RFC 5432
Quality of Service (QoS) Mechanism Selection in the Session Description Protocol (SDP)
Pages: 9
Proposed Standard
Proposed Standard
Network Working Group J. Polk Request for Comments: 5432 S. Dhesikan Category: Standards Track Cisco Systems G. Camarillo Ericsson March 2009 Quality of Service (QoS) Mechanism Selection in the Session Description Protocol (SDP) Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. 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. 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.
Abstract
The offer/answer model for the Session Description Protocol (SDP) assumes that endpoints somehow establish the Quality of Service (QoS) required for the media streams they establish. Endpoints in closed environments typically agree out-of-band (e.g., using configuration information) regarding which QoS mechanism to use. However, on the Internet, there is more than one QoS service 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 4.4. Subsequent Offer/Answer Exchanges ..........................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
1. Introduction
The offer/answer model [RFC3264] for SDP [RFC4566] does not provide any mechanism for endpoints to negotiate the QoS mechanism to be used for a particular media stream. Even when QoS preconditions [RFC3312] are used, the choice of the QoS mechanism is left unspecified and is 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) [RFC2205] and NSIS (Next Steps in Signaling) [QoS-NSLP]. This document defines a mechanism that allows endpoints to negotiate the QoS mechanism to be used for a particular media stream. However, the fact that endpoints agree on a particular QoS mechanism does not imply that that particular mechanism is supported by the network. Discovering which QoS mechanisms are supported at the network layer is out of the scope of this document. In any case, the information the endpoints exchange to negotiate QoS mechanisms, as defined in this document, can be useful for a network operator to resolve a subset of the QoS interoperability problem -- namely, to ensure that a mechanism commonly acceptable to the endpoints is chosen and make it possible to debug potential misconfiguration situations.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 [RFC2119].3. SDP Attributes Definition
This document defines the 'qos-mech-send' and 'qos-mech-recv' session and media-level SDP [RFC4566] attributes. The following is their Augmented Backus-Naur Form (ABNF) [RFC5234] syntax, which is based on the SDP [RFC4566] grammar: attribute =/ qos-mech-send-attr attribute =/ qos-mech-recv-attr qos-mech-send-attr = "qos-mech-send" ":" [[SP] qos-mech *(SP qos-mech)] qos-mech-recv-attr = "qos-mech-recv" ":" [[SP] qos-mech *(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
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 'qos-mech-send' and 'qos-mech-recv' attributes are not
interdependent; one can be used without the other.
The following is an example of an 'm' line with 'qos-mech-send' and
'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
Through the use of the 'qos-mech-send' and 'qos-mech-recv'
attributes, an offer/answer exchange 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 [RFC3312]. 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 (in order of preference) that are
supported in the send direction. Similarly, offerers include a
'qos-mech-recv' attribute with the tokens corresponding to the QoS
mechanisms (in order of preference) that are supported in the receive
direction.
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 that 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 that 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 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 in 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 unsuccessfully tries 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.4.4. Subsequent Offer/Answer Exchanges
If, during an established session for which the QoS mechanism to be used for a given direction was agreed upon using the mechanism defined in this specification, an endpoint receives a subsequent offer that does not contain the QoS selection attribute corresponding to that direction (i.e., the 'qos-mech-send' or 'qos-mech-recv' attribute is missing), the endpoints SHOULD continue using the same QoS mechanism used up to that moment.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 omitted 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
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
IANA has registered 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
IANA has registered 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
Allowed attribute values: IANA Registered Tokens
6.3. Registry for QoS Mechanism Tokens
The IANA has created 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 as follows:
QoS Mechanism Reference
---------------------------- ---------
rsvp RFC 5432
nsis RFC 5432
As per the terminology in [RFC5226], 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 and
authenticity protection be applied to SDP session descriptions
carrying these attributes. For session descriptions carried in SIP
[RFC3261], S/MIME [RFC3851] is the natural choice to provide such
end-to-end integrity protection, as described in [RFC3261]. Other
applications MAY use a different form of integrity protection.
8. Acknowledgements
Dave Oran helped form this effort. Flemming Andreasen and Magnus
Westerlund provided useful comments on this specification.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1 Message Specification", RFC 3851, July 2004. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.9.2. Informative References
[QoS-NSLP] Manner, J., Karagiannis, G., and A. McDonald, "NSLP for Quality-of-Service Signaling", Work in Progress, February 2008. [RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S. Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1 Functional Specification", RFC 2205, September 1997. [RFC3261] 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. [RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg, "Integration of Resource Management and Session Initiation Protocol (SIP)", RFC 3312, October 2002.
Authors' Addresses
James 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