Internet DRAFT - draft-cheshire-dnssd-roadmap
draft-cheshire-dnssd-roadmap
Internet Engineering Task Force S. Cheshire
Internet-Draft Apple Inc.
Intended status: Informational October 23, 2018
Expires: April 26, 2019
Service Discovery Road Map
draft-cheshire-dnssd-roadmap-03
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 26, 2019.
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
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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. They 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 communication 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 specification [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 some 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 a concrete 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. Depending on the
specific use case, this step may be performed only rarely. For
example, a user may do this just one once, the first time they
configure their computer to use their preferred printer, and never
again.
Once a desired printer has been chosen and configured, subsequent
printing of documents is an operation performing the third step.
This step may be done frequently, perhaps multiple times per day.
This third step is important because, in a world of DHCP, IPv6
Stateless Autoconfiguration, and similar dynamic address allocation
schemes, a printer's IP address could change from day to day, and to
use the printer, its current address must be known. However, this
third step need not be performed for every printer on the network,
just the specific printer that is about to be used. Also, it is not
necessary to repeat the second step again, learning the names of
every printer on the network, if the client device already knows the
name of the printer 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
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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.
A final requirement is that the service discovery protocol should not
only perform discovery at a single moment in time, but should also
provide ongoing change notification (sometimes called "Publish &
Subscribe"). Clients need to be notified in a timely fashion when
new data of interest appears, when data of interest changes, and,
equally importantly, when data of interest goes away ("goodbye
packets"). 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 creation, update, and deletion, 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 determine the
appropriate 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 broadly in Section 4 below.
Client operation is then discussed in detail in Section 5, and server
operation is discussed in detail in Section 6.
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4. Service Discovery Namespace
When used with Multicast DNS [RFC6762] Service Discovery queries
necessarily use the ".local" parent domain reserved for this purpose
[SUDN].
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 advanced service discovery to be
available even to people who haven't taken the step of registering
and paying annually 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" [RFC8375]. Services published in this factory-
default service discovery domain are not globally unique or globally
resolvable, but they can 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] on the local link, queries are
necessarily performed in the ".local" parent domain reserved for this
purpose [SUDN].
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.
A decade of operational experience has shown that an asynchronous
change notification protocol built on TCP is preferable for a variety
of reasons, so the IETF is has developed 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 trivial 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] by hand, 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 technique in the direction of automatically populating the
DNS namespace is the Service Discovery Proxy [DisProx]. This
technology works 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 with a presence 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 communicating with 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 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 technique 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 to the basic DNS Update protocol is the introduction of
a lifetime on DNS Updates, using 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 DHCP
lease lifetime, and if the device fails to renew the DHCP 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 for address assignment, 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 to the basic DNS Update protocol is the
introduction of information, carried using the EDNS(0) OWNER Option
[Owner], that tells the Service Registration server that the device
will be going to sleep to save power, and how the Service
Registration server can wake it up again on demand when needed. 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.
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 some mesh
networks.
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>.
[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>.
[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>.
[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>.
[RFC8375] Pfister, P. and T. Lemon, "Special-Use Domain
'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018,
<https://www.rfc-editor.org/info/rfc8375>.
[Broker] Cheshire, S. and T. Lemon, "Service Discovery Broker",
drdraft-sctl-discovery-broker-00 (work in progress), July
2017.
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[DisProx] Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", draft-ietf-dnssd-hybrid-08 (work in
progress), March 2018.
[DNS-LLQ] Sekar, K., "DNS Long-Lived Queries", draft-sekar-dns-
llq-01 (work in progress), August 2006.
[DNS-UL] Sekar, K., "Dynamic DNS Update Leases", draft-sekar-dns-
ul-01 (work in progress), August 2006.
[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.
[Owner] Cheshire, S. and M. Krochmal, "EDNS0 OWNER Option", draft-
cheshire-edns0-owner-option-01 (work in progress), July
2017.
[Push] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
draft-ietf-dnssd-push-14 (work in progress), March 2018.
[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.
[STR] "Service Name and Transport Protocol Port Number
Registry", <http://www.iana.org/assignments/
service-names-port-numbers/>.
[SUDN] "Special-Use Domain Names Registry",
<https://www.iana.org/assignments/
special-use-domain-names/>.
[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 talented 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 either way the
process by which clients query to discover services is the same.
This appendix gives a detailed step-by step account of how this
client query process works. It starts with a client joining the Wi-
Fi network and doing a DHCP request, and ends with paper coming out
of the printer. The reason the explanation is gives the specific
details of every step is to avoid inadvertently having a hand-waving
"and then a miracle occurs" part, which misses out some important
detail. 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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A.1. Domain Enumeration using PTR queries
Our DHCP domain name is meeting.ietf.org. Does meeting.ietf.org
recommend that we look in any Wide Area Service Discovery domains?
This step is called Domain Enumeration [RFC6763], and is performed
using a DNS PTR query for a name with the special prefix "lb._dns-
sd._udp":
% 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 in the Answer Section you'll see that the answer
to the PTR query 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 PTR 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 Answer Section 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.
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A.2. Instance Enumeration using PTR queries on a macOS computer
Once one or more service discovery domains have been determined, the
client then looks for instances of the desired service type. This
step is called Instance Enumeration and is also performed using a DNS
PTR queries, using a name with a prefix indicating the type of
service that is being sought.
A macOS computer with appropriate printer drivers installed will look
for instances of the service type "_pdl-datastream._tcp" in the
domain "meeting.ietf.org", as shown below. This is typically
performed just once, the first time the macOS computer is set up to
use that printer.
% 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.
A.3. Printing from a macOS computer
When the user actually prints something, macOS sends a DNS SRV query
for the printer name learned in the previous Instance Enumeration
step, to learn the target host and port for the service. This DNS
SRV query is then followed by address queries for the target host's
IPv4 and/or IPv6 addresses. The necessary address records are
usually included in the Additional Section of the reply to the SRV
query, so that these address queries can be answered from the local
cache, without resulting in additional packets over the air.
% 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.
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A.4. Instance Enumeration using PTR queries on an iOS device
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).
Consequently, the iOS device sends its Instance Enumeration DNS PTR
queries using the prefix "_universal._sub._ipp._tcp" to indicate that
it is looking for the subset of IPP printers that support 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 URF-capable 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.
A.5. Printing from an iOS device
When the user prints from their iPhone or iPad using AirPrint, iOS
does these DNS SRV and address 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 from the macOS example,
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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