[Sip] RPORT and TCP

Hi, 
RFC 3581-Symmetric Response Routing specifies that the rport parameter should be used only if the transport is unreliable.

"When a server attempts to send a response, it examines the topmost
   Via header field value of that response.  If the "sent-protocol"
   component indicates an unreliable unicast transport protocol, such as
   UDP, and there is no "maddr" parameter, but there is both a
   "received" parameter and an "rport" parameter, the response MUST be
   sent to the IP address listed in the "received" parameter, and the
   port in the "rport" parameter.  The response MUST be sent from the
   same address and port that the corresponding request was received on.
   This effectively adds a new processing step between bullets two and
   three in Section 18.2.2 of SIP [1]."

As I see it, the same problem that rport solves in UDP can happen in TCP.
This can happen if the connection is broken between the request and the response.

If for example the Via header in the request is:

Via: SIP/2.0/TCP proxy.example.com;branch=z9hG4bKkjsh77

and proxy.example.com was resolved to a port different then 5060, the server will not be able to 
establish a new connection in order to send the response since 5060 will be used.

Why do we have the UDP limitation in the standard?
How can such a problem be solved.

Regards,
Sarit.

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