From Oracle FAQ
Starting with Oracle 9i, the protocol.ora file is no longer being used. All (or, most?) protocol.ora parameters were moved to the sqlnet.ora file. These parameters enable you to configure database access and TCP/IP related settings. They include:
TCP.NODELAY TCP.EXCLUDED_NODES TCP.INVITED_NODES TCP.VALIDNODE_CHECKING
This file was located in $ORACLE HOME/network/admin on UNIX systems, and in ORACLE HOME\network\admin on Windows.
Oracle's Net Manager (netmgr), when first started, will automatically move protocol.ora parameters into your sqlnet.ora file.