15 744 Computer Networking L16 Multicast Reliability and

15 -744: Computer Networking L-16 Multicast Reliability and Congestion Control © Srinivasan Seshan, 2001 LH-1; 1 -15 -00

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; R 1 requests resend to Source and Receivers Resend request S Packet 1 is resent; R 2 and R 3 no longer have to request a resend Resent packet 1 2 X 1 R 1 R 2 R 3 © Srinivasan Seshan, 2002 S Delay varies by distance R 2 R 3 L -16; 11 -5 -02 X X 5

Deterministic Suppression 3 d d data Time 2 d d nack 3 d repair = Sender = Repairer d 4 d = Requestor d Delay = C 1 d. S, R © Srinivasan Seshan, 2002 L -16; 11 -5 -02 6

SRM Star Topology Packet 1 is lost; All Receivers request resends Resend request S Packet 1 is resent to all Receivers Resent packet S 1 2 X R 2 R 3 1 R 4 R 2 R 3 R 4 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 2 d = Sender Delay = U[0, D 2] d. S, 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? • 100 Mb/s R 100 Mb/s S ? ? ? Mb/s 1 Mb/s R 1 Mb/s 56 Kb/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 R 2 R 1 10 Mbps S R 10 Mbps 512 Kbps R R 1 joins layer 1, joins layer 2 joins layer 3 R 2 join layer 1, join layer 2 fails at layer 3 128 Kbps R 3 © Srinivasan Seshan, 2002 L -16; 11 -5 -02 R 3 joins layer 1, fails at layer 2 15

Drop Policies for Layered Multicast • Priority • • • Uniform (e. g. , drop tail, RED) • • Packets for low bandwidth layers are kept, drop queued packets for higher layers Requires router support 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 © Srinivasan Seshan, 2002 L -16; 11 -5 -02 17

RLM Intuition • Uniform • • • Priority • • • Better incentives to well-behaved users If oversend, performance rapidly degrades Clearer congestion signal Allows shared learning 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? • • Reduce # of experiments per host? • • More likely to conflict (false signals) Network spends more time congested 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 • • • [SWKA 00] Practical Network Support for IP Traceback [B 89] Security Problems in the TCP/IP Protocol Suite © Srinivasan Seshan, 2002 L -16; 11 -5 -02 22