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Advantages of persistent connections

From: J.P. Martin-Flatin <martin-flatin@epfl.ch>
Date: Mon, 15 Jun 1998 01:41:30 +0200
Message-Id: <199806142341.BAA20879@tcomhp31.epfl.ch>
To: http-wg@cuckoo.hpl.hp.com
Cc: martin-flatin@epfl.ch
Regarding the advantages of persistent connections listed in section 
8.1.1, under heading "Persistent HTTP connections have a number of 
advantages", one is missing (latency), and one is not always true (memory 
savings). I therefore suggest 2 changes.

1) Add a bullet between current 3rd and 4th bullet:

    .  Latency is reduced because network congestion is reduced, so
       less retransmissions are needed, so the elapsed transmission
       time is shorter.

2) Replace:

    .  By opening and closing fewer TCP connections, CPU time is saved,
       and memory used for TCP protocol control blocks is also saved.

with:

    .  By opening and closing fewer TCP connections, CPU time is saved
       in routers and hosts (clients, servers, proxies, gateways, tunnels,
       or caches), and memory used for TCP protocol control blocks can
       be saved in hosts.


Regarding memory savings, the original statement is not true in its 
generality. The number of TCP control blocks using up memory at any point 
in time depends on many parameters, among which we find:

* timeout value of persistent connections
* average number of simultaneous users
* activity of users:
    - whether they typically retrieve lots of objects (pages, inline
      images) and then become quiet
    - whether they typically download a few objects every N minutes
      with N inferior to the TCP connection timeout value
    - whether they typically download a few objects every N minutes
      with N superior to the TCP connection timeout value
    - whether they typically poll a server (e.g., to get the latest
      results of a sporting event)
* etc.

By varying these values, one can easily show that the number of TCP control blocks can be higher with persistent connections in some cases, smaller in others. Persistent connections often allow to save memory, but not always.

Jean-Philippe Martin-Flatin
Received on Sunday, 14 June 1998 16:44:10 EDT

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