Introduction to Networks

1-1 Introduction to Networks

1-1 Introduction to Networks

Before you learn Cisco Internet working, it is important to understand what a network is and the importance of networks themselves. Simply put, a network is a collection of interconnected devices (such as computers, printers, etc.). To understand the importance of networks, let us look at how things worked before networks were created. For this, consider a large multinational company that sells food products in a time when networks did not exist.

Let us call this company ABC Inc. Imagine the amount of information such as sales, inventory, etc. required by the management of the company to make everyday decisions. To get this information they will need to call their local offices. Their local offices will need to mail (postal!) or fax printed reports or even send media (floppies!) though the postal service. By the time the mail is received, the data is already days old. Even if reports are faxed, it will be a cumbersome task to consolidate all reports. This task also increases chance of human error since large numbers of reports are manually collated. This is just one part of the equation. You also need to consider the information required by the local offices. They also need various data from the head office and other offices around the world.

Now consider the same company, but in the present time with all their offices interconnected. They would use a single application around the world that takes advantage of their global network. The data from all offices would be instantly stored at the central location and with a single click, the management team can see data from around the world in any format they like. This data would also be real-time. This means that they see it as its happening. Since the data is centralized, any office location can see data pertaining to any location.

As you can see, the cost, time and effort involved in transferring data was much higher without networks. So networks decrease cost, time, and effort and thereby increase productivity. They also help in resource optimization by helping to share resources. A simple example of resource sharing is a printer in a typical office. Without networks, each computer would require a dedicated printer. However with a network, the printer can be shared between many different computers.

Now that you know how beneficial networks are, its time to look at how networks work. Figure 1-1 shows the most basic form of a network. This figure shows two hosts (end-user devices such as computers are commonly called hosts in networking terms) directly connected to each other using a networking cable. Today every host has aNetwork Interface Card (NIC) that is used to connect it to a network.

Figure 1-1 Most basic form of Network

 

One end of the network cable connects to the NIC on a host and the other connects to the network. In this case, the cable directly connects to another host. At this stage do not worry about network cables and how the hosts communicate across the network. This will be covered in detail later in the chapter. At this stage it is important to understand how hosts connect to a network.

In Figure 1-1, the hosts are “networked” and can share information. This network is effective, but not scalable. If you have more than 2 hosts to this “network”, it will not work without a separate NIC card for each connection and that is not scalable or realistic. For more than 2 hosts to be networked, you require a network device such as a hub. Figure 1-2 shows three hosts connected to a hub.

Figure 1-2 Network with a Hub

A hub is a network device that repeats information received from a host to all other connects hosts. In Figure 1-2 the hub will relay any information received from HostA to HostB and HostC. This means that all the three hosts can communicate with each other. Communication between hosts can be classified into three types:

• Unicast – Communication from one host to another host only.
• Broadcast – Communication from one host to all the hosts in the network.
• Multicast – Communication from one host to few hosts only.

When a hub is used to network hosts, there are two problems that arise:

1. A hub repeats information received from one host to all the other hosts. To understand this, consider HostA in Figure 1-2 sending a unicast message to HostB. When the hub receives this message; it will relay the message to both HostB and HostC. Even though the message was a unicast intended only for HostB, HostC also receives it. It is up to HostC to read the message and discard it after seeing that the message was not intended for it.
2. A hub creates a shared network medium where only a single host can send packets at a time. If another host attempts to send packets at the same time, a collision will occur. Then each device will need to resend their packets and hope not to have a collision again. This shared network medium is called a single collision domain. Imagine the impact of having a single collision domain where 50 or 100 hosts are connected to hubs that are interconnected and they are all trying to send data. That is just a recipe for many collisions and an inefficient network.

The problems associated with hubs can cause severe degradation of a network. To overcome these, switches are used instead of hubs. Like hubs, switches are used to connect hosts in a network but switches break up collision domain by providing a single collision domain for every port. This means that every host (one host connects to one port on the switch) gets its own collision domain thereby eliminating the collisions in the network. With switches, each host can transmit data anytime. Switches simply “switch” the data from one port to another in the switched network. Also, unlike hubs, switches do not flood every packet out all ports. They switch a unicast packet to the port where the destination host resides. They only flood out a broadcast packet. Figure 1-3 shows a switched network.

Figure 1-3 A switched network

  

Remember that each host in Figure 1-3 is in its own collision domain and if HostA sends a packet to HostC, HostB will not receive it.

Figure 1-4 and 1-5 show two networks. See if you can figure out how many collision domains exist in them.

Figure 1-4 Collision Domains – 1

 

Figure 1-5 Collision Domains – 2

  

If you answered 5 for Figure 1-4, then you are absolutely correct since each port of the Switches represent a single collision domain. If you answered more than 5 then you need to remember that a hub does not break collision domains. Similarly, Figure 1-5 has 7 collision domains.

Now that you know how a switch works and improves a network, consider the one problem associated with a switched network. Earlier, you learned that hubs flood out all packets, even the unicast ones. A switch does not flood out unicast packets but it does flood out a broadcast packet. All hosts connected to a switched network are said to be in the same broadcast domain. All hosts connected to it will receive any broadcast sent out in this domain. While broadcasts are useful and essential for network operations, in a large switched network too many broadcasts will slow down the network. To remedy this situation, networks are broken into smaller sizes and these separate networks are interconnected using routers. Routers do not allow broadcasts to be transmitted across different networks it interconnects and hence effectively breaks up a broadcast domain. Figure 1-6 shows three switched networks interconnected by a router. 

Figure 1-6 Router in an Internetwork

 

In the network shown in Figure 1-6, broadcasts from hosts connected to Switch1 will not reach hosts connected to Switch2 or Switch3. This is because the router will drop the broadcast on its receiving interface.

In addition to breaking up broadcast domains, routers also perform the following four essential functions in your network:

• Packet Switching – At the barest minimum, routers are like switches because they essentially switch packets between networks.
• Communication between Networks – As shown in Figure 1-6, routers allow communication between networks connected to it.
• Path Selection – Routers can talk to each other to learn about all the networks connected to various routers and then select the best path to reach a network. This is function is discussed in detail later in the book.
• Packet Filtering – Routers can drop or forward packets based on certain criteria like their source and destination. This is also discussed in detail later in the book.

 Exam Alert: Remember that switches break collision domains and routers break broadcast domains. In addition to that it is important to remember the functions of a router for your CCNA certification exam.

Now that you know what a network is and what various network devices do, its time to learn about various network types followed by networking models.

1-2 Networking Types

As you know a network is a collection of devices connected together. Networks are further classified into various types depending on their size, expanse, security, purpose and many other parameters. While covering all these classifications is beyond the scope of the CCNA exam, there are two important network classifications that you need to know about for the exam. In fact a large part of the CCNA exam revolves around these two types of networks:

•  Local Area Network (LAN) – This is a term used to describe a network covering a limited geographical area such as a floor, building or a campus. LAN usually has a high data-transfer rate. The Ethernet standard is the most commonly used technology in LANs. Ethernet is so common that it is almost synonymous with LAN today. As of late, wireless technology is also becoming increasingly common for a local LAN. Both these standards are covered in depth further in the book.
• Wide Area Network (WAN) – This is a term used to describe a network covering a large geographical area such as a multiple cities, a country or even across the world. They are used to connect LANs across the area they cover. A typical example would be the LANs at various offices of a company connected by WAN. Various technology standards used in WAN will be covered later in the book.

Internetworking Models

As the importance of computers grew, vendors recognized the need for networking them. They created various protocols whose specifications were not made public. Hence each vendor had different ways of networking computers and these ways were not compatible to each other. This means that computers of one vendor could not be networked with another vendor’s computers. Slowly these specifications were made public and some inter-vendor compatibility was created but this still represented too many complications. In 1977 the International Organization for Standardization (ISO) started working on an open standard networking model that all vendors would support to promote inter-operability. This standard was published in 1984 and was known as the Open Systems Interconnection (OSI). During the same time period (1973 to 1985) another effort by the Defense Advanced Research Projects Agency (DAPRA) was underway to create an open standard network model. This network model came to be known as the TCP/IP Model. By 1985, the TCP/IP model started gaining more prominence and support from vendors and eventually replaced the OSI model.

This section starts by discussing the OSI Reference model in some depth before moving into a deep discussion on the TCP/IP model and its protocols.

1-3 OSI Reference Model

As discussed earlier, the OSI model was created to promote communication between devices of various vendors. It also promotes communication between disparate hosts such as hosts using different operating platforms (Windows, OSX, Linux, etc.). Remember that you are very unlikely to ever work on a system that uses protocols conforming to the OSI reference model.  But it is essentially to know the model and its terminology because other models such as the TCP/IP model are often compared to the OSI reference model. Hence the discussion on this model will be limited compared to the discussion on the TCP/IP model.

The OSI reference model, like most other network models, divides the functions, protocols, and devices of a network into various layers. The layered approach provides many benefits, some of which are:

•  Communication is divided into smaller and simpler components. This makes designing, developing and troubleshooting easier.
• Since it is a layered approach, the vendors write to a common input and output specification per layer.  The guts of their products functions in between the input and output code of that layer.
• Changes in one layer do not affect other layers. Hence development in one layer is not bound by limitations of other layers. For example, wireless technologies are new but old applications run seamless over them without any changes.
• It is easier to standardize functions when they are divided into smaller parts like this.
• It allows various types of hardware and software, both new and old to communicate with each other seamlessly.

The OSI reference model has seven such layers that can be divided into two groups. The upper layers (Layers 7, 6 and 5) define how applications interact with the host interface, with each other, and the user. The lower four layers (Layers 4, 3, 2 and 1) define how data is transmitted between hosts in a network. Figure 1-7 shows the seven layers and a summary of their functions.

Figure 1-7 Seven Layers of OSI Reference Model

 

he sections below discuss each layer in detail.

 Application Layer

The Application Layer provides the interface between the software application on a system and the network. Remember that this layer does not include the application itself, but provides services that an application requires. One of the easiest ways to understand this layer’s function is to look at how a Web Browser such as Internet Explorer or Firefox works. IE or FF is the application. When it needs to fetch a webpage, it uses the HTTP protocol to send the request and receive the page contents.  This protocol resides at the application layer and can be used by an application such as IE or FF to get webpages from web servers across the network. On the other side, the web server application such as Apache or IIS interacts with the HTTP protocol on the Application layer to receive the HTTP request and send the response back.

Presentation Layer

As the name suggest, this layer presents data to the Application layer. The Presentation Layer is responsible for data translation and encoding. It will take the data from the Application layer and translate it into a generic format for transfer across the network. At the receiving end the Presentation layer takes in generically formatted data and translates into the format recognized by the Application layer. An example of this is an EBCDIC to ASCIItranslation. The OSI model has protocol standards that define how data should be formatted. This layer is also involved in data compression, decompression, encryption, and decryption.

Session Layer

In a host, different applications or even different instances of the same application might request data from across the network. It is the Sessions layer’s responsibility to keep the data from each session separate. It is responsible for setting up, managing and tearing down sessions. It also provides dialog control and coordinates communication between the systems.

Transport Layer

Where the upper layers are related to applications and data within the host, the transport layer is concerned with the actual end-to-end transfer of the data across the network. This layer establishes a logical connection between the two communicating hosts and provides reliable or unreliable data delivery and can provide flow control and error recovery. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, typical examples of Layer 4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). These protocols will be discussed in great detail later in this chapter.

Network Layer

To best understand what the Network layer does, consider what happens when you write a letter and use the postal service to send the letter. You put the letter in an envelope and write the destination address as well as your own address so that an undelivered letter can be returned back to you. In network terms, this address is called a logical address and is unique in the network. Each host has a logical address. When the post office receives this letter, it has to ascertain the best path for this letter to reach the destination. Similarly in a network, a router needs to determine the best path to a destination address. This is called path determination. Finally the post office sends the letter out the best path and it moves from post office to post office before finally being delivered to the destination address. Similarly data is moved across network mainly by routers before being finally delivered to the destination.

All these three functions – logical addressing, path determination and forwarding – are done at the Network Layer. Two types of protocols are used for these functions – routed protocols are used for logical addressing and forwarding while routing protocols are used for path determinations. There are many routed protocols and routing protocols available. Some of the common ones are discussed in great detail later the book. Routers function at this layer. Remember that routers only care about the destination network. They do not care about the destination host itself. The task of delivery to the destination host lies on the Data Link Layer.

Data Link Layer

While the Network layer deals with data moving across networks using logical addresses, Data Link layer deals with data moving within a local network using physical addresses. Each host has a logical address and a physical address. The physical address is only locally significant and is not used beyond the network boundaries (across a router). This layer also defines protocols that are used to send and receive data across the media. You will remember from earlier in the chapter that only a single host can send data at a time in a collision domain or else packets will collide and cause a host to back off for sometime. The Data Link layer determines when the media is ready for the host to send the data and also detects collisions and other errors in received data. Switches function at this layer.

Physical Layer

This layer deals with the physical transmission medium itself. It activates, maintains and deactivates the physical link between systems (host and switch for example). This is where the connectors, pin-outs, cables, electrical currents etc. are defined. Essentially this layer puts the data on the physical media as bits and receives it in the same way. Hubs work at this layer.

Data Encapsulation

In the previous sections you learned about various layers of the OSI reference model. Each layer has its distinct function and it interacts with the corresponding layer at the remote end. For example, the transport layer at the source will interact with the transport layer of the destination. For this interaction, each layer adds a header in front of the data from the previous layer. This header contains control information related to the protocol being used at that layer. This process is called encapsulation. This header and the data being sent from one layer to the next lower layer is called a Protocol Data Unit (PDU). Figure 1-8 shows how data gets encapsulated as it travels from layer 7 down to layer 1.

 Figure 1-8 Encapsulation in OSI Reference Model

 

As shown in Figure 1-8, The Application layer adds its protocol dependent header to the data and creates the Layer 7 PDU which is then passed down to the Presentation Layer. This layer then adds its header to the Layer 7 PDU to create the Layer 6 PDU and sends it down to the Session layer. This goes on till Layer 2 receives the Layer 3 PDU. Layer 2 adds a header and a trailer to the Layer 3 PDU to create the Layer 2 PDU that is then sent to Layer 1 for transmission.

At the receiving end, Layer 1 takes the data off the wire and sends it to Layer 2. Here the Layer 2 header and trailer are examined and removed. The resulting Layer 3 PDU is sent to Layer 3. Layer 3 in turn examines the header in the PDU and removes it. The resulting Layer 4 PDU is sent to Layer 4. Similarly, each layer removes the header added by the corresponding layer at the source before sending the data to the upper layer. Finally the Application layer removes the Layer 7 header and sends the data to the application. This process of examining, processing and removing the header is known as decapsulation.

Exam Alert: It is very important to remember the Layer names, their functions and the encapsulation process. You can use a common mnemonic to remember the layer names and their sequence – All PeopleSeem To Need Data Processing.  This is an important concept on your CCNA exam.

1-4 TCP/IP Model

As mentioned earlier, the OSI reference model and the TCP/IP model are two open standard networking models that are very similar.  However, the latter has found more acceptance today and the TCP/IP protocol suite is more commonly used. Just like the OSI reference model, the TCP/IP model takes a layered approach. In this section we will look at all the layers of the TCP/IP model and various protocols used in those layers.

The TCP/IP model is a condensed version of the OSI reference model consisting of the following 4 layers:

•  Application Layer
• Transport Layer
• Internet Layer
• Network Access Layer

 The functions of these four layers are comparable to the functions of the seven layers of the OSI model. Figure 1-9 shows the comparison between the layers of the two models.

The following sections discuss each of the four layers and protocols in those layers in detail.

 Figure 1-9 Comparison between TCP/IP and OSI models

 

Application Layer

 The Application Layer of the TCP/IP Model consists of various protocols that perform all the functions of the OSI model’s Application, Presentation and Session layers. This includes interaction with the application, data translation and encoding, dialogue control and communication coordination between systems.

The following are few of the most common Application Layer protocols used today:

Telnet – Telnet is a terminal emulation protocol used to access the resourses of a remote host. A host, called the Telnet server, runs a telnet server application (or daemon in Unix terms) that receives a connection from a remote host called the Telnet client. This connection is presented to the operating system of the telnet server as though it is a terminal connection connected directly (using keyboard and mouse). It is a text-based connection and usually provides access to the command line interface of the host. Remember that the application used by the client is usually named telnet also in most operating systems. You should not confuse the telnet application with the Telnet protocol.

HTTP – The Hypertext Transfer Protocol is foundation of the World Wide Web. It is used to transfer Webpages and such resources from the Web Server or HTTP server to the Web Client or the HTTP client. When you use a web browser such as Internet Explorer or Firefox, you are using a web client. It uses HTTP to transfer web pages that you request from the remote servers.

FTP – File Transfer Protocol is a protocol used for transferring files between two hosts. Just like telnet and HTTP, one host runs the FTP server application (or daemon) and is called the FTP server while the FTP client runs the FTP client application. A client connecting to the FTP server may be required to authenticate before being given access to the file structure. Once authenticated, the client can view directory listings, get and send files, and perform some other file related functions. Just like telnet, the FTP client application available in most operating systems is calledftp. So the protocol and the application should not be confused.

SMTP – Simple Mail Transfer Protocol is used to send e-mails. When you configure an email client to send e-mails you are using SMTP. The mail client acts as a SMTP client here. SMTP is also used between two mails servers to send and receive emails. However the end client does not receive emails using SMTP. The end clients use thePOP3 protocol to do that.

TFTP – Trivial File Transfer Protocol is a stripped down version of FTP. Where FTP allows a user to see a directory listing and perform some directory related functions, TFTP only allows sending and receiving of files. It is a small and fast protocol, but it does not support authentication. Because of this inherent security risk, it is not widely used.

DNS – Every host in a network has a logical address called the IP address (discussed later in the chapter). These addresses are a bunch of numbers. When you go to a website such as www.cisco.com you are actually going to a host which has an IP address, but you do not have to remember the IP Address of every WebSite you visit. This is because Domain Name Service (DNS) helps map a name such as www.cisco.com to the IP address of the host where the site resides. This obviously makes it easier to find resources on a network. When you type in the address of a website in your browser, the system first sends out a DNS query to its DNS server to resolve the name to an IP address. Once the name is resolved, a HTTP session is established with the IP Address.

DHCP – As you know, every host requires a logical address such as an IP address to communicate in a network. The host gets this logical address either by manual configuration or by a protocol such as Dynamic Host Configuration Protocol (DHCP). Using DHCP, a host can be provided with an IP address automatically. To understand the importance of DHCP, imagine having to manage 5000 hosts in a network and assigning them IP address manually! Apart from the IP address, a host needs other information such as the address of the DNS server it needs to contact to resolve names, gateways, subnet masks, etc. DHCP can be used to provide all these information along with the IP address.

Transport Layer

The protocols discussed above are few of the protocols available in the Application layer. There are many more protocols available. All of them take the user data and add a header and pass it down to the Transport layer to be sent across the network to the destination. The TCP/IP transport layer’s function is same as the OSI layer’s transport layer. It is concerned with end-to-end transportation of data and setups up a logical connection between the hosts.

Two protocols available in this layer are Transmission Control Protocol (TCP) and User Datagram Protocol  (UDP). TCP is a connection oriented and reliable protocol that uses windowing to control the flow and providesordered delivery of the data in segments. On the other hand, UDP simply transfers the data without the bells and whistles. Though these two protocols are different in many ways, they perform the same function of transferring data and they use a concept called port numbers to do this. The following sections cover port numbers before looking into TCP and UDP in detail.

Port Numbers

A host in a network may send traffic to or receive from multiple hosts at the same time. The system would have no way to know which data belongs to which application. TCP and UDP solve this problem by using port numbers in their header. Common application layer protocols have been assigned port numbers in the range of 1 to 1024. These ports are known as well-known ports. Applications implementing these protocols listen on these port numbers. TCP and UDP on the receiving host know which application to send the data to based on the port numbers received in the headers.

On the source host each TCP or UDP session is assigned a random port number above the range of 1024. So that returning traffic from the destination can be identified as belonging to the originating application. Combination of the IP address, Protocol (TCP or UDP) and the Port number forms a socket at both the receiving and sending hosts. Since each socket is unique, an application can send and receive data to and from multiple hosts. Figure 1-10 shows two hosts communicating using TCP. Notice that the hosts on the left and right are sending traffic to the host in the center and both of them are sending traffic destined to Port 80, but from different source ports. The host in the center is able to handle both the connections simultaneously because the combination of IP address, Port numbers and Protocols makes each connection different.

 Figure 1-10 Multiple Sessions using Port Numbers

 

Table 1-1 shows the transport layer protocol and port numbers used by different common application layer protocols.

 Table 1-1 Well-known Port Numbers

Application Protocol	Transport Protocol	Port Number
HTTP	TCP	80
HTTPS	TCP	443
FTP (control)	TCP	21
FTP (data)	TCP	20
SSH	TCP	22
Telnet	TCP	23
DNS	TCP, UDP	53
SMTP	TCP	25
TFTP	UDP	69

Exam Alert: It is important to remember the well-know port numbers and which application layer protocol they are assigned to as you will see this on your CCNA exam in a multiple choice question or an access-list question.

Transport Control Protocol (TCP)

TCP is one of the original protocols designed in the TCP/IP suite and hence the name of the model. When the application layer needs to send large amount of data, it sends the data down to the transport layer for TCP or UDP to transport it across the network. TCP first sets up a virtual-circuit between the source and the destination in a process called three-way handshake. Then it breaks down the data into chunks called segments, adds a header to each segment and sends them to the Internet layer.

The TCP header is 20 to 24 bytes in size and the format is shown in Figure 1-11. It is not necessary to remember all fields or their size but most of the fields are discussed below.

 Figure 1-11 TCP header

When the Application layer sends data to the transport layer, TCP sends the data across using the following sequence:

Connection Establishment – TCP uses a process called three-way handshake to establish a connection or virtual-circuit with the destination. The three-way handshake uses the SYN and ACK flags in the Code Bits section of the header. This process is necessary to initialize the sequence and acknowledgement number fields. These fields are important for TCP and will be discussed below.

 Figure 1-12 TCP three-way handshake

As shown in Figure 1-12, the source starts the three-way handshake by sending a TCP header to the destination with the SYN flag set. The destination responds back with the SYN and ACK flag sent. Notice in the figure that destination uses the received sequence number plus 1 as the Acknowledgement number. This is because it is assumed that 1 byte of data was contained in the exchange. In the final step, the source responds back with only the ACK bit set. After this, the data flow can commence.

Data Segmentation – The size of data that can be sent across in a single Internet layer PDU is limited by the protocol used in that layer. This limit is called the maximum transmission unit (MTU). The application layer may send data much larger than this limit; hence TCP has to break down the data into smaller chucks called segments. Each segment is limited to the MTU in size. Sequence numbers are used to identify each byte of data. The sequence number in each header signifies the byte number of the first byte in that segment.

Flow Control – The source starts sending data in groups of segments. The Window bit in the header determines the number of segments that can be sent at a time. This is done to avoid overwhelming the destination. At the start of the session the window in small but it increases over time. The destination host can also decrease the window to slow down the flow. Hence the window is called the sliding window. When the source has sent the number of segments allowed by the window, it cannot send any further segments till an acknowledgement is received from the destination. Figure 1-13 shows how the window increases during the session. Notice the Destination host increasing the Window from 1000 to 1100 and then to 1200 when it sends an ACK back to the source.

Figure 1-13 TCP Sliding Window and Reliable delivery

 

Reliable Delivery with Error recovery – When the destination receives the last segment in the agreed window, it has to send an acknowledgement to the source. It sets the ACK flag in the header and the acknowledgement number is set as the sequence number of the next byte expected. If the destination does not receive a segment, it does not send an acknowledgement back. This tells the source that some segments have been lost and it will retransmit the segments. Figure 1-13 shows how windowing and acknowledgement is used by TCP. Notice that when source does not receive acknowledgement for the segment with sequence number 2000, it retransmits the data. Once it receives the acknowledgement, it sends the next sequence according to the window size.

 Ordered Delivery – TCP transmits data in the order it is received from the application layer and uses sequence number to mark the order. The data may be received at the destination in the wrong order due to network conditions. Thus TCP at the destination orders the data according to the sequence number before sending it to the application layer at its end. This order delivery is part of the benefit of TCP and one of the purposes of the Sequence Number.

Connection Termination – After all data has been transferred, the source initiates a four-way handshake to close the session. To close the session, the FIN and ACK flags are used.

Exam Alert: TCP is one of the most important protocols you will learn about while preparing for the CCNA exam. Understanding how TCP works is very important and you will more than likely see an ACK question on the exam!

User Datagram Protocol (UDP)

The only thing common between TCP and UDP is that they use port numbers to transport traffic. Unlike TCP, UDP neither establishes a connection nor does it provide reliable delivery. UDP is connectionless and unreliableprotocol that delivers data without overheads associated with TCP. The UDP header contains only four parameters (Source port, Destination Port, Length and Checksum) and is 8 bytes in size.

At this stage you might think that TCP is a better protocol than UDP since it is reliable.  However you have to consider that networks now are far more stable than when these protocols where conceived. TCP has a higher overhead with a larger header and acknowledgements. The source also holds data till it receives acknowledgement. This creates a delay. Some applications, especially those that deal with voice and video, require fast transport and take care of the reliability themselves at the application layer. Hence in lot of cases UDP is a better choice than TCP.

Internet Layer

Once TCP and UDP have segmented the data and have added their headers, they send the segment down to the Network layer. The destination host may reside in a different network far from the host divided by multiple routers. It is the task of the Internet Layer to ensure that the segment is moved across the networks to the destination network.

 The Internet layer of the TCP/IP model corresponds to the Network layer of the OSI reference model in function. It provides logical addressing, path determination and forwarding.

The Internet Protocol (IP) is the most common protocol that provides these services. Also working at this layer are routing protocols which help routers learn about different networks they can reach and the Internet Control Message Protocol (ICMP) that is used to send error messages across at this layer.

Almost half of the book is dedicated IP and Routing protocols so they will be discussed in detail in later chapters, but the following sections discuss these protocols in brief.

 Internet Protocol (IP)

The Internet layer in the TCP/IP model is dominated by IP with other protocols supporting its purpose. Each host in a network and all interfaces of a router have a logical address called the IP address. All hosts in a network are grouped in a single IP address range similar to a street address with each host having a unique address from that range similar to a house or mailbox address. Each network has a different address range and routers that operate on layer 3 connect these different networks.

As IP receives segments from TCP or UDP, it adds a header with source IP address and destination IP address amongst other information. This PDU is called a packet. When a router receives a packet, it looks at the destination address in the header and forwards it towards the destination network. The packet may need to go through multiple routers before it reaches the destination network. Each router it has to go through is called a hop.

 Figure 1-14 Packet flow in internetwork

Consider the Internetwork shown in Figure 1-14 to understand the routing process better. When Host1 needs to send data to Host2, it does not get routed because the hosts are in the same network range. The Data Link layer takes care of this. Now consider Host1 sending data to Host3. Host1 will recognize that it needs to reach a host in another network and will forward the packet to Router1. Router1 checks the destination address and knows that the destination network is toward Router2 and hence forwards it to Router2. Similarly Router 2 forwards the packet to Router3. Router3 is directly connected to the destination network. Here the data link layer takes care of the delivery to the destination host. As you can see, the IP address fields in the IP header play a very important role in this process. In fact IP addresses are so important in a network that the next Chapter is entirely dedicated to it!

 Figure 1-15 IPv4 Header

 

 There are various versions of the Internet Protocol. Version 4 is the one used today and version 6 is slowly starting to replace it which is why it’s presence has increased on the CCNA Routing & Switching 200-120 exam compared to previous CCNA exam versions. Figure 1-15 shows the header structure of IPv4. The following fields make up the header:

Version – IP version number. For IPv4 this value is 4.

 Header Length – This specifies the size of the header itself. The minimum size is 20 bytes. The figure does not show the rarely used options field that is of a variable length. Most IPv4 headers are 20 bytes in length.

DS Field – The differentiated Services field is used for marking packets. Different Quality-Of-Service (QoS) levels can be applied on different markings. For example, data belonging to voice and video protocols have no tolerance for delay. The DS field is used to mark packets carrying data belonging to these protocols so that they get priority treatment through the network. On the other hand, peer-to-peer traffic is considered a major problem and can be marked down to give in best effort treatment.

 Total Length – This field specifies the size of the packet. This means the size of the header plus the size of the data.

 Identification – When IP receives a segment from TCP or UDP; it may need to break the segment into chucks called fragments before sending it out to the network. Identification fields serves to identify the fragments that make up the original segment. Each fragment of a segment will have the same identification number.

 Flags – Used for fragmentation process.

 Fragment Offset – This field identifies the fragment number and is used by hosts to reassemble the fragments in the correct order.

 Time to Live – The Time to Live (TTL) value is set at the originating host. Each router that the packet passes through reduces the TTL by one. If the TTL reaches 0 before reaching the destination, the packet is dropped. This is done to prevent the packet from moving around the network endlessly.

 Protocol – This field identifies the protocol to which the data it is carrying belongs. For example a value of 6 implies that the data contains a TCP segment while a value of 17 signifies a UDP segment. Apart from TCP and UDP there are many protocols whose data can be carried in an IP packet.

 Header Checksum – This field is used to check for errors in the header. At each router and at the destination, a cyclic redundancy check performed on the header and the result should match the value stored in this field. If the value does not match, the packet is discarded.

 Source IP address – This field stores the IP address of the source of the packet.

 Destination IP address – This field stores the IP address of the destination of the packet.

 Figure 1-16 Source and Destination IP address

 

Figure 1-16 shows how Source and Destination IP address is used in an IP packet. Notice how the source and destination addresses changed during the exchange between HostA and HostB

 Routing Protocols

In Figure 1-14, Router1 knew that it needed to send the packet destined to Host3 toward Router2. Router2 in turn knew that the packet needed to go toward Router3. To make these decisions, the routers need to build their routing table. This is a table of all networks known by it and all the routers in the internetwork. The table also lists the next router towards the destination network. To build this table dynamically, routers use routing protocols. There are many routing protocols and their sole purpose is to ensure that routers know about all the networks and the best path to any network. Chapter 4 and Chapter 5 discuss the routing process and some routing protocols in detail.

 Internet Control Message Protocol (ICMP)

ICMP is essentially a management protocol and messaging service for IP. Whenever IP encounters an error, it sends ICMP data as an IP packet.  Some of the reasons why an ICMP message can be generated are:

 Destination Network Unreachable – If a packet cannot be routed to the network in which the destination address resides, the router will drop the packet and generate an ICMP message back to the source informing that the destination network is unreachable.

Time Exceeded – If the TTL of a packet expiries (reduces to zero), the router will drop it and generate an ICMP message back to the source informing it that the time exceeded and the packet could not be delivered.

Echo Reply – ICMP can be used to check network connectivity. Popular utility called Ping is used to send Echo Requests to a destination. In reply to the request, the destination will send back an Echo reply back to the source. Successful receipt of Echo reply shows that the destination host is available and reachable from the source.

Network Access Layer

The Network Access layer of the TCP/IP model corresponds with the Data Link and Physical layers of the OSI reference model. It defines the protocols and hardware required to connect a host to a physical network and to deliver data across it. Packets from the Internet layer are sent down the Network Access layer for delivery within the physical network. The destination can be another host in the network, itself, or a router for further forwarding. So the Internet layer has a view of the entire Internetwork whereas the Network Access layer is limited to the physical layer boundary that is often defined by a layer 3 device such as a router.

The Network Access layer consists of a large number of protocols. When the physical network is a LAN, Ethernetat its many variations are the most common protocols used. On the other hand when the physical network is a WAN, protocols such as the Point-to-Point Protocol (PPP) and Frame Relay are common. In this section we take a deep look at Ethernet and its variations. WAN protocols are covered in detail in Chapter 11.

Before we explore Ethernet remember that:

Network Access layer uses a physical address to identify hosts and to deliver data.

• The Network Access layer PDU is called a frame. It contains the IP packet as well as a protocol header and trailer from this layer.
• The Network Access layer header and trailer are only relevant in the physical network. When a router receives a frame, it strips of the header and trailer and adds a new header and trailer before sending it out the next physical network towards the destination.

1-5 Ethernet Technologies and Cabling

Ethernet in the term used for a family of standards that define the Network Access layer of the most common type of LAN used today. The various standards differ in terms of speeds supported, cable types and the length of cables.The Institute of Electrical and Electronics Engineers (IEEE) is responsible for defining the various standards since it took over the process in 1980.

To make it easier to understand Ethernet, its functions will be discussed in terms of the OSI reference models’ Data Link and Physical layers. (Remember that Network Access Layer is a combination of these two layers).

IEEE defines various standards at the physical layer while it divides the Data Link functions into the following two sublayers:

•  The 802.3 Media Access Control (MAC) sublayer
• The 802.2 Logical Link Control (LLC) sublayer

Even though various physical layer standards are different and require changes at the layer, each of them use the same 802.3 header and the 802.2 LLC sublayer.

The following sections look at the collision detection mechanism used by Ethernet and how Ethernet functions at both the layers.

 Collision Detection in Ethernet

Ethernet is a contention media access method that allows all hosts in a network to share the available bandwidth. This means that multiple hosts try to use the media to transfer traffic. If multiple hosts send traffic at the same time, a collision can occur resulting in loss of the frames that collided. Ethernet cannot prevent such collision but it can detect them and take corrective actions to resolve. It uses the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol to do so. This is how CSMA/CD works:

1.  Hosts looking to transmit a frame listen until Ethernet is not busy.
2. When Ethernet is not busy, hosts start sending the frame.
3. The source listens to make sure no collision occurred.
4. If a collision occurs, the source hosts send a jamming signal to notify all hosts of the collision.
5. Each source host randomizes a timer and waits that long before resending the frame that collided.

CSMA/CD works well but it does create some performance issues because:

1.     Hosts must wait till the Ethernet media is not busy before sending frames. This means only one host can send frames at a time in a collision domain (such as in the case of a network connected to a hub). This also means that a host can either send or receive at one time. This logic is called half-duplex.
2.    During a collision, no frame makes it across the network. Also, the offending hosts must wait a random time before they can start to resend the frames.

Many networks suffered this sort of performance degradation due to the use of hubs until switches became affordable. In fact, statistics showed that anything over 30 percent utilization caused performance degradation in Ethernet.

Remember that switches break collision domains by providing a dedicated port to each host. This means that hosts connected to a switch only need to wait if the switch is sending frames destined to the host itself.

 Half and Full Duplex Ethernet

In the previous section, you learned about the logic called Half Duplex in which a host can only send or receive at one time. In a hub-based network, hosts are connected in a half-duplex mode because they must be able to detect collisions.

When hosts are connected to a switch, they can operate at Full duplex. This means they can send and receive at the same time without worrying about collisions. This is possible because full duplex uses two pairs of wire instead of one pair. Using the two pairs, a point-to-point connection is created between the transmitter of the host to the receiver of the switch and vice versa. So the host sends and receives frames via different pairs of wires and hence need to listed to see if it send frames or not. You should note that CSMA/CD is disabled at both ends when full duplex is used.

 Figure 1-17 Full Duplex

 

 Apart from eliminating collisions, each device actually gets to use twice the bandwidth available because it now has same bandwidth on both pairs of wire and each pair is used separately for sending and receiving.

 Figure 1-17 shows how the transmitter on the host’s interface card is connected to the receiver on the switch interface while the receiver on the host interface is connected to the transmitter on the switch interface. Now traffic sent by the host and traffic sent to the host both have a dedicated path with equal bandwidth. If each path has a bandwidth of 100Mbps, the host gets 200Mpbs of dedicated bandwidth to the switch. In case of half-duplex, there would have been only a single path of 100Mbps that would have been used for both receiving and sending traffic.

 Ethernet at the Data Link Layer

Ethernet at Data Link layer is responsible for addressing as well as framing the packets received from Network Layer and preparing them for the actual transmission.

 Ethernet Addressing

Ethernet Addressing identifies either a single device or a group of devices on a LAN and is called a MAC address. MAC address is 48 bits (6 bytes) long and is written is hexadecimal format. Cisco devices typically write it in a group of four hex digits separated by period while most operating systems write it in groups of two digits separated by a colon. For example, Cisco devices would write a MAC address as 5022.ab5b.63a9 while most operating systems would write it as 50:22:ab:5b:63:a9.

A unicast address identifies a single device. This address is used to identify the source and destination in a frame. Each LAN interface card has a globally unique MAC address. The IEEE defines the format and the assignment of addresses.

 Figure 1-18 48bit MAC address

 

To keep addresses unique, each manufacturer of LAN cards is assigned a code called the organizationally unique identifier (OUI). The first half of every MAC address is the OUI of the manufacturer. The manufacturer assigns the second half of the address while ensuring that the number is not used for any other card. The complete MAC address is then encoded into a ROM chip in the card. Figure 1-18 shows the composition of a MAC address.

 MAC address can also identify a group of devices. These are called group addresses. IEEE defines the following two types of group addresses:

•  Broadcast Address – This address has a value of  FFFF.FFFF.FFFF and means that all devices in the network should process the frame.
• Multicast Address – Multicast addresses are used when a frame needs to go to a group of hosts in the network. When IP multicast packets need to travel over Ethernet a multicast address of 0100.5exx.xxxx is used where xx.xxxx can be any value.

 Ethernet Framing

When the Data Link layer receives a packet from the Network layer for transmission, it has to encapsulate the packet in frames. These frames are used to identify the source and destination device by the switch. It also tells the receiving host how to interpret the bits received by the physical layer.

 Figure 1-19 IEEE Frame (1997)

 

The framing used by Ethernet has changed few times over the year. Xerox defined the original frame. When IEEE took over Ethernet in early 1980s it defined a new frame. In 1997 IEEE finalized the Ethernet frame that took a few components from the Xerox definition and a few from IEEE’s original frame. The finalized frame is shown in Figure 1-19. Table 1-2 lists the fields in the frame, their size and a brief description.

 Table 1-2 Frame Fields

Field	Length in bytes	Description
Preamble	7	It is used for synchronization. It tells the received device where the header starts.
SFD	1	Start Frame Delimiter (SFD) tells the receiving device that the next byte is the destination address
Destination Address	6	Identifies the intended destination of the frame.
Source Address	6	Identifies the source of the frame.
Length	2	Contains the length of the data field of the frame. (This field can either be length or type but not both)
Type	2	Identifies the Network layer protocol whose data is contained in the frame. (This field can either be length or type but not both)
Data	46-1500	The Network layer data.
FCS	4	Stores the CRC value which is used to check for errors in transmission.

The Length/Type field is something you need to understand a little more about. The type field is very important because it tells the receiving end about the protocol whose data is contained in the frame. If the value of the field is less than a hex value of 0600 (decimal value 1536), it signifies that the field is used as a length field in that frame. For cases where this field is used as a length field, either one or two additional headers are added after the Ethernet 802.3 header, but before the layer 3 header. When IP packets are being carried, the Ethernet frame has the following two additional headers:

•  An IEEE 802.2 Logical Link Control (LLC) header.
• An IEEE Subnetwork Access Protocol (SNAP) header.

Figure 1-20 shows an Ethernet frame with these two additional headers.

 Figure 1-20 802.3 Frame with LLC and SNAP header

Exam Alert: It is not necessary to remember the fields of the frame. Just remember why LLC and SNAP headers are used for your CCNA exam.

Ethernet at the Physical Layer

Ethernet was originally implemented by a group comprised of Digital, Xerox and Intel (DIX). IEEE then took over and created the 802.3 standard. This was a 10Mbps Ethernet that used co-axial cables.

Exam Alert: Ethernet is used to describe the family of standard that includes FastEthernet, Gigabit Ethernet etc. It is also used to describe the 10Mpbs variant also which is simply noted as Ethernet.

IEEE then extended the 802.3 committee to two new committees known as the 802.3u (FastEthernet) and 802.3ab (Gigabit Ethernet on category 5 cable). Then it created another committee known as the 802.3ae (10Gbps over fiber and co-axial).

On the other hand the Electronics Industries Association and the newer Telecommunication Industries Alliance(EIA/TIA) is the standards body that creates the physical layer specifications for Ethernet. It specifies that aregistered jack (RJ) connector with a 4 5 wiring sequence on an unshielded twisted-pair (UTP) cabling should be used with Ethernet. This cable comes in categories where higher category has less of the following two problems associated with them:

•  Attenuation – This is the loss of signal strength as it travels the length of the cable. It is measured in decibels.
• Crosstalk – This is the unwanted signal interference from adjacent pairs in the cable.

 What this means is that category 5 cable has lesser attenuation and crosstalk than category 3 cables.

Now that you know about the standards bodies involved and what they have done, it is time to look at the various Ethernet standards. Table 1-3 lists the original 3 standards. Remember that each standard is different in terms of Speed, Cable and the Maximum Length of cables.

 Table 1-3 Original Ethernet Standards

Name	Speed	Cable Type	Max Cable length	Connector	Description
10Base2	10Mbps	Coaxial	185 meters	AUI	Known as thinnet, it can support up to 30 hosts in a single segment. A single collision domain across the network.
10Base5	10Mbps	Coaxial	500 meters	AUI	Known as thicknet, it can support up to 100 users in a single segment. A single collision domain across the network.
10BaseT	10Mbps	UTP	100 meters	RJ45	The first standard to use UTP cable with RJ45. A single host can be connected to a segment or wire. It required use of hubs to connect multiple hosts.

Table 1-4 shows the extended Ethernet Standards.

Table 1-4 Extended Ethernet Standards

Name	Speed	Cable Type	Maximum Cable Length	Connector
100BaseTX (IEEE 802.3u)	100 Mbps	UTP cat. 5, 6 or 7 two-pair wiring	100 meters	RJ45
100BaseFX (IEEE 802.3u)	100Mbps	Multimode Fiber	412 meters	ST or SC connector
1000BaseCX(IEEE 802.3z)	1000Mpbs	Copper twisted pair called twinax	25 meters	DE-9 or 8P8C
1000BaseSX(IEEE 802.3z)	1000Mbps	Multimode Fiber	220 meters	ST or SC connector
1000BaseLX(IEEE 802.3z)	1000Mpbs	Single mode Fiber	5km	ST or SC connector
1000BaseT(IEEE 802.3ab)	1000Mpbs	Cat 5 UTP	100 meters	RJ45

Ethernet Cabling

When connecting different kinds of devices to each other, different kinds of cabling is used. The following three types of Ethernet cablings exist:

•  Straight-through cable (a normal patch cable)
• Crossover cable
• Rolled cable

The three cabling types are discussed below:

Straight-Though – A UTP cable has 8 wires. A straight-through uses 4 out of these 8 wires. Figure 1-21 shows the configuration of the wire on both ends in a straight-through cable. Notice that only wires 1, 2, 3 and 6 are used and they connect straight to corresponding number on the other end.

 Figure 1-21 Wire configuration in Straight-Through cable

 

Note: If you are wondering why the wire configuration is important remember that the transmitter on one end needs to connect to the receiver on the other end. If wiring configuration is incorrect, bits sent from one end will not be received at the other end.

Crossover – Crossover cable also uses the same four wires that are used in straight-through cable but different pins are connected here. Figure 1-22 shows the configuration of the wires in a crossover cable.

Figure 1-22 Wire configuration in Crossover cable

Crossover cable is used to connect:

•  Host to Host
• Switch to Switch
• Hub to Hub
• Switch to Hub
• Router to a host

Any easy way to remember this is that similar devices are connected to each other using crossover cables.

Rolled Cable – A rolled cable cannot be used for any Ethernet connection.  It is used for connecting to a router’s or a switch’s console port from your host’s serial communication (com) port.  Every Cisco router and switch has a console port that is used for initial configuration. All 8 wires are used in this cable and each wire connects to the opposite number on the end (1 to 8, 2 to 7, 3 to 6 etc). Figure 1-23 shows the wire configuration.

 Figure 1-23 Wire configuration in Crossover cable

Exam Alert: Cable types and where they are used is a very important topic not only for the CCNA Exam as you will see questions on it, but also for your networking career as well.

Data Encapsulation in TCP/IP Model

 The last thing you need to know about TCP/IP model is the Data encapsulation process and PDUs. As in case of the OSI reference model, the data is encapsulated in a header (and trailer in case of Network layer) to create a Protocol Data Unit (PDU) and is passed down to the next layer. Though you are aware of the process, you must know the names of each layer’s PDU. The PDU in TCP/IP model are:

•  Transport Layer -> Segment
• Internet Layer -> Packet
• Network Access Layer -> Frame

Figure 1-24 shows the encapsulation process in TCP/IP model.

Figure 1-24 Data encapsulation in TCP/IP Model

 

1-6 Cisco 3 Layer Model

In a large organization it is common to see large and complicated networks consisting of many locations, devices, services, and protocols. It can be cumbersome to manage and troubleshoot such networks. In addition to that as technologies evolve, the network has to evolve also. Making changes to a complex network is often difficult. Cisco with its years of experience in network equipment as well as managing its own network has defined a Three-layer hierarchical model. This model provides a hierarchical and modular method of building networks that makes it easy to implement, manage, scale and troubleshoot networks.

The model breaks an internetwork down to the following three layers:

•  The Core layer
• The Distribution layer
• The Access layer

These layers are logical and not physical. They have specific functions in an internetwork which are discussed below:

The Core Layer – This layer is the backbone of an internetwork. It is the simplest yet the most critical layer whose sole function is to transport large amount of data fast. It gets data from the distribution layer and sends it back to the distribution layer after transportation. Speed and fault tolerance are the two major requirements of this layer because it has to transport large amount of data and any fault at this layer will impact every user. Considering the functions of this layer, the following should be avoided at this layer:

•  Any thing that can slow down the traffic. For example, packet filtering, inter-VLAN routing etc.
• Direct user connections
• Direct server connections
• Complex service policies

While designing the core, the following should be kept in mind:

•  Routing protocol should have low convergence time.
• Network Access layer technologies should be fast with low latency
• Redundancy should be built into this layer.

The Distribution Layer – This layer acts as an interface between the Core and the Access layers. The primary function of the distribution layer is to provide routing, filtering, and WAN access and to determine how packets can access the core, if needed. Path determination is the most important function at the layer. It has to select the fastest way an access request can be completed. This layer also acts as the convergence point for all access layer switches. Hence it is generally the best place to apply most of the policies. The following are generally done at this layer:

•  Routing between subnets and VLANs and route distribution between routing protocols
• Implementation of security policies, including firewalls, address translations, packet filtering, etc.
• Breaking broadcast domains

The Access Layer – This layer is the edge of the network where wide variety of devices such as PCs, printers, iPads etc. connects to the network. Common resources needed by users are available at this layer while access request to remote resources are sent to the distribution layer. This layer is also known as the desktop layer. The following are generally done at this layer:

•  Access control and policies in addition to what exists in the distribution layer.
• Dynamic configuration mechanisms
• Breaking collision domains
• Ethernet switching and static routing

1-7 Summary

Though this chapter was long, it helped lay the foundation of your CCNA networking knowledge. The importance of understanding every topic in this chapter cannot be stressed enough. I would strongly suggest going through the chapter again to reinforce the basics.

The chapter started off with the importance of networks, basic network devices and network types and collision and broadcast domains.

Then the seven-layered OSI model was discussed. It is important to remember the functions of all the layers and how they map to the TCP/IP model. Remember that hubs work at Physical Layer, switches at Data-Link Layer and routers at the Network Layer of the OSI model.

The chapter then covered a long discussion on the TCP/IP model and its many protocols. Remember that TCP/IP and Ethernet form a major part of the CCNA exam and have a few chapters dedicated to them.

Lastly, the chapter covered the Cisco three-layer hierarchical model and how it is designed to help implement and manage a complex network.

The next chapter looks at IP addressing. Before heading to it, we suggest you review the CCNA Exam Alerts scattered through this chapter to recap the various important concepts.