Semester 3 - Computer Networks - Important Topics Part 1

Network Protocols

• These are rules for how devices communicate over a network. They’re used in both software and hardware.

  • Network Communications Protocols: Allow devices to communicate over networks.(e.g., IP, TCP, HTTP)

  • Network Security Protocols: Secure data for authentication, integrity, and encryption. (e.g., SSH, SSL, TLS).

  • Routing Protocols: Help routers exchange route information and select the best path. (e.g., OSPF, BGP)

  • Service Discovery Protocols: Used for automatic detection of devices or services. (eg DHCP for IP address allocation, DNS for name-to-IP address translation)

• Network Protocol Functions: These protocols perform specific functions necessary for communication.

  • Addressing: Identifies sender and receiver of the message. (e.g., Ethernet, IPv4, IPv6)

  • Reliability: Provides guaranteed delivery if messages are lost or corrupted. (e.g., TCP)

  • Flow control: Ensures efficient data flow between two devices. Also provided by TCP. (e.g., TCP)

  • Sequencing: Labels each transmitted segment of data for correct reassembly. Useful if data is lost or received out-of-order. (e.g., TCP)

  • Error Detection: Determines if data was corrupted during transmission. (e.g., Ethernet, IPv4, IPv6, TCP)

  • Application Interface: Contains information for process-to-process communications between network applications. HTTP or HTTPS are used for accessing a web page. (e.g., HTTP, HTTPS)

• Protocol Interaction: A message sent over a network typically requires several protocols, each with its own function.

  • HTTP: Governs how a web server and client interact. Defines content and formatting of requests and responses.

  • TCP: Manages conversations, guarantees reliable information delivery and manages flow control.

  • IP: Delivers messages from sender to receiver. Used by routers to forward messages across networks.

  • Ethernet: Delivers messages within the same Ethernet local area network (LAN).

Models

• OSI and TCP/IP models help understand network concepts.
• They ensure different vendor products can work together.
• OSI model is great for explaining network concepts and troubleshooting.
• TCP/IP suite protocols are the current standards for networks.
• It’s key to understand both models and how they relate.
• Usually, when discussing layers, we refer to the OSI model.
• When discussing protocols, we typically refer to the TCP/IP model.

OSI

• The OSI (Open Systems Interconnection) model is a conceptual framework used to understand and describe how different network protocols interact and work together to provide network services.

• The OSI model consists of seven different layers described below.

OSI Model Layer	Description
7 - Application	Where network-aware applications, such as web browsers or email clients, interact with the network.
6 - Presentation	Translates the data format for the application, like converting text files from one code page to another, or encrypting and decrypting data.
5 - Session	Establishes, manages, and terminates ‘conversations’ between networked applications.
4 - Transport	Ensures data gets to the correct application at the destination and checks that all data has arrived.
3 - Network	Determines the best path for data to take through the network from source to destination.
2 - Data Link	Controls how data is sent to and received from the network, including error checking and correction.
1 - Physical	Handles the physical transmission of data, including the network cabling and electrical signals.

TCP/IP

• TCP/IP stands for Transmission Control Protocol/Internet Protocol.
• TCP/IP is the communication protocol for connecting network devices on the internet.
• It’s an open standard, meaning no single company controls it.
• Unlike OSI, it was developed from real network implementations, making it the basis for the modern internet.
• The TCP/IP model has four key functional areas, each crucial for successful communication.

TCP/IP Layer	Function	Example Protocols
Application	Represents data to the user and controls dialogue	DNS, Telnet, SMTP, POP3, IMAP, DHCP, HTTP, FTP, SNMP
Transport	Supports communication between diverse devices across diverse networks	TCP, UDP
Internet	Determines the best path through the network	IP, ARP, ICMP
Network Access	Controls the hardware devices and media that make up the network	Ethernet, Wireless

Comparision - OSI vs TCP/IP

---

OSI model	TCP/IP model	Protocol data unit (PDU)	Example	Equipment operating on given layer	Domain
Application	Application	Data	HTTP, FTP, SMTP, DNS	Firewall, Load Balancer	-
Presentation	-	-	ASCII, JPEG, MPEG	-	-
Session	-	-	NetBIOS, SAP	-	-
Transport	Transport	Segment	TCP, UDP	Router w/NAT, Firewall	-
Network	Network	Packet	IP, ICMP	Router	Subnet
Data-link	Data-link	Frame	Ethernet, 802.11	Switch	Broadcast domain
Physical	Physical	Bit/Symbol	UTP, Single-mode fiber, Multi-mode fiber	Hub, Repeater	Collision domain

Physical Layer

Characteristics

• Physical Layer Standards
  • Developed by various organizations such as ISO, TIA/EIA, ITU, ANSI, IEEE.
  • Standards govern hardware components like electronic circuitry, media, connectors.
• Physical Components
  • Examples: NICs, interfaces, connectors, cable materials.
  • Transmit signals representing bits.
  • Cisco 1941 router ports and interfaces are physical components specified by standards.
• Encoding
  • Examples: Manchester encoding, 4B/5B encoding, 8B/10B encoding.
  • Converts data bits into predefined code or pattern.
  • Enables recognition of data by sender and receiver.
• Signaling
  • Examples: Electrical, optical, or wireless signals.
  • Represents “1” and “0” on the media.
  • Signal types defined by standards (e.g., voltage levels, pulse durations).
• Bandwidth
  • Refers to data capacity of a medium.
  • Examples: kbps, Mbps, Gbps.
  • Determined by physical media properties and signaling technologies.

Network Media Forms and Standards

Three basic forms of network media exist:

• Copper cable: The signals are patterns of electrical pulses.

• Fiber-optic cable: The signals are patterns of light.

• Wireless: The signals are patterns of microwave transmissions.

1. Copper Cabling:
   • Characteristics: Copper cabling is affordable and easy to install for network connections.
   • Types: Unshielded twisted-pair (UTP) and shielded twisted-pair (STP) cables.

   • Example: Ethernet cables commonly use UTP for connecting devices.

   • UTP Cabling: (optional)
     • Properties: UTP cables are flexible, cost-effective, and use RJ-45 connectors.
     • Standards and Connectors: TIA/EIA-568 standards define wiring schemes. RJ-45 connectors terminate UTP cables.
     • Example: Straight-through cables connect different devices (e.g., computer to switch), while crossover cables connect similar devices (e.g., computer to computer).
2. Fiber-Optic Cabling:
   • Characteristics: Fiber-optic cables use light for high-speed and long-distance data transmission.
   • Types: Single-mode fiber (SMF) for long distances and multimode fiber (MMF) for shorter distances.
   • Example: Fiber-optic cables are commonly used in high-speed networks and long-distance communication.
3. Wireless Media:
   • Characteristics: Wireless media allows data transmission without cables, providing mobility.
   • Types: Wi-Fi for local area networks, cellular networks for wider coverage.
   • Example: Wireless LANs use Wi-Fi and access points for wireless device connectivity.

Media	Physical Components	Frame Encoding Technique	Signaling Methods
Copper cable	UTP	Manchester encoding	Changes in the electromagnetic field.
	Coaxial	Nonreturn to zero (NRZ)	Intensity of the electromagnetic field.
		4B/5B codes with MLT-3 signaling	Phase of the electromagnetic wave.
Fiber-optic cable	Single-mode fiber	PAM5	Pulses of light
	Multimode fiber	8B/10B	Wavelength multiplexing using different colors
Wireless	Access points	Direct Sequence Spread Spectrum (DSSS)	Radio waves
	NICs	Orthogonal Frequency Division Multiplexing (OFDM)
	Radio
	Antennas

Data Link Layer Overview

The Data Link Layer is the second layer in the OSI model, and it consists of two sub-layers:

1. Logical Link Control (LLC) sub-layer: Handles error correction, flow regulation, data framing, and addressing within the MAC sub-layer.
2. Media Access Control (MAC) sub-layer: Manages access to shared media, such as Token passing or Ethernet.

Key Functions of Data Link Layer :

• Framing: Transforms bit streams from the network layer into manageable frames.
• Physical Addressing: Inserts a header for identifying sender/receiver addresses when routing frames.
• Flow Control: Balances sender/receiver transmission rates, preventing receiver congestion.
• Error Control: Adds a trailer to frames for accuracy and avoidance of frame duplication.
• Access Control: Determines which device controls the link when multiple devices are connected.

Data Link Layer

Data Link Frame

• Data Link Layer wraps data in a ‘frame’ with a header and trailer for local media transport.
• A frame has three parts: Header, Data, Trailer.
• Frame structure can change based on different protocols.
• The type and amount of control information in a frame depend on media and topology needs.

Frame Fields include the following

• Start and stop flags: Mark the frame boundaries.
• Addressing: Shows where the frame comes from and where it’s going.
• Type: Tells us the Layer 3 protocol in the data field.
• Control: Points out special flow control services, like QoS.
• Data: The actual information being transported.
• Error Detection: Checks if the frame arrived without mistakes.

Layer 2 Addresses

• Layer 2 uses physical addresses to transport a frame across local media.
• These addresses are unique to each device and don’t show network location.
• Layer 2 addresses work within the same shared media and IP network.

Difference between UDP and TCP

	TCP	UDP
Connection	Connection-oriented	Connectionless
Suitability	High reliability applications, not critical of transmission delays (e.g., HTTP, HTTPs, FTP, SMTP, Telnet)	Fast, efficient transmission applications (e.g., DNS, DHCP, TFTP, SNMP, RIP, VOIP)
Ordering of Data Packets	Rearranges data packets in order	No inherent order; managed by the application layer
Speed and Reliability	Slower, but guarantees packet delivery	Faster, but no guarantee of delivery
Header Size	20 bytes	8 bytes
Connection Setup	Requires three packets to set up a socket connection	Lightweight, no connection tracking
Flow Control	Handles flow control, reliability, and congestion control	No option for flow control
Error Checking	Error checking and recovery present; retransmits erroneous packets	Error checking present, but discards erroneous packets without recovery
Handshake	Includes handshake process (SYN, SYN-ACK, ACK)	No handshake (connectionless protocol)

TCP Handshake Procedure

• SYN (Synchronize): Initiates a connection.
• ACK (Acknowledgment): Acknowledges a received packet or confirms connection establishment.
• FIN : Terminates or closes a connection.

TCP connection establishment (three-way handshake):

• Client sends a TCP segment with a SYN flag
• Server responds with a TCP segment with SYN and ACK flags
• Client responds with a TCP segment with an ACK flag.

TCP four-way termination sequence

• Host A sends a TCP segment with ACK and FIN flags
• Host B responds with a TCP segment with an ACK flag
• Host B sends a TCP segment with ACK and FIN flags
• Host A responds with a TCP segment with an ACK flag