7: Wireless Ad Hoc Networks7-1 Chapter 7 Wireless Ad Hoc Networks.
Special Topics on Wireless Ad-hoc Networks
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Transcript of Special Topics on Wireless Ad-hoc Networks
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Special TopicsSpecial Topics on on
Wireless Ad-hoc Wireless Ad-hoc NetworksNetworks
University of TehranDept. of EE and Computer Engineering
By:Dr. Nasser Yazdani
Lecture 5: TCP for Ad-hoc TCP for Ad-hoc NetworksNetworks
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Covered topicCovered topic Problems with TCP in MANET
How to improve TCP in MANET
References Chapter 9 of the book ATP: A Reliable Transport protocol for Ad-
hoc Networks.
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OutlinesOutlines Problems with TCP in MANET
How to improve TCP in MANET
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Issues of TCP in Ad hocIssues of TCP in Ad hoc1. Induced traffic: Link level transmission affect
neighbors of sender and receiver.2. Induced Throughput unfairness:
throughput/delay unfairness in link layer3. Separation of congestion control, realiability
and flow control4. Power and bandwidth constrains.5. Misinterpretation of congestion:6. Completely decoupled transport layer: 7. Dynamic topology.
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Performance of TCPPerformance of TCPSeveral factors affect TCP performance in
MANET: Wireless transmission errors Multi-hop routes on shared wireless
medium For instance, adjacent hops typically cannot
transmit simultaneously Route failures due to mobility
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Random ErrorsRandom Errors If number of errors is small, they may be
corrected by an error correcting code Excessive bit errors result in a packet being
discarded, possibly before it reaches the transport layer
Problems Fast retransmission of lost packet reduction in congestion window Reduction in congestion window reduces the
throughput Burst Errors May Cause Timeouts
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Impact of Multi-Hop Impact of Multi-Hop Wireless PathsWireless Paths
0
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1 2 3 4 5 6 7 8 9 10
Number of hops
TCPThroughtput(Kbps)
TCP Throughput using 2 Mbps 802.11 MAC
•Connections over multiple hops are at a disadvantage compared to shorter connections, because they have to contend for wireless access at each hopExtent of packet delay or drop
increases with number of hops
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Mobile Ad Hoc NetworksMobile Ad Hoc Networks[IETF-MANET][IETF-MANET]
Mobility causes route changes
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Throughput Degradations Throughput Degradations withwithIncreasing Number of Increasing Number of HopsHops
Packet transmission can occur on at most one hop among three consecutive hops Increasing the number of hops from 1 to 2, 3 results
in increased delay, and decreased throughput Increasing number of hops beyond 3 allows
simultaneous transmissions on more than one link, however, degradation continues due to contention between TCP Data and Acks traveling in opposite directions
When number of hops is large enough, the throughput stabilizes due to effective pipelining
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Ideal ThroughputIdeal Throughput
f(i) = fraction of time for which shortest path length between sender and destination is I
T(i) = Throughput when path length is I
Ideal throughput = f(i) * T(i)
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Impact of MobilityImpact of Mobility
Ideal throughput (Kbps)
Act
ual t
hrou
ghpu
t
2 m/s 10 m/s
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Impact of MobilityImpact of Mobility
Ideal throughput
Act
ual t
hrou
ghpu
t
20 m/s 30 m/s
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Throughput degrades with Throughput degrades with
increasing speed increasing speed ……
Speed (m/s)
AverageThroughputOver 50 runs
Ideal
Actual
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But But notnot always always ……
Mobility pattern #
Actualthroughput
20 m/s
30 m/s
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mobility causeslink breakage,resulting in routefailure
TCP data and acksen route discarded
Why Does Throughput Why Does Throughput Degrade?Degrade?
TCP sender times out.Starts sending packets again
Route isrepaired
No throughput
No throughputdespite route repair
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mobility causeslink breakage,resulting in routefailure
TCP data and acksen route discarded
Why Does Throughput Why Does Throughput Degrade?Degrade?
TCP sendertimes out.Backs off timer.
Route isrepaired
TCP sendertimes out.Resumessending
Larger route repair delaysespecially harmful
No throughput
No throughput
despite route repair
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Low Speed ScenarioLow Speed Scenario
C
B
D
A
C
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D
A
C
B
D
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1.5 second route failure
Route from A to D is broken for ~1.5 second.
When TCP sender times after 1 second, route still broken.
TCP times out after another 2 seconds, and only then resumes.
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Higher (double) Speed Higher (double) Speed ScenarioScenario
C
B
D
A
C
B
D
A
C
B
D
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0.75 second route failure
Route from A to D is broken for ~ 0.75 second.
When TCP sender times after 1 second, route is repaired.
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General PrincipleGeneral Principle The previous two slides show a plausible
cause for improved throughput TCP timeout interval somewhat (not
entirely) independent of speed Network state at higher speed, when
timeout occurs, may be more favorable than at lower speed
Network state Link/route status Route caches Congestion
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Improve ThroughputImprove Throughput(Closer to Ideal)(Closer to Ideal)
Network feedback Inform TCP of route failure by
explicit message Let TCP know when route is repaired
Probing Explicit notification
Reduces repeated TCP timeouts and backoff
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Performance ImprovementPerformance Improvement
Without networkfeedback
Ideal throughput 2 m/s speed
With feedback
Act
ua
l thr
oug
hpu
t
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Performance ImprovementPerformance Improvement
Without networkfeedback
With feedback
Ideal throughput 30 m/s speed
Act
ua
l thr
oug
hpu
t
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Performance with Explicit Performance with Explicit NotificationNotification
0
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mean speed (m/s)
thro
ug
hp
ut
as a
fra
ctio
n o
f id
eal
Base TCP
With explicitnotification
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IssuesIssuesNetwork FeedbackNetwork Feedback
Network knows best (why packets are lost)+ Network feedback beneficial- Need to modify transport & network layer to
receive/send feedback Need mechanisms for information exchange
between layers [Holland99] discusses alternatives for
providing feedback (when routes break and repair) [Chandran98] also presents a feedback scheme
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Impact of CachingImpact of Caching Route caching has been suggested as a
mechanism to reduce route discovery overhead [Broch98]
Each node may cache one or more routes to a given destination
When a route from S to D is detected as broken, node S may: Use another cached route from local cache, or Obtain a new route using cached route at
another node
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To Cache or Not to CacheTo Cache or Not to Cache
Average speed (m/s)Ac t
ual t
hrou
ghpu
t (a s
fra
ctio
n of
exp
ecte
d th
roug
hput
)
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Why Performance Why Performance Degrades With CachingDegrades With Caching
When a route is broken, route discovery returns a cached route from local cache or from a nearby node
After a time-out, TCP sender transmits a packet on the new route.However, the cached route has also broken after it was cached
Another route discovery, and TCP time-out interval
Process repeats until a good route is found
timeout dueto route failure
timeout, cachedroute is broken
timeout, second cachedroute also broken
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IssuesIssuesTo Cache or Not to CacheTo Cache or Not to Cache
Caching can result in faster route “repair”
Faster does not necessarily mean correct
If incorrect repairs occur often enough, caching performs poorly
Need mechanisms for determining when cached routes are stale
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IssuesIssues Window Size After Route Window Size After Route RepairRepair
Same as before route break: may be too optimistic
Same as startup: may be too conservative
Better be conservative than overly optimistic Reset window to small value after route
repair Let TCP figure out the suitable window size Impact low on paths with small delay-bw
product
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IssuesIssuesRTO After Route RepairRTO After Route Repair
Same as before route break If new route long, this RTO may be too small, leading to
timeouts Same as TCP start-up (6 second)
May be too large May result in slow response to next packet loss
Another plausible approach: new RTO = function of old RTO, old route length, and new route length Example: new RTO = old RTO * new route length / old
route length Not evaluated yet Pitfall: RTT is not just a function of route length
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Out-of-Order Packet Out-of-Order Packet DeliveryDelivery
Out-of-order (OOO) delivery may occur due to: Route changes Link layer retransmissions schemes that deliver OOO
Significantly OOO delivery confuses TCP, triggering fast retransmit
Potential solutions: Deterministically prefer one route over others, even if
multiple routes are known Reduce OOO delivery by re-ordering received packets
can result in unnecessary delay in presence of packet loss Turn off fast retransmit
can result in poor performance in presence of congestion
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Impact of Impact of AcknowledgementsAcknowledgements
TCP Acks (and link layer acks) share the wireless bandwidth with TCP data packets
Data and Acks travel in opposite directions In addition to bandwidth usage, acks
require additional receive-send turnarounds, which also incur time penalty
To reduce frequency of send-receive turnaround and contention between acks and data
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Impact of Acks: MitigationImpact of Acks: Mitigation Piggybacking link layer acks with data Sending fewer TCP acks - ack every d-
th packet (d may be chosen dynamically)
but need to use rate control at sender to reduce burstiness (for large d)
Ack filtering - Gateway may drop an older ack in the queue, if a new ack arrives reduces number of acks that need to be
delivered to the sender
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TCP in Mobile Ad Hoc TCP in Mobile Ad Hoc NetworksNetworksIssues Issues (summary)(summary)
Route changes due to mobility Wireless transmission errors
problem compounded with multiple hops Out-of-order packet delivery
frequent route changes may cause out-of-order delivery TCP does not perform well if packets are significantly OOO
Multiple access protocol choice of MAC protocol can impact TCP performance
significantly Half-duplex radios
cannot send and receive packets simultaneously changing mode (send or receive) incurs overhead
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Impact of MAC - Impact of MAC - Delay Delay VariabilityVariability
As wireless medium is shared between multiple sources, the round-trip delay is variable
Also, on slow wireless networks, delay is large made larger by send-receive turnaround time
Large and variable delays result in larger RTO On packet loss, timeout takes much longer to
occur Idle source (waiting for timeout to occur) lowers
TCP throughput
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Impact of MAC - Impact of MAC - Delay Delay VariabilityVariability
Several techniques may be used to mitigate problem, based on minimizing ack transmissions to reduce frequency of send-receive turnaround and
contention between acks and data Piggybacking link layer acks with data Sending fewer TCP acks - ack every d-th packet (d
may be chosen dynamically) but need to use rate control at sender to reduce burstiness (for
large d) Ack filtering - Gateway may drop an older ack in
the queue, if a new ack arrives reduces number of acks that need to be delivered to the
sender
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Header Compression for Header Compression for Wireless NetworksWireless Networks
In TCP packet stream, most header bits are identical Van Jacobson’s scheme exploits this observation to compress
headers, by only sending the “delta” between the previous and current header
Packet losses result in inefficiency, as headers cannot be reconstructed due to lost information
Packet losses likely on wireless links [Degermark96] proposes a technique that works well despite
single packet loss “twice” algorithm if current packet fails TCP checksum, assume that a single packet is lost apply delta for the previous packet twice to the current header, and test
checksum again
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Automatic TCP Buffer Automatic TCP Buffer TuningTuning
Using too small buffers can yield poor performance
Using too large buffers can limit number of open connections
Automatic mechanisms to choose buffer size dynamically would be useful
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Sources of problems?Sources of problems? Misinterpretation of packet loss Frequent path breaks Effect of path length Misinterpretation of congestion window Asymmetric link behavior: location dependent
contention Uni-directional path: each Ack requires RTS-CTS-
data-Ack, more than 70 bytes of overhead Multipath routing: out of order packet and dupAck. Network partitioning and remerging Sliding Window: size of the window
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Design goalsDesign goals1. Maximize throughput per connection2. Throughput fairness3. Minimum connection setup and maintenance overhead.4. Mechanisms for congestion and flow controls5. Reliable and unreliable connections6. Adapt to the dynamics of the network.7. Resource, Bandwidth, used efficiently.8. Aware of resource, battery, constraints.9. End to end semantics10. Use information from the lower layer11. Well-defined cross-layer interaction for effective and
scalable protocol
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Different approachesDifferent approaches Adapt TCP for Ad hoc
Split-TCP End to end approaches Mostly the same problems discussed in
Wireless TCP New Transport protocols
Application Control Transport Protocol (ACTP)
Ad Hoc Transport Protocol (ATP)
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ACTP protocolACTP protocol A light weight transport layer protocol Reliability is left to the application It is like UDP with feedback It supports priority Advantages
It is scalable No congestion window, then, path break does not
affect much thoughput Disadvantages:
Not compatible with TCP Might cause congestion in very large ad hoc Networks
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ATP protocolATP protocol A reliable transport layer protocol Coordination between different layers of the
protocol stack. Rate based Transmission
Transmission is scheduled by timer No need for self clocking Avoid burstiness
Decoupling of congestion control and reliability. Assisted Congestion control: regulate rate based on
the feedback from network TCP friendliness and fairness
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ATP protocolATP protocol Intermediate node: keep Qt and Tt as the average
queuing and transmission delay. They are computed over all packet
Each packet has a D field, rate feedback, which is the maximum of Qt + Tt in the upstream nodes the packet travese.
Receiver: send periodic feedback to the source with value of D. it runs an epoch time of period E
Rate Feedback: an exponential averaging of D, Reliability Feedback: use SACK with bigger blocks (20
here) Flow Control feedback: done by received rate.
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ATP protocol (cont)ATP protocol (cont) Sender: Perform quick start, congestion control,
reliability and connection management Quick start: Send a probe packet to calculate
available BW by considering D and the rate Three congestion phases: Increase, decrease and
maintains Increase: done more aggressively. If the current rate was
less the calculated one Decrease: less conservative. Maitain: if the currently rate was close to the feedback rate.
Reliability: receiver sends hole periodicly
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Issues for Further Issues for Further InvestigationInvestigation
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Link Layer ProtocolsLink Layer Protocols “Pure” link layer designs that support
higher transport performance some recent work in this area as noted
earlier Identifying scenarios where link layer
solutions are inadequate If TCP-awareness is absolutely needed,
provide an interface that can be used by other transport protocols too
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End-to-End TechniquesEnd-to-End Techniques Existing techniques typically require
cooperation from intermediate nodes. Such techniques often not applicable
encrypted TCP headers TCP data and acks do not go through same base
station End-to-end techniques would rely on
information available only at end nodes Harder to design due to lack of complete
information about errors Explicit Notifications may make that easier
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Impact of Congestion Impact of Congestion LossesLosses
Past work typically evaluates performance in absence of congestion
Relative performance improvement may change substantially when congestion occurs performance improvement may reduce if
congestion is dominant, or if RTO becomes larger due to wireless errors
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Multiple TCP TransfersMultiple TCP Transfers Past work typically measures a single TCP
connection over wireless TCP throughput is the metric of choice
When multiple connections share a wireless link, other performance metrics may make sense fairness aggregate throughput
Relative performance improvements of various schemes may change when multiple connections are considered
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TCP Window & RTO TCP Window & RTO Settings After a MoveSettings After a Move
Congestion window & RTO size at connection open are chosen to be conservative
When a route change occurs due to mobility, which values to use? Same as before route change ---- may be too
aggressive Same as at connection open ---- may be too
conservative Answer unclear
some proposals attempt to use same values as before route change, but not clear if that is the best alternative
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TCP for Mobile Ad Hoc TCP for Mobile Ad Hoc NetworksNetworks
Much work on routing in ad hoc networks
Some recent work on TCP for ad hoc networks
Need to investigate many issues MAC-TCP interaction routing-TCP interaction impact of route changes on window
size, RTO choice after move