Understanding Data Link Layer, Transport Layer, Network Security, And Routing Tables

Data Link Layer

The layer is responsible for communication between machines connected in the same network for example home network such as WIFI, Satellites, WAN. The layer interface between Network layer and physical layer. It is responsible for exchanging data between locally connected devices.

They are two parts of Data Link Layer:

• Logical Link Layer. It is the interface with upper layer. It allows IPV4, IPV6 and other protocol transaction concurrently.
• Media Access Control The purpose of this is to send traffic and receives traffics from the network.

It provides a connection to the network. The following devices have network interface cards include:

• Network Printers
• Network Scanners
• Bluetooth Connection
• Routers

Network Card have 48 bit MAC address which is unique globally. Data link exchanges data using source and destination MAC address.  Mac address consist of two parts include:

• Organization Unique Identifier which is 24bit
• Unique 24 bit number 24 bit

• Windows – use ipconfig /all and in output locate a Physical address in command prompt.
• Linux – use ifconfig  output will display the mac address of the machine]

Network interface Cards the Smart Devices

NIC inspects every packets in the network therefore able to identify the packet and discard those which does not belong to the host machine.

Processing performance. Since the NIC is able to identify packets belonging to the specific host hence discard packets belonging to other host reducing the computer workload which could have been used to process every packet.

Error checking. With the help of Cyclic Redundancy Check CRC which is appended by the source of a packet, the destination computer will be able to crosscheck the checksum of packet. If the packet checksum does not the same as checksum in the packet header the packet will be dropped.

• Source Mac Address – Front of Ethernet frame
• Destination Mac Address
• Type
• Data
• CRC which 32 Bit Frame Check Sequence

The frame which contains the destination address must be process by all the networks machines.

Media Access Control

It concerns with arbitrating with network access.

• Controlled Access – One station can transmit at a time example Token Ring and FDDI
• Contention Based Access – Used in shared networks example of protocols CSMA/CD for Ethernet networks, CSMA/CA for 802.11 Wireless networks. The disadvantage of this controlled is that collision can occur in the network.

This is Media Access Protocol used by the Ethernet. Ethernet is widely used to connect devices to the network. The media access allow multiple devices to use a common network device. When two machines in the same network sends packets simultaneously collision will occur.

How CSMA/CD Works

NIC listen to the network media to see if another device is transmitting, if it is then it wait for the network media to be available so that it transmit.  If collision occurs both the stations transmitting will detect and stop transmitting. They both wait for some period of time before retransmission.

CSMA/CA

The station reserves the network media to avoid packet collisions.

How CSMA/CA Works

Before station transmitting a data frame it send a request to send a frame to the wireless access point. The wireless will send a response of a Clear To Send (CTS) frame to the wireless network. The CTS will reserve the network for the requesting station for a specific period of time. Then the requesting stations sends the frame without fear of collision without fear of collision.

MAC addresses

Address Resolution Protocol is used to determine the MAC Address of specific machine on the network.

Network operating system maintains a table in RAM that contains the IP address and Mac address of machines that have been contacted recently. In RAM the IP address and Mac Address are store in ARP cache. These data expires after some period of time. When machine want to contact another machine in the network it look ARP cache. If the entry is not found it sends a broadcast to the network asking who has the IP in the network. The machine with that IP address will respond with its MAC address. Therefore the Ethernet has all the information need to send the frame through network.

Transport Layer

Role of Transport Layer

• It ensures that each application has the access to the network
• It ensures that each application can be distinguished from other applications that are running on the computer
• Ensures that application data are transported without errors.

Segmentation and Reassembly

Application sends large streams of data to be delivered. The transport layer break this data into manageable segments so that the network layer are able to handle it. At the destination, the segmented data will be reassembled by the transport layer back to the format sent by the sending application.

Identifications of Application (Sockets)

In a computer several applications run concurrently and each of the application. Transport layer assigned each application a port number which distinguished from another application. Each host is identified by the IP address and the client application running on host is identified by port number. The combination of the IP address and port number is called socket. Each IP Addresses is associated with a port number. Port number ranges from numbered 0 – 65535. To view active sockets on windows we use netstat command.

• Port 0-1023 are reserved ports for server daemon process. They are called well-known ports.

User Application Port Numbers

User applications dynamically choose port above 1023 when they connect to the network therefore each application will have a different port numbers and sessions.

Transport Services with UDP

It provides basic transport services to the network applications. UDP header have the following include:

• Source Address
• Destination Address
• Port numbers.

The cons of this UDP is that they do not support check for checksum errors hence its ideal for video and audio streaming since it is very fast and efficient. It is a stateless protocol.

A stateless protocol is a communications protocol that treats each request as an independent transaction that is unrelated to any previous request. The request and responses are independent.

UDP requests must be fully specified to what they want the destination to do.

Transport Services with TCP

TCP header contains the following headers include:

• Sequence Number
• Acknowledge Number
• Flags – Indicate the function being performed by TCP segment
• Windows Size – Implements the flow control between source and destination

Processing performance

Connection Oriented protocol. Guarantee reliable delivery of data between sender and receiver.

Connection set up. Prior to sending and receiving data, connection must be set it up to allow exchange of sequence number which tracks data transfer throughout the session. 

Acknowledgements. Throughout the sessions the acknowledgements are used to confirm successfully delivery of data. The sequence numbers and associated acknowledgements are the basis of reliable communications.

The cons of the TCP protocol are slow as compared to UDP protocol since the connection must be established and pass acknowledgement messages between sender and receiver.

TCP Timers

TCP uses timers to detect lost Transport Layer Segments. Sender starts the timer each time it sends a TCP segment. If acknowledgement is not receive by the sender within a specified timeout period TCP will count a segment as a lost and it resend the segment again. This can lead to duplicate segments in the network.

TCP Flags

TCP header contains numerous flags that indicate the function of segments. TCP Flags include:

• SYN – Synchronize sequence numbers
• ACK – Acknowledgement segment
• FIN – Close connection
• RST – Abort connection due to error
• PSH – Don’t buffer data instead send it immediately
• URG – Prioritize this packet

TCP Connection establishment and Termination

Connection between client and server can last duration of the session until it get terminated. TCP connection are set-up using 3 handshakes:

• Facilitate two ends of connection to establish communication settings for upcoming data transfer.
• Establish Initial Sequence Number between client and server
• Establish a different Initial Sequence Number in opposite direction

TCP Flow Control

Flow control is required to throttle flow of data from fast sender. Windows size implements the flow control. The value here specifies the maximum number of outstanding Bytes a sender can send without receiving a corresponding acknowledgement. A Receiver can slow or stop a sender from sending data by delaying acknowledgements. This mechanism is called sliding windows flow control.

Networking and Security

Cloud computing. The process of using remote resources through internet through subscription. These resources includes network, hardware, storage etc.
Fed Uni Cloud Services

Amazon

• Corporate Web Site.

Azure

• Work station management
• Identity management

Cloud Service Pros

• Immediate deployment of service
• Illusion of infinite resources
• Relieved employed technical staff that do technical maintenance.
• Stable virtualization

Cloud Service Cons

• Comes at a cost
• Cost depend on the specific components

Cloud Virtualization relies on Virtualization

• Host – Hardware machines on which virtualization software are running on it.
• Guest – Virtual Operating system which can be server or guest.
• Type 1 or Bare metal Virtualization – Virtualization product that runs directly on hardware.
• Type 2 – Virtualization product that run on host operating system like windows mac and Linux.

Mobile Computing

Mobile technologies have undergone several revolution since its introduction in 1980. Different technologies have been added in every generations.

• Analog network. This was the first mobile phone network that supported only voice.
• 2G network –Based on GSM. It has ability to handle data SMS, MMS.
• 3G –Introduce high data speed and were based on CDMA.
• 4G Have much data rates.The main technology is LTE which is based in Internet Protocol.
• 5G is under development.

• Mobile Switching Center Controls transition of mobile phone as it moves from one cell to another. Also it control the interface between internet and public telephone network.
• Base Station Control communication with mobile devices in its cell. Each device use one radio frequency for transmission and another to receive. 

TOPIC 3

1. Describe why Peer-to-Peer networks are less than ideal for campus-sized networks.

Campus-sized networks have centralized roles and permissions which are managed in a central point and each user cannot allocate roles on his own. Also in campus resources are shared to specific users through a dedicated server which is available throughout.

1. You have just created an Application Layer client program that uses a new protocol (named ITECH1102) you have developed. Assuming it is a TCP/IP application show the structure of a Data Link frame. What is held in the first 12 bytes of the frame mentioned in the last question?

• Sequence Number
• Acknowledge Number
• Flags – Indicate the function being performed by TCP segment
• Windows Size – Implements the flow control between source and destination

1. Describe how the ping program works.

The Internet Ping program works much like a sonar echo-location, sending a small packet of information containing an ICMP ECHO_REQUEST to a specified computer, which then sends an ECHO_REPLY packet in return [7].

1. Use the Internet to determine more details about the Protocol that the Ping program uses.

• Internet Control Message Protocol [ICMP] for status messaging [7].

TOPIC 5

1. If the Network layer settings suggest the source and destination computers of a web interaction are on different network what MAC address does the Data Link use as the source and destination of the transmission.

• Data link layer will use remote source and destination address for request and response.

1. Use a diagram to show how two machines on a Private network use the one public address to communicate with Internet hosts. How are port addresses used in such interactions? 

1. The following diagram shows two routers and the associated interface IP addresses and network addresses.

(i) With reference to the Routing table describe what the interface entry contain?

• 132.32.1
• .132.128.0

(ii) What does the Next Hop entry contain?

• Source IP
• Mac Address
• Destination Address

(iii) Complete the associated routing table for Router 1.

• 132.128.0 – 141.132.128.1

(iii) Create the routing table for Router 2.

• 132.128.1v – 141.132.64.1 

1. If your local DNS server cannot resolve an IP address query locally, how does it proceed to fulfil the request?

The local DNS will send a query to internet service providers asking the remote DNS servers.

TOPIC 7

1. Describe the difference between Half duplex and full duplex communications.

• Half Duplex. This involves one side talking at a time. Once one side has finished talking the other side started talking. If both sides talks collision will occur [1].
• Full Duplex. Communication where both sides are able to send and receive data at the same time. There is no collision of data [1]. 

1. Describe why statistical TDM can achieve higher throughput than normal TDM.

• Stations transmitting at a faster data rate will be given longer slots

1. Describe the advantages of Manchester encoding.

• Signal synchronizes itself minimizes error rate and optimizes readability [6].

1. Under what circumstances would terrestrial microwave link be appropriate?

• Where there is a need of large information to be moved (Broadband) [3].
• Where it is impractical to run cables [4].
• Where there is no atmospheric interference [5]. 

References

[1]”The difference between half-duplex and full-duplex”, SearchNetworking, 2018. [Online]. Available: https://searchnetworking.techtarget.com/answer/The-difference-between-half-duplex-and-full-duplex. [Accessed: 17- May- 2018].

[2]”PERFORMANCE ANALYSIS OF STATISTICAL TIME DIVISION MULTIPLEXING (STDM)SYSTEMS”, Lejpt.academicdirect.org, 2018. [Online]. Available: https://lejpt.academicdirect.org/A16/get_htm.php?htm=151_166. [Accessed: 17- May- 2018].

[3]”Microwave Link Networks – Engineering and Technology History Wiki”, Ethw.org, 2018. [Online]. Available: https://ethw.org/Microwave_Link_Networks. [Accessed: 17- May- 2018].

[4]”Terrestrial Microwave”, Vclab.tu-sofia.bg, 2018. [Online]. Available: https://vclab.tu-sofia.bg/AVC%20Network%20Lessons/MCSE-NE/5a6bc9c.htm. [Accessed: 17- May- 2018].

[5]”Terrestrial Microwave”, Vclab.tu-sofia.bg, 2018. [Online]. Available: https://vclab.tu-sofia.bg/AVC%20Network%20Lessons/MCSE-NE/5a6bc9c.htm. [Accessed: 17- May- 2018].

[6]”What is Manchester encoding? – Definition from WhatIs.com”, SearchNetworking, 2018. [Online]. Available: https://searchnetworking.techtarget.com/definition/Manchester-encoding. [Accessed: 17- May- 2018].

[7]”Internet Ping Command, Test”, Livinginternet.com, 2018. [Online]. Available: https://www.livinginternet.com/i/ia_tools_ping.htm. [Accessed: 17- May- 2018].