C-DNS: A DNS Packet Capture Format
draft-ietf-dnsop-dns-capture-format-09

Document Type Active Internet-Draft (dnsop WG)
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Replaces draft-dickinson-dnsop-dns-capture-format
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dnsop                                                       J. Dickinson
Internet-Draft                                                  J. Hague
Intended status: Standards Track                            S. Dickinson
Expires: June 3, 2019                                         Sinodun IT
                                                            T. Manderson
                                                                 J. Bond
                                                                   ICANN
                                                       November 30, 2018

                   C-DNS: A DNS Packet Capture Format
                 draft-ietf-dnsop-dns-capture-format-09

Abstract

   This document describes a data representation for collections of DNS
   messages.  The format is designed for efficient storage and
   transmission of large packet captures of DNS traffic; it attempts to
   minimize the size of such packet capture files but retain the full
   DNS message contents along with the most useful transport metadata.
   It is intended to assist with the development of DNS traffic
   monitoring applications.

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 http://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 June 3, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Data collection use cases . . . . . . . . . . . . . . . . . .   5
   4.  Design considerations . . . . . . . . . . . . . . . . . . . .   7
   5.  Choice of CBOR  . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  C-DNS format conceptual overview  . . . . . . . . . . . . . .   9
     6.1.  Block Parameters  . . . . . . . . . . . . . . . . . . . .  13
     6.2.  Storage Parameters  . . . . . . . . . . . . . . . . . . .  13
       6.2.1.  Optional data items . . . . . . . . . . . . . . . . .  14
       6.2.2.  Optional RRs and OPCODEs  . . . . . . . . . . . . . .  15
       6.2.3.  Storage flags . . . . . . . . . . . . . . . . . . . .  15
       6.2.4.  IP Address storage  . . . . . . . . . . . . . . . . .  16
   7.  C-DNS format detailed description . . . . . . . . . . . . . .  16
     7.1.  Map quantities and indexes  . . . . . . . . . . . . . . .  16
     7.2.  Tabular representation  . . . . . . . . . . . . . . . . .  17
     7.3.  "File"  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.4.  "FilePreamble"  . . . . . . . . . . . . . . . . . . . . .  18
       7.4.1.  "BlockParameters" . . . . . . . . . . . . . . . . . .  19
       7.4.2.  "CollectionParameters"  . . . . . . . . . . . . . . .  22
     7.5.  "Block" . . . . . . . . . . . . . . . . . . . . . . . . .  24
       7.5.1.  "BlockPreamble" . . . . . . . . . . . . . . . . . . .  24
       7.5.2.  "BlockStatistics" . . . . . . . . . . . . . . . . . .  25
       7.5.3.  "BlockTables" . . . . . . . . . . . . . . . . . . . .  26
     7.6.  "QueryResponse" . . . . . . . . . . . . . . . . . . . . .  32
       7.6.1.  "ResponseProcessingData"  . . . . . . . . . . . . . .  34
       7.6.2.  "QueryResponseExtended" . . . . . . . . . . . . . . .  34
     7.7.  "AddressEventCount" . . . . . . . . . . . . . . . . . . .  35
     7.8.  "MalformedMessage"  . . . . . . . . . . . . . . . . . . .  36
   8.  Versioning  . . . . . . . . . . . . . . . . . . . . . . . . .  37
   9.  C-DNS to PCAP . . . . . . . . . . . . . . . . . . . . . . . .  37
     9.1.  Name compression  . . . . . . . . . . . . . . . . . . . .  38
   10. Data collection . . . . . . . . . . . . . . . . . . . . . . .  39
     10.1.  Matching algorithm . . . . . . . . . . . . . . . . . . .  40
     10.2.  Message identifiers  . . . . . . . . . . . . . . . . . .  42
       10.2.1.  Primary ID (required)  . . . . . . . . . . . . . . .  42
       10.2.2.  Secondary ID (optional)  . . . . . . . . . . . . . .  43
     10.3.  Algorithm parameters . . . . . . . . . . . . . . . . . .  43
     10.4.  Algorithm requirements . . . . . . . . . . . . . . . . .  43
     10.5.  Algorithm limitations  . . . . . . . . . . . . . . . . .  43

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     10.6.  Workspace  . . . . . . . . . . . . . . . . . . . . . . .  44
     10.7.  Output . . . . . . . . . . . . . . . . . . . . . . . . .  44
     10.8.  Post processing  . . . . . . . . . . . . . . . . . . . .  44
   11. Implementation guidance . . . . . . . . . . . . . . . . . . .  44
     11.1.  Optional data  . . . . . . . . . . . . . . . . . . . . .  45
     11.2.  Trailing bytes . . . . . . . . . . . . . . . . . . . . .  45
     11.3.  Limiting collection of RDATA . . . . . . . . . . . . . .  45
     11.4.  Timestamps . . . . . . . . . . . . . . . . . . . . . . .  45
   12. Implementation status . . . . . . . . . . . . . . . . . . . .  46
     12.1.  DNS-STATS Compactor  . . . . . . . . . . . . . . . . . .  46
   13. IANA considerations . . . . . . . . . . . . . . . . . . . . .  47
   14. Security considerations . . . . . . . . . . . . . . . . . . .  47
   15. Privacy considerations  . . . . . . . . . . . . . . . . . . .  47
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  48
   17. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  48
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     18.2.  Informative References . . . . . . . . . . . . . . . . .  52
     18.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  54
   Appendix A.  CDDL . . . . . . . . . . . . . . . . . . . . . . . .  55
   Appendix B.  DNS Name compression example . . . . . . . . . . . .  65
     B.1.  NSD compression algorithm . . . . . . . . . . . . . . . .  66
     B.2.  Knot Authoritative compression algorithm  . . . . . . . .  67
     B.3.  Observed differences  . . . . . . . . . . . . . . . . . .  67
   Appendix C.  Comparison of Binary Formats . . . . . . . . . . . .  67
     C.1.  Comparison with full PCAP files . . . . . . . . . . . . .  70
     C.2.  Simple versus block coding  . . . . . . . . . . . . . . .  71
     C.3.  Binary versus text formats  . . . . . . . . . . . . . . .  71
     C.4.  Performance . . . . . . . . . . . . . . . . . . . . . . .  71
     C.5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . .  72
     C.6.  Block size choice . . . . . . . . . . . . . . . . . . . .  72
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  73

1.  Introduction

   There has long been a need for server operators to collect DNS
   queries and responses on authoritative and recursive name servers for
   monitoring and analysis.  This data is used in a number of ways
   including traffic monitoring, analyzing network attacks and "day in
   the life" (DITL) [ditl] analysis.

   A wide variety of tools already exist that facilitate the collection
   of DNS traffic data, such as DSC [dsc], packetq [packetq], dnscap
   [dnscap] and dnstap [dnstap].  However, there is no standard exchange
   format for large DNS packet captures.  The PCAP [pcap] or PCAP-NG
   [pcapng] formats are typically used in practice for packet captures,
   but these file formats can contain a great deal of additional
   information that is not directly pertinent to DNS traffic analysis

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   and thus unnecessarily increases the capture file size.  Additionally
   these tools and formats typically have no filter mechanism to
   selectively record only certain fields at capture time, requiring
   post-processing for anonymization or pseudonymization of data to
   protect user privacy.

   There has also been work on using text based formats to describe DNS
   packets such as [I-D.daley-dnsxml], [RFC8427], but these are largely
   aimed at producing convenient representations of single messages.

   Many DNS operators may receive hundreds of thousands of queries per
   second on a single name server instance so a mechanism to minimize
   the storage and transmission size (and therefore upload overhead) of
   the data collected is highly desirable.

   The format described in this document, C-DNS (Compacted-DNS),
   focusses on the problem of capturing and storing large packet capture
   files of DNS traffic with the following goals in mind:

   o  Minimize the file size for storage and transmission.

   o  Minimize the overhead of producing the packet capture file and the
      cost of any further (general purpose) compression of the file.

   This document contains:

   o  A discussion of some common use cases in which DNS data is
      collected, see Section 3.

   o  A discussion of the major design considerations in developing an
      efficient data representation for collections of DNS messages, see
      Section 4.

   o  A description of why CBOR [RFC7049] was chosen for this format,
      see Section 5.

   o  A conceptual overview of the C-DNS format, see Section 6.

   o  The definition of the C-DNS format for the collection of DNS
      messages, see Section 7.

   o  Notes on converting C-DNS data to PCAP format, see Section 9.

   o  Some high level implementation considerations for applications
      designed to produce C-DNS, see Section 10.

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2.  Terminology

   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.

   "Packet" refers to an individual IPv4 or IPv6 packet.  Typically
   packets are UDP datagrams, but may also be part of a TCP data stream.
   "Message", unless otherwise qualified, refers to a DNS payload
   extracted from a UDP datagram or a TCP data stream.

   The parts of DNS messages are named as they are in [RFC1035].
   Specifically, the DNS message has five sections: Header, Question,
   Answer, Authority, and Additional.

   Pairs of DNS messages are called a Query and a Response.

3.  Data collection use cases

   From a purely server operator perspective, collecting full packet
   captures of all packets going in or out of a name server provides the
   most comprehensive picture of network activity.  However, there are
   several design choices or other limitations that are common to many
   DNS installations and operators.

   o  DNS servers are hosted in a variety of situations:

      *  Self-hosted servers

      *  Third party hosting (including multiple third parties)

      *  Third party hardware (including multiple third parties)

   o  Data is collected under different conditions:

      *  On well-provisioned servers running in a steady state

      *  On heavily loaded servers

      *  On virtualized servers

      *  On servers that are under DoS attack

      *  On servers that are unwitting intermediaries in DoS attacks

   o  Traffic can be collected via a variety of mechanisms:

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      *  Within the name server implementation itself

      *  On the same hardware as the name server itself

      *  Using a network tap on an adjacent host to listen to DNS
         traffic

      *  Using port mirroring to listen from another host

   o  The capabilities of data collection (and upload) networks vary:

      *  Out-of-band networks with the same capacity as the in-band
         network

      *  Out-of-band networks with less capacity than the in-band
         network

      *  Everything being on the in-band network

   Thus, there is a wide range of use cases from very limited data
   collection environments (third party hardware, servers that are under
   attack, packet capture on the name server itself and no out-of-band
   network) to "limitless" environments (self hosted, well provisioned
   servers, using a network tap or port mirroring with an out-of-band
   networks with the same capacity as the in-band network).  In the
   former, it is infeasible to reliably collect full packet captures,
   especially if the server is under attack.  In the latter case,
   collection of full packet captures may be reasonable.

   As a result of these restrictions, the C-DNS data format is designed
   with the most limited use case in mind such that:

   o  data collection will occur on the same hardware as the name server
      itself

   o  collected data will be stored on the same hardware as the name
      server itself, at least temporarily

   o  collected data being returned to some central analysis system will
      use the same network interface as the DNS queries and responses

   o  there can be multiple third party servers involved

   Because of these considerations, a major factor in the design of the
   format is minimal storage size of the capture files.

   Another significant consideration for any application that records
   DNS traffic is that the running of the name server software and the

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   transmission of DNS queries and responses are the most important jobs
   of a name server; capturing data is not.  Any data collection system
   co-located with the name server needs to be intelligent enough to
   carefully manage its CPU, disk, memory and network utilization.  This
   leads to designing a format that requires a relatively low overhead
   to produce and minimizes the requirement for further potentially
   costly compression.

   However, it is also essential that interoperability with less
   restricted infrastructure is maintained.  In particular, it is highly
   desirable that the collection format should facilitate the re-
   creation of common formats (such as PCAP) that are as close to the
   original as is realistic given the restrictions above.

4.  Design considerations

   This section presents some of the major design considerations used in
   the development of the C-DNS format.

   1.  The basic unit of data is a combined DNS Query and the associated
       Response (a "Q/R data item").  The same structure will be used
       for unmatched Queries and Responses.  Queries without Responses
       will be captured omitting the response data.  Responses without
       queries will be captured omitting the Query data (but using the
       Question section from the response, if present, as an identifying
       QNAME).

       *  Rationale: A Query and Response represents the basic level of
          a client's interaction with the server.  Also, combining the
          Query and Response into one item often reduces storage
          requirements due to commonality in the data of the two
          messages.

       In the context of generating a C-DNS file it is assumed that only
       those DNS payloads which can be parsed to produce a well-formed
       DNS message are stored in the C-DNS format and that all other
       messages will be (optionally) recorded as malformed messages.
       Parsing a well-formed message means as a minimum:

       *  The packet has a well-formed 12 byte DNS Header with a
          recognised OPCODE.

       *  The section counts are consistent with the section contents.

       *  All of the resource records can be fully parsed.

   2.  All top level fields in each Q/R data item will be optional.

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       *  Rationale: Different operators will have different
          requirements for data to be available for analysis.  Operators
          with minimal requirements should not have to pay the cost of
          recording full data, though this will limit the ability to
          perform certain kinds of data analysis and also to reconstruct
          packet captures.  For example, omitting the resource records
          from a Response will reduce the C-DNS file size; in principle
          responses can be synthesized if there is enough context.
          Operators may have different policies for collecting user data
          and can choose to omit or anonymize certain fields at capture
          time e.g. client address.

   3.  Multiple Q/R data items will be collected into blocks in the
       format.  Common data in a block will be abstracted and referenced
       from individual Q/R data items by indexing.  The maximum number
       of Q/R data items in a block will be configurable.

       *  Rationale: This blocking and indexing provides a significant
          reduction in the volume of file data generated.  Although this
          introduces complexity, it provides compression of the data
          that makes use of knowledge of the DNS message structure.

       *  It is anticipated that the files produced can be subject to
          further compression using general purpose compression tools.
          Measurements show that blocking significantly reduces the CPU
          required to perform such strong compression.  See
          Appendix C.2.

       *  Examples of commonality between DNS messages are that in most
          cases the QUESTION RR is the same in the query and response,
          and that there is a finite set of query signatures (based on a
          subset of attributes).  For many authoritative servers there
          is very likely to be a finite set of responses that are
          generated, of which a large number are NXDOMAIN.

   4.  Traffic metadata can optionally be included in each block.
       Specifically, counts of some types of non-DNS packets (e.g.
       ICMP, TCP resets) sent to the server may be of interest.

   5.  The wire format content of malformed DNS messages may optionally
       be recorded.

       *  Rationale: Any structured capture format that does not capture
          the DNS payload byte for byte will be limited to some extent
          in that it cannot represent malformed DNS messages.  Only
          those messages that can be fully parsed and transformed into
          the structured format can be fully represented.  Note,
          however, this can result in rather misleading statistics.  For

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          example, a malformed query which cannot be represented in the
          C-DNS format will lead to the (well formed) DNS responses with
          error code FORMERR appearing as 'unmatched'.  Therefore it can
          greatly aid downstream analysis to have the wire format of the
          malformed DNS messages available directly in the C-DNS file.

5.  Choice of CBOR

   This document presents a detailed format description using CBOR, the
   Concise Binary Object Representation defined in [RFC7049].

   The choice of CBOR was made taking a number of factors into account.

   o  CBOR is a binary representation, and thus is economical in storage
      space.

   o  Other binary representations were investigated, and whilst all had
      attractive features, none had a significant advantage over CBOR.
      See Appendix C for some discussion of this.

   o  CBOR is an IETF specification and familiar to IETF participants.
      It is based on the now-common ideas of lists and objects, and thus
      requires very little familiarization for those in the wider
      industry.

   o  CBOR is a simple format, and can easily be implemented from
      scratch if necessary.  More complex formats require library
      support which may present problems on unusual platforms.

   o  CBOR can also be easily converted to text formats such as JSON
      ([RFC8259]) for debugging and other human inspection requirements.

   o  CBOR data schemas can be described using CDDL
      [I-D.ietf-cbor-cddl].

6.  C-DNS format conceptual overview

   The following figures show purely schematic representations of the
   C-DNS format to convey the high-level structure of the C-DNS format.
   Section 7 provides a detailed discussion of the CBOR representation
   and individual elements.

   Figure 1 shows the C-DNS format at the top level including the file
   header and data blocks.  The Query/Response data items, Address/Event
   Count data items and Malformed Message data items link to various
   Block tables.

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                   +-------+
                   + C-DNS |
                   +-------+--------------------------+
                   | File type identifier             |
                   +----------------------------------+
                   | File preamble                    |
                   | +--------------------------------+
                   | | Format version info            |
                   | +--------------------------------+
                   | | Block parameters               |
                   +-+--------------------------------+
                   | Block                            |
                   | +--------------------------------+
                   | | Block preamble                 |
                   | +--------------------------------+
                   | | Block statistics               |
                   | +--------------------------------+
                   | | Block tables                   |
                   | +--------------------------------+
                   | | Query/Response data items      |
                   | +--------------------------------+
                   | | Address/Event Count data items |
                   | +--------------------------------+
                   | | Malformed Message data items   |
                   +-+--------------------------------+
                   | Block                            |
                   | +--------------------------------+
                   | | Block preamble                 |
                   | +--------------------------------+
                   | | Block statistics               |
                   | +--------------------------------+
                   | | Block tables                   |
                   | +--------------------------------+
                   | | Query/Response data items      |
                   | +--------------------------------+
                   | | Address/Event Count data items |
                   | +--------------------------------+
                   | | Malformed Message data items   |
                   +-+--------------------------------+
                   | Further Blocks...                |
                   +----------------------------------+

                        Figure 1: The C-DNS format.

   Figure 2 shows some more detailed relationships within each block,
   specifically those between the Query/Response data item and the
   relevant Block tables.

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   +----------------+
   | Query/Response |
   +-------------------------+
   | Time offset             |
   +-------------------------+             +------------------+
   | Client address          |------------>| IP address array |
   +-------------------------+             +------------------+
   | Client port             |
   +-------------------------+             +------------------+
   | Transaction ID          |     +------>| Name/RDATA array |<------+
   +-------------------------+     |       +------------------+       |
   | Query signature         |--+  |                                  |
   +-------------------------+  |  |       +-----------------+        |
   | Client hoplimit (q)     |  +--)------>| Query Signature |        |
   +-------------------------+     |       +-----------------+------+ |
   | Response delay (r)      |     |       | Server address         | |
   +-------------------------+     |       +------------------------+ |
   | Query name              |--+--+       | Server port            | |
   +-------------------------+  |          +------------------------+ |
   | Query size (q)          |  |          | Transport flags        | |
   +-------------------------+  |          +------------------------+ |
   | Response size (r)       |  |          | QR type                | |
   +-------------------------+  |          +------------------------+ |
   | Response processing (r) |  |          | QR signature flags     | |
   | +-----------------------+  |          +------------------------+ |
   | | Bailiwick index       |--+          | Query OPCODE (q)       | |
   | +-----------------------+             +------------------------+ |
   | | Flags                 |             | QR DNS flags           | |
   +-+-----------------------+             +------------------------+ |
   | Extra query info (q)    |             | Query RCODE (q)        | |
   | +-----------------------+             +------------------------+ |
   | | Question              |--+---+   +--+-Query Class/Type (q)   | |
   | +-----------------------+      |   |  +------------------------+ |
   | | Answer                |--+   |   |  | Query QD count (q)     | |
   | +-----------------------+  |   |   |  +------------------------+ |
   | | Authority             |--+   |   |  | Query AN count (q)     | |
   | +-----------------------+  |   |   |  +------------------------+ |
   | | Additional            |--+   |   |  | Query NS count (q)     | |
   +-+-----------------------+  |   |   |  +------------------------+ |
   | Extra response info (r) |  |-+ |   |  | Query EDNS version (q) | |
   | +-----------------------+  | | |   |  +------------------------+ |
   | | Answer                |--+ | |   |  | EDNS UDP size (q)      | |
   | +-----------------------+  | | |   |  +------------------------+ |
   | | Authority             |--+ | |   |  | Query Opt RDATA (q)    | |
   | +-----------------------+  | | |   |  +------------------------+ |
   | | Additional            |--+ | |   |  | Response RCODE (r)     | |
   +-+-----------------------+    | |   |  +------------------------+ |
                                  | |   |                             |

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                                  | |   |                             |
   + -----------------------------+ |   +----------+                  |
   |                                |              |                  |
   | + -----------------------------+              |                  |
   | |  +---------------+  +----------+            |                  |
   | +->| Question list |->| Question |            |                  |
   |    | array         |  | array    |            |                  |
   |    +---------------+  +----------+--+         |                  |
   |                       | Name        |--+------)------------------+
   |                       +-------------+  |      |  +------------+
   |                       | Class/type  |--)---+--+->| Class/Type |
   |                       +-------------+  |   |     | array      |
   |                                        |   |     +------------+--+
   |                                        |   |     | Class         |
   |    +---------------+  +----------+     |   |     +---------------+
   +--->| RR list array |->| RR array |     |   |     | Type          |
        +---------+-----+  +----------+--+  |   |     +---------------+
                           | Name        |--+   |
                           +-------------+      |
                           | Class/type  |------+
                           +-------------+

       Figure 2: The Query/Response data item and subsidiary tables.

   In Figure 2 data items annotated (q) are only present when a query/
   response has a query, and those annotated (r) are only present when a
   query/response response is present.

   A C-DNS file begins with a file header containing a File Type
   Identifier and a File Preamble.  The File Preamble contains
   information on the file Format Version and an array of Block
   Parameters items (the contents of which include Collection and
   Storage Parameters used for one or more blocks).

   The file header is followed by a series of data Blocks.

   A Block consists of a Block Preamble item, some Block Statistics for
   the traffic stored within the Block and then various arrays of common
   data collectively called the Block Tables.  This is then followed by
   an array of the Query/Response data items detailing the queries and
   responses stored within the Block.  The array of Query/Response data
   items is in turn followed by the Address/Event Counts data items (an
   array of per-client counts of particular IP events) and then
   Malformed Message data items (an array of malformed messages that
   stored in the Block).

   The exact nature of the DNS data will affect what block size is the
   best fit, however sample data for a root server indicated that block

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   sizes up to 10,000 Q/R data items give good results.  See
   Appendix C.6 for more details.

   This design exploits data commonality and block based storage to
   minimise the C-DNS file size.  As a result C-DNS cannot be streamed
   below the level of a block.

6.1.  Block Parameters

   The details of the Block Parameters items are not shown in the
   diagrams but are discussed here for context.

   An array of Block Parameters items is stored in the File Preamble
   (with a minimum of one item at index 0); a Block Parameters item
   consists of a collection of Storage and Collection Parameters that
   applies to any given Block.  An array is used in order to support use
   cases such as wanting to merge C-DNS files from different sources.
   The Block Preamble item then contains an optional index for the Block
   Parameters item that applies for that Block; if not present the index
   defaults to 0.  Hence, in effect, a global Block Parameters item is
   defined which can then be overridden per Block.

6.2.  Storage Parameters

   The Block Parameters item includes a Storage Parameters item - this
   contains information about the specific data fields stored in the
   C-DNS file.

   These parameters include:

   o  The sub-second timing resolution used by the data.

   o  Information (hints) on which optional data are omitted.  See
      Section 6.2.1.

   o  Recorded OPCODES [opcodes] and RR types [rrtypes].  See
      Section 6.2.2.

   o  Flags indicating, for example, whether the data is sampled or
      anonymized.  See Section 6.2.3 and Section 15.

   o  Client and server IPv4 and IPv6 address prefixes.  See
      Section 6.2.4

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6.2.1.  Optional data items

   To enable implementations to store data to their precise requirements
   in as space-efficient manner as possible, all fields in the following
   arrays are optional:

   o  Query/Response

   o  Query Signature

   o  Malformed messages

   In other words, an implementation can choose to omit any data item
   that is not required for its use case.  In addition, implementations
   may be configured to not record all RRs, or only record messages with
   certain OPCODES.

   This does, however, mean that a consumer of a C-DNS file faces two
   problems:

   1.  How can it quickly determine if a file definitely does not
       contain the data items it requires to complete a particular task
       (e.g. reconstructing query traffic or performing a specific piece
       of data analysis)?

   2.  How can it determine if a data item is not present because it
       was:

       *  explicitly not recorded or

       *  the data item was not available/present.

   For example, capturing C-DNS data from within a nameserver
   implementation makes it unlikely that the Client Hoplimit can be
   recorded.  Or, if there is no query ARCount recorded and no query OPT
   RDATA [RFC6891] recorded, is that because no query contained an OPT
   RR, or because that data was not stored?

   The Storage Parameters therefore also contains a Storage Hints item
   which specifies which items the encoder of the file omits from the
   stored data and will therefore never be present.  (This approach is
   taken because a flag that indicated which items were included for
   collection would not guarantee that the item was present, only that
   it might be.)  An implementation decoding that file can then use
   these to quickly determine whether the input data is rich enough for
   its needs.

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6.2.2.  Optional RRs and OPCODEs

   Also included in the Storage Parameters are explicit arrays listing
   the RR types and the OPCODEs to be recorded.  These remove any
   ambiguity over whether messages containing particular OPCODEs or are
   not present because they did not occur, or because the implementation
   is not configured to record them.

   In the case of OPCODEs, for a message to be fully parsable, the
   OPCODE must be known to the collecting implementation.  Any message
   with an OPCODE unknown to the collecting implementation cannot be
   validated as correctly formed, and so must be treated as malformed.
   Messages with OPCODES known to the recording application but not
   listed in the Storage Parameters are discarded by the recording
   application during C-DNS capture (regardless of whether they are
   malformed or not).

   In the case of RR records, each record in a message must be fully
   parsable, including parsing the record RDATA, as otherwise the
   message cannot be validated as correctly formed.  Any RR record with
   an RR type not known to the collecting implementation cannot be
   validated as correctly formed, and so must be treated as malformed.

   Once a message is correctly parsed, an implementation is free to
   record only a subset of the RR records present.

6.2.3.  Storage flags

   The Storage Parameters contains flags that can be used to indicate
   if:

   o  the data is anonymized,

   o  the data is produced from sample data, or

   o  names in the data have been normalized (converted to uniform
      case).

   The Storage Parameters also contains optional fields holding details
   of the sampling method used and the anonymization method used.  It is
   RECOMMENDED these fields contain URIs [RFC3986] pointing to resources
   describing the methods used.  See Section 15 for further discussion
   of anonymization and normalization.

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6.2.4.  IP Address storage

   The format can store either full IP addresses or just IP prefixes,
   the Storage Parameters contains fields to indicate if only IP
   prefixes were stored.

   If the IP address prefixes are absent, then full addresses are
   stored.  In this case the IP version can be directly inferred from
   the stored address length and the fields "qr-transport-flags" in
   QueryResponseSignature and "mm-transport-flags" in
   MalformedMessageData (which contain the IP version bit) are optional.

   If IP address prefixes are given, only the prefix bits of addresses
   are stored.  In this case the fields "qr-transport-flags" in
   QueryResponseSignature and "mm-transport-flags" in
   MalformedMessageData MUST be present, so that the IP version can be
   determined.  See Section 7.5.3.2 and Section 7.5.3.5.

   As an example of storing only IP prefixes, if a client IPv6 prefix of
   48 is specified, a client address of 2001:db8:85a3::8a2e:370:7334
   will be stored as 0x20010db885a3, reducing address storage space
   requirements.  Similarly, if a client IPv4 prefix of 16 is specified,
   a client address of 192.0.2.1 will be stored as 0xc000 (192.0).

7.  C-DNS format detailed description

   The CDDL definition for the C-DNS format is given in Appendix A.

7.1.  Map quantities and indexes

   All map keys are integers with values specified in the CDDL.  String
   keys would significantly bloat the file size.

   All key values specified are positive integers under 24, so their
   CBOR representation is a single byte.  Positive integer values not
   currently used as keys in a map are reserved for use in future
   standard extensions.

   Implementations may choose to add additional implementation-specific
   entries to any map.  Negative integer map keys are reserved for these
   values.  Key values from -1 to -24 also have a single byte CBOR
   representation, so such implementation-specific extensions are not at
   any space efficiency disadvantage.

   An item described as an index is the index of the data item in the
   referenced array.  Indexes are 0-based.

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7.2.  Tabular representation

   The following sections present the C-DNS specification in tabular
   format with a detailed description of each item.

   In all quantities that contain bit flags, bit 0 indicates the least
   significant bit, i.e. flag "n" in quantity "q" is on if "(q & (1 <<
   n)) != 0".

   For the sake of readability, all type and field names defined in the
   CDDL definition are shown in double quotes.  Type names are by
   convention camel case (e.g.  "BlockTable"), field names are lower-
   case with hyphens (e.g. "block-tables").

   For the sake of brevity, the following conventions are used in the
   tables:

   o  The column M marks whether items in a map are mandatory.

      *  X - Mandatory items.

      *  C - Conditionally mandatory item.  Such items are usually
         optional but may be mandatory in some configurations.

      *  If the column is empty, the item is optional.

   o  The column T gives the CBOR data type of the item.

      *  U - Unsigned integer

      *  I - Signed integer (i.e.  CBOR unsigned or negative integer)

      *  B - Boolean

      *  S - Byte string

      *  T - Text string

      *  M - Map

      *  A - Array

   In the case of maps and arrays, more information on the type of each
   value, include the CDDL definition name if applicable, is given in
   the description.

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7.3.  "File"

   A C-DNS file has an outer structure "File", a map that contains the
   following:

   +---------------+---+---+-------------------------------------------+
   | Field         | M | T | Description                               |
   +---------------+---+---+-------------------------------------------+
   | file-type-id  | X | T | String "C-DNS" identifying the file type. |
   |               |   |   |                                           |
   | file-preamble | X | M | Version and parameter information for the |
   |               |   |   | whole file. Map of type "FilePreamble",   |
   |               |   |   | see Section 7.4.                          |
   |               |   |   |                                           |
   | file-blocks   | X | A | Array of items of type "Block", see       |
   |               |   |   | Section 7.5. The array may be empty if    |
   |               |   |   | the file contains no data.                |
   +---------------+---+---+-------------------------------------------+

7.4.  "FilePreamble"

   Information about data in the file.  A map containing the following:

   +----------------------+---+---+------------------------------------+
   | Field                | M | T | Description                        |
   +----------------------+---+---+------------------------------------+
   | major-format-version | X | U | Unsigned integer '1'. The major    |
   |                      |   |   | version of format used in file.    |
   |                      |   |   | See Section 8.                     |
   |                      |   |   |                                    |
   | minor-format-version | X | U | Unsigned integer '0'. The minor    |
   |                      |   |   | version of format used in file.    |
   |                      |   |   | See Section 8.                     |
   |                      |   |   |                                    |
   | private-version      |   | U | Version indicator available for    |
   |                      |   |   | private use by implementations.    |
   |                      |   |   |                                    |
   | block-parameters     | X | A | Array of items of type             |
   |                      |   |   | "BlockParameters", see Section     |
   |                      |   |   | 7.4.1. The array must contain at   |
   |                      |   |   | least one entry. (The "block-      |
   |                      |   |   | parameters-index" item in each     |
   |                      |   |   | "BlockPreamble" indicates which    |
   |                      |   |   | array entry applies to that        |
   |                      |   |   | "Block".)                          |
   +----------------------+---+---+------------------------------------+

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7.4.1.  "BlockParameters"

   Parameters relating to data storage and collection which apply to one
   or more items of type "Block".  A map containing the following:

   +-----------------------+---+---+-----------------------------------+
   | Field                 | M | T | Description                       |
   +-----------------------+---+---+-----------------------------------+
   | storage-parameters    | X | M | Parameters relating to data       |
   |                       |   |   | storage in a "Block" item.  Map   |
   |                       |   |   | of type "StorageParameters", see  |
   |                       |   |   | Section 7.4.1.1.                  |
   |                       |   |   |                                   |
   | collection-parameters |   | M | Parameters relating to collection |
   |                       |   |   | of the data in a "Block" item.    |
   |                       |   |   | Map of type                       |
   |                       |   |   | "CollectionParameters", see       |
   |                       |   |   | Section 7.4.2.                    |
   +-----------------------+---+---+-----------------------------------+

7.4.1.1.  "StorageParameters"

   Parameters relating to how data is stored in the items of type
   "Block".  A map containing the following:

   +------------------+---+---+----------------------------------------+
   | Field            | M | T | Description                            |
   +------------------+---+---+----------------------------------------+
   | ticks-per-second | X | U | Sub-second timing is recorded in       |
   |                  |   |   | ticks. This specifies the number of    |
   |                  |   |   | ticks in a second.                     |
   |                  |   |   |                                        |
   | max-block-items  | X | U | The maximum number of items stored in  |
   |                  |   |   | any of the arrays in a "Block" item    |
   |                  |   |   | (Q/R items, address event counts or    |
   |                  |   |   | malformed messages). An indication to  |
   |                  |   |   | a decoder of the resources needed to   |
   |                  |   |   | process the file.                      |
   |                  |   |   |                                        |
   | storage-hints    | X | M | Collection of hints as to which fields |
   |                  |   |   | are omitted in the arrays that have    |
   |                  |   |   | optional fields. Map of type           |
   |                  |   |   | "StorageHints", see Section 7.4.1.1.1. |
   |                  |   |   |                                        |
   | opcodes          | X | A | Array of OPCODES [opcodes] (unsigned   |
   |                  |   |   | integers, each in the range 0 to 15    |
   |                  |   |   | inclusive) recorded by the collection  |
   |                  |   |   | implementation. See Section 6.2.2.     |

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   |                  |   |   |                                        |
   | rr-types         | X | A | Array of RR types [rrtypes] (unsigned  |
   |                  |   |   | integers, each in the range 0 to 65535 |
   |                  |   |   | inclusive) recorded by the collection  |
   |                  |   |   | implementation. See Section 6.2.2.     |
   |                  |   |   |                                        |
   | storage-flags    |   | U | Bit flags indicating attributes of     |
   |                  |   |   | stored data.                           |
   |                  |   |   | Bit 0. 1 if the data has been          |
   |                  |   |   | anonymized.                            |
   |                  |   |   | Bit 1. 1 if the data is sampled data.  |
   |                  |   |   | Bit 2. 1 if the names have been        |
   |                  |   |   | normalized (converted to uniform       |
   |                  |   |   | case).                                 |
   |                  |   |   |                                        |
   | client-address   |   | U | IPv4 client address prefix length, in  |
   | -prefix-ipv4     |   |   | the range 1 to 32 inclusive. If        |
   |                  |   |   | specified, only the address prefix     |
   |                  |   |   | bits are stored.                       |
   |                  |   |   |                                        |
   | client-address   |   | U | IPv6 client address prefix length, in  |
   | -prefix-ipv6     |   |   | the range 1 to 128 inclusive. If       |
   |                  |   |   | specified, only the address prefix     |
   |                  |   |   | bits are stored.                       |
   |                  |   |   |                                        |
   | server-address   |   | U | IPv4 server address prefix length, in  |
   | -prefix-ipv4     |   |   | the range 1 to 32 inclusive. If        |
   |                  |   |   | specified, only the address prefix     |
   |                  |   |   | bits are stored.                       |
   |                  |   |   |                                        |
   | server-address   |   | U | IPv6 server address prefix length, in  |
   | -prefix-ipv6     |   |   | the range 1 to 128 inclusive. If       |
   |                  |   |   | specified, only the address prefix     |
   |                  |   |   | bits are stored.                       |
   |                  |   |   |                                        |
   | sampling-method  |   | T | Information on the sampling method     |
   |                  |   |   | used. See Section 6.2.3.               |
   |                  |   |   |                                        |
   | anonymization    |   | T | Information on the anonymization       |
   | -method          |   |   | method used. See Section 6.2.3.        |
   +------------------+---+---+----------------------------------------+

7.4.1.1.1.  "StorageHints"

   An indicator of which fields the collecting implementation omits in
   the maps with optional fields.  A map containing the following:

   +------------------+---+---+----------------------------------------+

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   | Field            | M | T | Description                            |
   +------------------+---+---+----------------------------------------+
   | query-response   | X | U | Hints indicating which "QueryResponse" |
   | -hints           |   |   | fields are candidates for capture or   |
   |                  |   |   | omitted, see section Section 7.6. If a |
   |                  |   |   | bit is unset, the field is omitted     |
   |                  |   |   | from the capture.                      |
   |                  |   |   | Bit 0. time-offset                     |
   |                  |   |   | Bit 1. client-address-index            |
   |                  |   |   | Bit 2. client-port                     |
   |                  |   |   | Bit 3. transaction-id                  |
   |                  |   |   | Bit 4. qr-signature-index              |
   |                  |   |   | Bit 5. client-hoplimit                 |
   |                  |   |   | Bit 6. response-delay                  |
   |                  |   |   | Bit 7. query-name-index                |
   |                  |   |   | Bit 8. query-size                      |
   |                  |   |   | Bit 9. response-size                   |
   |                  |   |   | Bit 10. response-processing-data       |
   |                  |   |   | Bit 11. query-question-sections        |
   |                  |   |   | Bit 12. query-answer-sections          |
   |                  |   |   | Bit 13. query-authority-sections       |
   |                  |   |   | Bit 14. query-additional-sections      |
   |                  |   |   | Bit 15. response-answer-sections       |
   |                  |   |   | Bit 16. response-authority-sections    |
   |                  |   |   | Bit 17. response-additional-sections   |
   |                  |   |   |                                        |
   | query-response   | X | U | Hints indicating which                 |
   | -signature-hints |   |   | "QueryResponseSignature" fields are    |
   |                  |   |   | candidates for capture or omitted, see |
   |                  |   |   | section Section 7.5.3.2. If a bit is   |
   |                  |   |   | unset, the field is omitted from the   |
   |                  |   |   | capture.                               |
   |                  |   |   | Bit 0. server-address                  |
   |                  |   |   | Bit 1. server-port                     |
   |                  |   |   | Bit 2. qr-transport-flags              |
   |                  |   |   | Bit 3. qr-type                         |
   |                  |   |   | Bit 4. qr-sig-flags                    |
   |                  |   |   | Bit 5. query-opcode                    |
   |                  |   |   | Bit 6. dns-flags                       |
   |                  |   |   | Bit 7. query-rcode                     |
   |                  |   |   | Bit 8. query-class-type                |
   |                  |   |   | Bit 9. query-qdcount                   |
   |                  |   |   | Bit 10. query-ancount                  |
   |                  |   |   | Bit 11. query-nscount                  |
   |                  |   |   | Bit 12. query-arcount                  |
   |                  |   |   | Bit 13. query-edns-version             |
   |                  |   |   | Bit 14. query-udp-size                 |
   |                  |   |   | Bit 15. query-opt-rdata                |

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   |                  |   |   | Bit 16. response-rcode                 |
   |                  |   |   |                                        |
   | rr-hints         | X | U | Hints indicating which optional "RR"   |
   |                  |   |   | fields are candidates for capture or   |
   |                  |   |   | omitted, see Section 7.5.3.4. If a bit |
   |                  |   |   | is unset, the field is omitted from    |
   |                  |   |   | the capture.                           |
   |                  |   |   | Bit 0. ttl                             |
   |                  |   |   | Bit 1. rdata-index                     |
   | other-data-hints | X | U | Hints indicating which other data      |
   |                  |   |   | types are omitted. If a bit is unset,  |
   |                  |   |   | the the data type is omitted from the  |
   |                  |   |   | capture.                               |
   |                  |   |   | Bit 0. malformed-messages              |
   |                  |   |   | Bit 1. address-event-counts            |
   +------------------+---+---+----------------------------------------+

7.4.2.  "CollectionParameters"

   Parameters providing information to how data in the file was
   collected (applicable for some, but not all collection environments).
   The values are informational only and serve as hints to downstream
   analysers as to the configuration of a collecting implementation.
   They can provide context when interpreting what data is present/
   absent from the capture but cannot necessarily be validated against
   the data captured.

   These parameters have no default.  If they do not appear, nothing can
   be inferred about their value.

   A map containing the following items:

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   +------------------+---+---+----------------------------------------+
   | Field            | M | T | Description                            |
   +------------------+---+---+----------------------------------------+
   | query-timeout    |   | U | To be matched with a query, a response |
   |                  |   |   | must arrive within this number of      |
   |                  |   |   | seconds.                               |
   |                  |   |   |                                        |
   | skew-timeout     |   | U | The network stack may report a         |
   |                  |   |   | response before the corresponding      |
   |                  |   |   | query. A response is not considered to |
   |                  |   |   | be missing a query until after this    |
   |                  |   |   | many micro-seconds.                    |
   |                  |   |   |                                        |
   | snaplen          |   | U | Collect up to this many bytes per      |
   |                  |   |   | packet.                                |
   |                  |   |   |                                        |
   | promisc          |   | B | "true" if promiscuous mode             |
   |                  |   |   | [pcap-options] was enabled on the      |
   |                  |   |   | interface, "false" otherwise.          |
   |                  |   |   |                                        |
   | interfaces       |   | A | Array of identifiers (of type text     |
   |                  |   |   | string) of the interfaces used for     |
   |                  |   |   | collection.                            |
   |                  |   |   |                                        |
   | server-addresses |   | A | Array of server collection IP          |
   |                  |   |   | addresses (of type byte string). Hint  |
   |                  |   |   | for downstream analysers; does not     |
   |                  |   |   | affect collection.                     |
   |                  |   |   |                                        |
   | vlan-ids         |   | A | Array of identifiers (of type unsigned |
   |                  |   |   | integer, each in the range 1 to 4094   |
   |                  |   |   | inclusive) of VLANs [IEEE802.1Q]       |
   |                  |   |   | selected for collection. VLAN IDs are  |
   |                  |   |   | unique only within an administrative   |
   |                  |   |   | domain.                                |
   |                  |   |   |                                        |
   | filter           |   | T | "tcpdump" [pcap-filter] style filter   |
   |                  |   |   | for input.                             |
   |                  |   |   |                                        |
   | generator-id     |   | T | Implementation specific human-readable |
   |                  |   |   | string identifying the collection      |
   |                  |   |   | method.                                |
   |                  |   |   |                                        |
   | host-id          |   | T | String identifying the collecting      |
   |                  |   |   | host. Empty if converting an existing  |
   |                  |   |   | packet capture file.                   |
   +------------------+---+---+----------------------------------------+

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7.5.  "Block"

   Container for data with common collection and storage parameters.  A
   map containing the following:

   +--------------------+---+---+--------------------------------------+
   | Field              | M | T | Description                          |
   +--------------------+---+---+--------------------------------------+
   | block-preamble     | X | M | Overall information for the "Block"  |
   |                    |   |   | item. Map of type "BlockPreamble",   |
   |                    |   |   | see Section 7.5.1.                   |
   |                    |   |   |                                      |
   | block-statistics   |   | M | Statistics about the "Block" item.   |
   |                    |   |   | Map of type "BlockStatistics", see   |
   |                    |   |   | Section 7.5.2.                       |
   |                    |   |   |                                      |
   | block-tables       |   | M | The arrays containing data           |
   |                    |   |   | referenced by individual             |
   |                    |   |   | "QueryResponse" or                   |
   |                    |   |   | "MalformedMessage" items. Map of     |
   |                    |   |   | type "BlockTables", see Section      |
   |                    |   |   | 7.5.3.                               |
   |                    |   |   |                                      |
   | query-responses    |   | A | Details of individual DNS Q/R data   |
   |                    |   |   | items. Array of items of type        |
   |                    |   |   | "QueryResponse", see Section 7.6. If |
   |                    |   |   | present, the array must not be       |
   |                    |   |   | empty.                               |
   |                    |   |   |                                      |
   | address-event      |   | A | Per client counts of ICMP messages   |
   | -counts            |   |   | and TCP resets. Array of items of    |
   |                    |   |   | type "AddressEventCount", see        |
   |                    |   |   | Section 7.7. If present, the array   |
   |                    |   |   | must not be empty.                   |
   |                    |   |   |                                      |
   | malformed-messages |   | A | Details of malformed DNS messages.   |
   |                    |   |   | Array of items of type               |
   |                    |   |   | "MalformedMessage", see Section 7.8. |
   |                    |   |   | If present, the array must not be    |
   |                    |   |   | empty.                               |
   +--------------------+---+---+--------------------------------------+

7.5.1.  "BlockPreamble"

   Overall information for a "Block" item.  A map containing the
   following:

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   +------------------+---+---+----------------------------------------+
   | Field            | M | T | Description                            |
   +------------------+---+---+----------------------------------------+
   | earliest-time    | C | A | A timestamp (2 unsigned integers,      |
   |                  |   |   | "Timestamp") for the earliest record   |
   |                  |   |   | in the "Block" item. The first integer |
   |                  |   |   | is the number of seconds since the     |
   |                  |   |   | POSIX epoch [posix-time] ("time_t"),   |
   |                  |   |   | excluding leap seconds. The second     |
   |                  |   |   | integer is the number of ticks (see    |
   |                  |   |   | Section 7.4.1.1) since the start of    |
   |                  |   |   | the second. This field is mandatory    |
   |                  |   |   | unless all block items containing a    |
   |                  |   |   | time offset from the start of the      |
   |                  |   |   | block also omit that time offset.      |
   |                  |   |   |                                        |
   | block-parameters |   | U | The index of the item in the "block-   |
   | -index           |   |   | parameters" array (in the "file-       |
   |                  |   |   | premable" item) applicable to this     |
   |                  |   |   | block. If not present, index 0 is      |
   |                  |   |   | used. See Section 7.4.1.               |
   +------------------+---+---+----------------------------------------+

7.5.2.  "BlockStatistics"

   Basic statistical information about a "Block" item.  A map containing
   the following:

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   +---------------------+---+---+-------------------------------------+
   | Field               | M | T | Description                         |
   +---------------------+---+---+-------------------------------------+
   | processed-messages  |   | U | Total number of DNS messages        |
   |                     |   |   | processed from the input traffic    |
   |                     |   |   | stream during collection of data in |
   |                     |   |   | this "Block" item.                  |
   |                     |   |   |                                     |
   | qr-data-items       |   | U | Total number of Q/R data items in   |
   |                     |   |   | this "Block" item.                  |
   |                     |   |   |                                     |
   | unmatched-queries   |   | U | Number of unmatched queries in this |
   |                     |   |   | "Block" item.                       |
   |                     |   |   |                                     |
   | unmatched-responses |   | U | Number of unmatched responses in    |
   |                     |   |   | this "Block" item.                  |
   |                     |   |   |                                     |
   | discarded-opcode    |   | U | Number of DNS messages processed    |
   |                     |   |   | from the input traffic stream       |
   |                     |   |   | during collection of data in this   |
   |                     |   |   | "Block" item but not recorded       |
   |                     |   |   | because their OPCODE is not in the  |
   |                     |   |   | list to be collected.               |
   |                     |   |   |                                     |
   | malformed-items     |   | U | Number of malformed messages found  |
   |                     |   |   | in input for this "Block" item.     |
   +---------------------+---+---+-------------------------------------+

7.5.3.  "BlockTables"

   Map of arrays containing data referenced by individual
   "QueryResponse" or "MalformedMessage" items in this "Block".  Each
   element is an array which, if present, must not be empty.

   An item in the "qlist" array contains indexes to values in the "qrr"
   array.  Therefore, if "qlist" is present, "qrr" must also be present.
   Similarly, if "rrlist" is present, "rr" must also be present.

   The map contains the following items:

   +-------------------+---+---+---------------------------------------+
   | Field             | M | T | Description                           |
   +-------------------+---+---+---------------------------------------+
   | ip-address        |   | A | Array of IP addresses, in network     |
   |                   |   |   | byte order (of type byte string). If  |
   |                   |   |   | client or server address prefixes are |
   |                   |   |   | set, only the address prefix bits are |
   |                   |   |   | stored. Each string is therefore up   |

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   |                   |   |   | to 4 bytes long for an IPv4 address,  |
   |                   |   |   | or up to 16 bytes long for an IPv6    |
   |                   |   |   | address. See Section 7.4.1.1.         |
   |                   |   |   |                                       |
   | classtype         |   | A | Array of RR class and type            |
   |                   |   |   | information. Type is "ClassType", see |
   |                   |   |   | Section 7.5.3.1.                      |
   |                   |   |   |                                       |
   | name-rdata        |   | A | Array where each entry is the         |
   |                   |   |   | contents of a single NAME or RDATA in |
   |                   |   |   | wire format (of type byte string).    |
   |                   |   |   | Note that NAMEs, and labels within    |
   |                   |   |   | RDATA contents, are full domain names |
   |                   |   |   | or labels; no [RFC1035] name          |
   |                   |   |   | compression is used on the individual |
   |                   |   |   | names/labels within the format.       |
   |                   |   |   |                                       |
   | qr-sig            |   | A | Array Q/R data item signatures. Type  |
   |                   |   |   | is "QueryResponseSignature", see      |
   |                   |   |   | Section 7.5.3.2.                      |
   |                   |   |   |                                       |
   | qlist             |   | A | Array of type "QuestionList". A       |
   |                   |   |   | "QuestionList" is an array of         |
   |                   |   |   | unsigned integers, indexes to         |
   |                   |   |   | "Question" items in the "qrr" array.  |
   |                   |   |   |                                       |
   | qrr               |   | A | Array of type "Question". Each entry  |
   |                   |   |   | is the contents of a single question, |
   |                   |   |   | where a question is the second or     |
   |                   |   |   | subsequent question in a query. See   |
   |                   |   |   | Section 7.5.3.3.                      |
   |                   |   |   |                                       |
   | rrlist            |   | A | Array of type "RRList". An "RRList"   |
   |                   |   |   | is an array of unsigned integers,     |
   |                   |   |   | indexes to "RR" items in the "rr"     |
   |                   |   |   | array.                                |
   |                   |   |   |                                       |
   | rr                |   | A | Array of type "RR". Each entry is the |
   |                   |   |   | contents of a single RR. See Section  |
   |                   |   |   | 7.5.3.4.                              |
   |                   |   |   |                                       |
   | malformed-message |   | A | Array of the contents of malformed    |
   | -data             |   |   | messages.  Array of type              |
   |                   |   |   | "MalformedMessageData", see Section   |
   |                   |   |   | 7.5.3.5.                              |
   +-------------------+---+---+---------------------------------------+

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7.5.3.1.  "ClassType"

   RR class and type information.  A map containing the following:

               +-------+---+---+--------------------------+
               | Field | M | T | Description              |
               +-------+---+---+--------------------------+
               | type  | X | U | TYPE value [rrtypes].    |
               |       |   |   |                          |
               | class | X | U | CLASS value [rrclasses]. |
               +-------+---+---+--------------------------+

7.5.3.2.  "QueryResponseSignature"

   Elements of a Q/R data item that are often common between multiple
   individual Q/R data items.  A map containing the following:

   +--------------------+---+---+--------------------------------------+
   | Field              | M | T | Description                          |
   +--------------------+---+---+--------------------------------------+
   | server-address     |   | U | The index in the item in the "ip-    |
   | -index             |   |   | address" array of the server IP      |
   |                    |   |   | address. See Section 7.5.3.          |
   |                    |   |   |                                      |
   | server-port        |   | U | The server port.                     |
   |                    |   |   |                                      |
   | qr-transport-flags | C | U | Bit flags describing the transport   |
   |                    |   |   | used to service the query. Same      |
   |                    |   |   | definition as "mm-transport-flags"   |
   |                    |   |   | in Section 7.5.3.5, with an          |
   |                    |   |   | additional indicator for trailing    |
   |                    |   |   | bytes, see Appendix A.               |
   |                    |   |   | Bit 0. IP version. 0 if IPv4, 1 if   |
   |                    |   |   | IPv6. See Section 6.2.4.             |
   |                    |   |   | Bit 1-4. Transport. 4 bit unsigned   |
   |                    |   |   | value where 0 = UDP, 1 = TCP, 2 =    |
   |                    |   |   | TLS, 3 = DTLS [RFC7858], 4 = DoH     |
   |                    |   |   | [RFC8484]. Values 5-15 are reserved  |
   |                    |   |   | for future use.                      |
   |                    |   |   | Bit 5. 1 if trailing bytes in query  |
   |                    |   |   | packet. See Section 11.2.            |
   |                    |   |   |                                      |
   | qr-type            |   | U | Type of Query/Response transaction.  |
   |                    |   |   | 0 = Stub. A query from a stub        |
   |                    |   |   | resolver.                            |
   |                    |   |   | 1 = Client. An incoming query to a   |
   |                    |   |   | recursive resolver.                  |
   |                    |   |   | 2 = Resolver. A query sent from a    |

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   |                    |   |   | recursive resolver to an authorative |
   |                    |   |   | resolver.                            |
   |                    |   |   | 3 = Authorative. A query to an       |
   |                    |   |   | authorative resolver.                |
   |                    |   |   | 4 = Forwarder. A query sent from a   |
   |                    |   |   | recursive resolver to an upstream    |
   |                    |   |   | recursive resolver.                  |
   |                    |   |   | 5 = Tool. A query sent to a server   |
   |                    |   |   | by a server tool.                    |
   |                    |   |   |                                      |
   | qr-sig-flags       |   | U | Bit flags explicitly indicating      |
   |                    |   |   | attributes of the message pair       |
   |                    |   |   | represented by this Q/R data item    |
   |                    |   |   | (not all attributes may be recorded  |
   |                    |   |   | or deducible).                       |
   |                    |   |   | Bit 0. 1 if a Query was present.     |
   |                    |   |   | Bit 1. 1 if a Response was present.  |
   |                    |   |   | Bit 2. 1 if a Query was present and  |
   |                    |   |   | it had an OPT Resource Record.       |
   |                    |   |   | Bit 3. 1 if a Response was present   |
   |                    |   |   | and it had an OPT Resource Record.   |
   |                    |   |   | Bit 4. 1 if a Query was present but  |
   |                    |   |   | had no Question.                     |
   |                    |   |   | Bit 5. 1 if a Response was present   |
   |                    |   |   | but had no Question (only one query- |
   |                    |   |   | name-index is stored per Q/R item).  |
   |                    |   |   |                                      |
   | query-opcode       |   | U | Query OPCODE.                        |
   |                    |   |   |                                      |
   | qr-dns-flags       |   | U | Bit flags with values from the Query |
   |                    |   |   | and Response DNS flags. Flag values  |
   |                    |   |   | are 0 if the Query or Response is    |
   |                    |   |   | not present.                         |
   |                    |   |   | Bit 0. Query Checking Disabled (CD). |
   |                    |   |   | Bit 1. Query Authenticated Data      |
   |                    |   |   | (AD).                                |
   |                    |   |   | Bit 2. Query reserved (Z).           |
   |                    |   |   | Bit 3. Query Recursion Available     |
   |                    |   |   | (RA).                                |
   |                    |   |   | Bit 4. Query Recursion Desired (RD). |
   |                    |   |   | Bit 5. Query TrunCation (TC).        |
   |                    |   |   | Bit 6. Query Authoritative Answer    |
   |                    |   |   | (AA).                                |
   |                    |   |   | Bit 7. Query DNSSEC answer OK (DO).  |
   |                    |   |   | Bit 8. Response Checking Disabled    |
   |                    |   |   | (CD).                                |
   |                    |   |   | Bit 9. Response Authenticated Data   |
   |                    |   |   | (AD).                                |

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   |                    |   |   | Bit 10. Response reserved (Z).       |
   |                    |   |   | Bit 11. Response Recursion Available |
   |                    |   |   | (RA).                                |
   |                    |   |   | Bit 12. Response Recursion Desired   |
   |                    |   |   | (RD).                                |
   |                    |   |   | Bit 13. Response TrunCation (TC).    |
   |                    |   |   | Bit 14. Response Authoritative       |
   |                    |   |   | Answer (AA).                         |
   |                    |   |   |                                      |
   | query-rcode        |   | U | Query RCODE. If the Query contains   |
   |                    |   |   | OPT [RFC6891], this value            |
   |                    |   |   | incorporates any                     |
   |                    |   |   | EXTENDED_RCODE_VALUE [rcodes].       |
   |                    |   |   |                                      |
   | query-classtype    |   | U | The index to the item in the the     |
   | -index             |   |   | "classtype" array of the CLASS and   |
   |                    |   |   | TYPE of the first Question. See      |
   |                    |   |   | Section 7.5.3.                       |
   |                    |   |   |                                      |
   | query-qd-count     |   | U | The QDCOUNT in the Query, or         |
   |                    |   |   | Response if no Query present.        |
   |                    |   |   |                                      |
   | query-an-count     |   | U | Query ANCOUNT.                       |
   |                    |   |   |                                      |
   | query-ns-count     |   | U | Query NSCOUNT.                       |
   |                    |   |   |                                      |
   | query-ar-count     |   | U | Query ARCOUNT.                       |
   |                    |   |   |                                      |
   | edns-version       |   | U | The Query EDNS version.              |
   |                    |   |   |                                      |
   | udp-buf-size       |   | U | The Query EDNS sender's UDP payload  |
   |                    |   |   | size.                                |
   |                    |   |   |                                      |
   | opt-rdata-index    |   | U | The index in the "name-rdata" array  |
   |                    |   |   | of the OPT RDATA. See Section 7.5.3. |
   |                    |   |   |                                      |
   | response-rcode     |   | U | Response RCODE. If the Response      |
   |                    |   |   | contains OPT [RFC6891], this value   |
   |                    |   |   | incorporates any                     |
   |                    |   |   | EXTENDED_RCODE_VALUE [rcodes].       |
   +--------------------+---+---+--------------------------------------+

7.5.3.3.  "Question"

   Details on individual Questions in a Question section.  A map
   containing the following:

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   +-----------------+---+---+-----------------------------------------+
   | Field           | M | T | Description                             |
   +-----------------+---+---+-----------------------------------------+
   | name-index      | X | U | The index in the "name-rdata" array of  |
   |                 |   |   | the QNAME. See Section 7.5.3.           |
   |                 |   |   |                                         |
   | classtype-index | X | U | The index in the "classtype" array of   |
   |                 |   |   | the CLASS and TYPE of the Question. See |
   |                 |   |   | Section 7.5.3.                          |
   +-----------------+---+---+-----------------------------------------+

7.5.3.4.  "RR"

   Details on individual Resource Records in RR sections.  A map
   containing the following:

   +-----------------+---+---+-----------------------------------------+
   | Field           | M | T | Description                             |
   +-----------------+---+---+-----------------------------------------+
   | name-index      | X | U | The index in the "name-rdata" array of  |
   |                 |   |   | the NAME. See Section 7.5.3.            |
   |                 |   |   |                                         |
   | classtype-index | X | U | The index in the "classtype" array of   |
   |                 |   |   | the CLASS and TYPE of the RR. See       |
   |                 |   |   | Section 7.5.3.                          |
   |                 |   |   |                                         |
   | ttl             |   | U | The RR Time to Live.                    |
   |                 |   |   |                                         |
   | rdata-index     |   | U | The index in the "name-rdata" array of  |
   |                 |   |   | the RR RDATA. See Section 7.5.3.        |
   +-----------------+---+---+-----------------------------------------+

7.5.3.5.  "MalformedMessageData"

   Details on malformed message items in this "Block" item.  A map
   containing the following:

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   +--------------------+---+---+--------------------------------------+
   | Field              | M | T | Description                          |
   +--------------------+---+---+--------------------------------------+
   | server-address     |   | U | The index in the "ip-address" array  |
   | -index             |   |   | of the server IP address. See        |
   |                    |   |   | Section 7.5.3.                       |
   |                    |   |   |                                      |
   | server-port        |   | U | The server port.                     |
   |                    |   |   |                                      |
   | mm-transport-flags | C | U | Bit flags describing the transport   |
   |                    |   |   | used to service the query, see       |
   |                    |   |   | Section 6.2.4.                       |
   |                    |   |   | Bit 0. IP version. 0 if IPv4, 1 if   |
   |                    |   |   | IPv6                                 |
   |                    |   |   | Bit 1-4. Transport. 4 bit unsigned   |
   |                    |   |   | value where 0 = UDP, 1 = TCP, 2 =    |
   |                    |   |   | TLS, 3 = DTLS [RFC7858], 4 = DoH     |
   |                    |   |   | [RFC8484]. Values 5-15 are reserved  |
   |                    |   |   | for future use.                      |
   |                    |   |   |                                      |
   | mm-payload         |   | S | The payload (raw bytes) of the DNS   |
   |                    |   |   | message.                             |
   +--------------------+---+---+--------------------------------------+

7.6.  "QueryResponse"

   Details on individual Q/R data items.

   Note that there is no requirement that the elements of the "query-
   responses" array are presented in strict chronological order.

   A map containing the following items:

   +----------------------+---+---+------------------------------------+
   | Field                | M | T | Description                        |
   +----------------------+---+---+------------------------------------+
   | time-offset          |   | U | Q/R timestamp as an offset in      |
   |                      |   |   | ticks (see Section 7.4.1.1) from   |
   |                      |   |   | "earliest-time". The timestamp is  |
   |                      |   |   | the timestamp of the Query, or the |
   |                      |   |   | Response if there is no Query.     |
   |                      |   |   |                                    |
   | client-address-index |   | U | The index in the "ip-address"      |
   |                      |   |   | array of the client IP address.    |
   |                      |   |   | See Section 7.5.3.                 |
   |                      |   |   |                                    |
   | client-port          |   | U | The client port.                   |
   |                      |   |   |                                    |

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   | transaction-id       |   | U | DNS transaction identifier.        |
   |                      |   |   |                                    |
   | qr-signature-index   |   | U | The index in the "qr-sig" array of |
   |                      |   |   | the "QueryResponseSignature" item. |
   |                      |   |   | See Section 7.5.3.                 |
   |                      |   |   |                                    |
   | client-hoplimit      |   | U | The IPv4 TTL or IPv6 Hoplimit from |
   |                      |   |   | the Query packet.                  |
   |                      |   |   |                                    |
   | response-delay       |   | I | The time difference between Query  |
   |                      |   |   | and Response, in ticks (see        |
   |                      |   |   | Section 7.4.1.1). Only present if  |
   |                      |   |   | there is a query and a response.   |
   |                      |   |   | The delay can be negative if the   |
   |                      |   |   | network stack/capture library      |
   |                      |   |   | returns packets out of order.      |
   |                      |   |   |                                    |
   | query-name-index     |   | U | The index in the "name-rdata"      |
   |                      |   |   | array of the item containing the   |
   |                      |   |   | QNAME for the first Question. See  |
   |                      |   |   | Section 7.5.3.                     |
   |                      |   |   |                                    |
   | query-size           |   | U | DNS query message size (see        |
   |                      |   |   | below).                            |
   |                      |   |   |                                    |
   | response-size        |   | U | DNS response message size (see     |
   |                      |   |   | below).                            |
   |                      |   |   |                                    |
   | response-processing  |   | M | Data on response processing. Map   |
   | -data                |   |   | of type "ResponseProcessingData",  |
   |                      |   |   | see Section 7.6.1.                 |
   |                      |   |   |                                    |
   | query-extended       |   | M | Extended Query data. Map of type   |
   |                      |   |   | "QueryResponseExtended", see       |
   |                      |   |   | Section 7.6.2.                     |
   |                      |   |   |                                    |
   | response-extended    |   | M | Extended Response data. Map of     |
   |                      |   |   | type "QueryResponseExtended", see  |
   |                      |   |   | Section 7.6.2.                     |
   +----------------------+---+---+------------------------------------+

   The "query-size" and "response-size" fields hold the DNS message
   size.  For UDP this is the size of the UDP payload that contained the
   DNS message.  For TCP it is the size of the DNS message as specified
   in the two-byte message length header.  Trailing bytes in UDP queries
   are routinely observed in traffic to authoritative servers and this
   value allows a calculation of how many trailing bytes were present.

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7.6.1.  "ResponseProcessingData"

   Information on the server processing that produced the response.  A
   map containing the following:

   +------------------+---+---+----------------------------------------+
   | Field            | M | T | Description                            |
   +------------------+---+---+----------------------------------------+
   | bailiwick-index  |   | U | The index in the "name-rdata" array of |
   |                  |   |   | the owner name for the response        |
   |                  |   |   | bailiwick. See Section 7.5.3.          |
   |                  |   |   |                                        |
   | processing-flags |   | U | Flags relating to response processing. |
   |                  |   |   | Bit 0. 1 if the response came from     |
   |                  |   |   | cache.                                 |
   +------------------+---+---+----------------------------------------+

7.6.2.  "QueryResponseExtended"

   Extended data on the Q/R data item.

   Each item in the map is present only if collection of the relevant
   details is configured.

   A map containing the following items:

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   +------------------+---+---+----------------------------------------+
   | Field            | M | T | Description                            |
   +------------------+---+---+----------------------------------------+
   | question-index   |   | U | The index in the "qlist" array of the  |
   |                  |   |   | entry listing any second and           |
   |                  |   |   | subsequent Questions in the Question   |
   |                  |   |   | section for the Query or Response. See |
   |                  |   |   | Section 7.5.3.                         |
   |                  |   |   |                                        |
   | answer-index     |   | U | The index in the "rrlist" array of the |
   |                  |   |   | entry listing the Answer Resource      |
   |                  |   |   | Record sections for the Query or       |
   |                  |   |   | Response. See Section 7.5.3.           |
   |                  |   |   |                                        |
   | authority-index  |   | U | The index in the "rrlist" array of the |
   |                  |   |   | entry listing the Authority Resource   |
   |                  |   |   | Record sections for the Query or       |
   |                  |   |   | Response. See Section 7.5.3.           |
   |                  |   |   |                                        |
   | additional-index |   | U | The index in the "rrlist" array of the |
   |                  |   |   | entry listing the Additional Resource  |
   |                  |   |   | Record sections for the Query or       |
   |                  |   |   | Response. See Section 7.5.3. Note that |
   |                  |   |   | Query OPT RR data can be optionally    |
   |                  |   |   | stored in the QuerySignature.          |
   +------------------+---+---+----------------------------------------+

7.7.  "AddressEventCount"

   Counts of various IP related events relating to traffic with
   individual client addresses.  A map containing the following:

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   +------------------+---+---+----------------------------------------+
   | Field            | M | T | Description                            |
   +------------------+---+---+----------------------------------------+
   | ae-type          | X | U | The type of event. The following       |
   |                  |   |   | events types are currently defined:    |
   |                  |   |   | 0. TCP reset.                          |
   |                  |   |   | 1. ICMP time exceeded.                 |
   |                  |   |   | 2. ICMP destination unreachable.       |
   |                  |   |   | 3. ICMPv6 time exceeded.               |
   |                  |   |   | 4. ICMPv6 destination unreachable.     |
   |                  |   |   | 5. ICMPv6 packet too big.              |
   |                  |   |   |                                        |
   | ae-code          |   | U | A code relating to the event. For ICMP |
   |                  |   |   | or ICMPv6 events, this MUST be the     |
   |                  |   |   | ICMP [RFC0792] or ICMPv6 [RFC4443]     |
   |                  |   |   | code. For other events the contents    |
   |                  |   |   | are undefined.                         |
   |                  |   |   |                                        |
   | ae-address-index | X | U | The index in the "ip-address" array of |
   |                  |   |   | the client address. See Section 7.5.3. |
   |                  |   |   |                                        |
   | ae-count         | X | U | The number of occurrences of this      |
   |                  |   |   | event during the block collection      |
   |                  |   |   | period.                                |
   +------------------+---+---+----------------------------------------+

7.8.  "MalformedMessage"

   Details of malformed messages.  A map containing the following:

   +----------------------+---+---+------------------------------------+
   | Field                | M | T | Description                        |
   +----------------------+---+---+------------------------------------+
   | time-offset          |   | U | Message timestamp as an offset in  |
   |                      |   |   | ticks (see Section 7.4.1.1) from   |
   |                      |   |   | "earliest-time".                   |
   |                      |   |   |                                    |
   | client-address-index |   | U | The index in the "ip-address"      |
   |                      |   |   | array of the client IP address.    |
   |                      |   |   | See Section 7.5.3.                 |
   |                      |   |   |                                    |
   | client-port          |   | U | The client port.                   |
   |                      |   |   |                                    |
   | message-data-index   |   | U | The index in the "malformed-       |
   |                      |   |   | message-data" array of the message |
   |                      |   |   | data for this message. See Section |
   |                      |   |   | 7.5.3.                             |
   +----------------------+---+---+------------------------------------+

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8.  Versioning

   The C-DNS file preamble includes a file format version; a major and
   minor version number are required fields.  The document defines
   version 1.0 of the C-DNS specification.  This section describes the
   intended use of these version numbers in future specifications.

   It is noted that version 1.0 includes many optional fields and
   therefore consumers of version 1.0 should be inherently robust to
   parsing files with variable data content.

   Within a major version, a new minor version MUST be a strict superset
   of the previous minor version, with no semantic changes to existing
   fields.  New keys MAY be added to existing maps, and new maps MAY be
   added.  A consumer capable of reading a particular major.minor
   version MUST also be capable of reading all previous minor versions
   of the same major version.  It SHOULD also be capable of parsing all
   subsequent minor versions ignoring any keys or maps that it does not
   recognise.

   A new major version indicates changes to the format that are not
   backwards compatible with previous major versions.  A consumer
   capable of only reading a particular major version (greater than 1)
   is not required to and has no expectation to be capable of reading a
   previous major version.

9.  C-DNS to PCAP

   It is possible to re-construct PCAP files from the C-DNS format in a
   lossy fashion.  Some of the issues with reconstructing both the DNS
   payload and the full packet stream are outlined here.

   The reconstruction depends on whether or not all the optional
   sections of both the query and response were captured in the C-DNS
   file.  Clearly, if they were not all captured, the reconstruction
   will be imperfect.

   Even if all sections of the response were captured, one cannot
   reconstruct the DNS response payload exactly due to the fact that
   some DNS names in the message on the wire may have been compressed.
   Section 9.1 discusses this is more detail.

   Some transport information is not captured in the C-DNS format.  For
   example, the following aspects of the original packet stream cannot
   be re-constructed from the C-DNS format:

   o  IP fragmentation

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   o  TCP stream information:

      *  Multiple DNS messages may have been sent in a single TCP
         segment

      *  A DNS payload may have been split across multiple TCP segments

      *  Multiple DNS messages may have been sent on a single TCP
         session

   o  Malformed DNS messages if the wire format is not recorded

   o  Any Non-DNS messages that were in the original packet stream e.g.
      ICMP

   Simple assumptions can be made on the reconstruction: fragmented and
   DNS-over-TCP messages can be reconstructed into single packets and a
   single TCP session can be constructed for each TCP packet.

   Additionally, if malformed messages and Non-DNS packets are captured
   separately, they can be merged with packet captures reconstructed
   from C-DNS to produce a more complete packet stream.

9.1.  Name compression

   All the names stored in the C-DNS format are full domain names; no
   [RFC1035] name compression is used on the individual names within the
   format.  Therefore when reconstructing a packet, name compression
   must be used in order to reproduce the on the wire representation of
   the packet.

   [RFC1035] name compression works by substituting trailing sections of
   a name with a reference back to the occurrence of those sections
   earlier in the message.  Not all name server software uses the same
   algorithm when compressing domain names within the responses.  Some
   attempt maximum recompression at the expense of runtime resources,
   others use heuristics to balance compression and speed and others use
   different rules for what is a valid compression target.

   This means that responses to the same question from different name
   server software which match in terms of DNS payload content (header,
   counts, RRs with name compression removed) do not necessarily match
   byte-for-byte on the wire.

   Therefore, it is not possible to ensure that the DNS response payload
   is reconstructed byte-for-byte from C-DNS data.  However, it can at
   least, in principle, be reconstructed to have the correct payload
   length (since the original response length is captured) if there is

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   enough knowledge of the commonly implemented name compression
   algorithms.  For example, a simplistic approach would be to try each
   algorithm in turn to see if it reproduces the original length,
   stopping at the first match.  This would not guarantee the correct
   algorithm has been used as it is possible to match the length whilst
   still not matching the on the wire bytes but, without further
   information added to the C-DNS data, this is the best that can be
   achieved.

   Appendix B presents an example of two different compression
   algorithms used by well-known name server software.

10.  Data collection

   This section describes a non-normative proposed algorithm for the
   processing of a captured stream of DNS queries and responses and
   production of a stream of query/response items, matching queries/
   responses where possible.

   For the purposes of this discussion, it is assumed that the input has
   been pre-processed such that:

   1.  All IP fragmentation reassembly, TCP stream reassembly, and so
       on, has already been performed.

   2.  Each message is associated with transport metadata required to
       generate the Primary ID (see Section 10.2.1).

   3.  Each message has a well-formed DNS header of 12 bytes and (if
       present) the first Question in the Question section can be parsed
       to generate the Secondary ID (see below).  As noted earlier, this
       requirement can result in a malformed query being removed in the
       pre-processing stage, but the correctly formed response with
       RCODE of FORMERR being present.

   DNS messages are processed in the order they are delivered to the
   implementation.

   It should be noted that packet capture libraries do not necessarily
   provide packets in strict chronological order.  This can, for
   example, arise on multi-core platforms where packets arriving at a
   network device are processed by different cores.  On systems where
   this behaviour has been observed, the timestamps associated with each
   packet are consistent; queries always have a timestamp prior to the
   response timestamp.  However, the order in which these packets appear
   in the packet capture stream is not necessarily strictly
   chronological; a response can appear in the capture stream before the

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   query that provoked the response.  For this discussion, this non-
   chronological delivery is termed "skew".

   In the presence of skew, a response packets can arrive for matching
   before the corresponding query.  To avoid generating false instances
   of responses without a matching query, and queries without a matching
   response, the matching algorithm must take account of the possibility
   of skew.

10.1.  Matching algorithm

   A schematic representation of the algorithm for matching Q/R data
   items is shown in Figure 3.  It takes individual DNS query or
   response messages as input, and outputs matched Q/R items.  The
   numbers in the figure identify matching operations listed in Table 1.
   Specific details of the algorithm, for example queues, timers and
   identifiers, are given in the following sections.

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                         .----------------------.
                         | Process next message |<------------------+
                         `----------------------'                   |
                                     |                              |
                     +------------------------------+               |
                     | Generate message identifiers |               |
                     +------------------------------+               |
                                     |                              |
                            Response | Query                        |
                     +--------------< >---------------+             |
                     |                                |             |
           +--------------------+           +--------------------+  |
           | Find earliest QR   |           | Create QR item [2] |  |
           | item in OFIFO [1]  |           +--------------------+  |
           +--------------------+                     |             |
                      |                        +---------------+    |
                Match | No match               | Append new QR |    |
            +--------< >------+                | item to OFIFO |    |
            |                 |                +---------------+    |
      +-----------+      +--------+                   |             |
      | Update QR |      | Add to |          +-------------------+  |
      | item [3]  |      | RFIFO  |          | Find earliest QR  |  |
      +-----------+      +--------+          | item in RFIFO [1] |  |
            |                 |              +-------------------+  |
            +-----------------+                       |             |
                      |                               |             |
                      |     +----------------+  Match | No match    |
                      |     | Remove R       |-------< >-----+      |
                      |     | from RFIFO [3] |               |      |
                      |     +----------------+               |      |
                      |              |                       |      |
                      +--------------+-----------------------+      |
                                     |                              |
              +----------------------------------------------+      |
              | Update all timed out (QT) OFIFO QR items [4] |      |
              +----------------------------------------------+      |
                                     |                              |
                     +--------------------------------+             |
                     | Remove all timed out (ST) R    |             |
                     | from RFIFO, create QR item [5] |             |
                     +--------------------------------+             |
                 ____________________|_______________________       |
                /                                            /      |
               /  Remove all consecutive done entries from  /-------+
              /   front of OFIFO for further processing    /
             /____________________________________________/

                Figure 3: Query/Response matching algorithm

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            +-----+-------------------------------------------+
            | Ref | Operation                                 |
            +-----+-------------------------------------------+
            | [1] | Find earliest QR item in FIFO where:      |
            |     | * QR.done = false                         |
            |     | * QR.Q.PrimaryID == R.PrimaryID           |
            |     | and, if both QR.Q and R have SecondaryID: |
            |     | * QR.Q.SecondaryID == R.SecondaryID       |
            |     |                                           |
            | [2] | Set:                                      |
            |     | QR.Q := Q                                 |
            |     | QR.R := nil                               |
            |     | QR.done := false                          |
            |     |                                           |
            | [3] | Set:                                      |
            |     | QR.R := R                                 |
            |     | QR.done := true                           |
            |     |                                           |
            | [4] | Set:                                      |
            |     | QR.done := true                           |
            |     |                                           |
            | [5] | Set:                                      |
            |     | QR.Q := nil                               |
            |     | QR.R := R                                 |
            |     | QR.done := true                           |
            +-----+-------------------------------------------+

            Table 1: Operations used in the matching algorithm

10.2.  Message identifiers

10.2.1.  Primary ID (required)

   A Primary ID is constructed for each message.  It is composed of the
   following data:

   1.  Source IP Address

   2.  Destination IP Address

   3.  Source Port

   4.  Destination Port

   5.  Transport

   6.  DNS Message ID

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10.2.2.  Secondary ID (optional)

   If present, the first Question in the Question section is used as a
   secondary ID for each message.  Note that there may be well formed
   DNS queries that have a QDCOUNT of 0, and some responses may have a
   QDCOUNT of 0 (for example, responses with RCODE=FORMERR or NOTIMP).
   In this case the secondary ID is not used in matching.

10.3.  Algorithm parameters

   1.  Query timeout, QT.  A query arrives with timestamp t1.  If no
       response matching that query has arrived before other input
       arrives timestamped later than (t1 + QT), a query/response item
       containing only a query item is recorded.  The query timeout
       value is typically of the order of 5 seconds.

   2.  Skew timeout, ST.  A response arrives with timestamp t2.  If a
       response has not been matched by a query before input arrives
       timestamped later than (t2 + ST), a query/response item
       containing only a response is recorded.  The skew timeout value
       is typically a few microseconds.

10.4.  Algorithm requirements

   The algorithm is designed to handle the following input data:

   1.  Multiple queries with the same Primary ID (but different
       Secondary ID) arriving before any responses for these queries are
       seen.

   2.  Multiple queries with the same Primary and Secondary ID arriving
       before any responses for these queries are seen.

   3.  Queries for which no later response can be found within the
       specified timeout.

   4.  Responses for which no previous query can be found within the
       specified timeout.

10.5.  Algorithm limitations

   For cases 1 and 2 listed in the above requirements, it is not
   possible to unambiguously match queries with responses.  This
   algorithm chooses to match to the earliest query with the correct
   Primary and Secondary ID.

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10.6.  Workspace

   The algorithm employs two FIFO queues:

   o  OFIFO, an output FIFO containing Q/R items in chronological order,

   o  RFIFO, a FIFO holding responses without a matching query in order
      of arrival.

10.7.  Output

   The output is a list of Q/R data items.  Both the Query and Response
   elements are optional in these items, therefore Q/R data items have
   one of three types of content:

   1.  A matched pair of query and response messages

   2.  A query message with no response

   3.  A response message with no query

   The timestamp of a list item is that of the query for cases 1 and 2
   and that of the response for case 3.

10.8.  Post processing

   When ending capture, all items in the responses FIFO are timed out
   immediately, generating response-only entries to the Q/R data item
   FIFO.  These and all other remaining entries in the Q/R data item
   FIFO should be treated as timed out queries.

11.  Implementation guidance

   Whilst this document makes no specific recommendations with respect
   to Canonical CBOR (see Section 3.9 of [RFC7049]) the following
   guidance may be of use to implementors.

   Adherence to the first two rules given in Section 3.9 of [RFC7049]
   will minimise file sizes.

   Adherence to the last two rules given in Section 3.9 of [RFC7049] for
   all maps and arrays would unacceptably constrain implementations, for
   example, in the use case of real-time data collection in constrained
   environments where outputting block tables after query/response data
   and allowing indefinite length maps and arrays could reduce memory
   requirements.

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11.1.  Optional data

   When decoding C-DNS data some of the items required for a particular
   function that the consumer wishes to perform may be missing.
   Consumers should consider providing configurable default values to be
   used in place of the missing values in their output.

11.2.  Trailing bytes

   A DNS query message in a UDP or TCP payload can be followed by some
   additional (spurious) bytes, which are not stored in C-DNS.

   When DNS traffic is sent over TCP, each message is prefixed with a
   two byte length field which gives the message length, excluding the
   two byte length field.  In this context, trailing bytes can occur in
   two circumstances with different results:

   1.  The number of bytes consumed by fully parsing the message is less
       than the number of bytes given in the length field (i.e. the
       length field is incorrect and too large).  In this case, the
       surplus bytes are considered trailing bytes in an analogous
       manner to UDP and recorded as such.  If only this case occurs it
       is possible to process a packet containing multiple DNS messages
       where one or more has trailing bytes.

   2.  There are surplus bytes between the end of a well-formed message
       and the start of the length field for the next message.  In this
       case the first of the surplus bytes will be processed as the
       first byte of the next length field, and parsing will proceed
       from there, almost certainly leading to the next and any
       subsequent messages in the packet being considered malformed.
       This will not generate a trailing bytes record for the processed
       well-formed message.

11.3.  Limiting collection of RDATA

   Implementations should consider providing a configurable maximum
   RDATA size for capture, for example, to avoid memory issues when
   confronted with large XFR records.

11.4.  Timestamps

   The preamble to each block includes a timestamp of the earliest
   record in the block.  As described in Section 7.5.1, the timestamp is
   an array of 2 unsigned integers.  The first is a POSIX "time_t"
   [posix-time].  Consumers of C-DNS should be aware of this as it
   excludes leap seconds and therefore may cause minor anomalies in the
   data e.g. when calculating query throughput.

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12.  Implementation status

   [Note to RFC Editor: please remove this section and reference to
   [RFC7942] prior to publication.]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

12.1.  DNS-STATS Compactor

   ICANN/Sinodun IT have developed an open source implementation called
   DNS-STATS Compactor.  The Compactor is a suite of tools which can
   capture DNS traffic (from either a network interface or a PCAP file)
   and store it in the Compacted-DNS (C-DNS) file format.  PCAP files
   for the captured traffic can also be reconstructed.  See Compactor
   [1].

   This implementation:

   o  covers the whole of the specification described in the -03 draft
      with the exception of support for malformed messages and pico
      second time resolution.  (Note: this implementation does allow
      malformed messages to be recorded separately in a PCAP file).

   o  is released under the Mozilla Public License Version 2.0.

   o  has a users mailing list available, see dns-stats-users [2].

   There is also some discussion of issues encountered during
   development available at Compressing Pcap Files [3] and Packet
   Capture [4].

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   This information was last updated on 3rd of May 2018.

13.  IANA considerations

   None

14.  Security considerations

   Any control interface MUST perform authentication and encryption.

   Any data upload MUST be authenticated and encrypted.

15.  Privacy considerations

   Storage of DNS traffic by operators in PCAP and other formats is a
   long standing and widespread practice.  Section 2.5 of
   [I-D.bortzmeyer-dprive-rfc7626-bis] is an analysis of the risks to
   Internet users of the storage of DNS traffic data in servers
   (recursive resolvers, authoritative and rogue servers).

   Section 5.2 of [I-D.dickinson-dprive-bcp-op] describes mitigations
   for those risks for data stored on recursive resolvers (but which
   could by extension apply to authoritative servers).  These include
   data handling practices and methods for data minimization, IP address
   pseudonymization and anonymization.  Appendix B of that document
   presents an analysis of 7 published anonymization processes.  In
   addition, RSSAC have recently published RSSAC04: [5] "
   Recommendations on Anonymization Processes for Source IP Addresses
   Submitted for Future Analysis".

   The above analyses consider full data capture (e.g using PCAP) as a
   baseline for privacy considerations and therefore this format
   specification introduces no new user privacy issues beyond those of
   full data capture (which are quite severe).  It does provides
   mechanisms to selectively record only certain fields at the time of
   data capture to improve user privacy and to explicitly indicate that
   data is sampled and or anonymized.  It also provide flags to indicate
   if data normalization has been performed; data normalization
   increases user privacy by reducing the potential for fingerprinting
   individuals, however, a trade-off is potentially reducing the
   capacity to identify attack traffic via query name signatures.
   Operators should carefully consider their operational requirements
   and privacy policies and SHOULD capture at source the minimum user
   data required to meet their needs.

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16.  Acknowledgements

   The authors wish to thank CZ.NIC, in particular Tomas Gavenciak, for
   many useful discussions on binary formats, compression and packet
   matching.  Also Jan Vcelak and Wouter Wijngaards for discussions on
   name compression and Paul Hoffman for a detailed review of the
   document and the C-DNS CDDL.

   Thanks also to Robert Edmonds, Jerry Lundstroem, Richard Gibson,
   Stephane Bortzmeyer and many other members of DNSOP for review.

   Also, Miek Gieben for mmark [6]

17.  Changelog

   draft-ietf-dnsop-dns-capture-format-09

   o  Editorial changes arising from IESG review

   o  *-transport-flags and may be mandatory in some configurations

   o  Mark fields that are conditionally mandatory

   o  Change `promisc' flag CDDL data type to boolean

   o  Add ranges to configuration quantities where appropriate

   draft-ietf-dnsop-dns-capture-format-08

   o  Convert diagrams to ASCII

   o  Describe versioning

   o  Fix unused group warning in CDDL

   draft-ietf-dnsop-dns-capture-format-07

   o  Resolve outstanding questions and TODOs

   o  Make RR RDATA optional

   o  Update matching diagram and explain skew

   o  Add count of discarded messages to block statistics

   o  Editorial clarifications and improvements

   draft-ietf-dnsop-dns-capture-format-06

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   o  Correct BlockParameters type to map

   o  Make RR ttl optional

   o  Add storage flag indicating name normalization

   o  Add storage parameter fields for sampling and anonymization
      methods

   o  Editorial clarifications and improvements

   draft-ietf-dnsop-dns-capture-format-05

   o  Make all data items in Q/R, QuerySignature and Malformed Message
      arrays optional

   o  Re-structure the FilePreamble and ConfigurationParameters into
      BlockParameters

   o  BlockParameters has separate Storage and Collection Parameters

   o  Storage Parameters includes information on what optional fields
      are present, and flags specifying anonymization or sampling

   o  Addresses can now be stored as prefixes.

   o  Switch to using a variable sub-second timing granularity

   o  Add response bailiwick and query response type

   o  Add specifics of how to record malformed messages

   o  Add implementation guidance

   o  Improve terminology and naming consistency

   draft-ietf-dnsop-dns-capture-format-04

   o  Correct query-d0 to query-do in CDDL

   o  Clarify that map keys are unsigned integers

   o  Add Type to Class/Type table

   o  Clarify storage format in section 7.12

   draft-ietf-dnsop-dns-capture-format-03

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   o  Added an Implementation Status section

   draft-ietf-dnsop-dns-capture-format-02

   o  Update qr_data_format.png to match CDDL

   o  Editorial clarifications and improvements

   draft-ietf-dnsop-dns-capture-format-01

   o  Many editorial improvements by Paul Hoffman

   o  Included discussion of malformed message handling

   o  Improved Appendix C on Comparison of Binary Formats

   o  Now using C-DNS field names in the tables in section 8

   o  A handful of new fields included (CDDL updated)

   o  Timestamps now include optional picoseconds

   o  Added details of block statistics

   draft-ietf-dnsop-dns-capture-format-00

   o  Changed dnstap.io to dnstap.info

   o  qr_data_format.png was cut off at the bottom

   o  Update authors address

   o  Improve wording in Abstract

   o  Changed DNS-STAT to C-DNS in CDDL

   o  Set the format version in the CDDL

   o  Added a TODO: Add block statistics

   o  Added a TODO: Add extend to support pico/nano.  Also do this for
      Time offset and Response delay

   o  Added a TODO: Need to develop optional representation of malformed
      messages within C-DNS and what this means for packet matching.
      This may influence which fields are optional in the rest of the
      representation.

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   o  Added section on design goals to Introduction

   o  Added a TODO: Can Class be optimised?  Should a class of IN be
      inferred if not present?

   draft-dickinson-dnsop-dns-capture-format-00

   o  Initial commit

18.  References

18.1.  Normative References

   [I-D.ietf-cbor-cddl]
              Birkholz, H., Vigano, C., and C. Bormann, "Concise data
              definition language (CDDL): a notational convention to
              express CBOR and JSON data structures", draft-ietf-cbor-
              cddl-06 (work in progress), November 2018.

   [pcap-filter]
              tcpdump.org, "Manpage of PCAP-FILTER", 2017,
              <http://www.tcpdump.org/manpages/pcap-filter.7.html>.

   [pcap-options]
              tcpdump.org, "Manpage of PCAP", 2018,
              <http://www.tcpdump.org/manpages/pcap.3pcap.html>.

   [posix-time]
              The Open Group, "Section 4.16, Base Definitions, Standard
              for Information Technology - Portable Operating System
              Interface (POSIX(R)) Base Specifications, Issue 7", IEEE
              Standard 1003.1 2017 Edition,
              DOI 10.1109/IEEESTD.2018.8277153, 2017.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013, <https://www.rfc-
              editor.org/info/rfc6891>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

18.2.  Informative References

   [ditl]     DNS-OARC, "DITL", 2016, <https://www.dns-
              oarc.net/oarc/data/ditl>.

   [dnscap]   DNS-OARC, "DNSCAP", 2016, <https://www.dns-oarc.net/tools/
              dnscap>.

   [dnstap]   dnstap.info, "dnstap", 2016, <http://dnstap.info/>.

   [dsc]      Wessels, D. and J. Lundstrom, "DSC", 2016,
              <https://www.dns-oarc.net/tools/dsc>.

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   [I-D.bortzmeyer-dprive-rfc7626-bis]
              Bortzmeyer, S. and S. Dickinson, "DNS Privacy
              Considerations", draft-bortzmeyer-dprive-rfc7626-bis-01
              (work in progress), July 2018.

   [I-D.daley-dnsxml]
              Daley, J., Morris, S., and J. Dickinson, "dnsxml - A
              standard XML representation of DNS data", draft-daley-
              dnsxml-00 (work in progress), July 2013.

   [I-D.dickinson-dprive-bcp-op]
              Dickinson, S., Overeinder, B., Rijswijk-Deij, R., and A.
              Mankin, "Recommendations for DNS Privacy Service
              Operators", draft-dickinson-dprive-bcp-op-01 (work in
              progress), July 2018.

   [IEEE802.1Q]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Bridges and Bridged Networks",
              DOI 10.1109/IEEESTD.2014.6991462, 2014.

   [opcodes]  IANA, "DNS OpCodes", 2018,
              <http://www.iana.org/assignments/dns-parameters/
              dns-parameters.xhtml#dns-parameters-5>.

   [packetq]  .SE - The Internet Infrastructure Foundation, "PacketQ",
              2014, <https://github.com/dotse/PacketQ>.

   [pcap]     tcpdump.org, "PCAP", 2016, <http://www.tcpdump.org/>.

   [pcapng]   Tuexen, M., Risso, F., Bongertz, J., Combs, G., and G.
              Harris, "pcap-ng", 2016, <https://github.com/pcapng/
              pcapng>.

   [rcodes]   IANA, "DNS RCODEs", 2018,
              <http://www.iana.org/assignments/dns-parameters/
              dns-parameters.xhtml#dns-parameters-6>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017, <https://www.rfc-
              editor.org/info/rfc8259>.

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   [RFC8427]  Hoffman, P., "Representing DNS Messages in JSON",
              RFC 8427, DOI 10.17487/RFC8427, July 2018,
              <https://www.rfc-editor.org/info/rfc8427>.

   [rrclasses]
              IANA, "DNS CLASSes", 2018,
              <http://www.iana.org/assignments/dns-parameters/
              dns-parameters.xhtml#dns-parameters-2>.

   [rrtypes]  IANA, "Resource Record (RR) TYPEs", 2018,
              <http://www.iana.org/assignments/dns-parameters/
              dns-parameters.xhtml#dns-parameters-4>.

18.3.  URIs

   [1] https://github.com/dns-stats/compactor/wiki

   [2] https://mm.dns-stats.org/mailman/listinfo/dns-stats-users

   [3] https://www.sinodun.com/2017/06/compressing-pcap-files/

   [4] https://www.sinodun.com/2017/06/more-on-debian-jessieubuntu-
       trusty-packet-capture-woes/

   [5] https://www.icann.org/en/system/files/files/rssac-
       040-07aug18-en.pdf

   [6] https://github.com/miekg/mmark

   [7] https://www.nlnetlabs.nl/projects/nsd/

   [8] https://www.knot-dns.cz/

   [9] https://avro.apache.org/

   [10] https://developers.google.com/protocol-buffers/

   [11] http://cbor.io

   [12] https://github.com/kubo/snzip

   [13] http://google.github.io/snappy/

   [14] http://lz4.github.io/lz4/

   [15] http://www.gzip.org/

   [16] http://facebook.github.io/zstd/

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   [17] http://tukaani.org/xz/

   [18] https://github.com/dns-stats/draft-dns-capture-
        format/blob/master/file-size-versus-block-size.png

   [19] https://github.com/dns-stats/draft-dns-capture-
        format/blob/master/file-size-versus-block-size.svg

Appendix A.  CDDL

   This appendix gives a CDDL [I-D.ietf-cbor-cddl] specification for
   C-DNS.

   CDDL does not permit a range of allowed values to be specified for a
   bitfield.  Where necessary, those values are given as a CDDL group,
   but the group definition is commented out to prevent CDDL tooling
   from warning that the group is unused.

   ; CDDL specification of the file format for C-DNS,
   ; which describes a collection of DNS messages and
   ; traffic meta-data.

   ;
   ; The overall structure of a file.
   ;
   File = [
       file-type-id  : "C-DNS",
       file-preamble : FilePreamble,
       file-blocks   : [* Block],
   ]

   ;
   ; The file preamble.
   ;
   FilePreamble = {
       major-format-version => 1,
       minor-format-version => 0,
       ? private-version    => uint,
       block-parameters     => [+ BlockParameters],
   }
   major-format-version = 0
   minor-format-version = 1
   private-version      = 2
   block-parameters     = 3

   BlockParameters = {
       storage-parameters      => StorageParameters,
       ? collection-parameters => CollectionParameters,

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   }
   storage-parameters    = 0
   collection-parameters = 1

     IPv6PrefixLength = 1..128
     IPv4PrefixLength = 1..32
     OpcodeRange = 0..15
     RRTypeRange = 0..65535

     StorageParameters = {
         ticks-per-second             => uint,
         max-block-items              => uint,
         storage-hints                => StorageHints,
         opcodes                      => [+ OpcodeRange],
         rr-types                     => [+ RRTypeRange],
         ? storage-flags              => StorageFlags,
         ? client-address-prefix-ipv4 => IPv4PrefixLength,
         ? client-address-prefix-ipv6 => IPv6PrefixLength,
         ? server-address-prefix-ipv4 => IPv4PrefixLength,
         ? server-address-prefix-ipv6 => IPv6PrefixLength,
         ? sampling-method            => tstr,
         ? anonymisation-method       => tstr,
     }
     ticks-per-second           = 0
     max-block-items            = 1
     storage-hints              = 2
     opcodes                    = 3
     rr-types                   = 4
     storage-flags              = 5
     client-address-prefix-ipv4 = 6
     client-address-prefix-ipv6 = 7
     server-address-prefix-ipv4 = 8
     server-address-prefix-ipv6 = 9
     sampling-method            = 10
     anonymisation-method       = 11

       ; A hint indicates if the collection method will output the
       ; item or will ignore the item if present.
       StorageHints = {
           query-response-hints           => QueryResponseHints,
           query-response-signature-hints =>
               QueryResponseSignatureHints,
           rr-hints                       => RRHints,
           other-data-hints               => OtherDataHints,
       }
       query-response-hints           = 0
       query-response-signature-hints = 1
       rr-hints                       = 2

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       other-data-hints               = 3

         QueryResponseHintValues = &(
             time-offset                  : 0,
             client-address-index         : 1,
             client-port                  : 2,
             transaction-id               : 3,
             qr-signature-index           : 4,
             client-hoplimit              : 5,
             response-delay               : 6,
             query-name-index             : 7,
             query-size                   : 8,
             response-size                : 9,
             response-processing-data     : 10,
             query-question-sections      : 11,    ; Second & subsequent
                                                   ; questions
             query-answer-sections        : 12,
             query-authority-sections     : 13,
             query-additional-sections    : 14,
             response-answer-sections     : 15,
             response-authority-sections  : 16,
             response-additional-sections : 17,
         )
         QueryResponseHints = uint .bits QueryResponseHintValues

         QueryResponseSignatureHintValues = &(
             server-address     : 0,
             server-port        : 1,
             qr-transport-flags : 2,
             qr-type            : 3,
             qr-sig-flags       : 4,
             query-opcode       : 5,
             dns-flags          : 6,
             query-rcode        : 7,
             query-class-type   : 8,
             query-qdcount      : 9,
             query-ancount      : 10,
             query-arcount      : 11,
             query-nscount      : 12,
             query-edns-version : 13,
             query-udp-size     : 14,
             query-opt-rdata    : 15,
             response-rcode     : 16,
         )
         QueryResponseSignatureHints =
             uint .bits QueryResponseSignatureHintValues

         RRHintValues = &(

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             ttl         : 0,
             rdata-index : 1,
         )
         RRHints = uint .bits RRHintValues

         OtherDataHintValues = &(
             malformed-messages   : 0,
             address-event-counts : 1,
         )
         OtherDataHints = uint .bits OtherDataHintValues

       StorageFlagValues = &(
           anonymised-data      : 0,
           sampled-data         : 1,
           normalised-names     : 2,
       )
       StorageFlags = uint .bits StorageFlagValues

    ; Hints for later analysis.
    VLANIdRange = 1..4094

    CollectionParameters = {
         ? query-timeout      => uint,
         ? skew-timeout       => uint,
         ? snaplen            => uint,
         ? promisc            => bool,
         ? interfaces         => [+ tstr],
         ? server-addresses   => [+ IPAddress],
         ? vlan-ids           => [+ VLANIdRange],
         ? filter             => tstr,
         ? generator-id       => tstr,
         ? host-id            => tstr,
     }
     query-timeout      = 0
     skew-timeout       = 1
     snaplen            = 2
     promisc            = 3
     interfaces         = 4
     server-addresses   = 5
     vlan-ids           = 6
     filter             = 7
     generator-id       = 8
     host-id            = 9

   ;
   ; Data in the file is stored in Blocks.
   ;
   Block = {

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       block-preamble          => BlockPreamble,
       ? block-statistics      => BlockStatistics, ; Much of this
                                                   ; could be derived
       ? block-tables          => BlockTables,
       ? query-responses       => [+ QueryResponse],
       ? address-event-counts  => [+ AddressEventCount],
       ? malformed-messages    => [+ MalformedMessage],
   }
   block-preamble        = 0
   block-statistics      = 1
   block-tables          = 2
   query-responses       = 3
   address-event-counts  = 4
   malformed-messages    = 5

   ;
   ; The (mandatory) preamble to a block.
   ;
   BlockPreamble = {
       ? earliest-time          => Timestamp,
       ? block-parameters-index => uint .default 0,
   }
   earliest-time          = 0
   block-parameters-index = 1

   ; Ticks are subsecond intervals. The number of ticks in a second is
   ; file/block metadata. Signed and unsigned tick types are defined.
   ticks = int
   uticks = uint

   Timestamp = [
       timestamp-secs   : uint,
       timestamp-uticks : uticks,
   ]

   ;
   ; Statistics about the block contents.
   ;
   BlockStatistics = {
       ? processed-messages  => uint,
       ? qr-data-items       => uint,
       ? unmatched-queries   => uint,
       ? unmatched-responses => uint,
       ? discarded-opcode    => uint,
       ? malformed-items     => uint,
   }
   processed-messages  = 0
   qr-data-items       = 1

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   unmatched-queries   = 2
   unmatched-responses = 3
   discarded-opcode    = 4
   malformed-items     = 5

   ;
   ; Tables of common data referenced from records in a block.
   ;
   BlockTables = {
       ? ip-address             => [+ IPAddress],
       ? classtype              => [+ ClassType],
       ? name-rdata             => [+ bstr],    ; Holds both Names
                                                ; and RDATA
       ? qr-sig                 => [+ QueryResponseSignature],
       ? QuestionTables,
       ? RRTables,
       ? malformed-message-data => [+ MalformedMessageData],
   }
   ip-address             = 0
   classtype              = 1
   name-rdata             = 2
   qr-sig                 = 3
   qlist                  = 4
   qrr                    = 5
   rrlist                 = 6
   rr                     = 7
   malformed-message-data = 8

   IPv4Address = bstr .size 4
   IPv6Address = bstr .size 16
   IPAddress = IPv4Address / IPv6Address

   ClassType = {
       type  => uint,
       class => uint,
   }
   type  = 0
   class = 1

   QueryResponseSignature = {
       ? server-address-index  => uint,
       ? server-port           => uint,
       ? qr-transport-flags    => QueryResponseTransportFlags,
       ? qr-type               => QueryResponseType,
       ? qr-sig-flags          => QueryResponseFlags,
       ? query-opcode          => uint,
       ? qr-dns-flags          => DNSFlags,
       ? query-rcode           => uint,

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       ? query-classtype-index => uint,
       ? query-qd-count        => uint,
       ? query-an-count        => uint,
       ? query-ns-count        => uint,
       ? query-ar-count        => uint,
       ? edns-version          => uint,
       ? udp-buf-size          => uint,
       ? opt-rdata-index       => uint,
       ? response-rcode        => uint,
   }
   server-address-index  = 0
   server-port           = 1
   qr-transport-flags    = 2
   qr-type               = 3
   qr-sig-flags          = 4
   query-opcode          = 5
   qr-dns-flags          = 6
   query-rcode           = 7
   query-classtype-index = 8
   query-qd-count        = 9
   query-an-count        = 10
   query-ns-count        = 12
   query-ar-count        = 12
   edns-version          = 13
   udp-buf-size          = 14
   opt-rdata-index       = 15
   response-rcode        = 16

     ; Transport gives the values that may appear in bits 1..4 of
     ; TransportFlags. There is currently no way to express this in
     ; CDDL, so Transport is unused. To avoid confusion when used
     ; with CDDL tools, it is commented out.
     ;
     ; Transport = &(
     ;     udp               : 0,
     ;     tcp               : 1,
     ;     tls               : 2,
     ;     dtls              : 3,
     ;     doh               : 4,
     ; )

     TransportFlagValues = &(
         ip-version         : 0,     ; 0=IPv4, 1=IPv6
     ) / (1..4)
     TransportFlags = uint .bits TransportFlagValues

     QueryResponseTransportFlagValues = &(
         query-trailingdata : 5,

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     ) / TransportFlagValues
     QueryResponseTransportFlags =
         uint .bits QueryResponseTransportFlagValues

     QueryResponseType = &(
         stub      : 0,
         client    : 1,
         resolver  : 2,
         auth      : 3,
         forwarder : 4,
         tool      : 5,
     )

     QueryResponseFlagValues = &(
         has-query               : 0,
         has-reponse             : 1,
         query-has-opt           : 2,
         response-has-opt        : 3,
         query-has-no-question   : 4,
         response-has-no-question: 5,
     )
     QueryResponseFlags = uint .bits QueryResponseFlagValues

     DNSFlagValues = &(
         query-cd   : 0,
         query-ad   : 1,
         query-z    : 2,
         query-ra   : 3,
         query-rd   : 4,
         query-tc   : 5,
         query-aa   : 6,
         query-do   : 7,
         response-cd: 8,
         response-ad: 9,
         response-z : 10,
         response-ra: 11,
         response-rd: 12,
         response-tc: 13,
         response-aa: 14,
     )
     DNSFlags = uint .bits DNSFlagValues

   QuestionTables = (
       qlist => [+ QuestionList],
       qrr   => [+ Question]
   )

     QuestionList = [+ uint]           ; Index of Question

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     Question = {                      ; Second and subsequent questions
         name-index      => uint,      ; Index to a name in the
                                       ; name-rdata table
         classtype-index => uint,
     }
     name-index      = 0
     classtype-index = 1

   RRTables = (
       rrlist => [+ RRList],
       rr     => [+ RR]
   )

     RRList = [+ uint]                     ; Index of RR

     RR = {
         name-index      => uint,          ; Index to a name in the
                                           ; name-rdata table
         classtype-index => uint,
         ? ttl           => uint,
         ? rdata-index   => uint,          ; Index to RDATA in the
                                           ; name-rdata table
     }
     ; Other map key values already defined above.
     ttl         = 2
     rdata-index = 3

   MalformedMessageData = {
       ? server-address-index   => uint,
       ? server-port            => uint,
       ? mm-transport-flags     => TransportFlags,
       ? mm-payload             => bstr,
   }
   ; Other map key values already defined above.
   mm-transport-flags      = 2
   mm-payload              = 3

   ;
   ; A single query/response pair.
   ;
   QueryResponse = {
       ? time-offset              => uticks,     ; Time offset from
                                                 ; start of block
       ? client-address-index     => uint,
       ? client-port              => uint,
       ? transaction-id           => uint,
       ? qr-signature-index       => uint,
       ? client-hoplimit          => uint,

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       ? response-delay           => ticks,
       ? query-name-index         => uint,
       ? query-size               => uint,       ; DNS size of query
       ? response-size            => uint,       ; DNS size of response
       ? response-processing-data => ResponseProcessingData,
       ? query-extended           => QueryResponseExtended,
       ? response-extended        => QueryResponseExtended,
   }
   time-offset              = 0
   client-address-index     = 1
   client-port              = 2
   transaction-id           = 3
   qr-signature-index       = 4
   client-hoplimit          = 5
   response-delay           = 6
   query-name-index         = 7
   query-size               = 8
   response-size            = 9
   response-processing-data = 10
   query-extended           = 11
   response-extended        = 12

   ResponseProcessingData = {
       ? bailiwick-index  => uint,
       ? processing-flags => ResponseProcessingFlags,
   }
   bailiwick-index = 0
   processing-flags = 1

     ResponseProcessingFlagValues = &(
         from-cache : 0,
     )
     ResponseProcessingFlags = uint .bits ResponseProcessingFlagValues

   QueryResponseExtended = {
       ? question-index   => uint,       ; Index of QuestionList
       ? answer-index     => uint,       ; Index of RRList
       ? authority-index  => uint,
       ? additional-index => uint,
   }
   question-index   = 0
   answer-index     = 1
   authority-index  = 2
   additional-index = 3

   ;
   ; Address event data.
   ;

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   AddressEventCount = {
       ae-type          => &AddressEventType,
       ? ae-code        => uint,
       ae-address-index => uint,
       ae-count         => uint,
   }
   ae-type          = 0
   ae-code          = 1
   ae-address-index = 2
   ae-count         = 3

   AddressEventType = (
       tcp-reset              : 0,
       icmp-time-exceeded     : 1,
       icmp-dest-unreachable  : 2,
       icmpv6-time-exceeded   : 3,
       icmpv6-dest-unreachable: 4,
       icmpv6-packet-too-big  : 5,
   )

   ;
   ; Malformed messages.
   ;
   MalformedMessage = {
       ? time-offset           => uticks,   ; Time offset from
                                            ; start of block
       ? client-address-index  => uint,
       ? client-port           => uint,
       ? message-data-index    => uint,
   }
   ; Other map key values already defined above.
   message-data-index = 3

Appendix B.  DNS Name compression example

   The basic algorithm, which follows the guidance in [RFC1035], is
   simply to collect each name, and the offset in the packet at which it
   starts, during packet construction.  As each name is added, it is
   offered to each of the collected names in order of collection,
   starting from the first name.  If labels at the end of the name can
   be replaced with a reference back to part (or all) of the earlier
   name, and if the uncompressed part of the name is shorter than any
   compression already found, the earlier name is noted as the
   compression target for the name.

   The following tables illustrate the process.  In an example packet,
   the first name is foo.example.

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          +---+-------------+--------------+--------------------+
          | N | Name        | Uncompressed | Compression Target |
          +---+-------------+--------------+--------------------+
          | 1 | foo.example |              |                    |
          +---+-------------+--------------+--------------------+

   The next name added is bar.example.  This is matched against
   foo.example.  The example part of this can be used as a compression
   target, with the remaining uncompressed part of the name being bar.

        +---+-------------+--------------+-----------------------+
        | N | Name        | Uncompressed | Compression Target    |
        +---+-------------+--------------+-----------------------+
        | 1 | foo.example |              |                       |
        | 2 | bar.example | bar          | 1 + offset to example |
        +---+-------------+--------------+-----------------------+

   The third name added is www.bar.example.  This is first matched
   against foo.example, and as before this is recorded as a compression
   target, with the remaining uncompressed part of the name being
   www.bar.  It is then matched against the second name, which again can
   be a compression target.  Because the remaining uncompressed part of
   the name is www, this is an improved compression, and so it is
   adopted.

      +---+-----------------+--------------+-----------------------+
      | N | Name            | Uncompressed | Compression Target    |
      +---+-----------------+--------------+-----------------------+
      | 1 | foo.example     |              |                       |
      | 2 | bar.example     | bar          | 1 + offset to example |
      | 3 | www.bar.example | www          | 2                     |
      +---+-----------------+--------------+-----------------------+

   As an optimization, if a name is already perfectly compressed (in
   other words, the uncompressed part of the name is empty), then no
   further names will be considered for compression.

B.1.  NSD compression algorithm

   Using the above basic algorithm the packet lengths of responses
   generated by NSD [7] can be matched almost exactly.  At the time of
   writing, a tiny number (<.01%) of the reconstructed packets had
   incorrect lengths.

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B.2.  Knot Authoritative compression algorithm

   The Knot Authoritative [8] name server uses different compression
   behavior, which is the result of internal optimization designed to
   balance runtime speed with compression size gains.  In brief, and
   omitting complications, Knot Authoritative will only consider the
   QNAME and names in the immediately preceding RR section in an RRSET
   as compression targets.

   A set of smart heuristics as described below can be implemented to
   mimic this and while not perfect it produces output nearly, but not
   quite, as good a match as with NSD.  The heuristics are:

   1.  A match is only perfect if the name is completely compressed AND
       the TYPE of the section in which the name occurs matches the TYPE
       of the name used as the compression target.

   2.  If the name occurs in RDATA:

       *  If the compression target name is in a query, then only the
          first RR in an RRSET can use that name as a compression
          target.

       *  The compression target name MUST be in RDATA.

       *  The name section TYPE must match the compression target name
          section TYPE.

       *  The compression target name MUST be in the immediately
          preceding RR in the RRSET.

   Using this algorithm less than 0.1% of the reconstructed packets had
   incorrect lengths.

B.3.  Observed differences

   In sample traffic collected on a root name server around 2-4% of
   responses generated by Knot had different packet lengths to those
   produced by NSD.

Appendix C.  Comparison of Binary Formats

   Several binary serialisation formats were considered, and for
   completeness were also compared to JSON.

   o  Apache Avro [9].  Data is stored according to a pre-defined
      schema.  The schema itself is always included in the data file.

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      Data can therefore be stored untagged, for a smaller serialisation
      size, and be written and read by an Avro library.

      *  At the time of writing, Avro libraries are available for C,
         C++, C#, Java, Python, Ruby and PHP.  Optionally tools are
         available for C++, Java and C# to generate code for encoding
         and decoding.

   o  Google Protocol Buffers [10].  Data is stored according to a pre-
      defined schema.  The schema is used by a generator to generate
      code for encoding and decoding the data.  Data can therefore be
      stored untagged, for a smaller serialisation size.  The schema is
      not stored with the data, so unlike Avro cannot be read with a
      generic library.

      *  Code must be generated for a particular data schema to read and
         write data using that schema.  At the time of writing, the
         Google code generator can currently generate code for encoding
         and decoding a schema for C++, Go, Java, Python, Ruby, C#,
         Objective-C, Javascript and PHP.

   o  CBOR [11].  Defined in [RFC7049], this serialisation format is
      comparable to JSON but with a binary representation.  It does not
      use a pre-defined schema, so data is always stored tagged.
      However, CBOR data schemas can be described using CDDL
      [I-D.ietf-cbor-cddl] and tools exist to verify data files conform
      to the schema.

      *  CBOR is a simple format, and simple to implement.  At the time
         of writing, the CBOR website lists implementations for 16
         languages.

   Avro and Protocol Buffers both allow storage of untagged data, but
   because they rely on the data schema for this, their implementation
   is considerably more complex than CBOR.  Using Avro or Protocol
   Buffers in an unsupported environment would require notably greater
   development effort compared to CBOR.

   A test program was written which reads input from a PCAP file and
   writes output using one of two basic structures; either a simple
   structure, where each query/response pair is represented in a single
   record entry, or the C-DNS block structure.

   The resulting output files were then compressed using a variety of
   common general-purpose lossless compression tools to explore the
   compressibility of the formats.  The compression tools employed were:

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   o  snzip [12].  A command line compression tool based on the Google
      Snappy [13] library.

   o  lz4 [14].  The command line compression tool from the reference C
      LZ4 implementation.

   o  gzip [15].  The ubiquitous GNU zip tool.

   o  zstd [16].  Compression using the Zstandard algorithm.

   o  xz [17].  A popular compression tool noted for high compression.

   In all cases the compression tools were run using their default
   settings.

   Note that this draft does not mandate the use of compression, nor any
   particular compression scheme, but it anticipates that in practice
   output data will be subject to general-purpose compression, and so
   this should be taken into consideration.

   "test.pcap", a 662Mb capture of sample data from a root instance was
   used for the comparison.  The following table shows the formatted
   size and size after compression (abbreviated to Comp. in the table
   headers), together with the task resident set size (RSS) and the user
   time taken by the compression.  File sizes are in Mb, RSS in kb and
   user time in seconds.

   +-------------+-----------+-------+------------+-------+-----------+
   | Format      | File size | Comp. | Comp. size |   RSS | User time |
   +-------------+-----------+-------+------------+-------+-----------+
   | PCAP        |    661.87 | snzip |     212.48 |  2696 |      1.26 |
   |             |           | lz4   |     181.58 |  6336 |      1.35 |
   |             |           | gzip  |     153.46 |  1428 |     18.20 |
   |             |           | zstd  |      87.07 |  3544 |      4.27 |
   |             |           | xz    |      49.09 | 97416 |    160.79 |
   |             |           |       |            |       |           |
   | JSON simple |   4113.92 | snzip |     603.78 |  2656 |      5.72 |
   |             |           | lz4   |     386.42 |  5636 |      5.25 |
   |             |           | gzip  |     271.11 |  1492 |     73.00 |
   |             |           | zstd  |     133.43 |  3284 |      8.68 |
   |             |           | xz    |      51.98 | 97412 |    600.74 |
   |             |           |       |            |       |           |
   | Avro simple |    640.45 | snzip |     148.98 |  2656 |      0.90 |
   |             |           | lz4   |     111.92 |  5828 |      0.99 |
   |             |           | gzip  |     103.07 |  1540 |     11.52 |
   |             |           | zstd  |      49.08 |  3524 |      2.50 |
   |             |           | xz    |      22.87 | 97308 |     90.34 |
   |             |           |       |            |       |           |

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   | CBOR simple |    764.82 | snzip |     164.57 |  2664 |      1.11 |
   |             |           | lz4   |     120.98 |  5892 |      1.13 |
   |             |           | gzip  |     110.61 |  1428 |     12.88 |
   |             |           | zstd  |      54.14 |  3224 |      2.77 |
   |             |           | xz    |      23.43 | 97276 |    111.48 |
   |             |           |       |            |       |           |
   | PBuf simple |    749.51 | snzip |     167.16 |  2660 |      1.08 |
   |             |           | lz4   |     123.09 |  5824 |      1.14 |
   |             |           | gzip  |     112.05 |  1424 |     12.75 |
   |             |           | zstd  |      53.39 |  3388 |      2.76 |
   |             |           | xz    |      23.99 | 97348 |    106.47 |
   |             |           |       |            |       |           |
   | JSON block  |    519.77 | snzip |     106.12 |  2812 |      0.93 |
   |             |           | lz4   |     104.34 |  6080 |      0.97 |
   |             |           | gzip  |      57.97 |  1604 |     12.70 |
   |             |           | zstd  |      61.51 |  3396 |      3.45 |
   |             |           | xz    |      27.67 | 97524 |    169.10 |
   |             |           |       |            |       |           |
   | Avro block  |     60.45 | snzip |      48.38 |  2688 |      0.20 |
   |             |           | lz4   |      48.78 |  8540 |      0.22 |
   |             |           | gzip  |      39.62 |  1576 |      2.92 |
   |             |           | zstd  |      29.63 |  3612 |      1.25 |
   |             |           | xz    |      18.28 | 97564 |     25.81 |
   |             |           |       |            |       |           |
   | CBOR block  |     75.25 | snzip |      53.27 |  2684 |      0.24 |
   |             |           | lz4   |      51.88 |  8008 |      0.28 |
   |             |           | gzip  |      41.17 |  1548 |      4.36 |
   |             |           | zstd  |      30.61 |  3476 |      1.48 |
   |             |           | xz    |      18.15 | 97556 |     38.78 |
   |             |           |       |            |       |           |
   | PBuf block  |     67.98 | snzip |      51.10 |  2636 |      0.24 |
   |             |           | lz4   |      52.39 |  8304 |      0.24 |
   |             |           | gzip  |      40.19 |  1520 |      3.63 |
   |             |           | zstd  |      31.61 |  3576 |      1.40 |
   |             |           | xz    |      17.94 | 97440 |     33.99 |
   +-------------+-----------+-------+------------+-------+-----------+

   The above results are discussed in the following sections.

C.1.  Comparison with full PCAP files

   An important first consideration is whether moving away from PCAP
   offers significant benefits.

   The simple binary formats are typically larger than PCAP, even though
   they omit some information such as Ethernet MAC addresses.  But not
   only do they require less CPU to compress than PCAP, the resulting
   compressed files are smaller than compressed PCAP.

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C.2.  Simple versus block coding

   The intention of the block coding is to perform data de-duplication
   on query/response records within the block.  The simple and block
   formats above store exactly the same information for each query/
   response record.  This information is parsed from the DNS traffic in
   the input PCAP file, and in all cases each field has an identifier
   and the field data is typed.

   The data de-duplication on the block formats show an order of
   magnitude reduction in the size of the format file size against the
   simple formats.  As would be expected, the compression tools are able
   to find and exploit a lot of this duplication, but as the de-
   duplication process uses knowledge of DNS traffic, it is able to
   retain a size advantage.  This advantage reduces as stronger
   compression is applied, as again would be expected, but even with the
   strongest compression applied the block formatted data remains around
   75% of the size of the simple format and its compression requires
   roughly a third of the CPU time.

C.3.  Binary versus text formats

   Text data formats offer many advantages over binary formats,
   particularly in the areas of ad-hoc data inspection and extraction.
   It was therefore felt worthwhile to carry out a direct comparison,
   implementing JSON versions of the simple and block formats.

   Concentrating on JSON block format, the format files produced are a
   significant fraction of an order of magnitude larger than binary
   formats.  The impact on file size after compression is as might be
   expected from that starting point; the stronger compression produces
   files that are 150% of the size of similarly compressed binary
   format, and require over 4x more CPU to compress.

C.4.  Performance

   Concentrating again on the block formats, all three produce format
   files that are close to an order of magnitude smaller that the
   original "test.pcap" file.  CBOR produces the largest files and Avro
   the smallest, 20% smaller than CBOR.

   However, once compression is taken into account, the size difference
   narrows.  At medium compression (with gzip), the size difference is
   4%.  Using strong compression (with xz) the difference reduces to 2%,
   with Avro the largest and Protocol Buffers the smallest, although
   CBOR and Protocol Buffers require slightly more compression CPU.

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   The measurements presented above do not include data on the CPU
   required to generate the format files.  Measurements indicate that
   writing Avro requires 10% more CPU than CBOR or Protocol Buffers.  It
   appears, therefore, that Avro's advantage in compression CPU usage is
   probably offset by a larger CPU requirement in writing Avro.

C.5.  Conclusions

   The above assessments lead us to the choice of a binary format file
   using blocking.

   As noted previously, this draft anticipates that output data will be
   subject to compression.  There is no compelling case for one
   particular binary serialisation format in terms of either final file
   size or machine resources consumed, so the choice must be largely
   based on other factors.  CBOR was therefore chosen as the binary
   serialisation format for the reasons listed in Section 5.

C.6.  Block size choice

   Given the choice of a CBOR format using blocking, the question arises
   of what an appropriate default value for the maximum number of query/
   response pairs in a block should be.  This has two components; what
   is the impact on performance of using different block sizes in the
   format file, and what is the impact on the size of the format file
   before and after compression.

   The following table addresses the performance question, showing the
   impact on the performance of a C++ program converting "test.pcap" to
   C-DNS.  File size is in Mb, resident set size (RSS) in kb.

              +------------+-----------+--------+-----------+
              | Block size | File size |    RSS | User time |
              +------------+-----------+--------+-----------+
              |       1000 |    133.46 | 612.27 |     15.25 |
              |       5000 |     89.85 | 676.82 |     14.99 |
              |      10000 |     76.87 | 752.40 |     14.53 |
              |      20000 |     67.86 | 750.75 |     14.49 |
              |      40000 |     61.88 | 736.30 |     14.29 |
              |      80000 |     58.08 | 694.16 |     14.28 |
              |     160000 |     55.94 | 733.84 |     14.44 |
              |     320000 |     54.41 | 799.20 |     13.97 |
              +------------+-----------+--------+-----------+

   Increasing block size, therefore, tends to increase maximum RSS a
   little, with no significant effect (if anything a small reduction) on
   CPU consumption.

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   The following figure plots the effect of increasing block size on
   output file size for different compressions.

   Figure showing effect of block size on file size (PNG) [18]

   Figure showing effect of block size on file size (SVG) [19]

   From the above, there is obviously scope for tuning the default block
   size to the compression being employed, traffic characteristics,
   frequency of output file rollover etc.  Using a strong compression
   scheme, block sizes over 10,000 query/response pairs would seem to
   offer limited improvements.

Authors' Addresses

   John Dickinson
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   United Kingdom

   Email: jad@sinodun.com

   Jim Hague
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   United Kingdom

   Email: jim@sinodun.com

   Sara Dickinson
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   United Kingdom

   Email: sara@sinodun.com

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   Terry Manderson
   ICANN
   12025 Waterfront Drive
   Suite 300
   Los Angeles  CA 90094-2536

   Email: terry.manderson@icann.org

   John Bond
   ICANN
   12025 Waterfront Drive
   Suite 300
   Los Angeles  CA 90094-2536

   Email: john.bond@icann.org

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