Download Transport Issues in Ad Hoc and Sensor Networks

Transport Issues in Ad Hoc
and Sensor Networks
Παναγιώτης Παναγιώτου
Τμήμα Ηλεκτρολόγων Μηχανικών και Μηχανικών Υπολογιστών
Πολυτεχνική Σχολή
Πανεπιστήμιο Κύπρου
Computer Networks (ECE654) Project Presentation
19 November 2004
Ad Hoc Networks
 Wireless networks that operate without any
centralized structure used for directing
communication.
 Similar to users of cellular phones
communicating without the aid of a phone
tower or laptop users communicating with
one another without a wireless access point
available.
Ad Hoc Networks
 Nodes send and receive messages via end-toend
communication and also act as forwarding agents
for other mobile stations.
 The multi-hop network consists of
autonomous,mobile nodes that act as relay
stations for paths of communication.
 Because there are no centralized routers or base
stations, the nodes must also be able to handle
constant route changes and terminations
Ad Hoc Networks
Transport Layer
 Intended to provide a more reliable method of
communication that is not offered by the network layer.
 Transport layer protocols were initially intended to offer
services regardless of the underlying network.
 Receives data from the user and transmits it to the receiver
in order and without error.
 Makes it possible for the users to send and receive data
without concern of network congestion or loss of packets.
 Ability to dynamically adjust window size to provide better
flow control.
Common Transport Protocols
UDP:
 User Datagram Protocol
 Connectionless protocol that does not offer
guaranteed packet delivery.
Common Transport Protocols
Traditional TCP:
 Transmission Control Protocol
 Most commonly used transport protocol.
 End to end protocol that provides reliable, in-order
transmission over the unreliable IP.
 Operates independently of the routing mechanisms
implemented in the network layer.
 Traditional TCP was designed to be used on wired
networks.
 As it is, does not work well in wireless environments.
Traditional TCP Problem
 It assumes every packet loss to be due to
congestion even though the loss could
be due to a number of possibilities.
 Mechanisms:
 Slow Start
 Congestion Avoidance
Congestion Control in TCP
– Threshold := CongestionWindow/2
– CongestionWindow := 1
TCP vs. Ad Hoc Networks
 Bit Errors
– TCP immediately reduces its transmission
speed to avoid further congestion.
– A reduction in transmission speed is the exact
opposite of what needs to be done to remedy
the problem.
– The optimal way would be to immediately
resend the packet to the receiver.
TCP vs. Ad Hoc Networks
 Path Change
– In ad hoc networks, the mobility of the nodes
causes the probability of path change to
increase.
– TCP will timeout and incorrectly assume
network congestion
– The optimal way would be to stop transmitting
until the network has found a new path and then
resume sending at the rate determined by the
new node(s) and the network layer.
Important Issues in
Ad Hoc Networks





Variable link quality
Multi-user interference
Power consumption
Topological changes
Propagation path loss
Link Quality
 Quality of transmitters and receivers
 Distance from the Mobile Station to the
receiver
 The degradation of each link can have an
enormous effect on throughput.
Multi user interference
 Wired networks: there is not really any threat of multiple
nodes receiving packets even if they were not meant for
them.
 Ad hoc wireless networks: transmitters do not have a
pinpoint location to send data to.
 they must broadcast data / request to send data to
every node in range.
 Need to decipher whether or not the broadcasted message
is for the node.
– YES  accept the message and read it.
– NO  ignore it and go on.
 These problems can be seen in both the hidden and
exposed station problems.
Power Consumption
 An important issue that all mobile stations have.
 Not consistent or stable source of power.
 Wired networks: transmitting power does not
really need to change.
 An ad hoc wireless networks: power consumption
changes per mobile station
 Transmitting power needs to be adjusted
depending on that distance which can
continuously change with the inconsistent topology
of ad hocwireless networks.
Power Consumption: Distance
 At t1 mobile station A needs power P1 to
transmit to mobile station B successfully.
 At t2 Y mobile station A needs power P2 to
transmit successfully.
 Mobile station B has either moved closer or
farther away from mobile station A.
Path Loss
 Attenuation undergone by an electromagnetic wave in
transit between a transmitter and a receiver.
 Many of the transport protocols treat path loss as
congestion andimplement traditional congestion control
mechanisms to compensate.
 Ad hoc wireless networks: path loss does not necessarily
mean congestion.
 The destination node has moved out of range of the sender
or is now on a different path.
 Congestion Control in this case is not useful and wastes
bandwidth and power (this is not needed).
TCP based modifications
 Wired TCP:
 TCP-Reno
 TCP-Vegas
 TCP-Tahoe
 TCP-Sack
 Ad Hoc TCP:
 TCP-BuS
 TCP-F
TCP-BuS
 Proposed by Dongkyun Kim, C.K. Toh, and
Yanghee Choi.
 Main differentiating property is the
introduction of buffering capabilities in the
mobile nodes.
 Also does feedback information for detecting
route disconnection.
Modifications
1. Explicit notifications for route failures and
route reestablishment


ERDN (explicit route disconnection
notification)  Stop Transmission
ERSN (explicit route successful
notification)  Start Transmission
Modifications


Use of extended timeout values
Packets are still sent by the sender and the node
that sent the ERDN message buffers these
packets.
 Problem: the time it takes for a new path to be
discovered might be smaller than the time the
sender waits before thinking that the packet has
been lost.
 The timeout value for the buffered packets is
doubled.
Modifications
3. Selective retransmission of lost packets.
4. Avoidance of unnecessary requests for
fast transmission.
5. Reliable transmission of control messages.

2 timers: ERDN_RET_TIMER and
ERSN_RET_TIMER
TCP-F
 Created by Chandran, Raghunathan, Venkatesan,
and Prakash
 Uses feedback to overcome the limitations of TCP
 Goal: "source informed of route failure so that is
does not unnecessarily invoke congestion control
and can refrain from sending any further packets
until the route is restored."
TCP-F
 When an intermediate node in the path between the
sender and the receiver encounters a route disruption it
propagates a route failure notification (RFN) packet to
the source and records the occurrence.
 Each node that receives the RFN invalidates the particular
route and prevents incoming packets intended for the
destination from passing though that route.
 If one of the intermediate nodes knows an alternate route,
the RFN can be discarded and communication can begin
on the new route.
 If not,the RFN gets propagated all the way back to the
sender.
Snooze State
 When sender receives a RFM, it goes into a "snooze"
state:
1. Ceases sending all packets.
2. Marks all timers as invalid.
3. Freezes the packet window, the retransmit timers, and
window size.
4. Starts a route failure timer.
It remains in a snooze state until it receives notification of a
reestablished route by a RRN (route reestablishment
notification packet).
If a node discovers a working route,it propagates the
message back towards the sender.
All further RRNs for the same source-destination combination
are then discarded.
ATCP
Ad hoc Transmission Control Protocol
Acts as an intermediary between the
network layer and standard TCP
It is important for applications to be able to
use standard TCP without any
modifications.
ATCP is a thin layer between the network
layer and TCP.
Packet Loss
 When ATCP detects packet loss due to a route
failure or path change, it does not notify TCP of
the packet loss.
 Instead, it places TCP into persist mode
by changing the receiver’s advertised window size
to 0.
 This causes TCP to temporarily stop sending
without attempting congestion control.
 Once ATCP has detected that the network layer
has reestablished a path, it will notify TCP to begin
sending again by setting the receiver’s window to
the previous size.
High Bit Error Rate
 Same technique for high bit error rate.
 When ATCP detects that a packet has been
corrupted, it places TCP in persist mode and
retransmits the packet itself without notifying
TCP of the packet loss.
Advantages
 Current method of congestion control (TCP) is
available without any modifications.
 When ATCP detects packet loss due to
congestion, it simply steps aside and lets TCP
handle it as it would in a wired network.
 It supports the use of current TCP
implementations, which will allow better
interoperability with pre-existing applications.
 Programmers will not need to learn a new protocol
to use for ad hoc network applications.
Data Flow
ATP (1)
Application controlled Transport Protocol
 The application should be able to decide the QoS offered by the
transport layer.
 ATP also allows for the priority levels for different applications to be
determined dynamically, allowing for more flexibility in multiple
application environments.
 ATP is based on a good idea that applications should be able
todynamically set their priority.
 Example: parametic needs to look up patient data and communicate
with another doctor at the same time.
Disadvantages
 It omits much of the functionality required by an
ideal transport layer protocol.
 Reliable delivery is left to the user application,
which can query ATP to see if an ACK has arrived
but must determine itself when to retransmit.
 It completely overlooks congestion control, packet
errors, and path loss. These issues are
 left to the user application for implementation.
ENIC
ENhanced Inter-layer Communication and control
 The OSI model layers should have better
communication with the other layers.
 In order to avoid the hidden station and exposed
station problem in wireless networks, the ENIC
protocol utilizes Request-To-Send (RTS) and
Clear-To-Send (CTS) packets.
Hidden and Exposed Station
A
Δ
B
Γ
Ε
Ζ Η
ENIC
 Ability to freeze the TCP state when a route has failed.
 The network layers in ENIC will monitor the connections
between the sender and receiver and will notify the
transport layer of a failure with an Explicit Route State
Notification (ERSN).
 At this point, the protocol will save the current state of its
variables (retransmission time, congestion window, etc.)
until the network layer has determined a new route.
 The transport layer will periodically probe the network layer
for a new route by using a Route Recovery Timer (RRT).
 ENIC handles route change and path loss without a
serious reduction in throughput.
 In the case ofnetwork congestion, ENIC will use standard
TCP slow-start algorithm.
ATP (2)
Ad hoc Transport Protocol
 Communication between layers needed for startup rate
estimation, congestion control, and path loss/failure.
 This is a common theme among the ad hoc protocols, so
most lower layers should support this communication.
 The intermediate nodes are required to piggyback data
rates onto packets to the nodes. This allows the sender to
determine the amount of congestion in the network.
Sensor Networks
Transport Protocols
 PSFQ: Pump Slowly and Fetch Quickly
 ESRT Event-to-Sink Reliable Transport for
Wireless Sensor Networks
 CODA: Congestion Detection and
Avoidance in Sensor Networks
Sensor Networks
 Expectations for transport protocol for
sensor networks
 Reliability, congestion control
 Constraints
Resource constraints – power, storage,
computation complexity, data rates
 These constraints are application
specific.
Sensor Networks
General notion for sensor networks
Low data Rate
Power limited
Storage limited
High data Rate
Not power limited
Not storage limited
User
Sink
PSFQ
Pump Slowly and Fetch Quickly
 A Reliable Transport Protocol for Wireless
Sensor Networks
Multi-Hop Packet Forwarding
1
2
1
3
4
1
1
2
2
2
3
3
3
When No Link Loss – Multi-Hop Forwarding takes place
Recovering from Errors
1
3
2
1
1
4
1
2 lost
3
3
3
Recover 2
Recover 2
Recover 2
Error Recovery Control Messages are wasted
How PSFQ Recovers from Errors
“Store and Forward”
1
3
2
1
2
1
2 lost
4
1
3
Recover 2
2
2
3
2
3
No wastage of the Error Recovery control messages
PSFQ Pump Schedule
1
2
1
Tmin
Tmax
t
1
Tmin
Tmax
1
If not duplicate and in-order and TTL not 0
Cache and Schedule for Forwarding at time t (Tmin<t<Tmax)
“Fetch Quickly” Operation
1
2
1
1
2
2 lost
Tr
3
Recover 2
2
Tr
Tmin
Tmax
2
“Proactive Fetch”
1
2
last-1
last
Tproc
last
CODA
Congestion Detection and Avoidance in
Sensor Networks
 Energy efficient congestion control scheme
 Three mechanisms are involved
– Congestion Detection
– Open-loop hop-by-hop backpressure
– Closed-loop multi-source regulation
Congestion Detection
 Accurate and efficient congestion detection
is important
– Buffer queue length or Buffer occupancy – not a
good measure of the congestion.
– Channel loading – sample channel at
appropriate time to detect congestion.
– Report rate/Fidelity measurement – slow,
observed over a longer period
Open-Loop Hop-by-Hop
Backpressure
1
2
3
4
5
6
Congestion
detected
Closed Loop Multi-Source
Regulation
1
2
1,2,3
Regulate
bit is set
ACK
4,5,6
7,8
ACK
Congestio
n detected
ESRT
Event-to-Sink Reliable Transport for Wireless
Sensor Networks
ESRT
 Event-to-sink reliability
S
 Self-configuration
 Energy awareness [low power
consumption requirement!]
 Congestion Control
 Variation in complexity at source and
sink. [computation complexity]
ESRT’s Definition of Reliability
 Reliability is measured in terms of the
number of packets received. Or reporting
frequency (number of packets / decision
interval).
 Observed reliability: number of received
data packets in decision interval at the sink.
 Desired reliability: number of packets
required for reliable event detection.
 Normalized reliability is observed reliability
/ desired reliability.
Algorithm for ESRT
 If congestion and low reliability: decrease reporting
frequency aggressively. (exponential decrease)
 If congestion and high reliability: decrease
reporting to relieve congestion. No compromise on
reliability (multiplicative increase)
 If no congestion and low reliability: increase
reporting frequency aggressively (multiplicative
increase)
 If no congestion and high reliability: decrease
reporting slowing (half the slope)
References
1.
2.
3.
4.
Tanenbaum, A. Computer Networks, Third Edition.
Prentice Hall
John Calagaz, Wade Chatam, Brian Eoff and John A.
Hamilton, Jr. On the Current State of Transport Layer
Protocols in Mobile Ad hoc Networks.
Liu, J., Singh, S. ATCP: TCP for mobile ad hoc networks.
Selected Areas in Communications, IEEE Journal on,
Volume: 19 Issue: 7, (July 2001).
Dongkyun K., Toh, C.K., and Choi, Y. TCP-BuS:
improving TCP performance in wireless ad hoc networks.
In Communications, 2000 IEEE International Conference
on, Volume: 3, (18-22 June 2000), 1707 -1713.
References
5.
6.
7.
Sun, D., and Man, H. Performance comparison of
transport control protocols over mobile ad hoc networks.
In Personal, Indoor and Mobile Radio Communications
12th IEEE International Symposium on, Volume: 2, (30
Sept.-3 Oct. 2001), G-83 -G-87.
Chandran, K., Sudarshan, R., Venkatesan, S., Prakash,
R. A Feedback-Based Scheme for Improving TCP
Performance in Ad Hoc Wireless Networks. In IEEE
Personal Communications, (February 2001).
Holland, G., Vaidya, N. Analysis of TCP performance
over mobile ad hoc networks, Wireless Networks,
Volume 8 Issue 2/3 March 2002.
References
8.
Liu, J. and Singh, S., ATP: Application controlled
Transport Protocol for Mobile Ad Hoc Networks. Wireless
Communications and Networking Conference, 1999.
IEEE, (21-24 Sept. 1999) 1318 -1322 vol.3.
9. Sundaresan, K., Anantharaman, V., Hsieh, H., and
Sivakumar, R. ATP: a reliable transport protocol for adhoc networks. Proceedings of the 4th ACM international
symposium on Mobile ad hoc networking & computing.
(June 2003).
10. Sun, D., and Man, H. ENIC - an improved reliable
transport scheme for mobile ad hoc networks. Global
Telecommunications Conference, 2001. GLOBECOM
'01. IEEE, Volume: 5, (25-29 Nov. 2001).
End of presentation
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