Computer Networking Eliezer Dor eliezer dorgmail com Teaching

Computer Networking Eliezer Dor (eliezer dor@gmail. com) Teaching Assistant: Allon Wagner Ch. 1: Introduction 1

Course Information Lectures: Recitation: Wed Mon 12 – 15 10 – 11 / 11 – 12 Orenstein 103 Kaplun 118 Web site: http: //www. cs. tau. ac. il/~allonwag/comnet 2012 A/index. html Main Book: • Kurose-Ross: A Top-down Approach to Computer Networking Additional Books: 1. Keshav : An Engineering Approach to Computer Networking 2. Tanenbaum : Computer Networks 3. Bertsekas and Gallager : Data Networks Ch. 1: Introduction 2/71

Practical Information Homework assignments: Mandatory Both theoretical and programming Grades: Final Exam: theory exercises: Programming exercises: Ch. 1: Introduction 60% 20% 3/71

Chapter 1 Introduction Ch. 1: Introduction 4

Chapter 1 Contents n n n n Course Info General introduction Introduction to Layering The 5 layers of the Internet: basics What is a protocol Layer models: more detail Extra slides Ch. 1: Summary 1 -3 4 -18 19 -31 32 -36 37 -42 43 -63 64 -83 5/3

Motivation n 1 st stage society: Agriculture, handicraft 2 nd stage society: Industrial, labor intensive Today’s society: n n automated industry with sophisticated logistics information intensive: n n business, knowledge, advertising, news, social interaction, recreation Future society is likely to be even more information-dominated Ch. 1: Introduction 6/71

The Purpose of the Network serves network applications residing in hosts applications at distinct hosts need to co-ordinate actions / co-operate thus they need to communicate information to each other network must deliver that information to the right host to the right application process / thread network serves applications which serve users Ch. 1: Introduction 7/71

Information n n A representation of knowledge Examples: n n n Can be represented in two ways n n n text, music, video, technical specifications software, instructions, reports, alarms analog (pictures / ideograms) digital (bits) the Digital Revolution n n convert information as pictures to information as bits networks move around bits instead of pictures Ch. 1: Introduction 8/71

Challenges make order in the jungle of applications organize information into manageable units keep track of info units sent/ moving/ received take account of errors / misunderstandings etc. move the bits through the network u find the destination host in the network jungle u u using an efficient path learn automatically the current network topology make efficient use of link / router capacities resolve competition for use of same resource Cheaply, Securely, with Quality of Service, Ch. 1: Introduction 9/71

Internet Physical Infrastructure HOST = Workstation OR Server Ch. 1: Introduction 10/71

This course’s Challenge n To discuss this complexity in an organized way, so that we n n n understand the issues / alternatives can follow/design/troubleshoot processes Need to divide the job into functional layers Understand the interrelation between them These problems are beyond a specific technology Ch. 1: Introduction 11/71

Early communications systems n n n telegraph, telephone first used direct point to point links when number of users grew: introduced switching points/ configurable circuits each call had a dedicated circuit for its duration k n u r t up o r g phone line Switched connection Ch. 1: Introduction 12/71

Data Networks n n n set of interconnected nodes exchanging information links are common usage switching node must: n n choose for each data unit a link bringing it closer to dest. schedule their transmission on the common usage links (resolve the competition for the usage of the link) Ch. 1: Introduction 13/71

Qwest backbone Ch. 1: Introduction 14/71 http: //www. qwest. com/largebusiness/enterprisesolutions/network. Maps/preloader. swf

Networking Tasks – phone net. sol’n Addressing - Identify the end user phone number 1 -201 -222 -2673 = country code + region code + exchange + number Routing – Find route from source to destination. determined from phone number by static routing tables Forwarding – How information is moved circuit switching: : a fixed circuit along path to destination Information Units - How information is sent voice samples; no addressing attached samples sent continuously , 8000/sec network must prepare source-dest. circuit in advance Ch. 1: Introduction 15/71

Networking Tasks – Internet Solution Addressing - Identify the end user IP addresses 132. 66. 48. 37, = network number || host # Routing- Find route from source to destination routers learn automatically network topology build routing tables / updated frequently Forwarding – How information is moved packet switching: move packets 1 by 1 through routers. Information Units - How information is sent. self-descriptive packet = data + header contains destination address Ch. 1: Introduction 16/71

Telephone network supports a high end-to-end quality of service, but is expensive Internet supports no quality of service but is flexible and cheap Future networks will have to support a wide range of service qualities at a reasonable cost Ch. 1: Introduction 17/71

Cerf and Kahn’s internetworking principles: n n autonomy - no internal changes required to interconnect networks best effort service model stateless routers decentralized control Defines today’s Internet architecture Ch. 1: Introduction 18/71

Why Layering n n n Communication is a very complex task What we need is: communication btw applications at distant hosts What is reasonably feasible in one piece is: the ability to transfer a series of bits over a link We need to bridge between very sophisticated applications and very primitive physical layer What is needed is to divide the task’s functionality into well chosen parts n n each part should be reasonably ‘easy’ to do they should work well together Ch. 1: Introduction 19/71

How to do Layering n Define a conceptual Layering Model n n Set principles for proper usage of the model Build protocols for each layer n n n means: what is the function of each layer how they cooperate / use each other’s services protocol is between same layer entities @ distinct nodes there may be several protocols in each layer providing different type service for the layer’s function Define interfaces between layers n interface (here) is between distinct layer entities at same node (computing device) Ch. 1: Introduction 20/71

Layering Principles Modularity n each layer works independently of the others n n n this means: n n information exchange only according to Interfaces defined in the Model analogous to the Object Oriented principle in S/W eng. don’t change/peek into internal variables of other layers modularity is bypassed very seldom n only when there is no other solution to a problem Transparency n layering should be invisible to user Ch. 1: Introduction 21/71

Layering Benefits Layering enables: n discussion/understanding of the issues n n n efficient development of protocols n n n enables clear visualizing of relationships btw. functions it’s impossible to think about all layers @ once each layer has a different functional focus no need to think other layers when designing it easy replacement/maintenance of protocols n as long as modularity & interfaces are adhered to Layering is a good reference model for discussion Ch. 1: Introduction 22/71

A mail system layering model Ch. 1: Introduction 23/71

How do we Communicate? n Send a mail from Alice to Bob n n Bob Alice in Champaign, Bob in Hollywood Example: n US Postal Service Alice Hollywood, California Champaign, Illinois Ch. 1: Introduction 24/71

What does Alice do? Alice 200 Cornfield Rd. Champaign, IL 61820 Bob 100 Santa Monica Blvd. Hollywood, CA 90028 n n Bob’s address (his mailbox) Bob’s name – in case people share mailbox Postage – have to pay! Alice’s own name and address n n in case Bob wants to return a message In case the mail has to be returned. Ch. 1: Introduction 25/71

What does Bob do? Alice 200 Cornfield Rd. Champaign, IL 61820 Bob 100 Santa Monica Blvd. Hollywood, CA 90028 n n Install a mailbox Receive the mail Get rid of envelope Read the message Ch. 1: Introduction 26/71

Layers Peer entities Champaign Hollywood User Post office (P. O) give parcel to P. O counter handling pick up parcel at P. O put parcel in mailbox Ground transfer: Airport transfer: Airplane routing on truck to airport from airport to dest. P. O loading on airplane take off the airplane from source to destination each layer implements a service n via its own internal-layer actions n relying on services provided by layer below Qn: Find scenarios justifying adding extra layers to the mail model. n Name the layers and specify their place in model Ch. 1: Introduction 27/71

What Layers are Necessary? msg Application: I received your msg … I want you to do … Host A Router Transmitter: 1011001… Physical Layer Ch. 1: Introduction how to get it to B ? Application NETWORK how to make sense? 1. Host B Receiver 2. 28/71

What Layers are Necessary? msg Application: I received your msg … I want you to do … Host A Router Frame Link Layer format a frame Transmitter: 1011001… Physical Layer Ch. 1: Introduction how to get it to B ? Application NETWORK how to make Host B sense? Frame Link L. V 1. 1011001… Receiver 2. 29/71

What Layers are Necessary? msg Application: I received your msg … too many I want you to do … details !! NETWORK 3. Net Layer: I can route it! packet Link Layer Frame 1011001… Transmitter: how to Physical Layer get it to B ? Ch. 1: Introduction V 2. 4. Host B Router Host A Application too many pieces!! packet Net L. … Frame Link L. 1011001… Receiver 30/71

What Layers are Necessary? msg Application: I received your msg … I want you to do … Transport NETWORK I’ll look at details! V Net Layer packet Host A Link Layer packet Router Frame Transmitter: 1011001… Physical Layer Ch. 1: Introduction V Application Transport I’ll reassemble it! Net L. … 1011001… Host B Frame Link L. 1011001… Receiver THE FIVE LAYER MODEL 31/71

Application Layer n Tasks: n write messages serving needs of application n n n n L 5 proper type of semantics (meaning, information) appropriate syntax/format, so that semantics is understood keep track of the interaction process / state machine Focus: on needs of a specific application type Data unit: Message Peer: the Application Layer at destination host Uses: the Transport Layer Used by: the application itself Run by: the application Ch. 1: Introduction 32/71

Transport Layer n Main Tasks: n prepare data for transfer n n n n n L 4 fragment data into proper size segments / reassemble at dest. add header which enables delivery to the correct appl. process optional: error- /flow- /congestion-control Data Unit: Segment Focus: on control of End-to-End data transfer Peer: the Transport Layer at destination host Uses: the Network Layer Used by: the Application Layer Run by: the OS of the host Ch. 1: Introduction 33/71

Network Layer n Main Tasks: n n n n n L 3 learn network topology in real time prepare routing tables for fast usage in forwarding data network layer (WAN) addressing forward data from source to destination Data Unit: Datagram / “packet” Focus: on network and data fowarding Peers: the Network Layer along the whole path Uses: the Link Layer Used by: the Transport Layer Run by: the OS of the host, the router S/W Ch. 1: Introduction 34/71

Link Layer n Main Tasks: n insert delimiters so start/end of frame can be known n n n n L 2 physical layer may transfer an endless stream of bits this is part of the task of the Link header and Link trailer in LAN, access control/ link layer addressing Data Unit: Frame Focus: data transfer over a link Peer: Link Layer at the other end of the link Uses: the Physical Layer Used by: the Network Layer Run by: the NIC (Network interface card, )כרטיס רשת Ch. 1: Introduction 35/71

Physical Layer n Main Tasks: n n n n n L 1 transmit signals that encode bits 1 and 0 receive such signals and decode bits from them synchronize the bit rate clocks of the peer nodes Data Unit: Bit Focus: bit transfer over a link Peer: the Physical Layer @ other end of the link Uses: the raw media: cable/ space Used by: the Link Layer Run by: transmitter/ receiver /wave propagation Ch. 1: Introduction 36/71

Why do we need Protocols n Communication is between applications or other S/W entities n Its objective: enable cooperation on a common task n Need protocols to understand each other n Semantics: what I report/ want of you to do n Syntax/ format: how write/ read this info Ch. 1: Introduction 37/71

Open/ Proprietary Protocols n Open protocol can be used by anyone n n n it is published by a standards organization or a public consortium e. g. draft standard Proprietary protocol is owned by a company n may be used subject to company’s agreement Ch. 1: Introduction 38/71

Why do we need Standards n Communication happens between entities n n n Hosts (personal computers, servers) Routers H/W entities produced by different vendors S/W applications/ OS entities also Need agreement to ensure correct, efficient and meaningful communication n this is called Interworking Ch. 1: Introduction 39/71

Organizations that Issue Standards n IETF (Internet Engineering Task Force) n IEEE (Institute for Electrical and Electronic Engineers) n ITU (International Telecommunications Union) n ISO (International Organization for Standardization) n W 3 C (World Wide Web Consortium) Ch. 1: Introduction 40/71

Protocols n A protocol is a set of rules and formats that govern the communication between communicating peers n n n set of valid message formats - syntax meaning of each message - semantics Necessary for any function that requires cooperation between peers Ch. 1: Introduction 41/71

Protocols n A protocol provides a service n n Peer entities use a protocol to provide a service to a higher-level peer entity n n For example: the post office “registered” protocol for reliable parcel transfer service for example, truck drivers use a protocol to present post offices with the abstraction of an unreliable parcel transfer service In the layering model: n each layer gives service to next higher layer Ch. 1: Introduction 42/71

ISO OSI reference model n Reference model n n Service architecture n n formally defines what is meant by a layer, a service etc. describes the services provided by each layer and the service access point Protocol architecture n n set of protocols that implement the service architecture compliant service architectures may still use noncompliant protocol architectures Ch. 1: Introduction 43/71

The seven/five Layers There are only 5 (!!) in most architectures Application Presentation Session Transport Network Data Link Physical Application Transport End system Intermediate system Ch. 1: Introduction Network Data Link Physical Application Presentation Session Transport Network Data Link Physical End system 44/71

The seven Layers - protocol stack data Application Presentation Session Transport Network Data Link Physical n. Session AH PH data SH TH Network Data Link Physical data NH data DH+data+DT bits Application Presentation Session Transport Network Data Link Physical and presentation layers are not so important, and are often ignored Ch. 1: Introduction 45/71

Postal network n n n Application: people using the postal system Session and presentation: chief clerk sends some priority mail, and some by regular mail ; translator translates letters going abroad. Transport layer: mail clerk sends a message, retransmits if not acked Network layer: postal system computes a route and forwards the letters Datalink layer: letters loaded on planes, trains, trucks Physical layer: the driver/pilot carrying letters in sack Ch. 1: Introduction 46/71

Internet protocol stack n n application: supporting network applications n ftp, smtp, http transport: host-host data transfer n tcp, udp network: routing of datagrams from source to destination n ip, routing protocols link: data transfer between neighboring network elements n ppp, ethernet, Wi. Fi, token ring physical: bits “on the wire” Network access n Ch. 1: Introduction application transport network link physical 47/71

source host message M segment Ht M datagram/ H H n t M packet frame Hl Hn Ht M Tl application transport network link physical Encapsulation 1011……… M – message Ht – transport header Hn – network header Hl – link header Tl – link trailer destination host M Hn Ht M Hl ’ Hn Ht M Tl’ application transport network link physical Ch. 1: Introduction Hn Ht Hl ’ Hn Ht M M Tl’ network link physical Hn Ht M Hl Hn Ht M router 1 -48 Tl

Protocols and Interfaces Ch. 1: Introduction 1 -49

Service & protocol at layer k to layer k+1 Service received by layer k from layer k-1 Ch. 1: Introduction 1 -50

Packet structure: sending host view n n L 5: application layer generates a message and passes it to transport layer L 4: transport layer adds its header (H 4=Ht) n n L 3: network layer adds its header (H 3=Hn) n n this generates a datagram, which is passed to link layer L 2: link layer adds header (H 2=Hl), trailer (T 2=Tl) n n this generates a segment which is passed to netwk layer (one message may be fragmented into several segments) this generates a frame which is passed to physical layer L 1: physical layer sends the frame as a sequence of bytes is on link H 2 H 3 H 4 Message T 2 frame datagram Ch. 1: Introduction segment 1 -51

Ch. 1: Introduction Packet structure: router view Receiving stage: • L 1: physical layer receives frame, passes it to L 2 • L 2: link layer checks H 2+T 2 and removes them – this makes a datagram which is passed to network layer H 2 H 3 L 3 payload datagram T 2 frame Sending stage: L 3: network layer decides on which link to send q transfers datagram to L 2 m L 2: link layer adds new H 2+T 2 and makes a frame q frame is passed to L 1 which sends its bits on link H 2* H 3 datagram L 3 payload T 2* m frame 1 -52

Packet structure: destination view n n L 1: physical layer receives frame from link L 2: link layer recognizes frame boundaries n n n L 3: network layer checks H 3 and removes it n n this generates a segment, passed to transport layer L 4: transport layer checks H 4 and removes it n n checks H 2+T 2 and removes them this makes a datagram, passed to network layer this leaves the message which is saved in receive buffer L 5: application layer takes message from buffer H 2 H 3 H 4 Message T 2 frame datagram Ch. 1: Introduction segment 1 -53

Physical layer n Link Types: n n n shapes/sizes/material of connectors and cables/media coding scheme to represent a bit-level synchronization Interconnecting Nodes: n n n Point to Point (usually continuous transmission) LAN/multiple access (intermittent transmission) What is contained in a standard: n n L 1 Repeater Hub (on LAN only) Located: in transmitter/receiver of NIC ( )כרטיס רשת Ch. 1: Introduction 54/71

Datalink layer n Protocol Types n n n header & trailer format indication of start & end frame (delimitation) in cont. transmission links: filler frame/marker format in LAN: media access (MAC) rules, addressing rules Interconnecting Nodes (in LAN only) n n PTP protocols (HDLC, PPP, LAPD) LAN protocols: MAC prot. ’s: Ethernet, Token Ring, Wi. FI) n Auxiliary protocols: STP, DHCP, ARP What is contained in a protocol n n L 2 Bridge, (L 2 -) Switch Located: in NIC of hosts, routers, switches Ch. 1: Introduction 55/71

Network layer n Network Types n n n n header format, address formats forwarding rules (how to use routing tables) What is contained in a routing protocol: n n Circuit-switching / Packet-switching(datagram or VC) Protocol Types: Routing/Forwarding/Auxiliary(IPCP) What is contained in a forwarding protocol: n n L 3 rules/ messages for learning topology info rules for building routing tables Interconnecting Nodes: Router Metaphor: Gives a Host To Host End. To. End virtual channel Located: in Host OS, in Router CPU (part may sit in NIC) Ch. 1: Introduction 56/71

Network layer: structure L 3 n In datagram networks n n In connection-oriented* network n n same network forwards routing msgs & data msgs separate data plane and control plane data plane only schedules and forwards data control plane prepares (virtual) circuits before data is sent (routing rules required for control msgs) Internet n n forwarding by IP protocol (a datagram protocol) n best effort service (no reliability tools) several routing protocols (RIP, OSFP, BGP) * i. e Circuit Switching OR Virtual Circuit networks Ch. 1: Introduction 57/71

Network layer: action n n L 3 At end-systems: n mainly hides details of datalink layer from L 4 n segments and reassembles n detects errors At intermediate systems: n participates in routing protocol to create routing tables n forwards packets (fragments them if needed) n schedules the transmission order of packets n chooses which packets to drop Ch. 1: Introduction 58/71

Transport layer n Protocol Types: n n n L 4 Reliable stream protocols (TCP, SCTP, SSL) Unreliable datagram protocols (UDP) What is contained a protocol header format n user-process multiplexing rules (using port numbers) in Reliable protocols, has also: n error control (ack, seq. #s, retransmission, timers) n flow control (don’t overwhelm destination) n congestion control (don’t overload network) n n n Metaphor: Gives a Process to Process ETE channel Location: Hosts only (located in OS) Ch. 1: Introduction 59/71

Application layer n Application Types: n User-oriented applications (Web, Mail, File xfer. . ) n n n Infrastructure applications (DNS, NTP) Each specific application type has a separate protocol What is contained a protocol n n Protocols: HTTP, SMTP+POP, FTP Protocols: n n L 5 header format rules for mutual interaction of peer processes Metaphor: Talks to peer application about common job Location: Hosts only, run by the application S/W Ch. 1: Introduction 60/71

Layer Model at a Glance L Task Unit Peer @ By @ 5 Tell peer what Message application needs Destination Application Host only 4 ETE control of data transfer Segment Destination OS Host only 3 Route data thru Network Datagram All nodes along path OS All nodes 2 Delimit packet; Link xfer control Frame Neighbor node NIC All nodes 1 Transfer bits over link Bit Neighbor node NIC All nodes Ch. 1: Summary 61/3

Layer Model Issues & details L Types Ex. Node 5 1. User oriented 2. Infrastructure 1. http, ftp 2. Dns Applicat’n gateway 4 1. Reliable 2. Best effort 1. TCP 2. UDP 3 1. Circuit Sw. 2. Packet Sw. 1. Phone 2. tcp/ip Router Rtg algo. , address, forwarding rules 2 1. PTP protocol 2. LAN protocol 1. hdlc, ppp 2. ethernet Bridge L 2 -switch Delimitation 1. filler, 2. MAC 1 1. PTP link 2. shared link Repeater Hub Cable spec/ bit coding /clock sync Ch. 1: Summary Defines Interaction rules, message semantic and format Appl. proc. multiplex error/flow/cong ctrl 62/3

Layer Model: Discussion n n Studied (basically) the Internet 5 Layer Model OSI model (defined earlier, by ISO) n Contains 2 more layers: n n n Duplex ctrl, Data priority, Special session controls Presentation layer n n Application Layer is pushed to Layer 7 Not used in the Internet Session layer n n Layer 5 (Sessiion) Layer 6 (Presentation) Data structure standardization, encoding, encryption see Extra slides for more details Ch. 1: Introduction 63/71

EXTRA SLIDES Ch. 1: Introduction 64

History 1961 -1972: Early packet-switching principles 1961: Kleinrock - queuing theory shows effectiveness of packet-switching 1964: Baran - packet-switching in military networks 1967: ARPAnet – conceived by Advanced Research Projects Agency 1969: first ARPAnet node operational 1972: ARPAnet demonstrated publicly – NCP (Netwk Control Protocol) 1 st host-host protocol – first e-mail program – ARPAnet has 15 nodes Ch. 1: Introduction 65/71

History 1972 -1980: Internetworking, new and proprietary nets 1970: ALOHAnet satellite network in Hawaii 1973: Metcalfe’s Ph. D thesis proposes Ethernet 1974: Cerf and Kahn - architecture for interconnecting networks late 70’s: proprietary architectures: DECnet, SNA, XNA late 70’s: switching fixed length packets (ATM precursor) 1979: ARPAnet has 200 nodes Ch. 1: Introduction 66/71

Cerf and Kahn’s internetworking principles: – minimalism, autonomy - no internal changes required to interconnect networks – best effort service model – stateless routers – decentralized control Defines today’s Internet architecture Ch. 1: Introduction 67/71

History 1980 -1990: new protocols, proliferation of networks 1983: deployment of TCP/IP 1982: SMTP e-mail protocol defined 1983: DNS defined for name-to-IP-address translation 1985: FTP protocol defined 1988: TCP congestion control new national networks: CSnet, BITnet, NSFnet, Minitel 100, 000 hosts connected to confederation of networks Ch. 1: Introduction 68/71

History 1990 - : commercialization and WWW early 1990’s: ARPAnet decommissioned 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995) early 1990 s: WWW hypertext [Bush 1945, Nelson 1960’s] HTML, http: Berners-Lee 1994: Mosaic, later Netscape late 1990’s: commercialization of WWW Ch. 1: Introduction 69/71

Demand Supply • Huge growth in users – The introduction of the web • Faster home access – Better user experience. • Infrastructure – Significant portion of telecommunication. • New evolving industries – Although, sometimes temporary setbacks Ch. 1: Introduction 70/71

Internet: Users Ch. 1: Introduction 71/71

Penetration around the Globe (2009) Ch. 1: Introduction 72/71 http: //www. internetworldstats. com/stats. htm

Users around the Globe (2002/5/9) Ch. 1: Introduction 73/71

Technology: Modem speed Ch. 1: Introduction 74/71

Today’s options • • Modem: 56 K ISDN: 64 K – 128 K OBSOLETE Frame Relay: 56 K ++ Today High Speed Connections – Cable, ADSL, Satellite. – All are available at • 5 Mb (2005) • 30 Mb (2009) Ch. 1: Introduction 75/71

Coming soon (1999) Ch. 1: Introduction 76/71

Today (2005) Ch. 1: Introduction 77/71

Session layer (L 5 of OSI) • Not common • Provides full-duplex service, expedited data delivery, and session synchronization • Internet – doesn’t have a standard session layer Ch. 1: Introduction 78/71

Session layer (cont. ) • Duplex – if transport layer is simplex, concatenates two transport endpoints together • Expedited data delivery – allows some messages to skip ahead in end-system queues, by using a separate low-delay transport layer endpoint • Synchronization – allows users to place marks in data stream and to roll back to a prespecified mark Ch. 1: Introduction 79/71

Presentation layer (OSI L 6) • Usually ad hoc • Touches the application data (Unlike other layers which deal with headers) • Hides data representation differences between applications – characters (ASCII, unicode, EBCDIC. ) • Can also encrypt data • Internet – no standard presentation layer – only defines network byte order for 2 - and 4 -byte integers Ch. 1: Introduction 80/71

עיקרון השכבות Destination Source Vo. IP Email(smtp) UDP TCP ftp Transport Network (IPv 4) Modem Ethernet Application Network Wi. Fi Data-Link Network Ch. 1: Introduction 81/71

עיקרון השכבות Destination Source app 1 UDP app 2 app 3 app 1 TCP UDP Network (IPv 4) Modem Ethernet app 2 app 3 TCP Network (IPv 4) Wi. Fi Modem Ethernet Network Ch. 1: Introduction 82/71 Wi. Fi

Discussion • Layers break a complex problem into smaller, simpler pieces. • Why seven layers? – Need a top and a bottom 2 – Need to hide physical link; so need datalink 3 – Need both end-to-end and hop-by-hop actions; so need at least the network and transport layers 5 Ch. 1: Introduction 83/71