Re: MTU and TCP transmit offload.

[Ben Greear <greearb@candelatech.com>]

On 09/21/2011 03:11 PM, Rick Jones wrote:
> On 09/21/2011 02:06 PM, Ben Greear wrote:
>> We saw something interesting while doing some testing
>> on 3.0.4.
>>
>> We configured 2 Ethernet NICs with standard 1500 MTU, and added
>> a mac-vlan on each, with MTU of 300. The goal was to generate as
>> many ~300 byte TCP packets as possible, for load testing purposes.
>> We configured our tool to open sockets on the mac-vlans and send/receive
>> TCP (IPv4) traffic.
>
> Presumably one could instead set static PathMTU entries in the routing tables and accomplish the same thing as you did with the mac-vlans?
>
>> This actually seems to work quite nicely, allowing user-space to
>> do large writes (24k in our case), and it appears have lots of
>> small packets on the wire. We still need to sniff with external
>> system to verify this..but packets-per-second counters look good.
>>
>> Evidently this all works because macvlans know that the NIC
>> can do TSO, and the '300' MTU is passed in the big packet
>> given to the NIC.
>>
>> This got me thinking...at least for my purposes, it would be
>> nice to have a per-socket 'MTU' setting. The idea is that
>> you could ask the NIC to do the TSO at whatever 'mtu' you
>> wanted, without having to resort to mac-vlans with artificially
>> small MTU.
>>
>> So, is there any interest in supporting such a socket option?
>>
>> I can't think of any use besides TCP traffic load testing, but
>> perhaps someone else can think of one? Or, is load-testing
>> enough?
>
> Isn't that covered by setsockopt() support for TCP_MAX_SEG? With TSO what gets passed to the NIC isn't the MTU, but the connection's MSS derived (in part at
> least) from the MTU of the egress interface. If one had made a setsockopt(TCP_MAX_SEG) call prior to the connect() or listen() call, presumably that would have
> influenced the MSS exchange at connection establishment.

Ohh, that looks promising!

I'll give that a try.

Thanks,
Ben

>
> rick jones


-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com