Subject: Re: bad tcp sums
To: Martin Husemann <>
From: Andrew Brown <>
List: tech-net
Date: 04/15/2001 07:55:17
>I have a problem connecting to a pop3 server when connected over PPPoE
>(MTU 1492), while everything works using an ISDN connection (MTU 1500).
>I get bad tcp cksums for fragmented packets.

off the top of my head, i'd say that you should expect to.  fragments
are necessarily incomplete so the total checksum will be not
calculable.  only after reassembly should you be able to check the
checksum.  this is ip fragmentation, after all, not tcp fragmentation.

>Forcing the MSS of the connection to 1412 fixes the problem, but a router
>probably should not mess with MSSs negotiated by the peers, or do I
>misunderstand this? (The rp-pppoe package has an otpion to clamp MSS.)
>Does this ring a bell for someone? I'm suspecting a broken stack on the
>remote router (probably a Linux system). The NetBSD client is running

sorry...i've no idea.  just wanted to toss in my two cents above.

|-----< "CODE WARRIOR" >-----|             * "ah!  i see you have the internet (Andrew Brown)                that goes *ping*!"       * "information is power -- share the wealth."