Specify Span ID creation with sampling.

CHANGELOG.md

@@ -105,6 +105,8 @@ Updates:
 - SDK: Specify known values, as well as basic error handling for OTEL_PROPAGATORS.
   ([#962](https://github.com/open-telemetry/opentelemetry-specification/pull/962))
   ([#995](https://github.com/open-telemetry/opentelemetry-specification/pull/995))
+- SDK: Specify when to generate new IDs with sampling
+  ([#998](https://github.com/open-telemetry/opentelemetry-specification/pull/998))
 - Remove custom header name for Baggage, use official header
   ([#993](https://github.com/open-telemetry/opentelemetry-specification/pull/993))
 - Trace API: Clarifications for `Span.End`, e.g. IsRecording becomes false after End

specification/trace/sdk.md

@@ -60,11 +60,24 @@ The following table summarizes the expected behavior for each combination of
 The SDK defines the interface [`Sampler`](#sampler) as well as a set of
 [built-in samplers](#built-in-samplers) and associates a `Sampler` with each [`TracerProvider`].
 
-When asked to create a Span, the SDK MUST query the `Sampler`'s [`ShouldSample`](#shouldsample) method before actually creating the span, and act accordingly:
-see description of [`ShouldSample`'s](#shouldsample) return value below for how to set `IsRecording` and `Sampled` on the Span,
-and the [table above](#recording-sampled-reaction-table) on whether to pass the `Span` to `SpanProcessor`s.
-A non-recording span MAY be implemented using the same mechanism as when a `Span` is created with no API-implementation installed
-(sometimes called a `NoOpSpan` or `DefaultSpan`).
+### SDK Span creation
+
+When asked to create a Span, the SDK MUST act as if doing the following in order:
+
+1. If there is a valid parent trace ID, use it. Otherwise generate a new one (
+   note: this must be done before calling `ShouldSample`, because it expects
+   a valid trace Id as input).
+2. Query the `Sampler`'s [`ShouldSample`](#shouldsample) method.
+3. If the decision is `DROP` and there is a valid parent span ID, use it.
+   Otherwise (if the decision is not `DROP` or there was no valid parent span ID)
+   a new span ID MUST be generated.
+4. Create a span depending on the decision returned by `ShouldSample`:
+   see description of [`ShouldSample`'s](#shouldsample) return value below
+   for how to set `IsRecording` and `Sampled` on the Span,
+   and the [table above](#recording-sampled-reaction-table) on whether
+   to pass the `Span` to `SpanProcessor`s.
+   A non-recording span MAY be implemented using the same mechanism as when a
+   `Span` is created with no API-implementation installed (i.e., a [Propagated Span](api.md#propagated-span-creation)).
 
 ### Sampler