As per RFC 2866
This attribute is a unique Accounting ID to make it easy to match start and stop records in a log file. The start and stop records for a given session MUST have the same Acct-Session-Id. An Accounting-Request packet MUST have an Acct-Session-Id. An Access-Request packet MAY have an Acct-Session-Id; if it does, then the NAS MUST use the same Acct-Session-Id in the Accounting-Request packets for that session. The Acct-Session-Id SHOULD contain UTF-8 encoded 10646 [7] characters.
For example, one implementation uses a string with an 8-digit upper case hexadecimal number, the first two digits increment on each reboot (wrapping every 256 reboots) and the next 6 digits counting from 0 for the first person logging in after a reboot up to 2^24-1, about 16 million. Other encodings are possible.
A summary of the Acct-Session-Id attribute format is shown below. The fields are transmitted from left to right.
Type (IETF Number) = 44 for Acct-Session-Id.
Length = >= 3
String = The String field SHOULD be a string of UTF\-8 encoded 10646 \[7] characters.