[e2e] TCP in outer space
fred at cisco.com
Mon Apr 9 15:01:32 PDT 2001
There are at least two things worth pointing out here.
One is that the delay issues being raised in space are exactly those looked
at by the end2end Research Group and the TCPSAT Working Group a few years
ago, which resulted in a recommendation that initial TCP windows be
enlarged to at least two and perhaps 3 MTUs. Bottom line, if you have a
fair bit of data to send, starting it with a single data item per round
trip wastes significant time and bandwidth.
The other is that both HTTP and SMTP are fairly chatty transaction
protocols, which kill time for several RTTs before doing much of anything.
HTTP is materially improved by the use of tcp pipelining, which is commonly
implemented in servers but is not often used by browsers. SMTP generally is
implemented in a chit-chat fashion - one says "EHELO" and waits, then sends
the "FROM", then says (often individually) who the message is to, and
finally sends the (usually one or two) segments of data. There is a round
trip in between each, and if it is on the order of 560 ms as Mark suggests,
that will dominate the other link characteristics. In the general case, all
of these commands generally "work", so it would be sufficient to send all
the commands in a batch, receive all the responses, and determine what to
do next. I'd suggest this as an option for space-based mailers, both
directions, and would suggest that POP/IMAP services be to a space-borne
server, not a server on the ground.
In other words, don't blame TCP. The applications mentioned, the
applications would behave the same regardless of the transport they used.
As to whether another protocol could be used or is in use, yes, there are
other protocols involved, but not TCP variants. http://www.ccsds.org
describes the architecture.
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