[mpls] A comment on the TTL in draft-dong-mpls-mna-encaps-00

[Stewart Bryant <[e-mail]>]

3 <https://datatracker.ietf.org/doc/html/draft-dong-mpls-mna-encaps#section-3>.  MNA Indicator

   The MNA Indicator is introduced to indicate the presence of any MNA
   information in the packet.  It can be used to indicate the existence
   of MNA actions and the optional associated data in the ISD, or the
   PSD or both.  Since this indicator is generic for all types of MPLS
   network actions, it is reasonable to allocate a basic Special Purpose
   MPLS label (bSPL) for it.  The TC and TTL fields of the MNA Indicator
   are redefined as flags.

   The format of the MNA Indicator is shown as below:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       MNA Indicator=SPL (TBA)         |H|I|P|S|ISF|    RSV    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 1. The format of MNA Indicator

   Where:

<snip>


   *  RSV: The rest 6 bits in the original TTL field are reserved for
      future use.  They MUST be set to 0 on transmit and MUST be ignored
      on receipt.

SB> There might be some merit in setting the RSV to 0b111111

SB> Then if some router receiving the SPL predecremented the TTL (not unknown in hardware) the upper bits of the TTL and hence the indicators would be unchanged

- Stewart