Managing Client-Initiated Connections in the Session Initiation Protocol (SIP)
RFC 5626

Network Working Group                                   C. Jennings, Ed.
Request for Comments: 5626                                 Cisco Systems
Updates: 3261, 3327                                         R. Mahy, Ed.
Category: Standards Track                                   Unaffiliated
                                                           F. Audet, Ed.
                                                              Skype Labs
                                                            October 2009

                 Managing Client-Initiated Connections
                in the Session Initiation Protocol (SIP)

Abstract

   The Session Initiation Protocol (SIP) allows proxy servers to
   initiate TCP connections or to send asynchronous UDP datagrams to
   User Agents in order to deliver requests.  However, in a large number
   of real deployments, many practical considerations, such as the
   existence of firewalls and Network Address Translators (NATs) or the
   use of TLS with server-provided certificates, prevent servers from
   connecting to User Agents in this way.  This specification defines
   behaviors for User Agents, registrars, and proxy servers that allow
   requests to be delivered on existing connections established by the
   User Agent.  It also defines keep-alive behaviors needed to keep NAT
   bindings open and specifies the usage of multiple connections from
   the User Agent to its registrar.

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.

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Jennings, et al.            Standards Track                     [Page 1]
RFC 5626          Client-Initiated Connections in SIP       October 2009

   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  5
     2.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Summary of Mechanism . . . . . . . . . . . . . . . . . . .  6
     3.2.  Single Registrar and UA  . . . . . . . . . . . . . . . . .  7
     3.3.  Multiple Connections from a User Agent . . . . . . . . . .  8
     3.4.  Edge Proxies . . . . . . . . . . . . . . . . . . . . . . . 10
     3.5.  Keep-Alive Technique . . . . . . . . . . . . . . . . . . . 11
       3.5.1.  CRLF Keep-Alive Technique  . . . . . . . . . . . . . . 12
       3.5.2.  STUN Keep-Alive Technique  . . . . . . . . . . . . . . 12
   4.  User Agent Procedures  . . . . . . . . . . . . . . . . . . . . 13
     4.1.  Instance ID Creation . . . . . . . . . . . . . . . . . . . 13
     4.2.  Registrations  . . . . . . . . . . . . . . . . . . . . . . 14
       4.2.1.  Initial Registrations  . . . . . . . . . . . . . . . . 14
       4.2.2.  Subsequent REGISTER Requests . . . . . . . . . . . . . 16
       4.2.3.  Third-Party Registrations  . . . . . . . . . . . . . . 17
     4.3.  Sending Non-REGISTER Requests  . . . . . . . . . . . . . . 17
     4.4.  Keep-Alives and Detecting Flow Failure . . . . . . . . . . 18
       4.4.1.  Keep-Alive with CRLF . . . . . . . . . . . . . . . . . 19