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rfc:rfc1821

Network Working Group M. Borden Request for Comments: 1821 E. Crawley Category: Informational Bay Networks

                                                              B. Davie
                                                              Bellcore
                                                            S. Batsell
                                                                   NRL
                                                           August 1995
Integration of Real-time Services in an IP-ATM Network Architecture

Status of the Memo

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

Abstract

 The IETF is currently developing an integrated service model which is
 designed to support real-time services on the Internet.
 Concurrently, the ATM Forum is developing Asynchronous Transfer Mode
 networking which similarly provides real-time networking support. The
 use of ATM in the Internet as a link layer protocol is already
 occurring, and both the IETF and the ATM Forum are producing
 specifications for IP over ATM. The purpose of this paper is to
 provide a clear statement of what issues need to be addressed in
 interfacing the IP integrated services environment with an ATM
 service environment so as to create a seamless interface between the
 two in support of end users desiring real-time networking services.

Table of Contents

 1.0 Introduction                                                2
 2.0 Problem Space Overview                                      3
 2.1 Initial Assumptions                                         3
 2.2 Topologies Under Consideration                              4
 2.3 Providing QoS in IP over  ATM - a walk-though               5
 3.0 Service Model Issues                                        6
 3.1 Traffic Characterization                                    7
 3.2 QoS Characterization                                        8
 4.0 Resource Reservation Styles                                10
 4.1 RSVP                                                       10
 4.2 ST-II                                                      13
 4.3 Mapping IP flows to ATM Connections                        15
 5.0 End System Issues                                          16
 6.0 Routing Issues                                             16

Borden, et al Informational [Page 1] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 6.1 Multicast routing                                          17
 6.2 QoS Routing                                                17
 6.3 Mobile Routing                                             18
 7.0 Security Issues                                            19
 8.0 Future Directions                                          20
 9.0 References                                                 22
 10.0 Authors' Addresses                                        24

1.0 Introduction

 The traditional network service on the Internet is best-effort
 datagram transmission. In this service, packets from a source are
 sent to a destination, with no guarantee of delivery. For those
 applications that require a guarantee of delivery, the TCP protocol
 will trade packet delay for correct reception by retransmitting those
 packets that fail to reach the destination. For traditional
 computer-communication applications such as FTP and Telnet in which
 correct delivery is more important than timeliness, this service is
 satisfactory. However, a new class of application which uses multiple
 media (voice, video, and computer data) has begun to appear on the
 Internet. Examples of this class of application are video
 teleconferencing, video-on-demand, and distributed simulation. While
 these applications can operate to some extent using best-effort
 delivery, trading packet delay for correct reception is not an
 acceptable trade-off. Operating in the traditional mode for these
 applications results in reduced quality of the received information
 and, potentially, inefficient use of bandwidth. To remedy this
 problem the IETF is developing a real-time service environment in
 which multiple classes of service are offered [6]. This environment
 will greatly extend the existing best-effort service model to meet
 the needs of multimedia applications with real-time constraints.
 At the same time that this effort is underway in the IETF,
 Asynchronous Transfer Mode (ATM) is being developed, initially as a
 replacement for the current telephone network protocols, but more
 recently as a link-layer protocol for computer communications. As it
 was developed from the beginning with telephone voice applications in
 mind, a real-time service environment is an integral part of the
 protocol. With the approval of UNI 3.1 by the ATM Forum, the ATM
 standards now have several categories of service. Given the wide
 acceptance of ATM by the long-line carriers, the use of ATM in the
 Internet is, if not guaranteed, highly likely. The question now
 becomes, how can we successfully interface between the real-time
 services offered by ATM and the new,integrated service environment
 soon to be available in the IP protocol suite. The current IP over
 ATM standards assume no real-time IP protocols. It is the purpose of
 this RFC to clearly delineate what the issues are in integrating
 real-time services in an IP-over-ATM network [10,15,19,20,21].

Borden, et al Informational [Page 2] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 In the IP-over-ATM environment, as in many others, multicast routing
 adds an additional set of challenges. While the major focus of this
 paper is quality of service (QoS) issues, it is unwise at best to
 ignore multicast when considering these issues, especially since so
 many of the applications that motivate the provision of real time QoS
 also require efficient multicast support. We will therefore try to
 keep considerations of multicast in the foreground in the following
 discussion.
 One of the primary motivations for this document is a belief by the
 authors that ATM should, if possible, be used as more than a leased
 line replacement. That is to say, while it is possible for the
 Internet to be overlaid on constant bit rate (CBR), permanent virtual
 circuits (PVCs), thus reducing IP over ATM to a previously solved
 problem, we believe that this is unlikely to be the most efficient
 way to use ATM services as they are offered by carriers or as they
 appear in LANs. For example, a carrier offering a CBR service must
 assume that the peak bit rate can be used continuously with no
 degradation in quality and so resources must be allocated to the
 connection to provide that service, even if the peak rate is in fact
 rarely used. This is likely to make a CBR service more expensive that
 a variable bit rate service of the same peak capacity.  Another way
 to view this is that the new IP service model will allow us to
 associate information about the bandwidth requirements of
 applications with individual flows; surely it is not wise to discard
 this information when we request a service from an ATM subnet.
 While we believe that there is a range of capabilities in ATM
 networks that can be effectively used by a real-time Internet, we do
 not believe that just because ATM has a capability, the Internet must
 use it. Thus, our goal in this RFC is to begin to explore how an
 Internet with real time service capability might make most effective
 use of ATM networks.  Since there are a number of problems to be
 resolved to achieve this effective use, our major goal at this point
 is to describe the scope of the problems that need to be addressed.

2.0 Problem Space Overview

 In this section we aim to describe in high level terms the scope of
 the problem that will be explored in more detail in later sections.

2.1 Initial Assumptions

 We begin by assuming that an Integrated Services Internet, i.e., an
 Internet with a range of qualities of service to support both real-
 time and non-real-time applications, will eventually happen. A number
 of working groups are trying to make this happen, notably

Borden, et al Informational [Page 3] RFC 1821 Real-time Service in IP-ATM Networks August 1995

  • the Integrated Services group (int-serv), which is working to define

a new IP service model, including a set of services suited to a

   range of real-time applications;
  • the Resource reservation Setup Protocol group (rsvp), which is

defining a resource reservation protocol [7] by which the

   appropriate service for an application can be requested from the
   network;
  • the Internet Streams Protocol V2 group (ST-II), which is updating

[27], a stream-oriented internet protocol that provides a range of

   service qualities.
 In addition, the IETF IP over ATM working group and the ATM Forum
 Multiprotocol over ATM group are working to define a model for
 protocols to make use of the ATM layer.
 Since these groups have not yet generated standards, we will need to
 do some amount of extrapolation to predict the problems that may
 arise for IP over ATM. We also assume that the standards being
 developed in the ATM Forum will largely determine the service model
 for ATM. Again, some extrapolation may be needed. Given these
 assumptions, this paper aims to explore ways in which a future
 Integrated Services Internet might make effective use of ATM as it
 seems likely to be deployed.

2.2 Topologies Under Consideration

 Figure 1 shows a generic internetwork that includes ATM and non-ATM
 subnetworks. This paper aims to outline the problems that must be
 addressed to enable suitable quality of service to be provided end-
 to-end across such a network. The problem space, therefore, includes
  • communication across an 'ATM-only' network between two hosts

directly connected to the ATM network;

  • communication between ATM-connected hosts which involves traversing

some non-ATM subnets;

  • communication between a host on a non-ATM subnet and a host directly

connected to ATM;

  • communication between two hosts, neither of which has a direct ATM

connection, but which may make use of one or more ATM networks for

   some part of the path.

Borden, et al Informational [Page 4] RFC 1821 Real-time Service in IP-ATM Networks August 1995

                   [H]
                    |                           [H]
            ________|________________________    |
            |                                |   |
    ________|__                        ______|___|____
    |         |                        |             |
    |  ATM   [R]                      [R]  ATM       |
    |  Cloud  |                        |   Cloud     |___[H]
    |         |     Non-ATM Internet   |             |
    |         |                       [R]            |
    |________[R]                       |_____________|
     |      |                                |
     |      |                                |
    [H]     |________________________________|
                                      |
                                      |
                                     [H]
 [H] = Host
 [R] = Router
                            Figure 1
 In the last case, the entities connected to the ATM network are IP
 routers, and it is their job to manage the QoS provided by the ATM
 network(s) in such a way that the desired end-to-end QoS is provided
 to the hosts. While we wish to describe the problem space in a way
 that covers all of these scenarios, the last is perhaps the most
 general, so we will use it for most illustrative purposes. In
 particular, we are explicitly not interested in ways of providing QoS
 that are applicable only to a subset of these situations. We claim
 that addressing these four situations is sufficiently general to
 cover other situations such as those in which several ATM and non-ATM
 networks are traversed.
 It is worth mentioning that the ATM networks in this case might be
 local or wide area, private or public. In some cases, this
 distinction may be significant, e.g., because there may be economic
 implications to a particular approach to providing QoS.

2.3 Providing QoS in IP over ATM - a walk-through

 To motivate the following discussion, this section walks through an
 example of what might happen when an application with a certain set
 of QoS needs starts up. For this example, we will use the fourth case
 mentioned above, i.e., two hosts connected to non-ATM networks,
 making use of an ATM backbone.

Borden, et al Informational [Page 5] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 A generic discussion of this situation is made difficult by the fact
 that the reservation of resources in the Internet may be sender or
 receiver initiated, depending on the specifics of the setup protocol.
 We will attempt to gloss over this distinction for now, although we
 will return to it in Section 4. We will assume a unicast application
 and that the traffic characteristics and the QoS requirements (such
 as delay, loss, throughput) of the application are known to at least
 one host.  That host launches a request for the desired QoS and a
 description of the expected traffic into the network; at some point
 this request hits a router at the edge of the ATM network. The router
 must examine the request and decide if it can use an existing
 connection over the ATM network to honor the request or whether it
 must establish a new connection. In the latter case, it must use the
 QoS and traffic characterizations to decide what sort of ATM
 connection to open and to describe the desired service to the ATM
 network. It must also decide where to open the connection to. Once
 the connection is opened, the request is forwarded across the ATM
 network to the exit router and then proceeds across the non-ATM part
 of the network by the normal means.
 We can see from the above description that there are several sets of
 issues to be discussed:
  • How does the IP service model, with certain service classes and

associated styles of traffic and QoS characterization, map onto

   the ATM service model?
  • How does the IP reservation model (whatever it turns out to be) map

onto ATM signalling?

  • How does IP over ATM routing work when service quality is added to

the picture?

 These issues will be discussed in the following sections.

3.0 Service Model Issues

 There are several significant differences between the ways in which
 IP and ATM will provide QoS.  When IP commits to provide a certain
 QoS to an application according to the Internet service model, it
 must be able to request an appropriate QoS from the ATM network using
 the ATM service model. Since these service models are by no means the
 same, a potentially complex mapping must be performed for the IP
 layer to meet its commitments.  The details of the differences
 between ATM and IP and the problems presented by these differences
 are described below.

Borden, et al Informational [Page 6] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 We may think of a real-time service model as containing the following
 components:
  • a way to characterize traffic (sometimes called the Tspec);
  • a way to characterize the desired quality of service (the Rspec).
 We label these components as traffic characterization and QoS
 characterization.  Each of these components is discussed in turn in
 the following sections.
 As well as these aspects of the service model, both ATM and IP will
 have a number of mechanisms by which the model is implemented. The
 mechanisms include admission control, policing, and packet
 scheduling. A particularly important mechanism is the one by which
 end-nodes communicate their QoS needs and traffic characteristics to
 the network, and the network communicates admission control decisions
 to the end-nodes. This is referred to as resource reservation or
 signalling, and is the subject of Section 4. In fact, it seems to be
 the only mechanism where significant issues of IP/ATM integration
 arise. The details of admission control, policing and packet
 scheduling are largely internal to a single network element and we do
 not foresee significant problems caused by the integration of IP and
 ATM. For example, while there may be plenty of challenges in
 designing effective approaches to admission control for both IP and
 ATM, it is not apparent that there are any special challenges for the
 IP over ATM environment. As the walk-through of Section 2.3
 described, a reservation request from a host would at some point
 encounter the edge of the ATM cloud. At this point, either a new
 connection needs to be set up across the ATM cloud, or the router can
 decide to carry the requested traffic over an existing virtual
 circuit. If the ATM cloud cannot create a new connection as
 requested, this would presumably result in an admission control
 failure which would cause the router to deny the reservation request.

3.1 Traffic Characterization

 The traffic characterization provided by an application or user is
 used by the network to make decisions about how to provide the
 desired quality of service to this application and to assess the
 effect the new flow will have on the service provided to existing
 flows. Clearly this information feeds into the admission control
 decision process.
 In the Internet community, it is assumed that traffic will in general
 be bursty and that bursty traffic can be characterized by a `token
 bucket'.  While ATM does not expect all traffic to be bursty (the
 Continuous Bit Rate class being defined specifically for non-bursty

Borden, et al Informational [Page 7] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 traffic), it uses an essentially equivalent formulation for the
 characterization of traffic that is bursty, referred to as the
 Generic Cell Rate Algorithm (GCRA). However, ATM in some classes also
 requires specification of peak cell rate, whereas peak rates are not
 currently included in the IP traffic characterizations. It may be
 possible to use incoming interface speeds to determine an approximate
 peak rate.
 One of the functions that must be performed in order to carry IP
 traffic over an ATM network is therefore a mapping from the
 characterization of the traffic as supplied to IP to a
 characterization that is acceptable for ATM. While the similarity of
 the two characterizations suggests that this is straightforward,
 there is considerable flexibility in the mapping of parameters from
 IP to ATM. As an extreme example, a router at the edge of an ATM
 cloud that expects to receive bursts of IP packets on a non-ATM
 interface, with the bursts described by some token bucket parameters,
 could actually inject ATM cells at a constant rate into the ATM
 network. This may be achieved without significant buffering if the
 ATM link speed is faster than the point-to-point link speed;
 alternatively, it could be achieved by buffering out the burstiness
 of the arriving traffic. It seems more reasonable to map an IP flow
 (or a group of flows) with variable bandwidth requirements onto an
 ATM connection that accommodates variable bit rate traffic.
 Determining how best to map the IP traffic to ATM connections in this
 way is an area that warrants investigation.
 A potential complication to this process is the fact that the token
 bucket parameters are specified at the edge of the IP network, but
 that the specification of the GCRA parameters at the entry to an ATM
 network will frequently happen at a router in the middle of an IP
 network. Thus the actual burstiness that is encountered at the router
 may differ from that described by the IP token bucket parameters, as
 the burstiness changes as the traffic traverses a network. The
 seriousness of this problem needs to be understood to permit
 efficient resource utilization.

3.2 QoS Characterization

 In addition to specifying the traffic that they will submit to the
 network, applications must specify the QoS they require from the
 network. Since the goal is to carry IP efficiently over ATM networks,
 it is necessary to establish mechanisms by which QoS specifications
 for IP traffic can be translated into QoS specifications that are
 meaningful for an ATM network.

Borden, et al Informational [Page 8] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 The proposed method of QoS specification for the Internet is to
 specify a `service class' and some set of parameters, depending on
 the service class. The currently proposed service classes are
  • guaranteed, which provides a mathematically guaranteed delay

bound [23];

  • predictive delay, which provides a probabilistic delay bound

[24];and

  • controlled delay, which merely tries to provide several levels of

delay which applications may choose between [25].

 These are in addition to the existing `best-effort' class. More IP
 service classes are expected in the future. ATM has five service
 classes:
  • CBR (constant bit rate), which emulates a leased line, providing

very tightly constrained delay and designed for applications which

    can use a fixed bandwidth pipe;
  • VBR (variable bit rate)-real-time which attempts to constrain delay

for applications whose bandwidth requirements vary;

  • VBR-non-real-time, intended for variable bandwidth applications

without tight delay constraints;

  • UBR (unspecified bit rate) which most closely approximates the best

effort service of traditional IP;

  • ABR (available bit rate) which uses a complex feedback mechanism

to control loss.

 Each class requires some associated parameters to be specified, e.g.,
 CBR requires a peak rate. Observe that these classes are by no means
 in direct correspondence with the IP classes. In some cases, ATM
 classes require parameters which are not provided at the IP level,
 such as loss rate, to be specified. It may be necessary to assume
 reasonable default values in these cases.
 The major problem here is this: given traffic in a particular IP
 service class with certain QoS parameters, how should it be sent
 across an ATM network in such a way that it both meets its service
 commitments and makes efficient use of the ATM network's resources?
 For example, it would be possible to transport any class of IP
 traffic over an ATM network using the constant bit rate (CBR) ATM
 class, thus using the ATM network like a point-to-point link. This
 would allow IP to meet its service commitments, but would be an

Borden, et al Informational [Page 9] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 inefficient use of network resources in any case where the IP traffic
 was at all bursty (which is likely to be most cases). A more
 reasonable approach might be to map all IP traffic into a variable
 bit rate (VBR) class; certainly this class has the flexibility to
 accommodate bursty IP traffic more efficiently than CBR.
 At present, the IETF is not working on any service classes in which
 loss rate is considered as part of the QoS specification. As long as
 that is the case, the fact that ATM allows target loss rates to be
 specified is essentially not an issue. However, we may certainly
 expect that as the IP service model is further refined, service
 classes that include specifications of loss may be defined. At this
 point, it will be necessary to be able to map between loss rates at
 the IP level and loss rates at the ATM level. It has already been
 shown that relatively small loss rates in an ATM network can
 translate to high loss rates in IP due to the fact that each lost
 cell can cause the loss of an entire IP packet. Schemes to mitigate
 this problem, which include the proposed approach to implementing the
 ABR class, as well as other solutions [22], have been proposed. This
 is clearly likely to be an important issue in the future.

4.0 Resource Reservation Styles

 ATM uses a signalling protocol (Q.2931) both to establish virtual
 connections and to allocate resources to those connections. It has
 many of the characteristics of a 'conventional' signalling protocol,
 such as being sender-driven and relying on hard-state in switches to
 maintain connections. Some of the key characteristics are listed in
 the table below. In the current standards, the QoS associated with a
 connection at setup time cannot be changed subsequently (i.e., it is
 static); in a unicast connection, resources are allocated in both
 directions along the path, while in the multicast case, they are
 allocated only from the sender to the receivers. In this case, all
 senders receive the same QoS.
 Two protocols have been proposed for resource reservation in IP. The
 first (chronologically) is ST-II, the other is RSVP. Each of these,
 and its relationship to ATM, is discussed in the following sections.

4.1 RSVP

 IP has traditionally provided connectionless service. To support
 real-time services in a connectionless world, RSVP has been proposed
 to enable network resources to be reserved for a connectionless data
 stream. ATM, on the other hand, provides a connection-oriented
 service, where resource reservations are made at connection setup
 time, using a user-network interface (UNI) and a network-network
 interface (NNI) signalling protocol.

Borden, et al Informational [Page 10] RFC 1821 Real-time Service in IP-ATM Networks August 1995

  1. —————————————————————-

| Category | RSVP | ATM (UNI 3.0) |

  1. —————————————————————-

| | | |

   | Orientation  | Receiver-based       |       Sender-based      |
   |              |                      |                         |
    ----------------------------------------------------------------
   |              |                      |                         |
   |     State    |      Soft state      |       Hard state        |
   |              |  (refresh/time-out)  |   (explicit delete)     |
   -----------------------------------------------------------------
   |              |                      |                         |
   |QoS SetupTime |   Separate from      |    Concurrent with      |
   |              | route establishment  |   route establishment   |
   -----------------------------------------------------------------
   |              |                      |                         |
   |QoS Changes?  | Dynamic QoS          |       Static QoS        |
   |              |                      |  (Fixed at setup time)  |
   -----------------------------------------------------------------
   |              |                      | Bidirectional allocation|
   |Directionality|  Unidirectional      |  for unicast            |
   |              |resource allocation   |Unidirectional allocation|
   |              |                      |  for multicast          |
   -----------------------------------------------------------------
   |              |                      |                         |
   |Heterogeneity |   Receiver           |    Uniform QoS to       |
   |              |  heterogeneity       |    all receivers        |
   -----------------------------------------------------------------
 The principles used in the design of RSVP differ from those of ATM in
 the following respects:
  • Resource reservations in IP hosts and routers are represented by

soft state, i.e., reservations are not permanent, but time out

    after some period. Reservations must be refreshed to prevent
    time-out, and may also be explicitly deleted. In ATM, resources are
    reserved for the duration of a connection, which must be explicitly
    and reliably deleted.
  • The soft state approach of RSVP allows the QoS reserved for a flow

to be changed at any time, whereas ATM connections have a static

    QoS that is fixed at setup time.
  • RSVP is a simplex protocol, i.e., resources are reserved in one

direction only. In ATM, connections (and associated reservations)

    are bi-directional in point-to-point calls and uni-directional in
    point-to-multipoint calls.

Borden, et al Informational [Page 11] RFC 1821 Real-time Service in IP-ATM Networks August 1995

  • Resource reservation is receiver-initiated in RSVP. In ATM,

resources are reserved by the end system setting up the connection.

    In point-to-multipoint calls, connection setup (and hence resource
    reservation) must be done by the sender.
  • RSVP has explicit support for sessions containing multiple senders,

namely the ability to select a subset of senders, and to

    dynamically switch between senders. No such support is provided
    by ATM.
  • RSVP has been designed independently of other architectural

components, in particular routing. Moreover, route setup and

    resource reservation are done at different times.  In ATM, resource
    reservation and route setup are done at the same time (connection
    setup time).
 The differences between RSVP and ATM state establishment, as
 described above, raise numerous problems. For example, since point-
 to-point connections are bidirectional in ATM, and since reservations
 can be made in both directions, receiver-initiated resource
 reservations in RSVP can be simulated in ATM by having the receiver
 set up the connection and reserve resources in the backward direction
 only.  However, this is potentially wasteful of connection resources
 since connections are only ever used to transfer data in one
 direction even though communication between the two parties may be
 bidirectional. One option is to use a `point-to-multipoint' ATM
 connection with only one receiver. Of course, the fact that the RSVP
 reservation request is made by the receiver(s) means that this
 request must be somehow communicated to the sender on the ATM
 network. This is somewhat analogous to the receiver-oriented join
 operation of IP multicast and the problems of implementing it over
 ATM, as discussed in Section 6. In general, the efficiency of any
 proposed connection management scheme needs to be investigated in
 both unicast and multicast contexts for a range of application
 requirements, especially at a large scale.
 The use by RSVP of `soft state' as opposed to explicit connections
 means that routers at the ATM network's edges need to manage the
 opening and closing of ATM connections when RSVP reservations are
 made and released (or time out).  The optimal scheme for connection
 setup and tear-down will depend on the cost of setting up a
 connection versus the cost of keeping the connection open for
 possible future use by another stream, and is likely to be service
 class-dependent. For example, connections may be left open for reuse
 by best-effort traffic (subject to sufficient connections being
 available), since no resources are explicitly reserved. On the other
 hand, connections supporting the real-time service classes are likely
 to be expensive to leave open since resources may be allocated even

Borden, et al Informational [Page 12] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 when the connection is idle. Again, the cost incurred will depend on
 the class. For example, the cost of an open, idle `guaranteed' QoS
 connection is likely to be significantly more expensive than a
 connection providing predictive or controlled delay service. Note
 that connections can be reused for traffic of the same class with
 compatible QoS requirements, and that it may sometimes be possible to
 use a `higher quality' class to substitute for a lower quality one.
 Another characteristic of RSVP which presents problems for ATM is the
 use of PATH messages to convey information to receivers before any
 reservation is made. This works in IP because routing is performed
 independently of reservation. Delivery of PATH messages across an ATM
 network is therefore likely to require a mechanism for setting up
 connections without reservations being made. The connection also
 needs to be of sufficient quality to deliver PATH messages fairly
 reliably; in some circumstances, a low quality best effort service
 may be inadequate for this task. A related issue is the problem of
 advertising services prior to reservations. The OPWA model (one pass
 with advertising) requires network elements to advertise the QoS that
 they are able to provide so that receivers can decide what level of
 reservation to request. Since these advertisements may be made prior
 to any resources having been reserved in the ATM network, it is not
 clear how to make meaningful advertisements of the QoS that might be
 provided across the ATM cloud.
 Finally, the multiparty model of communication is substantially
 different in  RSVP and ATM. Emulating RSVP receiver-initiation using
 ATM point-to-multipoint connections is likely to cause severe scaling
 problems as the number of receivers becomes large. Also, some
 functions of RSVP are not currently provided by ATM. For example,
 there is no support for different receiver requirements and
 capabilities-all receivers in a session receive the same QoS, which
 is fixed at the time the first receiver is added to the multicast
 tree. It is likely that ATM support for multi-party sessions will be
 enhanced in later versions of the standards. It is necessary for such
 support to evolve in a manner compatible with RSVP and IP multicast
 routing protocols if large ATM clouds are to be deployed
 successfully.

4.2 ST-II

 ST-II [27] and ST2+ [12] (referred to generically as ST hereafter)
 have data distribution and resource reservation schemes that are
 similar to ATM in many respects.
  • ST is connection oriented using "hard state". Senders set up

simplex data flows to all receivers closely matching point-to-

   multipoint connections in ATM. Routing decisions are made when

Borden, et al Informational [Page 13] RFC 1821 Real-time Service in IP-ATM Networks August 1995

   the connection is made and are not changed unless there is a
   failure in the path. Positive acknowledgment is required from all
   receivers. ST2+ [12] adds a receiver-based JOIN mechanism that can
   reduce the burden on senders to track all receivers.
  • ST reserves network resources at connection setup time. The ST

CONNECT message contains a flowspec indicating the resources to be

   reserved for the stream. Agents along the path may change the
   flowspec based on restrictions they may need to impose on the
   stream. The final flowspec is returned to the sender in the ACCEPT
   message from each receiver or target.
  1. —————————————————————-

| Category | RSVP | ATM (UNI 3.0) |

  1. —————————————————————-

| | | |

   | Orientation  |   Sender-based       |       Sender-based      |
   |              |                      |                         |
    ----------------------------------------------------------------
   |              |                      |                         |
   |     State    |      Hard state      |       Hard state        |
   |              | (explicit disconnect)|   (explicit delete)     |
   -----------------------------------------------------------------
   |              |                      |                         |
   |QoS SetupTime |   Concurrent with    |    Concurrent with      |
   |              |     stream setup     |   route establishment   |
   -----------------------------------------------------------------
   |              |                      |                         |
   |QoS Changes?  | Dynamic QoS          |       Static QoS        |
   |              |                      |  (Fixed at setup time)  |
   -----------------------------------------------------------------
   |              |                      | Bidirectional allocation|
   |Directionality|  Unidirectional      |  for unicast            |
   |              |resource allocation   |Unidirectional allocation|
   |              |                      |  for multicast          |
   -----------------------------------------------------------------
   |              |                      |                         |
   |Heterogeneity |   Receiver           |    Uniform QoS to       |
   |              |  heterogeneity       |    all receivers        |
   -----------------------------------------------------------------
 These similarities make mapping ST services to ATM simpler than RSVP
 but the mapping is still not trivial.  The task of mapping the ST
 flowspec into an ATM service class still has to be worked out.  There
 may be policy issues related to opening a new VC for each stream
 versus aggregating flows over an existing VC.

Borden, et al Informational [Page 14] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 Additionally, ST has some differences with UNI 3.1 that can cause
 problems when integrating the two protocols:
  • In ST, changes to active stream reservations are allowed. For

example, if the flowspec received from the target is not sufficient

    for the stream, the sender can send a CHANGE message, requesting a
    different QoS. UNI 3.1 does not allow changes to the QoS of a VC
    after it is set up. Future ATM UNI specifications are contemplating
    allowing changes to a VC after set up but this is still preliminary.
    In the meantime, policies for over reservation or aggregation onto
    a larger VC may be needed.
  • ST uses simplex streams that flow in only one direction. This is

fine for UNI 3.1 point-to-multipoint connections since the data flow

   is only in one direction.  When mapping a point-to-point ST
   connection to a standard point-to-point ATM VC, the reverse flow
   connection is wasted.
 This can be solved simply by using only point-to-multipoint VCs, even
 if there is only one receiver.

4.3 Mapping IP flows to ATM connections

 In general, there will be a great deal of flexibility in how one maps
 flows at the IP level to connections at the ATM level. For example,
 one could imagine setting up an ATM connection when a reservation
 message arrives at the edge of an ATM cloud and then tearing it down
 as soon as the reservation times out. However, to minimize latency or
 perhaps for economic reasons, it may be preferable to keep the ATM
 connection up for some period in case it is needed. Similarly, it may
 be possible or desirable to map multiple IP flows to a single ATM
 connection or vice versa.
 An interesting situation arises when a reservation request is
 received for an existing route across the cloud but which, when added
 to the existing reservations using that connection, would exceed the
 capacity of that connection. Since the current  ATM standards do not
 allow the QoS of a connection to be changed, there are two options:
 tear down the old connection and create a new one with the new,
 larger allocation of resources, or simply add a new connection to
 accommodate the extra traffic. It is possible that the former would
 lead to more efficient resource utilization. However, one would not
 wish to tear down the first connection before the second was
 admitted, and the second might fail admission control because of the
 resources allocated to the first. The difficulties of this situation
 seem to argue for evolution of ATM standards to support QoS
 modification on an existing connection.

Borden, et al Informational [Page 15] RFC 1821 Real-time Service in IP-ATM Networks August 1995

5.0 End System Issues

 In developing an integrated IP-ATM environment the applications need
 to be as oblivious as possible of the details of the environment: the
 applications should not need to know about the network topology to
 work properly. This can be facilitated first by a common application
 programing interface (API) and secondly by common flow and filter
 specifications [18].
 An example of a common API that is gaining momentum is the BSD
 sockets interface. This is a UNIX standard and, with Winsock2, has
 also become a PC standard. With the IETF integrated service
 environment just beginning to appear in the commercial marketplace,
 the ability to standardize on one common interface for both IP and
 ATM applications is still possible and must be seriously and quickly
 pursued to insure interoperability.
 Since the IP integrated service and ATM environments offer different
 QoS service types, an application should specify sufficient
 information in its flow specification so that regardless of the
 topology of the network, the network can choose an acceptable QoS
 type to meet the applicationUs needs. Making the application provide
 sufficient information to quantify a QoS service and allowing the
 network to choose the QoS service type is essential to freeing the
 application from requiring a set network topology and allowing the
 network to fully utilize the features of IP and ATM.

6.0 Routing Issues

 There is a fundamental difference between the routing computations
 for IP and ATM that can cause problems for real-time IP services.
 ATM computes a route or path at connection setup time and leaves the
 path in place until the connection is terminated or there is a
 failure in the path.  An ATM cell only carries information
 identifying the connection and no information about the actual source
 and destination of the cell.  In order to forward cells, an ATM
 device needs to consult a list of the established connections that
 map to the next hop device, without checking the final destination.
 In contrast, routing decisions in IP are based on the destination
 address contained in every packet. This means that an IP router, as
 it receives each packet,  has to consult a table that contains the
 routes to all possible destinations and the routing decision is made
 based on the final destination of the packet.  This makes IP routing
 very robust in the face of path changes and link failures at the
 expense of the extra header information and the potentially larger
 table lookup.  However, if an IP path has been selected for a given
 QoS, changes in the route may mean a change in the QoS of the path.

Borden, et al Informational [Page 16] RFC 1821 Real-time Service in IP-ATM Networks August 1995

6.1 Multicast routing

 Considerable research has gone into overlaying IP multicast models
 onto ATM.  In the MARS (Multicast Address Resolution Server) model
 [1], a server is designated for the Logical IP Subnet (LIS) to supply
 the ATM addresses of the hosts in the IP multicast group, much like
 the ATM ARP server [15].  When a host or router wishes to send to a
 multicast group on the LIS, a query is made to the MARS and a list of
 the ATM address of the hosts or routers in the group is returned. The
 sending host can then set up point-to-point or point-to-multipoint
 VCs to the other group members. When a host or a router joins an IP
 multicast group, it notified the MARS. Each of the current senders to
 the group is then notified of the new group member so that the new
 member can be added to the point to multipoint VC's.
 As the number of LIS hosts and multicast groups grows, the number of
 VCs needed for a one-to-one mapping of VCs to multicast groups can
 get very large.  Aggregation of multicast groups onto the same VC may
 be necessary to avoid VC explosion.  Aggregation  is further
 complicated by the QoS that may be needed for particular senders in a
 multicast group.  There may be a need to aggregate all the multicast
 flows requiring a certain QoS to a set of VCs, and parallel VCs may
 be necessary to add flows of the same QoS.

6.2 QoS Routing

 Most unicast and multicast IP routing protocols compute the shortest
 path to a destination based solely on a hop count or metric.  OSPF
 [16] and MOSPF [17] allow computation based on different IP Type of
 Service (TOS) levels as well as link metrics, but no current IP
 routing protocols take into consideration the wide range of levels of
 quality of service that are available in ATM or in the Integrated
 Services models.  In many routing protocols, computing all the routes
 for just the shortest path for a large network is computationally
 expensive so repeating this process for multiple QoS levels might be
 prohibitively expensive.
 In ATM, the Private Network-to-Network Interface (PNNI) protocol [13]
 communicates QoS information along with routing information, and the
 network nodes can utilize this information to establish paths for the
 required QoS. Integrated PNNI (I-PNNI) [9] has been proposed as a way
 to pass the QoS information available in ATM to other routing
 protocols in an IP environment.
 Wang & Crowcroft [28] suggest that only bandwidth and delay metrics
 are necessary for QoS routing and this would work well for computing
 a route that required a particular QoS at some setup time, but this
 goes against the connectionless Internet model. One possible solution

Borden, et al Informational [Page 17] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 to the exhaustive computation of all possible routes with all
 possible QoS values would be to compute routes for a common set of
 QoS values and only then compute routes for uncommon QoS values as
 needed, extracting a performance penalty only on the first packets of
 a flow with an uncommon QoS.  Sparse multicast routing protocols that
 compute a multicast path in advance or on the first packets from a
 sender (such as CBT [5] and MOSPF [17]) could also use QoS routing
 information to set up a delivery tree that will have adequate
 resources.
 However, no multicast routing protocols allow the communication of
 QoS information at tree setup time.  Obtaining a tree with suitable
 QoS is intended to be handled by RSVP, usually after the distribution
 tree has been set up, and may require recomputation of the
 distribution tree to provide the requested QoS.One way to solve this
 problem is to add some "hints" to the multicast routing protocols so
 they can get an idea of the QoS that the multicast group will require
 at group initiation time and set up a distribution tree to support
 the desired QoS. The CBT protocol [5] has some TBD fields in its
 control headers to support resource reservation. Such information
 could also be added to a future IGMP [11] JOIN message that would
 include information on the PIM Rendezvous Point (RP) or CBT Core.
 Another alternative is to recompute the multicast distribution tree
 based on the RSVP messages but this has the danger of losing data
 during the recomputation. However, this can leave a timing window
 where other reservations can come along during the tree recomputation
 and use the resources of the new path as well as the old path,
 leaving the user with no path to support the QoS desired.
 If unicast routing is used to support multicast routing, we have the
 same problem of only knowing a single path to a given destination
 with no QoS information. If the path suggested by unicast routing
 does not have the resources to support the QoS desired, there are few
 choices available. Schemes that use an alternate route to "guess" at
 a better path have been suggested and can work for certain topologies
 but an underlying routing protocol that provides QoS information is
 necessary for a complete solution.  As mentioned earlier, I-PNNI has
 the potential to provide enough information to compute paths for the
 requested QoS.

6.3 Mobile Routing

 In developing an integrated IP-ATM network, potential new growth
 areas need to be included in the planning stages. One such area is
 mobile networking. Under the heading of mobile networks are included
 satellite extensions of the ATM cloud, mobile hosts that can join an
 IP subnetwork at random, and a true mobile network in which all

Borden, et al Informational [Page 18] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 network components including routers and/or switches are mobile.
 The IP-ATM real-time service environment must be extended to include
 mobile networks so as to allow mobile users to access the same
 services as fixed network users. In doing so, a number of problems
 exist that need to be addressed. The principle problems are that
 mobile networks have constrained bandwidth compared to fiber and
 mobile links and are less stable than fixed fiber links. The impact
 of these limitations affect IP and ATM differently.  In introducing
 one or more constrained components into the ATM cloud,the effects on
 congestion control in the overall network are unknown. One can
 envision significant buffering problems when a disadvantaged user on
 a mobile link attempts to access information from a high speed data
 stream. Likewise, as ATM uses out of band signalling to set up the
 connection, the stability of the mobile links that may have
 significant fading or complete loss of connectivity could have a
 significant effect on ATM performance.
 For QoS, fading on a link will appear as a varying channel capacity.
 This will result in time-dependent fluctuations of available links to
 support a level of service. Current routing protocols are not
 designed to operate in a rapidly changing topology. QoS routing
 protocols that can operate in a rapidly changing topology are
 required and need to be developed.

7.0 Security Issues

 In a quality of service environment where network resources are
 reserved, hence potentially depriving other users access to these
 resources for some time period, authentication of the requesting host
 is essential. This problem is greatly increased in a combined IP-ATM
 topology where the requesting host can access the network either
 through the IP or the ATM portion of the network. Differences in the
 security architectures between IP and ATM can lead to opportunities
 to reserve resources without proper authorization to do so.  A common
 security framework over the combined IP-ATM topology would be
 desirable. In lieu of this, the use of trusted edge devices
 requesting the QoS services are required as a near term solution.
 Significant progress in developing a common security framework for IP
 is underway in the IETF [2]. The use of authentication headers in
 conjunction with appropriate key management is currently being
 considered as a long range solution to providing QoS security [3,8].
 In developing this framework, the reality of ATM portions of the
 Internet should be taken into account. Of equal importance, the ATM
 Forum ad-hoc security group should take into account the current work
 on an IP security architecture to ensure compatibility.

Borden, et al Informational [Page 19] RFC 1821 Real-time Service in IP-ATM Networks August 1995

8.0 Future Directions

 Clearly, there are some challenging issues for real-time IP-ATM
 services and some areas are better understood than others. For
 example, mechanisms such as policing, admission control and packet or
 cell scheduling can be dealt with mostly independently within IP or
 ATM as appropriate.  Thus, while there may be hard problems to be
 solved in these areas that need to be addressed in either the IP or
 ATM communities, there are few serious problems that arise
 specifically in the IP over ATM environment. This is because IP does
 not particularly care what mechanisms a network element (such as an
 ATM network) uses to provide a certain QoS; what matters is whether
 the ATM service model is capable of offering services that can
 support the end-to-end IP service model. Most of the hard problems
 for IP over ATM therefore revolve around the service models for IP
 and ATM.  The one piece of mechanism that is important in an IP/ATM
 context is signalling or resource reservation, a topic we return to
 below.
 The following paragraphs enumerate some of the areas in which we
 believe significant work is needed. The work falls into three areas:
 extending the IP over ATM standards; extensions to the ATM service
 model; and extensions to the IP service model. In general, we expect
 that practical experience with providing IP QoS over ATM will suggest
 more enhancements to the service models.
 We need to define ways of mapping the QoS and traffic
 characterizations (Tspecs and Rspecs) of IP flows to suitable
 characterizations for ATM connections.  An agreement is needed so
 that some sort of uniform approach is taken. Whatever agreement is
 made for such mappings, it needs to be done so that when traversing
 several networks, the requested QoS is obtained end-to-end (when
 admission is possible). Practical experience should be gained with
 these mappings to establish that the ATM service classes can in fact
 provide suitable QoS to IP flows in a reasonably efficient way.
 Enhancement of the ATM service classes may be necessary, but
 experience is needed to determine what is appropriate.
 We need to determine how the resource reservation models of IP (RSVP
 and ST-II) interact with ATM signalling. Mechanisms for establishing
 appropriate connection state with suitable QoS in ATM networks that
 are part of a larger integrated services Internet need to be defined.
 It is possible that the current IP/ATM mechanisms such as ARP servers
 and MARS can be extended to help to manage this state.
 There is a need for better QoS routing.  While this functionality is
 needed even in the pure ATM or pure IP environment, there is also an
 eventual need for integrated QoS routing between ATM and IP.  Further

Borden, et al Informational [Page 20] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 research and practical experience is needed in the areas of QoS
 routing in IP in order to support more than the shortest best-effort
 path, especially when this path may traverse ATM networks.  In many
 IP networks, there are multiple paths between a given source and
 destination pair but current routing technologies only pay attention
 to the current shortest path. As resources on the shortest path are
 reserved, it will be necessary and viable to explore other paths in
 order to provide QoS to a flow.
 Enrichment of the ATM model to support dynamic QoS would greatly help
 the IP over ATM situation. At present, the QoS objectives for ATM are
 established at call set-up and then fixed for the duration of a call.
 It would be advantageous to have the ability to provide a dynamic QoS
 in ATM, so that an existing call could be modified to provide altered
 services.
 Another possible area of enhancement to the ATM service model is in
 the area of multicasting. The multicast QoS offered is equal for all
 receivers, and thus may be determined by the least favorable path
 through the tree or by the most demanding receiver. Furthermore,
 there is no current provision for multipoint to multipoint
 connections. This limitation may rule out some of the services
 envisioned in the IP service model.
 There are areas of potential enrichment of the IP model as well.
 While the receiver-based approach of RSVP has nice scaling properties
 and handles receiver heterogeneity well, it is not clear that it is
 ideal for all applications or for establishing state in ATM networks.
 It is possible that a sender-oriented mode for RSVP might ease the
 IP/ATM integration task.
 Since the widespread availability of QoS raises new security concerns
 (e.g., denial of service by excessive resource reservation), it seems
 prudent that the IP and ATM communities work closely to adopt
 compatible approaches to handling these issues.
 This list is almost certainly incomplete. As work progresses to
 define IP over ATM standards to support QoS and to implement
 integrated services internetworks that include ATM, more issues are
 likely to arise. However, we believe that this paper has described
 the major issues that need to be taken into consideration at this
 time by those who are defining the standards and building
 implementations.

Borden, et al Informational [Page 21] RFC 1821 Real-time Service in IP-ATM Networks August 1995

9.0 References

 1.  Armitage, G., "Support for Multicast over UNI 3.1 based ATM
     Networks", Work in Progress, Bellcore, February 1995.
 2.  Atkinson,  R., "Security Architecture for the Internet Protocol",
     RFC 1825, NRL, August 1995.
 3.  Atkinson, R., "IP Authentication Header", RFC 1826, NRL,
     August 1995.
 4.  Ballardie, A., and J. Crowcroft, "Multicast-Specific Security
     Threats and Counter-Measures", Proceedings of ISOC Symposium on
     Network and Distributed System Security, San Diego, Feb. 1995,
     pp. 2-16.
 5.  Ballardie, T., Jain, N., Reeve, S. "Core Based Trees (CBT)
     Multicast, Protocol Specification", Work In Progress, University
     College London, Bay Networks, June, 1995.
 6.  Braden, R., Clark, D., and S. Shenker, "Integrated Services in
     the Internet Architecture: an Overview", RFC 1633, ISI/MIT/Xerox
     PARC, July 1994.
 7.  Braden, R., Zhang, L., Estrin, Herzog, D., and S. Jamin,
     "Resource ReSerVation Protocol (RSVP) - Version 1 Functional
     Specification", Work in Progress, ISI/PARC/UCS, July 1995.
 8.  Braden, R., Clark, D., Crocker, S., and C. Huitema, "Report of IAB
     Workshop on Security in the Internet Architecture", RFC 1636, ISI,
     MIT, TIS, INRIA, June 1994.
 9.  Callon, R., and B. Salkewicz, An Outline for Integrated PNNI for
     IP Routing", ATM Forum/ 95-0649, Bay Networks, July 1995.
 10. Cole, R., Shur, D., and C. Villamizar, "IP over ATM: A Framework
     Document", Work in Progress, AT&T Bell Laboratories/ ANS, April
     1995.
 11. Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
     1112, Stanford University, August 1989.
 12. Delgrossi, L., and L. Berger, Editors, "Internet Stream Protocol
     Version 2 (ST-2) Protocol Specification - Version ST2+", RFC 1819,
     ST2 Working Group, August 1995.
 13. Dykeman, D., Ed., "PNNI Draft Specification", ATM Forum/94-0471R8,
     IBM Zurich Research Lab, May 1995.

Borden, et al Informational [Page 22] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 14. Goyal, P., Lam, S., and Vin, H., "Determining End-to-End Delay
     Bounds in Heterogeneous Networks," 5th International Workshop on
     Network and Operating System Support for Digital Audio and Video,
     April, 1995.(Available via URL http://www.cs.utexas.edu/users/dmcl)
 15. Laubach, M., "Classical IP and ARP over ATM", RFC 1577, HP,
     January 1994.
 16. Moy, J., "OSPF Version 2", RFC 1583, Proteon, March 1994.
 17. Moy, J., "Multicast Extensions to OSPF," RFC 1584, Proteon, March
     1994.
 18. Partridge, C., "A  Proposed Flow Specification", RFC 1363, BBN,
     September 1992.
 19. Perez, M., Liaw, F., Mankin, A., Hoffman, E., Grossman, D. and
     A. Malis, "ATM Signaling Support for IP over ATM", RFC 1755,
     ISI, Fore, Motorola Codex, Ascom Timeplex, February 1995.
 20. Perkins, D., and Liaw, Fong-Ching, "Beyond Classical IP-Integrated
     IP and ATM Architecture Overview", ATM Forum/94-0935, Fore Systems,
     September 1994.
 21. Perkins, D. and Liaw, Fong-Ching, "Beyond Classical IP-Integrated
     IP and ATM Protocol Specifications", ATM Forum/94-0936, Fore
     Systems, September 1994.
 22. Romanow, A., and S. Floyd, "The Dynamics of TCP Traffic over ATM
     Networks", Proceedings of ACM SIGCOMM U94, London, August 1994,
     pp.79-88.
 23. Shenker, S., and C. Partridge. "Specification of Guaranteed Quality
     of Service", Work in Progress, Xerox/BBN, July 1995.
 24. Shenker, S., and C. Partridge. "Specification of Predictive Quality
     of Service", Work in Progress, Xerox/BBN, March 1995.
 25. Shenker, S., C. Partridge and J. Wroclawski. "Specification of
     Controlled Delay Quality of Service", Work in Progress,
     Xerox/BBN/MIT, June 1995.
 26. Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP:
     A Transport Protocol for Real-time Applications", Work in Progress,
     GMD/ISI/Xerox/LBL, March 1995.
 27. Topolcic, C., "Experimental Internet Stream Protocol, Version 2
     (ST-II)", RFC 1190, BBN, October 1990.

Borden, et al Informational [Page 23] RFC 1821 Real-time Service in IP-ATM Networks August 1995

 28. Wang, Z., and J. Crowcroft, "QoS Routing for Supporting Resource
     Reservation", University College of London white paper, 1995.

10. Authors' Addresses

 Eric S. Crawley
 Marty Borden
 Bay Networks
 3 Federal Street
 Billerica, Ma 01821
 508-670-8888
 esc@baynetworks.com
 mborden@baynetworks.com
 Bruce S. Davie
 Bellcore
 445 South Street
 Morristown, New Jersey 07960-6438
 201-829-4838
 bsd@bellcore.com
 Stephen G. Batsell
 Naval Research Laboratory
 Code 5521
 Washington, DC 20375-5337
 202-767-3834
 sgb@saturn.nrl.navy.mil

Borden, et al Informational [Page 24]

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