1. 2: Application Layer1 Chapter 2 Application Layer Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002. 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 1996-2002 Material edited 2/03 by MLH, AHR, AK J.F Kurose and K.W. Ross, All Rights Reserved

2. 2: Application Layer2 Chapter 2 outline r 2.1 Principles of app layer protocols m clients and servers m app requirements r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail m SMTP, POP3, IMAP r 2.5 DNS r 2.6 Socket programming with TCP r 2.7 Socket programming with UDP r 2.8 Building a Web server r 2.9 Content distribution m Network Web caching m Content distribution networks m P2P file sharing

3. 2: Application Layer3 DNS: Domain Name System People: many identifiers: m SSN, name, passport # Internet hosts, routers: m IP address (32 bit) - used for addressing datagrams m “name”, e.g., gaia.cs.umass.edu - used by humans Domain Name System: r distributed database implemented in hierarchy of many name servers r application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) m note: core Internet function, implemented as application- layer protocol m complexity at network’s “edge”

4. 2: Application Layer4 DNS name servers r no server has all name-to-IP address mappings local name servers: m each ISP, company has local (default) name server m host DNS query first goes to local name server authoritative name server: m for a host: stores that host’s IP address, name m can perform name/address translation for that host’s name Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance doesn’t scale!

5. 2: Application Layer5 DNS: Root name servers r contacted by local name server that can not resolve name r root name server: m contacts authoritative name server if name mapping not known m gets mapping m returns mapping to local name server b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA i NORDUnet Stockholm k RIPE London m WIDE Tokyo a NSI Herndon, VA c PSInet Herndon, VA d U Maryland College Park, MD g DISA Vienna, VA h ARL Aberdeen, MD j NSI (TBD) Herndon, VA 13 root name servers worldwide

6. 2: Application Layer6 Simple DNS 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 authorititive name server dns.umass.edu local name server dns.eurecom.fr 1 2 3 4 5 6

7. 2: Application Layer7 DNS example Root name server: r may not know authoritative name server r may know intermediate name server: who to contact to find authoritative name server requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr 1 2 3 4 5 6 authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8

8. 2: Application Layer8 DNS: iterated queries recursive query: r server A makes a request to server B r server B obtains the requested mapping on behalf of A r forwards it’s mapping to A iterated query : r contacted server replies with name of server to contact r “I don’t know this name, but ask this server” requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr 1 2 3 4 5 6 authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8 iterated query applet: http://wps.aw.com/wps/media/objects/221 /227091/applets/dns/dns.html

9. 2: Application Layer9 DNS: caching and updating records r once (any) name server learns mapping, it caches mapping m cache entries timeout (disappear) after some time r update/notify mechanisms under design by IETF m RFC 2136 m http://www.ietf.org/html.charters/dnsind-charter.html

10. 2: Application Layer10 DNS records DNS: distributed db storing resource records (RR) r Type=NS  name is domain (e.g. foo.com)  value is IP address of authoritative name server for this domain RR format: (name, value, type,ttl) r Type=A  name is hostname  value is IP address r Type=CNAME  name is alias name for some “cannonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com  value is cannonical name r Type=MX  value is name of mailserver associated with name

11. 2: Application Layer11 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header r identification: 16 bit # for query, reply to query uses same # r flags: m query or reply m recursion desired m recursion available m reply is authoritative

12. 2: Application Layer12 DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative servers additional “helpful” info that may be used

13. 2: Application Layer13 Chapter 2 outline r 2.1 Principles of app layer protocols m clients and servers m app requirements r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail m SMTP, POP3, IMAP r 2.5 DNS r 2.6 Socket programming with TCP r 2.7 Socket programming with UDP r 2.8 Building a Web server r 2.9 Content distribution m Network Web caching m Content distribution networks m P2P file sharing

14. 2: Application Layer14 Electronic Mail Three major components: r user agents r mail servers r simple mail transfer protocol: SMTP User Agent r a.k.a. “mail reader” r composing, editing, reading mail messages r e.g., Eudora, Outlook, Groupwise, Netscape Messenger r 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

15. 2: Application Layer15 Electronic Mail: mail servers Mail Servers r mailbox contains incoming messages for user r message queue of outgoing (to be sent) mail messages r SMTP protocol between mail servers to send email messages m client: sending mail server m “server”: receiving mail server mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP

16. 2: Application Layer16 Electronic Mail: SMTP [RFC 2821] r uses TCP to reliably transfer email message from client to server, port 25 r direct transfer: sending server to receiving server r three phases of transfer m handshaking (greeting) m transfer of messages m closure r command/response interaction m commands: ASCII text m response: status code and phrase r messages must be in 7-bit ASCII

17. 2: Application Layer17 Scenario: Alice sends message to Bob 1) Alice uses UA to compose message and “to” bob@someschool.edu 2) Alice’s UA sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Bob’s mail server 4) SMTP client sends Alice’s message over the TCP connection 5) Bob’s mail server places the message in Bob’s mailbox 6) Bob invokes his user agent to read message user agent mail server mail server user agent 1 2 3 4 5 6

18. 2: Application Layer18 Sample SMTP interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 alice@crepes.fr... Sender ok C: RCPT TO: S: 250 bob@hamburger.edu... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C:. S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

19. 2: Application Layer19 SMTP: final words r SMTP uses persistent connections r SMTP requires message (header & body) to be in 7- bit ASCII  SMTP server uses CRLF.CRLF to determine end of message Comparison with HTTP: r HTTP: pull r SMTP: push r both have ASCII command/response interaction, status codes r HTTP: each object encapsulated in its own response msg r SMTP: multiple objects sent in multipart msg

20. 2: Application Layer20 Mail message format SMTP: protocol for exchanging email msgs RFC 822: standard for text message format: r header lines, e.g., m To: m From: m Subject: different from SMTP commands! (This is not a handshake) r body m the “message”, ASCII characters only header body blank line

21. 2: Application Layer21 Received message format Received: from crepes.fr by hamburger.edu; 12 Oct 98 15:27:39 GMT From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data....................................base64 encoded data SMTP “receiving” server adds this line. Additional timestamps are possible.

22. 2: Application Layer22 Message format: multimedia extensions r MIME: multimedia mail extension, RFC 2045, 2056 r additional lines in msg header declare MIME content type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data....................................base64 encoded data multimedia data type, subtype, parameter declaration method used to encode data MIME version encoded data

23. 2: Application Layer23 MIME types Content-Type: type/subtype; parameters Text  example subtypes: plain, html Image  example subtypes: jpeg, gif Audio  exampe subtypes: basic (8- bit mu-law encoded), 32kadpcm (32 kbps coding) Video  example subtypes: mpeg, quicktime Application r other data that must be processed by reader before “viewable”  example subtypes: msword, octet-stream

24. 2: Application Layer24 Multipart Type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=StartOfNextPart --StartOfNextPart Dear Bob, Please find a picture of a crepe. --StartOfNextPart Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data....................................base64 encoded data --StartOfNextPart Do you want the recipe?

25. 2: Application Layer25 Mail access protocols r SMTP: delivery/storage to receiver’s server r Mail access protocol: retrieval from server m POP: Post Office Protocol [RFC 1939] authorization (agent server) and download m IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server m HTTP: Hotmail, Yahoo! Mail, etc. user agent sender’s mail server user agent SMTP access protocol receiver’s mail server

26. 2: Application Layer26 POP3 protocol authorization phase r client commands:  user: declare username  pass: password r server responses m +OK  -ERR transaction phase, client:  list: list message numbers  retr: retrieve message by number  dele: delete r quit C: list S: 1 498 S: 2 912 S:. C: retr 1 S: S:. C: dele 1 C: retr 2 S: S:. C: dele 2 C: quit S: +OK POP3 server signing off S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on

27. 2: Application Layer27 POP3 (more) and IMAP More about POP3 r Previous example uses “download and delete” mode. r Bob cannot re-read e-mail if he changes client r “Download-and-keep”: copies of messages on different clients r POP3 is stateless across sessions IMAP r Keep all messages in one place: the server r Allows user to organize messages in folders r IMAP keeps user state across sessions: m names of folders and mappings between message IDs and folder name

28. 2: Application Layer28 Chapter 2 outline r 2.1 Principles of app layer protocols m clients and servers m app requirements r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail m SMTP, POP3, IMAP r 2.5 DNS r 2.6 Socket programming with TCP r 2.7 Socket programming with UDP r 2.8 Building a Web server r 2.9 Content distribution m Network Web caching m Content distribution networks m P2P file sharing

29. 2: Application Layer29 DNS: Domain Name System People: many identifiers: m SSN, name, passport # Internet hosts, routers: m IP address (32 bit) - used for addressing datagrams m “name”, e.g., gaia.cs.umass.edu - used by humans Domain Name System: r distributed database implemented in hierarchy of many name servers r application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) m note: core Internet function, implemented as application- layer protocol m complexity at network’s “edge”

30. 2: Application Layer30 DNS name servers r no server has all name-to-IP address mappings local name servers: m each ISP, company has local (default) name server m host DNS query first goes to local name server authoritative name server: m for a host: stores that host’s IP address, name m can perform name/address translation for that host’s name Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance doesn’t scale!

31. 2: Application Layer31 DNS: Root name servers r contacted by local name server that can not resolve name r root name server: m contacts authoritative name server if name mapping not known m gets mapping m returns mapping to local name server b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA i NORDUnet Stockholm k RIPE London m WIDE Tokyo a NSI Herndon, VA c PSInet Herndon, VA d U Maryland College Park, MD g DISA Vienna, VA h ARL Aberdeen, MD j NSI (TBD) Herndon, VA 13 root name servers worldwide

32. 2: Application Layer32 Simple DNS 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 authorititive name server dns.umass.edu local name server dns.eurecom.fr 1 2 3 4 5 6

33. 2: Application Layer33 DNS example Root name server: r may not know authoritative name server r may know intermediate name server: who to contact to find authoritative name server requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr 1 2 3 4 5 6 authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8

34. 2: Application Layer34 DNS: iterated queries recursive query: r server A makes a request to server B r server B obtains the requested mapping on behalf of A r forwards it’s mapping to A iterated query : r contacted server replies with name of server to contact r “I don’t know this name, but ask this server” requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr 1 2 3 4 5 6 authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8 iterated query applet: http://wps.aw.com/wps/media/objects/221 /227091/applets/dns/dns.html

35. 2: Application Layer35 DNS: caching and updating records r once (any) name server learns mapping, it caches mapping m cache entries timeout (disappear) after some time r update/notify mechanisms under design by IETF m RFC 2136 m http://www.ietf.org/html.charters/dnsind-charter.html

36. 2: Application Layer36 DNS records DNS: distributed db storing resource records (RR) r Type=NS  name is domain (e.g. foo.com)  value is IP address of authoritative name server for this domain RR format: (name, value, type,ttl) r Type=A  name is hostname  value is IP address r Type=CNAME  name is alias name for some “cannonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com  value is cannonical name r Type=MX  value is name of mailserver associated with name

37. 2: Application Layer37 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header r identification: 16 bit # for query, reply to query uses same # r flags: m query or reply m recursion desired m recursion available m reply is authoritative

38. 2: Application Layer38 DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative servers additional “helpful” info that may be used