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Chapter 1 Introduction 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 Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. What’s the Internet: “nuts and bolts” view • millions of connected computing devices: hosts = end systems • running network apps • communication links router server mobile local ISP regional ISP – fiber, copper, radio, satellite – transmission rate = bandwidth • routers: forward packets (chunks of data) workstation company network 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” router server mobile local ISP – loosely hierarchical – public Internet versus private intranet regional ISP • Internet standards – RFC: Request for comments – IETF: Internet Engineering Task Force workstation company network 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 What’s a protocol? a human protocol and a computer network protocol: Hi TCP connection request Hi TCP connection response Got the time? Get http://www.awl.com/kurose-ross 2:00 <file> time Q: Other human protocols? 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 “Cool” internet appliances Web-enabled toaster + weather forecaster IP picture frame http://www.ceiva.com/ World’s smallest web server http://www-ccs.cs.umass.edu/~shri/iPic.html Internet phones 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 source message segment Ht datagram Hn Ht frame Hl Hn Ht M M M M Encapsulation application transport network link physical Hl Hn Ht M link physical Hl Hn Ht M switch destination M Ht M Hn Ht Hl Hn Ht M M application transport network link physical Hn Ht Hl Hn Ht M M network link physical Hn Ht Hl Hn Ht M M router 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 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 HTTP overview • HTTP: hypertext transfer protocol • Web’s application layer protocol • client/server model PC running Explorer – client: browser that requests, receives, “displays” Web objects – server: Web server sends objects in response to requests Server running Apache Web server • HTTP 1.0: RFC 1945 – http://www.rfc-editor.org/rfc/rfc1945.txt • HTTP 1.1: RFC 2068 – http://www.rfc-editor.org/rfc/rfc2068.txt • HTTP state management (cookies): RFC 2109 – http://www.rfc-editor.org/rfc/rfc2109.txt Mac running Navigator 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 aside 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 URL (Uniform Resource Locator) Way of identifying and accessing a web page: Example http://www.lclark.edu/~jmache/index.html “where” “how” Address or name of server Type of transaction “what” (protocol) Resource requested URI (Uniform Resource Identifier) Identifies a resource and includes URLs, but broader in context. See http://www.w3.org/Addressing/ for more details 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 <tag_name> and a matching end tag </tag_name> . Example <title> CS 393 home page </title> 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. HTML page format <HTML> <HEAD> </HEAD> <BODY> </BODY> </HTML> 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 HTML Tags • Tags specify details such as type of text. Example <B> to start bold text </B> to end bold text <I> to start italic text </I> to end italic text HTML page <HTML> <HEAD> </HEAD> <BODY> Hello world <I> My name is <B>Jens</B> </I> </BODY> </HTML> Question What does the previous HTML page display? Answer Hello World My name is Jens HTML page <HTML> <HEAD> </HEAD> <BODY> Line break tag - some tags in <BR> HTML are not in pairs Hello world <P> <I> My name is <B>Jens</B> </I> </BODY> </HTML> Attributes Many tags can have attributes which specify something about the body between tag pair. Example Attributes <FONT COLOR=red SIZE=3 FACE=Times> This text is displayed in red in Times font, about 12 pt. </FONT> HTTP request message • two types of HTTP messages: request, response • HTTP request message: – ASCII request line (human-readable format) (GET, POST, GET /somedir/page.html HTTP/1.1 HEAD commands) Host: www.someschool.edu User-agent: Mozilla/4.0 header Connection: close lines Accept-language:fr Carriage return, line feed indicates end of message (extra carriage return, line feed) HTTP request message: general format 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 HTTP request line (cont.) • URI – Object to retrieve • E.g. http://www.cs.pdx.edu/index.html with a proxy • E.g. /index.html if no proxy • HTTP version – Version being used – HTTP 1.1 • Host: header required • Connection: header supported 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 Other HTTP request headers • Authorization – Authentication info for HTTP authentication • From – User email (when privacy is disabled) Rest of HTTP request Blank-line Separate request headers from POST information End of request Body If POST, send POST information Handling user input (forms) GET method: • Input is uploaded in URL field of request line POST method: • Input is uploaded to server in entity body GET search?name=george&animal=monkey HTTP/1.1 Host: www.somesite.com POST search HTTP/1.1 Host: www.somesite.com Content-type: application/x-www-form-urlencoded name=george&animal=monkey HTTP response message status line (protocol status code status phrase) header lines data, e.g., requested HTML file 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 ... HTTP response message: general format 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 HTTP response status codes A few sample 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 Common HTTP response headers • Server – server software • Content-Encoding – x-gzip • • • • • Content-Length Content-Type Expires Last-Modified ETag Other HTTP response headers • Location – redirection • WWW-Authenticate – request for authentication • Allow – list of methods supported (GET, HEAD, etc) Rest of HTTP response Blank-line Separate headers from data Body Data being returned to client HTTP headers by function • Authentication – Client • Authorization, ProxyAuthorization – 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, IfUnmodified-Since, IfMatch – Server • Last-Modified, Expires, ETag, Age Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet cis.poly.edu 80 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 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! 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 ecommerce 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 Cookies: keeping “state” (cont.) client Cookie file server usual http request msg usual http response + ebay: 8734 Cookie file amazon: 1678 ebay: 8734 Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg one week later: Cookie file amazon: 1678 ebay: 8734 usual http request msg cookie: 1678 usual http response msg server creates ID 1678 for user cookiespecific action cookiespectific action Cookies (continued) What cookies can bring: • authorization • shopping carts • Site preferences • recommendations • user session state (Web e-mail) aside 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 Web caches (proxy server) Goal: satisfy client request without involving origin server (i.e. do not send content that has not changed) Why Web caching? Info on web caching http://www.ircache.net/ • Reduce response time for client http://www.squid.org request. ICP • Reduce traffic on an institution’s http://www.rfcaccess link. editor.org/rfc/rfc2186.txt • Reduce load on servers. http://www.rfc• Enables “poor” content providers to editor.org/rfc/rfc2187.txt effectively deliver content (but so does P2P file sharing) 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 origin server client Proxy server • 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, client origin server 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 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache 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 10 Mbps access link institutional network 10 Mbps LAN institutional cache Caching example (cont) Install cache • suppose hit rate is .4 origin servers public Internet 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 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache Conditional GET • Goal: don’t send object if cache cache has up-to-date cached HTTP request msg If-modified-since: version <date> • cache: specify date of cached copy in HTTP HTTP response HTTP/1.0 request If-modified-since: <date> • server: response contains no object if cached copy is upto-date: HTTP/1.0 304 Not Modified server object not modified 304 Not Modified HTTP request msg If-modified-since: <date> HTTP response HTTP/1.0 200 OK <data> object modified 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 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 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) (RedHat/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"