1. VL2 – A Scalable & Flexible Data Center Network Authors: Greenberg et al Presenter: Syed M Irteza – CS @ LUMS CS678: 2 April 2013

2. Brief Overview VL2: A data center network architecture, that aims to remedy problems related to existing data center designs, such as oversubscription, collateral damage, fragmentation, etc. 2

3. Related Efforts BCube: Server-centric network structure - servers forward packets on behalf of other servers. Servers connect to multiple layers of switches. Monsoon: Multi-Homed Tree, using commodity switches, special servers manage load balancing SEATTLE: like Monsoon, but it uses a Distributed Hash Table to store location of each of the servers. PortLand (contemporary): based on Fat-Tree, geared to easy VM migration, use of hierarchical Pseudo MACs. 3

4. VL2: Contributions Detailed Data Center Traffic Analysis – Unpredictable traffic. Network oscillates between 50 to 60 patterns in a day, most of the time, each pattern does not last longer than 100 seconds. – 0.09% failures last over 10 days! Design & Deployment of VL2 – Shuffled 2.7 TB of data between 75 servers in 395 seconds. Compare cost of a VL2 network with equivalent networks based on existing designs 4

5. Problems with Existing Design Limited server-to-server capacity – Technical / financial barriers lead to heavy over- subscription at highest levels (near core router) Fragmentation of resources: – Different VLANs, cannot shift a VM across VLAN borders without reconfiguration (which takes time), thus congestion in one part of DCN, even though other areas are idle Poor reliability and utilization: – Layer 2 domain, uses Spanning Tree, thus only 1 path is used, even if multiple exist 5

6. Goals Uniform high capacity Performance isolation Layer-2 semantics 6

7. Conventional Topology 7

8. VL2: 3-level Clos Topology Scale-out (broader network of inexpensive devices) rather than scale-up (increase capacity and features of individual devices). 8

9. Routing Design Switches operate as layer-3 routers – routing tables are calculated by OSPF Multiple paths available, use Valiant Load Balancing (each flow’s packets bounce “off” of a randomly chosen intermediate switch at top-level of Clos) The packets are further load balanced along the two segments (source to randomly chosen switch, then switch to destination) using ECMP (Equal Cost Multi Path) routing 9

10. Addressing LA: Location-specific IP addresses AA: Application-specific IP addresses Separation was needed to enable easy VM migration, which is needed because we want to be able to pool large shared server pool among unpredictable independent services 10

11. Addressing 11 -Directory Service stores this mapping (LA-AA) - this is realized on servers, not switches. -Eliminates ARP and DHCP bottlenecks -Directory service can enforce access control policies -Shim layer (layer 2.5) invokes the directory service

12. Anycast We assign the same LA address to all Intermediate switches The directory system returns this anycast address to agents upon lookup 12

13. Directory Service 13

14. Evaluation: Goodput 14

15. Evaluation: VLB Fairness 15

16. Performance Isolation 16

17. Questions/Possible Issues Elephant flows may disrupt work of VLB 17