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Networking Fundamentals · Intermediate

Window Size

The TCP header value advertising how many bytes the receiver is currently willing to accept.

Definition

The 16-bit window field allows up to 65,535 bytes, and the window scale option (RFC 7323) multiplies it to support large windows on fast, long-delay networks. A receiver with a full buffer advertises a zero window, pausing the sender. Throughput is limited roughly to window size divided by round-trip time.

Example

A capture showing "TCP ZeroWindow" means the receiving application is not reading data fast enough.

Exam tip

Max single-flow TCP throughput is about window size divided by RTT.

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