Extending equation-based congestion control to multicast applications

Abstract

In this paper we introduce TFMCC, an equation-based multicast congestion control mechanism that extends the TCP-friendly TFRC protocol from the unicast to the multicast domain. The key challenges in the design of TFMCC lie in scalable round-trip time measurements, appropriate feedback suppression, and in ensuring that feedback delays in the control loop do not adversely affect fairness towards competing flows. A major contribution is the feedback mechanism, the key component of end-to-end multicast congestion control schemes. We improve upon the well-known approach of using exponentially weighted random timers by biasing feedback in favor of low-rate receivers while still preventing a response implosion. We evaluate the design using simulation, and demonstrate that TFMCC is both TCP-friendly and scales well to multicast groups with thousands of receivers. We also investigate TFMCC's weaknesses and scaling limits to provide guidance as to application domains for which it is well suited.

Negative-acknowledgment (NACK)-Oriented Reliable Multicast (NORM) Protocol

“…Additionally, for congestion control, this document includes a baseline congestion control mechanism (NORM-CC) based on the TCP-Friendly Multicast Congestion Control (TFMCC) scheme described in [19].…”

“…The "grtt" field contains a non-linear quantized representation of the sender's current estimate of group round-trip time (GRTT) (this is also referred to as R_max in [19]). This value is used to control timing of the NACK repair process and other aspects of protocol operation as described in this document.…”

“…A baseline NORM congestion control scheme (NORM-CC), based on the TCP-Friendly Multicast Congestion Control (TFMCC) scheme of [19] is described in Section 5.5.2 of this document. The NORM_CMD(CC) message is usually transmitted as part of NORM-CC congestion control operation.…”

“…For NORM-CC operation, it is used to provide functionality equivalent to the "feedback round number" (fb_nr)described in [19]. The most recently received "cc_sequence" value is recorded by receivers and can be fed back to the sender in congestion control feedback generated by the receivers for that sender.…”

A model of periodic acknowledgement

“…Many issues have been studied in this framework such as routing protocols (DVMRP, PIM), mechanisms to enforce reliability (scalable reliable multicast (SRM) [4], RAMP [5], RMTP [6], MTP [7], etc.) or congestion control (PGMCC [8], ECAM [9], TFMCC [10]). In this paper, we focus on the issue of multicast reliability as pioneered by XTP [11].…”