Internet DRAFT - draft-lynn-mdnsext-requirements
draft-lynn-mdnsext-requirements
DNS-SD/mDNS Extensions K. Lynn, Ed.
Internet-Draft Consultant
Intended status: Informational S. Cheshire
Expires: January 17, 2014 Apple, Inc.
July 16, 2013
Requirements for DNS-SD/mDNS Extensions
draft-lynn-mdnsext-requirements-02
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 January 17, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Internationalization Considerations . . . . . . . . . . . . . 5
5. Namespace Considerations . . . . . . . . . . . . . . . . . . 5
6. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
DNS-based service discovery [DNS-SD]/[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 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's 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 are
effectively multi-link subnets where multicasts must be routed 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 DNS-SD/mDNS Extensions.
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2. Problem Statement
Service discovery beyond the local link is probably the most
important feature missing currently in the DNS-SD/mDNS framework.
The following describes some of the issues.
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 Current products advertising services such as printing and
multimedia streaming via DNS-SD/mDNS do not work when devices are
on different links. It is common for enterprise-grade wireless
and wired networks in the institutions to utilize different links.
DNS-SD used with conventional unicast DNS does work when devices
are on different links, but the records need to get into the
unicast DNS namespace somehow.
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 address of
devices may change, as is common when DHCP is used.
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 proved to be overly
onerous.
o Therefore, a mechanism is desired that populates the unicast DNS
namespace with the appropriate DNS-SD records with less manual
administration.
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 negatively impact network traffic (wired
or wireless).
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o It must be easily manageable at an enterprise scale.
o If it requires a separate hardware solution, the solution must be
enterprise grade (rack mountable, dual power supplies, etc.)
o It must be provided at a reasonable cost.
2.2. 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 is often very low and 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.
To improve transmission reliability, the IEEE 802.11 MAC requires
positive acknowledgement of unicast frames. It does not however,
require positive acknowledgement of multicast frames. As a result,
it is common to observe much higher loss of multicast frames on
802.11 than other IEEE 802 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 RPL/6LoWPAN
[RFC4944] [RFC6550] present several challenges for the current DNS-
SD/mDNS design. First, "local link" is defined as a node's one-hop
neighbors. This effectively means that a mesh is a multi-link
single-prefix subnet and that link-local multicast scope is
insufficient to span it.
Not only is subnet-scoped multicast difficult on such networks, but
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. Use Cases
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The following use cases are defined with different constraints to
help distinguish and classify the target requirements. [This is a
strawman proposal. MB]
(A) Personal Area networks, e.g., one laptop and one printer.
This is the simplest example of an mDNS network.
(B) 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) Like B but may have a tree of links behind the single exit
router. However, the forwarding nodes are almost self-configured
and do not require routing protocol administrators.
(D) Enterprise networks, consisting of:
* Any depth of the forwarding tree, under a single administrative
domain. The large majority of the forwarding and security
devices are configured.
(E) Higher Education networks, consisting of:
* Any depth of the forwarding tree, core network under a central
administrative domain but leaf networks under multiple
administrative entities. The large majority of the forwarding
and security devices are configured.
(F) Mesh networks such as RPL/6LoWPAN, multi-link but single
prefix networks.
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
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The unicast DNS namespace is (somewhat) global. Naming services over
a local scope is local. 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
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.
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.
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]
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.
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9. Acknowledgments
We gratefully acknowledge contributions and review comments made by
Marc Blanchet, Tim Chown, Ralph Droms, Educause, Matthew Gast, and
Thomas Narten.
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10. References
10.1. Normative References
[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
[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>.
[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
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Stuart Cheshire
Apple, Inc.
1 Infinite Loop
Cupertino , California 95014
USA
Phone: +1 408 974 3207
Email: cheshire@apple.com
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