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<rfc category="std" submissionType="IETF" docName="draft-ietf-dnssd-srp-25" number="9665" ipr="trust200902" xmlns:xi="http://www.w3.org/2001/XInclude" obsoletes="" updates="" version="3"
     scripts="Common,Latin" sortRefs="false" sortRefs="true" consensus="true" symRefs="true" tocDepth="4" tocInclude="true" xml:lang="en">
  <front>
    <title abbrev='Service abbrev="Service Registration Protocol'>Service Protocol">Service Registration Protocol for DNS-Based Service Discovery</title>
    <seriesInfo name="RFC" value="9665"/>
    <author initials="T" initials="T." surname="Lemon" fullname="Ted Lemon">
      <organization>Apple Inc.</organization>
      <address>
	<postal>
          <street>One Apple Park Way</street>
          <city>Cupertino</city>
          <region>California</region>
          <region>CA</region>
          <code>95014</code>
          <country>USA</country>
          <country>United States of America</country>
        </postal>
        <email>mellon@fugue.com</email>
      </address>
    </author>

    <author initials='S' surname='Cheshire' fullname='Stuart Cheshire'> initials="S." surname="Cheshire" fullname="Stuart Cheshire">
      <organization>Apple Inc.</organization>
      <address>
        <postal>
          <street>One Apple Park Way</street>
          <city>Cupertino</city>
          <region>California</region>
          <region>CA</region>
          <code>95014</code>
          <country>USA</country>
          <country>United States of America</country>
        </postal>
        <phone>+1 408 974 3207</phone>
        <email>cheshire@apple.com</email>
      </address>
    </author>

    <date>March 4, 2024</date>
    <area>Internet</area>
    <workgroup>Internet Engineering Task Force</workgroup>

    <date month="October" year="2024"/>
    <area>INT</area>
    <workgroup>dnssd</workgroup>
    <keyword>Multicast DNS</keyword>
    <keyword>DNS-Based Service Discovery</keyword>
    <keyword>DNS Update</keyword>
    <keyword>SIG(0)</keyword>

    <abstract>
      <t>

        The
      <t>The Service Registration Protocol (SRP) for DNS-Based DNS-based Service Discovery (DNS-SD)
        uses the standard DNS Update mechanism to enable DNS-Based
        Service Discovery DNS-SD using only unicast packets.  This makes it possible to
        deploy DNS Service Discovery DNS-SD without multicast, which greatly improves
        scalability and improves performance on networks where multicast
        service is not an optimal choice, particularly IEEE 802.11 (Wi&nbhy;Fi)
        (Wi-Fi) and IEEE 802.15.4 networks.  DNS&nbhy;SD  DNS-SD Service
        registration uses public keys and SIG(0) to allow services to defend
        their registrations.
      </t>
    </abstract>
    <note removeInRFC="true">
      <name>About This Document</name>
      <t>
        The latest revision of this draft can be found at <eref target="https://dnssd-wg.github.io/draft-ietf-dnssd-srp/draft-ietf-dnssd-srp.html"/>.
        Status information for this document may be found at <eref target="https://datatracker.ietf.org/doc/draft-ietf-dnssd-srp/"/>.
      </t>
      <t>
        Discussion of this document takes place on the
        DNS-SD Working Group mailing list (<eref target="mailto:dnssd@ietf.org"/>),
        which is archived at <eref target="https://mailarchive.ietf.org/arch/browse/dnssd/"/>.
        Subscribe at <eref target="https://www.ietf.org/mailman/listinfo/dnssd/"/>.
      </t>
      <t>Source for this draft and an issue tracker can be found at
        <eref target="https://github.com/dnssd-wg/draft-ietf-dnssd-srp"/>.</t>
    </note>
  </front>

  <middle>

    <section>
      <name>Introduction</name>
      <t>
        DNS-SD (see <xref target="RFC6763">DNS-Based Service Discovery</xref> target="RFC6763"></xref>) is a component of Zero Configuration Networking
        (see <xref target="RFC6760"/> target="RFC6760"/>, <xref target="ZC"/> target="ZC"/>, and <xref target="I-D.cheshire-dnssd-roadmap"/>.</t> target="I-D.cheshire-dnssd-roadmap"/>).</t>
      <t>
        This document describes an enhancement to <xref target="RFC6763">DNS-Based Service Discovery</xref> (DNS&nbhy;SD) DNS-SD that
	allows servers to register the services they offer using the DNS protocol rather than using Multicast DNS (mDNS) (see <xref target="RFC6762">Multicast
	DNS</xref> (mDNS). target="RFC6762"></xref>).  There is already a large installed base of DNS&nbhy;SD clients that can discover services using the DNS
	protocol (e.g. (e.g., Android, Windows, Linux, Apple).</t>
      <t>
        This document is intended for three audiences: implementors of software that provides services that should be advertised
        using DNS&nbhy;SD, implementors of DNS servers that will be used in contexts where DNS&nbhy;SD registration is needed, and
        administrators of networks where DNS&nbhy;SD service is required.  The document is expected to provide sufficient
        information to allow interoperable implementation of the registration protocol.</t>
	<t>

<!--[rfced] Should "services" be "servers" here to match previous,
     similar text?  And perhaps avoiding the two "provide" uses so
     close together would be helpful for the reader?

Original:
   DNS-Based Service Discovery (DNS&nbhy;SD) (DNS-SD) allows services to advertise
   the fact that they provide service, and to provide the
   information required to access that service.

Perhaps:
   DNS-SD allows servers to advertise the fact that they provide
   service and to share the information required to access that
   service.

-->

        DNS&nbhy;SD allows services to advertise the fact that they provide service and to provide
        the information required to access that service.  DNS&nbhy;SD clients can then discover the set of services of a particular
        type that are available.  They can then select a service from among those that are available and obtain the information
        required to use it.  Although DNS Service Discovery (DNS-SD) DNS-SD using the DNS protocol (as opposed to mDNS) can be more efficient and versatile, it is
        not common in practice, practice because of the difficulties associated with updating authoritative DNS services with service
        information.</t>

      <t>
	Existing
	The existing practice for updating DNS zones is to either to manually enter new data, data or else to use a DNS Update
      (see <xref target="RFC2136"/>. Unfortunately target="RFC2136"/>). Unfortunately, a DNS Update requires either that either:</t>
      <ul>
	<li>that the authoritative DNS server automatically trust
	updates, or else that
	updates or</li>
	<li>that the DNS Update requestor have some kind of shared secret or public key that is known to the DNS server
	and can be used to authenticate the update.  Furthermore, update.</li></ul>
	<t>Furthermore, the DNS Update can be a fairly chatty process, requiring multiple
	round trips
	roundtrips with different conditional predicates to complete the update process.</t>

      <t>
	The Service Registration Protocol (SRP) adds a set of default heuristics for processing DNS updates that eliminates the need for DNS update
	conditional predicates: instead, DNS-update-conditional predicates. Instead, the SRP registrar (a DNS server that supports SRP updates) has a set of default predicates
	that are applied to the update, update; and the update either succeeds entirely, entirely or fails in a way that allows the requestor to know
	what went wrong and construct a new update.</t>

      <t>
	SRP also adds a feature called First-Come, First-Served (FCFS) Naming, "First Come, First Served Naming" (or "FCFS Naming"), which allows the requestor to claim to:</t>
	<ul><li>claim a name that is
	not yet in use, and, using and</li>
	<li>using SIG(0) <xref target="RFC2931"/>, to (<xref target="RFC2931"/>), authenticate both the initial claim and subsequent
	updates. This
	updates.</li></ul>
	<t>This prevents name conflicts, since a second SRP requestor attempting to claim the same name will not possess the
	SIG(0) key used by the first requestor to claim it, and it: so its claim will be rejected rejected, and the second requestor will have to
	choose a new name.</t>

      <t>
	It is important to understand that "authenticate" here just means that we can tell that an update came from the same source
	as the original registration. We have not established trust. This has important implications for what we can and can't do
	with data the client sends us. You will notice as you read this document that we only support adding a very restricted set
	of records, and the content of those records is further constrained.</t>

      <t>
	The reason for this is precisely that we have not established trust. So So, we can only publish information that we feel safe in
	publishing even though we do not have any basis for trusting the requestor. We reason that mDNS <xref target="RFC6762"/> (<xref target="RFC6762"/>)
	allows arbitrary hosts on a single IP link to advertise services <xref target="RFC6763"/>, (<xref target="RFC6763"/>), relying on whatever service is
	advertised to provide authentication as a part of its protocol rather than in the service advertisement.</t>

      <t>
	This is considered reasonably safe because it requires physical presence on the network in order to advertise. An off-network
	mDNS attack is simply not possible. Our goal with this specification is to impose similar constraints. Because of this Therefore, you will
	see in <xref target="add_validation"/> that a very restricted set of records with a very restricted set of relationships are
	allowed. You will also see in <xref target="source_validation"/> that we give advice on how to prevent off-network attacks.</t>

      <t>
	This leads us to the disappointing observation that this protocol is not a mechanism for adding arbitrary information to
	DNS zones. We have not evaluated the security properties of adding, for example, an SOA record, an MX record, or a CNAME
	record, and so
	record; therefore, these are forbidden. A future protocol specification might include analyses for other records, records and extend
	the set of records that can be registered here. Or it might require establishment of trust, and add an authorization model
	to the authentication model we now have. But this is work for a future document.</t>

      <t>
	Finally, SRP adds the concept of a 'lease,' "lease", similar to leases in Dynamic Host Configuration Protocol
	<xref target="RFC8415"/>. DHCP
	(<xref target="RFC8415"/>).  The SRP registration itself has a lease which that may be on the order of an hour; if the requestor
	does not renew the lease before it has elapsed, the registration is removed.  The claim on the name can have a longer
	lease,
	lease so that another requestor cannot claim the name, even though the registration has expired.</t>

      <t>
        The Service Registration Protocol SRP for DNS&nbhy;SD (SRP), DNS-SD specified in this document, document provides a reasonably secure mechanism
        for publishing this information.  Once published, these services can be readily discovered by DNS&nbhy;SD DNS-SD clients using
        standard DNS lookups.</t>
      <t>
        The DNS&nbhy;SD DNS-SD specification (<xref (see <xref target="RFC6763" section="10" sectionFormat="comma"/>, “Populating the DNS with
        Information”), sectionFormat="of"/> briefly discusses ways that servers can publish their information in the DNS namespace.  In the case of
        mDNS, it allows servers to publish their information on the local link, using names in the ".local" namespace, which makes
        their services directly discoverable by peers attached to that same local link.</t>
      <t>
        RFC6763
        RFC 6763 also allows clients to discover services using <xref target="RFC1035">the the DNS protocol</xref>. protocol (see <xref target="RFC1035"></xref>).  This can be done by
        having a system administrator manually configure service information in the DNS, but DNS; however, manually populating DNS authoritative
        server databases is costly and potentially error-prone, error-prone and requires a knowledgeable network administrator.  Consequently,
        although all DNS&nbhy;SD DNS-SD client implementations of which we are aware support DNS&nbhy;SD DNS-SD using DNS queries, in practice practice, it
        is used much less frequently than mDNS.</t>
      <t>
        The Discovery Proxy (see <xref target="RFC8766">Discovery Proxy</xref> target="RFC8766"></xref>) provides one way to automatically populate the DNS
        namespace,
        namespace but is only appropriate on networks where services are easily advertised using mDNS.  This  The present document describes a
        solution more suitable for networks where multicast is inefficient, inefficient or where sleepy devices are common, common by supporting both the
        offering of services, services and the discovery of services, services using unicast.</t>
    </section>

    <section>
      <name>Conventions and Terminology Used in This Document</name>
        <t>
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
	"MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
    NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
    described in BCP 14 BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
    when, and only when, they appear in all capitals, as shown here.
        </t>

    </section>

    <section>
      <name>Service Registration Protocol</name>
      <t>
        Services that implement SRP use DNS Update (see <xref target="RFC2136"/> and <xref target="RFC3007"/> target="RFC3007"/>) to publish service information
        in the DNS.  Two variants exist, exist: one for full-featured hosts, hosts and one for devices designed for "Constrained-Node Networks"
        <xref target="RFC7228"/>. Constrained-Node Networks (CNNs)
        (<xref target="RFC7228"/>). An SRP registrar is most likely an authoritative DNS server, server or else is updating an authoritative
	DNS server. There is no requirement that the server that is receiving SRP updates be the same server that is answering
	queries that return records that have been registered.</t>
      <section>
	<name>Protocol Variants</name>
	<section>
	  <name>Full-featured
	  <name>Full-Featured Hosts</name>
	  <t>
            Full-featured hosts either are configured manually with a registration domain, domain or discover the default registration
	    domain as described in <xref target="RFC6763" section="11" sectionFormat="of"/>.  If this process does not produce a
	    default registration domain, the Service Registration protocol SRP is not discoverable on the local network using this
	    mechanism. Other discovery mechanisms are possible, but they are out of scope for this document.</t>
	  <t>
            Manual configuration of the registration domain can be done either by either:</t>
	    <ul><li>by querying the list of available registration
            domains ("r._dns&nbhy;sd._udp") and allowing the user to select one from the UI, or by UI or</li>
	    <li>by any other means appropriate to
            the particular use case being addressed.  Full-featured addressed.</li></ul>
	    <t>Full-featured devices construct the names of the SRV, TXT, and PTR records
            describing their service(s) service or services as subdomains of the chosen service registration domain.  For these names names, they then discover
            the zone apex of the closest enclosing DNS zone using SOA queries (see <xref target="RFC8765" section="6.1"/>. section="6.1"/>).  Having
            discovered the enclosing DNS zone, they query for the "_dnssd&nbhy;srp._tcp.&lt;zone&gt;" SRV record to discover the
            server to which they can send SRP updates.  Hosts that support SRP Updates using TLS use the
            "_dnssd&nbhy;srp&nbhy;tls._tcp.&lt;zone&gt;" SRV record instead.</t>
	  <t>
	    Examples of full-featured hosts include devices such as home computers, laptops, powered peripherals with network
	    connections such (such as printers, home routers, and even battery-operated devices such as mobile phones that have
	    long battery lives. lives).
	  </t>
	</section>
	<section>
	  <name>Constrained Hosts</name>
	  <t>
            For devices designed for Constrained-Node Networks <xref target="RFC7228"/> CNNs (<xref target="RFC7228"/>), some simplifications are available.  Instead of
            being configured with (or discovering) the service registration domain, the special-use domain name (see
            <xref target="RFC6761"/>) "default.service.arpa" is used.  The details of how SRP registrar(s) registrars are discovered will be specific
            to the constrained network, and therefore network; therefore, we do not suggest a specific mechanism here.</t>
	  <t>
            SRP requestors on constrained networks are expected to receive receive, from the network network, a list of SRP registrars with which to register.
            It is the responsibility of a Constrained-Node Network CNN supporting SRP to provide one or more registrar addresses.  It is
            the responsibility of the registrar supporting a Constrained-Node Network CNN to handle the updates appropriately.  In some
            network environments, updates may be accepted directly into a local "default.service.arpa" zone, which has only local
            visibility.  In other network environments, updates for names ending in "default.service.arpa" may be rewritten by the registrar
            to names with broader visibility.</t>
	</section>
	<section>
	  <name>Why two variants?</name>
	  <t>
            The reason for these different variants is that low-power devices that typically use Constrained-Node Networks CNNs may have
            very limited battery storage.  The series of DNS lookups required to discover an SRP registrar and then communicate with
            it will increase the energy required to advertise a service; for low-power devices, the additional flexibility this
            provides does not justify the additional use of energy.  It is also fairly typical of such networks that some network
            service information is obtained as part of the process of joining the network, and so network; thus, this can be relied upon to provide
            nodes with the information they need.</t>
	  <t>
            Networks that are not constrained networks can have more complicated topologies at the IP layer. Nodes connected
            to such networks can be assumed to be able to do DNS-SD service registration domain discovery. Such networks are
            generally able to provide registration domain discovery and routing.  This creates the possibility of off-network
	    spoofing, where a device from a foreign network registers a service on the local network in order to attack devices
	    on the local network. To prevent such spoofing, TCP is required for such networks.
	  </t>
	</section>
      </section>
      <section>
	<name>Protocol Details</name>
	<t>
We will discuss several parts to this process: how process:</t>

<ul>
  <li>how to know what to publish, how publish (see <xref target="what"/>),</li>
  <li>how to know where to publish it (under what
          name), how name) (see <xref target="where"/>),</li>

<li>how to publish it, and how it (see <xref target="how"/>),</li>

<li>how to secure its publication. In publication (see <xref target="maintenance"/>, we specify how target="how-to-secure"/>), and</li>

<li>how to maintain
          the information once published.</t>

	<section> published (see <xref target="maintenance"/>).</li></ul>

	<section anchor="what">
	  <name>What to publish</name> Publish</name>
          <t>
            SRP Updates are sent by SRP requestors to SRP registrars.  Three types of instructions appear in an SRP update: Service
	    Discovery instructions, Service Description instructions, and Host Description instructions. These instructions are made
	    up of DNS Update RRs Resource Records (RRs) that are either adds or deletes. The types of records that are added, updated updated, and removed in each
	    of these instructions, as well as the constraints that apply to them, are described in <xref target="server_behavior"/>.
	    An SRP Update is a DNS Update message that is constructed so as to meet the constraints described in that section. The
	    following is a brief overview of what is included in a typical SRP Update:
	  </t>
          <ul spacing="compact"> spacing="normal">

            <li>
              PTR Resource Record (RR) RR for services, which map from a generic service type (or subtype) name to a specific
              Service Instance Name.</li> Name (<xref target="RFC6763" section="4.1" sectionFormat="of"/>).</li>
            <li>
	      For any Service Instance Name (<xref target="RFC6763" section="4.1" sectionFormat="comma"/>), Name, an SRV RR, one or more
	      TXT RRs, and a KEY RR. Although Although, in principle principle, DNS-SD Service Description records can include other record types with
	      the same Service Instance Name, in practice practice, they rarely do. SRP does not permit other record types. The KEY RR is used
	      to support FCFS naming, naming and has no specific meaning for DNS-SD lookups. SRV records for all services described in an
	      SRP update point to the same hostname.</li>
            <li>
	      There is never more than one hostname in a single SRP update. The hostname has one or more address RRs (AAAA or A) and
              a KEY RR (used for FCFS naming). Depending on the use case, an SRP requestor may be required to suppress some
              addresses that would not be usable by hosts discovering the service through the SRP registrar. The exact address
              record suppression behavior required may vary for different types of SRP requestors. An example of such advice can be
              found in <xref target="RFC8766" section="5.5.2" sectionFormat="of"/>.
	    </li>
	  </ul>
          <t>
            <xref target="RFC6763"/> describes the details of what each of these types of RR RRs mean, with the exception of
            the KEY RR, which is defined in <xref target="RFC2539"/>. These RFCs should be considered the definitive source sources for
            information about what to publish; the reason for summarizing this here is to provide the reader with enough information
            about what will be published that the service registration process can be understood at a high level without first
            learning the full details of DNS&nbhy;SD. DNS-SD.  Also, the "Service Instance Name" is an important aspect of FCFS
            naming, which we describe later on in this document.</t>
	</section>

	<section>

	<section anchor="where">
	  <name>Where to publish it</name> Publish It</name>
          <t>
            Multicast DNS (mDNS) uses a single namespace, ".local", which namespace that is valid on the local link. link called ".local".  This convenience is not available for
            DNS&nbhy;SD
            DNS-SD using the DNS protocol: services must exist in some specific DNS namespace that is chosen either by the
	    network operator, operator or automatically.</t>
          <t>
            As described above, full-featured devices are responsible for knowing the domain in which to register their services.
	    Such devices MAY <bcp14>MAY</bcp14> optionally support configuration of a registration domain by the operator of the device. However,
	    such devices MUST <bcp14>MUST</bcp14> support registration domain discovery as described in <xref target="RFC6763" section="11" sectionFormat="of"/>,
	    "Discovery of Browsing and Registration Domains". sectionFormat="of"/>.
	  </t>
	  <t>
            Devices made for Constrained-Node Networks CNNs register in the special use special-use domain name <xref target="RFC6761"/>
            "default.service.arpa", (<xref target="RFC6761"/>)
            "default.service.arpa" and let the SRP registrar handle rewriting that to a different domain if necessary.</t>
	</section>

	<section>

	<section anchor="how">
	  <name>How to publish it</name> Publish It</name>
          <t>
            It is possible to issue a DNS Update that does several things at once; this means once: meaning that it's possible to do all the work of
            adding a PTR resource record RR to the PTR RRset on the Service Name, Name and creating or updating the Service Instance Name and
            Host Description, Description in a single transaction.</t>
          <t>
            An SRP Update takes advantage of this: it is implemented as a single DNS Update message that contains a service's Service
            Discovery records, Service Description records, and Host Description records.</t>
          <t>
            Updates done according to this specification are somewhat different than regular DNS Updates as defined in
	    <xref target="RFC2136"/>.  The <xref target="RFC2136"/> where the update process can could involve many update attempts: you attempts. You might first
	    attempt to add a name if it doesn't exist; if that fails, then in a second message you might update the name if it does
	    exist but matches certain preconditions.  Because the registration protocol described in this document uses a single transaction, some of this
	    adaptability is lost.</t>
          <t>
            In order to allow updates to happen in a single transaction, SRP Updates do not include update prerequisites.  The
            requirements specified in <xref target="server_behavior"/> are implicit in the processing of SRP Updates, and so Updates; thus, there is
            no need for the SRP requestor to put in any explicit prerequisites.</t>

          <section>
	    <name>How the DNS&nbhy;SD DNS-SD Service Registration process differs Process Differs from the DNS Update as specified Specified in RFC2136</name> RFC 2136</name>
            <t>
              DNS&nbhy;SD
              DNS-SD Service Registration is based on the standard RFC2136 DNS Update, Update specified in <xref target="RFC2136"/>, with some differences:</t>
            <ul spacing="compact"> spacing="normal">
              <li>
		It implements first-come first-served FCFS name allocation, protected using SIG(0) <xref target="RFC2931"/>.</li> (<xref target="RFC2931"/>).</li>
              <li>
		It enforces policy about what updates are allowed.</li>
              <li>
		It optionally performs rewriting of "default.service.arpa" to some other domain.</li>
              <li>
		It optionally performs automatic population of the address-to-name reverse mapping domains.</li>
              <li>
		An SRP registrar is not required to implement general DNS Update prerequisite processing.</li>
              <li>
		Constrained-Node SRP requestors are allowed to send updates to the generic domain "default.service.arpa."</li> "default.service.arpa.".</li>
            </ul>
          </section>
	  <section>
	    <name>Retransmission Strategy</name>
	    <t>The DNS protocol, including DNS updates, can operate over UDP or TCP. When using UDP, reliable
	      transmission must be guaranteed by retransmitting if a DNS UDP message is not acknowledged in a
	      reasonable interval. <xref target="RFC1035" section="4.2.1" sectionFormat="of"/> provides some
	      guidance on this topic, as does <xref target="RFC1536" section="1" sectionFormat="of"/>.
	      <xref target="RFC8085" section="3.1.3" sectionFormat="of"/> also provides useful guidance that
	      is particularly relevant to DNS.</t>
	  </section>

	  <section>
	    <name>Successive Updates</name>
	    <t>Service Registration Protocol
	    <t>SRP does not require that every update contain the same information.
	      When an SRP requestor needs to send more than one SRP update to the SRP registrar, it MUST <bcp14>MUST</bcp14> send
	      these sequentially: until an earlier update has been successfully acknowledged, the requestor
	      MUST NOT
	      <bcp14>MUST NOT</bcp14> begin sending a subsequent update.</t>
	  </section>
	</section>

	<section anchor="how-to-secure">
	  <name>How to secure it</name> Secure It</name>
          <t>
            A DNS update update, as described in <xref target="RFC2136"/> target="RFC2136"/>, is secured using Secret Key Transaction Signatures,
            <xref target="RFC8945"/>, which secret key transaction signatures
            (<xref target="RFC8945"/>) that uses a secret key shared between the DNS Update requestor (which issues the update) and
            the server (which authenticates it).  This model does not work for automatic service registration.</t>
          <t>
            The goal of securing the DNS&nbhy;SD DNS-SD Registration Protocol is to provide the best possible security given the constraint
            that service registration has to be automatic.  It is possible to layer more operational security on top of what we
            describe here, but FCFS naming is already an improvement over the security of mDNS.</t>

          <section anchor="fcfs">
	    <name>First-Come First-Served
	    <name>FCFS Naming</name>
            <t>
              First-Come First-Serve

<!--[rfced] To what does "that" refer in this sentence?

Original:
 As long as the registration service remembers the name and the key
 used to register that name, no other server can add or update the
 information associated with that.

Perhaps:
 As long as the registration service remembers the name and the key
 used to register that name, no other server can add or update the
 information associated with them.

Perhaps:
 As long as the registration service remembers the name and the key
 used to register that name, no other server can add or update the
 information associated with that pair.

-->

              FCFS naming provides a limited degree of security: a security. A server that registers its service using
              DNS&nbhy;SD the
              DNS-SD Registration protocol Protocol is given ownership of a name for an extended period of time based on a lease
              specific to the key used to authenticate the DNS Update, which may be longer than the lease associated with the
              registered records.  As long as the registration service remembers the name and
              the key used to register that name, no other server can add or update the information associated with that.  If the
              server fails to renew its service registration before the KEY lease (<xref target="I-D.ietf-dnssd-update-lease" (see <xref target="RFC9664"
              section="4"/>) expires, its name is no longer protected.  FCFS naming is used to protect both the Service Description
              and the Host Description.</t>
	  </section>
	</section>

        <section>
	  <name>SRP Requestor Behavior</name>
	  <section>
	    <name>Public/Private key pair generation Key Pair Generation and storage</name> Storage</name>
            <t>
	      The requestor generates a public/private key pair (See (see <xref target="rsa"/>).  This key pair MUST <bcp14>MUST</bcp14> be stored in stable
	      storage; if there is no writable stable storage on the SRP requestor, the SRP requestor MUST <bcp14>MUST</bcp14> be pre-configured preconfigured with a
	      public/private key pair in read-only storage that can be used.  This key pair MUST <bcp14>MUST</bcp14> be unique to the device. A device
	      with rewritable storage SHOULD <bcp14>SHOULD</bcp14> retain this key indefinitely.  When the device changes ownership, it may be appropriate
	      for the former owner to erase the old key pair, which would then require the new owner to install a new
	      one. Therefore, the SRP requestor on the device SHOULD <bcp14>SHOULD</bcp14> provide a mechanism to erase the key, for example key (for example, as the
	      result of a "factory reset," reset") and to generate a new key.</t>
	    <t>
	      The policy described here for managing keys assumes that the keys are only used for SRP. If a key that is used for SRP
	      is also used for other purposes, the policy described here is likely to be insufficient. The policy stated here is NOT
	      RECOMMENDED <bcp14>NOT
	      RECOMMENDED</bcp14> in such a situation: a policy appropriate to the full set of uses for the key must be chosen. Specifying
	      such a policy is out of scope for this document.</t>
            <t>
	      When sending DNS updates, the requestor includes a KEY record containing the public portion of the key in each Host
	      Description Instruction and each Service Description Instruction.  Each KEY record MUST <bcp14>MUST</bcp14> contain the same public key.
	      The update is signed using SIG(0), using the private key that corresponds to the public key in the KEY record.  The
	      lifetimes of the records in the update is set using the EDNS(0) Extension Mechanisms for DNS  (EDNS(0)) Update Lease option
	      (see <xref target="I-D.ietf-dnssd-update-lease"/>.</t> target="RFC9664"/>).</t>
	    <t>
	      The format of the KEY resource record in the SRP Update is defined in <xref target="RFC3445"/>. Because the KEY RR
	      used in TSIG is not a zone-signing key, the flags field in the KEY RR MUST <bcp14>MUST</bcp14> be all zeroes.</t>
            <t>
	      The KEY record in Service Description updates MAY <bcp14>MAY</bcp14> be omitted for brevity; if it is omitted, the SRP registrar MUST <bcp14>MUST</bcp14> behave
	      as if the same KEY record that is given for the Host Description is also given for each Service Description for which
	      no KEY record is provided.  Omitted KEY records are not used when computing the SIG(0) signature.</t>
	  </section>
	  <section>
	    <name>Name Conflict Handling</name>
	    <t>
	      Both
	      Adds for both Host Description RR adds RRs and Service Description RR adds RRs can have names that result in name conflicts.
	      Service Discovery record adds cannot have name conflicts. If any Host Description or Service Description record
	      is found by the SRP registrar to have a conflict with an existing name, the registrar will respond to the SRP Update
	      with a YXDomain RCODE (<xref target="RFC2136" section="2.2" sectionFormat="of"/>).  In this case, the
	      requestor MUST <bcp14>MUST</bcp14> choose a new name or give up.</t>
	    <t>
	      There is no specific requirement for how this is done; typically, done. Typically, however, the requestor will append a number to the
	      preferred name. This number could be sequentially increasing, increasing or could be chosen randomly. One existing implementation
	      attempts several sequential numbers before choosing randomly. So for For instance, it might try host.default.service.arpa,
	      then host-1.default.service.arpa, then host-2.default.service.arpa, then host-31773.default.service.arpa.</t>
	  </section>
	  <section>
	    <name>Record Lifetimes</name>
	    <t>
	      The lifetime of the <xref target="RFC6763">DNS&nbhy;SD DNS-SD PTR, SRV, A, AAAA AAAA, and TXT records</xref> records (see <xref target="RFC6763"></xref>) uses the LEASE field
	      of the Update Lease option, option and is typically set to two hours.  This means that  Thus, if a device is disconnected from the
	      network, it does not appear in the user interfaces of devices looking for services of that type for too long.</t>
	    <t>
	      The lifetime of the KEY records is set using the KEY-LEASE field of the Update Lease Option, Option and SHOULD <bcp14>SHOULD</bcp14> be set to a
	      much longer time, typically 14 days.  The result of this is being that even though a device may be temporarily unplugged, unplugged --
	      disappearing from the network for a few days, days -- it makes a claim on its name that lasts much longer.</t>
	    <t>
	      This means that
	      Therefore, even if a device is unplugged from the network for a few days, and its services are not available for
	      that time, no other device can come along and claim its name the moment it disappears from the network.  In the event
	      that a device is unplugged from the network and permanently discarded, then its name is eventually cleaned up and made
	      available for re-use.</t> reuse.</t>
	  </section>
	  <section>
	    <name>Compression in SRV records</name> Records</name>
	    <t>
	      Although <xref target="RFC2782"/> requires that the target name in the SRV record not be compressed, an SRP requestor
	      MAY
	      <bcp14>MAY</bcp14> compress the target in the SRV record. The motivation for <em>not</em> compressing in <xref target="RFC2782"/>
	      is not stated, stated but is assumed to be because a caching resolver that does not understand the format of the SRV record
	      might store it as binary data and thus return an invalid pointer in response to a query. This does not apply in the
	      case of SRP: an SRP. An SRP registrar needs to understand SRV records in order to validate the SRP Update. Compression of the
	      target can save space in the SRP Update, so we want clients to be able to assume that the registrar will handle
	      this. Therefore, SRP registrars MUST <bcp14>MUST</bcp14> support compression of SRV RR targets.</t>
	      <t>

<!--[rfced] How might we clarify "this" for the ease of the reader
     (especially as this sentence is the first of the paragraph)?

Original:
   Note that this does not update <xref target="RFC2782"/>: [RFC2782]: DNS servers still MUST
   NOT compress SRV record targets.
-->

	      Note that this does not update <xref target="RFC2782"/>: DNS servers still <bcp14>MUST NOT</bcp14> compress SRV record targets. The
	      requirement to accept compressed SRV records in updates only applies to SRP registrars, and SRP registrars that are
	      also DNS servers still MUST NOT <bcp14>MUST NOT</bcp14> compress SRV record targets in DNS responses. We note also that
	      <xref target="RFC6762"/> recomments recommends that SRV records be compressed in mDNS messages, so <xref target="RFC2782"/> does
	      not apply to mDNS messages.</t>
	    <t>
	      In addition, we note that an implementor of an SRP requestor might update existing code that creates SRV records
	      or compresses DNS messages so that it compresses the target of an SRV record. Care must be taken if such code is
	      used both in requestors and in DNS servers that the code only compresses in the case where a requestor is generating
	      an SRP update.</t>
          </section>
          <section anchor="remove">
	    <name>Removing published services</name> Published Services</name>
	    <section anchor="zero-lease">
	      <name>Removing all published services</name> All Published Services</name>
              <t>
		To remove all the services registered to a particular host, the SRP requestor transmits an SRP update for that host
		with an Update Lease option that has a LEASE value of zero. If the registration is to be permanently removed,
		KEY-LEASE SHOULD <bcp14>SHOULD</bcp14> also be zero. Otherwise, it SHOULD <bcp14>SHOULD</bcp14> be set to the same value it had previously; this holds the name
		in reserve for when the SRP requestor is once again able to provide the service.</t>
              <t>
		SRP requestors are normally expected to remove all service instances when removing a host.  However, in some cases cases, an SRP
		requestor may not have retained sufficient state to know that some service instance is pointing to a host that it is
		removing.  This method of removing services is intended for the case where the requestor is going offline and does
		not want its services advertised. Therefore, it is sufficient for the requestor to send the
		<xref target="hdi">Host Host Description Instruction</xref>. Instruction (see <xref target="hdi"></xref>).
	      </t>
	      <t>
		To support this, when removing services based on the lease time being zero, an SRP registrar MUST <bcp14>MUST</bcp14> remove all service
		instances pointing to a host when a host is removed, even if the SRP requestor doesn't list them explicitly. If the
		KEY lease time is nonzero, the SRP registrar MUST NOT <bcp14>MUST NOT</bcp14> delete the KEY records for these SRP requestors.
	      </t>
	    </section>
	    <section>
	      <name>Removing some published services</name> Some Published Services</name>
	      <t>
		In some use cases cases, a requestor may need to remove some a specific service, service without removing its other services.  This can
be accomplished in one of two ways. To ways:</t>
<ol><li>To simply remove a specific service, the requestor sends a valid SRP Update where
		the <xref target="servdis">Service Service Discovery Instruction</xref> Instruction (see <xref target="servdis"></xref>) contains a single Delete an "Delete An RR from an RRset From An RRset" update
		(<xref target="RFC2136" section="2.5.4" sectionFormat="comma"/>) update sectionFormat="of"/>) that deletes the PTR record whose target is
		the service instance name. The <xref target="servdesc">Service Description Instruction</xref> in In this case case, the Service Description Instruction (see <xref target="servdesc"></xref>) contains
		a single Delete all "Delete All RRsets from a Name From A Name" update (<xref target="RFC2136" section="2.5.3" sectionFormat="comma"/>) update sectionFormat="of"/>) to
		the service instance name.
	      </t>
	      <t>
		The second </li>
	      <li>
		This alternative is used when some service is being replaced by a different service with a different service
		instance name. In this case, the old service is deleted as in the first alternative. The new service is added, just
		as it would be in an update that wasn't deleting the old service. Because both the removal of the old service and
		the add of the new service consist of a valid Service Discovery Instruction and a valid Service Description
		Instruction, the update as a whole is a valid SRP Update, Update and will result in the old service being removed and the
		new one added, added; or, to put it differently, the update will result in the old service being replaced by the new service.
	      </t>
	      </li></ol>
	      <t>
		It is perhaps worth noting that that, if a service is being updated without the service instance name changing, that situation will
		look very much like the second alternative above. The difference is that because the target for the PTR record in
		the Service Discovery Instruction is the same for both the Delete "Delete An RR From An RRset RRset" update and the Add "Add
		To An RRSet
		update, RRset" update (<xref target="RFC2136" section="2.5.1" sectionFormat="of"/>), there is no way to tell whether they were intended to be one or two Instructions. The same would be true of
		the Service Description Instruction.
	      </t>
	      <t>
		Whichever of these two alternatives is used, the host lease will be updated with the lease time provided in the SRP
		update. In neither of these cases is it permissible to delete the host. All services must point to a host. If a host
		is to be deleted, this must be done using the method described in <xref target="zero-lease"/>, which deletes the
		host and all services that have that host as their target.
	      </t>
	    </section>
          </section>
      </section></section>

      <section anchor="server_behavior">
	<name>Validation and Processing of SRP Updates</name>
        <section anchor="add_validation">
	  <name>Validation of DNS Update Add and Delete RRs</name>
          <t>
	    The SRP registrar first validates that the DNS Update is a syntactically and semantically valid DNS Update according to
	    the rules specified in <xref target="RFC2136"/>.</t>
          <t>
	    SRP Updates consist of a set of <em>instructions</em> that together add or remove one or more services. Each instruction
	    consists of some combination of delete updates and add updates. When an instruction contains a delete and an add, the
	    delete MUST <bcp14>MUST</bcp14> precede the add.</t>
          <t>
	    The SRP registrar checks each instruction in the SRP Update to see that it is either a Service Discovery Instruction, a
	    Service Description Instruction, or a Host Description Instruction.  Order matters in DNS updates.  Specifically,
	    deletes must precede adds for records that the deletes would affect; otherwise otherwise, the add will have no effect.  This is the
	    only ordering constraint; constraint: aside from this constraint, updates may appear in whatever order is convenient when
	    constructing the update.</t>
          <t>
	    Because the SRP Update is a DNS update, it MUST <bcp14>MUST</bcp14> contain a single question that indicates the zone to be updated.
	    Every delete and update in an SRP Update MUST <bcp14>MUST</bcp14> be within the zone that is specified for the SRP Update.</t>

	  <section anchor="servdis">
	    <name>Service Discovery Instruction</name>
            <t>An instruction is a Service Discovery Instruction if it:</t>

<!-- [rfced] FYI - we updated the list as follows for clarity. Please let us
know if there are any objections.

Original:
   An instruction is a Service Discovery Instruction if it contains</t>
            <ul spacing="compact">
	      <li>exactly contains

   *  exactly one "Add to an RRSet" (<xref target="RFC2136" section="2.5.1" sectionFormat="comma"/>) ([RFC2136], Section 2.5.1) or
      exactly one "Delete an RR from an RRSet" (<xref target="RFC2136" section="2.5.4" sectionFormat="comma"/>) ([RFC2136],
      Section 2.5.4) RR update,</li>
	      <li>which update,
   *  which updates a PTR RR,</li>
	      <li>the RR,
   *  the target of which is a Service Instance Name</li>
	      <li><t>for Name
   *  for which name a Service Description Instruction is present in the
      SRP Update, and:</t>
		<ul spacing="compact">
		  <li>if and:
      -  if the RR Update is an "Add to an RRSet" instruction, that
         Service Description Instruction contains an "Add to an RRset"
         RR update for the SRV RR describing that service and no other
         "Delete from an RRset" instructions for that Service Instance
         Name; or</li>
		  <li>if or
      -  if the RR Update is a "Delete an RR from an RRSet" instruction,
         that Service Description Instruction contains a "Delete from an
         RRset" RR update and no other "Add to an RRset" instructions
         for that Service Instance
		    Name.</li></ul></li>
	      <li>and Name.
   *  and contains no other add or delete RR updates for the same name
      as the PTR RR Update.

Current:
   An instruction is a Service Discovery Instruction if it:

   *  Contains exactly one "Add to an RRSet" (Section 2.5.1 of
      [RFC2136]) or exactly one "Delete an RR from an RRSet"
      (Section 2.5.4 of [RFC2136]) RR update, which updates a PTR RR;
      the target of which is a Service Instance Name for which name a
      Service Description Instruction is present in the SRP Update.
      Additionally:

      -  If the RR Update is an "Add to an RRSet" instruction, that
         Service Description Instruction contains an "Add to an RRset"
         RR update for the SRV RR describing that service and no other
         "Delete from an RRset" instructions for that Service Instance
         Name.
      -  If the RR Update is a "Delete an RR from an RRSet" instruction,
         that Service Description Instruction contains a "Delete from an
         RRset" RR update and no other "Add to an RRset" instructions
         for that Service Instance Name.

   *  Contains no other add or delete RR updates for the same name as
      the PTR RR Update.
-->
            <ul spacing="normal">
	      <li><t>Contains exactly one "Add To An RRset" RR update (<xref
	      target="RFC2136" section="2.5.1" sectionFormat="of"/>) or
	      exactly one "Delete An RR From An RRset" RR update (<xref target="RFC2136"
	      section="2.5.4" sectionFormat="of"/>), which updates a
	      PTR RR; the target of which is a Service Instance Name for which
	      name a Service Description Instruction is present in the SRP
	      Update. Additionally:</t>
		<ul spacing="compact">
		  <li>If the RR Update is an "Add To An RRset" instruction,
		  that Service Description Instruction contains an "Add To An
		  RRset" RR update for the SRV RR describing that service and
		  no other "Delete From An RRset" instructions for that
		  Service Instance Name.</li>
		  <li>If the RR Update is a "Delete An RR From An RRset"
		  instruction, that Service Description Instruction contains a
		  "Delete From An RRset" RR update and no other "Add To An
		  RRset" instructions for that Service Instance
		  Name.</li></ul></li>
	      <li>Contains no other add or delete RR updates for the same name
	      as the PTR RR Update.</li>
            </ul>
	    <t>
	      Note that there can be more than one Service Discovery
	      Instruction for the same name if the SRP requestor is
	      advertising more than one service of the same type, type or is
	      changing the target of a PTR RR. This is also true for SRP
	      subtypes (<xref target="RFC6763" section="7.1"/>). section="7.1"
	      sectionFormat="of"/>). For each such PTR RR add or delete, the
	      above constraints must be met.</t>
	  </section>

	  <section anchor="servdesc">
	    <name>Service Description Instruction</name>

            <t>An instruction is a Service Description Instruction if, for the
            appropriate Service Instance Name, the following are true:</t>
            <ul spacing="compact"> spacing="normal">
	      <li>
		It contains exactly one "Delete all All RRsets from a name" From A Name" update
		for the service instance name
		(<xref (see <xref target="RFC2136"
		section="2.5.3" sectionFormat="comma"/>),</li> sectionFormat="of"/>).</li>
	      <li>
		It contains zero or one "Add to an To An RRset" SRV RR,</li> RR.</li>
	      <li>
		It contains zero or one "Add to an To An RRset" KEY RR that, if
		present, contains the public key corresponding to the private
		key that was used to sign the message (if present, the KEY MUST
		<bcp14>MUST</bcp14> match the KEY RR given in the Host Description),</li>
		Description).</li>
	      <li>
		It contains zero or more "Add to an To An RRset" TXT RRs,</li> RRs.</li>
	      <li>
		If there is one "Add to an To An RRset" SRV update, there MUST
		<bcp14>MUST</bcp14> be at least one "Add to an To An RRset" TXT
		update.</li>
		<li>
		The

		<t>The target of the SRV RR Add, if present present, points to a
		hostname for which there is a Host Description Instruction in
		the SRP Update, or</li>
	      <li>
		If Update; or if there is no "Add to an To An RRset" SRV RR,
		then either:</li>
	      <li><ul> either:</t>
		<ul spacing="normal">
		  <li>the name to which the "Delete all All RRsets from a name" From A Name"
		  applies does not exist, or</li>
		  <li>there is an existing KEY RR on that name, which name that matches
		  the key with which the SRP Update was signed.</li></ul></li>
	      <li>
		No other resource records on the Service Instance Name are
		modified.</li>
            </ul>
	    <t>An SRP registrar MUST <bcp14>MUST</bcp14> correctly handle compressed names in the SRV target.</t>
	  </section>

	  <section anchor="hdi">
	    <name>Host Description Instruction</name>

            <t>An instruction is a Host Description Instruction if, for the appropriate hostname, it contains</t> contains the following:</t>
            <ul spacing="compact"> spacing="normal">
	      <li>
		exactly one "Delete all All RRsets from a name" From A Name" RR,</li>
	      <li>
		one or more "Add to an To An RRset" RRs of type A and/or AAAA,</li> AAAA, and </li>
	      <li>
		exactly one "Add to an To An RRset" RR that adds a KEY RR that
		contains the public key corresponding to the private key that
		was used to sign the message,</li>
	      <li>
		Host message</li>
	    </ul>

	    <t>Host Description Instructions do not modify any other resource records.</li>
            </ul> records.</t>

	    <t>
	      A and/or AAAA records that are not of sufficient scope to be
	      validly published in a DNS zone MAY <bcp14>MAY</bcp14> be ignored by
	      the SRP registrar, which could result in a host description
	      effectively containing zero reachable addresses even when it
	      contains one or more addresses.</t>

	    <t>
	      For example, if a link-scope address or IPv4 autoconfiguration address is provided by the SRP requestor, the SRP
	      registrar could not publish this in a DNS zone. However, in some situations, the registrar might make the records
	      available through a mechanism such as an advertising proxy only on the specific link from which the SRP update
	      originated; in
	      originated. In such a situation, locally-scoped locally scoped records are still valid.</t>
	  </section>
	</section>

	<section>
	  <name>Valid SRP Update Requirements</name>
          <t>
	    An SRP Update MUST <bcp14>MUST</bcp14> contain exactly one Host Description Instruction. In addition, there MUST NOT <bcp14>MUST NOT</bcp14> be any Service
	    Description Instruction to which no Service Discovery Instruction points.  A DNS Update that contains any additional
	    adds or deletes that cannot be identified as Service Discovery, Service Description Description, or Host Description Instructions is
	    not an SRP Update. A DNS update that contains any prerequisites is not an SRP Update.</t>
	  <t>An SRP Update MUST <bcp14>MUST</bcp14> include an EDNS(0) Update Lease option
	    (see <xref target="I-D.ietf-dnssd-update-lease"/>. target="RFC9664"/>). The LEASE time specified in the Update Lease option MUST <bcp14>MUST</bcp14> be less than
	    or equal to the KEY-LEASE time. A DNS update that does not include the Update Lease option, or that includes a
	    KEY-LEASE value that is less than the LEASE value, is not an SRP update.</t>
	  <t>When an SRP registrar receives a DNS Update that is not an SRP
	  update, it MAY <bcp14>MAY</bcp14> process the update as regular RFC2136 updates, updates
	  described in <xref target="RFC2136" format="default"/>, including
	  access control checks and constraint checks, if supported. Otherwise Otherwise,
	  the SRP registrar MUST <bcp14>MUST</bcp14> reject the DNS Update with the
	  Refused RCODE.</t>
          <t>
	    If the definitions of each of these instructions are followed carefully and the update requirements are validated
	    correctly, many DNS Updates that look very much like SRP Updates nevertheless will fail to validate.  For example, a DNS
	    update that contains an Add to an RRset "Add To An RRset" instruction for a Service Name and an Add to an RRset instruction for a Service
	    Instance Name, where the PTR record added to the Service Name does not reference the Service Instance Name, is not a
	    valid SRP Update message, message but may be a valid RFC2136 update.</t> update as described in <xref target="RFC2136" format="default"/>.</t>
	</section>
	<section>
	  <name>FCFS Name And and Signature Validation</name>

<!--[rfced] For the ease of the reader, might we clarify what "these
     conditions" are?

Original:
   Assuming that a DNS Update message has been validated with these
   conditions and is a valid SRP Update, the SRP registrar checks that
   the name in the Host Description Instruction exists.

Perhaps:
   Assuming that a DNS Update message has been validated with an FCFS name
   and signature and is a valid SRP Update, the SRP registrar checks that
   the name in the Host Description Instruction exists.
-->

          <t>
	    Assuming that a DNS Update message has been validated with these conditions and is a valid SRP Update, the SRP registrar
	    checks that the name in the Host Description Instruction exists.  If so, then the registrar checks to see if the KEY
	    record on that name is the same as the KEY record in the Host Description Instruction.  The registrar performs the same
	    check for the KEY records in any Service Description Instructions.  For KEY records that were omitted from Service
	    Description Instructions, the KEY from the Host Description Instruction is used.  If any existing KEY record
	    corresponding to a KEY record in the SRP Update does not match the KEY record in the SRP Update (whether provided
	    or taken from the Host Description Instruction), then the SRP registrar MUST <bcp14>MUST</bcp14> reject the SRP Update with the YXDomain
	  RCODE.</t>

<!--[rfced] Please review this transition sentence. Because it is
     placed at the beginning of a new paragraph, the "Otherwise" might
     be a bit jarring to the reader.  (Our suggestion is likely weak,
     but for demonstrative purposes...)

Original:
   Otherwise, the SRP registrar validates the SRP Update using SIG(0)
   against the public key in the KEY record of the Host Description
   Instruction.

Perhaps:
   If the above steps are not taken, the SRP registrar validates the
   SRP Update using SIG(0) against the public key in the KEY record of
   the Host Description Instruction.
-->

          <t>
	    Otherwise, the SRP registrar validates the SRP Update using SIG(0) against the public key in the KEY record of the Host
	    Description Instruction.  If the validation fails, the registrar MUST <bcp14>MUST</bcp14> reject the SRP Update with the Refused RCODE.
	    Otherwise, the SRP Update is considered valid and authentic, authentic and is processed according to the method described in
	    RFC2136.</t>
	    <xref target="RFC2136" format="default"/>.</t>
          <t>
	    KEY record updates omitted from Service Description Instruction are processed as if they had been explicitly present:
	    every Service Description that is updated MUST, after present.
	    After the SRP Update has been applied, every Service Description that is updated <bcp14>MUST</bcp14> have a KEY RR, RR: and it must be the
	    same KEY RR that is present in the Host Description to which the Service Description refers.</t>
	  <t>
	    <xref target="RFC3445"/> states that the flags field in the KEY RR MUST <bcp14>MUST</bcp14> be zero except for bit 7, which can
	    be one in the case of a zone key. However, the SRP registrar MUST NOT <bcp14>MUST NOT</bcp14> validate the flags field.</t>
	</section>
	<section>
	  <name>Handling of Service Subtypes</name>
	  <t>
	    SRP registrars MUST <bcp14>MUST</bcp14> treat the update instructions for a service type and all its subtypes as atomic. That is, when a
	    service and its subtypes are being updated, whatever information appears in the SRP Update is the entirety of
	    information about that service and its subtypes. If any subtype appeared in a previous update but does not appear in
	    the current update, then the SRP registrar MUST <bcp14>MUST</bcp14> remove that subtype.
	  </t>
	  <t>
	    Similarly, there is no mechanism for deleting subtypes. A delete of a service deletes all of its subtypes. To delete an
	    individual subtype, an SRP Update must be constructed that contains the service type and all subtypes for that service
	    except for the one to be deleted.
	  </t>
	</section>
	<section>
	  <name>SRP Update response</name> Response</name>
          <t>
	    The status that is returned depends on the result of processing the update, update and can be either NoError, ServFail, Refused Refused,
	    or YXDomain: all YXDomain. All other possible outcomes will already have been accounted for when applying the constraints that
	    qualify the update as an SRP Update. The meanings of these responses are explained in <xref target="RFC2136"
	    section="2.2"/>.</t>
	  <t>
	    In the case of a response other than NoError, <xref target="RFC2136" section="3.8"/> specifies that the server is permitted
            to respond either with no RRs or to copy the RRs sent by the client into the response. The SRP Requestor MUST NOT requestor <bcp14>MUST NOT</bcp14> attempt
	    to validate any RRs that are included in the response. It is possible that a future SRP extension may include per-RR
	    indications as to why the update failed, but at present the time of writing this is not specified, so specified. So, if a client were to attempt to validate
            the RRs in the response, it might reject such a response, response since it would contain RRs, RRs but probably not a set of RRs
	    identical to what was sent in the SRP Update.</t>
	</section>
	<section>
	  <name>Optional Behavior</name>

          <t>
	    The SRP registrar MAY <bcp14>MAY</bcp14> add a Reverse Mapping (<xref (see <xref target="RFC1035" section="3.5"/>, section="3.5"/> and <xref target="RFC3596" section="2.5"/>)
	    that corresponds to the Host Description.  This is not required because the Reverse Mapping reverse mapping serves no protocol function,
	    but it may be useful for debugging, e.g. e.g., in annotating network packet traces or logs.  In order for the registrar to do
	    a reverse mapping update, it must be authoritative for the zone that would need to be updated, updated or have credentials to do
	    the update.  The SRP requestor MAY <bcp14>MAY</bcp14> also do a reverse mapping update if it has credentials to do so.</t>
          <t>
	    The SRP registrar MAY <bcp14>MAY</bcp14> apply additional criteria when accepting updates.  In some networks, it may be possible to do
	    out-of-band registration of keys, keys and only accept updates from pre-registered preregistered keys.  In this case, an update for a key
	    that has not been registered SHOULD <bcp14>SHOULD</bcp14> be rejected with the Refused RCODE.</t>
          <t>
	  There are at least two benefits to doing this rather than simply using normal SIG(0) DNS updates.  First, the updates:</t>
	  <ol><li>The same
	    registration protocol can be used in both cases, so both use cases can be addressed by the same SRP requestor
	    implementation.  Second, the
	    implementation.</li>
	    <li>The registration protocol includes maintenance functionality not present with normal DNS
	    updates.</t>
	    updates.</li></ol>
          <t>
	    Note that the semantics of using SRP in this way are different than for typical RFC2136 implementations: the implementations described in <xref target="RFC2136" format="default"/>. The KEY used
	    to sign the SRP Update only allows the SRP requestor to update records that refer to its Host Description.  RFC2136
	    implementations
	    Implementations specific to <xref target="RFC2136" format="default"/> do not normally provide a way to enforce a constraint of this type.</t>
          <t>
	    The SRP registrar could also have a dictionary of names or name patterns that are not permitted.  If such a list is used,
	    updates for Service Instance Names that match entries in the dictionary are rejected with a Refused RCODE.</t>
	</section>
      </section>
    </section>

    <section>
      <name>TTL Consistency</name>
      <t>
	All RRs within an RRset are required to have the same TTL
	(<xref
	(see <xref target="RFC2181" section="5.2" sectionFormat="comma"> Clarifications to the DNS Specification</xref>). sectionFormat="of"/>).
	In order to avoid inconsistencies, SRP places restrictions on TTLs sent by requestors and requires that SRP registrars enforce
	consistency.</t>
      <t>
	Requestors sending SRP Updates MUST <bcp14>MUST</bcp14> use consistent TTLs in all RRs within the SRP Update.</t>
      <t>
	SRP registrars MUST <bcp14>MUST</bcp14> check that the TTLs for all RRs within the SRP Update are the same.  If they are not, the SRP
	update MUST <bcp14>MUST</bcp14> be rejected with a Refused RCODE.</t>
      <t>
	Additionally, when adding RRs to an RRset, for example RRset (for example, when processing Service Discovery records, records), the SRP registrar MUST <bcp14>MUST</bcp14> use the
	same TTL on all RRs in the RRset.  How this consistency is enforced is up to the implementation.</t>
      <t>
	TTLs sent in SRP Updates are advisory: they indicate the SRP requestor's guess as to what a good TTL would be.  SRP registrars may
	override these TTLs.  SRP registrars SHOULD <bcp14>SHOULD</bcp14> ensure that TTLs are reasonable: neither too long nor too short.  The TTL SHOULD NOT <bcp14>SHOULD NOT</bcp14>
	ever be longer than the lease time (<xref target="stale"/>).  Shorter TTLs will result in more frequent data refreshes;
	this increases latency on the DNS-SD client side, increases load on any caching resolvers and on the authoritative server,
	and also increases network load, which may be an issue for constrained networks.  Longer TTLs will increase the likelihood
	that data in caches will be stale.  TTL minimums and maximums SHOULD <bcp14>SHOULD</bcp14> be configurable by the operator of the SRP registrar.
      </t>
    </section>

    <section anchor="maintenance">
      <name>Maintenance</name>
      <section anchor="stale">
	<name>Cleaning up stale data</name> Up Stale Data</name>
	<t>Because the DNS&nbhy;SD DNS-SD registration protocol is automatic, automatic and not managed by humans,
          some additional bookkeeping is required.  When an update is constructed by the SRP requestor,
          it MUST <bcp14>MUST</bcp14> include an EDNS(0) Update Lease Option (see <xref target="I-D.ietf-dnssd-update-lease"/>. target="RFC9664"/>).
          The Update Lease Option contains two lease times: the Lease Time and the KEY
          Lease Time.</t>

	<t>These leases are promises, similar

	<t>Similar to DHCP leases (see <xref target="RFC2131">DHCP leases</xref>, target="RFC2131"></xref>), these leases are promises from the SRP requestor that it will send a new update for the service registration before the
          lease time expires.  The Lease time is chosen to represent the time after the
          update during which the registered records other than the KEY record can be assumed
          to be valid.  The KEY lease time represents the time after the update during
          which the KEY record can be assumed to be valid.</t>

	<t>The reasoning behind the different lease times is discussed in the section on FCFS naming
          (<xref target="fcfs"/>). <xref target="fcfs" format="default"/>.  SRP registrars may be configured with limits for these values.  A  At the time of writing, a default limit of two hours for
          the Lease and 14 days for the SIG(0) KEY are currently thought to be good choices.  Constrained devices with limited
          battery that wake infrequently are likely to request longer leases; registrars that support such devices may need to set
          higher limits.  SRP requestors that are going to continue to use names on which they hold leases SHOULD <bcp14>SHOULD</bcp14> update well before
          the lease ends, ends in case the registrar is unavailable or under heavy load.</t>

	<t>
	  The lease time applies specifically to the host. All service instances, and all service entries for such service
	  instances, depend on the host. When the lease on a host expires, the host and all services that reference it MUST <bcp14>MUST</bcp14> be
	  removed at the same time&mdash;it time: it is never valid for a service instance to remain when the host it references has been
	  removed. If the KEY record for the host is to remain, the KEY record for any services that reference it MUST <bcp14>MUST</bcp14> also
	  remain. However, the service PTR record MUST <bcp14>MUST</bcp14> be removed, removed since it has no key associated with it, it and since it is never
	  valid to have a service PTR record for which there is no service instance on the target of the PTR record.
	</t>

	<t>
	  SRP registrars MUST <bcp14>MUST</bcp14> also track a lease time per service instance. The reason for doing this is being that a requestor may
	  re-register a host with a different set of services, services and not remember that some different service instance had previously
	  been registered. In this case, when that service instance lease expires, the SRP registrar MUST <bcp14>MUST</bcp14> remove the service
	  instance (although the KEY record for the service instance SHOULD <bcp14>SHOULD</bcp14> be retained until the KEY lease on that service
	  expires). This is beneficial because otherwise because, otherwise, if the SRP requestor continues to renew the host, host but never mentions the
	  stale service again, the stale service will continue to be advertised.
	</t>

	<t>The SRP registrar MUST <bcp14>MUST</bcp14> include an EDNS(0) Update Lease option in the
          response if the lease time proposed by the requestor has been shortened or lengthened by the registrar.  The requestor
          MUST
          <bcp14>MUST</bcp14> check for the EDNS(0) Update Lease option in the response and MUST <bcp14>MUST</bcp14> use the lease
          times from that option in place of the options that it sent to the registrar when
          deciding when to renew its registration.   The times may be shorter or longer than
          those specified in the SRP Update; Update: the SRP requestor must honor them in either case.</t>

	<t>SRP

<!-- [rfced] In Section 5.1, we see both "N" and "'n'". Please review
     and let us know if/how we may update for uniformity.

Original "N":
   SRP requestors SHOULD assume that each lease ends N seconds after the
   update was first transmitted, where N is the lease duration.

Original "'n'":
   The lease time is never sent as a TTL; its
   sole purpose is to determine when the authoritative DNS server will
   delete stale records.  It is not an error to send a DNS response with
   a TTL of 'n' when the remaining time on the lease is less than 'n'.
-->

	<t>SRP requestors <bcp14>SHOULD</bcp14> assume that each lease ends N
	seconds after the update was first transmitted (where N is the lease
	duration).  SRP Registrars SHOULD registrars <bcp14>SHOULD</bcp14> assume that each lease
	ends N seconds after the update that was successfully processed was
	received.  Because the registrar will always receive the update after
	the SRP requestor sent it, this avoids the possibility of
	misunderstandings.</t>

	<t>SRP registrars MUST <bcp14>MUST</bcp14> reject updates that do not
	include an EDNS(0) Update Lease option.  DNS authoritative servers
	that allow both SRP and non-SRP DNS updates MAY <bcp14>MAY</bcp14> accept
	updates that don't include leases, but SHOULD they <bcp14>SHOULD</bcp14>
	differentiate between SRP Updates and other updates, updates and MUST
	<bcp14>MUST</bcp14> reject updates that would otherwise be SRP Updates
	if they do not include leases.</t>

	<t>Lease times have a completely different function than TTLs.  On an
	authoritative DNS server, the TTL on a resource record is a constant: whenever
	constant. Whenever that RR is served in a DNS response, the TTL value
	sent in the answer is the same.  The lease time is never sent as a
	TTL; its sole purpose is to determine when the authoritative DNS
	server will delete stale records.  It is not an error to send a DNS
	response with a TTL of 'n' when the remaining time on the lease is
	less than 'n'.</t>
      </section>
    </section>

    <section>
      <name>Security Considerations</name>
      <section anchor="source_validation">
	<name>Source Validation</name>
	<t>SRP Updates have no authorization semantics other than
	  FCFS.   This means that   Thus, if an attacker from outside of the administrative
	  domain of the SRP registrar knows the registrar's IP address, it can can, in principle principle, send updates to the registrar
	  that will be processed successfully.   Therefore, SRP Registrars SHOULD therefore registrars <bcp14>SHOULD</bcp14> be configured to reject updates
	  from source addresses outside of the administrative domain of the registrar.</t>

	<t>For TCP updates, the initial SYN-SYN+ACK handshake prevents updates being forged by an off-network attacker.  In order to
	  ensure that this handshake happens, SRP registrars relying on three-way-handshake validation MUST NOT <bcp14>MUST NOT</bcp14> accept TCP Fast Open payloads
	  (see <xref target="RFC7413"/> payloads. target="RFC7413"/>).  If the network infrastructure allows it, an SRP registrar MAY <bcp14>MAY</bcp14> accept TCP Fast Open payloads if all such packets
	  are validated along the path, and the network is able to reject this type of spoofing at all ingress points.</t>

	<t>For UDP updates from constrained devices, spoofing would have to be prevented with appropriate source address filtration
	  on routers <xref target="RFC2827"/>. (<xref target="RFC2827"/>). This would ordinarily be accomplished by measures such as are those described in
	  <xref
	  (<xref target="RFC7084" section="4.5" sectionFormat="of"/>. sectionFormat="of"/>). For example, a stub router <xref target="I-D.ietf-snac-simple"/> (<xref target="I-D.ietf-snac-simple"/>)
	  for a constrained network might only accept UDP updates from source addresses known to be on-link on that stub network, network and might
	  further validate that the UDP update was actually received on the stub network interface and not the interface connected to
	  the adjacent infrastructure link.</t>
      </section>
      <section>
	<name>Other DNS updates</name> Updates</name>
	<t>Note that these rules only apply to the validation of SRP Updates.
	  A server that accepts updates from SRP
	  requestors may also accept other DNS updates, and those DNS updates may be validated
	  using different rules.   However, in the case of a DNS server that accepts SRP
	  updates, the intersection of the SRP Update rules and
	  whatever other update rules are present must be considered very carefully.</t>

	<t>For example, a normal, normal authenticated DNS update to any RR that was added using SRP, but that is authenticated using a
	  different key, could be used to override a promise made by the SRP registrar to an SRP requestor, requestor by replacing all or part of
	  the service registration information with information provided by an authenticated DNS update requestor.  An implementation
	  that allows both kinds of updates SHOULD NOT <bcp14>SHOULD NOT</bcp14> allow DNS Update requestors that are using different authentication and
	  authorization credentials to update records added by SRP requestors.</t>
      </section>
      <section>
	<name>Risks of allowing arbitrary names Allowing Arbitrary Names to be registered Registered in SRP updates</name> Updates</name>
	<t>It is possible to set up SRP updates for a zone that is used for non-DNSSD services. For example, imagine that you set
	  up SRP service for example.com. SRP hosts can now register names like "www" or "mail" or "smtp" in this domain. In addition,
	  SRP updates using FCFS naming can insert names that are obscene or offensive into the zone. There is no simple solution to
	  these problems. We However, we have two recommendations to address this problem, however:</t> problem:</t>
	<ul spacing="compact"> spacing="normal">
	  <li>Do not provide SRP service in organization-level zones. Use subdomains of the organizational domain for DNS service
	    discovery. DNS-SD.  This does not prevent registering names as mentioned above, above but does ensure that genuinely important names
	    are not accidentally reserved for SRP clients. So So, for example, the zone "dnssd.example.com" could be used instead of
	    "example.com" for SRP updates. Because of the way that DNS browsing DNS-browsing domains are discovered, there is no need for the
	  DNSSD discovery zone that is updated by SRP to have a user-friendly or important-sounding name.</li>

	  <li>Configure a dictionary of names that are prohibited. Dictionaries of common obscene and offensive names are no doubt
	    available,
	    available and can be augmented with a list of typical "special" names like "www", "mail", "smtp" "smtp", and so on. Lists of
	    names are generally available, available or can be constructed manually.</li>
	</ul>
      </section>
      <section>
	<name>Security of local service discovery</name> Local Service Discovery</name>
	<t>Local links can be protected by managed services such as RA Router Advertisement Guard (see <xref target="RFC6105"/>, target="RFC6105"/>), but multicast services like
	  DHCP <xref target="RFC2131"/>, DHCPv6 <xref target="RFC8415"/>
	  DHCP, DHCPv6, and IPv6 Neighbor Discovery (see <xref target="RFC4861"/> are target="RFC2131"/>, <xref target="RFC8415"/>, and <xref target="RFC4861"/>, respectively) are,
	  in most cases cases, not authenticated and can't be controlled on unmanaged networks, such as home networks and small-office small office
	  networks where no network management staff are present. In such situations, the SRP service has comparatively fewer
	  potential security exposures and hence and, hence, is not the weak link. This is discussed in more detail in
	  <xref target="how-to-secure"/>.</t>
	<t>The fundamental protection for networks of this type is the user's choice of what devices to add to the network. Work is
	  being done in other working groups and standards bodies to improve the state of the art for network on-boarding and device
	  isolation (e.g., <xref target="RFC8520"/> provides a means for constraining what behaviors are allowed for a device in an
	  automatic way), but such work is out of scope for this document.</t>
      </section>
      <section>
	<name>SRP Registrar Authentication</name>
	<t>This specification does not provide a mechanism for validating responses from SRP Registrars registrars to
	  SRP requestors.   In principle, a KEY RR could be used by
	  a non-constrained SRP requestor to validate responses from the registrar, but this is not required,
	  nor do we specify a mechanism for determining which key to use.</t>
	<t>In addition, for DNS-over-TLS connections, out-of-band key pinning as described in
	  <xref target="RFC7858" section="4.2" sectionFormat="comma"/> sectionFormat="of"/> could be used for authentication of the SRP registrar,
	  e.g.
	  e.g., to prevent man-in-the-middle attacks. However However, the use of such keys is impractical for an unmanaged service
	  registration protocol, and hence protocol; hence, it is out of scope for this document.</t>
      </section>
      <section anchor="rsa">
	<name>Required Signature Algorithm</name>
	<t>
	  For validation, SRP registrars MUST <bcp14>MUST</bcp14> implement the ECDSAP256SHA256 signature algorithm.  SRP registrars SHOULD <bcp14>SHOULD</bcp14> implement the
	  algorithms that are specified in <xref target="RFC8624" section="3.1" sectionFormat="comma"/>, sectionFormat="of"/>, in the validation column of the
	  table, that are numbered 13 or higher higher, and that have a "MUST", "RECOMMENDED", "<bcp14>MUST</bcp14>", "<bcp14>RECOMMENDED</bcp14>", or "MAY" "<bcp14>MAY</bcp14>" designation in the validation column of
	  the table.
	  SRP requestors MUST NOT <bcp14>MUST NOT</bcp14> assume that any algorithm numbered lower than 13 is
	  available for use in validating SIG(0) signatures.</t>
      </section>
    </section>
    <section>
      <name>Privacy Considerations</name>
      <t>
	Because DNS-SD SRP Updates can be sent off-link, the privacy implications of SRP are different than for multicast DNS mDNS
	responses.  Host implementations that are using TCP SHOULD <bcp14>SHOULD</bcp14> also use TLS if available.  SRP Registrar registrar implementations MUST <bcp14>MUST</bcp14> offer
	TLS support.  The use of TLS with DNS is described in <xref target="RFC7858"/>. Because there is no mechanism for sharing
	keys, validation of DNS-over-TLS keys is not possible; DNS-over-TLS is used only as described in
	<xref target="RFC7858" section="4.1" sectionFormat="comma"/> sectionFormat="of"/>.
      </t>
      <t>
	Hosts that implement TLS support SHOULD NOT <bcp14>SHOULD NOT</bcp14> fall back to TCP; since TCP. Since SRP registrars are required to support
	TLS, it is entirely up to the host implementation whether to use it.
      </t>
      <t>
	Public keys can be used as identifiers to track hosts. SRP registrars MAY <bcp14>MAY</bcp14> elect not to return KEY records for queries for
	SRP registrations. To avoid DNSSEC validation failures, an SRP registrar that signs the zone for DNSSEC but refuses to return
	a KEY record MUST NOT <bcp14>MUST NOT</bcp14> store the KEY record in the zone itself. Because the KEY record isn't in the zone, the nonexistance nonexistence of
	the KEY record can be validated. If the zone is not signed, the server MAY <bcp14>MAY</bcp14> instead return a negative non-error response
	(either NXDOMAIN or no data).
      </t>
    </section>

    <section>
      <name>Domain Name Reservation Considerations</name>
      <t>This section specifies considerations for systems involved in domain name resolution when resolving queries for names
	ending with '.service.arpa.'. ".service.arpa.".  Each item in this section addresses some aspect of the DNS or the process of resolving domain
	names that would be affected by this special-use allocation.  Detailed explanations of these items can be found in Section 5
	of <xref target="RFC6761"/>.</t> target="RFC6761" sectionFormat="of" section="5"/>.</t>
      <section>
	<name>Users</name>
	<t>The current proposed use for 'service.arpa' "service.arpa" does not require special knowledge on the part of the user. While the
	  'default.service.arpa.'
	  "default.service.arpa." subdomain is used as a generic name for registration, users are not expected to see this name in
	  user interfaces. In the event that it does show up in a user interface, it is just a domain name, name and requires no special
	  treatment by the user. Users are not expected to see this name in user interfaces, although it's certainly possible that
	  they might. If they do, they are not expected to treat it specially.</t>
      </section>
      <section>
	<name>Application Software</name>
	<t>
	  Application software does not need to handle subdomains of 'service.arpa' "service.arpa" specially.  'service.arpa' SHOULD NOT  "service.arpa" <bcp14>SHOULD NOT</bcp14> be treated
	  as more trustworthy than any other insecure DNS domain, simply because it is locally-served locally served (or for any other reason). It
	  is not possible to register a PKI certificate for a subdomain of 'service.arpa.' "service.arpa." because it is a locally-served locally served domain
	  name. So So, no such subdomain can be considered as to be uniquely identifying a particular host, as would be required for such a
	  PKI cert certificate to be issued. If a subdomain of 'service.arpa.' "service.arpa." is returned by an API or entered in an input field of an
	  application, PKI authentication of the endpoint being identified by the name will not be possible. Alternative methods
	  and practices for authenticating such endpoints are out of scope for this document.</t>
      </section>
      <section>
	<name>Name Resolution APIs and Libraries</name>
	<t>Name resolution APIs and libraries MUST NOT <bcp14>MUST NOT</bcp14> recognize names that end in '.service.arpa.' "service.arpa." as special and MUST NOT <bcp14>MUST NOT</bcp14> treat
	  them as having special significance, except that it may be necessary that such APIs not bypass the locally configured
	  recursive resolvers.</t>
	<t>One or more IP addresses for recursive DNS servers will usually be supplied to the client through router advertisements
	  or DHCP.  For an administrative domain that uses subdomains of 'service.arpa.', "service.arpa.", the recursive resolvers provided by that
	  domain will be able to answer queries for subdomains of 'service.arpa.'; other "service.arpa.". Other (non-local) resolvers will not, or they
	  will provide answers that are not correct within that administrative domain.</t>
	<t>A host that is configured to use a resolver other than one that has been provided by the local network may not be unable able to
	  resolve,
	  resolve or may receive incorrect results for, for subdomains of 'service.arpa.'. "service.arpa.".  In order to avoid this, it is permissible
	  that hosts use the resolvers that are locally provided for resolving 'service.arpa.', "service.arpa.", even when they are configured to
	  use other resolvers.</t>
      </section>
      <section>
	<name>Caching DNS Servers</name>
	<t>There

<!-- [rfced] In the following text, before the two numbered points,
     the text reads "There are three considerations". Should we update
     "three" to "two", or is there another point that the text is
     missing?

Current:
   There are three considerations for caching DNS servers that follow
   this specification:</t>
	<ol>
	  <li>For specification:

   1.  For correctness, recursive resolvers at sites using
       'service.arpa.' must must, in practice practice, transparently support DNSSEC
       queries: queries for DNSSEC records and queries with the DNSSEC
       OK (DO) bit set (<xref target="RFC4035" section="3.2.1"
            sectionFormat="of"/>). (Section 3.2.1 of [RFC4035]).  DNSSEC validation
       is a Best Current Practice <xref target="RFC9364"/>: ([RFC9364]): although validation is not
       required, a caching recursive resolver that does not validate
       answers that can be validated may cache invalid data.
            This, in  In turn,
       this would prevent validating stub resolvers from successfully
       validating answers.  Hence, as a practical matter, recursive
       resolvers at sites using 'service.arpa' should do DNSSEC validation.</li>
	  <li>
	    <t>Unless
       validation.

   2.  Unless configured otherwise, recursive resolvers and DNS proxies
       MUST behave as described in Locally Served Zones,
	      <xref target="RFC6303" section="3" sectionFormat="of"/>. Zones (Section 3 of
       [RFC6303]).  That is, queries for 'service.arpa.' and subdomains
       of 'service.arpa.'  MUST NOT be forwarded, with one important
       exception: a query for a DS record with the DO bit set MUST
       return the correct answer for that question, including correct
       information in the authority section that proves that the record
       is nonexistent.</t>
            <t>So, nonexistent.

       So, for example, a query for the NS record for 'service.arpa.'
       MUST NOT result in that query being forwarded to an upstream
       cache nor to the authoritative DNS server for '.arpa.'.  However,
       as necessary to provide accurate authority information, a query
       for the DS record MUST result in forwarding whatever queries are
       necessary.  Typically, this will just be a query for the DS
       record since the necessary authority information will be included
       in the authority section of the response if the DO bit is set.
-->

	<t>There are three considerations for caching DNS servers that
	follow this specification:</t>

<!--[rfced In the following, is the intention to talk about the
     document status of RFC 9365 or to talk about the concept of
     DNSSEC validation as being a best current practice in the general
     sense?

Original:
DNSSEC validation is a Best Current Practice [RFC9364]:

Perhaps A:
"DNS Security Extensions (DNSSEC)" is a Best Current Practice
([RFC9364]) that describes DNSSEC validation:

Perhaps B:
DNSSEC (see [RFC9364]) validation is a best current practice:
-->

	<ol spacing="normal">
	  <li>For correctness, recursive resolvers at sites using
	  'service.arpa.' must, in practice, transparently support DNSSEC
	  queries: queries for DNSSEC records and queries with the DNSSEC OK
	  (DO) bit set (<xref target="RFC4035" section="3.2.1"
	  sectionFormat="of"/>).  DNSSEC validation is a Best Current Practice
	  (<xref target="RFC9364"/>): although validation is not required, a
	  caching recursive resolver that does not validate answers that can
	  be validated may cache invalid data.  In turn, this would prevent
	  validating stub resolvers from successfully validating
	  answers. Hence, as a practical matter, recursive resolvers at sites
	  using "service.arpa" should do DNSSEC validation.</li>
	  <li>
	    <t>Unless configured otherwise, recursive resolvers and DNS
	    proxies <bcp14>MUST</bcp14> behave as described in Locally Served
	    Zones (<xref target="RFC6303" section="3" sectionFormat="of"/>).
	    That is, queries for "service.arpa." and subdomains of
	    "service.arpa."  <bcp14>MUST NOT</bcp14> be forwarded, with one
	    important exception: a query for a DS record with the DO bit set
	    <bcp14>MUST</bcp14> return the correct answer for that question,
	    including correct information in the authority section that proves
that the record is nonexistent.</t>

<!--[rfced] Is this text redundant (with two uses of necessary)?  Does our
     suggestion change your intended meaning?

Original:
However, as necessary to provide accurate authority information, a
query for the DS record MUST result in forwarding whatever queries are
necessary; typically, ...

Perhaps:
However, to provide accurate authority information, a
query for the DS record MUST result in forwarding whatever queries are
necessary.
-->

            <t>So, for example, a query for the NS record for "service.arpa."
            <bcp14>MUST NOT</bcp14> result in that query being forwarded to an
            upstream cache nor to the authoritative DNS server for ".arpa.".
            However, as necessary to provide accurate authority information, a
            query for the DS record <bcp14>MUST</bcp14> result in forwarding
            whatever queries are necessary. Typically, this will just be a
            query for the DS record, record since the necessary authority information
            will be included in the authority section of the response if the
            DO bit is set.</t>
	  </li>
	</ol>
      </section>
      <section>
	<name>Authoritative DNS Servers</name>
	<t>No special processing of 'service.arpa.' "service.arpa." is required for authoritative DNS server implementations.  It is possible that an
	  authoritative DNS server might attempt to check the authoritative servers for 'service.arpa.' "service.arpa." for a delegation beneath that
	  name before answering authoritatively for such a delegated name.  In such a case, because the name always has only local
	  significance, there will be no such delegation in the 'service.arpa.' zone, and so "service.arpa." zone; therefore, the server would refuse to answer
	  authoritatively for such a zone.  A server that implements this sort of check MUST <bcp14>MUST</bcp14> be configurable so that either it does
	  not do this check for the 'service.arpa.' "service.arpa." domain or it ignores the results of the check.</t>
      </section>
      <section>

<!--[rfced] We are having trouble parsing this sentence. Is there text
     missing?

Original:
The operator for the DNS servers authoritative for 'service.arpa.' in
the global DNS will configure any such servers as described in Section
9.

Perhaps:
The operator for the DNS servers that are authoritative for "service.arpa." in
the global DNS will configure any such servers as described in Section
9.

-->

	<name>DNS Server Operators</name>
	<t>DNS server operators MAY <bcp14>MAY</bcp14> configure an authoritative server for 'service.arpa.' "service.arpa." for use with SRP.  The operator for the
	  DNS servers authoritative for 'service.arpa.' "service.arpa." in the global DNS will configure any such servers as described in
	  <xref target="delegation"/>.</t>
      </section>
      <section>
	<name>DNS Registries/Registrars</name>
	<t>'service.arpa.'
	<t>"service.arpa." is a subdomain of the 'arpa' "arpa" top-level domain, which is operated by IANA under the authority of the
	  Internet Architecture Board (IAB) according to the rules established in [RFC3172]. <xref target="RFC3172" format="default"/>.  There are no other DNS registrars for
	  '.arpa'.</t>
	  ".arpa".</t>
      </section>
    </section>

    <section anchor="delegation">
      <name>Delegation of 'service.arpa.'</name> "service.arpa."</name>
      <t>In order to be fully functional, the owner of the 'arpa.' "arpa." zone must add a delegation of 'service.arpa.' "service.arpa." in the '.arpa.' ".arpa."
	zone (see <xref target="RFC3172"/>. target="RFC3172"/>). This delegation is to be set up as was done for 'home.arpa', "home.arpa", as a result of the
	specification in <xref target="RFC8375" section="7" sectionFormat="of"/>. This is currently the responsibility of the IAB
	(see <xref target="IAB-ARPA"/></t> target="IAB-ARPA"/>).</t>
    </section>

    <section>
      <name>IANA Considerations</name>
      <section>
	<name>Registration

<!--[rfced] We have some questions about Section 10.1 in the IANA
     Considerations:

a) We see the title of the section is related to the first paragraph
only.  May we move the second paragraph to its own subsection?  If so,
please let us know how you would like the text to appear using
Old/New.

Original:

10.1.  Registration and Delegation of 'service.arpa' as a Special-Use
       Domain Name</name>
	<t>IANA Name

   IANA is requested to record the domain name 'service.arpa.' in the
   Special-Use Domain Names registry
	  <xref target="SUDN"/>. [SUDN].  IANA is requested, with
   the approval of IAB, to implement the delegation requested in
	  <xref target="delegation"/>.</t>

	<t>IANA
   Section 9.

   IANA is further requested to add a new entry to the "Transport-Independent "Transport-
   Independent Locally-Served Zones" subregistry of the "Locally-Served
   DNS Zones" registry <xref target="LSDZ"/>. [LSDZ].  The entry will be for the domain
   'service.arpa.' with the description "DNS&nbhy;SD "DNS-SD Service Registration
   Protocol Special-Use Domain", and listing this document as the
   reference.

b) The first paragraph of Section 10.1 mentions Section 9, which
states:

Original:
9.  Delegation of 'service.arpa.'

   In order to be fully functional, the owner of the 'arpa.' zone must
   add a delegation of 'service.arpa.' in the '.arpa.' zone [RFC3172].
   This delegation is to be set up as was done for 'home.arpa', as a
   result of the specification in Section 7 of [RFC8375].  This is
   currently the responsibility of the IAB [IAB-ARPA]

Should Section 9 be updated as follows since this action has been
taken?  Also, please review whether this information actually belongs
in the IANA section.  If so, please let us know (using old/new) how to
update.

9.  Delegation of "service.arpa."

The owner of the 'arpa.' zone, at the time of writing the IAB [IAB-ARPA],
has added a delegation of 'service.arpa.' in the '.arpa.' zone
[RFC3172], following the guidance provided in Section 7 of [RFC8375].

-->

	<name>Registration and Delegation of "service.arpa" as a Special-Use Domain Name</name>
	<t>IANA has recorded the domain name "service.arpa." in the "Special-Use Domain Names" registry
	  (see <xref target="SUDN"/>). IANA has implemented the delegation requested in
	  <xref target="delegation"/>.</t>

	<t>IANA has also added a new entry to the "Transport-Independent Locally-Served Zones Registry" registry of
	  the "Locally-Served DNS Zones" group (see <xref target="LSDZ"/>).  The entry is for the domain "SERVICE.ARPA" with the
	  description "DNS-SD Service Registration Protocol Special-Use Domain" and lists this document as the reference.</t>
      </section>

      <section anchor="subdomains">
<name>Subdomains of 'service.arpa.'</name> "service.arpa."</name>

	<t>This document only makes use of the 'default.service.arpa' "default.service.arpa" subdomain of 'service.arpa.' "service.arpa." Other subdomains are reserved for
	  future use by DNS-SD or related work. The IANA is requested to create a registry, has created the "service.arpa Subdomain" registry. registry (see <xref target="SUB"/>).
	  The IETF shall have has change control for this registry. New entries may be added either as a result of Standards Action
	  <xref
	  (<xref target="RFC8126" section="4.9"/> section="4.9" sectionFormat="of"/>) or with IESG approval <xref Approval (<xref target="RFC8126" section="4.10"/>, section="4.10" sectionFormat="of"/>), provided that a
	  specification exists <xref (<xref target="RFC8126" section="4.6"/>. section="4.6"/>).
	</t>
        <t>
	  The
	  IANA shall group has grouped the "service.arpa Subdomain" registry with the "Locally-Served DNS Zones" registry. group.
          The registry shall be is a table with three columns: the subdomain name (expressed as a fully-qualified fully qualified domain
	  name), a brief description of how it is used, and a reference to the document that describes its use in detail.
	</t>
	<t>
	  This initial contents of this registry shall begin are as the following table: follows:
	</t>
	<table>
	  <thead>
	    <tr>
	      <th>Subdomain Name</th>
	      <th>Description</th>
	      <th>reference</th>
	      <th>Reference</th>
	    </tr>
	  </thead>
	  <tbody>
	    <tr>
	      <td>default.service.arpa.</td>
	      <td>Default domain for SRP updates</td>
	      <td>[THIS DOCUMENT]</td>
	      <td>RFC 9665</td>
	    </tr>
	  </tbody>
	</table>
      </section>

      <section>
	<name>Service Name registrations</name> Registrations</name>
	<t>IANA is requested to add has added two new entries to the Service Names "Service Name and Transport Protocol Port Numbers registry. Number Registry" (see <xref target="PORT"/>). The following sections subsections
	  contain tables with the fields required by <xref target="RFC6335" section="8.1.1" sectionFormat="of"/>.</t>
      </section>

      <section>
<name>'dnssd-srp' Service Name</name>

	<table>
	  <thead><tr><td>Field Name</td><td>Value</td></tr></thead>
	  <thead><tr><th>Field Name</th><th>Value</th></tr></thead>
	  <tbody>
	    <tr><td>  Service Name       </td><td>  dnssd-srp                         </td></tr>
	    <tr><td>  Transport Protocol </td><td>  TCP  tcp                               </td></tr>
	    <tr><td>  Assignee           </td><td>  IESG &lt;iesg@ietf.org&gt;        </td></tr>
	    <tr><td>  Contact            </td><td>  IETF Chair &lt;chair@ietf.org&gt; </td></tr>
	    <tr><td>  Description        </td><td>  DNS-SD Service Registration Discovery          </td></tr>
	    <tr><td>  Reference          </td><td>  this document  RFC 9665                          </td></tr>
	    <tr><td>  Port Number        </td><td>  None                              </td></tr>
	    <tr><td>  Service Code       </td><td>  None                              </td></tr>
	  </tbody>
	</table>
      </section>
      <section>
	<name>'dnssd-srp-tls' Service Name</name>
	<table>
	  <thead><tr><td>Field Name</td><td>Value</td></tr></thead>
	  <thead><tr><th>Field Name</th><th>Value</th></tr></thead>
	  <tbody>
	    <tr><td>  Service Name       </td><td>  dnssd-srp-tls                     </td></tr>
	    <tr><td>  Transport Protocol </td><td>  TCP  tcp                               </td></tr>
	    <tr><td>  Assignee           </td><td>  IESG &lt;iesg@ietf.org&gt;                             </td></tr>
	    <tr><td>  Contact            </td><td>  IETF Chair Chair&lt;chair@ietf.org&gt;                        </td></tr>
	    <tr><td>  Description        </td><td>  DNS-SD Service Registration Discovery (TLS) </td></tr>
	    <tr><td>  Reference          </td><td>  this document  RFC 9665                          </td></tr>
	    <tr><td>  Port Number        </td><td>  None                              </td></tr>
	    <tr><td>  Service Code       </td><td>  None                              </td></tr>
	  </tbody>
	</table>
</section>
</section>
      <section>
	<name>Anycast Address</name>
	<t>IANA is requested to allocate has allocated an IPv6 Anycast address from the "IANA IPv6 Special-Purpose Address Registry, Registry" (see <xref target="IPv6"/>), similar to the Port
	  Control Protocol anycast address, address: 2001:1::1. The value TBD is to be replaced with the actual allocation in the table that
	  follows. The purpose of this allocation is to provide a fixed anycast address that can be commonly used as a destination for
	  SRP updates when no SRP registrar is explicitly configured. The initial values for the registry are:</t> are as follows:</t>

	<table>
	  <thead>
	    <tr><td>Attribute</td>                    <td>value</td></tr>
	    <tr><th>Attribute</th>                    <th>Value</th></tr>
	  </thead>
	  <tbody>
	    <tr><td>Address Block</td>                <td>2001:1::TBD/128</td></tr>                <td>2001:1::3/128</td></tr>
	    <tr><td>Name</td>                         <td>DNS-SD Service Registration Protocol Anycast Address</td></tr>
	    <tr><td>RFC</td>                          <td>[this document]</td></tr>                          <td>RFC 9665</td></tr>
	    <tr><td>Allocation Date</td>              <td>[date of allocation]</td></tr>              <td>2024-04</td></tr>
	    <tr><td>Termination Date</td>             <td>N/A</td></tr>
	    <tr><td>Source</td>                       <td>True</td></tr>
	    <tr><td>Destination</td>                  <td>True</td></tr>
	    <tr><td>Forwardable</td>                  <td>True</td></tr>
	    <tr><td>Global</td>
	    <tr><td>Globally Reachable</td>                       <td>True</td></tr>
	    <tr><td>Reserved-by-protocol</td>
	    <tr><td>Reserved-by-Protocol</td>         <td>False</td></tr>
	  </tbody>
	</table>
      </section>
    </section>

    <section>
      <name>Implementation Status</name>
      <t>[Note to the RFC Editor: please remove this section prior to publication.]</t>
      <t>
	This section records the status of known implementations of the protocol defined by this specification at the time of
	posting of this Internet-Draft, and is based on a proposal described in RFC 7942.  The description of implementations in
	this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs.  Please note that the
	listing of any individual implementation here does not imply endorsement by the IETF.  Furthermore, no effort has been spent
	to verify the information presented here that was supplied by IETF contributors.  This is not intended as, and must not be
	construed to be, a catalog of available implementations or their features.  Readers are advised to note that other
	implementations may exist.
      </t>
      <t>
	According to RFC 7942, "this will allow reviewers and working groups to assign due consideration to documents that have the
	benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented
	protocols more mature.  It is up to the individual working groups to use this information as they see fit".
      </t>
      <t>
	There are two known independent implementations of SRP requestors:
      </t>
      <ul>
	<li>SRP Client for OpenThread: https://github.com/openthread/openthread/pull/6038</li>
	<li>mDNSResponder open source project: https://github.com/Abhayakara/mdnsresponder</li>
      </ul>
      <t>
	There are two related implementations of an SRP registrar. One acts as a DNS Update proxy, taking an SRP Update and applying it
	to the specified DNS zone using DNS update. The other acts as an Advertising Proxy
	<xref target="I-D.ietf-dnssd-advertising-proxy"/>. Both are included in the mDNSResponder open source project mentioned above.
      </t>
    </section>

    <section>
      <name>Acknowledgments</name>
      <t>Thanks to <contact fullname="Toke Høiland-Jørgensen"/>, Jonathan Hui, Esko Dijk, Kangping Dong and Abtin Keshavarzian for
	their thorough technical reviews. Thanks to Kangping and Abtin as well for testing the document by doing an independent
	implementation. Thanks to Tamara Kemper for doing a nice developmental edit, Tim Wattenberg for doing an SRP requestor
	proof-of-concept implementation at the Montreal Hackathon at IETF 102, and Tom Pusateri for reviewing during the hackathon
	and afterwards. Thanks to Esko for a really thorough second last call review. Thanks also to Nathan Dyck, Gabriel
	Montenegro, Kangping Dong, Martin Turon, and Michael Cowan for their detailed second last call reviews. Thanks to Patrik
	Fältström, Dhruv Dhody, David Dong, Joey Salazar, Jean-Michel Combes, and Joerg Ott for their respective directorate
	reviews. Thanks to Paul Wouters for a <em>really</em> detailed IESG review! Thanks also to the other IESG members who
	provided comments or simply took the time to review the document.</t>
    </section>

  </middle>

  <back>
    <displayreference target="I-D.cheshire-dnssd-roadmap" to="ROADMAP"/>
    <displayreference target="I-D.ietf-dnssd-advertising-proxy" to="AP"/>

    <!-- <displayreference target="I-D.ietf-dnssd-hybrid" to="I-D.ietf-dnssd-hybrid"/> appears to not work in xml2rfc 2.6.2 --> target="I-D.ietf-snac-simple" to="SNAC-SIMPLE"/>

    <references>
      <name>References</name>
      <references>
      <name>Normative References</name>

<!-- [I-D.ietf-dnssd-update-lease]  companion document RFC 9664-->
      <reference anchor="RFC9664" target="https://www.rfc-editor.org/info/rfc9664">
	<front>
	  <title>An EDNS(0) Option to Negotiate Leases on DNS Updates</title>
	  <author fullname="Stuart Cheshire" initials="S." surname="Cheshire">
	    <organization>Apple Inc.</organization>
	  </author>
	  <author fullname="Ted Lemon" initials="T." surname="Lemon">
	    <organization>Apple Inc</organization>
	  </author>
	  <date month="October" year="2024"/>
	</front>
	<seriesInfo name="RFC" value="9664"/>
	<seriesInfo name="DOI" value="10.17487/RFC9664"/>
      </reference>

      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-dnssd-update-lease.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1035.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.1035.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1536.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.1536.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2136.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2136.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2181.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2181.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2539.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2539.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2782.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2782.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2931.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2931.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3172.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3172.xml"/>

<!-- [rfced] Normative reference RFC 3445 has been obsoleted by
RFC 4033.  We will update to the latter unless we hear objection.
-->

      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3445.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3445.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3596.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3596.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4035.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4035.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6303.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6303.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6763.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6763.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7858.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7858.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8085.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8085.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8375.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8375.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8624.xml"/> href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8624.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8765.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8765.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.9364.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9364.xml" />
    </references>

    <references>
      <name>Informative References</name>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2131.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2131.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2827.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2827.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3007.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.3007.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4861.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4861.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6105.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6105.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6335.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6335.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6760.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6760.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6761.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6761.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6762.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6762.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7084.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7084.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7228.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7228.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7413.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7413.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8415.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8415.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8520.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8520.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8766.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8766.xml" />
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8945.xml" href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8945.xml" />

<!-- [I-D.cheshire-dnssd-roadmap] IESG state: Expired as of 07/15/24-->
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml3/reference.I-D.cheshire-dnssd-roadmap.xml"/>
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-dnssd-advertising-proxy.xml"/> href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.cheshire-dnssd-roadmap.xml"/>

<!-- [I-D.ietf-snac-simple] IESG state: I-D Exists as of 07/15/24-->
      <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-snac-simple.xml"/> href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.ietf-snac-simple.xml"/>

      <reference anchor="SUDN" target="https://www.iana.org/assignments/special-use-domain-names/special-use-domain-names.xhtml"> target="https://www.iana.org/assignments/special-use-domain-names">
        <front>
          <title>Special-Use Domain Names Registry</title>
          <author/>
          <date month="July" year="2012"/> Names</title>
          <author>
	    <organization>IANA</organization>
	  </author>
        </front>
      </reference>

      <reference anchor="LSDZ" target="https://www.iana.org/assignments/locally-served-dns-zones/locally-served-dns-zones.xhtml"> target="https://www.iana.org/assignments/locally-served-dns-zones">
        <front>
          <title>Locally-Served DNS Zones Zones</title>
          <author>
	    <organization>IANA</organization>
	  </author>
        </front>
      </reference>

      <reference anchor="SUB" target="https://www.iana.org/assignments/locally-served-dns-zones/locally-served-dns-zones">
        <front>
          <title>service.arpa Subdomain</title>
          <author>
	    <organization>IANA</organization>
	  </author>
        </front>
</reference>

<reference anchor="PORT" target="https://www.iana.org/assignments/service-names-port-numbers">
        <front>
          <title>Service Name and Transport Protocol Port Number Registry</title>
          <author/>
          <date month="July" year="2011"/>
          <author>
	    <organization>IANA</organization>
	  </author>
        </front>
      </reference>

      <reference anchor="IPv6" target="https://www.iana.org/assignments/iana-ipv6-special-registry">
        <front>
          <title>IANA IPv6 Special-Purpose Address Registry</title>
          <author>
	    <organization>IANA</organization>
	  </author>
        </front>
      </reference>

      <reference anchor="IAB-ARPA" target="https://www.iab.org/documents/correspondence-reports-documents/2017-2/iab-statement-on-the-registration-of-special-use-names-in-the-arpa-domain/">
        <front>
          <title>Internet Architecture Board statement on the registration of special use names in the ARPA domain</title>
          <author/>
          <date month="March" year="2017"/>
        </front>
      </reference>

      <reference anchor="ZC">
        <front>
          <title>Zero Configuration Networking: The Definitive Guide</title>
          <author initials="S." surname="Cheshire" fullname="Stuart Cheshire"/>
          <author initials="D.H." surname="Steinberg" fullname="Daniel H. Steinberg"/>
          <author initials="S." surname="Cheshire" fullname="Stuart Cheshire"/>
          <date year="2005" month="December"/>
        </front>
        <seriesInfo name="O'Reilly
        <refcontent>O'Reilly Media, Inc." value=""/> Inc.</refcontent>
        <seriesInfo name="ISBN" value="0-596-10100-7"/> value="9780596101008"/>
      </reference>

    </references>
  </references>

  <!--[rfced] Might this be an agreeable update to the title of
       Appendix A (to avoid double -ing words in the beginning?)?

Original:

Appendix A.  Testing Using Standard DNS Servers Compliant with RFC
2136

Perhaps:

Appendix A.  Testing the Use of Standard DNS Servers Compliant with RFC
2136

Perhaps:

Appendix A.  Testing Standard DNS Servers Compliant with RFC 2136
-->

    <section>
      <name>Testing using standard RFC2136-compliant Using Standard DNS servers</name> Servers Compliant with RFC 2136</name>
      <t>
        It may be useful to set up an authoritative DNS server for testing that does not implement SRP.  This can be done by configuring the
        server to listen on the anycast address, address or by advertising it in the _dnssd&nbhy;srp._tcp.&lt;zone&gt; SRV and
        _dnssd&nbhy;srp&nbhy;tls._tcp.&lt;zone&gt; record.  It must be configured to be authoritative for
        "default.service.arpa",
        "default.service.arpa" and to accept updates from hosts on local networks for names under "default.service.arpa"
        without authentication, authentication since such servers will not have support for FCFS authentication (<xref target="fcfs"/>).</t>
      <t>
        An authoritative DNS server configured in this way will be able to successfully accept and process SRP Updates from requestors that send SRP
        updates.  However, no prerequisites will be applied, and applied; this means that the test server will accept internally
        inconsistent SRP Updates, Updates and will not stop two SRP Updates, Updates sent by different services, services that claim the same name(s), name or names
        from overwriting each other.</t>
      <t>
        Since SRP Updates are signed with keys, validation of the SIG(0) algorithm used by the requestor can be done by manually
        installing the requestor's public key on the DNS server that will be receiving the updates.  The key can then be used to
        authenticate the SRP update, update and can be used as a requirement for the update.  An example configuration for testing SRP
        using BIND 9 is given in <xref target="bind-example"/>.</t>
    </section>

    <section>
      <name>How to allow Allow SRP requestors Requestors to update standard RFC2136-compliant servers</name> Update Standard Servers Compliant with RFC 2136</name>
      <t>
        Ordinarily
        Ordinarily, SRP Updates will fail when sent to an RFC 2136-compliant a server compliant with <xref target="RFC2136"/> that does not implement SRP because the zone
        being updated is "default.service.arpa", "default.service.arpa" and because no DNS server that is not an SRP registrar would normally be configured to be
        authoritative for "default.service.arpa".  Therefore, a requestor that sends an SRP Update can tell that the receiving server
        does not support SRP, SRP but does support RFC2136, <xref target="RFC2136"/> because the RCODE will either be NotZone, NotAuth NotAuth, or Refused, Refused or because
        there is no response to the update request (when using the anycast address)</t> address).</t>
      <t>
        In this case case, a requestor MAY <bcp14>MAY</bcp14> attempt to register itself using regular RFC2136 DNS updates. updates described in <xref target="RFC2136"/>. To do so, it must discover the
        default registration zone and the DNS server designated to receive updates for that zone, as described earlier, using the
        _dns&nbhy;update._udp SRV record.  It can then send the update to the port and host pointed to by the SRV record, and it is
        expected to use appropriate prerequisites to avoid overwriting competing records.  Such updates are out of scope for SRP,
        and a requestor that implements SRP MUST <bcp14>MUST</bcp14> first attempt to use SRP to register itself, itself and only attempt to use RFC2136 backwards compatibility capability with <xref target="RFC2136"/>
        if that fails.  Although the owner name for the SRV record specifies the UDP protocol for updates,
        it is also possible to use TCP, and TCP SHOULD <bcp14>SHOULD</bcp14> be required to prevent spoofing.</t>
    </section>

    <section anchor="bind-example">
      <name>Sample BIND9 configuration Configuration for default.service.arpa.</name> "default.service.arpa."</name>

      <figure title="Zone Configuration in named.conf"><artwork><![CDATA[ named.conf">
<artwork><![CDATA[
zone "default.service.arpa." {
  type primary;
  file "/etc/bind/primary/service.db";
  allow-update { key demo.default.service.arpa.; };
};
                ]]></artwork></figure>
};]]></artwork>
</figure>

      <figure title="Example Zone file"><artwork><![CDATA[ File">
<artwork><![CDATA[
$ORIGIN .
$TTL 57600  ; 16 hours
default.service.arpa IN SOA          ns3.default.service.arpa.
                                     postmaster.default.service.arpa. (
                2951053287 ; serial
                3600       ; refresh (1 hour)
                1800       ; retry (30 minutes)
                604800     ; expire (1 week)
                3600       ; minimum (1 hour)
)
                        NS           ns3.default.service.arpa.
                        SRV 0 0 53   ns3.default.service.arpa.
$ORIGIN default.service.arpa.
$TTL 3600   ; 1 hour
_ipps._tcp              PTR          demo._ipps._tcp
$ORIGIN _ipps._tcp.default.service.arpa.
demo                    TXT          "0"
                        SRV 0 0 9992 demo.default.service.arpa.
$ORIGIN _udp.default.service.arpa.
$TTL 3600   ; 1 hour
_dns-update             PTR          ns3.default.service.arpa.
$ORIGIN _tcp.default.service.arpa.
_dnssd-srp              PTR          ns3.default.service.arpa.
$ORIGIN default.service.arpa.
$TTL 300    ; 5 minutes
ns3                     AAAA         2001:db8:0:1::1
$TTL 3600   ; 1 hour
demo                    AAAA         2001:db8:0:2::1
                        KEY 0 3 13 (
                           qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU
                           9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg==
                        ); alg = ECDSAP256SHA256 ; key id = 15008
                        AAAA    ::1
]]></artwork></figure>
]]></artwork>
</figure>

    </section>

    <section numbered="false">
      <name>Acknowledgments</name>
      <t>Thanks to <contact fullname="Toke Høiland-Jørgensen"/>, <contact
      fullname="Jonathan Hui"/>, <contact fullname="Esko Dijk"/>, <contact
      fullname="Kangping Dong"/>, and <contact fullname="Abtin Keshavarzian"/>
      for their thorough technical reviews. Thanks to <contact
      fullname="Kangping"/> and <contact fullname="Abtin"/> as well for
      testing the document by doing an independent implementation. Thanks to
      <contact fullname="Tamara Kemper"/> for doing a nice developmental edit,
      <contact fullname="Tim Wattenberg"/> for doing an SRP requestor
      proof-of-concept implementation at the Montreal Hackathon at IETF 102,
      and <contact fullname="Tom Pusateri"/> for reviewing during the
      hackathon and afterwards. Thanks to <contact fullname="Esko"/> for a
      really thorough second Last Call review. Thanks also to <contact
      fullname="Nathan Dyck"/>, <contact fullname="Gabriel Montenegro"/>,
      <contact fullname="Kangping Dong"/>, <contact fullname="Martin Turon"/>,
      and <contact fullname="Michael Cowan"/> for their detailed second last
      call reviews. Thanks to <contact fullname="Patrik Fältström"/>, <contact
      fullname="Dhruv Dhody"/>, <contact fullname="David Dong"/>, <contact
      fullname="Joey Salazar"/>, <contact fullname="Jean-Michel Combes"/>, and
      <contact fullname="Joerg Ott"/> for their respective directorate
      reviews. Thanks to <contact fullname="Paul Wouters"/> for a
      <em>really</em> detailed IESG review! Thanks also to the other IESG
      members who provided comments or simply took the time to review the
      document.</t>
    </section>

  </back>
</rfc>

<!-- Keep this comment [rfced] We had some questions about abbreviations:

a) Should "DNSSD" (in "non-DNSSD services" and "DNSSD discovery zone")
be updated to "DNS-SD" (hyphen) or "dnssd" (lowercase) to match prior
usage in the document?

b) Is the "Service" (or "Service Description") redundant here and in
similar cases throughout the document (as SD = Service Discovery)?
That is, just examples below, more cases exist.

Original:
DNS-SD Service registration uses public keys and SIG(0) to allow
services to defend their registrations.

Original:
Although in principle DNS-SD Service Description records can
include other record types with the same Service Instance Name, in
practice they rarely do.

c) For "TSIG", would you like us to expand to "transaction signature"
upon first usage to match RFC 8945?

Original:
   The format of the KEY resource record in the SRP Update is defined in
   [RFC3445].  Because the KEY RR used in TSIG is not a zone-signing
   key, the flags field in the KEY RR MUST be all zeroes.

d) Throughout the document, "SRP update" is used, and there is only
one instance of "SRV update". We wanted to make sure that "SRV" was
indeed intended and not "SRP".

Original:
   *  If there is one "Add to an RRset" SRV update, there MUST be at
   least one "Add to an RRset" TXT update.

e) We have updated to use the abbreviation CNN for Constrained-Node
Network (to match its use in RFC 7228).  Please review and let us know
any objections.  Further, please review uses of "constrained network"
and let us know if any of these should be updated to CNN as well.
-->

<!-- [rfced] We had some questions regarding capitalization of certain terms:

a) We see instances of "Anycast" (capitalized) and "anycast"
(lowercase) throughout the document, but we are unsure if certain
instances are part of proper names while other instances are more
generic. Please let us know if these need to be made more consistent
or if they are accurate as they currently are. We've listed a few
instances below.

Anycast vs. anycast:
   IPv6 Anycast address
   Port Control Protocol anycast address
   fixed anycast address
   anycast address

b) We see the following similar terms.  Please review and let us know
if/how to make these terms consistent.

   service instance name
   Service Instance Name
   "Service Instance Name"
   service instance
   Service Name

c) We see the following similar terms.  Please let us know how to
update for consistency.

BIND 9 vs. BIND9

d) We have updated the quoted terms that correspond to Sections 2.5.1
- 2.5.4 of RFC 2136 to appear consistently in double quotes and with
capitalization that matches those section titles.  Please let us know
any objections.

We further wondered if the following update should be made:

Original:
The target of the SRV RR Add...

Perhaps:
The "Add To An RRset" SRV update

Please review other terms similar to these titles if they exist and
let us know if any further changes should be made.

e) The NoError status names are in all caps in Section 2.2 of RFC
2136.  Should the following updates be made to match?

ServFail to SERVFAIL
Refused to REFUSED
YXDomain to YXDOMAIN

f) Regarding the terms “Service Description”, Service Discovery, and
“Host Description”.

- We see both Instruction and instruction when following these terms.
If/How may we make this uniform?

- Should “instruction” or the like should be inserted after instances
of these terms?  Sometimes they are followed by "record" or "update",
if they appear without a label, might this be confusing to the reader?

Example:
   The KEY record in Service Description updates MAY be omitted for
   brevity; if it is omitted, the SRP registrar MUST behave as if the
   same KEY record that is given for the Host Description is also given
   for each Service Description for which no KEY record is provided.

g) Please review the following similar terms and let us know if/how
they should be made uniform with regard to quotes and ending with a
period (note that this term would have IANA implications):

"default.service.arpa"
"default.service.arpa."
host.default.service.arpa
host-1.default.service.arpa
host-2.default.service.arpa
host-31773.default.service.arpa. (at end of sentence)
".service.arpa."
"service.arpa"
"service.arpa."

Further note that we have updated from single to double quotes around
terms that were quoted in the file
     Local variables:
     mode: sgml
     fill-column:132
     sgml-omittag:t
     sgml-shorttag:t
     sgml-namecase-general:t
     sgml-general-insert-case:lower
     sgml-minimize-attributes:nil
     sgml-always-quote-attributes:t
     sgml-indent-step:2
     sgml-indent-data:t
     sgml-parent-document:nil
     sgml-exposed-tags:nil
     sgml-local-catalogs:nil
     sgml-local-ecat-files:nil
     End: original consistently.  Please review
and let us know if further updates are necessary.

h) Please review the following for the use of quotes and consistent
use of SRV record.  Please let us know if/how to update.

"_dnssd-srp._tcp.<zone>" SRV record vs.  _dnssd-srp._tcp.<zone> SRV
"_dnssd-srp-tls._tcp.<zone>"  SRV record vs. _dnssd-srp-tls._tcp.<zone> record
_dns-update._udp SRV

-->

<!-- [rfced] Please review each artwork element in Appendix C in case
     they should be tagged as sourcecode or another element.
-->

<!-- [rfced] Please review the "Inclusive Language" portion of the
     online Style Guide
     <https://www.rfc-editor.org/styleguide/part2/#inclusive_language>
     and let us know if any changes are needed.

Note that our script did not flag any words in particular, but this
should still be reviewed as a best practice.
-->

</rfc>