Improving TCP Performance over
Mobile Networks
Zahra Imanimehr
Rahele Salari
Outline
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Problems with TCP
Class of solutions
Review some of the proposals
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TCP Reno
3-dupacks (3DA)
Freeze TCP
ATCP
References
Mobile Networks Topology
FH
BS1
BS2
FH – Fixed Host
BS – Base Station
MH – Mobile Host
MH
MH
Wireless Networks
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Communication characterized by
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sporadic high bit-error rates (10-4 to 10-6)
disconnections
intermittent connectivity due to handoffs
Limited and variable bandwidth
TCP Performance with BER
BER = 10-5 BER = 10-6
Throughput
(pkts/sec)
39.439
87.455
Success
Probability
0.9892
0.999
Transfer time of
5000 pkts. in
secs.
123.847
58.032
The congestion control in regular TCP
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Assume congestion to be the primary cause
for packet losses and unusual delays
Invoke congestion control and avoidance
algorithms, resulting in significant degraded
end-to-end performance and very high
interactive delays.
The congestion control Algorithm
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Slow start
Congestion avoidance
Fast Retransmit
Classification of Schemes
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End-to-End protocols
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loss recovery handled by sender
Link-layer protocols
Split-connection protocols
Link-Layer Protocols
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Hides the characteristics of the wireless link
from the transport layer and tries to solve
the problem at the link layer
Uses technique like forward error correction
(FEC)
Snoop, AIRMAIL(Asymmetric Reliable
Mobile Access In Link-layer)
Link-layer Protocols (cont.)
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Advantages:
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The wireless link is made more reliable
Doesn’t change the semantics of TCP
Fits naturally into the layered structure of
network protocols
Disadvantage:
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If the wireless link is very lossy, sender timesout waiting for ACK, and congestion control
algorithm starts
Split Connection Protocols
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Split the TCP connection into two separate
connections.
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1st connection: a wired connection between
fixed host and base station
2nd connection: a wireless connection between
base station and mobile host
Split Connection
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Advantages:
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Isolate mobility and wireless related problems
from the existing network protocols
Better throughput can be achieved by fine
tuning the wireless protocol link.
Disadvantages:
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Violates the semantics of TCP.
Extra copying at the Base station.
Proposed Protocols
Link Layer
RLP
EBSN
AIRMAIL
FR
End to End
Snoop
New Reno
Split Connection
MTC
SACK
Reno
Freeze
I-TCP
M-TCP
3-dupacks
WAP
ATCP
TCP Reno
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TCP Reno is like regular TCP except it includes
fast recovery
Fast recovery:
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Reduces the value of the congestion window (cwnd) by
half
Increments cwnd by one for each duplicate
acknowledgement received
When a “new” ACK is received, the sender exits fast
recovery, sets cwnd to ssthresh and enters
the congestion avoidance phase
3-dupacks
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After disconnection and upon subsequent
reconnection, the MH sends three duplicate
acknowledgements to the fixed host
These dupacks cause the TCP sender at the
FH immediately enter the fast recovery
phase
Freeze TCP
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Upon receipt of an indication of impending
disconnection, Freeze TCP at the MH sends
a zero window advertisement to the FH
Upon reconnection, it uses 3DA to restart
transmission
ATCP
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Unlike earlier work, ATCP improve the
performance when the TCP sender is FH or
MH
ATCP assumes that the network layer sends
connection event signal and disconnection
event signal to TCP
ATCP (cont.)
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MH to FH data transfer:
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Upon disconnection event:
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If sending window is open, ATCP does not
wait for packet sent before disconnection and cancel
the retransmission timer (RTX).
If sending window is closed and it was waiting for
ACKs, ATCP does not cancel RTX but waits for the
occurrence of an RTO event.
ATCP (cont.)
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Upon connection event
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If the sending window is open, ATCP sends data and sets a new
RTX
If the sending window is closed and RTO has occurred, ATCP
retransmits
If RTO has not occurred, ATCP waits for an RTO
Upon RTO event
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If a disconnection has occurred, ATCP sets ssthresh to the
value of cwnd at the time of disconnection and sets cwnd to
one
If MH is connected, ATCP retransmits lost packet
ATCP (cont.)
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FH to MH data transfer:
ATCP delays the ACK for the last two bytes
by d milliseconds.
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Upon disconnection event:
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the network connectivity status is updated
Upon disconnection event:
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ATCP ACKs the first of these bytes with zero
window advertisement and ACKs the second byte
with the full window
advertisement
Comparison
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MH to FH transfer:
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A percentage improvement of up to 40% is
observed for short RTT connections over TCP
Reno
An improvement of up to 150% is observed for
long RTT connections over TCP Reno
Comparison (cont.)
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FH to MH transfer:
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ATCP shows uniform improvement in throughput over
TCP Reno and 3DA
In WLAN environment, the performance of ATCP is
very close to that of Freeze TCP
In WWAN environment, the performance of ATCP is
very close to Freeze TCP for small disconnection
intervals but for longer disconnection intervals Freeze
TCP work better than ATCP