Core: Data Plane Transport

Transport protocols overview - TCP, UDP, QUIC

Miro Kosut

Miro Kosut

— 2 min read
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In one line

The transport layer decides reliability, ordering, congestion behaviour, and how visible traffic is to the network - and that shapes buffering, load-balancing, and security design.

The three you must reason about

  • TCP - connection-oriented, reliable, ordered, congestion-controlled. The workhorse for web/APIs, storage, and bulk transfer; sensitive to loss and RTT (see TCP behavior and the network).
  • UDP - connectionless, no reliability or congestion control (the app does it). Used for DNS, VoIP/RTP, streaming, gaming - and as the substrate for tunnels and QUIC.
  • QUIC - connection-oriented over UDP, with mandatory TLS 1.3, multiplexed streams (no head-of-line blocking), userspace congestion control, and connection migration. Carries HTTP/3 and is now a large share of web traffic.

Comparison

transport-protocol-matrix

Why a network designer cares

L4 behaviour drives several design choices downstream: TCP's loss sensitivity affects buffer/QoS design; QUIC/UDP single flows can't be split across links and hide entropy (Load balancing and hashing - ECMP and entropy); pervasive encryption (QUIC always, TLS often) erodes network visibility and changes the security model (Transport security and encrypted-traffic implications).

IPv6 / dual-stack note

All three run over v4 and v6; QUIC's connection migration is especially handy for dual-stack/mobile clients that change address.

Spaced repetition

In one phrase each: TCP is [...]; UDP is [...]; QUIC is [...].

In one phrase each: TCP is reliable, ordered, congestion-controlled (connection-oriented); UDP is connectionless, no reliability/congestion control; QUIC is reliable multiplexed streams over UDP with mandatory TLS 1.3 and connection migration.

QUIC's two standout advantages over TCP for design purposes are [...].

QUIC's two standout advantages over TCP for design purposes are no head-of-line blocking (independent streams) and connection migration (survives IP changes).

Why does the transport layer matter to a network designer at all?

It drives buffering/QoS (loss sensitivity), load-balancing/hashing (flow splittability + entropy), and security/visibility (encryption).

Sources

  • RFC 9000 (QUIC); Cisco Press, End-to-End QoS (transport behaviour).

domain: Core · blueprint-ref: Core 4.1/4.2 Areas of application, characteristics · type: concept · status: complete · tags: [core, core/dataplane, tradeoff/operability]

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