Internet DRAFT - draft-bhandari-dnssd-mdns-gateway
draft-bhandari-dnssd-mdns-gateway
dnssd S. Bhandari
Internet-Draft B. Fajalia
Intended status: Informational R. Schmieder
Expires: April 23, 2014 S. Orr
A. Dutta
Cisco
October 20, 2013
Extending multicast DNS across local links in Campus and Enterprise
networks
draft-bhandari-dnssd-mdns-gateway-00
Abstract
This document describes the requirements for extending multicast DNS
in enterprise networks. It provides an overview of a solution to
extend multicast DNS services across links that have been implemented
in routers, switches and wireless LAN controllers.
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 RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 23, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
Bhandari, et al. Expires April 23, 2014 [Page 1]
Internet-Draft mDNS across local links in campus network October 2013
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
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology Used in this Document . . . . . . 4
3. Solution overview . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Service Cache . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Service Filters . . . . . . . . . . . . . . . . . . . . . 6
3.3. Service Announcement . . . . . . . . . . . . . . . . . . . 6
3.4. Service Query . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Service Probing . . . . . . . . . . . . . . . . . . . . . 7
3.6. Service update, Service withdrawal . . . . . . . . . . . . 7
3.7. Service Refresh . . . . . . . . . . . . . . . . . . . . . 7
3.8. mDNS Gateway for Wireless Network . . . . . . . . . . . . 8
3.8.1. Advertising services on wireless networks . . . . . . 8
3.8.2. Device Tracking . . . . . . . . . . . . . . . . . . . 8
3.8.3. Mobility Considerations . . . . . . . . . . . . . . . 9
3.8.4. mDNS traffic optimization . . . . . . . . . . . . . . 9
4. Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Future work . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Bhandari, et al. Expires April 23, 2014 [Page 2]
Internet-Draft mDNS across local links in campus network October 2013
1. Introduction
Service discovery using multicast DNS (mDNS) as defined in [RFC6762]
is limited in scope to L3 boundaries due to the use of link-local
scoped multicast addresses. Networks are partitioned into multiple
segments by means of virtual local area networks (VLANs) or subnet
creation for various reasons. The need for network wide, seamless
service discovery demands the extension of the discovery protocol
beyond the L3 boundary. There are also challenges in wireless
networks (802.11, 802.15.4 etc) where a large number multicast
messages can impact wireless performance.
Enabling Service Discovery across L3 boundaries can be accomplished
in one of the following ways using existing, unmodified protocols:
1. Unicast DNS-SD only: Use of DNS servers and allowing clients to
use dynamic DNS updates and Long Lived Queries (LLQs) to announce
and learn services dynamically [I-D.sekar-dns-llq]
2. mDNS only: Defining a mDNS gateway entity at the L3 boundaries
extending service advertisements/discovery across the links it is
attached to
3. Combination of unicast DNS and mDNS - Hybrid proxy approach as
described in [I-D.cheshire-mdnsext-hybrid]
4. mDNS utilizing extended scope multicast - Modifying mDNS to use a
wider scope multicast address for message exchange
As a first step, this draft lists out the approach to use a mDNS
gateway on a network element (2) to extend the service advertisement/
discovery across network segments attached to the element. While
this approach does not preclude (1) or (3), it allows the extension
of service discovery in a limited number of segments with minimal
provisioning. Approach (4) is not explored further as it would add
to the flood of service discovery messages in the scope defined by
the multicast address and it would also require changes on mDNS
clients, which is undesirable.
1.1. Requirements
This section describes requirements for extending multicast DNS in an
enterprise environment:
1. Extend service discovery across L3 boundaries
2. Defining and enforcing a policy to selectively filter services
that are to be extended based on service type, service instance,
Bhandari, et al. Expires April 23, 2014 [Page 3]
Internet-Draft mDNS across local links in campus network October 2013
location of the provider/user, role of the device or user
accessing/offering the service.
3. Defining and enforcing a policy to selectively filter queries
and announcements from specific clients or over specific network
links
4. Filtering of link-local-only information - Services that resolve
to IPv4 and IPv6 link-local addresses only must not be extended
beyond the local link. Suppression of resource records that
contain link-local-only addresses from propagation beyond the
local link
5. Optimization of mDNS queries/advertisements in wireless networks
6. Effectively handle roaming of mobile devices (changes in the
Point of Attachment). Especially if those devices advertise
services
7. Limit the services in response to queries with a subset of the
services by geographic proximity
8. Handle conflict resolution of service instances and host names
across the links where the service is extended
9. Protection of resources (memory and CPU) of the network element
that participates in extending multicast DNS
10. Audit, logging of services that are denied based on policy
2. Conventions and Terminology Used in this Document
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].
This document uses the multicast DNS and DNS terminology conventions
from [RFC6762] and [RFC6763]. It uses the same convention for
services on the same link as defined in
[I-D.cheshire-mdnsext-hybrid], repeated here for quick reference:
Multicast DNS works between a hosts on the same link. A set of hosts
is considered to be "on the same link", if:
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-
Bhandari, et al. Expires April 23, 2014 [Page 4]
Internet-Draft mDNS across local links in campus network October 2013
layer packet payload arrives unmodified, and
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.
o mDNS gateway - An application that listens to services and extends
the services across links
3. Solution overview
The solution introduces the mDNS gateway function which is co-located
on the network element that connects to multiple links, typically an
IP router. The mDNS gateway function will be responsible for:
o Caching - Learn and cache services. Maintain services in the
cache according to service announcements, service removals and the
TTL of the records.
o Respond to queries - Advertise in response to queries with
services in the cache that are not in the same link-local domain
where the query is received.
o Service filtering - Filter services to be added to the cache and
to be included in the advertisements as per configured policies.
o Service redistribution - forwarding of unsolicited service
announcements across links based on configuration
o Active query - Service queries sent by the mDNS gateway itself to
learn about services or keep services 'fresh' in the cache. Can
be sent on one or more of the links the gateway is attached to.
3.1. Service Cache
The mDNS gateway maintains a database of DNS Resource Records (RR)
required to advertise and resolve services. At a minimum, the cache
will contain PTR, SRV,TXT and A/AAAA RRs for each service with NSEC
Bhandari, et al. Expires April 23, 2014 [Page 5]
Internet-Draft mDNS across local links in campus network October 2013
RR support for optimization. In addition, the link on which the
service and host originate is also maintained in the cache. Records
in the cache are refreshed based on TTL expiry.
3.2. Service Filters
A service filtering policy is configured with an action to permit or
deny services into the cache or to filter services included in the
response/advertisement messages based on matching criteria. The
matching criteria can be defined based on:
o Service type
o Service instance names
o Link on which the message is received
o Type of message - query or advertisement
o Location of the host querying or advertising a service
A Service filtering policy is applied for incoming and outgoing
messages. A unique filtering policy can be applied Globally or per
link.
When a mDNS message is received by the mDNS gateway matching an
action set for the link, the policy match is then executed. The
incoming advertisement is processed against the mDNS gateway inbound
filtering policy applied on the link where the advertisement is
received. If the action is 'permit' the service is added to the
cache. If a response or advertisement is to be sent out, the
outbound filtering policy applied applied on the interface is
processed and if the resulting action is 'deny' then the service and
its corresponding RRs are not included in the message sent out.
3.3. Service Announcement
The mDNS gateway listens for all service announcements. When a
service announcement is received, the announcement and all the
additional RRs learnt are added to the cache or ignored based on the
result of the configured inbound filter policy.
The RRs containing link-local information e.g. A or AAAA RRs that
contain link-local scoped IPv4 or IPv6 addresses are not stored in
the cache.
When the mDNS gateway learns a service it can also forward the
advertisement on other attached links.
Bhandari, et al. Expires April 23, 2014 [Page 6]
Internet-Draft mDNS across local links in campus network October 2013
3.4. Service Query
The mDNS gateway processes all queries against the configured
filtering policy. If the response to the query is permitted then it
constructs the answers and additional records required to resolve the
service from its cache for the services that are permitted. Services
that reside on the same link where the query is received are not
included as the owner of the service will also see the query and
would send the response directly. Only services learnt from
different links are considered in the response.
Any query received for additional RRs to resolve the service e.g.
query for SRV, A, AAAA etc are responded to in the same way. If the
records do not exist in the cache due to expiry or purging of cache
for any other reason, mDNS gateway sends out an explicit query to
fetch the records on the link where the service resides.
3.5. Service Probing
According to [RFC6762] before registering a service, RR probing is
performed to ensure unique names. As the mDNS gateway maintains a
cache of all the RRs that are extended across the links it responds
to probe records like any other query. This will help in detecting
and resolving name space conflicts across links where service
discovery has been extended.
3.6. Service update, Service withdrawal
When the mDNS gateway receives a service update or withdrawal it
updates or removes the service and all corresponding records from its
cache. It forwards the withdraw messages to other attached links.
3.7. Service Refresh
The RRs describing the service and resolving it have a TTL that
defines the validity of the RR. The mDNS gateway can continuously
refresh each of the RRs in the cache as per the TTL rules. For the
purpose of optimization, the mDNS gateway can rely on the host
interested in the RRs to trigger a refresh by setting the TTLs in the
response to the time remaining since the record was learnt by the
mDNS gateway. If a client is interested in the RR then it would
trigger a refresh when a fraction of the TTL is reached. While
responding to queries from hosts, the mDNS gateway inturn sends out
queries to refresh the records that are about to expire on the source
link where the records were learnt.
Bhandari, et al. Expires April 23, 2014 [Page 7]
Internet-Draft mDNS across local links in campus network October 2013
3.8. mDNS Gateway for Wireless Network
Deploying the mDNS gateway in wireless networks has a few additional
requirements w.r.t to multicast radio optimization and mobility
aspects. This section describes some additional capabilities added
to the mDNS gateway to satisfy these requirements.
3.8.1. Advertising services on wireless networks
In order to conserve wireless bandwidth, the mDNS gateway sends
service advertisements as L2 unicast messages to wireless devices .
In a wireless network, the mDNS gateway co-located on the network
element that is providing the wireless service can act as a passive
device and respond only if wireless clients send a mDNS query. When
bridging is turned off the mDNS gateway and the Layer 2 optimazation
is enabled, the mDNS gateway will need to send the query responseas
layer 2 unicast messages even when the provider is on the same link
as the requestor. Bridging of mDNS messages can be turned off based
on configuration. This is useful in the following scenario:
1. Save computation resources on the device which are used to
replicate the multicast packet as a L2 unicast for all wireless
clients.
2. If the wireless client is in power saving mode, sending a mDNS
advertisement as a L2 unicast would forcefully awake the client
and it would result into more power consumption by the wireless
client.
mDNS functionality is not impacted by acting as a passive gateway
because the client would always send the mDNS query when inquiring
for a service.
3.8.2. Device Tracking
Wireless clients are mobile in nature. The mDNS gateway should learn
the service instance only from the authenticated wireless client.
The mDNS gateway should tag each service instance from a wireless
client with the client's MAC address. This MAC address should be
used for device tracking. If the wireless client leaves the network,
the mDNS gateway should not wait until the TTL expires but it should
actively clean up the service instance provided by that wireless
client. This is done to protect the mDNS gateway cache resources as
well as to keep other clients from hearing about services that are no
longer connected to the network..
Bhandari, et al. Expires April 23, 2014 [Page 8]
Internet-Draft mDNS across local links in campus network October 2013
3.8.3. Mobility Considerations
Wireless deployments supports seamless mobility. In such a scenario,
the mDNS gateway needs to be aware of the client location. If the
location changes, the mDNS gateway needs to update its mDNS cache.
The mDNS gateway should tag each service instance with the device
location. The device location can be derived based on the Access
Point (AP) to which the wireless client is attached. If the client,
which is providing any service, changes its location, this change
needs to be reflected in the mDNS gateway. If the client roams from
one mDNS gateway to another mDNS gateway, then the old gateway should
provide the service instance information pertaining to the roamed
client to the new gateway and then it must clear the mDNS cache for
that particular client. If the mDNS gateway is not acting as a
passive gateway, it may choose to update the network about the new
service instance it has learnt.
3.8.4. mDNS traffic optimization
All mDNS packets are sent to the multicast link-local IP address.
When the mDNS gateway starts forwarding the mDNS advertisements
across L3 boundaries then the number of such advertisement on any
network would increase. Wireless networks should be optimized for
the increase in mulitcast traffic that will be generated by extending
the service advertisement domain. If there are many mDNS packets
going on air then it would impact other data traffic. Hence mDNS
traffic optimization is required. One method of optimization the
mDNS gateway could implement is sending the query reponse back as a
L2 unicast to the requesting client.
When services are advertised, each record has an associated TTL
value. When the TTL expires, the gateway needs to send a query (at
85%, 90% and 95% of the TTL) for that record to confirm its validity.
If the TTL value of each record is different, then mDNS gateway needs
to send a query for individual records. To minimize the mDNS
traffic, queries for multiple RRs for that service record set can be
initiated towards the source of the service. Such a query can be
sent with the QU bit set as described in [RFC6762] to solicit a
unicast response.
The mDNS gateway for wireless networks should act as a passive
gateway as explained in Section 3.8.1. When it is acting as a
passive gateway and bridging of mDNS packets is turned off it has to
respond to queries on the link even when the provider of the service
resides on the same link.
Bhandari, et al. Expires April 23, 2014 [Page 9]
Internet-Draft mDNS across local links in campus network October 2013
4. Challenges
This section lists out limitations and challenges faced as part of
the the solution described in this draft.
1. Name conflict resolution across links: Name conflict resolution
depends on probing followed by service registration. This is
done by the host which is providing the service. Name conflict
resolution across links depends on the mDNS gateway cache to have
a conclusive list of names already present to be able to
authoritatively respond to probe requests. However, this may not
always be posible due to timing issues when the cache gets
updated, records having expired from the cache etc.
2. Multi-homed hosts: There is also the case of a multihomed host
connected via multiple links to the same mDNS gateway that may
end up wrongly assuming conflict and getting into a continuous
renaming loop.
3. Multiple mDNS gateways on the link: If there are multiple mDNS
gateways enabled on the same link queries may get duplicate
responses.
4. Loops in the network: If there is a loop in the network with
multiple mDNS gateways enabled in such a topology it may end up
continuously cycling the service around the loop and keeping the
RRs alive forever.
5. Refreshing resource records: Balancing an excessive number of
queries to maintain the records in the cache vs. having the cache
up-to-date with all the known record names requires optimizations
that may lead to corner cases where wrong results or conflicts
arise.
5. Future work
The solution documented here is limited to extending services across
links attached to a single network element or mDNS gateway. For a
broader application, the service discovery solution described in
[I-D.cheshire-mdnsext-hybrid] should be realized with any
provisioning as needed.
Similar to auto provisioning and realization of the hybrid proxy
approach for homenet as described in
[I-D.stenberg-homenet-dnssdext-hybrid-proxy-ospf] a solution needs to
be built for enterprise and campus networks extending what has been
described in this draft.
Bhandari, et al. Expires April 23, 2014 [Page 10]
Internet-Draft mDNS across local links in campus network October 2013
There are other considerations such as including the location
information so that services can be ordered based on proximity of the
service.
6. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
7. Security Considerations
N/A
8. Acknowledgements
9. Normative References
[I-D.cheshire-mdnsext-hybrid]
Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service
Discovery", draft-cheshire-mdnsext-hybrid-02 (work in
progress), July 2013.
[I-D.sekar-dns-llq]
Sekar, K., "DNS Long-Lived Queries",
draft-sekar-dns-llq-01 (work in progress), August 2006.
[I-D.stenberg-homenet-dnssdext-hybrid-proxy-ospf]
Stenberg, M., "Hybrid Unicast/Multicast DNS-Based Service
Discovery Auto-Configuration Using OSPFv3",
draft-stenberg-homenet-dnssdext-hybrid-proxy-ospf-00 (work
in progress), June 2013.
[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.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
February 2013.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Bhandari, et al. Expires April 23, 2014 [Page 11]
Internet-Draft mDNS across local links in campus network October 2013
Discovery", RFC 6763, February 2013.
Authors' Addresses
Shwetha Bhandari
Cisco Systems, Inc.
Cessna Business Park, Sarjapura Marathalli Outer Ring Road
Bangalore, KARNATAKA 560 087
India
Email: shwethab@cisco.com
Bhavik Fajalia
Cisco Systems, Inc.
Cessna Business Park, Sarjapura Marathalli Outer Ring Road
Bangalore, KARNATAKA 560 087
India
Email: bfajalia@cisco.com
Ralph Schmieder
Cisco Systems, Inc.
City Plaza - 4th Floor
Stuttgart, BADEN-WURTTEMBERG 70178
Germany
Email: rschmied@cisco.com
Stephen Orr
Cisco Systems, Inc.
1 Paragon Drive
Montvale, NJ 07645
USA
Email: sorr@cisco.com
Bhandari, et al. Expires April 23, 2014 [Page 12]
Internet-Draft mDNS across local links in campus network October 2013
Amit Dutta
Cisco Systems, Inc.
Cessna Business Park, Sarjapura Marathalli Outer Ring Road
Bangalore, KARNATAKA 560 087
India
Email: amdutta@cisco.com
Bhandari, et al. Expires April 23, 2014 [Page 13]
