Re: [DNA] Text on Router Reachability for DNA Protocol Specification

[Brett Pentland <brett.pentland@eng.monash.edu.au>]

Hi James,

The proposed text looks very good (except for a swapped RS/RA - see
below).

An observation I have is that it seems that a single lost RA from the
default router that is "currently" being used would cause the host to
swap to a new default router, since the current one would be STALE and
there may be new ones that are REACHABLE.  Presumably this causes some
sort of (minor) disruption to the packet flow.

Would there be any value in having some sort of configuration variable
on the host to specify the number of missed RAs before switching to a
new default router when DNA otherwise indicates no movement?

Regards,
Brett.

James Kempf wrote:
> -----------------------------
> 
> Remove the last paragraph in Section 5.2.5, put in the following new 
> section
> after Section 5.2.5.1.
> 
> 5.2.5.2 Router Reachability Detection and Default Router Selection
> 
> The receipt of a unicast RA from a router in response to a multicast RS
> indicates that the host has bi-directional reachability with the routers
> that responded. Such reachability is necessary for the host to use a router
> as a default router, in order to have packets routed off the host's current
> link. If the destination address of the received RA is a unicast address,
> the host knows the router heard its RS, and therefore that the host
> has reachability with the router.
> 
> Prior to sending a DNA RS in response to an indication of link change,
> the host SHOULD set all Neighbor Cache entries for routers on its Default
> Router List to STALE. When the host receives an RS in reply to the RA, the
                                                   ^^                 ^^

> host SHOULD mark that router's Neighbor Cache Entry [3] as REACHABLE,
> or add a Neighbor Cache Entry in the REACHABLE state if one does not
> currently exist.
> 
> The host SHOULD also update its Default Router List in the following
> fashion. If any of the routers returning RAs are already on the default
> router list, the host SHOULD use the information in the RA to update the
> Default Route List entry with the new information. The host SHOULD
> add entries to the Default Router List for any routers returning RAs that
> are not on the list.The host SHOULD confine selection of a router from the
> Default Router List to those routers whose Neighbor Cache entries are in 
> the
> REACHABLE state. Note that the Default Router List SHOULD be updated as
> described here regardless of whether the RA indicates that the host has
> changed to a new IP link, since changes in router reachability are possible
> on some link types even if the host remains on the same IP link.
> 
> Note that this procedure does not prevent a MN from sending packets to its
> current default router while the RA solicitation is in progress and if
> reachability with the current default router is unchanged, there should
> be no change in default router after the RA solicitation completes. If the
> current default router is still reachable, it will forward the packets.
> 
> 
>