To find options on any Junos command just use one ? at the command string.
In your particular case:
admin@yourSRX# set security nat source rule-set xxx rule xxx match protocol ?
Possible completions:
<number> Numeric protocol value (0 .. 255)
[ Open a set of values
ah IP Security authentication header
egp Exterior gateway protocol
esp IPSec Encapsulating Security Payload
gre Generic routing encapsulation
icmp Internet Control Message Protocol
icmp6 Internet Control Message Protocol Version 6
igmp Internet Group Management Protocol
ipip IP in IP
ospf Open Shortest Path First
pim Protocol Independent Multicast
rsvp Resource Reservation Protocol
sctp Stream Control Transmission Protocol
tcp Transmission Control Protocol
udp User Datagram Protocol.
Regards
Leon Smirnov
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