IP over 802.3
From: David Wilburn (dwilburn_at_mNOSPAMitre.org)
Date: 08/31/03
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Date: Sun, 31 Aug 2003 18:57:40 +0200
All,
I've got a problem that I believe is related to data link layer
encapsulation from a WAP device trying to talk IP to a linux box over
802.3 encapsulation.
I recently purchased a Belkin 802.11G WAP and connected it up to my
wired LAN. My wired LAN consists of a PC running RH8 (2.4.20-20.8 stock
RH kernel) and a PC running Win2k, both with Intel Etherexpress Pros in
them. My laptop is connecting via Netgear 802.11b wireless card, and is
also running RH8 with the same kernel. Connecting the whole mess to the
Internet is an LG LAM200R DSL modem/router/switch/NAT combo device.
My laptop talks just fine with the DSL modem, the Win2k box, and the
Internet at large. It does NOT play nicely with the other wired HR8
box, however.
I think that I've traced the problem to a data link layer encapsulation
problem. The wired network RH8 PC can see the packets from the laptop,
but ignores them (this is NOT a firewall issue). The laptop can see the
packets from the workstation and replies to them just fine, although the
replies are once again seen but not processed by the workstation. I
took a peek from each system with Ethereal. The laptop sees all packets
as Ethernet II-encapsulated packets. The win2k box (winpcap and
ethereal equipped) sees its own packets as Ethernet II, but sees the
laptop's packets as 802.3-encapsulated. The RH8 workstation sees the
same thing that the Win2k workstation does. The difference is that the
Win2k box can handle the 802.3-encapsulated packets, while the RH8 box
chooses to ignore them.
There are basically two flavors of Ethernet out there (correct me if I'm
wrong), the RFC 894-defined encapsulation that is used for IP and
related protocols like ARP. Then there's the IEEE-defined 802.3
encapsulation, which is basically for everything else. I don't recall
ever having seen 802.3-encapsulated IP packets in the wild before, and I
don't know whether it's even allowed or not. I believe that the
Ethernet II data link layer identification in Ethereal corresponds to
RFC 894 spec.
So it would appear that the Belkin WAP is taking the RFC
894-encapsulated IP packets and is de-encapsulating them and then
reencapsulating them in 802.3 to feed it over to the wired side.
Is 802.3-encapsulated IP "correct?" If not, where can I find this
stated somewhere authoritative, so that I can quote it to Belkin
engineers chapter and versus if necessary?
Is the Linux system "correct" in refusing to process 802.3-encapsulated
IP traffic?
Has anyone else seen this kind of goofy encapsulation work from other WAPs?
Thanks!
-Dave Wilburn
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