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Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Informational                              July 2, 2017
Expires: January 3, 2018


                       Service Discovery Road Map
                    draft-cheshire-dnssd-roadmap-00

Abstract

   Over the course of several years, a rich collection of technologies
   has developed around DNS-Based Service Discovery, described across
   multiple documents.  This "Road Map" document gives an overview of
   how these separate but related technologies (and their documents) fit
   together, to facilitate Service Discovery in various environments.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 3, 2018.

Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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1.  Road Map

   DNS-Based Service Discovery [RFC6763] is a component of Zero
   Configuration Networking [RFC6760] [ZC] [Roadmap].

   Over the course of several years, a rich collection of technologies
   has developed around DNS-Based Service Discovery.  These various
   separate but related technologies are described across multiple
   documents.  This "Road Map" document gives an overview of how these
   technologies (and their documents) fit together to facilitate Service
   Discovery across a broad range of operating environments, from small
   scale zero-configuration networks to large scale administered
   networks, from local area to wide area, and from low-speed wireless
   links in the kb/s range to high-speed wired links operating at
   multiple Gb/s.

   Not all of the available components are necessary or appropriate in
   all scenarios.  One goal of this "Road Map" document is to provide
   guidance about which components to use depending on the problem being
   solved.































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2.  Service Type Namespace

   The single most important concept in Service Discovery is the
   namespace specifying how different service types are identified.
   This is how a client communicates what it needs, and how a server
   communicates what it offers.  For a client to discover a server,
   client and server need to use the same namespace of service types,
   otherwise they may actually speak the same application protocol over
   the air or on the wire, and may in fact be completely compatible, and
   yet may be unable to detect this because they are using different
   names to refer to the same actual service.  Hence, having a
   consistent namespace for referring to service types is vital.

   IANA manages the registry of Service Types [RFC6335][SN].  This
   registry of Service Types can (and should) be used in any Service
   Discovery protocol as the vocabulary for describing *all* IP-based
   services, not only DNS-Based Service Discovery [RFC6763].

   In this document we focus on the use of the IANA Service Type
   Registry [SN] in conjunction with DNS-Based Service Discovery, though
   that should not be taken in any way to imply any criticism of other
   Service Discovery protocols sharing the same namespace of service
   types.  In different circumstances different Service Discovery
   protocols are appropriate.

   For example, for Service Discovery of services potentially available
   via a Wi-Fi access point, prior to association with that Wi-Fi access
   point, when no IP link has yet been established, a Service Discovery
   protocol may use raw 802.11 frames, not necessarily IP, UDP, or DNS-
   formatted messages.  For Service Discovery using peer-to-peer Wi-Fi
   technologies, without any Wi-Fi access point at all, it may also be
   preferable to use raw 802.11 frames instead of IP, UDP, or DNS-
   formatted messages.  Service Discovery using IEEE 802.15.4 radios may
   use yet another over-the-air protocol.  What is important is that
   they all share the same vocabulary to describe all IP-based services,
   so that client and server software, using agnostic APIs to consume
   and offer services on the network, has a common language to identify
   those services, independent of the medium or the particular Service
   Discovery protocol in use on that medium.  Just as TCP/IP runs on
   many different link layers, and the concept of using an IP address to
   identify a particular peer is consistent across many different link
   layers, the concept of using a name from the IANA Service Type
   Registry to identify a particular service type also needs to be
   consistent across all IP-supporting link layers.







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3.  Service Discovery Operational Model

   The three principal Service Discovery operations utilizing service
   types in the IANA Service Type Registry [SN] are:

   1.  Offer
   2.  Discover/Enumerate
   3.  Use

   The first step, "Offer", is when a server is offering a service using
   some application-layer protocol on a listening TCP or UDP (or other
   transport protocol) port, and wishes to make that known to other
   devices.

   The second step, "Discover", sometimes called, "Enumerate", is when a
   client device wishes to perform some action, but does not yet know
   which particular service instance will be used to perform that
   action.  For example, when a user taps the "AirPrint" button on an
   iPhone, the iPhone knows that the user wishes to print, but not which
   particular printer to use.  The desired *function* is known (IPP
   printing), but not the particular instance.  In this case, the client
   device needs to enumerate the list of available service instances
   that are able to perform the desired task.  In most cases this list
   of service instances is presented to a human user to choose from; in
   some cases it is software that examines the list of available service
   instances and determines the best one to use.

   The third step, "Use", is when particular service instance has been
   selected, and the client wants to make use of that service instance,
   by opening a TCP connection to it or by sending UDP datagrams.

   The second and third steps are intentionally separate.  In the second
   step, a limited amount of information (typically just the name) is
   requested about a large number of service instances.  In the third
   step more detailed information (e.g, target host IP address, port
   number, etc.) is requested about one specific service instance.
   Requesting all the detailed information about all available service
   instances would be inefficient and wasteful on the network.  If the
   information about services on the network is imagined as a table,
   then the second step is requesting just one column from that table
   (the names) and the third step is requesting just one row from that
   table (the information pertaining to just one named service
   instance).

   To give an example, clicking the "+" button in the printer settings
   on macOS is an operation performing the second step.  It is
   requesting the names of all available printers.  Once a print queue
   has been configured for the chosen printer, subsequent printing of



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   documents is an operation performing the third step.  It only needs
   to request information about the specific printer in question.  It is
   not necessary to repeatedly discover the list of every printer on the
   network if the device already knows which one it intends to use.

   DNS-Based Service Discovery [RFC6763] implements these three
   principal Service Discovery operations using DNS records and queries,
   either using Multicast DNS [RFC6762] (for queries limited to the
   local link) or conventional unicast DNS [RFC1034] [RFC1035] (for
   queries beyond the local link).

   Other Service Discovery protocol achieve the same semantics using
   different packet formats and mechanisms.

   One incidental benefit of using DNS as the foundation layer is that
   Multicast DNS and conventional unicast DNS are also used provide name
   resolution (mapping host names to IP addresses) so there is some
   efficiency and code reuse in using the same underlying protocol for
   both naming and service discovery.

   A final requirement is that the Service Discovery protocol perform
   not only discovery at a single moment in time, but also ongoing
   change notification (sometimes called "Publish & Subscribe").
   Without support for ongoing change notification, clients would be
   forced to resort to polling to keep data up to date, which is
   inefficient and wasteful on the network.

   Multicast DNS [RFC6762] implicitly includes change notification by
   virtue of announcing record changes via IP Multicast, which allows
   these changes to be seen by all peers on the same link (broadcast
   domain).

   Conventional unicast DNS [RFC1034] [RFC1035] has historically not had
   broad support for change notification.  This capability is added via
   the new mechanism for DNS Push Notifications [Push].

   When using DNS-Based Service Discovery [RFC6763] there are two
   aspects to consider: firstly how the clients choose what DNS names to
   query, and what query mechanisms to use, and secondly how the
   relevant information got into the DNS namespace in the first place,
   so as to be available when clients query for it.

   The available namespaces are discussed below in Section 4.  Client
   operation is discussed in Section 5 and server operation is discussed
   in Section 6.






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4.  Service Discovery Namespace

   When used with Multicast DNS [RFC6762] queries are automatically
   performed in the ".local" parent domain.

   When used with conventional unicast DNS [RFC1034] [RFC1035] some
   other domain must be used.

   For individuals and organizations with a globally-unique domain name
   registered to them, their globally-unique domain name, or a subdomain
   of it, can be used for service discovery.

   However, it would be convenient for capable service discovery to be
   available even to people who haven't taken the step of registering
   and paying for a globally-unique domain name.  For these people it
   would be useful if devices arrived preconfigured with some suitable
   factory-default service discovery domain, such as "services.homenet"
   [I-D.ietf-homenet-dot].  Services published in this factory-default
   service discovery domain would not be globally unique or globally
   resolvable, but they could have scope larger than the single link
   provided by Multicast DNS.






























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5.  Client Configuration and Operation

   When using DNS-Based Service Discovery [RFC6763], clients have to
   choose what DNS names to query.

   When used with Multicast DNS [RFC6762] queries are automatically
   performed in the ".local" parent domain.

   For discovery beyond the local link, a unicast DNS domain must be
   used.  This unicast DNS domain can be configured manually by the
   user, or it can be learned dynamically from the network (as has been
   done for many years at IETF meetings to facilitate discovery of the
   IETF Terminal Room printer from outside the IETF Terminal Room
   network).  In the DNS-SD specification [RFC6763] section 11,
   "Discovery of Browsing and Registration Domains (Domain
   Enumeration)", describes how a client device learns one or more
   recommended service discovery domains from the network, using the
   special "lb._dns-sd._udp" query.

   Given the service type that the user or client device is seeking (see
   Section 2) and one or more service discovery domains to look in, the
   client then sends its DNS queries, and processes the responses.

   For some uses one-shot conventional DNS queries and responses are
   perfectly adequate, but for service discovery, where a list may be
   displayed on a screen for a user to see, it is desirable to keep that
   list up to date without the user having to repeatedly tap a "refresh"
   button, and without the software repeatedly polling the network on
   the user's behalf.

   And early solution to provide asynchronous change notifications for
   unicast DNS was the UDP-based protocol DNS Long-Lived Queries
   [DNS-LLQ].  This was used, among other things, by Apple's Back to My
   Mac Service [RFC6281] introduced in Mac OS X 10.5 Leopard in 2007.

   Recent experience has shown that an asynchronous change notification
   protocol built on TCP would be preferable, so the IETF is now
   developing DNS Push Notifications [Push].

   Because DNS Push Notifications is built on top of a DNS TCP
   connection, rather than inventing its own session signaling
   mechanisms, DNS Push Notifications adopts the conventions specified
   by DNS Session Signaling [S-Sig].








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6.  Server Configuration and Operation

   Section 5 above describes how clients perform their queries.  The
   related question is how the relevant information got into the DNS
   namespace in the first place, so as to be available when clients
   query for it.

   One way that relevant service discovery information can get into the
   DNS namespace is simply via manual configuration, creating the
   necessary PTR, SRV and TXT records [RFC6763], and indeed this is how
   the IETF Terminal Room printer has been advertised to IETF meeting
   attendees for many years.  While this is easy for the experienced
   network operators at the IETF, it can be onerous to others less
   familiar with how to set up DNS-SD records.

   Hence it would be convenient to automate this process of populating
   the DNS namespace with relevant service discovery information.  Two
   efforts are underway to address this need, the Service Discovery
   Proxy [DisProx] and the Service Registration Protocol [RegProt].

   The first effort is the Service Discovery Proxy [DisProx].  This
   technology is designed to work with today's devices that advertise
   services using Multicast DNS only (such as almost all network
   printers sold in the last decade).  A Service Discovery Proxy is a
   device colocated on the same link as the devices we wish to be able
   to discover from afar.  A remote client sends unicast queries to the
   Discovery Proxy, which performs local Multicast DNS queries on behalf
   of the remote client, and then sends back the answers it discovers.

   Because the time it takes to receive Multicast DNS responses is
   uncertain, this mechanism benefits from being able to deliver
   asynchronous change notifications as new answers come in, using DNS
   Long-Lived Queries [DNS-LLQ] or the newer DNS Push Notifications
   [Push] on top of DNS Session Signaling [S-Sig].

   As an alternative to having to be physically connected to the desired
   network link, a Service Discovery Proxy [DisProx] can use a Multicast
   DNS Discovery Relay [Relay] to give it a 'virtual' presence on a
   remote link.  Indeed, when using Discovery Relays, a single Discovery
   Proxy can have a 'virtual' presence on hundreds of remote links.  A
   single Discovery Proxy in the data center can serve the needs of an
   entire enterprise.  This is modeled after the DHCP protocol.  In
   simple residential scenarios the DHCP server resides on the local
   link.  In complex enterprise networks, a single DHCP server resides
   in the data center, using simple lightweight BOOTP relay agents
   colocated with the routers on each physical link.





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   Finally, when clients are making TCP connections to multiple Service
   Discovery Proxies at the same time, this can be burdensome for the
   clients (which may be mobile and battery powered) and for the the
   Service Discovery Proxies (which may have to serve hundreds of
   clients).  This situation is remedied by use of a Service Discovery
   Broker [Broker].  A Service Discovery Broker is an intermediary
   between client and server.  A client can issue a single query to the
   Service Discovery Broker and have the Service Discovery Broker do the
   hard work of issuing multiple queries on behalf of the client.  And a
   Service Discovery Broker can shield a Service Discovery Proxy from
   excessive load by colapsing multiple duplicate queries from different
   client down to a single query to the Service Discovery Proxy.

   The second effort in this space, tacking the chalenge of automating
   the process of populating the DNS namespace with relevant service
   discovery information, is the Service Registration Protocol
   [RegProt].  This technology is designed to work with future devices
   that explicitly cooperate with the network to advertise their
   services.

   The Service Registration Protocol is effectively DNS Update, with
   some minor additions.

   One addition is the introduction of a lifetime on DNS Updates, using
   the the Dynamic DNS Update Lease EDNS(0) option [DNS-UL].

   The second addition is the introduction of information that tells the
   Service Registration server that the device will be going to sleep to
   save power, combined with information specifying how to wake it up
   again on demand, using the EDNS(0) OWNER Option [Owner].

   The use of an explicit Service Registration Protocol is beneficial in
   networks where multicast is expensive, inefficient, or outright
   blocked, such as many Wi-Fi networks.  An explicit Service
   Registration Protocol is also beneficial in networks where multicast
   and broadcast are supported poorly, if at all, such as mesh networks
   like those using IEEE 802.15.4.

   The use of power management information in the Service Registration
   messages allows devices to sleep to save power, which is especially
   beneficial for battery-powered devices in the home.










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7.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <http://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

   [RFC6281]  Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
              "Understanding Apple's Back to My Mac (BTMM) Service",
              RFC 6281, DOI 10.17487/RFC6281, June 2011,
              <http://www.rfc-editor.org/info/rfc6281>.

   [RFC6760]  Cheshire, S. and M. Krochmal, "Requirements for a Protocol
              to Replace the AppleTalk Name Binding Protocol (NBP)",
              RFC 6760, DOI 10.17487/RFC6760, February 2013,
              <http://www.rfc-editor.org/info/rfc6760>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <http://www.rfc-editor.org/info/rfc6762>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <http://www.rfc-editor.org/info/rfc6763>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <http://www.rfc-editor.org/info/rfc6335>.

   [I-D.ietf-homenet-dot]
              Pfister, P. and T. Lemon, "Special Use Domain
              '.home.arpa'", draft-ietf-homenet-dot-06 (work in
              progress), June 2017.

   [DisProx]  Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", draft-ietf-dnssd-hybrid-06 (work in
              progress), March 2017.

   [Push]     Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-12 (work in progress), July 2017.





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   [S-Sig]    Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Mankin, A., and T. Pusateri, "DNS Session Signaling",
              draft-ietf-dnsop-session-signal-03 (work in progress),
              July 2017.

   [DNS-UL]   Sekar, K., "Dynamic DNS Update Leases", draft-sekar-dns-
              ul-01 (work in progress), August 2006.

   [DNS-LLQ]  Sekar, K., "DNS Long-Lived Queries", draft-sekar-dns-
              llq-01 (work in progress), August 2006.

   [Roadmap]  Cheshire, S., "Service Discovery Road Map", draft-
              cheshire-dnssd-roadmap-00 (work in progress), July 2017.

   [Owner]    Cheshire, S. and M. Krochmal, "EDNS0 OWNER Option", draft-
              cheshire-edns0-owner-option-01 (work in progress), July
              2017.

   [RegProt]  Cheshire, S. and T. Lemon, "Service Registration Protocol
              for DNS-Based Service Discovery", draft-sctl-service-
              registration-00 (work in progress), July 2017.

   [Relay]    Cheshire, S. and T. Lemon, "Multicast DNS Discovery
              Relay", draft-sctl-dnssd-mdns-relay-00 (work in progress),
              July 2017.

   [Broker]   Cheshire, S. and T. Lemon, "Service Discovery Broker",
              drdraft-sctl-discovery-broker-00 (work in progress), July
              2017.

   [SN]       "Service Name and Transport Protocol Port Number
              Registry", <http://www.iana.org/assignments/
              service-names-port-numbers/>.

   [ZC]       Cheshire, S. and D. Steinberg, "Zero Configuration
              Networking: The Definitive Guide", O'Reilly Media, Inc. ,
              ISBN 0-596-10100-7, December 2005.

Author's Address

   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014
   USA

   Phone: +1 408 974 3207
   Email: cheshire@apple.com



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