RTT: how it works

[Kevin Darcy]

jefferson.f.moreira at hsbc.com.br wrote:

> Hi all.
>
> I have a doubt on DNS servers behavior regarding RTT.
>
> Consider the following diagram:
>
>       cwb.somenet.net.br                  cwb.somenet.net.br
>             [cs1]                                [cs2]
>               |                                    |
>               |                                    |
>      +------[Sw1]------+                  +------[Sw2]-----+
>      /      /   \       \                /       /   \      \
>     /      /     \       \              /       /     \      \
>  (NS1)  [Srv1]   [Srv2]   \            /    [Srv3]   [Srv4]  (NS2)
>                            \          /
>                         (Router)---(Router)
>                              \      /
>                               \    /
>                              (Router)
>                                 |
> [ Corp NS ]---------------------+
>
> CS1/2 =3D Content Service Switches with DNS service
> Sw1 =3D Switches
> NS1/2 =3D Two Win2k Name servers. They=B4re not primary and secondary,
> but two servers with the same db.
>
> These questions apply to a query from an external NS (let=B4s say, a
> corporate NS)querying NS1 that will delegate it to CS1.
>
> q1: (this applies to the CSSs)
> When there are 2 name servers (SOAs) for one domain, how is calculated
> the Round Trip Time? At exactly what moment will be calculated this RTT
> and how often?

The RTT will be recalculated every time the nameserver is used. In the event
that a particular nameserver doesn't answer (this leads into your next
question), the RTT value for that nameserver will be severely penalized. But
that nameserver will still be tried occasionally, in case it has recovered.

> q2: in the event of one of the CSx failing, will we have that 75s time
> out? How to overcome that? What is the default behavior?

As long as both of your nameservers are published nameservers for the zone,
then other nameservers should quickly fail over when one of them becomes
unavailable for whatever reason.

What is this "75s" timeout you're referring to? That sounds more like a client
timeout, e.g. a time it takes a browser to fail over to another web server
address. Hopefully you realize that the RTT calculations only apply to
nameserver-to-nameserver interactions and have *nothing* to do with
client/server interactions. The CSS technology (which I'm vaguely familiar
with, since we use them here) sometimes tends to blur the distinction between
the two...


- Kevin