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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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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Transmission round

cong

estion

windo

w size

(seg

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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