MMUSIC Working Group R. Kumar Internet Draft F. Andreasen Document: draft-rajeshkumar-mmusic-gpmd-02.txt Cisco Systems Category: Informational Expires: August 2003 February 2003 SDP attribute for Qualifying Media Formats with Generic Parameters Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. Abstract This document defines a new SDP attribute called general-purpose media descriptor (gpmd). The gpmd attribute allows the use of new informative parameters, gpmd parameters, to qualify existing media formats. These gpmd parameters are not part of the standard (e.g., MIME) definition of the media format and support for them with a given media format can therefore not be assumed. Their use is therefore limited to cases where they provide information that may be of use to the other party in a session but is not critical to the use of the particular media format. This document also defines a specific gpmd parameter, voice-band data, which can be used to describe a media format as carrying voice-band data. This enables the receiver to optimize its handling of the media received. Kumar & Andreasen Expires - August 2003 [Page 1] Internet-Draft SDP General Purpose Media Descriptor February 2003 Conventions used in this document 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]. Table of Contents 1. Introduction...................................................2 2. Definition of the "gpmd" Attribute.............................3 2.1 General Semantics..........................................4 2.2 Offer/Answer Support.......................................4 2.3 ABNF Grammar for "gpmd" Attribute..........................5 2.4 Defining gpmd Parameters...................................6 3. The Voice-Band Data gpmd Parameter.............................6 4. Contrasting fmtp with gpmd.....................................7 5. Security Considerations........................................8 6. IANA considerations............................................8 6.1 Registration of the SDP attribute "gpmd"...................8 6.2 Creation of New SDP Sub-Registry for "gpmd" Parameters.....9 6.3 IANA Registration of the "gpmd" Parameter "vbd"............9 7. Normative References...........................................9 8. Informative References........................................10 Acknowledgments..................................................10 Authors' Addresses...............................................10 1. Introduction When a media format is defined and registered as a MIME type, part of the definition includes a list of required and optional parameters that can be supplied for the particular media format. For example, the registration of the "audio/PCMU" MIME type in [5] defines "rate" as a required parameter and "channels", "ptime", and "maxptime" as optional parameters that qualify the operation of the PCMU encoding method. Some media parameters may be common among different encoding methods while others may be specific to a particular encoding method. In either case, the parameters in question must be defined as part of the definition and registration of the particular media format; new parameters cannot be used without providing a new definition and registration of the media format. However, some applications can benefit from qualifying existing well- defined media formats with new informative generic parameters that are not critical to the use of that media format, but nevertheless would be beneficial for the other side to know. For example, an endpoint might benefit from knowing that an RTP payload type will be treated, for the duration of a session, as containing a modulated data signal. Other such cases might arise in the future. Kumar & Andreasen Expires - August 2003 [Page 2] Internet-Draft SDP General Purpose Media Descriptor February 2003 When using the Session Description Protocol (SDP) [3] to describe media streams, media format specific parameters, that are not already defined in SDP, are signaled through the use of the "fmtp" attribute. The use of the "fmtp" attribute parameter however is limited to media format parameters that are defined as part of the media format registration; new informative generic parameters cannot be provided with the "fmtp" parameter. We therefore need to provide the following: 1. An SDP mechanism to bind, on a per-session basis, values of one or more informative generic parameters to a media format. 2. A mechanism for defining and registering such parameters. In this document, we define a new SDP attribute called general- purpose media descriptor (gpmd) that can provide one or more of these informative generic parameters for a media format. An endpoint chooses which generic parameters to advertise using this general- purpose media descriptor. 2. Definition of the "gpmd" Attribute The "gpmd" attribute is a media-level attribute; it MUST NOT be provided at the session-level. The attribute is used when an endpoint wants to provide one or more informative generic parameters for a given media format in a media stream, a.k.a. a general-purpose media descriptor (gpmd). The attribute is defined as: a=gpmd: where is represented as a semicolon- separated list of case-sensitive = pairs (a formal ABNF grammar for the "gpmd" attribute is provided in Section 2.3). For example, the generic parameter "vbd" (see Section 3) is a boolean which may be associated with any RTP payload type. Thus, m=audio 3456 RTP/AVP 0 15 a=gpmd:0 vbd=yes enables voice-band data treatment for the static RTP payload type of 0 (PCMU). Note that need not be an RTP payload type. It is permissible to use any value of that can appear in an SDP "m=" line, as long as the definition of each generic parameter allows Kumar & Andreasen Expires - August 2003 [Page 3] Internet-Draft SDP General Purpose Media Descriptor February 2003 it to be used to qualify that media format. Thus, a hypothetical generic parameter, "someParameter", may be enclosed in a "gpmd" attribute associated with the format "wb" (whiteboard) as follows: m=application 3456 udp wb a=gpmd:wb someParameter=someValue The "gpmd" attribute is charset-independent. This property is inherited by all parameters enclosed within the "gpmd" attribute. The gpmd framework described in the rest of this section does not define any specific gpmd parameters. Instead we define general requirements and procedures for gpmd parameters. In Section 3, we define a specific voice-band data gpmd parameter, and in Section 6, we provide the IANA registration procedures for gpmd parameters. 2.1 General Semantics A general-purpose media descriptor contains one or more gpmd parameters. A gpmd parameter applies to one media format in one media stream, just like an "fmtp" parameter. In the following, all statements regarding gpmd parameters are to be taken in the context of a given media format in a given media stream unless otherwise noted. A gpmd parameter MUST by definition be informative and support for it by any party in a session MUST be optional. Correct operation of a given media stream MUST NOT depend on one or more participants either supporting or not supporting a given gpmd parameter. A gpmd parameter can be thought of as providing an optimization hint to the other party. If the other party support it, improved operation may result, but if the other party does not support it, the operation MUST NOT be any worse, than if the gpmd parameter(s) had not been indicated in the first place. A gpmd parameter can by default be used with any media format, however the definition of particular gpmd parameter MAY limit its scope to a subset of media formats. If the parameter is used with an unsupported media format, the parameter SHOULD be ignored. Like other new SDP attributes, support for the "gpmd" attribute is optional. SDP parsers that do not understand it MUST ignore it. The same is true for each generic parameter enclosed within the "gpmd" attribute; SDP parsers MUST ignore unsupported and erroneous gpmd parameters. 2.2 Offer/Answer Support A gpmd parameter can be either "unilateral" or "bilateral" as explained below. Support for and use of the gpmd attribute and gpmd parameters is specified in terms of the offer/answer model [4]: Kumar & Andreasen Expires - August 2003 [Page 4] Internet-Draft SDP General Purpose Media Descriptor February 2003 A bilateral gpmd parameter can always be present in an offer, however it MUST only be present in an answer, if it was present in the corresponding offer. If an offer contains a bilateral gpmd parameter which the answerer does not want to support, the answer MUST NOT include this gpmd parameter. If the answerer does not want to support any of the gpmd parameters offered, the answer MUST NOT include the gpmd attribute. The operation associated with a bilateral gpmd parameter MUST only be performed if both the offer and the answer indicated support for this parameter, i.e. the two sides have agreed to use it. In all other cases, operation MUST be as if the gpmd parameter had not been included in the first place. The only exception to this rule is in the period between the offer being issued and the answer being received; during that time, the offerer MAY use the operation associated with the offered gpmd parameter for any media received for that offer. A unilateral gpmd parameter can always be present in either an offer or an answer. Unilateral parameters can be thought of as being advertised rather than negotiated. They convey information that may be of interest to the other party, but there is no need for the two sides to negotiate or agree on this information. Thus, if an offer contains a unilateral gpmd parameter, the answerer MAY choose to adjust it's local operation in accordance with that parameter. The offerer however can neither depend on nor assume that the answerer will do so, irrespective of whether the parameter is included in the answer or not. If the answer does include the parameter, it is completely independent of the parameter provided in the offer, i.e., the parameter is truly unilateral. The answer operation is similar to the offer operation. Note that unilateral gpmd parameters can be provided in an answer that did not contain a corresponding parameter in the offer, and that any such parameters are unidirectional, i.e., they apply only to the party that sends it and are not negotiated between the two parties. 2.3 ABNF Grammar for "gpmd" Attribute This section provides an Augmented BNF (ABNF) grammar [2] for the "gpmd" attribute. Here, generic parameters that may be enclosed within the "gpmd" attribute are referred to as "subparameters". The "subparameters" are case-sensitive. RFC 2327 [3] generically defines SDP attributes as follows: attribute-fields = *("a=" attribute CRLF) attribute = (att-field ":" att-value) | att-field For the "gpmd" attribute, a special case of this ABNF syntax may be stated as follows: Kumar & Andreasen Expires - August 2003 [Page 5] Internet-Draft SDP General Purpose Media Descriptor February 2003 gpmd-attribute-field = "a=" gpmd-attribute CRLF gpmd-attribute = "gpmd" ":" gpmd-format SP gpmd-value gpmd-format = token ; for audio media, this is ; typically an RTP payload type gpmd-value = gpmd-subparameter *(";" gpmd-subparameter) gpmd-subparameter = gpmd-subpar-name "=" gpmd-subpar-value gpmd-subpar-name = 1*(alpha-numeric) gpmd-subpar-value = 1*(gpmd-val-char) alpha-numeric = ALPHA / DIGIT ; ALPHA and DIGIT from RFC 2234 [2] gpmd-val-char = %x21-3A / %x3C-7E ; VCHAR [2], except semi-colon (";") 2.4 Defining gpmd Parameters When defining a gpmd parameter, the following MUST be specified: * A descriptive name and a general description of what the gpmd parameter conveys MUST be provided. In so doing, it MUST be ensured, that the gpmd parameter does not violate the semantic requirements specified in Section 2.1. In particular, the requirement that correct operation of a given media stream does not depend on either party supporting the parameter MUST be honored. * The actual gpmd parameter name used in the gpmd attribute and the possible values MUST be specified according to the grammar defined in Section 2.3. * The offer/answer model, i.e., unilateral or bilateral as specified in Section 2.2) to be used for this gpmd parameter SHOULD be specified. If it is not specified, it is assumed that the bilateral model is used. * The definition MAY include a description of the media formats the gpmd parameter is expected to be used with, e.g. all audio codecs, or one or more specific codecs. Use of the gpmd parameter with other media formats does not constitute an error, but it may not result in any meaningful optimization either. 3. The Voice-Band Data gpmd Parameter This section defines a new gpmd parameter called Voice-Band Data ("vbd"). The registration of this parameter is addressed in Section 6.3. The "vbd" gpmd parameter, indicates voice-band data. It is a boolean that may be assigned the value "yes" or "no" (without the quotes). Using this generic parameter, any RTP audio payload type may be declared to carry voice-band data. In practice, most applications Kumar & Andreasen Expires - August 2003 [Page 6] Internet-Draft SDP General Purpose Media Descriptor February 2003 are likely to use one of the following audio media formats for the carriage of voice band data signals: PCMU, PCMA, G726-40 and G726-32. The "vbd" parameter is a bilateral gpmd parameter, and hence support for it MUST be negotiated in order to use any optimizations associated with it in either direction. The exact optimizations to be used are implementation dependent, however one example is to increase the size of the jitter buffer when voice-band data is present. Note that in a given session, a single RTP format (e.g., PCMU) may be associated with multiple payload types. For a given RTP format, one of these may be a static payload type, while the others are dynamically assigned payload types. Regardless of whether a given payload type is static or dynamic, it may be declared to carry voice- band data. Without the parameter "vbd", the payload type, whether static or dynamic, is insufficient to determine whether a modem signal is being encoded or not. Instead, the contents of the incoming media stream would have to be analyzed. The following example shows a static and a dynamic payload type associated with the PCMU format. Only the dynamic payload type, 98, is earmarked for voice-band data treatment. m=audio 3456 RTP/AVP 0 15 98 a=rtpmap:98 PCMU/8000 a=gpmd:98 vbd=yes The following example reverses the association of payload type with voice-band data. Of the static and dynamic payload types associated with PCMU, only the static payload type, 0, is earmarked for voice- band data treatment. m=audio 3456 RTP/AVP 0 15 98 a=gpmd:0 vbd=yes a=rtpmap:98 PCMU/8000 4. Contrasting fmtp with gpmd The SDP "fmtp" parameter and the "gpmd" parameter defined in this document may seem very similar. Although we have explained the rationale behind the gpmd parameter in Section 1, we here provide a quick overview of the difference between the fmtp and the gpmd parameters for easy reference: Parameters provided within the "fmtp" attribute must be provided in the standard definition (e.g. MIME registration) of that media format. Some of these parameters are mandatory, while others are optional. It is only parameters provided in this definition that can be used with the "fmtp" parameter. The reason for this is, that the Kumar & Andreasen Expires - August 2003 [Page 7] Internet-Draft SDP General Purpose Media Descriptor February 2003 "fmtp" parameter is intended to provide codec specific parameters that the SDP parser and higher level application may not fully understand, however if the codec in question is supported, it is guaranteed that any such parameters will be supported as well. By contrast, the parameters that may be provided within a "gpmd" line are not part of the standard definition (e.g. MIME registration) of the media format. These parameters are maintained in a separate IANA registry (Section 6.2). The definition of a "gpmd" parameter may indicate the range of media formats which may be qualified by it (all media formats by default). All "gpmd" enclosed parameters are optional at the SDP parser level, though some applications may negotiate particular parameters. When a given codec is supported, gpmd parameters for that codec may or may not be supported. 5. Security Considerations The general purpose media descriptor defined in this document is informative in nature and furthermore not critical to the use of any particular media format. Presence or absence of a gpmd parameter may however lead to use of or lack of certain optimizations on either the sending or receiving side. An attacker altering the presence or absence of one or more gpmd parameters in a session description may therefore negatively influence the resulting media stream. It is possible to protect against such attacks by employing integrity protection mechanism on the protocol(s) used to exchange the session description or by use of integrity protection mechanism on the session description itself. 6. IANA considerations There are three IANA actions in this document: * Registration of "gpmd" as a new SDP attribute (Section 8.1). * Creation of a new SDP sub-registry for gpmd parameters (Section 8.2). * Registration of "vbd" as a generic parameter within the SDP sub- registry for "gpmd" (Section 8.3). 6.1 Registration of the SDP attribute "gpmd" This document defines a new SDP attribute, which the IANA is hereby requested to register as follows: Attribute name: gpmd Kumar & Andreasen Expires - August 2003 [Page 8] Internet-Draft SDP General Purpose Media Descriptor February 2003 Long form name: General-purpose media descriptor Attribute type: Media-level Subject to charset: No Purpose: Informative generic parameters for media formats Attribute values: As defined in Section 2 of this document. 6.2 Creation of New SDP Sub-Registry for "gpmd" Parameters The IANA is hereby requested to create a new SDP sub-registry for gpmd parameters. Each entry in this registry shall contain the following: * A descriptive name for the gpmd parameter. * An alphanumeric character string defining the token for the parameter name in a gpmd attribute (as specified in Section 2). * A reference to the document defining the gpmd parameter. In order for the IANA to register a gpmd parameter defined outside of this document, the parameter MUST be defined in a Standards Track RFC in accordance with the requirements defined in Section 2.4. 6.3 IANA Registration of the "gpmd" Parameter "vbd" The IANA is hereby requested to register a new SDP gpmd parameter as follows: * Descriptive Name: Voice-band data * Token: vbd 7. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Crocker, D. and Overell, P.(Editors), "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium and Demon Internet Ltd., November 1997. [3] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [4] Rosenberg, J., and H. Schulzrinne, "An Offer/Answer Model with the Session Description Protocol (SDP)", RFC 3264, June 2002. Kumar & Andreasen Expires - August 2003 [Page 9] Internet-Draft SDP General Purpose Media Descriptor February 2003 8. Informative References [5] Casner, S., and P. Hoschka, "MIME Type Registration of RTP Payload Formats", Work in progress. Acknowledgments Henning Schulzrinne provided the original inspiration for this draft by pointing out the inherent simplicity and self-synchronizing nature of changing payload types. Steven Casner, Henning Schulzrinne and Colin Perkins all helped refine the idea of this new, generic SDP attribute. Colin Perkins suggested the IANA registry mechanism needed with the "gpmd" attribute. Bill Foster pursued the logical antecedents of this draft. Magnus Westerlund reviewed and corrected an earlier version of the ABNF grammar. Authors' Addresses Rajesh Kumar Cisco Systems 170 West Tasman Dr San Jose, CA Email: rkumar@cisco.com Flemming Andreasen Cisco Systems 499 Thornall Street, 8th Floor Edison, NJ Email: fandreas@cisco.com Kumar & Andreasen Expires - August 2003 [Page 10]