Issue cloudevents#331: Clarify scope of source and id uniqueness

Signed-off-by: Alan Conway <aconway@redhat.com>

spec.md

@@ -117,7 +117,7 @@ The following abstract data types are available for use in attributes.
 - `Map` - `String`-indexed dictionary of `Any`-typed values.
 - `Any` - Either a `Binary`, `Integer`, `Map` or `String`.
 - `URI-reference` - String expression conforming to `URI-reference`
-  as defined in
+  (a URL, URI or relative reference) as defined in
   [RFC 3986 §4.1](https://tools.ietf.org/html/rfc3986#section-4.1).
 - `Timestamp` - String expression as defined in
   [RFC 3339](https://tools.ietf.org/html/rfc3339).
@@ -202,11 +202,13 @@ help intermediate gateways determine how to route the events.
 
 ### source
 * Type: `URI-reference`
-* Description: This describes the event producer. Often this will include
-  information such as the type of the event source, the organization
-  publishing the event, the process that produced the event, and some unique
-  identifiers. The exact syntax and semantics behind the data encoded in the URI
-  is event producer defined.
+* Description: Identifies the event producer.
+  The source MAY include a URI authority
+  [RFC3986  §3.2](https://tools.ietf.org/html/rfc3986#section-3.2)
+  associated with the producer, providing an internet-wide unique identifier.
+  If not, it is unique within some application-specific scope.
+  The details of a source URI are determined by the producer, the consumer
+  SHOULD treat it as an opaque identifier.
 * Constraints:
   * REQUIRED
 * Examples
@@ -224,7 +226,12 @@ help intermediate gateways determine how to route the events.
 * Constraints:
   * REQUIRED
   * MUST be a non-empty string
-  * MUST be unique within the scope of the producer
+  * MUST be unique within the scope of the producer. A producer MUST
+    NOT generate events with identical "source" and "id" values,
+    unless it is generating an identical duplicate of an event that
+    may have been lost due to a transport failure.  A consumer SHOULD
+    ignore events that have the same "source" and "id" as an event
+    already processed.
 
 ### time
 * Type: `Timestamp`