EE 122 HyperText Transfer Protocol HTTP Ion Stoica Nov 25 2002 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 istoica cs berkeley edu 2 Basics Client server architecture Synchronous request reply protocol Stateless Uses unicast Implemented on top of TCP IP istoica cs berkeley edu 3 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 istoica cs berkeley edu 4 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 istoica cs berkeley edu 5 Client Request Steps to get the resource http www eecs berkeley edu index html 1 Use DNS to obtain the IP address of www eecs berkeley edu A 2 Send to A an HTTP request GET index html HTTP 1 0 3 Server response see next slide istoica cs berkeley edu 6 Server Response HTTP 1 0 200 OK Content Type text html Content Length 1234 Last Modified Mon 19 Nov 2001 15 31 20 GMT HTML HEAD TITLE EECS Home Page TITLE HEAD BODY HTML istoica cs berkeley edu 7 Big Picture Client Establish connection Client request Request response TCP Syn Server TCP syn ack T CP a ck H T TP G ET Close connection istoica cs berkeley edu 8 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 istoica cs berkeley edu 9 Response Codes 1x informational 2x success 3x redirection 4x client error in request 5x server error can t satisfy the request istoica cs berkeley edu 10 HTTP 1 0 Example Server Client Request image 1 age 1 im r e f s n a r T Request image 2 age 2 Transfer im Request text Transfer text Finish display page istoica cs berkeley edu 11 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 istoica cs berkeley edu 12 Web Proxies Intermediaries between client and server Client 1 Client 2 Proxy Proxy Server Client N istoica cs berkeley edu 13 Web Proxies cont d 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 istoica cs berkeley edu 14 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 istoica cs berkeley edu 15 HTTP 1 1 Performance Persistent connections Pipelined requests responses Support for virtual hosting Efficient caching support istoica cs berkeley edu 16 Persistent Connections Allow multiple transfers over one connection Avoid multiple TCP connection setups Avoid multiple TCP slow starts istoica cs berkeley edu 17 Pipelined Requests Responses 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 Server Client istoica cs berkeley edu Reques t1 Reques t2 Reques t3 r1 Transfe r2 Transfe r3 Transfe 18 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 istoica cs berkeley edu 19 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 istoica cs berkeley edu 20 HTTP 1 1 Caching cont d 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 istoica cs berkeley edu 21 HTTP 1 1 Caching cont d Cache Control no cache get resource only from server only if cached obtain resource only from cache no store don t allow caches to store request response max age response s 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 istoica cs berkeley edu 22 HTTP 1 1 Caching cont d 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 HTTP 1 1 200 OK Vary Accept Language Request will contain Accept Language en us Cache return the response that has Accept Language en us istoica cs berkeley edu 23 Summary HTTP the backbone of WWW Evolution of HTTP has concentrated on increasing the performance Next generations HTTP NG concentrate on increasing extensibility istoica cs berkeley edu 24
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