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Old one: https://www.juniper.net/documentation/en_US/junose15.1/topics/concept/nat-traversal-overview.html
IPsec VPN: https://www.juniper.net/documentation/en_US/day-one-books/DO_IPsec_VPNs_2018.pdf
NAT-T for IKE peer: NAT-T is used when there is a network device between the
two tunnel end-points that enforce NAT.
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