In MX304 platforms, the CLI commands for the following DDoS protection protocols are not operational:

show ddos-protection protocols <>

filter-action
virtual-chassis
mld 
ttl
redirect
firewall-host
re-services
re-services-v6 
l2pt
amtv4 
amtv6 
syslog 
pim-ctrl
proto-802-1x
rejectv6

Note that DDoS is fully supported for the protocols, but the CLI is not properly enabled.

"Command not valid" errors will appear after executing these commands:

MX304-re0> show ddos-protection protocols ttl

error: command is not valid on the mx304

This issue has been fixed via PR1816565:

https://prsearch.juniper.net/problemreport/PR1816565

Below is the related doc:

https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/command/show-ddos-protocols.html