IBM i - Network Performance

Network Tuning

Network performance is influenced by the Maximum Transmission Unit size, send and receive buffer size, port filtering, duplex settings and Domain Name lookup.


TCP/IP Buffer Size

Server performance can be improved by tuning the buffer size that is used by TCP/IP.

If your network is very reliable, try increasing the buffer size from the default value.

If your network experiences a significant amount of collisions or congestion, you may be able to improve performance by decreasing the TCP/IP send and receive buffers. This is because it will take less time to detect a bad packet, and less data will need to be re-transmitted.

Select a size in the range of 32K to 128K and use the same size for both buffers.

Setting the buffer size to a large value, 8388608 for example, can cause storage allocation problems on your machine. This is because every TCP/IP connection allocates 8MB of storage for the receive buffer.

In most network environments, performance will be improved by increasing these buffers to a minimum of 65535.


Domain Name Server Lookup

TCP/IP connection performance can be improved if the registered DNS servers are available and performing well.

The JSM_OPEN Built-In Function performs a DNS lookup to resolve the host name address. Using a dotted decimal IP address does not get around this DNS lookup.

If the JSM_OPEN Built-In Function is slow to open a connection, then you could have a DNS lookup issue. Add an entry to the local host table to improve performance.


Change TCP/IP Domain

Host name search priority           *LOCAL

Domain name server

Internet address          



If the first Domain Name Server (DNS) in the list does not respond, the second DNS server in the list will be contacted. If the second DNS server does not respond, the third DNS server will be contacted.