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Versions: 00 01 02 03 RFC 3451

Internet Engineering Task Force                                   RMT WG
INTERNET-DRAFT                                   M.Luby/Digital Fountain
draft-ietf-rmt-bb-lct-03.txt                         J.Gemmell/Microsoft
                                                        L.Vicisano/Cisco
                                            L.Rizzo/ACIRI and Univ. Pisa
                                                         M.Handley/ACIRI
                                                        J. Crowcroft/UCL
                                                        21 November 2001
                                                       Expires: May 2002


                       Layered Coding Transport:
     A massively scalable content delivery 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.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
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Internet-Drafts are valid for a maximum of six months and may be
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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


     This document describes Layered Coding Transport, a massively
     scalable reliable content delivery and stream delivery



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     transport, hereafter referred to as LCT.  LCT can be used for
     multi-rate delivery to large sets of receivers.  In LCT,
     scalability and congestion control are supported through the
     use of layered coding techniques. Coding techniques can be
     combined with LCT to provide support for reliability.

     Congestion control is receiver driven, and can be achieved by
     sending packets in the session to multiple ``LCT channels'',
     and having receivers join and leave LCT channels (thus
     adjusting their reception rate) in reaction to network
     conditions in a manner that is network friendly.








































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                           Table of Contents


     1. Introduction. . . . . . . . . . . . . . . . . . . . . .   4
      1.1. Related Documents. . . . . . . . . . . . . . . . . .   6
      1.2. Environmental Requirements and Considerations. . . .   7
     2. General Architecture. . . . . . . . . . . . . . . . . .   9
      2.1. Delivery service models. . . . . . . . . . . . . . .  11
      2.2. Congestion Control . . . . . . . . . . . . . . . . .  12
     3. LCT header. . . . . . . . . . . . . . . . . . . . . . .  12
      3.1. Default LCT header format. . . . . . . . . . . . . .  13
      3.2. Header-Extension Fields. . . . . . . . . . . . . . .  18
     4. Procedures. . . . . . . . . . . . . . . . . . . . . . .  21
      4.1. Sender Operation . . . . . . . . . . . . . . . . . .  21
      4.2. Receiver Operation . . . . . . . . . . . . . . . . .  23
     5. Security Considerations . . . . . . . . . . . . . . . .  24
     6. IANA Considerations . . . . . . . . . . . . . . . . . .  24
     7. Intellectual Property Issues. . . . . . . . . . . . . .  24
     8. Acknowledgments . . . . . . . . . . . . . . . . . . . .  25
     9. References. . . . . . . . . . . . . . . . . . . . . . .  25
     10. Authors' Addresses . . . . . . . . . . . . . . . . . .  26
     11. Full Copyright Statement . . . . . . . . . . . . . . .  28





























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1.  Introduction

This document describes a massively scalable reliable content delivery
and stream delivery transport, Layered Coding Transport (LCT), for
multi-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.

LCT is a building block as defined in RFC3048.  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.  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.

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 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.

Packets with LCT headers are carried in LCT channels. An LCT channel is
defined by the combination of a sender and an address associated with
the channel by the sender.  A receiver may join a channel to start
receiving the data packets sent to the channel by the sender, and a
receiver may leave a channel to stop receiving data packets from the
channel.

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.  Congestion control that conforms to RFC2357 must
be used 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 multi-rate or a single-rate congestion control protcol can be used
with LCT.  For multi-rate protocols, a session typically consists of



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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
multi-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 multi-rate
protocol is preferable to a single-rate protocol in a heterogeneous
receiver environment, but only if it can be achieved without sacrificing
scalability.  Some possible multi-rate congestion control protocols are
described in [11] and [1]. A possible single-rate congestion control
protocol is described in [10].

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 multi-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 multi-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 [9] [7] [3] [11] [2]. By



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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.
More details on the use of FEC for reliable content delivery can be
found in [5].  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, but scalability was not a design goal for many of them.

Two of the key difficulties in scaling reliable content delivery using
IP multicast are dealing with the amount of data that flows from
receivers back to the sender, and the associated response (generally
data retransmissions) from the sender.  Protocols that avoid any such
feedback, and minimize the amount of retransmissions, can be massively
scalable.  LCT can be used in conjunction with FEC codes or a layered
codec to achieve reliability with little or no feedback.

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 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.

In this document we present the architecture and abstract LCT header
structure, and describe its support for congestion control.

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.


1.1.  Related Documents

As described in RFC3048, 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.

A more in-depth description of the use of FEC in Reliable Multicast



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Transport (RMT) protocols is given in [5]. Some of the FEC codecs that
may be used in conjunction with LCT for reliable content delivery are
specified in [6]. 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.

Implementors of protocol instantiations that use LCT must also implement
congestion control in accordance to RFC2357, where the congestion
control is over the entire session.  Some possible schemes are specified
in [11] and [1]. 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.

Generic Router Assist may be used in conjunction with LCT.

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 [8].

1.2.  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 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.

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



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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, then the 16 bit
UDP source port number may serve as the TSI for the session.  If Generic
Router Assist (GRA) is being used then additional dependencies may be
introduced by GRA on the TSI field.  GRA work is ongoing within the RMT
working group at this time.  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.

There are currently two models of multicast delivery, the Any-Source
Multicast (ASM) model as defined in RFC1112 and the Source-Specific
Multicast (SSM) model as defined in [4]. 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 sources.

LCT also requires receivers to obtain Session Description Information,
as described in Section 4.1. The session description could be in a form
such as SDP as defined in RFC2327, or XML metadata, or HTTP/Mime headers
as defined in RFC2068, and distributed with SAP as defined in RFC2974,
using HTTP, or in other ways.

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.



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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.

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.

Some protocol instantiations that use LCT may require the generation of
feedback from the receivers to the sender.  For example, Generic Router
Assist may be used to help in passing real-time statistics in a scalable
manner from receivers back to the sender. However, the mechanism for
doing this is outside the scope of LCT.


2.  General Architecture

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.

For 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
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



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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 [6]
could be used.

For LCT, when multi-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 [11] and [1]. Other congestion
controls may be suitable when LCT is used for a streaming application.

LCT can be used with other congestion control protocols such as the one
described in [11], or Generic Router Assist schemes where the selection
of which packets to forward is performed by routers. This latter
approach potentially allows for finer grain congestion control and a
faster reaction to network congestion, but requires changes to the
router infrastructure.  We do not discuss this approach further in this
document.

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 out of
the scope of this document."


2.1.  Delivery service models

LCT can support several different delivery service models. Two examples
are briefly described here.



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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
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



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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.


2.2.  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 [11] and [1]. Different delivery service
models might require different congestion control protocols.


3.  LCT header

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.








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3.1.  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  |H|S| O |T|R|A|B|   HDR_LEN     | Codepoint (CP)|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Congestion Control Information (CCI)              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   CCI continued (if C = 1)                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  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




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      Indicates the LCT version number.  The LCT version number for this
      specification is 0.


  Congestion control flag (C): 1 bit

      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.


  Reserved (r): 3 bits

      Reserved for future use. A sender must set these bits to zero and
      a receiver must ignore these 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.


  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.


  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.



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  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 session.




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  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 RFC1889.


  Congestion Control Information (CCI): 32 or 64 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.


  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 must always uniquely identify 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 Generic Router Assist (GRA) is
      being used then additional dependencies may be introduced by GRA
      on the TSI field.  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.



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      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
      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 must 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 must 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





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      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.


3.2.  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
      version 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.
              If present, the format of this Header Extension and its
              processing must 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 must be performed before
              accepting the packet and performing any congestion
              control-related action on it.
              Some packet authentication schemes impose a delay of
              several seconds between when a packet is received and when
              the packet 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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4.  Procedures


4.1.  Sender Operation

A sender using LCT must make a session description available to clients
that want to join an LCT session.  This information could include, but
is not limited to:

  o The number of LCT channels;

  o The addresses, port numbers and data rates used for each LCT
    channel;

  o The formats of any other headers.  For example, an FEC header as
    described in [6] could be such an other header.  Then for example
    the information could include the mapping of codepoints used in the
    session to FEC codec types and parameters;

  o The format and lengths of the packet payload;

  o The Transport Session ID (TSI) to be used for the session;

  o Whether or not Generic Router Assist (GRA) is being used;

  o The congestion control scheme being used;

  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;

  o The packet authentication scheme being used, and all relevant
    information which is necessary for client packet authentication
    purposes;


Some of the session description information must be obtained by
receivers before they connect to the session.  This includes the number
and addresses of the LCT channels, the TSI value for the session, the
formats of any other headers, the congestion control scheme being used
and the packet authentication scheme if it is used.  Some of the session
description information may be obtained by receivers while they are
connected to the session, e.g., information relevant to objects being
transported within the session such as their TOI, when they are
available within the session, for how long, and their lengths.




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The session description could be in a form such as SDP as defined in
RFC2327, XML metadata, HTTP/Mime headers, etc. It might be carried in a
session announcement protocol such as SAP as defined in RFC2974,
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 a format defined in the session description.  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.

Multiple 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 [11] and [1].  A sender of



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packets using LCT must implement the sender-side part of one of the
congestion control schemes that is in accordance with RFC2357 using the
Congestion Control Information field provided in the LCT header, and the
corresponding receiver congestion control scheme must be communicated
out of band and implemented by any receivers participating in the
session.


4.2.  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 first obtain the
relevant session description information as listed in Section 4.1.

If packet authentication information is present in an LCT header, it
should be used as specified in Section 3.2.  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 itself. For a layered transmission on 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.

Multiple 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



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one, at least for some amount of time.

A receiver may be concurrently joined to multiple LCT sessions from one
or more senders. The receiver must perform congestion control on each
such LCT session.  If the congestion control protocol allows the
receiver some flexibility in terms of its actions within a session then
the receiver may make choices to optimize the packet flow performance
across the multiple LCT sessions, as long as the receiver still adheres
to the congestion control rules for each LCT session individually.


5.  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 of large
portions of the objects.

The same exact problems are present in TCP, where an attacker can forge
packets and either slow down or increase the throughput of the session,
or replace parts of the data stream with forged data. If the stream is
carrying compressed or otherwise coded data, even a single forged packet
could also cause incorrect reconstruction of the rest of the data
stream.

It is therefore recommended that protocol instantiations that use LCT
implement some form of packet authentication to protect themselves
against such attacks.


6.  IANA Considerations

No information in this specification is subject to IANA registration.

Building blocks used in conjunction with LCT may introduce additional
IANA considerations.


7.  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.




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8.  Acknowledgments

Thanks to Vincent Roca for detailed comments and contributions to this
document.  Thanks also to Bruce Lueckenhoff, Hayder Radha and Justin
Chapweske for detailed comments on this document.


9.  References


[1] Byers, J.W., Frumin, M., Horn, G., Luby, M., Mitzenmacher, M.,
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.

[2] 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.

[3] Gemmell, J., Schooler, E., and Gray, J., "Fcast Multicast File
Distribution", IEEE Network, Vol. 14, No. 1, pp. 58-68, January 2000.

[4] Holbrook, H. W., "A Channel Model for Multicast," Ph.D.
Dissertation, Stanford University, Department of Computer Science,
Stanford, California, August 2001.

[5] 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-01.txt, October 2001,
a work in progress.

[6] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., Handley, M.,
Crowcroft, J., "RMT BB Forward Error Correction Codes", Internet Draft
draft-ietf-rmt-bb-fec-04.txt, October 2001.

[7] Rizzo, L., "Effective Erasure Codes for Reliable Computer
Communication Protocols", ACM SIGCOMM Computer Communication Review,
Vol.27, No.2, pp.24-36, Apr 1997.

[8] 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.

[9] 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.



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[10] Rizzo, L, "PGMCC: A TCP-friendly single-rate multicast congestion
control scheme", Proceedings of SIGCOMM 2000, Stockholm Sweden, August
2000.

[11] 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.



10.  Authors' Addresses

   Michael Luby
   luby@digitalfountain.com
   Digital Fountain
   39141 Civic Center Drive
   Suite 300
   Fremont, CA, USA, 94538

   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



Luby/Gemmell/Vicisano/Rizzo/Handley/Crowcroft     Section 10.  [Page 26]

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   Department of Computer Science
   University College London
   Gower Street,
   London WC1E 6BT, UK















































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11.  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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