CompTIA Network+ Certification Study Guide part 27 ppsx

Other network/transport protocols, such as NetBIOS Extended User Interface NetBEUI, do not have a sophisti-cated addressing scheme nor the programming intelligence of high OSI model laye

TCP/IP protocols on the Internet and the IPX addresses used by the IPX/SPX protocols on NetWare networks are examples of logical addresses These

protocol stacks are referred to as routable because they include

address-ing schemes that identify both the network or subnet and the particular client on that network or subnet Other network/transport protocols, such

as NetBIOS Extended User Interface (NetBEUI), do not have a sophisti-cated addressing scheme (nor the programming intelligence of high OSI model layers such as network and transport layers), thus crippling it and not allowing it to be routed across different networks To make sure you understand what is meant by this, view Figure 6.7 Here, you see a network subdivided by different IP subnets (this will be covered in greater depth in Chapter 7)

You can see that each local area network (LAN) is connected to each other via a WAN, using Frame Relay (both of which will be covered in depth

in Chapter 7) The most critical fact here is that all of this logical address-ing and routaddress-ing are done at the network layer of the OSI model Each sub-net must be unique, and each LAN will need to know how to get to the other LANs That’s where the WAN and the routers come in, acting as the default gateway for your network Also, you need to understand that logical addressing (such as the 10.1.1.1 255.255.255.0 address being assigned to the router on the LAN as the default gateway) is important; it defines how and where the packets are sent and so on So, now that you have assigned the IP address, how does the MAC address tie in? Well, a TCP/IP protocol

called Address Resolution Protocol (ARP) will help map an IP address to a

physical machine address

The network layer is also responsible for creating a virtual circuit (a logical connection, not a physical connection) between points or nodes

A node is a device that has a MAC address, which typically includes

Note

To understand the difference between physical and logical addresses, consider this analogy: if you buy a house, it has a physical address that identifies exactly where it is located on the earth, at a specific latitude and longitude This never changes (unless you have a mobile home that can be moved from one plot of land to another) This is like the MAC address on a NIC Your house also has a logical address assigned to it by the post office, consisting of a street number and street name The city can (and occasionally does) change the names of streets or renumber the houses located on them This is like the IP address assigned to a network interface.

computers, printers, and routers This layer is also responsible for routing,

Layer 3 switching (which is nothing more than a Layer 2 switch with a

Layer 3 router built into it) and the forwarding of packets

Routing refers to forwarding packets from one network or subnet to

another Without routing, computers can communicate only with other

computers that are on the same network via ARP broadcasts Routing makes

it possible for computers to send data through many networks to other

computers that are on the other side of the world Routing is the key to the

FIGurE 6.7 TCP/IP Networks Subdivided and Connected via Routers.

global Internet and is one of the most important duties of the network layer Easy to remember, routing is simple to understand If you start with a LAN that has the 10.1.1.0 255.255.255.0 network and you wanted to get to the 10.1.2.0 255.255.255.0 network (which has a different network number in the third octet), you would need a router with a routing table (so it knows where to send the packet) to get it there

Finally, the network layer provides additional levels of flow control and error control As mentioned earlier, from this point on, the primary methods

of implementing the OSI model architecture involve software rather than hardware

Devices that operate at this layer include, most prominently, routers and Layer 3 switches

layer 4: Transport

Layer 4 is the transport layer As the name implies, it is responsible for

transporting the data from one node to another It provides transparent data transfer between nodes and manages the end-to-end flow control, error detection, and error recovery

The transport layer protocols initiate contact between host computers and set up a virtual circuit The transport protocols on each host computer verify that the application sending the data is authorized to access the network and that both ends are ready to initiate the data transfer When this synchronization is complete, the data can be sent As the data is being transmitted, the transport protocol on each host monitors the data flow and watches for transport errors If transport errors are detected, the transport protocol can provide error recovery

The functions performed by the transport layer are very important to network communication Just as the data link layer provides lower-level reliability and connection-oriented or connectionless communications, the transport layer does the same thing at a higher level In fact, the two protocols most commonly associated with the transport layer are defined by their connection state: TCP, which is connection-oriented, whereas UDP, which is connectionless

What else does the transport layer do? It handles another aspect of logical addressing: ports If you think of a computer’s IP address as analogous to the street address of a building, you can think of a port as a suite number

or apartment number within that building It further defines exactly where the data should go

A computer might have several network applications running at the same time: a Web browser sending a request to a Web server for a Web page,

an e-mail client sending and receiving mail, and a file transfer program

uploading or downloading information to and from an FTP server There

must be some mechanism to determine which incoming data packets

belong to which application, and that’s the function of port numbers The

FTP protocol used by that program is assigned a particular port, whereas

the Web browser and e-mail clients use different protocols (HTTP and Post

Office Protocol [POP3] or Internet Message Access Protocol [IMAP]) that

have their own assigned ports Thus the information that is intended for

the Web browser doesn’t go to the e-mail program by mistake Port numbers

are used by the transport layer protocols (TCP and UDP)

Finally, the transport layer deals with name resolution Because human

beings prefer to identify computers by names instead of IP addresses (after

all, it’s easier to remember “www.microsoft.com” for Microsoft’s Web

server than “207.46.249.222,” for example), but computers know only

how to interpret numbers (and binary numbers, at that), there must be a

way for names to be matched with numerical addresses so that people and

computers don’t drive one another crazy Name resolution methods such

as the domain name system (DNS) solve this problem, and they generally

operate at the upper layers of the OSI model

hEAd oF ThE ClASS…

different Switches for different layers

Troubleshooting network problems requires that you

understand which protocols and devices operate at

which layers of the networking model It’s important

to understand that all switches are not created equal

There are actually several different types of devices

that are called switches and they operate at different

layers of the OSI model.

Layer 2 switches are sometimes called standard

switches They operate at the data link layer and

func-tion like sophisticated hubs When a computer sends

data to a hub, the hub sends it back out on all ports, to

all the connected computers A switch sends the data

only out the port to which the destination computer

(based on the addressing information in the headers)

is attached This decreases the amount of unnecessary

traffic on the network and also increases security.

Layer 3 switches operate at the network layer and are really a specialized type of router They’re

sometimes called switched routers Layer 3 switches

use the information in the packet headers to apply policies, in addition to performing normal routing functions.

Layer 4 switches operate at the transport layer (in addition to the lower layers) and can use the port

number information from TCP or User Datagram

Protocol (UDP) headers They can provide access

control lists (ACLs) to filter traffic for better security and are able to control bandwidth allocation for load balancing purposes Some routers also function as Layer 4 switches These devices can help to identify application layer (Layer 7) protocols, such as capable Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), and so on.

layer 5: Session

After the transport layer has established the virtual connection, a communi-cation session can be established A communicommuni-cation session occurs between

two processes on two different computers The session layer is responsible

for establishing, monitoring, and terminating sessions, using the virtual circuits established by the transport layer

The session layer is also responsible for putting header information into data packets to indicate where the message begins and ends Once header information is attached to the data packets, the session layer performs synchronization between the sender’s session layer and the receiver’s session layer The use of acknowledgement (ACK) messages helps coordinate transfer of data at the session layer

A very important function of the session layer is controlling whether the communications within a session are sent as full-duplex or half-duplex messages Half-duplex communication goes in both directions between the communicating computers, but information can travel in only one direction at a time (as with walkie-talkie radio communications, in which you have to hold down the microphone button to transmit and cannot hear the person on the other end when you do) With full-duplex communica-tion, information can be sent in both directions at the same time (as in a regular telephone conversation, in which both parties can talk and hear one another at the same time)

hEAd oF ThE ClASS…

Connection-oriented versus Connectionless

What’s the difference between a connection-oriented

and a connectionless protocol? A connection-oriented

protocol such as TCP creates a connection between

the two computers before actually sending the data and

then verifies that the data has reached their destination

by using acknowledgements (messages sent back to

the sending computer from the receiving computer that

acknowledge receipt) Connectionless protocols send the

data and trust that it will reach the proper destination.

Consider an analogy: you need to send a very

impor-tant letter to a business associate, containing valuable

papers that must not get lost along the way You call

him before mailing the letter, to let him know he should expect it (establishing the connection) You might even insure it or send it via certified mail After a few days have passed, your friend calls you back to let you know that he did receive the letter or you get back the return receipt that you requested (acknowledgement) This is the way a connection-oriented communication works It’s different from mailing a relatively unimportant item, such as a postcard to a friend when you’re on vacation

In that case, you just drop it in the mailbox and hope

it gets to the addressee You don’t expect or require any acknowledgement This is like a connectionless communication.

Although the transport layer establishes a connection between two

machines, the session layer establishes a connection between two processes

A process is a defined task related to an application An application may run

many processes simultaneously to accomplish the work of the application

These processes are small executable files that together do the work

required by the application You can view the processes running on your

Windows-based computer by pressing CTLALTDEL, selecting Task

Manager, and then clicking the Processes tab You’ll notice you have far

more processes running than applications since each application typically

runs more than one process at a time

The session layer, then, is responsible for setting up the connection

between an application process on one computer and an application process

on another computer, after the transport layer has established the connection

between the two machines

There are many important protocols that operate at the session layer,

including Windows Sockets (the Winsock interface) and NetBIOS (the

Network Basic Input/Output interface)

layer 6: presentation

Data translation is the primary activity of Layer 6, the presentation

layer When data is sent from sender to receiver, the data is translated at

the presentation layer The sender’s application passes data down to the

presentation layer, where it is put into a common format When the data

is received on the other end, the presentation layer changes the data from

the common format back into a format that is useable by the application

Protocol translation, the conversion of data from one protocol to another so

that it can be exchanged between computers that use different platforms or

operating systems, takes place here

This is the layer at which many gateway services operate Gateways

are connection points between networks that use different platforms or

Note

Earlier in this chapter, we mentioned multiplexing Computer communications can be in

half-duplex or full-duplex mode Simplex, or unidirectional (one-way) communication,

generally, is not used in computer networking It is the type of communication used for

radio and over-the-air TV broadcasts (many cable television [CATV]) transmissions now

use two-way signaling to allow for interactive TV).

applications Examples include e-mail gateways (which allow for com-munications between two different e-mail programs using a common protocol such as Simple Mail Transfer Protocol [SMTP]), Systems Network Architecture (SNA) gateways (which allow PCs to communicate with mainframe computers), and gateways that cross platforms or file systems (for example, allowing Microsoft clients that use the Server Message Block (SMB) protocol for file sharing to access files on NetWare servers that use NetWare Core Protocol) Gateways are usually implemented via software, such as the Gateway Services for NetWare (GSNW) Software redirectors also operate at this layer

This layer is also where data compression can take place, to minimize the actual number of bits that must be transmitted on the network media to the receiver Data encryption and decryption take place in the presentation layer as well

layer 7: Application

The application layer is the point at which the user application program

interacts with the network This layer of the OSI model should not be confused with the application itself This is very important to understand and remember, as they share the same name Application processes, such as file transfers or e-mail, are initiated within a user application (for example,

an e-mail program) Then the data created by that process are handed to the application layer of the networking software Everything that occurs at this level is application-specific File sharing, remote printer access, network monitoring and management, Remote Procedure Calls (RPCs), and all forms

of electronic messaging occur at this level

Both FTP (a common way of transferring files across a network) and

Telnet function within the application layer, as do SMTP, POP3, and

IMAP4, all of which are used for sending or receiving e-mail There are many other application layer protocols, including HTTP, Network News

Test day Tip

Although it’s important to understand the details of the OSI model for the exam, you’re

likely to run into a limited number of questions related to the specific layers of the model Understanding the basic functions of each layer will help you easily identify correct answers to the questions you may see on the exam It is especially important to remember that, when troubleshooting, you should start with Layer 1 (physical) and work your way up A common error among technicians and network administrators is starting

to troubleshoot at Layer 7 Greater detail about troubleshooting with the OSI model can

be found in Chapter 11, “Network Troubleshooting Tools.”

Transfer Protocol (NNTP), and Simple Network Management Protocol

(SNMP)

Be sure to distinguish between the protocols mentioned and applications

that may bear the same names There are many different FTP programs

made by different software vendors, but all of them use the FTP protocol to

transfer files

Encapsulation of Data

One last item to cover before we move on to new material is that you should

make sure you understand what encapsulation is and how it works Notice

that each layer in Figure 6.8 adds a header to the data packet so that by the

time it reaches the physical layer (the last one on the bottom), it is much

longer than when it started at the application layer When data is received by

the receiving host, the headers are stripped off as the data moves back up the

stack, one layer at a time, by the layer that corresponds to the one that added

it This means that each layer on the sending computer communicates only

with the layer of the same name on the receiving machine

The Microsoft Model

Prior to the release of Windows NT 3.1, users who wanted to connect to

a network had to obtain the TCP/IP protocol suite from a third party and

install it TCP/IP did not come bundled with the software At times, the

TCP/IP software that was purchased didn’t work well with the operating

system (OS) because it handled various tasks of network communication in

a slightly different way than did the operating system This sometimes led to

intermittent network problems or time-spent troubleshooting TCP/IP and

operating system interoperability

With the release of Windows

NT 3.1, TCP/IP was built into the operating system, providing a seamless integration of network-ing functionality in the OS Since that time, it has become standard

to provide TCP/IP with the operating system because many computers today connect to a net-work in one form or another

The Microsoft model as

seen in Figure 6.9 provides a standard platform for application developers

FIGurE 6.8 Data Moving through the OSI Layers.

This modular design enables the developer to rely upon the underlying services of the OS through the use of standard interfaces (Sound familiar to the discussion we had earlier on the DoD and OSI models?) These interfaces provide specific functionality developers can use as building blocks to develop

an application This makes development time shorter and provides common interfaces for users, making learning and using new applications easier

FIGurE 6.9 The Microsoft Model.

Though the Microsoft model is used primarily by programmers, it’s

important to understand the framework we explore, of how TCP/IP works

on a Microsoft Windows-based computer

Understanding the Function of Boundary Layers

The Microsoft model describes software and hardware components and

the connections between them that facilitate computer networking This

modular approach both allows and encourages hardware and software

vendors to develop products that work together through the Microsoft

operating system Boundary layers are interfaces that reside at the

boundar-ies of functionality They interact with the layer below and the layer above,

providing an interface from one layer to the next

Within each layer, various components perform the tasks defined at

the layer A variety of components can provide similar functionality at any

given layer This modular approach provides flexibility for developers while

providing common interfaces that reduce development time and cost

A vendor can provide new functionality at any of these layers, knowing their

products will integrate with the other layers to provide seamless network

com-munications The interfaces defined by Microsoft are the Network Driver

Inter-face Specification (NDIS), Transport Driver InterInter-face (TDI), and the application

program interface (API) Figure 6.9 shows the relationship of these boundary

layers to both the OSI model and to the Microsoft Architecture

The Windows OS is divided into three primary areas: the User, the

Executive, and the Kernel The Kernel is the core of the Microsoft operating

system architecture and it manages the most basic operations including

interacting with the hardware abstraction layer that interacts with the

hardware (CPU, memory, etc.) The Kernel also synchronizes activities with

the Executive level, which includes the Input/Output (I/O) Manager and the

Process Manager The User level interacts with the Executive level; this is

the level at which most applications and user interfaces reside

the NDIs boundary Layer

The NDIS works at the bottom of the networking architecture and maps to

the data link layer of the OSI model and the Network Interface layer of the

DARPA model The NDIS layer is the boundary between the physical network

(physical layer of the OSI model) and the higher-level transport protocols This

layer provides the standardized functions that allow various transport protocols

to use any network device driver that is compatible with the specifications of

this layer, providing both flexibility and reliability to developers The earliest

versions of NDIS were developed by a Microsoft and 3Com joint effort Later,

NDIS versions are proprietary to Microsoft operating systems