Java Network Programming, Second Edition

The DatagramPacket Class

UDP datagrams add very little to the IP datagrams they sit on top of. Figure 13.1 shows a typical UDP datagram. The UDP header adds only eight bytes to the IP header. The UDP header includes source and destination port numbers, the length of everything that follows the IP header, and an optional checksum. Since port numbers are given as a 2-byte unsigned integer, 65,536 different possible UDP ports are available per host. These are distinct from the 65,536 different TCP ports per host. Since the length is also a 2-byte unsigned integer, the number of bytes in a datagram is limited to 65,536 minus the 8 bytes for the header. However, this is redundant with the datagram length field of the IP header, which limits datagrams to from 65,467 to 65,507 bytes. (The exact number depends on the size of the IP header.) The checksum field is optional and not used in or accessible from application layer programs. If the checksum for the data fails, the native network software will silently discard the datagram; neither the sender nor the receiver is notified. UDP is an unreliable protocol, after all.

Figure 13-1. The structure of a UDP datagram

Although the theoretical maximum amount of data in a UDP datagram is 65,507 bytes, in practice there is almost always much less. On many platforms, the actual limit is more likely to be 8,192 bytes (8K). And implementations are ...