rfc3208.txt

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   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.



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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.




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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.



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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.







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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.






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      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.








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RFC 3208            PGM Reliable Transport Protocol        December 2001