Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                            Jul 11, 2013
Expires: January 12, 2014

          Hybrid Unicast/Multicast DNS-Based Service Discovery


   Performing DNS-Based Service Discovery using purely Multicast DNS
   allows discovery only of services present on 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
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   This Internet-Draft will expire on January 12, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal

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   Provisions Relating to IETF Documents
   ( 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions and Terminology Used in this Document . . . . . . . 3
   3.  Hybrid Proxy Operation  . . . . . . . . . . . . . . . . . . . . 4
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . . . 6
   5.  IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Intelectual Property Rights . . . . . . . . . . . . . . . . . . 7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     10.1.  Normative References . . . . . . . . . . . . . . . . . . . 8
     10.2.  Informative References . . . . . . . . . . . . . . . . . . 9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9

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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].

   Section 10 ("Populating the DNS with Information") of the DNS-SD
   specification [RFC6763] discusses possible ways that a service's PTR,
   SRV, TXT and address records can make their way into the DNS
   namespace, including manual zone file configuration [RFC1034]
   [RFC1035], DNS Update [RFC2136] [RFC3007] and proxies.

   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.

2.  Conventions and Terminology Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

   Multicast DNS works between a 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 TTL or otherwise
   modifies the IP header.

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3.  Hybrid Proxy Operation

   In its simplest form, each local link in an organization is assigned
   a unique Unicast DNS domain name, such as "Building"
   or "4th Floor.Building"  (Grouping multiple local
   links under the same 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.)

   Each link in an organization has a Hybrid Proxy which serves it.
   This 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, multiple
   links.  In the organization's DNS server, NS records are used to
   delegate ownership of each defined link name (e.g., "Building") to the Hybrid Proxy which serves that link.

   Domain Enumeration PTR records [RFC6763] are also created to inform
   clients of available Device Discovery domains, e.g.,:  PTR Building

   When a DNS-SD client issues a Unicast DNS query to discover services
   in a particular Unicast DNS (e.g., "_printer._tcp.Building  PTR ?") the normal DNS delegation mechanism results
   in that query being served from the delegated authoritative name
   server for that subdomain, namely the Hybrid Proxy on the link in
   question.  Although a Hybrid Proxy implements the usual Unicast DNS
   protocol, in contrast to a conventional Unicast DNS server that
   generates answers according to data in its manually-configured zone
   file, a Hybrid Proxy gets its data by performing a Multicast DNS
   query (e.g., "_printer._tcp.local.  PTR ?") on its local link, and
   then, from the Multicast DNS replies it receives, it generates a
   corresponding Unicast DNS reply.

   Generating the corresponding Unicast DNS reply involves, at the very
   least, rewriting the "local" suffix to the appropriate Unicast DNS
   domain (e.g., "Building").

   In addition it would be desirable to suppress Unicast DNS replies 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 [RFC1918] realms, 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 DNS names with DNS SRV
   records that reference target host names that have no addresses
   usable by the requester should be also be suppressed.

   The same reachability requirement for advertised services also
   applies to the Hybrid Proxy itself.  The mechanism specified in this
   document only works if the Hybrid Proxy is reachable from the client
   making the request.

   In a simple analysis, this simple approach is 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 replies it needs to form a
   complete Unicast DNS reply.  If it waits too little time, then it
   risks its Unicast DNS reply being incomplete.  If it waits too long,
   then it creates a poor user experience at the client end.

   This dilemma is solved by use of DNS Long-Lived Queries (DNS LLQ)
   [I-D.sekar-dns-llq].  The Hybrid Proxy replies 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 additional Multicast DNS records it does not already have
   in its cache (including the case where it has *no* appropriate
   records in its cache).  Should additional Multicast DNS replies be
   received, these are then delivered to the client using DNS LLQ update
   events.  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 reply to allow for the expected worst-case delay
   receiving a Multicast DNS reply.  Even in the event that a Multicast
   DNS device takes even longer than the expected worst-case time, its
   reply is not lost; it is delivered when it arrives, in the form of a
   subsequent DNS LLQ update.

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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 discovery 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 discovery 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

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4.3.  Not Yet Implemented

   The translating/filtering Hybrid Proxy specified in this document.
   Once implemented, such a Hybrid Proxy will immediately make wide-area
   discovery available with today's existing clients and devices.

   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.

6.  Security Considerations

   A service proves its presence on a local 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 instead authenticated secure DNS Update should be used
   [RFC2136] [RFC3007].

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].

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8.  IANA Considerations

   This document has no IANA Considerations.

9.  Acknowledgments

   [To be filled in later.]

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.

   [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.

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

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10.2.  Informative References

   [IPR2119]  "Apple Inc.'s Statement about IPR related to

   [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

   Phone: +1 408 974 3207

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