6.1.6 TCP Window Probes

6.1.6 TCP Window Probes

When the TCP data receiver advertises a zero window, the TCP data sender sends window probes to determine if the receiver's window has increased. A window probe packet contains no user data except for the sequence number, which is one byte. If a window probe packet is dropped in the network, the receiver does not detect this loss. Therefore, the TCP data sender MUST NOT set the ECT codepoint or the CWR bit on window probe packets.

However, because window probes use the exact sequence number, they cannot be easily forged in a denial-of-service attack. Therefore, if a window probe arrives with the CE codepoint set, then the receiver SHOULD respond to the ECN indication.