





Network Working Group                                            M. Luby
Request for Comments: 3450                              Digital Fountain
Category: Experimental                                        J. Gemmell
                                                               Microsoft
                                                             L. Vicisano
                                                                   Cisco
                                                                L. Rizzo
                                                              Univ. Pisa
                                                            J. Crowcroft
                                                         Cambridge Univ.
                                                           December 2002


        Asynchronous Layered Coding (ALC) Protocol Instantiation

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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.

Table of Contents

   1. Introduction.................................................2
     1.1 Delivery service models...................................3
     1.2 Scalability...............................................5
     1.3 Environmental Requirements and Considerations.............6
   2. Architecture Definition......................................8
     2.1 LCT building block........................................9
     2.2 Multiple rate congestion control building block..........10
     2.3 FEC building block.......................................11
     2.4 Session Description......................................13



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     2.5 Packet authentication building block.....................14
   3. Conformance Statement.......................................14
   4. Functionality Definition....................................14
     4.1 Packet format used by ALC................................15
     4.2 Detailed Example of Packet format used by ALC............16
     4.3 Header-Extension Fields..................................23
     4.4 Sender Operation.........................................26
     4.5 Receiver Operation.......................................27
   5. Security Considerations.....................................29
   6. IANA Considerations.........................................31
   7. Intellectual Property Issues................................31
   8. Acknowledgments.............................................31
   9. References..................................................31
   Authors' Addresses.............................................33
   Full Copyright Statement.......................................34

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





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   using the packets to reliably reconstruct objects.  All information
   flow in an ALC session is in the form of data packets sent by a
   single sender to channels that receivers join to receive data.

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

   This document is a product of the IETF RMT WG and follows the general
   guidelines provided in RFC 3269 [8].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [2].

   Statement of Intent

      This memo contains part of the definitions necessary to fully
      specify a Reliable Multicast Transport protocol in accordance with
      RFC2357.  As per RFC2357, the use of any reliable multicast
      protocol in the Internet requires an adequate congestion control
      scheme.

      While waiting for such a scheme to be available, or for an
      existing scheme to be proven adequate, the Reliable Multicast
      Transport working group (RMT) publishes this Request for Comments
      in the "Experimental" category.

      It is the intent of RMT to re-submit this specification as an IETF
      Proposed Standard as soon as the above condition is met.

1.1 Delivery service models

   ALC can support several different reliable content delivery service
   models.  Some examples are briefly described here.

   Push service model.

   A push model is a sender initiated concurrent delivery of objects to
   a selected set of receivers.  A push service model can be used for
   example for reliable delivery of a large object such as a 100 GB
   file.  The sender could send a Session Description announcement to a



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   control channel and receivers could monitor this channel and join a
   session whenever a Session Description of interest arrives.  Upon
   receipt of the Session Description, each receiver could join the
   session to receive packets until enough packets have arrived to
   reconstruct the object, at which point the receiver could report back
   to the sender that its reception was completed successfully.  The
   sender could decide to continue sending packets for the object to the
   session until all receivers have reported successful reconstruction
   or until some other condition has been satisfied.  In this example,
   the sender uses ALC to generate packets based on the object and send
   packets to channels associated with the session, and the receivers
   use ALC to receive packets from the session and reconstruct the
   object.

   There are several features ALC provides to support the push model.
   For example, the sender can optionally include an Expected Residual
   Time (ERT) in the packet header that indicates the expected remaining
   time of packet transmission for either the single object carried in
   the session or for the object identified by the Transmission Object
   Identifier (TOI) if there are multiple objects carried in the
   session.  This can be used by receivers to determine if there is
   enough time remaining in the session to successfully receive enough
   additional packets to recover the object.  If for example there is
   not enough time, then the push application may have receivers report
   back to the sender to extend the transmission of packets for the
   object for enough time to allow the receivers to obtain enough
   packets to reconstruct the object.  The sender could then include an
   ERT based on the extended object transmission time in each subsequent
   packet header for the object.  As other examples, the LCT header
   optionally can contain a Close Session flag that indicates when the
   sender is about to end sending packet to the session and a Close
   Object flag that indicates when the sender is about to end sending
   packets to the session for the object identified by the Transmission
   Object ID.  However, these flags are not a completely reliable
   mechanism and thus the Close Session flag should only be used as a
   hint of when the session is about to close and the Close Object flag
   should only be used as a hint of when transmission of packets for the
   object is about to end.

   The push model is particularly attractive in satellite networks and
   wireless networks.  In these environments a session may include one
   channel and a sender may send packets at a fixed rate to this
   channel, but sending at a fixed rate without congestion control is
   outside the scope of this document.







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   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 a
   single object.  For example a popular software update might be
   transmitted using ALC 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 at any point in time when it is active.
   Receivers leave the session when they have received enough packets to
   recover the object.  The receivers, for example, obtain a Session
   Description by contacting a web server.

   Other service models.

   There may be other reliable content delivery service models that can
   be supported by ALC.  The description of the potential applications,
   the appropriate delivery service model, and the additional mechanisms
   to support such functionalities when combined with ALC is beyond the
   scope of this document.

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.





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

   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 [11], the FEC building block [10], the
   multiple rate congestion control building block and to any additional
   building blocks that ALC uses also apply to ALC.

   ALC requires connectivity between a sender and receivers, but does
   not require connectivity from receivers to a sender.  ALC 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 ALC is unlimited.
   However, ALC requires receivers to obtain the Session Description
   out-of-band before joining a session and some implementations of this
   may limit scalability.

   If a receiver is joined to multiple ALC sessions then the receiver
   MUST be able to uniquely identify and demultiplex packets to the
   correct session.  The Transmission Session Identifier (TSI) that MUST
   appear in each packet header is used for this purpose.  The TSI is
   scoped by the IP address of the sender, and the IP address of the
   sender together with the TSI uniquely identify the session.  Thus,
   the demultiplexing MUST be done on the basis of the IP address of the
   sender and the TSI of the session from that sender.

   ALC is presumed to be used with an underlying IP multicast 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.  There are
   currently two models of multicast delivery, the Any-Source Multicast
   (ASM) model as defined in RFC 1112 [3] and the Source-Specific
   Multicast (SSM) model as defined in [7].  ALC 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 an ALC channel address consists
   of the pair (S,G), where S is the IP address of the sender and G is a





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   multicast group address.  When using SSM, a sender S sends packets to
   an SSM channel (S,G), and an ALC channel address coincides with the
   SSM channel address.

   A sender can locally allocate unique SSM channel addresses, and this
   makes allocation of ALC channel addresses easy with SSM.  To allocate
   ALC channel addresses using ASM, the sender must uniquely choose the
   ASM multicast group address across the scope of the group, and this
   makes allocation of ALC channel addresses more difficult with ASM.

   ALC channels and SSM channels coincide, and thus the receiver will
   only receive packets sent to the requested ALC channel.  With ASM,
   the receiver joins an ALC 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.

   Other issues specific to ALC with respect to ASM is the way the
   multiple rate congestion control building block interacts with ASM.
   The congestion control building block may use the measured difference
   in time between when a join to a channel is sent and when the first
   packet from the channel arrives in determining the receiver reception
   rate.  The congestion control building block may also 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.

   Some networks are not amenable to some congestion control protocols
   that could be used with ALC.  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.

   ALC is compatible with either IPv4 or IPv6 as no part of the packet
   is IP version specific.





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2.  Architecture Definition

   ALC uses the LCT building block [11] to provide in-band session
   management functionality.  ALC uses a multiple rate congestion
   control building block that is compliant with RFC 2357 [12] 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 [10] 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 RFC 3048 [16].  This
   document describes version 1 of ALC which MUST use version 1 of LCT
   described in [11].  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 [15] 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 [11] followed by the
   FEC Payload ID that is described in [10].  The Congestion Control
   Information field within the LCT header carries the required



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   Congestion Control Information that is described in the multiple rate
   congestion control building block specified that is compliant with
   RFC 2357 [12].  The packet payload that follows the ALC packet header
   consists of encoding symbols that are identified by the FEC Payload
   ID as described in [10].

   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 [10]
   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 [10] 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 default LCT header from version 1 of the LCT building block [11]
   MUST be used.

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 RFC 2357
   [12].  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.





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2.3  FEC building block

   The FEC building block [10] provides reliable object delivery within
   an ALC session.  Each object sent in the session is independently
   encoded using FEC codes as described in [9], 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 building
   block [10].  The FEC Payload ID uniquely identifies the encoding
   symbols that constitute the payload of each packet, and the receiver
   MUST use the FEC Payload ID to determine how the encoding symbols
   carried in the payload of the packet were generated from the object
   as described in the FEC building block.

   As described in [10], a receiver is REQUIRED to obtain the FEC Object
   Transmission Information for each object for which data packets are
   received from the session.  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 length of the object in
       bytes.

     o The additional required FEC Object Transmission Information for
       the FEC Encoding ID as prescribed in the FEC building block [10].
       For example, when the FEC Encoding ID is 128, the required FEC
       Object Transmission Information is the number of source blocks
       that the object is partitioned into and the length of each source
       block in bytes.

   Some of the FEC Object Transmission Information MAY be implicit based
   on the 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.





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   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
   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.










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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.

   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.






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   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
   RFC 2327 [5], or XML metadata as defined in RFC 3023 [13], or
   HTTP/Mime headers as defined in RFC 2068 [4], etc.  It might be
   carried in a session announcement protocol such as SAP as defined in
   RFC 2974 [6], 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
   example of a possibly suitable scheme is described in [14].  Packet
   authentication in ALC, if used, is to be integrated through the
   Header Extension support for packet authentication provided in the
   LCT building block.

3.  Conformance Statement

   This Protocol Instantiation document, in conjunction with the LCT
   building block [11], the FEC building block [10] and with a multiple
   rate congestion control building block completely specifies a working
   reliable multicast transport protocol that conforms to the
   requirements described in RFC 2357 [12].

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.





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4.1  Packet format used by ALC

   The packet format used by ALC is the UDP header followed by the
   default LCT header followed by the FEC Payload ID followed by the
   packet payload.  The default LCT header is described in the LCT
   building block [11] and the FEC Payload ID is described in the FEC
   building block [10].  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.  This
   coincides with version 1 of the LCT building block [11] used in this
   specification.  The LCT version number field should be interpreted as
   the ALC version number field.

   The overall ALC packet format is depicted in Figure 1.  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.  The default LCT header MUST be used by ALC and this default
   is described in detail in the LCT building block [11].

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

                   Figure 1 - Overall ALC packet format






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   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.

4.2  Detailed Example of Packet format used by ALC

   A detailed example of an ALC packet starting with the LCT header is
   shown in Figure 2.  In the example, the LCT header is the first 5
   32-bit words, the FEC Payload ID is the next 2 32-bit words, and the
   remainder of the packet is the payload.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   1   | 0 | 0 |1| 1 |0|1|0|0|0|       5       |      128      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Congestion Control Information (CCI, length = 32 bits)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Transport Session Identifier (TSI, length = 32 bits)         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Transport Object Identifier (TOI, length = 32 bits)         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Sender Current Time                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Source Block Number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Encoding Symbol ID                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Encoding Symbol(s)                         |
    |                          ...                                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 2 - A detailed example of the ALC packet format

   The LCT portion of the overall ALC packet header is of variable size,
   which is specified by a length field in the third byte of the 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).





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   The function and length and particular setting of the value for each
   field in this detailed example of the header is the following,
   described in the order of their appearance in the header.

     ALC version number (V): 4 bits

         Indicates the ALC version number.
         The ALC version number for this specification is 1 as shown.
         This is also the LCT version number.

     Congestion control flag (C): 2 bits

         The Congestion Control Information (CCI) field specified by the
         multiple rate congestion control building block is a multiple
         of 32-bits in length.  The multiple rate congestion control
         building block MUST specify a format for the CCI.  The
         congestion control building block MAY specify formats for
         different CCI lengths, where the set of possible lengths is 32,
         64, 96 or 128 bits.  The value of C MUST match the length of
         exactly one of the possible formats for the congestion control
         building block, and this format MUST be used for the CCI field.
         The value of C MUST be the same for all packets sent to a
         session.

         C=0 indicates the 32-bit CCI field format is to be used.
         C=1 indicates the 64-bit CCI field format is to be used.
         C=2 indicates the 96-bit CCI field format is to be used.
         C=3 indicates the 128-bit CCI field format is to be used.

         In the example C=0 indicates that a 32-bit format is to be
         used.

     Reserved (r): 2 bits

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

         As required, these bits are set to 0 in the example.

     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.  For ALC the length of
         the TSI field is REQUIRED to be non-zero.  This implies that
         the setting S=0 and H=0 MUST NOT be used.

         In the example S=1 and H=0, and thus the TSI is 32-bits in
         length.



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     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.  If more than one
         object is to be delivered in the session then the TOI MUST be
         used, in which case the setting O=0 and H=0 MUST NOT be used.

         In the example O=1 and H=0, and thus the TOI is 32-bits in
         length.

     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.

         In the example H=0 which indicates that both TSI and TOI are
         both multiples of 32-bits in length.

     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.

         In the example T=1, which indicates that the SCT is carried in
         this packet.

     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 packets will be sent to the session for either the
         single object carried in the session or for the object
         identified by the TOI if there are multiple objects carried in
         the session.  Senders MUST NOT set R = 1 when the ERT for the
         object is more than 2^32-1 time units (approximately 49 days),
         where time is measured in units of milliseconds.

         In the example R=0, which indicates that the ERT is not carried
         in this packet.



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     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.

         In the example A=0, and thus this packet does not indicate the
         close of 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.

         In the example B=0, and thus this packet does not indicate the
         end of sending data packets for the object.

     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., the FEC Payload ID if the packet



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         contains a payload, or the end of the packet if the packet
         contains no payload.

         In the example HDR_LEN=5 to indicate that the length of the LCT
         header portion of the overall ALC is 5 32-bit words.

     Codepoint (CP): 8 bits

         This field is used by ALC to carry the mapping that identifies
         settings for portions of the Session Description that can
         change within the session.  The mapping between Codepoint
         values and the settings for portions of the Session Description
         is to be communicated out-of-band.

         In the example the portion of the Session Description that can
         change within the session is the FEC Encoding ID, and the
         identity mapping is used between Codepoint values and FEC
         Encoding IDs.  Thus, CP=128 identifies FEC Encoding ID 128, the
         "Small Block, Large Block and Expandable FEC Codes" as
         described in the FEC building block [10].  The FEC Payload ID
         associated with FEC Encoding ID 128 is 64-bits in length.

     Congestion Control Information (CCI): 32, 64, 96 or 128 bits

         This is field contains the Congestion Control Information as
         defined by the specified multiple rate congestion control
         building block.  The format of this field is determined by the
         multiple rate congestion control building block.

         This field MUST be 32 bits if C=0.
         This field MUST be 64 bits if C=1.
         This field MUST be 96 bits if C=2.
         This field MUST be 128 bits if C=3.

         In the example, the CCI is 32-bits in length.  The format of
         the CCI field for the example MUST correspond to the format for
         the 32-bit version of the CCI specified in the multiple rate
         congestion control building block.

     Transport Session Identifier (TSI): 16, 32 or 48 bits

         The TSI uniquely identifies a session among all sessions from a
         particular sender.  The TSI is scoped by the sender IP address,
         and thus the (sender IP address, TSI) pair uniquely identify
         the session.  For ALC, 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 is outside the scope of this document and is to be
         done out-of-band.

         The length of the TSI field is 32*S + 16*H bits.  Note that the
         aggregate lengths of the TSI field plus the TOI field is a
         multiple of 32 bits.

         In the example the TSI is 32 bits in length.

     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 output of a hash function applied to the
         object.  The mapping of TOI field values to objects is outside
         the scope of this document and is to be done out-of-band.  The
         TOI field MUST be used in all packets if more than one object
         is to be transmitted in a session, i.e., the TOI field is
         either present in all the packets of a session or is never
         present.

         The length of the TOI field is 32*O + 16*H bits.  Note that the
         aggregate lengths of the TSI field plus the TOI field is a
         multiple of 32 bits.

         In the example the TOI is 32 bits in length.







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     Sender Current Time (SCT): 0 or 32 bits

         This field represents the current clock of 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.

         In this example the SCT is present.

     Expected Residual Time (ERT): 0 or 32 bits

         This field represents the sender expected residual transmission
         time of packets for either the single object carried in the
         session or for the object identified by the TOI if there are
         multiple objects carried in the session.

         This field MUST NOT be present if R=0 and MUST be present if
         R=1.

         In this example the ERT is not present.

     FEC Payload ID: X bits

         The length and format of the FEC Payload ID depends on the FEC
         Encoding ID as described in the FEC building block [10].  The
         FEC Payload ID format is determined by the FEC Encoding ID that
         MUST be communicated in the Session Description.  The Session
         Description MAY specify that more than one FEC Encoding ID is
         used in the session, in which case the Session Description MUST
         contain a mapping that identifies which Codepoint values
         correspond to which FEC Encoding IDs.  This mapping, if used,
         is outside the scope of this document.

         The example packet format corresponds to the format for "Small
         Block, Large Block and Expandable FEC Codes" as described in
         the FEC building block, for which the associated FEC Encoding
         ID 128.  For FEC Encoding ID 128, the FEC Payload ID consists
         of the following two fields that in total are X = 64 bits in
         length:

         Source Block Number (SBN): 32 bits

            The Source Block Number identifies from which source block
            of the object the encoding symbol(s) in the payload are
            generated.  These blocks are numbered consecutively from




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            0 to N-1, where N is the number of source blocks in the
            object.

         Encoding Symbol ID (ESI): 32 bits

            The Encoding Symbol ID identifies which specific encoding
            symbol(s) generated from the source block are carried in the
            packet payload.  The exact details of the correspondence
            between Encoding Symbol IDs and the encoding symbol(s) in
            the packet payload are dependent on the particular encoding
            algorithm used as identified by the FEC Encoding ID and by
            the FEC Instance ID.

   Encoding Symbol(s): Y bits

         The encoding symbols are what the receiver uses to reconstruct
         an object.  The total length Y of the encoding symbol(s) in the
         packet can be determined by the receiver of the packet by
         computing the total length of the received packet and
         subtracting off the length of the headers.

4.3  Header-Extension Fields

   Header Extensions can be used to extend the LCT header portion of the
   ALC header to accommodate optional header fields that are not always
   used or have variable size.  Header Extensions are not used in the
   example ALC packet format shown in the previous subsection.  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 ALC without
   changing the ALC version number.  Non backward-compatible Header
   Extensions CANNOT be introduced without changing the ALC version
   number.





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   There are two formats for Header Extension fields, as depicted below.
   The first format is used for variable-length extensions, with Header
   Extension Type (HET) values between 0 and 127.  The second format is
   used for fixed length (one 32-bit word) extensions, using HET values
   from 127 to 255.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  HET (<=127)  |       HEL     |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    .                                                               .
    .              Header Extension Content (HEC)                   .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  HET (>=128)  |       Header Extension Content (HEC)          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3 - Format of additional headers

   The explanation of each sub-field is the following.

     Header Extension Type (HET): 8 bits

         The type of the Header Extension.  This document defines a
         number of possible types.  Additional types may be defined in
         future versions of this specification.  HET values from 0 to
         127 are used for variable-length Header Extensions.  HET values
         from 128 to 255 are used for fixed-length 32-bit Header
         Extensions.

     Header Extension Length (HEL): 8 bits

         The length of the whole Header Extension field, expressed in
         multiples of 32-bit words.  This field MUST be present for
         variable-length extensions (HET between 0 and 127) and MUST NOT
         be present for fixed-length extensions (HET between 128 and
         255).

     Header Extension Content (HEC): variable length

         The content of the Header Extension.  The format of this sub-
         field depends on the Header Extension type.  For fixed-length
         Header Extensions, the HEC is 24 bits.  For variable-length
         Header Extensions, the HEC field has variable size, as



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         specified by the HEL field.  Note that the length of each
         Header Extension field MUST be a multiple of 32 bits.  Also
         note that the total size of the LCT header, including all
         Header Extensions and all optional header fields, cannot exceed
         255 32-bit words.

   Header Extensions are further divided between general LCT extensions
   and Protocol Instantiation specific extensions (PI-specific).
   General LCT extensions have HET in the ranges 0:63 and 128:191
   inclusive.  PI-specific extensions have HET in the ranges 64:127 and
   192:255 inclusive.

   General LCT extensions are intended to allow the introduction of
   backward-compatible enhancements to LCT without changing the LCT
   version number.  Non backward-compatible Header Extensions CANNOT be
   introduced without changing the LCT version number.

   PI-specific extensions are reserved for PI-specific use with semantic
   and default parsing actions defined by the PI.

   The following general LCT Header Extension types are defined:

   EXT_NOP=0     No-Operation extension.
                 The information present in this extension field MUST be
                 ignored by receivers.

   EXT_AUTH=1    Packet authentication extension
                 Information used to authenticate the sender of the
                 packet.  The format of this Header Extension and its
                 processing is outside the scope of this document and is
                 to be communicated out-of-band as part of the Session
                 Description.

                 It is RECOMMENDED that senders provide some form of
                 packet authentication.  If EXT_AUTH is present,
                 whatever packet authentication checks that can be
                 performed immediately upon reception of the packet
                 SHOULD be performed before accepting the packet and
                 performing any congestion control-related action on it.
                 Some packet authentication schemes impose a delay of
                 several seconds between when a packet is received and
                 when the packet is fully authenticated.  Any congestion
                 control related action that is appropriate MUST NOT be
                 postponed by any such full packet authentication.

   All senders and receivers implementing ALC 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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   For this version of ALC, the following PI-specific extension is
   defined:

   EXT_FTI=64    FEC Object Transmission Information extension
                 The purpose of this extension is to carry in-band the
                 FEC Object Transmission Information for an object.  The
                 format of this Header Extension and its processing is
                 outside the scope of this document and is to be
                 communicated out-of-band as part of the Session
                 Description.

4.4  Sender Operation

   The sender operation when using ALC includes all the points made
   about the sender operation when using the LCT building block [11],
   the FEC building block [10] 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.

   Objects MAY be transmitted sequentially within a session, and they
   MAY be transmitted concurrently.  However, 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.  However, there are no rules
   with respect to mixing packets for different objects carried within
   the session.  Although this issue affects the efficiency of the



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   protocol, it does not affect the correctness nor the inter-
   operability of ALC between senders and receivers.

   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.

   It is RECOMMENDED that packet authentication be used.  If packet
   authentication is used then the Header Extensions described in
   Section 4.3 MUST be used to carry the authentication.

   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 the multiple rate congestion
   control building block.

4.5  Receiver Operation

   The receiver operation when using ALC includes all the points made
   about the receiver operation when using the LCT building block [11],
   the FEC building block [10] 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
   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.

   To be able to participate in a session, a receiver MUST implement the
   multiple rate congestion control building block using the Congestion
   Control Information field provided in the LCT header.  If a receiver
   is not able to implement the multiple rate congestion control
   building block it MUST NOT join the session.





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   Several objects can be carried either sequentially or concurrently
   within the same session.  In this case, each object is identified by
   a unique TOI.  Note that even if a sender 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 channels,
   and thus receivers SHOULD NOT assume that the reception of a packet
   for a new object means that there are no more packets in transit for
   the previous one, at least for some amount of time.

   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.

   A receiver MAY concurrently join multiple ALC sessions from one or
   more senders.  The receiver MUST perform congestion control on each
   such session.  The receiver MAY make choices to optimize the packet
   flow performance across multiple sessions, as long as the receiver
   still adheres to the multiple rate congestion control building block
   for each session individually.

   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.



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   (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
   reception rate using the multiple rate congestion control.

   Some packet authentication schemes such as TESLA [14] 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.

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



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   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 [14] 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
   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 [14] 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.

   A receiver with an incorrect or corrupted implementation of the
   multiple rate congestion control building block may affect health of
   the network in the path between the sender and the receiver, and may
   also affect the reception rates of other receivers joined to the
   session.  It is therefore RECOMMENDED that receivers be required to
   identify themselves as legitimate before they receive the Session
   Description needed to join the session.  How receivers identify
   themselves as legitimate is outside the scope of this document.





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   Another vulnerability of ALC is the potential of receivers obtaining
   an incorrect Session Description for the session.  The consequences
   of this could be that legitimate receivers with the wrong Session
   Description are unable to correctly receive the session content, or
   that receivers inadvertently try to receive at a much higher rate
   than they are capable of, thereby disrupting traffic in portions of
   the network.  To avoid these problems, it is RECOMMENDED that
   measures be taken to prevent receivers from accepting incorrect
   Session Descriptions, e.g., by using source authentication to ensure
   that receivers only accept legitimate Session Descriptions from
   authorized senders.  How this is done is outside the scope of this
   document.

6.  IANA Considerations

   No information in this specification is directly subject to IANA
   registration.  However, building blocks components used by ALC may
   introduce additional IANA considerations.  In particular, the FEC
   building block used by ALC does require IANA registration of the FEC
   codecs used.

7.  Intellectual Property Issues

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed
   rights.

8.  Acknowledgments

   Thanks to Vincent Roca, Justin Chapweske and Roger Kermode for their
   detailed comments on this document.

9.  References

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

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

   [3]  Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
        1112, August 1989.

   [4]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
        Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
        2616, January 1997.




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

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

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

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

   [9]  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.

   [10] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, M., and
        J.  Crowcroft, "Forward Error Correction (FEC) Building Block",
        RFC 3452, December 2002.

   [11] Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., Handley, M. and
        J.  Crowcroft, "Layered Coding Transport (LCT) Building Block",
        RFC 3451 December 2002.

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

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

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

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

   [16] 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.







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Authors' Addresses

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

   EMail: luby@digitalfountain.com


   Jim Gemmell
   Microsoft Research
   455 Market St. #1690
   San Francisco, CA, 94105

   EMail: jgemmell@microsoft.com


   Lorenzo Vicisano
   cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA, USA, 95134

   EMail: lorenzo@cisco.com


   Luigi Rizzo
   Dip. Ing. dell'Informazione,
   Univ. di Pisa
   via Diotisalvi 2, 56126 Pisa, Italy

   EMail: luigi@iet.unipi.it


   Jon Crowcroft
   Marconi Professor of Communications Systems
   University of Cambridge
   Computer Laboratory
   William Gates Building
   J J Thomson Avenue
   Cambridge CB3 0FD, UK

   EMail: Jon.Crowcroft@cl.cam.ac.uk







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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  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
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   The limited permissions granted above are perpetual and will not be
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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