2005/12/14 1 Improving TCP Performance over Mobile Ad Hoc Networks by Exploiting Cross-Layer Information Awareness Xin Yu Department of Computer Science Courant Institute of Mathematical Sciences New York University, New York, NY

2005/12/14 2 Outline Introduction(3) Background(5) How mobility affect TCP(5) EPLN and BEAD(6) PERFORMANCE EVALUATION(4) CONCLUSIONS(1)

2005/12/14 3 Introduction(1/3) TCP performance degrades significantly in mobile ad hoc networks. Route failures due to mobility are the primary reason for most of packet losses. TCP assumes that packet losses occur because of congestion.

2005/12/14 4 Introduction(2/3) ELFN (Explicit Link Failure Notification) ， notify the TCP sender about the link failure. Inability of a TCP sender’s routing protocol to quickly recognize and remove stale routes from its cache. DSR (Dynamic Source Routing protocol)

2005/12/14 5 Introduction(3/3) To make routing protocols aware of lost data packets and ACKs and help reduce TCP timeouts for mobility-induced losses. –Early packet loss notification (EPLN) –Best effort ACK delivery (BEAD).

2005/12/14 6 Background(1/5) DSR (Dynamic Source Routing) : route- discovery phase(has no route) Route Request

2005/12/14 7 Background(2/5) DSR: route-reply phase(has route)

2005/12/14 8 Background(3/5) DSR: Route-maintenance phase

2005/12/14 9 Background(4/5) DSR: Route-maintenance phase

2005/12/14 10 Background(5/5) Simulation Environment –2Mbps transmission rate and a nominal transmission range of 250m. –Interface uses IEEE DCF MAC protocol. –random waypoint model in a rectangular field. –random position, andom destination, randomly chosen speed, and pause time 0 s for all simulations. –1500m × 1000m field with 50 nodes and 2200m × 600m field with 100 nodes. –the packet size of 1460 bytes. The maximum size of both congestion window and receiver’s advertised window is 8. –FTP is the application that we used over TCP.

2005/12/14 11 How mobility affect TCP(1/6) ELFN in a network with 50 nodes and one TCP connection. Three issues –How to Set RTO and cwnd after Congestion Control Mechanisms are Restored? –When should TCP be Frozen? –Unaware of Lost Data Packets and ACKs

2005/12/14 12 How mobility affect TCP(2/6) How to Set RTO and cwnd after Congestion Control Mechanisms are Restored?

2005/12/14 13 How mobility affect TCP(3/6) Two choices for setting RTOand congestion window size, cwnd. –One choice is to use the default value 6 s for RTO and 2 for cwnd; –The other choice is to keep TCP’s state the same as it was when TCP was frozen.

2005/12/14 14 How mobility affect TCP(4/6) In our example, the next packet to be sent, 47, is larger than the highest sequence number of acknowledged packets, 39,TCP will not send any packet until an ACK arrives. However, the packets with sequence numbers from 40 to 46 were dropped, and thus no ACK will arrive. Here, reducing cwnd causes TCP to enter an idle state; if the packets already sent are lost, TCP has to wait for timeouts.

2005/12/14 15 How mobility affect TCP(5/6) When should TCP be Frozen? –In ELFN, TCP will be frozen when a TCP sender initiates a route discovery. –When a TCP sender receives an ELFN message indicating a link failure.

2005/12/14 16 How mobility affect TCP(6/6) Unaware of Lost Data Packets and ACKs –ELFN does not indicate whether the packet is lost. –Drops all the packets with the same next hop in the network interface queue. –Since TCP does not know about these losses, it has to time out. If an intermediate node notifies. –TCP senders will retransmit lost packets earlier and thus avoid waiting for timeouts.

2005/12/14 17 EPLN and BEAD(1/6) It is important for the network layer to be aware of lost data packets and ACKs and to help reduce TCP timeouts for mobility- induced losses. –early packet loss notification (EPLN) and –best-effort ACK delivery(BEAD).

2005/12/14 18 EPLN and BEAD(2/6) The key idea is that intermediate nodes notify TCP senders by extensively using cached routes. Route caches play an important role.

2005/12/14 19 EPLN and BEAD(3/6) early packet loss notification (EPLN)

2005/12/14 20 EPLN and BEAD(4/6) early packet loss notification (EPLN) If Broken Route cache new route or not??? NO Record drop packets in drop_list Drop all packets Send meg to TCP Sender Ask Sender retransmite Yes Drop packets or not??? No Continue transmit Yes Send msg to TCP Sender Tell Sender new router Record drop packets in drop_list Ask Sender retransmite

2005/12/14 21 EPLN and BEAD(5/6) best-effort ACK delivery(BEAD)

2005/12/14 22 EPLN and BEAD(6/6) best-effort ACK delivery(BEAD) If Broken Route cache new router or not??? NO Send ROUTE ERROR(with ACK) to the Sender or intermediate node Check new route or not??? Yes Continue transmit Yes No

2005/12/14 23 PERFORMANCE EVALUATION

2005/12/14 24 PERFORMANCE EVALUATION

2005/12/14 25 PERFORMANCE EVALUATION

2005/12/14 26 PERFORMANCE EVALUATION

2005/12/14 27 CONCLUSIONS(1) Our results lead to the following conclusions: –It is necessary for the network layer to notify TCP senders about lost packets and to retransmit ACKs for lost ACKs, so that TCP reacts quickly to frequent packet losses and is unaware of lost ACKs. –It is important to make route caches adapt fast to topology changes, because the validity of cached routes affects not only TCP performance but also the effectiveness of the mechanisms used to improve TCP performance.