What is a sliding window?

A sliding window is a TCP mechanism that lets a sender send several segments in a row without waiting for each one to be acknowledged, while not overloading the receiver.

Simply put: a sliding window is a limit on how much data can be sent "ahead" without waiting for an ACK.

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#Why a sliding window is needed

Without a window, TCP would work like this:

• send a segment
• wait for an acknowledgment
• send the next one

This would be very slow.

A sliding window allows:

• transferring data as a stream
• using the network's bandwidth efficiently

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#How a sliding window works

#1. The receiver announces the window size

In every ACK, the receiver reports:

• Window Size - how many bytes it is ready to accept

This protects the receiver from being overloaded.

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#2. The sender can send data within the window

The sender:

• can send several segments
• as long as their total size does not exceed the window size

Not every segment needs to be acknowledged immediately.

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#3. The window "slides"

When the receiver:

• acknowledges part of the data

the window:

• shifts forward
• freeing up space for new data

Hence the name - sliding window.

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#An example to understand it

• Window size: 10 bytes
• The sender can send bytes 1-10
• The receiver acknowledges bytes 1-5
• The window shifts forward → now bytes 11-15 can be sent

Transmission continues without stopping.

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#What a sliding window actually controls

Important. A sliding window in TCP is responsible for:

• flow control
• not for routing
• not for correcting errors directly

Loss is handled separately, through:

• ACKs
• timeouts
• retransmissions

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#Relationship to network congestion

TCP has two limits:

1. The receiver window

• how much the receiver can accept

2. The congestion window

• how much the network allows

In practice, the sender uses:

the minimum of these two values

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#Why this matters for applications

A sliding window:

• increases transmission speed
• makes TCP scalable
• lets it work equally well:
  • on a local network
  • and over the Internet