PSN 7 End-to-End question.


Robert Allen (robert@spam.istc.sri.com)
Thu, 28 Jan 88 11:13:51 PST


    Recently I've been thinking about the changes in host connections to
    the ARPAnet, and some questions have come up I can't answer for myself.
    If anyone can answer these publicly or privately I'd appreciate it. If
    the questions are to basic for this group any pointers to RFCs which are
    pertinent would be welcome. Thanks,
        Robert Allen, robert@spam.istc.sri.com
        415-859-2143 (work phone, days)

    ----------------------------------------------------------

    The questions are primarily two: first, why was X.25 chosen as the
    replacement for 1822 for all new IMP connections? second, why was
    it deemed necessary for an End-to-End protocol in the network?

    Regarding the first question: What exactly are the options for X.25
    connections to the ARPAnet? Is HDLC *or* LAPB acceptable, or only
    HDLC? Is LAPB deemed necessary for a reliable connection or is HDLC
    sufficient? Simply stated, are two reliable layers under IP needed
    or is 1 sufficient?

    Regarding the second question: First, I assume that the the ETE protocol
    is IMP-to-IMP. In the older releases of IMP software I assumed that it
    operated on a reliable link-to-link basis (except in cases of extreme
    problems), in other words, my concept of the IMP model comes from chapter
    5 of the Tannenbaum "Computer Networks" book. Under the ETE model as I
    understand it, there is an 'ETE ACK' for the packets sent. Questions
    I have include

        "is the ACK on a per-IP-packet basis or on a per-IMP-packet basis?",

        "if ETE is used on an per-IP-packet basis can fragmentation occur?",

        "is the ETE protocol used on all packets, or just those that request/
            require it?",

    and of course

        "why was the ETE protocol deemed necessary in a network that already
            had IMP-to-IMP reliablity (this leads me to believe that ETE is
            only used for some packets, namely those that require extreme
            reliability)?"

    Picture:
        host IMP IMP host
        ---- ---- ---- ----
        | |====| |<---------->NETWORK<--------------->| |====| |
        ---- ---- ---- ----
           |X.25|~~~~~~~~~~~~~~~~~ETE protocol~~~~~~~~~~~~~|X.25|



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