Reading through the IOS documentation it seems that I could use the CISCO router to do the following : SUN Unix <---TCP----> Router <----X.25----> Service provider Here is what I want to do 1) The SUN workstation will run a TCP software (not Telnet) that will connect to the ROuter (on a specific port number). 2) This will result in the Router originating an X25 Call to the service provider. 3) Once the X.25 call is established the router completes the TCP connection. (If the X.25 call is not completed TCP connection will time-out, if the X25 call is rejected TCP connection establishment will report an error). 4) TCP software on SUN will send messages. ROuter will convert the TCP message to X.25 Data packets (maybe requiring segmentation). 5) Similarly X.25 Data packets received by router will be sent as TCP messages over the established connection. Is this scenario feasible? If so what happens in step 4 if there is an error in sending X.25 data packets, will the TCP end be notified. Also what happens if at a later time the service provider terminates the X.25 session, will the TCP connection be terminated. Thanks in advance for any information. JR
Yes, this will work, depedning on exactly what you mean in request 4. Theremay be problems if the X.25 provider is particular about the size of the packets or expects them to arrive in a specified form (for example, all data portions of an X.25 packet are always the exact same size, or always end in the same delimiter character), of if the TCP application is choosy in the same way. This is RARELY a problem, but some poorly written X.25 or applications put undue restrictions on the form on the X.3 PAD data, ond refuse to let telnet or X.25 doe their job 8-(. No need to. X.25 has its own error recovery and retranmission methods, so TCP needs not know any thing of data errors since they are corrected locally in the TCP or X.25 stream. The applications at both end of the raw connection need to be tolerant of an ungraceful disconnect, but no data will be acknowledged on the TCP sideor X.25 side until the other side acknowledges it. Yes. Then the call is closed, the TCP connection will FIN. In the event of an ungraceful close, the TCP will RST. There is a big state machine on this whole thing . It is a very powerful feature, with a lot of field testing.
I assume there is another TCP capable device on the other side of the service provider. It's a while since I played with TCP to X.25, but as far as I can remember, your scenario is perfectly feasible. The TCP messages are encapsulated inside X.25 packets, with segmentation occurring if necessary. The X.25 protocol has error recovery built in so, in theory, you shouldn't lose any X.25 packets. If the TCP packet times out, that should retransmit anyway. I'm sure the TCP connection is closed by the router if the X.25 connection drops, but I can't remember the exact sequence of events in terms of FINs, ACKs etc. Pete
