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RFC 1821 - Integration of Real-time Services in an IP-ATM Networ


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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

   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].

   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

   * 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.

                     [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.

   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.

   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

   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.

   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

   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.

     -----------------------------------------------------------------
     |   Category   |      RSVP            |       ATM (UNI 3.0)     |
     -----------------------------------------------------------------
     |              |                      |                         |
     | 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.

   *  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

   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

     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.

     -----------------------------------------------------------------
     |   Category   |      RSVP            |       ATM (UNI 3.0)     |
     -----------------------------------------------------------------
     |              |                      |                         |
     | 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.

   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.

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.

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

   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

   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.

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

   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.

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.

   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.

   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

 

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