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PA2AGA > TCPDIG 27.07.97 06:13l 112 Lines 4624 Bytes #-10277 (0) @ EU
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Message-Id: <tcp_97_71>
From: pa2aga
To: tcp_broadcast@pa2aga
Subject: TCP-Group Digest 97/71
X-BBS-Msg-Type: B
TCP-Group Digest Fri, 25 Jul 97 Volume 97 : Issue 71
Today's Topics:
An experiment?
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Date: Thu, 24 Jul 1997 09:53:29 -0400
From: "Fred R. Goldstein" <fgoldstein@bbn.com>
Subject: An experiment?
At 04:37 AM 7/24/97 GMT, David Mackay wrote:
>Due to an imminent change in licensing conditions in the UK
>it appears that at present those of us with "Notices of Variation"
>to our licenses will be expected to try to run TCP/IP access on a
>frequency shared with vanilla AX25 and quite probably variants thereof.
>Certainly "wild nodes" "KA-Nodes" and others. We did try this from
>about five or six years back to the recent past and the results were
>not pretty to behold. TCP/IP got a 'bad name' and it was a real
>setback to progress in radio networking and the echoes still linger!
>
>I contend that nothing has changed in the basic MO of either TCP/IP
>or vanilla AX25 and variants thus the situation remains much as it was.
Of course nothing has changed. TCP's definition was firmed up in the
1980s when the Van Jacobson Slow Start algorithm was added, with subsequent
options not being the norm on the radio. VJSS is an absolute must for
coping with congestion, as it provides the flexible, automatic backoff
needed to prevent congestion collapse from occurring. Also TCP's backoff
on retransmission (separate from its VJSS dynamic window size) is useful for
shared media, like radio.
AX.25 in "vanilla packet" uses LAP-B procedures, which were developed for
wireline point-to-point use, where there is no possibility of congestion.
They are absolutely unworkable otherwise, and packet radio channels routinely
display congestion collapse behavior, in which retransmissions cause
loss which causes retransmissions etc. in a positive feedback loop, and
actual throughput goes towards nil.
Given this, it's obvious that TCP/IP is better at channel-sharing than
AX.25. But when you combine the two, the AX.25 users, who do NOT use the
appropriate backoff behavior, will keep the channel clobbered and cause
TCP/IP (which will slow down more) to get a disproportionately small share
of whatever gets through.
Truth is, the standard single-frequency radio technique (essentially Aloha)
is hopeless to begin with, so what I said above is actually only roughly
accurate since other factors are also at work, but the generalization should
hold.
>However the person responsible for issuing these "Notices of Variation"
>essential to comply with UK law. (I can hardly credit that I have to
>- write this btw. ;) Simply poo-poo's the idea of less than optimum
>working for both modes on a common frequency as I've attempted to
>describe above. His contention is simply that "it works" and he has
>expressed his intention to "experiment" with his idea.
Must be that old-time ham (antiquarian radio) regulatory attitude which the
ARRL so fiercely represents here in the states; however, the FCC isn't
quite as responsive to it since "deregulation" is a mantra. Quite depressing,
actually. Let's "experiment" with forcing vacuum-tube oscillators, coherers,
and other equally inappropriate modes, eh?
___
Fred R. Goldstein fgoldstein@bbn.com
BBN Corp. Cambridge MA USA +1 617 873 3850
------------------------------
End of TCP-Group Digest V97 #71
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