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INTERNET-DRAFT          Expires  1997                 INTERNET-DRAFT

Transport Services Area (TSV)                         Allison Mankin
INTERNET-DRAFT                                        USC/ISI,
Category: Informational                               Allyn Romanow
                                                      Sun Microsystems,
                                                      With the TSV
                                                      Area Directorate

                                                      November 1996


      IETF Criteria For Evaluating Reliable Multicast Transport
                   and Application Protocols

Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months.  Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a
   "working draft" or "work in progress."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the internet-drafts Shadow
   Directories on:

         ftp.is.co.za (Africa)
         nic.nordu.net (Europe)
         ds.internic.net (US East Coast)
         ftp.isi.edu (US West Coast)
         munnari.oz.au (Pacific Rim)


   This memo describes the procedures for reviewing reliable multicast
   protocols within the Transport Area (TSV) of the IETF.
   They are temporary procedures.  The Area Directors expect to
   be able to charter one or more WGs for standards track reliable
   multicast within a year or 18 months, as the procedures described
   herein help the transport and applications communities to identify
   and resolve the most serious technical problems with reliable

   Within today's Internet, important applications exist for
   a reliable multicast service.  Some examples that are pulling
   reliable multicast technology are collaborative workspaces (such
   as whiteboard), data and software distribution, and (more speculatively)
   web caching protocols.  However, the technology for accomplishing
   reliable multicast in the Internet is not well-understood at the current
   time.  Due to the nature of the technical issues, a single commonly
   accepted technical solution that solves all the demands for reliable
   multicast is infeasible.

   A number of reliable multicast protocols have already been
   invented to solve a variety of problems for various types
   of applications (See Bibliography). How should these protocols
   be treated within the IETF and how should the IETF guide
   the development of reliable multicast in a direction beneficial
   for the general Internet?

   The TSV Area Directors and their Directorate have outlined a set of
   review procedures that address these questions and set criteria
   and processes for the publication as RFCs of internet-drafts on
   reliable multicast transport or reliable multicast applications.

1.0 Background on IETF Processes and Procedures

   In the IETF, work in an area is directed and managed by the Area
   Directors (ADs), who have authority over the chartering of
   working groups (WGs).

   In addition, ADs review individually submitted (not by WGs)
   internet-drafts about work that is relevant to their area
   prior to publication as RFCs (Experimental,
   Informational or, in rare cases, Standards Track). The review
   is done according to the guidelines set out in the Internet
   Standards Process, RFC 2026 [InetStdProc96].

   The purpose of this Internet-Draft is to present
   in advance the criteria that will be used by the
   TSV ADs in reviewing reliable multicast internet-drafts.

2.0 Introduction

   There is a strong application pull for reliable multicast.
   Widespread use of the Internet makes the economy of multicast
   transport important.  Current Internet multicast offers
   a best-effort many-to-many delivery
   service and offers no guarantees. While many types of
   applications lend themselves to the group-delivery model of
   multicast, they require some kind of reliability, which is not
   offered by IP multicast. Applications wishing to use a
   reliable multicast include collaborative applications,
   distributed interactive simulations, and distribution services
   (e.g. netnews).

   To meet the growing demand for reliable multicast, many reliable
   multicast protocols have been proposed [Obraczka96, Multicast
   Transport Protocols, Reliable Multicast Protocols, Overview of RM].

   However, as we discuss in Section 3, the issues raised by reliable
   multicast are considerably more complex than those related to
   reliable unicast.  In particular, reliable multicast protocols can
   do damage in the Internet, through causing congestion disasters
   if they are used popularly before they are well-engineered.  Because
   of the complexity of the technical issues, and the abundance of
   proposed solutions, we are putting in place more well-defined review
   procedures than usual.  We compare this action with an IESG action
   taken in 1991, RFC 1264 [], when community experience with standard
   Internet dynamic routing protocols was still limited, and extra
   review was deemed necessary to assure that the protocols introduced
   would be effective, correct and robust.

   Section 3 describes in detail the nature of the particular challenges
   posed by reliable multicast. Section 4 describes the process for
   considering reliable multicast solutions. Section 5 details the
   additional requirements that need to be met by proposals to be advanced
   to RFC status.

3.0 Issues in Reliable Multicast

   Two aspects of reliable multicast make standardization particularly
   challenging. First, the meaning of reliability varies in the context of
   different applications. Secondly, if special care is not taken, reliable
   multicast protocols can cause a particular threat to the operation
   of the global Internet. These issues are discussed in detail in this

   3.1  One or Many Reliable Multicast Protocols or Frameworks?

   Unlike reliable unicast, where a single transport protocol (TCP) is
   currently used to meet the reliable delivery needs of a wide range of
   applications, reliable multicast does not necessarily lend itself to
   a single application interface or to a single underlying set of
   mechanisms.  For unicast transport, the requirements for reliable,
   sequenced data delivery are fairly general.  TCP, the primary
   transport protocol for reliable unicast, is a mature protocol with
   delivery semantics that suit a wide range of applications.

   In contrast, different multicast applications have widely different
   requirements for reliability.  For example, some applications
   require that message delivery obey a total ordering while others do
   not.  Some applications have many or all the members sending data
   while others have only one data source.  Some applications have
   replicated data, for example in an n-redundant file store, so that
   several members are capable of transmitting a data item while for
   others all data originates at a single source.  Some applications
   are restricted to small fixed-membership multicast groups, while
   other applications need to scale dynamically to thousands or tens of
   thousands of members (or possibly more).  Some applications have
   stringent delay requirements, while others do not.  Some
   applications such as file-transfer are high-bandwidth, while other
   applications such as interactive collaboration tools are more likely
   to be bursty but use low bandwidth overall.  These requirements each
   impact the design of a reliable multicast protocol in a different

   In addition, even for a specific application where the application's
   requirements for reliable multicast are well understood, there are
   many open questions about the underlying mechanisms for providing
   reliable multicast.  A key question concerns the robustness of the
   underlying reliable multicast mechanisms as the number of senders or
   the membership of the multicast group grows.

   One challenge to the IETF is to end up with the right match between
   applications' requirements and reliable multicast mechanisms.  While
   there is general agreement that a single reliable multicast protocol
   or framework is not likely to meet the needs of all Internet
   applications, there is less understanding and agreement about the
   exact relationship between application-specific requirements and
   more generic underlying reliable mutlicast protocols or mechanisms.
   There are also open questions about the appropriate integration
   between an application and an underlying reliable multicast
   framework, and potential generality of a single applications
   interface for that framework.

   3.2  Congestion Control

   A particular concern for the IETF (and a dominant concern
   for the Transport Services Area) is the impact of reliable
   multicast traffic on other traffic in the Internet in times of
   congestion (more specifically, the effect of reliable multicast
   traffic on competing TCP traffic).  The success of the Internet
   relies on the fact that best-effort traffic responds to congestion
   on a link (as currently indicated by packet drops) by reducing the
   load presented on that link.  Congestion collapse in today's
   Internet is prevented only by the congestion control mechanisms in
   TCP [Jacobson88, HostReq89, Stevens96].

   There are a number of reasons to be particularly attentive to the
   congestion-related issues raised by reliable multicast proposals.
   Multicast applications in general have the potential to do more
   congestion-related damage to the Internet than do unicast
   applications.  This is because a single multicast flow can be
   distributed along a large, global multicast tree reaching throughout
   the entire Internet.

   Further, reliable multicast applications have the potential to do
   more congestion-related damage than do unreliable multicast
   applications.  First, unreliable multicast applications such as
   audio and video are, at the moment, usually accompanied by a person
   at the receiving end, and people typically unsubscribe from a
   multicast group if congestion is so heavy that the audio or video
   stream is unintelligible.  Reliable multicast applications such as
   group file transfer applications, on the other hand, are likely to
   be between computers, with no humans in attendance monitoring
   congestion levels.

   In addition, reliable multicast applications do not necessarily have
   the natural time limitations typical of current unreliable multicast
   applications.  For a file transfer application, for example, the
   data transfer might continue until all of the data is transferred to
   all of the intended receivers, resulting in a potentially-unlimited
   duration for an individual flow.  Reliable multicast applications
   also have to contend with a potential explosion of control traffic
   (e.g., ACKs, NACKs, status messages), and with control traffic
   issues in general that may be more complex than for unreliable
   multicast traffic.

   The design of congestion control mechanisms for reliable multicast
   for large multicast groups is currently an area of active research.
   The challenge to the IETF is to encourage research and
   implementations of reliable multicast, and to enable the
   needs of applications for reliable multicast to be met as expeditously
   as possible, while at the same time protecting the Internet from
   the congestion disaster or collapse that could result from the
   widespread use of applications with inappropriate reliable multicast
   mechanisms.  Because of the setbacks  and costs that could result
   from the widespread deployment of reliable multicast with inadequate
   congestion control, the IETF must exercise care in the standardization
   of a reliable multicast protocol that might see widespread use.

   The careful review and cautious acceptance procedures for proposals
   submitted as internet-drafts reflects our concern to meet the
   challenges described here.

 4. IETF Process for Review and Publication of Reliable Multicast
      Protocol Specifications

   In the general case of individually submitted internet-drafts
   (proposals not produced by an IETF WG), the process of publication
   as some type of RFC is described in RFC 2026 (4.2.3) [InetStdProc96].
   This specifies that if the submitted internet-draft is closely
   related to work being done or expected to be done in the IETF, the ADs
   may recommend that the document be brought within the IETF and
   progressed in the IETF context.  Otherwise, the ADs may recommend
   that the Internet-Draft be published as an Experimental or Informa-
   tional RFC, with or without an IESG annotation of its relationship
   to the IETF context.

   The procedure for Reliable Multicast proposal publication will
   have as its default RFC status Experimental, when the technical
   criteria listed in Section 5 are deemed to be fulfilled.
   Both the criteria and the procedure reflect the ADs's
   technical assessment of the current state of reliable multicast
   technology.  It does not reflect the origins of the proposals, which
   we expect will be equally from commercial vendors with
   initial products and from researchers.

   Work on the development and engineering of protocols that may
   eventually meet the review criteria could take place either in
   an IRTF Research Group or a focused short IETF WG with an
   Experimental product.

   When the work in reliable multicast technology has matured enough
   to be considered for standardization within the IETF, the TSV Area
   will charter appropriate standards track WGs.  The criteria for
   evaluation of standards track products will be at least as
   stringent as those described herein (next section).

5. Technical Criteria for Reliable Multicast

   The internet-draft must:

     a. Analyze the behavior of the protocol.

        The vulnerabilities and performance problems must be
        shown through analysis. Especially the protocol behavior
        must be explained in detail with respect to scalability,
        congestion control, error recovery, and robustness.

        For example the following questions should be answered:

                How scalable is the protocol to the number of users
                in a group, number of groups, wide dispersion of
                group members? If appropriate, how scalable is the
                protocol to the number of senders?

                Identify the mechanisms which limit scalabilty and
                estimate those limits.

                How does the protocol protect the Internet from
                congestion? How well does it perform? When does it

                Under what circumstances will the protocol
                fail to perform the functions needed by the
                applications it serves?

                Is there a congestion control mechanism?
                How well does it perform? When does it fail?

     b. Include a description of trials and/or simulations
        which support the development of the protocol and the
        answers to the above questions.

    c.  Include an analysis of whether the protocol has
        congestion avoidance mechanisms strong enough to cope
        with deployment in the Global Internet, and if not, clearly
        document the circumstances in which congestion harm can
        occur.  How are these circumstances to be prevented?

     d. Include a description of any mechanisms which contain the
        the protocol within limited network environments.  It is
        likely that some answers to a. and c. will mean that such
        mechanisms are required.  We recognize that the confinement
        of internet applications is an open research area.

     e. Show that the protocol can use IPSEC or other mechanisms for
        secure operation.  (General requirement with specific
        ramifications for reliable multicast that are outside the
        scope of this memo).

    Comments on these criteria, additions to them, or refinements
    are welcome, and should be sent to the Transport Services
    Area (trans-area@isi.edu).

6. References

    [Floyd95] Floyd, S., Jacobson, V., Liu, C., McCanne, S., and Zhang, L.,
    A Reliable Multicast Framework for Light-weight Sessions and
    Application Level Framing. Submitted to IEEE/ACM Transactions on
    Networking. An earlier version of this paper appeared in ACM SIGCOMM
    95, August 1995, pp. 342-356.

    [HostReq89] R. Braden, Ed., Requirements for Internet Hosts --
    Communication Layers, RFC 1122, October 1989.

    [InetStdProc96] Bradner, S., The Internet Standards Process --
    Revision 3, RFC 2026, October 1996.

    [Jacobson 1988]  Jacobson, V.,
    Congestion Avoidance and Control,
    Proceedings of SIGCOMM '88, August 1988, pp. 314-329.
    An updated version of this paper is available at

    [Multicast Transport Protocols], Multicast Transport Protocols
    Web Page, URL http://www.roads.lut.ac.uk/DS-Archive/MTP.html.

    [Obraczka96] Obraczka, K., Multicast Transport Mechanisms: A Survey,
    October 1996.

    [Overview of RM] Overview of Reliable Multicast Protocols Web Page,
    URL http://gaia.cs.umass.edu/sigcomm_mcast/talk1.html.

    [Reliable Multicast Protocols] Reliable Multicast Protocols Web Page,
    URL http://www.tascnets.com/mist/doc/mcpCompare.html.

    [Stevens96] Stevens, W. R., TCP Slow Start, Congestion Avoidance,
    Fast Retransmit, and Fast Recovery Algorithms, Work in Progress,
    March 1996.

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