Internet-Draft Bulk DAP Submission October 2024
Thomson & Koshelev Expires 21 April 2025 [Page]
Workgroup:
Privacy Preserving Measurement
Internet-Draft:
draft-thomson-ppm-dap-bulk-00
Published:
Intended Status:
Informational
Expires:
Authors:
M. Thomson
Mozilla
A. Koshelev
Meta

Bulk Report Submission for Distributed Aggregation Protocol (DAP)

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.

About This Document

This note is to be removed before publishing as an RFC.

The latest revision of this draft can be found at https://martinthomson.github.io/dap-bulk/draft-thomson-ppm-dap-bulk.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-thomson-ppm-dap-bulk/.

Discussion of this document takes place on the Privacy Preserving Measurement Working Group mailing list (mailto:ppm@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/ppm/. Subscribe at https://www.ietf.org/mailman/listinfo/ppm/.

Source for this draft and an issue tracker can be found at https://github.com/martinthomson/dap-bulk.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 21 April 2025.

Table of Contents

1. Introduction

The Distributed Aggregation Protocol (DAP) [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.

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. 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 (Section 4). A bulk request is equivalent to multiple separate submissions.

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

4. Bulk Submission 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;
Figure 1: 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.

The header is followed by any number of reports, which are encoded exactly as described in [DAP], except as noted in Section 4.1. The use of REPORT_COUNT in Figure 1 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.

4.1. Necessary Changes to DAP Report Formats

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 Figure 2.

struct {
  ReportID report_id;
  Time time;
  Extension extensions<0..2^16-1>; /* new */
} ReportMetadata;
Figure 2: Proposed Report Metadata Format

This would be sufficient for many extensions, such as those that are defined in [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.

4.2. Optional Removal of Existing Report Extensions

The existing report extensions could also potentially be removed, as shown in Figure 3.

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;
*/
Figure 3: 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.

5. Security Considerations

Report metadata, which would include the extensions if the recommendations in Section 4.1 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.

6. IANA Considerations

IANA is requested to register at time of publication the "application/dap-bulk-report" media type in the "Media Types" registry at <https://iana.org/assignments/media-types>, 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 Section 5

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

7. References

7.1. Normative References

[DAP]
Geoghegan, T., Patton, C., Pitman, B., Rescorla, E., and C. A. Wood, "Distributed Aggregation Protocol for Privacy Preserving Measurement", Work in Progress, Internet-Draft, draft-ietf-ppm-dap-12, , <https://datatracker.ietf.org/doc/html/draft-ietf-ppm-dap-12>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC6838]
Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, , <https://www.rfc-editor.org/rfc/rfc6838>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.

7.2. Informative References

[DAP-DP-EXT]
Thomson, M., "Distributed Aggregation Protocol (DAP) Extensions for Improved Application of Differential Privacy", Work in Progress, Internet-Draft, draft-thomson-ppm-dap-dp-ext-00, , <https://datatracker.ietf.org/doc/html/draft-thomson-ppm-dap-dp-ext-00>.
[RFC9458]
Thomson, M. and C. A. Wood, "Oblivious HTTP", RFC 9458, DOI 10.17487/RFC9458, , <https://www.rfc-editor.org/rfc/rfc9458>.

Acknowledgments

TODO acknowledge.

Authors' Addresses

Martin Thomson
Mozilla
Alex Koshelev
Meta