Reliable Data Transport in Wireless Networks

1
Reliable Data Transport in Wireless Networks

• Anna Brunstrom
• Dept. of Computer Science
• Karlstad University

2
Outline

• Introduction
• TCP Basics
• Challanges and Proposed Enhancements
• Personal Reflections
• SCTP
• Conclusions

3
Introduction

• Explosive growth of the Internet
• Thin waist behind the success
• Design based on end-to-end principle
• TCP dominant transport protocol

4
TCP Basics
5
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

6
TCP Flow and Congestion Control

• Window based control
• Send rate limited by minimum of
• Receivers advertised window
• Congestion window
• Estimates bandwidth based on probing
• Assumes that packet loss indicates congestion

7
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

8
Timeout Scenario
After timeout
cwnd 20
ssthresh 10
ssthresh 8
9
Fast Retransmit Scenario
After fast recovery
Rwnd
10
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

11
Challanges and Proposed Enhancements
The information superhighway of wireline
communications
The rocky road of mobile radio communications
vs
12
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

13
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

14
Proposed Optimizations

• At the link layer
• Split connection
• Explicit notification/ cross layer
• End to end

15
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 TCPs retransmission timeout is large enough
  to accommodate for variable delays due to link
  layer retransmissions

16
Impact of Link Layer ARQ
15 packet loss ?
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Matching TCP Retransmissions
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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

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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

20
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

21
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

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Explicit notification Examples

• Partial Acknowledgements Biaz97
• Sender gets partial ack from base station, and
  normal ack from receiver -gt 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 DSRs route failure
  message to sender
• TCP sender disables congestion control until
  route is fixed

23
Checksum Based LN Example
Bandwidth 1 Mbps, 10 ms delay
24
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

25
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

26
Personal Reflections
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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

28
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

29
Example Buffering in GPRS
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Consider Short Flows

• Common assumption of bulk transfers
• Most TCP flows are short
• Loss recovery can be more important than
  congestion control

31
SCTP
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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

33
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

34
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
35
Conclusions
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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

37
?
38
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.
  469481.
• 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. 2624283.
• 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
  AM 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.
  27588.

39
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. 75766
• T. Goff et al., Freeze-TCP A True End-to-End
  TCP Enhancement Mechanism for Mobile
  Environments, IEEE INFOCOM, vol. 3, Apr. 2000,
  pp. 153745.
• 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. 23753.
• 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