Flow Control in TCP (rwnd)

Flow control ensures the sender doesn't overwhelm the receiver by sending data faster than the receiver can process it.

The Problem

Fast Sender ---- too much data ----> Slow Receiver
                (buffer overflow → packets dropped)

Solution — Receive Window (rwnd)

• Receiver tells sender how much buffer space is available.
• This is the rwnd (Receive Window) field in the TCP header.
• Sender must not have more than rwnd bytes of unacknowledged data in flight at any time.

How rwnd Works

Receiver Buffer = 4000 bytes

Step 1: Receiver → "rwnd = 4000" (I can accept 4000 bytes)
Step 2: Sender sends 4000 bytes
Step 3: Receiver processes 2000 bytes → "rwnd = 2000"
Step 4: Sender sends only 2000 more bytes
Step 5: Receiver processes all → "rwnd = 4000" again

Sliding Window Mechanism

• TCP uses a sliding window to allow multiple packets in flight without waiting for each ACK.
• Window slides forward as ACKs are received.
• Allows continuous data flow without stop-and-wait.
• Window size = rwnd (adjusted dynamically).

Sequence: 1  2  3  4  5  6  7  8  9  10
          [ sent ][ in flight ][ not sent ]
                ↑ window slides as ACKs arrive

Normal Operation

Sender                                Receiver
  |------- 1000 bytes -------------->|  rwnd = 4000
  |------- 1000 bytes -------------->|  rwnd = 3000
  |------- 1000 bytes -------------->|  rwnd = 2000
  |<------ ACK, rwnd=4000 ------------|  (receiver processed data)
  |------- 1000 bytes -------------->|

Zero Window (Receiver Buffer Full)

Sender                                Receiver
  |------- data --------------------->|
  |<------ ACK, rwnd=0 ----------------|  "STOP! Buffer full"
  |         (sender pauses)            |
  |<------ ACK, rwnd=2000 -------------|  "OK resume now"
  |------- data --------------------->|

When rwnd = 0, sender stops transmitting. Sender sends periodic Window Probe packets to check if window opened.

rwnd in TCP Header

• Window field = rwnd — 16 bits → max 65,535 bytes.
• Can be scaled using TCP Window Scaling option (up to 1GB).

Flow Control vs Congestion Control

	Flow Control	Congestion Control
Problem	Receiver too slow	Network too congested
Controlled by	rwnd (receiver)	cwnd (sender)
Who decides	Receiver	Sender
Mechanism	Sliding window	Slow start, AIMD
Goal	Protect receiver buffer	Protect network

Effective Window = min(rwnd, cwnd)

rwnd = receiver's available buffer (flow control)
cwnd = congestion window (congestion control)
Actual data in flight ≤ min(rwnd, cwnd)

Silly Window Syndrome — inefficiency when tiny rwnd causes tiny packets → solved by Nagle's Algorithm.