Charles M. Kozierok The TCP-IP Guide

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Figure 16: OSI Reference Model PDU and SDU Encapsulation

This example shows in more detail how OSI PDUs and SDUs are created and encapsulated. A TCP segment

(layer 4 PDU) becomes a layer 3 SDU, which is encapsulated into a layer 3 PDU through the addition of an IP

header. This becomes the payload of an Ethernet frame, which is a layer 2 PDU containing an Ethernet

header, layer 2 SDU (the IP datagram) and Ethernet footer. The receiving device extracts the IP datagram

from the Ethernet header and passes it to layer 3; the IP software extracts the TCP segment and passes it up

to the TCP software.

Layer 2 PDU (Ethernet Frame)

1 1

1 011011001001 011

Layer 3 PDU (IP Datagram)

Layer2

(Ethernet)

Header

Layer 2 SDU

Layer 2

(Ethernet)

Footer

Layer 3

(IP)

Header

Layer 3 SDU

Layer 4 PDU (TCP Segment)

Layer

4 (TCP)

Header

Upper Layer SDU

(Data)

Layer

4 (TCP)

Header

Upper Layer SDU

(Data)

Layer 3

(IP)

Header

Layer 3 SDU

Layer

4 (TCP)

Header

Upper Layer SDU

(Data)

Key Concept: The message used to communicate information for a particular

protocol is called its protocol data unit (PDU) in OSI model terminology. That PDU is

passed down to the next lower layer for transmission; since that layer is providing the

service of handling that PDU, it is called the lower layer’s service data unit (SDU). The SDU

is encapsulated into that layer’s own PDU and in turn sent to the next lower layer in the

stack, proceeding until the physical layer is reached. The process is reversed on the

recipient device. In summary: a layer N PDU is a layer N-1 SDU, which is encapsulated into

a layer N-1 PDU.

Indirect Device Connection and Message Routing

Most of the explanations that I have provided in the other topics of this section have

discussed the mechanisms by which machines connect to each other over a network

directly. However, one of the most powerful aspects of networking is that it is possible to

create internetworks—networks of networks—which allow devices to be connected

indirectly. For example, machine “A” may send a message to machine “B” without really

even knowing where it is on the network at all.

If a message is being sent between devices that are not on the same network, then it must

be passed between directly-connected networks until it reaches its final destination. The

process of transmitting a message from one network to another is called forwarding, and

the collective process of forwarding from one device to another is routing. These concepts

are fundamental to all internetworking, including the Internet itself. Every time you access

an Internet resource such as a Web site, you are sending messages that get routed to that

site, and the responses you receive get routed back.

Note: Even though the technically-correct term for moving a message from one

network to an adjacent network is “forwarding”, over time the term “routing” has

come to often be used both for a single network-to-network transfer as well as the

overall process of transmitting a message from one device to another.

In the context of the OSI Reference Model, routing is an activity that generally takes place

at the network layer—layer 3. Recall that data encapsulation causes a higher-layer

message to be surrounded by headers and/or footers at the lower layers. When a message

is routed, here's what happens:

☯ A high-level application on a machine decides to send a datagram to a distant

computer. The datagram is packaged, and then passed down vertically through the

protocol stack on the originating machine. Each layer encapsulates the data as

described earlier. The datagram is addressed to the final destination device. When the

message gets to the network layer and below, however, it is not packaged for local

delivery directly to its ultimate destination, but rather to an intermediate device. This is

the device that is responsible for routing to that destination network. The message is

passed down to the data link layer and then the physical layer for transmission to that

intermediate device.

☯ The intermediate device (often called a router) receives the message at the physical

layer. It is passed up to the data link layer, where it is processed, checked for errors

and so on, and the data link layer headers are removed. The resulting packet is

passed up to the network layer. There, the intermediate device determines if the desti-

nation machine is on its local network, or if it needs to be forwarded to another

intermediate device. It then repackages the message and passes it back down to the

data link layer to be sent on the next leg of its journey.

☯ After several potential intermediate devices “handle” the message, it eventually

reaches its destination. Here, it travels back up the protocol stack until it reaches the

same layer as the one of the application that generated the message on the originating

machine.

The key to this description is that in the intermediate devices, the message travels back up

the OSI layers only to the network layer. It is then repackaged and sent back along its way.

The higher layers are only involved on the source and destination devices. The protocol

used at layer 3 must be common across the internetwork but each individual network can

be different. This demonstrates some of the power of layering, by enabling even rather

dissimilar physical networks to be connected together. The process is illustrated in Figure

17.

Key Concept: In the OSI model, the process of routing occurs when data is sent not

directly from transmitter to ultimate recipient, but indirectly through the use of an

intermediate system. That device, normally called a router, connects to two or more

physical networks and thus has multiple interfaces to layer two. When it receives data, the

data passes up only to the network layer, where it is repackaged and then sent on the next

leg of its journey over the appropriate layer two interface.

Figure 17: Message Routing in the OSI Reference Model

This diagram shows how routing is accomplished conceptually in the OSI model. The intermediate device

connects the networks of the message transmitter and recipient. When data is sent, it is passed up to the

network layer on the intermediate device, where it is repackaged and sent back down the stack for the next

leg of its transmission. Note that the intermediate device actually has two different layer 1 and 2 implementa-

tions; one for the interface to each network. Also note that while the layer 3 protocol must be the same across

the internetwork, each network can use different technologies at layers 1 and 2.

Layer 3 Protocol Layer 3 Protocol

Layer 2 Protocol "A" Layer 2 Protocol "B"

Layer 2 Protocol "A" Layer 1 Protocol "B"

Intermediate Device

Understanding The OSI Reference Model: An Analogy

I have attempted in this discussion of the OSI Reference Model to provide as much “plain

English” explanation of how it works as I could. However, there are situations where a good

analogy can accomplish what lots of descriptions cannot. So, I am going to attempt to illus-

trate the key OSI model concepts (layers, vertical communication, horizontal

communication, data encapsulation and message routing) by way of a real-life analogy. You

can be the judge of whether it is a good analogy or not. Just remember that no analogy is

perfect.

Our scenario seems relatively simple and common: the CEO of a Fortune 500 company

needs to send a letter to the CEO of another large company. Simple, right? Just like firing

up your browser and connecting to your favorite Web site is simple. However, in both cases,

a lot goes on “behind the scenes” to make the communication happen. In the analogy

shown in Table 18 below, I compare these real-world and "cyber-world” communications.

Note: (Yes, the first CEO could fly to the other one's town in his Lear Jet, take him

out for a lobster-and-martini dinner and hand him the letter there. Please play

along, will you? Oh, and sorry to any CEOs for the blatant stereotyping. ☺

Table 18: OSI Reference Model Real-World Analogy (Page 1 of 3)

Phase

OSI

Layer

CEO Letter Web Site Connection (Simplified)

Transmission

The CEO of a company in Phoenix

decides he needs to send a letter to a

peer of his in Albany. He dictates the

letter to his administrative assistant.

You decide you want to connect to the

web server at IP address 10.0.12.34,

which is within your organization but

not on your local network. You type the

address into your browser.

The administrative assistant

transcribes the dictation into writing.

(Generally, with a web site connection,

nothing happens at this layer, but

format translation may be done in

some cases.)

The administrative assistant puts the

letter in an envelope and gives it to the

mail room. The assistant doesn't

actually know how the letter will be

sent, but he knows it is urgent so he

says, “get this to its destination

quickly”.

The request is sent via a call to an

application program interface (API), to

issue the command necessary to

contact the server at that address.

The mail room must decide how to get

the letter where it needs to go. Since it

is a rush, the people in the mail room

decide they must use a courier. The

envelope is given to the courier

company to send.

The Transmission Control Protocol

(TCP) is used to create a segment to

be sent to IP address 10.0.12.34.

Routing

The courier company receives the

envelope, but it needs to add its own

handling information, so it places the

smaller envelope in a courier envelope

(encapsulation). The courier then

consults its airplane route information

and determines that to get this

envelope to Albany, it must be flown

through its hub in Chicago. It hands

this envelope to the workers who load

packages on airplanes.

Your computer creates an IP datagram

encapsulating the TCP datagram

created above. It then addresses the

packet to 10.0.12.34. but discovers

that it is not on its local network. So

instead, it realizes it needs to send the

message to its designated routing

device at IP address 10.0.43.21. It

hands the packet to the driver for your

Ethernet card (the software that inter-

faces to the Ethernet hardware).

The workers take the courier envelope

and put on it a tag with the code for

Chicago. They then put it in a handling

box and then load it on the plane to

Chicago.

The Ethernet card driver forms a frame

containing the IP datagram and

prepares it to be sent over the network.

It packages the message and puts the

address 10.0.43.21 (for the router) in

the frame.

1 The plane flies to Chicago.

The frame is sent over the twisted pair

cable that connects your local area

network. (I'm ignoring overhead, colli-

sions, etc. here, but then I also ignored

the possibility of collisions with the

plane. ☺)

In Chicago, the box is unloaded, and

the courier envelope is removed from it

and given to the people who handle

routing in Chicago.

The Ethernet card at the machine with

IP address 10.0.43.21 receives the

frame, strips off the frame headers and

hands it up to the network layer.

The tag marked “Chicago” is removed

from the outside of the courier

envelope. The envelope is then given

back to the airplane workers to be sent

to Albany.

The IP datagram is processed by the

router, which realizes the destination

(10.0.12.34) can be reached directly. It

passes the datagram back down to the

Ethernet driver.

The envelope is given a new tag with

the code for Albany, placed in another

box and loaded on the plane to Albany.

The Ethernet driver creates a new

frame and prepares to send it to the

device that uses IP address

10.0.12.34.

1 The plane flies to Albany. The frame is sent over the network.

The box is unloaded and the courier

envelope is removed from the box. It is

given to the Albany routing office.

The Ethernet card at the device with IP

address 10.0.12.34 receives the

frame, strips off the headers and

passes it up the stack.

Table 18: OSI Reference Model Real-World Analogy (Page 2 of 3)

Phase

OSI

Layer

CEO Letter Web Site Connection (Simplified)

As you can see, the processes have a fair bit in common. The vertical communication and

encapsulation are pretty obvious, as is the routing. Also implied is the horizontal communi-

cation that occurs logically—the two CEOs seem to be “connected” despite all that happens

to enable this to occur. Similarly, the two assistants are logically connected as well, in a

way, even though they never actually converse. Of course, this example is highly simplified

in just about every way imaginable, so please don’t use it as a way of trying to learn about

how TCP/IP works—or courier services, for that matter.

Anyway, I hope the example helps make some sense of all of this OSI stuff!

Reception

The courier company in Albany sees

that the destination is in Albany, and

delivers the envelope to the destination

CEO's company.

The IP headers are removed from the

datagram and the TCP segment

handed up to TCP.

The mail room removes the inner

envelope from the courier envelope

and delivers it to the destination CEO's

assistant.

TCP removes its headers and hands

the data up to the drivers on the desti-

nation machine.

The assistant takes the letter out of the

envelope.

The request is sent to the Web server

software for processing.

The assistant reads the letter and

decides whether to give the letter to

the CEO, transcribe it to email, call the

CEO on her cell phone, or whatever.

(Again, in this example nothing

probably happens at the Presentation

layer.)

The second CEO receives the

message that was sent by the first one.

The Web server receives and

processes the request.

Table 18: OSI Reference Model Real-World Analogy (Page 3 of 3)

Phase

OSI

Layer

CEO Letter Web Site Connection (Simplified)

OSI Reference Model Layers

Finally, after much ado—hopefully not too much!—it is time to take a look at the actual

individual layers of the OSI Reference Model. As discussed in the section on OSI model

concepts, each layer has certain characteristics that define it, and also various protocols

normally associated with it. Understanding the nuances of each layer will help you under-

stand all the technologies that use them.

In this section, I describe each of the OSI Reference Model layers individually. For each one

I provide its name and layer number, describe its general function in the OSI layer stack,

and outline the specific types of activities for which each is normally responsible. I also

provide some examples of the technologies and protocols that reside at each layer. Keep in

mind, however, that the descriptions in this section are generic. To really comprehend the

various layers and how they are uses, there is no substitute for reading the details of the

individual protocols that function at each layer, covered elsewhere in this Guide.

Related Information: For assistance in remembering the correct order of the

layers, see the topic that describes common mnemonics used for the OSI

Reference Model. To easily compare the key characteristics of the seven layers,

refer to the OSI model layer summary.

Physical Layer (Layer 1)

The lowest layer of the OSI Reference Model is layer 1, the physical layer; it is commonly

abbreviated “PHY”. The physical layer is special compared to the other layers of the model,

because it is the only one where data is physically moved across the network interface. All

of the other layers perform useful functions to create messages to be sent, but they must all

be transmitted down the protocol stack to the physical layer, where they are actually sent

out over the network.

Note: The physical layer is also “special” in that it is the only layer that really does

not apply specifically to TCP/IP. Even in studying TCP/IP, however, it is still

important to understand its significance and role in relation to the other layers

where TCP/IP protocols reside.

Understanding the Role of the Physical Layer

The name “physical layer” can be a bit problematic. Because of that name, and because of

what I just said about the physical layer actually transmitting data, many people who study

networking get the impression that the physical layer is only about actual network hardware.

Some people may say the physical layer is “the network interface cards and cables”. This is

not actually the case, however. The physical layer defines a number of network functions,

not just hardware cables and cards.

A related notion is that “all network hardware belongs to the physical layer”. Again, this isn't

strictly accurate. All hardware must have some relation to the physical layer in order to

send data over the network, but hardware devices generally implement multiple layers of

the OSI model, including the physical layer but also others. For example, an Ethernet

network interface card performs functions at both the physical layer and the data link layer.

Physical Layer Functions

The following are the main responsibilities of the physical layer in the OSI Reference Model:

☯ Definition of Hardware Specifications: The details of operation of cables,

connectors, wireless radio transceivers, network interface cards and other hardware

devices are generally a function of the physical layer (although also partially the data

link layer; see below).

☯ Encoding and Signaling: The physical layer is responsible for various encoding and

signaling functions that transform the data from bits that reside within a computer or

other device into signals that can be sent over the network.

☯ Data Transmission and Reception: After encoding the data appropriately, the

physical layer actually transmits the data, and of course, receives it. Note that this

applies equally to wired and wireless networks, even if there is no tangible cable in a

wireless network!

☯ Topology and Physical Network Design: The physical layer is also considered the

domain of many hardware-related network design issues, such as LAN and WAN

topology.

In general, then, physical layer technologies are ones that are at the very lowest level and

deal with the actual ones and zeroes that are sent over the network. For example, when

considering network interconnection devices, the simplest ones operate at the physical

layer: repeaters, conventional hubs and transceivers. These devices have absolutely no

knowledge of the contents of a message. They just take input bits and send them as output.

Devices like switches and routers operate at higher layers and look at the data they receive

as being more than voltage or light pulses that represent one or zero.

Relationship Between the Physical Layer and Data Link Layer

It's important to point out that while the physical layer of a network technology primarily

defines the hardware it uses, the physical layer is closely related to the data link layer.

Thus, it is not generally possible to define hardware at the physical layer “independently” of

the technology being used at the data link layer. For example, Ethernet is a technology that

describes specific types of cables and network hardware, but the physical layer of Ethernet

can only be isolated from its data link layer aspects to a point. While Ethernet cables are

“physical layer”, for example, their maximum length is related closely to message format

rules that exist at the data link layer.

Furthermore, some technologies perform functions at the physical layer that are normally

more closely associated with the data link layer. For example, it is common to have the

physical layer perform low-level (bit level) repackaging of data link layer frames for trans-

mission. Error detection and correction may also be done at layer 1 in some cases. Most

people would consider these “layer two functions”.

In many technologies, a number of physical layers can be used with a data link layer. Again

here, the classic example is Ethernet, where dozens of different physical layer implementa-

tions exist, each of which uses the same data link layer (possibly with slight variations.)

Physical Layer Sublayers

Finally, many technologies further subdivide the physical layer into sublayers. In order to

increase performance, physical layer encoding and transmission methods have become

more complex over time. The physical layer may be broken into layers to allow different

network media to be supported by the same technology, while sharing other functions at the

physical layer that are common between the various media. A good example of this is the

physical layer architecture used for Fast Ethernet, Gigabit Ethernet and 10-Gigabit

Ethernet.

Note: In some contexts, the physical layer technology used to convey bits across

a network or communications line is called a transport method. Don't confuse this

with the functions of the OSI transport layer (layer 4).

Key Concept: The lowest layer in the OSI Reference Model is the physical layer. It is

the realm of networking hardware specifications, and is the place where technologies

reside that perform data encoding, signaling, transmission and reception functions.

The physical layer is closely related to the data link layer.

Data Link Layer (Layer 2)

The second-lowest layer (layer 2) in the OSI Reference Model stack is the data link layer,

often abbreviated “DLL” (though that abbreviation has other meanings as well in the

computer world). The data link layer, also sometimes just called the link layer, is where

many wired and wireless local area networking (LAN) technologies primarily function. For

example, Ethernet, Token Ring, FDDI and 802.11 (“wireless Ethernet” or “Wi-Fi’) are all

sometimes called “data link layer technologies”. The set of devices connected at the data

link layer is what is commonly considered a simple “network”, as opposed to an

internetwork.