Making TCP mandatory for the UA is a significant change from RFC 2543. It’s emerged from this should handle bigger messages, which MUST use TCP, as reviewed under. So, even if an element never sends large messages, then it can receive one and has in order to take care of these.
When a petition is within 200 bytes of the path MTU, or whether this really is more than 1300 bytes and the path MTU is unknown, then the petition has to be routed employing an (RFC 2914) congestion-controlled transport protocol, including TCP. If it induces a switch in the transport protocol by the one indicated in the top Via, the price at the top Via has to be shifted. But, implementations need to be able to manage messages up into the maximum datagram packet size.
You will find still experimental IETF Drafts around SIP More than STCP.
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