On Thu, Jul 08, 2004 at 07:54:55AM -0700, Tejas Vora wrote: > Hi, > > I am facing a starnge problem regaridng PPP connection > from Windows machine to the linux server. <snip> > The problem is once in a while on the client machine I > see lots of packet with Incorrect TCP checksum error. > Normally I receive the TCP packets properly, but > suddenly it starts showing the packet with having > WRONG TCP checksum. <snip> > I read so many mails in the previous mailing list and > found out that this problem is due to the VJ > Compression - and if I turn of this feature in Windows > machine - then everything seems to be working fine - > except little bit lower downloading speed. > > Now this is a very strange problem. The project we are > working requires the FTP DOWNLOADING TESTING with IP > Compression on on the client side while connecting to > the server. So I have to resolve this problem without > disabling the IP Compression on Windows client > machine. Basic problem. VJ Header Compression doesn't work on a medium that reorders or loses packets (ethernet). It basically replaces bits of the IP header with "copy this from the previous packet". But if the packets get out of order or lost, you get the wrong header. BOOM. There is no way around this, except run PPP over a medium that notices when a packet is lost or reordered and sends a message back to reset the compression algorithm when necessary. We see the same problem with L2TP over ethernet and we just disable VJ compression. There, in principle, it could be solved by preprocessing the packets, but we havn't done that yet. Curious as to why the timestamps would fix it, but I imagine that has some interaction with the header compression, but who knows. Hope this helps, -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a > tool for doing 5% of the work and then sitting around waiting for someone > else to do the other 95% so you can sue them.
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