MBoneD Working Group                                          Bob Quinn
Internet Engineering Task Force                         Stardust Forums                          Celox Networks
INTERNET-DRAFT                                           Kevin Almeroth
25 June 1999                                                       UCSB
March 2001                                             UC-Santa Barbara
Expires December 1999 September 2001

                         IP Multicast Applications:
                          Challenges and Solutions

                <draft-ietf-mboned-mcast-apps-01.txt>

                   <draft-ietf-mboned-mcast-apps-02.txt>

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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Abstract

   This document describes the challenges involved with designing and
   implementing multicast applications.  It is an introductory guide for
   application developers that highlights the unique considerations of
   multicast applications as compared to unicast applications.

   To this end, the document presents a taxonomy of multicast
   application I/O models and examples of the services they can support.
   It then describes the service requirements of these multicast
   applications, and the recent and ongoing efforts to build protocol
   solutions to support these services.

   Copyright (C) The Internet Society (1999). (2001).  All Rights Reserved.

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Table of Contents

   Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . .     1

   1. Introduction. . . . . . . . . . . . . . . . . . . . . . . .     3
     1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . .     3
     1.2 Focus and Scope. . . . . . . . . . . . . . . . . . . . .     4

   2. IP Multicast-enabled Network. . . . . . . . . . . . . . . .     4
     2.1 Essential Protocol Components. . . . . . . . . . . . . .     5
       2.1.1 Expedient Joins and Leaves . . . . . . . . . . . . .     5
       2.1.2 Send without a Join. . . . . . . . . . . . . . . . .     6

   3. IP Multicast Application Taxonomy . . . . . . . . . . . . .     6
     3.1 One-to-Many Applications . . . . . . . . . . . . . . . .     8
     3.2 Many-to-Many Applications. . . . . . . . . . . . . . . .     9
     3.3 Many-to-One Applications . . . . . . . . . . . . . . . .    10

   4. Common Multicast Service Requirements . . . . . . . . . . .    12    13
     4.1 Bandwidth Requirements . . . . . . . . . . . . . . . . .    12    13
     4.2 Delay Requirements . . . . . . . . . . . . . . . . . . .    13

   5. Unique Multicast Service Requirements . . . . . . . . . . .    14
     5.1 Address Management . . . . . . . . . . . . . . . . . . .    15
       5.1.1 Scope Discovery .  . . . . . . . . . . . . . . . . .    16
     5.2 Session Management . . . . . . . . . . . . . . . . . . .    16    17
     5.3 Heterogeneous Receiver Support . . . . . . . . . . . . .    17    18
     5.4 Reliable Data Delivery . . . . . . . . . . . . . . . . .    19    20
     5.5 Security . . . . . . . . . . . . . . . . . . . . . . . .    20    21
     5.6 Synchronized Play-Out. . . . . . . . . . . . . . . . . .    22    23

   6. Service APIs. . . . . . . . . . . . . . . . . . . . . . . .    22    23

   7. Security Considerations . . . . . . . . . . . . . . . . . .    23    24

   8. Acknowledgements. . . . . . . . . . . . . . . . . . . . . .    23    24

   9. References. . . . . . . . . . . . . . . . . . . . . . . . .    23    24

  10. Authors' Addresses. . . . . . . . . . . . . . . . . . . . .    26    28

  11. Full Copyright Statement. . . . . . . . . . . . . . . . . .    27    28

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1. Introduction

   IP Multicast will play a prominent role on the Internet in the coming
   years.  It is a requirement, not an option, if the Internet is going
   to scale.  Multicast allows application developers "to add more
   functionality without significantly impacting the network"
   [Bradner].

   Developing multicast-enabled applications is ostensibly simple.
   Having datagram access allows any application to send to a multicast
   address. A multicast application need only increase the Internet
   Protocol (IP) time-to-live (TTL) value to more than 1 (the default
   value) to allow outgoing datagrams to traverse routers.  To receive a
   multicast datagram, applications join the multicast group, which
   transparently generates an [IGMPV2] [IGMPv2, IGMPv3] group membership report.

   This apparent simplicity is deceptive, however.  Enabling multicast
   support in applications and protocols that can scale well on a
   heterogeneous network is a significant challenge.  Specifically,
   sending constant bit rate datastreams, reliable data delivery,
   security, and managing many-to-many communications all require
   special consideration.  Some solutions are available, but many of
   these services are still active research areas.

1.1 Motivation

   The purpose of this document is to provide a framework for
   understanding the challenges of designing and implementing multicast
   applications. In order to use multicast communications correctly,
   application developers must first understand the various I/O models
   and the network services (in addition to basic multicast
   communication) that are required.  Secondly, application developers
   need to be aware of efforts underway to provide these services.  Such
   efforts range in maturity from deployed commercial products to basic
   research efforts to evaluate feasibility.

   Multicast-based applications and services will play an important role
   in the future of the Internet as continued multicast deployment
   encourages their use and development.  It is important that
   developers be aware of the issues and solutions available--and
   especially of their limitations--in order to avoid protocols that
   negatively impact networks (thereby counter-acting the benefits of
   multicast) or wasting their efforts "re-inventing the wheel."

   The hope is that by raising developers' awareness, we can adjust
   their expectations of finding solutions and lead them to successful,
   scalable, and "network-friendly" development efforts.

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1.2 Focus and Scope

   Our initial premise is that the multicast infrastructure is
   transparent to applications, so it is not directly relevant to this
   discussion.  Our focus here is on multicast application protocol
   services, so this document explicitly avoids any discussion of
   multicast routing issues.  We identify and describe the multicast-
   specific issues involved with developing applications.

   We assume the reader has a general understanding of the mechanics of
   multicast, and in this respect we intend to compliment other
   introductory documents [Maufer, [BeauW, Maufer, Miller].  Since this is an
   introductory survey rather than a comprehensive examination, we refer
   readers to other multicast application requirements descriptions [LSMA, [RM,
   LSMA, Miller] for more detail.

   In the remainder of this document we first define the term "IP
   multicast enabled network," the multicast infrastructure and
   essential multicast services.  Next we describe the types of new
   functionality that multicast applications can enable and their
   requirements.  We then examine the services that satisfy these
   requirements, the challenges they present, and provide a brief survey
   of the solutions available or under development.  We wrap up with a
   discussion of application programming interfaces (APIs) for multicast
   services.

2. IP Multicast Enabled Network

   An "IP multicast-enabled network" provides end-to-end services in the
   IP network infrastructure to allow any IP host to send datagrams to
   an IP multicast address that any number of other IP hosts widely
   dispersed can receive.

   There are two kinds of multicast-enabled networks available.  The
   first is based on the original multicast service model as defined in
   RFC 1112 [Deering].  In this model, a receiver simply joins the group
   and does not need to know the identity of the source(s).  This model
   is known by a number of names including Internet Standard Multicast
   (ISM), Internet Traditional Multicast (ITM), Deering multicast, etc.
   The second kind of multicast modifies the original service model such
   that in addition to knowing the group address, a receiver must know
   the set of relevant sources.  This type of multicast is called Source
   Specific Multicast (SSM) [SSM].  It becomes the applicationĂs
   responsibility of knowing what kind of multicast capability the
   network provides.  Currently, the only way for an application to know
   the type of multicast is based on the group address.  If the group is
   in the 232/8 range, the application should assume SSM is the service
   model.  Otherwise, the application should assume source-generic
   multicast is the service model.

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   At the time of this writing writing, end-to-end "global" multicast service is
   not yet available, but the size of the "multicast-enabled" Internet
   is growing.  Recent development and deployment of interdomain
   multicast routing protocols and multicast-friendly Internet exchanges
   [MIX] have enabled peering between major ISPs.  This, along with the
   increasing availability of compelling content, is spurring deployment
   and availability of the IP Multicast Enabled Network.

   In the remainder of this document we assume that the multicast-
   enabled network is already ubiquitous.  Since such a large and
   growing portion of the global Internet is IP multicast-enabled now,
   and many enterprise networks (intranets) are also, this perspective
   is relevant today.

2.1 Essential Protocol Components

   An IP multicast enabled network requires two essential protocol
   components:

     1) An IP host-based protocol to allow a receiver application to
        notify a local router(s) that it has joined the group, and
        initiate the data flow from all sender(s) within the scope

     2) An IP router-based protocol to allow any routers with multicast
        group members (receivers) on their local networks to communicate
        with other routers to ensure that all datagrams sent to the
        group address are forwarded to all receivers within the intended
        scope

   Ideally, these protocol components are transparent to multicast
   applications.  However, there are two aspects of their functionality
   requirements that are worth mentioning specifically, since they
   affect application performance and design.  These are the multicast
   application requirements for:

     - Expedient Joins and Leaves
     - Sends without a Join

2.1.1 Expedient Joins and Leaves

   Some applications require expedient group joins and leaves, as their
   usability or functionality are sensitive to the latency involved with
   joining and leaving a group.

     Join Latency: The time it takes for data to begin flowing after an
        application issues a command to join a multicast group

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     Leave Latency: The time it takes for data to stop flowing after an
        application issues a command to leave a multicast group
        [IGMPv2]
        [IGMPv2,IGMPv3]

   For example, using distributed a/v as a multicast-based "television"
   must allow users to "channel surf"--changing channels frequently.
   Each channel change generates a group leave and group join, so delays
   in either will affect usability.  In a sense, this is more of a user
   requirement than an application requirement.

   The functionality of distributed interactive simulations [DIS] is
   often sensitive to join/leave latency.  This is particularly true
   when trying to exchange information to represent fast moving objects
   that quickly enter and exit the scope of a simulated environment
   (e.g. low-flying, fast-moving aircraft).  If the join latency is too
   long, the information provided may be old by the time it is received.

   A fast leave phase is highly desirable both for effective congestion
   control mechanisms, to stop undesirable flows quickly, and for the
   network in general, to better filter unwanted traffic [Rizzo].
   Applications cannot affect control over either join or leave latency,
   but are dependent on the multicast infrastructure to enable expedient
   operations.  This is a basic multicast service requirement.

2.1.2 Sends without a Join

   Applications that send to a multicast address should be able to start
   sending immediately, without having to join the destination group
   first.  This is important for embedded devices and bursty-
   source bursty-source
   applications with low-delay delivery requirements.

   The current IGMP-based multicast host model and all current
   implementations allow senders to send to a group without joining it
   as a standard feature.

3. IP Multicast Application Taxonomy

   With an IP multicast-enabled network available, some unique and
   powerful applications and application services are possible.
   "Multicast enables coordination - it is well suited to loosely
   coupled distributed systems (of people, servers, databases,
   processes, devices...)" [Estrin].

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   A "multicast application" is simply defined as any application that
   sends to and/or receives from an IP multicast address.  These may or
   may not also reference IP unicast addresses, as we describe later.

   What differentiates IP multicast applications from one-to-one unicast
   applications are the other sender and receiver relationships
   multicast enables.  There are three general categories of multicast
   applications:

     One-to-Many (1toM): A single host sending to two or more (n)
        receivers

     Many-to-Many (MtoM): Any number of hosts sending to the same
        multicast group address, as well as receiving from it

     Many-to-One (Mto1): Any number of receivers sending data back to a
        (source) sender via unicast or multicast

                              +-----------------------------------+
                              |        Host 2->n ("many")         |
                              +-------------+---------------------+
                              |   One-Way   |       Two-Way       |
                              +-------------+---------------------|
                              |  A      B   |   C      D      E   |
                  +-----------+-------------+---------------------+
                  |    I/O    |             |  S(m)/  S(u)/  S(m)/|
                  | Operations| S(m)   R(m) |  R(m)   R(m)   R(u) |
      +-------+---+-----------+-------------+---------------------|
      |       | 1 | S(m)      |        1toM |  MtoM               |
      | Host  | 2 | R(m)      | Mto1        |  MtoM               |
      |       +---+-----------+-------------+                     |
      |  1    | 3 | S(m)/R(m) | Mto1   1toM    MtoM               |
      |       | 4 | S(m)/R(u) |                       Mto1        |
      |("one")| 5 | S(u)/R(m) |                              Mto1 |
      +-------+---+-----------+-----------------------------------+

            Legend:    S: "Send"          (u): "unicast"
            ------     R: "Receive"       (m): "multicast"

     Table 1: Application types characterized by I/O relationships
              and destination address types (multicast or unicast)

   Table 1 defines these application types in terms of the I/O
   relationships they represent.  These categories are defined in terms
   of the combination of communication mechanisms they use.  At the IP
   level, all multicast I/O is only 1toM or MtoM and unicast is always
   one-to-one (1to1).  The Mto1 category, for example, refers to several
   possible combinations of IP-level relationships, including unicast.
   We created the Mto1 category to help differentiate between the many
   applications and services that use multicast.

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             1toM:          MtoM:             Mto1:
                  R1             S1/R1             S1
                 /               / | \               \
                S-R2         S2/R2-+-S3/R3         S2-R
                 \...            \ | /            .../
                  Rn             Sn/Rn             Sn

                Legend:  S: "Sender"
                ------   R: "Receiver"

      Figure 1: Generalization of the three application categories

   Figure 1 illustrates the general model for each of the three
   multicast application categories.  Again it is worth emphasizing that
   Mto1 is an artificial category defined by the application-level
   relationship between sender(s) and receiver.  At the IP-level,
   multicast does not provide an Mto1 I/O mechanism, since it does not
   allow senders to limit receivers, nor even know who they are.
   Receiver information and limitations are enabled at the application
   level, as required by the multicast application.

   We describe each of these general application types in more detail
   and provide application examples of each in the sub-sections below.
   The list of examples is not comprehensive, but attempts to define the
   prominent multicast application and service types in each of the
   three general categories.  We reference the items in these lists in
   the remainder of this document as we describe their specific service
   requirements, define the challenges they present, and reference
   solutions available or under development.

3.1 One-to-Many Applications

   One-to-Many (1toM) applications have a single sender, and multiple
   simultaneous receivers.  Entry B1 in Table 1 represents the classic
   1toM relationship.  Entry B3 differs only slightly, as the sender
   also acts as receiver (i.e. it has loopback enabled).

   When people think of multicast, they most often think of broadcast-
   based multimedia applications: television (video) and radio (audio).
   This is a reasonable analogy and indeed these are significant
   multicast applications, but these are far from the extent of
   applications that multicast can enable.  Audio/Video distribution
   represents a fraction of the multicast application possibilities, and
   most do not have analogs in today's consumer broadcast industry.

     a) Scheduled audio/video (a/v) distribution: Lectures,
        presentations, meetings, or any other type of scheduled event
        whose multimedia coverage could benefit an audience (i.e.

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        television and radio "broadcasts").  One or more constant-bit-
        rate (CBR) datastreams and relatively high-bandwidth demands
        characterize these applications.  When more than one datastream
        is present--as with an audio/video combination--the two are
        synchronized and one typically has a higher priority than the
        other(s).  For example, in an a/v combination it is more
        important to ensure a legible audio stream, than perfect video.

     b) Push media: News headlines, weather updates, sports scores, or
        other types of non-essential dynamic information.  "Drip-feed,"
        relatively low-bandwidth data characterize these applications.

     c) File Distribution and Caching: Web site content, executable
        binaries, and other file-based updates sent to distributed end-
        user or replication/caching sites

     d) Announcements: Network time, multicast session schedules, random
        numbers, keys, configuration updates, (scoped) network locality
        beacons, or other types of information that are commonly useful.
        Their bandwidth demands can vary, but generally they are very
        low bandwidth.

     e) Monitoring: Stock prices, Sensor equipment (seismic activity,
        telemetry, meteorological or oceanic readings), security
        systems, manufacturing or other types of real-time information.
        Bandwidth demands vary with sample frequency and resolution, and
        may be either constant-bit-rate or bursty (if event-
        driven). event-driven).

3.2 Many-to-Many Applications

   In many-to-Many (MtoM) applications two or more of the receivers also
   act as senders.  In other words, MtoM applications are characterized
   by two-way multicast communications.

   The many-to-many capabilities of IP multicast enable the most unique
   and powerful applications.  Each host running an MtoM application may
   receive data from multiple senders while it also sends data to all of
   them.  As a result, many-to-many applications often present complex
   coordination and management challenges.

     f) Multimedia Conferencing: Audio/Video and whiteboard comprise the
        classic conference application.  Having multiple datastreams
        with different priorities characterizes this type of
        application.  Co-ordination issues--such as determining who gets
        to talk when--complicate their development and usability.  There
        are common heuristics and "rules of play", but no standards
        exist for managing conference group dynamics.

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     g) Synchronized Resources: Shared distributed databases of any type
        (schedules, directories, as well as traditional Information
        System databases).

     h) Concurrent Processing: Distributed parallel processing.

     i) Collaboration: Shared document editing.

     j) Distance Learning: This is a one-to-many a/v distribution
        application with "upstream" capability that allows receivers to
        question the speaker(s).

     k) Chat Groups: These are like text-based conferences, but may also
        provide simulated representations ("avatars") for each "speaker"
        in simulated environments.

     l) Distributed Interactive Simulations [DIS]: Each object in a
        simulation multicasts descriptive information (e.g. telemetry)
        so all other objects can render the object, and interact as
        necessary.  The bandwidth demands for these can be tremendous,
        as the number of objects and the resolution of descriptive
        information increases.

     m) Multi-player Games: Many multi-player games are simply
        distributed interactive simulations, and may include chat group
        capabilities.  Bandwidth usage can vary widely, although today's
        first-generation multi-player games attempt to minimize
        bandwidth usage to increase the target audience (many of whom
        still use dial-up modems).

     n) Jam Sessions: Shared encoded audio (e.g. music).  The bandwidth
        demands vary based on the encoding technique, sample rate,
        sample resolution, number of channels, etc.

3.3 Many-to-One Applications

   Unlike the one-to-many and many-to-many application categories, the
   many-to-one (Mto1) category does not represent a communications
   mechanism at the IP layer.  Mto1 applications have multiple senders
   and one (or a few) receiver(s), as defined by the application layer.
   Table 1 shows that Mto1 applications can be one-way or use a two-way
   request/response type protocol, where either senders or receiver(s)
   may generate the request.  Figure 2 characterizes the I/O
   relationship possibilities in Mto1 applications:

      1)  S1        2)  S1            3)  S1           4)  S1
            \             \                 \                \
          S2-R          S2-R              S2-R             S2-R
         .../          .../              .../             .../
          Sn            Sn                Sn               Sn

         Data(m)     Request(m)       Request(m)       Request(u)
         ------>     ---------->     <----------       ---------->
                     Response(u)      Response(u)      Response(m)
                    <-----------      ----------->    <----------

          Figure 2: Characterization of Mto1 I/O possibilities

   Mto1 applications have many scaling issues.  Too many simultaneous
   senders can potentially overwhelm receiver(s), a condition
   characterized as an "implosion problem."   Another considerable
   scaling problem is the large amount of state in the network that
   having many multicast senders generates.  This is largely transparent
   to applications and the effect may be diminished in the future with
   the use of bidirectional bi-directional trees in multicast routing protocols, but
   nonetheless it should be considered by application designers.

   No standards yet exist for alternate and equivalent Mto1 application
   designs, but there are a number of possibilities to consider [HNRS].
   Since the main advantage of using multicast in a Mto1 application is
   that senders need not know the receiver(s) unicast address(es), one
   alternative is for the each receiver to advertise its unicast address
   via multicast.  However, since this strategy still leaves the
   potential for implosion on each receiver, additional strategies may
   be needed to distribute the load.  For example, it may be possible to
   increase the number of receivers (in a "flat" receiver topology) or
   establish localized receivers (in a "hierarchical" topology) as used
   in "local recovery" (section 5.3).  Such methods have coordination
   issues, and although standard solutions have not yet been identified,
   Mto1 application developers should consider their alternatives
   carefully.

     o) Resource Discovery: Service Location, for example, leverages IP
        Multicast to enable something like a "host anycasting service"
        capability [AnyCast]: A multicast receiver to send a query to a
        group address, to elicit responses from the closest host so they
        can satisfy the request.  The responses might also contain
        information that allows the receiver to determine the most
        appropriate (e.g. closest) service provider to use.

        In Table 1, this application is entry D4.  It is also
        illustrated in Figure 2 by possibility number 3.  Alternately,
        the response could be to a multicast rather than unicast
        address, although technically that would make it an MtoM
        application type (this is how the . Service Location Protocol
        [SLP] operates, when a user agent attempts to locate a directory
        agent).

     p) Data Collection: This is the converse of a one-to-many
        "monitoring" application described earlier.  In this case there
        may be any number of distributed "sensors" that send data to a
        data collection host.  The sensors might send updates in
        response to a request from the data collector, or send
        continuously at regular intervals, or send spontaneously when a
        pre-defined event occurs.  Bandwidth demands can vary based on
        sample frequency and resolution.

        This is illustrated in Table 1 by entries A1 and A3, the only
        difference being that A3 has a loopback interface.  In Figure 2,
        this is possibility number 1.  Since the number of receivers can
        easily be more than one, this is really an MtoM application.

     q) Auctions: The "auctioneer" starts the bidding by describing
        whatever it is for sale (product or service or whatever), and
        receivers send their bids privately or publicly (i.e. to a
        unicast or multicast address).

        This is possibility number 2 in Figure 2, and D5 in Table 1.
        The response could be sent to a multicast address, although
        technically that would make it an MtoM application.

     r) Polling: The "pollster" sends out a question, and the "pollees"
        respond with answers.  This is possibility number 2 in Figure 2,
        and could also be characterized as an MtoM application if the
        response is to a multicast address.

     s) Juke Box: Allows near-on-demand a/v playback.  Receivers use an
        "out-of-band" protocol mechanism (via web, email, unicast or
        multicast requests, etc.) to send their playback request into a
        scheduling queue [IMJ].

        This is characterized by possibility number 4 in Figure 2, and
        entry D4 in Table 1.  The initial unicast request is the only
        difference between this type of application and a typical 1toM.
        If that initial request were sent to a multicast address, this
        would effectively be an MtoM application.

     t) Accounting: This is basically data collection but is worth
        separating since it is such an important application. In some
        multicast applications it is imperative to know information
        about each receiver, possibly in real-time. But such information
        can be overwhelming. overwhelming[MRM]. Current mechanisms, like RTCP (which
        is actually MtoM since it is multicast but could be made Mto1),
        use scaling techniques but trade-off information granularity. As
        a group grows the total amount of feedback is constant but each
        receiver sends less.

4. Common Multicast Service Requirements

   Some multicast application service requirements are common to unicast
   network applications as well.  We characterize two of them
   here--bandwidth here--
   bandwidth and delay requirements.

4.1 Bandwidth Requirements

   Figure 3 shows multicast applications approximate bandwidth
   requirements.

   Unicast and multicast applications both need to design applications
   to adapt to the variability of network conditions.  But as we
   describe in section 4.1, it is the need to accommodate multiple
   heterogeneous multicast receivers--with their diversity of bandwidth
   capacity and delivery delays--that presents the unique challenge for
   multicast applications to satisfy these requirements.

          |
     1toM |     b, d          c, e               a
          |
     MtoM |       k           g, i        f, h, j, l, m, n
          |
     Mto1 |   o, q, r       p, q, t              s
          |
          +-----------------------------------------------
            Low Bandwidth                  High Bandwidth

           Figure 3: Bandwidth Requirements of applications

4.2 Delay Requirements

   Aside from those with time-sensitive data (e.g. stock prices, and
   real-time monitoring information), most one-to-many applications have
   a high tolerance for delay and delay variance (jitter).  Constant bit-rate bit-
   rate (CBR) data--such as streaming media (audio/video)-
   -are (audio/video)--are sensitive
   to jitter, but applications commonly counteract the effects by
   buffering data and delaying playback.

   Most many-to-one and many-to-many multicast applications are
   intolerant of delays because they are bidirectional, interactive and
   request/response dependent.  As a result, delays should be minimized,
   since they can adversely affect the application's usability.

   This need to minimize delays is most evident in (two-way) conference
   applications, where users cannot converse effectively if the audio
   or video is delayed more than 500 milliseconds.  For this and other
   examples see Figure 4, which plots multicast applications on a
   (coarse) scale of sensitivity to delivery delays.

          |
     1toM |     b, c         a, d                e
          |
     MtoM |               g, i, j, k       f, h, l, m, n
          |
     Mto1 |      r        o, p, s, t             q
          |
          +-----------------------------------------------
            Delay Tolerant                Delay Intolerant

           Figure 4: Delay tolerance of application types

   For delay-intolerant multicast (or unicast) applications, quality of
   service (QoS) is the only option.  IP networks currently provide only
   "best effort" delivery, so data are subject to variable router
   queuing delays and loss due to network congestion (router queue
   overflows).  IP QoS standards do exist now [RSVP] and efforts to
   enable end-to-end QoS support in the Internet are underway [E2EQOS].

   However, QoS support is an IP network infrastructure consideration.
   Although there are multicast-specific challenges for implementing QoS
   in the network for multicast flows, they are beyond the control of
   applications, so further discussion of the QoS protocols and services
   is beyond the scope of this document.

5. Unique Multicast Service Requirements

   The three application categories described earlier are very general
   in nature.  Within each category and even among each of the
   application types, specific application instances have a variety of
   application requirements.  One-to-many application types are
   relatively simple to develop, but as we pointed out there are
   challenges involved with developing many-to-one and many-to-many
   applications.  Some of these have requirements bandwidth and delay
   requirements, similar to unicast applications.

   Multicast applications can be further differentiated from unicast
   applications and from each other by the services they require.  In
   this section we provide a survey of the various services that have
   unique requirements for multicast applications.

       +------------------------------------------------------+

    +--------------------------------------------------------------+
    |                  Multicast Application                       |
    +--------------------------------------+   +-----------+   +-------------------+
    +-------------------------------------+|   |+----++----+   |+--------++--------+
    |          Multicast Security         ||   ||        ||        |
    +----------------------+   +----------+|   ||Sys-||Co-   || System ||        |
    +----------++---------+|   |+---------+|   ||tem ||decs|   ||  Time  || Codecs |
    | Reliable || Address ||   || Session ||   ||Time||   ||        ||        |
    | Delivery ||  Mgmnt.   Mgt   ||   ||  Mgmnt.   Mgt   ||   ||        ||        |
       +----------++---------++---++---------++---++----++----+
       +----------------------------------------++------------+
    +----------++---------++---++---------++---++--------++--------+
    +----------------------------------------++--------------------+
    |     Basic IP Multicast Service         ||     IP Unicast     |
    |       (e.g. UDP and IGMPv2) IGMPv2/v3)         ||      Service       |
       +----------------------------------------++------------+
    +----------------------------------------++--------------------+

            Figure 5: Multicast service requirements summary

   Here's the list of multicast application service requirements:

     Address Management - Coordinated ű Selection and coordinated of address allocation service that
        provides some
        allocation.  The need is provide assurances against "address collisions". ˘address
        collision÷ and provide address ownership.

     Session Management - Making session descriptions available (via
        advertisements and explicit queries) within appropriate scopes,
        and also enabling registration of new sessions ű Perform application-layer services on top of
        multicast transport.  These services depend heavily on the
        application but include functions like session advertisement,
        billing, group member monitoring, key distribution, etc.

     Heterogeneous Receiver Support - Sending to receivers with a wide
        variety of bandwidth capacities, latency characteristics, and
        network congestion requires feedback to monitor receiver
        performance.

     Reliable Data Delivery - Ensuring that all data sent is received by
        all receivers receivers.

     Security - Ensuring content privacy among dynamic multicast group
        memberships, and limiting senders senders.

     Synchronized Play-Out - Allow multiple receivers to "replay" data
        received in synchronized fashion fashion.

   In the remainder of this section, we describe each of these
   application services in more detail, the challenges they present, and
   the status of standardized solutions.

5.1 Address Management

   One of the first questions facing a multicast application developer
   is what multicast address to use.  Multicast addresses are not
   assigned to individual hosts, assignments can change dynamically, and
   addresses sometimes have semantics of their own (e.g
   Administrative (e.g. Admin Scoping).
   Multicast applications require an address management service that
   provides address allocation or assignment
   queries: queries.  There are a
   number of ways for applications to learn about multicast addresses:

     Hard-Coded: Software configuration, encoded in a binary executable,
        or burned into ROM in embedded devices.  These applications
        typically reference IANA statically allocated multicast
        addresses (including relative addresses).

     Advertised: Session announcements (as described in the next
        section), or via another "out-of-band" query or discovery
        protocol mechanism.

     Algorithmically Derived: Using a programmatic algorithm to allocate
        a statistically random (unused) address.

          |
     1toM |    c, e          a, b                d
          |
     MtoM |               f, j, k, n        g, h, i, l, m
          |
     Mto1 |    r            o, p, s             q, t
          |
          +-----------------------------------------------
            Hard-Coded       Advertised      Algorithmic

        Figure 6: Multicast address usage for application types

5.1.1 Scope Discovery

   Scope Discovery is a function of address management required by some
   applications.  An option for [MADCAP] allows clients to learn which
   scopes nest inside each other, for the purpose of executing
   expanding ring searches (among other things) [Kermode].   Scoped
   Address Discovery Protocol [SADP] allows applications to discover
   the administratively scoped

   In almost all cases, application designers should assume that
   multicast addresses already allocated are to
   a session within one or more administrative scopes in a hierarchy be dynamic.  Very little of
   nested scopes.  These protocols assume the use of Multicast Zone
   Announcement Protocol [MZAP] as a _back-end_ (transparent to
   applications), since it enables multicast
   address space is available for static assignment by IANA [MADDR].
   Also, given the creation of such hierarchies. host-specific addressing available with SSM, Internet-
   wide, static address assignment is expected to be very rare.

5.2 Session Management

   Multicast applications need

   Session management is one of the most misunderstood services with
   respect to multicast.  Most application developers assume that
   multicast will provide services like security, encryption,
   reliability, session advertisement, monitoring, billing, etc.  In
   fact, multicast is simply a "namespace" transport mechanism that provides session
   directory end-to-
   end delivery.  All of the other services are application-layer
   services that must be provided by each particular application.
   Furthermore, in most cases there are not defined standards for how
   these functions should be provided.  The particular functions are
   dependent on the particular needs of the application, and no single
   method (or standard) can be used made to co-ordinate application
   schedules be sufficient for all cases.

   While there are no generic solutions which provide all session
   management functions, there are some protocols and resources, common techniques
   that provide support for some of the functions.  Techniques for
   congestion control and describe heterogeneous receiver support are discussed
   in Section 5.3.  Protocols for reliability are discussed in Section
   5.4.  Security considerations are discussed in Section 5.5.

   With respect to session attributes.  These map
   multicast address and port combinations advertisement, there are a number of
   mechanisms for advertising sessions.  One commonly used technique is
   to advertise sessions via the WWW.  Users can join a date and time, content
   description, and other session attributes (e.g. bandwidth group by
   clicking on URLs, and delay
   requirements, encoding, security then having a response returned to the user
   that includes the group address and authorization methods, etc.).

   The maybe information about group
   source(s).  Another mechanism is the session description protocol [SDP] is designed
   [SDP].  It provides a format for this purpose, representing information about
   sessions, but it does not provide the transport for dissemination of
   these session descriptions, nor does it enable the provide address allocation
   and management.  SDP only provides the syntax for describing session
   attributes.

   SDP session descriptions may be conveyed publicly or privately by
   means of any number of transports including web (HTTP) and MIME
   encoded email.  The session announcement protocol [SAP] is the de
   facto standard transport and many multicast-enabled applications
   currently use it.  SAP limits distribution via multicast scoping, but
   the current protocol definition has scaling issues that need to be
   addressed.  Specifically, the initialization latency for a session
   directory can be quite long, and it increases in proportion to the
   number of session announcements.   This is to an extent a multicast
   infrastructure issue, however, as this level of protocol detail
   should be transparent to applications.

   The session management service needs to:
     -       Advertise scheduled sessions
     -       Provide a query mechanism for retrieving information about
       session schedules

5.3 Heterogeneous Receiver Support

   The Internet is a network of networks.  IP's strength is its ability
   to enable seamless interoperability between hosts on disparate
   network media, the heterogeneous network.

   When two hosts communicate via unicast--one-to-one--across an IP
   network, it is relatively easy for senders to adapt to varying
   network conditions.  The Transmission Control Protocol (TCP) provides
   reliable data transport, and is the model of "network friendly"
   adaptability.

   TCP receivers send acknowledgements back to the sender for data
   delivered.  A TCP sender detects data loss from the data sent that is
   not acknowledged.  When it detects data loss, TCP infers that there
   is network congestion or a low-bandwidth link, and adapts by
   throttling down its send rate [SlowStart].

   User Datagram Protocol (UDP) does not enable a receiver feedback loop
   the way TCP does, since UDP does not provide reliable data delivery
   service.  As a result, it also does not have a loss detection and
   adaptive congestion control mechanism as TCP does.  However, it is
   possible for a unicast UDP application to enable similar adaptive
   algorithms to achieve the same result, or even improve on it.

   A unicast UDP application that uses a feedback mechanism to detect
   data loss and adapt the send rate, can do so better than TCP.  TCP
   automatically reduces the "congestion window" when data loss is
   detected, although the updated send rate may be slower than a CBR
   audio/video stream requires.  When a UDP application detects loss, it
   can adapt the data itself to accommodate the lower send rate.  For
   example, a UDP application can:

     -  Reduce the data resolution (e.g. send lower fidelity audio/video
        by reducing sample frequency or frame rate) to reduce data rate.

     -  Modify the data encoding to add redundant data (e.g. forward
        error correction) offset in time to avoid fate sharing.  This
        could also be "layered", so a percentage of data loss will
        simply reduce fidelity rather than corrupt the data.

     -  Reduce the send rate of one datastream in order to favor another
        of higher priority (e.g. sacrifice video in order to ensure
        audio delivery).

     -  Send data at a lower rate (i.e. with a different encoding) on a
        separate multicast address and/or port number for high-loss
        receivers.

   However, with multicast applications--one-to-many or many-to-many--
   which have multiple receivers, the feedback loop design needs
   modification.  If all receivers return data loss reports
   simultaneously, the sender is easily overwhelmed in the storm of
   replies.  This is known as the "implosion problem."

   Another problem is that heterogeneous receiver capabilities can vary
   widely due to the wide range of (static) network media bandwidth
   capabilities and dynamically due to transient traffic conditions.  If
   a sender adapts its send rate and data resolution based on the loss
   rate of its worst receiver(s), then it can only service the lowest
   common denominator.  Hence, a single "crying baby" can spoil it for
   all other receivers.

   Strategies exist for dealing with these heterogeneous receiver
   problems.  Here are two examples:

     Shared Learning - When loss is detected (i.e. a sequenced packet
        isn't received), a receiver starts a random timer.  If it
        receives a data loss report sent by another receiver as it waits
        for the timer to expire, it stops the timer and does not send a
        report.  Otherwise, it sends a report when the timer expires.
        The Real-Time Protocol and its feedback-loop counterpart Real-Time Real-
        Time Control Protocol [RTP/RTCP] employ a strategy similar to
        this to keep feedback traffic to 5 percent or less than the
        overall session traffic.  This technique was originally utilized
        in IGMP.

     Local Recovery - Some receivers may be designated as local
        distribution points or "transcoders" that either re-send data
        locally (possibly via unicast) when loss is reported or they
        re-encode re-
        encode the data for lower bandwidth receivers before re-
        sending. re-sending.
        No standards exist for these strategies, although "local
        recovery" is used by several reliable multicast protocols.

   Adaptive multicast application design for heterogeneous receivers is
   still an active area of research.  The fundamental requirements are
   to maximize application usability, while accommodating network
   conditions in a "network friendly" manner.  In other words,
   congestion detection and avoidance are (at least) as important in
   protocol design as the user experience.  The adaptive mechanisms must
   also be stable, so they do not adapt too quickly--changing encoding
   and rates based on too little information about what may be a
   transient condition--to avoid oscillation.

   This "feedback loop" service necessary for support of heterogeneous
   receivers is not illustrated in the services summary in Figure 4,
   although it could be added alongside "Reliable Transport" and the
   others.  This service could be implemented within an application or
   accessed externally, as provided by the operating system or a third
   party.  See [HNRS] for a taxonomy of strategies for providing
   feedback for multicast, with the ultimate goal of developing a common
   multicast feedback protocol.

5.4 Reliable Data Delivery

   Many of the multicast application examples in our list--like
   audio/video distribution--have loss-tolerant data content.  In other
   words, the data content itself can remain useful even if some of it
   is lost.  For example, audio might have a short gap or lower fidelity
   but will remain legible despite some data loss.

   Other application examples--like caching and synchronized resources-
   -require resources--
   require reliable data delivery.  They deliver content that must be
   complete, unchanged, in sequence, and without duplicates.  The "Loss
   Intolerant" column in Figure 7 shows a list of applications with this
   requirement, while the others can tolerate varying levels of data
   loss.  The tolerance levels are typically determined by the nature of
   the data and the encoding in use.

        |
   1toM |     b             a, d               c, e
        |
   MtoM |             f, j, k, l, m, n       g, h, i
        |
   Mto1 |                o, p, r, s, t          q
        |
        +------------------------------------------------
          Loss Tolerant                   Loss Intolerant

      Figure 7: Reliability Requirements of Application types

   Some of the challenges involved with enabling reliable multicast
   transport are the same as those of sending to heterogeneous
   receivers, and some solutions are similar also.  For example, many
   reliable multicast transport protocols avoid the implosion problem by
   using negative acknowledgements (NAKs) from receivers to indicate
   what was lost.  They also use "shared learning" whereby receivers
   listen to others' NAKs and then listen for the resulting
   retransmission of data, rather than requesting retransmission by
   sending a NAK themselves.

   Although reliable delivery cannot change the data sent--except,
   perhaps, to use a loss-less data compression algorithm--they can use
   other adaptive techniques like sending redundant data, or adjusting
   the send rate.

   Although many reliable multicast protocol implementations exist
   [Obraczka], and a few are already available in commercial products,
   none of them are standardized.  Work is ongoing in the "Reliable
   Multicast" research group of the Internet Research Task Force [IRTF]
   to provide a better definition of the problem, the multicast
   transport requirements, and protocol mechanisms.  The IETF Relialbe
   Multicast Transport (RMT) working group is focusing on designing
   some working solutions in recognition of the urgent need for
   scalable reliable multicast transport [RMT BLOCKS, RMT DESIGN].

   Scalability is the paramount concern, and it implies the general need
   for "network friendly" protocols that detect and avoid congestion as
   they provide reliable delivery.  Other considerations are protocol
   robustness, support for "late joins", group management and security
   (which we discuss next).

   The current consensus is that due to the wide variety of multicast
   application requirements--some of which are at odds--no single
   multicast transport will likely be appropriate for all applications.
   As a result, most believe that we will eventually standardize a
   number of reliable multicast protocols, rather than a single one.
   one[BULK].

5.5 Security

   For any IP network application--unicast or multicast--security is
   necessary because networks comprise users with different levels of
   trust.

   Network application security is challenging, even for unicast.   And
   as the need for security increases--gauged by the risks of being
   without it--the challenges increase also.  Security system complexity
   and overhead is commensurate with the protection it provides. "No one
   can guarantee 100% security. But we can work toward 100% risk
   acceptance ...Strong cryptography can withstand targeted attacks up
   to a point--the point at which it becomes easier to get the
   information some other way ...A good design starts with a threat
   model: what the system is designed to protect, from whom, and for how
   long." [Schneier]

   Multicast applications are no different than unicast applications
   with respect to their need for security, and they require the same
   basic security services: user authentication, data integrity, data
   privacy and user privacy (anonymity).  However, enabling security for
   multicast applications is even more of a challenge than for unicast.
   Having multiple receivers makes a difference, as does their
   heterogeneity and the dynamic nature of multicast group memberships.

   Multicast security requirements can include any combination of the
   following services:

     Limiting Senders   - Controlling who can send to group addresses

     Limiting Receivers - Controlling who can receive

     Limiting Access    - Controlling who can "read" multicast content
        either by encrypting content or limiting receivers (which isn't
        possible yet)

     Verifying Content  - Ensuring that data originated from an
        authenticated sender and was not altered en route

     Protecting Receiver Privacy - Controlling whether sender(s) or
        other receivers know receiver identity

     Firewall Traversal - Proxying outgoing "join" requests through
        firewalls, allowing incoming or outgoing traffic through, and
        (possibly) authenticating receivers for filtering purposes and
        security [Chouinard, Finlayson].

   This list is not comprehensive, but includes the most commonly needed
   security services.  Different multicast applications and different
   application contexts can have very different needs with respect to
   these services, and others. "Two main issues emerge, where the
   performance of current solutions leaves much to be desired"
   [Canetti]:

     Individual authentication - how is sender identity verified for
        each multicast datagram received?

     Membership revocation - how is further group access disabled for
        group members that leave the group (e.g. encryption keys in
        their possession disabled)?

   Performance is largely a factor when a user joins or leaves a group.
   For example, methods used to authenticate potential group members
   during joins or re-keying current members after a member leaves can
   involve significant processing and protocol overhead and result in
   significant delays that affect usability.

   Like reliable multicast, secure multicast is also still under
   investigation in the Internet Research Task Force [IRTF].  Protocol
   mechanisms for many of the most important of these services--such as
   limiting senders--have not yet been defined, let alone developed and
   deployed.

   As is true for reliable multicast, the current consensus is that no
   single security protocol will satisfy the wide diversity of
   sometimes-contradictory sometimes-
   contradictory requirements among multicast applications.  Hence,
   multicast security will also likely require a number of different
   protocols.

5.6 Synchronized Play-Out

   This refers to having all receivers simultaneously play-out the
   multicast data they received.  This may be necessary for fairness--
   playing-out prices for auctions, or stock-prices--or to ensure
   synchronization with other receivers, such as when playing music.

   Here is an analogy to illustrate: Imagine a multi-speaker stereo
   system that is wired throughout a home (via analog).  With the stereo
   playing on all speaker sets, you will hear continuous music as you
   walk from room-to-room.

   Now imagine a house full of multi-media and network enabled computer
   systems.  Although they will all receive the same music datastream
   simultaneously via multicast, they will provide discontinuous music
   playback as you walk room-to-room.

   To provide synchronized playback that would enable continuous music
   from room-to-room would require three things:

    1)      system clocks on all systems should be synchronized
    2)      datastreams must be framed with timestamps
    3)      the playback latency of the multimedia hardware

   The third of these is the most difficult to achieve at this time.
   Hardware and drivers don't provide any mechanism for retrieving this
   information, although different audio and video devices have a wide-
   range of performance.

6. Service APIs

   In some cases, the protocol services mentioned in this document can
   be enabled transparently by passive configuration mechanisms and
   "middleware." For example, it is conceivable that a UDP
   implementation could implicitly enable a reliable multicast protocol
   without the explicit interaction of the application.

   Sometimes, however, applications need explicit access to these
   services for flexibility and control.  For example, an adaptive
   application sending to a heterogeneous group of receivers using RTP
   may need to process RTCP reports from receivers in order to adapt
   accordingly (by throttling send rate or changing data encoders, for
   example) [RTP API].  Hence, there is often a need for service APIs
   that allow an application to qualify and initiate service requests,
   and receive event notifications.  In Figure 4, the top edge of the
   box for each service effectively represents its API.

   Network APIs generally reflect the protocols they support.  Their
   functionality and argument values are a (varying) subset of protocol
   message types, header fields and values.  Although some protocol
   details and actions may not be exposed in APIs--since many protocol
   mechanics need not be exposed--others are crucial to efficient and
   flexible application operation.

   A more complete examination of the application services described in
   this document might also identify the protocol features that could be
   mapped to define a (generic) API definition for that service.   APIs
   are often controversial, however.  Not only are there many language
   differences, but it is also possible to create different APIs by
   exposing different levels of detail in trade-offs between flexibility
   and simplicity.

7. Security Considerations

   See section 4.4

8. Acknowledgements

   The author would like to acknowledge and thank the following
   individuals for their helpful feedback: Ran Canetti, Brian Haberman,
   Eric A. Hall, Kenneth C. Miller, and Dave Thaler.

9. References

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              Taxonomy", IEEE Communications Magazine, Vol. 36 No. 1,
              January 1998

  [Rizzo]     L. Rizzo, "Fast Group management in IGMP", Hipparc HIPPARC 98
              workshop, June 1998, UCL London
              http://www.iet.unipi.it/~luigi/hipparc98.ps.gz

  [RM]        A. Mankin, A. Romanow, S. Bradner, V. Paxson, "IETF
              Criteria for Evaluating Reliable Multicast Transport and
              Application Protocols", RFC 2357, June 1998

  [RM BLOCKS] B. Whetten, L. Vicisano, R. Kermode, M. Handley,
              S. Floyd, "Reliable Multicast Transport Building Blocks
              for One-to-Many Bulk-Data Transfer", <draft-ietf-rmt-
              buildingblocks-00.txt>, June 1999, Work in Progress

  [RM DESIGN] M. Handley, B. Whetten, R. Kermode, S. Floyd,
              L. Vicisano, "The Reliable Multicast Design Space for
              Bulk Data Transfer", <draft-ietf-rmt-design-space-00.txt>
              June 1999, Work in Progress

  [RSVP]      J. Wroclawski, "The Use of RSVP with IETF Integrated
              Services", RFC 2210, September 1997

  [RTP API]   J. Rosenberg, "Columbia RTP Library API Specification,"
              (Note: Does not include RTCP processing), February 1997
  [RTP/RTCP]  H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
              "RTP: A Transport Protocol for Real-Time Applications",
              RFC 1889, January 1996

  [SAP]       M. Handley, "SAP: Session Announcement Protocol", <draft-
              ietf-mmusic-sap-00.txt>, November 1996, Work in Progress

  [SADP]      R. Kermode, D. Thaler, "Scoped Address Discovery Protocol
              (SADP) ", <draft-ietf-mboned-sadp-01.txt>, Jan 1999, Work
              in Progress

  [SDP]       M. Handley, V. Jacobson, "SDP: Session Description
              Protocol", RFC 2327, April 1998

  [Schneier]  B. Schneier, "Why Cryptography Is Harder Than It Looks",
              December 1996, http://www.counterpane.com/whycrypto.html

  [SlowStart] W. Stevens, "TCP Slow Start, Congestion Avoidance, Fast
              Retransmit, and Fast Recovery Algorithms", RFC 2001,
              January 1997

  [SLP]       J. Veizades, E. Guttman, C. Perkins, S. Kaplan, "Service
              Location Protocol", RFC 2165, June 1997

   [SSM]       H. Holbrook, B. Cain, ˘Specific Multicast for IP÷, <draft-
              holbrook-ssm-arch-01.txt>, Nov 2000, Work in Progress

10. Authors' Addresses

   Bob Quinn
   IP Multicast Initiative (IPMI)
   Stardust Forums, Inc.
   1901 S. Bascom Ave. #333
   Campbell, CA 95008
   Celox Networks
   2 Park Central Drive
   Southborough, MA 01772

   +1 408 879 8080
   rcq@ipmulticast.com 508 305 7000
   bquinn@celoxnetworks.com

   Kevin Almeroth
   Department of Computer Science
   Office:  2113, Engineering I
   University of California
   Santa Barbara, CA 93106-5110

   +1 805 893 2777
   almeroth@cs.ucsb.edu

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