[vortex] 802.1q VLAN problem

[Bogdan Costescu]

On Mon, 15 Apr 2002, SCHIMMEL Guillermo     TECHTEL wrote:

> 	Recentrly we have started tests in order to develop a vlan based
> network around the 3com vortex card.
> 	The problem is that the driver doesn't like the oversized 1504 bytes
> packets that the 802.1q module gives to him.
> 
> 	Are you aware of that problem? Do you intend to patch the driver?

Yes and maybe. First of all, you didn't say which cards/chips you are 
using; both Cyclone (905B) and Tornado (905C) have some support for 
over-sized frames, but I don't know about the older ones (as the 
expression "3Com vortex card" covers quite a lot of them).

I don't have the documentation at hand so the following is just from 
memory:

Cyclone and Tornado have support for up to FDDI (4.5 Kb) frames; 
there is one bit which enables this behaviour. However, I don't know if 
the VLAN support really needs to have over-sized frame support activated. 
There is support for ignoring the 4 bytes VLAN tag when computing 
checksums (in the Rx path or zero-copy Tx path) and this can also be 
enabled easily.

There were some messages on netdev list some weeks ago (unfortunately 
the archive at http://oss.sgi.com/projects/netdev/archive/ seems to be 
dead, maybe you can find another one) related to this which included a 
link to a patch.

My feelings on the matter are mixed: I do understand that VLANs are useful 
(or even required) in some situations or that generally speaking 
over-sized frames might be useful for more efficient transmission (less 
header overhead). However, enabling over-sized frames (there is nothing 
like allow over-size frames up to a variable limit; you can only turn on 
or off the over-sized frame support) can have bad effects due to: limited 
FIFO buffers on the card (2 Kib), limited buffers on the switch (today's 
switches are almost all store-and-forward), incompatibilities which can 
appear if the recipient(s) of the packet do(es) not have support for 
over-sized frames (I think that there are switches that do GB Jumbo -> 
Ethernet conversions, but there are not some to do arbitrary size -> 
Ethernet conversions). In any case, I'd be willing to spend some time 
helping adding support to the driver.

Bogdan Costescu

IWR - Interdisziplinaeres Zentrum fuer Wissenschaftliches Rechnen
Universitaet Heidelberg, INF 368, D-69120 Heidelberg, GERMANY
Telephone: +49 6221 54 8869, Telefax: +49 6221 54 8868
E-mail: Bogdan.Costescu@IWR.Uni-Heidelberg.De