Add constraint for UDP's length field

[lemmy]

https://www.ietf.org/rfc/rfc768.txt

Length is the length in octets of this user datagram including this header and the data. (This means the minimum value of the length is eight.)

https://en.wikipedia.org/wiki/User_Datagram_Protocol#UDP_datagram_structure

This field specifies the length in bytes of the UDP header and UDP data. The minimum length is 8 bytes, the length of the header. The field size sets a theoretical limit of 65,535 bytes (8-byte header + 65,527 bytes of data) for a UDP datagram. However, the actual limit for the data length, which is imposed by the underlying IPv4 protocol, is 65,507 bytes (65,535 bytes − 8-byte UDP header − 20-byte IP header).[5]
Using IPv6 jumbograms it is possible to have UDP datagrams of size greater than 65,535 bytes. The length field is set to zero if the length of the UDP header plus UDP data is greater than 65,535.[6]