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ATP: A Reliable Transport Protocol for Ad-hoc Networks

Sundaresan, Anantharam, Hseih, Sivakumar

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TCP in MANET

TCP performance degrades significantly in MANET– TCP assumes the packet losses as an indication of network

congestion.– But link failures due to mobility are the primary reason for

most of packet losses. Some approaches as TCP variation to alleviate the

performance degradation– Use cross layer notifications to report route failure– TCP-ELFN: Explicit Link Failure Notification– TCP-EPLN&BEAD

The authors have more to say:– TCP mechanisms are fundamentally inappropriate for ad-hoc

networks

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Outline

TCP in mobile ad-hoc networks and drawbacks

ATP (Ad-hoc Transport Protocol) Performance evaluation

– TCP, TCP-ELFN, ATP Conclusions

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Reliable Transportation in TCP (1)

Window based transmission Slow-start Loss based congestion detection Linear increase multiplicative decrease Dependence on Acks

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Reliable Transportation in TCP (2)

Window Based Transmissions– Flow control: rwind

How many more packets can the receiver handle?– Congestion control: cwind

How many more packets can the network handle?– Sending rate is determined by min( rwind, cwind )

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Window Based Transmissions - Burstiness

Leads to burstiness Bunch of ACKs arrive together is quite common in

MANET– Short-term unfairness of the CSMA/CA mechanism– Burstiness: bunch of ACKs trigger bunch of packets in a

very short period. Why is burstiness bad?

– Varying round-trip time estimates cause RTO inflation Potentially leads to delayed loss recovery

– Bursty transmissions can result in higher contention at MAC layer; pose a problem especially under the heavy-load conditions

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Burstiness in RTT measurements

The value of RTT changes dramatically.

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Slow Start

Exponential growth to available capacity– It takes several RTT periods to reach available bandwidth

Slow start is not a serious problem for wireline network – associations are expected in the congestion avoidance phase for

most of the time. Not good for wireless ad hoc network

– Due to the dynamic nature of ad hoc networkfrequent packet losses → frequent timeouts → more slow start

phasesAgain, packet loss does not necessarily mean congestion. – During the lifetime of an association, considerable amount of

time is spent in slow start phase– Under-utilization of network resources

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Time spent in Slow-Start phase

Average time in Slow-Start phase.Total simulation time: 100sTCP New Reno

1. The time spent in Slow-Start increases with the increasing of the mobility.

2. The proportion of time goes above 50% for the higher load situation.

3. The connections spend a large portion of the lifetime probing for the available bandwidth.

50%

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Loss Based Congestion Indication

TCP detects congestion through the occurrence of losses– Either three duplicate ACKs or a timeout– Congestion is by far the main source of packet

losses in wireline network.

Losses in ad-hoc networks can occur due to either congestion or route failures

Loss on wireless links means try harder, loss on wired means backoff

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Losses due to route failure

Significant portion of losses “perceived” to be due to route failures

50%

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Multiplicative Decrease

Multiplicative decrease on congestion window when TCP detects congestion

In MANET, losses can happen by route failure– A new route might be used instead– Slow start to reach available bandwidth

TCP-ELFN freezes the TCP sender while a new route is being calculated, but still uses the old congestion windows state after freezing

– Congestion window state for previous route is not appropriate for the new route

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Dependence on ACKs

TCP relies on ACK very much– The acknowledgement of the correct receiving.– The progression of its congestion window

Acks can amount to 10-20% of data stream rate

Large volume of ACKs introduces more contention in MAC layer if the same path is used as reverse path.

Large volume of ACKs increases the probability of experiencing route failures (loss of ACKs) if different path is used.

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Outline

TCP in mobile ad-hoc networks and drawbacks

ATP (Ad-hoc Transport Protocol) Performance evaluations

– TCP, TCP-ELFN, ATP Conclusions

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Key design elements of ATP

Cross layer coordination

Rate based transmissions– This is the core of ATP

Decoupled congestion control and reliability

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Layer Coordination

Similar to TCP-ELFN

– Utilize explicit feedback from intermediate nodes.

ATP uses layer coordination for – Path failure notification– Initiating a sending-rate estimation for the new

route

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Rate based transmissions

What is rate based transmission– Transmit fixed size of data in each time interval.

GSM example, 260bits from speech codec in every 20ms– Use timer to clock the new data, not the sending window

Avoids drawbacks due to burstiness

The need for self-clocking by the arrival of ACKs is eliminated

– Allows decoupling of congestion control mechanism from the reliability mechanism

Timer granularity in low bandwidth MANETS large enough to be realized without significant overheads

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Decoupling of Congestion Control & Reliability

For congestion control:– Intermediate nodes provide the feedback of

available rate.– The feedback is piggybacked on forward path and

sent back from receiver to sender.– The sender adjusts the sending rate accordingly.

For reliability:– The receiver uses SACK to report any new holes

in the data stream.– Only use SACK, no cumulative ACK

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Detailed ATP

ATP Intermediate Node ATP Receiver ATP Sender

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ATP Intermediate Node (1)

Two parameters are measured– Qt , the average queuing delay per packet at the node itself– Tt , the average transmission delay at the node’s transmitter– Qt and Tt are maintained on a per-node basis– Update after every instantaneous measurement

– D = Qt +Tt , the total delay at current node, 1/D is the rate that the current node can handle.

(1 )

(1 )t t sample

t t sample

Q Q Q

T T T

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ATP Intermediate Node (2)

ATP header has an additional field other than TCP header: the rate feedback D

Update the D in each outgoing packet

Source Destination

Node i-1

Node i

Node i+1

Di-1

Di

1

1 1

( ) ( )( )

i t t i t t i i

i i t t i i

D Q T if Q T DD D if Q T D

Max Delay

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ATP Receiver (1)

The receiver provides periodic feedback to the sender for reliability and congestion control purpose

– Feedback is triggered by an epoch timer of period E Congestion & Flow Control feedback

– Calculate the average delay for one flow

– Application reading rate Rapp --- flow control– Send rate feedback to the sender

(1 )avg avgD D D

1/1/avg avg app

app

D if D RR otherwise

The value of feedback

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ATP Receiver (2)

Reliability feedback– Selective ACKs– Send out periodically to the sender– Contain at most 20 SACK blocks in every

feedback packet (report 20 holes in the received data stream).

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ATP Sender (1)

Quick Start– Send a probe packet to the receiver in order to

get back the initial sending rate– After rate feedback comes back, the sender

adjusts the sending rate accordingly.– The sender reaches available rate after 1 RTT.– Quick-start is performed both during

Associate establishment New path takes place of the original path due to link

failures.

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ATP Sender (2)

Congestion Control– Three phases: (rate) increase phase, decrease

phase, maintain phase– Update the sending rate S

R SS S if S R Sk

S R else if S Runchange otherwise

R : the new feedback rate; Φ : a small constant used to prevent fluctuations;k : a scale to reduce the increasing amount since increasing 1 pkt per sec in upper layer can introduce more control pkts in MAC layer.

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ATP Sender (3)

In case of the loss of feedback packets– Multiplicative decrease of sending rate if feedback

does not appear in every epoch time– If no feedback till the end of the third epoch, send

a probe packet to the receiver

Reliability: Process the SACK information– Mark the packets for retransmission accordingly.– Data for retransmission have higher preference

than new data.

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Performance Evaluation

Simulation environment– ns2 simulator– 1000m*1000m square, 100 nodes– Random way point mobility with 3 speeds

1 m/s, 10 m/s, 20 m/s– DSR routing– IEEE802.11b MAC– Network load: 1 flow, 5 flows and 25 flows, resp.– Packet size: 512 bytes– Epoch time: 1 sec

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Congestion window/rate progression vs. time (1 flow)

Default TCP TCP-ELFN

ATP

Route failure

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Congestion window/rate progression vs. time (25 flow)

Default TCPTCP-ELFN

ATP

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Reasoning

ATP does not decrease its rate on route failures unless dicated by the rate feedback mechanism

Owing to quick-start, it quickly catches upto the available bandwidth

Once it reaches avalable capacity, it maintains a steady rate

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Throughput vs. Mobility

5 flows 25 flows

ATP

It would be interesting to see the comparison between ATP and TCP-EPLN&BEAD.

TCP defaultTCP ELFN

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Conclusions

TCP not appropriate for MANETs

ATP looks promising – Rate based transmissions– Quick start– Decoupling of congestion control and reliability– Layer coordination– Avoid use of retransmission timeouts

What about VANETs?