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