CAP for Mobility Support - CS | Computer...

CAP for Mobility SupportYuanjie Li1, Zengwen Yuan1, Chunyi Peng2, Songwu Lu1

1University of California, Los Angeles2The Ohio State University

New Mobile Services in Future 5G

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As the technologies become mature and start to become available at low cost, increased number of devices and things are connected to provide rich information about individual users as well as the surroundings without human intervention. Although the data being sent and received by the individual devices belong to this category is expected to be best-effort basis and relatively small amount, this service requires the underlying future 5G communication system to be capable of handling massive number of potentially simultaneous control plane connectivity.

3.1.3. Mission-Critical Machine Type Communication [ST3]

As the technologies used to implement mobile networks and how the networks are managed for high performance, reliability, and availability are better understood over time, various mission critical services (e.g., remote machinery, vehicle, drone controlling, public safety networks, etc.) start to be implemented on top of mobile networks. The capability to send and receive data and control information wirelessly across large metro areas reliably with minimal end-to-end latency makes the future mobile network an ideal network solution, and this demand seems to only increase in 5G. These services require the underlying communication system to be highly reliable and available even in the case of large scale natural disasters.

Massive-connectivity machine type communication service requires the underlying future 5G communication system to be capable of handling massive number of potentially simultaneous connectivity. Some example services in this service type are: y Smart city, home, and

life y Smart metering y Smart environment

management y Smart wearables

Mission-critical machine type communication service type requires the underlying communication system to be highly reliable and available even in the case of large scale natural disasters. Some example services are: y Remote machinery,

vehicle, drone controlling

y Smart eHealth y Public safety

networks y Tactile Internet

Real-time virtual reality

Mission-critical communication

Remote mobile healthcare Emergency communication

Safe autonomous driving High-speed mobility2

Key Enablers for 5G Mobile ServicesAdvanced

wireless technologiesAdvanced

mobility support

3

Wide-Area Mobility via Cellular Networks

Cellular Networks (e.g., 3G/4G)

...

Page 10 of 45

As the technologies become mature and start to become available at low cost, increased number of devices and things are connected to provide rich information about individual users as well as the surroundings without human intervention. Although the data being sent and received by the individual devices belong to this category is expected to be best-effort basis and relatively small amount, this service requires the underlying future 5G communication system to be capable of handling massive number of potentially simultaneous control plane connectivity.

3.1.3. Mission-Critical Machine Type Communication [ST3]

As the technologies used to implement mobile networks and how the networks are managed for high performance, reliability, and availability are better understood over time, various mission critical services (e.g., remote machinery, vehicle, drone controlling, public safety networks, etc.) start to be implemented on top of mobile networks. The capability to send and receive data and control information wirelessly across large metro areas reliably with minimal end-to-end latency makes the future mobile network an ideal network solution, and this demand seems to only increase in 5G. These services require the underlying communication system to be highly reliable and available even in the case of large scale natural disasters.

Massive-connectivity machine type communication service requires the underlying future 5G communication system to be capable of handling massive number of potentially simultaneous connectivity. Some example services in this service type are: y Smart city, home, and

life y Smart metering y Smart environment

management y Smart wearables

Mission-critical machine type communication service type requires the underlying communication system to be highly reliable and available even in the case of large scale natural disasters. Some example services are: y Remote machinery,

vehicle, drone controlling

y Smart eHealth y Public safety

networks y Tactile Internet

4

Tracking Area 2Tracking Area 1

How does Mobility Support Work in 4G LTE?• Span on multiple network nodes• Involve multiple control procedures

Base station(eNB)

Gateway(SGW/PGW)

Mobility controller(MME)

User profile database(HSS)

Radio conn. setup

Session state transfer

Routing update

User profile update

Location update accept

Internet

Location update request

Data-planeControl-plane

5

Three Expectations for Mobility Support

Immediatedata service

Correctdata service

Continuousdata service

Once route isconnected

Forward packetsunder right policies

Even in presenceof failures

Can we always achieve all of them?6

This talk• A formal analysis of fundamental limits in mobility support• From the distributed system perspective

• Fundamental limits: CAP theorem on generic mobility support

• Rethink the tradeoff between different properties• Argument: 5G needs a paradigm shift in its mobility support

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Outline• Q1: What properties do we want?

• Q2: Can we achieve all properties simultaneously?

• Q3: How to balance different properties?

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What properties do we want?From mobile user demands to formalization

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What Properties do We Want?

Base station(eNB)

Gateway(SGW/PGW)



Internet Data-planeControl-plane

• Correctness: Forward every packet under correct policies

10



Base station(eNB)

Gateway(SGW/PGW)



Radio conn. setup


Routing update

User profile update


Internet

Location update request

Data-planeControl-plane

• Correctness: Forward every packet under correct policies• 4G LTE: correctness via sequential consistency


Base station(eNB)

Gateway(SGW/PGW)



Radio conn. setupLocation update request


Routing update

User profile update


• Correctness: Forward every packet under correct policies• 4G LTE: correctness via sequential consistency



Base station(eNB)

Gateway(SGW/PGW)




• Correctness: Forward every packet under correct policies• Availability: start data service once route is connected

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Base station(eNB)

Gateway(SGW/PGW)





• Correctness: Forward every packet under correct policies• Availability: start data service once route is connected• Partition tolerance: continuous operation once route is connected

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Can we achieve all properties?CAP theorem on mobility support

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For any mobility support, it is impossible to alwaysguarantee sequential consistency, availability, andcontrol-plane partition tolerance simultaneously.

CAP Theorem for Mobility

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Example: Control-Plane Partition Between 4G Tracking Areas

Base station(eNB)

Gateway(SGW/PGW)





How should I operate now?

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Option 1 (4G LTE): Consistency Over Availability• Suspend data service until the partition is recovered• Guaranteed correctness

Base station(eNB)

Gateway(SGW/PGW)





✓✓

✓ ✓

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Option 1 (4G LTE): How Much Delay in Reality?• Comparable to radio latency L: 193.8ms-6.4s (29.8-69.6%)

0 25 50 75 1000

500

1,000

1,500

2,000

Normalized sorted sample (%)

Susp

ensio

n (m

s)

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500

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Susp

ensio

n (m

s) Total suspension

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ensio

n (m

s) Total suspensionRouting updateState transfer (ctrl→ctrl)

0 25 50 75 1000

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1,000

1,500

2,000


Susp

ensio

n (m

s) Total suspensionRouting updateState transfer (ctrl→ctrl)Radio conn. setup

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Option 1 (4G LTE): How Much Delay in Reality?• Comparable to radio latency L: 193.8ms-6.4s (29.8-69.6%)• Failure recovery incurs extra delay L: 500ms-30s

0 25 50 75 1000

500

1,000

1,500

2,000


Susp

ensio

n (m

s)

0 25 50 75 1000

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1,000

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Susp

ensio

n (m

s) Total suspension

0 25 50 75 1000

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Susp

ensio

n (m

s) Total suspensionRouting updateState transfer (ctrl→ctrl)

0 0.5 1 1.5 20

0.5

1

1.5

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Extra suspension for failure recovery (s)

Norm

al da

ta s

uspe

nsio

n (s)

0 0.5 1 1.5 20

0.5

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1.5

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worse


Norm

al da

ta s

uspe

nsio

n (s)

0 0.5 1 1.5 20

0.5

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worse


Norm

al da

ta s

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nsio

n (s)

T-Mobile AT&T Sprint Project-Fi

0 25 50 75 1000

500

1,000

1,500

2,000


Susp

ensio

n (m

s) Total suspensionRouting updateState transfer (ctrl→ctrl)Radio conn. setup

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Option 2: Availability Over Consistency• Delivery user data without latest data session states• No guaranteed correctness: sequential consistency violated

Base station(eNB)

Gateway(SGW/PGW)





? ?

? ?

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Summary: Impact of CAP Theorem

Consistency over Availability• Guaranteed correctness J• Long data service delay L

Availability over Consistency• Correctness unguaranteed• Immediate data service J

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How to Balance Properties?Between availability, consistency and fault tolerance

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Correctness, Not Sequential Consistency!

Availability Correctness PartitionTolerance

SequentialConsistency

≠

• Sequential consistency is more than necessary for correctness

Moreflexibleconsistency model to achieve all!

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What Consistency is Required for Correctness?• Function 1: Data forwarding to correct location• Only a subset of network nodes need latest user location

Base station(eNB)

Gateway(SGW/PGW)




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What Consistency is Required for Correctness?• Function 2: Volume-based data billing• Decouple packet counting and billing policy• Packet counting can be performed together with data forwarding

Base station(eNB)

Gateway(SGW/PGW)



Internet Data-planeControl-plane01 1 + ?

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Other Cellular Functions in Paper• Radio access control

• Cryptography over the air

• QoS policy

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Future Directions• What is the minimal consistency model for mobility support?

• Can we have a continuous (thus tunable) balancing mechanism?

• Will unreliable wireless connectivity affect the tradeoffs?

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Conclusion• No free lunch for perfect mobility support

• Sequential consistency is more than necessary for correctness

• More flexible balance between C, A and P can benefits 5G

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Backup

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What Consistency is Required for Correctness?• Function 3: Radio access control• Observation: per-group access control in 4G LTE• No need to wait for per-device radio access control list

4G Radio Access Control List GranularityForbidden Tracking Area List (TS24.301) Per tracking area

Forbidden PLMN list (TS24.301) Per roaming networkClosed subscriber group (TS29.281) Per cell (private Femtocell)

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