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Robust Transport Protocol for Dynamic High-Speed Networks:

enhancing XCP approach

Dino M. Lopez Pacheco INRIA RESO/LIP, ENS of Lyon, FranceCongduc Pham LIUPPA, University of Pau, France

MICC-ICON 2005

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Variable bandwidth environment networks

● In High-Speed networks, the available best-effort bandwidth can vary over time for many reasons:

– non regulated flows (UDP).

– QoS Mechanisms.

– Resources reservation.

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Transport control protocols

At the moment, there are many TCP versions to take advantage of the network resources, for example:

➔ TCP Vegas, FAST TCP➔ TCP Westwood➔ SCTP, HSTCP

Other propositions very different from TCP, such as:➔ CADPC/PTP➔ XCP

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The XCP Protocol

● XCP is a router-assisted solution (XCP routers)

● Routers can compute the available bandwidth by monitoring the input traffic rate Ir, the output link capacity Or, and the persistent queue size Q.

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The XCP Protocol (2)

In this way:

● The feedback is computed by the XCP routers and sent back to the sender in the ACK packets.

● The sender receives the feedback and adds this value to his congestion window size.

● The sender does not make any assumption on the state of the network. This information is provided by the XCP routers.

Therefore, XCP will be able to find the optimal congestion window size, even though the load of the network changes over time.

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The simulation topology

● 1,..., 10 XCP senders/receivers.

● 2 routers (R1, R2).

● 200Mb – 1Gb of link capacity

● Classical dumpbell topology.

● 17 UDP senders/receivers.

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XCP in variable bandwidth environments (case 1)

Very good performance

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XCP in variable bandwidth environments (case 1)

Very good performance

Congestion window size evolution according to the available bandwidth

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XCP in variable bandwidth environment (case 2)

Good fairness and stability even in

unfriendly environments

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Loss of ACK packets in XCP networks

The loss of packets is common in the networks. In the case of ACK losses inside of an XCP network:● if the bandwidth does not vary over time, there is no problem because:

✔when the throughput is very slow, the feedback will be large enough to get the maximum capacity.✔when the throughput is optimal, the loss of very small feedbacks will not have any effect.

● if the bandwidth varies over time, and the ACKs are lost when the bandwidth decreases, this losses will produce a wrong calculus in the sender's congestion window size, increasing the probability of data packets loss.

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Timeouts introduced by the ACK loss (wrong cwnd value computed)

Effects of a 30% ACK loss rate in the XCP network (1 flow)

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Timeouts introduced by the ACK loss (wrong cwnd value computed)

Effects of a 30% ACK loss rate in the XCP network (1 flow)

Very important inactivity period (almost 5s)

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Timeouts at 5s and 30s

Effects of a 30% ACK loss rate in the XCP networks (3 flows)

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Timeouts at 5s and 30s

Effects of a 30% ACK loss rate in the XCP networks (3 flows)

Unfairness and difficulty in restarting the connection

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XCP-r, an approach to solve the problem

Since the ACK losses produce a wrong calculus of the congestion window size in the sender, we have proposed to compute this value in the receiver and send it to the sender inside the ACK packets.

We have called this approach XCP-r (XCP in the receiver)

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Some technical aspects in XCP-r

● The receiver will create an equivalent variable to cwnd (cwnd') when a connection request will be received.

● The receiver will have to create one cwnd' for each connection established.

● After a congestion problem, if the sender modifies its congestion window size, it will notify the change and the receiver will update the cwnd’ variable .

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XCP-r in variable bandwidth environments

Without ACK packets losses, the simulations results are similar to XCP

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XCP-r in variable bandwidth environments

Without ACK packets losses, the simulations results are similar to XCP

Without ACK packets losses, the simulations results are similar to XCP

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very fast recovery after a timeout

Effects of a 30% ACK loss rate in the XCP-r networks (1 flow)

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very fast recovery after a timeout

Effects of a 30% ACK loss rate in the XCP-r networks (1 flow)

the inactivity period is almost negligible

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Effects of a 30% ACK loss rate in the XCP-r networks (3 flow)

Fast recovery after the timeouts problems

Excellent fairness level

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Effects of a 30% ACK loss rate in the XCP-r networks (3 flow)

Fast recovery after the timeouts problems

No timeout registered

Excellent fairness level

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XCP and XCP-r in a High Speed Network (12% ACK loss rate)

Small changes in the bandwidth can introduce many problems in XCP

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XCP and XCP-r in a High Speed Network (12% ACK loss rate)

Small changes in the bandwidth can introduce many problems in XCPXCP-r

remarkably improves the performance!

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Conclusion

XCP is a promising router-assisted approach for high-speed networks, but :

➢The high dependence of the information contained in the ACK could make XCP very unstable in very dynamic high speed networks.

XCP-r diminishes this problem➢It provides a high level of performance in dynamic high speed networks.➢Suitable in very high-speed networking infrastructures like computational grids.