[Sip] a question on section 18.2.2 in RFC 3261

["Wang, Jin-Gen" <Jin-Gen.Wang@Level3.com>]

Section 18.2.2 specifies:

"18.2.2 Sending Responses

   The server transport uses the value of the top Via header field in
   order to determine where to send a response.  It MUST follow the
   following process:

      o  If the "sent-protocol" is a reliable transport protocol such as
         TCP or SCTP, or TLS over those, the response MUST be sent using
         the existing connection to the source of the original request
         that created the transaction, if that connection is still open.
         This requires the server transport to maintain an association
         between server transactions and transport connections.  If that
         connection is no longer open, the server SHOULD open a
         connection to the IP address in the "received" parameter, if
         present, using the port in the "sent-by" value, or the default
         port for that transport, if no port is specified.  If that
         connection attempt fails, the server SHOULD use the procedures
         in [4] for servers in order to determine the IP address and
         port to open the connection and send the response to.
"

Since the Via header has enough information (IP address + port + transport protocol) for an application to send back response messages, what is the design idea behind the above decision to request an application to send back responses using an existing connection if the connection is still open, other than (may be) optimization in implementation? In other words, from SIP protocol point of view, why is such request a MUST requirement?

Thanks.

Jin-Gen Wang
Level 3 Communications, Inc.

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