challenges in mobile mobile ad hoc wireless environments...

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CS680: Wireless Networks and Applications

William RegliDepartment of Computer Science

Drexel University

Cellular Wireless• Single hop wireless connectivity to the wired world

– Space divided into cells– A base station is responsible to communicate with hosts in its

cell– Mobile hosts can change cells while communicating– Hand-off occurs when a mobile host starts communicating via a

new base station

Challenges in Mobile Environments

• Limitations of the Wireless Network– packet loss due to transmission errors– variable capacity links– frequent disconnections/partitions– limited communication bandwidth – Broadcast nature of the communications

• Limitations Imposed by Mobility– dynamically changing topologies/routes– lack of mobility awareness by system/applications

• Limitations of the Mobile Computer– short battery lifetime– limited capacities

Mobile Ad Hoc Wireless Networks (aka MANETs)

• Mobile– Nodes are in motion– Typically implies nodes are “untethered”

• Handhelds, laptops, ruggedized PCs in a squad car, etc

• Ad Hoc– No fixed infrastructure– No access points, no “routers”

• Note: DNS, traditional internet routing, etc. are not designed for “mobility”

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Multi-Hop Wireless• In MANETs, packes will need to traverse multiple

network hops to reach their destination

• Node mobility causes topology and route changes

Obvious Motivations for Ad Hoc Networks

• Setting up of fixed access points and backbone infrastructure is not always viable– Infrastructure may not be present in a disaster area or

war zone– Infrastructure may not be practical for short-range

radios; Bluetooth (range ~ 10m)

• Ad hoc networks– Do not need backbone infrastructure support– Are “easy” to deploy– Useful when infrastructure is absent, destroyed or

impractical

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Obvious Applications for MANETs

• Personal area networking– cell phone, laptop, ear phone, wrist watch

• Military environments– soldiers, tanks, planes

• Civilian environments– taxi cab network– meeting rooms– sports stadiums– boats, small aircraft

• Emergency operations– search-and-rescue– policing and fire fighting

Ad Hoc Networking

• From “IEEE Explore” – Number of papers on Ad Hoc networks: 500

• Over 100 published routing protocols• Nearly all of this work is based on

simulation with NS-2 or OpNet• Several protocols are undergoing

standardization by the IETF

Partial Taxonomy of Routing Protocols

• Several survey papers and “special issues” exist• Broad categories of algorithm include

Reactive Proactive HybridLocation-based Hierarchical

IETF MANET Working Group

• Reactive– Dynamic Source Routing (DSR)– Ad-hoc On-Demand Distance Vector Routing

(AODV)• Pro-active

– Optimized Link State Routing Protocol (OLSR)

– Topology Broadcast based on Reverse Path Forwarding (TBRPF)

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Issues with Routing

• Scalability, number of nodes• Routing table convergence time• Administration • Resource-usage

– Bandwidth overhead– Power

• Security– Byzantine failure among other issues

Issues with Mobility• Frequent route changes

– amount of data transferred between route changes may be much smaller than traditional networks

• Route changes may be related to host movement

• Low bandwidth links

Goal of routing protocols– decrease routing-related overhead– find short routes– find “stable” routes (despite mobility)

Reactive and Proactive Routing• Reactive protocols

– Determine route if and when needed• Proactive protocols

– Traditional distributed shortest-path protocols– Routes are maintained by default

• Hybrid protocols– Adaptive; Combination of proactive and reactive

• Location-based– Uses GPS information about hosts to drive routing

decisions• Hierarchical

General Properties of Proactive Protocols

• Updates routing table proactively– before routes our used, they are available– Downside: routes that aren’t used are also available

• Routing table updates creates more overhead– Upside: no delay for route discovery

• Scalablity helped if zoned or nodes given different roles

• Larger signalling traffic and power consumption

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General Properties of Reactive Protocols

• Attempts to discover routes only on-demand by flooding• Updates routes among nodes only when data is to be

transferred– This reduces need for routing table updates and traffic overhead

• Updating typically done with controlled ’flooding requests’ into the wireless network

• Possible long delay for packet transmission if entry in routing-table has timed-out or gone offline

• Smaller signalling traffic and power consumption.• A long delay for application when no route to the

destination available

Distance Vector & Link State Routing

• Both assume router knows– address of each neighbor– cost of reaching each neighbor

• Both allow a router to determine global routing information by talking to its neighbors

• Distance Vector– router knows cost to each destination

• Link State– router knows entire network topology and computes

shortest path

IETF Routing Protocols

• Ad Hoc On Demand Distance Vector (AODV) Routing (RFC 3561)

• Optimized Link State Routing Protocol (RFC 3626)

• Topology Dissemination Based on Reverse-Path Forwarding (TBRPF) (RFC 3684)

• The Dynamic Source Routing (DSR) Protocol for Mobile Ad Hoc Networks (DSR)

IETF Routing Protocols

• Ad Hoc On Demand Distance Vector (AODV) Routing (RFC 3561)

• Optimized Link State Routing Protocol (RFC 3626)

• Topology Dissemination Based on Reverse-Path Forwarding (TBRPF) (RFC 3684)

• The Dynamic Source Routing (DSR) Protocol for Mobile Ad Hoc Networks (DSR)

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AODV

Ad-hoc On demand Distance Vector (AODV) Routing

Overview:• Reactive protocol• Protocol mechanics

– RREQ - broadcast route discovery message– RREP - unicast message to validate a path– RERR - Error message to inform nodes of link failure

• Routing information stored at the nodes• Routing table stores only active routes,

unused routes are removed• Timers and sequence number are used to avoid

stale routes

AODV

• When a node re-broadcasts a Route Request (RREQ), it sets up a reverse path pointing towards the source– AODV assumes symmetric (bi-directional) links

• When the intended destination receives a Route Request, it replies by sending a Route Reply (RREP)

• Route Reply travels along the reverse path set-up when Route Request is forwarded

Route Requests in AODV

B

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Represents a node that has received RREQ for D from S

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B

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Represents transmission of RREQ

Z

YBroadcast transmission

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B

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D

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Represents links on Reverse Path

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Reverse Path Setup in AODV

B

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IK

• Node C receives RREQ from G and H, but does not forwardit again, because node C has already forwarded RREQ once

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• Node D does not forward RREQ, because node Dis the intended target of the RREQ

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Forward Path Setup in AODV

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Forward links are setup when RREP travels alongthe reverse path

Represents a link on the forward path

Route Request and Route Reply

• Route Request (RREQ) includes the last known sequence number for the destination

• An intermediate node may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender

• Intermediate nodes that forward the RREP, also record the next hop to destination

• A routing table entry maintaining a reverse path is purged after a timeout interval

• A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval

Link Failure in AODV • A neighbor of node X is considered active for a routing

table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry

• Neighboring nodes periodically exchange HELLOs• When the next hop link in a routing table entry breaks, all

active neighbors are informed• Link failures are propagated by means of Route Error

(RERR) messages, which also update destination sequence numbers

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Route Error in AODV• When node X is unable to forward packet P (from node

S to node D) on link (X,Y), it generates a RERR message

• Node X increments the destination sequence number for D cached at node X

• The incremented sequence number N is included in the RERR

• When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N

• When node D receives the route request with destination sequence number N, node D will set its sequence number to N, unless it is already larger than N

AODV Summary• Nodes maintain routing tables containing entries

only for routes that are in active use• At most one next-hop per destination maintained

at each node• Sequence numbers are used to avoid old/broken

routes• Sequence numbers prevent formation of routing

loops• Unused routes expire even if topology does not

change

OLSR

Optimized Link State Routing (OLSR)

• A link state protocol optimization• Introduces serial numbers to handle stale routes• Selective Flooding

– Only Multipoint Relays (MPR) retransmit control messages

• Minimize flooding• Suitable for large and dense networks

• The goal is to– Reduce the flooding of topology control messages– Reduce size of topology control messages

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OLSR’s Key Components• Multipoint Relays (MPRs)

– MPRs relay all traffic from a node– Each node advertises the nodes that have selected it

as an MPR– Used to build a "last hop" table and routing table

• Topology Control (TC) messages– Neighbour sensing information base– Topology information base– Routing table

• TC messages are sent unicast from nodes and then multicast by MPRs

Multipoint Relays

4 retransmission to diffuse a message up to 2 hops

MPR(Retransmission node)

Multipoint Relays (MPRs)• MPRs = A set of selected neighbor nodes

– Goal: Minimize the flooding of broadcast packets

• Each node selects its own MPRs among its “one hop”neighbors– The set covers routing to all the nodes <= two hops away

• MPR Selector = node which has selected node as MPR– Nodes select nodes to serve as their MPRs

• Information required to calculate the multipoint relays– The set of one-hop neighbors and the two-hop neighbors

• Set of MPRs is able to transmit to all two-hop neighbors• Link between node and it’s MPR is bidirectional

Multipoint Relays (MPRs)

• A subset of 1-hop neighbors are selected as MPRs

• The set of MPRsmust cover all 2-hop neighbors

v

Retransmitting nodeor multipoint relay

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Computing Multipoint Relays

• Obtain information about one-hop neighbors– HELLO messages

(received by all one-hop neighbors)• Obtain the information about two-hop neighbors

– Each node attaches the list of its own neighbors to the HELLO messages

• When nodes have one and two-hop neighbors– Select a MPRs which covers all two-hop neighbors

• All nodes that are neighbors of my neighbors

Multipoint Relay Selection• Each node selects own set of multipoint relays• Multipoint relays are declared in the transmitted HELLO

messages• Multipoint relay set is re-calculated when

– A change in the neighborhood (neighbor is failed or add new neighbor )

– A change in the two-hop neighbor set

• Each node also construct its MPR Selector table with information obtained from the HELLO message

• A node updates its MPR Selector set with information in the received HELLO messages

Multipoint Relays (MPRs)

Two-hop node

One-hop node

MPR node

originating node1

2

3

5

4

MPR(1)={2,3,4,5}

OLSR Neighbor Sensing• Each node periodically broadcasts HELLO messages for

link sensing– Contains information about its neighbors and their link status– HELLO messages are received by all one-hop neighbors

• Everyone within radio range

• HELLO message contains– List of addresses of the neighbors to which there exists a valid

bi-directional link– List of addresses of the neighbors which are heard by node ( a

HELLO has been received )• But link is not yet validated as bi-directional

• On the reception of HELLO message– Each node constructs its MPR Selector table

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OLSR Neighbor Sensing

• In the neighbor table for each node– Information about its on hop neighbor and a

list of two hop neighbors– Holding time

• Upon expiry of holding time, removed– Sequence number

• value which specifies the most recent MPR set• Every time updates its MPR set, this sequence

number is incremented

OLSR Neighbor Sensing

• Example of neighbor table

One-hop neighbors

……

MPRC

UnidirectionalG

BidirectionalB

State of LinkNeighbor’s id

Two-hop neighbors

……

CD

CE

Access thoughNeighbor’s id

MPR Information Declaration: Topology Control Messages

• TC = Topology Control message– Builds intra-forwarding database– Only MPR nodes forward periodically to declare its

MPR Selector set – Message might not be sent if there are no updates– Contains:

• MPR Selector • Sequence number

• Each node maintains a Topology Table based on TC messages– Routing Tables are calculated using Topology tables

MPR Information DeclarationHolding timeMPR Selector

sequence number

Destination’s MPR

Destination address

MPR Selector in the received TC message

Last-hop node to the destination.

Originator of TC message

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MPR Information Declaration

• Nodes, upon receipt of TC message:– If there exist some entry to the same destination with

higher Sequence Number, the TC message is ignored– If there exist some entry to the same destination with

lower Sequence Number, the topology entry is removed and the new one is recorded

– If the entry is the same as in TC message, the holding time of this entry is refreshed

– If there are no corresponding entry, the new entry is recorded

Routing Table Calculation• Each node maintains its own routing table to all known

destinations in the network• After each node TC message receives, store connected

pairs of form ( last-hop, node) • Routing table is based on the information contained in

the neighbor table and the topology table • Routing table consists of:

– Destination address– Next Hop address– Distance

• Routing Table is recalculated after every change in neighbor table or in topology table

Routing Table Calculation

Source

Destination

(last-hop, destination)




OLSR Summary

• OLSR protocol is proactive or table driven• Advantages

– Route immediately available– Minimize flooding by using MPR

• OLSR protocol is suitable for– large networks– dense networks

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Discussion of AODV and OLSR Routing Algorithms

Comparison• Reactive

– Suitable for sparse networks with small subset of nodes communicating

– High latency when establishing new routes– Low storage and message overhead– Difficult to implement QoS

• Pro-active– Suitable for networks with large subset of nodes

communicating– Low latency when establishing connections– Overhead to maintain routes to all destinations– Relatively easy to implement QoS

Protocols• AODV

– Next-hop routing towards destination

– One route to each destination

– Timers and sequence numbers to avoid stale routes

– High routing load for small numbers of nodes

– Better performance in larger network

• OLSR– Suitable for dense

networks– Use of MPRs reduce

the set of active nodes– Small packet size– Dependent on special

HELLO messages

Protocol Trade-offs• Proactive protocols

– Always maintain routes– Little or no delay for route determination– Consume bandwidth to keep routes up-to-date– Maintain routes which may never be used

• Reactive protocols– Lower overhead since routes are determined on demand– Significant delay in route determination– Employ flooding (global search)– Control traffic may be bursty

• Which approach achieves a better trade-off depends on the traffic and mobility patterns

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Theoretical Comparisons of “Flat”Routing Protocols

Quick Overview of Other Protocol Options

Temporally-Ordered Routing Algorithm(TORA) [Park97Infocom]

• Route optimality is considered of secondary importance; longer routes may be used

• At each node, a logically separate copy of TORA is run for each destination, that computes the height of the node with respect to the destination

• Height captures number of hops and next hop• Route discovery is by using query and update packets• TORA modifies the partial link reversal method to be

able to detect partitions• When a partition is detected, all nodes in the partition are

informed, and link reversals in that partition cease

Location-Aided Routing (LAR) [Ko98Mobicom]

• Exploits location information to limit scope of route request flood– Location information may be obtained using GPS

• Expected Zone is determined as a region that is expected to hold the current location of the destination– Expected region determined based on potentially old

location information, and knowledge of the destination’s speed

• Route requests limited to a Request Zone that contains the Expected Zone and location of the sender node

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Request Zone• Define a Request Zone• LAR is same as flooding, except that only nodes in

request zone forward route request• Smallest rectangle including S and expected zone for D

S

Request ZoneD

Expected Zone

xY

Location Aided Routing (LAR)

• Advantages– reduces the scope of route request flood– reduces overhead of route discovery

• Disadvantages– Nodes need to know their physical locations– Does not take into account possible existence

of obstructions for radio transmissions

Other Protocols• Power-Aware Routing [Singh98Mobicom]

– Assign a weight to each link: function of energy consumed when transmitting a packet on that link, as well as the residual energy level

– Modify DSR to incorporate weights and prefer a route with the smallest aggregate weight

• Associativity-Based Routing (ABR) [Toh97]– Only links that have been stable for some minimum duration are utilized– Nodes increment the associativity ticks of neighbors by using periodic

beacons• Signal Stability Based Adaptive Routing (SSA) [Dube97]

– A node X re-broadcasts a Route Request received from Y only if the (X,Y) link has a strong signal stability

– Signal stability is evaluated as a moving average of the signal strength of packets received on the link in recent past

Asymmetric AlgorithmsAsymmetric => unidirectional links are allowed• Clusterhead Gateway Switch Routing (CGSR)

– All nodes within a cluster communicate with a clusterhead– Routing uses a hierarchical clusterhead-to-gateway approach

• Core-Extraction Distributed Ad Hoc Routing (CEDAR) [Sivakumar99]– A subset of nodes in the network is identified as the core– Each node in the network must be adjacent to at least one node

in the core– Each core node determines paths to nearby core nodes by

means of a localized broadcast

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CGSR CEDAR

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A core node

Node E is the dominator for nodes D, F and K

Destination-Sequenced Distance-Vector (DSDV) [Perkins94Sigcomm]

• Each node maintains a routing table which stores– next hop, cost metric towards each destination– a sequence number that is created by the destination itself

• Each node periodically forwards routing table to neighbors– Each node increments and appends its sequence number when

sending its local routing table• Each route is tagged with a sequence number; routes with greater

sequence numbers are preferred• Each node advertises a monotonically increasing even sequence

number for itself• When a node decides that a route is broken, it increments the

sequence number of the route and advertises it with infinite metric• Destination advertises new sequence number

Destination-Sequenced Distance-Vector (DSDV)

• When X receives information from Y about a route to Z– Let destination sequence number for Z at X be S(X), S(Y) is sent from Y

– If S(X) > S(Y), then X ignores the routing information received from Y – If S(X) = S(Y), and cost of going through Y is smaller than the route

known to X, then X sets Y as the next hop to Z– If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated

to equal S(Y)

X Y Z

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Zone Routing Protocol (ZRP) [Haas98]

• ZRP combines proactive and reactive approaches• All nodes within hop distance at most d from a node X

are said to be in the routing zone of node X• All nodes at hop distance exactly d are said to be

peripheral nodes of node X’s routing zone• Intra-zone routing: Proactively maintain routes to all

nodes within the source node’s own zone.• Inter-zone routing: Use an on-demand protocol (similar

to DSR or AODV) to determine routes to outside zone.

Zone Routing Protocol (ZRP)

Radius of routing zone = 2

Hierarchical Routing Techniques

• Nodes have different roles in the network

• Some serve as backbones

Comparisons of Hierarchical Routing Protocols

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Other Issues

Routing Summary• Protocols

– Typically divided into proactive, reactive and hybrid – Plenty of routing protocols. Discussion here is far from exhaustive

• Performance Studies– Typically studied by simulations using ns, discrete event simulator– Nodes (10-30) remains stationary for pause time seconds (0-900s) and

then move to a random destination (1500m X300m space) at a uniform speed (0-20m/s). CBR traffic sources (4-30 packets/sec, 64-1024 bytes/packet)

– Attempt to estimate latency of route discovery, routing overhead …• Actual trade-off depends a lot on traffic and mobility patterns

– Higher traffic diversity (more source-destination pairs) increases overhead in on-demand protocols

– Higher mobility will always increase overhead in all protocols

Issues for MANETs at all OSI Layers

• Application– new applications and adaptations

• Transport– congestion and flow control

• Network– addressing and routing

• Link– media access and handoff

• Physical– transmission errors and interference

Which protocol is “best”?

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A Simulation Comparison of TBRPF, OLSR, and AODV

from SRI

Protocols Simulated• AODV - for ns-2.1b8a

– Available from http://www.cs.ucsb.edu/~eroyer/aodv.html• TBRPF - for ns-2.1b8 (compliant with version 04 draft)

– Available from http://www.erg.sri.com/projects/tbrpf/sourcecode.html– Uses same ‘C’ module as our Linux implementation.

• OLSR - for ns-2.1b7 (compliant with version 03 draft)– Available from http://hipercom.inria.fr/olsr/– Bug affecting packet size was fixed.

• In all protocols, link-layer failure notification was NOT used, and packets could not be retrieved from the link layer (interface queue).

• ARP was bypassed for both TBRPF and OLSR, since MAC addresses can be obtained from received routing packets.

Simulation Model• ns-2 version 2.1b8• WaveLAN IEEE 802.11 MAC with rate 2Mb/s and range 250m• 50 and 100 nodes• Two area sizes: 670m x 670m and 1500m x 300m• Mobility model: No mobility and random waypoint model with 0

pause time and maximum speed 20 m/s.• 20 simultaneous CBR traffic streams:

– Source and destination of each stream are selected randomly – Duration of each stream is 30 seconds– Size of each data packet is 512 bytes– Packet generation rate per stream is increased from 2 to 8 packets/s

• Each simulation was run for 500 simulated seconds.

Performance Measures

• The following measures were plotted for increasing packet generation rates:– percent of data packets delivered– average end-to-end delay– total routing control traffic in kbytes, including

IP/MAC headers (divide by 500 to get kbytes/sec).

– normalized path length (average ratio of actual hops to minimum hops)

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100 nodes, 20 sources, 670x670 area, no mobility

DELAYPERCENTDELIVERED

ROUTINGLOAD PATH

LENGTH

100 nodes, 20 sources, 670x670 area, max speed 20mps

DELAYPERCENTDELIVERED

ROUTINGLOAD

PATHLENGTH


DELAY

PERCENTDELIVERED

ROUTINGLOAD

PATHLENGTH


DELAY

PERCENTDELIVERED

ROUTINGLOAD PATH

LENGTH

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DELAY

PERCENTDELIVERED

ROUTINGLOAD

PATHLENGTH


DELAY

PERCENTDELIVERED

ROUTINGLOAD

PATHLENGTH


PATHLENGTH

DELAY

PERCENTDELIVERED

ROUTINGLOAD


DELAY

PERCENTDELIVERED

ROUTINGLOAD

PATHLENGTH

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Summary of simulation results• In every scenario, TBRPF achieved a higher delivery percentage (up to

15% higher) than OLSR. TBRPF also achieved a higher delivery percentage (up to 15% higher) than AODV in all scenarios with no mobility, and in all scenarios using the square (670x670) area with the lower packet rates (2 and 4 packets/s). For the long rectangular (1500x300) area, AODV achieved a higher delivery percentage (up to 5% higher) than TBRPF.

• In every scenario, TBRPF generated less routing control traffic than the other protocols: up to 60% less than OLSR and up to 48% less than AODV. This is despite the fact that TBRPF sends HELLOs twice as frequently as OLSR.

• In every scenario, TBRPF used the shortest paths (except nearly shortest in some cases with the higher packet rates). In every scenario, AODV used paths that were 12-20% longer on average than TBRPF.

• TBRPF usually had the best or nearly best delay.

Remarks• This study did not use link-layer notification. It is important to also

compare the protocols with link-layer notification, since this allows one to achieve a delivery fraction close to 100%.

• A comparison of TBRPF with AODV using link-layer notification can also be found at the TBRPF web site. That comparison also compares the protocols with 40 sources (which gives TBRPF more advantage).

• Although control traffic is not a quality-of-service measure, a routing protocol that generates less control traffic and uses shorter paths:– uses less energy,– is more likely to scale to a larger number of nodes,– is more likely to perform well in lower bandwidth radios such as those

used by the military.

Protocol parameters used

• TBRPF used the following parameter values:– HELLO_INTERVAL = 1 – NBR_HOLD_COUNT = 3– NBR_HOLD_TIME = 3– HELLO_ACQUIRE_COUNT = 2– HELLO_ACQUIRE_WINDOW = 3– MIN_UPDATE_INTERVAL = 2– PERIODIC_UPDATE_INTERVAL = 6 – TOP_HOLD_TIME = 15– NON_REPORT_PENALTY = 1.01

Protocol parameters used (cont.)

• OLSR used the following parameter values:– HELLO_INTERVAL = 2 – TC_INTERVAL = 5– PACKET_JITTER = HELLO_JITTER = TC_JITTER =

0.5– NEIGHBOR_HOLD_TIME = 6– HELLO_ACQUIRE_COUNT = 2– TOPOLOGY_HOLD_TIME = 16– DUPLICATE_HOLD_TIME = 60– HOLDBACK_TIME = 0

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Discussion

Discussion

• Does the choice of what ad-hoc routing algorithm to use is depend on network layout and traffic characteristics?

• What about power?– What affects power?

• At what layer do we implement MANET?– Link layer, but what do we need to know

about the MAC layer?

Transport Layer Issues in MANETs

User Datagram Protocol (UDP)• Studies comparing different routing protocols for MANET

typically measure UDP performance• Several performance metrics are used

– routing overhead per data packet– packet delivery delay– throughput/loss

• Many variables affect performance – Traffic characteristics– Mobility characteristics– Node capabilities

• Difficult to identify a single scheme that will perform well in all environments

• Several relevant studies [Broch98Mobicom, Das9ic3n, Johansson99Mobicom, Das00Infocom, Jacquet00Inria]

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Transmission Control Protocol (TCP)

• Reliable ordered delivery– Reliability achieved by means of retransmissions if necessary

• End-to-end semantics– Receiver sends cumulative acknowledgements for in-sequence packets– Receiver sends duplicate acknowledgements for out-of-sequence

packets• Implements congestion avoidance and control using sliding-window

– Window size is minimum of• receiver’s advertised window - determined by available buffer space at the

receiver• congestion window - determined by the sender, based on feedback from the

network– Congestion window size bounds the amount of data that can be sent

per round-trip time

Detection of Packet Loss in TCP• Retransmission timeout (RTO)

– sender sets retransmission timer for only one packet– if ACK not received before timer expiry, the packet is

assumed lost– RTO dynamically calculated, doubles on each timeout

• Duplicate ACKS– sender assumes packet loss if it receives three

consecutive duplicate acknowledgements (dupacks)• On detecting a packet loss, TCP sender

assumes that network congestion has occurred and drastically reduces the congestion window

TCP in MANET

• Factors affecting TCP performance in MANET:– Wireless transmission errors

• may cause fast retransmit, which results in– retransmission of lost packet– reduction in congestion window

• reducing congestion window in response to errors is unnecessary

– Multi-hop routes on shared wireless medium• Longer connections are at a disadvantage compared to

shorter connections, because they have to contend for wireless access at each hop

– Route failures due to mobility

Impact of Multi-hop Wireless Paths

• TCP throughput degrades with increase in 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 (>6), throughput stabilizes [Holland99]

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mobility causeslink breakage,resulting in routefailure

TCP data and acksen route discarded

Impact of Node Mobility

TCP sender times out.Starts sending packets again

Route isrepaired

No throughput

No throughputdespite route repair

TCP throughput degrades with increase in mobility but not always

Larger route repairdelays are especially harmful

Improved Throughput with Increased Mobility

Low speed: (Route from A to D is broken for ~1.5 seconds)•When TCP sender times after 1 second, route still broken.•TCP times out after another 2 seconds, and only then resumes

High speed: (Route from A to D is broken for ~0.75 seconds)•When TCP sender times out after 1 second, route is repaired

TCP timeout interval somewhat (not entirely) independent of speedNetwork state at higher speed may sometimes be more favorable than

lower speed

C

B

D

A

C

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D

A

C

B

D

A

Impact of Route Caching• TCP performance typically degrades when caches are

used for route repair• 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, typically 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

Caching and TCP performance

• Caching can result in faster route repair– Faster does not necessarily mean correct– If incorrect repairs occur often enough, caching

performs poorly• If cache accuracy is not high enough, gains in

routing overhead may be offset by loss of TCP performance due to multiple time-outs

• Need mechanisms for determining when cached routes are stale

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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

• Reduction of contention between data and ACKS, and frequency of send-receive turnaround

• Mitigation [Balakrishnan97]– Piggybacking link layer ACKS with data– Sending fewer TCP ACKS - ACK every d-th packet (d may be

chosen dynamically)– ACK filtering - Gateway may drop an older ack in the queue, if a

new ack arrives

TCP Parameters after Route Repair

• 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; let TCP learn the

window size• Retransmission Timeout (RTO) after route repair

– Same as before route break: may be too small for long routes

– Same as TCP start-up: may be too large and respond slowly to packet loss

– new RTO could be made a function of old RTO and route lengths

Improving TCP Throughput• Network feedback

– Network knows best (why packets are lost)– Need to modify transport & network layer to

receive/send feedback– Need mechanisms for information exchange between

layers• Inform TCP of route failure by explicit message• Let TCP know when route is repaired

– Probing– Explicit notification– Better route caching mechanisms

• Reduces repeated TCP timeouts and backoff

END

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