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Internet Draft                                                 S. Berson
Expiration: September 1997                                           ISI
File: draft-ietf-issll-atm-support-03.txt                      L. Berger
                                                            FORE Systems



               IP Integrated Services with RSVP over ATM



                             March 26, 1997

Status of Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-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."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).

Abstract

   This draft describes a method for providing IP Integrated Services
   with RSVP over ATM switched virtual circuits (SVCs).  It provides an
   overall approach to the problem as well as a specific method for
   running over today's ATM networks.  There are two parts of this
   problem.  This draft provides guidelines for using ATM VCs with QoS
   as part of an Integrated Services Internet.  A related draft[6]
   describes service mappings between IP Integrated Services and ATM
   services.


Authors' Note

   The postscript version of this document contains figures that are not
   included in the text version, so it is best to use the postscript
   version.  Figures will be converted to ASCII in a future version.



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Table of Contents


   1. Introduction ........................................................3
      1.1 Terms ...........................................................4
      1.2 Assumptions .....................................................4
   2. Policy ..............................................................6
   3. Data VC Management ..................................................7
      3.1 Reservation to VC Mapping .......................................7
      3.2 Heterogeneity ...................................................8
      3.3 Multicast End-Point Identification ..............................12
      3.4 Multicast Data Distribution .....................................13
      3.5 Receiver Transitions ............................................14
      3.6 Dynamic QoS .....................................................14
         4.   Tear down old VC ............................................16
         5.   Activate timer ..............................................16
   6. Security ............................................................21
   7. Future Work .........................................................21
   8. Authors' Addresses ..................................................23
































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1. Introduction

   The Internet currently has one class of service normally referred to
   as "best effort."  This service is typified by first-come, first-
   serve scheduling at each hop in the network. Best effort service has
   worked  well for electronic mail, World Wide Web (WWW) access, file
   transfer (e.g. ftp), etc.  For real-time traffic such as voice and
   video, the current Internet has performed well only across unloaded
   portions of the network. In order to provide guaranteed quality
   real-time traffic, new classes of service and a QoS signalling
   protocol are being introduced in the Internet[7,18,17], while
   retaining the existing best effort service.  The QoS signalling
   protocol is RSVP[8,19], the Resource ReSerVation Protocol.

   ATM is rapidly becoming an important link layer technology.  One of
   the important features of ATM technology is the ability to request a
   point-to-point Virtual Circuit (VC) with a specified Quality of
   Service (QoS). An additional feature of ATM technology is the ability
   to request point-to-multipoint VCs with a specified QoS.  Point-to-
   multipoint VCs allows leaf nodes to be added and removed from the VC
   dynamically and so provide a mechanism for supporting IP multicast.
   It is only natural that RSVP and the Internet Integrated Services
   (IIS) model would like to utilize the QoS properties of any
   underlying link layer including ATM.

   Classical IP over ATM[11] has solved part of this problem, supporting
   IP unicast best effort traffic over ATM. Classical IP over ATM is
   based on a Logical IP Subnetwork (LIS), which is a separately
   administered IP sub-network.  Hosts within a LIS communicate using
   the ATM network, while hosts from different sub-nets communicate only
   by going through an IP router (even though it may be possible to open
   a direct VC between the two hosts over the ATM network).  Classical
   IP over ATM provides an Address Resolution Protocol (ATMARP) for ATM
   edge devices to resolve IP addresses to native ATM addresses.  For
   any pair of IP/ATM edge devices (i.e. hosts or routers), a single VC
   is created on demand and shared for all traffic between the two
   devices.  A second part of the RSVP and IIS over ATM problem, IP
   multicast, is close to being solved with MARS[1], the Multicast
   Address Resolution Server.  MARS compliments ATMARP by allowing an IP
   address to resolve into a list of native ATM addresses, rather than
   just a single address.

   A key remaining issue for IP over ATM is the integration of RSVP
   signalling and ATM signalling in support of the Internet Integrated
   Services (IIS) model.  There are two main areas involved in
   supporting the IIS model, QoS translation and VC management.  QoS
   translation concerns mapping a QoS from the IIS model to a proper ATM
   QoS, while VC management concentrates on how many VCs are needed and



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   which traffic flows are routed over which VCs.  Mapping of IP QoS to
   ATM QoS is the subject of a companion draft[6].

   This draft concentrates on VC management (and we assume in this draft
   that the QoS for a single reserved flow can be acceptably translated
   to an ATM QoS).  Two types of VCs need to be managed, data VCs which
   handle the actual data traffic, and control VCs which handle the RSVP
   signalling traffic.  Several VC management schemes for both data and
   control VCs are described in this draft.  For each scheme, there are
   two major issues - (1) heterogeneity and (2) dynamic behavior.
   Heterogeneity refers to how requests for different QoS's are handled,
   while dynamic behavior refers to how changes in QoS and changes in
   multicast group membership are handled.  These schemes will be
   evaluated in terms of the following metrics - (1) number of VCs
   needed to implement the scheme, (2) bandwidth wasted due to duplicate
   packets, and (3) flexibility in handling heterogeneity and dynamic
   behavior. Examples where each scheme is most applicable are given.

   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).

   1.2 Assumptions

      The following assumptions are made:


      o    Support for IPv4 and IPv6 best effort in addition to QoS



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      o    Use RSVP with policy control as signalling protocol

      o    Assume UNI 3.x and 4.0 ATM services

      o    VCs initiation by sub-net senders

      1.2.1 IPv4 and IPv6

         Currently IPv4 is the standard protocol of the Internet which
         now provides only best effort service.  We assume that best
         effort service will continue to be supported while introducing
         new types of service according to the IP Integrated Services
         model.  We also assume that IPv6 will be supported as well as
         IPv4.

      1.2.2 RSVP and Policy

         We assume RSVP as the Internet signalling protocol which is
         described in [19].  The reader is assumed to be familiar with
         [19].

         IP Integrated Services discriminates between users by providing
         some users better service at the expense of others.  For ATM,
         preferential service reflects when a new VC is created for a
         user rather than joining an existing VC.  Policy determines how
         preferential services are allocated while allowing network
         operators maximum flexibility to provide value-added services
         for the marketplace.  Mechanisms need to be be provided to
         enforce access policies.  These mechanisms may include such
         things as permissions and/or billing.

         For scaling reasons, policies based on bilateral agreements
         between neighboring providers are considered.  The bilateral
         model has similar scaling properties to multicast while
         maintaining no global information.  Policy control is currently
         being developed for RSVP (see [10] for details).

      1.2.3 ATM

         We assume ATM defined by UNI 3.x and 4.0, plus TM 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 Integrated Services support issues
         are similar to point-to-point links.  This draft describes
         schemes for supporting Integrated Services using SVCs.



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      1.2.4 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 sub-net sender.

         For data flows, this means that sub-net senders will establish
         all QoS VCs and the sub-net 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
         sub-net senders will interoperate.  Figure  shows this model of
         data flow VC initiation.

         [Figure goes here]
                   Figure 1: Data Flow VC Initiation


         The use of the reverse path provided by point-to-point VCs by
         receivers is for further study. There are two related issues.
         The first is that use of the reverse path requires the VC
         initiator to set appropriate reverse path QoS parameters.  The
         second issue is that reverse paths are not available with
         point-to-multipoint VCs, so reverse paths could only be used to
         support unicast RSVP reservations.  Receivers initiating VCs
         via the reverse path mechanism provided by point-to-point VCs
         is also for future study.

2. Policy

   RSVP allows for local policy control [10] as well as admission
   control.  Thus a user can request a reservation with a specific QoS
   and with a policy object that, for example, offers to pay for
   additional costs setting up a new reservation.  The policy module at
   the entry to a provider can decide how to satisfy that request -
   either by merging the request in with an existing reservation or by
   creating a new reservation for this (and perhaps other) users.  This
   policy can be on a per user-provider basis where a user and a
   provider have an agreement on the type of service offered, or on a
   provider-provider basis, where two providers have such an agreement.
   With the ability to do local policy control, providers can offer
   services best suited to their own resources and their customers
   needs.



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   Policy is expected to be provided as a generic API which will return
   values indicating what action should be taken for a specific
   reservation request.  The API is expected to have access to the
   reservation tables with the QoS for each reservation.  The RSVP
   Policy and Integrity objects will be passed to the policy() call.
   Four possible return values are expected.  The request can be
   rejected.  The request can be accepted as is.  The request can be
   accepted but at a different QoS.  The request can cause a change of
   QoS of an existing reservation.  The information returned from this
   call will be used to call the admission control interface.  As this
   area is critical to deployment, progress will need to be made in this
   area.

3. Data VC Management

   Any RSVP over ATM implementation must map RSVP and RSVP associated
   data flows to ATM Virtual Circuits (VCs). LAN Emulation [2],
   Classical IP [11] and, more recently, NHRP [12] discuss mapping IP
   traffic onto ATM SVCs, but they only cover a single QoS class, i.e.,
   best effort traffic. When QoS is introduced, VC mapping must be
   revisited. For RSVP controlled QoS flows, one issue is VCs to use for
   QoS data flows.

   In the Classic IP over ATM and current NHRP models, a single point-
   to-point VC is used for all traffic between two ATM attached hosts
   (routers and end-stations).  It is likely that such a single VC will
   not be adequate or optimal when supporting data flows with multiple
   QoS types. RSVP's basic purpose is to install support for flows with
   multiple QoS types, so it is essential for any RSVP over ATM solution
   to address VC usage for QoS data flows.

   This section describes issues and methods for management of VCs
   associated with QoS data flows. When establishing and maintaining
   VCs, the sub-net sender will need to deal with several complicating
   factors including multiple QoS reservations, requests for QoS
   changes, ATM short-cuts, and several multicast specific issues.  The
   multicast specific issues result from the nature of ATM connections.
   The key multicast related issues are heterogeneity, data
   distribution, receiver transitions, and end-point identification.

   3.1 Reservation to VC Mapping

      There are various approaches available for mapping reservations on
      to VCs.  A distinguishing attribute of all approaches is how
      reservations are combined on to individual VCs.  When mapping
      reservations on to VCs, individual VCs can be used to support a
      single reservation, or reservation can be combined with others on
      to "aggregate" VCs.  In the first case, each reservation will be



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      supported by one or more VCs.  Multicast reservation requests may
      translate into the setup of multiple VCs as is described in more
      detail in section 3.2.  Unicast reservation requests will always
      translate into the setup of a single QoS VC.  In both cases, each
      VC will only carry data associated with a single reservation.  The
      greatest benefit if this approach is ease of implementation, but
      it comes at the cost of increased (VC) setup time and the
      consumption of  greater number of VC and associated resources.

      We refer to the  other case, when reservations are not combined,
      as the "aggregation" model.  With this model, large VCs could be
      set up between IP routers and hosts in an ATM network.  These VCs
      could be managed much like IP Integrated Service (IIS) point-to-
      point links (e.g. T-1, DS-3) are managed now.  Traffic from
      multiple sources over multiple RSVP sessions might be multiplexed
      on the same VC.  This approach has a number of advantages.  First,
      there is typically no signalling latency as VCs would be in
      existence when the traffic started flowing, so no time is wasted
      in setting up VCs.  Second, the heterogeneity problem (section
      3.2) in full over ATM has been reduced to a solved problem.
      Finally, the dynamic QoS problem (section 3.6) for ATM has also
      been reduced to a solved problem.  This approach can be used with
      point-to-point and point-to-multipoint VCs.  The problem with the
      aggregation approach is that the choice of what QoS to use for
      which of the VCs is difficult, but is made easier since the VCs
      can be changed as needed.  The advantages of this scheme makes
      this approach an item for high priority study.

   3.2 Heterogeneity

      Heterogeneity occurs when receivers request different QoS's within
      a single session.  This means that the amount of requested
      resources differs on a per next hop basis.  A related type of
      heterogeneity occurs due to best-effort receivers. In any IP
      multicast group, it is possible that some receivers will request
      QoS (via RSVP) and some receivers will not.  Both types of
      heterogeneity are shown in figure .  In shared media, like
      Ethernet, receivers that have not requested resources can
      typically be given identical service to those that have without
      complications. This is not the case with ATM.  In ATM networks,
      any additional end-points of a VC must be explicitly added.  There
      may be costs associated with adding the best-effort receiver, and
      there might not be adequate resources.  An RSVP over ATM solution
      will need to support heterogeneous receivers even though ATM does
      not currently provide such support directly.






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      [Figure goes here]
              Figure 2: Types of Multicast Receivers



      RSVP heterogeneity is supported over ATM in the way RSVP
      reservations are mapped into ATM VCs.  There are multiple models
      for supporting RSVP heterogeneity over ATM.  Section 3.2.1
      examines the multiple VCs per RSVP reservation (or full
      heterogeneity) model where a single reservation can be forwarded
      into several VCs each with a different QoS.  Section 3.2.2
      presents a limited heterogeneity model where exactly one QoS VC is
      used along with a best effort VC.  Section 3.2.3 examines the VC
      per RSVP reservation (or homogeneous) model, where each RSVP
      reservation is mapped to a single ATM VC.  Section 3.2.4 describes
      the aggregation model allowing aggregation of multiple RSVP
      reservations into a single VC.  Further study is being done on the
      aggregation model.

      3.2.1 Full Heterogeneity Model

         We define the "full heterogeneity" model as providing a
         separate VC for each distinct QoS for a multicast session
         including best effort and one or more QoS's.  This is shown in
         figure  where S1 is a sender, R1-R3 are receivers, r1-r4 are IP
         routers, and s1-s2 are ATM switches.  Receivers R1 and R3 make
         reservations with different QoS while R2 is a best effort
         receiver.  Three point-to-multipoint VCs are created for this
         situation, each with the requested QoS.  Note that any leafs
         requesting QoS 1 or QoS 2 would be added to the existing QoS
         VC.

         [Figure goes here]
                   Figure 3: Full heterogeneity



         Note that while full heterogeneity gives users exactly what
         they request, it requires more resources of the network than
         other possible approaches.  In figure , three copies of each
         packet are sent on the link from r1 to s1.  Two copies of each
         packet are then sent from s1 to s2.  The exact amount of
         bandwidth used for duplicate traffic depends on the network
         topology and group membership.







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      3.2.2 Limited Heterogeneity Model

         An important special case of full heterogeneity is limited
         heterogeneity.  We define the "limited heterogeneity" model as
         the case where the receivers of a multicast session are limited
         to use either best effort service or a single alternate quality
         of service.  The alternate QoS can be chosen either by higher
         level protocols or by dynamic renegotiation of QoS as described
         below.

         [Figure goes here]
                Figure 4: Limited heterogeneity



         In order to support limited heterogeneity, each ATM edge device
         participating in a session would need at most two VCs.  One VC
         would be a point-to-multipoint best effort service VC and would
         serve all best effort service IP destinations for this RSVP
         session.  The other VC would be a point to multipoint VC with
         QoS and would serve all IP destinations for this RSVP session
         that have an RSVP reservation established. This is shown in
         figure  where there are three receivers, R2 requesting best
         effort service, while R1 and R3 request distinct reservations.
         Whereas, in figure , R1 and R3 have a separate VC, so each
         receives precisely the resources requested, in figure , R1 and
         R3 share the same VC (using the maximum of R1 and R3 QoS)
         across the ATM network.  Note that though the VC and hence the
         QoS for R1 and R3 are the same within the ATM cloud, the
         reservation outside the ATM cloud (from router r4 to receiver
         R3) uses the QoS actually requested by R3.

         As with full heterogeneity, a disadvantage of the limited
         heterogeneity scheme is that each packet will need to be
         duplicated at the network layer and one copy sent into each of
         the 2 VCs. Again, the exact amount of excess traffic will
         depend on the network topology and group membership.  Looking
         at figure , there are two VCs going from router r1 to switch
         s1.  Two copies of every packet will traverse the r1-s1 link.
         Another disadvantage of limited heterogeneity is that a
         reservation request can be rejected even when the resources are
         available.  This occurs when a new receiver requests a larger
         QoS.  If any of the existing QoS VC end-points cannot upgrade
         to the new QoS, then the new reservation fails though the
         resources exist for the new receiver.






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      3.2.3 Homogeneous and Modified Homogeneous Models

         We define the "homogeneous" model as the case where all
         receivers of a multicast session use a single quality of
         service VC. Best-effort receivers also use the single RSVP
         triggered QoS VC.  The single VC can be a point-to-point or
         point-to-multipoint as appropriate.  The QoS VC is sized to
         provide the maximum resources requested by all RSVP next-hops.

         This model matches the way the current RSVP specification
         addresses heterogeneous requests. The current processing rules
         and traffic control interface describe a model where the
         largest requested reservation for a specific outgoing interface
         is used in resource allocation, and traffic is transmitted at
         the higher rate to all next-hops. This approach would be the
         simplest method for RSVP over ATM implementations.

         While this approach is simple to implement, providing better
         than best-effort service may actually be the opposite of what
         the user desires since in providing ATM QoS. There may be
         charges incurred or resources that are wrongfully allocated.
         There are two specific problems.  The first problem is that a
         user making a small or no reservation would share a QoS VC
         resources without making (and perhaps paying for) an RSVP
         reservation.  The second problem is that a receiver may not
         receive any data.  This may occur when there is insufficient
         resources to add a receiver.  The rejected user would not be
         added to the single VC and it would not even receive traffic on
         a best effort basis.

         Not sending data traffic to best-effort receivers because of
         another receiver's RSVP request is clearly unacceptable.  The
         previously described limited heterogeneous model ensures that
         data is always sent to both QoS and best-effort receivers, but
         it does so by requiring replication of data at the sender in
         all cases.  It is possible to extend the homogeneous model to
         both ensure that data is always sent to best-effort receivers
         and also to avoid replication in the normal case.  This
         extension is to add special handling for the case where a
         best-effort receiver cannot be added to the QoS VC.  In this
         case, a best-effort VC can be established to any receivers that
         could not be added to the QoS VC.  Only in this special error
         case would senders be required to replicate data.  We define
         this approach as the "modified homogeneous" model.







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      3.2.4 Aggregation

         The last scheme is the multiple RSVP reservations per VC (or
         aggregation) model.  With this model, large VCs could be set up
         between IP routers and hosts in an ATM network.  These VCs
         could be managed much like IP Integrated Service (IIS) point-
         to-point links (e.g. T-1, DS-3) are managed now.  Traffic from
         multiple sources over multiple RSVP sessions might be
         multiplexed on the same VC.  This approach has a number of
         advantages.  First, there is typically no signalling latency as
         VCs would be in existence when the traffic started flowing, so
         no time is wasted in setting up VCs.  Second, the heterogeneity
         problem in full over ATM has been reduced to a solved problem.
         Finally, the dynamic QoS problem for ATM has also been reduced
         to a solved problem.  This approach can be used with point-to-
         point and point-to-multipoint VCs.  The problem with the
         aggregation approach is that the choice of what QoS to use for
         which of the VCs is difficult, but is made easier since the VCs
         can be changed as needed.  The advantages of this scheme makes
         this approach an item for high priority study.

         Multiple options for mapping reservations onto VCs have been
         discussed.  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 over
         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.

   3.3 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 provide QoS end-points, but not best-effort end-points.

      Another issue is identifying end-points of multicast traffic



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      handled by non-RSVP capable next-hops.  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 over a different route than
      what the data is using.  The source will get the RESV message, but
      will not know which egress router needs the QoS.  For unicast
      sessions, there is no problem since the ATM end-point will be the
      IP next-hop router.  Unfortunately, multicast routing may not be
      able to uniquely identify the IP next-hop router.  So it is
      possible that a multicast end-point can not be identified.

      In the most common case, MARS will be used to identify all end-
      points of a multicast group.  In the router to router case, a
      multicast routing protocol may provide all next-hops for a
      particular multicast group.  In either case, RSVP over ATM
      implementations must obtain a full list of end-points, both QoS
      and non-QoS, using the appropriate mechanisms.  The full list can
      be compared against the RSVP identified end-points to determine
      the list of best-effort receivers.

      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 routing protocols that
      support unique end-point identification.  In cases where such
      routing protocols are unavailable, all IP routers that will be
      used to support RSVP over ATM should support RSVP.

   3.4 Multicast Data Distribution

      Two models are planned 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  shows
      data flow for both modes of IP multicast data distribution.  RSVP
      over ATM solutions must ensure that IP multicast data is
      distributed with appropriate QoS.

      [Figure goes here]
         Figure 5: IP Multicast Data Distribution Over ATM



      In the Classical IP context, multicast server support is provided



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      via MARS[1].  MARS does not currently provide a way to communicate
      QoS requirements to a MARS multicast server (MCS).  When using
      multicast servers that do not support QoS requests, a sender must
      set the service, not global, break bit(s).

   3.5 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.  The issue is when to start send data
      on the new 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.  This means 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 lose information.
      So, the issue comes down to whether to send to both the old and
      new VCs, or to send to just one of the VCs.  In one case duplicate
      information will be received, in the other some information 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 very similar.  It both, it is possible to
      send data on the partially completed new VC, and the issue of
      duplicate versus lost information is the same.

      The last case is 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 information.  If the drop is requested first, then the
      end-point may loose information.

      In order to ensure predictable behavior and delivery of data to
      all receivers, data can only be sent on a new VCs once all parties
      have been added.  This will ensure that all data is only delivered
      once to all receivers.  This approach does not quite apply for the
      last case. In the last case, the add should be completed first,
      then the drop.  This means that receivers must be prepared to
      receive some duplicate packets at times of QoS setup.

   3.6 Dynamic QoS

      RSVP provides dynamic quality of service (QoS) in that the
      resources that are requested may change at any time.  There are
      several common reasons for a change of reservation QoS.  First, an



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      existing receiver can request a new larger (or smaller) QoS if the
      current received quality is unacceptable.  Second, a sender may
      change its traffic specification (TSpec), which can trigger a
      change in the reservation requests of the receivers.  Third, a new
      sender can start sending to a multicast group with a larger
      traffic specification than existing senders, triggering larger
      reservations.  Finally, a new receiver can make a reservation that
      is larger than existing reservations. If the limited heterogeneity
      model is being used and the merge node for the larger reservation
      is an ATM edge device, a new larger reservation must be set up
      across the ATM network.

      Since ATM service, as currently defined in UNI 3.x and UNI 4.0,
      does not allow renegotiating the QoS of a VC, dynamically changing
      the reservation means creating a new VC with the new QoS, and
      tearing down an established VC.  Tearing down a VC and setting up
      a new VC in ATM are complex operations that involve a non-trivial
      amount of processor time, and may have a substantial latency.

      There are several options for dealing with this mismatch in
      service. A specific approach will need to be a part of any RSVP
      over ATM solution.

      One 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 the
      old VC is then closed.

      If setup of the replacement VC fails, then the old QoS VC should
      continue to be used. When the new reservation is greater than the
      old reservation, the reservation request should be answered with
      an error. When the new reservation is less than the old
      reservation, the request should 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.
      Implementations should retry replacing the 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 is released and the whole VC replacement
      process is restarted.

      To limit the number of changes and to avoid excessive signalling



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      load, implementations may limit the number of changes that will be
      processed in a given period.  One implementation approach would
      have each ATM edge device configured with a time parameter tau
      (which can change over time) that gives the minimum amount of time
      the edge device will wait between successive changes of the QoS of
      a particular VC.  Thus if the QoS of a VC is changed at time t,
      all messages that would change the QoS of that VC that arrive
      before time t+tau would be queued.  If several messages changing
      the QoS of a VC arrive during the interval, redundant messages can
      be discarded.  At time t+tau, the remaining change(s) of QoS, if
      any, can be executed.

      The sequence of events for a single VC would be


      1.   Wait if timer is active

      2.   Establish VC with new QoS

      3.   Remap data traffic to new VC

      4.   Tear down old VC

      5.   Activate timer

      There is an interesting interaction between heterogeneous
      reservations and dynamic QoS.  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 signalling is minimized when the setup to
      the new next-hop fails.

   3.7 Short-Cuts

      Short-cuts [12] allow ATM attached routers and hosts to directly
      establish point-to-point VCs across LIS boundaries, i.e., the VC
      end-points are on different IP sub-nets.  The ability for short-
      cuts and RSVP to interoperate has been raised as a general
      question.  The area of concern is the ability to handle asymmetric
      short-cuts.  Specifically how RSVP can handle the case where a
      downstream short-cut may not have a matching upstream short-cut.
      In this case, which is shown in figure , PATH and RESV messages



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      following different paths.

      [Figure goes here]
      Figure 6: Asymmetric RSVP Message Forwarding With ATM Short-Cuts



      Examination of RSVP shows that the protocol already includes
      mechanisms that will support short-cuts.  The mechanism is the
      same one used to support RESV messages arriving at the wrong
      router and the wrong interface.  The key aspect of this mechanism
      is RSVP only processing messages that arrive at the proper
      interface and RSVP forwarding of messages that arrive on the wrong
      interface.  The proper interface is indicated in the NHOP object
      of the message.  So, existing RSVP mechanisms will support
      asymmetric short-cuts.

      The short-cut model of VC establishment still poses several issues
      when running with RSVP. The major issues are dealing with
      established best-effort short-cuts, when to establish short-cuts,
      and QoS only short-cuts. These issues will need to be addressed by
      RSVP implementations.

   3.8 VC Teardown

      RSVP can identify from either explicit messages or timeouts when a
      data VC is no longer needed.  Therefore, data VCs set up to
      support RSVP controlled flows should only be released at the
      direction of RSVP. 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[14], 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[15]
      states that inactivity timers must not be used at the VC receiver.

      When this timeout occurs for an RSVP initiated VC, a valid VC with
      QoS will be torn down unexpectedly.  While this behavior is
      acceptable for best-effort traffic, it is important that RSVP
      controlled VCs not be torn down.  If there is no choice about the
      VC being torn down, the RSVP daemon must be notified, so a
      reservation failure message can be sent.

4. RSVP Control VC Management

   One last important issue is providing a data path for the RSVP
   messages themselves.  There are two main types of messages in RSVP,



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   PATH and RESV.  PATH messages are sent to a multicast address, while
   RESV messages are sent to a unicast address.  Other RSVP messages are
   handled similar to either PATH or RESV [Note 1] So ATM VCs used for
   RSVP signalling messages need to provide both unicast and multicast
   functionality.

   There are several different approaches for how to assign VCs to use
   for RSVP signalling messages.  The main approaches are:

   o    use same VC as data

   o    single VC per session

   o    single point-to-multipoint VC multiplexed among sessions

   o    multiple point-to-point VCs multiplexed among sessions

   There are several different issues that affect the choice of how to
   assign VCs for RSVP signalling.  One issue is the number of
   additional VCs needed for RSVP signalling.  Related to this issue is
   the degree of multiplexing on the RSVP VCs.  In general more
   multiplexing means less VCs.  An additional issue is the latency in
   dynamically setting up new RSVP signalling VCs.  A final issue is
   complexity of implementation.  The remainder of this section
   discusses the issues and tradeoffs among these different approaches
   and suggests guidelines for when to use which alternative.

   4.1 Mixed data and control traffic

      In this scheme RSVP signalling messages are sent on the same VCs
      as is the data traffic.  The main advantage of this scheme is that
      no additional VCs are needed beyond what is needed for the data
      traffic.  An additional advantage is that there is no ATM
      signalling latency for PATH messages (which follow the same
      routing as the data messages).  However there can be a major
      problem when data traffic on a VC is nonconforming.  With
      nonconforming traffic, RSVP signalling messages may be dropped.
      While RSVP is resilient to a moderate level of dropped messages,
      excessive drops would lead to repeated tearing down and re-
      establishing QoS VCs, a very undesirable behavior for ATM.  Due to
      these problems, this is not a good choice for providing RSVP
      signalling messages, even though the number of VCs needed for this
      scheme is minimized.

      One variation of this scheme is to use the best effort data path
_________________________
[Note 1] This can be slightly more complicated for RERR messages




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      for signalling traffic.  In this scheme, there is no issue with
      nonconforming traffic, but there is an issue with congestion in
      the ATM network.

      RSVP provides some resiliency to message loss due to congestion,
      but RSVP control messages should be offered a preferred class of
      service.  A related variation of this scheme that is hopeful but
      requires further study is to have a packet scheduling algorithm
      (before entering the ATM network) that gives priority to the RSVP
      signalling traffic.  This can be difficult to do at the IP layer.
      One possible approach at the ATM layer would be to use the Cell
      Loss Priority (CLP) bit for RSVP signalling traffic to ensure
      better service.

   4.2 Single RSVP VC per RSVP Reservation

      In this scheme, there is a parallel RSVP signalling VC for each
      RSVP reservation.  This scheme results in twice the minimum number
      of VCs, but means that RSVP signalling messages have the advantage
      of a separate VC.  This separate VC means that RSVP signalling
      messages have their own traffic contract and compliant signalling
      messages are not subject to dropping due to other noncompliant
      traffic (such as can happen with the scheme in section 4.1).  The
      advantage of this scheme is its simplicity - whenever a data VC is
      created, a separate RSVP signalling VC is created.  The
      disadvantage of the extra VC is that extra ATM signalling needs to
      be done.

      Additionally, this scheme requires twice the minimum number of VCs
      and also additional latency, but is quite simple.  This approach
      would tend to work well on hosts.

   4.3 Multiplexed point-to-multipoint RSVP VCs

      In this scheme, there is a single point-to-multipoint RSVP
      signalling VC for each unique ingress router and unique set of
      egress routers.  This scheme allows multiplexing of RSVP
      signalling traffic that shares the same ingress router and the
      same egress routers.  This can save on the number of VCs, by
      multiplexing, but there are problems when the destinations of the
      multiplexed point-to-multipoint VCs are changing.  Several
      alternatives exist in these cases, that have applicability in
      different situations.  First, when the egress routers change, the
      ingress router can check if it already has a point-to-multipoint
      RSVP signalling VC for the new list of egress routers.  If the
      RSVP signalling VC already exists, then the RSVP signalling
      traffic can be switched to this existing VC.  If no such VC
      exists, one approach would be to create a new VC with the new list



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      of egress routers.  Other approaches include modifying the
      existing VC to add an egress router or using a separate new VC for
      the new egress routers.  When a destination drops out of a group,
      an alternative would be to keep sending to the existing VC even
      though some traffic is wasted.

      The number of VCs used in this scheme is a function of traffic
      patterns across the ATM network, but is always less than the
      number used with the Single RSVP VC per data VC.  In addition,
      existing best effort data VCs could be used for RSVP signalling.
      Reusing best effort VCs saves on the number of VCs at the cost of
      higher probability of RSVP signalling packet loss.  One possible
      place where this scheme will work well is in the core of the
      network where there is the most opportunity to take advantage of
      the savings due to multiplexing.  The exact savings depend on the
      patterns of traffic and the topology of the ATM network.

   4.4 Multiplexed point-to-point RSVP VCs

      In this scheme, multiple point-to-point RSVP signalling VCs are
      used for a single point-to-multipoint data VC.  This scheme allows
      multiplexing of RSVP signalling traffic but requires the same
      traffic to be sent on each of several VCs.  This scheme is quite
      flexible and allows a large amount of multiplexing.  Since point-
      to-point VCs can set up a reverse channel at the same time as
      setting up the forward channel, this scheme could save
      substantially on signalling cost.  In addition, signalling traffic
      could share existing best effort VCs.  Sharing existing best
      effort VCs reduces the total number of VCs needed, but might cause
      signalling traffic drops if there is congestion in the ATM
      network.

      This point-to-point scheme would work well in the core of the
      network where there is much opportunity for multiplexing.  Also in
      the core of the network, RSVP VCs can stay permanently established
      either as Permanent Virtual Circuits (PVCs) or as long lived
      Switched Virtual Circuits (SVCs).  The number of VCs in this
      scheme will depend on traffic patterns, but in the core of a
      network would be approximately n(n-1)/2 where n is the number of
      IP nodes in the network.  In the core of the network, this will
      typically be small compared to the total number of VCs.

   4.5 QoS for RSVP VCs

      There is an issue for what QoS, if any, to assign to the RSVP VCs.
      Three solutions have been covered in section 4.1 and in the shared
      best effort VC variations in sections 4.4 and 4.3.  For other RSVP
      VC schemes, a QoS (possibly best effort) will be needed.  What QoS



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      to use partially depends on the expected level of multiplexing
      that is being done on the VCs, and the expected reliability of
      best effort VCs.  Since RSVP signalling is infrequent (typically
      every 30 seconds), only a relatively small QoS should be needed.
      This is important since using a larger QoS risks the VC setup
      being rejected for lack of resources.  Falling back to best effort
      when a QoS call is rejected is possible, but if the ATM net is
      congested, there will likely be problems with RSVP packet loss on
      the best effort VC also.  Additional experimentation is needed in
      this area.

5. Encapsulation

   Since RSVP is a signalling protocol used to control flows of IP data
   packets, encapsulation for both RSVP packets and associated IP data
   packets must be defined. There are currently two encapsulation
   options for running IP over ATM, RFC 1483 and LANE.  There is also
   the possibility of future encapsulation options, such as MPOA[3].
   The first option is described in RFC 1483[9] and is currently used
   for "Classical" IP over ATM and NHRP.

   The second option is LAN Emulation, as described in [2].  LANE
   encapsulation does not currently include a QoS signalling interface.
   If LANE encapsulation is needed, LANE QoS signalling would first need
   to be defined by the ATM Forum.  It is possible that LANE 2.0 will
   include the required QoS support.

6. Security

   The same considerations stated in [8] and [14] apply to this
   document.  There are no additional security issues raised in this
   document.

7. Future Work

   We have described a set of schemes for deploying RSVP over IP over
   ATM.  There are a number of other issues that are subjects of
   continuing research.  These issues (and others) are covered in [5],
   and are briefly repeated here.

   A major issue is providing policy control for ATM VC creation.  There
   is work going on in the RSVP working group [8] on defining an
   architecture for policy support.  Further work is needed in defining
   an API and policy objects.  As this area is critical to deployment,
   progress will need to be made in this area.

   NHRP provides advantages in allowing short-cuts across 2 or more
   LIS's.  Short cutting router hops can lead to more efficient data



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   delivery.  Work on NHRP is on-going, but currently provides only a
   unicast delivery service.  Further study is needed to determine how
   NHRP can be used with RSVP and ATM.  Future work depends on the
   development of NHRP for multicast.

   Furthermore, when using RSVP it may be desirable to establish
   multiple short-cut VCs, to use these VCs for specific QoS flows, and
   to use the hop-by-hop path for other QoS and non-QoS flows. The
   current NHRP specification [12] does not preclude such an approach,
   but nor does it explicitly support it. We believe that explicit
   support of flow based short-cuts would improve RSVP over ATM
   solutions. We also believe that such support may require the ability
   to include flow information in the NHRP request.

   There is work in the ION working group on MultiCast Server (MCS)
   architectures for MARS.  An MCS provides savings in the number of VCs
   in certain situations.  When using a multicast server, the sub-
   network sender could establish a point-to-point VC with a specific
   QoS to the server, but there is not current mechanism to relay QoS
   requirements to the MCS.  Future work includes providing RSVP and ATM
   support over MARS MCS's.

   Unicast ATM VCs are inherently bi-directional and have the capability
   of supporting a "reverse channel".  By using the reverse channel for
   unicast VCs, the number of VCs used can potentially be reduced.
   Future work includes examining how the reverse VCs can be used most
   effectively.

   Current work in the ATM Forum and ITU promises additional advantages
   for RSVP and ATM including renegotiating QoS parameters and
   variegated VCs.  QoS renegotiation would be particularly beneficial
   since the only option available today for changing VC QoS parameters
   is replacing the VC.  It is important to keep current with changes in
   ATM, and to keep this document up-to-date.

   Scaling of the number of sessions is an issue.  The key ATM related
   implication of a large number of sessions is the number of VCs and
   associated (buffer and queue) memory. The approach to solve this
   problem is aggregation either at the RSVP layer or at the ISSLL layer
   (or both).

   This document describes approaches that can be used with ATM UNI4.0,
   but does not make use of the available leaf-initiated join, or LIJ,
   capability.  The use of LIJ may be useful in addressing scaling
   issues.  The coordination of RSVP with LIJ remains a research issue.

   Lastly, it is likely that LANE 2.0 will provide some QoS support
   mechanisms, including proper QoS allocation for multicast traffic.



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   It is important to track developments, and develop suitable RSVP over
   ATM LANE at the appropriate time.

8. Authors' Addresses

      Steven Berson
      USC Information Sciences Institute
      4676 Admiralty Way
      Marina del Rey, CA 90292

      Phone: +1 310 822 1511
      EMail: berson@isi.edu

      Lou Berger
      FORE Systems
      6905 Rockledge Drive
      Suite 800
      Bethesda, MD 20817

      Phone: +1 301 571 2534
      EMail: lberger@fore.com

REFERENCES

[1] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
    Networks," Internet Draft, February 1996.

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

[3] The ATM Forum, "MPOA Baseline Version 1", 95-0824r9, September 1996.

[4] Berson, S., "`Classical' RSVP and IP over ATM," INET '96, July 1996.

[5] Borden, M., Crawley, E., Krawczyk, J, Baker, F., and Berson, S.,
    "Issues for RSVP and Integrated Services over ATM," Internet Draft,
    February 1996.

[6] Borden, M., and Garrett, M., "Interoperation of Controlled-Load and
    Guaranteed-Service with ATM," Internet Draft, June 1996.

[7] Braden, R., Clark, D., Shenker, S.  "Integrated Services in the
    Internet Architecture: an Overview," RFC 1633, June 1994.

[8] Braden, R., Zhang, L., Berson, S., Herzog, S., and Jamin, S.,
    "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
    Specification," Internet Draft, November 1996.

[9] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation Layer



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    5," RFC 1483.

[10] Herzog, S., "Accounting and Access Control Policies for Resource
    Reservation Protocols," Internet Draft, June 1996.

[11] Laubach, M., "Classical IP and ARP over ATM," RFC 1577, January
    1994.

[12] Luciani, J., Katz, D., Piscitello, D., Cole, B., "NBMA Next Hop
    Resolution Protocol (NHRP)," Internet Draft, June 1996.

[13] Onvural, R., Srinivasan, V., "A Framework for Supporting RSVP Flows
    Over ATM Networks," Internet Draft, March 1996.

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

[15] Perez, M., Mankin, A.  "ATM Signalling Support for IP over ATM -
    UNI 4.0 Update" Internet Draft, November 1996.

[16] "ATM User-Network Interface (UNI) Specification - Version 3.1",
    Prentice Hall.

[17] Shenker, S., Partridge, C., Guerin, R., "Specification of
    Guaranteed Quality of Service," Internet Draft, August 1996.

[18] Wroclawski, J., "Specification of the Controlled-Load Network
    Element Service," Internet Draft, August, 1996.

[19] Zhang, L., Deering, S., Estrin, D., Shenker, S., Zappala, D.,
    "RSVP: A New Resource ReSerVation Protocol," IEEE Network, September
    1993.



















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