UNIT 1

CHAPTER 2
REFERENCE MODELS

SHRUTHI K ANCHAN
REFERENCE MODELS
We will discuss two important network architectures: the OSI reference model and the
TCP/IP reference model. Although the protocols associated with the OSI model are not used any
more, the model itself is actually quite general and still valid, and the features discussed at each layer
are still very important. The TCP/IP model has the opposite properties: the model itself is not of much
use but the protocols are widely used. For this reason we will look at both of them in detail.
The OSI Reference Model
The OSI model (minus the physical medium) is shown in Fig. 1-20. The model is called the
OSI(Open Systems Interconnection) Reference Model because it deals with connecting open systems—
that is, systems that are open for communication with other systems. We will just call it the OSI model for
short.
The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be briefly
summarized as follows:
 The Physical Layer
The physical layer is concerned with transmitting raw bits over a communication channel. The
design issues have to do with making sure that when one side sends a 1 bit it is received by the other side
as a 1 bit, not as a 0 bit. Typical questions here are what electrical signals should be used to represent a 1
and a 0, how many nanoseconds a bit lasts, whether transmission may proceed simultaneously in both
directions, how the initial connection is established, how it is torn down when both sides are finished, how
many pins the network connector has, and what each pin is used for. These design issues largely deal with
mechanical, electrical, and timing interfaces, as well as the physical transmission medium, which lies below
the physical layer.
 The Data Link Layer
The main task of the data link layer is to transform a raw transmission facility into a line that appears
free of undetected transmission errors. It does so by masking the real errors so the network layer does not see
them. It accomplishes this task by having the sender break up the input data into data frames (typically a few
hundred or a few thousand bytes) and transmit the frames sequentially. If the service is reliable, the receiver
confirms correct receipt of each frame by sending back an acknowledgement frame.
Another issue that arises in the data link layer (and most of the higher layers as well) is how to keep a
fast transmitter from drowning a slow receiver in data. Some traffic regulation mechanism may be needed to
let the transmitter know when the receiver can accept more data.
Broadcast networks have an additional issue in the data link layer: how to control access to the
shared channel. A special sublayer of the data link layer, the medium access control sublayer, deals with this
problem.
 The Network Layer
The network layer controls the operation of the subnet. A key design issue is determining how packets
are routed from source to destination. Routes can be based on static tables that are ‘‘wired into’’ the network
and rarely changed, or more often they can be updated automatically to avoid failed components. They can also
be determined at the start of each conversation. Finally, they can be highly dynamic, being determined a new for
each packet to reflect the current network load.
If too many packets are present in the subnet at the same time, they will get in one another’s way,
forming bottlenecks. Handling congestion is also a responsibility of the network layer, in conjunction with
higher layers that adapt the load they place on the network. More generally, the quality of service provided
(delay, transit time, jitter, etc.) is also a network layer issue.
 When a packet has to travel from one network to another to get to its destination, many problems
can arise. The addressing used by the second network may be different from that used by the first one.
The second one may not accept the packet at all because it is too large. The protocols may differ, and so
on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be
interconnected.
In broadcast networks, the routing problem is simple, so the network layer is often thin or even
non-existent.
 The Transport Layer
The basic function of the transport layer is to accept data from above it, split it up into smaller units if
need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end.
Furthermore, all this must be done efficiently and in a way that isolates the upper layers from the inevitable
changes in the hardware technology over the course of time.
The transport layer also determines what type of service to provide to the session layer, and,
ultimately, to the users of the network. The most popular type of transport connection is an error-free point-
to-point channel that delivers messages or bytes in the order in which they were sent. However, other
possible kinds of transport service exist, such as the transporting of isolated messages with no guarantee
about the order of delivery, and the broadcasting of messages to multiple destinations. The type of service is
determined when the connection is established. (As an aside, an error-free channel is completely impossible
to achieve; what people really mean by this term is that the error rate is low enough to ignore in practice.)
 The transport layer is a true end-to-end layer; it carries data all the way from the source to the
destination. In other words, a program on the source machine carries on a conversation with a similar
program on the destination machine, using the message headers and control messages. In the lower layers,
each protocols is between a machine and its immediate neighbors, and not between the ultimate source and
destination machines, which may be separated by many routers. The difference between layers 1 through 3,
which are chained, and layers 4 through 7, which are end-to-end, is illustrated in Fig. 1-20.
 The Session Layer
The session layer allows users on different machines to establish sessions between them. Sessions
offer various services, including dialog control (keeping track of whose turn it is to transmit), token
management (preventing two parties from attempting the same critical operation simultaneously), and
synchronization (checkpointing long transmissions to allow them to pick up from where they left off in the
event of a crash and subsequent recovery).
 The Presentation Layer
Unlike the lower layers, which are mostly concerned with moving bits around, the presentation
layer is concerned with the syntax and semantics of the information transmitted. In order to make it possible
for computers with different internal data representations to communicate, the data structures to be
exchanged can be defined in an abstract way, along with a standard encoding to be used ‘‘on the wire.’’ The
presentation layer manages these abstract data structures and allows higher-level data structures (e.g.,
banking records) to be defined and exchanged.
 The Application Layer
The application layer contains a variety of protocols that are commonly needed by users. One
widely used application protocol is HTTP (Hyper Text Transfer Protocol), which is the basis for the World
Wide Web. When a browser wants a Web page, it sends the name of the page it wants to the server
hosting the page using HTTP. The server then sends the page back. Other application protocols are used
for file transfer, electronic mail, and network news
The TCP/IP Reference Model
Let us now turn from the OSI reference model to the reference model used in the grandparent of all
wide area computer networks, the ARPANET, and its successor, the worldwide Internet. Although we will give
a brief history of the ARPANET later, it is useful to mention a few key aspects of it now. It eventually connected
hundreds of universities and government installations, using leased telephone lines. When satellite and
radio networks were added later, the existing protocols had trouble interworking with them, so a new reference
architecture was needed. Thus, from nearly the beginning, the ability to connect multiple networks in a
seamless way was one of the major design goals. This architecture later became known as the TCP/IP
Reference Model, after its two primary protocols.
 The Link Layer
The lowest layer in the model, the link layer describes what links such as serial lines and classic
Ethernet must do to meet the needs of this connectionless internet layer. It is not really a layer at all, in the
normal sense of the term, but rather an interface between hosts and transmission links.
 The Internet Layer
The internet layer is the linchpin that holds the whole architecture together. It is shown in Fig. 1-21
as corresponding roughly to the OSI network layer. Its job is to permit hosts to inject packets into any network
and have them travel independently to the destination (potentially on a different network). They may even
arrive in a completely different order than they were sent, in which case it is the job of higher layers to
rearrange them, if in-order delivery is desired. Note that ‘‘internet’’ is used here in a generic sense, even
though this layer is present in the Internet.
 The analogy here is with the (snail) mail system. A person can drop a sequence of international
letters into a mailbox in one country, most of them will be delivered to the correct address in the destination
country. The letters will probably travel through one or more international mail gateways along the way, but
this is transparent to the users. Furthermore, that each country (i.e., each network) has its own stamps,
preferred envelope sizes, and delivery rules is hidden from the users.
The internet layer defines an official packet format and protocol called IP (Internet Protocol), plus a
companion protocol called ICMP (Internet Control Message Protocol) that helps it function. The job of the
internet layer is to deliver IP packets where they are supposed to go. Packet routing is clearly a major issue
here, as is congestion (though IP has not proven effective at avoiding congestion).
 The Transport Layer
The layer above the internet layer in the TCP/IP model is now usually called the transport layer. It
is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in
the OSI transport layer. Two end-to-end transport protocols have been defined here.
The first one, TCP (Transmission Control Protocol), is a reliable connection-oriented protocol that allows
a byte stream originating on one machine to be delivered without error on any other machine in the
internet. It segments the incoming byte stream into discrete messages and passes each one on to the
internet layer. At the destination, the receiving TCP process reassembles the received messages into
the output stream. TCP also handles flow control to make sure a fast sender cannot swamp a slow
receiver with more messages than it can handle.
 The second protocol in this layer, UDP (User Datagram Protocol), is an unreliable,
connectionless protocol for applications that do not want TCP’s sequencing or flow control and wish to
provide their own. It is also widely used for one-shot, client-server-type request-reply queries and
applications in which prompt delivery is more important than accurate delivery, such as transmitting speech
or video. The relation of IP, TCP, and UDP is shown in Fig. 1-22. Since the model was developed, IP has
been implemented on many other networks.
 The Application Layer
The TCP/IP model does not have session or presentation layers. No need for them was perceived.
Instead, applications simply include any session and presentation functions that they require. Experience
with the OSI model has proven this view correct: these layers are of little use to most applications.
On top of the transport layer is the application layer. It contains all the higher-level protocols. The
early ones included virtual terminal (TELNET), file transfer (FTP), and electronic mail (SMTP). Many other
protocols have been added to these over the years. Some important ones that we will study, shown in Fig.
1-22, include the Domain Name System (DNS), for mapping host names onto their network addresses,
HTTP, the protocol for fetching pages on the World Wide Web, and RTP, the protocol for delivering real-
time media such as voice or movies.
A Comparison of the OSI and TCP/IP Reference Models
Three concepts are central to the OSI model:
1. Services.
2. Interfaces.
3. Protocols.
Probably the biggest contribution of the OSI model is that it makes the distinction between these
three concepts explicit. Each layer performs some services for the layer above it.
The service definition tells what the layer does, not how entities above it access it or how the layer
works. It defines the layer’s semantics.
A layer’s interface tells the processes above it how to access it. It specifies what the parameters are
and what results to expect. It, too, says nothing about how the layer works inside.
Finally, the peer protocols used in a layer are the layer’s own business. It can use any protocols it
wants to, as long as it gets the job done (i.e., provides the offered services). It can also change them at will
without affecting software in higher layers
 The TCP/IP model did not originally clearly distinguish between services,
interfaces, and protocols, although people have tried to retrofit it after the fact to make it more OSI-like.
For example, the only real services offered by the internet layer are SEND IP PACKET and RECEIVE IP
PACKET. As a consequence, the protocols in the OSI model are better hidden than in the TCP/IP model
and can be replaced relatively easily as the technology changes. Being able to make such changes
transparently is one of the main purposes of having layered protocols in the first place.
 The OSI reference model was devised before the corresponding protocols were invented. This
ordering meant that the model was not biased toward one particular set of protocols, a fact that made it quite
general. The downside of this ordering was that the designers did not have much experience with the subject
and did not have a good idea of which functionality to put in which layer.
For example, the data link layer originally dealt only with point-to-point networks. When
broadcast networks came around, a new sublayer had to be hacked into the model. Furthermore, when
people started to build real networks using the OSI model and existing protocols, it was discovered that
these networks did not match the required service specifications (wonder of wonders), so convergence
sublayers had to be grafted onto the model to provide a place for papering over the differences. Finally, the
committee originally expected that each country would have one network, run by the government and using
the OSI protocols, so no thought was given to internetworking. To make a long story short, things did not turn
out that way.
 An obvious difference between the two models is the number of layers: the OSI model has seven
layers and the TCP/IP model has four. Both have (inter)network, transport, and application layers, but the
other layers are different.
Another difference is in the area of connectionless versus connection-oriented communication.
The OSI model supports both connectionless and connection oriented communication in the network layer,
but only connection-oriented communication in the transport layer, where it counts (because the transport
service is visible to the users). The TCP/IP model supports only one mode in the network layer
(connectionless) but both in the transport layer, giving the users a choice. This choice is especially important
for simple request-response protocols.