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TCP, QUIC, and End-to-End Transport Control

Manual: General · Subject: Networking

Examine reliable transport, congestion control, and modern transport protocol design.

Transport Protocols in Practice

Transport-layer responsibilities

Transport protocols provide multiplexing, reliability, flow control, congestion control, and sometimes security or migration support. TCP is the classical Internet transport, while QUIC integrates transport and security in user space over UDP.

Core mechanisms

Sliding window
Limits the amount of unacknowledged data in flight
Congestion window cwndcwnd
Sender-side limit based on estimated network capacity
ACK
Acknowledgment indicating successful receipt
Fast retransmit
Recovery triggered by duplicate acknowledgments
RTT
Round-trip time used in timeout and control estimation

Congestion control as feedback

Congestion control is a feedback system: the sender probes available capacity, observes delay or loss, and adapts its sending rate. Stability, fairness, and responsiveness are central design goals, but they often trade off against one another.

TCP and QUIC

TCP

  • Kernel-integrated on many systems
  • Reliable byte stream semantics
  • Widely deployed but sometimes constrained by middleboxes

QUIC

  • Runs in user space over UDP
  • Combines transport with TLS-based encryption
  • Designed for faster evolution and reduced handshake latency

What is the main purpose of the congestion window?

Why can QUIC evolve faster than TCP in some environments?

A transport-protocol design checklist

  1. 1

    Define the service model: stream, datagram, or message.

  2. 2

    Specify reliability and ordering guarantees.

  3. 3

    Choose flow and congestion control strategies.

  4. 4

    Account for latency, fairness, and loss recovery.

  5. 5

    Evaluate performance under reordering, path changes, and middlebox constraints.

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Important distinction

Flow control protects the receiver from overload; congestion control protects the network from overload.

Which statement is most accurate about flow control?