VL2: A Scalable and Flexible Data Center Network

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Networking the Cloud
Presenter: b97901184 電機三 姜慧如
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Data Centers holding tens to hundreds of
thousands of servers.
Concurrently supporting a large number of
distinct services.
Economies of scale.
Dynamically reallocate servers among
services as workload pattern changes.
High utilization is needed.  Agility!
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Agility:
Any machine to be able to play any role.
“Any Service, any Server.”
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Ugly Secret: 30% utilization is considered good.
Uneven application fit:
-- Each server has CPU, memory, disk: most applications exhaust one
resource, stranding the others.
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Long provisioning timescales:
-- New servers purchased quarterly at best.
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Uncertainty in demand:
-- Demand for a new service can spike quickly.
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Risk management:
-- Not having spare servers to meet demand brings failure just when
success is at hand.
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Session state and storage constraints:
-- If the world were stateless servers….
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Workload management
-- Means for rapidly installing a service’s code on a
server.  Virtual Machines, disk images.
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Storage management
-- Means for a server to access persistent data.
Distributed filesystems.
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Network
-- Means for communicating with other servers,
regardless of where they are in the data center.
But:
 Today’s data center network prevent agility in
several ways.
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Static network assignment
Fragmentation of resource
I have spare ones,
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but…
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Poor server to server connectivity
Traffics affects each other
Poor reliability and utilization
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Achieve scale by assigning servers
topologically related IP addresses and dividing
servers among VLANs.
 Limited utility of VMs, cannot migrate out the
original VLAN while keeping the same IP address.
  Fragmentation of address space.
  Configuration needed when reassigned to different
services.
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1. L2 semantics
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2. Uniform high
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capacity
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3. Performance
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isolation
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Developers want a mental model where all
their servers, and only their servers, are
plugged into an Ethernet switch.
1. Layer-2 semantics
-- Flat addressing, so any server can have any IP Address.
-- Server configuration is the same as in a LAN.
-- VM keeps the same IP address even after migration
2. Uniform high capacity
-- Capacity between servers limited only by their NICs.
-- No need to consider topology when adding servers.
3. Performance isolation
-- Traffic of one service should be unaffected by others.
Objective
1. Layer-2 semantics
Approach
Employ flat addressing
Solution
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
Enforce hose model using
existing mechanisms only
TCP
Isolation
“Hose”: each node has ingress/egress bandwidth constraints
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Ethernet switching (layer 2)
 Cheaper switch equipment
 Fixed addresses and auto-configuration
 Seamless mobility, migration, and failover
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IP routing (layer 3)
 Scalability through hierarchical addressing
 Efficiency through shortest-path routing
 Multipath routing through equal-cost multipath
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So, like in enterprises…
 Data centers often connect layer-2 islands by IP routers
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Data-Center traffic analysis:
DC traffic != Internet traffic
 Traffic volume between servers to entering/leaving data center is 4:1
 Demand for bandwidth between servers growing faster
 Network is the bottleneck of computation
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Flow distribution analysis:
 Majority of flows are small, biggest flow size is 100MB
 The distribution of internal flows is simpler and more uniform
 50% times of 10 concurrent flows, 5% greater than 80 concurrent
flows
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Traffic matrix analysis:
 Poor summarizing of traffic patterns
 Instability of traffic patterns
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Failure characteristics:
 Pattern of networking equipment failures: 95% < 1min, 98% < 1hr,
99.6% < 1 day, 0.09% > 10 days
 No obvious way to eliminate all failures from the top of the hierarchy
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Flat Addressing:
Allow service instances (ex. virtual machines) to be placed
anywhere in the network.
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Valiant Load Balancing:
(Randomly) Spread network traffic uniformly across network
paths.
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End-system based address resolution:
To scale to large server pools, without introducing complexity
to the network control plane.
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Design principle:
 Randomizing to cope with volatility:
▪ Using Valiant Load Balancing (VLB) to do destination independent
traffic spreading across multiple intermediate nodes
 Building on proven networking technology:
▪ Using IP routing and forwarding technologies available in commodity
switches
 Separating names from locators:
▪ Using directory system to maintain the mapping between names and
locations
 Embracing end systems:
▪ A VL2 agent at each server
Offer huge aggr capacity & multi paths at modest cost
Int
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Aggr
K aggr switches with D ports
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TOR
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Servers
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20*(DK/4) Servers
Cope with arbitrary TMs with very little overhead
IANY
IANY
IANY
Links used
for up paths
Links used
for down paths
[ ECMP + IP Anycast ]
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Harness huge bisection bandwidth
Obviate esoteric traffic engineering or optimization
Ensure robustness to failures
Work with switch mechanisms available today
T1
IANY T35 zy
T2
payload
x
20
T3
T4
T5
T6
1. Must
spread
Equal
Cost
Multitraffic
Path Forwarding
y2. Must ensure dst
z independence
IANY
IANY
IANY
Links used
for up paths
Links used
for down paths
T1
IANY T53
T2
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x
y
T4
T5
yz payload
z
T6
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How Smart servers use Dumb switches– Encapsulation.
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Commodity switches have simple forwarding primitives.
Complexity moved to servers -- computing the headers.
RSM
RSM
Servers
3. Replicate
RSM
RSM
4. Ack
(6. Disseminate)
2. Set
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DS
DS
2. Reply
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DS
2. Reply
1. Lookup
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5. Ack
1. Update
Agent
Agent
“Lookup”
“Update”
Directory
Servers
Switches run link-state routing and
maintain only switch-level
topology
LAs
ToR1 . . .
ToR3 y payload
ToR34 z payload
AAs
ToR2
x
...
ToR3
y,yz
Servers use flat
names
...
Directory
Service
ToR4
…
x  ToR2
y  ToR3
z  ToR34
…
z
Lookup &
Response
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Data center Oses already heavily modified for
VMs, storage clouds, etc.
No change to apps or clients outside DC.
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Uniform high capacity:
 All-to-all data shuffle stress test:
▪ 75 servers, deliver 500MB
▪ Maximal achievable goodput is 62.3
▪ VL2 network efficiency as 58.8/62.3 = 94%
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Performance isolation:
 Two types of services:
▪ Service one: 18 servers do single TCP transfer all the time
▪ Service two: 19 servers starts a 8GB transfer over TCP every 2 seconds
▪ Service two: 19 servers burst short TCP connections
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Convergence after link failures
 75 servers
 All-to-all data shuffle
 Disconnect links between intermediate and aggregation switches

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