When does screening occur in the flow module?

In Juniper SRX flow-based packet processing, theflow moduleis responsible for security functions such as screening, session management, NAT, and policy enforcement. The processing order is critical:

Screens are applied before any session lookup.This ensures that packets are inspected for anomalies, floods, or protocol violations before consuming resources for session management. Examples of these screens include TCP SYN flood protection, ICMP flood protection, and port scanning protection.

After screening, thesession lookupoccurs. At this point, the firewall checks whether the packet belongs to an existing session in the session table. If a matching session is found, the packet bypasses policy evaluation and is forwarded according to the session state.

If no existing session is found, the packet continues throughroute lookup, NAT processing, and security policy evaluationbefore a new session is created.

Thus,screening occurs before the session lookup, protecting the system early in the flow process. This design ensures efficiency by dropping malicious or malformed traffic before allocating session resources.