Document: draft-cheshire-dnsext-dns-sd-04.txt            Stuart Cheshire
Internet-Draft                                             Marc Krochmal
Category: Standards Track                           Apple Computer, Inc.
Expires 10th February 2007                              10th August 2006

                      DNS-Based Service Discovery

                 <draft-cheshire-dnsext-dns-sd-04.txt>

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   For the purposes of this document, the term "BCP 79" refers
   exclusively to RFC 3979, "Intellectual Property Rights in IETF
   Technology", published March 2005.

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Abstract

   This document describes a convention for naming and structuring DNS
   resource records. Given a type of service that a client is looking
   for, and a domain in which the client is looking for that service,
   this convention allows clients to discover a list of named instances
   of that desired service, using only standard DNS queries. In short,
   this is referred to as DNS-based Service Discovery, or DNS-SD.


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Table of Contents

   1.   Introduction...................................................3
   2.   Conventions and Terminology Used in this Document..............4
   3.   Design Goals...................................................4
   4.   Service Instance Enumeration...................................5
   4.1  Structured Instance Names......................................5
   4.2  User Interface Presentation....................................7
   4.3  Internal Handling of Names.....................................7
   4.4  What You See Is What You Get...................................8
   4.5  Ordering of Service Instance Name Components...................9
   5.   Service Name Resolution.......................................11
   6.   Data Syntax for DNS-SD TXT Records............................12
   6.1  General Format Rules for DNS TXT Records......................12
   6.2  DNS TXT Record Format Rules for use in DNS-SD.................13
   6.3  DNS-SD TXT Record Size........................................14
   6.4  Rules for Names in DNS-SD Name/Value Pairs....................14
   6.5  Rules for Values in DNS-SD Name/Value Pairs...................16
   6.6  Example TXT Record............................................17
   6.7  Version Tag...................................................17
   7.   Application Protocol Names....................................18
   7.1  Selective Instance Enumeration................................19
   7.2  Service Name Length Limits....................................20
   8.   Flagship Naming...............................................22
   9.   Service Type Enumeration......................................23
   10.  Populating the DNS with Information...........................24
   11.  Relationship to Multicast DNS.................................24
   12.  Discovery of Browsing and Registration Domains................25
   13.  DNS Additional Record Generation..............................26
   14.  Comparison with Alternative Service Discovery Protocols.......27
   15.  Real Examples.................................................29
   16.  User Interface Considerations.................................30
   16.1 Service Advertising User-Interface Considerations.............30
   16.2 Client Browsing User-Interface Considerations.................31
   17.  IPv6 Considerations...........................................34
   18.  Security Considerations.......................................34
   19.  IANA Considerations...........................................34
   20.  Acknowledgments...............................................35
   21.  Deployment History............................................35
   22.  Copyright Notice..............................................36
   23.  Normative References..........................................37
   24.  Informative References........................................37
   25.  Authors' Addresses............................................38


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1. Introduction

   This document describes a convention for naming and structuring DNS
   resource records. Given a type of service that a client is looking
   for, and a domain in which the client is looking for that service,
   this convention allows clients to discover a list of named instances
   of a that desired service, using only standard DNS queries. In short,
   this is referred to as DNS-based Service Discovery, or DNS-SD.

   This document proposes no change to the structure of DNS messages,
   and no new operation codes, response codes, resource record types,
   or any other new DNS protocol values. This document simply proposes
   a convention for how existing resource record types can be named and
   structured to facilitate service discovery.

   This proposal is entirely compatible with today's existing unicast
   DNS server and client software.

   Note that the DNS-SD service does NOT have to be provided by the same
   DNS server hardware that is currently providing an organization's
   conventional host name lookup service (the service we traditionally
   think of when we say "DNS"). By delegating the "_tcp" subdomain,
   all the workload related to DNS-SD can be offloaded to a different
   machine. This flexibility, to handle DNS-SD on the main DNS server,
   or not, at the network administrator's discretion, is one of the
   things that makes DNS-SD so compelling.

   Even when the DNS-SD functions are delegated to a different machine,
   the benefits of using DNS remain: It is mature technology, well
   understood, with multiple independent implementations from different
   vendors, a wide selection of books published on the subject, and an
   established workforce experienced in its operation. In contrast,
   adopting some other service discovery technology would require every
   site in the world to install, learn, configure, operate and maintain
   some entirely new and unfamiliar server software. Faced with these
   obstacles, it seems unlikely that any other service discovery
   technology could hope to compete with the ubiquitous deployment
   that DNS already enjoys.

   This proposal is also compatible with (but not dependent on) the
   proposal outlined in "Multicast DNS" [mDNS].


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2. Conventions and Terminology Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in "Key words for use in
   RFCs to Indicate Requirement Levels" [RFC 2119].


3. Design Goals

   A good service discovery protocol needs to have many properties,
   three of which are mentioned below:

   (i) The ability to query for services of a certain type in a certain
   logical domain and receive in response a list of named instances
   (network browsing, or "Service Instance Enumeration").

   (ii) Given a particular named instance, the ability to efficiently
   resolve that instance name to the required information a client needs
   to actually use the service, i.e. IP address and port number, at the
   very least (Service Name Resolution).

   (iii) Instance names should be relatively persistent. If a user
   selects their default printer from a list of available choices today,
   then tomorrow they should still be able to print on that printer --
   even if the IP address and/or port number where the service resides
   have changed -- without the user (or their software) having to repeat
   the network browsing step a second time.

   In addition, if it is to become successful, a service discovery
   protocol should be so simple to implement that virtually any
   device capable of implementing IP should not have any trouble
   implementing the service discovery software as well.

   These goals are discussed in more detail in the remainder of this
   document. A more thorough treatment of service discovery requirements
   may be found in "Requirements for a Protocol to Replace AppleTalk
   NBP" [NBP]. That document draws upon examples from two decades of
   operational experience with AppleTalk Name Binding Protocol to
   develop a list of universal requirements which are broadly
   applicable to any potential service discovery protocol.


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4. Service Instance Enumeration

   DNS SRV records [RFC 2782] are useful for locating instances of a
   particular type of service when all the instances are effectively
   indistinguishable and provide the same service to the client.

   For example, SRV records with the (hypothetical) name
   "_http._tcp.example.com." would allow a client to discover a list of
   all servers implementing the "_http._tcp" service (i.e. Web servers)
   for the "example.com." domain. The unstated assumption is that all
   these servers offer an identical set of Web pages, and it doesn't
   matter to the client which of the servers it uses, as long as it
   selects one at random according to the weight and priority rules
   laid out in RFC 2782.

   Instances of other kinds of service are less easily interchangeable.
   If a word processing application were to look up the (hypothetical)
   SRV record "_ipp._tcp.example.com." to find the list of IPP printers
   at Example Co., then picking one at random and printing on it would
   probably not be what the user wanted.

   The remainder of this section describes how SRV records may be used
   in a slightly different way to allow a user to discover the names
   of all available instances of a given type of service, in order to
   select the particular instance the user desires.


4.1 Structured Instance Names

   This document borrows the logical service naming syntax and semantics
   from DNS SRV records, but adds one level of indirection. Instead of
   requesting records of type "SRV" with name "_ipp._tcp.example.com.",
   the client requests records of type "PTR" (pointer from one name to
   another in the DNS namespace).

   In effect, if one thinks of the domain name "_ipp._tcp.example.com."
   as being analogous to an absolute path to a directory in a file
   system then the PTR lookup is akin to performing a listing of that
   directory to find all the files it contains. (Remember that domain
   names are expressed in reverse order compared to path names: An
   absolute path name is read from left to right, beginning with a
   leading slash on the left, and then the top level directory, then
   the next level directory, and so on. A fully-qualified domain name is
   read from right to left, beginning with the dot on the right -- the
   root label -- and then the top level domain to the left of that, and
   the second level domain to the left of that, and so on. If the fully-
   qualified domain name "_ipp._tcp.example.com." were expressed as a
   file system path name, it would be "/com/example/_tcp/_ipp".)


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   The result of this PTR lookup for the name "<Service>.<Domain>" is a
   list of zero or more PTR records giving Service Instance Names of the
   form:

      Service Instance Name = <Instance> . <Service> . <Domain>

   The <Instance> portion of the Service Instance Name is a single DNS
   label, containing arbitrary precomposed UTF-8-encoded text [RFC
   3629]. It is a user-friendly name, meaning that it is allowed to
   contain any characters, without restriction, including spaces, upper
   case, lower case, punctuation -- including dots -- accented
   characters, non-roman text, and anything else that may be represented
   using UTF-8. DNS recommends guidelines for allowable characters for
   host names [RFC 1033][RFC 1034][RFC 1035], but Service Instance Names
   are not host names. Service Instance Names are not intended to ever
   be typed in by a normal user; the user selects a Service Instance
   Name by selecting it from a list of choices presented on the screen.

   Note that just because this protocol supports arbitrary UTF-8-encoded
   names doesn't mean that any particular user or administrator is
   obliged to make use of that capability. Any user is free, if they
   wish, to continue naming their services using only letters, digits
   and hyphens, with no spaces, capital letters, or other punctuation.

   DNS labels are currently limited to 63 octets in length. UTF-8
   encoding can require up to four octets per Unicode character, which
   means that in the worst case, the <Instance> portion of a name could
   be limited to fifteen Unicode characters. However, the Unicode
   characters with longer UTF-8 encodings tend to be the more obscure
   ones, and tend to be the ones that convey greater meaning per
   character.

   Note that any character in the commonly-used 16-bit Unicode space
   can be encoded with no more than three octets of UTF-8 encoding. This
   means that an Instance name can contain up to 21 Kanji characters,
   which is a sufficiently expressive name for most purposes.

   The <Service> portion of the Service Instance Name consists of a pair
   of DNS labels, following the established convention for SRV records
   [RFC 2782], namely: the first label of the pair is the Application
   Protocol Name, and the second label is either "_tcp" or "_udp",
   depending on the transport protocol used by the application.
   More details are given in Section 7, "Application Protocol Names".

   The <Domain> portion of the Service Instance Name specifies the DNS
   subdomain within which the service names are registered. It may be
   "local", meaning "link-local Multicast DNS" [mDNS], or it may be
   a conventional unicast DNS domain name, such as "apple.com.",
   "cs.stanford.edu.", or "eng.us.ibm.com." Because service names are
   not host names, they are not constrained by the usual rules for host


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   names [RFC 1033][RFC 1034][RFC 1035], and rich-text service
   subdomains are allowed and encouraged, for example:

     Building 2, 1st Floor.apple.com.
     Building 2, 2nd Floor.apple.com.
     Building 2, 3rd Floor.apple.com.
     Building 2, 4th Floor.apple.com.

   In addition, because Service Instance Names are not constrained by
   the limitations of host names, this document recommends that they
   be stored in the DNS, and communicated over the wire, encoded as
   straightforward canonical precomposed UTF-8, Unicode Normalization
   Form C [UAX15]. In cases where the DNS server returns an NXDOMAIN
   error for the name in question, client software MAY choose to retry
   the query using "Punycode" [RFC 3492] encoding, if possible.


4.2 User Interface Presentation

   The names resulting from the PTR lookup are presented to the user in
   a list for the user to select one (or more). Typically only the first
   label is shown (the user-friendly <Instance> portion of the name). In
   the common case, the <Service> and <Domain> are already known to the
   user, these having been provided by the user in the first place, by
   the act of indicating the service being sought, and the domain in
   which to look for it. Note: The software handling the response
   should be careful not to make invalid assumptions though, since it
   *is* possible, though rare, for a service enumeration in one domain
   to return the names of services in a different domain. Similarly,
   when using subtypes (see "Selective Instance Enumeration") the
   <Service> of the discovered instance my not be exactly the same as
   the <Service> that was requested.

   Having chosen the desired named instance, the Service Instance
   Name may then be used immediately, or saved away in some persistent
   user-preference data structure for future use, depending on what is
   appropriate for the application in question.


4.3 Internal Handling of Names

   If the <Instance>, <Service> and <Domain> portions are internally
   concatenated together into a single string, then care must be taken
   with the <Instance> portion, since it is allowed to contain any
   characters, including dots.

   Any dots in the <Instance> portion should be escaped by preceding
   them with a backslash ("." becomes "\."). Likewise, any backslashes
   in the <Instance> portion should also be escaped by preceding them
   with a backslash ("\" becomes "\\"). Having done this, the three
   components of the name may be safely concatenated. The backslash-


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   escaping allows literal dots in the name (escaped) to be
   distinguished from label-separator dots (not escaped).

   The resulting concatenated string may be safely passed to standard
   DNS APIs like res_query(), which will interpret the string correctly
   provided it has been escaped correctly, as described here.


4.4 What You See Is What You Get

   Some service discovery protocols decouple the true service identifier
   from the name presented to the user. The true service identifier used
   by the protocol is an opaque unique id, often represented using a
   long string of hexadecimal digits, and should never be seen by the
   typical user. The name presented to the user is merely one of the
   ephemeral attributes attached to this opaque identifier.

   The problem with this approach is that it decouples user perception
   from reality:

   * What happens if there are two service instances, with different
     unique ids, but they have inadvertently been given the same
     user-visible name? If two instances appear in an on-screen list
     with the same name, how does the user know which is which?

   * Suppose a printer breaks down, and the user replaces it with
     another printer of the same make and model, and configures the
     new printer with the exact same name as the one being replaced:
     "Stuart's Printer". Now, when the user tries to print, the
     on-screen print dialog tells them that their selected default
     printer is "Stuart's Printer". When they browse the network to see
     what is there, they see a printer called "Stuart's Printer", yet
     when the user tries to print, they are told that the printer
     "Stuart's Printer" can't be found. The hidden internal unique id
     that the software is trying to find on the network doesn't match
     the hidden internal unique id of the new printer, even though its
     apparent "name" and its logical purpose for being there are the
     same. To remedy this, the user typically has to delete the print
     queue they have created, and then create a new (apparently
     identical) queue for the new printer, so that the new queue will
     contain the right hidden internal unique id. Having all this hidden
     information that the user can't see makes for a confusing and
     frustrating user experience, and exposing long ugly hexadecimal
     strings to the user and forcing them to understand what they mean
     is even worse.

   * Suppose an existing printer is moved to a new department, and given
     a new name and a new function. Changing the user-visible name of
     that piece of hardware doesn't change its hidden internal unique
     id. Users who had previously created print queues for that printer
     will still be accessing the same hardware by its unique id, even


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     though the logical service that used to be offered by that hardware
     has ceased to exist.

   To solve these problems requires the user or administrator to be
   aware of the supposedly hidden unique id, and to set its value
   correctly as hardware is moved around, repurposed, or replaced,
   thereby contradicting the notion that it is a hidden identifier that
   human users never need to deal with. Requiring the user to understand
   this expert behind-the-scenes knowledge of what is *really* going on
   is just one more burden placed on the user when they are trying to
   diagnose why their computers and network devices are not working as
   expected.

   These anomalies and counter-intuitive behaviors can be eliminated by
   maintaining a tight bidirectional one-to-one mapping between what
   the user sees on the screen and what is really happening "behind
   the curtain". If something is configured incorrectly, then that is
   apparent in the familiar day-to-day user interface that everyone
   understands, not in some little-known rarely-used "expert" interface.

   In summary: The user-visible name is the primary identifier for a
   service. If the user-visible name is changed, then conceptually
   the service being offered is a different logical service -- even
   though the hardware offering the service stayed the same. If the
   user-visible name doesn't change, then conceptually the service being
   offered is the same logical service -- even if the hardware offering
   the service is new hardware brought in to replace some old equipment.

   There are certainly arguments on both sides of this debate.
   Nonetheless, the designers of any service discovery protocol have
   to make a choice between between having the primary identifiers be
   hidden, or having them be visible, and these are the reasons that
   we chose to make them visible. We're not claiming that there are no
   disadvantages of having primary identifiers be visible. We considered
   both alternatives, and we believe that the few disadvantages
   of visible identifiers are far outweighed by the many problems
   caused by use of hidden identifiers.


4.5 Ordering of Service Instance Name Components

   There have been questions about why services are named using DNS
   Service Instance Names of the form:

      Service Instance Name = <Instance> . <Service> . <Domain>

   instead of:

      Service Instance Name = <Service> . <Instance> . <Domain>


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   There are three reasons why it is beneficial to name service
   instances with the parent domain as the most-significant (rightmost)
   part of the name, then the abstract service type as the next-most
   significant, and then the specific instance name as the
   least-significant (leftmost) part of the name:


4.5.1. Semantic Structure

   The facility being provided by browsing ("Service Instance
   Enumeration") is effectively enumerating the leaves of a tree
   structure. A given domain offers zero or more services. For each
   of those service types, there may be zero or more instances of
   that service.

   The user knows what type of service they are seeking. (If they are
   running an FTP client, they are looking for FTP servers. If they have
   a document to print, they are looking for entities that speak some
   known printing protocol.) The user knows in which organizational or
   geographical domain they wish to search. (The user does not want a
   single flat list of every single printer on the planet, even if such
   a thing were possible.) What the user does not know in advance is
   whether the service they seek is offered in the given domain, or if
   so, how many instances are offered, and the names of those instances.
   Hence having the instance names be the leaves of the tree is
   consistent with this semantic model.

   Having the service types be the terminal leaves of the tree would
   imply that the user knows the domain name, and already knows the
   name of the service instance, but doesn't have any idea what the
   service does. We would argue that this is a less useful model.


4.5.2. Network Efficiency

   When a DNS response contains multiple answers, name compression works
   more effectively if all the names contain a common suffix. If many
   answers in the packet have the same <Service> and <Domain>, then each
   occurrence of a Service Instance Name can be expressed using only
   the <Instance> part followed by a two-byte compression pointer
   referencing a previous appearance of "<Service>.<Domain>". This
   efficiency would not be possible if the <Service> component appeared
   first in each name.


4.5.3. Operational Flexibility

   This name structure allows subdomains to be delegated along logical
   service boundaries. For example, the network administrator at Example
   Co. could choose to delegate the "_tcp.example.com." subdomain to a
   different machine, so that the machine handling service discovery


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   doesn't have to be the same as the machine handling other day-to-day
   DNS operations. (It *can* be the same machine if the administrator so
   chooses, but the point is that the administrator is free to make that
   choice.) Furthermore, if the network administrator wishes to delegate
   all information related to IPP printers to a machine dedicated to
   that specific task, this is easily done by delegating the
   "_ipp._tcp.example.com." subdomain to the desired machine. It is
   also convenient to set security policies on a per-zone/per-subdomain
   basis. For example, the administrator may choose to enable DNS
   Dynamic Update [RFC 2136] [RFC 3007] for printers registering
   in the "_ipp._tcp.example.com." subdomain, but not for other
   zones/subdomains. This easy flexibility would not exist if the
   <Service> component appeared first in each name.


5. Service Name Resolution

   Given a particular Service Instance Name, when a client needs to
   contact that service, it sends a DNS query for the SRV record of
   that name.

   The result of the DNS query is a SRV record giving the port number
   and target host where the service may be found.

   The use of SRV records is very important. There are only 65535 TCP
   port numbers available. These port numbers are being allocated
   one-per-application-protocol at an alarming rate. Some protocols
   like the X Window System have a block of 64 TCP ports allocated
   (6000-6063). If we start allocating blocks of 64 TCP ports at a time,
   we will run out even faster. Using a different TCP port for each
   different instance of a given service on a given machine is entirely
   sensible, but allocating large static ranges, as was done for X, is a
   very inefficient way to manage a limited resource. On any given host,
   most TCP ports are reserved for services that will never run on that
   particular host. This is very poor utilization of the limited port
   space. Using SRV records allows each host to allocate its available
   port numbers dynamically to those services running on that host that
   need them, and then advertise the allocated port numbers via SRV
   records. Allocating the available listening port numbers locally
   on a per-host basis as needed allows much better utilization of the
   available port space than today's centralized global allocation.

   In some environments there may be no compelling reason to assign
   managed names to every host, since every available service is
   accessible by name anyway, as a first-class entity in its own right.
   However, the DNS packet format and record format still require a host
   name to link the target host referenced in the SRV record to the
   address records giving the IPv4 and/or IPv6 addresses for that
   hardware. In the case where no natural host name is available, the
   SRV record may give its own name as the name of the target host, and
   then the requisite address records may be attached to that same name.


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   It is perfectly permissible for a single name in the DNS hierarchy
   to have multiple records of different type attached. (The only
   restriction being that a given name may not have both a CNAME record
   and other records at the same time.)

   In the event that more than one SRV is returned, clients MUST
   correctly interpret the priority and weight fields -- i.e. Lower
   numbered priority servers should be used in preference to higher
   numbered priority servers, and servers with equal priority should be
   selected randomly in proportion to their relative weights. However,
   in the overwhelmingly common case, a single advertised DNS-SD service
   instance is described by exactly one SRV record, and in this common
   case the priority and weight fields of the SRV record SHOULD both be
   set to zero.


6. Data Syntax for DNS-SD TXT Records

   Some services discovered via Service Instance Enumeration may need
   more than just an IP address and port number to properly identify the
   service. For example, printing via the LPR protocol often specifies a
   queue name. This queue name is typically short and cryptic, and need
   not be shown to the user. It should be regarded the same way as the
   IP address and port number -- it is one component of the addressing
   information required to identify a specific instance of a service
   being offered by some piece of hardware. Similarly, a file server may
   have multiple volumes, each identified by its own volume name. A Web
   server typically has multiple pages, each identified by its own URL.
   In these cases, the necessary additional data is stored in a TXT
   record with the same name as the SRV record. The specific nature of
   that additional data, and how it is to be used, is service-dependent,
   but the overall syntax of the data in the TXT record is standardized,
   as described below. Every DNS-SD service MUST have a TXT record in
   addition to its SRV record, with same name, even if the service has
   no additional data to store and the TXT record contains no more than
   a single zero byte.


6.1 General Format Rules for DNS TXT Records

   A DNS TXT record can be up to 65535 (0xFFFF) bytes long. The total
   length is indicated by the length given in the resource record header
   in the DNS message. There is no way to tell directly from the data
   alone how long it is (e.g. there is no length count at the start, or
   terminating NULL byte at the end). (Note that when using Multicast
   DNS [mDNS] the maximum packet size is 9000 bytes, which imposes an
   upper limit on the size of TXT records of about 8800 bytes.)

   The format of the data within a DNS TXT record is one or more
   strings, packed together in memory without any intervening gaps
   or padding bytes for word alignment.


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   The format of each constituent string within the DNS TXT record is a
   single length byte, followed by 0-255 bytes of text data.

   These format rules are defined in Section 3.3.14 of RFC 1035, and are
   not specific to DNS-SD. DNS-SD simply specifies a usage convention
   for what data should be stored in those constituent strings.

   An empty TXT record containing zero strings is disallowed by RFC
   1035. DNS-SD implementations MUST NOT emit empty TXT records.
   DNS-SD implementations receiving empty TXT records MUST treat them
   as equivalent to a one-byte TXT record containing a single zero byte
   (i.e. a single empty string).


6.2 DNS TXT Record Format Rules for use in DNS-SD

   DNS-SD uses DNS TXT records to store arbitrary name/value pairs
   conveying additional information about the named service. Each
   name/value pair is encoded as its own constituent string within the
   DNS TXT record, in the form "name=value". Everything up to the first
   '=' character is the name. Everything after the first '=' character
   to the end of the string (including subsequent '=' characters, if
   any) is the value. Specific rules governing names and values are
   given below. Each author defining a DNS-SD profile for discovering
   instances of a particular type of service should define the base set
   of name/value attributes that are valid for that type of service.

   Using this standardized name/value syntax within the TXT record makes
   it easier for these base definitions to be expanded later by defining
   additional named attributes. If an implementation sees unknown
   attribute names in a service TXT record, it MUST silently ignore
   them.

   The TCP (or UDP) port number of the service, and target host name,
   are given in the SRV record. This information -- target host name and
   port number -- MUST NOT be duplicated using name/value attributes in
   the TXT record.

   The intention of DNS-SD TXT records is to convey a small amount of
   useful additional information about a service. Ideally it SHOULD NOT
   be necessary for a client to retrieve this additional information
   before it can usefully establish a connection to the service. For a
   well-designed TCP-based application protocol, it should be possible,
   knowing only the host name and port number, to open a connection
   to that listening process, and then perform version- or feature-
   negotiation to determine the capabilities of the service instance.
   For example, when connecting to an AppleShare server over TCP, the
   client enters into a protocol exchange with the server to determine
   which version of the AppleShare protocol the server implements, and
   which optional features or capabilities (if any) are available. For a
   well-designed application protocol, clients should be able to connect


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   and use the service even if there is no information at all in the TXT
   record. In this case, the information in the TXT record should be
   viewed as a performance optimization -- when a client discovers many
   instances of a service, the TXT record allows the client to know some
   rudimentary information about each instance without having to open a
   TCP connection to each one and interrogate every service instance
   separately. Extreme care should be taken when doing this to ensure
   that the information in the TXT record is in agreement with the
   information retrieved by a client connecting over TCP.

   There are legacy protocols which provide no feature negotiation
   capability, and in these cases it may be useful to convey necessary
   information in the TXT record. For example, when printing using the
   old Unix LPR (port 515) protocol, the LPR service provides no way
   for the client to determine whether a particular printer accepts
   PostScript, or what version of PostScript, etc. In this case it is
   appropriate to embed this information in the TXT record, because the
   alternative is worse -- passing around written instructions to the
   users, arcane manual configuration of "/etc/printcap" files, etc.


6.3 DNS-SD TXT Record Size

   The total size of a typical DNS-SD TXT record is intended to be small
   -- 200 bytes or less.

   In cases where more data is justified (e.g. LPR printing), keeping
   the total size under 400 bytes should allow it to fit in a single
   standard 512-byte DNS message. (This standard DNS message size is
   defined in RFC 1035.)

   In extreme cases where even this is not enough, keeping the size of
   the TXT record under 1300 bytes should allow it to fit in a single
   1500-byte Ethernet packet.

   Using TXT records larger than 1300 bytes is NOT RECOMMENDED at this
   time.


6.4 Rules for Names in DNS-SD Name/Value Pairs

   The "Name" MUST be at least one character. Strings beginning with an
   '=' character (i.e. the name is missing) SHOULD be silently ignored.

   The characters of "Name" MUST be printable US-ASCII values
   (0x20-0x7E), excluding '=' (0x3D).

   Spaces in the name are significant, whether leading, trailing, or in
   the middle -- so don't include any spaces unless you really intend
   that!


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   Case is ignored when interpreting a name, so "papersize=A4",
   "PAPERSIZE=A4" and "Papersize=A4" are all identical.

   If there is no '=', then it is a boolean attribute, and is simply
   identified as being present, with no value.

   A given attribute name may appear at most once in a TXT record.
   The reason for this simplifying rule is to facilitate the creation
   of client libraries that parse the TXT record into an internal data
   structure, such as a hash table or dictionary object that maps from
   names to values, and then make that abstraction available to client
   code. The rule that a given attribute name may not appear more than
   once simplifies these abstractions because they aren't required to
   support the case of returning more than one value for a given key.

   If a client receives a TXT record containing the same attribute name
   more than once, then the client MUST silently ignore all but the
   first occurrence of that attribute. For client implementations that
   process a DNS-SD TXT record from start to end, placing name/value
   pairs into a hash table, using the name as the hash table key, this
   means that if the implementation attempts to add a new name/value
   pair into the table and finds an entry with the same name already
   present, then the new entry being added should be silently discarded
   instead. For client implementations that retrieve name/value pairs by
   searching the TXT record for the requested name, they should search
   the TXT record from the start, and simply return the first matching
   name they find.

   When examining a TXT record for a given named attribute, there are
   therefore four broad categories of results which may be returned:

   * Attribute not present (Absent)

   * Attribute present, with no value
     (e.g. "Anon Allowed" -- server allows anonymous connections)

   * Attribute present, with empty value (e.g. "Installed PlugIns=" --
     server supports plugins, but none are presently installed)

   * Attribute present, with non-empty value
     (e.g. "Installed PlugIns=JPEG,MPEG2,MPEG4")

   Each author defining a DNS-SD profile for discovering instances of a
   particular type of service should define the interpretation of these
   different kinds of result. For example, for some keys, there may be
   a natural true/false boolean interpretation:

   * Present implies 'true'
   * Absent implies 'false'


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   For other keys it may be sensible to define other semantics, such as
   value/no-value/unknown:

   * Present with value implies that value.
     E.g. "Color=4" for a four-color ink-jet printer,
     or "Color=6" for a six-color ink-jet printer.

   * Present with empty value implies 'false'. E.g. Not a color printer.

   * Absent implies 'Unknown'. E.g. A print server connected to some
     unknown printer where the print server doesn't actually know if the
     printer does color or not -- which gives a very bad user experience
     and should be avoided wherever possible.

   (Note that this is a hypothetical example, not an example of actual
   name/value keys used by DNS-SD network printers.)

   As a general rule, attribute names that contain no dots are defined
   as part of the open-standard definition written by the person or
   group defining the DNS-SD profile for discovering that particular
   service type. Vendor-specific extensions should be given names of the
   form "keyname.company.com=value", using a domain name legitimately
   registered to the person or organization creating the vendor-specific
   key. This reduces the risk of accidental conflict if different
   organizations each define their own vendor-specific keys.


6.5 Rules for Values in DNS-SD Name/Value Pairs

   If there is an '=', then everything after the first '=' to the end
   of the string is the value. The value can contain any eight-bit
   values including '='. Leading or trailing spaces are part of the
   value, so don't put them there unless you intend them to be there.
   Any quotation marks around the value are part of the value, so don't
   put them there unless you intend them to be part of the value.

   The value is opaque binary data. Often the value for a particular
   attribute will be US-ASCII (or UTF-8) text, but it is legal for a
   value to be any binary data. For example, if the value of a key is an
   IPv4 address, that address should simply be stored as four bytes of
   binary data, not as a variable-length 7-15 byte ASCII string giving
   the address represented in textual dotted decimal notation.

   Generic debugging tools should generally display all attribute values
   as a hex dump, with accompanying text alongside displaying the UTF-8
   interpretation of those bytes, except for attributes where the
   debugging tool has embedded knowledge that the value is some other
   kind of data.

   Authors defining DNS-SD profiles SHOULD NOT convert binary attribute
   data types into printable text (e.g. using hexadecimal, Base-64 or UU


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   encoding) merely for the sake of making the data be printable text
   when seen in a generic debugging tool. Doing this simply bloats the
   size of the TXT record, without actually making the data any more
   understandable to someone looking at it in a generic debugging tool.


6.6 Example TXT Record

   The TXT record below contains three syntactically valid name/value
   pairs. (The meaning of these name/value pairs, if any, would depend
   on the definitions pertaining to the service in question that is
   using them.)

   ---------------------------------------------------------------
   | 0x0A | name=value | 0x08 | paper=A4 | 0x0E | DNS-SD Is Cool |
   ---------------------------------------------------------------


6.7 Version Tag

   It is recommended that authors defining DNS-SD profiles include an
   attribute of the form "txtvers=xxx" in their definition, and require
   it to be the first name/value pair in the TXT record. This
   information in the TXT record can be useful to help clients maintain
   backwards compatibility with older implementations if it becomes
   necessary to change or update the specification over time. Even if
   the profile author doesn't anticipate the need for any future
   incompatible changes, having a version number in the specification
   provides useful insurance should incompatible changes become
   unavoidable. Clients SHOULD ignore TXT records with a txtvers number
   higher (or lower) than the version(s) they know how to interpret.

   Note that the version number in the txtvers tag describes the version
   of the TXT record specification being used to create this TXT record,
   not the version of the application protocol that will be used if the
   client subsequently decides to contact that service. Ideally, every
   DNS-SD TXT record specification starts at txtvers=1 and stays that
   way forever. Improvements can be made by defining new keys that older
   clients silently ignore. The only reason to increment the version
   number is if the old specification is subsequently found to be so
   horribly broken that there's no way to do a compatible forward
   revision, so the txtvers number has to be incremented to tell all the
   old clients they should just not even try to understand this new TXT
   record.

   If there is a need to indicate which version number(s) of the
   application protocol the service implements, the recommended key
   name for this is "protovers".


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7. Application Protocol Names

   The <Service> portion of a Service Instance Name consists of a pair
   of DNS labels, following the established convention for SRV records
   [RFC 2782], namely: the first label of the pair is an underscore
   character followed by the Application Protocol Name, and the second
   label is either "_tcp" or "_udp".

   Application Protocol Names may be no more than fourteen characters
   (not counting the mandatory underscore), conforming to normal DNS
   host name rules: Only lower-case letters, digits, and hyphens; must
   begin and end with lower-case letter or digit.

   Wise selection of an Application Protocol Name is very important,
   and the choice is not always as obvious as it may appear.

   In some cases, the Application Protocol Name merely names and refers
   to the on-the-wire message format and semantics being used. FTP is
   "ftp", IPP printing is "ipp", and so on.

   However, it is common to "borrow" an existing protocol and repurpose
   it for a new task. This is entirely sensible and sound engineering
   practice, but that doesn't mean that the new protocol is providing
   the same semantic service as the old one, even if it borrows the same
   message formats. For example, the local network music playing
   protocol implemented by iTunes on Macintosh and Windows is little
   more than "HTTP GET" commands. However, that does *not* mean that it
   is sensible or useful to try to access one of these music servers by
   connecting to it with a standard web browser. Consequently, the
   DNS-SD service advertised (and browsed for) by iTunes is "_daap._tcp"
   (Digital Audio Access Protocol), not "_http._tcp". Advertising
   "_http._tcp" service would cause iTunes servers to show up in
   conventional Web browsers (Safari, Camino, OmniWeb, Opera, Netscape,
   Internet Explorer, etc.) which is little use since it offers no pages
   containing human-readable content. Similarly, browsing for
   "_http._tcp" service would cause iTunes to find generic web servers,
   such as the embedded web servers in devices like printers, which is
   little use since printers generally don't have much music to offer.

   Similarly, NFS is built on top of SUN RPC, but that doesn't mean it
   makes sense for an NFS server to advertise that it provides "SUN RPC"
   service. Likewise, Microsoft SMB file service is built on top of
   Netbios running over IP, but that doesn't mean it makes sense for
   an SMB file server to advertise that it provides "Netbios-over-IP"
   service. The DNS-SD name of a service needs to encapsulate both the
   "what" (semantics) and the "how" (protocol implementation) of the
   service, since knowledge of both is necessary for a client to
   usefully use the service. Merely advertising that a service was
   built on top of SUN RPC is no use if the client has no idea what
   the service actually does.


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   Another common mistake is to assume that the service type advertised
   by iTunes should be "_daap._http._tcp." This is also incorrect.
   Similarly, a protocol designer implementing a network service that
   happens to use Simple Object Access Protocol [SOAP] should not feel
   compelled to have "_soap" appear somewhere in the Application
   Protocol Name. Part of the confusion here is that the presence of
   "_tcp" or "_udp" in the <Service> portion of a Service Instance Name
   has led people to assume that the structure of a service name has to
   reflect the internal structure of how the protocol was implemented.
   This is not correct. All that is required is that the service be
   identified by a unique Application Protocol Name. Making the
   Application Protocol Name at least marginally descriptive of
   what the service does is desirable, though not essential.

   The "_tcp" or "_udp" should be regarded as little more than
   boilerplate text, and care should be taken not to attach too much
   importance to it. Some might argue that the "_tcp" or "_udp" should
   not be there at all, but this format is defined by RFC 2782, and
   that's not going to change. In addition, the presence of "_tcp" has
   the useful side-effect that it provides a convenient delegation point
   to hand off responsibility for service discovery to a different DNS
   server, if so desired.


7.1. Selective Instance Enumeration

   This document does not attempt to define an arbitrary query language
   for service discovery, nor do we believe one is necessary.

   However, there are some circumstances where narrowing the list of
   results may be useful. A hypothetical Web browser client that is able
   to retrieve HTML documents via HTTP and display them may also be able
   to retrieve HTML documents via FTP and display them, but only in the
   case of FTP servers that allow anonymous login. For that Web browser,
   discovering all FTP servers on the network is not useful. The Web
   browser only wants to discover FTP servers that it is able to talk
   to. In this case, a subtype of "_ftp._tcp" could be defined. Instead
   of issuing a query for "_ftp._tcp.<Domain>", the Web browser issues a
   query for "_anon._sub._ftp._tcp.<Domain>", where "_anon" is a defined
   subtype of "_ftp._tcp". The response to this query only includes the
   names of SRV records for FTP servers that are willing to allow
   anonymous login.

   Note that the FTP server's Service Instance Name is unchanged -- it
   is still something of the form "The Server._ftp._tcp.example.com."
   The subdomain in which FTP server SRV records are registered defines
   the namespace within which FTP server names are unique. Additional
   subtypes (e.g. "_anon") of the basic service type (e.g. "_ftp._tcp")
   serve to narrow the list of results, not to create more namespace.


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   Subtypes are appropriate when it is desirable for different kinds
   of clients to be able to browse for services at two levels of
   granularity. In the example above, we hypothesize two classes of
   ftp client: clients that can provide username and password when
   connecting, and clients that can only do anonymous login. The set of
   ftp servers on the network is the same in both cases; the difference
   is that the more capable client wants to discover all of them,
   whereas the more limited client only wants to find the subset of
   those ftp servers that it can talk to. Subtypes are only appropriate
   in two-level scenarios such as this one, where some clients want to
   find the full set of services of a given type, and at the same time
   other clients only want to find some subset. Generally speaking, if
   there is no client that wants to find the entire set, then it's
   neither necessary nor desirable to use the subtype mechanism. If all
   clients are browsing for some particular subtype, and no client
   exists that browses for the parent type, then an Application Protocol
   Name representing the logical service should be defined, and software
   should simply advertise and browse for that particular service type
   directly. In particular, just because a particular network service
   happens to be implemented in terms of some other underlying protocol,
   like HTTP, Sun RPC, or SOAP, doesn't mean that it's sensible for that
   service to be defined as a subtype of "_http", "_sunrpc", or "_soap".
   That would only be useful if there were some class of client for
   which it is sensible to say, "I want to discover a service on the
   network, and I don't care what it does, as long as it does it using
   the SOAP XML RPC mechanism."

   As with the TXT record name/value pairs, the list of possible
   subtypes, if any, are defined and specified separately for each basic
   service type. Note that the example given here using "_ftp" is a
   hypothetical one. The "_ftp" service type does not (currently) have
   any subtypes defined. Subtypes are currently a little-used feature
   of DNS-SD, and experience will show whether or not they ultimately
   prove to have broad applicability.


7.2 Service Name Length Limits

   As described above, application protocol names are allowed to be up
   to fourteen characters long. The reason for this limit is to leave
   as many bytes of the domain name as possible available for use
   by both the network administrator (choosing service domain names)
   and the end user (choosing instance names).

   A domain name may be up to 255 bytes long, including the final
   terminating root label at the end. Domain names used by DNS-SD
   take the following forms:

      <Instance>.<app>._tcp.<servicedomain>.<parentdomain>.
      <sub>._sub.<app>._tcp.<servicedomain>.<parentdomain>.


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   The first example shows a service instance name, i.e. the name of the
   service's SRV and TXT records. The second shows a subtype browsing
   name, i.e. the name of a PTR record pointing to service instance
   names (see "Selective Instance Enumeration").

   The instance name <Instance> may be up to 63 bytes. Including the
   length byte used by the DNS format when the name is stored in a
   packet, that makes 64 bytes.

   When using subtypes, the subtype identifier is allowed to be up to
   63 bytes, plus the length byte, making 64. Including the "_sub"
   and its length byte, this makes 69 bytes.

   The application protocol name <app> may be up to 14 bytes, plus the
   underscore and length byte, making 16. Including the "_udp" or "_tcp"
   and its length byte, this makes 21 bytes.

   Typically, DNS-SD service records are placed into subdomains of their
   own beneath a company's existing domain name. Since these subdomains
   are intended to be accessed through graphical user interfaces, not
   typed on a command-line they are frequently long and descriptive.
   Including the length byte, the user-visible service domain may be up
   to 64 bytes.

   The terminating root label at the end counts as one byte.

   Of our available 255 bytes, we have now accounted for 69+21+64+1 =
   155 bytes. This leaves 100 bytes to accommodate the organization's
   existing domain name <parentdomain>. When used with Multicast DNS,
   <parentdomain> is "local", which easily fits. When used with parent
   domains of 100 bytes or less, the full functionality of DNS-SD is
   available without restriction. When used with parent domains longer
   than 100 bytes, the protocol risks exceeding the maximum possible
   length of domain names, causing failures. In this case, careful
   choice of short <servicedomain> names can help avoid overflows.
   If the <servicedomain> and <parentdomain> are too long, then service
   instances with long instance names will not be discoverable or
   resolvable, and applications making use of long subtype names
   may fail.

   Because of this constraint, we choose to limit Application Protocol
   Names to 14 characters or less. Allowing more characters would not
   add to the expressive power of the protocol, and would needlessly
   lower the limit on the maximum <parentdomain> length that may be
   safely used.


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8. Flagship Naming

   In some cases, there may be several network protocols available
   which all perform roughly the same logical function. For example,
   the printing world has the LPR protocol, and the Internet Printing
   Protocol (IPP), both of which cause printed sheets to be emitted
   from printers in much the same way. In addition, many printer vendors
   send their own proprietary page description language (PDL) data
   over a TCP connection to TCP port 9100, herein referred to as the
   "pdl-datastream" protocol. In an ideal world we would have only one
   network printing protocol, and it would be sufficiently good that no
   one felt a compelling need to invent a different one. However, in
   practice, multiple legacy protocols do exist, and a service discovery
   protocol has to accommodate that.

   Many printers implement all three printing protocols: LPR, IPP, and
   pdl-datastream. For the benefit of clients that may speak only one of
   those protocols, all three are advertised.

   However, some clients may implement two, or all three of those
   printing protocols. When a client looks for all three service types
   on the network, it will find three distinct services -- an LPR
   service, an IPP service, and a pdl-datastream service -- all of which
   cause printed sheets to be emitted from the same physical printer.

   In the case of multiple protocols like this that all perform
   effectively the same function, the client should suppress duplicate
   names and display each name only once. When the user prints to a
   given named printer, the printing client is responsible for choosing
   the protocol which will best achieve the desired effect, without, for
   example, requiring the user to make a manual choice between LPR and
   IPP.

   As described so far, this all works very well. However, consider some
   future printer that only supports IPP printing, and some other future
   printer that only supports pdl-datastream printing. The name spaces
   for different service types are intentionally disjoint -- it is
   acceptable and desirable to be able to have both a file server called
   "Sales Department" and a printer called "Sales Department". However,
   it is not desirable, in the common case, to have two different
   printers both called "Sales Department", just because those printers
   are implementing different protocols.

   To help guard against this, when there are two or more network
   protocols which perform roughly the same logical function, one of
   the protocols is declared the "flagship" of the fleet of related
   protocols. Typically the flagship protocol is the oldest and/or
   best-known protocol of the set.

   If a device does not implement the flagship protocol, then it instead
   creates a placeholder SRV record (priority=0, weight=0, port=0,


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   target host = hostname of device) with that name. If, when it
   attempts to create this SRV record, it finds that a record with the
   same name already exists, then it knows that this name is already
   taken by some entity implementing at least one of the protocols from
   the class, and it must choose another. If no SRV record already
   exists, then the act of creating it stakes a claim to that name so
   that future devices in the same class will detect a conflict when
   they try to use it. The SRV record needs to contain the target host
   name in order for the conflict detection rules to operate. If two
   different devices were to create placeholder SRV records both using a
   null target host name (just the root label), then the two SRV records
   would be seen to be in agreement so no conflict would be registered.

   By defining a common well-known flagship protocol for the class,
   future devices that may not even know about each other's protocols
   establish a common ground where they can coordinate to verify
   uniqueness of names.

   No PTR record is created advertising the presence of empty flagship
   SRV records, since they do not represent a real service being
   advertised.


9. Service Type Enumeration

   In general, clients are not interested in finding *every* service on
   the network, just the services that the client knows how to talk to.
   (Software designers may *think* there's some value to finding *every*
   service on the network, but that's just wooly thinking.)

   However, for problem diagnosis and network management tools, it may
   be useful for network administrators to find the list of advertised
   service types on the network, even if those service names are just
   opaque identifiers and not particularly informative in isolation.

   For this reason, a special meta-query is defined. A DNS query for
   PTR records with the name "_services._dns-sd._udp.<Domain>" yields
   a list of PTR records, where the rdata of each PTR record is the
   name of a service type. A subsequent query for PTR records with
   one of those names yields a list of instances of that service type.


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10. Populating the DNS with Information

   How the SRV and PTR records that describe services and allow them to
   be enumerated make their way into the DNS is outside the scope of
   this document. However, it can happen easily in any of a number of
   ways, for example:

   On some networks, the administrator might manually enter the records
   into the name server's configuration file.

   A network monitoring tool could output a standard zone file to be
   read into a conventional DNS server. For example, a tool that can
   find Apple LaserWriters using AppleTalk NBP could find the list
   of printers, communicate with each one to find its IP address,
   PostScript version, installed options, etc., and then write out a
   DNS zone file describing those printers and their capabilities using
   DNS resource records. That information would then be available to
   DNS-SD clients that don't implement AppleTalk NBP, and don't want to.

   Future IP printers could use Dynamic DNS Update [RFC 2136] to
   automatically register their own SRV and PTR records with the DNS
   server.

   A printer manager device which has knowledge of printers on the
   network through some other management protocol could also use Dynamic
   DNS Update [RFC 2136].

   Alternatively, a printer manager device could implement enough of
   the DNS protocol that it is able to answer DNS queries directly,
   and Example Co.'s main DNS server could delegate the
   _ipp._tcp.example.com subdomain to the printer manager device.

   Zeroconf printers answer Multicast DNS queries on the local link
   for appropriate PTR and SRV names ending with ".local." [mDNS]


11. Relationship to Multicast DNS

   DNS-Based Service Discovery is only peripherally related to Multicast
   DNS, in that the standard unicast DNS queries used by DNS-SD may also
   be performed using multicast when appropriate, which is particularly
   beneficial in Zeroconf environments [ZC].


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12. Discovery of Browsing and Registration Domains (Domain Enumeration)

   One of the main reasons for DNS-Based Service Discovery is so that
   when a visiting client (e.g. a laptop computer) arrives at a new
   network, it can discover what services are available on that network
   without manual configuration. This logic that applies to discovering
   services without manual configuration also applies to discovering the
   domains in which services are registered without requiring manual
   configuration.

   This discovery is performed recursively, using Unicast or Multicast
   DNS. Five special RR names are reserved for this purpose:

                      b._dns-sd._udp.<domain>.
                     db._dns-sd._udp.<domain>.
                      r._dns-sd._udp.<domain>.
                     dr._dns-sd._udp.<domain>.
                     lb._dns-sd._udp.<domain>.

   By performing PTR queries for these names, a client can learn,
   respectively:

    o A list of domains recommended for browsing

    o A single recommended default domain for browsing

    o A list of domains recommended for registering services using
      Dynamic Update

    o A single recommended default domain for registering services.

    o The final query shown yields the "legacy browsing" domain.
      Sophisticated client applications that care to present choices
      of domain to the user, use the answers learned from the previous
      four queries to discover those domains to present. In contrast,
      many current applications browse without specifying an explicit
      domain, allowing the operating system to automatically select an
      appropriate domain on their behalf. It is for this class of
      application that the "legacy browsing" query is provided, to allow
      the network administrator to communicate to the client operating
      systems which domain should be used for these applications.

   These domains are purely advisory. The client or user is free to
   browse and/or register services in any domains. The purpose of these
   special queries is to allow software to create a user-interface that
   displays a useful list of suggested choices to the user, from which
   they may make a suitable selection, or ignore the offered suggestions
   and manually enter their own choice.


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   The <domain> part of the name may be "local" (meaning "perform the
   query using link-local multicast) or it may be learned through some
   other mechanism, such as the DHCP "Domain" option (option code 15)
   [RFC 2132] or the DHCP "Domain Search" option (option code 119)
   [RFC 3397].

   The <domain> part of the name may also be derived from the host's IP
   address. The host takes its IP address, and calculates the logical
   AND of that address and its subnet mask, to derive the 'base' address
   of the subnet. It then constructs the conventional DNS "reverse
   mapping" name corresponding to that base address, and uses that
   as the <domain> part of the name for the queries described above.
   For example, if a host has address 192.168.12.34, with subnet mask
   255.255.0.0, then the 'base' address of the subnet is 192.168.0.0,
   and to discover the recommended legacy browsing domain for devices
   on this subnet, the host issues a DNS PTR query for the name
   "lb._dns-sd._udp.0.0.168.192.in-addr.arpa."

   Sophisticated clients may perform domain enumeration queries both in
   "local" and in one or more unicast domains, and then present the user
   with an aggregate result, combining the information received from all
   sources.


13. DNS Additional Record Generation

   DNS has an efficiency feature whereby a DNS server may place
   additional records in the Additional Section of the DNS Message.
   These additional records are typically records that the client did
   not explicitly request, but the server has reasonable grounds to
   expect that the client might request them shortly.

   This section recommends which additional records should be generated
   to improve network efficiency for both unicast and multicast DNS-SD
   responses.


13.1 PTR Records

   When including a PTR record in a response packet, the
   server/responder SHOULD include the following additional records:

   o The SRV record(s) named in the PTR rdata.
   o The TXT record(s) named in the PTR rdata.
   o All address records (type "A" and "AAAA") named in the SRV rdata.


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13.2 SRV Records

   When including an SVR record in a response packet, the
   server/responder SHOULD include the following additional records:

   o All address records (type "A" and "AAAA") named in the SRV rdata.


13.3 TXT Records

   When including a TXT record in a response packet, no additional
   records are required.


13.4 Other Record Types

   In response to address queries, or other record types, no additional
   records are required by this document.


14. Comparison with Alternative Service Discovery Protocols

   Over the years there have been many proposed ways to do network
   service discovery with IP, but none achieved ubiquity in the
   marketplace. Certainly none has achieved anything close to the
   ubiquity of today's deployment of DNS servers, clients, and other
   infrastructure.

   The advantage of using DNS as the basis for service discovery is
   that it makes use of those existing servers, clients, protocols,
   infrastructure, and expertise. Existing network analyzer tools
   already know how to decode and display DNS packets for network
   debugging.

   For ad-hoc networks such as Zeroconf environments, peer-to-peer
   multicast protocols are appropriate. The Zeroconf host profile [ZCHP]
   requires the use of a DNS-like protocol over IP Multicast for host
   name resolution in the absence of DNS servers. Given that Zeroconf
   hosts will have to implement this Multicast-based DNS-like protocol
   anyway, it makes sense for them to also perform service discovery
   using that same Multicast-based DNS-like software, instead of also
   having to implement an entirely different service discovery protocol.

   In larger networks, a high volume of enterprise-wide IP multicast
   traffic may not be desirable, so any credible service discovery
   protocol intended for larger networks has to provide some facility to
   aggregate registrations and lookups at a central server (or servers)
   instead of working exclusively using multicast. This requires some
   service discovery aggregation server software to be written,
   debugged, deployed, and maintained. This also requires some service
   discovery registration protocol to be implemented and deployed for


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   clients to register with the central aggregation server. Virtually
   every company with an IP network already runs a DNS server, and DNS
   already has a dynamic registration protocol [RFC 2136]. Given that
   virtually every company already has to operate and maintain a DNS
   server anyway, it makes sense to take advantage of this instead of
   also having to learn, operate and maintain a different service
   registration server. It should be stressed again that using the
   same software and protocols doesn't necessarily mean using the same
   physical piece of hardware. The DNS-SD service discovery functions
   do not have to be provided by the same piece of hardware that
   is currently providing the company's DNS name service. The
   "_tcp.<Domain>" subdomain may be delegated to a different piece of
   hardware. However, even when the DNS-SD service is being provided
   by a different piece of hardware, it is still the same familiar DNS
   server software that is running, with the same configuration file
   syntax, the same log file format, and so forth.

   Service discovery needs to be able to provide appropriate security.
   DNS already has existing mechanisms for security [RFC 2535].

   In summary:

      Service discovery requires a central aggregation server.
      DNS already has one: It's called a DNS server.

      Service discovery requires a service registration protocol.
      DNS already has one: It's called DNS Dynamic Update.

      Service discovery requires a query protocol
      DNS already has one: It's called DNS.

      Service discovery requires security mechanisms.
      DNS already has security mechanisms: DNSSEC.

      Service discovery requires a multicast mode for ad-hoc networks.
      Zeroconf environments already require a multicast-based DNS-like
      name lookup protocol for mapping host names to addresses, so it
      makes sense to let one multicast-based protocol do both jobs.

   It makes more sense to use the existing software that every network
   needs already, instead of deploying an entire parallel system just
   for service discovery.


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15. Real Examples

   The following examples were prepared using standard unmodified
   nslookup and standard unmodified BIND running on GNU/Linux.

   Note: In real products, this information is obtained and presented to
   the user using graphical network browser software, not command-line
   tools, but if you wish you can try these examples for yourself as you
   read along, using the command-line tools already available on your
   own Unix machine.

15.1 Question: What FTP servers are being advertised from dns-sd.org?

   nslookup -q=ptr _ftp._tcp.dns-sd.org.
   _ftp._tcp.dns-sd.org
            name = Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org
   _ftp._tcp.dns-sd.org
            name = Microsoft\032Developer\032Files._ftp._tcp.dns-sd.org
   _ftp._tcp.dns-sd.org
            name = Registered\032Users'\032Only._ftp._tcp.dns-sd.org

   Answer: There are three, called "Apple QuickTime Files",
   "Microsoft Developer Files" and "Registered Users' Only".

   Note that nslookup escapes spaces as "\032" for display purposes,
   but a graphical DNS-SD browser does not.

15.2 Question: What FTP servers allow anonymous access?

   nslookup -q=ptr _anon._sub._ftp._tcp.dns-sd.org
   _anon._sub._ftp._tcp.dns-sd.org
            name = Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org
   _anon._sub._ftp._tcp.dns-sd.org
            name = Microsoft\032Developer\032Files._ftp._tcp.dns-sd.org

   Answer: Only "Apple QuickTime Files" and "Microsoft Developer Files"
   allow anonymous access.

15.3 Question: How do I access "Apple QuickTime Files"?

   nslookup -q=any "Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org."
   Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org
             text = "path=/quicktime"
   Apple\032QuickTime\032Files._ftp._tcp.dns-sd.org
             priority = 0, weight = 0, port= 21 host = ftp.apple.com
   ftp.apple.com   internet address = 17.254.0.27
   ftp.apple.com   internet address = 17.254.0.31
   ftp.apple.com   internet address = 17.254.0.26

   Answer: You need to connect to ftp.apple.com, port 21, path
   "/quicktime". The addresses for ftp.apple.com are also given.


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16. User Interface Considerations

   DNS-Based Service Discovery was designed by first giving careful
   consideration to what constitutes a good user experience for service
   discovery, and then designing a protocol with the features necessary
   to enable that good user experience. This section covers two issues
   in particular: Choice of factory-default names (and automatic
   renaming behavior) for devices advertising services, and the
   "continuous live update" user-experience model for clients
   browsing to discover services.


16.1 Service Advertising User-Interface Considerations

   When a DNS-SD service is advertised using Multicast DNS [mDNS],
   automatic name conflict and resolution will occur if there is already
   another service of the same type advertising with the same name.
   As described in the Multicast DNS specification [mDNS], upon a
   conflict, the service should:
   
   1. Automatically select a new name (typically by appending
      or incrementing a digit at the end of the name),
   2. try advertising with the new name, and
   3. upon success, record the new name in persistent storage.

   This renaming behavior is very important, because it is the key
   to providing user-friendly service names in the out-of-the-box
   factory-default configuration. Some product developers may not
   have realized this, because there are some products today where
   the factory-default name is distinctly unfriendly, containing
   random-looking strings of characters, like the device's Ethernet
   address in hexadecimal. This is unnecessary, and undesirable, because
   the point of the user-visible name is that it should be friendly and
   useful to human users. If the name is not unique on the local network
   the protocol will rememdy this as necessary. It is ironic that many
   of the devices with this mistake are network printers, given that
   these same printers also simultaneously support AppleTalk-over-
   Ethernet, with nice user-friendly default names (and automatic
   conflict detection and renaming). Examples of good factory-default
   names are as follows:

      Brother 5070N
      Canon W2200                            [ Apologies to makers of ]
      HP LaserJet 4600                       [ DNS-SD/mDNS printers   ]
      Lexmark W840                           [ not listed. Email      ]
      Okidata C5300                          [ the authors and we'll  ]
      Ricoh Aficio CL7100                    [ add you to the list.   ]
      Xerox Phaser 6200DX


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   To complete the case for why adding long ugly serial numbers to
   the end of names is neither necessary nor desirable, consider
   the cases where the user has (a) only one network printer,
   (b) two network printers, and (c) many network printers.

   (a) In the case where the user has only one network printer, a simple
       name like (to use a vendor-neutral example) "Printer" is more
       user-friendly than an ugly name like "Printer 0001E68C74FB".
       Appending ugly hexadecimal goop to the end of the name to make
       sure the name is unique is irrelevant to a user who only has one
       printer anyway.

   (b) In the case where the user gets a second network printer,
       having it detect that the name "Printer" is already in use
       and automatically instead name itself "Printer (2)" provides a
       good user experience. For the users, remembering that the old
       printer is "Printer" and the new one is "Printer (2)" is easy
       and intuitive. Seeing two printers called "Printer 0001E68C74FB"
       and "Printer 00306EC3FD1C" is a lot less helpful.

   (c) In the case of a network with ten network printers, seeing a
       list of ten names all of the form "Printer xxxxxxxxxxxx" has
       effectively taken what was supposed to be a list of user-friendly
       rich-text names (supporting mixed case, spaces, punctuation,
       non-Roman characters and other symbols) and turned it into
       just about the worst user-interface imaginable: a list of
       incomprehensible random-looking strings of letters and digits.
       In a network with a lot of printers, it would be desirable for
       the people setting up the printers to take a moment to give each
       one a descriptive name, but in the event they don't, presenting
       the users with a list of sequentially-numbered printers is a much
       more desirable default user experience than showing a list of raw
       Ethernet addresses.


16.2 Client Browsing User-Interface Considerations

   Of particular concern in the design of DNS-SD was the dynamic nature
   of service discovery in a changing network environment. Other service
   discovery protocols have been designed with an implicit unstated
   assumption that the usage model is:

      (a) client calls the service discovery code
      (b) client gets list of discovered services
          as of a particular instant in time, and then
      (c) client displays list for user to select from

   Superficially this usage model seems reasonable, but the problem is
   that it's too optimistic. It only considers the success case, where
   the user successfully finds the service they're looking for. In the


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   case where the user is looking for (say) a particular printer, and
   that printer's not turned on or not connected, the user first has
   to attempt to remedy the problem, and then has to click a "refresh"
   button to retry the service discovery (or, worse, dismiss the
   browsing window entirely, and open a new one to initiate a new
   network search attempt) to find out whether they were successful.
   Because nothing happens instantaneously in networking, and packets
   can be lost, necessitating some number of retransmissions, a service
   discovery search typically takes a few seconds. A fairly typical user
   experience model is:

      (a) display an empty window,
      (b) display some animation like a searchlight
          sweeping back and forth for ten seconds, and then
      (c) at the end of the ten-second search, display
          a static list showing what was discovered.

   Every time the user clicks the "refresh" button they have to endure
   another ten-second wait, and every time the discovered list is
   finally shown at the end of the ten-second wait, the moment it's
   displayed on the screen it's already beginning to get stale and
   out-of-date.

   The service discovery user experience that the DNS-SD designers had
   in mind has some rather different properties:

   1. Displaying a list of discovered services should be effectively
      instantaneous -- i.e. typically 1/10 second, not 10 seconds.

   2. The list of discovered services should not be getting stale
      and out-of-date from the moment it's displayed. The list
      should be 'live' and should continue to update as new services
      are discovered. Because of the delays, packet losses, and
      retransmissions inherent in networking, it is to be expected
      that sometimes, after the initial list is displayed showing
      the majority of discovered services, a few remaining stragglers
      may continue to trickle in during the subsequent few seconds.
      Even after this initial stable list has been built and displayed,
      the list should remain 'live' and should continue to update.
      At any future time, be it minutes, hours, or even days later,
      if a new service of the desired type is discovered, it should be
      displayed in the list automatically, without the user having to
      click a "refresh" button or take any other explicit action to
      update the display.

   3. With users getting to be in the habit of leaving service discovery
      windows open, and coming to expect to be able to rely on them
      to show a continuous 'live' view of current network reality,
      this creates a new requirement for us: deletion of stale services.
      When a service discovery list shows just a static snapshot at a
      moment in time, then the situation is simple: either a service was


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      discovered and appears in the list, or it was not, and does not.
      However, when our list is live and updates continuously with the
      discovery of new services, then this implies the corollary: when
      a service goes away, it needs to *disappear* from the service
      discovery list. Otherwise, the result would be unacceptable: the
      service discovery list would simply grow monotonically over time,
      and would require a periodic "refresh" (or complete dismissal and
      recreation) to clear out old stale data.

   4. With users getting to be in the habit of leaving service discovery
      windows open, these windows need to update not only in response
      to services coming and going, but also in response to changes
      in configuration and connectivity of the client machine itself.
      For example, if a user opens a service discovery window when no
      Ethernet cable is connected to the client machine, and the window
      appears empty with no discovered services, then when the user
      connects the cable the window should automatically populate with
      discovered services without requiring any explicit user action.
      If the user disconnects the Ethernet cable, all the services
      discovered via that network interface should automatically
      disappear. If the user switches from one 802.11 wireless base
      station to another, the service discovery window should
      automatically update to remove all the services discovered
      via the old wireless base station, and add all the services
      discovered via the new one.

   If these requirements seem to be setting an arbitrary and
   unreasonably high standard for service discovery, bear in mind that
   while it may have seemed that way to some, back in the 1990s when
   these ideas were first proposed, in the years since then Apple and
   other companies have shipped multiple implementations of DNS-SD/mDNS
   that meet and exceed these requirements. In the years since Apple
   shipped Mac OS X 10.2 Jaguar with the Open Source mDNSResponder
   daemon, this service discovery "live browsing" paradigm has been
   adopted and implemented in a wide range of Apple and third-party
   applications, including printer discovery, Safari discovery of
   devices with embedded web servers (for status and configuration),
   iTunes music sharing, iPhoto photo sharing, the iChat Bonjour buddy
   list, SubEthaEdit multi-user document editing, etc.

   With so many different applications demonstrating that the "live
   browsing" paradigm is clearly achievable, these four requirements
   should not be regarded as idealistic unattainable goals, but
   instead as the bare minimum baseline functionality that any
   credible service discovery protocol needs to achieve.


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17. IPv6 Considerations

   IPv6 has no significant differences, except that the address of the
   SRV record's target host is given by the appropriate IPv6 address
   records instead of the IPv4 "A" record.


18. Security Considerations

   DNSSEC [RFC 2535] should be used where the authenticity of
   information is important. Since DNS-SD is just a naming and usage
   convention for records in the existing DNS system, it has no specific
   additional security requirements over and above those that already
   apply to DNS queries and DNS updates.


19. IANA Considerations

   This protocol builds on DNS SRV records [RFC 2782], and similarly
   requires IANA to assign unique application protocol names.
   Unfortunately, the "IANA Considerations" section of RFC 2782 says
   simply, "The IANA has assigned RR type value 33 to the SRV RR.
   No other IANA services are required by this document."
   Due to this oversight, IANA is currently prevented from carrying
   out the necessary function of assigning these unique identifiers.

   This document proposes the following IANA allocation policy for
   unique application protocol names:

   Allowable names:
     * Must be no more than fourteen characters long
     * Must consist only of:
       - lower-case letters 'a' - 'z'
       - digits '0' - '9'
       - the hyphen character '-'
     * Must begin and end with a lower-case letter or digit.
     * Must not already be assigned to some other protocol in the
       existing IANA "list of assigned application protocol names
       and port numbers" [ports].

   These identifiers are allocated on a First Come First Served basis.
   In the event of abuse (e.g. automated mass registrations, etc.),
   the policy may be changed without notice to Expert Review [RFC 2434].

   The textual nature of service/protocol names means that there are
   almost infinitely many more of them available than the finite set of
   65535 possible port numbers. This means that developers can produce
   experimental implementations using unregistered service names with
   little chance of accidental collision, providing service names are
   chosen with appropriate care. However, this document strongly


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   advocates that on or before the date a product ships, developers
   should properly register their service names.

   Some developers have expressed concern that publicly registering
   their service names (and port numbers today) with IANA before a
   product ships may give away clues about that product to competitors.
   For this reason, IANA should consider allowing service name
   applications to remain secret for some period of time, much as US
   patent applications remain secret for two years after the date of
   filing.

   This proposed IANA allocation policy is not in force until this
   document is published as an RFC. In the meantime, unique application
   protocol names may be registered according to the instructions at
   <http://www.dns-sd.org/ServiceTypes.html>. As of August 2006, there
   are roughly 300 application protocols in currently shipping products
   that have been so registered as using DNS-SD for service discovery.


20. Acknowledgments

   The concepts described in this document have been explored, developed
   and implemented with help from Richard Brown, Erik Guttman, Paul
   Vixie, and Bill Woodcock.

   Special thanks go to Bob Bradley, Josh Graessley, Scott Herscher,
   Roger Pantos and Kiren Sekar for their significant contributions.


21. Deployment History

   The first implementations of DNS-Based Service Discovery and
   Multicast DNS were initially developed during the late 1990s,
   but the event that put them into the media spotlight was Steve Jobs
   demonstrating it live on stage in his keynote presentation opening
   Apple's annual Worldwide Developers Conference in May 2002, and
   announcing Apple's adoption of the technology throughout its hardware
   and software product line. Three months later, in August 2002, Apple
   shipped Mac OS X 10.2 Jaguar, and millions of end-users got their
   first exposure to Zero Configuration Networking with DNS-SD/mDNS
   in applications like Safari, iChat, and printer setup. A month later,
   in September 2002, Apple released the entire source code for the
   mDNS Responder daemon under its Darwin Open Source project, with
   code not just for Mac OS X, but also for a range of other platforms
   including Windows, VxWorks, Linux, Solaris, FreeBSD, etc.

   Many hardware makers were quick to see the benefits of Zero
   Configuration Networking. Printer makers especially were enthusiastic
   early adopters, and within a year every major printer manufacturer
   was shipping DNS-SD/mDNS-enabled network printers. If you've bought
   any network printer at all in the last few years, it was probably one


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   that supports DNS-SD/mDNS, even if you didn't know that at the time.
   For Mac OS X users, telling if you have DNS-SD/mDNS printers on your
   network is easy because they automatically appear in the "Bonjour"
   submenu in the "Print" dialog of every Mac application. Microsoft
   Windows users can get a similar experience by installing Bonjour for
   Windows (takes about 90 seconds, no restart required) and running the
   Bonjour for Windows Printer Setup Wizard [B4W].

   The Open Source community has produced several independent
   implementations of DNS-Based Service Discovery and Multicast DNS,
   some in C like Apple's mDNSResponder daemon, and others in a variety
   of different languages including Java, Python, Perl, and C#/Mono.


22. Copyright Notice

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights. For the purposes of this document,
   the term "BCP 78" refers exclusively to RFC 3978, "IETF Rights
   in Contributions", published March 2005.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


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23. Normative References

   [ports]    IANA list of assigned application protocol names and port
              numbers <http://www.iana.org/assignments/port-numbers>

   [RFC 1033] Lottor, M., "Domain Administrators Operations Guide",
              RFC 1033, November 1987.

   [RFC 1034] Mockapetris, P., "Domain Names - Concepts and
              Facilities", STD 13, RFC 1034, November 1987.

   [RFC 1035] Mockapetris, P., "Domain Names - Implementation and
              Specifications", STD 13, RFC 1035, November 1987.

   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.

   [RFC 2782] Gulbrandsen, A., et al., "A DNS RR for specifying the
              location of services (DNS SRV)", RFC 2782, February 2000.

   [RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 3629, November 2003.

   [UAX15]    "Unicode Normalization Forms"
              http://www.unicode.org/reports/tr15/


24. Informative References

   [B4W]      Bonjour for Windows <http://www.apple.com/bonjour/>

   [mDNS]     Cheshire, S., and M. Krochmal, "Multicast DNS",
              Internet-Draft (work in progress),
              draft-cheshire-dnsext-multicastdns-06.txt, August 2006.

   [NBP]      Cheshire, S., and M. Krochmal,
              "Requirements for a Protocol to Replace AppleTalk NBP",
              Internet-Draft (work in progress),
              draft-cheshire-dnsext-nbp-05.txt, August 2006.

   [RFC 2132] Alexander, S., and Droms, R., "DHCP Options and BOOTP
              Vendor Extensions", RFC 2132, March 1997.

   [RFC 2136] Vixie, P., et al., "Dynamic Updates in the Domain Name
              System (DNS UPDATE)", RFC 2136, April 1997.

   [RFC 2434] Narten, T., and H. Alvestrand, "Guidelines for Writing
              an IANA Considerations Section in RFCs", RFC 2434,
              October 1998.


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   [RFC 2535] Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [RFC 3007] Wellington, B., et al., "Secure Domain Name System (DNS)
              Dynamic Update", RFC 3007, November 2000.

   [RFC 3397] Aboba, B., and Cheshire, S., "Dynamic Host Configuration
              Protocol (DHCP) Domain Search Option", RFC 3397, November
              2002.

   [SOAP]     Nilo Mitra, "SOAP Version 1.2 Part 0: Primer",
              W3C Proposed Recommendation, 24 June 2003
              http://www.w3.org/TR/2003/REC-soap12-part0-20030624

   [ZC]       Williams, A., "Requirements for Automatic Configuration
              of IP Hosts", Internet-Draft (work in progress),
              draft-ietf-zeroconf-reqts-12.txt, September 2002.

   [ZCHP]     Guttman, E., "Zeroconf Host Profile Applicability
              Statement", Internet-Draft (work in progress),
              draft-ietf-zeroconf-host-prof-01.txt, July 2001.


25. Authors' Addresses

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

   Phone: +1 408 974 3207
   EMail: rfc [at] stuartcheshire [dot] org


   Marc Krochmal
   Apple Computer, Inc.
   1 Infinite Loop
   Cupertino
   California 95014
   USA

   Phone: +1 408 974 4368
   EMail: marc [at] apple [dot] com


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