Re: [DNA] Re: LinkID v.s. Landmark Prefix

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

> Prefix Landmarks
> ----------------
> 
> 
> A Host H, exists and moves around the Internet.
> 
> On an link unvisited by the host, Link L, a prefix, P is
> retracted by one of the routers Router1. Subsequent to this,
> the actual lowest prefix on the link is Prefix Q.
> 
> 
> Another router, Router2 fails to see the prefix retraction.
> (Perhaps RAs are lost, or Router1 didn't comply with 
> section 6.2.6 of RFC2461).
> 
> The prefix is then reassigned within its previous 
> lifetime to another link, link M, whose lowest prefix
> was prefix T.  Link M, has also not been visited by
> host H.
> 
> Prefix Landmarking is in use on both Link L and
> Link M.  On Link M, routers advertise either P or T 
> but are aware of both prefixes' presence.
> 
> 
> On Link L, Router2 advertises prefix Q, but also
> believes prefix P is on-link.
> 
> The host H arrives on Link L, and learns that link's 
> prefixes  (for example, configuring an address from Q).
> The host then moves to Link M and solicits
> the routers and asks if one of the previous link's
> prefixes is available here.
> 
> If it asks about P - 'Is P on this Link?', then 
> the routers will answer (correctly) 'Yes'.
> In this case, no change will be detected upon arrival to 
> link M, and the previous prefix will remain configured.
> 
> If the host asks 'Is Q on this Link?', then it would
> receive a 'No' answer, and would detect change.

This case couldn't actually happen, because the draft requires
that the landmark be a prefix that the host has an address
configured with.  It couldn't get this from link L.

This is only a minor point though, because the following case
could occur because the host might configure an address with
prefix P when it visited link M and then router 2 would answer
yes if it asked about prefix P when it moved to link L.

Brett.

> 
> Alternatively, if the host instead visited Link M and then
> Link L, the host would similarly have had problems if 
> it asked (on Link L) 
> 
> 'Is P on this link?" but not if it asked 'Is T on this link?'
> Querying the moved prefix will be unsuccessful, even though 
> on link M, the prefix is advertised natively in PIOs.