RE: CPE router acting as host on its WAN interface (RE: draft-ietf-v6ops-ipv6-cpe-router-03.txt WGLC)

["Hemant Singh (shemant)" <shemant@cisco.com>]

-----Original Message-----
>From: owner-v6ops@ops.ietf.org [mailto:owner-v6ops@ops.ietf.org] On
Behalf Of Templin, Fred L
>Sent: Monday, January 04, 2010 8:01 PM
>To: Fred Baker (fred); v6ops@ops.ietf.org
>Cc: kurtis@kurtis.pp.se; rbonica@juniper.net
>Subject: CPE router acting as host on its WAN interface (RE:
draft-ietf-v6ops-ipv6-cpe-router-03.txt WGLC)

>Fred,

>A concern I raised on the list a while back centers around
>the behavior of the CPE router acting as a host on its WAN
>interface per section 4.1:

6man hasn't even agreed with you as to what is the problem with your
IsRouter flag discussion.

>  "When the router is attached to the WAN interface link it must act as
>   an IPv6 host for the purposes of stateless or stateful interface
>   address assignment ([RFC4862]/[RFC3315])."

>and per WPD-3:

>  "WPD-3:  Absent of other routing information the IPv6 CE router MUST
>           use Router Discovery as specified in [RFC4861] to discover a
>           default router and install a default route in its routing
>           table with the discovered router's address as the next-hop."

>To my understanding, this behavior would involve the CPE
>router sending Router Solicitation (RS) messages on its
>WAN interface in order to receive Router Advertisement (RA)
>messages. According to Section 6.2.6 of RFC4861, however:

>  "Whether or not a Source Link-Layer
>   Address option is provided, if a Neighbor Cache entry for the
>   (RS)'s sender exists (or is created) the entry's IsRouter flag
>   MUST be set to FALSE."

>RFC4861 goes to some level of detail to specify the setting
>of the IsRouter flag under various circumstances (including
>the RS case), but it only says what actions should be taken
>based on the flag as result of receiving Neighbor Advertisement
>messages. Other actions based on the IsRouter flag setting do
>not seem to be specified.

Not quite.  By default the IsRouter flag is set of FALSE.  Also, see
Appendix D of RFC 4861.

>In the linux kernel, it appears that the kernel will in some
>circumstances garbage-collect FIB entries that have a nexthop
>with the IsRouter flag set to FALSE. It is not clear what other
>router implementations would do based on the IsRouter setting,
>but it seems odd that the IsRouter flag in neighbor cache
>entries corresponding to CPE routers would be set to FALSE
>which in the linux case at least may lead to interoperability
issues.

What specific interoperability issues? What specific circumstances?
Further, since the FIB includes the Destination Cache, note that NUD
purges entries in the Destination Cache. Garbage collection can remove
an entry from the Default Router List - every entry in the Default
Router List is a router.  NUD can also purge entries in the Neighbor
cache.  If the CE Router sends an RS from its WAN interface, the router
receiving such and RS sets the IsRouter flag for the CE Router entry to
FALSE.  So what?  When the CE Router requests a delegated prefix, the
delegating router will know the CE Router is requesting router. 

Hemant