Networks II (Course Outline)

What this course covers Internet Basics:

The ISO layers model. The TCP/IP Protocol Stack. Basic Queuing Theory:

Notation: Poisson, Deterministic and General queues. Little’s Theorem, Markov Chains, Birth Death Processes, Generating

Functions. Routing:

Network Structures, Dijsktra, Bellman-Ford and Frank-Wolfe Algorithms.

Statistics in Networks Traffic Assumptions (Poisson, Heavy-Tail Distributions, Long-Range

Dependence)

Networks II (Course Aim)

By the end of this course you should: Have a working knowledge of how things find their way about

the internet. Be able to understand the mathematics of queuing systems and

routing. Understand research in the area of Network Engineering. Know some handy ways to investigate networks.

This course will not teach you: The practicalities of wiring networks or administrating

networked computers. How to program networked applications.

Networks II: Recommended Texts

Data Networks – Bertsekas/Gallager Becoming out of date but a good introduction to networking

with a mathematical bent. (Course recommended text). Computer Networks – Tanenbaum

Well known introductory text, more up to date but without the mathematical depth of the previous.

Queueing Systems (I and II) – Kleinrock A classic text introducing the heavy duty mathematics of

Queuing Theory. TCP/IP Illustrated (I and II) – Stephens

The classic text if you actually need to understand and program using internet based protocols.

This Lecture – Internet Basics

Basic terms we need to understand. The OSI/ISO (Open Systems

Internconnection/International Standards Office) “layers model” of computer networks. The standard model to describe how computer

networks should work. The TCP/IP (Transmission Control

Protocol/Internet Protocol) Protocol Stack The standard model which is how computer networks

actually 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 are

out of date and some are just jokes). IETF: (Internet Engineering Task Force)

http://www.ietf.org/ Course texts:

Bertsekas/Gallager: Layers Model: Section 1.3 IP: Section 2.8+ 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 interface usually (though not necessarily) between one computer and another.

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 8

bits – but may, rarely, be 7, 16 or 32). Octet: A collection of 8 bits. Packet: A collection of bits in order assembled for

transmission. Header: Part of packet with info about contents. Bandwidth: The amount of data which can be sent on a

channel. 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 retrieve

the data sent.

Models of the Internet

OSI/ISO Reference Model TCP/IP Reference Model

Open Systems Interconnection (International Standards Office)

ApplicationPresentationSessionTransportNetworkData LinkPhysical

Model Layers

Application

TransportInternetHost-to-network

Transmission Control Protocol/Internet Protocol

1) Physical layer

Purpose: Necessary infrastructure. Think "wires in the ground and switches

connecting 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 my neighbour”

Send raw packets between hosts. Basic error checking for lost data. In TCP/IP the "Physical layer" and the "Data

Link" layer are grouped together and called the host-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 make

sure that the data gets there”. Ensures a data gets between source and

destination. 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 be

synchronised. 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 to

ensure that character sets match – e.g. that Chinese characters are correctly received by the sends.

Again not discussed much as it is never implemented.

7) Application layer

Purpose: The computer programs which actually do things 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

richard@manor.york.ac.uk (email)

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

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

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

144.32.100.24

148.122.211.110

195.224.53.39

62.250.7.101

These are the “real” IP addressesof the above sites. IP addressesare 32 bits grouped into 4 octets.(Octet = 8 bits – a number from0-255)

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 bitwise

AND’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 subnet mask 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 later

reassembly) 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 not corrupted.

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).

Next Lecture

IP tells us how to get a message from A to B. However, the IP protocol is lossy (it doesn’t

guarantee that anything will actually “get there”). In the next lecture we will look at TCP/IP and

UDP/IP which sit on top of IP and deal with the sending of the messages.