The vast existing wireless infrastructure features a variety of systems and standards. It is of significant practical value to introduce new features and devices without changing the physical layer/hardware infrastructure, but upgrade it only in software. A way to achieve it is to apply protocol coding: encode information in the actions taken by a certain (existing) communication protocol. In this work we investigate strategies for protocol coding via combinatorial ordering of the labelled user resources (packets, channels) in an existing, primary system. Such a protocol coding introduces a new secondary communication channel in the existing system, which has been considered in the prior work exclusively in a steganographic context. Instead, we focus on the use of secondary channel for reliable communication with newly introduced secondary devices, that are low-complexity versions of the primary devices, capable only to decode the robustly encoded header information in the primary signals. We introduce a suitable communication model, capable to capture the constraints that the primary system operation puts on protocol coding. We have derived the capacity of the secondary channel under arbitrary error models. The insights from the information-theoretic analysis are used in Part II of this work to design practical error-correcting mechanisms for secondary channels with protocol coding.