Apache Reference: mod_unique_id
mod_unique_id - Generation of Unique Identifiers by Request
Since Apache 1.3, src/modules/standard/mod_unique_id.c
Dean Gaudet, Alvaro Martinez Echevarria (1997)
mod_unique_id performs a simple but sometimes useful task: it generates a magic token for each HTTP request that is guaranteed to be unique across all requests under very specific conditions. The identifier will even be unique across multiple machines in a properly configured cluster of machines. It is exported to the SSI/CGI environment as a variable.