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Improving TCP/IP Performance
When connecting to the AS/400 using TCP/IP, you may be able to improve performance with the following simple changes to the TCP/IP configuration on the AS/400:
- Change the default used for the Maximum Transmission Unit
- Change the send and receive buffer size
Change the default for the Maximum Transmission Unit
Currently the AS/400 system defaults to a Maximum Transmission Unit (MTU) of 576 when you add a route to the configuration (through CFGTCP option 2 or ADDTCPRTE). This value ensures packets will not be dropped over this route because all TCP/IP implementations have to support at least a 576-byte transmission unit.
In many cases, however, this value is unnecessarily small because this route will only be used on this Ethernet or token-ring, and there are no intermediate hops that only support a 576-byte packet. If this is the case, you should change the Route Maximum Transmission Unit size to *IFC. This will change the MTU on the Route to the Interface MTU size which defaults to the Line Description Frame Size. This defaults to approximately 2000 for token-ring and 1500 for Ethernet. There are also cases where the 576-byte size can cause adapter overruns that put TCP/IP in retransmit mode and slow things down considerably.
This change often results in a considerable increase in your throughput over TCP/IP.
To change the default used for the MTU:
- On the AS/400 command line type:
CFGTCP
Press the Enter key.
- Select
Option 2 (Work with TCP/IP routes).
- Type
2 (Change) next to the route you want to change.
- Type
*IFC in the Maximum transmission unit field. Press the Enter key to make the change.
Change the TCP/IP Send and Receive Buffer Size
The default size for the TCP send and receive buffers is 8192 bytes. Consider changing the default size for both the send and receive buffers to a larger value, for example, 64384 bytes.
- TCP receive buffer size: this value specifies the amount of data the remote system can send before being read by the local application. If a large number of buffer overruns are occurring on the network adapter, decreasing this value, instead of increasing it, could improve performance.
-
TCP send buffer size: this value provides a limit on the number of outgoing bytes that are buffered by TCP. If TCP has to resend the data many times due to a buffer overrun condition of a network adapter, decreasing this value, instead of increasing it, could help performance.
To change the defaults used for the TCP send and receive buffer size:
- On the AS/400 command line type:
CFGTCP
Press the Enter key.
- Select
Option 3 (Change TCP/IP attributes).
- Specify a new value for the
TCP send buffer size and the
TCP receive buffer size fields.
- Press the Enter key to make the changes.
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