DNSEXT Working Group                                            J. Damas
Internet-Draft                                                  M. Graff
Obsoletes: 2671, 2673                                           P. Vixie
(if approved)                                Internet Systems Consortium
Intended status: Standards Track                        February 7, 2012
Expires: August 10, 2012

                  Extension Mechanisms for DNS (EDNS0)


   The Domain Name System's wire protocol includes a number of fixed
   fields whose range has been or soon will be exhausted and does not
   allow requestors to advertise their capabilities to responders.  This
   document describes backward compatible mechanisms for allowing the
   protocol to grow.

   This document updates the EDNS0 specification (RFC 2671) based on
   feedback from deployment experience in several implementations.  It
   also closes the IANA registry for extended labels created as part of
   RFC 2671 and obsoletes RFC 2673 ("Binary Labels in the Domain Name
   System") which depends on the existence of extended labels.

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 August 10, 2012.

Copyright Notice

   Copyright (c) 2012 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

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   Provisions Relating to IETF Documents
   (http://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
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   material may not have granted the IETF Trust the right to allow
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  EDNS Support Requirement . . . . . . . . . . . . . . . . . . .  4
   4.  DNS Message changes  . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Message Header . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Label Types  . . . . . . . . . . . . . . . . . . . . . . .  5
     4.3.  UDP Message Size . . . . . . . . . . . . . . . . . . . . .  5
   5.  Extended Label Types . . . . . . . . . . . . . . . . . . . . .  6
   6.  The OPT pseudo-RR  . . . . . . . . . . . . . . . . . . . . . .  6
     6.1.  OPT Record Definition  . . . . . . . . . . . . . . . . . .  6
       6.1.1.  Basic elements . . . . . . . . . . . . . . . . . . . .  6
       6.1.2.  Wire Format  . . . . . . . . . . . . . . . . . . . . .  7
       6.1.3.  OPT Record TTL Field Use . . . . . . . . . . . . . . .  8
       6.1.4.  Flags  . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.2.  Behaviour  . . . . . . . . . . . . . . . . . . . . . . . .  9
       6.2.1.  Cache behaviour  . . . . . . . . . . . . . . . . . . .  9
       6.2.2.  Fallback . . . . . . . . . . . . . . . . . . . . . . .  9
       6.2.3.  Requestor's Payload Size . . . . . . . . . . . . . . .  9
       6.2.4.  Responder's Payload Size . . . . . . . . . . . . . . . 10
       6.2.5.  Payload Size Selection . . . . . . . . . . . . . . . . 10
       6.2.6.  Support in MiddleBoxes . . . . . . . . . . . . . . . . 11
   7.  OPT Option Code Allocation Procedure . . . . . . . . . . . . . 11
   8.  Transport Considerations . . . . . . . . . . . . . . . . . . . 11
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     11.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Document Editing History  . . . . . . . . . . . . . . 14
     A.1.  Changes since RFC2671  . . . . . . . . . . . . . . . . . . 14
     A.2.  Changes since -02  . . . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

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1.  Introduction

   DNS [RFC1035] specifies a Message Format and within such messages
   there are standard formats for encoding options, errors, and name
   compression.  The maximum allowable size of a DNS Message over UDP
   not using the extensions described in this document is limited to 512
   bytes.  Many of DNS's protocol limits, such as the maximum message
   size over UDP, are too small to efficiently support the additional
   information that can be conveyed in the DNS (e.g. several IPv6
   addresses or DNSSEC signatures).  Finally, RFC 1035 does not define
   any way for implementations to advertise their capabilities to any of
   the other actors they interact with.

   [RFC2671] added an extension mechanism to DNS.  This mechanism is
   widely supported and a number of new DNS uses and protocol extensions
   depend on the presence of these extensions.  This memo refines that
   specification and obsoletes [RFC2671].

   Unextended agents will not know how to interpret the protocol
   extensions defined in [RFC2671] and restated here.  Extended agents
   MUST be prepared for handling the interactions with unextended
   clients in the face of new protocol elements, and fall back
   gracefully to unextended DNS.

   [RFC2671] specified extended label types.  The only one proposed was
   in [RFC2673] for a label type called "Bitstring Labels."  For various
   reasons introducing a new label type was found to be extremely
   difficult, and [RFC2673] was moved to Experimental.  This document
   deprecates Extended Labels, and therefore Binary Labels, obsoleting

2.  Terminology

   "Requestor" is the side which sends a request.  "Responder" is an
   authoritative, recursive resolver, or other DNS component which
   responds to questions.  Other terminology is used as per its use in
   the references (e.g. middleboxes as in [RFC5625])

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  EDNS Support Requirement

   EDNS provides a mechanism to improve the scalability of DNS as its
   uses get more diverse on the Internet.  It does this by enabling the

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   use of UDP transport for DNS messages with sizes beyond the limits
   specified in RFC 1035 as well as providing extra data space for
   additional flags and return codes (RCODEs).

   With time, some applications of DNS have made EDNS a requirement for
   their deployment.  For instance, DNSSEC uses the additional flag
   space introduced in EDNS to signal the request to include DNSSEC data
   in a DNS response.

   Given the increase in DNS response sizes when including larger data
   items such as AAAA Records, DNSSEC information (e.g.  RRSIG or
   DNSKEY) or large TXT Records, the additional UDP payload capabilities
   provided by EDNS can help improve the scalability of the DNS by
   avoiding generalized use of TCP for DNS transport.

4.  DNS Message changes

4.1.  Message Header

   The DNS Message Header's second full 16-bit word is divided into a
   4-bit OPCODE, a 4-bit RCODE, and a number of 1-bit flags (see ,
   section 4.1.1 [RFC1035]).  Some of these were marked for future use,
   and most these have since been allocated.  Also, most of the RCODE
   values are now in use.  The OPT pseudo-RR specified below contains
   extensions to the RCODE bit field as well as additional flag bits.

4.2.  Label Types

   The first two bits of a wire format domain label are used to denote
   the type of the label.  [RFC1035] allocates two of the four possible
   types and reserves the other two.  More label types were defined in
   [RFC2671].  This document obsoletes the use of the 2-bit combination
   defined by [RFC2671] to identify extended label types.

4.3.  UDP Message Size

   Traditional DNS Messages are limited to 512 octets in size when sent
   over UDP [RFC1035].  Fitting the increasing amounts of data that can
   be transported in DNS in this 512-byte limit is becoming more
   difficult.  For instance, inclusion of DNSSEC records frequently
   requires a much larger response than a 512 byte message can hold.

   EDNS0 is intended to provide support for transporting these larger
   packet sizes while continuing to use UDP.  It specifies a way to
   advertise additional features such as larger response size
   capability, which is intended to help avoid truncated UDP responses
   which then cause retry over TCP.

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5.  Extended Label Types

   The first octet in the on-the-wire representation of a DNS label
   specifies the label type; the basic DNS specification [RFC1035]
   dedicates the two most significant bits of that octet for this

   [RFC2671] defined DNS label type 0b01 for use as an indication for
   Extended Label Types.  A specific Extended Label Type is selected by
   the 6 least significant bits of the first octet.  Thus, Extended
   Label Types are indicated by the values 64-127 (0b01xxxxxx) in the
   first octet of the label.

   Extended Label Types are difficult to use due to support in clients
   and intermediate gateways as described in [RFC3364] which moves them
   to experimental status and [RFC3363], which describes the pros and

   Therefore, this document moves them from experimental to historical,
   making them deprecated.

   Implementations MUST NOT generate or pass Extended Labels in their
   communications.  Additionally, no further registrations of Extended
   Label Types are permitted.

6.  The OPT pseudo-RR

6.1.  OPT Record Definition

6.1.1.  Basic elements

   An OPT pseudo-RR (sometimes called a meta-RR) MAY be added to the
   additional data section of a request.

   The OPT RR has RR type 41.

   If present in requests, compliant responders MUST include an OPT
   record in their respective responses.

   An OPT record does not carry any DNS data.  It is used only to
   contain control information pertaining to the question and answer
   sequence of a specific transaction.  OPT RRs MUST NOT be cached,
   forwarded, or stored in or loaded from master files.

   The OPT RR MAY be placed anywhere within the additional data section.
   No more than one OPT RR MUST be included within any DNS message.  If
   a query message with more than one OPT RR is received, a FORMERR

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   (RCODE=1) MUST be returned.  The placement flexibility for the OPT RR
   does not override the need for the TSIG or SIG(0) RRs to be the last
   in the additional section whenever they are present.

6.1.2.  Wire Format

   An OPT RR has a fixed part and a variable set of options expressed as
   {attribute, value} pairs.  The fixed part holds some DNS meta data
   and also a small collection of basic extension elements which we
   expect to be so popular that it would be a waste of wire space to
   encode them as {attribute, value} pairs.

   The fixed part of an OPT RR is structured as follows:

       | Field Name | Field Type   | Description                  |
       | NAME       | domain name  | Must be 0 (root domain)      |
       | TYPE       | u_int16_t    | OPT (41)                     |
       | CLASS      | u_int16_t    | requestor's UDP payload size |
       | TTL        | u_int32_t    | extended RCODE and flags     |
       | RDLEN      | u_int16_t    | length of all RDATA          |
       | RDATA      | octet stream | {attribute,value} pairs      |

                               OPT RR Format

   The variable part of an OPT RR may contain zero or more options in
   the RDATA.  Each option MUST be treated as binary.  Each option is
   encoded as:

                  +0 (MSB)                            +1 (LSB)
    0: |                          OPTION-CODE                          |
    2: |                         OPTION-LENGTH                         |
    4: |                                                               |
       /                          OPTION-DATA                          /
       /                                                               /

         Assigned by Expert Review.

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         Size (in octets) of OPTION-DATA.

         Varies per OPTION-CODE.  MUST be treated as binary.

   The order of appearance of option tuples is not defined.  If one
   option modifies the behavior of another or multiple options are
   related to one another in some way, they have the same effect
   regardless of ordering in the RDATA wire encoding.

   Any OPTION-CODE values not understood by a responder or requestor
   MUST be ignored.  Specifications of such options might wish to
   include some kind of signaled acknowledgement.  For example, an
   option specification might say that if a responder sees option XYZ,
   it MUST include option XYZ in its response.

6.1.3.  OPT Record TTL Field Use

   The extended RCODE and flags (which OPT stores in the RR TTL field)
   are structured as follows:

                  +0 (MSB)                            +1 (LSB)
    0: |         EXTENDED-RCODE        |            VERSION            |
    2: | DO|                           Z                               |

         Forms upper 8 bits of extended 12-bit RCODE (together with the
         4 bits defined in [RFC1035].  Note that EXTENDED-RCODE value 0
         indicates that an unextended RCODE is in use (values 0 through

         Indicates the implementation level of the setter.  Full
         conformance with this specification is indicated by version
         ``0.''  Requestors are encouraged to set this to the lowest
         implemented level capable of expressing a transaction, to
         minimize the responder and network load of discovering the
         greatest common implementation level between requestor and
         responder.  A requestor's version numbering strategy MAY
         ideally be a run time configuration option.
         If a responder does not implement the VERSION level of the
         request, then it MUST respond with RCODE=BADVERS.  All
         responses MUST be limited in format to the VERSION level of the
         request, but the VERSION of each response SHOULD be the highest

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         implementation level of the responder.  In this way a requestor
         will learn the implementation level of a responder as a side
         effect of every response, including error responses and
         including RCODE=BADVERS.

6.1.4.  Flags

         DNSSEC OK bit as defined by [RFC3225].

         Set to zero by senders and ignored by receivers, unless
         modified in a subsequent specification.

6.2.  Behaviour

6.2.1.  Cache behaviour

   The OPT record MUST NOT be cached.

6.2.2.  Fallback

   If a requestor detects that the remote end does not support EDNS0, it
   MAY issue queries without an OPT record.  It MAY cache this knowledge
   for a brief time in order to avoid fallback delays in the future.
   However, if DNSSEC or any future option using EDNS is required, no
   fallback should be performed as they are only signaled through EDNS0.
   If an implementation detects that some servers for the zone support
   EDNS(0) while others would force the use of TCP to fetch all data,
   preference SHOULD be given to those support EDNS(0).

6.2.3.  Requestor's Payload Size

   The requestor's UDP payload size (encoded in the RR CLASS field) is
   the number of octets of the largest UDP payload that can be
   reassembled and delivered in the requestor's network stack.  Note
   that path MTU, with or without fragmentation, could be smaller than

   Values lower than 512 MUST be treated as equal to 512.

   The requestor SHOULD place a value in this field that it can actually
   receive.  For example, if a requestor sits behind a firewall which
   will block fragmented IP packets, a requestor SHOULD NOT choose a
   value which will cause fragmentation.  Doing so will prevent large
   responses from being received, and can cause fallback to occur.  This
   knowledge may be auto-detected by the implementation or provided by a
   human administrator.

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   Note that a 512-octet UDP payload requires a 576-octet IP reassembly
   buffer.  Choosing between 1280 and 1410 bytes for IP (v4 or v6) over
   Ethernet would be reasonable.

   Bigger values SHOULD be considered where fragmentation is not a

   Choosing a very large value will guarantee fragmentation at the IP
   layer, and may prevent answers from being received due to a single
   fragment loss or misconfigured firewalls.

   The requestor's maximum payload size can change over time.  It MUST
   NOT be cached for use beyond the transaction in which it is

6.2.4.  Responder's Payload Size

   The responder's maximum payload size can change over time, but can be
   reasonably expected to remain constant between two closely spaced
   sequential transactions; for example, an arbitrary QUERY used as a
   probe to discover a responder's maximum UDP payload size, followed
   immediately by an UPDATE which takes advantage of this size.  This is
   considered preferable to the outright use of TCP for oversized
   requests, if there is any reason to suspect that the responder
   implements EDNS, and if a request will not fit in the default 512
   payload size limit.

6.2.5.  Payload Size Selection

   Due to transaction overhead, it is not recommended to advertise an
   architectural limit as a maximum UDP payload size.  Even on system
   stacks capable of reassembling 64KB datagrams, memory usage at low
   levels in the system will be a concern.  A good compromise may be the
   use of about 4KB of state memory per ongoing transaction, or a EDNS
   maximum payload size of 4096 octets.

   A requestor MAY choose to implement a fallback to smaller advertised
   sizes to work around firewall or other network limitations.  A
   requestor SHOULD choose to use a fallback mechanism which begins with
   a large size, such as 4096.  If that fails, a fallback around the
   1280-1410 byte range SHOULD be tried, as it has a reasonable chance
   to fit within a single Ethernet frame.  Failing that, a requestor MAY
   choose a 512 byte packet, which with large answers may cause a TCP

   Values of less than 512 bytes MUST be treated as equal to 512 bytes.

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6.2.6.  Support in MiddleBoxes

   In a network that carries DNS traffic there could be active equipment
   other than that participating directly in the DNS resolution process
   (stub and caching resolvers, authoritative servers) that affect the
   transmission of DNS messages (e.g. firewalls, load balancers,
   proxies, etc) referred to here as MiddleBoxes.

   Conformant MiddleBoxes MUST NOT limit DNS messages over UDP to 512

   MiddleBoxes which simply forward requests to a recursive resolver
   MUST NOT modify and MUST NOT delete the OPT record contents in either

   MiddleBoxes which have additional functionality, such as answering
   queries or acting as intelligent forwarders, SHOULD understand the
   OPT record.  These boxes MUST consider the incoming request and any
   outgoing requests as separate transactions if the characteristics of
   the messages are different.

   A more in depth discussion of this type of equipment and other
   considerations regarding their interaction with DNS traffic is found
   in [RFC5625]

7.  OPT Option Code Allocation Procedure

   Allocations are assigned by expert review.

   Assignment of Option Codes should be liberal, but duplicate
   functionality is to be avoided.

8.  Transport Considerations

   The presence of an OPT pseudo-RR in a request should be taken as an
   indication that the requestor fully implements the given version of
   EDNS, and can correctly understand any response that conforms to that
   feature's specification.

   Lack of presence of an OPT record in a request MUST be taken as an
   indication that the requestor does not implement any part of this
   specification and that the responder MUST NOT include an OPT record
   in its response.

   Responders which choose not to implement the protocol extensions
   defined in this document MUST respond with a return code (RCODE) of

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   FORMERR to messages containing an OPT RR in the additional section
   and MUST NOT include an OPT record in the response.

   If there is a problem with processing the OPT record itself, such as
   an option value that is badly formatted or includes out of range
   values, a FORMERR MUST be returned.  If this occurs the response MUST
   include an OPT record.  This is intended to allow the requestor to
   distinguish between servers which do not implement EDNS and format
   errors within EDNS.

   The minimal response MUST be the DNS header, question section, and an
   OPT record.  This MUST also occur when an truncated response (using
   the DNS header's TC bit) is returned.

9.  Security Considerations

   Requestor-side specification of the maximum buffer size may open a
   DNS denial of service attack if responders can be made to send
   messages which are too large for intermediate gateways to forward,
   thus leading to potential ICMP storms between gateways and

   Announcing very large UDP buffer sizes may result in dropping by
   middleboxes (see Section 6.2.6).  This could cause retransmissions
   with no hope of success.  Some devices have been found to reject
   fragmented UDP packets.

   Announcing too small UDP buffer sizes may result in fallback to TCP
   with a corresponding load impact on DNS servers.  This is especially
   important with DNSSEC, where answers are much larger.

10.  IANA Considerations

   The IANA has assigned RR type code 41 for OPT.

   [RFC2671] specified a number of IANA sub-registries within "DOMAIN

   o  DNS EDNS0 Options

   o  EDNS Version Number

   o  EDNS Header Flags

   Additionally, several entries were generated in existing registries:

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   EDNS Extended Label Type in the DNS Label Types Registry

   Bad OPT Version in the DNS RCODES registry

   IANA is advised to udpates references to [RFC2671] in these entries
   and sub-registries to this document.

   [RFC2671] created the "EDNS Extended Label Type Registry".  We
   request that this registry be closed.

   This document assigns option code 65535 in the "EDNS Option Codes"
   registry to "Reserved for future expansion."

   [RFC2671] expands the RCODE space from 4 bits to 12 bits.  This
   allows more than the 16 distinct RCODE values allowed in [RFC1035].
   IETF Standards Action is required to add a new RCODE.  Adding new
   RCODEs should be avoided due to the difficulty in upgrading the
   installed base.

   This document assigns EDNS Extended RCODE 16 to "BADVERS" in the DNS
   RCODES registry.

   [RFC2671] called for the recording of assignment of extended label
   types 0bxx111111 as "Reserved for future extended label types".  This
   request was implicitly a request to open a new registry for Extended
   Label Types but due to possible ambiguous text registrations were
   instead made within the general "DNS Label Types" registry which also
   registers entries originally defined by [RFC1035].

   This document requests IANA to close registration of further Extended
   Label Types in the "DNS Label Types" Registry.

   IETF Standards Action is required for assignments of new EDNS0 flags.
   Flags SHOULD be used only when necessary for DNS resolution to
   function.  For many uses, a EDNS Option Code may be preferred.

   IETF Standards Action is required to create new entries in the EDNS
   Version Number registry.  Expert Review is required for allocation of
   an EDNS Option Code.

11.  References

11.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

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   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.

   [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC",
              RFC 3225, December 2001.

11.2.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2673]  Crawford, M., "Binary Labels in the Domain Name System",
              RFC 2673, August 1999.

   [RFC3363]  Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
              Hain, "Representing Internet Protocol version 6 (IPv6)
              Addresses in the Domain Name System (DNS)", RFC 3363,
              August 2002.

   [RFC3364]  Austein, R., "Tradeoffs in Domain Name System (DNS)
              Support for Internet Protocol version 6 (IPv6)", RFC 3364,
              August 2002.

   [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",
              BCP 152, RFC 5625, August 2009.

Appendix A.  Document Editing History

   Following is a list of high-level changes made to the original

A.1.  Changes since RFC2671

   o  Support for the OPT record is now mandatory.

   o  Extended label types obsoleted and the registry is closed.

   o  The bitstring label type, which was already moved from draft to
      experimental, is requested to be moved to historical.

   o  Changes in how EDNS buffer sizes are selected, with
      recommendations on how to select them.

   o  Front material (IPR notice and such) was updated to current

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A.2.  Changes since -02

   o  Specified the method for allocation of constants.

   o  Cleaned up a lot of wording, along with quite a bit of document
      structure changes.

Authors' Addresses

   Joao Damas
   Internet Systems Consortium
   950 Charter Street
   Redwood City, California  94063

   Phone: +1 650.423.1312
   Email: joao@isc.org

   Michael Graff
   Internet Systems Consortium
   950 Charter Street
   Redwood City, California  94063

   Phone: +1 650.423.1304
   Email: mgraff@isc.org

   Paul Vixie
   Internet Systems Consortium
   950 Charter Street
   Redwood City, California  94063

   Phone: +1 650.423.1301
   Email: vixie@isc.org

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