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Versions: (draft-cheshire-dnssd-hybrid) 00 01 02 03 04 05 06 07 08 09 10

Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                       November 10, 2014
Expires: May 14, 2015


          Hybrid Unicast/Multicast DNS-Based Service Discovery
                       draft-ietf-dnssd-hybrid-00

Abstract

   Performing DNS-Based Service Discovery using purely link-local
   Multicast DNS enables discovery of services that are on the local
   link, but not (without some kind of proxy or similar special support)
   of services that are outside the local link.  Using a very large
   local link with thousands of hosts improves service discovery, but at
   the cost of large amounts of multicast traffic.

   Performing DNS-Based Service Discovery using purely Unicast DNS is
   more efficient, but requires configuration of DNS Update keys on the
   devices offering the services, which can be onerous for simple
   devices like printers and network cameras.

   Hence a compromise is needed, that provides easy service discovery
   without requiring either large amounts of multicast traffic or
   onerous configuration.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on May 14, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions and Terminology Used in this Document  . . . . . .  4
   3.  Hybrid Proxy Operation . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Domain Enumeration . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Delegated Subdomain for LDH Host Names . . . . . . . . . .  7
     3.3.  Delegated Subdomain for Reverse Mapping  . . . . . . . . .  9
     3.4.  Data Translation . . . . . . . . . . . . . . . . . . . . . 10
       3.4.1.  DNS TTL limiting . . . . . . . . . . . . . . . . . . . 10
       3.4.2.  Suppressing Unusable Records . . . . . . . . . . . . . 10
       3.4.3.  Application-Specific Data Translation  . . . . . . . . 11
     3.5.  Answer Aggregation . . . . . . . . . . . . . . . . . . . . 12
       3.5.1.  Discovery of LLQ Service . . . . . . . . . . . . . . . 14
   4.  Implementation Status  . . . . . . . . . . . . . . . . . . . . 15
     4.1.  Already Implemented and Deployed . . . . . . . . . . . . . 15
     4.2.  Partially Implemented  . . . . . . . . . . . . . . . . . . 15
     4.3.  Not Yet Implemented  . . . . . . . . . . . . . . . . . . . 16
   5.  IPv6 Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
     6.1.  Authenticity . . . . . . . . . . . . . . . . . . . . . . . 17
     6.2.  Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.3.  Denial of Service  . . . . . . . . . . . . . . . . . . . . 17
   7.  Intelectual Property Rights  . . . . . . . . . . . . . . . . . 18
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     10.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19










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1.  Introduction

   Multicast DNS [RFC6762] and its companion technology DNS-based
   Service Discovery [RFC6763] were created to provide IP networking
   with the ease-of-use and autoconfiguration for which AppleTalk was
   well known [RFC6760] [ZC].

   For a small network consisting of just a single link (or several
   physical links bridged together to appear as a single logical link to
   IP) Multicast DNS [RFC6762] is sufficient for client devices to look
   up the dot-local host names of peers on the same home network, and
   perform DNS-Based Service Discovery (DNS-SD) [RFC6763] of services
   offered on that home network.

   For a larger network consisting of multiple links that are
   interconnected using IP-layer routing instead of link-layer bridging,
   link-local Multicast DNS alone is insufficient because link-local
   Multicast DNS packets, by design, do not cross between links.
   (This was a deliberate design choice for Multicast DNS, since even on
   a single link multicast traffic is expensive -- especially on Wi-Fi
   links -- and multiplying the amount of multicast traffic by flooding
   it across multiple links would make that problem even worse.)
   In this environment, Unicast DNS would be preferable to Multicast
   DNS.  (Unicast DNS can be used either with a traditionally assigned
   globally unique domain name, or with a private local unicast domain
   name such as ".home" [HOME].)

   To use Unicast DNS, the names of hosts and services need to be made
   available in the Unicast DNS namespace.  In the DNS-SD specification
   [RFC6763] Section 10 ("Populating the DNS with Information")
   discusses various possible ways that a service's PTR, SRV, TXT and
   address records can make their way into the Unicast DNS namespace,
   including manual zone file configuration [RFC1034] [RFC1035],
   DNS Update [RFC2136] [RFC3007] and proxies of various kinds.

   This document specifies a type of proxy called a Hybrid Proxy that
   uses Multicast DNS [RFC6762] to discover Multicast DNS records on its
   local link, and makes corresponding DNS records visible in the
   Unicast DNS namespace.












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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", "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" [RFC2119].

   The Hybrid Proxy builds on Multicast DNS, which works between hosts
   on the same link.  A set of hosts is considered to be "on the same
   link" if:

   o  when any host A from that set sends a packet to any other host B
      in that set, using unicast, multicast, or broadcast, the entire
      link-layer packet payload arrives unmodified, and

   o  a broadcast sent over that link by any host from that set of hosts
      can be received by every other host in that set

   The link-layer *header* may be modified, such as in Token Ring Source
   Routing [802.5], but not the link-layer *payload*.  In particular, if
   any device forwarding a packet modifies any part of the IP header or
   IP payload then the packet is no longer considered to be on the same
   link.  This means that the packet may pass through devices such as
   repeaters, bridges, hubs or switches and still be considered to be on
   the same link for the purpose of this document, but not through a
   device such as an IP router that decrements the IP TTL or otherwise
   modifies the IP header.
























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3.  Hybrid Proxy Operation

   In its simplest form, each physical link in an organization is
   assigned a unique Unicast DNS domain name, such as
   "Building 1.example.com" or "4th Floor.Building 1.example.com".
   Grouping multiple links under a single Unicast DNS domain name is to
   be specified in a future companion document, but for the purposes of
   this document, assume that each link has its own unique Unicast DNS
   domain name.  In a graphical user interface these names are not
   displayed as strings with dots as shown above, but something more
   akin to a typical file browser graphical user interface (which is
   harder to illustrate in a text-only document) showing folders,
   subfolders and files in a file system.

   Each named link in an organization has a Hybrid Proxy which serves
   it.  This Hybrid Proxy function could be performed by a router on
   that link, or, with appropriate VLAN configuration, a single Hybrid
   Proxy could have a logical presence on, and serve as the Hybrid Proxy
   for, many links.  In the parent domain, NS records are used to
   delegate ownership of each defined link name
   (e.g., "Building 1.example.com") to the Hybrid Proxy that serves the
   named link.  In other words, the Hybrid Proxy is the authoritative
   name server for that subdomain.

   When a DNS-SD client issues a Unicast DNS query to discover services
   in a particular Unicast DNS subdomain
   (e.g., "_printer._tcp.Building 1.example.com. PTR ?") the normal DNS
   delegation mechanism results in that query being forwarded until it
   reaches the delegated authoritative name server for that subdomain,
   namely the Hybrid Proxy on the link in question.  Like a conventional
   Unicast DNS server, a Hybrid Proxy implements the usual Unicast DNS
   protocol [RFC1034] [RFC1035] over UDP and TCP.  However, unlike a
   conventional Unicast DNS server that generates answers from the data
   in its manually-configured zone file, a Hybrid Proxy generates
   answers using Multicast DNS.  A Hybrid Proxy does this by consulting
   its Multicast DNS cache and/or issuing Multicast DNS queries for the
   corresponding Multicast DNS name, type and class, (e.g., in this
   case, "_printer._tcp.local. PTR ?").  Then, from the received
   Multicast DNS data, the Hybrid Proxy synthesizes the appropriate
   Unicast DNS response.

   Naturally, the existing Multicast DNS caching mechanism is used to
   avoid issuing unnecessary Multicast DNS queries on the wire.  The
   Hybrid Proxy is acting as a client of the underlying Multicast DNS
   subsystem, and benefits from the same caching and efficiency measures
   as any other client using that subsystem.





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3.1.  Domain Enumeration

   The administrator creates Domain Enumeration PTR records [RFC6763] to
   inform clients of available service discovery domains, e.g.,:

       b._dns-sd._udp.example.com.    PTR   Building 1.example.com.
                                      PTR   Building 2.example.com.
                                      PTR   Building 3.example.com.
                                      PTR   Building 4.example.com.

       db._dns-sd._udp.example.com.   PTR   Building 1.example.com.

       lb._dns-sd._udp.example.com.   PTR   Building 1.example.com.

   The "b" ("browse") records tell the client device the list of
   browsing domains to display for the user to select from and the "db"
   ("default browse") record tells the client device which domain in
   that list should be selected by default.  The "lb" ("legacy browse")
   record tells the client device which domain to automatically browse
   on behalf of applications that don't implement UI for multi-domain
   browsing (which is most of them, today).  The "lb" domain is usually
   the same as the "db" domain.

   DNS responses are limited to a maximum size of 65535 bytes.  This
   limits the maximum number of domains that can be returned for a
   Domain Enumeration query, as follows:

   A DNS response header is 12 bytes.  That's typically followed by a
   single qname (up to 256 bytes) plus qtype (2 bytes) and qclass
   (2 bytes), leaving 65275 for the Answer Section.

   An Answer Section Resource Record consists of:
   o  Owner name, encoded as a two-byte compression pointer
   o  Two-byte rrtype (type PTR)
   o  Two-byte rrclass (class IN)
   o  Four-byte ttl
   o  Two-byte rdlength
   o  rdata (domain name, up to 256 bytes)

   This means that each Resource Record in the Answer Section can take
   up to 268 bytes total, which means that the Answer Section can
   contain, in the worst case, no more than 243 domains.

   In a more typical scenario, where the domain names are not all
   maximum-sized names, and there is some similarity between names so
   that reasonable name compression is possible, each Answer Section
   Resource Record may average 140 bytes, which means that the Answer
   Section can contain up to 466 domains.



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3.2.  Delegated Subdomain for LDH Host Names

   The rules for DNS-SD service instance names and domains are more
   permissive than the traditional rules for host names.

   Users typically interact with DNS-SD by viewing a list of discovered
   service instance names on the display and selecting one of them by
   pointing, touching, or clicking.  Similarly, in software that
   provides a multi-domain DNS-SD user interface, users view a list of
   offered domains on the display and select one of them by pointing,
   touching, or clicking.  To use a service, users don't have to
   remember domain or instance names, or type them; users just have to
   be able to recognize what they see on the display and click on the
   thing they want.

   In contrast, host names are often remembered and typed.  Also, host
   names are often used in command-line interfaces where spaces can be
   inconvenient.  For this reason, host names have traditionally been
   restricted to letters, digits and hyphens, with no spaces or other
   punctuation.

   While we still want to allow rich text for DNS-SD service instance
   names and domains, it is advisable, for maximum compatibility with
   existing software, to restrict host names to the traditional letter-
   digit-hyphen rules.  This means that while a service name
   "My Printer._ipp._tcp.Building 1.example.com" is acceptable and
   desirable (it is displayed in a graphical user interface as an
   instance called "My Printer" in the domain "Building 1" at
   "example.com"), a host name "My-Printer.Building 1.example.com" is
   not advisable (because of the space in "Building 1").

   To accomodate this difference in allowable characters, a Hybrid Proxy
   MUST support having two subdomains delegated to it, one to be used
   for host names (names of 'A' and 'AAAA' address records), which is
   restricted to the traditional letter-digit-hyphen rules, and another
   to be used for other records (including the PTR, SRV and TXT records
   used by DNS-SD), which is allowed to be arbitrary Net-Unicode text
   [RFC5198].













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   For example, a Hybrid Proxy could have the two subdomains
   "Building 1.example.com" and "bldg1.example.com" delegated to it.
   The Hybrid Proxy would then translate these two Multicast DNS
   records:

      My Printer._ipp._tcp.local. SRV 0 0 631 prnt.local.
      prnt.local.                 A   10.0.1.2

   into Unicast DNS records as follows:

      My Printer._ipp._tcp.Building 1.example.com.
                                  SRV 0 0 631 prnt.bldg1.example.com.
      prnt.bldg1.example.com.     A   10.0.1.2

   Note that the SRV record name is translated using the rich-text
   domain name ("Building 1.example.com") and the address record name is
   translated using the LDH domain ("bldg1.example.com").


































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3.3.  Delegated Subdomain for Reverse Mapping

   A Hybrid Proxy can facilitate easier management of reverse mapping
   domains, particularly for IPv6 addresses where manual management may
   be more onerous than it is for IPv4 addresses.

   To achieve this, in the parent domain, NS records are used to
   delegate ownership of the appropriate reverse mapping domain to the
   Hybrid Proxy.  In other words, the Hybrid Proxy becomes the
   authoritative name server for the reverse mapping domain.

   For example, if a given link is using the IPv4 subnet 10.1/16, then
   the domain "1.10.in-addr.arpa" is delegated to the Hybrid Proxy for
   that link.

   If a given link is using the IPv6 prefix 2001:0DB8/32, then the
   domain "8.b.d.0.1.0.0.2.ip6.arpa" is delegated to the Hybrid Proxy
   for that link.

   When a reverse mapping query arrives at the Hybrid Proxy, it issues
   the identical query on its local link as a Multicast DNS query.
   (In the Apple "/usr/include/dns_sd.h" APIs, using ForceMulticast
   indicates that the DNSServiceQueryRecord() call should perform the
   query using Multicast DNS.)  When the host owning that IPv4 or IPv6
   address responds with a name of the form "something.local", the
   Hybrid Proxy rewrites that to use its configured LDH host name domain
   instead of "local" and returns the response to the caller.

   For example, a Hybrid Proxy with the two subdomains
   "1.10.in-addr.arpa" and "bldg1.example.com" delegated to it would
   translate this Multicast DNS record:

      3.2.1.10.in-addr.arpa. PTR prnt.local.

   into this Unicast DNS response:

      3.2.1.10.in-addr.arpa. PTR prnt.bldg1.example.com.

   Subsequent queries for the prnt.bldg1.example.com address record,
   falling as it does within the bldg1.example.com domain, which is
   delegated to the Hybrid Proxy, will arrive at the Hybrid Proxy, where
   they are answered by issuing Multicast DNS queries and using the
   received Multicast DNS answers to synthesize Unicast DNS responses,
   as described above.







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3.4.  Data Translation

   Generating the appropriate Multicast DNS queries involves, at the
   very least, translating from the configured DNS domain
   (e.g., "Building 1.example.com") on the Unicast DNS side to "local"
   on the Multicast DNS side.

   Generating the appropriate Unicast DNS responses involves translating
   back from "local" to the configured DNS Unicast domain.

   Other beneficial translation and filtering operations are described
   below.

3.4.1.  DNS TTL limiting

   For efficiency, Multicast DNS typically uses moderately high DNS TTL
   values.  For example, the typical TTL on DNS-SD PTR records is 75
   minutes.  What makes these moderately high TTLs acceptable is the
   cache coherency mechanisms built in to the Multicast DNS protocol
   which protect against stale data persisting for too long.  When a
   service shuts down gracefully, it sends goodbye packets to remove its
   PTR records immediately from neighbouring caches.  If a service shuts
   down abruptly without sending goodbye packets, the Passive
   Observation Of Failures (POOF) mechanism described in Section 10.5 of
   the Multicast DNS specification [RFC6762] comes into play to purge
   the cache of stale data.

   A Unicast DNS client on a remote link does not get to participate in
   these Multicast DNS cache coherency mechanisms on the local link.
   For Unicast DNS requests received without any LLQ option the DNS TTLs
   reported in the resulting Unicast DNS response SHOULD be capped to be
   no more than ten seconds.  For received Unicast DNS requests that
   contain an LLQ option, the Multicast DNS record's TTL SHOULD be
   returned unmodified, because the LLQ notification channel exists to
   inform the remote client as records come and go.  For further details
   about the LLQ option, see Section 3.5.

3.4.2.  Suppressing Unusable Records

   A Hybrid Proxy SHOULD suppress Unicast DNS answers for records that
   are not useful outside the local link.  For example, DNS A and AAAA
   records for IPv4 link-local addresses [RFC3927] and IPv6 link-local
   addresses [RFC4862] should be suppressed.  Similarly, for sites that
   have multiple private address realms [RFC1918], private addresses
   from one private address realm should not be communicated to clients
   in a different private address realm.

   By the same logic, DNS SRV records that reference target host names



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   that have no addresses usable by the requester should be suppressed,
   and likewise, DNS PTR records that point to unusable SRV records
   should be similarly be suppressed.

3.4.3.  Application-Specific Data Translation

   There may be cases where Application-Specific Data Translation is
   appropriate.

   For example, AirPrint printers tend to advertise fairly verbose
   information about their capabilities in their DNS-SD TXT record.
   This information is a legacy from LPR printing, because LPR does not
   have in-band capability negotiation, so all of this information is
   conveyed using the DNS-SD TXT record instead.  IPP printing does have
   in-band capability negotiation, but for convenience printers tend to
   include the same capability information in their IPP DNS-SD TXT
   records as well.  For local mDNS use this extra TXT record
   information is inefficient, but not fatal.  However, when a Hybrid
   Proxy aggregates data from multiple printers on a link, and sends it
   via unicast (via UDP or TCP) this amount of unnecessary TXT record
   information can result in large responses.  Therefore, a Hybrid Proxy
   that is aware of the specifics of an application-layer protocol such
   as Apple's AirPrint (which uses IPP) can elide unnecessary key/value
   pairs from the DNS-SD TXT record for better network efficiency.

   Note that this kind of Application-Specific Data Translation is
   expected to be very rare.  It is the exception, rather than the rule.
   This is an example of a common theme in computing.  It is frequently
   the case that it is wise to start with a clean, layered design, with
   clear boundaries.  Then, in certain special cases, those layer
   boundaries may be violated, where the performance and efficiency
   benefits outweigh the inelegance of the layer violation.

   As in other similar situations, these layer violations optional.
   They are done only for efficiency reasons, and are not required for
   correct operation.  A Hybrid Proxy can operate solely at the mDNS
   layer, without any knowledge of semantics at the DNS-SD layer or
   above.













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3.5.  Answer Aggregation

   In a simple analysis, simply gathering multicast answers and
   forwarding them in a unicast response seems adequate, but it raises
   the question of how long the Hybrid Proxy should wait to be sure that
   it has received all the Multicast DNS answers it needs to form a
   complete Unicast DNS response.  If it waits too little time, then it
   risks its Unicast DNS response being incomplete.  If it waits too
   long, then it creates a poor user experience at the client end.  In
   fact, there may no time which is both short enough to produce a good
   user experience and at the same time long enough to reliably produce
   complete results.

   Similarly, the Hybrid Proxy -- the authoritative name server for the
   subdomain in question -- needs to decide what DNS TTL to report for
   these records.  If the TTL is too long then the recursive (caching)
   name servers issuing queries on behalf of their clients risk caching
   stale data for too long.  If the TTL is too short then the amount of
   network traffic will be more than necessary.  In fact, there may no
   TTL which is both short enough to avoid undesirable stale data and at
   the same time long enough to be efficient on the network.

   These dilemmas are solved by use of DNS Long-Lived Queries (DNS LLQ)
   [I-D.sekar-dns-llq].  The Hybrid Proxy responds immediately to the
   Unicast DNS query using the Multicast DNS records it already has in
   its cache (if any).  This provides a good client user experience by
   providing a near-instantaneous response.  Simultaneously, the Hybrid
   Proxy issues a Multicast DNS query on the local link to discover if
   there are any additional Multicast DNS records it did not already
   know about.  Should additional Multicast DNS responses be received,
   these are then delivered to the client using DNS LLQ update messages.
   The timeliness of such LLQ updates is limited only by the timeliness
   of the device responding to the Multicast DNS query.  If the
   Multicast DNS device responds quickly, then the LLQ update is
   delivered quickly.  If the Multicast DNS device responds slowly, then
   the LLQ update is delivered slowly.  The benefit of using LLQ is that
   the Hybrid Proxy can respond promptly because it doesn't have to
   delay its unicast response to allow for the expected worst-case delay
   for receiving all the Multicast DNS responses.  Even if a proxy were
   to try to provide reliability by assuming an excessively pessimistic
   worst-case time (thereby giving a very poor user experience) there
   would still be the risk of a slow Multicast DNS device taking even
   longer than that (e.g, a device that is not even powered on until ten
   seconds after the initial query is received) resulting in incomplete
   responses.  Using LLQs solves this dilemma: even very late responses
   are not lost; they are delivered in subsequent LLQ update messages.





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   There are two factors that determine specifically how responses are
   generated:

   The first factor is whether the query from the client included the
   LLQ option (typical with long-lived service browsing PTR queries) or
   not (typical with one-shot operations like SRV or address record
   queries).  Note that queries containing the LLQ option are received
   directly from the client (see Section 3.5.1).  Queries containing no
   LLQ option are generally received via the client's configured
   recursive (caching) name server.

   The second factor is whether the Hybrid Proxy already has at least
   one record in its cache that positively answers the question.

   o  No LLQ option; no answer in cache:
      Do local mDNS query up to three times, return answers if received,
      otherwise return negative response if no answer after three tries.
      DNS TTLs in responses are capped to at most ten seconds.

   o  No LLQ option; at least one answer in cache:
      Send response right away to minimise delay.
      DNS TTLs in responses are capped to at most ten seconds.
      No local mDNS queries are performed.
      (Reasoning: Given RRSet TTL harmonisation, if the proxy has one
      Multicast DNS answer in its cache, it can reasonably assume that
      it has all of them.)

   o  Query contains LLQ option; no answer in cache:
      As above, do local mDNS query up to three times, and return
      answers if received.
      If no answer after three tries, return negative response.
      (Reasoning: We don't need to rush to send an empty answer.)
      In both cases the query remains active for as long as the client
      maintains the LLQ state, and if mDNS answers are received later,
      LLQ update messages are sent.
      DNS TTLs in responses are returned unmodified.

   o  Query contains LLQ option; at least one answer in cache:
      As above, send response right away to minimise delay.
      The query remains active for as long as the client maintains the
      LLQ state, and if additional mDNS answers are received later, LLQ
      update messages are sent.
      (Reasoning: We want UI that is displayed very rapidly, yet
      continues to remain accurate even as the network environment
      changes.)
      DNS TTLs in responses are returned unmodified.

   Note that the "negative responses" referred to above are "no error no



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   answer" negative responses, not NXDOMAIN.  This is because the Hybrid
   Proxy cannot know all the Multicast DNS domain names that may exist
   on a link at any given time, so any name with no answers may have
   child names that do exist, making it an "empty nonterminal" name.

3.5.1.  Discovery of LLQ Service

   To issue LLQ queries, clients need to communicate directly with the
   authoritative Hybrid Proxy.  The procedure by which the client
   locates the authoritative Hybrid Proxy is described in the LLQ
   specification [I-D.sekar-dns-llq].

   Briefly, the procedure is as follows: To discover the LLQ service for
   a given domain name, a client first performs DNS zone apex discovery,
   and then, having discovered <apex>, the client then issues a DNS
   query for the SRV record with the name _dns-llq._udp.<apex> to find
   the target host and port for the LLQ service for that zone.  By
   default LLQ service runs on port 5352, but since SRV records are
   used, the LLQ service can be offered on any port.

   A client performs DNS zone apex discovery using the procedure below:

   1.  The client issues a DNS query for the SOA record with the given
       domain name.

   2.  A conformant recursive (caching) name server will either send a
       positive response, or a negative response containing the SOA
       record of the zone apex in the Authority Section.

   3.  If the name server sends a negative response that does not
       contain the SOA record of the zone apex, the client trims the
       first label off the given domain name and returns to step 1 to
       try again.

   By this method, the client iterates until it learns the name of the
   zone apex, or (in pathological failure cases) reaches the root and
   gives up.

   Normal DNS caching is used to avoid repetitive queries on the wire.












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4.  Implementation Status

   Some aspects of the mechanism specified in this document already
   exist in deployed software.  Some aspects are new.  This section
   outlines which aspects already exist and which are new.

4.1.  Already Implemented and Deployed

   Domain enumeration by the client (the "b._dns-sd._udp" queries) is
   already implemented and deployed.

   Unicast queries to the indicated discovery domain is already
   implemented and deployed.

   These are implemented and deployed in Mac OS X 10.4 and later
   (including all versions of Apple iOS, on all iPhone and iPads), in
   Bonjour for Windows, and in Android 4.1 "Jelly Bean" (API Level 16)
   and later.

   Domain enumeration and unicast querying have been used for several
   years at IETF meetings to make Terminal Room printers discoverable
   from outside the Terminal room.  When you Press Cmd-P on your Mac, or
   select AirPrint on your iPad or iPhone, and the Terminal room
   printers appear, that is because your client is doing unicast DNS
   queries to the IETF DNS servers.

4.2.  Partially Implemented

   The current APIs make multiple domains visible to client software,
   but most client UI today lumps all discovered services into a single
   flat list.  This is largely a chicken-and-egg problem.  Application
   writers were naturally reluctant to spend time writing domain-aware
   UI code when few customers today would benefit from it.  If Hybrid
   Proxy deployment becomes common, then application writers will have a
   reason to provide better UI.  Existing applications will work with
   the Hybrid Proxy, but will show all services in a single flat list.
   Applications with improved UI will group services by domain.

   The Long-Lived Query mechanism [I-D.sekar-dns-llq] referred to in
   this specification exists and is deployed, but has not been
   standardized by the IETF.  It is possible that the IETF may choose to
   standardize a different or better Long-Lived Query mechanism.  In
   that case, the pragmatic deployment approach would be for vendors to
   produce Hybrid Proxies that implement both the deployed Long-Lived
   Query mechanism [I-D.sekar-dns-llq] (for today's clients) and a new
   IETF Standard Long-Lived Query mechanism (as the future long-term
   direction).




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   The translating/filtering Hybrid Proxy specified in this document.
   Implementations are under development, and operational experience
   with these implementations has guided updates to this document.

4.3.  Not Yet Implemented

   A mechanism to 'stitch' together multiple ".local." zones so that
   they appear as one.  Such a mechanism will be specified in a future
   companion document.


5.  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.

   It will be desirable to have a mechanism to 'stitch' together these
   two unrelated ".local." zones so that they appear as one.  Such
   mechanism will need to be able to differentiate between a dual-stack
   (v4/v6) host participating in both ".local." zones, and two different
   hosts, one IPv4-only and the other IPv6-only, which are both trying
   to use the same name(s).  Such a mechanism will be specified in a
   future companion document.





















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6.  Security Considerations

6.1.  Authenticity

   A service proves its presence on a link by its ability to answer
   link-local multicast queries on that link.  If greater security is
   desired, then the Hybrid Proxy mechanism should not be used, and
   something with stronger security should be used instead, such as
   authenticated secure DNS Update [RFC2136] [RFC3007].

6.2.  Privacy

   The Domain Name System is, generally speaking, a global public
   database.  Records that exist in the Domain Name System name
   hierarchy can be queried by name from, in principle, anywhere in the
   world.  If services on a mobile device (like a laptop computer) are
   made visible via the Hybrid Proxy mechanism, then when those services
   become visibile in a domain such as "My House.example.com" that might
   indicate to (potentially hostile) observers that the mobile device is
   in my house.  When those services disappear from
   "My House.example.com" that change could be used by observers to
   infer when the mobile device (and possibly its owner) may have left
   the house.  The privacy of this information may be protected using
   techniques like firewalls and split-view DNS, as are customarily used
   today to protect the privacy of corporate DNS information.

6.3.  Denial of Service

   A remote attacker could use a rapid series of unique Unicast DNS
   queries to induce a Hybrid Proxy to generate a rapid series of
   corresponding Multicast DNS queries on one or more of its local
   links.  Multicast traffic is expensive -- especially on Wi-Fi links
   -- which makes this attack particularly serious.  To limit the damage
   that can be caused by such attacks, a Hybrid Proxy (or the underlying
   Multicast DNS subsystem which it utilizes) MUST implement Multicast
   DNS query rate limiting appropriate to the link technology in
   question.  For Wi-Fi links the Multicast DNS subsystem SHOULD NOT
   issue more than 20 Multicast DNS query packets per second.  On other
   link technologies like Gigabit Ethernet higher limits may be
   appropriate.











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7.  Intelectual Property Rights

   Apple has submitted an IPR disclosure concerning the technique
   proposed in this document.  Details are available on the IETF IPR
   disclosure page [IPR2119].


8.  IANA Considerations

   This document has no IANA Considerations.


9.  Acknowledgments

   Thanks to Markus Stenberg for helping develop the policy regarding
   the four styles of unicast response according to what data is
   immediately available in the cache.  Thanks to Andrew Yourtchenko for
   comments about privacy issues.  [Partial list; more names to be
   added.]


10.  References

10.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              May 2005.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.




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   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              December 2012.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, December 2012.

   [I-D.sekar-dns-llq]
              Sekar, K., "DNS Long-Lived Queries",
              draft-sekar-dns-llq-01 (work in progress), August 2006.

10.2.  Informative References

   [HOME]     Cheshire, S., "Special Use Top Level Domain 'home'",
              draft-cheshire-homenet-dot-home (work in progress),
              November 2014.

   [IPR2119]  "Apple Inc.'s Statement about IPR related to Hybrid
              Unicast/Multicast DNS-Based Service Discovery",
              <https://datatracker.ietf.org/ipr/2119/>.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, November 2000.

   [RFC6760]  Cheshire, S. and M. Krochmal, "Requirements for a Protocol
              to Replace the AppleTalk Name Binding Protocol (NBP)",
              RFC 6760, December 2012.

   [ZC]       Cheshire, S. and D. Steinberg, "Zero Configuration
              Networking: The Definitive Guide", O'Reilly Media, Inc. ,
              ISBN 0-596-10100-7, December 2005.


Author's Address

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

   Phone: +1 408 974 3207
   Email: cheshire@apple.com





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