Internet Engineering Task Force                              S. Cheshire
Internet-Draft                                                Apple Inc.
Intended status: Standards Track                            Jan 25, 2013
Expires: July 29, 2013

          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

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 29, 2013.

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
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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.  IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  Intelectual Property Rights . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

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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",
   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.

   By the same logic DNS SRV records that reference target host names
   that have *only* link-local addresses should be suppressed, and
   likewise, DNS PTR records that point to DNS names with DNS SRV
   records that reference target host names that have *only* link-local

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   addresses should be also be suppressed.

   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.

4.  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

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   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.

5.  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 teh Hybrid Proxy mechanism should not be
   used, and instead authenticated secure DNS Update should be used
   [RFC2136] [RFC3007].

6.  Intelectual Property Rights

   Apple may have patents or patent applications relating to the
   techniques described in this document.  Apple is working on preparing
   formal IETF IPR disclosures for such patents or patent applications,
   including licensing terms, which will be provided as soon as
   possible, but in the interest of expediency this proposal is being
   published so that its technical merits may be discussed independently
   of IPR licensing issues.

7.  IANA Considerations

   This document has no IANA Considerations.

8.  Acknowledgments

   [To be filled in later.]

9.  References

9.1.  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.

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

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   [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.

9.2.  Informative References

   [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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