Re: Am I imagining things?

The lost Bostonian (
Sat, 12 Jul 86 02:47:44 PDT

In article <In article <8606302206.AA07389@nrl-aic>, hemphill@NRL-AIC.ARPA (Gavin Hemphill) writes:
> Sorry about the previous empty message -- fumble fingers strikes again.
> Is it my imagination or are other people experiencing problems as
> well. I have noticed lately that I can't FTP files bigger than 20 or 30 KB.
> The symptoms are that after 6 or 7 minutes (clock time) the FTP connections
> (to wherever from nrl-aic) break, there are no messages or indications from
> the remote server just the "ftp>" prompt. Secondary to this when I try to
> open a new connection to the same host I'll get the "host unreachable"
> message, but if I telnet to any other host -- and from there FTP or telnet
> to my original destination I can get through, and after backing out of that
> a direct FTP will again get through. Anybody else had similar problems? If
> so, any pointers would be appreciated.
> G++

Sounds like a couple of problems. The "host unreachable" message could be
caused, if you are talking to an imp, by the bug in the imp driver that
causes a destination host to be marked down because no packets can be deliv-
ered to it. This can be fixed by changing the value of HOSTTIMER in
/sys/netimp/if_imp.h from 128 to a smaller number (I used 1 instead of 0
because our imp was crashing and I did not want to deliver packets too
quickly to it).

The ftp lockup problems sound like the "bad file number" problems we were
having a few months ago and a related problem with getting "connection
error" messages telnetting from a tiu on a prnet to a vax on the arpanet.
I discovered that the connection errors were coming when a gateway from
the arpanet to the prnet was having trouble fragmenting a packet of odd
byte length > 254. By changing the arpanet mtu on the vax from 982 (act-
ually arpanet-pli mtu) to 254 the problem went away. Likewise, many of
our bad file number and ftp lockup problems went away when I had the
ethernet mtu changed from 1500 to 576.

Next time this happens try and figure out to where you were trying to con-
nect, and what (if any) gateways you had to go through to get there. If
possible, monitor the tcp connection looking for a reset (by doing netstats
this will show as a connection going from ESTABLISHED to not showing up at
all). If you can turn on tcp debugging, look for a RST packet coming in.

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