1. Important r On Friday, could you ask students to please email me their groups (one email per group) for Project 2 so we can assign IP addresses. I’ll send a message to the google group tomorrow as well. Thanks, -Marcel

2. 2 Outline r Principles of network applications m App architectures m App requirements r Web and HTTP r FTP

3. 3 FTP: the file transfer protocol r transfer file to/from remote host r client/server model m client: side that initiates transfer (either to/from remote) m 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

4. 4 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 opens 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 Server opens a second TCP data connection to transfer another file. Control connection: “out of band” FTP server maintains “state”: current directory, earlier authentication

5. 5 Application layer r Electronic Mail m SMTP, POP3, IMAP r DNS r P2P file sharing

6. 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., Outlook, elm, Mozilla Thunderbird, iPhone mail client 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 6

7. 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 7

8. 8 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

9. 9 SMTP: final words r SMTP uses persistent connections r SMTP requires message (header & body) to be in 7- bit ASCII 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

10. 10 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! r body m the “message”, ASCII characters only header body blank line

11. 11 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

12. 12 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

13. 13 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

14. 14 Outline r Electronic Mail m SMTP, POP3, IMAP r DNS r P2P file sharing

15. 15 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., www.yahoo.com - 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) m note: core Internet function, implemented as application-layer protocol m complexity at network’s “edge”

16. 16 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 m Canonical and alias names r Mail server aliasing r Load distribution m Replicated Web servers: set of IP addresses for one canonical name

17. 17 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 m Canonical and alias names r Mail server aliasing r Load distribution m Replicated Web servers: set of IP addresses for one canonical name

18. 18 Root DNS Servers com DNS servers org DNS serversedu DNS servers poly.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 www.amazon.com; 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 www.amazon.com

19. 19 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 13 root name servers worldwide 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)

20. 20 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. m Network solutions maintains servers for com TLD m 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). m Can be maintained by organization or service provider

21. 21 Local Name Server r Does not strictly belong to hierarchy r Each ISP (residential ISP, company, university) has one. m Also called “default name server” r When a host makes a DNS query, query is sent to its local DNS server m Acts as a proxy, forwards query into hierarchy.

22. 22 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu 1 2 3 4 5 6 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

23. 23 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu 1 2 4 5 6 authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server 3 Recursive queries recursive query: rputs burden of name resolution on contacted name server iterated query: rcontacted server replies with name of server to contact r“I don’t know this name, but ask this server”

24. 24 DNS: caching and updating records r once (any) name server learns mapping, it caches mapping m cache entries timeout (disappear) after some time m TLD servers typically cached in local name servers Thus root name servers not often visited r update/notify mechanisms under design by IETF m RFC 2136 m http://www.ietf.org/html.charters/dnsind-charter.html

25. 25 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) rType=A  name is hostname  value is IP address rType=CNAME  name is alias name for some “cannonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com  value is cannonical name rType=MX  value is Canon. name of mailserver associated with alias name

26. 26 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header ridentification: 16 bit # for query, reply to query uses same # rflags: m query or reply m recursion desired m recursion available m reply is authoritative

27. 27 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