Topology:

Traffic Flow Overview

When a packet enters the SRX on the ingress interface, the configured DSCP classifier examines the packet's DSCP value and maps it to a forwarding class. 
The packet retains its original DSCP marking while traversing the device. On the egress interface, the scheduler map places the packet into the appropriate queue and applies the configured scheduling behavior before transmission.

1. Configure DSCP Classification - Map DSCP EF traffic to the expedited-forwarding forwarding class on ingress.

set class-of-service classifiers dscp DSCP-EF-CLASSIFIER forwarding-class expedited-forwarding loss-priority low code-points ef
set class-of-service interfaces ge-0/0/0 unit 0 classifiers dscp DSCP-EF-CLASSIFIER

2. Configure Schedulers

set class-of-service schedulers EF-SCHEDULER transmit-rate 50m
set class-of-service schedulers EF-SCHEDULER buffer-size percent 20
set class-of-service schedulers EF-SCHEDULER priority strict-high

set class-of-service schedulers NC-SCHEDULER transmit-rate 1m
set class-of-service schedulers NC-SCHEDULER buffer-size percent 5
set class-of-service schedulers NC-SCHEDULER priority high

set class-of-service schedulers DEFAULT-SCHEDULER transmit-rate remainder
set class-of-service schedulers DEFAULT-SCHEDULER buffer-size remainder
set class-of-service schedulers DEFAULT-SCHEDULER priority low

3. Create Scheduler Map

set class-of-service scheduler-maps QOS-SCHEDULER-MAP forwarding-class expedited-forwarding scheduler EF-SCHEDULER
set class-of-service scheduler-maps QOS-SCHEDULER-MAP forwarding-class network-control scheduler NC-SCHEDULER
set class-of-service scheduler-maps QOS-SCHEDULER-MAP forwarding-class best-effort scheduler DEFAULT-SCHEDULER
set class-of-service scheduler-maps QOS-SCHEDULER-MAP forwarding-class assured-forwarding scheduler DEFAULT-SCHEDULER

4. Apply Scheduler Map on Egress Interface

set class-of-service interfaces ge-0/0/2 unit 0 shaping-rate 250m
set class-of-service interfaces ge-0/0/2 unit 0 scheduler-map QOS-SCHEDULER-MAP

5. Interface Configuration

set interfaces ge-0/0/0 unit 0 family inet address <ingress-ip>
set interfaces ge-0/0/2 per-unit-scheduler
set interfaces ge-0/0/2 unit 0 family inet address <egress-ip>

Validation - EF-marked traffic was replayed through the SRX and verified using:

>show interfaces queue ge-0/0/2 forwarding-class expedited-forwarding

Note: Junos CoS classifiers map incoming packets to forwarding classes based on packet markings such as DSCP values. Scheduler maps then associate those forwarding classes with specific schedulers and egress queues, allowing differentiated treatment of traffic.