UDP - Maximum Datagram Size

The UDP Transport Layer protocol, unlike TCP, does not create sessions, and thus does not negotiate an MSS.

 The UDP header has a Length field that specifies the total length of the UDP datagram (header + payload), and that field is 16 bits in length. So theoretically, whole UDP datagram can be up to 65535 bytes in length. However, there are other factors that make the datagram size much smaller in real-world scenarios.

For UDP, it is the applications themselves, as well as the operation of the NIC cards of the end hosts that determine the actual size of a datagram. These processes take into account the underlying IPv4/IPv6 and Ethernet MTUs as well, to ensure an efficient transmission of data. But even if the datagram size is large, fragmentation will take place to ensure that the data “fits” in the payload of the underlying protocols.