Transport Protocols for Wireless Networks

1. Transport Protocols for Wireless Networks CMPE 293 - Spring 2001 Marcelo M. de Carvalho

2. Outline • Overview: • Transport Protocols & TCP • Limitations & Problems in Wireless • TCP for Single-Hop Networks • Improving the Performance for TCP: classes of protocols • TCP for Multi-Hop Networks • TCP for MANETs

3. Traditional Transport Protocols • Reliable transport protocols have been tuned for networks composed of wired links and stationary hosts. • They adapt to prevailing end-to-end delay conditions throughout the life of a connection; • Main Assumption: Increases in delay are interpreted as packet losses caused by congestion.

4. Sources of Errors in Wireless Links • Pauses due to handoff between cells; • Packet losses due to futile transmissions: mobile host out of reach of other transceivers (little or no overlap between cells); • Packet losses due to transmission errors in wireless links.

5. How does TCP work? • TCP continually measure how long acknowledgments take to return; • If • Retransmit packet; • Initiate congestion control procedure: • Drop transmission window size; • Activate slow-start algorithm; • Reset retransmission timer to a backoff interval that doubles with each consecutive time-out.

6. Improving the Performance of TCP SH MSS 1 MSS 2 MH Cell 2 Cell 1

7. Smooth Handoff • Cellular networks should strive to provide smooth handoffs in order to eliminate packet losses during cell crossings. • No overlaps are also good!!! • High aggregate bandwidth: adjacent cells can use the same portion of the spectrum; • Support for low-powered mobile receivers; • Accurate location information

8. Retransmission Timers • Long pauses are partly due to inaccurate retransmission timers. • TCP implementations have coarse timers (300- to 500-millisecond resolution); • Small timeout: • multiple reductions of the slow-start threshold; • multiple backoffs of the retransmission timer; • multiple retransmissions before the routes become consistent.

9. Fast Retransmissions • IDEA: Resume communication immediately after handoffs complete, without waiting for a retransmission timeout. • Modern TCPs: activated when a transmitter receives triplicate acknowledgments from a receiver; • Once a greeting arrives at the MH, TCP invokes the fast retransmission procedure.

11. Comparision of Mechanisms • End-to-end protocols • Split-connection protocols • Link-layer protocols • Hybrid protocols

12. End-to-end Protocols • Sender is aware of the existence of wireless hops. • Selective Acknowledgments (SACKs): sender can recover from multiple packet losses without resorting to a coarse timeout. • Explicit Loss Notification (ELFN): the sender can distinguish between congestion and other forms of losses.

13. Split-connection Protocols • Goal: to hide any non-congestion-related losses from the TCP sender. • TCP connection is split between a sender and receiver into two separate connections at the base station: • TCP connection over wired link; • Specialized protocol over wireless link.

14. I-TCP: Indirect TCP I-TCP TCP MSR MH FH • MH = Mobile Host • MSR = Mobile Support Router • FH = Fixed Host

15. TCP/IP in Mobile Environment • Main reason for throughput degradation: • Loss of TCP segments during cell crossovers, especially with non-overlapped cells. • Effects: • Lost segments trigger exponential back off and congestion control at the transmitting host. • Congestion recovery phase may last for several seconds.

16. Indirect Protocol • Different flow control and congestion control for wireless and wired links; • Separate transport protocol supports disconnections, moves and other wireless related features; • MSR manages much of the overhead; • Faster reaction to mobility due to proximity between MSR and MH.

17. MH socket MH socket MH MH MSR-1 MSR-1 FH I-TCP Basics move Wireless TCP I-TCP Handoff MSR1 fhsocket MSR2 fhsocket Regular TCP MSR-2 MSR1 mhsocket MSR2 mhsocket FH socket

18. Link-layer Protocols • Two main classes: • Error correction using techniques such as Forward Error Correction; • Retransmission of lost packets in response to automatic repeat request (ARQ) messages. • Tuned to the characteristics of the wireless link.

19. Hybrid Protocols: The Snoop Prootocol • An agent monitors every packet and maintains a cache of TCP segments that have not yet been acknowledged. • Packet loss is detected by the arrival of a small number of duplicate acks or by a local timeout. • The agent retransmits the lost packet and suppresses the duplicate acks.

20. Observations • TCP-aware link-layer protocol with selective acknowledgments performs the best; • Split-connection approaches is not a requirement for good performance. • Selective acknowledgment is very useful in lossy links, especially for burst losses. • Explicit Loss Notification is worth to try.

21. TCP Performance over MANETs • Goals: I • nvestigate the impact of link failures due to mobility on TCP performance; • Define expected throughput; • Enhance throughput with Explicity Link Failure Notification (ELFN).

22. NS Network Simulator; TCP-Reno over IP on an 802.11 wireless network; Dynamic Source Routing (DSR) Protocol; BSD ARP protocol (to resolve IP addresses to MAC addresses); 30 nodes in a 1500 X 300 meter area moving according to the random waypoint mobility model. Simulation Environment

23. Expected Throughput • ti = duration of time for which the shortest path from the sender to receiver contains i hops. • Ti = throughput obtained over a linear chain using i hops.

24. TCP with ELFN • Implementation: • Use ICMP message as a notice to the TCP sender; • If the routing protocol sends a route failure message to the sender, then the notice can be piggy-backed on it. • TCP’s response: disable congestion control mechanism until route has been restored.

25. Observations • Routing protocol has a significant impact on TCP performance (cache and propagation of stale routes); • More aggressive cache management protocols are needed.

26. Thank you !!!