Let’s now take a look “inside” an SA. To make the discussion tangible and concrete,
let’s do this in the context of an SA from router R1 to router R2 in Figure 8.28.
(You can think of Router R1 as the headquarters gateway router and Router R2 as
the branch office gateway router from Figure 8.27.) Router R1 will maintain state
information about this SA, which will include: