H3C Technologies H3C SR8800 - Manual (Page 58)

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Figure 16 Set up an LSP tunnel
The following is a simplified procedure for setting up an LSP tunnel with RSVP:
1. The ingress LSR sends a Path message that carries the label request information, and then forwards
the message along the path calculated by CSPF hop-by-hop towards the egress LSR.
2. After receiving the Path message, the egress generates a Resv message carrying the reservation
information and label and then forwards the message towards the ingress along the reverse
direction of the path along which the Path message travels. The LSRs that the Resv message
traverses along the path reserve resources as required.
3. When the ingress LSR receives the Resv message, LSP is established.
As resources are reserved on the LSRs along the path for the LSP established using RSVP-TE, services
transmitted on the LSP are guaranteed.
RSVP refresh mechanism
RSVP maintains path and reservation state by periodically retransmitting two types of messages: Path
and Resv. These periodically retransmitted Path and Resv messages are called refresh messages. They are
sent along the path that the last Path or Resv message travels to synchronize state between RSVP
neighbors and recover lost RSVP messages.
When many RSVP sessions are present, periodically sent refresh messages become a network burden. In
addition, for some delay sensitive applications, the refreshing delay they must wait for recovering lost
RSVP messages may be unbearable. As tuning refresh intervals is not adequate to address the two
problems, the refreshing mechanism was extended in RFC 2961 RSVP Refresh Overhead Reduction
Extensions as follows to address the problems:
1. Message_ID extension
RSVP itself uses Raw IP to send messages. The Message_ID extension mechanism defined in RFC
2961 adds objects that can be carried in RSVP messages. Of them, the Message_ID object and
the Message_ID_ACK object are used to acknowledge RSVP messages, thus improving
transmission reliability.
On an interface enabled with the Message_ID mechanism, you may configure RSVP message
retransmission. After the interface sends an RSVP message, it waits for acknowledgement. If no
ACK is received before the initial retransmission interval (Rf seconds for example) expires, the
interface resends the message. After that, the interface resends the message at an exponentially
increased retransmission interval equivalent to (1 + Delta) × Rf seconds.
2. Summary refresh extension
Send summary refreshes (Srefreshes) rather than retransmit standard Path or Resv messages to
refresh related RSVP state. This reduces refresh traffic and allows nodes to make faster processing.
To use summary refresh, you must use the Message_ID extension. Only states advertised using
MESSAGE_ID included Path and Resv messages can be refreshed using summary refreshes.
PSB, RSB, and BSB timeouts
To create an LSP tunnel, the sender sends a Path message with a LABEL_REQUEST object. After receiving
this Path message, the receiver assigns a label for the path and puts the label binding in the LABEL object
in the returned Resv message.