Re: Gateway Selection Procedure

7 Sep 1986 05:40-EDT


A fast reading of your message, especially the last part characterizing the DCD
X.25, suggest you are in trouble.

The IMP does NOT, so far as I know, examine the IP for an address. The host
or front end doing the IP has to map the internet address into an appropriate
X.25 address (to the destination or to the gateway on the X.25 network).

It is not necessarily a requirement that each TCP connection be mapped into
a distinct X.25 virtual circuit. Some implementations map all TCP connections
into a single X.25 virtual circuit between source and destination or source
and gateway. This could result in poorer throughput as the number of TCP
connections increases and also result in some interference between the varius
communicating processes sharing the common virtual circuit. Since there is
no easy way to tell IP that it is handling a new TCP connection, this is an
area of performance concern which may have implementation and/or application
specific consequences and require specific remedies.

BBN has developed software which permits 1822 and X.25 interoperability;
however, there are two modes of X.25 interface on the network: standard
and basic. The basic mode is incapable of dealing with the X.25/1822
mapping since it is really just an end/end X.25 pipe - the commercial
interface to X.25 that BBN sells. The Standard mode (DoD standard) will
support 1822/X.25 interfacing. ALL of X.25 is terminated in the IMP
at the interface. At the destination, if the interface is also Standard
X.25 (not BASIC X.25), the full X.25 is RECREATED. If the destination
is 1822, it will see a fabricated 1822 exchange. In the STANDARD case,
X.25 and 1822 addresses are mapped back and forth.

There was indication at the Monterey meeting that the STANDARD X.25 mode
had not necessarily been installed on both the ARPANET and MILNET -
the DDN NIC would be a good place to query in this regard.

The principle issue is whether the typical commercial implementations
of X.25 are compatible with (interoperate with) the DoD Standard X.25.
They certainly SHOULD interoperate with BASIC X.25.

The reason there are two X.25 style interfaces is that the BASIC mode was
readily available and permitted hosts with vendor-specific protocols to
use the MILNET for communications, independent of interoperability with
other hosts on the system.

If all hosts used X.25 interfaces, I believe there would be no need for
the Standard X.25 interface with its conversion facility - but this is
not the case.

I'm sure you'll get other comments on the TCP-IP distribution.

Maybe we need an RFC laying all this out...


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