RSVP summary refresh (Srefresh) ensures RSVP state updates while reducing the number of signaling packets that need to be transmitted and processed for maintaining RSVP states, optimizing network bandwidth resource usage.
RSVP Refresh messages are used to synchronize path state block (PSB) and reservation state block (RSB) information between nodes. They can also be used to monitor the reachability between RSVP neighbors and maintain RSVP neighbor relationships. As the sizes of Path and Resv messages are larger, sending many messages to establish many CR-LSPs causes increased consumption of network resources. RSVP Srefresh can be used to address this problem.
Message_ID extension and retransmission extension
Srefresh extension builds on Message_ID extension. According to the Message_ID extension mechanism defined in relevant standards, RSVP messages carry extended objects, including Message_ID and Message_ID_ACK objects. The two objects are used to confirm RSVP messages, thereby making RSVP message delivery more reliable.
The Message_ID object can also be used to provide the RSVP retransmission mechanism. For example, a node initializes a retransmission interval as Rf seconds after it sends an RSVP message carrying the Message_ID object. If the node does not receive ACK message within Rf seconds, the node retransmits the message after (1 + Delta) x Rf seconds. The Delta determines the increased rate of the transmission interval set by the sender. The node keeps retransmitting the message until it receives an ACK message or the retransmission times reach the threshold (called a retransmission increment value).
Srefresh can refresh the RSVP status without transmitting standard Path or Resv messages. It reduces the amount of information that needs to be transmitted and processed to maintain RSVP states. Regular refresh messages are suppressed when RSVP status is updated with Srefresh.
Each Srefresh message carries a series of Message_ID objects to identify the Path and Resv states to be refreshed. Srefresh requires use with Message_ID extension Only the states that have been advertised through Path and Resv messages carrying Message_ID can be refreshed through the Srefresh mechanism.
After a node receives a Srefresh message, the node compares the Message_ID with that saved in a local state block (PSB or RSB). If they match, the local state is updated as if a standard RSVP Refresh message was received. If they do not match, the node sends a NACK message to the sender, refreshes the PSB or RSB based on the Path or Resv message, and updates the Message_ID.
Message_ID objects contain Message_ID sequence numbers. If an LSP changes, the associated Message_ID sequence number increases. When receiving a Path message, the node compares the sequence number of the Message_ID with the sequence number of the Message_ID saved in its local state block. If they are the same, the node does not change the state; if the received sequence number is greater than the local one, the state has been updated.