Abstract: Malicious network node activity and, in particular, denial of service attacks, may be mitigated by one or more practical mitigation mechanisms and mitigation mechanism combinations. Suitable protocol messages may be challenged with a challenge probe. A response to the challenge probe may be utilized to determine if received protocol messages are illegitimate, that is, originated by a malicious network node. Received protocol messages may be classified as questionable protocol messages. For efficiency, protocol message challenges may be limited to protocol message classified as questionable. A sequence number limit may be calculated as a function of receive window size. Transmission control protocol messages may be determined to be illegitimate by comparing the acknowledgement number field with the calculated sequence number limit.