2.3 Soft State

RSVP takes a "soft state" approach to managing the reservation state in routers and hosts. RSVP soft state is created and periodically refreshed by Path and Resv messages. The state is deleted if no matching refresh messages arrive before the expiration of a "cleanup timeout" interval. State may also be deleted by an explicit "teardown" message, described in the next section. At the expiration of each "refresh timeout" period and after a state change, RSVP scans its state to build and forward Path and Resv refresh messages to succeeding hops.

Path and Resv messages are idempotent. When a route changes, the next Path message will initialize the path state on the new route, and future Resv messages will establish reservation state there; the state on the now-unused segment of the route will time out. Thus, whether a message is "new" or a "refresh" is determined separately at each node, depending upon the existence of state at that node.

RSVP sends its messages as IP datagrams with no reliability enhancement. Periodic transmission of refresh messages by hosts and routers is expected to handle the occasional loss of an RSVP message. If the effective cleanup timeout is set to K times the refresh timeout period, then RSVP can tolerate K-1 successive RSVP packet losses without falsely deleting state. The network traffic control mechanism should be statically configured to grant some minimal bandwidth for RSVP messages to protect them from congestion losses.

The state maintained by RSVP is dynamic; to change the set of senders Si or to change any QoS request, a host simply starts sending revised Path and/or Resv messages. The result will be an appropriate adjustment in the RSVP state in all nodes along the path; unused state will time out if it is not explicitly torn down.

In steady state, state is refreshed hop-by-hop to allow merging. When the received state differs from the stored state, the stored state is updated. If this update results in modification of state to be forwarded in refresh messages, these refresh messages must be generated and forwarded immediately, so that state changes can be propagated end-to-end without delay. However, propagation of a change stops when and if it reaches a point where merging causes no resulting state change. This minimizes RSVP control traffic due to changes and is essential for scaling to large multicast groups.

State that is received through a particular interface I* should never be forwarded out the same interface. Conversely, state that is forwarded out interface I* must be computed using only state that arrived on interfaces different from I*. A trivial example of this rule is illustrated in Figure 10, which shows a transit router with one sender and one receiver on each interface (and assumes one next/previous hop per interface). Interfaces (a) and (c) serve as both outgoing and incoming interfaces for this session. Both receivers are making wildcard-style reservations, in which the Resv messages are forwarded to all previous hops for senders in the group, with the exception of the next hop from which they came. The result is independent reservations in the two directions.

There is an additional rule governing the forwarding of Resv messages: state from Resv messages received from outgoing interface Io should be forwarded to incoming interface Ii only if Path messages from Ii are forwarded to Io.

                         ________________
                      a |                | c
      ( R1, S1 ) <----->|     Router     |<-----> ( R2, S2 )
                        |________________|

             Send                |        Receive
                                 |
        WF( *{3B}) <-- (a)       |     (c) <-- WF( *{3B})
                                 |
             Receive             |          Send
                                 |
        WF( *{4B}) --> (a)       |     (c) --> WF( *{4B})
                                 |
             Reserve on (a)      |        Reserve on (c)
              __________         |        __________
             |  * {4B}  |        |       |   * {3B} |
             |__________|        |       |__________|
                                 |

                     Figure 10: Independent Reservations