Internet DRAFT - draft-cheshire-dnssd-hybrid
draft-cheshire-dnssd-hybrid
Internet Engineering Task Force S. Cheshire
Internet-Draft Apple Inc.
Intended status: Standards Track January 22, 2014
Expires: July 26, 2014
Hybrid Unicast/Multicast DNS-Based Service Discovery
draft-cheshire-dnssd-hybrid-01
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.
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This Internet-Draft will expire on July 26, 2014.
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
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(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology Used in this Document . . . . . . 3
3. Hybrid Proxy Operation . . . . . . . . . . . . . . . . . . . . 4
4. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
5. IPv6 Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Intelectual Property Rights . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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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",
"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].
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 1.example.com."
or "4th Floor.Building 1.example.com." (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
1.example.com.") 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.,:
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.
lb._dns-sd._udp.example.com. PTR Building 1.example.com.
When a DNS-SD client issues a Unicast DNS query to discover services
in a particular Unicast DNS (e.g., "_printer._tcp.Building
1.example.com. 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. 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 by performing a Multicast DNS query
(e.g., "_printer._tcp.local. PTR ?") on its local link, and then,
from the data in the Multicast DNS replies it receives, generating
the corresponding Unicast DNS reply.
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3.1. Data Translation
Generating the corresponding Unicast DNS reply involves, at the very
least, rewriting the "local" suffix to the appropriate Unicast DNS
domain (e.g., "Building 1.example.com").
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 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
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.
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3.1.1. 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
put in 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 replies. 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 DNS-SD semantics, or of any DNS-SD
client semantics.
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3.2. Answer Aggregation
In a simple analysis, simply gathering multicast answers and
forwarding them in a unicast reply 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 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 any additional Multicast DNS records it did not already
know about. Should additional Multicast DNS replies 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 reply to allow for the expected worst-case delay
for receiving all the Multicast DNS replies. 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
replies. Using LLQs solves this dilemma: even very late replies are
not lost; they are delivered in subsequent LLQ update messages.
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There are two factors that determine specifically how replies are
generated. The first factor is whether the Hybrid Proxy already has
at least one record in its cache that positively answers the
question. The second factor is whether the query from the client
includes the LLQ option (typical with long-lived service browsing PTR
queries) or not (typical with one-shot operations like SRV or address
record queries).
o No answer in cache; no LLQ option: Do local mDNS query three
times, and then return NXDOMAIN if no answer after three tries.
o No answer in cache; with LLQ option: As above, do local mDNS query
three times, and then return NXDOMAIN if no answer after three
tries. However, 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. (Reasoning: We don't need to
rush to send an empty answer.)
o At least one answer in cache; no LLQ option: Send reply right away
to minimise delay. No local mDNS queries are performed.
(Reasoning: Given RRSet TTL harmonisation, if the proxy has one
answer in its cache, it should have all of them.)
o At least one answer in cache; with LLQ option: As above, send
reply right away to minimise delay. However, 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.)
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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
direction).
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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.
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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 something with stronger security should be used instead,
such as authenticated secure DNS Update [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].
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 reply.
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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.
[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
[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
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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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