Quality of Service (QoS) Mechanism Selection in the Session Description Protocol (SDP)
RFC 5432

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.

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RFC 5432             QoS Mechanism Selection in SDP           March 2009

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

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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

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   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.

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   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.

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      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

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   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.

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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.

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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

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