CBLM - Cheap Bastard Latency Monitor
  CBLM is a high performance latency (one-way and round-trip), packet loss and jitter monitoring probe. When run on two or more servers, a full mesh of connections is automatically setup between the probes. The full mesh of connections are used to transmit UDP packets between the probes. Statistics are collected and stored within a MySQL database.
Uses for CBLM
Measuring network performance
CBLM keeps a constant stream of traffic flowing between probes and tracks the real time performance statistics of that traffic. This data can be used to monitor for positive or negative changes in your network or service provider. Though the use of the PHP GUI this data can be monitored via graph and a matrix view. Since CBLM uses a MySQL database to store it's data, it's easy to build your own tools around. Additionally, CBLM is integrated with SellaNMS, allowing for exception based alarming.
Detecting network problems and configuration errors
CBLM tracks round trip time (RTT), one-way trip time (OTT), jitter, packet loss, CoS (DSCP and VLAN priority) markings, out of order packets, and duplicate packets. The round trip time, one-way trip time and jitter values allow you to see how your network performs over time. The packet loss values allow you to watch for congestion and circuit issues. The out of order and duplicate packet counts let you watch for network convergence issues and CBLM's tracking of CoS marking can be used to determine if your networks CoS configuration is consistent from end to end.