IPsec has been implemented almost completely within level 3 of the OSI model, i.e. in the network layer. The transfer of data packets using the IP protocol is realized on level 3 of IP networks.
IPsec thus replaces the IP protocol. Under IPsec, the packets have a different internal structure than IP packets. Their external structure remains fully compatible to IP, however. IPsec packets can therefore be transported without problems by existing IP networks. The devices in the network responsible for the transport of the packets cannot distinguish IPsec packets from IP packets on the basis of their exterior structure.
The exceptions in this case are certain firewalls and proxy servers that access the contents of the packets. Problems can arise from the (often function dependent) incompatibilities of these devices to the existing IP standard. These devices must therefore be adapted to IPsec.
IPsec will be firmly implemented in the next generation of the IP standard (IPv6). For this reason, we can assume that IPsec will remain the most important standard for virtual private networks in the future.
