Subject: [nemo] MR definition

From: "Pekka Pääkkönen" <[email-masked]>
Date: Mon, 9 Dec 2002 12:52:17 +0200

Hi all,

Please correct me if I have understood the definition of MR wrong.

MR is :

A router as defined in RFC 2460 : "a node that forwards IPv6 packets not explicitly addressed to itself."

In addition to this MR's relation to Neighbor Discovery (ND) is (RFC 2461):

- MR has host specific Neighbor Discovery behaviour in one or more interfaces as defined in RFC 2461. (MR's egress interfaces)
- MR has router specific Neighbor Discovery behaviour in one or more interfaces as defined in RFC 2461. (MR's ingress interfaces)

This would mean that MR would have different ND behaviour depending on the 
interface.

---

From: Mattias Pettersson <[email-masked]>
Date: Mon, 09 Dec 2002 15:37:15 +0100

Hi Pekka,

Pekka Pääkkönen wrote:
> 
> Hi all,
> This would mean that MR would have different ND behaviour depending on
> the interface.

Yes. Cf. with a multilink-subnet router, it has a proxy-mode interface
and router-mode interfaces.

/Mattias

---

From: "T.J. Kniveton" <[email-masked]>
Date: Mon, 09 Dec 2002 12:10:04 -0800

> Pekka Pddkkvnen wrote:

Yes, this is the strict definition. There is a very fine line between a router
and host when you look at it this way -- by changing one bit in my kernel
sysctl variables, I can change my "host" into a "router".

Most people also consider other factors, such as whether the node runs routing
protocols, whether it is administratively responsible for certain prefixes,
whether it is considered by other routers as the next hop to get to a prefix,
etc.

> - MR has host specific Neighbor Discovery behaviour in one or more interfaces
> as defined in RFC 2461. (MR's egress interfaces)
> - MR has router specific Neighbor Discovery behaviour in one or more
> interfaces as defined in RFC 2461. (MR's ingress interfaces)

The second statement is certainly true.

The first statement may or may not be true. In the Tunneled Mobile Router draft
(draft-kniveton-mobrtr), there are two cases defined: Consumer and
Fully-Enabled. In the consumer case, the MR may not be a trusted entity in the
visited domain. In that case, it has no choice but to act like a host (lowest
common denominator) in the visited domain to cooperate with the policies there,
and use its COA to create a virtual link to its Home Agent, so that it can have
the privelege of routing "upstream" through its home network.

In the Fully-Enabled case, the MR is considered as a full-fledged router
(beyond the ability to simply forward packets from one interface to another),
and thus it would behave just like any other router in most respects, including
neighbor discovery.

> This would mean that MR would have different ND behaviour depending on the

May be true. A MR has to be versatile enough to use Mobile IP, and tunneling,
to create a virtual link to its home network. That intelligence might include
using host-like ND procedures.

-TJ

---

From: Mattias Pettersson <[email-masked]>
Date: Tue, 10 Dec 2002 13:15:45 +0100

Hi again,

Pekka Pääkkönen wrote:
> - MR has host specific Neighbor Discovery behaviour in one or more
> interfaces as defined in RFC 2461. (MR's egress interfaces)
> the

I thought about this a little more. The MR may change its behaviour on
its egress interface depending on where it is attached. For instance, if
MR is at home the egress interface can be in router mode, but when MR is
away the egress interface can be in host mode (and the router-mode
functionality is moved to the bi-dir tunnel interface MRHA).

Is the changing of behaviour on a particular interface something we
desire?

/Mattias

---

From: Alexandru Petrescu<[email-masked]>
Date: 10 Dec 2002 14:23:48 +0100

> functionality is moved to the bi-dir tunnel interface MRHA).

So the description would work fine with many things, and less fine
with other things:

If we exemplify with "routing on the egress interface": when the MR is
at home, it is in "router mode" and routes through the egress
interface (it has entries in the routing table that point to the
egress interace); when MR is not at home, it's in host mode and
doesn't route through the egress interface (it has _not_ entries in
the routing table pointing to the egress interface).

But, if exemplify the above with the RA behaviour.  When MR is at
home, it sends RAs on the egress interface.  When the MR is not at
home, it is in "host mode", it sends RAs on the bi-dir tunnel
interface MRHA.  Maybe the home link doesn't need those RAs when the
MR is not at home?

> Is the changing of behaviour on a particular interface something we

I would say yes, but it's only one oppinion.

Alex

---

From: Vijay Devarapalli <[email-masked]>
Date: Tue, 10 Dec 2002 09:56:04 -0800

Alexandru Petrescu wrote:
> When the MR is not at

the home link does not need the RAs from the MR, when the MR is
not at home. the MR shouldnt tunnel the RAs through the 
bi-directional tunnel. 

Vijay

---

From: Alexandru Petrescu<[email-masked]>
Date: 10 Dec 2002 20:46:20 +0100

> not at home. the MR shouldnt tunnel the RAs through the 

Yes, that is reasonable.

Should the MR send ICMP redirects when it is not at home, and through
the MRHA tunnel, towards the home link?  Probably not.

If a host on the home link sends an RS on the home link towards the
all-routers multicast address, and the MR is not at home, should the
MR obtain that RS throught the MRHA tunnel?  Should MR respond to that
RS with an RA?

If MR is at home and is subscribed to the all-routers multicast
address with link scope, should this subscription continue when the MR
is not at home?

If MR is at home and is subscribed to the all-routers multicast
address with scope other than link, should this subscription continue
when the MR is not at home?

Should it be possible for the MR to be renumbered when it is not at
home, as if it were at home?

Alex

---

From: Vijay Devarapalli <[email-masked]>
Date: Tue, 10 Dec 2002 19:13:33 -0800

hi Alex,

before tunneling a packet through the bi-directional tunnel,
we have to see if the packet serves any purpose. for example
it makes no sense for the MR to send a router advert through
the bi-directional tunnel in response to a router solicitation
when it is away from the home. the tunneled router advert is
of no use to anyone. the HA shouldnt have tunneled the router 
solicitation to the MR in the first place.

Alexandru Petrescu wrote:

> MR obtain that RS throught the MRHA tunnel?  

no.

> Should MR respond to that
> RS with an RA?

no.

> address with link scope, should this subscription continue when the MR

no. if it is useful then maybe yes. do you have a scenario
where it would be useful?

> address with scope other than link, should this subscription continue

what is this multicast address used for?

> home, as if it were at home?

maybe yes. it can be done if we tunnel Router Renumbering Command/Result
messages through the bi-directional tunnel.

but I see your point. it can get quite complicated if the HA has
to inspect every packet before tunneling it to the MR. 

Vijay

---

From: Mattias Pettersson <[email-masked]>
Date: Wed, 11 Dec 2002 10:38:07 +0100

Hi Vijay,

> not at home. the MR shouldnt tunnel the RAs through the

I agree. I also would ask: why should MR send RAs when at home? It is
not a default router of the home link, as it will not be able to route
towards the Internet. The only other network MR is connected to is a
leaf network (the mobile network).

/Mattias

---

From: Alexandru Petrescu<[email-masked]>
Date: 11 Dec 2002 11:14:05 +0100

> leaf network (the mobile network).

RAs help setting a default route, and it is probably not necessary for
MRs to advertise themselves as default routers on the home link.  But
RAs also advertise the prefixes of the home link, in the home link,
and this is be helpful for address autoconfiguration.  For example, if
several MRs are at home and advertising RAs (w/ exponential backoff or
something to avoid peak sudden network loads) then another MR coming
back home will have more chances to identify it is back home based on
info from those RAs.  If it's only the BR/HA that sends these RAs then
maybe an MR coming back home will take more time to realize it is back
home.

So, maybe useful, or maybe not.  So, "MR MAY advertise RAs when at
home"?  Or, "MRs SHOULD NOT advertise RAs when at home because they
are not default routers at home"?

Alex

---

From: Alexandru Petrescu<[email-masked]>
Date: 11 Dec 2002 14:28:55 +0100

> what is this multicast address used for?

For example router renumbering?  For example a central administration
point in the home domain decides to renumber all routers belonging to
that home and thus sends a message to all-routers multicast address
within that domain.

Alex

---

From: Vijay Devarapalli <[email-masked]>
Date: Wed, 11 Dec 2002 09:56:59 -0800

> maybe an MR coming back home will take more time to realize it is back

this is a wrong mechanism for an MR to figure out that it is 
home. configure your HA or any router on the home link to
send out more frequent router advertisements. or better still
have some triggers on MR which sends out a router solicitation
as soon as it detects a link up.

> Or, "MRs SHOULD NOT advertise RAs when at home because they

on the egress interface, yes.

Vijay

---

From: Alexandru Petrescu<[email-masked]>
Date: 11 Dec 2002 19:28:41 +0100

> as soon as it detects a link up.

Yes, these are two more performant alternatives.

With respect to the first approach, one can argue that same machine
sending frequent RAs is higher load than same task performed by a set
of routers sending RAs less frequently.  But you're right that this
might be an configurational issue only.

In the second approach, the high constant load is eliminated and
offers an RA only when needed, thus looks very interesting.
Unfortunately, routers do not send RSs.  An MR (its egress) can send
RSs only if it acts like a router, and it logically does so only if it
is at home.

Following this, maybe we can suggest text where MR always acts like a
host, always sends RSs.  Except when it knows it is at home, it no
longer sends RSs.

If on the other hand MR would use "RA listening" to detect it is at
home, the above suggestion is worthless (we can get away without the
above text), because both hosts and routers listen to RAs.

Right?

Alex