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Versions: (draft-cheshire-dnsext-nias) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 6763

Document: draft-cheshire-dnsext-dns-sd-00.txt            Stuart Cheshire
Category: Standards Track                           Apple Computer, Inc.
Expires 20th June 2003                                20th December 2002

                      DNS-Based Service Discovery

                 <draft-cheshire-dnsext-dns-sd-00.txt>


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF),
   its areas, and its working groups.  Note that other groups may
   also distribute working documents as Internet-Drafts.

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

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   Distribution of this memo is unlimited.

   This document updates the document previously titled
   "Discovering Named Instances of Abstract Services using DNS"
   (draft-cheshire-dnsext-nias-00.txt)


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 a that desired service, using only standard DNS queries. In short,
   this is referred to as DNS-based Service Discovery, or DNS-SD.


Acknowledgements

   This concepts described in this document have been explored and
   developed with help from Erik Guttman, Paul Vixie, Bill Woodcock,
   and others.







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Table of Contents

   1.  Introduction....................................................3
   2.  Conventions and Terminology Used in this Document...............3
   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....................................7
   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..................12
   6.3 DNS-SD TXT Record Size.........................................13
   6.4 Rules for Names in DNS-SD Name/Value Pairs.....................14
   6.5 Rules for Values in DNS-SD Name/Value Pairs....................15
   6.6 Example TXT Record.............................................16
   6.7 Version Tag....................................................16
   7.  Selective Instance Enumeration.................................17
   8.  Flagship Naming................................................17
   9.  Service Type Enumeration.......................................19
   10. Populating the DNS with Information............................19
   11. Relationship to Multicast DNS..................................20
   12. Comparison with Alternative Service Discovery Protocols........20
   13. Real Example...................................................22
   14. IPv6 Considerations............................................23
   15. Security Considerations........................................23
   16. IANA Considerations............................................23
   17. Copyright......................................................23
   18. Normative References...........................................24
   19. Informative References.........................................25
   20. Author's Address...............................................25



















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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.

   This proposal is also compatible with (but not dependent on) the
   proposal for Multicast DNS outlined in "Performing DNS queries via IP
   Multicast" [mDNS-SC].


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].
























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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 the Replacement of AppleTalk Name
   Binding Protocol" [NBP]. That document draws upon examples from a
   decade-and-a-half 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 UTF-8-encoded text [RFC 2279]. 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]. The first label of the service pair is the application
   protocol name, as recorded in the IANA list of assigned application
   protocol names and port numbers [ports]. The second label of the
   service pair is either "_tcp" or "_udp", depending on the transport
   protocol used by the application.

   The <Domain> portion of the Service Instance Name is a conventional
   DNS domain name, consisting of as many labels as appropriate. For
   example, "apple.com.", "cs.stanford.edu.", and "eng.us.ibm.com." are
   all valid domain names for the <Domain> portion of the Service
   Instance Name.



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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).
   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.

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



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     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
     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. Requiring the user to 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 behaviours 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 service -- even if the hardware


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   in question has been recycled from some other part of the
   organization where it was previously in use providing the same type
   of service to a different group of people, or for a different
   purpose. If the user-visible name stays the same, then conceptually
   the service being offered is the same service -- even if the hardware
   in question is new hardware brought in to replace some old equipment
   that was broken, worn out, or out-of-date.


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>

   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 nextmost
   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 they 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



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      name of the service instance, but doesn't have any idea what the
      service does. Clearly 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 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.

















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5. Service Name Resolution

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

   The result of the DNS request 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.
   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.







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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.


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).

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

   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.


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 it's 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.


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   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 SHOULD 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 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 an 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
   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 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
   -- 100 bytes or less.

   In cases where more data is justified (e.g. LPR printing), keeping


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   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 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!

   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.

   When examining a TXT record for a given named attribute, there are
   therefore four broad categories of result 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")

   Unless specified otherwise by a particular DNS-SD profile, a given
   attribute name may appear at most once in a TXT record. If a client
   receives a TXT record containing the same attribute name more than
   once, then the client SHOULD 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


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   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.

   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 boolean interpretation:

   * Absent implies 'false'
   * Present with no value implies 'true'

   For other keys it may be sensible to define other semantics, such as:

   * 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 real
   name/value keys for printing.)

   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.



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   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, Base64 or UU
   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 truly 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 | 0x10 | Zeroconf Is Cool |
   -----------------------------------------------------------------


6.7 Version Tag

   It is recommended that authors defining DNS-SD profiles include an
   attribute of the form "version=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 help clients maintain
   backwards compatibility with older implementations if 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 version number
   higher (or lower) than the version(s) they know how to interpret.







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7. 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 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._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. "_tcp._tcp")
   serve to narrow the list of results, not to create more namespace.

   As for the TXT record name/value pairs, the list of possible
   subtypes, if any, are defined and specified separately for each basic
   service type.


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


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   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", even if 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 an empty SRV record (priority=0, weight=0, port=0, target
   host = null) with that name. If, when it attempts to create this SRV
   record, it finds that one with that name already exists, then it
   knows that this name is already taken by some entity implementing
   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 not
   try to use it.

   By defining a common well-known flagship protocol for the class,
   future devices that may not even know about each other's protocols
   have 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.






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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._mdns._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.

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 requests 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 requests on the local link
   for appropriate PTR and SRV names ending with ".local." [mDNS-SC].

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11. Relationship to Multicast DNS

   DNS-Based Service Discovery is not strictly related to Multicast DNS,
   but the two are highly complementary, particularly in Zeroconf
   environments [ZC].

   Lookups for PTR records of the form "<Service>.local." are defined to
   use multicast, and return a list of named instances of the form
   "<Instance>.<Service>.local."


12. 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 analyser 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. It is almost certain that the
   Zeroconf host profile [ZCHP] will specify 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
   clients to register with the central aggregation server. Virtually
   every company with an IP network already runs 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


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   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.
      DNS doesn't have one right now, but it will soon, to meet Zeroconf
      requirements.

   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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13. Real Example

   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.


13.1 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.dns-sd.org
   _ftp._tcp.dns-sd.org name=Microsoft\032Developer\032Files.dns-sd.org
   _ftp._tcp.dns-sd.org name=Registered\032Users'\032Only.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.


13.2 Question: What FTP servers allow anonymous access?

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

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


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

   nslookup -q=any "Apple\032QuickTime\032Files.dns-sd.org."
   Apple\032QuickTime\032Files.dns-sd.org  text = "path=/quicktime"
   Apple\032QuickTime\032Files.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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14. 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.


15. 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.


16. IANA Considerations

   The IANA will continue having to allocating symbolic service/protocol
   names, just as they do today every time someone requests a TCP or UDP
   port number. In future, as more applications start using DNS SRV
   records, it may make sense for IANA to start allocating symbolic
   service/protocol names without an associated hard-coded port number.

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

   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 their
   competition. 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.


17. Copyright

   Copyright (C) The Internet Society 20th December 2002.
   All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any


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   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.


18. 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 2279] Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 2279, January 1998.

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









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19. Informative References

   [mDNS-SC]  Cheshire, S., "Performing DNS queries via IP Multicast",
              Internet-Draft (work in progress),
              draft-cheshire-dnsext-multicastdns-01.txt, December 2002.

   [NBP]      Cheshire, S., "Requirements for the Replacement of
              AppleTalk Name Binding Protocol", Internet-Draft (work
              in progress), draft-cheshire-dnsext-nbp-01.txt, December
              2002.

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

   [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.

   [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.


20. Author's Address

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

   Phone: +1 408 974 3207
   EMail: rfc@stuartcheshire.org













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