[e2e] UDP length field

[Greg Minshall]

Michael,

i would assume that most UDPs get the IP length field as part of their 
interface with IP.  how they validity check the two length fields is an open 
question, though Craig gave a reasonable one (UDPlength <= IPlength).

(i assume the *2 bytes* used for UDP length in the UDP header are there for 
alignment, though i don't know why they were used to carry redundant 
information rather than "reserved".)

Greg Minshall