Loss of TCP connections--either the dropping of existing connections or the inability to access new connections--means the SUT has reached capacity. Capacity can be defined as bits per second, sessions per second, concurrent sessions or a combination.
Note that transactional generators also track transaction state and success/failure. Transactions can become latent and be lost even though the target TCP stack will continue to accept incoming connections. This indicates the application is overwhelmed, while the underlying OS may still be chugging along. So transactional generators track similar metrics, but the context of the metric dictates its meaning.
Other Tools
We keep a host of other gadgets in our toolbox to augment the test bed or aid in troubleshooting. Some of these are commercial products, but many are open source and just as feature-rich as their commercial counterparts. Here are a few of our favorites:
Although we typically talk about application performance on a clean test bed, network applications often break down when contending with packet loss, latency and fragmentation. However, testing on degraded links makes repeatability difficult at best. We use products from Shunra to model a WAN with varying link characteristics, such as bandwidth, latency, loss and jitter (see "File Distribution Across the WAN"). Application-simulation tools, such as NetIQ Chariot, are good at testing Layer 3/4 devices--but because the transactions are simulated, they don't work as well at the Session, Presentation and Application layers, which are static.
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