The dominant form of host authentication in today's Internet is address-based. That is, hosts often decide to trust other hosts based on their IP addresses. To the extent that it does work, address-based authentication relies on the implied accuracy of the return route. That is, though it is easy to inject packets with a false source address, replies will generally follow the usual routing patterns, and be sent to the real host with that address. This frustrates most, though not all, attempts at impersonation.
Problems can arise if source-routing is used.
A source route, which must be reversed for reply packets, overrides the usual routing mechanism, and hence destroys the security of address-based authentication. For this reason, many organizations disable source- routing, at least at their border routers. To the extent this is used, it is a breaks address-based authentication. But it is vital that a more secure cryptographic authentication protocol be defined and deployed before any substantial cutover to source routing, if SIPP is adopted. Accounting An significant part of the world wishes to do usage-sensitive accounting.