Bt0072 computer networks 2

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Computer Networking
BT0072 part-2
By Milan K Antony

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1. Define the term routing. Differentiate static and
dynamic routing.
Routing (or routeing) is the process of selecting paths
in a network along which to send network traffic. Routing is
performed for many kinds of networks, including the telephone
network, electronic data networks (such as the Internet), and
transportation networks. In packet switching networks, routing
directs packet forwarding, the transit of logically addressed
packets from their source toward their ultimate destination
through intermediate nodes; typically hardware devices called
routers, bridges, gateways, firewalls, or switches
Static Routing
Static routing is not really a protocol, simply the process of
manually entering routes into the routing table via a
configuration file that is loaded when the routing device starts
up. As an alternative, these routes can be enterd by a
network administrator who configures the routes. Since these
routes don't change after they are configured (unless a
human changes them) they are called 'static' routes.
Static routing is the simplest form of routing, but it is a
manual process and does not work well when the routing
information has to be changed frequently or needs to be
configured on a large number of routing devices (routers).

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Static routing also does not handle outages or down
connections well because any route that is configured manually
must be reconfigured manually to fix or repair any lost
connectivity.
Dynamic Routing
Dynamic routing protocols are software applications that
dynamically discover network destinations and how to get
to them.
A router will 'learn' routes to all directly connected networks
first. It will then learn routes from other routers that run the
same routing protocol. The router will then sort through it's
list of routes and select one or more 'best' routes for each
network destination it knows or has learned.
Dynamic protocols will then distribute this 'best route'
information to other routers running the same routing
protocol, thereby extending the information on what networks
exist and can be reached. This gives dynamic routing protocols
the ability to adapt to logical network topology changes,
equipment failures or network outages 'on the fly'.
2. Differentiate IPv4 and IPv6 addressing schemes.

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Pv6 is based on IPv4, it is an evolution of IPv4. So many
things that we find with IPv6 are familiar to us. The main
differences are:
1.Simplified header format. IPv6 has a fixed length header,
which does not include most of the options an IPv4 header can
include. Even though the IPv6 header contains two 128 bit
addresses (source and destination IP address) the whole
header has a fixed length of 40 bytes only. This allows for
faster processing.
Options are dealt with in extension headers, which are only
inserted after the IPv6 header if needed. So for instance if a
packet needs to be fragmented, the fragmentation header is
inserted after the IPv6 header. The basic set of extension
headers is defined in RFC 2460.
2.Address extended to 128 bits. This allows for hierarchical
structure of the address space and provides enough addresses
for almost every 'grain of sand' on the earth. Important for
security and new services/devices that will need multiple IP
addresses and/or permanent connectivity.
3.A lot of the new IPv6 functionality is built into ICMPv6 such
as Neighbor Discovery, Autoconfiguration, Multicast Listener
Discovery, Path MTU Discovery.
4.Enhanced Security and QoS Features.
IPv4 means Internet Protocol version 4, whereas IPv6 means
Internet Protocol version 6. IPv4 is 32 bits IP address that
we use commonly, it can be 192.168.8.1, 10.3.4.5 or other 32
bits IP addresses. IPv4 can support up to 2
32
addresses,

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however the 32 bits IPv4 addresses are finishing to be used in
near future, so IPv6 is developed as a replacement. IPv6 is
128 bits, can support up to 2
128
addresses to fulfill future
needs with better security and network related features. Here
are some examples of IPv6 address: 1050:0:0:0:5:600:300c:326b
ff06::c3
0:0:0:0:0:0:192.1.56.10
3. Describe the following:
a. Congestion
Network congestion occurs when a queue buffer of a
network node is full and starts to drop packets. Automatic
repeat request may keep the network in a congested state.
This situation can be avoided by adding congestion avoidance
to the flow control, including slow-start. This keeps the
bandwidth consumption at a low level in the beginning of the
transmission, or after packet retransmission.
b. Congestion Control
One of the main principles for congestion control is
avoidance. TCP tries to detect signs of congestion before it
happens and to reduce or increase the load into the network
accordingly. The alternative of waiting for congestion and
then reacting is much worse because once a network saturates,
it does so at an exponential growth rate and reduces overall
throughput enormously. It takes a long time for the queues to
drain, and then all senders again repeat this cycle. By taking a

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proactive congestion avoidance approach, the pipe is kept as
full as possible without the danger of network saturation. The
key is for the sender to understand the state of the network
and client and to control the amount of traffic injected into
the system. Flow control is accomplished by the receiver
sending back a window to the sender. The size of this window,
called the receive window, tells the sender how much data to
send. Often, when the client is saturated, it might not be able
to send back a receive window to the sender to signal it to
slow down transmission. However, the sliding windows protocol
is designed to let the sender know, before reaching a
meltdown, to start slowing down transmission by a steadily
decreasing window size. At the same time these flow control
windows are going back and forth, the speed at which ACKs
come back from the receiver to the sender provides additional
information to the sender that caps the amount of data to
send to the client. This is computed indirectly.
4. Discuss the Remote Procedure Calls and their
importance in Session Layer.
Remote Procedure Call (RPC) is a protocol which works in
session layer of OSI model and in the Application Layer of
TCP/IP model. It is useful in developing network applications
which need services from a remote computer in the network.
With the help of Remote Procedure Call the programmer need
not worry about the complex structure of OSI layer.

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Working of Remote Procedure Call
1. RPC is working as a client server model
2. RPC uses different Authentication methods to validate
the client request.
3. RPC protocol is independent of transport layer
protocols.
4. RPC reuests can use both UDP and TCP but prefer
UDP format
RPC uses the client/server model. The requesting
program is a client and the service-providing program
is the server. First, the caller process sends a call
message that includes the procedure parameters to
the server process. Then, the caller process waits for
a reply message (blocks). Next, a process on the
server side, which is dormant until the arrival of the
call message, extracts the procedure parameters,
computes the results, and sends a reply message. The
server waits for the next call message. Finally, a
process on the caller receives the reply message,
extracts the results of the procedure, and the caller
resumes execution.

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5. Discuss Simple Mail Transfer Protocol in detail.
Simple Mail Transfer Protocol (SMTP), documented in RFC 821,
is Internet's standard host-to-host mail transport protocol
and traditionally operates over TCP, port 25. In other words,
a UNIX user can type telnet hostname 25 and connect with an
SMTP server, if one is present. SMTP uses a style of
asymmetric request-response protocol popular in the early
1980s, and still seen occasionally, most often in mail protocols.
The protocol is designed to be equally useful to either a
computer or a human, though not too forgiving of the human.
From the server's viewpoint, a clear set of commands is
provided and well-documented in the RFC. For the human, all
the commands are clearly terminated by newlines and a HELP
command lists all of them. From the sender's viewpoint, the
command replies always take the form of text lines, each
starting with a three-digit code identifying the result of the
operation, a continuation character to indicate another lines
following, and then arbitrary text information designed to be
informative to a human.
If mail delivery fails, sendmail (the most important SMTP
implementation) will queue mail messages and retry delivery
later. However, a backoff algorithm is used, and no mechanism
exists to poll all Internet hosts for mail, nor does SMTP
provide any mailbox facility, or any special features beyond
mail transport. For these reasons, SMTP isn't a good choice
for hosts situated behind highly unpredictable lines (like
modems). A better-connected host can be designated as a

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DNS mail exchanger, then arrange for a relay scheme.
Currently, there two main configurations that can be used. One
is to configure POP mailboxes and a POP server on the
exchange host, and let all users use POP-enabled mail clients.
The other possibility is to arrange for a periodic SMTP mail
transfer from the exchange host to another, local SMTP
exchange host which has been queuing all the outbound mail. Of
course, since this solution does not allow full-time Internet
access, it is not too preferred. RFC 1869 defined the capability
for SMTP service extensions, creating Extended SMTP, or
ESMTP. ESMTP is by definition extensible, allowing new service
extensions to be defined and registered with IANA. Probably
the most important extension currently available is Delivery
Status Notification (DSN), defined in RFC 1891.
6. Explain any two static routing algorithms.
Dijkstra's algorithm, conceived by Dutch computer
scientist Edsger Dijkstra in 1959,
[1]
is a graph search algorithm
that solves the single-source shortest path problem for a
graph with nonnegative edge path costs, producing a shortest
path tree. This algorithm is often used in routing. An
equivalent algorithm was developed by Edward F. Moore in 1957.
Let the node at which we are starting be called the initial
node. Let the distance of node Y be the distance from the
initial node to Y. Dijkstra's algorithm will assign some initial
distance values and will try to improve them step-by-step.

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1. Assign to every node a distance value. Set it to zero
for our initial node and to infinity for all other nodes.
2. Mark all nodes as unvisited. Set initial node as current.
3. For current node, consider all its unvisited neighbors and
calculate their distance (from the initial node). For
example, if current node (A) has distance of 6, and an
edge connecting it with another node (B) is 2, the
distance to B through A will be 6+2=8. If this distance is
less than the previously recorded distance (infinity in the
beginning, zero for the initial node), overwrite the
distance.
4. When we are done considering all neighbors of the
current node, mark it as visited. A visited node will not
be checked ever again; its distance recorded now is final
and minimal.
5. If all nodes have been visited, finish. Otherwise, set the
unvisited node with the smallest distance (from the initial
node) as the next "current node" and continue from
step 3.
Suppose you want to find the shortest path between two
intersections on a map, a starting point and a destination. To
accomplish this, you could highlight the streets (tracing the
streets with a marker) in a certain order, until you have a
route highlighted from the starting point to the destination.
The order is conceptually simple: at each iteration, create a
set of intersections consisting of every unmarked intersection
that is directly connected to a marked intersection, this will be

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your set of considered intersections. From that set of
considered intersections, find the closest intersection to the
destination (this is the "greedy" part, as described above)
and highlight it and mark that street to that intersection, draw
an arrow with the direction, then repeat. In each stage mark
just one new intersection. When you get to the destination,
follow the arrows backwards. There will be only one path back
against the arrows, the shortest one.
A flooding algorithm is an algorithm for distributing material
to every part of a connected network. The name derives from
the concept of inundation by a flood.Flooding algorithms are
used in systems such as Usenet and peer-to-peer file sharing
systems and as part of some routing protocols, including OSPF,
DVMRP, and those used in ad-hoc wireless networks.There are
several variants of flooding algorithm: most work roughly as
follows.
Each node acts as both a transmitter and a receiver.
1. Each node tries to forward every message to every one
of its neighbors except the source node.
This results in every message eventually being delivered to all
reachable parts of the network Real-world flooding algorithms
have to be more complex than this, since precautions have to
be taken to avoid wasted duplicate deliveries and infinite
loops, and to allow messages to eventually expire from the
system. Flooding algorithms are also useful for solving many

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mathematical problems, including maze problems and many
problems in graph theory.
7. Discuss IPv4 addressing schemes.
Internet Protocol version 4 (IPv4) is the fourth
revision in the development of the Internet Protocol (IP) and
it is the first version of the protocol to be widely deployed.
Together with IPv6, it is at the core of standards-based
internetworking methods of the Internet. IPv4 is still by far
the most widely deployed Internet Layer protocol. As of 2010,
IPv6 deployment is still in its infancy. IPv4 uses 32-bit
(four-byte) addresses, which limits the address space to
4,294,967,296 (2
32
) possible unique addresses. However, some
are reserved for special purposes such as private networks (~18
million addresses) or multicast addresses (~270 million
addresses). This reduces the number of addresses that can
potentially be allocated for routing on the public Internet. As
addresses are being incrementally delegated to end users, an
IPv4 address shortage has been developing, however network
addressing architecture redesign via classful network design,
Classless Inter-Domain Routing, and network address
translation (NAT) has significantly delayed the inevitable
exhaustion.This limitation has stimulated the development of
IPv6, which is currently in the early stages of deployment, and
is the only long-term solution.Pv4 addresses are usually written
in dot-decimal notation, which consists of the four octets of
the address expressed in decimal and separated by periods.

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This is the base format used in the conversion in the following
table:
Notation Value Conversion from dot-decimal
Dot-decimal
notation
192.0.2.235 N/A
Dotted
Hexadecimal
0xC0.0x00.0x02
.0xEB
Each octet is individually
converted to hexadecimal form
Dotted Octal
0300.0000.0002.
0353
Each octet is individually
converted into octal
Hexadecimal 0xC00002EB
Concatenation of the octets from
the dotted hexadecimal
Decimal 3221226219
The 32-bit number expressed in
decimal
Octal 030000001353
The 32-bit number expressed in
octal
Most of these formats should work in all browsers.
Additionally, in dotted format, each octet can be of any of
the different bases. For example, 192.0x00.0002.235 is a valid
(though unconventional) equivalent to the above addresses.A
final form is not really a notation since it is rarely written in
an ASCII string notation. That form is a binary form of the
hexadecimal notation in binary. This difference is merely the
representational difference between the string "0xCF8E83EB"
and the 32-bit integer value 0xCF8E83EB. This form is used
for assigning the source and destination fields in a software
program.

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8. Discuss Congestion Avoidance in Transport Layer.
 Open loop solutions solve the problem by good
design, in essence, to make sure the problem does not
occur in the first place.
Tools include deciding when to accept new traffic, when
to discard packets and which ones, and how to schedule
packets at various points in the network. A common fact:
they make decisions without regard to the current state
of the network.
 Closed loop solutions are based on the concept of a
feedback loop, which consists of the following three
parts:
1. Monitor the system to detect when and where
congestion occurs.
2. Pass this information to places where actions can be
taken.
3. Adjust system operation to correct the problem.

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9. Discuss various design issues of Session Layer.
The session layer allows users on different machines to
establish sessions between them. A session allows ordinary data
transport, as does the transport layer, but it also provides
enhanced services useful in some applications.
Some of these services are:
 Dialog control - session can allow traffic to go in both
directions at the same time, or in only one direction at a
time. If traffic can go only in one way at a time, the
session layer can help to keep track of whose turn it is.
Simplex Dialogs - One way transfers, similar to an old
computer/printer. Data could be sent to the printer but
the printer could not communicate back.
 Half-Duplex Dialogs - Two-way transfers. Each device
must take their turn. Both cannot transfer at the same
time.
 Full-Duplex Dialogs - Two-way simultaneous data
transfers. Each device has it's own channel. Telephones
are full duplex, as are most modems. Full duplex is the
most expensive to implement.

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 Token management - for some protocols it is essential
that both sides do not attempt the same operation at the
same time. The session layer provides tokens that can be
exchanged. Only the side holding the token may perform
the critical action.
 Synchronization - by inserting checkpoints into the data
stream the layer eliminates problems with potential crashes
at long operations. After a crash, only the data
transferred after the last checkpoint have to be
repeated.
The user of the session layer is in similar position as the user
of the transport layer but having larger possibilities. Session
Administration: A session is a dialog or conversation between
a service requester and a service provider. There are three
phases:
Connection Establishment
Several tasks can be done at this phase:
 Specification of required services
 Login authentication and other security procedures
 Protocol negotiation and parameters
 Notification of connection IDs
 Establishment of dialog control, as well as acknowledgment
of numbering, and retransmission procedures.

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Data Transfer
Once the connection has been established, the devices exchange
data, acknowledgments, and other control data that manage
the dialog. The session layer can also incorporate protocols to
resume broken dialogs. If a connection has not been properly
released, the session can be resumed without re-establishment.
The devices have a specified time period to do so.
Connection Release
Consists of shutting down communications and releasing
resources on the service provider in an orderly fashion.
10. Describe the Multipurpose Internet Mail Extensions.
Multipurpose Internet Mail Extensions (MIME) is an
Internet standard that extends the format of e-mail to
support:
 Text in character sets other than ASCII
 Non-text attachments
 Message bodies with multiple parts
 Header information in non-ASCII character sets
MIME's use, however, has grown beyond describing the content
of e-mail to describing content type in general, including for
the web (see Internet media type).Virtually all human-written
Internet e-mail and a fairly large proportion of automated
e-mail is transmitted via SMTP in MIME format. Internet e-

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mail is so closely associated with the SMTP and MIME standards
that it is sometimes called SMTP/MIME e-mail.The content
types defined by MIME standards are also of importance
outside of e-mail, such as in communication protocols like
HTTP for the World Wide Web. HTTP requires that data be
transmitted in the context of e-mail-like messages, although
the data most often is not actually e-mail. The basic Internet
e-mail transmission protocol, SMTP, supports only 7-bit ASCII
characters (see also 8BITMIME). This effectively limits Internet
e-mail to messages which, when transmitted, include only the
characters sufficient for writing a small number of languages,
primarily English. Other languages based on the Latin alphabet
typically include diacritics not supported in 7-bit ASCII,
meaning text in these languages cannot be correctly
represented in basic e-mail. MIME defines mechanisms for
sending other kinds of information in e-mail. These include
text in languages other than English using character encodings
other than ASCII, and 8-bit binary content such as files
containing images, sounds, movies, and computer programs.
MIME is also a fundamental component of communication
protocols such as HTTP, which requires that data be
transmitted in the context of e-mail-like messages even
though the data might not (and usually doesn't) actually have
anything to do with e-mail. Mapping messages into and out of
MIME format is typically done automatically by an e-mail client
or by mail servers when sending or receiving Internet
(SMTP/MIME) e-mail.

Bt0072 computer networks 2

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