Specify allowed characters for Baggage keys and values

CHANGELOG.md

@@ -7,7 +7,10 @@ release.
 
 ## Unreleased
 
-### Context
+### Context & Baggage
+
+- Specify allowed characters for Baggage keys and values.
+  ([#3801](https://github.com/open-telemetry/opentelemetry-specification/pull/3801))
 
 ### Traces
 

specification/baggage/api.md

@@ -29,6 +29,19 @@ metrics, traces, and logs. It is a set of name/value pairs describing
 user-defined properties. Each name in `Baggage` MUST be associated with
 exactly one value.
 
+Baggage **names** are strings that satisfy the `token` definition from
+[RFC7230, Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6).
+Alpha-numeric names are strongly recommended. Language API MAY return
+an error when given a baggage name that is not a `token`.
+
+Baggage **values** are any valid UTF-8 strings. Language API MUST accept
+any valid UTF-8 string as baggage value. Language API MUST guarantee that
+`Get` and `Set` operations are symmetrical with respect to name and value,
+i.e. `Get` MUST return the same **value** string as received in `Set`.
+
+Language API MUST treat both baggage names and values as case sensitive.
+See also [W3C Baggage -- Case sensitivity of keys](https://github.com/w3c/baggage/blob/main/baggage/HTTP_HEADER_FORMAT_RATIONALE.md#case-sensitivity-of-keys).
+
 The Baggage API consists of:
 
 - the `Baggage`