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DNS-SD Compatible Service Discovery in GeneRic Autonomic Signaling Protocol (GRASP)

Document Type Active Internet-Draft (individual)
Authors Toerless Eckert , Mohamed Boucadair , Christian Jacquenet , Michael H. Behringer
Last updated 2024-01-05
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ANIMA WG                                                   T.T.E. Eckert
Internet-Draft                                                 Futurewei
Intended status: Standards Track                            M. Boucadair
Expires: 8 July 2024                                        C. Jacquenet
                                                            M. Behringer
                                                          5 January 2024

   DNS-SD Compatible Service Discovery in GeneRic Autonomic Signaling
                            Protocol (GRASP)


   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.

Note to the RFC Editor

   Please replace all occurrences of rfcXXXX with the RFC number
   assigned to this document.

Status of This Memo

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

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

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

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   This Internet-Draft will expire on 8 July 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Service and Name Objectives . . . . . . . . . . . . . . .   6
     3.2.  Objective Value Reuseable Elements Structure  . . . . . .   6
     3.3.  Reuseable Elements  . . . . . . . . . . . . . . . . . . .   8
       3.3.1.  Sender Loop Count . . . . . . . . . . . . . . . . . .   8
       3.3.2.  Service Element . . . . . . . . . . . . . . . . . . .   8
       3.3.3.  Name Element  . . . . . . . . . . . . . . . . . . . .  11
   4.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Using GRASP Service Announcements . . . . . . . . . . . .  13
     4.2.  Further Comparison with DNS-SD  . . . . . . . . . . . . .  15
     4.3.  Open Issues . . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  Change log [RFC Editor: Please remove]  . . . . . . . . . . .  17
     9.1.  05  . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.2.  06 - Refresh  . . . . . . . . . . . . . . . . . . . . . .  17
     9.3.  05 - Refresh  . . . . . . . . . . . . . . . . . . . . . .  17
     9.4.  04 - Refresh  . . . . . . . . . . . . . . . . . . . . . .  17
     9.5.  03 - Refresh  . . . . . . . . . . . . . . . . . . . . . .  17
     9.6.  02 - Revived after charter round 1 finished . . . . . . .  17
     9.7.  01 -  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.8.  00 - Initial version  . . . . . . . . . . . . . . . . . .  18
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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1.  Overview

   GeneRic Autonomic Signaling Protocol (GRASP) [RFC8990] 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).

   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.

   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.

   DNS-SD via mDNS [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.

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   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.

   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-

   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.

   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.

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   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.

   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.

   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

   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.

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2.  Terminology

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

   This document makes use of terms and concepts defined in [RFC8990].

3.  Specification

3.1.  Service and Name Objectives

   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 [RFC8990]).  Therefore, a scheme is required to
   indicate services via 'objective-names'.

      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.

   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.<service-name>'.  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.

   When an objective is meant announcement and discovery of a DNS
   compatible <name> such as "www-internal" in "www-", the objective SHOULD use an objective-name of
   NAME.<name>.  See Section 3.3.3 for more details.

3.2.  Objective Value Reuseable Elements Structure

   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.

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   objective-value  /= { 1*elements }
   elements        //= ( @rfcXXXX: { 1*relement } )

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

   If an objective relies upon reusable elements, the 'objective-value'
   MUST be a CBOR map and the reusable elements are found under the key

   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

   The value of "@rfcXXXX" is a map of reusable elements.  Each
   'relement' has an IANA registered element-name and codepoint (see
   Section 6).  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.

   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.

   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

   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.

   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.

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   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.

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

   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.

3.3.  Reuseable Elements

3.3.1.  Sender Loop Count

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

   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.

3.3.2.  Service Element

   The srv-element (service element) is a reusable element to request or
   announce a service instance or to request and list service instance

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   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 )

   Service:  A service name registered according to RFC6335.  If it is
      not present, then objective-name MUST be SRV.<service-name> where
      <service-name> is the service-name.

   Instance:  The <Instance> of a DNS-SD Service Instance Name (
      <Instance> . <Service> .  <Domain>).  It is optional, see
      Section 4.2.

   Domain:  The equivalent of the <Domain> 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.

   Priority, Weight:  Service Instance selection criteria as defined in
      RFC2782.  If either one is not present, its value defaults to 0.

   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.

   Range:  Allows to flexibly combine distance and priority/weight based
      service selection according to the definition of distance in
      Section 3.3.1.

      If min-distance is the distance of the closest service announcer,

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      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.

      If range is announced, the sender-loop-count element MUST also be

   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.

      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.

      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 "".

      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).

   Msg-type  Type (aka: intention) of the srv-element.  If not present,
      it is assumed to be "describe".

   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-

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      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"

   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.

      "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

   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.

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

   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

3.3.3.  Name Element

   The NAME,<name> 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.

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   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:

   For example consider a GRASP (ACP) domain of "".  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).

   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

   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.

   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.

   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.

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   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.

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

4.  Theory of Operation

4.1.  Using GRASP Service Announcements

   TBD: This section contains a range of details that should become
   normative in later versions.

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

   The most simple method to use GRASP service discovery is to select
   (and if still necessary, register) a <service-name> 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.<service-name> 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.

   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 & 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.

   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

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   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 <
   range < 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).

   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).

   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 <service-name> 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.

   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.

   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

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   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).

   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.

4.2.  Further Comparison with DNS-SD

   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.

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

   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"

4.3.  Open Issues

   TBD: Examine limitations mentioned in "in this version of the text/

   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 ?).

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   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.

   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.

   TBD: Is there a way in CDDL to formally define the string names of
   the relement-codepoint's ?

5.  Security Considerations


   GRASP-related security issues are discussed in Section 3 of

6.  IANA Considerations

   This document requests IANA to create a new "GRASP Objective Value
   Standard Elements" subregistry under the "GeneRic Autonomic Signaling
   Protocol (GRASP) Parameters" registry.

   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 [RFC8126].  The
   numerical value is simply to be assigned sequentially.  The following
   initial values are assigned by this document:

   sender-loop-count 1 [defined in rfcXXXX]

   srv-element 2 [defined in rfcXXXX]

   name-element 3 [defined in rfcXXXX]

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

   Assignments for objective-names of the form "SRV.<text>" and
   "NAME.<text>" are special.

   Assignment of "SRV.<text>" can only be requested if <text> 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.

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   Registration of "SRV.<text>" 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 <-> 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.<text>" is also an update to
   that specification.

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

7.  Acknowledgements

8.  Contributors

   Brian Carpenter

9.  Change log [RFC Editor: Please remove]

9.1.  05

   Rewrote overview section in response to review comments by Peter vdS
   and Esko (hopefully better justification/explanation).  Thanks!

9.2.  06 - Refresh

9.3.  05 - Refresh

9.4.  04 - Refresh

9.5.  03 - Refresh

9.6.  02 - Revived after charter round 1 finished

   Reviving after ANIMA charter 01 is finished, adding new co-authors,

   Textual improvements, updating references.

9.7.  01 -

   Only refreshing, no changes since -00.

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9.8.  00 - Initial version

10.  References

10.1.  Normative References

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

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

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

   [RFC8990]  Bormann, C., Carpenter, B., Ed., and B. Liu, Ed., "GeneRic
              Autonomic Signaling Protocol (GRASP)", RFC 8990,
              DOI 10.17487/RFC8990, May 2021,

10.2.  Informative References

   [RFC8994]  Eckert, T., Ed., Behringer, M., Ed., and S. Bjarnason, "An
              Autonomic Control Plane (ACP)", RFC 8994,
              DOI 10.17487/RFC8994, May 2021,

Authors' Addresses

   Toerless Eckert
   Futurewei Technologies USA Inc.
   2220 Central Expressway
   Santa Clara,  95050
   United States of America

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   Mohamed Boucadair
   35000 Rennes

   Christian Jacquenet
   35000 Rennes

   Michael H. Behringer

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