RE: [mpls] For your review - Issues/errors/clarifications in RFC3 036

[Nick Weeds <[e-mail]>]

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?

Second, the LDP protocol can fail if a Label Mapping crosses with a Label
Release.
This was raised by Kishore Tiruveedhula [[e-mail]] 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.

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.

(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: [e-mail] 
> [mailto:[e-mail]]On
> Behalf Of Ina Minei
> Sent: 27 September 2004 19:31
> To: [e-mail]
> 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:	[phone]
Fax:	[phone]
Email:	[e-mail]
Web:	http://www.dataconnection.com

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