Protocols

Defines:

Protocols as State Machines

A fully-defined protocol must provide a proper action for any event in any state. Most protocols can be modelled in terms of state machines.

Each interaction can have its own state machine.

Open Protocols
- Examples: HTTP, SMTP, SSH
- Usually defined in RFC documents (normally via some sort of working group or task force)
- Different implementations
- Allows a community, generally good!
Proprietary Protocols
- Examples: Skype, iCloud, Zoom
- Only one implementation

Generally includes a few states
- Idle - waiting for something to be initiated
- Waiting - waiting for a response
Edges are actions (e.g. receives a response of a certain type) or timeouts
Edge cases
- Timeout too soon! (may result in getting two ACKs)
Timeout formulas
- Assuming measured round trip time (RTT) of $t$
- Estimated RTT
  - $ERT T_{i} = (1 - α) ERT T_{i - 1} + α t$
- Deviation of RTT (captures jitter):
  - $Δ RT T_{i} = (1 - β) Δ RT T_{i - 1} + β ∣ t - ERT T_{i - 1} ∣$
- Timeout:
  - $ERT T_{i} + 4Δ RT T_{i}$
- Suggested: $α = 0.125$ , $β = 0.25$