Question: What is the primary function of the Security Policy Database (SPD) in the context of Network Domain Security (NDS), as it pertains to IP network layer security?
Option 1: To manage encryption keys for IPsec protocols across all network domains.
Option 2: To monitor and log all inbound and outbound IP traffic for security threats.
Option 3: To determine which security services are applied to both inbound and outbound traffic, including decisions on IPsec protection, bypass, or discard.
Option 4: To negotiate roaming agreements with external security domains for secure data exchange.
Explanation: The correct answer, Option 3, accurately reflects the SPD's role as outlined in the context. It emphasizes the SPD's decision-making process regarding the application of security services (e.g., IPsec) to both inbound and outbound traffic, including scenarios where traffic may not require protection or should be discarded. This aligns with the SPD's central role in defining security policies within and towards external domains, without specifically referencing roaming agreements as its primary function (which is more related to external interactions rather than the core function of the SPD).