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RMT Working Group     Generic Router Assist (GRA)              Brad Cain
INTERNET-DRAFT     for Multicast Transport Protocols     Cereva Networks
                                                           Tony Speakman
Expires January 2002                                               cisco
                                                             Don Towsley
                                                                   UMASS

                                                            20 July 2001


               Generic Router Assist (GRA) Building Block
                      Motivation and Architecture
                    <draft-ietf-rmt-gra-arch-02.txt>


   Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   Abstract

   Generic Router Assist (GRA) is a network-based service that enables
   end-to-end multicast transport protocols to take advantage of infor-
   mation distributed across the network elements in a given multicast
   distribution tree.  The service consists of a canonical set of simple
   functions which network elements may apply to selected packets in the
   transport session as they traverse the distribution tree.  The choice
   of function and the packet parameters to which it is applied can be
   defined and customized for a given transport session in a highly con-
   trolled fashion that still permits enough flexibility for GRA to be
   used to address a wide range of multicast transport problems not



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   amenable to end-to-end solution.  This document provides the motiva-
   tion and an architecture for GRA.

















































Cain/Speakman/Towsley                                           [Page 2]


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                             Table of Contents


   1.  Introduction ...............................................    4
   2.  Scope of Generic Router Assist .............................    6
   3.  Canonical Services and Functional Models/Examples ..........   11
   4.  Implementation Considerations ..............................   19
   Abbreviations ..................................................   22
   References .....................................................   23
   Revision History ...............................................   24
   Authors' Addresses .............................................   24








































Cain/Speakman/Towsley                                           [Page 3]


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   1.  Introduction

   The development of scalable end-to-end multicast protocols poses a
   tremendous challenge to network protocol designers. For example the
   development of reliable multicast protocols has received considerable
   attention in recent years. Most protocols are based on an end-to-end
   solution [SRM, RMTP, TKP] and have found the problem of scaling to
   1000s or even 100s of receivers daunting. The primary obstacles to
   the development of scalable protocols have been feedback implosion
   and transmission isolation. The first of these concerns the diffi-
   culty of a large multicast application to limit feedback from
   receivers to a data source or to each other. The second concerns the
   difficulty of limiting the transmission of data to the subset of a
   multicast group that requires it.

   Several proposals have been made to add functionality to routers for
   the purpose of improving the performance of multicast applications,
   particularly reliable multicast. Papadopoulos and Parulkar [LSM]
   introduced additional forwarding functionality to a router which
   would allow each router to identify a "special outgoing interface"
   over which to transmit a particular class of packets. They showed how
   this "turning point functionality" could be used to improve the per-
   formance of reliable multicast protocols. Levine and Garcia-Luna-
   Aceves [LABEL] proposed the addition of "routing labels" to routing
   tables which could be used to direct packets over specific inter-
   faces. One of these, called a distance label, was shown to be quite
   useful in reliable multicast for directing requests for repairs to
   nearby repair servers. The third and, perhaps most relevant proposal
   is the PGM protocol [PGM]. Briefly, PGM is a reliable multicast pro-
   tocol which uses negative acknowledgements (NAKs). The PGM protocol
   is an end-to-end transport protocol that contains a router component
   which performs NAK elimination and retransmission subcasting func-
   tionality. This proposal, like others [GMTS, BFS], is primarily
   motivated by PGM and the recognition of the benefits of exporting a
   set of flexible, simple router-based functionality for the purpose of
   multicast protocol design. Such functionality can significantly sim-
   plify the design of a large class of scalable multicast transport
   protocols.

   In this draft, we present Generic Router Assist (GRA) functionality
   intended to help protocol designers deal with the two problems of
   feedback implosion and transmission isolation. This functionality is
   designed to assist in the scaling of receiver feedback information
   and in providing subcasting for large multicast groups. It consists
   of simple filtering functions that reside within routers.

   Signaling protocols are used by hosts to set up and invoke this func-
   tionality. Briefly, a data source first initializes one or more



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   desired services on its multicast tree using GRA setup messages.  The
   GRA-capable routers on the tree then selectively eliminate feedback
   from receivers and/or isolate transmissions through the use of
   filters set by either the sender or the receivers. For robustness,
   periodic transmissions of setup messages on the multicast tree are
   used to refresh GRA state in the face of routing changes and other
   possible errors. It should be stressed that GRA services are only
   invoked for certain packets; data packets are usually not treated any
   differently and will not cause any additional processing in routers.

   GRA is not intended to provide sophisticated services which are dif-
   ficult or impossible to implement in routers.  GRA functionality is
   implemented at the IP layer and provides unreliable, best-effort ser-
   vices.  Transport protocols which make use of GRA must be robust in
   the face of failures and the absence of GRA-capable routers in the
   network.

   Before describing the details of GRA, we present a simple example in
   the context of a PGM-like reliable multicast protocol.

   Consider a NAK-based reliable multicast protocol which places the
   responsibility of packet loss detection on each receiver.  Each time
   that a receiver detects a loss (based on a gap in the sequence
   numbers of the packets that it receives), it unicasts a request for a
   repair (NAK) to the sender. Upon receipt of a NAK for a specific
   packet, the sender retransmits the packet to all receivers.

   This protocol faces considerable challenges in dealing with multiple
   NAKs for the same packet. First, there is the problem of the sender
   having to process many NAKs. Second, there is the problem of limiting
   the number of retransmissions to the same packet. GRA can be used to
   address these two problems. Prior to the transfer of any data, the
   application sets up a NAK elimination service at each GRA-capable
   router using a setup message. This service is set up to eliminate
   NAKs generated for the same packet. In addition, the service main-
   tains information regarding the interfaces over which it has received
   NAKs so that it can subcast the retransmission on the portion of the
   multicast tree that contains receivers requiring a retransmission of
   the packet.

   In Figure 1, we show how GRA can be used to eliminate feedback infor-
   mation in a reliable multicast transport protocol. In this figure, a
   multicast source (Src 1) transmits to two receivers (Rec 1 and Rec
   2). The data packets from Src 1 are treated as regular multicast
   packets and forwarded accordingly. On the link between router R1 and
   router R2, a data packet is lost. Assuming a NAK based reliable mul-
   ticast protocol, this loss causes the receivers to each send a NAK to
   the source for the packet that was lost. In the example, receivers



Cain/Speakman/Towsley                                           [Page 5]


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   include a GRA header in NAKs sent to the source.  Router R2 treats
   these NAKs in a special manner, elminating the redundant NAKs to the
   source. Therefore, only one NAK arrives at the source. We see from
   this example that only certain types of packets require additional
   processing at GRA routers and that the majority of end-to-end packets
   are forwarded according to normal multicast forwarding rules (i.e.
   without additional router processing).



                                          Src 1
                                          ^ | |
                                          | | | data packet
                                          | | |
                                           R1 |
                                          ^ | |
                                          | | V
                          R2 eliminates   | | X data packet dropped
                          duplicate  +--->R2 <---+
                          NAK        |  ---  ---  |
                                     | |        | |
                                     | |        | |
                                     | |        | |
                                      R3        R4
                                GRA  ^ |        | ^  GRA
                                NAK  | |        | |  NAK
                                     | |        | |
                                    Rec 1     Rec 2

       Figure 1. Example of network support for transport protocols.

   GRA may be used on all types of multicast forwarding trees.  However,
   GRA state is per-transport-session state and so requires per-
   transport-session state in routers in addition to the underlying mul-
   ticast routing state.

   2.  Scope of Generic Router Assist

   The types of services implemented with GRA are bounded by constraints
   and limitations of routing devices.  In this section we explicitly
   describe the limitations and constraints of routing devices and
   explain some reasonable services to implement in routers.

   We specifically describe router limitations in order to limit the
   scope of GRA.  A small set of GRA services in routers can assist in
   scaling many of the problems in end-to-end multicast.  Previous
   proposals[ACTIVE] have proposed complex elements that reside in
   routers to provide sophisticated capabilities.  These are probably



Cain/Speakman/Towsley                                           [Page 6]


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   unreasonable for current generation routers.  The approach of GRA is
   to provide a simple fixed set of services which give the maximum
   benefit with the least cost.

   2.1.  Service Properties

   GRA services are performed on a subset of packets sent between end-
   to-end transport protocols on the multicast distribution tree between
   a GRA Source and the set of GRA receivers.  Only routers on the dis-
   tribution tree for a particular GRA source act upon GRA packets.  The
   advantages of GRA type services can only be realized when the actions
   are performed on packets that are directly on the forwarding tree of
   a multicast group.

   In order to describe the requirements of GRA, we first describe the
   properties of appropriate services.

      Fixed: by fixed services we mean those which are statically part
      of router software or hardware.  We DO NOT mean dynamically load-
      able modules.  A fixed set of simple services will probably suf-
      fice for most of the scaling issues in transport protocols.

      Simple: We include only those services that are reasonable to be
      implemented in the forwarding path in routers.  These are services
      which can be performed with minimum CPU and memory overhead.

      Short Term: We provide services for which state and processing
      overhead is short lived.  GRA makes use of soft-state design prin-
      ciples.

   2.2.  GRA Requirements

   When considering the service and architecture of GRA, we adhere to
   the following principles:

      1. GRA services should be simple and must be fixed.  They must not
      require excessive processing in routing devices and they must not
      buffer packets.

      2. GRA services are not substitutes for well-engineered end-to-end
      protocol designs.  We support the end-to-end design principles of
      transport protocols.  GRA is an assist service which is designed
      to assist protocols in scaling aspects.

      3. GRA services will not take on active networking attributes such
      as dynamically uploadable modules or programming language propo-
      sals.




Cain/Speakman/Towsley                                           [Page 7]


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      4. GRA services should not directly participate in transport pro-
      tocols.  GRA should not be required for any transport protocols
      nor should any GRA services directly support a particular proto-
      col.

      5. GRA services should be those which may assist all or a reason-
      able subset of transport protocols.

      6. GRA services should be used for assisting in control packet
      operations.  GRA services should not be for the majority of pack-
      ets in a multicast group.

   2.3.  Constraints of networking devices

   Current generation routers perform processing of packets and execute
   routing and signaling protocols.  Routers perform fast packet for-
   warding on the forwarding path.  This is usually performed by
   hardware and software specifically designed for the forwarding of
   packets (and may include other functions such as policing, shaping,
   etc).  This is in contrast to the control plane of a router where
   control protocols are run in protocol-specific control components.
   Examples of these types of protocols are routing, management and sig-
   naling protocols.

   We now describe the role and impact of GRA services on these two
   planes of a router.

      Forwarding path: A router forwarding path usually consists of spe-
      cialized hardware and software which is designed specifically for
      the purpose of forwarding packets.  Newer routers also have abili-
      ties to perform other actions such as marking or policing for ser-
      vices such as QoS.  In general, the forwarding path of routers is
      very limited in the amount of state and complexity of processing
      that can be performed.  The processing complexity and state over-
      head of GRA services may limit the extent to which they may be
      implemented in the forwarding path.

      Control plane: Router control planes run embedded or general
      operating systems.  On top of these operating systems are imple-
      mentations of IP control protocols such as routing, signaling, and
      network management.  The processing power and memory of a control
      plane depends highly on the hardware design of the router.  Most
      routers use general purpose CISC or RISC processors for running
      the control plane operating system and protocols.  The control
      plane is limited in processing by its hardware and the load gen-
      erated from the other processes or tasks running.  We expect that
      complex or stateful GRA operations will be performed in the con-
      trol plane where state and processing power are more readily



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      available.  However, we do stress that routers generally have
      fairly slow control plane hardware.  This is to keep the cost low
      and because the processing required for control plane protocols is
      usually low.

   2.4.  State Constraints

   As we evaluate particular services which are candidates for inclusion
   in GRA, constraints in router state are an important consideration.
   We should select services which will not create substantial or long-
   lived state.

   2.4.1.  Session State

   Routers which perform multicast forwarding contain per-tree forward-
   ing state.  For trees rooted at multicast sources, this amounts to
   per-source, per-group forwarding entries.  This state must be kept in
   both the forwarding as well as the control plane.  GRA services are
   per-session, or per-source.  This is simply because GRA services are
   per-transport-session.

   The session state of GRA imposes additional state on routers.  Each
   session requires a state block describing the desired services.
   Other types of state may also be created during the course of a GRA
   session.  The GRA session state is the only state required for the
   length of a session; other state is set up and torn down in smaller
   intervals.

   2.4.2.  Per-Packet State

   The implementation of particular services may require per-packet
   state in GRA routers.  Services which require per-packet state should
   use short-lived timers to tear down this state so as to avoid an
   explosion in the amount of state at routers.  An example of a service
   which causes per-packet state is NAK elimination.  The use of small
   timers should minimize problems in state growth.

   2.4.3.  Buffering

   GRA services must not buffer packets.  GRA services may drop or
   modify packets in transit, but they will never buffer packets.

   The buffering of packets in routing devices is generally unacceptable
   due to the unpredictable behavior of such a service.  In addition
   this is, in general, an unreasonable service for routers to support
   without a significant payback in end-to-end scaling.





Cain/Speakman/Towsley                                           [Page 9]


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   2.5.  Processing Constraints

   GRA services may require differing amount of computation.  As a gen-
   eral rule, GRA services should require minimal computation and packet
   manipulation.

   2.5.1.  Computation

   Most GRA services should require minimal computation.  Services which
   require minimal computation are reasonable to implement in routing
   devices and minimize security risk.  Examples of some appropriate
   operations are:

      Comparison Operations: comparison operations may be performed on
      GRA packets against the GRA session state in the router in order
      to determine whether a service should be invoked.  Examples of
      comparison operations are: equal to, less than, greater than, etc.

      Update Operations: When receiving a GRA packet, a router may be
      required to update its GRA session state.  The operations required
      for updating the state should remain simple.  Example of simple
      operations are addition, subtraction, etc.

   In order to limit the computational complexity of GRA services, GRA
   operations should remain singular.  The combination of predicates and
   operations creates problems in both router processing and in GRA
   specifications themselves.  This restriction will avoid problems
   regarding operator and action precedence.

   2.5.2.  Buffer Operations

   One can define services requiring extensive packet manipulation by
   GRA routers.  This is probably expensive and therefore unreasonable
   for routers.  GRA services should be invoked without extensive mani-
   pulation of packets.  Services which update or overwrite fields are
   acceptable. Services which require the formation of new packets or
   accumulate information into new packets are unacceptable.

   2.6.  Examples of Reasonable Services

   In this section we briefly describe two GRA services and the ways in
   which they meet the above requirements.  The two example services are
   those that provide general functions which do not incur large amounts
   of state or processing.

      Elimination: Elimination is the selected dropping of redundant
      signaling.  An example of elimination is NAK elimination in a
      reliable multicast protocol.  Elimination is a service which



Cain/Speakman/Towsley                                          [Page 10]


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      requires little computational overhead.  It does however, require
      per-packet (or per-sequence-block) state.  This state can be con-
      trolled by the use of short soft state timers.

      Subcasting: Subcasting is the forwarding of a multicast packet to
      a subset of the multicast tree.  Subcasting is useful for a
      variety of multicast protocols.  Subcasting does not require a
      large amount of processing.  The state required is an identifier
      for the subtree and a list of outgoing interfaces.  This state is
      similar to multicast forwarding tree state.

   3.  Canonical Services and Functional Models/Examples

   While a variety of mechanisms must come together to enable a specific
   GRA service in a distribution tree (session path messages to estab-
   lish session parameters and neighbour information, a control protocol
   to define, enable, and disable specific filtering services, etc.),
   the basic mechanism of GRA consists of router based services that can
   be described in the language of filters, keys, and conditional func-
   tions (binary predicates and their outcomes).

   Using the example of reliable multicast presented in Figure 1., this
   section expands the model and terminology of filters to describe the
   full flexibility of GRA.  The intent here is to generalize the
   mechanism fully enough to be able to accommodate currently unspeci-
   fied requirements for multicast transport services as they emerge.

   Revisiting the example in Figure 1., note that the receipt of a loss
   report at a router implies several things.  The packet type implies
   that a type of filter should be established for the transport ses-
   sion, that the filter key is (a sequence number) of a particular
   length at a particular offset, that the loss report in hand should be
   forwarded, that the interface upon which the loss report was received
   should be recorded and associated with the key in the filter, and
   that subsequent matching loss reports on any interface should be
   eliminated.  The filter itself has other implied characteristics.  It
   eliminates only for a certain interval after forwarding any subse-
   quent loss report, and it has an implied lifetime after which it is
   locally expired by the router.

   Similarly in the case of sub-casting, the receipt of a retransmis-
   sion at a router implies several things.  The packet type implies
   that the packet should be matched against an existing filter type
   based on a key of a particular length and at a particular offset,
   that the packet itself should be forwarded on all interfaces for
   which a loss report associated with the key was recorded, and that
   the key's state should be discarded.  By implication, unmatched
   retransmissions should not be forwarded.



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   From this example some generalities emerge which, once they are
   extricated from their specific semantics, can be re-assembled in a
   variety of useful ways to provide router-based assistance for a
   broader class of transport services.

   3.1.  General Model

   The general model is one in which packets carry one of a tightly con-
   strained set of signals that alert the router to apply a pre-defined
   filtering service.

   The packet-borne signal conveys

      a filter type,
      an associated action,
      a key,
      and possible packet operands.

   The filter type specifies a particular router-based service.  The
   associated action specifies a particular function to carry out in the
   context of the service.  The key and any packet operands specify
   values upon which to operate in the context of the action.

   Canonical filtering services in routers can be defined by

      a filter type,
      associated supported actions,
      predicated on specific conditions,
      and three functions gated on that predicate whether TRUE or FALSE:

         f(p): how to dispose of the packet,
         f(s): how to transform the key's state, and
         f(v): how to transform the outgoing interface (OIF) list
               associated with the key.

   All of these as well as the offsets and lengths of the key and any
   packet operands for each supported action constitute the definition
   of the filtering service.

   3.2.  An Example of Elimination and Subcasting for ARQ

   Given this model, the handling of retransmissions in PGM can be
   described as a predicate eliminating and subcasting filter.

   Let IIF be the interface on which a packet is received.  Let RETX
   denote whether a packet retransmission has been requested either in a
   loss report (RQST RETX), as interface state (OIF RETX), or as key
   state (KEY RETX).  Let ET be the elimination timer for a particular



Cain/Speakman/Towsley                                          [Page 12]


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   key's state, and LT be the life timer for a particular key's state.

   The filtering service in the router supports two actions, one inbound
   (RCVR_UPDATE) and one outbound (FORWARD).

   3.2.1.  RCVR_UPDATE

   For RCVR_UPDATE, in addition to a transport session identifier, the
   following are defined for the signal in the packet:

      ELIM_SUBC   is the filter type
      RCVR_UPDATE is the action
      SQN         is the key
      RETX        is a packet-borne operand
                  (value of one in the PGM example)

   For RCVR_UPDATE, the following are defined for the filtering service
   in the router:

      In case the key fails to match existing key state:

         predicate: NOOP
         f(v):      OIF RETX = 1 for IIF
         f(s):      start ET, start LT, KEY RETX = RQST RETX
         f(p):      reverse forward to upstream neighbour

      In case the key matches existing key state:

         predicate: NOOP
         f(v):      OIF RETX = 1 for IIF
         f(s):      restart LT
         f(p):      discard

   3.2.2.  FORWARD

   For FORWARD, in addition to a transport session identifier, the fol-
   lowing are defined for the signal in the packet:

      ELIM_SUBC is the filter type
      FORWARD   is the action
      SQN       is the key

   For FORWARD, the following are defined for the filtering service in
   the router:

      In case the key fails to match existing key state:

         predicate: NOOP



Cain/Speakman/Towsley                                          [Page 13]


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         f(v):      NOOP
         f(s):      NOOP
         f(p):      discard

      In case the key matches existing key state:

         predicate: for all OIF RETXs, OIF RETX NE 0

         In case the predicate is TRUE:

            f(v): decrement OIF RETX
            f(s): NOOP
            f(p): forward on OIF

         In case the predicate is FALSE:

            f(v): NOOP
            f(s): NOOP
            f(p): discard

   Associated with the filtering service would be an additional house-
   keeping function which would discard a key's state if either LT
   expired or all the OIF COUNTs were zero.

   The point of this and subsequent examples is to demonstrate that this
   model for GRA can accommodate a highly functional set of router-based
   services which, given a transport-layer- independent implementation,
   could be provided by routers for general deployment by transport pro-
   tocols engineered to signal the routers in a generic way.  We now
   present a refinement of the PGM example adding forward error correc-
   tion.

   3.3.  An Example of Elimination and Subcasting with FEC

   In this example, a packet operand is used to carry a count of parity
   packets requested with the additional implication that the interface
   upon which the loss report was received along with the count of par-
   ity packets requested should be recorded and associated with the key
   in the filter, and that subsequent matching loss reports on any
   interface should be eliminated unless they request a larger number of
   parity packets than has been requested on any interface.

   Similarly upon forwarding parity packets implies that the packet
   itself should be forwarded on all interfaces with non-zero counts
   recorded as state associated with the key, that the corresponding
   count should be decremented by 1 per interface until it reaches 0,
   and that the key's state should be discarded once all counts are
   zero.



Cain/Speakman/Towsley                                          [Page 14]


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   The handling of parity NAKs and parity retransmissions in PGM can be
   described as a predicate eliminating and subcasting filter augmented
   by a packet operand, the number of parity packets requested.

   Let IIF be the interface on which a packet is received.  Let COUNT be
   the number of parity packets requested and recorded either in the
   loss report (RQST COUNT), as interface state (OIF COUNT), or as key
   state (HIGH COUNT).  Let ET be the elimination timer for a particular
   key's state, and LT be the life timer for a particular key's state.

   The filtering service in the router supports two actions, one inbound
   (RCVR_UPDATE) and one outbound (FORWARD).

   3.3.1.  RCVR_UPDATE

   For RCVR_UPDATE, in addition to a transport session identifier, the
   following are defined for the signal in the packet:

      ELIM_SUBC   is the filter type
      RCVR_UPDATE is the action
      SQN         is the key
      COUNT       is a packet-borne operand

   For RCVR_UPDATE, the following are defined for the filtering service
   in the router:

      In case the key fails to match existing key state:

         predicate: NOOP
         f(v):      OIF COUNT for IIF = MAX(RQST COUNT, 0)
         f(s):      start ET, start LT, HIGH COUNT = RQST COUNT
         f(p):      reverse forward to upstream neighbour

      In case the key matches existing key state:

         predicate: ET is running or RQST COUNT LEQ HIGH COUNT

         In case the predicate is TRUE:

            f(v): OIF COUNT for IIF = MAX(RQST COUNT, OIF COUNT for IIF)
            f(s): restart LT
            f(p): discard

         In case the predicate is FALSE:

            f(v): OIF COUNT for IIF = MAX(RQST COUNT, OIF COUNT for IIF)
            f(s): restart ET, HIGH COUNT = RQST COUNT
            f(p): reverse forward to upstream neighbour



Cain/Speakman/Towsley                                          [Page 15]


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   3.3.2.  FORWARD

   For FORWARD, in addition to a transport session identifier, the fol-
   lowing are defined for the signal in the packet:

      ELIM_SUBC is the filter type
      FORWARD   is the action
      SQN       is the key

   For FORWARD, the following are defined for the filtering service in
   the router:

      In case the key fails to match existing key state:

         predicate: NOOP
         f(v):      NOOP
         f(s):      NOOP
         f(p):      discard

      In case the key matches existing key state:

         predicate: for all OIF COUNTs, OIF COUNT NE 0

         In case the predicate is TRUE:

            f(v): decrement OIF COUNT
            f(s): NOOP
            f(p): forward on OIF

         In case the predicate is FALSE:

            f(v): NOOP
            f(s): NOOP
            f(p): discard

   Associated with the filtering service would be an additional house-
   keeping function which would discard a key's state if either LT
   expired or all the OIF COUNTs were zero.

   3.4.  An Example of a Surrogate Service for Subcasting

   The "turning point functionality" in LMS [LSM] cited above can be
   conceived of as enlisting the help of a willing surrogate to provide
   funtionality that may not supported natively in the network but may
   be supported by distinguished servers at the edge of the network
   (such as retransmitters).

   This example describes how surrogates can be supported in GRA. A



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   router first establishes state that enables a surrogate. This state
   is maintained per TSI and consists of the network-layer unicast
   address of the surrogate, the surrogate interface, and the surrogate
   current cost.  The state is established after receiving advertise-
   ments from surrogates. It is assumed that surrogates send such adver-
   tisements periodically. The router selects the surrogate that adver-
   tises the lowest cost.

   When a packet arrives requesting a service provided by the surrogate,
   the router forwards the packet as follows: if the packet came from
   the surrogate interface or no surrogate is known, it is forwarded
   upstream.  Otherwise, the packet is unicast to a known surrogate
   after recording the address of the incoming interface and the address
   of the surrogate interface in the GRA header.

   Finally, the surrogate unicasts a response packet to the router to be
   multicast on the interface on which the original request arrived.

   For each transport session, let SURR_ADDR be the network layer uni-
   cast address of the surrogate (initially NULL), let SURR_IF be the
   surrogate interface (initially NULL), and let SURR_COST be the cost
   of the current surrogate interface (initially infinity).  Let IIF be
   the interface on which a packet is received.

   The filtering service in the router supports three actions:
   SURR_UPDATE. FWD_TP, and FWD_OIF.

   SURR_UPDATE is the surrogate election action, FWD_TP is the turning
   point locator service, and FWD_OIF is the subcast service.

   Note that a packet using the service FWD_TP does not carry the turn-
   ing point Information (i.e the PKT_IIF and PKT_OIF are NULL).


   3.4.1.

   For SURR_UPDATE, the following are defined for the signal in the
   packet:

      SURROGATE   is the filter type
      SURR_UPDATE is the action
      ADDR        is a packet-borne operand
      COST        is a packet-borne operand

   For SURR_UPDATE, the following are defined for the filtering service
   at the router:

      predicate: COST < SURR_COST



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      In case the predicate is true:

         f(v): NOOP
         f(s): SURR_ADDR = ADDR, SURR_IF = IIF, SURR_COST = COST
         f(p): reverse forward to upstream neighbor

      In case the predicate is false:

         f(v): NOOP
         f(s): NOOP
         f(p): discard

   3.4.2.

   For FWD_TP, the following are defined for the signal in the packet.

      SURROGATE        is the filter type
      FWD_TP           is the action
      PKT_IIF, PKT_OIF are packet_borne variables

   For FWD_TP, the following are defined for the filtering service at
   the router:

      predicate: ((SIF != NULL) && (IIF != SIF))

      In case the predicate is TRUE

         f(v): NOOP
         f(s): NOOP
         f(p): PKT_IIF = IIF and PKT_OIF = Surrogate IF
               (i.e Insert the Turning Point information)
               Unicast to SURR_ADDR

      In case the predicate is FALSE

         f(v): NOOP
         f(s): NOOP
         f(p): Unicast to upstream GRA neighbour

   3.4.3.

   For FWD_OIF the following are defined for the signal in the packet:

      SURROGATE is the filter type
      FWD_OIF   is the action
      PKT_OIF   is a packet-borne operand

   For FWD_OIF, the following are defined for the filtering service at



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   the router:

      predicate: for all interfaces in PKT_OIF list:
      f(v): NOOP
      f(s): NOOP
      f(p): Multicast the packet on PKT_OIF
            to the S,G associated with the TSI

   3.5.  Summary

   The point of these examples is to demonstrate that this model for GRA
   can accommodate a highly functional set of router-based services
   which, given a transport-layer-independent implementation, could be
   provided by routers for general deployment by transport protocols
   engineered to signal the routers in a generic way.  Now that we have
   established the context and a model for GRA, the next section
   discusses implementation considerations in the interest of making the
   mechanisms of GRA more concrete, and highlighting their practical and
   performance consequences to traditional multicast forwarding.

   4.  Implementation Considerations

   4.1.  Signalling Protocol - GRA service indicators

   A consideration of the implementation issues attending GRA strongly
   determines the class of functions that may be realized with this
   mechanism.  These considerations relate to both security of the
   router and performance in the forwarding path.  The former will be
   dealt with in another section.  This section outlines the time and
   space scaling consequences of GRA to the forwarding path.

   To be a generic network layer service, it's clear that some minimal
   indicator is required in the network layer to signal the presence of
   GRA signal on a packet.  As has been previously noted, remember that
   the GRA indicator is typically NOT borne by basic data packets.
   These are switched without exception in the usual forwarding path.
   In this section, packets bearing a GRA indicator will be referred to
   as GRA packets.

   A network layer indicator frees forwarding engines from the burden of
   having to walk into and parse transport headers on every switched
   packet.  It's highly efficient to encode the GRA indicator on the
   network layer header since that header is typically already within
   the grasp of the forwarding engine.  In PGM, this indicator is imple-
   mented with an IP Router Alert option, but a single bit in the basic
   IP header would function just as well (even better, actually, for
   legacy routers).




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   Once GRA packets can be detected in the forwarding path, the next
   step is to locate and parse the GRA parameters:  the filter type, the
   action, the key, and the operands from above.  These should be
   encoded as TLVs located somewhere between the end of the network
   layer header and the beginning of the transport layer payload or SDU.

   It's tempting in the case of the key and the packet operands to con-
   sider encoding their offsets and lengths in a TLV that could be used
   to locate the actual values themselves in the transport header or,
   more wildly, in the SDU, but this would amount to providing a near
   programmatic directive to the network element which is fraught with
   risk.  So note that, in the example of elimination above, the number
   of loss reports requested would, in this model, be encoded both in
   its natural place in the transport header, and also as a GRA-specific
   operand in a separate GRA TLV.

   An alternative is to establish the location and length of keys and
   packet operands as attributes of the filtering service defined in the
   network element itself so that the only vulnerability to the network
   elements would derive from abuse of the setup protocol in the control
   plane in place of vulnerability in the forwarding path or data plane.

   The location of GRA TLVs before, inside, or after the transport layer
   header is at the crux of whether GRA is regarded as integral to a
   specific layer or as a shim layer between layers.  The answer to the
   question determines not just where to locate the TLVs but also where
   to implement the service in host protocol stacks.

   As for forwarding-time implementation of GRA services, we take it as
   a requirement for this specification that some services must be
   light-weight enough to be candidates for optimized implementations in
   hardware or firmware, while others may be sufficiently complex to
   warrant software implementations.  Soft-state-based services that do
   not maintain per-packet state may be in the first class; services
   that maintain per-packet state and therefor require a
   sorted/searchable data structure may be in the second class.

   Beyond these very GRA-specific issues are the more general control
   protocol issues of how to interoperate network elements with
   disparate GRA capabilities, and all of the specifics of defining,
   enabling, and disabling filters themselves.  These issues are best
   treated once a solid model of the GRA mechanism itself is esta-
   blished.

   4.2.  Control Protocol - Filter definition, enabling, and disabling






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   4.3.  Control Protocol - Session path messages and neighbour informa-
   tion

















































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   Abbreviations

   IIF     Incoming Interface
   NAK     Negative Acknowledgement
   NOOP    No Operation
   OIF     Outgoing Interface
   SDU     Service Data Unit
   SQN     Sequence Number
   TLV     Type Length Value










































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   References

   [ACTIVE] L.H. Lehman, S.J. Garland, D. L. Tennenhouse. "Active Reli-
   able Multicast", Proc. INFOCOM'98, 1998.

   [BFS] Koichi Yano, Steven McCanne, "The Breadcrumb Forwarding Service
   and the Digital Fountain Rainbow: Toward a TCP-Friendly Reliable Mul-
   ticast", UCB/CSD Technical Report No. UCB//CSD-99-1068

   [GMTS] Brad Cain, Don Towsley, "Generic Multicast Transport Services
   (GMTS)", Nortel Networks Technical Report, December 1998.

   [KKT] S. Kasera, J. Kurose, D. Towsley. "A Comparison of Server-Based
   and Receiver-Based Local Recovery Approaches for Scalable Reliable
   Multicast", Proc. INFOCOM'98, 1998.

   [LABEL] B.N. Levine, J.J. Garcia-Luna-Aceves. "Improving Internet
   Multicast with Routing Labels", Proc. ICNP-97, pp. 241-250, Oct.
   1997.

   [LSM] C. Papadopoulos, G. Parulkar. "An Error Control Scheme for
   Large-Scale Multicast Applications", Proc. INFOCOM'98.

   [PGM] T. Speakman, D. Farinacci, S. Lin, A. Tweedly. "PGM Reliable
   Transport Protocol", IETF draft-speakman-pgm-spec-03.txt, June, 1999.

   [RMTP] J. Lin, S. Paul. "RMTP: A reliable multicast transport proto-
   col", Proc. of IEEE INFOCOM'95, 1995.

   [SRM] S. Floyd, V. Jacobson, S. McCanne, C. Lin, L. Zhang. "A Reli-
   able Multicast Framework for Light-weight Sessions and Application
   Level Framing", IEEE/ACM Trans on Networking, vol. 5, 784-803, Dec.
   1997.

   [TKP] D. Towsley, J. Kurose, S. Pingali. "A comparison of sender-
   initiated and receiver-initiated reliable multicast protocols" IEEE
   JSAC, April 1997.














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   Revision History

      draft-ietf-rmt-gra-00.txt October 1999

         Original draft.

      draft-ietf-rmt-gra-01.txt March 2000

         Added the example of simple elmination and subcasting.

      draft-ietf-rmt-gra-02.txt July 2001

         Replaced references to "suppression" and "aggregation" with
         "elimination".
         Replaced references to "aggregate" with "accumulate".
         Changed "should" to "must" in principle #1.
         Added the surrogate example.

   Authors' Addresses

           Brad Cain
           bcain@cereva.com

           Tony Speakman
           speakman@cisco.com

           Don Towsley
           towsley@cs.umass.edu























Cain/Speakman/Towsley                                          [Page 24]


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