Re: More notes from Haifa

[Brent Callaghan <Brent.Callaghan@eng.sun.com>]

Matt Wakely writes:
> These issues are a real detriment to the use of TCP as an iSCSI
> transport.  The benefits of TCP are it's "reliable" delivery and
> congestion avoidance.  The disadvantage is there is no "framing".

In practice, this needn't be a problem.

The NFS protocol started out as a UDP protocol, but now more
commonly runs over TCP connections because TCP is so much
better at managing congestion and reliable delivery.

NFS is built on an RPC protocol that assumes discrete messages
being passed back and forth on the TCP streams.  The RPC messages
are mapped onto the stream with a simple record marking protocol
(see RFC 1831 Section 10. "Record Marking Standard").

The record marking is simple, each message has a 32 bit
value prepended that gives the length of the message.
That's it.

TCP's reliable delivery makes certain that there's never a
sync problem.  If the TCP connection is lost, for whatever
reason, then you a bit of higher level code to
retransmit any unacknowledged message.  This works very
well for NFS - clients and servers crash, connections are
lost, but there's never a sync problem.

	Brent