Glossary

The networking terms stay in English throughout, because that is how every article and every RFC you meet next will name them. Here is what each one means. Anywhere they appear on the site, the dotted underline gives you this definition again.

flow

One connection. One download, one video stream, one upload — one sender talking to one receiver.

bottleneck

The slowest link on the path. It is the only one that matters: it sets the speed, and it is where the queue forms.

segment

One packet of data, as TCP counts them. Windows and queues are measured in these as often as in bytes, because what matters is how many separate things are in flight.In this model every segment is exactly one MSS, which a real stack does not guarantee.

MSS

The largest amount of data one packet can carry — 1,448 bytes here. It is the unit the window grows in: "one more segment per round trip" means one more of these.Maximum segment size. Set by the path's smallest link; 1,448 bytes is typical Ethernet minus headers.

ACK

The receiver saying "got it". Every acknowledgement that comes back is also the sender's permission to send one more — so the rhythm of packets going out is a copy of the rhythm of ACKs coming in, which is a copy of the rhythm the slow point allowed. Nobody measures anything; the connection paces itself.Acknowledgement. Three duplicate ACKs in a row is how a sender infers that one packet went missing.

buffer

The waiting room at the slow point of the path. When packets arrive faster than the link can send them, they queue here instead of being thrown away — until it is full.Always shown here as a multiple of BDP, because the byte count alone predicts nothing.

RTT

How long a packet takes to get there and be acknowledged back. This is what you feel as lag.Round-trip time. Base RTT is the wire alone; anything above it is time spent waiting in a queue.

BDP

How much data fits inside the wire itself. A fast, long link is a long pipe: you can have a lot in flight before the first byte has arrived anywhere.Bandwidth-delay product: capacity × round-trip time. The right amount to keep in flight.

packet loss

A packet that never arrived. On a wired path it almost never means damage — it means a queue was full. That is why TCP treats loss as the signal to slow down.

congestion window

The sender’s own guess at how much it is allowed to have in flight at once. Nobody tells it the answer, so it raises the guess while things go well and cuts it when a packet is lost.Written cwnd. This is the quantity drawn in teal, and the reason the trace has a sawtooth in it.

ssthresh

The line between the two growth rules. Below it the window doubles every round trip; above it it adds one segment per round trip. A loss sets it to half the current window — which is why the climb out of a drop is a straight line and the very first climb is a curve.Slow-start threshold. Drawn as the dashed grey line in the window panel.

slow start

How a connection begins: send a little, and double it every round trip until something breaks. It is slow for a moment and then very fast, which is how it overshoots.

AIMD

The rule that makes the sawtooth: climb by a fixed step when things go well, halve when a packet is lost. Climb slowly, back off hard.Additive increase, multiplicative decrease. It is also what makes competing flows converge on a fair split.

sawtooth

The shape a TCP connection traces out over time: climb, drop, climb, drop. Nobody designed the shape — it is what AIMD looks like when you plot it.

throughput

How much data actually gets through per second.

goodput

Throughput with the retransmissions taken out — the data that arrived and counted. A run can be busy and still deliver little, if it keeps sending the same packets twice.

utilisation

How much of the link was actually used. Below 100% the wire sat idle at some point, usually because the sender had just cut its window after a loss.

bufferbloat

A buffer so big that it hides the congestion instead of signalling it. Nothing is lost, so the sender keeps pushing, the queue stays permanently full, and every packet waits behind it. The download finishes at the same speed and everything else on the connection feels broken.

AQM

A queue that drops a packet early, on purpose, before it is full — so the sender is told to slow down while the waiting room is still mostly empty.Active queue management. RED and CoDel are the two modelled here.

queue discipline

The rule the waiting room uses to decide which packet to throw away, and when.

timeout

The expensive kind of loss. No acknowledgement came back at all, so the sender waits, gives up, and starts again almost from scratch. One timeout costs more than several ordinary drops.

Jain's fairness index

One number for how evenly the link was shared. 1.00 is a dead-even split; 0.50 between two flows means one of them got essentially everything.

seed

Which set of random numbers this run used. Same seed, same settings, byte-identical result — so a link you share shows exactly what you saw.