CSE 561 – Multicast Applications David Wetherall Spring 2000

djw // CS 561, Spring 2000 L10.2 This Lecture Floyd, Jacobson, Liu, McCanne, Zhang –“A Reliable Multicast Framework for Lightweight Sessions and Application Level Framing” –IEEE/ACM Transactions on Networking, McCanne and Jacobson –“Receiver-driven Layered Multicast” –SIGCOMM 1996.

djw // CS 561, Spring 2000 L10.3 Context You know about multicast addressing and routing … Now focus on how to extend to multicast applications –Video broadcasting, shared whiteboard –Adding congestion control and reliability to multicast

djw // CS 561, Spring 2000 L10.4 Why is this difficult? Heterogeneity –Of network paths: one size doesn’t fit all Scalability –Implosion, Growth of control traffic Group membership –Dynamic set w/ late joiners etc. Application types –Many-to-many vs. one-to-many

djw // CS 561, Spring 2000 L10.5 RLM Paper Focus on rate adaptation of video transmission in the presence of bandwidth heterogeneity –Non-TCP multicast congestion control TCP probes the network to track the congested link between one sender and one receiver Now we need to infer the points of congestion for multiple receivers … difficult problem

djw // CS 561, Spring 2000 L10.6 Previous Approaches Sender adapts transmission rate based on receiver feedback a la TCP –Feedback needed despite unreliable transport –Typically track slowest receiver How to gather feedback in a scalable manner? –Implosion problem: source overwhelmed by near simultaneous messages from receivers  decorrelate over an interval –Group size problem: decorrelation not enough  make interval depend on group size and pick a random sample

djw // CS 561, Spring 2000 L10.7 Layered Video One source rate not right for all receivers Use layered video to efficiently support multiple rates –Different receivers can receive different rates Q: How to selectively forward flows in network? –Network support –Multiple multicast groups

djw // CS 561, Spring 2000 L10.8 Priority vs. Uniform Drop See S. Bajaj, L. Breslau, S. Shenker, “Uniform Versus Priority Drop for Layered Video”, SIGCOMM 98 RED also useful for RLM

djw // CS 561, Spring 2000 L10.9 RLM algorithms Receivers run join experiments –How do we determine if it succeeds? –Backoff exponentially if they fail Experiments are shared for scalability –How does this help? –How does it scale?

djw // CS 561, Spring 2000 L10.10 SRM Paper Adding the word “reliable” to multicast Focus on scalability over the wide-area Protocol works for many-to-many distribution Protocol tied to application characteristics (ALF)

djw // CS 561, Spring 2000 L10.11 Reliable Multicast Approaches Token-ring Sender-based Receiver-based Tree-based

djw // CS 561, Spring 2000 L10.12 SRM Design Receiver and “NACK”-based and relies on co-operation –NACKs triggered by hole in sequence space or session messages Data, requests and repairs multicast to entire group Timers bias who sends requests and repairs: –To avoid implosion –To have a neighbor respond/repair loss quickly Ideally exactly one request and response per loss

djw // CS 561, Spring 2000 L10.13 SRM and ALF Clark and Tennenhouse, “Architectural Considerations for a New Generation of Protocols”, SIGCOMM 90 ALF = Application Level Framing Looks at interaction of network and application protocols and argues network protocol should be designed with application needs in mind.

djw // CS 561, Spring 2000 L10.14 Scalability of SRM What happens as group size increases? –To request/repair timers –To expected number of messages Large groups with uncorrelated loss neighborhoods lead to poor scaling –There is a strong need for localized recovery See M. Yajnik, J. Kurose, D. Towsley, "Packet Loss Correlation in the MBone Multicast Network“ for experimental data

djw // CS 561, Spring 2000 L10.15 Local Recovery in SRM Want for form appropriate loss neighborhoods (related to topology) and send repairs only to the right group SRM suggests the use of TTL-scoped multicast Q: How well does this work? Alternatively use multiple multicast groups Q: How well does this work?

djw // CS 561, Spring 2000 L10.16 Other Strategies: FEC FEC (or parity) on the data stream –Allows single receiver to tolerate some loss –Allows multiple receivers to each tolerate different loss! “FEC” (or parity) as the repair stream –Allows one message to repair different losses at each receiver

djw // CS 561, Spring 2000 L10.17 Other strategies: Network support Mismatch between multicast delivery primitive and need for scoping Q: Can we change multicast delivery? Cisco’s PGM, Papadopoulos’s Repliers, various active … Deployment at strategic point an example of network- based support for “discontinuities”