FreeBSD Security Advisory FreeBSD-SA-06:08.sack – SACK (Selective Acknowledgment) is an extension to the TCP/IP protocol that allows hosts to acknowledge the receipt of some, but not all, of the packets sent, thereby reducing the cost of retransmissions. When insufficient memory is available to handle an incoming selective acknowledgment, the TCP/IP stack may enter an infinite loop.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/43545/FreeBSD-SA-06-08.sack.txt
Source: https://packetstormsecurity.com/files/43545/FreeBSD-SA-06-08.sack.txt.html