Subject: RE: Reference model for H-VPLS node

From: Olen Stokes <[email-masked]>
Date: Mon, 28 Jan 2008 15:19:24 -0800

Following off-line exchanges, I would like to draw attention on the mailing list to several points raised in this thread so far.  I would also like to request comments from the list concerning these topics.

The VPLS reference model described in RFC 4664 refers to an IEEE 802.1ad bridge module.  As such, this discussion involves both IETF and IEEE concepts and terminology.  The discussion points are therefore divided into IETF-centric and IEEE-centric sections.

The following IETF-centric topics were raised:

1) Marteen makes the statement, "In tree-structured mode the VPLS Forwarder has only 1 PW associated with the in/output port it has with the bridge function; it essentially becomes a null function and can be omitted"

This is the critical difference between the two models shown in Figure A and in Figure B in my original email.  In this case, I believe that Maarten's reference model agrees with the reference model in Ali's presentation.  In this H-VPLS model, a packet from a MTU-s passes through the bridge module in a PE-rs.

2) Maarten also makes the statement, "the set of PWs within a MPLS tunnel that carry VPLS (i.e. VLAN) signals constitute a virtual bridge port."

I believe that Maarten's and Ali's model differ on this point.  Ali's charts show a "virtual bridge port" at the LAN emulation interface between the bridge module and the VPLS forwarders.  Maarten appears to be describing a virtual bridge port on the other side of the VPLS forwarders.  This difference could potentially have significance when 802.1ag maintenance points are discussed.

Please comment on the mailing list your agreement or disagreement on these points.  These comments are needed in order to address OAM for L2VPNs using an accepted reference model.  Please remember to use IETF terminology whenever possible.

The IEEE-centric topics listed below were also raised in the attached email.  They are listed here because they may influence the IETF reference model for a H-VPLS node.  Comments are requested on these points as required in order to reach agreement on the H-VPLS model questions presented above.

a) Maarten states that "The Bridge and VPLS Forwarder functions are in terms of 802.1Q part of the MAC Relay function."

b) Maarten also states that 'For a full-mesh VPLS, the 802.1Q MAC Relay function combines the regular any-to-any MAC forwarding within the (Independent VLAN Learning (IVL) mode) VLAN with the "Asymmetric-VLANs in SVL mode".'

c) The above points demonstrate a need to understand where in a 802.1ad node is the location of the emulated LAN interface shown in Figure 3 of RFC 4664.

Cheers,
Olen

-----Original Message-----
From: VISSERS Maarten [mailto:[email-masked]]
Sent: Wednesday, December 12, 2007 12:24 PM
To: Olen Stokes
Cc: [email-masked]
Subject: RE: Reference model for H-VPLS node

Olen,

I made the following reference model for this figure 2, assuming the
there are VPLSs between the MTU and the PE-rs. I have included all PW's
that terminate in the PE1-rs and PE3-rs nodes; i.e. also PW-12 and
PW-23.

                                     PW-23-|
                               |-PW-12     |
             |--- PW-1 ---|    ||- PW-13 -||    |--- PW-3 ---|
   Emulated  |            |    ||         ||    |            |
    LAN      |            |    ||         ||    |            |
   .....................................................................
   .         |            |    ||         ||    |            |         .
   . |-------|-----|  |---|----||---| |---||----|---|  |-----|-------| .
   . | ----------- |  | ----- ----- | | ----- ----- |  | ----------- | .
   . |     VPLS    |  | VPLS| |VPLS | | VPLS| |VPLS |  |     VPLS    | .
   . |  Forwarder  |  | Fwdr| |Fwdr | | Fwdr| |Fwdr |  |  Forwarder  | .
   . | -----|----- |  | --|-- --|-- | | --|-- --|-- |  | -----|----- | .
   ..|.................................................|................
     |      |      |  |   |     |   | |   |     |   |  |      |      |
     |      |      |  |   |     |   | |   |     |   |  |      |      |
     | -----|----- |  | --|-----|-- | | --|-----|-- |  | -----|----- |
     | |         | |  | |         | | | |         | |  | |         | |
     | | Bridge  | |  | | Bridge  | | | | Bridge  | |  | | Bridge  | |
     | |         | |  | |         | | | |         | |  | |         | |

         MTU1-s            PE1-rs          PE3-rs           MTU3-s

                                        FIGURE A

In your model, you assume that there are no VPLSs present between MTU
and PE-rs. This would then result in the following model (Figure B), in
which also the PW-12 and PW-23 are included.

                                     PW-23-|
                               |-PW-12     |
             |--- PW-1 ---|    ||- PW-13 -||    |--- PW-3 ---|
   Emulated  |            |    ||         ||    |            |
    LAN      |            |    ||         ||    |            |
   .....................................................................
   .         |            |    ||         ||    |            |         .
   . |-------|-----|  |---|----||---| |---||----|---|  |-----|-------| .
   . |       |     |  |   |   ----- | | -----   |   |  |     |       | .
   . |       |     |  |   |   |VPLS | | VPLS|   |   |  |     |       | .
   . |       |     |  |   |   |Fwdr | | Fwdr|   |   |  |     |       | .
   . |       |     |  |   |   --|-- | | --|--   |   |  |     |       | .
   ..|.................................................|................
     |       |     |  |   |     |   | |   |     |   |  |     |       |
     |       |     |  |   |     |   | |   |     |   |  |     |       |
     | ------|---- |  | --|-----|-- | | --|-----|-- |  | ----|------ |
     | |         | |  | |         | | | |         | |  | |         | |
     | | Bridge  | |  | | Bridge  | | | | Bridge  | |  | | Bridge  | |
     | |         | |  | |         | | | |         | |  | |         | |

         MTU1-s            PE1-rs          PE3-rs           MTU3-s

                                        FIGURE B

The Bridge and VPLS Forwarder functions are in terms of 802.1Q part of
the MAC Relay function. We have to consider now the different VPLS
modes: tree-structured and full-mesh.

In tree-structured mode the VPLS Forwarder has only 1 PW associated with
the in/output port it has with the bridge function; it essentially
becomes a null function and can be omitted, as is done in the case of
PW-1 and PW-3.

In full-mesh mode the VPLS Forwarder provides a split-horizon
functionality, which is similar to the MAC Relay functionality specified
for "Asymmetric-VLANs in Shared VLAN Learning (SVL) mode" as specified
in clause B.1.3/802.1Q.

For a full-mesh VPLS, the 802.1Q MAC Relay function combines the regular
any-to-any MAC forwarding within the (Independent VLAN Learning (IVL)
mode) VLAN with the "Asymmetric-VLANs in SVL mode". I.e. some of the
ports of the VLAN on this MAC RElay (encompassing the bridge and VPLS
Forwarder functionality) are more special (split-horizon) than other
ports of the VLAN.

>From an 802.1Q/ag perspective, this will then result in the following
location of Ethernet MEPs and MIPs (see Figure C), assuming that the
MTU-s's have a MEP at MD Level 4. Consequently, the Ethernet MIPs that
are located between the PW and Bridge (PW-1, PW-3), or the PW and the
VPLS Forwarder (PW-12,PW-13,PW-23) are also having MD Level 4.

                                     PW-23-|
                               |-PW-12     |
             |--- PW-1 ---|    ||- PW-13 -||    |--- PW-3 ---|
   Emulated  |            |    ||         ||    |            |
    LAN      |            |    ||         ||    |            |
   .....................................................................
   . |-------|-----|  |---|----||---| |---||----|---|  |-----|-------| .
     |  MIP X MD   |  |MD X MIPXX MD| |MD XXMIP X MD|  | MIP X MD    |
     |      | L4   |  |L4 |    || L4| |L4 ||    | L4|  |     | L4    |
     |      |      |  |   |    ||   | |   ||    |   |  |     |       |
   . |      |      |  |   |    ||   | |   ||    |   |  |     |       |
.
   . |      |      |  |   |   ----- | | -----   |   |  |     |       | .
   . |      |      |  |   |   |VPLS | | VPLS|   |   |  |     |       | .
   . |      |      |  |   |   |Fwdr | | Fwdr|   |   |  |     |       | .
   . |      |      |  |   |   --|-- | | --|--   |   |  |      |      | .
   ..|.................................................|................
     |      |      |  |   |     |   | |   |     |   |  |      |      |
     |      |      |  |   |     |   | |   |     |   |  |      |      |
     | -----|----- |  | --|-----|-- | | --|-----|-- |  | -----|----- |
     | |         | |  | |         | | | |         | |  | |         | |
     | | Bridge  | |  | | Bridge  | | | | Bridge  | |  | | Bridge  | |
     | |         | |  | |         | | | |         | |  | |         | |
     | |    |    | |  | |         | | | |         | |  | |    |    | |
     | |MEP X MD | |  | |         | | | |         | |  | |MEP X MD | |
     | |    | L4 | |  | |         | | | |         | |  | |    | L4 | |

         MTU-1s           PE1-rs           PE3-rs          MTU-3s

                                        FIGURE C

>From an 802.1Q perspective, the set of PWs within a MPLS tunnel that
carry VPLS (i.e. VLAN) signals constitute a virtual bridge port. Such
virtual bridge port has an 802.1Q EISS SAP with the VPLS Forwarder
fucntion (as it is part of the MAC Relay). Over this EISS SAP flows
EM_UNITDATA.indication and EM_UNITDATA.request information.

The endpoint of the set of PWs provide from an 802.1Q perspective a
"support of the EISS" function. Such function adds in a pure 802.1Q
environment a VLAN Tag or a Backbone Service Instance Tag, while in the
hybrid 802.1Q/MPLS environment it adds a PW label to carry the
EM_UNITDATA.request information over the MPLS tunnel.

In this model, the MPLS PW label stack entry provides from the viewpoint
of the 802.1Q environment a Tag which has a 20-bit "vlan identifier" (to
carry the EM_UNITDATA.request.vlan_identifier parameter information) and
a 3-bit "priority code point" (to carry the EM_UNITDATA.request.priority
and EM_UNITDATA.request.drop_eligible parameter information. From the
MPLS environment viewpoint, the PW label provides a 20-bit label and a
3-bit EXP field carrying PHB information.

In this model, the VPLS looks from the 802.1Q viewpoint a VLAN with a
"MPLS Label Stack Entry based VLAN Tag" (i.e. an "M-VLAN Tag" or
"M-Tag").

As the PW label value for every PW in the VPLS may be different, and as
the PW label value between MTU-s and PE-rs may be different, the 802.1Q
equivalent model of the VPLS contains a "VID Translation Table" function
as specified in 802.1ad.

In this model, the MPLS tunnels provide point-to-point virtual interface
ports between MAC Relay functions.

----------------

There is a second 802.1Q equivalent model possible for VPLS. In this
model each PW endpoint provides a virtual bridge port supporting a
single (untagged) VLAN. The interconnect between the PW endpoint and the
VPLS Forwarder/Bridge function is then an ISS SAP and M_UNITDATA.request
and M_UNITDATA.indication information is passed over it.

The 1-to-1 nature of the VLAN-PW relationship allows to carry the VLAN
untagged. In such case the M_UNITDATA.request.priority and
M_UNITDATA.request.drop_eligible information is either dropped (no means
to carry it), or is carried via the PW label (note that in this case the
PW label is not longer performing the "M-Tag" function).

It is obvious that the number of virtual bridge ports in this second
model will be much larger than in the first model. From the 802.1Q
environment viewpoint there will now be many parallel virtual links
between two adjacent MAC Relay functions. This seems more complex then
the first model.

Regards,
Maarten

-------- Originele bericht --------
Onderwerp:      Reference model for H-VPLS node
Datum:  Wed, 12 Dec 2007 06:12:42 -0800
Van:    Olen Stokes <[email-masked]>
Aan:    [email-masked]

There was significant discussion at the last IETF about the correct
reference model to be used when discussing OAM for a H-VPLS node.  For
the H-VPLS network in Figure 2 of
draft-stokes-l2vpn-cfm-vccv-oam-00.txt, the reference model described in
that draft is shown below in Figure A:

         |--- PW-1 ----|  |--- PW-13 ----|  |---- PW-3 ----|
Emulated |             |  |              |  |              |
LAN      |             |  |              |  |              |
....................................................................
.        |             |  |              |  |              |       .
.|-------|-----|  |----|--|-----|  |-----|--|----|  |------|------|.
.| ----------- |  | ----------- |  | ----------- |  | ----------- |.
.|     VPLS    |  |     VPLS    |  |     VPLS    |  |    VPLS     |.
.|  Forwarder  |  |  Forwarder  |  |  Forwarder  |  |  Forwarder  |.
.| ------|---- |  | -----|----- |  | -----|----- |  | -----|----- |.
.|.............|..|................|................................
 |       |     |  |      |      |  |      |      |  |      |      |
 |       |     |  |      |      |  |      |      |  |------|------|
 | ------|---- |  | -----|----- |  | -----|----- |  | -----|----- |
 | |         | |  | |         | |  | |         | |  | |         | |
 | | Bridge  | |  | | Bridge  | |  | |  Bridge | |  | |  Bridge | |
 | |         | |  | |         | |  | |         | |  | |         | |
 | |  |      | |  | |         | |  | |         | |  | |      |  | |
 | ---|------- |  | ----------- |  | ----------- |  | -------|--- |
|----|--------|  |-------------|  |-------------|  |--------|----|
      |                                                      |
      |                                                      |
     CE-1                                                  CE-3

     MTU1-s            PE1-rs           PE3-rs           MTU3-s

     Figure A: H-VPLS node using only VPLS Forwarder connections

An alternative model based on
http://www3.ietf.org/proceedings/04nov/slides/l2vpn-9.pdf is shown below
in Figure B:

                          |--- PW-13 ----|
                Emulated  |              |
                 LAN      |              |
                ....................................
                .         |              |         .
                . |-------|-----|  |-----|-------| .
                . | ----------- |  | ----------- | .
                . |     VPLS    |  |     VPLS    | .
                . |  Forwarder  |  |  Forwarder  | .
                . | -----|----- |  | -----|----- | .
                ..|................|................
                  |      |      |  |      |      |
 |-------------|  |      |      |  |      |      |  |-------------|
 | ----------- |  | -----|----- |  | -----|----- |  | ----------- |
 | |         | |  | |         | |  | |         | |  | |         | |
 | |  Bridge | |  | | Bridge  | |  | |  Bridge | |  | |  Bridge | |
 | |         | |  | |         | |  | |         | |  | |         | |
 | |  |   *  | |  | |    *    | |  | |    *    | |  | |  *   |  | |
 | ---|---|--- |  | -----|----- |  | -----|----- |  | ---|---|--- |
|----|---|----|  |------|------|  |------|------|  |----|---|----|
      |   |              |                |              |   |
      |   |---- PW-1 ----|                |----- PW-3 ---|   |
     CE-1                                                  CE-3

     MTU1-s            PE1-rs           PE3-rs           MTU3-s

Note: "*" = emulated bridge port

    Figure B: H-VPLS node using emulated bridge port connections

Please note that Figure B is my interpretation of Ali's charts from his
November, 2004 presentation.

There are definite implications based on the differences of the two
models.
The VPLS forwarder only model in Figure A treats a H-VPLS network with
PW access the same as a simple full mesh VPLS network.  The bridge model
sees a single emulated LAN interface to the (H)VPLS network and
therefore is not concerned with the structure of that (H)VPLS network.

The emulated bridge port model treats a H-VPLS network with PW access
the same as a H-VPLS network with Ethernet access.  In effect, it
"separates"
the network into a full mesh VPLS emulated LAN and individual access
networks that can be PW or Ethernet.  This "separation" is a result of
the presence of the bridge module between these connections.

One difference from an OAM perspective is that the emulated bridge port
model allows 802.1ag MPs to be associated with PW access connections
such as
PW-1 in Figure B above.

There may be implications to other protocols such as spanning tree.  In
the emulated bridge port model, PW-1 is seen by the bridge module and
could be considered separately should the bridge module in PE1-rs
participate in spanning tree.  However, in the VPLS forwarder only
model, only the single emulated LAN would be included in spanning tree.

As I said during the L2 VPN meeting, I believe that choosing the correct
reference model is a decision to be made by the working group.
Therefore, please use the mailing list to state your views on the
correct reference model to be used for a H-VPLS node.

Cheers,
Olen