What is the maximum UDP payload size and what fragmentation considerations apply?

• A. 512 bytes payload
• B. 65535 bytes payload
• C. 65507 bytes payload; UDP datagrams may be fragmented by IP; PMTUD helps avoid fragmentation.
• D. 1460 bytes payload

Answer: (C)

UDP payload size is limited by the 16-bit length field in the UDP header, which counts the header plus data. The maximum UDP datagram length is 65535, and with an IPv4 header typically 20 bytes, plus an 8-byte UDP header, the largest payload you can carry in a single UDP datagram is 65535 − 20 − 8 = 65507 bytes. That’s why 65507 bytes is the commonly cited maximum UDP payload.

Fragmentation comes into play because if a UDP datagram is larger than the path MTU, IP can fragment it into smaller pieces so it can traverse the network. Fragmentation has downsides: fragments can be lost independently, reassembly can fail, and many networks drop fragments, which breaks the UDP message. Path MTU Discovery helps avoid this by learning the smallest MTU along the route and keeping datagrams small enough to fit, so they don’t need to be fragmented. If the DF (Don't Fragment) bit is set, routers will drop oversized packets and signal back a “Packet Too Big” ICMP message, which PMTUD uses to adjust payload size.