networking

OSI Model

Components

• Local networking - Ethernet
• Routing
• Segmenting, ports and sessions
• Applications

Key features

• Higher layer built on lower layer, adding features and capabilities
• This bottom-up approach illustrates the limitations of each lower layer and how each subsequent upper layer was developed to address and overcome those limitations

Layer 1 - Physical - Bit

Key features

• Layer 1 (Physical) specifications define the transmission and reception of raw bit streams between a device and a shared physical medium
• It defines things like voltage levels, timing, rates, distances, modulation, and connectors
• Physical medium can be copper (electrical), fibre (light), or wifi (RF)

Physical HUB

• Connects two more devices

Combinations

• We can mix and match multiple components depending on the network's needs
  • Cables and NICs
  • Cables and transceivers
  • Cables and repeaters and hubs
  • Cables and wireless access points

Summary

• Physical shared medium
• Standards for transmitting onto the medium
• Standards for receiving from the medium
• No access control
• No uniquely identified devices
• No devices → Device communications

Layer 2 - Data Link - Frame

Components on frame

• Preamble and SFD: Define the start of the frame
• MAC header: Contains the destination and source MAC addresses and the EtherType field (layer 3 protocol)
• Payload: The data encapsulated within the frame
• FCS: Check to detect any transmission errors

CSMA/CD - Carrier-sense Multiple Access/Collision Detection

• Challenge

• Solution

Switch - Layer 2 device

• The NIC handles the conversion of a structured frame (Layer 2) into raw bits encoded as physical signals (Layer 1) for transmission
• Problem

• Solution

Summary

• Identifiable devices
• Media access control (sharing)
• Collision detection
• Unicast 1:1
• Broadcast 1:ALL
• Switches - Like Hubs with Super powers (Layer 2)

Layer 3 - Network - Packet

Decimal and binary

• Human vs. computer

• Conversion: Decimal to binary

• Conversion: Binary to decimal

Layer 3

IPv4 and IPv6

• IPv4

Subnet mask

• /16 in CIDR notation means the first 16 bits of an IP address are the network portion

Route tables and routes

• Each router typically has a single routing table

Address resolution protocol (ARP)

How routing works?

• During routing, the IP packet's core content (source/destination IP, payload) typically remains unchanged, but fields like TTL and checksum are modified
• Routers work at layer 3 but need MACs for layer 2
• MAC address is in frame header, not packet

Summary

• IP addresses (IPv4/IPv6) enable cross-network addressing
• ARP maps an IP address to its corresponding MAC address
• A route determines where to forward a packet
• Routing tables store multiple routes for packet forwarding
• Routers move packets, encapsulated in Layer 2 frames
• Devices communicate over the Internet using IP addresses
• IP uses source and destination IP addresses for communication
• IP packets may be delivered out of order

Layer 4 - Transport - Segment

Problems - Layer 3

TCP and UDP

• TCP segments
  • Source port: Identifies the port number of the sending application, enabling the recipient to know which application sent the data
  • Destination port: Identifies the port number of the receiving application, allowing the recipient's operating system to direct the segment to the correct application
  • Sequence number: A 32-bit number used to ensure correct ordering of data segments and to acknowledge received data
  • Acknowledgment number: A 32-bit number used by the receiver to indicate the next sequence number it expects to receive, confirming receipt of previous segments
  • Flags 'N' things (*): A set of 1-bit flags (e.g., SYN, ACK, FIN, RST, PSH, URG) that control the state and management of the TCP connection
  • Window: A 16-bit field that specifies the number of data bytes the sender of this segment is willing to accept from the other end (flow control)
  • Checksum: A 16-bit field used for error checking of the TCP header and data to ensure integrity
  • Urgent pointer: A 16-bit field that, if the URG flag is set, indicates the offset from the current sequence number to the last byte of urgent data
  • Options: A variable-length field used to convey additional information not covered in the standard header, such as Maximum Segment Size (MSS)
  • Padding: Zero bits added to the end of the options field to ensure the TCP header ends on a 32-bit boundary

• TCP

Layer 5 - Session

TCP connection 3-way handshake

Session and state

Network Address Translation (NAT)