Network Working Group G. Somogyi Internet-Draft Nokia Siemens Networks Updates: 4867 (if approved) November 21, 2008 Intended status: Standards Track Expires: May 25, 2009 draft-somogyi-sdp-amr-bicc-like-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. This Internet-Draft will expire on May 25, 2009. Abstract This document describes and update to File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs. The new format allows better interworking with widely deployed mobile telecommunication switching systems. This document updates RFC 4867 [RFC4867]. Somogyi Expires May 25, 2009 [Page 1] Internet-Draft SDP format for AMR and AMR-WB November 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3 2. Shortcomings of the original format . . . . . . . . . . . . . . 3 2.1. Information loss . . . . . . . . . . . . . . . . . . . . . 3 2.2. Significant signaling overhead . . . . . . . . . . . . . . 4 3. SDP format extension . . . . . . . . . . . . . . . . . . . . . 5 3.1. New parameters . . . . . . . . . . . . . . . . . . . . . . 5 3.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Backward compatibility . . . . . . . . . . . . . . . . . . . . 6 4.1. Full backward compatibility . . . . . . . . . . . . . . . . 6 4.2. Partial backward compatibility . . . . . . . . . . . . . . 7 4.3. Deciding backward compatibility mode . . . . . . . . . . . 7 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 8.2. Informative References . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 Intellectual Property and Copyright Statements . . . . . . . . . . 9 Somogyi Expires May 25, 2009 [Page 2] Internet-Draft SDP format for AMR and AMR-WB November 2008 1. Introduction The SDP [RFC4566] format description of AMR and AMR-WB codecs are described in RFC 4867 [RFC4867]. That format causes trouble for SIP [RFC3261] based mobile switching systems, as the described SDP format did not contain all the necessary information for proper codec negotiation. And that format caused significant signaling overhead. The aim of this document to address these issues, while remaining backward compatible. This document introduces new parameters, allowing describing codecs of mobile world in full detail. The new parameters make the SDP even larger. But it makes several payload types of the same codec redundant, thus those could be omitted. Omitting those payload types limits backward compatibility, and therefore optional. The new syntax elements were designed to fully support 3GPP Transcoder Free Operation (TrFO; 3GPP TS 23.153 [3GPP23153]) and Tandem Free Operation (TFO; 3GPP TS 28.062 [3GPP28062]). Compliance with 3GPP IP Multimedia Subsystem (IMS; TS 26.114[3GPP26114]) was taken care of. Although 3GPP Release 8 standards were used as reference, the descriptions apply to earlier and possibly later releases, as well. Besides mobile switching systems this document was created with the intention to be able to be used within any kind of SIP systems. 1.1. Requirements Language 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 inRFC 2119 [RFC2119]. 2. Shortcomings of the original format 2.1. Information loss The old syntax did not provide a way to represent all the AMR codec information defined for mobile switches at 3GPP TS 23.153 [3GPP23153]. The information loss could result extra transcoding and speech quality degradation. Using the syntax defined in RFC 4867 only the following 3GPP single codec format parameter is presented in SDP in full detail: Somogyi Expires May 25, 2009 [Page 3] Internet-Draft SDP format for AMR and AMR-WB November 2008 ACS: Active Codec mode Set is presented by "mode-set" parameter. Using the syntax defined in RFC 4867 the following 3GPP single codec format parameters are NOT presented in full detail: SCS: Supported Codec mode Set is the list of all supported modes. It is a superset of ACS. Using codec negotiation methods ACS can be modified to use any of the supported modes. As the syntax defined in RFC 4867 does not have parameter representing SCS, several distinct payload types present the most preferred mode sets, in accordance with 3GPP TS 29.163 [3GPP29163]. Therefore only a fraction of the mode combinations are presented, compared to what is intended. MACS: Maximal number of codec modes in the ACS shows the maximum number of codec modes that may be selected for the ACS at a time during speech codec negotiation. OM: Optimization Mode flag shows whether optimization (modification) of Active Codec mode Set is supported or not. If optimization is not supported, SCS and MACS have no meaning for the relevant codec. Codec type: There are numerous AMR codec types, described at 3GPP TS 26.103 [3GPP26103]. Those form two groups, narrow-band (NB) and wide-band (WB) AMR codecs. In SDP representation there is one NB and one WB codec type. As codecs within one group have different properties, those properties are described by additional codec parameters in SDP. According to 3GPP TS 29.163 [3GPP29163] when only one mode is defined in "mode-set" parameter, "mode-change-period" and "mode-change-neighbor" parameters should not be present. That causes trouble, as in single mode configuration "UMTS AMR" and "FR AMR" codecs become indistinguishable, even though they are not compatible, according to 3GPP TS 23.153 [3GPP23153]. 2.2. Significant signaling overhead As described beforehand, RFC 4867 [RFC4867] does not provide a mechanism to signal the SCS, MACS or OM parameters in SDP. Therefore codecs with OM allowed should have been translated into a list of SDP Somogyi Expires May 25, 2009 [Page 4] Internet-Draft SDP format for AMR and AMR-WB November 2008 payload formats, where each includes a "mode-set" parameter with a unique value derived from the ACS, SCS and MACS. That resulted in quite huge SDPs. Regarding signaling that significantly increased the traffic, required significantly mode processing resources to parse and significant extra memory to store the information elements of SDP. 3. SDP format extension 3.1. New parameters The following new codec format parameters are introduced: extra-mode-set: Lists supported modes that are not listed in mode-set (ACS). Value syntax is the same as for parameter "mode-set", defined at RFC 4867 [RFC4867]. If optimization of ACS is supported and SCS differs from ACS, then the parameter MUST be present, otherwise it SHOULD NOT be present. macs: Defines MACS. Valid values between 1 and 8. (Note, that 3GPP TS 26.103 [3GPP26103] defines range between 0 and 7, value zero meaning eight. Here "8" represents value eight.) If optimization of ACS is supported, then the parameter MUST be present, otherwise it MUST NOT be present. Consequently, the presence of "macs" parameter indicates the status of OM. codec-type: Codec type. Valid values are capitalized names of AMR codecs defined in 3GPP TS 26.103 [3GPP26103], spaces replaced by underscores. Namely FR_AMR, HR_AMR, UMTS_AMR, UMTS_AMR_2, OHR_AMR, FR_AMR-WB, UMTS_AMR-WB, OFR_AMR-WB and OHR_AMR-WB. This parameter MAY be omitted if the codec type is obvious from mode change parameters defined at RFC 4867 [RFC4867]. In such case omitting codec type is RECOMMENDED. Codec type MUST NOT conflict with mode change parameters. 3.2. Example The example shows a possible SDP representation of FR AMR codec with ACS=4,5,6,7; SCS=2,3,4,5,6,7; OM=supported; MACS=4. Note: There is an extra line-break in the SDP example at extra-mode- set attribute. That was added so that to match traditional RFC Somogyi Expires May 25, 2009 [Page 5] Internet-Draft SDP format for AMR and AMR-WB November 2008 formatting. Real SDP does not contain such line break. v=0 o=- 0 1 IN IP4 1.2.3.4 s=- c=IN IP4 5.6.7.8 t=0 0 m=audio 6000 RTP/AVP 96 97 98 99 100 b=AS:16 a=rtpmap:96 AMR/8000 a=fmtp:96 mode-set=4,5,6,7; mode-change-period=2; extra-mode-set=2,3; macs=4 a=rtpmap:97 AMR/8000 a=fmtp:97 mode-set=2,4,5,6; mode-change-period=2 a=rtpmap:98 AMR/8000 a=fmtp:98 mode-set=2,4,6; mode-change-period=2 a=rtpmap:99 AMR/8000 a=fmtp:99 mode-set=3,6; mode-change-period=2 a=rtpmap:100 AMR/8000 a=fmtp:100 mode-set=3; codec-type=FR_AMR Explanation on RTP payload type descriptions: 96: All codec data present. Codec type is obvious, so not explicitly stated. 97-99: These lines are not necessary to be present, if full backward compatibility is not needed. Codec type is obvious, so not explicitly stated. These lines do not contain any new parameter defined in this document, as the extra information is available at the definition of payload 96. 100: This line is not necessary to be present, if full backward compatibility is not needed. As codec type is not obvious, it is explicitly stated here. 4. Backward compatibility 4.1. Full backward compatibility Full backward compatibility with RFC 4867 [RFC4867] can be achieved by enumerating possible mode sets, as before. Extra parameters must not cause any problem for older implementations. Somogyi Expires May 25, 2009 [Page 6] Internet-Draft SDP format for AMR and AMR-WB November 2008 4.2. Partial backward compatibility The aim of providing only partial compatibility is to reduce signaling bandwidth, CPU and RAM usage. Partial compatibility is achieved by not presenting redundant information (payload type description 97-100 in the example). Older implementations interpret the received payload type description as usual, but they are provided with less rich set of choices. 4.3. Deciding backward compatibility mode It is up to the implementation which approach to take. For example the compatibility mode can be fixed, configurable, or even route specific decisions could be made. 5. Acknowledgements Andras Stefan (Nokia Siemens Networks) provided great support in writing this document by explaining his viewes on issues of AMR codecs. 6. IANA Considerations This memo includes no request to IANA. Two media types (audio/AMR and audio/AMR-WB) have already been registered. 7. Security Considerations There are no specific security issues regarding AMR codec syntax extension. 8. References 8.1. Normative References [3GPP23153] 3rd Generation Partnership Project, 3GPP., "3GPP TS 23.153 Out of band transcoder control; Stage 2, version 8.0.0", 2007-12. [3GPP26103] 3rd Generation Partnership Project, 3GPP., "3GPP TS 26.103 Speech codec list for GSM and UMTS, version 8.0.0", 2008-09. Somogyi Expires May 25, 2009 [Page 7] Internet-Draft SDP format for AMR and AMR-WB November 2008 [3GPP29163] 3rd Generation Partnership Project, 3GPP., "3GPP TS 29.163 Interworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networks, version 8.1.0", 2007-12. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie, "RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs", RFC 4867, April 2007. 8.2. Informative References [3GPP26114] 3rd Generation Partnership Project, 3GPP., "3GPP TS 26.114 IP Multimedia Subsystem (IMS); Multimedia Telephony; Media handling and interaction, version 8.0.0", 2008-09. [3GPP28062] 3rd Generation Partnership Project, 3GPP., "3GPP TS 28.062 Inband Tandem Free Operation (TFO) of speech codecs; Service description; Stage 3, version 7.0.0", 2007-06. [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. [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006. Author's Address Gabor Somogyi Nokia Siemens Networks Koztelek u. 6. Budapest 1092 Hungary Email: gabor.somogyi@nsn.com Somogyi Expires May 25, 2009 [Page 8] Internet-Draft SDP format for AMR and AMR-WB November 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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