PSN 6.0 Battle Scars

Ron Stoughton (rms@ACC-SB-UNIX.ARPA)
Tue, 17 Jun 86 00:10:09 pdt

There have been some inquiries lately regarding the inter-operability
of X.25 with 1822 using PSN 6.0 in Standard mode. I believe a
subsequent reply stated that PSN 6.0 supports such inter-operability.
Recent testing we have done at customer sites has confirmed this, but
some problems appear to exist in the PSN 6.0 software. A couple of
these are described below in an effort to assist users attempting to
connect to MILNET using Standard mode X.25. We would appreciate
hearing from anyone experiencing similar problems since debugging of
such can be very time consuming.

Battle #1:

This configuration consisted of our IBM MVS host in Columbia connected
to the same IMP as our VAX running 4.2 BSD Unix. The VAX system was
connected via an ACC IF-11/HDH interface running HDH (1822-J) packet
mode at 19.2K bps. The IBM system was connected via an ACC IF-370/DDN
interface running X.25 Standard at 57.6K bps. The problem occurred when
attempting to log onto the IBM system from a VAX terminal using Telnet
and manifested itself has a permanent hang condition during the logon
sequence. The hang occurred when the IBM system sent a long message
prompting for input. This message was sent across the X.25 interface
as a 2-packet sequence (128-byte packet size) and was sent across the
HDH interface as a 3-packet sequence (leader packet plus two data
packets). However, the middle packet was 134 bytes in length which
is a violation of the HDH protocol which specifies a middle packet
length in the range of 2 to 126 bytes. This oversize packet was
dutifully discarded by the HDH interface, thus causing a subsequent
retransmission by TCP which repeated the error.

Battle #2:

This configuration consisted of an IBM VM host located at the Navy
Yard (NARDAC) connected to a PSN 6.0 IMP via an ACC IF-370/DDN
interface running X.25 Standard at 9.6K bps. It was discovered
that FTPing the host table from the NIC took an extraordinarily long
time (it actually appeared to be hung) and caused X.25 packet-level
errors at our interface. However, transferring the same file from
our IBM host in Columbia completed as expected and without error.
In this case our IBM host in Columbia was connected via an ACC
IF-370/1822 interface running distant host 1822 across a 9.6K bps
ECU connection to the PSN 5.0 IMP at NIH. The NIC is connected to
its local MILNET PSN 5.0 IMP via a BBN 1822 interface running in
local or distant mode. Being a local IMP connection, the NIC interface
will run at the asynchronous speed of the C/30's 1822 interface. We
investigated this further with a Chameleon X.25 Data Analyzer and
found the PSN 6.0 IMP was transmitting several IP datagrams
concatenated together into a single X.25 packet sequence (i.e., the
M-bit was set for several [how about 14?] successive X.25 packets).
Each full-size (576-byte) IP datagram was also truncated to two

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