THE OSI MODEL Application Presentation Session Transport Network

THE OSI MODEL Application Presentation Session Transport Network Data-Link Physical Semesters 1 & 2 Concept Review Chapter 1—Review By: Allan Johnson

Table of Contents Go There! • Review the OSI Model Go There! • LAN Devices & Technologies Go There! • IP Addressing Go There! • CIDR Notation Go There! • Routing Go There! • Transport Layer

Arhitectura generala a Internetului

Arhitectura Internet - 2

Componente Internet

Protocol de retea - definitie • un protocol defineşte formatul şi ordinea mesajelor schimbate între două sau mai multe entităţi ce comunică între ele, precum şi acţiunile ce sunt întreprinse odată cu transmiterea sau recepţia unui mesaj sau a unui alt eveniment.

THE OSI MODEL Application Presentation Review The Model Session Transport Network Data-Link Physical Open Systems Interconnected Reference Model Table of Contents

Why A Layered Model? Application Presentation Session Transport Network Data-Link Physical • Reduces complexity • Standardizes interfaces • Facilitates modular engineering • Ensures interoperable technology • Accelerates evolution • Simplifies teaching & learning

Application Layer Application Presentation Session Transport Network Data-Link Physical § Provides network services (processes) to applications. § For example, a computer on a LAN can save files to a server using a network redirector supplied by NOSs like Novell. § Network redirectors allow applications like Word and Excel to “see” the network.

Presentation Layer Application Presentation Session Transport Network Data-Link Physical § Provides data representation and code formatting. § Code formatting includes compression and encryption § Basically, the presentation layer is responsible for representing data so that the source and destination can communicate at the application layer.

Session Layer Application Presentation Session Transport Network Data-Link Physical § Provides inter-host communication by establishing, maintaining, and terminating sessions. § Session uses dialog control and dialog separation to manage the session § Some Session protocols: ü ü ü NFS (Network File System) SQL (Structured Query Language) RCP (Remote Call Procedure) ASP (Apple. Talk Session Protocol) SCP (Session Control Protocol) X-window

Transport Layer Application Presentation Session Transport Network Data-Link Physical § Provides reliability, flow control, and error correction through the use of TCP. § TCP segments the data, adding a header with control information for sequencing and acknowledging packets received. § The segment header also includes source and destination ports for upper-layer applications § TCP is connection-oriented and uses windowing. § UDP is connectionless. UDP does not acknowledge the receipt of packets.

Network Layer Application Presentation Session Transport Network Data-Link Physical § Responsible for logically addressing the packet and path determination. § Addressing is done through routed protocols such as IP, IPX, Apple. Talk, and DECnet. § Path Selection is done by using routing protocols such as RIP, IGRP, EIGRP, OSPF, and BGP. § Routers operate at the Network Layer

Data-Link Layer Application Presentation Session Transport Network Data-Link Physical § Provides access to the media § Handles error notification, network topology issues, and physically addressing the frame. § Media Access Control through either. . . ü Deterministic—token passing ü Non-deterministic—broadcast topology (collision domains) § Important concept: CSMA/CD

Physical Layer Application Presentation Session Transport Network Data-Link Physical § Provides electrical, mechanical, procedural and functional means for activating and maintaining links between systems. § Includes the medium through which bits flow. Media can be. . . ü ü CAT 5 cable Coaxial cable Fiber Optics cable The atmosphere

Peer-to-Peer Communications • Peers communicate using the PDU of their layer. For example, the network layers of the source and destination are peers and use packets to communicate with each other. Application Data Application Presentation Data Presentation Session Transport Data Segments Transport Network Packets Network Data-Link Frames Data-Link Physical Bits Physical

Encapsulation Example Application Presentation Session Transport Network Data-Link Physical • You type an email message. SMTP takes the data and passes it to the Presentation Layer. • Presentation codes the data as ASCII. • Session establishes a connection with the destination for the purpose of transporting the data.

Encapsulation Example Application Presentation Session Transport Network Data-Link Physical • Transport segments the data using TCP and hands it to the Network Layer for addressing • Network addresses the packet using IP. • Data-Link then encaps. the packet in a frame and addresses it for local delivery (MACs) • The Physical layer sends the bits down the wire.

THE OSI MODEL Application Presentation Session Transport Network Data-Link LAN Devices & Technologies The Data-Link & Physical Layers Physical Table of Contents

Devices What layer device? • What does it do? § Connects LAN segments; § Filters traffic based on MAC addresses; and § Separates collision domains based upon MAC addresses.

Devices • What does it do? What layer device? § Since it is a multi-port bridge, it can also ü Connect LAN segments; ü Filter traffic based on MAC addresses; and ü Separate collision domains § However, switches also offer full-duplex, dedicated bandwidth to segments or desktops.

Devices What layer device? • What does it do? § Concentrates LAN connections from multiple devices into one location § Repeats the signal (a hub is a multi-port repeater)

Devices • What does it do? What layer device? § Interconnects networks and provides broadcast control § Determines the path using a routing protocol or static route § Re-encapsulates the packet in the appropriate frame format and switches it out the interface § Uses logical addressing (i. e. IP addresses) to determine the path

Media Types

LAN Technologies Three Most Common Used Today in Networking

Ethernet/802. 3 • Cable Specifications: § 10 Base 2 ü Called Thinnet; uses coax ü Max. distance = 185 meters (almost 200) § 10 Base 5 ü Called Thicknet; uses coax ü Max. distance = 500 meters § 10 Base. T ü Uses Twisted-pair ü Max. distance = 100 meters § 10 means 10 Mbps

Ethernet/802. 3 • Ethernet is broadcast topology. § What does that mean? ü Every devices on the Ethernet segment sees every frame. ü Frames are addressed with source and destination ______ addresses. ü When a source does not know the destination or wants to communicate with every device, it encapsulates the frame with a broadcast MAC address: FFFF § What is the main network traffic problem caused by Ethernet broadcast topologies?

Ethernet/802. 3 • Ethernet topologies are also shared media. • That means media access is controlled on a “first come, first serve” basis. • This results in collisions between the data of two simultaneously transmitting devices. • Collisions are resolved using what method?

Ethernet/802. 3 • CSMA/CD (Carrier Sense Multiple Access with Collision Detection) • Describe how CSMA/CD works: § A node needing to transmit listens for activity on the media. If there is none, it transmits. § The node continues to listen. A collision is detected by a spike in voltage (a bit can only be a 0 or a 1 --it cannot be a 2) § The node generates a jam signal to tell all devices to stop transmitting for a random amount of time (back-off algorithm). § When media is clear of any transmissions, the node can attempt to retransmit.

Address Resolution Protocol • In broadcast topologies, we need a way to resolve unknown destination MAC addresses. • ARP is protocol where the sending device sends out a broadcast ARP request which says, “What’s you MAC address? ” • If the destination exists on the same LAN segment as the source, then the destination replies with its MAC address. • However, if the destination and source are separated by a router, the router will not forward the broadcast (an important function of routers). Instead the router replies with its own MAC address.

THE OSI MODEL Application Presentation IP Addressing Session Transport Network Subnetting Review Data-Link Physical Table of Contents

Logical Addressing • At the network layer, we use logical, hierarchical addressing. • With Internet Protocol (IP), this address is a 32 -bit addressing scheme divided into four octets. • Do you remember the classes 1 st octet’s value? § § § Class Class A: 1 - 126 B: 128 - 191 C: 192 - 223 D: 224 - 239 (multicasting) E: 240 - 255 (experimental)

Network vs. Host Class A: 27 = 126 networks; 224 > 16 million hosts N Class B : H H 214 = 16, 384 networks; 216 > 65, 534 hosts N Class C : H N H H 221 > 2 million networks; 28 = 254 hosts N N N H

Why Subnet? • Remember: we are usually dealing with a broadcast topology. • Can you imagine what the network traffic overhead would be like on a network with 254 hosts trying to discover each others MAC addresses? • Subnetting allows us to segment LANs into logical broadcast domains called subnets, thereby improving network performance.

Four Subnetting Steps • To correctly subnet a given network address into subnet addresses, ask yourself the following questions: 1. How many bits do I need to borrow? 2. What’s the subnet mask? 3. What’s the “magic number” or multiplier? 4. What are the first three subnetwork addresses? • Let’s look at each of these questions in detail

1. How many bits to borrow? • First, you need to know how many bits you have to work with. • Second, you must know either how many subnets you need or how many hosts per subnet you need. • Finally, you need to figure out the number of bits to borrow.

1. How many bits to borrow? • How many bits do I have to work with? § Depends on the class of your network address. ü Class C: 8 host bits ü Class B: 16 host bits ü Class A: 24 host bits § Remember: you must borrow at least 2 bits for subnets and leave at least 2 bits for host addresses. § 2 bits borrowed allows 22 - 2 = 2 subnets

1. How many bits to borrow? • How many subnets or hosts do I need? • A simple formula: § Total Bits = Bits Borrowed + Bits Left § TB = BB + BL • I need x subnets: • I need x hosts: • Remember: we need to subtract two to provide for the subnetwork and broadcast addresses.

1. How many bits to borrow? • Class C Example: 210. 93. 45. 0 • Design goals specify at least 5 subnets so how many bits do we borrow? • How many bits in the host portion do we have to work with (TB)? • What’s the BB in our TB = BB + BL formula? (8 = BB + BL) • 2 to the what power will give us at least 5 subnets? 3 2 - 2 = 6 subnets

1. How many bits to borrow? • How many bits are left for hosts? TB = BB + BL 8 = 3 + BL BL = 5 • So how many hosts can we assign to each subnet? 5 2 - 2 = 30 hosts

1. How many bits to borrow? • Class B Example: 185. 75. 0. 0 • Design goals specify no more than 126 hosts per subnet, so how many bits do we need to leave (BL)? • How many bits in the host portion do we have to work with (TB)? • What’s the BL in our TB = BB + BL formula? (16 = BB + BL) • 2 to the what power will insure no more than 126 hosts per subnet and give us the most subnets? 7 2 - 2 = 126 hosts

1. How many bits to borrow? • How many bits are left for subnets? TB = BB + BL 16 = BB + 7 BL = 9 • So how many subnets can we have? 9 2 - 2 = 510 subnets

2. What’s the subnet mask? • We determine the subnet mask by adding up the decimal value of the bits we borrowed. • In the previous Class C example, we borrowed 3 bits. Below is the host octet showing the bits we borrowed and their decimal values. 1 128 64 32 16 8 4 2 1 We add up the decimal value of these bits and get 224. That’s the last non-zero octet of our subnet mask. So our subnet mask is 255. 224

3. What’s the “magic number? ” • To find the “magic number” or the multiplier we will use to determine the subnetwork addresses, we subtract the last non-zero octet from 256. • In our Class C example, our subnet mask was 255. 224 is our last non-zero octet. • Our magic number is 256 - 224 = 32

Last Non-Zero Octet • Memorize this table. You should be able to: § Quickly calculate the last non-zero octet when given the number of bits borrowed. § Determine the number of bits borrowed given the last non-zero octet. § Determine the amount of bits left over for hosts and the number of host addresses available.

4. What are the subnets? • We now take our “magic number” and use it as a multiplier. • Our Class C address was 210. 93. 45. 0. • We borrowed bits in the fourth octet, so that’s where our multiplier occurs § 1 st subnet: 210. 93. 45. 32 § 2 nd subnet: 210. 93. 45. 64 § 3 rd subnet: 210. 93. 45. 96 • We keep adding 32 in the fourth octet to get all six available subnet addresses.

Host & Broadcast Addresses • Now you can see why we subtract 2 when determining the number of host address. • Let’s look at our 1 st subnet: 210. 93. 45. 32 • What is the total range of addresses up to our next subnet, 210. 93. 45. 64? • 210. 93. 45. 32 to 210. 93. 45. 63 or 32 addresses • . 32 cannot be assigned to a host. Why? • . 63 cannot be assigned to a host. Why? • So our host addresses are. 33 -. 62 or 30 host addresses--just like we figured out earlier.

THE OSI MODEL Application Presentation CIDR Notation Session Transport Network Data-Link A Different Way to Represent a Subnet Mask Physical Table of Contents

CIDR Notation • Classless Interdomain Routing is a method of representing an IP address and its subnet mask with a prefix. • For example: 192. 168. 50. 0/27 • What do you think the 27 tells you? § 27 is the number of 1 bits in the subnet mask. Therefore, 255. 224 § Also, you know 192 is a Class C, so we borrowed 3 bits!! § Finally, you know the magic number is 256 - 224 = 32, so the first useable subnet address is 197. 168. 50. 32!! • Let’s see the power of CIDR notation.

202. 151. 37. 0/26 • Subnet mask? § 255. 192 • Bits borrowed? § Class C so 2 bits borrowed • Magic Number? § 256 - 192 = 64 • First useable subnet address? § 202. 151. 37. 64 • Third useable subnet address? § 64 + 64 = 192, so 202. 151. 37. 192

198. 53. 67. 0/30 • Subnet mask? § 255. 252 • Bits borrowed? § Class C so 6 bits borrowed • Magic Number? § 256 - 252 = 4 • Third useable subnet address? § 4 + 4 = 12, so 198. 53. 67. 12 • Second subnet’s broadcast address? § 4 + 4 - 1 = 11, so 198. 53. 67. 11

200. 39. 89. 0/28 • What kind of address is 200. 39. 89. 0? § § Class C, so 4 bits borrowed Last non-zero octet is 240 Magic number is 256 - 240 = 16 32 is a multiple of 16 so 200. 39. 89. 32 is a subnet address--the second subnet address!! • What’s the broadcast address of 200. 39. 89. 32? § 32 + 16 -1 = 47, so 200. 39. 89. 47

194. 53. 45. 0/29 • What kind of address is 194. 53. 45. 26? § § § Class C, so 5 bits borrowed Last non-zero octet is 248 Magic number is 256 - 248 = 8 Subnets are. 8, . 16, . 24, . 32, ect. So 194. 53. 45. 26 belongs to the third subnet address (194. 53. 45. 24) and is a host address. • What broadcast address would this host use to communicate with other devices on the same subnet? § It belongs to. 24 and the next is. 32, so 1 less is. 31 (194. 53. 45. 31)

No Worksheet Needed! • After some practice, you should never need a subnetting worksheet again. • The only information you need is the IP address and the CIDR notation. • For example, the address 221. 39. 50/26 • You can quickly determine that the first subnet address is 221. 39. 50. 64. How? § Class C, 2 bits borrowed § 256 - 192 = 64, so 221. 39. 50. 64 • For the rest of the addresses, just do multiples of 64 (. 64, . 128, . 192).

The Key!! • MEMORIZE THIS TABLE!!!

Practice On Your Own • Below are some practice problems. Take out a sheet of paper and calculate. . . § § § 1. 2. 3. 4. 5. 6. 7. Bits borrowed Last non-zero octet Second subnet address and broadcast address 192. 168. 15. 0/26 220. 75. 32. 0/30 200. 39. 79. 0/29 195. 50. 120. 0/27 202. 139. 67. 0/28 Challenge: 132. 59. 0. 0/19 Challenge: 64. 0. 0. 0/16 Answers

THE OSI MODEL Application Presentation Routing Basics Session Transport Network Data-Link Path Determination & Packet Switching Physical Table of Contents

A Router’s Functions • A router is responsible for determining the packet’s path and switching the packet out the correct port. • A router does this in five steps: 1. De-encapsulates the packet 2. Performs the ANDing operation 3. Looks for entry in routing table 4. Re-encapsulates packet into a frame 5. Switches the packet out the correct interface

Routed v. Routing Protocols • What is a routed protocol? § Routed protocols are protocols that enable data to be transmitted across a collection of networks or internetworks using a hierarchical addressing scheme. § Examples include IP, IPX and Apple. Talk. § A routable protocol provides both a network and node number to each device on the network. Routers AND the address to discover the network portion of the address. § An example of a protocol that is not routable is Net. BEUI because it does not have a network/node structure.

Routed v. Routing Protocols • What is a routing protocol? § A routing protocol is a protocol that determines the path a routed protocol will follow to its destination. § Routers use routing protocols to create a map of the network. These maps allow path determination and packet switching. Maps become part of the router’s routing table. § Examples of routing protocols include: RIP, IGRP, EIGRP, & OSPF

Multi-protocol Routing • Routers are capable of running multiple routing protocols (RIP, IGRP, OSPF, etc. ) as well as running multiple routed protocols (IP, IPX, Apple. Talk). • For a router to be able use different routing and routing protocols, you must enable the protocols using the appropriate commands.

Dynamic v. Static Routing • Dynamic routing refers to the process of allowing the router to determine the path to the destination. • Routing protocols enable dynamic routing where multiple paths to the same destination exist.

Dynamic v. Static Routing • Static routing means that the network administrator directly assigns the path router are to take to the destination. • Static routing is most often used with stub networks where only one path exists to the destination.

Default Routes • A default route is usually to a border or gateway router that all routers on a network can send packets to if they do not know the route for a particular network.

Routing Protocol Classes • Routing protocols can be divided into three classes: § Distance–vector: determines the route based on the direction (vector) and distance to the destination § Link-state: opens the shortest path first to the destination by recreating an exact topology of the network in its routing table § Hybrid: combines aspects of both

Convergence • Convergence means that all routers share the same information about the network. In other words, each router knows its neighbor routers routing table • Every time there is a topology change, routing protocols update the routers until the network is said to have converged again. • The time of convergence varies depending upon the routing protocol being used.

Distance-vector Routing • Each router receives a routing table periodically from its directly connected neighboring routers. • For example, in the graphic, Router B receives information from Router A. Router B adds a distance-vector number (such as a number of hops), and then passes this new routing table to its other neighbor, Router C.

Link-state Routing • Link-state protocols maintain complex databases that summarize routes to the entire network. • Each time a new route is added or a route goes down, each router receives a message and then recalculates a spanning tree algorithm and updates its topology database.

Comparing the Two DISTANCE-VECTOR LINK-STATE Views network topology from neighbor’s perspective Gets common view of entire network topology Adds distance vectors from router to router Calculates the shortest path to other routers Frequent, periodic updates: slow convergence Event triggered updates: fast convergence Passes copies of routing tables Passes link-state routing updates to neighbors to all routers in the system.

Hybrid Routing • Cisco’s proprietary routing protocol, EIGRP, is considered a hybrid. • EIGRP uses distance-vector metrics. However, it uses event-triggered topology changes instead of periodic passing of routing tables.

THE OSI MODEL Application Presentation Transport Layer Session Transport Network A Quick Review Data-Link Physical Table of Contents

Transport Layer Functions • Synchronization of the connection § Three-way handshake • Flow Control § “Slow down, you’re overloading my memory buffer!!” • Reliability & Error Recovery § Windowing: “How much data can I send before getting an acknowledgement? ” § Retransmission of lost or unacknowledged segments

Transport’s Two Protocols • TCP § Transmission Control Protocol § Connection-oriented § Acknowledgment & Retransmission of segments § Windowing § Applications: ü Email ü File Transfer ü E-Commerce • UDP § User Datagram Protocol § Connectionless § No Acknowledgements § Applications: ü ü Routing Protocols Streaming Audio Gaming Video Conferencing