When combined with the modern PPP serial line protocol, the use of header compression is automatic and invisible to the user. Unfortunately, many sites have existing users of the SLIP described in  which doesn't allow for different protocol types to distinguish header compressed packets from IP packets or for version numbers or an option exchange that could be used to automatically negotiate header compression.
The author has used the following tricks to allow header compressed SLIP to interoperate with the existing servers and clients. Note that these are hacks for compatibility with past mistakes and should be offensive to any right thinking person. They are offered solely to ease the pain of running SLIP while users wait patiently for vendors to release PPP.