Overlay Network and Data Transmission Over Wireless

1. Overlay Network and Data Transmission Over Wireless For EE290T Minghua Chen EECS@UC, Berkeley

2. Outline • Overlay network & virtual overlay network • Data transmission over wireless • Improving TCP performance over wireless • Improving video performance over wireless

3. Overlay Network • What is overlay network? • “A configuration within which a base network is used to support some second network, “layered” upon the underlying infrastructure” • Another layer, logical network • Virtual overlay network – the interface used to provide service are virtual • Send(): reliable_send(), fast_send()… • Why overlay network? • Deploy/testing new protocol/service with minimal affecting the lower IP infrastructure • IPv6 TestBed • Multicast • QoS • Content Addressable Network (CAN, a hash lookup system)

4. Example – v6 Over v4 Overlay “Backbone” Network

5. The Challenges • Assume underlying infrastructure can guarantee the bandwidth (even latency) between overlay nodes • Still need trust between AS • Management • Combine all extended services overlay into one overlay • Heterogeneous multimedia distribution overlay network • Multicast overlay network

6. ON’s Advantages & Disadvantages • Advantages • Can apply RSVP, providing isolation (e.g., flow isolation • Flexible security approach (IPSec, PK/SK) • New service (anycast etc) • Disadvantages • Efficiency • Latency • How many nodes need to be modified? • What if one day the underlying network changes to support all the functionalities?

7. Improving TCP Performance over Wireless • Problem • Packet may get loss on wireless due to channel error or hand-off • TCP treat packet loss as hint for congestionnot necessarily decrease congestion window size  low bandwidth usage efficiency • Solution: shield sender from error-based packet loss • Split connection • End-to-End with error loss notification (ELN) • Need some point on route to set the ELN bit • Link Layer based solution • LL retransmission • In WLAN, After LL retransmission, packet loss rate ~ 3%* • LL-SMART-TCP-AWARED (snoop+sack) • +30% improvement compare to LL retransmission *:A TRACE-BASED APPROACH FOR MODELING WIRELESS CHANNEL BEHAVIOR -- G. Nguyen et.al @1996

8. Architecture

9. Comparison

10. What Improves TCP Over Wireless • Shield sender from the effect of the packet loss due to wireless channel error loss • LL retransmission • TCP-AWARED • Prepare for hand-off • Multicast the data on fly to nearby BS when the MS is in hand-off period • Do this in LL? • Drawbacks of LL-SMART-TCP-AWARED • Need to modify the Link layer • What if there is another hop between BS and MH? • Can’t distinguish packet loss with wireless link congestion • Has reached the end? • How about ad-hoc

11. One Point of View LL-TCP-AWARED sits here 1 Relay S BS R 1 1 Packet No. 1 arrived BS, but loss at Relay point due to congestion on Relay point -- When the BS notice there is a packet loss between BS—Relay, he could not distinguish this loss between congestion-based or channel-error-based

12. Improving Multimedia Transmission Performance over Wireless • Use FEC • Real-time multimedia may not afford long-time retransmission • Modify UDP protocol • Traditional UDP: small part error in packet  drop the entire packet – there are still some useful information in the corrupted packet • UDP-lite: pass the partly corrupted packet to upper layer • CUDP: Vertical Packet Coding (VPC, similar to interleaving) + improved UDP-lite (use the frame error information to help decoder to locate the error position (thus can do error concealment) • But need to modify the layers to enable information exchange between the layers

13. CUDP result

14. Improving Multimedia Transmission Performance over Wireless • Scheduling: • Instead of sending packets only in deadline order, try to send them in app-specified order (e.g., importance or deadline order) • When need scheduling: in-order delivery could not guarantee all the packets meet its deadline (e.g., when bandwidth decrease due to channel error) • How to scheduling • Important packet first • Send lower important packet iff • Important packets would meet their deadline • Current lower important packet won’t miss the deadline

15. Scheduling Sending reference value: S(i) = F(I_index(i), D_index(i)) F – function I_index(i) – importance index D_index(i) – deadline index *Send out packet in increasing order of S(i)* Importance index (I_index) 1 2 3 4 5 6 7 Pkt No.

16. An Example [46] Transmit this layer 3 packet if and only if all the layer0/1/2 packets in following frames will “successful” transmit over wireless link; “successful” is in a probabilistic means.

17. So… • All these works on multimedia over wireless focus on • How to combat with channel error • What should we do when packet loss happens • But one step back • @ what rate should we send multimedia data? • Send @ maximum available rate, how? • Over wired network: TCP-friendly • Over wireless network?... • Could we reduce the number of packet loss in case of hand-off, temporary out-of-range and channel error? • LL retransmission – cost: out of order delivery + delay • How about hand-off?

18. Summary • Overlay network shows some good properties. But not sure about its future, whether it is a temporary solution or it will be a long-lived service-providing logic infrastructure • Data transmission over wireless • Reliable data transmission: cake has gone… • Real-time data transmission: is there a cake?