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Service binding and parameter specification via the DNS (DNS SVCB and HTTPSSVC)

The information below is for an old version of the document.
Document Type This is an older version of an Internet-Draft whose latest revision is Replaced
Authors Benjamin M. Schwartz , Mike Bishop , Erik Nygren
Last updated 2020-03-09
Replaces draft-nygren-dnsop-svcb-httpssvc
Replaced by draft-ietf-dnsop-svcb-https
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DNSOP Working Group                                          B. Schwartz
Internet-Draft                                                    Google
Intended status: Standards Track                               M. Bishop
Expires: September 10, 2020                                    E. Nygren
                                                     Akamai Technologies
                                                           March 9, 2020

 Service binding and parameter specification via the DNS (DNS SVCB and


   This document specifies the "SVCB" and "HTTPSSVC" DNS resource record
   types to facilitate the lookup of information needed to make
   connections for origin resources, such as for HTTPS URLs.  SVCB
   records allow an origin to be served from multiple network locations,
   each with associated parameters (such as transport protocol
   configuration and keying material for encrypting TLS SNI).  They also
   enable aliasing of apex domains, which is not possible with CNAME.
   The HTTPSSVC DNS 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 proposal is inspired by and based on recent DNS
   usage proposals such as ALTSVC, ANAME, and ESNIKEYS (as well as long
   standing desires to have SRV or a functional equivalent implemented
   for HTTP).  These proposals each provide an important function but
   are potentially incompatible with each other, such as when an origin
   is load-balanced across multiple hosting providers (multi-CDN).
   Furthermore, these each add potential cases for adding additional
   record lookups in-addition to AAAA/A lookups.  This design attempts
   to provide a unified framework that encompasses the key functionality
   of these proposals, as well as providing some extensibility for
   addressing similar future challenges.

   TO BE REMOVED: The specific name for this RR type is an open topic
   for discussion.  "SVCB" and "HTTPSSVC" are meant as placeholders as
   they are easy to replace.  Other names might include "B", "SRV2",

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

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Status of This Memo

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   This Internet-Draft will expire on September 10, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Example: Protocol enhancements  . . . . . . . . . . . . .   5
     1.2.  Example: Apex aliasing  . . . . . . . . . . . . . . . . .   5
     1.3.  Example: Parameter binding  . . . . . . . . . . . . . . .   6
     1.4.  Example: Non-HTTPS uses . . . . . . . . . . . . . . . . .   6
     1.5.  Goals of the SVCB RR  . . . . . . . . . . . . . . . . . .   7
     1.6.  Overview of the SVCB RR . . . . . . . . . . . . . . . . .   8
     1.7.  Parameter for ESNI  . . . . . . . . . . . . . . . . . . .   9
     1.8.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   9
   2.  The SVCB record type  . . . . . . . . . . . . . . . . . . . .   9
     2.1.  Presentation format . . . . . . . . . . . . . . . . . . .  10
       2.1.1.  Presentation format for SvcFieldValue key=value pairs  10
     2.2.  SVCB RDATA Wire Format  . . . . . . . . . . . . . . . . .  11
     2.3.  SVCB owner names  . . . . . . . . . . . . . . . . . . . .  12

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     2.4.  SvcRecordType . . . . . . . . . . . . . . . . . . . . . .  13
     2.5.  SVCB records: AliasForm . . . . . . . . . . . . . . . . .  13
     2.6.  SVCB records: ServiceForm . . . . . . . . . . . . . . . .  14
       2.6.1.  Special handling of "." for SvcDomainName in
               ServiceForm . . . . . . . . . . . . . . . . . . . . .  14
       2.6.2.  SvcFieldPriority  . . . . . . . . . . . . . . . . . .  14
   3.  Client behavior . . . . . . . . . . . . . . . . . . . . . . .  14
     3.1.  Handling resolution failures  . . . . . . . . . . . . . .  15
     3.2.  Clients using a Proxy . . . . . . . . . . . . . . . . . .  16
   4.  DNS Server Behavior . . . . . . . . . . . . . . . . . . . . .  16
     4.1.  Authoritative servers . . . . . . . . . . . . . . . . . .  16
     4.2.  Recursive resolvers . . . . . . . . . . . . . . . . . . .  16
     4.3.  General requirements  . . . . . . . . . . . . . . . . . .  17
   5.  Performance optimizations . . . . . . . . . . . . . . . . . .  18
     5.1.  Optimistic pre-connection and connection reuse  . . . . .  18
     5.2.  Generating and using incomplete responses . . . . . . . .  18
     5.3.  Structuring zones for performance . . . . . . . . . . . .  19
   6.  Initial SvcParamKeys  . . . . . . . . . . . . . . . . . . . .  19
     6.1.  "alpn" and "no-default-alpn"  . . . . . . . . . . . . . .  19
     6.2.  "port"  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     6.3.  "esniconfig"  . . . . . . . . . . . . . . . . . . . . . .  21
     6.4.  "ipv4hint" and "ipv6hint" . . . . . . . . . . . . . . . .  21
   7.  Using SVCB with HTTPS and HTTP  . . . . . . . . . . . . . . .  22
     7.1.  Owner names for HTTPSSVC records  . . . . . . . . . . . .  23
     7.2.  Relationship to Alt-Svc . . . . . . . . . . . . . . . . .  23
       7.2.1.  ALPN usage  . . . . . . . . . . . . . . . . . . . . .  23
       7.2.2.  Untrusted channel . . . . . . . . . . . . . . . . . .  24
       7.2.3.  TTL and granularity . . . . . . . . . . . . . . . . .  24
     7.3.  Interaction with Alt-Svc  . . . . . . . . . . . . . . . .  24
     7.4.  HTTP Strict Transport Security  . . . . . . . . . . . . .  25
   8.  SVCB/HTTPSSVC parameter for ESNI keys . . . . . . . . . . . .  25
     8.1.  Client behavior . . . . . . . . . . . . . . . . . . . . .  26
     8.2.  Deployment considerations . . . . . . . . . . . . . . . .  26
   9.  Interaction with other standards  . . . . . . . . . . . . . .  26
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     11.1.  New registry for Service Parameters  . . . . . . . . . .  27
       11.1.1.  Procedure  . . . . . . . . . . . . . . . . . . . . .  27
       11.1.2.  Initial contents . . . . . . . . . . . . . . . . . .  28
     11.2.  Registry updates . . . . . . . . . . . . . . . . . . . .  28
   12. Acknowledgments and Related Proposals . . . . . . . . . . . .  29
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     13.2.  Informative References . . . . . . . . . . . . . . . . .  32
     13.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  33
   Appendix A.  Comparison with alternatives . . . . . . . . . . . .  33
     A.1.  Differences from the SRV RR type  . . . . . . . . . . . .  33
     A.2.  Differences from the proposed HTTP record . . . . . . . .  33

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     A.3.  Differences from the proposed ANAME record  . . . . . . .  33
     A.4.  Differences from the proposed ESNI record . . . . . . . .  34
   Appendix B.  Design Considerations and Open Issues  . . . . . . .  34
     B.1.  Record Name . . . . . . . . . . . . . . . . . . . . . . .  34
     B.2.  Generality  . . . . . . . . . . . . . . . . . . . . . . .  34
     B.3.  Wire Format . . . . . . . . . . . . . . . . . . . . . . .  34
     B.4.  Whether to include Weight . . . . . . . . . . . . . . . .  35
   Appendix C.  Change history . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   The SVCB and HTTPSSVC RRs provide clients with complete instructions
   for access to an origin.  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 [ESNI]).
   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-trip and lookups required to learn this additional

   The SVCB and HTTPSSVC RRs also help when the operator of an origin
   wishes to delegate operational control to one or more other domains,
   e.g. delegating the origin resource "" to a
   service operator endpoint at "".  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 HTTPSSVC RR is then defined
   as a special case of SVCB that improves efficiency and convenience
   for use with HTTPS (Section 7) by avoiding the need for an [Attrleaf]
   label (Section 7.1).  Other protocols with similar needs may follow
   the pattern of HTTPSSVC and assign their own SVCB-compatible RR

   All behaviors described as applying to the SVCB RR also apply to the
   HTTPSSVC RR unless explicitly stated otherwise.  Section 7 describes

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   additional behaviors specific to the HTTPSSVC record.  Apart from
   Section 7 and introductory examples, much of this document refers
   only to the SVCB RR, but those references should be taken to apply to
   SVCB, HTTPSSVC, and any future SVCB-compatible RR types.

   The SVCB RR has two forms: 1) the "Alias Form" simply delegates
   operational control for a resource; 2) the "Service Form" binds
   together configuration information for a service endpoint.  The
   Service Form provides additional key=value parameters within each
   RDATA set.

   TO BE REMOVED: If we use this for providing configuration for DNS
   authorities, it is likely we'd specify a distinct "NS2" RR type that
   is an instantiation of SVCB for authoritative nameserver delegation
   and parameter specification, similar to HTTPSSVC.

   TO BE REMOVED: Another open question is whether SVCB records should
   be self-descriptive and include the service name (eg, "https") in the
   RDATA section to avoid ambiguity.  Perhaps this could be included as
   a svc="baz" parameter for protocols that are not the default for the
   RR type?  Current inclination is to not do so.

1.1.  Example: Protocol enhancements

   Consider a simple zone of the form

   simple.example. 300 IN A
                          AAAA 2001:db8::1

   The domain owner could add a record like

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

   The presence of this record indicates to clients that simple.example
   supports HTTPS, and the key=value pairs indicate that it supports
   QUIC in addition to HTTPS over TLS (an implicit default).  The record
   could also include other information (e.g. non-standard port, ESNI

1.2.  Example: Apex aliasing

   Consider a zone that is using CNAME aliasing:

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   $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
                        IN AAAA  2001:db8::1

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

   .               7200 IN HTTPSSVC 0 pool.svc.example.

   With this record in place, HTTPSSVC-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-HTTPSSVC-aware
   clients will just ignore the new record.

   Similar to CNAME, HTTPSSVC has 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.

1.3.  Example: 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 HTTPSSVC 1 h3pool alpn=h2,h3 esniconfig="123..."
                 HTTPSSVC 2 .      alpn=h2 esniconfig="abc..."
   pool   300 IN A
                 AAAA     2001:db8::2
   h3pool 300 IN A
                 AAAA     2001:db8::3

   This configuration is entirely compatible with the "Apex aliasing"
   example, whether the client supports HTTPSSVC or not.  If the client
   does support HTTPSSVC, 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, ESNI
   configuration, etc.

1.4.  Example: Non-HTTPS uses

   For services other than HTTPS, the SVCB RR and an [Attrleaf] label
   will be used.  For example, to reach an example resource of
   "baz://", the following Alias Form SVCB record

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   would be used to delegate to "" which in-turn
   could return AAAA/A records and/or SVCB records in ServiceForm. 7200 IN SVCB 0

   HTTPSSVC records use similar [Attrleaf] labels if the origin contains
   a non-default port.

1.5.  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 service endpoints authoritative for the service, along
      with parameters associated with each of these endpoints.

   o  Does not assume that all alternative service 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 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
      "") for participating protocols, and generally enables
      delegation of operational authority for an origin within the DNS
      to an alternate name.

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

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

   o  Obtain the [ESNI] keys associated with an alternative service

   o  Support non-default TCP and UDP ports

   o  Address a set of long-standing issues due to HTTP(S) clients not
      implementing support for SRV records, as well as due to a
      limitation that a DNS name can not have both CNAME and NS RRs (as
      is the case for zone apex names)

   o  Provide an HSTS-like indication signaling for the duration of the
      DNS RR TTL that the HTTPS scheme should be used instead of HTTP
      (see Section 7.4).

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1.6.  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
   HTTPSSVC RR which shares the same encoding, format, and high-level

   The SVCB RR has two forms: AliasForm, which aliases a name to another
   name, and ServiceForm, 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

   1.  SvcFieldPriority: The priority of this record (relative to
       others, with lower values preferred).  A value of 0 indicates
       AliasForm.  (Described in Section 2.6.2.)

   2.  SvcDomainName: The domain name of either the alias target (for
       AliasForm) or the alternative service endpoint (for ServiceForm).

   3.  SvcFieldValue (optional): A list of key=value pairs describing
       the alternative service endpoint for the domain name specified in
       SvcDomainName (only used in ServiceForm and otherwise ignored).
       Described in Section 2.1.1.

   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 must 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 may attach in-bailiwick SVCB,
   A, AAAA, and CNAME records in the Additional Section to responses for
   an SVCB query.

   When in the ServiceForm, the SvcFieldValue of the SVCB RR provides an
   extensible data model for describing network endpoints that are
   authoritative for the origin, along with parameters associated with
   each of these endpoints.

   For the HTTPS use-case, the HTTPSSVC RR enables many of the benefits
   of [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.

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1.7.  Parameter for ESNI

   This document also defines a parameter for Encrypted SNI [ESNI] keys.
   See Section 8.

1.8.  Terminology

   For consistency with [AltSvc], we adopt the following definitions:

   o  An "origin" is an information source as in [RFC6454].  For
      services other than HTTPS, the exact definition will need to be
      provided by the document mapping that service onto the SVCB RR.

   o  The "origin server" is the server that the client would reach when
      accessing the origin in the absence of the SVCB record or an HTTPS

   o  An "alternative service" is a different server that can serve the
      origin over a specified protocol.

   For example within HTTPS, the origin consists of a scheme (typically
   "https"), a host name, and a port (typically "443").

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

   SVCB is a contraction of "service binding".  HTTPSSVC is a
   contraction of "HTTPS service".  SVCB, HTTPSSVC, 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",
   "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 ???) is used to
   locate endpoints that can service an origin.  There is special
   handling for the case of "https" origins.

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

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2.1.  Presentation format

   The presentation format of the record is:

   Name TTL IN SVCB SvcFieldPriority SvcDomainName SvcFieldValue

   The SVCB record is defined specifically within the Internet ("IN")
   Class ([RFC1035]).  SvcFieldPriority is a number in the range
   0-65535, SvcDomainName is a domain name, and SvcFieldValue is a set
   of key=value pairs present for the ServiceForm.  Each key SHALL
   appear at most once in a SvcFieldValue.  The SvcFieldValue is empty
   for the AliasForm.

2.1.1.  Presentation format for SvcFieldValue key=value pairs

   In ServiceForm, the SvcFieldValue consists of zero or more elements
   separated by whitespace.  Each element represents a key=value pair.

   Keys are IANA-registered SvcParamKeys (Section 11.1) with both a
   case-insensitive string representation and a numeric representation
   in the range 0-65535.  Registered key names should only contain
   characters from the ranges "a"-"z", "0"-"9", and "-".  In ABNF

   ALPHA_LC    = %x61-7A   ;  a-z
   key         = ALPHA_LC / DIGIT / "-"
   display-key = ALPHA / DIGIT / "-"

   Values are in a format specific to the SvcParamKey.  Their definition
   should specify both their presentation format and wire encoding
   (e.g., domain names, binary data, or numeric values).

   The presentation format for SvcFieldValue is a whitespace-separated
   list of elements representing a key-value pair, with an absent value
   or "=" indicating an empty value.  Each element is presented in the
   following form:

   ; basic-visible is VCHAR minus DQUOTE, ";", and "\"
   basic-visible = %x21 / %x23-3A / %x3C-5B / %x5D-7E
   escaped-char  = "\" (VCHAR / WSP)
   contiguous    = *(basic-visible / escaped-char)
   quoted-string = DQUOTE *(contiguous / WSP) DQUOTE
   value         = quoted-string / contiguous
   pair          = display-key "=" value
   element       = display-key / pair

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   The value format is intended to match the definition of <character-
   string> in [RFC1035] Section 5.1.  (Unlike <character-string>, the
   length of a value is not limited to 255 characters.)

   Unrecognized keys are represented in presentation format as
   "keyNNNNN" where NNNNN is the numeric value of the key type without
   leading zeros.  In presentation format, values of unrecognized keys
   SHALL be represented in wire format, using decimal escape codes (e.g.
   \255) when necessary.

   Elements in presentation format MAY appear in any order.

2.2.  SVCB RDATA Wire Format

   The RDATA for the SVCB RR consists of:

   o  a 2 octet field for SvcFieldPriority as an integer in network byte

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

   o  the SvcFieldValue byte string, consuming the remainder of the
      record (so smaller than 65535 octets and constrained by the RDATA
      and DNS message sizes).

   AliasForm is defined by SvcFieldPriority being 0.

   When SvcFieldValue is non-empty (ServiceForm), 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.

   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 the length defined by the previous field.

   SvcParamKeys SHALL appear in increasing numeric order.

   Clients MUST consider an RR malformed if

   o  the parser reaches the end of the RDATA while parsing a

   o  SvcParamKeys are not in strictly increasing numeric order.

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   o  a SvcParamValue for a known SvcParamKey does not have the expected

   Note that the second condition implies that there are no duplicate

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

   TODO: decide if we want special handling for any SvcParamKey ranges?
   For example: range for greasing; experimental range; range-of-
   mandatory-to-use-the-RR vs range of ignore-just-param-if-unknown.

2.3.  SVCB owner names

   When querying the SVCB RR, an origin is typically translated into a
   QNAME by prefixing the port and scheme with "_", then concatenating
   them with the host name, resulting in a domain name like

   Protocol mappings for SVCB MAY remove the port or replace it with
   other protocol-specific information, but MUST retain the scheme in
   the QNAME.  RR types other than SVCB can define additional behavior
   for translating origins to QNAMEs.  See Section 7.1 for the HTTPSSVC

   When a prior CNAME or SVCB record has aliased to an 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, clients MUST continue to validate
   TLS certificate hostnames based on the origin host.

   As an example: 7200 IN SVCB 0  7200  IN SVCB 3 (
       alpn="bar" port="8004" esniconfig="..." )

   would indicate that "foo://" is aliased to the
   service endpoints offered at "" on port number 8004,
   which support 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).)

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2.4.  SvcRecordType

   The SvcRecordType is indicated by the SvcFieldPriority, and defines
   the form of the SVCB RR.  When SvcFieldPriority is 0, the SVCB
   SvcRecordType is defined to be in AliasForm.  Otherwise, the SVCB
   SvcRecordType is defined to be in ServiceForm.

   Within an SVCB RRSet, all RRs should have the same SvcRecordType.  If
   an RRSet contains a record in AliasForm, the client MUST ignore any
   records in the set with ServiceForm.

2.5.  SVCB records: AliasForm

   When SvcRecordType is AliasForm, the SVCB record is to be treated
   similar to a CNAME alias pointing to SvcDomainName.  SVCB RRSets
   SHOULD only have a single resource record in this form.  If multiple
   are present, clients or recursive resolvers SHOULD pick one at

   The AliasForm's primary purpose is to allow aliasing at the zone
   apex, where CNAME is not allowed.  For example, if an operator of wanted to point HTTPS requests to a service
   operating at, they would publish a record such as: 3600 IN SVCB 0

   The SvcDomainName MUST point to a domain name that contains another
   SVCB record, address (AAAA and/or A) records, or both address records
   and a ServiceForm SVCB record.

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

   Due to the risk of loops, clients and recursive resolvers MUST
   implement loop detection.  Chains of consecutive SVCB and CNAME
   records SHOULD be limited to (8?) prior to reaching terminal address

   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.

   Note that SVCB AliasForm RRs do not alias to RR types other than
   address records (AAAA and A), CNAMEs, and ServiceForm SVCB records.

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   For example, an AliasForm SVCB record does not alias to an HTTPSSVC
   record, nor vice-versa.

2.6.  SVCB records: ServiceForm

   When SvcRecordType is the ServiceForm, the combination of
   SvcDomainName and SvcFieldValue parameters within each resource
   record associates an alternative service location with its connection

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

2.6.1.  Special handling of "." for SvcDomainName in ServiceForm

   For ServiceForm SVCB RRs, if SvcDomainName has the value ".", then
   the owner name of this record MUST be used as the effective
   SvcDomainName.  (The SvcDomainName of an SVCB RR in AliasForm MUST
   NOT have this value.)

   For example, in the following example "" is the
   effective SvcDomainName:  7200  IN HTTPSSVC  7200  IN CNAME 7200  IN HTTPSSVC 1 . port=8002 esniconfig="..." 300   IN A 300   IN AAAA     2001:db8::2

2.6.2.  SvcFieldPriority

   As RRs within an RRSet are explicitly unordered collections, the
   SvcFieldPriority value serves to indicate priority.  SVCB RRs with a
   smaller SvcFieldPriority value SHOULD be given preference over RRs
   with a larger SvcFieldPriority value.

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

3.  Client behavior

   An SVCB-aware client resolves an origin HOST by attempting to
   determine the preferred SvcFieldValue and IP addresses for its
   service, using the following procedure:

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   1.  Issue parallel AAAA/A and SVCB queries for the name HOST.  The
       answers for these may or may not include CNAME pointers before
       reaching one or more of these records.

   2.  If an SVCB record of AliasForm SvcRecordType is returned for
       HOST, clients MUST loop back to step 1 replacing HOST with
       SvcDomainName, subject to loop detection heuristics.

   3.  If one or more SVCB records of ServiceForm SvcRecordType are
       returned for HOST, clients should select the highest-priority
       option with acceptable parameters, and resolve AAAA and/or A
       records for its SvcDomainName if they are not already available.
       These are the preferred SvcFieldValue and IP addresses.  If the
       connection fails, the client MAY try to connect using values from
       a lower-priority record.  If none of the options succeed, the
       client SHOULD connect to the origin server directly.

   4.  If an SVCB record for HOST does not exist, the received AAAA and/
       or A records are the preferred IP addresses and there is no

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

   When selecting between AAAA and A records to use, clients may use an
   approach such as [HappyEyeballsV2].

   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 an SVCB query results in a SERVFAIL error, transport error, or
   timeout, and DNS exchanges between the client and the recursive
   resolver are cryptographically protected (e.g. using TLS [RFC7858] or
   HTTPS [RFC8484]), the client MUST NOT fall back to non-SVCB
   connection establishment.  This ensures that an active attacker
   cannot mount a downgrade attack by denying the user access to the
   SVCB information.

   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.

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   Similarly, if the client enforces DNSSEC validation on A/AAAA
   responses, it MUST NOT fall back to non-SVCB connection establishment
   if the SVCB response fails to validate.

3.2.  Clients using a Proxy

   Clients using a domain-oriented transport proxy like HTTP CONNECT
   ([RFC7231] Section 4.3.6) or SOCKS5 ([RFC1928]) SHOULD disable SVCB
   support if performing SVCB queries would violate the client's privacy

   If the client can safely perform SVCB queries (e.g. via the proxy or
   an affiliated resolver), the client SHOULD follow the standard SVCB
   resolution process, selecting the highest priority option that is
   compatible with the client and the proxy.  The client SHOULD provide
   the final SvcDomainName and port (if present) to the proxy as the
   destination host and port.

   Providing the proxy with the final SvcDomainName has several

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

   o  It allows the origin 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 an SVCB query, authoritative DNS servers SHOULD
   return A, AAAA, and SVCB records (as well as any relevant CNAME or
   [DNAME] records) in the Additional Section for any in-bailiwick

4.2.  Recursive resolvers

   Recursive resolvers that are aware of SVCB SHOULD ensure that the
   client can execute the procedure in Section 3 without issuing a
   second round of queries, by incorporating all the necessary
   information into a single response.  For the initial SVCB record
   query, this is just the normal response construction process (i.e.

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   unknown RR type resolution under [RFC3597]).  For followup
   resolutions performed during this procedure, we define incorporation
   as adding all Answer and Additional RRs to the Additional section,
   and all Authority RRs to the Authority section, without altering the
   response code.

   Upon receiving an 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.

   2.  If any of the resolved SVCB records are in AliasForm, choose an
       AliasForm record at random, and resolve SVCB, A, and AAAA records
       for its SvcDomainName.

       *  If any SVCB records are resolved, go to step 1, subject to
          loop detection heuristics.

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

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

   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]).

4.3.  General requirements

   All DNS servers SHOULD treat the SvcParam portion of the SVCB RR as
   opaque and SHOULD NOT try to alter their behavior based on its

   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).

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5.  Performance optimizations

   For optimal performance (i.e. minimum connection setup time), clients
   SHOULD issue address (AAAA and/or A) and SVCB queries simultaneously,
   and SHOULD implement a client-side DNS cache.  Responses in the
   Additional section of an SVCB response SHOULD be placed in cache
   before performing any followup queries.  With these optimizations in
   place, and conforming DNS servers, using SVCB does not add network
   latency to connection setup.

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 "esniconfig" value of an 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].

   An SVCB record is consistent with a connection if the client would
   attempt an equivalent connection when making use of that record.  If
   an 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: 300 IN SVCB 1 (
       esniconfig="111..." ipv6hint=2001:db8::1 port=1234 ... )
                                  SVCB 2 (
       esniconfig="222..." ipv6hint=2001:db8::2 port=1234 ... )

   If the client has an in-progress TCP connection to
   "[2001:db8::2]:1234", it MAY proceed with TLS on that connection
   using "esniconfig="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 must proceed as described in Step 3 of
   the procedure 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 might be appropriate when the maximum
   response size is reached, or when responding before fully chasing

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   dependencies would improve performance.  When omitting certain
   RRSets, recursive resolvers SHOULD prioritize information from higher
   priority ServiceForm records over lower priority ServiceForm records.

   As discussed in Section 3, clients MUST be able fetch additional
   information that is required to use an 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.

5.3.  Structuring zones for performance

   To avoid a delay for clients using a nonconforming recursive
   resolver, domain owners SHOULD use a single SVCB record whose
   SvcDomainName is "." if possible.  This will ensure that the required
   address records are already present in the client's DNS cache as part
   of the responses to the address queries that were issued in parallel.

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.

6.1.  "alpn" and "no-default-alpn"

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

   As with [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).

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

   alpn-id = 1*255(OCTET)

   The presentation value of "alpn" is a comma-separated list of one or
   more "alpn-id"s.  Any commas present in the protocol-id are escaped
   by a backslash:

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   escaped-octet = %x00-2b / "\," / %x2d-5b / "\\" / %5d-%FF
   escaped-id = 1*255(escaped-octet)
   alpn-value = escaped-id *("," escaped-id)

   In the wire format for "alpn", each ALPN identifier ("alpn-id") is
   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

   For "no-default-alpn", the presentation and wire format values MUST
   be empty.

   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 "ALPN set"), the client
   parses the set of ALPN identifiers in the "alpn" parameter, and then
   adds the default set unless the "no-default-alpn" SvcParamKey is
   present.  The presence of a value in the alpn set indicates that this
   service endpoint, described by SvcDomainName and the other parameters
   (e.g. "port") offers service with the protocol suite associated with
   the ALPN ID.

   ALPN IDs that do not uniquely identify a protocol suite (e.g. an ID
   that can be used with both TLS and DTLS) are not compatible with this
   SvcParamKey and MUST NOT be included in the ALPN set.

   Clients SHOULD NOT attempt connection to a service endpoint whose
   ALPN set does not contain any compatible protocol suites.  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.

   Clients MUST include an "application_layer_protocol_negotiation"
   extension in their ClientHello with a ProtocolNameList that includes
   at least one ID from the ALPN set.  Clients SHOULD also include any
   other values that they support and could negotiate on that connection
   with equivalent or better security properties.  For example, if the
   ALPN set only contains "http/1.1", the client could include
   "http/1.1" and "h2" in the ProtocolNameList.

   Once the client has formulated the ClientHello, protocol negotiation
   on that connection proceeds as specified in [ALPN], without regard to

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   the SVCB ALPN set.  To preserve the security guarantees of this
   process, clients MUST consolidate all compatible ALPN IDs into a
   single ProtocolNameList.

6.2.  "port"

   The "port" SvcParamKey defines the TCP or UDP port that should be
   used to contact this alternative service.

   The presentation format of the SvcParamValue is a numeric value
   between 0 and 65535 inclusive.  Any other values (e.g. the empty
   value) are syntax errors.

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

6.3.  "esniconfig"

   The SvcParamKey for ESNI is "esniconfig".  Its value is defined in
   Section 8.  It is applicable to most TLS-based protocols.

   When publishing a record containing an "esniconfig" parameter, the
   publisher MUST ensure that all IP addresses of SvcDomainName
   correspond to servers that have access to the corresponding private
   key or are authoritative for the fallback domain.  (See [ESNI] for
   more details about the fallback domain.)  This yields an anonymity
   set of cardinality equal to the number of ESNI-enabled server domains
   supported by a given client-facing server.  Thus, even with SNI
   encryption, an attacker who can enumerate the set of ESNI-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 ESNI-
   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 represent IP address hints for the
   service.  If A and AAAA records for SvcDomainName are locally
   available, the client SHOULD ignore these hints.  Otherwise, clients
   SHOULD perform A and/or AAAA queries for SvcDomainName 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 SvcDomainName.  Failure to use A and/or AAAA response
   addresses may negatively impact load balancing or other geo-aware
   features and thereby degrade client performance.

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   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 [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 an SVCB record.  For best
   performance, server operators SHOULD include an "ipv6hint" parameter
   whenever they include an "ipv4hint" parameter.

   The presentation format for each parameter is a comma-separated list
   of IP addresses in standard textual format [RFC5952].

   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 SvcDomainName is ".",
   server operators SHOULD NOT include these hints, because they are
   unlikely to convey any performance benefit.

7.  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

   To enable special handling for the HTTPS and HTTP use-cases, the
   HTTPSSVC RR type is defined as an 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 HTTPSSVC wire format and presentation format are identical to
   SVCB, and both share the SvcParamKey registry.  SVCB semantics apply
   equally to HTTPSSVC unless specified otherwise.

   All the SvcParamKeys defined in Section 6 are permitted for use in
   HTTPSSVC.  The default set of ALPN IDs is the single value

   The presence of an HTTPSSVC record for an origin also indicates that
   all HTTP resources are available over HTTPS, as discussed in
   Section 7.4.  This allows HTTPSSVC RRs to apply to pre-existing
   "http" scheme URLs, while ensuring that the client uses a secure and
   authenticated HTTPS connection.

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   The HTTPSSVC RR parallels the concepts introduced in the HTTP
   Alternative Services proposed standard [AltSvc].  Clients and servers
   that implement HTTPSSVC are NOT REQUIRED to implement Alt-Svc.

7.1.  Owner names for HTTPSSVC records

   The HTTPSSVC RR extends the behavior for determining a QNAME
   specified above in Section 2.3.  In particular, if the scheme is
   "https" with port 443 then the client's original QNAME is equal to
   the origin host name.

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

   For HTTPS origins with ports other than 443, the port and scheme
   continue to be prefixed to the hostname as described in Section 2.3.
   Following of HTTPSSVC AliasForm and CNAME aliases is also unchanged
   from SVCB.

   Clients always convert "http" URLS to "https" before performing an
   HTTPSSVC query using the process described in Section 7.4, so domain
   owners MUST NOT publish HTTPSSVC records 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 host.

7.2.  Relationship to Alt-Svc

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

7.2.1.  ALPN usage

   Unlike Alt-Svc Field Values, HTTPSSVC records can contain multiple
   ALPN IDs, and clients are encouraged to offer additional ALPNs that
   they support (subject to security constraints).

   TO BE REMOVED: The ALPN semantics in [AltSvc] are ambiguous, and
   problematic in some interpretations.  We should update [AltSvc] to
   give it well-defined semantics that match HTTPSSVC.

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7.2.2.  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

7.2.3.  TTL and granularity

   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 will typically be similar to the "ma" value
   used for Alt-Svc, but may vary 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
   HTTPSSVC records 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

   Sending Alt-Svc over HTTP allows the server to tailor the Alt-Svc
   Field Value specifically to the client.  When using an HTTPSSVC DNS
   record, groups of clients will necessarily receive the same
   SvcFieldValue.  Therefore, HTTPSSVC is not suitable for uses that
   require single-client granularity.

7.3.  Interaction with Alt-Svc

   Clients that do not implement support for ESNI MAY skip the HTTPSSVC
   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
   HTTPSSVC client connection procedure (Section 3).

   For clients that implement support for ESNI, the interaction between
   HTTPSSVC and Alt-Svc is described in Section 8.1.

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

7.4.  HTTP Strict Transport Security

   By publishing an HTTPSSVC record, the server operator indicates that
   all useful HTTP resources on that origin are reachable over HTTPS,
   similar to HTTP Strict Transport Security [HSTS].  When an HTTPSSVC
   record is present for an origin, all "http" scheme requests for that
   origin SHOULD logically be redirected to "https".

   Prior to making an "http" scheme request, the client SHOULD perform a
   lookup to determine if an HTTPSSVC record is available 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 HTTPSSVC record is present for this "https" URL, the client
   should treat this as the equivalent of receiving an HTTP "307
   Temporary Redirect" redirect to the "https" URL.  Because HTTPSSVC is
   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 making an "https" scheme request to an origin with an HTTPSSVC
   record, either directly or via the above redirect, the client SHOULD
   terminate the connection if there are any errors with the underlying
   secure transport, such as errors in certificate validation.  This
   aligns with Section 8.4 and Section 12.1 of [HSTS].

8.  SVCB/HTTPSSVC parameter for ESNI keys

   The SVCB "esniconfig" parameter is defined for conveying the ESNI
   configuration of an alternative service.  The value of the parameter
   is an ESNIConfig structure [ESNI].  In presentation format, the
   structure is encoded in [base64].  The SVCB SvcParamValue wire format
   is the octet string containing the binary ESNIConfig structure.

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8.1.  Client behavior

   The general client behavior specified in Section 3 permits clients to
   retry connection with a less preferred alternative if the preferred
   option fails, including falling back to a direct connection if all
   SVCB options fail.  This behavior is not suitable for ESNI, because
   fallback would negate the privacy benefits of ESNI.  Accordingly,
   ESNI-capable clients SHALL implement the following behavior for
   connection establishment.

   1.  Perform connection establishment using HTTPSSVC as described in
       Section 3, but do not fall back to the origin's A/AAAA records.
       If all the HTTPSSVC RRs have esniconfig, and they all fail,
       terminate connection establishment.

   2.  If the client implements Alt-Svc, try to connect using any
       entries from the Alt-Svc cache.

   3.  Fall back to the origin's A/AAAA records if necessary.

   As a latency optimization, clients MAY prefetch DNS records for later
   steps before they are needed.

8.2.  Deployment considerations

   An HTTPSSVC RRSet containing some RRs with esniconfig 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 esniconfig as more preferred (i.e. smaller
   SvcFieldPriority) than those without, in order to maximize the
   likelihood that ESNI will be used in the absence of an active

9.  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
   [RFC7858] or DNS over HTTPS [RFC8484]).  However, performance
   improvements, and some modest privacy improvements, are possible
   without the use of those standards.

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   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.

10.  Security Considerations

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

   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.

11.  IANA Considerations

11.1.  New registry for Service Parameters

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

   ACTION: create and include a reference to this registry.

11.1.1.  Procedure

   A registration MUST include the following fields:

   o  Name: Service parameter key name

   o  SvcParamKey: Service parameter key numeric identifier (range

   o  Meaning: a short description

   o  Pointer to specification text

   Values to be added to this name space require Expert Review (see
   [RFC5226], Section 4.1).  Apart from the initial contents, the name
   MUST NOT start with "key".

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11.1.2.  Initial contents

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

   | SvcParamKey | NAME            | Meaning              | Reference  |
   | 0           | (no name)       | Reserved for         | (This      |
   |             |                 | internal use         | document)  |
   |             |                 |                      |            |
   | 1           | alpn            | Additional supported | (This      |
   |             |                 | protocols            | document)  |
   |             |                 |                      |            |
   | 2           | no-default-alpn | No support for       | (This      |
   |             |                 | default protocol     | document)  |
   |             |                 |                      |            |
   | 3           | port            | Port for alternative | (This      |
   |             |                 | service              | document)  |
   |             |                 |                      |            |
   | 4           | ipv4hint        | IPv4 address hints   | (This      |
   |             |                 |                      | document)  |
   |             |                 |                      |            |
   | 5           | esniconfig      | Encrypted SNI        | (This      |
   |             |                 | configuration        | document)  |
   |             |                 |                      |            |
   | 6           | ipv6hint        | IPv6 address hints   | (This      |
   |             |                 |                      | document)  |
   |             |                 |                      |            |
   | 65280-65534 | keyNNNNN        | Private Use          | (This      |
   |             |                 |                      | document)  |
   |             |                 |                      |            |
   | 65535       | key65535        | Reserved             | (This      |
   |             |                 |                      | document)  |

   TODO: do we also want to reserve a range for greasing?

11.2.  Registry updates

   Per [RFC6895], please add the following entry 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)     |
   |          |                                        |               |
   | HTTPSSVC | 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       |
       | HTTPSSVC | _https     | HTTPS SVCB info | (This document) |

12.  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.draft-ietf-dnsop-aname-03], 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, and others for their feedback and suggestions on this

13.  References

13.1.  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, <>.

              Crocker, D., "DNS Scoped Data Through "Underscore" Naming
              of Attribute Leaves", draft-ietf-dnsop-attrleaf-16 (work
              in progress), November 2018.

   [base64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,

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   [DNAME]    Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,

   [ESNI]     Rescorla, E., Oku, K., Sullivan, N., and C. Wood,
              "Encrypted Server Name Indication for TLS 1.3", draft-
              ietf-tls-esni-05 (work in progress), November 2019.

              Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,

   [HSTS]     Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", draft-ietf-quic-http-20 (work in progress),
              April 2019.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [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,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC",
              RFC 3225, DOI 10.17487/RFC3225, December 2001,

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <>.

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   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [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,

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,

   [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,

   [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,

   [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,

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   [RFC7858]  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, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

13.2.  Informative References

   [AltSvc]   Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <>.

   [DNSTerm]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <>.

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

              Finch, T., Hunt, E., Dijk, P., Eden, A., and W. Mekking,
              "Address-specific DNS aliases (ANAME)", draft-ietf-dnsop-
              aname-03 (work in progress), April 2019.

   [RFC2782]  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,

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <>.

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13.3.  URIs


Appendix A.  Comparison with alternatives

   The SVCB and HTTPSSVC record 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.

A.1.  Differences from the SRV RR type

   An SRV record [RFC2782] 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

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

A.2.  Differences from the proposed HTTP record

   Unlike [I-D.draft-bellis-dnsop-http-record-00], this approach is
   extensible to cover Alt-Svc and ESNI 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.

A.3.  Differences from the proposed ANAME record

   Unlike [I-D.draft-ietf-dnsop-aname-03], this approach is extensible
   to cover Alt-Svc and ESNI use-cases.  This approach also does not
   require any changes or special handling on either authoritative or
   master servers, beyond optionally returning in-bailiwick additional

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

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   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.

A.4.  Differences from the proposed ESNI record

   Unlike [ESNI], this approach is extensible and covers the Alt-Svc
   case as well as addresses the zone apex CNAME challenge.

   By mirroring the Alt-Svc model we also provide a way to solve the
   ESNI multi-CDN challenges in a general case.

   Unlike ESNI, SVCB allows specifying different ESNI configurations for
   different protocols and ports, rather than applying a single
   configuration to all ports on a domain.

Appendix B.  Design Considerations and Open Issues

   This draft is intended to be a work-in-progress for discussion.  Many
   details are expected to change with subsequent refinement.  Some
   known issues or topics for discussion are listed below.

B.1.  Record Name

   Naming is hard.  "SVCB" and "HTTPSSVC" are proposed as placeholders
   that are easy to search for and replace when a final name is chosen.
   Other names for this record might include B, ALTSVC, HTTPS, HTTPSSRV,
   HTTPSSVC, SVCHTTPS, or something else.

B.2.  Generality

   The SVCB record was designed as a generalization of HTTPSSVC, based
   on feedback requesting a solution that applied to protocols pther
   than HTTP.  Past efforts to over-generalize have not met with broad
   success, but we hope that HTTPSSVC and SVCB have struck an acceptable
   balance between generality and focus.

B.3.  Wire Format

   Advice from experts in DNS wire format best practices would be
   greatly appreciated to refine the proposed details, overall.

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B.4.  Whether to include Weight

   Some other similar mechanisms such as SRV have a weight in-addition
   to priority.  That is excluded here for simplicity.  It could always
   be added as an optional SVCB parameter.

Appendix C.  Change history

   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.

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

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

      *  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

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      *  Replace "hq" with "h3".

   o  draft-nygren-httpbis-httpssvc-01

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

   o  draft-nygren-httpbis-httpssvc-00

      *  Initial version

Authors' Addresses

   Ben Schwartz


   Mike Bishop
   Akamai Technologies


   Erik Nygren
   Akamai Technologies


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