5. IP Traffic Processing

As mentioned in Section 4.4.1, "The Security Policy Database (SPD)", the SPD (or associated caches) MUST be consulted during the processing of all traffic that crosses the IPsec protection boundary, including IPsec management traffic. If no policy is found in the SPD that matches a packet (for either inbound or outbound traffic), the packet MUST be discarded.

SPD Cache Mechanism

To simplify processing, and to allow for very fast SA lookups (for SG/BITS/BITW), this document introduces the notion of an SPD cache for all outbound traffic (SPD-O plus SPD-S), and a cache for inbound, non-IPsec-protected traffic (SPD-I). (As mentioned earlier, the SAD acts as a cache for checking the selectors of inbound IPsec-protected traffic arriving on SAs.)

There is nominally one cache per SPD. For the purposes of this specification, it is assumed that each cached entry will map to exactly one SA. Note, however, exceptions arise when one uses multiple SAs to carry traffic of different priorities (e.g., as indicated by distinct DSCP values) but the same selectors.

Decorrelated SPD

Since SPD entries may overlap, one cannot safely cache these entries in general. Simple caching might result in a match against a cache entry, whereas an ordered search of the SPD would have resulted in a match against a different entry. But, if the SPD entries are first decorrelated, then the resulting entries can safely be cached.

Note: It is assumed that one starts with a correlated SPD because that is how users and administrators are accustomed to managing these sorts of access control lists or firewall filter rules. Then the decorrelation algorithm is applied to build a list of cache-able SPD entries. The decorrelation is invisible at the management interface.

Traffic Processing Directions

• 5.1 Outbound IP Traffic Processing (protected-to-unprotected)
• 5.2 Inbound IP Traffic Processing (unprotected-to-protected)

For inbound IPsec traffic, the SAD entry selected by the SPI serves as the cache for the selectors to be matched against arriving IPsec packets, after AH or ESP processing has been performed.