PGMCC :A TCP-friendly Single-rate Multicast Congestion Control Scheme

1. PGMCC :A TCP-friendly Single-rate Multicast Congestion ControlScheme Author - Luigi Rizzo CS 590F Seminar Presenter - Ameya Varde

2. Terminology • Multicast - data transfer from a single sender to multiple receivers. • Group - set of receivers. • Acker - slowest receiver within the group. • Single rate multicast - • All receivers get the same data rate. • Sender adapts to the slowest receiver. • A single slow receiver can drag down the data rate for the whole group. • Scheme is relatively easy and consumes smaller bandwidth. • TCP friendly - sender transmits no faster than the TCP fair rate at the slowest receiver.

3. Objective of PGMCC • Prevent congestion in the multicast session. • To do so, we need to - • Find the slowest receiver (acker). • Ackers may change over time. • Transmit no faster than the acker can accept.

4. Working of PGMCC • Acker election and tracking. • PGMCC makes use of NAK's sent by receivers to decide the acker. • Congestion control. • The acker then supplies ACK's to the sender as an indication of successful data packet transfer. • A window-based congestion controller scheme similar to the one used in TCP is used between the sender and acker.

5. Packet Formats

6. Acker Election and Tracking • Two parameters (present in the NAK) decide the acker. • rxw_lead Sender computes RTT. RTT = (most recent seq_no) – rxw_lead • rx_loss Receiver computes loss rate p. p = W x pprev + (1-W) x seq_no • Throughput T  1 / (RTT x p) • Acker is the receiver with the lowest T. • Receiver i becomes the new acker if • Ti < c x Tacker c  0.6-0.8 • c is used to reduce the number of acker switches.

7. Congestion Control • Sender maintains two state variables, window W and token count T. • W is an estimate of the number of packets in transit. • T regulates the generation of data packets. • on session start, W=1, T=1. • on transmit, T=T-1 (i.e. consume one token). • on ACK, W=W +1/W, T=T+1+1/W. • on NAK, W=W/2, T unchanged for next W/2 ACK's.

8. PGMCC enabled routers • NAK suppression. • Filters repeated NAK's coming from receivers to sender. • Stores state. • Maintains rx_id, rxw_lead. • Selective Forwarding. • Forwards re-transmitted packets only to expecting receivers • Problem: • Could result in sender not receiving a NAK from a potential acker • Solution: • Maintain rxw_loss in router as well. • For a repeated NAK, compare incoming rxw_loss with the stored one. If lesser, forward NAK to sender.

9. Experimental Results

10. Advantages of PGMCC • RTT computed without the need for globally synchronized clocks. • Sender is not hindered by acker switches. • Works fine even for unreliable protocols. • Works with or without router support. • Responsive to uncorrelated losses . • Sender uses loss rates (rx_loss) computed by receivers. • Avoids "drop-to-zero" problem. • Achieves intra protocol and inter protocol fairness. • Scaleable.

11. Disadvantages and Future work Disadvantages • Requires loss reports from receivers. • Most calculations are approximate. Future work • Use of alternate methods to compute loss rate at the receivers. • Computing throughput more accurately. • Use adaptive rather than a fixed value (6) for slow start threshold.

12. Conclusions • PGMCC has been implemented by the author in the PGM protocol on BSD Unix. • Achieves the desired results in the configurations that were tested. • Author believes that PGMCC can be reliably used for multicast communication over the Internet on a small-medium scale.