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Versions: 00 01

Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Informational                            March 18, 2018
Expires: September 19, 2018


                       Service Discovery Road Map
                    draft-cheshire-dnssd-roadmap-01

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 related but separate 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
   Task Force (IETF).  Note that other groups may also distribute
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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 September 19, 2018.

Copyright Notice

   Copyright (c) 2018 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   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].

   Over the course of several years, a rich collection of technologies
   has developed around DNS-Based Service Discovery.  These various
   related but separate 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.

2.  Namespace of Service Types

   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, the
   client and server need to have a common language to describe what
   they need and what they offer.  The 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 of service types is the essential
   prerequisite for any useful service discovery.

   IANA manages the registry of Service Types [RFC6335][STR].  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 [STR] 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.




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   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.
   Using the same service type vocabulary means 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.

   Originally, the IANA Service Type Registry [RFC6335][STR] used the
   term "Service Name" rather than "Service Type".  Later it became
   clear that this term could be ambiguous.  For a given service
   instance on the network, there is the machine-visible name of the
   type of service it provides, and the human-visible name of the
   particular instance of that type of service.  For clarity, this
   document and related specifications use the term "Service Type" to
   denote the machine-visible name of the type of service, and the term
   "Instance Name" to denote the human-visible name of a particular
   instance.




















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3.  Service Discovery Operational Model

   The original DNS-Based Service Discovery specifications [RFC6763]
   used the terms "register" (advertise a service), "browse" (discover
   service instances), and "resolve" (get IP address and port for a
   specific service instance).  This terminology is reflective of the
   thinking at the time, which viewed service discovery as a new and
   separate step, added to existing networking code.  For example, a
   server would first open a listening socket as it always had, and then
   "register" that listening socket with the service discovery engine.
   Similarly, a client would first "resolve" a service instance to an IP
   address and port, and then, having done that, "connect" to that IP
   address and port.

   More recent thinking in this area [RFC8305] has come to the
   conclusion that it is preferable wherever possible to insulate
   application software from networking details like having to decide
   between IPv4 and IPv6, having to decide among multiple IP addresses
   of either or both address families, and having to decide among
   multiple available network interfaces.  Consequently this document
   and related specifications adopt newer terminology as follows:

   1.  Offer
   2.  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.  This encompasses both making a listening socket (or the
   equivalent concept in whatever underlying networking API is being
   used) and advertising the existence of that listening socket via a
   service discovery mechanism.

   The second step, "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 or iPad, the iPhone or
   iPad 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.  This second step is the
   operation that was called "browsing" in the original specifications.




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   The third step, "Use", is when particular service instance has been
   selected, and the client wants to make use of that service instance.
   This encompasses both the "resolve" step (finding IP address(es) and
   port(s) for the service instance) and the subsequent steps to
   establish communication with it, which may include details like
   address family selection, interface selection, transport protocol
   selection, etc.  Ideally, application-layer code should never be
   exposed to IP addresses at all, just as application-layer code today
   is generally not exposed to details like MAC addresses [RFC8305].

   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 name column) 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 desired
   printer has been chosen and configured, subsequent printing of
   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 client 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 for
   service discovery, in cases where that makes sense, is that both
   Multicast DNS and conventional unicast DNS are also used provide name
   resolution (mapping host names to IP addresses).  There is some
   efficiency and code reuse gained by using the same underlying
   protocol for both service discovery and naming.



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   A final requirement is that the service discovery protocol perform
   discovery not only 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 (i.e., same
   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.home.arpa" [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).  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.
   All of the details from that specification are not repeated here.
   A walk-through describing one real-world example of how this works,
   using discovery of the IETF Terminal Room printer as a specific
   concrete case study, is given in Appendix A.

   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, DNS Push Notifications adopts the conventions specified
   by DNS Stateful Operations [DSO] rather than inventing its own
   session management mechanisms.





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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] (see Section 6.1) and the Service Registration
   Protocol [RegProt] (see Section 6.4).

6.1.  Service Discovery Proxy

   The first effort in the direction of automatically populating the DNS
   namespace is the Service Discovery Proxy [DisProx].  This technology
   is designed to work with today's existing 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 Stateful Operations [DSO].













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6.2.  Multicast DNS Discovery Relay

   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 in the home
   gateway, which is physically attached to the (single) local link.  In
   complex enterprise networks, it is common to have a single
   centralized DHCP server, which resides in the data center and
   communicates with a multitude of simple lightweight BOOTP relay
   agents, implemented in the routers on each physical link.

6.3.  Service Discovery Broker

   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 collapsing multiple duplicate queries from
   different client down to a single query to the Service Discovery
   Proxy.




















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6.4.  Service Registration Protocol

   The second effort in the direction of automatically populating the
   DNS namespace is the Service Registration Protocol [RegProt].  This
   technology is designed to enable future devices that will explicitly
   cooperate with the network infrastructure 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].  This
   option has similar semantics to a DHCP address lease, where a device
   is granted an address with with a certain lease lifetime, and if the
   device fails to renew the lease before it expires then the address
   will be reclaimed and become available to be allocated to a different
   device.  In cases where DHCP is being used, a device will generally
   request a DNS Update Lease with the same expiration time as its DHCP
   address lease.  This way, if the device is abruptly disconnected from
   the network, around the same time as its address gets reclaimed its
   DNS records will also be garbage collected.

   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.

7.  Security Considerations

   As an informational document, this document introduces no new
   Security Considerations of its own.  The various referenced documents
   each describe their own relevant Security Considerations as
   appropriate.






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8.  Informative References

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

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc6760>.

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

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://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,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [I-D.ietf-homenet-dot]
              Pfister, P. and T. Lemon, "Special Use Domain
              'home.arpa.'", draft-ietf-homenet-dot-14 (work in
              progress), September 2017.

   [DisProx]  Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", draft-ietf-dnssd-hybrid-08 (work in
              progress), March 2018.



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   [Push]     Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-14 (work in progress), March 2018.

   [DSO]      Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Lemon, T., and T. Pusateri, "DNS Stateful Operations",
              draft-ietf-dnsop-session-signal-07 (work in progress),
              March 2018.

   [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.

   [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-04 (work in progress),
              March 2018.

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

   [STR]      "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.














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Appendix A.  IETF Terminal Room Printer Discovery Walk-through

   For about a decade now, the capable IETF network staff have provided
   off-link DNS Service Discovery for the Terminal Room printer at IETF
   meetings three times a year.  In the case of the IETF meetings the
   necessary DNS records are entered manually, whereas this document
   advocates for increased automation of that task, but the process by
   which clients query to discover services is the same either way.

   This appendix gives a detailed step-by step account of how this
   works.  It starts with joining the Wi-Fi network and doing a DHCP
   request, and ends with paper coming out of the printer.  The reason
   the explanation is so detailed is to avoid inadvertently having a
   hand-waving "and then a miracle occurs" part, which skips over
   important details.  And one of the reasons for asking the IETF
   network team to set this up for IETF meetings is that operational use
   is an important reality check.  When standing in front of a room,
   giving a presentation, if you miss out some vital step, people may
   not notice.  When running an actual service used by actual people, if
   you miss out some vital step, no paper comes out of the printer, and
   everyone notices.

   Using a macOS computer, at an IETF meeting, you can repeat the steps
   illustrated here to see exactly how it works.  Or you can simply
   press Cmd-P in any application and see that "term-printer" appears as
   an available printer, to confirm that it does in fact work.

   First, let's see what the macOS computer learned from the local DHCP
   server:

     % scutil
     > list
      ...
      subKey [74] = State:/Network/Service/21B5304C...54B28F4CA1D2/DHCP
      ...

     > show State:/Network/Service/21B5304C...54B28F4CA1D2/DHCP
     <dictionary> {
      Option_15 : <data> 0x6d656574696e672e696574662e6f7267
      ...
     }

   Option_15 is Domain Name.  To see what domain name, we need to decode
   the hexadecimal data to ASCII.

     % echo 6d656574696e672e696574662e6f7267 0A | xxd -r -p
     meeting.ietf.org




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   Our DHCP domain name is meeting.ietf.org.  Does meeting.ietf.org
   recommend that we look in any Wide Area Service Discovery domains?

     % dig lb._dns-sd._udp.meeting.ietf.org. ptr

     ; <<>> DiG 9.6-ESV-R4-P3 <<>> lb._dns-sd._udp.meeting.ietf.org. ptr
     ;; global options: +cmd
     ;; Got answer:
     ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35624
     ;; flags: qr aa rd ra;
                        QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 4

     ;; QUESTION SECTION:
     ;lb._dns-sd._udp.meeting.ietf.org. IN PTR

     ;; ANSWER SECTION:
     lb._dns-sd._udp.meeting.ietf.org. 3600 IN PTR meeting.ietf.org.

     ...

     ;; Query time: 8 msec
     ;; SERVER: 130.129.5.6#53(130.129.5.6)
     ;; WHEN: Wed Mar 13 10:16:40 2013
     ;; MSG SIZE  rcvd: 188

   In the middle there you'll see that the answer is "meeting.ietf.org".
   In this case the answer is self-referential -- "meeting.ietf.org" is
   inviting us to look for services in "meeting.ietf.org", but the PTR
   record(s) could equally well point at any other domain, such as
   "services.ietf.org", or anything else.





















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   Note that this answer does not depend on the client device being "on"
   the IETF meeting network, which is in any case a loosely defined
   concept at best.  Nor does it depend on sending the DNS query to a
   DNS server that is "on" the IETF meeting network.  Any capable DNS
   recursive resolver anywhere on the planet will give the same answer.
   We can test this by sending the same DNS query to Google's 8.8.8.8
   public resolver:

     % dig @8.8.8.8 lb._dns-sd._udp.meeting.ietf.org. ptr

     ; <<>> DiG 9.6-ESV-R4-P3 <<>>
                          @8.8.8.8 lb._dns-sd._udp.meeting.ietf.org. ptr
     ; (1 server found)
     ;; global options: +cmd
     ;; Got answer:
     ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 24571
     ;; flags: qr rd ra; QUERY:1, ANSWER:1, AUTHORITY:0, ADDITIONAL:0

     ;; QUESTION SECTION:
     ;lb._dns-sd._udp.meeting.ietf.org. IN PTR

     ;; ANSWER SECTION:
     lb._dns-sd._udp.meeting.ietf.org. 1532 IN PTR meeting.ietf.org.

     ;; Query time: 21 msec
     ;; SERVER: 8.8.8.8#53(8.8.8.8)
     ;; WHEN: Wed Mar 13 10:18:27 2013
     ;; MSG SIZE  rcvd: 64

   In the middle there you'll see that the answer is still
   "meeting.ietf.org".




















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   In this example, this particular test was done at the 86th IETF in
   Orlando, Florida, in March 2013.  The Google 8.8.8.8 public resolver
   still gave the correct answer, even though it was 13 hops away:

     % traceroute -q 1 8.8.8.8
     traceroute to 8.8.8.8 (8.8.8.8), 64 hops max, 52 byte packets
     1  rtra (130.129.80.2)  1.369 ms
     2  75-112-170-148.net.bhntampa.com (75.112.170.148)  14.494 ms
     3  bun2.tamp20-car1.bhn.net (71.44.3.73)  19.558 ms
     4  hun0-0-0-0-tamp20-cbr1.bhn.net (72.31.117.156)  20.730 ms
     5  xe-8-2-0.bar1.tampa1.level3.net (4.53.172.9)  13.052 ms
     6  ae-5-5.ebr1.miami1.level3.net (4.69.148.213)  27.413 ms
     7  ae-1-51.edge1.miami2.level3.net (4.69.138.75)  15.552 ms
     8  google-inc.edge1.miami2.level3.net (4.59.240.26)  48.852 ms
     9  209.85.253.118 (209.85.253.118)  21.118 ms
     10  216.239.48.192 (216.239.48.192)  21.890 ms
     11  216.239.48.192 (216.239.48.192)  23.221 ms
     12  *
     13  google-public-dns-a.google.com (8.8.8.8)  32.961 ms

   For the rest of this example we use the Google 8.8.8.8 public
   resolver for all the queries.

   In the case of IETF meetings the PTR is self-referential --
   meeting.ietf.org is advising us to look in meeting.ietf.org, but it
   could easily be set up to direct us elsewhere.  However, since it's
   suggesting we look for services in meeting.ietf.org, we'll do that.

   A macOS computer with appropriate printer drivers installed will look
   for instances of the "_pdl-datastream._tcp" service type at
   "meeting.ietf.org":

     % dig +short @8.8.8.8 _pdl-datastream._tcp.meeting.ietf.org. ptr
     term-printer._pdl-datastream._tcp.meeting.ietf.org.

   There's one printing service available here, called "term-printer".
   That's what you see when you press the "+" button in the Print & Fax
   Preference Pane on macOS.













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   When the user actually prints something, macOS does these queries:

     % dig +short @8.8.8.8 \
                 term-printer._pdl-datastream._tcp.meeting.ietf.org. srv
     0 0 9100 term-printer.meeting.ietf.org.

     % dig +short @8.8.8.8 term-printer.meeting.ietf.org. AAAA
     2001:df8::48:200:74ff:fee0:6cf8

   This tells the computer that to use this printer, it must connect to
   [2001:df8::48:200:74ff:fee0:6cf8]:9100, using the installed printer
   driver, which speaks the appropriate vendor-specific printing
   protocol for that printer.

   Printing from an iPhone or iPad is similar, except there are no
   vendor-specific printer drivers installed.  Instead, printing from an
   iPhone or iPad uses the IETF Standard IPP printing protocol, using an
   IPP printer that supports at least URF (Universal Raster Format):

     % dig +short @8.8.8.8 \
                         _universal._sub._ipp._tcp.meeting.ietf.org. ptr
     term-printer._ipp._tcp.meeting.ietf.org.

   An iPhone or iPad will discover that there's one IPP-based printing
   service available here, called "term-printer".  It has the same name
   as the pdl-datastream printing service, and exists on the same
   physical hardware, but uses a different printing protocol.

   When the user prints from their iPhone or iPad using AirPrint, iOS
   does these queries:

     % dig +short @8.8.8.8 term-printer._ipp._tcp.meeting.ietf.org. srv
     0 0 631 term-printer.meeting.ietf.org.

     % dig +short @8.8.8.8 term-printer.meeting.ietf.org. aaaa
     2001:df8::48:200:74ff:fee0:6cf8

   Note that the "_ipp._tcp" service has the same target hostname and
   IPv6 address as the "_pdl-datastream" service, but is accessed at a
   different TCP port on that hardware device.

   To use this printer, the iPhone or iPad connects to
   [2001:df8::48:200:74ff:fee0:6cf8]:631, and uses IPP to print.








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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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