Passive monitoring

Passive monitoring is a technique used to capture traffic from a network by generating a copy of that traffic, often from a span port or mirror port or via a network tap. Once the data (a stream of frames or packets) has been extracted, it can be used in many ways.

*It can be analyzed in a sniffer such as Wireshark
*It can be examined for flows of traffic, providing information on "top talkers" in a network as well as TCP round-trip time.
*It can be reassembled according to an application's state machine into end-user activity (for example, into database queries, e-mail messages, and so on.) This kind of technology is common in Real User Monitoring when applied to the http protocol in web applications.
*In some cases, http reassembly is further analyzed for web analytics

Passive monitoring can be very helpful in troubleshooting performance problems once they have occurred. Passive monitoring differs from synthetic monitoring in that it relies on actual inbound web traffic to take measurements, so problems can only be discovered after they have occurred.

While initially viewed as competitive to synthetic monitoring approaches, most networking professionals now recognize that passive and synthetic monitoring are complementary.