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
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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
4.4.2. Keep-Alive with STUN . . . . . . . . . . . . . . . . . 21
4.5. Flow Recovery . . . . . . . . . . . . . . . . . . . . . . 21
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