Extended Secure RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)
RFC 5124

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Network Working Group                                             J. Ott
Request for Comments: 5124             Helsinki University of Technology
Category: Standards Track                                     E. Carrara
                                                                     KTH
                                                           February 2008

                    Extended Secure RTP Profile for
 Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)

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.

Abstract

   An RTP profile (SAVP) for secure real-time communications and another
   profile (AVPF) to provide timely feedback from the receivers to a
   sender are defined in RFC 3711 and RFC 4585, respectively.  This memo
   specifies the combination of both profiles to enable secure RTP
   communications with feedback.

Ott & Carrara               Standards Track                     [Page 1]
RFC 5124                                                   February 2008

Table of Contents

   1. Introduction ....................................................3
      1.1. Definitions ................................................4
      1.2. Terminology ................................................4
   2. SAVPF Rules .....................................................4
      2.1. Packet Formats .............................................5
      2.2. Extensions .................................................5
      2.3. Implications from Combining AVPF and SAVP ..................6
   3. SDP Definitions .................................................6
      3.1. Profile Definition .........................................6
      3.2. Attribute Definitions ......................................6
      3.3. Profile Negotiation ........................................6
           3.3.1. Offer/Answer-Based Negotiation of Session
                  Descriptions ........................................7
           3.3.2. RTSP-Based Negotiation of Session Descriptions ......8
           3.3.3. Announcing Session Descriptions .....................9
           3.3.4. Describing Alternative Session Profiles .............9
      3.4. Examples ..................................................10
   4. Interworking of AVP, SAVP, AVPF, and SAVPF Entities ............13
   5. Security Considerations ........................................14
   6. IANA Considerations ............................................15
   7. Acknowledgements ...............................................15
   8. References .....................................................15
      8.1. Normative References ......................................15
      8.2. Informative References ....................................16

Ott & Carrara               Standards Track                     [Page 2]
RFC 5124                                                   February 2008

1.  Introduction

   The Real-time Transport Protocol, the associated RTP Control Protocol
   (RTP/RTCP, RFC 3550) [1], and the profile for audiovisual
   communications with minimal control (RFC 3551) [2] define mechanisms
   for transmitting time-based media across an IP network.  RTP provides
   means to preserve timing and detect packet losses, among other
   things, and RTP payload formats provide for proper framing of
   (continuous) media in a packet-based environment.  RTCP enables
   receivers to provide feedback on reception quality and allows all
   members of an RTP session to learn about each other.

   The RTP specification provides only rudimentary support for
   encrypting RTP and RTCP packets.  Secure RTP (RFC 3711) [4] defines
   an RTP profile ("SAVP") for secure RTP media sessions, defining
   methods for proper RTP and RTCP packet encryption, integrity, and
   replay protection.  The initial negotiation of SRTP and its security
   parameters needs to be done out-of-band, e.g., using the Session
   Description Protocol (SDP, RFC 4566) [6] together with extensions for
   conveying keying material (RFC 4567 [7], RFC 4568 [8]).

   The RTP specification also provides limited support for timely
   feedback from receivers to senders, typically by means of reception
   statistics reporting in somewhat regular intervals depending on the
   group size, the average RTCP packet size, and the available RTCP
   bandwidth.  The extended RTP profile for RTCP-based feedback ("AVPF")
   (RFC 4585, [3]) allows session participants statistically to provide
   immediate feedback while maintaining the average RTCP data rate for
   all senders.  As for SAVP, the use of AVPF and its parameters needs