Reliable Data Transport in
Wireless Networks
Anna Brunstrom
Dept. of Computer Science
Karlstad University
Outline
•
•
•
•
•
•
Introduction
TCP Basics
Challanges and Proposed Enhancements
Personal Reflections
SCTP
Conclusions
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Networks
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Introduction
• Explosive growth of
the Internet
• ”Thin waist” behind
the success
• Design based on endto-end principle
• TCP dominant
transport protocol
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Application
Transport - TCP/UDP
Network – IP
Reliable Data Transport in Wireless
Networks
Data link
Physical
3
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
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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
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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
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After timeout
25
cwnd = 20
20
15
10
ssthresh = 10
5
ssthresh = 8
25
22
20
15
12
9
6
3
0
0
Congestion window (segments)
Timeout Scenario
Time (round trips)
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Window size (segments)
Fast Retransmit Scenario
10
After fast recovery
8
Rwnd
6
4
2
0
0
2
4
6
8
10 12 14
Time (round trips)
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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
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Challanges and Proposed
Enhancements
The ”information
superhighway” of
wireline communications
vs
The ”rocky road”
of mobile radio
communications
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
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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
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Proposed Optimizations
•
•
•
•
At the link layer
Split connection
Explicit notification/
cross layer
End to end
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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
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Impact of Link Layer ARQ
Transmission of 3 Mb bulk data
40.00 k
35.00 k
Throughput (Bps)
30.00 k
25.00 k
20.00 k
15.00 k
15% packet loss →
10.00 k
TCP-L, AM, PredErr, Levels for NMSE0.1
5.00 k
TCP-WW, AM, PredErr, Levels for NMSE0.1
TCP, AM, PredErr, Levels for NMSE0.1
0.00
5
10
15
20
25
30
Maximum allowed number of link layer 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
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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
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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 -> 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
fixed
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Checksum Based LN: Example
Bandwidth 1 Mbps,
10 ms delay
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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
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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
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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
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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]
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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
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SCTP
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
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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
Interface 1
ISP
ISP
Interface 1
Internet
Host A
Interface 2
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ISP
ISP
Reliable Data Transport in Wireless
Networks
Interface 2
Host B
33
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
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[Casetti06]
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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
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?
References
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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.
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References
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•
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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, 2629 June 2006
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