MMUSIC James Polk
Internet-Draft Subha Dhesikan
Expires: April 9, 2007 Cisco Systems
Gonzalo Camarillo
Ericsson
October 6, 2006
Quality of Service (QoS) Mechanism Selection in the Session Description
Protocol (SDP)
draft-polk-mmusic-qos-mechanism-identification-02.txt
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Copyright (C) The Internet Society (2006).
Abstract
The offer/answer model for SDP assumes that endpoints establish,
somehow, the QoS required for the media streams they establish.
Endpoints in closed environments typically agree out of band (e.g.,
using configuration information) which QoS mechanism to use.
However, on the Internet, there is more than one QoS service
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available. Consequently, there is a need for a mechanism to
negotiate which QoS mechanism to use for a particular media stream.
This document defines such a mechanism.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. SDP Attributes Definition . . . . . . . . . . . . . . . . . . 3
4. Offer/answer Behavior . . . . . . . . . . . . . . . . . . . . 4
4.1. Offerer Behavior . . . . . . . . . . . . . . . . . . . . . 4
4.2. Answerer Behavior . . . . . . . . . . . . . . . . . . . . 4
4.3. Resource Reservation . . . . . . . . . . . . . . . . . . . 5
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6.1. Registration of the SDP 'qos-mech-send' Attribute . . . . 6
6.2. Registration of the SDP 'qos-mech-recv' Attribute . . . . 6
6.3. Registry for QoS Mechanism Tokens . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
The offer/answer model [1] for SDP [2] does not provide any mechanism
for endpoints to negotiate the QoS mechanism to be used for a
particular media stream. Even when QoS preconditions [10] are used,
the choice of the QoS mechanism is left unspecified, up to the
endpoints.
Endpoints that support more than one QoS mechanism need a way to
negotiate which one to use for a particular media stream. Examples
of QoS mechanisms are RSVP (Resource Reservation Protocol) [3] and
NSIS (Next Steps in Signaling) [4].
This document defines a mechanism that allows endpoints to negotiate
the QoS mechanism to be used for a particular media stream.
Section 3 defines the 'qos-mech-send' and 'qos-mech-recv' SDP
attributes. Section 4 specifies the use of these new SDP attributes
with the offer/answer model. Section 5 provides an example of an
offer/answer exchanges that uses these attributes.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [5] and indicate requirement levels for
compliant implementations.
3. SDP Attributes Definition
This document defines the 'qos-mech-send' and 'qos-mech-recv' session
and media-level SDP [2] attributes. The following is their ABNF
syntax [6], which is based on the SDP [2] grammar:
attribute = qos-mech-send-attr
attribute = qos-mech-recv-attr
qos-mech-send-attr = "qos-mech-send" ":" *(SP qos-mech)
qos-mech-recv-attr = "qos-mech-recv" ":" *(SP qos-mech)
qos-mech = rsvp / nsis / extension-mech
extension-mech = token
The 'qos-mech' token identifies a QoS mechanism that is supported by
the entity generating the session description. A token that appears
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in a 'qos-mech-send' attribute identifies a QoS mechanism that can be
used to reserve resources for traffic sent by the entity generating
the session description. A token that appears in a 'qos-mech-recv'
attribute identifies a QoS mechanism that can be used to reserve
resources for traffic received by the entity generating the session
description.
The following is an example of an 'm' line with a 'qos-mech-send' and
a 'qos-mech-recv' attributes:
m=audio 50000 RTP/AVP 0
a=qos-mech-send:rsvp nsis
a=qos-mech-recv:rsvp nsis
4. Offer/answer Behavior
An offer/answer exchange using the 'qos-mech-send' and 'qos-mech-
recv' attributes allows endpoints to come up with a list of common
QoS mechanisms sorted by preference. However, note that endpoints
negotiate in which direction QoS is needed using other mechanisms,
such as preconditions [10]. Endpoints may also use other mechanisms
to negotiate, if needed, the parameters to use with a given QoS
mechanism (e.g., bandwidth to be reserved).
4.1. Offerer Behavior
Offerers include a 'qos-mech-send' attribute with the tokens
corresponding to the QoS mechanisms supported in the send direction
in order of preference. Similarly, offerers include a 'qos-mech-
recv' attribute with the tokens corresponding to the QoS mechanisms
supported in the receive direction in order of preference.
4.2. Answerer Behavior
On receiving an offer with a set of tokens in a 'qos-mech-send'
attribute, the answerer takes those tokens corresponding to QoS
mechanisms it supports in the receive direction and includes them, in
order of preference, in a 'qos-mech-recv' attribute in the answer.
On receiving an offer with a set of tokens in a 'qos-mech-recv'
attribute, the answerer takes those tokens corresponding to QoS
mechanisms it supports in the send direction and includes them, in
order of preference, in a 'qos-mech-send' attribute in the answer.
When ordering the tokens in a 'qos-mech-send' or a 'qos-mech-recv'
attribute by preference, the answerer may take into account its own
preferences and those expressed in the offer. However, the exact
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algorithm to be used to order such token lists is outside the scope
of this specification.
Note that if the answerer does not have any QoS mechanism in common
with the offerer, it will return empty 'qos-mech-send' and 'qos-mech-
recv' attributes.
4.3. Resource Reservation
Once the offer/answer exchange completes, both offerer and answerer
use the token lists in the 'qos-mech-send' and 'qos-mech-recv'
attributes of the answer to perform resource reservations. Offerers
and answerers SHOULD attempt to use the QoS mechanism with highest
priority in each direction first. If an endpoint (the offerer or the
answerer) does not succeed using the mechanism with highest priority
in a given direction, it SHOULD attempt to use the next QoS mechanism
in order of priority in that direction, and so on.
If an endpoint tries unsuccessfully all the common QoS mechanisms for
a given direction, the endpoint MAY attempt to use additional QoS
mechanisms not supported by the remote endpoint. This is because
there may be network entities out of the endpoint's control (e.g., an
RSVP proxy) that make those mechanisms work.
5. Example
The following is an offer/answer exchange between two endpoints using
the 'qos-mech-send' and 'qos-mech-recv' attributes. Parts of the
session descriptions are ommitted for clarity purposes.
The offerer generates the following session description listing both
RSVP and NSIS for both directions. The offerer would prefer to use
RSVP and, thus, includes it before NSIS.
m=audio 50000 RTP/AVP 0
a=qos-mech-send: rsvp nsis
a=qos-mech-recv: rsvp nsis
The answerer supports NSIS in both directions, but not RSVP.
Consequently, it returns the following session description:
m=audio 55000 RTP/AVP 0
a=qos-mech-send: nsis
a=qos-mech-recv: nsis
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6. IANA Considerations
This specification registers two new SDP attributes and creates a new
registry for QoS mechanisms.
6.1. Registration of the SDP 'qos-mech-send' Attribute
This section instructs the IANA to register the following SDP att-
field under the Session Description Protocol (SDP) Parameters
registry:
Contact name: Gonzalo.Camarillo@ericsson.com
Attribute name: qos-mech-send
Long-form attribute name: QoS Mechanism for the Send Direction
Type of attribute Session and Media levels
Subject to charset: No
Purpose of attribute: To list QoS mechanisms supported in the send
direction.
Allowed attribute values: IANA Registered Tokens
6.2. Registration of the SDP 'qos-mech-recv' Attribute
This section instructs the IANA to register the following SDP att-
field under the Session Description Protocol (SDP) Parameters
registry:
Contact name: Gonzalo.Camarillo@ericsson.com
Attribute name: qos-mech-recv
Long-form attribute name: QoS Mechanism for the Receive Direction
Type of attribute Session and Media levels
Subject to charset: No
Purpose of attribute: To list QoS mechanisms supported in the
receive direction.
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Allowed attribute values: IANA Registered Tokens
6.3. Registry for QoS Mechanism Tokens
The IANA is requested to create a subregistry for QoS mechanism token
values to be used in the 'qos-mech-send' and 'qos-mech-recv'
attributes under the Session Description Protocol (SDP) Parameters
registry. The initial values for the subregistry are presented in
the following, and IANA is requested to add them into its database:
QoS Mechanism Reference
---------------------------- ---------
rsvp RFC 2205
nsis RFC 4080
As per the terminology in RFC 2434 [7], the registration policy for
new QoS mechanism token values shall be 'Specification Required'.
7. Security Considerations
An attacker may attempt to add, modify, or remove 'qos-mech-send' and
'qos-mech-recv' attributes from a session description. This could
result in an application behaving in a non-desirable way. For
example, the endpoints under attack may not be able to find a common
QoS mechanism to use.
Consequently, it is strongly RECOMMENDED that integrity protection be
applied to SDP session descriptions carrying these attributes. For
session descriptions carried in SIP [8], S/MIME [9] is the natural
choice to provide such end-to-end integrity protection, as described
in [8]. Other applications MAY use a different form of integrity
protection.
8. Acknowledgements
Dave Oran helped form this effort. Flemming Andreasen provided
useful comments on this specification.
9. References
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9.1. Normative References
[1] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[2] Handley, M., "SDP: Session Description Protocol",
draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.
[3] Braden, B., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
"Resource ReSerVation Protocol (RSVP) -- Version 1 Functional
Specification", RFC 2205, September 1997.
[4] Hancock, R., Karagiannis, G., Loughney, J., and S. Van den
Bosch, "Next Steps in Signaling (NSIS): Framework", RFC 4080,
June 2005.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[7] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[8] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[9] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851,
July 2004.
9.2. Informative References
[10] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)",
RFC 3312, October 2002.
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Authors' Addresses
James M. Polk
Cisco Systems
3913 Treemont Circle
Colleyville, Texas 76034
USA
Phone: +1-817-271-3552
Email: jmpolk@cisco.com
Subha Dhesikan
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: sdhesika@cisco.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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