Internet DRAFT - draft-ietf-l3vpn-mvpn-global-table-mcast
draft-ietf-l3vpn-mvpn-global-table-mcast
L3VPN Working Group Jeffrey Zhang
Internet Draft Lenny Giuliano
Intended Status: Standards Track Juniper Networks, Inc.
Expires: December 24, 2014
Eric C. Rosen
Karthik Subramanian
Cisco Systems, Inc.
Dante J. Pacella
Verizon
Jason Schiller
Google
June 24, 2014
Global Table Multicast with BGP-MVPN Procedures
draft-ietf-l3vpn-mvpn-global-table-mcast-00.txt
Abstract
RFC6513, RFC6514, and other RFCs describe protocols and procedures
which a Service Provider (SP) may deploy in order offer Multicast
Virtual Private Network (Multicast VPN or MVPN) service to its
customers. Some of these procedures use BGP to distribute VPN-
specific multicast routing information across a backbone network.
With a small number of relatively minor modifications, the very same
BGP procedures can also be used to distribute multicast routing
information that is not specific to any VPN. Multicast that is
outside the context of a VPN is known as "Global Table Multicast", or
sometimes simply as "Internet multicast". In this document, we
describe the modifications that are needed to use the MVPN BGP
procedures for Global Table Multicast.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright and License Notice
Copyright (c) 2014 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.
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Table of Contents
1 Introduction .......................................... 4
2 Adapting MVPN Procedures to GTM ....................... 6
2.1 Use of Route Distinguishers ........................... 7
2.2 Use of Route Targets .................................. 7
2.3 UMH-eligible Routes ................................... 9
2.3.1 Routes of SAFI 1, 2 or 4 with MVPN ECs ................ 10
2.3.2 MVPN ECs on the Route to the Next Hop ................. 11
2.3.3 Non-BGP Routes as the UMH-eligible Routes ............. 12
2.3.4 Why SFS Does Not Apply to GTM ......................... 13
2.4 Inclusive and Selective Tunnels ....................... 14
2.5 I-PMSI A-D Routes ..................................... 14
2.5.1 Intra-AS I-PMSI A-D Routes ............................ 14
2.5.2 Inter-AS I-PMSI A-D Routes ............................ 15
2.6 S-PMSI A-D Routes ..................................... 15
2.7 Leaf A-D Routes ....................................... 15
2.8 Source Active A-D Routes .............................. 15
2.8.1 Finding the Originator of an SA A-D Route ............. 15
2.8.2 Optional Additional Constraints on Distribution ....... 16
2.9 C-multicast Source/Shared Tree Joins .................. 17
3 Differences from other MVPN-like GTM Procedures ....... 18
4 IANA Considerations ................................... 19
5 Security Considerations ............................... 19
6 Additional Contributors ............................... 20
7 Acknowledgments ....................................... 20
8 Authors' Addresses .................................... 21
9 References ............................................ 22
9.1 Normative References .................................. 22
9.2 Informative References ................................ 22
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1. Introduction
[RFC4364] specifies architecture, protocols, and procedures that a
Service Provider (SP) can use to provide Virtual Private Network
(VPN) service to its customers. In that architecture, one or more
Customer Edge (CE) routers attach to a Provider Edge (PE) router.
Each CE router belongs to a single VPN, but CE routers from several
VPNs may attach to the same PE router. In addition, CEs from the
same VPN may attach to different PEs. BGP is used to carry VPN-
specific information among the PEs. Each PE router maintains a
separate Virtual Routing and Forwarding table (VRF) for each VPN to
which it is attached.
[RFC6513] and [RFC6514] extend the procedures of [RFC4364] to allow
the SP to provide multicast service to its VPN customers. The
customer's multicast routing protocol (e.g., PIM) is used to exchange
multicast routing information between a CE and a PE. The PE stores a
given customer's multicast routing information in the VRF for that
customer's VPN. BGP is used to distribute certain multicast-related
control information among the PEs that attach to a given VPN, and BGP
may also be used to exchange the customer multicast routing
information itself among the PEs.
While this multicast architecture was originally developed for VPNs,
it can also be used (with a small number of modifications to the
procedures) to distribute multicast routing information that is not
specific to VPNs. The purpose of this document is to specify the way
in which BGP MVPN procedures can be adapted to support non-VPN
multicast.
Multicast routing information that is not specific to VPNs is stored
in a router's "global table", rather than in a VRF; hence it is known
as "Global Table Multicast" (GTM). GTM is sometimes more simply
called "Internet multicast". However, we will avoid that term
because it suggests that the multicast data streams are available on
the "public" Internet. The procedures for GTM can certainly be used
to support multicast on the public Internet, but they can also be
used to support multicast streams that are not public, e.g., content
distribution streams offered by content providers to paid
subscribers. For the purposes of this document, all that matters is
that the multicast routing information is maintained in a global
table rather than in a VRF.
This architecture does assume that the network over which the
multicast streams travel can be divided into a "core network" and one
or more non-core parts of the network, which we shall call
"attachment networks". The multicast routing protocol used in the
attachment networks may not be the same as the one used in the core,
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so we consider there to be a "protocol boundary" between the core
network and the attachment networks. We will use the term "Protocol
Boundary Router" (PBR) to refer to the core routers that are at the
boundary. We will use the term "Attachment Router" (AR) to refer to
the routers that are not in the core but that attach to the PBRs.
This document does not make any particular set of assumptions about
the protocols that the ARs and the PBRs use to exchange unicast and
multicast routing information with each other. For instance,
multicast routing information could be exchanged between an AR and a
PBR via PIM, IGMP, or even BGP. Multicast routing also depends on an
exchange of routes that are used for looking up the path to the root
of a multicast tree. This routing information could be exchanged
between an AR and a PBR via IGP, via EBGP, or via IBGP ([RFC6368]).
Note that if IBGP is used, the [RFC6368] "push/pop procedures" are
not necessary.
The PBRs are not necessarily "edge" routers, in the sense of
[RFC4364]. For example, they may be both be Autonomous System Border
Routers (ASBR). As another example, an AR may be an "access router"
attached to a PBR that is an OSPF Area Border Router (ABR). Many
other deployment scenarios are possible. However, the PBRs are
always considered to be delimiting a "backbone" or "core" network. A
multicast data stream from an AR is tunneled over the core network
from an Ingress PBR to one or more Egress PBRs. Multicast routing
information that a PBR learns from the ARs attached to it is stored
in the PBR's global table. The PBRs use BGP to distribute multicast
routing and auto-discovery information among themselves. This is
done following the procedures of [RFC6513], [RFC6514], and other MVPN
specifications, as modified in this document.
In general, PBRs follow the same MVPN/BGP procedures that PE routers
follow, except that these procedures are adapted to be applicable to
the global table rather than to a VRF. Details are provided in
subsequent sections of this document.
By supporting GTM using the BGP procedures designed for MVPN, one
obtains a single control plane that governs the use of both VPN and
non-VPN multicast. Most of the features and characteristics of MVPN
carry over automatically to GTM. These include scaling, aggregation,
flexible choice of tunnel technology in the SP network, support for
both segmented and non-segmented tunnels, ability to use wildcards to
identify sets of multicast flows, support for the Any Source
Multicast (ASM), Single Source Multicast (SSM), and Bidirectional
(bidir) multicast paradigms, support for both IPv4 and IPv6 multicast
flows over either an IPv4 or IPv6 SP infrastructure, support for
unsolicited flooded data (including support for BSR as RP-to-group
mapping protocols), etc.
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This document not only uses MVPN procedures for GTM, but also,
insofar as possible, uses the same protocol elements, encodings, and
formats. The BGP Updates for GTM thus use the same Subsequent
Address Family Identifier (SAFI), and have the same Network Layer
Reachability Information (NLRI) format, as the BGP Updates for MVPN.
Details for supporting MVPN (either IPv4 or IPv6 MVPN traffic) over
an IPv6 backbone network can be found in [RFC6515]. The procedures
and encodings described therein are also applicable to GTM.
The document [SEAMLESS-MCAST] extends [RFC6514] by providing
procedures that allow tunnels through the core to be "segmented" at
ABRs within the core. The ABR segmentation procedures are also
applicable to GTM as defined in the current document. In general,
the MVPN procedures of [SEAMLESS-MCAST], adapted as specified in the
current document, are applicable to GTM.
The document [SEAMLESS-MCAST] also defines a set of procedures for
GTM. Those procedures are different from the procedures defined in
the current document, and the two sets of procedures are not
interoperable with each other. The two sets of procedures can co-
exist in the same network, as long as they are not applied to the
same multicast flows or to the same multicast group addresses. See
section 3 for more details.
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. Adapting MVPN Procedures to GTM
In general, PBRs support Global Table Multicast by using the
procedures that PE routers use to support VPN multicast. For GTM,
where [RFC6513] and [RFC6514] talk about the "PE-CE interface", one
should interpret that to mean the interface between the AR and the
PBR. For GTM, where [RFC6513] and [RFC6514] talk about the
"backbone" network, one should interpret that to mean the part of the
network that is delimited by the PBRs.
A few adaptations to the procedures of [RFC6513] and [RFC6514] need
to be made. Those adaptations are described in the following sub-
sections.
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2.1. Use of Route Distinguishers
The MVPN procedures require the use of BGP routes, defined in
[RFC6514], that have a SAFI value of 5 ("MCAST-VPN"). We refer to
these simply as "MCAST-VPN routes". [RFC6514] defines the Network
Layer Reachability Information (NLRI) format for MCAST-VPN routes.
The NLRI field always begins with a "Route Type" octet, and,
depending on the route type, may be followed by a "Route
Distinguisher" (RD) field.
When a PBR originates an MCAST-VPN route in support of GTM, the RD
field (for those routes types where it is defined) of that route's
NLRI MUST be set to zero (i.e., to 64 bits of zero). Since no VRF
may have an RD of zero, this allows "MCAST-VPN" routes that are
"about" GTM to be distinguished from MCAST-VPN routes that are about
VPNs.
2.2. Use of Route Targets
The MVPN procedures require all MCAST-VPN routes to carry Route
Targets (RTs). When a PE router receives an MCAST-VPN route, it
processes the route in the context of a particular VRF if and only if
the route is carrying an RT that is configured as one of that VRF's
"import RTs".
There are two different "kinds" of RT used in MVPN.
- One kind of RT is carried only by the following MCAST-VPN route
types: C-multicast Shared Tree Joins, C-multicast Source Tree
Joins, and Leaf A-D routes. This kind of RT identifies the PE
router that has been selected by the route's originator as the
"Upstream PE" or as the "Upstream Multicast Hop" (UMH) for a
particular (set of) multicast flow(s). Per [RFC6514] and
[RFC6515], this RT must be an IPv4-address-specific or
IPv6-address-specific Extended Community (EC), whose "Global
Administrator" field identifies the Upstream PE or the UMH. If
the Global Administrator field identifies the Upstream PE, the
"Local Administrator" field identifies a particular VRF in that
PE.
The GTM procedures of this document require the use of this type
of RT, in exactly the same situations where it is used in the
MVPN specification. However, one adaptation is necessary: the
"Local Administrator" field of this kind of RT MUST always be set
to zero, thus implicitly identifying the global table, rather
than identifying a VRF. We will refer to this kind of RT as a
"PBR-identifying RT".
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- The other kind of RT is the conventional RT first specified in
[RFC4364]. It does not necessarily identify a particular router
by address, but is used to constrain the distribution of VPN
routes, and to ensure that a given VPN route is processed in the
context of a given VRF if and only if the route is carrying an RT
that has been configured as one of that VRF's "import RTs".
Whereas every VRF must be configured with at least one import RT,
there is heretofore no requirement to configure any RTs for the
global table of any router. As stated above, this document makes
the use of PBR-identifying RTs mandatory for GTM. This document
makes the use of non-PBR-identifying RTs OPTIONAL for GTM.
The procedures for the use of RTs in GTM are the following:
- If the global table of a particular PBR is NOT configured with
any import RTs, then a received MCAST-VPN route is processed in
the context of the global table only if it is carrying no RTs, or
if it is carrying a PBR-identifying RT whose Global Administrator
field identifies that PBR.
- The global table in each PBR MAY be configured with (a) a set of
export RTs to be attached to MCAST-VPN routes that are originated
to support GTM, and (b) with a set of import RTs for GTM.
If the global table of a given PBR has been so configured, the
PBR will process a received MCAST-VPN route in the context of the
global table if and only if the route carries an RT that is one
of the global table's import RTs, or if the route carries a PBR-
identifying RT whose global administrator field identifies the
PBR.
If the global tables are configured with RTs, care must be taken
to ensure that the RTs configured for the global table are
distinct from any RTs used in support of MVPN (except in the case
where it is actually intended to create an "extranet"
[MVPN-extranet] in which some sources are reachable in global
table context while others are reachable in VPN context.)
The "RT Constraint" procedures of [RFC4684] MAY be used to constrain
the distribution of MCAST-VPN routes (or other routes) that carry RTs
that have been configured as import RTs for GTM. (This includes the
PBR-identifying RTs.)
In [RFC6513], the UMH-eligible routes (see section 5.1 of [RFC6513],
"Eligible Routes for UMH Selection") are generally routes of SAFI 128
(Labeled VPN-IP routes) or 129 (VPN-IP multicast routes), and are
required to carry RTs. These RTs determine which VRFs import which
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such routes. However, for GTM, when the UMH-eligible routes may be
routes of SAFI 1, 2, or 4, the routes are not required to carry RTs.
This document does NOT specify any new rules for determine whether a
SAFI 1, 2, or 4 route is to be imported into the global table of any
PBR.
2.3. UMH-eligible Routes
[RFC6513] section 5.1 defines procedures by which a PE router
determines the "C-root", the "Upstream Multicast Hop" (UMH), the
"Upstream PE", and the "Upstream RD" of a given multicast flow. (In
non-VPN multicast documents, the UMH of a multicast flow at a
particular router is generally known as the "RPF neighbor" for that
flow.) It also defines procedures for determining the "Source AS" of
a particular flow. Note that in GTM, the "Upstream PE" is actually
the "Upstream PBR".
The definition of the C-root of a flow is the same for GTM as for
MVPN.
For MVPN, to determine the UMH, Upstream PE, Upstream RD, and Source
AS of a flow, one looks up the C-root of the flow in a particular
VRF, and finds the "UMH-eligible" routes (see section 5.1.1 of
[RFC6513]) that "match" the C-root. From among these, one is chosen
as the "selected UMH route".
For GTM, the C-root is of course looked up in the global table,
rather than in a VRF. For MVPN, the UMH-eligible routes are routes
of SAFI 128 or 129. For GTM, the UMH-eligible routes are routes of
SAFI 1, SAFI 4, or SAFI 2. If the global table has imported routes
of SAFI 2, then these are the UMH-eligible routes. Otherwise, routes
of SAFI 1 or SAFI 4 are the UMH-eligible routes. For the purpose of
UMH determination, if a SAFI 1 route and a SAFI 4 route contain the
same IP prefix in their respective NLRI fields, then the two routes
are considered by the BGP bestpath selection process to be
comparable.
[RFC6513] defines procedures for determining which of the UMH-
eligible routes that match a particular C-root is to become the
"Selected UMH route". With one exception, these procedures are also
applicable to GTM. The one exception is the following. Section
9.1.2 of [RFC6513] defines a particular method of choosing the
Upstream PE, known as "Single Forwarder Selection" (SFS). This
procedure MUST NOT be used for GTM (see section 2.3.4 for an
explanation of why the SFS procedure cannot be applied to GTM).
In GTM, the "Upstream RD" of a multicast flow is always considered to
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be zero, and is NOT determined from the Selected UMH route.
The MVPN specifications require that when BGP is used for
distributing multicast routing information, the UMH-eligible routes
MUST carry the VRF Route Import EC and the Source AS EC. To
determine the Upstream PE and Source AS for a particular multicast
flow, the Upstream PE and Source AS are determined, respectively,
from the VRF Route Import EC and the Source AS EC of the Selected UMH
route for that flow. These ECs are generally attached to the UMH-
eligible routes by the PEs that originate the routes.
In GTM, there are certain situations in which it is allowable to omit
the VRF Route Import EC and/or the Source AS EC from the UMH-eligible
routes. The following sub-sections specify the various options for
determining the Upstream PBR and the Source AS in GTM.
The procedures in sections 2.3.1 MUST be implemented. The procedures
in sections 2.3.2 and 2.3.3 are OPTIONAL to implement. It should be
noted that while the optional procedures may be useful in particular
deployment scenarios, there is always the potential for
interoperability problems when relying on OPTIONAL procedures.
2.3.1. Routes of SAFI 1, 2 or 4 with MVPN ECs
If the UMH-eligible routes have a SAFI of 1, 2 or 4, then they MAY
carry the VRF Route Import EC and/or the Source AS EC. If the
selected UMH route is a route of SAFI 1, 2 or 4 that carries the VRF
Route Import EC, then the Upstream PBR is determined from that EC.
Similarly, if the selected UMH route is a route of SAFI 1, 2, or 4
route that carries the Source AS EC, the Source AS is determined from
that EC.
When the procedure of this section is used, a PBR that distributes a
UMH-eligible route to other PBRs is responsible for ensuring that the
VRF Route Import and Source AS ECs are attached to it.
If the selected UMH-eligible route has a SAFI of 1, 2 or 4, but is
not carrying a VRF Route Import EC, then the Upstream PBR is
determined as specified in section 2.3.2 or 2.3.3 below.
If the selected UMH-eligible route has a SAFI of 1, 2 or 4, but is
not carrying a Source AS EC, then the Source AS is considered to be
the local AS.
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2.3.2. MVPN ECs on the Route to the Next Hop
Some service providers may consider it to be undesirable to have the
PBRs put the VRF Route Import EC on all the UMH-eligible routes. Or
there may be deployment scenarios in which the UMH-eligible routes
are not advertised by the PBRs at all. The procedures described in
this section provide an alternative that can be used under certain
circumstances.
The procedures of this section are OPTIONAL.
In this alternative procedure, each PBR MUST originate a BGP route of
SAFI 1, 2 or 4 to itself. This route MUST carry a VRF Route Import
EC that identifies the PBR. The address that appears in the Global
Administrator field of that EC MUST be the same address that appears
in the NLRI and in the Next Hop field of that route. This route MUST
also carry a Source AS EC identifying the AS of the PBR.
Whenever the PBR distributes a UMH-eligible route for which it sets
itself as next hop, it MUST use this same IP address as the Next Hop
of the UMH-eligible route that it used in the route discussed in the
prior paragraph.
When the procedure of his section is used, then when a PBR is
determining the Selected UMH Route for a given multicast flow, it may
find that the Selected UMH Route has no VRF Route Import EC. In this
case, the PBR will look up (in the global table) the route to the
Next Hop of the Selected UMH route. If the route to the Next Hop has
a VRF Route Import EC, that EC will be used to determine the Upstream
PBR, just as if the EC had been attached to the Selected UMH Route.
If recursive route resolution is required in order to resolve the
next hop, the Upstream PBR will be determined from the first route
with a VRF Route Import EC that is encountered during the recursive
route resolution process. (The recursive route resolution process
itself is not modified by this document.)
The same procedure can be applied to find the Source AS, except that
the Source AS EC is used instead of the VRF Route Import EC.
Note that this procedure is only applicable in scenarios where it is
known that the Next Hop of the UMH-eligible routes is not be changed
by any router that participates in the distribution of those routes;
this procedure MUST NOT be used in any scenario where the next hop
may be changed between the time one PBR distributes the route and
another PBR receives it. The PBRs have no way of determining
dynamically whether the procedure is applicable in a particular
deployment; this must be made known to the PBRs by provisioning.
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Some scenarios in which this procedure can be used are:
- all PBRs are in the same AS, or
- the UMH-eligible routes are distributed among the PBRs by a Route
Reflector (that does not change the next hop), or
- the UMH-eligible routes are distributed from one AS to another
through ASBRs that do not change the next hop.
If the procedures of this section are used in scenarios where they
are not applicable, GTM will not function correctly.
2.3.3. Non-BGP Routes as the UMH-eligible Routes
In particular deployment scenarios, there may be specific procedures
that can be used, in those particular scenarios, to determine the
Upstream PBR for a given multicast flow.
Suppose the PBRs neither put the VRF Route Import EC on the UMH-
eligible routes, nor do they distribute BGP routes to themselves. It
may still be possible to determine the Upstream PBR for a given
multicast flow, using specific knowledge about the deployment.
For example, suppose it is known that all the PBRs are in the same
OSPF area. It may be possible to determine the Upstream PBR for a
given multicast flow by looking at the link state database to see
which router is attached to the flow's C-root.
As another example, suppose it is known that the set of PBRs is fully
meshed via Traffic Engineering (TE) tunnels. When a PBR looks up, in
its global table, the C-root of a particular multicast flow, it may
find that the next hop interface is a particular TE tunnel. If it
can determine the identify of the router at the other end of that TE
tunnel, it can deduce that that router is the Upstream PBR for that
flow.
This is not an exhaustive set of examples. Any procedure that
correctly determines the Upstream PBR in a given deployment scenario
MAY be used in that scenario.
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2.3.4. Why SFS Does Not Apply to GTM
To see why the SFS procedure cannot be applied to GTM, consider the
following example scenario. Suppose some multicast source S is homed
to both PBR1 and PBR2, and suppose that both PBRs export a route (of
SAFI 1, 2, or 4) whose NLRI is a prefix matching the address of S.
These two routes will be considered comparable by the BGP decision
process. A route reflector receiving both routes may thus choose to
redistribute just one of the routes to S, the one chosen by the
bestpath algorithm. Different route reflectors may even choose
different routes to redistribute (i.e., one route reflector may
choose the route to S via PBR1 as the bestpath, while another chooses
the route to S via PBR2 as the bestpath). As a result, some PBRs may
receive only the route to S via PBR1 and some may receive only the
route to S via PBR2. In that case, it is impossible to ensure that
all PBRs will choose the same route to S.
The SFS procedure works in VPN context as along the following
assumption holds: if S is homed to VRF-x in PE1 and to VRF-y in PE2,
then VRF-x and VRF-y have been configured with different RDs. In VPN
context, the route to S is of SAFI 128 or 129, and thus has an RD in
its NLRI. So the route to S via PE1 will not have the same NLRI as
the route to S via PE2. As a result, all PEs will see both routes,
and the PEs can implement a procedure that ensures that they all pick
the same route to S.
That is, the SFS procedure of [RFC6513] relies on the UMH-eligible
routes being of SAFI 128 or 129, and relies on certain VRFs being
configured with distinct RDs. Thus the procedure cannot be applied
to GTM.
One might think that the SFS procedure could be applied to GTM as
long as the procedures defined in [ADD-PATH] are applied to the UMH-
eligible routes. Using the [ADD-PATH] procedures, the BGP speakers
could advertise more than one path to a given prefix. Typically
[ADD-PATH] is used to report the n best paths, for some small value
of n. However, this is not sufficient to support SFS, as can be seen
by examining the following scenario.
AS-X | AS-Y | AS-Z
| |
S--PBR1---ASBR1--|--ASBR2--|---ASBR5
| \______/ | |
| / \ | |
|--PBR2---ASBR3--|--ASBR4--|---ASBR6
| |
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In AS-X, PBR1 reports to both ASBR1 and ASBR3 that it has a route to
S. Similarly, PBR2 reports to both ASBR1 and ASBR3 that it has a
route to S. Using [ADD-PATH], ASBR1 reports both routes to ASBR2,
and ASBR3 reports both routes to ASBR4. Now AS-Y sees 4 paths to S.
The AS-Z ASBRs will each see eight paths (four via ASBR2 and four via
ASBR4). To avoid this explosion in the number of paths, a BGP
speaker that uses [ADD-PATH] is usually considered to report only the
n best paths. However, there is then no guarantee that the reported
set of paths will contain at least one path via PBR1 and at least one
path via PBR2. Without such a guarantee, the SFS procedure will not
work.
2.4. Inclusive and Selective Tunnels
The MVPN specifications allow multicast flows to be carried on either
Inclusive Tunnels or on Selective Tunnels. When a flow is sent on an
Inclusive Tunnel of a particular VPN, it is sent to all PEs in that
VPN. When sent on a Selective Tunnel of a particular VPN, it may be
sent to only a subset of the PEs in that VPN.
This document allows the use of either Inclusive Tunnels or Selective
Tunnels for GTM. However, any service provider electing to use
Inclusive Tunnels for GTM should carefully consider whether sending a
multicast flow to ALL its PBRs would result in problems of scale.
There are potentially many more MBRs for GTM than PEs for a
particular VPN. If the set of PBRs is large and growing, but most
multicast flows do not need to go to all the PBRs, the exclusive use
of Selective Tunnels may be a better option.
2.5. I-PMSI A-D Routes
2.5.1. Intra-AS I-PMSI A-D Routes
Per [MVPN-BGP}, there are certain conditions under which is it NOT
required for a PE router implementing MVPN to originate one or more
Intra-AS I-PMSI A-D routes. These conditions apply as well to PBRs
implementing GTM.
In addition, a PBR implementing GTM is NOT required to originate an
Intra-AS I-PMSI A-D route if both of the following conditions hold:
- The PBR is not using Inclusive Tunnels for GTM, and
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- The distribution of the C-multicast Shared Tree Join and C-
multicast Source Tree Join routes is done in such a manner that
the next hop of those routes does not change.
Please see also the sections on RD and RT usage.
2.5.2. Inter-AS I-PMSI A-D Routes
There are no GTM-specific procedures for the origination,
distribution, and processing of these routes, other than those
specified in the sections on RD and RT usage.
2.6. S-PMSI A-D Routes
There are no GTM-specific procedures for the origination,
distribution, and processing of these routes, other than those
specified in the sections on RD and RT usage.
2.7. Leaf A-D Routes
There are no GTM-specific procedures for the origination,
distribution, and processing of these routes, other than those
specified in the sections on RD and RT usage.
2.8. Source Active A-D Routes
Please see the sections on RD and RT usage for information applies to
the origination and distribution of Source Active A-D routes.
Additional procedures governing the use of Source Active A-D routes
are given in the sub-sections of this section.
2.8.1. Finding the Originator of an SA A-D Route
To carry out the procedures specified in [RFC6514] (e.g., in Section
13.2 of that document), it is sometimes necessary for an egress PE to
determine the ingress PE that originated a given Source Active A-D
route. The procedure used in [RFC6514] to find the originator of a
Source Active A-D route assumes that no two routes have the same RD
unless they have been originated by the same PE. However, this
assumption is not valid in GTM, because each Source Active A-D route
used for GTM will have an RD of 0, and all the UMH-eligible routes
also have an RD of 0. So GTM requires a different procedure for
determining the originator of a Source Active A-D route.
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In GTM, the procedure for determining the originating PE of a Source
Active A-D route is the following:
- When a Source Active A-D route is originated, the originating PE
MAY attach a VRF Route Import Extended Community to the route.
- When a Source Active A-D route is distributed by one BGP speaker
to another, then
* if the Source Active A-D route does not carry the VRF Route
Import EC, the BGP speaker distributing the route MUST NOT
change the route's next hop field;
* if the Source Active A-D route does carry the VRF Route
Import EC, the BGP speaker distributing the route MAY change
the route's next hop field to itself.
- When an egress PE needs to determine the originator of a Source
Active A-D route, then
* if the Source Active A-D route carries the VRF Route Import
EC, the originating PE is the PE identified in the Global
Administrator field of that EC;
* if the Source Active A-D route does not carry the VRF Route
Import EC, the originating PE is the PE identified in the
route's next hop field.
2.8.2. Optional Additional Constraints on Distribution
If some site has receivers for a particular ASM group G, then it is
possible (by the procedures of [RFC6514]) that every PBR attached to
a site with a source for group G will originate a Source Active A-D
route whose NLRI identifies that source and group. These Source
Active A-D routes may be distributed to every PBR. If only a
relatively small number of PBRs are actually interested in traffic
from group G, but there are many sources for group G, this could
result in a large number of (S,G) Source Active A-D routes being
installed in a large number of PBRs that have no need of them.
For GTM, it is possible to constrain the distribution of (S,G) Source
Active A-D routes to those PBRs that are interested in GTM traffic to
group G. This can be done using the following OPTIONAL procedures:
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- If a PBR originates a C-multicast Shared Tree Join whose NLRI
contains (RD=0,*,G), then it dynamically creates an import RT for
its global table, where the Global Administrator field of the RT
contains the group address G, and the Local Administrator field
contains zero. (Note that an IPv6-address-specific RT would need
to be used if the group address is an IPv6 address.)
- When a PBR creates such an import RT, it uses "RT Constraint"
[RFC4684] procedures to advertise its interest in routes that
carry this RT.
- When a PBR originates a Source Active A-D route from its global
table, it attaches the RT described above.
- When the C-multicast Shared Tree Join is withdrawn, so is the
corresponding RT constrain route, and the corresponding RT is
removed as an import RT of its global table.
These procedures enable a PBR to automatically filter all Source
Active A-D routes that are about multicast groups in which the PBR
has no interest.
This procedure does introduce the overhead of distributing additional
"RT Constraint" routes, and therefore may not be cost-effective in
all scenarios, especially if the number of sources per ASM group is
small. This procedure may also result in increased join latency.
2.9. C-multicast Source/Shared Tree Joins
[RFC6514] section 11.1.3 has the following procedure for determining
the IP-address-specific RT that is attached to a C-multicast route:
(a) determine the upstream PE, RD, AS, (b) find the proper Inter-AS
or Intra-AS I-PMSI A-D route based on (a), (c) find the next hop of
that A-D route, (d) base the RT on that next hop.
However, for GTM, in environments where it is known a priori that
that the next hop of the C-multicast Source/Shared Tree Joins does
not change during the distribution of those routes, the proper
procedure for creating the IP-address-specific RT is to just put the
IP Address of the Upstream PBR in the Global Administrator field of
the RT. In other scenarios, the procedure of the previous paragraph
(as modified by this document's sections on "RD usage" and "RT
usage") is applied by the PBRs.
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3. Differences from other MVPN-like GTM Procedures
The document [SEAMLESS-MCAST] also defines a procedure for GTM that
is based on the BGP procedures that were developed for MVPN.
However, the GTM procedures of [SEAMLESS-MCAST] are different than
and are NOT interoperable with the procedures defined in this
document.
The two sets of procedures can co-exist in the same network, as long
as they are not applied to the same multicast flows or to the same
ASM multicast group addresses.
Some of the major differences between the two sets of procedures are
the following;
- The [SEAMLESS-MCAST] procedures for GTM do not use C-multicast
Shared Tree Joins or C-multicast Source Tree Joins at all. The
procedures of this document use these C-multicast routes for GTM,
setting the RD field of the NLRI to zero.
- The [SEAMLESS-MCAST] procedures for GTM use Leaf A-D routes
instead of C-multicast Shared/Source Tree Join routes. Leaf A-D
routes used in that manner can be distinguished from Leaf A-D
routes used as specified in [RFC6514] by means of the NLRI
format; [SEAMLESS-MCAST] defines a new NLRI format for Leaf A-D
routes. Whether a given Leaf A-D route is being used according
to the [SEAMLESS-MCAST] procedures or not can be determined from
its NLRI. (See [SEAMLESS-MCAST] section "Leaf A-D Route for
Global Table Multicast".)
- The Leaf A-D routes used by the current document contain an NLRI
that is in the format defined in [RFC6514], NOT in the format as
defined in [SEAMLESS-MCAST]. The procedures assumed by this
document for originating and processing Leaf A-D routes are as
specified in [RFC6514], NOT as specified in [SEAMLESS-MCAST].
- The current document uses an RD value of zero in the NLRI in
order to indicate that a particular route is "about" a Global
Table Multicast, rather than a VPN multicast. No other semantics
are inferred from the fact that RD is zero. [SEAMLESS-MCAST]
uses two different RD values in its GTM procedures, with semantic
differences that depend upon the RD values.
- In order for both sets of procedures to co-exist in the same
network, the PBRs MUST be provisioned so that for any given IP
group address in the global table, all egress PBRs use the same
set of procedures for that group address (i.e., for group G,
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either all egress PBRs use the GTM procedures of this document or
all egress PBRs use the GTM procedures of [SEAMLESS-MCAST].
4. IANA Considerations
This document has no IANA considerations.
5. Security Considerations
The security considerations of this document are primarily the
security considerations of the base protocols, as discussed in
[RFC6514], [RFC4601], and [RFC5294].
This document makes use of a BGP SAFI (MCAST-VPN routes) that was
originally designed for use in VPN contexts only. It also makes use
of various BGP path attributes and extended communities (VRF Route
Import Extended Community, Source AS Extended Community, Route Target
Extended Community) that were originally intended for use in VPN
contexts. If these routes and/or attributes leak out into "the
wild", multicast data flows may be distributed in an unintended
and/or unauthorized manner.
Internet providers often make extensive use of BGP communities (ie,
adding, deleting, modifying communities throughout a network). As
such, care should be taken to avoid deleting or modifying the VRF
Route Import Extended Community and Source AS Extended Community.
Incorrect manipulation of these ECs may result in multicast streams
being lost or misrouted.
The procedures of this document require certain BGP routes to carry
IP multicast group addresses. Generally such group addresses are
only valid within a certain scope. If a BGP route containing a group
address is distributed outside the boundaries where the group address
is meaningful, unauthorized distribution of multicast data flows may
occur.
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6. Additional Contributors
Zhenbin Li
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: lizhenbin@huawei.com
Wei Meng
ZTE Corporation
No.50 Software Avenue, Yuhuatai District
Nanjing
China
Email: meng.wei2@zte.com.cn,vally.meng@gmail.com
Cui Wang
ZTE Corporation
No.50 Software Avenue, Yuhuatai District
Nanjing
China
Email: wang.cui1@zte.com.cn
Shunwan Zhuang
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Beijing 100095
China
Email: zhuangshunwan@huawei.com
7. Acknowledgments
The authors and contributors would like to thank Rahul Aggarwal,
Huajin Jeng, Hui Ni, Yakov Rekhter, and Samir Saad for their
contributions to this work.
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8. Authors' Addresses
Lenny Giuliano
Juniper Networks
2251 Corporate Park Drive
Herndon, VA 20171
US
Email: lenny@juniper.net
Dante J. Pacella
Verizon
Verizon Communications
22001 Loudoun County Parkway
Ashburn, VA 20147
US
Email: dante.j.pacella@verizonbusiness.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA, 01719
US
Email: erosen@cisco.com
Jason Schiller
Google
1818 Library Street
Suite 400
Reston, VA 20190
US
Email: jschiller@google.com
Karthik Subramanian
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
US
Email: kartsubr@cisco.com
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Jeffrey Zhang
Juniper Networks
10 Technology Park Dr.
Westford, MA 01886
US
Email: zzhang@juniper.net
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4364], Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks", RFC 4364, February 2006.
[RFC6513] Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP VPNs",
RFC 6513, February 2012.
[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP VPNs", RFC
6514, February 2012.
[RFC6515] Aggarwal, R., and E. Rosen, "IPv4 and IPv6 Infrastructure
Addresses in BGP Updates for Multicast VPN", RFC 6515, February 2012.
9.2. Informative References
[ADD-PATH] "Advertisement of Multiple Paths in BGP", D. Walton, A.
Retana, E. Chen, J. Scudder, draft-ietf-idr-add-paths-09.txt, October
2013.
[RFC6368] Marques, P., Raszuk, R., Patel, K., Kumaki, K., and T.
Yamagata, "Internal BGP as the Provider/Customer Edge Protocol for
BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 6368, September
2011.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
Specification (Revised)", RFC 4601, August 2006.
[RFC4684] P. Marques, et. al., "Constrained Route Distribution for
Border Gateway Protocol/MultiProtocol Label Switching (BGP/MPLS)
Internet Protocol (IP) Virtual Private Networks (VPNs)", RFC 4684,
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November 2006.
[RFC5294] Savola, P. and J. Lingard, "Host Threats to Protocol
Independent Multicast (PIM)", RFC 5294, August 2008.
[MVPN-extranet] Rekhter, Y. and E. Rosen (editors), "Extranet
Multicast in BGP/IP MPLS VPNs", draft-ietf-l3vpn-mvpn-
extranet-04.txt, March 2014
[SEAMLESS-MCAST] Rekhter, Y., Aggarwal, R., Morin, T., Grosclaude,
I., Leymann, N., and S. Saad,"Inter-Area P2MP Segmented LSPs", draft-
ietf-mpls-seamless-mcast-13.txt, June 2014
Zhang, et al. [Page 23]
