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Versions: 00 01 02 03                                                   
Internet Draft                                                 S. Berson
Expiration: March 1997                                               ISI
File: draft-ietf-issll-atm-support-01.ps                       L. Berger
                                                            FORE Systems



               IP Integrated Services with RSVP over ATM



                           September 24, 1996

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
   time.  It is inappropriate to use Internet-Drafts as reference
   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[12]
   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 ................................................5
   2. Policy .........................................................6
      2.1 Implementation Guidelines ..................................7
   3. Data VC Management .............................................7
      3.1 Heterogeneity ..............................................7
      3.2 Multicast Data Distribution ................................11
      3.3 Receiver Transitions .......................................12
      3.4 Multicast End-Point Identification .........................13
      3.5 Reservation to VC Mapping ..................................14
      3.6 Dynamic QoS ................................................15
   4.   Tear down old VC .............................................16
   5.   Activate timer ...............................................16
      5.1 Implementation Guidelines ..................................22
   6. Security .......................................................23
   7. Future Work ....................................................23
   8. Authors' Addresses .............................................24






























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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[13,16,15], while
   retaining the existing best effort service.  The QoS signalling
   protocol is RSVP[5,17], 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[14] 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[12].

   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.

   The general issues related to running RSVP[5,17] over ATM have been
   covered in several papers including [2,3,10].  This document will
   review key issues that must be addressed by any RSVP over ATM UNI
   solution.  It will discuss  advantages and disadvantages of different
   methods for running RSVP over ATM.  It will also provide specific
   guidelines to implementors using ATM UNI3.x and 4.0. These guidelines
   are intended to provide a baseline set of functionality, while
   allowing for more sophisticated approaches.  We expect some vendors
   to also provide some of the more sophisticated approaches described
   below, and some networks to only make use of such approaches.

   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.  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 [5] for details of
           each.   Each 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



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           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

      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 [17].  The reader is assumed to be familiar with
         [17].

         IP Integrated Services discriminates between users by providing
         some users better service at the expense of others.  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 [8] for details).

      1.2.3 ATM

         We assume ATM defined by UNI 3.x and 4.0.  ATM provides both
         point-to-point and point-to-multipoint Virtual Circuits (VCs)



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         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.

      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. 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 [8] 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 service 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



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   such an agreement.  With the ability to do local policy control,
   service providers can offer services best suited to their own
   resources and their customers needs.

   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.

   2.1 Implementation Guidelines

      Currently, the contents of policy data objects is not specified.
      So specifics of policy implementation are not defined at this
      time.

3. Data VC Management

   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 issues.

   There are several aspects to running RSVP over ATM that are
   particular to multicast sessions. These issues result from the nature
   of ATM connections. The key issues are heterogeneity, data
   distribution, receiver transitions, and end-point identification.

   3.1 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



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      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.

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



      There are multiple models for supporting RSVP heterogeneity over
      ATM.  Section 3.1.1 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
      3.1.2 presents a limited heterogeneity model where exactly one QoS
      VC is used along with a best effort VC.  Section 3.1.3 examines
      the VC per RSVP reservation (or single VC) model, where each RSVP
      reservation is mapped to a single ATM VC.  Section 3.1.4 describes
      the aggregation model allowing aggregation of multiple RSVP
      reservations into a single VC.  Further study is being done on the
      aggregation model.

      3.1.1 Many VCs per RSVP reservation

         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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      3.1.2 Two VCs per RSVP reservation

         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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      3.1.3 Single VC per RSVP Reservation

         An even simpler approach for mapping RSVP reservations into VCs
         is to have a single VC for each RSVP reservation.  This ATM VC
         can be a point-to-point or point-to-multipoint as appropriate.
         In this approach even the best-effort receivers use the RSVP
         triggered QoS VC.  The QoS VC is sized to handle the maximum of
         the requested resources of all the receivers of a session.
         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.

      3.1.4 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.

      3.1.5 Implementation Guidelines

         Multiple options for mapping reservations onto VCs have been
         discussed.  The key issue to be addressed is providing
         requested QoS downstream.  Currently, the aggregation approach
         is for high priority study, so RSVP over ATM implementations



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         should use one of the other approaches.

         The current RSVP specification addresses heterogeneous
         requests, but not within an ATM specific context. 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
         delivered at the higher rate to all next-hops. The simplest
         approach for RSVP over ATM will be to emulate this approach
         even though this approach may be undesirable in certain
         circumstances. So,  RSVP over ATM implementations
         **should/must** [Note 1]
          be able to support heterogeneity in QoS requests by providing
         the largest requested QoS to all next hops using a single QoS
         VC as described in sections 3.1.2 and 3.1.3.  Implementations,
         may also support heterogeneity through some other mechanism,
         e.g., using multiple appropriately sized VCs.

         The other type of heterogeneity to be addressed is best-effort
         receivers.  Two possible approaches for handling best-effort
         receivers are using a single QoS VC as described in section
         3.1.3 or using two VCs, as described in section 3.1.2.
         Unfortunately, neither of these approaches is the right answer
         for all cases. For some networks, e.g. LANs, it is likely that
         the single VC approach will be desired. In other networks, e.g.
         public WANs, it is likely that the multiple approach will be
         desired.  Each sub-network sender (router, or host) may choose
         how traffic is mapped onto VCs. For this reason, baseline RSVP
         over ATM implementations **should/must** [Note 2]

         support best-effort multicast receivers either using the single
         QoS VC or the limited heterogeneity approach. Implementations
         should support both approaches and provide the ability to
         select which method is actually used, but are not required to
         do so.

   3.2 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"
_________________________
[Note 1] The working group must decide  if  this  is  requirement  or  a
recommendation.
[Note 2] The working group must decide  if  this  is  requirement  or  a
recommendation.




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      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



      3.2.1 Implementation Guidelines

         In the Classical IP context, multicast server support is
         provided via MARS[1].  MARS does not currently provide a way to
         communicate QoS requirements to a MARS multicast server.
         Therefore, RSVP over ATM implementations **must/should**  [Note
         3]
           support "mesh-mode" distribution for RSVP controlled
         multicast flows.

   3.3 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 one or both of the new
      QoS VC and the old VC.  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
_________________________
[Note 3] The working group must decide  if  this  is  requirement  or  a
recommendation.




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      duplicate versus lost information is the same.

      The last case is when an end-point must be added 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.

      3.3.1 Implementation Guidelines

         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.

   3.4 Multicast End-Point Identification

      One basic issue is how to identify the 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
      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.

      3.4.1 Implementation Guidelines

         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



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         can be compared against with 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.

   3.5 Reservation to VC Mapping

      There is a basic need to map from IP and RSVP to ATM Virtual
      Circuits (VCs).  LAN Emulation [7], Classical IP [14] and, more
      recently, NHRP [9] 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. RSVP
      reservation styles will also need to be taken into account in any
      VC usage strategy.

      There are multiple options for mapping flows onto VCs.  The key
      issue to be addressed is providing requested QoS downstream.  This
      can be done by mapping each reservation into a single VC or
      through more aggregation schemes as discussed in section 3.1.4.

      3.5.1 Minimum Implementation

         While it is possible to send multiple flows and multiple
         distinct reservations (FF) over single VCs, implementation of
         such approaches is a matter for further study. So, baseline
         RSVP over ATM implementations **may/must** [Note 4]
           allow for the use of a single VC to support each RSVP
         reservation. By using independent VCs per reservation, delivery
         of requested resources to the associated QoS data flow can be
         assured. This approach does not preclude support for multiple
_________________________
[Note 4] The working group must decide  if  this  is  requirement  or  a
suggestion.  The appropriate wording will be used based on the result.




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         flows per VC.

   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
      existing receiver can request a new larger (or smaller) QoS.
      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
      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.

      3.6.1 Implementation Guidelines

         The proposed approach 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 is 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.




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         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 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 set-up to the new next-hop fails.






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   3.7 Short-Cuts

      Short-cuts [9] 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
      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.

      3.7.1 Implementation Guidelines

         The key issue to be addressed by the baseline RSVP over ATM
         solution is when to establish a short-cut for a QoS data flow.
         The proposed approach is to simply follow best-effort traffic.
         When a short-cut has been established for best-effort traffic
         to a destination or next-hop, that same end-point should be
         used when setting up RSVP triggered VCs for QoS traffic to the
         same destination or next-hop. This will happen naturally when
         PATH messages are forwarded over the best-effort short-cut.
         Note that in this approach when best-effort short-cuts are
         never established, RSVP triggered QoS short-cuts will also
         never be established.





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   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[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. 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.  The RSVP daemon must
      also be notified whenever a VC is torn down without direction from
      RSVP.

      3.8.1 Implementation Guidelines

         For VCs initiated at the request of RSVP, the configurable
         inactivity timer mentioned in [11] must be set to "infinite".
         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.  To properly set the timer it is necessary to
         identify an incoming VC setup as RSVP initiated.  We propose to
         make this identification as part of the negotiation of
         encapsulation.  Specifically, to indicate in the B-LLI IE in
         the SETUP message that the associated VC is controlled by an
         internet layer signalling protocol and should not be timed out.

         The format of the B-LLI IE is [Note 5] :

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,
   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 6] So ATM VCs used for
_________________________
[Note 5] This will be defined in a future version
[Note 6] This can be slightly more complicated for RERR messages



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   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
      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



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      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.

   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.

   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
      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.



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      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
      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.



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   4.6 Implementation Guidelines

      Implementations **will/should** [Note 7] , at a minimum, be able
      to send RSVP control (messages) over the best effort data path,
      see figure . The specific best effort paths that will be used by
      RSVP are: for unicast, the same VC used to reach the unicast
      destination; and for multicast, the same VC that is used for best
      effort traffic destined to the IP multicast group.  Note that
      there may be another best effort VC that is used to carry session
      data traffic.

      [Figure goes here]
              Figure 7: RSVP Control Message VC Usage



      An issue with this approach is that best effort VCs may not
      provide the reliability that RSVP needs.  However RSVP allows for
      a certain amount of packet loss without any loss of state
      synchronization.  And in all cases, RSVP control traffic should be
      offered a preferred class of service.

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 two encapsulation options for
   running IP over ATM, RFC 1483 and LANE.  The first option is
   described in RFC 1483[6] and is currently used for "Classical" IP
   over ATM and NHRP.

   The second option is LAN Emulation, as described in [7].  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.

   5.1 Implementation Guidelines

      While it is possible to use different encapsulations for RSVP
      packets and associated IP data packets, this does not seem to make
      sense. So, the same encapsulation must be used for each.

      The choice of encapsulation options is clear.  Currently LANE
_________________________
[Note 7] The working group must decide  if  this  is  requirement  or  a
recommendation.




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      doesn't have a QoS control interface and there is no way to
      communicate QoS requirements to the LANE BUS.  Since QoS control
      is needed to make RSVP over ATM useful, RFC 1483 encapsulation
      must be used by RSVP over ATM.

6. Security

   The same considerations stated in [5] and [11] 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 [3],
   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
   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 [9] 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.




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   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.
   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













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

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

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

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

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

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

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

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

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

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

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

[13] Braden, R., Clark, D., Shenker, S.  "Integrated Services in the



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    Internet Architecture: an Overview," RFC 1633, June 1994.

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

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

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

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





































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