Hello,
If the message size is less than 1300 bytes as described in RFC 3261 and RFC 3263, the system uses UDP. If the message size is greater than 1300 bytes, the system uses TCP; however, if TCP fails, the system attempts to use UDP.
Now coming to your point you can configure your system that can use TCP if the MTU size being is more 1300 bytes. also check what is transport being used .
qouting RFC from 3216
The client side of the transport layer is responsible for sending the
request and receiving responses. The user of the transport layer
passes the client transport the request, an IP address, port,
transport, and possibly TTL for multicast destinations.
If a request is within 200 bytes of the path MTU, or if it is larger
than 1300 bytes and the path MTU is unknown, the request MUST be sent
using an RFC 2914 [43] congestion controlled transport protocol, such
as TCP. If this causes a change in the transport protocol from the
one indicated in the top Via, the value in the top Via MUST be
changed. This prevents fragmentation of messages over UDP and
provides congestion control for larger messages. However,
implementations MUST be able to handle messages up to the maximum
datagram packet size. For UDP, this size is 65,535 bytes, including
IP and UDP headers
Regards
Nadeem
Br,
Nadeem
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