csg@pyramid.pyramid.com (Carl S. Gutekunst) (04/23/88)
[Followups redirected to comp.protocols.iso] >> The 7-bit-printable-ASCII restriction comes from international X.25 gateways, >> many of which insist on swiping the eigth bit for parity or somesuch. > >It's difficult to believe CCITT is so stupid to allow this in X.25 VCs. Ah, but this has nothing to do with X.25, and is completely outside the realm of CCITT. X.25 only describes the connection between a DTE (that is, a host) and a DCE (a network), and *some* of the behavior between two DTEs (two hosts, with a network inbetween). The standards deliberately say nothing about what protocols are used internally in the network, and here is where you get into trouble. Not only do you have gateways, but you also have network proprietary protocols. This is very different from ARPANet, where you have TCP/IP riding on top of an IMP-to-IMP protocol. The DCEs on many public data networks do a protocol conversion, from X.25 to something deemed more suitable for long-haul packet switching, or -- more likely -- whatever protocol the network was using before they started using X.25 to talk to DTEs. >What happens if one wants to run IP, DECNET, or OSI across such a gateway? >I guess you don't. That's right. Note that this *is* changing; I believe we can now run 8 bits among the U.S. (Tymnet and Telenet) the U.K. (PSS) and Canada (Datapac). But for amusement I tried Germany again yesterday. All my 8th-bit-on words were stripped. Curious thing, too -- you *need* all eight bits to get X.25 packet header through, to say nothing of the 16-bit FCS. But if the network has done a protocol conversion, almost anything can happen.... <csg>