It is difficult to measure the maintainability of a software system early in the development life cycle from its requirement descriptions written in a natural language because informal specifications cannot be analyzed. With the uses of formal description techniques (FDTs) in the communication protocol area since the mid-1980s, avenues have been opened up for a system to be analyzed early in the specification phase. Quantitative measures on its maintainability can then be extracted from such a formal specification, so that we can develop easily maintainable communication software systems and further reduce the increasingly high cost of software maintenance. To date, there is hardly any work done on measuring the maintainability of a system early in its specification phase. This paper presents a method for measuring the maintainability of a communication by using maintainability metrics derived from its formal specification written in Estelle. The methodology for building the Estelle maintainability metrics hierarchy is presented. We have also developed an automated tool, called PSAMS, to automate the calculation of the maintainability indices. We also found that there is a significant correlation between the specification metrics proposed and the widely adopted implementation metrics, thus demonstrating that our proposed metrics are a reliable means of measuring the maintainability of a communication protocol early in the specification phase.