Organization: Swedish Institute of Computer Science, Kista
Date: Mon, 9 Nov 1992 18:17:08 GMT
Lines: 42
goldstein@carafe.enet.dec.com (Fred R. Goldstein) writes:
> Just in case anyone's wondering, SSCOP (service-specific
> connection-oriented protocol) is the service-specific sublayer that sits
> above AAL3/4 OR AAL 5 to create a connection-oriented data link
> service.
> ...
> Or if you want "end-to-end ATM", it can be used to deliver
> a transport-like service.
Sigh... I thought everyone had learned about the end-to-end argument by
now. The link layer (ATM) has no business providing a function that higher
layers also have to provide. If this synopsis is accurate, SSCOP sounds like
a remarkably dumb idea.
> Personally, I think datagramme services can use it too: If you have a
> lot of loss in the ATM layer, then TCP recovery (single selective
> recovery, at best) will not be very effective at high speeds over
> multi-hop (or other long delay) links. For pure local use, it might be
> overkill, but it's not so hard to build so why not?
TCP will run just fine over multihop gigabit links. There's some
misunderstanding here.
Most TCPs currently rexmit one segment per rtt for two reasons: (1) the
receiving TCP doesn't use selective acks, so all that is known is that a segmentat the edge of the window was lost. So the only segment that the sending
TCP knows needs retransmission is the first segment, so that's the one to send;
(2) the usual cause of loss right now is congestion, which makes it all the
more important to only launch segments known to be missing (to avoid
overloading the network further with useless duplicates).
Another problem for TCP with high-bandwidth paths is that the window size
is too small.
However, there are standard TCP extensions for high-bandwidth paths that
extend the window size and provide for selective acks and better RTT timing.
(See RFC 1323 -- which is a proposed standard and note these extensions are
deployed in some implementations, like Cray's). Testing to date shows that
this TCP (which interoperates with older versions) runs at gigabit speeds
