View source for UDP

IPv4 protocol 17, Berkeley Socket type <tt>SOCK_DGRAM</tt>. Header is eight bytes:

* source port
* dest port
* length (header + payload, minimum 8)
* checksum

If no response is expected, the source port can be set to 0. Under IPv4, the checksum can be set to 0 to indicate that it is unused (the checksum is mandatory in IPv6). When using IPv6 jumbograms, and the length is greater than 65,535, the length filed ought be set to 0.