The MPLS WG Archive

[Abhijit Gadgil <gabhijit@ee.iitb.ac.in>]

John.Brennen@marconi.com wrote :

>  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.

Key point missed here is, LRl. 1. and the note at the end of it.

>
>The problem seems independent of distribution, control, or retention
>modes.

Not really, if B is using liberal retention it wont release the mappings
received as soon as next-hop changes.

>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.
>

-- 
-abhijit

Abhijit Gadgil
Graduate Student,
Dept. of Electrical Engineering,
IIT, Bombay.