tcp opening a connection and closing a connection

TCP opening a connection and closing a connection

SYN sent from client

SYN/ACK sent from server

ACK sent from client

FIN sent from client

FIN/ACK sent from server

ACK sent from client

(1) TCPs three-way handshake in opening a connection

(2) TCPs three-way handshake in closing a connection

7 Layer OSI Model

OSI Layer Function provided

Application Network application such as file transfer and terminal emulation

Presentation Data formatting and encryption.

Session Establishment and maintenance of sessions.

Transport Provision for end-to-end reliable delivery.

Network Delivery of packets of information, which includes routing.

Data Link Transfer of units of information, framing and error checking.

Physical Transmission of binary data of a medium.

Circuit Switching, Message Switching and Packet Switching

Circuit Switching : Establishes end-to-end network path before any data is sent. Network path once set up, is not shared with other users. Other nodes have to wait until the transmission is over to the circuit to be released.

Message Switching : No network path is established. Entire message is sent to the destination via network paths that are shared with other users. Congestions are likely to occur depending on the size of the message.

Circuit Switching, Message Switching and Packet Switching

Packet Switching : No network path is established. Messages are broken down in to smaller size manageable packets before being sent. Packets are being sent to the destination over network paths shared with other traffic. Destination assembles the packets retrieving the original message.

Virtual Circuit Switching : Packet switching network that emulates circuit switching by establishing a virtual circuit before the packets are sent. All the packets are delivered using the same virtual

circuit.

Question 13)

1 4 5

2 3

6

1

2

3

3

1

1

2

5

5

Find the least cost path from node (1) to node (6) using forward search algorithm.

Network : Network is a set of interconnected computers

Local Area Network : A computer network covering a small geographic area ( usually less than 1 km2 )

Wide Area Network : A computer network that covers a broad geographic area ( usually a collection of LANs )

A network switch constructs its switching table by extracting the source MAC address from the received frames. If the entry does not exist the switch will forward the frames to all its ports.

Prone to ARP poisoning attacks. Cain and Able

Hub : A Hub is used in a wired network to connected Ethernet cables from number of workstations together. Data packets sent from one machine are connected to rest of the machines. The each machine check the header for the destination address. The intended machine retrieves the packet while other machines discard it. Hubs are prone to packet sniffing attacks. TCP dump/Net stumbler/ Wireshark(Ethereal) – A hub operates on layer 1 : physical layer

Switch : A switch is a intelligent hub that forwards incoming frames to a specific port that will take data to its intended destination. Reads the intended MAC address from the received data frame and determines the forwarding port from the Switching Table and connect the received frame to the intended port. Switch operates on MAC sub layer of the layer 2 : the data link layer.

Bridge : A bridge is used to partition busy networks into several collision domains. Bridge operates on the MAC layer, sub layer of the layer 2 (data link layer). Bridge reads the destination MAC address from the frame header and decides which partition the frame should be relayed to.

Once switched on the bridge learns which computer is connected in each collision domain. The learning process occurs as the bridge encounters traffic. The bridge extracts the source MAC address from received frames to determine collision domain that the extracted MAC address is connected to and constructs a bridging table. If the bridging table doesn’t have an entry for the MAC address, bridge will forward the frame to all the domains connected.

Router : A router is an internetworking device commonly used to connect different network types together. A router has two or more interface, each interface connecting a different network type and forwards packets according to its destination address. Router can be seen as a layer 3 switch (network layer ). Router accepts incoming packets from one network interface and forwards towards its intended destination. Routers are the basic building block holding the internet in place. Routers uses IP address instead of MAC addresses and constructs routing table from the received packets.

Computer A

Computer B

Modem : A modem is a communication device that converts one form of signal to another that is suitable for transmission over communication network such as telephone lines, typically from digital to analogue and from analogue to digital.

Therefore average time for transmitting one frame -

Sending time - Time to detect collision - 2

Propagation delay to frame length ratio - Ta Transmitted frame length -

Number of retries needed -

vt

K

T

KT 21

KTtv 2

KaT 211

KTT 211 vtTU

KatTU v 2111

Utilization factor -

Advantages of wireless LAN -

Increased mobility of users

Increased flexibility

Instant networking

Availability of LAN technology

Wireless LAN - A computer network covering a small geographic area that communicates with each other without wires (wireless)

Disadvantages of wireless LAN -

Higher cost

Lower performance

Lower reliability

Multiple standards

Poor security (WEP-Wired Equivalent Privacy- open system authentication- no real authentication but clients need correct key to encrypt data, shared key authentication – shared key authentication can easily be cracked)

WPA- Wifi Protected Access – can be cracked passively or actively. Needs several thousand authentication packets to crack pre shared key. Alternatively you send deauthentication signals to existing clients, forcing them to reauthenticate with the AP, accelerating the cracking process.

Functionality – Speed and Reliability, The network must work with reasonable speed and reliability

Scalability – Ability to grow without major changes, The network must be able to grow without any major changes to the overall design

Adaptability – Easily implements new technologies, The network should include no element that would limit the use of new technologies as they become available

Manageability – Facilitates monitoring and ease of Management, The network must be able to allow ease of monitoring and managing

Criteria for LAN Design

7 Layer OSI Model

OSI Layer Function provided

Application Network application such as file transfer and terminal emulation

Presentation Data formatting and encryption.

Session Establishment and maintenance of sessions.

Transport Provision for end-to-end reliable delivery.

Network Delivery of packets of information, which includes routing.

Data Link Transfer of units of information, framing and error checking.

Physical Transmission of binary data of a medium.

IP Address is 32 bits in length and consists of two parts, prefix denoting network id. and suffix denoting host id. Since internet might include networks including from few computers to hundreds of thousands we divide the 32 bit IP address space into three primary classes with different prefix and suffix sizes

IP addresses

0 prefix suffix

0 1 2 3 4 8 16 24 31

Class A

Class B 1 prefix suffix0

Class C 1 prefix suffix1 0

0Class D 1 multicast address1 1

1Class E 1 Reserved for future use1 1

Primary Classes

Net id

Although IP addresses are 4-octets, 32 bit numbers we generally represent each octet by its decimal value separated by a dot. This is known as dotted decimal notation.

10000000 00001010 00000010 00000011 128 . 10 . 2 . 3

32 bit binary number Dotted decimal notation

The class of the network must be identified from the first octet.

Class Range of Values

A 0 through 127

B 128 through 191

C 192 through 223

D 224 through 239

E 240 though 255

Multicast addresses : To use IP multicasting set of hosts must agree to share a multicast address. Once the multicast group has been established, a copy of any packet sent to the multicast address will be delivered to each host in the set.

IP Address designing parameters

Address Class Bits in PrefixMaximum Number

of NetworksBits in Suffix

Maximum Number of Hosts in the

Network

A 7 128 24 16777216

B 14 16384 16 65536

C 21 2097152 8 256

Subnet MaskingRather than allocating address blocks in eight bit boundaries, which in certain cases could be a waste of address space, arbitrary prefix and suffix lengths are introduced using a subnet mask. Subnet mask is a 32 bit number in which left hand side ones correspond to prefix and right hand side zeros correspond to suffix.

255.255.255.0 11111111 11111111 11111111 00000000Subnet Mask

IP Address 128.10.2.3 10000000 00001010 00000010 00000011

Prefix – first 24 bits – 128.10.2.

Suffix – last 8 bits - 3

This corresponds to a class C network

CIDR : Classless Inter-Domain Routing

Rather than allocating address blocks in eight bit boundaries forcing 8, 16, 24 bit prefixes, it uses arbitrary length prefixes.

192.168.0.3 / 16CIDR Notation Prefix length in bits

32bit IP address

Example :- Consider a private intranet of a large organization which consists of 4 physical networks. One small, two medium sized and one extremely large network. Design suitable IP address scheme.

Medium sized network 1 Medium sized network 2

Large Network

Small network

128.10.0.1 128.10.0.2128.11.0.1

128.11.0.2

192.168.0.1 192.168.0.2 10.10.0.1 10.10.0.2

***

ARP : Address Resolution Protocol

ARP is the standard method of finding out hosts layer 2 MAC address when only its layer 3 IP address is known. If host ‘A’ needs to send a a data packet to host ’B’, host ‘A’ broadcasts ARP request packet containing ‘A’s MAC address. Host ‘B’ upon receiving the ARP request replies with its MAC address directly (unicast) to host ‘A’ while other stations discard the ARP request. Host ‘A’ now having the MAC address of host ‘B’ can directly send the data packet.

RARP is the reverse process of ARP. It’s the process of finding out the layer 3 network address from layer 2 MAC address. RARP was subsequently replaced by DHCP.

RARP : Reverse Address Resolution Protocol

DHCP : Dynamic Host Configuration Protocol

DHCP is a protocol used by clients to obtain various necessary parameters for its operation. DHCP allows clients to be configured automatically over the network. New machines can be added to the network more easily. Less chance of error.

DNS : Domain Name System

DNS associates domain names with difficult to remember IP address. Mapping domain name to an IP address is called domain name translation. When a host requires a domain name translation it makes a request to its assigned DNS server.

Parameters802.16a (WiMax)

802.11 (WLAN)

802.15 (Bluetoo

th)

Frequency Band 2-11GHz 2.4GHz Varies

Range ~31miles~100meter

s~10meters

Data transfer rate 70 Mbps11 Mbps – 55 Mbps

20Kbps – 55 Mbps

Number of Users Thousands DozensDozens

WiMax, WLAN and Bluetooth Comparison

Aloha System

G ≥ N

Aloha key concept : If you have data send data. If the message collides try resending the data later. The waiting time must be random or the same frames will collide over and over again, in a deadlock.

N – Number of frames generated by all the users connected per frame time

G – Total Number of frames generated per frame time (New and retransmitted)

S – Channel throughput per frame time, number of successful transmission per frame time

Channel throughput is equal to number of total packets generated in the system per given time into the probability of a frame does not suffer collision

G is also known as the channel traffic

ALOHA was a pioneering computer networking system developed at the University of Hawaii. Although the network itself is no longer used, the ALOHA project is quite important as one of the core concepts in the ALOHA net is the basis for widely used Ethernet technology.

Aloha CapacityS – Channel throughput per frame time, number of successful transmission per frame time


P0 – Probability of a frame does not suffer collision

S=G. P0If (number of users connected) → ∞, N and G takes the shape of Poisson distribution. P0 = e-2G

Therefore S = G.e-2G

Aloha CapacityS – Channel throughput per frame time, number of successful transmission per frame time


P0 – Probability of a frame does not suffer collision

S=G. P0If (number of users connected) → ∞, N and G takes the shape of Poisson distribution. P0 = e-2G

Therefore S = G.e-2G

Capacity is the maximum throughput

We can see the throughput is maximum at G = 0.5

S=0.5 x e-2x0.5 = 0.1839

The channel Utilization is Approximately 18%

Slotted AlohaSlotted Aloha divides the time into discrete time intervals, each interval corresponding to one frame time. Rather than trying to send data at any given time, users are forced to send data only at the beginning of a time slot. This minimises probability of collision and increase the channel throughput. The users must know the beginning of the time slot to begin transmitting data. The system uses a pilot signal to inform the workstations of the beginning of the time slot.

S=G. P0

Therefore S = G.e-G


We can see the throughput is maximum at G = 1

S=1 x e-1 = .368


The channel throughputP0 = e-G

In slotted Aloha the probability of frame not colliding improves

The channel capacity doubles


We can see the throughput is maximum at G = 0.5

S=0.5 x e-2x0.5 = 0.1839


Pure ALOHA system has a utilization factor of 18%, and therefore can reach a maximum capacity of 14400x0.1839 = 2648 bits/s

If an ALOHA system has a total generated capacity of 14400 bits/s,

a.) Compute the maximum capacity a pure ALOHA system can reach

b.) Compute the maximum capacity a slotted ALOHA system can reach

Pure ALOHA system has a utilization factor of 37%, and therefore can reach a maximum capacity of 14400x0.386 = 5558 bits/s

tcp opening a connection and closing a connection

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