1. Scaling up DNS-based service discovery
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Scaling up DNS-based service discovery
18/09/2018
Dr Tim Chown, Jisc TNC, Prague, 15th June 2016

2. What is DNS-based service discovery?
DNS-based service discovery (RFC 6763) was designed for zero configuration networking within a local subnet
Allows devices to automatically discover services on other devices that are ‘nearby’
Driven by Apple, commonly referred to as ‘Bonjour’
Works with link-local addressing and naming (mDNS, under .local, as per RFC 6762), and DNS TXT/PTR/SRV records to describe the services
Service discovery messages are sent as link scope multicast using well-known addresses, on UDP port 5353
IPv4 :
IPv6 : ff02::fb
May be user to device, or device to device, with an ever wider variety of services for which discovery is desirable
15/06/2016
Scaling up DNS-based service discovery

3. OK, so what is the problem?
The messages are constrained to a local subnet
Because link scope multicast is not forwarded by routers
So discovery only works for devices on the same link
But increasingly they are not, e.g., devices in a campus network, or emerging multi-link home networks
A screen in the same room, but on a wired network while you are on WiFi
Two WiFi devices in different eduroam subnets
Frustrating when you can see the device you want to discover!
So how do we solve this?
15/06/2016
Scaling up DNS-based service discovery

4. Requirements?
We’d like a scalable DNS-based service discovery mechanism
Capable of running over / spanning multiple subnets
Applicable to personal, home, campus, enterprise and other network scenarios
Backwardly compatible with mDNS/DNS-SD
And supporting incremental deployment
May need to be be zero configuration, e.g., in the home
Or may benefit from supporting administrator configuration, e.g., in a campus environment
The IETF dnssd WG has been formed to address this problem, and it has already documented requirements
Published as RFC 7558 –
15/06/2016
Scaling up DNS-based service discovery

5. What about solution approaches?
What could we do?
Define a new “xmDNS” that operates by extending mDNS to use site scope multicast messages
Not scalable, or interoperable with existing mDNS
Bridge queries and responses
Beware loops. And no use when subnets are disjoint
Make the network flat
But a campus may have 20,000++ devices…
Hybrid Unicast/Multicast DNS-Based Service Discovery
Leaf routers act as proxies to their local links
Clients query a list of proxies, and aggregate the results
mDNS / DNS-SD runs as normal in local subnets
15/06/2016
Scaling up DNS-based service discovery

6. How far has the dnssd WG got?
Produced a draft documenting the hybrid proxy solution
Produced a draft on DNS Push mechanism
Subscription-based DNS updates (for timeliness…)
Identified issues with label interoperability between mDNS and DNS name space (UTF-8 vs IDNA2008)
interop-02
Considering privacy issues
Still non-standards work to be done as well…
What does the user interface look like?
15/06/2016
Scaling up DNS-based service discovery

7. How you can get involved…
Take a look at the IETF dnssd WG Charter:
Check the status pages for existing dnssd drafts:
Join the mail list, join discussions:
To join:
Explore early implementations
See section 6 of the hybrid proxy draft
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Scaling up DNS-based service discovery

8. Find out more… Contact: Dr Tim Chown Senior Network Services Developer
jisc.ac.uk
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15/06/2016
Scaling up DNS-based service discovery