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EE 122: Lecture 21 HyperText Transfer Protocol HTTP

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Steps to get the resource: http://www.eecs.berkeley.edu/index.html ... Change http requests to ftp requests ... In HTTP/1.0 server indicate the end of dynamic ... – PowerPoint PPT presentation

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Title: EE 122: Lecture 21 HyperText Transfer Protocol HTTP


1
EE 122 Lecture 21 (HyperText Transfer Protocol -
HTTP)
  • Ion Stoica
  • Nov 20, 2001

()
2
Background
  • World Wide Web (WWW) a set of cooperating
    clients and servers that communicate through
    HTTP
  • HTTP history
  • First HTTP implementation - 1990
  • Tim Berners-Lee at CERN
  • HTTP/0.9 1991
  • Simple GET command for the Web
  • HTTP/1.0 1992
  • Client/Server information, simple caching
  • HTTP/1.1 - 1996

3
Basics
  • Client-server architecture
  • Synchronous request/reply protocol
  • Stateless
  • Uses unicast
  • Implemented on top of TCP/IP

4
Terminology
  • Resource file or service (e.g., dynamic results
    from the execution of a script)
  • Entity information transferred in a request or
    response
  • Entity Tag unique identifier for a resource

5
Universal Resource Locator
  • An address or location of a resource, e.g.,
    http//www.eecs.berkeley.edu/index.html
  • Prefix up to represents the protocol to be
    used to obtain the resource

6
Client Request
  • Steps to get the resource http//www.eecs.berkele
    y.edu/index.html
  • Use DNS to obtain the IP address of
    www.eecs.berkeley.edu, A
  • Send to A an HTTP request
  • Server response (see next slide)

GET /index.html HTTP/1.0
7
Server Response
HTTP/1.0 200 OK Content-Type text/html Content-
Length 1234 Last-Modified Mon, 19 Nov 2001 153
120 GMT EECS Home PageITLE …
8
Big Picture
Client
Server
TCP Syn
Establish connection
TCP syn ack
Client request
TCP ack HTTP GET
Request response
Close connection
9
Request Methods
  • GET transfer resource from given URL
  • HEAD GET resource metadata (headers) only
  • PUT store/modify resource under the given URL
  • DELETE remove resource
  • POST provide input for a process identified by
    the given URL (usually used to post CGI
    parameters)

10
Response Codes
  • 1x informational
  • 2x success
  • 3x redirection
  • 4x client error in request
  • 5x server error cant satisfy the request

11
HTTP/1.0 Example
Server
Client
Request image 1
Transfer image 1
Request image 2
Transfer image 2
Request text
Transfer text
Finish display page
12
HHTP/1.0 Performance
  • Create a new TCP connection for each resource
  • Large number of embedded objects in a web page
  • Many short lived connections
  • TCP transfer
  • Too slow for small object
  • May never exit slow-start phase

13
Web Proxies
  • Intermediaries between client and server

Client 1
Client 2
Proxy
Proxy
Server
Client N
14
Web Proxies (contd)
  • Location close to the server, client, or in the
    network
  • Functions
  • Filter requests/responses
  • Modify requests/responses
  • Change http requests to ftp requests
  • Change response content, e.g., transcoding to
    display data efficiently on a Palm Pilot
  • Provide better privacy
  • Caching

15
HTTP/1.0 Caching
  • A request directive
  • Pragma no-cache ignore all caches and get
    resource directly from server
  • A modifier to the GET request
  • If-modified-since return a not modified
    response if resource was not modified since
    specified time
  • A response header
  • Expires specify to the client for how long it
    is safe to cache the resource

16
HTTP/1.1
  • Performance
  • Persistent connections
  • Pipelined requests/responses
  • Chunked transfer encoding
  • Compression of data types
  • …
  • Support for virtual hosting
  • Efficient caching support

17
Persistent Connections
  • Allow multiple transfers over one connection
  • Avoid multiple TCP connection setups
  • Avoid multiple TCP slow starts

18
Pipelined Requests/Responses
Server
Client
  • Buffer requests and responses to reduce the
    number of packets
  • Multiple requests can be contained in one TCP
    segment
  • Note order of responses has to be maintained

Request 1
Request 2
Request 3
Transfer 1
Transfer 2
Transfer 3
19
Chunked Transfer Encoding
  • In HTTP/1.0 server indicate the end of dynamic
    content by closing connection
  • Persistent connections not possible! Why?
  • In HTTP/1.1 server splits dynamic content in
    chunks
  • Size of chunk in hex followed by semicolon
  • Dynamic content transfer series of chunks
    followed by a chunk of size 0

20
Compression of Data Types
  • Enables transport compression of data types to
    decrease payload size
  • Example
  • Server sends in response Content-Encoding
    gzip
  • Client sends Accept-Encodinggzip

21
Support for Virtual Hosting
  • Problem recall that a request to get
    http//www.foo.com/index.html has in its header
    only
  • GET /index.html HTTP/1.0
  • It is not possible to run two web servers at the
    same IP address, because GET is ambiguous
  • This is useful when outsourcing web content,
    i.e., company foo asks company outsource to
    manage its content
  • HTTP/1.1 addresses this problem by mandating
    Host header line, e.g.,

GET /index.html HTTP/1.1 Host www.foo.com
22
HTTP/1.1 - Caching
  • HTTP/1.1 provides better support for caching
  • Separate what to cache and whether a cache
    response can be used safely
  • Allow server to provide more info on resource
    cacheability
  • A cache does not return unknowingly a stale
    resources
  • Not depending on absolute timestamps

23
HTTP/1.1 - Caching (contd)
  • Four new headers associated to caching age
    header, entity tags, cache-control, and vary
  • Age Header the amount of time that is known to
    have passed since the response message was
    retrieved
  • Entity tags unique tags to differentiate
    between different cached versions of the same
    resource

24
HTTP/1.1 - Caching (contd)
  • Cache-Control
  • no-cache get resource only from server
  • only-if-cached obtain resource only from cache
  • no-store dont allow caches to store
    request/response
  • max-age responses should be no grater than this
    value
  • max-stale expired response OK but not older than
    staled value
  • min-fresh response should remain fresh for at
    least stated value
  • no-transform proxy should not change media type

25
HTTP/1.1 Caching (contd)
  • Vary
  • Accommodate multiple representations of the same
    resource
  • Used to list a set of request headers to be used
    to select the appropriate representation
  • Example
  • Server sends the following response
  • Request will contain
  • Cache return the response that has

HTTP/1.1 200 OK … Vary Accept-Language
Accept-Language en-us
Accept-Language en-us
26
Summary
  • HTTP the backbone of WWW
  • Evolution of HTTP has concentrated on increasing
    the performance
  • Next generations (HTTP/NG) concentrate on
    increasing extensibility
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