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Quality of Service in MANETs
Part 7Mobile Ad Hoc NetworkingWuhan University
Quality of Service – QoSWhat does the Internet provide?n Best-effort servicew Guarantee only one thing – delivery of data
n That is, making the “best of the best effort”n But, many things required by the application layer
can’t be guaranteed if lower layers do notw E.g. delay and bandwidth may be important to
multimedia related applications
What is QoS?n Mechanism to provide certain service grade
regardless network conditions
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QoS Metrics
Available bandwidthPacket loss rateEstimated delayPacket jitterHop countPath reliability
QoS in the InternetQoS Modelsn IntServ (Integrated Services)n Resource reservationn DiffServ (Differentiated Services)n Traffic classification and policingn MPLS (Multi-Protocol Label Switching)
QoS routingn How do we find good paths with good QoS
properties?
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IntServ[Braden, Clark & Shenker RFC1633 1994]
All flows start after resource reservationOnly two classesn Guaranteed or controlled-load
Componentsn RSVP (Resource Reservation Protocol)w [Braden, Zhang, Berson, Herzog & Jamin RFC2205 1997]w On per-flow basis – routers maintain flow-specific info
n Admission controln Classificationn Packet scheduling
Disadvantages?
DiffServ[Blake, Black, Carlson, Davies, Wang & Weiss RFC2475 1998]
Packets are divided into classesn The TOS (type of services) byte in IPv4 header is now called
DiffServ Fieldn Packet forwarding treatment – PHB (per-hop behavior)
Essentially a relative-priority schemeSLA (Service Level Agreement) – a contract between customer and ISPn Service classes supportedn Amount of traffic allowed in each class
Customer (or its leaf router) marks packets individuallyIngress (edge) router of ISP classifies and polices the packet flowsCore routers forward (and possibly drop in case of congestion) packets according DiffServ Field marks
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Diffserv Architecture IllustratedEdge router:- per-flow traffic management
- marks packets as in-profile and out-profile
Core router:
- per class traffic management
- buffering and scheduling
based on marking at edge- preference given to in-profile packets- Assured Forwarding
scheduling
...
r
b
marking
Scheduling and Dropping Mechanisms
FIFO (FCFS)Priority queuingRound robin and WFQ (Weighted Fair
Queuing)
Drop tailRED (Random Early Detection)RIO (RED with In and Out)
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RED Packet Drop Explained
Use exponential average of queue length to determine when to dropn avoid overly penalizing short-term burstsn React to longer term trends
Tie drop prob. to weighted avg. queue lengthn avoids over-reaction to mild overload conditions
Maxthreshold
Minthreshold
Average queue length
Forced drop
Probabilisticearly dropNo drop
Time
Drop probabilityMax
queue length
Why QoS Routing?Causes of congestionn Lack of network resourcesn Uneven traffic load distributionw Direct results of many dynamic routing protocols
such as RIP, OSPF, IS-IS etc.
Goal – Arrange how traffic flows through the network so that congestion caused by uneven network utilization is avoidedConstraint-based routing – calculate
routes subject to multiple constraints
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Constraint-Based RoutingGoalsn Select routes that meet QoS requirementsn Increase network utilization
In addition to topology, routers should known Flow requirementsn Routing policiesn Route metricsw E.g. monetary cost, hop-count, bandwidth, delay,
reliability, jitter
Routing algorithms select routes that optimize one or more of these metrics
Route ComputationTypes of route metrics
Computing optimal routes subject to constraints of two or more additive and/or multiplicative metrics is NP-completeSorts of heuristics or approximationsCan be computation-intensive
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Flexible QoS Model for MANETs[Xiao, Seah, Lo & Chua 2000]
FQMM (Flexible QoS Model for MANETs)n Consider characteristics of MANETsn Combination of IntServ and DiffServPer-flown As in IntServn Tries to preserve per-flow granularity for a small
portion of trafficPer-classn As in DiffServn Done for the mass majority of the traffic
QoS in MANETsSignalingn INSIGNIAn dRSVP (dynamic RSVP)
Routingn CEDAR (Core Extraction Distributed Ad Hoc Routing)n Ticket-based routingn Bandwidth routing
Link layern MACA/PRn IEEE 802.11e
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INSIGNIA[Ahn, Campbell, Lee & Zhang 1998]
Per-flowIn-band signaling for less overheadn Unlike RSVP which has an explicit resource reservation phase
before data transportationn Info used for resource reservation is piggybacked on data
packetsn No flow rejected – if QoS requirement satisfied, forward as RES
(reservation); otherwise, forward as BE (best effort)
Soft-stateQoS reporting – periodic report from destination to source about statsRestoration – depending on underlying routing protocol
CEDAR[Sivakumar, Sinha & Bharghavan 1999]
Core extractionn Core – a small dominating set of nodesn Core graph – graph on the dominatorsn Distributed algorithm for small dominating setsn For local topology control and smart broadcastLink state propagationn Significant bandwidth changes are broadcast (by
dominators) throughout networkRoute computationn On-demand source routingn Routes established first within core graph
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IEEE 802.11 Standard Family
802.11 is Best EffortDCF’s limitations
PCF’s limitations
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Transport Layer Issues in MANETs
Part 8CS-6777 Mobile Ad Hoc NetworkingMemorial University of Newfoundland
Transport Layer Services
End-to-end logical communications between two “processes” n Demux through port numbers
Two styles of servicesn Fast and unreliablen Mild and reliable
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UDP & TCPUDP – User Datagram Protocoln No error controln No flow/congestion controln Short header – how many bytes?TCP – Transmission Control Protocoln Error controln In-order deliveryn Flow/congestion controln Longer header – how many bytes?
TCP Features
AIMD n Additive increase
multiplicative decrease
Retransmit timern Time-out interval and
exponential backoff
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4
68
10
1214
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Transmission round
cong
estion
windo
w size
(seg
men
ts)
Series1 Series2
thresholdTCP
Tahoe
TCPReno
TimeOut
Slow startn Versus fast recovery in response of segment loss
ACK-clocking
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What is Wrong with TCP? (I)Misinterpretation of segment losses
Performing congestion control in cases of losses not induced by network congestionn Many losses in MANETs are not caused by
congestionWhere are the losses from?n Lossy channelsn Path asymmetry – bandwidth, loss rate, routen Power constraintsn Route failuren Network partitionn Multi-path routing and out-of-order (OOO)
What is Wrong with TCP? (II)Oversized maximum congestion window size
In high speed networksn “bandwidth-delay product”n [RFC 1323 - TCP Extensions for High
Performance]In a chain of 802.11 nodesn Much smaller due to interference among
neighboring nodes
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Solutions
Overhaul
Fix/tweak existing protocolsn Cross layern Layered
TCP – Network Cross Layer
Use feedback from network layer to clarify cause of packet losses to avoid starting congestion control blindly
Even to break the end-to-end semantics of TCP
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TCP-F[Chandran, Raghuanathan, Venkatesan and Prakash 1998]
Feedback basedn To minimize throughput degradation resulting from
path breaks
TCP sender gets notified of route failuresn In TCP-F, when an intermediate node (called failure
point, FP) detects a path failure (how?), it sends a Route Failure Notification (RFN) to the TCP sender
n Upstream node may discard the RFN if knowing alternative routes
n After TCP sender receives RFN, it enters a snooze state by freezing all TCP variables
TCP-F (cont’d)When broken link is fixed, FP sends a Route Re-establishment Notification (RRN) to TCP sender, which in turn returns to normal state TCP sender may also return to normal state after a timeout
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TCP-ELFN[Holland and Vaidya 2002]
Similar feedback based approachn TCP with Explicit Link Failure Notificationn TCP sender snoozes when notified of route
breakWhen in snooze state, TCP sender periodically
probes the destination to see if break has been fixed so as to return to normal state
TCP-BuS[Kim, Toh and Choi 2001]
Similar feedback based approachn TCP with Buffering capability and Sequence
informationTCP sender freezes when notified of
route breakIntermediate nodes buffer segments until
route reconstructedUtilize the SACK option of TCP s.t. sender
does not need to resend segments
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ATCP [Liu and Singh 2001]
Ad hoc TCPn Uses Explicit Congestion Notification (ECN) of IP
[RFC 3168]n ECN is part of Active Queue Management (AQM) of
IP, in contrast to Random Early Detection (RED)
A layer inserted between TCP and network layers of the sendern No modification to TCP
TCP sender starts congestion control when receiving ECN; freezes on DUACKs and TIMEOUTs
TCP-Split [Kopparty, Krishnamurthy, Faloutous, and Tripathi, 2002]
Split long TCP connections into short onesn Observing that TCP throughput decreases rapidly as
hop length increases (why?)
Sender → proxies → destinationn Local ACK (LACK) vs. End-to-end (EACK)n Sender maintains two CW’s while proxies maintain one
Separate error control from congestion control
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Network – PHY Cross LayerPreemptive routingn [Goff, Abu-Ghazaleh, Phatak and Kahvecioglu 2001]n To reduce route failuresn Routing module finds and switches to new routes
when capacity of a link on current route drops below a threshold
Signal strength based link managementn [Klemm, Krishnamurthy and Tripathi, 2003]n Similar to above, when locating new route, increase
transmission power to keep the link alive temporarily
TCP Layer ProposalsDynamic delayed ACKn [Altman and Jimenez 2003]n Reduce the number of ACKs to reduce
channel contentionwMay cause sender burst (why?)
Adaptive CWL settingn [Chen, Yue and Nahrstedt 2003]n Set contention window length (upper limit
of CW) according to path length, e.g. 1/5 of RTHC (round trip hop count)
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Link Layer ProposalsLink REDn [Fu, Zerfos, Luo, Lu, Zhang and Gerla 2003]n Drop/mark frames (as in AQM) when link
quality drops below threshold
Neighborhood REDn [Xu, Gerla, Qi and Shu 2003]n Refer to (up/down stream) neighbors when
calculating drop/mark probability
Non-TCP Transport Layer ProtocolsACTP – application controlled transport
protocoln [Liu and Singh 1999]n Like UDP with feedback and state maintenancen No congestion window maintainedn Application layer responsible for retransmission
ATP – ad hoc transport protocoln [Samdaresan, Anantharaman, Hsieh and Sivakumar
2003]n A light-weight TCP like protocoln Uses feedbacks from layers below
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Cross-Layer Design