Internet Routing Architectures, Second Edition

Symmetry

Symmetry refers to when traffic leaving the AS from a given exit point comes back through the same point. This is easy to achieve if a single exit and entrance point exist. However, given the mandates of redundancy and the presence of multiple connections, traffic tends to be asymmetrical. When traffic is asymmetrical, customers and providers notice a lack of control over how traffic flows into and out of their ASs. Traffic leaving the AS from the East Coast might end up taking the "scenic route," coming back from the West Coast and traveling inside the AS multiple hops before returning to its origin. This is usually the result of closest-exit routing, as discussed in Chapter 6.

In reality, this is not as bad as it sounds. In some ...