The MPLS WG Archive

[<neil.2.harrison@bt.com>]

Thanks Huub....back in 2002 when I wrote this I did flag up the issue of
misconnectivity even then......but reading my own words I was only
thinking about a bottom of stack problem, ie S=1 in preceding header.
The text Stewart picked out of Y.1720/Appendix II seems to imply that
this can also apply to an LSP in the *middle* of an arbitrary
stack....ergo the questions wrt:
-	what are the S bit settings in this stack?
-	what happens if misconnectivity occurs, ie could such an LSP
arriving unexpectedly at some LSR end-up parsing the sequence number
field assuming it had the 'normal' MPLS 4 octet header structure (ie
label/S/EXP/TTL)?.....though as I noted in a previous mail, there is an
even larger issue here that the fields in traffic units should have
consistent semantics network-wide (any mode/technology)

regards, Neil

> -----Original Message-----
> From: Huub van Helvoort [mailto:hhelvoort@chello.nl] 
> Sent: 26 January 2007 12:10
> To: Harrison,N,Neil,JCGA1 R
> Cc: mpls@ietf.org; stbryant@cisco.com; pwe3@ietf.org; 
> ghani.abbas@ericsson.com
> Subject: [PWE3] Re: Y.1720 liased text
> 
> 
> Hello Neil,
> 
> You responded:
> 
> > Thanks Huub,
> > 
> > Can you clarify how the S bit is set when this 1+1 seq no. field is 
> > used?
> 
> I checked the appendix II twice (because my left eye is still 
> 10%), I could not find any text referring to setting the S 
> bit. I only see that the 4 octet sequence number is placed 
> right after the shim header.
> 
> > What is puzzling me is this sentence in the text Stewart extracted 
> > from Y.1720/Appendix 11:
> > 
> > "Note that packet 1+1 can be provided at any level of the 
> hierarchy of 
> > a nested LSP."
> > 
> > I sort-of read this as there can be a set of nested LSPs (hence 
> > concatenated MPLS headers) and that the protection mechanism can be 
> > applied to any one of these within such a stack.  Am I reading this 
> > right?
> 
> I agree with your reading.
> 
> > If I am, then please see my concerns of yesterday on this 
> > thread....which, in essence, is querying what happens if there is 
> > misconnectivity and the LSP carrying the 1+1 Seq No. stuff 
> arrives at 
> > some arbitrary LSR.....how will that LSR parse/interpret 
> the fields it 
> > sees?
> > 
> > I have no background on Y.1720, so my apologies if I am 
> asking a dumb 
> > question.
> 
> There are no dumb questions, only dumb answers. Maybe this is one:
> 
> This morning I did some further investigating where the text 
> originates and found it in the IETF archives: 
> http://www.watersprings.org/pub/id/draft-nagarajan-ccamp-mpls-
packet-protection-00.txt

It was also discussed on the MPLS WG list and there I found the answer
to your concerns expressed above (provided by yourself ;-)

http://cell.onecall.net/mhonarc/mpls/2002-Apr/msg00211.html

Cheers, Huub.

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