Internet DRAFT - draft-ietf-rmt-bb-lct
draft-ietf-rmt-bb-lct
Internet Engineering Task Force RMT WG
INTERNET-DRAFT M.Luby/Digital Fountain
draft-ietf-rmt-bb-lct-04.txt J.Gemmell/Microsoft
L.Vicisano/Cisco
L.Rizzo/ACIRI and Univ. Pisa
M.Handley/ACIRI
J. Crowcroft/UCL
8 February 2002
Expires: August 2002
Layered Coding Transport building block
Status of this Document
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026 [1]. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
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This document is a product of the IETF RMT WG. Comments should be
addressed to the authors, or the WG's mailing list at rmt@lbl.gov.
Abstract
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
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that use unicast. LCT is compatible with congestion control
that provides muliple rate delivery to receivers and is also
compatible with coding techniques that provide reliable
delivery of content.
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Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . 4
2. Rationale . . . . . . . . . . . . . . . . . . . . . . . 4
3. Functionality . . . . . . . . . . . . . . . . . . . . . 5
4. Applicability . . . . . . . . . . . . . . . . . . . . . 8
4.1. Environmental Requirements and Considerations. . . . 9
4.2. Delivery service models. . . . . . . . . . . . . . . 11
4.3. Congestion Control . . . . . . . . . . . . . . . . . 12
5. Packet Header Fields. . . . . . . . . . . . . . . . . . 12
5.1. Default LCT header format. . . . . . . . . . . . . . 13
5.2. Header-Extension Fields. . . . . . . . . . . . . . . 19
6. Operations. . . . . . . . . . . . . . . . . . . . . . . 22
6.1. Sender Operation . . . . . . . . . . . . . . . . . . 22
6.2. Receiver Operation . . . . . . . . . . . . . . . . . 24
7. Requirements from Other Building Blocks . . . . . . . . 25
8. Security Considerations . . . . . . . . . . . . . . . . 26
9. IANA Considerations . . . . . . . . . . . . . . . . . . 27
10. Intellectual Property Issues . . . . . . . . . . . . . 27
11. Acknowledgments. . . . . . . . . . . . . . . . . . . . 27
12. References . . . . . . . . . . . . . . . . . . . . . . 27
13. Authors' Addresses . . . . . . . . . . . . . . . . . . 29
14. Full Copyright Statement . . . . . . . . . . . . . . . 31
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1. Introduction
This document describes a massively scalable reliable content delivery
and stream delivery transport level support, Layered Coding Transport
(LCT), for multiple rate content delivery primarily designed to be used
with the IP multicast network service, but may also be used with other
basic underlying network or transport services such as unicast UDP. LCT
supports both reliable and unreliable delivery.
This document describes a building block as defined in RFC3048 [26].
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 RFC2119 [2].
2. Rationale
LCT provides transport level support for massively scalable protocols
using the IP multicast network service. The support that LCT provides
is common to a variety of very important applications, including
reliable content delivery and streaming applications.
An LCT session comprises multiple channels originating at a single
sender that are used for some period of time to carry packets pertaining
to the transmission of one or more objects that can be of interest to
receivers. The logic behind defining a session as originating from a
single sender is that this is the right granularity to regulate packet
traffic via congestion control. One rationale for using multiple
channels within the same session is that there are massively scalable
congestion control protocols that use multiple channels per session.
These congestion control protocols are considered to be layered because
a receiver joins and leaves channels in a layered order during its
participation in the session. The use of layered channels is also
useful for streaming applications.
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 streaming applications.
LCT avoids providing functionality that is not massively scalable. For
example, LCT does not provide any mechanisms for sending information
from receivers to senders, although this does not rule out protocols
that both use LCT and do require sending information from receivers to
senders.
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LCT includes general support for congestion control that must be used
but does not 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, and thus it must
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 thus
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. Thus, LCT provides
many of the commonly used fields and support for functionality required
by many protocols.
3. Functionality
An LCT session consists of a set of logically grouped LCT channels
associated with a single sender carrying packets with LCT headers for
one or more objects. An LCT channel is defined by the combination of a
sender and an address associated with the channel by the sender. A
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
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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 other 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 continue 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
authentication.
LCT provides support for congestion control. Congestion control MUST be
used that conforms to RFC2357 [16] 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 protcol 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 receivers.
For single rate protocols, a session typically consists of one channel
and the sender sends packets to the channel at variable rates over time
depending on feedback from receivers. Each receiver remains joined to
the channel during its participation in the session. Thus, for single
rate protocols, the reception rate of each receiver may vary dynamically
but in coordination with all receivers. Generally, a multiple rate
protocol is preferable to a single rate protocol in a heterogeneous
receiver environment, since generally it more easily achieves
scalability to many receivers and provides higher throughput to each
individual receiver. Some possible multiple rate congestion control
protocols are described in [25] and [3]. A possible single rate
congestion control protocol is described in [22].
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Layered coding refers to the ability to produce a coded stream of
packets that can be partioned into an ordered set of layers. The coding
is meant to provide some form of reliability, and the layering is meant
to allow the receiver experience (in terms of quality of playout, or
overall transfer speed) to vary in a predictable way depending on how
many consecutive layers of packets the receiver is receiving.
Layered coding can be naturally combined with multiple rate congestion
control. For example, the sender could send the packets for each layer
to a separate channel associated with the session, and then receivers
dynamically join and leave channels to adjust their reception rate
according to the multiple rate congestion control protocol.
Layered coding can also be combined with single rate congestion control.
For example, the sender could dynamically vary how many layers are sent
to the channel associated with the session as the rate of transmission
varies according to the single rate congestion control protocol.
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 [23], [21], [7], [25] and [4]. 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 [14].
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
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receivers back to the sender, and the associated response (generally
data retransmissions) from the sender. Protocols that avoid any such
feedback, and minimize the amount of retransmissions, can be massively
scalable. LCT can be used in conjunction with FEC codes or a layered
codec to achieve reliability with little or no feedback.
Protocol instantiations 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 RFC2357 [16]. 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.
4. Applicability
An LCT session comprises a logically related set of one or more LCT
channels originating at a single sender that are used for some period of
time to carry packets containing LCT headers pertaining 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 of substantial
length, i.e., objects or streams that range in length from hundreds of
kilobytes to many gigabytes, and whose transfer time is in the order of
tens of seconds or more.
As an example, an LCT session could be used to deliver a TV program
using three LCT channels. Receiving packets from the first LCT channel
could allow black and white reception, receiving the first two LCT
channels could also permit color reception, whereas receiving all three
channels could allow HDTV quality reception. Objects in this example
could correspond to individual TV programs being transmitted.
As another example, a reliable LCT session could be used to reliably
deliver hourly-updated weather maps (objects) using ten LCT channels at
different rates, using FEC coding. A receiver may join and concurrently
receive packets from subsets of these channels, until it has enough
packets in total to recover the object, then leave the session (or
remain connected listening for session description information only)
until it is time to receive the next object. In this case, the quality
metric is the time required to receive each object.
Before joining a session, the receivers MUST obtain enough of the
session description to start the session. This MUST include the
relevant session parameters needed by a receiver to participate in the
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session, including all information relevant to congestion control. The
session description is determined by the sender, and is typically
communicated to the receivers out-of-band. In some cases, as described
later, parts of the session description that are not required to
initiate a session MAY be included in the LCT header or communicated to
a receiver out-of-band after the receiver has joined the session.
An encoder MAY be used to generate the data that is placed in the packet
payload in order to provide reliability. A suitable decoder is used to
reproduce the original information from the packet payload. There MAY
be a reliability header that follows the LCT header if such an encoder
and decoder is used. The reliability header helps to describe the
encoding data carried in the payload of the packet. The format of the
reliability header depends on the coding used, and this is negotiated
out-of-band. As an example, one of the FEC headers described in [15]
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 [25] and [3]. 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
information).
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
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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 RFC768 [19], 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, 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 RFC1112 [5] is such a "best
effort" network service. While the basic service provided by RFC1112 is
largely scalable, providing congestion control or reliability should be
done carefully to avoid severe scalability limitations, especially in
presence of heterogeneous sets of receivers.
There are currently two models of multicast delivery, the Any-Source
Multicast (ASM) model as defined in RFC1112 [5] and the Source-Specific
Multicast (SSM) model as defined in [10]. 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.
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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 sources.
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 (and the reception quality)
according to the available bandwidth. Receivers then drop from the
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session when they have received enough packets to recover the object.
As an example, assume that an object is 50 MB. The sender could send 1
KB packets to the first LCT channel at 50 packets per second, so that
receivers using just this LCT channel could complete reception of the
object in 1,000 seconds in absence of loss, and would be able to
complete reception even in presence of some substantial amount of losses
with the use of coding for reliability. Furthermore, the sender could
use a number of LCT channels such that the aggregate rate of 1 KB
packets to all LCT channels is 1,000 packets per second, so that a
receiver could be able to complete reception of the object in as little
50 seconds (assuming no loss and that the congestion control mechanism
immediately converges to the use of all LCT channels).
Other service models.
There are many other delivery service models 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. The 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 transport.
4.3. Congestion Control
The specific congestion control protocol to be used for LCT sessions
depends on the type of content to be delivered. While the general
behavior of the congestion control protocol is to reduce the throughput
in presence of congestion and gradually increase it in the absence of
congestion, the actual dynamic behavior (e.g. response to single losses)
can vary.
Some possible congestion control protocols for reliable content delivery
using LCT are described in [25] and [3]. Different delivery service
models might require different congestion control protocols.
5. Packet Header Fields
Packets sent to an LCT session MUST include an "LCT header". The LCT
header format described below is the default format, and this is the
format that is recommended for use by protocol instantiations to ensure
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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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| V | C | 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.
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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
32-bits.
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
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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 session.
LCT header length (HDR_LEN): 8 bits
Total length of the LCT header in units of 32-bit words. The
length of the LCT header MUST be a multiple of 32-bits. This
field can be used to directly access the portion of the packet
beyond the LCT header, i.e., to the first other header if it
exists, or to the packet payload if it exists and there is no
other header, or to the end of the packet if there are no other
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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 RFC1889 [24].
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 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
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set of channels. A receiver that connects and remains connected
to the first session during this sender activity could possibly
accept packets from the second session as belonging to the first
session if the TSI for the two sessions were identical. The
mapping of TSI field values to sessions is outside the scope of
this document and is to be done out-of-band.
The length of the TSI field is 32*S + 16*H bits. Note that the
aggregate lengths of the TSI field plus the TOI field is a
multiple of 32 bits.
Transport Object Identifier (TOI): 0, 16, 32, 48, 64, 80, 96 or 112
bits.
This field indicates 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 ouptut
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 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 T=1.
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 R=1.
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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 Extension
fields.
If present, Header Extensions MUST be processed to ensure that they are
recognized before performing any congestion control procedure or
otherwise accepting a packet. The default action for unrecognized header
extensions is to ignore them. This allows the future introduction of
backward-compatible enhancements to LCT without changing the LCT version
number. Non backward-compatible header extensions CANNOT be introduced
without changing the LCT version number.
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.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET (<=127) | HEL | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
. .
. Header Extension Content (HEC) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HET (>=128) | Header Extension Content (HEC) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5 - Format of additional headers
The explanation of each sub-field is the following.
Header Extension Type (HET): 8 bits
The type of the Header Extension. This document defines a number
of possible types. Additional types may be defined in future
versions of this specification. HET values from 0 to 127 are used
for variable-length Header Extensions. HET values from 128 to 255
are used for fixed-length 32-bit Header Extensions.
Header Extension Length (HEL): 8 bits
The length of the whole Header Extension field, expressed in
multiples of 32-bit words. This field MUST be present for
variable-length extensions (HET between 0 and 127) and MUST NOT be
present for fixed-length extensions (HET between 128 and 255).
Header Extension Content (HEC): variable length
The content of the Header Extension. The format of this sub-field
depends on the Header Extension type. For fixed-length Header
Extensions, the HEC is 24 bits. For variable-length Header
Extensions, the HEC field has variable size, as specified by the
HEL field. Note that the length of each Header Extension field
MUST be a multiple of 32 bits. Also note that the total size of
the LCT header, including all Header Extensions and all optional
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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 192:255 inclusive.
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
description.
It is RECOMMENDED that senders provide some form of packet
authentication. If EXT_AUTH is present, whatever packet
authentication checks that can be performed immediately
upon reception of the packet SHOULD be performed before
accepting the packet and performing any congestion
control-related action on it.
Some packet authentication schemes impose a delay of
several seconds between when a packet is received and when
the packet is fully authenticated. Any congestion control
related action that is appropriate 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.
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6. Operations
6.1. Sender Operation
Before joining an LCT session a receiver MUST obtain a session
description. The session description MUST include:
o The sender IP address;
o The number of LCT channels;
o The addresses and port numbers used for each LCT channel;
o The Transport Session ID (TSI) to be used for the session;
o Enough information to determine the congestion control protocol
being used;
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
RFC2327 [8], or XML metadata as defined in RFC3023 [17], or HTTP/Mime
headers as defined in RFC2068 [6], etc. It might be carried in a
session announcement protocol such as SAP as defined in RFC2974 [9],
obtained using a proprietary session control protocol, located on a Web
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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 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 fragmentation coupled with packet loss
might introduce severe inefficiency in the transmission.
It is recommended that all packets have the same or very similar sizes,
as this can have a severe impact on the effectiveness of congestion
control schemes such as the ones described in [25] and [3]. A sender of
packets using LCT MUST implement the sender-side part of one of the
congestion control schemes that is in accordance with RFC2357 [16] 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
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participating in the session.
6.2. Receiver Operation
Receivers can operate differently depending on the delivery service
model. For example, for an on demand service model receivers may join a
session, obtain the necessary encoding symbols 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
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such LCT session. If the congestion control protocol allows the
receiver some flexibility in terms of its actions within a session then
the receiver MAY make choices to optimize the packet flow performance
across the multiple LCT sessions, as long as the receiver still adheres
to the congestion control rules for each LCT session individually.
7. Requirements from Other Building Blocks
As described in RFC3048 [26], 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 building block.
The congestion control MUST be applied to the LCT session as an entity,
i.e., over the aggregate of the traffic carried by all of the LCT
channels associated with the LCT session. Some possible schemes are
specified in [25] and [3]. 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 [14]. Some of the FEC codecs that
MAY be used in conjunction with LCT for reliable content delivery are
specified in [15]. 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 RFC2327 [8], or XML metadata as defined in
RFC3023 [17], or HTTP/Mime headers as defined in RFC2068 [6], and
distributed with SAP as defined in RFC2974 [9], using HTTP, or in other
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ways. It is RECOMMENDED that an authentication protocol such as IPSEC
[11] 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 [18].
Some protocol instantiations that use LCT MAY use building blocks that
require the generation of feedback from the receivers to the sender.
However, the mechanism for doing this is outside the scope of LCT.
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 the 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 content before delivery to an
application, e.g., by appending an MD5 hash [20] to the content before
it is sent and then computing the MD5 hash once the content is
reconstructed to ensure it is the same as the sent content. 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 [18]
to protect against such attacks. Furthermore, 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 the receiver authenticate the
session description, for example by using either the ESP (with enabled
authentication service) [13] or AH [12] protocols of IPSEC [11] to
ensure the authenticity of the session description.
A receiver with an incorrect or corrupted implementation of the
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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 identify themselves as legitimate
before they receive the session description needed to join the session.
9. IANA Considerations
No information in this specification is subject to IANA registration.
Building blocks used in conjunction with LCT MAY introduce additional
IANA considerations.
10. Intellectual Property Issues
No specific reliability protocol or congestion control protocol is
specified or referenced as mandatory in this document.
LCT MAY be used with congestion control protocols and other protocols,
such as reliability protocols, which are proprietary or have pending or
granted patents.
11. Acknowledgments
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
document.
12. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3",
RFC2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC2119, March 1997.
[3] Byers, J.W., Frumin, M., Horn, G., Luby, M., Mitzenmacher, M.,
Luby/Gemmell/Vicisano/Rizzo/Handley/Crowcroft Section 12. [Page 27]
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Roetter, A., and Shaver, W., "FLID-DL: Congestion Control for Layered
Multicast", Proceedings of Second International Workshop on Networked
Group Communications (NGC 2000), Palo Alto, CA, November 2000.
[4] Byers, J.W., Luby, M., Mitzenmacher, M., and Rege, A., "A Digital
Fountain Approach to Reliable Distribution of Bulk Data", Proceedings
ACM SIGCOMM '98, Vancouver, Canada, September 1998.
[5] Deering, S., "Host Extensions for IP Multicasting", RFC1112, August
1989.
[6] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Berners-Lee, T.,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC2068, January 1997.
[7] Gemmell, J., Schooler, E., and Gray, J., "Fcast Multicast File
Distribution", IEEE Network, Vol. 14, No. 1, pp. 58-68, January 2000.
[8] Handley, M., Jacobson, V., "SDP: Session Description Protocol",
RFC2327, April 1998.
[9] Handley, M., Perkins, C., Whelan, E., "Session Announcement
Protocol", RFC2974, October 2000.
[10] Holbrook, H. W., "A Channel Model for Multicast", Ph.D.
Dissertation, Stanford University, Department of Computer Science,
Stanford, California, August 2001.
[11] Kent, S., Atkinson, R., "Security Architecture for the Internet
Protocol", RFC2401, November 1998.
[12] Kent, S., Atkinson, R., "IP Authentication Header", RFC2406,
November 1998.
[13] Kent, S., Atkinson, R., "IP Encapsulating Security Payload (ESP)",
RFC2406, November 1998.
[14] Luby, M., Gemmell, Vicisano, L., J., Rizzo, L., Handley, M.,
Crowcroft, J., "The use of Forward Error Correction in Reliable
Multicast", Internet Draft draft-ietf-rmt-info-fec-02.txt, January 2002,
a work in progress.
[15] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., Handley, M.,
Crowcroft, J., "Forward Error Correction building block", Internet Draft
draft-ietf-rmt-bb-fec-06.txt, January 2002.
[16] Mankin, A., Romanow, A., Bradner, S., Paxson V., "IETF Criteria for
Evaluating Reliable Multicast Transport and Application Protocols",
RFC2357, June 1998.
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[17] Murata, M., St.Laurent, S., Kohn, D., "XML Media Types", RFC3023,
January 2001.
[18] Perrig, A., Canetti, R., Song, D., Tygar, J.D., "Efficient and
Secure Source Authentication for Multicast", Network and Distributed
System Security Symposium, NDSS 2001, pp. 35-46, February 2001.
[19] Postel, J., "User Datagram Protocol", RFC768, August 1980.
[20] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, April
1992.
[21] Rizzo, L., "Effective Erasure Codes for Reliable Computer
Communication Protocols", ACM SIGCOMM Computer Communication Review,
Vol.27, No.2, pp.24-36, Apr 1997.
[22] Rizzo, L, "PGMCC: A TCP-friendly single-rate multicast congestion
control scheme", Proceedings of SIGCOMM 2000, Stockholm Sweden, August
2000.
[23] Rizzo, L, and Vicisano, L., "Reliable Multicast Data Distribution
protocol based on software FEC techniques", Proceedings of the Fourth
IEEES Workshop on the Architecture and Implementation of High
Performance Communication Systems, HPCS'97, Chalkidiki Greece, June
1997.
[24] Schulzrinne, H., Casner, S., Frederick, R., Jacobson, V., "RTP: A
Transport Protocol for Real-Time Applications", RFC1889, January 1996.
[25] Vicisano, L., Rizzo, L., Crowcroft, J., "TCP-like Congestion
Control for Layered Multicast Data Transfer", IEEE Infocom '98, San
Francisco, CA, Mar.28-Apr.1 1998.
[26] Whetten, B., Vicisano, L., Kermode, R., Handley, M., Floyd, S.,
Luby, M., "Reliable Multicast Transport Building Blocks for One-to-Many
Bulk-Data Transfer", RFC3048, January 2001.
13. Authors' Addresses
Michael Luby
luby@digitalfountain.com
Digital Fountain
39141 Civic Center Drive
Suite 300
Fremont, CA, USA, 94538
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Jim Gemmell
jgemmell@microsoft.com
Microsoft Research
301 Howard St., #830
San Francisco, CA, USA, 94105
Lorenzo Vicisano
lorenzo@cisco.com
cisco Systems, Inc.
170 West Tasman Dr.,
San Jose, CA, USA, 95134
Luigi Rizzo
luigi@iet.unipi.it
ACIRI/ICSI,
1947 Center St, Berkeley, CA, USA, 94704
and
Dip. Ing. dell'Informazione,
Univ. di Pisa
via Diotisalvi 2, 56126 Pisa, Italy
Mark Handley
mjh@aciri.org
ACIRI,
1947 Center St,
Berkeley, CA, USA, 94704
Jon Crowcroft
J.Crowcroft@cs.ucl.ac.uk
Department of Computer Science
University College London
Gower Street,
London WC1E 6BT, UK
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14. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works. However, this document itself
may not be modified in any way, such as by removing the copyright notice
or references to the Internet Society or other Internet organizations,
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an "AS
IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT not
LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL not
INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR
FITNESS FOR A PARTICULAR PURPOSE."
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