1. ADVANCED COMPUTER NETWORKSCS 6390.521 1 ACTIVE RELIABLE MULTICAST by Li-wei H. Lehman, Stephan J. Garland, and David L. Tennenhouse MIT Laboratory for Computer Science. Presented By Ravikiran Tunuguntla Asutosh Regulagadda Prasanth Jain

2. ADVANCED COMPUTER NETWORKSCS 6390.521 2 Challenges for Reliable Multicast NACK Implosion Problem. Variable packet loss rates at receiver end. Entire group retransmission. Non static group memberships. Retransmission only by source.

3. ADVANCED COMPUTER NETWORKSCS 6390.521 3 Existing Schemes of Multicast SRM: Scalable Reliable Multicast Uses randomized back off to avoid NACK implosion. NACKs are mcast to a group with TTL. TTL limits NACK delivery scope. Any member with data can respond a NACK. LBRM:Log Based Receiver Reliable Multicast. RMTP:Reliable Multicast Transport Protocol

4. ADVANCED COMPUTER NETWORKSCS 6390.521 4 Difficulties with Existing Schemes Provides approximate solutions to scoped recovery. No shield to bottle neck links from repair traffic. Less fault tolerance. Less Robust to topology changes. Group topology must be known. Local recovery is still an open issue (SRM).

5. ADVANCED COMPUTER NETWORKSCS 6390.521 5 The Network Model Similar to traditional packet networks. Provides “best effort” service model. Some intermediate routers may be active. Active routers provide Best effort soft state storage. Flush their caches periodically to remove items with expired life time Perform customized computation based on packet types. End points specify TTL of cached items. No Assumption about # of active nodes. Does not fully rely on Active nodes for error recovery.

6. ADVANCED COMPUTER NETWORKSCS 6390.521 6 The Network Model End points compensate for flushed caches & Router failures Timeouts. Retransmissions. Operates correctly even in face of router failure. Provides IP-multicast style multicast routing. Tree rooted at sender formed for mcast packets. Paths for mcast routing  path for ucast routing. ARM is Receiver reliable.

7. ADVANCED COMPUTER NETWORKSCS 6390.521 7 Active Reliable Multicast Receiver detect losses by sequence gaps or no data arrived after T max Single sender and multiple Receivers. Multiple NACKS are fused at active nodes. Sender responds only to first NACK by mcasting Repair to the group. NACK count is maintained to request for repair. No assumption about end point behavior such as Sending Rate. Time out schemes. Actions performed by active intermediate routers. Data caching for local Retransmission. NACK fusion/Suppression. Partial mcast for scoped retransmission.

8. ADVANCED COMPUTER NETWORKSCS 6390.521 8 Active Reliable Multicast A.Caching Data at Routers (Data Packet): Group address:Multicast group address Cache TTL:Useful life of DP @ active node. NACK count:Significant only for repair packets TTL depends on “farthest” receiver down stream TTL should be bound by sequence # cycle. Cache packets only at strategic locations. In active router simply forwards DP to receiver. NACK count indicates # of receiver requests for repair

9. ADVANCED COMPUTER NETWORKSCS 6390.521 9 Active Reliable Multicast A.Caching Data at Routers: Significance of Caching are End-to-end latency over WAN can be reduced. Distribution of load of retransmission over multicast tree. Reduces recovery latency for distant receivers. Protects sender and bottle neck links from repair traffic. Strategic location of active nodes helps recover latency. Examples of strategic locations are Edge of a backbone network. Node before more lossy link(air in wireless network).

10. ADVANCED COMPUTER NETWORKSCS 6390.521 10 Active Reliable Multicast B.NACK Suppression and Local Retransmission: 3 records maintained at ARM active node are A NACK Record for DP with “NACK count” received for packet. “Subscription bitmap” for outgoing links. A REPAIR Record for DP (p) contains Vector of outgoing links on repair for (p). REPAIR record used to suppress NACK sent by receivers before they receive a repair packet. A Datapacket itself. The above 3 uniquely identifies group addr, source addr, seq# of lost DP.

11. ADVANCED COMPUTER NETWORKSCS 6390.521 11 Active Reliable Multicast B.NACK Suppression and Local Retransmission: NACK packet form receiver to ARM active node Significance of processing NACK packet Suppresses duplicate NACKS. Prepares for necessary scoped retransmission.

12. ADVANCED COMPUTER NETWORKSCS 6390.521 12 Active Reliable Multicast B.NACK Suppression and Local Retransmission:

13. ADVANCED COMPUTER NETWORKSCS 6390.521 13 Active Reliable Multicast B.NACK Suppression and Local Retransmission: First checks if requested repair has been sent to to link where NACK is arrived If yes router drops NACK. If No and Repair is In cache, Router retransmits REPAIR. not in cache, subscribes for retransmission and forwards 1 NACK upstream A NACK with higher NACK count is sent after repair timeout. An in-active router just forwards towards sender. REPAIR record cache TTL  RTT from sender to farthest receiver.

14. ADVANCED COMPUTER NETWORKSCS 6390.521 14 Active Reliable Multicast C.Scoped Retransmissions: Algorithm for repair packet RP: Look up unexpired NACK record and REPAIR record RR for (RP.group, RP.Source, RP.seqnumber); If (cache available at this node)  Store RP in cache; Set RP’s cache TTL to RP.t; If (NR found)  Forward RP down each subscribed link in NR; Remove NR;   Else  For each (outgoing link)  If (downstream receivers are subscribed to RP.group)  Forward RP down link;  If (RR not found)  Create REPAIR record RR for (RP.group, RP.source, RP.seqnumber); Set RR’s cache TTL to RP.t;  For (Each link I on which RP was forwarded)  Set NACK count for I in RR to RP.nackCount; 

15. ADVANCED COMPUTER NETWORKSCS 6390.521 15 Active Reliable Multicast C.Scoped Retransmissions: ARM process REPAIR Packets same as Data Packets. Retransmissions is done to portions of mcast groups By seeing “subscription bitmap”. Which are suffering with losses. Find relevant subscription information If yes, router forwards REPAIR only to subscribed links. If no, Router caches REPAIR. If No cache, Forward REPAIR to all links. Advantageous to cache REPAIR packets than all Data Packets.

16. ADVANCED COMPUTER NETWORKSCS 6390.521 16 Implementation Implemented ARM prototype in JAVA on SPARC under Solaris 2.5 Prototype built using Active Node Transport System (ANTS) ANTS provides a set of JAVA classes which include “capsule” class. Capsule attaches code fragment to Data packets. CCF can be demand loaded at Active Nodes Dynamically. Different processing for Different ARM capsule types. ANTS provide soft-state storage for transient data at ARMs.

17. ADVANCED COMPUTER NETWORKSCS 6390.521 17 Implementation Emulated mcast by creating many ANTS nodes. Work stations are connected to many ANTS nodes by 100MB Ethernet. ANTS nodes communicate using UDP. ARM increases only negligible processing. Efficient implementation of ARM adds 200  s latency. Priority is to cache REPAIRs than Data Packets. Small portion of overall traffic requires Active processing

18. ADVANCED COMPUTER NETWORKSCS 6390.521 18 Simulations Simulated ARM in order to evaluate performance. Simulated varying Active Routers in network. Measured tradeoffs between storage/processing and bandwidth/latency. Results show ARM performs well w.r.t following metrics Recovery Latency. Implosion Control Bandwidth Consumption Compared ARM with SRM using simulation results. Did not considered loss of NACKs, REPAIRs in addition to Data.

19. ADVANCED COMPUTER NETWORKSCS 6390.521 19 Simulations A.Recovery Latency: Compares loss recovery delay (LRD) for ARM to SRM. LRD: Time from detecting packet loss to receive repair. Worst Case Delay measured in units of RTT Group Size Range from 10 to 100. Adaptive in SRM adjusts for Delays. SRM worst Case Latency for Non Adaptive 2.5 RTT. Adaptive 1.4 RTT. ARM Worst case delay is 0.2 RTT. Local Retransmission in ARM (0.2RTT).

20. ADVANCED COMPUTER NETWORKSCS 6390.521 20 Simulations A.Recovery Latency: All non leaf nodes are active and does NACK suppression. Scoped Retransmission. # of routers cache fresh data range 0%-100%. Recovery latency cut 1/2 by 20% of cached data ARM improves recovery latency by Caching mcast data. Caching at small # of strategic active routers

21. ADVANCED COMPUTER NETWORKSCS 6390.521 21 Simulations B.Implosion Control: Measures effectiveness of ARM Vs SRM for NACK implosion. Max # NACKS handled by routers or end points to handle single loss. Shows what happens when packet is lost near source.

22. ADVANCED COMPUTER NETWORKSCS 6390.521 22 Simulations B.Implosion Control: Total # of hops NACKS go before recovery. ARM performance  Nodes performing NACK suppression. ARM achieves benefits with < 50% of active nodes at strategic locations.

23. ADVANCED COMPUTER NETWORKSCS 6390.521 23 Simulations B.Implosion Control: # of strategically placed nodes/ Total # of non-leaf nodes in mcast tree. Group size taken is up to 100. < 50% of nodes are strategic in mcast tree

24. ADVANCED COMPUTER NETWORKSCS 6390.521 24 Simulations B.Implosion Control: Compares SARM-SRM-RARM where SARM: Strategically active ARM. RARM: Randomly active ARM. SARM approaches All active nodes in ARM.

25. ADVANCED COMPUTER NETWORKSCS 6390.521 25 Simulations C.Bandwidth Consumption: Shows bandwidth consumed by REPAIR. Normal distrib of total # of hops in randomly generated mcast tree. Dose not cache fresh data. 40% losses require REPAIR to go 10 hops. 80% losses require REPAIR to go 12 hops. When only 25% of nodes are active 40% losses require 1/3 mcast tree traversal. 80% losses require 1/2 mcast tree traversal. Group size is fixed at 100.

26. ADVANCED COMPUTER NETWORKSCS 6390.521 26 Simulations C.Bandwidth Consumption: Shows effectiveness of scoped retransmission for Different group sizes. Different # of active nodes in mcast tree. SRM recovery bandwidth  group size. ARM is effective in limiting repair traffic.

27. ADVANCED COMPUTER NETWORKSCS 6390.521 27 Simulations C.Bandwidth Consumption: All non leaf nodes are active, but # of cache capable varies. Most benefit on bandwidth occurs when only 40% of nodes cache repairs.

28. ADVANCED COMPUTER NETWORKSCS 6390.521 28 Simulations C.Bandwidth Consumption: When all nodes are cache capable ARM achives perfect scoping of REPAIRS. If 40% can cache it shields 90% of affected nodes.

29. ADVANCED COMPUTER NETWORKSCS 6390.521 29 Related Work Existing work on Wide area Reliable Multicast is 2 types SRM. Hierarchical such as RMTP, TMTP, LBRM. Both approaches don’t give satisfactory solution. Hierarchical approaches perform better for localized recovery. General idea to send NACK upstream till “turning point”. Difficult to decide location of “turning points”. When ARM compares to SRM, has Low recovery latency. Provides specific solution. ARM is flexible to hierarchical approaches. ARM does not require all active nodes.

30. ADVANCED COMPUTER NETWORKSCS 6390.521 30 Conclusions ARM uses intermediate nodes to reduce NACKs. Repair Traffic. Robust to dynamic changes in group membership. ARM performs well in terms of Recovery latency. Implosion Control. Repair Bandwidth. ARM performance degrades gracefully as # of active nodes decreases.