SDP QoS Selection: Quality of Service (QoS) Mechanism Selection in the Session Description Protocol (SDP)
Two SDP attributes that let endpoints agree on which QoS mechanism, such as RSVP or NSIS, to use for a media stream.
Polk, J., Dhesikan, S., and G. Camarillo, "Quality of Service (QoS) Mechanism Selection in the Session Description Protocol (SDP)", RFC 5432, DOI 10.17487/RFC5432, March 2009, <https://www.rfc-editor.org/info/rfc5432>.
@misc{rfc5432,
series = {Request for Comments},
number = 5432,
howpublished = {RFC 5432},
publisher = {RFC Editor},
doi = {10.17487/RFC5432},
url = {https://www.rfc-editor.org/info/rfc5432},
author = {J. Polk and S. Dhesikan and G. Camarillo},
title = {{Quality of Service (QoS) Mechanism Selection in the Session Description Protocol (SDP)}},
pagetotal = 9,
year = 2009,
month = mar,
} RFC text
Contents
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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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
AI ELI5
On this page
§1The Problem and Its Limits
The offer/answer model (RFC 3264) for SDP (RFC 4566) gives endpoints no way to negotiate the Quality of Service (QoS) mechanism for a media stream. Even with QoS preconditions (RFC 3312), the choice of mechanism is left to the endpoints. Endpoints in closed environments typically agree out of band, but on the Internet more than one QoS service is available.
Examples of QoS mechanisms are RSVP (RFC 2205) and NSIS. The RFC is clear about its limits: agreeing on a mechanism does not mean the network supports it, and discovering what the network supports is out of scope. The exchanged information can still help a network operator choose a mechanism acceptable to both endpoints and debug misconfiguration.
There is more than one way to reserve a fast lane for a call. Before this, the two callers had no standard way to say which way they know how to use.
§3The Two SDP Attributes
The document defines qos-mech-send and qos-mech-recv, both usable at the session and media level. Each holds a list of qos-mech tokens: rsvp, nsis or an extension token.
A token in qos-mech-send names a mechanism that can reserve resources for traffic sent by the entity that generated the session description. A token in qos-mech-recv does the same for traffic it receives. The two attributes are not interdependent, so one can be used without the other.
The RFC's example media line is m=audio 50000 RTP/AVP 0 followed by a=qos-mech-send: rsvp nsis and a=qos-mech-recv: rsvp nsis.
There are two little labels. One says how I can reserve the lane for what I send, and the other says how I can reserve the lane for what I receive.
§4Offer/Answer Behavior
The offerer puts in each attribute the mechanisms it supports for that direction, in order of preference. The answerer mirrors the directions: tokens from the offer's qos-mech-send that it supports in the receive direction go into its qos-mech-recv, and tokens from the offer's qos-mech-recv that it supports for sending go into its qos-mech-send. It may order the tokens by its own preferences and the offer's, but the ordering algorithm is out of scope. If nothing is in common, it returns empty attributes.
Which direction needs QoS is negotiated by other means, such as preconditions, and so are mechanism parameters like the bandwidth to reserve.
After the exchange, both sides use the answer's lists. They SHOULD try the highest-priority mechanism in each direction first, then the next one, and so on. If all common mechanisms fail, an endpoint MAY try mechanisms the other side does not support, because a network entity out of its control, such as an RSVP proxy, may make them work.
If a later offer in an established session lacks the attribute for a direction, the endpoints SHOULD keep using the mechanism already in use.
One side shows its list of tricks in order of favorite. The other side crosses out the ones it cannot do and sends the list back. Then both try the top trick first, and the next one if that fails.
§5Example Exchange
The offerer lists both RSVP and NSIS for both directions and puts RSVP first, because it prefers it. The answerer supports NSIS in both directions but not RSVP, so its answer lists only nsis in qos-mech-send and qos-mech-recv. The endpoints therefore use NSIS.
I say I can do trick A or trick B and I like A more. You can only do B, so you answer with B, and we both use B.
- OFFERqos-mech-send: rsvp nsis; qos-mech-recv: rsvp nsis
- ANSWERqos-mech-send: nsis; qos-mech-recv: nsis
§6IANA Registrations
IANA registered the two attributes under the SDP Parameters registry, both at session and media level and not subject to charset. It also created a subregistry of QoS mechanism tokens with initial values rsvp and nsis, both referencing RFC 5432. New tokens follow the 'Specification Required' policy from RFC 5226.
A public list was started that holds the names of the allowed tricks. New names can be added only when someone writes down how the trick works.
§7Security Considerations
An attacker may add, modify or remove the qos-mech-send and qos-mech-recv attributes in a session description. The endpoints might then be unable to find a common QoS mechanism, and the application could behave badly.
The RFC therefore strongly RECOMMENDS integrity and authenticity protection of SDP carrying these attributes. For SDP carried in SIP (RFC 3261), S/MIME is the natural choice for end-to-end integrity protection. Other applications MAY use a different form of integrity protection.
If a sneaky person scribbles on the list while it travels, the two callers might think they have no trick in common. Sealing the message stops that.