The remote host is missing an update to strongswan announced via advisory DSA 1759-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%201759-1

Gerd v. Egidy discovered that the Pluto IKE daemon in strongswan, an IPSec implementation for linux, is prone to a denial of service attack via a malicious packet. For the stable distribution (lenny), this problem has been fixed in version 4.2.4-5+lenny1. For the oldstable distribution (etch), this problem has been fixed in version 2.8.0+dfsg-1+etch1. For the testing distribution (squeeze) and the unstable distribution (sid), this problem will be fixed soon. We recommend that you upgrade your strongswan packages.