transport protocols

[Rod Van Meter <rdv@Network-Alchemy.COM>]

Sorry to be so late getting in on this conversation.  There was a
discussion of transport protocols a couple of weeks ago, and I wanted
to contribute my $0.02.

When I was at Quantum, we investigated heavily a number of transport
protocols for possible use over IP:

* FC classes 2 & 3
* TCP
* T/TCP
* XTP
* VMTP
* ST
* lightweight reliability over UDP

I no longer have my notes (they stayed at Quantum), but if memory
serves:

* FC 2 & 3: forget it.  Too dependent on a reliable lower layer.  All
  2 really gets you is faster notification of errors -- you still fail
  out to app-level recovery on dropped packets.

* TCP: the best choice.  The problem is efficiently multiplexing
  multiple commands and finding message boundaries -- issues the TCP
  option draft here addressed.

* T/TCP: I like it.  There are apparently some security concerns.  For
  me the show-stopper, though, is that it's designed for
  single-threaded RPC.  A T/TCP connection starts, does its thing,
  shuts down, then is ready for the next one.  If you want two
  concurrent I/Os, you need multiple T/TCP streams.

* XTP: another good option, with a few advantages over TCP (simplicity
  was an XTP goal, partially reached), but not compelling enough.
  Inadequate congestion control; by the time you add it, you're
  probably close to TCP in complexity.  Not firewall friendly?

* VMTP: a little long in the tooth -- would need a serious
  update. Rate-based flow control not useful over today's Internet
  infrastructure, could still be made to work in a LAN.  Forwarded
  RPCs (command to node A, response from node B) is quite intriguing
  for more complex storage systems, but not really needed for SCSI
  over IP.  Not firewall friendly.

* ST: a nice, lightweight protocol, but I'm concerned about a)
  assumptions about a reliable link layer, b) latency sensitivity, c)
  congestion control.  Probably fine in a LAN, not useful in a WAN.


I've got a long list of references on the above, plus more, if
anyone's interested.

		--Rod