From owner-freebsd-net  Fri Jun 14 11:20:21 2002
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Date: Fri, 14 Jun 2002 11:19:02 -0700 (PDT)
From: Julian Elischer <julian@elischer.org>
To: John Polstra <jdp@polstra.com>
Cc: net@freebsd.org, spe@selectbourse.net
Subject: Re: Problem using ng_ether
In-Reply-To: <200206141650.g5EGo6c12182@vashon.polstra.com>
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On Fri, 14 Jun 2002, John Polstra wrote:

> 
> I'm not Julian or Archie, but I think I know what the problem is.
> The xl interface supports hardware checksum offloading, while the rl
> interface does not.  When checksum offloading is used, the NIC itself
> checks the checksums of received packets, and calculates the checksums
> of transmitted packets.
> 
> When transmitting packets the TCP/IP/UDP stacks set flags in the mbuf
> headers which tell the interface to calculate the checksums itself,
> if the interface has reported that it is capable of that.  In that
> case, the TCP/IP/UDP layers don't bother to calculate the checksums
> in software.  If you pass the mbufs directly from the upper hook
> to the lower hook inside the kernel, the relevant mbuf flags are
> preserved, and it all works as one would hope.  But since you're doing
> the forwarding between hooks at the application layer, the mbuf flags
> are lost.  The TCP/IP/UDP stacks assume the NIC will calculate the
> checksums of transmitted packets (because it said it could).  But the
> flags telling the NIC to do so are lost.

I think you are 'spot on'

I guess we need to figure out a way make the activation of
the netgraph hooks automatically disable H/W checksumming by default.
(or at least on how to codify metadata such as that so it can be
passed through user space successfully)

Julian



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