Session Description Protocol (SDP) Security Descriptions for Media Streams
RFC 4568

Network Working Group                                       F. Andreasen
Request for Comments: 4568                                    M. Baugher
Category: Standards Track                                        D. Wing
                                                           Cisco Systems
                                                               July 2006

                   Session Description Protocol (SDP)
                Security Descriptions for Media Streams

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.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document defines a Session Description Protocol (SDP)
   cryptographic attribute for unicast media streams.  The attribute
   describes a cryptographic key and other parameters that serve to
   configure security for a unicast media stream in either a single
   message or a roundtrip exchange.  The attribute can be used with a
   variety of SDP media transports, and this document defines how to use
   it for the Secure Real-time Transport Protocol (SRTP) unicast media
   streams.  The SDP crypto attribute requires the services of a data
   security protocol to secure the SDP message.

Table of Contents

   1. Introduction ....................................................3
   2. Notational Conventions ..........................................5
   3. Applicability ...................................................5
   4. SDP "Crypto" Attribute and Parameters ...........................5
      4.1. Tag ........................................................6
      4.2. Crypto-Suite ...............................................6
      4.3. Key Parameters .............................................7
      4.4. Session Parameters .........................................8
      4.5. Example ....................................................8
   5. General Use of the crypto Attribute .............................9
      5.1. Use with Offer/Answer ......................................9
           5.1.1. Generating the Initial Offer - Unicast Streams ......9

Andreasen, et al.           Standards Track                     [Page 1]
RFC 4568               SDP Security Descriptions               July 2006

           5.1.2. Generating the Initial Answer - Unicast Streams ....10
           5.1.3. Processing of the Initial Answer - Unicast
                  Streams ............................................11
           5.1.4. Modifying the Session ..............................11
      5.2. Use Outside Offer/Answer ..................................11
      5.3. General Backwards Compatibility Considerations ............12
   6. SRTP Security Descriptions .....................................12
      6.1. SRTP Key Parameter ........................................13
      6.2. Crypto-Suites .............................................16
           6.2.1. AES_CM_128_HMAC_SHA1_80 ............................16
           6.2.2. AES_CM_128_HMAC_SHA1_32 ............................17
           6.2.3. F8_128_HMAC_SHA1_80 ................................17
           6.2.4. Adding New Crypto-Suite Definitions ................17
      6.3. Session Parameters ........................................17
           6.3.1. KDR=n ..............................................18
           6.3.2. UNENCRYPTED_SRTCP and UNENCRYPTED_SRTP .............18
           6.3.3. UNAUTHENTICATED_SRTP ...............................18
           6.3.4. FEC_ORDER=order ....................................19
           6.3.5. FEC_KEY=key-params .................................19
           6.3.6. Window Size Hint (WSH) .............................19
           6.3.7. Defining New SRTP Session Parameters ...............20
      6.4. SRTP Crypto Context Initialization ........................20
           6.4.1. Late Binding of One or More SSRCs to a
                  Crypto Context .....................................21
           6.4.2. Sharing Cryptographic Contexts among
                  Sessions or SSRCs ..................................22
      6.5. Removal of Crypto Contexts ................................23
   7. SRTP-Specific Use of the Crypto Attribute ......................23
      7.1. Use with Offer/Answer .....................................23
           7.1.1. Generating the Initial Offer - Unicast Streams .....23
           7.1.2. Generating the Initial Answer - Unicast Streams ....24
           7.1.3. Processing of the Initial Answer - Unicast