Subject: [GROW] I-D Action:draft-ietf-grow-va-auto-00.txt

From: [email-masked]
Date: Mon, 19 Oct 2009 06:45:01 -0700 (PDT)

A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft is a work item of the Global Routing Operations Working Group of the IETF.

	Title           : Proposal for Auto-Configuration in Virtual Aggregation
	Author(s)       : P. Francis, et al.
	Filename        : draft-ietf-grow-va-auto-00.txt
	Pages           : 11
	Date            : 2009-10-18

Virtual Aggregation as specified in [I-D.ietf-grow-va] requires a
certain amount of configuration, namely virtual prefixes (VP), a VP
list, type of tunnel, and popular prefixes.  This draft proposes
optional approaches to auto-configuration of popular prefixes and the
VP list, and discusses the pros and cons of each.  If these proposals
are accepted, they will be incorporated into [I-D.ietf-grow-va].

A URL for this Internet-Draft is:
http://www.ietf.org/internet-drafts/draft-ietf-grow-va-auto-00.txt

Internet-Drafts are also available by anonymous FTP at:
ftp://ftp.ietf.org/internet-drafts/

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Internet-Draft.

---

From: Paul Francis <[email-masked]>
Date: Tue, 20 Oct 2009 18:55:01 +0200

Hi Joel,

I'm quite embarrassed---I don't see the loop you are referring to. Could 
you spell it out for me?

FAIW, here is my analysis (within which I don't see a looping 
situation...what am I missing?).  I'm happy to clean up this analysis and 
include it in the draft if that is what you are asking for.

First, lets consider the static configured VP-list approach (currently 
described in draft-ietf-grow-va-00).

There are two misconfig possibilities:
V1:  A router thinks a VP exists that doesn't actually (i.e. its VP-list 
claims that a VP exists, but there is no router that itself acts as the 
APR for that VP).
In this case, the router expects to see a route to the VP.  If it does not 
see the route, then it either installs the sub-prefixes, in which case 
routing works fine, or it does not, in which case it drops the packets. 
(Which it does is a policy decision, subject to available FIB space.) 
Either way, no loop.
V2:  A router does not think a VP exists that actually does.  In this 
case, the router will FIB-install the sub-prefixes and routing works fine.

Next, lets consider VP-route tagging approach (described in 
draft-ietf-grow-va-auto-00).

Here, there is no static configuration of VPs per se.  Rather, the only 
configuration is that of the APR.  When an APR advertises a VP-route, 
presumably it is either prepared to forward packets (i.e. it has 
FIB-installed the sub-prefix), in which case all is well, or it is has for 
some reason not FIB-installed one or more sub-prefixes, in which case it 
should drop the packet.  (There should never be a case where an APR 
forwards a packet to another APR just cause it doesn't have a route to a 
specific IP address.  This is a router functionality issue, not a 
configuration issue.) 

Lets consider the non-APRs (for a given prefix).  When it receives a 
VP-route, the route is tagged, and so the non-APR knows it is a VP.  It 
will forward packets to the VP (for FIB-suppressed sub-prefixes), at which 
point the behavior in the above paragraph applies (the APR either forwards 
for drops the packet, but no loops).  If there is no VP-route anywhere for 
a given VP, the non-APR simply assumes that it is not a VP, and either 
installs the sub-prefixes or not, but either way no loop.

If a non-APR is doing graceful restart, then until the RIB converges, it 
might have incorrect information, as follows:
GR1:  It thinks there is a VP when there isn't.
In this case, it will tunnel packets to what it thinks is the APR for the 
VP (say R1).  R1 will then tunnel the packet to the correct APR (if there 
is one), or drop it (if there isn't).  Is there a possible scenario 
whereby R1 thinks that some router R2 is the APR, and R2 thinks that some 
router R1 is the APR?  It is hard to imagine one.  Lets imagine, for 
instance, that both R1 and R2 are APRs at time T1.  At T1, the admin 
reconfigures them both to be non-APRs.  Then they both at the same time go 
into GR.  But this shouldn't happen, because when a router is told that it 
is no longer an APR, it will first withdraw the VP route, then wait a 
while (for the withdraw info to propagate), and only then FIB-suppress the 
sub-prefixes.   So both R1 and R2 should be able to forward to 
sub-prefixes during the GR period, and so no loop should form.  (As a 
further safety, we could have a rule that says that a router never 
forwards a received tunneled packet to an APR, but this seems unnecessary 
in practice.)

GR2:  It does not know of a VP that exists.
Here, the non-APR should simply drop the packets.  No loop.

Finally, lets consider the can-suppress approach specified in 
draft-ietf-grow-va-auto-00.
Here the concern is that the configured VP-range table does not match 
reality.  Again, there are two cases of concern:
CS1:  The router thinks that an address range has a VP when there is in 
fact none.
Here the router will tag sub-prefix routes as can-suppress.  Routers that 
see this (which are non-APR routers) will FIB-suppress the sub-prefixes, 
and packets will be dropped, not looped.

CS2:  The router thinks that an address range has no VP when there is one.
In this case, the sub-prefix routes will not be tagged as can-suppress. 
Other routers will FIB-install the sub-prefixes (if there is FIB space), 
and packets will be forwarded correctly.  If there is not FIB space, then 
the packets will correctly be forwarded to the VP. 

PF

"Joel M. Halpern" <[email-masked]> wrote on 10/20/2009 05:20:54 PM:

> I am glad to see this draft, as I believe this problem is a critical 
> obstacle in the way of the community giving VA serious consideration.
> 
> I believe that there is one class of issue that is alluded to briefly in 

> the document, but not discussed, which needs attention for credibility.
> That is the issue of inconsistent information.
> As written, there is no discussion about what happens if during 
> transitions, routers have different vies of the VP routes and the 
> suppress prefix sets.  It appears that under such circumstances large 
> collections of traffic may loop.  (Double-tunneling would prevent this, 
> but that is not currently mandated by the base VA draft, for good 
> reason.)  Without some discussion of the problem, there will be a 
> tendency for readers to conclude that it is a major issue.
> 
> 
> As a lesser topic, it seems to me that the inability to install prefixes 

> if the VP routers are down seems like a serious flaw in the can-suppress 

> approach.  If it is somehow not an issue, then the problems raised in 
> the VP-route tag discussion would seem to be even less important. 
> Either way, it seems that VP-route tag is simply cleaner.
> 
> Yours,
> Joel
> 
> 
> [email-masked] wrote:
------------------------------------------------------------------------
> > 
> > _______________________________________________
> > GROW mailing list
> > [email-masked]
> > https://www.ietf.org/mailman/listinfo/grow

---

From: "Joel M. Halpern" <[email-masked]>
Date: Tue, 20 Oct 2009 15:55:27 -0400

First, to be clear, the loop conditions I was raising for the 
autoconfiguration are transient.  They are not persisetnt loops.  I 
would ignore them except for the large number of prefixes (and therefore 
potentially large amount of traffic) that could be affected.

The basic situation is, given a chain of routers
A - B - C - D - E ...
Suppose that for some prefix(es), B is the VP Aggregator, and the 
correct exit is E or past E.
Now suppose that C happens to be a busy router, doing other control 
computations.  If necessary, assume additional connectivity for the 
announcements getting to C and D (maybe B, C, and D all talk to a route 
reflector F.)
Suppose that C and D have gotten the advertisement telling them that the 
individual paths are reachable via E or something past E.
Suppose that C and D both get the VP-tagged router advertisement from B.
But C is busy and does not update his FIB for many seconds.  D however 
happens to get his timing better aligned with the advertisements, and 
updates his FIB.

At that point, any packets destined to the covered prefixes, which 
arrive at D will be sent to C, towards the VP Aggregator B.
However, C still thinks that the exit is somewhere past E.  So it sends 
the packets right back to D.
(This is somewhat akin to the micro-loop problem in fast IP rerouting.)

I am presuming for now that we are prepared to disregard the case of C 
not having been upgraded to understand the community.  We have to 
presume that all the routers have been upgraded before the feature is 
turned on.  Otherwise, the problem above persists.

Yours,
Joel

---

From: Xu Xiaohu <[email-masked]>
Date: Wed, 21 Oct 2009 12:16:08 +0800

Hi Joel,

> -----邮件原件-----
> 发件人: Joel M. Halpern [mailto:[email-masked]]
> 发送时间: 2009年10月21日 3:55
> 收件人: Paul Francis
> 抄送: [email-masked]; [email-masked]
> 主题: Re: [GROW] I-D Action:draft-ietf-grow-va-auto-00.txt
> 

Yes, this transient loop will occur as long as the routes (either on control
plane or data plane) on different routers are not synchronized, no matter
whether the VA feature is turned on or not.

> turned on.  Otherwise, the problem above persists.

In order to allow VA-enable routers and legacy routers to coexist within the
same AS, the packets matching the VP route should be tunneled to the
corresponding VP aggregator.

Best wishes,
Xiaohu

---

From: Paul Francis <[email-masked]>
Date: Thu, 22 Oct 2009 19:27:08 +0200

Just in case Xiaohu's point is not clear....the exact looping problem you 
describe does not occur because packets are tunneled to APRs.  This is 
made clear in draft-ietf-grow-va-00.

PF

From:
Xu Xiaohu <[email-masked]>
To:
"'Joel M. Halpern'" <[email-masked]>, 'Paul Francis' 
<[email-masked]>
Cc:
[email-masked], [email-masked]
Date:
10/21/2009 06:16 AM
Subject:
答复: [GROW] I-D Action:draft-ietf-grow-va-auto-00.txt

Hi Joel,

> (This is somewhat akin to the micro-loop problem in fast IP rerouting.)

Yes, this transient loop will occur as long as the routes (either on 
control
plane or data plane) on different routers are not synchronized, no matter
whether the VA feature is turned on or not.

> turned on.  Otherwise, the problem above persists.

In order to allow VA-enable routers and legacy routers to coexist within 
the
same AS, the packets matching the VP route should be tunneled to the
corresponding VP aggregator.

Best wishes,
Xiaohu

> > reconfigures them both to be non-APRs.  Then they both at the same 
time
go
> > into GR.  But this shouldn't happen, because when a router is told 
that
it

---

From: "Joel M. Halpern" <[email-masked]>
Date: Thu, 22 Oct 2009 14:10:57 -0400

Mandating tunnels from the ingress BGP speaker to the APR is a
significant change since the version of the document I read carefully.
My apologies for not having noticed that.  Earlier, you were using
normal forwarding from each router to the APR, and tunneling from the
APR to the correct egress.  There was mention that tunnels could be used
for the first leg, but it was not mandated.

Yes, if you have decided to switch to mandating double-tunneling, first
from the edge to the APR, and then from the APR to the egress, there is
no loop.  I am sorry if I missed that change.  Double-tunneling is much
more robust, and is probably a good choice.  Given that we have to
assume we can cope with the MTU issues for the tunnels anyway (in order
to tunnel from the APR to the egress, this makes sense to me.

It seems to me that if we are going to double-tunnel, then it is
important that the Virtual Aggregates be marked with a specific
community, since the router using those routes need to know that it
should tunnel that traffic.  (While such may be inferable from the
can-suppress, being explicit is much better than drawing conclusions.)

Yours,
Joel

---

From: Paul Francis <[email-masked]>
Date: Fri, 23 Oct 2009 23:06:21 +0200

> can-suppress, being explicit is much better than drawing conclusions.)

Now that you point this out, I think this is a bug in the "can suppress" 
method.  The configured VP-list approach, as well as the VP-route tag make 
it clear what the VPs are, so that tunneling to the APR can be done.  The 
"can suppress" approach removes knowledge of the VPs per se, so we lose 
the ability to really know when we need to tunnel.

Xiaohu, do you have any thoughts?

PF

> >>> situation...what am I missing?).  I'm happy to clean up this 
analysis
> > and
> >>> In this case, the router expects to see a route to the VP.  If it 
does
> > not
> >>> FIB-installed the sub-prefix), in which case all is well, or it is 
has
> > for
> >>> some reason not FIB-installed one or more sub-prefixes, in which 
case 
> >>> forwards a packet to another APR just cause it doesn't have a route 
to 
> > a
> >>> for drops the packet, but no loops).  If there is no VP-route 
anywhere
> > for
> >>> If a non-APR is doing graceful restart, then until the RIB 
converges, 
> >>> In this case, it will tunnel packets to what it thinks is the APR 
for
> >>> while (for the withdraw info to propagate), and only then 
FIB-suppress
> >>> CS1:  The router thinks that an address range has a VP when there is 

> > in
> >>> CS2:  The router thinks that an address range has no VP when there 
is
> > one.
> >>> "Joel M. Halpern" <[email-masked]> wrote on 10/20/2009 05:20:54 

> > PM:
> >>>> I believe that there is one class of issue that is alluded to 
briefly
> > in

---

From: Xu Xiaohu <[email-masked]>
Date: Mon, 26 Oct 2009 12:10:04 +0800

Sorry for late response.

IMHO, if the packets need to be tunneled to the APRs by the ingress routers
(e.g., in case that legacy BGP routers and VA-enable BGP routers coexist
within a given AS), one can adopt the methods used for BGP free core (e.g.,
the BGP Encapsulation Extended Community defined in
[draft-ietf-softwire-encaps-safi] ) to realize it. If all the BGP routers
within a given AS are VA-enabled, it is not necessary to use tunnels between
the ingress routers and the APRs.

Yes, we need to state it clearly in the next version of the related draft.

Xiaohu

> -----邮件原件-----
> 发件人: Paul Francis [mailto:[email-masked]]
> 发送时间: 2009年10月24日 5:06
> 收件人: Joel M. Halpern
> 抄送: [email-masked]; Xu Xiaohu
> 主题: Re: 答复: [GROW] I-D Action:draft-ietf-grow-va-auto-00.txt
>