unit1- data communication

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Data Communication

Components

Data Representation

Direction of Data Flow



Five components of data communication



Simplex



Half-duplex



Full-duplex



Networks

Distributed Processing

Network Criteria

Physical Structures

Categories of Networks



Point-to-point connection



Multipoint connection



Categories of topology



Figure 1.8 Fully connected mesh topology (for five devices)



Mesh.

• A network that uses a mesh topology provides a direct connection

between each pair of computers. The chief disadvantage of a mesh

arises from the cost: a mesh network connecting n computers

requires:

• The important point is that the number of connections needed for a

mesh network grows faster than the number of computers. Because

connections are expensive, few LANs employ a mesh topology.



Mesh Topology

• Every Link carries its own

data avoids traffic

problems.

• Robust as alternative

path is available

• Privacy & Security as

dedicated link is there

only desired user can see

the message

• Troubleshooting is easy

• Large amount of cabling.

• No. of I/O ports requirdare more (N-1).

• Installation and

Reconnection are difficult

(scalability).

• Expensive.

• In a limited application or very good for to be used

as backbone.

S l



Star topology

A network uses a star topology when all computers attach to a central point.

Because a star-shaped network resembles the spokes of a wheel, the

center of a star network is often called a hub. A typical hub consists of an

electronic device that accepts data from a sending computer and delivers it

to the appropriate destination. In practice, star networks seldom have a

symmetric shape in which the hub is located an equal distance from all

computers. Instead, a hub often resides in a location separate from the

computers attached to it. For example, computers can reside in individualoffices, while the hub resides in a location accessible to an organization’s

networking staff.



Star Topology

• Less Expensive one link

and one I/O port only per

device.

• Easy installation ,

reconfiguration, addition

and deletion of nodes.Less cabling

• Robust- only affected link

is inactive.

• Easy troubleshooting.

• Works effectively as long

as hub works.

• More cabling as

compared to ring or bus.

B t l



Bus topology

A network that uses a bus topology usually consists of asingle cable to which computers attach†. Any computer

attached to a bus can send a signal down the cable, and all

computers receive the signal. Because all computers attachdirectly to the cable, any computer can send data to any

other computer. Of course, the computers attached to a bus

network must coordinate to ensure that only one computer

sends a signal at any time.

B T l



Bus Topology• Ease of installation.

• Less cabling efficientbackbone.

• Due to tap signal becomesweaker and weaker as ittravels farther & farther.Hence limitation of distance

and no. of nodes.• Includes difficult

reconnection (splicing).

• Difficult Troubleshooting.• Addition, Deletion is difficult.Signal reflection at tapdegrades quality may

involve backbone cablereplacement.

• A fault or break in cable

causes reflection noise andinterruption in transmission.

Ring topology



Ring topology

A network that uses a ring topology arranges for computers to be connected

in a closed loop — a cable connects the first computer to a second computer,

another cable connects the second computer to a third, and so on, until a

cable connects the final computer back to the first. Some technologies that

use a ring topology require a computer to connect to a small device that

forms the ring. The advantage of using a separate device lies in the ability of

the ring to continue operation even if some of the computers are

disconnected. The name ring arises because one can imagine the computers

and the cables connecting them arranged in a circle as Figure 13.7 illustrates.

In practice, the cables in a ring network do not form a circle. Instead, they run

along hallways or rise vertically from one floor of a building to another.



Ring Topology• Media and Traffic

consideration constraints.

• Maximum ring length maytake time to reach signal

to destination. The

requisite node can raise

an alarm.

• A break in ring disrupts

entire network , solved by

using dual ring.

• Easier installation ,

reconfiguration.

• Addition and Deletion iseasier.

• Fault isolation is easier.

Reason for Multiple Topologies



Reason for Multiple Topologies• Each topology has advantages and disadvantages. A

ring topology makes it easy for computers to coordinateaccess and to detect whether the network is operatingcorrectly. However, an entire ring network is disabled if one of the cables is cut. A star topology helps protect thenetwork from damage to a single cable because eachcable connects only one machine. A bus requires fewer wires than a star, but has the same disadvantage as a

ring: a network is disabled if someone accidentally cutsthe main cable. Later chapters that describe specificnetwork technologies provide additional details aboutdifferences. For now, it is sufficient to understand:

• Networks are classified into broad categories according to their general shape. Although a mesh topology is possible, the primary topologies used with LANs are star,

ring, and bus; each has advantages and disadvantages.



Exercise

• Draw a hybrid topology with a Star withtwo Buses each with 3 Rings of 4 devices

each



Exercise

• For n devices in a network the number of

cable links required for Mesh, Ring , Busand Star topology is…

• Mesh: n (n – 1) / 2• Star: n

• Ring: n – 1

• Bus: one backbone and n drop lines



Exercise

• Mesh: secure

• Bus: easy installation

• Star: robust

• Ring: easy fault isolation

• Mesh: easy installation

• Bus: easy fault isolation

• Star: secure

• Ring: robust

Categories of networks



g f



Network Types

LAN



LAN (Continued)



Figure 1.14 MAN



WAN



Network: Links & switches

• Circuit consists of dedicated resources in

sequence of links & switches across network

• Circuit switch connects input links to output linksSwitch

Network Control

12

3

N

12

3

N

Connection

of inputs

to outputs … …User 1

SwitchLink

User n

User n – 1



Switching Methods

• Circuit Switching.

• Packet Switching.



Circuit Switching.• The term circuit switching refers to a

communication mechanism that establishes a

path between a sender and receiver withguaranteed isolation from paths used by other

pairs of senders and receivers. Circuit switching

is usually associated with telephone technologybecause a telephone system provides a

dedicated connection between two telephones.

In fact, the term originated with early dialuptelephone networks that used electromechanical

switching devices to form a physical circuit.



Circuit Switching



Circuit Switching• Currently, circuit switching networks use electronic

devices to establish circuits. Furthermore, instead of having each circuit correspond to a physical path,

multiple circuits are multiplexed over shared media,and the result is known as a virtual circuit . Thus, thedistinction between circuit switching and other forms

of networking does not arise from the existence of separate physical paths. Instead, three generalproperties define a circuit switched paradigm:

• Point-to-point communication• Separate steps for circuit creation, use, andtermination

• Performance equivalent to an isolated physical path



Circuit Switching

• The first property means that a circuit is formedbetween exactly two endpoints, and the second

property distinguishes circuits that are switched (i.e., established when needed) from circuits thatare permanent (i.e., always remain in place

ready for use). Switched circuits use a three-step process analogous to placing a phone call.In the first step, a circuit is established. In the

second, the two parties use the circuit tocommunicate, and in the third, the two partiesterminate use.



Circuit Switching

• The third property provides a crucial distinctionbetween circuit switched networks and other

types. Circuit switching means that thecommunication between two parties is notaffected in any way by communication amongother parties, even if all communication ismultiplexed over a common medium. Inparticular, circuit switching must provide theillusion of an isolated path for each pair of

communicating entities. Thus, techniques suchas frequency division multiplexing or synchronous time division multiplexing must beused to multiplex circuits over a shared medium.



Conclusion

• Circuit switching provides the illusion of an

isolated physical path between a pair of communicating entities; a path is created

when needed, and discontinued after use.



Packet Switching

• The main alternative to circuit switching, packet switching , forms the basis for the Internet. A packet

switching system uses statistical multiplexing in whichcommunication from multiple sources competes for theuse of shared media. The chief difference betweenpacket switching and other forms of statistical

multiplexing arises because a packet switching systemrequires a sender to divide each message into blocks of data that are known as packets. The size of a packet

varies; each packet switching technology defines amaximum packet size.

Packet Switching



• Arbitrary, asynchronous communication

• No set-up required before communication begins• Performance varies due to statistical multiplexing among

packets

• The first property means that packet switching can allowa sender to communicate with one recipient or multiplerecipients, and a given recipient can receive messagesfrom one sender or multiple senders. Furthermore,communication can occur at any time, and a sender candelay arbitrarily long between successivecommunications. The second property means that,unlike a circuit switched system, a packet switchedsystem remains ready to deliver a packet to any

destination at any time. Thus, a sender does not need toperform initialization before communicating, and doesnot need to notify the underlying system whencommunication terminates.

P k t S it hi



Packet Switching

• The third property means that multiplexing

occurs among packets rather than among bits or

bytes. That is, once a sender gains access to

the underlying channel, the sender transmits an

entire packet, and then allows other senders totransmit a packet. When no other senders are

ready to transmit a packet, a single sender can

transmit repeatedly. However, if N senders eachhave a packet to send, a given sender will

transmit approximately 1/N of all packets.

P k t S it hi



Packet Switching

• One of the chief advantages of packet

switching is the lower cost that arises fromsharing. To provide communication among

N computers, a circuit-switched network

must have a connection for each computer plus at least N/ 2 independent paths. With

packet switching, a network must have aconnection for each computer, but only

requires one path that is shared.

C l i



Conclusion

• Packet switching, which forms the basis of

the Internet, is a form of statistical multiplexing that permits many-to-many

communication. A sender must divide a

message into a set of packets; after transmitting a packet, a sender allows

other senders to transmit beforetransmitting a successive packet.

Comparison



Comparison



Comparison

• Propagation delay. The time it takes a signal topropagate from one node to the next. This time

is generally negligible. The speed of electromagnetic signals through a wire medium,for example, is typically 2 X lo8 mts.

• Transmission time. The time it takes for atransmitter to send out a block of data. For example, it takes 1 s to transmit a 10,000-bitblock of data onto a 10-kbps line.

• Node delay. The time it takes for a node toperform the necessary processing as it switchesdata.

Ci it S it h T



Circuit Switch Types

• Space-Division switches

– Provide separate physical connection

between inputs and outputs

– Crossbar switches

– Multistage switches

• Time-Division switches

– Time-slot interchange technique – Time-space-time switches

• Hybrids combine Time & Space switching



Space Division Switching

Crossbar Space Switch



Crossbar Space Switch

• N x N array of

crosspoints

• Connect an input

to an output by

closing a

crosspoint

• Nonblocking: Any

input can connectto idle output

• Complexity: N 2

N

1 2

1

N

2

N –1

…

…

crosspoints

Multistage Space Switch• Large switch built from multiple stages of small switches



• Large switch built from multiple stages of small switches

• The n inputs to a first-stage switch share k paths throughintermediate crossbar switches

• Larger k (more intermediate switches) means more paths tooutput

• In 1950s, Clos asked, “How many intermediate switchesrequired to make switch nonblocking?”

2(N/n)nk + k (N/n)2 crosspoints

n×k

n×k

n×k

n×k

N/n × N/n

N/n × N/n

N/n × N/n

k ×n1

2

N/n

N

inputs

1

2

3 3

N/n

N

outputs

1

2

k

k ×n

k ×n

k ×n

… … …

Clos Non-Blocking Condition: k=2n-1



Clos Non Blocking Condition: k 2n 1

nxk

nxk

nxk

N/n x N/n

N/n x N/n

N/n x N/n

k xn1

N/n

Desired

input

1

j m

N/n

Desired

output

1

2n -1

k xn

k xn

n-1

N/n x N/nn+1

N/n x N/n2n-2

Free path Free path

n-1

busy

n-1

busy

…

… …

…

• Request connection from last input to input switch j to last output in outputswitch m

• Worst Case: All other inputs have seized top n-1 middle switches AND allother outputs have seized next n-1 middle switches

• If k=2n-1, there is another path left to connect desired input to desired output

# internal links =2x # external links

Minimum Complexity Clos Switch



Minimum Complexity Clos Switch

C(n) = number of crosspoints in Clos switch

= 2Nk + k ( )2 = 2N (2n – 1)+(2n – 1)( )2

Differentiate with respect to n:

0 = = 4N – + ≈ 4N – ==> n ≈ √

The minimized number of crosspoints is then:

C* = (2N + )(2( )1/2 – 1) ≈ 4N √ 2N = 4 √ 2N 1.5

This is lower than N 2 for large N

N 2

N /2

2N 2

n2

2N 2

n3

2N 2

n2

N

2

δC

δn

N

n

N

n

N 2

Example: Clos Switch Design



Example: Clos Switch Design

• Circa 2002, Mindspeed offered a Crossbar chip with the following specs:

– 144 inputs x 144 outputs, 3.125

Gbps/line – Aggregate Crossbar chip throughput:450 Gbps

• Clos Nonblocking Design for 1152x1152switch

– N=1152, n=8, k=16

– N/n=144 8x16 switches in first stage

– 16 144x144 in centre stage

– 144 16x8 in third stage

– Aggregate Throughput: 3.6 Tbps!

– Note: the 144x144 crossbar can bepartitioned into multiple smaller switches

8x16

8x16

8x16

8x16

144×144

144x144

144x144

16x8 1

2

144

1 1 5 2

i n p u t s

1

2

3 3

N/n

1 1 5 2

o u

t p u t s

1

2

16

16x8

16x8

16x8

… … …

Ti Di i i S it hi



Time Division Switching

Space

SwitchTime

Switch

Hybrid

•Time Division switches are cost effective as

compared to space switches.

Time Division Space Switching



Time Division Space Switching•Each inlet & outlet is asingle subscriber line

•Sampling rate is 8 Khz.

hence T= 125 µs.•Simultaneous conversationsor switching capacity = 125µs/ ts , where ts is time

required for transfer of onevalue.

•A single switching elementand a bus is time shared

and a physical connection isestablished between oneinlet & outlet for time tshence is TD Space switch

1

.

.

N

1

.

.

N

Counter – N-

Decoder 2N

Non blocking

No full availability :- i.e. any inlet cannot be connected to any outlet.

• Thus to achieve full availability



• Thus to achieve full availability

following types of TDS switches canbe used,

Input controlled TDS switch.

Output controlled TDS switch.

Memory controlled TDS switch

Input Controlled TDS Switch



Input Controlled TDS Switch1

.

.

N

1

.

.

N

Counter – N-

Decoder 2N

MAR

5

Time-Slot Interchange (TSI)Switching



1

23

22

23

24

Write

slots inorder ofarrival

Read slots

according toconnectionpermutation

24 23 12

Time-slot interchange

24 23 12abcd b a d c

a

b

c

d

… …

• Write bytes from arriving TDM stream into memory

• Read bytes in permuted order into outgoing TDMstream

• Max # slots = 125 µsec / (2 x memory cycle time)

Switching

Incoming TDM

stream

Outgoing TDM

stream

Time-Space-Time Hybrid Switch



Time Space Time Hybrid Switch• Use TSI in first & third stage; Use crossbar in middle

nxk

nxk

nxk

nxk

N/n x N/n k xn1

2

N/n

N inputs

1

3

1

…

1

2

n

Time-slot interchange

Input TDMframe with

n slots

Output TDM

frame with k

slots

n … 2 1 k … 2 1

• Replace n input x k output space switch by TSI switch that takes n-slot input frame and switches it to k-slot output frame

Flow of time slots between



n × k N/n × N/n

N/n × N/n

N/n × N/n

k × n1 1

2

N/n

1

2

k

k × n

k × n

n × k 2

n × k N/n

First slot

k th slot

First slot

k th slot

… … …

switches

• Only one space switch active in each time slot

Time-Share the Crossbar Switch



nxk

nxk

nxk

nxk

N/n x N/n

Time-shared

space switch

k xn

1

2

N/n

N

inputs

1

2

3 3

N/n

N

outputs

TDMn slots

n slots

n slots

n slots

k xn

k xn

k xn

TDMk slots

TDMk slots

TSI stage TSI stageSpace stage

… …

• Interconnection pattern of space switch isreconfigured every time slot

• Very compact design: fewer lines because of TDM & less space because of time-sharedcrossbar

Example: A→3, B→4, C→1, D→3



A

B

C

D

(a)C

A

D

B

p , , ,

3-stage

SpaceSwitch

2x3

2x3

3x21

2

1

23x2D1

B1 A1B2 A2

C1D2 C2

B1 A1

C1D1

A1

B1

C1

D1

A1 C1

B1 D1

(b)

Equivalent TST Switch

Example: T-S-T Switch Design



For N = 960• Single stage space switch ~ 1 million

crosspoints

• T-S-T – Let n = 120 N/n = 8 TSIs

– k = 2n – 1 = 239 for non-blocking

– Pick k = 240 time slots – Need 8x8 time-multiplexed space switch

For N = 96,000• T-S-T

– Let n = 120 k = 239

– N / n = 800 – Need 800x800 space switch

Available TSI Chips circa 2002



p

• OC-192 SONET Framer Chips

– Decompose 192 STS1s and perform(restricted) TSI

• Single-chip TST

– 64 inputs x 64 outputs

– Each line @ STS-12 (622 Mbps) – Equivalent to 768x768 STS-1 switch

Pure Optical Switching



• Pure Optical switching: light-in, light-out, withoutoptical-to-electronic conversion

• Space switching theory can be used to design

optical switches – Multistage designs using small optical switches

– Typically 2x2 or 4x4

– MEMs and Electro-optic switching devices

• Wavelength switches

– Very interesting designs when space switching iscombined with wavelength conversion devices

Exercise



• Construct A three stage switch with N = 16,

n = 4, k = 2.

• Repeat all questions for k=4 and k=10.

• What is the maximum number of connections that can be

supported at any given time?



pp y g

– Thus, the second stage is the bottleneck, and blocking can occur inthe first stage. Thus, eight connections can be supported at a time. If

k = 4, then blocking will occur if we are not allowed to rearrange

connections. It can be shown that in this case blocking can be

avoided if we are allowed to rearrange the connection pattern everytime a new connection request is made. If k = 10, then there are ten

4 x 4 switches in the second stage. Since there are only 16 inputs

and 16 outputs, the switch can accommodate any set of connections

without blocking.

• For a given set of input-output pairs, is there more than

one way to arrange the connections over the multistage

switch? – As shown in the picture in part (a), it is clear that each input-output

pair can be connected through any one of the k second-stage

switches. Thus, here are k ways to arrange the connections over amulti-stage switch.

Exercise



• What is the delay incurred in traversing a TSI

switch? – The arriving frame is written onto the switch

register and after the frame is completely

written in, it needs to be read out in permutedorder.

Protocols



• A protocol is a set of rules that governshow two or more communicating entities ina layer are to interact

• Messages that can be sent and received

• Actions that are to be taken when a certainevent occurs, e.g. sending or receivingmessages, expiry of timers

• The purpose of a protocol is to providea service to the layer above

Layers



• A set of related communication functions thatcan be managed and grouped together

• Application Layer: communications functionsthat are used by application programs – HTTP, DNS, SMTP (email)

• Transport Layer: end-to-end communicationsbetween two processes in two machines – TCP, User Datagram Protocol (UDP)

• Network Layer: node-to-node communicationsbetween two machines – Internet Protocol (IP)

Summary



• Layers: related communications functions – Application Layer: HTTP, DNS

– Transport Layer: TCP, UDP

– Network Layer: IP• Services: a protocol provides a communications

service to the layer above – TCP provides connection-oriented reliable byte transfer

service – UDP provides best-effort datagram service

• Each layer builds on services of lower layers

– HTTP builds on top of TCP – DNS builds on top of UDP

– TCP and UDP build on top of IP



Applications and LayeredArchitectures

OSI Reference Model

Why Layering?



• Layering simplifies design, implementation, andtesting by partitioning overall communications

process into parts• Protocol in each layer can be designed

separately from those in other layers

• Protocol makes “calls” for services from layer below

• Layering provides flexibility for modifying andevolving protocols and services without havingto change layers below

• Monolithic non-layered architectures are costly,inflexible, and soon obsolete

Open Systems Interconnection



• Network architecture:

– Definition of all the layers

– Design of protocols for every layer

• By the 1970s every computer vendor haddeveloped its own proprietary layered networkarchitecture

• Problem: computers from different vendors

could not be networked together • Open Systems Interconnection (OSI) was an

international effort by the International

Organization for Standardization (ISO) to enablemultivendor computer interconnection

OSI Reference Model



• Describes a seven-layer abstract referencemodel for a network architecture

• Purpose of the reference model was to provide aframework for the development of protocols

• OSI also provided a unified view of layers,

protocols, and services which is still in use in thedevelopment of new protocols

• Detailed standards were developed for eachlayer, but most of these are not in use

• TCP/IP protocols preempted deployment of OSIprotocols

7-Layer OSI Reference Model



Application

Layer

Presentation

Layer

Session

Layer

TransportLayer

Network

Layer

Data LinkLayer

Physical

Layer

Application

Layer

Presentation

Layer

Session

Layer

TransportLayer

Network

Layer

Data LinkLayer

Physical

Layer

Network

Layer

Application Application

Data LinkLayer

Physical

Layer

Network

Layer

Data LinkLayer

Physical

Layer

Communicating End SystemsOne or More Network Nodes

End-to-End Protocols

Physical Layer



• Transfers bits across link

• Definition & specification of the physicalaspects of a communications link – Mechanical: cable, plugs, pins...

– Electrical/optical: modulation, signal strength, voltage

levels, bit times, … – functional/procedural: how to activate, maintain, and

deactivate physical links…

• Ethernet, DSL, cable modem, telephonemodems…

• Twisted-pair cable, coaxial cable optical fiber,

radio, infrared, …

Data Link Layer



• Transfers frames across direct connections

• Groups bits into frames

• Detection of bit errors; Retransmission of frames• Activation, maintenance, & deactivation of data link

connections

• Medium access control for local area networks

• Flow control

Data Link

Layer

Physical

Layer

Data Link

Layer

Physical

Layer

frames

bits

Network Layer



• Transfers packets across multiple links and/or

multiple networks

• Addressing must scale to large networks

• Nodes jointly execute routing algorithm to

determine paths across the network• Forwarding transfers packet across a node

• Congestion control to deal with traffic surges

• Connection setup, maintenance, and teardown

when connection-based

Internetworking



• Internetworking is part of network layer andprovides transfer of packets across multiple

possibly dissimilar networks• Gateways (routers) direct packets across

networks

G = gateway

H = host

Net 1

Net 5

Net 3

Net 2

HNet 3

G

H

H

H

GG

GG

G

Net 1

Net 2 Net 4

Net 5

Ethernet LAN

ATM

Switch

ATM

Switch

ATMSwitch

ATM

Switch

ATM

Network

Transport Layer



• Transfers data end-to-end from process in amachine to process in another machine

• Reliable stream transfer or quick-and-simplesingle-block transfer

• Port numbers enable multiplexing

• Message segmentation and reassembly

• Connection setup, maintenance, and release

TransportLayer

Network

Layer

TransportLayer

Network

Layer Network

Layer

Network

Layer

Communication Network

Application & Upper Layers



• Application Layer: Providesservices that are frequently

required by applications:DNS, web acess, file transfer,email…

• Presentation Layer: machine-independent representation of data…

• Session Layer: dialogmanagement, recovery fromerrors, …

Application

Layer

Presentation

Layer

Session

Layer

TransportLayer

Application

Application

Layer

TransportLayer

Application

Incorporated intoApplication Layer

Headers & Trailers



• Each protocol uses a header that carries addresses,sequence numbers, flag bits, length indicators, etc…

• CRC check bits may be appended for error detection

Application

Layer

Transport

Layer

NetworkLayer

Data Link

Layer

PhysicalLayer

Application

Layer

Transport

Layer

NetworkLayer

Data Link

Layer

PhysicalLayer

Application ApplicationAPP DATA

AH APP DATA

TH AH APP DATA

NH TH AH APP DATA

DH NH TH AH APP DATA CRC

bits

OSI Unified View: Protocols



• Layer n in one machine interacts with layer n inanother machine to provide a service to layer n

+1• The entities comprising the corresponding layers

on different machines are called peer processes.

• The machines use a set of rules andconventions called the layer-n protocol .

• Layer-n peer processes communicate byexchanging Protocol Data Units (PDUs)

n

Entityn

Entity

n-PDUs

Layer n peer protocol

OSI Unified View: Services



• Communication between peer processes is virtualand actually indirect

• Layer n+1 transfers information by invoking theservices provided by layer n

• Services are available at Service Access Points( SAP’s)

• Each layer passes data & control information to thelayer below it until the physical layer is reached andtransfer occurs

• The data passed to the layer below is called aService Data Unit (SDU)

• SDU’s are encapsulated in PDU’s

Layers, Services & Protocols



n+1

entity

n-SAP

n+1

entity

n-SAP

n entity n entity

n-SDU

n-SDU

n-SDU

H

H n-SDU

n-PDU

Interlayer Interaction



layer

N+1 user N provider N provider N+1 user

Request Ind i c at i o n

R e s p o n

s e

C o n f i r m

System A System B

Connectionless & Connection-

Oriented Services



• Connection-Oriented

– Three-phases:

1. Connection setup

between two SAPs

to initialize state

information2. SDU transfer

3. Connection release

– E.g. TCP, ATM

• Connectionless

– Immediate SDU

transfer – No connection setup

– E.g. UDP, IP

• Layered servicesneed not be of sametype

– TCP operates over IP – IP operates over ATM

Segmentation & Reassembly



• A layer may impose alimit on the size of a datablock that it can transfer for implementation or other reasons

• Thus a layer-n SDU may

be too large to behandled as a single unitby layer-(n-1)

• Sender side: SDU issegmented into multiplePDUs

• Receiver side: SDU is

reassembled fromsequence of PDUs

n-SDU

n-PDU n-PDU n-PDU

Segmentation(a)

Reassembly(b)

n-PDU

n-SDU

n-PDU n-PDU

Multiplexing



• Sharing of layer n service by multiple layer n+1 users

• Multiplexing tag or ID required in each PDU to determine

which users an SDU belongs to

n+1entity

n+1entity

n+1entity

n+1entity

n entity n entity

n-SDUn-SDU

n-SDUH

Hn-SDU

n-PDU

Summary

Layers: related communications functions



• Layers: related communications functions – Application Layer: HTTP, DNS

– Transport Layer: TCP, UDP

– Network Layer: IP• Services: a protocol provides a communicationsservice to the layer above – TCP provides connection-oriented reliable byte transfer

service – UDP provides best-effort datagram service

• Each layer builds on services of lower layers

– HTTP builds on top of TCP – DNS builds on top of UDP

– TCP and UDP build on top of IP

Summary of duties




TCP/IP Architecture

How the Layers Work Together

Why Internetworking?

• To build a “network of networks” or internet



• To build a network of networks or internet – operating over multiple, coexisting, different network

technologies

– providing ubiquitous connectivity through IP packettransfer

– achieving huge economies of scale

G

G

G

GG

G

H

Net 5Net 5

HNet 5Net 2

H

Net 5Net 3H

Net 5Net 1

Net 5Net 4


• To provide universal communication services



• To provide universal communication services – independent of underlying network technologies

– providing common interface to user applications

G

G

G

GG

G

H

Net 5Net 5

HNet 5Net 2

H

Net 5Net 3H

Net 5Net 1

Net 5Net 4

Reliable Stream Service

User Datagram Service


• To provide distributed applications



• To provide distributed applications – Any application designed to operate based on Internet

communication services immediately operates across

the entire Internet – Rapid deployment of new applications

• Email, WWW, Peer-to-peer

– Applications independent of network technology• New networks can be introduced below

• Old network technologies can be retired

Internet Protocol Approach IP packets transfer information across Internet



IP packets transfer information across InternetHost A IP → router → router…→ router → Host B IP

IP layer in each router determines next hop (router)

Network interfaces transfer IP packets across networks

Router

InternetLayer

Network

Interface

TransportLayer

Internet

Layer

Network

Interface

TransportLayer

Internet

Layer

Network

Interface

Host A Host B

Net 5Net 1

Net 5Net 2 Net 5Net 3

Router

Internet

Layer

NetworkInterface

Router

Internet

Layer

Network

Interface

Net 5Net 4

TCP/IP Protocol Suite

Distributed

HTTP SMTP RTPDNS



Reliable

streamservice

Distributedapplications User

datagramservice

(ICMP, ARP)

TCP UDP

IP

Network

interface 1

Network

interface 3

Network

interface 2

Best-effortconnectionlesspacket transfer

Diverse network technologies

Internet Names & Addresses

I t t N Internet Addresses



Internet Names• Each host a a unique name

– Independent of physicallocation

– Facilitate memorization byhumans

– Domain Name

– Organization under singleadministrative unit

• Host Name

– Name given to host computer

• User Name

– Name assigned to user

[email protected]

Internet Addresses

• Each host has globally uniquelogical 32 bit IP address

• Separate address for eachphysical connection to anetwork

• Routing decision is done

based on destination IPaddress

• IP address has two parts:

– netid and hostid – netid unique

– netid facilitates routing

• Dotted Decimal Notation:

int1.int2.int3.int4DNS resolves IP name to IP address

Physical Addresses

LAN ( d th t k ) i h i l



• LANs (and other networks) assign physicaladdresses to the physical attachment to the

network• The network uses its own address to transfer

packets or frames to the appropriate destination

• IP address needs to be resolved to physicaladdress at each IP network interface

• Example: Ethernet uses 48-bit addresses

– Each Ethernet network interface card (NIC) hasglobally unique Medium Access Control (MAC) or physical address

– First 24 bits identify NIC manufacturer; second 24 bitsare serial number

Intel

Example internet

(2 1)

PCServer

Router



(1,1) s

(2,1)

(1,3) r (2,2)PPP

Netid=2w

Ethernet(netid=1)

*PPP does not use addressesWorkstation

(1,2)

netid hostidPhysical

address

router 2 1 -

PC 2 2 -

server 1 1 s

workstation 1 2 w

router 1 3 r

Encapsulation



IP

header IP Payload

Ethernetheader FCSIP

header IP Payload

Ethernet header contains: source and destination physical addresses

network protocol type (e.g. IP)

IP packet from workstation to

server PCServer

Router



(1,1) s

w

(2,1)

(1,3) r (2,2)PPP

Ethernet

Router

(1,2), (1,1)w, s

Workstation

(1,2)

1. IP packet has (1,2) IP address for source and (1,1) IP address for destination

2. IP table at workstation indicates (1,1) connected to same network, so IP

packet is encapsulated in Ethernet frame with addresses w and s3. Ethernet frame is broadcast by workstation NIC and captured by server

NIC

4. NIC examines protocol type field and then delivers packet to its IP layer

IP packet from server to PC

(2 1)

PCServer Router

(1 1) (2 2)



(1,1) s

w

(2,1)

(1,3) r

(1,1), (2,2)s, r

(1,1), (2,2)

(2,2)

Workstation

(1,2)

1. IP packet has (1,1) and (2,2) as IP source and destination addresses

2. IP table at server indicates packet should be sent to router, so IP packetis encapsulated in Ethernet frame with addresses s and r

3. Ethernet frame is broadcast by server NIC and captured by router NIC

4. NIC examines protocol type field and then delivers packet to its IP layer

5. IP layer examines IP packet destination address and determines IPpacket should be routed to (2,2)

6. Router’s table indicates (2,2) is directly connected via PPP link

7. IP packet is encapsulated in PPP frame and delivered to PC

8. PPP at PC examines rotocol t e field and delivers acket to PC IP

How the layers work together PCServer

Router(a)



(1,1) s

(2,1)

(1,3) r (2,2)PPP

Router

Ethernet

Server (b)

Network interface

IP

TCP

HTTP

Network interface

IP

Network interface

IP

TCP

HTTP

Ethernet PPPRouter

PC

TCP uses node-to-node

Unreliable packet transfer of IP

Server IP address & PC IP address

Internet

HTTP uses process-to-process

Reliable byte stream transfer of

TCP connection:

Server socket: (IP Address, 80)

PC socket (IP Address, Eph. #)

Encapsulation

TCP Header contains



source & destination

port numbers

IP Header contains

source and destination

IP addresses;

transport protocol type

Ethernet Header contains

source & destination MAC

addresses;network protocol type

HTTP Request

TCP

header HTTP Request

IP

header

TCP

header HTTP Request

Ethernet

header

IP

header

TCP

header HTTP Request FCS

How the layers work together:Network Analyzer Example



Internet

• User clicks on http://www.nytimes.com/• Ethereal network analyzer captures all frames

observed by its Ethernet NIC

• Sequence of frames and contents of frame canbe examined in detail down to individual bytes

Ethereal windows

Top Pane

shows

frame/packet

sequence

Middle Pane

shows

encapsulation for a given frame

http://www.nytimes.com/




q a given frame

Bottom Pane shows hex & text

Top pane: frame sequence

DNSQuery

TCP

ConnectionSetup

HTTP



Query SetupRequest &

Response

Middle pane: Encapsulation



Ethernet Frame

EthernetDestination and

Source

Addresses

Protocol Type


And a lot of

other stuff!



IP Packet

IP Source and

Destination

Addresses

Protocol Type




TCP Segment

Source and

Destination PortNumbers

HTTP

Request

GET

Summary

• Encapsulation is key to layering



Encapsulation is key to layering

• IP provides for transfer of packets across

diverse networks

• TCP and UDP provide universal

communications services across the Internet

• Distributed applications that use TCP and UDP

can operate over the entire Internet

• Internet names, IP addresses, port numbers,sockets, connections, physical addresses




Sockets

Socket API

• API (Application Programming Interface)P id t d d t f f ti th t b ll d



– Provides a standard set of functions that can be calledby applications

• Berkeley UNIX Sockets API – Abstraction for applications to send & receive data

– Applications create sockets that “plug into” network

– Applications write/read to/from sockets

– Implemented in the kernel

– Facilitates development of network applications

– Hides details of underlying protocols & mechanisms

• Also in Windows, Linux, and other OS’s

Communications through Socket

InterfaceClient Server

A li ti 1Socket A li ti 2Socket



descriptor descriptor

• Application references a

socket through a descriptor • Socket bound to a port number port number port number

Application 1

Socket

Socket

interface

User

Kernel

Application 2

User

Kernel

Underlying

communication

protocols

Underlying

communication

protocols

Communications

network

Socket

Socket

interface

Stream mode of service

Connection-oriented • Connectionless



• First, setup connectionbetween two peer application processes

• Then, reliablebidirectional in-sequence

transfer of byte stream(boundaries notpreserved in transfer)

• Multiple write/readbetween peer processes

• Finally, connectionrelease

• Uses TCP

• Immediate transfer of oneblock of information(boundaries preserved)

• No setup overhead &delay

• Destination address witheach block

• Send/receive to/from

multiple peer processes• Best-effort service only

– Possible out-of-order

– Possible loss• Uses UDP

Client & Server Differences

• Server



– Specifies well-known port # when creating socket

– May have multiple IP addresses (net interfaces) – Waits passively for client requests

• Client

– Assigned ephemeral port #

– Initiates communications with server

– Needs to know server’s IP address & port #• DNS for URL & server well-known port #

– Server learns client’s address & port #

Socket Calls for Connection-

Oriented Mode

Server Server does Passive Open socket creates socket to listen for connection



socket()

socket()

bind()

read()

close()

Data

Data

Clientlisten()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

soc et c eates soc et to ste o co ect orequests

Server specifies type: TCP (stream)

socket call returns: non-negative integer descriptor ;or -1 if unsuccessful

close()

Socket Calls for Connection-

Oriented Mode

Server Server does Passive Open bind assigns local address & port # to socket with



socket()

socket()

bind()

read()

close()

Data

Data

Clientlisten()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

g pspecified descriptor

Can wildcard IP address for multiple net interfaces

bind call returns: 0 (success); or -1 (failure)

Failure if port # already in use or if reuse option notset

close()

Socket Calls for Connection-Oriented Mode

Server Server does Passive Open listen indicates to TCP readiness to receive



socket()

socket()

bind()

read()

close()

Data

Data

Clientlisten()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

connection requests for socket with given descriptor

Parameter specifies max number of requests that maybe queued while waiting for server to accept them

listen call returns: 0 (success); or -1 (failure)

close()


Server Server does Passive Open Server calls accept to accept incoming requests



socket()

socket()

bind()

read()

close()

Data

Data

Clientlisten()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

accept blocks if queue is empty

close()


Server Client does Active Open socket creates socket to connect to server



socket()

socket()

bind()

read()

close()

Data

Data

Clientlisten()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

close()

Client specifies type: TCP (stream)

socket call returns: non-negative integer descriptor ;or -1 if unsuccessful


Server Client does Active Open connect establishes a connection on the local socket



socket()

socket()

bind()

read()

close()

Data

Data

Clientlisten()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

close()

with the specified descriptor to the specified remoteaddress and port #

connect returns 0 if successful; -1 if unsuccessful

Note: connect

initiates TCP three-way

handshake


Server accept wakes with incoming connection request accept fills client address & port # into address structure



socket()

socket()

bind()

read()

close()

Data

Data

Client

listen()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

accept call returns: descriptor of new connection socket (success); or -1 (failure)

Client & server use new socket for data transfer

Original socket continues to listen for new requests

close()


Server Data Transfer Client or server call write to transmit data into a

t d k t



socket()

socket()

bind()

read()

close()

Data

Data

Client

listen()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

connected socket

write specifies: socket descriptor; pointer to a buffer;

amount of data; flags to control transmission behavior write call returns: # bytes transferred (success); or -1

(failure); blocks until all data transferred

close()


Server Data Transfer Client or server call read to receive data from a

t d k t



socket()

socket()

bind()

read()

close()

Data

Data

Client

listen()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

connected socket

read specifies: socket descriptor; pointer to a buffer;

amount of data read call returns: # bytes read (success); or -1 (failure);

blocks if no data arrives

close()

Note: write and read

can be called multiple

times to transfer byte

streams in bothdirections


Server Connection Termination Client or server call close when socket is no longer

needed



socket()

socket()

bind()

read()

close()

Data

Data

Client

listen()

accept()

Blocks

write()

connect()

Connect

negotiation

write()

read()

needed

close specifies the socket descriptor

close call returns: 0 (success); or -1 (failure)

Note: close initiates

TCP graceful close

sequence

close()

Example: TCP Echo Server /* Bind an address to the socket */

bzero((char *)&server, sizeof(struct sockaddr_in));

server.sin_family = AF_INET;

server.sin_port = htons(port);

server.sin_addr.s_addr = htonl(INADDR_ANY);

if (bind(sd, (struct sockaddr *)&server,

i

/* A simple echo server using TCP */

#include <stdio.h>

#include <sys/types.h>

#include <sys/socket.h>

#include <netinet/in.h>



sizeof(server)) == -1) {

fprintf(stderr, "Can't bind name to socket\n");

exit(1);

}

/* queue up to 5 connect requests */

listen(sd, 5);

while (1) {

client_len = sizeof(client);

if ((new_sd = accept(sd, (struct sockaddr *)&client,

&client_len)) == -1) {

fprintf(stderr, "Can't accept client\n");

exit(1);

}

bp = buf;

bytes_to_read = BUFLEN;

while ((n = read(new_sd, bp, bytes_to_read)) > 0) {

bp += n;

bytes_to_read -= n;

}

printf("Rec'd: %s\n", buf);

write(new_sd, buf, BUFLEN);printf("Sent: %s\n", buf);

close(new_sd);

}

close(sd);

return(0);

}

#define SERVER_TCP_PORT 3000

#define BUFLEN 256

int main(int argc, char **argv)

{

int n, bytes_to_read;

int sd, new_sd, client_len, port;

struct sockaddr_in server, client;

char *bp, buf[BUFLEN];

switch(argc) {

case 1:

port = SERVER_TCP_PORT;

break;

case 2:port = atoi(argv[1]);

break;

default:

fprintf(stderr, "Usage: %s [port]\n", argv[0]);

exit(1);

}

/* Create a stream socket */

if ((sd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {

fprintf(stderr, "Can't create a socket\n");exit(1);

}

Example: TCP Echo Client/* A simple TCP client */

#include <stdio.h>

#include <netdb.h>




bzero((char *)&server, sizeof(struct sockaddr_in));server.sin_family = AF_INET;


if ((hp = gethostbyname(host)) == NULL) {

fprintf(stderr, "Can't get server's address\n");

exit(1);



#define SERVER_TCP_PORT 3000

#define BUFLEN 256

int main(int argc, char **argv){

int n, bytes_to_read;

int sd, port;

struct hostent *hp;

struct sockaddr_in server;

char *host, *bp, rbuf[BUFLEN], sbuf[BUFLEN];

switch(argc) {

case 2:

host = argv[1];port = SERVER_TCP_PORT;

break;

case 3:

host = argv[1];

port = atoi(argv[2]);

break;

default:

fprintf(stderr, "Usage: %s host [port]\n", argv[0]);

exit(1);

}

/* Create a stream socket */

if ((sd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {

fprintf(stderr, "Can't create a socket\n");

exit(1);

}

}

bcopy(hp->h_addr, (char *)&server.sin_addr, hp->h_length);

/* Connecting to the server */

if (connect(sd, (struct sockaddr *)&server,sizeof(server)) == -1) {

fprintf(stderr, "Can't connect\n");

exit(1);

}

printf("Connected: server's address is %s\n", hp->h_name);

printf("Transmit:\n");

gets(sbuf);

write(sd, sbuf, BUFLEN);

printf("Receive:\n");

bp = rbuf;

bytes_to_read = BUFLEN;

while ((n = read(sd, bp, bytes_to_read)) > 0) {

bp += n;

bytes_to_read -= n;

}

printf("%s\n", rbuf);

close(sd);return(0);

}

Socket Calls for Connection-LessMode

Server Server started socket creates socket of type UDP (datagram)

k t call returns: descriptor; or 1 if unsuccessful



socket()

sendto()

socket()

recvfrom()

Data

Client

bind()

Blocks until server

receives data from

client

recvfrom()

close()close()

Data

sendto()

socket call returns: descriptor ; or -1 if unsuccessful

bind assigns local address & port # to socket withspecified descriptor; Can wildcard IP address


Server

recvfrom copies bytes received in specified socketinto a specified location

recvfrom blocks until data arrives



socket()

sendto()

socket()

recvfrom()

Data

Client

bind()

Blocks until server

receives data from

client

recvfrom()

close()close()

Data

sendto()

recvfrom blocks until data arrives


Server Client started socket creates socket of type UDP (datagram)

socket call returns: descriptor; or 1 if unsuccessful



sendto()

socket()

sendto()

socket()

recvfrom()

Data

Client

bind()

Blocks until server

receives data from

client

recvfrom()

close()close()

Data

socket call returns: descriptor ; or -1 if unsuccessful


Server Client started sendto transfer bytes in buffer to specified socket

sendto specifies: socket descriptor; pointer to a



sendto()

socket()

sendto()

socket()

recvfrom()

Data

Client

bind()

Blocks until server

receives data from

client

recvfrom()

close()close()

Data

sendto specifies: socket descriptor; pointer to abuffer; amount of data; flags to control transmission

behavior; destination address & port #; length of destination address structure

sendto returns: # bytes sent; or -1 if unsuccessful


Server

recvfrom wakes when data arrives recvfrom specifies: socket descriptor; pointer to a

buffer to put data; max # bytes to put in buffer; control



socket()

sendto()

socket()

recvfrom()

Data

Client

bind()

Blocks until server

receives data from

client

recvfrom()

close()close()

Data

sendto()

buffer to put data; max # bytes to put in buffer; controlflags; copies: sender address & port #; length of

sender address structure recvfrom returns # bytes received or -1 (failure)

Note: receivefrom

returns data from atmost one send, i.e.

from one datagram


Server Socket Close Client or server call close when socket is no longer

needed



socket()

sendto()

socket()

recvfrom()

Data

Client

bind()

Blocks until server

receives data from

client

recvfrom()

close()close()

Data

sendto()

close specifies the socket descriptor

close call returns: 0 (success); or -1 (failure)

Example: UDP Echo Server

/* Echo server using UDP */

#include <stdio.h>




/* Bind an address to the socket */

bzero((char *)&server, sizeof(server));



server.sin_addr.s_addr = htonl(INADDR_ANY);

if (bind(sd, (struct sockaddr *)&server,



#define SERVER_UDP_PORT 5000

#define MAXLEN 4096

int main(int argc, char **argv){

int sd, client_len, port, n;

char buf[MAXLEN];

struct sockaddr_in server, client;

switch(argc) {

case 1:

port = SERVER_UDP_PORT;

break;

case 2:port = atoi(argv[1]);

break;

default:

fprintf(stderr, "Usage: %s [port]\n", argv[0]);

exit(1);

}

/* Create a datagram socket */

if ((sd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {

fprintf(stderr, "Can't create a socket\n");exit(1);

}

if (bind(sd, (struct sockaddr )&server,

sizeof(server)) == -1) {

fprintf(stderr, "Can't bind name to socket\n");

exit(1);

}

while (1) {

client_len = sizeof(client);

if ((n = recvfrom(sd, buf, MAXLEN, 0,

(struct sockaddr *)&client, &client_len)) < 0) {

fprintf(stderr, "Can't receive datagram\n");

exit(1);

}

if (sendto(sd, buf, n, 0,(struct sockaddr *)&client, client_len) != n) {

fprintf(stderr, "Can't send datagram\n");

exit(1);

}

}

close(sd);

return(0);

}

Example: UDP Echo Client#include <stdio.h>

#include <string.h>

#include <sys/time.h>

#include <netdb.h>




#define SERVER_UDP_PORT 5000

#define MAXLEN 4096

#define DEFLEN 64

l d l ( i l 1 i l 2)

else {

fprintf(stderr,

"Usage: %s [-s data_size] host [port]\n", pname);

exit(1);

}

if ((sd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {

fprintf(stderr, "Can't create a socket\n");

exit(1);

}



long delay(struct timeval t1, struct timeval t2)

{

long d;

d = (t2.tv_sec - t1.tv_sec) * 1000;

d += ((t2.tv_usec - t1.tv_usec + 500) / 1000);

return(d);

}

int main(int argc, char **argv)

{

int data_size = DEFLEN, port = SERVER_UDP_PORT;

int i, j, sd, server_len;

char *pname, *host, rbuf[MAXLEN], sbuf[MAXLEN];

struct hostent *hp;

struct sockaddr_in server;struct timeval start, end;

unsigned long address;

pname = argv[0];

argc--;

argv++;

if (argc > 0 && (strcmp(*argv, "-s") == 0)) {

if (--argc > 0 && (data_size = atoi(*++argv))) {

argc--;

argv++;}

else {

fprintf(stderr,

"Usage: %s [-s data_size] host [port]\n", pname);

exit(1);

}

}

if (argc > 0) {

host = *argv;

if (--argc > 0)port = atoi(*++argv);

}

}

bzero((char *)&server, sizeof(server));



if ((hp = gethostbyname(host)) == NULL) {

fprintf(stderr, "Can't get server's IP address\n");

exit(1);

}

bcopy(hp->h_addr, (char *) &server.sin_addr, hp->h_length);

if (data_size > MAXLEN) {

fprintf(stderr, "Data is too big\n");

exit(1);

}for (i = 0; i < data_size; i++) {

j = (i < 26) ? i : i % 26;

sbuf[i] = 'a' + j;

}

gettimeofday(&start, NULL); /* start delay measurement */

server_len = sizeof(server);

if (sendto(sd, sbuf, data_size, 0, (struct sockaddr *)

&server, server_len) == -1) {

fprintf(stderr, "sendto error\n");

exit(1);}

if (recvfrom(sd, rbuf, MAXLEN, 0, (struct sockaddr *)

&server, &server_len) < 0) {

fprintf(stderr, "recvfrom error\n");

exit(1);

}

gettimeofday(&end, NULL); /* end delay measurement */

if (strncmp(sbuf, rbuf, data_size) != 0)

printf("Data is corrupted\n");

close(sd);return(0);

}




Application Layer Protocols &IP Utilities

Telnet (RFC 854)

• Provides general bi-directional byte-orientedTCP-based communications facility (Network



Virtual Terminal)

• Initiating machine treated as local to the remote

host

• Used to connect to port # of other servers and tointeract with them using command line

NVTNVT

Server

process

Network Virtual Terminal

• Network Virtual Terminal

• Lowest common denominator terminal



• Lowest common denominator terminal

• Each machine maps characteristics to NVT

• Negotiate options for changes to the NVT

• Data input sent to server & echoed back• Server control functions : interrupt, abort output,

are-you-there, erase character, erase line

• Default requires login & password

telnet

• A program that uses the Telnet protocol

• Establishes TCP socket



• Establishes TCP socket

• Sends typed characters to server

• Prints whatever characters arrive

• Try it to retrieve a web page (HTTP) or to

send an email (SMTP)

File Transfer Protocol (RFC 959)

• Provides for transfer of file from one machine toanother machine



• Designed to hide variations in file storage• FTP parameter commands specify file info

– File Type: ASCII , EBCDIC, image, local.

– Data Structure: file, record, or page – Transmission Mode: stream, block, compressed

• Other FTP commands

– Access Control: USER, PASS, CWD, QUIT, … – Service: RETR, STOR, PWD, LIST, …

FTP File Transfer

User

interface



User PI

User DTP

PI = Protocol interface

DTP = Data transfer process

User FTP

Server PI

Server DTP

Server FTP

Control

connection

Data

connection

Two TCP Connections

Control connection – Set up using Telnet

protocol on well-known port

Data connection – To perform file transfer,

obtain lists of files



protocol on well known port

21 – FTP commands & replies

between protocol

interpreters

– PIs control the data transfer

process

– User requests close of

control connection; server performs the close

obtain lists of files,

directories – Each transfer requires

new data connection

– Passive open by user PIwith ephemeral port #

– Port # sent over controlconnection

– Active open by server using port 20

FTP RepliesReply Meaning

1yz Positive preliminary reply (action has begun, but wait for another reply before

sending a new command).

2yz Positive completion reply (action completed successfully; new command may be



sent).

3yz Positive intermediary reply (command accepted, but action cannot be performed

without additional information; user should send a command with the

necessary information).

4yz Transient negative completion reply (action currently cannot be performed;

resend command later).5zy Permanent negative completion reply (action cannot be performed; do not

resend it).x0z Syntax errors.

x1z Information (replies to requests for status or help).

x2z Connections (replies referring to the control and data connections).

x3z Authentication and accounting (replies for the login process and accounting

procedures).

x4z Unspecified.

x5z File system status.

FTP Client (192.168.1.132: 1421) establishesControl Connection to FTP Server

(128.100.132.23: 21)



.

in control connection (frame 47 request); Filetransfer on new data connection (port 1423, fr.

48, 49, 51)



Hypertext Transfer Protocol

• RFC 1945 (HTTP 1.0), RFC 2616 (HTTP 1.1)

• HTTP provides communications between web



p

browsers & web servers• Web: framework for accessing documents &

resources through the Internet

• Hypertext documents: text, graphics, images,

hyperlinks

• Documents prepared using Hypertext MarkupLanguage (HTML)

HTTP Protocol

• HTTP servers use well-known port 80

• Client request / Server reply



Client request / Server reply

• Stateless: server does not keep any

information about client

• HTTP 1.0 new TCP connection per request/reply (non-persistent)

• HTTP 1.1 persistent operation is default

HTTP Typical Exchange



HTTP Message Formats

• HTTP messages written in ASCII text

• Request Message Format

R t Li (E h li d ith i t )



– Request Line (Each line ends with carriage return)

• Method URL HTTP-Version \r\n

• Method specifies action to apply to object

• URL specifies object

– Header Lines (Ea. line ends with carriage return)

• Attribute Name: Attribute Value

• E.g. type of client, content, identity of requester, …

• Last header line has extra carriage return)

– Entity Body (Content)

• Additional information to server

HTTP Request Methods

Requestmethod

Meaning

GET Retrieve information (object) identified by the URL.



HEAD Retrieve meta-information about the object, but do nottransfer the object; Can be used to find out if a document

has changed.

POST Send information to a URL (using the entity body) and retrieve

result; used when a user fills out a form in a browser.

PUT Store information in location named by URL

DELETE Remove object identified by URL

TRACE Trace HTTP forwarding through proxies, tunnels, etc.

OPTIONS Used to determine the capabilities of the server, or

characteristics of a named resource.

Universal Resource Locator

• Absolute URL

– scheme://hostname[:port]/path



– http://www.nytimes.com/

• Relative URL

– /path

– /

HTTP Request Message





HTTP Response Message

• Response Message Format

– Status Line



• HTTP-Version Status-Code Message• Status Code: 3-digit code indicating result

• E.g. HTTP/1.0 200 OK

– Headers Section• Information about object transferred to client

• E.g. server type, content length, content type, …

– Content • Object (document)

HTTP Response Message



HTTP Proxy Server & Caching

• Web users generate large traffic volumes• Traffic causes congestion & delay



• Can improve delay performance and reducetraffic in Internet by moving content to servers

closer to the user

• Web proxy servers cache web information – Deployed by ISPs

– Customer browsers configured to first access ISPs

proxy servers

– Proxy replies immediately when it has requested

object or retrieves the object if it does not

Cookies and Web Sessions

• Cookies are data exchanged by clients &servers as header lines

• Since HTTP stateless, cookies can provide



context for HTTP interaction• Set cookie header line in reply message from

server + unique ID number for client

• If client accepts cookie, cookie added to client’scookie file (must include expiration date)

• Henceforth client requests include ID• Server site can track client interactions, storethese in a separate database, and access

database to prepare appropriate responses

Cookie Header Line;ID is 24 hexadecimal numeral



PING

• Application to determine if host is reachable

• Based on Internet Control Message Protocol



– ICMP informs source host about errors encountered inIP packet processing by routers or by destination host

– ICMP Echo message requests reply from destination

host• PING sends echo message & sequence #

• Determines reachability & round-trip delay

• Sometimes disabled for security reasons

PING from NAL host

Microsoft(R) Windows DOS(c)Copyright Microsoft Corp 1990-2001.

C:\DOCUME~1\1>ping nal.toronto.edu



Pinging nal.toronto.edu [128.100.244.3] with 32 bytes of data:

Reply from 128.100.244.3: bytes=32 time=84ms TTL=240Reply from 128.100.244.3: bytes=32 time=110ms TTL=240Reply from 128.100.244.3: bytes=32 time=81ms TTL=240Reply from 128.100.244.3: bytes=32 time=79ms TTL=240

Ping statistics for 128.100.244.3:Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds: Minimum = 79ms, Maximum = 110ms, Average = 88ms

C:\DOCUME~

1\1>

Traceroute

• Find route from local host to a remote host

• Time-to-Live (TTL)– IP packets have TTL field that specifies maximum #



IP packets have TTL field that specifies maximum #hops traversed before packet discarded

– Each router decrements TTL by 1

– When TTL reaches 0 packet is discarded

• Traceroute – Send UDP to remote host with TTL=1

– First router will reply ICMP Time Exceeded Msg – Send UDP to remote host with TTL=2, …

– Each step reveals next router in path to remote host

Traceroute from home PC touniversity host

Tracing route to www.comm.utoronto.ca [128.100.11.60]

over a maximum of 30 hops:

1 1 ms <10 ms <10 ms 192.168.2.1

2 3 ms 3 ms 3 ms 10.202.128.1

3 4 ms 3 ms 3 ms gw04.ym.phub.net.cable.rogers.com [66.185.83.142]

4 * * * Request timed out.

Home Network



4 Request timed out.

5 47 ms 59 ms 66 ms gw01.bloor.phub.net.cable.rogers.com [66.185.80.230]

6 3 ms 3 ms 38 ms gw02.bloor.phub.net.cable.rogers.com [66.185.80.242]

7 8 ms 3 ms 5 ms gw01.wlfdle.phub.net.cable.rogers.com [66.185.80.2]

8 8 ms 7 ms 7 ms gw02.wlfdle.phub.net.cable.rogers.com [66.185.80.142]

9 4 ms 10 ms 4 ms gw01.front.phub.net.cable.rogers.com [66.185.81.18]

10 6 ms 4 ms 5 ms ra1sh-ge3-4.mt.bigpipeinc.com [66.244.223.237]11 16 ms 17 ms 13 ms rx0sh-hydro-one-telecom.mt.bigpipeinc.com [66.244.223.246]

12 7 ms 14 ms 8 ms 142.46.4.2

13 10 ms 7 ms 6 ms utorgw.onet.on.ca [206.248.221.6]

14 7 ms 6 ms 11 ms mcl-gateway.gw.utoronto.ca [128.100.96.101]

15 7 ms 5 ms 8 ms sf-gpb.gw.utoronto.ca [128.100.96.17]

16 7 ms 7 ms 10 ms bi15000.ece.utoronto.ca [128.100.96.236]

17 7 ms 9 ms 9 ms www.comm.utoronto.ca [128.100.11.60]

Trace complete.

Rogers Cable

ISP

Ontario Net

University of

Toronto

Hydro OneShaw Net

ipconfig

• Utility in Microsoft® Windows to display

TCP/IP information about a host



• Many options – Simplest: IP address, subnet mask, default

gateway for the host – Information about each IP interface of a host

• DNS hostname, IP addresses of DNS servers,

physical address of network card, IP address, …

– Renew IP address from DHCP server

netstat

• Queries a host about TCP/IP network

status



• Status of network drivers & their interfacecards

– #packets in, #packets out, errored packets, …• State of routing table in host

• TCP/IP active server processes• TCP active connections

netstat protocol statistics

IPv4 Statistics

Packets Received = 71271

Received Header Errors = 0

Received Address Errors = 9

Datagrams Forwarded = 0

Unknown Protocols Received = 0

ICMPv4 Statistics

Received Sent

Messages 10 6

Errors 0 0

Destination Unreachable 8 1

Time Exceeded 0 0



Received Packets Discarded = 0

Received Packets Delivered = 71271

Output Requests = 70138

Routing Discards = 0

Discarded Output Packets = 0

Output Packet No Route = 0

Reassembly Required = 0Reassembly Successful = 0

Reassembly Failures = 0

Datagrams Successfully Fragmented = 0

Datagrams Failing Fragmentation = 0

Fragments Created = 0

UDP Statistics for IPv4

Datagrams Received = 6810

No Ports = 15

Receive Errors = 0

Datagrams Sent = 6309

Parameter Problems 0 0

Source Quenches 0 0

Redirects 0 0

Echos 0 2

Echo Replies 2 0

Timestamps 0 0

Timestamp Replies 0 0Address Masks 0 0

Address Mask Replies 0 0

TCP Statistics for IPv4

Active Opens = 798

Passive Opens = 17Failed Connection Attempts = 13

Reset Connections = 467

Current Connections = 0

Segments Received = 64443

Segments Sent = 63724

Segments Retransmitted = 80

tcpdump and Network ProtocolAnalyzers

• tcpdump program captures IP packets on anetwork interface (usually Ethernet NIC)

• Filtering used to select packets of interest



• Packets & higher-layer messages can bedisplayed and analyzed

• tcpdump basis for many network protocolanalyzers for troubleshooting networks

• We use the open source Ethereal analyzer to

generate examples – www.ethereal.com

Standards

• New technologies very costly and risky

• Standards allow players to share risk andbenefits of a new market

http://www.ethereal.com/

http://www.ethereal.com/



– Reduced cost of entry

– Interoperability and network effect

– Compete on innovation

– Completing the value chain• Chips, systems, equipment vendors, service providers

• Example – 802.11 wireless LAN products

Standards Bodies

• Internet Engineering Task Force – Internet standards development

– Request for Comments (RFCs): www.ietf.org

http://www.ietf.org/

http://www.ietf.org/



• International Telecommunications Union – International telecom standards

• IEEE 802 Committee – Local area and metropolitan area network standards

• Industry Organizations

– MPLS Forum, WiFi Alliance, World Wide WebConsortium

A humorous illustration of aprotocol stack as depicted by

various

d d i i



standards organizations.