draft-eckert-anima-grasp-dnssd-05.xml

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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc toc="yes"?>
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<?rfc symrefs="yes"?>
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<?rfc sortrefs="yes"?>
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<?rfc compact="yes"?>
<rfc category="std" docName="draft-eckert-anima-grasp-dnssd-05"
     ipr="trust200902">
  <front>
    <title abbrev="DNS-SD via GRASP">DNS-SD Compatible Service Discovery in
    GeneRic Autonomic Signaling Protocol (GRASP)</title>

    <author fullname="Toerless Eckert" initials="T.T.E." surname="Eckert">
      <organization abbrev="Futurewei">Futurewei Technologies USA
      Inc.</organization>

      <address>
        <postal>
          <street>2220 Central Expressway</street>

          <city>Santa Clara</city>

          <code>95050</code>

          <country>USA</country>
        </postal>

        <email>tte+ietf@cs.fau.de</email>
      </address>
    </author>

    <author fullname="Mohamed Boucadair" initials="M." surname="Boucadair">
      <organization>Orange</organization>

      <address>
        <postal>
          <street></street>

          <city>Rennes</city>

          <code>35000</code>

          <country>France</country>
        </postal>

        <email>mohamed.boucadair@orange.com</email>
      </address>
    </author>

    <author fullname="Christian Jacquenet" initials="C." surname="Jacquenet">
      <organization>Orange</organization>

      <address>
        <postal>
          <street></street>

          <city>Rennes</city>

          <code>35000</code>

          <country>France</country>
        </postal>

        <email>christian.jacquenet@orange.com</email>
      </address>
    </author>

    <author fullname="Michael H. Behringer" initials="M." surname="Behringer">
      <address>
        <email>michael.h.behringer@gmail.com</email>
      </address>
    </author>


    <area>Operations and Management</area>

    <workgroup>ANIMA WG</workgroup>

    <abstract>
      <t>DNS Service Discovery (DNS-SD) defines a framework for applications
      to announce and discover services. This includes service names, service
      instance names, common parameters for selecting a service instance
      (weight or priority) as well as other service-specific parameters. For
      the specific case of autonomic networks, GeneRic Autonomic Signaling
      Protocol (GRASP) intends to be used for service discovery in addition to
      the setup of basic connectivity. Reinventing advanced service discovery
      for GRASP with a similar set of features as DNS-SD would result in
      duplicated work. To avoid that, this document defines how to use GRASP
      to announce and discover services relying upon DNS-SD features while
      maintaining the intended simplicity of GRASP. To that aim, the document
      defines name discovery and schemes for reusable elements in GRASP
      objectives. </t>
    </abstract>

    <note title="Note to the RFC Editor">
      <t>Please replace all occurrences of rfcXXXX with the RFC number
      assigned to this document.</t>
    </note>
  </front>

  <middle>
    <section anchor="overview" title="Overview">
<t>GeneRic Autonomic Signaling Protocol (GRASP) <xref target="RFC8990"></xref>
is intended to be used for Service Announcement, Discovery and Selection especially
in network or for network services intended to be deployable without dependencies
against centralized "server" entities, such as fully autonomous networks or 
Autonomous Service Agents (ASA).</t>

<t>To support these goals, GRASP provides a hop-by-hop network wide flooding of
announcement or discover messages reliably and secured and without looping messages.
This flooding is achieved with a per-hop GRASP agent responsible for per-hop flooding
of GRASP messages.</t>

<t>While such flooding based procedures do not necessarily scale to arbitrarily
large number of services or services instances, it is easy to calculate how many
service anouncement and/or discovery messages can be supported in a target network
without exceeding reasonable limits on those service messages use of network 
resources. Typically, all services required by the network infrastructure, as well
as core application services will scale perfectly well with this model and eradicate
the requirement for provisioning of centralized entities and building redundancy
for them.</t>

<t>DNS-SD via mDNS <xref target="RFC6763"/> was introduced with the same purposes,
but does not have a solid multi-hop flooding modely to rely on because it solely relies
on ASM IP Multicast, and there is no IETF standards track solution through which this
service can be autonomously provided. Instead, it would have to rely on protocols
such as PIM-SM or Bidir-PIM which all require careful planning of centralized service
entities called Rendesvous points - as well as planning and deployment redundancy
for them. The non-ability to use his service for DNS-SD with mDNS first lead to
attempts building flooding for mDNS messages without an underlying IP multicast
service as an mDNS message flooding through various commercial vendors, but these
solutions all suffered from the problem, that mDNS messages themselves do not provide
the means for loop detection.</t>

<t>Ultimately, mDNS today is strongly recommended to
only be used within IP subnets, and no expectation of reach beyond a single subnet.
Instead, any larger-scale network deployments of mDNS would rely on mDNS to unicast
DNS proxies which in turn depend on explicitly provisioned and "centralized" deployed
DNS servers. Which is not a well enough feasible solution for service that easily
could and should operate autonomously: Just plug a few routers together, have services
on them be able to run and be used by any other client in the network without any
configuration. This is what ANIMA ANI achieves to deliver, but this is also what
very ilghtweight implementations of only GRASP on every router can deliver - without
necessarily requirring the rest of ANI - BRSKI or ACP.</t>

<t>What GRASP itself does not define though is what DNS-SD defines very well, and
that is the nature of what a service announcement/discover is: What is the name of
a service ? When there are multiple instances (entities) that offer the service,
how are they distinguished from one another (service-instance names) ? How should a client
for a service determine, which service instance to use ? Some services may be high
priority than others. Other instances may be equally well usebable but have different
performances and load sharing by clients is desired. These and others are all
questions and requirements for any service announcement/discovery/selection mechanism,
and DNS-SD has well defined them. So it seems frivolous to have to reinvent all
these solutions, especially when it would lead to useless duplication of IANA
registries such as service registries already existing for use with any service
discovery mechanism, but primarily used for DNS-SD.</t>

<t>When attempting to thus reuse what was well defined for DNS-SD, the first idea
coming to mind is likely to simply encapsulate mDNS messages into GRASP, but that
wold simply create a lot of unnecessary overhead on the wire as well as unnecessary
processing.</t>

<t>As RFC6763 explains, DNS-SD itself is not necessarily the ideal way to define
signalling for service announcement/discovery/selection, but it is based on decades long
experience in Apple with the (proprietary) Name Binding Protocol (NBP), and DNS-SD
was merely the approach on how to map the information required for services into DNS. Both
DNS unicast, as well as DNS multicast (mDNS). This effectively lead to a whole layer
of complexity, which is to split of the information required for a single service into
multiple DNS Resource Records (DNS-RR) because that is how DNS operates. In result, a
single DNS-SD service instance consists of a SRV RR, PTR RR, TXT RR, A and/or AAAA
RR.</t>

<t>None of this complexity is necessary in GRASP, because in GRASP it is very simple
to define a CBOR structure carrying all the desired information elements for a
service instance announcement and/or discovery, and this document is exactly doing this:
Specifying a direct binding from the service instance information elements as
specified in RFC6760 and then detailled in DNS-SD (RFC6763) into a single type of
GRASP message (GRASP objective) so that there can be a single consistent service instance
definition with its information elements, but two different mappings into separate
underlying "protocol machineries": DNS-SD into DNS (unicast/multicast) and this document
defining mapping into GRASP.</t>

<t>One of the big benefits of this approach is that it also allows to easily convert
DNS-SD service information into  GRASP and vice versa. For example via proxies. It is
equally possible to build APIs for applications that only need to be concerned with
the service information elements and let the underlying SDK determine whether to use
DNS-SD and/or GRASP to signal it.</t>

<t>While the focus of this document is to define GRASP service data encoding and signaling
primarily for the flooding based methods in GRASP, they can equally be applied to the
unicast signaling methods of GRASP. However, this document (in this version) does not
aim to provide a 100% mapping of all features of DNS-SD. This may change inf future revisions,
but for now, the document concentrates on service announcement and discovery within a
single local domain. Somthing which in DNS is covered via domain ".local" in mDNS and
an appropriate mapping into some named local domain in unicast DNS. The reason for this
limitation is simply that there is as of today no well developed structuring of flooding
GRASP, and as such the best constraint to be put onto the use of GRASP for flooded service
announcemenet/discovery is by constraining it to the equivalent of ".local".</t>

<t>To not limit deployment of solutions in need of broader DNS services,
the mechanisms in this document allows for automatically discovering DNS-SD servers
via GRASP and thus easy building of hybrid solutions leveraging the best of GRASP
and DNS: Use GRASP for local domain (but potentially large scale) flooding based
discovery/selection via GRASP eliminating multicast-DNS and need for DNS servers,
and use unicast-DNS for any services that can not be deployed without dependency against
centralized DNS servers anyhow.</t>

    </section>

    <section anchor="notation" title="Terminology">
      <t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
      "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
      "OPTIONAL" in this document are to be interpreted as described in BCP 14
      <xref target="RFC2119"></xref> <xref target="RFC8174"></xref> when, and
      only when, they appear in all capitals, as shown here.</t>

      <t>This document makes use of terms and concepts defined in <xref
      target="RFC8990"></xref>.</t>
    </section>

    <section anchor="normative" title="Specification">
      <section anchor="service-objectives" title="Service and Name Objectives">
        <t>Unsolicited, flooded announcements (M_FLOOD) in GRASP and solicited
        flooded discovery (M_DISCOVERY) operate on the unit of GRASP technical
        objectives (identified by 'objective-names' as discussed in Section
        2.10 of <xref target="RFC8990"></xref>). Therefore, a scheme is
        required to indicate services via 'objective-names'. </t>

        <t><list style="empty">
            <t>Note: Future work may want to reuse the encodings related to
            services (defined below in this document) inside other (multicast
            or unicast only) objective exchanges, in which case the service
            names are not impacted.</t>
          </list></t>

        <t>When a technical objective (simply referred to as objective) is
        meant to be solely about a service name, the objective MUST uses an
        'objective-name' of 'SRV.&lt;service-name&gt;'. This naming scheme is
        meant to avoid creating duplicates and, potentially, inconsistent name
        registrations for those objectives vs. registrations done, for
        example, for DNS-SD. </t>

        <t>When an objective is meant announcement and discovery of a DNS
        compatible &lt;name&gt; such as "www-internal" in
        "www-internal.example.com", the objective SHOULD use an objective-name
        of NAME.&lt;name&gt;. See <xref target="name-resolution"></xref> for
        more details.</t>
      </section>

      <section anchor="elements-structure"
               title="Objective Value Reuseable Elements Structure">
        <t>Because service discovery, as explained in the prior section, needs
        to utilize different objectives, it requires cross-objective
        standardized encoding of the elements of services. GRASP does not
        define standardized message elements for the message body (called
        "objective-value") of GRASP messages. Therefore, this document
        introduces such a feature.</t>

        <t><figure>
            <artwork><![CDATA[         
objective-value  /= { 1*elements }
elements        //= ( @rfcXXXX: { 1*relement } )

relement  = ( relement-codepoint => relement-value )
relement-codepoint = uint
relement-value     = any

]]></artwork>
          </figure></t>

        <t>If an objective relies upon reusable elements, the
        'objective-value' MUST be a CBOR map and the reusable elements are
        found under the key "@rfcXXXX". </t>

        <t>Objectives that do not want reusable elements may use any
        objective-value format including a CBOR map, but they can not use the
        "@rfcXXXX" key if they use a map. This approach was chosen as the
        hopefully least intrusive mechanism given how by nature all of
        "objective-value" is meant to be defined by individual objective
        definitions.</t>

        <t>The value of "@rfcXXXX" is a map of reusable elements. Each
        'relement' has an IANA registered element-name and codepoint (see
        <xref target="iana-considerations"></xref>). The element-name is for
        documentation purposes only, CBOR encodings only use the numeric
        codepoint for encoding efficiency to minimize the risk for this
        solution to not be applicable to low-bitrate networks such as in
        IoT.</t>

        <t>Format and semantic of the relement-value is determined by the
        specification of the reusable element as is the fact whether more than
        one instances of the same reusable element are permitted.</t>

        <t>Reusable elements should be defined to be extensible. The methods
        used depend on the complexity of the element and the likely need to
        extend/modify the element with backward or non-backward compatible
        information. The following is a set of initial options to choose
        from:</t>

        <t>Element values that are a map MUST permit and reserve key value 0
        (numerical) for private extensions of the element defined by the
        individual objective.</t>

        <t>Element values that are a map MUST NOT use bareword key values
        starting with a "_". These too are for private extensions defined by
        the individual objective.</t>

        <t>Element values SHOULD be defined so that additional keys in maps
        and additional elements at the end of arrays can be ignored by prior
        versions of the definition. Whenever a newer definition is made for an
        element where this rule is violated, the element SHOULD be changed in
        a way for older version recipients to recognize that it is not
        compatible with it.</t>

        <t>One method to indicate compatibility is a traditional version
        "&lt;mayor&gt;.&lt;minor&gt;". Within the same &lt;mayor&gt; version
        number, increasing &lt;minor&gt; version numbers must be backward
        compatible. Different &lt;mayor&gt; version numbers are not expected
        to be compatible with each other. If they are, then this can be
        indicated by including multiple version numbers.</t>

        <t>A compressed form of version compatibility information is the use
        of a simple bitmask element where each bit indicates a version that
        the represented data is compatible with.</t>
      </section>

      <section anchor="standard-elements" title="Reuseable Elements">
        <section anchor="sender-loop-count" title="Sender Loop Count">
          <t><figure>
              <artwork><![CDATA[

                
relement-codepoint //= ( &(sender-loop-count:1) => 1..255 )


]]></artwork>
            </figure></t>

          <t>Sender-loop-count is set by the sender of an objective message to
          the same value as the loop-count of the message. On receipt,
          distance = ( sender-loop-count - loop-count ) is the distance of the
          sender from the receiver in hops. This element can be used for
          informational purposes in M_FLOOD and M_DISCOVERY messages and may
          be required to be used in these messages by the specification of
          other elements (such as the service element described below). This
          element MUST occur at most once. If a receiver expects to use the
          distance but sender-loop-count was not announced, then distance
          SHOULD be assumed to be 255 by the receiver.</t>
        </section>

        <section anchor="service-element" title="Service Element">
          <t>The srv-element (service element) is a reusable element to
          request or announce a service instance or to request and list
          service instance names.</t>

          <t><figure>
              <artwork><![CDATA[

                
relement-codepoint //= ( &(srv-element:2) => context-element )

context-element  =  {
     ?( &(private:0)      => any),
     ?( &(msg-type:1      => msg-type),
     ?( &(service:2)      => tstr),
     *( &(instance:3)     => tstr),
     ?( &(domain:4)       => tstr),
     ?( &(priority:5)     => 0..65535 ),
     ?( &(weight:6)       => 0..65535 ),
     *( &(kvpairs:7)      => { *(tstr: any) },
     ?( &(range:8)        => 0..255 ),
     *( &(clocator:9)     => clocator),
    }
clocator = [ context, locator-option ]
context = cstr
locator-option = ; from GRASP

msg-type = &( describe: 0, describe-request:1,
             enumerate:2, enumerate-request:3 )



]]></artwork>
            </figure></t>

          <t><list style="hanging">
              <t hangText="Service:">A service name registered according to
              RFC6335. If it is not present, then objective-name MUST be
              SRV.&lt;service-name&gt; where &lt;service-name&gt; is the
              service-name.</t>

              <t hangText="Instance:">The &lt;Instance&gt; of a DNS-SD Service
              Instance Name ( &lt;Instance&gt; . &lt;Service&gt; .
              &lt;Domain&gt;). It is optional, see <xref
              target="dnssd-comparison"></xref>.</t>

              <t hangText="Domain:">The equivalent of the &lt;Domain&gt; field
              of a DNS-SD Service Instance Name. If domain is not present,
              this is equivalent to ".local" in DNS (as introduced by mDNS)
              and implies the unnamed "local" domain, which is the GRASP
              domain across which the message is transmitted.</t>

              <t hangText="Priority, Weight:">Service Instance selection
              criteria as defined in RFC2782. If either one is not present,
              its value defaults to 0.</t>

              <t hangText="Kvpairs:">Map of key/value pairs that are service
              parameters in the same format as the key/value pairs in TXT
              field(s) of DNS-SD TXT records as defined in RFC6763, section
              6.3.</t>

              <t hangText="Range:">Allows to flexibly combine distance and
              priority/weight based service selection according to the
              definition of distance in <xref
              target="sender-loop-count"></xref>.</t>

              <t>If min-distance is the distance of the closest service
              announcer, and min-range the range announced by it, then the
              recipient MUST consider the priority/weight of all service
              announcers that are not further away than (min-distance +
              min-range). If not included, range defaults to 255.</t>

              <t>If range is announced, the sender-loop-count element MUST
              also be announced.</t>

              <t hangText="Clocator:">The "contextual locator" allows to
              indicate zero or more locators for the indicated service
              instance. The context element indicates in which context the
              locator-option is to be resolved. The reserved context value of
              "" (empty string) indicates the GRASP domain used, aka: the
              "local" context in which the service announcement is made. The
              reserved context value of "0" indicates the default routing
              context of the announcing node. This is often called "global
              table", "VRF 0" or "default VRF" on nodes using the "VRF"
              abstraction. Any other value is a string specifying a context
              such as another VRF.</t>

              <t>The mechanism by which originator and recipient of the
              srv-element agree on common naming for contexts is outside the
              scope of this specification. The context therefore allows to
              indicate locators both for the context through which the GRASP
              message distributed the srv-element (GRASP domain) as well as
              that for other contexts. Assume the GRASP domain is the ACP,
              then clocators in ACP would have a context of "", clocators in
              the global routing table (part of the data-plane) a context of
              "0", and clocators on other VRFs (also part of data-plane) a
              clocator that is their string name.</t>

              <t>If no locators are indicated, then the locator of the
              service(s) is the optional locator-option of the GRASP message
              in which the objective is contained meant to be used for the
              service(s) indicated and the clocator implied is "".</t>

              <t>If locator(s) are indicated, the messages location-option
              must be ignored for the service (but may be necessary to be
              present for other purposes of the objective).</t>

              <t hangText="Msg-type">Type (aka: intention) of the srv-element.
              If not present, it is assumed to be "describe".</t>
            </list></t>

          <t><list style="hanging">
              <t hangText="Describe:">Describes one service instance. At least
              one clocator is required for a positive response, all other
              fields are permitted, but optional. "Describe" is used in
              M_FLOOD for unsolicited announcements of services (flooded), in
              M_RESPONSE messages for solicited announcements of a service and
              in M_NEGOTIATE for negotiated announcements (both unicasted). If
              clocator is not included, then all fields except service and
              instance (and msg-type and private) must not be included and the
              srv-element provides a negative reply: No information about this
              service/service instance. This is only permitted in unicasted
              "describe" messages.</t>

              <t hangText="Describe-request:">Request for a "describe" reply.
              It is used in M_DISCOVERY (flooded) for solicited discovery of
              services or in M_REQ_SYN (unicasted) for negotiated discovery of
              service instance(s). In "describe-request", only service is
              mandatory (but can be provided via the objective-name field of
              the message), and domain is optional. "Instance" is optional. If
              provided, then the recipient is asked to provide information
              about the named instance only. All other fields of srv-element
              are to be ignored by the receiver in this specification, but a
              semantic for setting them may be introduced in follow-up work,
              specifically to filter replies by the indicated fields.</t>

              <t>"Describe-request" without instance MAY be answered by
              "Enumerate" (see below) if the responder has so many instances
              that it thinks the initiator should rather first select one or
              fewer instances and ask for their description. The sender of te
              "Describe-request" MUST be prepared to accept that answer and as
              necessary follow up with "Describe-request" with the instance
              names of interest.</t>

              <t hangText="Enumerate:">Used in the same GRASP messages as
              "describe", but instead of providing information about one
              service instance, it is listing service instance names. The
              purpose of enumerate is the same as browsing a service in
              DNS-SD. It would be followed by some human or automated
              selection of one or more instances and then a "describe"
              M_REQ_SYN request for those instances sent to the source of the
              "enumerate" to learn about the locators and other parameters of
              the service instances.</t>

              <t>In this specification, all fields other than service,
              instance and domain (and msg-type and private) must be unset in
              "enumerate".</t>

              <t hangText="Enumerate-request:">Requests an "enumerate" reply.
              It is used in the same way as "Describe-request" except that
              instance would usually not be set (because in that case it is
              more useful to send a "Describe-request").</t>
            </list></t>
        </section>

        <section anchor="name-resolution" title="Name Element">
          <t>The NAME,&lt;name&gt; elements is meant to provide basic name
          resolution comparable to mDNS name resolution for GRASP domains
          where this is desirable and no better name resolution exist - for
          example in the ACP where there is no requirement for DNS.</t>

          <t>Because the GRASP service lookup (unlike) DNS does not mandate
          that nodes have names (not even service instance names), the use of
          names is primarily meant to support legacy software. New designs
          should instead look up only services and service instance names, and
          nodes should announce their names as service instance names for the
          services they offer:</t>

          <t>For example consider a GRASP (ACP) domain of "example.com". The
          node providing some "www" service could have a name "www-internal"
          which means GRASP objective NAME.www-internal, that objective value
          would include primarily the nodes IP address(es) and the port number
          for the www service would have to be guessed (80). Better, the node
          would announce GRASP objective SRV.www and the objective value would
          include the service instance name www-internal and the (TCP) port
          information (80 or a non-default port).</t>

          <t><figure>
              <artwork><![CDATA[

                
relement-codepoint //= ( &(name-element:3) => context-element )

context-element //= {
     *( &name:10)         => tstr),
    }

ipv6-address-option = [O_IPv4_ADDRESS, ipv6-address]
ipv4-address-option = [O_IPv6_ADDRESS, ipv6-address]
locator-option /= ipv4-address-option
locator-option /= ipv6-address-option



]]></artwork>
            </figure></t>

          <t>Name information is carried in the name-element relement. It is a
          context-element like the one used for srv-element except that it
          adds the name component and that it does not permit the service and
          instance components and that it allows only describe and
          describe-request values in the msg-type. Clocators MUST use the
          ipv6-address-option or ipv4-address-option in the locator-option
          component.</t>

          <t>TBD: Unclear if/how we should best formalize the differences in
          the context element permitted information between services and
          names. The above is quite informal.</t>

          <t>Priority, weight, kvpairs, range (and of course private) MAY be
          used in describe messages to support multiple instances of the same
          name, as used for name anycast/prioritycast.</t>

          <t>Nodes may have multiple names. These can be listed in the name
          component. If a nodes names have the notion of a primary name and
          secondary names then the primary name should be the first in the
          list of names. In DNS-SD, the name pointed to by CNAME RRs can be
          considered to be the primary name. A describe-request for a
          non-primary name SHOULD return in the list of names the requested
          name and the primary name.</t>

          <t>Note that there is no reverse lookup defined in this version of
          the document (no lookup from IP address to name).</t>
        </section>
      </section>
    </section>

    <section anchor="explanations" title="Theory of Operation">
      <t></t>

      <section anchor="usage" title="Using GRASP Service Announcements">
        <t>TBD: This section contains a range of details that should become
        normative in later versions.</t>

        <t>This section provides a step by step walk-through of how to use
        GRASP service announcements and compares it to DNS-SD.</t>

        <t>The most simple method to use GRASP service discovery is to select
        (and if still necessary, register) a &lt;service-name&gt; and start
        one or more agents (e.g.: ASAs) announcing their service instance(s)
        via GRASP. At minimum, an agent should periodically (default 60
        seconds) announce the service instance via GRASP M_FLOOD messages as
        an objective SRV.&lt;service-name&gt; with a srv-element and a
        sender-loop-count element (default 255). The ttl of the GRASP message
        should be 3.5 times the announcement period, e.g.: 210000 msec.</t>

        <t>Consumers of the service will use GRASP to learn of the service
        instances and select one. This approach is most similar to the use of
        DNS-SD with mDNS except that the scope of the announcement is a whole
        GRASP domain (such as the ACP) as opposed to a single IP subnet in
        mDNS and that mDNS primarily relies on request &amp; reply but in its
        standard not on periodic unsolicited announcements. We describe here
        the unsolicited flooding option via M_FLOOD first because it is
        recommended for services with a dense population of service consumers
        and it is most simple to describe.</t>

        <t>On the service announcer, the parameters priority, weight and range
        of the service instance can be selected from intent or configuration -
        or left at default. The default range 255 will result in selection of
        a random target of the service like in DNS-SD. Setting priority/weight
        allows to prioritize and weigh the selection as in DNS-SD. Setting
        range to 0 allows to select the closest target, priority/weight are
        only compared between targets of the same shortest distance. Distance
        based options are not available in DNS-SD because it does not expect
        that network distance is available to arbitrary DNS-SD client. It is
        available to GRASP clients though. Using 0 &lt; range &lt; 255 allows
        for a hybrid priority/weight and distance based service selection
        (e.g.: Select the highest priority instance within a range of 5
        hops).</t>

        <t>If the service is a non-GRASP service, then the result of the
        service discovery has to be a transport locator to which the client
        can open a connection and talk the protocol implied by the service.
        This transport locator(s) have to be put into the clocator parameter.
        The context of the clocator would normally be "", aka: the transport
        locator is in the IP reachability associated with the GRASP domain
        (e.g.: IPv6 of the ACP for ACP GRASP domain).</t>

        <t>If an ACP service is announced via ACP GRASP, then the locator(s)
        can be O_IPv6_LOCATOR or O_FQDN_LOCATOR. The O_IPv6_LOCATOR is used if
        the service is defined to be available via some transport layer port
        (TCP, UDP or other). The determination of the actual transport
        connection to be used is the same as in DNS-SD: If the transport
        protocol is not TCP or UDP, it has to be implied by the specification
        of &lt;service-name&gt; or can be detailed in kvpairs which carries
        the same information as DNS-TXT TXT RRs of the service. Alternatively,
        the transport-proto field of the locator can contain any valid IP
        protocol directly (TBD), which is not possible in DNS-SD.</t>

        <t>Like DNS-SD, service discovery via GRASP does not require
        allocation and use of well-known ports for services. Unlike DNS-SD,
        there is no need in GRASP to define service instance names or target
        names. In DNS SD, PTR RRs resolve from a service name to a set of
        service instance named. SRV and TXT RRs resolve from service instance
        names to service instance parameters including the target. A target is
        the DNS host name of the service instance. It gets resolved via A/AAAA
        RRs to IPv4/IPv6 addresses of the target. In GRASP service discovery,
        host names are not used. Service instance names are optional too.
        Service instance names are useful for human diagnostics and human
        selection of service instances. In fully automated environments, they
        can be are less important. For diagnostic purposes, it is recommended
        to give service instances service instance names in GRASP service
        announcements.</t>

        <t>A locator with O_URI_LOCATOR type can be used in GRASP to indicate
        a URI for the transport method for a service instance. If the URI
        includes a host part, care must be taken to use only IP addresses in
        the host part if the context of the GRASP domain does not support host
        name resolution - such as the ACP - or to use the GRASP name
        resolution mechanisms described elsewhere in this document. And that
        the addresses indicated are also reachable in the GRASP domain. For
        example, in service announcements across a DULL GRASP domain, only the
        IPv6 link-local addresses on that subnet must be used (this applies
        equally when using the O_IPv6_LOCATOR).</t>

        <t>Instead of using M_FLOOD to periodically announce service
        instances, M_DISCOVERY can be used to actively query for service
        instances. The msg-type type must then be "describe-request". Because
        no periodic flooding is necessary, this solution is more lightweight
        for the network when the number of requesting clients is small. Note
        though that the M_DISCOVERY will terminate as soon as a provider of
        the objective is found, so the service instances found will be based
        on distance and therefore selection of instance by priority and weight
        will not work equally well as with M_FLOOD. Consider for example a
        central service instance in the NOC that should always be used (for
        example for centralized operational diagnostics) unless the WAN
        connection is broken, in which case distributed backup service
        instances should be used. With the current logic of M_DISCOVERY this
        is not possible.</t>
      </section>

      <section anchor="dnssd-comparison"
               title="Further Comparison with DNS-SD">
        <t>Neither the GRASP SRV.* objective-name, the service name nor any
        other parameter explicitly indicate the second label "_tcp" or "_udp"
        of DNS-SD entries. DNS-SD, RFC6763 explains how this is an
        unnecessary, historic artifact.</t>

        <t>This version of the document does not define an equivalent to
        "_sub" structuring of service enumeration.</t>

        <t>This version of the document does not define mechanisms for reverse
        resolution of arbitrary services: An inquirer may unicast M_SYNC_REC
        to a node with a series of objectives with specific service names of
        interest and describe-request, but there is no indication of "ANY"
        service.</t>
      </section>

      <section anchor="open-issues" title="Open Issues">
        <t>TBD: Examine limitations mentioned in "in this version of the
        text/document".</t>

        <t>TBD: The GRASP specification does currently only permit TCP and UDP
        for the transport-proto element. This draft should expand the GRASP
        definitions to permit any valid IP protocol. We just need to decide
        whether this should only apply to the locator in the srv element or
        also retroactive to the locator-option in GRASP messages (maybe not
        there ?).</t>

        <t>TBD: A fitting CBOR representation for a kvpair key without value
        needs to be specified so that it can be distinguished from an empty
        value as outlined in RFC6763 section 6.4.</t>

        <t>TBD: In this version, every service/service-instance is an element
        by itself. Future versions of this document may add more encoding
        options to allow more compact encoding of recurring fields.</t>

        <t>TBD: Is there a way in CDDL to formally define the string names of
        the relement-codepoint's ?</t>
      </section>
    </section>

    <section anchor="security" title="Security Considerations">
      <t>TBD.</t>

      <t>GRASP-related security issues are discussed in Section 3 of <xref
      target="RFC8990"></xref>.</t>
    </section>

    <!-- security -->

    <section anchor="iana-considerations" title="IANA Considerations">
      <t>This document requests IANA to create a new "GRASP Objective Value
      Standard Elements" subregistry under the "GeneRic Autonomic Signaling
      Protocol (GRASP) Parameters" registry. </t>

      <t>The values in this table are names and a unique numerical value
      assigned to each name. Future values MUST be assigned using the RFC
      Required policy as dedfined in Section 4.7 of <xref
      target="RFC8126"></xref>. The numerical value is simply to be assigned
      sequentially. The following initial values are assigned by this
      document:</t>

      <t>sender-loop-count 1 [defined in rfcXXXX]</t>

      <t>srv-element 2 [defined in rfcXXXX]</t>

      <t>name-element 3 [defined in rfcXXXX]</t>

      <t>This document updates the handling of the "GRASP Objective Names"
      Table introduced in the GRASP IANA considerations as follows:</t>

      <t>Assignments for objective-names of the form "SRV.&lt;text&gt;" and
      "NAME.&lt;text&gt;" are special.</t>

      <t>Assignment of "SRV.&lt;text&gt;" can only be requested if
      &lt;text&gt; is also a registered service-name according to RFC6335. The
      specification required for registration of a "GRASP Objective Name" MUST
      declare that the intended use of the objective name in GRASP is intended
      to be compatible with the indented use of the registered service
      name.</t>

      <t>Registration of "SRV.&lt;text&gt;" in the "GRASP Objective Name"
      table is optional, but recommended for all new service-names that are
      meant to be used with GRASP. Non-registration can for example happen
      with DNS-SD &lt;-&gt; GRASP gateways that inject pre-existing
      service-names into GRASP. Note that according to the GRASP RFC,
      registration is mandatory, so this exemption for "SRV.&lt;text&gt;" is
      also an update to that specification.</t>

      <t>There MUST NOT be any assignment for objective names of the form
      "NAME.&lt;text&gt;". These names are simply used by GRASP nodes without
      registration (just like names in mDNS).</t>
    </section>

    <section anchor="ack" title="Acknowledgements">
      <t></t>
    </section>

    <!-- ack -->

    <section anchor="contributors" title="Contributors">
      <t>Brian Carpenter</t>

    <!--

    <author fullname="Brian Carpenter" initials="B." role="editor"
            surname="Carpenter">
      <organization abbrev="Univ. of Auckland"></organization>

      <address>
        <postal>
          <street>School of Computer Science</street>

          <street>University of Auckland</street>

          <street>PB 92019</street>

          <city>Auckland</city>

          <code>1142</code>

          <country>New Zealand</country>
        </postal>

        <email>brian.e.carpenter@gmail.com</email>
      </address>
    </author>
    -->

    </section>


    <section anchor="changes" title="Change log [RFC Editor: Please remove]">
      <section title="05">
      <t>Rewrote overview section in response to review comments by Peter vdS and Esko (hopefully better justification/explanation). Thanks!</t>
      </section>
      <section title="04 - Refresh">
      </section>
      <section title="03 - Refresh">
      </section>

      <section title="02 - Revived after charter round 1 finished">
        <t>Reviving after ANIMA charter 01 is finished, adding new co-authors, contributors.</t>
        <t>Textual improvements, updating references.</t>
      </section>

      <section title="01 - ">
        <t>Only refreshing, no changes since -00.</t>
      </section>

      <section title="00 - Initial version"></section>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include="reference.RFC.8990"?>

      <?rfc include='reference.RFC.2119'?>

      <?rfc include='reference.RFC.8174'?>

      <?rfc include='reference.RFC.6763'?>

      <?rfc include='reference.RFC.8126'?>
    </references>

    <references title="Informative References">
      <?rfc include="reference.RFC.8994"?>
    </references>
  </back>
</rfc>