Mark,
Thank you for your message:
... how does the end host figure out which situation (congestion
or outage) it is in?
There are two end hosts. Only one of them has a good chance of
knowing, and the other doesn't usually care these days.
I agree that a well-designed signaling method (whether by TCP
extensions or ICMP extensions) is a good idea, and I am sure
that will be better-used in the future.
For now, though, in the situation most nets are in, if people
want services that can be accessed reliably by wireless
communications, they might want to at least lower their TCP
maximum retransmition timeout values, consider lowering their
RTO maximum, read Reiner's comments on the DoCoMo draft:
http://pilc.grc.nasa.gov/pilc/list/archive/0996.html
and have a look at his TCP-Eifel page:
http://iceberg.cs.berkeley.edu/downloads/tcp-eifel
Now, I wonder which cellphone will be the first to play MIME
audio/mpeg attachments AND have PPP available on a DB-9 RS-232.
Cheers,
James