This Data Center Network Architecture introduces a virtual layer 2.5 in the protocol stack of hosts and uses a directory service to achieve efficient forwarding. It uses separate location/identifier IPs

1. VL2: A Scalable and Flexible
Data Center Network
Microsoft Research
Presented by: Ankita Mahajan

2. INTRODUCTION
• Cloud services need data centers with
hundreds of thousands of servers and that
concurrently support a large number of
distinct services.
• To be profitable, DC must achieve high
utilization, and key to this is agility — the
capacity to assign any server to any service.
• Agility promises: improved risk management
and cost savings.

3. Agility in today’s DCN
• Designs for today’s DCN prevent agility in
several ways:
• Oversubscription : Existing architectures do not
provide enough capacity between the servers
they interconnect
• Traffic flood in 1 service affects other services.
• Topologically significant IP addresses.
• Dividing servers among VLANs: reates
Fragmentation of Address Space.

4. Objectives of building VL2
To overcome limitations we need a n/w with
following objectives:
• Uniform high capacity: Hot-spot free
• Performance isolation
• Layer-2 semantics: Just as if the servers were
on a LAN it should be easy to assign any server
to any service and configure that server with
whatever IP address the service expects.

5. • 20 to 40 servers per rack, each singly connected to
• a Top of Rack (ToR) switch with a 1 Gbps link.
• ToRs connect to two aggregation switches for redundancy, and
• these switches aggregate further connecting to access routers.
• At the top, core routers carry traffic between access routers.
• All links use Ethernet as a physical-layer protocol
• To limit overheads (e.g., packet flooding and ARP broadcasts) and to isolate
different services servers are partitioned into virtual LANs (VLANs)

6. Limitations
3 fundamental Limitations:
• Limited server-to-server capacity: ToRs are
1:5 to 1:20 oversubscribed and paths through
the highest layer can be 1:240 oversubscribed.
• Fragmentation of resources: spare capacity is
reserved by individual services.
• Poor reliability and utilization: Resilience
model forces each device and link to be run up
to at most 50% of its maximum utilization.

7. Data-Center Traffic Analysis
1. The ratio of traffic volume between servers into
data centers, to, traffic entering/leaving data
centers is currently around 4:1
2. data-center computation is focused where high
speed access to data on memory or disk is fast
and cheap.
3. The demand for bandwidth between servers
inside a data center is growing faster than the
demand for bandwidth to external hosts.
4. The network is a bottleneck to computation.

8. Flow Distribution Analysis:
Distribution of flow sizes:
• Similar to Internet traffic, 99% of flows are
smaller than 100 MB.
• But the distribution is simpler and more
uniform than Internet.
• More than 90% of bytes are in flows between
100MB and 1 GB.

9. Flow Distribution Analysis:
Number of Concurrent Flows:
• More than 50% of the time, an average machine has
about ten concurrent flows.
• At least 5% of the time it has greater than 80
concurrent ows.
• We almost never see more than 100 concurrent ows.
Both the above Flow Distribution Analysis imply that VLB
will perform well on this traffic. Since even big flows are
only 100 MB.
adaptive routing schemes may be dicult to implement in
the data center since any reactive traffic engineering will
need to run at least once a second if it wants to react to
individual flows.

10. Traffic Matrix Analysis
• Poor summarizability of trac patterns:
Is there regularity in the trac that might be
exploited through careful measurement and trac
engineering?
TM(t)ij clustering: a day’s worth of trac in the
datacenter, even when approximating with 50-60
clusters, the fitting error remains 60%
• Instability of trac patterns: how predictable is the
trac in the next interval given the current trac?

11. Failure Characteristics
1. pattern of networking equipment failures:
• Most failures are small in size: 50% of network
device failures involve < 4 devices and 95% of
network device failures involve < 20 devices
• Large correlated failures are rare: the largest
correlated failure involved 217 switches
• downtimes can be signicant: 95% of failures are
resolved in 10min, 98% in < 1 hr, 99.6% in < 1 day,
but 0.09% last > 10 days.

12. impact of networking equipment
failure?
• in 0.3% of failures all redundant components in a
network device group became unavailable
• The main causes of these downtimes are network
miscongurations, firmware bugs, and faulty
components (e.g., ports).
• With no obvious way to eliminate all failures from
the top of the hierarchy, VL2’sapproach is to
broaden the topmost levels of the network so
that the impact of failures is muted and
performance degrades gracefully,
• moving from 1:1 redundancy to n:m redundancy.

13. Terminology
• Goodput: useful information delivered per
second to the application layer.
• VLB: each server independently picks a path at
random through the network for each of the
flows it sends to other servers in the data
center.
• ECMP: distributes traffic across equal-cost
paths
• anycast addresses for the Directory System