1. Special Topics on Wireless Ad-hoc Networks
Lecture 5: TCP for Ad-hoc Networks
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
Univ. of Tehran
Computer Network

2. Covered 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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3. Outlines Problems with TCP in MANET How to improve TCP in MANET
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4. Issues of TCP in Ad hoc
Induced traffic: Link level transmission affect neighbors of sender and receiver.
Induced Throughput unfairness: throughput/delay unfairness in link layer
Separation of congestion control, realiability and flow control
Power and bandwidth constrains.
Misinterpretation of congestion:
Completely decoupled transport layer:
Dynamic topology.
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5. 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
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6. 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
Reduction in congestion window reduces the throughput
Burst Errors May Cause Timeouts
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7. Impact of 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
Extent of packet delay or drop
increases with number of hops
TCP Throughput using
2 Mbps MAC
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8. Mobile Ad Hoc Networks [IETF-MANET]
Mobility causes route changes
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9. Throughput Degradations with Increasing 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
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10. Ideal 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 = S f(i) * T(i)
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11. Impact of Mobility 2 m/s 10 m/s Actual throughput
Ideal throughput (Kbps)
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12. Impact of Mobility 20 m/s 30 m/s Actual throughput Ideal throughput
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13. Throughput degrades with increasing speed …
Ideal
Average
Throughput
Over
50 runs
Actual
Speed (m/s)
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14. But not always … 30 m/s 20 m/s Actual throughput Mobility pattern #
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15. Why Does Throughput Degrade?
mobility causes
link breakage,
resulting in route
failure
TCP data and acks
en route discarded
Route is
repaired
TCP sender times out.
Starts sending packets again
No throughput
despite route repair
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16. Why Does Throughput Degrade?
TCP sender
times out.
Resumes
sending
Larger route repair delays
especially harmful
mobility causes
link breakage,
resulting in route
failure
TCP data and acks
en route discarded
TCP sender
times out.
Backs off timer.
Route is
repaired
No throughput
despite route repair
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17. Low Speed Scenario D D D C C C B B B A A A 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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18. Higher (double) Speed Scenario
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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19. General 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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20. Improve Throughput (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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21. Performance Improvement
Without network
feedback
With feedback
Actual throughput
Ideal throughput
2 m/s speed
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22. Performance Improvement
Without network
feedback
With feedback
Actual throughput
Ideal throughput
30 m/s speed
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23. Performance with Explicit Notification
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24. 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
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25. 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
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26. To Cache or Not to Cache Average speed (m/s)
Actual throughput (as fraction of expected throughput)
Average speed (m/s)
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27. 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 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 due
to route failure
timeout, cached
route is broken
timeout, second cached
route also broken
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28. Issues 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
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29. Issues Window Size After Route Repair
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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30. Issues RTO 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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31. Out-of-Order Packet Delivery
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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32. Impact of Acknowledgements
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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33. Impact 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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34. TCP in Mobile Ad Hoc Networks Issues (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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35. Impact of MAC - Delay Variability
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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36. Impact of MAC - Delay Variability
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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37. Header Compression for Wireless 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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38. Automatic TCP Buffer Tuning
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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39. 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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40. Design goals Maximize throughput per connection Throughput fairness
Minimum connection setup and maintenance overhead.
Mechanisms for congestion and flow controls
Reliable and unreliable connections
Adapt to the dynamics of the network.
Resource, Bandwidth, used efficiently.
Aware of resource, battery, constraints.
End to end semantics
Use information from the lower layer
Well-defined cross-layer interaction for effective and scalable protocol
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41. Different approaches Adapt TCP for Ad hoc New Transport protocols
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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42. ACTP 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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43. ATP 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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44. ATP 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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45. 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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46. Issues for Further Investigation
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47. Link 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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48. End-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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49. Impact of Congestion Losses
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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50. Multiple 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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51. TCP Window & RTO Settings 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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52. TCP for Mobile Ad Hoc Networks
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
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