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https://www.juniper.net/documentation/us/en/software/junos/vpn-ipsec/topics/topic-map/security-route-based-and-policy-based-vpns-with-nat-t.html

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2x NAT-D ( 1st= remote IP@/port and 2nd= local IP@/port)
>> solve the questions: direction and existance of NAT .
If both ends calculate those hashes and get same result, they know there is no NAT between.
If the hashes do not match, somebody has translated the address or port.

Payload encapsulation

Signaling

in ISAKMP ( IKE phase1 )

https://www.rfc-editor.org/rfc/rfc3947

1st/2nd packet

in the 3rd and 4th packet

payload: NAT-Discovery

Compare the NAT-D ( IP@ + port)

with the packet received

Phase 2 or Quick Mode

To perform incremental TCP checksum updates, both peers may need to
   know the original IP addresses used by their peers when those peers
   constructed the packet

Code Block

title	AO-

 Example 2:

         Initiator  ------> NAT1  ---------> NAT2  -------> Responder
                  ^             ^           ^              ^
                Iaddr        Nat1Pub     Nat2Pub          (Raddr)

         Initiator <------  NAT1 <---------  NAT2 <-------  Responder
                  ^             ^           ^              ^
               (Iaddr)        Nat1Pub     Nat2Pub         Raddr

   Here, NAT2 "publishes" Nat2Pub for Responder and forwards all traffic
   to that address to Responder.

   Initiator:
                     NAT-OAi = Iaddr
                     NAT-OAr = Nat2Pub ( responder of the IPsec public IP address )

   Responder:
                     NAT-OAi = Nat1Pub ( initiator of the IPsec public IP address )
                     NAT-OAr = Raddr

NAT-T for IKE peer: NAT-T is used when there is a network device between the two tunnel end-points that enforce NAT.

all the IPSEC tunnerl to traverse the NATing equipement

Challenge: Any changes to the IP addressing, which is the function of NAT, causes IKE to discard packets

Solution: ESP over UDP/4500

After detecting one or more NAT devices ( by the firwall) along the datapath during Phase 1 exchanges,

>> NAT-T adds a layer of User Datagram Protocol (UDP) encapsulation to IPsec packets so they are not discarded after address translation.

edit security ike gateway gateway-name

[edit security ike gateway IKE-GW1]

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