kern/91032: invalid IP checksum under if_bridge(4)+em(4) combination

[Ralf S. Engelschall]

The following reply was made to PR kern/91032; it has been noted by GNATS.

From: "Ralf S. Engelschall" <rse at FreeBSD.org>
To: Gleb Smirnoff <glebius at FreeBSD.org>
Cc: FreeBSD-gnats-submit at FreeBSD.org
Subject: Re: kern/91032: invalid IP checksum under if_bridge(4)+em(4) combination
Date: Wed, 28 Dec 2005 22:05:24 +0100

 On Wed, Dec 28, 2005, Gleb Smirnoff wrote:
 
 > On Wed, Dec 28, 2005 at 09:45:38PM +0100, Ralf S. Engelschall wrote:
 > R> Another workaround would have been to put into the box a 100baseTX NIC
 > R> driven by fxp(4) instead of the 1000baseTX NIC driven by em(4). Because
 > R> the combination of if_bridge(4) and fxp(4) I've running fine with mostly
 > R> the same configuration on another server.
 >
 > Not all fxp(4) cards can do checksum offloading. If you put the one
 > that can't, then the seconds workaround doesn't differ from the first
 > one. An problem ins't em specific in this case probably.
 
 Yes, of course. I think we actually have two problems here: first
 if_bridge(4) unconditionally clears the checksum, although I see no
 reason for doing this in case of non-generated and just forwarded
 packets. And the second problem (that the checksum is not re-calculated)
 is more em(4) related or at least related to the packet flow through the
 system in case a network device is attached to if_bridge(4).
 
 > So important questions is: does the fxp card you used can do offloading?
 
 According to the ifconfig(8) output on that box and the fact that I do
 not see any "TXCSUM" in the "options", I would say this particular card
 is not capable of doing "checksum offloading":
 
 | net0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
 |         options=8<VLAN_MTU>
 |         inet 172.16.120.130 netmask 0xffff0000 broadcast 172.16.255.255
 |         ether 00:07:e9:d9:4d:43
 |         media: Ethernet autoselect (100baseTX <full-duplex>)
 |         status: active
 
 Ok, then it is clear why the bridging works fine on this FreeBSD
 6.0-STABLE box while it was broken (and required the "ifconfig em0
 -txcsum") on the other box.
 
 --
 rse at FreeBSD.org                        Ralf S. Engelschall
 FreeBSD.org/~rse                       rse at engelschall.com
 FreeBSD committer                      www.engelschall.com