Global Opaque Block for IOAM Pre-allocated Trace Option

Extensible metadata carried within the IOAM Pre-allocated Trace Option (PTO) can support packet-level, flow-level, or path-level processing beyond per-node trace data. This document defines the Global Opaque Block (GOB), an extension to the IOAM PTO that introduces a single pre-allocated global metadata region placed between the PTO fixed header and the node data list. The GOB carries an explicit length and schema identifier, preserves the pre-allocated PTO processing model, and can be used to transport Extensible In-band Processing (EIP) Information Elements or other structured metadata formats.