tcp over adhoc networks

8/12/2019 Tcp Over Adhoc Networks

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TCP Over Wireless Ad-hocNetworks


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Vinay Sridhara

[email protected]

Nagendra Subramanya

[email protected]


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In wired LANs, an address is equivalent to a

physical location. This is implicitly assumed in the

design of the wired LANs. In wireless, the

addressable unit is the station. The station is a

message destination, but not a fixed location.

Use a medium that has neither absolute nor readily

observable boundaries outside of which stations

with conformant physical transceivers are known

to be unable to receive network frames

Wired Vs Wireless


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Continued..

They are unprotected from outside signals . Have dynamic topologies.

They lack full connectivity and therefore the

assumption that one station can hear every otherstation is invalid.

They have time varying and asymmetric

propagation properties.


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802.11 Main purpose is to provide the MAC and physical

layer specification for wireless. Permits the operation of an IEEE 802.11

conformant device within a wireless local area

network that may coexist with multipleoverlapping IEEE 802.11 conformant devices.

Describes the requirements and procedures toprovide privacy of user information being

transferred over the wireless medium andauthentication of IEEE 802.11 conformantdevices.


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802.11 in ISO/OSI Model.


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A few terminologies of 802.11

STA Any mobile, portableor stationary terminal.

BSS Basic service set.

The BSS is an entity which

consists of several mobile

stations that can interact with

each other.

STA1

STA2

STA1

STA2

BSS1

BSS2

802.11 Components


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Continued. A BSS maybe comprised of only two mobile

stations.

This is called a IBSS (Independent Basic Service

Set).

This is the one that is called Ad Hoc Networks.

A network composed solely of stations within mutual

communication range of each other via the wireless

medium

network is typically created in a spontaneous manner The principal distinguishing char of an Ad Hoc network

is its limited temporal and spatial extent


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Continued. Instead of existing

independently,

multiple BSSs mayform a network.

The architectural

component used tointerconnect BSSs isthe Distributionsystem (DS).

The access points arethe stations used toconnect BSSs to DS.


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Continued. BSSs may generally

overlap to provide

continuous coveragein physical volume.

The BSSs could be

physically disjoint.

Logically there is nolimit to the distance

between BSSs.

STA1 STA-C STA1

STA1 STA2

STA2STA1


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Continued.

ESS - The DS and the

BSSs allow the IEEE

802.11 to create a wirelessnetwork of arbitrary size

and complexity.

Stations within an ESS

may communicate and

Mobile stations may move

from one BSS to anothertransparently to LLC.


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Extended Service Set

The stations communicate to Access pointswhich are part of a Distribution System

Access point serves the stations in a BSS

The set of BSSs are called Extended Service

Set (ESS)


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Communication with external world

Portal Is the logicalpoint at which the MSDUs

enter and leave the IEEE802.11 network.

This portal provideslogical integrationbetween the IEEE 802.11architecture and theexisting wired LANs

The stations act as bothAP and portal when theDS is a wired network.


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The Big Picture


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Services

The services provided by the 802.11 are dividedinto two.

Those that are part of every STA, called Station

services Authentication

De-Authentication

Privacy

MSDU Delivery

The services provided by the DS are known as the

Distribution system service.

Association

Disassociation

Distribution

Integration

Reassociation


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MAC layer Services Services provided by the MAC layer

Security Services Security is provided by the WEP (Wired equivalent privacy)

The security provided is limited to Station-to-Station exchangeof data

The privacy service offered by the IEEE 802.11 WEP is theencryption of the MSDU

The security services provided are Confidentiality

Authentication

Access control in conjunction with layer management.


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Continued.

Asynchronous data services

Provided by the following three primitives.

MA-UNITDATA.request generated by the LLC when anMSDU is to transmitted to the peer LLC. The format is asshow below.

MA-UNITDATA.request{

Source address,

Destination address,

Routing information,Data,

Priority,

Service class

}


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MA-UNITDATA.indication generated by the MAC sublayerentity to indicate to the LLC the arrival of a MSDU.

MA-UNITDATA.indication

{

Source address,


Routing information,

Data,

Reception status,

Priority,

Service class

}


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MA-UNITDATA-STATUS.indication - provides the LLC

sublayer with status information of the corresponding MA-

UNITDATA.request primitive.

MA-UNITDATA-STATUS.indication

{

Source address,


Transmission status,

Provided priority,Provided service class

}


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MAC header

The four address fields in the MAC header are used to

indicate the BSSID, Source address, destination address,transmitting station address and the receiving stationaddress.

The frame control field has the following format


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Collision Avoidance - 802.11 802.11 uses a protocol scheme know as carrier-sense,

multiple access, collision avoidance (CSMA/CA).

Avoidance scheme is used because it is difficult to detect

collisions in the RF media.

Hence is interoperates with the physical layer by sampling

the transmitted energy over the medium transmitting data. The physical layer uses an algorithm for clear channel

assessment (CCA) to determine if the channel is clear.

If the received signal is below a certain threshold the

channel is declared clear.


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Why Do We Need RTS/CTS ? Used because of the Hidden terminal problem.

It occurs when there is a station in a service setthat cannot detect the transmission of another

station, and thus cannot detect that the media is

busy

Station C

Station A

Station B


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What Does RTS/CTS Do?

Communications is established when one of thewireless nodes sends a short message RTS frame.

The message duration is known as the networkallocation vector (NAV). This alerts to back offduring the transmission time.

The receiving station issues a CTS frame whichechoes the senders address and the NAV.

If the CTS frame is not received, it is assumed thata collision occurred and the RTS process startsover.


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Characteristics of Wireless Networks

Limited bandwidth

High latencies and bit-error rates

Mobility


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Difference between General wireless

networks and Ad-Hoc Networks ?

Absence of a central base station

Frequent route re-computation

Network partitions

Multi-path routing algorithms


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What is an Ad-Hoc Network?

AD-HOC network is a collection of mobile nodes with wireless

network infrastructure capable of organizing themselves into a

temporary network without the aid of any centralized network

manager.

According to the IETF definition, a mobile Ad Hoc network is an

autonomous system of mobile routers (and associated hosts) connected

by wireless links--the union of which form an arbitrary graph.


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AD-HOC Networks

SOURCE

DESTINATION


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Implications of using NormalTCP over Ad-Hoc Wireless N/W

TCP does not distinguish between congestion andpacket loss due to transmission errors and route

failures

This inability results in performance degradation

in ad hoc networks


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Implications of using NormalTCP over Ad-Hoc Wireless N/W

Route re-computation takes a finite amount of time

During this time no packet can reach the destination

through the existing route

Packets and ACKs may get queued and possibly dropped

In turn leads to timeouts at the source, which ismisinterpreted as congestion


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Consequences

Source retransmits unACKed packets

Invokes congestion control

Enters slow start recovery

These are undesirable because?

Why retransmit when there is no route

Retransmission wastes power and bandwidth

Low throughput as a result of slow start recovery after route restoration

(this is actually desirable. Why?)


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Why use TCP at all in such cases ?

For seamless portability to applications like file

transfer, e-mail and browsers which use standard

TCP


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Indirect TCP (I-TCP)

Connection between end points split into separate connections

+ No need of changes in TCP on wired hosts

+ Transmission errors on wireless link not propagated to wired network

+ Different optimal transport layer protocol between BS and MH

- Loss of E2E semantics of TCP

- Increased hand-off latency

- Copying overhead


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Snoop TCP

BS caches packets from senderPerforms local re-transmissions

+ Preserves E2E TCP semantics

+ Only BS needs to be changed

- Doesnt isolate behavior of wireless link as good as I-TCP

- MH may need to be modified to accept NACKs

- Snooping and caching may fail if E2E encryption schemes are used


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M-TCP

Same as I-TCP, but the BS is known as SH

SH holds back ACK to the last byte

Uses this ACK to freeze the sender when disconnection is detected

+ Maintains E2E semantics

+ Avoids useless re-transmissions and slow-starts

+ SH doesnt buffer data

- Needs a bandwidth manager to implement fair sharing over the wireless link


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Freeze-TCP

The client freezes the sender when it detects possible hand-

off or predicts a temporary disconnection

+ No need of base station

+ Only mobile clients TCP needs to be changed

+ Can be used with encrypted data

- MAC has to detect future interruptions

- Freezing fails when encryption scheme uses timestamps

- Re-transmission restarted with old CWND


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Network state after a while

SOURCE

DESTINATION

X


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TCP-Feedback (TCP-F)

K. Chandran, S. Raghunathan, S. Venkatesan and R.

Prakash


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TCP-F: Introduction

ECN is not used. Why?

Instead uses Route Failure Notification (RFN)

packet to inform source when route is disrupted

And Route Reestablishment Notification (RRN)

packet informs the source when route isreestablished


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TCP-F: Protocol

Failure Point (intermediate node) detects the routedisruption

Explicitly sends RFN to source and records theevent

Each intermediate node that receives the RFN:

Invalidates the particular route

If it knows of an alternate route, that route is used forfurther communication and RFN is discarded

Else, RFN is propagated towards source


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TCP-F: Protocol (contd.)

On receiving RFN, source goes into snooze state


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Source when in snooze state

Stops sending further packets

All existing timers are marked invalid

Send window and other state variables (RTO, etc) are frozen

Starts a route failure timer whose timeout = worst case route

reestablishment time. Why?

Stays in this state till it receives an RRN packet


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One of the intermediate nodes that had previously

forwarded RFN learns about a new route Sends an RRN to the source

Further RRNs received by this node for the same source-

destination pair are discarded

Other nodes simply forward RRN towards the source

Source on receiving RRN Changes to active state

Flushes out all unACKed packets in its current window


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TCP-F: Conclusion

Communication resumes at the same rate as before theroute failure occurred There is no unnecessary loss of throughput

Is this ok?

Use of an additional packet RRN. Is it really needed?

Simulation environment

Based on a simple one-hop network Links failed/recovered according to an exponential model

Routing protocol was not simulated


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TCP-F: Conclusion

Overhead on routers

Detect route failures and reestablishments

Provide feedback to the source

Store the source id after forwarding an RFN so that it

can send an RRN on finding a route

The paper does not discuss about multiple flows

Enhancements

Buffering at intermediate nodes


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TCP with Explicit Link FailureNotification (ELFN)

G. Holland and N.G. Holland and N. VaidyaVaidya


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ELFN: Introduction

DSR complicates the situation stale routes may be

cached and propagated

Throughput is degraded

Turning of caching works only with single TCP connection

What if there are more sources?


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ELFN

Similar to TCP-F

Expected throughput throughput of a static wireless

network

ELFN is piggy-backed to DSRs route failure message

ELFN has a payload similar to that of host unreachableICMP message


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ELFN: Protocol

TCP sender receives ELFN

Disables congestion control mechanism, enters stand-by

mode

Sends probes

On receiving an ACK for a probe, leaves stand-by mode


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ELFN: Variations

Time interval between probe packets

greater it is, slower is the route discovery

smaller it is, causes congestion

best would be to have it as a function of RTT


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ELFN: Variations

Adjusting CWND and/or RTO after route restoration no changes

CWND = 1

RTO = default initial value and CWND = 1

Resetting RTO degrades throughput due to frequent routebreaks and ARPs tendency to silently drop packets

CWND = 1 caused no significant change, because optimalwindow is relatively small Things have improved now bandwidth/delay products are larger


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ELFN: Variations

Choice of probe packet First packet in congestion window

Lowest sequence numbered packet that was lost

indicated by ELFN

If forward and reverse routes are different , both

approaches are same


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ELFN: Limitations

Base TCP outperforms TCP+ELFN on static networks

Why? Slow-start results in high network buffer utilization

Causes large RTT => large RTOs => long timeouts =>badthroughput

Abrupt reduction of CWND to 1 => several RTTs before sender

reaches avoidance phase

CWND worth of data lost due to route failures. What if smallernetwork buffers are used?


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ELFN: Limitations

Channel characteristics of ad hoc networks Available bandwidth is greatly reduced

Packets from same flow contend with each other

Every node gets a share of the channel based on the fairnessproperties of the MAC layer

In static networks, channel contention results in frequentfalse route error messages

Time wasted in computing new route => throughputdegrades


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ELFN: Limitations

Even a single flow can create enough contention

Contention among data packets

Contention caused by ACK packets

MAC contention algorithm

TCPs saw-tooth behavior during congestion avoidance

In mobile scenario, there will be lot of high priority routingpackets in the network resulting in congestion


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ELFN: Limitations

Large number of routing packets force TCP packets to

reside in network buffers for long intervals

Data packets can get delayed or dropped due to change in

network configuration.

Queue management is thus very critical


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ELFN: Limitation Fairness Issues

One flow reduces its CWND, allowing another to maintain

a larger window and send more packets

Short range flows have fewer timeouts and a larger CWNDcausing them to contend more aggressively.

Both ELFN and plain TCP result in unfair networkresource distribution due to congestion at intermediate

hops.

Rate control mechanisms are more suited for ad hocnetworks.


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ELFN: Rate Control Methods

End-to-end rate control

could depend on network feedback

leads to large buffer buildups

longer flows reduce their sending rate when congestion is observedin any part of the path

takes a few RTTs to adjust the outgoing rate

intermediate nodes dont store state of flows


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ELFN: Rate Control Methods

Hop-by-hop rate control

each node individually controls the outgoing rates of all flows

prevents buffers from getting large

congestion causes only the immediate hop to reduce sending rate

more responsive to changes in connectivity , congestion or number of

contending nodes

can reduce number of packets stored at a disconnected node in a staleroute


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ATCP TCP for Mobile Ad HocNetworks

J. Liu and S. Singh


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ATCP TCP for Ad Hoc Networks

Thin layer between TCP and IP

Treats loss due to congestion and medium differently

Functions efficiently even with high bit error rates

Handles network partition gracefully

Maintains end-to-end TCP semantics ?


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Modes of Operation

ATCP

Normal

Congested

Loss

Disconnected

TCP Depending on its mode of operation ATCP sets TCP in one of the

following modes

Normal

Congested

Persist


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Comparison with ELFN andTCP-F

Old CWND usedOne of the variations

suggests changing CWND

to 1

Reset for each new routeCWND

Not handledNot handledSender invokes

congestion control on

receiving an ECN

Congestion

Not handledNot handledATCP reorders packets so

that TCP does not

generate duplicates

Packet reordering

TCP-F freezes sender

state

ELFN freezes sender stateICMP Destination

Unreachable puts sender

into persist mode

Network Partition and

Route Changes

Not handledNot handledATCP retransmits, TCP

does not invoke

congestion control

Packet loss due to high bit

error rate

TCP-FELFNATCPEvent


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Some Issues

ECNs may not reach the sender

Sender keeps re-transmitting, congesting the network

further

When would this happen?

Possible solution


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Issues

RTO generally indicates congestion more than

packet loss due to high BER

Why?


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References

IEEE 802.11 Specifications

H. Balakrishnan, V. N. Padmanabhan, S. Seshan and R. Katz, A Comparison

of Mechanisms for Improving TCP Performance over Wireless Links,

IEEE/ACM Transactions on Networking, Vol. 5, No. 6, December 1997.

K. Brown and S. Singh, M-TCP: TCP for Mobile Cellular Networks, ACM

Computer Communications Review, 1997.

T. Goff et al., Freeze-TCP: A True End-to-End TCP Enhancement mechanism

for Mobile environments, Proc. IEEE INFOCOM, 2000.


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References

K. Chandran, S. Raghunathan, S. Venkatesan and R. Prakash, A Feedback-

Based Scheme for Improving TCP Performance in Ad Hoc WirelessNetworks, Proc. IEEE ICDCS, Amsterdam, The Netherlands, May 1998.

G. Holland and Nitin H. Vaidya, Analysis of TCP Performance over Mobile

Ad Hoc Networks, Proc. IEEE MOBICOM, Seattle, Aug 1999.

J. Monks, P. Sinha and V. Bharghavan, Limitations of TCP-ELFN for Ad

hoc Networks, MOMUC, Tokyo, Japan, October 2000.

J. Liu, S. Singh, ATCP: TCP for mobile Ad Hoc networks, IEEE Journal ofSelected Areas in Communications, July 2001.


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Thank you

tcp over adhoc networks

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