Peter Osborne wrote:

>Does anyone know why NSVT runs over both ports 1537 & 1570? I find it very
>odd, I use xhpterm for the most part, which happens to run over 1570 except
>for one customer that only lets us in through 1537. I solved this by
>modifying the xhpterm source but I'm wondering why there's 2 ports used
>anyway, was one not enough?

The two ports host slightly different protocols. Port 1537 was for the
original protocol, which uses "record" mode. This means that the protocol
assumes that VT packets are guaranteed not to be fragmented -- in effect,
treating TCP/IP like a record-based service. On the MPE/V network stack,
TCP just happened to work this way -- one packet sent becomes one packet
received -- and nobody used the Internet as a connection medium, which
meant that there were no routers or firewalls in the middle that might
break up or aggregate packets. But this doesn't always work on the TCP/IP
Internet.

Port 1570 is "streams"-mode VT. This eliminates the dependence on the
record-based behavior of the MPE/V network stack and VT implementation.
The streams-mode VT client and server take care of their own packet
assembly, so it doesn't matter how packets get fragmented along the way.
Since the two strategies can't interoperate reliably, they're treated as
different protocols with different port numbers. TCP *is* a stream
service, so this is the way it should have been designed in the first
place. But the VT protocol was developed a very long time ago, before the
implications of this design decision were obvious.

MPE/V supports only record-mode VT, on port 1537. MPE/iX supports both
streams-mode and record-mode VT (for compatibility).

-- Bruce


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