Grid computing has attracted many researchers over a few years, and as a result many new protocols have emerged and also evolved since its inception a decade ago. Grid protocols play major role in implementing services that facilitate coordinated resource sharing across diverse organizations. In this paper, we provide comprehensive coverage of different core Grid protocols that can be used in Global Grid Computing. We establish the classification of core Grid protocols into i) Grid network communication and Grid data transfer protocols, ii) Grid information security protocols, iii) Grid resource information protocols, iv) Grid management protocols, and v) Grid interface protocols, depending upon the kind of activities handled by these protocols. All the classified protocols are also organized into layers of the Hourglass model of Grid architecture to understand dependency among these protocols. We also present the characteristics of each protocol. For better understanding of these protocols, we also discuss applied protocols as examples from either Globus toolkit or other popular Grid middleware projects. We believe that our classification and characterization of Grid protocols will enable better understanding of core Grid protocols and will motivate further research in the area of Global Grid Computing.