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Re: HTTP/1.1 Pipelining

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Mon, 14 Apr 97 16:34:47 MDT
Message-Id: <9704142334.AA12871@acetes.pa.dec.com>
To: http-wg@cuckoo.hpl.hp.com
    When a client is talking to a proxy server and is pipelining requests,
    should it use a single pipeline connection to issue requests to 
    different origin servers ?
    ie should GET http://www.ups.com/ HTTP/1.1
       and    GET http://www.fedex.com/ HTTP/1.1 
    be sent over the same pipeline or should a new connection be established.
    A common occurence of this is when advertisement gifs come from a 
    different server than the html file.
    
    I guess the question becomes :
    Can requests in the same pipeline be to different origin servers?
    
In the case where you have an HTML file coming from one server, and
an imbedded GIF from another, I think the "use two connections"
rule applies.  I.e., the client has two connections to the proxy,
uses the first connection to fetch the HTML file, and the other
for the imbedded images.

Note that this is actually not a particularly hard example; you
can't actually start fetching the image until you have at least
some of the HTML file (since you don't have the IMG SRC until then).

A harder question is "what if the HTML file has lots of IMG SRCs,
which come from more than one origin server?"  Then you could get
into a situation where you request several different images over
one connection, and some might be blocked waiting for earlier ones
to complete.

I think the best answer is that as long as the images are ultimately
being used to construct the same layout, then you should stick
to using as few connections as possible.  Ultimately, you can't
finish the layout until you have most or all of the images, and so
while there might be some minor benefit in trying to get them in
"fastest first" order, this probably isn't a big deal.

On the other hand, if one browser program has two or more independent
Web-browser windows open (I often do this, especially when following
up on search-engine results), then it might be best to act as if
these are separate instances of the application.  I.e., each window
gets its own pair of TCP connections, so that the two windows can
be operated without interfering with each other.

Although the use of lots of parallel connections has taken some
criticism (including from me), the real problem is not having lots
of TCP connections open per se.  The two bad things to do are
(1) open and close connections frequently, resulting in lots of
overhead at the server, and (2) run lots packets simultaneously
over parallel connections, which defeats the TCP congestion
control algorthms.  If you get into a situation where you have
two proxy connections open, but both of them appear stalled, it might
make sense to open another connection *if* you are going to use
it to get responses from a different origin server.

And if you have multiple origin servers to load from at once,
it probably pays to distribute the URLs among your TCP connections
so that all of the URLs from one server are fetched on one connection.
I.e., if the servers are A and B, do this:
	connection 1: A A A A A A
	connection 2: B B B B B B
instead of this
	connection 1: A B A B A B
	connection 2: A B A B A B
since if server B is slow or dead, the latter approach will cause
a lot more trouble.  It might even cause more TCP opening/closing,
if the user gets tired and hits "stop" ... since if a request is
in progress when that happens, you pretty much have to close the
TCP connection to recover.

Bottom line: minimize the number of (1) simultaneously active
TCP connections and (2) the number of TCP opens and closes.
But don't make the user wait because you've done things in the
wrong order.

    With the advent of pipelined persistent connections ( and to a lesser
    extend 1.0 keep alives ), the distinction of 'who the client is
    talking to' is confusing to me.  Since while the client may be
    pipelining to a proxy, and the proxy can go ahead and do an
    old style connection to an origin server, how does the client deal
    with old responses?
    Assuming the answer to the previous question,  is yes...
    
    IE: client pipelines:
      GET http://www.foo.com/ HTTP/1.1
      GET http://www.bar.com/ HTTP/1.1
    what happens if foo.com is a 1.1 server ( the proxy can do a
    persistent conenction ) and bar.com is 1.0 (proxy cannot)?
    
It's important to understand that persistent connections are
"hop-by-hop", NOT "end-to-end".  I.e., what goes on between
proxy and client (as far as TCP connections are concerned)
is almost entirely unrelated to what goes on between proxy
and origin server; at a higher level, the proxy is always just
forwarding requests and responses.  If it has to use a separate
TCP connection for each request/response, because its peer
speaks HTTP/1.0, so be it.

The "almost entirely" in that paragraph is because if the user
hits "stop", the only way for the user agent to stop a transfer
in progress is to close the TCP connection.  And since we want the
proxy to pass this along (e.g., to stop wasting Internet bandwidth
that's being used to load a large response from the origin server),
then this causes the connection-close to propagate all the way through
the chain of proxies.  Which is sad, but we've always said that
TCP connections are not immortal.

-Jeff
Received on Monday, 14 April 1997 16:41:15 EDT

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