Computer Network Basics

Components of Any Computer

Processor (active)

Computer

Control(“brain”)

Datapath(“brawn”)

Memory(passive)

(where programs, data live whenrunning)

Devices

Input

Output

Keyboard, Mouse

Display, Printer

Disk,Network

Communication Devices

Synchronous communication uses a clock signal separate from the data signal- communication can only happen during the ‘tick’ of the timing cycle

Asynchronous communication does not use a clock signal- rather, it employs a start and stop bit to begin and end the irregular transmission of data

Connecting to Networks (and Other I/O)

Bus - shared medium of communication that can connect to many devices

Hierarchy of Buses in a PC

Operating Systems

Developer or manufacturer Operating system

Apple Computers Inc. Mac OS 8/9/X

AT&T Bell Laboratories Unix

Be Inc. beOS

Berkeley University BSD, FreeBSD

Carnegie-Mellon University Mach 3.0

Cisco Systems Inc. IOS

HP HP-UX

IBM AIX and OS/2

Linus Thorvald Linux

Microsoft Windows XP, Vista

Novell NetWare

Santa Cruz Operation Inc. (SCO) SCO XENIX, SCO UNIX, SCO MPX

Siemens SINIX

Silicon Graphics IRIX

Sun Microsystems Solaris, SunOS, JavaOS

Operating Systems Developed for Portable Devices

Developer or manufacturer Operating system

Microsoft Windows CE

Microsoft Windows Mobile 6.0

Palm PalmOS

Symbian Symbian OS

RIM (Research In Motion Limited) RIM

A Closer Look at Network Structure

network edge: applications and hosts

network core: routersnetwork of

networks

General Architecture of Computer Networks

Cloud

External nodes

Internal nodes

(or stations)

(swithing devices)

The Network Core

mesh of interconnected routers

the fundamental question: how is data transferred through net? circuit switching:

dedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”

Connection of Networks

networks or subnetworks

router orgateway

node(host,

station)

Network Topology

a) bus, b) star, c) ring, d) tree structure

a) b) c) d)

Classification of the networks according to the connection

establishing

Line switched network Packet switched network Radiating/data disseminating systems Point-to-point connected networks

Wired Media

Telephone line Thin Coax Thick Coax Unshielded Twisted Pair (UTP) Shielded Twisted Pair (STP) Fibre

(Data) Reliability

A network service is (data) reliable if the sender application can rely on the error-free and ordered delivery of the data to the destination

In the Internet the reliability can obtained mainly by acknowledgements and retransmission

In such a way the losses in the underlying layers can be retrieved

Flow-control and Congestion Prevention

Flow-control: to protect the receiver against the overload I.e.: the sender (source) sends more

data than the receiver can process it is mainly necessary in link and

transport level Congestion prevention: to

prevent the intermediate nodes against the overload it is mainly necessary in network

level

Overload and Congestion

Overload: Too many packets occur in a subnetwork in the same time, which prevent each other and in such a way the throughput decreases

Congestion: the queues in the routers are too long, the buffers are full. As a consequence some packages are

dropped if the buffers of the routers are overloaded

In extreme case: grid-lock, lock-up

Deadlock

Deadlock: the most serious situation of the congestion, the routers wait for each other

Direct store and forward deadlock: the buffers of two neighbouring routers are full with the packets to be sent to the other router

Indirect store and forward deadlock: the deadlock occurred not between two neighbouring routers but in a subnetwork, where any of the routers has not free buffer space for accepting packets

Network: physical connection that allows two computers to communicate

Packet: unit of transfer, bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives

Protocol: agreement between two parties as to how information is to be transmitted

Broadcast Network: Shared Communication Medium Delivery: How does a receiver know who packet is for?

Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target

Arbitration: Act of negotiating use of shared medium Point-to-point network: a network in which every

physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus

(broadcast) configuration into a point-to-point network Router: a device that acts as a junction between two

networks to transfer data packets among them

Networking Definitions

The Need for a Protocol Architecture

Procedures to exchange data between devices can be complex

High degree of cooperation required between communicating systems destination addressing, path readiness to receive file formats, structure of data how commands are sent/received and

acknowledged etc.

Layered Protocol Architecture

Modules arranged in a vertical stack Each layer in stack:

Performs related functions Relies on lower layer for more primitive

functions Provides services to next higher layer Communicates with corresponding peer

layer of neighboring system using a protocol

Network Layering Layering: building complex services from simpler ones

Each layer provides services needed by higher layers by utilizing services provided by lower layers

The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU:

often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a

switch Our goal in the following is to show how to construct a secure,

ordered, message service routed to anywhere:

Physical Reality: Packets Abstraction: Messages

Limited Size Arbitrary Size

Unordered (sometimes) Ordered

Unreliable Reliable

Machine-to-machine Process-to-process

Only on local area net Routed anywhere

Asynchronous Synchronous

Insecure Secure

Key Features of a Protocol

Set of rules or conventions to exchange blocks of formatted data

Syntax: data format Semantics: control information

(coordination, error handling) Timing: speed matching, sequencing Actions: what happens when an event

occurs

Operation of Protocols

(interlayer) protocol layerprotocol

Host Host

Physical connection

(n-1). layerprotocol entity

(n-1). layerprotocol entity

(n+1). layerprotocol entity

n. layerprotocol entity

(n+1). layerprotocol entity

n. layerprotocol entity

... ...

The OSI Model

Physical Layer (Data) Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer

Physical Layer

Transmission of energy onto the medium Collection of energy from the medium This layer is concerned with the

physical transmission of raw bits This bits are transmitted through

mechanical, electrical, and procedural interfaces which include• interface card standard• modem standards• certain portions of the ISDN and LAN MAN

standards

(Data) Link Layer

Transmission of frames over one link or network Often subdivided into the MAC and LLC It receives bits from the physical layer, converting

bits to frames frame boundaries

Using protocols (e.g. HDLC), this layer corrects errors that might have occurred during transmission across a link

In addition this layer provides an “error-free” transmission channel to the next layer known as the network layer: error control ARQ duplicates

Flow control

The previous two layers were concerned with getting error-free data across a link

The network layer establishes connections between nodes, routes data packets through the network, and accounts for them

End-to-end transmission of packets (possibly over multiple links)

Controls the operation of the subnet Routing

static dynamic

Congestion control At this stage, there may be congestion due to many packets

waiting to be routed Some packets may be lost during congestion

Network Layer I

Network Layer II Accounting

packets bytes etc.

Internetworking This layer is also concerned with internetworking

where there is ‘talking’ between technologies, such as the traditional Internet connected to ATM

segmentation addressing sequencing accounting

Broadcast subnets: thin network layer

Transport Layer I

This layer presumes the ability to pass through a network and provides additional services to end-users, such as and-to-and packet reliability

End-to-end delivery of a complete message (end-to-end communication path, usually reliable)

Isolation from “hardware” Multiplexing/demultiplexing Divide message into packets Reassemble (possibly out of order packets)

into the original message of the distant end

Transport Layer II

End-to-end flow control Acknowledgments Types of service

error-free, point-to-point, in sequence, flow controlled

no correctness guarantees no sequencing

Establishing/terminating connections naming/addressing intra-host addressing (process, ports)

This layer enables users to establish sessions across a network between machines

In addition, it offers session management services Set up and management of end-to-end conversation Establish and terminate sessions

superset of connections Assignment of logical ports Dialogue control Token management

for critical operations Synchronization

checkpoints/restarts

Session Layer

Presentation Layer

This layer is concerned with the syntax and semantics of messages, code conversions between machines, and other data conversion services

Some of these services are data compression and data encryption

Interface between lower layers and application Formatting Syntax & semantics of messages Data encoding (e.g.: ASCII to EBCDIC) Compression Encryption/Decryption Authentication

Application Layer

This layer provides support for the user's network applications

Some application layer services have been standardized, e.g.: File Transfer and Management (FTAM) Message Handling Services for electronic mail (X.400) Directory Services (X.500) Electronic Data Interchange (EDI)

Program you’re running,applications file transfer, access & management e-mail virtual terminals WWW

The OSI Protocol Stack

Operation of the model

Virtualtransmission

Real datatransmission

Applicationlayer entity

Session layerentity

Transportlayer entity

Networklayer entity

Datalinklayer entity

Presentationlayer entity

Intermediate

Networklayer entity

Datalinklayer entity

Intermediate

Networklayer entity

Datalinklayer entity

Endsystem

Applicationlayer entity

Session layerentity

Transportlayer entity

Networklayer entity

Presentationlayer entity

Datalinklayer entity

Physical medium

Endsystem

Physical layerentity

Physical layerentity

Physical layerentity

Physical layerentity

Names of the Nodes, Connections and Data Units

Layer name Node Connection Data unit

Application layer application network service e.g. file (ADU)

Presentation layer host session data structure (PPDU)

Session layer host transport connection message (SPDU)

Transport layer host network path message (TPDU)

Network layer host, router line (data)packet (NPDU)

(Data)link layer station (physical) channel (data)frame (LLC PDU)

Physical layer switch physical transmissionmedium

bit

Communication among the layers

Connection oriented network service (virtual circuits, eg. ATM) • Reliable transport service• Unreliable transport service

Connectionless network service (datagram service, eg. IP)• Reliable transport service (eg. TCP)• Unreliable transport service (eg. UDP)

Network Tools

Repeater: connects network segments logically to one network

Hub: multiport repeater Bridge: datalink level connection of two

networks Switch: multiport bridge Router: connects networks that are

compatible in transport level subnetworks are connected to the interfaces

of the repeater Gateway (proxy server): router

between two individual network. The “Way Out”

Physical Layer Devices

Repeater

Hub “dumb” level-1 hub multi-port repeater

Data Link Layer Devices

Bridge Cascaded vs. Backbone Single Multiple

Switch (switched hub)

Routers

Provide link between networks Accommodate network differences:

Addressing schemes Maximum packet sizes Hardware and software interfaces Network reliability

Congestion/Traffic Management

Devices of the Network Connection

Application layer

Presentation layer

Session layer

Transport layer

Datalink layer

Network layer

Physical layer

Gateway

or

Proxy server

Router or Gateway

Bridge or Switch

Repeater or Hub

Application layer

Presentation layer

Session layer

Transport layer

Datalink layer

Network layer

Physical layer

Architectural Implementation of the LANs

Ethernet (IEEE 802.3)FDDIGigabit EthernetToken Bus (IEEE 802.4)Token Ring (IEEE 802.5)

Characteristics of High-Speed LANs

Fast Ethernet Gigabit Ethernet Fibre Channel Wireless LAN

Data Rate 100 Mbps 1 Gbps, 10 Gbps100 Mbps – 3.2

Gbps1 Mbps – 2 Gbps

Transmission Mode

UTP,STP, Optical Fiber

UTP, shielded cable, optical

fiber

Optical fiber, coaxial cable, STP

2.4 GHz, 5 GHz Microwave

Access Method CSMA/CD CSMA/CD Switched CSMA/CA Polling

Supporting Standard

IEEE 802.3 IEEE 802.3Fibre Channel Association

IEEE 802.11

Wide Area Network Connections

Solutions for connecting LANs to the Internet Ethernet (ring or star topology) Managed Leased Line Network (MLLN) ATM (Asynchronous Transfer Mode) Switched line ISDN line

Soft and Hard States

State: the data collection, which are necessary for keeping the connection between two protocol entities

Hard state If the connection is established once, it is never timed out,

even if it is not in usage To cancel the connection one of the participants of the

connection must explicitly close it The history of the state is stored

Soft state To keep the connection the participants must send

occasionally keep-alive messages, since without keep-alive message the state information is timed out after a certain period

The state is called as “soft” since in the ordinary operation the state can change easily

The history of the state is not stored

Packet switching versus circuit switching

Great for bursty data resource sharing no call setup (less start-up delay)

However… Packets can experience delays, so not for “real-time”

applications excessive congestion leads to packet delay and loss

• protocols (like TCP) are needed for reliable data transfer, and congestion control

Is packet switching best in every case?

Performance Considerations Before continue, need some performance metrics

Overhead: CPU time to put packet on wire Throughput: Maximum number of bytes per second

• Depends on “wire speed”, but also limited by slowest router (routing delay) or by congestion at routers

Latency: time until first bit of packet arrives at receiver• Raw transfer time + overhead at each routing hop

Contributions to Latency Wire latency: depends on speed of light on wire

• about 1–1.5 ns/foot Router latency: depends on internals of router

• Could be < 1 ms (for a good router)

Router Router

LR1 LR2LW1 LW2 Lw3

Delay in packet-switched networks

packets experience delay on end-to-end path

four sources of delay at each hop

Nodal processing: check bit errors determine output link

Queueing: time waiting at output

link for transmission depends on

congestion level of router

A

B

propagation

transmission

nodalprocessing queueing

Delay in packet-switched networks

Transmission delay: R=link bandwidth

(bps) L=packet length

(bits) time to send bits into

link = L/R

Propagation delay: d = length of physical

link s = propagation speed

in medium (~2x108 m/sec)

propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

Queueing delay

R=link bandwidth (bps)

L=packet length (bits)

a=average packet arrival ratetraffic intensity = La/R

La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can

be serviced, average delay infinite!

Internet protocol stack

Application: supporting network applications ftp, smtp, http

Transport: host-host data transfer tcp, udp

Network: routing of datagrams from source to destination ip, routing protocols

Network access: data transfer between neighboring network elements ppp, ethernet

Physical: bits “on the wire”

Layering: logical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

E.g.: transport take data from

app add addressing,

reliability check info to form “datagram”

send datagram to peer

wait for peer to ack receipt

analogy: post office

data

transport

transport

ack

Layering: physical communication

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

networklink

physical

data

data

Protocol layering and data

Each layer takes data from above adds header information to create new data

unit passes new data unit to layer below

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnHl

M

M

M

M

Ht

HtHn

HtHnHl

message

segment

datagram

frame

IP over ATM

ATM Adaptation Layer (AAL): interface to upper layers end-system segmentation/

reassembly ATM Layer: cell

switching Physical

AAL5ATM

physical

AAL5ATM

physical

AAL5ATM

physical

AAL5ATM

physical

ATMphysical

applicationTCP/UDP

IP

applicationTCP/UDP

IP

applicationTCP/UDP

IP

applicationTCP/UDP

IP

Physical

Data Link

Network

Transport

Session

Presentation

Application

Network Access

IP

TCP UDP

Application

Sockets

The Internet Protocol Stack

Network Protocols Protocol: Agreement between two parties as to how

information is to be transmitted Example: system calls are the protocol between the operating

system and application Networking examples: many levels

• Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented)

• Link level: packet formats/error control (for instance, the CSMA/CD protocol)

• Network level: network routing, addressing• Transport Level: reliable message delivery

Protocols on today’s Internet:

Ethernet ATM Packet radio

IP

UDP TCP

RPCNFS WWW e-mail ssh

Physical/Link

Network

Transport

Building a messaging serviceProcess to process communication

Basic routing gets packets from machinemachineWhat we really want is routing from processprocess

• Example: ssh, email, ftp, web browsingSeveral IP protocols include notion of a “port”,

which is a 16-bit identifiers used in addition to IP addresses

• A communication channel (connection) defined by 5 items:

[source address, source port, dest address, dest port, protocol]

UDP: The User Datagram Protocol UDP layered on top of basic IP (IP Protocol 17)

• Unreliable, unordered, user-to-user communication

UDP Data

16-bit UDP length 16-bit UDP checksum16-bit source port 16-bit destination port

IP Header(20 bytes)

Building a messaging service (con’t)UDP: The Unreliable Datagram Protocol

Datagram: an unreliable, unordered, packet sent from source user dest user (Call it UDP/IP)

Important aspect: low overhead!• Often used for high-bandwidth video streams• Many uses of UDP considered “anti-social” – none of the

“well-behaved” aspects of (say) TCP/IPBut we need ordered messages

Create ordered messages on top of unordered ones• IP can reorder packets! P0,P1 might arrive as P1,P0

How to fix this? Assign sequence numbers to packets

• 0,1,2,3,4…..• If packets arrive out of order, reorder before delivering to

user application• For instance, hold onto #3 until #2 arrives, etc.

Sequence numbers are specific to particular connection

Message

TCP/IP packet, Ethernet frame Application sends

message

TCP data

TCP Header

IP Header

IP DataEH

Ethernet Hdr

Ethernet Hdr

TCP breaks into 64KB segments, adds 20B header

IP adds 20B header, sends to network

If Ethernet, broken into 1500B frames with headers, trailers (24B)

All Headers, trailers have length field, destination, ...