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Re: wm* hardware assist breaking gif tunnels

Subject: Re: wm* hardware assist breaking gif tunnels
From: Darren Reed
Date: Mon, 10 Sep 2007 02:27:24 UTC
Newsgroups: fa.netbsd.tech.net

David Young wrote:
On Thu, Sep 06, 2007 at 05:43:00PM +0000, John Klos wrote:
> Any suggestions?

Taking a guess, maybe wm h/w sets received packets' embedded checksums
to 0 (or anything not equal to the actual checksum) when h/w assist
is active?  Maybe wm h/w expects for an embedded checksum to be 0 in
transmitted packets, also?  I have seen wm(4) lose in a bridge until I
turned off h/w checksum; it may have been for that very reason.
And I see this comment in ip_output(),

                 * Always initialize the sum to 0!  Some HW assisted
                 * checksumming requires this.
                ip->ip_sum = 0;
Correct.  For some NICs, the hardware checksum is computed by specifying
a start and end offset: if ip_sum isn't 0, the start is the beginning of the IP header and the end is, well, at the end, the checksum placed in ip_sum will be wrong.

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