Typically, the Maximum Transmission Unit (MTU) value for the Internet is 1500 bytes (this is the maximum size of an IP packet). SIP protocol does not support message fragmentation at the SIP protocol layer, so if a SIP message does not fit into a single packet, this may cause issues with calls.
SIP messages can be sent over the UDP protocol when it is known that their size is less than the MTU value of the IP network (e.g., these are simple SIP messages).
However, there may be a need to send large SIP messages (containing multiple message bodies and large header fields, e.g., with a large list of codecs). In this case, SIP messages must be sent over TCP protocol which is more reliable but takes more time to process.
Note that also, proxies may reject requests with large message bodies with a “413 Request Entity Too Large” response, since processing large messages can load a server.
Starting from MR103, the SIP messages sent by PortaSIP are optimized to decrease their size (e.g., PortaSIP strips the service attributes for internal usage). This significantly reduces the chance of issues caused by the SIP message size and, thus, allows you to ensure service quality in your network, e.g., if your equipment uses the UDP protocol.
