IPP Mail Archive: RE: IPP> machine readable etc. - why Harry

IPP Mail Archive: RE: IPP> machine readable etc. - why Harry

RE: IPP> machine readable etc. - why Harry is right

From: Carl Kugler/Boulder/IBM (kugler@us.ibm.com)
Date: Tue Aug 22 2000 - 15:52:10 EDT

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    Paul wrote:
    >I felt that the nail in the coffin of multi-part response was the radical
    >in the data flow - it was too much of a change (at least to my mental
    model of
    Hmmm, I'm not sure what you mean by a radical change in the data flow.
    It's a perfectly normal HTTP request/response that happens to have
    "unusual" (but still legal) timing in delivery of the response. For
    example, you do a Print-Job request, and get back a chunked response. The
    first chunk contains the normal Print-Job response. Additional chunks
    follow (later), containing notifications about the job progress. This is
    efficient enough for page-by-page notifications.

    >Regarding the problems you cite
    >1. What is a buffering proxy. If it doesnt allow what I described then it will
    >break instant messegner systems and hence wont survive in the market place very
    >long. IN fact this proposal has the same requirements as instant messgener
    >systems (and they definitely work).
    It's a mythical beast, but theoretically a standard-compliant HTTP proxy
    implementation. A proxy might buffer up a complete response from a server
    before relaying it to a client. For example, a proxy sitting between an
    HTTP/1.1 server and an HTTP/1.0 client might use this algorithm (from
    section 19.4.6. of RFC 2616) to buffer up a chunked response in order to
    relay it to the client with Content-Length.

    A process for decoding the "chunked" transfer-coding (section 3.6) can be
    represented in pseudo-code as:

           length := 0
           read chunk-size, chunk-extension (if any) and CRLF
           while (chunk-size > 0) {
              read chunk-data and CRLF
              append chunk-data to entity-body
              length := length + chunk-size
              read chunk-size and CRLF
           read entity-header
           while (entity-header not empty) {
              append entity-header to existing header fields
              read entity-header
           Content-Length := length
           Remove "chunked" from Transfer-Encoding

    The reason I say that this kind of proxy doesn't exist is also to be found
    in the HTTP spec: "Note: HTTP/1.1 does not define any means to limit the
    size of a chunked response such that a client can be assured of buffering
    the entire response." (That and the extremely poor responsiveness, for
    interactive applications.)

    >2. Ditto
    Some HTTP proxies will drop an idle connection after 5-15 minutes. There
    is no spec for this, but it seems to be common practice. (The spec just
    says, in essence, anyone can drop a connection any time for any reason.)

    >3. What does that means -proxies are limited to 2 per server (by spec). Do you
    >mean HTTP proxies?
    Re: 2 per server, I just looked it up, and it's not quite as bad as I
    thought. From HTTP spec, "A proxy SHOULD use up to 2*N connections to
    another server or proxy, where N is the number of simultaneously active
    users. " So each user is limited to two connections through a proxy to a
    particular Printer.

    >Well this wont be using HTTP proxies - they will have to be
    >direct (socks or winsock) proxies.
    So much for "native" IPP notifications! I agree, though, if you get away from HTTP, most of these problems go away.

    >4. is a good point. But then the printer probably isnt going to have a lot of
    >clients talking directly to it.
    We have this recurring scenario of a lightweight printer shared by 1000
    users, each of which wants to get Printer notifications (out of paper, out
    of toner, paper jam, etc.). I don't believe in it, myself.

    >"Carl Kugler/Boulder/IBM" <kugler@us.ibm.com> on 08/22/2000 09:35:44 AM
    >To: pmoore@peerless.com
    >cc: "Harry Lewis/Boulder/IBM" <harryl@us.ibm.com>, bwagner@digprod.com,
    > ipp@pwg.org, "Herriot, Robert" <Robert.Herriot@pahv.xerox.com> (bcc: Paul
    > Moore/AUCO/US)
    >Subject: RE: IPP> machine readable etc. - why Harry is right
    >Paul wrote:
    >>There are two workable possibilities - one is ipp-get. This already exists but
    >>is polling - and people have a visceral dislike of polling to say the least!
    >It is a little better than polling, in that it introduces some message
    >queuing. With Get-Job-Attributes polling, you just get a periodic snapshot
    >of the state. With ipp-get, you can reconstruct the sequence of events
    >(e.g., Job history), even if you don't get them in real time.
    >>The other alternative is a tweak to indp. Instead of the client sending in a
    >>and the printer connecting to that URL we could have a client-opened
    >>i.e. the client opens the connection
    >>it subscribes on the 'normal' operation url,
    >>in the subsription the notification url is 'indp:'
    >>the lack of an address in the url causes the printer to use the exisitng
    >>connection rather than open a new one.
    >This solution has all the problems that killed the multi-part response solution:
    >1. Buffering proxies. (Although I strongly doubt their existence.)
    >2. Proxy time outs. (Although this could be handled with reconnects and
    >sequence numbers.)
    >3. Proxies being limited (by spec) to two(?) connections to any particular
    >4. Tiny printers that can't handle more than a few active connections at a
    >Ipp-get might be as good as it gets.
    > -Carl

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