TTL in DNS part 2

In part 1 of this series, we discussed what TTL is, and provided a few exercises to help strengthen your understanding of TTL.  We'll pick up where we left off with our same DNS zone for domain1.com:

domain1.com. 86400 IN NS ns1.domain1s-ISP.com. 
domain1.com. 86400 IN NS ns2.domain1s-ISP.com. 

domain1.com. 14400 IN MX mail.domain1.com. 

mail.domain1.com. 3600  IN A  10.11.12.13.

As we discussed, the NS records will expire 1 day after they were last cached, the MX records will expire 4 hours after they were last cached, and the A record will expire 60 minutes after it was last cached.  What does this mean for you and your planned migration?

A common misconception about TTL is that it's the amount of time the Name Server requires to update the record.  This is absolutely not the case.  Do you remember what RFC 1034 said a TTL is?  A TTL is:

...primarily used by resolvers when they cache RRs.  The TTL describes how long a RR can be cached before it should be* discarded.

Where in there does it say that a TTL is how long a name server takes to update?  What does it say?  It says that a DNS resolver (which is anything that resolves DNS, your computer has a DNS resolver in it) caches the records, and the TTL describes how long the DNS resolver should cache the record before discarding it (and then re-requesting it).

What this means is that (for the sake of simplicity) every computer connected to the internet has it's own local cache of websites it's visited, mail servers it's talked to, etc. and will not re-request those records until it's own, local, cache expires.  TTL is how long it should* keep the records until it discards them and re-requests them.

Object lesson time...

UserPC1.ValuedReader.com visits www.TheEmailAuthority.com at 2:30 PM.  www.TheEmailAuthority.com has a 1 hour TTL for it's A record.  If I move the website at 3 PM, and the old IP stops serving the site, how long would it be until UserPC1.ValuedReader.com can read my site again?

UserPC-2.ValuedReader.com visits www.TheEmailAuthority.com at 3:10 PM How long will it be until they can read the site?

The answers and explanations will be in part three.