netdev - 802.1AD packets - Kernel changes ether type from 88A8 to 8100 on all packets.

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Date:	Fri, 11 Apr 2014 11:01:21 +0000
From:	Dennis Punjabi <linuxdev@...s.bz>
To:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: 802.1AD packets - Kernel changes ether type from 88A8 to 8100 on
 all packets.

Background:
1) I need to setup a Linux software bridge and ensure that packets coming in on one bridge port exit unchanged on another bridge port, like a normal HW bridge.
2) I need to use the new vlan filtering capability of the bridge. Good news of this feature is made in the kernel announcement of release 3.9 @ http://kernelnewbies.org/Linux_3.9
3) I need to test what presently happens to 802.1AD tagged packets when bridge vlan filtering is enabled because I also need support for Q-in-Q.

I am using a packet generator (Ostinato) and creating 802.1AD packets as follows:

(MAC_HDR)           (8021AD_HDR)   (8021Q HDR)  (PAYLOAD)

(DA | SA | TYPE)    (TCI | TPID)        (TCI | TPID)    (TEST PAYLOAD - ARP BCAST PACKET)

(DA | SA | 88A8)    (TCI | 8100)         (TCI | 0806)    (TEST PAYLOAD - ARP BCAST PACKET)

(hopefully the alignment is ok above, if not I apologize and hope its clear enough)

Even before setting up the the bridge, I want to ensure that packets are being received properly to my system.
I do a tcpdump output for the port receiving the generated packets.

What I notice is that the 88A8 TPIC/ether type get changed to 8100.
Ie, all packets with ether type 802.1AD get changed to 802.1Q type, ie 8100.
I initially tested this on a 3.14 vanilla kernel.

I recompiled my 8139too.c driver and printed printk the packets in the driver before they are handed over to netif_receive_skb.
Before being passed to the kernel networking stack the packet is as it is supposed to be, ie it has the correct 88A8 ether type.

I went ahead and tested this on kernel 3.12, 3.11, 3.10. I Saw the same result.

I also tested it on 3.9, 3.8 and 2.6 and these kernels did not change the 88A8 to 8100.

Is this intentional or a bug? Does this have something to do with the addition of the new feature (vlan: Add 802.1ad support) announced in kernel 3.10? http://kernelnewbies.org/Linux_3.10

I have been following the net-dev mailing lists and want to test the bridge 802.1AD filtering feature discussed here http://www.spinics.net/lists/linux-ethernet-bridging/msg05207.html but cannot do so if the bridge does not even get the 802.1AD tagged packets to begin with.

Assuming I don't want to use filtering and just want a simple bridge. The expected behaviour is that the bridge must not change my packet and should simply pass it along. Is my thinking correct?

Thanks for any help provided.

Dennis
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