Networks II (Course Outline)

1. 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)

2. 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.

3. Networks II: Recommended Texts • Data Networks – Bertsekas/Gallager • Becoming out of date but a good introduction to networking with a mathematical bent.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.

4. 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.

5. 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which define the internet: • http://www.rfc-editor.org/ • RFCs define how things work (but some are spurious, some are out of date and some are just jokes).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

6. 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”: • Knock on the door. • Wait for someone to say “Come in.” • If someone says “Come in.” then open the door and enter. • 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.

7. 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.

8. 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.

9. 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.

10. Models of the Internet OSI/ISO Reference Model TCP/IP Reference Model Application Transport Internet Host-to-network Application Presentation Session Transport Network Data Link Physical Model Layers Open Systems Interconnection (International Standards Office) Transmission Control Protocol/ Internet Protocol

11. 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.

12. 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.

13. 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.

14. 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).

15. 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.

16. 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.

17. 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.

18. TCP/IP model in summary

19. 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 addresses of the above sites. IP addresses are 32 bits grouped into 4 octets. (Octet = 8 bits – a number from 0-255)

20. 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.

21. IP Networks(2) • IP networks were originally subdivided into class A, B, C, D and E networks.

22. 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.

23. The IP header • IP packets all have a header as shown

24. 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

25. 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.

26. 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).

27. 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.