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 Host BS – Base Station MH – Mobile Host FH BS1BS2 MH

Wireless Networks Communication characterized by sporadic high bit-error rates (10 -4 to ) disconnections intermittent connectivity due to handoffs Limited and variable bandwidth

TCP Performance with BER BER = BER = Throughput (pkts/sec) Success Probability Transfer time of 5000 pkts. in secs

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. 1 st connection: a wired connection between fixed host and base station 2 nd 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 EndSplit Connection Link Layer RenoNew Reno Proposed Protocols RLP AIRMAILSnoop SACKFREBSN MTCI-TCPM-TCP WAP Freeze3-dupacksATCP

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