[e2e] UDP length field

[Joe Touch]

Greg Minshall wrote:
> 
> Joe,
> 
> >       3. in the case where IP options add trailers
> >               in this case, there is insufficient information in the IP_LEN
> >               and IP_HLEN fields to determine where the UDP data ends
> 
> foo.  if an IP option adds a trailer, it needs to give an indication of how to
> find that trailer, which is in "IP land", not "UDP/TCP/whatever land" (note
> that in rfc893, "how to find the trailer" is actually in "ethernet land"; that
> works, since rfc893 is effectively describing an ethernet option that adds a
> trailer).

I suspected as much. However, if that information is in a trailer-specific
location in IP land, then UDP would have to know about all the possible
IP trailers in order to determine the payload length, unless that info
were passed explicitly.

This way UDP has the information more readily available, and is not
dependent on an IP/UDP interface.

(it's a thin justification, admittedly - esp. that there was only
one trailer spec'd and it's been deprecated).

Joe