1. Mobility in a Publish Subscribe Internetwork
George Xylomenos
Mobile Multimedia Laboratory
Dept. of Informatics
Athens University of Economics and Business
PURSUIT Summer School, August

2. Outline
Introduction
Why PSI for mobility?
The PSI Architecture
Caching in PSI
Mobility in PSI: Reactive
Mobility in PSI: Proactive
Conclusion
Credits

3. Introduction
End-to-end (E2E) Internet Architecture
Endpoint centric, based on telephony
Accused as the root of all evil (NAT, CDN, MIP)
Publish/Subscribe (Pub/Sub) Internet Architecture
Information Centric Networking (ICN)
Information decoupled from location
The PSI Architecture
The Pub/Sub architecture of PSIRP/PURSUIT
Supports multicast, anonymity and asynchrony
Seamlessly supports mobile nodes (MNs)

4. What is the difference with mobile IP?
Handover in mobile IP
Data sent during disconnection are lost
Mobile IP extensions reduce handover delay
Micro mobility keeps handovers local
Context transfer allows fast reconnections
Handover in mobile PSI
The built in PSI mechanisms simplify things
Asynchrony: no need for rendezvous in time
Data can be cached during disconnection
Multicast: allows multicast assisted mobility
Target multiple Access Points (APs)
Anonymity: data can be provided by anyone

5. The PSI Architecture
Three main entities
Publisher: advertises availability of information items
Issues publication messages
Sends information items upon request
Information items may have many publishers
Subscriber: expresses interest on information items
Issues subscription messages
Receives information items from some publisher
Event notification service
Brings publishers and subscribers together

6. Information identifiers: Statistically unique pair
The PSI Architecture
Information identifiers: Statistically unique pair
Rendezvous ID (RID): application derived
Scope ID (SID): access control and policies
Example: Scope_A/ Scope_B/…/Scope_Z/RID
Rendezvous Network (RENE)
Provides the event notification service
Consists of Rendezvous Nodes (RN)
Each information item handled by an RN
The Rendezvous Point (RVP) of the item
RENE matches publishers and subscribers
Instructs publishers to start forwarding data

7. Forwarding ID (FID): sent by RVP to the publisher
The PSI Architecture
Forwarding ID (FID): sent by RVP to the publisher
Source routing path to subscribers (multicast)
Each link has a statistically unique Link ID
Within its AS or network area
A path is encoded as a Bloom filter of Link IDs
Each node tests its Link IDs against the FID
Native multicast without extra cost
Publishers do not know subscribers (anonymity)
FIDs do not betray location
FIDs can be frequently changed/invalidated
Pub/Sub decoupled in time (asynchrony)

8. The PSI Architecture RENE <Bob_SId, Bob_RId, metadata> RV Point
Router
AP1
AP2
<Bob_SId, Bob_RId>
Alice

9. The PSI Architecture RENE RV Point Bob Fid (Bob, Alice@AP1) Router AP2

10. In PSI caches are just alternative publishers Transparent caching
Caching in PSI
Caching in PSI
In PSI caches are just alternative publishers
The RVP may point at cached data copies
Transparent caching
The cache stores in transit data
The RVP is notified about the cached data
Non-transparent caching
The cache intercepts subscription requests
Information explicitly delivered in two stages
One FID from the publisher to the cache
Another FID from the cache to the MN

11. Caching in PSI RENE <Bob_SId, Bob_RId, metadata> RV Point Bob
Router
AP1
AP2
<Bob_SId, Bob_RId>
Alice

12. Caching in PSI RENE RV Point Bob Fid (Bob, Router) Router
AP1
AP2
Fid (Router,
Alice

13. Mobility in PSI
What happens when the MN moves?
Two generic approaches: reactive and proactive
Assume that each MN is served by a broker
Reactive approach
On disconnection the old broker starts caching data
On reconnection data are available from there
Proactive approach
Old broker selects candidate neighbor brokers
On disconnection candidates cache data for the MN
On reconnection the new broker already has the data

14. Mobility in PSI: Reactive
PSI Reactive approach
A Smart Cache (SC) is selected for the MN
The SC caches data non-transparently
It explicitly mediates on behalf of the MN
The RENE is aware of the SC
MN disconnection: the SC starts caching data
Data sent during disconnection are not lost
MN reconnection: the SC makes data available
The RENE uses the SC as the source of cached data
No data loss and quick recovery
The FID to the SC does not change
Rerouting is a local procedure

15. Mobility in PSI: Reactive
<Bob_SId, Bob_RId,
metadata>
RV Point
RENE
Bob
<Bob_SId, Bob_RId>
<Bob_SId, Bob_RId,
metadata>
Router
Smart
Cache
AP2
<Bob_SId, Bob_RId>
AP1
Alice

16. Mobility in PSI: Reactive
RV Point
Bob
Fid (Bob, SC)
RENE
Router
Smart
Cache
AP2
Fid (SC,
AP1
Alice

17. Mobility in PSI: Reactive
RV Point
RENE
Bob
Router
Smart
Cache
Fid (SC,
Alice’s new
position
Alice’s previous
position

18. Mobility in PSI: Reactive
Smart Caches are not an extension to PSI
SCs are simply alternative publishers
MNs benefit by a built-in feature of PSI
How to select a good SC?
Ideally it should serve the MN for many handovers
SC selection makes the scheme proactive/reactive
RVP forecasting (RVPf)
RVP is aware of topology around the MN
The RVP forecasts future MN positions
SCs are designated in advance by the RVP

19. Mobility in PSI: Reactive
AP based (APb)
APs forecast handovers via signal strength
Works only for wireless networks
Mobility is not limited to wireless!
The AP notifies the RVP before the handover
The AP sends information about nearby APs
The RVP selects the SC dynamically
No need to know the topology in advance
Also supports dynamic (ad-hoc) topologies
Extra control messaging compared to RVPf
May delay handovers

20. Mobility in PSI: Proactive
PSI Proactive approach
A broker serves each MN
Could be the AP or a nearby cache
The broker selects candidate neighbor brokers
Need an algorithm to select the best candidates
MN disconnection
The old broker notifies the selected neighbors
The selected neighbor brokers start caching data
MN reconnection
The new broker becomes known on reconnection
Data are already available at the new broker

21. Mobility in PSI: Proactive
<Bob_SId, Bob_RId,
metadata>
RV Point
RENE
Bob
<Bob_SId, Bob_RId>
<Bob_SId, Bob_RId,
metadata>
Router
AP1
AP2
<Bob_SId, Bob_RId>
Alice

22. Mobility in PSI: Proactive
RV Point
RENE
Bob
Fid (Bob, Router)
Router
Fid (Router,AP1)
AP1
AP2
Alice

23. Mobility in PSI: Proactive
RV Point
RENE
Bob
Fid (Bob, Router)
Router
Fid (Router,AP1|AP2)
AP1
AP2
Alice

24. Mobility in PSI: Proactive
RV Point
RENE
Bob
Router
Alice’s new
position
Alice’s previous
position

25. Mobility in PSI: Proactive
How to select the candidate neighbors?
Each neighbor incurs some caching costs
Candidate brokers k m l i
Current broker j MN

26. Mobility in PSI: Proactive
Conventions
S: the set of neighboring brokers
S*: the set of selected candidate brokers
Phit: probability that the MN will handover in S*
Chit: cost to serve the MN from a broker in S*
Cmiss: cost to serve the MN from the publisher
Ccache: cost to cache data during MN disconnection
|S*|: number of selected candidate brokers
Input: handover probabilities
Output: the optimal set S*

27. Mobility in PSI: Proactive
Select subset S* to minimize cost
Phit*Chit+(1-Phit)*Cmiss + |S*|*Ccache
Evaluation procedure
Assume pik is the probability to handover to k
Order the neighbors k in descending pik order
Add the next k to S*
Add pik to Phit and increment |S*|
Evaluate the cost function
Select the S* that minimizes the cost function
Delivery cost
Caching cost

28. Mobility in PSI: Proactive
How to evaluate the pik values?
Past handover behavior
The broker tracks actual handovers
Probabilities take into account actual handovers
MN mobility context
Each MN has a mobility context
The context can be used to predict direction
Signal strength measurements
The MN sends to the broker its measurements
The broker predicts the most likely new brokers

29. Conclusion
Mobility handling in PSI
Based on multicast, anonymity, asynchrony
Exploits embedded support for caching
Does not require any new mechanisms
Specifically based on information centrism
Open issues
How to evaluate mobility solutions for PSI?
Traffic models for pub/sub?
Mobility patterns for pub/sub?
Evaluation metrics for pub/sub?

30. Credits Authors of papers used for this presentation Paris Flegkas
Nikos Fotiou
Varvara Giannaki
Konstantinos Katsaros
George Parisis
George C. Polyzos
Mikko Sarela
Vasilios A. Siris
Vasilis Sourlas
Charilaos Stais
Dirk Trossen
Xenofon Vasilakos