PART III NETWORK LAYER

1

Networking and

Internetworking Devices

Connecting Devices

Following are the networking devices mostly used

Connecting Devices and the OSI Model

A Repeater in the OSI Model :Repeater is a regenerator not an amplifier

A Repeater

Function of a Repeater

Bridge : Operates in both physical and data link layer

A Bridge

Function of a Bridge

Multiport Bridge

Router: Router is Network layer device Router relay packets among multiple

interconnected networks

Gateway :Is a protocol convertor A gateway accept a packet formatted for

one protocol and convert it to packet formatted for another protocol

A Gateway

Single-Protocol: Designd to route packet belonging to same protocol

Multiprotocol Router:Designd to route packet belonging to two or more protocol

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NETWORK LAYER DUTIES

INTERNETWORKING

The Motivation for Internetworking There are many different LAN and WAN technologies In real world, computers are connected by many different

technologies Any system that spans a large organization must accommodate

multiple technologies

Universal services Telephones are useful because any telephone can reach any other

telephone Universal service among computers greatly increases the usefulness

of each computer Providing universal service requires interconnecting networks

employing different technologies

WHAT IS INTERNETWORKING?

A scheme that provides universal service among heterogeneous networks

Hardware Used to interconnect a set of physical networks

Software Provide universal services

Internet System of connected physical networks

INTERNETWORK

NETWORK LAYER IN AN INTERNETWORK

IPv4 ADDRESSESIPv4 ADDRESSES

An An IPv4 addressIPv4 address is a is a 32-bit32-bit address that uniquely and address that uniquely and universally defines the connection of a device (for universally defines the connection of a device (for example, a computer or a router) to the Internet.example, a computer or a router) to the Internet.

Address SpaceNotationsClassful AddressingClassless AddressingNetwork Address Translation (NAT)

Topics discussed in this section:Topics discussed in this section:

FINDING THE ADDRESS CLASS

FINDING THE CLASS IN DECIMAL NOTATION

Example 4Example 4

Find the class of each address:

a. 227.12.14.87

b. 252.5.15.111

c. 134.11.78.56

SolutionSolutiona. The first byte is 227 (between 224 and 239); the class is D.b. The first byte is 252 (between 240 and 255); the class is E.c. The first byte is 134 (between 128 and 191); the class is B.

NETID AND HOSTID

NETWORK ADDRESS

Example 5Example 5

Given the address 23.56.7.91, find the network address.

SolutionSolution

The class is A. Only the first byte defines the netid. We can find the network address by replacing the hostid bytes (56.7.91) with 0s. Therefore, the network address is 23.0.0.0.

Example 6Example 6

Given the address 132.6.17.85, find the network address.

SolutionSolution

The class is B. The first 2 bytes defines the netid. We can find the network address by replacing the hostid bytes (17.85) with 0s. Therefore, the network address is 132.6.0.0.

Example 7Example 7

Given the network address 17.0.0.0, find the class.

SolutionSolution

The class is A because the netid is only 1 byte.

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A NETWORK WITH TWO LEVELS OF HIERARCHY

• IP addresses are designed with two levels of hierarchy.

31

A NETWORK WITH THREE LEVELS OF HIERARCHY

ADDRESSES IN A NETWORK WITH AND WITHOUT SUBNETTING

SUBNET MASK

Dotted-decimal notation and binary notation for an IPv4 address

Change the following IPv4 addresses from binary notation to dotted-decimal notation.

SolutionWe replace each group of 8 bits with its equivalent decimal number (see Appendix B) and add dots for separation.

Change the following IPv4 addresses from dotted-decimal notation to binary notation.

SolutionWe replace each decimal number with its binary equivalent (see Appendix B).

Finding the classes in binary and dotted-decimal notation

Find the class of each address.a. 00000001 00001011 00001011 11101111b. 11000001 10000011 00011011 11111111c. 14.23.120.8d. 252.5.15.111

Solutiona. The first bit is 0. This is a class A address.b. The first 2 bits are 1; the third bit is 0. This is a class C address.c. The first byte is 14; the class is A.d. The first byte is 252; the class is E.

Default masks for classful addressing

Mask Address

shows a block of addresses, in both binary and dotted-decimal notation, granted to a small business that needs 16 addresses.

We can see that the restrictions are applied to this block. The addresses are contiguous. The number of addresses is a power of 2 (16 = 24), and the first address is divisible by 16. The first address, when converted to a decimal number, is 3,440,387,360, which when divided by 16 results in 215,024,210.

A block of 16 addresses granted to a small organization

A block of addresses is granted to a small organization. We know that one of the addresses is 205.16.37.39/28. What is the first address in the block?

SolutionThe binary representation of the given address is

11001101 00010000 00100101 00100111If we set 32−28 rightmost bits to 0, we get

11001101 00010000 00100101 0010000 or

205.16.37.32. This is actually the block shown in Figure 19.3.

Find the last address for the block in the above example

SolutionThe binary representation of the given address is

11001101 00010000 00100101 00100111If we set 32 − 28 rightmost bits to 1, we get

11001101 00010000 00100101 00101111 or

205.16.37.47This is actually the block shown in Figure 19.3.

b. The last address can be found by ORing the given addresses with the complement of the mask. ORing here is done bit by bit. The result of ORing 2 bits is 0 if both bits are 0s; the result is 1 otherwise. The complement of a number is found by changing each 1 to 0 and each 0 to 1.

c. The number of addresses can be found by complementing the mask, interpreting it as a decimal number, and adding 1 to it.

Configuration and addresses in a subnetted network

Three-level hierarchy in an IPv4 address

Network and Host Addresses

A Network with Two Levels of Hierarchy

A Network with Three Levels of Hierarchy

Addresses with and without Subnetting

Masking

IP Datagram

Detail of IP datagram

Version Number- Four bit version number defines the version of IP Header length- Defines the total length of the datagram header Service type-How the datagram should be handelled Total length – Defines total length of IP datagram Identification,Flags and fragmentation offset- These three fields are related to the fragmentation of the IP datagram when the size of the datagram is larger than the underplaying network can carry Time-to-alive- Is used to control the maximum number of hopes visited by the router

Continue…

Protocol – In TCP/IP ,the data section of a packet ,called the payload ,carries the whole packet from another protocol Header checksum – Checking of header of IP is the responsibility of header checksum as IP is not a reliable protocol Source and Destination address- IP address of source machine and destination machine Option – A datagram header can have up to 40 bytes of optiones Payload – Is data coming from its higher layer

ARP

Routing Algorithm

Two common methods are used to calculate shortest path between

two routers :Distance vector routing

Link state routing

Example of an Internet

Distance Vector routing

Three keys to understand how this algorithm works1.Knowledge about the whole network –Each routers shares its knowledge about the entire network2.Routing only to its neighbor-Each router periodically shows its knowledge about the network only to those routers to which it has direct link .3.Information sharing at regular interval –For example ,every 30 seconds router send its information about the whole network to its neighbor

The Concept of Distance Vector Routing

Distance Vector Routing Table

Routing Table Distribution

Updating Routing Table for Router A

Final Routing Tables

1. Router transmits its distance vector to each of its neighbors.

2. Each router receives and saves the most recently received distance vector from each of its neighbors.

3. A router recalculates its distance vector when:

a. It receives a distance vector from a neighbor containing different information than before.

b. It discovers that a link to a neighbor has gone down (i.e., a topology change).

The DV calculation is based on minimizing the cost to each destination.

Algorithm :Distance Vector Routing Algorithm :Distance Vector Routing

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Link State Routing

The following are true related to link state routing 1.Knowledge about the neighborhood 2.To all routers –Each router send this information to every other router on the internetwork not just to its neighbor .3.Information sharing when there is change –Each router send out information about the neighbors when there is change.

Algorithm :Link State routing Algorithm :Link State routing

1. Each router is responsible for meeting its neighbors and learning their names.

2. Each router constructs a link state packet (LSP) which consists of a list of names and cost to reach each of its neighbors.

3. The LSP is transmitted to ALL other routers. Each router stores the most recently generated LSP from each other router.

4. Each router uses complete information on the network topology to compute the shortest path route to each destination node.

Concept of Link State Routing

Cost in Link State Routing

Link State Packet

Flooding of A’s LSP

Link State Database

Initially mark all nodes (except source) with infinite distance.

working node = source nodeSink node = destination nodeWhile the working node is not equal to the sink 1. Mark the working node as permanent. 2. Examine all adjacent nodes in turn

If the sum of label on working node plus distance from working node to adjacent node is less than current labeled distance on the adjacent node, this implies a shorter path. Relabel the distance on the adjacent node and label it with the node from which the probe was made.

3. Examine all tentative nodes (not just adjacent nodes) and mark the node with the smallest labeled value as permanent. This node becomes the new working node.

Reconstruct the path backwards from sink to source.

Dijkstra’s Shortest Path Algorithm

Costs in the Dijkstra Algorithm

Shortest Path Calculation, Part I

Shortest Path Calculation, Part II

Shortest Path Calculation, Part III

Shortest Path Calculation, Part IV

Shortest Path Calculation, Part V

Shortest Path Calculation, Part VI

Shortest Path Calculation, Part VII

Shortest Path Calculation, Part VIII

Shortest Path Calculation, Part IX

Shortest Path Calculation, Part X

Shortest Path Calculation, Part XI

Shortest Path Calculation, Part XII

Shortest Path Calculation, Part XIII

Routing Table for Router A

Multiprotocol Router Vs Single protocol Router

90

Single protocol Router: (Unicast Routing protocol ) : One to one source and destination network RIP :Routing Information Protocol OSPF : Open Shortest Path First BGP4 :Boarder Gateway Protocol 4

Multiprotocol Router (Multicast routing Protocol) One source and group of destination (One to many ) Protocols are DVMRP :Distance Vector Multicast routing protocol MOSF :Multicast open Shortest Path First PIM :Protocol Independent Multicast

Part-IV Transport Layer

Transport Layer protocol

Two protocols : UDP and TCP UDP is connectionless protocol while TCP is connection oriented protocol

Duties of transport layer Duties of transport layer

Process-to-Process Communication Stream Delivery Service Full-Duplex Communication Multiplexing and Demultiplexing

UDP Format

15-2 TCP FEATURES

To provide the services mentioned in the previous section, TCP has several features that are briefly summarized in this section and discussed later in detail.

The bytes of data being transferred in each connection are numbered by TCP.

The numbering starts with an arbitrarily generated number.

Note

Suppose a TCP connection is transferring a file of 5,000 bytes. The first byte is numbered 10,001. What are the sequence numbers for each segment if data are sent in five segments, each carrying 1,000 bytes?

SolutionThe following shows the sequence number for each segment:

The value in the sequence number field of a segment defines the number

assigned to the first data byte contained in that segment.

Note

Stream delivery

Sending and receiving buffers

Stream of bytes

TCP segments

Segment 1

H

Segment N

H

The value of the acknowledgment field in a segment defines the number of the next

byte a party expects to receive.

The acknowledgment number is cumulative.

Note

SEGMENT

Before discussing TCP in more detail, let us discuss the TCP packets themselves. A packet in TCP is called a segment.

TCP segment format

Control field

TCP header fields106

Port Number: A port number identifies the endpoint of a

connection. A pair <IP address, port number> identifies one

endpoint of a connection. Two pairs <client IP address, server port number> and <server IP address, server port number> identify a TCP connection.

TCP

IP

Applications

23 10480Ports:

TCP

IP

Applications

7 1680 Ports:

TCP header fields107

Sequence Number (SeqNo): Sequence number is 32 bits long. So the range of SeqNo is

0 <= SeqNo <= 232 -1 4.3 Gbyte

Each sequence number identifies a byte in the byte stream

Initial Sequence Number (ISN) of a connection is set during connection establishment

TCP header fields108

Acknowledgement Number (AckNo): Acknowledgements are piggybacked, I.e

a segment from A -> B can contain an acknowledgement for a segment sent in the B -> A direction.

A hosts uses the AckNo field to send acknowledgements. (If a host sends an AckNo in a segment it sets the “ACK flag”)

The AckNo contains the next SeqNo that a hosts wants to receiveExample: The acknowledgement for a segment with sequence number 0 and 1500 data bytes is AckNo=1500+1

TCP header fields109

Acknowledge Number (cont’d) TCP uses the sliding window flow protocol to

regulate the flow of traffic from sender to receiver TCP uses the following variation of sliding window:

no NACKs (Negative ACKnowledgement) only cumulative ACKs

Example: Assume: Sender sends two segments with “0..1500”

and “1501..3000”, but receiver only gets the second segment.

In this case, the receiver cannot acknowledge the second packet. It can only send AckNo=0+1

TCP header fields110

Header Length ( 4bits): Length of header in 32-bit words Note that TCP header has variable length

(with minimum 20 bytes)

TCP header fields111

Flag bits: URG: Urgent pointer is valid

If the bit is set, the following bytes contain an urgent message in the range:

SeqNo <= urgent message <= SeqNo+urgent pointer

ACK: Acknowledgement Number is valid PSH: PUSH Flag

Notification from sender to the receiver that the receiver should pass all data that it has to the application.

Normally set by sender when the sender’s buffer is empty

TCP header fields112

Flag bits: RST: Reset the connection

The flag causes the receiver to reset the connection

Receiver of a RST terminates the connection and indicates higher layer application about the reset

SYN: Synchronize sequence numbers Sent in the first packet when initiating a connection

FIN: Sender is finished with sending Used for closing a connection Both sides of a connection must send a FIN

TCP header fields113

Window Size: Each side of the connection advertises the

window size Window size is the maximum number of

bytes that a receiver can accept. Maximum window size is 216-1= 65535 bytes

TCP Checksum: TCP checksum covers TCP segment and IP

pseudo header (see discussion on UDP). Urgent Pointer:

Only valid if URG flag is set

TCP header fields114

Options: NOP is used to pad TCP header to multiples

of 4 bytes Maximum Segment Size Window Scale Options

Increases the TCP window from 16 to 32 bits This option can only be used in the SYN

segment (first segment) during connection establishment time

Timestamp Option Can be used for roundtrip measurements

Services of TCP115

Connection Management (TCP transmission policy) :To establish connection before transmitting data

Congestion Management Timer Management for sending lost

frame and others

Connection Management in TCP

116

Opening a TCP Connection Closing a TCP Connection Special Scenarios State Diagram

TCP Connection Establishment

117

TCP uses a three-way handshake to open a connection:(1) ACTIVE OPEN: Client sends a segment with

SYN bit set port number of client initial sequence number (ISN) of client

(2) PASSIVE OPEN: Server responds with a segment with

SYN bit set initial sequence number of server ACK for ISN of client

(3) Client acknowledges by sending a segment with:

ACK ISN of server

A TCP CONNECTION

TCP is connection-oriented. It establishes a virtual path between the source and destination. All of the segments belonging to a message are then sent over this virtual path. You may wonder how TCP, which uses the services of IP, a connectionless protocol, can be connection-oriented. The point is that a TCP connection is virtual, not physical. TCP operates at a higher level. TCP uses the services of IP to deliver individual segments to the receiver, but it controls the connection itself. If a segment is lost or corrupted, it is retransmitted.

Connection establishment using three-way handshake

Connectionopened

PassiveopenActive

open

SYN

U A P R S F

seq: 8000

SYN + ACKU A P R S F

seq: 15000

ack: 8001rwnd: 5000

ACK

U A P R S F

seq: 8000ack: 15001

rwnd: 10000

Means “no data” !

A SYN segment cannot carry data, but it consumes one sequence number.

A SYN + ACK segment cannot carry data, but does consume one

sequence number.

An ACK segment, if carrying no data, consumes no sequence number.

WINDOWS IN TCP

Before discussing data transfer in TCP and the issues such as flow, error, and congestion control, we describe the windows used in TCP. TCP uses two windows (send window and receive window) for each direction of data transfer, which means four windows for a bidirectional communication. To make the discussion simple, we make an assumption that communication is only unidirectional; the bidirectional communication can be inferred using two unidirectional communications with piggybacking.

Topics Discussed in the SectionTopics Discussed in the Section

Send Window Receive Window

Send window in TCP

Receive window in TCP

FLOW CONTROL

As discussed in, flow control balances the rate a producer creates data with the rate a consumer can use the data. TCP separates flow control from error control. In this section we discuss flow control, ignoring error control. We temporarily assume that the logical channel between the sending and receiving TCP is error-free. Figure 15.24 shows unidirectional data transfer between a sender and a receiver; bidirectional data transfer can be deduced from unidirectional one as discussed in Chapter 13.

TCP/IP protocol suite

Messagesare pushed

1

Segements are pushed

2

Messagesare pulled

3

Flow control feedback

4

Flow controlfeedback5

ERROR CONTROL

TCP is a reliable transport layer protocol. This means that an application program that delivers a stream of data to TCP relies on TCP to deliver the entire stream to the application program on the other end in order, without error, and without any part lost or duplicated.

Error control in TCP is achieved through the use of three tools: checksum, acknowledgment, and time-out.

CONGESTION CONTROL

Congestion control in TCP is based on both open loop and closed-loop mechanisms. TCP uses a congestion window and a congestion policy that avoid congestion and detect and alleviate congestion after it has occurred.

TCP TIMERS: Timer Management

To perform its operation smoothly, most TCP implementations use at least four timers as shown in Figure 15.38 (slide 83).

TCP timers

TCP TimerTCP Timer

Retransmission Timer(RTT):To retransmit lost segments TCP employs one retransmission timer

Rules of RTT

1.When TCP send the segment in front of the sending queue ,it starts the timer

2.When the timer expires TCP send first segment in front of the queue and restart the timer

3.When the segments are cumulatively acknowledged segment or segments are purged from the queue

4.If the queue is empty ,TCP stops the timer ,otherwise TCP restarts the timer

Persistence Timer :To deal with zero window size advertise TCP uses other timer known as persistence timer .If the receiving TCP announces a window size of zero ,the sending TCP stops transmitting segments until the receiving TCP sends an ACK segment announcing non zero size of window Keepalive Timer: Is used in some implementation to prevent long idle connection between two TCPs.Suppose a client open a connection with server and transfer data and become silent . TIME-WAIT Timer:Is used during connection termination

Part-V Presentation and Application Layer

Session Layer Dialog

Session-to-Transport Layer Communication

Synchronization Points

Presentation Layer Functions

Direct and Indirect Methods of Translation

Concept of Encryption and Decryption

Encryption/Decryption Methods

Monoalphabetic Substitution

Polyalphabetic Substitution

Transpositional Encryption

Encoding/Decoding

Permutation

Substitution

Product

Exclusive OR

Rotation

DES

Subkey Generation in DES

One of the 16Steps in DES

Public Key Encryption

RSA

RSA Encryption and Decryption

Security of RSA

Signature Authentication

Application Layer

Client-Server Model

DNS in the Internet

Generic Domains

Country Domains

Inverse Domain

Local Login

Remote Login

Concept of NVT

FTP

SMTP Concept

UAs and MTs

Relay MTAs

Mail Gateways

E-mail Address

The Entire E-mail System

MIME

POP3 and SMTP

SNMP Concept

Internet Management Components

MIB

SNMP Messages