Subject: [Simple] Overlapped MESSAGE transactions in chatting

From: Loh Kok Jeng <[email-masked]>
Date: Thu, 5 Sep 2002 20:43:58 +0800

Dear all,

In a chatting scenario where the UA sends MESSAGEs with the same Call-ID but
with increasing CSeq in quick succession, how should a stateful proxy handle
these MESSAGEs?  

The diagram below is an illustration of the scenario mentioned above: 

  +----+            +-------+
  | UA |            | Proxy |
  +----+            +-------+
    |                   |
    |  MESSAGE 1        |
    |------------------>|
    |  MESSAGE 2        |
    |------------------>|
    |  MESSAGE 3        |
    |------------------>|
    |                   |

In draft-ietf-sip-message-06.txt, Section 9., it is stated that:

   A UAC MUST NOT initiate a new out-of-dialog MESSAGE transaction to a
   given URI if there is a previous out-of-dialog transaction pending
   for the same URI.  Similarly, A UAC SHOULD NOT initiate overlapping
   MESSAGE transactions inside a dialog, and MUST NOT do so unless the
   route set for that dialog uses a congestion-controlled transport at
   every hop.  UACs SHOULD NOT set the T1 timer value to less than 500
   ms for MESSAGE transactions.  UACs may use smaller T1 values if they
   know that that the next hop latency warrants it.

      The prohibition againt overlapping MESSAGE request provides some
      degree of congestion-safe behavior.  A request and its associated
      response must each cross the full path between the UAC and the
      UAS.  The time required for this will increase as networks become
      congested.  This provides an adaptive mechanism to slow the
      introduction of new MESSAGE requests to the same destination.

Does it mean that the UA must wait for a final response before sending the
next MESSAGE?  If so, should a stateful proxy drop the subsequent MESSAGEs
before it receives a final response for the first MESSAGE?

regards,
KJ
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From: Jonathan Rosenberg <[email-masked]>
Date: Thu, 05 Sep 2002 10:43:37 -0400

inline.

> with increasing CSeq in quick succession, how should a stateful proxy handle

The proxy shouldn't provide any special handling. It should properly 
proxy all messages. This is the case for stateless and stateful proxies 
alike. It is not the role of the proxy to police the client to ensure 
they are consistent with the congestion control requirements in the 
specification.

-Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.                72 Eagle Rock Ave.
Chief Scientist                             First Floor
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