2.1 Chapter 2 Network Models Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2.1

Chapter 2

Network Models

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2.2

2-1 LAYERED TASKS2-1 LAYERED TASKS

We use the concept of We use the concept of layerslayers in our daily life. As an in our daily life. As an example, let us consider two friends who communicate example, let us consider two friends who communicate through postal mail. The process of sending a letter to a through postal mail. The process of sending a letter to a friend would be complex if there were no services friend would be complex if there were no services available from the post office. available from the post office.

Sender, Receiver, and CarrierHierarchy

Topics discussed in this section:Topics discussed in this section:

2.3

Figure 2.1 Tasks involved in sending a letter

2.4

2-2 THE OSI MODEL2-2 THE OSI MODEL

Established in 1947, the International Standards Organization (Established in 1947, the International Standards Organization (ISOISO) is a ) is a multinational body dedicated to worldwide agreement on international multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (communications is the Open Systems Interconnection (OSIOSI) model. It was ) model. It was first introduced in the late 1970s. first introduced in the late 1970s.

an ISO (International Standard Organization) standard that covers all aspects of network communications

•An open system is a model that allows any two different systems to communicate regardless of their underlying architecture•Purpose of OSI model is to open communication between different systems without requiring changes to the logic of the underlying hardware and software

•a reference model for understanding and designing a network architecture that is flexible, robust, interoperable

Layered ArchitecturePeer-to-Peer ProcessesEncapsulation

2.5

ISO is the organization.OSI is the model.

Note

2.6

Figure 2.2 Seven layers of the OSI model

•A layered framework that allows for communication across all types of computers•Consists of seven separate but related layers – defining a segment of process of moving information across network

2.7

Each layer defines a family of functions (or services) distinct from those of the other layers an architecture that is modular,

comprehensive, flexible The OSI model allows complete

transparency between otherwise incompatible systems

2.8

Figure 2.3 The interaction between layers in the OSI model

2.9

Each layer communicates with the peer layer by means of a protocol an agreed-upon series of rules and conventions

Communication between machines is peer-to-peer process using protocols at any given layer

Each layer adds information to the data – Headers are added to the data at layers 6, 5, 4, 3 and 2. Trailers are usually added at layer 2

Each layer calls upon of the services of the layer below it by means of an interface Interface defines what information and services a layer

must provide for the layer above it As long as a layer provides expected services, specific

functions can be modified and replaced without requiring changes to other layers

2.10

Figure 2.4 An exchange using the OSI model

Encapsulation

data will be encapsulated with headers and trailers by the senders

headers and trailers will be stripped off by the receiver leaving the data intact

2.11

2-3 LAYERS IN THE OSI MODEL2-3 LAYERS IN THE OSI MODEL

In this section we briefly describe the functions of each In this section we briefly describe the functions of each layer in the OSI model.layer in the OSI model.

Physical LayerData Link LayerNetwork LayerTransport LayerSession LayerPresentation LayerApplication Layer

Topics discussed in this section:Topics discussed in this section:

2.12

Figure 2.5 Physical layer

Major duties of the physical layer:

• Physical characteristics of interfaces and media.

• Representation of bits 0 – encode into signals (electrical or optical) and how 0s and 1s are changed into signals.

• Data rate – the transmission rate: the number of bits sent each second.

• synchronization of bits – sender and receiver must use the same bit rate (their clock must be synchronized)

2.13

The physical layer is responsible for movements ofindividual bits from one hop (node) to the next.

Note

2.14

Figure 2.6 Data link layer

•Framing – divides the stream of bits received from the network layer into data units called frames.

•Physical addressing – define a sender and receiver.

•Flow control – imposed a mechanism to prevent overwhelming the receiver.

•Error control

•Access control

2.15

The data link layer is responsible for moving frames from one hop (node) to the next.

Note

2.16

Figure 2.7 Hop-to-hop delivery

2.17

Figure 2.8 Network layer

Responsible for : source-to-destination delivery across multiple networks.

Needs for delivering a packet to different networks with connecting devices between the networks. (Local delivery Vs global delivery)

Major duties:

•Logical addressing – adds a header to the packet coming from the upper layer (logical addresses of the sender and receiver)

•Routing – works at the connecting devices (routers)

2.18

The network layer is responsible for the delivery of individual packets from

the source host to the destination host.

Note

2.19

Figure 2.9 Source-to-destination delivery

2.20

Figure 2.10 Transport layer

Responsible for: process-to-process delivery of the entire message.

•Service point addressing– include a port address in the header (forward the packet to the correct process).

•Segmentation and reassembly – Sender; message is divided into transmittable segments, each segment containing a sequence number. Destination; reassemble the message based on seq. number (identify and replace packet that were lost in transmission).

•Connection control - Connection-oriented or connectionless.

•Flow control – performed end-to-end

•Error control – entire message arrives without error. Error correction achieved through retransmission.

2.21

The transport layer is responsible for the delivery of a message from one process to another.

Note

2.22

Figure 2.11 Reliable process-to-process delivery of a message

2.23

Figure 2.12 Session layer

Session layer is the network dialog controller Session Layer Responsibilities:

Dialog control – establishes, maintains, terminates dialog between communicating systems. Communication between two process can be either half-duplex or full-duplex

Synchronization – allows a process to add checkpoints (synchronization points) into a stream of data (for efficient retransmission if necessary).

2.24

The session layer is responsible for dialog control and synchronization.

Note

2.25

Figure 2.13 Presentation layer

Presentation Layer – concerns with the syntax and semantics of the information exchange

2.26

The presentation layer is responsible for translation, compression, and encryption.

Presentation Layer Responsibilities: Translation – information in the form of character

strings, numbers, etc need to be encoded to bit streams before being transmitted; presentation layer is responsible for interoperability between different encoding systems; possible different sender-dependent format and receiver-dependent format need to be encoded and decoded.

Encryption – encryption and decryption may be necessary for sensitive information.

Compression – compression and decompression if required will reduce the number of bits transmitted. Important in transmission of multimedia such as text, audio and video

2.27

Figure 2.14 Application layer

provides interfaces and support to various applications, e-mails, remote file access and transfer, shared data base management, etc. Example:X.500 (directory services), X.400 (message handling), FTAM (file transfer access and management)

2.28

The application layer is responsible for providing services to the user.

Application Layer (cont) Network virtual terminal – allows user to log on

to a remote host via terminal emulation software

File transfer, access and management (FTAM) – allows user to access (read, make changes), retrieve, send, manage files on a remote computer

Mail services – e-mail forwarding and storage

Directory services – provides distributed database source and access for global information about various objects and services

2.29

Figure 2.15 Summary of layers

2.30

2-4 TCP/IP PROTOCOL SUITE2-4 TCP/IP PROTOCOL SUITEThe layers in the The layers in the TCP/IP protocol suiteTCP/IP protocol suite do not exactly match do not exactly match those in the OSI model. The original TCP/IP protocol suite was those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: defined as having four layers: host-to-networkhost-to-network, , internetinternet, , transporttransport, and , and applicationapplication. .

However, when TCP/IP is compared to OSI, we can say that the However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: TCP/IP protocol suite is made of five layers: physicalphysical, , data linkdata link, , networknetwork, , transporttransport, and , and applicationapplication

At transport layer, TCP/IP defines two protocol – TCP and UDP. At network layer, the main protocol defined by TCP/IP is IP.

Physical and Data Link LayersNetwork LayerTransport LayerApplication Layer

Topics discussed in this section:Topics discussed in this section:

2.31

Figure 2.16 TCP/IP and OSI model

2.32

2-5 ADDRESSING2-5 ADDRESSING

Four levels of addresses are used in an internet employing Four levels of addresses are used in an internet employing the TCP/IP protocols: the TCP/IP protocols: physicalphysical, , logicallogical, , portport, and , and specificspecific..

Physical AddressesLogical AddressesPort AddressesSpecific Addresses

Topics discussed in this section:Topics discussed in this section:

2.33

Figure 2.17 Addresses in TCP/IP

Address of node defined by LAN or WAN

IP Address for universal communications32 bit IP address that is unique

Labels assign to processes

16 bit

Examples: EmailURL

2.34

Figure 2.18 Relationship of layers and addresses in TCP/IP

2.35

In Figure 2.19 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.

Example 2.1

2.36

Figure 2.19 Physical addresses

2.37

As we will see in Chapter 13, most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:

Example 2.2

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.

2.38

Figure 2.20 shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection.

Example 2.3

2.39

Figure 2.20 IP addresses

2.40

Figure 2.21 shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination.

Example 2.4

2.41

Figure 2.21 Port addresses

2.42

The physical addresses will change from hop to hop,but the logical addresses usually remain the same.

Note

2.43

Example 2.5

As we will see in Chapter 23, a port address is a 16-bit address represented by one decimal number as shown.

753

A 16-bit port address represented as one single number.

2.44

The physical addresses change from hop to hop,but the logical and port addresses usually remain the same.

Note

Extra: The Client-Server Model

Client and server processes are considered to be in the Application layer.

the device requesting the information is called a client the device responding to the request is called a server. Application layer protocols describe the format of the requests and responses between clients and servers.

One example of a client/server network is a corporate environment where employees use a company e-mail server to send, receive and store e-mail.

The e-mail client on an employee computer issues a request to the e-mail server for any unread mail. The server responds by sending the requested e-mail to the client.

Data is typically flowing from the server to the client, some data always flows from the client to the server.

For example, a client may transfer a file to the server for storage purposes (upload). Data from a server to a client as a download.

Extra: Servers

In a general networking context, any device that responds to requests from client applications is functioning as a server.

A server is usually a computer that contains information to be shared with many client systems. For example, web pages, documents, databases, pictures, video, and audio files can all be stored on a server and delivered to requesting clients. In other cases, such as a network printer, the print server delivers the client print requests to the specified printer.Some servers may require authentication of user account information to verify if the user has permission to access the requested data or to use a particular operation.

if you request to upload data to the FTP server, you may have permission to write to your individual folder but not to read other files on the site.

Extra: Servers

In a client/server network, the server runs a service, or process, sometimes called a server daemon.

Like most services, daemons typically run in the background and are not under an end user's direct control. Daemons are described as "listening" for a request from a client, because they are programmed to respond whenever the server receives a request for the service provided by the daemon. When a daemon "hears" a request from a client, it exchanges appropriate messages with the client, as required by its protocol, and proceeds to send the requested data to the client in the proper format.

Extra: The Peer-to-Peer Model In addition to the client/server model for networking,

there is also a peer-to-peer model. Peer-to-peer networking involves two distinct forms: peer-to-peer network design and peer-to-peer applications (P2P).

Peer-to-Peer NetworksIn a peer-to-peer network, two or more computers are connected via a network and can share resources (such as printers and files) without having a dedicated server. Every connected end device (known as a peer) can function as either a server or a client.

One computer might assume the role of server for one transaction while simultaneously serving as a client for another.

A simple home network with two computers sharing a printer is an example of a peer-to-peer network.

Each person can set his or her computer to share files, enable networked games, or share an Internet connection.

Because peer-to-peer networks usually do not use centralized user accounts, permissions, or monitors

it is difficult to enforce security

Extra: Peer-to-Peer Applications Peer-to-Peer (P2P) Applications

A P2P application, allows a device to act as both a client and a server within the same communication. However, peer-to-peer applications require that each end device provide a user interface and run a background service.

When you launch a specific P2P application it invokes the required user interface and background services.

Some P2P applications use a hybrid system where resource sharing is decentralized but the indexes that point to resource locations are stored in a centralized directory.

In a hybrid system, each peer accesses an index server to get the location of a resource stored on another peer. The index server can also help connect two peers, but once connected, the communication takes place between the two peers without additional communication to the index server.

Peer-to-peer applications can be used on peer-to-peer networks, client/server networks, and across the Internet.

Extra: Application Layer Protocols

The widely-known Application layer protocols are those that provide the exchange of information.

Among these TCP/IP protocols are:Domain Name Service Protocol (DNS) is used to resolve Internet names to IP addresses.Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the Web pages of the World Wide Web.Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and attachments.Telnet, a terminal emulation protocol, is used to provide remote access to servers and networking devices.File Transfer Protocol (FTP) is used for interactive file transfer between systems.

The protocols in the TCP/IP suite are generally defined by Requests for Comments (RFCs).

The Internet Engineering Task Force maintains the RFCs as the standards for the TCP/IP suite.

Extra: Services and Protocol: Port Number

As we will see later in this course, the Transport layer uses an addressing scheme called a port number.

Port numbers identify applications and Application layer services that are the source and destination of data. Server programs generally use predefined port numbers that are commonly known by clients. As we examine the different TCP/IP Application layer protocols and services, we will be referring to the TCP and UDP port numbers associated with these services.

Some of these services are:Domain Name System (DNS) - TCP/UDP Port 53Hypertext Transfer Protocol (HTTP) - TCP Port 80Simple Mail Transfer Protocol (SMTP) - TCP Port 25Post Office Protocol (POP) - UDP Port 110Telnet - TCP Port 23Dynamic Host Configuration Protocol - UDP Port 67File Transfer Protocol (FTP) - TCP Ports 20 and 21

Extra: DNS In data networks, devices are labeled with

numeric IP addresses, so that they can participate in sending and receiving messages over the network.

However, most people have a hard time remembering this numeric address. Hence, domain names were created to convert the numeric address into a simple, recognizable name.

On the Internet these domain names, such as www.cisco.com, are much easier for people to remember than 198.133.219.25, which is the actual numeric address for this server.

Also, if Cisco decides to change the numeric address, it is transparent to the user, since the domain name will remain www.cisco.com. The new address will simply be linked to the existing domain name and connectivity is maintained.

The DNS was created for domain name to address resolution for these networks.

DNS uses a distributed set of servers to resolve the names associated with these numbered addresses.

Extra: DNS Services and Protocol

DNS is a client/server service; It differs from the other client/server services that we are examining. While other services use a client that is an application (such as web browser), the DNS client runs as a service itself.

The DNS client, sometimes called the DNS resolver, supports name resolution for our other network applications and other services that need it.

Computer operating systems also have a utility called nslookup that allows the user to manually query the name servers to resolve a given host name.

This utility can also be used to troubleshoot name resolution issues and to verify the current status of the name servers. In the first query in the figure, a query is made for www.cisco.com. The responding name server provides the address of 198.133.219.25.

Extra: WWW Service and HTTP When a web address (or URL) is typed into a

web browser, the web browser establishes a connection to the web service running on the server using the HTTP protocol.

The http://www.cisco.com/index.html example http (the protocol or scheme) www.cisco.com (the server name) A web page named index.html on a server.

The browser then checks with a name server to convert www.cisco.com into a numeric address, which it uses to connect to the server. Using the HTTP protocol, the browser sends a GET request to the server asks for file index.html. The server in turn sends the HTML code for this web page to the browser. Finally, the browser deciphers the HTML code and formats the page for the browser window.

Other types of data, may require another service or program, typically referred to as plug-ins

Extra: WWW Service and HTTP HTTP is not a secure protocol.

The POST messages upload information to the server in plain text that can be intercepted and read. Similarly, the server responses, typically HTML pages, are also unencrypted.

For secure communication across the Internet, the HTTP Secure (HTTPS) protocol is used for accessing or posting web server information.

HTTPS can use authentication and encryption to secure data as it travels between the client and server. HTTPS specifies additional rules for passing data between the Application layer and the Transport Layer.

Extra: E-mail Services and SMTP/POP3

User composes an e-mail using an application called a mail user agent (MUA) or e-mail client

Client sends e-mails to a server using Simple Mail Transfer Protocol (SMTP) and receives e-mails using Post Office Protocol version 3 (POP3)

• SMTP uses TCP port 25

• POP uses UDP port 110

Extra: File Transfer Protocol (FTP) The FTP is a Application layer protocol.

FTP was developed to allow for file transfers between a client and a server. An FTP client is an application that runs on a computer that is used to push and pull files from a FTP server.

The client can download (pull) file from server

or, the client can upload (push) file to server.

To transfer files, FTP requires two connections between client and server:

The client establishes the first connection to the server on TCP port 21.

It consists of client commands and server replies.

The client establishes the second connection to the server over TCP port 20.

This connection is for the actual file transfer and is created every time there is a file transferred.

Extra: Dynamic Host Configuration Protocol (DHCP) The DHCP service enables devices on a

network to obtain IP addresses and other information from a DHCP server.

This service automates the assignment of IP addresses, subnet masks, gateway and other IP networking parameters.

When the DHCP server is contacted and an address requested.

The DHCP server chooses an address from a configured range of addresses called a pool and assigns ("leases") it to the host for a set period.If the host is powered down or taken off the network, the address is returned to the pool for reuse. This is especially helpful with mobile users that come and go on a network.

Extra: P2P Service and Gnutella Protocol Sharing files over the Internet has become

extremely popular. With P2P applications based on the Gnutella protocol, people can make files on their hard disks available to others for downloading.

pronounced /nʊˈtɛlə/ with a silent g, Gnutella-compatible client software allows users to connect to Gnutella services over the Internet and to locate and access resources shared by other Gnutella peers. Many client applications are available for accessing the Gnutella network, including: BearShare, Gnucleus, LimeWire, Morpheus, WinMX and XoloX (see a screen capture of LimeWire in the figure).

Extra: P2P Service and Gnutella Protocol Many P2P applications do not use a central

database to record all the files available on the peers.

Instead, the devices on the network each tell the other what files are available when queried and use the Gnutella protocol and services to support locating resources.

When a user is connected to a Gnutella service, the client applications will search for other Gnutella nodes to connect to.

These nodes handle queries for resource locations and replies to those requests. They also govern control messages, which help the service discover other nodes. The actual file transfers usually rely on HTTP services.

The Gnutella protocol defines five different packet types:

ping - for device discovery pong - as a reply to a pingquery - for file locationquery hit - as a reply to a query push - as a download request

Ch 3 - 61

Extra: Telnet

Telnet uses TCP port 23 Provides a method of emulating text-based terminals over

the network allows a local device to access a remote device as if the

keyboard and monitor are connected to the remote device directly

A connection using Telnet is called a virtual terminal (VTY) session

• The Telnet server runs a service called the Telnet daemon