[e2e] TCP in outer space

[Fred Baker]

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.