Subject: [Sipping] I-Ds concerning problems with response codes

From: [email-masked]
Date: Thu, 1 Mar 2007 10:30:54 -0500

I've submitted a set of Internet-Drafts about three problems with SIP
response codes:

(1) the current response codes are not in some instances fine-grained
enough to support networks in which there are redundant paths to
destination UAs,

(2) the 6xx class responses are (as defined) harmful in most
applications and should not be generated by UAs, and

(3) the response codes in the category "no usable destination for this
URI" are muddled and need careful redefinition.

The abstracts are:

   Classifying Response Codes to Support Multiple Path Routing in the
   Session Initiation Protocol (SIP)
   draft-worley-redundancy-response-00

   http://www.ietf.org/internet-drafts/draft-worley-redundancy-response-00.txt

   An increasing number of SIP architectures implement multiple path
   routing (MPR), which is the providing of more than one path for a
   call to reach a destination user agent (UA).  To implement MPR
   well, the proxy which forks a request onto the paths needs to be
   able to determine if a fork that failed reached the destination UA
   and was rejected by the UA (and so an alternate path should not be
   tried), or if the fork failed before reaching the UA (and so an
   alternate path should be attempted).  In order that the proxy can
   distinguish these two situations, response codes should
   unambiguously identify which of these situations applies.  This
   Internet-Draft begins classifying the current usage of response
   codes, their implications for MPR, and possible improvements.

   6xx-Class Responses Considered Harmful in the Session Initiation
   Protocol (SIP)
   draft-worley-6xx-considered-harmful-00

   http://www.ietf.org/internet-drafts/draft-worley-6xx-considered-harmful-00.txt

   The specification of the Session Initiation Protocol (SIP) permits a
   user agent (UA) that receives a call to generate a response in the
   "6xx class" that not only rejects the call but also requires the
   termination of all attempts to route the call to alternative
   destinations.  As the call may have reached the UA through multiple
   forwardings whose meanings cannot be known by the UA, the global
   termination of alternative routing can only rarely be correctly
   requested by the UA.  Because such responses are almost never
   appropriate, ability of a UA to generate such responses is harmful,
   and all 6xx class responses should be replaced with 4xx class
   responses with similar meanings.  However, there is no 4xx class
   response similar to "603 Decline", and so one needs to be created.

   Response Codes Regarding URIs that Cannot Be Routed to a
   Destination in the Session Initiation Protocol (SIP)
   draft-worley-ex-uri-00

   http://www.ietf.org/internet-drafts/draft-worley-ex-uri-00.txt

   The Session Initiation Protocol (SIP) provides a number of response
   codes which a SIP agent can use to indicate that it cannot find a
   suitable destination to which to send a request.  Unfortunately, this
   set of response codes was not carefully designed, making it difficult
   in many cases for a proxy to select a response code that accurately
   reflects the proxy's knowledge about the request URI.  This document
   describes these response codes and their current usage.

Feel free to contact me with any comments.

Dale

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