For the first packet of a new session (first path processing) on a Juniper SRX Series device, the processing order is critical for security and efficiency. The packet is first subjected to stateless screen protections applied to the ingress interface. These screens check for anomalies and potential attacks before consuming further resources. Next, the device identifies the ingress security zone based on the interface the packet arrived on. Finally, after a route lookup determines the egress zone, the device evaluates stateful security policies to decide whether to permit or deny the creation of a session between the source and destination zones.

A. security policies –> screens –> zones: Incorrect. Screens are stateless and applied first, before zone identification and stateful policy evaluation.

B. screens –> security policies –> zones: Incorrect. The security zone must be identified before a zone-based security policy can be evaluated.

D. security policies –> zones –> screens: Incorrect. This order is fundamentally wrong; screens are the initial check, not the final one.

1. Juniper Networks TechLibrary

"Flow-Based and Packet-Based Processing": This official documentation details the packet processing sequence. In the "First Packet Processing" section

it shows that screens are applied at the ingress interface before the packet is associated with a zone

and policy lookup occurs after the from-zone and to-zone are known.

URL: https://www.juniper.net/documentation/us/en/software/junos/security-policies/topics/concept/security-flow-based-processing-understanding.html

Specific Section: See the diagram and description under "First Packet Processing".

2. Juniper Networks TechLibrary

"Understanding Session Creation": This document reinforces the flow

stating

"When a packet arrives at the device

the device checks it against existing sessions... If no session exists... the device performs a series of checks on it... These checks include... applying security screens... determining the zone the packet is coming from... [and] applying the security policy for that context."

URL: https://www.juniper.net/documentation/us/en/software/junos/flow-packet-processing/topics/concept/session-understanding.html

Specific Section: "Session Creation" details the sequence of events for a new flow.