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

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

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.

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.

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.

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

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

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.

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.

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

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.

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.

I-TCP: Indirect TCP MH MSR FH MH = Mobile Host MSR = Mobile Support Router FH = Fixed Host I-TCPTCP

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.

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.

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

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.

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.

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.

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).

Simulation Environment NS Network Simulator; TCP-Reno over IP on an 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.

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

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.

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