MBONED Working Group H. Asaeda
Internet-Draft Keio University
Intended status: Informational V. Roca
Expires: April 29, 2010 INRIA
October 26, 2009
Requirements for IP Multicast Session Announcement
draft-ietf-mboned-session-announcement-req-02
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. 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.
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
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 29, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
Asaeda & Roca Expires April 29, 2010 [Page 1]
Internet-Draft Req. IP Multicast Session Announcement October 2009
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Asaeda & Roca Expires April 29, 2010 [Page 2]
Internet-Draft Req. IP Multicast Session Announcement October 2009
Abstract
The Session Announcement Protocol (SAP) [2] was used to announce
information for all available IP multicast sessions to the
prospective receiver in an experimental network. It is easy to use,
but not scalable and difficult to control the SAP message
transmission in a wide area network. This document describes the
issues and the requirements for multicast session announcement in the
global Internet.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Potential Problems in SAP . . . . . . . . . . . . . . . . . . 6
3.1. Announcement Interval vs. Latency . . . . . . . . . . . . 6
3.2. Difficulties in Scope Definition . . . . . . . . . . . . . 6
3.3. ASM Dependency . . . . . . . . . . . . . . . . . . . . . . 7
4. Lack of Sender and Receiver Control in Announcement . . . . . 9
5. Potential Problems in Central Server Model . . . . . . . . . . 10
6. Potential Problems in Discovery Model . . . . . . . . . . . . 11
7. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Asaeda & Roca Expires April 29, 2010 [Page 3]
Internet-Draft Req. IP Multicast Session Announcement October 2009
1. Introduction
IP multicast session or channel information is described with the
Session Description Protocol (SDP) [3] syntax or written in a
metafile.
The Session Announcement Protocol (SAP) [2] was used 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 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
scalability.
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 and other systems have 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 [6].
Asaeda & Roca Expires April 29, 2010 [Page 4]
Internet-Draft Req. IP Multicast Session Announcement October 2009
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].
Asaeda & Roca Expires April 29, 2010 [Page 5]
Internet-Draft Req. IP Multicast Session Announcement October 2009
3. Potential Problems in SAP
3.1. 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 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
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. 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
Asaeda & Roca Expires April 29, 2010 [Page 6]
Internet-Draft Req. IP Multicast Session Announcement October 2009
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 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
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 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.
3.3. ASM 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 224.2.127.254
(without specifying any source address). This is another major
limitation of SAP since some Internet Service Providers (ISPs) may
Asaeda & Roca Expires April 29, 2010 [Page 7]
Internet-Draft Req. IP Multicast Session Announcement October 2009
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 including multicast routers listening SAP messages take in
the stream and their networks may be corrupted or destroyed.
Asaeda & Roca Expires April 29, 2010 [Page 8]
Internet-Draft Req. IP Multicast Session Announcement October 2009
4. Lack of Sender and Receiver Control in Announcement
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 3.2.
Asaeda & Roca Expires April 29, 2010 [Page 9]
Internet-Draft Req. IP Multicast Session Announcement October 2009
5. Potential Problems in Central Server Model
Emails, RSS (Rich Site Summary or Really Simple Syndication), and the
Web are the alternative ways of conveying session descriptions.
These applications are of wide use and can be used to carry many
kinds of information. However, to provide a multicast announcement
function, these approaches would have to rely on a central server or
a central management system. This server-based approach reduces
flexibility of fine-grained user and session management.
Session announcement should be decided by data senders or
administrators policy, such as scoping policy [4], or content-level
or user-level access control, to define "who can access which
contents". Defining and applying such site-local policy or user
management would be very difficult or impossible on a single server
in the global Internet. This condition contradicts the requirements
experienced in the traditional MBone and expected in current or
future use.
In addition, emails and the RSS feed are implemented with a
"subscription model". The subscription model requires end users to
know the address of service providers and have subscribed to the
services for getting session information prior to receiving the
contents information. This condition is not reasonable for session
announcement, because end users do not always know potential data
senders.
Finally, server-based systems may require a large amount of
operational costs or cause scalability problems for the fine-grained
user and session management and session announcement, especially when
the systems need to support a large number of users and contents
information.
Asaeda & Roca Expires April 29, 2010 [Page 10]
Internet-Draft Req. IP Multicast Session Announcement October 2009
6. Potential Problems in Discovery Model
Session information discovery is another possible approach to
retrieve session information. Currently, there are information
discovery systems largely deployed in the Internet. However, an
information discovery system usually adopts crawling method to
discover information. If an information discovery system is used for
session information discovery, it not only causes a number of traffic
but also takes time for gathering all available session information
in the entire Internet or updating the collected session information.
This is a drawback for searching the available IP multicast session
information, because many of IP multicast sessions are possibly
launched and terminated highly dynamically.
Another issue resided in an information discovery system is that it
is difficult to enable a scoping function on it, as each site-local
operator or administrator does not control the service, especially
when the system is implemented with the server-based approach as
described in Section 5.
Asaeda & Roca Expires April 29, 2010 [Page 11]
Internet-Draft Req. IP Multicast Session Announcement October 2009
7. Requirements
According to the analyses 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
information.
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 Sender control: Administrators must be able to allow to announce
multicast sessions only from approved multicast senders.
o User access control: Administrators or data senders must be able
to configure approved multicast data receivers. They must be able
to filter out malicious users.
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 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.
Asaeda & Roca Expires April 29, 2010 [Page 12]
Internet-Draft Req. IP Multicast Session Announcement October 2009
8. References
8.1. 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.
8.2. 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.
Asaeda & Roca Expires April 29, 2010 [Page 13]
Internet-Draft Req. IP Multicast Session Announcement October 2009
Authors' Addresses
Hitoshi Asaeda
Keio University
Graduate School of Media and Governance
5322 Endo
Fujisawa, Kanagawa 252-8520
Japan
Email: asaeda@wide.ad.jp
URI: http://www.sfc.wide.ad.jp/~asaeda/
Vincent Roca
INRIA
Planete Research Team
655, Avenue de l'Europe
Montbonnot - Saint Martin, Saint Ismier 38334
France
Email: vincent.roca@inrialpes.fr
URI: http://planete.inrialpes.fr/~roca/
Asaeda & Roca Expires April 29, 2010 [Page 14]
