Subject: RE: Review of"Use of /127 Prefix Length Between Routers Considered Harmful"

From: "Michel Py" <[email-masked]>
Date: Wed, 12 Jun 2002 01:21:12 -0700

> Alain Durand wrote:
> I still do not understand why this draft does not simply advocate
> using 2 /128.

There is something that makes me scream at the thought of not having
both ends in the same subnet and/or not having a route for that subnet.
Possibly some frame-relay flashback? Anyway, this sounds to me as bad as
using unnumbered. B.A.D.

Michel.

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From: Alain Durand <[email-masked]>
Date: Wed, 12 Jun 2002 01:36:51 -0700

> Possibly some frame-relay flashback? Anyway, this sounds to me as bad as

This sounds like F.U.D. to me. Why do you think it is B.A.D.?

	- Alain.

---

From: Antonio Querubin <[email-masked]>
Date: Tue, 11 Jun 2002 23:07:32 -1000 (HST)

> Possibly some frame-relay flashback? Anyway, this sounds to me as bad as

Why?  Using unnumbered on point to point links works quite well.  The only
place where I've found a numbered link to be more useful on a point to
point link is when BGP peering needs to be established across the link and
the numbered link will prevent the need to create a static route to the
remote neighbor address.

---

From: Robert Elz <[email-masked]>
Date: Wed, 12 Jun 2002 17:02:35 +0700

Date:        Wed, 12 Jun 2002 01:36:51 -0700
    From:        Alain Durand <[email-masked]>
    Message-ID:  <[email-masked]>

  | This sounds like F.U.D. to me. Why do you think it is B.A.D.?

Why are we even having this discussion (at this level)?

Different people like to number p2p links different ways, and there's
no reason to prohibit, or insist on, any of them.  Which is the best
is very much a matter of opinion, they all have some advantages and
some drawbacks.

Personally, I don't see nearly as many problems with /127 as the
draft seems to - but on the other hand, I also see no possible
potential drawbacks from just using /112 instead - it is just absurd
to imagine that the difference in address space to number all the
p2p's between /112 and /127 is ever going to matter to anyone.
I have been using /112 on these things for a while now (I used to use /127).

The draft is just fine, with or without the extra mention of 2 * /128
as yet another way to "number" a p2p link (a link numbered that way
is generally just the same as a unnumbered link - the link itself has
no prefix assigned).   Let's just agree to ship it.

kre

---

From: Jun-ichiro itojun Hagino <[email-masked]>
Date: Wed, 12 Jun 2002 19:29:31 +0900

>the numbered link will prevent the need to create a static route to the

	checking - "unnumbered" meaning "link-local address only", am I
	correct?  yes, p2p link without global address works just fine.

itojun

---

From: Antonio Querubin <[email-masked]>
Date: Wed, 12 Jun 2002 00:42:16 -1000 (HST)

> 	correct?  yes, p2p link without global address works just fine.

No, I meant unnumbered which has no requirement on the scope.  In either
case though you're correct in that a point to point link doesn't really
need a global address in many cases.

---

From: Robert Elz <[email-masked]>
Date: Wed, 12 Jun 2002 17:39:03 +0700

Date:        Wed, 12 Jun 2002 19:29:31 +0900
    From:        Jun-ichiro itojun Hagino <[email-masked]>
    Message-ID:  <[email-masked]>

  | 	checking - "unnumbered" meaning "link-local address only", am I
  | 	correct?  yes, p2p link without global address works just fine.

As I just said, I don't really think we need to be having this discussion.

But a question for those who like unnumbered links - do they work if
a host (not being a router) that has a p2p link implements the "strong"
model of addressing?   If so, how?   (Assuming the host wants to be
reached via a global address of course, and assuming it desires to be
reached using the p2p link - that being the purpose of installing it).

kre

---

From: Robert Elz <[email-masked]>
Date: Wed, 12 Jun 2002 17:57:41 +0700

Date:        Wed, 12 Jun 2002 00:42:16 -1000 (HST)
    From:        Antonio Querubin <[email-masked]>
    Message-ID:  <[email-masked]>

  | a point to point link doesn't really
  | need a global address in many cases.

Assuming this is true, the inference is that there are some cases
where a global address is required.

That's true: I know of one - munnari.oz.au has a P2P link for IPv6,
and that's its only IPv6 connectivity.   Assuming that it is to have
a global v6 address (and it does) it has to be assigned to the P2P
link, there is nothing else to assign it to (it isn't a router, just
a host, and the p2p link is a tunnel, of course, not that that matters).

Given that, the question is how the global address space should be
used on P2P links when addresses are required, surely?   The question
of what to do when they're not required (and not wanted either) is
not relevant, is it?

So, can we please get off the rat hole issue of whether or not p2p
links should be numbered, and return to discussing the draft, which
gives some guidelines on how to number them (and how not to) when they
are being numbered?   (With, as I recall, though it has been a while
since I looked, no statements at all saying that p2p links actually
should be numbered).

kre

ps: there's no v6 DNS entry for munnari yet, so don't bother looking.
Its v6 connectivity is weird, and I don't want to subject that path to
the kind of traffic volumes that munnari tends to attract - and perhaps
would, even over the 6bone.   Its A6 record will appear once I get better
connectivity, which means when the local academic network people manage
to get themselves connected (they have the APNIC prefix ready...)

---

From: Jun-ichiro itojun Hagino <[email-masked]>
Date: Thu, 13 Jun 2002 13:04:57 +0900

>of what to do when they're not required (and not wanted either) is

	i usually use /64 for p2p link.  it works just fine, it supports
	temporary address (RFC3041) if you desire, i can think of no reason to
	use somethiing other than /64.

itojun

---

From: Robert Elz <[email-masked]>
Date: Thu, 13 Jun 2002 16:53:41 +0700

Date:        Thu, 13 Jun 2002 13:04:57 +0900
    From:        Jun-ichiro itojun Hagino <[email-masked]>
    Message-ID:  <[email-masked]>

  | 	i usually use /64 for p2p link.  it works just fine, it supports
  | 	temporary address (RFC3041) if you desire,

Yes, no argument with any of that.  As I recall, using /64 is one of
the alternatives (to using /127 which this draft was written to discourage
I believe) that is given.

Of course, on a P2P link, any prefix length (< about 126) will support
temporary addresses, the only difference is how many of them exist to
choose between (which can't be detected from outside, which makes the
problem of having a smaller set fairly irrelevant).   That is, there's
only one other node with which to possibly clash, and that one usually
has no motivation to use temporary addresses on the P2P link.

  |	i can think of no reason to use somethiing other than /64.

The obvious one is to conserve address space.   Another is to make it
easier to see what are p2p links from their addresses (all of an organisation's
p2p links can easily be numbered out of one /64).

In any case, unless there's something that will be broken by using a
longer prefix, there's no reason to forbid it, is there?

kre

---

From: Jun-ichiro itojun Hagino <[email-masked]>
Date: Wed, 12 Jun 2002 19:29:31 +0900

itojun

---

From: [email-masked]
Date: Thu, 13 Jun 2002 22:05:32 +0900

>has no motivation to use temporary addresses on the P2P link.

	read RFC3041 again carefully.  it assumes 64bit interface identifier,
	therefore it assumes /64 subnet prefix.

itojun

---

From: Antonio Querubin <[email-masked]>
Date: Thu, 13 Jun 2002 09:29:50 -1000 (HST)

> longer prefix, there's no reason to forbid it, is there?

Concur on both points.  Just because one can assign a /64 and get millions
of addresses on a p2p link doesn't mean one should.  Furthermore, I can
understand the arguments for avoiding /127 or /128 but in the cases where
the lost capability is a non-issue why bother forbidding even those 2
prefix lengths?

---

From: [email-masked]
Date: Fri, 14 Jun 2002 08:09:55 +0900

>	use somethiing other than /64.

	let me be clear - I usually use either /64 global address prefix, or no
	global address (link-local only), on p2p link.  both works just fine.

	i assign /64 global prefix when we need to monitor router p2p interface
	from remote (by ping6), for instance.  in other cases, i don't
	assign /64 global addess prefix.

itojun

---

From: Randy Bush <[email-masked]>
Date: Thu, 13 Jun 2002 16:13:48 -0700

> 	let me be clear - I usually use either /64 global address prefix, or
> 	no global address (link-local only), on p2p link.  both works just
> 	fine.

traceroute is my friend.  this is an enemy of traceroute.  and enemy of
my friend is my enemy.

> 	i assign /64 global prefix when we need to monitor router p2p
> 	interface from remote (by ping6), for instance.  in other cases, i
> 	don't assign /64 global addess prefix.

this is like assigning 1918 addresses to v4 interface, only the filtering
is more assured.

does not make me comfortable.

randy

---

From: [email-masked]
Date: Fri, 14 Jun 2002 08:18:17 +0900

>traceroute is my friend.  this is an enemy of traceroute.  and enemy of

	as long as the following two condition holds, traceroute6 work fine.
	- intermediate routers are using weak host model
	- intermediate routers have at least one global address

itojun

---

From: Robert Elz <[email-masked]>
Date: Wed, 12 Jun 2002 17:57:41 +0700

kre

---

From: Pekka Savola <[email-masked]>
Date: Fri, 14 Jun 2002 10:49:31 +0300 (EEST)

> 	- intermediate routers have at least one global address

Up to some level of "fine".

Practically, this works if the conditions hold:

 - network is small
 - you know the topology very well

Reason for this is that global address assigned to loopback does not give 
as much information in traceroute as point-to-point links.

For example, we embed the names of both ends' routers, the type of the
link (implicitly its speed) and the interface slot numer etc. in reverse
lookups for the P-t-P addresses, like:

router1-ge321-router2 or router3-a1031-router4 

(this is at router1's end connecting to router2, on GigaEthernet3/2/1;  
router3's end connecting to router4, with ATM1/0/3, subinterface 1).

There is _no way_ loopbacks are going to give that debugging/tracing
information to us :-).

I guess there are many ways to use traceroute, and global addresses, 
coupled with nice reverse lookups are quite invaluable.

In small networks especially if you know the topology, this feature is 
rather useless and link-locals + 1 global is definitely the easiest 
approach.

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords

---

From: Robert Elz <[email-masked]>
Date: Fri, 14 Jun 2002 17:26:10 +0700

Date:        Thu, 13 Jun 2002 22:05:32 +0900
    From:        [email-masked]
    Message-ID:  <[email-masked]>

  | 	read RFC3041 again carefully.  it assumes 64bit interface identifier,
  | 	therefore it assumes /64 subnet prefix.

I know.

But I said nothing about 3041, all I said was "temporary address".

The changes needed to 3041 to handle other sizes for temporary address
allocation would be trivial, there's nothing in 3041 that actually
depends upon having 64 bits to assign (other than the number of bits
and the birthday paradox combining to make the chances of an addr
clash very small - which on a p2p link is meaningless).  That is, if
there was any point in actually standardising a method to use for this,
which there almost certainly isn't.

kre