RR and rrset

[Paul Vixie]

bobvance at alumni.caltech.edu ("Bob Vance") writes:

> What's the difference between the terms
>     "RR"
> and "rrset"
> ?

A resource record (RR) is the indivisible unit of change in DNS.  Here's
an example:

caltech.edu.            2D IN NS        MERCUTIO.NI.caltech.edu.

A resource record set (RRset) is the indivisible unit of transfer in DNS.
Here's an example, from which the above RR is drawn:

caltech.edu.            2D IN NS        MERCUTIO.NI.caltech.edu.
caltech.edu.            2D IN NS        TEPID.NI.caltech.edu.
caltech.edu.            2D IN NS        TYBALT.caltech.edu.
caltech.edu.            2D IN NS        NSX.LBL.GOV.

By "indivisible" I mean that nothing smaller is ever done.  You can change
(or add or delete) a single RR but you always transfer (either in a zone
transfer or a query or an update) whole RRsets.  RRsets are said to be
"atomic" which means their TTL (time to live) is "minimized".  If in a zone
file an RRset is presented with unequal TTL's, then the lowest TTL is used
for all RRs in the RRset.  If in a DNS cache somewhere an RRset's RRs have
unequal TTL's, then when the lowest TTL is reached, the whole RRset is
dropped and refetched.