Ethereal-users: [Ethereal-users] TCP checksum problems... (fwd)

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From: Jens Petersohn <jkp@xxxxxxx>
Date: Wed, 6 Jun 2001 13:13:22 -0500 (CDT)
Here's a dump of one of the packets that ethereal claims bad
TCP checksums on.

Frame 10 (60 on wire, 60 captured)
    Arrival Time: Jun  6, 2001 13:09:36.7119
    Time delta from previous packet: 0.000156 seconds
    Time relative to first packet: 5.386718 seconds
    Frame Number: 10
    Packet Length: 60 bytes
    Capture Length: 60 bytes
Ethernet II
    Destination: 00:00:0c:07:ac:b8 (00:00:0c:07:ac:b8)
    Source: 08:00:69:0b:8f:48 (08:00:69:0b:8f:48)
    Type: IP (0x0800)
    Trailer: 00000008BF09
Internet Protocol
    Version: 4
    Header length: 20 bytes
    Differentiated Services Field: 0x10 (DSCP 0x04: Unknown DSCP; ECN: 0x00)
        0001 00.. = Differentiated Services Codepoint: Unknown (0x04)
        .... ..0. = ECN-Capable Transport (ECT): 0
        .... ...0 = ECN-CE: 0
    Total Length: 40
    Identification: 0x1b9c
    Flags: 0x04
        .1.. = Don't fragment: Set
        ..0. = More fragments: Not set
    Fragment offset: 0
    Time to live: 60
    Protocol: TCP (0x06)
    Header checksum: 0xa8f8 (correct)
    Source: tardis.americas.sgi.com (128.162.184.103)
    Destination: 24-240-40-50.hsacorp.net (24.240.40.50)
Transmission Control Protocol, Src Port: 21443 (21443), Dst Port: 22 (22), Seq: 1680596439, Ack: 120121820
    Source port: 21443 (21443)
    Destination port: 22 (22)
    Sequence number: 1680596439
    Acknowledgement number: 120121820
    Header length: 20 bytes
    Flags: 0x0010 (ACK)
        0... .... = Congestion Window Reduced (CWR): Not set
        .0.. .... = ECN-Echo: Not set
        ..0. .... = Urgent: Not set
        ...1 .... = Acknowledgment: Set
        .... 0... = Push: Not set
        .... .0.. = Reset: Not set
        .... ..0. = Syn: Not set
        .... ...0 = Fin: Not set
    Window size: 49152
    Checksum: 0xb932 (incorrect, should be 0xeec6)

   0  0000 0c07 acb8 0800 690b 8f48 0800 4510   ........i..H..E. 
  10  0028 1b9c 4000 3c06 a8f8 80a2 b867 18f0   .(..@.<......g.. 
  20  2832 53c3 0016 642b ddd7 0728 e9dc 5010   (2S...d+...(..P. 
  30  c000 b932 0000 0000 0008 bf09             ...2........   

> Hi everyone,
> 
> I'm building Ethereal 0.8.18 sources on MIPS/IRIX. Eventhough
> the program builds fine and works great, the TCP checksum
> verification seems broken. The content of the packet are
> OK, I can pick out the data just fine.
> Approximately 75% of TCP packets complain of an invalid
> checksum.
> MIPS/IRIX is a big endian system that doesn't permit
> unaligned access.
> 
> Thanks in advance,
> 
> Jens Petersohn