10.4. Transport protocols
The IMS puts no further restrictions on the transport protocol for SIP used between the UE and the P-CSCF. In this example it is assumed that the User Datagram Protocol (UDP) is the default transport protocol. UDP will be used for the transport of SIP messages that are sent between Tobias's UE and the P-CSCF as long as these messages do not exceed 1,300 bytes. When they exceed this limit, the Transmission Control Protocol (TCP) must be used. Due to the fact that SIP also allows a lot of content in the SIP message body (e.g., pictures can be attached to the body of a MESSAGE request), it is likely that both UDP and TCP will be used in parallel while a user is registered.
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