Writing a very quick reply as at work and not so much time left these days. The primary problem I found when testing on DCCP over Internet to Arnaldo in Brazil was that NAT would rewrite the IP address which would make the checksum wrong. Firewalls/NAT know about TCP/UDP and rewrite checksums. The checksum for DCCP encapsulated needs to be protected or implemented in a manner that survives UDP header rewrites. Your emails may well cover this but as said just skim reading at work and just thought I'd mention our implementation experience... 2009/11/23 Phelan, Tom <tphelan@xxxxxxxxxxxx>: > [Subject changed to focus on sub-thread] > > Hi Michael, > > I've been thinking about a slight variation of option 3 below for > dealing with partial checksums. Note that the intent of option 3 was > _not_ IP/UDP/UDP-Lite/DCCP as was mentioned in another thread. That > just creates a turtles-all-the-way-down problem -- you still have the > top-level UDP checksum to work around. The intent of option 3 is to > make UDP-Lite changes to UDP. Basically, just redefine the UDP length > field to the semantics of the checksum coverage field in UDP-Lite. > > The variation I'm thinking of is to say this -- when a UDP port is > offering the DCCP_NAT service, the length field is redefined to checksum > coverage. To use DCCP partial checksum, set the (redefined) length > field to the portion of the datagram that needs protection, as > negotiated via the DCCP Partial Checksum feature. This might get around > the giant can of worms that redefining the length field everywhere would > open. > > I'm not sure what setting the length field to less than the total packet > size would do to existing end system and NAT implementations. Looking > at the Linux code, UDP and UDP-Lite are integrated, so it doesn't barf > on UDP length less than packet length, but it looks like you can't use > the socket option to accept less checksum coverage unless it's a > UDP-Lite socket. > > But I don't think it matters if end systems and NATs need to be upgraded > to support this, since links already need to be upgraded to understand > partial checksums. > > Opinions? > > Tom P. > >> -----Original Message----- >> From: Michael Welzl [mailto:michawe@xxxxxxxxxx] >> Sent: Friday, November 20, 2009 11:59 AM >> To: Phelan, Tom >> Cc: gorry@xxxxxxxxxxxxxx; DCCP working group >> Subject: Re: Soliciting input on UDP encapsulation for DCCP >> >> > 3) Define UDP-Lite-in-UDP, then DCCP partial checksum becomes use >> > UDP-Lite and ensure that the DCCP header is covered. I can imagine >> > that >> > this would open quite the can of worms :-). >> >> Haha, I love that idea! >> >> But seriously, I do, it's a good one I think! >> >> Cheers, >> Michael > >
