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
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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.
Asaeda & Roca Expires September 7, 2011 [Page 1]
Internet-Draft Limitations of SAP March 2011
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Asaeda & Roca Expires September 7, 2011 [Page 2]
Internet-Draft Limitations of SAP March 2011
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
Asaeda & Roca Expires September 7, 2011 [Page 3]
Internet-Draft Limitations of SAP March 2011
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
Asaeda & Roca Expires September 7, 2011 [Page 4]
Internet-Draft Limitations of SAP March 2011
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
Asaeda & Roca Expires September 7, 2011 [Page 5]
Internet-Draft Limitations of SAP March 2011
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
Asaeda & Roca Expires September 7, 2011 [Page 6]
Internet-Draft Limitations of SAP March 2011
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
Asaeda & Roca Expires September 7, 2011 [Page 7]
Internet-Draft Limitations of SAP March 2011
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.
Asaeda & Roca Expires September 7, 2011 [Page 8]
Internet-Draft Limitations of SAP March 2011
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.
Asaeda & Roca Expires September 7, 2011 [Page 9]
Internet-Draft Limitations of SAP March 2011
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/
Asaeda & Roca Expires September 7, 2011 [Page 10]