/usr/lib/libpacketdump/link_3.protocol is in libpacketdump3 3.0.18-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 | # ATM
le12	integer "ATM: VPI"
le16	integer "ATM: VCI"
le3	hex 	"ATM: Payload Type"
le1	flag	"ATM: Cell loss priority"
be8	hex 	"LLC: DSAP"
be8	hex 	"LLC: SSAP"
be8	hex 	"LLC: Control"
be24	hex 	"SNAP: OUI"
be16	hex 	"SNAP: Type"
next	"eth"	"SNAP: Type"
 |