Qian ZhangDepartment of Computer Science
HKUST
Advanced Topics in Next-Generation Wireless Networks
Transport Protocols in Ad hoc Networks
Transmission Control Protocol (TCP)
• Reliable ordered delivery
• Implements congestion avoidance and control
• Reliability achieved by means of retransmissions if necessary
• End-to-end semantics– Acknowledgements sent to TCP sender confirm
delivery of data received by TCP receiver– Ack for data sent only after data has reached
receiver
Overview of TCP concepts
• Conventional TCP• Sending rate is controlled by
– Congestion window (cwnd): limits the – Slow-start threshold (ssthresh): when
• Loss detection– 3 duplicate ACKs (faster, more efficient)– Retransmission timer expires (slower,
less efficient)
• Overview of congestion control mechanisms– Slow-start phase: cwnd start from 1
and increase exponentially– Congestion avoidance (CA): increase
linearly– Fast retransmit and fast recovery:
Trigger by 3 duplicate ACKs
Slo
w s
tart
Slo
w s
tart
Congestion
avoidance
Congestiondetected
Congestion
avoidance
Fast retransmit/fast recovery
1
2
3
4
threshold
threshold
Time
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 220
2
4
6
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10
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18
20
22
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28
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32
34
Con
gesti
on
win
dow
s s
ize
Overview
Slow-start Congestionavoidance
– Tahoe, Reno, New-Reno, SACK
# of packets in flightCA start
Challenges
• Throughput unfairness– Unfairness at MAC layer– Transport layer should take this into account
• Resource constraints– Power and bandwidth constraints
• Separation of congestion control and reliability control
• Completely decoupled transport layer– Wired network: transport protocol completely separated
from underlying layer– Ad hoc network: interaction with network and MAC layer
is expected for adaptivity
Challenges (Cont.)
• Misinterpretation of congestion– Traditional mechanism: packet loss, timeout– Ad hoc loss/delay due to
• High bit error rate due to varying link condition• Packet collisions due to contention and hidden
terminal• Path breaks due to node mobility
• Dynamically changing topology– Frequent path breaks– Partitioning and merging of networks– High delay in reestablishment of path
Performance of TCP
• Several 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
Random 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
reduces the throughput
– Burst errors may cause timeouts
Throughput over Multi-Hop Wireless Paths
• Connections over multiple hops are at a disadvantage compared to shorter connections, because they have to contend for wireless access at each hop
0
200
400
600
800
1000
1200
1400
1600
1 2 3 4 5 6 7 8 9 10
Number of hops
TCPThroughtput(Kbps)
TCP Throughput using 2 Mbps 802.11 MAC
Throughput Degradations withIncreasing Number of Hops
• 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
Performance Metric: Expected Throughput
• Compute the expected throughput as
where
– ti: the duration for which the shortest path from the sender to the receiver contains i hops
– Ti : the throughput obtained in the fixed, linear network with i hops
Impact of Mobility
measured TCP Reno throughput
Throughput Degrades with Increasing Speed …
But NOT Always …
mobility causeslink breakage,resulting in routefailure
TCP data and acksen route discarded
Why Does Throughput Degrade?
TCP sender times out.Starts sending packets again
Route isrepaired
No throughput
No throughputdespite route repair
mobility causeslink breakage,resulting in routefailure
TCP data and acksen route discarded
TCP sendertimes out.Backs off timer.
Route isrepaired
TCP sendertimes out.Resumessending
Larger route repair delaysespecially harmful
No throughput
No throughput
despite route repair
Why Does Throughput Degrade?
C
B
D
A
C
B
D
A
C
B
D
A
1.5 second route failure
Route from A to D is broken for ~1.5 second
When TCP sender times out after 1 second, route still broken
TCP times out after another 2 seconds, and only then resumes
Low Speed Scenario
C
B
D
A
C
B
D
A
C
B
D
A
0.75 second route failure
Route from A to D is broken for ~ 0.75 second
When TCP sender times out after 1 second, route is repaired
Higher (double) Speed Scenario
Impact 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
Impact to TCP
Average speed (m/s)
Act
ual t
hrou
ghpu
t (a
s fr
actio
n of
exp
ecte
d th
roug
hput
)
Why Performance Degrades 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 routeHowever, 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
To 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
Caching and TCP performance
• Caching can reduce overhead of route discovery even if cache accuracy is not very high
• But if cache accuracy is not high enough, gains in routing overhead may be offset by loss of TCP performance due to multiple time-outs
How to Improve Throughput(Bring 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
TCP with ELFN
• Explicit Link Failure Notification – Not totally new, e.g., ECN bits in TCP
– To provide the TCP sender with information about link and route failures, so that it can avoid responding to the failures as if congestion has occurred
• How does it work?– When a TCP sender receives an ELFN: disables its
retransmission timers and enters a “stand-by” mode
– While on standby: A packet is sent at periodic intervals to probe the network to see if a route has been established
– If an acknowledgment is received: leaves stand-by mode and restores the retransmission timers
Measured TCP Reno w/ELFN throughput
Performance with Explicit Notification
0
0.2
0.4
0.6
0.8
1
2 10 20 30
mean speed (m/s)
thro
ug
hp
ut
as a
fra
ctio
n o
f id
eal
Base TCP
With explicitnotification
Issues: Network 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