Subject: Re: Support for draft-kompella-ppvpn-vpls-01.txt

From: Eric Rosen <[email-masked]>
Date: Tue, 29 Apr 2003 11:36:30 -0400

Eric> Could you tell  us exactly which features of BGP  you are referring to
Eric> here?  Please be specific. 

Mourad> VPLS autodiscovery and signaling over 2 different networks

Well, no  one is proposing to use  LDP for VPLS autodiscovery.   And one can
use LDP between two different networks  as easily as one can use BGP between
two different networks.

Mourad> If  the forwarding  plane is  MPLS then  LDP could  be used  as MPLS
Mourad> signaling  protocol   but  in  this   case  LDP  is  not   used  for
Mourad> signaling/controling IPv4 routes, LDP isn t a routing protocol

Certainly it's true  that LDP is NOT  a routing protocol, and that  BGP IS a
routing  protocol.   It's   also  true  that  both  are   used,  in  varying
circumstances, for  distributing MPLS  labels.  As VPLS  does not  require a
routing protocol,  I'm not sure  how one is  supposed to conclude  from this
that BGP is a better signaling protocol for VPLS.

---

From: Yakov Rekhter <[email-masked]>
Date: Tue, 29 Apr 2003 09:07:52 -0700

Eric,

> Well, no  one is proposing to use  LDP for VPLS autodiscovery.   

From draft-lasserre-vkompella-ppvpn-vpls-04.txt (please pay attention
to "[LDP-DISC]"): 

  6.  Discovery
   
   Currently, no discovery mechanism has been prescribed for VPLS.
   There are three potential candidates, [BGP-DISC], [DNS-DISC], [LDP- 
   DISC].

> And one can

Not really. One difference is in the issue of using TCP MD5 for
authentication.

> that BGP is a better signaling protocol for VPLS. 

As I (as well as other folks) said several times before, it is
not that meaningful to compare LDP vs BGP just for signaling - one
needs to take the whole system into perspective. And the whole
system includes not just signaling but autodiscovery as well.  So,
a meanigful comparison should be BGP for both signaling and
autodiscovery vs BGP for autodiscovery + LDP for signaling.

Yakov.

---

From: Eric Rosen <[email-masked]>
Date: Tue, 29 Apr 2003 12:38:05 -0400

Eric> Well, no one is proposing to use LDP for VPLS autodiscovery.  

Yakov> please pay attention to "[LDP-DISC]"

Okay, it serves me right for saying "no one" ;-) How about "almost no one"? 

Yakov> One difference is in the issue of using TCP MD5 for authentication. 

Last  I  heard,  LDP used  TCP  as  its  transport,  and  used TCP  MD5  for
authentication.    I  don't   believe   that  there   is  any   BGP-specific
authentication mechanism in use.

Robert> there could  be a  slight difference in  number of TCP  sessions you
Robert> maintain in the BGP versus LDP proposals (especially those TCP which
Robert> do cross inter-as  boundary), hence number of TCP  peers you need to
Robert> somehow agree on using the same keys. 

That's one  of the  issues that led  me to  the suggestion of  confining the
inter-AS LDP control connections to the border routers (as in section 5.5 of
you know what draft); then you  have the same number of inter-AS connections
whether you use  LDP or BGP.  But even  if you didn't do that,  I'm not sure
you'd really need a distinct key for each pair of peers. 

Vach> Sure, but  the number of TCP  connections inter-AS is as  many as LDP,
Vach> unless you are telling me that eBGP uses RR! 

I think Vach's  model is the following.  Each VPLS has,  within each AS, one
or more  "hub PEs". If more  than one, they  are fully meshed.  If  the VPLS
extends over two  ASes, there is a point-to-point link between  a hub in one
AS and  a hub in  the other.  Each  hub sees the  other as a  spoke, meaning
primarily that neither applies split horizon to the pw that leads out of the
AS.  It  is correct that this  is a way  of reducing the number  of inter-AS
LDP connections so  that there aren't any more of them  than would be needed
with BGP.   It's not completely  clear to me  that this is analogous  to the
infamous option 10a from 2547bis, but  it does require an extra level of MAC
lookup.

Yakov> one needs to take the whole system into perspective. 

I thought that was  my point ;-) And that means we  don't just consider what
happens at time  zero, that we consider what  happens in various operational
fault scenarios, and  that we also consider, as part  of "the whole system",
VPWS services and p2p layer 2 services generally.

---

From: Mourad BERKANE <[email-masked]>
Date: Tue, 29 Apr 2003 17:23:28 +0200

Eric> Could you tell us exactly which features of BGP you are referring to
here? 
Eric> Please be specific. 

VPLS autodiscovery and signaling over 2 differents networks  

Mourad> LDP is not used as a signaling protocol for IPv4, 

Eric> I don't know  why you would say  this.  LDP is primarily used  today
to bind
Eric> MPLS labels to IPv4  routes.  I'm not sure why you say it  is "not
used as a
Eric> signaling protocol for IPv4".

by VPLS signaling i mean the protocol used for VPLS control plane (LDP or
BGP)
by IPv4 signaling i mean the protocol used for IPv4 control plane (IGP or
BGP)

If the forwarding plane is MPLS then LDP could be used as MPLS signaling
protocol but in this case LDP is not used for signaling/controling IPv4
routes, LDP isn t a routing protocol

Mourad

-----Message d'origine-----
De : Eric Rosen [mailto:[email-masked]]
Envoyé : mardi 29 avril 2003 16:34
À : Mourad BERKANE
Cc : [email-masked]'; [email-masked];
[email-masked]
Objet : Re: Support for draft-kompella-ppvpn-vpls-01.txt 

Mourad> MP-BGP is  much more  designed for a  *public* inter-AS  session for
Mourad> VPLS. 

Could you tell us exactly which features of BGP you are referring to here? 
Please be specific. 

Mourad> Even if LDP/RSVP  already present for the forwarding  plane inside a
Mourad> MPLS  domain, why should  i used  LDP for  VPLS signalling  when BGP
Mourad> already do the job (IPv4,v6,rfc2547bis,...)??? 

In  several previous messages,  I tried  to give  some specific  examples of
signaling interactions that BGP cannot do or cannot do well. 

Mourad> LDP is not used as a signaling protocol for IPv4, 

I don't know  why you would say  this.  LDP is primarily used  today to bind
MPLS labels to IPv4  routes.  I'm not sure why you say it  is "not used as a
signaling protocol for IPv4".  

Mourad> or  rfc2547bis :-)  BGP is  in charge  of it  (and do  the  job very
Mourad> well). 

So, if BGP is  good for 2547, it must be good for  VPLS too?  Isn't that the
issue under discussion?

---

From: Rajiv Asati <[email-masked]>
Date: Tue, 29 Apr 2003 13:09:04 -0400

Yakov,

>Not really. One difference is in the issue of using TCP MD5 for

And why would "TCP MD5 for authentication" become an issue for LDP !!

Cheers,
Rajiv

At 12:07 PM 4/29/2003, Yakov Rekhter wrote:
> > Mourad> 
> signaling  protocol   but  in  this   case  LDP  is  not   used  for

---

From: "Vach Kompella" <[email-masked]>
Date: Tue, 29 Apr 2003 10:21:04 -0700

Yakov,

I asked Hector but he didn't answer this.  There is some magic here in the
auto-discovery.  How does a PE know its site ID?  Does it auto-discover it?  Is
it configured?  If it is configured, how can you guarantee the uniqueness?  With
or without reference to all the other PEs in the VPLS?  If with reference to
them, then what's the point of auto-discovery?

Are site IDs reused?  Can one PE be site ID 1 in one VPN and site ID n (!= 1) in
another?

If I understand correctly, the real identifier for a PE in an L2VPN is:
VPNID.PE.siteId.  Without that, it won't work.  (And I'm not talking of the
remote PE's site ID which is in the NLRI, I'm talking of the local PE's site
ID).

If you remove a site from the VPLS, do you renumber site IDs?  Or waste labels?

If you remove a range of sites from the NLRI, do you advertise dummy label
ranges to avoid renumbering sites?

The auto-discovery for 2547 is pretty straightforward because all you need to
know is the VPN ID and the PE, which comes from the NLRI.  That comes because
the label is "advertised" not "signaled" with the distinction being that the
former is useable across the PEs in question and the latter is per-PE.  You can
just use loopback address based RDs to ensure route uniqueness, and the only
common piece of info that you need for correlation is the VPN ID.

-Vach

---

From: Yakov Rekhter <[email-masked]>
Date: Tue, 29 Apr 2003 10:39:04 -0700

Rajiv,

> And why would "TCP MD5 for authentication" become an issue for LDP !!

Because of the difference between the number of LDP sessions
vs BGP sessions.

Yakov.

---

From: Rajiv Asati <[email-masked]>
Date: Tue, 29 Apr 2003 13:56:37 -0400

Yakov ,

So the issue is not "TCP MD5 authentication", rather no of sessions.

Rajiv

At 01:39 PM 4/29/2003, Yakov Rekhter wrote:
> > > > Mourad> If  the forwarding  plane is  MPLS then  LDP could  be 
> used  as
> > > MPLS
> > > > Mourad>
> > > > Certainly it's true  that LDP is NOT  a routing protocol, and 
> that  BGP
> > > IS a
> > > >
> > > 
> > > > routing protocol,  I'm not sure  how one is  supposed to 
> conclude  from
> > > this