Exploring Efficient and Scalable Multicast Routing in Future Data Center Networks

1. Exploring Efficient and Scalable Multicast Routing in Future Data Center Networks Dan Li, Jiangwei Yu, Junbiao Yu, Jianping Wu Tsinghua University Presented by DENG Xiang 1

2. Outline I Introduction and background II Build an efficient multicast tree III Make multicast routing scalable IV Evaluation V Conclusion

3. Introduction and background Data Centers • the core of cloud services • online cloud applications • back-end infrastructural computations • servers and switches • popularity of group communication

4. Multicast • save network traffic • improve application throughput

5. Internet-orieted Multicast is successful.

6. When Multicast meets data center networks... Problem A: Data center topologies usually expose high link density and traditional technologies can result in severe link waste. Problem B: Low-end commodity switches are largely used in most data center designs for economic and scalability consideration.

7. Build an efficient Multicast tree Data Center Network Architecture • BCube • Portland • VL2 (similar to Portland)

8. BCube • constructed recursively: BCube(n,0), BCube(n,1)...BCube(n,k) • each server has k+1 ports • each switch has n ports • number of servers: nk+1

9. Portland • three-level and n pods • aggregation level and edge level: n/2 switches with n ports • core level: (n/2)2 switches with n ports • number of servers: n3/4

10. Consistent themes lie in them • use low-end switches in the view of expense • high link density exists • data center structure is built in a hierarchical and regular way

11. In order to save network traffic, how to build an efficient Multicast tree • traditional receiver-driven Multicast routing protocols originally for the Internet, such as PIM • approximate algorithm of Steiner tree Steiner tree problem: to build a Multicast tree with the lowest cost covering the given nodes • source-driven tree building algorithm the proposed algorithm

12. group spanning graph • each hop is a stage • stage 0 includes the sender only • stage d is composed of receivers • d is the diameter of data center topology

15. Build Multicast tree in a source-to-receiver expansion way upon the group spanning graph, with the tree node set from each stage strictly covering downstream receivers definition of cover: A covers B if and only if for each node in B, there exists a directed path from a node in A A strictly covers B when A covers B and any subset of A does not cover B.

16. algorithm details in BCube: • select the set of servers(assume the set is E) from stage 2 which are covered by sender s and a single switch in stage 1(assume it is W) • |E| of the BCube(n,k-1)s has a server in E as the source p, and the receiver set in stage 2*(k+1) covered by p. • the other BCube(n,k-1) has s as the source and receivers in stage 2*k covered by s but not by W as the receiver set

17. algorithm details in Portland: • From the first stage to the stage of core-level switches, any single path can be chosen, because any single core-level switch can cover the downstream receivers. • From the stage of core-level switches to the final stage of receivers, the paths are fixed due to the interconnection rule in PortLand.

18. Make Multicast routing scalable a mechanism of packet forward to support massive Multicast group is necessary: • in-packet Bloom Filter For only in-packet Bloom Filter, bandwidth waste is significant for large groups. • in-switch forwarding table For only in-switch forwarding table, very large memory space is needed.

19. The bandwidth waste ofin-packet Bloom Filter comes from: • theBloom Filter field in the packet brings networkbandwidthcost. • false-positive forwarding by Bloom Filter causestrafficleakage. • switches receiving packets by false-positive forwardingmay further forward packets to otherswitches, incurringnot only additional traffic leakage but alsopossible loops.

20. we define Bandwidth Overhead Ratior to decribe in- packet Bloom Filter: p--the packet length (including the Bloom Filterfield) f--the length of the in-packet Bloom Filter field t--the number of links in the Multicast tree c--the numberof actual links covered by Bloom Filter based forwarding

21. with the packet size as 1500 bytes, the relation among r, f and group size: BCube(8,3) Portland with 48-port switches

22. In-packet Bloom Filter does not accommodate large-size group. So a combination routing scheme is proposed. a) in-packet Bloom Filters are used for small-sized groups to save routing space in switches, while routing entries are installed into switches for large groups to alleviate bandwidth overhead. b) Intermediate switches/servers receiving the Multicast packet check a special TAG in the packet to determine whether to forward the packet via in-packet Bloom Filter or looking up the in-switch forwarding table.

23. two ways of in-packet Bloom Filter • node-based encoding elements are the tree nodes, including switches and servers and it is chosen. • link-based encoding elements are the directed physical links

24. false-positive forwarding caused by in-packet Bloom Filter may result in loops. the solution: When a node onlyforwards the packet to its neighboring nodes (withinthe BloomFilter) whose distances to source are larger than itself.

25. Evaluation evaluation of souce-driven tree buiding algorithm: • BCube(8,3) and 48-port-switch Portland; • 1Gbps link speed; • 200 random-sized groups; • number of links in the tree • computation time

26. BCube Portland

27. BCube Portland

28. evaluation of combination forwarding scheme with 32-byte Bloom Filter:

29. Conclusion Efficient and Scalable Multicast Routing in Future Data Center Networks • an efficient Multicast tree building algorithm • a combination forwarding scheme for salable Multicast routing

30. Thank You！