Last lesson Moving beyond individual computers Networks Layer - PDF document

11/21/2009 Last lesson  Moving beyond individual computers  Networks  Layer model  The Internet  IP addresses  Domain names 1

11/21/2009 This week  Application Protocols  Email  SMTP, POP, IMAP  The Web  HTTP The Layer Model 2

11/21/2009 Application protocols  Protocol: set of rules for communication  Messages over Internet via application layer  Examples... Email  Message headers  From, To, Subject, Date, ...  Message body  Actual text, plus attachments 3

11/21/2009 MIME Multipu tipurpose ose Internet et Mail Exten ensio sions  Emails encoded in ASCII  Non-ASCII character sets?  Non-text attachments? (e.g. images)  Non-ASCII data can be MIME encoded  Translated to ASCII  Transmitted as part of email header/body  Header gives decoding information SMTP Simple e Mail Transf sfer er Prot otoc ocol  Used to send emails  Extended protocol ESMTP  Client-server communication  Client makes requests, server responds  1. User’s client to local server  2. Local server (as client) to destination server 4

11/21/2009 SMTP Finding ding the Destination tination  account@domain  DNS: MX record for domain is mail server  mas02jg@gold.ac.uk  Domain: gold.ac.uk  MX record: mailhub.gold.ac.uk  Sends to destination server via SMTP SMTP Request uests & Responses onses Client Requests Server Responses  End: QUIT  2xx (Accept)  Greeting: HELO, (ESMTP: EHLO)  3xx, 4xx (Temporary Reject)  Envelope: MAIL FROM, RCPT TO  5xx (Permanent Reject)  Data: DATA 5

11/21/2009 <C initiates connection> S: 220 mail.gold.ac.uk ESMTP Goldsmiths C: EHLO gmail.com S: 250 Hello gmail.com C: MAIL FROM andy@gmail.com S: 250 OK C: RCPT TO: bob@gold.ac.uk S: 250 Accepted C: DATA S: 354 Enter message, ending with “.” <C sends email headers and body> S: 250 OK id=1L4kge-0001ud-SC C: QUIT S: 221 mail.gold.ac.uk closing connection POP Post st Office ce Prot otoc ocol ol  Used to receive emails  Mail server places emails into account’s mailbox  Email client accesses mailbox via POP  Typically move all messages to email client  Some servers allow mail to be left there  But no information kept about emails 6

11/21/2009 POP Request uest & Responses onses Client Requests Server Responses  Security: USER, PASS,  Accept: +OK APOP  Error: – ERR  Info: STAT, LIST, UIDL  Fetch: RETR, DELE  QUIT <C initiates connection> S: +OK POP3 server ready pop.gold.ac.uk C: USER mas03jg S: +OK Name is a valid mailbox C: PASS mypassword S: +OK Mailbox locked and ready C: LIST S: +OK 1 message (321 octets) S: 1 321 S: . C: RETR 1 S: +OK Message follows <S sends message 1> C: DELE 1 S: +OK message 1 deleted C: QUIT S: +OK POP3 server signing off (maildrop empty) 7

11/21/2009 IMAP Inter erne net t Message age Acces ess s Prot otocol ocol  Used to receive emails (alternative to POP)  Emails remain on mail server  Organised in IMAP mail folders  Knows which have been read or flagged  Multiple email clients can access server  Can access all your email from different locations A sends an email to B and C 8

11/21/2009 Hypermedia  Interlinked documents  Text, images, audio/video  User chooses own path by selecting links  Since 1960s, e.g. NLS  Many single-machine systems, e.g. Hypercard The World Wide Web  A distributed collection of hypermedia documents  accessed over the Internet  Invented in 1990 by Tim Berners-Lee at CERN  URLs, HTTP, HTML  Estimated 100 million pages by 2008 9

11/21/2009 URL Un Uniform orm Resour urce ce Locator or  Every web page has a URL (an address)  http://doc.gold.ac.uk/~mas02jg/fy04/labs/lab s08.html  Protocol: http  Domain name of web server: doc.gold.ac.uk  Directory path: ~mas02jg/fy04/labs/  Document file name: labs08.html  Web browser uses URL to fetch page from server HTTP Hyper erText xt Transf sfer er Prot otocol ocol  Used to access documents over the Web  Stateless protocol  each request/response is independent  protocol does not remember previous 10

11/21/2009 HTTP Client ent Requests uests (Meth ethods) ods)  GET (gets a resource)  HEAD (gets resource header only)  PUT (uploads a resource)  DELETE (deletes a resource)  POST (submits data)  TRACE (echoes request)  OPTIONS (displays server options)  CONNECT (used to make connection secure) HTTP Server r Responses onses (Statu tus s Codes)  1xx (Informational)  2xx (Success): 200 OK  3xx (Redirect)  301 Moved Permanently  303 See Other 304 Not Modified  4xx (Client Error)  403 Forbidden  404 Not Found  5xx (Server Error) 11

11/21/2009 HTTP Headers ers Request headers Response headers  Host: domain name of  Content-Type: e.g. server text/html, image/gif  Referer: address of  Location: for redirect linking page  Date: when served  User-Agent: details of  ETag: resource ID browser, OS  ....  .... C: GET /~mas02jg/fy04/labs/lab08.html HTTP/1.1 C: Host: doc.gold.ac.uk S: HTTP/1.1 200 OK S: Content-Type: text/html S: Content-Length: 2339 <S sends rest of header and lab08.html text> C: GET /~mas02jg/fy04/labs/lab80.html HTTP/1.1 C: Host: doc.gold.ac.uk S: HTTP/1.1 404 Not Found S: Content-Type: text/html; charset=iso-8859-1 <S sends rest of header and 404 page text> 12

11/21/2009 Summary  Application protocols  Emails have headers, body; MIME  SMTP sends emails  POP, IMAP receive emails  HTTP transfers web pages  Client-server protocols  Reading: Brookshear 4.2, 4.3 (also White 30) 13