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

Yann Droneaud
2015-07-30T13:58:13Z

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.

It sound like it's a departure from the behavior one can expect from an
IPv4 network stack.

> 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:

- What will see tool such as wireshark/tcpdump when sniffing checksum
-less IPv4 packets sent/received on IPoIB interface ?

- What might happen if such checksum-less IPv4 packet is later routed to a different IPv4 network ?

> With this enhancement throughput is increased by 60%.
>

> Signed-off-by: Yuval Shaia <yuval.shaia-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>

Regards.

--
Yann Droneaud
OPTEYA