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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 5775

Reliable Multicast Transport (RMT)                                  Luby
Working Group                                                     Watson
Internet-Draft                                                  Vicisano
Obsoletes: 3450 (if approved)                           Digital Fountain
Intended status: Standards Track                       February 22, 2007
Expires: August 26, 2007


        Asynchronous Layered Coding (ALC) Protocol Instantiation
                    draft-ietf-rmt-pi-alc-revised-04

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on August 26, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).












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Abstract

   This document describes the Asynchronous Layered Coding (ALC)
   protocol, a massively scalable reliable content delivery protocol.
   Asynchronous Layered Coding combines the Layered Coding Transport
   (LCT) building block, a multiple rate congestion control building
   block and the Forward Error Correction (FEC) building block to
   provide congestion controlled reliable asynchronous delivery of
   content to an unlimited number of concurrent receivers from a single
   sender.  This document obsoletes RFC3450.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Delivery service models  . . . . . . . . . . . . . . . . .  4
     1.2.  Scalability  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Environmental Requirements and Considerations  . . . . . .  5
   2.  Architecture Definition  . . . . . . . . . . . . . . . . . . .  6
     2.1.  LCT building block . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Multiple rate congestion control building block  . . . . .  9
     2.3.  FEC building block . . . . . . . . . . . . . . . . . . . .  9
     2.4.  Session Description  . . . . . . . . . . . . . . . . . . . 11
     2.5.  Packet authentication building block . . . . . . . . . . . 12
   3.  Conformance Statement  . . . . . . . . . . . . . . . . . . . . 14
   4.  Functionality Definition . . . . . . . . . . . . . . . . . . . 15
     4.1.  Packet format used by ALC  . . . . . . . . . . . . . . . . 15
     4.2.  LCT Header-Extension Fields  . . . . . . . . . . . . . . . 16
     4.3.  Sender Operation . . . . . . . . . . . . . . . . . . . . . 17
     4.4.  Receiver Operation . . . . . . . . . . . . . . . . . . . . 17
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   8.  Changes from RFC3450 . . . . . . . . . . . . . . . . . . . . . 24
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     9.1.  Normative references . . . . . . . . . . . . . . . . . . . 25
     9.2.  Informative references . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
   Intellectual Property and Copyright Statements . . . . . . . . . . 28












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1.  Introduction

   This document describes a massively scalable reliable content
   delivery protocol, Asynchronous Layered Coding (ALC), for multiple
   rate congestion controlled reliable content delivery.  The protocol
   is specifically designed to provide massive scalability using IP
   multicast as the underlying network service.  Massive scalability in
   this context means the number of concurrent receivers for an object
   is potentially in the millions, the aggregate size of objects to be
   delivered in a session ranges from hundreds of kilobytes to hundreds
   of gigabytes, each receiver can initiate reception of an object
   asynchronously, the reception rate of each receiver in the session is
   the maximum fair bandwidth available between that receiver and the
   sender, and all of this can be supported using a single sender.

   Because ALC is focused on reliable content delivery, the goal is to
   deliver objects as quickly as possible to each receiver while at the
   same time remaining network friendly to competing traffic.  Thus, the
   congestion control used in conjunction with ALC should strive to
   maximize use of available bandwidth between receivers and the sender
   while at the same time backing off aggressively in the face of
   competing traffic.

   The sender side of ALC consists of generating packets based on
   objects to be delivered within the session and sending the
   appropriately formatted packets at the appropriate rates to the
   channels associated with the session.  The receiver side of ALC
   consists of joining appropriate channels associated with the session,
   performing congestion control by adjusting the set of joined channels
   associated with the session in response to detected congestion, and
   using the packets to reliably reconstruct objects.  All information
   flow in an ALC session is in the form of data packets sent by a
   single sender to channels that receivers join to receive data.

   ALC does specify the Session Description needed by receivers before
   they join a session, but the mechanisms by which receivers obtain
   this required information is outside the scope of ALC.  An
   application that uses ALC may require that receivers report
   statistics on their reception experience back to the sender, but the
   mechanisms by which receivers report back statistics is outside the
   scope of ALC.  In general, ALC is designed to be a minimal protocol
   instantiation that provides reliable content delivery without
   unnecessary limitations to the scalability of the basic protocol.

   This document is a product of the IETF RMT WG and follows the general
   guidelines provided in [RFC3269].

   RFC3450 [RFC3450], which is obsoleted by this document, contained a



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   previous versions of the protocol.  RFC3450 was published in the
   "Experimental" category.  It was the stated intent of the RMT working
   group to re-submit these specifications as an IETF Proposed Standard
   in due course.

   This Proposed Standard specification is thus based on and backwards
   compatible with the protocol defined in RFC3450 [RFC3450] updated
   according to accumulated experience and growing protocol maturity
   since its original publication.  Said experience applies both to this
   specification itself and to congestion control strategies related to
   the use of this specification.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, [RFC2119].

1.1.  Delivery service models

   ALC can support several different reliable content delivery service
   models as described in [I-D.ietf-rmt-bb-lct-revised].

1.2.  Scalability

   Massive scalability is a primary design goal for ALC.  IP multicast
   is inherently massively scalable, but the best effort service that it
   provides does not provide session management functionality,
   congestion control or reliability.  ALC provides all of this on top
   of IP multicast without sacrificing any of the inherent scalability
   of IP multicast.  ALC has the following properties:

   o  To each receiver, it appears as if though there is a dedicated
      session from the sender to the receiver, where the reception rate
      adjusts to congestion along the path from sender to receiver.

   o  To the sender, there is no difference in load or outgoing rate if
      one receiver is joined to the session or a million (or any number
      of) receivers are joined to the session, independent of when the
      receivers join and leave.

   o  No feedback packets are required from receivers to the sender.

   o  Almost all packets in the session that pass through a bottleneck
      link are utilized by downstream receivers, and the session shares
      the link with competing flows fairly in proportion to their
      utility.

   Thus, ALC provides a massively scalable content delivery transport
   that is network friendly.



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   ALC intentionally omits any application specific features that could
   potentially limit its scalability.  By doing so, ALC provides a
   minimal protocol that is massively scalable.  Applications may be
   built on top of ALC to provide additional features that may limit the
   scalability of the application.  Such applications are outside the
   scope of this document.

1.3.  Environmental Requirements and Considerations

   All of the environmental requirements and considerations that apply
   to the LCT building block [I-D.ietf-rmt-bb-lct-revised], the FEC
   building block [I-D.ietf-rmt-fec-bb-revised], the multiple rate
   congestion control building block and to any additional building
   blocks that ALC uses also apply to ALC.

   One issues that is specific to ALC with respect to the Any- Source
   Multicast (ASM) model of IP multicast as defined in RFC 1112
   [RFC1112] is the way the multiple rate congestion control building
   block interacts with ASM.  The congestion control building block may
   use the measured difference in time between when a join to a channel
   is sent and when the first packet from the channel arrives in
   determining the receiver reception rate.  The congestion control
   building block may also uses packet sequence numbers per channel to
   measure losses, and this is also used to determine the receiver
   reception rate.  These features raise two concerns with respect to
   ASM: The time difference between when the join to a channel is sent
   and when the first packet arrives can be significant due to the use
   of Rendezvous Points (RPs) and the MSDP protocol, and packets can be
   lost in the switch over from the (*,G) join to the RP and the (S,G)
   join directly to the sender.  Both of these issues could potentially
   substantially degrade the reception rate of receivers.  To ameliorate
   these concerns, it is RECOMMENDED that the RP be as close to the
   sender as possible.  SSM does not share these same concerns.  For a
   fuller consideration of these issues, consult the multiple rate
   congestion control building block.
















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2.  Architecture Definition

   ALC uses the LCT building block [I-D.ietf-rmt-bb-lct-revised] to
   provide in-band session management functionality.  ALC uses a
   multiple rate congestion control building block that is compliant
   with [RFC2357] to provide congestion control that is feedback free.
   Receivers adjust their reception rates individually by joining and
   leaving channels associated with the session.  ALC uses the FEC
   building block [I-D.ietf-rmt-fec-bb-revised] to provide reliability.
   The sender generates encoding symbols based on the object to be
   delivered using FEC codes and sends them in packets to channels
   associated with the session.  Receivers simply wait for enough
   packets to arrive in order to reliably reconstruct the object.  Thus,
   there is no request for retransmission of individual packets from
   receivers that miss packets in order to assure reliable reception of
   an object, and the packets and their rate of transmission out of the
   sender can be independent of the number and the individual reception
   experiences of the receivers.

   The definition of a session for ALC is the same as it is for LCT.  An
   ALC session comprises multiple channels originating at a single
   sender that are used for some period of time to carry packets
   pertaining to the transmission of one or more objects that can be of
   interest to receivers.  Congestion control is performed over the
   aggregate of packets sent to channels belonging to a session.  The
   fact that an ALC session is restricted to a single sender does not
   preclude the possibility of receiving packets for the same objects
   from multiple senders.  However, each sender would be sending packets
   to a a different session to which congestion control is individually
   applied.  Although receiving concurrently from multiple sessions is
   allowed, how this is done at the application level is outside the
   scope of this document.

   ALC is a protocol instantiation as defined in [RFC3048].  This
   document describes version 1 of ALC which MUST use version 1 of LCT
   described in [I-D.ietf-rmt-bb-lct-revised].  Like LCT, ALC is
   designed to be used with the IP multicast network service.  This
   specification defines ALC as payload of the UDP transport protocol
   [RFC0768] that supports IP multicast delivery of packets.  Future
   versions of this specification, or companion documents may extend ALC
   to use the IP network layer service directly.  ALC could be used as
   the basis for designing a protocol that uses a different underlying
   network service such as unicast UDP, but the design of such a
   protocol is outside the scope of this document.

   An ALC packet header immediately follows the UDP header and consists
   of the default LCT header that is described in
   [I-D.ietf-rmt-bb-lct-revised] followed by the FEC Payload ID that is



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   described in [I-D.ietf-rmt-fec-bb-revised].  The Congestion Control
   Information field within the LCT header carries the required
   Congestion Control Information that is described in the multiple rate
   congestion control building block specified that is compliant with
   [RFC2357].  The packet payload that follows the ALC packet header
   consists of encoding symbols that are identified by the FEC Payload
   ID as described in [I-D.ietf-rmt-fec-bb-revised].

   Each receiver is required to obtain a Session Description before
   joining an ALC session.  As described later, the Session Description
   includes out-of-band information required for the LCT, FEC and the
   multiple rate congestion control building blocks.  The FEC Object
   Transmission Information specified in the FEC building block
   [I-D.ietf-rmt-fec-bb-revised] required for each object to be received
   by a receiver can be communicated to a receiver either out-of-band or
   in-band using a Header Extension.  The means for communicating the
   Session Description and the FEC Object Transmission Information to a
   receiver is outside the scope of this document.

2.1.  LCT building block

   LCT requires receivers to be able to uniquely identify and
   demultiplex packets associated with an LCT session, and ALC inherits
   and strengthens this requirement.  A Transport Session Identifier
   (TSI) MUST be associated with each session and MUST be carried in the
   LCT header of each ALC packet.  The TSI is scoped by the sender IP
   address, and the (sender IP address, TSI) pair MUST uniquely identify
   the session.

   The LCT header contains a Congestion Control Information (CCI) field
   that MUST be used to carry the Congestion Control Information from
   the specified multiple rate congestion control protocol.  There is a
   field in the LCT header that specifies the length of the CCI field,
   and the multiple rate congestion control building block MUST uniquely
   identify a format of the CCI field that corresponds to this length.

   The LCT header contains a Codepoint field that MAY be used to
   communicate to a receiver the settings for information that may vary
   during a session.  If used, the mapping between settings and
   Codepoint values is to be communicated in the Session Description,
   and this mapping is outside the scope of this document.  For example,
   the FEC Encoding ID that is part of the FEC Object Transmission
   Information as specified in the FEC building block
   [I-D.ietf-rmt-fec-bb-revised] could vary for each object carried in
   the session, and the Codepoint value could be used to communicate the
   FEC Encoding ID to be used for each object.  The mapping between FEC
   Encoding IDs and Codepoints could be, for example, the identity
   mapping.



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   If more than one object is to be carried within a session then the
   Transmission Object Identifier (TOI) MUST be used in the LCT header
   to identify which packets are to be associated with which objects.
   In this case the receiver MUST use the TOI to associate received
   packets with objects.  The TOI is scoped by the IP address of the
   sender and the TSI, i.e., the TOI is scoped by the session.  The TOI
   for each object is REQUIRED to be unique within a session, but MAY
   NOT be unique across sessions.  Furthermore, the same object MAY have
   a different TOI in different sessions.  The mapping between TOIs and
   objects carried in a session is outside the scope of this document.

   If only one object is carried within a session then the TOI MAY be
   omitted from the LCT header.

   The LCT header from version 1 of the LCT building block
   [I-D.ietf-rmt-bb-lct-revised] MUST be used.

   The LCT Header includes a two-bit Protocol Specific Indication (PSI)
   field in bits 6 and 7 of the first word of the LCT header.  These two
   bits are used by ALC as follows:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                   +-+-+
                ...|A|B|...
                   +-+-+

                   Figure 1: PSI bits within LCT Headder

      PSI bit A - Source Packet Indicator (SPI)

      PSI bit B - Reserved

   The Source Packet Indicator is used with systematic FEC Schemes which
   define a different FEC Payload ID format for packets containing only
   source data compared to the FEC Payload ID format for packets
   containing repair data.  For such FEC Schemes, then the SPI MUST be
   set to 1 when the FEC Payload ID format for packets containing only
   source data is used and the SPI MUST be set to zero, when the FEC
   Payload ID for packerts containing repair data is used.  In the case
   of FEC Schemes which define only a single FEC Payload ID format, then
   the SPI MUST be set to zero by the sender and MUST be ignored by the
   receiver.

   Support of two FEC Payload ID formats allows FEC Payload ID
   information which is only of relevance when FEC decoding is to be
   performed to be provided in the FEC Payload ID format for packets
   containing repair data.  This information need not be processed by



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   receivers which do not perform FEC decoding (either because no FEC
   decoding is required or because the receiver does not support FEC
   decoding).

2.2.  Multiple rate congestion control building block

   Implementors of ALC MUST implement a multiple rate feedback-free
   congestion control building block that is in accordance to [RFC2357].
   Congestion control MUST be applied to all packets within a session
   independently of which information about which object is carried in
   each packet.  Multiple rate congestion control is specified because
   of its suitability to scale massively and because of its suitability
   for reliable content delivery.  The multiple rate congestion control
   building block MUST specify in-band Congestion Control Information
   (CCI) that MUST be carried in the CCI field of the LCT header.  The
   multiple rate congestion control building block MAY specify more than
   one format, but it MUST specify at most one format for each of the
   possible lengths 32, 64, 96 or 128 bits.  The value of C in the LCT
   header that determines the length of the CCI field MUST correspond to
   one of the lengths for the CCI defined in the multiple rate
   congestion control building block, this length MUST be the same for
   all packets sent to a session, and the CCI format that corresponds to
   the length as specified in the multiple rate congestion control
   building block MUST be the format used for the CCI field in the LCT
   header.

   When using a multiple rate congestion control building block a sender
   sends packets in the session to several channels at potentially
   different rates.  Then, individual receivers adjust their reception
   rate within a session by adjusting which set of channels they are
   joined to at each point in time depending on the available bandwidth
   between the receiver and the sender, but independent of other
   receivers.

2.3.  FEC building block

   The FEC building block [I-D.ietf-rmt-fec-bb-revised] provides
   reliable object delivery within an ALC session.  Each object sent in
   the session is independently encoded using FEC codes as described in
   [RFC3453], which provide a more in-depth description of the use of
   FEC codes in reliable content delivery protocols.  All packets in an
   ALC session MUST contain an FEC Payload ID in a format that is
   compliant with the FEC Scheme in use.  The FEC Payload ID uniquely
   identifies the encoding symbols that constitute the payload of each
   packet, and the receiver MUST use the FEC Payload ID to determine how
   the encoding symbols carried in the payload of the packet were
   generated from the object as described in the FEC building block.




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   As described in [I-D.ietf-rmt-fec-bb-revised], a receiver is REQUIRED
   to obtain the FEC Object Transmission Information for each object for
   which data packets are received from the session.  In the context of
   ALC, the FEC Object Transmission Information includes:

   o  The FEC Encoding ID.

   o  If an Under-Specified FEC Encoding ID is used then the FEC
      Instance ID associated with the FEC Encoding ID.

   o  For each object in the session, the transfer length of the object
      in bytes.

   Additional FEC Object Transmission Information may be required
   depending on the FEC Scheme that is used (identified by the FEC
   Encoding ID).

   Some of the FEC Object Transmission Information MAY be implicit based
   on the FEC Scheme and/or implementation.  As an example, source block
   lengths may be derived by a fixed algorithm from the object length.
   As another example, it may be that all source blocks are the same
   length and this is what is passed out-of-band to the receiver.  As
   another example, it could be that the full sized source block length
   is provided and this is the length used for all but the last source
   block, which is calculated based on the full source block length and
   the object length.  As another example, it could be that the same FEC
   Encoding ID and FEC Instance ID are always used for a particular
   application and thus the FEC Encoding ID and FEC Instance ID are
   implicitly defined.

   Sometimes the objects that will be sent in a session are completely
   known before the receiver joins the session, in which case the FEC
   Object Transmission Information for all objects in the session can be
   communicated to receivers before they join the session.  At other
   times the objects may not know when the session begins, or receivers
   may join a session in progress and may not be interested in some
   objects for which transmission has finished, or receivers may leave a
   session before some objects are even available within the session.
   In these cases, the FEC Object Transmission Information for each
   object may be dynamically communicated to receivers at or before the
   time packets for the object are received from the session.  This may
   be accomplished using either an out-of-band mechanism, in-band using
   the Codepoint field or a Header Extension, or any combination of
   these methods.  How the FEC Object Transmission Information is
   communicated to receivers is outside the scope of this document.

   If packets for more than one object are transmitted within a session
   then a Transmission Object Identifier (TOI) that uniquely identifies



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   objects within a session MUST appear in each packet header.  Portions
   of the FEC Object Transmission Information could be the same for all
   objects in the session, in which case these portions can be
   communicated to the receiver with an indication that this applies to
   all objects in the session.  These portions may be implicitly
   determined based on the application, e.g., an application may use the
   same FEC Encoding ID for all objects in all sessions.  If there is a
   portion of the FEC Object Transmission Information that may vary from
   object to object and if this FEC Object Transmission Information is
   communicated to a receiver out-of-band then the TOI for the object
   MUST also be communicated to the receiver together with the
   corresponding FEC Object Transmission Information, and the receiver
   MUST use the corresponding FEC Object Transmission Information for
   all packets received with that TOI.  How the TOI and corresponding
   FEC Object Transmission Information is communicated out-of-band to
   receivers is outside the scope of this document.

   It is also possible that there is a portion of the FEC Object
   Transmission Information that may vary from object to object that is
   carried in-band, for example in the CodePoint field or in Header
   Extensions.  How this is done is outside the scope of this document.
   In this case the FEC Object Transmission Information is associated
   with the object identified by the TOI carried in the packet.

2.4.  Session Description

   The Session Description that a receiver is REQUIRED to obtain before
   joining an ALC session MUST contain the following information:

   o  The multiple rate congestion control building block to be used for
      the session;

   o  The sender IP address;

   o  The number of channels in the session;

   o  The address and port number used for each channel in the session;

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

   o  An indication of whether or not the session carries packets for
      more than one object;

   o  If Header Extensions are to be used, the format of these Header
      Extensions.

   o  Enough information to determine the packet authentication scheme
      being used, if it is being used.



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   How the Session Description is communicated to receivers is outside
   the scope of this document.

   The Codepoint field within the LCT portion of the header CAN be used
   to communicate in-band some of the dynamically changing information
   within a session.  To do this, a mapping between Codepoint values and
   the different dynamic settings MUST be included within the Session
   Description, and then settings to be used are communicated via the
   Codepoint value placed into each packet.  For example, it is possible
   that multiple objects are delivered within the same session and that
   a different FEC encoding algorithm is used for different types of
   objects.  Then the Session Description could contain the mapping
   between Codepoint values and FEC Encoding IDs.  As another example,
   it is possible that a different packet authentication scheme is used
   for different packets sent to the session.  In this case, the mapping
   between the packet authentication scheme and Codepoint values could
   be provided in the Session Description.  Combinations of settings can
   be mapped to Codepoint values as well.  For example, a particular
   combination of a FEC Encoding ID and a packet authentication scheme
   could be associated with a Codepoint value.

   The Session Description could also include, but is not limited to:

   o  The mappings between combinations of settings and Codepoint
      values;

   o  The data rates used for each channel;

   o  The length of the packet payload;

   o  Any information that is relevant to each object being transported,
      such as the Object Transmission Information for each object, when
      the object will be available within the session and for how long.

   The Session Description could be in a form such as SDP as defined in
   [RFC2327], or XML metadata as defined in [RFC3023], or HTTP/Mime
   headers as defined in [RFC2616], 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 formats and
   methods for communication of Session Descriptions to receivers is
   beyond the scope of this document.

2.5.  Packet authentication building block

   It is RECOMMENDED that implementors of ALC use some packet
   authentication scheme to protect the protocol from attacks.  An



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   example of a possibly suitable scheme is described in [PER2001].
   Packet authentication in ALC, if used, is to be integrated through
   the Header Extension support for packet authentication provided in
   the LCT building block.















































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3.  Conformance Statement

   This Protocol Instantiation document, in conjunction with the LCT
   building block [I-D.ietf-rmt-bb-lct-revised], the FEC building block
   [I-D.ietf-rmt-fec-bb-revised] and with a multiple rate congestion
   control building block completely specifies a working reliable
   multicast transport protocol that conforms to the requirements
   described in [RFC2357].











































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4.  Functionality Definition

   This section describes the format and functionality of the data
   packets carried in an ALC session as well as the sender and receiver
   operations for a session.

4.1.  Packet format used by ALC

   The packet format used by ALC is the UDP header followed by the LCT
   header followed by the FEC Payload ID followed by the packet payload.
   The LCT header is defined in the LCT building block
   [I-D.ietf-rmt-bb-lct-revised] and the FEC Payload ID is described in
   the FEC building block [I-D.ietf-rmt-fec-bb-revised].  The Congestion
   Control Information field in the LCT header contains the REQUIRED
   Congestion Control Information that is described in the multiple rate
   congestion control building block used.  The packet payload contains
   encoding symbols generated from an object.  If more than one object
   is carried in the session then the Transmission Object ID (TOI)
   within the LCT header MUST be used to identify which object the
   encoding symbols are generated from.  Within the scope of an object,
   encoding symbols carried in the payload of the packet are identified
   by the FEC Payload ID as described in the FEC building block.

   The version number of ALC specified in this document is 1.  The
   version number field of the LCT header MUST be interpreted as the ALC
   version number field.  This version of ALC implicitly makes use of
   version 1 of the LCT building block defined in
   [I-D.ietf-rmt-bb-lct-revised].

   The overall ALC packet format is depicted in Figure 2.  The packet is
   an IP packet, either IPv4 or IPv6, and the IP header precedes the UDP
   header.  The ALC packet format has no dependencies on the IP version
   number.


















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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         UDP header                            |
       |                                                               |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                         LCT header                            |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       FEC Payload ID                          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Encoding Symbol(s)                        |
       |                           ...                                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 2: Overall ALC packet format

   In some special cases an ALC sender may need to produce ALC packets
   that do not contain any payload.  This may be required, for example,
   to signal the end of a session or to convey congestion control
   information.  These data-less packets do not contain the FEC Payload
   ID either, but only the LCT header fields.  The total datagram
   length, conveyed by outer protocol headers (e.g., the IP or UDP
   header), enables receivers to detect the absence of the ALC payload
   and FEC Payload ID.

   For ALC the length of the TSI field within the LCT header is REQUIRED
   to be non-zero.  This implies that the sender MUST NOT set both the
   LCT flags S and H to zero.

4.2.  LCT Header-Extension Fields

   All senders and receivers implementing ALC MUST support the EXT_NOP
   Header Extension and MUST recognize EXT_AUTH, but MAY NOT be able to
   parse its content.  The EXT_NOP and EXT_AUTH LCT Header Extensions
   are defined in [I-D.ietf-rmt-bb-lct-revised]

   This specification defines a new LCT Header Extension, "EXT_FTI", for
   the purpose of communicating the FEC Object Transmission Information
   in association with data packets of an object.  The LCT Header
   Extension Type for EXT_FTI is 64.

   The Header Extension Content (HEC) field of the EXT_FTI LCT Header
   Extension contains the encoded FEC Object Transmission Information as
   defined in [I-D.ietf-rmt-fec-bb-revised].  The format of the encoded
   FEC Object Transmission Information is dependent on the FEC Scheme in



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   use and so is outside the scope of this document.

4.3.  Sender Operation

   The sender operation when using ALC includes all the points made
   about the sender operation when using the LCT building block
   [I-D.ietf-rmt-bb-lct-revised], the FEC building block
   [I-D.ietf-rmt-fec-bb-revised] and the multiple rate congestion
   control building block.

   A sender using ALC MUST make available the required Session
   Description as described in Section 2.4.  A sender also MUST make
   available the required FEC Object Transmission Information as
   described in Section 2.3.

   Within a session a sender transmits a sequence of packets to the
   channels associated with the session.  The ALC sender MUST obey the
   rules for filling in the CCI field in the packet headers and MUST
   send packets at the appropriate rates to the channels associated with
   the session as dictated by the multiple rate congestion control
   building block.

   The ALC sender MUST use the same TSI for all packets in the session.
   Several objects MAY be delivered within the same ALC session.  If
   more than one object is to be delivered within a session then the
   sender MUST use the TOI field and each object MUST be identified by a
   unique TOI within the session, and the sender MUST use corresponding
   TOI for all packets pertaining to the same object.  The FEC Payload
   ID MUST correspond to the encoding symbol(s) for the object carried
   in the payload of the packet.

   It is RECOMMENDED that packet authentication be used.  If packet
   authentication is used then the Header Extensions described in
   Section 4.2 MUST be used to carry the authentication.

4.4.  Receiver Operation

   The receiver operation when using ALC includes all the points made
   about the receiver operation when using the LCT building block
   [I-D.ietf-rmt-bb-lct-revised], the FEC building block
   [I-D.ietf-rmt-fec-bb-revised] and the multiple rate congestion
   control building block.

   To be able to participate in a session, a receiver MUST obtain the
   REQUIRED Session Description as listed in Section 2.4.  How receivers
   obtain a Session Description is outside the scope of this document.

   To be able to be a receiver in a session, the receiver MUST be able



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   to process the ALC 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 the ALC header, it MUST drop
   from the session.

   As described in Section 2.3, a receiver MUST obtain the required FEC
   Object Transmission Information for each object for which the
   receiver receives and processes packets.

   Upon receipt of each packet the receiver proceeds with the following
   steps in the order listed.

   1.  The receiver MUST parse the packet header and verify that it is a
       valid header.  If it is not valid then the packet MUST be
       discarded without further processing.  If multiple packets are
       received that cannot be parsed then the receiver SHOULD leave the
       session.

   2.  The receiver MUST verify that the sender IP address together with
       the TSI carried in the header matches one of the (sender IP
       address, TSI) pairs that was received in a Session Description
       and that the receiver is currently joined to.  If there is not a
       match then the packet MUST be discarded without further
       processing.  If multiple packets are received with non-matching
       (sender IP address, TSI) values then the receiver SHOULD leave
       the session.  If the receiver is joined to multiple ALC sessions
       then the remainder of the steps are performed within the scope of
       the (sender IP address, TSI) session of the received packet.

   3.  The receiver MUST process and act on the CCI field in accordance
       with the multiple rate congestion control building block.

   4.  If more than one object is carried in the session, the receiver
       MUST verify that the TOI carried in the LCT header is valid.  If
       the TOI is not valid, the packet MUST be discarded without
       further processing.

   5.  The receiver SHOULD process the remainder of the packet,
       including interpreting the other header fields appropriately, and
       using the FEC Payload ID and the encoding symbol(s) in the
       payload to reconstruct the corresponding object.

   It is RECOMMENDED that packet authentication be used.  If packet
   authentication is used then it is RECOMMENDED that the receiver
   immediately check the authenticity of a packet before proceeding with
   step (3) above.  If immediate checking is possible and if the packet
   fails the check then the receiver MUST discard the packet and reduce
   its reception rate to a minimum before continuing to regulate its



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   reception rate using the multiple rate congestion control.

   Some packet authentication schemes such as TESLA [PER2001] do not
   allow an immediate authenticity check.  In this case the receiver
   SHOULD check the authenticity of a packet as soon as possible, and if
   the packet fails the check then it MUST be discarded before step (5)
   above and reduce its reception rate to a minimum before continuing to
   regulate its reception rate using the multiple rate congestion
   control.










































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5.  Security Considerations

   The same security consideration that apply to the LCT, FEC and the
   multiple rate congestion control building blocks also apply to ALC.

   Because of the use of FEC, ALC is especially vulnerable to denial-
   of-service attacks by attackers that try to send forged packets to
   the session which would prevent successful reconstruction or cause
   inaccurate reconstruction of large portions of the object by
   receivers.  ALC is also particularly affected by such an attack
   because many receivers may receive the same forged packet.  There are
   two ways to protect against such attacks, one at the application
   level and one at the packet level.  It is RECOMMENDED that prevention
   be provided at both levels.

   At the application level, it is RECOMMENDED that an integrity check
   on the entire received object be done once the object is
   reconstructed to ensure it is the same as the sent object.  Moreover,
   in order to obtain strong cryptographic integrity protection a
   digital signature verifiable by the receiver SHOULD be used to
   provide this application level integrity check.  However, if even one
   corrupted or forged packet is used to reconstruct the object, it is
   likely that the received object will be reconstructed incorrectly.
   This will appropriately cause the integrity check to fail and in this
   case the inaccurately reconstructed object SHOULD be discarded.
   Thus, the acceptance of a single forged packet can be an effective
   denial of service attack for distributing objects, but an object
   integrity check at least prevents inadvertent use of inaccurately
   reconstructed objects.  The specification of an application level
   integrity check of the received object is outside the scope of this
   document.

   At the packet level, it is RECOMMENDED that a packet level
   authentication be used to ensure that each received packet is an
   authentic and uncorrupted packet containing FEC data for the object
   arriving from the specified sender.  Packet level authentication has
   the advantage that corrupt or forged packets can be discarded
   individually and the received authenticated packets can be used to
   accurately reconstruct the object.  Thus, the effect of a denial of
   service attack that injects forged packets is proportional only to
   the number of forged packets, and not to the object size.  Although
   there is currently no IETF standard that specifies how to do
   multicast packet level authentication, TESLA [PER2001] is a known
   multicast packet authentication scheme that would work.

   In addition to providing protection against reconstruction of
   inaccurate objects, packet level authentication can also provide some
   protection against denial of service attacks on the multiple rate



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   congestion control.  Attackers can try to inject forged packets with
   incorrect congestion control information into the multicast stream,
   thereby potentially adversely affecting network elements and
   receivers downstream of the attack, and much less significantly the
   rest of the network and other receivers.  Thus, it is also
   RECOMMENDED that packet level authentication be used to protect
   against such attacks.  TESLA [PER2001] can also be used to some
   extent to limit the damage caused by such attacks.  However, with
   TESLA a receiver can only determine if a packet is authentic several
   seconds after it is received, and thus an attack against the
   congestion control protocol can be effective for several seconds
   before the receiver can react to slow down the session reception
   rate.

   Reverse Path Forwarding checks SHOULD be enabled in all network
   routers and switches along the path from the sender to receivers to
   limit the possibility of a bad agent injecting forged packets into
   the multicast tree data path.

































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6.  IANA Considerations

   This specification registers the following LCT Header Extensions
   Types in namespace ietf:rmt:lct:headerExtensionTypes:variableLength:

                 +-------+---------+--------------------+
                 | Value | Name    | Reference          |
                 +-------+---------+--------------------+
                 | 64    | EXT_FTI | This specification |
                 +-------+---------+--------------------+









































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7.  Acknowledgments

   This specification is substantially based on RFC3450 [RFC3450] and
   thus credit for the authorship of this document is primarily due to
   the authors of RFC3450: Mike Luby, Jim Gemmel, Lorenzo Vicisano,
   Luigi Rizzo and Jon Crowcroft.  Vincent Roca, Justin Chapweske and
   Roger Kermode also contributed to RFC3450.  Additional thanks are due
   to Vincent Roca and Rod Walsh for contributions to this update to
   Proposed Standard.










































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8.  Changes from RFC3450

   This section summarises the changes that were made from the
   Experimental version of this specification published as RFC3450
   [RFC3450]:

   o  Update all references to the obsoleted RFC 2068 to RFC 2616

   o  Removed the 'Statement of Intent' from the introduction (The
      statement of intent was meant to clarify the "Experimental" status
      of RFC3450.)

   o  Removed the 'Intellectual Property Issues' Section and replaced
      with a standard IPR Statement

   o  Remove material duplicated in LCT

   o  Update references for LCT and FEC Building Block to new versions
      and align with changes in the FEC Building Block.

   o  Split normative and informative references

   o  Material applicable in a general LCT context, not just for ALC has
      been moved to LCT

   o  The first bit of the "Protocol Specific Indication" in the LCT
      Headert is defined as a "Source Packet Indication".  This is used
      in the case that an FEC Scheme defines two FEC Payload ID formats,
      one of which is for packets containing only source symbols which
      can be processed by receivers that do not support FEC Decoding.

   o  Definition and IANA registration of the EXT_FTI LCT Header
      Extension


















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9.  References

9.1.  Normative references

   [I-D.ietf-rmt-bb-lct-revised]
              Luby, M., "Layered Coding Transport (LCT) Building Block",
              draft-ietf-rmt-bb-lct-revised-04 (work in progress),
              June 2006.

   [I-D.ietf-rmt-fec-bb-revised]
              Watson, M., "Forward Error Correction (FEC) Building
              Block", draft-ietf-rmt-fec-bb-revised-04 (work in
              progress), September 2006.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2327]  Handley, M. and V. Jacobson, "SDP: Session Description
              Protocol", RFC 2327, April 1998.

   [RFC2357]  Mankin, A., Romanov, A., Bradner, S., and V. Paxson, "IETF
              Criteria for Evaluating Reliable Multicast Transport and
              Application Protocols", RFC 2357, June 1998.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, October 2000.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

9.2.  Informative references

   [HOL2001]  Holbrook, H., "A Channel Model for Multicast",  Ph.D.
              Dissertation, Stanford University, Department of Computer
              Science, Stanford, CA , August 2001.



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   [PER2001]  Perrig, A., Canetti, R., Song, D., and J. Tygar,
              "Efficient and Secure Source Authentication for
              Multicast", Network and Distributed System Security
              Symposium, NDSS 2001, pp. 35-46 , February 2001.

   [RFC3048]  Whetten, B., Vicisano, L., Kermode, R., Handley, M.,
              Floyd, S., and M. Luby, "Reliable Multicast Transport
              Building Blocks for One-to-Many Bulk-Data Transfer",
              RFC 3048, January 2001.

   [RFC3269]  Kermode, R. and L. Vicisano, "Author Guidelines for
              Reliable Multicast Transport (RMT) Building Blocks and
              Protocol Instantiation documents", RFC 3269, April 2002.

   [RFC3450]  Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and J.
              Crowcroft, "Asynchronous Layered Coding (ALC) Protocol
              Instantiation", RFC 3450, December 2002.

   [RFC3453]  Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley,
              M., and J. Crowcroft, "The Use of Forward Error Correction
              (FEC) in Reliable Multicast", RFC 3453, December 2002.






























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Authors' Addresses

   Michael Luby
   Digital Fountain
   39141 Civic Center Dr.
   Suite 300
   Fremont, CA  94538
   US

   Email: luby@digitalfountain.com


   Mark Watson
   Digital Fountain
   39141 Civic Center Dr.
   Suite 300
   Fremont, CA  94538
   US

   Email: mark@digitalfountain.com


   Lorenzo Vicisano
   Digital Fountain
   39141 Civic Center Dr.
   Suite 300
   Fremont, CA  94538
   US

   Email: lorenzo@digitalfountain.com





















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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


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   http://www.ietf.org/ipr.

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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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