Scalable, low-overhead network delay estimation

The round trip network delay times between pairs of nodes in a multicast session is a key parameter; it is used in suppressing the implosion of request and repair packets, in detecting congestion, etc. This paper presents a protocol to estimate the delays between every pair of nodes in a multicast group. While existing protocols use O(n) multicast messages with O(n) size each (total of O(n²) bits), our protocol requires O(n) multicasts with only O(1) size each (total of O(n) bits); here, n is the session size. It does not require clocks to be synchronized, or any knowledge of network topology and the session size. We incorporate our protocol into the SRM protocol and show by our simulations that (i) our delay estimation protocol is reasonably accurate, (ii) it is just as effective as existing delay estimation protocols in suppressing repair and request traffic in SRM, and (iii) it has significantly lower overhead than the existing delay estimation protocols. Furthermore, our protocol can be extended to run within a hierarchically structured multicast protocol such as RMTP. Here the goal is to estimate round trip delays from a sender to each receiver. Our protocol ensures that only the immediate children of the sender send feedback messages directly to the sender, thereby avoiding feedback implosion. Our delay estimation protocols therefore significantly enhance the scalability of unstructured multicast protocols such as SRM, as well as tree-structured multicast protocols such as RMTP.