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Versions: 00                                                            
INTERNET-DRAFT                                        Werner Almesberger
                                                     Jean-Yves Le Boudec
                                                   EPFL ICA, Switzerland
                                                         Tiziana Ferrari
                                     DEIS, Uni Bologna, INFN/CNAF, Italy
                                                              March 1998

              Encoding of SRP packet types in the DS byte

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its
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   We propose an encoding of the packet types used by SRP (Scalable
   Reservation Protocol) in the DS byte under study by the
   Differentiated Services working group.

1. Introduction

   SRP [1] is a light-weight reservation protocol designed to provide
   dependable reservations for the Internet. SRP aggregates flows in
   routers to achieve good scalability to very large numbers of
   individual flows.

   The semantics of the actual reservation protocol can be expressed on
   a per-packet and per-hop basis. We describe the desired behaviour of
   routers and define a possible encoding of the SRP packet types in the
   "TOS octet" or "DS byte" as defined in the context of the IETF
   Differentiated Services (diffserv) working group [2].

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   Although diffserv targets slightly different semantics when using the
   TOS octet, we believe this octet to be a highly appropriate location
   for indicating the use of other, packet-based mechanisms to control
   QoS. With this draft, we intend to advocate that the specification
   produced by the diffserv working group should allow for such uses,
   and that it should provide room for the corresponding code points.

   We base our description on the initial draft proposal for using the
   TOS octet [3] and we acknowledge that the exact layout of the TOS
   octet is still subject to ongoing discussion and may change in the
   future. We plan to update this document accordingly.

   Note that our description of possible uses of the DS byte only
   reflects our own design ideas and shall not be interpreted as
   implying any endorsement by the diffserv working group or by related

2. Reservations

   SRP sets up reservations for aggregated flows. The exact definition
   of what constitutes an aggregated flow is a local issue of each
   router. Simple definitions may be obtained by considering all flows
   as aggregates, which:

     - leave the router at the same port, or
     - traverse the router through the same pair of ports.

   A reservation means that a router has set aside sufficient resources
   to process and forward conforming packets using the reservation, and
   to do so in timely way. A packet conforms to a reservation if

     - it traverses the router through the same pair of ports as the
       packet(s) that have set up the reservation,
     - the source has shaped it according to the shaping requirements
       for sources (TBD) and all routers have forwarded it according to
       rules specified in this document, and
     - it does not request any other service(s) from the router (by
       whatever means the router supports) than the one(s) used for the
       packet(s) that have set up the reservation.

3. Packet types

   We distinguish three types of packets:

     - RESERVED packets use a previously agreed-upon reservation.
       Sources may emit RESERVED packets only if a reservation was
       granted and only according to the shaping requirements. After
       optionally performing policing, routers must include such packets
       in their estimate of current resource use, and forward them with
       priority over best-effort.

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     - flows of REQUEST packets have the effect of setting up a new
       reservation or increasing an existing one. This specification
       does not impose any restrictions on when or how often a source
       may send REQUEST packets. When receiving a REQUEST packet, a
       router must decide if it has sufficient resources to increase the
       present reservation. If yes, it updates its estimate accordingly
       and forwards REQUEST the packet like it forwards RESERVED
       packets. Otherwise, it degrades the packet to DENIED and
       processes it accordingly.
     - DENIED packets are treated similar to best-effort, but a router
       may discard DENIED packets even with higher probability than
       normal best-effort, because DENIED packets will typically not be
       congestion-controlled by the source. SRP does not allow a source
       to emit packets of type DENIED (but, if the encoding of the
       DENIED packet type coincides with a packet type defined
       elsewhere, a source may of course emit such packets according to
       that other specification)

   As a local option, routers may entrust other routers to make
   acceptance decisions for them. Doing so does not remove their
   obligation to forward RESERVED and REQUEST packets according to this

   Estimator algorithms are not specified in this document. We study
   examples for such algorithms in [4].

   Destinations identify the types of incoming packets and generate
   feedback for the source. Further details of this are TBD.

4. Encoding of packet types

   We propose two possible ways for encoding SRP packet types in the DS
   byte. Criteria for deciding which of the proposals is appropriate are
   discussed below.

4.1 Proposal 1

      Assuming a PHB (per-hop behaviour) code point is assigned to SRP,
      we define the encoding of SRP packet types in values of the DS
      byte as follows:

      SRP packet |     IN        PHB         CU
      type       | (in/out of  (per-hop  (currently
                 |  profile)  behaviour)  unused)
      RESERVED   |   in(1)       SRP         -
      REQUEST    |   out(0)      SRP         -
      DENIED     |   out(0)       DE         -

      Where DE is the default best-effort forwarding described in [3].

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4.2 Proposal 2

      Recent discussion in the diffserv group has indicated that perhaps
      only selected routers may be allowed to change the DS byte. Not
      allowing every router to change the DS byte would conflict with
      SRP's potential need to perform acceptance control at every
      router, and consequently to degrade some REQUEST packets to
      DENIED. If such restrictions should be put on the use of the DS
      byte, we propose using the ECN bit (Explicit Congestion
      Notification, [5]), which - much like SRP packet types - may be
      set at any router, and which carries similar information. Then,
      the encoding would be as follows:

      SRP packet |   IN   PHB   ECN   ECN
      type       |            Capable
      RESERVED   | in(1)  SRP   TBD   TBD
      REQUEST    | out(0) SRP   TBD    0
      DENIED     | out(0) SRP   TBD    1

      Note that the use of the ECN bits as described in this second
      proposal does not interfere with employing them for implementing
      congestion control on top of reservations (like it is done in
      Frame Relay [6]), if a need for this should arise.

      Because the exact use of ECN and the relation between DS and ECN
      are still the subject of vivid discussion, this latter encoding
      proposal should be considered as even more volatile than the
      previous one.

5. References

     [1]  Almesberger, Werner; Ferrari, Tiziana; Le Boudec, Jean-Yves.
       SRP essentials (work in progress), Internet Draft
       draft-watfjyl-srp-00.txt, March 1998.
     [2]  IETF, Differentiated Services (diffserv) working group.
     [3]  Nichols, Kathleen (Ed.); Blake, Steven (Ed.).  Differentiated
       Services Operational Model and Definitions (work in progress),
       Internet Draft draft-nichols-dsopdef-00.txt, February 1998.
     [4]  Almesberger, Werner; Ferrari, Tiziana; Le Boudec, Jean-Yves.
       SRP: a Scalable Resource Reservation Protocol for the Internet,
       Technical Report SSC/1998/009, EPFL, March 1998.
     [5]  Ramakrishnan, Kadangode K.; Floyd, Sally.  A Proposal to add
       Explicit Congestion Notification (ECN) to IPv6 and to TCP (work
       in progress), Internet Draft draft-kksjf-ecn-00.txt, November
     [6]  ITU-T Recommendation X.36.  Interface between data terminal
       equipment (DTE) and data circuit-terminating equipment (DCE) for
       public data networks providing frame relay data transmission

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       service by dedicated circuit, ITU, April 1995.

6. Author's address

   Werner Almesberger
   Jean-Yves Le Boudec
   Institute for computer Communications and Applications
   Swiss Federal Institute of Technology (EPFL)
   CH-1015 Lausanne
   email: Werner.Almesberger,Leboudec@epfl.ch

   Tiziana Ferrari
   DEIS, University of Bologna
   viale Risorgimento, 2
   I-40136 Bologna
   Italian National Inst. for Nuclear Physics/CNAF
   viale Berti Pichat, 6/2
   I-40127 Bologna
   email: Tiziana.Ferrari@cnaf.infn.it

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