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Persistent HTTP
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Motivation
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A Typical Application of HTTP

images/http-over-tcp.gif
A Client-Side "Fix"
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  • GET the HTML file using one connection.
  • Parse the HTML file and use "parallel" connections for the subsequent GET requests.
Problems with this Approach
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  • Multiple TCP Connections:
    • Each of which requires a 3-way handshake
  • Short TCP Connections:
    • Stuck in slow start
    • Small congestion window
    • Bad round-trip-time estimates
Persistence
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  • The Idea:
    • Use one TCP connection for multiple GET requests
  • The Process:
    • Server leaves the connection open after sending a response
    • Client uses an existing connection if there is one
Persistent HTTP
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An HTML File with One IMG Element

images/http-persistent.gif
Persistence and Pipelining
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  • Without Pipelining:
    • Client issues a new request after the previous response has been received
  • With Pipelining:
    • Client sends request immediately
    • Server must handle responses in the order of the requests
There's Always More to Learn
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