Internet Engineering Task Force (IETF)                       B. Schwartz
Request for Comments: 9460                          Meta Platforms, Inc.
Category: Standards Track                                      M. Bishop
ISSN: 2070-1721                                                E. Nygren
                                                    Akamai Technologies
                                                          November 2023


Service Binding and Parameter Specification via the DNS (SVCB and HTTPS
                          Resource Records)

Abstract

  This document specifies the "SVCB" ("Service Binding") 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 are extensible to support
  future uses (such as 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 (see
  RFC 9110, "HTTP Semantics").  By providing more information to the
  client before it attempts to establish a connection, these records
  offer potential benefits to both performance and privacy.

Status of This Memo

  This is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc9460.

Copyright Notice

  Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  in the Revised BSD License.

Table of Contents

  1.  Introduction
    1.1.  Goals
    1.2.  Overview of the SVCB RR
    1.3.  Terminology
  2.  The SVCB Record Type
    2.1.  Zone-File Presentation Format
    2.2.  RDATA Wire Format
    2.3.  SVCB Query Names
    2.4.  Interpretation
      2.4.1.  SvcPriority
      2.4.2.  AliasMode
      2.4.3.  ServiceMode
    2.5.  Special Handling of "." in TargetName
      2.5.1.  AliasMode
      2.5.2.  ServiceMode
  3.  Client Behavior
    3.1.  Handling Resolution Failures
    3.2.  Clients Using a Proxy
  4.  DNS Server Behavior
    4.1.  Authoritative Servers
    4.2.  Recursive Resolvers
      4.2.1.  DNS64
    4.3.  General Requirements
    4.4.  EDNS Client Subnet (ECS)
  5.  Performance Optimizations
    5.1.  Optimistic Pre-connection and Connection Reuse
    5.2.  Generating and Using Incomplete Responses
  6.  SVCB-Compatible RR Types
  7.  Initial SvcParamKeys
    7.1.  "alpn" and "no-default-alpn"
      7.1.1.  Representation
      7.1.2.  Use
    7.2.  "port"
    7.3.  "ipv4hint" and "ipv6hint"
    7.4.  "mandatory"
  8.  ServiceMode RR Compatibility and Mandatory Keys
  9.  Using Service Bindings with HTTP
    9.1.  Query Names for HTTPS RRs
    9.2.  Comparison with Alt-Svc
      9.2.1.  ALPN Usage
      9.2.2.  Untrusted Channels
      9.2.3.  Cache Lifetime
      9.2.4.  Granularity
    9.3.  Interaction with Alt-Svc
    9.4.  Requiring Server Name Indication
    9.5.  HTTP Strict Transport Security (HSTS)
    9.6.  Use of HTTPS RRs in Other Protocols
  10. Zone Structures
    10.1.  Structuring Zones for Flexibility
    10.2.  Structuring Zones for Performance
    10.3.  Operational Considerations
    10.4.  Examples
      10.4.1.  Protocol Enhancements
      10.4.2.  Apex Aliasing
      10.4.3.  Parameter Binding
      10.4.4.  Multi-CDN Configuration
      10.4.5.  Non-HTTP Uses
  11. Interaction with Other Standards
  12. Security Considerations
  13. Privacy Considerations
  14. IANA Considerations
    14.1.  SVCB RR Type
    14.2.  HTTPS RR Type
    14.3.  New Registry for Service Parameters
      14.3.1.  Procedure
      14.3.2.  Initial Contents
    14.4.  Other Registry Updates
  15. References
    15.1.  Normative References
    15.2.  Informative References
  Appendix A.  Decoding Text in Zone Files
    A.1.  Decoding a Comma-Separated List
  Appendix B.  HTTP Mapping Summary
  Appendix C.  Comparison with Alternatives
    C.1.  Differences from the SRV RR Type
    C.2.  Differences from the Proposed HTTP Record
    C.3.  Differences from the Proposed ANAME Record
    C.4.  Comparison with Separate RR Types for AliasMode and
          ServiceMode
  Appendix D.  Test Vectors
    D.1.  AliasMode
    D.2.  ServiceMode
    D.3.  Failure Cases
  Acknowledgments and Related Proposals
  Authors' Addresses

1.  Introduction

  The SVCB ("Service Binding") and HTTPS resource records (RRs) provide
  clients with complete instructions for access to a service.  This
  information enables improved performance and privacy by avoiding
  transient connections to a suboptimal 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., aliasing 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 RR
  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 and 2) "ServiceMode", which binds
  together configuration information for a service endpoint.
  ServiceMode provides additional key=value parameters within each
  RDATA set.

1.1.  Goals

  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 Content Delivery Networks (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
     extending operational authority for a service identified by a
     domain name to other instances with alternate names.

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

  *  Connecting directly to HTTP/3 (QUIC transport) alternative
     endpoints [HTTP/3].

  *  Supporting non-default TCP and UDP ports.

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

  *  Providing an indication signaling that the "https" scheme should
     be used instead of "http" for all HTTP requests to this host and
     port, similar to HTTP Strict Transport Security [HSTS] (see
     Section 9.5).

  *  Enabling the conveyance of Encrypted ClientHello keys [ECH]
     associated with an alternative endpoint.

1.2.  Overview of the SVCB RR

  This subsection briefly describes the SVCB RR with forward references
  to the full exposition of each component.  (As discussed in
  Section 6, this all applies equally to the HTTPS RR, which shares the
  same encoding, format, and high-level semantics.)

  The SVCB RR has two modes: 1) AliasMode (Section 2.4.2), which
  aliases a name to another name and 2) ServiceMode (Section 2.4.3),
  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 (Section 2.3).

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

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

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

  SvcParams (optional):  A list of key=value pairs describing the
     alternative endpoint at TargetName (only used in ServiceMode and
     otherwise ignored).  SvcParams are 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 (Section 4.2).  Clients either use
  responses included in the Additional section returned by the
  recursive resolver or perform necessary SVCB, A, and AAAA record
  resolutions (Section 3).  DNS authoritative servers can attach in-
  bailiwick SVCB, A, AAAA, and CNAME records in the Additional section
  to responses for a SVCB query (Section 4.1).

  In ServiceMode, the SvcParams of the SVCB RR provide an extensible
  data model for describing alternative endpoints that are
  authoritative for a service, along with parameters associated with
  each of these alternative endpoints (Section 7).

  For HTTP use cases, the HTTPS RR (Section 9) enables many of the
  benefits of Alt-Svc [AltSvc] without waiting for a full HTTP
  connection initiation (multiple round trips) before learning of the
  preferred alternative, and without necessarily revealing the user's
  intended destination to all entities along the network path.

1.3.  Terminology

  Terminology in this document 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 a 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 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 RR 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.

  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.  Each SvcParam has a specified
  presentation format (used in zone files) and wire encoding (e.g.,
  domain names, binary data, or numeric values).  The initial
  SvcParamKeys and their formats are defined in Section 7.

2.1.  Zone-File Presentation Format

  The presentation format <RDATA> of the record ([RFC1035],
  Section 5.1) has the form:

  SvcPriority TargetName SvcParams

  The SVCB record is defined specifically within the Internet ("IN")
  Class ([RFC1035], Section 3.2.4).

  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 registered by IANA (Section 14.3).

  Each SvcParamKey SHALL appear at most once in the SvcParams.  In
  presentation format, SvcParamKeys are lowercase alphanumeric strings.
  Key names 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 ; See Appendix A.
  value         = *OCTET ; Value before key-specific parsing

  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 optional "=" and SvcParamValue are 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.

  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 per 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 14.3.2 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 a SvcParamKey does not have the expected
     format.

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

  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 11).

  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.
  This document terms this behavior "Port Prefix Naming" and uses it in
  the examples throughout.

  See Section 9.1 for information regarding 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, as in ordinary CNAME
  processing ([RFC1034], Section 3.6.2).  For details, see the
  recommendations regarding aliases for clients (Section 3), servers
  (Section 4), and zones (Section 10).

  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.

  This record would 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" )

  This record would 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, per Section 5.1 of [RFC1035].)

2.4.  Interpretation

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.  A smaller SvcPriority
  indicates that the domain owner recommends the use of this record
  over ServiceMode 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 RR in AliasMode.  If multiple
  AliasMode RRs 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 (see, for example, [RFC1912],
  Section 2.4).  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.)  Unlike CNAME, AliasMode
  records do not affect the resolution of other RR types and apply only
  to a specific service, not an entire domain name.

  The AliasMode TargetName SHOULD NOT be equal to the owner name, as
  this would result in a loop.  In AliasMode, recipients MUST ignore
  any SvcParams that are present.  Zone-file parsers MAY emit a warning
  if an AliasMode record has SvcParams.  The use of SvcParams in
  AliasMode records is currently not defined, but a future
  specification could extend AliasMode records to include SvcParams.

  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 at the same owner
  name.

  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.

  Zones that require following multiple AliasMode records could
  encounter compatibility and performance issues.

  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 or vice
  versa.

2.4.3.  ServiceMode

  In ServiceMode, the TargetName and SvcParams within each RR 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 other's requirements.  Clients MUST reject any RR
  whose recognized SvcParams are not self-consistent and MAY reject the
  entire RRset.  To help zone operators avoid this condition, zone-file
  implementations SHOULD enforce self-consistency as well.

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.
  If the record has a wildcard owner name in the zone file, the
  recipient SHALL use the response's synthesized owner name as the
  effective TargetName.

  Here, for 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
  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 and ordering the
  available 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.  Sort the
      records by ascending SvcPriority.

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

  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; otherwise, it is called "SVCB-reliant".
  Clients for pre-existing protocols (e.g., HTTP) SHALL implement SVCB-
  optional behavior (except as noted in Section 3.1 or when modified by
  future specifications).

  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, if at
  least one AliasMode record was processed, SVCB-optional clients SHALL
  append to the list of endpoints 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 this 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 now attempts to use 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 its attempt to reach the service, 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
  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.  Otherwise, an
  attacker could defeat the A/AAAA protection by forging SVCB responses
  that direct the client to other IP addresses.

  If DNS responses are not cryptographically protected, clients MAY
  treat SVCB resolution failure as fatal or nonfatal.

  If the client is unable to complete SVCB resolution due to its chain
  length limit, the client MUST fall back to the authority endpoint, as
  if the service'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 of
  using named destinations, in which case the client does not perform
  any A or AAAA queries for destination domains.  If the client is
  configured to use 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
  and not just the proxy.  Clients in this configuration SHOULD arrange
  for a separate SVCB resolution procedure with appropriate privacy
  properties.  If this is not possible, SVCB-optional clients MUST
  disable SVCB resolution entirely, and SVCB-reliant clients MUST treat
  the configuration as invalid.

  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., supported
        protocols) and privacy.

     -  It allows a service on an apex domain to use aliasing.

  *  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.

     -  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 DNSSEC records authenticating the existence or
  nonexistence of these records in the Additional section.

  See Section 4.4 for exceptions.

4.2.  Recursive Resolvers

  Whether the recursive resolver is aware of SVCB or not, the normal
  response construction process used for unknown RR types [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 recursive
      resolver's local chain length limit (which may be different from
      the client's 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.

  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.

4.2.1.  DNS64

  DNS64 resolvers synthesize responses to AAAA queries for names that
  only have an A record (Section 5.1.7 of [RFC6147]).  SVCB-aware DNS64
  resolvers SHOULD apply the same synthesis logic when resolving AAAA
  records for the TargetName for inclusion in the Additional section
  (Step 2 in Section 4.2) and MAY omit the A records from this section.

  DNS64 resolvers MUST NOT extrapolate the AAAA synthesis logic to the
  IP hints in the SvcParams (Section 7.3).  Modifying the IP hints
  would break DNSSEC validation for the SVCB record and would not
  improve performance when the above recommendation is implemented.

4.3.  General Requirements

  Recursive resolvers MUST be able to convey SVCB records with
  unrecognized SvcParamKeys.  Resolvers MAY accomplish this by treating
  the entire SvcParams portion of the record as opaque, even if the
  contents are invalid.  If a recognized SvcParamKey is followed by a
  value that is invalid according to the SvcParam's specification, a
  recursive resolver MAY report an error such as SERVFAIL instead of
  returning the record.  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 (ECS) option [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.

  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 set to 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 follow-up
  query that can receive a properly tailored ECS.  (This is similar to
  the usage of CNAME with the ECS option as discussed in [RFC7871],
  Section 7.2.1.)

  Authoritative servers that omit Additional records can avoid the
  added latency of a follow-up query by following the advice in
  Section 10.2.

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 follow-up queries.  With this behavior, and
  with 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 10.2).

  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, future
  SvcParamKeys could be defined that alter the TLS ClientHello.

  Clients implementing this optimization SHOULD wait for 50
  milliseconds before starting optimistic pre-connection, as per the
  guidance in [HappyEyeballsV2].

  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. (
      ipv6hint=2001:db8::1 port=1234 )
                                 SVCB 2 svc2.example.net. (
      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, 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 (Step 1 in Section 4.2) 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 RR Types

  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

  *  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
  follow-up 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 expected to be generally
  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").

  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 in ALPN IDs 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
  ALPN IDs that are supported by nearly all clients and servers; this
  set MAY be empty.  To determine the SVCB ALPN set, the client starts
  with the list of alpn-ids from the "alpn" SvcParamKey, and it adds
  the default set unless the "no-default-alpn" SvcParamKey is present.

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
  compatible RRs indicate "no-default-alpn", even if connection could
  have succeeded using a non-default ALPN protocol.

  Zone operators SHOULD ensure that at least one RR in each RRset
  supports the default transports.  This enables compatibility with the
  greatest number of clients.

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
  SvcParamValue 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.  "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 per 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
  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] [RFC4001].  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 (for the respective address family).  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, NAT64 clients may synthesize IPv6 addresses as
  specified in [RFC7050] or ignore the "ipv4hint" key and wait for AAAA
  resolution (Section 3).  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
  service name 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.

7.4.  "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.

  A ServiceMode RR is considered "compatible" by a client if the client
  recognizes all the mandatory keys and their values indicate that
  successful connection establishment is possible.  Incompatible RRs
  are ignored (see step 5 of the procedure defined in Section 3).

  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:

  ipv6hint=... key65333=ex1 key65444=ex2 mandatory=key65444,ipv6hint

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

  The use of any protocol with SVCB requires a protocol-specific
  mapping specification.  This section specifies the mapping for the
  "http" and "https" URI schemes [HTTP].

  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

  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 "HTTP Alternative
  Services" [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.

  The procedure for following HTTPS AliasMode RRs and CNAME aliases is
  unchanged from SVCB (as described in Sections 2.4.2 and 3).

  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.

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 are discussed in Section 7.1.2.

9.2.2.  Untrusted Channels

  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.

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 and
  are making a connection based on a cached Alt-Svc response 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
  (per 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 foo=...
  alt2.example.             IN HTTPS 1 alt2b.example. alpn=h3 foo=...
  _8443._https.example.com. IN HTTPS 1 alt3.example. (
      port=9443 alpn=h2,h3 foo=... )

  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 client's non-Alt-Svc connection behavior

  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 ID consistent with both
     the HTTPS record and Alt-Svc)

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

  Suppose that "foo" is a SvcParamKey that renders the client SVCB-
  reliant.  The following Alt-Svc-only connection attempts would be
  allowed only if the client does not support "foo", as they rely on
  SVCB-optional fallback behavior:

  *  HTTP/2 to alt2.example:443

  *  HTTP/3 to example.com:8443

  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 support for privacy features that are available
  on the authority endpoint.

9.4.  Requiring Server Name Indication

  Clients MUST NOT use an HTTPS RR response unless the client supports
  the TLS Server Name Indication (SNI) extension and indicates the
  origin name in the TLS ClientHello (which might be encrypted via a
  future specification such as [ECH]).  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 (HSTS)

  An HTTPS RR directs the client to communicate with this host only
  over a secure transport, similar to HSTS [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], Step 5.

  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 (Temporary Redirect) response
  over cleartext HTTP.

  Publishing an HTTPS RR can potentially lead to unexpected results or
  a loss in functionality in cases where the "http" resource neither
  redirects to the "https" resource nor references the same underlying
  resource.

  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 Sections 8.4 and
  12.1 of [HSTS].

9.6.  Use of HTTPS RRs in Other Protocols

  All HTTP connections to named origins are eligible to use HTTPS RRs,
  even when HTTP is used as part of another protocol or without an
  explicit HTTP-related URI scheme (Section 4.2 of [HTTP]).  For
  example, clients that support HTTPS RRs and implement [WebSocket]
  using the altered opening handshake from [FETCH-WEBSOCKETS] SHOULD
  use HTTPS RRs for the requestURL.

  When HTTP is used in a context where URLs or redirects are not
  applicable (e.g., connections to an HTTP proxy), clients that find a
  corresponding HTTPS RR SHOULD implement security upgrade behavior
  equivalent to that specified in Section 9.5.

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

10.  Zone Structures

10.1.  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.
  When multiple port numbers are in use, it may be helpful to repeat
  the prefix labels in the alias target name (e.g.,
  "_1234._foo.svc.example.net").

10.2.  Structuring Zones for Performance

  To avoid a delay for clients using a non-conforming 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.

  $ORIGIN example.com. ; Origin
  foo                  3600 IN CNAME foosvc.example.net.
  _8080._foo.foo       3600 IN CNAME foosvc.example.net.
  bar                   300 IN AAAA 2001:db8::2
  _9090._bar.bar       3600 IN SVCB 1 bar key65444=...

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

          Figure 1: "foo://foo.example.com:8080" Is Available at
     "foosvc.example.net", but "bar://bar.example.com:9090" Is Served
                                 Locally

  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 eight aliases
  (counting both AliasMode and CNAME records) are NOT RECOMMENDED.

10.3.  Operational Considerations

  Some authoritative DNS servers may not allow A or AAAA records on
  names starting with an underscore (e.g., [BIND-CHECK-NAMES]).  This
  could create an operational issue when the TargetName contains an
  Attrleaf label, or when using a TargetName of "." if the owner name
  contains an Attrleaf label.

10.4.  Examples

10.4.1.  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

  This record would indicate that "https://simple.example" supports
  QUIC in addition to HTTP/1.1 over TLS over TCP (the implicit
  default).  The record could also include other information (e.g., a
  non-standard port).  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".

10.4.2.  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.

10.4.3.  Parameter Binding

  Suppose that svc.example's primary server pool supports HTTP/3 but
  its backup server pool does not.  This can be expressed in the
  following form:

  $ORIGIN svc.example. ; A hosting provider
  pool  7200 IN HTTPS 1 . alpn=h2,h3
                HTTPS 2 backup alpn=h2 port=8443
  pool   300 IN A        192.0.2.2
                AAAA     2001:db8::2
  backup 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, port
  number, etc.

10.4.4.  Multi-CDN Configuration

  The HTTPS RR is intended to support HTTPS services operated by
  multiple independent entities, such as different 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
                    HTTPS 2 . alpn=h2
                    A    192.0.2.2
                    AAAA 2001:db8:192::4
   h3pool 300 IN A 192.0.2.3
              AAAA 2001:db8:192:7::3

   ; 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
             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 cdn3.svc3.example
   ; path use these records.
   ; Note that this CDN has no HTTPS records.
   $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
  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 a desired protocol or feature and CDN 2 does
     not, the client is vulnerable to 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
     resolvers may observe and follow different CNAMEs to different
     CDNs.  Clients may thus find that the A and AAAA responses do not
     correspond to the TargetName in the HTTPS response; these clients
     will need to perform additional queries to retrieve the correct IP
     addresses.  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)
     but could receive A/AAAA records from a different CDN.  Clients
     will attempt to connect to this CDN without the benefit of its
     HTTPS records.

10.4.5.  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.

11.  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 Online Certificate Status
  Protocol (OCSP) Stapling (i.e., Certificate Status Request in
  Section 8 of [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 the use of SVCB with a protocol MUST have an
  entry for its scheme under the SVCB RR type in the IANA DNS
  "Underscored and Globally Scoped DNS Node Names" registry [Attrleaf].
  The scheme MUST have an entry in the "Uniform Resource Identifier
  (URI) Schemes" registry [RFC7595] and MUST have a defined
  specification for use with SVCB.

12.  Security Considerations

  SVCB/HTTPS RRs permit 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.

  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 can disable any other protections coordinated via
  SvcParams.  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 authoritative server 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, the attacker could gain access to the
  internal service.  (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.

13.  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.

14.  IANA Considerations

14.1.  SVCB RR Type

  IANA has registered the following new DNS RR type in the "Resource
  Record (RR) TYPEs" registry on the "Domain Name System (DNS)
  Parameters" page:

  Type:  SVCB
  Value:  64
  Meaning:  General-purpose service binding
  Reference:  RFC 9460

14.2.  HTTPS RR Type

  IANA has registered the following new DNS RR type in the "Resource
  Record (RR) TYPEs" registry on the "Domain Name System (DNS)
  Parameters" page:

  Type:  HTTPS
  Value:  65
  Meaning:  SVCB-compatible type for use with HTTP
  Reference:  RFC 9460

14.3.  New Registry for Service Parameters

  IANA has created the "Service Parameter Keys (SvcParamKeys)" registry
  in the "Domain Name System (DNS) Parameters" category on a new page
  entitled "DNS Service Bindings (SVCB)".  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.

14.3.1.  Procedure

  A registration MUST include the following fields:

  Number:  Wire-format numeric identifier (range 0-65535)
  Name:  Unique presentation name
  Meaning:  A short description
  Reference:  Location of specification or registration source
  Change Controller:  Person or entity, with contact information if
     appropriate

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

  The registration policy for new entries is Expert Review ([RFC8126],
  Section 4.5).  The designated expert MUST ensure that the reference
  is stable and publicly available and that it specifies how to convert
  the SvcParamValue's presentation format to wire format.  The
  reference MAY be any individual's Internet-Draft or a document from
  any other source with similar assurances of stability and
  availability.  An entry MAY specify a 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.

14.3.2.  Initial Contents

  The "Service Parameter Keys (SvcParamKeys)" registry has been
  populated with the following initial registrations:

  +===========+=================+================+=========+==========+
  |   Number  | Name            | Meaning        |Reference|Change    |
  |           |                 |                |         |Controller|
  +===========+=================+================+=========+==========+
  |     0     | mandatory       | Mandatory      |RFC 9460,|IETF      |
  |           |                 | keys in this   |Section 8|          |
  |           |                 | RR             |         |          |
  +-----------+-----------------+----------------+---------+----------+
  |     1     | alpn            | Additional     |RFC 9460,|IETF      |
  |           |                 | supported      |Section  |          |
  |           |                 | protocols      |7.1      |          |
  +-----------+-----------------+----------------+---------+----------+
  |     2     | no-default-alpn | No support     |RFC 9460,|IETF      |
  |           |                 | for default    |Section  |          |
  |           |                 | protocol       |7.1      |          |
  +-----------+-----------------+----------------+---------+----------+
  |     3     | port            | Port for       |RFC 9460,|IETF      |
  |           |                 | alternative    |Section  |          |
  |           |                 | endpoint       |7.2      |          |
  +-----------+-----------------+----------------+---------+----------+
  |     4     | ipv4hint        | IPv4 address   |RFC 9460,|IETF      |
  |           |                 | hints          |Section  |          |
  |           |                 |                |7.3      |          |
  +-----------+-----------------+----------------+---------+----------+
  |     5     | ech             | RESERVED       |N/A      |IETF      |
  |           |                 | (held for      |         |          |
  |           |                 | Encrypted      |         |          |
  |           |                 | ClientHello)   |         |          |
  +-----------+-----------------+----------------+---------+----------+
  |     6     | ipv6hint        | IPv6 address   |RFC 9460,|IETF      |
  |           |                 | hints          |Section  |          |
  |           |                 |                |7.3      |          |
  +-----------+-----------------+----------------+---------+----------+
  |65280-65534| N/A             | Reserved for   |RFC 9460 |IETF      |
  |           |                 | Private Use    |         |          |
  +-----------+-----------------+----------------+---------+----------+
  |   65535   | N/A             | Reserved       |RFC 9460 |IETF      |
  |           |                 | ("Invalid      |         |          |
  |           |                 | key")          |         |          |
  +-----------+-----------------+----------------+---------+----------+

                                 Table 1

14.4.  Other Registry Updates

  Per [Attrleaf], the following entry has been added to the DNS
  "Underscored and Globally Scoped DNS Node Names" registry:

                  +=========+============+===========+
                  | RR Type | _NODE NAME | Reference |
                  +=========+============+===========+
                  | HTTPS   | _https     | RFC 9460  |
                  +---------+------------+-----------+

                                Table 2

15.  References

15.1.  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/info/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/info/rfc8552>.

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

  [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/info/rfc7858>.

  [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/info/rfc8305>.

  [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
             Ed., "HTTP Semantics", STD 97, RFC 9110,
             DOI 10.17487/RFC9110, June 2022,
             <https://www.rfc-editor.org/info/rfc9110>.

  [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
             STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
             <https://www.rfc-editor.org/info/rfc1034>.

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

  [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/info/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/info/rfc2119>.

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

  [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC",
             RFC 3225, DOI 10.17487/RFC3225, December 2001,
             <https://www.rfc-editor.org/info/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/info/rfc3597>.

  [RFC4001]  Daniele, M., Haberman, B., Routhier, S., and J.
             Schoenwaelder, "Textual Conventions for Internet Network
             Addresses", RFC 4001, DOI 10.17487/RFC4001, February 2005,
             <https://www.rfc-editor.org/info/rfc4001>.

  [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/info/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/info/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/info/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/info/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/info/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/info/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/info/rfc7595>.

  [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/info/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/info/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/info/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/info/rfc8446>.

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

15.2.  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/info/rfc7838>.

  [ANAME-DNS-RR]
             Finch, T., Hunt, E., van Dijk, P., Eden, A., and W.
             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>.

  [BIND-CHECK-NAMES]
             Internet Systems Consortium, "BIND v9.19.11 Configuration
             Reference: "check-names"", September 2023,
             <https://bind9.readthedocs.io/en/v9.19.11/
             reference.html#namedconf-statement-check-names>.

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

  [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/info/rfc8499>.

  [ECH]      Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
             Encrypted Client Hello", Work in Progress, Internet-Draft,
             draft-ietf-tls-esni-17, 9 October 2023,
             <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
             esni-17>.

  [FETCH]    WHATWG, "Fetch Living Standard", October 2023,
             <https://fetch.spec.whatwg.org/>.

  [FETCH-WEBSOCKETS]
             WHATWG, "WebSockets Living Standard", September 2023,
             <https://websockets.spec.whatwg.org/>.

  [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/info/rfc6797>.

  [HTTP-DNS-RR]
             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>.

  [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
             June 2022, <https://www.rfc-editor.org/info/rfc9114>.

  [RFC1912]  Barr, D., "Common DNS Operational and Configuration
             Errors", RFC 1912, DOI 10.17487/RFC1912, February 1996,
             <https://www.rfc-editor.org/info/rfc1912>.

  [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
             DOI 10.17487/RFC6454, December 2011,
             <https://www.rfc-editor.org/info/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/info/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/info/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,
  according to an algorithm defined in Section 5.1 of [RFC1035].  The
  following summarizes some allowed inputs 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,
  using quoting to group values (e.g., those with internal whitespace),
  and escaping to represent each non-printable octet as a single
  escaped sequence.  In this document, this algorithm is referred to as
  "character-string decoding", because Section 5.1 of [RFC1035] uses
  this algorithm to produce a <character-string>.  Note that while the
  length of a <character-string> is limited to 255 octets, the
  character-string decoding algorithm can produce output of any length.

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.  This corresponds to
  the simple-comma-separated syntax:

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

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

  item            = 1*OCTET
  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.

           +--------------------------+----------------------+
           | *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*      | Upgrade from HTTP to |
           |                          | HTTPS                |
           +--------------------------+----------------------+
           | *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.  One complaint regarding
  all of the alternatives is that web clients have seemed
  unenthusiastic about implementing them.  The hope here is that an
  extensible solution that solves multiple problems will overcome this
  inertia and have a path to achieve client implementation.

C.1.  Differences from the SRV RR Type

  An SRV record [SRV] can perform a function similar to that of 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).

  *  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 [HTTP-DNS-RR], 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 [ANAME-DNS-RR], this approach is extensible to cover Alt-Svc
  and Encrypted ClientHello 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 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 query
  and an AliasMode query to the authoritative DNS server.  Thus,
  splitting the RR type would double, or in some cases triple, the load
  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 wire format 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 2: AliasMode

D.2.  ServiceMode

  example.com.   SVCB   1 .

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

  \x00\x01   # priority
  \x00       # target (root label)

                       Figure 3: TargetName Is "."

  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
  \x00\x35                                           # value

                        Figure 4: 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 5: A Generic Key and Unquoted Value

  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 6: 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 7: Two Quoted IPv6 Hints

  example.com.   SVCB   1 example.com. (
                          ipv6hint="2001:db8:122:344::192.0.2.33"
                          )
  \# 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
  20 01 0d b8 01 22 03 44 00 00 00 00 c0 00 02 21    ; address
  )

  \x00\x01                                           # priority
  \x07example\x03com\x00                             # target
  \x00\x06                                           # key 6
  \x00\x10                                           # length 16
  \x20\x01\x0d\xb8\x01\x22\x03\x44
       \x00\x00\x00\x00\xc0\x00\x02\x21              # address

          Figure 8: An IPv6 Hint Using the Embedded IPv4 Syntax

  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 9: SvcParamKey Ordering Is Arbitrary in Presentation
                     Format but Sorted in Wire Format

  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 10: An "alpn" Value with an Escaped Comma and an Escaped
                  Backslash in Two Presentation Formats

D.3.  Failure Cases

  This subsection contains test vectors that 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 11: 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 12: Missing SvcParamValues That Must Be Non-Empty

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

     Figure 13: The "no-default-alpn" SvcParamKey Value Must Be Empty

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

                Figure 14: A Mandatory SvcParam Is Missing

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

      Figure 15: The "mandatory" SvcParamKey Must Not Be Included in
                            the Mandatory List

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

       Figure 16: Multiple Instances of the Same SvcParamKey in the
                              Mandatory List

Acknowledgments and Related Proposals

  Over the years, IETF participants have proposed a wide range of
  solutions to the "CNAME at the zone apex" challenge, including
  [HTTP-DNS-RR], [ANAME-DNS-RR], and others.  The authors are grateful
  for their work to elucidate the problem and identify promising
  strategies to address it, some of which are reflected in this
  document.

  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 document.

Authors' Addresses

  Ben Schwartz
  Meta Platforms, Inc.
  Email: [email protected]


  Mike Bishop
  Akamai Technologies
  Email: [email protected]


  Erik Nygren
  Akamai Technologies
  Email: [email protected]

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