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RFC 2382 - A Framework for Integrated Services and RSVP over ATM


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Network Working Group                                 E. Crawley, Editor
Request for Comments: 2382                                Argon Networks
Category: Informational                                        L. Berger
                                                            Fore Systems
                                                               S. Berson
                                                                    ISI
                                                                F. Baker
                                                           Cisco Systems
                                                               M. Borden
                                                            Bay Networks
                                                             J. Krawczyk
                                               ArrowPoint Communications
                                                             August 1998

         A Framework for Integrated Services and RSVP over ATM

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

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

Abstract

   This document outlines the issues and framework related to providing
   IP Integrated Services with RSVP over ATM. It provides an overall
   approach to the problem(s) and related issues.  These issues and
   problems are to be addressed in further documents from the ISATM
   subgroup of the ISSLL working group.

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 quality real-time
   traffic, new classes of service and a QoS signalling protocol are

   being introduced in the Internet [1,6,7], while retaining the
   existing best effort service.  The QoS signalling protocol is RSVP
   [1], the Resource ReSerVation Protocol and the service models

   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 provides 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, and this memo concentrates on
   ATM.

   Classical IP over ATM [10] 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 subnetwork.  Hosts within an LIS
   communicate using the ATM network, while hosts from different subnets
   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 being solved with MARS [5],
   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.

   The ATM Forum's LAN Emulation (LANE) [17, 20] and Multiprotocol Over
   ATM (MPOA) [18] also address the support of IP best effort traffic
   over ATM through similar means.

   A key remaining issue for IP in an ATM environment 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
   which traffic flows are routed over which VCs.

1.1 Structure and Related Documents

   This document provides a guide to the issues for IIS over ATM.  It is
   intended to frame the problems that are to be addressed in further
   documents. In this document, the modes and models for RSVP operation
   over ATM will be discussed followed by a discussion of management of
   ATM VCs for RSVP data and control. Lastly, the topic of
   encapsulations will be discussed in relation to the models presented.

   This document is part of a group of documents from the ISATM subgroup
   of the ISSLL working group related to the operation of IntServ and
   RSVP over ATM.  [14] discusses the mapping of the IntServ models for
   Controlled Load and Guaranteed Service to ATM.  [15 and 16] discuss
   detailed implementation requirements and guidelines for RSVP over
   ATM, respectively.  While these documents may not address all the
   issues raised in this document, they should provide enough
   information for development of solutions for IntServ and RSVP over
   ATM.

1.2 Terms

   Several term used in this document are used in many contexts, often
   with different meaning.  These terms are used in this document with
   the following meaning:

   - Sender is used in this document to mean the ingress point to the
     ATM network or "cloud".
   - Receiver is used in this document to refer to the egress point from
     the ATM network or "cloud".
   - 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 [1] for details of each. Each new
     request is referred to in this document as an RSVP reservation, or
     simply reservation.
   - 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).

2. Issues Regarding the Operation of RSVP and IntServ over ATM

   The issues related to RSVP and IntServ over ATM fall into several
   general classes:

   - How to make RSVP run over ATM now and in the future
   - When to set up a virtual circuit (VC) for a specific Quality of
     Service (QoS) related to RSVP
   - How to map the IntServ models to ATM QoS models
   - How to know that an ATM network is providing the QoS necessary for
     a flow
   - How to handle the many-to-many connectionless features of IP
     multicast and RSVP in the one-to-many connection-oriented world of
     ATM

2.1 Modes/Models for RSVP and IntServ over ATM

   [3] Discusses several different models for running IP over ATM
   networks.  [17, 18, and 20] also provide models for IP in ATM
   environments.  Any one of these models would work as long as the RSVP
   control packets (IP protocol 46) and data packets can follow the same
   IP path through the network.  It is important that the RSVP PATH
   messages follow the same IP path as the data such that appropriate
   PATH state may be installed in the routers along the path.  For an
   ATM subnetwork, this means the ingress and egress points must be the
   same in both directions for the RSVP control and data messages.  Note
   that the RSVP protocol does not require symmetric routing.  The PATH
   state installed by RSVP allows the RESV messages to "retrace" the
   hops that the PATH message crossed.  Within each of the models for IP
   over ATM, there are decisions about using different types of data
   distribution in ATM as well as different connection initiation.  The
   following sections look at some of the different ways QoS connections
   can be set up for RSVP.

2.1.1 UNI 3.x and 4.0

   In the User Network Interface (UNI) 3.0 and 3.1 specifications [8,9]
   and 4.0 specification, both permanent and switched virtual circuits
   (PVC and SVC) may be established with a specified service category
   (CBR, VBR, and UBR for UNI 3.x and VBR-rt and ABR for 4.0) and
   specific traffic descriptors in point-to-point and point-to-
   multipoint configurations.  Additional QoS parameters are not
   available in UNI 3.x and those that are available are vendor-
   specific.  Consequently, the level of QoS control available in
   standard UNI 3.x networks is somewhat limited.  However, using these
   building blocks, it is possible to use RSVP and the IntServ models.
   ATM 4.0 with the Traffic Management (TM) 4.0 specification [21]
   allows much greater control of QoS.  [14] provides the details of
   mapping the IntServ models to UNI 3.x and 4.0 service categories and
   traffic parameters.

2.1.1.1 Permanent Virtual Circuits (PVCs)

   PVCs emulate dedicated point-to-point lines in a network, so the
   operation of RSVP can be identical to the operation over any point-
   to-point network.  The QoS of the PVC must be consistent and
   equivalent to the type of traffic and service model used.  The
   devices on either end of the PVC have to provide traffic control
   services in order to multiplex multiple flows over the same PVC.
   With PVCs, there is no issue of when or how long it takes to set up
   VCs, since they are made in advance but the resources of the PVC are
   limited to what has been pre-allocated.  PVCs that are not fully
   utilized can tie up ATM network resources that could be used for
   SVCs.

   An additional issue for using PVCs is one of network engineering.
   Frequently, multiple PVCs are set up such that if all the PVCs were
   running at full capacity, the link would be over-subscribed.  This
   frequently used "statistical multiplexing gain" makes providing IIS
   over PVCs very difficult and unreliable.  Any application of IIS over
   PVCs has to be assured that the PVCs are able to receive all the
   requested QoS.

2.1.1.2 Switched Virtual Circuits (SVCs)

   SVCs allow paths in the ATM network to be set up "on demand".  This
   allows flexibility in the use of RSVP over ATM along with some
   complexity.  Parallel VCs can be set up to allow best-effort and
   better service class paths through the network, as shown in Figure 1.
   The cost and time to set up SVCs can impact their use.  For example,
   it may be better to initially route QoS traffic over existing VCs
   until a SVC with the desired QoS can be set up for the flow.  Scaling
   issues can come into play if a single RSVP flow is used per VC, as
   will be discussed in Section 4.3.1.1. The number of VCs in any ATM
   device may also be limited so the number of RSVP flows that can be
   supported by a device can be strictly limited to the number of VCs
   available, if we assume one flow per VC.  Section 4 discusses the
   topic of VC management for RSVP in greater detail.

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

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

                    Figure 1: Data Flow VC Initiation

   While RSVP is receiver oriented, ATM is sender oriented.  This might
   seem like a problem but the sender or ingress point receives RSVP
   RESV messages and can determine whether a new VC has to be set up to
   the destination or egress point.

2.1.1.3 Point to MultiPoint

   In order to provide QoS for IP multicast, an important feature of
   RSVP, data flows must be distributed to multiple destinations from a
   given source.  Point-to-multipoint VCs provide such a mechanism.  It
   is important to map the actions of IP multicasting and RSVP (e.g.
   IGMP JOIN/LEAVE and RSVP RESV/RESV TEAR) to add party and drop party
   functions for ATM.  Point-to-multipoint VCs as defined in UNI 3.x and
   UNI 4.0 have a single service class for all destinations.  This is
   contrary to the RSVP "heterogeneous receiver" concept.  It is
   possible to set up a different VC to each receiver requesting a
   different QoS, as shown in Figure 2. This again can run into scaling
   and resource problems when managing multiple VCs on the same
   interface to different destinations.

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

                    Figure 2: Types of Multicast Receivers

   RSVP sends messages both up and down the multicast distribution tree.
   In the case of a large ATM cloud, this could result in a RSVP message
   implosion at an ATM ingress point with many receivers.

   ATM 4.0 expands on the point-to-multipoint VCs by adding a Leaf
   Initiated Join (LIJ) capability. LIJ allows an ATM end point to join
   into an existing point-to-multipoint VC without necessarily
   contacting the source of the VC.  This can reduce the burden on the
   ATM source point for setting up new branches and more closely matches
   the receiver-based model of RSVP and IP multicast.  However, many of
   the same scaling issues exist and the new branches added to a point-
   to-multipoint VC must use the same QoS as existing branches.

2.1.1.4 Multicast Servers

   IP-over-ATM has the concept of a multicast server or reflector that
   can accept cells from multiple senders and send them via a point-to-
   multipoint VC to a set of receivers.  This moves the VC scaling
   issues noted previously for point-to-multipoint VCs to the multicast
   server.  Additionally, the multicast server will need to know how to
   interpret RSVP packets or receive instruction from another node so it
   will be able to provide VCs of the appropriate QoS for the RSVP
   flows.

2.1.2 Hop-by-Hop vs. Short Cut

   If the ATM "cloud" is made up a number of logical IP subnets (LISs),
   then it is possible to use "short cuts" from a node on one LIS
   directly to a node on another LIS, avoiding router hops between the
   LISs. NHRP [4], is one mechanism for determining the ATM address of
   the egress point on the ATM network given a destination IP address.
   It is a topic for further study to determine if significant benefit
   is achieved from short cut routes vs. the extra state required.

2.1.3 Future Models

   ATM is constantly evolving.  If we assume that RSVP and IntServ
   applications are going to be wide-spread, it makes sense to consider
   changes to ATM that would improve the operation of RSVP and IntServ
   over ATM.  Similarly, the RSVP protocol and IntServ models will
   continue to evolve and changes that affect them should also be
   considered.  The following are a few ideas that have been discussed
   that would make the integration of the IntServ models and RSVP easier
   or more complete.  They are presented here to encourage continued
   development and discussion of ideas that can help aid in the
   integration of RSVP, IntServ, and ATM.

2.1.3.1 Heterogeneous Point-to-MultiPoint

   The IntServ models and RSVP support the idea of "heterogeneous
   receivers"; e.g., not all receivers of a particular multicast flow
   are required to ask for the same QoS from the network, as shown in
   Figure 2.

   The most important scenario that can utilize this feature occurs when
   some receivers in an RSVP session ask for a specific QoS while others
   receive the flow with a best-effort service.  In some cases where
   there are multiple senders on a shared-reservation flow (e.g., an
   audio conference), an individual receiver only needs to reserve
   enough resources to receive one sender at a time.  However, other
   receivers may elect to reserve more resources, perhaps to allow for
   some amount of "over-speaking" or in order to record the conference
   (post processing during playback can separate the senders by their
   source addresses).

   In order to prevent denial-of-service attacks via reservations, the
   service models do not allow the service elements to simply drop non-
   conforming packets.  For example, Controlled Load service model [7]
   assigns non-conformant packets to best-effort status (which may
   result in packet drops if there is congestion).

   Emulating these behaviors over an ATM network is problematic and
   needs to be studied.  If a single maximum QoS is used over a point-
   to-multipoint VC, resources could be wasted if cells are sent over
   certain links where the reassembled packets will eventually be
   dropped.  In addition, the "maximum QoS" may actually cause a
   degradation in service to the best-effort branches.

   The term "variegated VC" has been coined to describe a point-to-
   multipoint VC that allows a different QoS on each branch.  This
   approach seems to match the spirit of the Integrated Service and RSVP
   models, but some thought has to be put into the cell drop strategy
   when traversing from a "bigger" branch to a "smaller" one.  The
   "best-effort for non-conforming packets" behavior must also be
   retained.  Early Packet Discard (EPD) schemes must be used so that
   all the cells for a given packet can be discarded at the same time
   rather than discarding only a few cells from several packets making
   all the packets useless to the receivers.

2.1.3.2 Lightweight Signalling

   Q.2931 signalling is very complete and carries with it a significant
   burden for signalling in all possible public and private connections.
   It might be worth investigating a lighter weight signalling mechanism
   for faster connection setup in private networks.

2.1.3.3 QoS Renegotiation

   Another change that would help RSVP over ATM is the ability to
   request a different QoS for an active VC.  This would eliminate the
   need to setup and tear down VCs as the QoS changed.  RSVP allows
   receivers to change their reservations and senders to change their
   traffic descriptors dynamically.  This, along with the merging of
   reservations, can create a situation where the QoS needs of a VC can
   change.  Allowing changes to the QoS of an existing VC would allow
   these features to work without creating a new VC.  In the ITU-T ATM
   specifications [24,25], some cell rates can be renegotiated or
   changed.  Specifically, the Peak Cell Rate (PCR) of an existing VC
   can be changed and, in some cases, QoS parameters may be renegotiated
   during the call setup phase. It is unclear if this is sufficient for
   the QoS renegotiation needs of the IntServ models.

2.1.3.4 Group Addressing

   The model of one-to-many communications provided by point-to-
   multipoint VCs does not really match the many-to-many communications
   provided by IP multicasting.  A scaleable mapping from IP multicast
   addresses to an ATM "group address" can address this problem.

2.1.3.5 Label Switching

   The MultiProtocol Label Switching (MPLS) working group is discussing
   methods for optimizing the use of ATM and other switched networks for
   IP by encapsulating the data with a header that is used by the
   interior switches to achieve faster forwarding lookups.  [22]
   discusses a framework for this work.  It is unclear how this work
   will affect IntServ and RSVP over label switched networks but there
   may be some interactions.

2.1.4 QoS Routing

   RSVP is explicitly not a routing protocol.  However, since it conveys
   QoS information, it may prove to be a valuable input to a routing
   protocol that can make path determinations based on QoS and network
   load information.  In other words, instead of asking for just the IP
   next hop for a given destination address, it might be worthwhile for
   RSVP to provide information on the QoS needs of the flow if routing
   has the ability to use this information in order to determine a
   route.  Other forms of QoS routing have existed in the past such as
   using the IP TOS and Precedence bits to select a path through the
   network.  Some have discussed using these same bits to select one of
   a set of parallel ATM VCs as a form of QoS routing.  ATM routing has
   also considered the problem of QoS routing through the Private
   Network-to-Network Interface (PNNI) [26] routing protocol for routing
   ATM VCs on a path that can support their needs.  The work in this
   area is just starting and there are numerous issues to consider.
   [23], as part of the work of the QoSR working group frame the issues
   for QoS Routing in the Internet.

2.2 Reliance on Unicast and Multicast Routing

   RSVP was designed to support both unicast and IP multicast
   applications.  This means that RSVP needs to work closely with
   multicast and unicast routing.  Unicast routing over ATM has been
   addressed [10] and [11].  MARS [5] provides multicast address
   resolution for IP over ATM networks, an important part of the
   solution for multicast but still relies on multicast routing
   protocols to connect multicast senders and receivers on different
   subnets.

2.3 Aggregation of Flows

   Some of the scaling issues noted in previous sections can be
   addressed by aggregating several RSVP flows over a single VC if the
   destinations of the VC match for all the flows being aggregated.
   However, this causes considerable complexity in the management of VCs
   and in the scheduling of packets within each VC at the root point of

   the VC.  Note that the rescheduling of flows within a VC is not
   possible in the switches in the core of the ATM network. Virtual
   Paths (VPs) can be used for aggregating multiple VCs. This topic is
   discussed in greater detail as it applies to multicast data
   distribution in section 4.2.3.4

2.4 Mapping QoS Parameters

   The mapping of QoS parameters from the IntServ models to the ATM
   service classes is an important issue in making RSVP and IntServ work
   over ATM.  [14] addresses these issues very completely for the
   Controlled Load and Guaranteed Service models.  An additional issue
   is that while some guidelines can be developed for mapping the
   parameters of a given service model to the traffic descriptors of an
   ATM traffic class, implementation variables, policy, and cost factors
   can make strict mapping problematic.  So, a set of workable mappings
   that can be applied to different network requirements and scenarios
   is needed as long as the mappings can satisfy the needs of the
   service model(s).

2.5 Directly Connected ATM Hosts

   It is obvious that the needs of hosts that are directly connected to
   ATM networks must be considered for RSVP and IntServ over ATM.
   Functionality for RSVP over ATM must not assume that an ATM host has
   all the functionality of a router, but such things as MARS and NHRP
   clients would be worthwhile features.  A host must manage VCs just
   like any other ATM sender or receiver as described later in section
   4.

2.6 Accounting and Policy Issues

   Since RSVP and IntServ create classes of preferential service, some
   form of administrative control and/or cost allocation is needed to
   control access.  There are certain types of policies specific to ATM
   and IP over ATM that need to be studied to determine how they
   interoperate with the IP and IntServ policies being developed.
   Typical IP policies would be that only certain users are allowed to
   make reservations.  This policy would translate well to IP over ATM
   due to the similarity to the mechanisms used for Call Admission
   Control (CAC).

   There may be a need for policies specific to IP over ATM.  For
   example, since signalling costs in ATM are high relative to IP, an IP
   over ATM specific policy might restrict the ability to change the
   prevailing QoS in a VC.  If VCs are relatively scarce, there also
   might be specific accounting costs in creating a new VC.  The work so
   far has been preliminary, and much work remains to be done.  The

   policy mechanisms outlined in [12] and [13] provide the basic
   mechanisms for implementing policies for RSVP and IntServ over any
   media, not just ATM.

3. Framework for IntServ and RSVP over ATM

   Now that we have defined some of the issues for IntServ and RSVP over
   ATM, we can formulate a framework for solutions.  The problem breaks
   down to two very distinct areas; the mapping of IntServ models to ATM
   service categories and QoS parameters and the operation of RSVP over
   ATM.

   Mapping IntServ models to ATM service categories and QoS parameters
   is a matter of determining which categories can support the goals of
   the service models and matching up the parameters and variables
   between the IntServ description and the ATM description(s).  Since
   ATM has such a wide variety of service categories and parameters,
   more than one ATM service category should be able to support each of
   the two IntServ models.  This will provide a good bit of flexibility
   in configuration and deployment.  [14] examines this topic
   completely.

   The operation of RSVP over ATM requires careful management of VCs in
   order to match the dynamics of the RSVP protocol.  VCs need to be
   managed for both the RSVP QoS data and the RSVP signalling messages.
   The remainder of this document will discuss several approaches to
   managing VCs for RSVP and [15] and [16] discuss their application for
   implementations in term of interoperability requirement and
   implementation guidelines.

4. RSVP VC Management

   This section provides more detail on the issues related to the
   management of SVCs for RSVP and IntServ.

4.1 VC Initiation

   As discussed in section 2.1.1.2, there is an apparent mismatch
   between RSVP and ATM. Specifically, RSVP control is receiver oriented
   and ATM control is sender oriented.  This initially may seem like a
   major issue, but really is not.  While RSVP reservation (RESV)
   requests are generated at the receiver, actual allocation of
   resources takes place at the subnet sender. For data flows, this
   means that subnet senders will establish all QoS VCs and the subnet
   receiver must be able to accept incoming QoS VCs, as illustrated in
   Figure 1.  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.

   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.

4.2 Data VC Management

   Any RSVP over ATM implementation must map RSVP and RSVP associated
   data flows to ATM Virtual Circuits (VCs). LAN Emulation [17],
   Classical IP [10] and, more recently, NHRP [4] 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
   .bp 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, as shown in Figure
   1.

   RSVP reservation styles must also be taken into account in any VC
   usage strategy.

   This section describes issues and methods for management of VCs
   associated with QoS data flows. When establishing and maintaining
   VCs, the subnet 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.

4.2.1 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
   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 4.2.2.  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.

   When multiple reservations are combined onto a single VC, it is
   referred to 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 4.2.2) in full over ATM
   has been reduced to a solved problem. Finally, the dynamic QoS
   problem (section 4.2.7) for ATM has also been reduced to a solved
   problem.

   The aggregation model can be used with point-to-point and point-to-
   multipoint VCs.  The problem with the aggregation model is that the
   choice of what QoS to use for the VCs may be difficult, without
   knowledge of the likely reservation types and sizes but is made
   easier since the VCs can be changed as needed.

4.2.2 Unicast Data VC Management

   Unicast data VC management is much simpler than multicast data VC
   management but there are still some similar issues.  If one considers
   unicast to be a devolved case of multicast, then implementing the
   multicast solutions will cover unicast.  However, some may want to
   consider unicast-only implementations.  In these situations, the
   choice of using a single flow per VC or aggregation of flows onto a
   single VC remains but the problem of heterogeneity discussed in the
   following section is removed.

4.2.3 Multicast Heterogeneity

   As mentioned in section 2.1.3.1 and shown in figure 2, multicast
   heterogeneity occurs when receivers request different qualities of
   service 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. In shared media networks,
   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.

   RSVP heterogeneity is supported over ATM in the way RSVP reservations
   are mapped into ATM VCs.  There are four alternative approaches this
   mapping. There are multiple models for supporting RSVP heterogeneity
   over ATM.  Section 4.2.3.1 examines the multiple VCs per RSVP
   reservation (or full heterogeneity) model where a single reservation
   can be forwarded onto several VCs each with a different QoS. Section
   4.2.3.2 presents a limited heterogeneity model where exactly one QoS
   VC is used along with a best effort VC.  Section 4.2.3.3 examines the
   VC per RSVP reservation (or homogeneous) model, where each RSVP
   reservation is mapped to a single ATM VC.  Section 4.2.3.4 describes
   the aggregation model allowing aggregation of multiple RSVP
   reservations into a single VC.

4.2.3.1 Full Heterogeneity Model

   RSVP supports heterogeneous QoS, meaning that different receivers of
   the same multicast group can request a different QoS.  But
   importantly, some receivers might have no reservation at all and want
   to receive the traffic on a best effort service basis.  The IP model
   allows receivers to join a multicast group at any time on a best
   effort basis, and it is important that ATM as part of the Internet
   continue to provide this service. 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 qualities of
   service.

   Note that while full heterogeneity gives users exactly what they
   request, it requires more resources of the network than other
   possible approaches. The exact amount of bandwidth used for duplicate
   traffic depends on the network topology and group membership.

4.2.3.2 Limited Heterogeneity Model

   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.

   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.

   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. 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.

4.2.3.3 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.  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.

4.2.3.4 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 if the VCs can be changed
   as needed.

4.2.4 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 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.  To ensure proper
   behavior, implementations should, by default, only establish RSVP-
   initiated VCs to RSVP capable end-points.

4.2.5 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. RSVP over ATM solutions must
   ensure that IP multicast data is distributed with appropriate QoS.

   In the Classical IP context, multicast server support is provided via
   MARS [5].  MARS does not currently provide a way to communicate QoS
   requirements to a MARS multicast server.  Therefore, RSVP over ATM
   implementations must, by default, support "mesh-mode" distribution
   for RSVP controlled multicast flows.  When using multicast servers
   that do not support QoS requests, a sender must set the service, not
   global, break bit(s).

4.2.6 Receiver Transitions

   When setting up a point-to-multipoint VCs for multicast RSVP
   sessions, 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 can 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
   - 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 operation should be completed first, then
   the drop operation.  This means that receivers must be prepared to
   receive some duplicate packets at times of QoS setup.

4.2.7 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.

   1. An existing receiver can request a new larger (or smaller) QoS.
   2. A sender may change its traffic specification (TSpec), which can
      trigger a change in the reservation requests of the receivers.
   3. A new sender can start sending to a multicast group with a larger
      traffic specification than existing senders, triggering larger
      reservations.
   4. 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 processing
   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.

   The default 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 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 T
   (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+T would be queued. If several messages changing the QoS of a VC
   arrive during the interval, redundant messages can be discarded. At
   time t+T, the remaining change(s) of QoS, if any, can be executed.
   This timer approach would apply more generally to any network
   structure, and might be worthwhile to incorporate into RSVP.

   The sequence of events for a single VC would be

   - Wait if timer is active
   - Establish VC with new QoS
   - Remap data traffic to new VC
   - Tear down old VC
   - 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.

4.2.8 Short-Cuts

   Short-cuts [4] 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 subnets.  The ability for short-cuts
   and RSVP to interoperate has been raised as a general question.  An
   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, PATH
   and RESV messages following different paths.

   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.

   The key issue to be addressed by any RSVP over ATM solution is when
   to establish a short-cut for a QoS data flow. The default behavior 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.  More study is expected in this area.

4.2.9 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.   Although,
   section 3.1 of the update to RFC 1755 [11] 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.

   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,
   and would require some mechanism to signal that an incoming VC was
   RSVP initiated.  To avoid this complexity and to conform to [11]
   implementations must not use an inactivity timer to clear received
   connections.

4.3 RSVP Control 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 unicast or multicast
   addresses, while RESV messages are sent only to unicast addresses.
   Other RSVP messages are handled similar to either PATH or RESV,
   although this might be more complicated for RERR messages.  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:

      - use same VC as data
      - single VC per session
      - single point-to-multipoint VC multiplexed among sessions
      - 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
   fewer 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.

4.3.1 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 of QoS VCs, a very undesirable behavior for
   ATM. Due to these problems, this may not be 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 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.

4.3.1.1 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 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 4.3.1). 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.

4.3.1.2 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. 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.

4.3.1.3 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.

4.3.2 QoS for RSVP VCs

   There is an issue of what QoS, if any, to assign to the RSVP
   signalling VCs. 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.

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. The methods for transmitting IP packets over
   ATM (Classical IP over ATM[10], LANE[17], and MPOA[18]) are all based
   on the encapsulations defined in RFC1483 [19].  RFC1483 specifies two
   encapsulations, LLC Encapsulation and VC-based multiplexing.  The
   former allows multiple protocols to be encapsulated over the same VC

   and the latter requires different VCs for different protocols.

   For the purposes of RSVP over ATM, any encapsulation can be used as
   long as the VCs are managed in accordance to the methods outlined in
   Section 4.  Obviously, running multiple protocol data streams over
   the same VC with LLC encapsulation can cause the same problems as
   running multiple flows over the same VC.

   While none of the transmission methods directly address the issue of
   QoS, RFC1755 [11] does suggest some common values for VC setup for
   best-effort traffic.  [14] discusses the relationship of the RFC1755
   setup parameters and those needed to support IntServ flows in greater
   detail.

6. Security Considerations

   The same considerations stated in [1] and [11] apply to this
   document.  There are no additional security issues raised in this
   document.

7. References

   [1] Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
       "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
       Specification", RFC 2209, September 1997.

   [2] Borden, M., Crawley, E., Davie, B., and S. Batsell, "Integration
       of Realtime Services in an IP-ATM Network Architecture", RFC
       1821, August 1995.

   [3] Cole, R., Shur, D., and C. Villamizar, "IP over ATM: A Framework
       Document", RFC 1932, April 1996.

   [4] Luciani, J., Katz, D., Piscitello, D., Cole, B., and N.
       Doraswamy, "NBMA Next Hop Resolution Protocol (NHRP)", RFC 2332,
       April 1998.

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

   [6] Shenker, S., and C. Partridge, "Specification of Guaranteed
       Quality of Service", RFC 2212, September 1997.

   [7] Wroclawski, J., "Specification of the Controlled-Load Network
       Element Service", RFC 2211, September 1997.

   [8] ATM Forum. ATM User-Network Interface Specification Version 3.0.
       Prentice Hall, September 1993.

   [9] ATM Forum. ATM User Network Interface (UNI) Specification Version
       3.1. Prentice Hall, June 1995.

   [10] Laubach, M., "Classical IP and ARP over ATM", RFC 2225, April
        1998.

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

   [12] Herzog, S., "RSVP Extensions for Policy Control", Work in
        Progress.

   [13] Herzog, S., "Local Policy Modules (LPM): Policy Control for
        RSVP", Work in Progress.

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

   [15] Berger, L., "RSVP over ATM Implementation Requirements", RFC
        2380, August 1998.

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

   [17] ATM Forum Technical Committee. LAN Emulation over ATM, Version
        1.0 Specification, af-lane-0021.000, January 1995.

   [18] ATM Forum Technical Committee. Baseline Text for MPOA, af-95-
        0824r9, September 1996.

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

   [20] ATM Forum Technical Committee. LAN Emulation over ATM Version 2
        - LUNI Specification, December 1996.

   [21] ATM Forum Technical Committee. Traffic Management Specification
        v4.0, af-tm-0056.000, April 1996.

   [22] Callon, R., et al., "A Framework for Multiprotocol Label
        Switching, Work in Progress.

   [23] Rajagopalan, B., Nair, R., Sandick, H., and E. Crawley, "A
        Framework for QoS-based Routing in the Internet", RFC 2386,
        August 1998.

   [24] ITU-T. Digital Subscriber Signaling System No. 2-Connection
        modification: Peak cell rate modification by the connection
        owner, ITU-T Recommendation Q.2963.1, July 1996.

   [25] ITU-T. Digital Subscriber Signaling System No. 2-Connection
        characteristics negotiation during call/connection establishment
        phase, ITU-T Recommendation Q.2962, July 1996.

   [26] ATM Forum Technical Committee. Private Network-Network Interface
        Specification v1.0 (PNNI), March 1996.

8. Authors' Addresses

   Eric S. Crawley
   Argon Networks
   25 Porter Road
   Littleton, Ma 01460

   Phone: +1 978 486-0665
   EMail: esc@argon.com

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

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

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

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

   Fred Baker
   Cisco Systems
   519 Lado Drive
   Santa Barbara, California 93111

   Phone: +1 805 681-0115
   EMail: fred@cisco.com

   Marty Borden
   Bay Networks
   125 Nagog Park
   Acton, MA 01720

   Phone: +1 978 266-1011
   EMail: mborden@baynetworks.com

   John J. Krawczyk
   ArrowPoint Communications
   235 Littleton Road
   Westford, Massachusetts 01886

   Phone: +1 978 692-5875
   EMail: jj@arrowpoint.com

9.  Full Copyright Statement

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   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
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   included on all such copies and derivative works.  However, this
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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