Subject: [Autoconf] Clarification of the formulation of the /128 recommendation

From: Alexandru Petrescu <[email-masked]>
Date: Mon, 27 Jul 2009 18:32:27 +0200

Current DT draft recommends:
>    o  A subnet prefix configured on this interface should be of length
>       /128.

I am trying to understand this formulation.

What does it mean in practice?

Does it mean that when I type 'ifconfig' all shown addresses will be 
suffixed by "/128"?  (and not by any other length, like /64, or /48 or 
such).

Does it mean that when I type 'route -n' all entries in the routing 
table have prefix length 128? (and not /64, nor /48, or similar).

There was quick discussion on the jabber room at the end of the meeting, 
making think that "/128" may be a red herring, and that the "/128" 
recommendation may not conflict with existing specs.  These statements 
are rather surprising to me.  For clarification, I think formulations in 
terms of practical screen dump of issued commands may help, at least 
myself, thank you.

Or do most people think that this "/128" recommendation is mostly clear 
and is understood?

Alex

---

From: Carlos Jesús Bernardos Cano <[email-masked]>
Date: Mon, 27 Jul 2009 19:14:27 +0200

Hi Alex,

> >    o  A subnet prefix configured on this interface should be of length

I guess next version will say "A Prefix", instead of "A subnet prefix".

> suffixed by "/128"?  (and not by any other length, like /64, or /48 or 

I think so, for all ULAs/global addresses.

> table have prefix length 128? (and not /64, nor /48, or similar).

I think so, for all routes added to the table because of addresses (ULAs
and globals) configured on MANET interfaces.

> Or do most people think that this "/128" recommendation is mostly clear 

What I understand from that formulation is that that by configuring
these addresses, a node will not assume that another address from the
same prefix would be directly reachable (within one IP hop) through the
interface where the address is configured.

Carlos

> 
> Alex
> 
> _______________________________________________
> Autoconf mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/autoconf
-- 
Carlos Jesús Bernardos Cano     http://www.netcoms.net
GPG FP: D29B 0A6A 639A A561 93CA  4D55 35DC BA4D D170 4F67

---

From: Rafael Roque <[email-masked]>
Date: Mon, 27 Jul 2009 14:17:35 -0300

Hi Alexandru,

i'm still trying to understand all impacts of /32 and /128 fields.
I thing that it is not your case, but for those that are a little lost,
draft-clausen-manet-autoconf-recommendations-00<http://tools.ietf.org/html/draft-clausen-manet-autoconf-recommendations-00>provides
good discussions about the topic.

On Mon, Jul 27, 2009 at 1:32 PM, Alexandru Petrescu <
[email-masked]> wrote:

> Does it mean that when I type 'route -n' all entries in the routing table
> have prefix length 128? (and not /64, nor /48, or similar).
>
> recommendation may not conflict with existing specs.  These statements are
> rather surprising to me.  For clarification, I think formulations in terms
> of practical screen dump of issued commands may help, at least myself, thank
> you.
>
> Or do most people think that this "/128" recommendation is mostly clear and
> is understood?
>
> Alex
>

-- 
Rafael Roque Aschoff
GPRT/UFPE

---

From: Alexandru Petrescu <[email-masked]>
Date: Mon, 27 Jul 2009 21:45:06 +0200

> I guess next version will say "A Prefix", instead of "A subnet prefix".

Will it say "for all addresses configured on this interface, the 
respective prefix will be /128"?  Or just for some?

> I think so, for all ULAs/global addresses.

I hope that instead of "for all" it will be "for some".

> and globals) configured on MANET interfaces.

I think so too.

> interface where the address is configured.

We don't understand the same thing.

Would a routing table entry in the node look like this:

Destination           Gateway              Iface
2001:db0:1::1/128     2001:db0:2::2        eth0

Maybe this is what is meant by the "/128" requirement?  The Gateway 
address is directly reachable from the node.

Setting a host-based route (/128) can not mean that the Gateway address 
is not directly reachable.

But, if node has the routing table entry like this:

Destination           Gateway              Iface
2001:db0:1::1/128     ::                   eth0

Then this may mean something different - go through self to reach some 
outter destination.  To clarify this difference, the draft could require 
that in addition to the /128 host-based route, the routing table entry 
MUST use an unspecified gateway address - ::.  I guess this is what 
happens in some MANET implementations (guessing).  If so, it should be 
reflected in the draft.

[Another thing, if the "/128" requirement is actually the host-based
  route in the routing table, then
  it may pose a problem reaching
  hosts behind a MANET router having 2 interfaces, because the source
  would
  have to have a host-based route for each host behind that
  MANET router, whereas a shorter-than-128 prefix route would suffice.]

Alex

---

From: Erik Nordmark <[email-masked]>
Date: Mon, 27 Jul 2009 23:21:40 +0200

> Or do most people think that this "/128" recommendation is mostly clear 

To me it is merely restating that there are no on-link prefixes (except 
the link-local prefix), which is clear to me.

    Erik

---

From: Carlos Jesús Bernardos Cano <[email-masked]>
Date: Tue, 28 Jul 2009 09:08:23 +0200

> respective prefix will be /128"?  Or just for some?

I think it should say for all the addresses configured by autoconf.

> I hope that instead of "for all" it will be "for some".

Agree.

---

From: Ulrich Herberg <[email-masked]>
Date: Tue, 28 Jul 2009 09:38:18 +0200

Alex,

On Mon, Jul 27, 2009 at 9:45 PM, Alexandru Petrescu <
[email-masked]> wrote:

> [...]
> Will it say "for all addresses configured on this interface, the respective
> prefix will be /128"?  Or just for some?
>

For all addresses that we configure in autoconf.

> [...]

Why?

> [...]
> Maybe this is what is meant by the "/128" requirement?  The Gateway address
> is directly reachable from the node.

Yes, that looks valid for me.

>
>
> Setting a host-based route (/128) can not mean that the Gateway address is
> not directly reachable.

Why should it?

> that in addition to the /128 host-based route, the routing table entry MUST
> use an unspecified gateway address - ::.  I guess this is what happens in
> some MANET implementations (guessing).  If so, it should be reflected in the
> draft.

No, I don't think that's a necessity.

>  MANET router, whereas a shorter-than-128 prefix route would suffice.]

Keep in mind that we are talking about MANET interfaces on a MANET router
only. If there is other stuff (e.g. hosts) behind the MANET router (so, on a
non-MANET interface of that MANET router), they are configured as usual. We
are not talking about how to configure these hosts behind the MANET router.

Ulrich

---

From: Alexandru Petrescu <[email-masked]>
Date: Tue, 28 Jul 2009 11:02:46 +0200

Ulrich Herberg a écrit :
> Alex,
> 
> On Mon, Jul 27, 2009 at 9:45 PM, Alexandru Petrescu 
> <[email-masked] <mailto:[email-masked]>> wrote:
> 
>     [...]

Ok.

>             [...]
>             Does it mean that when I type 'ifconfig' all shown addresses
>             will be suffixed by "/128"?  (and not by any other length,
>             like /64, or /48 or such).
> 
> 

I think it would be good if it were only for all addresses AUTOCONFed, 
not for the ones manually configured.

>     Setting a host-based route (/128) can not mean that the Gateway
>     address is not directly reachable.
> 
> 
> Why should it?

So you mean that the Gateway address is _not_ directly reachable?  If 
so, then the packet to that Destination will be dropped, because NS/NA 
for Gateway address will timeout.

> Keep in mind that we are talking about MANET interfaces on a MANET 
> router only. If there is other stuff (e.g. hosts) behind the MANET 
> router (so, on a non-MANET interface of that MANET router), they are 
> configured as usual. We are not talking about how to configure these 
> hosts behind the MANET router.

    ------        ------------          ----------
   |Source|------|MANET Router|----+---|Otherhost1|
    ------        ------------     |    ----------
                                   |    ----------
                                   +---|Otherhost2|
                                        ----------

Does Source have two host based /128 entries in its routing table for 
each Otherhost?  Or does it have a single /64 entry covering all 
Otherhosts behind MANET Router?

Alex

---

From: Ulrich Herberg <[email-masked]>
Date: Tue, 28 Jul 2009 13:02:54 +0200

Alex,

On Tue, Jul 28, 2009 at 11:02 AM, Alexandru Petrescu <
[email-masked]> wrote:

> [..]
> I think it would be good if it were only for all addresses AUTOCONFed, not
> for the ones manually configured.

What is the difference? It does not matter how to configure these addresses.
And again, the draft says: "A subnet prefix configured on this interface
*should* be of length /128" and "When more specific assumptions can be made
regarding the connectivity to other interfaces on the link, these SHOULD be
considered when configuring subnet prefixes." So, if you know better, you
*can* configure another address (possibly with a shorter prefix) on the
interface (manually or autoconf'ed, doesn't matter)

> So you mean that the Gateway address is _not_ directly reachable?  If so,
> then the packet to that Destination will be dropped, because NS/NA for
> Gateway address will timeout.

That's not what I meant. Maybe I misunderstood the two "not"s in your
sentence :-) Can you rephrase the sentence, please?

> Does Source have two host based /128 entries in its routing table for each
> Otherhost?  Or does it have a single /64 entry covering all Otherhosts
> behind MANET Router?

Since these hosts are behind the MANET router (i.e. on a non-MANET
interface), routing can be done as usual. In the draft, we do not talk about
how these hosts are configured (not necessarily with a /128 prefix!), we
only talk about the interface of the router. So, depending how these hosts
are configured, aggregation could be used for avoiding having two entries in
the routing tables.

Ulrich

---

From: Alexandru Petrescu <[email-masked]>
Date: Tue, 28 Jul 2009 14:13:26 +0200

Ulrich,

Sorry, my fault.  I think I am inconsciously diverging the discussion 
away from the topic of the draft under discussion.

You are right about the questions you raise below and the normal routing.

I will wait to see other's comments, the new DT draft when it comes out, 
and remark on it then.

Thanks,

Alex

Ulrich Herberg a écrit :
> Alex,
> 
> On Tue, Jul 28, 2009 at 11:02 AM, Alexandru Petrescu 
>     I think it would be good if it were only for all addresses
>     AUTOCONFed, not for the ones manually configured.
> 
> 
> 
> What is the difference? It does not matter how to configure these 
> addresses. And again, the draft says: "A subnet prefix configured on 
> this interface *should* be of length /128" and "When more specific 
> assumptions can be made regarding the connectivity to other interfaces 
> on the link, these SHOULD be considered when configuring subnet 
> prefixes." So, if you know better, you *can* configure another address 
> (possibly with a shorter prefix) on the interface (manually or 
> autoconf'ed, doesn't matter)
> 
> 
> 
>     So you mean that the Gateway address is _not_ directly reachable?
>      If so, then the packet to that Destination will be dropped, because
>     NS/NA for Gateway address will timeout.
> 
> 
>         Keep in mind that we are talking about MANET interfaces on a
>         MANET router only. If there is other stuff (e.g. hosts) behind
>         the MANET router (so, on a non-MANET interface of that MANET
>         router), they are configured as usual. We are not talking about
>         how to configure these hosts behind the MANET router.
> 
> 
>     Does Source have two host based /128 entries in its routing table
>     for each Otherhost?  Or does it have a single /64 entry covering all
> interface), routing can be done as usual. In the draft, we do not talk 
> about how these hosts are configured (not necessarily with a /128 
> prefix!), we only talk about the interface of the router. So, depending 
> how these hosts are configured, aggregation could be used for avoiding 
> having two entries in the routing tables.
> 
> Ulrich
>