IPv6 Routing Header - How does it work?

[mcmanus at ducksong.com (Patrick R. McManus)]

[Vriz: Wed, Apr 17, 2002 at 09:49:23AM +0800]

> IPv6's Routing Header plays the same role as the source routing option of
> IPv4, in which the source host specify a list of hops that the packet must
> traverse. However, could anyone advise on how the list of hops is chosen?
> Thank you.

the list of hops comes from some external source. For example, the -g
option to traceroute.

LSRR (loose source combined with record route) is a reasonably common
peering requirment between network providers. It is used to verify
each provider is obeying the agreed upon peering policy. Suppose
network A is peered with network B and their peering policy says
simply A may send traffic destined to 1.0.0.0/8 to B and B may send
traffic destined to 17.0.0.0/8 to A via the peering circuit. As a
diagnostic/verification, A could address a traceroute to destination
204.0.0.1 but source route it via B's router on the peering
circuit. This will let A see how B handles a packet destined for a
third party network - what A wants to make sure of is that it does
*not* go back through his own network (i.e. that B is not abusing the
peering circuit to force A to carry traffic that isn't his
responsibility.)

The use of "traceroute servers" can also provide this information -
but some companies insist on lsrr because it is a harder thing to
fake, and the purpose is for verification.

Even away from peering routers, LS is enabled more commonly than this
thread would lead you to believe. 

-P