Subject: Ad Review of Draft-ietf-l2vpn-vpls-mcast

From: Stewart Bryant <[email-masked]>
Date: Fri, 31 May 2013 18:47:39 +0100

I apologize for the length of time that I have taken with
this draft, however I found that it was a particularly
difficult text to read and understand, particularly the
earlier text which has the purpose of setting out the
solution for the reader.

I recognize that the draft describes an existing, deployed
technology, and as such I have no desire to change the
definition of the technology. However I do have
significant concerns over the earlier sections of
the text. The purpose of a Standards Track
RFC is to describe a technology such that a competent
engineer that did not take part in the IETF process can
read the it, together with its normative references,
and then correctly and safely implement or operate
the technology, in a way that is interoperable with
other equipments and deployments. It is my opinion
this in this regard that the text falls short.

When I first reviewed the draft I found it pretty much
impenetrable, mainly due to its density, repetition and
lack of precise consistent definitions, and on a
second reading reached the same conclusion. I have
commissioned a early Routing Directorate review of the
draft, and although the review is not yet complete
preliminary discussions with the reviewer indicate
concurrence with this view. Thus I fear that if
we plough on with the publication process these
concerns will be echoed in the reviews that follow
AD evaluation.

I read the text a third time and I provide my own
detailed review comments inline. A revised ID is
certainly needed and I have changed the state to
indicate that. I hope that the shepherd will work
with you in the resolving these comments, and I would
suggest that you engage the L2VPN working group in
the verification of the correctness of the changes
and the clarity of any new or revised text.

Regards

Stewart

========

Draft-ietf-l2vpn-vpls-mcast-13.txt

========
Talking to the chairs we seems that this draft
has a number of components:

- The framework
- An underlay system with a number of options
- An overlay for L2
- An overlay for IP.

Now my view is that this might have been more manageable
and easier for a new reader if it had been written as
perhaps four (or more) smaller drafts with the above
structure.

However I assume that we are to continue with a single
draft and I suspect that it would be a lot more readable
if the was more obviously structured in a format similar
to the above so the reader knew more clearly which aspect
of the problem they were dealing with and could better
focus on the elements of the networking problem that
they were concerned with at the time.

Also I think that you need to give serious consideration
to providing a manageability section. From previous experience with
Ops Directorate reviews I think that you are going to be asked
for more text on operations and management.

=======

Abstract

This document describes a solution for overcoming a subset of the
limitations of existing VPLS multicast solutions.
SB> I think that it would be would be better for the reader if you
SB> started with a succinct definition of the problem.
SB>
SB> VPLS is not an IETF "known" term. The RFC Editor has a list
SB> of terms that it assumes everyone knows. All other terms
SB> need to be expanded on first use, and in the case of this
SB> draft could usefully be include in the terminology section.
SB>
SB> If we don't do this it will be a lot of work for the RFCeditor
SB> and then the authors will need to select or verify the proposed
SB> expansions in Auth48, so please scrub the document for unexpanded
SB> terms.

It describes
procedures for VPLS multicast that utilize multicast trees in the
sevice provider (SP) network. One such multicast tree can be shared
SB> sevice - typo
between multiple VPLS instances. Procedures by which a single
multicast tree in the SP network can be used to carry traffic
belonging only to a specified set of one or more IP multicast streams
from one or more VPLSes are also described.

SB> s/VPLSes/VPLS instances/

Internet Draft draft-ietf-l2vpn-vpls-mcast-13.txt January 2013

Table of Contents

1 Specification of requirements ......................... 4
2 Contributors .......................................... 4
3 Terminology ........................................... 5
4 Introduction .......................................... 5
5 Existing Limitations of VPLS Multicast ................ 6
6 Overview .............................................. 6
6.1 Inclusive and Selective Multicast Trees ............... 6
6.2 BGP-Based VPLS Membership Auto-Discovery .............. 8
6.3 IP Multicast Group Membership Discovery ............... 8
6.4 Advertising P-Multicast Tree to VPLS/C-Multicast Binding ..9
6.5 Aggregation ........................................... 10
6.6 Inter-AS VPLS Multicast ............................... 10
7 Intra-AS Inclusive P-Multicast Tree Auto-discovery/Binding11
7.1 Originating intra-AS VPLS A-D routes .................. 12
7.2 Receiving intra-AS VPLS A-D routes .................... 12
8 Demultiplexing P-Multicast Tree Traffic ............... 14
8.1 One P-Multicast Tree - One VPLS Mapping ............... 14
8.2 One P-Multicast Tree - Many VPLS Mapping .............. 14
9 Establishing P-Multicast Trees ........................ 15
9.1 Common Procedures ..................................... 15
9.2 RSVP-TE P2MP LSPs ..................................... 16
9.2.1 P2MP TE LSP - VPLS Mapping ............................ 16
9.3 Receiver Initiated MPLS Trees ......................... 17
9.3.1 P2MP LSP - VPLS Mapping ............................... 17
9.4 Encapsulation of Aggregate P-Multicast Trees .......... 17
10 Inter-AS Inclusive P-Multicast Tree A-D/Binding ....... 17
10.1 VSIs on the ASBRs ..................................... 18
10.1.1 Option (a): VSIs on the ASBRs ......................... 18
10.1.2 Option (e): VSIs on the ASBRs ......................... 18
10.2 Option (b) - Segmented Inter-AS Trees ................. 19
10.2.1 Segmented Inter-AS Trees VPLS Inter-AS A-D/Binding .... 19
10.2.2 Propagating BGP VPLS A-D routes to other ASes: Overview ..20
10.2.2.1 Propagating Intra-AS VPLS A-D routes in EBGP .......... 21
10.2.2.2 Inter-AS A-D route received via EBGP .................. 22
10.2.2.3 Leaf A-D Route received via EBGP ...................... 24
10.2.2.4 Inter-AS A-D Route received via IBGP .................. 24
10.3 Option (c): Non-Segmented Tunnels ..................... 25
11 Optimizing Multicast Distribution via Selective Trees
. 26
11.1 Protocol for Switching to Selective Trees ............. 27
11.2 Advertising (C-S, C-G) Binding to a Selective Tree .... 28
11.3 Receiving S-PMSI A-D routes by PEs .................... 30
11.4 Inter-AS Selective Tree ............................... 32
11.4.1 VSIs on the ASBRs ..................................... 33
11.4.1.1 VPLS Inter-AS Selective Tree A-D Binding .............. 33
11.4.2 Inter-AS Segmented Selective Trees .................... 33
11.4.2.1 Handling S-PMSI A-D routes by ASBRs ................... 34
11.4.2.1.1 Merging Selective Tree into an Inclusive Tree ......... 35
11.4.3 Inter-AS Non-Segmented Selective trees ................ 36
12 BGP Extensions ........................................ 36
12.1 Inclusive Tree/Selective Tree Identifier .............. 36
12.2 MCAST-VPLS NLRI ....................................... 37
12.2.1 S-PMSI A-D route ...................................... 38
12.2.2 Leaf A-D route ........................................ 39
13 Aggregation Considerations ............................ 39
14 Data Forwarding ....................................... 40
14.1 MPLS Tree Encapsulation ............................... 40
14.1.1 Mapping multiple VPLS instances to a P2MP LSP ......... 40
14.1.2 Mapping one VPLS instance to a P2MP LSP ............... 41
15 VPLS Data Packet Treatment ............................ 42
16 Security Considerations ............................... 43
17 IANA Considerations ................................... 44
18 Acknowledgments ....................................... 44
19 Normative References .................................. 44
20 Informative References ................................ 45
21 Author's Address ...................................... 46

<snip>

3. Terminology

This document uses terminology described in [RFC4761] And [RFC4762].

In this document we refer to various auto-discovery routes, as "A-D
routes".

SB> As far as I can tell you have not precisely alignmed the
SB> terminology with the terminology in the texts that you reference.
SB>
SB> In any case there seem to be terms in here that you assume are
SB> defined over there, but which are not over there. I think that
SB> you need a scub of the terminology to make sure that if it is
SB> over there you reference it as such (and that it is precisely
SB> aligned). If it is not in your references you need a precise
SB> definition in this section.
SB>
SB> Additionally it would be useful to the raeder to at least list
SB> the terms which are used from these RFCs

4. Introduction

SB> For someone who is not familiar with Inclusive, Aggregate
SB> Inclusive, Selective and Aggregate Selective, this section is
SB> very difficult to follow. Starting off with some general
SB> definitions of these terms or a reference to other RFCs (may be
SB> under the terminology section) would certainly help the reader.
SB>
SB> Alternatively you could include a forward reference to
SB> section 6.1, but that is less helpful to the reader than
SB> moving them earlier in the draft.

[RFC4761] and [RFC4762] describe a solution for VPLS multicast that
relies on the use of P2P RSVP-TE or MP2P LDP LSPs, referred to as
Ingress Replication In this document.

SB> Ingress Replication does not seem to be defined either in this
SB> text or the referenced text.

This solution

SB> "This solution" - Do you mean or the solution defined here?
SB> Do you mean: “for the use of pseudowires transported over P2P
SB> RSVP-TE or MP2P …

has certain
limitations for certain VPLS multicast traffic profiles. For example,
it may result in highly non-optimal bandwidth utilization in the MPLS
network when large amount of multicast traffic is to be transported.

SB> You say plural limitations but you only specify one.

This document describes procedures for overcoming the limitations of
existing VPLS multicast solutions. It describes procedures for VPLS
multicast that utilize multicast trees in the Service Provider (SP)
network. The procedures described in this document are applicable to
both [RFC4761] and [RFC4762].

It provides mechanisms that allow a single multicast distribution
SB> Please say what "it" is.

tree in the Service Provider (SP) network to carry all the multicast
traffic from one or more VPLS sites connected to a given PE,
irrespective of whether these sites belong to the same or different
VPLSes. Such a tree is referred to as an "Inclusive tree" and more
SB> Since the term VPLS has multiple definitions (solution, network, 
service),
SB> perhaps it is better to use VPLS instances here?
specifically as an "Aggregate Inclusive tree" when the tree is used
to carry multicast traffic from more than one VPLS.

SB> You need to define the difference between an IT and an AIT?
SB> s/VPLS/VPLS instances/

This document also provides procedures by which a single multicast
distribution tree in the SP network can be used to carry traffic
belonging only to a specified set of IP multicast streams, originated
in one or more VPLS sites connected to a given PE, irrespective of
whether these sites belong to the same or different VPLSes.

SB> It might be useful to introduce the problem before introducing
SB> the solution - minor reordering.
SB> Given the association of VPLS with Ethernet, why do you only
SB> provide this service for IP packets?

Such a
tree is referred to as a "Selective tree" and more specifically as an
"Aggregate Selective tree" when the IP multicast streams belong to
different VPLSes. This allows multicast traffic, by default, to be
carried on an Inclusive tree, while traffic from some specific
multicast streams, e.g., high bandwidth streams, could be carried on
one of the "Selective trees".

SB> Some figures/reference models in the draft would really help
SB> the reader to understand the problem. They need to be early
SB> in the text, such as here.

5. Existing Limitations of VPLS Multicast

One of the limitations of existing VPLS multicast solutions described
in [RFC4761] and [RFC4762] is that they rely on ingress replication.
Thus, the ingress PE replicates the multicast packet for each egress
PE and sends it to the egress PE using a unicast tunnel.

SB> This text would have been useful earlier.
SB> Also much of this text seems to repeat earlier text and I would
SB> suggest that you remove it.

Ingress Replication may be an acceptable model when the bandwidth of
the multicast traffic is low or/and the number of replications
performed on average on each outgoing interface for a particular
customer VPLS multicast packet is small.
SB> Is this a run on sentence?
If this is not the case it
is desirable to utilize multicast trees in the SP network to transmit
VPLS multicast packets [RFC5501].
SB> There’s no explicit mention of “broadcast”. Would it fit under the
SB> multicast or unicast treatment?

Note that unicast packets that are
flooded to each of the egress PEs, before the ingress PE learns the
destination MAC address of those unicast packets, MAY still use
ingress replication.

SB> Again, I think that a consistent single intro to the problem
SB> would be much more helpful to the reader in getting up to speed
SB> with the applicability of the solution you are about to describe.

6. Overview

This document describes procedures for using multicast trees in the
SP network to transport VPLS multicast data packets.

SB> This text seems to repeat text that you wrote earlier.
SB> At this stage I was confused as to whether or not
SB> you were only concerned with MPLS m/c trees. You clarify
SB> this later, but I think that the raeder would find it
SB> useful if you cleared this up earlier?

RSVP-TE P2MP
LSPs described in [RFC4875] are an example of such multicast trees.
The use of multicast trees in the SP network can be beneficial when
the bandwidth of the multicast traffic is high or when it is
desirable to optimize the number of copies of a multicast packet
transmitted on a given link.

SB> Again this seems to be repeated text.

This comes at a cost of state in the SP
network to build multicast trees and overhead to maintain this state.
This document describes procedures for using multicast trees for VPLS
multicast when the provider tunnels are P2MP LSPs signaled by either
P2MP RSVP-PE or mLDP [RFC6388].

SB> Again I think this is repeated text.
SB> You seem to be saying m/c trees are a state problem
SB> so we are going to use them, when think you mean to say
SB> m/c trees are a state problem but we are going to show how
SB> to use them efficiently. Please consider integrating
SB> this text with 6.1.

This document uses the prefix 'C' to refer to the customer control or
data packets and 'P' to refer to the provider control or data
packets.

SB> This notation needs to be much easier to find - perhaps in
SB> it should be in the terminology section?

An IP (multicast source, multicast group) tuple is
abbreviated to (S, G).

SB> Again this is almost certainly better placed in terminology.

6.1. Inclusive and Selective Multicast Trees

SB> You strongly hint at this before. It would be better to unify
SB> the definitions of the m/c tree types so as to make a
SB> more efficient description that the reader can get their
SB> heads around the first time they meet this.

Multicast trees used for VPLS can be of two types:

SB> It would be useful if things like Inc P-m/c tree were in terminology.

+ Inclusive trees. This option supports the use of a single
multicast distribution tree, referred to as an Inclusive P-
Multicast tree, in the SP network to carry all the multicast
traffic from a specified set of VPLS sites connected to a given
PE. There is no assumption made with respect to whether this
traffic is IP encapsulated or not. A particular P-Multicast tree
can be set up to carry the traffic originated by sites belonging
to a single VPLS, or to carry the traffic originated by sites
belonging to different VPLSes. The ability to carry the traffic
of more than one VPLS on the same tree is termed Aggregation. The
tree needs to include every PE that is a member of any of the
VPLSes that are using the tree. This implies that a PE may
receive multicast traffic for a multicast stream even if it
doesn't have any receivers that are interested in receiving
traffic for that stream.

SB> This would have been better earlier in place of the partial
SB> definitions above.

An Inclusive P-Multicast tree as defined in this document is a
P2MP tree. A P2MP tree is used to carry traffic only from VPLS
sites that are connected to the PE that is the root of the tree.

SB> This seems to be repeated text, and does the following text.

+ Selective trees. A Selective P-Multicast tree is used by a PE to
send IP multicast traffic for one or more specific IP multicast
streams, received by a PE over PE-CE interfaces that belong to
the same or different VPLSes, to a subset of the PEs that belong
to those VPLSes. Each of the PEs in the subset should be on the
SB> Please clarify which PEs?
path to a receiver of one or more multicast streams that are
mapped onto the tree. The ability to use the same tree for
multicast streams that belong to different VPLSes is termed
Aggregation. The reason for having Selective P-Multicast trees is
to provide a PE the ability to create separate SP multicast trees
for specific multicast streams, e.g. high bandwidth multicast
streams. This allows traffic for these multicast streams to reach
only those PE routers that have receivers in these streams. This
SB> s/in/for/
avoids flooding other PE routers in the VPLS.

SB> Again this seems to be repeating earlier text

SB> I think you are saying that an ST is an IT with IP m/c filtering
SB? is that correct?

A SP can use both Inclusive P-Multicast trees and Selective P-
Multicast trees or either of them for a given VPLS on a PE, based on
local configuration. Inclusive P-Multicast trees can be used for
both IP and non-IP data multicast traffic, while Selective P-
Multicast trees must be used only for IP multicast data traffic. The
use of Selective P-Multicast trees for non-IP multicast traffic is
outside the scope of this document.

SB> The normal ordering is A or B or A+B, but it's up to you.

A variety of transport technologies may be used in the SP network.

SB> I am not sure these are transport technologies. They do not
SB> fit the ITU use of the term nor the layer four use. I think
SB> you really mean network layer technologies.

For inclusive P-Multicast trees, these transport technologies include
point-to-multipoint LSPs created by RSVP-TE or mLDP. For selective P-
Multicast trees, unicast PE-PE tunnels (using MPLS or IP/GRE

SB> Why unicast? You said you were using p2mp in the SP layer!

encapsulation) and P2MP LSPs are supported, and the supported P2MP
LSP signaling protocols are RSVP-TE, and mLDP. Other transport
technologies are outside the scope of this document.

SB> You only support MPLS - is that correct? If so where does
SB> IP/GRE come into it?

This document also describes the data plane encapsulations for
supporting the various SP multicast transport options.

SB> Again I am confused over the term "transport".

6.2. BGP-Based VPLS Membership Auto-Discovery

In order to establish Inclusive P-Multicast trees for one or more
VPLSes, when aggregation is performed (with either mLDP or P2MP RSVP-
TE as the tunneling technology), or when the tunneling technology is
P2MP RSVP-TE, the PE acting as a root of a P2MP LSP must be able to
discover the other PEs that have membership in one or more of these
VPLSes (these other PEs act as leaves of that P2MP LSP). This
document uses the BGP-based procedures described in [RFC4761] and
[RFC6074] for discovering the VPLS membership of all PEs. When no
aggregation is performed and the tunneling technology is mLDP, then
the root of the P2MP LSP need not discover the other PEs that are the
leaves of that LSP.

SB> I do not understand the logic that gets you to the above
SB> statement or why the different behaviors?

SB> Re the text up to PSMP LSP)
SB> Please consider revising, too long to follow.
SB>
SB> This is confusing: when mLDP is used, the root doesn’t need to
SB> discover the leaf nodes for inclusive multicast. However,
SB> this sentence implies otherwise.

The leaves of the Inclusive P-Multicast tree must also be able to
auto-discover the identifier of the tree (note that this applies when
the tree is established by either mLDP or P2MP RSVP-TE). This is
described in section 6.4.

SB> It is the procedures that is described in 6.4. I assume that you
SB> are saying that you always need to discover the tree identity.
SB> The way that you express it leaves a mild doubt.

6.3. IP Multicast Group Membership Discovery

The setup of a Selective P-Multicast tree for one or more IP
multicast (C-S, C-G)s, requires the ingress PE to learn the PEs that
have receivers in one or more of these (C-S, C-G)s, in the following
cases:

+ When aggregation is used OR

SB> Why in the case of mLDP - this is not clear?

+ When the tunneling technology is P2MP RSVP-TE

+ If ingress replication is used and the ingress PE wants to send
traffic for (C-S, C-G)s to only those PEs that are on the path to
receivers for the (C-S,C-G)s.

SB> It would be helpful if you explained the overarching reason
SB> and then perhaps a list rather than leaving the reader to puzzle.

For discovering the IP multicast group membership, this document
describes procedures that allow an ingress PE to enable explicit
tracking.

SB> Tracking of what?

Thus an ingress PE can request the IP multicast membership
from egress PEs for one or more C-multicast streams. These procedures
are described in section "Optimizing Multicast Distribution via
Selective Trees".

SB> You should consider rewording the above two sentences for clarity.

These procedures are applicable when IGMP is used as the multicast
signaling protocol between the VPLS CEs. They are also applicable
when PIM as specified in [RFC4601] is used as the multicast routing
protocol between the VPLS CEs and PIM join suppression is disabled on
all the CEs. However these procedures do not apply when PIM is used
as the multicast routing protocol between the VPLS CEs and PIM join
suppression is not disabled on all the CEs. Procedures for this case
are for further study.

SB> Why is this? The rationale for all of these conditions is not
SB> captured. A reader who was not involved in writing this draft has
SB> no idea as to why these restrictions are in place.

The leaves of the Selective P-Multicast trees must also be able to
discover the identifier of the tree. This is described in section
6.4.

SB> Identifier of which tree?

6.4. Advertising P-Multicast Tree to VPLS/C-Multicast Binding

This document describes procedures based on BGP VPLS Auto-Discovery
(A-D)

SB> This needs a reference and is also a good candidate for the terminology
SB> section.

that are used by the root of an Aggregate P-Multicast tree to
advertise the Inclusive or Selective P-Multicast tree binding and the
de-multiplexing information to the leaves of the tree. This document
uses the PMSI Tunnel attribute [RFC6514] for this purpose.
SB> PMSI needs expanding

Once a PE decides to bind a set of VPLSes or customer multicast

SB> You need to be clear whether this is an ingress, egress or
SB> either type of PE, and how the decision to bind is taken.

groups to an Inclusive P-Multicast tree or a Selective P-Multicast
tree, it needs to announce this binding to other PEs in the network.
This procedure is referred to as Inclusive P-Multicast tree or
Selective P-Multicast tree binding distribution and is performed
using BGP.

When an Aggregated Inclusive P-Multicast tree is used by an ingress
PE, this discovery implies that an ingress PE MUST announce the
SB> What discovery?
binding of all VPLSes bound to the Inclusive P-Multicast tree to the
other PEs.
SB> Why to all?

The inner label assigned by the ingress PE for each VPLS
MUST be included, if more than one VPLS is bound to the same P-
Multicast tree. The Inclusive P-Multicast tree Identifier MUST be
included.

For a Selective P-Multicast tree this discovery implies announcing
all the specific <C-S, C-G> entries bound to this P-Multicast tree
along with the Selective P-Multicast tree Identifier. The inner label
assigned for each <C-S, C-G> MUST be included if <C-S, C-G>s from
different VPLSes are bound to the same P-Multicast tree. The labels
MUST be distinct on a per VPLS basis and MAY be distinct on a per <C-
S, C-G> basis. The Selective P-Multicast tree Identifier MUST be
included.

6.5. Aggregation

As described above the ability to carry the traffic of more than one
VPLS on the same P-Multicast tree is termed 'Aggregation'. Both
Inclusive and Selective P-Multicast trees support aggregation.

Aggregation enables the SP to place a bound on the amount of
multicast tree forwarding and control plane state which the P routers
must have.

SB> Re all of the above, this seems similar to text earlier in the draft.

Let us call the number of VPLSes aggregated onto a single
P-Multicast tree as the "Aggregation Factor". When Inclusive source
P-Multicast trees are used the number of trees that a PE is the root
of is proportional to:

+ (Number of VPLSes on the PE / Aggregation Factor).

In this case the state maintained by a P router, is proportional to:

+ ((Average number of VPLSes on a PE / Aggregation Factor) * number
of PEs) / (Average number of P-Multicast trees that transit a
given P router)

Thus, the state does not grow linearly with the number of VPLSes.

SB> Isn't the above sort of obvious? It might be useful to comment
SB> on the expected range of AFs.

Aggregation requires a mechanism for the egresses of the P-Multicast
tree to demultiplex the multicast traffic received over the P-
Multicast tree. To enable the egress nodes to perform this
demultiplexing, upstream-assigned labels [RFC5331] MUST be assigned
and distributed by the root of the aggregate P-multicast tree."

SB> Spurious """

6.6. Inter-AS VPLS Multicast

This document supports four models of inter-AS VPLS service, option
(a), (b), (c), and (e).

SB> You do not explain to the reader
SB>
SB> 1) Where on earth options a..b,e come from.
SB> 2) What they are in a more meaningful name.

Options (a), (b), and (c) are very similar
conceptually to options (a), (b) and (c) specified in [RFC4364]

SB> I just grepped [RFC4364] and the only time "option" occurs
SB> is in conjunction with MD5 at least in the normative text.
SB> You either need a concise definition here and a pointer
SB< to a precise definition, or a precise definition.
SB>
SB> This is an important point because you can't follow
SB> a lot of what follows without these definitions.

For
IP VPNs. These options described here are also similar to the three
options described in [RFC4761], Which in turn extends the concepts of
[RFC4364] to inter-AS VPLS.

SB> Again the definitions in tbe referenced text seems to be
SB> missing.

For option (a) and option (b) support, this document specifies a
model where Inter-AS VPLS service can be offered without requiring a
single P-Multicast tree to span multiple ASes. There are two variants
of this model and they are described in section 10.

SB> Again the above is impossible to understand because the reader has no
SB> idea what option(a/b) is at this point.

For option (c) support this document specifies a model where Inter-AS
VPLS service is offered by requiring a single P-Multicast tree to

span multiple ASes. This is because in the case of option (c) the
ASBRs do not exchange BGP-VPLS NLRIs or A-D routes.

In addition, this document also specifies option (e), which one may
think of as a variant of option (a).

SB> So option(e) is not defined in any of the referenced text and the
SB> reader so far has no idea at all what it is.

SB> It's not clear where you use the option notation until you list it
SB> all again much later.

7. Intra-AS Inclusive P-Multicast Tree Auto-discovery/Binding

This section specifies procedures for the intra-AS auto-discovery of
VPLS membership and the distribution of information used to
instantiate P-Multicast Tunnels.

VPLS auto-discovery/binding consists of two components: intra-AS and
inter-AS. The former provides VPLS auto-discovery/binding within a
single AS. The latter provides VPLS auto-discovery/binding across
multiple ASes. Inter-AS auto-discovery/binding is described in
section 10.

VPLS auto-discovery using BGP as described in [RFC4761, RFC6074]
enables a PE to learn the VPLS membership of other PEs. A PE that
belongs to a particular VPLS announces a BGP Network Layer
Reachability Information (NLRI) that identifies the Virtual Switch
Instance (VSI). This NLRI is constructed from the <Route-
Distinguisher (RD), VPLS Edge Device Identifier (VE-ID)> tuple. The
NLRI defined in [RFC4761] comprises the <RD, VE-ID> tuple and label
blocks for PW signaling. The VE-ID in this case is a two octet
number. The NLRI defined in [RFC6074] comprises only the <RD, VE-ID>
where the VE-ID is a four octet number.

The procedures for constructing Inclusive intra-AS and inter-AS trees
as specified in this document require the BGP A-D NLRI to carry only
the <RD, VE-ID>. Hence these procedures can be used for both BGP-VPLS
and LDP-VPLS with BGP A-D.

It is to be noted that BGP A-D is an inherent feature of BGP-VPLS.
However it is not an inherent feature of LDP-VPLS. In fact there are
deployments and/or implementations of LDP-VPLS that require
configuration to enable a PE in a particular VPLS to determine other
PEs in the VPLS and exchange PW labels using FEC 128 [RFC4447]. The
use of BGP A-D for LDP-VPLS [RFC6074], to enable automatic setup of
PWs, requires FEC 129 [RFC4447]. However FEC 129 is not required in
order to use procedures in this document for LDP-VPLS. An LDP-VPLS
implementation that supports this document MUST support the BGP A-D
procedures to setup P-Multicast trees, as described here, and it MAY
support FEC 129 to automate the signaling of PWs.

SB> Please look to see if the above can be expressed more clearly.

7.1. Originating intra-AS VPLS A-D routes

To participate in the VPLS auto-discovery/binding a PE router that
has a given VSI of a given VPLS originates an BGP VPLS intra-AS A-D
SB> s/an/a/
route and advertises this route in Multi-Protocol (MP) IBGP. The
route is constructed as described in [RFC4761] and [RFC6074].

SB> This seems to be a repetition of earlier text

The route carries a single L2VPN NLRI with the RD set to the RD of
the VSI, and the VE-ID set to the VE-ID of the VSI. The route also
carries one or more Rout Targets (RTs) as specified in [RFC4761] and
[RFC6074].

SB>s/Rout/Route/

If an Inclusive P-Multicast tree is used to instantiate the provider
tunnel for VPLS multicast on the PE, the advertising PE MUST
advertise the type and the identity of the P-Multicast tree in the
PMSI Tunnel attribute [RFC6514]. This attribute is described in
section 12.1.

A PE that uses an Inclusive P-Multicast tree to instantiate the
provider tunnel MAY aggregate two or more VPLSes present on the PE
onto the same tree. If the PE decides to perform aggregation after it
has already advertised the intra-AS VPLS A-D routes for these VPLSes,
then aggregation requires the PE to re-advertise these routes. The
re-advertised routes MUST be the same as the original ones, except
for the PMSI Tunnel attribute.
SB> Why the MUST?

If the PE has not previously
advertised intra-AS A-D routes for these VPLSes, then the aggregation
requires the PE to advertise (new) intra-AS A-D routes for these
VPLSes.
SB> This seems obvious, I am wondering whether the order of cases
SB> called up is counter intuitive which then causes this to grate a bit.

The PMSI attribute in the newly advertised/re-advertised
routes MUST carry the identity of the P-Multicast tree that
aggregates the VPLSes, as well as an MPLS upstream-assigned label
[RFC5331]. Each re-advertised route MUST have a distinct label.

SB> Distinct over what space?
SB> Re "re-advertised"
SB> Even a new route must have a distinct label

Discovery of PE capabilities in terms of what tunnels types they
support is outside the scope of this document. Within a given AS PEs
participating in a VPLS are expected to advertise tunnel bindings
whose tunnel types are supported by all other PEs that are
participating in this VPLS and are part of the same AS.

7.2. Receiving intra-AS VPLS A-D routes

When a PE receives a BGP Update message that carries an intra-AS A-D
route such that (a) the route was originated by some other PE within
the same AS as the local PE, (b) at least one of the Route Targets of
the route matches one of the import Route Targets configured for a
particular VSI on the local PE, (c) the BGP route selection
determines that this is the best route with respect to the NLRI
carried by the route, and (d) the route carries the PMSI Tunnel
attribute, the PE performs the following.

If the route carries the PMSI Tunnel attribute then:
SB> We only got here because Condition (d) above confirmed
SB> that the route did indeed carry this attribute. So, this check
SB> is not needed.

+ If the Tunnel Type in the PMSI Tunnel attribute is set to LDP
P2MP LSP, the PE SHOULD join the P-Multicast tree whose identity
is carried in the PMSI Tunnel attribute.

+ If the Tunnel Type in the PMSI Tunnel attribute is set to RSVP-TE
P2MP LSP,

SB> Didn't you say earlier: "Discovery of PE capabilities in terms of what
SB> tunnels types they support is outside the scope of this document."

the receiving PE has to establish the appropriate state
to properly handle the traffic received over that LSP. The PE
that originated the route MUST establish an RSVP-TE P2MP LSP with
the local PE as a leaf. This LSP MAY have been established before
the local PE receives the route.

SB> Re whole para
SB> It might make sense to defer this bullet till after the very last
SB> paragraph of this section, i.e. the one that starts with “If the local
SB> PE uses RSVP-TE P2MP LSP for sending (multicast) traffic.” That,
SB> I think, provides a more logical flow.

+ If the PMSI Tunnel attribute does not carry a label, then all
packets that are received on the P-Multicast tree, as identified
by the PMSI Tunnel attribute, are forwarded using the VSIs that
have at least one of its import Route Targets that matches one of
the Route Targets of the received A-D route.

+ If the PMSI Tunnel attribute has the Tunnel Type set to LDP P2MP
LSP or RSVP-TE P2MP LSP, and the attribute also carries an MPLS
label, then the egress PE MUST treat this as an upstream-assigned
label, and all packets that are received on the P-Multicast tree,
as identified by the PMSI Tunnel attribute, with that upstream
label are forwarded using the VSIs that have at least one of its
import Route Target that matches one of the Route Targets of the
received intra-AS A-D route.

If the local PE uses RSVP-TE P2MP LSP for sending (multicast)
traffic, originated by VPLS sites connected to the PE, to the sites
attached to other PEs then the local PE MUST use the Originating
Router's IP address information carried in the intra-AS A-D route to
add the PE, that originated the route, as a leaf node to the LSP.
This MUST be done irrespective of whether the received Intra-AS A-D
route carries the PMSI Tunnel attribute or not.

8. Demultiplexing P-Multicast Tree Traffic

Demultiplexing received VPLS traffic requires the receiving PE to
determine the VPLS instance the packet belongs to. The egress PE can
then perform a VPLS lookup to further forward the packet. It also
requires the egress PE to determine the identity of the ingress PE
for MAC learning, as described in section 15.

8.1. One P-Multicast Tree - One VPLS Mapping

When a P-Multicast tree is mapped to only one VPLS, determining the
tree on which the packet is received is sufficient to determine the
VPLS instance on which the packet is received. The tree is determined
based on the tree encapsulation. If MPLS encapsulation is used,
e.g.,: RSVP-TE P2MP LSPs, the outer MPLS label is used to determine
the tree. Penultimate-hop-popping MUST be disabled on the MPLS LSP
(RSVP-TE P2MP LSP or LDP P2MP LSP).

8.2. One P-Multicast Tree - Many VPLS Mapping

As traffic belonging to multiple VPLSes can be carried over the same
tree, there is a need to identify the VPLS the packet belongs to.
This is done by using an inner label that determines the VPLS for
which the packet is intended. The ingress PE uses this label as the
inner label while encapsulating a customer multicast data packet.
Each of the egress PEs must be able to associate this inner label
with the same VPLS and use it to demultimplex the traffic received
SB> s/demultimplex/demultiplex/
over the Aggregate Inclusive tree or the Aggregate Selective tree.

SB> Again this seems to repeat earlier text

If traffic from multiple VPLSes is carried on a single tree,
upstream-assigned labels [RFC5331] MUST be used. Hence the inner
label is assigned by the ingress PE. When the egress PE receives a
packet over an Aggregate tree, the outer encapsulation (in the case
of MPLS P2MP LSPs, the outer MPLS label) specifies the label space to
perform the inner label lookup. The same label space MUST be used by
the egress PE for all P-Multicast trees that have the same root
[RFC5331].

If the tree uses MPLS encapsulation, as in RSVP-TE P2MP LSPs, the
outer MPLS label and optionally the incoming interface provides the
label space of the label beneath it. This assumes that penultimate-
hop-popping is disabled. The egress PE MUST NOT advertise IMPLICIT
NULL or EXPLICIT NULL for that tree once it is known to the egress PE
that the tree is bound to one or more VPLSes. Once the label
representing the tree is popped off the MPLS label stack, the next
label is the demultiplexing information that allows the proper VPLS
instance to be determined.

The ingress PE informs the egress PEs about the inner label as part
of the tree binding procedures described in section 12.

9. Establishing P-Multicast Trees

This document supports only P2MP P-Multicast trees wherein it is
possible for egress PEs to identify the ingress PE to perform MAC
learning. Specific procedures are specified only for RSVP-TE P2MP
LSPs and LDP P2MP LSPs. An implementation that supports this document
MUST support RSVP-TE P2MP LSPs and LDP P2MP LSPs.

A P2MP tree is used to carry traffic originated in sites connected to
the PE which is the root of the tree. These sites MAY belong to
different VPLSes or the same VPLS.

SB> Again this seems to repeat earlier text

9.1. Common Procedures

The following procedures apply to both RSVP-TE P2MP and LDP P2MP
LSPs.

Demultiplexing the C-multicast data packets at the egress PE requires
that the PE must be able to determine the P2MP LSP that the packets
are received on. This enables the egress PE to determine the VPLS
instances that the packet belongs to. To achieve this the LSP MUST be
signaled with penultimate-hop-popping (PHP) off and a non-reserved
MPLS label off as described in section 8. In other words an egress PE
MUST NOT advertise IMPLICIT NULL or EXPLICIT NULL for a P2MP LSP that
is carrying traffic for one or more VPLSes.

SB> The above seems to be a universal statement, in which case perhaps
SB> it could be stated once as a reference to the whole document

This is because the
egress PE needs to rely on the MPLS label, that it advertises to its
upstream neighbor, to determine the P2MP LSP that a C-multicast data
packet is received on.

The egress PE also needs to identify the ingress PE to perform MAC
learning. When P2MP LSPs are used as P2MP trees, determining the
P2MP LSP that the packets are received on, is sufficient to determine
the ingress PE. This is because the ingress PE is the root of the
P2MP LSP.

The egress PE relies on receiving the PMSI Tunnel attribute in BGP to
determine the VPLS instance to P2MP LSP mapping.

SB> It seems to me that there are a few key invariants that could be
SB> called out such as no PHP and then the rest of the text would
SB> be simpler.
SB>
SB> Is the problem that you have two (or more) classes of solutions that 
you
SB> are trying to force into a single text?

9.2. RSVP-TE P2MP LSPs

This section describes procedures that are specific to the usage of
RSVP-TE P2MP LSPs for instantiating a P-Multicast tree. Procedures in
[RFC4875] are used to signal the P2MP LSP. The LSP is signaled as the
root of the P2MP LSP discovers the leaves. The egress PEs are
discovered using the procedures described in section 7. Aggregation
as described in this document is supported.

9.2.1. P2MP TE LSP - VPLS Mapping

P2MP TE LSP to VPLS mapping is learned at the egress PEs using BGP
based advertisements of the P2MP TE LSP - VPLS mapping. They require
that the root of the tree include the P2MP TE LSP identifier as the
tunnel identifier in the BGP advertisements. This identifier contains
the following information elements:
- The type of the tunnel is set to RSVP-TE P2MP LSP
- RSVP-TE P2MP LSP's SESSION Object

This Tunnel Identifier is described in section 12.1.

Once the egress PE receives the P2MP TE LSP to VPLS mapping:

+ If the egress PE already has RSVP-TE state for the P2MP TE LSP,
it MUST begin to assign an MPLS label from the non-reserved label
SB> Reserved labels are now called special purpose labels.

range, for the P2MP TE LSP and signal this to the previous hop of
the P2MP TE LSP. Further it MUST create forwarding state to
forward packets received on the P2MP LSP.

+ If the egress PE does not have RSVP-TE state for the P2MP TE LSP,
it MUST retain this mapping. Subsequently when the egress PE
receives the RSVP-TE P2MP signaling message, it creates the RSVP-
TE P2MP LSP state. It MUST then assign an MPLS label from the
non-reserved label range, for the P2MP TE LSP, and signal this to
the previous hop of the P2MP TE LSP.

Note that if the signaling to set up an RSVP-TE P2MP LSP is
completed before a given egress PE learns, via a PMSI Tunnel
attribute, of the VPLS or set of VPLSes to which the LSP is
bound, the PE MUST discard any traffic received on that LSP until
the binding is received. In order for the egress PE to be able to
discard such traffic it needs to know that the LSP is associated
with one or more VPLSes and that the VPLS A-D route that binds
the LSP to a VPLS has not yet been received. This is provided by
extending [RFC4875] with [RFC6511].

9.3. Receiver Initiated MPLS Trees

Receiver initiated P2MP MPLS trees signaled using LDP [RFC6388] can
also be used. Procedures in [RFC6388] MUST be used to signal the P2MP
LSP. The LSP is signaled once the leaves receive the LDP FEC for the
tree from the root as described in section 7. An ingress PE is
required to discover the egress PEs when aggregation is used and this
is achieved using the procedures in section 7.

SB> Sentence starting "An ingress PE..."
SB> Why? This is the second time that this is being asserted, in the 
document,
SB> without providing a reason or rationale to the reader.

9.3.1. P2MP LSP - VPLS Mapping

P2MP LSP to VPLS mapping is learned at the egress PEs using BGP based
advertisements of the P2MP LSP - VPLS mapping. They require that the
root of the tree include the P2MP LSP identifier as the tunnel
identifier in the BGP advertisements. This identifier contains the
following information elements:
- The type of the tunnel is set to LDP P2MP LSP
- LDP P2MP FEC which includes an identifier generated by the
root.

Each egress PE SHOULD "join" the P2MP MPLS tree by sending LDP label
mapping messages for the LDP P2MP FEC, that was learned in the BGP
advertisement, using procedures described in [RFC6388].

9.4. Encapsulation of Aggregate P-Multicast Trees

An Aggregate Inclusive P-Multicast tree or an Aggregate Selective P-
Multicast tree MUST use MPLS encapsulation. The protocol type in the
data link header is as described in [RFC5332].

10. Inter-AS Inclusive P-Multicast Tree A-D/Binding

SB> Same comments as when I last saw options a..c, e - please
SB> provide a precise definition and maybe even a comparison
SB> table.

This document supports four options of inter-AS VPLS service, option
(a), (b), (c) and (e). Of these option (a), (b) and (c) are very
similar conceptually to option (a), (b) and (c) specified in
[RFC4364] for IP VPNs. These three options are also similar to the
three options described in [RFC4761], which in turn extend the
concepts of [RFC4364] to inter-AS VPLS. An implementation MUST
SB> Why a MUST? Too strong of a requirement. Why not give vendors
SB> greater flexibility
support all three of these options. When there are multiple ways for
implementing one of these options this section specifies which one is
mandatory. Option (e) is described further down in section 10.1.2.

For option (a), (b) and (e) support this section specifies a model
where inter-AS VPLS service can be offered without requiring a single
P-Multicast tree to span multiple ASes. This allows individual ASes
to potentially use different P-tunneling technologies. There are two
variants of this model. One that requires MAC lookup on the ASBRs and
applies to option (a) and (e). The other is one that does not require
MAC lookup on the ASBRs and instead builds segmented inter-AS
Inclusive or Selective trees. This applies only to option (b).

For option (c) support this document specifies a model where Inter-AS
VPLS service is offered by requiring a single Inclusive P-Multicast
tree to span multiple ASes. This is referred to as a non-segmented P-
Multicast tree. This is because in the case of option (c) the ASBRs
do not exchange BGP-VPLS NLRIs or VPLS A-D routes. Selective inter-AS
trees for option (c) support may be segmented or non-segmented.

SB> It be so useful in understanding the above to have names other
SB> than option(x).

SB> Additionally the following would be so much easier to follow if
SB> there were some reference models/figs somewhere.

10.1. VSIs on the ASBRs

When VSIs are configured on ASBRs, the ASBRs MUST perform a MAC
lookup, in addition to any MPLS lookups, to determine the forwarding
decision on a VPLS packet. The P-Multicast trees are confined to an
AS. An ASBR on receiving a VPLS packet from another ASBR is required
to perform a MAC lookup to determine how to forward the packet. Thus
an ASBR is required to keep a VSI for the VPLS and MUST be configured
with its own VE ID for the VPLS. The BGP VPLS A-D routes generated by
PEs in an AS MUST NOT be propagated outside the AS.

10.1.1. Option (a): VSIs on the ASBRs

When VSIs are configured on ASBRs and option (a) is used then an ASBR
in one AS treats an adjoining ASBR in another AS as a CE and
determines the VSI for packets received from that ASBR based on the
incoming Ethernet interface or VLAN ID. In option (a) the ASBRs do
not exchange VPLS A-D routes.

An implementation MUST support option (a).

SB> Please condider find a way to structure this discussion in
SB> a less confusing way.

10.1.2. Option (e): VSIs on the ASBRs

The VSIs on the ASBRs scheme can be used such that the interconnect
between the ASBRs is a PW and MPLS encapsulation is used between the
ASBRs. An ASBR in one AS treats an adjoining ASBR in another AS as a
CE and determines the VSI for packets received from another ASBR

SB> Should this be U-PE? VPLS doesn’t define a model where PWs are used
SB> between a PE and a CE. It seems to me that this model makes each ASBR
SB> treat its counterpart as a U-PE with a spoke PW.

based on the incoming MPLS encapsulation. This is referred to as
option (e). The only VPLS A-D routes that are propagated outside the
AS are the ones originated by ASBRs. This MPLS PW connects the VSIs
on the ASBRs and MUST be signaled using the procedures defined in
[RFC4761] or [RFC4762].

The P-Multicast trees for a VPLS are confined to each AS and the VPLS
auto-discovery/binding MUST follow the intra-AS procedures described
in section 8. An implementation MAY support option (e).

10.2. Option (b) - Segmented Inter-AS Trees

In this model, an inter-AS P-Multicast tree, rooted at a particular
PE for a particular VPLS instance, consists of a number of
"segments", one per AS, which are stitched together at ASBRs. These
are known as "segmented inter-AS trees". Each segment of a segmented
inter-AS tree may use a different multicast transport technology. In
this model, an ASBR is not required to keep a VSI for the VPLS and is
not required to perform a MAC lookup in order to forward the VPLS
packet. This implies that an ASBR is not required to be configured
with a VE ID for the VPLS. This model is applicable to option (b).

SB> Aren't you describing option (b)?

An
implementation MUST support option (b) using this model.

The construction of segmented Inter-AS trees requires the BGP-VPLS A-
D NLRI described in [RFC4761, RFC6074]. A BGP VPLS A-D route for a
<RD, VE ID> tuple advertised outside the AS, to which the originating
PE belongs, will be referred to as an inter-AS VPLS A-D route (Though
this route is originated by a PE as an intra-AS route and is referred
to as an inter-AS route outside the AS).

In addition to this, segmented inter-AS trees require support for the
PMSI Tunnel attribute described in section 12.1. They also require
additional procedures in BGP to signal leaf A-D routes between ASBRs
as explained in subsequent sections.

10.2.1. Segmented Inter-AS Trees VPLS Inter-AS A-D/Binding

This section specifies the procedures for inter-AS VPLS A-D/binding
for segmented inter-AS trees.

An ASBR must be configured to support a particular VPLS as follows:

+ An ASBR MUST be configured with a set of (import) Route Targets
(RTs) that specify the set of VPLSes supported by the ASBR. These
Route Targets control acceptance of BGP VPLS auto-discovery
routes by the ASBR. Note that instead of being configured, the
ASBR MAY obtain this set of (import) Route Targets (RTs) by using
Route Target Constrain [RFC4684].

+ The ASBR MUST be configured with the tunnel types for the intra-
AS segments of the VPLSes supported by the ASBR, as well as
(depending on the tunnel type) the information needed to create
the PMSI Tunnel attribute for these tunnel types. Note that
instead of being configured, the ASBR MAY derive the tunnel types
from the intra-AS A-D routes received by the ASBR from the PEs in
its own AS.

SB> What about the inter-AS segments, how do the ASBRs determine the tunnel
SB> types for those? In a later section, it is mentioned that the inter-AS
SB> tunnel types are set to ingress replication. But the reader is left
SB> wondering here…

If an ASBR is configured to support a particular VPLS, the ASBR MUST
participate in the intra-AS VPLS auto-discovery/binding procedures
for that VPLS within the ASBR's own AS, as defined in this document.

Moreover, in addition to the above the ASBR performs procedures
specified in the next section.

SB> Always best to specify the section number - it may change

10.2.2. Propagating BGP VPLS A-D routes to other ASes: Overview

SB> The first two paragraphs of this section are cryptic. A diagram
SB> and thoroughly detailed example can help clear it up. There are
SB> terms being used that do not align with the rest of the document.
R> For e.g., see the next comment.

An A-D route for a given VPLS, originated by an ASBR within a given
SB> re ASBR Shouldn’t this be “PE” instead?
AS, is propagated via BGP to other ASes. The precise rules for
distributing and processing the inter-AS A-D routes are given in
subsequent sections.

Suppose that an ASBR A receives and installs an A-D route for VPLS
"X" and VE ID "V" that originated at a particular PE, PE1. The BGP
next hop of that received route becomes A's "upstream neighbor" on a
multicast distribution tree for (X, V) that is rooted at PE1. When
the A-D routes have been distributed to all the necessary ASes, they
define a "reverse path" from any AS that supports VPLS X and VE ID V
back to PE1. For instance, if AS2 supports VPLS X, then there will be
a reverse path for VPLS X and VE ID V from AS2 to AS1. This path is a
sequence of ASBRs, the first of which is in AS2, and the last of
which is in AS1. Each ASBR in the sequence is the BGP next hop of the
previous ASBR in the sequence.

SB> Re the sentence starting "the BGP next hop..."
SB> What does this mean? How is the tree for a VPLS AND a VE ID?
SB>
SB> What does “upstream neighbor” mean here? Are PE1 and A in the same
SB> AS? Ifo so, then A will become the next hop when the route is
SB> re-advertised by A.

This reverse path information can be used to construct a
unidirectional multicast distribution tree for VPLS X and VE ID V,
containing all the ASes that support X, and having PE1 at the root.
We call such a tree an "inter-AS tree". Multicast data originating in
VPLS sites for VPLS X connected to PE1 will travel downstream along
the tree which is rooted at PE1.

The path along an inter-AS tree is a sequence of ASBRs. It is still
necessary to specify how the multicast data gets from a given ASBR to
the set of ASBRs which are immediately downstream of the given ASBR
along the tree. This is done by creating "segments": ASBRs in
adjacent ASes will be connected by inter-AS segments, ASBRs in the
same AS will be connected by "intra-AS segments".

For a given inter-AS tree and a given AS there MUST be only one ASBR
within that AS that accepts traffic flowing on that tree. Further for
a given inter-AS tree and a given AS there MUST be only one ASBR in
that AS that sends the traffic flowing on that tree to a particular
adjacent AS. The precise rules for accomplishing this are given in
subsequent sections.

An ASBR initiates creation of an intra-AS segment when the ASBR
receives an inter-AS A-D route from an EBGP neighbor. Creation of
the segment is completed as a result of distributing, via IBGP, this
route within the ASBR's own AS.

For a given inter-AS tunnel each of its intra-AS segments could be
constructed by its own independent mechanism. Moreover, by using
upstream-assigned labels within a given AS multiple intra-AS segments
of different inter-AS tunnels of either the same or different VPLSes
may share the same P-Multicast tree.

If the P-Multicast tree instantiating a particular segment of an
inter-AS tunnel is created by a multicast control protocol that uses
receiver-initiated joins (e.g, mLDP), and this P-Multicast tree does
not aggregate multiple segments, then all the information needed to
create that segment will be present in the inter-AS A-D routes
received by the ASBR from the neighboring ASBR. But if the P-
Multicast tree instantiating the segment is created by a protocol
that does not use receiver-initiated joins (e.g., RSVP-TE, ingress
unicast replication), or if this P-Multicast tree aggregates multiple
segments (irrespective of the multicast control protocol used to
create the tree), then the ASBR needs to learn the leaves of the
segment. These leaves are learned from A-D routes received from other
PEs in the AS, for the same VPLS as the one that the segment belongs
to.

The following sections specify procedures for propagation of inter-AS
A-D routes across ASes in order to construct inter-AS segmented
trees.

10.2.2.1. Propagating Intra-AS VPLS A-D routes in EBGP

For a given VPLS configured on an ASBR when the ASBR receives intra-
AS A-D routes originated by PEs in its own AS, the ASBR MUST
propagate each of these route in EBGP. This procedure MUST be
performed for each of the VPLSes configured on the ASBR. Each of
these routes is constructed as follows:

+ The route carries a single BGP VPLS A-D NLRI with the RD and VE
ID being the same as the NLRI in the received intra-AS A-D route.

+ The Next Hop field of the MP_REACH_NLRI attribute is set to a
routable IP address of the ASBR.

+ The route carries the PMSI Tunnel attribute with the Tunnel Type
set to Ingress Replication; the attribute carries no MPLS labels.

+ The route MUST carry the export Route Target used by the VPLS.

10.2.2.2. Inter-AS A-D route received via EBGP

When an ASBR receives from one of its EBGP neighbors a BGP Update
message that carries an inter-AS A-D route, if (a) at least one of
the Route Targets carried in the message matches one of the import
Route Targets configured on the ASBR, and (b) the ASBR determines
that the received route is the best route to the destination carried
in the NLRI of the route, the ASBR re-advertises this inter-AS A-D
route to other PEs and ASBRs within its own AS. The best route
selection procedures MUST ensure that for the same destination, all
ASBRs in an AS pick the same route as the best route. The best route
selection procedures are specified in [RFC4761] and clarified in
[MULTI-HOMING]. The best route procedures ensure that if multiple
ASBRs, in an AS, receive the same inter-AS A-D route from their EBGP
neighbors, only one of these ASBRs propagates this route in IBGP.
This ASBR becomes the root of the intra-AS segment of the inter-AS
tree and ensures that this is the only ASBR that accepts traffic into
this AS from the inter-AS tree.

When re-advertising an inter-AS A-D route the ASBR MUST set the Next
Hop field of the MP_REACH_NLRI attribute to a routable IP address of
the ASBR.

Depending on the type of a P-Multicast tunnel used to instantiate the
intra-AS segment of the inter-AS tunnel, the PMSI Tunnel attribute of
the re-advertised inter-AS A-D route is constructed as follows:

+ If the ASBR uses ingress replication to instantiate the intra-AS
segment of the inter-AS tunnel, the re-advertised route MUST NOT
carry the PMSI Tunnel attribute.

SB> It is not clear why not include the PMSI tunnel attribute with Tunnel
SB> Type set to “ingress replication? As a reader, I’m left wondering
SB> about the reason…

+ If the ASBR uses a P-Multicast tree to instantiate the intra-AS
segment of the inter-AS tunnel, the PMSI Tunnel attribute MUST
contain the identity of the tree that is used to instantiate the
segment (note that the ASBR could create the identity of the tree
prior to the actual instantiation of the segment). If in order to
instantiate the segment the ASBR needs to know the leaves of the
tree, then the ASBR obtains this information from the A-D routes
received from other PEs/ASBRs in ASBR's own AS.

+ An ASBR that uses a P-Multicast tree to instantiate the intra-AS
segment of the inter-AS tunnel MAY aggregate two or more VPLSes
present on the ASBR onto the same tree. If the ASBR already
advertises inter-AS A-D routes for these VPLSes, then aggregation
requires the ASBR to re-advertise these routes. The re-advertised
routes MUST be the same as the original ones, except for the PMSI
Tunnel attribute. If the ASBR has not previously advertised
inter-AS A-D routes for these VPLSes, then the aggregation
requires the ASBR to advertise (new) inter-AS A-D routes for
these VPLSes. The PMSI Tunnel attribute in the newly
advertised/re-advertised routes MUST carry the identity of the P-
Multicast tree that aggregates the VPLSes, as well as an MPLS
upstream-assigned label [RFC5331]. Each re-advertised route MUST
have a distinct label.

SB> Each re-advertised or new

In addition the ASBR MUST send to the EBGP neighbor, from whom it
receives the inter-AS A-D route, a BGP Update message that carries a
leaf A-D route. The exact encoding of this route is described in
section 12. This route contains the following information elements:

+ The route carries a single NLRI with the Route Key field set to
the <RD, VE ID> tuple of the BGP VPLS A-D NLRI of the inter-AS A-
D route received from the EBGP neighbor. The NLRI also carries
the IP address of the ASBR (this MUST be a routable IP address).

+ The leaf A-D route MUST include the PMSI Tunnel attribute with
the Tunnel Type set to Ingress Replication, and the Tunnel
Identifier set to a routable address of the advertising router.
The PMSI Tunnel attribute MUST carry a downstream assigned MPLS
label that is used to demultiplex the VPLS traffic received over
a unicast tunnel by the advertising router.
SB> Re unicast tunnel
SB> This should be the inter-AS segment of the multicast tunnel.

+ The Next Hop field of the MP_REACH_NLRI attribute of the route
SHOULD be set to the same IP address as the one carried in the
Originating Router's IP Address field of the route.

+ To constrain the distribution scope of this route the route MUST
carry the NO_ADVERTISE BGP community ([RFC1997]).

+ The ASBR constructs an IP-based Route Target extended community
by placing the IP address carried in the next hop of the received
Inter-AS VPLS A-D route in the Global Administrator field of the
community, with the Local Administrator field of this community
set to 0, and sets the Extended Communities attribute of the leaf
A-D route to that community. Note that this Route Target is the
same as the ASBR Import RT of the EBGP neighbor from which the
ASBR received the inter-AS VPLS A-D route.

10.2.2.3. Leaf A-D Route received via EBGP

When an ASBR receives via EBGP a leaf A-D route, the ASBR accepts the
route only if (a) at least one of the Route Targets carried in the
message matches one of the import Route Targets configured on the
ASBR, and (b) the ASBR determines that the received route is the best
route to the destination carried in the NLRI of the route.

If the ASBR accepts the leaf A-D route, the ASBR looks for an
existing A-D route whose BGP-VPLS A-D NLRI has the same value as the
<RD, VE-ID> field of the leaf A-D route just accepted. If such an A-D
route is found, then the MPLS label carried in the PMSI Tunnel
attribute of the leaf A-D route is used to stitch a one hop ASBR-ASBR
LSP to the tail of the intra-AS tunnel segment associated with the
found auto- discovery route.

10.2.2.4. Inter-AS A-D Route received via IBGP

In the context of this section we use the term "PE/ASBR router" to
denote either a PE or an ASBR router.

Note that a given inter-AS A-D route is advertised within a given AS
by only one ASBR as described above.

SB> Please elaborated the contest better. I had to read this
SB> section, then go back to section 10.2.2.2 and then come back and
SB> reread this to understand the context. It would be great to
SB> clarify here that the ASBR advertises the inter-AS AD route within
SB> its local AS as a result of receiving an inter-AS AD route via
SB> EBGP as was discussed in section 10.2.2.2.

When a PE/ASBR router receives from one of its IBGP neighbors a BGP
Update message that carries an inter-AS A-D route, if (a) at least
one of the Route Targets carried in the message matches one of the
import Route Targets configured on the PE/ASBR, and (b) the PE/ASBR
determines that the received route is the best route to the
destination carried in the NLRI of the route, the PE/ASBR performs
the following operations. The best route determination is based as
described in [RFC4761] and clarified in [MULTI-HOMING].

If the router is an ASBR then the ASBR propagates the route to its
EBGP neighbors. When propagating the route to the EBGP neighbors the
ASBR MUST set the Next Hop field of the MP_REACH_NLRI attribute to a
routable IP address of the ASBR.

If the received inter-AS A-D route carries the PMSI Tunnel attribute
with the Tunnel Type set to LDP P2MP LSP, the PE/ASBR SHOULD join the
P-Multicast tree whose identity is carried in the PMSI Tunnel
attribute.

If the received inter-AS A-D route carries the PMSI Tunnel attribute
with the Tunnel Identifier set to RSVP-TE P2MP LSP, then the ASBR
that originated the route MUST establish an RSVP-TE P2MP LSP with the
local PE/ASBR as a leaf. This LSP MAY have been established before
the local PE/ASBR receives the route, or MAY be established after the
local PE receives the route.

If the received inter-AS A-D route carries the PMSI Tunnel attribute
with the Tunnel Type set to LDP P2MP LSP, or RSVP-TE P2MP LSP, but
the attribute does not carry a label, then the P-Multicast tree, as
identified by the PMSI Tunnel attribute, is an intra-AS LSP segment
that is part of the inter-AS Tunnel for the <VPLS, VE ID> advertised
by the inter-AS A-D route and rooted at the PE that originated the A-
D route. If the PMSI Tunnel attribute carries a (upstream-assigned)
label, then a combination of this tree and the label identifies the
intra-AS segment. If the receiving router is an ASBR, this intra-AS
segment may further be stitched to ASBR-ASBR inter-AS segment of the
inter-AS tunnel. If the PE/ASBR has local receivers in the VPLS,
packets received over the intra-AS segment must be forwarded to the
local receivers using the local VSI.

10.3. Option (c): Non-Segmented Tunnels

In this model, there is a multi-hop EBGP peering between the PEs (or
a Route Reflector) in one AS and the PEs (or Route Reflector) in
another AS. The PEs exchange BGP-VPLS NLRI or BGP-VPLS A-D NLRI,
along with PMSI Tunnel attribute, as in the intra-AS case described
in section 8. An implementation MUST support this model.

The PEs in different ASes use a non-segmented inter-AS P2MP tunnel
for VPLS multicast. A non-segmented inter-AS tunnel is a single
tunnel which spans AS boundaries. The tunnel technology cannot change
from one point in the tunnel to the next, so all ASes through which
the tunnel passes must support that technology. In essence, AS
boundaries are of no significance to a non-segmented inter-AS P2MP
tunnel.

This model requires no VPLS A-D routes in the control plane or VPLS
MAC address learning in the data plane on the ASBRs. The ASBRs only
need to participate in the non-segmented P2MP tunnel setup in the
control plane, and do MPLS label forwarding in the data plane.

The setup of non-segmented inter-AS P2MP tunnels MAY require the P-
routers in one AS to have IP reachability to the loopback addresses
of the PE routers in another AS, depending on the tunneling
technology chosen. If this is the case, reachability to the loopback
addresses of PE routers in one AS MUST be present in the IGP in
another AS.

SB> re technology chosen:
SB> A listing of which tunneling technologies impose this restriction
SB> would be helpful here.

The data forwarding in this model is the same as in the intra-AS case
described in section 8.

11. Optimizing Multicast Distribution via Selective Trees

Whenever a particular multicast stream is being sent on an Inclusive
P-Multicast tree, it is likely that the data of that stream is being
sent to PEs that do not require it as the sites connected to these
PEs may have no receivers for the stream. If a particular stream has
a significant amount of traffic, it may be beneficial to move it to a
Selective P-Multicast tree which has at its leaves only those PEs,
connected to sites that have receivers for the multicast stream (or
at least includes fewer PEs that are attached to sites with no
receivers compared to an Inclusive tree).

A PE connected to the multicast source of a particular multicast
stream may be performing explicit tracking - i.e., it may know the
PEs that have receivers in the multicast stream. Section 11.3
describes procedures that enable explicit tracking. If this is the
case Selective P-Multicast trees can also be triggered on other
criteria. For instance there could be a "pseudo wasted bandwidth"
criteria: switching to a Selective tree would be done if the
bandwidth multiplied by the number of "uninterested" PEs (PEs that
are receiving the stream but have no receivers) is above a specified
threshold. The motivation is that (a) the total bandwidth wasted by
many sparsely subscribed low-bandwidth groups may be large, and (b)
there's no point to moving a high-bandwidth group to a Selective tree
if all the PEs have receivers for it.

Switching a (C-S, C-G) stream to a Selective P-Multicast tree may
require the root of the tree to determine the egress PEs that need to
receive the (C-S, C-G) traffic. This is true in the following cases:

+ If the tunnel is a P2MP tree, such as a RSVP-TE P2MP Tunnel, the
PE needs to know the leaves of the tree before it can instantiate
the Selective tree.

+ If a PE decides to send traffic for multicast streams, belonging
to different VPLSes, using one P-Multicast Selective tree, such a
tree is termed an Aggregate tree with a selective mapping. The
setting up of such an Aggregate tree requires the ingress PE to
know all the other PEs that have receivers for multicast groups
that are mapped onto the tree.

SB> If mLDP is used to set up the tree, why does the root need to know
SB> the leaves?

+ If ingress replication is used and the ingress PE wants to send
traffic for (C-S, C-G)s to only those PEs that are on the path to
receivers to the (C-S,C-G)s.

For discovering the IP multicast group membership, for the above
cases, this document describes procedures that allow an ingress PE to
enable explicit tracking. Thus an ingress PE can request the IP
multicast membership from egress PEs for one or more C-multicast
streams. These procedures are described in section 11.3.

The root of the Selective P-Multicast tree MAY decide to do explicit
tracking of the IP multicast stream only after it has determined to
move the stream to a Selective tree, or it MAY have been doing
explicit tracking all along. This document also describes explicit
tracking for a wildcard source and/or group in section 11.3, which
facilitates a Selective P-Multicast tree only mode in which IP
multicast streams are always carried on a Selective P-Multicast tree.
In the description on Selective P-Multicast trees the notation C-S,
is intended to represent either a specific source address or a
wildcard. Similarly C-G is intended to represent either a specific
group address or a wildcard.

The PE at the root of the tree MUST signal the leaves of the tree
that the (C-S, C-G) stream is now bound to the Selective Tree. Note
that the PE could create the identity of the P-Multicast tree prior
to the actual instantiation of the tunnel.

If the Selective tree is instantiated by a RSVP-TE P2MP LSP the PE at
the root of the tree MUST establish the P2MP RSVP-TE LSP to the
leaves. This LSP MAY have been established before the leaves receive
the Selective tree binding, or MAY be established after the leaves
receive the binding. A leaf MUST NOT switch to the Selective tree
until it receives the binding and the RSVP-TE P2MP LSP is setup to
the leaf.

11.1. Protocol for Switching to Selective Trees

Selective trees provide a PE the ability to create separate P-
Multicast trees for certain <C-S, C-G> streams. The source PE, that
originates the Selective tree, and the egress PEs, MUST use the
Selective tree for the <C-S, C-G> streams that are mapped to it. This
may require the source and egress PEs to switch to the Selective tree
from an Inclusive tree if they were already using an Inclusive tree
for the <C-S, C-G> streams mapped to the Selective tree.

Once a source PE decides to setup a Selective tree, it MUST announce
the mapping of the <C-S, C-G> streams (which may be in different
VPLSes) that are mapped to the tree to the other PEs using BGP. After
the egress PEs receive the announcement they setup their forwarding
path to receive traffic on the Selective tree if they have one or
more receivers interested in the <C-S, C-G> streams mapped to the
tree. Setting up the forwarding path requires setting up the
demultiplexing forwarding entries based on the top MPLS label (if
there is no inner label) or the inner label (if present) as described
in section 9. The egress PEs MAY perform this switch to the Selective
tree once the advertisement from the ingress PE is received or wait
for a preconfigured timer to do so, after receiving the
advertisement, when the P2MP LSP protocol is mLDP. When the P2MP LSP
protocol is P2MP RSVP-TE an egress PE MUST perform this switch to the
Selective tree only after the advertisement from the ingress PE is
received and the RSVP-TE P2MP LSP has been setup to the egress PE.
This switch MAY be done after waiting for a preconfigured timer after
these two steps have been accomplished.

A source PE MUST use the following approach to decide when to start
transmitting data on the Selective tree, if it was already using an

SB> I think you mean "if it is currently using"

Inclusive tree. A certain pre-configured delay after advertising the
<C-S, C-G> streams mapped to an Selective tree, the source PE begins
to send traffic on the Selective tree.

SB> The ops area is going to ask you a whole load of questions about
SB> seting, maintaining etc that delay and what happens if you get it
SB> wrong.

At this point it stops to send
traffic for the <C-S, C-G> streams, that are mapped on the Selective
tree, on the Inclusive tree. This traffic is instead transmitted on
the Selective tree.

11.2. Advertising (C-S, C-G) Binding to a Selective Tree

The ingress PE informs all the PEs that are on the path to receivers
of the (C-S, C-G) of the binding of the Selective tree to the (C-S,
C-G), using BGP. The BGP announcement is done by sending update for
the MCAST-VPLS address family using what is referred to as the S-PMSI
A-D route.
SB> Please provide a reference?

The format of the NLRI is described in section 12.1. The
NLRI MUST be constructed as follows:

+ The RD MUST be set to the RD configured locally for the VPLS.

SB> Have you defined RD yet - I don't think so - please check?

This is required to uniquely identify the <C-S, C-G> as the
addresses could overlap between different VPLSes. This MUST be
the same RD value used in the VPLS auto-discovery process.

+ The Multicast Source field MUST contain the source address
associated with the C-multicast stream, and the Multicast Source
Length field is set appropriately to reflect this. If the source
address is a wildcard the source address is set to 0.

+ The Multicast Group field MUST contain the group address
associated with the C-multicast stream, and the Multicast Group
Length field is set appropriately to reflect this. If the group
address is a wildcard the group address is set to 0.

+ The Originating Router's IP Address field MUST be set to the IP
address that the (local) PE places in the BGP next-hop of the
BGP-VPLS A-D routes. Note that the <RD, Originating Router's IP
address> tuple uniquely identifies a given VPLS instance on a PE.

The PE constructs the rest of the Selective A-D route as follows.

Depending on the type of a P-Multicast tree used for the P-tunnel,
the PMSI tunnel attribute of the S-PMSI A-D route is constructed as
follows:

+ The PMSI tunnel attribute MUST contain the identity of the P-
Multicast tree (note that the PE could create the identity of the
tree prior to the actual instantiation of the tree).

+ If in order to establish the P-Multicast tree the PE needs to
know the leaves of the tree within its own AS, then the PE
obtains this information from the leaf A-D routes received from
other PEs/ASBRs within its own AS (as other PEs/ASBRs originate
leaf A-D routes in response to receiving the S-PMSI A-D route) by
setting the Leaf Information Required flag in the PMSI Tunnel
attribute to 1. This enables explicit tracking for the multicast
stream(s) advertised by the S-PMSI A-D route.

+ If a PE originates S-PMSI A-D routes with the Leaf Information
Required flag in the PMSI Tunnel attribute set to 1, then the PE
MUST be (auto)configured with an import Route Target, which
controls acceptance of leaf A-D routes by the PE. (Procedures for
originating leaf A-D routes by the PEs that receive the S-PMSI A-
D route are described in section "Receiving S-PMSI A-D routes by
PEs.")

This Route Target is IP address specific. The Global
Administrator field of this Route Target MUST be set to the IP
address carried in the Next Hop of all the S-PMSI A-D routes
advertised by this PE (if the PE uses different Next Hops, then
the PE MUST be (auto)configured with multiple import RTs, one per
each such Next Hop). The Local Administrator field of this Route
Target MUST be set to 0.

If the PE supports Route Target Constrain [RFC4684], the PE
SHOULD advertise this import Route Target within its own AS using
Route Target Constrains. To constrain distribution of the Route
Target Constrain routes to the AS of the advertising PE these
routes SHOULD carry the NO_EXPORT Community ([RFC1997]).

+ A PE MAY aggregate two or more S-PMSIs originated by the PE onto
the same P-Multicast tree. If the PE already advertises S-PMSI A-
D routes for these S-PMSIs, then aggregation requires the PE to
re-advertise these routes. The re-advertised routes MUST be the
same as the original ones, except for the PMSI tunnel attribute.
If the PE has not previously advertised S-PMSI A-D routes for
these S-PMSIs, then the aggregation requires the PE to advertise
(new) S-PMSI A-D routes for these S-PMSIs. The PMSI Tunnel
attribute in the newly advertised/re-advertised routes MUST carry
the identity of the P-Multicast tree that aggregates the S-PMSIs.
If at least some of the S-PMSIs aggregated onto the same P-
Multicast tree belong to different VPLSes, then all these routes
MUST carry an MPLS upstream assigned label [RFC5331]. If all
these aggregated S-PMSIs belong to the same VPLS, then the routes
MAY carry an MPLS upstream assigned label [RFC5331]. The labels
MUST be distinct on a per VPLS basis, and MAY be distinct on a
per route basis.

The Next Hop field of the MP_REACH_NLRI attribute of the route SHOULD
be set to the same IP address as the one carried in the Originating
Router's IP Address field.

By default the set of Route Targets carried by the route MUST be the
same as the Route Targets carried in the BGP-VPLS A-D route
originated from the VSI. The default could be modified via
configuration.

11.3. Receiving S-PMSI A-D routes by PEs

Consider a PE that receives an S-PMSI A-D route. If one or more of
the VSIs on the PE have their import Route Targets that contain one
or more of the Route Targets carried by the received S-PMSI A-D
route, then for each such VSI the PE performs the following.

Procedures for receiving an S-PMSI A-D route by a PE (both within and
outside of the AS of the PE that originates the route) are the same
as specified in Section "Inter-AS A-D route received via IBGP" except
that (a) instead of Inter-AS A-D routes the procedures apply to S-
PMSI A-D routes, and (b) the rules for determining whether the
received S-PMSI A-D route is the best route to the destination
carried in the NLRI of the route, are the same as BGP path selection
rules and may be modified by policy, and (c) a PE performs procedures
specified in that section only if in addition to the criteria
specified in that section the following is true:

+ If as a result of multicast state snooping on the PE-CE
interfaces, the PE has snooped state for at least one multicast
join that matches the multicast source and group advertised in
the S-PMSI A-D route. Further if the oifs (outgoing interfaces)
for this state contains one or more interfaces to the locally
attached CEs. When the multicast signaling protocol among the CEs
is IGMP, then snooping and associated procedures are defined in
[RFC4541]. The snooped state is determined using these
procedures. When the multicast signaling protocol among the CEs
is PIM, the procedures in [RFC4541] are not sufficient to
determine the snooped state. The additional details required to
determine the snooped state when CE-CE protocol is PIM are for
further study. When such procedures are defined it is expected
that the procedures in this section will apply to the snooped
state created as a result of PIM as PE-CE protocol.

The snooped state is said to "match" the S-PMSI A-D route if any of
the following is true:

+ The S-PMSI A-D route carries (C-S, C-G) and the snooped state is
for (C-S, C-G) or for (C-*, C-G), OR

+ The S-PMSI A-D route carries (C-*, C-G) and (a) the snooped
state is for (C-*, C-G) OR (b) the snooped state is for at least
one multicast join with the multicast group address equal to C-G
and there doesn't exist another S-PMSI A-D route that carries (C-
S, C-G) where C-S is the source address of the snooped state.

+ The S-PMSI A-D route carries (C-S, C-*) and (a) the snooped
state is for at least one multicast join with the multicast
source address equal to C-S, and (b) there doesn't exist another
S-PMSI A-D route that carries (C-S, C-G) where C-G is the group
address of the snooped state.

+ The S-PMSI A-D route carries (C-*, C-*) and there is no other S-
PMSI A-D route that matches the snooped state as per the above
conditions.

Note if the above conditions are true, and if the received S-PMSI A-D
route has a PMSI Tunnel attribute with the Leaf Information Required
flag set to 1, then the PE originates a leaf A-D route, constructed
as follows:

+ The route carries a single MCAST-VPN NLRI with the Route Key
field set to the MCAST-VPN NLRI of the received S-PMSI A-D route.

SB> The route name in the BGP section later on is MCAST-VPLS

+ The Originating Router's IP address set to the IP address of the
PE (this MUST be a routable IP address).

+ The PE constructs an IP-based Route Target Extended Community by
placing the IP address carried in the Next Hop of the received S-
PMSI A-D route in the Global Administrator field of the
Community, with the Local Administrator field of this Community
set to 0 and setting the Extended Communities attribute of the
leaf A-D route to that Community.

+ The Next Hop field of the MP_REACH_NLRI attribute of the route
MUST be set to the same IP address as the one carried in the
Originating Router's IP Address field of the route.

+ To constrain the distribution scope of this route, the route
MUST carry the NO_EXPORT Community [RFC1997], except for the
inter-AS scenario with option (c).

Once the leaf A-D route is constructed, the PE advertises this route
into IBGP.

In addition to the procedures specified in Section "Inter-AS A-D
route received via IBGP" the PE MUST set up its forwarding path to
receive traffic, for each multicast stream in the matching snooped
state, from the tunnel advertised by the S-PMSI A-D route (the PE
MUST switch to the Selective tree).

When a new snooped state is created by a PE then the PE MUST first
determine if there is a S-PMSI route that matches the snooped state
as per the conditions described above. If such a S-PMSI route is
found then the PE MUST follow the procedures described in this
section, for that particular S-PMSI route.

SB> What happens when the snoop multicast state ages out or is
SB> removed, shouldn’t the leaf AD route be withdrawn? There’s
SB> no discussion of that case.

11.4. Inter-AS Selective Tree

Inter-AS Selective trees support all three options of inter-AS VPLS
service, option (a), (b) and (c), that are supported by Inter-AS
Inclusive trees. They are constructed in a manner that is very
similar to Inter-AS Inclusive trees.

For option (a) and option (b) support inter-AS Selective trees are
constructed without requiring a single P-Multicast tree to span
multiple ASes. This allows individual ASes to potentially use
different P-tunneling technologies. There are two variants of this.

One that requires MAC and IP multicast lookup on the ASBRs and
another that does not require MAC/IP multicast lookup on the ASBRs
and instead builds segmented inter-AS Selective trees.

Segmented Inter-AS Selective trees can also be used with option (c),
unlike Segmented Inter-AS Inclusive trees. This is because the S-PMSI
A-D routes can be exchanged via ASBRs (even though BGP VPLS A-D
routes are not exchanged via ASBRs).

In the case of Option (c) an Inter-AS Selective tree may also be a
non-segmented P-Multicast tree that spans multiple ASes.

11.4.1. VSIs on the ASBRs

The requirements on ASBRs, when VSIs are present on the ABSRs,
include the requirements presented in section 10. The source ASBR
(that receives traffic from another AS) may independently decide
whether it wishes to use Selective trees or not. If it uses Selective
trees the source ASBR MUST perform a MAC lookup to determine the
Selective tree to forward the VPLS packet on.

11.4.1.1. VPLS Inter-AS Selective Tree A-D Binding

The mechanisms for propagating S-PMSI A-D routes are the same as the
intra-AS case described in section 12.2. The BGP Selective tree A-D
routes generated by PEs in an AS MUST NOT be propagated outside the
AS.

11.4.2. Inter-AS Segmented Selective Trees

Inter-AS Segmented Selective trees MUST be used when option (b) is
used to provide the inter-AS VPLS service. They MAY be used when
option (c) is used to provide the inter-AS VPLS service.

A Segmented inter-AS Selective Tunnel is constructed similar to an
inter-AS Segmented Inclusive Tunnel. Namely, such a tunnel is
constructed as a concatenation of tunnel segments. There are two
types of tunnel segments: an intra-AS tunnel segment (a segment that
spans ASBRs within the same AS), and inter-AS tunnel segment (a
segment that spans adjacent ASBRs in adjacent ASes). ASes that are
spanned by a tunnel are not required to use the same tunneling
mechanism to construct the tunnel - each AS may pick up a tunneling
mechanism to construct the intra-AS tunnel segment of the tunnel, in
its AS.

The PE that decides to set up a Selective tree, advertises the
Selective tree to multicast stream binding using an S-PMSI A-D route
as per procedures in section 11.2, to the routers in its own AS.

An S-PMSI A-D route advertised outside the AS, to which the
originating PE belongs, will be referred to as an inter-AS Selective
Tree A-D route (although this route is originated by a PE as an
intra-AS route it is referred to as an inter-AS route outside the
AS).

11.4.2.1. Handling S-PMSI A-D routes by ASBRs

Procedures for handling an S-PMSI A-D route by ASBRs (both within and
outside of the AS of the PE that originates the route) are the same
as specified in Section "Propagating VPLS BGP A-D routes to other
ASes", except that instead of Inter-AS BGP-VPLS A-D routes and the
BGP-VPLS A-D NLRI these procedures apply to S-PMSI A-D routes and the
S-PMSI A-D NLRI.

In addition to these procedures an ASBR advertises a leaf A-D route
in response to an S-PMSI A-D route only if:

+ The S-PMSI A-D route was received via EBGP from another ASBR and
the ASBR merges the S-PMSI A-D route into an Inter-AS BGP VPLS A-
D route as described in the next section. OR

+ The ASBR receives a leaf A-D route from a downstream PE or ASBR
in response to the S-PMSI A-D route, received from an upstream PE
or ASBR, that the ASBR propagated inter-AS to downstream ASBRs
and PEs.

+ The ASBR has snooped state from local CEs that matches the NLRI
carried in the S-PMSI A-D route as per the following rules:

i) The NLRI encodes (C-S, C-G) which is the same as the snooped
(C-S, C-G)

ii) The NLRI encodes (*, C-G) and there is snooped state for at
least one (C-S, C-G) and there is no other matching S-PMSI A-D
route for (C-S, C-G) OR there is snooped state for (*, C-G)

iii) The NLRI encodes (*, *) and there is snooped state for at
least one (C-S, C-G) or (*, C-G) and there is no other matching
S-PMSI A-D route for that (C-S, C-G) or (*, C-G) respecively.

The C-multicast data traffic is sent on the Selective tree by the
originating PE. When it reaches an ASBR that is on the Inter-AS
segmented tree, it is delivered to local receivers, if any. It is
then forwarded on any inter-AS or intra-AS segments that exist on the
Inter-AS Selective Segmented tree. If the Inter-AS Segmented
Selective Tree is merged onto an Inclusive tree, as described in the
next section, the data traffic is forwarded onto the Inclusive tree.

11.4.2.1.1. Merging Selective Tree into an Inclusive Tree

Consider the situation where:

+ An ASBR is receiving (or expecting to receive) inter-AS (C-S, C-
G) data from upstream via a Selective tree.

+ The ASBR is sending (or expecting to send) the inter-AS (C-S, C-
G) data downstream via an Inclusive tree.

This situation may arise if the upstream providers have a policy of
using Selective trees but the downstream providers have a policy of
using Inclusive trees. To support this situation, an ASBR MAY, under
certain conditions, merge one or more upstream Selective trees into a
downstream Inclusive tree. Note that this can be the case only for
option (b) and not for option (c) as for option (c) the ASBRs do not
have Inclusive tree state.

A Selective tree (corresponding to a particular S-PMSI A-D route) MAY
be merged by a particular ASBR into an Inclusive tree (corresponding
to a particular Inter-AS BGP VPLS A-D route) if and only if the
following conditions all hold:

+ The S-PMSI A-D route and the Inter-AS BGP VPLS A-D route
originate in the same AS. The Inter-AS BGP VPLS A-D route carries
the originating AS in the AS_PATH attribute of the route. The S-
PMSI A-D route carries the originating AS in the AS_PATH
attribute of the route.

+ The S-PMSI A-D route and the Inter-AS BGP VPLS A-D route have
exactly the same set of RTs.

An ASBR performs merging by stitching the tail end of the P-tunnel,
as specified in the PMSI Tunnel attribute of the S-PMSI A-D route
received by the ASBR, to the head of the P-tunnel, as specified in
the PMSI Tunnel attribute of the Inter-AS BGP VPLS A-D route re-
advertised by the ASBR.

An ASBR that merges an S-PMSI A-D route into an Inter-AS BGP VPLS A-D
route MUST NOT re-advertise the S-PMSI A-D route.

11.4.3. Inter-AS Non-Segmented Selective trees

Inter-AS Non-segmented Selective trees MAY be used in the case of
option (c).

In this method, there is a multi-hop EBGP peering between the PEs (or
a Route Reflector) in one AS and the PEs (or Route Reflector) in
another AS. The PEs exchange BGP Selective tree A-D routes, along
with PMSI Tunnel attribute, as in the intra-AS case described in
section 10.3.

The PEs in different ASes use a non-segmented Selective inter-AS P2MP
tunnel for VPLS multicast.

This method requires no VPLS information (in either the control or
the data plane) on the ASBRs. The ASBRs only need to participate in
the non-segmented P2MP tunnel setup in the control plane, and do MPLS
label forwarding in the data plane.

The data forwarding in this model is the same as in the intra-AS case
described in section 9.

12. BGP Extensions

This section describes the encoding of the BGP extensions required by
this document.

SB> Has this section been reviewed by the IDR WG?

12.1. Inclusive Tree/Selective Tree Identifier

Inclusive P-Multicast tree and Selective P-Multicast tree
advertisements carry the P-Multicast tree identifier.

This document reuses the BGP attribute, called PMSI Tunnel attribute
that is defined in [RFC6514].

This document supports only the following Tunnel Types when PMSI
Tunnel attribute is carried in VPLS A-D or VPLS S-PMSI A-D routes:

+ 0 - No tunnel information present
+ 1 - RSVP-TE P2MP LSP
+ 2 - LDP P2MP LSP
+ 6 - Ingress Replication

12.2. MCAST-VPLS NLRI

This document defines a new BGP NLRI, called the MCAST-VPLS NLRI.

Following is the format of the MCAST-VPLS NLRI:

+-----------------------------------+
| Route Type (1 octet) |
+-----------------------------------+
| Length (1 octet) |
+-----------------------------------+
| Route Type specific (variable) |
+-----------------------------------+

The Route Type field defines encoding of the rest of MCAST-VPLS NLRI
(Route Type specific MCAST-VPLS NLRI).

The Length field indicates the length in octets of the Route Type
specific field of MCAST-VPLS NLRI.

This document defines the following Route Types for A-D routes:
+ 3 - Selective Tree A-D route;
+ 4 - Leaf A-D route.

The MCAST-VPLS NLRI is carried in BGP using BGP Multiprotocol
Extensions [RFC4760] with an AFI of 25 (L2VPN AFI), and an SAFI of
MCAST-VPLS [To be assigned by IANA].

SB> If this has been implemented (as stated) - how has this been done
SB> without an IANA assignment?

The NLRI field in the
MP_REACH_NLRI/MP_UNREACH_NLRI attribute contains the MCAST-VPLS NLRI
(encoded as specified above).

In order for two BGP speakers to exchange labeled MCAST-VPLS NLRI,
they must use BGP Capabilities Advertisement to ensure that they both
are capable of properly processing such NLRI. This is done as
specified in [RFC4760], by using capability code 1 (multiprotocol
BGP) with an AFI of 25 and an SAFI of MCAST-VPLS.

The following describes the format of the Route Type specific MCAST-
VPLS NLRI for various Route Types defined in this document.

12.2.1. S-PMSI A-D route

An S-PMSI A-D route type specific MCAST-VPLS NLRI consists of the
following:

+-----------------------------------+
| RD (8 octets) |
+-----------------------------------+
| Multicast Source Length (1 octet) |
+-----------------------------------+
| Multicast Source (Variable) |
+-----------------------------------+
| Multicast Group Length (1 octet) |
+-----------------------------------+
| Multicast Group (Variable) |
+-----------------------------------+
| Originating Router's IP Addr |
+-----------------------------------+

The RD is encoded as described in [RFC4364].

The Multicast Source field contains the C-S address i.e the address
of the multicast source. If the Multicast Source field contains an
IPv4 address, then the value of the Multicast Source Length field is
32. If the Multicast Source field contains an IPv6 address, then the
value of the Multicast Source Length field is 128. The value of the
Multicast Source Length field may be set to 0 to indicate a wildcard.

The Multicast Group field contains the C-G address i.e. the address
of the multicast group. If the Multicast Group field contains an IPv4
address, then the value of the Multicast Group Length field is 32.
If the Multicast Group field contains an IPv6 address, then the value
of the Multicast Group Length field is 128. The Multicast Group
Length field may be set to 0 to indicate a wildcard.

Whether the Originating Router's IP Address field carries an IPv4 or
IPv6 address is determined from the value of the Length field of the
MCAST-VPLS NLRI. If the Multicast Source field contains an IPv4
address and the Multicast Group field contains an IPv4 address, then
the value of the Length field is 22 bytes if the Originating Router's
IP address carries an IPv4 address and 34 bytes if it is an IPv6
address. If the Multicast Source and Multicast Group fields contain
IPv6 addresses, then the value of the Length field is 46 bytes if the
Originating Router's IP address carries an IPv4 address and 58 bytes
if it is an IPv6 address. The following table summarizes the above.

Multicast Multicast Originating Router's Length
Source Group IP Address

IPv4 IPv4 IPv4 22
IPv4 IPv4 IPv6 34
IPv6 IPv6 IPv4 46
IPv6 IPv6 IPv6 58

Usage of Selective Tree A-D routes is described in Section
11.

12.2.2. Leaf A-D route

A leaf A-D route type specific MCAST-VPLS NLRI consists of the
following:

+-----------------------------------+
| Route Key (variable) |
+-----------------------------------+
| Originating Router's IP Addr |
+-----------------------------------+

Whether the Originating Router's IP Address field carries an IPv4 or
IPv6 address is determined from the Length field of the MCAST-VPLS
NLRI and the length of the Route Key field. From these two length
fields one can compute the length of the Originating Router's IP
Address. If this computed length is 4 then the address is an IPv4
address and if its 16 then the address is an IPv6 address.

Usage of leaf A-D routes is described in sections "Inter-AS Inclusive
P-Multicast tree A-D/Binding" and "Optimizing Multicast Distribution
via Selective trees".

13. Aggregation Considerations

In general the heuristic used to decide which VPLS instances or (C-S,
C-G) entries to aggregate is implementation dependent. It is also
conceivable that offline tools can be used for this purpose. This
section discusses some tradeoffs with respect to aggregation.

The "congruency" of aggregation is defined by the amount of overlap
in the leaves of the client trees that are aggregated on an SP tree.
For Aggregate Inclusive trees the congruency depends on the overlap
in the membership of the VPLSes that are aggregated on the Aggregate
Inclusive tree. If there is complete overlap aggregation is perfectly
congruent. As the overlap between the VPLSes that are aggregated
reduces, the congruency reduces.

If aggregation is done such that it is not perfectly congruent a PE
may receive traffic for VPLSes to which it doesn't belong. As the
amount of multicast traffic in these unwanted VPLSes increases
aggregation becomes less optimal with respect to delivered traffic.
Hence there is a tradeoff between reducing multicast state in the
core and delivering unwanted traffic.

An implementation should provide knobs to control the congruency of
aggregation.

SB> Do you mean congurency here? Surely you are talking about the
SB> the extent of wasted b/w. Delivered traffic MUST be congruent
SB> with the tree.
SB> In any case I am not sure you actually control the congruency
SB> you control criteria for creating or merging trees.
SB>
SB> Expect a bunch of questions from the ops area reviewer
SB> on this.

This will allow an SP to deploy aggregation depending on
the VPLS membership and traffic profiles in its network. If
different PEs or shared roots are setting up Aggregate Inclusive
trees this will also allow an SP to engineer the maximum amount of
unwanted VPLSes that a particular PE may receive traffic for.

SB> I would have thought bw was more of an issue than number of
SB> VPLSes

The state/bandwidth optimality trade-off can be further improved by
having a versatile many-to-many association between client trees and
provider trees. Thus a VPLS can be mapped to multiple Aggregate
trees. The mechanisms for achieving this are for further study. Also
it may be possible to use both ingress replication and an Aggregate
tree for a particular VPLS. Mechanisms for achieving this are also
for further study.

14. Data Forwarding

14.1. MPLS Tree Encapsulation

14.1.1. Mapping multiple VPLS instances to a P2MP LSP

The following diagram shows the progression of the VPLS multicast
packet as it enters and leaves the SP network when MPLS trees are
being used for multiple VPLS instances. RSVP-TE P2MP LSPs are
examples of such trees.

Packets received Packets in transit Packets forwarded
at ingress PE in the service by egress PEs
provider network

+---------------+
|MPLS Tree Label|
+---------------+
| VPLS Label |
++=============++ ++=============++ ++=============++
||C-Ether Hdr || || C-Ether Hdr || || C-Ether Hdr ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-IP Header || || C-IP Header || || C-IP Header ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-Payload || || C-Payload || || C-Payload ||
++=============++ ++=============++ ++=============++

The receiver PE

SB> You mean the egress PE surely? Both PEs receive. In any case
SB> receiver PE is not defined in the above

does a lookup on the outer MPLS tree label and
determines the MPLS forwarding table in which to lookup the inner
MPLS label.

SB> Much better to use the names of the labels rather than inner
SB> outer.

This table is specific to the tree label space. The inner
label is unique within the context of the root of the tree (as it is
assigned by the root of the tree, without any coordination with any
other nodes). Thus it is not unique across multiple roots. So, to
unambiguously identify a particular VPLS one has to know the label,
and the context within which that label is unique. The context is
provided by the outer MPLS label [RFC5331].

The outer MPLS label is stripped. The lookup of the resulting MPLS
label determines the VSI in which the receiver PE needs to do the C-
multicast data packet lookup. It then strips the inner MPLS label and
sends the packet to the VSI for multicast data forwarding.

SB> The above is correct, but I am sure it can be written with
SB> much greater precision.

SB> Where do you describe the imposition logic?
SB> Same question for following sub-sections.

SB> The term is surely pops not strips?

14.1.2. Mapping one VPLS instance to a P2MP LSP

The following diagram shows the progression of the VPLS multicast
packet as it enters and leaves the SP network when a given MPLS tree
is being used for a single VPLS instance. RSVP-TE P2MP LSPs are
examples of such trees.

Packets received Packets in transit Packets forwarded
at ingress PE in the service by egress PEs
provider network

+---------------+
|MPLS Tree Label|
++=============++ ++=============++ ++=============++
||C-Ether Hdr || || C-Ether Hdr || || C-Ether Hdr ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-IP Header || || C-IP Header || || C-IP Header ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-Payload || || C-Payload || || C-Payload ||
++=============++ ++=============++ ++=============++

The receiver PE does a lookup on the outer MPLS tree label and
SB> Surely the egress PE

determines the VSI in which the receiver PE needs to do the C-
multicast data packet lookup. It then strips the inner MPLS label and
SB> The term is surely pops, not strips?
sends the packet to the VSI for multicast data forwarding.

15. VPLS Data Packet Treatment

If the destination MAC address of a VPLS packet received by a PE from
a VPLS site is a multicast address, a P-Multicast tree SHOULD be used
to transport the packet, if possible. If the packet is an IP
multicast packet and a Selective tree exists for that multicast
stream, the Selective tree MUST be used. Else if a (C-*, C-*)
Selective tree exists for the VPLS it SHOULD be used. Else if an
Inclusive tree exists for the VPLS, it SHOULD be used.

If the destination MAC address of a VPLS packet is a broadcast
address, it is flooded. If a (C-*, C-*) Selective tree exists for the
VPLS the PE SHOULD flood over it. Else if Inclusive tree exists for
the VPLS the PE SHOULD flood over it. Else the PE MUST flood over
multiple PWs, based on [RFC4761] or [RFC4762].

If the destination MAC address of a packet is a unicast address and
it has not been learned, the packet MUST be sent to all PEs in the
VPLS. Inclusive P-Multicast trees or a Selective P-Multicast tree
bound to (C-*, C-*) SHOULD be used for sending unknown unicast MAC
packets to all PEs. When this is the case the receiving PEs MUST
support the ability to perform MAC address learning for packets
received on a multicast tree. In order to perform such learning, the
receiver PE MUST be able to determine the sender PE when a VPLS
packet is received on a P-Multicast tree. This further implies that
the MPLS P-Multicast tree technology MUST allow the egress PE to
determine the sender PE from the received MPLS packet.

When a receiver PE receives a VPLS packet with a source MAC address,
that has not yet been learned, on a P-Multicast tree, the receiver PE
determines the PW to the sender PE. The receiver PE then creates
forwarding state in the VPLS instance with a destination MAC address
being the same as the source MAC address being learned, and the PW
being the PW to the sender PE.

It should be noted that when a sender PE that is sending packets
destined to an unknown unicast MAC address over a P-Multicast tree
learns the PW to use for forwarding packets destined to this unicast
MAC address, it might immediately switch to transport such packets
over this particular PW. Since the packets were initially being
forwarded using a P-Multicast tree, this could lead to packet
reordering. This constraint should be taken into consideration if
unknown unicast frames are forwarded using a P-Multicast tree,
instead of multiple PWs based on [RFC4761] or [RFC4762].

An implementation MUST support the ability to transport unknown
unicast traffic over Inclusive P-Multicast trees. Further an
implementation MUST support the ability to perform MAC address
learning for packets received on a P-Multicast tree.

SB> Why not a SHOULD? Especially given the packet reordering potential.

16. Security Considerations

Security considerations discussed in [RFC4761] and [RFC4762] apply to
this document. This section describes additional considerations.

As mentioned in [RFC4761], there are two aspects to achieving data
privacy in a VPLS: securing the control plane and protecting the
forwarding path. Compromise of the control plane could result in a PE
sending multicast data belonging to some VPLS to another VPLS, or
black-holing VPLS multicast data, or even sending it to an
SB> Black holing is a security issue, but not a privacy issue.
eavesdropper; none of which are acceptable from a data privacy point
of view. The mechanisms in this document use BGP for the control
plane. Hence techniques such as in [RFC5925] help authenticate BGP
messages, making it harder to spoof updates (which can be used to
divert VPLS traffic to the wrong VPLS) or withdrawals (denial-of-
service attacks).
SB> DOS is not a privacy issue.

In the multi-AS methods (b) and (c) described in
Section 11, this also means protecting the inter-AS BGP sessions,
between the ASBRs, the PEs, or the Route Reflectors.

Note that [RFC5925] will not help in keeping MPLS labels, associated
with P2MP LSPs or the upstream MPLS labels used for aggregation,
private -- knowing the labels, one can eavesdrop on VPLS traffic.

SB> Surely only on an otherwise trusted node?

However, this requires access to the data path within a Service
Provider network.

SB> Don't you also require trusted signalling access?

One of the requirements for protecting the data plane is that the
MPLS labels are accepted only from valid interfaces. This applies
both to MPLS labels associated with P2MP LSPs and also applies to the
upstream assigned MPLS labels. For a PE, valid interfaces comprise
links from P routers.
SB> Surely also from peer PEs in a multi-domain case?

For an ASBR, valid interfaces comprise links
from P routers and links from other ASBRs in ASes that have instances
of a given VPLS.
SB> ... and the PEs attached?

It is especially important in the case of multi-AS
VPLSes that one accept VPLS packets only from valid interfaces.

17. IANA Considerations

This document defines a new NLRI, called MCAST-VPLS, to be carried in
BGP using multiprotocol extensions. It requires assignment of a new
SAFI. This is to be assigned by IANA.

SB> See earlier puzzlement - I thought this was already implimented.
SB> Is there an un-noted early allocation?

This document defines a BGP optional transitive attribute, called
PMSI attribute. This is the same attribute as the one defined in
[RFC6514] and the code point for this attribute has already been
assigned by IANA as 22 [BGP-IANA]. Hence no further action is
required from IANA regarding this attribute.

<end>

---

From: "Bitar, Nabil N" <[email-masked]>
Date: Fri, 31 May 2013 15:38:09 -0400

Stewart,
Thanks you for the feedback. I will take a look through and work with the
authors as you indicated.

Thanks,
Nabil

On 5/31/13 1:47 PM, "Stewart Bryant" <[email-masked]> wrote:

>SB> Do you mean: ³for the use of pseudowires transported over P2P
>SB> RSVP-TE or MP2P Š
>
>has certain
>SB> There¹s no explicit mention of ³broadcast². Would it fit under the
>SB> This is confusing: when mLDP is used, the root doesn¹t need to
>SB> paragraph of this section, i.e. the one that starts with ³If the local
>SB> PE uses RSVP-TE P2MP LSP for sending (multicast) traffic.² That,
>SB> Should this be U-PE? VPLS doesn¹t define a model where PWs are used
>SB> wondering hereŠ
>SB> re ASBR Shouldn¹t this be ³PE² instead?
>SB> What does ³upstream neighbor² mean here? Are PE1 and A in the same
>SB> Type set to ³ingress replication? As a reader, I¹m left wondering
>SB> about the reasonŠ
>
>SB> removed, shouldn¹t the leaf AD route be withdrawn? There¹s