10.1 Fragmentation

Fragmentation in IP can also be used for UDP datagrams, but we should do our best to avoid fragmentation in UDP.

DNS could serve as a classic example. A DNS client sends a query via UDP to a server. If the server’s answer exceeds the 512 B limit, all the extra data is cut short and the TC Flag in application data is set to indicate this message truncation (TC). Should the client request more information, it repeats the request using TCP which will also be used by the server for returning a complete answer.

An example of UDP datagram:

+ FRAME: Base frame properties
+ ETHERNET: ETYPE = 0x0800 : Protocol = IP: DOD Internet Protocol
IP: ID = 0x9CCE; Proto = UDP; Len: 74
IP: Version = 4 (0x4)
IP: Header Length = 20 (0x14)
+ IP: Service Type = 0 (0x0)
IP: Total Length = 74 (0x4A)
IP: Identification = 40142 (0x9CCE)
+ IP: Flags Summary = 0 (0x0)
IP: Fragment Offset = 0 (0x0) bytes
IP: Time to Live = 30 (0x1E)
IP: Protocol = UDP - User Datagram
IP: Checksum = 0x803D
IP...