🚀 go-pugleaf

Disambiguation of ISO CLNP ("ISO IP") and DOD IP is done using next lower
layer information.  In the DDN backbones this would specifically mean
the Call User Data field of the X.25 Call Request.  For IP, the first
octet is agreed to be set to hex CC.  For ISO CLNP the dust has not yet
settled in the DDN world, because there was a sort of interim decision at
one point to use hex CD, but since then the ISO standards have specified
hex 81 for this use.  I tend to think DDN will eventually officially
adopt 81 because that would fit in with the general trend of policy
nowadays.  The CC and CD values are taken from the "bilateral agreement"
range specified in X.224, and thus there is no guarantee in principle
that different subscribers won't assign the same values to different
protocols.  So the use of CC for DOD IP is really an agreement among
some subscribers rather than a recognized CCITT/ISO/etc. assigned value.

The DDN also has a notion of Basic and Standard service.  Basic is like
commercial X.25 and is specified by DOD policy to be used for OSI
services in DDN.  Standard is really a somewhat special mode with
defined semantics for interoperation with AHIP (1822) interfaced
hosts.  (Among other things, the X.25 CUD protocol identification is
mapped to an AHIP "link number" via a table.)  DOD IP on DDN is run
using Standard service.  The distinction between the DDN-private "Type
of Service" selection facility and the Call User Data protocol
designation I mentioned above is that the Standard Service facility is
considered a signal from a subscriber to the network, whereas the Call
User Data is a signal from a subscriber to another subscriber.  That's
why I said that you disambiguate based on the CUD, *NOT* on the Type of
Service facility.  Implementations running DOD IP on generic X.25
networks such as public data networks will use the CC CUD, but not the
DDN TOS facility.

For other types of network technology the details of protocol selection
vary, but the same general idea holds; there are Ethernet types, IEEE
802.x SAPs (or are they SNAPs), and so forth, and distinct values are
used to distinguish the next higher layer as IP, CLNP, ARP, ES-IS, or
whatever.

Bill Barns / MITRE-Washington / barns@gateway.mitre.org