It is impossible to deploy RSVP (or any new protocol) at the same moment throughout the entire Internet. Furthermore, RSVP may never be deployed everywhere. RSVP must therefore provide correct protocol operation even when two RSVP-capable routers are joined by an arbitrary "cloud" of non-RSVP routers. Of course, an intermediate cloud that does not support RSVP is unable to perform resource reservation. However, if such a cloud has sufficient capacity, it may still provide useful realtime service.
RSVP is designed to operate correctly through such a non-RSVP cloud. Both RSVP and non-RSVP routers forward Path messages towards the destination address using their local uni-/multicast routing table. Therefore, the routing of Path messages will be unaffected by non-RSVP routers in the path. When a Path message traverses a non-RSVP cloud, it carries to the next RSVP-capable node the IP address of the last RSVP-capable router before entering the cloud. An Resv message is then forwarded directly to the next RSVP-capable router on the path(s) back towards the source.
Even though RSVP operates correctly through a non-RSVP cloud, the non-RSVP-capable nodes will in general perturb the QoS provided to a receiver. Therefore, RSVP passes a `NonRSVP' flag bit to the local traffic control mechanism when there are non-RSVP-capable hops in the path to a given sender. Traffic control combines this flag bit with its own sources of information, and forwards the composed information on integrated service capability along the path to receivers using Adspecs [RFC 2210].
Some topologies of RSVP routers and non-RSVP routers can cause Resv messages to arrive at the wrong RSVP-capable node, or to arrive at the wrong interface of the correct node. An RSVP process must be prepared to handle either situation. If the destination address does not match any local interface and the message is not a Path or PathTear, the message must be forwarded without further processing by this node. To handle the wrong interface case, a "Logical Interface Handle" (LIH) is used. The previous hop information included in a Path message includes not only the IP address of the previous node but also an LIH defining the logical outgoing interface; both values are stored in the path state. A Resv message arriving at the addressed node carries both the IP address and the LIH of the correct outgoing interface, i.e, the interface that should receive the requested reservation, regardless of which interface it arrives on.
The LIH may also be useful when RSVP reservations are made over a complex link layer, to map between IP layer and link layer flow entities.