If there will be more than two simultaneous user connections, install a Dynamic VPN license in the device. Dynamic VPN is a licensed feature for SRX-Branch devices. By default, a two user evaluation license is provided free of cost and it does not expire.
Note: If using only two users, you will not see an entry for the dynamic-vpn license. By default, it will allow you to connect with two dynamic vpn. In cases where there are more than two users that need to connect concurrently, a license is required.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB17292&actp=search
http://www.juniper.net/documentation/en_US/junos12.1x44/topics/example/vpn-security-dynamic-example-configuring.html
http://www.juniper.net/documentation/en_US/junos12.1x44/topics/concept/security-license-overview.html
-Python
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