Internet Engineering Task Force (IETF) K. Lynn
Request for Comments: 7558 Verizon Labs
Category: Informational S. Cheshire
ISSN: 2070-1721 Apple, Inc.
M. Blanchet
Viagenie
D. Migault
Ericsson
July 2015
Requirements for Scalable DNS-Based Service
Discovery (DNS-SD) / Multicast DNS (mDNS) Extensions
Abstract
DNS-based Service Discovery (DNS-SD) over Multicast DNS (mDNS) is
widely used today for discovery and resolution of services and names
on a local link, but there are use cases to extend DNS-SD/mDNS to
enable service discovery beyond the local link. This document
provides a problem statement and a list of requirements for scalable
DNS-SD.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7558.
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Copyright Notice
Copyright (c) 2015 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
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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Basic Use Cases . . . . . . . . . . . . . . . . . . . . . . . 6
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Namespace Considerations . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
Acknowedgements . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
DNS-based Service Discovery [DNS-SD] in combination with its
companion technology Multicast DNS [mDNS] is widely used today for
discovery and resolution of services and names on a local link. As
users move to multi-link home or campus networks, however, they find
that mDNS (by design) does not work across routers. DNS-SD can also
be used in conjunction with conventional unicast DNS to enable
wide-area service discovery, but this capability is not yet widely
deployed. This disconnect between customer needs and current
practice has led to calls for improvement, such as the Educause
petition [EP].
In response to this and similar evidence of market demand, several
products now enable service discovery beyond the local link using
different ad hoc techniques. As of yet, no consensus has emerged
regarding which approach represents the best long-term direction for
DNS-based Service Discovery protocol development.
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Multicast DNS in its present form is also not optimized for network
technologies where multicast transmissions are relatively expensive.
Wireless networks such as Wi-Fi [IEEE.802.11] may be adversely
affected by excessive mDNS traffic due to the higher network overhead
of multicast transmissions. Wireless mesh networks such as IPv6 over
Low-Power Wireless Personal Area Network (6LoWPAN) [RFC4944] are
effectively multi-link subnets [RFC4903] where multicasts must be
forwarded by intermediate nodes.
It is in the best interests of end users, network administrators, and
vendors for all interested parties to cooperate within the context of
the IETF to develop an efficient, scalable, and interoperable
standards-based solution.
This document defines the problem statement and gathers requirements
for scalable DNS-SD/mDNS extensions.
1.1. Terminology and Acronyms
Service: A listening endpoint (host and port) for a given application
protocol. Services are identified by Service Instance Names.
DNS-SD: DNS-based Service Discovery [DNS-SD] is a conventional
application of DNS resource records and messages to facilitate the
naming, discovery, and location of services. When used alone, the
term generally refers to the wide-area unicast protocol.
mDNS: Multicast DNS [mDNS] is a mechanism that facilitates
distributed DNS-like capabilities (including DNS-SD) on a local link
without need of traditional DNS infrastructure.
SSD: Scalable Service Discovery (or Scalable DNS-SD) is a future
extension of DNS-SD (and perhaps mDNS) that meets the requirements
set forth in this document.
Scope of Discovery: A subset of a local or global namespace, e.g., a
DNS subdomain, that is the target of a given SSD query.
Zero Configuration: A deployment of SSD that requires no
administration (although some administration may be optional).
Incremental Deployment: An orderly transition, as a network
installation evolves, from DNS-SD/mDNS to SSD.
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2. Problem Statement
Service discovery beyond the local link is perhaps the most important
feature currently missing from the DNS-SD-over-mDNS framework (also
written as "DNS-SD over mDNS" or "DNS-SD/mDNS"). Other issues and
requirements are summarized below.
2.1. Multi-link Naming and Discovery
A list of desired DNS-SD/mDNS improvements from network
administrators in the research and education community was issued in
the form of the Educause petition [EP]. The following is a summary
of their technical issues:
o It is common practice for enterprises and institutions to use
wireless links for client access and wired links for server
infrastructure; typically, they are on different subnets.
Products that advertise services such as printing and multimedia
streaming via DNS-SD over mDNS are not currently discoverable by
client devices on other links. DNS-SD used with conventional
unicast DNS does work when servers and clients are on different
links, but the resource records that describe the services must
somehow be entered into the unicast DNS namespace.
o DNS-SD resource records may be entered manually into a unicast DNS
zone file [STATIC], but this task must be performed by a DNS
administrator. It is labor intensive and brittle when IP
addresses of devices change dynamically, as is common when DHCP is
used.
o Automatically adding DNS-SD records using DNS Update works, but it
requires that the DNS server be configured to allow DNS Updates
and that devices be configured with the DNS Update credentials to
permit such updates, which has proven to be onerous.
Therefore, a mechanism is desired that populates the DNS namespace
with the appropriate DNS-SD records with less manual administration
than is typically needed for a conventional unicast DNS server.
The following is a summary of technical requirements:
o It must scale to a range of hundreds to thousands of DNS-SD/mDNS-
enabled devices in a given environment.
o It must simultaneously operate over a variety of network link
technologies, such as wired and wireless networks.
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o It must not significantly increase network traffic (wired or
wireless).
o It must be cost-effective to manage at up to enterprise scale.
2.2. IEEE 802.11 Wireless LANs
Multicast DNS was originally designed to run on Ethernet - the
dominant link layer at the time. In shared-medium Ethernet networks,
multicast frames place little additional demand on the shared network
medium compared to unicast frames. In IEEE 802.11 networks, however,
multicast frames are transmitted at a low data rate supported by all
receivers. In practice, this data rate leads to a larger fraction of
airtime being devoted to multicast transmission. Some network
administrators block multicast traffic or use access points that
transmit multicast frames using a series of link-layer unicast
frames.
Wireless links may be orders of magnitude less reliable than their
wired counterparts. To improve transmission reliability, the IEEE
802.11 Medium Access Control (MAC) requires positive acknowledgement
of unicast frames. It does not, however, support positive
acknowledgement of multicast frames. As a result, it is common to
observe higher loss rates of multicast frames on wireless network
technologies than on wired network technologies.
Enabling service discovery on IEEE 802.11 networks requires that the
number of multicast frames be restricted to a suitably low value or
replaced with unicast frames to use the MAC's reliability features.
2.3. Low-Power and Lossy Networks (LLNs)
Emerging wireless mesh networking technologies such as the Routing
Protocol for LLNs (RPL) [RFC6550] and 6LoWPAN present several
challenges for the current DNS-SD/mDNS design. First, link-local
multicast scope [RFC4291] is defined as a single-hop neighborhood. A
wireless mesh network representing a single logical subnet may often
extend to multiple hops [RFC4903]; therefore, a larger multicast
scope is required to span it [RFC7346]. Multicast DNS was
intentionally not specified for greater than link-local scope because
of the additional off-link multicast traffic that it would generate.
Additionally, low-power nodes may be offline for significant periods
either because they are "sleeping" or due to connectivity problems.
In such cases, LLN nodes might fail to respond to queries or defend
their names using the current design.
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3. Basic Use Cases
The following use cases are defined with different characteristics to
help motivate, distinguish, and classify the target requirements.
They cover a spectrum of increasing deployment and administrative
complexity.
(A) Personal Area Networks (PANs): The simplest example of a
network may consist of a single client and server, e.g., one
laptop and one printer, on a common link. PANs that do not
contain a router may use Zero Configuration Networking [ZC] to
self-assign link-local addresses [RFC3927] [RFC4862] and Multicast
DNS [mDNS] to provide naming and service discovery, as is
currently implemented and deployed in Mac OS X, iOS, Windows
[B4W], and Android [NSD].
(B) Classic home or 'hotspot' networks, with the following
properties:
* Single exit router: The network may have one or more upstream
providers or networks, but all outgoing and incoming traffic
goes through a single router.
* One-level depth: A single physical link, or multiple physical
links bridged to form a single logical link, that is connected
to the default router. The single logical link provides a
single broadcast domain, facilitating use of link-local
Multicast DNS, and also ARP, which enables the home or
'hotspot' network to consist of just a single IPv4 subnet.
* Single administrative domain: All nodes under the same
administrative authority. Note that this does not necessarily
imply a network administrator.
(C) Advanced home and small business networks [RFC7368]:
Like B, but consists of multiple wired and/or wireless links,
connected by routers, generally behind a single exit router.
However, the forwarding nodes are largely self-configuring and do
not require routing protocol administration. Such networks should
also not require DNS administration.
(D) Enterprise networks:
Consists of arbitrary network diameter under a single
administrative authority. A large majority of the forwarding and
security devices are configured, and multiple exit routers are
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more common. Large-scale conference-style networks, which are
predominantly wireless access, e.g., as available at IETF
meetings, also fall within this category.
(E) Higher-Education networks:
Like D, but the core network may be under a central administrative
authority while leaf networks are under local administrative
authorities.
(F) Mesh networks such as RPL/6LoWPAN:
Multi-link subnets with prefixes defined by one or more border
routers. May comprise any part of networks C, D, or E.
4. Requirements
Any successful SSD solution(s) will have to strike the proper balance
between competing goals such as scalability, deployability, and
usability. With that in mind, none of the requirements listed below
should be considered in isolation.
REQ1: For use cases A, B, and C, there should be a Zero
Configuration mode of operation. This implies that servers
and clients should be able to automatically determine a
default scope of discovery in which to advertise and discover
services, respectively.
REQ2: For use cases C, D, and E, there should be a way to configure
scopes of discovery that support a range of topologically
independent zones (e.g., from department to campus wide).
This capability must exist in the protocol; individual
operators are not required to use this capability in all
cases -- in particular, use case C should support Zero
Configuration operation where that is desired. If multiple
scopes are available, there must be a way to enumerate the
choices from which a selection can be made. In use case C,
either Zero Configuration (one flat list of resources) or
configured (e.g., resources sorted by room) modes of
operation should be available.
REQ3: As stated in REQ2 above, the discovery scope need not be
aligned to network topology. For example, it may instead be
aligned to physical proximity (e.g., building) or
organizational structure (e.g., "Sales" vs. "Engineering").
REQ4: For use cases C, D, and E, there should be an incremental way
to deploy the solution.
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REQ5: SSD should leverage and build upon current link scope DNS-SD/
mDNS protocols and deployments.
REQ6: SSD must not adversely affect or break any other current
protocols or deployments.
REQ7: SSD must be capable of operating across networks that are not
limited to a single link or network technology, including
clients and services on non-adjacent links.
REQ8: It is desirable that a user or device be able to discover
services within the sites or networks to which the user or
device is connected.
REQ9: SSD should operate efficiently on common link layers and link
types.
REQ10: SSD should be considerate of networks where power consumption
is a critical factor; for example, nodes may be in a low-
power or sleeping state.
REQ11: SSD must be scalable to thousands of nodes with minimal
configuration and without degrading network performance. A
possible figure of merit is that, as the number of services
increases, the amount of traffic due to SSD on a given link
remains relatively constant.
REQ12: SSD should enable a way to provide a consistent user
experience whether local or remote services are being
discovered.
REQ13: The information presented by SSD should closely reflect the
current state of discoverable services on the network. That
is, new information should be available within a few seconds
and stale information should not persist indefinitely. In
networking, all information is necessarily somewhat out of
date by the time it reaches the receiver, even if only by a
few microseconds or less. Thus, timeliness is always an
engineering trade-off against efficiency. The engineering
decisions for SSD should appropriately balance timeliness
against network efficiency.
REQ14: SSD should operate over existing networks (as described by
use cases A through F above) without requiring changes to the
network at the physical, link, or internetworking layers.
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REQ15: The administrator of an advertised service should be able to
control whether the service is advertised beyond the local
link.
5. Namespace Considerations
The traditional unicast DNS namespace contains, for the most part,
globally unique names. Multicast DNS provides every link with its
own separate link-local namespace, where names are unique only within
the context of that link. Clients discovering services may need to
differentiate between local and global names and may need to
determine when names in different namespaces identify the same
service.
Devices on different links may have the same mDNS name (perhaps due
to vendor defaults) because link-local mDNS names are only guaranteed
to be unique on a per-link basis. This may lead to a local label
disambiguation problem when results are aggregated (e.g., for
presentation).
SSD should support rich internationalized labels within Service
Instance Names, as DNS-SD/mDNS does today. SSD must not negatively
impact the global DNS namespace or infrastructure.
The problem of publishing local services in the global DNS namespace
may be generally viewed as exporting local resource records and their
associated labels into some DNS zone. The issues related to defining
labels that are interoperable between local and global namespaces are
discussed in a separate document [INTEROP-LABELS].
6. Security Considerations
Insofar as SSD may automatically gather DNS-SD resource records and
publish them over a wide area, the security issues are likely to
include the union of those discussed in the Multicast DNS [mDNS] and
DNS-based Service Discovery [DNS-SD] specifications. The following
sections highlight potential threats that are posed by deploying DNS-
SD over multiple links or by automating DNS-SD administration.
6.1. Scope of Discovery
In some scenarios, the owner of the advertised service may not have a
clear indication of the scope of its advertisement.
For example, since mDNS is currently restricted to a single link, the
scope of the advertisement is limited, by design, to the shared link
between client and server. If the advertisement propagates to a
larger set of links than expected, this may result in unauthorized
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clients (from the perspective of the owner) discovering and then
potentially attempting to connect to the advertised service. It also
discloses information (about the host and service) to a larger set of
potential attackers.
Note that discovery of a service does not necessarily imply that the
service is reachable by, or can be connected to, or can be used by, a
given client. Specific access-control mechanisms are out of scope of
this document.
If the scope of the discovery is not properly set up or constrained,
then information leaks will happen outside the appropriate network.
6.2. Multiple Namespaces
There is a possibility of conflicts between the local and global DNS
namespaces. Without adequate feedback, a discovering client may not
know if the advertised service is the correct one, therefore enabling
potential attacks.
6.3. Authorization
DNSSEC can assert the validity but not the accuracy of records in a
zone file. The trust model of the global DNS relies on the fact that
human administrators either (a) manually enter resource records into
a zone file or (b) configure the DNS server to authenticate a trusted
device (e.g., a DHCP server) that can automatically maintain such
records.
An impostor may register on the local link and appear as a legitimate
service. Such "rogue" services may then be automatically registered
in unicast DNS-SD.
6.4. Authentication
Up to now, the "plug-and-play" nature of mDNS devices has relied only
on physical connectivity. If a device is visible via mDNS, then it
is assumed to be trusted. This is not likely to be the case in
foreign networks.
If there is a risk that clients may be fooled by the deployment of
rogue services, then application-layer authentication should be
considered as part of any security solution. Authentication of any
particular service is outside the scope of this document.
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6.5. Access Control
Access Control refers to the ability to restrict which users are able
to use a particular service that might be advertised via DNS-SD. In
this case, "use" of a service is different from the ability to
"discover" or "reach" a service.
While controlling access to an advertised service is outside the
scope of DNS-SD, we note that access control today often is provided
by existing site infrastructure (e.g., router access-control lists,
firewalls) and/or by service-specific mechanisms (e.g., user
authentication to the service). For example, networked printers can
control access via a user ID and password. Apple's software supports
such access control for USB printers shared via Mac OS X Printer
Sharing, as do many networked printers themselves. So the reliance
on existing service-specific security mechanisms (i.e., outside the
scope of DNS-SD) does not create new security considerations.
6.6. Privacy Considerations
Mobile devices such as smart phones or laptops that can expose the
location of their owners by registering services in arbitrary zones
pose a risk to privacy. Such devices must not register their
services in arbitrary zones without the approval ("opt-in") of their
users. However, it should be possible to configure one or more
"safe" zones in which mobile devices may automatically register their
services.
7. References
7.1. Normative References
[DNS-SD] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
[mDNS] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
DOI 10.17487/RFC3927, May 2005,
<http://www.rfc-editor.org/info/rfc3927>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
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[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<http://www.rfc-editor.org/info/rfc4862>.
[RFC4903] Thaler, D., "Multi-Link Subnet Issues", RFC 4903,
DOI 10.17487/RFC4903, June 2007,
<http://www.rfc-editor.org/info/rfc4903>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<http://www.rfc-editor.org/info/rfc4944>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<http://www.rfc-editor.org/info/rfc6550>.
[RFC7346] Droms, R., "IPv6 Multicast Address Scopes", RFC 7346,
DOI 10.17487/RFC7346, August 2014,
<http://www.rfc-editor.org/info/rfc7346>.
[RFC7368] Chown, T., Ed., Arkko, J., Brandt, A., Troan, O., and J.
Weil, "IPv6 Home Networking Architecture Principles", RFC
7368, DOI 10.17487/RFC7368, October 2014,
<http://www.rfc-editor.org/info/rfc7368>.
7.2. Informative References
[B4W] "Bonjour (software)",
<http://en.wikipedia.org/wiki/Bonjour_(software)>.
[EP] Badman, L., "Petitioning Apple: From Educause Higher Ed
Wireless Networking Admin Group", July 2012,
<https://www.change.org/p/from-educause-higher-ed-
wireless-networking-admin-group>.
[IEEE.802.11]
IEEE Computer Society, "IEEE Standard for Information
technology - Telecommunications and information exchange
between systems Local and metropolitan area networks -
Specific requirements Part 11: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) Specifications",
IEEE Std 802.11,
<http://standards.ieee.org/about/get/802/802.11.html>.
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[INTEROP-LABELS]
Sullivan, A., "On Interoperation of Labels Between mDNS
and DNS", Work in Progress,
draft-sullivan-dnssd-mdns-dns-interop-01, October 2014.
[NSD] Android, "NsdManager",
<http://developer.android.com/reference/android/net/nsd/
NsdManager.html>.
[STATIC] "Manually Adding DNS-SD Service Discovery Records to an
Existing Name Server", July 2013,
<http://www.dns-sd.org/ServerStaticSetup.html>.
[ZC] Cheshire, S. and D. Steinberg, "Zero Configuration
Networking: The Definitive Guide", O'Reilly Media, Inc.,
ISBN 0-596-10100-7, December 2005.
Acknowedgements
We gratefully acknowledge contributions and review comments made by
RJ Atkinson, Tim Chown, Guangqing Deng, Ralph Droms, Educause, David
Farmer, Matthew Gast, Thomas Narten, Doug Otis, David Thaler, and
Peter Van Der Stok.
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Authors' Addresses
Kerry Lynn
Verizon Labs
50 Sylvan Road
Waltham, MA 95014
United States
Phone: +1 781 296 9722
Email: kerry.lynn@verizon.com
Stuart Cheshire
Apple, Inc.
1 Infinite Loop
Cupertino, CA 95014
United States
Phone: +1 408 974 3207
Email: cheshire@apple.com
Marc Blanchet
Viagenie
246 Aberdeen
Quebec, QC G1R 2E1
Canada
Email: Marc.Blanchet@viagenie.ca
URI: http://viagenie.ca
Daniel Migault
Ericsson
8400 Boulevard Decarie
Montreal, QC H4P 2N2
Canada
Phone: +1 514 452 2160
Email: daniel.migault@ericsson.com
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