Download Part III Network Layer

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
NETWORK LAYER DUTIES
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INTERNETWORKING
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The Motivation for Internetworking
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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

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
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?
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A scheme that provides universal service among
heterogeneous networks
Hardware
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Software
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Used to interconnect a set of physical networks
Provide universal services
Internet
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System of connected physical networks
INTERNETWORK
NETWORK LAYER IN AN
INTERNETWORK
IPv4 ADDRESSES
An IPv4 address is a 32-bit address that uniquely and
universally defines the connection of a device (for
example, a computer or a router) to the Internet.
Topics discussed in this section:
Address Space
Notations
Classful Addressing
Classless Addressing
Network Address Translation (NAT)
FINDING THE ADDRESS CLASS
FINDING THE CLASS IN DECIMAL NOTATION
Example 4
Find the class of each address:
a.
227.12.14.87
b.
252.5.15.111
c.
134.11.78.56
Solution
a.
b.
c.
The first byte is 227 (between 224 and 239); the class is D.
The first byte is 252 (between 240 and 255); the class is E.
The first byte is 134 (between 128 and 191); the class is B.
NETID AND HOSTID
NETWORK ADDRESS
Example 5
Given the address 23.56.7.91, find the network address.
Solution
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 6
Given the address 132.6.17.85, find the network address.
Solution
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 7
Given the network address 17.0.0.0, find the class.
Solution
The class is A because the netid is only 1 byte.
A NETWORK WITH TWO LEVELS OF HIERARCHY
• IP addresses are designed with two levels of hierarchy.
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A NETWORK WITH THREE LEVELS OF HIERARCHY
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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.
Solution
We 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.
Solution
We 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 11101111
b. 11000001 10000011 00011011 11111111
c. 14.23.120.8
d. 252.5.15.111
Solution
a. 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.
Mask Address
Default masks for classful addressing
shows a block of addresses, in both binary and dotteddecimal 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?
Solution
The binary representation of the given address is
11001101 00010000 00100101 00100111
If 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
Solution
The binary representation of the given address is
11001101 00010000 00100101 00100111
If we set 32 − 28 rightmost bits to 1, we get
11001101 00010000 00100101 00101111
or
205.16.37.47
This 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 works
1. Knowledge about the whole network –Each routers
shares its knowledge about the entire network
2. 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
Algorithm :Distance Vector Routing
1.
2.
3.
Router transmits its distance vector to each of its
neighbors.
Each router receives and saves the most recently
received distance vector from each of its neighbors.
A router recalculates its distance vector when:
a.
b.
It receives a distance vector from a neighbor containing
different information than before.
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.
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.
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Algorithm :Link State routing
1.
2.
3.
4.
Each router is responsible for meeting its neighbors
and learning their names.
Each router constructs a link state packet (LSP) which
consists of a list of names and cost to reach each of its
neighbors.
The LSP is transmitted to ALL other routers. Each router
stores the most recently generated LSP from each
other router.
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
Dijkstra’s Shortest Path Algorithm
Initially mark all nodes (except source) with infinite distance.
working node = source node
Sink node = destination node
While 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.
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
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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
connection oriented protocol
while
TCP
is
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.
Note
The bytes of data being transferred in each
connection are numbered by TCP.
The numbering starts with an arbitrarily
generated number.
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?
Solution
The following shows the sequence number for each segment:
Note
The value in the sequence number
field of a segment defines the number
assigned to the first data byte
contained in that segment.
Stream delivery
Sending and receiving buffers
Stream of bytes
TCP segments
Segment N
H
Segment 1
H
Note
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.
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 fields
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
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.
Applications
Applications
Ports:
23 80 104
7
80 16
TCP
TCP
IP
IP
Ports:
TCP header fields
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
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 fields
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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
receive
Example:
The acknowledgement for a segment with
sequence number 0 and
1500 data bytes is
AckNo=1500+1
TCP header fields
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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:
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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 fields
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
Header Length ( 4bits):
 Length
of header in 32-bit words
 Note that TCP header has variable length (with
minimum 20 bytes)
TCP header fields
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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 fields
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Flag bits:
 RST:


 SYN:

 FIN:


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
Synchronize sequence numbers
Sent in the first packet when initiating a connection
Sender is finished with sending
Used for closing a connection
Both sides of a connection must send a FIN
TCP header fields
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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 fields
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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 TCP
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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
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Opening a TCP Connection
Closing a TCP Connection
Special Scenarios
State Diagram
TCP Connection Establishment
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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
seq: 8000
UAPRS F
SYN
seq: 15000
ack: 8001
nd: 5000
U A P R S F rw
SYN + ACK
seq: 8000
ack: 15001
UAPRS F
rwnd: 10000
ACK
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 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
Messages
are pushed
1
5
Flow control
feedback
3 Messages
are pulled
2
Segements are pushed
4
Flow control feedback
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 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 16
Steps 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