Lecture Notes for Client-Server Interfaces

30 March 2004 - The Hypertext Transfer Protocol (HTTP)

What is HTTP?

• The HyperText Transfer Protocol.

  • There are two versions of HTTP: 1.0 (rfc 1945) and 1.1 (rfc 2616).

• A protocol for requesting and delivering resources.

  • A good example of a protocol hierarchy: HTTP on TCP on IP.

  • HTTP is also good for building more protocols on top.

• A request-response, client-server protocol.

  • An RPC protocol (in HTTP 1.0, anyway).

• What is a resource?

  • Anything that can be identified by a uniform resource locator (URI).

  • Anything that can be encoded in a HTTP response.

  • The client and server must agree on what a resource is.

    • The Multipurpose Intermet Mail Extensions (MIME, rfc 2045 through rfc 2049) help them agree.

HTTP Transactions

• Client connects and requests, server replies and disconnects.

  • The server can keep-alive the connection in HTTP 1.1.

  • HTTP 1.0 is an enormously expensive protocol.

    • Every URL requires a separate TCP connection.

• Any reliable transport protocol will do.

  • Although it should probably look a lot like TCP.

    • Addressing, reliable byte streams.

HTTP Messages

• The requests sent and the replies returned.

• Text and byte based.

• Same general format for requests and responses.

  • A single identification text line.

  • Zero or more header text lines.

  • An empty line.

  • Zero or more bytes of arbitrary other data.

• Text lines end in CRLF (ascii 13₁₀, ascii 10₁₀)

HTTP Identifier Lines

• Identifier lines differ for requests and responses.

  • Requests: method uri protocol

    • The GET, HEAD, and POST methods, and others .

    • The uniform resource identifier (URI).

    • The protocol (HTTP/1.0 or HTTP/1.1).

    • Example - HEAD /rclayton/web-pages/me/weather.html HTTP/1.0

  • Responses: protocol status-code description

    • The protocol (HTTP/1.0 or HTTP/1.1).

    • A three digit status code.

      • Five families ; the dreaded 404, the desirable 200.

    • A human readable description of the code.

    • Example - HTTP/1.1 200 OK

HTTP Header Lines

• Have the format tag: rest of line.

• Header lines starting with whitespace are a continuation of the previous header line - the same format as rfc 822.

• HTTP 1.0 defines 16 header lines, none required

• HTTP 1.1 defines 46 header lines, one required (Host:)

• The trailing blank line is required.

HTTP Message Bodies

• Can be any sequence of 8-bit bytes.

• Described by the Content-Type: and Content-Length: header lines.

• Content description related to MIME (rfc 2045).

HTTP Methods

• GET, HEAD, and POST.

• The GET method is the simplest form of request response.

  • The requested resource is given by the uri in the get line.

  • The resource is returned in the reply - the header lines contain information describing the content.

• HEAD is like GET but the resource isn't returned

• POST is like GET except it allows the client to send arbitrary data to the server - forms, file uploading

HTTP Communication

• Clients and servers only have to agree to use HTTP.

• Clients don't have to be browsers - spiders.

• Servers don't have to be web servers - proxies

HTTP 1.1

• A performance-enhanced HTTP 1.0 (rfc 2616)

  • Persistent connections.

  • Cache support.

  • Piece-by-piece responses.

This page last modified on 6 April 2004.