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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
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
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
7
Outline• Q1: What properties do we want?
• Q2: Can we achieve all properties simultaneously?
• Q3: How to balance different properties?
8
What properties do we want?From mobile user demands to formalization
9
Tracking Area 2Tracking Area 1
What Properties do We Want?
Base station(eNB)
Gateway(SGW/PGW)
Mobility controller(MME)
User profile database(HSS)
Internet Data-planeControl-plane
• Correctness: Forward every packet under correct policies
10
Tracking Area 2Tracking Area 1
What Properties do We Want?
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
• Correctness: Forward every packet under correct policies• 4G LTE: correctness via sequential consistency
What Properties do We Want?
Base station(eNB)
Gateway(SGW/PGW)
Mobility controller(MME)
User profile database(HSS)
Radio conn. setupLocation update request
Session state transfer
Routing update
User profile update
Location update accept
• Correctness: Forward every packet under correct policies• 4G LTE: correctness via sequential consistency
Tracking Area 2Tracking Area 1
What Properties do We Want?
Base station(eNB)
Gateway(SGW/PGW)
Mobility controller(MME)
User profile database(HSS)
Internet Data-planeControl-plane
• Correctness: Forward every packet under correct policies• Availability: start data service once route is connected
13
Tracking Area 2Tracking Area 1
What Properties do We Want?
Base station(eNB)
Gateway(SGW/PGW)
Mobility controller(MME)
User profile database(HSS)
Internet Data-planeControl-plane
Session state transfer
• Correctness: Forward every packet under correct policies• Availability: start data service once route is connected• Partition tolerance: continuous operation once route is connected
14
Can we achieve all properties?CAP theorem on mobility support
15
For any mobility support, it is impossible to alwaysguarantee sequential consistency, availability, andcontrol-plane partition tolerance simultaneously.
CAP Theorem for Mobility
16
Tracking Area 2Tracking Area 1
Example: Control-Plane Partition Between 4G Tracking Areas
Base station(eNB)
Gateway(SGW/PGW)
Mobility controller(MME)
User profile database(HSS)
Session state transfer
Internet Data-planeControl-plane
How should I operate now?
17
Tracking Area 2Tracking Area 1
Option 1 (4G LTE): Consistency Over Availability• Suspend data service until the partition is recovered• Guaranteed correctness
Base station(eNB)
Gateway(SGW/PGW)
Mobility controller(MME)
User profile database(HSS)
Session state transfer
Internet Data-planeControl-plane
✓✓
✓ ✓
18
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)
0 25 50 75 1000
500
1,000
1,500
2,000
Normalized sorted sample (%)
Susp
ensio
n (m
s) Total suspension
0 25 50 75 1000
500
1,000
1,500
2,000
Normalized sorted sample (%)
Susp
ensio
n (m
s) Total suspensionRouting updateState transfer (ctrl→ctrl)
0 25 50 75 1000
500
1,000
1,500
2,000
Normalized sorted sample (%)
Susp
ensio
n (m
s) Total suspensionRouting updateState transfer (ctrl→ctrl)Radio conn. setup
19
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
Normalized sorted sample (%)
Susp
ensio
n (m
s)
0 25 50 75 1000
500
1,000
1,500
2,000
Normalized sorted sample (%)
Susp
ensio
n (m
s) Total suspension
0 25 50 75 1000
500
1,000
1,500
2,000
Normalized sorted sample (%)
Susp
ensio
n (m
s) Total suspensionRouting updateState transfer (ctrl→ctrl)
0 0.5 1 1.5 20
0.5
1
1.5
2
Extra suspension for failure recovery (s)
Norm
al da
ta s
uspe
nsio
n (s)
0 0.5 1 1.5 20
0.5
1
1.5
2
worse
Extra suspension for failure recovery (s)
Norm
al da
ta s
uspe
nsio
n (s)
0 0.5 1 1.5 20
0.5
1
1.5
2
worse
Extra suspension for failure recovery (s)
Norm
al da
ta s
uspe
nsio
n (s)
T-Mobile AT&T Sprint Project-Fi
0 25 50 75 1000
500
1,000
1,500
2,000
Normalized sorted sample (%)
Susp
ensio
n (m
s) Total suspensionRouting updateState transfer (ctrl→ctrl)Radio conn. setup
20
Tracking Area 2Tracking Area 1
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)
Mobility controller(MME)
User profile database(HSS)
Session state transfer
Internet Data-planeControl-plane
? ?
? ?
21
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
22
How to Balance Properties?Between availability, consistency and fault tolerance
23
Correctness, Not Sequential Consistency!
Availability Correctness PartitionTolerance
SequentialConsistency
≠
• Sequential consistency is more than necessary for correctness
Moreflexibleconsistency model to achieve all!
24
Tracking Area 2Tracking Area 1
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)
Mobility controller(MME)
User profile database(HSS)
Internet Data-planeControl-plane
25
Tracking Area 2Tracking Area 1
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)
Mobility controller(MME)
User profile database(HSS)
Internet Data-planeControl-plane01 1 + ?
26
Other Cellular Functions in Paper• Radio access control
• Cryptography over the air
• QoS policy
27
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?
28
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
29
Backup
30
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)
31