Transcript ppt
15-744: Computer Networking L-16 Multicast Reliability and Congestion Control Multicast Issues • Reliable multicast • Multicast congestion control • Assigned reading • [F+97] A Reliable Multicast Framework for Light-Weight Sessions and Application Level Framing © Srinivasan Seshan, 2002 L -16; 11-5-02 2 Overview • Scalable Reliable Multicast • Congestion Control © Srinivasan Seshan, 2002 L -16; 11-5-02 3 SRM • Originally designed for wb • Receiver-reliable • NACK-based • Every member may multicast NACK or retransmission © Srinivasan Seshan, 2002 L -16; 11-5-02 4 SRM Request Suppression Packet 1 is lost; R1 requests resend to Source and Receivers Resend request Packet 1 is resent; R2 and R3 no longer have to request a resend Resent packet S X1 2 1 R1 R1 R2 R3 © Srinivasan Seshan, 2002 S Delay varies by distance R2 R3 L -16; 11-5-02 X X 5 Deterministic Suppression 3d d data Time 2d d d d nack 3d repair = Sender = Repairer d 4d = Requestor d Delay = C1dS,R © Srinivasan Seshan, 2002 L -16; 11-5-02 6 SRM Star Topology Packet 1 is lost; All Receivers request resends Resend request Resent packet S X1 R2 Packet 1 is resent to all Receivers S 2 R3 1 R4 R2 R3 R4 Delay is same length © Srinivasan Seshan, 2002 L -16; 11-5-02 7 SRM: Stochastic Suppression 0 d d Time data 1 repair session msg d 2 NACK d 3 2d = Sender Delay = U[0,D2] dS,R = Repairer = Requestor © Srinivasan Seshan, 2002 L -16; 11-5-02 8 SRM (Summary) • NACK/Retransmission suppression • Delay before sending • Delay based on RTT estimation • Deterministic + Stochastic components • Periodic session messages • Full reliability • Estimation of distance matrix among members © Srinivasan Seshan, 2002 L -16; 11-5-02 9 What’s Missing? • Losses at link (A,C) causes retransmission to the whole group • Only retransmit to those members who lost the packet • [Only request from the nearest responder] S 0 0 B A 0.99 C 0 0 D Sender © Srinivasan Seshan, 2002 L -16; 11-5-02 E 0 F Receiver 10 Local Recovery • Different techniques in various systems • Application-level hierarchy • Fixed v.s. dynamic • TTL scoped multicast • Router supported © Srinivasan Seshan, 2002 L -16; 11-5-02 11 Overview • Scalable Reliable Multicast • Congestion Control © Srinivasan Seshan, 2002 L -16; 11-5-02 12 Multicast Congestion Control • What if receivers have very different bandwidths? • Send at max? • Send at min? • Send at avg? 100Mb/s 100Mb/s R S 1Mb/s ???Mb/s R 1Mb/s 56Kb/s © Srinivasan Seshan, 2002 L -16; 11-5-02 R R 13 Video Adaptation: RLM • • • • • • Receiver-driven Layered Multicast Layered video encoding Each layer uses its own mcast group On spare capacity, receivers add a layer On congestion, receivers drop a layer Join experiments used for shared learning © Srinivasan Seshan, 2002 L -16; 11-5-02 14 Layered Media Streams R2 R1 R1 joins layer 1, joins layer 2 joins layer 3 10Mbps 10Mbps S R 10Mbps R2 join layer 1, join layer 2 fails at layer 3 512Kbps R 128Kbps R3 © Srinivasan Seshan, 2002 L -16; 11-5-02 R3 joins layer 1, fails at layer 2 15 Drop Policies for Layered Multicast • Priority • Packets for low bandwidth layers are kept, drop queued packets for higher layers • Requires router support • Uniform (e.g., drop tail, RED) • Packets arriving at congested router are dropped regardless of their layer • Which is better? • Intuition vs. reality! © Srinivasan Seshan, 2002 L -16; 11-5-02 16 RLM Intuition Uniform vs. Priority Dropping 70 Performance 60 50 Uniform Priority 40 30 20 10 0 © Srinivasan Seshan, 2002 Offered load L -16; 11-5-02 17 RLM Intuition • Uniform • • • • Better incentives to well-behaved users If oversend, performance rapidly degrades Clearer congestion signal Allows shared learning • Priority • Can waste upstream resources • Hard to deploy • RLM approaches optimal operating point • Uniform is already deployed • Assume no special router support © Srinivasan Seshan, 2002 L -16; 11-5-02 18 RLM Join Experiment • Receivers periodically try subscribing to higher layer • If enough capacity, no congestion, no drops Keep layer (& try next layer) • If not enough capacity, congestion, drops Drop layer (& increase time to next retry) • What about impact on other receivers? © Srinivasan Seshan, 2002 L -16; 11-5-02 19 Join Experiments Layer 4 3 2 1 Time © Srinivasan Seshan, 2002 L -16; 11-5-02 20 RLM Scalability? • What happens with more receivers? • Increased frequency of experiments? • More likely to conflict (false signals) • Network spends more time congested • Reduce # of experiments per host? • Takes longer to converge • Receivers coordinate to improve behavior © Srinivasan Seshan, 2002 L -16; 11-5-02 21 Next Lecture: Security • • • • Denial of service IPSec Firewalls Assigned reading • [SWKA00] Practical Network Support for IP Traceback • [B89] Security Problems in the TCP/IP Protocol Suite © Srinivasan Seshan, 2002 L -16; 11-5-02 22