xref: /external/avahi/specs/draft-cheshire-dnsext-multicastdns-05.txt

Document: draft-cheshire-dnsext-multicastdns-05.txt      Stuart Cheshire
Category: Standards Track                           Apple Computer, Inc.
Expires 7th December 2005                                  Marc Krochmal
                                                    Apple Computer, Inc.
                                                           7th June 2005

                             Multicast DNS

               <draft-cheshire-dnsext-multicastdns-05.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 have been or will be disclosed, and any of which he or she
   become aware will be disclosed, in accordance with RFC 3979.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts.

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Abstract

   As networked devices become smaller, more portable, and more
   ubiquitous, the ability to operate with less configured
   infrastructure is increasingly important. In particular, the ability
   to look up DNS resource record data types (including, but not limited
   to, host names) in the absence of a conventional managed DNS server,
   is becoming essential.

   Multicast DNS (mDNS) provides the ability to do DNS-like operations
   on the local link in the absence of any conventional unicast DNS
   server. In addition, mDNS designates a portion of the DNS namespace
   to be free for local use, without the need to pay any annual fee, and
   without the need to set up delegations or otherwise configure a
   conventional DNS server to answer for those names.

   The primary benefits of mDNS names are that (i) they require little
   or no administration or configuration to set them up, (ii) they work
   when no infrastructure is present, and (iii) they work during
   infrastructure failures.




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Table of Contents

   1.   Introduction...................................................3
   2.   Conventions and Terminology Used in this Document..............4
   3.   Multicast DNS Names............................................5
   4.   Source Address Check...........................................8
   5.   Reverse Address Mapping........................................9
   6.   Querying.......................................................9
   7.   Duplicate Suppression.........................................13
   8.   Responding....................................................15
   9.   Probing and Announcing on Startup.............................18
   10.  Conflict Resolution...........................................22
   11.  Resource Record TTL Values and Cache Coherency................23
   12.  Special Characteristics of Multicast DNS Domains..............28
   13.  Multicast DNS for Service Discovery...........................30
   14.  Enabling and Disabling Multicast DNS..........................30
   15.  Considerations for Multiple Interfaces........................30
   16.  Multicast DNS and Power Management............................31
   17.  Multicast DNS Character Set...................................32
   18.  Multicast DNS Message Size....................................34
   19.  Multicast DNS Message Format..................................34
   20.  Choice of UDP Port Number.....................................37
   21.  Summary of Differences Between Multicast DNS and Unicast DNS..38
   22.  Benefits of Multicast Responses...............................38
   23.  IPv6 Considerations...........................................39
   24.  Security Considerations.......................................40
   25.  IANA Considerations...........................................41
   26.  Acknowledgments...............................................42
   27.  Copyright.....................................................42
   28.  Normative References..........................................42
   29.  Informative References........................................43
   30.  Authors' Addresses............................................44























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1. Introduction

   When reading this document, familiarity with the concepts of Zero
   Configuration Networking [ZC] and automatic link-local addressing
   [RFC 2462] [RFC 3927] is helpful.

   This document proposes no change to the structure of DNS messages,
   and no new operation codes, response codes, or resource record types.
   This document simply discusses what needs to happen if DNS clients
   start sending DNS queries to a multicast address, and how a
   collection of hosts can cooperate to collectively answer those
   queries in a useful manner.

   There has been discussion of how much burden Multicast DNS might
   impose on a network. It should be remembered that whenever IPv4 hosts
   communicate, they broadcast ARP packets on the network on a regular
   basis, and this is not disastrous. The approximate amount of
   multicast traffic generated by hosts making conventional use of
   Multicast DNS is anticipated to be roughly the same order of
   magnitude as the amount of broadcast ARP traffic those hosts already
   generate.

   New applications making new use of Multicast DNS capabilities for
   unconventional purposes may generate more traffic. If some of those
   new applications are "chatty", then work will be needed to help them
   become less chatty. When performing any analysis, it is important to
   make a distinction between the application behavior and the
   underlying protocol behavior. If a chatty application uses UDP, that
   doesn't mean that UDP is chatty, or that IP is chatty, or that
   Ethernet is chatty. What it means is that the application is chatty.
   The same applies to any future applications that may decide to layer
   increasing portions of their functionality over Multicast DNS.























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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].

   This document uses the term "host name" in the strict sense to mean a
   fully qualified domain name that has an address record. It does not
   use the term "host name" in the commonly used but incorrect sense to
   mean just the first DNS label of a host's fully qualified domain
   name.

   A DNS (or mDNS) packet contains an IP TTL in the IP header, which
   is effectively a hop-count limit for the packet, to guard against
   routing loops. Each Resource Record also contains a TTL, which is
   the number of seconds for which the Resource Record may be cached.

   In any place where there may be potential confusion between these two
   types of TTL, the term "IP TTL" is used to refer to the IP header TTL
   (hop limit), and the term "RR TTL" is used to refer to the Resource
   Record TTL (cache lifetime).

   When this document uses the term "Multicast DNS", it should be taken
   to mean: "Clients performing DNS-like queries for DNS-like resource
   records by sending DNS-like UDP query and response packets over IP
   Multicast to UDP port 5353."

   This document uses the terms "shared" and "unique" when referring to
   resource record sets.

   A "shared" resource record set is one where several Multicast DNS
   responders may have records with that name, rrtype, and rrclass, and
   several responders may respond to a particular query.

   A "unique" resource record set is one where all the records with that
   name, rrtype, and rrclass are under the control or ownership of a
   single responder, and at most one responder should respond to any
   given query. Before claiming ownership of a unique resource record
   set, a responder MUST probe to verify that no other responder
   already claims ownership of that set, as described in Section 9.1
   "Probing".

   Strictly speaking the terms "shared" and "unique" apply to resource
   record sets, not to individual resource records, but it is sometimes
   convenient to talk of "shared resource records" and "unique resource
   records". When used this way, the terms should be understood to mean
   a record that is a member of a "shared" or "unique" resource record
   set, respectively.






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3. Multicast DNS Names

   This document proposes that the DNS top-level domain ".local." be
   designated a special domain with special semantics, namely that any
   fully-qualified name ending in ".local." is link-local, and names
   within this domain are meaningful only on the link where they
   originate. This is analogous to IPv4 addresses in the 169.254/16
   prefix, which are link-local and meaningful only on the link where
   they originate.

   Any DNS query for a name ending with ".local." MUST be sent
   to the mDNS multicast address (224.0.0.251 or its IPv6 equivalent
   FF02::FB).

   It is unimportant whether a name ending with ".local." occurred
   because the user explicitly typed in a fully qualified domain name
   ending in ".local.", or because the user entered an unqualified
   domain name and the host software appended the suffix ".local."
   because that suffix appears in the user's search list. The ".local."
   suffix could appear in the search list because the user manually
   configured it, or because it was received in a DHCP option, or via
   any other valid mechanism for configuring the DNS search list. In
   this respect the ".local." suffix is treated no differently to any
   other search domain that might appear in the DNS search list.

   DNS queries for names that do not end with ".local." MAY be sent to
   the mDNS multicast address, if no other conventional DNS server is
   available. This can allow hosts on the same link to continue
   communicating using each other's globally unique DNS names during
   network outages which disrupt communication with the greater
   Internet. When resolving global names via local multicast, it is even
   more important to use DNSSEC or other security mechanisms to ensure
   that the response is trustworthy. Resolving global names via local
   multicast is a contentious issue, and this document does not discuss
   it in detail, instead concentrating on the issue of resolving local
   names using DNS packets sent to a multicast address.

   A host which belongs to an organization or individual who has control
   over some portion of the DNS namespace can be assigned a globally
   unique name within that portion of the DNS namespace, for example,
   "cheshire.apple.com." For those of us who have this luxury, this
   works very well. However, the majority of home customers do not have
   easy access to any portion of the global DNS namespace within which
   they have the authority to create names as they wish. This leaves the
   majority of home computers effectively anonymous for practical
   purposes.

   To remedy this problem, this document allows any computer user to
   elect to give their computers link-local Multicast DNS host names of
   the form: "single-dns-label.local." For example, a laptop computer
   may answer to the name "cheshire.local." Any computer user is granted
   the authority to name their computer this way, provided that the
   chosen host name is not already in use on that link. Having named


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   their computer this way, the user has the authority to continue using
   that name until such time as a name conflict occurs on the link which
   is not resolved in the user's favour. If this happens, the computer
   (or its human user) SHOULD cease using the name, and may choose to
   attempt to allocate a new unique name for use on that link. These
   conflicts are expected to be relatively rare for people who choose
   reasonably imaginative names, but it is still important to have a
   mechanism in place to handle them when they happen.

   The point made in the previous paragraph is very important and bears
   repeating. It is easy for those of us in the IETF community who run
   our own name servers at home to forget that the majority of computer
   users do not run their own name server and have no easy way to create
   their own host names. When these users wish to transfer files between
   two laptop computers, they are frequently reduced to typing in
   dotted-decimal IP addresses because they simply have no other way for
   one host to refer to the other by name. This is a sorry state of
   affairs. What is worse, most users don't even bother trying to use
   dotted-decimal IP addresses. Most users still move data between
   machines by copying it onto a floppy disk or similar removable media.

   In a world of gigabit Ethernet and ubiquitous wireless networking it
   is a sad indictment of the networking community that the preferred
   communication medium for most computer users is still the floppy
   disk.

   Allowing ad-hoc allocation of single-label names in a single flat
   ".local." namespace may seem to invite chaos. However, operational
   experience with AppleTalk NBP names [NBP], which on any given link
   are also effectively single-label names in a flat namespace, shows
   that in practice name collisions happen extremely rarely and are not
   a problem. Groups of computer users from disparate organizations
   bring Macintosh laptop computers to events such as IETF Meetings, the
   Mac Hack conference, the Apple World Wide Developer Conference, etc.,
   and complaints at these events about users suffering conflicts and
   being forced to rename their machines have never been an issue.

   Enforcing uniqueness of host names (i.e. the names of DNS address
   records mapping names to IP addresses) is probably desirable in the
   common case, but this document does not mandate that. It is
   permissible for a collection of coordinated hosts to agree to
   maintain multiple DNS address records with the same name, possibly
   for load balancing or fault-tolerance reasons. This document does not
   take a position on whether that is sensible. It is important that
   both modes of operation are supported. The Multicast DNS protocol
   allows hosts to verify and maintain unique names for resource records
   where that behavior is desired, and it also allows hosts to maintain
   multiple resource records with a single shared name where that
   behavior is desired. This consideration applies to all resource
   records, not just address records (host names). In summary: It is
   required that the protocol have the ability to detect and handle name
   conflicts, but it is not required that this ability be used for every
   record.


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3.1 Governing Standards Body

   Note that this use of the ".local." suffix falls under IETF
   jurisdiction, not ICANN jurisdiction. DNS is an IETF network
   protocol, governed by protocol rules defined by the IETF. These IETF
   protocol rules dictate character set, maximum name length, packet
   format, etc. ICANN determines additional rules that apply when the
   IETF's DNS protocol is used on the public Internet. In contrast,
   private uses of the DNS protocol on isolated private networks are not
   governed by ICANN. Since this proposed change is a change to the core
   DNS protocol rules, it affects everyone, not just those machines
   using the ICANN-governed Internet. Hence this change falls into the
   category of an IETF protocol rule, not an ICANN usage rule.

3.2 Private DNS Namespaces

   Note also that the special treatment of names ending in ".local." has
   been implemented in Macintosh computers since the days of Mac OS 9,
   and continues today in Mac OS X. There are also implementations for
   Linux and other platforms [dotlocal]. Operators setting up private
   internal networks ("intranets") are advised that their lives may be
   easier if they avoid using the suffix ".local." in names in their
   private internal DNS server. Alternative possibilities include:

      .intranet
      .internal
      .private
      .corp
      .home

   Another alternative naming scheme, advocated by Professor D. J.
   Bernstein, is to use a numerical suffix, such as ".6." [djbdl].

3.3 Maximum Multicast DNS Name Length

   RFC 1034 says:

     "the total number of octets that represent a domain name (i.e.,
     the sum of all label octets and label lengths) is limited to 255."

   This text implies that the final root label at the end of every name
   is included in this count (a name can't be represented without it),
   but the text does not explicitly state that. Implementations of
   Multicast DNS MUST include the label length byte of the final root
   label at the end of every name when enforcing the rule that no name
   may be longer than 255 bytes. For example, the length of the name
   "apple.com." is considered to be 11, which is the number of bytes it
   takes to represent that name in a packet without using name
   compression:

     ------------------------------------------------------
     | 0x05 | a | p | p | l | e | 0x03 | c | o | m | 0x00 |
     ------------------------------------------------------


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4. Source Address Check

   All Multicast DNS responses (including responses sent via unicast)
   SHOULD be sent with IP TTL set to 255. This is recommended to provide
   backwards-compatibility with older Multicast DNS clients that check
   the IP TTL on reception to determine whether the packet originated
   on the local link. These older clients discard all packets with TTLs
   other than 255.

   A host sending Multicast DNS queries to a link-local destination
   address (including the 224.0.0.251 link-local multicast address)
   MUST only accept responses to that query that originate from the
   local link, and silently discard any other response packets. Without
   this check, it could be possible for remote rogue hosts to send
   spoof answer packets (perhaps unicast to the victim host) which the
   receiving machine could misinterpret as having originated on the
   local link.

   The test for whether a response originated on the local link
   is done in two ways:

   * All responses sent to the link-local multicast address 224.0.0.251
     are necessarily deemed to have originated on the local link,
     regardless of source IP address. This is essential to allow devices
     to work correctly and reliably in unusual configurations, such as
     multiple logical IP subnets overlayed on a single link, or in cases
     of severe misconfiguration, where devices are physically connected
     to the same link, but are currently misconfigured with completely
     unrelated IP addresses and subnet masks.

   * For responses sent to a unicast destination address, the source IP
     address in the packet is checked to see if it is an address on a
     local subnet. An address is determined to be on a local subnet if,
     for (one of) the address(es) configured on the interface receiving
     the packet, (I & M) == (P & M), where I and M are the interface
     address and subnet mask respectively, P is the source IP address
     from the packet, '&' represents the bitwise logical 'and'
     operation, and '==' represents a bitwise equality test.

   Since queriers will ignore responses apparently originating outside
   the local subnet, responders SHOULD avoid generating responses that
   it can reasonably predict will be ignored. This applies particularly
   in the case of overlayed subnets. If a responder receives a query
   addressed to the link-local multicast address 224.0.0.251, from a
   source address not apparently on the same subnet as the responder,
   then even if the query indicates that a unicast response is preferred
   (see Section 6.5, "Questions Requesting Unicast Responses"), the
   responder SHOULD elect to respond by multicast anyway, since it can
   reasonably predict that a unicast response with an apparently
   non-local source address will probably be ignored.





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5. Reverse Address Mapping

   Like ".local.", the IPv4 and IPv6 reverse-mapping domains are also
   defined to be link-local.

   Any DNS query for a name ending with "254.169.in-addr.arpa." MUST
   be sent to the mDNS multicast address 224.0.0.251. Since names under
   this domain correspond to IPv4 link-local addresses, it is logical
   that the local link is the best place to find information pertaining
   to those names. As an optimization, these queries MAY be first
   unicast directly to the address in question, but if this query is not
   answered, the query MUST also be sent via multicast, to accommodate
   the case where the machine in question is not answering for itself
   (for example, because it is currently sleeping).

   Likewise, any DNS query for a name ending with "0.8.e.f.ip6.arpa."
   MUST be sent to the IPv6 mDNS link-local multicast address FF02::FB,
   with or without an optional initial query unicast directly to the
   address in question.


6. Querying

   There are three kinds of Multicast DNS Queries, one-shot queries of
   the kind made by today's conventional DNS clients, one-shot queries
   accumulating multiple responses made by multicast-aware DNS clients,
   and continuous ongoing Multicast DNS Queries used by IP network
   browser software.

   A Multicast DNS Responder that is offering records that are intended
   to be unique on the local link MUST also implement a Multicast DNS
   Querier so that it can first verify the uniqueness of those records
   before it begins answering queries for them.


6.1 One-Shot Queries

   An unsophisticated DNS client may simply send its DNS queries
   blindly to the 224.0.0.251 multicast address, without necessarily
   even being aware what a multicast address is.

   Such an unsophisticated DNS client may not get ideal behavior. Such
   a client may simply take the first response it receives and fail to
   wait to see if there are more, but in many instances this may not be
   a serious problem. If a user types "http://cheshire.local." into
   their Web browser and gets to see the page they were hoping for,
   then the protocol has met the user's needs in this case.








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6.2 One-Shot Queries, Accumulating Multiple Responses

   A more sophisticated DNS client should understand that Multicast DNS
   is not exactly the same as unicast DNS, and should modify its
   behavior in some simple ways.

   As described above, there are some cases, such as looking up the
   address associated with a unique host name, where a single response
   is sufficient, and moreover may be all that is expected. However,
   there are other DNS queries where more than one response is
   possible, and for these queries a more sophisticated Multicast DNS
   client should include the ability to wait for an appropriate period
   of time to collect multiple responses.

   A naive DNS client retransmits its query only so long as it has
   received no response. A more sophisticated Multicast DNS client is
   aware that having received one response is not necessarily an
   indication that it might not receive others, and has the ability to
   retransmit its query an appropriate number of times at appropriate
   intervals until it is satisfied with the collection of responses it
   has gathered.

   A more sophisticated Multicast DNS client that is retransmitting
   a query for which it has already received some responses, MUST
   implement Known Answer Suppression, as described below in Section
   7.1. This indicates to responders who have already replied that their
   responses have been received, and they don't need to send them again
   in response to this repeated query. In addition, the interval between
   the first two queries SHOULD be one second, and the intervals between
   subsequent queries SHOULD double.


6.3 Continuous Querying

   In One-Shot Queries, with either a single or multiple responses,
   the underlying assumption is that the transaction begins when the
   application issues a query, and ends when all the desired responses
   have been received. There is another type of operation which is more
   akin to continuous monitoring.

   Macintosh users are accustomed to opening the "Chooser" window,
   selecting a desired printer, and then closing the Chooser window.
   However, when the desired printer does not appear in the list, the
   user will typically leave the "Chooser" window open while they go and
   check to verify that the printer is plugged in, powered on, connected
   to the Ethernet, etc. While the user jiggles the wires, hits the
   Ethernet hub, and so forth, they keep an eye on the Chooser window,
   and when the printer name appears, they know they have fixed whatever
   the problem was. This can be a useful and intuitive troubleshooting
   technique, but a user who goes home for the weekend leaving the
   Chooser window open places a non-trivial burden on the network.




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   With continuous querying, multiple queries are sent over a long
   period of time, until the user terminates the operation. It is
   important that an IP network browser window displaying live
   information from the network using Multicast DNS, if left running
   for an extended period of time, should generate significantly less
   multicast traffic on the network than the old AppleTalk Chooser.
   Therefore, the interval between the first two queries SHOULD be one
   second, the intervals between subsequent queries SHOULD double, and
   the querier MUST implement Known Answer Suppression, as described
   below in Section 7.1. When the interval between queries reaches or
   exceeds 60 minutes, a querier MAY cap the interval to a maximum of 60
   minutes, and perform subsequent queries at a steady-state rate of one
   query per hour.

   When a Multicast DNS Querier receives an answer, the answer contains
   a TTL value that indicates for how many seconds this answer is valid.
   After this interval has passed, the answer will no longer be valid
   and SHOULD be deleted from the cache. Before this time is reached, a
   Multicast DNS Querier with an ongoing interest in that record SHOULD
   re-issue its query to determine whether the record is still valid,
   and if so update its expiry time.

   To perform this cache maintenance, a Multicast DNS Querier should
   plan to re-query for records after at least 50% of the record
   lifetime has elapsed. This document recommends the following
   specific strategy:

   The Querier should plan to issue a query at 80% of the record
   lifetime, and then if no answer is received, at 85%, 90% and 95%. If
   an answer is received, then the remaining TTL is reset to the value
   given in the answer, and this process repeats for as long as the
   Multicast DNS Querier has an ongoing interest in the record. If after
   four queries no answer is received, the record is deleted when it
   reaches 100% of its lifetime.

   To avoid the case where multiple Multicast DNS Queriers on a network
   all issue their queries simultaneously, a random variation of 2% of
   the record TTL should be added, so that queries are scheduled to be
   performed at 80-82%, 85-87%, 90-92% and then 95-97% of the TTL.


6.4 Multiple Questions per Query

   Multicast DNS allows a querier to place multiple questions in the
   Question Section of a single Multicast DNS query packet.

   The semantics of a Multicast DNS query packet containing multiple
   questions is identical to a series of individual DNS query packets
   containing one question each. Combining multiple questions into a
   single packet is purely an efficiency optimization, and has no other
   semantic significance.




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   A useful technique for adaptively combining multiple questions into a
   single query is to use a Nagle-style algorithm: When a client issues
   its first question, a Query packet is immediately built and sent,
   without delay. If the client then continues issuing a rapid series of
   questions they are held until either the first query receives at
   least one answer, or 100ms has passed, or there are enough questions
   to fill the Question Section of a Multicast DNS query packet. At this
   time, all the held questions are placed into a Multicast DNS query
   packet and sent.

6.5 Questions Requesting Unicast Responses

   Sending Multicast DNS responses via multicast has the benefit that
   all the other hosts on the network get to see those responses, and
   can keep their caches up to date, and detect conflicting responses.

   However, there are situations where all the other hosts on the
   network don't need to see every response. One example is a laptop
   computer waking from sleep. At that instant it is a brand new
   participant on a new network. Its Multicast DNS cache is empty, and
   it has no knowledge of its surroundings. It may have a significant
   number of queries that it wants answered right away to discover
   information about its new surroundings and present that information
   to the user. As a new participant on the network, it has no idea
   whether the exact same questions may have been asked and answered
   just seconds ago. In this case, trigging a large sudden flood of
   multicast responses may impose an unreasonable burden on the network.
   To avoid this, the Multicast DNS Querier SHOULD set the top bit in
   the class field of its DNS question(s), to indicate that it is
   willing to accept unicast responses instead of the usual multicast
   responses. These questions requesting unicast responses are referred
   to as "QU" questions, to distinguish them from the more usual
   questions requesting multicast responses ("QM" questions).

   When retransmitting a question more than once, the 'unicast response'
   bit SHOULD be set only for the first question of the series. After
   the first question has received its responses, the querier should
   have a large known-answer list (see "Known Answer Suppression" below)
   so that subsequent queries should elicit few, if any, further
   responses. Reverting to multicast responses as soon as possible is
   important because of the benefits that multicast responses provide
   (see "Benefits of Multicast Responses" below).

   When receiving a question with the 'unicast response' bit set, a
   responder SHOULD usually respond with a unicast packet directed back
   to the querier. If the responder has not multicast that record
   recently (within one quarter of its TTL), then the responder SHOULD
   instead multicast the response so as to keep all the peer caches up
   to date, and to permit passive conflict detection.

   Unicast replies are subject to all the same packet generation rules
   as multicast replies, including the cache flush bit (see Section
   11.3, "Announcements to Flush Outdated Cache Entries") and randomized
   delays to reduce network collisions (see Section 8, "Responding").

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6.6 Suppressing Initial Query

   If a query is issued for which there already exist one or more
   records in the local cache, and those record(s) were received with
   the cache flush bit set (see Section 11.3, "Announcements to Flush
   Outdated Cache Entries"), indicating that they form a unique RRSet,
   then the host SHOULD suppress its initial "QU" query, and proceed to
   issue a "QM" query. To avoid the situation where a group of hosts
   are synchronized by some external event and all perform the same
   query simultaneously, a host suppressing its initial "QU" query
   SHOULD impose a random delay from 500-1000ms before transmitting its
   first "QM" query for this question. This means that when the first
   host (selected randomly by this algorithm) transmits its "QM" query,
   all the other hosts that were about to transmit the same query can
   suppress their superfluous query, as described in "Duplicate
   Question Suppression" below.

7. Duplicate Suppression

   A variety of techniques are used to reduce the amount of redundant
   traffic on the network.

7.1 Known Answer Suppression

   When a Multicast DNS Querier sends a query to which it already knows
   some answers, it populates the Answer Section of the DNS message with
   those answers.

   A Multicast DNS Responder SHOULD NOT answer a Multicast DNS Query if
   the answer it would give is already included in the Answer Section
   with an RR TTL at least half the correct value. If the RR TTL of the
   answer as given in the Answer Section is less than half of the true
   RR TTL as known by the Multicast DNS Responder, the responder MUST
   send an answer so as to update the Querier's cache before the record
   becomes in danger of expiration.

   Because a Multicast DNS Responder will respond if the remaining TTL
   given in the known answer list is less than half the true TTL, it is
   superfluous for the Querier to include such records in the known
   answer list. Therefore a Multicast DNS Querier SHOULD NOT include
   records in the known answer list whose remaining TTL is less than
   half their original TTL. Doing so would simply consume space in the
   packet without achieving the goal of suppressing responses, and would
   therefore be a pointless waste of network bandwidth.

   A Multicast DNS Querier MUST NOT cache resource records observed in
   the Known Answer Section of other Multicast DNS Queries. The Answer
   Section of Multicast DNS Queries is not authoritative. By placing
   information in the Answer Section of a Multicast DNS Query the
   querier is stating that it *believes* the information to be true.
   It is not asserting that the information *is* true. Some of those
   records may have come from other hosts that are no longer on the
   network. Propagating that stale information to other Multicast DNS
   Queriers on the network would not be helpful.

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7.2 Multi-Packet Known Answer Suppression

   Sometimes a Multicast DNS Querier will already have too many answers
   to fit in the Known Answer Section of its query packets. In this
   case, it should issue a Multicast DNS Query containing a question and
   as many Known Answer records as will fit. It MUST then set the TC
   (Truncated) bit in the header before sending the Query. It MUST then
   immediately follow the packet with another query packet containing no
   questions, and as many more Known Answer records as will fit. If
   there are still too many records remaining to fit in the packet, it
   again sets the TC bit and continues until all the Known Answer
   records have been sent.

   A Multicast DNS Responder seeing a Multicast DNS Query with the TC
   bit set defers its response for a time period randomly selected in
   the interval 400-500ms. This gives the Multicast DNS Querier time to
   send additional Known Answer packets before the Responder responds.
   If the Responder sees any of its answers listed in the Known Answer
   lists of subsequent packets from the querying host, it SHOULD delete
   that answer from the list of answers it is planning to give, provided
   that no other host on the network is also waiting to receive the same
   answer record.

   Previous versions of this draft specified a delay of 20-120ms before
   answering queries with multi-packet Known Answer lists. However,
   operational experience showed that, while this works well on
   Ethernet, on very busy 802.11 networks, it is not uncommon to observe
   consecutively sent packets arriving separated by as much as
   200-400ms.


7.3 Duplicate Question Suppression

   If a host is planning to send a query, and it sees another host on
   the network send a query containing the same question, and the Known
   Answer Section of that query does not contain any records which this
   host would not also put in its own Known Answer Section, then this
   host should treat its own query as having been sent. When multiple
   clients on the network are querying for the same resource records,
   there is no need for them to all be repeatedly asking the same
   question.


7.4 Duplicate Answer Suppression

   If a host is planning to send an answer, and it sees another host on
   the network send a response packet containing the same answer record,
   and the TTL in that record is not less than the TTL this host would
   have given, then this host should treat its own answer as having been
   sent. When multiple responders on the network have the same data,
   there is no need for all of them to respond.




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   This feature is particularly useful when multiple Sleep Proxy Servers
   are deployed (see Section 16, "Multicast DNS and Power Management").
   In the future it is possible that every general-purpose OS (Mac,
   Windows, Linux, etc.) will implement Sleep Proxy Service as a matter
   of course. In this case there could be a large number of Sleep Proxy
   Servers on any given network, which is good for reliability and
   fault-tolerance, but would be bad for the network if every Sleep
   Proxy Server were to answer every query.


8. Responding

   When a Multicast DNS Responder constructs and sends a Multicast DNS
   response packet, the Answer Section of that packet must contain only
   records for which that Responder is explicitly authoritative. These
   answers may be generated because the record answers a question
   received in a Multicast DNS query packet, or at certain other times
   that the responder determines than an unsolicited announcement is
   warranted. A Multicast DNS Responder MUST NOT place records from its
   cache, which have been learned from other responders on the network,
   in the Answer Section of outgoing response packets. Only an
   authoritative source for a given record is allowed to issue responses
   containing that record.

   The determination of whether a given record answers a given question
   is done using the standard DNS rules: The record name must match the
   question name, the record rrtype must match the question qtype
   (unless the qtype is "ANY"), and the record rrclass must match the
   question qclass (unless the qclass is "ANY").

   A Multicast DNS Responder MUST only respond when it has a positive
   non-null response to send. Error responses must never be sent. The
   non-existence of any name in a Multicast DNS Domain is ascertained by
   the failure of any machine to respond to the Multicast DNS query, not
   by NXDOMAIN errors.

   Multicast DNS Responses MUST NOT contain any questions in the
   Question Section. Any questions in the Question Section of a received
   Multicast DNS Response MUST be silently ignored. Multicast DNS
   Queriers receiving Multicast DNS Responses do not care what question
   elicited the response; they care only that the information in the
   response is true and accurate.

   A Multicast DNS Responder on Ethernet [IEEE802] and similar shared
   multiple access networks SHOULD have the capability of delaying its
   responses by up to 500ms, as determined by the rules described below.
   If multiple Multicast DNS Responders were all to respond immediately
   to a particular query, a collision would be virtually guaranteed. By
   imposing a small random delay, the number of collisions is
   dramatically reduced. On a full-sized Ethernet using the maximum
   cable lengths allowed and the maximum number of repeaters allowed, an
   Ethernet frame is vulnerable to collisions during the transmission of
   its first 256 bits. On 10Mb/s Ethernet, this equates to a vulnerable


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   time window of 25.6us. On higher-speed variants of Ethernet, the
   vulnerable time window is shorter.

   In the case where a Multicast DNS Responder has good reason to
   believe that it will be the only responder on the link with a
   positive non-null response, it SHOULD NOT impose any random delay
   before responding, and SHOULD normally generate its response within
   at most 10ms. In particular, this applies to responding to probe
   queries. Since receiving a probe query gives a clear indication that
   some other Responder is planning to start using this name in the very
   near future, answering such probe queries to defend a unique record
   is a high priority and needs to be done immediately, without delay. A
   probe query can be distinguished from a normal query by the fact that
   a probe query contains a proposed record in the Authority Section
   which answers the question in the Question Section (for more details,
   see Section 9.1, "Probing").

   To generate immediate responses safely, it MUST have previously
   verified that the requested name, rrtype and rrclass in the DNS query
   are unique on this link. Responding immediately without delay is
   appropriate for things like looking up the address record for a
   particular host name, when the host name has been previously verified
   unique. Responding immediately without delay is *not* appropriate for
   things like looking up PTR records used for DNS Service Discovery
   [DNS-SD], where a large number of responses may be anticipated.

   In any case where there may be multiple responses, such as queries
   where the answer is a member of a shared resource record set, each
   responder SHOULD delay its response by a random amount of time
   selected with uniform random distribution in the range 20-120ms.

   In the case where the query has the TC (truncated) bit set,
   indicating that subsequent known answer packets will follow,
   responders SHOULD delay their responses by a random amount of time
   selected with uniform random distribution in the range 400-500ms,
   to allow enough time for all the known answer packets to arrive.

   Except when a unicast reply has been explicitly requested via the
   "unicast reply" bit, Multicast DNS Responses MUST be sent to UDP port
   5353 (the well-known port assigned to mDNS) on the 224.0.0.251
   multicast address (or its IPv6 equivalent FF02::FB). Operating in a
   Zeroconf environment requires constant vigilance. Just because a name
   has been previously verified unique does not mean it will continue to
   be so indefinitely. By allowing all Multicast DNS Responders to
   constantly monitor their peers' responses, conflicts arising out of
   network topology changes can be promptly detected and resolved.

   Sending all responses by multicast also facilitates opportunistic
   caching by other hosts on the network.

   To protect the network against excessive packet flooding due to
   software bugs or malicious attack, a Multicast DNS Responder MUST NOT
   multicast a given record on a given interface if it has previously


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   multicast that record on that interface within the last second. A
   legitimate client on the network should have seen the previous
   transmission and cached it. A client that did not receive and cache
   the previous transmission will retry its request and receive a
   subsequent response. Under no circumstances is there any legitimate
   reason for a Multicast DNS Responder to multicast a given record more
   than once per second on any given interface.


8.1 Legacy Unicast Responses

   If the source UDP port in a received Multicast DNS Query is not port
   5353, this indicates that the client originating the query is a
   simple client that does not fully implement all of Multicast DNS. In
   this case, the Multicast DNS Responder MUST send a UDP response
   directly back to the client, via unicast, to the query packet's
   source IP address and port. This unicast response MUST be a
   conventional unicast response as would be generated by a conventional
   unicast DNS server; for example, it MUST repeat the query ID and the
   question given in the query packet.

   The resource record TTL given in a legacy unicast response SHOULD NOT
   be greater than ten seconds, even if the true TTL of the Multicast
   DNS resource record is higher. This is because Multicast DNS
   Responders that fully participate in the protocol use the cache
   coherency mechanisms described in Section 13 to update and invalidate
   stale data. Were unicast responses sent to legacy clients to use the
   same high TTLs, these legacy clients, which do not implement these
   cache coherency mechanisms, could retain stale cached resource record
   data long after it is no longer valid.

   Having sent this unicast response, if the Responder has not sent this
   record in any multicast response recently, it SHOULD schedule the
   record to be sent via multicast as well, to facilitate passive
   conflict detection. "Recently" in this context means "if the time
   since the record was last sent via multicast is less than one quarter
   of the record's TTL".


8.2 Multi-Question Queries

   Multicast DNS Responders MUST correctly handle DNS query packets
   containing more than one question, by answering any or all of the
   questions to which they have answers. Any (non-defensive) answers
   generated in response to query packets containing more than one
   question SHOULD be randomly delayed in the range 20-120ms, or
   400-500ms if the TC (truncated) bit is set, as described above.
   (Answers defending a name, in response to a probe for that name,
   are not subject to this delay rule and are still sent immediately.)






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8.3 Response Aggregation

   When possible, a responder SHOULD, for the sake of network
   efficiency, aggregate as many responses as possible into a single
   Multicast DNS response packet. For example, when a responder has
   several responses it plans to send, each delayed by a different
   interval, then earlier responses SHOULD be delayed by up to an
   additional 500ms if that will permit them to be aggregated with
   other responses scheduled to go out a little later.


9. Probing and Announcing on Startup

   Typically a Multicast DNS Responder should have, at the very least,
   address records for all of its active interfaces. Creating and
   advertising an HINFO record on each interface as well can be useful
   to network administrators.

   Whenever a Multicast DNS Responder starts up, wakes up from sleep,
   receives an indication of an Ethernet "Link Change" event, or has any
   other reason to believe that its network connectivity may have
   changed in some relevant way, it MUST perform the two startup steps
   below.


9.1 Probing

   The first startup step is that for all those resource records that a
   Multicast DNS Responder desires to be unique on the local link, it
   MUST send a Multicast DNS Query asking for those resource records, to
   see if any of them are already in use. The primary example of this is
   its address record which maps its unique host name to its unique IP
   address. All Probe Queries SHOULD be done using the desired resource
   record name and query type T_ANY (255), to elicit answers for all
   types of records with that name. This allows a single question to be
   used in place of several questions, which is more efficient on the
   network. It also allows a host to verify exclusive ownership of a
   name, which is desirable in most cases. It would be confusing, for
   example, if one host owned the "A" record for "myhost.local.", but a
   different host owned the HINFO record for that name.

   The ability to place more than one question in a Multicast DNS Query
   is useful here, because it can allow a host to use a single packet
   for all of its resource records instead of needing a separate packet
   for each. For example, a host can simultaneously probe for uniqueness
   of its "A" record and all its SRV records [DNS-SD] in the same query
   packet.

   When ready to send its mDNS probe packet(s) the host should first
   wait for a short random delay time, uniformly distributed in the
   range 0-250ms. This random delay is to guard against the case where a
   group of devices are powered on simultaneously, or a group of devices
   are connected to an Ethernet hub which is then powered on, or some


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   other external event happens that might cause a group of hosts to all
   send synchronized probes.

   250ms after the first query the host should send a second, then
   250ms after that a third. If, by 250ms after the third probe, no
   conflicting Multicast DNS responses have been received, the host may
   move to the next step, announcing. (Note that this is the one
   exception from the normal rule that there should be at least one
   second between repetitions of the same question, and the interval
   between subsequent repetitions should double.)

   If any conflicting Multicast DNS responses are received, then the
   probing host MUST defer to the existing host, and MUST choose new
   names for some or all of its resource records as appropriate, to
   avoid conflict with pre-existing hosts on the network. In the case
   of a host probing using query type T_ANY as recommended above, any
   answer containing a record with that name, of any type, MUST be
   considered a conflicting response and handled accordingly.

   If fifteen failures occur within any ten-second period, then the host
   MUST wait at least five seconds before each successive additional
   probe attempt. This is to help ensure that in the event of software
   bugs or other unanticipated problems, errant hosts do not flood the
   network with a continuous stream of multicast traffic. For very
   simple devices, a valid way to comply with this requirement is to
   always wait five seconds after any failed probe attempt.

   If a responder knows by other means, with absolute certainty, that
   its unique resource record set name, rrtype and rrclass cannot
   already be in use by any other responder on the network, then it MAY
   skip the probing step for that resource record set. For example, when
   creating the reverse address mapping PTR records, the host can
   reasonably assume that no other host will be trying to create those
   same PTR records, since that would imply that the two hosts were
   trying to use the same IP address, and if that were the case, the two
   hosts would be suffering communication problems beyond the scope of
   what Multicast DNS is designed to solve.


9.2 Simultaneous Probe Tie-Breaking

   The astute reader will observe that there is a race condition
   inherent in the previous description. If two hosts are probing for
   the same name simultaneously, neither will receive any response to
   the probe, and the hosts could incorrectly conclude that they may
   both proceed to use the name. To break this symmetry, each host
   populates the Authority Section of its queries with records giving
   the rdata that it would be proposing to use, should its probing be
   successful. The Authority Section is being used here in a way
   analogous to the Update Section of a DNS Update packet [RFC 2136].

   When a host that is probing for a record sees another host issue a
   query for the same record, it consults the Authority Section of that


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   query. If it finds any resource record there which answers the query,
   then it compares the data of that resource record with its own
   tentative data. The lexicographically later data wins. This means
   that if the host finds that its own data is lexicographically later,
   it simply ignores the other host's probe. If the host finds that its
   own data is lexicographically earlier, then it treats this exactly
   as if it had received a positive answer to its query, and concludes
   that it may not use the desired name.

   The determination of 'lexicographically later' is performed by first
   comparing the record class, then the record type, then raw comparison
   of the binary content of the rdata without regard for meaning or
   structure. If the record classes differ, then the numerically greater
   class is considered 'lexicographically later'. Otherwise, if the
   record types differ, then the numerically greater type is considered
   'lexicographically later'. If the rrtype and rrclass both match then
   the rdata is compared.

   In the case of resource records containing rdata that is subject to
   name compression, the names MUST be uncompressed before comparison.
   (The details of how a particular name is compressed is an artifact of
   how and where the record is written into the DNS message; it is not
   an intrinsic property of the resource record itself.)

   The bytes of the raw uncompressed rdata are compared in turn,
   interpreting the bytes as eight-bit UNSIGNED values, until a byte
   is found whose value is greater than that of its counterpart (in
   which case the rdata whose byte has the greater value is deemed
   lexicographically later) or one of the resource records runs out
   of rdata (in which case the resource record which still has
   remaining data first is deemed lexicographically later).

   The following is an example of a conflict:

   cheshire.local. A 169.254.99.200
   cheshire.local. A 169.254.200.50

   In this case 169.254.200.50 is lexicographically later (the third
   byte, with value 200, is greater than its counterpart with value 99),
   so it is deemed the winner.

   Note that it is vital that the bytes are interpreted as UNSIGNED
   values, or the wrong outcome may result. In the example above, if
   the byte with value 200 had been incorrectly interpreted as a
   signed value then it would be interpreted as value -56, and the
   wrong address record would be deemed the winner.


9.3 Announcing

   The second startup step is that the Multicast DNS Responder MUST send
   a gratuitous Multicast DNS Response containing, in the Answer
   Section, all of its resource records (both shared records, and unique


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   records that have completed the probing step). If there are too many
   resource records to fit in a single packet, multiple packets should
   be used.

   In the case of shared records (e.g. the PTR records used by DNS
   Service Discovery [DNS-SD]), the records are simply placed as-is
   into the Answer Section of the DNS Response.

   In the case of records that have been verified to be unique in the
   previous step, they are placed into the Answer Section of the DNS
   Response with the most significant bit of the rrclass set to one.
   The most significant bit of the rrclass for a record in the Answer
   Section of a response packet is the mDNS "cache flush" bit and is
   discussed in more detail below in Section 11.3 "Announcements to
   Flush Outdated Cache Entries".

   The Multicast DNS Responder MUST send at least two gratuitous
   responses, one second apart. A Responder MAY send up to ten
   gratuitous Responses, provided that the interval between gratuitous
   responses doubles with every response sent.

   A Multicast DNS Responder SHOULD NOT continue sending gratuitous
   Responses for longer than the TTL of the record. The purpose of
   announcing new records via gratuitous Responses is to ensure that
   peer caches are up to date. After a time interval equal to the TTL of
   the record has passed, it is very likely that old stale copies of
   that record in peer caches will have expired naturally, so subsequent
   announcements serve little purpose.

   A Multicast DNS Responder MUST NOT send announcements in the absence
   of information that its network connectivity may have changed in some
   relevant way. In particular, a Multicast DNS Responder MUST NOT send
   regular periodic announcements as a matter of course.

   Whenever a Multicast DNS Responder receives any Multicast DNS
   response (gratuitous or otherwise) containing a conflicting resource
   record, the conflict MUST be resolved as described below in "Conflict
   Resolution".

9.4 Updating

   At any time, if the rdata of any of a host's Multicast DNS records
   changes, the host MUST repeat the Announcing step described above to
   update neighboring caches. For example, if any of a host's IP
   addresses change, it MUST re-announce those address records.

   In the case of shared records, a host MUST send a 'goodbye'
   announcement with TTL zero (see Section 11.2 "Goodbye Packets")
   for the old rdata, to cause it to be deleted from peer caches,
   before announcing the new rdata. In the case of unique records,
   a host SHOULD omit the 'goodbye' announcement, since the cache
   flush bit on the newly announced records will cause old rdata
   to be flushed from peer caches anyway.


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   A host may update the contents of any of its records at any time,
   though a host SHOULD NOT update records more frequently than ten
   times per minute. Frequent rapid updates impose a burden on the
   network. If a host has information to disseminate which changes more
   frequently than ten times per minute, then it may be more appropriate
   to design a protocol for that specific purpose.


10. Conflict Resolution

   A conflict occurs when a Multicast DNS Responder has a unique record
   for which it is authoritative, and it receives, in the Answer Section
   of a Multicast DNS response another record with the same name, rrtype
   and rrclass, but inconsistent rdata. What may be considered
   inconsistent is context sensitive, except that resource records with
   identical rdata are never considered inconsistent, even if they
   originate from different hosts. This is to permit use of proxies and
   other fault-tolerance mechanisms that may cause more than one
   responder to be capable of issuing identical answers on the network.

   A common example of a resource record type that is intended to be
   unique, not shared between hosts, is the address record that maps a
   host's name to its IP address. Should a host witness another host
   announce an address record with the same name but a different IP
   address, then that is considered inconsistent, and that address
   record is considered to be in conflict.

   Whenever a Multicast DNS Responder receives any Multicast DNS
   response (gratuitous or otherwise) containing a conflicting resource
   record in the Answer Section, the Multicast DNS Responder MUST
   immediately reset its conflicted unique record to probing state, and
   go through the startup steps described above in Section 9. "Probing
   and Announcing on Startup". The protocol used in the Probing phase
   will determine a winner and a loser, and the loser MUST cease using
   the name, and reconfigure.

   It is very important that any host receiving a resource record that
   conflicts with one of its own MUST take action as described above.
   In the case of two hosts using the same host name, where one has been
   configured to require a unique host name and the other has not, the
   one that has not been configured to require a unique host name will
   not perceive any conflict, and will not take any action. By reverting
   to Probing state, the host that desires a unique host name will go
   through the necessary steps to ensure that a unique host is obtained.

   The recommended course of action after probing and failing is as
   follows:

   o Programmatically change the resource record name in an attempt to
     find a new name that is unique. This could be done by adding some
     further identifying information (e.g. the model name of the
     hardware) if it is not already present in the name, appending the
     digit "2" to the name, or incrementing a number at the end of the
     name if one is already present.

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   o Probe again, and repeat until a unique name is found.

   o Record this newly chosen name in persistent storage so that the
     device will use the same name the next time it is power-cycled.

   o Display a message to the user or operator informing them of the
     name change. For example:

        The name "Bob's Music" is in use by another iTunes music
        server on the network. Your music has been renamed to
        "Bob's Music (G4 Cube)". If you want to change this name,
        use [describe appropriate menu item or preference dialog].

   How the user or operator is informed depends on context. A desktop
   computer with a screen might put up a dialog box. A headless server
   in the closet may write a message to a log file, or use whatever
   mechanism (email, SNMP trap, etc.) it uses to inform the
   administrator of other error conditions. On the other hand a headless
   server in the closet may not inform the user at all -- if the user
   cares, they will notice the name has changed, and connect to the
   server in the usual way (e.g. via Web Browser) to configure a new
   name.

   The examples in this section focus on address records (i.e. host
   names), but the same considerations apply to all resource records
   where uniqueness (or maintenance of some other defined constraint)
   is desired.



11. Resource Record TTL Values and Cache Coherency

   As a general rule, the recommended TTL value for Multicast DNS
   resource records with a host name as the resource record's name
   (e.g. A, AAAA, HINFO, etc.) or contained within the resource record's
   rdata (e.g. SRV, reverse mapping PTR record, etc.) is 120 seconds.

   The recommended TTL value for other Multicast DNS resource records
   is 75 minutes.

   A client with an active outstanding query will issue a query packet
   when one or more of the resource record(s) in its cache is (are) 80%
   of the way to expiry. If the TTL on those records is 75 minutes,
   this ongoing cache maintenance process yields a steady-state query
   rate of one query every 60 minutes.

   Any distributed cache needs a cache coherency protocol. If Multicast
   DNS resource records follow the recommendation and have a TTL of 75
   minutes, that means that stale data could persist in the system for
   a little over an hour. Making the default TTL significantly lower
   would reduce the lifetime of stale data, but would produce too much
   extra traffic on the network. Various techniques are available to
   minimize the impact of such stale data.


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11.1 Cooperating Multicast DNS Responders

   If a Multicast DNS Responder ("A") observes some other Multicast DNS
   Responder ("B") send a Multicast DNS Response packet containing a
   resource record with the same name, rrtype and rrclass as one of A's
   resource records, but different rdata, then:

   o If A's resource record is intended to be a shared resource record,
     then this is no conflict, and no action is required.

   o If A's resource record is intended to be a member of a unique
     resource record set owned solely by that responder, then this
     is a conflict and MUST be handled as described in Section 10
     "Conflict Resolution".

   If a Multicast DNS Responder ("A") observes some other Multicast DNS
   Responder ("B") send a Multicast DNS Response packet containing a
   resource record with the same name, rrtype and rrclass as one of A's
   resource records, and identical rdata, then:

   o If the TTL of B's resource record given in the packet is at least
     half the true TTL from A's point of view, then no action is
     required.

   o If the TTL of B's resource record given in the packet is less than
     half the true TTL from A's point of view, then A MUST mark its
     record to be announced via multicast. Clients receiving the record
     from B would use the TTL given by B, and hence may delete the
     record sooner than A expects. By sending its own multicast response
     correcting the TTL, A ensures that the record will be retained for
     the desired time.

   These rules allow multiple Multicast DNS Responders to offer the same
   data on the network (perhaps for fault tolerance reasons) without
   conflicting with each other.


11.2 Goodbye Packets

   In the case where a host knows that certain resource record data is
   about to become invalid (for example when the host is undergoing a
   clean shutdown) the host SHOULD send a gratuitous announcement mDNS
   response packet, giving the same resource record name, rrtype,
   rrclass and rdata, but an RR TTL of zero. This has the effect of
   updating the TTL stored in neighboring hosts' cache entries to zero,
   causing that cache entry to be promptly deleted.

   Clients receiving a Multicast DNS Response with a TTL of zero SHOULD
   NOT immediately delete the record from the cache, but instead record
   a TTL of 1 and then delete the record one second later. In the case
   of multiple Multicast DNS Responders on the network described in
   Section 11.1 above, if one of the Responders shuts down and
   incorrectly sends goodbye packets for its records, it gives the other


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   cooperating Responders one second to send out their own response to
   "rescue" the records before they expire and are deleted.

   Generally speaking, it is more important to send goodbye packets for
   shared records than unique records. A given shared record name (such
   as a PTR record used for DNS Service Discovery [DNS-SD]) by its
   nature often has many representatives from many different hosts, and
   tends to be the subject of long-lived ongoing queries. Those
   long-lived queries are often concerned not just about being informed
   when records appear, but also about being informed if those records
   vanish again. In contrast, a unique record set (such as an SRV
   record, or a host address record), by its nature, often has far fewer
   members than a shared record set, and is usually the subject of
   one-shot queries which simply retrieve the data and then cease
   querying once they have the answer they are seeking. Therefore,
   sending a goodbye packet for a unique record set is likely to offer
   less benefit, because it is likely at any given moment that no one
   has an active query running for that record set. One example where
   goodbye packets for SRV and address records are useful is when
   transferring control to a Sleep Proxy Server (see Section 16,
   "Multicast DNS and Power Management").


11.3 Announcements to Flush Outdated Cache Entries

   Whenever a host has a resource record with potentially new data (e.g.
   after rebooting, waking from sleep, connecting to a new network link,
   changing IP address, etc.), the host MUST send a series of gratuitous
   announcements to update cache entries in its neighbor hosts. In
   these gratuitous announcements, if the record is one that is intended
   to be unique, the host sets the most significant bit of the rrclass
   field of the resource record. This bit, the "cache flush" bit, tells
   neighboring hosts that this is not a shared record type. Instead of
   merging this new record additively into the cache in addition to any
   previous records with the same name, rrtype and rrclass, all old
   records with that name, type and class that were received more than
   one second ago are declared invalid, and marked to expire from the
   cache in one second.

   The semantics of the cache flush bit are as follows: Normally when a
   resource record appears in the Answer Section of the DNS Response, it
   means, "This is an assertion that this information is true." When a
   resource record appears in the Answer Section of the DNS Response
   with the "cache flush" bit set, it means, "This is an assertion that
   this information is the truth and the whole truth, and anything you
   may have heard more than a second ago regarding records of this
   name/rrtype/rrclass is no longer valid".

   To accommodate the case where the set of records from one host
   constituting a single unique RRSet is too large to fit in a single
   packet, only cache records that are more than one second old are
   flushed. This allows the announcing host to generate a quick burst of
   packets back-to-back on the wire containing all the members


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   of the RRSet. When receiving records with the "cache flush" bit set,
   all records older than one second are marked to be deleted one second
   in the future. One second after the end of the little packet burst,
   any records not represented within that packet burst will then be
   expired from all peer caches.

   Any time a host sends a response packet containing some members of a
   unique RRSet, it SHOULD send the entire RRSet, preferably in a single
   packet, or if the entire RRSet will not fit in a single packet, in a
   quick burst of packets sent as close together as possible. The host
   SHOULD set the cache flush bit on all members of the unique RRSet.
   In the event that for some reason the host chooses not to send the
   entire unique RRSet in a single packet or a rapid packet burst,
   it MUST NOT set the cache flush bit on any of those records.

   The reason for waiting one second before deleting stale records from
   the cache is to accommodate bridged networks. For example, a host's
   address record announcement on a wireless interface may be bridged
   onto a wired Ethernet, and cause that same host's Ethernet address
   records to be flushed from peer caches. The one-second delay gives
   the host the chance to see its own announcement arrive on the wired
   Ethernet, and immediately re-announce its Ethernet interface's
   address records so that both sets remain valid and live in peer
   caches.

   These rules apply regardless of *why* the response packet is being
   generated. They apply to startup announcements as described in
   Section 9.3, and to responses generated as a result of receiving
   query packets.

   The "cache flush" bit is only set in records in the Answer Section of
   Multicast DNS responses sent to UDP port 5353. The "cache flush" bit
   MUST NOT be set in any resource records in a response packet sent in
   legacy unicast responses to UDP ports other than 5353.

   The "cache flush" bit MUST NOT be set in any resource records in the
   known-answer list of any query packet.

   The "cache flush" bit MUST NOT ever be set in any shared resource
   record. To do so would cause all the other shared versions of this
   resource record with different rdata from different Responders to be
   immediately deleted from all the caches on the network.

   The "cache flush" bit does apply to questions listed in the Question
   Section of a Multicast DNS packet. The top bit of the rrclass field
   in questions is used for an entirely different purpose (see Section
   6.5, "Questions Requesting Unicast Responses").

   Note that the "cache flush" bit is NOT part of the resource record
   class. The "cache flush" bit is the most significant bit of the
   second 16-bit word of a resource record in the Answer Section of
   an mDNS packet (the field conventionally referred to as the rrclass
   field), and the actual resource record class is the least-significant


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   fifteen bits of this field. There is no mDNS resource record class
   0x8001. The value 0x8001 in the rrclass field of a resource record in
   an mDNS response packet indicates a resource record with class 1,
   with the "cache flush" bit set. When receiving a resource record with
   the "cache flush" bit set, implementations should take care to mask
   off that bit before storing the resource record in memory.


11.4 Cache Flush on Topology change

   If the hardware on a given host is able to indicate physical changes
   of connectivity, then when the hardware indicates such a change, the
   host should take this information into account in its mDNS cache
   management strategy. For example, a host may choose to immediately
   flush all cache records received on a particular interface when that
   cable is disconnected. Alternatively, a host may choose to adjust the
   remaining TTL on all those records to a few seconds so that if the
   cable is not reconnected quickly, those records will expire from the
   cache.

   Likewise, when a host reboots, or wakes from sleep, or undergoes some
   other similar discontinuous state change, the cache management
   strategy should take that information into account.


11.5 Cache Flush on Failure Indication

   Sometimes a cache record can be determined to be stale when a client
   attempts to use the rdata it contains, and finds that rdata to be
   incorrect.

   For example, the rdata in an address record can be determined to be
   incorrect if attempts to contact that host fail, either because
   ARP/ND requests for that address go unanswered (for an address on a
   local subnet) or because a router returns an ICMP "Host Unreachable"
   error (for an address on a remote subnet).

   The rdata in an SRV record can be determined to be incorrect if
   attempts to communicate with the indicated service at the host and
   port number indicated are not successful.

   The rdata in a DNS-SD PTR record can be determined to be incorrect if
   attempts to look up the SRV record it references are not successful.

   In any such case, the software implementing the mDNS resource record
   cache should provide a mechanism so that clients detecting stale
   rdata can inform the cache.

   When the cache receives this hint that it should reconfirm some
   record, it MUST issue two or more queries for the resource record in
   question. If no response is received in a reasonable amount of time,
   then, even though its TTL may indicate that it is not yet due to
   expire, that record SHOULD be promptly flushed from the cache.


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   The end result of this is that if a printer suffers a sudden power
   failure or other abrupt disconnection from the network, its name may
   continue to appear in DNS-SD browser lists displayed on users'
   screens. Eventually that entry will expire from the cache naturally,
   but if a user tries to access the printer before that happens, the
   failure to successfully contact the printer will trigger the more
   hasty demise of its cache entries. This is a sensible trade-off
   between good user-experience and good network efficiency. If we were
   to insist that printers should disappear from the printer list within
   30 seconds of becoming unavailable, for all failure modes, the only
   way to achieve this would be for the client to poll the printer at
   least every 30 seconds, or for the printer to announce its presence
   at least every 30 seconds, both of which would be an unreasonable
   burden on most networks.


11.6 Passive Observation of Failures

   A host observes the multicast queries issued by the other hosts on
   the network. One of the major benefits of also sending responses
   using multicast is that it allows all hosts to see the responses (or
   lack thereof) to those queries.

   If a host sees queries, for which a record in its cache would be
   expected to be given as an answer in a multicast response, but no
   such answer is seen, then the host may take this as an indication
   that the record may no longer be valid.

   After seeing two or more of these queries, and seeing no multicast
   response containing the expected answer within a reasonable amount of
   time, then even though its TTL may indicate that it is not yet due to
   expire, that record MAY be flushed from the cache. The host SHOULD
   NOT perform its own queries to re-confirm that the record is truly
   gone. If every host on a large network were to do this, it would
   cause a lot of unnecessary multicast traffic. If host A sends
   multicast queries that remain unanswered, then there is no reason to
   suppose that host B or any other host is likely to be any more
   successful.

   The previous section, "Cache Flush on Failure Indication", describes
   a situation where a user trying to print discovers that the printer
   is no longer available. By implementing the passive observation
   described here, when one user fails to contact the printer, all hosts
   on the network observe that failure and update their caches
   accordingly.


12. Special Characteristics of Multicast DNS Domains

   Unlike conventional DNS names, names that end in ".local.",
   "254.169.in-addr.arpa." or "0.8.e.f.ip6.arpa." have only local
   significance. Conventional DNS seeks to provide a single unified
   namespace, where a given DNS query yields the same answer no matter


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   where on the planet it is performed or to which recursive DNS server
   the query is sent. (However, split views, firewalls, intranets and
   the like have somewhat interfered with this goal of DNS representing
   a single universal truth.) In contrast, each IP link has its own
   private ".local.", "254.169.in-addr.arpa." and "0.8.e.f.ip6.arpa."
   namespaces, and the answer to any query for a name within those
   domains depends on where that query is asked.

   Multicast DNS Domains are not delegated from their parent domain via
   use of NS records. There are no NS records anywhere in Multicast DNS
   Domains. Instead, all Multicast DNS Domains are delegated to the IP
   addresses 224.0.0.251 and FF02::FB by virtue of the individual
   organizations producing DNS client software deciding how to handle
   those names. It would be extremely valuable for the industry if this
   special handling were ratified and recorded by IANA, since otherwise
   the special handling provided by each vendor is likely to be
   inconsistent.

   The IPv4 name server for a Multicast DNS Domain is 224.0.0.251. The
   IPv6 name server for a Multicast DNS Domain is FF02::FB. These are
   multicast addresses; therefore they identify not a single host but a
   collection of hosts, working in cooperation to maintain some
   reasonable facsimile of a competently managed DNS zone. Conceptually
   a Multicast DNS Domain is a single DNS zone, however its server is
   implemented as a distributed process running on a cluster of loosely
   cooperating CPUs rather than as a single process running on a single
   CPU.

   No delegation is performed within Multicast DNS Domains. Because the
   cluster of loosely coordinated CPUs is cooperating to administer a
   single zone, delegation is neither necessary nor desirable. Just
   because a particular host on the network may answer queries for a
   particular record type with the name "example.local." does not imply
   anything about whether that host will answer for the name
   "child.example.local.", or indeed for other record types with the
   name "example.local."

   Multicast DNS Zones have no SOA record. A conventional DNS zone's
   SOA record contains information such as the email address of the zone
   administrator and the monotonically increasing serial number of the
   last zone modification. There is no single human administrator for
   any given Multicast DNS Zone, so there is no email address. Because
   the hosts managing any given Multicast DNS Zone are only loosely
   coordinated, there is no readily available monotonically increasing
   serial number to determine whether or not the zone contents have
   changed. A host holding part of the shared zone could crash or be
   disconnected from the network at any time without informing the other
   hosts. There is no reliable way to provide a zone serial number that
   would, whenever such a crash or disconnection occurred, immediately
   change to indicate that the contents of the shared zone had changed.

   Zone transfers are not possible for any Multicast DNS Zone.



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13. Multicast DNS for Service Discovery

   This document does not describe using Multicast DNS for network
   browsing or service discovery. However, the mechanisms this document
   describes are compatible with (and support) the browsing and service
   discovery mechanisms proposed in "DNS-Based Service Discovery"
   [DNS-SD].


14. Enabling and Disabling Multicast DNS

   The option to fail-over to Multicast DNS for names not ending in
   ".local." SHOULD be a user-configured option, and SHOULD
   be disabled by default because of the possible security issues
   related to unintended local resolution of apparently global names.

   The option to lookup unqualified (relative) names by appending
   ".local." (or not) is controlled by whether ".local." appears
   (or not) in the client's DNS search list.

   No special control is needed for enabling and disabling Multicast DNS
   for names explicitly ending with ".local." as entered by the user.
   The user doesn't need a way to disable Multicast DNS for names ending
   with ".local.", because if the user doesn't want to use Multicast
   DNS, they can achieve this by simply not using those names. If a user
   *does* enter a name ending in ".local.", then we can safely assume
   the user's intention was probably that it should work. Having user
   configuration options that can be (intentionally or unintentionally)
   set so that local names don't work is just one more way of
   frustrating the user's ability to perform the tasks they want,
   perpetuating the view that, "IP networking is too complicated to
   configure and too hard to use." This in turn perpetuates the
   continued use of protocols like AppleTalk. If we want to retire
   AppleTalk, NetBIOS, etc., we need to offer users equivalent IP
   functionality that they can rely on to, "always work, like
   AppleTalk." A little Multicast DNS traffic may be a burden on the
   network, but it is an insignificant burden compared to continued
   widespread use of AppleTalk.


15. Considerations for Multiple Interfaces

   A host should defend its host name (FQDN) on all active interfaces on
   which it is answering Multicast DNS queries.

   In the event of a name conflict on *any* interface, a host should
   configure a new host name, if it wishes to maintain uniqueness of its
   host name.

   A host may choose to use the same name for all of its address records
   on all interfaces, or it may choose to manage its Multicast DNS host
   name(s) independently on each interface, potentially answering to
   different names on different interfaces.


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   When answering a Multicast DNS query, a multi-homed host with a
   link-local address (or addresses) should take care to ensure that
   any address going out in a Multicast DNS response is valid for use
   on the interface on which the response is going out.

   Just as the same link-local IP address may validly be in use
   simultaneously on different links by different hosts, the same
   link-local host name may validly be in use simultaneously on
   different links, and this is not an error. A multi-homed host with
   connections to two different links may be able to communicate with
   two different hosts that are validly using the same name. While this
   kind of name duplication should be rare, it means that a host that
   wants to fully support this case needs network programming APIs that
   allow applications to specify on what interface to perform a
   link-local Multicast DNS query, and to discover on what interface a
   Multicast DNS response was received.


16. Multicast DNS and Power Management

   Many modern network devices have the ability to go into a low-power
   mode where only a small part of the Ethernet hardware remains
   powered, and the device can be woken up by sending a specially
   formatted Ethernet frame which the device's power-management hardware
   recognizes.

   To make use of this in conjunction with Multicast DNS, we propose a
   network power management service called Sleep Proxy Service. A device
   that wishes to enter low-power mode first uses DNS-SD to determine if
   Sleep Proxy Service is available on the local network. In some
   networks there may be more than one piece of hardware implementing
   Sleep Proxy Service, for fault-tolerance reasons.

   If the device finds the network has Sleep Proxy Service, the device
   transmits two or more gratuitous mDNS announcements setting the TTL
   of its relevant resource records to zero, to delete them from
   neighboring caches. The relevant resource records include address
   records and SRV records, and other resource records as may apply to a
   particular device. The device then communicates all of its remaining
   active records, plus the names, rrtypes and rrclasses of the deleted
   records, to the Sleep Proxy Service(s), along with a copy of the
   specific "magic packet" required to wake the device up.

   When a Sleep Proxy Service sees an mDNS query for one of the
   device's active records (e.g. a DNS-SD PTR record), it answers on
   behalf of the device without waking it up. When a Sleep Proxy Service
   sees an mDNS query for one of the device's deleted resource
   records, it deduces that some client on the network needs to make an
   active connection to the device, and sends the specified "magic
   packet" to wake the device up. The device then wakes up, reactivates
   its deleted resource records, and re-announces them to the network.
   The client waiting to connect sees the announcements, learns the
   current IP address and port number of the desired service on the
   device, and proceeds to connect to it.

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   The connecting client does not need to be aware of how Sleep Proxy
   Service works. Only devices that implement low power mode and wish to
   make use of Sleep Proxy Service need to be aware of how that protocol
   works.

   The reason that a device using a Sleep Proxy Service should send more
   than one goodbye packet is to ensure deletion of the resource records
   from all peer caches. If resource records were to inadvertently
   remain in some peer caches, then those peers may not issue any query
   packets for those records when attempting to access the sleeping
   device, so the Sleep Proxy Service would not receive any queries for
   the device's SRV and/or address records, and the necessary wake-up
   message would not be triggered.

   The full specification of mDNS / DNS-SD Sleep Proxy Service
   is described in another document [not yet published].


17. Multicast DNS Character Set

   Unicast DNS has been plagued by the lack of any support for non-US
   characters. Indeed, conventional DNS is usually limited to just
   letters, digits and hyphens, with no spaces or other punctuation.
   Attempts to remedy this for unicast DNS have been badly constrained
   by the need to accommodate old buggy legacy DNS implementations.
   In reality, the DNS specification actually imposes no limits on what
   characters may be used in names, and good DNS implementations handle
   any arbitrary eight-bit data without trouble. However, the old rules
   for ARPANET host names back in the 1980s required names to be just
   letters, digits, and hyphens [RFC 1034], and since the predominant
   use of DNS is to store host address records, many have assumed that
   the DNS protocol itself suffers from the same limitation. It would be
   more accurate to say that certain bad implementations may not handle
   eight-bit data correctly, not that the protocol doesn't support it.

   Multicast DNS is a new protocol and doesn't (yet) have old buggy
   legacy implementations to constrain the design choices. Accordingly,
   it adopts the simple obvious elegant solution: all names in Multicast
   DNS are encoded using precomposed UTF-8 [RFC 3629]. The
   characters SHOULD conform to Unicode Normalization Form C (NFC): Use
   precomposed characters instead of combining sequences where possible,
   e.g. use U+00C4 ("Latin capital letter A with diaeresis") instead of
   U+0041 U+0308 ("Latin capital letter A", "combining diaeresis").

   For names that are restricted to letters, digits and hyphens, the
   UTF-8 encoding is identical to the US-ASCII encoding, so this is
   entirely compatible with existing host names. For characters outside
   the US-ASCII range, UTF-8 encoding is used.

   Multicast DNS implementations MUST NOT use any other encodings apart
   from precomposed UTF-8 (US-ASCII being considered a compatible subset
   of UTF-8).



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   This point bears repeating: After many years of debate, as a result
   of the need to accommodate certain DNS implementations that
   apparently couldn't handle any character that's not a letter, digit
   or hyphen (and apparently never will be updated to remedy this
   limitation) the unicast DNS community settled on an extremely baroque
   encoding called "Punycode" [RFC 3492]. Punycode is a remarkably
   ingenious encoding solution, but it is complicated, hard to
   understand, and hard to implement, using sophisticated techniques
   including insertion unsort coding, generalized variable-length
   integers, and bias adaptation. The resulting encoding is remarkably
   compact given the constraints, but it's still not as good as simple
   straightforward UTF-8, and it's hard even to predict whether a given
   input string will encode to a Punycode string that fits within DNS's
   63-byte limit, except by simply trying the encoding and seeing
   whether it fits. Indeed, the encoded size depends not only on the
   input characters, but on the order they appear, so the same set of
   characters may or may not encode to a legal Punycode string that fits
   within DNS's 63-byte limit, depending on the order the characters
   appear. This is extremely hard to present in a user interface that
   explains to users why one name is allowed, but another name
   containing the exact same characters is not. Neither Punycode nor any
   other of the "Ascii Compatible Encodings" proposed for Unicast DNS
   may be used in Multicast DNS packets. Any text being represented
   internally in some other representation MUST be converted to
   canonical precomposed UTF-8 before being placed in any Multicast DNS
   packet.

   The simple rules for case-insensitivity in Unicast DNS also apply in
   Multicast DNS; that is to say, in name comparisons, the lower-case
   letters "a" to "z" (0x61 to 0x7A) match their upper-case equivalents
   "A" to "Z" (0x41 to 0x5A). Hence, if a client issues a query for an
   address record with the name "cheshire.local", then a responder
   having an address record with the name "Cheshire.local" should
   issue a response. No other automatic equivalences should be assumed.
   In particular all UTF-8 multi-byte characters (codes 0x80 and higher)
   are compared by simple binary comparison of the raw byte values.

   No other automatic character equivalence is defined in Multicast DNS.
   For example, accented characters are not defined to be automatically
   equivalent to their unaccented counterparts. Where automatic
   equivalences are desired, this may be achieved through the use of
   programmatically-generated CNAME records. For example, if a responder
   has an address record for an accented name Y, and a client issues a
   query for a name X, where X is the same as Y with all the accents
   removed, then the responder may issue a response containing two
   resource records: A CNAME record "X CNAME Y", asserting that the
   requested name X (unaccented) is an alias for the true (accented)
   name Y, followed by the address record for Y.







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18. Multicast DNS Message Size

   RFC 1035 restricts DNS Messages carried by UDP to no more than 512
   bytes (not counting the IP or UDP headers). For UDP packets carried
   over the wide-area Internet in 1987, this was appropriate. For
   link-local multicast packets on today's networks, there is no reason
   to retain this restriction. Given that the packets are by definition
   link-local, there are no Path MTU issues to consider.

   Multicast DNS Messages carried by UDP may be up to the IP MTU of the
   physical interface, less the space required for the IP header (20
   bytes for IPv4; 40 bytes for IPv6) and the UDP header (8 bytes).

   In the case of a single mDNS Resource Record which is too large to
   fit in a single MTU-sized multicast response packet, a Multicast DNS
   Responder SHOULD send the Resource Record alone, in a single IP
   datagram, sent using multiple IP fragments. Resource Records this
   large SHOULD be avoided, except in the very rare cases where they
   really are the appropriate solution to the problem at hand.
   Implementers should be aware that many simple devices do not
   re-assemble fragmented IP datagrams, so large Resource Records SHOULD
   NOT be used except in specialized cases where the implementer knows
   that all receivers implement reassembly.

   A Multicast DNS packet larger than the interface MTU, which is sent
   using fragments, MUST NOT contain more than one Resource Record.

   Even when fragmentation is used, a Multicast DNS packet, including IP
   and UDP headers, MUST NOT exceed 9000 bytes.


19. Multicast DNS Message Format

   This section describes specific restrictions on the allowable
   values for the header fields of a Multicast DNS message.

19.1. ID (Query Identifier)

   Multicast DNS clients SHOULD listen for gratuitous responses
   issued by hosts booting up (or waking up from sleep or otherwise
   joining the network). Since these gratuitous responses may contain a
   useful answer to a question for which the client is currently
   awaiting an answer, Multicast DNS clients SHOULD examine all received
   Multicast DNS response messages for useful answers, without regard to
   the contents of the ID field or the Question Section. In Multicast
   DNS, knowing which particular query message (if any) is responsible
   for eliciting a particular response message is less interesting than
   knowing whether the response message contains useful information.

   Multicast DNS clients MAY cache any or all Multicast DNS response
   messages they receive, for possible future use, provided of course
   that normal TTL aging is performed on these cached resource records.



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   In multicast query messages, the Query ID SHOULD be set to zero on
   transmission.

   In multicast responses, including gratuitous multicast responses, the
   Query ID MUST be set to zero on transmission, and MUST be ignored on
   reception.

   In unicast response messages generated specifically in response to a
   particular (unicast or multicast) query, the Query ID MUST match the
   ID from the query message.


19.2. QR (Query/Response) Bit

   In query messages, MUST be zero.

   In response messages, MUST be one.


19.3. OPCODE

   In both multicast query and multicast response messages, MUST be zero
   (only standard queries are currently supported over multicast, unless
   other queries are allowed by future IETF Standards Action).


19.4. AA (Authoritative Answer) Bit

   In query messages, the Authoritative Answer bit MUST be zero on
   transmission, and MUST be ignored on reception.

   In response messages for Multicast Domains, the Authoritative Answer
   bit MUST be set to one (not setting this bit implies there's some
   other place where "better" information may be found) and MUST be
   ignored on reception.


19.5. TC (Truncated) Bit

   In query messages, if the TC bit is set, it means that additional
   Known Answer records may be following shortly. A responder MAY choose
   to record this fact, and wait for those additional Known Answer
   records, before deciding whether to respond. If the TC bit is clear,
   it means that the querying host has no additional Known Answers.

   In multicast response messages, the TC bit MUST be zero on
   transmission, and MUST be ignored on reception.

   In legacy unicast response messages, the TC bit has the same meaning
   as in conventional unicast DNS: it means that the response was too
   large to fit in a single packet, so the client SHOULD re-issue its
   query using TCP in order to receive the larger response.



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19.6. RD (Recursion Desired) Bit

   In both multicast query and multicast response messages, the
   Recursion Desired bit SHOULD be zero on transmission, and MUST be
   ignored on reception.


19.7. RA (Recursion Available) Bit

   In both multicast query and multicast response messages, the
   Recursion Available bit MUST be zero on transmission, and MUST be
   ignored on reception.


19.8. Z (Zero) Bit

   In both query and response messages, the Zero bit MUST be zero on
   transmission, and MUST be ignored on reception.


19.9. AD (Authentic Data) Bit [RFC 2535]

   In query messages the Authentic Data bit MUST be zero on
   transmission, and MUST be ignored on reception.

   In response messages, the Authentic Data bit MAY be set. Resolvers
   receiving response messages with the AD bit set MUST NOT trust the AD
   bit unless they trust the source of the message and either have a
   secure path to it or use DNS transaction security.


19.10. CD (Checking Disabled) Bit [RFC 2535]

   In query messages, a resolver willing to do cryptography SHOULD set
   the Checking Disabled bit to permit it to impose its own policies.

   In response messages, the Checking Disabled bit MUST be zero on
   transmission, and MUST be ignored on reception.


19.11. RCODE (Response Code)

   In both multicast query and multicast response messages, the Response
   Code MUST be zero on transmission. Multicast DNS messages received
   with non-zero Response Codes MUST be silently ignored.


19.12. Repurposing of top bit of qclass in Question Section

   In the Question Section of a Multicast DNS Query, the top bit of the
   qclass field is used to indicate that unicast responses are preferred
   for this particular question.



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19.12. Repurposing of top bit of rrclass in Answer Section

   In the Answer Section of a Multicast DNS Response, the top bit of the
   rrclass field is used to indicate that the record is a member of a
   unique RRSet, and the entire RRSet has been sent together (in the
   same packet, or in consecutive packets if there are too many records
   to fit in a single packet).


20. Choice of UDP Port Number

   Arguments were made for and against using Multicast on UDP port 53.
   The final decision was to use UDP port 5353. Some of the arguments
   for and against are given below.


20.1 Arguments for using UDP port 53:

   * This is "just DNS", so it should be the same port.

   * There is less work to be done updating old clients to do simple
     mDNS queries. Only the destination address need be changed.
     In some cases, this can be achieved without any code changes,
     just by adding the address 224.0.0.251 to a configuration file.


20.2 Arguments for using a different port (UDP port 5353):

   * This is not "just DNS". This is a DNS-like protocol, but different.

   * Changing client code to use a different port number is not hard.

   * Using the same port number makes it hard to run an mDNS Responder
     and a conventional unicast DNS server on the same machine. If a
     conventional unicast DNS server wishes to implement mDNS as well,
     it can still do that, by opening two sockets. Having two different
     port numbers is important to allow this flexibility.

   * Some VPN software hijacks all outgoing traffic to port 53 and
     redirects it to a special DNS server set up to serve those VPN
     clients while they are connected to the corporate network. It is
     questionable whether this is the right thing to do, but it is
     common, and redirecting link-local multicast DNS packets to a
     remote server rarely produces any useful results. It does mean, for
     example, that the user becomes unable to access their local network
     printer sitting on their desk right next to their computer. Using
     a different UDP port eliminates this particular problem.

   * On many operating systems, unprivileged clients may not send or
     receive packets on low-numbered ports. This means that any client
     sending or receiving mDNS packets on port 53 would have to run as
     "root", which is an undesirable security risk. Using a higher-
     numbered UDP port eliminates this particular problem.


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   Continuing the previous point, since using an unprivileged port
   allows normal user-level code to bind, a given machine may have more
   than one such user-level application running at a time. Because of
   this, any code binding to UDP port 5353 MUST use the SO_REUSEPORT
   option, so as to be a good citizen and not block other clients on the
   machine from also binding to that port.


21. Summary of Differences Between Multicast DNS and Unicast DNS

   The value of Multicast DNS is that it shares, as much as possible,
   the familiar APIs, naming syntax, resource record types, etc., of
   Unicast DNS. There are of course necessary differences by virtue of
   it using Multicast, and by virtue of it operating in a community of
   cooperating peers, rather than a precisely defined authoritarian
   hierarchy controlled by a strict chain of formal delegations from the
   top. These differences are listed below:

   Multicast DNS...
   * uses multicast
   * uses UDP port 5353 instead of port 53
   * operates in well-defined parts of the DNS namespace
   * uses UTF-8, and only UTF-8, to encode resource record names
   * defines a clear limit on the maximum legal domain name (255 bytes)
   * allows larger UDP packets
   * allows more than one question in a query packet
   * uses the Answer Section of a query to list Known Answers
   * uses the TC bit in a query to indicate additional Known Answers
   * uses the Authority Section of a query for probe tie-breaking
   * ignores the Query ID field (except for generating legacy responses)
   * doesn't require the question to be repeated in the response packet
   * uses gratuitous responses to announce new records to the peer group
   * defines a "unicast response" bit in the rrclass of query questions
   * defines a "cache flush" bit in the rrclass of response answers
   * uses DNS TTL 0 to indicate that a record has been deleted
   * monitors queries to perform Duplicate Question Suppression
   * monitors responses to perform Duplicate Answer Suppression...
   * ... and Ongoing Conflict Detection
   * ... and Opportunistic Caching


22. Benefits of Multicast Responses

   Some people have argued that sending responses via multicast is
   inefficient on the network. In fact the benefits of using multicast
   responses result in a net lowering of overall multicast traffic, for
   a variety of reasons.

   * One multicast response can update the cache on all machines on the
     network. If another machine later wants to issue the same query, it
     already has the answer in its cache, so it may not need to even
     transmit that multicast query on the network at all.



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   * When more than one machine has the same ongoing long-lived query
     running, every machine does not have to transmit its own
     independent query. When one machine transmits a query, all the
     other hosts see the answers, so they can suppress their own
     queries.

   * When a host sees a multicast query, but does not see the
     corresponding multicast response, it can use this information to
     promptly delete stale data from its cache. To achieve the same
     level of user-interface quality and responsiveness without
     multicast responses would require lower cache lifetimes and more
     frequent network polling, resulting in a significantly higher
     packet rate.

   * Multicast responses allow passive conflict detection. Without this
     ability, some other conflict detection mechanism would be needed,
     imposing its own additional burden on the network.

   * When using delayed responses to reduce network collisions, clients
     need to maintain a list recording to whom each answer should be
     sent. The option of multicast responses allows clients with limited
     storage, which cannot store an arbitrarily long list of response
     addresses, to choose to fail-over to a single multicast response in
     place of multiple unicast responses, when appropriate.

   * In the case of overlayed subnets and other misconfigurations,
     multicast responses allow a receiver to know with certainty that
     a response originated on the local link, even when its source
     address may apparently suggest otherwise. This can be extremely
     helpful when diagnosing and rectifying network problems, since
     it facilitates a direct communication channel between client and
     server that works without reliance on ARP, IP routing tables, etc.
     Being able to discover what IP address a device has (or thinks it
     has) is frequently a very valuable first step in diagnosing why
     it unable to communicate on the local network.


23. IPv6 Considerations

   An IPv4-only host and an IPv6-only host behave as "ships that pass in
   the night". Even if they are on the same Ethernet, neither is aware
   of the other's traffic. For this reason, each physical link may have
   *two* unrelated ".local." zones, one for IPv4 and one for IPv6.
   Since for practical purposes, a group of IPv4-only hosts and a group
   of IPv6-only hosts on the same Ethernet act as if they were on two
   entirely separate Ethernet segments, it is unsurprising that their
   use of the ".local." zone should occur exactly as it would if
   they really were on two entirely separate Ethernet segments.

   A dual-stack (v4/v6) host can participate in both ".local."
   zones, and should register its name(s) and perform its lookups both
   using IPv4 and IPv6. This enables it to reach, and be reached by,
   both IPv4-only and IPv6-only hosts. In effect this acts like a


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   multi-homed host, with one connection to the logical "IPv4 Ethernet
   segment", and a connection to the logical "IPv6 Ethernet segment".

23.1 IPv6 Multicast Addresses by Hashing

   Some discovery protocols use a range of multicast addresses, and
   determine the address to be used by a hash function of the name being
   sought. Queries are sent via multicast to the address as indicated by
   the hash function, and responses are returned to the querier via
   unicast. Particularly in IPv6, where multicast addresses are
   extremely plentiful, this approach is frequently advocated.

   There are some problems with this:

   * When a host has a large number of records with different names, the
     host may have to join a large number of multicast groups. This can
     place undue burden on the Ethernet hardware, which typically
     supports a limited number of multicast addresses efficiently. When
     this number is exceeded, the Ethernet hardware may have to resort
     to receiving all multicasts and passing them up to the host
     software for filtering, thereby defeating the point of using a
     multicast address range in the first place.

   * Multiple questions cannot be placed in one packet if they don't all
     hash to the same multicast address.

   * Duplicate Question Suppression doesn't work if queriers are not
     seeing each other's queries.

   * Duplicate Answer Suppression doesn't work if responders are not
     seeing each other's responses.

   * Opportunistic Caching doesn't work.

   * Ongoing Conflict Detection doesn't work.


24. Security Considerations

   The algorithm for detecting and resolving name conflicts is, by its
   very nature, an algorithm that assumes cooperating participants. Its
   purpose is to allow a group of hosts to arrive at a mutually disjoint
   set of host names and other DNS resource record names, in the absence
   of any central authority to coordinate this or mediate disputes. In
   the absence of any higher authority to resolve disputes, the only
   alternative is that the participants must work together cooperatively
   to arrive at a resolution.

   In an environment where the participants are mutually antagonistic
   and unwilling to cooperate, other mechanisms are appropriate, like
   manually administered DNS.




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   In an environment where there is a group of cooperating participants,
   but there may be other antagonistic participants on the same physical
   link, the cooperating participants need to use IPSEC signatures
   and/or DNSSEC [RFC 2535] signatures so that they can distinguish mDNS
   messages from trusted participants (which they process as usual) from
   mDNS messages from untrusted participants (which they silently
   discard).

   When DNS queries for *global* DNS names are sent to the mDNS
   multicast address (during network outages which disrupt communication
   with the greater Internet) it is *especially* important to use
   DNSSEC, because the user may have the impression that he or she is
   communicating with some authentic host, when in fact he or she is
   really communicating with some local host that is merely masquerading
   as that name. This is less critical for names ending with ".local.",
   because the user should be aware that those names have only local
   significance and no global authority is implied.

   Most computer users neglect to type the trailing dot at the end of a
   fully qualified domain name, making it a relative domain name (e.g.
   "www.example.com"). In the event of network outage, attempts to
   positively resolve the name as entered will fail, resulting in
   application of the search list, including ".local.", if present.
   A malicious host could masquerade as "www.example.com" by answering
   the resulting Multicast DNS query for "www.example.com.local."
   To avoid this, a host MUST NOT append the search suffix
   ".local.", if present, to any relative (partially qualified)
   domain name containing two or more labels. Appending ".local." to
   single-label relative domain names is acceptable, since the user
   should have no expectation that a single-label domain name will
   resolve as-is.


25. IANA Considerations

   IANA has allocated the IPv4 link-local multicast address 224.0.0.251
   for the use described in this document.

   IANA has allocated the IPv6 multicast address set FF0X::FB for the
   use described in this document. Only address FF02::FB (Link-Local
   Scope) is currently in use by deployed software, but it is possible
   that in future implementers may experiment with Multicast DNS using
   larger-scoped addresses, such as FF05::FB (Site-Local Scope).

   When this document is published, IANA should designate a list of
   domains which are deemed to have only link-local significance, as
   described in Section 12 of this document ("Special Characteristics of
   Multicast DNS Domains").

   The re-use of the top bit of the rrclass field in the Question and
   Answer Sections means that Multicast DNS can only carry DNS records
   with classes in the range 0-32767. Classes in the range 32768 to
   65535 are incompatible with Multicast DNS. However, since to-date


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   only three DNS classes have been assigned by IANA (1, 3 and 4),
   and only one (1, "Internet") is actually in widespread use, this
   limitation is likely to remain a purely theoretical one.

   No other IANA services are required by this document.


26. Acknowledgments

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

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


27. Copyright

   Copyright (C) The Internet Society 2005.
   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
   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.


28. Normative References

   [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.

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

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


29. Informative References

   [dotlocal] <http://www.dotlocal.org/>

   [djbdl]    <http://cr.yp.to/djbdns/dot-local.html>

   [DNS-SD]   Cheshire, S., and M. Krochmal, "DNS-Based Service
              Discovery", Internet-Draft (work in progress),
              draft-cheshire-dnsext-dns-sd-03.txt, June 2005.

   [IEEE802]  IEEE Standards for Local and Metropolitan Area Networks:
              Overview and Architecture.
              Institute of Electrical and Electronic Engineers,
              IEEE Standard 802, 1990.

   [NBP]      Cheshire, S., and M. Krochmal,
              "Requirements for a Protocol to Replace AppleTalk NBP",
              Internet-Draft (work in progress),
              draft-cheshire-dnsext-nbp-04.txt, June 2005.

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

   [RFC 2462] S. Thomson and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

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

   [RFC 3492] Costello, A., "Punycode: A Bootstring encoding of
              Unicode for use with Internationalized Domain Names
              in Applications (IDNA)", RFC 3492, March 2003.

   [RFC 3927] Cheshire, S., B. Aboba, and E. Guttman,
              "Dynamic Configuration of IPv4 Link-Local Addresses",
              RFC 3927, May 2005.

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








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30. Authors' Addresses

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

   Phone: +1 408 974 3207
   EMail: rfc (a] stuartcheshire.org


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

   Phone: +1 408 974 4368
   EMail: marc (a] apple.com

































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