inline aggregation job resp validation

The section describing how the Leader validates an `AggregationJobResp`
only contained a single criteria, and referring to it from multiple
places made it unnecessarily complicated. We now simply restate the
singular requirement in two sections, which should make this more clear
for implementers.

Resolves #473

draft-ietf-ppm-dap.md

@@ -1304,9 +1304,11 @@ Finally, the Leader sends a PUT request to
 "application/dap-aggregation-job-init-req".
 
 The Helper's response will be an `AggregationJobResp` message (see
-{{aggregation-helper-init}}, which the Leader validates according to the
-criteria in {{aggregation-job-validation}}. If the message is invalid, the
-Leader MUST abort the aggregation job.
+{{aggregation-helper-init}}. The response's `preapre_resps` must include exactly
+the same report IDs in the same order as the Leader's `AggregationJobInitReq`.
+Otherwise, the Leader MUST abort the aggregation job.
+
+[[OPEN ISSUE: consider relaxing this ordering constraint. See issue#217.]]
 
 Otherwise, the Leader proceeds as follows with each report:
 
@@ -1591,23 +1593,6 @@ following checks:
 
 If all of the above checks succeed, the input share is not marked as invalid.
 
-#### Aggregation Job Validation {#aggregation-job-validation}
-
-During the aggregation job initialization ({{leader-init}}) or continuation
-({{agg-continue-flow}}) phases, the Leader will receive an `AggregationJobResp`
-message from the Helper, which needs to be validated before the Leader can move
-to the next phase of the aggregation protocol.
-
-An `AggregationJobResp` is valid only if it satisfies the following requirement:
-
-* The Helper's `prepare_resps` MUST include exactly the same report IDs in the
-  same order as either the `prepare_inits` in the Leader's
-  `AggregationJobInitReq` (if this is the first step of the aggregation job) or
-  the `prepare_continues` in the Leader's `AggregationJobContinueReq` (if this
-  is a subsequent step).
-
-[[OPEN ISSUE: consider relaxing this ordering constraint. See issue#217.]]
-
 ### Aggregate Continuation {#agg-continue-flow}
 
 In the continuation phase, the Leader drives the VDAF preparation of each share
@@ -1653,9 +1638,12 @@ preparation continuation messages constructed in the previous step. The
 `PrepareContinue`s MUST be in the same order as the previous aggregate request.
 
 The Helper's response will be an `AggregationJobResp` message (see
-{{aggregation-helper-init}}), which the Leader validates according to the
-criteria in {{aggregation-job-validation}}. If the message is invalid, the
-Leader MUST abort the aggregation job.
+{{aggregation-helper-init}}). The response's `prepare_resps` must include
+exactly the same report IDs in the same order as the Leader's
+`AggregationJobContinueReq`. Otherwise, the Leader MUST abort the aggregation
+job.
+
+[[OPEN ISSUE: consider relaxing this ordering constraint. See issue#217.]]
 
 Otherwise, the Leader proceeds as follows with each report: