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rfc:rfc6891

Internet Engineering Task Force (IETF) J. Damas Request for Comments: 6891 Bond Internet Systems STD: 75 M. Graff Obsoletes: 2671, 2673 Category: Standards Track P. Vixie ISSN: 2070-1721 Internet Systems Consortium

                                                            April 2013
               Extension Mechanisms for DNS (EDNS(0))

Abstract

 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 Extension Mechanisms for DNS (EDNS(0))
 specification (and obsoletes RFC 2671) based on feedback from
 deployment experience in several implementations.  It also obsoletes
 RFC 2673 ("Binary Labels in the Domain Name System") and adds
 considerations on the use of extended labels in the DNS.

Status of This Memo

 This is an Internet Standards Track document.
 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).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6891.

Damas, et al. Standards Track [Page 1] RFC 6891 EDNS(0) Extensions April 2013

Copyright Notice

 Copyright (c) 2013 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
 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.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 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.

Damas, et al. Standards Track [Page 2] RFC 6891 EDNS(0) Extensions April 2013

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
 2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
 3.  EDNS Support Requirement . . . . . . . . . . . . . . . . . . .  5
 4.  DNS Message Changes  . . . . . . . . . . . . . . . . . . . . .  5
   4.1.  Message Header . . . . . . . . . . . . . . . . . . . . . .  5
   4.2.  Label Types  . . . . . . . . . . . . . . . . . . . . . . .  5
   4.3.  UDP Message Size . . . . . . . . . . . . . . . . . . . . .  6
 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 . . . . . . . . . . . . . . .  9
     6.1.4.  Flags  . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.2.  Behaviour  . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.2.1.  Cache Behaviour  . . . . . . . . . . . . . . . . . . . 10
     6.2.2.  Fallback . . . . . . . . . . . . . . . . . . . . . . . 10
     6.2.3.  Requestor's Payload Size . . . . . . . . . . . . . . . 10
     6.2.4.  Responder's Payload Size . . . . . . . . . . . . . . . 11
     6.2.5.  Payload Size Selection . . . . . . . . . . . . . . . . 11
     6.2.6.  Support in Middleboxes . . . . . . . . . . . . . . . . 11
 7.  Transport Considerations . . . . . . . . . . . . . . . . . . . 12
 8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
 9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   9.1.  OPT Option Code Allocation Procedure . . . . . . . . . . . 15
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
   10.2. Informative References . . . . . . . . . . . . . . . . . . 15
 Appendix A.  Changes since RFCs 2671 and 2673  . . . . . . . . . . 16

Damas, et al. Standards Track [Page 3] RFC 6891 EDNS(0) Extensions April 2013

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 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 DNS
 Security (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 extension mechanisms to DNS.  These mechanisms are
 widely supported, and a number of new DNS uses and protocol
 extensions depend on the presence of these extensions.  This memo
 refines and obsoletes [RFC2671].
 Unextended agents will not know how to interpret the protocol
 extensions defined in [RFC2671] and restated here.  Extended agents
 need to be prepared for handling the interactions with unextended
 clients in the face of new protocol elements and fall back gracefully
 to unextended DNS.
 EDNS is a hop-by-hop extension to DNS.  This means the use of EDNS is
 negotiated between each pair of hosts in a DNS resolution process,
 for instance, the stub resolver communicating with the recursive
 resolver or the recursive resolver communicating with an
 authoritative server.
 [RFC2671] specified extended label types.  The only such label
 proposed was in [RFC2673] for a label type called "Bit-String Label"
 or "Binary Labels", with this latest term being the one in common
 use.  For various reasons, introducing a new label type was found to
 be extremely difficult, and [RFC2673] was moved to Experimental.
 This document obsoletes [RFC2673], deprecating Binary Labels.
 Extended labels remain defined, but their use is discouraged due to
 practical difficulties with deployment; their use in the future
 SHOULD only be considered after careful evaluation of the deployment
 hindrances.

2. Terminology

 "Requestor" refers to the side that sends a request.  "Responder"
 refers to an authoritative, recursive resolver or other DNS component
 that responds to questions.  Other terminology is used here as
 defined in the RFCs cited by this document.

Damas, et al. Standards Track [Page 4] RFC 6891 EDNS(0) Extensions April 2013

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 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
 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).  However, implementation
 experience indicates that adding new RCODEs should be avoided due to
 the difficulty in upgrading the installed base.  Flags SHOULD be used
 only when necessary for DNS resolution to function.
 For many uses, an EDNS Option Code may be preferred.
 Over 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 widespread 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 of [RFC1035]).  Some of these flag values were marked for
 future use, and most of 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 2 bits of a wire format domain label are used to denote the
 type of the label.  [RFC1035] allocates 2 of the 4 possible types and
 reserves the other 2.  More label types were defined in [RFC2671].
 The use of the 2-bit combination defined by [RFC2671] to identify
 extended label types remains valid.  However, it has been found that
 deployment of new label types is noticeably difficult and so is only

Damas, et al. Standards Track [Page 5] RFC 6891 EDNS(0) Extensions April 2013

 recommended after careful evaluation of alternatives and the need for
 deployment.

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.
 EDNS(0) specifies a way to advertise additional features such as
 larger response size capability, which is intended to help avoid
 truncated UDP responses, which in turn cause retry over TCP.  It
 therefore provides support for transporting these larger packet sizes
 without needing to resort to TCP for transport.

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 2 most significant bits of that octet for this purpose.
 [RFC2671] defined DNS label type 0b01 for use as an indication for
 extended label types.  A specific extended label type was selected by
 the 6 least significant bits of the first octet.  Thus, extended
 label types were indicated by the values 64-127 (0b01xxxxxx) in the
 first octet of the label.
 Extended label types are extremely difficult to deploy due to lack of
 support in clients and intermediate gateways, as described in
 [RFC3363], which moved [RFC2673] to Experimental status; and
 [RFC3364], which describes the pros and cons.  As such, proposals
 that contemplate extended labels SHOULD weigh this deployment cost
 against the possibility of implementing functionality in other ways.
 Finally, implementations MUST NOT generate or pass Binary Labels in
 their communications, as they are now deprecated.

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.

Damas, et al. Standards Track [Page 6] RFC 6891 EDNS(0) Extensions April 2013

 The OPT RR has RR type 41.
 If an OPT record is present in a received request, 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.
 When an OPT RR is included within any DNS message, it MUST be the
 only OPT RR in that message.  If a query message with more than one
 OPT RR is received, a FORMERR (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 metadata,
 and also a small collection of basic extension elements that 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

Damas, et al. Standards Track [Page 7] RFC 6891 EDNS(0) Extensions April 2013

 The variable part of an OPT RR may contain zero or more options in
 the RDATA.  Each option MUST be treated as a bit field.  Each option
 is encoded as:
                +0 (MSB)                            +1 (LSB)
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  0: |                          OPTION-CODE                          |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  2: |                         OPTION-LENGTH                         |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  4: |                                                               |
     /                          OPTION-DATA                          /
     /                                                               /
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 OPTION-CODE
    Assigned by the Expert Review process as defined by the DNSEXT
    working group and the IESG.
 OPTION-LENGTH
    Size (in octets) of OPTION-DATA.
 OPTION-DATA
    Varies per OPTION-CODE.  MUST be treated as a bit field.
 The order of appearance of option tuples is not defined.  If one
 option modifies the behaviour 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 and supports
 option XYZ, it MUST include option XYZ in its response.

Damas, et al. Standards Track [Page 8] RFC 6891 EDNS(0) Extensions April 2013

6.1.3. OPT Record TTL Field Use

 The extended RCODE and flags, which OPT stores in the RR Time to Live
 (TTL) field, are structured as follows:
                +0 (MSB)                            +1 (LSB)
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  0: |         EXTENDED-RCODE        |            VERSION            |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
  2: | DO|                           Z                               |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
 EXTENDED-RCODE
    Forms the 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
    15).
 VERSION
    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 minimise 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 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

 DO
    DNSSEC OK bit as defined by [RFC3225].
 Z
    Set to zero by senders and ignored by receivers, unless modified
    in a subsequent specification.

Damas, et al. Standards Track [Page 9] RFC 6891 EDNS(0) Extensions April 2013

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 EDNS(0),
 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 these options are only
 signaled through EDNS.  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 MAY be given to servers that
 support EDNS(0).  Implementers SHOULD analyse this choice and the
 impact on both endpoints.

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
 this.
 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 that
 will block fragmented IP packets, a requestor SHOULD NOT choose a
 value that 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.
 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.
 Where fragmentation is not a concern, use of bigger values SHOULD be
 considered by implementers.  Implementations SHOULD use their largest
 configured or implemented values as a starting point in an EDNS
 transaction in the absence of previous knowledge about the
 destination server.

Damas, et al. Standards Track [Page 10] RFC 6891 EDNS(0) Extensions April 2013

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

6.2.4. Responder's Payload Size

 The responder's maximum payload size can change over time but can
 reasonably be 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 that 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-byte 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 64 KB datagrams, memory usage at low
 levels in the system will be a concern.  A good compromise may be the
 use of an EDNS maximum payload size of 4096 octets as a starting
 point.
 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 that begins with
 a large size, such as 4096.  If that fails, a fallback around the
 range of 1280-1410 bytes 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 retry.
 Values of less than 512 bytes MUST be treated as equal to 512 bytes.

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 affects the transmission of DNS messages (e.g.,
 firewalls, load balancers, proxies, etc.), referred to here as
 "middleboxes".

Damas, et al. Standards Track [Page 11] RFC 6891 EDNS(0) Extensions April 2013

 Conformant middleboxes MUST NOT limit DNS messages over UDP to 512
 bytes.
 Middleboxes that simply forward requests to a recursive resolver MUST
 NOT modify and MUST NOT delete the OPT record contents in either
 direction.
 Middleboxes that have additional functionality, such as answering
 queries or acting as intelligent forwarders, SHOULD be able to
 process the OPT record and act based on its contents.  These
 middleboxes 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. 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.
 Extended agents MUST be prepared for handling interactions with
 unextended clients in the face of new protocol elements and fall back
 gracefully to unextended DNS when needed.
 Responders that choose not to implement the protocol extensions
 defined in this document MUST respond with a return code (RCODE) of
 FORMERR to messages containing an OPT record 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 that 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 that do not implement EDNS and format
 errors within EDNS.

Damas, et al. Standards Track [Page 12] RFC 6891 EDNS(0) Extensions April 2013

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

8. 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 that are too large for intermediate gateways to forward,
 thus leading to potential ICMP storms between gateways and
 responders.
 Announcing very large UDP buffer sizes may result in dropping of DNS
 messages 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 UDP buffer sizes that are too small 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.

9. IANA Considerations

 The IANA has assigned RR type code 41 for OPT.
 [RFC2671] specified a number of IANA subregistries within "DOMAIN
 NAME SYSTEM PARAMETERS":
 o  DNS EDNS(0) Options
 o  EDNS Version Number
 o  EDNS Header Flags
 Additionally, two entries were generated in existing registries:
 o  EDNS Extended Label Type in the DNS Label Types registry
 o  Bad OPT Version in the DNS RCODES registry
 IANA has updated references to [RFC2671] in these entries and
 subregistries to this document.
 [RFC2671] created the DNS Label Types registry.  This registry is to
 remain open.
 The registration procedure for the DNS Label Types registry is
 Standards Action.

Damas, et al. Standards Track [Page 13] RFC 6891 EDNS(0) Extensions April 2013

 This document assigns option code 65535 in the DNS EDNS0 Options
 registry to "Reserved for future expansion".
 The current status of the IANA registry for EDNS Option Codes at the
 time of publication of this document is
 o  0-4 assigned, per references in the registry
 o  5-65000 Available for assignment, unassigned
 o  65001-65534 Local/Experimental use
 o  65535 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 Review is required to add a new RCODE.
 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"; the
 IANA registry currently contains "Reserved for future expansion".
 This request implied, at that time, a request to open a new registry
 for extended label types, but due to the possibility of ambiguity,
 new text registrations were instead made within the general DNS Label
 Types registry, which also registers entries originally defined by
 [RFC1035].  There is therefore no Extended Label Types registry, with
 all label types registered in the DNS Label Types registry.
 This document deprecates Binary Labels.  Therefore, the status for
 the DNS Label Types registration "Binary Labels" is now "Historic".
 IETF Standards Action is required for assignments of new EDNS(0)
 flags.  Flags SHOULD be used only when necessary for DNS resolution
 to function.  For many uses, an EDNS Option Code may be preferred.
 IETF Standards Action is required to create new entries in the EDNS
 Version Number registry.  Within the EDNS Option Code space, Expert
 Review is required for allocation of an EDNS Option Code.  Per this
 document, IANA maintains a registry for the EDNS Option Code space.

Damas, et al. Standards Track [Page 14] RFC 6891 EDNS(0) Extensions April 2013

9.1. OPT Option Code Allocation Procedure

 OPT Option Codes are assigned by Expert Review.
 Assignment of Option Codes should be liberal, but duplicate
 functionality is to be avoided.

10. References

10.1. Normative References

 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
            RFC 2671, August 1999.
 [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC",
            RFC 3225, December 2001.

10.2. Informative References

 [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.

Damas, et al. Standards Track [Page 15] RFC 6891 EDNS(0) Extensions April 2013

Appendix A. Changes since RFCs 2671 and 2673

 Following is a list of high-level changes to RFCs 2671 and 2673.
 o  Support for the OPT record is now mandatory.
 o  Extended label types remain available, but their use is
    discouraged as a general solution due to observed difficulties in
    their deployment on the Internet, as illustrated by the work with
    the "Binary Labels" type.
 o  RFC 2673, which defined the "Binary Labels" type and is currently
    Experimental, is requested to be moved to Historic.
 o  Made changes in how EDNS buffer sizes are selected, and provided
    recommendations on how to select them.

Authors' Addresses

 Joao Damas
 Bond Internet Systems
 Av Albufera 14
 S.S. Reyes, Madrid  28701
 ES
 Phone: +1 650.423.1312
 EMail: joao@bondis.org
 Michael Graff
 EMail: explorer@flame.org
 Paul Vixie
 Internet Systems Consortium
 950 Charter Street
 Redwood City, California  94063
 US
 Phone: +1 650.423.1301
 EMail: vixie@isc.org

Damas, et al. Standards Track [Page 16]

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