Subject: Re: Ethernet over MPLS

From: Eric Rosen <[email-masked]>
Date: Thu, 13 Mar 2003 12:50:11 -0500

Peter> I thought that the main benefit  of the tagged mode was that it makes
Peter> VLAN tags only locally significant.  

I think this issue is orthogonal.  Irrespective of the PW mode, one can have
an NSP function which manipulates the tags according to some local policy.

Peter> It seems to make more sense to  me to use the inner label to identify
Peter> the customer and to use the additional VLAN tag to identify a VLAN on
Peter> that customers network.  

Use raw mode for this, along with an NSP function for translating tags. 

This does  show a problem with Andy's  statement that in raw  mode "tags are
ignored by the PEs".  It would be more precise to say that the tags (if any)
which are carried in raw mode are not used by the PE to choose the PW. 

Even more precise might be: if a frame received by its ingress PE has one or
more (in  the case  of q-in-q, say)  VLAN tags,  then we have  the following
cases:

- The VLAN tag (or outer VLAN tag,  in the case of q-in-q) is used to choose
  the PW.  This breaks down into two sub-cases: 

  * The  VLAN tag (or outer  VLAN tag) is carried  on the PW  along with the
    packet: use tagged mode. 

  * The VLAN tag (or outer VLAN tag) is not carried on the PW along with the
    packet: use raw mode. 

- No VLAN tag is present, or the  VLAN tag is not used to choose the PW: use
  raw mode. 

I also don't  think it would make any  sense to use tagged mode  but to omit
the VLAN tag, as this would be no different than raw mode.

---

From: "Vach Kompella" <[email-masked]>
Date: Thu, 13 Mar 2003 10:01:05 -0800

Assume a PSN function "strip" that takes a VLAN-tagged packet associates the
packet with a VC *and* strips the tag.

Take n streams of customer packets that you want in a single VC.  Tag each
stream with its own "service-delimiting" VLAN tag.  This is over and above
whatever customer VLAN tag (if any) exists on the customer packet.

Now use the plain ethernet VC type, with PSN function strip.  The n streams are
mapped to the single VC.

Alternatively, use the VLAN VC type, and implement your mapping function from
VLAN tag to VC to handle many-to-one, but you have to disable VLAN rewriting.

-Vach

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of
> Busschbach, Peter B (Peter)
> Sent: Thursday, March 13, 2003 9:24 AM
> To: 'Andrew G. Malis'; Shahram Davari
> Cc: [email-masked]'
> Subject: RE: [PWE3] Ethernet over MPLS
>
>
> Hi Andy,
>
> I think I have the same question as Shahram ("why don't we have
> something like ATM n:1 mode?")
>
> I don't see why the fact that the PE interprets the VLAN tag implies
> that there needs to be a one-to-one association between tag and inner
> VC label. As you write yourself, it makes the VLAN tag redundant.
>
> I thought that the main benefit of the tagged mode was that it makes
> VLAN tags only locally significant. I.e. a specific tag is valid in
> one metro network and can be translated to another tag if the traffic
> is forwarded to another metro network, hence increasing the
> scaleability of the service. It seems to make more sense to me to use
> the inner label to identify the customer and to use the additional
> VLAN tag to identify a VLAN on that customers network. In other
> words, use a one-to-one association between customer and inner label
> but an N:1 association between VLAN tags and inner label.
>
> Peter
>
> -----Original Message-----
> From: Andrew G. Malis [mailto:[email-masked]]
> Sent: Thursday, March 13, 2003 10:12 AM
> To: Shahram Davari
> Cc: [email-masked]'
> Subject: Re: [PWE3] Ethernet over MPLS
>
>
> Shahram,
>
> I agree that the draft could do a better job explaining the difference
> between raw mode and tagged mode.  The distinction really is how the VLAN
> tags are treated by the network.
>
> In tagged mode, the PEs interpret the VLAN tag and have a one-to-one
> association between tags and inner VC labels.  Since the ingress PE is
> demultiplexing ingress frames by VLAN tag in order to associate them with a
> particular VC label, a priori all of the frames sent using a particular VC
> label must have the same tag.  This is similar in concept to ATM VPI/VCI mode.
>
> In raw mode, VLAN tags are ignored by the PEs (LERs) and are just passed
> transparently if they are in the frames.  If they exist, they only have
> significance to the CEs. An inner LSP can mix both tagged and non-tagged
> frames, and different tags can used in the frames.  This is very much like
> ATM port mode.
>
> A very good question is whether the VLAN tags should be stripped in tagged
> mode, since they are carrying redundant information and are thus just
> additional overhead.  I personally wouldn't mind that, but the consensus
> among the authors was that the additional processing at the ingress wasn't
> worth it, especially since there is a good chance that a new tag will be
> imposed at the egress.  If you transmit the tag, then you just have to
> overwrite it at the egress rather than have to add it to the frame.  But
> you may have to strip it at the egress anyway.
>
> Cheers,
> Andy
>
> -------
>
> At 3/12/2003 11:24 AM -0800, Shahram Davari wrote:
>
> >Hi,
> >
> >Why does draft-ietf-pwe3-ethernet-encap-02.txt requires that in tagged
> >mode all frames inside a single PW have the same VLAN tag?
> >
> >
> >Thanks,
> > > __________________________________________
> > > Shahram Davari, B.Eng., M.Sc.
> > > Senior Product Research Engineer
> > > PMC_Sierra, Inc.
> > > Tel: (613)271-4018
> > > Fax: (613)271-6468
> > > MailTo: [email-masked]
> > > __________________________________________
> > >
> > >
> >_______________________________________________
> >pwe3 mailing list
> >[email-masked]
> >https://www1.ietf.org/mailman/listinfo/pwe3
>
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>

---

From: "Andrew G. Malis" <[email-masked]>
Date: Thu, 13 Mar 2003 15:03:24 -0500

Eric and Vach,

Thanks for the clarifications.  One thing that's perfectly clear :-) is 
that we need to better specify the external functionality and internal 
behavior on the wire of these modes before the Ethernet spec makes it to 
RFC-hood.

As Eric points out, you should use a combination of raw mode on the wire 
and ingress classification in the PE to produce "N-to-1 mode".  And Vach is 
also correct that you most certainly don't want to rewrite the tags.

So "tagged" mode remains the only mode where you can rewrite existing tags 
on the egress, because you absolutely know that you're not losing any 
information.

>which are carried in raw mode are not used by the PE to choose the PW.

Not exactly (see above).  It would be more precise to say that raw mode 
requires the transparent transport of any and all VLAN tags at the ingress, 
and at the egress tags are not rewritten; while with tagged mode, all of 
the tags are the same, and this allows tags to be rewritten at the egress.

As Vach pointed out, for the most part it becomes VPLS functionality when 
you start pushing and popping tags, but there is one case where PWE3 label 
pushing and popping should be allowed, and that is to allow the 
interoperation of 802.1Q on one side of the pseudo-wire and "legacy" 802.1 
(non-Q) on the other.  The canonical example is the interconnection of a 
multi-VLAN port (using tags to distinguish between VLANs) to a bunch of 
individual non-tagged Ethernet ports elsewhere in the network, with each 
non-tagged Ethernet port corresponding to a VLAN at the 802.1Q port.  In 
this case tagged mode would be used for the pseudo-wire.  I suppose this 
could also be generalized to Q-in-Q by supporting a single port with N+1 
tags on one side of the pseudo-wires and a bunch of ports with N tags on 
the other side of the PWs.

>the VLAN tag, as this would be no different than raw mode.

The point here was that in tagged mode, you don't lose any information if 
you strip the outermost tag before transmitting on the wire, so it's an 
implementation ease vs. transmission efficiency issue.

Cheers,
Andy

---

From: Ali Sajassi <[email-masked]>
Date: Thu, 13 Mar 2003 14:17:51 -0800

>this case tagged mode would be used for the pseudo-wire.

If a single PW is used to carry all the tagged frames from the .1q port, 
then raw mode needs to be used. However, if multiple PWs are used, then you 
have a choice of  using either raw mode (stripping the VLAN tag at the 
ingress and then pass it over the PW) or tagged mode (passing the VLAN tag 
over the PW and then stripping it at the egress before delivery to 
non-tagged Ethernet port). The functionality of pushing/popping tags is a 
local mater which should be handled within the NSP.
Basically, raw mode PW can handle all types of scenarios and tag mode is 
needed for the scenarios where the PE can not do pushing/popping of VLAN tags.

-Ali