Re: Hop problem

Phil R. Karn (thumper!
2 Feb 88 01:19:26 GMT

> the "packet radio" referenced was not the DARPA packet radio but the
> amateur packet radio. I don't know how those systems handle the TTL
> field...

The "amateur Internet" is much like the real Internet -- a mixture of IP
gateways and "digipeaters". Digipeaters are single frequency
store-and-forward repeaters that operate on the source routing fields in
the "AX.25" amateur link level protocol. Since digipeaters do not
understand IP, they do not modify TTL fields even though they may
introduce considerable delay. Most amateur packet radio channels
operate at 1200 bps synchronous, with substantial contention and keyup
delays, plus high collision rates due to hidden transmitters.

In general, the TCP/IP nodes on the amateur Internet are PCs running my
TCP/IP implementation (the "KA9Q" package). This includes end-nodes as
well as IP gateways. At present, my code decrements the TTL field by 1
for each pass through a gateway module, including the end hosts.

One of the interesting observations to come out of our experiments so far is
the interaction between the retransmission algorithms in the TCP
(transport) and AX.25 (link) layers. It definitely does *not* pay to try
"too hard" at the link layer. If it takes several extra retransmissions
to get a packet through, chances are that a TCP retransmission will
occur in the meantime and you'll just have to do the same all over again
with another copy. It may be worthwhile to allow control over the "beta"
constant in TCP, perhaps in combination with the "reliability" bit in the
IP header (my gateway code uses this bit to override the per-link default
setting as to whether to use link level acknowledgements or not).

I think we've shown pretty convincingly, though, that you can't use link
level ARQ to sweep fundamental inadequacies in the lower layers under
the rug. This is especially true with channel access algorithms. Our
experiments are now moving toward multi-frequency full duplex gateways
that will eliminate contention and improve channel utilization markedly.


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