RSVP Summary Refresh
RSVP summary refresh (Srefresh) function enables a node to send digests of RSVP Refresh messages to maintain RSVP soft states and respond to RSVP soft state changes, which reduces signaling packets used to maintain the RSVP soft states and optimizes bandwidth allocation.
Background
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.
Implementation
Message_ID extension and retransmission extension
The Srefresh extension builds on the 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 and support reliable RSVP message delivery.
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 receives no ACK message within Rf seconds, the node retransmits an RSVP 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).
Summary Refresh extension
The Summary Refresh extension supports Srefresh messages to update the RSVP status, without the transmission of standard Path or Resv messages.
Each Srefresh message carries a Message_ID object. Each object contains multiple messages IDs, each of which identifies a Path or Resv state to be refreshed. If a CR-LSP changes, its message ID value increases.
Only the state that was previously advertised by Path and Resv messages containing Message_ID objects can be refreshed using the Srefresh extension.
After a node receives an Srefresh message, the node compares the Message_ID with that saved in a local state block. If they match, the node does not change the state. If the Message_ID is greater than that saved in the local state block, 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 sequence numbers of Message_ID objects. If a CR-LSP changes, the associated Message_ID sequence number increases. When receiving an Srefresh message, the node compares the sequence number of the Message_ID with the sequence number of the Message_ID saved in the PSB. 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.
