Subject: path vectors !!

From: Manoj Dutta <[email-masked]>
Date: Wed, 16 May 2001 00:51:32 -0700

Hi,

In RFC 3036, section A.1.1 (receive label request processing), there is
a parameter RAttributes which is basically the attributes received in
the label request message. Now in subsection LRq.9, this same
RAttributes is being used for creating label mapping attributes which
will be sent back to the requesting LSR.
Now, consider the following case :

LSR A is configured to send path vectors in label request messages. It
will include its own lsr id in the request sent to LSR B.  Now
LSR B uses this same path vector and adds its lsr id in the lable
mapping message sent back to LSR A. LSR A will find its own lsr id in
the mapping received from LSR B and declare that a loop has been
detected which is incorrect.

I thnk LSR B, in this case, should be using the attributes which it
might have received from its next hop ,if any, while preparing label
mapping attributes to respond to request from LSR A. Please let me know
if this is right.

Thanks in advance
manoj

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From: Eric Gray <[email-masked]>
Date: Wed, 16 May 2001 10:16:22 -0400

Manoj,

    The procedures in the LDP appendix may be somewhat difficult to
plow through.  In order to understand what goes on with RAttributes,
we need to read through 'Common Label Distribution Procedures' in
Appendix section A.2 - in particular:

    Prepare_Label_Request_Attributes
    Prepare_Label_Mapping_Attributes

    I believe your question shows that you did read through, but possibly
did not see some of the implications of these defintions.

    Although it may not be clear at first, the definitions for RAttributes and
SAttributes in the above common procedures imply that the RAttributes
argument in the Prepare_Label_Mapping_Attributes procedure is intended
to be that which was previously retained (see FEC.1 [2 places], LRq.9
[2 places] and LMp.16) or recently received (see LMp.16, LMp.23 and
LMp.29 [2 places]) from a Label Mapping which is being propagated.
Alternatively (LRq.9), it is possible that the downstream LSR is the egress
or is operating in DoD, independent distribution mode - in which case, if
there is no downstream mapping, RAttributes would be InitAttributes with
a hop count of unknown (if DoD, independent, intermediate LSR) or one
(if LSR is egress) and a Path Vector including only the LSR ID of sending
(downstream) LSR (only if loop detection is enabled).

    In returning a Label Mapping in a direct response to a Label Request, the
RAttributes of the Label Request would never be used.  I believe the use of
RAttributes is clear enough in the case where a Label Mapping is being
propagated as a result of receiving a Label Mapping.  Hopefully this may
be clearer now...

--
Eric Gray