# Appendix 2.A Prior Art

_Peios / Advanced Peios / PCDS / GUID_

> Where the Peios GUID comes from — MS-DTYP §2.3.4, RFC 4122 and DCE RPC — and the one place Peios normalises differently.

## 2.A.1 MS-DTYP

The GUID type defined in this document derives from the Microsoft
Data Types specification (MS-DTYP), §2.3.4. The Peios GUID binary
format is identical to the MS-DTYP GUID. The Peios GUID string
format follows the same hyphenated hex convention but normalises to
lowercase hex digits on output, where Microsoft implementations
typically produce uppercase.

## 2.A.2 RFC 4122

RFC 4122 ("A Universally Unique IDentifier (UUID) URN Namespace")
defines the UUID format and generation algorithms. The GUID
binary layout used by Microsoft and adopted by Peios is the
mixed-endian variant of the RFC 4122 UUID: the first three fields
are little-endian integers and the last field is a raw byte array.
This differs from the RFC 4122 network byte order representation
where all fields are big-endian.

Peios generates version 4 (random) GUIDs as defined in RFC 4122 §4.4.

## 2.A.3 DCE RPC

The GUID structure originates from the DCE 1.1 RPC specification,
which defined the `uuid_t` type with the same field layout. The
mixed-endian encoding reflects the DCE convention of encoding integer
fields in the sender's native byte order (little-endian on x86).