Internet Engineering Task Force (IETF) P. Hoffman

Internet Engineering Task Force (IETF) P. Hoffman
Request for Comments: 8427 ICANN
Category: Informational July 2018
ISSN: 2070-1721

Representing DNS Messages in JSON

Abstract

Some applications use DNS messages, or parts of DNS messages, as
data. For example, a system that captures DNS queries and responses
might want to be able to easily search them without having to decode
the messages each time. Another example is a system that puts
together DNS queries and responses from message parts. This document
describes a general format for DNS message data in JSON. Specific
profiles of the format in this document can be described in other
documents for specific applications and usage scenarios.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8427.

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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
(https://trustee.ietf.org/license-info) in effect on the date of
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
1.1. Design of the Format . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. JSON Format for DNS Messages . . . . . . . . . . . . . . . . 5
2.1. Message Object Members . . . . . . . . . . . . . . . . . 5
2.2. Resource Record Object Members . . . . . . . . . . . . . 6
2.3. Specific RDATA Field Members . . . . . . . . . . . . . . 7
2.4. The Message and Its Parts as Octets . . . . . . . . . . . 8
2.5. Additional Message Object Members . . . . . . . . . . . . 8
2.6. Name Fields . . . . . . . . . . . . . . . . . . . . . . . 9
3. JSON Format for a Paired DNS Query and Response . . . . . . . 9
4. Streaming DNS Objects . . . . . . . . . . . . . . . . . . . . 9
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Example of the Format of a DNS Query . . . . . . . . . . 10
5.2. Example of the Format of a Paired DNS Query and Response 11
6. Local Format Policy . . . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7.1. Media Type Registration of application/dns+json . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . 15
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15

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1. Introduction

The DNS message format is defined in [RFC1035]. DNS queries and DNS
responses have exactly the same structure. Many of the field names
and data type names given in [RFC1035] are commonly used in
discussions of DNS. For example, it is common to hear things like
"the query had a QNAME of 'example.com'" or "the RDATA has a simple
structure".

There are hundreds of data-interchange formats for serializing
structured data. Currently, JSON [RFC8259] is quite popular for many
types of data, particularly data that has named subfields and
optional parts.

This document uses JSON to describe DNS messages. It also defines
how to describe a paired DNS query and response and how to stream DNS
objects.

1.1. Design of the Format

There are many ways to design a data format. This document uses a
specific design methodology based on the DNS format.

o The format is based on JSON objects in order to allow a writer to
include or exclude parts of the format at will. No object members
are ever required.

o This format is purposely overly general. A protocol or
application that uses this format is expected to use only a subset
of the items defined here; it is expected to define its own
profile from this format.

o The format allows transformation through JSON that would permit
re-creation of the wire content of the message.

o All members whose values are always 16 bits or shorter are
represented by JSON numbers with no minus sign, no fractional part
(except in fields that are specifically noted below), and no
exponent part. One-bit values are represented as JSON numbers
whose values are either 0 or 1. See Section 6 of [RFC8259] for
more detail on JSON numbers.

o The JSON representation of the objects described in this document
is limited to the UTF-8 codepoints from U+0000 to U+007F. This is
done to prevent an attempt to use a different encoding such as
UTF-8 for octets in names or data.

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o Items that have string values can have "HEX" appended to their
names to indicate a non-ASCII encoding of the value. Names that
end in "HEX" have values stored in base16 encoding (hex with
uppercase letters) defined in [RFC4648]. This is particularly
useful for RDATA that is binary.

o All field names in this format are used as in [RFC1035], including
their capitalization. Names not defined in [RFC1035] generally
use "camel case".

o The same data may be represented in multiple object members
multiple times. For example, there is a member for the octets of
the DNS message header, and there are members for each named part
of the header. A message object can thus inadvertently have
inconsistent data, such as a header member whose value does not
match the value of the first bits in the entire message member.

o It is acceptable that there are multiple ways to represent the
same data. This is done so that application designers can choose
what fields are best for them and even so that they are able to
allow multiple representations. That is, there is no "better" way
to represent DNS data, so this design doesn't prefer specific
representations.

o The design explicitly allows for the description of malformed DNS
messages. This is important for systems that are logging messages
seen on the wire, particularly messages that might be used as part
of an attack. A few examples of malformed DNS messages include:

* a resource record (RR) that has an RDLENGTH of 4 but an RDATA
whose length is longer than 4 (if it is the last RR in a
message)

* a DNS message whose QDCOUNT is 0

* a DNS message whose ANCOUNT is large but there are insufficient
bytes after the header

* a DNS message whose length is less than 12 octets, meaning it
doesn't even have a full header

o An object in this format can have zero or more of the members
defined here; that is, no members are required by the format
itself. Instead, profiles that use this format might have
requirements for mandatory members, optional members, and
prohibited members from the format. Also, this format does not
prohibit members that are not defined in this format; profiles of
the format are free to add new members in the profile.

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o This document defines DNS messages, not the zone files described
in [RFC1035]. A different specification could be written to
extend it to represent zone files. Note that DNS zone files allow
escaping of octet values using "\DDD" notation, but this
specification does not allow that; when encoding from a zone file
to this JSON format, you need to do a conversion for many types of
values.

1.2. 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.

2. JSON Format for DNS Messages

The following gives all of the members defined for a DNS message. It
is organized approximately by levels of the DNS message.

2.1. Message Object Members

o ID - Integer whose value is 0 to 65535

o QR - Boolean

o Opcode - Integer whose value is 0 to 15

o AA - Boolean

o TC - Boolean

o RD - Boolean

o RA - Boolean

o AD - Boolean

o CD - Boolean

o RCODE - Integer whose value is 0 to 15

o QDCOUNT - Integer whose value is 0 to 65535

o ANCOUNT - Integer whose value is 0 to 65535

o NSCOUNT - Integer whose value is 0 to 65535

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o ARCOUNT - Integer whose value is 0 to 65535

o QNAME - String of the name of the first Question section of the
message; see Section 2.6 for a description of the contents

o compressedQNAME - Object that describes the name with two optional
values: "isCompressed" (with a value of 0 for no and 1 for yes)
and "length" (with an integer giving the length in the message)

o QTYPE - Integer whose value is 0 to 65535, of the QTYPE of the
first Question section of the message

o QTYPEname - String whose value is from the IANA "Resource Record
(RR) TYPEs" registry or has the format in [RFC3597]; this is case
sensitive, so "AAAA", not "aaaa"

o QCLASS - Integer whose value is 0 to 65535, of the QCLASS of the
first Question section of the message

o QCLASSname - String whose value is "IN", "CH", or "HS" or that has
the format in [RFC3597]

o questionRRs - Array of zero or more resource records or rrSet
objects in the Question section

o answerRRs - Array of zero or more resource records or rrSet
objects in the Answer section

o authorityRRs - Array of zero or more resource records or rrSet
objects in the Authority section

o additionalRRs - Array of zero or more resource records or rrSet
objects in the Additional section

2.2. Resource Record Object Members

A resource record is represented as an object with the following
members.

o NAME - String of the NAME field of the resource record; see
Section 2.6 for a description of the contents

o compressedNAME - Object that describes the name with two optional
values: "isCompressed" (with a value of 0 for no and 1 for yes)
and "length" (with an integer giving the length in the message)

o TYPE - Integer whose value is 0 to 65535

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o TYPEname - String whose value is from the IANA "Resource Record
(RR) TYPEs" registry or has the format in [RFC3597]; this is case
sensitive, so "AAAA", not "aaaa"

o CLASS - Integer whose value is 0 to 65535

o CLASSname - String whose value is "IN", "CH", or "HS" or has the
format in [RFC3597]

o TTL - Integer whose value is -2147483648 to 2147483647 (it will
only be 0 to 2147483647 in normal circumstances)

o RDLENGTH - Integer whose value is 0 to 65535. Applications using
this format are unlikely to use this value directly, and instead
calculate the value from the RDATA.

o RDATAHEX - Hex-encoded string (base16 encoding, described in
[RFC4648]) of the octets of the RDATA field of the resource
record. The data in some common RDATA fields are also described
in their own members; see Section 2.3

o rrSet - List of objects that have RDLENGTH and RDATA members

A Question section can be expressed as a resource record. When doing
so, the TTL, RDLENGTH, and RDATA members make no sense.

2.3. Specific RDATA Field Members

The following are common RDATA types and how to specify them as JSON
members. The name of the member contains the name of the RDATA type.
The data type for each of these members is a string. Each name is
prefaced with "rdata" to prevent a name collision with fields that
might later be defined that have the same name as the raw type name.

o rdataA - IPv4 address, such as "192.168.33.44"

o rdataAAAA - IPv6 address, such as "fe80::a65e:60ff:fed6:8aaf", as
defined in [RFC5952]

o rdataCNAME - A domain name

o rdataDNAME - A domain name

o rdataNS - A domain name

o rdataPTR - A domain name

o rdataTXT - A text value

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In addition, each of the following members has a value that is a
space-separated string that matches the display format definition in
the RFC that defines that RDATA type. It is not expected that every
receiving application will know how to parse these values.

rdataCDNSKEY, rdataCDS, rdataCSYNC, rdataDNSKEY, rdataHIP,
rdataIPSECKEY, rdataKEY, rdataMX, rdataNSEC, rdataNSEC3,
rdataNSEC3PARAM, rdataOPENPGPKEY, rdataRRSIG, rdataSMIMEA, rdataSPF,
rdataSRV, rdataSSHFP, rdataTLSA

2.4. The Message and Its Parts as Octets

The following can be members of a message object. These members are
all encoded in base16 encoding, described in [RFC4648]. All these
items are strings.

o messageOctetsHEX - The octets of the message

o headerOctetsHEX - The first 12 octets of the message (or fewer, if
the message is truncated)

o questionOctetsHEX - The octets of the Question section

o answerOctetsHEX - The octets of the Answer section

o authorityOctetsHEX - The octets of the Authority section

o additionalOctetsHEX - The octets of the Additional section

The following can be a member of a resource record object.

o rrOctetsHEX - The octets of a particular resource record

The items in this section are useful in applications to canonically
reproduce what appeared on the wire. For example, an application
that is converting wire-format requests and responses might do
decompression of names, but the system reading the converted data may
want to be sure the decompression was done correctly. Such a system
would need to see the part of the message where the decompressed
labels resided, such as in one of the items in this section.

2.5. Additional Message Object Members

The following are members that might appear in a message object:

o dateString - The date that the message was sent or received, given
as a string in the standard format described in [RFC3339] and
refined by Section 3.3 of [RFC4287].

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o dateSeconds - The date that the message was sent or received,
given as a JSON number that is the number of seconds since
1970-01-01T00:00Z in UTC time; this number can be fractional.
This number must have no minus sign, can have an optional
fractional part, and can have no exponent part.

o comment - An unstructured comment as a string.

2.6. Name Fields

Names are represented by JSON strings. The rules for how names are
encoded are described in Section 1.1. (To recap: it is limited to
the UTF-8 codepoints from U+0000 to U+007F.) The contents of these
fields are always uncompressed; that is, after [RFC1035], name
compression has been removed.

There are two encodings for names:

o If the member name does not end in "HEX", the value is a domain
name encoded as DNS labels consisting of UTF-8 codepoints from
U+0000 to U+007F. Within a label, codepoints above U+007F and the
codepoint U+002E (ASCII period) MUST be expressed using JSON's
escaping rules within this set of codepoints. Separation between
labels is indicated with a period (codepoint U+002E).
Internationalized Domain Name (IDN) labels are always expressed in
their A-label form, as described in [RFC5890].

o If the member name ends in "HEX", the value is the wire format for
an entire domain name stored in base16 encoding, which is
described in [RFC4648].

3. JSON Format for a Paired DNS Query and Response

A paired DNS query and response is represented as an object. Two
optional members of this object are named "queryMessage" and
"responseMessage", and each has a value that is a message object.
This design was chosen (as compared to the more obvious array of two
values) so that a paired DNS query and response could be
differentiated from a stream of DNS messages whose length happens to
be two.

4. Streaming DNS Objects

Streaming of DNS objects is performed using JSON text sequences
[RFC7464].

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5. Examples

5.1. Example of the Format of a DNS Query

The following is an example of a query for the A record of
example.com.

{ "ID": 19678, "QR": 0, "Opcode": 0,
"AA": 0, "TC": 0, "RD": 0, "RA": 0, "AD": 0, "CD": 0, "RCODE": 0,
"QDCOUNT": 1, "ANCOUNT": 0, "NSCOUNT": 0, "ARCOUNT": 0,
"QNAME": "example.com", "QTYPE": 1, "QCLASS": 1
}

As stated earlier, all members of an object are optional. This
example object could have one or more of the following members as
well:

"answerRRs": []
"authorityOctetsHEX": ""
"comment": "Something pithy goes here"
"dateSeconds": 1408504748.657783
"headerOctetsHEX": "4CDE00000001000000000000"
"QNAMEHEX": "076578616D706C6503636F6D00",
"compressedQNAME": { "isCompressed": 0 },
"messageOctetsHEX":
"4CDE00000001000000000000076578616D706C6503636F6D0000010001"
"questionOctetsHEX": "076578616D706C6503636F6D0000010001"
"questionRRs": [ { "NAMEHEX": "076578616D706C6503636F6D00",
"TYPE": 1, "CLASS": 1, "hostNAME" : "example.com." } ]
"questionRRs": [ { "NAME": "example.com.", "TYPE": 1,
"CLASS": 1, } ]

(Note that this is an incomplete list of what else could be in the
object.)

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5.2. Example of the Format of a Paired DNS Query and Response

The following is a paired DNS query and response for a query for the
A record of example.com.

{
"queryMessage": { "ID": 32784, "QR": 0, "Opcode": 0, "AA": 0,
"TC": 0, "RD": 0, "RA": 0, "AD": 0, "CD": 0,
"RCODE": 0, "QDCOUNT": 1, "ANCOUNT": 0,
"NSCOUNT": 0, "ARCOUNT": 0,
"QNAME": "example.com.",
"QTYPE": 1, "QCLASS": 1 },
"responseMessage": { "ID": 32784, "QR": 1, "AA": 1, "RCODE": 0,
"QDCOUNT": 1, "ANCOUNT": 1, "NSCOUNT": 1,
"ARCOUNT": 0,
"answerRRs": [ { "NAME": "example.com.",
"TYPE": 1, "CLASS": 1,
"TTL": 3600,
"RDATAHEX": "C0000201" },
{ "NAME": "example.com.",
"TYPE": 1, "CLASS": 1,
"TTL": 3600,
"RDATAHEX": "C000AA01" } ],
"authorityRRs": [ { "NAME": "ns.example.com.",
"TYPE": 1, "CLASS": 1,
"TTL": 28800,
"RDATAHEX": "CB007181" } ]
}
}

The Answer section could instead be given with an rrSet:

"answerRRs": [ { "NAME": "example.com.",
"TYPE": 1, "CLASS": 1,
"TTL": 3600,
"rrSet": [ { "RDATAHEX": "C0000201" },
{ "RDATAHEX": "C000AA01" } ] } ],

(Note that this is an incomplete list of what else could be in the
Answer section.)

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6. Local Format Policy

Systems using the format in this document will likely have policy
about what must be in the objects. Those policies are outside the
scope of this document.

For example, passive DNS systems such as those described in
[PASSIVE-DNS] cover just DNS responses. Such a system might have a
policy that makes QNAME, QTYPE, and answerRRs mandatory. That
document also describes two mandatory times that are not in this
format, so the policy would possibly also define those members and
make them mandatory. The policy could also define additional members
that might appear in a record.

As another example, a program that uses this format for configuring
what a test client sends on the wire might have a policy of "each
record object can have as few members as it wants; all unstated
members are filled in from previous records".

7. IANA Considerations

7.1. Media Type Registration of application/dns+json

Type name: application

Subtype name: dns+json

Required parameters: N/A

Optional parameters: N/A

Encoding considerations: Encoding considerations are identical to
those specified for the "application/json" media type.

Security considerations: This document specifies the security
considerations for the format.

Interoperability considerations: This document specifies format of
conforming messages and the interpretation thereof.

Published specification: This document

Applications that use this media type: Systems that want to exchange
DNS messages

Fragment identifier considerations: None

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Additional information:

Deprecated alias names for this type: N/A

Magic number(s): N/A

File extension(s): This document uses the media type to refer to
protocol messages and thus does not require a file extension.

Macintosh file type code(s): N/A

Person & email address to contact for further information:
Paul Hoffman, [email protected]

Intended usage: COMMON

Restrictions on usage: N/A

Author: Paul Hoffman, [email protected]

Change controller: Paul Hoffman, [email protected]

8. Security Considerations

As described in Section 1.1, a message object can have inconsistent
data, such as a message with an ANCOUNT of 1 but that has either an
empty answerRRs array or an answerRRs array that has two or more RRs.
Other examples of inconsistent data would be resource records whose
RDLENGTH does not match the length of the decoded value in the
RDATAHEX member, a record whose various header fields do not match
the value in headerOctetsHEX, and so on. A reader of this format
must never assume that all of the data in an object are all
consistent with each other.

This document describes a format, not a profile of that format. Lack
of profile can lead to security issues. For example, if a system has
a filter for JSON representations of DNS packets, that filter needs
to have the same semantics for the output JSON as the consumer has.
Unless the profile is quite tight, this can lead to the producer
being able to create fields with different contents (using the HEX
and regular formats), fields with malformed lengths, and so on.

Numbers in JSON do not have any bounds checking. Thus, integer
values in a record might have invalid values, such as an ID field
whose value is greater than or equal to 2^16, a QR field that has a
value of 2, and so on.

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9. Privacy Considerations

The values that can be contained in this format may contain privacy-
sensitive information. For example, a profile of this format that is
used for logging queries sent to recursive resolvers might have
source IP addresses that could identify the location of the person
who sent the DNS query.

10. References

10.1. Normative References

[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>.

[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.

[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <https://www.rfc-editor.org/info/rfc3597>.

[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.

[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.

[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.

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[RFC7464] Williams, N., "JavaScript Object Notation (JSON) Text
Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015,
<https://www.rfc-editor.org/info/rfc7464>.

[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>.

[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>.

10.2. Informative References

[DNS-QUERY]
Parthasarathy, M. and P. Vixie, "Representing DNS messages
using XML", Work in Progress,
draft-mohan-dns-query-xml-00, September 2011.

[DNSXML] Daley, J., Morris, S., and J. Dickinson, "dnsxml - A
standard XML representation of DNS data", Work in
Progress, draft-daley-dnsxml-00, July 2013.

[PASSIVE-DNS]
Dulaunoy, A., Kaplan, A., Vixie, P., and H. Stern,
"Passive DNS - Common Output Format", Work in Progress,
draft-dulaunoy-dnsop-passive-dns-cof-04, June 2018.

Acknowledgements

Some of the ideas in this document were inspired by earlier,
abandoned work such as [DNSXML], [DNS-QUERY], and [PASSIVE-DNS]. The
document was also inspired by early ideas from Stephane Bortzmeyer.
Many people in the Domain Name System Operations (DNSOP) and DNS Over
HTTPS (DOH) working groups contributed very useful ideas (even though
this was not a WG work item).

Author's Address

Paul Hoffman
ICANN

Email: [email protected]

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