This document defines an extension to the Session Initiation Protocol
(SIP) that provides for persistent and reusable connections between
SIP User Agents and SIP Proxy Servers. In particular, this 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.
Working Group Summary
The working group process on this document was exceptionally long. The
first WG version of the draft appeared in the summer of 2005. Working
group last call initiated in the summer of 2006 and extended until the
summer of 2008, requiring several iterations of the draft and the
assignment of Francois Audet as a "process champion" for the draft
within the working group. Most delays seem to have been related to
slow cycle time on the part of the authors, but the process was also
delayed by a number of changes occurring during the review cycle.
Particular sticking points included the keepalive mechanism and a
requirement for binding to multiple outbound proxies if so
configured. Both were eventually resolved by a widely-accepted
compromises. Additional work is being pursued in SIPCORE to allow
the use of the keepalive mechanism independently of the outbound
There have been implementations of previous versions of this
draft testing at SIPit events since 2005.
Dean Willis is the document shepherd with significant assistance
from Francois Audet. Robert Sparks is the responsible area director.