Subject: Ch 3.5.3 LDP Spec v6 - ATM VP

From: Michelle Burns <[email-masked]>
Date: Thu, 07 Oct 1999 16:44:39 +0200

ATM Session Parameters:
"When peer LSRs are connected indirectly by means of an ATM VP,
the sending LSR should set the Minimum and Maximum VPI fields to
0, and the receiving LSR must ignore the Minimum and Maximum VPI
fields."

Setting the Minimum and Maximum VPI fields to 0 is also valid 
for VPI/VCI significant labels. It should not imply VCI-only 
labels. 

How does LDP ensure that both peers set the Minimum and 
Maximum VPI fields to 0? One peer could set 0 0 (ie. it intends 
to use VCI only) and the other could set 0 x (ie. it intends 
to use VPI/VCI). Both peers have different interpretations of 
the label format because the session negotiation did not 
detect this.

Could there be a V-bit field in the ATM Session Parameters TLV 
which indicates the use of the VPI and VCI? 

This would ensure that both peers are using the same V-bit. 

This would also mean that the V-bit could be removed from the 
Label TLV since it is now known from the session initialisation. 

Michelle Burns

---

From: Srinivasan Balaji <[email-masked]>
Date: Fri, 08 Oct 1999 10:29:10 -0400

Michelle,
    Your point seems to be very valid. With the current spec.,
the two LSRs might have different understanding of the
validity of the VPI value (one might think it is invalid & the
other might think that it should be 0).
  Only when one of  them receives a label map from the
other, & would see the V-bit, would it know that there is
a mismatch (& would end up releasing the LSP). Obviously
the best thing would have been the detection of this during
the session setup.
   There can be two ways of looking at this. One is that such
cases any way need careful configuration & hence happening
of this is not very common, so may be we can live with this.
If we want to fix this problem, then as you said we need to
keep the V-bit during session negotiation stage, but not
"instead" but "also" so as to keep all possibilities open (some
ranges have VPI valid & others not, for theoretical
possibilities).

Thanks
Balaji.

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