VL2 – A Scalable & Flexible Data
Center Network
Authors: Greenberg et al
Presenter: Syed M Irteza – CS @ LUMS
CS678: 2 April 2013
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
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
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
Problems with Existing Design
• Limited server-to-server capacity
– Technical / financial barriers lead to heavy oversubscription 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
Goals
• Uniform high capacity
• Performance isolation
• Layer-2 semantics
6
Conventional Topology
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VL2: 3-level Clos Topology
• Scale-out (broader network of inexpensive
devices) rather than scale-up (increase
capacity and features of individual devices).
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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
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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
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Addressing
- 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
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Anycast
• We assign the same LA address to all
Intermediate switches
• The directory system returns this anycast
address to agents upon lookup
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Directory Service
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Evaluation: Goodput
14
Evaluation: VLB Fairness
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Performance Isolation
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Questions/Possible Issues
• Elephant flows may disrupt work of VLB
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