The IP Security Protocol, Part 1

Privacy to ensure data confidentiality. In other words, it must not be possible for eavesdroppers to sniff your data over the network and read what you are sending. In one of its two working modes, IPSec also allows for traffic flow confidentiality; not only will the eavesdropper be prevented from reading your data, but he will not even know who you are communicating with.

Integrity to guarantee that data has not been tampered with. No one will be able to modify your data as it is flowing by without the legitimate receiver noticing it. An interesting property of IPSec is it provides connectionless integrity, in the sense that protection is applied to each individual IP datagram.

Authenticity to protect against identity spoofing. Attackers will no longer be able to pass themselves off as you by simply forging packets bearing your IP address or carrying data allegedly written by you. Each packet carries a type of "signature" that uniquely identifies you as the sender. This is also important for putting sophisticated access control mechanisms in place.

Robustness to prevent DoS and replay attacks. IPSec detects the arrival of duplicate IP datagrams, thus preventing attackers from recording a legitimate session and playing it back (by sending the packets again to the destination).

Host-to-host communication: IPSec is deployed on each host that requires secure communication services. Each host pair negotiates its own IPSec parameters and establishes its own connection. This is typically the case when two private users wish to share a private connection over the Internet.

Gateway-to-gateway communication: IPSec is deployed on network gateways (thus called security gateways), which can be either routers or special firewalls. Each pair of security gateways establishes a secure tunnel over which all the hosts in the LAN send protected packets. This is completely transparent to the hosts, hence it is well suited for connecting distant LANs over the Internet.

Host-to-gateway communication: IPSec is deployed both on a security gateway (as defined above) and on a host (typically, a mobile PC) that remotely connects to it. In this scenario, a remote user (e.g., a teleworker) is able to reach a private LAN (e.g., his office) without requiring a dedicated connection, such as a dial-up link. The IPSec connection between the PC and the gateway ensures that all the packets will be protected. This set up is often referred to as the "road warrior" scenario.