3.1. What IPsec Does

IPsec creates a boundary between unprotected and protected interfaces, for a host or a network (see Figure 1 below). Traffic traversing the boundary is subject to the access controls specified by the user or administrator responsible for the IPsec configuration. These controls indicate whether packets cross the boundary unimpeded, are afforded security services via AH or ESP, or are discarded.

IPsec security services are offered at the IP layer through selection of appropriate security protocols, cryptographic algorithms, and cryptographic keys. IPsec can be used to protect one or more "paths" (a) between a pair of hosts, (b) between a pair of security gateways, or (c) between a security gateway and a host. A compliant host implementation MUST support (a) and (c) and a compliant security gateway must support all three of these forms of connectivity, since under certain circumstances a security gateway acts as a host.

                    Unprotected
                     ^       ^
                     |       |
       +-------------|-------|-------+
       | +-------+   |       |       |
       | |Discard|<--|       V       |
       | +-------+   |B  +--------+  |
     ................|y..| AH/ESP |..... IPsec Boundary
       |   +---+     |p  +--------+  |
       |   |IKE|<----|a      ^       |
       |   +---+     |s      |       |
       | +-------+   |s      |       |
       | |Discard|<--|       |       |
       | +-------+   |       |       |
       +-------------|-------|-------+
                     |       |
                     V       V
                     Protected

        Figure 1.  Top Level IPsec Processing Model

In this diagram, "unprotected" refers to an interface that might also be described as "black" or "ciphertext". Here, "protected" refers to an interface that might also be described as "red" or "plaintext". The protected interface noted above may be internal, e.g., in a host implementation of IPsec, the protected interface may link to a socket layer interface presented by the OS. In this document, the term "inbound" refers to traffic entering an IPsec implementation via the unprotected interface or emitted by the implementation on the unprotected side of the boundary and directed towards the protected interface. The term "outbound" refers to traffic entering the implementation via the protected interface, or emitted by the implementation on the protected side of the boundary and directed toward the unprotected interface. An IPsec implementation may support more than one interface on either or both sides of the boundary.

Note the facilities for discarding traffic on either side of the IPsec boundary, the BYPASS facility that allows traffic to transit the boundary without cryptographic protection, and the reference to IKE as a protected-side key and security management function.

IPsec optionally supports negotiation of IP compression [SMPT01], motivated in part by the observation that when encryption is employed within IPsec, it prevents effective compression by lower protocol layers.