1. Introduction to Computer Networks Slides For Week 1

2. Course Goals  Understand the basic principles of computer networks  Understand the Internet and its protocols  Understand the key design principles used to build the Internet  Experience building network systems  Course is not about specific skills  E.g. configure a router from company X vs. learn principles of how all routers work  Success means you are confident to tackle a range of network programming, design and maintenance. 2Introduction to Computer Networks

3. Networks Fundamentals

4. Why Study Networks?  Integral part of society  Work, entertainment, community  Pervasive  Home, car, office, school, mall …  Huge impact on people and society  Person-to-person and group communication  email, blogs, chat  Form and strengthen communities  chat rooms, newsgroups...  Commerce, services, entertainment, socializing 4Introduction to Computer Networks

5. Concepts for this week  What is the Internet?  What is a network?  Core and Edge of the Internet  Circuit, message and packet switching  Layering and encapsulation  Network delay analysis 5Introduction to Computer Networks

6. What is the Internet? What is a network?

7. Lets start at the beginning… 7  What is a network for?  To allow two or more endpoints to communicate  Is Cable T.V. a network?  – yes.  What is a computer network?  Nodes connected by links  Nodes are computers:  Processing ability  Receive and transmit information (unlike televisions).  Is this a Network?  Yes, not an interesting one.. Introduction to Computer Networks

8. Other Networks (network topologies) 8 RingStar (Hub and Spoke) HierarchicalBroadcast Introduction to Computer Networks

9. Data Networks 9  Data networks are Packet Switching Networks  Data is carried in packets and is switched between routers  Every packet carries its destination address and is routed according to current network conditions.  There is no fixed route between every pair of end points.  Telephone networks are Circuit Switched Networks  Voice is carried over a predetermined circuit between two phones.  The route is established before any of the data is sent, and then the data is sent (like a phone call – you first call, have a connection, and then talk).  Data is also carried (e.g., fax). But it is not a data network! Introduction to Computer Networks

10. Data Types 10 Non-IsochronousIsochronous Examples: mail, web files, P2P traffic, etc.Examples: voice, live broadcast Arrival rate is undetermined, bursty. Jitter (variation of delay between subsequent packets) is unknown, and not important. Arrival rate is predetermined and jitter should be fixed. E.g., packet every 0.25 msec. Delay is possible. For example, your email cab be delayed by a few seconds. Delay is critical and bounded. Maximal possible delay is ~ 30-70 msecs. Packet loss can be taken care of by the network, and missing packets can be retransmitted, so that the receiving side receives the complete data sent. Packets lost cannot be resent due to the maximal delay limitation. Ideal for packet switching Ideal for circuit switching Introduction to Computer Networks

11. The Internet 11  Built initially for non-isochronous data  Hence, a packet switching architecture.  Today we see that the world moves towards integration of networks: carry data, voice, TV over the Internet.  Termed: everything over IP  (We will see later what that means)  Why not use circuit switching (telephone network)?  There was a big debate, of course, but telephone networks are less efficient for routing bursty data (Internet data).  In this course we study a packet switching network: The Internet. Introduction to Computer Networks

12. What is Internet Technology?  What is an internet?  Network of networks  What is the Internet?  A global internet based on the IP protocol  To what does “Internet technology” refer?  Architecture  Services  We will learn the protocols which enable the services, and understand the architecture it is built over. Protocols 12Introduction to Computer Networks

13. Architecture-wise Host: Machine running user application Media: Physical process used (copper wire, fiber optics, satellite link) Company A Company B Internet Service Provider 1 ISP 2 13Introduction to Computer Networks

14. 14 Architecture-wise Network : Collection of interconnected machines or interconnected LANs (subnetworks) Router: decide where to send data next Company A Company B Internet Service Provider 1 (ISP 1) ISP 2 Introduction to Computer Networks

15. 15 Architecture-wise Company A Company B ISP 2 Core Networks (ISP tiers) Tier 1: Biggest ISPs Tier 2 and 3: Regional and very small. Internet Service Provider 1 (ISP 1) Edge Networks: Companies, organizations with a “default route” Introduction to Computer Networks

16. Networks Today 16  Default Route Networks: Campus, Company  Local Area Networks (LANs) for labs, or certain floors  Virtual LANs  Edge Networks:  Small Internet Service Providers  Metropolitan  Core Networks:  Wide Area Network  The Internet: Core + Edge Networks Introduction to Computer Networks

17. Networks Legendary Scaling 17 Inter-processor Distance Processors Located In Same 1 mSquare meterPersonal Area Network 10m 100m 1km Room Building Campus Local Area Network 10 kmCity Metropolitan Area Network 100 km 1000km Country Continent Wide Area Network 10000 kmPlanetThe Internet Introduction to Computer Networks

18. Local Area Network (LAN) 18  Limited Size  Run at speeds of 10Mbs, 100 Mbs, 1Gbps, 10 Gbps  Legend topology: Ethernet (broadcast)  Example: PC farm with servers and printers.  Today: Switched computers and nodes. Introduction to Computer Networks

19. Service-wise (applications)  Electronic mail  Remote terminal  File transfer  Newsgroups  File sharing  Resource distribution  World Wide Web  Video conferencing  Games 19Introduction to Computer Networks

20. Enabling A Service 20  The user, at home, is requesting page www.mta.ac.ilwww.mta.ac.il  The network (Internet) should figure: 1. Which server is www.mta.ac.il? (all computers use numbers. Most people seem to prefer names).www.mta.ac.il 2. Which Route to use for user’s request? 3. What format to use for sender request and receiver’s reply? (maybe data should be encrypted) 4. What to do if the message gets corrupted along the way? 5. What to do if a routing computer fails along the way? 6. Maybe the server restricts access to this page, is this user ok? 7. What if the page content was moved to another server, or was deleted? 8. What if user logged off or pressed “refresh” before transmission was completed? …. Introduction to Computer Networks

21. Protocols  The Internet uses the TCP/IP protocol suite  Architecture  Service  Rules of communication  Divided to layers … FTPHTTPRTP TFTP TCP UDP IP Ethernet802.11 PPP CAT-5Single-Mode Fiber RS-232 21Introduction to Computer Networks

22. Layering 22  An important and basic principle in computer networks  Divide the work hierarchically: each layer is a separate entity that maintains its part. Application Transport Network Host-to-Net * (Link layer) Host The application layer receives or transmits the data to/from the application The Transport layer is responsible to packetize the data and transmit it without errors to the end host The Network layer is responsible to choose the next hop (a neighbor network node) and route the packets to it The Host-to-net layer is responsible to send the data over the attached physical link in its format and * host-to-net layer is also called the link layer Introduction to Computer Networks

23. TCP/IP Layering Architecture Application Protocol end-to-end communication Transport Protocol (TCP) end-to-end communication Application Layer Transport Layer Network Layer Link Layer Host A Host B Application Layer Transport Layer Network Layer Link Layer Network Layer Link Layer Network Layer Link Layer IP 23Introduction to Computer Networks

24. Layering and Encapsulation 24  Use layers to hide complexity, each layer implements a service:  Layer N uses service provided by layer N-1  layer N-1 provides a service to layer N  This is done using encapsulation:  Treat the neighbouring layer’s information as a “black box”, can’t look inside or break message  Sending: add information needed by the current layer “around” the higher layers’ data  headers in front  trailers in back  Receiving: Strip off headers and trailers before handing up the stack Introduction to Computer Networks

25. Layering ( cont.) 25  A layer N, to accomplish its task:  Receives information from layer N-1, termed Protocol Data Units (PDU).  Executes an algorithm with layer N in other stations by sending and receiving messages.  The algorithm and the set of rules of sending and receiving packets at each layer is termed Protocol.  PDU at the network layer is termed packet.  PDU at the link layer (host-to-net) is termed frame. Introduction to Computer Networks

26. Layering ( cont.) 26 data Transport layer header Network layer header data Transport layer header Network layer header data Transport layer header Host-to-net layer header At the sending host  A Packet  A Frame Host sends the frame on the attached link Introduction to Computer Networks

27. Layers ( cont.) 27  At the receiving host  The frame arrives on the attached link data Transport layer header Network layer header data Transport layer header Network layer header data Transport layer header Host-to-net layer header Data is delivered to the application at the receiving host Introduction to Computer Networks

28. ISO OSI Layering Architecture – The reference model Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Application Protocol Transport Protocol Presentation Protocol Session Protocol Host AHost B Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer Router 28Introduction to Computer Networks

29. ISO’s Design Principles  A layer should be created where a different level of abstraction is needed  Each layer should perform a well-defined function  The layer boundaries should be chosen to minimize information flow across the interfaces  The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy 29Introduction to Computer Networks

30. Who is who? 30  ISO - International Organization for Standardization  ISO 9000, etc.  IETF – The Internet Engineering Task Force: Near term, standardized oriented group, divided into disciplines such as: applications, routing, addressing, etc.  IRTF – The Internet Research Task Force: Pursue long term research projects  RFCs: Requests For Comment  Some become official standards  http://www.rfc-editor.org http://www.rfc-editor.org  Look for “aviation over IP” (aka birds)… Introduction to Computer Networks

31. Host-to-net Layer 31  Receives a data packet from the network layer  Creates a frame of bits  Signal the start and end of a frame (usually by a string of known bits)  Add error correcting codes to enable transmission even on noisy channels  Collision detection and retransmissions  What is the time it takes to transmit and receive a frame on a link? Introduction to Computer Networks

32. Link Delay 32  Two factors  Propagation delay = Distance / Speed of light Time for bits to “move across wires”  Transmission delay = Packet size / Bandwidth Time to “get bits on wires” Introduction to Computer Networks

33. Transmission vs. Prop. delay A single transmission link as a water pipe The thicker the pipe, the more water it can carry from one end to the other in each unit time Water is carried from one end of the pipe to the other at constant speed, no matter how thick the pipe is Water = Data bits Thickness of the pipe = Channel capacity (bandwidth) Speed of water through the pipe = Propagation speed (light speed) 33Introduction to Computer Networks

34. Transmission vs. Prop. Delay (cont) pipe 1. Propagation delay is how long takes to cross the pipe, irrespective of volume 2. Transmission (bandwidth delay) is related to how much water can be pushed in through the opening per unit time 34Introduction to Computer Networks

35. 1500 x 8 bits Transmission Time How long does it take A to transmit an entire packet on the link? Relevant information: packet length = 1500 bytes channel capacity = 100 Mbps Another way to ask this question: If the link can transmit 100 million bits in a second, how many seconds does it take to transmit 1500 bytes (8x1500 bits)? 100 Mbits 1 sec = t Solving for t… t = 0.00012 sec (or 120  sec) 35Introduction to Computer Networks

36. Propagation Delay How long does it take a single bit to travel on the link from A to B? Relevant information: link distance = 500 m prop. delay factor = 5  sec/km Another way to ask this question: If it takes a signal 5  sec to travel 1 kilometer, then how long does it take a signal to travel 500 meters? 5  sec 1000 m = 500 m t Solving for t… t = 2.5  sec 36Introduction to Computer Networks

37. Units  Bits are the units used to describe an amount of data in a network  1 kilobit (Kbit) ~1 x 10 3 bits = 1,000 bits  1 megabit (Mbit)~1 x 10 6 bits = 1,000,000 bits  1 gigabit (Gbit)~1 x 10 9 bits = 1,000,000,000 bits  Seconds are the units used to measure time  1 millisecond (msec)= 1 x 10 -3 seconds = 0.001 seconds  1 microsecond (  sec) = 1 x 10 -6 seconds = 0.000001 seconds  1 nanosecond (nsec) = 1 x 10 -9 seconds = 0.000000001 seconds  Bits per second are the units used to measure channel capacity/bandwidth and throughput  bit per second (bps)  kilobits per second (Kbps)  megabits per second (Mbps)  Bytes are units that describe a series of eight bits  1 Byte = 8 bits  Bytes per second (Bps) 37Introduction to Computer Networks

38. Processing Delay  Stylized format required to send data  Analogy: adding and removing envelopes to letters  How long does it take to execute all these layers? Application Layer Transport Layer Network Layer Host-to- Net Layer Host Why is this time important? Network Layer Host-to- Net Layer Router Application Layer Transport Layer Network Layer Host-to- Net Layer Host 38Introduction to Computer Networks

39. Example Protocol Processing Time = 40  sec packet length = 1500 bytes channel capacity = 100 Mbps propagation delay factor = 5  sec/km AB 500 m 1. How long to format the data? 2. How long does it take a single bit to travel on the link from A to B? 3. How long does it take A to transmit an entire packet onto the link? 39Introduction to Computer Networks

40. Timeline Method Time Protocol Delay Transmission time Propagation delay Protocol Delay Host AHost B 40 2.5 120 40 1 st bit last bit Total time: 40+120+2.5+40 = 202.5  sec 40Introduction to Computer Networks