1. Data Center Traffic Engineering

2. Cloud Computing Elastic resources Multi-tenancy
Expand and contract resources
Pay-per-use
Infrastructure on demand
Multi-tenancy
Multiple independent users
Security and resource isolation
Amortize the cost of the (shared) infrastructure
Flexibility service management
Resiliency: isolate failure of servers and storage
Workload movement: move work to other locations

3. Cloud Service Models Software as a Service Platform as a Service
Provider licenses applications to users as a service
E.g., customer relationship management, , ..
Avoid costs of installation, maintenance, patches, …
Platform as a Service
Provider offers software platform for building applications
E.g., Google’s App-Engine
Avoid worrying about scalability of platform
Infrastructure as a Service
Provider offers raw computing, storage, and network
E.g., Amazon’s Elastic Computing Cloud (EC2)
Avoid buying servers and estimating resource needs

4. Multi-Tier Applications
Applications consist of tasks
Many separate components
Running on different machines
Commodity computers
Many general-purpose computers
Not one big mainframe
Easier scaling
Front end Server
Aggregator
… …
Aggregator
Aggregator
Aggregator … …
Worker
Worker
Worker
Worker
Worker

5. Enabling Technology: Virtualization
Multiple virtual machines on one physical machine
Applications run unmodified as on real machine
VM can migrate from one computer to another

6. Data Center Network

7. Status Quo: Virtual Switch in Server

8. Top-of-Rack Architecture
Rack of servers
Commodity servers
And top-of-rack switch
Modular design
Preconfigured racks
Power, network, and storage cabling
Aggregate to the next level

9. Modularity, Modularity, Modularity
Containers
Many containers

10. Data Center Network Topology
Internet CR CR
. . . AR AR AR AR S S
. . . S S S S
Key
CR = Core Router
AR = Access Router
S = Ethernet Switch
A = Rack of app. servers A A … A A A … A
~ 1,000 servers/pod

11. Capacity Mismatch . . . ~ 200:1 ~ 40:1 ~ 5:1 … … … … CR CR AR AR AR AR

12. ~ 1,000 servers/pod == IP subnet
Data-Center Routing
Internet CR CR
DC-Layer 3
. . . AR AR AR AR
DC-Layer 2 S S S S S S S S
. . . S S S S
Key
CR = Core Router (L3)
AR = Access Router (L3)
S = Ethernet Switch (L2)
A = Rack of app. servers A A … A A A … A
~ 1,000 servers/pod == IP subnet

13. Reminder: Layer 2 vs. Layer 3
Ethernet switching (layer 2)
Cheaper switch equipment
Fixed addresses and auto-configuration
Seamless mobility, migration, and failover
IP routing (layer 3)
Scalability through hierarchical addressing
Efficiency through shortest-path routing
Multipath routing through equal-cost multipath
So, like in enterprises…
Data centers often connect layer-2 islands by IP routers

14. Load Balancers Spread load over server replicas
Present a single public address (VIP) for a service
Direct each request to a server replica
Virtual IP (VIP)

15. Data Center Costs (Monthly Costs)
Servers: 45%
CPU, memory, disk
Infrastructure: 25%
UPS, cooling, power distribution
Power draw: 15%
Electrical utility costs
Network: 15%
Switches, links, transit

16. Wide-Area Network . . . Internet Data Centers Servers Router
DNS Server
DNS-based
site selection
. . .
Servers
Internet
Clients
Data Centers

17. Wide-Area Network: Ingress Proxies
Router
Data Centers
. . .
Servers
Clients
Proxy

18. Data Center Traffic Engineering
Challenges and Opportunities

19. Traffic Engineering Challenges
Scale
Many switches, hosts, and virtual machines
Churn
Large number of component failures
Virtual Machine (VM) migration
Traffic characteristics
High traffic volume and dense traffic matrix
Volatile, unpredictable traffic patterns
Performance requirements
Delay-sensitive applications
Resource isolation between tenants

20. Traffic Engineering Opportunities
Efficient network
Low propagation delay and high capacity
Specialized topology
Fat tree, Clos network, etc.
Opportunities for hierarchical addressing
Control over both network and hosts
Joint optimization of routing and server placement
Can move network functionality into the end host
Flexible movement of workload
Services replicated at multiple servers and data centers
Virtual Machine (VM) migration

21. The Illusion of a Huge L2 Switch 3. Performance isolation
Virtual Layer 2 Switch
The Illusion of a Huge L2 Switch CR AR S
1. L2 semantics
. . .
2. Uniform high capacity
3. Performance isolation
. . . A A A A A … A A A A A A … A A A A A A A A A A … A A A A A A A A A A A A … A A A A

22. VL2 Goals and Solutions Objective Approach Solution
1. Layer-2 semantics
Employ flat addressing
Name-location separation & resolution service
2. Uniform high capacity between servers
Guarantee bandwidth for hose-model traffic
Flow-based random traffic indirection (Valiant LB)
3. Performance Isolation
Enforce hose model using existing mechanisms only
TCP
“Hose”: each node has ingress/egress bandwidth constraints

23. Name/Location Separation
Cope with host churns with very little overhead
VL2
Directory Service
Switches run link-state routing and maintain only switch-level topology
Allows to use low-cost switches
Protects network and hosts from host-state churn
Obviates host and switch reconfiguration …
x  ToR2
y  ToR3
z  ToR4 …
x  ToR2
y  ToR3
z  ToR3
ToR1
. . .
ToR2
. . .
ToR3
. . .
ToR4
Lazy update of stale cache entries when packet erroneously reaches the old ToR (who forwards to the directory server, who updates with unicast)
ToR3 y
payload
Lookup &
Response x y
y, z z
ToR3
ToR4 z z
payload
payload
Servers use flat names

24. Offer huge aggr capacity & multi paths at modest cost
Clos Network Topology
Offer huge aggr capacity & multi paths at modest cost
VL2
Int
. . .
D (# of 10G ports)
Max DC size
(# of Servers) 48
11,520 96
46,080
144
103,680
Aggr
. . .
K aggr switches with D ports
D/2 ports facing up, and D/2 ports facing down…
. . .
TOR
. . .
20 Servers
20*(DK/4) Servers

25. Valiant Load Balancing: Indirection
Cope with arbitrary TMs with very little overhead
Links used for up paths
Links used for down paths
IANY
IANY
IANY
[ ECMP + IP Anycast ]
Harness huge bisection bandwidth
Obviate esoteric traffic engineering or optimization
Ensure robustness to failures
Work with switch mechanisms available today T1 T2 T3 T4 T5 T6
IANY T3 T5 y z
payload
payload
1. Must spread traffic
2. Must ensure dst independence
Equal Cost Multi Path Forwarding x y z

26. VL2 vs. Seattle Similar “virtual layer 2” abstraction
Flat end-point addresses
Indirection through intermediate node
Enterprise networks (Seattle)
Hard to change hosts  directory on the switches
Sparse traffic patterns  effectiveness of caching
Predictable traffic patterns  no emphasis on TE
Data center networks (VL2)
Easy to change hosts  move functionality to hosts
Dense traffic matrix  reduce dependency on caching
Unpredictable traffic patterns  ECMP and VLB for TE

27. Research Questions What topology to use in data centers?
Reducing wiring complexity
Achieving high bisection bandwidth
Exploiting capabilities of optics and wireless
Routing architecture?
Flat layer-2 network vs. hybrid switch/router
Flat vs. hierarchical addressing
How to perform traffic engineering?
Over-engineering vs. adapting to load
Server selection, VM placement, or optimizing routing
Virtualization of NICs, servers, switches, …

28. Research Questions Rethinking TCP congestion control?
Low propagation delay and high bandwidth
“Incast” problem leading to bursty packet loss
Division of labor for TE, access control, …
VM, hypervisor, ToR, and core switches/routers
Reducing energy consumption
Better load balancing vs. selective shutting down
Wide-area traffic engineering
Selecting the least-loaded or closest data center
Security
Preventing information leakage and attacks