Internet DRAFT - draft-lynn-dnssd-requirements
draft-lynn-dnssd-requirements
DNS-SD/mDNS Extensions K. Lynn, Ed.
Internet-Draft Consultant
Intended status: Informational S. Cheshire
Expires: April 25, 2014 Apple, Inc.
October 22, 2013
Requirements for Scalable DNS-SD/mDNS Extensions
draft-lynn-dnssd-requirements-00
Abstract
DNS-SD/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.
Status of This Memo
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This Internet-Draft will expire on April 25, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Basic Use Cases . . . . . . . . . . . . . . . . . . . . . . . 5
4. Internationalization Considerations . . . . . . . . . . . . . 6
5. Namespace Considerations . . . . . . . . . . . . . . . . . . 6
6. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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.
However, as users move to multi-link home or campus networks they
find that mDNS 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. However, it is unclear which approach
represents the best long-term direction for DNS-based service
discovery protocol development.
DNS-SD/mDNS in its present form is also not optimized for network
technologies where multicast transmissions are relatively expensive.
Wireless networks such as [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 6LoWPAN
[RFC4944] are effectively multi-link subnets 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.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD 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].
1.2. Terminology [TBD]
Discovery Scope
Zero Configuration
Incremental Deployment
2. Problem Statement
Service discovery beyond the local link is perhaps the most important
feature currently missing from the DNS-SD/mDNS framework. The issues
and requirements are summarized below.
2.1. Multilink 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 technical
summary of the issues:
o Products that advertise services such as printing and multimedia
streaming via DNS-SD/mDNS are not currently discoverable by
devices on other links. It is common practice for enterprises and
institutions to use wireless links for client access and wired
networks for server infrastructure, typically on different
subnets. DNS-SD used with conventional unicast DNS does work when
devices are on different links, but the resource records that
describe the service must somehow be entered into the unicast DNS
namespace.
o Entering DNS-SD records manually into a unicast DNS zone file
works, (as has been done for many years for the Terminal Room
printers at IETF meetings) but requires the DNS administrator to
know how to do that [static] and is fragile when IP addresses of
devices may change, as is common when DHCP is used.
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o Automatically adding DNS-SD records using DNS Update works, but
requires that the DNS server be configured to allow DNS Updates,
and requires that devices be configured with the DNS Update
credentials to permit such updates, which has proven to be
onerous.
o Therefore, a mechanism is desired that populates the DNS namespace
with the appropriate DNS-SD records with less manual
administration than typically needed for a unicast DNS server.
The following is a technical summary of the requirements:
o It must scale to a range of hundreds or thousands of DNS-SD/mDNS
enabled devices in a given environment.
o It must work with wired and wireless networks from different
vendors.
o It must not significantly increase network traffic (wired or
wireless).
o It must be easily managed at an enterprise scale.
o It must be provided at a reasonable cost. [CapEx + OpEx. KEL]
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 Ethernet networks,
multicast frames place little additional demand on the shared network
medium above 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 convert it to a series of
link-layer unicast frames.
Wired links may be orders of magnitude less reliable than their wired
counterparts. To improve transmission reliability, the IEEE 802.11
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 much higher loss of multicast
frames on wireless as compared to 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.
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2.3. Low Power and Lossy Networks (LLNs)
Emerging wireless mesh networking technologies such as 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 single subnet prefix in a wireless mesh
network may often span multiple links, therefore a larger multicast
scope is required to span it [I-D.ietf-6man-multicast-scopes].
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.
3. Basic Use Cases
The following use cases are defined with different constraints to
help distinguish and classify the target requirements.
(A) Personal Area networks; e.g., one laptop and one printer.
This is the simplest example of a DNS-SD/mDNS network.
(B) Classic home networks, consisting of:
* Single exit router: the network may have multiple upstream
providers or networks, but all outgoing and incoming trafic
goes through a single router.
* One level depth: all links on the network are connected to the
same default router.
* Single administrative domain: all nodes under the same admin
entity.
(C) Advanced residential and small business networks
[I-D.ietf-homenet-arch]:
Like B but consist of two or more wired and/or wireless links,
connected by routers, behind the single exit router. However, the
forwarding nodes are largely self-configuring and do not require
routing protocol administration.
(D) Enterprise networks:
Like C but consist of arbitrary diameter under a single
administrative domain. A large majority of the forwarding and
security devices are configured.
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(E) Higher Education networks:
Like D but core network may be under a central administrative
domain while leaf networks are under local administrative domains.
(F) Mesh networks such as RPL/6LoWPAN:
Multi-link subnets with prefixes defined by one or more border
routers. May comprise network B and any part of networks C, D, or
E.
4. Internationalization Considerations
The solution should support rich international text, as do DNS-SD and
mDNS today. Users will not accept a solution that does not allow the
richness of service naming that they currently have with mDNS, manual
zone files, and DNS Update today.
5. Namespace Considerations
The unicast DNS namespace contains globally unique names. Naming
services over a local scope contain locally unique names. Clients
discovering services need to be able to differentiate global names
from local names.
6. Requirements
[This is a strawman proposal. MB]
REQ1: The scope of the discovery should be either automatically
found by the discovering devices and/or configured.
REQ2: For use cases A, B, and C, there should be a zero
configuration mode of operation.
REQ3: For use cases D and E, there should be a way to configure the
scope of the discovery and also support both smaller (ex:
department) and larger (ex: campus-wide) discovery scopes.
REQ4: For use cases D and E, there should be an incremental way to
deploy the solution.
REQ5: The new solution should integrate or at least should not break
any current link scope DNS-SD/mDNS protocols and deployments.
REQ6: The new solution MUST be capable of spanning multiple links
(hops) and network technologies.
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REQ7: The new solution MUST be scalable to thousands of servers with
minimal configuration and without degrading network performance.
REQ8: The new solution MUST provide a consistent user experience
whether local or global services are being discovered.
7. IANA Considerations
This document currently makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
8. Security Considerations
[Not complete - initial ideas. MB/KEL]
If the scope of the discovery is not properly setup or constrained,
then information leaks will happen outside the appropriate network.
Visiting nodes on a network may discover more services than desired
by the network policies, if filtering of discovery packets was not
properly setup. [Is this a NAC or DNS problem? KL]
Depending on the chosen solution, there is a possibility of name
space conflicts between the DNS tree and this solution. In this
case, a node may not know if the target node or service is the right
one, therefore enabling ground for various attacks.
The DNS-SD/mDNS framework security considerations also apply.
DNSSEC can assert the validity but not the veracity 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.
By contrast, the "plug-and-play" nature of mDNS devices has up to now
depended only on physical connectivity. If a device is visible via
mDNS then it is assumed to be trusted. This is no longer likely to
be the case in larger networks. Still, the new solution SHOULD
leverage existing security solutions and not invent new ones.
Mobile devices such as smart phones 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 of their operators. However, it
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SHOULD be possible to configure one or more "home" zones, e.g., based
on subnet prefix, in which mobile devices may automatically register
their services.
9. Acknowledgments
We gratefully acknowledge contributions and review comments made by
RJ Atkinson, Marc Blanchet, Tim Chown, Ralph Droms, Educause, David
Farmer, Matthew Gast, Peter Van Der Stok, and Thomas Narten.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC6550] Winter, T., Thubert, P., 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, March 2012.
[mDNS] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
February 2013.
[DNS-SD] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, February 2013.
10.2. Informative References
[I-D.ietf-6man-multicast-scopes]
Droms, R., "IPv6 Multicast Address Scopes", draft-ietf-
6man-multicast-scopes-00 (work in progress), August 2013.
[I-D.ietf-homenet-arch]
Chown, T., Arkko, J., Brandt, A., Troan, O., and J. Weil,
"Home Networking Architecture for IPv6", draft-ietf-
homenet-arch-10 (work in progress), August 2013.
[EP] "Educause Petition", https://www.change.org/petitions/
from-educause-higher-ed-wireless-networking-admin-group,
July 2012.
[IEEE.802.11]
"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 802.11-2012, 2012,
<http://standards.ieee.org/getieee802/download/
802.11-2012.pdf>.
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[static] "Manually Adding DNS-SD Service Discovery Records to an
Existing Name Server", July 2013,
<http://www.dns-sd.org/ServerStaticSetup.html>.
Authors' Addresses
Kerry Lynn (editor)
Consultant
Phone: +1 978 460 4253
Email: kerlyn@ieee.org
Stuart Cheshire
Apple, Inc.
1 Infinite Loop
Cupertino , California 95014
USA
Phone: +1 408 974 3207
Email: cheshire@apple.com
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