Transport Protocols for Wireless Networks

CMPE 293 - 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 }{2}{ delayVardelayEdelay – 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 1 MSS 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-TCP TCP

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 MSR-1

MSR1mhsocket

MSR1fhsocket

MSR2fhsocket

MSR2mhsocket

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 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.

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.

ThroughputExpected

1

1

ii

iii

t

Tt

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.