Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett

FIGURE 1.11

Transmitting segments with flow control

In a reliable, connection-oriented data transfer, datagrams are delivered

to the receiving host hopefully in the same sequence they’re transmitted.

A failure will occur if any data segments are lost, duplicated, or damaged

along the way—a problem solved by having the receiving host

acknowledge that it has received each and every data segment.

A service is considered connection-oriented if it has the following

characteristics:

A virtual circuit, or “three-way handshake,” is set up.

It uses sequencing.

It uses acknowledgments.

It uses flow control.

The types of flow control are buffering, windowing, and

congestion avoidance.

Windowing

Ideally, data throughput happens quickly and efficiently. And as you can

imagine, it would be painfully slow if the transmitting machine had to

actually wait for an acknowledgment after sending each and every

segment! The quantity of data segments, measured in bytes, that the

transmitting machine is allowed to send without receiving an

acknowledgment is called a window.

Windows are used to control the amount of outstanding,

unacknowledged data segments.

The size of the window controls how much information is transferred

from one end to the other before an acknowledgement is required. While

some protocols quantify information depending on the number of

packets, TCP/IP measures it by counting the number of bytes.

As you can see in

Figure 1.12

, there are two window sizes—one set to 1

and one set to 3.

FIGURE 1.12

Windowing

If you’ve configured a window size of 1, the sending machine will wait for

an acknowledgment for each data segment it transmits before

transmitting another one but will allow three to be transmitted before

receiving an acknowledgement if the window size is set to 3.

In this simplified example, both the sending and receiving machines are

workstations. Remember that in reality, the transmission isn’t based on

simple numbers but in the amount of bytes that can be sent!

If a receiving host fails to receive all the bytes that it should

acknowledge, the host can improve the communication session by

decreasing the window size.

Acknowledgments

Reliable data delivery ensures the integrity of a stream of data sent from

one machine to the other through a fully functional data link. It

guarantees that the data won’t be duplicated or lost. This is achieved

through something called positive acknowledgment with retransmission

—a technique that requires a receiving machine to communicate with the

transmitting source by sending an acknowledgment message back to the

sender when it receives data. The sender documents each segment

measured in bytes, then sends and waits for this acknowledgment before

sending the next segment. Also important is that when it sends a

segment, the transmitting machine starts a timer and will retransmit if it

expires before it gets an acknowledgment back from the receiving end.

Figure 1.13

shows the process I just described.

FIGURE 1.13

Transport layer reliable delivery

In the figure, the sending machine transmits segments 1, 2, and 3. The

receiving node acknowledges that it has received them by requesting

segment 4 (what it is expecting next). When it receives the

acknowledgment, the sender then transmits segments 4, 5, and 6. If

segment 5 doesn’t make it to the destination, the receiving node

acknowledges that event with a request for the segment to be re-sent. The

sending machine will then resend the lost segment and wait for an

acknowledgment, which it must receive in order to move on to the

transmission of segment 7.

The Transport layer, working in tandem with the Session layer, also

separates the data from different applications, an activity known as

session multiplexing, and it happens when a client connects to a server

with multiple browser sessions open. This is exactly what’s taking place

when you go someplace online like Amazon and click multiple links,

opening them simultaneously to get information when comparison

shopping. The client data from each browser session must be separate

when the server application receives it, which is pretty slick

technologically speaking, and it’s the Transport layer to the rescue for

that juggling act!

The Network Layer

The Network layer, or layer 3, manages device addressing, tracks the

location of devices on the network, and determines the best way to move

data. This means that it’s up to the Network layer to transport traffic

between devices that aren’t locally attached. Routers, which are layer 3

devices, are specified at this layer and provide the routing services within

an internetwork.

Here’s how that works: first, when a packet is received on a router

interface, the destination IP address is checked. If the packet isn’t

destined for that particular router, it will look up the destination network

address in the routing table. Once the router chooses an exit interface, the

packet will be sent to that interface to be framed and sent out on the local

network. If the router can’t find an entry for the packet’s destination

network in the routing table, the router drops the packet.

Data and route update packets are the two types of packets used at the

Network layer:

Data Packets These are used to transport user data through the

internetwork. Protocols used to support data traffic are called routed

protocols, and IP and IPv6 are key examples. I’ll cover IP addressing in

Chapter 3, “Introduction to TCP/IP,” and Chapter 4, “Easy Subnetting,”

and I’ll cover IPv6 in Chapter 14, “Internet Protocol Version 6 (IPv6).”

Route Update Packets These packets are used to update neighboring

routers about the networks connected to all routers within the

internetwork. Protocols that send route update packets are called routing

protocols; the most critical ones for CCNA are RIPv2, EIGRP, and OSPF.

Route update packets are used to help build and maintain routing tables.

Figure 1.14

shows an example of a routing table. The routing table each

router keeps and refers to includes the following information:

FIGURE 1.14

Routing table used in a router

Network Addresses Protocol-specific network addresses. A router

must maintain a routing table for individual routing protocols because

each routed protocol keeps track of a network with a different addressing

scheme. For example, the routing tables for IP and IPv6 are completely

different, so the router keeps a table for each one. Think of it as a street

sign in each of the different languages spoken by the American, Spanish,

and French people living on a street; the street sign would read

Cat/Gato/Chat.

Interface The exit interface a packet will take when destined for a

specific network.

Metric The distance to the remote network. Different routing protocols

use different ways of computing this distance. I’m going to cover routing

protocols thoroughly in Chapter 9, “IP Routing.” For now, know that

some routing protocols like the Routing Information Protocol, or RIP, use

hop count, which refers to the number of routers a packet passes through

en route to a remote network. Others use bandwidth, delay of the line, or

even tick count (1/18 of a second) to determine the best path for data to

get to a given destination.

And as I mentioned earlier, routers break up broadcast domains, which

means that by default, broadcasts aren’t forwarded through a router. Do

you remember why this is a good thing? Routers also break up collision

domains, but you can also do that using layer 2 (Data Link layer)

switches. Because each interface in a router represents a separate

network, it must be assigned unique network identification numbers, and

each host on the network connected to that router must use the same

network number.

Figure 1.15

shows how a router works in an

internetwork.

FIGURE 1.15

A router in an internetwork. Each router LAN interface is

a broadcast domain. Routers break up broadcast domains by default and

provide WAN services.

Here are some router characteristics that you should never forget:

Routers, by default, will not forward any broadcast or multicast

packets.

Routers use the logical address in a Network layer header to

determine the next-hop router to forward the packet to.

Routers can use access lists, created by an administrator, to control

security based on the types of packets allowed to enter or exit an

interface.

Routers can provide layer 2 bridging functions if needed and can

simultaneously route through the same interface.

Layer 3 devices—in this case, routers—provide connections between

virtual LANs (VLANs).

Routers can provide quality of service (QoS) for specific types of

network traffic.

The Data Link Layer

The Data Link layer provides for the physical transmission of data and

handles error notification, network topology, and flow control. This

means that the Data Link layer will ensure that messages are delivered to

the proper device on a LAN using hardware addresses and will translate

messages from the Network layer into bits for the Physical layer to

transmit.

The Data Link layer formats the messages, each called a data frame, and

adds a customized header containing the hardware destination and

source address. This added information forms a sort of capsule that

surrounds the original message in much the same way that engines,

navigational devices, and other tools were attached to the lunar modules

of the Apollo project. These various pieces of equipment were useful only

during certain stages of space flight and were stripped off the module and

discarded when their designated stage was completed. The process of

data traveling through networks is similar.

Figure 1.16

shows the Data Link layer with the Ethernet and IEEE

specifications. When you check it out, notice that the IEEE 802.2

standard is used in conjunction with and adds functionality to the other

IEEE standards. (You’ll read more about the important IEEE 802

standards used with the Cisco objectives in Chapter 2, “Ethernet

Networking and Data Encapsulation.”)

FIGURE 1.16

Data Link layer

It’s important for you to understand that routers, which work at the

Network layer, don’t care at all about where a particular host is located.

They’re only concerned about where networks are located and the best

way to reach them—including remote ones. Routers are totally obsessive

when it comes to networks, which in this case is a good thing! It’s the

Data Link layer that’s responsible for the actual unique identification of

each device that resides on a local network.

For a host to send packets to individual hosts on a local network as well

as transmit packets between routers, the Data Link layer uses hardware

addressing. Each time a packet is sent between routers, it’s framed with

control information at the Data Link layer, but that information is

stripped off at the receiving router and only the original packet is left

completely intact. This framing of the packet continues for each hop until

the packet is finally delivered to the correct receiving host. It’s really

important to understand that the packet itself is never altered along the

route; it’s only encapsulated with the type of control information required

for it to be properly passed on to the different media types.

The IEEE Ethernet Data Link layer has two sublayers:

Media Access Control (MAC) Defines how packets are placed on the

media. Contention for media access is “first come/first served” access

where everyone shares the same bandwidth—hence the name. Physical

addressing is defined here as well as logical topologies. What’s a logical

topology? It’s the signal path through a physical topology. Line discipline,

error notification (but not correction), the ordered delivery of frames, and

optional flow control can also be used at this sublayer.

Logical Link Control (LLC) Responsible for identifying Network layer

protocols and then encapsulating them. An LLC header tells the Data

Link layer what to do with a packet once a frame is received. It works like

this: a host receives a frame and looks in the LLC header to find out

where the packet is destined—for instance, the IP protocol at the Network

layer. The LLC can also provide flow control and sequencing of control

bits.

The switches and bridges I talked about near the beginning of the chapter

both work at the Data Link layer and filter the network using hardware

(MAC) addresses. I’ll talk about these next.

As data is encoded with control information at each layer of

the OSI model, the data is named with something called a protocol

data unit (PDU). At the Transport layer, the PDU is called a segment,

at the Network layer it’s a packet, at the Data Link a frame, and at the

Physical layer it’s called bits. This method of naming the data at each

layer is covered thoroughly in Chapter 2.

Switches and Bridges at the Data Link Layer

Layer 2 switching is considered hardware-based bridging because it uses

specialized hardware called an application-specific integrated circuit

(ASIC). ASICs can run up to high gigabit speeds with very low latency

rates.

Latency is the time measured from when a frame enters a

port to when it exits a port.

Bridges and switches read each frame as it passes through the network.

The layer 2 device then puts the source hardware address in a filter table

and keeps track of which port the frame was received on. This

information (logged in the bridge’s or switch’s filter table) is what helps

the machine determine the location of the specific sending device.

Figure

1.17

shows a switch in an internetwork and how John is sending packets

to the Internet and Sally doesn’t hear his frames because she is in a

different collision domain. The destination frame goes directly to the

default gateway router, and Sally doesn’t see John’s traffic, much to her

relief.

FIGURE 1.17

A switch in an internetwork

The real estate business is all about location, location, location, and it’s

the same way for both layer 2 and layer 3 devices. Though both need to be

able to negotiate the network, it’s crucial to remember that they’re

concerned with very different parts of it. Primarily, layer 3 machines

(such as routers) need to locate specific networks, whereas layer 2

machines (switches and bridges) need to eventually locate specific

devices. So, networks are to routers as individual devices are to switches

and bridges. And routing tables that “map” the internetwork are for

routers as filter tables that “map” individual devices are for switches and

bridges.

After a filter table is built on the layer 2 device, it will forward frames only

to the segment where the destination hardware address is located. If the

destination device is on the same segment as the frame, the layer 2 device

will block the frame from going to any other segments. If the destination

is on a different segment, the frame can be transmitted only to that

segment. This is called transparent bridging.

When a switch interface receives a frame with a destination hardware

address that isn’t found in the device’s filter table, it will forward the

frame to all connected segments. If the unknown device that was sent the

“mystery frame” replies to this forwarding action, the switch updates its

filter table regarding that device’s location. But in the event the

destination address of the transmitting frame is a broadcast address, the

switch will forward all broadcasts to every connected segment by default.

All devices that the broadcast is forwarded to are considered to be in the

same broadcast domain. This can be a problem because layer 2 devices

propagate layer 2 broadcast storms that can seriously choke performance,

and the only way to stop a broadcast storm from propagating through an

internetwork is with a layer 3 device—a router!

The biggest benefit of using switches instead of hubs in your internetwork

is that each switch port is actually its own collision domain. Remember

that a hub creates one large collision domain, which is not a good thing!

But even armed with a switch, you still don’t get to just break up

broadcast domains by default because neither switches nor bridges will

do that. They’ll simply forward all broadcasts instead.

Another benefit of LAN switching over hub-centered implementations is

that each device on every segment plugged into a switch can transmit

simultaneously. Well, at least they can as long as there’s only one host on

each port and there isn’t a hub plugged into a switch port! As you might

have guessed, this is because hubs allow only one device per network

segment to communicate at a time.

The Physical Layer

Finally arriving at the bottom, we find that the Physical layer does two

things: it sends bits and receives bits. Bits come only in values of 1 or 0—a

Morse code with numerical values. The Physical layer communicates

directly with the various types of actual communication media. Different

kinds of media represent these bit values in different ways. Some use

audio tones, while others employ state transitions—changes in voltage

from high to low and low to high. Specific protocols are needed for each

type of media to describe the proper bit patterns to be used, how data is

encoded into media signals, and the various qualities of the physical

media’s attachment interface.

The Physical layer specifies the electrical, mechanical, procedural, and

functional requirements for activating, maintaining, and deactivating a

physical link between end systems. This layer is also where you identify

the interface between the data terminal equipment (DTE) and the data

communication equipment (DCE). (Some old phone-company employees

still call DCE “data circuit-terminating equipment.”) The DCE is usually

located at the service provider, while the DTE is the attached device. The

services available to the DTE are most often accessed via a modem or

channel service unit/data service unit (CSU/DSU).

The Physical layer’s connectors and different physical topologies are

defined by the OSI as standards, allowing disparate systems to

communicate. The Cisco exam objectives are interested only in the IEEE

Ethernet standards.

Hubs at the Physical Layer

A hub is really a multiple-port repeater. A repeater receives a digital

signal, reamplifies or regenerates that signal, then forwards the signal out

the other port without looking at any data. A hub does the same thing

across all active ports: any digital signal received from a segment on a

hub port is regenerated or reamplified and transmitted out all other ports

on the hub. This means all devices plugged into a hub are in the same

collision domain as well as in the same broadcast domain.

Figure 1.18

shows a hub in a network and how when one host transmits, all other

hosts must stop and listen.

FIGURE 1.18

A hub in a network

Hubs, like repeaters, don’t examine any of the traffic as it enters or before

it’s transmitted out to the other parts of the physical media. And every

device connected to the hub, or hubs, must listen if a device transmits. A

physical star network, where the hub is a central device and cables extend

in all directions out from it, is the type of topology a hub creates. Visually,

the design really does resemble a star, whereas Ethernet networks run a

logical bus topology, meaning that the signal has to run through the

network from end to end.

Hubs and repeaters can be used to enlarge the area covered

by a single LAN segment, but I really do not recommend going with

this configuration! LAN switches are affordable for almost every

situation and will make you much happier.

Topologies at the Physical layer

One last thing I want to discuss at the Physical layer is topologies, both

physical and logical. Understand that every type of network has both a

physical and a logical topology.

The physical topology of a network refers to the physical layout of the

devices, but mostly the cabling and cabling layout.

The logical topology defines the logical path on which the signal will

travel on the physical topology.

Figure 1.19

shows the four types of topologies.

FIGURE 1.19

Physical vs. Logical Topolgies

Here are the topology types, although the most common, and pretty

much the only network we use today is a physical star, logical bus

technology, which is considered a hybrid topology (think Ethernet):

Bus: In a bus topology, every workstation is connected to a single

cable, meaning every host is directly connected to every other

workstation in the network.

Ring: In a ring topology, computers and other network devices are

cabled together in a way that the last device is connected to the first to

form a circle or ring.

Star: The most common physical topology is a star topology, which is

your Ethernet switching physical layout. A central cabling device

(switch) connects the computers and other network devices together.

This category includes star and extended star topologies. Physical

connection is commonly made using twisted-pair wiring.

Mesh: In a mesh topology, every network device is cabled together

with connection to each other. Redundant links increase reliability

and self-healing. The physical connection is commonly made using

fiber or twisted-pair wiring.

Hybrid: Ethernet uses a physical star layout (cables come from all

directions), and the signal travels end-to-end, like a bus route.