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Oracle SDU and TDU parameters

The session data unit  (SDU) specifies the size of the packets to send over the network. The maximum transmission unit (MTU) is a fixed value that depends on the actual network implementation used. Ideally, SDU should not surpass the size of the MTU. Oracle recommends that SDU be set equal to the MTU. The tnsnames.ora and listener.ora files house the SDU and TDU parameters.


To group data together, the TDU value is the default packet size used within Oracle*Net.  The default value for both SDU and TDU is 2,048 bytes and the maximum value is 32,767 bytes.  The TDU parameter should ideally be a multiple of the SDU parameter. For SDU and TDU, the following guidelines apply:

§       On fast network connections such as T1 or T3 lines, SDU and TDU should be set equal to the MTU for the network. On standard Ethernet networks, the default MTU size should be set to 1,514 bytes. On standard token ring networks, the default MTU size is 4,202 bytes.

§       If the users are connecting via dial-up modem lines, it may be desirable to set SDU and TDU to smaller values in consideration of the frequent resends that occur over modem connections.

§       The mts_dispatchers must also be set with the proper MTU - TDU configuration, if a Multi-Threaded Server (MTS) is used.

§       As a rule, SDU should not be set greater than the TDU because network resources will be wasted by shipping wasted space in each packet.

This brief review of network-related parameters was intended provide an introduction to the scope and complexity of network tuning. It is important to understand that Oracle*Net is simply a layer in the OSI model that is above the network-specific protocol stack, and therefore, virtually all network tuning is external to Oracle.



This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 50%-off and get instant access to the code depot of Oracle tuning scripts:




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