2: Application Layer1 Chapter 2: Application layer r 2.1 Principles of network applications r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail  SMTP, POP3, IMAP r 2.5 DNS r 2.6 P2P file sharing r 2.7 VOIP r 2.8 Socket programming with TCP r 2.9 Socket programming with UDP r 2.10 Building a Web server

2: Application Layer2 FTP: the file transfer protocol r transfer file to/from remote host r client/server model  client: side that initiates transfer (either to/from remote)  server: remote host r ftp: RFC 959 r ftp server: port 21 file transfer FTP server FTP user interface FTP client local file system remote file system user at host

2: Application Layer3 FTP: separate control, data connections r FTP client contacts FTP server at port 21, specifying TCP as transport protocol r Client obtains authorization over control connection r Client browses remote directory by sending commands over control connection. r When server receives a command for a file transfer, the server initiates a TCP data connection to client r After transferring one file, server closes connection. FTP client FTP server TCP control connection port 21 TCP data connection port 20 r Server initiates a second TCP data connection to transfer another file. r Control connection: “out of band” r FTP server maintains “state”: current directory, earlier authentication

2: Application Layer4 Chapter 2: Application layer r 2.1 Principles of network applications r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail  SMTP, POP3, IMAP r 2.5 DNS r 2.6 P2P file sharing r 2.7 VOIP r 2.8 Socket programming with TCP r 2.9 Socket programming with UDP r 2.10 Building a Web server

2: Application Layer5 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, elm, 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

2: Application Layer6 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 messages  client: sending mail server  “server”: receiving mail server mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP

2: Application Layer7 Electronic Mail: SMTP [RFC 2821] r uses TCP to reliably transfer message from client to server, port 25 r direct transfer: sending server to receiving server r three phases of transfer  handshaking (greeting)  transfer of messages  closure r command/response interaction  commands: ASCII text  response: status code and phrase r messages must be in 7-bit ASCII

2: Application Layer8 Scenario: Alice sends message to Bob 1) Alice uses UA to compose message and “to” 2) Alice’s UA sends message to her mail server; message placed in message queue (SMTP protocol) 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

2: Application Layer9 Sample SMTP interaction S: 220 longwood.cs.ucf.edu C: HELO fake.domain S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: subject: who am I? C: Do you like ketchup? C: How about pickles? C:. S: 250 Message accepted for delivery C: QUIT S: 221 longwood.cs.ucf.edu closing connection

2: Application Layer10 Try SMTP interaction for yourself:  telnet servername 25 r see 220 reply from server r enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands  “mail from” the domain may need to be existed  “rcpt to” the user needs to be existed  A mail server may or may not support “relay” above lets you send without using client (reader)

2: Application Layer11 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 one msg Problem: Only support ASCII content

2: Application Layer12 Message format: multimedia extensions r MIME (Multi-purpose Internet Mail Extensions)  multimedia mail extension, RFC 2045, 2056 r additional lines in msg header declare MIME content type From: To: 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 encoded data MIME version

2: Application Layer13 Mail access protocols r SMTP: deliver to receiver’s server r 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

2: Application Layer14 POP3 protocol (tcp: 110) authorization phase r client commands:  user: declare username  pass: password r server responses  +OK  -ERR transaction phase, client:  list: list message numbers  retr: retrieve message by number  dele: delete r quit C: list S: S: 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 pass hungry C: pass hungry S: +OK user successfully logged on Length(bytes) Telnet example

2: Application Layer15 POP3 (more) and IMAP More about POP3 r Read a message will get a copy of it into client machine  User can delete the message on server by choice r One Message as a unit  Cannot read a message without downloading all of it attachments  Trouble for low- bandwidth users IMAP r Keep all messages in one place: the server r Allows user to organize messages in folders  Create/delete folders on server r Can retrieve any parts of a message  Header only  Header + text content r More popular than POP3 Both have encrypted protocols

2: Application Layer16 Web-based r Hotmail, Yahoo mail, AOL, … r Convenient:  only need Internet Browser  User can access any where r Unified interface:  Same interface, folders, any where  Similar to IMAP on message organizing r Combination with other Web services  Hotmail+MSN messenger, AOL, new mail reminder r Free!

2: Application Layer17 Chapter 2: Application layer r 2.1 Principles of network applications r 2.2 Web and HTTP r 2.3 FTP r 2.4 Electronic Mail  SMTP, POP3, IMAP r 2.5 DNS r 2.6 P2P file sharing r 2.7 VOIP r 2.8 Socket programming with TCP r 2.9 Socket programming with UDP r 2.10 Building a Web server

2: Application Layer18 DNS: Domain Name System People: many identifiers:  SSN, name, passport # Internet hosts, routers:  IP address (32 bit) – unique ID  “name”, e.g., - used by humans Q: map between IP addresses and name ? 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)  note: core Internet function, implemented as application-layer protocol  complexity at network’s “edge” philosophy

2: Application Layer19 DNS Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance doesn’t scale! DNS services r Hostname to IP address translation r Host aliasing  Canonical and alias names  Many names for a single host r Mail server aliasing r Load distribution  Replicated Web servers: set of IP addresses for one canonical name

2: Application Layer20 Root DNS Servers com DNS servers org DNS serversedu DNS servers ucf.edu DNS servers umass.edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers Distributed, Hierarchical Database Client wants IP for 1 st approx: r Client queries a root server to find com DNS server r Client queries “com” DNS server to get amazon.com DNS server r Client queries amazon.com DNS server to get IP address for

2: Application Layer21 DNS: Root name servers b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) i Autonomica, Stockholm (plus 3 other locations) k RIPE London (also Amsterdam, Frankfurt) m WIDE Tokyo a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations) 13 root name servers worldwide

2: Application Layer22 TLD and Authoritative Servers r Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.  Network solutions maintains servers for com TLD  Educause for edu TLD r Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail).  Can be maintained by organization or service provider (paid by the organization)

2: Application Layer23 Local Name Server r Does not strictly belong to hierarchy r Each ISP (residential ISP, company, university) has one  Also called “default name server” r When a host makes a DNS query, query is sent to its local DNS server  Acts as a proxy (cache), forwards query into hierarchy

2: Application Layer24 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server Example r Host at cis.poly.edu wants IP address for gaia.cs.umass.edu

2: Application Layer25 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server 3 Recursive queries recursive query: r puts burden of name resolution on contacted name server r heavy load? iterated query: r contacted server replies with name of server to contact r “I don’t know this name, but ask this server”

2: Application Layer26 DNS: caching and updating records r once (any) name server learns mapping, it caches mapping  cache entries timeout (disappear) after some time (keep fresh copy)  TLD servers typically cached in local name servers Thus root name servers not often visited

2: Application Layer27 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 DNS 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 “canonical” (the real) name is really servereast.backup2.ibm.com  value is canonical name r Type=MX  value is name of mailserver associated with name

2: Application Layer28 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:  query or reply  recursion desired  recursion available  reply is authoritative

2: Application Layer29 DNS protocol, messages (UDP 53) Name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used Let’s check a web example using Ethereal!

2: Application Layer30 Inserting records into DNS r Example: just created startup “Network Utopia” r Register name networkuptopia.com at a registrar (e.g., Network Solutions)  Need to provide registrar with names and IP addresses of your authoritative name server (primary and secondary)  Registrar inserts two RRs into the com TLD server: (networkutopia.com, dns1.networkutopia.com, NS) (dns1.networkutopia.com, , A)  Put in authoritative server dns1.networkutopia.com  Type A record for  Type MX record for networkutopia.com ( ) r How do people get the IP address of your Web site?

2: Application Layer31 Chapter 2: Application layer r 2.1 Principles of network applications  app architectures  app requirements r 2.2 Web and HTTP r 2.4 Electronic Mail  SMTP, POP3, IMAP r 2.5 DNS r 2.6 P2P file sharing r 2.7 VOIP r 2.8 Socket programming with TCP r 2.9 Socket programming with UDP r 2.10 Building a Web server

2: Application Layer32 P2P: Example original “Napster” design 1) when peer connects, it informs central server:  IP address  content 2) Alice queries Central for “Hey Jude” 3) Central tells Alice that Bob has it and Bob’s IP 4) Alice downloads file from Bob (via HTTP, why?) 5) While Alice downloads, others download files from Alice’s computer, too centralized directory server peers Alice Bob

2: Application Layer33 Why using P2P? r Resource bottleneck in server r Take advantage of CPU, disk storage, bandwidth of millions of computers  Grid computing: distributed computation (CPU)  File sharing: (storage, bandwidth) r Direct communication (less delay)  Internet gaming, video/audio meeting r Copyright avoidance  Movie/music/software Piracy