TCP checksum offloads - slow internet response times

mbroberson1 · ‎10-21-2008

Has anyone seen an issue were a client was having internet slowness issues (with using a proxy server) and upon investigating a packet sniff revealed bad checksums and the possible cause is "TCP checksum offload"? The bad checksums are only sourced from the client side.

Thanks

mklute · ‎10-21-2008

Heyas,

That is a normal error when checksum offload is enabled on the NIC. In windows you can change this setting in the network card properties of most NICs and see if it makes a difference.

see http://wiki.wireshark.org/TCP_checksum_offload

mbroberson1 · ‎10-21-2008

Can is cause (being turned on or off) issues related to slowness?

Thanks

mklute · ‎10-21-2008

It is possible, but doubtful, especially on a desktop/laptop system.

my biggest issues are usualy duplex mismatches or bad cables

on windows try..

pathping proxy

should how if pakets are dropping or just being slow.

mbroberson1 · ‎10-21-2008

What if you had say a Microsoft ISA server and the internal nic had this enabled, but the external nic had it disabled or vice versa. Could this lead to an issue?

Thanks

mklute · ‎10-21-2008

On a server, yes it could be a larger problem. There were some issues with 2003sp2 like here, but ISA server is not mentioned...

http://support.microsoft.com/kb/945977/en-us

If in doubt turn it off.

If your server is not maxed out on proc usage, you most likely not notice if it is turned off.

mklute · ‎10-21-2008

fyi.... the tool to diagnose the issue described above is call poolmon, documented here....

http://technet.microsoft.com/en-us/library/cc737099.aspx

you would look for growth in the MmCm pool.