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MBONED Working Group                                           H. Asaeda
Internet-Draft                                           Keio University
Intended status: Informational                                   V. Roca
Expires: September 7, 2011                                         INRIA
                                                           March 6, 2011


           Limitations of Session Announcement Protocol (SAP)
                 draft-asaeda-mboned-sap-limitation-00

Abstract

   The Session Announcement Protocol (SAP) [2] has historically been
   used to announce information for all available IP multicast sessions
   to the prospective receivers in the experimental MBone.  Each
   receiver can then discover which sessions are available and which
   ones he may want to join.  Although SAP is easy to use, SAP is not
   scalable and controlling the SAP message transmission in a wide area
   network is not easy.  Therefore this document describes the
   limitations of SAP when used in the global Internet.  Furthermore,
   SAP has recently been used as a convenient method for conveying
   configuration information to a set of receivers that are already
   interested by a multicast session (e.g., to carry FEC Framework
   Configuration Information [7]).  This documents describes the
   limitations of SAP for this type of usage, since this latter is
   rather different from its original goals.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 7, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 4
     1.1.  SAP as a Component of a Session Discovery Mechanism . . . . 4
     1.2.  SAP as a Component of a Configuration Information
           Transport Mechanism . . . . . . . . . . . . . . . . . . . . 4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   3.  Potential Limitations with SAP  . . . . . . . . . . . . . . . . 5
     3.1.  Announcement Interval vs. Latency (SAP as a Session
           Discovery Mechanism)  . . . . . . . . . . . . . . . . . . . 5
     3.2.  Announcement Interval vs. Latency (SAP as a
           Configuration Information Transport Mechanism)  . . . . . . 6
     3.3.  Difficulties in Scope Definition (both SAP Uses)  . . . . . 6
     3.4.  ASM Dependency (both SAP Uses)  . . . . . . . . . . . . . . 7
     3.5.  Lack of Sender and Receiver Control during
           Announcements (both SAP Uses) . . . . . . . . . . . . . . . 8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 9
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9




























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1.  Introduction

1.1.  SAP as a Component of a Session Discovery Mechanism

   Session configuration information (e.g., IP multicast session or
   channel information) can be described with the Session Description
   Protocol (SDP) [3] syntax, or written in a metafile whose format has
   been defined elsewhere.  The Session Announcement Protocol (SAP) [2]
   has been used to announce information for all available multicast
   sessions to the prospective receivers in the experimental MBone.  In
   a SAP announcement procedure, the entire session information must be
   periodically transmitted and all active session descriptions 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 "soft-state" protocol).

   SAP enables to keep the session information active by periodically
   refreshing it, and it provides a robust and fault-tolerant system.
   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 significant
   overheads which further reduces its scalability.

   Another issue is closely related to a security or policy management.
   Indeed, using SAP and existing scoping techniques make it difficult
   to control precisely the amount of traffic distributed as well as the
   distribution area itself.

   This document describes the limitations of SAP when used in the
   global Internet, inspired by the work originally published by the
   authors in [6].

1.2.  SAP as a Component of a Configuration Information Transport
      Mechanism

   More recently SAP has been used as a convenient method for conveying
   configuration information to a set of receivers that are already
   interested in a multicast session (e.g., these receivers have
   obtained the content description through another mechanism and have
   decided to join the session).  For instance SAP can be used to convey
   the FEC Framework Configuration Information (FFCI) of a given
   FECFRAME Instance [7].  The FFCI is the information that the FEC
   Framework needs in order to apply FEC protection to the upper
   application flows [8].  Said differently, this FFCI defines the way
   the packets containing encoding symbols (e.g., result of a Reed-
   Solomon encoding) are generated from one or several upper application
   flows (e.g., an RTP stream containing video).  This use-case is
   significantly different from the traditional one since the receivers



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   have already expressed their interest in joining the FECFRAME
   Instance session and now need to collect additional information on
   how to exploit the associated flow(s).

   This document describes the limitations of SAP for this type of usage
   that is rather different from the original goals of SAP.


2.  Terminology

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


3.  Potential Limitations with SAP

3.1.  Announcement Interval vs. Latency (SAP as a Session Discovery
      Mechanism)

   SAP improves the robustness and data consistency in front of packet
   losses by periodically transmitting SAP messages.  However,
   transmitting a large number of SAP messages with active multicast
   session information in a flooding manner may cause major overheads.
   The solution defined in [2] 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



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   shorter latency for the sender-receiver communication.  However, it
   makes the total amount of SAP messages transmitted larger and may
   increase the probability of creating congestions.

3.2.  Announcement Interval vs. Latency (SAP as a Configuration
      Information Transport Mechanism)

   Using SAP as a configuration information transport mechanism raises
   the problem of choosing an appropriate announcement interval.  The
   goal of the algorithm introduced in [2] is to control SAP
   transmission overhead, in particular when the number of active
   sessions is high and generates a large number of announcements.  When
   SAP is used as a configuration information transport mechanism, the
   problem is totally different, since SAP is used within a given
   session and the goal is to ensure that all receivers, including late-
   comers, retrieve the configuration information (e.g., the FEC
   Framework Configuration Information) in a timely manner.  To achieve
   this goal it is desired to set up periodic transmissions.  For
   instance, [7] suggests a time interval in the range of 1 - 200
   seconds that defaults to 60 seconds (to be compared to the one hour
   minimum implicit timeout duration of SAP).  SAP SHOULD enable such a
   flexibility when defining the announcement interval strategy.

   A receiver SHOULD be able to determine the validity period of each
   SAP announcement, since SAP entries are cached by the reaceiver and
   are automatically discarded at timeout.  SAP specifies that the
   announcement interval can be predicted by the receiver and defines a
   minimum of one hour for an implicit timeout of the entries, with the
   goal to allow for transient network partitionings (as described in
   section 4 of [2]).  This approach contradicts the goal of precisely
   controlling the announcement interval strategy with a possibility to
   use intervals in the range of a few seconds.  Therefore, a solution
   could be for the SAP sender to communicate the announcement interval
   being used to the receivers.  The current SAP specification does not
   allow the time-interval to be signaled in the SAP header which
   requires to include this information within the payload itself (given
   in [7]), making the technique dependant on the configuration
   information being transported which is not a desired property.

3.3.  Difficulties in Scope Definition (both SAP Uses)

   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" and the end users who belong to this
   scope area are the only ones who 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



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   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
   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 [4] provides
   clear and simple semantics and scope boundaries are associated to
   multicast addresses.  With IPv4, packets addressed to the
   administratively scoped multicast address range 239/8 (i.e., from
   239.0.0.0 to 239.255.255.255) 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 that 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) [5] 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.

3.4.  ASM Dependency (both SAP Uses)

   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



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   sessions, all prospective receivers must join session 224.2.127.254
   (without specifying any source address).  This is another major
   limitation of SAP since some Internet Service Providers (ISPs) may
   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, 224.2.127.254, then all prospective
   receivers and multicast routers that listen to SAP messages will
   receive this high bandwidth flow which will impact their own
   performance and that of their network.

3.5.  Lack of Sender and Receiver Control during Announcements (both SAP
      Uses)

   Network administrators or service providers may want to define
   approved senders and restrict multicast data transmissions or
   announcement only from them.  However, in a spontaneous announcement
   protocol, it is impossible to allow to send announcement messages
   only from approved senders or make non-approved senders stop sending
   announcement messages.

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

   Conceptually, it is difficult to disallow non-approved data receivers
   to receive session information announced by an announcement protocol,
   if the announcement data is flooded to their network.  It is the
   basic concept that IP multicast requires scoping configuration to
   address this issue.  However, defining a fine-grained scope areas
   with using TTL or a multicast address range is a big challenge as
   described in Section 3.3.


4.  Security Considerations

   TBD


5.  IANA Considerations

   This document does not require any action from IANA.



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6.  Acknowledgments

   TBD


7.  References

7.1.  Normative References

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

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

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

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

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

7.2.  Informative References

   [6]  Asaeda, H. and V. Roca, "Policy and Scope Management for
        Multicast Channel Announcement", IEICE Trans. on Information and
        Systems, Vol.E88-D, No.7, pp.1638-1645, July 2005.

   [7]  Asati, R., "Methods to convey FEC Framework Configuration
        Information", draft-ietf-fecframe-config-signaling-04 (Work in
        Progress), January 2011.

   [8]  Watson, M., Begen, A., and V. Roca, "Forward Error Correction
        (FEC) Framework", draft-ietf-fecframe-framework-13 (Work in
        Progress), February 2011.














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Authors' Addresses

   Hitoshi Asaeda
   Keio University
   Graduate School of Media and Governance
   5322 Endo
   Fujisawa, Kanagawa  252-0882
   Japan

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


   Vincent Roca
   INRIA
   655, av. de l'Europe
   Inovalle; Montbonnot
   ST ISMIER cedex  38334
   France

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





























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