The most important protocol for a data network engineer?

What a wise pick for a topic ;) lets discuss about the protocols again. Needless to say, there are so many protocols out there and some Ipv6 protocols are still coming. Since i myself only working in data network, i can not discuss about voice or security or wireless, lets base on TCP/IP model also I do believe that Application Layer is less important then the others for a network operation engineer whi maintain and run a live network. However that was my earlier perspective and later I found out the other way round. I try to understand how the internal mechanism of most popular protocols are working, how they behave one another in different platforms, what m trying to say is for example, the famous routing protocol OSPF forming neighbors over LAN, and over WAN. But now I am quite surprise that these are not enough. There are quite a few numbers of protocols out there, most of them become IEEE standard while some of them are Cisco proprietary protocols, like routing protocol EIGRP and redundancy protocols like HSRP and GLBP(excuse me, m cisco student).
Anyway.. lets think again, u know what? I jst want to stop typing with my tiny iphone, this gonna b my first iphone blogging, gudnight!