Private Header (P-Header) Extension to the Session Initiation Protocol (SIP) for Authorization of Early Media
RFC 5009

• Status
• IESG evaluation record
• IESG writeups
• Email expansions
• History

• Versions
• 04

Network Working Group                                           R. Ejzak
Request for Comments: 5009                                Alcatel-Lucent
Category: Informational                                   September 2007

                Private Header (P-Header) Extension to
the Session Initiation Protocol (SIP) for Authorization of Early Media

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   This document describes a private Session Initiation Protocol (SIP)
   header field (P-header) to be used by the European Telecommunications
   Standards Institute (ETSI) Telecommunications and Internet-converged
   Services and Protocols for Advanced Networks (TISPAN) for the purpose
   of authorizing early media flows in Third Generation Partnership
   Project (3GPP) IP Multimedia Subsystems (IMS).  This header field is
   useful in any SIP network that is interconnected with other SIP
   networks and needs to control the flow of media in the early dialog
   state.

Ejzak                        Informational                      [Page 1]
RFC 5009                  P-Early-Media Header            September 2007

Table of Contents

   1. Introduction ....................................................2
   2. Applicability Statement .........................................3
   3. Conventions and Acronyms ........................................3
   4. Background on Early Media Authorization .........................4
      4.1. Backward Early Media .......................................5
      4.2. Forward Early Media ........................................5
   5. Applicability of RFC 3959 and RFC 3960 ..........................6
   6. Overview of Operation ...........................................6
   7. Limitations of the P-Early-Media Header Field ...................8
   8. The P-Early-Media Header Field ..................................8
      8.1. Procedures at the User Agent Client .......................10
      8.2. Procedures at the User Agent Server .......................10
      8.3. Procedures at the Proxy ...................................11
   9. Formal Syntax ..................................................11
   10. Security Considerations .......................................11
   11. IANA Considerations ...........................................12
      11.1. Registration of the "P-Early-Media" SIP Header Field .....12
   12. Acknowledgements ..............................................12
   13. References ....................................................12
      13.1. Normative References .....................................12
      13.2. Informative References ...................................13

1.  Introduction

   This document defines the use of the P-Early-Media header field for
   use within SIP [1] messages in certain SIP networks to authorize the
   cut-through of backward and/or forward early media when permitted by
   the early media policies of the networks involved.  The P-Early-Media
   header field is intended for use in a SIP network, such as a 3GPP IMS
   [13][14] that has the following characteristics: its early media
   policy prohibits the exchange of early media between end users; it is
   interconnected with other SIP networks that have unknown, untrusted,
   or different policies regarding early media; and it has the
   capability to "gate" (enable/disable) the flow of early media to/from
   user equipment.

   Within an isolated SIP network, it is possible to gate early media
   associated with all endpoints within the network to enforce a desired
   early media policy among network endpoints.  However, when a SIP
   network is interconnected with other SIP networks, only the boundary
   node connected to the external network can determine which early
   media policy to apply to a session established between endpoints on
   different sides of the boundary.  The P-Early-Media header field
   provides a means for this boundary node to communicate this early
   media policy decision to other nodes within the network.

Ejzak                        Informational                      [Page 2]
RFC 5009                  P-Early-Media Header            September 2007

2.  Applicability Statement

   The use of this extension is only applicable inside a "Trust Domain"
   as defined in RFC 3325 [6].  Nodes in such a Trust Domain are
   explicitly trusted by its users and end-systems to authorize early
   media requests only when allowed by early media policy within the
   Trust Domain.

   This document does NOT offer a general early media authorization
   model suitable for inter-domain use or use in the Internet at large.
   Furthermore, since the early media requests are not cryptographically
   certified, they are subject to forgery, replay, and falsification in
   any architecture that does not meet the requirements of the Trust