4000 Router Series MPLS Configuration Guide

Srefresh

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.

Reliable RSVP message delivery

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.

RSVP authentication

RSVP authentication ensures integrity of RSVP messages, and prevents false resource reservation requests

from occupying network resources.

With RSVP authentication, the sender uses the MD5 algorithm and the authentication key to calculate a

message digest for an RSVP message, and inserts the message digest to the RSVP message. When the

receiver receives the message, it performs the same calculation and compares the result with the message

digest. If they match, the receiver accepts the message. Otherwise, it drops the message.

By carrying a sequence number in a message, RSVP authentication can also prevent packet replay

attacks. The device records the sequence number of a received RSVP message, and determines whether

the subsequent messages are valid according to the recorded sequence number. If the sequence number

of a subsequent message is within the valid range, the device accepts the message. Otherwise, it drops

the message.

RSVP GR

RSVP GR preserves the soft state and label forwarding information when the signaling protocol or control

plane fails, so that LSRs can still forward packets according to forwarding entries.

RSVP GR defines two roles:

• GR restarter—Router that gracefully restarts due to a manually configured command or a fault. It

must be GR-capable.

• GR helper—Neighbor of the GR restarter. A GR helper maintains the neighbor relationship with the

GR restarter and helps the GR restarter restore its LFIB information. A GR helper must be

GR-capable.

The router can act only as a GR helper.

The RSVP GR function depends on the extended hello capability of RSVP. A GR-capable device

advertises its GR capability and relevant time parameters to its neighbors in RSVP hello packets. If a