None

  A set of rules (i.e., formats and procedures) to implement and control some type of association (e.g., communication) between systems.
Source(s):
CNSSI 4009-2015 IETF RFC 4949 Ver 2
NIST SP 800-82 Rev. 2 under Protocol RFC 4949
NISTIR 8183 under Protocol NIST SP 800-82, NIST SP 800-82 Rev. 2
NISTIR 8183A Vol. 1 under Protocol NIST SP 800-82
NISTIR 8183A Vol. 2 under Protocol NIST SP 800-82
NISTIR 8183A Vol. 3 under Protocol NIST SP 800-82

  A set of rules used by two or more communicating entities that describe the message order and data structures for information exchanged between the entities.
Source(s):
NIST SP 800-56B Rev. 2 under Protocol
NIST SP 800-175B Rev. 1 under Protocol

  A set of rules (i.e., formats and procedures) for communications that computers use when sending signals between themselves.
Source(s):
NISTIR 7316 under Protocol

  A special set of rules used by two or more communicating entities that describe the message order and data structures for information exchanged between the entities.
Source(s):
NIST SP 800-56B Rev. 1 under Protocol [Superseded]