4.2. SA Functionality

The set of security services offered by an SA depends on the security protocol selected, the SA mode, the endpoints of the SA, and the election of optional services within the protocol.

For example, both AH and ESP offer integrity and authentication services, but the coverage differs for each protocol and differs for transport vs. tunnel mode. If the integrity of an IPv4 option or IPv6 extension header must be protected en route between sender and receiver, AH can provide this service, except for IP or extension headers that may change in a fashion not predictable by the sender. However, the same security may be achieved in some contexts by applying ESP to a tunnel carrying a packet.

The granularity of access control provided is determined by the choice of the selectors that define each SA. Moreover, the authentication means employed by IPsec peers, e.g., during creation of an IKE (vs. child) SA also affects the granularity of the access control afforded.

Traffic Flow Confidentiality

If confidentiality is selected, then an ESP (tunnel mode) SA between two security gateways can offer partial traffic flow confidentiality. The use of tunnel mode allows the inner IP headers to be encrypted, concealing the identities of the (ultimate) traffic source and destination. Moreover, ESP payload padding also can be invoked to hide the size of the packets, further concealing the external characteristics of the traffic. Similar traffic flow confidentiality services may be offered when a mobile user is assigned a dynamic IP address in a dialup context, and establishes a (tunnel mode) ESP SA to a corporate firewall (acting as a security gateway). Note that fine-granularity SAs generally are more vulnerable to traffic analysis than coarse-granularity ones that are carrying traffic from many subscribers.

Note: A compliant implementation MUST NOT allow instantiation of an ESP SA that employs both NULL encryption and no integrity algorithm. An attempt to negotiate such an SA is an auditable event by both initiator and responder. The audit log entry for this event SHOULD include the current date/time, local IKE IP address, and remote IKE IP address. The initiator SHOULD record the relevant SPD entry.