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Chapter 2 outline

2.1 Principles of app layer protocols 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail 2.5 DNS. 2.6 Socket programming with TCP 2.7 Socket programming with UDP 2.8 Building a Web server 2.9 Content distribution Network Web caching Content distribution networks P2P file sharing.

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Chapter 2 outline

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  1. 2.1 Principles of app layer protocols 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail 2.5 DNS 2.6 Socket programming with TCP 2.7 Socket programming with UDP 2.8 Building a Web server 2.9 Content distribution Network Web caching Content distribution networks P2P file sharing Chapter 2 outline 2: Application Layer

  2. 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 FTP user interface FTP client FTP server local file system FTP: the file transfer protocol file transfer user at host remote file system 2: Application Layer

  3. FTP client contacts FTP server at port 21, specifying TCP as transport protocol Client obtains authorization over control connection Client browses remote directory by sending commands over control connection. When server receives a command for a file transfer, the server opens a TCP data connection to client After transferring one file, server closes connection. TCP control connection port 21 TCP data connection port 20 FTP client FTP server FTP: separate control, data connections • Server opens a second TCP data connection to transfer another file. • Control connection: “out of band” • FTP server maintains “state”: current directory, earlier authentication 2: Application Layer

  4. Sample commands: sent as ASCII text over control channel USER username PASS password LISTreturn list of file in current directory RETR filenameretrieves (gets) file STOR filenamestores (puts) file onto remote host Sample return codes status code and phrase (as in HTTP) 331 Username OK, password required 125 data connection already open; transfer starting 425 Can’t open data connection 452 Error writing file FTP commands, responses 2: Application Layer

  5. Three major components: user agents, mail servers, SMTP User Agent a.k.a. “mail reader” composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger Mail Servers mailbox contains incoming messages for user messagequeue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send email messages client: sending mail server “server”: receiving mail server user agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server outgoing message queue user mailbox Electronic Mail: components 2: Application Layer

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

  7. 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 user agent mail server mail server Electronic Mail: scenario 1 2 6 3 4 5 Q: Why not route mail hop by hop toward destination? 2: Application Layer

  8. Try SMTP interaction for yourself % telnet po.cwru.edu 25 S: 220 ims-msg.TIS.CWRU.Edu – Server ESMTP??? C: HELO po S: 250 ims-msg.TIS.CWRU.Edu OK, csa.bu.edu [128.197.12.3] C: MAIL FROM: <jins@cs.bu.edu> S: 250 2.5.0 Address Ok C: RCPT TO: <???@po.cwru.edu> S: 250 ??? 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 2.3.0 Bye received. Goodbye. Connection closed bu foreign host. 2: Application Layer

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

  10. SMTP: protocol for exchanging email msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: differentfrom SMTP commands! body the “message”, ASCII characters only Electronic Mail: message format header blank line body 2: Application Layer

  11. MIME: multimedia mail extension, RFC 2045, 2056 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 Electronic Mail: MIME MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data 2: Application Layer

  12. 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 Electronic Mail: MIME typesContent-Type: type/subtype; parameters 2: Application Layer

  13. 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 Originally designed more for connected operation HTTP: Hotmail , Yahoo! Mail, etc. user agent user agent sender’s mail server SMTP Electronic Mail: mail access protocols SMTP access protocol receiver’s mail server 2: Application Layer

  14. authorization phase client commands: user: declare username pass: password server responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit Electronic Mail: POP3 protocol S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off 2: Application Layer

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

  16. 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 doesn’t scale! DNS name servers 2: Application Layer

  17. contacted by local name server that can not resolve name root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server 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 k RIPE London i NORDUnet Stockholm m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA DNS: 13 root name servers 13 root name servers worldwide 2: Application Layer

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

  19. recursive query: puts burden of name resolution on contacted name server heavy load? iterative query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” local name server dns.eurecom.fr intermediate name server dns.umass.edu DNS: recursive versus iterative queries root name server iterative query 2 3 4 7 5 6 1 8 authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu 2: Application Layer

  20. DNS: distributed db storing resource records (RR) once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time update/notify mechanisms under design by IETF RFC 2136, http://www.ietf.org/html.charters/dnsind-charter.html 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) DNS: records, and caching/updating • Type=A • name is hostname • value is IP address • Type=CNAME • name is alias name for some “canonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com • value is canonical name • Type=MX • value is name of mailserver associated with name 2: Application Layer

  21. Queryand reply messages, both with same message format DNS protocol, messages msg header • identification: 16-bit #, reply uses same # • flags: • query or reply • recursion desired • recursion available • reply is authoritative Name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used 2: Application Layer

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