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Improving TCP Performance over Mobile Networks
Zahra Imanimehr
Rahele Salari
Outline
Problems with TCP Class of solutions Review some of the proposals
TCP Reno 3-dupacks (3DA) Freeze TCP ATCP
References
Mobile Networks Topology
FH – Fixed HostBS – Base StationMH – Mobile Host
FH
BS1 BS2
MH MH
Wireless Networks
Communication characterized by 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
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
Slow start Congestion avoidance Fast Retransmit
Classification of Schemes
End-to-End protocols loss recovery handled by sender
Link-layer protocols Split-connection protocols
Link-Layer Protocols
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.)
Advantages: The wireless link is made more reliable Doesn’t change the semantics of TCP Fits naturally into the layered structure of
network protocols Disadvantage:
If the wireless link is very lossy, sender times-out waiting for ACK, and congestion control algorithm starts
Split Connection Protocols
Split the TCP connection into two separate connections. 1st connection: a wired connection between
fixed host and base station 2nd connection: a wireless connection between
base station and mobile host
Split Connection
Advantages: Isolate mobility and wireless related problems
from the existing network protocols Better throughput can be achieved by fine
tuning the wireless protocol link. Disadvantages:
Violates the semantics of TCP. Extra copying at the Base station.
End to End Split ConnectionLink Layer
RenoNew Reno
Proposed Protocols
RLP AIRMAIL Snoop
SACKFREBSN
MTC I-TCP M-TCP WAP
Freeze 3-dupacks ATCP
TCP Reno TCP Reno is like regular TCP except it includes
fast recovery Fast recovery:
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
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
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
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.)
MH to FH data transfer: Upon disconnection event:
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.) Upon connection event
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
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.)
FH to MH data transfer: ATCP delays the ACK for the last two bytes
by d milliseconds. Upon disconnection event:
the network connectivity status is updated Upon disconnection event:
ATCP ACKs the first of these bytes with zero window advertisement and ACKs the second byte with the full window
advertisement
Comparison
MH to FH transfer: 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.)
FH to MH transfer: 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