HDH and X.25

G. B. Reilly (reilly@wharton-10)
21 Feb 87 22:59:00 EST

        A while ago, when NSF.ARPA was first being added to the
        Internet, it was noted that there were problems when
        a connection between an node that communicated with the PSN
        using X.25 and one that used an HDH to talk to the PSN.

        It appears this problem has occurred again. The system
        administrator at UPENN.ARPA (, which uses an HDH,
        has noticed that his system's connection to the Internet has
        been dropping each time NSF.ARPA connects to send mail to the

        The people at NSF can't do much about this - let them know
        if you believe that a connection from them is causing your
        HDH problems - send mail to postmaster@note.nsf.gov

Starting 2 or 3 days ago, we have been experiencing a very weird
problem. We (Think.COM, are getting hdh timeouts (resulting
in a hung interface) every time we try to communicate with one
particular host, NSF.ARPA (

If we ping (ICMP echo) them with 56 data bytes, everything is OK. If
we try to ping them with 100 data bytes (or more), we get hdh
timeouts. Probably has something to do with crossing the 128-byte
frame length.

We are running HDH in packet mode with ACC IF-11/HDH unibus interface.
No problems talking to any other hosts.

Any ideas or similar problems? This has been reported to NOC as SR#8700483.
Didn't I read here that new HDH code was installed on the PSNs recently?



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