IPP>MOD Should IPP notification use http as t

IPP>MOD Should IPP notification use http as t

IPP>MOD Should IPP notification use http as t

Carl Kugler kugler at us.ibm.com
Tue Aug 17 14:47:31 EDT 1999


http://www.pwg.org/hypermail/ipp/2842.html

> RE: IPP>MOD Should IPP notification use http as t
>
> Carl-Uno Manros (carl at manros.com)
> Tue, 17 Aug 1999 06:50:41 -0700
>
> Carl,
>
> I have a number of last minute questions about the proposed HTTP solution in
> time for our meeting tomorrow, hope you have time to respond today:

I wish I had time to write up a proper proposal for this. Sorry I've been so vague.

Let me point out that what I'm proposing is really just an optimization over polling (e.g., with
Get-Job-Attributes or Get-Printer-Attributes).  It doesn't solve all the general Notification
Requirements.  But I think it's a worthwhile optimization, because it takes a lot of load off the
server and the network, and provides much more timely notifications to the client.

> 1) Our current solution assumes a URI address for delivery of notifications.
> In your case you don't have one as far as I can understand? What do we put
> in the URI attribute?

In my proposed solution, you don't need a delivery address, because the notifications are part of
an extended response to the client resulting from some IPP operation.

> 2) How does the IPP client signal to the IPP server that it is ready to get
> notifications? A new Send-Notifications Operations to which the serever can
> then respond with a multipart MIME object?

I envision these notifications as extensions to the normal IPP response for existing IPP operations
like Print-Job.  For example, in response to a Print-Job request, the Printer sends a
mulipart/replace response.  The first "part" contains the normal IPP operation response.  The
connection remains open, and as the Job progresses the Printer sends a stream of additional parts
containing IPP notifications back to the client.  When the Job reaches a final state, the Printer
sends the terminating boundary string indicating the end of the response.

In order for this to work, the client must first indicate in the HTTP request heading that it is
willing to accept multipart responses, with an "Accept" header.  Then, we could make
"notify-events" an OPTIONAL operation attribute for every IPP operation.  Then, whenver a client
submits an IPP operation, it can track the progress and ultimate success or failure of that
operation.

We could still have a separate Send-Notifications operation for the specific purpose of subscribing
to notifications independent of any real operation.  Or perhaps we could modify Subscribe-Job,
Subscribe-Printer, Renew-Subscription, etc., to allow for notification via multi-part HTTP
response.


> 3) Are not all our operations synchronous? Does that not mean that while you
> are waiting for the potentially long response on your Send-Notification
> request, you cannot send any other operations? Can you abort the
> Send-Notification operation in order to do something else and then start a
> new Send-Operation again afterwards or do you always need a separate HTTP
> connection for the Send-Notification operation?

The client can always open another connection to the Printer in order to do other operations.  I
don't see any problem with that.

> 4) If the IPP client wants to go away before all notifications are
> delivered, what happens with the rest? Do they just get discarded or are
> they stored and delivered the next time you start up the Send-Notification
> operation?

I'd reword the question as, "what happens if the connection drops before all notifications are
delivered?"    I like the idea of having notification sequence numbers in addition to
subscription-ids, so that a client could issue Renew-Subsription or Resume-Subscription  (or
something like that) and pick up where it left off.  This should probably be optional, though,
since it might be a burden on Printers to store undelivered notifications for some length of time.
Simplest approach would be to discard notifications if there's nobody listening, but still allow
clients to reestablish a subsription by subscription-id for the lifetime of the lease.

    -Carl

>
>
> Carl-Uno
>
> > -----Original Message-----
> > From: owner-ipp at pwg.org [mailto:owner-ipp at pwg.org]On Behalf Of
> > kugler at us.ibm.com
> > Sent: Monday, August 16, 1999 3:14 PM
> > To: ipp at pwg.org
> > Subject: Re: IPP>MOD Should IPP notification use http as t
> >
> >
> > original article:http://www.egroups.com/group/ipp/?start=6169
> > > > I asked you about using http as the transport. You suggested that
> > I
> > > > send you email to get your comments on the pros and cons of http as
> > > > the event transport.
> > >
> > > basically, you don't want to use http (or any tcp based protocol)
> > > to send small payloads, because the connection setup overhead
> > > is fairly large, and also because tcp connections that are being
> > > established do not compete fairly with tcp connctions that are
> > > already established (under very loaded conditions, attempts to
> > > establish new connections can steal bandwidth from
> > > already-eastablished connections)
> > >
> > > but a lot depends on how you use http or tcp.
> > >
> > > if you know you're going to be sending several short status updates
> > > (say, per-page acknowledgements) and the duration of the session is
> > > such that you can probably keep a single tcp connection open for the
> > > entire time, and especially if you need reliable delivery of those
> > short
> > > messages (i.e. you can't afford to have a status update dropped
> > > or delivered out-of-order) it's probably reasonable to open a tcp
> > > connection and deliver each update over the connection. but you
> > > want to try to do this not only in a single tcp session, but also
> > > in a single http transaction. i.e. you should avoid introducing
> > > extra http round-trips. e.g. do something like this:
> > >
> > > (connection opened)
> > > >>> GET /printer/status/job#2343 HTTP/1.1
> > > >>>
> > > <<< Content-type: application/ipp-status-messages
> > > <<<
> > > (pause)
> > > <<< status: page 1 printed
> > > (pause)
> > > <<< status: page 2 printed
> > > (pause)
> > > <<< status: page 3 printed
> > > (pause)
> > > <<< status: out of paper
> > > (pause)
> > > <<< status: resumed
> > > (pause)
> > > <<< status: page 4 printed
> > > <<< status: job complete
> > > (connection closed)
> > >
> > > rather than sending a separate GET or POST or whatever for each
> > > transaction reported. and you really want to avoid
> > > opening up a new tcp connection for every page printed.
> > >
> > > you can save a bit more bandwidth by batching status updates,
> > > and only sending a single update (perhaps for multiple pages)
> > > every N seconds, or when an exception occurs, whichever
> > > is sooner.
> >
> > Can't we depend on TCP's Nagle algorithm to do this coalescing for us?
> >
> > >
> > > if you're doing per-page status updates, you're going to
> > > be sending at least one packet per page regardless of whether
> > > you use TCP or UDP. the difference would be that each packet
> > > sent in TCP would be acknowledged by the receiver, in
> > > most cases doubling the number of packets sent over the link.
> >
> > Why would the receiver acknowledge each packet? I thought the receiver
> > was supposed to ack the longest contiguous prefix of the stream that
> > has been received correctly -- a cumulative acknowledgement scheme.
> > Therefore there may only be one ACK for multiple segments transmitted.
> > I thought one ACK for two segments is typical.
> >
> > > for most kinds of links you are concerned about minimizing
> > > bandwidth (rather than packets) or delay (to improve response
> > > time for interactive applications). the latter doesn't apply
> > > in this case. as for users of links that charge per-packet,
> > > they can avoid the problem by not requesting per-page status updates.
> > >
> > > if you're doing less frequent notifications (like job completion
> > > notifications), the volume of data is less, but in this you probably
> > > do want reliable delivery and acknowledgement. an RPC-like protocol
> > > layered on UDP would work. but if you're only sending a single
> > > notification per print job, it's probably not worth the savings.
> > >
> > > so offhand, as long as IPP notifications can nearly always occur in a
> > > single TCP session and a single HTTP transaction per print job
> > > (and especially if it can be piggybacked in the same HTTP/TCP session
> > > used to submit the print job), I can't make a strong case for using
> > > UDP for IPP notifications, or even for supporting it as an option.
> > >
> > > of course, if there's an important case that I'm missing, let me know.
> > >
> > > (just in case the http session gets disconnected, you probably will
> > > want to have the capability of reconnecting, but it should be
> > > the exception rather than the rule. and you need to define what
> > > happens in this case - do you replay all status messages since
> > > the start of the print job or do you just replay those issued after
> > the
> > > start of that http session?)
> > >
> > > hope this helps,
> > >
> > > Keith
> >
> >
>






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