Transport Protocols and Congestion Control
From Reliability to Throughput
Transport Functions
Transport protocols provide multiplexing, reliability, ordering, flow control, and congestion control. Their design balances latency, bandwidth efficiency, robustness, and fairness across competing flows.
TCP as a Feedback System
Classic TCP increases sending rate cautiously and reduces it when loss or delay indicates congestion. Its dynamics can be modeled using control theory, where the congestion window acts like a control variable and queue occupancy provides feedback.
Why Loss Is Ambiguous
Packet loss may reflect congestion, corruption, wireless interference, or policy drops. Interpreting signals correctly is central to transport design.
Transport Design Axes
| Axis | Examples |
|---|---|
| Reliability | ARQ, FEC, selective acknowledgments |
| Congestion signaling | Loss, delay, explicit marks |
| Ordering | In-order delivery, partial ordering |
| Multiplexing | Per-flow, per-connection, multiplexed streams |
What is the main purpose of congestion control?
Congestion control aims to prevent overload by adapting sender behavior to available capacity.
Correct answer: Match sending rate to available network capacity
Why is transport often studied as a control problem?
Control-theoretic language helps analyze convergence, stability, and responsiveness.
Correct answer: Because it uses feedback from the network to adjust sending rates and maintain stable performance.
Modern Transport Directions
Recent research includes low-latency congestion control, model-based algorithms, QUIC transport evolution, multipath transport, and learned controllers that adapt to workload and network conditions.
Which signal is often used by modern congestion controllers in low-latency designs?
Delay and explicit congestion notification can provide earlier signals than loss alone.
Correct answer: Queueing delay or ECN marks
State one challenge in designing multipath transport.
Multipath adds coupling between subflows and complicates control and scheduling.
Correct answer: Coordinating path selection while avoiding reordering, instability, and unfairness.
Seminal Theme
The enduring question is how a distributed transport protocol can be both efficient and fair when each sender sees only partial, noisy feedback.