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rfc:rfc3208

Network Working Group T. Speakman Request for Comments: 3208 Cisco Systems Category: Experimental J. Crowcroft

                                                                   UCL
                                                            J. Gemmell
                                                             Microsoft
                                                          D. Farinacci
                                                      Procket Networks
                                                                S. Lin
                                                      Juniper Networks
                                                         D. Leshchiner
                                                        TIBCO Software
                                                               M. Luby
                                                      Digital Fountain
                                                         T. Montgomery
                                                  Talarian Corporation
                                                              L. Rizzo
                                                    University of Pisa
                                                            A. Tweedly
                                                            N. Bhaskar
                                                         R. Edmonstone
                                                       R. Sumanasekera
                                                           L. Vicisano
                                                         Cisco Systems
                                                         December 2001
           PGM Reliable Transport Protocol Specification

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 (2001).  All Rights Reserved.

Abstract

 Pragmatic General Multicast (PGM) is a reliable multicast transport
 protocol for applications that require ordered or unordered,
 duplicate-free, multicast data delivery from multiple sources to
 multiple receivers.  PGM guarantees that a receiver in the group
 either receives all data packets from transmissions and repairs, or
 is able to detect unrecoverable data packet loss.  PGM is

Speakman, et. al. Experimental [Page 1] RFC 3208 PGM Reliable Transport Protocol December 2001

 specifically intended as a workable solution for multicast
 applications with basic reliability requirements.  Its central design
 goal is simplicity of operation with due regard for scalability and
 network efficiency.

Table of Contents

 1.  Introduction and Overview ..................................    3
 2.  Architectural Description ..................................    9
 3.  Terms and Concepts .........................................   12
 4.  Procedures - General .......................................   18
 5.  Procedures - Sources .......................................   19
 6.  Procedures - Receivers .....................................   22
 7.  Procedures - Network Elements ..............................   27
 8.  Packet Formats .............................................   31
 9.  Options ....................................................   40
 10. Security Considerations ....................................   56
 11. Appendix A - Forward Error Correction ......................   58
 12. Appendix B - Support for Congestion Control ................   72
 13. Appendix C - SPM Requests ..................................   79
 14. Appendix D - Poll Mechanism ................................   82
 15. Appendix E - Implosion Prevention ..........................   92
 16. Appendix F - Transmit Window Example .......................   98
 17  Appendix G - Applicability Statement .......................  103
 18. Abbreviations ..............................................  105
 19. Acknowledgments ............................................  106
 20. References .................................................  106
 21. Authors' Addresses..........................................  108
 22. Full Copyright Statement ...................................  111

Nota Bene:

 The publication of this specification is intended to freeze the
 definition of PGM in the interest of fostering both ongoing and
 prospective experimentation with the protocol.  The intent of that
 experimentation is to provide experience with the implementation and
 deployment of a reliable multicast protocol of this class so as to be
 able to feed that experience back into the longer-term
 standardization process underway in the Reliable Multicast Transport
 Working Group of the IETF.  Appendix G provides more specific detail
 on the scope and status of some of this experimentation.  Reports of
 experiments include [16-23].  Additional results and new
 experimentation are encouraged.

Speakman, et. al. Experimental [Page 2] RFC 3208 PGM Reliable Transport Protocol December 2001

1. Introduction and Overview

 A variety of reliable protocols have been proposed for multicast data
 delivery, each with an emphasis on particular types of applications,
 network characteristics, or definitions of reliability ([1], [2],
 [3], [4]).  In this tradition, Pragmatic General Multicast (PGM) is a
 reliable transport protocol for applications that require ordered or
 unordered, duplicate-free, multicast data delivery from multiple
 sources to multiple receivers.
 PGM is specifically intended as a workable solution for multicast
 applications with basic reliability requirements rather than as a
 comprehensive solution for multicast applications with sophisticated
 ordering, agreement, and robustness requirements.  Its central design
 goal is simplicity of operation with due regard for scalability and
 network efficiency.
 PGM has no notion of group membership.  It simply provides reliable
 multicast data delivery within a transmit window advanced by a source
 according to a purely local strategy.  Reliable delivery is provided
 within a source's transmit window from the time a receiver joins the
 group until it departs.  PGM guarantees that a receiver in the group
 either receives all data packets from transmissions and repairs, or
 is able to detect unrecoverable data packet loss.  PGM supports any
 number of sources within a multicast group, each fully identified by
 a globally unique Transport Session Identifier (TSI), but since these
 sources/sessions operate entirely independently of each other, this
 specification is phrased in terms of a single source and extends
 without modification to multiple sources.
 More specifically, PGM is not intended for use with applications that
 depend either upon acknowledged delivery to a known group of
 recipients, or upon total ordering amongst multiple sources.
 Rather, PGM is best suited to those applications in which members may
 join and leave at any time, and that are either insensitive to
 unrecoverable data packet loss or are prepared to resort to
 application recovery in the event.  Through its optional extensions,
 PGM provides specific mechanisms to support applications as disparate
 as stock and news updates, data conferencing, low-delay real-time
 video transfer, and bulk data transfer.
 In the following text, transport-layer originators of PGM data
 packets are referred to as sources, transport-layer consumers of PGM
 data packets are referred to as receivers, and network-layer entities
 in the intervening network are referred to as network elements.

Speakman, et. al. Experimental [Page 3] RFC 3208 PGM Reliable Transport Protocol December 2001

 Unless otherwise specified, the term "repair" will be used to
 indicate both the actual retransmission of a copy of a missing packet
 or the transmission of an FEC repair packet.

Terminology

 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 RFC 2119 [14] and
 indicate requirement levels for compliant PGM implementations.

1.1. Summary of Operation

 PGM runs over a datagram multicast protocol such as IP multicast [5].
 In the normal course of data transfer, a source multicasts sequenced
 data packets (ODATA), and receivers unicast selective negative
 acknowledgments (NAKs) for data packets detected to be missing from
 the expected sequence.  Network elements forward NAKs PGM-hop-by-
 PGM-hop to the source, and confirm each hop by multicasting a NAK
 confirmation (NCF) in response on the interface on which the NAK was
 received.  Repairs (RDATA) may be provided either by the source
 itself or by a Designated Local Repairer (DLR) in response to a NAK.
 Since NAKs provide the sole mechanism for reliability, PGM is
 particularly sensitive to their loss.  To minimize NAK loss, PGM
 defines a network-layer hop-by-hop procedure for reliable NAK
 forwarding.
 Upon detection of a missing data packet, a receiver repeatedly
 unicasts a NAK to the last-hop PGM network element on the
 distribution tree from the source.  A receiver repeats this NAK until
 it receives a NAK confirmation (NCF) multicast to the group from that
 PGM network element.  That network element responds with an NCF to
 the first occurrence of the NAK and any further retransmissions of
 that same NAK from any receiver.  In turn, the network element
 repeatedly forwards the NAK to the upstream PGM network element on
 the reverse of the distribution path from the source of the original
 data packet until it also receives an NCF from that network element.
 Finally, the source itself receives and confirms the NAK by
 multicasting an NCF to the group.
 While NCFs are multicast to the group, they are not propagated by PGM
 network elements since they act as hop-by-hop confirmations.

Speakman, et. al. Experimental [Page 4] RFC 3208 PGM Reliable Transport Protocol December 2001

 To avoid NAK implosion, PGM specifies procedures for subnet-based NAK
 suppression amongst receivers and NAK elimination within network
 elements.  The usual result is the propagation of just one copy of a
 given NAK along the reverse of the distribution path from any network
 with directly connected receivers to a source.
 The net effect is that unicast NAKs return from a receiver to a
 source on the reverse of the path on which ODATA was forwarded, that
 is, on the reverse of the distribution tree from the source.  More
 specifically, they return through exactly the same sequence of PGM
 network elements through which ODATA was forwarded, but in reverse.
 The reasons for handling NAKs this way will become clear in the
 discussion of constraining repairs, but first it's necessary to
 describe the mechanisms for establishing the requisite source path
 state in PGM network elements.
 To establish source path state in PGM network elements, the basic
 data transfer operation is augmented by Source Path Messages (SPMs)
 from a source, periodically interleaved with ODATA.  SPMs function
 primarily to establish source path state for a given TSI in all PGM
 network elements on the distribution tree from the source.  PGM
 network elements use this information to address returning unicast
 NAKs directly to the upstream PGM network element toward the source,
 and thereby insure that NAKs return from a receiver to a source on
 the reverse of the distribution path for the TSI.
 SPMs are sent by a source at a rate that serves to maintain up-to-
 date PGM neighbor information.  In addition, SPMs complement the role
 of DATA packets in provoking further NAKs from receivers, and
 maintaining receive window state in the receivers.
 As a further efficiency, PGM specifies procedures for the constraint
 of repairs by network elements so that they reach only those network
 segments containing group members that did not receive the original
 transmission.  As NAKs traverse the reverse of the ODATA path
 (upward), they establish repair state in the network elements which
 is used in turn to constrain the (downward) forwarding of the
 corresponding RDATA.
 Besides procedures for the source to provide repairs, PGM also
 specifies options and procedures that permit designated local
 repairers (DLRs) to announce their availability and to redirect
 repair requests (NAKs) to themselves rather than to the original
 source.  In addition to these conventional procedures for loss
 recovery through selective ARQ, Appendix A specifies Forward Error
 Correction (FEC) procedures for sources to provide and receivers to
 request general error correcting parity packets rather than selective
 retransmissions.

Speakman, et. al. Experimental [Page 5] RFC 3208 PGM Reliable Transport Protocol December 2001

 Finally, since PGM operates without regular return traffic from
 receivers, conventional feedback mechanisms for transport flow and
 congestion control cannot be applied.  Appendix B specifies a TCP-
 friendly, NE-based solution for PGM congestion control, and cites a
 reference to a TCP-friendly, end-to-end solution for PGM congestion
 control.
 In its basic operation, PGM relies on a purely rate-limited
 transmission strategy in the source to bound the bandwidth consumed
 by PGM transport sessions and to define the transmit window
 maintained by the source.
 PGM defines four basic packet types:  three that flow downstream
 (SPMs, DATA, NCFs), and one that flows upstream (NAKs).

1.2. Design Goals and Constraints

 PGM has been designed to serve that broad range of multicast
 applications that have relatively simple reliability requirements,
 and to do so in a way that realizes the much advertised but often
 unrealized network efficiencies of multicast data transfer.  The
 usual impediments to realizing these efficiencies are the implosion
 of negative and positive acknowledgments from receivers to sources,
 repair latency from the source, and the propagation of repairs to
 disinterested receivers.

1.2.1. Reliability.

 Reliable data delivery across an unreliable network is conventionally
 achieved through an end-to-end protocol in which a source (implicitly
 or explicitly) solicits receipt confirmation from a receiver, and the
 receiver responds positively or negatively.  While the frequency of
 negative acknowledgments is a function of the reliability of the
 network and the receiver's resources (and so, potentially quite low),
 the frequency of positive acknowledgments is fixed at at least the
 rate at which the transmit window is advanced, and usually more
 often.
 Negative acknowledgments primarily determine repairs and reliability.
 Positive acknowledgments primarily determine transmit buffer
 management.
 When these principles are extended without modification to multicast
 protocols, the result, at least for positive acknowledgments, is a
 burden of positive acknowledgments transmitted to the source that
 quickly threatens to overwhelm it as the number of receivers grows.
 More succinctly, ACK implosion keeps ACK-based reliable multicast
 protocols from scaling well.

Speakman, et. al. Experimental [Page 6] RFC 3208 PGM Reliable Transport Protocol December 2001

 One of the goals of PGM is to get as strong a definition of
 reliability as possible from as simple a protocol as possible.  ACK
 implosion can be addressed in a variety of effective but complicated
 ways, most of which require re-transmit capability from other than
 the original source.
 An alternative is to dispense with positive acknowledgments
 altogether, and to resort to other strategies for buffer management
 while retaining negative acknowledgments for repairs and reliability.
 The approach taken in PGM is to retain negative acknowledgments, but
 to dispense with positive acknowledgments and resort instead to
 timeouts at the source to manage transmit resources.
 The definition of reliability with PGM is a direct consequence of
 this design decision.  PGM guarantees that a receiver either receives
 all data packets from transmissions and repairs, or is able to detect
 unrecoverable data packet loss.
 PGM includes strategies for repeatedly provoking NAKs from receivers,
 and for adding reliability to the NAKs themselves.  By reinforcing
 the NAK mechanism, PGM minimizes the probability that a receiver will
 detect a missing data packet so late that the packet is unavailable
 for repair either from the source or from a designated local repairer
 (DLR).  Without ACKs and knowledge of group membership, however, PGM
 cannot eliminate this possibility.

1.2.2. Group Membership

 A second consequence of eliminating ACKs is that knowledge of group
 membership is neither required nor provided by the protocol.
 Although a source may receive some PGM packets (NAKs for instance)
 from some receivers, the identity of the receivers does not figure in
 the processing of those packets.  Group membership MAY change during
 the course of a PGM transport session without the knowledge of or
 consequence to the source or the remaining receivers.

1.2.3. Efficiency

 While PGM avoids the implosion of positive acknowledgments simply by
 dispensing with ACKs, the implosion of negative acknowledgments is
 addressed directly.
 Receivers observe a random back-off prior to generating a NAK during
 which interval the NAK is suppressed (i.e. it is not sent, but the
 receiver acts as if it had sent it) by the receiver upon receipt of a
 matching NCF.  In addition, PGM network elements eliminate duplicate
 NAKs received on different interfaces on the same network element.

Speakman, et. al. Experimental [Page 7] RFC 3208 PGM Reliable Transport Protocol December 2001

 The combination of these two strategies usually results in the source
 receiving just a single NAK for any given lost data packet.
 Whether a repair is provided from a DLR or the original source, it is
 important to constrain that repair to only those network segments
 containing members that negatively acknowledged the original
 transmission rather than propagating it throughout the group.  PGM
 specifies procedures for network elements to use the pattern of NAKs
 to define a sub-tree within the group upon which to forward the
 corresponding repair so that it reaches only those receivers that
 missed it in the first place.

1.2.4. Simplicity

 PGM is designed to achieve the greatest improvement in reliability
 (as compared to the usual UDP) with the least complexity.  As a
 result, PGM does NOT address conference control, global ordering
 amongst multiple sources in the group, nor recovery from network
 partitions.

1.2.5. Operability

 PGM is designed to function, albeit with less efficiency, even when
 some or all of the network elements in the multicast tree have no
 knowledge of PGM.  To that end, all PGM data packets can be
 conventionally multicast routed by non-PGM network elements with no
 loss of functionality, but with some inefficiency in the propagation
 of RDATA and NCFs.
 In addition, since NAKs are unicast to the last-hop PGM network
 element and NCFs are multicast to the group, NAK/NCF operation is
 also consistent across non-PGM network elements.  Note that for NAK
 suppression to be most effective, receivers should always have a PGM
 network element as a first hop network element between themselves and
 every path to every PGM source.  If receivers are several hops
 removed from the first PGM network element, the efficacy of NAK
 suppression may degrade.

1.3. Options

 In addition to the basic data transfer operation described above, PGM
 specifies several end-to-end options to address specific application
 requirements.  PGM specifies options to support fragmentation, late
 joining, redirection, Forward Error Correction (FEC), reachability,
 and session synchronization/termination/reset.  Options MAY be
 appended to PGM data packet headers only by their original
 transmitters.  While they MAY be interpreted by network elements,
 options are neither added nor removed by network elements.

Speakman, et. al. Experimental [Page 8] RFC 3208 PGM Reliable Transport Protocol December 2001

 All options are receiver-significant (i.e., they must be interpreted
 by receivers).  Some options are also network-significant (i.e., they
 must be interpreted by network elements).
 Fragmentation MAY be used in conjunction with data packets to allow a
 transport-layer entity at the source to break up application-layer
 data packets into multiple PGM data packets to conform with the
 maximum transmission unit (MTU) supported by the network layer.
 Late joining allows a source to indicate whether or not receivers may
 request all available repairs when they initially join a particular
 transport session.
 Redirection MAY be used in conjunction with Poll Responses to allow a
 DLR to respond to normal NCFs or POLLs with a redirecting POLR
 advertising its own address as an alternative re-transmitter to the
 original source.
 FEC techniques MAY be applied by receivers to use source-provided
 parity packets rather than selective retransmissions to effect loss
 recovery.

2. Architectural Description

 As an end-to-end transport protocol, PGM specifies packet formats and
 procedures for sources to transmit and for receivers to receive data.
 To enhance the efficiency of this data transfer, PGM also specifies
 packet formats and procedures for network elements to improve the
 reliability of NAKs and to constrain the propagation of repairs.  The
 division of these functions is described in this section and expanded
 in detail in the next section.

2.1. Source Functions

    Data Transmission
       Sources multicast ODATA packets to the group within the
       transmit window at a given transmit rate.
    Source Path State
       Sources multicast SPMs to the group, interleaved with ODATA if
       present, to establish source path state in PGM network
       elements.

Speakman, et. al. Experimental [Page 9] RFC 3208 PGM Reliable Transport Protocol December 2001

    NAK Reliability
       Sources multicast NCFs to the group in response to any NAKs
       they receive.
    Repairs
       Sources multicast RDATA packets to the group in response to
       NAKs received for data packets within the transmit window.
    Transmit Window Advance
       Sources MAY advance the trailing edge of the window according
       to one of a number of strategies.  Implementations MAY support
       automatic adjustments such as keeping the window at a fixed
       size in bytes, a fixed number of packets or a fixed real time
       duration.  In addition, they MAY optionally delay window
       advancement based on NAK-silence for a certain period.  Some
       possible strategies are outlined later in this document.

2.2. Receiver Functions

    Source Path State
       Receivers use SPMs to determine the last-hop PGM network
       element for a given TSI to which to direct their NAKs.
    Data Reception
       Receivers receive ODATA within the transmit window and
       eliminate any duplicates.
    Repair Requests
       Receivers unicast NAKs to the last-hop PGM network element (and
       MAY optionally multicast a NAK with TTL of 1 to the local
       group) for data packets within the receive window detected to
       be missing from the expected sequence.  A receiver MUST
       repeatedly transmit a given NAK until it receives a matching
       NCF.
    NAK Suppression
       Receivers suppress NAKs for which a matching NCF or NAK is
       received during the NAK transmit back-off interval.

Speakman, et. al. Experimental [Page 10] RFC 3208 PGM Reliable Transport Protocol December 2001

    Receive Window Advance
       Receivers immediately advance their receive windows upon
       receipt of any PGM data packet or SPM within the transmit
       window that advances the receive window.

2.3. Network Element Functions

    Network elements forward ODATA without intervention.
    Source Path State
       Network elements intercept SPMs and use them to establish
       source path state for the corresponding TSI before multicast
       forwarding them in the usual way.
    NAK Reliability
       Network elements multicast NCFs to the group in response to any
       NAK they receive.  For each NAK received, network elements
       create repair state recording the transport session identifier,
       the sequence number of the NAK, and the input interface on
       which the NAK was received.
    Constrained NAK Forwarding
       Network elements repeatedly unicast forward only the first copy
       of any NAK they receive to the upstream PGM network element on
       the distribution path for the TSI until they receive an NCF in
       response.  In addition, they MAY optionally multicast this NAK
       upstream with TTL of 1.
    Nota Bene: Once confirmed by an NCF, network elements discard NAK
    packets; NAKs are NOT retained in network elements beyond this
    forwarding operation, but state about the reception of them is
    stored.
    NAK Elimination
       Network elements discard exact duplicates of any NAK for which
       they already have repair state (i.e., that has been forwarded
       either by themselves or a neighboring PGM network element), and
       respond with a matching NCF.

Speakman, et. al. Experimental [Page 11] RFC 3208 PGM Reliable Transport Protocol December 2001

    Constrained RDATA Forwarding
       Network elements use NAKs to maintain repair state consisting
       of a list of interfaces upon which a given NAK was received,
       and they forward the corresponding RDATA only on these
       interfaces.
    NAK Anticipation
       If a network element hears an upstream NCF (i.e., on the
       upstream interface for the distribution tree for the TSI), it
       establishes repair state without outgoing interfaces in
       anticipation of responding to and eliminating duplicates of the
       NAK that may arrive from downstream.

3. Terms and Concepts

 Before proceeding from the preceding overview to the detail in the
 subsequent Procedures, this section presents some concepts and
 definitions that make that detail more intelligible.

3.1. Transport Session Identifiers

 Every PGM packet is identified by a:
 TSI            transport session identifier
 TSIs MUST be globally unique, and only one source at a time may act
 as the source for a transport session.  (Note that repairers do not
 change the TSI in any RDATA they transmit).  TSIs are composed of the
 concatenation of a globally unique source identifier (GSI) and a
 source-assigned data-source port.
 Since all PGM packets originated by receivers are in response to PGM
 packets originated by a source, receivers simply echo the TSI heard
 from the source in any corresponding packets they originate.
 Since all PGM packets originated by network elements are in response
 to PGM packets originated by a receiver, network elements simply echo
 the TSI heard from the receiver in any corresponding packets they
 originate.

3.2. Sequence Numbers

 PGM uses a circular sequence number space from 0 through ((2**32) -
 1) to identify and order ODATA packets.  Sources MUST number ODATA
 packets in unit increments in the order in which the corresponding
 application data is submitted for transmission.  Within a transmit or

Speakman, et. al. Experimental [Page 12] RFC 3208 PGM Reliable Transport Protocol December 2001

 receive window (defined below), a sequence number x is "less" or
 "older" than sequence number y if it numbers an ODATA packet
 preceding ODATA packet y, and a sequence number y is "greater" or
 "more recent" than sequence number x if it numbers an ODATA packet
 subsequent to ODATA packet x.

3.3. Transmit Window

 The description of the operation of PGM rests fundamentally on the
 definition of the source-maintained transmit window.  This definition
 in turn is derived directly from the amount of transmitted data (in
 seconds) a source retains for repair (TXW_SECS), and the maximum
 transmit rate (in bytes/second) maintained by a source to regulate
 its bandwidth utilization (TXW_MAX_RTE).
 In terms of sequence numbers, the transmit window is the range of
 sequence numbers consumed by the source for sequentially numbering
 and transmitting the most recent TXW_SECS of ODATA packets.  The
 trailing (or left) edge of the transmit window (TXW_TRAIL) is defined
 as the sequence number of the oldest data packet available for repair
 from a source.  The leading (or right) edge of the transmit window
 (TXW_LEAD) is defined as the sequence number of the most recent data
 packet a source has transmitted.
 The size of the transmit window in sequence numbers (TXW_SQNS) (i.e.,
 the difference between the leading and trailing edges plus one) MUST
 be no greater than half the PGM sequence number space less one.
 When TXW_TRAIL is equal to TXW_LEAD, the transmit window size is one.
 When TXW_TRAIL is equal to TXW_LEAD plus one, the transmit window
 size is empty.

3.4. Receive Window

 The receive window at the receivers is determined entirely by PGM
 packets from the source.  That is, a receiver simply obeys what the
 source tells it in terms of window state and advancement.
 For a given transport session identified by a TSI, a receiver
 maintains:
 RXW_TRAIL      the sequence number defining the trailing edge of the
                receive window, the sequence number (known from data
                packets and SPMs) of the oldest data packet available
                for repair from the source

Speakman, et. al. Experimental [Page 13] RFC 3208 PGM Reliable Transport Protocol December 2001

 RXW_LEAD       the sequence number defining the leading edge of the
                receive window, the greatest sequence number of any
                received data packet within the transmit window
 The receive window is the range of sequence numbers a receiver is
 expected to use to identify receivable ODATA.
 A data packet is described as being "in" the receive window if its
 sequence number is in the receive window.
 The receive window is advanced by the receiver when it receives an
 SPM or ODATA packet within the transmit window that increments
 RXW_TRAIL.  Receivers also advance their receive windows upon receipt
 of any PGM data packet within the receive window that advances the
 receive window.

3.5. Source Path State

 To establish the repair state required to constrain RDATA, it's
 essential that NAKs return from a receiver to a source on the reverse
 of the distribution tree from the source.  That is, they must return
 through the same sequence of PGM network elements through which the
 ODATA was forwarded, but in reverse.  There are two reasons for this,
 the less obvious one being by far the more important.
 The first and obvious reason is that RDATA is forwarded on the same
 path as ODATA and so repair state must be established on this path if
 it is to constrain the propagation of RDATA.
 The second and less obvious reason is that in the absence of repair
 state, PGM network elements do NOT forward RDATA, so the default
 behavior is to discard repairs.  If repair state is not properly
 established for interfaces on which ODATA went missing, then
 receivers on those interfaces will continue to NAK for lost data and
 ultimately experience unrecoverable data loss.
 The principle function of SPMs is to provide the source path state
 required for PGM network elements to forward NAKs from one PGM
 network element to the next on the reverse of the distribution tree
 for the TSI, establishing repair state each step of the way.  This
 source path state is simply the address of the upstream PGM network
 element on the reverse of the distribution tree for the TSI.  That
 upstream PGM network element may be more than one subnet hop away.
 SPMs establish the identity of the upstream PGM network element on
 the distribution tree for each TSI in each group in each PGM network
 element, a sort of virtual PGM topology.  So although NAKs are
 unicast addressed, they are NOT unicast routed by PGM network
 elements in the conventional sense.  Instead PGM network elements use

Speakman, et. al. Experimental [Page 14] RFC 3208 PGM Reliable Transport Protocol December 2001

 the source path state established by SPMs to direct NAKs PGM-hop-by-
 PGM-hop toward the source.  The idea is to constrain NAKs to the pure
 PGM topology spanning the more heterogeneous underlying topology of
 both PGM and non-PGM network elements.
 The result is repair state in every PGM network element between the
 receiver and the source so that the corresponding RDATA is never
 discarded by a PGM network element for lack of repair state.
 SPMs also maintain transmit window state in receivers by advertising
 the trailing and leading edges of the transmit window (SPM_TRAIL and
 SPM_LEAD).  In the absence of data, SPMs MAY be used to close the
 transmit window in time by advancing the transmit window until
 SPM_TRAIL is equal to SPM_LEAD plus one.

3.6. Packet Contents

 This section just provides enough short-hand to make the Procedures
 intelligible.  For the full details of packet contents, please refer
 to Packet Formats below.

3.6.1. Source Path Messages

3.6.1.1. SPMs

 SPMs are transmitted by sources to establish source-path state in PGM
 network elements, and to provide transmit-window state in receivers.
 SPMs are multicast to the group and contain:
 SPM_TSI        the source-assigned TSI for the session to which the
                SPM corresponds
 SPM_SQN        a sequence number assigned sequentially by the source
                in unit increments and scoped by SPM_TSI
    Nota Bene: this is an entirely separate sequence than is used to
    number ODATA and RDATA.
 SPM_TRAIL      the sequence number defining the trailing edge of the
                source's transmit window (TXW_TRAIL)
 SPM_LEAD       the sequence number defining the leading edge of the
                source's transmit window (TXW_LEAD)
 SPM_PATH       the network-layer address (NLA) of the interface on
                the PGM network element on which the SPM is forwarded

Speakman, et. al. Experimental [Page 15] RFC 3208 PGM Reliable Transport Protocol December 2001

3.6.2. Data Packets

3.6.2.1. ODATA - Original Data

 ODATA packets are transmitted by sources to send application data to
 receivers.
 ODATA packets are multicast to the group and contain:
 OD_TSI         the globally unique source-assigned TSI
 OD_TRAIL       the sequence number defining the trailing edge of the
                source's transmit window (TXW_TRAIL)
                OD_TRAIL makes the protocol more robust in the face of
                lost SPMs.  By including the trailing edge of the
                transmit window on every data packet, receivers that
                have missed any SPMs that advanced the transmit window
                can still detect the case, recover the application,
                and potentially re-synchronize to the transport
                session.
 OD_SQN         a sequence number assigned sequentially by the source
                in unit increments and scoped by OD_TSI

3.6.2.2. RDATA - Repair Data

 RDATA packets are repair packets transmitted by sources or DLRs in
 response to NAKs.
 RDATA packets are multicast to the group and contain:
 RD_TSI         OD_TSI of the ODATA packet for which this is a repair
 RD_TRAIL       the sequence number defining the trailing edge of the
                source's transmit window (TXW_TRAIL).  This is updated
                to the most current value when the repair is sent, so
                it is not necessarily the same as OD_TRAIL of the
                ODATA packet for which this is a repair
 RD_SQN         OD_SQN of the ODATA packet for which this is a repair

3.6.3. Negative Acknowledgments

3.6.3.1. NAKs - Negative Acknowledgments

 NAKs are transmitted by receivers to request repairs for missing data
 packets.

Speakman, et. al. Experimental [Page 16] RFC 3208 PGM Reliable Transport Protocol December 2001

 NAKs are unicast (PGM-hop-by-PGM-hop) to the source and contain:
 NAK_TSI        OD_TSI of the ODATA packet for which a repair is
                requested
 NAK_SQN        OD_SQN of the ODATA packet for which a repair is
                requested
 NAK_SRC        the unicast NLA of the original source of the missing
                ODATA.
 NAK_GRP        the multicast group NLA

3.6.3.2. NNAKs - Null Negative Acknowledgments

 NNAKs are transmitted by a DLR that receives NAKs redirected to it by
 either receivers or network elements to provide flow-control feed-
 back to a source.
 NNAKs are unicast (PGM-hop-by-PGM-hop) to the source and contain:
 NNAK_TSI       NAK_TSI of the corresponding re-directed NAK.
 NNAK_SQN       NAK_SQN of the corresponding re-directed NAK.
 NNAK_SRC       NAK_SRC of the corresponding re-directed NAK.
 NNAK_GRP       NAK_GRP of the corresponding re-directed NAK.

3.6.4. Negative Acknowledgment Confirmations

3.6.4.1. NCFs - NAK confirmations

 NCFs are transmitted by network elements and sources in response to
 NAKs.
 NCFs are multicast to the group and contain:
 NCF_TSI        NAK_TSI of the NAK being confirmed
 NCF_SQN        NAK_SQN of the NAK being confirmed
 NCF_SRC        NAK_SRC of the NAK being confirmed
 NCF_GRP        NAK_GRP of the NAK being confirmed

Speakman, et. al. Experimental [Page 17] RFC 3208 PGM Reliable Transport Protocol December 2001

3.6.5. Option Encodings

 OPT_LENGTH      0x00 - Option's Length
 OPT_FRAGMENT    0x01 - Fragmentation
 OPT_NAK_LIST    0x02 - List of NAK entries
 OPT_JOIN        0x03 - Late Joining
 OPT_REDIRECT    0x07 - Redirect
 OPT_SYN         0x0D - Synchronization
 OPT_FIN         0x0E - Session Fin   receivers, conventional
                        feedbackish
 OPT_RST         0x0F - Session Reset
 OPT_PARITY_PRM  0x08 - Forward Error Correction Parameters
 OPT_PARITY_GRP  0x09 - Forward Error Correction Group Number
 OPT_CURR_TGSIZE 0x0A - Forward Error Correction Group Size
 OPT_CR          0x10 - Congestion Report
 OPT_CRQST       0x11 - Congestion Report Request
 OPT_NAK_BO_IVL  0x04 - NAK Back-Off Interval
 OPT_NAK_BO_RNG  0x05 - NAK Back-Off Range
 OPT_NBR_UNREACH 0x0B - Neighbor Unreachable
 OPT_PATH_NLA    0x0C - Path NLA
 OPT_INVALID     0x7F - Option invalidated

4. Procedures - General

 Since SPMs, NCFs, and RDATA must be treated conditionally by PGM
 network elements, they must be distinguished from other packets in
 the chosen multicast network protocol if PGM network elements are to
 extract them from the usual switching path.

Speakman, et. al. Experimental [Page 18] RFC 3208 PGM Reliable Transport Protocol December 2001

 The most obvious way for network elements to achieve this is to
 examine every packet in the network for the PGM transport protocol
 and packet types.  However, the overhead of this approach is costly
 for high-performance, multi-protocol network elements.  An
 alternative, and a requirement for PGM over IP multicast, is that
 SPMs, NCFs, and RDATA MUST be transmitted with the IP Router Alert
 Option [6].  This option gives network elements a network-layer
 indication that a packet should be extracted from IP switching for
 more detailed processing.

5. Procedures - Sources

5.1. Data Transmission

 Since PGM relies on a purely rate-limited transmission strategy in
 the source to bound the bandwidth consumed by PGM transport sessions,
 an assortment of techniques is assembled here to make that strategy
 as conservative and robust as possible.  These techniques are the
 minimum REQUIRED of a PGM source.

5.1.1. Maximum Cumulative Transmit Rate

 A source MUST number ODATA packets in the order in which they are
 submitted for transmission by the application.  A source MUST
 transmit ODATA packets in sequence and only within the transmit
 window beginning with TXW_TRAIL at no greater a rate than
 TXW_MAX_RTE.
 TXW_MAX_RTE is typically the maximum cumulative transmit rate of SPM,
 ODATA, and RDATA.  Different transmission strategies MAY define
 TXW_MAX_RTE as appropriate for the implementation.

5.1.2. Transmit Rate Regulation

 To regulate its transmit rate, a source MUST use a token bucket
 scheme or any other traffic management scheme that yields equivalent
 behavior.  A token bucket [7] is characterized by a continually
 sustainable data rate (the token rate) and the extent to which the
 data rate may exceed the token rate for short periods of time (the
 token bucket size).  Over any arbitrarily chosen interval, the number
 of bytes the source may transmit MUST NOT exceed the token bucket
 size plus the product of the token rate and the chosen interval.
 In addition, a source MUST bound the maximum rate at which successive
 packets may be transmitted using a leaky bucket scheme drained at a
 maximum transmit rate, or equivalent mechanism.

Speakman, et. al. Experimental [Page 19] RFC 3208 PGM Reliable Transport Protocol December 2001

5.1.3. Outgoing Packet Ordering

 To preserve the logic of PGM's transmit window, a source MUST
 strictly prioritize sending of pending NCFs first, pending SPMs
 second, and only send ODATA or RDATA when no NCFs or SPMs are
 pending.  The priority of RDATA versus ODATA is application
 dependent.  The sender MAY implement weighted bandwidth sharing
 between RDATA and ODATA.  Note that strict prioritization of RDATA
 over ODATA may stall progress of ODATA if there are receivers that
 keep generating NAKs so as to always have RDATA pending (e.g. a
 steady stream of late joiners with OPT_JOIN).  Strictly prioritizing
 ODATA over RDATA may lead to a larger portion of receivers getting
 unrecoverable losses.

5.1.4. Ambient SPMs

 Interleaved with ODATA and RDATA, a source MUST transmit SPMs at a
 rate at least sufficient to maintain current source path state in PGM
 network elements.  Note that source path state in network elements
 does not track underlying changes in the distribution tree from a
 source until an SPM traverses the altered distribution tree.  The
 consequence is that NAKs may go unconfirmed both at receivers and
 amongst network elements while changes in the underlying distribution
 tree take place.

5.1.5. Heartbeat SPMs

 In the absence of data to transmit, a source SHOULD transmit SPMs at
 a decaying rate in order to assist early detection of lost data, to
 maintain current source path state in PGM network elements, and to
 maintain current receive window state in the receivers.
 In this scheme [8], a source maintains an inter-heartbeat timer
 IHB_TMR which times the interval between the most recent packet
 (ODATA, RDATA, or SPM) transmission and the next heartbeat
 transmission.  IHB_TMR is initialized to a minimum interval IHB_MIN
 after the transmission of any data packet.  If IHB_TMR expires, the
 source transmits a heartbeat SPM and initializes IHB_TMR to double
 its previous value.  The transmission of consecutive heartbeat SPMs
 doubles IHB each time up to a maximum interval IHB_MAX.  The
 transmission of any data packet initializes IHB_TMR to IHB_MIN once
 again.  The effect is to provoke prompt detection of missing packets
 in the absence of data to transmit, and to do so with minimal
 bandwidth overhead.

Speakman, et. al. Experimental [Page 20] RFC 3208 PGM Reliable Transport Protocol December 2001

5.1.6. Ambient and Heartbeat SPMs

 Ambient and heartbeat SPMs are described as driven by separate timers
 in this specification to highlight their contrasting functions.
 Ambient SPMs are driven by a count-down timer that expires regularly
 while heartbeat SPMs are driven by a count-down timer that keeps
 being reset by data, and the interval of which changes once it begins
 to expire.  The ambient SPM timer is just counting down in real-time
 while the heartbeat timer is measuring the inter-data-packet
 interval.
 In the presence of data, no heartbeat SPMs will be transmitted since
 the transmission of data keeps setting the IHB_TMR back to its
 initial value.  At the same time however, ambient SPMs MUST be
 interleaved into the data as a matter of course, not necessarily as a
 heartbeat mechanism.  This ambient transmission of SPMs is REQUIRED
 to keep the distribution tree information in the network current and
 to allow new receivers to synchronize with the session.
 An implementation SHOULD de-couple ambient and heartbeat SPM timers
 sufficiently to permit them to be configured independently of each
 other.

5.2. Negative Acknowledgment Confirmation

 A source MUST immediately multicast an NCF in response to any NAK it
 receives.  The NCF is REQUIRED since the alternative of responding
 immediately with RDATA would not allow other PGM network elements on
 the same subnet to do NAK anticipation, nor would it allow DLRs on
 the same subnet to provide repairs.  A source SHOULD be able to
 detect a NAK storm and adopt countermeasure to protect the network
 against a denial of service.  A possible countermeasure is to send
 the first NCF immediately in response to a NAK and then delay the
 generation of further NCFs (for identical NAKs) by a small interval,
 so that identical NCFs are rate-limited, without affecting the
 ability to suppress NAKs.

5.3. Repairs

 After multicasting an NCF in response to a NAK, a source MUST then
 multicast RDATA (while respecting TXW_MAX_RTE) in response to any NAK
 it receives for data packets within the transmit window.
 In the interest of increasing the efficiency of a particular RDATA
 packet, a source MAY delay RDATA transmission to accommodate the
 arrival of NAKs from the whole loss neighborhood.  This delay SHOULD
 not exceed twice the greatest propagation delay in the loss
 neighborhood.

Speakman, et. al. Experimental [Page 21] RFC 3208 PGM Reliable Transport Protocol December 2001

6. Procedures - Receivers

6.1. Data Reception

 Initial data reception
 A receiver SHOULD initiate data reception beginning with the first
 data packet it receives within the advertised transmit window.  This
 packet's sequence number (ODATA_SQN) temporarily defines the trailing
 edge of the transmit window from the receiver's perspective.  That
 is, it is assigned to RXW_TRAIL_INIT within the receiver, and until
 the trailing edge sequence number advertised in subsequent packets
 (SPMs or ODATA or RDATA) increments past RXW_TRAIL_INIT, the receiver
 MUST only request repairs for sequence numbers subsequent to
 RXW_TRAIL_INIT.  Thereafter, it MAY request repairs anywhere in the
 transmit window.  This temporary restriction on repair requests
 prevents receivers from requesting a potentially large amount of
 history when they first begin to receive a given PGM transport
 session.
 Note that the JOIN option, discussed later, MAY be used to provide a
 different value for RXW_TRAIL_INIT.
 Receiving and discarding data packets
 Within a given transport session, a receiver MUST accept any ODATA or
 RDATA packets within the receive window.  A receiver MUST discard any
 data packet that duplicates one already received in the transmit
 window.  A receiver MUST discard any data packet outside of the
 receive window.
 Contiguous data
 Contiguous data is comprised of those data packets within the receive
 window that have been received and are in the range from RXW_TRAIL up
 to (but not including) the first missing sequence number in the
 receive window.  The most recently received data packet of contiguous
 data defines the leading edge of contiguous data.
 As its default mode of operation, a receiver MUST deliver only
 contiguous data packets to the application, and it MUST do so in the
 order defined by those data packets' sequence numbers.  This provides
 applications with a reliable ordered data flow.

Speakman, et. al. Experimental [Page 22] RFC 3208 PGM Reliable Transport Protocol December 2001

 Non contiguous data
 PGM receiver implementations MAY optionally provide a mode of
 operation in which data is delivered to an application in the order
 received.  However, the implementation MUST only deliver complete
 application protocol data units (APDUs) to the application.  That is,
 APDUs that have been fragmented into different TPDUs MUST be
 reassembled before delivery to the application.

6.2. Source Path Messages

 Receivers MUST receive and sequence SPMs for any TSI they are
 receiving.  An SPM is in sequence if its sequence number is greater
 than that of the most recent in-sequence SPM and within half the PGM
 number space.  Out-of-sequence SPMs MUST be discarded.
 For each TSI, receivers MUST use the most recent SPM to determine the
 NLA of the upstream PGM network element for use in NAK addressing.  A
 receiver MUST NOT initiate repair requests until it has received at
 least one SPM for the corresponding TSI.
 Since SPMs require per-hop processing, it is likely that they will be
 forwarded at a slower rate than data, and that they will arrive out
 of sync with the data stream.  In this case, the window information
 that the SPMs carry will be out of date.  Receivers SHOULD expect
 this to be the case and SHOULD detect it by comparing the packet lead
 and trail values with the values the receivers have stored for lead
 and trail.  If the SPM packet values are less, they SHOULD be
 ignored, but the rest of the packet SHOULD be processed as normal.

6.3. Data Recovery by Negative Acknowledgment

 Detecting missing data packets
 Receivers MUST detect gaps in the expected data sequence in the
 following manners:
    by comparing the sequence number on the most recently received
    ODATA or RDATA packet with the leading edge of contiguous data
    by comparing SPM_LEAD of the most recently received SPM with the
    leading edge of contiguous data
 In both cases, if the receiver has not received all intervening data
 packets, it MAY initiate selective NAK generation for each missing
 sequence number.

Speakman, et. al. Experimental [Page 23] RFC 3208 PGM Reliable Transport Protocol December 2001

 In addition, a receiver may detect a single missing data packet by
 receiving an NCF or multicast NAK for a data packet within the
 transmit window which it has not received.  In this case it MAY
 initiate selective NAK generation for the said sequence number.
 In all cases, receivers SHOULD temper the initiation of NAK
 generation to account for simple mis-ordering introduced by the
 network.  A possible mechanism to achieve this is to assume loss only
 after the reception of N packets with sequence numbers higher than
 those of the (assumed) lost packets.  A possible value for N is 2.
 This method SHOULD be complemented with a timeout based mechanism
 that handles the loss of the last packet before a pause in the
 transmission of the data stream.  The leading edge field in SPMs
 SHOULD also be taken into account in the loss detection algorithm.
 Generating NAKs
 NAK generation follows the detection of a missing data packet and is
 the cycle of:
    waiting for a random period of time (NAK_RB_IVL) while listening
    for matching NCFs or NAKs
    transmitting a NAK if a matching NCF or NAK is not heard
    waiting a period (NAK_RPT_IVL) for a matching NCF and recommencing
    NAK generation if the matching NCF is not received
    waiting a period (NAK_RDATA_IVL) for data and recommencing NAK
    generation if the matching data is not received
 The entire generation process can be summarized by the following
 state machine:

Speakman, et. al. Experimental [Page 24] RFC 3208 PGM Reliable Transport Protocol December 2001

                            |
                            | detect missing tpdu
                            |   - clear data retry count
                            |   - clear NCF retry count
                            V
    matching NCF |--------------------------|
 <---------------|   BACK-OFF_STATE         | <----------------------
 |               | start timer(NAK_RB_IVL)  |            ^          ^
 |               |                          |            |          |
 |               |--------------------------|            |          |
 |       matching |         | timer expires              |          |
 |         NAK    |         |   - send NAK               |          |
 |                |         |                            |          |
 |                V         V                            |          |
 |               |--------------------------|            |          |
 |               |    WAIT_NCF_STATE        |            |          |
 |  matching NCF | start timer(NAK_RPT_IVL) |            |          |
 |<--------------|                          |------------>          |
 |               |--------------------------| timer expires         |
 |                    |         |         ^    - increment NCF      |
 |    NAK_NCF_RETRIES |         |         |      retry count        |
 |       exceeded     |         |         |                         |
 |                    V         -----------                         |
 |                Cancelation      matching NAK                     |
 |                                   - restart timer(NAK_RPT_IVL)   |
 |                                                                  |
 |                                                                  |
 V               |--------------------------|                       |
 --------------->|   WAIT_DATA_STATE        |----------------------->
                 |start timer(NAK_RDATA_IVL)|  timer expires
                 |                          |   - increment data
                 |--------------------------|     retry count
                    |        |           ^
   NAK_DATA_RETRIES |        |           |
       exceeded     |        |           |
                    |         -----------
                    |          matching NCF or NAK
                    V            - restart timer(NAK_RDATA_IVL)
               Cancellation
 In any state, receipt of matching RDATA or ODATA completes data
 recovery and successful exit from the state machine.  State
 transition stops any running timers.
 In any state, if the trailing edge of the window moves beyond the
 sequence number, data recovery for that sequence number terminates.

Speakman, et. al. Experimental [Page 25] RFC 3208 PGM Reliable Transport Protocol December 2001

 During NAK_RB_IVL a NAK is said to be pending.  When awaiting data or
 an NCF, a NAK is said to be outstanding.
 Backing off NAK transmission
 Before transmitting a NAK, a receiver MUST wait some interval
 NAK_RB_IVL chosen randomly over some time period NAK_BO_IVL.  During
 this period, receipt of a matching NAK or a matching NCF will suspend
 NAK generation.  NAK_RB_IVL is counted down from the time a missing
 data packet is detected.
 A value for NAK_BO_IVL learned from OPT_NAK_BO_IVL (see 16.4.1 below)
 MUST NOT be used by a receiver (i.e., the receiver MUST NOT NAK)
 unless either NAK_BO_IVL_SQN is zero, or the receiver has seen
 POLL_RND == 0 for POLL_SQN =< NAK_BO_IVL_SQN within half the sequence
 number space.
 When a parity NAK (Appendix A, FEC) is being generated, the back-off
 interval SHOULD be inversely biased with respect to the number of
 parity packets requested.  This way NAKs requesting larger numbers of
 parity packets are likely to be sent first and thus suppress other
 NAKs.  A NAK for a given transmission group suppresses another NAK
 for the same transmission group only if it is requesting an equal or
 larger number of parity packets.
 When a receiver has to transmit a sequence of NAKs, it SHOULD
 transmit the NAKs in order from oldest to most recent.
 Suspending NAK generation
 Suspending NAK generation just means waiting for either NAK_RB_IVL,
 NAK_RPT_IVL or NAK_RDATA_IVL to pass.  A receiver MUST suspend NAK
 generation if a duplicate of the NAK is already pending from this
 receiver or the NAK is already outstanding from this or another
 receiver.
 NAK suppression
 A receiver MUST suppress NAK generation and wait at least
 NAK_RDATA_IVL before recommencing NAK generation if it hears a
 matching NCF or NAK during NAK_RB_IVL.  A matching NCF must match
 NCF_TSI with NAK_TSI, and NCF_SQN with NAK_SQN.
 Transmitting a NAK
 Upon expiry of NAK_RB_IVL, a receiver MUST unicast a NAK to the
 upstream PGM network element for the TSI specifying the transport
 session identifier and missing sequence number.  In addition, it MAY

Speakman, et. al. Experimental [Page 26] RFC 3208 PGM Reliable Transport Protocol December 2001

 multicast a NAK with TTL of 1 to the group, if the PGM parent is not
 directly connected.  It also records both the address of the source
 of the corresponding ODATA and the address of the group in the NAK
 header.
 It MUST repeat the NAK at a rate governed by NAK_RPT_IVL up to
 NAK_NCF_RETRIES times while waiting for a matching NCF.  It MUST then
 wait NAK_RDATA_IVL before recommencing NAK generation.  If it hears a
 matching NCF or NAK during NAK_RDATA_IVL, it MUST wait anew for
 NAK_RDATA_IVL before recommencing NAK generation (i.e. matching NCFs
 and NAKs restart NAK_RDATA_IVL).
 Completion of NAK generation
 NAK generation is complete only upon the receipt of the matching
 RDATA (or even ODATA) packet at any time during NAK generation.
 Cancellation of NAK generation
 NAK generation is cancelled upon the advancing of the receive window
 so as to exclude the matching sequence number of a pending or
 outstanding NAK, or NAK_DATA_RETRIES / NAK_NCF_RETRIES being
 exceeded.  Cancellation of NAK generation indicates unrecoverable
 data loss.
 Receiving NCFs and multicast NAKs
 A receiver MUST discard any NCFs or NAKs it hears for data packets
 outside the transmit window or for data packets it has received.
 Otherwise they are treated as appropriate for the current repair
 state.

7. Procedures - Network Elements

7.1. Source Path State

 Upon receipt of an in-sequence SPM, a network element records the
 Source Path Address SPM_PATH with the multicast routing information
 for the TSI.  If the receiving network element is on the same subnet
 as the forwarding network element, this address will be the same as
 the address of the immediately upstream network element on the
 distribution tree for the TSI.  If, however, non-PGM network elements
 intervene between the forwarding and the receiving network elements,
 this address will be the address of the first PGM network element
 across the intervening network elements.

Speakman, et. al. Experimental [Page 27] RFC 3208 PGM Reliable Transport Protocol December 2001

 The network element then forwards the SPM on each outgoing interface
 for that TSI.  As it does so, it encodes the network address of the
 outgoing interface in SPM_PATH in each copy of the SPM it forwards.

7.2. NAK Confirmation

 Network elements MUST immediately transmit an NCF in response to any
 unicast NAK they receive.  The NCF MUST be multicast to the group on
 the interface on which the NAK was received.
    Nota Bene: In order to avoid creating multicast routing state for
    PGM network elements across non-PGM-capable clouds, the network-
    header source address of NCFs transmitted by network elements MUST
    be set to the ODATA source's NLA, not the network element's NLA as
    might be expected.
 Network elements should be able to detect a NAK storm and adopt
 counter-measure to protect the network against a denial of service.
 A possible countermeasure is to send the first NCF immediately in
 response to a NAK and then delay the generation of further NCFs (for
 identical NAKs) by a small interval, so that identical NCFs are
 rate-limited, without affecting the ability to suppress NAKs.
 Simultaneously, network elements MUST establish repair state for the
 NAK if such state does not already exist, and add the interface on
 which the NAK was received to the corresponding repair interface list
 if the interface is not already listed.

7.3. Constrained NAK Forwarding

 The NAK forwarding procedures for network elements are quite similar
 to those for receivers, but three important differences should be
 noted.
 First, network elements do NOT back off before forwarding a NAK
 (i.e., there is no NAK_BO_IVL) since the resulting delay of the NAK
 would compound with each hop.  Note that NAK arrivals will be
 randomized by the receivers from which they originate, and this
 factor in conjunction with NAK anticipation and elimination will
 combine to forestall NAK storms on subnets with a dense network
 element population.
 Second, network elements do NOT retry confirmed NAKs if RDATA is not
 seen; they simply discard the repair state and rely on receivers to
 re-request the repair.  This approach keeps the repair state in the
 network elements relatively ephemeral and responsive to underlying
 routing changes.

Speakman, et. al. Experimental [Page 28] RFC 3208 PGM Reliable Transport Protocol December 2001

 Third, note that ODATA does NOT cancel NAK forwarding in network
 elements since it is switched by network elements without transport-
 layer intervention.
    Nota Bene: Once confirmed by an NCF, network elements discard NAK
    packets; they are NOT retained in network elements beyond this
    forwarding operation.
 NAK forwarding requires that a network element listen to NCFs for the
 same transport session.  NAK forwarding also requires that a network
 element observe two time out intervals for any given NAK (i.e., per
 NAK_TSI and NAK_SQN): NAK_RPT_IVL and NAK_RDATA_IVL.
 The NAK repeat interval NAK_RPT_IVL, limits the length of time for
 which a network element will repeat a NAK while waiting for a
 corresponding NCF.  NAK_RPT_IVL is counted down from the transmission
 of a NAK.  Expiry of NAK_RPT_IVL cancels NAK forwarding (due to
 missing NCF).
 The NAK RDATA interval NAK_RDATA_IVL, limits the length of time for
 which a network element will wait for the corresponding RDATA.
 NAK_RDATA_IVL is counted down from the time a matching NCF is
 received.  Expiry of NAK_RDATA_IVL causes the network element to
 discard the corresponding repair state (due to missing RDATA).
 During NAK_RPT_IVL, a NAK is said to be pending.  During
 NAK_RDATA_IVL, a NAK is said to be outstanding.
 A Network element MUST forward NAKs only to the upstream PGM network
 element for the TSI.
 A network element MUST repeat a NAK at a rate of NAK_RPT_RTE for an
 interval of NAK_RPT_IVL until it receives a matching NCF.  A matching
 NCF must match NCF_TSI with NAK_TSI, and NCF_SQN with NAK_SQN.
 Upon reception of the corresponding NCF, network elements MUST wait
 at least NAK_RDATA_IVL for the corresponding RDATA.  Receipt of the
 corresponding RDATA at any time during NAK forwarding cancels NAK
 forwarding and tears down the corresponding repair state in the
 network element.

7.4. NAK elimination

 Two NAKs duplicate each other if they bear the same NAK_TSI and
 NAK_SQN.  Network elements MUST discard all duplicates of a NAK that
 is pending.

Speakman, et. al. Experimental [Page 29] RFC 3208 PGM Reliable Transport Protocol December 2001

 Once a NAK is outstanding, network elements MUST discard all
 duplicates of that NAK for NAK_ELIM_IVL.  Upon expiry of
 NAK_ELIM_IVL, network elements MUST suspend NAK elimination for that
 TSI/SQN until the first duplicate of that NAK is seen after the
 expiry of NAK_ELIM_IVL.  This duplicate MUST be forwarded in the
 usual manner.  Once this duplicate NAK is outstanding, network
 elements MUST once again discard all duplicates of that NAK for
 NAK_ELIM_IVL, and so on.  NAK_RDATA_IVL MUST be reset each time a NAK
 for the corresponding TSI/SQN is confirmed (i.e., each time
 NAK_ELIM_IVL is reset).  NAK_ELIM_IVL MUST be some small fraction of
 NAK_RDATA_IVL.
 NAK_ELIM_IVL acts to balance implosion prevention against repair
 state liveness.  That is, it results in the elimination of all but at
 most one NAK per NAK_ELIM_IVL thereby allowing repeated NAKs to keep
 the repair state alive in the PGM network elements.

7.5. NAK Anticipation

 An unsolicited NCF is one that is received by a network element when
 the network element has no corresponding pending or outstanding NAK.
 Network elements MUST process unsolicited NCFs differently depending
 on the interface on which they are received.
 If the interface on which an NCF is received is the same interface
 the network element would use to reach the upstream PGM network
 element, the network element simply establishes repair state for
 NCF_TSI and NCF_SQN without adding the interface to the repair
 interface list, and discards the NCF.  If the repair state already
 exists, the network element restarts the NAK_RDATA_IVL and
 NAK_ELIM_IVL timers and discards the NCF.
 If the interface on which an NCF is received is not the same
 interface the network element would use to reach the upstream PGM
 network element, the network element does not establish repair state
 and just discards the NCF.
 Anticipated NAKs permit the elimination of any subsequent matching
 NAKs from downstream.  Upon establishing anticipated repair state,
 network elements MUST eliminate subsequent NAKs only for a period of
 NAK_ELIM_IVL.  Upon expiry of NAK_ELIM_IVL, network elements MUST
 suspend NAK elimination for that TSI/SQN until the first duplicate of
 that NAK is seen after the expiry of NAK_ELIM_IVL.  This duplicate
 MUST be forwarded in the usual manner.  Once this duplicate NAK is
 outstanding, network elements MUST once again discard all duplicates
 of that NAK for NAK_ELIM_IVL, and so on.  NAK_RDATA_IVL MUST be reset

Speakman, et. al. Experimental [Page 30] RFC 3208 PGM Reliable Transport Protocol December 2001

 each time a NAK for the corresponding TSI/SQN is confirmed (i.e.,
 each time NAK_ELIM_IVL is reset).  NAK_ELIM_IVL must be some small
 fraction of NAK_RDATA_IVL.

7.6. NAK Shedding

 Network elements MAY implement local procedures for withholding NAK
 confirmations for receivers detected to be reporting excessive loss.
 The result of these procedures would ultimately be unrecoverable data
 loss in the receiver.

7.7. Addressing NAKs

 A PGM network element uses the source and group addresses (NLAs)
 contained in the transport header to find the state for the
 corresponding TSI, looks up the corresponding upstream PGM network
 element's address, uses it to re-address the (unicast) NAK, and
 unicasts it on the upstream interface for the distribution tree for
 the TSI.

7.8. Constrained RDATA Forwarding

 Network elements MUST maintain repair state for each interface on
 which a given NAK is received at least once.  Network elements MUST
 then use this list of interfaces to constrain the forwarding of the
 corresponding RDATA packet only to those interfaces in the list.  An
 RDATA packet corresponds to a NAK if it matches NAK_TSI and NAK_SQN.
 Network elements MUST maintain this repair state only until either
 the corresponding RDATA is received and forwarded, or NAK_RDATA_IVL
 passes after forwarding the most recent instance of a given NAK.
 Thereafter, the corresponding repair state MUST be discarded.
 Network elements SHOULD discard and not forward RDATA packets for
 which they have no repair state.  Note that the consequence of this
 procedure is that, while it constrains repairs to the interested
 subset of the network, loss of repair state precipitates further NAKs
 from neglected receivers.

8. Packet Formats

 All of the packet formats described in this section are transport-
 layer headers that MUST immediately follow the network-layer header
 in the packet.  Only data packet headers (ODATA and RDATA) may be
 followed in the packet by application data.  For each packet type,
 the network-header source and destination addresses are specified in

Speakman, et. al. Experimental [Page 31] RFC 3208 PGM Reliable Transport Protocol December 2001

 addition to the format and contents of the transport layer header.
 Recall from General Procedures that, for PGM over IP multicast, SPMs,
 NCFs, and RDATA MUST also bear the IP Router Alert Option.
 For PGM over IP, the IP protocol number is 113.
 In all packets the descriptions of Data-Source Port, Data-Destination
 Port, Type, Options, Checksum, Global Source ID (GSI), and Transport
 Service Data Unit (TSDU) Length are:
    Data-Source Port:
       A random port number generated by the source.  This port number
       MUST be unique within the source.  Source Port together with
       Global Source ID forms the TSI.
    Data-Destination Port:
       A globally well-known port number assigned to the given PGM
       application.
    Type:
       The high-order two bits of the Type field encode a version
       number, 0x0 in this instance.  The low-order nibble of the type
       field encodes the specific packet type.  The intervening two
       bits (the low-order two bits of the high-order nibble) are
       reserved and MUST be zero.
       Within the low-order nibble of the Type field:
          values in the range 0x0 through 0x3 represent SPM-like
          packets (i.e., session-specific, sourced by a source,
          periodic),
          values in the range 0x4 through 0x7 represent DATA-like
          packets (i.e., data and repairs),
          values in the range 0x8 through 0xB represent NAK-like
          packets (i.e., hop-by-hop reliable NAK forwarding
          procedures),
          and values in the range 0xC through 0xF represent SPMR-like
          packets (i.e., session-specific, sourced by a receiver,
          asynchronous).

Speakman, et. al. Experimental [Page 32] RFC 3208 PGM Reliable Transport Protocol December 2001

    Options:
       This field encodes binary indications of the presence and
       significance of any options.  It also directly encodes some
       options.
       bit 0 set => One or more Option Extensions are present
       bit 1 set => One or more Options are network-significant
          Note that this bit is clear when OPT_FRAGMENT and/or
          OPT_JOIN are the only options present.
       bit 6 set => Packet is a parity packet for a transmission group
       of variable sized packets (OPT_VAR_PKTLEN).  Only present when
       OPT_PARITY is also present.
       bit 7 set => Packet is a parity packet (OPT_PARITY)
       Bits are numbered here from left (0 = MSB) to right (7 = LSB).
       All the other options (option extensions) are encoded in
       extensions to the PGM header.
    Checksum:
       This field is the usual 1's complement of the 1's complement
       sum of the entire PGM packet including header.
       The checksum does not include a network-layer pseudo header for
       compatibility with network address translation.  If the
       computed checksum is zero, it is transmitted as all ones.  A
       value of zero in this field means the transmitter generated no
       checksum.
       Note that if any entity between a source and a receiver
       modifies the PGM header for any reason, it MUST either
       recompute the checksum or clear it.  The checksum is mandatory
       on data packets (ODATA and RDATA).
    Global Source ID:
       A globally unique source identifier.  This ID MUST NOT change
       throughout the duration of the transport session.  A
       RECOMMENDED identifier is the low-order 48 bits of the MD5 [9]
       signature of the DNS name of the source.  Global Source ID
       together with Data-Source Port forms the TSI.

Speakman, et. al. Experimental [Page 33] RFC 3208 PGM Reliable Transport Protocol December 2001

    TSDU Length:
       The length in octets of the transport data unit exclusive of
       the transport header.
       Note that those who require the TPDU length must obtain it from
       sum of the transport header length (TH) and the TSDU length.
       TH length is the sum of the size of the particular PGM packet
       header (type_specific_size) plus the length of any options that
       might be present.
 Address Family Indicators (AFIs) are as specified in [10].

8.1. Source Path Messages

 SPMs are sent by a source to establish source path state in network
 elements and to provide transmit window state to receivers.
 The network-header source address of an SPM is the unicast NLA of the
 entity that originates the SPM.
 The network-header destination address of an SPM is a multicast group
 NLA.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Source Port           |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Type     |    Options    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Global Source ID                   ... |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | ...    Global Source ID       |           TSDU Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     SPM's Sequence Number                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Trailing Edge Sequence Number                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Leading Edge Sequence Number                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            NLA AFI            |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            Path NLA                     ...   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
 | Option Extensions when present ...                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+

Speakman, et. al. Experimental [Page 34] RFC 3208 PGM Reliable Transport Protocol December 2001

 Source Port:
    SPM_SPORT
    Data-Source Port, together with SPM_GSI forms SPM_TSI
 Destination Port:
    SPM_DPORT
    Data-Destination Port
 Type:
    SPM_TYPE = 0x00
 Global Source ID:
    SPM_GSI
    Together with SPM_SPORT forms SPM_TSI
 SPM's Sequence Number
    SPM_SQN
    The sequence number assigned to the SPM by the source.
 Trailing Edge Sequence Number:
    SPM_TRAIL
    The sequence number defining the current trailing edge of the
    source's transmit window (TXW_TRAIL).
 Leading Edge Sequence Number:
    SPM_LEAD
    The sequence number defining the current leading edge of the
    source's transmit window (TXW_LEAD).
    If SPM_TRAIL == 0 and SPM_LEAD == 0x80000000, this indicates that
    no window information is present in the packet.

Speakman, et. al. Experimental [Page 35] RFC 3208 PGM Reliable Transport Protocol December 2001

 Path NLA:
    SPM_PATH
    The NLA of the interface on the network element on which this SPM
    was forwarded.  Initialized by a source to the source's NLA,
    rewritten by each PGM network element upon forwarding.

8.2. Data Packets

 Data packets carry application data from a source or a repairer to
 receivers.
    ODATA:
       Original data packets transmitted by a source.
    RDATA:
       Repairs transmitted by a source or by a designated local
       repairer (DLR) in response to a NAK.
 The network-header source address of a data packet is the unicast NLA
 of the entity that originates the data packet.
 The network-header destination address of a data packet is a
 multicast group NLA.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Source Port           |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Type     |    Options    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Global Source ID                   ... |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | ...    Global Source ID       |           TSDU Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Data Packet Sequence Number                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Trailing Edge Sequence Number                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Option Extensions when present ...                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Data ...
 +-+-+- ...

Speakman, et. al. Experimental [Page 36] RFC 3208 PGM Reliable Transport Protocol December 2001

 Source Port:
    OD_SPORT, RD_SPORT
    Data-Source Port, together with Global Source ID forms:
    OD_TSI, RD_TSI
 Destination Port:
    OD_DPORT, RD_DPORT
    Data-Destination Port
 Type:
    OD_TYPE =  0x04 RD_TYPE =  0x05
 Global Source ID:
    OD_GSI, RD_GSI
    Together with Source Port forms:
    OD_TSI, RD_TSI
 Data Packet Sequence Number:
    OD_SQN, RD_SQN
    The sequence number originally assigned to the ODATA packet by the
    source.
 Trailing Edge Sequence Number:
    OD_TRAIL, RD_TRAIL
    The sequence number defining the current trailing edge of the
    source's transmit window (TXW_TRAIL).  In RDATA, this MAY not be
    the same as OD_TRAIL of the ODATA packet for which it is a repair.
 Data:
    Application data.

Speakman, et. al. Experimental [Page 37] RFC 3208 PGM Reliable Transport Protocol December 2001

8.3. Negative Acknowledgments and Confirmations

    NAK:
       Negative Acknowledgments are sent by receivers to request the
       repair of an ODATA packet detected to be missing from the
       expected sequence.
    N-NAK:
       Null Negative Acknowledgments are sent by DLRs to provide flow
       control feedback to the source of ODATA for which the DLR has
       provided the corresponding RDATA.
 The network-header source address of a NAK is the unicast NLA of the
 entity that originates the NAK.  The network-header source address of
 NAK is rewritten by each PGM network element with its own.
 The network-header destination address of a NAK is initialized by the
 originator of the NAK (a receiver) to the unicast NLA of the upstream
 PGM network element known from SPMs.  The network-header destination
 address of a NAK is rewritten by each PGM network element with the
 unicast NLA of the upstream PGM network element to which this NAK is
 forwarded.  On the final hop, the network-header destination address
 of a NAK is rewritten by the PGM network element with the unicast NLA
 of the original source or the unicast NLA of a DLR.
    NCF:
       NAK Confirmations are sent by network elements and sources to
       confirm the receipt of a NAK.
 The network-header source address of an NCF is the ODATA source's
 NLA, not the network element's NLA as might be expected.
 The network-header destination address of an NCF is a multicast group
 NLA.
 Note that in NAKs and N-NAKs, unlike the other packets, the field
 SPORT contains the Data-Destination port and the field DPORT contains
 the Data-Source port.  As a general rule, the content of SPORT/DPORT
 is determined by the direction of the flow: in packets which travel
 down-stream SPORT is the port number chosen in the data source
 (Data-Source Port) and DPORT is the data destination port number
 (Data-Destination Port).  The opposite holds for packets which travel
 upstream.  This makes DPORT the protocol endpoint in the recipient
 host, regardless of the direction of the packet.

Speakman, et. al. Experimental [Page 38] RFC 3208 PGM Reliable Transport Protocol December 2001

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Source Port           |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Type     |    Options    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Global Source ID                   ... |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | ...    Global Source ID       |           TSDU Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   Requested Sequence Number                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            NLA AFI            |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           Source NLA                    ...   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
 |            NLA AFI            |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Multicast Group NLA                ...   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
 | Option Extensions when present ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ...
 Source Port:
    NAK_SPORT, NNAK_SPORT
       Data-Destination Port
    NCF_SPORT
    Data-Source Port, together with Global Source ID forms NCF_TSI
 Destination Port:
    NAK_DPORT, NNAK_DPORT
       Data-Source Port, together with Global Source ID forms:
          NAK_TSI, NNAK_TSI
    NCF_DPORT
    Data-Destination Port

Speakman, et. al. Experimental [Page 39] RFC 3208 PGM Reliable Transport Protocol December 2001

 Type:
    NAK_TYPE =  0x08 NNAK_TYPE = 0x09
    NCF_TYPE =  0x0A
 Global Source ID:
    NAK_GSI, NNAK_GSI, NCF_GSI
    Together with Data-Source Port forms
       NAK_TSI, NNAK_TSI, NCF_TSI
 Requested Sequence Number:
    NAK_SQN, NNAK_SQN
    NAK_SQN is the sequence number of the ODATA packet for which a
    repair is requested.
    NNAK_SQN is the sequence number of the RDATA packet for which a
    repair has been provided by a DLR.
    NCF_SQN
    NCF_SQN is NAK_SQN from the NAK being confirmed.
 Source NLA:
    NAK_SRC, NNAK_SRC, NCF_SRC
    The unicast NLA of the original source of the missing ODATA.
 Multicast Group NLA:
    NAK_GRP, NNAK_GRP, NCF_GRP
    The multicast group NLA.  NCFs MAY bear OPT_REDIRECT and/or
    OPT_NAK_LIST

9. Options

 PGM specifies several end-to-end options to address specific
 application requirements.  PGM specifies options to support
 fragmentation, late joining, and redirection.

Speakman, et. al. Experimental [Page 40] RFC 3208 PGM Reliable Transport Protocol December 2001

 Options MAY be appended to PGM data packet headers only by their
 original transmitters.  While they MAY be interpreted by network
 elements, options are neither added nor removed by network elements.
 Options are all in the TLV style, or Type, Length, Value.  The Type
 field is contained in the first byte, where bit 0 is the OPT_END bit,
 followed by 7 bits of type.  The OPT_END bit MUST be set in the last
 option in the option list, whichever that might be.  The Length field
 is the total length of the option in bytes, and directly follows the
 Type field.  Following the Length field are 5 reserved bits, the
 OP_ENCODED flag, the 2 Option Extensibility bits OPX and the
 OP_ENCODED_NULL flag.  Last are 7 bits designated for option specific
 information which may be defined on a per-option basis.  If not
 defined for a particular option, they MUST be set to 0.
 The Option Extensibility bits dictate the desired treatment of an
 option if it is unknown to the network element processing it.
    Nota Bene:  Only network elements pay any attention to these bits.
    The OPX bits are defined as follows:
    00 -     Ignore the option
    01 -     Invalidate the option by changing the type to OPT_INVALID
             = 0x7F
    10 -     Discard the packet
    11 -     Unsupported, and reserved for future use
 Some options present in data packet (ODATA and RDATA) are strictly
 associated with the packet content (PGM payload), OPT_FRAGMENT being
 an example.  These options must be preserved even when the data
 packet that would normally contain them is not received, but its the
 payload is recovered though the use of FEC.  PGM specifies a
 mechanism to accomplish this that uses the F (OP_ENCODED) and U
 (OP_ENCODED_NULL) bits in the option common header.  OP_ENCODED and
 OP_ENCODED_NULL MUST be normally set to zero except when the option
 is used in FEC packets to preserve original options.  See Appendix A
 for details.
 There is a limit of 16 options per packet.

Speakman, et. al. Experimental [Page 41] RFC 3208 PGM Reliable Transport Protocol December 2001

 General Option Format
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|Opt. Specific|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Option Value                    ...    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+

9.1. Option extension length - OPT_LENGTH

 When option extensions are appended to the standard PGM header, the
 extensions MUST be preceded by an option extension length field
 specifying the total length of all option extensions.
 In addition, the presence of the options MUST be encoded in the
 Options field of the standard PGM header before the Checksum is
 computed.
 All network-significant options MUST be appended before any
 exclusively receiver-significant options.
 To provide an indication of the end of option extensions, OPT_END
 (0x80) MUST be set in the Option Type field of the trailing option
 extension.

9.1.1. OPT_LENGTH - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Option Type  | Option Length |  Total length of all options  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x00
 Option Length = 4 octets
 Total length of all options
    The total length in octets of all option extensions including
    OPT_LENGTH.
 OPT_LENGTH is NOT network-significant.

Speakman, et. al. Experimental [Page 42] RFC 3208 PGM Reliable Transport Protocol December 2001

9.2. Fragmentation Option - OPT_FRAGMENT

 Fragmentation allows transport-layer entities at a source to break up
 application protocol data units (APDUs) into multiple PGM data
 packets (TPDUs) to conform with the MTU supported by the network
 layer.  The fragmentation option MAY be applied to ODATA and RDATA
 packets only.
 Architecturally, the accumulation of TSDUs into APDUs is applied to
 TPDUs that have already been received, duplicate eliminated, and
 contiguously sequenced by the receiver.  Thus APDUs MAY be
 reassembled across increments of the transmit window.

9.2.1. OPT_FRAGMENT - Packet Extension Contents

 OPT_FRAG_OFF   the offset of the fragment from the beginning of the
                APDU
 OPT_FRAG_LEN   the total length of the original APDU

9.2.2. OPT_FRAGMENT - Procedures - Sources

 A source fragments APDUs into a contiguous series of fragments no
 larger than the MTU supported by the network layer.  A source
 sequentially and uniquely assigns OD_SQNs to these fragments in the
 order in which they occur in the APDU.  A source then sets
 OPT_FRAG_OFF to the value of the offset of the fragment in the
 original APDU (where the first byte of the APDU is at offset 0, and
 OPT_FRAG_OFF numbers the first byte in the fragment), and set
 OPT_FRAG_LEN to the value of the total length of the original APDU.

9.2.3. OPT_FRAGMENT - Procedures - Receivers

 Receivers detect and accumulate fragmented packets until they have
 received an entire contiguous sequence of packets comprising an APDU.
 This sequence begins with the fragment bearing OPT_FRAG_OFF of 0, and
 terminates with the fragment whose length added to its OPT_FRAG_OFF
 is OPT_FRAG_LEN.

Speakman, et. al. Experimental [Page 43] RFC 3208 PGM Reliable Transport Protocol December 2001

9.2.4. OPT_FRAGMENT - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    First Sequence Number                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            Offset                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            Length                             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x01
 Option Length = 12 octets
 First Sequence Number
    Sequence Number of the PGM DATA/RDATA packet containing the first
    fragment of the APDU.
 Offset
    The byte offset of the fragment from the beginning of the APDU
    (OPT_FRAG_OFF).
 Length
    The total length of the original APDU (OPT_FRAG_LEN).
 OPT_FRAGMENT is NOT network-significant.

9.3. NAK List Option - OPT_NAK_LIST

 The NAK List option MAY be used in conjunction with NAKs to allow
 receivers to request transmission for more than one sequence number
 with a single NAK packet.  The option is limited to 62 listed NAK
 entries.  The NAK list MUST be unique and duplicate free.  It MUST be
 ordered, and MUST consist of either a list of selective or a list of
 parity NAKs.  In general, network elements, sources and receivers
 must process a NAK list as if they had received individual NAKs for
 each sequence number in the list.  The procedures for each are
 outlined in detail earlier in this document.  Clarifications and
 differences are detailed here.

Speakman, et. al. Experimental [Page 44] RFC 3208 PGM Reliable Transport Protocol December 2001

9.3.1. OPT_NAK_LIST - Packet Extensions Contents

 A list of sequence numbers for which retransmission is requested.

9.3.2. OPT_NAK_LIST - Procedures - Receivers

 Receivers MAY append the NAK List option to a NAK to indicate that
 they wish retransmission of a number of RDATA.
 Receivers SHOULD proceed to back off NAK transmission in a manner
 consistent with the procedures outlined for single sequence number
 NAKs.  Note that the repair of each separate sequence number will be
 completed upon receipt of a separate RDATA packet.
 Reception of an NCF or multicast NAK containing the NAK List option
 suspends generation of NAKs for all sequence numbers within the NAK
 list, as well as the sequence number within the NAK header.

9.3.3. OPT_NAK_LIST - Procedures - Network Elements

 Network elements MUST immediately respond to a NAK with an identical
 NCF containing the same NAK list as the NAK itself.
 Network elements MUST forward a NAK containing a NAK List option if
 any one sequence number specified by the NAK (including that in the
 main NAK header) is not currently outstanding.  That is, it MUST
 forward the NAK, if any one sequence number does not have an
 elimination timer running for it.  The NAK must be forwarded intact.
 Network elements MUST eliminate a NAK containing the NAK list option
 only if all sequence numbers specified by the NAK (including that in
 the main NAK header) are outstanding.  That is, they are all running
 an elimination timer.
 Upon receipt of an unsolicited NCF containing the NAK list option, a
 network element MUST anticipate data for every sequence number
 specified by the NAK as if it had received an NCF for every sequence
 number specified by the NAK.

9.3.4. OPT_NAK_LIST - Procedures - Sources

 A source MUST immediately respond to a NAK with an identical NCF
 containing the same NAK list as the NAK itself.
 It MUST then multicast RDATA (while respecting TXW_MAX_RTE) for every
 requested sequence number.

Speakman, et. al. Experimental [Page 45] RFC 3208 PGM Reliable Transport Protocol December 2001

9.3.5. OPT_NAK_LIST - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Requested Sequence Number 1                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  .....                                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Requested Sequence Number N                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x02
 Option Length = 4 + (4 * number of SQNs) octets
 Requested Sequence Number
    A list of up to 62 additional sequence numbers to which the NAK
    applies.
 OPT_NAK_LIST is network-significant.

9.4. Late Joining Option - OPT_JOIN

 Late joining allows a source to bound the amount of repair history
 receivers may request when they initially join a particular transport
 session.
 This option indicates that receivers that join a transport session in
 progress MAY request repair of all data as far back as the given
 minimum sequence number from the time they join the transport
 session.  The default is for receivers to receive data only from the
 first packet they receive and onward.

9.4.1. OPT_JOIN - Packet Extensions Contents

 OPT_JOIN_MIN   the minimum sequence number for repair

9.4.2. OPT_JOIN - Procedures - Receivers

 If a PGM packet (ODATA, RDATA, or SPM) bears OPT_JOIN, a receiver MAY
 initialize the trailing edge of the receive window (RXW_TRAIL_INIT)
 to the given Minimum Sequence Number and proceeds with normal data
 reception.

Speakman, et. al. Experimental [Page 46] RFC 3208 PGM Reliable Transport Protocol December 2001

9.4.3. OPT_JOIN - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Minimum Sequence Number                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x03
 Option Length = 8 octets
 Minimum Sequence Number
    The minimum sequence number defining the initial trailing edge of
    the receive window for a late joining receiver.
 OPT_JOIN is NOT network-significant.

9.5. Redirect Option - OPT_REDIRECT

 Redirection MAY be used by a designated local repairer (DLR) to
 advertise its own address as an alternative to the original source,
 for requesting repairs.
 These procedures allow a PGM Network Element to use a DLR that is one
 PGM hop from it either upstream or downstream in the multicast
 distribution tree.  The former are referred to as upstream DLRs.  The
 latter are referred to as off-tree DLRs.  Off-Tree because even
 though they are downstream of the point of loss, they might not lie
 on the subtree affected by the loss.
 A DLR MUST receive any PGM sessions for which it wishes to provide
 retransmissions.  A DLR SHOULD respond to NCFs or POLLs sourced by
 its PGM parent with a redirecting POLR response packet containing an
 OPT_REDIRECT which provides its own network layer address.
 Recipients of redirecting POLRs MAY then direct NAKs for subsequent
 ODATA sequence numbers to the DLR rather than to the original source.
 In addition, DLRs that receive redirected NAKs for which they have
 RDATA MUST send a NULL NAK to provide flow control to the original
 source without also provoking a repair from that source.

Speakman, et. al. Experimental [Page 47] RFC 3208 PGM Reliable Transport Protocol December 2001

9.5.1. OPT_REDIRECT - Packet Extensions Contents

 OPT_REDIR_NLA  the DLR's own unicast network-layer address to which
                recipients of the redirecting POLR MAY direct
                subsequent NAKs for the corresponding TSI.

9.5.2. OPT_REDIRECT - Procedures - DLRs

 A DLR MUST receive any PGM sessions for which it wishes to provide a
 source of repairs.  In addition to acting as an ordinary PGM
 receiver, a DLR MAY then respond to NCFs or relevant POLLs sourced by
 parent network elements (or even by the source itself) by sending a
 POLR containing an OPT_REDIRECT providing its own network-layer
 address.
 If a DLR can provide FEC repairs it MUST denote this by setting
 OPT_PARITY in the PGM header of its POLR response.

9.5.2.1. Upstream DLRs

 If the NCF completes NAK transmission initiated by the DLR itself,
 the DLR MUST NOT send a redirecting POLR.
 When a DLR receives an NCF from its upstream PGM parent, it SHOULD
 send a redirecting POLR, multicast to the group.  The DLR SHOULD
 record that it is acting as an upstream DLR for the said session.
 Note that this POLR MUST have both the data source's source address
 and the router alert option in its network header.
 An upstream DLR MUST act as an ordinary PGM source in responding to
 any NAK it receives (i.e., directed to it).  That is, it SHOULD
 respond first with a normal NCF and then RDATA as usual.  In
 addition, an upstream DLR that receives redirected NAKs for which it
 has RDATA MUST send a NULL NAK to provide flow control to the
 original source.  If it cannot provide the RDATA it forwards the NAK
 to the upstream PGM neighbor as usual.
    Nota Bene: In order to propagate on exactly the same distribution
    tree as ODATA, RDATA and POLR  packets transmitted by DLRs MUST
    bear the ODATA source's NLA as the network-header source address,
    not the DLR's NLA as might be expected.

Speakman, et. al. Experimental [Page 48] RFC 3208 PGM Reliable Transport Protocol December 2001

9.5.2.2. Off-Tree DLRs

 A DLR that receives a POLL with sub-type PGM_POLL_DLR MUST respond
 with a unicast redirecting POLR if it provides the appropriate
 service.  The DLR SHOULD respond using the rules outlined for polling
 in Appendix D of this text.  If the DLR responds, it SHOULD record
 that it is acting as an off-tree DLR for the said session.
 An off-tree DLR acts in a special way in responding to any NAK it
 receives (i.e., directed to it).  It MUST respond to a NAK directed
 to it from its parent by unicasting an NCF and RDATA to its parent.
 The parent will then forward the RDATA down the distribution tree.
 The DLR uses its own and the parent's NLA addresses in the network
 header for the source and destination respectively.  The unicast NCF
 and RDATA packets SHOULD not have the router alert option.  In all
 other ways the RDATA header should be "as if" the packet had come
 from the source.
 Again, an off-tree DLR that receives redirected NAKs for which it has
 RDATA MUST originate a NULL NAK to provide flow control to the
 original source.  It MUST originate the NULL NAK before originating
 the RDATA.  This must be done to reduce the state held in the network
 element.
 If it cannot provide the RDATA for a given NAK, an off-tree DLR
 SHOULD confirm the NAK with a unicast NCF as normal, then immediately
 send a NAK for the said data packet back to its parent.

9.5.2.3. Simultaneous Upstream and Off-Tree DLR operation

 Note that it is possible for a DLR to provide service to its parent
 and to downstream network elements simultaneously.  A downstream loss
 coupled with a loss for the same data on some other part of the
 distribution tree served by its parent could cause this.  In this
 case it may provide both upstream and off-tree functionality
 simultaneously.
 Note that a DLR differentiates between NAKs from an NE downstream or
 from its parent by comparing the network-header source address of the
 NAK with it's upstream PGM parent's NLA.  The DLR knows the parent's
 NLA from the session's SPM messages.

Speakman, et. al. Experimental [Page 49] RFC 3208 PGM Reliable Transport Protocol December 2001

9.5.3. OPT_REDIRECT - Procedures - Network Elements

9.5.3.1. Discovering DLRs

 When a PGM router receives notification of a loss via a NAK, it
 SHOULD first try to use a known DLR to recover the loss.  If such a
 DLR is not known it SHOULD initiate DLR discovery.  DLR discovery may
 occur in two ways.  If there are upstream DLRs, the NAK transmitted
 by this router to its PGM parent will trigger their discovery, via a
 redirecting POLR.  Also, a network element SHOULD initiate a search
 for off-tree DLRs using the PGM polling mechanism, and the sub-type
 PGM_POLL_DLR.
 If a DLR can provide FEC repairs it will denote this by setting
 OPT_PARITY in the PGM header of its POLR response.  A network element
 SHOULD only direct parity NAKs to a DLR that can provide FEC repairs.

9.5.3.2. Redirected Repair

 When it can, a network element SHOULD use upstream DLRs.
 Upon receiving a redirecting POLR, network elements SHOULD record the
 redirecting information for the TSI, and SHOULD redirect subsequent
 NAKs for the same TSI to the network address provided in the
 redirecting POLR rather than to the PGM neighbor known via the SPMs.
 Note, however, that a redirecting POLR is NOT regarded as matching
 the NAK that provoked it, so it does not complete the transmission of
 that NAK.  Only a normal matching NCF can complete the transmission
 of a NAK.
 For subsequent NAKs, if the network element has recorded redirection
 information for the corresponding TSI, it MAY change the destination
 network address of those NAKs and attempt to transmit them to the
 DLR.  No NAK for a specific SQN SHOULD be sent to an off-tree DLR if
 a NAK for the SQN has been seen on the interface associated with the
 DLR.  Instead the NAK SHOULD be forwarded upstream.  Subsequent NAKs
 for different SQNs MAY be forwarded to the said DLR (again assuming
 no NAK for them has been seen on the interface to the DLR).
 If a corresponding NCF is not received from the DLR within
 NAK_RPT_IVL, the network element MUST discard the redirecting
 information for the TSI and re-attempt to forward the NAK towards the
 PGM upstream neighbor.

Speakman, et. al. Experimental [Page 50] RFC 3208 PGM Reliable Transport Protocol December 2001

 If a NAK is received from the DLR for a requested SQN, the network
 element MUST discard the redirecting information for the SQN and re-
 attempt to forward the NAK towards the PGM upstream neighbor.  The
 network element MAY still direct NAKs for different SQNs to the DLR.
 RDATA and NCFs from upstream DLRs will flow down the distribution
 tree.  However, RDATA and NCFs from off-tree DLRs will be unicast to
 the network element.  The network element will terminate the NCF, but
 MUST put the source's NLA and the group address into the network
 header and MUST add router alert before forwarding the RDATA packet
 to the distribution subtree.
 NULL NAKs from an off-tree DLR for an RDATA packet requested from
 that off-tree DLR MUST always be forwarded upstream.  The network
 element can assume that these will arrive before the matching RDATA.
 Other NULL NAKs are forwarded only if matching repair state has not
 already been created.  Network elements MUST NOT confirm or retry
 NULL NAKs and they MUST NOT add the receiving interface to the repair
 state.  If a NULL NAK is used to initially create repair state, this
 fact must be recorded so that any subsequent non-NULL NAK will not be
 eliminated, but rather will be forwarded to provoke an actual repair.
 State created by a NULL NAK exists only for NAK_ELIM_IVL.

9.5.4. OPT_REDIRECT - Procedures - Receivers

 These procedures are intended to be applied in instances where a
 receiver's first hop router on the reverse path to the source is not
 a PGM Network Element.  So, receivers MUST ignore a redirecting POLR
 from a DLR on the same IP subnet that the receiver resides on, since
 this is likely to suffer identical loss to the receiver and so be
 useless.  Therefore, these procedures are entirely OPTIONAL.  A
 receiver MAY choose to ignore all redirecting POLRs since in cases
 where its first hop router on the reverse path is PGM capable, it
 would ignore them anyway.  Also, note that receivers will never learn
 of off-tree DLRs.
 Upon receiving a redirecting POLR, receivers SHOULD record the
 redirecting information for the TSI, and MAY redirect subsequent NAKs
 for the same TSI to the network address provided in the redirecting
 POLR rather than to the PGM neighbor for the corresponding ODATA for
 which the receiver is requesting repair.  Note, however, that a
 redirecting POLR is NOT regarded as matching the NAK that provoked
 it, so it does not complete the transmission of that NAK.  Only a
 normal matching NCF can complete the transmission of a NAK.
 For subsequent NAKs, if the receiver has recorded redirection
 information for the corresponding TSI, it MAY change the destination
 network address of those NAKs and attempt to transmit them to the

Speakman, et. al. Experimental [Page 51] RFC 3208 PGM Reliable Transport Protocol December 2001

 DLR.  If a corresponding NCF is not received within NAK_RPT_IVL, the
 receiver MUST discard the redirecting information for the TSI and
 re-attempt to forward the NAK to the PGM neighbor for the original
 source of the missing ODATA.

9.5.5. OPT_REDIRECT - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            NLA AFI            |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           DLR's NLA                     ...   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
 Option Type = 0x07
 Option Length = 4 + NLA length
 DLR's NLA
    The DLR's own unicast network address to which recipients of the
    redirecting POLR may direct subsequent NAKs.
 OPT_REDIRECT is network-significant.

9.6. OPT_SYN - Synchronization Option

 The SYN option indicates the starting data packet for a session.  It
 must only appear in ODATA or RDATA packets.
 The SYN option MAY be used to provide a useful abstraction to
 applications that can simplify application design by providing stream
 start notification.  It MAY also be used to let a late joiner to a
 session know that it is indeed late (i.e. it would not see the SYN
 option).

9.6.1. OPT_SYN - Procedures - Receivers

 Procedures for receivers are implementation dependent.  A receiver
 MAY use the SYN to provide its applications with abstractions of the
 data stream.

Speakman, et. al. Experimental [Page 52] RFC 3208 PGM Reliable Transport Protocol December 2001

9.6.2. OPT_SYN - Procedures - Sources

 Sources MAY include OPT_SYN in the first data for a session.  That
 is, they MAY include the option in:
    the first ODATA sent on a session by a PGM source
    any RDATA sent as a result of loss of this ODATA packet
    all FEC packets for the first transmission group; in this case it
    is interpreted as the first packet having the SYN

9.6.3. OPT_SYN - Procedures - DLRs

    In an identical manner to sources, DLRs MUST provide OPT_SYN in
    any retransmitted data that is at the start of a session.

9.6.4. OPT_SYN - Packet Extension Format

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |E| Option Type | Option Length |Reserved |F|OPX|U|             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Option Type = 0x0D
    Option Length = 4
    OPT_SYN is NOT network-significant.

9.7. OPT_FIN - Session Finish Option

    This FIN option indicates the last data packet for a session and
    an orderly close down.
    The FIN option MAY be used to provide an abstraction to
    applications that can simplify application design by providing
    stream end notification.
    This option MAY be present in the last data packet or transmission
    group for a session.  The FIN PGM option MUST appear in every SPM
    sent after the last ODATA for a session.  The SPM_LEAD sequence
    number in an SPM with the FIN option indicates the last known data
    successfully transmitted for the session.

Speakman, et. al. Experimental [Page 53] RFC 3208 PGM Reliable Transport Protocol December 2001

9.7.1. OPT_FIN - Procedures - Receivers

    A receiver SHOULD use receipt of a FIN to let it know that it can
    tear down its data structures for the said session once a suitable
    time period has expired (TXW_SECS).  It MAY still try to solicit
    retransmissions within the existing transmit window.
    Other than this, procedures for receivers are implementation
    dependent.  A receiver MAY use the FIN to provide its applications
    with abstractions of the data stream and to inform its
    applications that the session is ending.
    9.7.2.  OPT_FIN - Procedures - Sources
    Sources MUST include OPT_FIN in every SPM sent after it has been
    determined that the application has closed gracefully.  If a
    source is aware at the time of transmission that it is ending a
    session the source MAY include OPT_FIN in,
    the last ODATA
    any associated RDATAs for the last data
    FEC packets for the last transmission group; in this case it is
    interpreted as the last packet having the FIN
 When a source detects that it needs to send an OPT_FIN it SHOULD
 immediately send it.  This is done either by appending it to the last
 data packet or transmission group or by immediately sending an SPM
 and resetting the SPM heartbeat timer (i.e. it does not wait for a
 timer to expire before sending the SPM).  After sending an OPT_FIN,
 the session SHOULD not close and stop sending SPMs until after a time
 period equal to TXW_SECS.

9.7.3. OPT_FIN - Procedures - DLRs

 In an identical manner to sources, DLRs MUST provide OPT_FIN in any
 retransmitted data that is at the end of a session.

Speakman, et. al. Experimental [Page 54] RFC 3208 PGM Reliable Transport Protocol December 2001

9.7.4. OPT_FIN - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x0E
 Option Length = 4
 OPT_FIN is NOT network-significant.

9.8. OPT_RST - Session Reset Option

 The RST option MAY appear in every SPM sent after an unrecoverable
 error is identified by the source.  This acts to notify the receivers
 that the session is being aborted.  This option MAY appear only in
 SPMs.  The SPM_LEAD sequence number in an SPM with the RST option
 indicates the last known data successfully transmitted for the
 session.

9.8.1. OPT_RST - Procedures - Receivers

 Receivers SHOULD treat the reception of OPT_RST in an SPM as an abort
 of the session.
 A receiver that receives an SPM with an OPT_RST with the N bit set
 SHOULD not send any more NAKs for the said session towards the
 source.  If the N bit (see 9.8.5) is not set, the receiver MAY
 continue to try to solicit retransmit data within the current
 transmit window.

9.8.2. OPT_RST - Procedures - Sources

 Sources SHOULD include OPT_RST in every SPM sent after it has been
 determined that an unrecoverable error condition has occurred.  The N
 bit of the OPT_RST SHOULD only be sent if the source has determined
 that it cannot process NAKs for the session.  The cause of the
 OPT_RST is set to an implementation specific value.  If the error
 code is unknown, then the value of 0x00 is used.  When a source
 detects that it needs to send an OPT_RST it SHOULD immediately send
 it.  This is done by immediately sending an SPM and resetting the SPM
 heartbeat timer (i.e. it does not wait for a timer to expire before
 sending the SPM).  After sending an OPT_RST, the session SHOULD not
 close and stop sending SPMs until after a time period equal to
 TXW_SECS.

Speakman, et. al. Experimental [Page 55] RFC 3208 PGM Reliable Transport Protocol December 2001

9.8.3. OPT_RST - Procedures - DLRs

 None.

9.8.4. OPT_RST - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|N|Error Code |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x0F
 Option Length = 4
 N bit
    The N bit is set to 1 to indicate that NAKs for previous ODATA
    will go unanswered from the source.  The application will tell the
    source to turn this bit on or off.
 Error Code
    The 6 bit error code field is used to forward an error code down
    to the receivers from the source.
    The value of 0x00 indicates an unknown reset reason.  Any other
    value indicates the application purposely aborted and gave a
    reason (the error code value) that may have meaning to the end
    receiver application.  These values are entirely application
    dependent.
 OPT_RST is NOT network-significant.

10. Security Considerations

 In addition to the usual problems of end-to-end authentication, PGM
 is vulnerable to a number of security risks that are specific to the
 mechanisms it uses to establish source path state, to establish
 repair state, to forward NAKs, to identify DLRs, and to distribute
 repairs.  These mechanisms expose PGM network elements themselves to
 security risks since network elements not only switch but also
 interpret SPMs, NAKs, NCFs, and RDATA, all of which may legitimately
 be transmitted by PGM sources, receivers, and DLRs.  Short of full
 authentication of all neighboring sources, receivers, DLRs, and
 network elements, the protocol is not impervious to abuse.

Speakman, et. al. Experimental [Page 56] RFC 3208 PGM Reliable Transport Protocol December 2001

 So putting aside the problems of rogue PGM network elements for the
 moment, there are enough potential security risks to network elements
 associated with sources, receivers, and DLRs alone.  These risks
 include denial of service through the exhausting of both CPU
 bandwidth and memory, as well as loss of (repair) data connectivity
 through the muddling of repair state.
 False SPMs may cause PGM network elements to mis-direct NAKs intended
 for the legitimate source with the result that the requested RDATA
 would not be forthcoming.
 False NAKs may cause PGM network elements to establish spurious
 repair state that will expire only upon time-out and could lead to
 memory exhaustion in the meantime.
 False NCFs may cause PGM network elements to suspend NAK forwarding
 prematurely (or to mis-direct NAKs in the case of redirecting POLRs)
 resulting eventually in loss of RDATA.
 False RDATA may cause PGM network elements to tear down legitimate
 repair state resulting eventually in loss of legitimate RDATA.
 The development of precautions for network elements to protect
 themselves against incidental or unsophisticated versions of these
 attacks is work outside of this spec and includes:
    Damping of jitter in the value of either the network-header source
    address of SPMs or the path NLA in SPMs.  While the network-header
    source address is expected to change seldom, the path NLA is
    expected to change occasionally as a consequence of changes in
    underlying multicast routing information.
 The extension of NAK shedding procedures to control the volume, not
 just the rate, of confirmed NAKs.  In either case, these procedures
 assist network elements in surviving NAK attacks at the expense of
 maintaining service.  More efficiently, network elements may use the
 knowledge of TSIs and their associated transmit windows gleaned from
 SPMs to control the proliferation of repair state.
 A three-way handshake between network elements and DLRs that would
 permit a network element to ascertain with greater confidence that an
 alleged DLR is identified by the alleged network-header source
 address, and is PGM conversant.

Speakman, et. al. Experimental [Page 57] RFC 3208 PGM Reliable Transport Protocol December 2001

11. Appendix A - Forward Error Correction

11.1. Introduction

 The following procedures incorporate packet-level Reed Solomon
 Erasure correcting techniques as described in [11] and [12] into PGM.
 This approach to Forward Error Correction (FEC) is based upon the
 computation of h parity packets from k data packets for a total of n
 packets such that a receiver can reconstruct the k data packets out
 of any k of the n packets.  The original k data packets are referred
 to as the Transmission Group, and the total n packets as the FEC
 Block.
 These procedures permit any combination of pro-active FEC or on-
 demand FEC with conventional ARQ (selective retransmission) within a
 given TSI to provide any flavor of layered or integrated FEC.  The
 two approaches can be used by the same or different receivers in a
 single transport session without conflict.  Once provided by a
 source, the actual use of FEC or selective retransmission for loss
 recovery in the session is entirely at the discretion of the
 receivers.  Note however that receivers SHOULD NOT ask for selective
 retransmissions when FEC is available, nevertheless sources MUST
 provide selective retransmissions in response to selective NAKs from
 the leading partial transmission group (i.e. the most recent
 transmission group, which is not yet full).  For any group that is
 full, the source SHOULD provide FEC on demand in response to a
 selective NAK.
 Pro-active FEC refers to the technique of computing parity packets at
 transmission time and transmitting them as a matter of course
 following the data packets.  Pro-active FEC is RECOMMENDED for
 providing loss recovery over simplex or asymmetric multicast channels
 over which returning repair requests is either impossible or costly.
 It provides increased reliability at the expense of bandwidth.
 On-demand FEC refers to the technique of computing parity packets at
 repair time and transmitting them only upon demand (i.e., receiver-
 based loss detection and repair request).  On-demand FEC is
 RECOMMENDED for providing loss recovery of uncorrelated loss in very
 large receiver populations in which the probability of any single
 packet being lost is substantial.  It provides equivalent reliability
 to selective NAKs (ARQ) at no more and typically less expense of
 bandwidth.
 Selective NAKs are NAKs that request the retransmission of specific
 packets by sequence number corresponding to the sequence number of
 any data packets detected to be missing from the expected sequence
 (conventional ARQ).  Selective NAKs can be used for recovering losses

Speakman, et. al. Experimental [Page 58] RFC 3208 PGM Reliable Transport Protocol December 2001

 occurring in leading partial transmission groups, i.e. in the most
 recent transmission group, which is not yet full.  The RECOMMENDED
 way of handling partial transmission groups, however, is for the data
 source to use variable-size transmission groups (see below).
 Parity NAKs are NAKs that request the transmission of a specific
 number of parity packets by count corresponding to the count of the
 number of data packets detected to be missing from a group of k data
 packets (on-demand FEC).
 The objective of these procedures is to incorporate these FEC
 techniques into PGM so that:
    sources MAY provide parity packets either pro-actively or on-
    demand, interchangeably within the same TSI,
    receivers MAY use either selective or parity NAKs interchangeably
    within the same TSI (however, in a session where on-demand parity
    is available receivers SHOULD only use parity NAKs).
    network elements maintain repair state based on either selective
    or parity NAKs in the same data structure, altering only search,
    RDATA constraint, and deletion algorithms in either case,
    and only OPTION additions to the basic packet formats are
    REQUIRED.

11.2. Overview

 Advertising FEC parameters in the transport session
 Sources add OPT_PARITY_PRM to SPMs to provide session-specific
 parameters such as the number of packets (TGSIZE == k) in a
 transmission group.  This option lets receivers know how many packets
 there are in a transmission group, and it lets network elements sort
 repair state by transmission group number.  This option includes an
 indication of whether pro-active and/or on-demand parity is available
 from the source.
 Distinguishing parity packets from data packets
 Sources send pro-active parity packets as ODATA (NEs do not forward
 RDATA unless a repair state is present) and on-demand parity packets
 as RDATA.  A source MUST add OPT_PARITY to the ODATA/RDATA packet
 header of parity packets to permit network elements and receivers to
 distinguish them from data packets.

Speakman, et. al. Experimental [Page 59] RFC 3208 PGM Reliable Transport Protocol December 2001

 Data and parity packet numbering
 Parity packets MUST be calculated over a fixed number k of data
 packets known as the Transmission Group.  Grouping of packets into
 transmission groups effectively partitions a packet sequence number
 into a high-order portion (TG_SQN) specifying the transmission group
 (TG), and a low-order portion (PKT_SQN) specifying the packet number
 (PKT-NUM in the range 0 through k-1) within that group.  From an
 implementation point of view, it's handy if k, the TG size, is a
 power of 2.  If so, then TG_SQN and PKT_SQN can be mapped side-by-
 side into the 32 bit SQN.  log2(TGSIZE) is then the size in bits of
 PKT_SQN.
 This mapping does not reduce the effective sequence number space
 since parity packets marked with OPT_PARITY allow the sequence space
 (PKT_SQN) to be completely reused in order to number the h parity
 packets, as long as h is not greater than k.
 In the case where h is greater than k, a source MUST add
 OPT_PARITY_GRP to any parity packet numbered j greater than k-1,
 specifying the number m of the group of k parity packets to which the
 packet belongs, where m is just the quotient from the integer
 division of j by k.  Correspondingly, PKT-NUM for such parity packets
 is just j modulo k.  In other words, when a source needs to generate
 more parity packets than there were original data packets (perhaps
 because of a particularly lossy line such that a receiver lost not
 only the original data but some of the parity RDATA as well), use the
 OPT_PARITY_GRP option in order to number and identify the
 transmission group of the extra packets that would exceed the normal
 sequential number space.
 Note that parity NAKs (and consequently their corresponding parity
 NCFs) MUST also contain the OPT_PARITY flag in the options field of
 the fixed header, and that in these packets, PKT_SQN MUST contain
 PKT_CNT, the number of missing packets, rather than PKT_NUM, the SQN
 of a specific missing packet.  More on all this later.
 Variable Transmission Group Size
 The transmission group size advertised in the OPT_PARITY_PRM option
 on SPMs MUST be a power of 2 and constant for the duration of the
 session.  However, the actual transmission group size used MAY not be
 constant for the duration of the session, and MAY not be a power of
 2.  When a TG size different from the one advertised in
 OPT_PARITY_PRM is used, the TG size advertised in OPT_PARITY_PRM MUST
 be interpreted as specifying the maximum effective size of the TG.

Speakman, et. al. Experimental [Page 60] RFC 3208 PGM Reliable Transport Protocol December 2001

 When the actual TG size is not a power of 2 or is smaller than the
 max TG size, there will be sparse utilization of the sequence number
 space since some of the sequence numbers that would have been
 consumed in numbering a maximum sized TG will not be assigned to
 packets in the smaller TG.  The start of the next transmission group
 will always begin on the boundary of the maximum TG size as though
 each of the sequence numbers had been utilized.
 When the source decides to use a smaller group size than that
 advertised in OPT_PARITY_PRM, it appends OPT_CURR_TGSIZE to the last
 data packet (ODATA) in the truncated transmission group.  This lets
 the receiver know that it should not expect any more packets in this
 transmission group, and that it may start requesting repairs for any
 missing packets.  If the last data packet itself went missing, the
 receiver will detect the end of the group when it receives a parity
 packet for the group, an SPM with SPM_LEAD equal to OD_SQN of the
 last data packet, or the first packet of the next group, whichever
 comes first.  In addition, any parity packet from this TG will also
 carry the OPT_CURR_TGSIZE option as will any SPM sent with SPM_LEAD
 equal to OD_SQN of the last data packet.
 Variable TSDU length
 If a non constant TSDU length is used within a given transmission
 group, the size of parity packets in the corresponding FEC block MUST
 be equal to the size of the largest original data packet in the
 block.  Parity packets MUST be computed by padding the original
 packets with zeros up to the size of the largest data packet.  Note
 that original data packets are transmitted without padding.
 Receivers using a combination of original packets and FEC packets to
 rebuild missing packets MUST pad the original packets in the same way
 as the source does.  The receiver MUST then feed the padded original
 packets plus the parity packets to the FEC decoder.  The decoder
 produces the original packets padded with zeros up to the size of the
 largest original packet in the group.  In order for the receiver to
 eliminate the padding on the reconstructed data packets, the original
 size of the packet MUST be known, and this is accomplished as
 follows:
    The source, along with the packet payloads, encodes the TSDU
    length and appends the 2-byte encoded length to the padded FEC
    packets.
    Receivers pad the original packets that they received to the
    largest original packet size and then append the TSDU length to
    the padded packets.  They then pass them and the FEC packets to
    the FEC decoder.

Speakman, et. al. Experimental [Page 61] RFC 3208 PGM Reliable Transport Protocol December 2001

    The decoder produces padded original packets with their original
    TSDU length appended.  Receivers MUST now use this length to get
    rid of the padding.
 A source that transmits variable size packets MUST take into account
 the fact that FEC packets will have a size equal to the maximum size
 of the original packets plus the size of the length field (2 bytes).
 If a fixed packet size is used within a transmission group, the
 encoded length is not appended to the parity packets.  The presence
 of the fixed header option flag OPT_VAR_PKTLEN in parity packets
 allows receivers to distinguish between transmission groups with
 variable sized packets and fixed-size ones, and behave accordingly.
 Payload-specific options
 Some options present in data packet (ODATA and RDATA) are strictly
 associated with the packet content (PGM payload), OPT_FRAGMENT being
 an example.  These options must be preserved even when the data
 packet that would normally contain them is not received, but its the
 payload is recovered though the use of FEC.
 To achieve this, PGM encodes the content of these options in special
 options that are inserted in parity packets.  Two flags present in
 the the option common-header are used for this process:  bit F
 (OP_ENCODED) and bit U (OP_ENCODED_NULL).
 Whenever at least one of the original packets of a TG contains a
 payload-specific option of a given type, the source MUST include an
 encoded version of that option type in all the parity packets it
 transmits.  The encoded option is computed by applying FEC encoding
 to the whole option with the exception of the first three bytes of
 the option common-header (E, Option Type, Option Length, OP_ENCODED
 and OPX fields).  The type, length and OPX of the encoded option are
 the same as the type, length and OPX in the original options.
 OP_ENCODED is set to 1 (all original option have OP_ENCODED = 0).
 The encoding is performed using the same process that is used to
 compute the payload of the parity packet. i.e. the FEC encoder is fed
 with one copy of that option type for each original packet in the TG.
 If one (or more) original packet of the TG does not contain that
 option type, an all zeroes option is used for the encoding process.
 To be able to distinguish this "dummy" option from valid options with
 all-zeroes payload, OP_ENCODED_NULL is used.  OP_ENCODED_NULL is set
 to 0 in all the original options, but the value of 1 is used in the
 encoding process if the option did not exist in the original packet.
 On the receiver side, all option with OP_ENCODED_NULL equal to 1 are
 discarded after decoding.

Speakman, et. al. Experimental [Page 62] RFC 3208 PGM Reliable Transport Protocol December 2001

 When a receiver recovers a missing packet using FEC repair packets,
 it MUST also recover payload-specific options, if any.  The presence
 of these can be unequivocally detected through the presence of
 encoded options in parity packets (encoded options have OP_ENCODED
 set to 1).  Receivers apply FEC-recovery to encoded options and
 possibly original options, as they do to recover packet payloads.
 The FEC decoding is applied to the whole option with the exception of
 the first three bytes of the option common-header (E, Option Type,
 Option Length, OP_ENCODED and OPX fields).  Each decoded option is
 associated with the relative payload, unless OP_ENCODED_NULL turns
 out to be 1, in which case the decoded option is discarded.
 The decoding MUST be performed using the 1st occurrence of a given
 option type in original/parity packets.  If one or more original
 packets do not contain that option type, an option of the same type
 with zero value must be used.  This option MUST have OP_ENCODED_NULL
 equal to 1.

11.3. Packet Contents

 This section just provides enough short-hand to make the Procedures
 intelligible.  For the full details of packet contents, please refer
 to Packet Formats below.
 OPT_PARITY        indicated in pro-active (ODATA) and on-demand
                   (RDATA) parity packets to distinguish them from
                   data packets.  This option is directly encoded in
                   the "Option" field of the fixed PGM header
 OPT_VAR_PKTLEN    MAY be present in pro-active (ODATA) and on-demand
                   (RDATA) parity packets to indicate that the
                   corresponding transmission group is composed of
                   variable size data packets.  This option is
                   directly encoded in the "Option" field of the fixed
                   PGM header
 OPT_PARITY_PRM    appended by sources to SPMs to specify session-
                   specific parameters such as the transmission group
                   size and the availability of pro-active and/or on-
                   demand parity from the source
 OPT_PARITY_GRP    the number of the group (greater than 0) of h
                   parity packets to which the parity packet belongs
                   when more than k parity packets are provided by the
                   source

Speakman, et. al. Experimental [Page 63] RFC 3208 PGM Reliable Transport Protocol December 2001

 OPT_CURR_TGSIZE   appended by sources to the last data packet and any
                   parity packets in a variable sized transmission
                   group to indicate to the receiver the actual size
                   of a transmission group.  May also be appended to
                   certain SPMs

11.3.1. Parity NAKs

 NAK_TG_SQN        the high-order portion of NAK_SQN specifying the
                   transmission group for which parity packets are
                   requested
 NAK_PKT_CNT       the low-order portion of NAK_SQN specifying the
                   number of missing data packets for which parity
                   packets are requested
    Nota Bene: NAK_PKT_CNT (and NCF_PKT_CNT) are 0-based counters,
    meaning that NAK_PKT_CNT = 0 means that 1 FEC RDATA is being
    requested, and in general NAK_PKT_CNT = k - 1 means that  k FEC
    RDATA are being requested.

11.3.2. Parity NCFs

 NCF_TG_SQN        the high-order portion of NCF_SQN specifying the
                   transmission group for which parity packets were
                   requested
 NCF_PKT_CNT       the low-order portion of NCF_SQN specifying the
                   number of missing data packets for which parity
                   packets were requested
    Nota Bene: NCF_PKT_CNT (and NAK_PKT_CNT) are 0-based counters,
    meaning that NAK_PKT_CNT = 0 means that 1 FEC RDATA is being
    requested, and in general NAK_PKT_CNT = k - 1 means that  k FEC
    RDATA are being requested.

11.3.3. On-demand Parity

 RDATA_TG_SQN      the high-order portion of RDATA_SQN specifying the
                   transmission group to which the parity packet
                   belongs
 RDATA_PKT_SQN     the low-order portion of RDATA_SQN specifying the
                   parity packet sequence number within the
                   transmission group

Speakman, et. al. Experimental [Page 64] RFC 3208 PGM Reliable Transport Protocol December 2001

11.3.4. Pro-active Parity

 ODATA_TG_SQN      the high-order portion of ODATA_SQN specifying the
                   transmission group to which the parity packet
                   belongs
 ODATA_PKT_SQN     the low-order portion of ODATA_SQN specifying the
                   parity packet sequence number within the
                   transmission group

11.4. Procedures - Sources

 If a source elects to provide parity for a given transport session,
 it MUST first provide the transmission group size PARITY_PRM_TGS in
 the OPT_PARITY_PRM option of its SPMs.  This becomes the maximum
 effective transmission group size in the event that the source elects
 to send smaller size transmission groups.  If a source elects to
 provide proactive parity for a given transport session, it MUST set
 PARITY_PRM_PRO in the OPT_PARITY_PRM option of its SPMs.  If a source
 elects to provide on-demand parity for a given transport session, it
 MUST set PARITY_PRM_OND in the OPT_PARITY_PRM option of its SPMs.
 A source MUST send any pro-active parity packets for a given
 transmission group only after it has first sent all of the
 corresponding k data packets in that group.  Pro-active parity
 packets MUST be sent as ODATA with OPT_PARITY in the fixed header.
 If a source elects to provide on-demand parity, it MUST respond to a
 parity NAK for a transmission group with a parity NCF.  The source
 MUST complete the transmission of the k original data packets and the
 proactive parity packets, possibly scheduled, before starting the
 transmission of on-demand parity packets.  Subsequently, the source
 MUST send the number of parity packets requested by that parity NAK.
 On-demand parity packets MUST be sent as RDATA with OPT_PARITY in the
 fixed header.  Previously transmitted pro-active parity packets
 cannot be reused as on-demand parity packets, these MUST be computed
 with new, previously unused, indexes.
 In either case, the source MUST provide selective retransmissions
 only in response to selective NAKs from the leading partial
 transmission group.  For any group that is full, the source SHOULD
 provide FEC on demand in response to a selective retransmission
 request.
 In the absence of data to transmit, a source SHOULD prematurely
 terminate the current transmission group by including OPT_CURR_TGSIZE
 to the last data packet or to any proactive parity packets provided.

Speakman, et. al. Experimental [Page 65] RFC 3208 PGM Reliable Transport Protocol December 2001

 If the last data packet has already been transmitted and there is no
 provision for sending proactive parity packets, an SPM with
 OPT_CURR_TGSIZE SHOULD be sent.
 A source consolidates requests for on-demand parity in the same
 transmission group according to the following procedures.  If the
 number of pending (i.e., unsent) parity packets from a previous
 request for on-demand parity packets is equal to or greater than
 NAK_PKT_CNT in a subsequent NAK, that subsequent NAK MUST be
 confirmed but MAY otherwise be ignored.  If the number of pending
 (i.e., unsent) parity packets from a previous request for on-demand
 parity packets is less than NAK_PKT_CNT in a subsequent NAK, that
 subsequent NAK MUST be confirmed but the source need only increase
 the number of pending parity packets to NAK_PKT_CNT.
 When a source provides parity packets relative to a transmission
 group with variable sized packets, it MUST compute parity packets by
 padding the smaller original packets with zeroes out to the size of
 the largest of the original packets.  The source MUST also append the
 encoded TSDU lengths at the end of any padding or directly to the end
 of the largest packet, and add the OPT_VAR_PKTLEN option as specified
 in the overview description.
 When a source provides variable sized transmission groups, it SHOULD
 append the OPT_CURR_TGSIZE option to the last data packet in the
 shortened group, and it MUST append the OPT_CURR_TGSIZE option to any
 parity packets it sends within that group.  In case the the last data
 packet is sent before a determination has been made to shorten the
 group and there is no provision for sending proactive parity packets,
 an SPM with OPT_CURR_TGSIZE SHOULD be sent.  The source MUST also add
 OPT_CURR_TGSIZE to any SPM that it sends with SPM_LEAD equal to
 OD_SQN of the last data packet.
 A receiver MUST NAK for the entire number of packets missing based on
 the maximum TG size, even if it already knows that the actual TG size
 is smaller.  The source MUST take this into account and compute the
 number of packets effectively needed as the difference between
 NAK_PKT_CNT and an offset computed as the difference between the max
 TG size and the effective TG size.

11.5. Procedures - Receivers

 If a receiver elects to make use of parity packets for loss recovery,
 it MUST first learn the transmission group size PARITY_PRM_TGS from
 OPT_PARITY_PRM in the SPMs for the TSI.  The transmission group size
 is used by a receiver to determine the sequence number boundaries
 between transmission groups.

Speakman, et. al. Experimental [Page 66] RFC 3208 PGM Reliable Transport Protocol December 2001

 Thereafter, if PARITY_PRM_PRO is also set in the SPMs for the TSI, a
 receiver SHOULD use any pro-active parity packets it receives for
 loss recovery, and if PARITY_PRM_OND is also set in the SPMs for the
 TSI, it MAY solicit on-demand parity packets upon loss detection.  If
 PARITY_PRM_OND is set, a receiver MUST NOT send selective NAKs,
 except in partial transmission groups if the source does not use the
 variable transmission-group size option.  Parity packets are ODATA
 (pro-active) or RDATA (on-demand) packets distinguished by OPT_PARITY
 which lets receivers know that ODATA/RDATA_TG_SQN identifies the
 group of PARITY_PRM_TGS packets to which the parity may be applied
 for loss recovery in the corresponding transmission group, and that
 ODATA/RDATA_PKT_SQN is being reused to number the parity packets
 within that group.  Receivers order parity packets and eliminate
 duplicates within a transmission group based on ODATA/RDATA_PKT_SQN
 and on OPT_PARITY_GRP if present.
 To solicit on-demand parity packets, a receiver MUST send parity NAKs
 upon loss detection.  For the purposes of soliciting on-demand
 parity, loss detection occurs at transmission group boundaries, i.e.
 upon receipt of the last data packet in a transmission group, upon
 receipt of any data packet in any subsequent transmission group, or
 upon receipt of any parity packet in the current or a subsequent
 transmission group.
 A parity NAK is simply a NAK with OPT_PARITY and NAK_PKT_CNT set to
 the count of the number of packets detected to be missing from the
 transmission group specified by NAK_TG_SQN.  Note that this
 constrains the receiver to request no more parity packets than there
 are data packets in the transmission group.
 A receiver SHOULD bias the value of NAK_BO_IVL for parity NAKs
 inversely proportional to NAK_PKT_CNT so that NAKs for larger losses
 are likely to be scheduled ahead of NAKs for smaller losses in the
 same receiver population.
 A confirming NCF for a parity NAK is a parity NCF with NCF_PKT_CNT
 equal to or greater than that specified by the parity NAK.
 A receiver's NAK_RDATA_IVL timer is not cancelled until all requested
 parity packets have been received.
 In the absence of data (detected from SPMs bearing SPM_LEAD equal to
 RXW_LEAD) on non-transmission-group boundaries, receivers MAY resort
 to selective NAKs for any missing packets in that partial
 transmission group.

Speakman, et. al. Experimental [Page 67] RFC 3208 PGM Reliable Transport Protocol December 2001

 When a receiver handles parity packets belonging to a transmission
 group with variable sized packets, (detected from the presence of the
 OPT_VAR_PKTLEN option in the parity packets), it MUST decode them as
 specified in the overview description and use the decoded TSDU length
 to get rid of the padding in the decoded packet.
 If the source was using a variable sized transmission group via the
 OPT_CURR_TGSIZE, the receiver might learn this before having
 requested (and received) any retransmission.  The above happens if it
 sees OPT_CURR_TGSIZE in the last data packet of the TG, in any
 proactive parity packet or in a SPM.  If the receivers learns this
 and determines that it has missed one or more packets in the
 shortened transmission group, it MAY then NAK for them without
 waiting for the start of the next transmission group.  Otherwise it
 will start NAKing at the start of the next transmission group.
 In both cases, the receiver MUST NAK for the number of packets
 missing assuming that the size of the transmission group is the
 maximum effective transmission group.  In other words, the receivers
 cannot exploit the fact that it might already know that the
 transmission group was smaller but MUST always NAK for the number of
 packets it believes are missing, plus the number of packets required
 to bring the total packets up to the maximum effective transmission
 group size.
 After the first parity packet has been delivered to the receiver, the
 actual TG size is known to him, either because already known or
 because discovered via OPT_CURR_TGSIZE contained in the parity
 packet.  Hence the receiver can decode the whole group as soon as the
 minimum number of parity packets needed is received.

11.6. Procedures - Network Elements

 Pro-active parity packets (ODATA with OPT_PARITY) are switched by
 network elements without transport-layer intervention.
 On-demand parity packets (RDATA with OPT_PARITY) necessitate modified
 request, confirmation and repair constraint procedures for network
 elements.  In the context of these procedures, repair state is
 maintained per NAK_TSI and NAK_TG_SQN, and in addition to recording
 the interfaces on which corresponding NAKs have been received,
 records the largest value of NAK_PKT_CNT seen in corresponding NAKs
 on each interface.  This value is referred to as the known packet
 count.  The largest of the known packet counts recorded for any
 interface in the repair state for the transmit group or carried by an
 NCF is referred to as the largest known packet count.

Speakman, et. al. Experimental [Page 68] RFC 3208 PGM Reliable Transport Protocol December 2001

 Upon receipt of a parity NAK, a network element responds with the
 corresponding parity NCF.  The corresponding parity NCF is just an
 NCF formed in the usual way (i.e., a multicast copy of the NAK with
 the packet type changed), but with the addition of OPT_PARITY and
 with NCF_PKT_CNT set to the larger of NAK_PKT_CNT and the known
 packet count for the receiving interface.  The network element then
 creates repair state in the usual way with the following
 modifications.
 If repair state for the receiving interface does not exist, the
 network element MUST create it and additionally record NAK_PKT_CNT
 from the parity NAK as the known packet count for the receiving
 interface.
 If repair state for the receiving interface already exists, the
 network element MUST eliminate the NAK only if NAK_ELIM_IVL has not
 expired and NAK_PKT_CNT is equal to or less than the largest known
 packet count.  If NAK_PKT_CNT is greater than the known packet count
 for the receiving interface, the network element MUST update the
 latter with the larger NAK_PKT_CNT.
 Upon either adding a new interface or updating the known packet count
 for an existing interface, the network element MUST determine if
 NAK_PKT_CNT is greater than the largest known packet count.  If so or
 if NAK_ELIM_IVL has expired, the network element MUST forward the
 parity NAK in the usual way with a value of NAK_PKT_CNT equal to the
 largest known packet count.
 Upon receipt of an on-demand parity packet, a network element MUST
 locate existing repair state for the corresponding RDATA_TSI and
 RDATA_TG_SQN.  If no such repair state exists, the network element
 MUST discard the RDATA as usual.
 If corresponding repair state exists, the largest known packet count
 MUST be decremented by one, then the network element MUST forward the
 RDATA on all interfaces in the existing repair state, and decrement
 the known packet count by one for each.  Any interfaces whose known
 packet count is thereby reduced to zero MUST be deleted from the
 repair state.  If the number of interfaces is thereby reduced to
 zero, the repair state itself MUST be deleted.
 Upon reception of a parity NCF, network elements MUST cancel pending
 NAK retransmission only if NCF_PKT_CNT is greater or equal to the
 largest known packet count.  Network elements MUST use parity NCFs to
 anticipate NAKs in the usual way with the addition of recording
 NCF_PKT_CNT from the parity NCF as the largest known packet count
 with the anticipated state so that any subsequent NAKs received with
 NAK_PKT_CNT equal to or less than NCF_PKT_CNT will be eliminated, and

Speakman, et. al. Experimental [Page 69] RFC 3208 PGM Reliable Transport Protocol December 2001

 any with NAK_PKT_CNT greater than NCF_PKT_CNT will be forwarded.
 Network elements which receive  a parity NCF with NCF_PKT_CNT larger
 than the largest known packet count MUST also use it to anticipate
 NAKs, increasing the largest known packet count to reflect
 NCF_PKT_CNT (partial anticipation).
 Parity NNAKs follow the usual elimination procedures with the
 exception that NNAKs are eliminated only if existing NAK state has a
 NAK_PKT_CNT greater than NNAK_PKT_CNT.
 Network elements must take extra precaution when the source is using
 a variable sized transmission group.  Network elements learn that the
 source is using a TG size smaller than the maximum from
 OPT_CURR_TGSIZE in parity RDATAs or in SPMs.  When this happens, they
 compute a TG size offset as the difference between the maximum TG
 size and the actual TG size advertised by OPT_CURR_TGSIZE.  Upon
 reception of parity RDATA, the TG size offset is used to update the
 repair state as follows:
    Any interface whose known packet count is reduced to the TG size
    offset is deleted from the repair state.
 This replaces the normal rule for deleting interfaces that applies
 when the TG size is equal to the maximum TG size.

11.7. Procedures - DLRs

 A DLR with the ability to provide FEC repairs MUST indicate this by
 setting the OPT_PARITY bit in the redirecting POLR.  It MUST then
 process any redirected FEC NAKs in the usual way.

11.8. Packet Formats

11.8.1. OPT_PARITY_PRM - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|         |P O|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Transmission Group Size                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x08
 Option Length = 8 octets
 P-bit (PARITY_PRM_PRO)

Speakman, et. al. Experimental [Page 70] RFC 3208 PGM Reliable Transport Protocol December 2001

    Indicates when set that the source is providing pro-active parity
    packets.
 O-bit (PARITY_PRM_OND)
    Indicates when set that the source is providing on-demand parity
    packets.
 At least one of PARITY_PRM_PRO and PARITY_PRM_OND MUST be set.
 Transmission Group Size (PARITY_PRM_TGS)
    The number of data packets in the transmission group over which
    the parity packets are calculated.  If a variable transmission
    group size is being used, then this becomes the maximum effective
    transmission group size across the session.
 OPT_PARITY_PRM MAY be appended only to SPMs.
 OPT_PARITY_PRM is network-significant.

11.8.2. OPT_PARITY_GRP - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Parity Group Number                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x09
 Option Length = 8 octets
 Parity Group Number (PRM_GROUP)
    The number of the group of k parity packets amongst the h parity
    packets within the transmission group to which the parity packet
    belongs, where the first k parity packets are in group zero.
    PRM_GROUP MUST NOT be zero.
 OPT_PARITY_GRP MAY be appended only to parity packets.
 OPT_PARITY_GRP is NOT network-significant.

Speakman, et. al. Experimental [Page 71] RFC 3208 PGM Reliable Transport Protocol December 2001

11.8.3. OPT_CURR_TGSIZE - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Actual Transmission Group Size                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x0A
 Option Length = 8 octets
 Actual Transmission Group Size (PRM_ATGSIZE)
    The actual number of data packets in this transmission group.
    This MUST be less than or equal to the maximum transmission group
    size PARITY_PRM_TGS in OPT_PARITY_PRM.
 OPT_CURR_TGSIZE MAY be appended to data and parity packets (ODATA or
 RDATA) and to SPMs.
 OPT_CURR_TGSIZE is network-significant except when appended to ODATA.

12. Appendix B - Support for Congestion Control

12.1. Introduction

 A source MUST implement strategies for congestion avoidance, aimed at
 providing overall network stability, fairness among competing PGM
 flows, and some degree of fairness towards coexisting TCP flows [13].
 In order to do this, the source must be provided with feedback on the
 status of the network in terms of traffic load.  This appendix
 specifies NE procedures that provide such feedback to the source in a
 scalable way.  (An alternative TCP-friendly scheme for congestion
 control that does not require NE support can be found in [16]).
 The procedures specified in this section enable the collection and
 selective forwarding of three types of feedback to the source:
    o Worst link load as measured in network elements.
    o Worst end-to-end path load as measured in network elements.
    o Worst end-to-end path load as reported by receivers.

Speakman, et. al. Experimental [Page 72] RFC 3208 PGM Reliable Transport Protocol December 2001

 This specification defines in detail NE procedures, receivers
 procedures and packet formats.  It also defines basic procedures in
 receivers for generating congestion reports.  This specification does
 not define the procedures used by PGM sources to adapt their
 transmission rates in response of congestion reports.  Those
 procedures depend upon the specific congestion control scheme.
 PGM defines a header option that PGM receivers may append to NAKs
 (OPT_CR).  OPT_CR carries congestion reports in NAKs that propagate
 upstream towards the source.
 During the process of hop-by-hop reverse NAK forwarding, NEs examine
 OPT_CR and possibly modify its contents prior to forwarding the NAK
 upstream.  Forwarding CRs also has the side effect of creating
 congestion report state in the NE.  The presence of OPT_CR and its
 contents also influences the normal NAK suppression rules.  Both the
 modification performed on the congestion report and the additional
 suppression rules depend on the content of the congestion report and
 on the congestion report state recorded in the NE as detailed below.
 OPT_CR contains the following fields:
 OPT_CR_NE_WL   Reports the load in the worst link as detected though
                NE internal measurements
 OPT_CR_NE_WP   Reports the load in the worst end-to-end path as
                detected though NE internal measurements
 OPT_CR_RX_WP   Reports the load in the worst end-to-end path as
                detected by receivers
 A load report is either a packet drop rate (as measured at an NE's
 interfaces) or a packet loss rate (as measured in receivers).  Its
 value is linearly encoded in the range 0-0xFFFF, where 0xFFFF
 represents a 100% loss/drop rate.  Receivers that send a NAK bearing
 OPT_CR determine which of the three report fields are being reported.
 OPT_CR also contains the following fields:
 OPT_CR_NEL     A bit indicating that OPT_CR_NE_WL is being reported.
 OPT_CR_NEP     A bit indicating that OPT_CR_NE_WP is being reported.
 OPT_CR_RXP     A bit indicating that OPT_CR_RX_WP is being reported.

Speakman, et. al. Experimental [Page 73] RFC 3208 PGM Reliable Transport Protocol December 2001

 OPT_CR_LEAD    A SQN in the ODATA space that serves as a temporal
                reference for the load report values.  This is
                initialized by receivers with the leading edge of the
                transmit window as known at the moment of transmitting
                the NAK.  This value MAY be advanced in NEs that
                modify the content of OPT_CR.
 OPT_CR_RCVR    The identity of the receiver that generated the worst
                OPT_CR_RX_WP.
 The complete format of the option is specified later.

12.2. NE-Based Worst Link Report

 To permit network elements to report worst link, receivers append
 OPT_CR to a NAK with bit OPT_CR_NEL set and OPT_CR_NE_WL set to zero.
 NEs receiving NAKs that contain OPT_CR_NE_WL process the option and
 update per-TSI state related to it as described below.  The ultimate
 result of the NEs' actions ensures that when a NAK leaves a sub-tree,
 OPT_CR_NE_WL contains a congestion report that reflects the load of
 the worst link in that sub-tree.  To achieve this, NEs rewrite
 OPT_CR_NE_WL with the worst value among the loads measured on the
 local (outgoing) links for the session and the congestion reports
 received from those links.
 Note that the mechanism described in this sub-section does not permit
 the monitoring of the load on (outgoing) links at non-PGM-capable
 multicast routers.  For this reason, NE-Based Worst Link Reports
 SHOULD be used in pure PGM topologies only.  Otherwise, this
 mechanism might fail in detecting congestion.  To overcome this
 limitation PGM sources MAY use a heuristic that combines NE-Based
 Worst Link Reports and Receiver-Based Reports.

12.3. NE-Based Worst Path Report

 To permit network elements to report a worst path, receivers append
 OPT_CR to a NAK with bit OPT_CR_NEP set and OPT_CR_NE_WP set to zero.
 The processing of this field is similar to that of OPT_CR_NE_WL with
 the difference that, on the reception of a NAK, the value of
 OPT_CR_NE_WP is adjusted with the load measured on the interface on
 which the NAK was received according to the following formula:
 OPT_CR_NE_WP = if_load + OPT_CR_NE_WP * (100% - if_loss_rate)
 The worst among the adjusted OPT_CR_NE_WP is then written in the
 outgoing NAK.  This results in a hop-by-hop accumulation of link loss
 rates into a path loss rate.

Speakman, et. al. Experimental [Page 74] RFC 3208 PGM Reliable Transport Protocol December 2001

 As with OPT_CR_NE_WL, the congestion report in OPT_CR_NE_WP may be
 invalid if the multicast distribution tree includes non-PGM-capable
 routers.

12.4. Receiver-Based Worst Report

 To report a packet loss rate, receivers append OPT_CR to a NAK with
 bit OPT_CR_RXP set and OPT_CR_RX_WP set to the packet loss rate.  NEs
 receiving NAKs that contain OPT_CR_RX_WP process the option and
 update per-TSI state related to it as described below.  The ultimate
 result of the NEs' actions ensures that when a NAK leaves a sub-tree,
 OPT_CR_RX_WP contains a congestion report that reflects the load of
 the worst receiver in that sub-tree.  To achieve this, NEs rewrite
 OTP_CR_RE_WP with the worst value among the congestion reports
 received on its outgoing links for the session.  In addition to this,
 OPT_CR_RCVR MUST contain the NLA of the receiver that originally
 measured the value of OTP_CR_RE_WP being forwarded.

12.5. Procedures - Receivers

 To enable the generation of any type of congestion report, receivers
 MUST insert OPT_CR in each NAK they generate and provide the
 corresponding field (OPT_CR_NE_WL, OPT_CR_NE_WP, OPT_CR_RX_WP).  The
 specific fields to be reported will be advertised to receivers in
 OPT_CRQST on the session's SPMs.  Receivers MUST provide only those
 options requested in OPT_CRQST.
 Receivers MUST initialize OPT_CR_NE_WL and OPT_CR_NE_WP to 0 and they
 MUST initialize OPT_CR_RCVR to their NLA.  At the moment of sending
 the NAK, they MUST also initialize OPT_CR_LEAD to the leading edge of
 the transmission window.
 Additionally, if a receiver generates a NAK with OPT_CR with
 OPT_CR_RX_WP, it MUST initialize OPT_CR_RX_WP to the proper value,
 internally computed.

12.6. Procedures - Network Elements

 Network elements start processing each OPT_CR by selecting a
 reference SQN in the ODATA space.  The reference SQN selected is the
 highest SQN known to the NE.  Usually this is OPT_CR_LEAD contained
 in the NAK received.
 They use the selected SQN to age the value of load measurement as
 follows:
    o  locally measured load values (e.g. interface loads) are
       considered up-to-date

Speakman, et. al. Experimental [Page 75] RFC 3208 PGM Reliable Transport Protocol December 2001

    o  load values carried in OPT_CR are considered up-to-date and are
       not aged so as to be independent of variance in round-trip
       times from the network element to the receivers
    o  old load values recorded in the NE are exponentially aged
       according to the difference between the selected reference SQN
       and the reference SQN associated with the old load value.
 The exponential aging is computed so that a recorded value gets
 scaled down by a factor exp(-1/2) each time the expected inter-NAK
 time elapses.  Hence the aging formula must include the current loss
 rate as follows:
    aged_loss_rate = loss_rate * exp( - SQN_difference * loss_rate /
    2)
 Note that the quantity 1/loss_rate is the expected SQN_lag between
 two NAKs, hence the formula above can also be read as:
    aged_loss_rate = loss_rate * exp( - 1/2 * SQN_difference /
    SQN_lag)
 which equates to (loss_rate * exp(-1/2)) when the SQN_difference is
 equal to expected SQN_lag between two NAKs.
 All the subsequent computations refer to the aged load values.
 Network elements process OPT_CR by handling the three possible types
 of congestion reports independently.
 For each congestion report in an incoming NAK, a new value is
 computed to be used in the outgoing NAK:
    o  The new value for OPT_CR_NE_WL is the maximum of the load
       measured on the outgoing interfaces for the session, the value
       of OPT_CR_NE_WL of the incoming NAK, and the value previously
       sent upstream (recorded in the NE).  All these values are as
       obtained after the aging process.
    o  The new value for OPT_CR_NE_WP is the maximum of the value
       previously sent upstream (after aging) and the value of
       OPT_CR_NE_WP in the incoming NAK adjusted with the load on the
       interface upon which the NAK was received (as described above).
    o  The new value for OPT_CR_RX_WP is the maximum of the value
       previously sent upstream (after aging) and the value of
       OPT_CR_RX_WP in the incoming NAK.

Speakman, et. al. Experimental [Page 76] RFC 3208 PGM Reliable Transport Protocol December 2001

    o  If OPT_CR_RX_WP was selected from the incoming NAK, the new
       value for OPT_CR_RCVR is the one in the incoming NAK.
       Otherwise it is the value previously sent upstream.
    o  The new value for OPT_CR_LEAD is the reference SQN selected for
       the aging procedure.

12.6.1. Overriding Normal Suppression Rules

 Normal suppression rules hold to determine if a NAK should be
 forwarded upstream or not.  However if any of the outgoing congestion
 reports has changed by more than 5% relative to the one previously
 sent upstream, this new NAK is not suppressed.

12.6.2. Link Load Measurement

 PGM routers monitor the load on all their outgoing links and record
 it in the form of per-interface loss rate statistics. "load" MUST be
 interpreted as the percentage of the packets meant to be forwarded on
 the interface that were dropped.  Load statistics refer to the
 aggregate traffic on the links and not to PGM traffic only.
 This document does not specify the algorithm to be used to collect
 such statistics, but requires that such algorithm provide both
 accuracy and responsiveness in the measurement process.  As far as
 accuracy is concerned, the expected measurement error SHOULD be
 upper-limited (e.g. less than than 10%).  As far as responsiveness is
 concerned, the measured load SHOULD converge to the actual value in a
 limited time (e.g. converge to 90% of the actual value in less than
 200 milliseconds), when the instantaneous offered load changes.
 Whenever both requirements cannot be met at the same time, accuracy
 SHOULD be traded for responsiveness.

Speakman, et. al. Experimental [Page 77] RFC 3208 PGM Reliable Transport Protocol December 2001

12.7. Packet Formats

12.7.1. OPT_CR - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|        L P R|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                Congestion Report Reference SQN                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        NE Worst Link          |        NE Worst Path          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Rcvr Worst Path         |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            NLA AFI            |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Worst Receiver's NLA                ...   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
 Option Type = 0x10
 Option Length = 20 octets + NLA length
    L OPT_CR_NEL bit : set indicates OPT_CR_NE_WL is being reported
    P OPT_CR_NEP bit : set indicates OPT_CR_NE_WP is being reported
    R OPT_CR_RXP bit : set indicates OPT_CR_RX_WP is being reported
 Congestion Report Reference SQN (OPT_CR_LEAD).
    A SQN in the ODATA space that serves as a temporal reference point
    for the load report values.
 NE Worst Link (OPT_CR_NE_WL).
    Reports the load in the worst link as detected though NE internal
    measurements
 NE Worst Path (OPT_CR_NE_WP).
    Reports the load in the worst end-to-end path as detected though
    NE internal measurements

Speakman, et. al. Experimental [Page 78] RFC 3208 PGM Reliable Transport Protocol December 2001

 Rcvr Worst Path (OPT_CR_RX_WP).
    Reports the load in the worst end-to-end path as detected by
    receivers
 Worst Receiver's NLA (OPT_CR_RCVR).
    The unicast address of the receiver that generated the worst
    OPT_CR_RX_WP.
 OPT_CR MAY be appended only to NAKs.
 OPT-CR is network-significant.

12.7.2. OPT_CRQST - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|        L P R|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x11
 Option Length = 4 octets
    L OPT_CRQST_NEL bit : set indicates OPT_CR_NE_WL is being
    requested
    P OPT_CRQST_NEP bit : set indicates OPT_CR_NE_WP is being
    requested
    R OPT_CRQST_RXP bit : set indicates OPT_CR_RX_WP is being
    requested
 OPT_CRQST MAY be appended only to SPMs.
 OPT-CRQST is network-significant.

13. Appendix C - SPM Requests

13.1. Introduction

 SPM Requests (SPMRs) MAY be used to solicit an SPM from a source in a
 non-implosive way.  The typical application is for late-joining
 receivers to solicit SPMs directly from a source in order to be able
 to NAK for missing packets without having to wait for a regularly
 scheduled SPM from that source.

Speakman, et. al. Experimental [Page 79] RFC 3208 PGM Reliable Transport Protocol December 2001

13.2. Overview

 Allowing for SPMR implosion protection procedures, a receiver MAY
 unicast an SPMR to a source to solicit the most current session,
 window, and path state from that source any time after the receiver
 has joined the group.  A receiver may learn the TSI and source to
 which to direct the SPMR from any other PGM packet it receives in the
 group, or by any other means such as from local configuration or
 directory services.  The receiver MUST use the usual SPM procedures
 to glean the unicast address to which it should direct its NAKs from
 the solicited SPM.

13.3. Packet Contents

 This section just provides enough short-hand to make the Procedures
 intelligible.  For the full details of packet contents, please refer
 to Packet Formats below.

13.3.1. SPM Requests

 SPMRs are transmitted by receivers to solicit SPMs from a source.
 SPMs are unicast to a source and contain:
 SPMR_TSI       the source-assigned TSI for the session to which the
                SPMR corresponds

13.4. Procedures - Sources

 A source MUST respond immediately to an SPMR with the corresponding
 SPM rate limited to once per IHB_MIN per TSI.  The corresponding SPM
 matches SPM_TSI to SPMR_TSI and SPM_DPORT to SPMR_DPORT.

13.5. Procedures - Receivers

 To moderate the potentially implosive behavior of SPMRs at least on a
 densely populated subnet, receivers MUST use the following back-off
 and suppression procedure based on multicasting the SPMR with a TTL
 of 1 ahead of and in addition to unicasting the SPMR to the source.
 The role of the multicast SPMR is to suppress the transmission of
 identical SPMRs from the subnet.
 More specifically, before unicasting a given SPMR, receivers MUST
 choose a random delay on SPMR_BO_IVL (~250 msecs) during which they
 listen for a multicast of an identical SPMR.  If a receiver does not
 see a matching multicast SPMR within its chosen random interval, it
 MUST first multicast its own SPMR to the group with a TTL of 1 before
 then unicasting its own SPMR to the source.  If a receiver does see a

Speakman, et. al. Experimental [Page 80] RFC 3208 PGM Reliable Transport Protocol December 2001

 matching multicast SPMR within its chosen random interval, it MUST
 refrain from unicasting its SPMR and wait instead for the
 corresponding SPM.
 In addition, receipt of the corresponding SPM within this random
 interval SHOULD cancel transmission of an SPMR.
 In either case, the receiver MUST wait at least SPMR_SPM_IVL before
 attempting to repeat the SPMR by choosing another delay on
 SPMR_BO_IVL and repeating the procedure above.
 The corresponding SPMR matches SPMR_TSI to SPMR_TSI and SPMR_DPORT to
 SPMR_DPORT.  The corresponding SPM matches SPM_TSI to SPMR_TSI and
 SPM_DPORT to SPMR_DPORT.

13.6. SPM Requests

    SPMR:
       SPM Requests are sent by receivers to request the immediate
       transmission of an SPM for the given TSI from a source.
 The network-header source address of an SPMR is the unicast NLA of
 the entity that originates the SPMR.
 The network-header destination address of an SPMR is the unicast NLA
 of the source from which the corresponding SPM is requested.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Source Port           |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Type     |    Options    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Global Source ID                   ... |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | ...    Global Source ID       |           TSDU Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Option Extensions when present ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ...
 Source Port:
    SPMR_SPORT
    Data-Destination Port

Speakman, et. al. Experimental [Page 81] RFC 3208 PGM Reliable Transport Protocol December 2001

 Destination Port:
    SPMR_DPORT
    Data-Source Port, together with Global Source ID forms SPMR_TSI
 Type:
    SPMR_TYPE =  0x0C
 Global Source ID:
    SPMR_GSI
    Together with Source Port forms
       SPMR_TSI

14. Appendix D - Poll Mechanism

14.1. Introduction

    These procedures provide PGM network elements and sources with the
    ability to poll their downstream PGM neighbors to solicit replies
    in an implosion-controlled way.
    Both general polls and specific polls are possible.  The former
    provide a PGM (parent) node with a way to check if there are any
    PGM (children) nodes connected to it, both network elements and
    receivers, and to estimate their number.  The latter may be used
    by PGM parent nodes to search for nodes with specific properties
    among its PGM children.  An example of application for this is DLR
    discovery.
    Polling is implemented using two additional PGM packets:
 POLL  a Poll Request that PGM parent nodes multicast to the group to
       perform the poll.  Similarly to NCFs, POLL packets stop at the
       first PGM node they reach, as they are not forwarded by PGM
       network elements.
 POLR a Poll Response that PGM children nodes (either network elements
       or receivers) use to reply to a Poll Request by addressing it
       to the NLA of the interface from which the triggering POLL was
       sent.

Speakman, et. al. Experimental [Page 82] RFC 3208 PGM Reliable Transport Protocol December 2001

 The polling mechanism dictates that PGM children nodes that receive a
 POLL packet reply to it only if certain conditions are satisfied and
 ignore the POLL otherwise.  Two types of condition are possible: a
 random condition that defines a probability of replying for the
 polled child, and a deterministic condition.  Both the random
 condition and the deterministic condition are controlled by the
 polling PGM parent node by specifying the probability of replying and
 defining the deterministic condition(s) respectively.  Random-only
 poll, deterministic-only poll or a combination of the two are
 possible.
 The random condition in polls allows the prevention of implosion of
 replies by controlling their number.  Given a probability of replying
 P and assuming that each receiver makes an independent decision, the
 number of expected replies to a poll is P*N where N is the number of
 PGM children relative to the polling PGM parent.  The polling node
 can control the number of expected replies by specifying P in the
 POLL packet.

14.2. Packet Contents

 This section just provides enough short-hand to make the Procedures
 intelligible.  For the full details of packet contents, please refer
 to Packet Formats below.

14.2.1. POLL (Poll Request)

 POLL_SQN       a sequence number assigned sequentially by the polling
                parent in unit increments and scoped by POLL_PATH and
                the TSI of the session.
 POLL_ROUND     a poll round sequence number.  Multiple poll rounds
                are possible within a POLL_SQN.
 POLL_S_TYPE    the sub-type of the poll request
 POLL_PATH      the network-layer address (NLA) of the interface on
                the PGM network element or source on which the POLL is
                transmitted
 POLL_BO_IVL    the back-off interval that MUST be used to compute the
                random back-off time to wait before sending the
                response to a poll.  POLL_BO_IVL is expressed in
                microseconds.
 POLL_RAND      a random string used to implement the randomness in
                replying

Speakman, et. al. Experimental [Page 83] RFC 3208 PGM Reliable Transport Protocol December 2001

 POLL_MASK      a bit-mask used to determine the probability of random
                replies
 Poll request MAY also contain options which specify deterministic
 conditions for the reply.  No options are currently defined.

14.2.2. POLR (Poll Response)

 POLR_SQN       POLL_SQN of the poll request for which this is a reply
 POLR_ROUND     POLL_ROUND of the poll request for which this is a
                reply
 Poll response MAY also contain options.  No options are currently
 defined.

14.3. Procedures - General

14.3.1. General Polls

 General Polls may be used to check for and count PGM children that
 are 1 PGM hop downstream of an interface of a given node.  They have
 POLL_S_TYPE equal to PGM_POLL_GENERAL.  PGM children that receive a
 general poll decide whether to reply to it only based on the random
 condition present in the POLL.
 To prevent response implosion, PGM parents that initiate a general
 poll SHOULD establish the probability of replying to the poll, P, so
 that the expected number of replies is contained.  The expected
 number of replies is N * P, where N is the number of children.  To be
 able to compute this number, PGM parents SHOULD already have a rough
 estimate of the number of children.  If they do not have a recent
 estimate of this number, they SHOULD send the first poll with a very
 low probability of replying and increase it in subsequent polls in
 order to get the desired number of replies.
 To prevent poll-response implosion caused by a sudden increase in the
 children population occurring between two consecutive polls with
 increasing probability of replying, PGM parents SHOULD use poll
 rounds.  Poll rounds allow PGM parents to "freeze" the size of the
 children population when they start a poll and to maintain it
 constant across multiple polls (with the same POLL_SQN but different
 POLL_ROUND).  This works by allowing PGM children to respond to a
 poll only if its POLL_ROUND is zero or if they have previously
 received a poll with the same POLL_SQN and POLL_ROUND equal to zero.

Speakman, et. al. Experimental [Page 84] RFC 3208 PGM Reliable Transport Protocol December 2001

 In addition to this PGM children MUST observe a random back-off in
 replying to a poll.  This spreads out the replies in time and allows
 a PGM parent to abort the poll if too many replies are being
 received.  To abort an ongoing poll a PGM parent MUST initiate
 another poll with different POLL_SQN.  PGM children that receive a
 POLL MUST cancel any pending reply for POLLs with POLL_SQN different
 from the one of the last POLL received.
 For a given poll with probability of replying P, a PGM parent
 estimates the number of children as M / P, where M is the number of
 responses received.  PGM parents SHOULD keep polling periodically and
 use some average of the result of recent polls as their estimate for
 the number of children.

14.3.2. Specific Polls

 Specific polls provide a way to search for PGM children that comply
 to specific requisites.  As an example specific poll could be used to
 search for down-stream DLRs.  A specific poll is characterized by a
 POLL_S_TYPE different from PGM_POLL_GENERAL.  PGM children decide
 whether to reply to a specific poll or not based on the POLL_S_TYPE,
 on the random condition and on options possibly present in the POLL.
 The way options should be interpreted is defined by POLL_S_TYPE.  The
 random condition MUST be interpreted as an additional condition to be
 satisfied.  To disable the random condition PGM parents MUST specify
 a probability of replying P equal to 1.
 PGM children MUST ignore a POLL packet if they do not understand
 POLL_S_TYPE.  Some specific POLL_S_TYPE may also require that the
 children ignore a POLL if they do not fully understand all the PGM
 options present in the packet.

14.4. Procedures - PGM Parents (Sources or Network Elements)

 A PGM parent (source or network element), that wants to poll the
 first PGM-hop children connected to one of its outgoing interfaces
 MUST send a POLL packet on that interface with:
 POLL_SQN       equal to POLL_SQN of the last POLL sent incremented by
                one.  If poll rounds are used, this must be equal to
                POLL_SQN of the last group of rounds incremented by
                one.
 POLL_ROUND     The round of the poll.  If the poll has a single
                round, this must be zero.  If the poll has multiple
                rounds, this must be one plus the last POLL_ROUND for
                the same POLL_SQN, or zero if this is the first round
                within this POLL_SQN.

Speakman, et. al. Experimental [Page 85] RFC 3208 PGM Reliable Transport Protocol December 2001

 POLL_S_TYPE    the type of the poll.  For general poll use
                PGM_POLL_GENERAL
 POLL_PATH      set to the NLA of the outgoing interface
 POLL_BO_IVL    set to the wanted reply back-off interval.  As far as
                the choice of this is concerned, using NAK_BO_IVL is
                usually a conservative option, however a smaller value
                MAY be used, if the number of expected replies can be
                determined with a good confidence or if a
                conservatively low probability of reply (P) is being
                used (see POLL_MASK next).  When the number of
                expected replies is unknown, a large POLL_BO_IVL
                SHOULD be used, so that the poll can be effectively
                aborted if the number of replies being received is too
                large.
 POLL_RAND      MUST be a random string re-computed each time a new
                poll is sent on a given interface
 POLL_MASK      determines the probability of replying, P,  according
                to the relationship P = 1 / ( 2 ^ B ), where B is the
                number of bits set in POLL_MASK [15].  If this is a
                deterministic poll, B MUST be 0, i.e. POLL_MASK MUST
                be a all-zeroes bit-mask.
    Nota Bene: POLLs transmitted by network elements MUST bear the
    ODATA source's network-header source address, not the network
    element's NLA.  POLLs MUST also be transmitted with the IP
    Router Alert Option [6], to be allow PGM network element to
    intercept them.
 A PGM parent that has started a poll by sending a POLL packet SHOULD
 wait at least POLL_BO_IVL before starting another poll.  During this
 interval it SHOULD collect all the valid response (the one with
 POLR_SQN and POLR_ROUND matching with the outstanding poll) and
 process them at the end of the collection interval.
 A PGM parent SHOULD observe the rules mentioned in the description of
 general procedures, to prevent implosion of response.  These rules
 should in general be observed both for generic polls and specific
 polls.  The latter however can be performed using deterministic poll
 (with no implosion prevention) if the expected number of replies is
 known to be small.  A PGM parent that issue a generic poll with the
 intent of estimating the children population size SHOULD use poll
 rounds to "freeze" the children that are involved in the measure
 process.  This allows the sender to "open the door wider" across

Speakman, et. al. Experimental [Page 86] RFC 3208 PGM Reliable Transport Protocol December 2001

 subsequent rounds (by increasing the probability of response),
 without fear of being flooded by late joiners.  Note the use of
 rounds has the drawback of introducing additional delay in the
 estimate of the population size, as the estimate obtained at the end
 of a round-group refers to the condition present at the time of the
 first round.
 A PGM parent that has started a poll SHOULD monitor the number of
 replies during the collection phase.  If this become too large, the
 PGM parent SHOULD abort the poll by immediately starting a new poll
 (different POLL_SQN) and specifying a very low probability of
 replying.
 When polling is being used to estimate the receiver population for
 the purpose of calculating NAK_BO_IVL, OPT_NAK_BO_IVL (see 16.4.1
 below) MUST be appended to SPMs, MAY be appended to NCFs and POLLs,
 and in all cases MUST have NAK_BO_IVL_SQN set to POLL_SQN of the most
 recent complete round of polls, and MUST bear that round's
 corresponding derived value of NAK_BAK_IVL.  In this way,
 OPT_NAK_BO_IVL provides a current value for NAK_BO_IVL at the same
 time as information is being gathered for the calculation of a future
 value of NAK_BO_IVL.

14.5. Procedures - PGM Children (Receivers or Network Elements)

 PGM receivers and network elements MUST compute a 32-bit random node
 identifier (RAND_NODE_ID) at startup time.  When a PGM child
 (receiver or network element) receives a POLL it MUST use its
 RAND_NODE_ID to match POLL_RAND of incoming POLLs.  The match is
 limited to the bits specified by POLL_MASK.  If the incoming POLL
 contain a POLL_MASK made of all zeroes, the match is successful
 despite the content of POLL_RAND (deterministic reply).  If the match
 fails, then the receiver or network element MUST discard the POLL
 without any further action, otherwise it MUST check the fields
 POLL_ROUND, POLL_S_TYPE and any PGM option included in the POLL to
 determine whether it SHOULD reply to the poll.
 If POLL_ROUND is non-zero and the PGM receiver has not received a
 previous poll with the same POLL_SQN and a zero POLL_ROUND, it MUST
 discard the poll without further action.
 If POLL_S_TYPE is equal to PGM_POLL_GENERAL, the PGM child MUST
 schedule a reply to the POLL despite the presence of PGM options on
 the POLL packet.

Speakman, et. al. Experimental [Page 87] RFC 3208 PGM Reliable Transport Protocol December 2001

 If POLL_S_TYPE is different from PGM_POLL_GENERAL, the decision on
 whether a reply should be scheduled depends on the actual type and on
 the options possibly present in the POLL.
 If POLL_S_TYPE is unknown to the recipient of the POLL, it MUST NOT
 reply and ignore the poll.  Currently the only POLL_S_TYPE defined
 are PGM_POLL_GENERAL and PGM_POLL_DLR.
 If a PGM receiver or network element has decided to reply to a POLL,
 it MUST schedule the transmission of a single POLR at a random time
 in the future.  The random delay is chosen in the interval [0,
 POLL_BO_IVL].  POLL_BO_IVL is the one contained in the POLL received.
 When this timer expires, it MUST send a POLR using POLL_PATH of the
 received POLL as destination address.  POLR_SQN MUST be equal to
 POLL_SQN and POLR_ROUND must be equal to POLL_ROUND.  The POLR MAY
 contain PGM options according to the semantic of POLL_S_TYPE or the
 semantic of PGM options possibly present in the POLL.  If POLL_S_TYPE
 is PGM_POLL_GENERAL no option is REQUIRED.
 A PGM receiver or network element MUST cancel any pending
 transmission of POLRs if a new POLL is received with POLL_SQN
 different from POLR_SQN of the poll that scheduled POLRs.

14.6. Constant Definition

 The POLL_S_TYPE values currently defined are:
    PGM_POLL_GENERAL  0
    PGM_POLL_DLR      1

14.7. Packet Formats

 The packet formats described in this section are transport-layer
 headers that MUST immediately follow the network-layer header in the
 packet.
 The descriptions of Data-Source Port, Data-Destination Port, Options,
 Checksum, Global Source ID (GSI), and TSDU Length are those provided
 in Section 8.

14.7.1. Poll Request

 POLL are sent by PGM parents (sources or network elements) to
 initiate a poll among their first PGM-hop children.

Speakman, et. al. Experimental [Page 88] RFC 3208 PGM Reliable Transport Protocol December 2001

 POLLs are sent to the ODATA multicast group.  The network-header
 source address of a POLL is the ODATA source's NLA.  POLL MUST be
 transmitted with the IP Router Alert Option.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Source Port           |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Type     |    Options    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Global Source ID                   ... |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | ...    Global Source ID       |           TSDU Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    POLL's Sequence Number                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         POLL's Round          |       POLL's Sub-type         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            NLA AFI            |          Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            Path NLA                     ...   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
 |                  POLL's  Back-off Interval                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Random String                          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Matching Bit-Mask                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Option Extensions when present ...                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Source Port:
    POLL_SPORT
    Data-Source Port, together with POLL_GSI forms POLL_TSI
 Destination Port:
    POLL_DPORT
    Data-Destination Port
 Type:
    POLL_TYPE = 0x01

Speakman, et. al. Experimental [Page 89] RFC 3208 PGM Reliable Transport Protocol December 2001

 Global Source ID:
    POLL_GSI
    Together with POLL_SPORT forms POLL_TSI
 POLL's Sequence Number
    POLL_SQN
    The sequence number assigned to the POLL by the originator.
 POLL's Round
    POLL_ROUND
    The round number within the poll sequence number.
 POLL's Sub-type
    POLL_S_TYPE
    The sub-type of the poll request.
 Path NLA:
    POLL_PATH
    The NLA of the interface on the source or network element on which
    this POLL was forwarded.
 POLL's Back-off Interval
    POLL_BO_IVL
    The back-off interval used to compute a random back-off for the
    reply, expressed in microseconds.
 Random String
    POLL_RAND
    A random string used to implement the random condition in
    replying.

Speakman, et. al. Experimental [Page 90] RFC 3208 PGM Reliable Transport Protocol December 2001

 Matching Bit-Mask
    POLL_MASK
    A  bit-mask used to determine the probability of random replies.

14.7.2. Poll Response

 POLR are sent by PGM children (receivers or network elements) to
 reply to a POLL.
 The network-header source address of a POLR is the unicast NLA of the
 entity that originates the POLR.  The network-header destination
 address of a POLR is initialized by the originator of the POLL to the
 unicast NLA of the upstream PGM element (source or network element)
 known from the POLL that triggered the POLR.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Source Port           |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Type     |    Options    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Global Source ID                   ... |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | ...    Global Source ID       |           TSDU Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    POLR's Sequence Number                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         POLR's Round          |           reserved            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Option Extensions when present ...                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Source Port:
    POLR_SPORT
    Data-Destination Port
 Destination Port:
    POLR_DPORT
    Data-Source Port, together with Global Source ID forms POLR_TSI

Speakman, et. al. Experimental [Page 91] RFC 3208 PGM Reliable Transport Protocol December 2001

 Type:
    POLR_TYPE = 0x02
 Global Source ID:
    POLR_GSI
    Together with POLR_DPORT forms POLR_TSI
 POLR's Sequence Number
    POLR_SQN
    The sequence number (POLL_SQN) of the POLL packet for which this
    is a reply.
 POLR's Round
    POLR_ROUND
    The round number (POLL_ROUND) of the POLL packet for which this is
    a reply.

15. Appendix E - Implosion Prevention

15.1. Introduction

 These procedures are intended to prevent NAK implosion and to limit
 its extent in case of the loss of all or part of the suppressing
 multicast distribution tree.  They also provide a means to adaptively
 tune the NAK back-off interval, NAK_BO_IVL.
 The PGM virtual topology is established and refreshed by SPMs.
 Between one SPM and the next, PGM nodes may have an out-of-date view
 of the PGM topology due to multicast routing changes, flapping, or a
 link/router failure.  If any of the above happens relative to a PGM
 parent node, a potential NAK implosion problem arises because the
 parent node is unable to suppress the generation of duplicate NAKs as
 it cannot reach its children using NCFs.  The procedures described
 below introduce an alternative way of performing suppression in this
 case.  They also attempt to prevent implosion by adaptively tuning
 NAK_BO_IVL.

Speakman, et. al. Experimental [Page 92] RFC 3208 PGM Reliable Transport Protocol December 2001

15.2. Tuning NAK_BO_IVL

 Sources and network elements continuously monitor the number of
 duplicated NAKs received and use this observation to tune the NAK
 back-off interval (NAK_BO_IVL) for the first PGM-hop receivers
 connected to them.  Receivers learn the current value of NAK_BO_IVL
 through OPT_NAK_BO_IVL appended to NCFs or SPMs.

15.2.1. Procedures - Sources and Network Elements

 For each TSI, sources and network elements advertise the value of
 NAK_BO_IVL that their first PGM-hop receivers should use.  They
 advertise a separate value on all the outgoing interfaces for the TSI
 and keep track of the last values advertised.
 For each interface and TSI, sources and network elements count the
 number of NAKs received for a specific repair state (i.e., per
 sequence number per TSI) from the time the interface was first added
 to the repair state list until the time the repair state is
 discarded.  Then they use this number to tune the current value of
 NAK_BO_IVL as follows:
    Increase the current value NAK_BO_IVL when the first duplicate NAK
    is received for a given SQN on a particular interface.
 Decrease the value of NAK_BO_IVL if no duplicate NAKs are received on
 a particular interface for the last NAK_PROBE_NUM measurements where
 each measurement corresponds to the creation of a new repair state.
 An upper and lower limit are defined for the possible value of
 NAK_BO_IVL at any time.  These are NAK_BO_IVL_MAX and NAK_BO_IVL_MIN
 respectively.  The initial value that should be used as a starting
 point to tune NAK_BO_IVL is NAK_BO_IVL_DEFAULT.  The policies
 RECOMMENDED for increasing and decreasing NAK_BO_IVL are multiplying
 by two and dividing by two respectively.
 Sources and network elements advertise the current value of
 NAK_BO_IVL through the OPT_NAK_BO_IVL that they append to NCFs.  They
 MAY also append OPT_NAK_BO_IVL to outgoing SPMs.
 In order to avoid forwarding the NAK_BO_IVL advertised by the parent,
 network elements must be able to recognize OPT_NAK_BO_IVL.  Network
 elements that receive SPMs containing OPT_NAK_BO_IVL MUST either
 remove the option or over-write its content (NAK_BO_IVL) with the
 current value of NAK_BO_IVL for the outgoing interface(s), before
 forwarding the SPMs.

Speakman, et. al. Experimental [Page 93] RFC 3208 PGM Reliable Transport Protocol December 2001

 Sources MAY advertise the value of NAK_BO_IVL_MAX and NAK_BO_IVL_MIN
 to the session by appending a OPT_NAK_BO_RNG to SPMs.

15.2.2. Procedures - Receivers

 Receivers learn the value of NAK_BO_IVL to use through the option
 OPT_NAK_BO_IVL, when this is present in NCFs or SPMs.  A value for
 NAK_BO_IVL learned from OPT_NAK_BO_IVL MUST NOT be used by a receiver
 unless either NAK_BO_IVL_SQN is zero, or the receiver has seen
 POLL_RND == 0 for POLL_SQN =< NAK_BO_IVL_SQN within half the sequence
 number space.  The initial value of NAK_BO_IVL is set to
 NAK_BO_IVL_DEFAULT.
 Receivers that receive an SPM containing OPT_NAK_BO_RNG MUST use its
 content to set the local values of NAK_BO_IVL_MAX and NAK_BO_IVL_MIN.

15.2.3. Adjusting NAK_BO_IVL in the absence of NAKs

 Monitoring the number of duplicate NAKs provides a means to track
 indirectly the change in the size of first PGM-hop receiver
 population and adjust NAK_BO_IVL accordingly.  Note that the number
 of duplicate NAKs for a given SQN is related to the number of first
 PGM-hop children that scheduled (or forwarded) a NAK and not to the
 absolute number of first PGM-hop children.  This mechanism, however,
 does not work in the absence of packet loss, hence a large number of
 duplicate NAKs is possible after a period without NAKs, if many new
 receivers have joined the session in the meanwhile.  To address this
 issue, PGM Sources and network elements SHOULD periodically poll the
 number of first PGM-hop children using the "general poll" procedures
 described in Appendix D.  If the result of the polls shows that the
 population size has increased significantly during a period without
 NAKs, they SHOULD increase NAK_BO_IVL as a safety measure.

15.3. Containing Implosion in the Presence of Network Failures

15.3.1. Detecting Network Failures

 In some cases PGM (parent) network elements can promptly detect the
 loss of all or part of the suppressing multicast distribution tree
 (due to network failures or route changes) by checking their
 multicast connectivity, when they receive NAKs.  In some other cases
 this is not possible as the connectivity problem might occur at some
 other non-PGM node downstream or might take time to reflect in the
 multicast routing table.  To address these latter cases, PGM uses a
 simple heuristic: a failure is assumed for a TSI when the count of
 duplicated NAKs received for a repair state reaches the value
 DUP_NAK_MAX in one of the interfaces.

Speakman, et. al. Experimental [Page 94] RFC 3208 PGM Reliable Transport Protocol December 2001

15.3.2. Containing Implosion

 When a PGM source or network element detects or assumes a failure for
 which it looses multicast connectivity to down-stream PGM agents
 (either receivers or other network elements), it sends unicast NCFs
 to them in response to NAKs.  Downstream PGM network elements which
 receive unicast NCFs and have multicast connectivity to the multicast
 session send special SPMs to prevent further NAKs until a regular SPM
 sent by the source refreshes the PGM tree.
 Procedures - Sources and Network Elements
 PGM sources or network elements which detect or assume a failure that
 prevents them from reaching down-stream PGM agents through multicast
 NCFs revert to confirming NAKs through unicast NCFs for a given TSI
 on a given interface.  If the PGM agent is the source itself, than it
 MUST generate an SPM for the TSI, in addition to sending the unicast
 NCF.
 Network elements MUST keep using unicast NCFs until they receive a
 regular SPM from the source.
 When a unicast NCF is sent for the reasons described above, it MUST
 contain the OPT_NBR_UNREACH option and the OPT_PATH_NLA option.
 OPT_NBR_UNREACH indicates that the sender is unable to use multicast
 to reach downstream PGM agents.  OPT_PATH_NLA carries the network
 layer address of the NCF sender, namely the NLA of the interface
 leading to the unreachable subtree.
 When a PGM network element receives an NCF containing the
 OPT_NBR_UNREACH option, it MUST ignore it if OPT_PATH_NLA specifies
 an upstream neighbour different from the one currently known to be
 the upstream neighbor for the TSI.  Assuming the network element
 matches the OPT_PATH_NLA of the upstream neighbour address, it MUST
 stop forwarding NAKs for the TSI until it receives a regular SPM for
 the TSI.  In addition, it MUST also generate a special SPM to prevent
 downstream receivers from sending more NAKs.  This special SPM MUST
 contain the OPT_NBR_UNREACH option and SHOULD have a SPM_SQN equal to
 SPM_SQN of the last regular SPM forwarded.  The OPT_NBR_UNREACH
 option invalidates the windowing information in SPMs (SPM_TRAIL and
 SPM_LEAD).  The PGM network element that adds the OPT_NBR_UNREACH
 option SHOULD invalidate the windowing information by setting
 SPM_TRAIL to 0 and SPM_LEAD to 0x80000000.
 PGM network elements which receive an SPM containing the
 OPT_NBR_UNREACH option and whose SPM_PATH matches the currently known
 PGM parent, MUST forward them in the normal way and MUST stop

Speakman, et. al. Experimental [Page 95] RFC 3208 PGM Reliable Transport Protocol December 2001

 forwarding NAKs for the TSI until they receive a regular SPM for the
 TSI.  If the SPM_PATH does not match the currently known PGM parent,
 the SPM containing the OPT_NBR_UNREACH option MUST be ignored.
 Procedures - Receivers
 PGM receivers which receive either an NCF or an SPM containing the
 OPT_NBR_UNREACH option MUST stop sending NAKs until a regular SPM is
 received for the TSI.
 On reception of a unicast NCF containing the OPT_NBR_UNREACH option
 receivers MUST generate a multicast copy of the packet with TTL set
 to one on the RPF interface for the data source.  This will prevent
 other receivers in the same subnet from generating NAKs.
 Receivers MUST ignore windowing information in SPMs which contain the
 OPT_NBR_UNREACH option.
 Receivers MUST ignore NCFs containing the OPT_NBR_UNREACH option if
 the OPT_PATH_NLA specifies a neighbour different than the one
 currently know to be the PGM parent neighbour.  Similarly receivers
 MUST ignore SPMs containing the OPT_NBR_UNREACH option if SPM_PATH
 does not match the current PGM parent.

15.4. Packet Formats

15.4.1. OPT_NAK_BO_IVL - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     NAK Back-Off Interval                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                   NAK Back-Off Interval SQN                   |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x04
 NAK Back-Off Interval
    The value of NAK-generation Back-Off Interval in microseconds.

Speakman, et. al. Experimental [Page 96] RFC 3208 PGM Reliable Transport Protocol December 2001

 NAK Back-Off Interval Sequence Number
    The POLL_SQN to which this value of NAK_BO_IVL corresponds.  Zero
    is reserved and means NAK_BO_IVL is NOT being determined through
    polling (see Appendix D) and may be used immediately.  Otherwise,
    NAK_BO_IVL MUST NOT be used unless the receiver has also seen
    POLL_ROUND = 0 for POLL_SQN =< NAK_BO_IVL_SQN within half the
    sequence number space.
 OPT_NAK_BO_IVL MAY be appended to NCFs, SPMs, or POLLs.
 OPT_NAK_BO_IVL is network-significant.

15.4.2. OPT_NAK_BO_RNG - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Maximum  NAK Back-Off Interval                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Minimum  NAK Back-Off Interval                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x05
 Maximum NAK Back-Off Interval
    The maximum value of NAK-generation Back-Off Interval in
    microseconds.
 Minimum NAK Back-Off Interval
    The minimum value of NAK-generation Back-Off Interval in
    microseconds.
 OPT_NAK_BO_RNG MAY be appended to SPMs.
 OPT_NAK_BO_RNG is network-significant.

15.4.3. OPT_NBR_UNREACH - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Speakman, et. al. Experimental [Page 97] RFC 3208 PGM Reliable Transport Protocol December 2001

    Option Type = 0x0B
    When present in SPMs, it invalidates the windowing information.
 OPT_NBR_UNREACH MAY be appended to SPMs and NCFs.
 OPT_NBR_UNREACH is network-significant.

15.4.4. OPT_PATH_NLA - Packet Extension Format

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |E| Option Type | Option Length |Reserved |F|OPX|U|             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                            Path NLA                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Option Type = 0x0C
 Path NLA
    The NLA of the interface on the originating PGM network element
    that it uses to send multicast SPMs to the recipient of the packet
    containing this option.
 OPT_PATH_NLA MAY be appended to NCFs.
 OPT_PATH_NLA is network-significant.

16. Appendix F - Transmit Window Example

    Nota Bene: The concept of and all references to the increment
    window (TXW_INC) and the window increment (TXW_ADV_SECS)
    throughout this document are for illustrative purposes only.  They
    provide the shorthand with which to describe the concept of
    advancing the transmit window without also implying any particular
    implementation or policy of advancement.
 The size of the transmit window in seconds is simply TXW_SECS.  The
 size of the transmit window in bytes (TXW_BYTES) is (TXW_MAX_RTE *
 TXW_SECS).  The size of the transmit window in sequence numbers
 (TXW_SQNS) is (TXW_BYTES / bytes-per-packet).
 The fraction of the transmit window size (in seconds of data) by
 which the transmit window is advanced (TXW_ADV_SECS) is called the
 window increment.  The trailing (oldest) such fraction of the
 transmit window itself is called the increment window.

Speakman, et. al. Experimental [Page 98] RFC 3208 PGM Reliable Transport Protocol December 2001

 In terms of sequence numbers, the increment window is the range of
 sequence numbers that will be the first to be expired from the
 transmit window.  The trailing (or left) edge of the increment window
 is just TXW_TRAIL, the trailing (or left) edge of the transmit
 window.  The leading (or right) edge of the increment window
 (TXW_INC) is defined as one less than the sequence number of the
 first data packet transmitted by the source TXW_ADV_SECS after
 transmitting TXW_TRAIL.
 A data packet is described as being "in" the transmit or increment
 window, respectively, if its sequence number is in the range defined
 by the transmit or increment window, respectively.
 The transmit window is advanced across the increment window by the
 source when it increments TXW_TRAIL to TXW_INC.  When the transmit
 window is advanced across the increment window, the increment window
 is emptied (i.e., TXW_TRAIL is momentarily equal to TXW_INC), begins
 to refill immediately as transmission proceeds, is full again
 TXW_ADV_SECS later (i.e., TXW_TRAIL is separated from TXW_INC by
 TXW_ADV_SECS of data), at which point the transmit window is advanced
 again, and so on.

16.1. Advancing across the Increment Window

 In anticipation of advancing the transmit window, the source starts a
 timer TXW_ADV_IVL_TMR which runs for time period TXW_ADV_IVL.
 TXW_ADV_IVL has a value in the range (0, TXW_ADV_SECS).  The value
 MAY be configurable or MAY be determined statically by the strategy
 used for advancing the transmit window.
 When TXW_ADV_IVL_TMR is running, a source MAY reset TXW_ADV_IVL_TMR
 if NAKs are received for packets in the increment window.  In
 addition, a source MAY transmit RDATA in the increment window with
 priority over other data within the transmit window.
 When TXW_ADV_IVL_TMR expires, a source SHOULD advance the trailing
 edge of the transmit window from TXW_TRAIL to TXW_INC.
 Once the transmit window is advanced across the increment window,
 SPM_TRAIL, OD_TRAIL and RD_TRAIL are set to the new value of
 TXW_TRAIL in all subsequent transmitted packets, until the next
 window advancement.
 PGM does not constrain the strategies that a source may use for
 advancing the transmit window.  The source MAY implement any scheme
 or number of schemes.  Three suggested strategies are outlined here.

Speakman, et. al. Experimental [Page 99] RFC 3208 PGM Reliable Transport Protocol December 2001

 Consider the following example:
    Assuming a constant transmit rate of 128kbps and a constant data
    packet size of 1500 bytes, if a source maintains the past 30
    seconds of data for repair and increments its transmit window in 5
    second increments, then
       TXW_MAX_RTE = 16kBps
       TXW_ADV_SECS = 5 seconds,
       TXW_SECS = 35 seconds,
       TXW_BYTES = 560kB,
       TXW_SQNS = 383 (rounded up),
    and the size of the increment window in sequence numbers
    (TXW_MAX_RTE * TXW_ADV_SECS / 1500) = 54 (rounded down).
 Continuing this example, the following is a diagram of the transmit
 window and the increment window therein in terms of sequence numbers.
     TXW_TRAIL                                     TXW_LEAD
        |                                             |
        |                                             |
     |--|--------------- Transmit Window -------------|----|
     v  |                                             |    v
        v                                             v
 n-1 |  n  | n+1 | ... | n+53 | n+54 | ... | n+381 | n+382 | n+383
                          ^
     ^                    |   ^
     |--- Increment Window|---|
                          |
                          |
                       TXW_INC
    So the values of the sequence numbers defining these windows are:
       TXW_TRAIL = n
       TXW_INC = n+53
       TXW_LEAD = n+382
    Nota Bene: In this example the window sizes in terms of sequence
    numbers can be determined only because of the assumption of a
    constant data packet size of 1500 bytes.  When the data packet
    sizes are variable, more or fewer sequence numbers MAY be consumed
    transmitting the same amount (TXW_BYTES) of data.
 So, for a given transport session identified by a TSI, a source
 maintains:

Speakman, et. al. Experimental [Page 100] RFC 3208 PGM Reliable Transport Protocol December 2001

 TXW_MAX_RTE    a maximum transmit rate in kBytes per second, the
                cumulative transmit rate of some combination of SPMs,
                ODATA, and RDATA depending on the transmit window
                advancement strategy
 TXW_TRAIL      the sequence number defining the trailing edge of the
                transmit window, the sequence number of the oldest
                data packet available for repair
 TXW_LEAD       the sequence number defining the leading edge of the
                transmit window, the sequence number of the most
                recently transmitted ODATA packet
 TXW_INC        the sequence number defining the leading edge of the
                increment window, the sequence number of the most
                recently transmitted data packet amongst those that
                will expire upon the next increment of the transmit
                window
 PGM does not constrain the strategies that a source may use for
 advancing the transmit window.  A source MAY implement any scheme or
 number of schemes.  This is possible because a PGM receiver must obey
 the window provided by the source in its packets.  Three strategies
 are suggested within this document.
 In the first, called "Advance with Time", the transmit window
 maintains the last TXW_SECS of data in real-time, regardless of
 whether any data was sent in that real time period or not.  The
 actual number of bytes maintained at any instant in time will vary
 between 0 and TXW_BYTES, depending on traffic during the last
 TXW_SECS.  In this case, TXW_MAX_RTE is the cumulative transmit rate
 of SPMs and ODATA.
 In the second, called "Advance with Data", the transmit window
 maintains the last TXW_BYTES bytes of data for repair.  That is, it
 maintains the theoretical maximum amount of data that could be
 transmitted in the time period TXW_SECS, regardless of when they were
 transmitted.  In this case, TXW_MAX_RTE is the cumulative transmit
 rate of SPMs, ODATA, and RDATA.
 The third strategy leaves control of the window in the hands of the
 application.  The API provided by a source implementation for this,
 could allow the application to control the window in terms of APDUs
 and to manually step the window.  This gives a form of Application
 Level Framing (ALF).  In this case, TXW_MAX_RTE is the cumulative
 transmit rate of SPMs, ODATA, and RDATA.

Speakman, et. al. Experimental [Page 101] RFC 3208 PGM Reliable Transport Protocol December 2001

16.2. Advancing with Data

 In the first strategy, TXW_MAX_RTE is calculated from SPMs and both
 ODATA and RDATA, and NAKs reset TXW_ADV_IVL_TMR.  In this mode of
 operation the transmit window maintains the last TXW_BYTES bytes of
 data for repair.  That is, it maintains the theoretical maximum
 amount of data that could be transmitted in the time period TXW_SECS.
 This means that the following timers are not treated as real-time
 timers, instead they are "data driven".  That is, they expire when
 the amount of data that could be sent in the time period they define
 is sent.  They are the SPM ambient time interval, TXW_ADV_SECS,
 TXW_SECS, TXW_ADV_IVL, TXW_ADV_IVL_TMR and the join interval.  Note
 that the SPM heartbeat timers still run in real-time.
 While TXW_ADV_IVL_TMR is running, a source uses the receipt of a NAK
 for ODATA within the increment window to reset timer TXW_ADV_IVL_TMR
 to TXW_ADV_IVL so that transmit window advancement is delayed until
 no NAKs for data in the increment window are seen for TXW_ADV_IVL
 seconds.  If the transmit window should fill in the meantime, further
 transmissions would be suspended until the transmit window can be
 advanced.
 A source MUST advance the transmit window across the increment window
 only upon expiry of TXW_ADV_IVL_TMR.
 This mode of operation is intended for non-real-time, messaging
 applications based on the receipt of complete data at the expense of
 delay.

16.3. Advancing with Time

 This strategy advances the transmit window in real-time.  In this
 mode of operation, TXW_MAX_RTE is calculated from SPMs and ODATA only
 to maintain a constant data throughput rate by consuming extra
 bandwidth for repairs.  TXW_ADV_IVL has the value 0 which advances
 the transmit window without regard for whether NAKs for data in the
 increment window are still being received.
 In this mode of operation, all timers are treated as real-time
 timers.
 This mode of operation is intended for real-time, streaming
 applications based on the receipt of timely data at the expense of
 completeness.

Speakman, et. al. Experimental [Page 102] RFC 3208 PGM Reliable Transport Protocol December 2001

16.4. Advancing under explicit application control

 Some applications may wish more explicit control of the transmit
 window than that provided by the advance with data / time strategies
 above.  An implementation MAY provide this mode of operation and
 allow an application to explicitly control the window in terms of
 APDUs.

17. Appendix G - Applicability Statement

 As stated in the introduction, PGM has been designed with a specific
 class of applications in mind in recognition of the fact that a
 general solution for reliable multicast has proven elusive.  The
 applicability of PGM is narrowed further, and perhaps more
 significantly, by the prototypical nature of at least four of the
 transport elements the protocol incorporates.  These are congestion
 control, router assist, local retransmission, and a programmatic API
 for reliable multicast protocols of this class.  At the same time as
 standardization efforts address each of these elements individually,
 this publication is intended to foster experimentation with these
 elements in general, and to inform that standardization process with
 results from practise.
 This section briefly describes some of the experimental aspects of
 PGM and makes non-normative references to some examples of current
 practise based upon them.
 At least 3 different approaches to congestion control can be explored
 with PGM: a receiver-feedback based approach, a router-assist based
 approach, and layer-coding based approach.  The first is supported by
 the negative acknowledgement mechanism in PGM augmented by an
 application-layer acknowledgement mechanism.  The second is supported
 by the router exception processing mechanism in PGM.  The third is
 supported by the FEC mechanisms in PGM.  An example of a receiver-
 feedback based approach is provided in [16], and a proposal for a
 router-assist based approach was proposed in [17].  Open issues for
 the researchers include how do each of these approaches behave in the
 presence of multiple competing sessions of the same discipline or of
 different disciplines, TCP most notably; how do each of them behave
 over a particular range of topologies, and over a particular range of
 loads; and how do each of them scale as a function of the size of the
 receiver population.
 Router assist has applications not just to implosion control and
 retransmit constraint as described in this specification, but also to
 congestion control as described above, and more generally to any
 feature which may be enhanced by access to per-network-element state
 and processing.  The full range of these features is as yet

Speakman, et. al. Experimental [Page 103] RFC 3208 PGM Reliable Transport Protocol December 2001

 unexplored, but a general mechanism for providing router assist in a
 transport-protocol independent way (GRA) is a topic of active
 research [18].  That effort has been primarily informed by the router
 assist component of PGM, and implementation and deployment experience
 with PGM will continue to be fed back into the specification and
 eventual standardization of GRA.  Open questions facing the
 researchers ([19], [20], [21]) include how router-based state scales
 relative to the feature benefit obtained, how system-wide factors
 (such as throughput and retransmit latency) vary relative to the
 scale and topology of deployed router assistance, and how incremental
 deployment considerations affect the tractability of router-assist
 based features.  Router assist may have additional implications in
 the area of congestion control to the extent that it may be applied
 in multi-group layered coding schemes to increase the granularity and
 reduce the latency of receiver based congestion control.
 GRA itself explicitly incorporates elements of active networking, and
 to the extent that the router assist component of PGM is reflected in
 GRA, experimentation with the narrowly defined network-element
 functionality of PGM will provide some of the first real world
 experience with this promising if controversial technology.
 Local retransmission is not a new idea in general in reliable
 multicast, but the specific approach taken in PGM of locating re-
 transmitters on the distribution tree for the session, diverting
 repair requests from network elements to the re-transmitters, and
 then propagating repairs downward from the repair point on the
 distribution tree raises interesting questions concerning where to
 locate re-transmitters in a given topology, and how network elements
 locate those re-transmitters and evaluate their efficiency relative
 to other available sources of retransmissions, most notably the
 source itself.  This particular aspect of PGM, while fully specified,
 has only been implemented on the network element side, and awaits a
 host-side implementation before questions like these can be
 addressed.
 PGM presents the opportunity to develop a programming API for
 reliable multicast applications that reflects both those
 applications' service requirements as well as the services provided
 by PGM in support of those applications that may usefully be made
 visible above the transport interface.  At least a couple of host-
 side implementations of PGM and a concomitant API have been developed
 for research purposes ([22], [23]), and are available as open source
 explicitly for the kind of experimentation described in this section.
 Perhaps the broadest experiment that PGM can enable in a community of
 researchers using a reasonable scale experimental transport protocol
 is simply in the definition, implementation, and deployment of IP

Speakman, et. al. Experimental [Page 104] RFC 3208 PGM Reliable Transport Protocol December 2001

 multicast applications for which the reliability provided by PGM is a
 significant enabler.  Experience with such applications will not just
 illuminate the value of reliable multicast, but will also provoke
 practical examination of and responses to the attendant policy issues
 (such as peering, billing, access control, firewalls, NATs, etc.),
 and, if successful, will ultimately encourage more wide spread
 deployment of IP multicast itself.

18. Abbreviations

 ACK     Acknowledgment
 AFI     Address Family Indicator
 ALF     Application Level Framing
 APDU    Application Protocol Data Unit
 ARQ     Automatic Repeat reQuest
 DLR     Designated Local Repairer
 GSI     Globally Unique Source Identifier
 FEC     Forward Error Correction
 MD5     Message-Digest Algorithm
 MTU     Maximum Transmission Unit
 NAK     Negative Acknowledgment
 NCF     NAK Confirmation
 NLA     Network Layer Address
 NNAK    Null Negative Acknowledgment
 ODATA   Original Data
 POLL    Poll Request
 POLR    Poll Response
 RDATA   Repair Data
 RSN     Receive State Notification
 SPM     Source Path Message
 SPMR    SPM Request
 TG      Transmission Group
 TGSIZE  Transmission Group Size
 TPDU    Transport Protocol Data Unit
 TSDU    Transport Service Data Unit
 TSI     Transport Session Identifier
 TSN     Transmit State Notification

Speakman, et. al. Experimental [Page 105] RFC 3208 PGM Reliable Transport Protocol December 2001

19. Acknowledgements

 The design and specification of PGM has been substantially influenced
 by reviews and revisions provided by several people who took the time
 to read and critique this document.  These include, in alphabetical
 order:
 Bob Albrightson
 Joel Bion
 Mark Bowles
 Steve Deering
 Tugrul Firatli
 Dan Harkins
 Dima Khoury
 Gerard Newman
 Dave Oran
 Denny Page
 Ken Pillay
 Chetan Rai
 Yakov Rekhter
 Dave Rossetti
 Paul Stirpe
 Brian Whetten
 Kyle York

20. References

 [1]   B. Whetten, T. Montgomery, S. Kaplan, "A High Performance
       Totally Ordered Multicast Protocol", in "Theory and Practice in
       Distributed Systems", Springer Verlag LCNS938, 1994.
 [2]   S. Floyd, V. Jacobson, C. Liu, S. McCanne, L. Zhang, "A
       Reliable Multicast Framework for Light-weight Sessions and
       Application Level Framing", ACM Transactions on Networking,
       November 1996.
 [3]   J. C. Lin, S. Paul, "RMTP: A Reliable Multicast Transport
       Protocol", ACM SIGCOMM August 1996.
 [4]   Miller, K., Robertson, K., Tweedly, A. and M. White, "Multicast
       File Transfer Protocol (MFTP) Specification", Work In Progress.
 [5]   Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
       1112, August 1989.
 [6]   Katz, D., "IP Router Alert Option", RFC 2113, February 1997.
 [7]   C. Partridge, "Gigabit Networking", Addison Wesley 1994.

Speakman, et. al. Experimental [Page 106] RFC 3208 PGM Reliable Transport Protocol December 2001

 [8]   H. W. Holbrook, S. K. Singhal, D. R. Cheriton, "Log-Based
       Receiver-Reliable Multicast for Distributed Interactive
       Simulation", ACM SIGCOMM 1995.
 [9]   Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
       1992.
 [10]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
       1700, October 1994.
 [11]  J. Nonnenmacher, E. Biersack, D. Towsley, "Parity-Based Loss
       Recovery for Reliable Multicast Transmission", ACM SIGCOMM
       September 1997.
 [12]  L. Rizzo, "Effective Erasure Codes for Reliable Computer
       Communication Protocols", Computer Communication Review, April
       1997.
 [13]  V. Jacobson, "Congestion Avoidance and Control", ACM SIGCOMM
       August 1988.
 [14]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP, 14, RFC 2119, March 1997.
 [15]  J. Bolot, T. Turletti, I. Wakeman, "Scalable Feedback Control
       for Multicast Video Distribution in the Internet", Proc.
       ACM/Sigcomm 94, pp.  58-67.
 [16]  L. Rizzo, "pgmcc: A TCP-friendly Single-Rate Multicast
       Congestion Control Scheme", Proc. of ACM SIGCOMM August 2000.
 [17]  M. Luby, L. Vicisano, T. Speakman. "Heterogeneous multicast
       congestion control based on router packet filtering", RMT
       working group, June 1999, Pisa, Italy.
 [18]  Cain, B., Speakman, T. and D. Towsley, "Generic Router Assist
       (GRA) Building Block, Motivation and Architecture", Work In
       Progress.
 [19]  C. Papadopoulos, and E. Laliotis,"Incremental Deployment of a
       Router-assisted Reliable Multicast Scheme,", Proc. of Networked
       Group Communications (NGC2000), Stanford University, Palo Alto,
       CA. November 2000.

Speakman, et. al. Experimental [Page 107] RFC 3208 PGM Reliable Transport Protocol December 2001

 [20]  C. Papadopoulos, "RAIMS: an Architecture for Router-Assisted
       Internet Multicast Services." Presented at ETH, Zurich,
       Switzerland, October 23 2000.
 [21]  J. Chesterfield, A. Diana, A. Greenhalgh, M. Lad, and M. Lim,
       "A BSD Router Implementation of PGM",
       http://www.cs.ucl.ac.uk/external/m.lad/rpgm/
 [22]  L. Rizzo, "A PGM Host Implementation for FreeBSD",
       http://www.iet.unipi.it/~luigi/pgm.html
 [23]  M. Psaltaki, R. Araujo, G. Aldabbagh, P. Kouniakis, and A.
       Giannopoulos, "Pragmatic General Multicast (PGM) host
       implementation for FreeBSD.",
       http://www.cs.ucl.ac.uk/research/darpa/pgm/PGM_FINAL.html

21. Authors' Addresses

 Tony Speakman
 EMail: speakman@cisco.com
 Dino Farinacci
 Procket Networks
 3850 North First Street
 San Jose, CA 95134
 USA
 EMail: dino@procket.com
 Steven Lin
 Juniper Networks
 1194 N. Mathilda Ave.
 Sunnyvale, CA 94086
 USA
 EMail: steven@juniper.net
 Alex Tweedly
 EMail: agt@cisco.com
 Nidhi Bhaskar
 EMail: nbhaskar@cisco.com
 Richard Edmonstone
 EMail: redmonst@cisco.com
 Rajitha Sumanasekera
 EMail: rajitha@cisco.com

Speakman, et. al. Experimental [Page 108] RFC 3208 PGM Reliable Transport Protocol December 2001

 Lorenzo Vicisano
 Cisco Systems, Inc.
 170 West Tasman Drive,
 San Jose, CA 95134
 USA
 EMail: lorenzo@cisco.com
 Jon Crowcroft
 Department of Computer Science
 University College London
 Gower Street
 London WC1E 6BT
 UK
 EMail: j.crowcroft@cs.ucl.ac.uk
 Jim Gemmell
 Microsoft Bay Area Research Center
 301 Howard Street, #830
 San Francisco, CA 94105
 USA
 EMail: jgemmell@microsoft.com
 Dan Leshchiner
 Tibco Software
 3165 Porter Dr.
 Palo Alto, CA 94304
 USA
 EMail: dleshc@tibco.com
 Michael Luby
 Digital Fountain, Inc.
 39141 Civic Center Drive
 Fremont CA  94538
 USA
 EMail: luby@digitalfountain.com
 Todd L. Montgomery
 Talarian Corporation
 124 Sherman Ave.
 Morgantown, WV 26501
 USA
 EMail: todd@talarian.com

Speakman, et. al. Experimental [Page 109] RFC 3208 PGM Reliable Transport Protocol December 2001

 Luigi Rizzo
 Dip. di Ing. dell'Informazione
 Universita` di Pisa
 via Diotisalvi 2
 56126 Pisa
 Italy
 EMail: luigi@iet.unipi.it

Speakman, et. al. Experimental [Page 110] RFC 3208 PGM Reliable Transport Protocol December 2001

22. Full Copyright Statement

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 This document and translations of it may be copied and furnished to
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 or assist in its implementation may be prepared, copied, published
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 Internet organizations, except as needed for the purpose of
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 followed, or as required to translate it into languages other than
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 The limited permissions granted above are perpetual and will not be
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 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
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Speakman, et. al. Experimental [Page 111]

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