Transcript Checksum Errors - The Technology Firm

TCP Checksum Errors
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What’s the Problem
 As I was using my Protocol Analyzer, I noticed a bunch of packets that had TCP
CRC errors.
 The packets were all from my PC, but everything was working fine.
 I noticed that there weren’t any IP checksum errors, UDP Errors or ICMP checksum
errors.
 I also noted that SYN, FIN TCP packets were fine as well.
Thanks goes to my friends at Wildpackets for informing me as to what was happening.
 I just thought I would put some more info together for myself and for all my faithful
readers.
 Enjoy
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TCP/IP Task Offload in NDIS 5
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From: http://www.microsoft.com/hwdev/tech/network/taskoffload.asp
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Network adapters with appropriate NDIS 5 (and later) miniport drivers can increase the system performance by
supporting hardware offload of Transmission Control Protocol/Internet Protocol (TCP/IP) CPU-intensive tasks. For
example, with TCP/IP checksum offload alone, up to 30 percent performance gain in CPU utilization has been
seen in testing at Microsoft. The NDIS interface and the TCP/IP transport have been enhanced to allow miniport
drivers to indicate hardware support for performing:
 TCP/IP checksum calculation.
 TCP/IP segmentation.
 Internet Protocol Security (IPSec) Encryption Ciphers and Message Digests.
During initialization or when an interface appears as a Plug and Play event, the TCP/IP driver will query the
miniport through the NdisRequest() mechanism with an object ID (OID) of OID_TCP_TASK_OFFLOAD in order to
find out which offload capabilities the network adapter supports.
For each task the network adapter can offload, it will return an NDIS_TASK_OFFLOAD structure that contains an
indication of the task supported and parameters specific to that task. The protocol then enables the appropriate
tasks by submitting a set request containing the NDIS_TASK_OFFLOAD structures for those tasks. At this point,
these tasks are enabled for offload. The network adapter will receive information specific to the task on a perpacket basis, along with each packet.
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Checksum Offload
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On the send side, the network adapter that supports this offload will calculate checksums that are needed and for
which it has indicated the capability of performing. For the TCP checksum, the Microsoft transport will calculate
the TCP pseudo header checksum and place this value in the checksum field, so that the network adapter can
calculate the correct TCP checksum without touching the IP header.
On the receive side, the network adapter will fill in a NDIS_TCP_IP_CHECKSUM_PACKET_INFO structure and
set the appropriate bits. If for any reason the network adapter cannot perform the checksum, it will not set any bits
and indicate the packet. TCP/IP will then look at this and calculate the appropriate checksum itself.
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What Does It Look Like With Wildpackets
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What Does It Look Like With NAI
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What Does It Look Like With Fluke
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What Does It Look Like With Ethereal
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Card Configuration Details
 In my case I had a 3COM 3C920 and I simply disabled the Tx Checksum Offload
and everything is back to normal.
© 2003, The Technology Firm
WWW.THETECHFIRM.COM