The MPLS WG Archive

[Janardan Ramesh <JRamesh@shastanets.com>]

 draft-ietf-mpls-bgp4-mpls-04.txt has the following sentence
for the NLRI.

Length(1), Label (3 - octest 1 or more labels), Prefix (variable)

The description of length says:

The length field indicates the length in bits of the address
prefix plus the label(s).


- First interpretation of this is that the label lengths are also
  encoded in bits.

  Take the example of 2 labels (48 bits) and a 26 bit prefix.
  The count will be 74. From the count it is not possible
   to say whether we have 2 labels (48 bits) and a 26 bit prefix or
   3 labels (72 bits) and a 4 bit prefix.

- Second interpretation of this is that the length field contains the
   label counbt and prefix length in bits.

   Take tha example of 2 labels and a 26 bit prefix. The length
   field will contain a count of 28. Assuming that there is
   always 1 label, this can mean that either we have 1 label
   and a 27 bir prefix or 2 labels and a 26 bit prefix.

It would be really helpful to expand the semantics of this field
and include a few examples to avoid ambiguity.

Regards
Ramesh