There are two configuration parameters associated with header compression: Whether or not compressed packets should be sent on a particular line and, if so, how many state slots (saved packet headers) to reserve. There is also one link-level configuration parameter, the maximum packet size or MTU, and one front-end configuration parameter, data compression, that interact with header compression. Compression configuration is discussed in this section. MTU and data compression are discussed in the next two sections.
There are some hosts (e.g., low end PCs) which may not have enough processor or memory resources to implement this compression. There are also rare link or application characteristics that make header compression unnecessary or undesirable. And there are many existing SLIP links that do not currently use this style of header compression. For the sake of interoperability, serial line IP drivers that allow header compression should include some sort of user configurable flag to disable compression (see appendix B.2)./32/
If compression is enabled, the compressor must be sure to never send a connection id (state index) that will be dropped by the decompressor. E.g., a black hole is created if the decompressor has sixteen slots and the compressor uses twenty./33/ Also, if the compressor is allowed too few slots, the LRU allocator will thrash and most packets will be sent as UNCOMPRESSED_TCP. Too many slots and memory is wasted.
Experimenting with different sizes over the past year, the author has found that eight slots will thrash (i.e., the performance degradation is noticeable) when many windows on a multi-window workstation are simultaneously in use or the workstation is being used as a gateway for three or more other machines. Sixteen slots were never observed to thrash. (This may simply be because a 9600 bps line split more than 16 ways is already so overloaded that the additional degradation from round-robbining slots is negligible.)
Each slot must be large enough to hold a maximum length TCP/IP header of 128 bytes/34/ so 16 slots occupy 2KB of memory. In these days of 4 Mbit RAM chips, 2KB seems so little memory that the author recommends the following configuration rules: