Security Preconditions for Session Description Protocol (SDP) Media Streams
RFC 5027

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Network Working Group                                       F. Andreasen
Request for Comments: 5027                                       D. Wing
Updates: 3312                                              Cisco Systems
Category: Standards Track                                   October 2007

                      Security Preconditions for
           Session Description Protocol (SDP) 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.

Abstract

   This document defines a new security precondition for the Session
   Description Protocol (SDP) precondition framework described in RFCs
   3312 and 4032.  A security precondition can be used to delay session
   establishment or modification until media stream security for a
   secure media stream has been negotiated successfully.

Table of Contents

   1. Introduction ....................................................2
   2. Notational Conventions ..........................................2
   3. Security Precondition Definition ................................2
   4. Examples ........................................................6
      4.1. SDP Security Descriptions Example ..........................6
      4.2. Key Management Extension for SDP Example ...................9
   5. Security Considerations ........................................11
   6. IANA Considerations ............................................13
   7. Acknowledgements ...............................................13
   8. Normative References ...........................................13
   9. Informative References .........................................14

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RFC 5027                 Security Preconditions             October 2007

1.  Introduction

   The concept of a Session Description Protocol (SDP) [RFC4566]
   precondition is defined in [RFC3312] as updated by [RFC4032].  A
   precondition is a condition that has to be satisfied for a given
   media stream in order for session establishment or modification to
   proceed.  When a (mandatory) precondition is not met, session
   progress is delayed until the precondition is satisfied or the
   session establishment fails.  For example, RFC 3312 defines the
   Quality-of-Service precondition, which is used to ensure availability
   of network resources prior to establishing (i.e., alerting) a call.

   Media streams can either be provided in cleartext and with no
   integrity protection, or some kind of media security can be applied,
   e.g., confidentiality and/or message integrity.  For example, the
   Audio/Video profile of the Real-Time Transfer Protocol (RTP)
   [RFC3551] is normally used without any security services whereas the
   Secure Real-time Transport Protocol (SRTP) [SRTP] is always used with
   security services.  When media stream security is being negotiated,
   e.g., using the mechanism defined in SDP Security Descriptions
   [SDESC], both the offerer and the answerer [RFC3264] need to know the
   cryptographic parameters being used for the media stream; the offerer
   may provide multiple choices for the cryptographic parameters, or the
   cryptographic parameters selected by the answerer may differ from
   those of the offerer (e.g., the key used in one direction versus the
   other).  In such cases, to avoid media clipping, the offerer needs to
   receive the answer prior to receiving any media packets from the
   answerer.  This can be achieved by using a security precondition,
   which ensures the successful negotiation of media stream security
   parameters for a secure media stream prior to session establishment
   or modification.

2.  Notational Conventions

   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 [RFC2119].

3.  Security Precondition Definition

   The semantics for a security precondition are that the relevant
   cryptographic parameters (cipher, key, etc.) for a secure media
   stream are known to have been negotiated in the direction(s)
   required.  If the security precondition is used with a non-secure
   media stream, the security precondition is by definition satisfied.
   A secure media stream is here defined as a media stream that uses
   some kind of security service (e.g., message integrity,

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RFC 5027                 Security Preconditions             October 2007

   confidentiality, or both), regardless of the cryptographic strength
   of the mechanisms being used.