|Title||Layered Coding Transport (LCT) Building Block
|Author||M. Luby, J. Gemmell,
L. Vicisano, L. Rizzo, M. Handley, J. Crowcroft
Network Working Group M. Luby
Request for Comments: 3451 Digital Fountain
Category: Experimental J. Gemmell
Layered Coding Transport (LCT) Building Block
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2002). All Rights Reserved.
Layered Coding Transport (LCT) provides transport level support for
reliable content delivery and stream delivery protocols. LCT is
specifically designed to support protocols using IP multicast, but
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.
Table of Contents
4.1 Environmental Requirements and Considerations...............8
4.2 Delivery service models....................................10
4.3 Congestion Control.........................................11
5. Packet Header Fields..........................................12
5.1 Default LCT header format..................................12
5.2 Header-Extension Fields....................................17
6.1 Sender Operation...........................................20
6.2 Receiver Operation.........................................22
7. Requirements from Other Building Blocks.......................23
8. Security Considerations.......................................24
9. IANA Considerations...........................................25
Full Copyright Statement.........................................29
Layered Coding Transport 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 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
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 .
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 .
Statement of Intent
This memo contains part of the definitions necessary to fully
specify a Reliable Multicast Transport protocol in accordance with
RFC 2357. As per RFC 2357, the use of any reliable multicast
protocol in the Internet requires an adequate congestion control
While waiting for such a scheme to be available, or for an
existing scheme to be proven adequate, the Reliable Multicast
Transport working group (RMT) publishes this Request for Comments
in the "Experimental" category.
It is the intent of RMT to re-submit this specification as an IETF
Proposed Standard as soon as the above condition is met.
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.
The use of layered channels is also useful for streaming
There are coding techniques that provide massively scalable
reliability and asynchronous delivery which 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
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 which session each received packet belongs
to. 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 which session they
belong to. As another example, LCT provides optional support for
identifying which object each packet is carrying information about.
Therefore, LCT provides many of the commonly used fields and support
for functionality required by many protocols.
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
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 which object the packet contains
data for. As other examples, 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 will be continued for, flags for indicating the close of the
session and the close of sending packets for an object, and header
extensions for fields that for example can be used for packet
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 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
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. Some possible
multiple rate congestion control protocols are described in ,
, and . A possible single rate congestion control protocol is
described in .
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
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 , , ,  and . 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 a receiver is receiving
from, the receiver can reduce 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 .
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
Protocol instantiations 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 an session
control protocol that is compatible with LCT, and it MAY include
other protocols such as security protocols.
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
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
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
Before joining a session, the receivers MUST obtain enough of the
session description to start the session. This MUST include 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
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  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 which 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
, , and . Other congestion controls may be suitable when
LCT is used for a streaming application.
This document does not specify and restrict the type of exchanges
between LCT (or any PI 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 PI 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
LCT can be used with both multicast and unicast delivery. LCT
requires connectivity between a sender and receivers but 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 which 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
 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 presence of heterogeneous sets
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 . 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 attacks. In either case,
receivers SHOULD use mechanisms to filter out packets from unwanted
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.
4.2 Delivery service models
LCT can support several different delivery service models. Two
examples are briefly described here.
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.
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, and dynamically adapt
the number of LCT channels they subscribe to 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
Other service models.
There are many other delivery service models that LCT can be used for
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 this document. This
document only attempts to describe the minimal common scalable
elements to these diverse applications using LCT as the delivery
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.
Some possible congestion control protocols for reliable content
delivery using LCT are described in , , and . Different
delivery service models might require different congestion control
5. Packet Header Fields
Packets sent to an LCT session MUST include an "LCT header". The LCT
header format described below is the default format, and this is the
format that is recommended for use by protocol instantiations to
ensure a uniform format across different protocol instantiations.
Other LCT header formats MAY be used by protocol instantiations, but
if the default LCT header format is not used by a protocol
instantiation that uses LCT, then the protocol instantiation MUST
specify the lengths and positions within the LCT header it uses of
all fields described in the default LCT header.
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 Default LCT header format
The default 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. Unless otherwise noted, numeric constants in this
specification are in decimal (base 10).
The format of the default LCT header is depicted in Figure 1.
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 | r |S| O |H|T|R|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) |
| ... |
| Sender Current Time (SCT, if T = 1) |
| Expected Residual Time (ERT, if R = 1) |
| 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. Fields marked as "1" mean that the corresponding bits
MUST be set to "1" by the sender. Fields marked as "r" or "0" mean
that the corresponding bits MUST be set to "0" by the sender.
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.
Reserved (r): 2 bits
Reserved for future use. A sender MUST set these bits to zero
and a receiver MUST ignore these bits.
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
Sender Current Time present flag (T): 1 bit
T = 0 indicates that the Sender Current Time (SCT) field is not
present. T = 1 indicates that the SCT field is present. The
SCT is inserted by senders to indicate to receivers how long
the session has been in progress.
Expected Residual Time present flag (R): 1 bit
R = 0 indicates that the Expected Residual Time (ERT) field is
not present. R = 1 indicates that the ERT field is present.
The ERT is inserted by senders to indicate to receivers how
much longer the session / object transmission will continue.
Senders MUST NOT set R = 1 when the ERT for the session is more
than 2^32-1 time units (approximately 49 days), where time is
measured in units of milliseconds.
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 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 packets
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
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 which 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 1889 .
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.
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 receivers are subscribed
to. 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
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
This field indicates which object within the session this
packet pertains to. 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 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 lengths of the TSI field plus the TOI field is a
multiple of 32 bits.
Sender Current Time (SCT): 0 or 32 bits
This field represents the current clock at the sender and at
the time this packet was transmitted, measured in units of 1ms
and computed modulo 2^32 units from the start of the session.
This field MUST NOT be present if T=0 and MUST be present if
Expected Residual Time (ERT): 0 or 32 bits
This field represents the sender expected residual transmission
time for the current session or for the transmission of the
current object, measured in units of 1ms. If the packet
containing the ERT field also contains the TOI field, then ERT
refers to the object corresponding to the TOI field, otherwise
it refers to the session.
This field MUST NOT be present if R=0 and MUST be present if
5.2 Header-Extension Fields
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.
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
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
Protocol instantiation MAY override this default behavior for PI-
specific extensions (see below).
There are two formats for Header Extension fields, as depicted below.
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.
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
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 (HET between 0 and 127) and MUST NOT
be present for fixed-length extensions (HET between 128 and
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.
Header Extensions are further divided between general LCT extensions
and Protocol Instantiation specific extensions (PI-specific).
General LCT extensions have HET in the ranges 0:63 and 128:191
inclusive. PI-specific extensions have HET in the ranges 64:127 and
General LCT extensions are intended to allow the 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.
PI-specific extensions are reserved for PI-specific use with semantic
and default parsing actions defined by the PI.
The following general LCT Header Extension types are defined:
EXT_NOP=0 No-Operation extension.
The information present in this extension field MUST be
ignored by receivers.
EXT_AUTH=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
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 MUST NOT be
postponed by any such full packet authentication.
All senders and receivers implementing LCT MUST support the EXT_NOP
Header Extension and MUST recognize EXT_AUTH, but MAY NOT be able to
parse its content.
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;
o Enough information to determine the congestion control protocol
o Enough information to determine the packet authentication scheme
being used if it 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
to be included in the session description.
The session description could be in a form such as SDP as defined in
RFC 2327 , or XML metadata as defined in RFC 3023 , or
HTTP/Mime headers as defined in RFC 2068 , 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
E-mail 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.
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 that they have no interest in.
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
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 , ,
and . 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
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.
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 a 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
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
LCT header MUST be used by any protocol instantiation that uses LCT,
and other building blocks MAY also be used, such as a reliability
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. Some possible schemes
are specified in , , and . 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 . Some of the FEC codecs
that MAY be used in conjunction with LCT for reliable content
delivery are specified in . 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 2327 , or XML metadata as defined in
RFC 3023 , or HTTP/Mime headers as defined in RFC 2068 , and
distributed with SAP as defined in RFC 2974 , using HTTP, or in
other ways. It is RECOMMENDED that an authentication protocol such
as IPSEC  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 .
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
8. Security Considerations
LCT can be subject to denial-of-service attacks by attackers which
try to confuse the congestion control mechanism, or send forged
packets to the session which would prevent successful reconstruction
or cause inaccurate reconstruction of large portions of an object by
receivers. LCT is particularly affected by such an attack since many
receivers may receive the same forged packet. It is therefore
RECOMMENDED that an integrity check be made on received objects
before delivery to an application, e.g., by appending an MD5 hash
 to an object before it is sent and then computing the MD5 hash
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
computed on top of such a hash value. It is also RECOMMENDED that
protocol instantiations that use LCT implement some form of packet
authentication such as TESLA  to protect against such attacks.
Finally, it is RECOMMENDED that Reverse Path Forwarding checks 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.
Another vulnerability of LCT is the potential of receivers obtaining
an incorrect session description for the session. The consequences
of this could be that legitimate receivers with the wrong session
description are 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. To avoid these problems, it is RECOMMENDED that
measures be taken to prevent receivers from accepting incorrect
Session Descriptions, e.g., by using source authentication to ensure
that receivers only accept legitimate Session Descriptions from
A receiver with an incorrect or corrupted implementation of the
multiple rate congestion control building block may affect health of
the network in the path between the sender and the receiver, and may
also affect the reception rates of other receivers joined to the
session. It is therefore RECOMMENDED that receivers be required to
identify themselves as legitimate before they receive the Session
Description needed to join the session. How receivers identify
themselves as legitimate is outside the scope of this document.
9. IANA Considerations
No information in this specification is subject to IANA registration.
Building blocks used in conjunction with LCT MAY introduce additional
Thanks to Vincent Roca and Roger Kermode for detailed comments and
contributions to this document. Thanks also to Bruce Lueckenhoff,
Hayder Radha and Justin Chapweske for detailed comments on this
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Dissertation, Stanford University, Department of Computer
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J. Crowcroft, "The Use of Forward Error Correction (FEC) in
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J. Crowcroft, "Forward Error Correction (FEC) Building Block",
RFC 3452, December 2002.
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Criteria for Evaluating Reliable Multicast Transport and
Application Protocols", RFC 2357, June 1998.
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3023, January 2001.
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Greece, June 1997.
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and M. Luby, "Reliable Multicast Transport Building Blocks for
One-to-Many Bulk-Data Transfer", RFC 3048, January 2001.
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Multicast Transport (RMT) Building Blocks and Protocol
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Dip. Ing. dell'Informazione,
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Berkeley, CA, USA, 94704
Marconi Professor of Communications Systems
University of Cambridge
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Cambridge CB3 0FD, UK
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