Re: [PATCH] IB/ipoib: CSUM support in connected mode

[Yuval Shaia <yuval.shaia-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

On Thu, Jul 30, 2015 at 03:58:13PM +0200, Yann Droneaud wrote:
> Hi,
> 
> Le jeudi 30 juillet 2015 à 04:46 -0700, Yuval Shaia a écrit :
> > This enhancement suggest the usage of IB CRC instead of CSUM in IPoIB 
> > CM. IPoIB CM uses RC (Reliable Connection) which guarantees the 
> > corruption free delivery of the packet.
> > 
> > InfiniBand uses 32b CRC which provides stronger data integrity 
> > protection compare to 16b IP Checksum.
> 
> InfiniBand 32b CRC <=> Ethernet 32b CRC, it's link layer, layer 2.
> 
> IPv4 checksum is at another level, it's internet layer, layer 3.
> 
> >  So, there is no added value that IP/TCP Checksum provides in the IB 
> > world.
> > 
> 
> Sure, IPv4 checksum is a thing of the past: checksum was dropped from
> IP header in IPv6: it assumes the lower layer, such as Ethernet,
> provides the required integrety check.
> 
> I think not checking the IPv4 checksum should be a choice, carefully
> thought, for inside a fabric, as I understand your proposal, packet
> with invalid checksum will be allowed to go in/out of the fabric.
Yes, this is why it is controlled by module parameter.
Maybe a better choice would be to default it to 0.
> 
> It sound like it's a departure from the behavior one can expect from an
> IPv4 network stack.
It should be considered as network-fine-tuning parameter so if admin knows his fabric he can use it.
> 
> > The proposal is to tell network stack that IPoIB-CM supports IP 
> > Checksum offload. This enables the kernel to save the time of 
> > checksum calculation of IPoIB CM packets. Network sends the IP packet 
> > without adding the IP Checksum to the header. On the receive side, 
> > IPoIB driver again tells the network stack that IP Checksum is good 
> > for the incoming packets and network stack avoids the IP Checksum 
> > calculations.
> > 
> > During connection establishment the driver determine if peer supports
> > IB CRC as checksum. This is done so driver will be able to calculate
> > checksum before transmiting the packet in case the peer does not 
> > support this feature.
> > 
> 
> Two questions:
Three :)
> 
> - What will see tool such as wireshark/tcpdump when sniffing checksum
Zero or what ever the networking layer puts in csum when H/W supports CSUM-offloading.
Please note that with this patch driver still supports backward computability (per connection).
This means that for connections with peer which does not support this functionality you expect to see this value filled with checksum.
> -less IPv4 packets sent/received on IPoIB interface ?
No
> 
> - What might happen if such checksum-less IPv4 packet is later routed to a different IPv4 network ?
As noted above, for network that is opened to outside world this feature should be blocked.
In general i would say that if a layer 2 terminator device (e.x router) exist in the fabric - this feature can't be used and must be blocked.
With this limitation it still worth use it because of the reason of increasing throughput
> 
> > With this enhancement throughput is increased by 60%.
> > 
> 
> > Signed-off-by: Yuval Shaia <yuval.shaia-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
> 
> Regards.
> 
> -- 
> Yann Droneaud
> OPTEYA
> 
> 
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