The MPLS WG Archive

[Eric Gray <ewgray2k@netscape.net>]

Title:

Nick,

    See in line.

Nick Weeds wrote:
> Ina,
>
> Just a couple of points on the update to RFC 3036...
>
> First, the list of issues not addressed says that RFC 3036 already says that
> loop detection should not be used in DU mode.  I can't find this statement
> in RFC 3036.  Which section is it in?

I'm pretty sure discussed this on the list, and determined that it is
more 
appropriate to address when a feature should or should not be used in 
an applicability document - such as RFC 3037.

I don't know that there is universal agreement that loop detection is
not
used in the DU mode, nor is it relevant from a specification perspective
whether or not it "should" be used - as long as there is agreement that
it
either is or is not required to be supported.
> Second, the LDP protocol can fail if a Label Mapping crosses with a Label
> Release.
> This was raised by Kishore Tiruveedhula [tiruveedhula@avici.com] on 25th
> August in connection with loop detection, but it is a more general problem
> with the protocol.
>
> The problem arises with the following sequence:
> (1) Router D sends a Label Mapping
> (2) Router U sends a Label Release
> (3) Independently router D sends an updated Label Mapping (same FEC and
> label, different details).
>
> If messages (2) and (3) cross in transit then the label mapping is
> programmed on U (on receipt of the updated Label Mapping) but released on D
> (on receipt of the Label Release).  Any data sent using the label will be
> lost.
>
> The problem can occur whenever the downstream router sends a Label Mapping
> message to update an existing mapping.  This is probably unusual in
> practice, but RFC 3036 allows it and indeed describes it for hop count
> changes when using independent control (see Kishore's email).  From a quick
> check, the MTU signaling extension
> (draft-ietf-mpls-ldp-mtu-extensions-03.txt) also seems to require Label
> Mapping updates.  I am not aware of other examples, but the problem is a
> potential trap for any LDP protocol extension.
This is not a problem with any reasonably robust implementation.
A "reasonably robust" implementation does something intelligent
with the knowledge that it is getting packets with labels it does not
know how to handle.  Something obvious, like sending a (possibly
redundant) label release to the router from which it receives them.

This is not as simple, if the labels it is getting are the result of
PHP.
But even this can be dealt with.

> It is unclear how to proceed on this, as potential fixes are likely to
> require protocol changes.  Perhaps it would be sufficient to explain the
> problem and suggest how to avoid it.
There is no "potential fix", and this has been discussed before. It is
not the task of the author(s) of an RFC to tell their competitors how
to build an implementation that works.

No number of messages - where each message is intended to be
an acknowledgement of the one it is  response to - can ever ensure
that two parties to an exchange are in possession of exactly the same 
awareness of the state of a particular exchange token. This is known
variously as the "two generals" or "common knowledge" problem.

> (This problem was drawn to my attention in discussions of C-bit negotiation
> in draft-ietf-pwe3-control-protocol-xx.txt.  This negotiation takes care to
> avoid Label Release messages in response to Label Withdraw "Wrong C-bit".
> The protocol problem in RFC 3036 was suggested as one reason for suppressing
> the Label Release, but there are probably other reasons too.)
>
>     Nick.
>
> > -----Original Message-----
> > From: mpls-bounces@lists.ietf.org 
> > [mailto:mpls-bounces@lists.ietf.org]On
> > Behalf Of Ina Minei
> > Sent: 27 September 2004 19:31
> > To: mpls@ietf.org
> > Subject: [mpls] For your review - Issues/errors/clarifications in
> > RFC3036
> >
> >
> >
> >    As part of the effort to move RFC3036 to draft standard, here is an
> > annotated version of RFC3036, with the changes enclosed by ###.
> > There is a separate section summarizing all changes towards 
> > the end of the
> > document.
> >
> >    Please review the changes and send comments to the list by October
> > 11th.
> >
> >    At the end of this mail is a list of issues that were raised but
> > were not included in the annotated RFC (with an explanation of
> > why).
> >
> >    Please review both the RFC changes and the list of issues not
> > included. Also let me know if I forgot to include any other issues
> > that were raised on the list.
> >
> >    Many thanks to all who contributed, and in particular to Bob Thomas
> > for maintaining an extensive list of errors/issues over the years.
> >
> >
> >             Thank you,
> >
> >             Ina
> >
> >
> > Issues that were raised but not included in the annotated RFC
> > =============================================================
> >
> > - Issue: discussion on the merits/drawbacks of independent-control and
> > ordered-control
> > - Issue: discussion on which FECs should be advertised
> > Reason why not included: The above two issues are either for the
> > applicability doc or for the "experiences with the protocol" document.
> >
> > - Issue:  minor optimization: if A is a stub node, i.e., only one LDP
> > session, does it really have to send a label mapping for 
> > every FEC that
> > it has, or can it do it lazily, e.g., when it has a second 
> > LDP session?
> > Reason why not included : It is not clear what problem this change in
> > behavior brings, and what would be the added benefit,the issue must
> > be discussed on the list first.
> >
> > - Issue: the ldp loop detection mechanisms don't make sense in DU.
> > We should add something explicitly that
> > says that these TLVs should not be used in DU mode. ( This is the
> > current practice in all implementations that I know of )
> > Why not included: RFC 3036 already states this in the section
> > describing these TLVs, when it talks about the usage of the TLVs.
> >
> > - Issue: the rfc should specifically say that  a  wildcard release
> > message should be sent only in response to a wildcard 
> > withdraw message.
> > Reason not included: it is covered in the rules in the appendix.
> >
> > - There is a long list of issues that was added to the "for future
> > study" area of the RFC. The list is quite long, we could probably
> > remove some of the items.
> >
> >   
>
> Nick Weeds
> Software Developer
> Network Protocols Group
> Data Connection Ltd
> Tel:    +44 1244 305200
> Fax:    +44 1244 312422
> Email:  Nick.Weeds@dataconnection.com
> Web:    http://www.dataconnection.com
>
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