The MPLS WG Archive

[John.Brennen@marconi.com]

Vijay wrote:
>I faced a similar problem some time back
>
>I think the problem comes because of the configuration of the LDP label
>distribution mode, control mode and Request mode.
>
>Particularly here the request node is REQUEST_NEVER though the distribution
>mode is DuS. If router B were 'Request When Needed' then it would have
>requested A for a label when A becomes its next Hop then the problem would
>have been overcome by A sending the mapping again after receiving the
>request. I wonder if DuS Ind control with 'Request Never' is not a good
>configuration choice
>
>Please correct me if I am wrong.

The problem also manifests itself if the session is
"Request-when-Needed" and "Downstream-on-Demand."  Example:

  Router A                          Router B
  --------                          --------

                                    Router A becomes the next hop
                                    Sends Label Request
  Receives Label Request
  Sends Label Mapping (Label 1)
                                    Router A becomes NOT the next hop
                                    Sends Label Abort Request
  Receives Label Abort Request
   (which is ignored)
                                    Receives Label Mapping (Label 1)
                                    Sends Label Release (Label 1)
                                     (because Router A is not the next hop)
  Sends Label Mapping (Label 1)
   (because the hop count changed)
                                    Router A becomes the next hop
                                    Sends Label Request
  Receives Label Release (Label 1)
                                    Receives Label Mapping (Label 1)
  Receives Label Request
  Sends Label Mapping (Label 2)
                                    Receives Label Mapping (Label 2)
                                    Sends Label Release (Label 2)
  Receives Label Release (Label 2)

The final outcome is the same.  Router B believes that a
Label Mapping exists; Router A believes that no Label Mapping exists.

The problem seems independent of distribution, control, or retention
modes.  The crux of the problem is that when Router A sends a
Label Mapping message which is an "update" message (the same FEC
and Label as a previous message, usually sent to change the
hop count and/or path vector) and then subsequently receives a
Label Release message for that FEC and Label, one of two
situations can exist:

  * If Router B sent the Label Release after receiving the
    "update" message, then Router B is not keeping the label

  * If Router B sent the Label Release before receiving the
    "update" message, then Router B may in fact keep the
    label if his situation changed between sending the
    Label Release and receiving the "update" message.
    (The most likely reason for the situation to change
    would be Router A becoming the next hop for the FEC.)

Router A simply cannot reliably determine which of these
two situations exists on Router B.