1.1

• OSI Model


  A model for learning purposes.
• TCP/IP model


  The model employed on the internet
• Why use the OSI model?

  A good reference for network architecture and troubleshooting.
• PDU


  Data used at the layer.
• Layer 1


  data in and data out.
• Layer 2 Purpose

  LAN connectivity.
• Layer 1 PDU

  bits
• Layer 2 Important Fields:

  MAC Address - Source and Destination
• Layer 2 PDU

  Frames
• Layer 3 Purpose

  WAN connectivity and finding location.
• Layer 3 PDU

  PDU changes depending on protocol.

  TCP = Segments

  UDP = Datagrams
• Layer 4 Important Information

  Ports (DAT): Destination and source ports.
• Layer 4 Purpose

  Delivery of packets.
• Layer 5, 6 and 7

  • Session layer:

  ○ E.g., multiple tabs open in the browser.

  • Presentation:

  ○ Encoding protocols (e.g., ASCII, UTF-16)

  • Application:

  Protocols (e.g., HTTP, SSH, etc)
• Encapsulation and decapsulation


  The process of encasing several PDUs as they move from one device to the other for transmitting data
• Encapsulation


  Process of adding headers and footers to data as it moves through network layers.
• Ethernet

  Layer 2 protocol
• Internet Protocol


  Protocol responsible for addressing and routing packets across networks.
• TCP


  connection-oriented protocol for reliable data delivery.
• UDP


  connectionless protocol for faster data delivery.
• 3-way handshake


  Process in TCP where devices establish a connection before data transfer.
• Connection-oriented


  Type of communication where a dedicated connection is established before data transfer.
• Connectionless


  Type of communication where data is sent without prior arrangement.
• Three-way handshake

  Consists of three packets: SYN flag, SYN/ACK flag, and ACK flag
• L4 Payload

  Contains Application Data
• L3 Payload

  Contains L4 Header + Application Data
• Payload


  Relative to the header of the data sent to the network.

  
• Headers merging


  Headers merge with the payload as they go down the layers
• Maximum Transmission Unit

  The maximum rate of data transfer
• MTU settings

  Must match on all devices between two points.
• Slowest MTU = MTU of all packets.
• Fragmentation


  Chopping a packet's size into smaller pieces

  Example: File of 3000 bytes fragmented into 2 packets of 1500 bytes each