introduction 1-1 lecture 2 computer networking: a top down approach 6 th edition jim kurose, keith...

Introduction 1-1

Lecture 2

Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith RossAddison-WesleyMarch 2012

CS3516:

These slides are generated from those made available by the authors of our text.

Introduction

Lecture 2: outline1.5 protocol layers, service models2.7 socket programming with UDP and

TCP.1.3 network core

packet switching, circuit switching, network structure

1-2

Introduction

Protocol “layers”Networks are

complex,with many

“pieces”: hosts routers links of

various media applications protocols hardware,

software

Question: is there any hope of

organizing structure of network?

…. or at least our discussion of

networks?

1-3

Introduction

Organization of air travel

a series of steps

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

1-4

Introduction

ticket (purchase)

baggage (check)

gates (load)

runway (takeoff)

airplane routing

departureairport

arrivalairport

intermediate air-trafficcontrol centers

airplane routing airplane routing

ticket (complain)

baggage (claim

gates (unload)

runway (land)

airplane routing

ticket

baggage

gate

takeoff/landing

airplane routing

Layering of airline functionality

layers: each layer implements a service via its own internal-layer actions relying on services provided by layer

below

1-5

Introduction

Why layering?dealing with complex systems: explicit structure allows identification,

relationship of complex system’s pieces layered reference model for discussion

modularization eases maintenance, updating of system change of implementation of layer’s service

transparent to rest of system e.g., change in gate procedure doesn’t

affect rest of system layering considered harmful?

1-6

Introduction

Internet protocol stack application: supporting

network applications FTP, SMTP, HTTP

transport: process-process data transfer TCP, UDP

network: routing of datagrams from source to destination IP, routing protocols

link: data transfer between neighboring network elements Ethernet, 802.111 (WiFi), PPP

physical: bits “on the wire”

application

transport

network

link

physical

1-7

Introduction

ISO/OSI reference model presentation: allow

applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions

session: synchronization, checkpointing, recovery of data exchange

Internet stack “missing” these layers! these services, if needed,

must be implemented in application

needed?

application

presentation

session

transport

network

link

physical

1-8

Introduction

source

applicationtransportnetwork

linkphysical

HtHn M

segment Ht

datagram

destination

applicationtransportnetwork

linkphysical

HtHnHl M

HtHn M

Ht M

M

networklink

physical

linkphysical

HtHnHl M

HtHn M

HtHn M

HtHnHl M

router

switch

Encapsulationmessage M

Ht M

Hn

frame

1-9

Application Layer 2-10

Lecture 2: outline

1.5 protocol layers, service models2.7 socket programming with UDP

and TCP. How does this relate to Project 1?

1.3 network core packet switching, circuit switching,

network structure


Socket programming

goal: learn how to build client/server applications that communicate using sockets

socket: door between application process and end-end-transport protocol

Internet

controlledby OS

controlled byapp developer

transport

application

physical

link

network

process

transport

application

physical

link

network

processsocket


Socket programming

Two socket types for two transport services:

UDP: unreliable datagram TCP: reliable, byte stream-oriented

Application Example:1. Client reads a line of characters (data)

from its keyboard and sends the data to the server.

2. The server receives the data and converts characters to uppercase.

3. The server sends the modified data to the client.

4. The client receives the modified data and displays the line on its screen.


Socket programming with UDPUDP: no “connection” between client &

server no handshaking before sending data sender explicitly attaches IP destination

address and port # to each packet rcvr extracts sender IP address and port#

from received packet

UDP: transmitted data may be lost or received out-of-order

Application viewpoint: UDP provides unreliable transfer of groups

of bytes (“datagrams”) between client and server

Client/server socket interaction: UDP

closeclientSocket

read datagram fromclientSocket

create socket:clientSocket =socket(AF_INET,SOCK_DGRAM)

Create datagram with server IP andport=x; send datagram viaclientSocket

create socket, port= x:serverSocket =socket(AF_INET,SOCK_DGRAM)

read datagram fromserverSocket

write reply toserverSocketspecifying client address,port number

Application 2-14

server (running on serverIP) client


Example app: UDP client

from socket import *

serverName = ‘hostname’

serverPort = 12000

clientSocket = socket(socket.AF_INET,

socket.SOCK_DGRAM)

message = raw_input(’Input lowercase sentence:’)

clientSocket.sendto(message,(serverName, serverPort))

modifiedMessage, serverAddress =

clientSocket.recvfrom(2048)

print modifiedMessage

clientSocket.close()

Python UDPClientinclude Python’s socket library

create UDP socket for server

get user keyboardinput

Attach server name, port to message; send into socket

print out received string and close socket

read reply characters fromsocket into string


Example app: UDP server


serverPort = 12000

serverSocket = socket(AF_INET, SOCK_DGRAM)

serverSocket.bind(('', serverPort))

print “The server is ready to receive”

while 1: message, clientAddress = serverSocket.recvfrom(2048) modifiedMessage = message.upper() serverSocket.sendto(modifiedMessage, clientAddress)

Python UDPServer

create UDP socket

bind socket to local port number 12000

loop forever

Read from UDP socket into message, getting client’s address (client IP and port)

send upper case string back to this client


Socket programming with TCPclient must contact

server server process must

first be running server must have

created socket (door) that welcomes client’s contact

client contacts server by:

Creating TCP socket, specifying IP address, port number of server process

when client creates socket: client TCP establishes connection to server TCP

when contacted by client, server TCP creates new socket for server process to communicate with that particular client allows server to talk

with multiple clients source port numbers

used to distinguish clients (more in Chap 3)

TCP provides reliable, in-orderbyte-stream transfer (“pipe”) between client and server

application viewpoint:


Client/server socket interaction: TCP

wait for incomingconnection requestconnectionSocket =serverSocket.accept()

create socket,port=x, for incoming request:serverSocket = socket()

create socket,connect to hostid, port=xclientSocket = socket()

server (running on hostid) client

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

closeconnectionSocket

read reply fromclientSocket

closeclientSocket


Example app: TCP client


serverName = ’servername’

serverPort = 12000

clientSocket = socket(AF_INET, SOCK_STREAM)

clientSocket.connect((serverName,serverPort))

sentence = raw_input(‘Input lowercase sentence:’)

clientSocket.send(sentence)

modifiedSentence = clientSocket.recv(1024)

print ‘From Server:’, modifiedSentence

clientSocket.close()

Python TCPClient

create TCP socket for server, remote port 12000

No need to attach server name, port


Example app: TCP server

from socket import *serverPort = 12000serverSocket = socket(AF_INET,SOCK_STREAM)serverSocket.bind((‘’,serverPort))serverSocket.listen(1)print ‘The server is ready to receive’while 1: connectionSocket, addr = serverSocket.accept() sentence = connectionSocket.recv(1024) capitalizedSentence = sentence.upper() connectionSocket.send(capitalizedSentence) connectionSocket.close()

Python TCPServer

create TCP welcomingsocket

server begins listening for incoming TCP requests

loop forever

server waits on accept()for incoming requests, new socket created on return

read bytes from socket (but not address as in UDP)

close connection to this client (but not welcoming socket)


Socket programming

Two socket types for two transport services:

UDP: unreliable datagram• Connectionless• Best effort• No checking to see if send was successful

TCP: reliable, byte stream-oriented • Connection established before data transfer can begin• The network can still garble, lose, or put packets out of

order. It’s up to TCP to make it right.• Guaranteed successful delivery.


Project 1

Your starter code for Project 1

Project1_thread_example.c

Project1_Starter_Client.c

AddressTranslation.c

Gettysburg.html Mickey.jpg

Use this code foryour server.

Use this code foryour client.

Support files.

Introduction

Lecture 2: roadmap1.5 protocol layers, service models2.7 socket programming with UDP and

TCP1.3 network core

packet switching, circuit switching, network structure

1-23

Introduction

mesh of interconnected routers

packet-switching: hosts break application-layer messages into packets forward packets from

one router to the next, across links on path from source to destination

each packet transmitted at full link capacity

The network core

1-24

Introduction

Packet-switching: store-and-forward

takes L/R seconds to transmit (push out) L-bit packet into link at R bps

store and forward: entire packet must arrive at router before it can be transmitted on next link

one-hop numerical example:

L = 7.5 Mbits R = 1.5 Mbps one-hop

transmission delay = 5 sec

more on delay shortly …1-25

sourceR bps destination

123

L bitsper packet

R bps

end-end delay = 2L/R (assuming zero propagation delay)

Introduction

Packet Switching: queueing delay, loss

A

B

CR = 100 Mb/s

R = 1.5 Mb/sD

Equeue of packetswaiting for output link

1-26

queuing and loss: If arrival rate (in bits) to link exceeds

transmission rate of link for a period of time: packets will queue, wait to be transmitted on

link packets can be dropped (lost) if memory

(buffer) fills up

Network Layer 4-27

Two key network-core functions

forwarding: move packets from router’s input to appropriate router output

routing: determines source-destination route taken by packets

routing algorithms

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

1

23

0111

dest address in arrivingpacket’s header

Introduction

Alternative core: circuit switchingend-end resources

allocated to, reserved for “call” between source & dest:

In diagram, each link has four circuits. call gets 2nd circuit in

top link and 1st circuit in right link.

dedicated resources: no sharing circuit-like

(guaranteed) performance

circuit segment idle if not used by call (no sharing)

Commonly used in traditional telephone networks

1-28

Introduction

Circuit switching: FDM versus TDM

FDM

frequency

timeTDM

frequency

time

4 users

Example:

1-29

Introduction

Packet switching versus circuit switching

example: 1 Mb/s link each user:

• 100 kb/s when “active”• active 10% of time

circuit-switching: 10 users

packet switching: with 35 users,

probability > 10 active at same time is less than .0004 *

packet switching allows more users to use network!

N users

1 Mbps link

Q: what happens if > 35 users ?

…..

1-30* Check out the online interactive exercises for more examples

http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/transmission/delay.html



Introduction

great for bursty data resource sharing simpler, no call setup

excessive congestion possible: packet delay and loss protocols needed for reliable data transfer,

congestion control Q: How to provide circuit-like behavior?

bandwidth guarantees needed for audio/video apps

still an unsolved problem (chapter 7)

is packet switching a “slam dunk winner?”

Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?

Packet switching versus circuit switching

1-31

Internet structure: network of networks

End systems connect to Internet via access ISPs (Internet Service Providers) Residential, company and university ISPs

Access ISPs in turn must be interconnected. So that any two hosts can send packets to

each other Resulting network of networks is very complex

Evolution was driven by economics and national policies

Let’s take a stepwise approach to describe current Internet structure


Question: given millions of access ISPs, how to connect them together?

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

…

………

…

…


Option: connect each access ISP to every other access ISP?

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

…

………

…

…

…

…

………

connecting each access ISP to each other directly doesn’t

scale: O(N2) connections.


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

…

………

…

…

Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement.

globalISP


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

…

………

…

…

But if one global ISP is viable business, there will be competitors ….

ISP B

ISP A

ISP C


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

…

………

…

…

But if one global ISP is viable business, there will be competitors …. which must be interconnected

ISP B

ISP A

ISP C

IXP

IXP

peering link

Internet exchange point


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

…

………

…

…

… and regional networks may arise to connect access nets to ISPS

ISP B

ISP A

ISP C

IXP

IXP

regional net


accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnet

accessnetaccess

net

accessnet

…

………

…

…

… and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users

ISP B

ISP A

ISP B

IXP

IXP

regional net

Content provider network

Introduction


at center: small # of well-connected large networks “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT),

national & international coverage content provider network (e.g, Google): private network

that connects it data centers to Internet, often bypassing tier-1, regional ISPs

1-40

accessISP

accessISP

accessISP

accessISP

accessISP

accessISP

accessISP

accessISP

Regional ISP Regional ISP

IXP

IXP

Tier 1 ISP Tier 1 ISP Google

IXP

Introduction

Tier-1 ISP: e.g., Sprint

…

to/from customers

peering

to/from backbone

…

………

POP: point-of-presence

1-41

introduction 1-1 lecture 2 computer networking: a top down approach 6 th edition jim kurose, keith...

Documents

network corepacket

introduction lecture

network applicationsftp

application process

baggage claim gates

layers service transparent

service models2

explicit structure