Chapter 1 OverviewReviewOSI reference modelTCP/IP (or Internet) reference modelCompare OSI and TCP/IP modelsFormat of Ethernet framesFunction of various network devicesOverview of demonstration network

OSI Reference ModelApplication: supporting network applications: FTP, SMTP, HTTP, etc.Presentation: handle different data representations (e.g., encryption)Session: connections between appsTransport: host-host: TCP, UDPNetwork: routing of datagrams from source to dest: IP, routing protocolsLink: data transfer between adjacent network elements: PPP, EthernetPhysical: bits on the wire

Application (7)Presentation (6)Session (5)Transport (4)Network (3)Data link (2)Physical (1)

OSI Reference ModelEach layer provides service to the layer aboveLink layer relies on physical layerNetwork layer relies on link layer, etc.Protocol stack is conceptualLayering is supposed to simplify networkingBelieve it or not!

Example NetworkHosts live at edge of networkComputers, servers, etc.Routers are at the coreThis network hasWANConnected by satellite2 LANs2 segments on LAN 1Connected by bridge

WAN

Physical LayerBits on the wireWe dont care much about thisRepeaters and hubs are physical layer devices

WAN

Data Link Layer2 sublayers:LLC sublayerLogical Link ControlMAC sublayerMedium Access ControlCSMA/CD (Ethernet)

WAN

Data Link LayerLink layer data unit is a frameRecall that a bridge operates at the link layerBridge looks at MAC address to decide which interface(s) to forward frameBridges are self-learningBridges separate segments (separate collision domains)

Frame StructurePreamble --- allows NIC to detect frameStart Frame Delimiter (SFD) --- marks start of frameDest. Address --- hardware address of destSource Address --- hardware address of source

7 bytes 1 byte 2 or 6 bytes 2 or 6 bytes 2 bytes var variable var 2 bytes

MAC Header

MAC

Trailer

Preamble SFD Destination Source Length LLC Packet PAD FCS

Address Address

Frame StructureLength --- number of bytes in packetLogical Link Control (LLC) --- For reliable communication on the same LANPacket --- data from link layer perspectivePAD --- pad frame if less than 64 bytesFrame Check Sequence (FCS) --- error checking

7 bytes 1 byte 2 or 6 bytes 2 or 6 bytes 2 bytes var variable var 2 bytes

MAC Header

MAC

Trailer

Preamble SFD Destination Source Length LLC Packet PAD FCS

Address Address

Network LayerRoute packets over the networkFrom source to destData unit is a packet or datagramRouters live at the network layerCore of the network

WAN

Transport LayerLogical host-to-host communicationTCP is reliableUDP is unreliableNote that SNMP uses UDPWhy?Transport layer operates in hostsRouters dont care about transport layer

Session LayerHandles multiple logical processes on hostsFor example, you can browse the Web, send email, FTP and telnet simultaneouslyMust keep these processes separateIs this really worth an entire layer?

Presentation LayerCompression, encryption, encoding, etc., are the job of presentation layerAlso deals with Abstract Syntax Notation One (ASN.1) which is discussed in Appendix BBasic Encoding Rules (BER) which is covered in Appendix D Is this really worth an entire layer?

Application LayerLike an API: transparently provides access to lower layer servicesNo formal layer above, but application layer provides service to applicationsSystem Management Application Entity (SMAE) provides support to network management applicationsRemote Operations Service Element (ROSE) is contained in SMAE

Application LayerCommon Management Information Protocol (CMIP) makes use of ROSENote that well be interested in SNMPSNMP is not part of the OSI model Instead, its part of the TCP/IP modelCMIP is the analog of SNMP in the OSI model

OSI Reference Model: The Bottom LineOSI model has everything and the kitchen sinkAs a result, its complexOver-engineered?OSI model took a long time to developSo people started using a simpler model: TCP/IP (or Internet) modelTCP/IP model applies to the Internet

TCP/IP Protocol Stackapplication: includes OSI application, presentation and session layers transport: same as OSInetwork: same as OSI link: same as OSIAuthor calls this Network Interface layer with MAC implementationphysical: same as OSI

Network Interface LayerNetwork interface layer implements Network Device Interface Specification (NDIS)NDIS is interfaces between higher layers and NIC Drivers for Ethernet, FDDI, etc.NDIS allows higher layers to be independent of media access method

Network Interface Layer

TCP/IP Higher Level Protocols

NDIS

Ethernet Token FDDI

Driver Ring DriverOther Drivers

Driver

Physical

Network

Interface

Layer

OSI vs TCP/IP StacksLink layer is slightly differentOSI designed with reliable link layerOSI model has it allBut more complexTCP/IP is good enoughSimpler

Demo NetworkLAN used in book

PSTN

Probe = Remote Monitor

WS = Workstation

PSTN = Public Switched Telephone

Network

NMS = Network Management System

= network links

= management links

Router

10BASE2 Segment

WS 1

NMS

LAN

SUBNET 1

SUBNET 2

SWITCH 2

SWITCH 1

PROBE 1

SERVER

WS 2

PROBE 2

HUB

WAN

ROUTER

Demo NetworkRepeater is a physical layer deviceAmplifies the signalCan extend the length of the LANHub is a repeater with multiple I/O portsA physical layer deviceDemo network has a hub in subnet 2Bridge is a link layer deviceOnly forwards frame onto appropriate link(s)Transparent since self-learning

Demo NetworkA switch is a fancy bridgeSo a switch is a layer 2 deviceIn switched Ethernet, can have simultaneous comm. between hosts on LAN without collisionsLayer 3 and layer 4 switches existThese switches can use info in higher layersEg, layer 4 switch could give retransmission priority to packets that require high QoS

Demo NetworkRoutersLayer 3 devicesLike bridges/switches, routers isolate collision domainsRouters also isolate broadcast domainsRouting tables use IP addressFor small network, static table is OKFor larger network, use RIP, OSPF, etc.Routers route between networks, not hosts

Demo NetworkProbesGather information and pass it to management stationManagement station analyzes the network trafficProbe also called a remote monitorIn demo network, Probe 1 can monitor 2 segments simultaneously

Chapter 1 SummaryOverview of networkingOSI reference modelTCP/IP protocol stackNetworking devicesDemo networkBook includes lots of pictures of hardware