Subject: [PWE3] LDP PW switching point TLV usage ambiguity?

From: Ville Hallivuori <[email-masked]>
Date: Mon, 27 Oct 2008 12:36:16 +0200

Hello all,

I have a question regarding ambiguity in
draft-ietf-pwe3-segmented-pw-09.txt regarding use of SWP TLVs.

LDP PW switching point TLV (0x096D) is used both with PW status TLVs
(in notify messages and in label mapping messages) and "appended to FEC" on
label mapping messages. If S-PE reports a problem during initial label
mapping, it might transmit a label mapping message with:
  -A FEC
  -A "stack" of SWP TLVs describing PW path taken so-far
  -A label
  -A PW status TLV indicating the initial state of PWE3 circuit
  (e.g. PSN tunnel is down)
  -A SWP TLV identifying the S-PE that is reporting the problem.

My question is: how does one separate between SWP TLVs that indicate
the MS-PW path and the SWP TLV that indicates S-PE that raised a bit in
PW Status TLV, if both kind of SWP TLVs are present in same label
mapping message?

One could try to rely on positioning of TLVs... after all ch 7.4 says:
  "The PW switching point TLV is appended to the PW FEC at each
   switching point. Each S-PE can append a PW switching point TLV, and
   the order of the PW switching point TLVs MUST be preserved."

However chapter 9.5 shows in a diagram that SWP TLV associated with PW
status (in notify message, however I would assume same concept applies in
label mapping) is also appended to FEC.

Perhaps a separate type value for SWP TLV associated with PW status
would be a more robust solution?

-- 
[Ville Hallivuori][[email-masked]][http://www.iki.fi/vph/]
[ID 30EC5DF0][FP20=3B02 94F1 FF6F 9CD0 EC81  0FAE 5B3B 3D21 30EC 5DF0]
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From: Luca Martini <[email-masked]>
Date: Mon, 27 Oct 2008 12:53:51 -0600

> PW Status TLV, if both kind of SWP TLVs are present in same label
Ville,

The initial PW status TLV included with the FEC is created by the T-PEs 
only. The S-PE cannot add status information during the PW setup phase ( 
to the label mapping message). If there is a fault, the S-PE can send a 
PW status notification message right after the PW is fully negotiated. ( 
both FECs have passed thought the S-PE ).

> label mapping) is also appended to FEC.
I'm not clear on this point. the diagram in chapter 9.5 does not apply 
to the "PW FEC" in a label mapping message.
> would be a more robust solution?
I think the confusion is coming from the fact that you assumed that the 
PW status message in the label mapping message can somehow also be used 
by the S-PEs ....
This is not the case.

If there is such a requirement I would be happy to figure out a solution 
, but I would need to understand this requirement better.

Thanks.
Luca