Types of Vulnerability Assessment - External Assessment External assessment examines the network from a hacker's point of view to identify exploits and vulnerabilities accessible to the outside world. These types of assessments use external devices such as firewalls, routers, and servers. An external assessment estimates the threat of network security attacks from outside the organization. It determines the level of security of the external network and firewall. (P.527/511) External assessment examines the network from a hacker's point of view to identify exploits and vulnerabilities accessible to the outside world. These types of assessments use external devices such as firewalls, routers, and servers. An external assessment estimates the threat of network security attacks from outside the organization. It determines the level of security of the external network and firewall.
The following are some of the possible steps in performing an external assessment:
o Determine a set of rules for firewall and router configurations for the external network o Check whether the external server devices and network devices are mapped o Identify open ports and related services on the external network o Examine the patch levels on the server and external network devices o Review detection systems such as IDS, firewalls, and application-layer protection systems o Get information on DNS zones o Scan the external network through a variety of proprietary tools available on the Internet o Examine Web applications such as e-commerce and shopping cart software for vulnerabilities

https://en.wikipedia.org/wiki/Network_Time_Protocol
The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks.
NTP is intended to synchronize all participating computers within a few milliseconds of Coordinated Universal Time (UTC). It uses the intersection algorithm, a modified version of Marzullo's algorithm, to select accurate time servers and is designed to mitigate variable network latency effects. NTP can usually maintain time to within tens of milliseconds over the public Internet and achieve better than one millisecond accuracy in local area networks. Asymmetric routes and network congestion can cause errors of 100 ms or more.
The protocol is usually described in terms of a client-server model but can easily be used in peer-to-peer relationships where both peers consider the other to be a potential time source. Implementations send and receive timestamps using the User Datagram Protocol (UDP) on port number 123.