the multiple rate congestion control building block.
2. Architecture Definition
ALC uses the LCT building block [RFC 5651] to provide in-band session
management functionality. ALC uses a multiple rate congestion
control building block that is compliant with [RFC 2357] to provide
congestion control that is feedback free. Receivers adjust their
reception rates individually by joining and leaving channels
associated with the session. ALC uses the FEC building block
[RFC 5052] to provide reliability. The sender generates encoding
symbols based on the object to be delivered using FEC codes and sends
them in packets to channels associated with the session. Receivers
Luby, et al. Standards Track PAGE 5
RFC 5775 ALC Protocol Instantiation April 2010
simply wait for enough packets to arrive in order to reliably
reconstruct the object. Thus, there is no request for retransmission
of individual packets from receivers that miss packets in order to
assure reliable reception of an object, and the packets and their
rate of transmission out of the sender can be independent of the
number and the individual reception experiences of the receivers.
The definition of a session for ALC is the same as it is for LCT. An
ALC 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. Congestion control is performed over the
aggregate of packets sent to channels belonging to a session. The
fact that an ALC session is restricted to a single sender does not
preclude the possibility of receiving packets for the same objects
from multiple senders. However, each sender would be sending packets
to a different session to which congestion control is individually
applied. Although receiving concurrently from multiple sessions is
allowed, how this is done at the application level is outside the
scope of this document.
ALC is a protocol instantiation as defined in [RFC 3048]. This
document describes version 1 of ALC, which MUST use version 1 of LCT
described in [RFC 5651]. Like LCT, ALC is designed to be used with
the IP multicast network service. This specification defines ALC as
payload of the UDP transport protocol [RFC 768] that supports the IP
multicast delivery of packets.
The ALC packet format is illustrated in Figure 1. An ALC packet
header immediately follows the IP/UDP header and consists of the
default LCT header that is described in [RFC 5651] followed by the FEC
Payload ID that is described in [RFC 5052]. The Congestion Control
Information field within the LCT header carries the required
Congestion Control Information that is described in the multiple rate
congestion control building block specified that is compliant with
[RFC 2357]. The packet payload that follows the ALC packet header
consists of encoding symbols that are identified by the FEC Payload
ID as described in [RFC 5052].
Luby, et al. Standards Track PAGE 6
RFC 5775 ALC Protocol Instantiation April 2010
+----------------------------------------+
| IP Header |
+----------------------------------------+
| UDP Header |
+----------------------------------------+
| LCT Header |
+----------------------------------------+
| FEC Payload Id |
+----------------------------------------+
| Encoding Symbols |
+----------------------------------------+
Figure 1: ALC Packet Format
Each receiver is required to obtain a Session Description before
joining an ALC session. As described later, the Session Description
includes out-of-band information required for the LCT, FEC, and the
multiple rate congestion control building blocks. The FEC Object
Transmission Information specified in the FEC building block
[RFC 5052] required for each object to be received by a receiver can
be communicated to a receiver either out-of-band or in-band using a
Header Extension. The means for communicating the Session
Description and the FEC Object Transmission Information to a receiver
is outside the scope of this document.
2.1. LCT Building Block
LCT requires receivers to be able to uniquely identify and
demultiplex packets associated with an LCT session, and ALC inherits
and strengthens this requirement. A Transport Session Identifier
(TSI) MUST be associated with each session and MUST be carried in the
LCT header of each ALC packet. The TSI is scoped by the sender IP
address, and the (sender IP address, TSI) pair MUST uniquely identify
the session.
The LCT header contains a Congestion Control Information (CCI) field
that MUST be used to carry the Congestion Control Information from
the specified multiple rate congestion control protocol. There is a
field in the LCT header that specifies the length of the CCI field,
and the multiple rate congestion control building block MUST uniquely
identify a format of the CCI field that corresponds to this length.
The LCT header contains a Codepoint field that MAY be used to
communicate to a receiver the settings for information that may vary
during a session. If used, the mapping between settings and
Codepoint values is to be communicated in the Session Description,
and this mapping is outside the scope of this document. For example,
the FEC Encoding ID that is part of the FEC Object Transmission
Luby, et al. Standards Track PAGE 7
RFC 5775 ALC Protocol Instantiation April 2010
Information, as specified in the FEC building block [RFC 5052], could
vary for each object carried in the session, and the Codepoint value
could be used to communicate the FEC Encoding ID to be used for each
object. The mapping between FEC Encoding IDs and Codepoints could
be, for example, the identity mapping.
If more than one object is to be carried within a session, then the
Transmission Object Identifier (TOI) MUST be used in the LCT header
to identify which packets are to be associated with which objects.
In this case, the receiver MUST use the TOI to associate received
packets with objects. The TOI is scoped by the IP address of the
sender and the TSI, i.e., the TOI is scoped by the session. The TOI
for each object is REQUIRED to be unique within a session, but is not
required be unique across sessions. Furthermore, the same object MAY
have a different TOI in different sessions. The mapping between TOIs
and objects carried in a session is outside the scope of this
document.
If only one object is carried within a session, then the TOI MAY be
omitted from the LCT header.
The LCT header from version 1 of the LCT building block [RFC 5651]
MUST be used.
The LCT Header includes a two-bit Protocol Specific Indication (PSI)
field in bits 6 and 7 of the first word of the LCT header. These two
bits are used by ALC 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
+-+-+
...|X|Y|...
+-+-+
Figure 2: PSI Bits within LCT Header
PSI bit X - Source Packet Indicator (SPI)
PSI bit Y - Reserved
The Source Packet Indicator is used with systematic FEC Schemes which
define a different FEC Payload ID format for packets containing only
source data compared to the FEC Payload ID format for packets
containing repair data. For such FEC Schemes, the SPI MUST be set to
1 when the FEC Payload ID format for packets containing only source
data is used, and the SPI MUST be set to zero when the FEC Payload ID
for packets containing repair data is used. In the case of FEC
Luby, et al. Standards Track PAGE 8
RFC 5775 ALC Protocol Instantiation April 2010
Schemes that define only a single FEC Payload ID format, the SPI MUST
be set to zero by the sender and MUST be ignored by the receiver.
Support of two FEC Payload ID formats allows FEC Payload ID
information that is only of relevance when FEC decoding is to be
performed to be provided in the FEC Payload ID format for packets
containing repair data. This information need not be processed by
receivers that do not perform FEC decoding (either because no FEC
decoding is required or because the receiver does not support FEC
decoding).
2.2. Multiple Rate Congestion Control Building Block
At a minimum, implementations of ALC MUST support [RFC 3738]. Note
that [RFC 3738] has been published in the "Experimental" category and
thus has lower maturity level than the present document. Caution
should be used since it may be less stable than this document.
Congestion control MUST be applied to all packets within a session
independently of which information about which object is carried in
each packet. Multiple rate congestion control is specified because
of its suitability to scale massively and because of its suitability
for reliable content delivery. [RFC 3738] specifies in-band
Congestion Control Information (CCI) that MUST be carried in the CCI
field of the LCT header.
Alternative multiple rate congestion control building blocks MAY be
supported, but only a single congestion control building block may be
used in a given ALC session. The congestion control building block
to be used in an ALC session is specified in the Session Description
(see Section 2.4). A multiple rate congestion control building block
MAY specify more than one format for the CCI field, but it MUST
specify at most one format for each of the possible lengths 32, 64,
96, or 128 bits. The value of C in the LCT header that determines
the length of the CCI field MUST correspond to one of the lengths for
the CCI defined in the multiple rate congestion control building
block; this length MUST be the same for all packets sent to a
session, and the CCI format that corresponds to the length as
specified in the multiple rate congestion control building block MUST
be the format used for the CCI field in the LCT header.
When using a multiple rate congestion control building block, a
sender sends packets in the session to several channels at
potentially different rates. Then, individual receivers adjust their
reception rate within a session by adjusting to which set of channels
they are joined at each point in time depending on the available
bandwidth between the receiver and the sender, but independent of
other receivers.
Luby, et al. Standards Track PAGE 9
RFC 5775 ALC Protocol Instantiation April 2010
2.3. FEC Building Block
The FEC building block [RFC 5052] provides reliable object delivery
within an ALC session. Each object sent in the session is
independently encoded using FEC codes as described in [RFC 3453],
which provide a more in-depth description of the use of FEC codes in
reliable content delivery protocols. All packets in an ALC session
MUST contain an FEC Payload ID in a format that is compliant with the
FEC Scheme in use. The FEC Payload ID uniquely identifies the
encoding symbols that constitute the payload of each packet, and the
receiver MUST use the FEC Payload ID to determine how the encoding
symbols carried in the payload of the packet were generated from the
object as described in the FEC building block.
As described in [RFC 5052], a receiver is REQUIRED to obtain the FEC
Object Transmission Information for each object for which data
packets are received from the session. In the context of ALC, the
FEC Object Transmission Information includes:
o The FEC Encoding ID.
o If an Under-Specified FEC Encoding ID is used, then the FEC
Instance ID associated with the FEC Encoding ID.
o For each object in the session, the transfer length of the object
in bytes.
Additional FEC Object Transmission Information may be required
depending on the FEC Scheme that is used (identified by the FEC
Encoding ID).
Some of the FEC Object Transmission Information MAY be implicit based
on the FEC Scheme and/or implementation. As an example, source block
lengths may be derived by a fixed algorithm from the object length.
As another example, it may be that all source blocks are the same
length and this is what is passed out-of-band to the receiver. As
another example, it could be that the full-sized source block length
is provided, and this is the length used for all but the last source
block, which is calculated based on the full source block length and
the object length. As another example, it could be that the same FEC
Encoding ID and FEC Instance ID are always used for a particular
application, and thus the FEC Encoding ID and FEC Instance ID are
implicitly defined.
Sometimes the objects that will be sent in a session are completely
known before the receiver joins the session, in which case the FEC
Object Transmission Information for all objects in the session can be
communicated to receivers before they join the session. At other
Luby, et al. Standards Track PAGE 10
RFC 5775 ALC Protocol Instantiation April 2010
times, the objects may not know when the session begins, receivers
may join a session in progress and may not be interested in some
objects for which transmission has finished, or receivers may leave a
session before some objects are even available within the session.
In these cases, the FEC Object Transmission Information for each
object may be dynamically communicated to receivers at or before the
time packets for the object are received from the session. This may
be accomplished using an out-of-band mechanism, in-band using the
Codepoint field or a Header Extension, or any combination of these
methods. How the FEC Object Transmission Information is communicated
to receivers is outside the scope of this document.
2.4. Session Description
Before a receiver can join an ALC session, the receiver needs to
obtain a Session Description that contains the following information:
o The multiple rate congestion control building block to be used for
the session;
o The sender IP address;
o The number of channels in the session;
o The address and port number used for each channel in the session;
o The Transport Session ID (TSI) to be used for the session;
o An indication of whether or not the session carries packets for
more than one object;
o If Header Extensions are to be used, the format of these Header
Extensions.
o Enough information to determine the packet authentication scheme
being used, if one is being used.
How the Session Description is communicated to receivers is outside
the scope of this document.
The Codepoint field within the LCT portion of the header CAN be used
to communicate in-band some of the dynamically changing information
within a session. To do this, a mapping between Codepoint values and
the different dynamic settings MUST be included within the Session
Description, and then settings to be used are communicated via the
Codepoint value placed into each packet. For example, it is possible
that multiple objects are delivered within the same session and that
a different FEC encoding algorithm is used for different types of
Luby, et al. Standards Track PAGE 11
RFC 5775 ALC Protocol Instantiation April 2010
objects. Then the Session Description could contain the mapping
between Codepoint values and FEC Encoding IDs. As another example,
it is possible that a different packet authentication scheme is used
for different packets sent to the session. In this case, the mapping
between the packet authentication scheme and Codepoint values could
be provided in the Session Description. Combinations of settings can
be mapped to Codepoint values as well. For example, a particular
combination of a FEC Encoding ID and a packet authentication scheme
could be associated with a Codepoint value.
The Session Description could also include, but is not limited to:
o The mappings between combinations of settings and Codepoint
values;
o The data rates used for each channel;
o The length of the packet payload;
o Any information that is relevant to each object being transported,
such as the Object Transmission Information for each object, when
the object will be available within the session, and for how long.
The Session Description could be in a form such as the Session
Description Protocol (SDP) as defined in [RFC 4566], XML metadata as
defined in [RFC 3023], or HTTP/MIME headers as defined in [RFC 2616],
etc. 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 formats and methods for communication of Session
Descriptions to receivers is beyond the scope of this document.
2.5. Packet Authentication Building Block
It is RECOMMENDED that implementors of ALC use some packet
authentication scheme to protect the protocol from attacks. Suitable
schemes are described in [RFC 5776] and [RMT-SIMPLE].
3. Conformance Statement
This Protocol Instantiation document, in conjunction with the LCT
building block [RFC 5651], the FEC building block [RFC 5052], and
[RFC 3738] completely specifies a working reliable multicast transport
protocol that conforms to the requirements described in [RFC 2357].
Luby, et al. Standards Track PAGE 12
RFC 5775 ALC Protocol Instantiation April 2010
4. Functionality Definition
This section describes the format and functionality of the data
packets carried in an ALC session as well as the sender and receiver
operations for a session.
4.1. Packet Format Used by ALC
The packet format used by ALC is the UDP header followed by the LCT
header followed by the FEC Payload ID followed by the packet payload.
The LCT header is defined in the LCT building block [RFC 5651] and the
FEC Payload ID is described in the FEC building block [RFC 5052]. The
Congestion Control Information field in the LCT header contains the
required Congestion Control Information that is described in the
multiple rate congestion control building block used. The packet
payload contains encoding symbols generated from an object. If more
than one object is carried in the session, then the Transmission
Object ID (TOI) within the LCT header MUST be used to identify from
which object the encoding symbols are generated. Within the scope of
an object, encoding symbols carried in the payload of the packet are
identified by the FEC Payload ID as described in the FEC building
block.
The version number of ALC specified in this document is 1. The
version number field of the LCT header MUST be interpreted as the ALC
version number field. This version of ALC implicitly makes use of
version 1 of the LCT building block defined in [RFC 5651].
The overall ALC packet format is depicted in Figure 3. The packet is
an IP packet, either IPv4 or IPv6, and the IP header precedes the UDP
header. The ALC packet format has no dependencies on the IP version
number.
Luby, et al. Standards Track PAGE 13
RFC 5775 ALC Protocol Instantiation April 2010
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Header |
| |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| LCT Header |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Payload ID |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol(s) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Overall ALC Packet Format
In some special cases an ALC sender may need to produce ALC packets
that do not contain any payload. This may be required, for example,
to signal the end of a session or to convey congestion control
information. These data-less packets do not contain the FEC Payload
ID either, but only the LCT header fields. The total datagram
length, conveyed by outer protocol headers (e.g., the IP or UDP
header), enables receivers to detect the absence of the ALC payload
and FEC Payload ID.
For ALC, the length of the TSI field within the LCT header is
REQUIRED to be non-zero. This implies that the sender MUST NOT set
both the LCT flags S and H to zero.
4.2. LCT Header Extension Fields
This specification defines a new LCT Header Extension, "EXT_FTI", for
the purpose of communicating the FEC Object Transmission Information
in association with data packets of an object. The LCT Header
Extension Type for EXT_FTI is 64.
The Header Extension Content (HEC) field of the EXT_FTI LCT Header
Extension contains the encoded FEC Object Transmission Information as
defined in [RFC 5052]. The format of the encoded FEC Object
Transmission Information is dependent on the FEC Scheme in use and so
is outside the scope of this document.
Luby, et al. Standards Track PAGE 14
RFC 5775 ALC Protocol Instantiation April 2010
4.3. Sender Operation
The sender operation, when using ALC, includes all the points made
about the sender operation when using the LCT building block
[RFC 5651], the FEC building block [RFC 5052], and the multiple rate
congestion control building block.
A sender using ALC should make available the required Session
Description as described in Section 2.4. A sender should also make
available the required FEC Object Transmission Information as
described in Section 2.3.
Within a session, a sender transmits a sequence of packets to the
channels associated with the session. The ALC sender MUST obey the
rules for filling in the CCI field in the packet headers, and it MUST
send packets at the appropriate rates to the channels associated with
the session as dictated by the multiple rate congestion control
building block.
The ALC sender MUST use the same TSI for all packets in the session.
Several objects MAY be delivered within the same ALC session. If
more than one object is to be delivered within a session, then the
sender MUST use the TOI field. Each object MUST be identified by a
unique TOI within the session, and the sender MUST use corresponding
TOI for all packets pertaining to the same object. The FEC Payload
ID MUST correspond to the encoding symbol(s) for the object carried
in the payload of the packet.
It is RECOMMENDED that packet authentication be used. If packet
authentication is used, then the Header Extensions described in
Section 4.2 MAY be used to carry the authentication.
4.4. Receiver Operation
The receiver operation, when using ALC, includes all the points made
about the receiver operation when using the LCT building block
[RFC 5651], the FEC building block [RFC 5052], and the multiple rate
congestion control building block.
To be able to participate in a session, a receiver needs to obtain
the required Session Description as listed in Section 2.4. How
receivers obtain a Session Description is outside the scope of this
document.
As described in Section 2.3, a receiver needs to obtain the required
FEC Object Transmission Information for each object for which the
receiver receives and processes packets.
Luby, et al. Standards Track PAGE 15
RFC 5775 ALC Protocol Instantiation April 2010
Upon receipt of each packet, the receiver proceeds with the following
steps in the order listed.
1. The receiver MUST parse the packet header and verify that it is a
valid header. If it is not valid, then the packet MUST be
discarded without further processing.
2. The receiver MUST verify that the sender IP address together with
the TSI carried in the header matches one of the (sender IP
address, TSI) pairs that was received in a Session Description
and to which the receiver is currently joined. If there is not a
match, then the packet MUST be silently discarded without further
processing. The remaining steps are performed within the scope
of the (sender IP address, TSI) session of the received packet.
3. The receiver MUST process and act on the CCI field in accordance
with the multiple rate congestion control building block.
4. If more than one object is carried in the session, the receiver
MUST verify that the TOI carried in the LCT header is valid. If
the TOI is not valid, the packet MUST be discarded without
further processing.
5. The receiver SHOULD process the remainder of the packet,
including interpreting the other header fields appropriately, and
using the FEC Payload ID and the encoding symbol(s) in the
payload to reconstruct the corresponding object.
It is RECOMMENDED that packet authentication be used. If packet
authentication is used, then it is RECOMMENDED that the receiver
immediately check the authenticity of a packet before proceeding with
step (3) above. If immediate checking is possible and if the packet
fails the check, then the receiver MUST silently discard the packet.
5. Security Considerations
The same security considerations that apply to the LCT, FEC, and the
multiple rate congestion control building blocks also apply to ALC.
ALC is especially vulnerable to denial-of-service attacks by
attackers that try to send forged packets to the session, which would
prevent successful reconstruction or cause inaccurate reconstruction
of large portions of the object by receivers. ALC is also
particularly affected by such an attack because many receivers may
receive the same forged packet. There are two ways to protect
against such attacks, one at the application level and one at the
packet level. It is RECOMMENDED that prevention be provided at both
levels.
Luby, et al. Standards Track PAGE 16
RFC 5775 ALC Protocol Instantiation April 2010
At the application level, it is RECOMMENDED that an integrity check
on the entire received object be done once the object is
reconstructed to ensure it is the same as the sent object. Moreover,
in order to obtain strong cryptographic integrity protection, a
digital signature verifiable by the receiver SHOULD be used to
provide this application-level integrity check. However, if even one
corrupted or forged packet is used to reconstruct the object, it is
likely that the received object will be reconstructed incorrectly.
This will appropriately cause the integrity check to fail and in this
case, the inaccurately reconstructed object SHOULD be discarded.
Thus, the acceptance of a single forged packet can be an effective
denial-of-service attack for distributing objects, but an object
integrity check at least prevents inadvertent use of inaccurately
reconstructed objects. The specification of an application-level
integrity check of the received object is outside the scope of this
document.
At the packet level, it is RECOMMENDED that a packet-level
authentication be used to ensure that each received packet is an
authentic and uncorrupted packet containing data for the object
arriving from the specified sender. Packet-level authentication has
the advantage that corrupt or forged packets can be discarded
individually and the received authenticated packets can be used to
accurately reconstruct the object. Thus, the effect of a denial-of-
service attack that injects forged packets is proportional only to
the number of forged packets, and not to the object size.
[RMT-SIMPLE]and [RFC 5776] described packet level authentication
schemes that can be used with the ALC protocol.
In addition to providing protection against reconstruction of
inaccurate objects, packet-level authentication can also provide some
protection against denial-of-service attacks on the multiple rate
congestion control. Attackers can try to inject forged packets with
incorrect congestion control information into the multicast stream,
thereby potentially adversely affecting network elements and
receivers downstream of the attack, and much less significantly the
rest of the network and other receivers. Thus, it is also
RECOMMENDED that packet-level authentication be used to protect
against such attacks. Timed Efficient Stream Loss-Tolerant
Authentication (TESLA) [RFC 5776] can also be used to some extent to
limit the damage caused by such attacks. However, with TESLA, a
receiver can only determine if a packet is authentic several seconds
after it is received, and thus an attack against the congestion
control protocol can be effective for several seconds before the
receiver can react to slow down the session reception rate.
Some packet authentication schemes such as TESLA [RFC 5776] do not
allow an immediate authenticity check. In this case, the receiver
Luby, et al. Standards Track PAGE 17
RFC 5775 ALC Protocol Instantiation April 2010
SHOULD check the authenticity of a packet as soon as possible, and if
the packet fails the check, then it MUST be silently discarded before
Step 5 above. It is RECOMMENDED that if receivers detect many
packets that fail authentication checks within a session, the above
procedure should be modified for this session so that Step 3 is
delayed until after the authentication check and only performed if
the check succeeds.
Reverse Path Forwarding checks SHOULD be enabled in all network
routers and switches along the path from the sender to receivers to
limit the possibility of a bad agent injecting forged packets into
the multicast tree data path.
5.1. Baseline Secure ALC Operation
This section describes a baseline mode of secure ALC protocol
operation based on application of the IPsec security protocol. This
approach is documented here to provide a reference of an
interoperable secure mode of operation. However, additional
approaches to ALC 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 ALC-specific
authentication or security encapsulation headers similar to those of
IPsec to be specified and inserted into the ALC/LCT protocol message
headers. This would allow header compression techniques to be
applied to IP and ALC protocol headers when needed in a similar
fashion to that of RTP [RFC 3550] and as preserved in the
specification for Secure Real Time Protocol (SRTP) [RFC 3711].
The baseline approach described is applicable to ALC operation
configured for SSM (or SSM-like) operation where there is a single
sender. The receiver set would maintain a single IPsec Security
Association (SA) for each ALC sender.
5.1.1. IPsec Approach
To support this baseline form of secure ALC one-to-many SSM
operation, each node SHALL be configured with a transport mode IPsec
Security Association and corresponding Security Policy Database (SPD)
entry. This entry will be used for sender-to-group multicast packet
authentication and optionally encryption.
The ALC sender SHALL use an IPsec SA configured for Encapsulating
Security Payload (ESP) protocol [RFC 4303] operation with the option
for data origination authentication enabled. It is also RECOMMENDED
that this IPsec ESP SA be also configured to provide confidentiality
protection for IP packets containing ALC protocol messages. This is
suggested to make the realization of complex replay attacks much more
Luby, et al. Standards Track PAGE 18
RFC 5775 ALC Protocol Instantiation April 2010
difficult. The encryption key for this SA SHALL be preplaced at the
sender and receiver(s) prior to ALC 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 that receivers may use the built-in IPsec replay attack
protection possible for an IPsec SA with a single source (the ALC
sender). Thus, the receivers SHALL enable replay attack protection
for this SA used to secure ALC sender traffic. The sender IPsec SPD
entry MUST be configured to process outbound packets to the
destination address and UDP port number of the applicable ALC
session.
The ALC receiver(s) MUST be configured with the SA and SPD entry to
properly process the IPsec-secured packets from the sender. Note
that use of ESP confidentiality, as RECOMMENDED, for secure ALC
protocol operation makes it more difficult for adversaries to conduct
effective replay attacks that may mislead receivers on content
reception. This baseline approach can be used for ALC protocol
sessions with multiple senders if a distinct SA is established for
each sender.
In early deployments of this baseline approach to ALC security, it is
anticipated that key management will be conducted out-of-band with
respect to ALC protocol operation. For ALC unidirectional operation,
it is possible that receivers may retrieve keying information from a
central server via out-of-band (with respect to ALC) communication as
needed or otherwise conduct group key updates with a similar
centralized approach. However, it may be possible with some key
management schemes for rekey messages to be transmitted to the group
as a message or transport object within the ALC reliable transfer
session. An additional specification is necessary to define an in-
band key management scheme for ALC sessions perhaps using the
mechanisms of the automated group key management specifications cited
in this document.
5.1.2. IPsec Requirements
In order to implement this secure mode of ALC protocol operation, the
following IPsec capabilities are required.
5.1.2.1. Selectors
The implementation MUST be able to use the source address,
destination address, protocol (UDP), and UDP port numbers as
selectors in the SPD.
Luby, et al. Standards Track PAGE 19
RFC 5775 ALC Protocol Instantiation April 2010
5.1.2.2. Mode
IPsec in transport mode MUST be supported. The use of IPsec
[RFC 4301] processing for secure ALC traffic SHOULD also be REQUIRED
such that unauthenticated packets are not received by the ALC
protocol implementation.
5.1.2.3. Key Management
An automated key management scheme for group key distribution and
rekeying such as the Group Domain of Interpretation (GDOI) [RFC 3547],
Group Secure Association Key Management Protocol (GSAKMP) [RFC 4535],
or Multimedia Internet KEYing (MIKEY) [RFC 3830] SHOULD be used.
Relatively short-lived ALC 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. It should also be noted that it may be possible for key update
messages (e.g., the GDOI GROUPKEY-PUSH message) to be included in the
ALC application reliable data transmission as transport objects if
appropriate interfaces were available between the ALC application and
the key management daemon.
5.1.2.4. Security Policy
Receivers SHOULD accept connections only from the designated,
authorized sender(s). It is expected that appropriate key management
will provide 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.
5.1.2.5. Authentication and Encryption
Large ALC group sizes may necessitate some form of key management
that does rely upon shared secrets. The GDOI and GSAKMP protocols
mentioned here allow for certificate-based authentication. These
certificates SHOULD use IP addresses for authentication. However, it
is likely that available group key management implementations will
not be ALC-specific.
5.1.2.6. Availability
The IPsec requirements profile outlined here is commonly available on
many potential ALC hosts. The principal issue is that configuration
and operation of IPsec typically requires privileged user
authorization. Automated key management implementations are
typically configured with the privileges necessary to allow the
needed system IPsec configuration.
Luby, et al. Standards Track PAGE 20
RFC 5775 ALC Protocol Instantiation April 2010
6. IANA Considerations
This specification registers one value in the LCT Header Extensions
Types namespace as follows:
+-------+---------+--------------------+
| Value | Name | Reference |
+-------+---------+--------------------+
| 64 | EXT_FTI | This specification |
+-------+---------+--------------------+
7. Acknowledgments
This specification is substantially based on RFC 3450 [RFC 3450] and
thus credit for the authorship of this document is primarily due to
the authors of RFC 3450: Mike Luby, Jim Gemmel, Lorenzo Vicisano,
Luigi Rizzo, and Jon Crowcroft. Vincent Roca, Justin Chapweske, and
Roger Kermode also contributed to RFC 3450. Additional thanks are
due to Vincent Roca and Rod Walsh for contributions to this update to
Proposed Standard.
8. Changes from RFC 3450
This section summarizes the changes that were made from the
"Experimental" version of this specification published as RFC 3450
[RFC 3450]:
o Updated all references to the obsoleted RFC 2068 to RFC 2616.
o Removed the 'Statement of Intent' from the introduction. (The
Statement of Intent was meant to clarify the "Experimental" status
of RFC 3450.)
o Removed the 'Intellectual Property Issues' Section and replaced
with a standard IPR Statement.
o Removed material duplicated in LCT.
o Updated references in this document to new versions of the LCT
Building Block and the FEC Building Block, and aligned this
document with changes in the new version of the FEC Building
Block.
o Split normative and informative references.
o Material applicable in a general LCT context, not just for ALC has
been moved to LCT.
Luby, et al. Standards Track PAGE 21
RFC 5775 ALC Protocol Instantiation April 2010
o The first bit of the "Protocol-Specific Indication" in the LCT
Header is defined as a "Source Packet Indication". This is used
in the case that an FEC Scheme defines two FEC Payload ID formats,
one of which is for packets containing only source symbols that
can be processed by receivers that do not support FEC Decoding.
o Definition and IANA registration of the EXT_FTI LCT Header
Extension.
9. References
9.1. 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 2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC 3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[RFC 3738] Luby, M. and V. Goyal, "Wave and Equation Based Rate
Control (WEBRC) Building Block", RFC 3738, April 2004.
[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 4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566, July 2006.
[RFC 5052] Watson, M., Luby, M., and L. Vicisano, "Forward Error
Correction (FEC) Building Block", RFC 5052,
August 2007.
[RFC 5651] Luby, M., Watson, M., and L. Vicisano, "Layered Coding
Transport (LCT) Building Block", RFC 5651,
October 2009.
Luby, et al. Standards Track PAGE 22
RFC 5775 ALC Protocol Instantiation April 2010
9.2. Informative References
[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 3450] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and J.
Crowcroft, "Asynchronous Layered Coding (ALC) Protocol
Instantiation", RFC 3450, 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 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 3569] Bhattacharyya, S., "An Overview of Source-Specific
Multicast (SSM)", RFC 3569, July 2003.
[RFC 3618] Fenner, B. and D. Meyer, "Multicast Source Discovery
Protocol (MSDP)", RFC 3618, October 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.
Luby, et al. Standards Track PAGE 23
RFC 5775 ALC Protocol Instantiation April 2010
[RFC 3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and
K. Norrman, "MIKEY: Multimedia Internet KEYing",
RFC 3830, August 2004.
[RFC 4535] Harney, H., Meth, U., Colegrove, A., and G. Gross,
"GSAKMP: Group Secure Association Key Management
Protocol", RFC 4535, June 2006.
[RFC 5776] Roca, V., Francillon, A., and S. Faurite, "Use of Timed
Efficient Stream Loss-Tolerant Authentication (TESLA)
in the Asynchronous Layered Coding (ALC) and NACK-
Oriented Reliable Multicast (NORM) Protocols",
RFC 5776, April 2010.
[RMT-SIMPLE] Roca, V., "Simple Authentication Schemes for the ALC
and NORM Protocols", Work in Progress, October 2009.
Authors' Addresses
Michael Luby
Qualcomm, Inc.
3165 Kifer Road
Santa Clara, CA 95051
US
EMail: luby@qualcomm.com
Mark Watson
Qualcomm, Inc.
3165 Kifer Road
Santa Clara, CA 95051
US
EMail: watson@qualcomm.com
Lorenzo Vicisano
Qualcomm, Inc.
3165 Kifer Road
Santa Clara, CA 95051
US
EMail: vicisano@qualcomm.com
Luby, et al. Standards Track PAGE 24
Asynchronous Layered Coding (ALC) Protocol Instantiation
RFC TOTAL SIZE: 59518 bytes
PUBLICATION DATE: Tuesday, April 6th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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