Glossary.md

@@ -61,6 +61,11 @@ Linktest benchmarks communications by repeating a communication many times. The
 # Number of Warm-Up Messages
 Linktest warms up connections by testing them multiple times before timing begins. Basically the same actions as during timing occur multiple times beforehand. This is often done because connections need to be first initialized, which means that sending a message the first time often takes longer than when it is sent the second time a short time afterwards. During the first time, sometimes couple of times, a message is sent over a network the network optimizes itself for the transmission of the message, i.e. it becomes primed for this message. As such it often makes sense to include at least one warm-up message before benchmarking a connection. For small message sizes more should be used, 3-5 work well. In Linktest this number of warm-up messages must be stipulated via the `--num-warmup-messages` command-line argument. 
 
+# Ping-Pong Testing
+Ping-Pong tests are a standard tool for network operators. They can thought of as an extension to the `ping` command used to test for the accessibility of machines for a given address, which is a ping test. In a ping test a message is sent from an origin to a destination and the time is taken at the origin till a receipt is received at the destination. Ping-pong tests extend this by timing at the origin till the original message is received back again, i.e. the pong in ping-pong testing. In birectional testing the sending of messages is done by the origin and destination congruently, see [Bidirectional Testing](#bidirectional-testing).
+
+Ping-pong testing is useful to measure network latency and bandwidth. It is also less susceptible to differences in speed in a given direction since only the time it took for the message to go in both directions is recorded.
+
 # Serial Testing
 By default Linktest tests as many connections as possible in parallel, this, however, can cause tests to interfere. This is sometimes desired, for example, if real-world network performance under a sustained network load is to be tested. In other cases peak performance without the interference of other parallel tests is desired. In this case serial testing is done, in which each connection between a pair of tasks is tested individually. This effectively serializes the test and will cause it to take significantly longer. Serial testing can be turned on in Linktest by using the `--serial` command-line option.