Translate from TCP to X.25

Discussion in 'Cisco' started by JR, Jan 19, 2004.

  1. JR

    JR Guest

    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
    (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
    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, Jan 19, 2004
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  2. 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 :cool:. It is a very powerful feature, with a lot of field
    Phillip Remaker, Jan 20, 2004
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  3. 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

    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 Mainwaring, Jan 20, 2004
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