TCP Flow Control & Sliding Window

核心 · Key Idea

In one line: in every ACK, the receiver tells the sender "I can accept N more bytes" (receive window, rwnd). The sender never exceeds rwnd — making receiver-buffer overflow structurally impossible.

What it is#

send window = min(rwnd, cwnd)
              ↑              ↑
       receiver capacity   sender's network estimate

The sender maintains a sliding window with four regions: acked / sent-but-unacked / sendable / not-yet-allowed. Each ACK slides it right.

Analogy#

打个比方 · Analogy

You're delivering buckets of water. Your friend says "I have room for 5 more" (rwnd=5), so you send 5. After they drink 3, they say "room for 3 more" (rwnd=3), and you continue. You never deliver more than the kitchen can hold.

Key concepts#

rwndReceive Window

Receiver-advertised free space in its receive buffer.

Window ScalingWindow Scaling

Plain rwnd is 16 bits (max 64KB); TCP option scales it up to 1GB.

Zero WindowZero Window

rwnd=0; sender pauses and probes via Window Probe.

Nagle's algorithmNagle

Coalesces small writes — saves overhead at the cost of **extra latency**.

Delayed ACKDelayed ACK

Receiver waits a bit before ACKing. Paired with Nagle, this causes the classic **deadlock-like stall**.

How it works#

The slower the receiver app reads, the smaller rwnd shrinks, and the sender slows down accordingly.

Practical notes#

Modern OSes enable Window Scaling by default — required for transoceanic "long fat pipes" (high BDP).

Tune server receive buffers:

sysctl -w net.core.rmem_max=16777216
sysctl -w net.ipv4.tcp_rmem='4096 87380 16777216'

Too-small buffers cap rwnd and starve the wire.

Slow read() in the app keeps rwnd at 0 for long periods, stalling the sender — looks like "packet loss" but is really "slow reader".
Disable Nagle (TCP_NODELAY) for latency-sensitive small writes (IM, games, interactive SSH).
Don't conflate rwnd with cwnd: rwnd is receiver capacity, cwnd is network capacity — the sender takes the smaller of the two.

Easy confusions#

rwnd is full

**Receiver app reads too slowly**.
Fix: bigger buffer / faster reader.

cwnd halved

**Network packet loss**.
Fix: better link / different congestion algo (e.g. BBR).