2. Chapter 1 Introduction Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright 1996-2005 J.F Kurose and K.W. Ross, All Rights Reserved

3. What’s the Internet: “nuts and bolts” view millions of connected computing devices: hosts = end systems running network apps communication links –fiber, copper, radio, satellite –transmission rate = bandwidth routers: forward packets (chunks of data) local ISP company network regional ISP router workstation server mobile

4. What’s the Internet: “nuts and bolts” view protocols control sending, receiving of msgs –e.g., TCP, IP, HTTP, FTP, PPP Internet: “network of networks” –loosely hierarchical –public Internet versus private intranet Internet standards –RFC: Request for comments –IETF: Internet Engineering Task Force local ISP company network regional ISP router workstation server mobile

5. What’s a protocol? human protocols: “what’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

6. What’s a protocol? a human protocol and a computer network protocol: Q: Other human protocols? Hi Got the time? 2:00 TCP connection request TCP connection response Get http://www.awl.com/kurose-ross time

7. Some network apps E-mail Web Instant messaging Remote login P2P file sharing Multi-user network games Streaming stored video clips Internet telephone Real-time video conference Massive parallel computing

8. “Cool” internet appliances World’s smallest web server http://www-ccs.cs.umass.edu/~shri/iPic.html IP picture frame http://www.ceiva.com/ Web-enabled toaster + weather forecaster Internet phones

9. Internet protocol stack application: (L7 & L6 of OSI) supporting network applications –FTP, SMTP, HTTP transport: (L5 & L4 of OSI) host-host data transfer –TCP, UDP network: routing of datagrams from source to destination –IP, routing protocols link: data transfer between neighboring network elements –PPP, Ethernet physical: bits “on the wire” application transport network link physical

10. message segment datagram frame source application transport network link physical HtHt HnHn HlHl M HtHt HnHn M HtHt M M destination application transport network link physical HtHt HnHn HlHl M HtHt HnHn M HtHt M M network link physical link physical HtHt HnHn HlHl M HtHt HnHn M HtHt HnHn HlHl M HtHt HnHn M HtHt HnHn HlHl M HtHt HnHn HlHl M router switch Encapsulation

11. Background: Addressing For a process to receive messages, it must have an identifier A host has a unique32-bit IP address Q: does the IP address of the host on which the process runs suffice for identifying the process? Answer: No, many processes can be running on same host Identifier includes both the IP address and port numbers associated with the process on the host. Example port numbers: –HTTP server: 80 –Mail server: 25 More on this later

12. Web and HTTP First some jargon Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio file,… Each object is addressable by a URL Web page consists of base HTML-file which includes several referenced objects Example URL: www.someschool.edu/someDept/pic.gif host name path name

13. HTTP overview HTTP: hypertext transfer protocol Web’s application layer protocol client/server model –client: browser that requests, receives, “displays” Web objects –server: Web server sends objects in response to requests HTTP 1.0: RFC 1945 –http://www.rfc-editor.org/rfc/rfc1945.txthttp://www.rfc-editor.org/rfc/rfc1945.txt HTTP 1.1: RFC 2068 –http://www.rfc-editor.org/rfc/rfc2068.txthttp://www.rfc-editor.org/rfc/rfc2068.txt HTTP state management (cookies): RFC 2109 –http://www.rfc-editor.org/rfc/rfc2109.txthttp://www.rfc-editor.org/rfc/rfc2109.txt PC running Explorer Server running Apache Web server Mac running Navigator HTTP request HTTP response

14. HTTP overview (continued) Uses TCP: client initiates bi-directional TCP connection (via socket) to server, port 80 server accepts TCP connection from client HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) –Messages encoded in text TCP connection closed HTTP is “stateless” server maintains no information about past client requests Protocols that maintain “state” are complex! past history (state) must be maintained if server/client crashes, their views of “state” may be inconsistent, must be reconciled aside

15. URL (Uniform Resource Locator) Way of identifying and accessing a web page: Example http://www.lclark.edu/~jmache/index.html “how” Type of transaction (protocol) “where” Address or name of server “what” Resource requested

16. URI (Uniform Resource Identifier) Identifies a resource and includes URLs, but broader in context. See http://www.w3.org/Addressing/ for more detailshttp://www.w3.org/Addressing/

17. Mark-up Languages A way of describing information in a document. Standard Generalized Mark-Up Language (SGML) - a specification for a mark-up language ratified in 1986. Key aspect - using pairs of tags that surround information - a begin tag and a matching end tag. Example CS 393 home page

18. HyperText Markup Language (HTML) A mark-up language used in web pages. “Hypertext” refers to the text’s ability to link to other documents. “Markup” refers to providing information to tell browser how to display page and other things.

19. HTML page format Signifies an HTML document Head section includes information about document - “metadata” Body section contains text and references to images to be displayed End of document

20. HTML Tags Tags specify details such as type of text. Example to start bold text to end bold text to start italic text to end italic text

21. HTML page Hello world My name is Jens

22. Question What does the previous HTML page display? Answer Hello World My name is Jens

23. HTML page Hello world My name is Jens Line break tag - some tags in HTML are not in pairs

24. Attributes Many tags can have attributes which specify something about the body between tag pair. Example This text is displayed in red in Times font, about 12 pt. Attributes

25. HTTP request message two types of HTTP messages: request, response HTTP request message: –ASCII (human-readable format) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) request line (GET, POST, HEAD commands) header lines Carriage return, line feed indicates end of message

26. HTTP request message: general format

27. HTTP request line (methods) HTTP/1.0 GET –Return object specified by URI POST –Send data to server (forms) HEAD –asks server to leave requested object out of response –Return headers only of GET 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 OPTIONS, TRACE, CONNECT

28. HTTP request line (cont.) URI –Object to retrieve E.g. http://www.cs.pdx.edu/index.html with a proxyhttp://www.cs.pdx.edu/index.html E.g. /index.html if no proxy HTTP version –Version being used –HTTP 1.1 Host: header required Connection: header supported

29. Common HTTP request headers Accept –Acceptable document types, encodings, languages, character sets If-Modified-Since –For use with caching Referer –URL which caused this page to be requested User-Agent Host –For multiple web sites hosted on same server Connection –Keep connection alive for subsequent request or close connection

30. Other HTTP request headers Authorization –Authentication info for HTTP authentication From –User email (when privacy is disabled) Rest of HTTP request r Blank-line  Separate request headers from POST information  End of request r Body  If POST, send POST information

31. Handling user input (forms) POST method: Input is uploaded to server in entity body GET method: Input is uploaded in URL field of request line GET search?name=george&animal=monkey HTTP/1.1 Host: www.somesite.comwww.somesite.com POST search HTTP/1.1 Host: www.somesite.comwww.somesite.com Content-type: application/x-www-form-urlencoded name=george&animal=monkey

32. HTTP response message HTTP/1.1 200 OK Connection close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data... status line (protocol status code status phrase) header lines data, e.g., requested HTML file

33. HTTP response message: general format

34. HTTP response format Status-line –HTTP version –3 digit response code 1XX – informational 2XX – success 3XX – redirection 4XX – client error 5XX – server error –Reason phrase

35. HTTP response status codes 200 OK –request succeeded, requested object later in this message 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 505 HTTP Version Not Supported A few sample codes:

36. Common HTTP response headers Server –server software Content-Encoding –x-gzip Content-Length Content-Type Expires Last-Modified ETag

37. Other HTTP response headers Location –redirection WWW-Authenticate –request for authentication Allow –list of methods supported (GET, HEAD, etc) Rest of HTTP response r Blank-line  Separate headers from data r Body  Data being returned to client

38. HTTP headers by function Authentication –Client Authorization, Proxy- Authorization –Server WWW-authenticate, Proxy-Authenticate User, server tracking –Client Cookie, Referer, From, User-agent –Server Set-cookie, Server Caching –General Cache-control, Pragma –Client If-Modified-Since, If- Unmodified-Since, If- Match –Server Last-Modified, Expires, ETag, Age

39. Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: Opens TCP connection to port 80 (default HTTP server port) at cis.poly.edu. Anything typed in sent to port 80 at cis.poly.edu telnet cis.poly.edu 80 2. Type in a GET HTTP request: GET /~ross/ HTTP/1.1 Host: cis.poly.edu By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server 3. Look at response message sent by HTTP server!

40. User-server state: cookies Many major Web sites use cookies Four components: 1) cookie header line of HTTP response message Set-cookie: 2) cookie header line in HTTP request message Cookie: 3) cookie file kept on user’s host, managed by user’s browser 4) back-end database at Web site Example: –Susan access Internet always from same PC –She visits a specific e- commerce site for first time –When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID

41. Cookies: keeping “state” (cont.) client server usual http request msg usual http response + Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg cookie- specific action cookie- spectific action server creates ID 1678 for user entry in backend database access Cookie file amazon: 1678 ebay: 8734 Cookie file ebay: 8734 Cookie file amazon: 1678 ebay: 8734 one week later:

42. Cookies (continued) What cookies can bring: authorization shopping carts Site preferences recommendations user session state (Web e-mail) Cookies and privacy: cookies permit sites to learn a lot about you you may supply name and e-mail to sites search engines use redirection & cookies to learn yet more advertising companies obtain info across sites aside

43. Web caches (proxy server) Why Web caching? Reduce response time for client request. Reduce traffic on an institution’s access link. Reduce load on servers. Enables “poor” content providers to effectively deliver content (but so does P2P file sharing) Goal: satisfy client request without involving origin server (i.e. do not send content that has not changed) Info on web caching r http://www.ircache.net/ http://www.ircache.net/ r http://www.squid.org http://www.squid.org r ICP r http://www.rfc- editor.org/rfc/rfc2186.txt http://www.rfc- editor.org/rfc/rfc2186.txt r http://www.rfc- editor.org/rfc/rfc2187.txt http://www.rfc- editor.org/rfc/rfc2187.txt

44. More about Web caching 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 Done directly at client –Via browser web cache Along path from client to origin server –Via proxy web cache –Proxy acts as both client and server –Typically cache is installed by ISP (university, company, residential ISP) client Proxy server client HTTP request HTTP response HTTP request HTTP response origin server origin server

45. Caching example Assumptions average object size = 100,000 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 origin servers public Internet institutional network 10 Mbps LAN 1.5 Mbps access link institutional cache

46. Caching example (cont) Possible solution increase bandwidth of access link to, say, 10 Mbps Consequences 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 origin servers public Internet institutional network 10 Mbps LAN 10 Mbps access link institutional cache

47. Caching example (cont) Install cache suppose hit rate is.4 Consequence 40% requests will be satisfied almost immediately 60% requests satisfied by origin server utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec) total avg delay = Internet delay + access delay + LAN delay =.6*(2.01) secs +.4*milliseconds < 1.4 secs origin servers public Internet institutional network 10 Mbps LAN 1.5 Mbps access link 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: server: response contains no object if cached copy is up- to-date: HTTP/1.0 304 Not Modified cache server HTTP request msg If-modified-since: HTTP response HTTP/1.0 304 Not Modified object not modified HTTP request msg If-modified-since: HTTP response HTTP/1.0 200 OK object modified

49. HTTP caching Additional caching methods –ETag and If-Match HTTP 1.1 has file signature as well When/how often should the original be checked for changes? –Check every time? –Check each session? Day? Etc? –Use Expires header If no Expires, often use Last-Modified as estimate

50. Example Cache Check Request GET / HTTP/1.1 Accept: */* Accept-Language: en-us Accept-Encoding: gzip, deflate If-Modified-Since: Mon, 29 Jan 2001 17:54:18 GMT If-None-Match: "7a11f-10ed-3a75ae4a" User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0) Host: www.cs.pdx.edu Connection: Keep-Alive

51. Example Cache Check Response HTTP/1.1 304 Not Modified Date: Tue, 27 Mar 2001 03:50:51 GMT Server: Apache/1.3.14 (Unix) (Red- Hat/Linux) mod_ssl/2.7.1 OpenSSL/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24 Connection: Keep-Alive Keep-Alive: timeout=15, max=100 ETag: "7a11f-10ed-3a75ae4a"