[6tsch] architecture with remote BBR

[Thomas Watteyne <[e-mail]>]

All,

There is a case I believe we are not covering explicitly in the
architecture.

The architecture draft now considers the following topology:

               ---+------------------------
                  |      External Network
                  |
               +-----+                  +-----+
               |     | Router           |     | PCE
               |     |                  |     |
               +-----+                  +-----+
                  |                        |
                  |     Subnet Backbone    |
            +--------------------+------------------+
            |                    |                  |
         +-----+             +-----+             +-----+
         |     | Backbone    |     | Backbone    |     | Backbone
    o    |     | router      |     | router      |     | router
         +-----+             +-----+             +-----+
    o                  o                   o                 o   o
        o    o   o         o   o  o   o         o  o   o    o
   o             o        o  LLN      o      o         o      o
      o   o    o      o      o o     o  o   o    o    o     o



The backbone needs to be one broadcast domain for the ND proxy operations
defines in draft-thubert-6lowpan-backbone-router-03 to work.

Now, let's consider a campus-wide deployment, where the requirements is
that (1) all the nodes use the same IPv6 prefix, and (2) all are managed by
the same PCE. Since BBRs are "far apart", they will not all live on the
same (broadcast) backbone.

This is a very realistic scenario that I have come across multiple times,
and which I believe 6TSCH group could/should address.

I can see the following options:

*Option 1*: "under-the-hood" tunneling

When installing the network, network administrators interconnect the
different pieces of the BB using some VLAN solution, essentially recreating
a broadcast domain.

pros:
- This option does not require any change the ND operation.
cons:
- IMO, in most multi-BBR deployments, the remote BBR case is the rule
rather than the exception. Using tunnels looks more like a "patch" which
might be seen as overly complex if it needs to be applied all the time.


               ---+------------------------
                  |      External Network
                  |
               +-----+                  +-----+
               |     | Router           |     | PCE
               |     |                  |     |
               +-----+                  +-----+
                  |                        |
                  |     Subnet Backbone    |    *==========*
            +--------------------+--------------  *TUNNEL*  ----+
            |                    |              *==========*    |
         +-----+             +-----+                       +-----+ (*remote*)
         |     | Backbone    |     | Backbone              |     | *Backbone*
    o    |     | router      |     | router                |     | *router*
         +-----+             +-----+                       +-----+
    o                  o                   o                 o   o
        o    o   o         o   o  o   o                  o  o   o    o
   o             o        o  LLN      o      o             o      o
      o   o    o      o      o o     o  o   o             o    o     o


*
*
*Option 2*: PCE responsible for forwarding to correct BBR

Each BBR establishes an explicit (and secure) connection to the PCE. Since
the PCE is aware of the nodes connected through each BBR, it can forward
some inbound packet to the appropriate BBR.

The functionality of the PCE and Router can be merged. The PCE/Router gets
a packet for a particular node, and forwards it to the appropriate BBR over
the explicit connection to that BBR.


                --------------+-------------------
                              |  External Network
                              |
                           +-----+
                           |     | PCE/Router
                           |     |
                           +-----+
                            ^ ^ ^

                            | | |

            +---------------+ | +----------------+
            |                 |                  |
            v                 v                  v

         +-----+           +-----+            +-----+
         |     | Backbone  |     | Backbone   |     | Backbone
    o    |     | router    |     | router     |     | router
         +-----+           +-----+            +-----+
    o                  o                   o                 o   o
        o    o   o         o   o  o   o         o  o   o    o
   o             o        o  LLN      o      o         o      o
      o   o    o      o      o o     o  o   o    o    o     o


*Option 3*: hybrid


This is the same as option 2, but the router and the PCE as separate.
The PCE acts as the ND proxy for all the nodes attached to all the
BBRs it is managing. The router is a regular router.


  ---+-----------------------

     |   External Network
     |
  +-----+                  +-----+
  |     | Router           |     | PCE
  |     |               +--|     |
  +-----+               |  +-----+

| | ^ ^ ^

     |                  |   | | |

  ------------------------  | | |
                            | | |

                            | | |

            +---------------+ | +----------------+
            |                 |                  |
            v                 v                  v

         +-----+           +-----+            +-----+
         |     | Backbone  |     | Backbone   |     | Backbone
    o    |     | router    |     | router     |     | router
         +-----+           +-----+            +-----+
    o                  o                   o                 o   o
        o    o   o         o   o  o   o         o  o   o    o
   o             o        o  LLN      o      o         o      o
      o   o    o      o      o o     o  o   o    o    o     o



Thoughts?


Thomas