You have already learned a fair bit about SRRD in Chapter 9, "Dynamic Routing Protocols?Interior Gateway Protocols." For purposes of this discussion, just be aware that SRRD can do without an external heartbeat mechanism and instead just requires Quagga Open Shortest Path First (OSPF) for signaling based on opaque link-state advertisements (LSAs). Undeservedly, this elegant concept is underrepresented in HA architectures. According to http://www.srrd.org, "SRRD is fully configurable over the web and supports SSL and PKI client and server authentication. It implements cluster server features like Service Groups, Service Dependencies, as well as Critical Services."