Service binding and parameter specification via the DNS (DNS SVCB and HTTPS RRs)
draft-ietf-dnsop-svcb-https-08

Document Type Active Internet-Draft (dnsop WG)
Authors Benjamin Schwartz  , Mike Bishop  , Erik Nygren 
Last updated 2021-10-13 (latest revision 2021-10-12)
Replaces draft-ietf-dnsop-svcb-httpssvc
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DNSOP Working Group                                          B. Schwartz
Internet-Draft                                                    Google
Intended status: Standards Track                               M. Bishop
Expires: 15 April 2022                                         E. Nygren
                                                     Akamai Technologies
                                                         12 October 2021

 Service binding and parameter specification via the DNS (DNS SVCB and
                               HTTPS RRs)
                     draft-ietf-dnsop-svcb-https-08

Abstract

   This document specifies the "SVCB" and "HTTPS" DNS resource record
   (RR) types to facilitate the lookup of information needed to make
   connections to network services, such as for HTTP origins.  SVCB
   records allow a service to be provided from multiple alternative
   endpoints, each with associated parameters (such as transport
   protocol configuration and keys for encrypting the TLS ClientHello).
   They also enable aliasing of apex domains, which is not possible with
   CNAME.  The HTTPS RR is a variation of SVCB for use with HTTP [HTTP].
   By providing more information to the client before it attempts to
   establish a connection, these records offer potential benefits to
   both performance and privacy.

   TO BE REMOVED: This document is being collaborated on in Github at:
   https://github.com/MikeBishop/dns-alt-svc
   (https://github.com/MikeBishop/dns-alt-svc).  The most recent working
   version of the document, open issues, etc. should all be available
   there.  The authors (gratefully) accept pull requests.

Status of This Memo

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

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

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

   This Internet-Draft will expire on 15 April 2022.

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Copyright Notice

   Copyright (c) 2021 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  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Goals of the SVCB RR  . . . . . . . . . . . . . . . . . .   5
     1.2.  Overview of the SVCB RR . . . . . . . . . . . . . . . . .   5
     1.3.  Parameter for Encrypted ClientHello . . . . . . . . . . .   6
     1.4.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
   2.  The SVCB record type  . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Zone file presentation format . . . . . . . . . . . . . .   8
     2.2.  RDATA wire format . . . . . . . . . . . . . . . . . . . .   9
     2.3.  SVCB query names  . . . . . . . . . . . . . . . . . . . .  10
     2.4.  Interpretation  . . . . . . . . . . . . . . . . . . . . .  10
       2.4.1.  SvcPriority . . . . . . . . . . . . . . . . . . . . .  11
       2.4.2.  AliasMode . . . . . . . . . . . . . . . . . . . . . .  11
       2.4.3.  ServiceMode . . . . . . . . . . . . . . . . . . . . .  12
     2.5.  Special handling of "." in TargetName . . . . . . . . . .  13
       2.5.1.  AliasMode . . . . . . . . . . . . . . . . . . . . . .  13
       2.5.2.  ServiceMode . . . . . . . . . . . . . . . . . . . . .  13
   3.  Client behavior . . . . . . . . . . . . . . . . . . . . . . .  13
     3.1.  Handling resolution failures  . . . . . . . . . . . . . .  14
     3.2.  Clients using a Proxy . . . . . . . . . . . . . . . . . .  15
   4.  DNS Server Behavior . . . . . . . . . . . . . . . . . . . . .  16
     4.1.  Authoritative servers . . . . . . . . . . . . . . . . . .  16
     4.2.  Recursive resolvers . . . . . . . . . . . . . . . . . . .  16
     4.3.  General requirements  . . . . . . . . . . . . . . . . . .  17
     4.4.  EDNS Client Subnet (ECS)  . . . . . . . . . . . . . . . .  17
   5.  Performance optimizations . . . . . . . . . . . . . . . . . .  18
     5.1.  Optimistic pre-connection and connection reuse  . . . . .  18
     5.2.  Generating and using incomplete responses . . . . . . . .  19
   6.  SVCB-compatible . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Initial SvcParamKeys  . . . . . . . . . . . . . . . . . . . .  20
     7.1.  "alpn" and "no-default-alpn"  . . . . . . . . . . . . . .  20
       7.1.1.  Representation  . . . . . . . . . . . . . . . . . . .  21
       7.1.2.  Use . . . . . . . . . . . . . . . . . . . . . . . . .  22

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     7.2.  "port"  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     7.3.  "ech" . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     7.4.  "ipv4hint" and "ipv6hint" . . . . . . . . . . . . . . . .  23
     7.5.  "mandatory" . . . . . . . . . . . . . . . . . . . . . . .  24
   8.  ServiceMode RR compatibility and mandatory keys . . . . . . .  24
   9.  Using Service Bindings with HTTP  . . . . . . . . . . . . . .  25
     9.1.  Query names for HTTPS RRs . . . . . . . . . . . . . . . .  26
     9.2.  Comparison with Alt-Svc . . . . . . . . . . . . . . . . .  27
       9.2.1.  ALPN usage  . . . . . . . . . . . . . . . . . . . . .  27
       9.2.2.  Untrusted channel . . . . . . . . . . . . . . . . . .  27
       9.2.3.  Cache lifetime  . . . . . . . . . . . . . . . . . . .  27
       9.2.4.  Granularity . . . . . . . . . . . . . . . . . . . . .  28
     9.3.  Interaction with Alt-Svc  . . . . . . . . . . . . . . . .  28
     9.4.  Requiring Server Name Indication  . . . . . . . . . . . .  29
     9.5.  HTTP Strict Transport Security  . . . . . . . . . . . . .  29
     9.6.  Use of HTTPS RRs in other protocols . . . . . . . . . . .  30
   10. SVCB/HTTPS RR parameter for ECH configuration . . . . . . . .  30
     10.1.  Client behavior  . . . . . . . . . . . . . . . . . . . .  31
     10.2.  Deployment considerations  . . . . . . . . . . . . . . .  31
   11. Zone Structures . . . . . . . . . . . . . . . . . . . . . . .  31
     11.1.  Structuring zones for flexibility  . . . . . . . . . . .  31
     11.2.  Structuring zones for performance  . . . . . . . . . . .  32
     11.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . .  32
       11.3.1.  Protocol enhancements  . . . . . . . . . . . . . . .  32
       11.3.2.  Apex aliasing  . . . . . . . . . . . . . . . . . . .  33
       11.3.3.  Parameter binding  . . . . . . . . . . . . . . . . .  33
       11.3.4.  Multi-CDN  . . . . . . . . . . . . . . . . . . . . .  34
       11.3.5.  Non-HTTP uses  . . . . . . . . . . . . . . . . . . .  36
   12. Interaction with other standards  . . . . . . . . . . . . . .  36
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  36
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  37
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
     15.1.  SVCB RRType  . . . . . . . . . . . . . . . . . . . . . .  37
     15.2.  HTTPS RRType . . . . . . . . . . . . . . . . . . . . . .  38
     15.3.  New registry for Service Parameters  . . . . . . . . . .  38
       15.3.1.  Procedure  . . . . . . . . . . . . . . . . . . . . .  38
       15.3.2.  Initial contents . . . . . . . . . . . . . . . . . .  39
     15.4.  Other registry updates . . . . . . . . . . . . . . . . .  40
   16. Acknowledgments and Related Proposals . . . . . . . . . . . .  41
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  41
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  41
     17.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Appendix A.  Decoding text in zone files  . . . . . . . . . . . .  45
     A.1.  Decoding a comma-separated list . . . . . . . . . . . . .  46
   Appendix B.  HTTP Mapping Summary . . . . . . . . . . . . . . . .  46
   Appendix C.  Comparison with alternatives . . . . . . . . . . . .  47
     C.1.  Differences from the SRV RR type  . . . . . . . . . . . .  47
     C.2.  Differences from the proposed HTTP record . . . . . . . .  48

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     C.3.  Differences from the proposed ANAME record  . . . . . . .  48
     C.4.  Comparison with separate RR types for AliasMode and
           ServiceMode . . . . . . . . . . . . . . . . . . . . . . .  48
   Appendix D.  Test vectors . . . . . . . . . . . . . . . . . . . .  49
     D.1.  AliasMode . . . . . . . . . . . . . . . . . . . . . . . .  49
     D.2.  ServiceMode . . . . . . . . . . . . . . . . . . . . . . .  49
     D.3.  Failure cases . . . . . . . . . . . . . . . . . . . . . .  54
   Appendix E.  Change history . . . . . . . . . . . . . . . . . . .  55
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  59

1.  Introduction

   The SVCB ("Service Binding") and HTTPS RRs provide clients with
   complete instructions for access to a service.  This information
   enables improved performance and privacy by avoiding transient
   connections to a sub-optimal default server, negotiating a preferred
   protocol, and providing relevant public keys.

   For example, HTTP clients currently resolve only A and/or AAAA
   records for the origin hostname, learning only its IP addresses.  If
   an HTTP client learns more about the origin before connecting, it may
   be able to upgrade "http" URLs to "https", enable HTTP/3 or Encrypted
   ClientHello [ECH], or switch to an operationally preferable endpoint.
   It is highly desirable to minimize the number of round-trips and
   lookups required to learn this additional information.

   The SVCB and HTTPS RRs also help when the operator of a service
   wishes to delegate operational control to one or more other domains,
   e.g. delegating the origin "https://example.com" to a service
   operator endpoint at "svc.example.net".  While this case can
   sometimes be handled by a CNAME, that does not cover all use-cases.
   CNAME is also inadequate when the service operator needs to provide a
   bound collection of consistent configuration parameters through the
   DNS (such as network location, protocol, and keying information).

   This document first describes the SVCB RR as a general-purpose
   resource record that can be applied directly and efficiently to a
   wide range of services (Section 2).  It also describes the rules for
   defining other SVCB-compatible RR types (Section 6), starting with
   the HTTPS RR type (Section 9), which provides improved efficiency and
   convenience with HTTP by avoiding the need for an Attrleaf label
   [Attrleaf] (Section 9.1).

   The SVCB RR has two modes: 1) "AliasMode", which simply delegates
   operational control for a resource; 2) "ServiceMode", which binds
   together configuration information for a service endpoint.
   ServiceMode provides additional key=value parameters within each
   RDATA set.

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1.1.  Goals of the SVCB RR

   The goal of the SVCB RR is to allow clients to resolve a single
   additional DNS RR in a way that:

   *  Provides alternative endpoints that are authoritative for the
      service, along with parameters associated with each of these
      endpoints.

   *  Does not assume that all alternative endpoints have the same
      parameters or capabilities, or are even operated by the same
      entity.  This is important, as DNS does not provide any way to tie
      together multiple RRSets for the same name.  For example, if
      www.example.com is a CNAME alias that switches between one of
      three CDNs or hosting environments, successive queries for that
      name may return records that correspond to different environments.

   *  Enables CNAME-like functionality at a zone apex (such as
      "example.com") for participating protocols, and generally enables
      delegation of operational authority for an origin within the DNS
      to an alternate name.

   Additional goals specific to HTTPS RRs and the HTTP use-cases
   include:

   *  Connect directly to HTTP/3 (QUIC transport) alternative endpoints
      [HTTP3]

   *  Obtain the Encrypted ClientHello [ECH] keys associated with an
      alternative endpoint

   *  Support non-default TCP and UDP ports

   *  Enable SRV-like benefits (e.g. apex delegation, as mentioned
      above) for HTTP, where SRV [SRV] has not been widely adopted

   *  Provide an HSTS-like indication [HSTS] signaling that the "https"
      scheme should be used instead of "http" for this request (see
      Section 9.5).

1.2.  Overview of the SVCB RR

   This subsection briefly describes the SVCB RR in a non-normative
   manner.  (As mentioned above, this all applies equally to the HTTPS
   RR which shares the same encoding, format, and high-level semantics.)

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   The SVCB RR has two modes: AliasMode, which aliases a name to another
   name, and ServiceMode, which provides connection information bound to
   a service endpoint domain.  Placing both forms in a single RR type
   allows clients to fetch the relevant information with a single query.

   The SVCB RR has two required fields and one optional.  The fields
   are:

   1.  SvcPriority: The priority of this record (relative to others,
       with lower values preferred).  A value of 0 indicates AliasMode.
       (Described in Section 2.4.1.)

   2.  TargetName: The domain name of either the alias target (for
       AliasMode) or the alternative endpoint (for ServiceMode).

   3.  SvcParams (optional): A list of key=value pairs describing the
       alternative endpoint at TargetName (only used in ServiceMode and
       otherwise ignored).  Described in Section 2.1.

   Cooperating DNS recursive resolvers will perform subsequent record
   resolution (for SVCB, A, and AAAA records) and return them in the
   Additional Section of the response.  Clients either use responses
   included in the additional section returned by the recursive resolver
   or perform necessary SVCB, A, and AAAA record resolutions.  DNS
   authoritative servers can attach in-bailiwick SVCB, A, AAAA, and
   CNAME records in the Additional Section to responses for a SVCB
   query.

   In ServiceMode, the SvcParams of the SVCB RR provide an extensible
   data model for describing alternative endpoints that are
   authoritative for the origin, along with parameters associated with
   each of these alternative endpoints.

   For HTTP use-cases, the HTTPS RR enables many of the benefits of Alt-
   Svc [AltSvc] without waiting for a full HTTP connection initiation
   (multiple roundtrips) before learning of the preferred alternative,
   and without necessarily revealing the user's intended destination to
   all entities along the network path.

1.3.  Parameter for Encrypted ClientHello

   This document also defines a parameter for Encrypted ClientHello
   [ECH] keys.  See Section 10.

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1.4.  Terminology

   Our terminology is based on the common case where the SVCB record is
   used to access a resource identified by a URI whose authority field
   contains a DNS hostname as the host.

   *  The "service" is the information source identified by the
      authority and scheme of the URI, capable of providing access to
      the resource.  For "https" URIs, the "service" corresponds to an
      "origin" [RFC6454].

   *  The "service name" is the host portion of the authority.

   *  The "authority endpoint" is the authority's hostname and a port
      number implied by the scheme or specified in the URI.

   *  An "alternative endpoint" is a hostname, port number, and other
      associated instructions to the client on how to reach an instance
      of service.

   Additional DNS terminology intends to be consistent with [DNSTerm].

   SVCB is a contraction of "service binding".  The SVCB RR, HTTPS RR,
   and future RR types that share SVCB's formats and registry are
   collectively known as SVCB-compatible RR types.  The contraction
   "SVCB" is also used to refer to this system as a whole.

   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.  The SVCB record type

   The SVCB DNS resource record (RR) type (RR type 64) is used to locate
   alternative endpoints for a service.

   The algorithm for resolving SVCB records and associated address
   records is specified in Section 3.

   Other SVCB-compatible resource record types can also be defined as-
   needed (see Section 6).  In particular, the HTTPS RR (RR type 65)
   provides special handling for the case of "https" origins as
   described in Section 9.

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   SVCB RRs are extensible by a list of SvcParams, which are pairs
   consisting of a SvcParamKey and a SvcParamValue.  Each SvcParamKey
   has a presentation name and a registered number.  Values are in a
   format specific to the SvcParamKey.  Their definition should specify
   both their presentation format and wire encoding (e.g., domain names,
   binary data, or numeric values).  The initial SvcParamKeys and
   formats are defined in Section 7.

2.1.  Zone file presentation format

   The presentation format of the record is:

   Name TTL IN SVCB SvcPriority TargetName SvcParams

   The SVCB record is defined specifically within the Internet ("IN")
   Class ([RFC1035]).

   SvcPriority is a number in the range 0-65535, TargetName is a
   <domain-name> ([RFC1035], Section 5.1), and the SvcParams are a
   whitespace-separated list, with each SvcParam consisting of a
   SvcParamKey=SvcParamValue pair or a standalone SvcParamKey.
   SvcParamKeys are subject to IANA control (Section 15.3).

   Each SvcParamKey SHALL appear at most once in the SvcParams.  In
   presentation format, SvcParamKeys are lower-case alphanumeric
   strings.  Key names should contain 1-63 characters from the ranges
   "a"-"z", "0"-"9", and "-".  In ABNF [RFC5234],

   alpha-lc      = %x61-7A   ;  a-z
   SvcParamKey   = 1*63(alpha-lc / DIGIT / "-")
   SvcParam      = SvcParamKey ["=" SvcParamValue]
   SvcParamValue = char-string
   value         = *OCTET

   The SvcParamValue is parsed using the character-string decoding
   algorithm (Appendix A), producing a value.  The value is then
   validated and converted into wire-format in a manner specific to each
   key.

   When the "=" is omitted, the value is interpreted as empty.

   Arbitrary keys can be represented using the unknown-key presentation
   format "keyNNNNN" where NNNNN is the numeric value of the key type
   without leading zeros.  A SvcParam in this form SHALL be parsed as
   specified above, and the decoded value SHALL be used as its wire
   format encoding.

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   For some SvcParamKeys, the value corresponds to a list or set of
   items.  Presentation formats for such keys SHOULD use a comma-
   separated list (Appendix A.1).

   SvcParams in presentation format MAY appear in any order, but keys
   MUST NOT be repeated.

2.2.  RDATA wire format

   The RDATA for the SVCB RR consists of:

   *  a 2 octet field for SvcPriority as an integer in network byte
      order.

   *  the uncompressed, fully-qualified TargetName, represented as a
      sequence of length-prefixed labels as in Section 3.1 of [RFC1035].

   *  the SvcParams, consuming the remainder of the record (so smaller
      than 65535 octets and constrained by the RDATA and DNS message
      sizes).

   When the list of SvcParams is non-empty, it contains a series of
   SvcParamKey=SvcParamValue pairs, represented as:

   *  a 2 octet field containing the SvcParamKey as an integer in
      network byte order.  (See Section 15.3.2 for the defined values.)

   *  a 2 octet field containing the length of the SvcParamValue as an
      integer between 0 and 65535 in network byte order.

   *  an octet string of this length whose contents are the
      SvcParamValue in a format determined by the SvcParamKey.

   SvcParamKeys SHALL appear in increasing numeric order.

   Clients MUST consider an RR malformed if:

   *  the end of the RDATA occurs within a SvcParam.

   *  SvcParamKeys are not in strictly increasing numeric order.

   *  the SvcParamValue for an SvcParamKey does not have the expected
      format.

   Note that the second condition implies that there are no duplicate
   SvcParamKeys.

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   If any RRs are malformed, the client MUST reject the entire RRSet and
   fall back to non-SVCB connection establishment.

2.3.  SVCB query names

   When querying the SVCB RR, a service is translated into a QNAME by
   prepending the service name with a label indicating the scheme,
   prefixed with an underscore, resulting in a domain name like
   "_examplescheme.api.example.com.".  This follows the Attrleaf naming
   pattern [Attrleaf], so the scheme MUST be registered appropriately
   with IANA (see Section 12).

   Protocol mapping documents MAY specify additional underscore-prefixed
   labels to be prepended.  For schemes that specify a port
   (Section 3.2.3 of [URI]), one reasonable possibility is to prepend
   the indicated port number if a non-default port number is specified.
   We term this behavior "Port Prefix Naming", and use it in the
   examples throughout this document.

   See Section 9.1 for the HTTPS RR behavior.

   When a prior CNAME or SVCB record has aliased to a SVCB record, each
   RR shall be returned under its own owner name.

   Note that none of these forms alter the origin or authority for
   validation purposes.  For example, TLS clients MUST continue to
   validate TLS certificates for the original service name.

   As an example, the owner of example.com could publish this record:

   _8443._foo.api.example.com. 7200 IN SVCB 0 svc4.example.net.

   to indicate that "foo://api.example.com:8443" is aliased to
   "svc4.example.net".  The owner of example.net, in turn, could publish
   this record:

   svc4.example.net.  7200  IN SVCB 3 svc4.example.net. (
       alpn="bar" port="8004" ech="..." )

   to indicate that these services are served on port number 8004, which
   supports the protocol "bar" and its associated transport in addition
   to the default transport protocol for "foo://".

   (Parentheses are used to ignore a line break in DNS zone file
   presentation format ([RFC1035], Section 5.1).)

2.4.  Interpretation

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2.4.1.  SvcPriority

   When SvcPriority is 0 the SVCB record is in AliasMode
   (Section 2.4.2).  Otherwise, it is in ServiceMode (Section 2.4.3).

   Within a SVCB RRSet, all RRs SHOULD have the same Mode.  If an RRSet
   contains a record in AliasMode, the recipient MUST ignore any
   ServiceMode records in the set.

   RRSets are explicitly unordered collections, so the SvcPriority field
   is used to impose an ordering on SVCB RRs.  SVCB RRs with a smaller
   SvcPriority value SHOULD be given preference over RRs with a larger
   SvcPriority value.

   When receiving an RRSet containing multiple SVCB records with the
   same SvcPriority value, clients SHOULD apply a random shuffle within
   a priority level to the records before using them, to ensure uniform
   load-balancing.

2.4.2.  AliasMode

   In AliasMode, the SVCB record aliases a service to a TargetName.
   SVCB RRSets SHOULD only have a single resource record in AliasMode.
   If multiple are present, clients or recursive resolvers SHOULD pick
   one at random.

   The primary purpose of AliasMode is to allow aliasing at the zone
   apex, where CNAME is not allowed.  In AliasMode, the TargetName will
   be the name of a domain that resolves to SVCB, AAAA, and/or A
   records.  (See Section 6 for aliasing of SVCB-compatible RR types.)
   The TargetName SHOULD NOT be equal to the owner name, as this would
   result in a loop.

   In AliasMode, records SHOULD NOT include any SvcParams, and
   recipients MUST ignore any SvcParams that are present.

   For example, the operator of foo://example.com:8080 could point
   requests to a service operating at foosvc.example.net by publishing:

   _8080._foo.example.com. 3600 IN SVCB 0 foosvc.example.net.

   Using AliasMode maintains a separation of concerns: the owner of
   foosvc.example.net can add or remove ServiceMode SVCB records without
   requiring a corresponding change to example.com.  Note that if
   foosvc.example.net promises to always publish a SVCB record, this
   AliasMode record can be replaced by a CNAME, which would likely
   improve performance.

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   AliasMode is especially useful for SVCB-compatible RR types that do
   not require an underscore prefix, such as the HTTPS RR type.  For
   example, the operator of https://example.com could point requests to
   a server at svc.example.net by publishing this record at the zone
   apex:

   example.com. 3600 IN HTTPS 0 svc.example.net.

   Note that the SVCB record's owner name MAY be the canonical name of a
   CNAME record, and the TargetName MAY be the owner of a CNAME record.
   Clients and recursive resolvers MUST follow CNAMEs as normal.

   To avoid unbounded alias chains, clients and recursive resolvers MUST
   impose a limit on the total number of SVCB aliases they will follow
   for each resolution request.  This limit MUST NOT be zero, i.e.
   implementations MUST be able to follow at least one AliasMode record.
   The exact value of this limit is left to implementations.

   For compatibility and performance, zone owners SHOULD NOT configure
   their zones to require following multiple AliasMode records.

   As legacy clients will not know to use this record, service operators
   will likely need to retain fallback AAAA and A records alongside this
   SVCB record, although in a common case the target of the SVCB record
   might offer better performance, and therefore would be preferable for
   clients implementing this specification to use.

   AliasMode records only apply to queries for the specific RR type.
   For example, a SVCB record cannot alias to an HTTPS record, nor vice-
   versa.

2.4.3.  ServiceMode

   In ServiceMode, the TargetName and SvcParams within each resource
   record associate an alternative endpoint for the service with its
   connection parameters.

   Each protocol scheme that uses SVCB MUST define a protocol mapping
   that explains how SvcParams are applied for connections of that
   scheme.  Unless specified otherwise by the protocol mapping, clients
   MUST ignore any SvcParam that they do not recognize.

   Some SvcParams impose requirements on other SvcParams in the RR.  A
   ServiceMode RR is called "self-consistent" if its SvcParams all
   comply with each others' requirements.  Zone-file implementations
   SHOULD enforce self-consistency.  Clients MUST reject any RR whose
   recognized SvcParams are not self-consistent, and MAY reject the
   entire RRSet.

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2.5.  Special handling of "." in TargetName

   If TargetName has the value "." (represented in the wire format as a
   zero-length label), special rules apply.

2.5.1.  AliasMode

   For AliasMode SVCB RRs, a TargetName of "." indicates that the
   service is not available or does not exist.  This indication is
   advisory: clients encountering this indication MAY ignore it and
   attempt to connect without the use of SVCB.

2.5.2.  ServiceMode

   For ServiceMode SVCB RRs, if TargetName has the value ".", then the
   owner name of this record MUST be used as the effective TargetName.

   For example, in the following example "svc2.example.net" is the
   effective TargetName:

   example.com.      7200  IN HTTPS 0 svc.example.net.
   svc.example.net.  7200  IN CNAME svc2.example.net.
   svc2.example.net. 7200  IN HTTPS 1 . port=8002 ech="..."
   svc2.example.net. 300   IN A     192.0.2.2
   svc2.example.net. 300   IN AAAA  2001:db8::2

3.  Client behavior

   "SVCB resolution" is the process of enumerating the priority-ordered
   endpoints for a service, as performed by the client.  SVCB resolution
   is implemented as follows:

   1.  Let $QNAME be the service name plus appropriate prefixes for the
       scheme (see Section 2.3).

   2.  Issue a SVCB query for $QNAME.

   3.  If an AliasMode SVCB record is returned for $QNAME (after
       following CNAMEs as normal), set $QNAME to its TargetName
       (without additional prefixes) and loop back to step 2, subject to
       chain length limits and loop detection heuristics (see
       Section 3.1).

   4.  If one or more "compatible" (Section 8) ServiceMode records are
       returned, these represent the alternative endpoints.

   5.  Otherwise, SVCB resolution has failed, and the list of known
       endpoints is empty.

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   This procedure does not rely on any recursive or authoritative DNS
   server to comply with this specification or have any awareness of
   SVCB.

   A client is called "SVCB-optional" if it can connect without the use
   of ServiceMode records, and "SVCB-reliant" otherwise.  Clients for
   pre-existing protocols (e.g.  HTTP) SHALL implement SVCB-optional
   behavior (except as noted in Section 3.1 and Section 10.1).

   SVCB-optional clients SHOULD issue in parallel any other DNS queries
   that might be needed for connection establishment if the SVCB record
   is absent, in order to minimize delay in that case and enable the
   optimizations discussed in Section 5.

   Once SVCB resolution has concluded, whether successful or not, SVCB-
   optional clients SHALL append to the priority list an endpoint
   consisting of the final value of $QNAME, the authority endpoint's
   port number, and no SvcParams.  (This endpoint will be attempted
   before falling back to non-SVCB connection modes.  This ensures that
   SVCB-optional clients will make use of an AliasMode record whose
   TargetName has A and/or AAAA records but no SVCB records.)

   The client proceeds with connection establishment using the resolved
   list of endpoints.  Clients SHOULD try higher-priority alternatives
   first, with fallback to lower-priority alternatives.  Clients resolve
   AAAA and/or A records for the selected TargetName, and MAY choose
   between them using an approach such as Happy Eyeballs
   [HappyEyeballsV2].

   If the client is SVCB-optional, and connecting using this list of
   endpoints has failed, the client SHOULD attempt non-SVCB connection
   modes.

   Some important optimizations are discussed in Section 5 to avoid
   additional latency in comparison to ordinary AAAA/A lookups.

3.1.  Handling resolution failures

   If DNS responses are cryptographically protected (e.g. using DNSSEC
   or TLS [DoT][DoH]), and SVCB resolution fails due to an
   authentication error, SERVFAIL response, transport error, or timeout,
   the client SHOULD abandon the connection attempt even if the client
   is SVCB-optional.  Otherwise, an active attacker could mount a
   downgrade attack by denying the user access to the SvcParams.

   A SERVFAIL error can occur if the domain is DNSSEC-signed, the
   recursive resolver is DNSSEC-validating, and the attacker is between
   the recursive resolver and the authoritative DNS server.  A transport

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   error or timeout can occur if an active attacker between the client
   and the recursive resolver is selectively dropping SVCB queries or
   responses, based on their size or other observable patterns.

   If the client enforces DNSSEC validation on A/AAAA responses, it
   SHOULD apply the same validation policy to SVCB.

   If the client is unable to complete SVCB resolution due to its chain
   length limit, the client SHOULD fall back to the authority endpoint,
   as if the origin's SVCB record did not exist.

3.2.  Clients using a Proxy

   Clients using a domain-oriented transport proxy like HTTP CONNECT
   ([RFC7231], Section 4.3.6) or SOCKS5 ([RFC1928]) have the option to
   use named destinations, in which case the client does not perform any
   A or AAAA queries for destination domains.  If the client is using
   named destinations with a proxy that does not provide SVCB query
   capability (e.g. through an affiliated DNS resolver), the client
   would have to perform SVCB resolution separately, likely disclosing
   the destinations to additional parties.  Clients that support such
   proxies SHOULD arrange for a separate SVCB resolution procedure with
   appropriate privacy properties, or disable SVCB resolution entirely
   if SVCB-optional.

   If the client does use SVCB and named destinations, the client SHOULD
   follow the standard SVCB resolution process, selecting the smallest-
   SvcPriority option that is compatible with the client and the proxy.
   When connecting using a SVCB record, clients MUST provide the final
   TargetName and port to the proxy, which will perform any required A
   and AAAA lookups.

   This arrangement has several benefits:

   *  Compared to disabling SVCB:

      -  It allows the client to use the SvcParams, if present, which
         are only usable with a specific TargetName.  The SvcParams may
         include information that enhances performance (e.g. alpn) and
         privacy (e.g. ech).

      -  It allows the service to delegate the apex domain.

   *  Compared to providing the proxy with an IP address:

      -  It allows the proxy to select between IPv4 and IPv6 addresses
         for the server according to its configuration.

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      -  It ensures that the proxy receives addresses based on its
         network geolocation, not the client's.

      -  It enables faster fallback for TCP destinations with multiple
         addresses of the same family.

4.  DNS Server Behavior

4.1.  Authoritative servers

   When replying to a SVCB query, authoritative DNS servers SHOULD
   return A, AAAA, and SVCB records in the Additional Section for any
   TargetNames that are in the zone.  If the zone is signed, the server
   SHOULD also include positive or negative DNSSEC responses for these
   records in the Additional section.

   See Section 4.4 for exceptions.

4.2.  Recursive resolvers

   Whether or not the recursive resolver is aware of SVCB, the normal
   response construction process (i.e. unknown RR type resolution under
   [RFC3597]) generates the Answer section of the response.  Recursive
   resolvers that are aware of SVCB SHOULD help the client to execute
   the procedure in Section 3 with minimum overall latency by
   incorporating additional useful information into the Additional
   section of the response as follows:

   1.  Incorporate the results of SVCB resolution.  If the chain length
       limit has been reached, terminate.

   2.  If any of the resolved SVCB records are in AliasMode, choose one
       of them at random, and resolve SVCB, A, and AAAA records for its
       TargetName.

       *  If any SVCB records are resolved, go to step 1.

       *  Otherwise, incorporate the results of A and AAAA resolution,
          and terminate.

   3.  All the resolved SVCB records are in ServiceMode.  Resolve A and
       AAAA queries for each TargetName (or for the owner name if
       TargetName is "."), incorporate all the results, and terminate.

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   In this procedure, "resolve" means the resolver's ordinary recursive
   resolution procedure, as if processing a query for that RRSet.  This
   includes following any aliases that the resolver would ordinarily
   follow (e.g.  CNAME, DNAME [DNAME]).  Errors or anomalies in
   obtaining additional records MAY cause this process to terminate, but
   MUST NOT themselves cause the resolver to send a failure response.

   See Section 2.4.2 for additional safeguards for recursive resolvers
   to implement to mitigate loops.

   See Section 5.2 for possible optimizations of this procedure.

4.3.  General requirements

   Recursive resolvers MUST be able to convey SVCB records with
   unrecognized SvcParamKeys, and MAY treat the entire SvcParams portion
   of the record as opaque, even if the contents are invalid.
   Alternatively, recursive resolvers MAY report an error such as
   SERVFAIL to avoid returning a SvcParamValue that is invalid according
   to the SvcParam's specification.  For complex value types whose
   interpretation might differ between implementations or have
   additional future allowed values added (e.g.  URIs or "alpn"),
   resolvers SHOULD limit validation to specified constraints.

   When responding to a query that includes the DNSSEC OK bit
   ([RFC3225]), DNSSEC-capable recursive and authoritative DNS servers
   MUST accompany each RRSet in the Additional section with the same
   DNSSEC-related records that they would send when providing that RRSet
   as an Answer (e.g.  RRSIG, NSEC, NSEC3).

   According to Section 5.4.1 of [RFC2181], "Unauthenticated RRs
   received and cached from ... the additional data section ... should
   not be cached in such a way that they would ever be returned as
   answers to a received query.  They may be returned as additional
   information where appropriate.".  Recursive resolvers therefore MAY
   cache records from the Additional section for use in populating
   Additional section responses, and MAY cache them for general use if
   they are authenticated by DNSSEC.

4.4.  EDNS Client Subnet (ECS)

   The EDNS Client Subnet option (ECS, [RFC7871]) allows recursive
   resolvers to request IP addresses that are suitable for a particular
   client IP range.  SVCB records may contain IP addresses (in ipv*hint
   SvcParams), or direct users to a subnet-specific TargetName, so
   recursive resolvers SHOULD include the same ECS option in SVCB
   queries as in A/AAAA queries.

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   According to Section 7.3.1 of [RFC7871], "Any records from [the
   Additional section] MUST NOT be tied to a network".  Accordingly,
   when processing a response whose QTYPE is SVCB-compatible, resolvers
   SHOULD treat any records in the Additional section as having SOURCE
   PREFIX-LENGTH zero and SCOPE PREFIX-LENGTH as specified in the ECS
   option.  Authoritative servers MUST omit such records if they are not
   suitable for use by any stub resolvers that set SOURCE PREFIX-LENGTH
   to zero.  This will cause the resolver to perform a followup query
   that can receive properly tailored ECS.  (This is similar to the
   usage of CNAME with ECS discussed in [RFC7871], Section 7.2.1.)

   Authoritative servers that omit Additional records can avoid the
   added latency of a followup query by following the advice in
   Section 11.2.

5.  Performance optimizations

   For optimal performance (i.e. minimum connection setup time), clients
   SHOULD implement a client-side DNS cache.  Responses in the
   Additional section of a SVCB response SHOULD be placed in cache
   before performing any followup queries.  With this behavior, and
   conforming DNS servers, using SVCB does not add network latency to
   connection setup.

   To improve performance when using a non-conforming recursive
   resolver, clients SHOULD issue speculative A and/or AAAA queries in
   parallel with each SVCB query, based on a predicted value of
   TargetName (see Section 11.2).

   After a ServiceMode RRSet is received, clients MAY try more than one
   option in parallel, and MAY prefetch A and AAAA records for multiple
   TargetNames.

5.1.  Optimistic pre-connection and connection reuse

   If an address response arrives before the corresponding SVCB
   response, the client MAY initiate a connection as if the SVCB query
   returned NODATA, but MUST NOT transmit any information that could be
   altered by the SVCB response until it arrives.  For example, a TLS
   ClientHello can be altered by the "ech" value of a SVCB response
   (Section 7.3).  Clients implementing this optimization SHOULD wait
   for 50 milliseconds before starting optimistic pre-connection, as per
   the guidance in [HappyEyeballsV2].

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   A SVCB record is consistent with a connection if the client would
   attempt an equivalent connection when making use of that record.  If
   a SVCB record is consistent with an active or in-progress connection
   C, the client MAY prefer that record and use C as its connection.
   For example, suppose the client receives this SVCB RRSet for a
   protocol that uses TLS over TCP:

   _1234._bar.example.com. 300 IN SVCB 1 svc1.example.net. (
       ech="111..." ipv6hint=2001:db8::1 port=1234 )
                                  SVCB 2 svc2.example.net. (
       ech="222..." ipv6hint=2001:db8::2 port=1234 )

   If the client has an in-progress TCP connection to
   [2001:db8::2]:1234, it MAY proceed with TLS on that connection using
   ech="222...", even though the other record in the RRSet has higher
   priority.

   If none of the SVCB records are consistent with any active or in-
   progress connection, clients proceed with connection establishment as
   described in Section 3.

5.2.  Generating and using incomplete responses

   When following the procedure in Section 4.2, recursive resolvers MAY
   terminate the procedure early and produce a reply that omits some of
   the associated RRSets.  This is REQUIRED when the chain length limit
   is reached (Section 4.2 step 1), but might also be appropriate when
   the maximum response size is reached, or when responding before fully
   chasing dependencies would improve performance.  When omitting
   certain RRSets, recursive resolvers SHOULD prioritize information for
   smaller-SvcPriority records.

   As discussed in Section 3, clients MUST be able to fetch additional
   information that is required to use a SVCB record, if it is not
   included in the initial response.  As a performance optimization, if
   some of the SVCB records in the response can be used without
   requiring additional DNS queries, the client MAY prefer those
   records, regardless of their priorities.

6.  SVCB-compatible

   An RR type is called "SVCB-compatible" if it permits an
   implementation that is identical to SVCB in its:

   *  RDATA presentation format

   *  RDATA wire format

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   *  IANA registry used for SvcParamKeys

   *  Authoritative server Additional Section processing

   *  Recursive resolution process

   *  Relevant Class (i.e.  Internet ("IN") [RFC1035])

   This allows authoritative and recursive DNS servers to apply
   identical processing to all SVCB-compatible RR types.

   All other behaviors described as applying to the SVCB RR also apply
   to all SVCB-compatible RR types unless explicitly stated otherwise.
   When following an AliasMode record (Section 2.4.2) of RR type $T ,
   the followup query to the TargetName MUST also be for type $T.

   This document defines one SVCB-compatible RR type (other than SVCB
   itself): the HTTPS RR type (Section 9), which avoids Attrleaf label
   prefixes [Attrleaf] in order to improve compatibility with wildcards
   and CNAMEs, which are widely used with HTTP.

   Standards authors should consider carefully whether to use SVCB or
   define a new SVCB-compatible RR type, as this choice cannot easily be
   reversed after deployment.

7.  Initial SvcParamKeys

   A few initial SvcParamKeys are defined here.  These keys are useful
   for the "https" scheme, and most are applicable to other schemes as
   well.

   Each new protocol mapping document MUST specify which keys are
   applicable and safe to use.  Protocol mappings MAY alter the
   interpretation of SvcParamKeys but MUST NOT alter their presentation
   or wire formats.

7.1.  "alpn" and "no-default-alpn"

   The "alpn" and "no-default-alpn" SvcParamKeys together indicate the
   set of Application Layer Protocol Negotiation (ALPN) protocol
   identifiers [ALPN] and associated transport protocols supported by
   this service endpoint (the "SVCB ALPN set").

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   As with Alt-Svc [AltSvc], each ALPN protocol identifier is used to
   identify the application protocol and associated suite of protocols
   supported by the endpoint (the "protocol suite").  The presence of an
   ALPN protocol identifier in the SVCB ALPN set indicates that this
   service endpoint, described by TargetName and the other parameters
   (e.g. "port") offers service with the protocol suite associated with
   this ALPN identifier.

   Clients filter the set of ALPN identifiers to match the protocol
   suites they support, and this informs the underlying transport
   protocol used (such as QUIC-over-UDP or TLS-over-TCP).  ALPN protocol
   identifiers that do not uniquely identify a protocol suite (e.g. an
   Identification Sequence that can be used with both TLS and DTLS) are
   not compatible with this SvcParamKey and MUST NOT be included in the
   SVCB ALPN set.

7.1.1.  Representation

   ALPNs are identified by their registered "Identification Sequence"
   (alpn-id), which is a sequence of 1-255 octets.

   alpn-id = 1*255OCTET

   For "alpn", the presentation value SHALL be a comma-separated list
   (Appendix A.1) of one or more alpn-ids.  Zone file implementations
   MAY disallow the "," and "\" characters instead of implementing the
   value-list escaping procedure, relying on the opaque key format (e.g.
   key1=\002h2) in the event that these characters are needed.

   The wire format value for "alpn" consists of at least one alpn-id
   prefixed by its length as a single octet, and these length-value
   pairs are concatenated to form the SvcParamValue.  These pairs MUST
   exactly fill the SvcParamValue; otherwise, the SvcParamValue is
   malformed.

   For "no-default-alpn", the presentation and wire format values MUST
   be empty.  When "no-default-alpn" is specified in an RR, "alpn" must
   also be specified in order for the RR to be "self-consistent"
   (Section 2.4.3).

   Each scheme that uses this SvcParamKey defines a "default set" of
   ALPNs that are supported by nearly all clients and servers, which MAY
   be empty.  To determine the SVCB ALPN set, the client starts with the
   list of alpn-ids from the "alpn" SvcParamKey, and adds the default
   set unless the "no-default-alpn" SvcParamKey is present.

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7.1.2.  Use

   To establish a connection to the endpoint, clients MUST

   1.  Let SVCB-ALPN-Intersection be the set of protocols in the SVCB
       ALPN set that the client supports.

   2.  Let Intersection-Transports be the set of transports (e.g.  TLS,
       DTLS, QUIC) implied by the protocols in SVCB-ALPN-Intersection.

   3.  For each transport in Intersection-Transports, construct a
       ProtocolNameList containing the Identification Sequences of all
       the client's supported ALPN protocols for that transport, without
       regard to the SVCB ALPN set.

   For example, if the SVCB ALPN set is ["http/1.1", "h3"], and the
   client supports HTTP/1.1, HTTP/2, and HTTP/3, the client could
   attempt to connect using TLS over TCP with a ProtocolNameList of
   ["http/1.1", "h2"], and could also attempt a connection using QUIC,
   with a ProtocolNameList of ["h3"].

   Once the client has constructed a ClientHello, protocol negotiation
   in that handshake proceeds as specified in [ALPN], without regard to
   the SVCB ALPN set.

   Clients MAY implement a fallback procedure, using a less-preferred
   transport if more-preferred transports fail to connect.  This
   fallback behavior is vulnerable to manipulation by a network attacker
   who blocks the more-preferred transports, but it may be necessary for
   compatibility with existing networks.

   With this procedure in place, an attacker who can modify DNS and
   network traffic can prevent a successful transport connection, but
   cannot otherwise interfere with ALPN protocol selection.  This
   procedure also ensures that each ProtocolNameList includes at least
   one protocol from the SVCB ALPN set.

   Clients SHOULD NOT attempt connection to a service endpoint whose
   SVCB ALPN set does not contain any supported protocols.  To ensure
   consistency of behavior, clients MAY reject the entire SVCB RRSet and
   fall back to basic connection establishment if all of the RRs
   indicate "no-default-alpn", even if connection could have succeeded
   using a non-default alpn.

   For compatibility with clients that require default transports, zone
   operators SHOULD ensure that at least one RR in each RRSet supports
   the default transports.

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7.2.  "port"

   The "port" SvcParamKey defines the TCP or UDP port that should be
   used to reach this alternative endpoint.  If this key is not present,
   clients SHALL use the authority endpoint's port number.

   The presentation value of the SvcParamValue is a single decimal
   integer between 0 and 65535 in ASCII.  Any other value (e.g. an empty
   value) is a syntax error.  To enable simpler parsing, this SvcParam
   MUST NOT contain escape sequences.

   The wire format of the SvcParamValue is the corresponding 2 octet
   numeric value in network byte order.

   If a port-restricting firewall is in place between some client and
   the service endpoint, changing the port number might cause that
   client to lose access to the service, so operators should exercise
   caution when using this SvcParamKey to specify a non-default port.

7.3.  "ech"

   The SvcParamKey to enable Encrypted ClientHello (ECH) is "ech".  Its
   value is defined in Section 10.  It is applicable to most TLS-based
   protocols.

   When publishing a record containing an "ech" parameter, the publisher
   MUST ensure that all IP addresses of TargetName correspond to servers
   that have access to the corresponding private key or are
   authoritative for the public name.  (See Section 7.2.2 of [ECH] for
   more details about the public name.)  This yields an anonymity set of
   cardinality equal to the number of ECH-enabled server domains
   supported by a given client-facing server.  Thus, even with an
   encrypted ClientHello, an attacker who can enumerate the set of ECH-
   enabled domains supported by a client-facing server can guess the
   correct SNI with probability at least 1/K, where K is the size of
   this ECH-enabled server anonymity set.  This probability may be
   increased via traffic analysis or other mechanisms.

7.4.  "ipv4hint" and "ipv6hint"

   The "ipv4hint" and "ipv6hint" keys convey IP addresses that clients
   MAY use to reach the service.  If A and AAAA records for TargetName
   are locally available, the client SHOULD ignore these hints.
   Otherwise, clients SHOULD perform A and/or AAAA queries for
   TargetName as in Section 3, and clients SHOULD use the IP address in
   those responses for future connections.  Clients MAY opt to terminate
   any connections using the addresses in hints and instead switch to
   the addresses in response to the TargetName query.  Failure to use A

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   and/or AAAA response addresses could negatively impact load balancing
   or other geo-aware features and thereby degrade client performance.

   The presentation value SHALL be a comma-separated list (Appendix A.1)
   of one or more IP addresses of the appropriate family in standard
   textual format [RFC5952].  To enable simpler parsing, this
   SvcParamValue MUST NOT contain escape sequences.

   The wire format for each parameter is a sequence of IP addresses in
   network byte order.  Like an A or AAAA RRSet, the list of addresses
   represents an unordered collection, and clients SHOULD pick addresses
   to use in a random order.  An empty list of addresses is invalid.

   When selecting between IPv4 and IPv6 addresses to use, clients may
   use an approach such as Happy Eyeballs [HappyEyeballsV2].  When only
   "ipv4hint" is present, IPv6-only clients may synthesize IPv6
   addresses as specified in [RFC7050] or ignore the "ipv4hint" key and
   wait for AAAA resolution (Section 3).  Recursive resolvers MUST NOT
   perform DNS64 ([RFC6147]) on parameters within a SVCB record.  For
   best performance, server operators SHOULD include an "ipv6hint"
   parameter whenever they include an "ipv4hint" parameter.

   These parameters are intended to minimize additional connection
   latency when a recursive resolver is not compliant with the
   requirements in Section 4, and SHOULD NOT be included if most clients
   are using compliant recursive resolvers.  When TargetName is the
   origin hostname or the owner name (which can be written as "."),
   server operators SHOULD NOT include these hints, because they are
   unlikely to convey any performance benefit.

7.5.  "mandatory"

   See Section 8.

8.  ServiceMode RR compatibility and mandatory keys

   In a ServiceMode RR, a SvcParamKey is considered "mandatory" if the
   RR will not function correctly for clients that ignore this
   SvcParamKey.  Each SVCB protocol mapping SHOULD specify a set of keys
   that are "automatically mandatory", i.e. mandatory if they are
   present in an RR.  The SvcParamKey "mandatory" is used to indicate
   any mandatory keys for this RR, in addition to any automatically
   mandatory keys that are present.

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   A ServiceMode RR is considered "compatible" with a client if the
   client recognizes all the mandatory keys, and their values indicate
   that successful connection establishment is possible.  If the SVCB
   RRSet contains no compatible RRs, the client will generally act as if
   the RRSet is empty.

   The presentation value SHALL be a comma-separated list (Appendix A.1)
   of one or more valid SvcParamKeys, either by their registered name or
   in the unknown-key format (Section 2.1).  Keys MAY appear in any
   order, but MUST NOT appear more than once.  For self-consistency
   (Section 2.4.3), listed keys MUST also appear in the SvcParams.

   To enable simpler parsing, this SvcParamValue MUST NOT contain escape
   sequences.

   For example, the following is a valid list of SvcParams:

   ech=... key65333=ex1 key65444=ex2 mandatory=key65444,ech

   In wire format, the keys are represented by their numeric values in
   network byte order, concatenated in ascending order.

   This SvcParamKey is always automatically mandatory, and MUST NOT
   appear in its own value-list.  Other automatically mandatory keys
   SHOULD NOT appear in the list either.  (Including them wastes space
   and otherwise has no effect.)

9.  Using Service Bindings with HTTP

   Use of any protocol with SVCB requires a protocol-specific mapping
   specification.  This section specifies the mapping for the "http" and
   "https" URI schemes [I-D.draft-ietf-httpbis-semantics].

   To enable special handling for HTTP use-cases, the HTTPS RR type is
   defined as a SVCB-compatible RR type, specific to the "https" and
   "http" schemes.  Clients MUST NOT perform SVCB queries or accept SVCB
   responses for "https" or "http" schemes.

   The presentation format of the record is:

   Name TTL IN HTTPS SvcPriority TargetName SvcParams

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   All the SvcParamKeys defined in Section 7 are permitted for use in
   HTTPS RRs.  The default set of ALPN IDs is the single value
   "http/1.1".  The "automatically mandatory" keys (Section 8) are
   "port" and "no-default-alpn".  (As described in Section 8, clients
   must either implement these keys or ignore any RR in which they
   appear.)  Clients that restrict the destination port in "https" URIs
   (e.g. using the "bad ports" list from [FETCH]) SHOULD apply the same
   restriction to the "port" SvcParam.

   The presence of an HTTPS RR for an origin also indicates that clients
   should connect securely and use the "https" scheme, as discussed in
   Section 9.5.  This allows HTTPS RRs to apply to pre-existing "http"
   scheme URLs, while ensuring that the client uses a secure and
   authenticated connection.

   The HTTPS RR parallels the concepts introduced in the HTTP
   Alternative Services proposed standard [AltSvc].  Clients and servers
   that implement HTTPS RRs are not required to implement Alt-Svc.

9.1.  Query names for HTTPS RRs

   The HTTPS RR uses Port Prefix Naming (Section 2.3), with one
   modification: if the scheme is "https" and the port is 443, then the
   client's original QNAME is equal to the service name (i.e. the
   origin's hostname), without any prefix labels.

   By removing the Attrleaf labels [Attrleaf] used in SVCB, this
   construction enables offline DNSSEC signing of wildcard domains,
   which are commonly used with HTTP.  Using the service name as the
   owner name of the HTTPS record, without prefixes, also allows the
   targets of existing CNAME chains (e.g.  CDN hosts) to start returning
   HTTPS RR responses without requiring origin domains to configure and
   maintain an additional delegation.

   Following of HTTPS AliasMode RRs and CNAME aliases is unchanged from
   SVCB.

   Clients always convert "http" URLs to "https" before performing an
   HTTPS RR query using the process described in Section 9.5, so domain
   owners MUST NOT publish HTTPS RRs with a prefix of "_http".

   Note that none of these forms alter the HTTPS origin or authority.
   For example, clients MUST continue to validate TLS certificate
   hostnames based on the origin.

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9.2.  Comparison with Alt-Svc

   Publishing a ServiceMode HTTPS RR in DNS is intended to be similar to
   transmitting an Alt-Svc field value over HTTP, and receiving an HTTPS
   RR is intended to be similar to receiving that field value over HTTP.
   However, there are some differences in the intended client and server
   behavior.

9.2.1.  ALPN usage

   Unlike Alt-Svc Field Values, HTTPS RRs can contain multiple ALPN IDs.
   The meaning and use of these IDs is discussed in Section 7.1.2.

9.2.2.  Untrusted channel

   HTTPS records do not require or provide any assurance of
   authenticity.  (DNSSEC signing and verification, which would provide
   such assurance, are OPTIONAL.)  The DNS resolution process is modeled
   as an untrusted channel that might be controlled by an attacker, so
   Alt-Svc parameters that cannot be safely received in this model MUST
   NOT have a corresponding defined SvcParamKey.  For example, there is
   no SvcParamKey corresponding to the Alt-Svc "persist" parameter,
   because this parameter is not safe to accept over an untrusted
   channel.

9.2.3.  Cache lifetime

   There is no SvcParamKey corresponding to the Alt-Svc "ma" (max age)
   parameter.  Instead, server operators encode the expiration time in
   the DNS TTL.

   The appropriate TTL value might be different from the "ma" value used
   for Alt-Svc, depending on the desired efficiency and agility.  Some
   DNS caches incorrectly extend the lifetime of DNS records beyond the
   stated TTL, so server operators cannot rely on HTTPS RRs expiring on
   time.  Shortening the TTL to compensate for incorrect caching is NOT
   RECOMMENDED, as this practice impairs the performance of correctly
   functioning caches and does not guarantee faster expiration from
   incorrect caches.  Instead, server operators SHOULD maintain
   compatibility with expired records until they observe that nearly all
   connections have migrated to the new configuration.

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9.2.4.  Granularity

   Sending Alt-Svc over HTTP allows the server to tailor the Alt-Svc
   Field Value specifically to the client.  When using an HTTPS RR,
   groups of clients will necessarily receive the same SvcParams.
   Therefore, HTTPS RRs are not suitable for uses that require single-
   client granularity.

9.3.  Interaction with Alt-Svc

   Clients that implement support for both Alt-Svc and HTTPS records
   SHOULD retrieve any HTTPS records for the Alt-Svc alt-authority, and
   ensure that their connection attempts are consistent with both the
   Alt-Svc parameters and any received HTTPS SvcParams.  If present, the
   HTTPS record's TargetName and port are used for connection
   establishment (as in Section 3).  For example, suppose that
   "https://example.com" sends an Alt-Svc field value of:

   Alt-Svc: h2="alt.example:443", h2="alt2.example:443", h3=":8443"

   The client would retrieve the following HTTPS records:

   alt.example.              IN HTTPS 1 . alpn=h2,h3 ech=...
   alt2.example.             IN HTTPS 1 alt2b.example. alpn=h3 ech=...
   _8443._https.example.com. IN HTTPS 1 alt3.example. (
       port=9443 alpn=h2,h3 ech=... )

   Based on these inputs, the following connection attempts would always
   be allowed:

   *  HTTP/2 to alt.example:443

   *  HTTP/3 to alt3.example:9443

   *  Fallback to the the client's non-Alt-Svc connection behavior

   ECH-capable clients would use ECH when establishing any of these
   connections.

   The following connection attempts would not be allowed:

   *  HTTP/3 to alt.example:443 (not consistent with Alt-Svc)

   *  Any connection to alt2b.example (no ALPN consistent with both the
      HTTPS record and Alt-Svc)

   *  HTTPS over TCP to any port on alt3.example (not consistent with
      Alt-Svc)

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   The following Alt-Svc-only connection attempts would be allowed only
   if the client does not support ECH, as they rely on SVCB-optional
   fallback behavior that the client will disable if it implements
   support for ECH and the "ech" SvcParam is present (Section 10.1):

   *  HTTP/2 to alt2.example:443

   *  HTTP/3 to example.com:8443

   Origins that publish an "ech" SvcParam in their HTTPS record SHOULD
   also publish an "ech" SvcParam for any alt-authorities.  Otherwise,
   clients might reveal the name of the server in an unencrypted
   ClientHello.  Similar consistency considerations could apply to
   future SvcParamKeys, so alt-authorities SHOULD carry the same
   SvcParams as the origin unless a deviation is specifically known to
   be safe.

   As noted in Section 2.4 of [AltSvc], clients MAY disallow any Alt-Svc
   connection according to their own criteria, e.g. disallowing Alt-Svc
   connections that lack ECH support when there is an active ECH-
   protected connection for this origin.

9.4.  Requiring Server Name Indication

   Clients MUST NOT use an HTTPS RR response unless the client supports
   TLS Server Name Indication (SNI) and indicates the origin name when
   negotiating TLS.  This supports the conservation of IP addresses.

   Note that the TLS SNI (and also the HTTP "Host" or ":authority") will
   indicate the origin, not the TargetName.

9.5.  HTTP Strict Transport Security

   An HTTPS RR directs the client to communicate with this host only
   over a secure transport, similar to HTTP Strict Transport Security
   [HSTS].  Prior to making an "http" scheme request, the client SHOULD
   perform a lookup to determine if any HTTPS RRs exist for that origin.
   To do so, the client SHOULD construct a corresponding "https" URL as
   follows:

   1.  Replace the "http" scheme with "https".

   2.  If the "http" URL explicitly specifies port 80, specify port 443.

   3.  Do not alter any other aspect of the URL.

   This construction is equivalent to Section 8.3 of [HSTS], point 5.

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   If an HTTPS RR query for this "https" URL returns any AliasMode HTTPS
   RRs, or any compatible ServiceMode HTTPS RRs (see Section 8), the
   client SHOULD behave as if it has received an HTTP 307 (Temporary
   Redirect) status code with this "https" URL in the "Location" field.
   (Receipt of an incompatible ServiceMode RR does not trigger the
   redirect behavior.)  Because HTTPS RRs are received over an often
   insecure channel (DNS), clients MUST NOT place any more trust in this
   signal than if they had received a 307 redirect over cleartext HTTP.
   If this redirection would result in a loss of functionality (e.g.
   important resources that are only available on the "http" origin),
   the operator MUST NOT publish an HTTPS RR.

   When an "https" connection fails due to an error in the underlying
   secure transport, such as an error in certificate validation, some
   clients currently offer a "user recourse" that allows the user to
   bypass the security error and connect anyway.  When making an "https"
   scheme request to an origin with an HTTPS RR, either directly or via
   the above redirect, such a client MAY remove the user recourse
   option.  Origins that publish HTTPS RRs therefore MUST NOT rely on
   user recourse for access.  For more information, see Section 8.4 and
   Section 12.1 of [HSTS].

9.6.  Use of HTTPS RRs in other protocols

   All protocols employing "http://" or "https://" URLs SHOULD respect
   HTTPS RRs.  For example, clients that support HTTPS RRs and implement
   the altered WebSocket [WebSocket] opening handshake from the W3C
   Fetch specification [FETCH] SHOULD use HTTPS RRs for the requestURL.

   Such protocols MAY define their own SVCB mappings, which MAY be
   defined to take precedence over HTTPS RRs.

10.  SVCB/HTTPS RR parameter for ECH configuration

   The SVCB "ech" parameter is defined for conveying the ECH
   configuration of an alternative endpoint.  In wire format, the value
   of the parameter is an ECHConfigList [ECH], including the redundant
   length prefix.  In presentation format, the value is a single
   ECHConfigList encoded in Base64 [base64].  Base64 is used here to
   simplify integration with TLS server software.  To enable simpler
   parsing, this SvcParam MUST NOT contain escape sequences.

   When ECH is in use, the TLS ClientHello is divided into an
   unencrypted "outer" and an encrypted "inner" ClientHello.  The outer
   ClientHello is an implementation detail of ECH, and its contents are
   controlled by the ECHConfig in accordance with [ECH].  The inner
   ClientHello is used for establishing a connection to the service, so
   its contents may be influenced by other SVCB parameters.  For

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   example, the requirements on the ProtocolNameList in Section 7.1
   apply only to the inner ClientHello.  Similarly, it is the inner
   ClientHello whose Server Name Indication identifies the desired
   service.

10.1.  Client behavior

   The SVCB-optional client behavior specified in Section 3 permits
   clients to fall back to a direct connection if all SVCB options fail.
   This behavior is not suitable for ECH, because fallback would negate
   the privacy benefits of ECH.  Accordingly, ECH-capable SVCB-optional
   clients MUST switch to SVCB-reliant connection establishment if SVCB
   resolution succeeded (following Section 3) and all alternative
   endpoints have an "ech" key.

   As a latency optimization, clients MAY prefetch DNS records that will
   only be used in SVCB-optional mode.

10.2.  Deployment considerations

   An HTTPS RRSet containing some RRs with "ech" and some without is
   vulnerable to a downgrade attack.  This configuration is NOT
   RECOMMENDED.  Zone owners who do use such a mixed configuration
   SHOULD mark the RRs with "ech" as more preferred (i.e. smaller
   SvcPriority) than those without, in order to maximize the likelihood
   that ECH will be used in the absence of an active adversary.

11.  Zone Structures

11.1.  Structuring zones for flexibility

   Each ServiceMode RRSet can only serve a single scheme.  The scheme is
   indicated by the owner name and the RR type.  For the generic SVCB RR
   type, this means that each owner name can only be used for a single
   scheme.  The underscore prefixing requirement (Section 2.3) ensures
   that this is true for the initial query, but it is the responsibility
   of zone owners to choose names that satisfy this constraint when
   using aliases, including CNAME and AliasMode records.

   When using the generic SVCB RR type with aliasing, zone owners SHOULD
   choose alias target names that indicate the scheme in use (e.g.
   foosvc.example.net for foo:// schemes).  This will help to avoid
   confusion when another scheme needs to be added to the configuration.

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11.2.  Structuring zones for performance

   To avoid a delay for clients using a nonconforming recursive
   resolver, domain owners SHOULD minimize the use of AliasMode records,
   and SHOULD choose TargetName according to a predictable convention
   that is known to the client, so that clients can issue A and/or AAAA
   queries for TargetName in advance (see Section 5).  Unless otherwise
   specified, the convention is to set TargetName to the service name
   for an initial ServiceMode record, or to "." if it is reached via an
   alias.  For foo://foo.example.com:8080, this might look like:

   $ORIGIN example.com. ; Origin
   foo                  3600 IN CNAME foosvc.example.net.
   _8080._foo.foo       3600 IN CNAME foosvc.example.net.

   $ORIGIN example.net. ; Service provider zone
   foosvc               3600 IN SVCB 1 . key65333=...
   foosvc                300 IN AAAA 2001:db8::1

   Domain owners SHOULD avoid using a TargetName that is below a DNAME,
   as this is likely unnecessary and makes responses slower and larger.
   Also, zone structures that require following more than 8 aliases
   (counting both AliasMode and CNAME records) are NOT RECOMMENDED.

11.3.  Examples

11.3.1.  Protocol enhancements

   Consider a simple zone of the form:

   $ORIGIN simple.example. ; Simple example zone
   @ 300 IN A    192.0.2.1
            AAAA 2001:db8::1

   The domain owner could add this record:

   @ 7200 IN HTTPS 1 . alpn=h3

   to indicate that https://simple.example supports QUIC in addition to
   TLS over TCP (the implicit default).  The record could also include
   other information (e.g. non-standard port, ECH configuration).  For
   https://simple.example:8443, the record would be:

   _8443._https 7200 IN HTTPS 1 . alpn=h3

   These records also respectively tell clients to replace the scheme
   with "https" when loading http://simple.example or
   http://simple.example:8443.

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11.3.2.  Apex aliasing

   Consider a zone that is using CNAME aliasing:

   $ORIGIN aliased.example. ; A zone that is using a hosting service
   ; Subdomain aliased to a high-performance server pool
   www             7200 IN CNAME pool.svc.example.
   ; Apex domain on fixed IPs because CNAME is not allowed at the apex
   @                300 IN A     192.0.2.1
                        IN AAAA  2001:db8::1

   With HTTPS RRs, the owner of aliased.example could alias the apex by
   adding one additional record:

   @               7200 IN HTTPS 0 pool.svc.example.

   With this record in place, HTTPS-RR-aware clients will use the same
   server pool for aliased.example and www.aliased.example.  (They will
   also upgrade "http://aliased.example/..." to "https".)  Non-HTTPS-RR-
   aware clients will just ignore the new record.

   Similar to CNAME, HTTPS RRs have no impact on the origin name.  When
   connecting, clients will continue to treat the authoritative origins
   as "https://www.aliased.example" and "https://aliased.example",
   respectively, and will validate TLS server certificates accordingly.

11.3.3.  Parameter binding

   Suppose that svc.example's default server pool supports HTTP/2, and
   it has deployed HTTP/3 on a new server pool with a different
   configuration.  This can be expressed in the following form:

   $ORIGIN svc.example. ; A hosting provider.
   pool  7200 IN HTTPS 1 h3pool alpn=h2,h3 ech="123..."
                 HTTPS 2 .      alpn=h2 ech="abc..."
   pool   300 IN A        192.0.2.2
                 AAAA     2001:db8::2
   h3pool 300 IN A        192.0.2.3
                 AAAA     2001:db8::3

   This configuration is entirely compatible with the "Apex aliasing"
   example, whether the client supports HTTPS RRs or not.  If the client
   does support HTTPS RRs, all connections will be upgraded to HTTPS,
   and clients will use HTTP/3 if they can.  Parameters are "bound" to
   each server pool, so each server pool can have its own protocol, ECH
   configuration, etc.

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11.3.4.  Multi-CDN

   The HTTPS RR is intended to support HTTPS services operated by
   multiple independent entities, such as different Content Delivery
   Networks (CDNs) or different hosting providers.  This includes the
   case where a service is migrated from one operator to another, as
   well as the case where the service is multiplexed between multiple
   operators for performance, redundancy, etc.

   This example shows such a configuration, with www.customer.example
   having different DNS responses to different queries, either over time
   or due to logic within the authoritative DNS server:

    ; This zone contains/returns different CNAME records
    ; at different points-in-time.  The RRset for "www" can
    ; only ever contain a single CNAME.

    ; Sometimes the zone has:
    $ORIGIN customer.example.  ; A Multi-CDN customer domain
    www 900 IN CNAME cdn1.svc1.example.

    ; and other times it contains:
    $ORIGIN customer.example.
    www 900 IN CNAME customer.svc2.example.

    ; and yet other times it contains:
    $ORIGIN customer.example.
    www 900 IN CNAME cdn3.svc3.example.

    ; With the following remaining constant and always included:
    $ORIGIN customer.example.  ; A Multi-CDN customer domain
    ; The apex is also aliased to www to match its configuration
    @     7200 IN HTTPS 0 www
    ; Non-HTTPS-aware clients use non-CDN IPs
                  A    203.0.113.82
                  AAAA 2001:db8:203::2

    ; Resolutions following the cdn1.svc1.example
    ; path use these records.
    ; This CDN uses a different alternative service for HTTP/3.
    $ORIGIN svc1.example.  ; domain for CDN 1
    cdn1     1800 IN HTTPS 1 h3pool alpn=h3 ech="123..."
                     HTTPS 2 . alpn=h2 ech="123..."
                     A    192.0.2.2
                     AAAA 2001:db8:192::4
    h3pool 300 IN A 192.0.2.3
               AAAA 2001:db8:192:7::3

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    ; Resolutions following the customer.svc2.example
    ; path use these records.
    ; Note that this CDN only supports HTTP/2.
    $ORIGIN svc2.example. ; domain operated by CDN 2
    customer 300 IN HTTPS 1 . alpn=h2 ech="xyz..."
              60 IN A    198.51.100.2
                    A    198.51.100.3
                    A    198.51.100.4
                    AAAA 2001:db8:198::7
                    AAAA 2001:db8:198::12

    ; Resolutions following the customer.svc2.example
    ; path use these records.
    ; Note that this CDN has no HTTPS records
    ; and thus no ECH support.
    $ORIGIN svc3.example. ; domain operated by CDN 3
    cdn3      60 IN A    203.0.113.8
                    AAAA 2001:db8:113::8

   Note that in the above example, the different CDNs have different ECH
   configurations and different capabilities, but clients will use HTTPS
   RRs as a bound-together unit.

   Domain owners should be cautious when using a multi-CDN
   configuration, as it introduces a number of complexities highlighted
   by this example:

   *  If CDN 1 supports ECH, and CDN 2 does not, the client is
      vulnerable to ECH downgrade by a network adversary who forces
      clients to get CDN 2 records.

   *  Aliasing the apex to its subdomain simplifies the zone file but
      likely increases resolution latency, especially when using a non-
      HTTPS-aware recursive resolver.  An alternative would be to alias
      the zone apex directly to a name managed by a CDN.

   *  The A, AAAA, and HTTPS resolutions are independent lookups, so
      clients may observe and follow different CNAMEs to different CDNs.
      Clients may thus find a TargetName pointing to a name other than
      the one which returned along with the A and AAAA lookups and will
      need to do an additional resolution for them.  Including ipv6hint
      and ipv4hint will reduce the performance impact of this case.

   *  If not all CDNs publish HTTPS records, clients will sometimes
      receive NODATA for HTTPS queries (as with cdn3.svc3.example
      above), and thus no "ech" SvcParam, but could receive A/AAAA
      records from a different CDN which does support ECH.  Clients will
      be unable to use ECH in this case.

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11.3.5.  Non-HTTP uses

   For protocols other than HTTP, the SVCB RR and an Attrleaf label
   [Attrleaf] will be used.  For example, to reach an example resource
   of "baz://api.example.com:8765", the following SVCB record would be
   used to alias it to "svc4-baz.example.net." which in-turn could
   return AAAA/A records and/or SVCB records in ServiceMode:

   _8765._baz.api.example.com. 7200 IN SVCB 0 svc4-baz.example.net.

   HTTPS RRs use similar Attrleaf labels if the origin contains a non-
   default port.

12.  Interaction with other standards

   This standard is intended to reduce connection latency and improve
   user privacy.  Server operators implementing this standard SHOULD
   also implement TLS 1.3 [RFC8446] and OCSP Stapling [RFC6066], both of
   which confer substantial performance and privacy benefits when used
   in combination with SVCB records.

   To realize the greatest privacy benefits, this proposal is intended
   for use over a privacy-preserving DNS transport (like DNS over TLS
   [DoT] or DNS over HTTPS [DoH]).  However, performance improvements,
   and some modest privacy improvements, are possible without the use of
   those standards.

   Any specification for use of SVCB with a protocol MUST have an entry
   for its scheme under the SVCB RR type in the IANA DNS Underscore
   Global Scoped Entry Registry [Attrleaf].  The scheme SHOULD have an
   entry in the IANA URI Schemes Registry [RFC7595].  The scheme SHOULD
   have a defined specification for use with SVCB.

13.  Security Considerations

   SVCB/HTTPS RRs are intended for distribution over untrusted channels,
   and clients are REQUIRED to verify that the alternative endpoint is
   authoritative for the service (similar to Section 2.1 of [AltSvc]).
   Therefore, DNSSEC signing and validation are OPTIONAL for publishing
   and using SVCB and HTTPS RRs.

   Clients MUST ensure that their DNS cache is partitioned for each
   local network, or flushed on network changes, to prevent a local
   adversary in one network from implanting a forged DNS record that
   allows them to track users or hinder their connections after they
   leave that network.

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   An attacker who can prevent SVCB resolution can deny clients any
   associated security benefits.  A hostile recursive resolver can
   always deny service to SVCB queries, but network intermediaries can
   often prevent resolution as well, even when the client and recursive
   resolver validate DNSSEC and use a secure transport.  These downgrade
   attacks can prevent the "https" upgrade provided by the HTTPS RR
   (Section 9.5), and disable the encryption enabled by the "ech"
   SvcParamKey (Section 10).  To prevent downgrades, Section 3.1
   recommends that clients abandon the connection attempt when such an
   attack is detected.

   A hostile DNS intermediary might forge AliasMode "." records
   (Section 2.5.1) as a way to block clients from accessing particular
   services.  Such an adversary could already block entire domains by
   forging erroneous responses, but this mechanism allows them to target
   particular protocols or ports within a domain.  Clients that might be
   subject to such attacks SHOULD ignore AliasMode "." records.

   A hostile DNS intermediary or origin can return SVCB records
   indicating any IP address and port number, including IP addresses
   inside the local network and port numbers assigned to internal
   services.  If the attacker can influence the client's payload (e.g.
   TLS session ticket contents), and an internal service has a
   sufficiently lax parser, it's possible that the attacker could gain
   unintended access.  (The same concerns apply to SRV records, HTTP
   Alt-Svc, and HTTP redirects.)  As a mitigation, SVCB mapping
   documents SHOULD indicate any port number restrictions that are
   appropriate for the supported transports.

14.  Privacy Considerations

   Standard address queries reveal the user's intent to access a
   particular domain.  This information is visible to the recursive
   resolver, and to many other parties when plaintext DNS transport is
   used.  SVCB queries, like queries for SRV records and other specific
   RR types, additionally reveal the user's intent to use a particular
   protocol.  This is not normally sensitive information, but it should
   be considered when adding SVCB support in a new context.

15.  IANA Considerations

15.1.  SVCB RRType

   This document defines a new DNS RR type, SVCB, whose value 64 has
   been allocated by IANA from the "Resource Record (RR) TYPEs"
   subregistry of the "Domain Name System (DNS) Parameters" registry:

   *  Type: SVCB

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   *  Value: 64

   *  Meaning: General Purpose Service Binding

   *  Reference: This document

15.2.  HTTPS RRType

   This document defines a new DNS RR type, "HTTPS", whose value 65 has
   been allocated by IANA from the "Resource Record (RR) TYPEs"
   subregistry of the "Domain Name System (DNS) Parameters" registry:

   *  Type: HTTPS

   *  Value: 65

   *  Meaning: Service Binding type for use with HTTP

   *  Reference: This document

15.3.  New registry for Service Parameters

   IANA is requested to create a new registry, entitled "Service
   Parameter Keys (SvcParamKeys)".  This registry defines the namespace
   for parameters, including string representations and numeric
   SvcParamKey values.  This registry is shared with other SVCB-
   compatible RR types, such as the HTTPS RR.

   ACTION: create this registry, on a new page entitled "DNS Service
   Bindings (SVCB)" under the "Domain Name System (DNS) Parameters"
   category.

15.3.1.  Procedure

   A registration MUST include the following fields:

   *  Number: wire format numeric identifier (range 0-65535)

   *  Name: unique presentation name

   *  Meaning: a short description

   *  Format Reference: pointer to specification text

   The characters in the registered Name MUST be lower-case alphanumeric
   or "-" (Section 2.1).  The name MUST NOT start with "key" or
   "invalid".

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   Entries in this registry are subject to a First Come First Served
   registration policy ([RFC8126], Section 4.4).  The Format Reference
   MUST specify how to convert the SvcParamValue's presentation format
   to wire format and MAY detail its intended meaning and use.  An entry
   MAY specify a Format Reference of the form "Same as (other key Name)"
   if it uses the same presentation and wire formats as an existing key.

   This arrangement supports the development of new parameters while
   ensuring that zone files can be made interoperable.

15.3.2.  Initial contents

   The "Service Binding (SVCB) Parameter Registry" shall initially be
   populated with the registrations below:

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   +=============+=================+================+=================+
   | Number      | Name            | Meaning        | Format          |
   |             |                 |                | Reference       |
   +=============+=================+================+=================+
   | 0           | mandatory       | Mandatory keys | (This document) |
   |             |                 | in this RR     | Section 8       |
   +-------------+-----------------+----------------+-----------------+
   | 1           | alpn            | Additional     | (This document) |
   |             |                 | supported      | Section 7.1     |
   |             |                 | protocols      |                 |
   +-------------+-----------------+----------------+-----------------+
   | 2           | no-default-alpn | No support for | (This document) |
   |             |                 | default        | Section 7.1     |
   |             |                 | protocol       |                 |
   +-------------+-----------------+----------------+-----------------+
   | 3           | port            | Port for       | (This document) |
   |             |                 | alternative    | Section 7.2     |
   |             |                 | endpoint       |                 |
   +-------------+-----------------+----------------+-----------------+
   | 4           | ipv4hint        | IPv4 address   | (This document) |
   |             |                 | hints          | Section 7.4     |
   +-------------+-----------------+----------------+-----------------+
   | 5           | ech             | Encrypted      | (This document) |
   |             |                 | ClientHello    | Section 7.3     |
   |             |                 | info           |                 |
   +-------------+-----------------+----------------+-----------------+
   | 6           | ipv6hint        | IPv6 address   | (This document) |
   |             |                 | hints          | Section 7.4     |
   +-------------+-----------------+----------------+-----------------+
   | 65280-65534 | N/A             | Private Use    | (This document) |
   +-------------+-----------------+----------------+-----------------+
   | 65535       | N/A             | Reserved       | (This document) |
   |             |                 | ("Invalid      |                 |
   |             |                 | key")          |                 |
   +-------------+-----------------+----------------+-----------------+

                                 Table 1

15.4.  Other registry updates

   Per [Attrleaf], please add the following entry to the DNS Underscore
   Global Scoped Entry Registry:

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       +=========+============+=================+=================+
       | RR TYPE | _NODE NAME | Meaning         | Reference       |
       +=========+============+=================+=================+
       | HTTPS   | _https     | HTTPS SVCB info | (This document) |
       +---------+------------+-----------------+-----------------+

                                 Table 2

16.  Acknowledgments and Related Proposals

   There have been a wide range of proposed solutions over the years to
   the "CNAME at the Zone Apex" challenge proposed.  These include
   [I-D.bellis-dnsop-http-record], [I-D.ietf-dnsop-aname], and others.

   Thank you to Ian Swett, Ralf Weber, Jon Reed, Martin Thomson, Lucas
   Pardue, Ilari Liusvaara, Tim Wicinski, Tommy Pauly, Chris Wood, David
   Benjamin, Mark Andrews, Emily Stark, Eric Orth, Kyle Rose, Craig
   Taylor, Dan McArdle, Brian Dickson, Willem Toorop, Pieter Lexis,
   Puneet Sood, Olivier Poitrey, Mashooq Muhaimen, Tom Carpay, and many
   others for their feedback and suggestions on this draft.

17.  References

17.1.  Normative References

   [ALPN]     Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.

   [Attrleaf] Crocker, D., "Scoped Interpretation of DNS Resource
              Records through "Underscored" Naming of Attribute Leaves",
              BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019,
              <https://www.rfc-editor.org/rfc/rfc8552>.

   [base64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [DNAME]    Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
              <https://www.rfc-editor.org/rfc/rfc6672>.

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

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   [DoT]      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/rfc/rfc7858>.

   [ECH]      Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-13, 12 August 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              esni-13>.

   [HappyEyeballsV2]
              Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8305>.

   [HSTS]     Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,
              <https://www.rfc-editor.org/rfc/rfc6797>.

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

   [I-D.draft-ietf-httpbis-semantics]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
              semantics-19>.

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

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 1928,
              DOI 10.17487/RFC1928, March 1996,
              <https://www.rfc-editor.org/rfc/rfc1928>.

   [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/rfc/rfc2119>.

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   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/rfc/rfc2181>.

   [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC",
              RFC 3225, DOI 10.17487/RFC3225, December 2001,
              <https://www.rfc-editor.org/rfc/rfc3225>.

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

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5234>.

   [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/rfc/rfc5952>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/rfc/rfc6066>.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,
              <https://www.rfc-editor.org/rfc/rfc6147>.

   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,
              <https://www.rfc-editor.org/rfc/rfc7050>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7231>.

   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,
              <https://www.rfc-editor.org/rfc/rfc7595>.

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   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,
              <https://www.rfc-editor.org/rfc/rfc7871>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/rfc/rfc8126>.

   [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/rfc/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [WebSocket]
              Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,
              <https://www.rfc-editor.org/rfc/rfc6455>.

17.2.  Informative References

   [AltSvc]   Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/rfc/rfc7838>.

   [DNSTerm]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/rfc/rfc8499>.

   [FETCH]    "Fetch Living Standard", May 2020,
              <https://fetch.spec.whatwg.org/>.

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              http-34>.

   [I-D.bellis-dnsop-http-record]
              Bellis, R., "A DNS Resource Record for HTTP", Work in
              Progress, Internet-Draft, draft-bellis-dnsop-http-record-
              00, 3 November 2018,
              <https://datatracker.ietf.org/doc/html/draft-bellis-dnsop-
              http-record-00>.

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   [I-D.ietf-dnsop-aname]
              Finch, T., Hunt, E., Dijk, P. V., Eden, A., and M.
              Mekking, "Address-specific DNS aliases (ANAME)", Work in
              Progress, Internet-Draft, draft-ietf-dnsop-aname-04, 8
              July 2019, <https://datatracker.ietf.org/doc/html/draft-
              ietf-dnsop-aname-04>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/rfc/rfc6454>.

   [SRV]      Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/rfc/rfc2782>.

   [URI]      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/rfc/rfc3986>.

Appendix A.  Decoding text in zone files

   DNS zone files are capable of representing arbitrary octet sequences
   in basic ASCII text, using various delimiters and encodings.  The
   algorithm for decoding these character-strings is defined in
   Section 5.1 of [RFC1035].  Here we summarize the allowed input to
   that algorithm, using ABNF:

   ; non-special is VCHAR minus DQUOTE, ";", "(", ")", and "\".
   non-special = %x21 / %x23-27 / %x2A-3A / %x3C-5B / %x5D-7E
   ; non-digit is VCHAR minus DIGIT
   non-digit   = %x21-2F / %x3A-7E
   ; dec-octet is a number 0-255 as a three-digit decimal number.
   dec-octet   = ( "0" / "1" ) 2DIGIT /
                 "2" ( ( %x30-34 DIGIT ) / ( "5" %x30-35 ) )
   escaped     = "\" ( non-digit / dec-octet )
   contiguous  = 1*( non-special / escaped )
   quoted      = DQUOTE *( contiguous / ( ["\"] WSP ) ) DQUOTE
   char-string = contiguous / quoted

   The decoding algorithm allows char-string to represent any *OCTET.
   In this document, this algorithm is referred to as "character-string
   decoding".  The algorithm is the same as used by <character-string>
   in RFC 1035, although the output length in this document is not
   limited to 255 octets.

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A.1.  Decoding a comma-separated list

   In order to represent lists of items in zone files, this
   specification uses comma-separated lists.  When the allowed items in
   the list cannot contain "," or "\", this is trivial.  (For
   simplicity, empty items are not allowed.)  A value-list parser that
   splits on "," and prohibits items containing "\" is sufficient to
   comply with all requirements in this document.

   For implementations that allow "," and "\" in item values, the
   following escaping syntax applies:

   item            = 1*OCTET
   ; item-allowed is OCTET minus "," and "\".
   item-allowed    = %x00-2B / %x2D-5B / %x5D-FF
   escaped-item    = 1*(item-allowed / "\," / "\\")
   comma-separated = [escaped-item *("," escaped-item)]

   Decoding of value-lists happens after character-string decoding.  For
   example, consider these char-string SvcParamValues:

   "part1,part2,part3\\,part4\\\\"
   part1\,\p\a\r\t2\044part3\092,part4\092\\

   These inputs are equivalent: character-string decoding either of them
   would produce the same value:

   part1,part2,part3\,part4\\

   Applying comma-separated list decoding to this value would produce a
   list of three items:

   part1
   part2
   part3,part4\

Appendix B.  HTTP Mapping Summary

   This table serves as a non-normative summary of the HTTP mapping for
   SVCB (Section 9).  Future protocol mappings may provide a similar
   summary table.

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            +==========================+======================+
            +==========================+======================+
            | *Mapped scheme*          | "https"              |
            +--------------------------+----------------------+
            | *Other affected schemes* | "http", "wss", "ws", |
            |                          | (other HTTP-based)   |
            +--------------------------+----------------------+
            | *RR type*                | HTTPS (65)           |
            +--------------------------+----------------------+
            | *Name prefix*            | None for port 443,   |
            |                          | else _$PORT._https   |
            +--------------------------+----------------------+
            | *Automatically Mandatory | port, no-default-    |
            | Keys*                    | alpn                 |
            +--------------------------+----------------------+
            | *SvcParam defaults*      | alpn: ["http/1.1"]   |
            +--------------------------+----------------------+
            | *Special behaviors*      | HTTP to HTTPS        |
            |                          | upgrade              |
            +--------------------------+----------------------+
            | *Keys that records must  | None                 |
            | include*                 |                      |
            +--------------------------+----------------------+

                                  Table 3

Appendix C.  Comparison with alternatives

   The SVCB and HTTPS RR types closely resemble, and are inspired by,
   some existing record types and proposals.  A complaint with all of
   the alternatives is that web clients have seemed unenthusiastic about
   implementing them.  The hope here is that by providing an extensible
   solution that solves multiple problems we will overcome the inertia
   and have a path to achieve client implementation.

C.1.  Differences from the SRV RR type

   An SRV record [SRV] can perform a similar function to the SVCB
   record, informing a client to look in a different location for a
   service.  However, there are several differences:

   *  SRV records are typically mandatory, whereas SVCB is intended to
      be optional when used with pre-existing protocols.

   *  SRV records cannot instruct the client to switch or upgrade
      protocols, whereas SVCB can signal such an upgrade (e.g. to
      HTTP/2).

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   *  SRV records are not extensible, whereas SVCB and HTTPS RRs can be
      extended with new parameters.

   *  SRV records specify a "weight" for unbalanced randomized load-
      balancing.  SVCB only supports balanced randomized load-balancing,
      although weights could be added via a future SvcParam.

C.2.  Differences from the proposed HTTP record

   Unlike [I-D.bellis-dnsop-http-record], this approach is extensible to
   cover Alt-Svc and Encrypted ClientHello use-cases.  Like that
   proposal, this addresses the zone apex CNAME challenge.

   Like that proposal, it remains necessary to continue to include
   address records at the zone apex for legacy clients.

C.3.  Differences from the proposed ANAME record

   Unlike [I-D.ietf-dnsop-aname], this approach is extensible to cover
   Alt-Svc and ECH use-cases.  This approach also does not require any
   changes or special handling on either authoritative or primary
   servers, beyond optionally returning in-bailiwick additional records.

   Like that proposal, this addresses the zone apex CNAME challenge for
   clients that implement this.

   However, with this SVCB proposal, it remains necessary to continue to
   include address records at the zone apex for legacy clients.  If
   deployment of this standard is successful, the number of legacy
   clients will fall over time.  As the number of legacy clients
   declines, the operational effort required to serve these users
   without the benefit of SVCB indirection should fall.  Server
   operators can easily observe how much traffic reaches this legacy
   endpoint, and may remove the apex's address records if the observed
   legacy traffic has fallen to negligible levels.

C.4.  Comparison with separate RR types for AliasMode and ServiceMode

   Abstractly, functions of AliasMode and ServiceMode are independent,
   so it might be tempting to specify them as separate RR types.
   However, this would result in a serious performance impairment,
   because clients cannot rely on their recursive resolver to follow
   SVCB aliases (unlike CNAME).  Thus, clients would have to issue
   queries for both RR types in parallel, potentially at each step of
   the alias chain.  Recursive resolvers that implement the
   specification would, upon receipt of a ServiceMode query, emit both a
   ServiceMode and an AliasMode query to the authoritative.  Thus,
   splitting the RR type would double, or in some cases triple, the load

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   on clients and servers, and would not reduce implementation
   complexity.

Appendix D.  Test vectors

   These test vectors only contain the RDATA portion of SVCB/HTTPS
   records in presentation format, generic format ([RFC3597]) and wire
   format.  The wire format uses hexadecimal (\xNN) for each non-ascii
   byte.  As the wireformat is long, it is broken into several lines.

D.1.  AliasMode

   example.com.   HTTPS   0 foo.example.com.

   \# 19 (
   00 00                                              ; priority
   03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
   )

   \x00\x00                                           # priority
   \x03foo\x07example\x03com\x00                      # target

                            Figure 1: AliasMode

D.2.  ServiceMode

   example.com.   SVCB   1 .

   \# 3 (
   00 01      ; priority
   00         ; target (root label)
   )

   \x00\x01   # priority
   \x00       # target, root label

                        Figure 2: TargetName is "."

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   example.com.   SVCB   16 foo.example.com. port=53

   \# 25 (
   00 10                                              ; priority
   03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
   00 03                                              ; key 3
   00 02                                              ; length 2
   00 35                                              ; value
   )

   \x00\x10                                           # priority
   \x03foo\x07example\x03com\x00                      # target
   \x00\x03                                           # key 3
   \x00\x02                                           # length: 2 bytes
   \x00\x35                                           # value

                         Figure 3: Specifies a port

   example.com.   SVCB   1 foo.example.com. key667=hello

   \# 28 (
   00 01                                              ; priority
   03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
   02 9b                                              ; key 667
   00 05                                              ; length 5
   68 65 6c 6c 6f                                     ; value
   )

   \x00\x01                                           # priority
   \x03foo\x07example\x03com\x00                      # target
   \x02\x9b                                           # key 667
   \x00\x05                                           # length 5
   hello                                              # value

                 Figure 4: A generic key and unquoted value

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   example.com.   SVCB   1 foo.example.com. key667="hello\210qoo"

   \# 32 (
   00 01                                              ; priority
   03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
   02 9b                                              ; key 667
   00 09                                              ; length 9
   68 65 6c 6c 6f d2 71 6f 6f                         ; value
   )

   \x00\x01                                           # priority
   \x03foo\x07example\x03com\x00                      # target
   \x02\x9b                                           # key 667
   \x00\x09                                           # length 9
   hello\xd2qoo                                       # value

       Figure 5: A generic key and quoted value with a decimal escape

   example.com.   SVCB   1 foo.example.com. (
                         ipv6hint="2001:db8::1,2001:db8::53:1"
                         )

   \# 55 (
   00 01                                              ; priority
   03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00 ; target
   00 06                                              ; key 6
   00 20                                              ; length 32
   20 01 0d b8 00 00 00 00 00 00 00 00 00 00 00 01    ; first address
   20 01 0d b8 00 00 00 00 00 00 00 00 00 53 00 01    ; second address
   )

   \x00\x01                                           # priority
   \x03foo\x07example\x03com\x00                      # target
   \x00\x06                                           # key 6
   \x00\x20                                           # length 32
   \x20\x01\x0d\xb8\x00\x00\x00\x00
        \x00\x00\x00\x00\x00\x00\x00\x01              # first address
   \x20\x01\x0d\xb8\x00\x00\x00\x00
        \x00\x00\x00\x00\x00\x53\x00\x01              # second address

                      Figure 6: Two quoted IPv6 hints

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   example.com.   SVCB   1 example.com. ipv6hint="::ffff:198.51.100.100"

   \# 35 (
   00 01                                              ; priority
   07 65 78 61 6d 70 6c 65 03 63 6f 6d 00             ; target
   00 06                                              ; key 6
   00 10                                              ; length 16
   00 00 00 00 00 00 00 00 00 00 ff ff c6 33 64 64    ; address
   )

   \x00\x01                                           # priority
   \x07example\x03com\x00                             # target
   \x00\x06                                           # key 6
   \x00\x10                                           # length 16
   \x00\x00\x00\x00\x00\x00\x00\x00
        \x00\x00\xff\xff\xc6\x33\x64\x64              # address

                Figure 7: An IPv6 hint in IPv4-mapped format

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   example.com.   SVCB   16 foo.example.org. (
                         alpn=h2,h3-19 mandatory=ipv4hint,alpn
                         ipv4hint=192.0.2.1
                         )

   \# 48 (
   00 10                                              ; priority
   03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 ; target
   00 00                                              ; key 0
   00 04                                              ; param length 4
   00 01                                              ; value: key 1
   00 04                                              ; value: key 4
   00 01                                              ; key 1
   00 09                                              ; param length 9
   02                                                 ; alpn length 2
   68 32                                              ; alpn value
   05                                                 ; alpn length 5
   68 33 2d 31 39                                     ; alpn value
   00 04                                              ; key 4
   00 04                                              ; param length 4
   c0 00 02 01                                        ; param value
   )

   \x00\x10                                           # priority
   \x03foo\x07example\x03org\x00                      # target
   \x00\x00                                           # key 0
   \x00\x04                                           # param length 4
   \x00\x01                                           # value: key 1
   \x00\x04                                           # value: key 4
   \x00\x01                                           # key 1
   \x00\x09                                           # param length 9
   \x02                                               # alpn length 2
   h2                                                 # alpn value
   \x05                                               # alpn length 5
   h3-19                                              # alpn value
   \x00\x04                                           # key 4
   \x00\x04                                           # param length 4
   \xc0\x00\x02\x01                                   # param value

        Figure 8: SvcParamKey ordering is arbitrary in presentation
                      format but sorted in wire format

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   example.com.   SVCB   16 foo.example.org. alpn="f\\\\oo\\,bar,h2"
   example.com.   SVCB   16 foo.example.org. alpn=f\\\092oo\092,bar,h2

   \# 35 (
   00 10                                              ; priority
   03 66 6f 6f 07 65 78 61 6d 70 6c 65 03 6f 72 67 00 ; target
   00 01                                              ; key 1
   00 0c                                              ; param length 12
   08                                                 ; alpn length 8
   66 5c 6f 6f 2c 62 61 72                            ; alpn value
   02                                                 ; alpn length 2
   68 32                                              ; alpn value
   )

   \x00\x10                                           # priority
   \x03foo\x07example\x03org\x00                      # target
   \x00\x01                                           # key 1
   \x00\x0c                                           # param length 12
   \x08                                               # alpn length 8
   f\oo,bar                                           # alpn value
   \x02                                               # alpn length 2
   h2                                                 # alpn value

        Figure 9: An alpn value with an escaped comma and an escaped
                   backslash in two presentation formats

D.3.  Failure cases

   This subsection contains test vectors which are not compliant with
   this document.  The various reasons for non-compliance are explained
   with each example.

   example.com.   SVCB   1 foo.example.com. (
                          key123=abc key123=def
                          )

           Figure 10: Multiple instances of the same SvcParamKey

   example.com.   SVCB   1 foo.example.com. mandatory
   example.com.   SVCB   1 foo.example.com. alpn
   example.com.   SVCB   1 foo.example.com. port
   example.com.   SVCB   1 foo.example.com. ipv4hint
   example.com.   SVCB   1 foo.example.com. ipv6hint

          Figure 11: Missing SvcParamValues that must be nonempty

   example.com.   SVCB   1 foo.example.com. no-default-alpn=abc

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      Figure 12: The "no-default-alpn" SvcParamKey value must be empty

   example.com.   SVCB   1 foo.example.com. mandatory=key123

                 Figure 13: A mandatory SvcParam is missing

   example.com.   SVCB   1 foo.example.com. mandatory=mandatory

       Figure 14: The "mandatory" SvcParamKey must not be included in
                             the mandatory list

   example.com.   SVCB   1 foo.example.com. (
                         mandatory=key123,key123 key123=abc
                         )

        Figure 15: Multiple instances of the same SvcParamKey in the
                               mandatory list

Appendix E.  Change history

   (This section to be removed by the RFC editor.)

   *  draft-ietf-dnsop-svcb-https-08

      -  Extensive structural and editorial adjustments based on area
         reviews, including:

         o  A new section on SVCB-compatible record types

         o  Reorganized description of client behavior

         o  Test vectors are now in titled figures

         o  Adjusted mapping summary

         o  Improve description of rules for resolver handling of
            invalid SvcParamValues.

      -  New text on cross-transport fallback (e.g.  QUIC vs. TCP)

      -  Improved explanation of use with domain-oriented transport
         proxies

      -  HTTP terminology adjusted to match draft-ietf-httpbis-semantics

      -  Improved and corrected IANA instructions

   *  draft-ietf-dnsop-svcb-https-07

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      -  Editorial improvements following AD review.

   *  draft-ietf-dnsop-svcb-https-06

      -  Add requirements for HTTPS origins that also use Alt-Svc

      -  Remove requirement for comma-escaping related to unusual ALPN
         values

      -  Allow resolvers to reject invalid SvcParamValues, with
         additional guidance

   *  draft-ietf-dnsop-svcb-https-05

      -  Specify interaction with EDNS Client Subnet and Additional
         section caching

      -  Rename "echconfig" to "ech"

      -  Add a suite of test vectors (both valid and invalid) and more
         examples

      -  Clarify requirements for resolvers' (non-)use of SvcParams

      -  Clarify guidance regarding default ALPN values

   *  draft-ietf-dnsop-svcb-https-04

      -  Simplify the IANA instructions (pure First Come First Served)

      -  Recommend against publishing chains of >8 aliases

      -  Clarify requirements for using SVCB with a transport proxy

      -  Adjust guidance for Port Prefix Naming

      -  Minor editorial updates

   *  draft-ietf-dnsop-svcb-https-03

      -  Simplified escaping of comma-separated values

      -  Reorganized client requirements

      -  Added a warning about port filtering for cross-protocol attacks

      -  Clarified self-consistency rules for SvcParams

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      -  Added a non-normative mapping summary table for HTTPS

   *  draft-ietf-dnsop-svcb-https-02

      -  Added a Privacy Considerations section

      -  Adjusted resolution fallback description

      -  Clarified status of SvcParams in AliasMode

      -  Improved advice on zone structuring and use with Alt-Svc

      -  Improved examples, including a new Multi-CDN example

      -  Reorganized text on value-list parsing and SvcPriority

      -  Improved phrasing and other editorial improvements throughout

   *  draft-ietf-dnsop-svcb-https-01

      -  Added a "mandatory" SvcParamKey

      -  Added the ability to indicate that a service does not exist

      -  Adjusted resolution and ALPN algorithms

      -  Major terminology revisions for "origin" and CamelCase names

      -  Revised ABNF

      -  Include allocated RR type numbers

      -  Various corrections, explanations, and recommendations

   *  draft-ietf-dnsop-svcb-https-00

      -  Rename HTTPSSVC RR to HTTPS RR

      -  Rename "an SVCB" to "a SVCB"

      -  Removed "design considerations and open issues" section and
         some other "to be removed" text

   *  draft-ietf-dnsop-svcb-httpssvc-03

      -  Revised chain length limit requirements

      -  Revised IANA registry rules for SvcParamKeys

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      -  Require HTTPS clients to implement SNI

      -  Update terminology for Encrypted ClientHello

      -  Clarifications: non-default ports, transport proxies, HSTS
         procedure, WebSocket behavior, wire format, IP hints, inner/
         outer ClientHello with ECH

      -  Various textual and ABNF corrections

   *  draft-ietf-dnsop-svcb-httpssvc-02

      -  All changes to Alt-Svc have been removed

      -  Expanded and reorganized examples

      -  Priority zero is now the definition of AliasForm

      -  Repeated SvcParamKeys are no longer allowed

      -  The "=" sign may be omitted in a key=value pair if the value is
         also empty

      -  In the wire format, SvcParamKeys must be in sorted order

      -  New text regarding how to handle resolution timeouts

      -  Expanded description of recursive resolver behavior

      -  Much more precise description of the intended ALPN behavior

      -  Match the HSTS specification's language on HTTPS enforcement

      -  Removed 'esniconfig=""' mechanism and simplified ESNI
         connection logic

   *  draft-ietf-dnsop-svcb-httpssvc-01

      -  Reduce the emphasis on conversion between HTTPSSVC and Alt-Svc

      -  Make the "untrusted channel" concept more precise.

      -  Make SvcFieldPriority = 0 the definition of AliasForm, instead
         of a requirement.

   *  draft-ietf-dnsop-svcb-httpssvc-00

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      -  Document an optimization for optimistic pre-connection.  (Chris
         Wood)

      -  Relax IP hint handling requirements.  (Eric Rescorla)

   *  draft-nygren-dnsop-svcb-httpssvc-00

      -  Generalize to an SVCB record, with special-case handling for
         Alt-Svc and HTTPS separated out to dedicated sections.

      -  Split out a separate HTTPSSVC record for the HTTPS use-case.

      -  Remove the explicit SvcRecordType=0/1 and instead make the
         AliasForm vs ServiceForm be implicit.  This was based on
         feedback recommending against subtyping RR type.

      -  Remove one optimization.

   *  draft-nygren-httpbis-httpssvc-03

      -  Change redirect type for HSTS-style behavior from 302 to 307 to
         reduce ambiguities.

   *  draft-nygren-httpbis-httpssvc-02

      -  Remove the redundant length fields from the wire format.

      -  Define a SvcDomainName of "." for SvcRecordType=1 as being the
         HTTPSSVC RRNAME.

      -  Replace "hq" with "h3".

   *  draft-nygren-httpbis-httpssvc-01

      -  Fixes of record name.  Replace references to "HTTPSVC" with
         "HTTPSSVC".

   *  draft-nygren-httpbis-httpssvc-00

      -  Initial version

Authors' Addresses

   Ben Schwartz
   Google

   Email: bemasc@google.com

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   Mike Bishop
   Akamai Technologies

   Email: mbishop@evequefou.be

   Erik Nygren
   Akamai Technologies

   Email: erik+ietf@nygren.org

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