iperf

Internet Protocol bandwidth measuring tool
Iperf is a modern alternative for measuring TCP and UDP bandwidth performance,
allowing the tuning of various parameters and characteristics.
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Features:
* Measure bandwidth, packet loss, delay jitter
* Report MSS/MTU size and observed read sizes.
* Support for TCP window size via socket buffers.
* Multi-threaded. Client and server can have multiple simultaneous
connections.
* Client can create UDP streams of specified bandwidth.
* Multicast and IPv6 capable.
* Options can be specified with K (kilo-) and M (mega-) suffices.
* Can run for specified time, rather than a set amount of data to transfer.
* Picks the best units for the size of data being reported.
* Server handles multiple connections.
* Print periodic, intermediate bandwidth, jitter, and loss reports at
specified intervals.
* Server can be run as a daemon.
* Use representative streams to test out how link layer compression affects
your achievable bandwidth.