UoL CS Notes
Menu

HTTP Overview & Persistent vs. Non-Persistent

COMP211 Lectures

A web page is a set of several referenced objects which are accessible by their URL:

\[\underbrace{\text{www.someschool.edu}}_\text{host name}\text{/}\underbrace{\text{someDept/pic.gif}}_\text{path name}\]

HTTP

This is the application layer protocol for the web:

  • It uses a client server model.
    • Clients request and receive, servers can return objects in response to a request.
  • HTTP uses TCP over port 80.

  • HTTP is stateless - No information about past client requests are held.

    Protocols that maintain state are complex. The past history must be maintained and if the client or server crashes their views of state may become inconsistent.

Types of Connections

There are two different types of HTTP connection:

Non-Persistent HTTP

  1. TCP connection opened.
  2. At most one object sent over the TCP connection.
  3. TCP connection closed.

Persistent HTTP

  1. TCP connection opened.
  2. Multiple objects can be sent over a single TCP connection.
  3. The connection is closed when finished.

Round Trip Time (RTT)

This is the time for a small packet to travel from client to server and back.

For HTTP 1.0 (non-persistent) the response time is:

\[\text{HTTP Response Time}=2\times \text{RTT}+\text{File Transmission Time}\]

from the following sequence diagram:

sequenceDiagram
Note left of client: Initiate TCP
client ->> server: Open TCP
activate server
server ->> client: TCP Open
Note left of client: Request File
client ->> server: Request File URL
server ->> client: Transmit File
deactivate server
Note left of client: File Received

This is an issue as there is additional overhead in order to open and close the single TCP connections.

In HTTP 1.1 (persistent) you can have as little as one RTT for all referenced objects as the connection is left open.