OSI/AS Configuration and Management Manual

Performance Considerations
OSI/AS Configuration and Management Manual—424119-001
6-8
Checksum Attributes
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Window-Size Attributes on page 6-9
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L4WINDOW (Layer 4)
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L3WINDOW (Layer 3, X25AM)
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L2WINDOW (X25AM SYSGEN)
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Protocol-Overhead Attributes on page 6-9
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EXTENDEDFORMAT (Layer 4)
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RECVSECURITY (Layer 4)
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SENDSECURITY (Layer 4)
Checksum Attributes
OSI/TS provides a checksum capability in the transport class 4 protocol (CHECKSUM),
the ES-IS routing exchange protocol (ESISCHECKSUM), and the IP (IPCHECKSUM).
You can turn the checksum capability on or off using SCF commands.
Checksums add a small amount of overhead for each PDU and incur some processing
overhead in the calculation of the checksum at both ends of the connection. In most
lightly loaded systems, this overhead probably is not noticeable. However, in heavily
loaded systems with a lot of small PDUs, checksums could become a potential factor in
high line utilizations.
Checksums are needed only when a line is unreliable. When considering the use of
checksums, you need to weigh the overhead incurred against the value of checksums to
detect errors. Since each of these three attributes generates a checksum on a different
part of the PDUs, you should set all three of these attributes on if your subsystem
requires the use of checksums.
Message-Size and PDU-Size Attributes
It is important to select the TPDU size and the X25AM packet size so that they
accommodate the message sizes of your application processes and of the network. You
should set these sizes large enough to minimize the number of short packets or TPDUs
sent. The smaller the number of short packets, including acknowledgment packets and
TPDUs, the lower the total protocol overhead. The larger the TPDU size and the fewer
the number of multiplexed transport connections, the lower the total number of transport
connections the TSP process can handle.
The dilemma, of course, is that most applications use varying message sizes. You
probably need to do some research into the application requirements on your system to
arrive at optimum sizes. For many users, the additional overhead may be insignificant.
For some users, especially where large volumes are sent through the network, the
additional protocol overhead can have a significant effect on CPU utilization and line
utilization, and can adversely affect response times. In addition, for X.25 networks, a
larger TPDU size requires additional buffer space.
You can set the X.25 packet size in SYSGEN using the PACKETSIZE attribute, and can
modify it using DSC. You can set the maximum TPDU size using SCF commands on
the TPDUSIZE attribute (maximum 8K, or 8192 bytes, except for class 0). However,
setting TPDUSIZE to the maximum value does not mean that the maximum size is
always used. The transport protocol accepts the lesser of two values configured for the