Pre$SDN era: network trends in data centre networking

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Pre$SDN era: network trends

in data centre networking

Zaheer Chothia – 27.02.2015

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Outline

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Challenges and New Requirements

History of Programmable Networks

Spanning Tree Protocol HSTPI

Network Overlay Technologies

Examples: SPB, TRILL, FabricPath, MPLS

Emerging Next$Generation Protocols

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Networks have not kept pace

“In the modern data center, traditional technologies

are

limiting the speed, flexibility, scalability, and

manageability

of application deployments.”

Cloud data center networks must contend with:

• huge numbers of attached devices Hboth physical and virtualI

• large numbers of isolated independent subnetworks

• multitenancy Hdifferent tenants collocated on a single hostI

• automated creation, deletion, and migration of virtual machines

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Requirements of Modern Data Center

4

GDeHcoupling

Scalability

Mobility

Others

•  Virtualized networks •  Optimized forwarding •  Cloud integration •  … and many more

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Requirements of Modern Data Center

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GDeHcoupling

Physical/logical connectivity Want to deploy and expand workloads anywhere

Can extend VLAN domain but this affects availability Hlarger fault domainI + reconfiguration and administrative overhead

Infrastructure and policy Common practice:

•  Group entities with like membership into a VLAN •  IP addressing schemes based

on subnet boundaries Results in many inefficiencies and limitations Hcyclic changeI

Scalability

More end hosts and isolated subnetworks

Forwarding tables Network uses end`host

information HIP/MAC addressI to make forwarding decisions Need to propagate this state

across entire data center fabric Network segments

Space limitations: 802.1Q supports at most 4,094 VLANs

Also desired:

•  traffic management •  secure segmentation •  performance isolation

Mobility

Use case: live migration of VMs Need to retain adequate

network state

Address of end host should be

independent of location in the network

Others

•  Virtualized networks •  Optimized forwarding •  Cloud integration •  … and many more

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20 years of development

• Active Networking

–  Mid`90s: Internet took off; standardization is too slow! –  Approach: programmable functions in the network

–  Capsule model Hcode to execute at nodes carried in`band in data packetsI

• Separating Control and Data Planes

–  Early 2000s: increasing traffic volumes and greater emphasis on network reliability, predictability, and performance

–  Desired: better control of paths used to deliver traffic Htraffic engineeringI –  Logically centralized control + open interfaces to routers and switches

• OpenFlow API and Network OSes

–  Network experimentation at scale Hencouraged by successes of PlanetLab/EmuLabI –  Pragmatism: limit flexibility Hbuild on existing switchesI, but immediate deployability –  First widespread adoption of an open interface

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The Road to SDN: An Intellectual 

History of Programmable Networks

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https://www.cs.princeton.edu/~jrex/papers/queue14.pdf https://www.youtube.com/watch?v=dkUDUb9GtH0

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Spanning Tree Protocol GSTPH

Purpose: Ensures loop`free topology by blocking redundant paths

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http://www.cisco.com/c/en/us/support/docs/lan`switching/spanning`tree`protocol/24248`147.html

Common Spanning Tree ?CST@

•  No load balancing possible; need to block one uplink for all VLANs

•  CPU is spared; only one instance needs to be computed

PerAVLAN Spanning Tree ?PVST@

•  Optimum load balancing Hodd`evenI

•  1000 separate instances Hfor each VLANI even though only two different final topologies

•  Considerably wastes CPU cycles for all of the switches in the network

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STP drawbacks

• Designed with maximum

stability and safety

in mind

• Blocking redundant ports:

squandered

aggregate bandwidth

• Cannot HeasilyI

segregate

into smaller domains 

Desirable for scalability, fault isolation, multi`tenancy

• Topology change: network

halts

while STP recalculates [see link below]

–  Bounded by Max_Age+2xForward_Time Htypical: 20 + 2*15 = 50 seconds!I

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http://blog.ine.com/wp`content/uploads/2011/11/understanding`stp`rstp`convergence.pdf

• Patchwork:

– 

Rapid STP: faster convergence after a topology

change Hwith active confirmationI

– 

Multiple STP: per`VLAN spanning tree + block all

but one of alternate paths within each

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Network Overlays

Benefits

•  Simplified management

•  Multi`tenancy: scalable beyond 4000 VLANs

•  Workload`anywhere Hmobility and reachabilityI

•  Arbitrary forwarding topologies over fixed underlay

Challenges

•  Decreased visibility: traceroute in overlay will not report individual underlay hop counts

•  Troubleshooting complexity:  need to investigate mapping of  virtual to physical topology

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Bag of protocols

• Designed to address shortcomings mentioned earlier

• Typically employ

encapsulation

for transparency from host

Hrather than for logical separationI

• Surveyed: layer`2 ECMP designs with

multipath

routing

– 

Shortest Path Bridging HSPBI

– 

Transparent Interconnection of Lots of Links HTRILLI

– 

Cisco FabricPath

–

 

Brocade: Virtual Cluster Switching HVCSI 

Based on TRILL Hdata planeI; doesn’t use IS`IS core

–

 

Juniper: QFabric Hshelved?I

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Shortest Path Bridging GSPBH

• Consolidate into a one link state protocol:

–  STP, Multiple STP, Rapid STP –  Multichassis Link Aggregation

–  Multiple MAC Registration Protocol HMMRPI

• Service identifiers HI`SIDI for independence from backbone MAC

address / VLAN IDs

• New device: learn its immediate neighbourhood, compute shortest

bidirectional paths using link metrics Hsuch as ECMPI

• End points are fully aware of entire traffic path Hunlike TRILLI

• Two modes

–  SPBV: use VLAN ID for delineation and load balancing  Learn MAC addresses on all bridges along shortest path

–  SPBM: uses B`MAC+B`VID HbackboneI to designate reachability 

C`MAC HclientI addresses are never learned or looked up in the core

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Transparent Interconnection of

Lots of Links GTRILLH

• Control plane: IS`IS for discovery and to distribute link`state

database Heach node has state of entire networkI

• Uses Routing Bridges HRBridgesI to provide:

– 

Shortest unicast paths Hunlike STP: no single tree constraintI

– 

Faster convergence times

– 

Load splitting over multiple paths

– 

Loop mitigation

– 

MAC scalability: relegated learning to the edge RBs

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•  Several possible multicast trees •  More active paths Hefficient use

of bandwidthI

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Cisco FabricPath

• Proprietary implementation of TRILL

– 

Control plane: utilizes TRILL Hincluding IS`IS for L2 multipath routingI

– 

Data plane: non`interoperable

• Forwarding tag GFTAGH

– 

for multi`destination, unicast frames;

– 

assigned on the edge port, honored throughout;

– 

selects one of multiple paths Hmax 1024I that the packet traverses

• Conversational MAC learning

– 

interface learns source MAC of ingress frame only if destination MAC is

already present in the table

– 

i.e. only learn if remote device is having a bidirectional conversation with a

locally connected device

– 

unknown unicast frames being flooded: no learning on edge switches

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Framing formats compared

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TRILL

Ethernet frame

FabricPath

Extended segment ID G24$bitsH 

Supports over 16 million virtual networks

Variant 1 Variant 2

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Bag of Protocols

• Keynote at 1

st

Open Networking Summit in 2011 

“The Future of Networking, and the Past of Protocols” – Scott Schenker

• Why does networking lag behind?

–

 

“… because of your great ability to master complexity”

• Future of networking lies in finding right abstractions

–

 

“

The era of ‘a new protocol per problem’ is over

”

• SDN is defined precisely by these three abstractions

–

 

“Distribution, forwarding, configuration”

• SDN not just a random good idea…

–

 

“... can be “derived” from decomposing network control”

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http://opennetsummit.org/archives/oct11/shenker`tue.pdf

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My take

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1.  Thankful I am not a protocol designer!

2.  Decoupling: limited integration between overlay and underlay

3.  Are the original problems fully addressed? 

HHard to judge from the fencepostI

Discussion

• Uninitiated / new format for me

• 6 critique / 2 in defense

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