Re: IPoIB-RC and Checksums

At 12:18 PM 11/18/2004, Yaron Haviv wrote:
>In GbE usually the NIC Tx Segmentation (large send) capability comes
>hand in hand with Checksum offload for greater efficiency (and zero
>copy)
>
>On UD we decided not to address checksum offloading, since we cannot
>guarantee that the node will not forward an un-checked packet
>
>Where as in RC we can have examples of devices that can guarantee
>checksum One example is an IB-IP gateway that always checksum outgoing and
>incoming packets, and can act as a remote IP NIC to the Host
>
>I suggest we include a checksum option in the CM Exchange  Where a node 
>can request that its peer will not checksum the packet for it And also 
>signal that he sends packets that are already checked  That can help 
>improve performance of IPoIB RC
>
>P.S. another note, we discussed in IETF was that we may want to
>mention/suggest (not mandate) use of SRQ for IPoIB-RC in order to
>preserve memory

I remain opposed to disabling checksums under any circumstances.  I do not 
believe there is a method to guarantee that a packet will not be routed by 
higher layers within the network stack without sticking one's nose way into 
the packet.  There is nothing that precludes an IB HCA from providing 
checksum off-load today through a private interface much as what is done 
with Ethernet today.   It isn't hard and presents no interoperability 
issues as it is a local optimization.  Attempting to do this as an 
optimization between endnode pairs makes this more complex and requires 
knowledge that may not be available between all combinations of endnode pairs.

Mike

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