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Sticky headers and pipelining (was: Sticky header draft -- as an attachment)

From: Paul Leach <paulle@microsoft.com>
Date: Mon, 5 Aug 1996 14:40:47 -0700
Message-Id: <c=US%a=_%p=msft%l=RED-77-MSG-960805214047Z-34049@tide19.microsoft.com>
To: 'Jeffrey Mogul' <mogul@pa.dec.com>
Cc: 'http-wg' <http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com>


>----------
>From: 	Jeffrey Mogul[SMTP:mogul@pa.dec.com]
>Subject: 	Re: Sticky header draft -- as an attachment 
>
>Your draft says:
>
>    The negotiation of the sticky headers option may take place on any
>    request sent over a persistent connection. The client may add the
>    connection-token "Sticky" to the Connection header in a request; if the
>    server accepts the use of sticky headers, it responds with the same
>    token in the Connection header of its response.
>    
>    Once the use of sticky headers has been negotiated, specified message-
>    headers (see section 4.1 and 4.2 of [1]) are remembered from message to
>    message, so that they need to be transmitted in a message only if they
>    have changed since the last message. The use of sticky headers continues
>    until the connection is closed, without further need for the "Sticky"
>    connection-token on each request.
>
>I think this leads to an ambiguous situation when the client is
>pipelining requests.

>  We identified this ambiguity at the meeting
>we had in January of the persistent-connections subgroup.
>
>Consider this **somewhat contrived** example
>
>     client:
>     ## requesting first object, negotiating sticky headers ##
>     GET / HTTP/1.1<CRLF>
>     Accept: text/html<CRLF>
>     Accept-Language: en<CRLF>
>     Connection: sticky<CRLF>
>     <CRLF>
>
>     server:
>     HTTP/1.1 200 OK<CRLF>
>     MIME-Version: 1.0<CRLF>
>     Connection: sticky<CRLF>
>     Content-Type: text/html<CRLF>
>     Content-Length:94<CRLF>
>     <CRLF>
>     <94 bytes body data here>
>
>     client:
>     ## requesting second object, changing one of the to-be-sticky headers ##
>     GET /a.gif HTTP/1.1<CRLF>
>     Accept: image/gif<CRLF>
>     <CRLF>
>     ## requesting 3rd object, again changing 1 of the to-be-sticky headers
>##
>     GET /b.jpeg HTTP/1.1<CRLF>
>     Accept: image/jpeg<CRLF>
>     ## requesting 4th object, not sending one of the to-be-sticky headers ##
>     GET /c.gif HTTP/1.1<CRLF>
>     <CRLF>
>
>OK, so what are the effective request headers for the 4th request?
>Your design implicitly disallows the client from assuming "stickiness"
>until it has received
>     Connection: sticky<CRLF>
>from the server.  However, because there is no way for the server
>to tell whether the client received that header before or after
>sending the fourth request, the server cannot unambiguously know
>what the client means (i.e., should the fourth request be interpreted
>as having "Accept: image/jpeg" or not?)

I don't see the ambiguity in this example. The fourth request should be
interpreted as having
	Accept: image/jpeg
and the second, third and fourh requests all should be interpreted as
having
	Accept-Language: en
The server doesn't need to know when the client received
	Connection: sticky
instead, the client has to send requests that are unambiguous. I.e.,
until it receives the
	Connection: sticky
it sends ALL headers that have ever been sent, which is unambiguous --
even if the server thinks that the client *could* be omitting headers,
none are actually omitted, so the server won't be confused. The server
should remember the headers, regardless of what it thinks the client
knows, so that when the client starts omitting headers, the server has
the previous value to add to the message.

I think maybe your example was just buggy (or not clear to me). If the
client had sent the second request _before_ it got the reply from the
server, then it would have been ambiguous as to whether
"Accept-Language: en" was omitted from the request. So, in such a case,
it should either send
	Accept-Language:
if there wasn't supposed to be any Accept-Language header, or
	Accept-Language: en
if it was supposed to have remained the same as the previous message.

>
>One could argue that this kind of thing could never happen in practice,
>and so my contrived example (which admittedly is pretty foolish on the
>part of the client) is not worth discussing.  But the race condition
>seems to be intrinsic in the current draft design, and I'd feel more
>confident if there were either a proof that it is not really a problem,
>or a design modification that prevented the ambiguity.

I agree that something exlicit should be said about pipelining.
>
>I can see several possible ways to solve this:
>	(1) Explicitly declare that the client, after having sent
>	"Connection: sticky", may start omitting headers as soon as it
>	receives the server's "Connection: sticky" response, but MUST
>	NOT change any such headers before receiving a response to the
>	first request sent *after* it has received that "Connection:
>	sticky" response from the server.  This rule has to be
>	interpreted "per connection".  (If you draw out the timing
>	diagram, you can see that this will always avoid the race
>	condition.)

I agree it needs to be per connection. I don't see why it has to wait
the extra round trip. I think the modification above solves the problem.
If a client were _required_ to omit headers that were the same as in the
last message once it had received the "Connection: sticky" from the
server, then I think there would need to be tighter synchorization.
>
Received on Monday, 5 August 1996 14:43:50 EDT

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