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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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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	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.

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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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