[e2e] purpose of pseudo header in TCP checksum

Michael Welzl michael.welzl at uibk.ac.at
Tue Feb 15 11:42:03 PST 2005

> IPv6 makes no mention of a problem with 'sending packets god knows 
> where', so long as they're checked at the receiver. Although IPv6 (at 
> least as I recall) omits the IP checksum because of sufficient link 
> checksums to detect corrupted packets, there's no requirement that link 
> layers have such checksums (as hinted in the UDP-Lite RFC) - at least 
> not in 2460 (anyone else know anywhere else a body is buried in this 
> regard?)

I also searched RFC 2460 for this once, but couldn't find anything.
Still, that doesn't make sense to me. What if you transfer an
IPv6 packet across a link layer that does *NOT* check your data -
why wouldn't this cause harm to the source / destination addresses,
and therefore cause wrong router behavior downstream? And
what about the other fields - say, "next header"; a checksum
error could turn a TCP packet into a UDP packet, change the
payload length field, whatever.

There must be a requirement for link layers to do a strong
check somewhere?!

> > So we need to specify that link layers DO check the headers
> > and just do NOT check the payload if it's a UDP Lite packet.
> > This is the part that I consider inevitable (given the
> > current situation) but ugly.
> The first part, not AFAICT.
> The second IS ugly. Complain to UDP-Lite - it's definitely reaching 
> across layers to coordinate the link checksum configuration with UDP's 
> presence or lack of checksumming.
> >>IP still works over everything; things that work over IP need to know 
> >>something about it, as always.
> > 
> > "over" is fine with me, but I don't like to have things
> > underneath know so much about it.
> Agreeed. IMO, that's UDP-Lite's problem, not IPv6.

What if we had a CRC-32 checksum in the IPv4 header, in
the IPv6 header, in UDP Lite's header, in TCP's header,
in the UDP header, well - just all over the place.

Then, we could just say "disable your checksums" to
link layers when they notice a UDP Lite packet. It would
even make sense to have a "corruption acceptable" bit
somewhere in the IP header in such a scenario (I know,
no bits to spare).

Wouldn't that be better design?


More information about the end2end-interest mailing list