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3.5.8. Label Request Message

An LSR sends the Label Request message to an LDP peer to request a binding (mapping) for a FEC.

The encoding for the Label 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 Request (0x0401)    |      Message Length           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Message ID                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     FEC TLV                                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Optional Parameters                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

FEC TLV The FEC for which a label is being requested. See Section "FEC TLV" for encoding.

Optional Parameters This variable length field contains 0 or more parameters, each encoded as a TLV. The optional parameters are:

  Optional Parameter     Length       Value
      Hop Count TLV          1            See below
      Path Vector TLV        variable     See below

The encodings for the Hop Count and Path Vector TLVs can be found in Section "TLV Encodings for Commonly Used Parameters".

Hop Count Specifies the running total of the number of LSR hops along the LSP being set up by the Label Request message. Section "Hop Count Procedures" describes how to handle this TLV.

Path Vector Specifies the LSRs along the LSR being set up by the Label Request message. Section "Path Vector Procedures" describes how to handle this TLV.

3.5.8.1. Label Request Message Procedures​

The Request message is used by an upstream LSR to explicitly request that the downstream LSR assign and advertise a label for a FEC.

An LSR may transmit a Request message under any of the following conditions:

  1. The LSR recognizes a new FEC via the forwarding table, and the next hop is an LDP peer, and the LSR doesn't already have a mapping from the next hop for the given FEC.

  2. The next hop to the FEC changes, and the LSR doesn't already have a mapping from that next hop for the given FEC.

    Note that if the LSR already has a pending Label Request message for the new next hop, it SHOULD NOT issue an additional Label Request in response to the next hop change.

  3. The LSR receives a Label Request for a FEC from an upstream LDP peer, the FEC next hop is an LDP peer, and the LSR doesn't already have a mapping from the next hop.

    Note that since a non-merge LSR must set up a separate LSP for each upstream peer requesting a label, it must send a separate Label Request for each such peer. A consequence of this is that a non-merge LSR may have multiple Label Request messages for a given FEC outstanding at the same time.

The receiving LSR SHOULD respond to a Label Request message with a Label Mapping for the requested label or with a Notification message indicating why it cannot satisfy the request.

When the FEC for which a label is requested is a Prefix FEC Element, the receiving LSR uses its routing table to determine its response. Unless its routing table includes an entry that exactly matches the requested Prefix, the LSR MUST respond with a No Route Notification message.

The message ID of the Label Request message serves as an identifier for the Label Request transaction. When the receiving LSR responds with a Label Mapping message, the mapping message MUST include a Label Request/Returned Message ID TLV optional parameter that includes the message ID of the Label Request message. Note that since LSRs use Label Request message IDs as transaction identifiers, an LSR SHOULD NOT reuse the message ID of a Label Request message until the corresponding transaction completes.

This version of the protocol defines the following Status Codes for the Notification message that signals a request cannot be satisfied:

No Route The FEC for which a label was requested includes a FEC Element for which the LSR does not have a route.

No Label Resources The LSR cannot provide a label because of resource limitations. When resources become available, the LSR MUST notify the requesting LSR by sending a Notification message with the Label Resources Available Status Code.

  An LSR that receives a No Label Resources response to a Label
  Request message MUST NOT issue further Label Request messages
  until it receives a Notification message with the Label
  Resources Available Status Code.

Loop Detected The LSR has detected a looping Label Request message.

See Appendix A, "LDP Label Distribution Procedures", for more details.