1. 1 HTTP - Hypertext Transfer Protocol Arthur : Yigal Eliaspur Date : 28.1.2001

2. 2 HTTP Overview n Web’s application-layer protocol n in use by the WWW since 1990 n client/server paradigm –in the web: clients : browsers (IExplorer,Netscape..) server : web servers (Apache,IIS..) n Request/Response Protocol: n Web servers usually using TCP port 80 response request SC

3. 3 HTTP Overview (cont.) n Stateless protocol - HTTP server maintains no information about the client.

4. 4 HTTP Versions n HTTP 0.9 Simple GET protocol for the Web limits on data transfer (1024 characters) n HTTP 1.0 Headers give information about the data transferred. Greater data type/quantity transfer in both directions n HTTP 1.1 Supports hierarchical proxy servers caching persistent connections

5. 5 HTTP 0.9 GET example n telnet www.cs.huji.ac.il 80 n GET /~dbsi/index.html n output :...................... Connection closed by foreign host

6. 6 HTTP 1.0 n developed between 1992 and 1996. n Exchange more than simple text  Headers allowed in both requests and responses  Extends GET request to allow headers  Adds HEAD request to get information  Adds POST request, sends information with the request

7. 7 Request message format

8. 8 Response message format

9. 9 HTTP Request/Response example

10. 10 Response-codes

11. 11 Headers types n General –Date, Pragma.. n Request –Authorization, From, If-Modifed-Since, Referer, User-Agent.. n Response –Location, Server, WWW-Authenticate... n Entity –Allow, Content-Encoding, Content-Length, Content-Type, Expires, Last-Modified, extension-header...

12. 12 POST & HEAD messages  POST  sends information with the request in the Entity Body.  Useful when the user fills out a form.  HEAD  return only the request result without the data itself (I.e. only the Status line and the Header lines)  use for debugging HTTP servers and for page update checking.

13. 13 Upgrading Header n allows the client to specify what additional communication protocols it supports n The server may choose to switch protocols, but this is not mandatory. n Example: Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

14. 14 Caching n Why? –Reduces response time  Request is satisfied from cache closest to browser  Takes less time to get the page and display it –Reduces traffic  Each page only accessed from the server once  Reduces bandwidth used by browser –Saves money if client is paying by traffic –Keeps bandwidth requirements down

15. 15 Caching (cont.) n Risks? –Might not be ``semantically transparent'’ the response is different from what would have been returned by the origin server.

16. 16 Caching in HTTP/1.0 n simple caching mechanism: Origin server may mark a response, using the Expires header cache validity checking using a ”conditional request“ which include : If-Modified-Since & Last- Modified headers. n server responds: –304 (Not Modified) –200 (OK) + the New entry.

17. 17 Caching in HTTP/1.0 (cont.) The Pragma: no-cache request Header indicate that a request should not be satisfied from a cache. n PROBLEM - origin servers/clients can’t give full and explicit instructions to caches (will be explained later)

18. 18 Caching in HTTP/1.1 n retains the basic HTTP/1.0 design –new features –more careful specifications of the existing features. Entry start as fresh. Become stale - when reaches its expiration time. must revalidate it with the origin server.

19. 19 Caching in HTTP/1.1 (cont.) cache validator string : entity tag. –two responses resource with the same entity tag must be identical. – Can include : fine-grained timestamp, internal database pointer... –If-None-Match header with one or more entity tags. Much stronger then If-Modified-Since.

20. 20 Caching in HTTP/1.1 (cont.) Cache-Control header –server/client implicit directives to caches –directives examples: max-age - relative expiration time. –HTTP/1.0 Expires header can lead to clock skew failure. no-transform - prevent proxies response transformations. – like reduce image complexity over a slow link (WAP) private & no-store - prevent the storage of some or all of a response.

21. 21 Caching in HTTP/1.1 (cont.) Vary header - include list of headers that identical the request beside the URL field. – For example : Accept-Language, Accept-Charset...

22. 22 Cooperative Cashing

23. 23 Cooperative Cashing (cont.) n Higher level cache ( e.g. national cash) –larger user population –higher hit rates. n Multiple Web cashes which cooperate => Improve overall performance. n Cooperative cashes usualy built from clusters –divide the traffic overhead –improve storage capacity

24. 24 Cooperative Cashing (cont.) –which of the cashes we sould ask for a particular doc? Hash routing (of URLs) - an object want be present in more then one cash. n HTTP/1.1 introduces the concept of hop-by-hop headers: –message headers that apply only to a given connection, and not to the entire path. –This enable much more power with proxies (cashes) usage.

25. 25 Cooperative Cashing (cont.) n HTTP 1.1 hop-by-hop headers: Connection –options that are desired for that particular connection (e.g connection:close.) Public –lists the set of methods supported by the server Proxy-Authenticate –enable authentication methods between two hops. Transfer-Encoding - –compression method between two hops. Upgrade –additional communication protocols supported.

26. 26 Persistent & Non Persistent Connections. n Persistent Connections: Opens new TCP connection for each request. For example : for a web page with 10 image - 11 new TCP connections is needed. Used in HTTP/1.0 n nonpersistent connections : one TCP connection can serve more then one request/response pair. Less connection establishing overhead, smaller slow-start delay. Used as default in HTTP/1.1

27. 27 Persistent & Non Persistent Connections.(cont.) n nonpersistent connections, two types: –without pipelining the client issues a new request only when the previous response has been arrived. – with pipelining client send the request as soon as it encounters a reference. Multiple request/response on the same TCP packet. Or on back-to-back packets.

28. 28 Compression n most image formats (GIF, JPEG, MPEG) are precompressed. n many other data types used in the Web are not. n compression could save almost 40% of the bytes sent via HTTP n need for negotiating the use of codings.

29. 29 Compression (cont.) Client send : Accept-Encoding header –indicate what content-codings it can handle, and which ones it prefers. n Server Send : –Content-Encoding header - for end-to-end coding indication. –Transfer-Encoding header - for hop-to-hop coding indication. (supported only in HTTP/1.1)

30. 30 W3C Performance Measurements n "Microscape" Benchmark, 43 inline images Scenarios  HTTP/1.0: using 4 simultaneous connections  HTTP/1.1: using 1 persistent connection  HTTP/1.1 pipeline: using 1 persistent connection  HTTP/1.1 pipeline + compression: using 1 connection

31. 31 W3C Performance Measurements (cont.)

32. 32 Authentication n Many sites require users to provide a username and password in order to access the documents housed on the server. n Provide mechanism for keeping track of users (more then security mechanism). n How does it’s work? Client send –ordinary request message server responds with –401 Authorization Required status code –WWW-Authenticate header which specified how to perform authentication

33. 33 Authentication (cont.) Client resend –the requested message but this time including Authorization header (e.g. user-name & password.) The client continue to add this header for each following request to that server.

34. 34 Cookies n Another site mechanism for keeping tracks of users. n Example: –Client contact a web site for the first time. –Server response with : Set-cookie : 1678453 header –client store the cookie value and the server name in a special “cookie file”. –For each further request for that server the client will add the Cookie : 1678453 header

35. 35 Cookies (cont.) n Usage: –server requires authentication but doesn’t want to hassle a user with a user-name and password. –Remembering user’s preferences for advertising. –Enable creating a virtual shopping cart. n Problems –users who accesses the same site from different machines.

36. 36 References n http://www.ietf.org/rfc/rfc2068.txt http://www.ietf.org/rfc/rfc2068.txt n http://www.ietf.org/rfc/rfc1945.txt http://www.ietf.org/rfc/rfc1945.txt n http://www.w3.org/Protocols/ http://www.w3.org/Protocols/ n http://www8.org/w8-papers/5c- protocols/key/key.html http://www8.org/w8-papers/5c- protocols/key/key.html n Computer Networks by Joames Fokurose & Keith W.Ross.