Basic Infrastructure for Electronic Business on the Internet Authors: Miroslava Mitrovic...

Basic Infrastructure Basic Infrastructure for Electronic Business for Electronic Business

on the Interneton the Internet

Authors:Miroslava Mitrovic ([email protected])Veljko Milutinovic ([email protected])

A network is a group of computers and peripheralsconnected together by some medium

The connection can be direct (through a cable) or indirect (through a modem)

The different devices on the networkcommunicate with each otherthrough a predefined set of rules (protocol)

NetworkNetwork

Workstation

Workstation

Laser printer

Workstation IBM Compatible

Network HardwareNetwork Hardware

Physically connects computers in the network together

Main hardware components:NodesCommunication equipment Additional componentsTopology

NodesNodes

Network node (computer, printers, routers) Computers in network

can be used as work stations or servers or both

PC computers need NIC (Network Interface Card) to be able to be part of LAN (Local Area Network)

Different standards, different network cards! (Ethernet, Token Ring, ARC net)

Communication equipmentCommunication equipment

Cables : (coaxial cables, fiber-optic cables, twisted pairs)

Wiring centers : (HUB , concentrator, MAU (Multi station Access Units))

Intranetwork links : (connectors, transceivers, repeaters)

Internetwork links : (bridges, gateways, routers, switches)

Wireless components

Additional ComponentsAdditional Components

Peripherals :(printers, fax machines, modems, cd readers)

Safety devices : UPS (Uninterruptible Power Supply), SPS (Standby Power Supply)

Network tools

TopologyTopology

Defines rules for connecting network nodes Logical topology differs from physical topology! Logical topology defines rules for

transmitting information's (bus, ring, star) Physical topology defines rules for

electrical connection components in network

TopologyTopology

Workstation

IBM Compatible

Workstation

Workstation

Star

Workstation

TopologyTopology

TokenRing

Workstation

Workstation

Workstation

IBM Compatible

Ring

Network SoftwareNetwork Software

Enables using hardware for communication and exchanging information

Protocols (set of rules) give formulas for passing messages, specifying the details of message formats, and for describing how to handle error conditions

InternettingInternetting

Connecting many disparate physical networks and make them function as a coordinate unit

Internet technology hides the details of network hardware and permits computers to communicate independent of their physical network connection

InternettingInternetting

Workstation

IBM Compatible

Mac II

IBM Compatible

Internet

Laptop computer

InternetInternet

Internet is network of all networks Interconnected computer networks

that communicate, usually via TCP/IP

Internet and internet Set of application programs

that use the network to carry out useful communication tasks (user’s point of view)

No one owns the Internet!

Internet architectureInternet architecture

How are networks interconnected to form a internet work?

Computers that interconnect two networks and pass packets from one to the other are called internet gateways or internet routers


G Net 2Net1

Two networks interconnectedby G, a gateway (router)


G1 Net 2Net1

Three networksinterconnected by two

gateways

Net 3G2


Gateways provide all interconnections among physical networks

Usually minicomputers Gateways route packets based on destination

network, not on destination host!


Internet

hosts

Users point of view


hosts

Structure of physical networkand gateways that provide interconnection

Open SystemsOpen Systems

Architecture is not secret The definition applies equally well

to software and hardware UNIX-classic example for

an open software platform Open system networking:

network based on well-known and understood protocol

Three most popular services:File Transfer

Electronic mailRemote login

The ISO OSI-RMThe ISO OSI-RMInternational Organization for StandardizationInternational Organization for StandardizationOpen System Interconnection Reference ModelOpen System Interconnection Reference Model

ISO has developed an architecture that defines seven layers of network protocol

Physical

Data Link

Network

Presentation

Transport

Session

Application

Grouping of OSI LayersGrouping of OSI Layers

Physical

Data Link

Network

Transport

Session

Presentation

Application

Upper Layers

Lower Layers

Middle Layer

Upper Layers Upper Layers Application/Presentation/SessionApplication/Presentation/Session

Applications oriented Presents the interface to the user Don’t care how data gets to the application!

Middle LayerMiddle LayerTransportTransport

Separate application oriented upper layers from network oriented lower layers

Lower LayersLower LayersNetwork/Data Link/PhysicalNetwork/Data Link/Physical

Oriented toward data transmission Packages, routes, verifies and transmits datagrams Type of data for application doesn't matter Has no application dependencies !

Communication in OSI-RMCommunication in OSI-RM

Physical Physical

Data Link

Network

Transport

Session

Presentation

Application

Internet

Transport

Application

Presentation

Session

Data Link

Application LayerApplication Layer

Highest layer in the OSI reference model Concerned with the user’s view of the network Handles network transparency, resource allocation

and problem partitioning Sends data and program tasks

to the presentation layer,which is responsible for coding data from application layer in the appropriate form

Presentation LayerPresentation Layer

The second highest layer in the OSI reference model Converts network data to application format Performs function as

text compression , code or format conversion to try to smooth out differences between hosts

Serves application layer, uses session layer

Session LayerSession Layer

The third highest layer in the OSI reference model Synchronized data exchange

between upper and lower layers Manages the current connection, or session,

between two computers Handles security and creation of the session Establishing connection Closes connection Uses the transport layer to establish a connection

between processes on different hosts Used by the presentation layer

Transport LayerTransport Layer

The middle layer in the OSI reference model Uses the network layer to provide a

virtual, error-free, point-to-point connection so that host A can send messages to host B and they will arrive uncorrupted and in the correct order

Hides details necessary to actually make a connection between two computers

Establishes and dissolves connections between hosts

Used by the session layer

Network LayerNetwork Layer

The third lowest layer in the OSI reference model Determines addresses Responsible for routing packets across the network Establishing logical link between two nodes Used by the transport layer, uses the data link layer

Data Link LayerData Link Layer

The second lowest layer in the OSI reference model Splits data into frames for sending

on the physical layer Receives acknowledgment frames Performs error checking and retransmission Provides an error free virtual channel

to the network layer Split into

an upper sublayer Logical Link Control (LLC)and lower sublayer Media Access Control (MAC)

Physical LayerPhysical Layer

The lowest layer in OSI model Transmits data How bits are passed over the network medium,

what control signals are used, and the mechanical properties of the network itself (cable size, connector, and so on)

Only layer in which actual communication occur Used by the data link layer

Data Transfer over the InternetData Transfer over the Internet

Physical Physical

Data Link

Network

Transport

Session

Presentation

Application

Internet

Transport

Application

Presentation

Session

Data Link

Network

Data Link

Physical

Network

Data Link

Physical

Subnetwork Subnetwork

GatewayGateway

Computer A Computer B

OSI vs. TCP/IPOSI vs. TCP/IP

PhysicalPhysical

Data Link

Network

Transport

Session

Presentation

Application

Internet

Transport

Application

The TCP/IP protocol familyThe TCP/IP protocol family

TCP UDP IP ICMP RIP OSPF ARP DNS RARP BOOTP

FTP Telnet EGP GGP IGP BGP NFS NIS RPC SMTP SNMP

TransportTransport

Transmission Control Protocol (TCP): connection-based services

User Datagram Protocol (UDP): connectionless services

RoutingRouting

Internet Protocol (IP): handles transmission of information

Internet Control Message Protocol (ICMP): handles status messages for IP

Routing Information Protocol (RIP): determines routing

Open Shortest Path First (OSPF):alternate protocol for determining routing

Network AddressesNetwork Addresses

Address Resolution Protocol (ARP): determines addresses

Domain Name System (DNS): determines addresses from machine names

Reverse Address Resolution Protocol (RARP): - determines addresses

User ServicesUser Services

Boot Protocol (BOOTP): starts up a network machine

File Transfer Protocol (FTP): transfers files

Telnet: allows remote logins

Gateway ProtocolsGateway Protocols

Exterior Gateway Protocol (EGP): transfers routing information for external networks

Gateway-to-Gateway Protocol (GGP): transfers routing information between gateways

Interior Gateway Protocol (IGP): transfers routing information for internal networks

Border Gateway Protocol (BGP)newer than EGP, used within an autonomous network

OthersOthers

Network File System (NFS): enables directories on one machine to be mounted on another

Network Information Service (NIS): maintains user accounts across networks

Remote Procedure Call (RPC): enables remote applications to communicate

Simple Mail Transfer Protocol (SMTP): transfers electronic mail

Simple Network Management Protocol (SNMP): sends status messages about the network

DatagramDatagram

Basic unit of data send thought an internet Moving packets between routers and/or hosts Consists of a header followed by the data Header:

information necessary to move the packetacross the Internet

Connectionless transport mechanism

DatagramDatagram

DatagramDatagram

Field Function

ver The version of the IP protocol that created the datagram.

IHL The length of the IP header in 32-bit words (the minimum value is 5).

Type of service

The quality of service (precedence, delay, throughput, and reliability) desired for the datagram.

Total length

The length of the datagram (both header and user data), in octets.

Identification

A 16-bit value assigned by the originator of the datagram, used during reassembly.

DatagramDatagram

Flags Control bits indicating whether the datagram may be fragmented, and if so, whether other later fragments exist.

Fragment offset

The offset in the original datagram of the data being carried in this datagram, for fragmented datagrams.

Time to live

The time in seconds the datagram is allowed to remain in the internet system.

Protocol The high level protocol used to create the message (analogous to the type field in an Ethernet packet).

Header checksum

A checksum of the header.

DatagramDatagram

Source IP address

32-bit IP address of the sender.

Destination IP address

32-bit IP address of the recipient.

Options An optional field primarily used for network testing or debugging.

Padding All bits set to zero - used to pad the datagram header to a length that is a multiple of 32 bits.

User data The actual data being sent.

Internet AddressesInternet Addresses

Globally accepted method of identifying computers Each host on a TCP/IP internet is assigned

a unique 32-bit address that is used in all communication with that host

Must not be confused with physical addresses! Each address is a pair (netid, hostid),

where netid identifies a network,and hostid identifies a host on that network

In practice, each IP address must have one of the first three forms shown in the picture that follows


Class Maximum number of

possible networks

Maximum number of

hosts per network

A 127 16,777,216

B 16,384 65,536

C 2,097,152 255

D Reserved Class

E Reserved Class



The IP address is expressed in dotted decimal notation (DDN) by taking the 32 binary bits and forming 4 groups of 8 bits, each separated by a dot - 10.4.8.2 is a class A address- 10 is the DDN assigned network number- .4.8 are (possibly) user assigned subnet numbers- .2 is the user assigned host number- 172.16.9.190 is a class B address- 172.16 is the DDN assigned network number- .9 is the user assigned subnet number- .190 is the user assigned host number


Host: any attached device on a subnet, including PC’s, mainframes and routers

Most hosts are connected to only one network,and they have a single IP address

Routers are connected to more than one network and they can have multiple IP addresses

Problem: IP type of addressing defines connection,not host!

Problem: what when an organization which has a class C address finds that they (.org) need to upgrade to class B

DNS – The Domain Name SystemDNS – The Domain Name System

Converts symbolic node name to IP address Uses distributed database Network Information Center (NIC) maintains names

and network gateway addresses Name Resolver on each network

tries to complete address from own database Examines Name Server for full NIC database

if unsuccessful resolution

ARP - Address Resolution Protocol ARP - Address Resolution Protocol

Allows host to find the physical address of a target host on the same physical network, given only the target’s IP address

It does this by sending out an ARP broadcast packet containing both the source and destination IP address

The broadcast is media-dependant For Ethernet LANs the broadcast address

is a packet whose destination address bits are all `1'

ARP - Address Resolution ProtocolARP - Address Resolution Protocol

All stations on the LAN receive this address, but only one host is able to recognize its own IP address

It replies, thereby giving the original host its physical address.


A X B Y

Host A broadcasts an ARP request containing IB to all machines


A X B Y

Host B responds with an ARP reply that contains the pair (Ib, Pb)

RARP - Reverse Address Resolution ProtocolRARP - Reverse Address Resolution Protocol

How does the diskless machine, one without access to secondary storage,determines its IP address at system startup?

A diskless machine uses RARP to obtain its IP address from a server


A B C D

Machine A broadcasts a RARP request specifying itself as a target


A B C D

Those machines authorized to supply the RARP service (C i D) replydirectly to A

IP - Internet ProtocolIP - Internet Protocol

Defines unreliable, connectionless delivery mechanism

IP provides three important definitions: - Defines the basic unit of data transfer used throughout a TCP/IP internet - Performs routing function,

choosing a path over which data will be sent - Includes a set of rules that embody

the idea of unreliable packet delivery

ICMP - Internet Control Message ProtocolICMP - Internet Control Message Protocol

Integral part of IP Allows gateways to send error or control messages

to other gateways or hosts Provides communication between

the Internet Protocol software on one machine and the Internet Protocol Software on another

Only reports error conditions to the original source! ICMP message is encapsuled in an IP datagram

ICMP-The Internet Control Message ProtocolICMP-The Internet Control Message Protocol

ICMP-The Internet Control Message ProtocolICMP-The Internet Control Message Protocol

Value Description

0 Echo Reply

3 Destination Not Reachable

4 Source Quench

5 Redirection Required

8 Echo Request

11 Time to Live Exceeded

12 Parameter Problem

13 Timestamp Request

14 Timestamp Reply

15 Information Request (now obsolete)

16 Information Reply (now obsolete)

17 Address Mask Request

18 Address Mask Reply

Type field

UDP - User Datagram ProtocolUDP - User Datagram Protocol

Connectionless protocol Provides the primary mechanism that application

programs use to send datagramsto other application programs

Provides protocol ports used to distinguish among multiple programs executing on a single machine

Uses IP to transport a message from one machine to another

Does not use acknowledgements to make sure messages arrive

UDP message is called a user datagram



Source port: An optional field with the port number If a port number is not specified, the field is set to 0

Destination port: The port on the destination machine

Length: The length of the datagram, including header and data

Checksum: A 16-bit one's complement of the one's complement sum of the datagram, including a pseudo header similar to that of TCP


Physical

Internet (IP)

UserDatagram(UDP)

Application

Conceptual Layering

TCP - Transmission Control ProtocolTCP - Transmission Control Protocol

TCP is a communication protocol, not a piece of software!

Provides reliable stream service Positive acknowledgement with retransmission Specifies the format of the data and

acknowledgements that two computers exchange to achieve a reliable transfer

Specifies the procedures the computers use to ensure that the data arrives correctly

Specifies how TCP software distinguishes among multiple destinations on given machine

Specifies how communicating machines recover from errors like lost or duplicated packets


Specifies how two computers initiate a TCP stream transfer and how they agree when it is complete

Does not dictate the details of the interface between an application program and TCP

Does not specify the exact application programs to access these operations

Reason: flexibility TCP uses the connection, not the protocol port,

as its fundamental abstraction Connections are identified by a pair of endpoints


TCP port number can be shared by a multiple connection on the same machine

The basic unit of transfer used by TCP is a segment Segments are used to pass control information or data


Send Packet 1

Receive ACK1

Send Packet 2

Receive ACK2

Receive Packet 1

Send ACK 1

Receive Packet 2

Send ACK2

Sender awaits an acknowledgement for each packet sent

Server Site Receiver SiteNetwork Messages


Send Packet1Start Timer

ACK would normallyArrive at this time

Retransmit Packet1Start Timer

Timer Expires

Packet should arriveACK should be sent

Receive Packet1

Receive ACK1Cancel Timer

Server Site Network MessagesReceiver Site

Packet lost

Send ACK1

Timeout and retransmission that occurs when a packet is lost


A simple positive acknowledgement protocol wastes a substantial amount of network bandwidth

Delays sending a new packet until it receives an acknowledgement for the previous packet

Sliding window protocol Keeps the network completely saturated with packets Higher throughput than

a simple positive acknowledgment protocol


Send Packet1

Send Packet2

Send Packet3

Receive ACK1

Receive ACK2

Receive ACK3

Receive Packet1Send ACK1



Sender Site Network Messages Receiver Site

Three packet transmitted using a sliding window protocol


Conceptual Layering

Physical

Internet (IP)

TCP

Application

UDP

Telnet – remote login protocolTelnet – remote login protocol

Allows user at one site to establish a TCP connection to a login server at another site, and then it passes keystrokes from the user’s terminal directly to the remote machine as if they had been typed at a terminal on the remote machine

Carries output from the remote machine back to the user’s terminal


The service is called transparent Telnet client software User specifies a remote machine

either by giving its domain name or IP address1. Telnet offers three basic services2. Defines a network virtual terminal (NVT)3. Includes a mechanism that allows

the client and server to negotiate options1. Treats both ends of the connection symmetrically


User's terminal

operating system

TELNETclient

TCP/IPinternet

operating system

TELNETserver

client readsfrom terminal client sends

to server

server receives from terminal

server sends topseudo terminal

FTP – File Transfer ProtocolFTP – File Transfer Protocol

Manages files across machines without remote Telnet session

Transfers files, manages directories, accesses e-mail Does not enable access

to remote machine for program execution Uses two channels

for simultaneous transfer of commands and data

Conducts all transfers in foreground Does not use queues or spoolers

so you are watching the transfer process in real time


Relies on TCP to provide connection management Protocol Interpreter PI transfers instructions

using TCP port 21 Data Transfer Process DTP transfers files

using TCP port 20


FTP channel connections


LoginVerifies user ID and passwordAnonymous FTP allows access without login accountAccess and file transfer requires proper permissions on the remote side

Define directoryIdentifies starting directoryLocal and remote file system

Define File Transfer ModeText uses ASCII characters separated by CR and NLBinary transfers without conversion or formatting


Start Data TransferEnables user commands

Stop Data TransferCloses connection

TFTP – Trivial File Transfer ProtocolTFTP – Trivial File Transfer Protocol

Does not log on to remote machine Uses UDP connectionless transport

TFTP transfer identifiers (TID’s) set ends of connectionSupport basic error messagesAny kind of error can result in transfer failure

Uses to boot diskless terminals and load applicationsSmall executable size and memory requirementsCannot execute FTP until loaded with OS

Lax access regulations pose security risks

Electronic MailElectronic Mail

The most widely used application service Differs fundamentally from other uses of network Usually timeout and retransmission

if no acknowledgement Sender don’t want to wait for

the remote machine to become available before continuing work!

User don't want to have transfer abort merely because communication with the remote machine becomes temporary unavailable!

Spooling

Conceptual components of an electronic mail systemConceptual components of an electronic mail system

userinter-face

outgoing mailspool area

mailboxes forincoming mail

server (toaccept mail)

client(background

transfer)

TCP connection

for incoming mail

TCP connection

for outgoing mail

user readsmail

user sendsmail

SMTP - Simple Mail Transfer ProtocolSMTP - Simple Mail Transfer Protocol

Standard for the exchange of mailbetween machines

Focuses specifically on how the underlying mail delivery systempasses messages across a link from one machine to another

Messages transferred as 7-bit ASCII Uses spools or queues Does not specify

how the mail system accepts mail from a user or how the user interface presents the user with incoming mail

SMTP - Simple Mail Transfer ProtocolSMTP - Simple Mail Transfer Protocol

Does not specify how mail is stored or how frequently the mail system attempts to send messages

Mail command identifies sender and provides message information

Response is acknowledgment with identification of recipient

Example of SMTP TransferExample of SMTP Transfer

S: 220 Beta.gov Simple Mail Transfer Service ReadyC: HELO Alpha.eduS: 250 Beta.gov

C: MAIL FROM: <[email protected]>S: 250 OK

C: RCPT TO: <[email protected]>S: 550 No such user here

Example of SMTP TransferExample of SMTP Transfer

C: RCPT TO: <[email protected]> S: 250 OK

C: DATAS: 354 Start mail input; end with <CR><LF>.<CR><LF>C: …sends body of mail message…C: …continues for as many lines as message containsC: <CR><LF>. <CR><LF>S: 250 OK

C: QUITS: 221 Beta.gov Service closing transmission channel

ReferencesReferences

The Internet Unleashed, SAMS Publishing Internetworking With TCP/IP, Douglas Comer Teach yourself TCP/IP in 14 days, Tim Parker Novell’s Complete Encyclopedia of Networking,

Werner Feibel http://www.ieee-occs.org

ConclusionConclusion

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