RSVP-TE State Refresh and Reliability

RSVP-TE employs refresh messages to synchronize state between neighbors and to recover from lost RSVP-TE messages of other types. RSVP-TE by default does not provide for reliable transmission. Each node is responsible for the transmission of RSVP-TE messages to its neighbors and relies on periodic state refreshes to recover from lost messages.

RSVP-TE refresh reduction provides a means to increase reliability while reducing message handling and synchronization overhead. Issuing the mpls rsvp refresh-reduction command enables the following features:

• The message ID object is included in RSVP-TE messages to provide a unique message ID on a per-hop basis. In refresh messages, it indicates to the receiving node that the message is a refresh message, eliminating the need for the node to completely analyze the message and thus reducing the processing overhead at the receiver.
• RSVP-TE uses a message acknowledgment mechanism to support reliable message delivery on a per-hop basis. Messages that include the message ID object also include a request for acknowledgment. Upon receipt, the receiving node returns a message ack object, enabling the sending node to determine whether a message was lost and triggering a retransmission as necessary.
• Summary refresh (srefresh) messages refresh the state previously advertised in path or resv messages that included message ID objects. The srefresh message carries the unique message ID as state identifier, eliminating the need to send whole refresh messages and reducing the overhead needed to maintain RSVP-TE state synchronization. This method maintains RSVP-TE's ability to indicate when the state is lost and to adjust to changes in routing. The srefresh message can carry message IDs for multiple RSVP-TE sessions.

Issuing the mpls rsvp message-bundling command enables RSVP-TE to use bundle messages, each of which includes multiple standard RSVP-TE messages, to reduce the overall message processing overhead.

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