Only the first two options mentioned by hangon would work.
IP addresses can be shared by multiple users using the same host and therefore will not serve as a unique session id.
The userid (retrieved with $ENV{REMOTE_USER}) is no good either because multiple users could share a userid and logon concurrently. Example: company has a subscription on a web magazine and multiple employees can view pages using the company userid.

In reply to Re^2: Session handling without using Cookie by varian
in thread Session handling without using Cookie by Anonymous Monk

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