Internet Protocol Stack

• Using models helps us organize and dedicate parts of networks
• Typically OSI or TCP

TCP/IP

• 5 layers, made by US DoD
  • Application: service to users
    • FTP, SMTP, HTTP, HTTPS, SSH, etc.
  • Transport: Process-process data transfer
    • TCP, UDP, etc.
  • Network: routing
    • IP, routing protocols
  • Data link: data transfer between neighboring network elements
    • PPP, Ethernet
  • Physical: bits "on the wire"
  • Application, transport, network, link, physical
  • Each layer communicates only w/ layers that it borders
• Encapsulation: wrapping data as it moves down the protocol stack

LAYER 5 (application): imagine a message, 10kb. Split it into application data units (ADUs)
LAYER 4 (transport): ADU moves into transport layer. A header is added. You now have a segment. Our header (i.e., that of the sender) communicates with the transport layer of the receiver. 
LAYER 3 (network): A header of the network layer is added. We now have a datagram
Layer 2 (data link): Creates a frame from the datagram, often adds metadata. Our "message" is now handed off to the next device (e.g., router)

Now, on the other device

• PDU: Protocol data unit (ADU, segment, datagram, datagram, frame)

QUIC (Quick UDP Internet Connections)

• Breaks traditional layering by running on top of UDP
• HTTP/3 -> QUIC -> UDP
• QUICK & UDP effectively function as transport layer

Layering

Pros	Cons
Modularity	Limited
Innovation	Redundancies
Interoperability	Violated everywhere: Firewalls (inspect data at many layers, which is typically only at application), NAT, DPI
Simplicity

• E2E Principle (highly controversial & influential, made by MIT researchers in '80s):
  • Don't implement function at lower levels of system UNLESS does one of the following:
    • Can be completely implemented at this level
    • Improves performance
  • Unless you can completely remove burden from end hosts, don't bother
  • Keep network simple & dump
    • Put intelligence on edges (endpoints)