W3C home > Mailing lists > Public > ietf-http-wg-old@w3.org > January to April 1998

Re: Confused about persistent connection for old clients

From: <Dominic.Chambers@mimesweeper.com>
Date: Fri, 17 Apr 1998 10:08:46 +0100 (BST)
Message-Id: <00001C72.eval@mimesweeper.com>
To: http-wg-request@cuckoo.hpl.hp.com
     
     >>>>> "EW" ==  <ewindes@spyglass.com> writes:
     
     EW> OK, I too, am confused about why proxies MUST NOT establish
     EW> persistent connections with 1.0 clients.  If the client and
     EW> origin server connections are handled separately, and if the
     EW> proxy understands the 1.0 Keep-alive, what's the danger?
     
     Persistent connections in HTTP/1.0 must be established on a hop-by-hop 
     basis, so that intermediaries (proxies) are not forced into persistent 
     connections if they do not understand them. The connection: keep-alive 
     header allowed a client to request a persistent connection from an 
     origin server without consideration for intermediaries (end-to-end).
     
     To prevent this problem, clients connecting to origin servers via 
     intermediaries were required to send a proxy-connection: keep-alive 
     header instead which the proxy could convert to a plain connection: 
     keep-alive header if it also implemented persistent connections. 
     Origin servers only respond to the plain connection: keep-alive 
     headers, knowing that if they receive a proxy-connection: keep-alive 
     header that the proxy does not support persistent connections. This 
     works fine in this scenario:
     
     C <--> P <--> OS
     
     Proxy chains were, in theory, to be supported by ensuring that only 
     proxies at the end of the chain could perform keep-alive header 
     conversions so that upstream proxies were not unwittingly forced into 
     agreeing a persistent connection. Where P* is a proxy that will 
     perform keep-alive header conversions, then a typical scenario might 
     be:
     
     C <--> P <--> P <--> P* <--> OS
     
     Finally, the reason origin servers should not provide persistent 
     connections to clients is that if the last proxy in the chain supports 
     persistent connections, then the entire connection will be persistent 
     regardless of whether all the downstream proxies support persistent 
     connections.
     
     Proxy chains constructed in this way will never work because any proxy 
     that does not support persistent connections will wait for an end of 
     connection from the upstream proxy that will never arrive. If the 
     first proxy in our chain did not support persistent connections the 
     data flow would be:
     
      _       _       _       __       __
     |C| --> |P| --> |P| --> |P*| --> |OS|
     | |     | | <-- | | <-- |  | <-- |  |
     
     Hope that makes sense (it's not easy to explain without a drawing 
     board).
     
     
     Cheers,
     
     
     Dominic
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Received on Friday, 17 April 1998 09:17:05 EDT

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