RFC 3608 (rfc3608) - Page 2 of 17

Session Initiation Protocol (SIP) Extension Header Field for Service Route Discovery During Registration

Alternative Format: Original Text Document
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RFC 3608       SIP Extension for Service Route Discovery    October 2003


   11. Intellectual Property Statement. . . . . . . . . . . . . . .  16
   12. Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  16
   13. Full Copyright Statement . . . . . . . . . . . . . . . . . .  17

1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119 [1].

2.  Background

   The Third Generation Partnership Project (3GPP) established a
   requirement for discovering home proxies during SIP registration and
   published this requirement in [6].  The 3GPP network dynamically
   assigns a home service proxy to each address-of-record (AOR).  This
   assignment may occur in conjunction with a REGISTER operation, or
   out-of-band as needed to support call services when the address-of-
   record has no registrations.  This home service proxy may provide
   both inbound (UA terminated) and outbound (UA originated) services.

   In the inbound case, the Request-Uniform Resource Identifier (URI) of
   incoming SIP requests matches the address-of-record of a user
   associated with the home service proxy.  The home service proxy then
   (in most cases) forwards the request to the registered contact
   address for that AOR.  A mechanism for traversing required proxies
   between the home service proxy and the registered UA is presented in
   [4].

   Outbound (UA originated) session cases raise another issue.
   Specifically, "How does the UA know which service proxy to use and
   how to get there?"

   Several mechanisms were proposed in list discussions, including:

   1. Configuration data in the UA.  This raises questions of UA
      configuration management and updating, especially if proxy
      assignment is very dynamic, such as in load-balancing scenarios.

   2. Use of some other protocol, such as HTTP, to get configuration
      data from a configuration server in the home network.  While
      functional, this solution requires additional protocol engines,
      firewall complexity, operations overhead, and significant
      additional "over the air" traffic.

   3. Use of lookup tables in the home network, as may be done for
      inbound requests in some 3G networks.  This has a relatively high
      overhead in terms of database operations.



Willis & Hoeneisen          Standards Track
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