Comp_Network_Intro [Compatibility Mode].pdf

7/27/2019 Comp_Network_Intro [Compatibility Mode].pdf

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This Lecture – Internet Basics

Basic terms we need to understand.

The OSI/ISO (Open SystemsInternconnection/International Standards Office)“layers model” of computer networks.

The standard model to describe how computernetworks should work.

The TCP/IP (Transmission ControlProtocol/Internet Protocol) Protocol Stack

The standard model which is how computer networksactually work.



Where to go for more information on

this lecture’s subjects RFCs: (Requests for Comments): The protocols which

define the internet: http://www.rfc-editor.org/

RFCs define how things work (but some are spurious, some areout of date and some are just jokes).

IETF: (Internet Engineering Task Force)

http://www.ietf.org/

Related texts:

Bertsekas/Gallager: Layers Model: Section 1.3 IP: Section2.8+ 2.9

Tanenbaum: Layers Model: Section 1.4 IP: Section 5.5



Basic Definitions: Protocol

Protocol: A formal specification of how things should

communicate. In networking a protocol defines an interfaceusually (though not necessarily) between one computer andanother.

A simple example of a protocol “Knock and Enter”:

1. Knock on the door.2. Wait for someone to say “Come in.”

3. If someone says “Come in.” then open the door and enter.

4. If you wait for five minutes then give up. We might want to combine this with a protocol for saying

“Come in” when you hear a knock.

Two computers need to use the same protocol to talk to one

another. The definition of protocols is critical to networking.



Basic Definitions: Bit, Byte, Octet,

Packet, Header, Bandwidth Bit: A 0 or a 1 – the basic unit of digital data.

Byte: A short collection of bits (usually assumed to be 8bits – but may, rarely, be 7, 16 or 32).

Octet: A collection of 8 bits.

Packet: A collection of bits in order assembled fortransmission.

Header: Part of packet with info about contents.

Bandwidth: The amount of data which can be sent on achannel. Usually bits per second – sometimes in bytes(octets) per second. (Yes this is confusing.)

KB = kilobytes. Kb = kilobits.



Basic Definitions: Host, Router,

Switch, Source, Destination

Host: A machine which is a point on a network

which packets travel through – a node in a graph.

Router: A host which finds a route for packets to

travel down – an intermediate point in a journey. Switch: Often used interchangeably with router

but implies that the routes are “fixed”.

Source: Where data is coming from.

Destination: (or sink) Where data is going to.



A Simple Model of Reliable Internet

Communications.

To send data to another computer:

Find the address of the computer you are sending to.

Break the data into manageable chunks (packets).

Put the address on each packet (packet heard) and also

your own address.

Send each packet in return to the receiving computer.

Get a receipt for each packet which has been sent.

Resend packets for which we do not have a receipt.

The receiver then reassembles the packets to retrievethe data sent.



Models of the Internet

OSI/ISO Reference Model TCP/IP Reference Model

Open Systems Interconnection

(International Standards Office)

ApplicationPresentation

SessionTransportNetwork Data Link

Physical

Model Layers

Application

TransportInternetHost-to-network

Transmission Control

Protocol/

Internet Protocol



1) Physical layer

Purpose: Necessary infrastructure.

Think "wires in the ground and switchesconnecting them".

This is the physical hardware of the internet. Wires/optical cables/wireless links and other

technologies provide a way for transmission of

raw bits (0s and 1s). Routers and switches connect these cables and

direct the traffic.



2) Data link layer

Purpose: Provides basic connection between two

logically connected machines.

Think: “I stuff packets down a wire to myneighbour”

Send raw packets between hosts.

Basic error checking for lost data.

In TCP/IP the "Physical layer" and the "DataLink" layer are grouped together and called thehost-to-network layer.



3) Network Layer/Internet Layer

Purpose: Provide end-to-end communication

between any two machines.

Think: “I try to get a packet to its destination”

Tells data which link to travel down. Addresses the problem known as routing.

Deals with the question "where do I go next to

get to my destination?" Ensures packets get from source A to destination

B.



4) Transport Layer

Purpose: Ensure that data gets between A and B.

Think: “From the source and destination, I makesure that the data gets there”.

Ensures a data gets between source anddestination.

If necessary ensure that connection is lossless

(resend missing data). Provides flow control if necessary (send data

faster or slower depending on the network

conditions).



5) Session Layer (not TCP/IP)

Purpose: Provides a single connection for one

application.

Think: “I am in charge of the entire message.”

This connection may be two way or may besynchronised.

Not discussed much as it is never implemented.



6) Presentation Layer (Not TCP/IP)

Purpose: Provides commonly used functions for

applications. Think: “I meet internationalisation standards”.

The main job of the presentation layer is toensure that character sets match – e.g. thatChinese characters are correctly received by thesends.

Again not discussed much as it is neverimplemented.



7) Application layer

Purpose: The computer programs which actually dothings with the network.

Think: “I deliver the mail, browse the web etc.”

For example, your email client program which will talk to the email server at the other end.

At this layer, we have many protocols (http, snmp,

smtp, ftp, telnet) which different bits of software use. We often talk in terms of client and server architecture

for the software.



TCP/IP model in summary



Internet (IP) addresses

xxx.lcsee.wvu.edu (email, or similar)

http://www.apoptygma.eu.org (www)

ftp://ftp.uk.debian.org (file transfer)

telnet://towel.blinkenlights.nl (telnet)

157.182.194.89

148.122.211.110195.224.53.39

62.250.7.101

These are the “real” IP addresses

of the above sites. IP addressesare 32 bits grouped into 4 octets.

(Octet = 8 bits – a number from

0-255)



IP Networks(1)

IP addresses use less significant bits first to

indicate sub-networks. IP address: 123.45.67.89

Netmask :255.255.255.0 (no holes allowed)

If two IP addresses are the same when bitwiseAND’d against the netmask then they are on the

same subnet. 123.45.67.?? is always on the same subnet in the

above example.



IP Networks(2)

IP networks were originally subdivided into class

A, B, C, D and E networks.

Start End Networks Hosts/network

A 1.0.0.0 127.255.255.255 126 16 million

B 128.0.0.0 191.255.255.255 16,382 64K

C 192.0.0.0 223.255.255.255 2 million 254

D 224.0.0.0 239.255.255.255 Multicast

E 240.0.0.0 247.255.255.255 Reserved



Subnet examples

IP Addresses: A= 132.128.208.32

10000100.10000000.11010000.00100000

B= 132.128.217.63

10000100.10000000.11011001.00111111

Subnet mask 1: 255.255.255.0 =

11111111.11111111.11111111.00000000

Subnet mask 2: 255.255.240.0 =

11111111.11111111.11110000.00000000

A and B would be on the same subnet if the subnetmask was 1 but different subnets if the mask was 2.



The IP header

IP packets all have a header as shown



About the IP header

Type of Service: (Best efforts, immediate

delivery etc) Total length (of whole packet)

Identification (number of packet for laterreassembly)

Fragment offset – sometimes the network splits a

packet into fragments. Flags (information about fragments). DF= Dont

Fragment MF= More Fragments to come



About the IP header (2)

Time To Live (TTL) – reduced by one every hop.

When it reaches zero packet is killed. (This is to

ensure that the network doesn’t fill up with lost

packets).

Protocol – identified by a number (usually TCP

or UDP).

Checksum – to ensure that the packet is notcorrupted.



IPv6

IPv4 allows over 4 billion computers (but not really) –

inefficient subnetting is using these up. IPv6 allows 16 octet addresses (4 octets in IPv4).

3x1038 addresses (> Avogadro’s number). 7x1023 IP

addresses per square meter of the earth’

s surface. Why so many? Electrical devices may want IP addresses

– your house could be its own subnetwork. Why NOT?

Better security than current IP(v4). Allow “roaming hosts”.

Pay more attention to type of service (for real time data).

Documents

Comp_Network_Intro [Compatibility Mode].pdf