The ASA builds a ICMp connection for the icmp echoes that is sees. It creates a session based on the identification/sequence number of the icmp header, it's source and destination ip addresses and its type and code. So, it knows what the icmp reply for that packet will look like and it will allow the return traffic.
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...