Network Working Group R. Aggarwal
Internet Draft Juniper Networks
Expiration Date: January 2006
Y. Kamite
NTT Communications
L. Fang
AT&T
July 2005
Multicast in VPLS
draft-raggarwa-l2vpn-vpls-mcast-01.txt
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Abstract
This document describes a solution for overcoming the limitations of
existing VPLS multicast solutions. It describes procedures for VPLS
multicast that utilize multicast trees in the sevice provider (SP)
network. One such multicast tree can be shared between multiple VPLS
instances. Procedures by which a single multicast tree in the
backbone can be used to carry traffic belonging only to a specified
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set of one or more multicast groups from one or more VPLSs are also
described.
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Table of Contents
1 Specification of requirements ......................... 4
2 Contributors .......................................... 4
3 Terminology ........................................... 4
4 Introduction .......................................... 4
5 Existing Limitation of VPLS Multicast ................. 5
6 Overview .............................................. 5
7 VPLS Multicast / Broadcast / Unknown Unicast Data Packet Treatment 7
8 Propagating Multicast Control Information ............. 7
9 Multicast Tree Leaf Discovery ......................... 8
9.1 Inclusive Tree Leaf Discovery ......................... 8
9.2 Selective Tree Leaf Discovery ......................... 8
10 Demultiplexing Multicast Tree Traffic ................. 8
10.1 One Multicast Tree - One VPLS Mapping ................. 8
10.1.1 One Multicast Tree - Many VPLS Mapping ................ 8
11 Establishing Multicast Trees .......................... 9
11.1 RSVP-TE P2MP LSPs ..................................... 10
11.1.1 P2MP TE LSP - VPLS Mapping ............................ 10
11.1.2 Demultiplexing C-Multicast Data Packets ............... 10
11.2 Receiver Initiated MPLS Trees ......................... 10
11.2.1 P2MP LSP - VPLS Mapping ............................... 11
11.2.2 Demultiplexing C-Multicast Data Packets ............... 11
11.3 PIM Based Trees ....................................... 11
11.4 Encapsulation of the Aggregate Inclusive Tree and Aggregate Selective Tree 11
12 Tree to VPLS / C-Multicast Stream Binding Distribution ....12
13 Switching to Aggregate Selective Trees ................ 12
14 BGP Advertisements .................................... 13
14.1 Information Elements .................................. 13
14.1.1 Inclusive Tree - VPLS Binding Advertisement ........... 13
14.1.2 Selective Tree - C-Multicast Stream Binding Advertisement .14
14.1.3 Inclusive Tree/Selective Tree Identifier .............. 14
14.2 Encoding .............................................. 15
15 Aggregation Methodology ............................... 15
16 Data Forwarding ....................................... 16
16.1 MPLS Tree Encapsulation ............................... 16
16.2 IP Tree Encapsulation ................................. 17
17 Security Considerations ............................... 18
18 Acknowledgments ....................................... 18
19 Normative References .................................. 18
20 Informative References ................................ 19
21 Author Information .................................... 19
22 Intellectual Property Statement ....................... 19
23 Full Copyright Statement .............................. 20
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1. Specification of requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Contributors
Rahul Aggarwal
Yakov Rekhter
Juniper Networks
Yuji Kamite
NTT Communications
Luyuan Fang
AT&T
Chaitanya Kodeboniya
Juniper Networks
3. Terminology
This document uses terminology described in [VPLS-BGP] and [VPLS-
LDP].
4. Introduction
[VPLS-BGP] and [VPLS-LDP] describe a solution for VPLS multicast that
relies on ingress replication. This solution has certain limitations
for certain VPLS multicast traffic profiles. 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 sevice provider (SP) network.
It provides mechanisms that allow a single multicast distribution
tree in the backbone to carry all the multicast traffic from a speci-
fied set of one or more VPLSs. Such a tree is referred to as an
"Inclusive Tree" and more specifically as an "Aggregate Inclusive
Tree" when the tree is used to carry multicast traffic from more than
VPLS.
This document also provides procedures by which a single multicast
distribution tree in the backbone can be used to carry traffic
belonging only to a specified set of one or more multicast groups,
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from one or more VPLSs. Such a tree is referred to as a "Selective
Tree" and more specifically as an "Aggregate Selective Tree" when the
multicast groups belong to different VPLSs. So traffic from most mul-
ticast groups could be carried by an Inclusive Tree, while traffic
from, e.g., high bandwidth groups could be carried in one of the
"Selective Trees".
5. Existing Limitation of VPLS Multicast
VPLS multicast solutions described in [VPLS-BGP] and [VPLS-LDP] 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 uni-
cast tunnel.
This is a reasonable model when the bandwidth of the multicast traf-
fic is low or/and the number of replications performed on an average
on each outgoing interface for a particular customer VPLS multicast
packet is small. If this is not the case it is desirable to utilize
multicast trees in the SP core to transmit VPLS multicast packets
[VPLS-MCAST-REQ]. Note that unicast packets that are flooded to each
of the egress PEs, before the ingress PE performs learning for those
unicast packets, MAY still use ingress replication.
6. Overview
This document describes procedures for using multicast trees in the
SP network to transport VPLS multicast data packets. RSVP-TE P2MP
LSPs described in [RSVP-P2MP] 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 desir-
able to optimize the number of copies of a multicast packet transmit-
ted by the ingress. This comes at a cost of state in the SP core to
build multicast trees and overhead to maintain this state. This docu-
ment places no restrictions on the protocols used to build SP multi-
cast trees.
Multicast trees used for VPLS can be of two types:
1. Inclusive Trees. A single multicast distribution tree in the
SP backbone is used to carry all the multicast traffic from a speci-
fied set of one or more VPLSs. These multicast distribution trees can
be set up to carry the traffic of a single VPLS, or to carry the
traffic of multiple VPLSs. The ability to carry the traffic of more
than one VPLS on the same tree is termed 'Aggregation'. The tree will
include every PE that is a member of any of the VPLSs that are using
the tree. This enables the SP to place a bound on the amount of mul-
ticast routing state which the P routers must have. This implies that
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a PE may receive multicast traffic for a multicast stream even if it
doesn't have any receivers on the path of that stream.
2. Selective Trees. A Selective Tree is used by a PE to send mul-
ticast traffic for one or more multicast streams, that belong to the
same or different VPLSs, to a subset of the PEs that belong to those
VPLSs. Each of the PEs in the subset are on the 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 VPLSs is termed 'Aggregation'. The reason for having Selec-
tive Trees is to provide a PE to have the ability to create separate
SP multicast trees for high bandwidth multicast groups. This allows
traffic for these multicast groups to reach only those PE routers
that have receivers in these groups. This avoids flooding other PE
routers in the VPLS.
A SP can use both Inclusive Trees and Selective Trees or either of
them for a given VPLS on a PE, based on local configuration. Inclu-
sive Trees can be used for both IP and non-IP data multicast traffic,
while Selective Trees can be used only for IP multicast data traffic.
In order to establish Inclusive and Selective multicast trees the
root of the tree must be able to discover the VPLS membership of all
the PEs and/or the multicast groups that each PE has receivers in.
This document describes procedures for doing this for Inclusive mul-
ticast trees. For discovering the IP multicast group membership pro-
cedures described in [VPLS-CTRL] are used. Procedures in [VPLS-CTRL]
can also be used with ingress replication to send traffic for a mul-
ticast stream to only those PEs that are on the path to receivers for
that stream. Aggregation also requires a mechanism for the egresses
of the tree to demultiplex the multicast traffic received over the
tree. This document describes how upstream label allocation by the
root of the tree can be used to perform this demultiplexing. This
document also describes procedures based on BGP that are used by the
root of an Aggregate Tree to advertise the Inclusive or Selective
tree binding and the demultiplexing information to the leaves of the
tree
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 pack-
ets.
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7. VPLS Multicast / Broadcast / Unknown Unicast Data Packet Treatment
If the destination MAC address of a VPLS packet received by a PE from
a VPLS site is a multicast adddress, a multicast tree SHOULD be used
to transport the packet, if possible. If the packet is an IP multi-
cast packet and a Selective tree exists for that multicast stream,
the Selective tree 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 Inclusive tree is already established, PE
floods over it. If Inclusive Tree cannot be used for some reason, PE
MUST flood over multiple unicast PWs, based on [VPLS-BGP] [VPLS-LDP].
If the destination MAC address of the packet has not been learned,
the flooding of the packet also occurs. Unlike broadcast case, it
should be noted that when a PE learns the MAC it might immediately
switch to transport over one particular PW. This implies that flood-
ing unknown unicast traffic over Inclusive Tree might lead to packet
reordering. This contraint should be taken into consideration if
unknown unicast frames are flooded using a Inclusive Tree, instead of
multiple unicast PWs based on [VPLS-BGP] [VPLS-LDP].
P-multicast trees are intended to be used only for VPLS C-multicast
data packets, not for control packets being used by a customer's
layer-2 and layer-3 control protocols. For instance, Bridge Protocol
Data Units (BPDUs) use an IEEE assigned all bridges multicast MAC
address, and OSPF uses OSPF routers multicast MAC address. P-multi-
cast trees SHOULD NOT be used for transporting these control packets.
8. Propagating Multicast Control Information
PEs participating in VPLS need to learn the <C-S, C-G> information
for two reasons:
1. With ingress replication, this allows a PE to send the IP mul-
ticast packet for a <C-S, C-G> only to other PEs in the VPLS
instance, that have receivers interested in that particular <C-S, C-
G>. This eliminates flooding.
2. It allows the construction of Aggregate Selective Trees.
Procedures for learning the <C-S, C-G> information are described in
[VPLS-CTRL].
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9. Multicast Tree Leaf Discovery
9.1. Inclusive Tree Leaf Discovery
VPLS auto-discovery as described in [VPLS-BGP, BGP-AUTO] or another
VPLS auto-discovery mechanism enables a PE to learn the VPLS member-
ship of other PEs. This is used by the root of the Tree to learn the
egresses of the tree.
9.2. Selective Tree Leaf Discovery
This is done using the C-Multicast control information propagation
described in [VPLS-CTRL].
10. Demultiplexing 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.
10.1. One Multicast Tree - One VPLS Mapping
When a 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, eg:
RSVP-TE P2MP LSPs, the outer MPLS label is used to determine the
tree. Penultimate-hop-popping MUST be disabled on the RSVP-TE P2MP
LSP.
10.1.1. One Multicast Tree - Many VPLS Mapping
As traffic belonging to multiple VPLSs 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 corresponds to 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
over the Aggregate Inclusive Tree or the Aggregate Selective Tree. If
downstream label assignment were used this would require all the
egress PEs in the VPLS to agree on a common label for the VPLS.
We propose a solution that uses upstream label assignment by the
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ingress PE [MPLS-UPSTREAM]. Hence the inner label is allocated by the
ingress PE. Each egress PE maintains a separate label space for every
other PE. The egress PEs create a forwarding entry for the inner VPN
label, allocated by the ingress PE, in this label space. Hence when
the egress PE receives a packet over an Aggregate Tree, the Tree
identifier specifies the label space to perform the inner label
lookup. The same label space may be used for all P-multicast trees
rooted at the same ingress PE, or an implementation may decide to use
a separate label space for every P-multicast tree.
When PIM based IP/GRE trees are used the root PE source address and
the tree P-group address identifies the tree interface. The label
space corresponding to the tree interface is the label space to per-
form the inner label lookup in. A lookup in this label space identi-
fies the VPLS in which the customer multicast lookup needs to be
done.
If the tree uses MPLS encapsulation the outer MPLS label and the
incoming interface provides the label space of the label beneath it.
This assumes that penultimate-hop-popping is disabled. An example of
this is RSVP-TE P2MP LSPs. The outer label and incoming interface
effectively identifies the Tree interface [MPLS-UPSTREAM, MPLS-
MCAST].
The ingress PE informs the egress PEs about the inner label as part
of the tree binding procedures described in section 12.
11. Establishing Multicast Trees
This document does not place any restrictions on the multicast tech-
nology used to setup P-multicast trees. However specific procedures
are specified currently only for RSVP-TE P2MP LSPs, LDP P2MP LSPs and
PIM-SM and PIM-SSM based trees.
A P-multicast tree can be either a source tree or a shared tree. A
source tree is used to carry traffic only for the VPLSs that exist
locally on the root of the tree i.e. for which the root has local
CEs. A shared tree on the other hand can be used to carry traffic
belonging to VPLSs that exist on other PEs as well. For example a RP
based PIM-SM Aggregate tree would be a shared tree. Another example
of a shared tree is a RSVP-TE P2MP LSP. The shared tree root partici-
pates in VPLS auto-discovery. Each of the PEs transport the VPLS
traffic to the shared tree root using ingress replication. The shared
root splices the traffic onto the shared tree.
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11.1. RSVP-TE P2MP LSPs
This section describes procedures that are specific to the usage of
RSVP-TE P2MP LSPs for instantiating a tree. The RSVP-TE P2MP LSP can
be either a source tree or a shared tree. Procedures in [RSVP-TE-
P2MP] are used to signal the LSP. The LSP is signaled after the root
of the LSP discovers the leaves. The egress PEs are discovered using
the procedures described in section 9. Aggregation as described in
this document is supported.
11.1.1. P2MP TE LSP - VPLS Mapping
P2MP TE LSP to VPLS mapping can be 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
- Optionally RSVP-TE P2MP LSP's SENDER_TEMPLATE Object
11.1.2. Demultiplexing C-Multicast Data Packets
Demultiplexing the C-multicast data packets at the egress PE require
that the PE be able to determine the P2MP TE LSP that the packets are
received on. The egress PE needs to determine the P2MP LSP to deter-
mine the VPLS that the packet belongs to, as described in section 10.
To achieve this the LSP must be signaled with penultimate-hop-popping
(PHP) off. 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.
11.2. Receiver Initiated MPLS Trees
Receiver initiated MPLS trees can also be used. An example of such
trees are LDP setup P2MP MPLS Trees [LDP-P2MP1, LDP-P2MP2].
The LDP P2MP LSP can be either a source tree or a shared tree. Proce-
dures in [LDP-P2MP1, LDP-P2MP2] are used to signal the LSP. The LSP
is signaled after the root of the LSP discovers the leaves and once
the leaves receive the LDP FEC for the tree from the root. The egress
PEs are discovered using the procedures described in section 9.
Aggregation as described in this document is supported.
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11.2.1. P2MP LSP - VPLS Mapping
P2MP LSP to VPLS mapping can be 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 tun-
nel 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 "joins" the P2MP MPLS tree by sending LDP label map-
ping messages for the LDP P2MP FEC, that was learned in the BGP
advertisement, using procedures described in [LDP-P2MP1, LDP-P2MP2].
11.2.2. Demultiplexing C-Multicast Data Packets
This follows the same procedures described above for RSVP-TE P2MP
LSPs.
11.3. PIM Based Trees
When PIM is used to setup multicast trees in the SP core the Aggre-
gate Inclusive Tree may be a shared tree, rooted at the RP, or a
shortest path tree. Aggregate Selective Tree is rooted at the PE that
is connected to the multicast traffic source. The root of the Aggre-
gate Tree or the Aggregate Selective Tree has to advertise the P-
Group address chosen by it for the tree to the PEs that are leaves of
the tree. These other PEs can then Join this tree. The announcement
of this address is done as part of the tree binding procedures
described in section 12.
11.4. Encapsulation of the Aggregate Inclusive Tree and Aggregate Selec-
tive Tree
An Aggregate Inclusive Tree or an Aggregate Selective Tree may use an
IP/GRE encapsulation or a MPLS encapsulation. The protocol type in
the IP/GRE header in the former case and the protocol type in the
data link header in the latter case are as described in [MPLS-MCAST].
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12. Tree to VPLS / C-Multicast Stream Binding Distribution
Once a PE sets up an Aggregate Inclusive Tree or an Aggregate Selec-
tive Tree it needs to announce the customer multicast groups being
mapped to this tree to other PEs in the network. This procedure is
referred to as Inclusive Tree or Selective Tree binding distribution
and is performed using BGP. For an Inclusive Tree this discovery
implies announcing the mapping of all VPLSs mapped to the Inclusive
Tree. The inner label allocated by the ingress PE for each VPLS is
included. The Inclusive Tree Identifier is also included. For an
Selective Tree this discovery implies announcing all the specific <C-
Source, C-Group> entries mapped to this tree along with the Selective
Tree Identifier. The inner label allocated for each <C-Source, C-
Group> is included. The Selective Tree Identifier is also included.
An Inclusive Tree by definition maps to all the <C-Source, C-Group>
entries belonging to all the VPLSs associated with the Inclusive
Tree. An Selective Tree maps to the specific <C-Source, C-Group>
associated with it.
When PIM or LDP is used to setup SP multicast trees, the egress PE
also Joins the P-Group Address or the LDP P2MP FEC corresponding to
the Inclusive or Selective tree. This results in setup of the
receiver driven multicast tree with IP or MPLS encapsulation.
13. Switching to Aggregate Selective Trees
Selective Trees provide a PE the ability to create separate SP multi-
cast trees for certain <C-S, C-G> entires. The source PE that origi-
nates the Selective Tree and the egress PEs have to switch to using
the Selective Tree for the <C-S, C-G> entries that are mapped to it.
Once a source PE decides to setup an Selective Tree, it announces the
mapping of the <C-S, C-G> entries that are mapped on the tree to the
other PEs using BGP. Depending on the SP multicast technology used,
this announcement may be done before or after setting up the Selec-
tive Tree. 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> entries
mapped to the tree. This implies setting up the demultiplexing for-
warding entries based on the inner label as described earlier. The
egress PEs may perform this switch to the Selective Tree once the
advertisement from the ingress PE is received or wait for a precon-
figured timer to do so.
A source PE may use one of two approaches to decide when to start
transmitting data on the Selective tree. In the first approach once
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the source PE sets up the Selective Tree, it starts sending multicast
packets for <C-S, C-G> entries mapped to the tree on both that tree
as well as on the Inclusive Tree. After some preconfigured timer the
PE stops sending multicast packets for <C-S, C-G> entries mapped on
the Selective Tree on the default tree. In the second approach a cer-
tain pre-configured delay after advertising the <C-S, C-G> entries
mapped to an Selective Tree, the source PE begins to send traffic on
the Selective Tree. At this point it stops to send traffic for the
<C-S, C-G> entries, that are mapped on the Selective Tree, on the
Inclusive Tree. This traffic is instead transmitted on the Selective
Tree.
14. BGP Advertisements
The procedures required in this document use BGP for P-Tree - VPLS
binding advertisements and P-Tree - Multicast stream binding adver-
tisement. This section first describes the information that needs to
be propagated in BGP for achieving the functional requirements. It
then describes a suggested encoding.
14.1. Information Elements
14.1.1. Inclusive Tree - VPLS Binding Advertisement
The root of an Aggregate Inclusive Tree maps one or more VPLS
instances to the Inclusive Tree. It announces this mapping in BGP.
Along with the VPLS instances that are mapped to the Inclusive Tree,
the Inclusive Tree identifier is also advertised in BGP.
The following information is required in BGP to advertise the VPLS
instance that is mapped to the Inclusive Tree:
1. The address of the router that is the root of the Inclusive
Tree.
2. The inner label allocated by the Inclusive Tree root for the
VPLS instance. The usage of this label is described in section 10.
When a PE distributes this information via BGP, it must include the
following:
1. An identifier of the Inclusive Tree.
2. Route Target Extended Communities attribute. This RT must be an
"Import RT" of each VSI in the VPLS. The BGP distribution procedures
used by [VPLS-BGP] or [BGP-AUTO] will then ensure that the advertised
information gets associated with the right VSIs.
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14.1.2. Selective Tree - C-Multicast Stream Binding Advertisement
The root of an Aggregate Selective Tree maps one or more <C-Source,
C-Group> entries to the tree. These entries are advertised in BGP
along with the the Selective Tree identifier to which these entries
are mapped.
The following information is required in BGP to advertise the <C-
Source, C-Group> entries that are mapped to the Selective Tree:
1. The RD configured for the VPLS instance. This is required to
uniquely identify the <C-Source, C-Group> as the addresses could
overlap between different VPLS instances.
2. The inner label allocated by the Selective Tree root for the
<C-Source, C-Group>. The usage of this label is described in section
10.
3. The C-Source address. This address can be a prefix in order to
allow a range of C-Source addresses to be mapped to the Selective
Tree.
4. The C-Group address. This address can be a range in order to
allow a range of C-Group addresses to be mapped to the Selective
Tree.
When a PE distributes this information via BGP, it must include the
following:
1. An identifier of the Selective Tree.
2. Route Target Extended Communities attribute. This is used as
described in section 14.1.1.
14.1.3. Inclusive Tree/Selective Tree Identifier
Inclusive Tree and Selective Tree advertisements carry the Tree iden-
tifier. The following information elements are needed in this identi-
fier.
1. Whether this is a shared Inclusive Tree or not.
2. The type of the tree. For example the tree may use PIM-SM or
PIM-SSM.
3. The identifier of the tree. For trees setup using PIM the
identifier is a (S, G) value.
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14.2. Encoding
Encoding details will be described later.
15. Aggregation Methodology
In general the herustics 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 a SP tree.
For Aggregate Inclusive Trees the congruency depends on the overlap
in the membership of the VPLSs that are aggregated on the Aggregate
Inclusive Tree. If there is complete overlap aggregation is perfectly
congruent. As the overlap between the VPLSs that are aggregated
reduces, the congruency reduces.
If aggregation is done such that it is not perfectly congruent a PE
may receive traffic for VPLSs to which it doesn't belong. As the
amount of multicast traffic in these unwanted VPLSs increases aggre-
gation becomes less optimal with respect to delivered traffic. Hence
there is a tradeoff between reducing state and delivering unwanted
traffic.
An implementation should provide knobs to control the congruency of
aggregation. This will allow a SP to deploy aggregation depending on
the VPLS membership and traffic profiles in its network. If differ-
ent PEs or shared roots' are setting up Aggregate Inclusive Trees
this will also allow a SP to engineer the maximum amount of unwanted
VPLSs that a particular PE may receive traffic for.
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.
Raggarwa, Kamite & Fang [Page 15]
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16. Data Forwarding
16.1. MPLS Tree Encapsulation
The following diagram shows the progression of the VPLS IP 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 exam-
ples of such trees.
Packets received Packets in transit Packets forwarded
at ingress PE in the service by egress PEs
provider network
+---------------+
|MPLS Tree Label|
+---------------+
| VPN Label |
++=============++ ++=============++ ++=============++
|| 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 deter-
mines the MPLS forwarding table in which to lookup the inner MPLS
label. 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 [MPLS-UPSTREAM].
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.
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16.2. IP Tree Encapsulation
The following diagram shows the progression of the packet as it
enters and leaves the SP network when the Aggregate MDT or Aggregate
Selective MDTs are being used for multiple VPLS instances. MPLS-in-
GRE [MPLS-IP] encapsulation is used to encapsulate the customer mul-
ticast packets.
Packets received Packets in transit Packets forwarded
at ingress PE in the service by egress PEs
provider network
+---------------+
| P-IP Header |
+---------------+
| GRE |
+---------------+
| VPN Label |
++=============++ ++=============++ ++=============++
|| C-IP Header || || C-IP Header || || C-IP Header ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-Payload || || C-Payload || || C-Payload ||
++=============++ ++=============++ ++=============++
The P-IP header contains the Aggregate Tree (or Aggregate Selective
Tree) P-group address as the destination address and the root PE
address as the source address. The receiver PE does a lookup on the
P-IP header and determines the MPLS forwarding table in which to
lookup the inner MPLS label. This table is specific to the Aggregate
Tree (or Aggregate Selective 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 unam-
biguously identify a particular VPLS one has to know the label, and
the context within which that label is unique. The context is pro-
vided by the P-IP header [MPLS-UPSTREAM].
The P-IP header and the GRE header 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.
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Internet Draft draft-raggarwa-l2vpn-vpls-mcast-01.txt July 2005
17. Security Considerations
Security considerations discussed in [VPLS-BGP] and [VPLS-LDP] apply
to this document.
18. Acknowledgments
Many thanks to Thomas Morin for his support of this work.
19. Normative References
[RFC2119] "Key words for use in RFCs to Indicate Requirement Lev-
els.", Bradner, March 1997
[RFC3107] Y. Rekhter, E. Rosen, "Carrying Label Information in
BGP-4", RFC3107.
[VPLS-BGP] K. Kompella, Y. Rekther, "Virtual Private LAN Service",
draft-ietf-l2vpn-vpls-bgp-02.txt
[VPLS-LDP] M. Lasserre, V. Kompella, "Virtual Private LAN Services
over MPLS", draft-ietf-l2vpn-vpls-ldp-03.txt
[MPLS-IP] T. Worster, Y. Rekhter, E. Rosen, "Encapsulating MPLS in IP
or Generic Routing Encapsulation (GRE)", draft-ietf-mpls-in-ip-or-
gre-08.txt
[BGP-AUTO] H. Ould-Brahim et al., "Using BGP as an Auto-Discovery
Mechanism for Layer-3 and Layer-2 VPNs", draft-ietf-l3vpn-bgpvpn-
auto-04.txt
[VPLS-CTRL] R. Aggarwal, Y. Kamite, L. Fang, "Propagation of VPLS IP
Multicast Group Membership Information", draft-raggarwa-l2vpn-vpls-
mcast-ctrl-00.txt
[MPLS-UPSTREAM] R. Aggarwal, Y. Rekhter, E. Rosen, "MPLS Upstream
Label Assignment and Context Specific Label Space", draft-raggarwa-
mpls-upstream-label-00.txt
[MPLS-MCAST] T. Eckert, E. Rosen, R. Aggarwal, Y. Rekhter, "MPLS Mul-
ticast Encapsulations", draft-rosen-mpls-multicast-encaps-00.txt
[VPLS-MCAST-REQ] Y. kamite, et. al., "Requirements for Multicast Sup-
port in Virtual Private LAN Services", draft-kamite-l2vpn-vpls-mcast-
reqts-00.txt
Raggarwa, Kamite & Fang [Page 18]
Internet Draft draft-raggarwa-l2vpn-vpls-mcast-01.txt July 2005
20. Informative References
[MVPN] E. Rosen, R. Aggarwal, "Multicast in 2547 VPNs", draft-ietf-
l3vpn-2547bis-mcast-00.txt"
[RSVP-P2MP] R. Aggarwal et. al, "Extensions to RSVP-TE for Point to
Multipoint TE LSPs", draft-ietf-mpls-rsvp-te-p2mp-02.txt
[LDP-P2MP1] I. Minei et. al, "Label Distribution Protocol Extensions
for Point-to-Multipoint Label Switched Paths", draft-minei-mpls-ldp-
p2mp-00.txt
[LDP-P2MP2] I. Wijnands et. al., "Multicast Extensions for LDP",
draft-wijnands-mpls-ldp-mcast-ext-00.txt
21. Author Information
Rahul Aggarwal Juniper Networks 1194 North Mathilda Ave. Sunnyvale,
CA 94089 Email: rahul@juniper.net
Yakov Rekhter Juniper Networks 1194 North Mathilda Ave. Sunnyvale,
CA 94089 Email: yakov@juniper.net
Yuji Kamite NTT Communications Corporation Tokyo Opera City Tower
3-20-2 Nishi Shinjuku, Shinjuku-ku, Tokyo 163-1421, Japan Email:
y.kamite@ntt.com
Luyuan Fang AT&T 200 Laurel Avenue, Room C2-3B35 Middletown, NJ 07748
Phone: 732-420-1921 Email: luyuanfang@att.com
Chaitanya Kodeboniya Juniper Networks 1194 North Mathilda Ave. Sun-
nyvale, CA 94089 Email: ck@juniper.net
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Raggarwa, Kamite & Fang [Page 19]
Internet Draft draft-raggarwa-l2vpn-vpls-mcast-01.txt July 2005
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Raggarwa, Kamite & Fang [Page 20]