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)