Delay Characteristics

Figure 11: Cascaded CLVL scenario using FEC+ARQ CLVLs: End-to-end ordering within OverQoS network has much better delay characteristics than hop-by-hop ordering.

This section answers the question: What is the delay cost of using OverQoS? A potential criticism of our algorithm is that it increases the delay observed by packets.³ There are two reasons for this increase in delay. First, if one or more packets in a window are lost, the recovery process will cause additional delays. Second, if OverQoS is required to support in-sequence delivery of packets, the loss of one packet can increase the delay of other packets. Our implementation showed that the additional delay incurred at a node due to processing overhead is negligible.

In OverQoS, we can support three different models for packet delivery: (a) No packet ordering; (b) End-to-end (E2E) ordering between first and last OverQoS node in a path; (c) Hop-by-hop ordering. We consider a simple scenario where an overlay path traverses multiple overlay nodes with each link having an RTT of $100$ msec and experiencing frequent losses ( $p_{avg}=4\%$). Figure 11 shows the distribution of the additional delay incurred due to loss recovery for each of the three packet delivery models. We consider a path consisting of up to three overlay links. We make three observations. First, end-to-end packet recovery has much better delay characteristics than hop-by-hop delay characteristics. Second, the additional delay incurred by adding new OverQoS nodes along a path is limited. Third, the additional delay is also dependent on the loss rate. The loss-rate dictates how frequently the loss recovery process is being invoked.