Reliable Data Transport in Wireless Networks

Anna Brunstrom

Dept. of Computer Science

Karlstad University

April 11, 2007 Reliable Data Transport in Wireless Networks

Outline

• Introduction

• TCP Basics

• Challanges and Proposed Enhancements

• Personal Reflections

• SCTP

• Conclusions

Introduction

• Explosive growth of the Internet

• ”Thin waist” behind the success

• Design based on end-to-end principle

• TCP dominant transport protocol

Application

Transport - TCP/UDP

Network – IP

Data link

Physical

TCP Basics

Reliable Data Transfer in TCP

• Based on retransmission of lost packets• A timer is started when a packet is sent

– Dynamically calculated based on RTT• When the packet arrives at the receiver a

cumulative acknowledgment (ACK) is sent – End-to-end semantics

• If an ACK is not received in time, then the packet is retransmitted – Exponential backoff of timer

• Fast retransmit rule - three duplicate ACKs trigger retransmission

TCP Flow and Congestion Control

• Window based control

• Send rate limited by minimum of– Receiver’s advertised window– Congestion window

• Estimates bandwidth based on probing– Assumes that packet loss indicates

congestion

TCP Flow and Congestion Control

• Slow start– Cwnd grows exponentially– Ends when cwnd reaches slow-start threshold

• Congestion avoidance– Cwnd grows linearly

• Fast recovery– Works in conjunction with fast retransmit– Avoids slow start after single packet loss

Timeout Scenario

Time (round trips)

ssthresh = 8ssthresh = 10

cwnd = 20

After timeout

Time (round trips)

ts)After fast recovery

Fast Retransmit Scenario

Many TCP Variants• TCP Tahoe

– Slow start, congestion avoidance, fast retransmit

• TCP Reno– Slow start, congestion avoidance, fast retransmit, fast recovery

• TCP New-Reno– Stay in fast recovery until all packet losses in window are

recovered– Can recover 1 packet loss per RTT without causing a timeout

• Selective Acknowledgements (SACK) – Provides information about out-of-order packets received by

receiver– Can recover multiple packet losses per RTT

The ”informationsuperhighway” ofwireline communications

The ”rocky road”of mobile radiocommunications

Challanges and Proposed Enhancements

Challenges

• Fundamental differences between wired and wireless communication– Channel errors

• Due to interference, multipath fading etc.

– Mobility• Handoff may cause packet loss and delay• Route failures and disconnections in MANETs

– Channel contention (Ad hoc networks)• Hidden terminal and exposed terminal problems

TCP suffers because…

• Transmission problems at the phys/link layer and handoff/route failures due to mobility can lead to packet loss

• Packet loss invokes congestion control at the sender– Exponential backoff entered for multiple losses

• Spurious timeouts may occur when RTT fluctuates

• High sending rate can cause channel contention

Proposed Optimizations

• At the link layer• Split connection• Explicit notification/

cross layer• End to end

Link Layer Error Recovery

• Shields upper layers (e.g. TCP) from errors that can be recovered at lower layers– Errors recovered locally

• Reliable link layer beneficial to TCP – If it provides (almost) in-order delivery– If TCP’s retransmission timeout is large

enough to accommodate for variable delays due to link layer retransmissions

Impact of Link Layer ARQ

5.00 k

10.00 k

15.00 k

20.00 k

25.00 k

30.00 k

35.00 k

40.00 k

5 10 15 20 25 30

Maximum allowed number of link layer retransmissions

Transmission of 3 Mb bulk data

TCP-L, AM, PredErr, Levels for NMSE0.1

TCP-WW, AM, PredErr, Levels for NMSE0.1

TCP, AM, PredErr, Levels for NMSE0.1

15% packet loss →

Link layer: Examples• Snoop [Balakrishnan95]

– TCP-aware link layer– Detect packet losses on wireless link, BS performs

local retransmissions– Prevents fast retransmit by supressing dupACKs at

BS– Does not work with encrypted IP-payload (e.g. IPSec)

• TCP SACK-Aware Snoop [Vangala03]• Reliable link layer available in most modern

wireless networks

Split connection

• E2E TCP connection split into one connection on wired part of route and one over wireless part– Hides transmission errors from sender, local

recovery– Primary responsibility at base station– If specialized transport protocol used on

wireless, then wireless host also needs modification

– Breaks end-to-end semantics

Split connection: Examples • Indirect TCP (I-TCP) [Bakre97]

– Standard TCP connection over both hops– Mainly used for mobility (transfer state between BSs’)

• WAP– Own networking stack to gateway/proxy– WAP2.0 can use native TCP

• Split TCP [Kopparty02]– Split long TCP connections into localized segments to

deal with frequent route failures in MANETs

Explicit notification• A node determines whether packets are lost for

reasons other than congestion and informs sender– Sender can retransmit packet without invoking

congestion control • Motivated by Explicit Congestion Notification

(ECN) proposal • Proposed solutions differ in

– Who sends explicit notification– How they decide to send explicit notification– What sender does on receiving notification

Explicit notification: Examples

• Partial Acknowledgements [Biaz97]– Sender gets partial ack from base station, and normal

ack from receiver -> wireless losses can be differentiated

• Checksum Based Loss Notification [Garcia02]– Detect corrupt TCP checksum, notify sender via new

TCP option • Explicit Link Failure Notification [Holland02]

– Targets ad hoc networks– Piggyback notification onto DSR’s route failure

message to sender– TCP sender disables congestion control until route is

Checksum Based LN: Example

Bandwidth 1 Mbps, 10 ms delay

End to end optimizations

• Only end node(s) are modified• Receiver-based scheme

– Receiver infers cause of packet loss or other event

– Explicitly or implicitly informs the sender

• Sender-based scheme– Sender attempts to determine cause of

packet loss

End to end optimizations: Examples

• TCP Westwood/Westwood+ [Casetti02]– Use rate of packets to estimate available bandwidth instead of

probing

• JTCP [Wu04]– Uses loss predictor based on the interarrival jitter to distinguish

congestion and wireless losses

• Freeze-TCP [Goff00]– When handover is about to happen, send a packet with receiver

window set to zero to “freeze” transmissions

• Dynamic Delayed ACK [Altman03]– Reduce contention on the wireless channel by reducing the

number of ACKs, ACK generation frequency set dynamically

Personal Reflections

Balanced View of Problem

• TCP works quite well in many wireless networks

• A reliable link layer goes a long way– TCP fairly robust to delay variations– Consider the time scales

• Requires– Accurate models of wireless networks– Accurate models of TCP

• Compare with relevant version

Impact of Buffering

• Not well understood– Often neglected in studies– Still influences results

• Trend towards smaller buffers• Some examples of work on this

– AQM method for 3G networks, aim for single packet drop in TCP window [Sågfors03]

– Apply RED to UMTS RLC buffer, pace ACKs depending on buffer occupancy [Alcaraz06]

Example: Buffering in GPRS

Consider Short Flows

• Common assumption of bulk transfers

• Most TCP flows are short

• Loss recovery can be more important than congestion control

Stream Control Transmission Protocol

• Standardized for carrying signaling traffic over IP

• Defined as a general purpose protocol

• Like TCP, SCTP – is connection-oriented (association)

– provides a reliable transport service

– uses window-based congestion control

• Unlike TCP, SCTP – is message oriented

– supports multiple concurrent data streams

– supports the concept of multihoming

– supports unordered messages

Multihoming• Multiple IP addresses at each endpoint for a single

association• Originally defined for link redundancy• Extensions: increased performance by load balancing

mobility management at transport layer

ISPHost B

Host A

Internet

ISPISP

Interface 1Interface 1

Interface 2Interface 2

AISLE (autonomic interface selection)

• When congestion detected: consider switching to secondary path

• Based on bandwidth and capacity estimates

• Time hysterisis to avoid ping-pong effects [Casetti06]

Conclusions

Conclusions• Wireless communication different from wireline

communications• A large number of TCP optimizations proposed

– Small subset illustrated– Based on some general principles– No single “right” solution– But after all, TCP works quite ok

• SCTP– Adds some new features– Multihoming

References• H. Balakrishnan, S. Seshan, and R. H. Katz, “Improving Reliable Transport and

Handoff Performance in Cellular Wireless Networks,” ACM Wireless Networks, vol. 1, no. 4, Nov. 1995, pp. 469–481.

• S. Vangala and M. Labrador, “The TCP SACK-Aware-Snoop Protocol for TCP over Wireless Networks,” IEEE VTC, Orlando, FL, vol. 4, Oct. 2003, pp. 2624–283.

• A. V. Bakre , B. R. Badrinath, Implementation and Performance Evaluation of Indirect TCP, IEEE Transactions on Computers, v.46 n.3, p.260-278, March 1997

• S. Kopparty, S.V. Krishnamurthy, M. Faloutsos, S.K. Tripathi, "Split-TCP for Mobile Ad Hoc Networks", Proceedings of IEEE GLOBECOM, Taipei 2002.

• S. Biaz, M. Mehta, S. West and N. Vaidya, "TCP over Wireless Networks Using Multiple Acknowledgments", Technical Report 97-001, Texas A&M University, Jan. 1997.

• J. Garcia and A. Brunstrom, “Checksum-based Loss Differentiation”, Proceedings 4th IEEE Conference on Mobile and Wireless Communications Networks (MWCN 2002), Stockholm, Sweden, September 2002.

• G. Holland and N. Vaidya, “Analysis of TCP Performance over Mobile Ad Hoc Networks,” ACM Wireless Networks, vol. 8, no. 2, Mar. 2002, pp. 275–88.

References• S. Mascolo, C. Casetti, M. Gerla, M. Y. Sanadidi and R. Wang, “TCP Westwood:

Bandwidth Estimation for Enhanced Transport over Wireless Links”, Proc. of the ACM Mobicom 2001, Rome, Italy, July 16-21 2001.

• E. H. K. Wu and M. Z. Chen, “JTCP: Jitter-Based TCP for Heterogeneous Wireless Networks,” IEEE JSAC, vol. 22, no. 4, May 2004, pp. 757–66

• T. Goff et al., “Freeze-TCP: A True End-to-End TCP Enhancement Mechanism for Mobile Environments,” IEEE INFOCOM, vol. 3, Apr. 2000, pp. 1537–45.

• E. Altman and T. Jimenez, “Novel Delayed ACK Techniques for Improving TCP Performance in Multihop Wireless Networks,” Proc. Pers. Wireless Commun., Venice, Italy, Sep. 2003, pp. 237–53.

• M. Sågfors, R. Ludwig, M. Meyer and J. Peisa, "Queue Management for TCP Traffic over 3G Links", IEEE WCNC 2003, New Orleans, USA, March 2003

• C. Casetti, C. F. Chiasserini, R. Fracchia, M. Meo, AISLE: Autonomic Interface SeLEction for Wireless Users, IEEE WoWMoM 2006, Niagara-Falls, Buffalo-NY, 26-29 June 2006

Reliable Data Transport in Wireless Networks

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