Specify expected behaviour when no traceparent is received (#475)

* Specify expected behaviour when no traceparent is received Resolves #442. * Be more precise in possible reason for not generating traceparent
w3c · Jan 4, 2022 · c899924 · c899924
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commit c899924
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diff --git a/spec/20-http_request_header_format.md b/spec/20-http_request_header_format.md
@@ -334,6 +334,8 @@ The version of `tracestate` is defined by the version prefix of `traceparent` he
 
 ## Mutating the traceparent Field
 
+A vendor receiving a request without a `traceparent` header SHOULD generate `traceparent` headers for outbound requests, effectively starting a new trace. A possible reason for not doing this could be a performance sensitive scenario when the vendor decides to not sample a request. Note that for most scenarios, vendors are expected to generate the header even when not sampling, to propagate the sampling decision downstream.
+
 A vendor receiving a `traceparent` request header MUST send it to outgoing requests. It MAY mutate the value of this header before passing it to outgoing requests.
 
 If the value of the `traceparent` field wasn't changed before propagation, `tracestate` MUST NOT be modified as well. Unmodified header propagation is typically implemented in pass-through services like proxies. This behavior may also be implemented in a service which currently does not collect distributed tracing information.