RSVP over ATM Reqs: RSVP over ATM Implementation Requirements

Network Working Group                                          L. Berger
Request for Comments: 2380                                  FORE Systems
Category: Standards Track                                    August 1998


               RSVP over ATM Implementation Requirements

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

   This memo presents specific implementation requirements for running
   RSVP over ATM switched virtual circuits (SVCs).  It presents
   requirements that ensure interoperability between multiple
   implementations and conformance to the RSVP and Integrated Services
   specifications.  A separate document [5] provides specific guidelines
   for running over today's ATM networks.  The general problem is
   discussed in [9].   Integrated Services to ATM service mappings are
   covered in [6].  The full set of documents present the background and
   information needed to implement Integrated Services and RSVP over
   ATM.

Table of Contents

   1. Introduction .................................................  2
      1.1 Terms ....................................................  2
      1.2 Assumptions ..............................................  3
   2. General RSVP Session Support .................................  4
      2.1 RSVP Message VC Usage ....................................  4
      2.2 VC Initiation ............................................  4
      2.3 VC Teardown ..............................................  5
      2.4 Dynamic QoS ..............................................  6
      2.5 Encapsulation ............................................  6
   3. Multicast RSVP Session Support ...............................  7
      3.1 Data VC Management for Heterogeneous Sessions ............  7
      3.2 Multicast End-Point Identification .......................  8
      3.3 Multicast Data Distribution ..............................  9
      3.4 Receiver Transitions ..................................... 11
   4. Security Considerations ...................................... 11
   5. Acknowledgments .............................................. 11
   6. Author's Address ............................................. 12
   REFERENCES ...................................................... 13
   FULL COPYRIGHT STATEMENT ........................................ 14

1. Introduction

   This memo discusses running IP over ATM in an environment where SVCs
   are used to support QoS flows and RSVP is used as the internet level
   QoS signaling protocol.  It applies when using CLIP/ION, LANE2.0 and
   MPOA [4] methods for supporting IP over ATM.  The general issues
   related to running RSVP [8] over ATM have been covered in several
   papers including [9] and other earlier work.  This document is
   intended as a companion to [9,5].  The reader should be familiar with
   both documents.

   This document defines the specific requirements for implementations
   using ATM UNI3.x and 4.0.  These requirements must be adhered to by
   all RSVP over ATM implementations to ensure interoperability.
   Further recommendations to guide implementers of RSVP over ATM are
   provided in [5].

   The rest of this section will define terms and assumptions. Section 2
   will cover implementation guidelines common to all RSVP session.
   Section 3 will cover implementation guidelines specific to multicast
   sessions.

1.1 Terms

   The terms "reservation" and "flow" are used in many contexts, often
   with different meaning.  These terms are used in this document with
   the following meaning:

   o    Reservation is used in this document to refer to an RSVP
        initiated request for resources.  RSVP initiates requests for
        resources based on RESV message processing.  RESV messages that
        simply refresh state do not trigger resource requests.  Resource
        requests may be made based on RSVP sessions and RSVP reservation
        styles. RSVP styles dictate whether the reserved resources are
        used by one sender or shared by multiple senders.  See [8] for
        details of each. Each new request is referred to in this
        document as an RSVP reservation, or simply reservation.

   o    Flow is used to refer to the data traffic associated with a
        particular reservation.  The specific meaning of flow is RSVP
        style dependent.  For shared style reservations, there is one
        flow per session.  For distinct style reservations, there is one
        flow per sender (per session).

   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 [7].

1.2 Assumptions

   The following assumptions are made:

   o    RSVP

        We assume RSVP as the internet signaling protocol which is
        described in [8].  The reader is assumed to be familiar with
        [8].

   o    IPv4 and IPv6

        RSVP support has been defined for both IPv4 and IPv6.  The
        guidelines in this document are intended to be used to support
        RSVP with either IPv4 or IPv6.  This document does not require
        one version over the other.

   o    Best effort service model

        The current Internet only supports best effort service.  We
        assume that as additional components of the Integrated Services
        model are defined, best effort service must continue to be
        supported.

   o    ATM UNI 3.x and 4.0

        We assume ATM service as defined by UNI 3.x and 4.0.  ATM
        provides both point-to-point and point-to-multipoint Virtual
        Circuits (VCs) with a specified Quality of Service (QoS).  ATM
        provides both Permanent Virtual Circuits (PVCs) and Switched
        Virtual Circuits (SVCs).  In the Permanent Virtual Circuit (PVC)
        environment, PVCs are typically used as point-to-point link
        replacements.  So the support issues are similar to point-to-
        point links.  This memo assumes that SVCs are used to support
        RSVP over ATM.

2. General RSVP Session Support

   This section provides implementation requirements that are common for
   all (both unicast and multicast) RSVP sessions.  The section covers
   VC usage, QoS VC initiation, VC teardown, handling requested changes
   in QoS, and encapsulation.

2.1 RSVP Message VC Usage

   There are several RSVP Message VC Usage options available to
   implementers.  Implementers must select which VC to use for RSVP
   messages and how to aggregate RSVP sessions over QoS VCs.  These
   options have been covered in [9] and some specific implementation
   guidelines are stated in [5].  In order to ensure interoperability
   between implementations that follow different options, RSVP over ATM
   implementations MUST NOT send RSVP (control) messages on the same QoS
   VC as RSVP associated data packets.  RSVP over ATM implementations
   MAY send RSVP messages on either the best effort data path or on a
   separate control VC.

   Since RSVP (control) messages and RSVP associated data packets are
   not sent on the same VCs, it is possible for a VC supporting one type
   of traffic to fail while the other remains in place.  When the VC
   associated with data packets fails and cannot be reestablished, RSVP
   SHOULD treat this as an allocation failure.  When the VC used to
   forward RSVP control messages is abnormally released and cannot be
   reestablished, the RSVP associated QoS VCs MUST also be released.
   The release of the associated data VCs is required to maintain the
   synchronization between forwarding and reservation states for the
   associated data flows.

2.2 VC Initiation

   There is an apparent mismatch between RSVP and ATM. Specifically,
   RSVP control is receiver oriented and ATM control is sender oriented.
   This initially may seem like a major issue but really is not.  While
   RSVP reservation (RESV) requests are generated at the receiver,
   actual allocation of resources takes place at the subnet sender.

   For data flows, this means that subnet senders MUST establish all QoS
   VCs and the RSVP enabled subnet receiver MUST be able to accept
   incoming QoS VCs.  These restrictions are consistent with RSVP
   version 1 processing rules and allow senders to use different flow to
   VC mappings and even different QoS renegotiation techniques without
   interoperability problems.  All RSVP over ATM approaches that have
   VCs initiated and controlled by the subnet senders will interoperate.
   Figure 1 shows this model of data flow VC initiation.

                              Data Flow ==========>

                      +-----+
                      |     |      -------------->  +----+
                      | Src |    -------------->    | R1 |
                      |    *|  -------------->      +----+
                      +-----+       QoS VCs
                           /\
                           ||
                       VC  ||
                       Initiator

                     Figure 1: Data Flow VC Initiation

   RSVP over ATM implementations MAY send data in the backwards
   direction on an RSVP initiated QoS point-to-point VC.  When sending
   in the backwards data path, the sender MUST ensure that the data
   conforms to the backwards direction traffic parameters.  Since the
   traffic parameters are set by the VC initiator, it is quite likely
   that no resources will be requested for traffic originating at the
   called party.  It should be noted that the backwards data path is not
   available with point-to-multipoint VCs.

2.3 VC Teardown

   VCs supporting IP over ATM data are typically torndown based on
   inactivity timers.  This mechanism is used since IP is connectionless
   and there is therefore no way to know when a VC is no longer needed.
   Since RSVP provides explicit mechanisms (messages and timeouts) to
   determine when an associated data VC is no longer needed, the
   traditional VC timeout mechanisms are not needed. Additionally, under
   normal operations RSVP implementations expect to be able to allocate
   resources and have those resources remain allocated until released at
   the direction of RSVP.  Therefore, data VCs set up to support RSVP
   controlled flows should only be released at the direction of RSVP.
   Such VCs must not be timed out due to inactivity by either the VC
   initiator or the VC receiver.  This conflicts with VCs timing out as
   described in RFC 1755 [11], section 3.4 on VC Teardown.  RFC 1755
   recommends tearing down a VC that is inactive for a certain length of
   time. Twenty minutes is recommended.  This timeout is typically
   implemented at both the VC initiator and the VC receiver.  Although,
   section 3.1 of the update to RFC 1755 [12] states that inactivity
   timers must not be used at the VC receiver.

   In RSVP over ATM implementations, the configurable inactivity timer
   mentioned in [11] MUST be set to "infinite" for VCs initiated at the
   request of RSVP.  Setting the inactivity timer value at the VC
   initiator should not be problematic since the proper value can be
   relayed internally at the originator.  Setting the inactivity timer
   at the VC receiver is more difficult, and would require some
   mechanism to signal that an incoming VC was RSVP initiated.  To avoid
   this complexity and to conform to [12], RSVP over ATM implementations
   MUST not use an inactivity timer to clear any received connections.

2.4 Dynamic QoS

   As stated in [9], there is a mismatch in the service provided by RSVP
   and that provided by ATM UNI3.x and 4.0.  RSVP allows modifications
   to QoS parameters at any time while ATM does not support any
   modifications to QoS parameters post VC setup.  See [9] for more
   detail.

   The method for supporting changes in RSVP reservations is to attempt
   to replace an existing VC with a new appropriately sized VC. During
   setup of the replacement VC, the old VC MUST be left in place
   unmodified. The old VC is left unmodified to minimize interruption of
   QoS data delivery.  Once the replacement VC is established, data
   transmission is shifted to the new VC, and only then is the old VC
   closed.

   If setup of the replacement VC fails, then the old QoS VC MUST
   continue to be used.  When the new reservation is greater than the
   old reservation, the reservation request MUST be answered with an
   error. When the new reservation is less than the old reservation, the
   request MUST be treated as if the modification was successful.  While
   leaving the larger allocation in place is suboptimal, it maximizes
   delivery of service to the user.  The behavior is also required in
   order to conform to RSVP error handling as defined in sections 2.5,
   3.1.8 and 3.11.2 of [8].  Implementations SHOULD retry replacing a
   too large VC after some appropriate elapsed time.

   One additional issue is that only one QoS change can be processed at
   one time per reservation. If the (RSVP) requested QoS is changed
   while the first replacement VC is still being setup, then the
   replacement VC SHOULD BE released and the whole VC replacement
   process is restarted.  Implementations MAY also limit number of
   changes processed in a time period per [9].

2.5 Encapsulation

   There are multiple encapsulation options for data sent over RSVP
   triggered QoS VCs.  All RSVP over ATM implementations MUST be able to
   support LLC encapsulation per RFC 1483 [10] on such QoS VCs.
   Implementations MAY negotiate alternative encapsulations using the
   B-LLI negotiation procedures defined in ATM Signalling, see [11] for
   details.  When a QoS VC is only being used to carry IP packets,
   implementations SHOULD negotiate VC based multiplexing to avoid
   incurring the overhead of the LLC header.

3. Multicast RSVP Session Support

   There are several aspects to running RSVP over ATM that are unique to
   multicast sessions.  This section addresses multicast end-point
   identification, multicast data distribution, multicast receiver
   transitions and next-hops requesting different QoS values
   (heterogeneity) which includes the handling of multicast best effort
   receivers.  Handling of best effort receivers is not strictly an RSVP
   issue, but needs to be addressed by any RSVP over ATM implementation
   in order to maintain expected best effort internet service.

3.1 Data VC Management for Heterogeneous Sessions

   The issues relating to data VC management of heterogeneous sessions
   are covered in detail in [9].  In summary, heterogeneity occurs when
   receivers request different levels of QoS within a single session,
   and also when some receivers do not request any QoS.  Both types of
   heterogeneity are shown in figure 2.

                                 +----+
                        +------> | R1 |
                        |        +----+
                        |
                        |        +----+
           +-----+ -----+   +--> | R2 |
           |     | ---------+    +----+        Receiver Request Types:
           | Src |                             ---->  QoS 1 and QoS 2
           |     | .........+    +----+        ....>  Best-Effort
           +-----+ .....+   +..> | R3 |
                        :        +----+
                    /\  :
                    ||  :        +----+
                    ||  +......> | R4 |
                    ||           +----+
                  Single
               IP Mulicast
                  Group

                 Figure 2: Types of Multicast Receivers

   [9] provides four models for dealing with heterogeneity: full
   heterogeneity, limited heterogeneity, homogeneous, and modified
   homogeneous models.  No matter which model or combination of models
   is used by an implementation, implementations MUST NOT normally send
   more than one copy of a particular data packet to a particular next-
   hop (ATM end-point).  Some transient duplicate transmission is
   acceptable, but only during VC setup and transition.

   Implementations MUST also ensure that data traffic is sent to best
   effort receivers.  Data traffic MAY be sent to best effort receivers
   via best effort or QoS VCs as is appropriate for the implemented
   model.  In all cases, implementations MUST NOT create VCs in such a
   way that data cannot be sent to best effort receivers.  This includes
   the case of not being able to add a best effort receiver to a QoS VC,
   but does not include the case where best effort VCs cannot be setup.
   The failure to establish best effort VCs is considered to be a
   general IP over ATM failure and is therefore beyond the scope of this
   document.

   There is an interesting interaction between dynamic QoS and
   heterogeneous requests when using the limited heterogeneity,
   homogeneous, or modified homogeneous models.  In the case where a
   RESV message is received from a new next-hop and the requested
   resources are larger than any existing reservation, both dynamic QoS
   and heterogeneity need to be addressed.  A key issue is whether to
   first add the new next-hop or to change to the new QoS.  This is a
   fairly straight forward special case.  Since the older, smaller
   reservation does not support the new next-hop, the dynamic QoS
   process SHOULD be initiated first. Since the new QoS is only needed
   by the new next-hop, it SHOULD be the first end-point of the new VC.
   This way signaling is minimized when the setup to the new next-hop
   fails.

3.2 Multicast End-Point Identification

   Implementations must be able to identify ATM end-points participating
   in an IP multicast group.  The ATM end-points will be IP multicast
   receivers and/or next-hops.  Both QoS and best effort end-points must
   be identified.  RSVP next-hop information will usually provide QoS
   end-points, but not best effort end-points.

   There is a special case where RSVP next-hop information will not
   provide the appropriate end-points.  This occurs when a next-hop is
   not RSVP capable and RSVP is being automatically tunneled. In this
   case a PATH message travels through a non-RSVP egress router on the
   way to the next-hop RSVP node.  When the next-hop RSVP node sends a
   RESV message it may arrive at the source via a different route than
   used by the PATH message.  The source will get the RESV message, but
   will not know which ATM end-point should be associated with the
   reservation. For unicast sessions, there is no problem since the ATM
   end-point will be the IP next-hop router.  There is a problem with
   multicast, since multicast routing may not be able to uniquely
   identify the IP next-hop router.  It is therefore possible for a
   multicast end-point to not be properly identified.

   In certain cases it is also possible to identify the list of all best
   effort end-points.  Some multicast over ATM control mechanisms, such
   as MARS in mesh mode, can be used to identify all end-points of a
   multicast group.  Also, some multicast routing protocols can  provide
   all next-hops for a particular multicast group.  In both cases, RSVP
   over ATM implementations can obtain a full list of end-points, both
   QoS and non-QoS, using the appropriate mechanisms.  The full list can
   then be compared against the RSVP identified end-points to determine
   the list of best effort receivers.

   While there are cases where QoS and best effort end-points can be
   identified, there is no straightforward solution to uniquely
   identifying end-points of multicast traffic handled by non-RSVP
   next-hops.  The preferred solution is to use multicast control
   mechanisms and routing protocols that support unique end-point
   identification.  In cases where such mechanisms and routing protocols
   are unavailable, all IP routers that will be used to support RSVP
   over ATM should support RSVP. To ensure proper behavior, baseline
   RSVP over ATM implementations MUST only establish RSVP-initiated VCs
   to RSVP capable end-points.  It is permissible to allow a user to
   override this behavior.

3.3 Multicast Data Distribution

   Two basic models exist for IP multicast data distribution over ATM.
   In one model, senders establish point-to-multipoint VCs to all ATM
   attached destinations, and data is then sent over these VCs.  This
   model is often called "multicast mesh" or "VC mesh" mode
   distribution.  In the second model, senders send data over point-to-
   point VCs to a central point and the central point relays the data
   onto point-to-multipoint VCs that have been established to all
   receivers of the IP multicast group.  This model is often referred to
   as "multicast server" mode distribution. Figure 3 shows data flow for
   both modes of IP multicast data distribution.

                            _________
                           /         \
                          / Multicast \
                          \   Server  /
                           \_________/
                             ^  |  |
                             |  |  +--------+
              +-----+        |  |           |
              |     | -------+  |           |         Data Flow:
              | Src | ...+......|....+      V         ---->  Server
              |     |    :      |    :    +----+      ....>  Mesh
              +-----+    :      |    +...>| R1 |
                         :      |         +----+
                         :      V
                         :    +----+
                         +..> | R2 |
                              +----+

             Figure 3: IP Multicast Data Distribution Over ATM

   The goal of RSVP over ATM solutions is to ensure that IP multicast
   data is distributed with appropriate QoS.  Current multicast servers
   [1,2] do not support any mechanisms for communicating QoS
   requirements to a multicast server.  For this reason, RSVP over ATM
   implementations SHOULD support "mesh-mode" distribution for RSVP
   controlled multicast flows.  When using multicast servers that do not
   support QoS requests, a sender MUST set the service, not global,
   break bit(s). Use of the service-specific break bit tells the
   receiver(s) that RSVP and Integrated Services are supported by the
   router but that the service cannot be delivered over the ATM network
   for the specific request.

   In the case of MARS [1], the selection of distribution modes is
   administratively controlled.  Therefore network administrators that
   desire proper RSVP over ATM operation MUST appropriately configure
   their network to support mesh mode distribution for multicast groups
   that will be used in RSVP sessions.  For LANE1.0 networks the only
   multicast distribution option is over the LANE Broadcast and Unknown
   Server which means that the break bit MUST always be set.  For
   LANE2.0 [3] there are provisions that allow for non-server solutions
   with which it may be possible to ensure proper QoS delivery.

3.4 Receiver Transitions

   When setting up a point-to-multipoint VCs there will be a time when
   some receivers have been added to a QoS VC and some have not.

   During such transition times it is possible to start sending data on
   the newly established VC. If data is sent both on the new VC and the
   old VC, then data will be delivered with proper QoS to some receivers
   and with the old QoS to all receivers.  Additionally, the QoS
   receivers would get duplicate data.  If data is sent just on the new
   QoS VC, the receivers that have not yet been added will miss data.
   So, the issue comes down to whether to send to both the old and new
   VCs, or to just send to one of the VCs.  In one case duplicate data
   will be received, in the other some data may not be received.  This
   issue needs to be considered for three cases: when establishing the
   first QoS VC, when establishing a VC to support a QoS change, and
   when adding a new end-point to an already established QoS VC.

   The first two cases are essentially the same.  In both, it is
   possible to send data on the partially completed new VC.  In both,
   there is the option of duplicate or lost data.  In order to ensure
   predictable behavior and to conform to the requirement to deliver
   data to all receivers, data MUST NOT be sent on new VCs until all
   parties have been added.  This will ensure that all data is only
   delivered once to all receivers.

   The last case differs from the others and occurs when an end-point
   must be added to an existing QoS VC.  In this case the end-point must
   be both added to the QoS VC and dropped from a best effort VC.  The
   issue is which to do first.  If the add is first requested, then the
   end-point may get duplicate data.  If the drop is requested first,
   then the end-point may miss data.  In order to avoid loss of data,
   the add MUST be completed first and then followed by the drop.  This
   behavior requires receivers to be prepared to receive some duplicate
   packets at times of QoS setup.

4. Security Considerations

   The same considerations stated in [8] and [11] apply to this
   document.  There are no additional security issues raised in this
   document.

5. Acknowledgments

   This work is based on earlier drafts and comments from the ISSLL
   working group.  The author would like to acknowledge their
   contribution, most notably Steve Berson who coauthored one of the
   drafts.

6. Author's Address

   Lou Berger
   FORE Systems
   1595 Spring Hill Road
   5th Floor
   Vienna, VA 22182

   Phone: +1 703-245-4527
   EMail: lberger@fore.com

REFERENCES

   [1] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
       Networks," RFC 2022, November 1996.

   [2] The ATM Forum, "LAN Emulation Over ATM Specification", Version
       1.0.

   [3] The ATM Forum, "LAN Emulation over ATM Version 2 - LUNI
       Specification", April 1997.

   [4] The ATM Forum, "MPOA Baseline Version 1", May 1997.

   [5] Berger, L., "RSVP over ATM Implementation Guidelines", BCP 24,
       RFC 2379, August 1998.

   [6] Borden, M., and M. Garrett, "Interoperation of Controlled-Load
       and Guaranteed-Service with ATM", RFC 2381, August 1998.

   [7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

   [8] Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
       "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
       Specification", RFC 2205, September 1997.

   [9] Crawley, E., Berger, L., Berson, S., Baker, F., Borden, M., and
       J. Krawczyk, "A Framework for Integrated Services and RSVP over
       ATM", RFC 2382, August 1998.

   [10] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
        Layer 5", RFC 1483, July 1993.

   [11] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E., and
        A. Malis, "ATM Signalling Support for IP over ATM", RFC 1755,
        February 1995.

   [12] Maher, M., "ATM Signalling Support for IP over ATM - UNI 4.0
        Update", RFC 2331, April 1998.

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