[nsp] Help: Packet TCP checksum errors (IP checksum is OK)

Goldberg Alain (IT) alain at towersemi.com
Thu Feb 12 03:33:58 EST 2004

Hello all,
My scenario is:
Host 1 & 2 are unix gears.
L1 & L3 are 100Mb Ethernet
L2 is 100Mb ethernet encapsulated by a HUNT to ATM over SDH (STM1) and
vice versa.
Reported problem: FTP file transfers fails from Host-1 to Host-2.
My investigation results:
0) Files transfers OK if not via L2 link.
1) The problem is consistent with specific files, other files have no
2) The same files can be transferred after compression.
3) Tried to send files filled with binary zero -> it works
4) Tried to send files filled with 0xff -> it works.
5) Used Ethereal to try to figure-out the problem, 
 - it appears that it fails at a consistent location (for each file)
 thus it's not related to the file length.
 - When it fails I see that the FTP sender side is sending some 3 or 4
 that arrive with correct IP checksum and incorrect TCP checsum, and
then the file transfer fails.
6) Sometime, FTP (when using ncftp) will finnaly succeed to transfer the
L2 is a leased line, the provider says he don't see any problem with the
the "hunt" is not under my management.
Any ideas what to investigate ? how to fix ? Workarounds ? 

Alain Goldberg - Network manager - CCDA/CCNA AGG-RIPE

Tower              Semiconductors           LTD.

Tel : 972-4-6506003        Fax : 972-4-6547788        Cell:



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