The MPLS WG Archive

["Vijayanand C - CTD, Chennai." <vijayc@ctd.hcltech.com>]

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.

Regards,
Vijay

-----Original Message-----
From: John.Brennen@marconi.com [mailto:John.Brennen@marconi.com]
Sent: Friday, January 18, 2002 8:53 AM
To: mpls@UU.NET
Subject: LDP protocol problem?


I think I have found a serious protocol problem in LDP (RFC 3036).
The best way to describe it is to give an example...


Consider the following set of events, in chronological order
 (all messages refer to the same FEC/Label pairing; the session
 supports unsolicited label mappings; and Router B is using
 conservative label retention):


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

  Sends Label Mapping
                                    Receives Label Mapping
                                    Sends Label Release
                                     (because Router A is not the next hop)
  Sends Label Mapping
   (because the hop count changed)
                                    Router A becomes the next hop
  Receives Label Release
                                    Receives Label Mapping


The problem here is that the Label Mapping message which updates the
hop count and the Label Release message are sent more or less
simultaneously.

The result in the scenario above is that Router A thinks that there is no
valid Label Mapping advertised to Router B.  Router B thinks that it
has a valid Label Mapping.

At least three problems can come from this.  First, Router B can put traffic
onto a label which Router A has reused for another purpose.

Second, Router A may try to readvertise the FEC in the future.
Router B will not accept any such Label Mappings (LMp.10, Appendix A,
RFC 3036), unless Router A is fortunate enough to use the same
label which was originally used.

Third, if Router B tries to release the label, Router A may
report an error.


Is this analysis correct?  Is this a real problem with the protocol?
If so, is it already known?  How can this problem be solved?

    Jack Brennen