RE: 6MAN WG Last Call:draft-ietf-6man-ipv6-subnet-model-00.txt

"MILES DAVID" <> Fri, 27 June 2008 00:57 UTC

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Subject: RE: 6MAN WG Last Call:draft-ietf-6man-ipv6-subnet-model-00.txt
Date: Fri, 27 Jun 2008 08:57:12 +0800
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From: "MILES DAVID" <>
To: "Wes Beebee \(wbeebee\)" <>, "Wojciech Dec \(wdec\)" <>, "Brian Haberman" <>, <>
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Wes & Hemant,

Can we walk through the situation of hosts without routers where you
suggest a possible issue?

HostA ----(link)---- HostB



Are we suggesting HostA may src from 2002:db8:200::1234 to
2002:db8:100::9999, ie, we are forgoing source-address selection and are
overriding this behaviour?
Assuming that is the case, and assuming HostB did in fact populate a
Neighbour Cache entry with 2002:db8:200::1234 - STALE, I cannot see how
this would affect HostB's next-hop/on-link determination.

According to the current text in RFC4861 HostB would perform the
following on the first packet to send to 2002:db8:200::1234:
1) Check destination cache (empty, never seen this destination before)
2) Check Prefix List for on-link determination (off-link) -
2002:db8:200::1234 is not in the Prefix List
3) If off-link, select a router from Default Router List and determine
next-hop IP (no default routers)
--end in our example as there are no routers--
4) Consult Neighbour Cache for link-layer address of next-hop, invoke
Address Resolution if needed
5) Cache result in destination cache

I understand Neighbour Cache is not consulted for next-hop/on-link
determination, and Destination Cache is updated with the result of the
Prefix-List lookup (next-hop determination). So while there is a STALE
entry in the Neighbour Cache, it is never queried. Similar to the case
of a router, I think we would need to update a host's equivalent (the
Prefix List) to affect forwarding.

It would be good to better understand the scenario you are seeing
between host and host. 

Best Regards,


-----Original Message-----
From: Wes Beebee (wbeebee) [] 
Sent: Friday, 27 June 2008 12:29 AM
To: Wojciech Dec (wdec); Brian Haberman;
Cc: MILES DAVID; Bob Hinden
Subject: RE: 6MAN WG Last Call:draft-ietf-6man-ipv6-subnet-model-00.txt

This rule derives directly from the Terminology section of RFC 4861
(definition of on-link).

Note that the presence of a bogus entry causes no harm (the routing
table takes precedence 
over the ND cache in this case).

However, the removal of the rule DOES cause harm in the case of
communication without routers.

Therefore, we currently see no reason to change the text.

- Wes & Hemant

-----Original Message-----
From: [] On Behalf Of
Wojciech Dec (wdec)
Sent: Thursday, June 26, 2008 10:05 AM
To: Brian Haberman;
Cc: MILES DAVID; Bob Hinden
Subject: RE: 6MAN WG Last Call:draft-ietf-6man-ipv6-subnet-model-00.txt

Based on a recent thread
( the
following paragraph from the draft appears to warrant some more thought
if not outright a revision

"   In addition to the Prefix List, individual addresses are on-link if
   they are the target of a Redirect Message indicating on-link, or the
   source of a valid Neighbor Solicitation or Neighbor Advertisement
   message.  Note that Redirect Messages can also indicate an address is
   off-link.  Individual address entries can be expired by the Neighbor
   Unreachability Detection mechanism."

Using unconditionally the source address of a neighbour solicitation or
NA to determine on-link would indeed appear to be undesirable, unless
the intent is allow some direct host-host cross subnet/prefix
communication without a router involved at any stage (this is not a good
idea IMO). A constraint could be introduced such as: A host only learns
on-link addresses from the source of NS and NA messages iff it already
has an on-link prefix that would cover that address. Learning from
Redirect messages would continue to be allowed.

My 2c.

> -----Original Message-----
> From: [] On Behalf 
> Of Brian Haberman
> Sent: 26 June 2008 14:17
> To:
> Cc: Bob Hinden
> Subject: 6MAN WG Last Call:draft-ietf-6man-ipv6-subnet-model-00.txt
> All,
>       This message starts a 3-week 6MAN Working Group Last Call on
> advancing:
>       Title     : IPv6 Subnet Model: the Relationship between
>                   Links and Subnet Prefixes
>       Author(s) : H. Singh, et al.
>       Filename  : draft-ietf-6man-ipv6-subnet-model-00.txt
>       Pages     : 8
>       Date      : 2008-05-08
> as a Proposed Standard.  Substantive comments and statements of 
> support for advancing this document should be directed to the mailing 
> list.
> Editorial suggestions can be sent to the document editor.  
> This last call will end on July 10, 2008.
> Regards,
> Brian & Bob
> 6MAN co-chairs
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