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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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IPsec VPN:  https://www.juniper.net/documentation/en_US/day-one-books/DO_IPsec_VPNs_2018.pdf


Payload encapsulation

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Signalingin ISAKMP ( IKE phase1 )

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


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1st/2nd packet

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in the 3rd and 4th packet


payload: NAT-Discovery


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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
titleAO-
 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 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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