Hi all, I have some questions about PMTU Black Hole Detection for Windows 7. (1) Was it enabled by default ? (2) I found this in one of the official doc of MSFT: "With this feature enabled, TCP tries to send segments without the Don’t Fragment flag set if several retransmissions of a segment go unacknowledged. If the segment is then acknowledged, the TCP maximum segment size (MSS) for the connection is decreased and the Don’t Fragment flag is set for subsequent segments. Enabling PMTU black hole detection increases the maximum number of retransmissions that are performed for a given segment." I don't understand the (2), does it mean if one of the segment go UNACK, then the retransmitted segments' size was reduced so that it can pass the black hole router, after that the latter segments still use the orignal size? That's why the doc said: "Enabling PMTU black hole detection increases the maximum number of retransmissions that are performed for a given segment." If it was true, the latter segments will forsure cannot pass the black hole router because the segment size was too big, then it still need to be cut to fit the black hole router's needs.

Hi, PMTU Black Hole Detection is not enabled by default. For more information, please refer to the link below: Path MTU Discovery Thanks,Novak

thanks Novak, but this article is for Win2k3, not for Win7 and it's also not answering my question.