[vortex] 802.1q VLAN problem

Ben Greear greearb@candelatech.com
Wed Apr 17 13:52:02 2002

Bogdan Costescu wrote:

> 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.

Just enabling the larger frame sizes does not mean that all of a sudden we'll
be using 4k packets, so I think many of your concerns relating to switch support
etc are not big problems.  Note that the standard VLAN setup does not increase
the MTU of the ethernet interface, so the max sized packet will be just 4 bytes
bigger than normal.

If there are performance tradeoffs, then just having a module variable to
toggle VLAN or regular support would be much better than nothing.


Ben Greear <greearb@candelatech.com>       <Ben_Greear AT excite.com>
President of Candela Technologies Inc      http://www.candelatech.com
ScryMUD:  http://scry.wanfear.com     http://scry.wanfear.com/~greear