[geonet/its] Comments on draft-petrescu-autoconf-ra-based-routing

[Emmanuel Thierry <[e-mail]>]

Hello,

I took a look to the I.D. draft-petrescu-autoconf-ra-based-routing and i have some comments and questions about it. I don't know if it is the best place to discuss, please redirect me to the appropriate list if not !


1/ RA Routing limit:
My understanding of this draft is that RAs will be "forwarded" across multiple links. A look to section 4.3 makes me think that, since Route Information are stored by the intermediate node (MR2), then resent to the next Mobile Router (either MR1 or MR3). However i don't see any limit. It may be a concern if there is a undiscontinuing queue of moving objects. Where this broadcast is supposed to be stopped ? Shouldn't there exist a TTL for that ?

2/ Fixed Scene:
The concept of "Fixed Scene" is quite abstract to me, i don't understand what it means and how it could be interpreted in a protocol standard. At no moment i envision in which Fixed Scene i am supposed to be. Could you elaborate on that ?

3/ Preference value:
You use a preference value of 0x09, so that this route is not preferred among others. I think there is a typo and it should be 0x03, as stated in RFC 4191.

4/ Default router
Even if you set the preference value to 0x03 to lower the preference of the advertising router, it remains a valid router for the receiver of this Router Advertisement. As a consequence, if there is no other router available (no road infrastructure), the Mobile Router which receives this Router Advertisement (or a node that don't understand the newly introduced "M" flag) may try to use it as a default router. Moreover, if a roadside router chooses to use the same preference value to lower its preference compared to other routers (for example a roadside router which serves as a backup router), a Mobile Router advertising its prefix through Router Advertisements may be preferred among this valid backup router.

It is unclear in section 4.3 if by "preference" and "lifetime", you mean the corresponding fields in the RA itself or in the RIO. I assumed you meant in the RA. The RFC 4861 section 4.2 clearly states that the proper way to indicate that an advertising router is not likely to forward traffic for the default route, is to set Router Lifetime to 0. So the Router lifetime shouldn't be used there. A Mobile Router (or roadside router) that wants to advertise specific prefixes may want to advertise it independently of whether the node is or isn't a router. The draft should allow to use these fields normally, i.e. set the Router Lifetime to 0 and Router preference to 0x00 if you don't forward traffic for ::/0, set the Router Lifetime and Router preference to an appropriate value if you forward traffic for ::/0.

In order to code lifetime, Mobile Routers should use the lifetime field in the RIO, and set the preference field in the RIO to an appropriate value, according to whether or not the node considers it is the best candidate for reaching the destination.

Best regards
Emmanuel Thierry