The correct answer is D. AppQoS. The requirement is not to block, permit, translate, or inspect SIP signaling; it is to prioritize VoIP traffic on a low-bandwidth branch link. Juniper Application QoS, or AppQoS, is designed exactly for this purpose. Juniper states that AppQoS provides the ability to prioritize and meter application traffic so business-critical or high-priority application traffic receives better service. It can classify traffic by application, assign forwarding classes, rewrite DSCP values, set loss priority, and apply rate limiters.
Option A, AppFW, is wrong because AppFW controls application access; it is used to permit, deny, reject, or log application traffic, not primarily to prioritize voice traffic. Option B, SIP ALG, is wrong because the SIP ALG helps inspect and handle SIP control traffic and related media pinholes, but it is not a QoS prioritization service. Option C, AppQoE, is not the correct SRX edge service being tested here. For cloud-based VoIP under a broad outbound permit rule, AppQoS is the correct service because it can identify the VoIP application and give it better treatment during congestion. Reference topics: AppQoS, application traffic control, VoIP prioritization, DSCP rewrite, forwarding class, rate limiting.
