Chapter 25 Internet Routing

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Chapter 25

Internet Routing

Internetworking Technology

How routing tables are built initially

How routing software updates the tables as needed.

Propagation of routing information in the global Internet

General concept of routing information exchange,

several routing update protocols used in the Internet.

Static Vs. Dynamic Routing

IP routing can be partitioned into two
broad categories.

Static routing: Routes are called

they do not change

a static routing
table is loaded with values when the
system starts and the routes do not
change unless an error is detected.

Dynamic routing: system that can change
routing table information over time.

Static Routing In Hosts And A Default

Static routing is easy to specify and does not require extra
routing software.

It does not consume bandwidth, and no CPU cycles are required
to propagate routing information.

However, it is relatively inflexible, it cannot accommodate
network failures or changes in topology.

Where is static routing used?

Most hosts use static routing, especially in cases where the host
has one network connection and a single router connects the
network to the rest of the Internet. The host’s routing table
contains two entries: one for the network to which the host
attaches and a default entry that directs all other traffic to a
specific router.

Static Routing In Hosts And A Default
Route (continued)

Dynamic Routing And Routers

To ensure that all routers maintain
information about how to reach each possible
destination, each router runs software that
learns about destinations other routers can
reach, and informs other routers about
destinations that it can reach.

The routing software uses incoming
information to update the local routing table

Dynamic Routing And Routers

Routing In The Global Internet

To limit routing traffic, the Internet uses a
two level routing hierarchy: Routers and
networks in the Internet are divided into
groups, where each group is known as an
autonomous system.

All routers within a group exchange routing

At least one router (possible more) in each group
summarizes the information before passing it to
other groups.

Routing In The Global Internet

How large is a group? What protocol do
routers use within a group? How is routing
information represented? What protocol do
routers use between groups?

The designers did not dictate an exact size nor
specify an exact data presentation or protocol.

The designers purposefully kept the
architecture flexible enough to handle a wide
variety of organizations

they decided to
permit each organization to choose a routing
protocol independently.

Autonomous System Concept

Autonomous System (AS): Concept of groups
and routers

A contiguous set of networks and routers all under
control of one administrative authority

Sufficiently flexible co accommodate many

EX: AS can correspond to an entire corporation or

AS size can be made for economic, technical, or
administrative reasons.

Two Types Of Internet
Routing Protocols

Interior Gateway Protocols (IGPs)

Routers within an autonomous system uses an Interior
Gateway Protocol (IGP) to exchange routing information

IGP is easy to install and operate, but may limit the size or
routing complexity.

Exterior Gateway Protocols (EGPs)

A router in one autonomous system uses an Exterior
Gateway Protocol (EGP) to exchange routing information
with a router in another autonomous system.

EGPs are more complex to install and operate than IGPs but
EGPs offer more flexibility and lower overhead (i.e., less

Two Types Of Internet
Routing Protocols (continued)

Routes And Data Traffic

Data traffic for a given destination flows
in exactly the opposite direction of
routing traffic

The Border Gateway Protocol

Version 4 is the current standard BGP, it is
the Exterior Gateway Protocol used to
exchange routing information among
autonomous systems in the global Internet.

ISPs use BGP
4 to obtain routing information
from each other and from an authoritative
route server

Because all ISPs participate, a datagram from
an arbitrary computer to an arbitrary
destination will be forwarded correctly.

The Routing Information
Protocol (RIP)

RIP Characteristics:

Routing within an autonomous system: RIP is
designed as an IGP used to pass information among
routers within an autonomous system

Hop Count Metric:

RIP measures distance in network

each network
between source and destination counts as single hop.

RIP uses origin
one counting, meaning that a directly
connected network is 1 hop away, not zero.

Unreliable Transport: RIP uses UDP for all message

RIP (continued)

Broadcast Or Multicast Delivery: RIP is
intended for use over LAN technologies that
support broadcast or multicast

Support For Default Route Propagation:
Allows router to advertise a default route

Distance Vector Algorithm: Uses distance
vector approach to routing defined in

Passive Version For Hosts: Allows a host to
listen passively and update its routing table.

The Open Shortest Path First
Protocol (OSPF)

Although RIP works well among a few
routers, it does not scale to a large
internet (since messages are large)

To satisfy demand for a routing protocol
that can scale to large organizations,
the IETF devised an IGP known as

OSPF Areas

One particular that makes OSPF more complex and
powerful is
hierarchical routing

To achieve a hierarchy, a manager divides routers
and networks into subsets that calls

OSPF allows communication between areas in
addition to exchanging information within an area.

One router in each area is configured to
communicate with a router in one or more other

The two routers summarize routing information they
have learned from other routers within their
respective area, and then exchange the summary.

Multicast Routing

Different between multicast routing and unicast
routing: Internet multicast allows dynamic group
membership and anonymous senders.

An IP multicast group is anonymous in two ways:

First, neither a sender nor a receiver knows (or can find out)
the identity or the number of group members

Second, routers and hosts do not know which applications
will send a datagram to a group because an arbitrary
application can send a datagram to any multicast group at
any time.

Chapter 25 Summary

Both hosts and routers contain an IP routing table.
Most hosts use static routing and some use dynamic

The Internet is divided into a set of autonomous

IGPs: exchange routing information within
autonomous system

EGPs: pass routes between autonomous system.

BGP: is the primary EGP in the Internet

Internet multicast allows dynamic group membership
and an arbitrary source can send to multicast group
without being a member.

Chapter 26


Server Interaction


server interaction

Basis forms for all network applications.

Basic client
server model

sever interaction arises from the
way networks protocols operate.

Example applications use the client
server paradigm.

Functional Application Software

Applications supply high
level services that
users access

Determine how users perceive the underlying

Determine the format which information is

Define symbolic names identifier for physical
and abstract resources on internet

The Client
Server Paradigm

The terms client and server refer to the two
applications involved in a communication:

The application that actively initiates contact is
called a

The application that passively waits for contact is
called a

Network applications use a form of
communication known as the client

Client Characteristics

Is an arbitrary application program that becomes a
client temporarily when remote access is needed.

Is invoked directly by a user, and executes only for
one section.

Runs locally on a user’s personal computer.

Actively initiates contact with a server

Can access multiple services as needed, but actively
contacts one remote server at a time.

Does not require special hardware or a sophisticated
operating system.

Server Characteristics

Is a special
purpose, privileged program dedicated to
providing one service, but can handle multiple
remote clients at the same time.

Is invoked automatically when a system boots, and
continues to execute through many sections.

Runs on a shared computer

Waits passively for contact from arbitrary remote

Accepts contact from arbitrary clients, but offers a
single service.

Requires powerful hardware and a sophisticated
operating system.

Requests, Responses, And
Direction of Data Flow

Information can pass in either or both
directions between a client and a server

A client sends a request (or a series or
requests) to a server, and the server returns
a response (or issues a series of responses)
to the client.

In other cases, the server provides
continuous output without any request

soon as the client contacts the server, the
server begins sending data.

Transport Protocols and
Server Interaction

As the figure shows, a client
or server application
interacts directly with a
layer protocol to
establish communication and
to send or receive

The transport protocol then
uses lower layer protocols to
send and receive individual

Multiple Services On One

Powerful computer system can run
multiple clients and servers at the same


The computer must have fast processor
and large memory

Must have an operating system that allows
multiple application programs to execute
concurrently (e.g., UNIX or Windows).

Multiple Services On One

Identifying A Particular Service

Transport protocols mechanism assigns
each service a unique identifier and
requires both the client and server to
use the identifier.

Protocol software uses the identifier to
direct each incoming request to the
correct server.

Dynamic Server Creation

Server is constructed in two parts:

One that accepts requests and creates new
thread for the request

Another consists of the code to handle
individual request.

Oriented And
Connectionless Transport

Transport protocols support two basic forms of
communication: connection
oriented or


requires explicit connection, provides reliable data delivery.

Clients and Server exchange data once connection is


Permits an application to send a message to any destination
at any time.

When using connectionless, sending application must specify
a destination with each message it sends.

Service Through Multiple

Servers need not choose between connectionless and
oriented transport

the same service can
be made available over two or more transport

There are two possible implementations of a
multiprotocol server:

First: two servers exist for the same service

one server
uses connectionless, the other uses connection

Second: a single server program interacts with two or more
transport protocols at the same time.

Complex Client

A client application is not restricted to accessing a
single service

An application can first become a client of one service, and
then become a client of another.

The client contacts a different server (perhaps on a different
computer) for each service.

A client application is not restricted a accessing a
single server for a given service. In some services,
each server provides different information than
servers running on other computers.

A server is not restricted from performing further
server interactions

a sever for one service
can become a client of the other.

Chapter 26 Summary

Communication across a network or internet all use a single
form of interaction. The interaction is called the client

A program that passively waits for contact is called a sever, and
a program that actively initiates contact with a server is called a

A client program is often invoked by a user, and usually
executes on the user’s private computer.

Sever program usually run on large, server
class computers that
have sophisticated operating systems.

Clients and servers use transport protocols to communicate

Chapter 26 Summary

Both client and server are required to
use identifier for the desired service.

server interaction can be

A single client can access more than one

A client can access server on multiple
machines, and a server for one service can
become a client for other services.