ATP: A Reliable Transport Protocol for
Ad-hoc Networks
Sundaresan, Anantharam,
Hseih, Sivakumar
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TCP in MANET
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TCP performance degrades significantly in MANET
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Some approaches as TCP variation to alleviate the
performance degradation
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Use cross layer notifications to report route failure
TCP-ELFN: Explicit Link Failure Notification
TCP-EPLN&BEAD
The authors have more to say:
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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.
TCP mechanisms are fundamentally inappropriate for adhoc networks
Outline
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TCP in mobile ad-hoc networks and
drawbacks
ATP (Ad-hoc Transport Protocol)
Performance evaluation
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TCP, TCP-ELFN, ATP
Conclusions
Reliable Transportation in TCP (1)
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Window based transmission
Slow-start
Loss based congestion detection
Linear increase multiplicative decrease
Dependence on Acks
Reliable Transportation in TCP (2)
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Window Based Transmissions
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Flow control: rwind
How many more packets can the receiver handle?
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Congestion control: cwind
How many more packets can the network handle?
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Sending rate is determined by min( rwind, cwind )
Window Based Transmissions Burstiness
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Leads to burstiness
Bunch of ACKs arrive together is quite common in
MANET
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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?
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Varying round-trip time estimates cause RTO inflation
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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
Burstiness in RTT measurements
The value of RTT changes dramatically.
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Slow Start
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Exponential growth to available capacity
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Slow start is not a serious problem for wireline network
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It takes several RTT periods to reach available bandwidth
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 network
frequent packet losses → frequent timeouts → more slow start
phases
Again, 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
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
50%
situation.
Average time in Slow-Start phase.
Total simulation time: 100s
TCP New Reno
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3. The connections spend a
large portion of the lifetime
probing for the available
bandwidth.
Loss Based Congestion Indication
 TCP detects congestion through the
occurrence of losses
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Either three duplicate ACKs or a timeout
Congestion is by far the main source of packet
losses in wireline network.
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Losses in ad-hoc networks can occur due to
either congestion or route failures
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Loss on wireless links means try harder, loss
on wired means backoff
Losses due to route failure
50%
Significant portion of losses “perceived” to be due to route failures
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Multiplicative Decrease
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Multiplicative decrease on congestion window when
TCP detects congestion
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In MANET, losses can happen by route failure
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TCP-ELFN freezes the TCP sender while a new
route is being calculated, but still uses the old
congestion windows state after freezing
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A new route might be used instead
Slow start to reach available bandwidth
Congestion window state for previous route is not
appropriate for the new route
Dependence on ACKs
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TCP relies on ACK very much
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The acknowledgement of the correct receiving.
The progression of its congestion window
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Acks can amount to 10-20% of data stream rate
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Large volume of ACKs introduces more contention in
MAC layer if the same path is used as reverse path.
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Large volume of ACKs increases the probability of
experiencing route failures (loss of ACKs) if different
path is used.
Outline
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TCP in mobile ad-hoc networks and
drawbacks
ATP (Ad-hoc Transport Protocol)
Performance evaluations
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TCP, TCP-ELFN, ATP
Conclusions
Key design elements of ATP
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Cross layer coordination
Rate based transmissions
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This is the core of ATP
Decoupled congestion control and reliability
Layer Coordination
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Similar to TCP-ELFN
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ATP uses layer coordination for
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Utilize explicit feedback from intermediate nodes.
Path failure notification
Initiating a sending-rate estimation for the new
route
Rate based transmissions
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What is rate based transmission
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Transmit fixed size of data in each time interval.
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Use timer to clock the new data, not the sending window
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Avoids drawbacks due to burstiness
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The need for self-clocking by the arrival of ACKs is
eliminated
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GSM example, 260bits from speech codec in every 20ms
Allows decoupling of congestion control mechanism from the
reliability mechanism
Timer granularity in low bandwidth MANETS large
enough to be realized without significant overheads
Decoupling of Congestion Control
& Reliability
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For congestion control:
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For reliability:
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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.
The receiver uses SACK to report any new holes
in the data stream.
Only use SACK, no cumulative ACK
Detailed ATP
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ATP Intermediate Node
ATP Receiver
ATP Sender
ATP Intermediate Node (1)
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Two parameters are measured
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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
Qt    Qt  (1   )  Qsample
Tt    Tt  (1   )  Tsample
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D = Qt +Tt , the total delay at current node, 1/D is the rate
that the current node can handle.
ATP Intermediate Node (2)
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ATP header has an additional field other than TCP
header: the rate feedback D
Update the D in each outgoing packet
Di
Source
Di-1
Node i
Destination
Node i+1
Node i-1
 Di  (Qt  Tt )i if (Qt  Tt )i  Di 1

if (Qt  Tt )i  Di 1
 Di  Di 1
Max Delay
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ATP Receiver (1)
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The receiver provides periodic feedback to the
sender for reliability and congestion control purpose
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Feedback is triggered by an epoch timer of period E
Congestion & Flow Control feedback
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Calculate the average delay for one flow
Davg    Davg  (1   )  D
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Application reading rate Rapp --- flow control
Send rate feedback to the sender
 Davg if Davg  1/ Rapp
The value of feedback 
otherwise
 1/ Rapp
ATP Receiver (2)
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Reliability feedback
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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).
ATP Sender (1)
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Quick Start
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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
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Associate establishment
New path takes place of the original path due to link
failures.
ATP Sender (2)
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Congestion Control
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Three phases: (rate) increase phase, decrease
phase, maintain phase
Update the sending rate S
RS
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S  S  k

 SR
 unchange

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if S  R    S
else if S  R
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.
ATP Sender (3)
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In case of the loss of feedback packets
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Reliability: Process the SACK information
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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
Mark the packets for retransmission accordingly.
Data for retransmission have higher preference
than new data.
Performance Evaluation
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Simulation environment
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ns2 simulator
1000m*1000m square, 100 nodes
Random way point mobility with 3 speeds
1 m/s,
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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
Congestion window/rate progression
vs. time (1 flow)
Route
failure
Default TCP
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TCP-ELFN
ATP
Congestion window/rate progression
vs. time (25 flow)
Default TCP
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TCP-ELFN
ATP
Reasoning
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ATP does not decrease its rate on route failures
unless dicated by the rate feedback mechanism
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Owing to quick-start, it quickly catches upto the
available bandwidth
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Once it reaches avalable capacity, it maintains a
steady rate
Throughput vs. Mobility
ATP
TCP ELFN
TCP default
5 flows
25 flows
It would be interesting to see the comparison between ATP and TCPEPLN&BEAD.
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Conclusions
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TCP not appropriate for MANETs
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ATP looks promising
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Rate based transmissions
Quick start
Decoupling of congestion control and reliability
Layer coordination
Avoid use of retransmission timeouts
What about VANETs?