Appendix E. Configurable Options

We've seen many features of TCP/IP that we've had to describe with the qualifier "it depends on the configuration." Typical examples are whether or not UDP checksums are enabled (Section 11.3), whether destination IP addresses with the same network ID but a different subnet ID are local or nonlocal (Section 18.4), and whether directed broadcasts are forwarded or not (Section 12.3). Indeed, many operating characteristics of a given TCP/IP implementation can be modified by the system administrator.

This appendix lists some of the configurable options for the various TCP/IP implementations that have been used throughout the text. As you might expect, every vendor does things differently from all others. Nevertheless, this appendix gives an idea of the types of parameters different implementations allow one to modify. A few options that are highly implementation specific, such as the low-water mark for the memory buffer pool, are not described.

These variables are described for informational purposes only. Their names , default values, or interpretation can change from one release to the next . Always check your vendor's documentation (or bug them for adequate documentation) for the final word on these variables.

This appendix does not cover the initialization that takes place every time the system is bootstrapped: the initialization of each network interface using ifconfig (setting the IP address, the subnet mask, etc.), entering static routes into the routing table, and the like. Instead, this appendix focuses on the configuration options that affect how TCP/IP operates.