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IETF RFC 7988
Last modified on Friday, October 7th, 2016
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Internet Engineering Task Force (IETF) E. Rosen, Ed.
Request for Comments: 7988 Juniper Networks, Inc.
Updates: 6513, 6514 K. Subramanian
Category: Standards Track Sproute Networks
ISSN: 2070-1721 Z. Zhang
Juniper Networks, Inc.
October 2016
Ingress Replication Tunnels in Multicast VPN
 Abstract
RFCs 6513, 6514, and other RFCs describe procedures by which a
Service Provider may offer Multicast VPN (MVPN) service to its
customers. These procedures create point-to-multipoint (P2MP) or
multipoint-to-multipoint (MP2MP) trees across the Service Provider's
backbone. One type of P2MP tree that may be used is known as an
"Ingress Replication (IR) tunnel". In an IR tunnel, a parent node
need not be directly connected to its child nodes. When a parent
node has to send a multicast data packet to its n child nodes, it
does not use Layer 2 multicast, IP multicast, or MPLS multicast to do
so. Rather, it makes n individual copies, and then unicasts each
copy, through an IP or MPLS unicast tunnel, to exactly one child
node. While the prior MVPN specifications allow the use of IR
tunnels, those specifications are not always very clear or explicit
about how the MVPN protocol elements and procedures are applied to IR
tunnels. This document updates RFCs 6513 and 6514 by adding
additional details that are specific to the use of IR tunnels.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7988.
Rosen, et al. Standards Track PAGE 1 
RFC 7988 IR Tunnels in MVPN October 2016
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. What is an IR P-tunnel? . . . . . . . . . . . . . . . . . . . 5
3. How are IR P-tunnels identified? . . . . . . . . . . . . . . 7
4. How to Join an IR P-Tunnel . . . . . . . . . . . . . . . . . 9
4.1. Advertised IR P-Tunnels . . . . . . . . . . . . . . . . . 9
4.1.1. If the Leaf Information Required Bit Is Set . . . . . 10
4.1.2. If the Leaf Information Required Bit Is Not Set . . . 10
4.2. Unadvertised IR P-Tunnels . . . . . . . . . . . . . . . . 11
5. The PTA's Tunnel Identifier Field . . . . . . . . . . . . . . 11
6. A Note on IR P-Tunnels and Discarding Packets from the Wrong
PE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. The PTA's MPLS Label Field . . . . . . . . . . . . . . . . . 14
7.1. Leaf A-D Route Originated by an Egress PE . . . . . . . . 14
7.2. Leaf A-D Route Originated by an Intermediate Node . . . . 16
7.3. Intra-AS I-PMSI A-D Route . . . . . . . . . . . . . . . . 17
8. How A Child Node Prunes Itself from an IR P-Tunnel . . . . . 17
9. Parent Node Actions upon Receiving Leaf A-D Route . . . . . . 18
10. Use of Timers When Switching UMH . . . . . . . . . . . . . . 19
11. Security Considerations . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
Rosen, et al. Standards Track PAGE 2
RFC 7988 IR Tunnels in MVPN October 2016
1. Introduction
RFCs 6513, 6514, and other RFCs describe procedures by which a
Service Provider (SP) may offer Multicast VPN (MVPN) service to its
customers. These procedures create point-to-multipoint (P2MP) or
multipoint-to-multipoint (MP2MP) tunnels, called "P-tunnels"
(provider tunnels), across the SP's backbone network. Customer
multicast traffic is carried through the P-tunnels.
A number of different P-tunnel technologies are supported. One of
the supported P-tunnel technologies is known as "ingress replication"
or "unicast replication". We will use the acronym "IR" to refer to
this P-tunnel technology.
An IR P-tunnel is a P2MP tree, but a given node on the tree is not
necessarily directly attached to its parent node or to its child
nodes. To send a multicast data packet from a parent node to one of
its child nodes, the parent node encapsulates the packet and then
unicasts it through a tunnel to the child node. The tunnel may be a
P2P or MP2P MPLS LSP (Label Switched Path) or a unicast IP tunnel.
If a node on an IR tree has n child nodes, and has a multicast data
packet that must be sent along the tree, the parent node makes n
individual copies of the data packet, and then sends each copy,
through a unicast tunnel, to exactly one child node. No lower-layer
multicast technology is used when sending traffic from a parent node
to a child node; therefore, multiple copies of the packet may be sent
out a single interface.
With the single exception of IR, the P-tunnel technologies supported
by the MVPN specifications are preexisting IP multicast or MPLS
multicast technologies. Each such technology has its own set of
specifications, its own setup and maintenance protocols, its own
syntax for identifying specific multicast trees, and its own
procedures for enabling a router to be added to or removed from a
particular multicast tree. For IR P-tunnels, on the other hand,
there is no prior specification for setting up and maintaining the
P2MP trees; the procedures and protocol elements used for setting up
and maintaining the P2MP trees are specified in the MVPN
specifications themselves, and all the signaling/setup is done by
using the BGP Auto-Discovery (A-D) routes that are defined in
[RFC 6514]. (The unicast tunnels used to transmit multicast data from
one node to another in an IR P-tunnel may of course have their own
setup and maintenance protocols, e.g., [RFC 5036], [RFC 3209].)
Since the transmission of a multicast data packet along an IR
P-tunnel is done by transmitting the packet through a unicast tunnel,
previous RFCs sometimes describe an IR P-tunnel as "consisting of" a
set of unicast tunnels. However, that description is not quite
Rosen, et al. Standards Track PAGE 3
RFC 7988 IR Tunnels in MVPN October 2016
accurate. For one thing, it obscures the fact that an IR P-tunnel is
really a P2MP tree, whose nodes must maintain multicast state in both
the control and data planes. For another, it obscures the fact the
unicast tunnels used by a particular IR P-tunnel need not be specific
to that P-tunnel; a single unicast tunnel can carry the multicast
traffic of many different IR P-tunnels (and can also carry unicast
traffic as well).
In this document, we provide a clearer and more explicit conceptual
model for IR P-tunnels, clarifying the relationship between an IR
P-tunnel and the unicast tunnels that are used for data transmission
along the IR P-tunnel.
Section 5 of [RFC 6514] defines a BGP Path Attribute known as the
"PMSI (Provider Multicast Service Interface) Tunnel attribute" (PTA).
This attribute contains a field known as the "Tunnel Identifier"
field. For most P-tunnel technologies, the PTA's "Tunnel Identifier"
field is used to identify a P-tunnel (i.e., to identify a P2MP or
MP2MP tree). However, when IR P-tunnels are used, the PTA "Tunnel
Identifier" field does not actually identify an IR P-tunnel. In some
cases, it identifies one of the P-tunnel's constituent unicast
tunnels; in other cases, it is not used to identify a tunnel at all.
In this document, we provide an explicit specification for how IR
P-tunnels are actually identified.
Some of the MVPN specifications specify procedures that require a PE
router to join the P-tunnel that has been identified in a particular
MVPN route. However, up to now, there has not been an explicit
specification of how to identify an IR P-tunnel, of how a router
joins such a P-tunnel, or of how a router prunes itself from such a
P-tunnel. In this document, we make these procedures more explicit.
[RFC 6514] does provide a method for binding an MPLS label to a
P-tunnel, but does not discuss the label allocation policies that are
needed for correct operation when the P-tunnel is an IR P-tunnel.
Those policies are discussed in this document.
This document does not provide any new protocol elements or any
fundamentally new procedures; its purpose is to make explicit just
how a router is to use the protocol elements and procedures of
[RFC 6513] and [RFC 6514] to identify an IR P-tunnel, to join an IR
P-tunnel, and to prune itself from an IR P-tunnel.
This document also discusses the MPLS label allocation policies that
need to be supported when binding MPLS labels to IR P-tunnels, and
the timer policies that need to be supported when switching a
customer multicast flow from one IR P-tunnel to another. These are
procedures that are not clearly specified in [RFC 6513] or [RFC 6514].
Rosen, et al. Standards Track PAGE 4
RFC 7988 IR Tunnels in MVPN October 2016
As the material in this document must be understood in order to
properly implement IR P-tunnels, this document updates [RFC 6513] and
[RFC 6514].
This document also discusses the application of "seamless multicast"
[RFC 7524] and "extranet" [RFC 7900] procedures to IR P-tunnels.
This document does not discuss the use of IR P-tunnels to support a
VPN customer's use of Bidirectional Protocol Independent Multicast
(BIDIR-PIM). [RFC 7740] explains how to adapt the procedures of
[RFC 6513], [RFC 6514], and [RFC 7582] so that a customer's use of
BIDIR-PIM can be supported by IR P-tunnels.
In the event of any conflict between this document and either
[RFC 6513] or [RFC 6514], this document takes precedence.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL", when and only when appearing in all capital letters, are
to be interpreted as described in [RFC 2119].
2. What is an IR P-tunnel?
An IR P-tunnel is a P2MP tree. Its nodes are BGP speakers that
support the MVPN procedures of [RFC 6514] and related RFCs. In
general, the nodes of an IR P-tunnel are either Provider Edge (PE)
routers, Autonomous System Border Routers (ASBRs), or (if [RFC 7524]
is supported) Area Border Routers (ABRs). (MVPN procedures are
sometimes used to support non-MVPN, or "global table" multicast; one
way of doing this is defined in [RFC 7524]. Another way is defined in
[RFC 7716]. In such cases, IR P-tunnels can be used outside the
context of MVPN.)
MVPN P-tunnels may be either "segmented" or "non-segmented" (as these
terms are defined in [RFC 6513] and [RFC 6514]).
A "non-segmented" IR P-tunnel is a two-level P2MP tree, consisting
only of a root node and a set of nodes that are children of the root
node. When used in an MVPN context, the root is an ingress PE, and
the child nodes of the root are the egress PEs.
In a segmented P-tunnel, IR may be used for some or all of the
segments. If a particular segment of a segmented P-tunnel uses IR,
then the root of that segment may have child nodes that are ABRs or
ASBRs, rather than egress PEs.
Rosen, et al. Standards Track PAGE 5
RFC 7988 IR Tunnels in MVPN October 2016
As with any type of P2MP tree, each node of an IR P-tunnel holds
"multicast state" for the P-tunnel. That is, each node knows the
identity of its parent node on the tree, and each node knows the
identities of its child nodes on the tree. In the MVPN specs, the
"parent" node is also known as the "Upstream Multicast Hop" or "UMH".
Note that the UMH may be a PE, an ASBR, or (if procedures from
[RFC 7524] are being used) an ABR. (In [RFC 7524], the term "upstream
node" is used instead of "UMH".)
What distinguishes an IR P-tunnel from any other kind of P2MP tree is
the method by which a data packet is transmitted from a parent node
to a child node. To transmit a multicast data packet from a parent
node to a child node along a particular IR P-tunnel, the parent node
does the following:
o It labels the packet with a label (call it a "P-tunnel label")
that the child node has assigned to that P-tunnel.
o It then places the packet in a unicast encapsulation and unicasts
the packet to the child node. That is, the parent node sends the
packet through a unicast tunnel to a particular child node. This
unicast tunnel need not be specially created to be part of the IR
P-tunnel; it can be any P2P or MP2P unicast tunnel that will get
the packets from the parent node to the child node. A single such
unicast tunnel may be carrying multicast data packets of several
different P2MP trees and may also be carrying unicast data
packets.
The parent node repeats this process for each child node, creating
one copy for each child node, and sending each copy through a unicast
tunnel to corresponding child node. It does not use Layer 2
multicast, IP multicast, or MPLS multicast to transmit packets to its
child nodes. As a result, multiple copies of each packet may be sent
out a single interface; this may happen, e.g., if that interface is
the next-hop interface, according to unicast routing, from the parent
node to several of the child nodes.
Since data traveling along an IR P-tunnel is always unicast from
parent node to child node, it can be convenient to think of an IR
P-tunnel as a P2MP tree whose arcs are unicast tunnels. However, it
is important to understand that the unicast tunnels need not be
specific to any particular IR P-tunnel. If R1 is the parent node of
R2 on two different IR P-tunnels, a single unicast tunnel from R1 to
R2 may be used to carry data along both IR P-tunnels. All that is
required is that when the data packets arrive at R2, R2 will see the
"P-tunnel label" at the top of the packets' label stack; R2's further
Rosen, et al. Standards Track PAGE 6
RFC 7988 IR Tunnels in MVPN October 2016
processing of the packets will depend upon that label. Note that the
same unicast tunnel between R1 and R2 may also be carrying unicast
data packets.
Typically, the unicast tunnels are the LSPs that already exist to
carry unicast traffic; either MP2P LSPs created by the Label
Distribution Protocol (LDP) [RFC 5036] or P2P LSPs created by Resource
Reservation Protocol - Traffic Engineering (RSVP-TE) [RFC 3209].
However, any other kind of unicast tunnel may be used. A unicast
tunnel may have an arbitrary number of intermediate routers; those
routers do not maintain any multicast state for the IR P-tunnel and,
in general, are not even aware of its existence.
As with all other P-tunnel types, an IR P-tunnel may be used to
instantiate either an Inclusive PMSI (I-PMSI) or a Selective PMSI
(S-PMSI). See Section 3.2 of [RFC 6513] for an explanation of those
concepts.
3. How are IR P-tunnels identified?
There are four MVPN BGP route types in which P-tunnels can be
identified: Intra-AS I-PMSI A-D routes, Inter-AS I-PMSI A-D routes,
S-PMSI A-D routes, and Leaf A-D routes. (These route types are all
defined in [RFC 6514]).
Whenever it is necessary to identify a P-tunnel in a route of one of
these types, a "PMSI Tunnel Attribute" (PTA) is added to the route.
As defined in Section 5 of [RFC 6514], the PTA contains four fields:
Tunnel Type, MPLS Label, Tunnel Identifier, and Flags. [RFC 6514]
defines only one bit in the Flags field, the Leaf Information
Required bit.
If a route identifies an IR P-tunnel, the Tunnel Type field of its
PTA is set to the value 6, meaning "Ingress Replication".
Most types of P-tunnel are associated with specific protocols that
are used to set up and maintain tunnels of that type. For example,
if the Tunnel Type field is set to 2, meaning "mLDP P2MP LSP", the
associated setup protocol is Multipoint LDP (mLDP) [RFC 6388]. The
associated setup protocol always has a method of identifying the
tunnels that it sets up. For example, mLDP uses an FEC element
(Forwarding Equivalence Class element) to identify a tree. If the
Tunnel Type field is set to 3, meaning "PIM-SSM Tree", where "SSM"
stands for Source-Specific Multicast, the associated setup protocol
is PIM, and "(S,G)" is used to identify the tree. In these cases,
the Tunnel Identifier field of the PTA carries a tree identifier as
defined by the setup protocol used for the particular tunnel type.
Rosen, et al. Standards Track PAGE 7
RFC 7988 IR Tunnels in MVPN October 2016
IR P-tunnels, on the other hand, are entirely setup and maintained by
the use of BGP A-D routes and are not associated with any other setup
protocol. (The unicast tunnels used to transmit multicast data along
an IR P-tunnel may have their own setup and maintenance protocols, of
course.) The means of identifying a P-tunnel is very different for
IR P-tunnels than for other types of P-tunnel:
When an IR P-tunnel is identified in an S-PMSI A-D route, an
Intra-AS I-PMSI A-D route, or an Inter-AS I-PMSI A-D route (we
will refer to these three route types as "advertising A-D
routes"), its identifier is hereby defined to be the NLRI (Network
Layer Reachability Information) of that route. See Sections 4.1,
4.2, and 4.3 of [RFC 6514] for the specification of these NLRIs.
Note that the IR P-tunnel identifier includes the Route Type and
Length fields (see Section 4 of [RFC 6514]) of the NLRI.
To reiterate:
The identifier of the IR P-tunnel does not appear in the PTA at
all; the Tunnel Identifier field of the PTA does not contain the
identifier of the IR P-tunnel.
Rather, the identifier of the IR P-tunnel appears in the Network
Layer Reachability Information (NLRI) field of the A-D routes that
are used to advertise and to setup the IR P-tunnel.
Note that an advertising A-D route is considered to identify an IR
P-tunnel only if it carries a PTA whose Tunnel Type field is set to
"IR".
When an IR P-tunnel is identified in an S-PMSI A-D route or in an
Inter-AS I-PMSI A-D route, the Leaf Information Required bit of the
Flags field of the PTA MUST be set.
In an advertising A-D route:
o If the Leaf Information Required bit of the Flags field of the PTA
is set, then the Tunnel Identifier field of the PTA has no
significance whatsoever and MUST be ignored upon reception.
Note that, per RFC 6514, the length of the Tunnel Identifier field
of the PTA is variable and is inferred from the length of the PTA.
Even when this field is of no significance, its length MUST be the
length of an IP address in the address space of the SP's backbone,
as specified in Section 4.2 of [RFC 6515]. In this case, it is
RECOMMENDED that it be set to a routable address of the router
that constructed the PTA. (While it might make more sense to
Rosen, et al. Standards Track PAGE 8
RFC 7988 IR Tunnels in MVPN October 2016
allow or even require the field to be omitted entirely, that might
raise issues of backwards compatibility with implementations that
were designed prior to the publication of this document.)
o If the Leaf Information Required bit is not set, the Tunnel
Identifier field of the PTA does have significance, but it does
not identify the IR P-tunnel. The use of the PTA's Tunnel
Identifier field in this case is discussed in Section 5 of this
document.
Note that according to the above definition, there is no way for two
different advertising A-D routes (i.e., two advertising A-D routes
with different NLRIs) to advertise the same IR P-tunnel. In the
terminology of [RFC 6513], an IR P-tunnel can instantiate only a
single PMSI. If an ingress PE, for example, wants to bind two
customer multicast flows to a single IR P-tunnel, it must advertise
that IR P-tunnel either in an I-PMSI A-D route or in an S-PMSI A-D
route whose NLRI contains wildcards [RFC 6625].
When an IR P-tunnel is identified in a Leaf A-D route, its identifier
is the Route Key field of the route's NLRI. See Section 4.4 of
[RFC 6514].
A Leaf A-D route is considered to identify an IR P-tunnel only if it
carries a PTA whose Tunnel Type field is set to "IR". In this type
of route, the Tunnel Identifier field of the PTA does have
significance, but it does not identify the IR P-tunnel. The use of
the PTA's Tunnel Identifier field in this case is discussed in
Section 5.
4. How to Join an IR P-Tunnel
The procedures for joining an IR P-tunnel depend upon whether the
P-tunnel has been previously advertised, and, if so, upon how the
P-tunnel was advertised. Note that joining an unadvertised IR
P-tunnel is only possible when using the "global table multicast"
procedures of [RFC 7524].
4.1. Advertised IR P-tunnels
The procedures in this section apply when the IR P-tunnel to be
joined has been advertised in an S-PMSI A-D route, an Inter-AS I-PMSI
A-D route, or an Intra-AS I-PMSI A-D route.
The procedures for joining an advertised IR P-tunnel depend upon
whether the A-D route that advertises the IR P-tunnel has the Leaf
Information Required bit set in its PTA.
Rosen, et al. Standards Track PAGE 9
RFC 7988 IR Tunnels in MVPN October 2016
4.1.1. If the Leaf Information Required Bit Is Set
The procedures in this section apply when the P-tunnel to be joined
has been advertised in a route whose PTA has the Leaf Information
Required bit set.
The router joining a particular IR P-tunnel must determine its UMH
for that P-tunnel. If the route that advertised the IR P-tunnel
contains a P2MP Segmented Next Hop Extended Community, the UMH is
determined from the value of this community (see [RFC 7524]).
Otherwise, the UMH is determined from the route's next hop (see
[RFC 6514]).
Once the UMH is determined, the router joining the IR P-tunnel
originates a Leaf A-D route. The NLRI of the Leaf A-D route is
formed following the procedures of [RFC 6514]. As a result, the NLRI
of the Leaf A-D route will contain the IR P-tunnel identifier defined
in Section 3 above as its "route key". The UMH MUST be identified by
attaching an "IP-address-specific Route Target" (or an "IPv6-address-
specific Route Target") to the Leaf A-D route. The IP address of the
UMH appears in the Global Administrator field of the Route Target
(RT). Details can be found in [RFC 6514] and [RFC 7524].
The Leaf A-D route MUST also contain a PTA whose fields are set as
follows:
o The Tunnel Type field is set to "IR".
o The Tunnel Identifier field is set as described in Section 5 of
this document. (Note that this field does not contain the IR
P-tunnel Identifier that is defined in Section 3.)
o The MPLS Label field is set to a non-zero value. This is the
"P-tunnel label". The value must be chosen so as to satisfy
various constraints, as discussed in Section 7 this document.
4.1.2. If the Leaf Information Required Bit Is Not Set
The procedures in this section apply when the IR P-tunnel to be
joined has been advertised in a route whose PTA does not have the
Leaf Information Required bit set. This can only be the case if the
IR P-tunnel was advertised in an Intra-AS I-PMSI A-D route.
If an IR P-tunnel is advertised in the Intra-AS I-PMSI A-D routes
originated by the PE routers of a given MVPN, the Intra-AS I-PMSI can
be thought of as being instantiated by a set of IR P-tunnels. Each
PE is the root of one such IR P-tunnel, and the other PEs are
Rosen, et al. Standards Track PAGE 10
RFC 7988 IR Tunnels in MVPN October 2016
children of the root. A PE simultaneously joins all these P-tunnels
by originating (if it hasn't already done so) an Intra-AS I-PMSI A-D
route with a PTA whose fields are set as follows:
o The Tunnel Type field is set to "IR".
o The Tunnel Identifier field is set as described in Section 5 of
this document. (Note that this field does not contain the IR
P-tunnel identifier that is defined in Section 3.)
o The MPLS Label field MUST be set to a non-zero value. This label
value will be used by the child node to associate a received
packet with the I-PMSI of a particular MVPN. The MPLS label
allocation policy must be such as to ensure that the binding from
label to I-PMSI is one to one.
The NLRI and the RTs of the originated I-PMSI A-D route are set as
specified in [RFC 6514].
4.2. Unadvertised IR P-Tunnels
In [RFC 7524], a procedure is defined for "global table multicast", in
which a P-tunnel can be joined even if the P-tunnel has not been
previously advertised. See Sections 6.2.2 and 6.2.3 of [RFC 7524]:
"Leaf A-D Route for Global Table Multicast" and "Constructing the
Rest of the Leaf A-D Route". The route key of the Leaf A-D route has
the form of the "S-PMSI Route-Type Specific NLRI" (see Section 4.3 of
[RFC 6514]) in this case, and that should be considered to be the IR
P-tunnel identifier. Note that the procedure for finding the UMH is
different in this case; the UMH is the next hop of the best UMH-
eligible route towards the "ingress PE". See Section 6.1 of
[RFC 7524], entitled "Determining the Upstream ABR/PE/ASBR (Upstream
Node)".
5. The PTA's Tunnel Identifier Field
As discussed in Section 1, when the Tunnel Type field of a PTA is set
to "IR", the Tunnel Identifier field of that PTA does not contain the
IR P-tunnel identifier. This section specifies the procedures for
setting the Tunnel Identifier field of the PTA when the Tunnel Type
field of the PTA is set to "IR".
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RFC 7988 IR Tunnels in MVPN October 2016
If the Tunnel Type field of a PTA is set to "IR", its Tunnel
Identifier field is significant only when one of the following two
conditions holds:
o The PTA is carried by a Leaf A-D route, or
o The Leaf Information Required bit of the Flags field of the PTA is
not set.
If one of these conditions holds, then the Tunnel Identifier field
must contain a routable IP address of the originator of the route.
(See Sections 9.2.3.2.1 and 9.2.3.4.1 of [RFC 6514] for the detailed
specification of the contents of this field.) This address is used
by the UMH to determine the unicast tunnel that it will use in order
to send data, along the IR P-tunnel identified by the route key, to
the originator of the Leaf A-D route.
The means by which the unicast tunnel is determined from this IP
address is outside the scope of this document. The means by which
the unicast tunnel is set up and maintained is also outside the scope
of this document.
Section 4 of [RFC 6515] MUST be applied when a PTA is carried in a
Leaf A-D route. It describes how to determine whether the Tunnel
Identifier field carries an IPv4 or an IPv6 address.
If neither of the above conditions hold, then the Tunnel Identifier
field is of no significance and MUST be ignored upon reception.
6. A Note on IR P-Tunnels and Discarding Packets from the Wrong PE
Section 9.1.1 of [RFC 6513] specifies a procedure known as "Discarding
Packets from the Wrong PE". When an egress PE receives a multicast
data packet, this procedure requires it to determine the packet's
ingress PE.
In this document, we assume that when a packet has reached an egress
PE via an IR P-tunnel, the egress PE will infer the identity of the
packet's ingress PE by examining the packet's P-tunnel label.
Section 7 specifies certain constraints on the way in which the
P-tunnel label is allocated for a given P-tunnel. In general, if
these constraints are followed, an egress PE will be able to infer
the identity of a packet's ingress PE from the P-tunnel label, and
hence will be able to apply the procedures of Section 9.1.1 of
[RFC 6513]. This method of identifying a packet's ingress PE works
exactly the same when the unicast tunnels are IP tunnels as it does
when the unicast tunnels are MPLS LSPs.
Rosen, et al. Standards Track PAGE 12
RFC 7988 IR Tunnels in MVPN October 2016
However, if the egress PE joined a particular IR P-tunnel using the
procedures of Section 4.1.2, then when the egress PE receives a
packet through that P-tunnel, it will not be able to infer the
identity of the packet's ingress PE from the P-tunnel label, and thus
will not be able to apply the procedures of Section 9.1.1 of
[RFC 6513].
One might think that if a particular IR P-tunnel uses IP unicast
tunnels rather than MPLS LSPs, an egress PE could identify the
ingress PE by inspecting the IP source address field of the
encapsulating IP header. However, there are several reasons why this
procedure is not desirable:
o When segmented P-tunnels are being used, the IP source address
field of the encapsulating IP header might not contain the address
of the ingress PE.
o Even if the IP source address field of the encapsulating IP header
does identify the ingress PE, there is no guarantee that the IP
source address in that header is the same as the IP address used
by the ingress PE for the MVPN signaling procedures.
o To apply the procedures of Section 9.1.1 of [RFC 6513] when
extranet functionality [RFC 7900] is supported, it is necessary to
infer a packet's ingress VRF (Virtual Routing and Forwarding
table), not merely its ingress PE. This can be inferred from the
P-tunnel label (assuming that the label is allocated following the
procedures of Section 7), but it cannot be inferred from the IP
source address of the encapsulating IP header.
We therefore assume in this document that if the procedures of
Section 9.1.1 of [RFC 6513] are to be applied to packets traveling
through IR P-tunnels, those procedures will be based on the P-tunnel
label, even if the IR P-tunnel is using IP unicast tunnels.
This means that if an egress PE joined a particular IR P-tunnel using
the procedures of Section 4.1.2, duplicate prevention on that IR
P-tunnel requires the use of either Single Forwarder Selection
(Section 9.1.2 of [RFC 6513]) or native PIM procedures (Section 9.1.3
of [RFC 6513]).
Rosen, et al. Standards Track PAGE 13
RFC 7988 IR Tunnels in MVPN October 2016
7. The PTA's MPLS Label Field
When the Tunnel Type field of a PTA is set to "IR", the MPLS Label
field is not always significant. It is significant only under the
following conditions:
1. Either the PTA is being carried in a Leaf A-D route, or
2. the Leaf Information Required flag of the PTA is NOT set.
Note that the Leaf Information Required flag of the PTA is always set
when a PTA specifying an IR P-tunnel is carried in an S-PMSI A-D
route or in an Inter-AS I-PMSI A-D route; thus, the MPLS Label field
of the PTA is never significant when the PTA is carried by one of
these route types. The MPLS Label field is significant only when the
PTA appears either in a Leaf A-D route or in an Intra-AS I-PMSI A-D
route that does not have the Leaf Information Required bit set. In
these cases, the MPLS label is the label that the originator of the
route is assigning to the IR P-tunnel(s) identified by the route's
NLRI. (That is, the MPLS label assigned in the PTA is what we have
called the "P-tunnel label".)
In those cases where the MPLS Label field is not significant, it
SHOULD be set to zero upon transmission and MUST be ignored upon
reception.
7.1. Leaf A-D Route Originated by an Egress PE
As previously stated, when a Leaf A-D route is used to join an IR
P-tunnel, the "route key" of the Leaf A-D route is the P-tunnel
identifier.
We now define the notion of the "root of an IR P-tunnel".
o If the identifier of an IR P-tunnel is of the form of an S-PMSI
NLRI, the "root" of the IR P-tunnel is the router identified in
the Originating Router's IP Address field of that NLRI.
o If the identifier of an IR P-tunnel is of the form specified in
Section 6.2.2 of [RFC 7524] ("Leaf A-D Route for Global Table
Multicast"), the "root" of the IR P-tunnel is the router
identified in the Ingress PE's IP Address field of that NLRI.
o If the identifier of an IR P-tunnel is of the form of an Intra-AS
I-PMSI NLRI, the "root" of the IR P-tunnel is the router
identified in the Originating Router's IP Address field of that
NLRI.
Rosen, et al. Standards Track PAGE 14
RFC 7988 IR Tunnels in MVPN October 2016
o If the identifier of an IR P-tunnel is of the form of an Inter-AS
I-PMSI NLRI, the "root" of the IR P-tunnel is same as the
identifier of the IR P-tunnel, i.e., the combination of a Route
Distinguisher (RD) and an AS.
Note that if an IR P-tunnel is segmented, the root of the IR
P-tunnel, by this definition, is actually the root of the entire
P-tunnel, not the root of the local segment. In this case, there may
be segments upstream that are not IR P-tunnels themselves. However,
the egress PE is aware only of the final segment of the P-tunnel, and
hence considers the P-tunnel to be an IR P-tunnel.
In order to apply the procedures of Section 9.1.1 of RFC 6513
("Discarding Packets from Wrong PE"), the following condition MUST be
met by the MPLS label allocation policy:
Suppose an egress PE originates two Leaf A-D routes, each with a
different route key in its NLRI, and each with a PTA specifying a
Tunnel Type field of "IR". Thus, each of the Leaf A-D routes
identifies a different IR P-tunnel. Suppose further that each of
those IR P-tunnels has a different root. Then, the egress PE MUST
NOT specify the same MPLS label in both PMSI Tunnel attributes.
That is, to apply the duplicate prevention procedures (in "Discarding
Packets from Wrong PE", Section 9.1.1 of [RFC 6513]), the same MPLS
label MUST NOT be assigned to two IR P-tunnels that have different
roots.
If segmented P-tunnels are in use, the above rule is necessary but
not sufficient to prevent a PE from forwarding duplicate data to the
CEs. For various reasons, a given egress PE or egress ABR or egress
ASBR may decide to change its parent node, on a given segmented
P-tunnel, from one router to another. It does this by changing the
RT of the Leaf A-D route that it originated in order to join that
P-tunnel. Once the RT is changed, there may be a period of time
during which the old parent node and the new parent node are both
sending data of the same multicast flow. To ensure that the egress
node not forward duplicate data, whenever the egress node changes the
RT that it attaches to a Leaf A-D route, it MUST also change the
"MPLS Label" specified in the Leaf A-D route's PTA. This allows the
egress router to distinguish between packets arriving on a given
P-tunnel from the old parent and packets arriving on that same
P-tunnel from the new parent. At any given time, a router MUST
consider itself to have only a single parent node on a given P-tunnel
and MUST discard traffic that arrives on that P-tunnel from a
different parent node.
Rosen, et al. Standards Track PAGE 15
RFC 7988 IR Tunnels in MVPN October 2016
If extranet functionality [RFC 7900] is not implemented in a
particular egress PE, or if an egress PE is provisioned with the
knowledge that extranet functionality is not needed, the PE may adopt
the policy of assigning a label that is unique for the ordered triple
<root, parent node, egress VRF>. This will enable the egress PE to
apply the duplicate prevention procedures discussed above and to
determine the VRF to which an arriving packet must be directed.
However, this policy is not sufficient to support the "Do Not Deliver
Packets from the Wrong P-tunnel" procedures that are specified in
Section 2.3.1 of [RFC 7900]. To support those procedures, the labels
specified in the PTA of Leaf A-D routes originated by a given egress
PE MUST be unique for the ordered triple <root, root RD, parent
node>, where the "root RD" is taken from the RD field of the IR
P-tunnel identifier. (All forms of IR P-tunnel identifier contain an
embedded RD field.) This policy is also sufficient for supporting
non-extranet cases, but, in some cases, may result in the use of more
labels than the policy of the preceding paragraph.
7.2. Leaf A-D Route Originated by an Intermediate Node
When a P-tunnel is segmented, there will be "intermediate nodes",
i.e., nodes that have a parent and also have children on the
P-tunnel. Each intermediate node is a leaf node of an "upstream
segment" and a root node of one or more "downstream segments". The
intermediate node needs to set up its forwarding state so that data
it receives on the upstream segment gets transmitted on the proper
downstream segments.
If the upstream segment is instantiated by IR, the intermediate node
will need to originate a Leaf A-D route to join that segment, and
will need to allocate a downstream-assigned MPLS label to advertise
in the MPLS Label field of the Leaf A-D route's PTA. Section 7.1
specifies constraints on the label allocation policy for egress PEs;
this section specifies constraints on the label allocation policy for
intermediate nodes.
Suppose intermediate node N originates two Leaf A-D routes, one whose
route key is K1, and one whose route key is K2, where K1 != K2. The
respective PTAs of these Leaf A-D routes MUST specify distinct non-
zero MPLS labels, UNLESS the following conditions all hold:
1. N's parent node for P-tunnel K1 is the same as N's parent node
for P-tunnel K2.
2. N's forwarding state is such that any packet it receives from
P-tunnel K1 is forwarded to the exact same set of downstream
neighbors as any packet it receives from P-tunnel K2.
Rosen, et al. Standards Track PAGE 16
RFC 7988 IR Tunnels in MVPN October 2016
3. For each downstream neighbor D to which N sends the packets it
receives from P-tunnels K1 and K2, N's forwarding state is such
that it applies the exact same encapsulation to packets it
forwards from either tunnel to D. (For example, if N uses MPLS
to forward the packets to D, it pushes the exact same set of
labels on packets from P-tunnel K1 as it pushes on packets from
P-tunnel K2.)
Of course, N MAY always specify distinct non-zero labels in each of
the Leaf A-D routes that it originates.
Note that the rules of this section apply whenever the upstream
P-tunnel segment is an IR P-tunnel. These rules hold whether or not
some or all of the downstream segments are other types of P-tunnels.
If the P-tunnels from N to a particular downstream neighbor D are IR
P-tunnels, then condition 3 above will hold with respect to D only if
the following conditions all hold as well:
o N has received and installed a Leaf A-D route from D, whose route
key is K1, and which carries an IP-address-specific RT identifying
N,
o N has received and installed a Leaf A-D route from D, whose route
key is K2, and which carries an IP-address-specific RT identifying
N,
o Those two Leaf A-D routes specify the same MPLS label in their
respective PTAs.
7.3. Intra-AS I-PMSI A-D Route
When a router joins a set of IR P-tunnels using the procedures of
Section 4.1.2 of this document, the procedures of Section 9.1.1 of
[RFC 6513] cannot be applied, no matter what the label allocation
policy is. In this case, the ingress PE is the same as the UMH, but
it is not possible to assign a label uniquely to a particular ingress
PE or UMH. However, the label in the MPLS Label field of the PTA
MUST NOT appear in the MPLS Label field of the PTA carried by any
other route originated by the same router.
8. How a Child Node Prunes Itself from an IR P-Tunnel
If a particular IR P-tunnel was joined via the procedures of
Section 4.1.2, a router can prune itself from the P-tunnel by
withdrawing the Intra-AS I-PMSI A-D route it used to join the
P-tunnel. This is not usually done unless the router is removing
itself entirely from a particular MVPN.
Rosen, et al. Standards Track PAGE 17
RFC 7988 IR Tunnels in MVPN October 2016
The procedures in the remainder of this section apply when a router
joined a particular IR P-tunnel by originating a Leaf A-D route (as
described in Sections 4.1.1 or 4.2).
If a router no longer has a need to receive any multicast data from a
given IR P-tunnel, it may prune itself from the P-tunnel by
withdrawing the Leaf A-D route it used to join the tunnel. This is
done, e.g., if the router no longer needs any of the flows traveling
over the P-tunnel, or if all the flows the router does need are being
received over other P-tunnels.
A router that is attached to a particular IR P-tunnel via a
particular parent node may determine that it needs to stay joined to
that IR P-tunnel but via a different parent node. This can happen,
for example, if there is a change in the Next Hop or the P2MP
Segmented Next-Hop Extended Community of the S-PMSI A-D route in
which that P-tunnel was advertised. In this case, the router changes
the Route Target of the Leaf A-D route it used to join the IR
P-tunnel, so that the Route Target now identifies the new parent
node.
A parent node must notice when a child node has been pruned from a
particular tree, as this will affect the parent node's multicast data
state. Note that the pruning of a child node may appear to the
parent node as the explicit withdrawal of a Leaf A-D route, or it may
appear as a change in the Route Target of a Leaf A-D route. If the
Route Target of a particular Leaf A-D route previously identified a
particular parent node, but changes so that it no longer does so, the
effect on the multicast state of the parent node is the same as if
the Leaf A-D route had been explicitly withdrawn.
9. Parent Node Actions upon Receiving Leaf A-D Route
These actions are detailed in [RFC 6514] and [RFC 7524]. Two points of
clarification are made:
o If a router R1 receives and installs a Leaf A-D route originated
by router R2, R1's multicast state is affected only if the Leaf
A-D route carries an "IP-address-specific RT" (or "IPv6-address-
specific RT") whose Global Administrator field identifies R1.
(This is as specified in [RFC 6514] and [RFC 7524].) If a Leaf A-D
route's RT does not identify R1, but then changes so that it does
identify R1, R1 must take the same actions it would take if the
Leaf A-D route were newly received.
Rosen, et al. Standards Track PAGE 18
RFC 7988 IR Tunnels in MVPN October 2016
o It is possible that router R1 will receive and install a Leaf A-D
route originated by router R2, where:
* the route's RT identifies R1,
* the route's NLRI contains a route key whose first octet
indicates that it is identifying a P-tunnel advertised in an
S-PMSI A-D route,
* R1 has neither originated nor installed any such S-PMSI A-D
route.
If at some later time, R1 installs the corresponding S-PMSI A-D
route, and the Leaf A-D route is still installed, and the Leaf A-D
route's RT still identifies R1, then R1 MUST follow the same
procedures it would have followed if the S-PMSI A-D route had been
installed before the Leaf A-D route was installed. Implementers must
not assume that events occur in the "usual" or "expected" order.
10. Use of Timers When Switching UMH
Consider a child node that has joined a particular IR P-tunnel via a
particular UMH. To do so, it will have originated a Leaf A-D route
with an RT that identifies the UMH. Suppose the child node now
determines (for whatever reason) that it needs to change its UMH for
that P-tunnel. It does this by:
o modifying the RT of the Leaf A-D route, so that the RT now
identifies the new parent rather than the old one, and by
o modifying the PTA of the Leaf A-D route, changing the MPLS Label
field as discussed in Section 7.
Note that, in accordance with the procedures of [RFC 6514] and of
Section 4 of this document, the NLRI of the Leaf A-D route is not
modified; only the RT and the PTA are changed.
It is desirable for such a "switch of UMH" to be done using a "make
before break" technique, so that the old UMH does not stop
transmitting packets of the given P-tunnel to the child until the new
UMH has a chance to start transmitting packets of the given P-tunnel
to the child. However, the control-plane operation (i.e., modifying
the RT and PTA of the Leaf A-D route) does not permit the child node
to first join the IR P-tunnel via the new UMH, and then later prune
itself from the old UMH. Rather, a single control-plane operation
has both effects.
Rosen, et al. Standards Track PAGE 19
RFC 7988 IR Tunnels in MVPN October 2016
Therefore, the old UMH MUST continue transmitting to the child node
for a period of time after it sees the child's Leaf A-D route being
withdrawn (or its RT changing to identify a different UMH). This
timer (the "parent-continues" timer) SHOULD have a default value of
60 seconds and SHOULD be configurable.
By the procedures of Section 7, the child node will have advertised a
different label for the IR P-tunnel to the new UMH than it had
advertised to the old UMH. This allows it to distinguish the packets
of that IR P-tunnel transmitted by the new UMH from packets of that
IR P-tunnel transmitted by the old UMH. At any given time, the child
node will accept packets of that IR P-tunnel from only one parent
node and will discard packets of that IR P-tunnel that are received
from the other. To achieve "make before break" functionality, the
child node needs to continue to accept packets from the old UMH for a
period of time. After this period, it will discard any packets from
the given IR P-tunnel that it receives from the old UMH and will only
accept such packets from the new UMH.
Once the child node modifies the RT of its Leaf A-D route, it MUST
run a timer (the "switch-parents-delay" timer). This timer SHOULD
default to 30 seconds and SHOULD be configurable. The child node
MUST continue to accept packets of the given IR P-tunnel from the old
UMH until the timer expires. However, once the child node receives a
packet of the given IR P-tunnel from the new UMH, it MAY consider the
"switch-parents-delay" timer to have expired.
The "parent-continues" timer MUST be longer than the "switch-parents-
delay" timer. Note that both timers are specific to a given IR
P-tunnel.
11. Security Considerations
No security considerations are raised by this document beyond those
already discussed in [RFC 6513] and [RFC 6514].
Rosen, et al. Standards Track PAGE 20
RFC 7988 IR Tunnels in MVPN October 2016
12. References
12.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 6513] Rosen, E., Ed., and R. Aggarwal, Ed., "Multicast in
MPLS/BGP IP VPNs", RFC 6513, DOI 10.17487/RFC 6513,
February 2012, <http://www.rfc-editor.org/info/RFC 6513>.
[RFC 6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC 6514, February 2012,
<http://www.rfc-editor.org/info/RFC 6514>.
[RFC 6515] Aggarwal, R. and E. Rosen, "IPv4 and IPv6 Infrastructure
Addresses in BGP Updates for Multicast VPN", RFC 6515,
DOI 10.17487/RFC 6515, February 2012,
<http://www.rfc-editor.org/info/RFC 6515>.
12.2. Informative References
[RFC 3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC 3209, December 2001,
<http://www.rfc-editor.org/info/RFC 3209>.
[RFC 5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC 5036,
October 2007, <http://www.rfc-editor.org/info/RFC 5036>.
[RFC 6388] Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
Thomas, "Label Distribution Protocol Extensions for Point-
to-Multipoint and Multipoint-to-Multipoint Label Switched
Paths", RFC 6388, DOI 10.17487/RFC 6388, November 2011,
<http://www.rfc-editor.org/info/RFC 6388>.
[RFC 6625] Rosen, E., Ed., Rekhter, Y., Ed., Hendrickx, W., and R.
Qiu, "Wildcards in Multicast VPN Auto-Discovery Routes",
RFC 6625, DOI 10.17487/RFC 6625, May 2012,
<http://www.rfc-editor.org/info/RFC 6625>.
Rosen, et al. Standards Track PAGE 21
RFC 7988 IR Tunnels in MVPN October 2016
[RFC 7524] Rekhter, Y., Rosen, E., Aggarwal, R., Morin, T.,
Grosclaude, I., Leymann, N., and S. Saad, "Inter-Area
Point-to-Multipoint (P2MP) Segmented Label Switched Paths
(LSPs)", RFC 7524, DOI 10.17487/RFC 7524, May 2015,
<http://www.rfc-editor.org/info/RFC 7524>.
[RFC 7582] Rosen, E., Wijnands, IJ., Cai, Y., and A. Boers,
"Multicast Virtual Private Network (MVPN): Using
Bidirectional P-Tunnels", RFC 7582, DOI 10.17487/RFC 7582,
July 2015, <http://www.rfc-editor.org/info/RFC 7582>.
[RFC 7716] Zhang, J., Giuliano, L., Rosen, E., Ed., Subramanian, K.,
and D. Pacella, "Global Table Multicast with BGP Multicast
VPN (BGP-MVPN) Procedures", RFC 7716,
DOI 10.17487/RFC 7716, December 2015,
<http://www.rfc-editor.org/info/RFC 7716>.
[RFC 7740] Zhang, Z., Rekhter, Y., and A. Dolganow, "Simulating
Partial Mesh of Multipoint-to-Multipoint (MP2MP) Provider
Tunnels with Ingress Replication", RFC 7740,
DOI 10.17487/RFC 7740, January 2016,
<http://www.rfc-editor.org/info/RFC 7740>.
[RFC 7900] Rekhter, Y., Ed., Rosen, E., Ed., Aggarwal, R., Cai, Y.,
and T. Morin, "Extranet Multicast in BGP/IP MPLS VPNs",
RFC 7900, DOI 10.17487/RFC 7900, June 2016,
<http://www.rfc-editor.org/info/RFC 7900>.
Rosen, et al. Standards Track PAGE 22
RFC 7988 IR Tunnels in MVPN October 2016
Acknowledgments
The authors wish to thank Yakov Rekhter for his contributions to this
work. We also wish to thank Huajin Jeng and Samir Saad for their
contributions, and to thank Thomas Morin for pointing out (both
before and after the document was written) some of the issues that
needed further elaboration. We also thank Lucy Yong for her review
and comments.
Section 7.1 discusses the importance of having an MPLS label
allocation policy that, when ingress replication is used, allows an
egress PE to infer the identity of a received packet's ingress PE.
This issue was first raised in earlier work by Xu Xiaohu.
Authors' Addresses
Eric C. Rosen (editor)
Juniper Networks, Inc.
10 Technology Park Drive
Westford, Massachusetts 01886
United States of America
Email: erosen@juniper.net
Karthik Subramanian
Sproute Networks
Email: karthik@sproute.com
Zhaohui Zhang
Juniper Networks, Inc.
10 Technology Park Drive
Westford, Massachusetts 01886
United States of America
Email: zzhang@juniper.net
Rosen, et al. Standards Track PAGE 23
RFC TOTAL SIZE: 55979 bytes
PUBLICATION DATE: Friday, October 7th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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