Subject: [nemo] Jim's comments on Basic Support

From: Vijay Devarapalli <[email-masked]>
Date: Wed, 19 Nov 2003 12:04:49 -0800

hi Jim,

I have filed your comments as Issues 20 and 21 at
http://people.nokia.net/vijayd/nemo/issues.html.

>    When the Mobile Router is at home, it MAY be configured to send
>    Router Advertisements and reply to Router Solicitations on the
>    interface attached to the home link.  The value of the Router
>    Lifetime field MUST be set to zero to prevent other nodes from
>    configuring the Mobile Router as the default router.
> 
> jak> What happens if the Mobile Router advertises prefix A on the 
> home link, then moves and advertises prefix B? This could happen 
> if the Mobile Router was configured for a particular prefix on the 
> home link, then received a separate prefix when it moved away.

the above text is for the egress interface (the interface
with which the Mobile Router is attached to the home/visited
link). if the Mobile Router is not at home, it must not send
any router advertisements on its egress interface. when at
home, it maybe be configured to send out router advertisements
on its egress interface.

as for as the ingress interface (the interface on the Mobile
Network), the Mobile Router always sends router advertisements
advertising the prefix that is currently delegated/configured
for the Mobile Network.

>    A Mobile Router SHOULD NOT send unsolicited Router Advertisements
>    and SHOULD NOT reply to Router Solicitations on any egress interface
>    when that interface is attached to a visited link.  However, the
>    Mobile Router SHOULD reply with Neighbor Advertisements to Neighbor
>    Solicitations received on the egress interface, for topologically
>    correct addresses.
> 
> jak> What happens to hosts that are utilizing the router due to it 
> having sent unsolicted RAs on the egress interface on the home link 
> when the router moves away? 

that is why we recommend setting the router lifetime to zero
so that these hosts do not pick the mobile router as the
default router. they should pick some other router on the home
link.

> Wouldn't it be better just to prohibit 
> the Mobile Router from responding to RSs and sending unsolicited RAs 
> on its egress interface?

sure we can prohibit. but should we? we can do without
prohibiting it. maybe somebody will come up with something which
might need these router advertisements. it is normal router
behavior to send router advertisements on all its interfaces.

> 
>    A Mobile Router MUST NOT ignore Router Advertisements received on
>    the egress interface.  The received Router Advertisements MAY be
>    used for address configuration, default router selection or movement
>    detection.
> 
> jak> In fact, it MUST utilize these as described in the base MIP6 
> protocol for movement detection.

okay. I will modify the text.

>    In order to be able to receive packets meant for the mobile network,
>    the Home Agent advertises reachability to the mobile network.  If the
>    Home Link is configured with a prefix that is an aggregation and if
>    the Mobile Network Prefix is aggregated under that prefix, then the
>    routing updates advertising reachability to the mobile network are
>    sent only on the Home Link.  If the Home Agent is the only default
>    router on the Home Link, routes to the Mobile Network Prefix get
>    aggregated naturally under the Home Agent and the Home Agent does not
>    have to do anything special.
> 
> jak>>I'd suggest requiring the prefix to be an aggregation, or have you 
> carefully worked through a case where it isn't?

IMO, that is a deployment issue. there is another draft, targeted
at Informational, which talks about the home network.
http://www.ietf.org/internet-drafts/draft-thubert-nemo-basic-usages-00.txt.
we can add more text in this. the concensus at the WG meeting last
was to make this a WG document.

> 
>    If the Mobile Router is not authorized to use this Home Address to
>    forward packets for one or more prefixes that are present in the
>    Binding Update, the Home Agent sets the status code in the Binding
>    Acknowledgement to '142' (Not Authorized for Prefix) in order to
>    indicate this.
> 
> jak>> How, specifically, is authorization set up for a prefix between 
> the MR and HA?

using the Prefix Table on the Home Agent. ofcourse there is the
issue of filling up the Prefix Table. if the Mobile Router is
manually configured with a Mobile Network Prefix (MNP), then you
have to manually add entries to the Prefix Table. if the Mobile
Router gets the MNP through DHCPv6 based prefix delegation, the
Home Agent always acts as a DHCP relay. it is then easy for the
Home Agent to figure out which Mobile Router was delegated which
prefix.

there is a draft by Raplh Droms on extending prefix delegation
for NEMO.
http://www.ietf.org/internet-drafts/draft-droms-nemo-dhcpv6-pd-00.txt

Vijay

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From: Alexandru Petrescu <[email-masked]>
Date: Wed, 19 Nov 2003 21:32:32 +0100

Vijay Devarapalli wrote:
>> home link, then moves and advertises prefix B? This could happen if
>>  the Mobile Router was configured for a particular prefix on the 

I agree there might be a problem if MR is allowed to advertise _any_
prefix (send RA's) on the visited link (be it prefix A, or a dynamically
acquired prefix B from HA).  I think that's why spec says MR SHOULD NOT
send RA's on the visited link.  What could be discussed is whether this
should be a "SHOULD NOT" or a "MUST NOT".

I suggest to keep it "SHOULD NOT".

>> and SHOULD NOT reply to Router Solicitations on any egress 
>> interface when that interface is attached to a visited link. 
>> However, the Mobile Router SHOULD reply with Neighbor 
>> Advertisements to Neighbor Solicitations received on the egress 
>> interface, for topologically correct addresses.
>> 
>>  having sent unsolicted RAs on the egress interface on the home
>> link when the router moves away?

So are you suggesting making it "MUST NOT"?

>> Wouldn't it be better just to prohibit the Mobile Router from 
>> responding to RSs and sending unsolicited RAs on its egress 
>> interface?

Prohibit that, maybe yes, but only when MR is away from home.

I suggest to keep it as SHOULD NOT, in order to allow for an extreme
case that is this:

MR attaches to AR.  A new MH attaches to same AR.  Fade AR disappear
away.  At this point, if MR does not send RA's on the visited link it
has no way of talking to MH.  This would be a limited case, and if MR
could send RA's in the ether (when AR is dead) then MH autoconfigures an
address and can talk directly to MR (of course, not using their home
addresses, and MH can not talk to LFN's inside the moving network).

I think this is the only reason that could be a "SHOULD NOT" instead of
a "MUST NOT".

>> used for address configuration, default router selection or 
>> movement detection.
>> 

What the MIPv6 spec does not observe (even if the MN terminology
suggests so) is that that MN could be an MR and send RA's natively.
Thinking in a MIPv6 for hosts context exclusively does not make it
obvious that that MH might be able to talk to another MH, when they're
both under the same AR and when that AR disappears.

Alex

---

From: "James Kempf" <[email-masked]>
Date: Thu, 20 Nov 2003 11:17:10 -0800

> So are you suggesting making it "MUST NOT"?

In fact, the router won't do this anyway if it is RFC 2461 compliant if the
mobile subnet is a stub subnet (as it is in the NEMO scenario), since the
mobile router will only be the first hop router for nodes within the mobile
subnet and not without ('cause the stub subnet doesn't lead to the
Internet). I'd suggest perhaps adding wording to that effect, just to
emphasize the fact.

RFC 2461 only allows a router to advertise if it is a first hop router. A
mobile router connected up to its home link or on a visited link is only a
first hop router to nodes within its subnet, not on the egress side. So it
should not be advertising there.

> both under the same AR and when that AR disappears.

I'm not sure I follow. My point was only that at first pass, I don't see any
reason why movement detection for routers should be any different than for
hosts. So the MIPv6 movement detection rules should be the same. Probing
more deeply, as the DNA BOF is doing, may turn up some corner cases, of
course.

            jak

---

From: "James Kempf" <[email-masked]>
Date: Thu, 20 Nov 2003 11:17:10 -0800

jak

---

From: Alexandru Petrescu <[email-masked]>
Date: Thu, 20 Nov 2003 20:43:57 +0100

James Kempf wrote:
>>>> A Mobile Router SHOULD NOT send unsolicited Router 
>>>> Advertisements and SHOULD NOT reply to Router Solicitations on 
>>>> any egress interface when that interface is attached to a 
>>>> visited link. However, the Mobile Router SHOULD reply with 
>>>> Neighbor Advertisements to Neighbor Solicitations received on 
>>>> the egress interface, for topologically correct addresses.
>>>> 
>>>> jak> What happens to hosts that are utilizing the router due to
>>>>  it having sent unsolicted RAs on the egress interface on the 
>>>> home link when the router moves away?
>> 
> In fact, the router won't do this anyway if it is RFC 2461 compliant 
> if the mobile subnet is a stub subnet (as it is in the NEMO 
> scenario), since the mobile router will only be the first hop router 
> for nodes within the mobile subnet and not without ('cause the stub 
> subnet doesn't lead to the Internet). I'd suggest perhaps adding 
> wording to that effect, just to emphasize the fact.

I think a short clarification adding that is writable and needed.

> RFC 2461 only allows a router to advertise if it is a first hop 
> router. A mobile router connected up to its home link or on a visited
>  link is only a first hop router to nodes within its subnet, not on 
> the egress side. So it should not be advertising there.

Seems logic, but.  MR at home _will_ become a first-hop for hosts on the
home link (towards hosts on the stub subnet) after FN receives an ICMP
Redirect from the current first-hop router.

(detailed scenario is this: host "FN" on the home link wants to talk to
LFN on the stub subnet, sends packet to HA which is currently its
first-hop, HA forwards packet to MR and simultaneously sends Redirect to
FN, FN adds a new route towards LFN through MR, thus MR becomes a
first-hop; at least that's the way I understand Redirects)

>>>> A Mobile Router MUST NOT ignore Router Advertisements received 
>>>> on the egress interface.  The received Router Advertisements 
>>>> MAY be used for address configuration, default router selection
>>>>  or movement detection.
>>>> 
>>>> jak> In fact, it MUST utilize these as described in the base 
>>>> MIP6 protocol for movement detection.
>> 
>>  obvious that that MH might be able to talk to another MH, when 
>> they're both under the same AR and when that AR disappears.
>> 
> 
> 
> I'm not sure I follow.

Err, yes, ok.  Just wanted to say that MIPv6 spec does not consider very 
much that MN is a router too.  Nevermind, not relevant.

> My point was only that at first pass, I don't see any reason why 
> movement detection for routers should be any different than for 
> hosts. So the MIPv6 movement detection rules should be the same. 
> Probing more deeply, as the DNA BOF is doing, may turn up some corner
>  cases, of course.

With respect to movement detection, I entirely agree with you.  MR
should movement detect exactly as MIPv6 spec says it, or as a potential
DNA spec will say it.  Maybe short specific text could be added saying
MR movement detects as MIPv6 says it to do, it's safer.

But I doubt that either the MIPv6 spec or the DNA spec will have
anything in their movement detection that influences MN
_sending_ RA's or _receiving_ RS's (I agree, it is based on _receiving_
RA's).

Alex

---

From: Alexandru Petrescu <[email-masked]>
Date: Thu, 20 Nov 2003 20:43:57 +0100

Alex

---

From: Vijay Devarapalli <[email-masked]>
Date: Fri, 21 Nov 2003 09:52:30 -0800

James Kempf wrote:
> Internet).

Jim, can you point me to the text in RFC 2461 which says a
router shouldnt sent a router advertisement if it is not a
first-hop router for the nodes on a link?

> I'd suggest perhaps adding wording to that effect, just to
> emphasize the fact.

currently, the NEMO spec says that the mobile router MAY be
configured to send router advertisements on its egress
interface when it is attached to the home link. and if the
mobile router is sending router advertisements, it should
set the router lifetime to 0, so that the hosts on the home
link do not configure the mobile router as a default router.

is this what you are suggesting?

    Since the Mobile Router is not a first hop router for nodes
    on the home link, it SHOULD NOT send any router
    advertisements on its egress interface when attached to the
    home link.

but I dont see RFC 2461 prohibiting such a thing.

Vijay

---

From: Vijay Devarapalli <[email-masked]>
Date: Fri, 21 Nov 2003 09:52:30 -0800

Vijay

---

From: "James Kempf" <[email-masked]>
Date: Fri, 21 Nov 2003 13:21:25 -0800

> first-hop router for the nodes on a link?

Well, there's this (Section 6, first paragraph):

    "...Routers send Router Advertisements that indicate whether
    the sender is willing to be a default router..."

But then there's this (Section 6.2.3, page 45):

    "A router might want to send Router Advertisements without
    advertising itself as a default router. For instance, a router
    might advertise prefixes for address autoconfiguration
    while not wishing to forward packets. Such a router
    sets the Router Lifetime field in outgoing advertisements
    to zero."

It is unclear to me from this whether or not a router can advertise itself
even if it is willing to forward packets but not act as a default router
(i.e. a gateway to a wider network, usually the Internet), rather than just
prefixes. Would a lifetime of zero also be valid if the router wanted to do
that? Typically, routing decisions after the first hop are made by the first
hop router, and the host's routing decision is restricted just to selecting
the first hop router, so I would say no, but the spec doesn't specifically
say that (at least, as far as I can tell).

So, I'd say 2461 is not particularly clear on this issue. Perhaps an issue
for the ongoing discussion in the IPv6 group about updating 2461?

However, 2461 is very clear about Redirects. This from Section 8, paragraph
2:

    "A router MUST be able to determine the link-local
    address for each of its neighboring routers in
    order to ensure that the target address in
    a Redirect message identifies the neighbor
    router by its link-local address..."

and later, under a list of items to check before accepting a Redirect:

    "-The IP source address of the Redirect is the same as the current
    first-hop router for the specified ICMP Destination Address".

so even if a host can utilize the mobile router to route directly to the
stub mobile subnet rather than go through the default router, a Redirect
can't be used to tell the host to move its routing if the mobile router
moves, and it couldn't be used by the HA in any event to do a proxy
redirect.

> is this what you are suggesting?

That is what 2461 suggests, but it is not clear from 2461 whether a host on
the home link could actually use the advertisement for routing, at least,
not to me.

I think the wording is OK if one believes that a router should be able to
use RAs to advertise itself to hosts as something other than a default
router.

> but I dont see RFC 2461 prohibiting such a thing.

You're right. But it is also not clear to me whether it should be
encouraged.

I think this is an issue that needs to be clarified in the revision of 2461.
I don't know what to suggest for the NEMO spec at this point, but I don't
believe the current wording is out of compliance with what 2461 says.

            jak

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From: "James Kempf" <[email-masked]>
Date: Fri, 21 Nov 2003 13:21:25 -0800

jak

---

From: Vijay Devarapalli <[email-masked]>
Date: Fri, 21 Nov 2003 13:41:42 -0800

> believe the current wording is out of compliance with what 2461 says.

cool. it might be a good idea to bring this up in 2461bis discussion.

Vijay

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From: Vijay Devarapalli <[email-masked]>
Date: Fri, 21 Nov 2003 13:41:42 -0800

Vijay