LinuxDig.com Request For Comments

RFC Number : 3208 Title : PGM Reliable Transport Protocol Specification. 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 on