ITI-510

ComputerNetworks ITI 510 – Computer Networks

Meeting 1

Rutgers University Internet InstituteInstructor: Chris Uriarte

(2002 UPS01)

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ComputerNetworks

Welcome…

ITI-510 Computer NetworksInstructor: Chris Uriarte (chris@cju.com)

Six Meetings, 18 Hours total

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About The Course…

• For Who?– Anyone who has an interest in, or wants to explore,

computer networks.

• Pre-requisites:– Basic knowledge of computers. User-level UNIX

and/or PC administration experience can help, but is not required.

– Use of Internet technologies like web and email.

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What We Explore…

• The concepts and theories behind computer networking.

• Network architectures• Network protocols and “packet-level” analysis• How network protocols and applications are used

in the “real world”• Introduction to network hardware components• Basic concepts in network troubleshooting and

support• Trends in computer networks today

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Specific Topics…

• OSI Model, network layers, Internet Protocol, Transmission Control Protocol, Link Layer Protocols, the Internet.

• ARP, RAPR, ICMP, IP Routing, CIDR, networking utilities, Routing algorithms and Protocols like RIPv1/2, OSPF, BGP, etc.

• TCP, UDP, TCP/IP and packet delivery • Application layers: DNS, FTP, HTTP, NNTP, SMTP,

SNMP• Multicast technologies and tools, MBONE• Security, MAC Protocols, Advanced topics• Operating system specifics, Microsoft NetBIOS• Overview of networking hardware

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How we learn…

• Lectures• Slides• Internet Resources• Book: Networking Explained, 2ed

Gallo and Hancock• Website:

Class notes and slides: http://www.cju.com/classes/

• Hands on exercises• By asking lots of questions….

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Instructor

• Christopher Uriarte– Email: chrisjur@cju.com or chrisjur@internet.

rutgers.edu– Office: 732-847-6249

• Call or email anytime

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Agenda: Meeting 1

• Introduction to networks• Network devices – high level overview• LAN vs. WAN• OSI• Packet Overview• IP – The Internet Protocol

• A few small Exercises

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What is a computer network?

• Formal Definition: Computer Network– A series of points or nodes interconnected

by communication paths. Networks can interconnect with other networks and contain subnetworks.

• Simple Definition: Computer Network– Connecting computers and/or devices in such a

way that they can interact with each other.

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Characterizing Networks

• Sometimes characterized by Topology– i.e. bus, star or ring network

• …by Spatial Distance– Wide Area Network, Local Area Network

• …by Type of Data Transmission or what it carries– IP Network, Voice Network, Data Network

• …by Type of Physical Link– Fiber Optic Network, Ethernet Network,

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Network Topology

• In the context of communication networks, a topology pictorially describes the configuration or arrangement of a network, including its nodes and connecting lines.

• Three general network topologies:– Bus– Star– Ring

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Bus Networks

• A bus network is a network topology in which all devices are directly attached to a line and all signals pass through each of the devices. Each device has a unique identity and can recognize those signals intended for it.

• “Single String” of network wire• Antiquated technologies such as 10Base2

are considered a bus network.

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Examples of Bus Networks

• A single wire or a group of small wires is used to create one data path that all traffic flows through.– 2 simple examples:

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Bus Networks: Advantages and Disadvantages

• Disadvantages:– If one single point in the network is severed,

hosts may experience connectivity loss– Possible bandwidth constraints

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Ring Networks

• Each device is attached along the same signal path to two other devices, forming a path in the shape of a ring.

• Each device in the ring has a unique address.

• Information usually flows in one direction and there is usually a controlling device that intercepts and manages the flow to and from the ring.

• Popular ring network technologies are Token Ring and FDDI

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Examples of Ring Networks

• Simple Example of a Ring Network:

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Ring Networks: Advantages and Disadvantages

• Advantages:– If a single point of the physical cable is

detached, traffic can begin to flow in an the opposite direction – no loss of connectivity.

• Disadvantages:– Possible bandwidth constraints – one single pipe

for all traffic– In most cases, every computer sees every bit of

traffic across the ring

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Star Topology

• Each device has a unique path to a central point that distributes data

• Each device “hangs” off of a piece of hardware, such as a hub or a switch

• Very popular today: Traditional 10BaseT, 100BaseT Ethernet networks use this topology.

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Example of Star Networks

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Advantages of Star Networks

• A single cable break will usually only disrupt service for a single host within a local network segment.

• Newer technologies allow you to dedicated and guarantee high bandwidth rates for each individual host or network hanging off of a central switch.

• The ability to eliminate packet broadcasts – every computers does not have to see every packet on the network.

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Where Networks are Going…

• 10 Years ago: The 80/20 Rule– 80% of all traffic stays on the LOCAL network

and only 20% of traffic is sent off to other networks or to the network “backbone”

– Typically describes the “workgroup” model of computing: access devices on your local network like file servers, printers, other workstations.

• Today: The 20/80 Rule– 20% or all traffic stays on the LOCAL network

and 80% of traffic is sent to other networks or the network backbone.

– Cause by the Increased use of WAN technologies and distributed computing models.

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LAN vs. WAN

• Local Area Networks (LANs): a group of computers and associated devices that share a common communications line and typically share the resources of a single processor or server within a small geographic area like an office building. Usually privately-owned.

• Wide Area Networks (WANs): a geographically dispersed network. It may be privately owned or rented, but the term usually connotes the inclusion of public (shared user) networks like the Internet or the PSTN (Public Switched Telephone Network)

• We may use different network technologies, protocols, hardware, etc. to connect devices within a WAN than we use when connecting devices in a LAN.

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The OSI Model

• OSI (Open Systems Interconnection) is a standard description or "reference model" for how messages should be transmitted between any two points in a telecommunication network.

• Its purpose is to guide product implementers so that their products will consistently work with other products.

• Developed by representatives of major computer and telecommunication companies in 1983 – now a standard way of examining computer network technologies.

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The OSI 7 Layer Model

• The general OSI model contains 7 layers (layers 1-7 respectively):– Physical– Data Link – Network – Transport– Session– Presentation– Application

• Each layer has a specific function

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Depiction of the 7 Layer Model

• Layer 7 (Application) is a “high layer”• Layer 1 (Physical is a low layer)

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATALINK

PHYSICAL

Netscape, Outlook, FTP Programs, Internet Explorer

HTTP, POP, SMTP

Application ports 25 (SMTP), 23 (Telnet) etc.

TCP, UDP

IP

Cables, ASDL, POTS, CAT5, FDDI, etc.

SLIP, PPP, Ethernet

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TCP/IP 5 Layer Model

• TCP/IP, a very popular protocol used in LANs, WANs and the Internet, usually groups the 7-layer model’s Application, Presentation and Session layers into one “Application” layer, resulting in a 5 layer model.

APPLICATION

TRANSPORT

NETWORK

DATALINK

PHYSICAL

Web Services, Email Services, News Services, etc.

TCP, UDP

IP

Cables, ASDL, POTS, CAT5, FDDI, etc.

SLIP, PPP, Ethernet

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Description of the 5 Main OSI Layers (5 Layer Model)

• Layer 5: The application layer...This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (e.g. Funcationality provided by web browsers, mail reader programs and their associated protocols like HTTP and SMTP)

• Layer 4: The transport layer...This layer manages the end-to-end control and error-checking of network traffic. It checks to see if all packets have arrived and ensures complete data transfer between parties. (e.g. TCP and UDP protcols)

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OSI Layers, con’t.

• Layer 3: The network layer...This layer handles the routing of outgoing data (making sure that a packet is sent to the right place) and also handles incoming data. (e.g. IP)

• Layer 2: The data-link layer... This layer defines the rules for sending and receiving data across the physical connection between two systems. (e.g. Ethernet, PPP, SLIP)

• Layer 1: The physical layer... This layer governs hardware connections and byte preparation for transmissions. It is the only layer that involves a physical transfer of information between network nodes. It’s usually responsible for translating electrical impulses into 1s and 0s.

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Sending and Receiving Data

• Layers only interact with other layers directly above and below them.

• When data is sent, it originates at the application layer and moves “down” the OSI layers until it is transmitted to another host.

• When data arrives, it originates at the physical layer and moves up the OSI model until it’s received by the application layer.

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Typical Flow From Layer to Layer: Sending Data

APPLICATION

TRANSPORT

NETWORK

DATALINK

PHYSICAL

You use MS Outlook to send an email to your friend, friend@hotmail.com. The Email “packets” are sent to the Transport Layer

The transport layer takes the email and packages it in a format that ensures it will be completely delivered. The Network layer makes sure the email knows how to get to the hotmail.com server

The physical layer creates the necessary electrical impulses and trasmits the data over the physical medium.

The DLL converts the information from the layers above into 1s and 0s that can be understood by a “peer” on the other end of the phone line or network connection (e.g. your ISP’s modem?)

Email Sent

Move from Top to Bottom

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Typical Flow from Layer to Layer: Receiving Data

APPLICATION

TRANSPORT

NETWORK

DATALINK

PHYSICAL

Your email server receives the full email from the Transport layer and you use a client program (Outlook, Eudora) to read it.

The transport ensures that all the pieces of the email have arrived. When it has, it’s passed to the application layer.

The Network layer verifies where the email originated from (e.g. What IP address?)

The physical layer decodes the electrical impulses it receives into 1s and 0s

The DLL converts 1s and 0s received from the physical layer and passes them onto the network layer

Email Arrives

Move from Bottom to Top

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OSI Example Diagram

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OSI Points

• The OSI model allows hardware and software manufacturers to keep a limited scope when developing and manufacturing

• A vendor only has to create a product that can function within its specific layer and interact with only the layers directly above and below– For example, a manufacturer of network cards need

only know how to operate within the Physical layer and how to pass data to the Data Link layer – the network card does not need to know anything about the network, transport or application layers.

– Example 2: If you are writing a web browser (Application layer), you only need to know how to interact with the Transport layer (usually referred to as the TCP Stack within an operating system)

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Introduction to the Internet

• The Internet is a global network that is comprised of smaller networks owned by commercial entities, educational institutions, government agencies, etc.

• No one “owns” the Internet.• Traffic is carried through the Internet using

a hardware (physical layer) and communication links (data link layer).

• Host-to-host communication is accomplished using TCP/IP or UDP/IP – the combination of the TCP or UDP transmission layer protocols and the IP (Internet Protocol) network-layer protocol.

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IP – The Internet Protocol

• In an IP network, individual hosts are distinguished by a unique address, known as an “IP address”

• An IP address is comprised of four Octals (8-bit numbers), separated by a decimal point, e.g.:– 126.14.34.18

• Each decimal number (126, 14, 34, 18, etc.) has a BINARY equivalent that is used many network equations.– 128.14.34.18 = 10000000.00001110.

00100010. 00010010

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IP Networks

• Internet service providers (ISPs) are assigned blocks of IP addresses, which they are free to use on their Internal networks.

• ISPs form “peering agreements” with other service providers so they have a pathway other other provider's networks.

• ISP networks are connected through hardware devices knows as routers, which are responsible for directing traffic to and from other networks.

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IP Networks - Example

Rutgers Network

128.6.*.* IP Block

UUNET

Network

63.*.*.* IP Block

SPRINT

Network

24.*.*.* IP Block

Rutgers obtains connectivity to the Internet from UUNET, their Internet Service Provider

UUNET peers with Sprint, which gives UUNET access to Sprint-connected networks and Sprint access to UUNET networks.

= Router

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IP Addresses: The Numbers Behind the Name

• The “common” internet hostnames we use everyday (www.yahoo.com, iti.rutgers.edu, etc.) all have corresponding IP addresses behind them.

• Routers move packets and messages from network to network based on IP address – not based on hostname.

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Class Exercise: Introduction to Binary Numbers…

• 1’s and 0’s = on and off• Question: What is:

1+ 1------- ???

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Bits and Bytes

• As you probably know, computers internally represent data in bits.

• While you and I may see count the number of fingers our hands as “10” a computer may represent that number as “1010”.

• The number of fingers on our hands = “10” only when we’ve agreed on a common way to represent numbers.

• People = represent numbers in decimal format

• Computers = represent numbers in binary format

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Binary Numbers

• Our base 10 number system, the decimal system, is based on ten numbers: 0 to 9.

• Computers work in a digital environment that has only two variables: 0 and 1. All numbers in the decimal system may be translated into 0's and 1's of the binary system

• For example, the number “9” is equivalent to “1001” in binary.

• Binary numbers are very important in computer networks, as the form the basis for the contents of network packets and serve as the backbone to addressing schemes in popular network protocols, such as TCP/IP.

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How Binary Counting Works

• Binary numbers are read from right to left, where each number represents a power of 2, starting with 20.

• As you move to the left of the number, each place represents one greater power of 2.

• For example, in the binary number “10”, the “0” ‘slot’ represents 20, while the “1” slot represents the 21

• The total value of the binary number is the sum of the 2x slots that are equal to “1”.

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Binary Counting Recipe

• Identify all slots with the number “1” and associated the correct 2x value with the slot.

• Compute the 2x value for each slot with “1”.

• Sum the results of the 2x values• The resulting sum is the value of your

binary number