To prevent off-path attackers from impersonating legitimate peers, clients require that the origin timestamp in a received response packet match the transmit timestamp from its last request to a given peer. Under assumption that only the recipient of the request packet will know the value of the transmit timestamp, this prevents an attacker from forging replies http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2015-8139
