OSI Layer

Overview

Layer 7 - Application

• HTTP, FTP, ..

Layer 6 - Presentation

• jpg, mpg

Layer 5 - Session

• NetBIOS

Layer 4 - Transport

• TCP/UDP
• Ports
• Segmentation (breaks large files into smaller segments)
• Segments
• Flow control - ensures that receiving host can handle the transfer (prevents overwhelming the receiver)
• Session multiplexing
• Stateful firewalls - return traffic is allowed by keeping track of src and dst ports on client end

• 3-way handshake
• TCP header

• UDP header

Layer 3 - Network

• IP addressing, logical addressing
• Routers
• Packets
• IP Header

• Maximum Transmission Unit (MTU)
  • Max packet size
  • Default: 1500 byte
  • Packets larger than MTU will be splitted into parts (fragments)
• Time to Live (TTL)
  • Lifetime of a packet
  • Every time a packet passes on a router, the router decrements this by 1
  • Prevents routing loop (e.g on broken network)
  • Once the TTL reaches 0, the router will discard/drop the packet and send back time exceeded message to original sender
• 8-bit Protocol - layer 4 information (TCP or UDP)
• Quality of Service (QoS)
• Connectionless protocol
• Example protocols: ICMP and IPSec

Layer 2 - Data Link

• MAC address
  • 48-bit hexadecimal address
  • first 24 bits - OUI (Organizationally Unique Identifier)
  • last 24 bits - vendor assigned
  • 248 = 281,474,976,710,656 MACs
• Switches
• Frames
• Error detection (e.g CRC)
• Ethernet Header

• Preamble - for synchronization between source and destination
• FCS - frame sequence (for data integrity and error checking)
• In wireshark, the length column is the frame length (may not include FCS and Preamble)

Layer 1 - Physical Layer

• Cables
• Hubs
• Voltage levels, max transmission distance
• Straight-Through cable - connects end device (e.g PC, laptop) to a switch
• Crossover cable - connects devices of same type (e.g PC to PC)

• Fiber cables - supports longer distances and higher bandwidth
  • Single mode vs Multi mode
• Power over Ethernet (POE) - e.g IP Phones

• Others:
  • Cat5 - up to 100 Mbps
  • Cat5e - up to 1000 Mbps