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Versions: 00 01 02 03                                                   
MBONED Working Group                                           H. Asaeda
Internet-Draft                                           Keio University
Expires: April 30, 2009                                          V. Roca
                                                        October 27, 2008

   Requirements for IP Multicast Session Announcement in the Internet

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Internet-Draft   Req. IP Multicast Session Announcement     October 2008


   The Session Announcement Protocol (SAP) [3] was used to announce
   information for all available multicast sessions to the prospective
   receiver in an experimental network.  It is useful and easy to use,
   but difficult to control the SAP message transmission in a wide area
   network.  This document describes the several major limitations SAP
   has and the requirements for multicast session announcement in the
   global Internet.

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Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   this document are to be interpreted as described in RFC 2119 [1].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Potential Problems in SAP  . . . . . . . . . . . . . . . . . .  5
     2.1.  Announcement Interval vs. Latency  . . . . . . . . . . . .  5
     2.2.  Difficulties in Scope Definition . . . . . . . . . . . . .  5
     2.3.  Communication Dependency . . . . . . . . . . . . . . . . .  6
     2.4.  Lack of Sender and Receiver Control  . . . . . . . . . . .  7
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Normative References . . . . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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1.  Introduction

   The Session Announcement Protocol (SAP) [3] was a necessary component
   to announce information for all available multicast sessions to the
   prospective receiver in the experimental MBone.  In a SAP
   announcement procedure, the entire session information must be
   periodically transmitted and all active session descriptions
   (described with the Session Description Protocol (SDP) [4] syntax)
   must be continuously refreshed.  If ever a session is no longer
   announced, its description eventually times out and is deleted from
   the available session list.  This is a major property of a "soft-
   state" protocol.

   SAP enables to keep the session information active and refresh it,
   and builds robust and fault-tolerant systems.  However, it requires
   the periodic message transmission (i.e. message flooding) that may
   cause major overheads or overloads.  Although this strategy keeps the
   implementation simple, it rises costs and further reduces its

   Another issue is closely related to a security or policy management.
   As with the above issue, it is difficult to control a data sender or
   a receiver and the amount of traffic or the data distribution area
   even with existing scoping techniques.

   This document explains the issues SAP has been raised and clarifies
   the requirements that should fulfill an ideal session announcement
   system.  This document describes work originally published by Asaeda
   and Roca in IEICE Transactions on Information and Systems [2].

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2.  Potential Problems in SAP

2.1.  Announcement Interval vs. Latency

   SAP improves the robustness and data consistency in front of packet
   losses by transmitting each message several times.  However,
   transmitting a large number of active multicast sesssion information
   in a flooding manner may cause major overheads.  The solution defined
   in [3] is the time period between repetitions of an announcement.
   This period is chosen such that the total bandwidth used by all
   announcements on a single SAP group remains below a preconfigured
   limit, and the bandwidth limit should be assumed to be 4000 bits per
   second, if not specified.

   However, this solution largely increases the latency experienced by
   end users especially when the number of sessions increases.  In its
   definition, since the minimum interval of SAP message transmission is
   200 seconds, end users experience a minimum waiting time of 200
   seconds to obtain the entire session list, irrespective of the number
   of observed multicast sessions, message size of multicast session
   information, and bandwidth SAP uses.  Let us assume the average
   message size of a single multicast session information is about 300
   bytes.  When there are more than 500 active multicast sessions, an
   interval time of each session announcement becomes greater than 200
   seconds and the average announcement interval increases accordingly.
   For instance, if 2000 multicast sessions are active in the Internet,
   each session announcement interval is between 800 seconds and 1600
   seconds.  In this case, if some SAP message is lost, users may need
   to wait 1600 seconds for the next announcement as maximum.

   Obviously, it is possible to make the announcement interval shorter
   by changing the SAP configuration on a sender side and provide
   shorter latency for the sender-receiver communication.  However, it
   makes the total ammount of SAP messages transmitted larger and may
   increase the probability of creating congestions.

2.2.  Difficulties in Scope Definition

   Multicast data senders or network administrators may want to define
   an area where data packets sent within a session will be confined.
   This area is called "scope area".  An end user who belongs to the
   scope area can receive the session data.

   When IP multicast was initially deployed in the MBone, the Time-To-
   Live (TTL) field of the IP header was used to control the
   distribution of multicast traffic.  A multicast router configured
   with a TTL threshold drops any multicast packet in which the TTL
   falls below the threshold.  For instance, a router at the boundary of

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   an organization configures the threshold to 32 which denotes an
   "organization" scope boundary.

   The drawbacks of this "TTL scoping" are: 1) the senders must be
   sufficiently aware of the network topology to determine the TTL value
   to use, and 2) complex scope areas cannot be defined (e.g., between
   overlapped areas).  Especially the first point becomes big obstacles
   for general end users to precisely set up the data distribution area.
   TTL scoping, which only defines a rough granularity, does not provide
   a complete solution.

   The "administratively scoped IP multicast" approach [5] provides
   clear and simple semantics such as scope boundaries are associated to
   multicast addresses.  With IPv4, packets addressed to the
   administratively scoped multicast address range 239/8 (i.e. from to cannot cross the configured
   administrative boundaries.  Since scoped addresses are defined
   locally, the same multicast address can be used in different non-
   overlapping areas.  Oppositely, an administrator can define multiple
   areas overlap by dividing the administratively scoped address range,
   which is not possible with TTL scoping.

   However, administrative scoping has several major limitations.  An
   administrator may want to partition the scope area to disjoint areas
   on a per receiver basis, or he may want to limit data distribution
   according to the transmission rate or the content category of each
   session, or he may want to use the data sender's address as a keyword
   to set up the scope.  Note that the latter aspect is nowadays
   feasible since Source-Specific Multicast (SSM) [6] requires that a
   join request specifies both the multicast and source addresses.

   SSM highlights another contradiction in the administrative scoping
   approach: the address range dedicated to SSM, 232/8 with IPv4, cannot
   cover the address range dedicated to administrative scoping, 239/8.
   Although the problem can be solved by defining yet another SSM
   specific administrative scoping address range, defining a new
   addressing architecture requires modifying application, end host, and
   router implementations or configurations.  Hence, using multicast
   addresses to define a scope is not a complete solution either.

2.3.  Communication Dependency

   SAP relies on the ASM model, since every SAP instance can send
   announcements in the SAP announcement group.  For instance, to
   receive SAP announcement messages for the global scope IPv4 multicast
   sessions, all prospective receivers must join session
   (without specifying any source address).  This is another major
   limitation of SAP since some Internet Service Providers (ISPs) may

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   want to provide only SSM multicast routing.  It is known that a
   versatile announcement protocol should not rely on any specific
   routing architecture.

   Moreover, this communication model is subject to a Denial-of-Service
   attack.  If malicious hosts flood high bandwidth stream to this
   global announcement address,, then all prospective
   receivers including multicast routers listening SAP messages take in
   the stream and their networks may be corrupted or destroyed.

2.4.  Lack of Sender and Receiver Control

   Network administrators or service providers may want to define
   approved senders and restrict multicast data transmissions or
   announcement only from them.  However, it is difficult to configure
   approved senders only who can send SAP messages, or non-approved
   senders who are disabled to send SAP messages.

   In addition, it is difficult to hide multicast session information
   announced by SAP from non-approved receivers if they are inside the
   scoped network.  SAP messages might be encrypted to prevent non
   authorized client from reading them.  However, it adds more
   complexity to SAP by combining with a key sharing mechanism.

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3.  Requirements

   According to the SAP analysis aforementioned, the requirements for IP
   multicast session announcement are defined as follows;

   o  Information consistency: Information consistency, which warrants
      that end users have a consistent view of session announcement, is
      of major importance.

   o  Low information update latency: IP multicast session would be
      fully dynamic.  The list of sessions should be updated rapidly
      after the creation, modification, or removal of the session

   o  Low bandwidth consumption: IP multicast session announcement
      should effectively consume the network bandwidth so that it does
      not affect other communications or services.

   o  Scalability: Session announcement can be used by a large number of
      end users spread throughout the Internet, and can manage a very
      large number of sessions.

   o  High availability: The scheme must be robust in front of host/link
      failures and packet losses.  This can be fulfilled either by
      transmitting messages periodically or by keeping track of failures
      and recovering them.

   o  Scope control: Scope control is required to preserve bandwidth
      resources and offer a certain level of confidentiality in IP
      multicast communication.

   o  No dependency on a routing architecture: The session announcement
      scheme must accommodate (or be independent of) any kind of
      multicast routing protocol or communication model.

   o  Sender and receiver control: Administrators must be able to allow
      to announce multicast sessions only from approved multicast
      senders and only to approved multicast data receivers in their
      network.  They must be able to filter out malicious users.

   o  Security consideration: In order to provide secure multicast
      communication, session announcement should have a function that
      enables to encrypt session information and distribute it to only
      the legitimate users.

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4.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to indicate requirement
        levels", RFC 2119, March 1997.

   [2]  Asaeda, H. and V. Roca, "Policy and Scope Management for
        Multicast Channel Announcement", IEICE Trans. on Information and
        Systems, Vol.E88-D, No.7, July 2005.

   [3]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement
        Protocol", RFC 2974, October 2000.

   [4]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
        Description Protocol", RFC 4566, July 2006.

   [5]  Mayer, D., "Administratively scoped IP multicast", RFC 2365,
        July 1998.

   [6]  Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
        RFC 4607, August 2006.

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Authors' Addresses

   Hitoshi Asaeda
   Keio University
   Graduate School of Media and Governance
   5322 Endo
   Fujisawa, Kanagawa  252-8520

   Email: asaeda@wide.ad.jp
   URI:   http://www.sfc.wide.ad.jp/~asaeda/

   Vincent Roca
   Planete Research Team
   655, Avenue de l'Europe
   Montbonnot - Saint Martin, Saint Ismier  38334

   Email: vincent.roca@inrialpes.fr
   URI:   http://planete.inrialpes.fr/~roca/

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