Ref: https://learn.cantrill.io/courses/2022818/lectures/45636950 and https://learn.cantrill.io/courses/2022818/lectures/45636949

YouTube: https://www.youtube.com/watch?v=MBLecyQuNqk and https://www.youtube.com/watch?v=drvbm6R0ONY

• Summary:
• Contents:

OSI L4 (Transport) - Key Concepts

• 🔧 Protocols for reliable and efficient data delivery over an L3 network
  • Responsible for device app connections → multiple apps can simultaneously use same device IPs
• Provides:
  • Peer-to-peer connections
  • Flow control
  • Error recovery
  • L4 data containers (e.g. TCP segments)
• Two main L4 protocols:
  1. **Transmission Control Protocol (TCP)** ← Will focus on this one
     • Connection-oriented protocol
       • Must set up a connection between devices before communicating
       • Established connection creates bidirectional channel of communications
         • 💡 A bidirectional communication is like a conversation
     • 👍 Reliable (has error correction, ordering of data…)
     • 👎 Slower (must ensure reliability)
     • Widely used for L7 App protocols that require reliable connections: HTTP(S), SSH…
     • 💡 TCP/IP = TCP running on top of IP
  2. User Datagram Protocol (UDP)
     • 👎 Less reliable
     • 👍 Faster, more performant (does not have reliability overhead)
     • Typical use case: stream video/audio (not so important if not absolutely all pixels or audio samples make it to destination)
  • 💡 Using TCP or UDP is a trade-off between reliability and speed/efficiency
• TCP vs UDP Diagram

Transmission Control Protocol (TCP)

TCP Segment (L4 Encapsulation)

Diagram: https://github.com/acantril/aws-sa-associate-saac03/blob/main/0400-TECH_FUNDAMENTALS/00_LEARNINGAIDS/OSI-LAYER4-TRANSORT-2.png

• SRC Port & DST Port
  • Combine with L3 SRC IP & DST IP to make multiple communication streams possible
    • 1 communication channel = <SRC_IP>:<SRC_PORT> + <DST_IP>:<DST_PORT>
  • e.g. tcp/443 (TCP port 443) is usually TCP port for HTTPS in a server → tcp/443 on Netflix server's IP is the way to reach Netflix via HTTPS
    • 💡 tcp/22 → SSH, tcp/80 → HTTP… (default ports in servers)
• Sequence number: position of the segment in the sequence
  • +1 increment with each segment sent → keeps order
  • Allows destination to:
    • Reorder segments as they arrive
    • Ask for retransmission of segment with sequence number that never arrived
• Acknowledgement (ACK): destination has received everything up to this sequence number
  • Every transmitted segment must be acknowledged → keeps reliability
• Flags & things: various connection controls
  • Used for e.g. establish & terminate connections, sync sequence numbers…
• Window: max number of bytes host can receive without acknowledgement
  • When reached, sender pauses until data has been acknowledged → keeps flow control
  • Trade-off: Small window → higher control of data flow; Big window → higher efficiency
• Checksum: error checking (ensures no data corruption)