1. Fall 2005 By: H. Veisi Computer networks course Olum-fonoon Babol Chapter 7 The Application Layer

2. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 2 Overview  Processes running in different hosts communicate with an application-layer protocol  Popular application-level protocols:  DNS  HTTP  FTP  SMTP / POP3 / IMAP  Programming network applications  socket API

3. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 3 DNS (1)  Programs could refer to hosts, mailboxes and other resources by their network IP.  These Numbers are difficult to remember by people?  Also, if moving the servers to different machine?  Use names instead of IP numbers.  Ex. ce.sharif.edu, veisi@yahoo.com  Need a mechanism to convert names to IP Number  Use a file: hosts.txt  As the number of PCs increase?  Size increase, Conflict of host names  Need a central manager:  DNS: Domain Name System

4. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 4 DNS (2)  DNS Name Space  For managing large and changing set of names, need a postal like management,  Ex. No. 4, Azadi St., Tehran, Iran  Hierarchical addressing  Top-level domain:  200 top-level domains

5. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 5  no server has all name-to-IP address mappings local name servers:  each ISP, company has local (default) name server  host DNS query first goes to local name server authoritative name server:  for a host: stores that host’s IP address, name  can perform name/address translation for that host’s name Why not centralize DNS?  single point of failure  traffic volume  distant centralized database  maintenance DNS (3)

6. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 6  Example: host surf.eurecom.fr wants IP address of gaia.cs.umass.edu 1. contacts its local DNS server, dns.eurecom.fr 2. dns.eurecom.fr contacts root name server, if necessary 3. root name server contacts authoritative name server, dns.umass.edu, if necessary requesting host surf.eurecom.fr gaia.cs.umass.edu root name server authoritative name server dns.umass.edu local name server dns.eurecom.fr 1 2 3 4 5 6 DNS (4)

7. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 7 DNS (5)  Once (any) name server learns mapping, it caches mapping  cache entries timeout (disappear) after some time  DNS Records  format: (name, TTL, value, type)  Records types:

8. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 8  Three major components:  user agents  mail servers  simple mail transfer protocol: SMTP  User Agent  Sometimes is called: “mail reader”  composing, editing, reading mail messages  e.g., Eudora, Outlook, elm, Netscape Messenger  outgoing, incoming messages stored on server user mailbox outgoing message queue mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP E-Mail (1)

9. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 9  Mail Servers  mailbox contains incoming messages for user  message queue of outgoing (to be sent) mail messages  SMTP protocol between mail servers to send email messages  “client”: sending mail server (sending agent)  “server”: receiving mail server (receiving agent) mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP E-Mail (2)

10. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 10  Message Format:  SMTP: protocol for exchanging email msgs  RFC 822: standard for text message format:  header lines, e.g.,  To:  From:  Subject:  different from SMTP commands!  body  the “message”, ASCII characters only header body blank line E-Mail (3)

11. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 11 E-Mail (4)  MIME: Multipurpose Internet Mail Extension  For multimedia mail extension  Text  example subtypes: plain, html  Image  example subtypes: jpeg, gif  Audio  example subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding)  Video  example subtypes: mpeg, quicktime  Application  other data that must be processed by reader before “viewable”  example subtypes: msword, octet-stream

12. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 12  SMTP: delivery/storage to receiver’s server  Mail access protocol: retrieval from server  POP: Post Office Protocol [RFC 1939]  authorization (agent server) and download  IMAP: Internet Mail Access Protocol [RFC 1730]  more features (more complex)  manipulation of stored msgs on server  HTTP: Hotmail, Yahoo! Mail, etc. user agent sender’s mail server user agent SMTP access protocol receiver’s mail server E-Mail (5)

13. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 13 E-Mail (6)  POP3  Previous example uses “download and delete” mode.  “Download” brings the mail into client computer.  “Delete” removes the mail from the mail server. Mr.X cannot re-read e-mail if he changes client computer.  “Download-and-keep”: leaves the message on the mail server.  POP3 is stateless across sessions.  IMAP  Keep all messages in one place: the server  Allows user to organize messages in folders  IMAP keeps user state across sessions:  names of folders and mappings between message IDs and folder name

14. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 14  transfer file to/from remote host  client/server model  client: side that initiates transfer (either to/from remote)  server: remote host  ftp: RFC 959  ftp server: port 21 file transfer FTP server FTP user interface FTP client local file system remote file system user at host FTP: File Transfer Protocol

15. H. Veisi Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 15 WWW: World Wide Web  In 1989, CERN: European Center of Nebular Research, By Berners-Lee.  Accessing linked documents, support Text, Image, Audio, Video, …  People think that this is “Internet” !  Hypertext Browsers:  Mosaic, Netscape, IE, …  Use HTML (Hyper-Text Markup Language) as a common language of web.