Align environment variable propagation names with POSIX and define normalization rules

CHANGELOG.md

@@ -9,6 +9,10 @@ release.
 
 ### Context
 
+- Align environment variable context propagation name restrictions with POSIX.1-2024
+  and define normalization behavior.
+  ([#4914](https://github.com/open-telemetry/opentelemetry-specification/issues/4914))
+
 ### Traces
 
 - Stabilize Tracer `enabled` operation

specification/context/env-carriers.md

@@ -77,12 +77,16 @@ configuration options to override the default environment variable.
 
 Environment variable names used for context propagation:
 
-- SHOULD use uppercase letters, digits, and underscores for maximum
-  cross-platform compatibility
-- MUST NOT include characters forbidden in environment variables per
-  platform-specific restrictions
-- SHOULD follow naming conventions that align with the propagation format
-  specification they're implementing (e.g., `TRACEPARENT` for W3C trace context)
+- MUST be normalized by:
+  - uppercasing ASCII letters,
+  - replacing every character that is not an ASCII letter, digit, or underscore
+    (`_`) with an underscore (`_`),
+  - prefixing the name with an underscore (`_`) if it would otherwise start with
+    an ASCII digit.
+
+> [!NOTE]
+> This normalization is consistent with the environment variable naming rules
+> defined in [POSIX.1-2024](https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap08.html).
 
 #### Value Restrictions