"Everyone on the Internet has at least one unique AS number, and they use BGP to advertise their networks to their peers. BGP is a path-vector protocol, because it advertises the paths required to get to a certain destination. BGP does not say anything about how a packet will get routed within the AS, nor does it know about the entire network as OSPF does. BGP can be called a distance-vector protocol, because it’s similar, excluding a few twists.
"BGP itself is a Layer 4 protocol that sits on top of TCP. BGP is much simpler than OSPF, because it doesn’t have to worry about the things TCP will handle. This works because BGP is very connection-oriented anyway, since it requires two manually configured peers, who configure their routers then exchange routes. BGP peers (neighbors) will generally be directly connected, but some masochists like to set up BGP sessions between multi-hop peers—which is okay, since BGP uses TCP (port 179) and doesn’t rely on broadcasts or link-local multicast."
