O Override flag. When set, the O-bit indicates that
the advertisement should override an existing cache
entry and update the cached link-layer address.
When it is not set the advertisement will not
update a cached link-layer address though it will
update an existing Neighbor Cache entry for which
no link-layer address is known. It SHOULD NOT be
set in solicited advertisements for anycast
addresses and in solicited proxy advertisements.
It SHOULD be set in other solicited advertisements
and in unsolicited advertisements.
Target link-layer address
The link-layer address for the target, i.e., the
sender of the advertisement. This option MUST be
included on link layers that have addresses when
responding to multicast solicitations. When
responding to a unicast Neighbor Solicitation this
option SHOULD be included.
The option MUST be included for multicast
solicitations in order to avoid infinite Neighbor
Solicitation "recursion" when the peer node does
not have a cache entry to return a Neighbor
Advertisements message. When responding to unicast
solicitations, the option can be omitted since the
sender of the solicitation has the correct link-
layer address; otherwise it would not have be able
to send the unicast solicitation in the first
place. However, including the link-layer address in
this case adds little overhead and eliminates a
potential race condition where the sender deletes
the cached link-layer address prior to receiving a
response to a previous solicitation.
Linux is does not seem to fulfull these recommendations. These seem to
affect USAGI too.
1) When soliciting for anycast address, override flag in the reply SHOULD
NOT be set. This may happen under certain circumstances. Proxy
advertisements are already handled as recommended.
There doesn't seem to be any checks for anycast in ndisc yet.. Probably
not too supported yet.
2) In neighbor solicitations to unicast addresses, lladdr is not added to
the router advertisement.
Adding lladdr could probably be always done, except when L2 medium
doesn't have lladdr. The only downside here is added space requirements
(see above for discussion) which are IMO negligible.
Warning: the patch has not been tested extensively, but compiles and
doesn't seem to cause too major side effects. Double check for
sanity / better way to do it.
Pekka Savola "Tell me of difficulties surmounted,
Netcore Oy not those you stumble over and fall"
Systems. Networks. Security. -- Robert Jordan: A Crown of Swords
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