Re: TCP speed

[Spencer Shepler <shepler@eng.sun.com>]

Y P Cheng wrote:
> 
> Julo wrote:
> >Our experience is the same. TCP is FAST.
> >The only remaining trouble is memory copy from TCP buffers
> >to application buffers. Unless
> >handled properly this may slow you down considerably.
> 
> The issue is not the TCP memory to memory copy speed, it is the latency time
> of receiving TCP acknowledges.  Between two endpoints of New York and Los
> Angeles, latency is in milliseconds if not in seconds.  On a one-gigabit
> network, for each millisecond there are 100K of data, or 66 1.5K datagrams
> being transferred.  In fibre channel, there is this EE-credit, End-to-End.
> If the sending party has 10 EE credits, it can't send more than 10
> datagrams.  EE-credit manages the TCP sliding window currently discussed in
> iSCSI.  After sending 10 datagrams, one must wait for acknowledges that may
> take several hundred milliseconds to come.
> 
> I do believe TCP is a wrong protocol for iSCSI.  A SCSI request from an
> initiator is inherently acknowledged by its response from a target.
> Therefore, UDP for iSCSI is a better choice.  NFS is implemented on UDP.

And without additionly significant work, NFS over UDP does not have
any form of congestion control and running NFS/UDP cross country should
never be done.  It bad news. :-)

The NFSv4 specification requires the use of a transport that provides
congestion control.  NFSv4 does not mandate the use of TCP but does 
mandate the use of congestion control for comforming implementations.

Spencer