The remote host is missing an update to the system as announced in the referenced advisory FreeBSD-SA-05:15.tcp.asc

Upgrade your system to the appropriate stable release or security branch dated after the correction date https://secure1.securityspace.com/smysecure/catid.html?in=FreeBSD-SA-05:15.tcp.asc

The Transmission Control Protocol (TCP) of the TCP/IP protocol suite provides a connection-oriented, reliable, sequence-preserving data stream service. TCP timestamps are used to measure Round-Trip Time and in the Protect Against Wrapped Sequences (PAWS) algorithm. TCP packets with the SYN flag set are used during setup of new TCP connections. Two problems have been discovered in the FreeBSD TCP stack. First, when a TCP packets containing a timestamp is received, inadequate checking of sequence numbers is performed, allowing an attacker to artificially increase the internal recent timestamp for a connection. Second, a TCP packet with the SYN flag set is accepted for established connections, allowing an attacker to overwrite certain TCP options.