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title: "Bulk Report Submission for Distributed Aggregation Protocol (DAP)" abbrev: "Bulk DAP Submission" category: info

docname: draft-thomson-ppm-dap-bulk-latest submissiontype: IETF number: date: consensus: true v: 3 area: "Security" workgroup: "Privacy Preserving Measurement" keyword:

author:

fullname: "Martin Thomson"
organization: Mozilla
email: "[email protected]"

normative:

informative:

--- abstract

A bulk report submission endpoint and format are described for the Distributed Aggregation Protocol (DAP). This provides modest, but meaningful, efficiency benefits over the core protocol for cases where an intermediary is able to collect large numbers of reports.

--- middle

Introduction

The Distributed Aggregation Protocol (DAP) {{!DAP=I-D.ietf-ppm-dap}} accepts reports from many clients and aggregates them in a manner that protects individual contributions. The core protocol requires that each report be submitted to the DAP leader.

The assumption implicit in this design is that reports are submitted directly by clients. This is not necessary for security reasons due to the encryption used (this encryption is necessary for the portion of reports that is intended for DAP helpers). Clients could instead pass their reports to an intermediary for forwarding.

Use of an intermediary reduces the availability requirements of aggregators and might remove the need for anonymizing proxy to protect client identity from the server (such as the use of Oblivious HTTP {{?RFC9458}} as described in {{Section 7.4 of DAP}}). It also creates an opportunity to amortize the overheads involved in report submission.

This document defines a bulk submissions endpoint for DAP and defines a report submission format for use with that endpoint.

Conventions and Definitions

{::boilerplate bcp14-tagged}

Bulk Submissions Endpoint

DAP defines the {leader}/tasks/{task-id}/reports resource for report submission for a task. This document extends DAP to define the {leader}/tasks/{task-id}/reports/bulk resource for bulk report submission for a task.

Clients can send a HTTP POST request to this resource with a payload in the bulk submission format ({{format}}). A bulk request is equivalent to multiple separate submissions.

A DAP leader MAY accept bulk submissions at the regular "reports" resource.

Bulk Submission Format {#format}

The "application/dap-bulk-report" format contains a header that encodes extensions that are common to all reports. The header is followed by any number of reports, from which those shared fields are removed.

struct {
  Extension common_extensions<0..2^16-1>;
  Report report[REPORT_COUNT];
} BulkReport;

{: #syn-bulk-report title="Bulk Report Format"}

The common_extensions field contains common extensions that are added to the set of public report extensions in each report that follows. Reports that include values for any common extension override the value in the common extension.

{:aside}

It is slightly more intrusive and fragile, but we should consider removing the extensions field from ReportMetadata as well. Repeating those two bytes isn't a huge problem, but it's pretty cheap. It also removes the last clause from the preceding paragraph.

The header is followed by any number of reports, which are encoded exactly as described in {{DAP}}, except as noted in {{changes}}. The use of REPORT_COUNT in {{syn-bulk-report}} is a small abuse of the TLS syntax to signify that any number of reports are included. This "value" could be any positive integer. Unlike a variable-length field, as denoted with &lt; and >, this format does not require that the size of all included reports be known before constructing a request.

Necessary Changes to DAP Report Formats {#changes}

DAP currently encodes report extensions in the plaintext of shares. For extensions that contain public information this is inefficient for a couple of reasons:

  • Multiple copies of the data is included.

  • The use of per-record encryption prevents compression.

This document recommends the addition of a new public report extensions field to the ReportMetadata structure. This would be modified to include extensions that are public, as shown in {{syn-metadata-change}}.

struct {
  ReportID report_id;
  Time time;
  Extension extensions<0..2^16-1>; /* new */
} ReportMetadata;

{: #syn-metadata-change title="Proposed Report Metadata Format"}

This would be sufficient for many extensions, such as those that are defined in {{?DAP-DP-EXT=I-D.thomson-ppm-dap-dp-ext}}. As with the current design ({{Section 4.5.3 of DAP}}), unknown extensions would result in the report being rejected. The primary difference being that the leader can determine this before initiating the preparation phase.

Optional Removal of Existing Report Extensions {#removal}

The existing report extensions could also potentially be removed, as shown in {{syn-plaintext-change}}.

opaque PlaintextInputShare<0..2^32-1>;

/* the old format, for reference:
struct {
  Extension extensions<0..2^16-1>;
  opaque payload<0..2^32-1>;
} PlaintextInputShare;
*/

{: #syn-plaintext-change title="Proposed Plaintext Share Format"}

These private extensions currently have no defined purpose and add two bytes per aggregator to every report. The risk of removing them is that a purpose for a generic extension is discovered at some point in the future. Though specific VDAF instantiations might define their own extension container, this decision might limit the availability of extensions that apply to any VDAF.

Security Considerations {#security}

Report metadata, which would include the extensions if the recommendations in {{changes}} are adopted, are included in the additional associated data for every report. Bulk submission is therefore strictly a performance optimization as far as the operation of DAP is concerned.

The potential for a single client to generate large amounts of work for a DAP service is a serious threat to service availability. Any DAP leader SHOULD implement measures to defend against resource exhaustion attacks through this interface. This might include strong authentication of the requester. The logical entity to make this request is a collector, which is likely to be known to the leader.

The addition of public extensions exposes more information to the leader. This might be used by a malicious leader to selectively remove reports. A leader is already able to do this without helper awareness, but the added information might allow this to be more selective.

IANA Considerations {#iana}

IANA is requested to register at time of publication the "application/dap-bulk-report" media type in the "Media Types" registry at {: brackets="angle"}, following the procedures of {{!RFC6838}}. That registration includes the following:

Type name:

: application

Subtype name:

: dap-bulk-report

Required parameters:

: N/A

Optional parameters:

: N/A

Encoding considerations:

: "binary"

Security considerations:

: See {{security}}

Interoperability considerations:

: N/A

Published specification:

: this document

Applications that use this media type:

: This type identifies a bulk report submission for the Distributed Aggregation Protocol.

Fragment identifier considerations:

: N/A

Additional information:

:

Magic number(s):
N/A
Deprecated alias names for this type:
N/A
File extension(s):
N/A
Macintosh file type code(s):
N/A

Person and email address to contact for further information:

:
See Authors' Addresses section

Intended usage:

: COMMON

Restrictions on usage:

: N/A

Author:

: See Authors' Addresses section

Change controller:

: IETF {: spacing="compact"}

--- back

Acknowledgments

{:numbered="false"}

TODO acknowledge.