Re: TCP limitations (was Re: ISCSI: Urgent Flag requirement violates TCP.)

[csapuntz@cisco.com]

> Question for you.. I remember reading over Costa's RDMA 
> proposal and it seems to me that he proposes using TCP options.
> How does this interact with the use of SACK TCP? I know the
> WG has discussed the need for SACK (or possibly not) and I
> am just curious... will the use of a RDMA option limit you from
> using SACK?
> 

I guess you were talking to Bernard, but I'll interrupt here.. :-)

My original proposal took the form of a TCP option. The RDMA option
was a large TCP option, about 12-16 bytes. Since the TCP option area
is only 40 bytes total, some were worried that the RDMA option would
crowd out/limit SACK.

Since the original proposal, Jim Williams of Giganet has shown that
a shim protocol is just as viable.  A shim protocol, unlike a TCP
option, appears in the TCP stream but under the application
protocol. Jim's VI/TCP <draft-dicecco-vitcp-01.txt> sits in a TCP
stream and encapsulates VI-style RDMAs and message.

Given that shim protocol requires no changes to the TCP in the sender,
it is currently my favorite way of doing RDMA.

-Costa