Ref: https://learn.cantrill.io/courses/2022818/lectures/45636948 ; https://learn.cantrill.io/courses/2022818/lectures/45636953 and https://learn.cantrill.io/courses/2022818/lectures/45636951

YouTube: https://www.youtube.com/watch?v=WAZxo2ObFIw ; https://www.youtube.com/watch?v=2MpRlYIYxz4 and https://www.youtube.com/watch?v=W-0ynJOsXvo

• Summary:
• Contents:

OSI L3 (Network) - Key Concepts

• 🔧 Protocols for end-to-end (E2E) data delivery across DIFFERENT network segments
  • Responsible for moving data between local networks (LANs) → “inter-networking”
• Provides:
  • Cross-network addressing and routing (e.g. IP addresses)
  • L3 data containers (e.g. IP packets)
• L2 NWs only use one single L2 protocol, can't mix L2 protocols (different frame formats)
  • A home LAN might use wired connections with Ethernet, but long distance NWs might use satellite connections with PPP, MPLS, ATM…
  • L3 allows connecting different L2 NWs, even if each uses a different L2 protocol
    • L3 peers don't necessarily have point-to-point (P2P) connections
    • Data is reframed/re-encapsulated as it hops from one L2 NW to another
• L3 protocol examples
  • Internet Protocol (IP) ← Very widespread, will focus on this one
  • IPsec
  • Internet Control Message Protocol (ICMP) → e.g. NW pings
    • 💡 Technically protocol between layers 3 and 4
  • Address Resolution Protocol (ARP)
    • 💡 Technically protocol between layers 2 and 3
  • …
• Example: two L2 NWs, on opposite ends of the US, connected via intermediary NWs

Internet Protocol (IP)

• Two versions:
  1. IPv4 (traditional, used for decades)
  2. IPv6 (bigger IP addresses) → more scalable

IP Packet (L3 Encapsulation)

• Source IP address (SRC IP) & Destination IP address (DST IP)
  • Larger in IPv6 than IPv4
• Protocol: number that indicates which L4 protocol inserts packet payload/data
  • 💡 ICMP = 1; TCP = 6; UDP = 17…
• Time To Live (TTL)/Hop Limit → Max number of hops/NW changes before packet expires
  • Stops packets looping around forever
• 💡 Packets are similar to frames… but keep them distinct!
  • Packets → global/remote NW; Frames → local NW
  • L3 packets get encapsulated inside L2 frames
  • ❗ Unlike frames, packets remain the same from SRC to DST of L3 NW