ormal processes for the signaling protocol and the switch is deprogrammed. 8.4.2 Soft-State Overhead Optical MPLS networks are typically stable and the LSPs are long lived and relatively unchanging. This stability puts extra focus on the overhead of running RSVP, which requires regular refreshes between each network node to keep an LSP alive. The overhead of running this soft-state mecha- nism is more obvious in stable networks because the benefit of automati- cally cleaning up expired state is less frequently needed. The following sections outline some approaches to reducing this overhead.5 All the approaches are general to RSVP-TE, but have particular applicability to optical networks because of their stability. Note that soft-state overhead is not an issue for LDP and CR-LDP because they are hard-state protocols. The other side of the coin for LDP and CR-LDP is that they require special measures to preserve LSPs when a signaling session is temporarily broken.4 Message IDs The soft-state nature of RSVP allows the loss of protocol messages to be handled by simply waiting for the next refresh interval, or in the case of teardown messages, waiting for the state to time out. A con- sequence of this is that there is a tension between choosing a short refresh interval for robustness against message loss and the increasing overhead of running RSVP that this entails. Message IDs provide an alternative approach to improving the robust- ness of message transmission that does not significantly increase the over- head of running RSVP. Each message includes a message ID, chosen by the sender, that uniquely identifies the message. Received message IDs are acknowledged by the receiver and are either piggybacked on other RSVP messages or sent in a lightweight ACK message if no other RSVP messages are available. The sender can then employ a retry timer for sent messages, resending messages until they're acknowledged by the recipient. As mes- sage loss is rare and ACKing message IDs is quick, message retries can be kept to a min