RFC7772 Unicast RAdv Replies

[Martin Weinelt]

Hi all,

we're users of Birds RAdv implementation in a mesh networking project
with many opportunistic WiFi users on mobile devices.

We and our users would likely profit from a reduced rate of router
advertisments that we see as a result from high client turnover.

RFC7772¹ describes in 5.1:

       Router manufacturers SHOULD allow network administrators to
       configure the routers to respond to Router Solicitations with
       unicast Router Advertisements if:

       *  The Router Solicitation's source address is not the
          unspecified address, and:

       *  The solicitation contains a valid Source Link-Layer Address
          option.

   2.  Administrators of networks that serve large numbers (tens or
       hundreds) of battery-powered devices SHOULD enable this behavior.

   3.  Networks that serve battery-powered devices SHOULD NOT send
       multicast RAs too frequently (see Section 4) unless the
       information in the RA packet has substantially changed.  If there
       is a desire to ensure that hosts pick up configuration changes
       quickly, those networks MAY send frequent Router Advertisements
       for a limited period of time (e.g., not more than one minute)
       immediately after a configuration change.


There is already an implementation for this behaviour in radvd² with the
peculiarity that they ignore missing SLLA, because little to no clients
actually sent them back in 2017. Note that the RFC does not make SLLA a
MUST. They justify unicast responses in absence of SLLA as follows:

   However, testing shows that many clients do not set the SLLA option:
   https://github.com/reubenhwk/radvd/issues/63#issuecomment-287172252
   As of 2017/03/16:
    - macOS 10.12.3 sierra - sends SLLA 2 times out of 4
    - iOS 10.2.1 (iPhone 5s) - no SLLA
    - Android 7.0 (sony xperia phone) - sends SLLA
    - Android 5.1 (nexus 7 tablet) - sends SLLA
    - Ubuntu 16.04.2 LTS w/ Network Manager, running 4.9 kernel (dell
laptop) - no SLLA
    - Windows 10 (dell laptop) - no SLLA

   We decide to ignore the SLLA option for now, and only require the
   unspecified address check. Clients that did not set the SLLA option
   will trigger a neighbour solicit to the solicited-node address trying
   to resolve the link-local address to, this would still be less
   traffic than the all-nodes multicast.


I'd be very happy if you would consider implementing this BCP and would
especially appreciate it if there were a knob to make Bird reply with
unicast radv replies to router solications and only use multicast
advertisments in larger intervals.

Thanks for your consideration!

Martin Weinelt


[1] https://tools.ietf.org/html/rfc7772
[2]
https://github.com/reubenhwk/radvd/commit/30328bdbfc1f08ea100de9a6011f981cc4d916f2

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