Lec 7(HTTP Protocol)
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This document summarizes a lecture on computer networks and the hypertext transfer protocol (HTTP). It first reviews the early history of computer networking and the development of the world wide web. It then provides details on HTTP, including requests and responses, methods, status codes, and cookies. It discusses how caching works to improve performance by satisfying requests locally when possible. Methods like If-Modified-Since are described which check if a cached object has been updated before retrieving from the origin server.
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Lec 7(HTTP Protocol)
- 1. Computer Networks – CSE331 Lecture 7 2 Hrs
- 2. First, a review… Until 1990s universities, researchers => remote loging, file transfer, email World wide web (www) in 1994 Tim Berners Lee web page consists of objects object can be HTML file, JPEG image, Java applet, audio file,… web page consists of base HTML-file which includes several referenced objects each object is addressable by a URL, e.g., www.someschool.edu/someDept/pic.gif host name path name
- 3. hypertext transfer protocol HTTP/Web Client: browser HTTP Server: apache, IIS Web page: index.html Object: picture, javascript file URL: http://www.uet.edu.pk/cs.html Uses TCP Stateless: server don’t remember Non-persistent: one connection per object Persistent: leave connection open Uses ASCII messages not binary PC running Firefox browser server running Apache Web server iphone running Safari browser
- 4. two types of HTTP messages: request, response HTTP request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) header lines carriage return, line feed at start of line indicates end of header lines GET /department/cs HTTP/1.1rn Host: www.rcet.uet.edu.pkrn User-Agent: Firefox/3.6.10rn Accept: text/html,application/xhtml+xmlrn Accept-Language: en-us,enrn Accept-Encoding: gzip,deflatern Accept-Charset: ISO-8859-1,utf-8rn Connection: keep-alivern rn carriage return character line-feed character
- 5. request line header lines body method sp sp cr lfversionURL cr lfvalueheader field name cr lfvalueheader field name ~~ ~~ cr lf entity body~~ ~~
- 6. HTTP/1.0: GET, POST HEAD asks server to leave requested object out of response HTTP/1.1: GET, POST, HEAD PUT uploads file in entity body to path specified in URL field DELETE deletes file specified in the URL field
- 8. status line (protocol status code status phrase) header lines data, e.g., requested HTML file HTTP/1.1 200 OKrn Date: Sun, 19 Feb 2017 18:41:45 GMT rn Server: Apache/2.2.27 (Unix) rn Expires: Thu, 19 Nov 1981 08:52:00 GMT Accept-Ranges: bytesrn Content-Length: 2652rn Keep-Alive: timeout=10, max=100rn Connection: Keep-Alivern Content-Type: text/html; charset=ISO-8859-1rn rn data data data data data ... persistent
- 9. 200 OK request succeeded, requested object later in this msg 301 Moved Permanently requested object moved, new location specified later in this msg (Location:) 400 Bad Request request msg not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported In 1st line in server-to-client response message. some sample codes:
- 10. many Web sites use cookies four components: 1) cookie header line of HTTP response message 2) cookie header line in next HTTP request message 3) cookie file kept on user’s host, managed by user’s browser 4) back-end database at Web site example: visits specific e-commerce site for first time when initial HTTP requests arrives at site, site creates: unique ID entry in backend database for ID
- 11. client server usual http response msg usual http response msg cookie file one week later: usual http request msg cookie: 1678 cookie- specific action access ebay 8734 usual http request msg Amazon server creates ID 1678 for user create entry usual http response set-cookie: 1678ebay 8734 amazon 1678 usual http request msg cookie: 1678 cookie- specific action access ebay 8734 amazon 1678 backend database
- 12. user sets browser: Web accesses via cache browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin server, then returns object to client goal: satisfy client request without involving origin server client proxy server client origin server origin server
- 13. cache acts as both client and server server for original requesting client client to origin server typically cache is installed by ISP (university, company, residential ISP) why Web caching? reduce response time for client request reduce traffic on an institution’s access link Internet dense with caches: enables “poor” content providers to effectively deliver content (so too does P2P file sharing)
- 14. origin serverspublic Internet institutional network 100 Mbps LAN 15 Mbps access link assumptions: avg object size: 1Mbits avg request rate from browsers to origin servers:15/sec RTT from institutional router to any origin server: 2 sec access link rate: 15 Mbps consequences: LAN utilization: 0.15 = 15% access link utilization = 1 = 100% total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + μsecs problem!
- 15. assumptions: avg object size: 1Mbits avg request rate from browsers to origin servers:15/sec RTT from institutional router to any origin server: 2 sec access link rate: 15 Mbps consequences: LAN utilization: 15% access link utilization = 100% total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + μsecs origin servers 15 Mbps access link 150 Mbps 150 Mbps msecs Cost: increased access link speed (not cheap!) 10% public Internet institutional network 100Mbps LAN
- 16. institutional network 100Mbps LAN origin servers 15 Mbps access link local web cache assumptions: avg object size: 1Mbits avg request rate from browsers to origin servers:15/sec RTT from institutional router to any origin server: 2 sec access link rate: 15 Mbps consequences: LAN utilization: 15% access link utilization = total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + μsecs ? ? How to compute link utilization, delay? Cost: web cache (cheap!) public Internet
- 17. Calculating access link utilization, delay with cache: suppose cache hit rate is 0.4 40% requests satisfied at cache, 60% requests satisfied at origin origin servers 15 Mbps access link access link utilization: 60% of requests use access link data rate to browsers over access link = 0.6*15 Mbps = 9 Mbps utilization = 9/15 = 0.6 total delay = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache) = 0.6 (2.01) + 0.4 (~msecs) = ~ 1.2 secs less than with 150 Mbps link (and cheaper too!) public Internet institutional network 100 Mbps LAN local web cache
- 18. Goal: don’t send object if cache has up-to-date cached version no object transmission delay lower link utilization cache: specify date of cached copy in HTTP request If-modified-since: <date> server: response contains no object if cached copy is up- to-date: HTTP/1.0 304 Not Modified HTTP request msg If-modified-since: <date> HTTP response HTTP/1.0 304 Not Modified object not modified before <date> HTTP request msg If-modified-since: <date> HTTP response HTTP/1.0 200 OK <data> object modified after <date> client server
- 20. Ross book 2.2 Forouzan book Chapter 22 HTTP methods example See attached file http_methods.pdf Key Differences between HTTP1.1 & 1.0 www8.org/w8-papers/5c-protocols/key/key.html