Multiple Dialog Usages in the Session Initiation Protocol
RFC 5057

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Network Working Group                                          R. Sparks
Request for Comments: 5057                              Estacado Systems
Category: Informational                                    November 2007

       Multiple Dialog Usages in the Session Initiation Protocol

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

   Several methods in the Session Initiation Protocol (SIP) can create
   an association between endpoints known as a dialog.  Some of these
   methods can also create a different, but related, association within
   an existing dialog.  These multiple associations, or dialog usages,
   require carefully coordinated processing as they have independent
   life-cycles, but share common dialog state.  Processing multiple
   dialog usages correctly is not completely understood.  What is
   understood is difficult to implement.

   This memo argues that multiple dialog usages should be avoided.  It
   discusses alternatives to their use and clarifies essential behavior
   for elements that cannot currently avoid them.

   This is an informative document and makes no normative statements of
   any kind.

Sparks                       Informational                      [Page 1]
RFC 5057                 Multiple Dialog Usages            November 2007

Table of Contents

   1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.  Examples of Multiple Usages  . . . . . . . . . . . . . . . . .  4
     3.1.  Transfer . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Reciprocal Subscription  . . . . . . . . . . . . . . . . .  6
   4.  Usage Creation and Destruction . . . . . . . . . . . . . . . .  9
     4.1.  Invite Usages  . . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Subscribe usages . . . . . . . . . . . . . . . . . . . . .  9
   5.  Proper Handling of Multiple Usages . . . . . . . . . . . . . .  9
     5.1.  A Survey of the Effect of Failure Responses on Usages
           and Dialogs  . . . . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Transaction Timeouts . . . . . . . . . . . . . . . . . . . 15
     5.3.  Matching Requests to Usages  . . . . . . . . . . . . . . . 16
     5.4.  Target Refresh Requests  . . . . . . . . . . . . . . . . . 17
     5.5.  Refreshing and Terminating Usages  . . . . . . . . . . . . 17
     5.6.  Refusing New Usages  . . . . . . . . . . . . . . . . . . . 18
     5.7.  Replacing Usages . . . . . . . . . . . . . . . . . . . . . 18
   6.  Avoiding Multiple Usages . . . . . . . . . . . . . . . . . . . 18
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   8.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24
   10. Informative References . . . . . . . . . . . . . . . . . . . . 24

1.  Overview

   This is an informative document.  It makes no normative statements of
   any kind.  This document refines the concept of a dialog usage in the
   Session Initiation Protocol (SIP [1]), and discusses what led to its
   existence.  It explores ambiguity associated with processing multiple
   dialog usages that share a dialog.  In particular, it surveys the
   effect of SIP failure responses on transaction, dialog usage, and
   dialog state.  This document will help the implementer understand
   what is required to process multiple dialog usages correctly, and
   will provide information for future standards-track work that will
   clarify RFC 3261 and other related documents.  Finally, the document
   explores single-usage dialog alternatives (using SIP extensions) to
   multiple dialog usages.

2.  Introduction

   Several methods in SIP can establish a dialog.  When they do so, they
   also establish an association between the endpoints within that
   dialog.  This association has been known for some time as a "dialog
   usage" in the developer community.  A dialog initiated with an INVITE
   request has an invite usage.  A dialog initiated with a SUBSCRIBE

Sparks                       Informational                      [Page 2]
RFC 5057                 Multiple Dialog Usages            November 2007

   request has a subscribe usage.  A dialog initiated with a REFER
   request has a subscribe usage.

   Dialogs with multiple usages arise when a usage-creating action
   occurs inside an existing dialog.  Such actions include accepting a
   REFER or SUBSCRIBE issued inside a dialog established with an INVITE
   request.  Multiple REFERs within a dialog create multiple
   subscriptions, each of which is a new dialog usage sharing common
   dialog state.  (Note that any REFER issued utilizing the
   subscription-suppression mechanism specified in [2] creates no new