Network Working Group Eric C. Rosen (Editor)
Internet Draft Cisco Systems, Inc.
Intended Status: Standards Track
Expires: July 14, 2008 Rahul Aggarwal (Editor)
Juniper Networks
January 14, 2008
Multicast in MPLS/BGP IP VPNs
draft-ietf-l3vpn-2547bis-mcast-06.txt
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Abstract
In order for IP multicast traffic within a BGP/MPLS IP VPN (Virtual
Private Network) to travel from one VPN site to another, special
protocols and procedures must be implemented by the VPN Service
Provider. These protocols and procedures are specified in this
document.
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Table of Contents
1 Specification of requirements ......................... 5
2 Introduction .......................................... 5
2.1 Optimality vs Scalability ............................. 5
2.1.1 Multicast Distribution Trees .......................... 7
2.1.2 Ingress Replication through Unicast Tunnels ........... 8
2.2 Overview .............................................. 8
2.2.1 Multicast Routing Adjacencies ......................... 8
2.2.2 MVPN Definition ....................................... 9
2.2.3 Auto-Discovery ........................................ 10
2.2.4 PE-PE Multicast Routing Information ................... 11
2.2.5 PE-PE Multicast Data Transmission ..................... 11
2.2.6 Inter-AS MVPNs ........................................ 12
2.2.7 Optionally Eliminating Shared Tree State .............. 12
3 Concepts and Framework ................................ 13
3.1 PE-CE Multicast Routing ............................... 13
3.2 P-Multicast Service Interfaces (PMSIs) ................ 14
3.2.1 Inclusive and Selective PMSIs ......................... 15
3.2.2 Tunnels Instantiating PMSIs ........................... 16
3.3 Use of PMSIs for Carrying Multicast Data .............. 18
3.3.1 MVPNs with MI-PMSIs ................................... 18
3.3.2 When MI-PMSIs are Required ............................ 19
3.3.3 MVPNs That Do Not Use MI-PMSIs ........................ 19
3.4 PE-PE Transmission of C-Multicast Routing ............. 19
3.4.1 PIM Peering ........................................... 20
3.4.1.1 Full Per-MVPN PIM Peering Across a MI-PMSI ............ 20
3.4.1.2 Lightweight PIM Peering Across a MI-PMSI .............. 20
3.4.1.3 Unicasting of PIM C-Join/Prune Messages ............... 21
3.4.2 Using BGP to Carry C-Multicast Routing ................ 21
4 BGP-Based Autodiscovery of MVPN Membership ............ 22
5 PE-PE Transmission of C-Multicast Routing ............. 25
5.1 Selecting the Upstream Multicast Hop (UMH) ............ 25
5.1.1 Eligible Routes for UMH Selection ..................... 26
5.1.2 Information Carried by Eligible UMH Routes ............ 26
5.1.3 Selecting the Upstream PE ............................. 27
5.1.4 Selecting the Upstream Multicast Hop .................. 29
5.2 Details of Per-MVPN Full PIM Peering over MI-PMSI ..... 29
5.2.1 PIM C-Instance Control Packets ........................ 30
5.2.2 PIM C-instance RPF Determination ...................... 30
5.2.3 Backwards Compatibility ............................... 31
5.3 Use of BGP for Carrying C-Multicast Routing ........... 31
5.3.1 Sending BGP Updates ................................... 31
5.3.2 Explicit Tracking ..................................... 33
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5.3.3 Withdrawing BGP Updates ............................... 33
6 I-PMSI Instantiation .................................. 33
6.1 MVPN Membership and Egress PE Auto-Discovery .......... 34
6.1.1 Auto-Discovery for Ingress Replication ................ 34
6.1.2 Auto-Discovery for P-Multicast Trees .................. 34
6.2 C-Multicast Routing Information Exchange .............. 35
6.3 Aggregation ........................................... 35
6.3.1 Aggregate Tree Leaf Discovery ......................... 35
6.3.2 Aggregation Methodology ............................... 36
6.3.3 Encapsulation of the Aggregate Tree ................... 37
6.3.4 Demultiplexing C-multicast traffic .................... 37
6.4 Mapping Received Packets to MVPNs ..................... 38
6.4.1 Unicast Tunnels ....................................... 38
6.4.2 Non-Aggregated P-Multicast Trees ...................... 39
6.4.3 Aggregate P-Multicast Trees ........................... 39
6.5 I-PMSI Instantiation Using Ingress Replication ........ 40
6.6 Establishing P-Multicast Trees ........................ 41
6.7 RSVP-TE P2MP LSPs ..................................... 42
6.7.1 P2MP TE LSP Tunnel - MVPN Mapping ..................... 42
6.7.2 Demultiplexing C-Multicast Data Packets ............... 42
7 Optimizing Multicast Distribution via S-PMSIs ......... 43
7.1 S-PMSI Instantiation Using Ingress Replication ........ 44
7.2 Protocol for Switching to S-PMSIs ..................... 44
7.2.1 A UDP-based Protocol for Switching to S-PMSIs ......... 44
7.2.1.1 Binding a Stream to an S-PMSI ......................... 45
7.2.1.2 Packet Formats and Constants .......................... 46
7.2.2 A BGP-based Protocol for Switching to S-PMSIs ......... 48
7.2.2.1 Advertising C-(S, G) Binding to a S-PMSI using BGP .... 48
7.2.2.2 Explicit Tracking ..................................... 49
7.2.2.3 Switching to S-PMSI ................................... 50
7.3 Aggregation ........................................... 50
7.4 Instantiating the S-PMSI with a PIM Tree .............. 51
7.5 Instantiating S-PMSIs using RSVP-TE P2MP Tunnels ...... 52
8 Inter-AS Procedures ................................... 52
8.1 Non-Segmented Inter-AS Tunnels ........................ 53
8.1.1 Inter-AS MVPN Auto-Discovery .......................... 53
8.1.2 Inter-AS MVPN Routing Information Exchange ............ 53
8.1.3 Inter-AS P-Tunnels .................................... 54
8.1.3.1 PIM-Based Inter-AS P-Multicast Trees .................. 54
8.2 Segmented Inter-AS Tunnels ............................ 55
8.2.1 Inter-AS MVPN Auto-Discovery Routes ................... 55
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8.2.1.1 Originating Inter-AS MVPN A-D Information ............. 56
8.2.1.2 Propagating Inter-AS MVPN A-D Information ............. 57
8.2.1.2.1 Inter-AS Auto-Discovery Route received via EBGP ....... 57
8.2.1.2.2 Leaf Auto-Discovery Route received via EBGP ........... 58
8.2.1.2.3 Inter-AS Auto-Discovery Route received via IBGP ....... 58
8.2.2 Inter-AS MVPN Routing Information Exchange ............ 60
8.2.3 Inter-AS I-PMSI ....................................... 60
8.2.3.1 Support for Unicast VPN Inter-AS Methods .............. 61
8.2.4 Inter-AS S-PMSI ....................................... 61
9 Duplicate Packet Detection and Single Forwarder PE .... 62
9.1 Multihomed C-S or C-RP ................................ 63
9.1.1 Single forwarder PE selection ......................... 64
9.2 Switching from the C-RP tree to C-S tree .............. 65
10 Eliminating PE-PE Distribution of (C-*,C-G) State ..... 66
10.1 Co-locating C-RPs on a PE ............................. 67
10.1.1 Initial Configuration ................................. 68
10.1.2 Anycast RP Based on Propagating Active Sources ........ 68
10.1.2.1 Receiver(s) Within a Site ............................. 68
10.1.2.2 Source Within a Site .................................. 68
10.1.2.3 Receiver Switching from Shared to Source Tree ......... 69
10.2 Using MSDP between a PE and a Local C-RP .............. 69
11 Encapsulations ........................................ 70
11.1 Encapsulations for Single PMSI per Tunnel ............. 70
11.1.1 Encapsulation in GRE .................................. 70
11.1.2 Encapsulation in IP ................................... 72
11.1.3 Encapsulation in MPLS ................................. 72
11.2 Encapsulations for Multiple PMSIs per Tunnel .......... 73
11.2.1 Encapsulation in GRE .................................. 73
11.2.2 Encapsulation in IP ................................... 73
11.3 Encapsulations Identifying a Distinguished PE ......... 74
11.3.1 For MP2MP LSP P-tunnels ............................... 74
11.3.2 For Support of PIM-BIDIR C-Groups ..................... 74
11.4 Encapsulations for Unicasting PIM Control Messages .... 75
11.5 General Considerations for IP and GRE Encaps .......... 75
11.5.1 MTU ................................................... 75
11.5.2 TTL ................................................... 76
11.5.3 Avoiding Conflict with Internet Multicast ............. 76
11.6 Differentiated Services ............................... 76
12 Support for PIM-BIDIR C-Groups ........................ 77
12.1 The VPN Backbone Becomes the RPL ...................... 78
12.1.1 Control Plane ......................................... 78
12.1.2 Data Plane ............................................ 79
12.2 Partitioned Sets of PEs ............................... 79
12.2.1 Partitions ............................................ 79
12.2.2 Using PE Labels ....................................... 80
12.2.3 Mesh of MP2MP P-Tunnels ............................... 81
13 Security Considerations ............................... 81
14 IANA Considerations ................................... 82
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15 Other Authors ......................................... 82
16 Other Contributors .................................... 82
17 Authors' Addresses .................................... 82
18 Normative References .................................. 84
19 Informative References ................................ 85
20 Full Copyright Statement .............................. 85
21 Intellectual Property ................................. 86
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. Introduction
[RFC4364] specifies the set of procedures which a Service Provider
(SP) must implement in order to provide a particular kind of VPN
service ("BGP/MPLS IP VPN") for its customers. The service described
therein allows IP unicast packets to travel from one customer site to
another, but it does not provide a way for IP multicast traffic to
travel from one customer site to another.
This document extends the service defined in [RFC4364] so that it
also includes the capability of handling IP multicast traffic. This
requires a number of different protocols to work together. The
document provides a framework describing how the various protocols
fit together, and also provides detailed specification of some of the
protocols. The detailed specification of some of the other protocols
is found in pre-existing documents or in companion documents.
2.1. Optimality vs Scalability
In a "BGP/MPLS IP VPN" [RFC4364], unicast routing of VPN packets is
achieved without the need to keep any per-VPN state in the core of
the SP's network (the "P routers"). Routing information from a
particular VPN is maintained only by the Provider Edge routers (the
"PE routers", or "PEs") that attach directly to sites of that VPN.
Customer data travels through the P routers in tunnels from one PE to
another (usually MPLS Label Switched Paths, LSPs), so to support the
VPN service the P routers only need to have routes to the PE routers.
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The PE-to-PE routing is optimal, but the amount of associated state
in the P routers depends only on the number of PEs, not on the number
of VPNs.
However, in order to provide optimal multicast routing for a
particular multicast flow, the P routers through which that flow
travels have to hold state which is specific to that flow. A
multicast flow is identified by the (source, group) tuple where the
source is the IP address of the sender and the group is the IP
multicast group address of the destination. Scalability would be
poor if the amount of state in the P routers were proportional to the
number of multicast flows in the VPNs. Therefore, when supporting
multicast service for a BGP/MPLS IP VPN, the optimality of the
multicast routing must be traded off against the scalability of the P
routers. We explain this below in more detail.
If a particular VPN is transmitting "native" multicast traffic over
the backbone, we refer to it as an "MVPN". By "native" multicast
traffic, we mean packets that a CE sends to a PE, such that the IP
destination address of the packets is a multicast group address, or
the packets are multicast control packets addressed to the PE router
itself, or the packets are IP multicast data packets encapsulated in
MPLS.
We say that the backbone multicast routing for a particular multicast
group in a particular VPN is "optimal" if and only if all of the
following conditions hold:
- When a PE router receives a multicast data packet of that group
from a CE router, it transmits the packet in such a way that the
packet is received by every other PE router which is on the path
to a receiver of that group;
- The packet is not received by any other PEs;
- While in the backbone, no more than one copy of the packet ever
traverses any link.
- While in the backbone, if bandwidth usage is to be optimized, the
packet traverses minimum cost trees rather than shortest path
trees.
Optimal routing for a particular multicast group requires that the
backbone maintain one or more source-trees which are specific to that
flow. Each such tree requires that state be maintained in all the P
routers that are in the tree.
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This would potentially require an unbounded amount of state in the P
routers, since the SP has no control of the number of multicast
groups in the VPNs that it supports. Nor does the SP have any control
over the number of transmitters in each group, nor of the
distribution of the receivers.
The procedures defined in this document allow an SP to provide
multicast VPN service without requiring the amount of state
maintained by the P routers to be proportional to the number of
multicast data flows in the VPNs. The amount of state is traded off
against the optimality of the multicast routing. Enough flexibility
is provided so that a given SP can make his own tradeoffs between
scalability and optimality. An SP can even allow some multicast
groups in some VPNs to receive optimal routing, while others do not.
Of course, the cost of this flexibility is an increase in the number
of options provided by the protocols.
The basic technique for providing scalability is to aggregate a
number of customer multicast flows onto a single multicast
distribution tree through the P routers. A number of aggregation
methods are supported.
The procedures defined in this document also accommodate the SP that
does not want to build multicast distribution trees in his backbone
at all; the ingress PE can replicate each multicast data packet and
then unicast each replica through a tunnel to each egress PE that
needs to receive the data.
2.1.1. Multicast Distribution Trees
This document supports the use of a single multicast distribution
tree in the backbone to carry all the multicast traffic from a
specified set of one or more MVPNs. Such a tree is referred to as an
"Inclusive Tree". An Inclusive Tree which carries the traffic of more
than one MVPN is an "Aggregate Inclusive Tree". An Inclusive Tree
contains, as its members, all the PEs that attach to any of the MVPNs
using the tree.
With this option, even if each tree supports only one MVPN, the upper
bound on the amount of state maintained by the P routers is
proportional to the number of VPNs supported, rather than to the
number of multicast flows in those VPNs. If the trees are
unidirectional, it would be more accurate to say that the state is
proportional to the product of the number of VPNs and the average
number of PEs per VPN. The amount of state maintained by the P
routers can be further reduced by aggregating more MVPNs onto a
single tree. If each such tree supports a set of MVPNs, (call it an
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"MVPN aggregation set"), the state maintained by the P routers is
proportional to the product of the number of MVPN aggregation sets
and the average number of PEs per MVPN. Thus the state does not grow
linearly with the number of MVPNs.
However, as data from many multicast groups is aggregated together
onto a single "Inclusive Tree", it is likely that some PEs will
receive multicast data for which they have no need, i.e., some degree
of optimality has been sacrificed.
This document also provides procedures which enable a single
multicast distribution tree in the backbone to be used to carry
traffic belonging only to a specified set of one or more multicast
groups, from one or more MVPNs. 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 MVPNs. By
default, traffic from most multicast 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". When setting up the
Selective Trees, one should include only those PEs which need to
receive multicast data from one or more of the groups assigned to the
tree. This provides more optimal routing than can be obtained by
using only Inclusive Trees, though it requires additional state in
the P routers.
2.1.2. Ingress Replication through Unicast Tunnels
This document also provides procedures for carry MVPN data traffic
through unicast tunnels from the ingress PE to each of the egress
PEs. The ingress PE replicates the multicast data packet received
from a CE and sends it to each of the egress PEs using the unicast
tunnels. This requires no multicast routing state in the P routers
at all, but it puts the entire replication load on the ingress PE
router, and makes no attempt to optimize the multicast routing.
2.2. Overview
2.2.1. Multicast Routing Adjacencies
In BGP/MPLS IP VPNs [RFC4364], each CE ("Customer Edge") router is a
unicast routing adjacency of a PE router, but CE routers at different
sites do not become unicast routing adjacencies of each other. This
important characteristic is retained for multicast routing -- a CE
router becomes a multicast routing adjacency of a PE router, but CE
routers at different sites do not become multicast routing
adjacencies of each other.
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The multicast routing protocol on the PE-CE link is presumed to be
PIM ("Protocol Independent Multicast") [PIM-SM]. The Sparse Mode,
Dense Mode, Single Source Mode, and Bidirectional Modes are
supported. A CE router exchanges "ordinary" PIM control messages with
the PE router to which it is attached.
The PEs attaching to a particular MVPN then have to exchange the
multicast routing information with each other. Two basic methods for
doing this are defined: (1) PE-PE PIM, and (2) BGP. In the former
case, the PEs need to be multicast routing adjacencies of each other.
In the latter case, they do not. For example, each PE may be a BGP
adjacency of a Route Reflector (RR), and not of any other PEs.
To support the "Carrier's Carrier" model of [RFC4364], mLDP or BGP
can be used on the PE-CE interface. This will be described in
subsequent versions of this document.
2.2.2. MVPN Definition
An MVPN is defined by two sets of sites, Sender Sites set and
Receiver Sites set, with the following properties:
- Hosts within the Sender Sites set could originate multicast
traffic for receivers in the Receiver Sites set.
- Receivers not in the Receiver Sites set should not be able to
receive this traffic.
- Hosts within the Receiver Sites set could receive multicast
traffic originated by any host in the Sender Sites set.
- Hosts within the Receiver Sites set should not be able to receive
multicast traffic originated by any host that is not in the
Sender Sites set.
A site could be both in the Sender Sites set and Receiver Sites set,
which implies that hosts within such a site could both originate and
receive multicast traffic. An extreme case is when the Sender Sites
set is the same as the Receiver Sites set, in which case all sites
could originate and receive multicast traffic from each other.
Sites within a given MVPN may be either within the same, or in
different organizations, which implies that an MVPN can be either an
Intranet or an Extranet.
A given site may be in more than one MVPN, which implies that MVPNs
may overlap.
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Not all sites of a given MVPN have to be connected to the same
service provider, which implies that an MVPN can span multiple
service providers.
Another way to look at MVPN is to say that an MVPN is defined by a
set of administrative policies. Such policies determine both Sender
Sites set and Receiver Site set. Such policies are established by
MVPN customers, but implemented/realized by MVPN Service Providers
using the existing BGP/MPLS VPN mechanisms, such as Route Targets,
with extensions, as necessary.
2.2.3. Auto-Discovery
In order for the PE routers attaching to a given MVPN to exchange
MVPN control information with each other, each one needs to discover
all the other PEs that attach to the same MVPN. (Strictly speaking,
a PE in the receiver sites set need only discover the other PEs in
the sender sites set and a PE in the sender sites set need only
discover the other PEs in the receiver sites set.) This is referred
to as "MVPN Auto-Discovery".
This document discusses two ways of providing MVPN autodiscovery:
- BGP can be used for discovering and maintaining MVPN membership.
The PE routers advertise their MVPN membership to other PE
routers using BGP. A PE is considered to be a "member" of a
particular MVPN if it contains a VRF (Virtual Routing and
Forwarding table, see [RFC4364]) which is configured to contain
the multicast routing information of that MVPN. This auto-
discovery option does not make any assumptions about the methods
used for transmitting MVPN multicast data packets through the
backbone.
- If it is known that the multicast data packets of a particular
MVPN are to be transmitted (at least, by default) through a non-
aggregated Inclusive Tree which is to be set up by PIM-SM or
BIDIR-PIM, and if the PEs attaching to that MVPN are configured
with the group address corresponding to that tree, then the PEs
can auto-discover each other simply by joining the tree and then
multicasting PIM Hellos over the tree.
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2.2.4. PE-PE Multicast Routing Information
The BGP/MPLS IP VPN [RFC4364] specification requires a PE to maintain
at most one BGP peering with every other PE in the network. This
peering is used to exchange VPN routing information. The use of Route
Reflectors further reduces the number of BGP adjacencies maintained
by a PE to exchange VPN routing information with other PEs. This
document describes various options for exchanging MVPN control
information between PE routers based on the use of PIM or BGP. These
options have different overheads with respect to the number of
routing adjacencies that a PE router needs to maintain to exchange
MVPN control information with other PE routers. Some of these options
allow the retention of the unicast BGP/MPLS VPN model letting a PE
maintain at most one BGP routing adjacency with other PE routers to
exchange MVPN control information. BGP also provides reliable
transport and uses incremental updates. Another option is the use of
the currently existing, "soft state" PIM standard [PIM-SM] that uses
periodic complete updates.
2.2.5. PE-PE Multicast Data Transmission
Like [RFC4364], this document decouples the procedures for exchanging
routing information from the procedures for transmitting data
traffic. Hence a variety of transport technologies may be used in the
backbone. For inclusive trees, these transport technologies include
unicast PE-PE tunnels (using MPLS or IP/GRE encapsulation), multicast
distribution trees created by PIM-SSM, PIM-SM, or BIDIR-PIM (using
IP/GRE encapsulation), point-to-multipoint LSPs created by RSVP-TE or
mLDP, and multipoint-to-multipoint LSPs created by mLDP. (However,
techniques for aggregating the traffic of multiple MVPNs onto a
single multipoint-to-multipoint LSP or onto a single bidirectional
multicast distribution tree are for further study.) For selective
trees, only unicast PE-PE tunnels (using MPLS or IP/GRE
encapsulation) and unidirectional single-source trees are supported,
and the supported tree creation protocols are PIM-SSM (using IP/GRE
encapsulation), RSVP-TE, and mLDP.
In order to aggregate traffic from multiple MVPNs onto a single
multicast distribution tree, it is necessary to have a mechanism to
enable the egresses of the tree to demultiplex the multicast traffic
received over the tree and to associate each received packet with a
particular MVPN. This document specifies a mechanism whereby
upstream label assignment [MPLS-UPSTREAM-LABEL] is used by the root
of the tree to assign a label to each flow. This label is used by
the receivers to perform the demultiplexing. This document also
describes procedures based on BGP that are used by the root of an
Aggregate Tree to advertise the Inclusive and/or Selective binding
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and the demultiplexing information to the leaves of the tree.
This document also describes the data plane encapsulations for
supporting the various SP multicast transport options.
This document assumes that when SP multicast trees are used, traffic
for a particular multicast group is transmitted by a particular PE on
only one SP multicast tree. The use of multiple SP multicast trees
for transmitting traffic belonging to a particular multicast group is
for further study.
2.2.6. Inter-AS MVPNs
[RFC4364] describes different options for supporting BGP/MPLS IP
unicast VPNs whose provider backbones contain more than one
Autonomous System (AS). These are know as Inter-AS VPNs. In an
Inter-AS VPN, the ASes may belong to the same provider or to
different providers. This document describes how Inter-AS MVPNs can
be supported for each of the unicast BGP/MPLS VPN Inter-AS options.
This document also specifies a model where Inter-AS MVPN service can
be offered without requiring a single SP multicast tree to span
multiple ASes. In this model, an inter-AS multicast tree consists of
a number of "segments", one per AS, which are stitched together at AS
boundary points. These are known as "segmented inter-AS trees". Each
segment of a segmented inter-AS tree may use a different multicast
transport technology.
It is also possible to support Inter-AS MVPNs with non-segmented
source trees that extend across AS boundaries.
2.2.7. Optionally Eliminating Shared Tree State
The document also discusses some options and protocol extensions
which can be used to eliminate the need for the PE routers to
distribute to each other the (*, G) and (*, G, RPT-bit) states when
there are PIM Sparse Mode multicast groups in the VPNs.
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3. Concepts and Framework
3.1. PE-CE Multicast Routing
Support of multicast in BGP/MPLS IP VPNs is modeled closely after
support of unicast in BGP/MPLS IP VPNs. That is, a multicast routing
protocol will be run on the PE-CE interfaces, such that PE and CE are
multicast routing adjacencies on that interface. CEs at different
sites do not become multicast routing adjacencies of each other.
If a PE attaches to n VPNs for which multicast support is provided
(i.e., to n "MVPNs"), the PE will run n independent instances of a
multicast routing protocol. We will refer to these multicast routing
instances as "VPN-specific multicast routing instances", or more
briefly as "multicast C-instances". The notion of a "VRF" ("Virtual
Routing and Forwarding Table"), defined in [RFC4364], is extended to
include multicast routing entries as well as unicast routing entries.
Each multicast routing entry is thus associated with a particular
VRF.
Whether a particular VRF belongs to an MVPN or not is determined by
configuration.
In this document, we will not attempt to provide support for every
possible multicast routing protocol that could possibly run on the
PE-CE link. Rather, we consider multicast C-instances only for the
following multicast routing protocols:
- PIM Sparse Mode (PIM-SM)
- PIM Single Source Mode (PIM-SSM)
- PIM Bidirectional Mode (BIDIR-PIM)
- PIM Dense Mode (PIM-DM)
In order to support the "Carrier's Carrier" model of [RFC4364], mLDP
or BGP will also be supported on the PE-CE interface. The use of mLDP
on the PE-CE interface is described in [MVPN-BGP]. The use of BGP on
the PE-CE interface is not described in this revision.
As the document only supports PIM-based C-instances, we will
generally use the term "PIM C-instances" to refer to the multicast C-
instances.
A PE router may also be running a "provider-wide" instance of PIM, (a
"PIM P-instance"), in which it has a PIM adjacency with, e.g., each
of its IGP neighbors (i.e., with P routers), but NOT with any CE
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routers, and not with other PE routers (unless another PE router
happens to be an IGP adjacency). In this case, P routers would also
run the P-instance of PIM, but NOT a C-instance. If there is a PIM
P-instance, it may or may not have a role to play in support of VPN
multicast; this is discussed in later sections. However, in no case
will the PIM P-instance contain VPN-specific multicast routing
information.
In order to help clarify when we are speaking of the PIM P-instance
and when we are speaking of a PIM C-instance, we will also apply the
prefixes "P-" and "C-" respectively to control messages, addresses,
etc. Thus a P-Join would be a PIM Join which is processed by the PIM
P-instance, and a C-Join would be a PIM Join which is processed by a
C-instance. A P-group address would be a group address in the SP's
address space, and a C-group address would be a group address in a
VPN's address space.
3.2. P-Multicast Service Interfaces (PMSIs)
Multicast data packets received by a PE over a PE-CE interface must
be forwarded to one or more of the other PEs in the same MVPN for
delivery to one or more other CEs.
We define the notion of a "P-Multicast Service Interface" (PMSI). If
a particular MVPN is supported by a particular set of PE routers,
then there will be a PMSI connecting those PE routers. A PMSI is a
conceptual "overlay" on the P network with the following property: a
PE in a given MVPN can give a packet to the PMSI, and the packet will
be delivered to some or all of the other PEs in the MVPN, such that
any PE receiving such a packet will be able to tell which MVPN the
packet belongs to.
As we discuss below, a PMSI may be instantiated by a number of
different transport mechanisms, depending on the particular
requirements of the MVPN and of the SP. We will refer to these
transport mechanisms as "tunnels".
For each MVPN, there are one or more PMSIs that are used for
transmitting the MVPN's multicast data from one PE to others. We
will use the term "PMSI" such that a single PMSI belongs to a single
MVPN. However, the transport mechanism which is used to instantiate
a PMSI may allow a single "tunnel" to carry the data of multiple
PMSIs.
In this document we make a clear distinction between the multicast
service (the PMSI) and its instantiation. This allows us to separate
the discussion of different services from the discussion of different
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instantiations of each service. The term "tunnel" is used to refer
only to the transport mechanism that instantiates a service.
3.2.1. Inclusive and Selective PMSIs
We will distinguish between three different kinds of PMSI:
- "Multidirectional Inclusive" PMSI (MI-PMSI)
A Multidirectional Inclusive PMSI is one which enables ANY PE
attaching to a particular MVPN to transmit a message such that it
will be received by EVERY other PE attaching to that MVPN.
There is at most one MI-PMSI per MVPN. (Though the tunnel or
tunnels that instantiate an MI-PMSI may actually carry the data
of more than one PMSI.)
An MI-PMSI can be thought of as an overlay broadcast network
connecting the set of PEs supporting a particular MVPN.
- "Unidirectional Inclusive" PMSI (UI-PMSI)
A Unidirectional Inclusive PMSI is one which enables a particular
PE, attached to a particular MVPN, to transmit a message such
that it will be received by all the other PEs attaching to that
MVPN. There is at most one UI-PMSI per PE per MVPN, though the
tunnel which instantiates a UI-PMSI may in fact carry the data of
more than one PMSI.
- "Selective" PMSI (S-PMSI).
A Selective PMSI is one which provides a mechanism wherein a
particular PE in an MVPN can multicast messages so that they will
be received by a subset of the other PEs of that MVPN. There may
be an arbitrary number of S-PMSIs per PE per MVPN. Again, the
tunnel which instantiates a given S-PMSI may carry data from
multiple S-PMSIs.
We will see in later sections the role played by these different
kinds of PMSI. We will use the term "I-PMSI" when we are not
distinguishing between "MI-PMSIs" and "UI-PMSIs".
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3.2.2. Tunnels Instantiating PMSIs
The tunnels which are used to instantiate PMSIs will be referred to
as "P-tunnels". A number of different tunnel setup techniques can be
used to create the P-tunnels that instantiate the PMSIs. Among these
are:
- PIM
A PMSI can be instantiated as (a set of) Multicast Distribution
Trees created by the PIM P-instance ("P-trees").
PIM-SSM, BIDIR-PIM, or PIM-SM can be used to create P-trees.
(PIM-DM is not supported for this purpose.)
A single MI-PMSI can be instantiated by a single shared P-tree,
or by a number of source P-trees (one for each PE of the MI-
PMSI). P-trees may be shared by multiple MVPNs (i.e., a given P-
tree may be the instantiation of multiple PMSIs), as long as the
encapsulation provides some means of demultiplexing the data
traffic by MVPN.
Selective PMSIs are instantiated by source P-trees, and are most
naturally created by PIM-SSM, since by definition only one PE is
the source of the multicast data on a Selective PMSI.
- MLDP
A PMSI may be instantiated as one or more mLDP Point-to-
Multipoint (P2MP) LSPs, or as an mLDP Multipoint-to-
MultiPoint(MP2MP) LSP. A Selective PMSI or a Unidirectional
Inclusive PMSI would be instantiated as a single mLDP P2MP LSP,
whereas a Multidirectional Inclusive PMSI could be instantiated
either as a set of such LSPs (one for each PE in the MVPN) or as
a single MP2MP LSP.
MLDP P2MP LSPs can be shared across multiple MVPNs.
- RSVP-TE
A PMSI may be instantiated as one or more RSVP-TE Point-to-
Multipoint (P2MP) LSPs. A Selective PMSI or a Unidirectional
Inclusive PMSI would be instantiated as a single RSVP-TE P2MP
LSP, whereas a Multidirectional Inclusive PMSI would be
instantiated as a set of such LSPs, one for each PE in the MVPN.
RSVP-TE P2MP LSPs can be shared across multiple MVPNs.
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- A Mesh of Unicast Tunnels.
If a PMSI is implemented as a mesh of unicast tunnels, a PE
wishing to transmit a packet through the PMSI would replicate the
packet, and send a copy to each of the other PEs.
An MI-PMSI for a given MVPN can be instantiated as a full mesh of
unicast tunnels among that MVPN's PEs. A UI-PMSI or an S-PMSI
can be instantiated as a partial mesh.
- Unicast Tunnels to the Root of a P-Tree.
Any type of PMSI can be instantiated through a method in which
there is a single P-tree (created, for example, via PIM-SSM or
via RSVP-TE), and a PE transmits a packet to the PMSI by sending
it in a unicast tunnel to the root of that P-tree. All PEs in
the given MVPN would need to be leaves of the tree.
When this instantiation method is used, the transmitter of the
multicast data may receive its own data back. Methods for
avoiding this are for further study.
It can be seen that each method of implementing PMSIs has its own
area of applicability. This specification therefore allows for the
use of any of these methods. At first glance, this may seem like an
overabundance of options. However, the history of multicast
development and deployment should make it clear that there is no one
option which is always acceptable. The use of segmented inter-AS
trees does allow each SP to select the option which it finds most
applicable in its own environment, without causing any other SP to
choose that same option.
Specifying the conditions under which a particular tree building
method is applicable is outside the scope of this document.
The choice of the tunnel technique belongs to the sender router and
is a local policy decision of the router. The procedures defined
throughout this document do not mandate that the same tunnel
technique be used for all PMSI tunnels going through a given provider
backbone. It is however expected that any tunnel technique that can
be used by a PE for a particular MVPN is also supported by other PE
having VRFs for the MVPN. Moreover, the use of ingress replication
by any PE for an MVPN, implies that all other PEs MUST use ingress
replication for this MVPN.
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3.3. Use of PMSIs for Carrying Multicast Data
Each PE supporting a particular MVPN must have a way of discovering:
- The set of other PEs in its AS that are attached to sites of that
MVPN, and the set of other ASes that have PEs attached to sites
of that MVPN. However, if segmented inter-AS trees are not used
(see section 8.2), then each PE needs to know the entire set of
PEs attached to sites of that MVPN.
- If segmented inter-AS trees are to be used, the set of border
routers in its AS that support inter-AS connectivity for that
MVPN
- If the MVPN is configured to use a MI-PMSI, the information
needed to set up and to use the tunnels instantiating the default
MI-PMSI,
- For each other PE, whether the PE supports Aggregate Trees for
the MVPN, and if so, the demultiplexing information which must be
provided so that the other PE can determine whether a packet
which it received on an aggregate tree belongs to this MVPN.
In some cases this information is provided by means of the BGP-based
auto-discovery procedures detailed in section 4. In other cases,
this information is provided after discovery is complete, by means of
procedures defined in section 6.1.2. In either case, the information
which is provided must be sufficient to enable the PMSI to be bound
to the identified tunnel, to enable the tunnel to be created if it
does not already exist, and to enable the different PMSIs which may
travel on the same tunnel to be properly demultiplexed.
3.3.1. MVPNs with MI-PMSIs
If an MVPN uses an MI-PMSI, then the MI-PMSI for that MVPN will be
created as soon as the necessary information has been obtained.
Creating a PMSI means creating the tunnel which carries it (unless
that tunnel already exists), as well as binding the PMSI to the
tunnel. The MI-PMSI for that MVPN is then used as the default method
of transmitting multicast data packets for that MVPN. In effect, all
the multicast streams for the MVPN are, by default, aggregated onto
the MI-MVPN.
If a particular multicast stream from a particular source PE has
certain characteristics, it can be desirable to migrate it from the
MI-PMSI to an S-PMSI. These characteristics and procedures for
migrating a stream from an MI-PMSI to an S-PMSI are discussed in
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section 7.
3.3.2. When MI-PMSIs are Required
MI-PMSIs are required under the following conditions:
- The MVPN is using PIM-DM, or some other protocol (such as BSR)
which relies upon flooding. Only with an MI-PMSI can the C-data
(or C-control-packets) received from any CE be flooded to all
PEs.
- If the procedure for carrying C-multicast routes from PE to PE
involves the multicasting of P-PIM control messages among the PEs
(see sections 3.4.1.1, 3.4.1.2, and 5.2).
3.3.3. MVPNs That Do Not Use MI-PMSIs
If a particular MVPN does not use a MI-PMSI, then its multicast data
may be sent on a set of UI-PMSIs.
It is also possible to send all the multicast data on a set of S-
PMSIs, omitting any usage of I-PMSIs. This prevents PEs from
receiving data which they don't need, at the cost of requiring
additional tunnels. However, cost-effective instantiation of S-PMSIs
is likely to require Aggregate P-trees, which in turn makes it
necessary for the transmitting PE to know which PEs need to receive
which multicast streams. This is known as "explicit tracking", and
the procedures to enable explicit tracking may themselves impose a
cost. This is further discussed in section 7.2.2.2.
3.4. PE-PE Transmission of C-Multicast Routing
As a PE attached to a given MVPN receives C-Join/Prune messages from
its CEs in that MVPN, it must convey the information contained in
those messages to other PEs that are attached to the same MVPN.
There are several different methods for doing this. As these methods
are not interoperable, the method to be used for a particular MVPN
must either be configured, or discovered as part of the auto-
discovery process.
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3.4.1. PIM Peering
3.4.1.1. Full Per-MVPN PIM Peering Across a MI-PMSI
If the set of PEs attached to a given MVPN are connected via a MI-
PMSI, the PEs can form "normal" PIM adjacencies with each other.
Since the MI-PMSI functions as a broadcast network, the standard PIM
procedures for forming and maintaining adjacencies over a LAN can be
applied.
As a result, the C-Join/Prune messages which a PE receives from a CE
can be multicast to all the other PEs of the MVPN. PIM "join
suppression" can be enabled and the PEs can send Asserts as needed.
This procedure is fully specified in section 5.2.
3.4.1.2. Lightweight PIM Peering Across a MI-PMSI
The procedure of the previous section has the following
disadvantages:
- Periodic Hello messages must be sent by all PEs.
Standard PIM procedures require that each PE in a particular MVPN
periodically multicast a Hello to all the other PEs in that MVPN.
If the number of MVPNs becomes very large, sending and receiving
these Hellos can become a substantial overhead for the PE
routers.
- Periodic retransmission of C-Join/Prune messages.
PIM is a "soft-state" protocol, in which reliability is assured
through frequent retransmissions (refresh) of control messages.
This too can begin to impose a large overhead on the PE routers
as the number of MVPNs grows.
The first of these disadvantages is easily remedied. The reason for
the periodic PIM Hellos is to ensure that each PIM speaker on a LAN
knows who all the other PIM speakers on the LAN are. However, in the
context of MVPN, PEs in a given MVPN can learn the identities of all
the other PEs in the MVPN by means of the BGP-based auto-discovery
procedure of section 4. In that case, the periodic Hellos would
serve no function, and could simply be eliminated. (Of course, this
does imply a change to the standard PIM procedures.)
When Hellos are suppressed, we may speak of "lightweight PIM
peering".
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The periodic refresh of the C-Join/Prunes is not as simple to
eliminate. If and when "refresh reduction" procedures are specified
for PIM, it may be useful to incorporate them, so as to make the
lightweight PIM peering procedures even more lightweight.
Lightweight PIM peering is not specified in this document.
3.4.1.3. Unicasting of PIM C-Join/Prune Messages
PIM does not require that the C-Join/Prune messages which a PE
receives from a CE to be multicast to all the other PEs; it allows
them to be unicast to a single PE, the one which is upstream on the
path to the root of the multicast tree mentioned in the Join/Prune
message. Note that when the C-Join/Prune messages are unicast, there
is no such thing as "join suppression". Therefore PIM Refresh
Reduction may be considered to be a pre-requisite for the procedure
of unicasting the C-Join/Prune messages.
When the C-Join/Prunes are unicast, they are not transmitted on a
PMSI at all. Note that the procedure of unicasting the C-Join/Prunes
is different than the procedure of transmitting the C-Join/Prunes on
an MI-PMSI which is instantiated as a mesh of unicast tunnels.
If there are multiple PEs that can be used to reach a given C-source,
procedures described in section 9 MUST be used to ensue that, at
least within a single AS, all PEs choose the same PE to reach the C-
source.
Procedures for unicasting the PIM control messages are not further
specified in this document.
3.4.2. Using BGP to Carry C-Multicast Routing
It is possible to use BGP to carry C-multicast routing information
from PE to PE, dispensing entirely with the transmission of C-
Join/Prune messages from PE to PE. This is specified in section 5.3.
Inter-AS procedures are described in section 8.
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4. BGP-Based Autodiscovery of MVPN Membership
BGP-based autodiscovery is done by means of a new address family, the
MCAST-VPN address family. (This address family also has other uses,
as will be seen later.) Any PE which attaches to an MVPN must issue
a BGP update message containing an NLRI in this address family, along
with a specific set of attributes. In this document, we specify the
information which must be contained in these BGP updates in order to
provide auto-discovery. The encoding details, along with the
complete set of detailed procedures, are specified in a separate
document [MVPN-BGP].
This section specifies the intra-AS BGP-based autodiscovery
procedures. When segmented inter-AS trees are used, additional
procedures are needed, as specified in section 8. Further detail may
be found in [MVPN-BGP]. (When segmented inter-AS trees are not used,
the inter-AS procedures are almost identical to the intra-AS
procedures.)
BGP-based autodiscovery uses a particular kind of MCAST-VPN route
known as an "auto-discovery routes", or "A-D route". In particular,
it uses two kinds of "A-D routes", the "Intra-AS A-D Route" and the
"Inter-AS A-D Route". (There are also additional kinds of A-D
routes, such as the Source Active A-D routes which are used for
purposes that go beyond auto-discovery. These are discussed in
subsequent sections.)
The Inter-AS A-D Route is used only when segmented inter-AS tunnels
are used, as specified in section 8.
The "Intra-AS A-D route" is originated by the PEs that are (directly)
connected to the site(s) of an MVPN. It is distributed to other PEs
that attach to sites of the MVPN. If segmented Inter-AS Tunnels are
used, then the Intra-AS A-D routes are not distributed outside the AS
where they originate; if segmented Inter-AS Tunnels are not used,
then the Intra-AS A-D routes are, despite their name, distributed to
all PEs attached to the VPN, no matter what AS the PEs are in.
The NLRI of an Intra-AS A-D route must contain the following
information:
- The route type (i.e., Intra-AS A-D route)
- The IP address of the originating PE
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- An RD configured locally for the MVPN. This is an RD which can
be prepended to that IP address to form a globally unique VPN-IP
address of the PE.
The A-D route must also carry the following attributes:
- One or more Route Target attributes. If any other PE has one of
these Route Targets configured for import into a VRF, it treats
the advertising PE as a member in the MVPN to which the VRF
belongs. This allows each PE to discover the PEs that belong to a
given MVPN. More specifically it allows a PE in the receiver
sites set to discover the PEs in the sender sites set of the MVPN
and the PEs in the sender sites set of the MVPN to discover the
PEs in the receiver sites set of the MVPN. The PEs in the
receiver sites set would be configured to import the Route
Targets advertised in the BGP Auto-Discovery routes by PEs in the
sender sites set. The PEs in the sender sites set would be
configured to import the Route Targets advertised in the BGP
Auto-Discovery routes by PEs in the receiver sites set.
- PMSI tunnel attribute. This attribute is present if and only if
either MI-PMSI is to be used for the MVPN, or UI-PMSI is to be
used for the MVPN on the PE that originates the intra-AS A-D
route. It contains the following information:
* whether the MI-PMSI is instantiated by
+ A BIDIR-PIM tree,
+ a set of PIM-SSM trees,
+ a set of PIM-SM trees
+ a set of RSVP-TE point-to-multipoint LSPs
+ a set of mLDP point-to-multipoint LSPs
+ an mLDP multipoint-to-multipoint LSP
+ a set of unicast tunnels
+ a set of unicast tunnels to the root of a shared tree (in
this case the root must be identified)
* If the PE wishes to setup a tunnel to instantiate the I-PMSI,
a unique identifier for the tunnel used to instantiate the I-
PMSI. This identifier depends on the tunnel technology used.
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All the PEs attaching to a given MVPN (within a given AS)
must have been configured with the same PMSI tunnel attribute
for that MVPN. They are also expected to know the
encapsulation to use.
Note that a tunnel can be identified at discovery time only
if the tunnel already exists (e.g., it was constructed by
means of configuration), or if it can be constructed without
each PE knowing the the identities of all the others. This is
obviously the case when the tunnel is constructed by a
receiver-initiated join technique such as PIM or mLDP. It is
also the case when the tunnel is an RSVP-TE P2MP LSP as the
tunnel identifier can be constructed without the head end
learning the identities of the other PEs.
In other cases, a tunnel cannot be identified until the PE
has discovered one or more of the other PEs. In these cases,
a PE will first send an A-D route without a tunnel
identifier, and then will send another one with a tunnel
identifier after discovering one or more of the other PEs.
All the PEs attaching to a given MVPN must be configured with
information specifying the encapsulation to use.
* Whether the tunnel used to instantiate the I-PMSI for this
MVPN is aggregating I-PMSIs from multiple MVPNs. This will
affect the encapsulation used. If aggregation is to be used,
a demultiplexor value to be carried by packets for this
particular MVPN must also be specified. The demultiplexing
mechanism and signaling procedures are described in section
6.
Further details of the use of this information are provided in
subsequent sections.
Sometimes it is necessary for one PE to advertise an upstream-
assigned MPLS label that identifies another PE. Under certain
circumstances to be discussed later, a PE which is the root of a
multicast P-tunnel will bind an MPLS label value to one or more
of the PEs that belong to the P-tunnel, and will distribute these
label bindings using A-D routes. The precise details of this
label distribution will be included in the next revision of this
document. We will refer to these as "PE Labels". A packet
traveling on the P-tunnel may carry one of these labels as an
indication that the PE corresponding to that label is special.
See section 11.3 for more details.
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5. PE-PE Transmission of C-Multicast Routing
As a PE attached to a given MVPN receives C-Join/Prune messages from
its CEs in that MVPN, it must convey the information contained in
those messages to other PEs that are attached to the same MVPN. This
is known as the "PE-PE transmission of C-multicast routing
information".
This section specifies the procedures used for PE-PE transmission of
C-multicast routing information. Not every procedure mentioned in
section 3.4 is specified here. Rather, this section focuses on two
particular procedures:
- Full PIM Peering.
This procedure is fully specified herein.
- Use of BGP to distribute C-multicast routing
This procedure is described herein, but the full specification
appears in [MVPN-BGP].
Those aspect of the procedures which apply to both of the above are
also specified fully herein.
Specification of other procedures is for future study.
5.1. Selecting the Upstream Multicast Hop (UMH)
When a PE receives a C-Join/Prune message from a CE, the message
identifies a particular multicast flow as belonging either to a
source tree (S,G) or to a shared tree (*,G). Throughout this
section, we use the term C-source to refer to S, in the case of a
source tree, or to the Rendezvous Point (RP) for G, in the case of
(*,G). If the route to the C-source is across the VPN backbone, then
the PE needs to find the "upstream multicast hop" (UMH) for the (S,G)
or (*,G) flow. The "upstream multicast hop" is either the PE at which
(S,G) or (*,G) data packets enter the VPN backbone, or else is the
Autonomous System Border Router (ASBR) at which those data packets
enter the local AS when traveling through the VPN backbone. The
process of finding the upstream multicast hop for a given C-source is
known as "upstream multicast hop selection".
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5.1.1. Eligible Routes for UMH Selection
In the simplest case, the PE does the upstream hop selection by
looking up the C-source in the unicast VRF associated with the PE-CE
interface over which the C-Join/Prune was received. The route that
matches the C-source will contain the information needed to select
the upstream multicast hop.
However, in some cases, the CEs may be distributing to the PEs a
special set of routes that are to be used exclusively for the purpose
of upstream multicast hop selection, and not used for unicast routing
at all. For example, when BGP is the CE-PE unicast routing protocol,
the CEs may be using SAFI 2 to distribute a special set of routes
that are to be used for, and only for, upstream multicast hop
selection. When OSPF is the CE-PE routing protocol, the CE may use
an MT-ID of 1 to distribute a special set of routes that are to be
used for, and only for, upstream multicast hop selection . When a CE
uses one of these mechanisms to distribute to a PE a special set of
routes to be used exclusively for upstream multicast hop selection,
these routes are distributed among the PEs using SAFI 129, as
described in [MVPN-BGP].
Whether the routes used for upstream multicast hop selection are (a)
the "ordinary" unicast routes or (b) a special set of routes that are
used exclusively for upstream multicast hop selection, is a matter of
policy. How that policy is chosen, deployed, or implemented is
outside the scope of this document. In the following, we will simply
refer to the set of routes that are used for upstream multicast hop
selection, the "Eligible UMH routes", with no presumptions about the
policy by which this set of routes was chosen.
5.1.2. Information Carried by Eligible UMH Routes
Every route which is eligible for UMH selection MUST carry a VRF
Route Import Extended Community [MVPN-BGP]. This attribute
identifies the PE that originated the route.
If BGP is used for carrying C-multicast routes, OR if "Segmented
Inter-AS Tunnels" (see section 8.2) are used, then every UMH route
MUST also carry a Source AS Extended Community [MVPN-BGP].
These two attributes are used in the upstream multicast hop selection
procedures described below.
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5.1.3. Selecting the Upstream PE
The first step in selecting the upstream multicast hop for a given C-
source is to select the upstream PE router for that C-source.
The PE that received the C-Join message from a CE looks in the VRF
corresponding to the interfaces over which the C-Join was received.
It finds the Eligible UMH route which is the best match for the C-
source specified in that C-Join. Call this the "Installed UMH
Route".
Note that the outgoing interface of the Installed UMH Route may be
one of the interfaces associated with the VRF, in which case the
upstream multicast hop is a CE and the route to the C-source is not
across the VPN backbone.
Consider the set of all VPN-IP routes that are: (a) eligible to be
imported into the VRF (as determined by their Route Targets), (b) are
eligible to be used for upstream multicast hop selection, and (c)
have exactly the same IP prefix (not necessarily the same RD) as the
installed UMH route.
For each route in this set, determine the corresponding upstream PE
and upstream RD. If a route has a VRF Route Import Extended
Community, the route's upstream PE is determined from it. If a route
does not have a VRF Route Import Extended Community, the route's
upstream PE is determined from the route's BGP next hop attribute.
In either case, the upstream RD is taken from the route's NLRI.
This results in a set of pairs of <route, upstream PE, upstream RD>.
Call this the "UMH Route Candidate Set." Then the PE MUST select a
single route from the set to be the "Selected UMH Route". The
corresponding upstream PE is known as the "Selected Upstream PE", and
the corresponding upstream RD is known as the "Selected Upstream RD".
There are several possible procedures that can be used by a PE to
select a single route from the candidate set.
The default procedure, which MUST be implemented, is to select the
route whose corresponding upstream PE address is numerically highest,
where a 32-bit IP address is treated as a 32 bit unsigned integer.
Call this the "default upstream PE selection". For a given C-source,
provided that the routing information used to create the candidate
set is stable, all PEs will have the same default upstream PE
selection. (Though different default upstream PE selections may be
chosen during a routing transient.)
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An alternative procedure which MUST be implemented, but which is
disabled by default, is the following. This procedure ensures that,
except during a routing transient, each PE chooses the same upstream
PE for a given combination of C-source and C-G.
1. The PEs in the candidate set are numbered from lower to higher
IP address, starting from 0.
2. The following hash is performed:
- A bytewise exclusive-or of all the bytes in the C-source
address and the C-G address is performed.
- The result is taken modulo n, where n is the number of PEs
in the candidate set. Call this result N.
The selected upstream PE is then the one that appears in position N
in the list of step 1.
Other hashing algorithms are allowed as well, but not required.
The alternative procedure allows a form of "equal cost load
balancing". Suppose, for example, that from egress PEs PE3 and PE4,
source C-S can be reached, at equal cost, via ingress PE PE1 or
ingress PE PE2. The load balancing procedure makes it possible for
PE1 to be the ingress PE for (C-S, C-G1) data traffic while PE2 is
the ingress PE for (C-S, C-G2) data traffic.
Another procedure, which SHOULD be implemented, is to use the
Installed UMH Route as the Selected UMH Route. If this procedure is
used, the result is likely to be that a given PE will choose the
upstream PE that is closest to it, according to the routing in the SP
backbone. As a result, for a given C-source, different PEs may
choose different upstream PEs. This is useful if the C-source is an
anycast address, and can also be useful if the C-source is in a
multihomed site (i.e., a site that is attached to multiple PEs).
However, this procedure is more likely to lead to steady state
duplication of traffic unless (a) PEs discard data traffic which
arrives from the "wrong" upstream PE, or (b) data traffic is carried
only in non-aggregated S-PMSIs . This issue is discussed at length
in section 9.
General policy-based procedures for selecting the UMH route are
allowed, but not required and are not further discussed in this
specification.
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5.1.4. Selecting the Upstream Multicast Hop
In certain cases, the selected upstream multicast hop is the same as
the selected upstream PE. In other cases, the selected upstream
multicast hop is the ASBR which is the "BGP next hop" of the Selected
UMH Route.
If the selected upstream PE is in the local AS, then the selected
upstream PE is also the selected upstream multicast hop. This is the
case if any of the following conditions holds:
- The selected UMH route has a Source AS Extended Community, and
the Source AS is the same as the local AS,
- The selected UMH route does not have a Source AS Extended
Community, but the route's BGP next hop is the same as the
upstream PE.
Otherwise, the selected upstream multicast hop is an ASBR. The
method of determining just which ASBR it is depends on the particular
inter-AS signaling method being used (PIM or BGP), and on whether
segmented or non-segmented inter-AS tunnels are used. These details
are presented in later sections.
5.2. Details of Per-MVPN Full PIM Peering over MI-PMSI
In this section, we assume that inter-AS MVPNs will be supported by
means of non-segmented inter-AS trees. Support for segmented inter-
AS trees with PIM peering is for further study.
When an MVPN uses an MI-PMSI, the C-instances of that MVPN can treat
the MI-PMSI as a LAN interface, and form either full PIM adjacencies
with each other over that "LAN interface".
To form a full PIM adjacency, the PEs execute the PIM LAN procedures,
including the generation and processing of PIM Hello, Join/Prune,
Assert, DF election and other PIM control packets. These are
executed independently for each C-instance. PIM "join suppression"
SHOULD be enabled.
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5.2.1. PIM C-Instance Control Packets
All PIM C-Instance control packets of a particular MVPN are addressed
to the ALL-PIM-ROUTERS (224.0.0.13) IP destination address, and
transmitted over the MI-PMSI of that MVPN. While in transit in the
P-network, the packets are encapsulated as required for the
particular kind of tunnel that is being used to instantiate the MI-
PMSI. Thus the C-instance control packets are not processed by the P
routers, and MVPN-specific PIM routes can be extended from site to
site without appearing in the P routers.
As specified in section 5.1.2, when a PE distributes VPN-IP routes
which are eligible for use as UMH routes, the PE MUST include a VRF
Route Import Extended Community with each route. For a given MVPN, a
single such IP address MUST be used, and that same IP address MUST be
used as the source address in all PIM control packets for that MVPN.
5.2.2. PIM C-instance RPF Determination
Although the MI-PMSI is treated by PIM as a LAN interface, unicast
routing is NOT run over it, and there are no unicast routing
adjacencies over it. It is therefore necessary to specify special
procedures for determining when the MI-PMSI is to be regarded as the
"RPF Interface" for a particular C-address.
The PE follows the procedures of section 5.1 to determine the
selected UMH route. If that route is NOT a VPN-IP route learned from
BGP as described in [RFC4364], or if that route's outgoing interface
is one of the interfaces associated with the VRF, then ordinary PIM
procedures for determining the RPF interface apply.
However, if the selected UMH route is a VPN-IP route whose outgoing
interface is not one of the interfaces associated with the VRF, then
PIM will consider the RPF interface to be the MI-PMSI associated with
the VPN-specific PIM instance.
Once PIM has determined that the RPF interface for a particular C-
source is the MI-PMSI, it is necessary for PIM to determine the "RPF
neighbor" for that C-source. This will be one of the other PEs that
is a PIM adjacency over the MI-PMSI. In particular, it will be the
"selected upstream PE" as defined in section 5.1.
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5.2.3. Backwards Compatibility
There are older implementations which do not use the VRF Route Import
Extended Community or any explicit mechanism for carrying information
to identify the originating PE of a selected UMH route.
For backwards compatibility, when the selected UMH route does not
have any such mechanism, the IP address from the "BGP Next Hop" field
of the selected UMH route will be used as the selected UMH address,
and will be treated as the address of the upstream PE. There is no
selected upstream RD in this case. However, use of this backwards
compatibility technique presupposes that:
- The PE which originated the selected UMH route placed the same IP
address in the BGP Next Hop field that it is using as the source
address of the PE-PE PIM control packets for this MVPN.
- The MVPN is not an Inter-AS MVPN that uses option b from section
10 of [RFC4364].
Should either of these conditions fail, interoperability with the
older implementations will not be achieved.
5.3. Use of BGP for Carrying C-Multicast Routing
It is possible to use BGP to carry C-multicast routing information
from PE to PE, dispensing entirely with the transmission of C-
Join/Prune messages from PE to PE. This section describes the
procedures for carrying intra-AS multicast routing information.
Inter-AS procedures are described in section 8. The complete
specification of both sets of procedures and of the encodings can be
found in [MVPN-BGP].
5.3.1. Sending BGP Updates
The MCAST-VPN address family is used for this purpose. MCAST-VPN
routes used for the purpose of carrying C-multicast routing
information are distinguished from those used for the purpose of
carrying auto-discovery information by means of a "route type" field
which is encoded into the NLRI. The following information is
required in BGP to advertise the MVPN routing information. The NLRI
contains:
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- The type of C-multicast route.
There are two types:
* source tree join
* shared tree join
- The RD configured, for the MVPN, on the PE that is advertising
the information. The RD is required in order to uniquely
identify the <C-Source, C-Group> when different MVPNs have
overlapping address spaces.
- The C-Group address.
- The C-Source address.
This field is omitted if the route type is "shared tree join".
In the case of a shared tree join, the C-source is a C-RP. The
address of the C-RP corresponding to the C-group address is
presumed to be already known (or automatically determinable) be
the other PEs, though means that are outside the scope of this
specification.
- The Selected Upstream RD corresponding to the C-source address
(determined by the procedures of section 5.1).
Whenever a C-multicast route is sent, it must also carry the Selected
Upstream Multicast Hop corresponding to the C-source address
(determined by the procedures of section 5.1). The selected upstream
multicast hop must be encoded as part of a Route Target Extended
Community, to facilitate the optional use of filters which can
prevent the distribution of the update to BGP speakers other than the
upstream multicast hop. See section 10.1.3 of [MVPN-BGP] for the
details.
There is no C-multicast route corresponding to the PIM function of
pruning a source off the shared tree when a PE switches from a <C-*,
C-G> tree to a <C-S, C-G> tree. Section 9 of this document specifies
a mandatory procedure that ensures that if any PE joins a <C-S, C-G>
source tree, all other PEs that have joined or will join the <C-*, C-
G> shared tree will also join the <C-S, C-G> source tree. This
eliminates the need for a C-multicast route that prunes C-S off the
<C-*, C-G> shared tree when switching from <C-*, C-G> to <C-S, C-G>
tree.
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5.3.2. Explicit Tracking
Note that the upstream multicast hop is NOT part of the NLRI in the
C-multicast BGP routes. This means that if several PEs join the same
C-tree, the BGP routes they distribute to do so are regarded by BGP
as comparable routes, and only one will be installed. If a route
reflector is being used, this further means that the PE which is used
to reach the C-source will know only that one or more of the other
PEs have joined the tree, but it won't know which one. That is, this
BGP update mechanism does not provide "explicit tracking". Explicit
tracking is not provided by default because it increases the amount
of state needed and thus decreases scalability. Also, as
constructing the C-PIM messages to send "upstream" for a given tree
does not depend on knowing all the PEs that are downstream on that
tree, there is no reason for the C-multicast route type updates to
provide explicit tracking.
There are some cases in which explicit tracking is necessary in order
for the PEs to set up certain kinds of P-trees. There are other
cases in which explicit tracking is desirable in order to determine
how to optimally aggregate multicast flows onto a given aggregate
tree. As these functions have to do with the setting up of
infrastructure in the P-network, rather than with the dissemination
of C-multicast routing information, any explicit tracking that is
necessary is handled by sending the "source active" A-D routes, that
are described in sections 9 and 10. Detailed procedures for turning
on explicit tracking can be found in [MVPN-BGP].
5.3.3. Withdrawing BGP Updates
A PE removes itself from a C-multicast tree (shared or source) by
withdrawing the corresponding BGP update.
If a PE has pruned a C-source from a shared C-multicast tree, and it
needs to "unprune" that source from that tree, it does so by
withdrawing the route that pruned the source from the tree.
6. I-PMSI Instantiation
This section describes how tunnels in the SP network can be used to
instantiate an I-PMSI for an MVPN on a PE. When C-multicast data is
delivered on an I-PMSI, the data will go to all PEs that are on the
path to receivers for that C-group, but may also go to PEs that are
not on the path to receivers for that C-group.
The tunnels which instantiate I-PMSIs can be either PE-PE unicast
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tunnels or P-multicast trees. When PE-PE unicast tunnels are used the
PMSI is said to be instantiated using ingress replication. The
instantiation of a tunnel for an I-PMSI is a matter of local policy
decision and is not mandatory. Even for a site attached to multicast
sources, transport of customer multicast traffic can be accommodated
with S-PMSI-bound tunnels only
6.1. MVPN Membership and Egress PE Auto-Discovery
As described in section 4 a PE discovers the MVPN membership
information of other PEs using BGP auto-discovery mechanisms or using
a mechanism that instantiates a MI-PMSI interface. When a PE supports
only a UI-PMSI service for an MVPN, it MUST rely on the BGP auto-
discovery mechanisms for discovering this information. This
information also results in a PE in the sender sites set discovering
the leaves of the P-multicast tree, which are the egress PEs that
have sites in the receiver sites set in one or more MVPNs mapped onto
the tree.
6.1.1. Auto-Discovery for Ingress Replication
In order for a PE to use Unicast Tunnels to send a C-multicast data
packet for a particular MVPN to a set of remote PEs, the remote PEs
must be able to correctly decapsulate such packets and to assign each
one to the proper MVPN. This requires that the encapsulation used for
sending packets through the tunnel have demultiplexing information
which the receiver can associate with a particular MVPN.
If ingress replication is being used for an MVPN, the PEs announce
this as part of the BGP based MVPN membership auto-discovery process,
described in section 4. The PMSI tunnel attribute specifies ingress
replication. The demultiplexor value is a downstream-assigned MPLS
label (i.e., assigned by the PE that originated the A-D route, to be
used by other PEs when they send multicast packets on a unicast
tunnel to that PE).
Other demultiplexing procedures for unicast are under consideration.
6.1.2. Auto-Discovery for P-Multicast Trees
A PE announces the P-multicast technology it supports for a specified
MVPN, as part of the BGP MVPN membership discovery. This allows other
PEs to determine the P-multicast technology they can use for building
P-multicast trees to instantiate an I-PMSI. If a PE has a tree
instantiation of an I-PMSI, it also announces the tree identifier as
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part of the auto-discovery, as well as announcing its aggregation
capability.
The announcement of a tree identifier at discovery time is only
possible if the tree already exists (e.g., a preconfigured "traffic
engineered" tunnel), or if the tree can be constructed dynamically
without any PE having to know in advance all the other PEs on the
tree (e.g., the tree is created by receiver-initiated joins).
6.2. C-Multicast Routing Information Exchange
When a PE doesn't support the use of a MI-PMSI for a given MVPN, it
MUST either unicast MVPN routing information using PIM or else use
BGP for exchanging the MVPN routing information.
6.3. Aggregation
A P-multicast tree can be used to instantiate a PMSI service for only
one MVPN or for more than one MVPN. When a P-multicast tree is shared
across multiple MVPNs it is termed an "Aggregate Tree". The
procedures described in this document allow a single SP multicast
tree to be shared across multiple MVPNs. The procedures that are
specific to aggregation are optional and are explicitly pointed out.
Unless otherwise specified a P-multicast tree technology supports
aggregation.
Aggregate Trees allow a single P-multicast tree to be used across
multiple MVPNs and hence state in the SP core grows per-set-of-MVPNs
and not per MVPN. Depending on the congruence of the aggregated
MVPNs, this may result in trading off optimality of multicast
routing.
An Aggregate Tree can be used by a PE to provide an UI-PMSI or MI-
PMSI service for more than one MVPN. When this is the case the
Aggregate Tree is said to have an inclusive mapping.
6.3.1. Aggregate Tree Leaf Discovery
BGP MVPN membership discovery allows a PE to determine the different
Aggregate Trees that it should create and the MVPNs that should be
mapped onto each such tree. The leaves of an Aggregate Tree are
determined by the PEs, supporting aggregation, that belong to all the
MVPNs that are mapped onto the tree.
If an Aggregate Tree is used to instantiate one or more S-PMSIs, then
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it may be desirable for the PE at the root of the tree to know which
PEs (in its MVPN) are receivers on that tree. This enables the PE to
decide when to aggregate two S-PMSIs, based on congruence (as
discussed in the next section). Thus explicit tracking may be
required. Since the procedures for disseminating C-multicast routes
do not provide explicit tracking, a type of A-D route known as a
"Leaf A-D Route" is used. The PE which wants to assign a particular
C-multicast flow to a particular Aggregate Tree can send an A-D route
which elicits Leaf A-D routes from the PEs that need to receive that
C-multicast flow. This provides the explicit tracking information
needed to support the aggregation methodology discussed in the next
section. For more details on Leaf A-D routes please refer to [MVPN-
BGP].
6.3.2. Aggregation Methodology
This document does not specify the mandatory implementation of any
particular set of rules for determining whether or not the PMSIs of
two particular MVPNs are to be instantiated by the same Aggregate
Tree. This determination can be made by implementation-specific
heuristics, by configuration, or even perhaps by the use of offline
tools.
It is the intention of this document that the control procedures will
always result in all the PEs of an MVPN to agree on the PMSIs which
are to be used and on the tunnels used to instantiate those PMSIs.
This section discusses potential methodologies with respect to
aggregation.
The "congruence" of aggregation is defined by the amount of overlap
in the leaves of the customer trees that are aggregated on a SP tree.
For Aggregate Trees with an inclusive mapping the congruence depends
on the overlap in the membership of the MVPNs that are aggregated on
the tree. If there is complete overlap i.e. all MVPNs have exactly
the same sites, aggregation is perfectly congruent. As the overlap
between the MVPNs that are aggregated reduces, i.e. the number of
sites that are common across all the MVPNs reduces, the congruence
reduces.
If aggregation is done such that it is not perfectly congruent a PE
may receive traffic for MVPNs to which it doesn't belong. As the
amount of multicast traffic in these unwanted MVPNs increases
aggregation becomes less optimal with respect to delivered traffic.
Hence there is a tradeoff between reducing state and delivering
unwanted traffic.
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An implementation should provide knobs to control the congruence of
aggregation. These knobs are implementation dependent. Configuring
the percentage of sites that MVPNs must have in common to be
aggregated, is an example of such a knob. This will allow a SP to
deploy aggregation depending on the MVPN membership and traffic
profiles in its network. If different PEs or servers are setting up
Aggregate Trees this will also allow a service provider to engineer
the maximum amount of unwanted MVPNs hat a particular PE may receive
traffic for.
6.3.3. Encapsulation of the Aggregate Tree
An Aggregate Tree may use an IP/GRE encapsulation or an 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 need
further explanation. This will be specified in a separate document.
6.3.4. Demultiplexing C-multicast traffic
When multiple MVPNs are aggregated onto one P-Multicast tree,
determining the tree over which the packet is received is not
sufficient to determine the MVPN to which the packet belongs. The
packet must also carry some demultiplexing information to allow the
egress PEs to determine the MVPN to which the packet belongs. Since
the packet has been multicast through the P network, any given
demultiplexing value must have the same meaning to all the egress
PEs. The demultiplexing value is a MPLS label that corresponds to
the multicast VRF to which the packet belongs. This label is placed
by the ingress PE immediately beneath the P-Multicast tree header.
Each of the egress PEs must be able to associate this MPLS label with
the same MVPN. If downstream label assignment were used this would
require all the egress PEs in the MVPN to agree on a common label for
the MVPN. Instead the MPLS label is upstream assigned [MPLS-UPSTREAM-
LABEL]. The label bindings are advertised via BGP updates originated
the ingress PEs.
This procedure requires each egress PE to support a separate label
space for every other PE. The egress PEs create a forwarding entry
for the upstream assigned MPLS label, allocated by the ingress PE, in
this label space. Hence when the egress PE receives a packet over an
Aggregate Tree, it first determines the tree that the packet was
received over. The tree identifier determines the label space in
which the upstream assigned MPLS label lookup has to be performed.
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.
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The support of aggregation for shared trees and MP2MP trees is
discussed in section 6.6.
The encapsulation format is either MPLS or MPLS-in-something (e.g.
MPLS-in-GRE [MPLS-IP]). When MPLS is used, this label will appear
immediately below the label that identifies the P-multicast tree.
When MPLS-in-GRE is used, this label will be the top MPLS label that
appears when the GRE header is stripped off.
When IP encapsulation is used for the P-multicast Tree, whatever
information that particular encapsulation format uses for identifying
a particular tunnel is used to determine the label space in which the
MPLS label is looked up.
If the P-multicast tree uses MPLS encapsulation, the P-multicast tree
is itself identified by an MPLS label. The egress PE MUST NOT
advertise IMPLICIT NULL or EXPLICIT NULL for that tree. Once the
label representing the tree is popped off the MPLS label stack, the
next label is the demultiplexing information that allows the proper
MVPN to be determined.
This specification requires that, to support this sort of
aggregation, there be at least one upstream-assigned label per MVPN.
It does not require that there be only one. For example, an ingress
PE could assign a unique label to each C-(S,G). (This could be done
using the same technique this is used to assign a particular C-(S,G)
to an S-PMSI, see section 7.3.)
6.4. Mapping Received Packets to MVPNs
When an egress PE receives a C-multicast data packet over a P-
multicast tree, it needs to forward the packet to the CEs that have
receivers in the packet's C-multicast group. In order to do this the
egress PE needs to determine the tunnel that the packet was received
on. The PE can then determine the MVPN that the packet belongs to and
if needed do any further lookups that are needed to forward the
packet.
6.4.1. Unicast Tunnels
When ingress replication is used, the MVPN to which the received C-
multicast data packet belongs can be determined by the MPLS label
that was allocated by the egress. This label is distributed by the
egress.
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6.4.2. Non-Aggregated P-Multicast Trees
If a P-multicast tree is associated with only one MVPN, determining
the P-multicast tree on which a packet was received is sufficient to
determine the packet's MVPN. All that the egress PE needs to know is
the MVPN the P-multicast tree is associated with.
There are different ways in which the egress PE can learn this
association:
a) Configuration. The P-multicast tree that a particular MVPN
belongs to is configured on each PE.
b) BGP based advertisement of the P-multicast tree - MPVN mapping
after the root of the tree discovers the leaves of the tree.
The root of the tree sets up the tree after discovering each of
the PEs that belong to the MVPN. It then advertises the P-
multicast tree - MVPN mapping to each of the leaves. This
mechanism can be used with both source initiated trees [e.g.
RSVP-TE P2MP LSPs] and receiver initiated trees [e.g. PIM
trees].
c) BGP based advertisement of the P-multicast tree - MVPN mapping
as part of the MVPN membership discovery. The root of the tree
advertises, to each of the other PEs that belong to the MVPN,
the P-multicast tree that the MVPN is associated with. This
implies that the root doesn't need to know the leaves of the
tree beforehand. This is possible only for receiver initiated
trees e.g. PIM based trees.
Both of the above require the BGP based advertisement to contain the
P-multicast tree identifier. This identifier is encoded as a BGP
attribute and contains the following elements:
- Tunnel Type.
- Tunnel identifier. The semantics of the identifier is determined
by the tunnel type.
6.4.3. Aggregate P-Multicast Trees
Once a PE sets up an Aggregate Tree it needs to announce the C-
multicast groups being mapped to this tree to other PEs in the
network. This procedure is referred to as Aggregate Tree discovery.
For an Aggregate Tree with an inclusive mapping this discovery
implies announcing:
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- The mapping of all MVPNs mapped to the Tree.
- For each MVPN mapped onto the tree the inner label allocated for
it by the ingress PE. The use of this label is explained in the
demultiplexing procedures of section 6.3.4.
- The P-multicast tree Identifier
The egress PE creates a logical interface corresponding to the tree
identifier. This interface is the RPF interface for all the <C-
Source, C-Group> entries mapped to that tree.
When PIM is used to setup P-multicast trees, the egress PE also Joins
the P-Group Address corresponding to the tree. This results in setup
of the PIM P-multicast tree.
6.5. I-PMSI Instantiation Using Ingress Replication
As described in section 3 a PMSI can be instantiated using Unicast
Tunnels between the PEs that are participating in the MVPN. In this
mechanism the ingress PE replicates a C-multicast data packet
belonging to a particular MVPN and sends a copy to all or a subset of
the PEs that belong to the MVPN. A copy of the packet is tunneled to
a remote PE over an Unicast Tunnel to the remote PE. IP/GRE Tunnels
or MPLS LSPs are examples of unicast tunnels that may be used. Note
that the same Unicast Tunnel can be used to transport packets
belonging to different MVPNs.
Ingress replication can be used to instantiate a UI-PMSI. The PE sets
up unicast tunnels to each of the remote PEs that support ingress
replication. For a given MVPN all C-multicast data packets are sent
to each of the remote PEs in the MVPN that support ingress
replication. Hence a remote PE may receive C-multicast data packets
for a group even if it doesn't have any receivers in that group.
Ingress replication can also be used to instantiate a MI-PMSI. In
this case each PE has a mesh of unicast tunnels to every other PE in
that MVPN.
However when ingress replication is used it is recommended that only
S-PMSIs be used. Instantiation of S-PMSIs with ingress replication is
described in section 7.1. Note that this requires the use of
explicit tracking, i.e., a PE must know which of the other PEs have
receivers for each C-multicast tree.
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6.6. Establishing P-Multicast Trees
It is believed that the architecture outlined in this document places
no limitations on the protocols used to instantiate P-multicast
trees. However, the only protocols being explicitly considered are
PIM-SM, PIM-SSM, BIDIR-PIM, RSVP-TE, and mLDP.
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 multicast VRFs that
exist locally on the root of the tree i.e. for which the root has
local CEs. The root is a PE router. Source P-multicast trees can be
instantiated using PIM-SM, PIM-SSM, RSVP-TE P2MP LSPs, and mLDP P2MP
LSPs.
A shared tree on the other hand can be used to carry traffic
belonging to VRFs that exist on other PEs as well. The root of a
shared tree is not necessarily one of the PEs in the MVPN. All PEs
that use the shared tree will send MVPN data packets to the root of
the shared tree; if PIM is being used as the control protocol, PIM
control packets also get sent to the root of the shared tree. This
may require an unicast tunnel between each of these PEs and the root.
The root will then send them on the shared tree and all the PEs that
are leaves of the shared tree will receive the packets. For example a
RP based PIM-SM tree would be a shared tree. Shared trees can be
instantiated using PIM-SM, PIM-SSM, BIDIR-PIM, RSVP-TE P2MP LSPs,
mLDP P2MP LSPs, and mLDP MP2MP LSPs.. Aggregation support for
bidirectional P-trees (i.e., BIDIR-PIM trees or mLDP MP2MP trees) is
for further study. Shared trees require all the PEs to discover the
root of the shared tree for a MVPN. To achieve this the root of a
shared tree advertises as part of the BGP based MVPN membership
discovery:
- The capability to setup a shared tree for a specified MVPN.
- A downstream assigned label that is to be used by each PE to
encapsulate a MVPN data packet, when they send this packet to the
root of the shared tree.
- A downstream assigned label that is to be used by each PE to
encapsulate a MVPN control packet, when they send this packet to
the root of the shared tree.
Both a source tree and a shared tree can be used to instantiate an I-
PMSI. If a source tree is used to instantiate an UI-PMSI for a MVPN,
all the other PEs that belong to the MVPN, must be leaves of the
source tree. If a shared tree is used to instantiate a UI-PMSI for a
MVPN, all the PEs that are members of the MVPN must be leaves of the
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shared tree.
6.7. RSVP-TE P2MP LSPs
This section describes procedures that are specific to the usage of
RSVP-TE P2MP LSPs for instantiating a UI-PMSI. The RSVP-TE P2MP LSP
can be either a source tree or a shared tree. Procedures in [RSVP-
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 MVPN membership procedures described in section 4. RSVP-TE P2MP
LSPs can optionally support aggregation.
6.7.1. P2MP TE LSP Tunnel - MVPN Mapping
P2MP TE LSP Tunnel to MVPN mapping can be learned at the egress PEs
using either option (a) or option (b) described in section 6.4.2.
Option (b) i.e. BGP based advertisements of the P2MP TE LSP Tunnel -
MPVN mapping require that the root of the tree include the P2MP TE
LSP Tunnel 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 Tunnel
- RSVP-TE P2MP Tunnel's SESSION Object
- Optionally RSVP-TE P2MP LSP's SENDER_TEMPLATE Object. This object
is included when it is desired to identify a particular P2MP TE
LSP.
6.7.2. Demultiplexing C-Multicast Data Packets
Demultiplexing the C-multicast data packets at the egress PE follow
procedures described in section 6.3.4. The RSVP-TE P2MP LSP Tunnel
must be signaled with penultimate-hop-popping (PHP) off. Signaling
the P2MP TE LSP Tunnel with PHP off requires an extension to RSVP-TE
which will be described later.
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7. Optimizing Multicast Distribution via S-PMSIs
Whenever a particular multicast stream is being sent on an I-PMSI, it
is likely that the data of that stream is being sent to PEs that do
not require it. If a particular stream has a significant amount of
traffic, it may be beneficial to move it to an S-PMSI which includes
only those PEs that are transmitters and/or receivers (or at least
includes fewer PEs that are neither).
If explicit tracking is being done, S-PMSI creation can also be
triggered on other criteria. For instance there could be a "pseudo
wasted bandwidth" criteria: switching to an S-PMSI would be done if
the bandwidth multiplied by the number of uninterested PEs (PE 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 an S-PMSI if all
the PEs have receivers for it.
Switching a (C-S, C-G) stream to an S-PMSI may require the root of
the S-PMSI 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 source initiated tree, such as a RSVP-TE P2MP
Tunnel, the PE needs to know the leaves of the tree before it can
instantiate the S-PMSI.
- If a PE instantiates multiple S-PMSIs, belonging to different
MVPNs, using one P-multicast 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.
The above two cases require that explicit tracking be done for the
(C-S, C-G) stream. The root of the S-PMSI MAY decide to do explicit
tracking of this stream only after it has determined to move the
stream to an S-PMSI, or it MAY have been doing explicit tracking all
along.
If the S-PMSI is instantiated by a P-multicast tree, 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 to the S-PMSI. Note that the PE could
create the identity of the P-multicast tree prior to the actual
instantiation of the tunnel.
If the S-PMSI is instantiated by a source-initiated P-multicast tree
(e.g., an RSVP-TE P2MP tunnel), the PE at the root of the tree must
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establish the source-initiated P-multicast tree to the leaves. This
tree MAY have been established before the leaves receive the S-PMSI
binding, or MAY be established after the leaves receives the binding.
The leaves MUST NOT switch to the S-PMSI until they receive both the
binding and the tree signaling message.
7.1. S-PMSI Instantiation Using Ingress Replication
As described in section 6.1.1, ingress replication can be used to
instantiate a UI-PMSI. However this can result in a PE receiving
packets for a multicast group for which it doesn't have any
receivers. This can be avoided if the ingress PE tracks the remote
PEs which have receivers in a particular C-multicast group. In order
to do this it needs to receive C-Joins from each of the remote PEs.
It then replicates the C-multicast data packet and sends it to only
those egress PEs which are on the path to a receiver of that C-group.
It is possible that each PE that is using ingress replication
instantiates only S-PMSIs. It is also possible that some PEs
instantiate UI-PMSIs while others instantiate only S-PMSIs. In both
these cases the PE MUST either unicast MVPN routing information using
PIM or use BGP for exchanging the MVPN routing information. This is
because there may be no MI-PMSI available for it to exchange MVPN
routing information.
Note that the use of ingress replication doesn't require any extra
procedures for signaling the binding of the S-PMSI from the ingress
PE to the egress PEs. The procedures described for I-PMSIs are
sufficient.
7.2. Protocol for Switching to S-PMSIs
We describe two protocols for switching to S-PMSIs. These protocols
can be used when the tunnel that instantiates the S-PMSI is a P-
multicast tree.
7.2.1. A UDP-based Protocol for Switching to S-PMSIs
This procedure can be used for any MVPN which has an MI-PMSI.
Traffic from all multicast streams in a given MPVN is sent, by
default, on the MI-PMSI. Consider a single multicast stream within a
given MVPN, and consider a PE which is attached to a source of
multicast traffic for that stream. The PE can be configured to move
the stream from the MI-PMSI to an S-PMSI if certain configurable
conditions are met. To do this, it needs to inform all the PEs which
attach to receivers for stream. These PEs need to start listening
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for traffic on the S-PMSI, and the transmitting PE may start sending
traffic on the S-PMSI when it is reasonably certain that all
receiving PEs are listening on the S-PMSI.
7.2.1.1. Binding a Stream to an S-PMSI
When a PE which attaches to a transmitter for a particular multicast
stream notices that the conditions for moving the stream to an S-PMSI
are met, it begins to periodically send an "S-PMSI Join Message" on
the MI-PMSI. The S-PMSI Join is a UDP-encapsulated message whose
destination address is ALL-PIM-ROUTERS (224.0.0.13), and whose
destination port is 3232.
The S-PMSI Join Message contains the following information:
- An identifier for the particular multicast stream which is to be
bound to the S-PMSI. This can be represented as an (S,G) pair.
- An identifier for the particular S-PMSI to which the stream is to
be bound. This identifier is a structured field which includes
the following information:
* The type of tunnel used to instantiate the S-PMSI
* An identifier for the tunnel. The form of the identifier
will depend upon the tunnel type. The combination of tunnel
identifier and tunnel type should contain enough information
to enable all the PEs to "join" the tunnel and receive
messages from it.
* Any demultiplexing information needed by the tunnel
encapsulation protocol to identify the particular S-PMSI.
This allows a single tunnel to aggregate multiple S-PMSIs.
If a particular tunnel is not aggregating multiple S-PMSIs,
then no demultiplexing information is needed.
A PE router which is not connected to a receiver will still receive
the S-PMSI Joins, and MAY cache the information contained therein.
Then if the PE later finds that it is attached to a receiver, it can
immediately start listening to the S-PMSI.
Upon receiving the S-PMSI Join, PE routers connected to receivers for
the specified stream will take whatever action is necessary to start
receiving multicast data packets on the S-PMSI. The precise action
taken will depend upon the tunnel type.
After a configurable delay, the PE router which is sending the S-PMSI
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Joins will start transmitting the stream's data packets on the S-
PMSI.
When the pre-configured conditions are no longer met for a particular
stream, e.g. the traffic stops, the PE router connected to the source
stops announcing S-PMSI Joins for that stream. Any PE that does not
receive, over a configurable interval, an S-PMSI Join for a
particular stream will stop listening to the S-PMSI.
7.2.1.2. Packet Formats and Constants
The S-PMSI Join message is encapsulated within UDP, and has the
following type/length/value (TLV) encoding:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (8 bits)
Length (16 bits): the total number of octets in the Type, Length, and
Value fields combined
Value (variable length)
Currently only one type of S-PMSI Join is defined. A type 1 S-PMSI
Join is used when the S-PMSI tunnel is a PIM tunnel which is used to
carry a single multicast stream, where the packets of that stream
have IPv4 source and destination IP addresses.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| C-source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| C-group |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| P-group |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (8 bits): 1
Length (16 bits): 16
Reserved (8 bits): This field SHOULD be zero when transmitted, and
MUST be ignored when received.
C-Source (32 bits): the IPv4 address of the traffic source in the
VPN.
C-Group (32 bits): the IPv4 address of the multicast traffic
destination address in the VPN.
P-Group (32 bits): the IPv4 group address that the PE router is going
to use to encapsulate the flow (C-Source, C-Group).
The P-group identifies the S-PMSI tunnel, and the (C-S, C-G)
identifies the multicast flow that is carried in the tunnel.
The protocol uses the following constants.
[S-PMSI_DELAY]:
the PE router which is to transmit onto the S-PMSI will delay
this amount of time before it begins using the S-PMSI. The
default value is 3 seconds.
[S-PMSI_TIMEOUT]:
if a PE (other than the transmitter) does not receive any packets
over the S-PMSI tunnel for this amount of time, the PE will prune
itself from the S-PMSI tunnel, and will expect (C-S, C-G) packets
to arrive on an I-PMSI. The default value is 3 minutes. This
value must be consistent among PE routers.
[S-PMSI_HOLDOWN]:
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if the PE that transmits onto the S-PMSI does not see any (C-S,
C-G) packets for this amount of time, it will resume sending (C-
S, C-G) packets on an I-PMSI.
This is used to avoid oscillation when traffic is bursty. The
default value is 1 minute.
[S-PMSI_INTERVAL]
the interval the transmitting PE router uses to periodically send
the S-PMSI Join message. The default value is 60 seconds.
7.2.2. A BGP-based Protocol for Switching to S-PMSIs
This procedure can be used for a MVPN that is using either a UI-PMSI
or a MI-PMSI. Consider a single multicast stream for a C-(S, G)
within a given MVPN, and consider a PE which is attached to a source
of multicast traffic for that stream. The PE can be configured to
move the stream from the MI-PMSI or UI-PMSI to an S-PMSI if certain
configurable conditions are met. Once a PE decides to move the C-(S,
G) for a given MVPN to a S-PMSI, it needs to instantiate the S-PMSI
using a tunnel and announce to all the egress PEs, that are on the
path to receivers of the C-(S, G), of the binding of the S-PMSI to
the C-(S, G). The announcement is done using BGP. Depending on the
tunneling technology used, this announcement may be done before or
after setting up the tunnel. The source and egress PEs have to switch
to using the S-PMSI for the C-(S, G).
7.2.2.1. Advertising C-(S, G) Binding to a S-PMSI using BGP
The ingress PE informs all the PEs that are on the path to receivers
of the C-(S, G) of the binding of the S-PMSI to the C-(S, G). The BGP
announcement is done by sending update for the MCAST-VPN address
family. An A-D route is used, containing the following information:
a) IP address of the originating PE
b) The RD configured locally for the MVPN. This is required to
uniquely identify the <C-Source, C-Group> as the addresses
could overlap between different MVPNs. This is the same RD
value used in the auto-discovery process.
c) The C-Source address.
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d) The C-Group address.
e) 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 auto-discovery 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 auto-discovery routes for these S-PMSIs, then the
aggregation requires the PE to advertise (new) S-PMSI auto-
discovery 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 MVPNs, then all these routes
MUST carry an MPLS upstream assigned label [MPLS-UPSTREAM-
LABEL, section 6.3.4]. If all these aggregated S-PMSIs belong
to the same MVPN, then the routes MAY carry an MPLS upstream
assigned label [MPLS-UPSTREAM-LABEL]. The labels MUST be
distinct on a per MVPN basis, and MAY be distinct on a per
route basis.
When a PE distributes this information via BGP, it must include the
following:
1. An identifier for the particular S-PMSI to which the stream is
to be bound. This identifier is a structured field which
includes the following information:
* The type of tunnel used to instantiate the S-PMSI
* An identifier for the tunnel. The form of the identifier
will depend upon the tunnel type. The combination of
tunnel identifier and tunnel type should contain enough
information to enable all the PEs to "join" the tunnel and
receive messages from it.
2. Route Target Extended Communities attribute. This is used as
described in section 4.
7.2.2.2. Explicit Tracking
If the PE wants to enable explicit tracking for the specified flow,
it also indicates this in the A-D route it uses to bind the flow to a
particular S-PMSI. Then any PE which receives the A-D route will
respond with a "Leaf A-D Route" in which it identifies itself as a
receiver of the specified flow. The Leaf A-D route will be withdrawn
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when the PE is no longer a receiver for the flow.
If the PE needs to enable explicit tracking for a flow before binding
the flow to an S-PMSI, it can do so by sending an A-D route
identifying the flow but not specifying an S-PMSI. This will elicit
the Leaf A-D Routes. This is useful when the PE needs to know the
receivers before selecting an S-PMSI.
7.2.2.3. Switching to S-PMSI
After the egress PEs receive the announcement they setup their
forwarding path to receive traffic on the S-PMSI if they have one or
more receivers interested in the <C-S, C-G> bound to the S-PMSI. This
involves changing the RPF interface for the relevant <C-S, C-G>
entries to the interface that is used to instantiate the S-PMSI. If
an Aggregate Tree is used to instantiate a S-PMSI this also implies
setting up the demultiplexing forwarding entries based on the inner
label as described in section 6.3.4. The egress PEs may perform the
switch to the S-PMSI once the advertisement from the ingress PE is
received or wait for a preconfigured timer to do so.
A source PE may use one of two approaches to decide when to start
transmitting data on the S-PMSI. In the first approach once the
source PE instantiates the S-PMSI, it starts sending multicast
packets for <C-S, C-G> entries mapped to the S-PMSI on both that as
well as on the I-PMSI, which is currently used to send traffic for
the <C-S, C-G>. After some preconfigured timer the PE stops sending
multicast packets for <C-S, C-G> on the I-PMSI. In the second
approach after a certain pre-configured delay after advertising the
<C-S, C-G> entry bound to a S-PMSI, the source PE begins to send
traffic on the S-PMSI. At this point it stops to send traffic for the
<C-S, C-G> on the I-PMSI. This traffic is instead transmitted on the
S-PMSI.
7.3. Aggregation
S-PMSIs can be aggregated on a P-multicast tree. The S-PMSI to C-(S,
G) binding advertisement supports aggregation. Furthermore the
aggregation procedures of section 6.3 apply. It is also possible to
aggregate both S-PMSIs and I-PMSIs on the same P-multicast tree.
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7.4. Instantiating the S-PMSI with a PIM Tree
The procedures of section 7.3 tell a PE when it must start listening
and stop listening to a particular S-PMSI. Those procedures also
specify the method for instantiating the S-PMSI. In this section, we
provide the procedures to be used when the S-PMSI is instantiated as
a PIM tree. The PIM tree is created by the PIM P-instance.
If a single PIM tree is being used to aggregate multiple S-PMSIs,
then the PIM tree to which a given stream is bound may have already
been joined by a given receiving PE. If the tree does not already
exist, then the appropriate PIM procedures to create it must be
executed in the P-instance.
If the S-PMSI for a particular multicast stream is instantiated as a
PIM-SM or BIDIR-PIM tree, the S-PMSI identifier will specify the RP
and the group P-address, and the PE routers which have receivers for
that stream must build a shared tree toward the RP.
If the S-PMSI is instantiated as a PIM-SSM tree, the PE routers build
a source tree toward the PE router that is advertising the S-PMSI
Join. The IP address root of the tree is the same as the source IP
address which appears in the S-PMSI Join. In this case, the tunnel
identifier in the S-PMSI Join will only need to specify a group P-
address.
The above procedures assume that each PE router has a set of group P-
addresses that it can use for setting up the PIM-trees. Each PE must
be configured with this set of P-addresses. If PIM-SSM is used to
set up the tunnels, then the PEs may be with overlapping sets of
group P-addresses. If PIM-SSM is not used, then each PE must be
configured with a unique set of group P-addresses (i.e., having no
overlap with the set configured at any other PE router). The
management of this set of addresses is thus greatly simplified when
PIM-SSM is used, so the use of PIM-SSM is strongly recommended
whenever PIM trees are used to instantiate S-PMSIs.
If it is known that all the PEs which need to receive data traffic on
a given S-PMSI can support aggregation of multiple S-PMSIs on a
single PIM tree, then the transmitting PE, may, at its discretion,
decide to bind the S-PMSI to a PIM tree which is already bound to one
or more other S-PMSIs, from the same or from different MVPNs. In
this case, appropriate demultiplexing information must be signaled.
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7.5. Instantiating S-PMSIs using RSVP-TE P2MP Tunnels
RSVP-TE P2MP Tunnels can be used for instantiating S-PMSIs.
Procedures described in the context of I-PMSIs in section 6.7 apply.
8. Inter-AS Procedures
If an MVPN has sites in more than one AS, it requires one or more
PMSIs to be instantiated by inter-AS tunnels. This document
describes two different types of inter-AS tunnel:
1. "Segmented Inter-AS tunnels"
A segmented inter-AS tunnel consists of a number of independent
segments which are stitched together at the ASBRs. There are
two types of segment, inter-AS segments and intra-AS segments.
The segmented inter-AS tunnel consists of alternating intra-AS
and inter-AS segments.
Inter-AS segments connect adjacent ASBRs of different ASes;
these "one-hop" segments are instantiated as unicast tunnels.
Intra-AS segments connect ASBRs and PEs which are in the same
AS. An intra-AS segment may be of whatever technology is
desired by the SP that administers the that AS. Different
intra-AS segments may be of different technologies.
Note that the intra-AS segments of inter-AS tunnels form a
category of tunnels that is distinct from simple intra-AS
tunnels; we will rely on this distinction later (see Section
9).
A segmented inter-AS tunnel can be thought of as a tree which
is rooted at a particular AS, and which has as its leaves the
other ASes which need to receive multicast data from the root
AS.
2. "Non-segmented Inter-AS tunnels"
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
tunnel.
Section 10 of [RFC4364] describes three different options for
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supporting unicast Inter-AS BGP/MPLS IP VPNs, known as options A, B,
and C. We describe below how both segmented and non-segmented inter-
AS trees can be supported when option B or option C is used. (Option
A does not pass any routing information through an ASBR at all, so no
special inter-AS procedures are needed.)
8.1. Non-Segmented Inter-AS Tunnels
In this model, the previously described discovery and tunnel setup
mechanisms are used, even though the PEs belonging to a given MVPN
may be in different ASes.
8.1.1. Inter-AS MVPN Auto-Discovery
The previously described BGP-based auto-discovery mechanisms work "as
is" when an MVPN contains PEs that are in different Autonomous
Systems. However, please note that, if non-segmented Inter-AS
Tunnels are to be used, then the "Intra-AS" A-D routes MUST be
distributed across AS boundaries!
8.1.2. Inter-AS MVPN Routing Information Exchange
When non-segmented inter-AS tunnels are used, MVPN C-multicast
routing information may be exchanged by means of PIM peering across
an MI-PMSI, or by means of BGP carrying C-multicast routes.
When PIM peering is used to distribute the C-multicast routing
information, a PE that sends C-PIM Join/Prune messages for a
particular C-(S,G) must be able to identify the PE which is its PIM
adjacency on the path to S. This is the "selected upstream PE"
described in section 5.1.
If BGP (rather than PIM) is used to distribute the C-multicast
routing information, and if option b of section 10 of [RFC4364] is in
use, then the C-multicast routes will be installed in the ASBRs along
the path from each multicast source in the MVPN to each multicast
receiver in the MVPN. If option b is not in use, the C-multicast
routes are not installed in the ASBRs. The handling of the C-
multicast routes in either case is thus exactly analogous to the
handling of unicast VPN-IP routes in the corresponding case.
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8.1.3. Inter-AS P-Tunnels
The procedures described earlier in this document can be used to
instantiate either an I-PMSI or an S-PMSI with inter-AS P-tunnels.
Specific tunneling techniques require some explanation.
If ingress replication is used, the inter-AS PE-PE tunnels will use
the inter-AS tunneling procedures for the tunneling technology used.
Procedures in [RSVP-P2MP] are used for inter-AS RSVP-TE P2MP P-
Tunnels.
Procedures for using PIM to set up the P-tunnels are discussed in
the next section.
8.1.3.1. PIM-Based Inter-AS P-Multicast Trees
When PIM is used to set up an inter-AS P-multicast tree, the PIM
Join/Prune messages used to join the tree contain the IP address of
the upstream PE. However, there are two special considerations that
must be taken into account:
- It is possible that the P routers within one or more of the ASes
will not have routes to the upstream PE. For example, if an AS
has a "BGP-free core", the P routers in an AS will not have
routes to addresses outside the AS.
- If the PIM Join/Prune message must travel through several ASes,
it is possible that the ASBRs will not have routes to he PE
routers. For example, in an inter-AS VPN constructed according
to "option b" of section 10 of [RFC4364], the ASBRs do not
necessarily have routes to the PE routers.
If either of these two conditions obtains, then "ordinary" PIM
Join/Prune messages cannot be routed to the upstream PE. Thus the
following information needs to be added to the PIM Join/Prune
messages: a "Proxy Address", which contains the address of the next
ASBR on the path to the upstream PE. When the PIM Join/Prune arrives
at the ASBR which is identified by the "proxy address", that ASBR
must change the proxy address to identify the next hop ASBR.
This information allows the PIM Join/Prune to be routed through an AS
even if the P routers of that AS do not have routes to the upstream
PE. However, this information is not sufficient to enable the ASBRs
to route the Join/Prune if the ASBRs themselves do not have routes to
the upstream PE.
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However, even if the ASBRs do not have routes to the upstream PE, the
procedures of this draft ensure that they will have A-D routes that
lead to the upstream PE. If non-segmented inter-AS MVPNs are being
used, the ASBRs (and PEs) will have Intra-AS A-D routes which have
been distributed inter-AS.
So rather than having the PIM Join/Prune messages routed by the ASBRs
along a route to the upstream PE, the PIM Join/Prune messages MUST
be routed along the path determined by the intra-AS A-D routes.
If the only intra-AS A-D route for a given MVPN is the "Intra-AS I-
PMSI Route", the PIM Join/Prunes will be routed along that. However,
if the PIM Join/Prune message is for a particular P-group address,
and there is an "Intra-AS S-PMSI Route" specifying that particular P-
group address as the P-tunnel for a particular S-PMSI, then the PIM
Join/Prunes MUST be routed along the path determined by those intra-
AS A-D routes.
The next revision of this document will provide the following
details:
- encoding of the proxy address in the PIM message (the PIM Join
Attribute [PIM-ATTRIB] will be used)
- encoding of any other information which may be needed in order to
enable the correct intra-AS route to be chosen.
Support for non-segmented inter-AS trees using BIDIR-PIM is for
further study.
8.2. Segmented Inter-AS Tunnels
8.2.1. Inter-AS MVPN Auto-Discovery Routes
The BGP based MVPN membership discovery procedures of section 4 are
used to auto-discover the intra-AS MVPN membership. This section
describes the additional procedures for inter-AS MVPN membership
discovery. It also describes the procedures for constructing
segmented inter-AS tunnels.
In this case, for a given MVPN in an AS, the objective is to form a
spanning tree of MVPN membership, rooted at the AS. The nodes of this
tree are ASes. The leaves of this tree are only those ASes that have
at least one PE with a member in the MVPN. The inter-AS tunnel used
to instantiate an inter-AS PMSI must traverse this spanning tree. A
given AS needs to announce to another AS only the fact that it has
membership in a given MVPN. It doesn't need to announce the
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membership of each PE in the AS to other ASes.
This section defines an inter-AS auto-discovery route as a route that
carries information about an AS that has one or more PEs (directly)
connected to the site(s) of that MVPN. Further it defines an inter-AS
leaf auto-discovery route in the following way:
- Consider a node which is the root of an an intra-AS segment of an
inter-AS tunnel. An inter-AS leaf autodiscovery route is used to
inform such a node of a leaf of that intra-AS segment.
8.2.1.1. Originating Inter-AS MVPN A-D Information
A PE in a given AS advertises its MVPN membership to all its IBGP
peers. This IBGP peer may be a route reflector which in turn
advertises this information to only its IBGP peers. In this manner
all the PEs and ASBRs in the AS learn this membership information.
An Autonomous System Border Router (ASBR) may be configured to
support a particular MVPN. If an ASBR is configured to support a
particular MVPN, the ASBR MUST participate in the intra-AS MVPN auto-
discovery/binding procedures for that MVPN within the AS that the
ASBR belongs to, as defined in this document.
Each ASBR then advertises the "AS MVPN membership" to its neighbor
ASBRs using EBGP. This inter-AS auto-discovery route must not be
advertised to the PEs/ASBRs in the same AS as this ASBR. The
advertisement carries the following information elements:
a. A Route Distinguisher for the MVPN. For a given MVPN each ASBR
in the AS must use the same RD when advertising this
information to other ASBRs. To accomplish this all the ASBRs
within that AS, that are configured to support the MVPN, MUST
be configured with the same RD for that MVPN. This RD MUST be
of Type 0, MUST embed the autonomous system number of the AS.
b. The announcing ASBR's local address as the next-hop for the
above information elements.
c. By default the BGP Update message MUST carry export Route
Targets used by the unicast routing of that VPN. The default
could be modified via configuration by having a set of Route
Targets used for the inter-AS auto-discovery routes being
distinct from the ones used by the unicast routing of that VPN.
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8.2.1.2. Propagating Inter-AS MVPN A-D Information
As an inter-AS auto-discovery route originated by an ASBR within a
given AS is propagated via BGP to other ASes, this results in
creation of a data plane tunnel that spans multiple ASes. This tunnel
is used to carry (multicast) traffic from the MVPN sites connected to
the PEs of the AS to the MVPN sites connected to the PEs that are in
the other ASes. Such tunnel consists of multiple intra-AS segments
(one per AS) stitched at ASBRs' boundaries by single hop <ASBR-ASBR>
LSP segments.
An ASBR originates creation of an intra-AS segment when the ASBR
receives an inter-AS auto-discovery 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 labels within a given AS multiple intra-AS segments of
different inter-AS tunnels of either the same or different MVPNs may
share the same P-Multicast Tree.
Since (aggregated) inter-AS auto-discovery routes have granularity of
<AS, MVPN>, an MVPN that is present in N ASes would have total of N
inter-AS tunnels. Thus for a given MVPN the number of inter-AS
tunnels is independent of the number of PEs that have this MVPN.
The following sections specify procedures for propagation of
(aggregated) inter-AS auto-discovery routes across ASes.
8.2.1.2.1. Inter-AS Auto-Discovery Route received via EBGP
When an ASBR receives from one of its EBGP neighbors a BGP Update
message that carries the inter-AS auto-discovery 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:
a) Re-advertises this inter-AS auto-discovery route within its own
AS.
If the ASBR uses ingress replication to instantiate the intra-
AS segment of the inter-AS tunnel, the re-advertised route
SHOULD carry a Tunnel attribute with the Tunnel Identifier set
to Ingress Replication, but no MPLS labels.
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If a P-Multicast Tree is used to instantiate the intra-AS
segment of the inter-AS tunnel, and in order to advertise the
P-Multicast tree identifier the ASBR doesn't need to know the
leaves of the tree beforehand, then the advertising ASBR SHOULD
advertise the P-Multicast tree identifier in the Tunnel
Identifier of the Tunnel attribute. This, in effect, creates a
binding between the inter-AS auto-discovery route and the P-
Multicast Tree.
If a P-Multicast Tree is used to instantiate the intra-AS
segment of the inter-AS tunnel, and in order to advertise the
P-Multicast tree identifier the advertising ASBR needs to know
the leaves of the tree beforehand, the ASBR first discovers the
leaves using the Auto-Discovery procedures, as specified
further down. It then advertises the binding of the tree to the
inter-AS auto-discovery route using the the original auto-
discovery route with the addition of carrying in the route the
Tunnel attribute that contains the type and the identity of the
tree (encoded in the Tunnel Identifier of the attribute).
b) Re-advertises the received inter-AS auto-discovery route to its
EBGP peers, other than the EBGP neighbor from which the best
inter-AS auto-discovery route was received.
c) Advertises to its neighbor ASBR, from which it received the
best inter-AS autodiscovery route to the destination carried in
the NRLI of the route, a leaf auto-discovery route that carries
an ASBR-ASBR tunnel binding with the tunnel identifier set to
ingress replication. This binding as described in section 6 can
be used by the neighbor ASBR to send traffic to this ASBR.
8.2.1.2.2. Leaf Auto-Discovery Route received via EBGP
When an ASBR receives via EBGP a leaf auto-discovery route, the ASBR
finds an inter-AS auto-discovery route that has the same RD as the
leaf auto-discovery route. The MPLS label carried in the leaf auto-
discovery route is used to stitch a one hop ASBR-ASBR LSP to the tail
of the intra-AS tunnel segment associated with the inter-AS auto-
discovery route.
8.2.1.2.3. Inter-AS Auto-Discovery Route received via IBGP
If a given inter-AS auto-discovery route is advertised within an AS
by multiple ASBRs of that AS, the BGP best route selection performed
by other PE/ASBR routers within the AS does not require all these
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PE/ASBR routers to select the route advertised by the same ASBR - to
the contrary different PE/ASBR routers may select routes advertised
by different ASBRs.
Further when a PE/ASBR receives from one of its IBGP neighbors a BGP
Update message that carries a AS MVPN membership tree , if (a) the
route was originated outside of the router's own AS, (b) at least one
of the Route Targets carried in the message matches one of the import
Route Targets configured on the PE/ASBR, and (c) the PE/ASBR
determines that the received route is the best route to the
destination carried in the NLRI of the route, if the router is an
ASBR then the ASBR propagates the route to its EBGP neighbors. In
addition the PE/ASBR performs the following.
If the received inter-AS auto-discovery route carries the Tunnel
attribute with the Tunnel Identifier set to LDP P2MP LSP, or PIM-SSM
tree, or PIM-SM tree, the PE/ASBR SHOULD join the P-Multicast tree
whose identity is carried in the Tunnel Identifier.
If the received source auto-discovery route carries the Tunnel
attribute with the Tunnel Identifier set to RSVP-TE P2MP LSP, then
the ASBR that originated the route MUST signal the local PE/ASBR as
one of leaf LSRs of the RSVP-TE P2MP LSP. This signaling MAY have
been completed before the local PE/ASBR receives the BGP Update
message.
If the NLRI of the route does not carry a label, then this tree is an
intra-AS tunnel segment that is part of the inter-AS Tunnel for the
MVPN advertised by the inter-AS auto-discovery route. If the NLRI
carries a (upstream) label, then a combination of this tree and the
label identifies the intra-AS segment.
If this 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 MVPN, packets received over the intra-AS
segment must be forwarded to the local receivers using the local VRF.
If the received inter-AS auto-discovery route either does not carry
the Tunnel attribute, or carries the Tunnel attribute with the Tunnel
Identifier set to ingress replication, then the PE/ASBR originates a
new auto-discovery route to allow the ASBR from which the auto-
discovery route was received, to learn of this ASBR as a leaf of the
intra-AS tree.
Thus the AS MVPN membership information propagates across multiple
ASes along a spanning tree. BGP AS-Path based loop prevention
mechanism prevents loops from forming as this information propagates.
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8.2.2. Inter-AS MVPN Routing Information Exchange
All of the MVPN routing information exchange methods specified in
section 5 can be supported across ASes.
The objective in this case is to propagate the MVPN routing
information to the remote PE that originates the unicast route to C-
S/C-RP, in the reverse direction of the AS MVPN membership
information announced by the remote PE's origin AS. This information
is processed by each ASBR along this reverse path.
To achieve this the PE that is generating the MVPN routing
advertisement, first determines the source AS of the unicast route to
C-S/C-RP. It then determines from the received AS MVPN membership
information, for the source AS, the ASBR that is the next-hop for the
best path of the source AS MVPN membership. The BGP MVPN routing
update is sent to this ASBR and the ASBR then further propagates the
BGP advertisement. BGP filtering mechanisms ensure that the BGP MVPN
routing information updates flow only to the upstream router on the
reverse path of the inter-AS MVPN membership tree. Details of this
filtering mechanism and the relevant encoding will be specified in a
separate document.
8.2.3. Inter-AS I-PMSI
All PEs in a given AS, use the same inter-AS heterogeneous tunnel,
rooted at the AS, to instantiate an I-PMSI for an inter-AS MVPN
service. As explained earlier the intra-AS tunnel segments that
comprise this tunnel can be built using different tunneling
technologies. To instantiate an MI-PMSI service for a MVPN there must
be an inter-AS tunnel rooted at each AS that has at least one PE that
is a member of the MVPN.
A C-multicast data packet is sent using an intra-AS tunnel segment by
the PE that first receives this packet from the MVPN customer site.
An ASBR forwards this packet to any locally connected MVPN receivers
for the multicast stream. If this ASBR has received a tunnel binding
for the AS MVPN membership that it advertised to a neighboring ASBR,
it also forwards this packet to the neighboring ASBR. In this case
the packet is encapsulated in the downstream MPLS label received from
the neighboring ASBR. The neighboring ASBR delivers this packet to
any locally connected MVPN receivers for that multicast stream. It
also transports this packet on an intra-AS tunnel segment, for the
inter-AS MVPN tunnel, and the other PEs and ASBRs in the AS then
receive this packet. The other ASBRs then repeat the procedure
followed by the ASBR in the origin AS and the packet traverses the
overlay inter-AS tunnel along a spanning tree.
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8.2.3.1. Support for Unicast VPN Inter-AS Methods
The above procedures for setting up an inter-AS I-PMSI can be
supported for each of the unicast VPN inter-AS models described in
[RFC4364]. These procedures do not depend on the method used to
exchange unicast VPN routes. For Option B and Option C they do
require MPLS encapsulation between the ASBRs.
8.2.4. Inter-AS S-PMSI
An inter-AS tunnel for an S-PMSI is constructed similar to an inter-
AS tunnel for an I-PMSI. 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 and
PEs 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 on its own.
The PE that decides to set up a S-PMSI, advertises the S-PMSI tunnel
binding using procedures in section 7.3.2 to the routers in its own
AS. The <C-S, C-G> membership for which the S-PMSI is instantiated,
is propagated along an inter-AS spanning tree. This spanning tree
traverses the same ASBRs as the AS MVPN membership spanning tree. In
addition to the information elements described in section 7.3.2
(Origin AS, RD, next-hop) the C-S and C-G is also advertised.
An ASBR that receives the AS <C-S, C-G> information from its upstream
ASBR using EBGP sends back a tunnel binding for AS <C-S, C-G>
information 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 instantiates a S-PMSI for the AS <C-S, C-G> it sends back a
downstream label that is used to forward the packet along its intra-
AS S-PMSI for the <C-S, C-G>. However the ASBR may decide to use an
AS MVPN membership I-PMSI instead, in which case it sends back the
same label that it advertised for the AS MVPN membership I-PMSI. If
the downstream ASBR instantiates a S-PMSI, it further propagates the
<C-S, C-G> membership to its downstream ASes, else it does not.
An AS can instantiate an intra-AS S-PMSI for the inter-AS S-PMSI
tunnel only if the upstream AS instantiates a S-PMSI. The procedures
allow each AS to determine whether it wishes to setup a S-PMSI or not
and the AS is not forced to setup a S-PMSI just because the upstream
AS decides to do so.
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The leaves of an intra-AS S-PMSI tunnel will be the PEs that have
local receivers that are interested in <C-S, C-G> and the ASBRs that
have received MVPN routing information for <C-S, C-G>. Note that an
AS can determine these ASBRs as the MVPN routing information is
propagated and processed by each ASBR on the AS MVPN membership
spanning tree.
The C-multicast data traffic is sent on the S-PMSI by the originating
PE. When it reaches an ASBR that is on the spanning tree, it is
delivered to local receivers, if any, and is also forwarded to the
neighbor ASBR after being encapsulated in the label advertised by the
neighbor. The neighbor ASBR either transports this packet on the S-
PMSI for the multicast stream or an I-PMSI, delivering it to the
ASBRs in its own AS. These ASBRs in turn repeat the procedures of the
origin AS ASBRs and the multicast packet traverses the spanning tree.
9. Duplicate Packet Detection and Single Forwarder PE
Consider the case of an egress PE that receives packets of a customer
multicast stream (C-S, C-G) over a non-aggregated S-PMSI. The
procedures described so far will never cause the PE to receive
duplicate copies of any packet in that stream. It is possible that
the (C-S, C-G) stream is carried in more than one S-PMSI; this may
happen when the site that contains C-S is multihomed to more than one
PE. However, a PE that needs to receive (C-S, C-G) packets only
joins one of these S-PMSIs, and so only receives one copy of each
packet.
However, if the data packets of stream (C-S, C-G) are carried in
either an I-PMSI or in an aggregated S-PMSI, then it the procedures
specified so far make it possible for an egress PE to receive more
than one copy of each data packet. In this section, we define
additional procedures to that an MVPN customer sees no multicast data
packet duplication.
This section covers the situation where the customer multicast tree
is unidirectional, i.e. with the C-G is either a "Sparse Mode" or a
"Single Source Mode" group. The case where the customer multicast
tree is bidirectional (the C-G is a BIDIR-PIM group) is considered
separately in section 12.
The first case when an egress PE may receive duplicate multicast data
packets, is the case where both (a) an MVPN site that contains C-S or
C-RP is multihomed to more than one PE, and (b) either an I-PMSI, or
an aggregated S-PMSI is used for carrying the packets originated by
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C-S. In this case, an egress PE may receive one copy of the packet
from each PE to which the site is homed.
The second case when an egress PE may receive duplicate multicast
data packets is when all of the following is true: (a) the IP
destination address of the customer packet is a C-G that is operating
in ASM mode, and whose C-multicast tree is set up using PIM-SM, (b)
an MI-PMSI is used for carrying the packets, and (c) a router or a CE
in a site connected to the egress PE switches from the C-RP tree to
C-S tree. In this case, it is possible to get one copy of a given
packet from the ingress PE attached to the C-RP's site, and one from
the ingress PE attached to the C-S's site.
9.1. Multihomed C-S or C-RP
In the first case for a given <C-S, C-G> an egress PE, say PE1,
expects to receive C-data packets from the upstream PE, say PE2,
which PE1 identified as the upstream multicast hop in the C-Multicast
Routing Update that PE1 sent in order to join <C-S, C-G>. If PE1 can
determine that a data packet for <C-S, C-G> was received from the
expected upstream PE, PE2, PE1 will accept and forward the packet.
Otherwise, PE1 will drop the packet; this means that the PE will see
a duplicate, but the duplicate will not get forwarded. (But see
section 10 for an exception case where PE1 will accept a packet even
if it is from an unexpected upstream PE.)
The method used by an egress PE to determine the ingress PE for a
particular packet, received over a particular PMSI, depends on the P-
tunnel technology that is used to instantiate the PMSI. If the P-
tunnel is a P2MP LSP, a PIM-SM or PIM-SSM tree, or a unicast tunnel,
then the tunnel encapsulation contains information which can be used
(possibly along with other state information in the PE) to determine
the ingress PE, as long as the P-tunnel is instantiating an intra-AS
PMSI, or an inter-AS PMSI which is supported by a non-segmented
inter-AS tunnel.
Even when inter-AS segmented tunnels are used, if an aggregated S-
PMSI is used for carrying the packets, the P-tunnel encapsulation
must have some information which can be used to identify the PMSI,
and that in turn implicitly identifies the ingress PE.
If an I-PMSI is used for carrying the packets, the I-PMSI spans
multiple ASes, and the I-PMSI is realized via segmented inter-AS
tunnels, if C-S or C-RP is multi-homed to different PEs, as long as
each such PE is in a different AS, the egress PE can detect duplicate
traffic as such duplicate traffic will arrive on a different (inter-
AS) tunnel. Specifically, if the PE was expecting the traffic on an
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particular inter-AS tunnel, duplicate traffic will arrive either on
an intra-AS tunnel [this is not an intra-AS tunnel segment, of an
inter-AS tunnel], or on some other inter-AS tunnel. Therefore, to
detect duplicates the PE has to keep track of which (inter-AS) auto-
discovery route the PE uses for sending MVPN multicast routing
information towards C-S/C-RP. Then the PE should receive (multicast)
traffic originated by C-S/C-RP only from the (inter-AS) tunnel that
was carried in the best Inter-AS auto-discovery route for the MVPN
and was originated by the AS that contains C-S/C-RP (where "the best"
is determined by the PE). The PE should discard, as duplicated, all
other multicast traffic originated by C-S/C-RP, but received on any
other tunnel.
9.1.1. Single forwarder PE selection
When for a given MVPN (a) MI-PMSI is used for carrying multicast data
packets, (b) C-S or C-RP is multi-homed to different PEs, and (c) at
least two of such PEs are in the same AS, then depending on the
tunneling technology used by the MI-PMSI it may not always be
possible for the egress PE to determine the upstream PE. Therefore,
when this determination may not be possible procedures are needed to
ensure that packets are received on an MI-PMSI at an egress PE only
from a single upstream PE. Furthermore, even if the determination is
possible, it may be preferable to send only one copy of each packet
to each egress PE, rather than sending multiple copies and having the
egress PE discard all but one.
Section 5.1 specifies a procedure for choosing a "default upstream PE
selection", such that (except during routing transients) all PEs will
choose the same default upstream PE. To ensure that duplicate
packets are not sent through the backbone (except during routing
transients), an ingress PE does not forward to the backbone any (C-S,
C-G) multicast data packet it receives from a CE, unless the PE is
the default upstream PE selection.
This procedure is optional whenever the P-tunnel technology that is
being used to carry the multicast stream in question allows the
egress PEs to determine the identity of the ingress PE. This
procedure is mandatory if the P-tunnel technology does not make this
determination possible.
The above procedure ensures that if C-S or C-RP is multi-homed to PEs
within a single AS, a PE will not receive duplicate traffic as long
as all the PEs are on either the C-S or C-RP tree. If some PEs are on
the C-S tree and some on the C-RP tree, however, packet duplication
is still possible. This is discussed in the next section.
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9.2. Switching from the C-RP tree to C-S tree
If some PEs are on the C-S tree and some on the R-RP tree then a PE
may also receive duplicate traffic during a <C-*, C-G> to <C-S, C-G>
switch. The issue and the solution are described next.
When for a given MVPN (a) MI-PMSI is used for carrying multicast data
packets, (b) C-S and C-RP are connected to PEs within the same AS,
and (c) the MI-PMSI tunneling technology in use does not allow the
egress PEs to identify the ingress PE, then having all the PEs select
the same PE to be the upstream multicast hop for C-S or C-RP is not
sufficient to prevent packet duplication.
The reason is that a single tunnel used by MI-PMSI may be carrying
traffic on both the (C-*, C-G) tree and the (C-S, C-G) tree. If some
of the egress PEs have joined the source tree, but others expect to
receive (C-S, C-G) packets from the shared tree, then two copies of
data packet will travel on the tunnel, and since due to the choice of
the tunneling technology the egress PEs have no way to identify the
ingress PE, the egress PEs will have no way to determine that only
one copy should be accepted.
To avoid this, it is necessary to ensure that once any PE joins the
(C-S, C-G) tree, any other PE that has joined the (C-*, C- G) tree
also switches to the (C-S, C-G) tree (selecting, of course, the same
upstream multicast hop, as specified above).
Whenever a PE creates an <C-S, C-G> state as a result of receiving a
C-multicast route for <C-S, C-G> from some other PE, and the C-G
group is a Sparse Mode group, the PE that creates the state MUST
originate a Source Active auto-discovery route (see [MVPN-BGP]
section 4.5) as specified below. The route is advertised using the
same procedures as the MVPN auto-discovery/binding (both intra-AS and
inter-AS) specified in this document with the following
modifications:
1. The Multicast Source field MUST be set to C-S. The Multicast
Source Length field is set appropriately to reflect this.
2. The Multicast Group field MUST be set to C-G. The Multicast
Group Length field is set appropriately to reflect this.
The route goes to all the PEs of the MVPN. When as a result of
receiving a new Source Active auto-discovery route a PE updates its
VRF with the route, the PE MUST check if the newly received route
matches any <C-*, C-G> entries. If (a) there is a matching entry, (b)
the PE does not have (C-S, C-G) state in its MVPN-TIB for (C-S, C-G)
carried in the route, and (c) the received route is selected as the
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best (using the BGP route selection procedures), then the PE sets up
its forwarding path to receive (C-S, C-G) traffic from the tunnel the
originator of the selected Source Active auto-discovery route uses
for sending (C-S, C-G). This procedures forces all the PEs (in all
ASes) to switch from the C-RP tree to the C-S tree for <C-S, C-G>.
(Additional uses of the Source Active A-D route are discussed in
section 10.)
Note that when a PE thus joins the <C-S, C-G> tree, it may need to
send a PIM (S,G,RPT-bit) prune to one of its CE PIM neighbors, as
determined by ordinary PIM procedures. (This will be the case if the
incoming interface for the (C-*, C-G) tree is one of the VRF
interfaces.) However, before doing this, it SHOULD run a timer to
help ensure that the source is not pruned from the shared tree until
all PEs have had time to receive the Source Active route.
Whenever the PE deletes the <C-S, C-G> state that was previously
created as a result of receiving a C-multicast route for <C-S, C-G>
from some other PE, the PE that deletes the state also withdraws the
auto-discovery route that was advertised when the state was created.
N.B.: SINCE ALL PEs WITH RECEIVERS FOR GROUP C-G WILL JOIN THE C-S
SOURCE TREE IF ANY OF THEM DO, IT IS NEVER NECESSARY TO DISTRIBUTE A
BGP C-MULTICAST ROUTE FOR THE PURPOSE OF PRUNING SOURCES FROM THE
SHARED TREE.
It is worth nothing that if a PE joins a source tree as a result of
this procedure, the UMH is not necessarily the same as it would be if
the PE had joined the source tree as a result of receiving a PIM Join
for the same source tree from a directly attached CE.
10. Eliminating PE-PE Distribution of (C-*,C-G) State
In sparse mode PIM, a node that wants to become a receiver for a
particular multicast group G first joins a shared tree, rooted at a
rendezvous point. When the receiver detects traffic from a
particular source it has the option of joining a source tree, rooted
at that source. If it does so, it has to prune that source from the
shared tree, to ensure that it receives packets from that source on
only one tree.
Maintaining the shared tree can require considerable state, as it is
necessary not only to know who the upstream and downstream nodes are,
but to know which sources have been pruned off which branches of the
share tree.
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The BGP-based signaling procedures defined in this document and in
[MVPN-BGP] eliminate the need for PEs to distribute to each other any
state having to do with which sources have been pruned off a shared
C-tree. Those procedures do still allow multicast data traffic to
travel on a shared C-tree, but they do not allow a situation in which
some CEs receive (S,G) traffic on a shared tree and some on a source
tree. This results in a considerable simplification of the PE-PE
procedures with minimal change to the multicast service seen within
the VPN. However, shared C-trees are still supported across the VPN
backbone. That is, (C-*, C-G) state is distributed PE-PE, but (C-*,
C-G, RPT-bit) state is not.
In this section, we specify a number of optional procedures which go
further, and which completely eliminate the support for shared C-
trees across the VPN backbone. In these procedures, the PEs keep
track of the active sources for each C-G. As soon as a CE tries to
join the (*,G) tree, the PEs instead join the (S,G) trees for all the
active sources. Thus all distribution of (C-*,C-G) state is
eliminated. These procedures are optional because they require some
additional support on the part of the VPN customer, and because they
are not always appropriate. (E.g., a VPN customer may have his own
policy of always using shared trees for certain multicast groups.)
There are several different options, described in the following sub-
sections.
10.1. Co-locating C-RPs on a PE
[MVPN-REQ] describes C-RP engineering as an issue when PIM-SM (or
BIDIR-PIM) is used in "Any Source Multicast (ASM) mode" [RFC4607] on
the VPN customer site. To quote from [MVPN-REQ]:
"In some cases this engineering problem is not trivial: for instance,
if sources and receivers are located in VPN sites that are different
than that of the RP, then traffic may flow twice through the SP
network and the CE-PE link of the RP (from source to RP, and then
from RP to receivers) ; this is obviously not ideal. A multicast VPN
solution SHOULD propose a way to help on solving this RP engineering
issue."
One of the C-RP deployment models is for the customer to outsource
the RP to the provider. In this case the provider may co-locate the
RP on the PE that is connected to the customer site [MVPN-REQ]. This
section describes how anycast-RP can be used for achieving this. This
is described below.
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10.1.1. Initial Configuration
For a particular MVPN, at least one or more PEs that have sites in
that MVPN, act as an RP for the sites of that MVPN connected to these
PEs. Within each MVPN all these RPs use the same (anycast) address.
All these RPs use the Anycast RP technique.
10.1.2. Anycast RP Based on Propagating Active Sources
This mechanism is based on propagating active sources between RPs.
10.1.2.1. Receiver(s) Within a Site
The PE which receives C-Join for (*,G) or (S,G) does not send the
information that it has receiver(s) for G until it receives
information about active sources for G from an upstream PE.
On receiving this (described in the next section), the downstream PE
will respond with Join for C-(S,G). Sending this information could be
done using any of the procedures described in section 5. If BGP is
used, the ingress address is set to the upstream PE's address which
has triggered the source active information. Only the upstream PE
will process this information. If unicast PIM is used then a unicast
PIM message will have to be sent to the PE upstream PE that has
triggered the source active information. If a MI-PMSI is used than
further clarification is needed on the upstream neighbor address of
the PIM message and will be provided in a future revision.
10.1.2.2. Source Within a Site
When a PE receives PIM-Register from a site that belongs to a given
VPN, PE follows the normal PIM anycast RP procedures. It then
advertises the source and group of the multicast data packet carried
in PIM-Register message to other PEs in BGP using the following
information elements:
- Active source address
- Active group address
- Route target of the MVPN.
This advertisement goes to all the PEs that belong to that MVPN. When
a PE receives this advertisement, it checks whether there are any
receivers in the sites attached to the PE for the group carried in
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the source active advertisement. If yes, then it generates an
advertisement for C-(S,G) as specified in the previous section.
Note that the mechanism described in section 7.3.2. can be leveraged
to advertise a S-PMSI binding along with the source active messages.
10.1.2.3. Receiver Switching from Shared to Source Tree
No additional procedures are required when multicast receivers in
customer's site shift from shared tree to source tree.
10.2. Using MSDP between a PE and a Local C-RP
Section 10.1 describes the case where each PE is a C-RP. This
enables the PEs to know the active multicast sources for each MVPN,
and they can then use BGP to distribute this information to each
other. As a result, the PEs do not have to join any shared C-trees,
and this results in a simplification of the PE operation.
In another deployment scenario, the PEs are not themselves C-RPs, but
use MSDP to talk to the C-RPs. In particular, a PE which attaches to
a site that contains a C-RP becomes an MSDP peer of that C-RP. That
PE then uses BGP to distribute the information about the active
sources to the other PEs. When the PE determines, by MSDP, that a
particular source is no longer active, then it withdraws the
corresponding BGP update. Then the PEs do not have to join any
shared C-trees, but they do not have to be C-RPs either.
MSDP provides the capability for a Source Active message to carry an
encapsulated data packet. This capability can be used to allow an
MSDP speaker to receive the first (or first several) packet(s) of an
(S,G) flow, even though the MSDP speaker hasn't yet joined the (S,G)
tree. (Presumably it will join that tree as a result of receiving
the SA message which carries the encapsulated data packet.) If this
capability is not used, the first several data packets of an (S,G)
stream may be lost.
A PE which is talking MSDP to an RP may receive such an encapsulated
data packet from the RP. The data packet should be decapsulated and
transmitted to the other PEs in the MVPN. If the packet belongs to a
particular (S,G) flow, and if the PE is a transmitter for some S-PMSI
to which (S,G) has already been bound, the decapsulated data packet
should be transmitted on that S-PMSI. Otherwise, if an I-PMSI exists
for that MVPN, the decapsulated data packet should be transmitted on
it. (If a MI-PMSI exists, this would typically be used.) If neither
of these conditions hold, the decapsulated data packet is not
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transmitted to the other PEs in the MVPN. The decision as to whether
and how to transmit the decapsulated data packet does not effect the
processing of the SA control message itself.
Suppose that PE1 transmits a multicast data packet on a PMSI, where
that data packet is part of an (S,G) flow, and PE2 receives that
packet from that PMSI. According to section 9, if PE1 is not the PE
that PE2 expects to be transmitting (S,G) packets, then PE2 must
discard the packet. If an MSDP-encapsulated data packet is
transmitted on a PMSI as specified above, this rule from section 9
would likely result in the packet's getting discarded. Therefore, if
MSDP-encapsulated data packets being decapsulated and transmitted on
a PMSI, we need to modify the rules of section 9 as follows:
1. If the receiving PE, PE2, has already joined the (S,G) tree,
and has chosen PE1 as the upstream PE for the (S,G) tree, but
this packet does not come from PE1, PE2 must discard the
packet.
2. If the receiving PE, PE2, has not already joined the (S,G)
tree, but is a PIM adjacency to a CE which is downstream on the
(*,G) tree, the packet should be forwarded to the CE.
11. Encapsulations
The BGP-based auto-discovery procedures will ensure that the PEs in a
single MVPN only use tunnels that they can all support, and for a
given kind of tunnel, that they only use encapsulations that they can
all support.
11.1. Encapsulations for Single PMSI per Tunnel
11.1.1. Encapsulation in GRE
GRE encapsulation can be used for any PMSI that is instantiated by a
mesh of unicast tunnels, as well as for any PMSI that is instantiated
by one or more PIM tunnels of any sort.
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Packets received Packets in transit Packets forwarded
at ingress PE in the service by egress PEs
provider network
+---------------+
| P-IP Header |
+---------------+
| GRE |
++=============++ ++=============++ ++=============++
|| C-IP Header || || C-IP Header || || C-IP Header ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-Payload || || C-Payload || || C-Payload ||
++=============++ ++=============++ ++=============++
The IP Protocol Number field in the P-IP Header must be set to 47.
The Protocol Type field of the GRE Header must be set to 0x800.
When an encapsulated packet is transmitted by a particular PE, the
source IP address in the P-IP header must be the same address that
the PE uses to identify itself in the VRF Route Import Extended
Communities that it attaches to any of VPN-IP routes eligible for UMH
determination that it advertises via BGP (see section 5.1).
If the PMSI is instantiated by a PIM tree, the destination IP address
in the P-IP header is the group P-address associated with that tree.
The GRE key field value is omitted.
If the PMSI is instantiated by unicast tunnels, the destination IP
address is the address of the destination PE, and the optional GRE
Key field is used to identify a particular MVPN. In this case, each
PE would have to advertise a key field value for each MVPN; each PE
would assign the key field value that it expects to receive.
[RFC2784] specifies an optional GRE checksum, and [RFC2890] specifies
an optional GRE sequence number fields.
The GRE sequence number field is not needed because the transport
layer services for the original application will be provided by the
C-IP Header.
The use of GRE checksum field must follow [RFC2784].
To facilitate high speed implementation, this document recommends
that the ingress PE routers encapsulate VPN packets without setting
the checksum, or sequence fields.
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11.1.2. Encapsulation in IP
IP-in-IP [RFC1853] is also a viable option. When it is used, the
IPv4 Protocol Number field is set to 4. The following diagram shows
the progression of the packet as it enters and leaves the service
provider network.
Packets received Packets in transit Packets forwarded
at ingress PE in the service by egress PEs
provider network
+---------------+
| P-IP Header |
++=============++ ++=============++ ++=============++
|| C-IP Header || || C-IP Header || || C-IP Header ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-Payload || || C-Payload || || C-Payload ||
++=============++ ++=============++ ++=============++
When an encapsulated packet is transmitted by a particular PE, the
source IP address in the P-IP header must be the same address that
the PE uses to identify itself in the VRF Route Import Extended
Communities that it attaches to any of VPN-IP routes eligible for UMH
determination that it advertises via BGP (see section 5.1).
11.1.3. Encapsulation in MPLS
If the PMSI is instantiated as a P2MP MPLS LSP or MP2MP LSP, MPLS
encapsulation is used. Penultimate-hop-popping must be disabled for
the P2MP MPLS LSP. If the PMSI is instantiated as an RSVP-TE P2MP
LSP, additional MPLS encapsulation procedures are used, as specified
in [RSVP-P2MP].
If other methods of assigning MPLS labels to multicast distribution
trees are in use, these multicast distribution trees may be used as
appropriate to instantiate PMSIs, and appropriate additional MPLS
encapsulation procedures may be used.
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Packets received Packets in transit Packets forwarded
at ingress PE in the service by egress PEs
provider network
+---------------+
| P-MPLS Header |
++=============++ ++=============++ ++=============++
|| C-IP Header || || C-IP Header || || C-IP Header ||
++=============++ >>>>> ++=============++ >>>>> ++=============++
|| C-Payload || || C-Payload || || C-Payload ||
++=============++ ++=============++ ++=============++
11.2. Encapsulations for Multiple PMSIs per Tunnel
The encapsulations for transmitting multicast data messages when
there are multiple PMSIs per tunnel are based on the encapsulation
for a single PMSI per tunnel, but with an MPLS label used for
demultiplexing.
The label is upstream-assigned and distributed via BGP as specified
in section 4. The label must enable the receiver to select the
proper VRF, and may enable the receiver to select a particular
multicast routing entry within that VRF.
11.2.1. Encapsulation in GRE
Rather than the IP-in-GRE encapsulation discussed in section 11.1.1,
we use the MPLS-in-GRE encapsulation. This is specified in [MPLS-
IP]. The GRE protocol type MUST be set to 0x8847. [The reason for
using the unicast rather than the multicast value is specified in
[MPLS-MCAST-ENCAPS].
11.2.2. Encapsulation in IP
Rather than the IP-in-IP encapsulation discussed in section 12.1.2,
we use the MPLS-in-IP encapsulation. This is specified in [MPLS-IP].
The IP protocol number MUST be set to the value identifying the
payload as an MPLS unicast packet. [There is no "MPLS multicast
packet" protocol number.]
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11.3. Encapsulations Identifying a Distinguished PE
11.3.1. For MP2MP LSP P-tunnels
As discussed in section 9, if a multicast data packet belongs to a
Sparse Mode or Single Source Mode multicast group, it is highly
desirable for the PE that receives the packet from a PMSI to be able
to determine the identity of the PE that transmitted the data packet
onto the PMSI. The encapsulations of the previous sections all
provide this information, except in one case. If a PMSI is being
instantiated by a MP2MP LSP, then the encapsulations discussed so far
do not allow one to determine the identity of the PE that transmitted
the packet onto the PMSI.
Therefore, when a packet that belongs to a Sparse Mode or Single
Source Mode multicast group is traveling on a MP2MP LSP P-tunnel, it
MUST carry, as its second label, a label which has been bound to the
packet's ingress PE. This label is an upstream-assigned label that
the LSP's root node has bound to the ingress PE and has distributed
via an A-D Route (see section 4; precise details of this distribution
procedure will be included in the next revision of this document).
This label will appear immediately beneath the labels that are
discussed in sections 11.1.3 and 11.2.
11.3.2. For Support of PIM-BIDIR C-Groups
As will be discussed in section 12, when a packet belongs to a PIM-
BIDIR multicast group, the set of PEs of that packet's VPN can be
partitioned into a number of subsets, where exactly one PE in each
partition is the upstream PE for that partition. When such packets
are transmitted on a PMSI, then unless the procedures of section
12.2.3 are being used, it is necessary for the packet to carry
information identifying a particular partition. This is done by
having the packet carry the PE label corresponding to the upstream PE
of one partition. For a particular P-tunnel, this label will have
been advertised by the node which is the root of that P-tunnel.
(Details of the procedure by which the PE labels are advertised will
be included in the next revision of this document.)
This label needs to be used whenever a packet belongs to a PIM-BIDIR
C-group, no matter what encapsulation is used by the P-tunnel. Hence
the encapsulations of section 11.2 MUST be used. If the tunnel
contains only one PMSI, the PE label replaces the label discussed in
section 11.2 If the tunnel contains multiple PMSIs, the PE label
follows the label discussed in section 11.2
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11.4. Encapsulations for Unicasting PIM Control Messages
When PIM control messages are unicast, rather than being sent on an
MI-PMSI, then the receiving PE needs to determine the particular MVPN
whose multicast routing information is being carried in the PIM
message. One method is to use a downstream-assigned MPLS label which
the receiving PE has allocated for this specific purpose. The label
would be distributed via BGP. This can be used with an MPLS, MPLS-
in-GRE, or MPLS-in-IP encapsulation.
A possible alternative to modify the PIM messages themselves so that
they carry information which can be used to identify a particular
MVPN, such as an RT.
This area is still under consideration.
11.5. General Considerations for IP and GRE Encaps
These apply also to the MPLS-in-IP and MPLS-in-GRE encapsulations.
11.5.1. MTU
It is the responsibility of the originator of a C-packet to ensure
that the packet is small enough to reach all of its destinations,
even when it is encapsulated within IP or GRE.
When a packet is encapsulated in IP or GRE, the router that does the
encapsulation MUST set the DF bit in the outer header. This ensures
that the decapsulating router will not need to reassemble the
encapsulating packets before performing decapsulation.
In some cases the encapsulating router may know that a particular C-
packet is too large to reach its destinations. Procedures by which
it may know this are outside the scope of the current document.
However, if this is known, then:
- If the DF bit is set in the IP header of a C-packet which is
known to be too large, the router will discard the C-packet as
being "too large", and follow normal IP procedures (which may
require the return of an ICMP message to the source).
- If the DF bit is not set in the IP header of a C-packet which is
known to be too large, the router MAY fragment the packet before
encapsulating it, and then encapsulate each fragment separately.
Alternatively, the router MAY discard the packet.
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If the router discards a packet as too large, it should maintain OAM
information related to this behavior, allowing the operator to
properly troubleshoot the issue.
Note that if the entire path of the tunnel does not support an MTU
which is large enough to carry the a particular encapsulated C-
packet, and if the encapsulating router does not do fragmentation,
then the customer will not receive the expected connectivity.
11.5.2. TTL
The ingress PE should not copy the TTL field from the payload IP
header received from a CE router to the delivery IP or MPLS header.
The setting of the TTL of the delivery header is determined by the
local policy of the ingress PE router.
11.5.3. Avoiding Conflict with Internet Multicast
If the SP is providing Internet multicast, distinct from its VPN
multicast services, and using PIM based P-multicast trees, it must
ensure that the group P-addresses which it used in support of MPVN
services are distinct from any of the group addresses of the Internet
multicasts it supports. This is best done by using administratively
scoped addresses [ADMIN-ADDR].
The group C-addresses need not be distinct from either the group P-
addresses or the Internet multicast addresses.
11.6. Differentiated Services
The setting of the DS field in the delivery IP header should follow
the guidelines outlined in [RFC2983]. Setting the EXP field in the
delivery MPLS header should follow the guidelines in [RFC3270]. An SP
may also choose to deploy any of additional Differentiated Services
mechanisms that the PE routers support for the encapsulation in use.
Note that the type of encapsulation determines the set of
Differentiated Services mechanisms that may be deployed.
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12. Support for PIM-BIDIR C-Groups
In BIDIR-PIM, each multicast group is associated with an RPA
(Rendezvous Point Address). The Rendezvous Point Link (RPL) is the
link that attaches to the RPA. Usually it's a LAN where the RPA is
in the IP subnet assigned to the LAN. The root node of a BIDIR-PIM
tree is a node which has an interface on the RPL.
On any LAN (other than the RPL) which is a link in a PIM-bidir tree,
there must be a single node that has been chosen to be the DF. (More
precisely, for each RPA there is a single node which is the DF for
that RPA.) A node which receives traffic from an upstream interface
may forward it on a particular downstream interface only if the node
is the DF for that downstream interface. A node which receives
traffic from a downstream interface may forward it on an upstream
interface only if that node is the DF for the downstream interface.
If, for any period of time, there is a link on which each of two
different nodes believes itself to be the DF, data forwarding loops
can form. Loops in a bidirectional multicast tree can be very
harmful. However, any election procedure will have a convergence
period. The BIDIR-PIM DF election procedures is very complicated,
because it goes to great pains to ensure that if convergence is not
extremely fast, then there is no forwarding at all until convergence
has taken place.
Other variants of PIM also have a DF election procedure for LANs.
However, as long as the multicast tree is unidirectional,
disagreement about who the DF is can result only in duplication of
packets, not in loops. Therefore the time taken to converge on a
single DF is of much less concern for unidirectional trees and it is
for bidirectional trees.
In the MVPN environment, if PIM signaling is used among the PEs, the
can use the standard LAN-based DF election procedure can be used.
However, election procedures that are optimized for a LAN may not
work as well in the MVPN environment. So an alternative to DF
election would be desirable.
If BGP signaling is used among the PEs, an alternative to DF election
is necessary. One might think that use the "single forwarder
selection" procedures described in sections 5 and 9 coudl be used to
choose a single PE "DF" for the backbone (for a given RPA in a given
MVPN). However, that is still likely to leave a convergence period
of at least several seconds during which loops could form, and there
could be a much longer convergence period if there is anything
disrupting the smooth flow of BGP updates. So a simple procedure
like that is not sufficient.
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The remainder of this section describes two different methods that
can be used to support BIDIR-PIM while eliminating the DF election.
12.1. The VPN Backbone Becomes the RPL
On a per MVPN basis, this method treats the whole service provider(s)
infrastructure as a single RPL (RP Link). We refer to such an RPL as
an "MVPN-RPL". This eliminates the need for the PEs to engage in any
"DF election" procedure, because PIM-bidir does not have a DF on the
RPL.
However, this method can only be used if the customer is
"outsourcing" the RPL/RPA functionality to the SP.
An MVPN-RPL could be realized either via an I-PMSI (this I-PMSI is on
a per MVPN basis and spans all the PEs that have sites of a given
MVPN), or via a collection of S-PMSIs, or even via a combination of
an I-PMSI and one or more S-PMSIs.
12.1.1. Control Plane
Associated with each MVPN-RPL is an address prefix that is
unambiguous within the context of the MVPN associated with the MVPN-
RPL.
For a given MVPN, each VRF connected to an MVPN-RPL of that MVPN is
configured to advertise to all of its connected CEs the address
prefix of the MVPN-RPL.
Since in PIM Bidir there is no Designated Forwarder on an RPL, in the
context of MVPN-RPL there is no need to perform the Designated
Forwarder election among the PEs (note there is still necessary to
perform the Designated Forwarder election between a PE and its
directly attached CEs, but that is done using plain PIM Bidir
procedures).
For a given MVPN a PE connected to an MVPN-RPL of that MVPN should
send multicast data (C-S,C-G) on the MVPN-RPL only if at least one
other PE connected to the MVPN-RPL has a downstream multicast state
for C-G. In the context of MVPN this is accomplished by requring a PE
that has a downstream state for a particular C-G of a particular VRF
present on the PE to originate a C-multicast route for (*, C-G). The
RD of this route should be the same as the RD associated with the
VRF. The RT(s) carried by the route should be the same as the one(s)
used for VPN-IPv4 routes. This route will be distributed to all the
PEs of the MVPN.
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12.1.2. Data Plane
A PE that receives (C-S,C-G) multicast data from a CE should forward
this data on the MVPN-RPL of the MVPN the CE belongs to only if the
PE receives at least one C-multicast route for (*, C-G). Otherwise,
the PE should not forward the data on the RPL/I-PMSI.
When a PE receives a multicast packet with (C-S,C-G) on an MVPN-RPL
associated with a given MVPN, the PE forwards this packet to every
directly connected CE of that MVPN, provided that the CE sends Join
(*,C-G) to the PE (provided that the PE has the downstream (*,C-G)
state). The PE does not forward this packet back on the MVPN-RPL. If
a PE has no downstream (*,C-G) state, the PE does not forward the
packet.
12.2. Partitioned Sets of PEs
This method does not require the use of the MVPN-RPL, and does not
require the customer to outsource the RPA/RPL functionality to the
SP.
12.2.1. Partitions
Consider a particular C-RPA, call it C-R, in a particular MVPN.
Consider the set of PEs that attach to sites that have senders or
receivers for a BIDIR-PIM group C-G, where C-R is the RPA for C-G.
(As always we use the "C-" prefix to indicate that we are referring
to an address in the VPN's address space rather than in the
provider's address space.)
Following the procedures of section 5.1, each PE in the set
independently chooses some other PE in the set to be its "upstream
PE" for those BIDIR-PIM groups with RPA C-R. Optionally, they can
all choose the "default selection" (described in section 5.1), to
ensure that each PE to choose the same upstream PE. Note that if a
PE has a route to C-R via a VRF interface, then the PE may choose
itself as the upstream PE.
The set of PEs can now be partitioned into a number of subsets.
We'll say that PE1 and PE2 are in the same partition if and only if
there is some PE3 such that PE1 and PE2 have each chosen PE3 as the
upstream PE for C-R. Note that each partition has exactly one
upstream PE. So it is possible to identify the partition by
identifying its upstream PE.
Consider packet P, and let PE1 be its ingress PE. PE1 will send the
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packet on a PMSI so that it reaches the other PEs that need to
receive it. This is done by encapsulating the packet and sending it
on a P-tunnel. If the original packet is part of a PIM-BIDIR group
(its ingress PE determines this from the packet's destination address
C-G), and if the VPN backbone is not the RPL, then the encapsulation
MUST carry information that can be used to identify the partition to
which the ingress PE belongs.
When PE2 receives a packet from the PMSI, PE2 must determine, by
examining the encapsulation, whether the packet's ingress PE belongs
to the same partition (relative to the C-RPA of the packet's C-G)
that PE2 itself belongs to. If not, PE2 discards the packet.
Otherwise PE2 performs the normal BIDIR-PIM data packet processing.
With this rule in place, harmful loops cannot be introduced by the
PEs into the customer's bidirectional tree.
Note that if there is more than one partition, the VPN backbone will
not carry a packet from one partition to another. The only way for a
packet to get from one partition to another is for it to go up
towards the RPA and then to go down another path to the backbone. If
this is not considered desirable, then all PEs should choose the same
upstream PE for a given C-RPA. Then multiple partitions will only
exist during routing transients.
12.2.2. Using PE Labels
If a given P-tunnel is to be used to carry packets belonging to a
bidirectional C-group, then, EXCEPT for the case described in section
12.2.3 the packets that travel on that P-tunnel MUST carry a PE label
(defined in section 4), using the encapsulation discussed in section
11.3.
When a given PE transmits a given packet of a bidirectional C-group
to the P-tunnel, the packet will carry the PE label corresponding to
the partition, for the C-group's C-RPA, that contains the
transmitting PE. This is the PE label that has been bound to the
upstream PE of that partition; it is not necessarily the label that
has been bound to the transmitting PE.
Recall that the PE labels are upstream-assigned labels that are
assigned and advertised by the node which is at the root of the P-
tunnel. (Procedures for PE label assignment when the P-tunnel is not
a multicast tree will be given is later revisions of this document.)
When a PE receives a packet with a PE label that does not identify
the partition of the receiving PE, then the receiving PE discards the
packet.
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Note that this procedure does not require the root of a P-tunnel to
assign a PE label for every PE that belongs to the tunnel, but only
for those PEs that might become the upstream PEs of some partition.
12.2.3. Mesh of MP2MP P-Tunnels
There is one case in which support for BIDIR-PIM C-groups does not
require the use of a PE label. For a given C-RPA, suppose a distinct
MP2MP LSP is used as the P-tunnel serving that partition. Then for a
given packet, a PE receiving the packet from a P-tunnel can be infer
the partition from the tunnel. So PE labels are not needed in this
case.
13. Security Considerations
This document describes an extension to the procedures of [RFC4364],
and hence shares the security considerations described in [RFC4364]
and [RFC4365].
When GRE encapsulation is used, the security considerations of [MPLS-
IP] are also relevant. The security considerations of [RFC4797] are
also relevant as it discusses implications on packet spoofing in the
context of 2547 VPNs.
The security considerations of [MPLS-HDR] apply when MPLS
encapsulation is used.
This document makes use of a number of control protocols: PIM [PIM-
SM], BGP MVPN-BGP], mLDP [MLDP], and RSVP-TE [RSVP-P2MP]. Security
considerations relevant to each protocol are discussed in the
respective protocol specifications.
If one uses the UDP-based protocol for switching to S-PMSI (as
specified in Section 7.2.1), then by default each PE router MUST
install packet filters that would result in discarding all UDP
packets with the destination port 3232 that the PE router receives
from the CE routers connected to the PE router.
The various procedures for P-tunnel construction have security issues
that are specific to the way in which the P-tunnels are used in this
document. When P-tunnels are constructed via such techniques as as
PIM, mLDP, or RSVP-TE, it is important for each P or PE router
receiving a control message to be sure that the control message comes
from another P or PE router, not from a CE router. This should not
be a problem, because mLDP or PIM or RSVP-TE control messages from CE
routers will never be interpreted as referring to P-tunnels.
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An ASBR may receive, from one SP's domain, an mLDP, PIM, or RSVP-TE
control message that attempts to extend a multicast distribution tree
from one SP's domain into another SP's domain. The ASBR should not
allow this unless explicitly configured to do so.
14. IANA Considerations
Section 7.2.1.1 defines the "S-PMSI Join Message", which is carried
in a UDP datagram whose port number is 3232. This port number is
already assigned by IANA to "MDT port". IANA should now have that
assignment reference this document.
IANA should create a registry for the "S-PMSI Join Message Type
Field". The value 1 should be registered with a reference to this
document. The description should read "PIM IPv4 S-PMSI
(unaggregated)".
15. Other Authors
Sarveshwar Bandi, Yiqun Cai, Thomas Morin, Yakov Rekhter, IJsbrands
Wijnands, Seisho Yasukawa
16. Other Contributors
Significant contributions were made Arjen Boers, Toerless Eckert,
Adrian Farrel, Luyuan Fang, Dino Farinacci, Lenny Guiliano, Shankar
Karuna, Anil Lohiya, Tom Pusateri, Ted Qian, Robert Raszuk, Tony
Speakman, Dan Tappan.
17. Authors' Addresses
Rahul Aggarwal (Editor)
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
Email: rahul@juniper.net
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Sarveshwar Bandi
Motorola
Vanenburg IT park, Madhapur,
Hyderabad, India
Email: sarvesh@motorola.com
Yiqun Cai
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
E-mail: ycai@cisco.com
Thomas Morin
France Telecom R & D
2, avenue Pierre-Marzin
22307 Lannion Cedex
France
Email: thomas.morin@francetelecom.com
Yakov Rekhter
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
Email: yakov@juniper.net
Eric C. Rosen (Editor)
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA, 01719
E-mail: erosen@cisco.com
IJsbrand Wijnands
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
E-mail: ice@cisco.com
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Seisho Yasukawa
NTT Corporation
9-11, Midori-Cho 3-Chome
Musashino-Shi, Tokyo 180-8585,
Japan
Phone: +81 422 59 4769
Email: yasukawa.seisho@lab.ntt.co.jp
18. Normative References
[MLDP] I. Minei, K., Kompella, I. Wijnands, B. Thomas, "Label
Distribution Protocol Extensions for Point-to-Multipoint and
Multipoint-to-Multipoint Label Switched Paths", draft-ietf-mpls-ldp-
p2mp-03, July 2007
[MPLS-HDR] E. Rosen, et. al., "MPLS Label Stack Encoding", RFC 3032,
January 2001
[MPLS-IP] T. Worster, Y. Rekhter, E. Rosen, "Encapsulating MPLS in IP
or Generic Routing Encapsulation (GRE)", RFC 4023, March 2005
[MPLS-MCAST-ENCAPS] T. Eckert, E. Rosen, R. Aggarwal, Y. Rekhter,
"MPLS Multicast Encapsulations", draft-ietf-mpls-multicast-
encaps-06.txt, July 2007
[MPLS-UPSTREAM-LABEL] R. Aggarwal, Y. Rekhter, E. Rosen, "MPLS
Upstream Label Assignment and Context Specific Label Space", draft-
ietf-mpls-upstream-label-02.txt, March 2007
[MVPN-BGP], R. Aggarwal, E. Rosen, T. Morin, Y. Rekhter, C.
Kodeboniya, "BGP Encodings for Multicast in MPLS/BGP IP VPNs", draft-
ietf-l3vpn-2547bis-mcast-bgp-04.txt, November 2007
[PIM-ATTRIB], A. Boers, IJ. Wijnands, E. Rosen, "Format for Using
TLVs in PIM Messages", draft-ietf-pim-join-attributes-03, May 2007
[PIM-SM] "Protocol Independent Multicast - Sparse Mode (PIM-SM)",
Fenner, Handley, Holbrook, Kouvelas, August 2006, RFC 4601
[RFC2119] "Key words for use in RFCs to Indicate Requirement
Levels.", Bradner, March 1997
[RFC4364] "BGP/MPLS IP VPNs", Rosen, Rekhter, et. al., February 2006
[RSVP-P2MP] R. Aggarwal, D. Papadimitriou, S. Yasukawa, et. al.,
"Extensions to RSVP-TE for Point-to-Multipoint TE LSPs", RFC 4875,
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May 2007
19. Informative References
[ADMIN-ADDR] D. Meyer, "Administratively Scoped IP Multicast", RFC
2365, July 1998
[MVPN-REQ] T. Morin, Ed., "Requirements for Multicast in L3 Provider-
Provisioned VPNs", RFC 4834, April 2007
[RFC1853] W. Simpson, "IP in IP Tunneling", October 1995
[RFC2784] D. Farinacci, et. al., "Generic Routing Encapsulation",
March 2000
[RFC2890] G. Dommety, "Key and Sequence Number Extensions to GRE",
September 2000
[RFC2983] D. Black, "Differentiated Services and Tunnels", October
2000
[RFC3270] F. Le Faucheur, et. al., "MPLS Support of Differentiated
Services", May 2002
[RFC4365], E. Rosen, " Applicability Statement for BGP/MPLS IP
Virtual Private Networks (VPNs)", February 2006
[RFC4607] H. Holbrook, B. Cain, "Source-Specific Multicast for IP",
August 2006
[RFC4797] Y. Rekhter, R. Bonica, E. Rosen, "Use of Provider Edge to
Provider Edge (PE-PE) Generic Routing Encapsulation (GRE) or IP in
BGP/MPLS IP Virtual Private Networks", January 2007
20. Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
21. Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
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