Hi Robert, If you're using DecIPTTL, that element already updates the IP checksum incrementally.
Eddie Robert Ross wrote: > We're modifying the TTL on outgoing packets, so I assumed we had to > modify the checksums. Now that I think about it, we probably only need > to set the IP checksum and not the UDP or TCP checksum. I'll try that > out... > > > -----Original Message----- > From: Eddie Kohler [mailto:[EMAIL PROTECTED] > Sent: Tue 5/1/2007 4:03 PM > To: Robert Ross > Cc: [email protected] > Subject: Re: [Click] SetUDPChecksum dropping packets > > Robert, > > Yes, it is impossible to set a UDP packet's checksum without the whole > packet. > Fragments won't work: not all the data is available. I've just > checked in a > change so that IP fragments, or too-short packets, are emitted on the 2nd > output, or dropped if there is no 2nd output. > > I am surprised you need to fix TCP and UDP checksums on input packets. > Usually that is not necessary for input packets; and in fact fixing the > checksums is a bad idea, because it can hide errors elsewhere. > SetUDPChecksum > is generally used when you're generating packets from scratch. > > Eddie > > > Robert Ross wrote: > > We have found what appears to be a bug (or undocumented feature) in > SetUDPChecksum where packets sent into this element are somehow > discarded. In the attached config, you will see an ElementClass called > FixChecksumsDebug which counts packets into each checksum fixer and > counts packets out the other side. We have found that, under specific > circumstances, packets sent into SetUDPChecksum never come out the other > side. This is true of the latest version from SVN. > > > > As to the particular situations for this problem, it seems to happen > only with fragmented packets. In a set of packet fragments, the first > portion (offset +0) always seems to be discarded when going through > SetUDPChecksum. There is no indication that I have found as to the > reason for the discard. > > > > Has anyone else seen this issue? Does SetUDPChecksum have a second > output that is undocumented? Any suggestions? > > > > Thanks, > > > > Robert Ross > > > > > > ------------------------------------------------------------------------ > > > > _______________________________________________ > > click mailing list > > [email protected] > > https://amsterdam.lcs.mit.edu/mailman/listinfo/click > _______________________________________________ click mailing list [email protected] https://amsterdam.lcs.mit.edu/mailman/listinfo/click
