Layer 4

Transport Layer

• "Shipping and receiving docks"
  • They help you with handling data and getting from one point to another reliably.
• End-to-end data transfer
  • Flow control
  • Segmentation / reassembly
  • Errors
    • TCP (Transmission Control Protocol) has built-in capabilities to handle errors.
    • UDP (User Datagram Protocol) is usually used for real time applications because i notices that there is an error and it keeps track of it, and if you have a lot of errors it can lower the quality to make it an easier stream to deliver, but for the most part errors are OK in UDP.
• Tunneling
  • IPSec ESP (Encapsulating Security Payload)
    • This is what we use to handle encrypted data
    • Confidential tunneling
  • GRE (Generic Router Encapsulation)
    • Clear text tunneling
• Management & diagnostics
  • ICMP is a diagnostic protocol, sometimes if you ever try to use ping, if it's there it comes back and tell us that's an ICMP request and an ICMP reply.

Data Transfer

• Applications use "sockets"

  • Apache: TCP / 443 / HTTPS

  • Bind: UDP / 53 / DNS

    • Bind is a Domain Name Service (DNS) working using UDP for velocity.

  • Transmission Control Protocol (TCP)

    • Connection-based
    • Reliable
    • Detail-oriented

  • User Datagram Protocol (UDP)

    • Connectionless
    • Unreliable
    • Low overhead
    • Typically lower the quality to prioritize speed

Tunneling

• Packets in packets
  • A tunnel is a connection between two sides
  • Back in the day we used huge and long cables to connect to part of the world
  • Nowadays we tunnel over the internet
  • Encrypted data is sent from one side to the other via tunneling.
  • When we encrypt this packages we are putting them into other packages.
• VPN = Tunneling
• Common protocols include:
  • ESP
    • Encapsulating Security Payload
      • IPsec VPN
    • If you need encryption or more privacy use ESP
  • GRE
    • Generic Routing Encapsulation
    • GRE Tunneling

Routing Protocols & Diagnostics

• Network Reachability
  • Open Shortest Path First (OSPF)
  • Enhanced Interior Gateway Routing Protocol (EIGRP)
    • The fastest, initially Cisco propietary.
  • TCP/ UDP (BGP/RIP)
    • BGP is the protocol of the Internet, the entire internet runs in BPG version 4, it's one big network that's it's protocol.
    • BGP is classified as an exterior gateway protocol.
      • Interior gateway is INSIDE your network where you'll usually use OSPF and EIGRP
  • Diagnostics
    • Internet Control Message Protocol (ICMP)