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

[[e-mail]]

Orange Restricted
From: mpls <[e-mail]> On Behalf Of Stewart Bryant
Sent: Thursday, August 4, 2022 3:18 PM
To: mpls <[e-mail]>
Cc: [e-mail]
Subject: [mpls] A comment on the TTL in draft-dong-mpls-mna-encaps-00








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

[Bruno] May be that would call for setting aside the last bit and always send it to 1. (hence reducing RSV to 5 bits)

[Bruno] Also if PHP is used, the penultimate node may recopy the TTL value from the top label to the SPL, which would erase fields.

[Bruno]  Another comment regarding the TC. If PHP is used, the egress would receive the MNA as top label and may select its CoS treatment based on the TC. So there might be some merit in not overloading this TC field.

[Bruno] In https://www.ietf.org/archive/id/draft-jags-mpls-mna-hdr-01.html#name-overview we intentionally did not repurpose those two fields and explained why.

--Bruno

- Stewart

_________________________________________________________________________________________________________________________

Ce message et ses pieces jointes peuvent contenir des informations confidentielles ou privilegiees et ne doivent donc
pas etre diffuses, exploites ou copies sans autorisation. Si vous avez recu ce message par erreur, veuillez le signaler
a l'expediteur et le detruire ainsi que les pieces jointes. Les messages electroniques etant susceptibles d'alteration,
Orange decline toute responsabilite si ce message a ete altere, deforme ou falsifie. Merci.

This message and its attachments may contain confidential or privileged information that may be protected by law;
they should not be distributed, used or copied without authorisation.
If you have received this email in error, please notify the sender and delete this message and its attachments.
As emails may be altered, Orange is not liable for messages that have been modified, changed or falsified.
Thank you.