Head of Line

Controls

Loss probability

How likely any given packet is to be dropped on the wire — both transports see the identical loss pattern for a fair comparison.

RTT

Round-trip time, which sets how long a lost packet takes to be detected and retransmitted.

Auto-play

Replays the scenario forward automatically; scrubbing manually pauses it.

Scrub

Drag to any point in the replay and watch both transports' state at that instant.

Pause / Play, Restart

Standard playback controls for the replay.

What to watch for

• Watch a loss hit stream A — in the TCP pane, ALL 4 lanes freeze and then burst forward together once the retransmit lands.
• In the QUIC pane at the same moment, only stream A's lane stalls; B, C, D keep delivering — just a little slower than before.
• Shared cwnd drops right when the loss happens, and QUIC advantage shows how much time QUIC saved the unaffected streams over TCP for this exact run.

Why it happens

TCP delivers one ordered byte-stream — if a packet is lost, everything sequenced after it is stuck behind it in that one queue, regardless of which logical stream it belongs to, until the gap is filled. QUIC gives each stream its own independent delivery order, so a loss on stream A only blocks stream A's own later data. But both protocols still share ONE congestion window for the whole connection — any loss anywhere forces that window, and therefore everyone's send rate, to shrink. QUIC fixes head-of-line blocking (ordering); it does not make you immune to a shared network event.