We implemented the OverQoS node architecture in about lines of C code. The communication between overlay nodes uses the UDP socket interface. For loss recovery, we use the FEC software library built by Rizzo et al. . Our implementation works on both Linux and FreeBSD platforms.
Figure 4 illustrates the structure of a single OverQoS node along a given path. An OverQoS node listens on a UDP socket for the arrival bundle and tunnels the traffic to the exit node as a UDP stream. The CLVL Encoder and Decoder modules implement the CLVL abstraction on top of the overlay link by adding the necessary level of redundancy to recover from packet losses.The decoder also provides loss feedback to the encoder for computing the optimal redundancy factor. The Traffic Management module implements per-flow or per-packet resource management. Different QoS schedulers and buffer management schemes like priority scheduling and smoothing losses is performed by this module. The rate estimator computes the CLVL parameters , and while the link estimator provides feedback to the transmitting OverQoS node about the virtual link characteristics comprising: (a) loss feedback for computing the loss distribution; (b) , the round trip time.
CLVLs along an overlay path can be stitched together (or cascaded) to provide end-to-end services. Cascaded CLVLs can introduce artificial losses at an overlay node if the available bandwidth on the incoming links is larger than the available bandwidth in the outgoing links. In order to avoid any artificial packet losses at an intermediary node in an overlay path, an OverQoS node uses to signal the maximum sending rate to its predecessor. This is illustrated in Figure 4.