I’ve read lots of comments about how people needed to change their MTU size to accomodate Verizion or others, but I haven’t understood why anyone needs to tune the MTU. Well, I have a better sense now that I’ve read RFC 3022, “Traditional IP Network Address Translator (Traditional NAT)”:
6.3. Translation of outbound TCP/UDP fragmented packets in NAPT setup
Translation of outbound TCP/UDP fragments (i.e., those originating from private hosts) in NAPT setup are doomed to fail.
You can’t have fragments and use NAT. If you decrease your MTU size, that decreases the chance that a router in the middle will want to fragment your packet.
(This is just my best guess, not an authoritative answer.)