1 Mobility Aware Server Selection for Mobile Streaming Multimedia CDN Muhammad Mukarram Bin Tariq, Ravi Jain, Toshiro Kawahara {tariq, jain, DoCoMo USA Labs. September 29, 2003

2 Summary We present a mobility-aware server selection scheme for content distribution networks. Our target is CDN with high density of servers, each server having a small coverage area. Mobile users can move out of such service areas in the duration of streaming media sessions, resulting potentially degraded QoS. Server Handoff can be performed to revive QoS, but it is expensive. We use user’s mobility along with traditional criteria such as proximity, server load etc., and assigns a server such that the probability of user moving out of coverage area of the assigned server is reduced, while meeting QoS criteria. Simulation results show up to 18 % reduction in number of server handoffs.

3 Outline Summary Introduction –Overview –Problem Statement Mobility Aware Server Selection –Assumed CDN topology –Gathering user mobility information and estimating residence time with servers. –Server Selection. Simulation –Mobility, Server Selection, Content Distribution. Results Conclusions

4 Introduction Multimedia has increasing share in overall traffic Fixed broadband has not harnessed Multimedia, how will mobile broadband? –Mobile phones are there all the time. –Usage scenarios: movies, songs, news, playing video games etc, while traveling CDN must meet the challenge of mobility, wireless and streaming media trio. –Our focus is the (Mobility + Streaming). Expected Future mobile communication market [Yumiba01] Year Market Size (traffic) Voice 20-30% Multimedia 70-80%

5 Streaming Media In Mobile Networks In previous work [Tariq02] we showed that server handoff is helpful for streaming content to mobile users. Localizes traffic, reduces delay, jitter, and load on the network. R R Subnets in a Mobile Network RRR RR R Logically Non Adjacent Subnets, (hot spots) R Server R Server Handoff Server

6 Naïve server handoff scheme has problems If the users move too fast, there would be too many server handoffs, which are expensive for the network. –Signaling, Content Placement Our mobility-aware server selection assigns right users to right servers, reducing the need for handoffs. –Reduce Number of Handoffs while meeting QoS criteria.

7 CDN Topology Allows: –Maximization of traffic localization –Obtain desired QoS ↔ Number of Handoffs tradeoff by choosing appropriate server tier. Access Network Subnets Tier 1 Server Coverage Area aka. server-zone Each has a RR Tier 2 Server Coverage Area Tier 3 Server Coverage Area Servers More Coverage Area Better QoS

8 Server Selection Process Server Tiers RR Server Capacity Information Move to higher tier if 1)Server Capacity Available 2)User is Moving Fast 3)QoS Diff is maintained Move to lower tier if 1)Server Capacity Available 2)Won’t increase handoffs 3)QoS Diff is maintained Mobility Information We introduce a Lazy Mode where we do not move users to lower tiers unless higher tiers are saturated!!!

9 Mobility Information Residence Time Trajectory of the client A subnet Client i Client maintains its average subnet residence time over k recent moves Mean residence time of all n clients in RR’s server-zone Mean Server Residence-time for each tier t RR uses the information to estimate a future residence time of client i with tier t We can make a high granularity estimate using subnet specific information, at cost of higher overhead.

10 Simulation Simulate realistic user movement in a large geographical area, collect movement events – we wrote a custom simulator for this. Simulate different server selection algorithms –Baseline, clients always assigned to default tier –Eager mode with both Low and High Granularity Mobility Information –Lazy Mode with both Low and High Granularity Mobility Information. See how we did in terms of delay and jitter experience by the users. Mobility Simulation Server Selection Simulation Content Distribution Simulation

11 Mobility Simulation Custom simulator to simulate realistic urban area user movement. –Cars, Trains, Streets, Freeways, Public Transport, Congestion, etc. –San Francisco Bay area, 3575 sq. miles. –Over laid with 189 base- stations, 59 subnets

12 Simulation Parameters CDN topology –34 servers arranged in 3 tiers, 21, 8 and 4 in tiers 1, 2, and 3 respectively. – The 3 tiers at 80ms, 160ms and 240ms respectively, from the edge. –Server Capacity, variable {50, 75, 100, 200, 300} simultaneous sessions Users –2500 users with three QoS class, {1, 2, 3}, users distributed across the three QoS classes proportionately to the number of servers at corresponding tier. –Session Durations, variable {50, 100, 200, 1000, 1500} seconds –Data rate per user: 64kbps, 20pps Selection Criteria –Desired QoS Separation between adjacent classes: 20 ms. –Server Overload threshold for Lazy mode. 10% of the maximum reported load allowance.

13 Simulation Results (1/2) Eager Mode Lazy Mode Low Granularity High Granularity Results for server capacity = 300 Results for server capacity = 100 More results in the paper…

14 Simulation Results (2/2) Desired Separation is maintained in all scenarios Eager mode is achieves better convergence and at lower overall value. Higher server capacity allows us to do more. Accuracy of estimation has little impact. Eager Mode QoS Class 1QoS Class 2QoS Class 3 Lazy Mode

15 Conclusions We have presented a mobility aware server selection scheme. –Up to 18% reduction in number of server handoffs. –Simple, Largely stateless –Relies on simple and manageable information – much of which is already available in the network. If you are eager, you better be sure – with eager mode, higher accuracy is crucial. Has applications beyond streaming media. –Anywhere that you want to make tradeoff with mobility by switching to wider-area systems. Open issues: –Improving while maintaining simplicity. –Manageability. –Bundling with other technologies.

16 Algorithm Details Task: Assign server to a client i of QoS class q and current/default server tier t selectedTier := t Find load allowance of next higher tier L t+1 If the client is in fastest L t+1 – true if is less than L t+1 here U j number of sessions of a client j If the delay separation will be maintained – true if. similarly for q,q-1 Assign Server Tier t+1. End-If Else-If Eager Mode or (Lazy Mode with L t+1 too low) – checking to see if we can move it to lower tier instead Make sure client won’t increase the number of handoffs i.e., Assign Server Tier t-1. End-If

17 IWCW 2003 Conference Report Muhammad Mukarram Bin Tariq DoCoMo USA Labs. October 8, 2003