Multiparty Multimedia Session T. Taylor Control (mmusic) Nortel Internet-Draft January 4, 2007 Updates: 4733 (if approved) Intended status: Standards Track Expires: July 8, 2007 Signalling the Ability To Understand Packing of Multiple Telephony Events Into One RTP Packet draft-taylor-mmusic-multev-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 July 8, 2007. Copyright Notice Copyright (C) The Internet Society (2007). Taylor Expires July 8, 2007 [Page 1] Internet-Draft Signalling Event Packing Capability January 2007 Abstract Section 2.5.1.5 of RFC 4733 specifies how an implementation of the telephony-event payload type can pack multiple short-duration event reports into one RTP packet. Because this capability was added to RFC 4733 in a fashion which is not backward compatible with RFC 2833, it is desirable that a sender have the means to determine whether the receiver understands such packets. This memo specifies a new SDP attribute, a=multev, to indicate that capability. Table of Contents 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Proposed New SDP Attribute a=multev . . . . . . . . . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 6. Normative References . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . . . 10 Taylor Expires July 8, 2007 [Page 2] Internet-Draft Signalling Event Packing Capability January 2007 1. Terminology In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119 [1]. Taylor Expires July 8, 2007 [Page 3] Internet-Draft Signalling Event Packing Capability January 2007 2. Introduction RFC 4733, recently published, has replaced RFC 2833. The latter is best known as the preferred mechanism for in-band transmission of DTMF, but also had other applications including the transmission of data modem signals over RTP. Section 2.5.1.5 of RFC 4733 introduced a new capability to optimize the usage of the telephone-event payload to carry a series of short-duration events such as those found in data modem signalling. This new capability allows the sender to pack multiple event reports into a single RTP packet, provided that they occur consecutively without a pause between them. Unfortunately, packets containing multiple event reports cannot be processed properly by implementations of RFC 2833. At best, an RFC 2833 receiver would handle the first event in the packet successfully, but would ignore the remaining events in the packet. At worst, the RFC 2833 receiver would identify the packet as malformed and discard it. In either case, meaningful information would fail to be transmitted. As a result, it is desirable for an RFC 4733 implementation to know in advance whether its peer acting as receiver has the capability to process multiple event reports in a single RTP packet. Taylor Expires July 8, 2007 [Page 4] Internet-Draft Signalling Event Packing Capability January 2007 3. Proposed New SDP Attribute a=multev To meet the need just described, this memo introduces the a=multev SDP attribute. If this attribute is present in a session description, it indicates that the originator of the session description can properly decode RTP packets containing multiple event reports as specified by RFC 4733 sections 2.5.1.5 and 2.5.2.4. The a=multev attribute MAY be present at either the session level or media level. At the session level, this attribute indicates that the capability to decode multiple event reports in one RTP packet is applicable to any media stream within the session which carries the audio/telephone-event payload type. At the media level, the a=multev attribute indicates the capability of decoding multiple event reports in an RTP packet for this particular stream (which typically will be limited to a specific set of events.) If the attribute is present at both levels, the media-level occurrences serve as hints as to the particular streams in which packing of multiple events is expected. An implementation of RFC 4733 MAY choose always to report just one event per RTP packet, to guarantee backward compatibility. In the alternative, an implementation of RFC 4733 that also supports the present memo MUST NOT encode multiple events into one RTP packet unless it has determined that its peer is able to decode those events properly. The receipt of a session description containing the a=multev attribute is one means of making such a determination. If this attribute is present only at the media level, the sender MUST NOT encode multiple events into one RTP packet for media streams other than those identified by the presence of the attribute. Taylor Expires July 8, 2007 [Page 5] Internet-Draft Signalling Event Packing Capability January 2007 4. Security Considerations The a=multev attribute introduces no new security threats, with the possible exception that a man-in-the-middle attacker could insert the attribute into messages containing SDP where it was absent. This would constitute a rather weak denial of service threat, since the SDP receiver might not choose to use the event packing capability even though the SDP sender seemingly signalled willingness to accept packed events. Since since such an attacker is in a position to introduce much more effective attacks, there is little point to taking special measures to protect against this one. In general, this points to a requirement to provide message integrity for signalling. Taylor Expires July 8, 2007 [Page 6] Internet-Draft Signalling Event Packing Capability January 2007 5. IANA Considerations This document registers an additional SDP attribute "multev" as defined in this document, within the registry for "att-field (both session and media level)". multev [RFCXXXX] NOTE TO THE RFC EDITOR: Please replace all occurrences of RFC XXXX by the RFC number assigned to this document. Taylor Expires July 8, 2007 [Page 7] Internet-Draft Signalling Event Packing Capability January 2007 6. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals", RFC 4733, December 2006. Taylor Expires July 8, 2007 [Page 8] Internet-Draft Signalling Event Packing Capability January 2007 Author's Address Tom Taylor Nortel 1852 Lorraine Ave Ottawa, Ontario K1H 6Z8 CA Email: taylor@nortel.com Taylor Expires July 8, 2007 [Page 9] Internet-Draft Signalling Event Packing Capability January 2007 Full Copyright Statement Copyright (C) The Internet Society (2007). 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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Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Taylor Expires July 8, 2007 [Page 10]