TCP and Explicit Congestion Notification (ECN bit)
When looking over an Ethereal capture, a student noticed the presence of two flags in addition to the six flags specified in RFC 793 for TCP. One of the bits indicated a change in window size, the other was called ECN (explicate Congestion Notification). Here is a well written article explaining this flag and the RFC references that discuss it further.
TCP and Explicit Congestion Notification
TCP and Explicit Congestion Notification
