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Don Burleson Blog 







Optimize Oracle Network Tips

Oracle Tips by Burleson Consulting

Nov 18, 2015, updated June 17, 2007

Oracle databases are often shared across geographical areas, so it's imperative that the Oracle professional understand how database performance is affected by network communications. The Transparent Network Substrate (TNS), provided by Oracle, allows distributed communications between databases.

The TNS serves as an insulator between Oracle's logical data request and the physical communications between the remote servers. As such, the network administrator is able to control much of the network performance tuning. The Oracle administrator, then, has little control over the network settings that can affect overall database performance.

The performance of distributed transactions can be improved using some important settings that I'll illustrate below. These include parameters within the sqlnet.ora, tnsnames.ora, and protocol.ora files. These parameters can be used to change the configuration and size of TCP packets, and adjusting these parameters can have a profound impact on the underlying network transport layer to improve the throughput of all Oracle transactions.

As I noted, Oracle*Net does not allow the Oracle professional the ability to tune the underlying network layer, and the majority of network traffic cannot be tuned from within the Oracle environment. Remember, Oracle Net is a layer in the OSI model that resides above the network-specific protocol stack.

The frequency and size of network packets, however, can be controlled by the Oracle DBA. Oracle has a wealth of tools to change packet frequency and size. A simple example involves changing the refresh interval for a snapshot to ship larger amounts at less frequent intervals.

Tuning for throughput vs response time

The DBA is faced with two approaches to network tuning, optimizing for either fast individual response time vs. optimizing packet shipment for maximum throughput:

  • Optimal network throughput:  Oracle suggests setting recv_buf_size and send_buf_size sqlnet.ora parameters equal to 3 times the Bandwidth Delay Product (BDP).  Also, set tcp.nodelay=yes.
  • Optimal network response time: For optimal response time for individual transactions, see the notes below for getting fast, small packets.

Oracle*Net connections between servers can be tuned using several parameters. Keep in mind, however, that network tuning is outside the scope of Oracle, and a qualified network administrator should be consulted for tuning the network. The frequency and size of packet shipping across the network can be affected by using settings contained in the following parameter files: 

  • The sqlnet.ora server file—The automatic_ipc parameter
  • The sqlnet.ora client file—The break_poll_skip parameter
  • The tnsnames.ora and listener.ora files—The SDU and TDU parameters
  • The protocol.ora file—The tcp.nodelay parameter

These tuning parameters will affect only the performance of the Oracle Net layer. Let's examine them in detail and see how they can be adjusted to improve Oracle Net throughput.

The tcp.nodelay parameter in the protocol.ora file

Oracle Net, by default, waits until the buffer is filled before transmitting data. Therefore, requests aren't always sent immediately to their destinations. This is most common when large amounts of data are streamed from one end to another, and Oracle Net does not transmit the packet until the buffer is full. Adding a protocol.ora file, and specifying a tcp.nodelay to stop buffer flushing delays, can remedy this problem.

The protocol.ora file can be specified to indicate no data buffering for all TCP/IP implementations. The parameter can be used on both the client and server. The protocol.ora statement is:

tcp.nodelay = yes

Specifying this parameter causes TCP buffering to be skipped so that every request is sent immediately. Keep in mind, however, that network traffic can increase due to smaller and more frequent packet transmission, causing slowdowns in the network.

The tcp.nodelay parameter should be used only if TCP timeouts are encountered. Setting tcp.nodelay can cause a huge improvement in performance when there is high-volume traffic between database servers.

The recv_buf_size and send_buf_size in the sqlnet.ora file

If you are on 10g and beyond and you are tuning your network to maximize throughput (as opposed to tuning for fast response time), Oracle suggests setting recv_buf_size and send_buf_size sqlnet.ora parameters equal to 3 times the Bandwidth Delay Product (BDP).   See setting recv_buf_size and send_buf_size.

As the name implies, the recv_buf_size and send_buf_size parameters in the sqlnet.ora file determine the size (in bytes) of the socket receive and send buffers for a SQL*Net connection.

To prevent network contention, the values for recv_buf_size and send_buf_size should be set to the maximum amount of data that might be sent, at least 3x the value of your bandwidth delay product (BDP).  

See these important notes on setting send_buf_size and recv_buf_size. and these tips for finding your Oracle BDP value.

The automatic_ipc parameter of the sqlnet.ora file

The automatic_ipc parameter bypasses the network layer, thereby speeding local connections to the database. When automatic_ipc=on, Oracle Net checks to see if a local database is defined by the same alias. If so, network layers are bypassed as the connection is translated directly to the local IPC connections. This is useful on database servers, but it's absolutely useless for Oracle Net clients.

The automatic_ipc parameter should be used only on the database server when an Oracle Net connection must be made to the local database. If local connections are not needed or required, set this parameter to off; with this setting, all Oracle Net clients can improve performance.

The SDU and TDU parameters in the tnsnames.ora file

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

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

Note: In Oracle 10g and beyond, Oracle recommends an SDU value of 32k (32767). 

The following guidelines apply to SDU and TDU:

  • The SDU should never be set greater than TDU because you'll waste network resources by shipping wasted space in each packet.
  • If your users are connecting via modem lines, you may want to set SDU and TDU to smaller values because of the frequent resends that occur over modem lines.
  • On fast network connections (T1 or T3 lines), you should set SDU and TDU equal to the MTU for your network. On standard Ethernet networks, the default MTU size is set to 1,514 bytes. On standard token ring networks, the default MTU size is 4,202.
  • If the Multi-Threaded Server (MTS) is used, you must also set the mts_dispatchers with the proper MTU TDU configuration.
  • For Streams replication and for using Oracle in a WAN environment, Oracle recommends that you increase the SDU to 32k.  Also, the Oracle docs note:

To take advantage of an increased SDU for Streams propagation, the receiving side sqlnet.ora file must include the DEFAULT_SDU_SIZE parameter. The receiving side listener.ora file must indicate the SDU change for the system identifier (SID). The sending side tnsnames.ora file connect string must also include the SDU modification for the particular service.

For Streams, the SEND_BUF_SIZE and RECV_BUF_SIZE parameters in the listener.ora file increase the performance of propagation on your system.  Oracle internal testing of Oracle Data Guard on a WAN has demonstrated that the maximum setting of 32K (32768) performs best on a WAN. The primary gain in performance when setting the SDU is a result of the reduced number of calls to packet the data.

In addition to setting the SDU parameter, network throughput can often be substantially improved by using the SQLNET.SEND_BUF_SIZE and SQLNET.RECV_BUF_SIZE Oracle Net parameters to increase the size of the network TCP send and receive I/O buffers.

The SDU and TDU settings are a direct function of the connection speed between the hosts. For fast T1 lines, set SDU=TDU=MTU. For slower modem lines, experiment with smaller values of SDU and TDU.

The queuesize parameter in the listener.ora file

The number of requests the listener can store while Oracle is working to establish a connection is determined by the undocumented queuesize parameter. This parameter is used only for very high-volume databases, where the listener spawns thousands of connections per hour. The number of expected simultaneous connections should be equal to the size of the queuesize parameter. Here's an example of this parameter in the listener.ora file:

           (ADDRESS =
             (PROTOCOL = TCP)
             (HOST = marvin)
             (PORT = 1521)
            (QUEUESIZE = 32)

A disadvantage of this parameter is that it pre-allocates resources for anticipated requests, therefore using more system memory and resources. You may want to consider using MTS and pre-spawned Oracle connections if you have high-volume connections into a dedicated listener. Also, note that some versions of UNIX do not allow queues greater than five, and there are some restrictions of the MTS queue size.


While most of the network packet traffic is tuned at the network level, proper settings for Oracle Net parameters can have a great impact on the performance of distributed systems. It's the job of the Oracle professional to fully understand and optimize these important parameters.

Oracle-l notes this on tcp.nodelay and it's relationship to the SDU and TDU parameters:

"Setting tcp.nodelay disables the Nagle algorithm in the tcp stack that tries to efficiently balance the data load of a packet with the delay in dispatching it.

Effectively, you're saying "to xxx with optimizing the data payload ... send those babies now!". The complete antithesis of what the SDU/TDU settings are trying to do for you.

You'll end up with a larger number of smaller packets on your WAN, and if it's latency that's your problem, this will make matters worse, not better."

For more information, check my notes on Oracle Network Tuning.

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