Service binding and parameter specification via the DNS (DNS SVCB and HTTPS RRs)
draft-ietf-dnsop-svcb-https-04

The information below is for an old version of the document
Document Type Active Internet-Draft (dnsop WG)
Authors Benjamin Schwartz  , Mike Bishop  , Erik Nygren 
Last updated 2021-03-18 (latest revision 2021-03-17)
Replaces draft-ietf-dnsop-svcb-httpssvc
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DNSOP Working Group                                          B. Schwartz
Internet-Draft                                                    Google
Intended status: Standards Track                               M. Bishop
Expires: September 18, 2021                                    E. Nygren
                                                     Akamai Technologies
                                                          March 17, 2021

 Service binding and parameter specification via the DNS (DNS SVCB and
                               HTTPS RRs)
                     draft-ietf-dnsop-svcb-https-04

Abstract

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

   TO BE REMOVED: This document is being collaborated on in Github at:
   https://github.com/MikeBishop/dns-alt-svc [1].  The most recent
   working version of the document, open issues, etc. should all be
   available there.  The authors (gratefully) accept pull requests.

Status of This Memo

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

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

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

   This Internet-Draft will expire on September 18, 2021.

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Goals of the SVCB RR  . . . . . . . . . . . . . . . . . .   5
     1.2.  Overview of the SVCB RR . . . . . . . . . . . . . . . . .   5
     1.3.  Parameter for Encrypted ClientHello . . . . . . . . . . .   6
     1.4.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
   2.  The SVCB record type  . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Zone file presentation format . . . . . . . . . . . . . .   8
     2.2.  RDATA wire format . . . . . . . . . . . . . . . . . . . .   9
     2.3.  SVCB query names  . . . . . . . . . . . . . . . . . . . .  10
     2.4.  Interpretation  . . . . . . . . . . . . . . . . . . . . .  10
       2.4.1.  SvcPriority . . . . . . . . . . . . . . . . . . . . .  11
       2.4.2.  AliasMode . . . . . . . . . . . . . . . . . . . . . .  11
       2.4.3.  ServiceMode . . . . . . . . . . . . . . . . . . . . .  12
     2.5.  Special handling of "." in TargetName . . . . . . . . . .  13
       2.5.1.  AliasMode . . . . . . . . . . . . . . . . . . . . . .  13
       2.5.2.  ServiceMode . . . . . . . . . . . . . . . . . . . . .  13
   3.  Client behavior . . . . . . . . . . . . . . . . . . . . . . .  13
     3.1.  Handling resolution failures  . . . . . . . . . . . . . .  14
     3.2.  Clients using a Proxy . . . . . . . . . . . . . . . . . .  15
   4.  DNS Server Behavior . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  Authoritative servers . . . . . . . . . . . . . . . . . .  15
     4.2.  Recursive resolvers . . . . . . . . . . . . . . . . . . .  16
     4.3.  General requirements  . . . . . . . . . . . . . . . . . .  16
   5.  Performance optimizations . . . . . . . . . . . . . . . . . .  17
     5.1.  Optimistic pre-connection and connection reuse  . . . . .  17
     5.2.  Generating and using incomplete responses . . . . . . . .  18
   6.  Initial SvcParamKeys  . . . . . . . . . . . . . . . . . . . .  18
     6.1.  "alpn" and "no-default-alpn"  . . . . . . . . . . . . . .  18
     6.2.  "port"  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     6.3.  "echconfig" . . . . . . . . . . . . . . . . . . . . . . .  21
     6.4.  "ipv4hint" and "ipv6hint" . . . . . . . . . . . . . . . .  21

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   7.  ServiceMode RR compatibility and mandatory keys . . . . . . .  22
   8.  Using SVCB with HTTPS and HTTP  . . . . . . . . . . . . . . .  23
     8.1.  Query names for HTTPS RRs . . . . . . . . . . . . . . . .  23
     8.2.  Relationship to Alt-Svc . . . . . . . . . . . . . . . . .  24
       8.2.1.  ALPN usage  . . . . . . . . . . . . . . . . . . . . .  24
       8.2.2.  Untrusted channel . . . . . . . . . . . . . . . . . .  24
       8.2.3.  Cache lifetime  . . . . . . . . . . . . . . . . . . .  25
       8.2.4.  Granularity . . . . . . . . . . . . . . . . . . . . .  25
     8.3.  Interaction with Alt-Svc  . . . . . . . . . . . . . . . .  25
     8.4.  Requiring Server Name Indication  . . . . . . . . . . . .  25
     8.5.  HTTP Strict Transport Security  . . . . . . . . . . . . .  26
     8.6.  HTTP-based protocols  . . . . . . . . . . . . . . . . . .  26
   9.  SVCB/HTTPS RR parameter for ECH configuration . . . . . . . .  27
     9.1.  Client behavior . . . . . . . . . . . . . . . . . . . . .  27
     9.2.  Deployment considerations . . . . . . . . . . . . . . . .  27
   10. Zone Structures . . . . . . . . . . . . . . . . . . . . . . .  27
     10.1.  Structuring zones for flexibility  . . . . . . . . . . .  28
     10.2.  Structuring zones for performance  . . . . . . . . . . .  28
     10.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . .  28
       10.3.1.  Protocol enhancements  . . . . . . . . . . . . . . .  28
       10.3.2.  Apex aliasing  . . . . . . . . . . . . . . . . . . .  29
       10.3.3.  Parameter binding  . . . . . . . . . . . . . . . . .  29
       10.3.4.  Multi-CDN  . . . . . . . . . . . . . . . . . . . . .  30
       10.3.5.  Non-HTTPS uses . . . . . . . . . . . . . . . . . . .  32
   11. Interaction with other standards  . . . . . . . . . . . . . .  32
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  32
   13. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  33
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
     14.1.  SVCB RRType  . . . . . . . . . . . . . . . . . . . . . .  34
     14.2.  HTTPS RRType . . . . . . . . . . . . . . . . . . . . . .  34
     14.3.  New registry for Service Parameters  . . . . . . . . . .  34
       14.3.1.  Procedure  . . . . . . . . . . . . . . . . . . . . .  34
       14.3.2.  Initial contents . . . . . . . . . . . . . . . . . .  35
     14.4.  Registry updates . . . . . . . . . . . . . . . . . . . .  36
   15. Acknowledgments and Related Proposals . . . . . . . . . . . .  37
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     16.2.  Informative References . . . . . . . . . . . . . . . . .  40
     16.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Appendix A.  Decoding text in zone files  . . . . . . . . . . . .  41
     A.1.  Decoding a comma-separated list . . . . . . . . . . . . .  41
   Appendix B.  HTTP Mapping Summary . . . . . . . . . . . . . . . .  42
   Appendix C.  Comparison with alternatives . . . . . . . . . . . .  43
     C.1.  Differences from the SRV RR type  . . . . . . . . . . . .  43
     C.2.  Differences from the proposed HTTP record . . . . . . . .  43
     C.3.  Differences from the proposed ANAME record  . . . . . . .  43
     C.4.  Comparison with separate RR types for AliasMode and
           ServiceMode . . . . . . . . . . . . . . . . . . . . . . .  44

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   Appendix D.  Change history . . . . . . . . . . . . . . . . . . .  44
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  47

1.  Introduction

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

   For example, when clients need to make a connection to fetch
   resources associated with an HTTPS URI, they currently resolve only A
   and/or AAAA records for the origin hostname.  This is adequate for
   services that use basic HTTPS (fixed port, no QUIC, no [ECH]).  Going
   beyond basic HTTPS confers privacy, performance, and operational
   advantages, but it requires the client to learn additional
   information, and it is highly desirable to minimize the number of
   round-trips and lookups required to learn this additional
   information.

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

   This document first describes the SVCB RR as a general-purpose
   resource record that can be applied directly and efficiently to a
   wide range of services (Section 2).  The HTTPS RR is then defined as
   a special case of SVCB that improves efficiency and convenience for
   use with HTTPS (Section 8) by avoiding the need for an Attrleaf label
   [Attrleaf] (Section 8.1).  Other protocols with similar needs may
   follow the pattern of HTTPS and assign their own SVCB-compatible RR
   types.

   All behaviors described as applying to the SVCB RR also apply to the
   HTTPS RR unless explicitly stated otherwise.  Section 8 describes
   additional behaviors specific to the HTTPS RR.  Apart from Section 8
   and introductory examples, much of this document refers only to the
   SVCB RR, but those references should be taken to apply to SVCB,
   HTTPS, and any future SVCB-compatible RR types.

   The SVCB RR has two modes: 1) "AliasMode" simply delegates
   operational control for a resource; 2) "ServiceMode" binds together

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   configuration information for a service endpoint.  ServiceMode
   provides additional key=value parameters within each RDATA set.

1.1.  Goals of the SVCB RR

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

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

   o  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 RRs for the same name.  For example, if
      www.example.com is a CNAME alias that switches between one of
      three CDNs or hosting environments, successive queries for that
      name may return records that correspond to different environments.

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

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

   o  Connect directly to HTTP3 (QUIC transport) alternative endpoints
      [HTTP3]

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

   o  Support non-default TCP and UDP ports

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

   o  Provide an HSTS-like indication [HSTS] signaling that the HTTPS
      scheme should be used instead of HTTP for this request (see
      Section 8.5).

1.2.  Overview of the SVCB RR

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

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

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

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

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

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

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

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

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

1.3.  Parameter for Encrypted ClientHello

   This document also defines a parameter for Encrypted ClientHello
   [ECH] keys.  See Section 9.

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1.4.  Terminology

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

   o  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
      HTTPS "origin" [RFC6454].

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

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

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

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

   SVCB is a contraction of "service binding".  The SVCB RR, HTTPS RR,
   and future RR types that share SVCB's format and registry are
   collectively known as SVCB-compatible RR types.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  The SVCB record type

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

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

   Other SVCB-compatible resource record types can also be defined as-
   needed.  In particular, the HTTPS RR (RR type 65) provides special
   handling for the case of "https" origins as described in Section 8.

   SVCB RRs are extensible by a list of SvcParams, which are pairs
   consisting of a SvcParamKey and a SvcParamValue.  Each SvcParamKey
   has a presentation name and a registered number.  Values are in a
   format specific to the SvcParamKey.  Their definition should specify

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   both their presentation format and wire encoding (e.g., domain names,
   binary data, or numeric values).  The initial SvcParamKeys and
   formats are defined in Section 6.

2.1.  Zone file presentation format

   The presentation format of the record is:

   Name TTL IN SVCB SvcPriority TargetName SvcParams

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

   SvcPriority is a number in the range 0-65535, TargetName is a
   "<domain-name>" ([RFC1035] Section 5.1), and the SvcParams are a
   whitespace-separated list, with each SvcParam consisting of a
   SvcParamKey=SvcParamValue pair or a standalone SvcParamKey.
   SvcParamKeys are subject to IANA control (Section 14.3).

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

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

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

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

   Unrecognized keys are represented in presentation format as
   "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).

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   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:

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

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

   o  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 (ServiceMode), it contains a
   series of SvcParamKey=SvcParamValue pairs, represented as:

   o  a 2 octet field containing the SvcParamKey as an integer in
      network byte order.  (See Section 14.3.2 for the defined values.)

   o  a 2 octet field containing the length of the SvcParamValue as an
      integer between 0 and 65535 in network byte order (but constrained
      by the RDATA and DNS message sizes).

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

   SvcParamKeys SHALL appear in increasing numeric order.

   Clients MUST consider an RR malformed if:

   o  the end of the RDATA occurs within a SvcParam.

   o  SvcParamKeys are not in strictly increasing numeric order.

   o  the SvcParamValue for an 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.

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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.
   We term this behavior "Port Prefix Naming", and use it in the
   examples throughout this document.

   See Section 8.1 for the HTTPS RR behavior.

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

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

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

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

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

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

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

   (Parentheses are used to ignore a line break ([RFC1035]
   Section 5.1).)

2.4.  Interpretation

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2.4.1.  SvcPriority

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

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

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

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

2.4.2.  AliasMode

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

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

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

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

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

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

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

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

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

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

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

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

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

2.4.3.  ServiceMode

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

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

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

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2.5.  Special handling of "." in TargetName

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

2.5.1.  AliasMode

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

2.5.2.  ServiceMode

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

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

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

3.  Client behavior

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

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

   2.  Issue a SVCB query for $QNAME.

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

   4.  If one or more "compatible" (Section 7) ServiceMode records are
       returned, these represent the alternative endpoints.

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

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

   Once SVCB resolution has concluded, the client proceeds with
   connection establishment.  Clients SHOULD try higher-priority
   alternatives first, with fallback to lower-priority alternatives.
   Clients issue AAAA and/or A queries for the selected TargetName, and
   MAY choose between them using an approach such as Happy Eyeballs
   [HappyEyeballsV2].

   A client is called "SVCB-optional" if it can connect without the use
   of ServiceMode records, and "SVCB-reliant" otherwise.  Clients for
   pre-existing protocols (e.g.  HTTPS) SHALL implement SVCB-optional
   behavior (except as noted in Section 3.1 and Section 9.1).

   SVCB-optional clients SHOULD issue in parallel any other DNS queries
   that might be needed for connection establishment.  SVCB-optional
   clients SHALL append an alternative endpoint consisting of the final
   value of $QNAME, the authority endpoint's port number, and no
   SvcParams, to the list of alternative endpoints, which is 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.

   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 SVCB resolution fails due to a SERVFAIL error, transport error, or
   timeout, and DNS exchanges between the client and the recursive
   resolver are cryptographically protected (e.g. using TLS [DoT] or
   HTTPS [DoH]), a SVCB-optional client SHOULD abandon the connection
   attempt like a SVCB-reliant client would.  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.

   Similarly, if the client enforces DNSSEC validation on A/AAAA
   responses, it SHOULD terminate the connection if a SVCB response
   fails to validate.

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

3.2.  Clients using a Proxy

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

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

   Providing the proxy with the final TargetName has several benefits:

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

   o  It allows the service to delegate the apex domain.

   o  It allows the proxy to select between IPv4 and IPv6 addresses for
      the server according to its configuration, and receive addresses
      based on its network geolocation.

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
   in-bailiwick TargetNames.  If the zone is signed, the server SHOULD
   also include positive or negative DNSSEC responses for these records
   in the Additional section.

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4.2.  Recursive resolvers

   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 response.  For
   the initial SVCB record query, this is just the normal response
   construction process (i.e. unknown RR type resolution under
   [RFC3597]).  For followup resolutions performed during this
   procedure, we define incorporation as adding all useful RRs from the
   response to the Additional section without altering the response
   code.

   Upon receiving a SVCB query, recursive resolvers SHOULD start with
   the standard resolution procedure, and then follow this procedure to
   construct the full response to the stub resolver:

   1.  Incorporate the results of SVCB resolution.  If the chain length
       limit has been reached, terminate successfully (i.e. a NOERROR
       response).

   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]).

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

   See Section 5.2 for possible optimizations of this procedure.

4.3.  General requirements

   Recursive resolvers SHOULD treat the SvcParams portion of the SVCB RR
   as opaque and SHOULD NOT try to alter their behavior based on its
   contents.

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   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).

5.  Performance optimizations

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

   To improve performance when using a non-conforming recursive
   resolver, clients SHOULD issue speculative A and/or AAAA queries in
   parallel with each SVCB query, based on a predicted value of
   TargetName (see Section 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, a TLS
   ClientHello can be altered by the "echconfig" value of a SVCB
   response (Section 6.3).  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. (
       echconfig="111..." ipv6hint=2001:db8::1 port=1234 )
                                  SVCB 2 svc2.example.net. (
       echconfig="222..." ipv6hint=2001:db8::2 port=1234 )

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

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

5.2.  Generating and using incomplete responses

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

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

6.  Initial SvcParamKeys

   A few initial SvcParamKeys are defined here.  These keys are useful
   for HTTPS, and most are applicable to other protocols 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.

6.1.  "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.

   As with Alt-Svc [AltSvc], the ALPN protocol identifier is used to
   identify the application protocol and associated suite of protocols
   supported by the endpoint (the "protocol suite").  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).

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

   alpn-id = 1*255OCTET

   The presentation "value" SHALL be a comma-separated list
   (Appendix A.1) of one or more "alpn-id"s.

   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
   supported ALPNs, which SHOULD NOT be empty.  To determine the set of
   protocol suites supported by an endpoint (the "SVCB ALPN set"), the
   client adds the default set to the list of "alpn-id"s unless the "no-
   default-alpn" SvcParamKey is present.  The presence of an ALPN
   protocol 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 protocol.

   ALPN protocol names 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.

   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

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   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.

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

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

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

6.2.  "port"

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

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

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

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

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6.3.  "echconfig"

   The SvcParamKey to enable Encrypted ClientHello (ECH) is "echconfig".
   Its value is defined in Section 9.  It is applicable to most TLS-
   based protocols.

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

6.4.  "ipv4hint" and "ipv6hint"

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

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

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

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

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   best performance, server operators SHOULD include an "ipv6hint"
   parameter whenever they include an "ipv4hint" parameter.

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

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

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

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

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

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

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

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

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8.  Using SVCB with HTTPS and HTTP

   Use of any protocol with SVCB requires a protocol-specific mapping
   specification.  This section specifies the mapping for HTTPS and
   HTTP.

   To enable special handling for the HTTPS and 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 HTTPS RR wire format and presentation format are identical to
   SVCB, and both share the SvcParamKey registry.  SVCB semantics apply
   equally to HTTPS RRs unless specified otherwise.  The presentation
   format of the record is:

   Name TTL IN HTTPS SvcPriority TargetName SvcParams

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

   All the SvcParamKeys defined in Section 6 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 7) are
   "port" and "no-default-alpn".  (As described in Section 7, clients
   must either implement these keys or ignore any RR in which they
   appear.)  Clients that restrict the HTTPS destination port (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 all
   HTTP resources are available over HTTPS, as discussed in Section 8.5.
   This allows HTTPS RRs to apply to pre-existing "http" scheme URLs,
   while ensuring that the client uses a secure and authenticated HTTPS
   connection.

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

8.1.  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.

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

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

   Clients always convert "http" URLs to "https" before performing an
   HTTPS RR query using the process described in Section 8.5, 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.

8.2.  Relationship to Alt-Svc

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

8.2.1.  ALPN usage

   Unlike Alt-Svc Field Values, HTTPS RRs can contain multiple ALPN IDs,
   and clients are encouraged to offer additional ALPNs that they
   support.

8.2.2.  Untrusted channel

   SVCB does not require or provide any assurance of authenticity.
   (DNSSEC signing and verification, which would provide such assurance,
   are OPTIONAL.)  The DNS resolution process is treated as an untrusted
   channel that learns only the QNAME, and is prevented from mounting
   any attack beyond denial of service.

   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.

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8.2.3.  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.

8.2.4.  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.

8.3.  Interaction with Alt-Svc

   Clients that do not implement support for Encrypted ClientHello MAY
   skip the HTTPS RR query if a usable Alt-Svc value is available in the
   local cache.  If Alt-Svc connection fails, these clients SHOULD fall
   back to the HTTPS RR client connection procedure (Section 3).

   Clients that implement support for ECH MUST perform the HTTPS RR
   query first, and MUST only make use of Alt-Svc when operating in
   SVCB-optional mode (see Section 9.1).

   This specification does not alter the DNS records used when
   connecting to an Alt-Svc hostname (typically A and/or AAAA only).

8.4.  Requiring Server Name Indication

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

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

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8.5.  HTTP Strict Transport Security

   By publishing a usable HTTPS RR, the server operator indicates that
   all useful HTTP resources on that origin are reachable over HTTPS,
   similar to HTTP Strict Transport Security [HSTS].

   Prior to making an "http" scheme request, the client SHOULD perform a
   lookup to determine if any HTTPS RRs exist for that origin.  To do
   so, the client SHOULD construct a corresponding "https" URL as
   follows:

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

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

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

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

   If an HTTPS RR query for this "https" URL returns any AliasMode HTTPS
   RRs, or any compatible ServiceMode HTTPS RRs (see Section 7), the
   client SHOULD act as if it has received an HTTP "307 Temporary
   Redirect" redirect to this "https" URL.  (Receipt of an incompatible
   ServiceMode RR does not trigger the redirect behavior.)  Because
   HTTPS RRs are received over an often insecure channel (DNS), clients
   MUST NOT place any more trust in this signal than if they had
   received a 307 redirect over cleartext HTTP.

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

8.6.  HTTP-based protocols

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

   An HTTP-based protocol MAY define its own SVCB mapping.  Such
   mappings MAY be defined to take precedence over HTTPS RRs.

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9.  SVCB/HTTPS RR parameter for ECH configuration

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

   When ECH is in use, the TLS ClientHello is divided into an
   unencrypted "outer" and an encrypted "inner" ClientHello.  The outer
   ClientHello is an implementation detail of ECH, and its contents are
   controlled by the ECHConfig in accordance with [ECH].  The inner
   ClientHello is used for establishing a connection to the service, so
   its contents may be influenced by other SVCB parameters.  For
   example, the requirements on the ProtocolNameList in Section 6.1
   apply only to the inner ClientHello.  Similarly, it is the inner
   ClientHello whose Server Name Indication identifies the desired
   service.

9.1.  Client behavior

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

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

9.2.  Deployment considerations

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

10.  Zone Structures

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10.1.  Structuring zones for flexibility

   Each ServiceForm 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.

10.2.  Structuring zones for performance

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

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

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

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

10.3.  Examples

10.3.1.  Protocol enhancements

   Consider a simple zone of the form:

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

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   The domain owner could add this record:

   simple.example. 7200 IN HTTPS 1 . alpn=h3

   to indicate that simple.example uses HTTPS, and supports QUIC in
   addition to HTTPS over TCP (an implicit default).  The record could
   also include other information (e.g. non-standard port, ECH
   configuration).

10.3.2.  Apex aliasing

   Consider a zone that is using CNAME aliasing:

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

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

   @               7200 IN HTTPS 0 pool.svc.example.

   With this record in place, HTTPS-RR-aware clients will use the same
   server pool for aliased.example and www.aliased.example.  (They will
   also upgrade to HTTPS on aliased.example.)  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.

10.3.3.  Parameter binding

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

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

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

10.3.4.  Multi-CDN

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

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

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

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

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

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

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

    ; Resolutions following the cdn1.svc1.example
    ; path use these records.
    ; This CDN uses a different alternative service for HTTP/3.
    $ORIGIN svc1.example.  ; domain for CDN 1

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    cdn1     1800 IN HTTPS 1 h3pool alpn=h3 echconfig="123..."
                     HTTPS 2 . alpn=h2 echconfig="123..."
                     A    192.0.2.2
                     AAAA 2001:db8:192::4
    h3pool 300 IN A 192.0.2.3
               AAAA 2001:db8:192:7::3

    ; 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 echconfig="xyz..."
              60 IN A    198.51.100.2
                    A    198.51.100.3
                    A    198.51.100.4
                    AAAA 2001:db8:198::7
                    AAAA 2001:db8:198::12

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

   Note that in the above example, the different CDNs have different
   echconfig 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:

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

   o  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.

   o  The A, AAAA, HTTPS resolutions are independent lookups so clients
      may observe and follow different CNAMEs to different CDNs.
      Clients may thus find a SvcDomainName pointing to a name other
      than the one which returned along with the A and AAAA lookups and
      will need to do an additional resolution for them.  Including

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      ipv6hint and ipv4hint will reduce the performance impact of this
      case.

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

10.3.5.  Non-HTTPS uses

   For services other than HTTPS, 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 OCSP Stapling [RFC6066], both of
   which confer substantial performance and privacy benefits when used
   in combination with SVCB records.

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

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

12.  Security Considerations

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

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   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 8.5), and disable the encryption enabled by the echconfig
   SvcParamKey (Section 9).  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 AliasForm "." 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 AliasForm "." records.

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

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.

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14.  IANA Considerations

14.1.  SVCB RRType

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

   Type: SVCB

   Value: 64

   Meaning: General Purpose Service Endpoints

   Reference: This document

14.2.  HTTPS RRType

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

   Type: HTTPS

   Value: 65

   Meaning: HTTPS Specific Service Endpoints

   Reference: This document

14.3.  New registry for Service Parameters

   The "Service Binding (SVCB) Parameter Registry" defines the namespace
   for parameters, including string representations and numeric
   SvcParamKey values.  This registry is shared with other SVCB-
   compatible RR types, such as the HTTPS RR.

   ACTION: create and include a reference to this registry.

14.3.1.  Procedure

   A registration MUST include the following fields:

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

   o  Name: unique presentation name

   o  Meaning: a short description

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   o  Format Reference: pointer to specification text

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

   Entries in this registry are subject to a First Come First Served
   registration policy ([RFC8126], Section 4.6).  The Format Reference
   MUST specify how to convert the SvcParamValue's presentation format
   to wire format and MAY detail its intended meaning and use.  An entry
   MAY specify a Format Reference of the form "Same as (other key Name)"
   if it uses the same presentation and wire formats as an existing key.

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

14.3.2.  Initial contents

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

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   +-------------+-----------------+------------------+----------------+
   | Number      | Name            | Meaning          | Format         |
   |             |                 |                  | Reference      |
   +-------------+-----------------+------------------+----------------+
   | 0           | mandatory       | Mandatory keys   | (This          |
   |             |                 | in this RR       | document)      |
   |             |                 |                  | Section 7      |
   |             |                 |                  |                |
   | 1           | alpn            | Additional       | (This          |
   |             |                 | supported        | document)      |
   |             |                 | protocols        | Section 6.1    |
   |             |                 |                  |                |
   | 2           | no-default-alpn | No support for   | (This          |
   |             |                 | default protocol | document)      |
   |             |                 |                  | Section 6.1    |
   |             |                 |                  |                |
   | 3           | port            | Port for         | (This          |
   |             |                 | alternative      | document)      |
   |             |                 | endpoint         | Section 6.2    |
   |             |                 |                  |                |
   | 4           | ipv4hint        | IPv4 address     | (This          |
   |             |                 | hints            | document)      |
   |             |                 |                  | Section 6.4    |
   |             |                 |                  |                |
   | 5           | echconfig       | Encrypted        | (This          |
   |             |                 | ClientHello info | document)      |
   |             |                 |                  | Section 6.3    |
   |             |                 |                  |                |
   | 6           | ipv6hint        | IPv6 address     | (This          |
   |             |                 | hints            | document)      |
   |             |                 |                  | Section 6.4    |
   |             |                 |                  |                |
   | 65280-65534 | N/A             | Private Use      | (This          |
   |             |                 |                  | document)      |
   |             |                 |                  |                |
   | 65535       | N/A             | Reserved         | (This          |
   |             |                 | ("Invalid key")  | document)      |
   +-------------+-----------------+------------------+----------------+

14.4.  Registry updates

   Per [RFC6895], please add the following entries to the data type
   range of the Resource Record (RR) TYPEs registry:

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   +-------+------------------------------------------+----------------+
   | TYPE  | Meaning                                  | Reference      |
   +-------+------------------------------------------+----------------+
   | SVCB  | Service Location and Parameter Binding   | (This          |
   |       |                                          | document)      |
   |       |                                          |                |
   | HTTPS | HTTPS Service Location and Parameter     | (This          |
   |       | Binding                                  | document)      |
   +-------+------------------------------------------+----------------+

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

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

15.  Acknowledgments and Related Proposals

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

   Thank you to Ian Swett, Ralf Weber, Jon Reed, Martin Thomson, Lucas
   Pardue, Ilari Liusvaara, Tim Wicinski, Tommy Pauly, Chris Wood, David
   Benjamin, Mark Andrews, Emily Stark, Eric Orth, Kyle Rose, Craig
   Taylor, Dan McArdle, Brian Dickson, and others for their feedback and
   suggestions on this draft.

16.  References

16.1.  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>.

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

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

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

   [ECH]      Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "TLS
              Encrypted Client Hello", draft-ietf-tls-esni-09 (work in
              progress), December 2020.

   [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>.

   [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>.

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", draft-ietf-quic-http-33 (work in progress),
              December 2020.

   [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>.

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   [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>.

   [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>.

   [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>.

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   [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>.

16.2.  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>.

   [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>.

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

   [I-D.bellis-dnsop-http-record]
              Bellis, R., "A DNS Resource Record for HTTP", draft-
              bellis-dnsop-http-record-00 (work in progress), November
              2018.

   [I-D.ietf-dnsop-aname]
              Finch, T., Hunt, E., Dijk, P., Eden, A., and W. Mekking,
              "Address-specific DNS aliases (ANAME)", draft-ietf-dnsop-
              aname-04 (work in progress), July 2019.

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

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://www.rfc-editor.org/info/rfc6895>.

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   [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>.

16.3.  URIs

   [1] https://github.com/MikeBishop/dns-alt-svc

Appendix A.  Decoding text in zone files

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

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

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

A.1.  Decoding a comma-separated list

   In order to represent lists of items in zone files, this
   specification uses comma-separated lists.  When "," is not escaped
   (by a preceding "\"), it separates items in the list.  (For
   simplicity, empty items are not allowed.)

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

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

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

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

   part1,part2,part3\,part4\\

   Applying comma-separated list decoding to this "value" would produce
   a list of three "item"s:

   part1
   part2
   part3,part4\

Appendix B.  HTTP Mapping Summary

   This table serves as a non-normative summary of the HTTP mapping for
   SVCB (Section 8).  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-alpn"           |
   | Keys*                       |                                     |
   |                             |                                     |
   | *SvcParam defaults*         | "alpn": ["http/1.1"]                |
   |                             |                                     |
   | *Special behaviors*         | HTTP to HTTPS upgrade               |
   +-----------------------------+-------------------------------------+

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   This table does not indicate any SvcParamKeys that servers are
   required to publish, or that clients are required to implement,
   because there are none in this mapping.

Appendix C.  Comparison with alternatives

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

C.1.  Differences from the SRV RR type

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

   o  SRV records are typically mandatory, whereas clients will always
      continue to function correctly without making use of SVCB.

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

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

   o  SVCB records use 16 bit for SvcPriority for consistency with SRV
      and other RR types that also use 16 bit priorities.

C.2.  Differences from the proposed HTTP record

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

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

C.3.  Differences from the proposed ANAME record

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

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

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

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

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

Appendix D.  Change history

   o  draft-ietf-dnsop-svcb-https-04

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

      *  Recommend against publishing chains of >8 aliases

      *  Clarify requirements for using SVCB with a transport proxy

      *  Adjust guidance for Port Prefix Naming

      *  Minor editorial updates

   o  draft-ietf-dnsop-svcb-https-03

      *  Simplified escaping of comma-separated values

      *  Reorganized client requirements

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

      *  Clarified self-consistency rules for SvcParams

      *  Added a non-normative mapping summary table for HTTPS

   o  draft-ietf-dnsop-svcb-https-02

      *  Added a Privacy Considerations section

      *  Adjusted resolution fallback description

      *  Clarified status of SvcParams in AliasMode

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

      *  Improved examples, including a new Multi-CDN example

      *  Reorganized text on value-list parsing and SvcPriority

      *  Improved phrasing and other editorial improvements throughout

   o  draft-ietf-dnsop-svcb-https-01

      *  Added a "mandatory" SvcParamKey

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

      *  Adjusted resolution and ALPN algorithms

      *  Major terminology revisions for "origin" and CamelCase names

      *  Revised ABNF

      *  Include allocated RR type numbers

      *  Various corrections, explanations, and recommendations

   o  draft-ietf-dnsop-svcb-https-00

      *  Rename HTTPSSVC RR to HTTPS RR

      *  Rename "an SVCB" to "a SVCB"

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

   o  draft-ietf-dnsop-svcb-httpssvc-03

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      *  Revised chain length limit requirements

      *  Revised IANA registry rules for SvcParamKeys

      *  Require HTTPS clients to implement SNI

      *  Update terminology for Encrypted ClientHello

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

      *  Various textual and ABNF corrections

   o  draft-ietf-dnsop-svcb-httpssvc-02

      *  All changes to Alt-Svc have been removed

      *  Expanded and reorganized examples

      *  Priority zero is now the definition of AliasForm

      *  Repeated SvcParamKeys are no longer allowed

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

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

      *  New text regarding how to handle resolution timeouts

      *  Expanded description of recursive resolver behavior

      *  Much more precise description of the intended ALPN behavior

      *  Match the HSTS specification's language on HTTPS enforcement

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

   o  draft-ietf-dnsop-svcb-httpssvc-01

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

      *  Make the "untrusted channel" concept more precise.

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

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   o  draft-ietf-dnsop-svcb-httpssvc-00

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

      *  Relax IP hint handling requirements.  (Eric Rescorla)

   o  draft-nygren-dnsop-svcb-httpssvc-00

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

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

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

      *  Remove one optimization.

   o  draft-nygren-httpbis-httpssvc-03

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

   o  draft-nygren-httpbis-httpssvc-02

      *  Remove the redundant length fields from the wire format.

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

      *  Replace "hq" with "h3".

   o  draft-nygren-httpbis-httpssvc-01

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

   o  draft-nygren-httpbis-httpssvc-00

      *  Initial version

Authors' Addresses

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   Ben Schwartz
   Google

   Email: bemasc@google.com

   Mike Bishop
   Akamai Technologies

   Email: mbishop@evequefou.be

   Erik Nygren
   Akamai Technologies

   Email: erik+ietf@nygren.org

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