Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!virtech!dennis From: dennis@virtech.UUCP (Dennis P. Bednar) Newsgroups: comp.dcom.lans Subject: Re: What services does X.25 provide? Summary: X.25 is not an end-to-end protocol Keywords: x.25, services, login, e-mail, file transfer, IPC Message-ID: <1167@virtech.UUCP> Date: 15 Sep 89 23:20:24 GMT References: <796@maxim.erbe.se> <3279@wasatch.utah.edu> <522@wet.UUCP> Organization: Virtual Technologies Inc Lines: 49 In article <522@wet.UUCP>, epsilon@wet.UUCP (Eric P. Scott) writes: > In article <3279@wasatch.utah.edu> haas@wasatch.utah.edu (Walt Haas) writes: > > But the X.25 standard defines reliable data > >transmission over virtual circuits, with a means of addressing, much as > >TCP/IP does. > > It doesn't meet the Internet's definition of reliable. If you > don't want to see your VCs reset everytime someone sneezes, you > should run a truly reliable protocol (e.g. TCP) on top of X.25. > (This also addresses all the concerns of the original query.) > > -=EPS=- This is correct. X.25 is specified as a protocol between a DTE (packet switching host) and a DCE (IMP or interface message processor a.k.a. packet switching node). While X.25 level 2 (the lowest protocol layer) has reliability built into it (checksums, acks, retransmissions, sequenced packets), the reliablility is only between the host and the IMP. There is no guarantee of end-to-end reliablity between two hosts: (local) (local) (remote) (remote) DTE DCE DCE DTE host <---> IMP <---> IMP <----> host In other words, when the local DTE on the left sends a level 3 DATA packet and receives a level 3 RR acknowledgement packet from the local IMP (on the left side of the above diagram), it only means that the local IMP has acked the packet, but not necessarily that the remove DTE in the right side of the figure has even received it. The virtual circuit could be closed or torn down by the portion of the network between the two IMPs shown above before the DATA packet reaches the remote DTE. What you need is a layer 4 transport protocol that speaks end-to-end (host-to-host) for the extra end-to-end reliability not provided by X.25. There are two ways to layer TCP over X.25: TCP/ IP / X.25 level 2 TCP/ IP / X.25 level 3 / X.25 level 2 The second way is the DDN interface specified by the Defense Department. -- Dennis Bednar uunet!virtech!dennis (703) 430-9247 Virtual Technologies Inc, P.O. Box 876, Sterling VA, 22170