Information Session

DNS Service level recommendations and experiences

What is a DNS Service Level (or SLA)?

SLAs are a measure of:

•The ability to resolve names in the name space

•The ability to obtain authoritative answers for domain delegations

•The time taken for changes in registry data to be reflected in the name servers

How to measure DNS Service Levels?

These service levels are measured in a number of ways. Each has its own metric and recommended level and each deals with vastly different areas that effect DNS availability.

Service Level Measures:

•Overall Availability

•Per Name Server Availability

•Round Trip Time - RTT

•Packet Loss

•Average Resolution Time

•Scheduled Down Time

•DNS updates

Service Level Measures - Overall Availability

This refers to the ability of an authoritative answer for DNS requests.

Recommended SLA: 100%

Service Level Measures – Per Name Server Availability

This refers to the availability of an authoritative answer for a DNS request from each name server individually.

Recommended SLA: 99.99%

Service Level Measures – Round Trip Time & Packet Loss

This refers to the ‘ping’ time and number of packets that ‘go missing’ in the network that the name server is connected to.

Recommended SLA: 300ms/10%

Service Level Measures – Average Resolution Times

This refers to the average response time for DNS queries from the time the query is received to the time the response is sent.

Recommended SLA: 250ms

Service Level Measures – Scheduled downtime

This refers to the amount of scheduled controlled total DNS downtime allowed.

Recommended SLA: No downtime

Service Level Measures – DNS updates

This refers to the addition, removal and modification of records in the name space, 5-10min updates are not uncommon, but now days they are average, with providers able to offer instant DNS updates, and propagation using IXFR.

Our Recommended SLA: 20mins Max

Recommended Service Levels Summary:

•100% Overall DNS Availability

•99.99% Per Name Server

•300ms RTT (round trip time)

•10% Packet Loss maximum

•250ms average resolution time

•No down time

•20 mins Max updates

How to confirm Service Levels are being met?

Normally the registry operator will perform there own SLA monitoring and report back any variations to the governing body. However you may like to measure these things yourself.

Service levels such as: •Overall Availability•Per Name Server Availability•Round Trip Time - RTT•Packet Loss•Average Resolution Time•DNS updates

Can be measured through ‘sampling’

Taking samples of each measurement over time periods is the easiest and most effective way to ensure service levels are being met. The frequency of these samples is flexible and can be decided between you and your registry operator.

Recommended sampling Frequency:

•Overall Availability ( 1 min )•Per Name Server Availability ( 1 min )•Round Trip Time – RTT ( 1 min )•Packet Loss ( 1 min)•Average Resolution Time ( 1 min)•DNS updates ( 10 min )

`Daily' Graph (5 Minute Average)

                                                                                          

Max Response

Time:

127.0 ms (0.4%)

Average Response

Time:

20.0 ms (0.1%)

Current Response

Time:

12.0 ms (0.0%)

Example: Using mrtg to ‘sample’ Average Response Time

How to maintain good Service Levels?

Choose excellent carry class data centres with redundant network connections, power etc•Run multiple secondaries in diverse locations and networks•Maintain a strong and up to date understanding of DNS•Use clustering at each location•Use quality hardware with different OS•Maintain industry best practices, such as those stated in RFC2182, 2870 etc

How to co-operate with users to maintain service levels?

•Target larger commercial ISPs for secondary services•Making use of the existing experienced DNS companies•Keep up to date, and provide discussion lists so that you can hear your communities needs.•Regularly participate in discussions and show you are taking responsibility for the Name space

In summary - key factors to maintaining SLAs are:

•Multiple secondaries•Network carrier diversity•Provider diversity•Monitoring•Accountability•Maintain industry best practice and leverage existing industry experience

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Thanks for listening

Any Questions?