Protocol overhead refers to metadata and network routing information sent by an application, which uses a portion of the available bandwidth of a communications protocol. This extra data, making up the protocol headers and application-specific information is referred to as overhead, since it does not contribute to the content of the message.
Protocol overhead can be expressed as a percentage of non-application bytes (protocol and frame synchronization) divided by the total number of bytes in the message.
References
Protocol overhead Wikipedia(Text) CC BY-SA