Administrative Things

1
Administrative Things

• Grader Yona Raekow
• Email yona.raekow_at_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

2
A review of last lecture

• HTTP
• FTP
• Protocols for Internet emails
• SMTP
• POP3
• IMAP
• HTTP

3
Internet email system
???
???
receivers mail server

• From senders agent to senders mail server
• ???
• From senders mail server to receivers mail
  server
• ???
• From receivers mail server to receivers agent
• ???

4
Internet email system
SMTP
POP3/IMAP/HTTP
receivers mail server

• From senders agent to senders mail server
• SMTP/HTTP
• From senders mail server to receivers mail
  server
• SMTP
• From receivers mail server to receivers agent
• POP3/IMAP/HTTP

5
Chapter 2 outline

• 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

6
DNS Domain Name System

• 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 resolve names
  (address/name translation)
• note core Internet function, implemented as
  application-layer protocol
• complexity at networks edge
• runs over UDP, port 53

7
DNS name servers

• 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 hosts IP address, name
• can perform name/address translation for that
  hosts name

• Why not centralize DNS?
• single point of failure
• traffic volume
• distant centralized database
• maintenance
• doesnt scale!

8
DNS Root name servers

• 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

13 root name servers worldwide (Feb. 2002)
9
Simple DNS example
root name server

• 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

2
4
3
5
authorititive name server dns.uconn.edu
1
6
requesting host surf.eurecom.fr
www.cse.uconn.edu
10
DNS example
root name server

• Root name server
• may not know authoritative name server
• may know intermediate name server who to contact
  to find authoritative 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
11
DNS iterated queries
root name server

• 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 dont know this name, but ask this 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
12
DNS caching and updating records

• 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.h
  tml

13
DNS records

• DNS distributed db storing resource records (RR)

• TypeA
• name is hostname
• value is IP address

• TypeCNAME
• name is alias name for some cannonical (the
  real) name
• www.ibm.com is really
• servereast.backup2.ibm.com
• value is cannonical name

• TypeNS
• name is domain (e.g. foo.com)
• value is IP address of authoritative name server
  for this domain

• TypeMX
• value is name of mailserver associated with name

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_at_uconn.edu ? yournetid_at_uconnvm.uconn.edu
• http//uconn.edu yourname_at_uconn.edu
• Load distribution
• CNAME type
• replicated web servers www.cnn.com

15
DNS protocol, messages

• DNS protocol query and reply messages, both
  with same message format

• msg header
• identification 16 bit for query, reply to
  query uses same (UDP)
• flags
• query or reply
• recursion desired
• recursion available
• reply is authoritative

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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
17
Questions on homework 1