Extending the Session Initiation Protocol (SIP) Reason Header for Preemption Events
RFC 4411

Network Working Group                                            J. Polk
Request for Comments: 4411                                 Cisco Systems
Category: Standards Track                                  February 2006

            Extending the Session Initiation Protocol (SIP)
                  Reason Header for Preemption Events

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 proposes an IANA Registration extension to the Session
   Initiation Protocol (SIP) Reason Header to be included in a BYE
   Method Request as a result of a session preemption event, either at a
   user agent (UA), or somewhere in the network involving a
   reservation-based protocol such as the Resource ReSerVation Protocol
   (RSVP) or Next Steps in Signaling (NSIS).  This document does not
   attempt to address routers failing in the packet path; instead, it
   addresses a deliberate tear down of a flow between UAs, and informs
   the terminated UA(s) with an indication of what occurred.

Polk                        Standards Track                     [Page 1]
RFC 4411        SIP Reason Header for Preemption Events    February 2006

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................4
   2. Access Preemption Events ........................................4
      2.1. Effects of Preemption at the User Agent ....................6
      2.2. Reason Header Requirements for Access Preemption Events ....6
   3. Network Preemption Events .......................................7
      3.1. Reason Header Requirements for Network Preemption Events ..10
   4. Including a Hybrid Infrastructure ..............................10
      4.1. Hybrid Infrastructure Requirements ........................11
   5. Preemption Reason Header Cause Codes and Semantics .............11
      5.1. Access Preemption Event Reason Code .......................12
           5.1.1. Access Preemption Event Call Flow ..................12
      5.2. Network Preemption Events Reason Code .....................14
           5.2.1. Network Preemption Event Call Flow .................15
      5.3. Generic Preemption Event Reason Code ......................16
      5.4. Non-IP Preemption Event Reason Code .......................16
           5.4.1. Non-IP Preemption Event Call Flow ..................17
   6. Security Considerations ........................................17
   7. IANA Considerations ............................................17
      7.1. "Preemption" Namespace Registry ...........................18
      7.2. Default Reason-Text IANA Registry for the SIP
           Reason Header .............................................20
   8. Contributions ..................................................20
   9. Acknowledgements ...............................................20
   10. References ....................................................21
      10.1. Normative References .....................................21
      10.2. Informative References ...................................21

1.  Introduction

   With the introduction of the SIP Resource-Priority (R-P) header [4],
   there became the possibility of sessions being torn down for (scarce)
   resource reasons, meaning there weren't enough resources for a
   particular session to continue.  Certain domains will implement this
   mechanism where resources may become constrained either at the user
   agent (UA) or at congested router interfaces where more important
   sessions are to be completed at the expense of less important
   sessions.  Which sessions are more or less important than others will
   not be discussed here.  What is proposed here is a SIP [2] extension
   to synchronize SIP elements as to why a preemption event occurred and
   which type of preemption event occurred, as viewed by the element
   that performed the preemption of a session.

   The SIP Reason Header is an application layer feedback mechanism to
   synchronize SIP elements of events; the particular event explained
   here deals with preemption of a session.  Q.850 [5] provides an

Polk                        Standards Track                     [Page 2]
RFC 4411        SIP Reason Header for Preemption Events    February 2006

   indication for preemption (cause=8) and for preemption "circuit