FreeBSD Security Advisory (FreeBSD-SA-06:08.sack.asc)

The remote host is missing an update to the system as announced in the referenced advisory FreeBSD-SA-06:08.sack.asc
Upgrade your system to the appropriate stable release or security branch dated after the correction date
SACK (Selective Acknowledgement) 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 acknowledgement, the TCP/IP stack may enter an infinite loop.