RSVP refresh mechanism

RSVP maintains resource reservation states on a node by periodically sending messages.

The resource reservation states include path states and reservation states. A path state is saved in a path state block (PSB), and a reservation state is saved in a reservation state block (RSB). A PSB is created by a Path message and saves the LABEL_REQUEST object. A RSB is created by a Resv message and saves the LABEL object.

The path states and reservation states are refreshed periodically by Path and Resv messages. A state is removed if no refresh messages for the state are received in a certain interval, and the CRLSP established based on this state is also removed.

The Path and Resv messages for refreshing the resource reservation states are collectively referred to as refresh messages. Refresh messages can also be used to recover from lost RSVP messages.

When multiple RSVP sessions exist on a network, a short refresh interval can cause network degradation, but a long refresh interval cannot meet the requirements of delay sensitive applications. To find an appropriate balance, you can use the summary refresh (Srefresh) and the reliable RSVP message delivery functions.

Srefresh is implemented by adding a Message_ID object to a Path or Resv message to uniquely identify the message. To refresh Path and Resv states, RSVP does not need to send standard Path and Resv messages. Instead, it sends an Srefresh message carrying a set of Message_ID objects that identify the Path and Resv states to be refreshed. The Srefresh function reduces the number of refresh messages on the network and speeds up refresh message processing.

An RSVP sender cannot know or retransmit lost RSVP messages. The reliable RSVP message delivery mechanism is designed to ensure reliable transmission.

This mechanism requires the peer device to acknowledge each RSVP message received from the local device. If no acknowledgement is received, the local device retransmits the message.

To implement reliable RSVP message delivery, a node sends an RSVP message that includes a Message_ID object in which the ACK_Desired flag is set. The receiver needs to confirm the delivery by sending back a message that includes the Message_ID_ACK object. If the sender does not receive a Message_ID_ACK within the retransmission interval (Rf), it retransmits the message when Rf expires and sets the next transmission interval to (1 + delta) × Rf. The sender repeats this process until it receives the Message_ID_ACK before the retransmission time expires or it has transmitted the message three times.