engleski

Integration of TCP and ATM Explicit Congestion Notification Schemes

TCP uses a packet drop as an indication of network congestion. While this reactive scheme has the advantage of not making any assumptions about the operation of the underlying network layer it is inadequate for high speed networking. Generally speaking, the separation of flow/congestion control into two independent parts, TCP's and network's, has major drawbacks on the performance of TCP. In order to achieve better performance the control has to be unified. The explicit congestion notification (ECN) [RFC2481] is a mechanism which provides IP networks with means to inform end-systems' TCPs of incipient congestion. By marking IP packets on their way to the destination, a router notifies the TCP receiver that the average queue size for the connection is increasing towards some predefined upper limit. The TCP receiver passes this information to the sender by setting a bit in the TCP header of an outgoing acknowledgment. Upon detection of congestion indication bit, TCP sender reduces the outgoing traffic flow and consequently maintains the router's queue within limits avoiding excessive delays and packet drops. The application of ECN mandates the usage of active queue management routers such as RED. For congestion notification in ATM networks, ATM switches can use the Explicit Forward Congestion Indication (EFCI) bit in an ATM cell header. Unfortunately, the setting of the EFCI bit is not based on the average queue size but on the instantaneous queue size. Because of this, the EFCI bit is generally considered not applicable for ECN. Nevertheless, this presentation explores possible ways of using EFCI in ECN scheme because such a configuration would greatly benefit TCP traffic passing through ATM network. The simulation results obtained by using the xGlobe network simulator [ATM Protocols Implementation and Evaluation Using xGlobe Simulator, 8th Intl. Conf. on Telecomm. Systems, Nashville, USA, March 2000] are analyzed. Further, the presentation discusses an implementation of the unified flow/congestion control in the Linux operating system. Analysis of the required changes to the kernel code is given. Also, preliminary support for ECN/EFCI is analyzed.

TCP; ATM; congestion control

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