Administrative Things

1. Administrative Things • Grader: Yona Raekow Email: yona.raekow@gmx.de Office hours: Wed. 1pm-3pm or by appointments (in the week when you get your returned homework) • Homeworks • Usually due on Tuesday (in class) • Returned to you on the following Tuesday • See Yona within one week if you have questions • Projects • See announcements • LATE homeworks or projects will NOT be accepted Lecture6

2. A review of last lecture • HTTP • FTP • Protocols for Internet emails • SMTP • POP3 • IMAP • HTTP Lecture6

3. From sender’s agent to sender’s mail server ??? From sender’s mail server to receiver’s mail server ??? From receiver’s mail server to receiver’s agent ??? user agent user agent sender’s mail server ??? Internet email system ??? ??? receiver’s mail server Lecture6

4. From sender’s agent to sender’s mail server SMTP/HTTP From sender’s mail server to receiver’s mail server SMTP From receiver’s mail server to receiver’s agent POP3/IMAP/HTTP user agent user agent sender’s mail server SMTP/HTTP Internet email system SMTP POP3/IMAP/HTTP receiver’s mail server Lecture6

5. 2.1 Principles of app layer protocols clients and servers app requirements 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 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 Lecture6

6. People: many identifiers: SSN, name, passport # Internet hosts, routers: IP address (32 bit) - used for addressing datagrams “name”, e.g., www.engr.uconn.edu - used by humans Q: map between IP addresses and name ? (required by HTTP, SMTP, FTP, etc.) 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” runs over UDP, port 53 DNS: Domain Name System Lecture6

7. 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 Lecture6

8. 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: Root name servers 13 root name servers worldwide (Feb. 2002) Lecture6

9. host surf.eurecom.fr wants IP address of www.cse.uconn.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.uconn.edu, if necessary local name server dns.eurecom.fr Simple DNS example root name server 2 4 3 5 authorititive name server dns.uconn.edu 1 6 requesting host surf.eurecom.fr www.cse.uconn.edu Lecture6

10. Root name server: may not know authoritative name server may know intermediate name server: who to contact to find authoritative name server local name server dns.eurecom.fr intermediate name server dns.uconn.edu DNS example root name server 6 2 3 7 5 4 1 8 authoritative name server dns.cse.uconn.edu requesting host surf.eurecom.fr www.cse.uconn.edu Lecture6

11. recursive query: puts burden of name resolution on contacted name server heavy load? iterated 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.uconn.edu DNS: iterated queries root name server iterated query 2 3 4 7 5 6 1 8 authoritative name server dns.cse.uconn.edu requesting host surf.eurecom.fr www.cse.uconn.edu Lecture6

12. once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time (often set to 2 days) content of each DNS server were configured statically until recently update/notify mechanisms under design by IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html DNS: caching and updating records Lecture6

13. DNS: distributed db storing resource records (RR) 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 • Type=A • name is hostname • value is IP address • 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 • Type=MX • value is name of mailserver associated with name Lecture6

14. Services provided by DNS • Hostname to IP address mapping • A and NS type • Host aliases • CNAME type • www.cse.uconn.edu  www.engr.uconn.edu/cse/ • Mail server aliases • MX type • yourname@uconn.edu  yournetid@uconnvm.uconn.edu • http://uconn.edu ; yourname@uconn.edu • Load distribution • CNAME type • replicated web servers: www.cnn.com Lecture6

15. DNS protocol :queryand reply messages, both with same message format DNS protocol, messages msg header • identification: 16 bit # for query, reply to query uses same # (UDP) • flags: • query or reply • recursion desired • recursion available • reply is authoritative Lecture6

16. 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 For more, see DNS RFCs: RFC 1034, RFC 1035 Lecture6

17. Questions on homework #1 Lecture6