Not sure exactly what you're asking here, but I think you're referring to Path MTU Discovery and how it utilises ICMP type 3 code 4 to determine the largest packet that can traverse a specific link. Because IPSec increases the original packet size it's handy to use this to avoid fragmentation of your original packets. Note that these ICMP types should really be allowed everywhere, not just over IPSec links as filtering them will stop PMTUD from working at all.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...