Re: IPoIB-RC and Checksums

Let me try to be clear with my understanding of the position of the
IESG.

Not sending IP/TCP header checksums in an IP packet is a non-starter.

Using checksum offload technologies to accelerate these computations is
one thing, sending the packets with a checksum value of 0 and not
checking on receive is another.

I talked with Allison Mankin in Washington D.C. and she was terrified of
"raw" IB packets (ie. RC/UD) getting out on the internet and messing
things up because there aren't congestion controls built into these
protocols that will behave correctly with IP.

I would caution the group in considering removing checksumming from
packets when it is relatively cheap for hardware to be added to
HCA/HBA's that can calculate the checksum before sending it on the wire.

Some comments inline.
Bill

On Thu, 2004-11-18 at 22:18 +0200, 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 
I do not believe either the IEEE or the IETF have ever addressed
checksum offloading.  I am not sure that there is a protocol piece to do
here - it is an implementation issue between the OS and the hardware.

> 
> 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  
Here you are talking about a different device.  And again - I am not
sure that there is an IETF standard here.  Much like the IETF does not
want to standardize iSER over IB (with no IP in the middle) I don't
believe it wants to standardize Host/OS <--> NIC interactions.  The
device you are proposing does not have (require might be a better word
here) an IP interaction between the HOST and an IP ofload NIC (I have
heard of several implementations of things called a VNIC or virtual NIC)

I do not believe there is a proposal to standardize a VNIC protocol -
and if there was, I do not believe that this is IETF work.
> 
> 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
> 
I believe this is a non-starter in the IESG - Margarete, can you confirm
this ?

> 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 
> 
Again, in the spirit of Wire protocol vs. Implementation.  I think this
is an implementation issue that will not change wire protocols at all.
Is there a point where using SRQ vs. Not Using SRQ would have to change
the wire protocol ?

If not - lets not say anything.
If there is - I would be very interested in understanding.

Bill