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3.5.9. Label Abort Request Message

The Label Abort Request message may be used to abort an outstanding Label Request message.

The encoding for the Label Abort Request message is:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|   Label Abort Req (0x0404)  |      Message Length           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Message ID                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     FEC TLV                                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Label Request Message ID TLV              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Optional Parameters                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Message ID 32-bit value used to identify this message.

FEC TLV Identifies the FEC for which the Label Request is being aborted.

Label Request Message ID TLV Specifies the message ID of the Label Request message to be aborted.

Optional Parameters No optional parameters are defined for the Label Abort Req message.

3.5.9.1. Label Abort Request Message Procedures​

An LSR Ru may send a Label Abort Request message to abort an outstanding Label Request message for a FEC sent to an LSR Rd in the following circumstances:

  1. Ru's next hop for the FEC has changed from LSR Rd to LSR X; or

  2. Ru is a non-merge, non-ingress LSR and has received a Label Abort Request for the FEC from an upstream peer Y.

  3. Ru is a merge, non-ingress LSR and has received a Label Abort Request for the FEC from an upstream peer Y and Y is the only (last) upstream LSR requesting a label for the FEC.

There may be other situations where an LSR may choose to abort an outstanding Label Request message in order to reclaim resource associated with the pending LSP. However, specification of general strategies for using the abort mechanism is beyond the scope of LDP.

When an LSR receives a Label Abort Request message, if it has not previously responded to the Label Request being aborted with a Label Mapping message or some other Notification message, it MUST acknowledge the abort by responding with a Label Request Aborted Notification message. The Notification MUST include a Label Request Message ID TLV that carries the message ID of the aborted Label Request message.

If an LSR receives a Label Abort Request Message after it has responded to the Label Request in question with a Label Mapping message or a Notification message, it ignores the abort request.

If an LSR receives a Label Mapping message in response to a Label Request message after it has sent a Label Abort Request message to abort the Label Request, the label in the Label Mapping message is valid. The LSR may choose to use the label or to release it with a Label Release message.

An LSR aborting a Label Request message may not reuse the Message ID for the Label Request message until it receives one of the following from its peer:

  • A Label Request Aborted Notification message acknowledging the abort;

  • A Label Mapping message in response to the Label Request message being aborted;

  • A Notification message in response to the Label Request message being aborted (e.g., Loop Detected, No Label Resources, etc.).

To protect itself against tardy peers or faulty peer implementations an LSR may choose to time out receipt of the above. The timeout period should be relatively long (several minutes). If the timeout period elapses with no reply from the peer, the LSR may reuse the Message ID of the Label Request message; if it does so, it should also discard any record of the outstanding Label Request and Label Abort messages.

Note that the response to a Label Abort Request message is never "ordered". That is, the response does not depend on the downstream state of the LSP setup being aborted. An LSR receiving a Label Abort Request message MUST process it immediately, regardless of the downstream state of the LSP, responding with a Label Request Aborted Notification or ignoring it, as appropriate.