Week3 lec 2
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Chapter 2Application Layer
Computer Networking: A Top Down Approach, 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.
What Transport Service does an Application need?
Data Loss Loss Tolerant Applications
Some apps (e.g., audio, VoIP) can tolerate some loss
2% tolerable for VoIP Other apps (e.g., file transfer,
email) require 100% reliable data transfer
Timing Application may require
timing guarantee Tight timing constraints
multiplayer games, VoIP, teleconferencing.
In Non-real time lower delays are preferred but no tight constraint on end-to-end delays.
Throughput Bandwidth sensitive
applications (e.g., multimedia) require minimum amount of throughput
Other apps (“elastic apps”) make use of whatever throughput they get
e.g .Email, file transferSecurity Encryption and decryption
Transport Service Requirements of Common Applications
Application
file transfere-mail
Web documentsreal-time audio/video
stored audio/videointeractive games
Data loss
no lossno lossno lossloss-tolerant
loss-tolerantloss-tolerant
Throughput
elasticelasticelasticaudio: 5kbps-1Mbpsvideo:10kbps-5Mbpssame as above few kbps -10kbps
Time Sensitive
nononoyes, 100’s msec
yes, few secsyes, 100’s msec
Internet transport protocols services
TCP service: connection-oriented: setup
required between client and server processes
reliable transport between sending and receiving process
flow control: sender won’t overwhelm receiver
congestion control: throttle sender when network overloaded
does not provide: timing, minimum throughput guarantees, security
UDP service: unreliable data transfer
between sending and receiving process
does not provide: connection setup, reliability, flow control, security
Throughput and timing guarantee not provided
Internet Applications: application, transport protocols
Application
e-mailremote terminal access
Web file transfer
streaming multimedia
Internet telephony
Applicationlayer protocol
SMTP [RFC 2821]Telnet [RFC 854]HTTP [RFC 2616]FTP [RFC 959]RTP [RFC 1889]
SIP, Skype
Underlyingtransport protocol
TCPTCPTCPTCPUDP
UDP
DNS: Domain Name System
People: many identifiers: SSN, name, passport
#
Internet hosts: IP address (32 bit) “name”, e.g.,
ww.yahoo.com - used by humans
Q: map between IP addresses and name ?
Domain Name System: Distributed Database
implemented in hierarchy of many DNS servers
An application-layer protocol
that allows hosts to query the distributed database
DNS uses UDP over port number 53.
RFC 1034 and RFC 1035
DNS
Simple design would have one DNS server that contains all the mappings
Why not centralize DNS? Single point of failure Traffic volume Distant centralized database Maintenance
A centralized database in a single DNS server doesn’t scale!
Root DNS Servers
com DNS servers org DNS servers edu DNS servers
poly.eduDNS servers
umass.eduDNS servers
yahoo.comDNS servers
amazon.comDNS servers
pbs.orgDNS servers
Distributed, Hierarchical Database
Client wants IP for www.amazon.com: Client first queries a root server The root server returns the IP addresses for TLD servers for
the top level domain com The client then contacts one of the TLD servers which
returns the IP address of an authoritative server for amazon.com
The authoritative server then returns the IP address for the host name www.amazon.com
DNS: Root Name Servers
13 root DNS servers world wide Each server is a cluster of replicated servers
security and reliability purposes. For more information see www.root-servers.org
b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA
e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 36 other locations)
i Autonomica, Stockholm (plus 28 other locations)
k RIPE London (also 16 other locations)
m WIDE Tokyo (also Seoul, Paris, SF)
a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 21 locations)
TLD and Authoritative Servers Top-level Domain (TLD) Servers:
Responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, jp.
Network Solutions maintains servers for com TLD
Educause for edu TLD ICANN: Internet Corporation for Assigned
Names and Numbers Authoritative DNS Servers:
Every organization with publicly accessible hosts provide accessible DNS records.That maps the names of those hosts to IP addresses
Authoritative DNS servers houses these DNS records
Local Name Server
Does not strictly belong to hierarchy Each company, university has one.
Also called “default name server” When host makes DNS query, query is
sent to its local DNS server acts as proxy, forwards query into hierarchy
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
1
23
4
5
6
authoritative DNS serverdns.cs.umass.edu
78
TLD DNS server
DNS name resolution example
Host at cis.poly.edu wants IP address for gaia.cs.umass.edu
Iterated query: Contacted server
replies with name of server to contact.
“I don’t know this name, but ask this server”.
requesting hostcis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS serverdns.poly.edu
1
2
45
6
authoritative DNS serverdns.cs.umass.edu
7
8
TLD DNS server
3Recursive Query: Puts burden of
name resolution on other server.
DNS Caching Extensively used Cache entries timeout
(disappear) after some time
TLD servers typically cached in local name servers Thus root name
servers not often visited
DNS name resolution example
Assignment 1
Statefulness in HTTP Web Caches (Conditional GET statement) Cookies (Set Cookie Header)