what does this trace mean?

What does this trace mean?

Where is it coming from?

Is it abnormal?

I have substituted aaa-bbb for the last portion of the ip
address

I have substituted "xxx" for the ip server domain
(mayby dumb because 12-203 is unique to it)

22:00:04.642207 12-203-aaa-bbb.client."xxx".com.1237 >
ns1."xxx".com.domain: 2517+ PTR? 170.7.203.12.in-addr.arpa.
(43) (DF)
22:00:04.679506 ns1."xxx".com.domain >
12-203-26-242.client."xxx".com.1237: 2517* 1/4/4 (258) (DF)

The trace repeats about once every 6 seconds.

Dan