CS 355. Computer Networking. Wei Lu, Ph.D., P.Eng.

Transcription

1 CS 355 Computer Networking Wei Lu, Ph.D., P.Eng.

2 Chapter 2: Application Layer Overview: Principles of network applications? Introduction to Wireshark Web and HTTP FTP Electronic Mail SMTP, POP3, IMAP DNS P2P applications Socket programming with TCP and UDP

3 Web and HTTP: HTTP message: request and response Web techniques: cookie and cache

4 HTTP Message: Request and Response

5 HTTP Request Message: Post POST used to submit content to web server request line header lines Entity body

6 HTTP Request Message: an example POST is one of the http request methods, used to submit web form to server /LoginServlet is the path of file on the server used to submit web content HTTP/1.1 indicates the HTTP version

7 HTTP Request Message: an example Host identify the host name and port of URL, here the default port is not 80, instead we use 8080 for http web service, thus the host information is: :8080

8 HTTP Request Message: an example User-Agent identify what browser user use, here is Mozilla/5.0

9 HTTP Request Message: an example ACCEPT is the file format that can be accepted by browser, here is text/html, application/xhtml+xml or application/xml

10 HTTP Request Message: an example Accept-Language defines the language that can be accepted by the browser, here is English

11 HTTP Request Message: an example Accept-Encoding identify the encoding method supported by the browser, here is gzip and deflate

12 HTTP Request Message: an example Accept-Charset identify the character encoding method that can be accepted by the browser, default one for English is ISO

13 HTTP Request Message: an example Keep-Alive determines how long keep-alive connections are kept alive; default time is 115 sec

14 HTTP Request Message: an example Connection determine the http connection is persistent

15 HTTP Request Message: an example Referer defines where the submit request come from, it identifies the relationship in between file loginservlet and web page:

16 HTTP Request Message: an example Content-Type defines content type that to be submitted to the server, here is the web page form of application

17 HTTP Request Message: an example Content-Length is the length of content to be submitted to server, here is 51 characters

18 HTTP Request Message: an example Entity-Body of POST message

19 HTTP request message: general format

20 Other Typical HTTP Request Messages HEAD request file header only, server doesn t return entity body, often used by search engine PUT uploads file in entity body to path specified in URL field DELETE deletes file specified in the URL field Refer RFC 2616 for HTTP 1.1 at For more details

21 HTTP response messages status line (protocol status code status phrase) header lines data, e.g., requested HTML file

22 HTTP Response Message: an example HTTP Response Status determine the status of the http response message, here status code 200 means request succeed, requested resource is found, everything is OK

23 HTTP Response Message: an example Content-Type determine the type of response content, here is text/html

24 HTTP Response Message: an example Content-Length is the length of the response content, here is the length of returned html file, date means the time the response generated, and server is type of web server

25 HTTP Response Message: an example Response Entity Body

26 HTTP response message: general format

27 Other Typical HTTP response status codes 301 Moved Permanently requested object moved, new location specified later in this message (Location:) 400 Bad Request request message not understood by server 404 Not Found requested document not found on this server 500 Internal Server Error 505 HTTP Version Not Supported Refer RFC 2616 for HTTP 1.1 at for more details

28 Web techniques: Cookies

29 What is a Cookie A cookie is a piece of text that a web server can store on the hard drive of your computer. Cookies allow a web site to store information on your computer and later retrieve it. Of course a piece of text doesn t look like a real cookie but we ll represent an internet cookie as follows text

30 When you see a particular web site in the first time Web browser client software What is a Cookie Get a web page Server create ID 1678 for client Create entry Client Server put a cookie on your computer text usual http response Set-cookie: 1678 Backend database

31 What is a Cookie The first time to visit Amazon, set cookie in response msg.

32 What is a Cookie The first time to visit Amazon, set cookie in client computer

33 Next time you visit this particular web site Web browser client software Get a web page using http request msg. by Cookie: 1678 What is a Cookie Cookie specification assess Client Server text usual http response Backend database

34 What is a Cookie Next time to visit Amazon, set cookie in request msg.

35 More about cookies A cookie is simply a piece of data with a name A cookie is not a program A cookie can t do anything When a website puts a cookie on your hard drive It can retrieve only the information that it has placed on your computer It cannot retrieve information from other cookie files It cannot retrieve any other information from your computer

36 Why cookies Quite often the cookie is something such as an ID unique to each visitor Your ID as a site visitor is put into a database used by the web site So when you next go to that web site, it can use the cookie to recognize that you were there before Web sites can count How many visitors it had How many are new versus repeat visitors How often any visitor has been to that site

37 Why cookies Cookies are used by web sites that sell things online As you click on various items to purchase which are usually on many different pages of the site, you are adding them to your cart The web site knows who the cart owner is because it left a cookie on your computer as an identifier So at checkout time, it knows which items you are purchasing, i.e. how one-click purchase works in Amazon text

38 Why cookies Cookies are stored on the computer s hard drive which means You can delete them whenever you want to A website will not be able to use the cookie it left on one computer to identify you on another computer

39 Why cookies Highest privacy level Question: if I select Block all Cookies can I still purchase items from a website? No, you wouldn t be able to purchase online, unless that website allows you to register and sign in

40 Why cookies Lowest privacy level Question: if I select Accept All Cookies is that dangerous? It may not be dangerous but you may get more tracking cookies than you want.

41 What is a Web Cache Cache is a place where temporary copies of objects are stored Cached information is generally closer to the requester than the permanent information is Objects -HTML pages, images, files

42 What is a Web Cache

43 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 Origin Server is the server on which the given resource resides or is to be created What is a Web Cache client client HTTP request HTTP response HTTP request HTTP response Proxy server HTTP request HTTP response origin server Proxy- An intermediary program which acts both as a server and a client which requests on behalf of the other clients Proxy is not necessarily a cache * Proxy does not always cache the replies passing through it * It may be used on a firewall to monitor accesses origin server

44 Why Web Cache World Wide Web is of an exponential growth in size, Web cache can: reduce latency, ie response time for client request reduce network traffic reduce load on origin servers isolate end users from network failures cache acts as both client and server typically cache is installed by ISP (university, company, residential ISP)

45 Why Web Cache: an example Assumptions average object size = 1,000,000 bits avg. request rate from institution s browsers to origin servers = 15/sec delay from institutional router to any origin server and back to router = 2 sec Consequences utilization on LAN = 15% utilization on access link = 100% total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds institutional network public Internet 15 Mbps access link 100 Mbps LAN origin servers institutional cache

46 Why Web Cache: an example possible solution increase bandwidth of access link to, say, 100 Mbps consequence utilization on LAN = 15% utilization on access link = 15% Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs often a costly upgrade institutional network public Internet 100 Mbps access link 100 Mbps LAN origin servers institutional cache

47 Why Web Cache: an example possible solution: install cache suppose hit rate is 0.4 consequence 40% requests will be satisfied almost immediately 60% requests satisfied by origin server utilization of access link reduced to 60%, resulting in a negligible delays (say 10 msec) institutional network public Internet 15 Mbps access link origin servers 100 Mbps LAN institutional cache

48 Conditional Get Goal: don t send object if cache has up-to-date cached version cache: specify date of cached copy in HTTP request If-modified-since: <date> server: response contains no object if cached copy is upto-date: HTTP/ Not Modified cache HTTP request msg If-modified-since: <date> HTTP response HTTP/ Not Modified HTTP request msg If-modified-since: <date> HTTP response HTTP/ OK <data> server object not modified object modified

49 Conditional Get: an example

50 Conditional Get: an example

51 Disadvantage of Web Cache With cached data there is always a chance of receiving stale information Content providers lose access counts when cache hits are served Manual configuration is often required Operation of cache requires additional resources In some situations the cache can be a single point of failure

52 Other Features of Web Cache Depending on the perspective the following may be good or bad * Cache requests on behalf of clients; the servers never see the clients IP addresses * Cache provides an easy opportunity to monitor and analyze browsing activities * Cache can be used to block certain requests

53 Thank you for your attendance Any questions?