Network Working Group Yakov Rekhter (Juniper Networks)
Internet Draft Rahul Aggarwal
Intended Status: Proposed Standard Nicolai Leymann (Deutsche Telekom)
Expiration Date: March 2014 August 2, 2013
Carrying PIM-SM in ASM mode Trees over P2MP mLDP LSPs
draft-rekhter-mpls-pim-sm-over-mldp-04.txt
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Abstract
When IP multicast trees created by PIM-SM in ASM mode need to pass
through an MPLS domain, it may be desirable to map such trees to
Point-to-Multipoint Label Switched Paths. This document describes how
to accomplish this in the case where such Point-to-Multipoint Label
Switches Paths are established using mLDP.
Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1. Introduction
When IP multicast trees need to pass through an MPLS domain, it may
be desirable to map such trees to Point-to-Multipoint Label Switched
Paths (P2MP LSPs). When P2MP LSPs are created by mLDP [mLDP], [MLDP-
IN-BAND-SIGNALLING] describes how to accomplish this when the trees
are created by PIM-SM in SSM mode [RFC4607], but does not describe
how to accomplish this when the trees are created by PIM-SM in ASM
mode [RFC4601].
This document describes how a combination of mLDP and BGP can be used
to accomplish this for multicast trees created by PIM-SM in ASM mode,
and P2MP LSPs created by mLDP. It describes two possible options for
doing this.
An implementation MAY support Option 1, as described in Section 2 of
this document. An implementation MUST support Option 2, as described
in Section 3 of this document.
This document uses BGP Source Active auto-discovery routes, as
defined in [MVPN-BGP].
Like [MLDP-IN-BAND-SIGNALLING], each IP multicast tree is mapped one-
to-one to a P2MP LSP in the MPLS network. This type of service works
well if the number of LSPs that are created is under control of the
MPLS network operator, or if the number of LSPs for a particular
service are known to be limited in number.
It is to be noted that the existing BGP MVPN [MVPN-BGP] procedures
may be used to map Internet IP multicast trees to P2MP LSPs. These
procedures would accomplish this for IP multicast trees created by
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PIM-SM in SSM mode as well as for IP multicast trees created by PIM-
SM in ASM mode. Furthermore, these procedures would also support the
ability to aggregate multiple IP multicast trees to one P2MP LSP in
the MPLS network. The details of this particular approach are out of
scope of this document.
2. Option 1
This option does not transit IP multicast shared trees over the MPLS
network. Therefore, when an LSR creates (*, G) state (as a result of
receiving PIM messages on one of its IP multicast interfaces), the
LSR does not propagate this state in mLDP.
2.1. Originating Source Active auto-discovery routes
Whenever (as a result of receiving either PIM Register or MSDP
messages) an RP discovers a new multicast source, the RP SHOULD
originate a BGP Source Active auto-discovery route. The route carries
a single MCAST-VPN NLRI constructed as follows:
+ The RD in this NLRI is set to 0.
+ The Multicast Source field MUST be set to S. The Multicast
Source Length field is set appropriately to reflect this.
+ The Multicast Group field MUST be set to G. The Multicast Group
Length field is set appropriately to reflect this.
To constrain distribution of the Source Active auto-discovery route
to the AS of the advertising RP this route SHOULD carry the NO_EXPORT
Community ([RFC1997]).
Using the normal BGP procedures the Source Active auto-discovery
route is propagated to all other LSRs within the AS.
Whenever the RP discovers that the source is no longer active, the RP
MUST withdraw the Source Active auto-discovery route, if such a route
was previousely advertised by the RP.
2.2. Receiving BGP Source Active auto-discovery route by LSR
Consider an LSR that has some of its interfaces capable of IP
multicast and some capable of MPLS multicast.
When as a result of receiving PIM messages on one of its IP multicast
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interfaces such LSR creates in its Tree Information Base (TIB) a new
(*, G) entry with a non-empty outgoing interface list that contains
one or more IP multicast interfaces, the LSR MUST check if it has any
Source Active auto-discovery routes for that G. If there is such a
route, S of that route is reachable via an MPLS interface, and the
LSR does not have (S, G) state in its TIB for (S, G) carried in the
route, then the LSR originates the mLDP Label Map with the Transit
IPv4/IPv6 Source TLV carrying (S,G), as specified in [MLDP-IN-BAND-
SIGNALLING].
When an LSR receives a new Source Active auto-discovery route, the
LSR MUST check if its TIB contains an (*, G) entry with the same G as
carried in the Source Active auto-discovery route. If such an entry
is found, S is reachable via an MPLS interface, and the LSR does not
have (S, G) state in its TIB for (S, G) carried in the route, then
the LSR originates an mLDP Label Map with the Transit IPv4/IPv6
Source TLV carrying (S,G), as specified in [MLDP-IN-BAND-SIGNALLING].
2.3. Handling (S, G, RPTbit) state
Creation and deletion of (S, G, RPTbit) state on a LSR that resulted
from receiving PIM messages on one of its IP multicast interfaces
does not result in any mLDP and/or BGP actions by the LSR.
3. Option 2
This option enables transit of IP multicast shared trees over the
MPLS network. Therefore, when an LSR creates (*, G) state as a result
of receiving PIM messages on one of its IP multicast interfaces, the
LSR does propagate this state in mLDP, as described below.
Note that in the deployment scenarios where for a given G none of the
PEs originate an (S, G) mLDP Label Map with the Transit IPv4/IPv6
Source TLV, no Source Active auto-discovery routes will be used. One
other scenario where no Source Active auto-discovery route will be
used is described in section "Originating Source Active auto-
discovery routes". In all these scenarios the only part of Option 2
that will be used is the in-band signaling for IP Multicast Shared
Tree, as described in the next section.
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3.1. In-band signaling for IP Multicast Shared Tree
To provide support for in-band mLDP signaling of IP multicast shared
trees this document defines two new mLDP TLVs: Transit IPv4 Shared
Tree TLV, and Transit IPv6 Shared Tree TLV.
These two TLVs have exactly the same encoding/format as the IPv4/IPv6
Source Tree TLVs defined in [MLDP-IN-BAND-SIGNALLING], except that
instead of the Source field they have the RP field, and this field
carries the address of the RP.
Procedures for in-band signaling for IP multicast shared trees with
mLDP follow the same procedures as for in-band signaling for IP
multicast source trees specified in [MLDP-IN-BAND-SIGNALLING], except
that while the latter signals (S,G) state using Transit IPv4/IPv6
Source TLVs, the former signals (*,G) state using Transit IPv4/IPv6
Shared Tree TLVs.
3.2. Originating Source Active auto-discovery routes
Consider an LSR that has some of its interfaces capable of IP
multicast and some capable of MPLS multicast.
Whenever such LSR creates an (S, G) state as a result of receiving an
mLDP Label Map with the Transit IPv4/IPv6 Source TLV for (S, G), if
all of the following are true:
+ S is reachable via one of the IP multicast capable interfaces,
+ the LSR determines that G is in the PIM-SM in ASM mode range, and
+ the LSR does not have an (*, G) state with one of the IP
multicast capable interfaces as an incoming interface (iif) for
that state
the LSR MUST originate a BGP Source Active auto-discovery route.
The route carries a single MCAST-VPN NLRI constructed as follows:
+ The RD in this NLRI is set to 0.
+ The Multicast Source field MUST be set to S. The Multicast Source
Length field is set appropriately to reflect this.
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+ The Multicast Group field MUST be set to G. The Multicast Group
Length field is set appropriately to reflect this.
To constrain distribution of the Source Active auto-discovery route
to the AS of the advertising LSR this route SHOULD carry the
NO_EXPORT Community ([RFC1997]).
Using the normal BGP procedures the Source Active auto-discovery
route is propagated to all other LSRs within the AS.
Whenever the LSR deletes the (S, G) state that was previously created
as a result of receiving an mLDP Label Map with the Transit IPv4/IPv6
Source TLV for (S, G), the LSR that deletes the state MUST also
withdraw the Source Active auto-discovery route, if such a route was
advertised when the state was created.
Note that whenever an LSR creates an (S, G) state as a result of
receiving an mLDP Label Map with the Transit IPv4/IPv6 Source TLV for
(S, G) with S reachable via one of the IP multicast capable
interfaces, and, as a result of receiving an mLDP Label Map with the
Transit IPv4/IPv6 Shared Tree TLV for (*, G), the LSR already has a
(*, G) state with RP reachable via one of the IP multicast capable
interfaces, the LSR does not originate a Source Active auto-discovery
route.
3.3. Receiving BGP Source Active auto-discovery route
Procedures for receiving BGP Source Active auto-discovery routes are
the same as with Option 1.
3.4. Pruning Sources off the Shared Tree
If after receiving a new Source Active auto-discovery route for (S,G)
the LSR determines that (a) it has the (*, G) entry in its TIB, (b)
the incoming interface list (iif) for that entry contains one of the
IP interfaces, (c) at least one of the MPLS interfaces is in the
outgoing interface list (oif) for that entry, and (d) the LSR does
not originate an mLDP Label Mapping message for (S,G) with the
Transit IPv4/IPv6 Source TLV, then the LSR MUST transition the
(S,G,rpt) downstream state to the Prune state. [Conceptually the PIM
state machine on the LSR will act "as if" it had received
Prune(S,G,Rpt) on one of its MPLS interfaces, without actually having
received one.] Depending on the (S,G,rpt) state on the iif, this may
result in the LSR using PIM procedures to prune S off the Shared
(*,G) tree.
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The LSR MUST keep the (S,G,rpt) downstream state machine in the Prune
state for as long as (a) the outgoing interface list (oif) for (*, G)
contains one of the MPLS interfaces, and (b) the LSR has at least one
Source Active auto-discovery route for (S,G), and (c) the LSR does
not originate the mLDP Label Mapping message for (S,G) with the
Transit IPv4/IPv6 Source TLV. Once either of these conditions become
no longer valid, the LSR MUST transition the (S,G,rpt) downstream
state machine to the NoInfo state.
Note that except for the scenario described in the first paragraph of
this section, in all other scenarios relying solely on PIM procedures
on the LSR is sufficient to ensure the correct behavior when pruning
sources off the shared tree.
3.5. More on handling (S, G, RPTbit) state
Creation and deletion of (S, G, RPTbit) state on a LSR that resulted
from receiving PIM messages on one of its IP multicast interfaces
does not result in any mLDP and/or BGP actions by the LSR.
4. IANA Considerations
This document defines two new mLDP TLVs: Transit IPv4 Shared Tree
TLV, and Transit IPv6 Shared Tree TLV.
5. Security Considerations
All the security considerations for mLDP apply here.
6. Acknowledgements
Use of Source Active auto-discovery routes was borrowed from [MVPN-
BGP]. Some text in this document was borrowed almost verbatim from
[MVPN-BGP].
Some of the text in this document was borrowed almost verbatim from
[MLDP-IN-BAND-SIGNALLING].
We would like to acknowledge Arkadiy Gulko for his review and
comments.
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7. Normative References
[mLDP] Minei, I., "Label Distribution Protocol Extensions for Point-
to- Multipoint and Multipoint-to-Multipoint Label Switched Paths",
draft-ietf-mpls-ldp-p2mp-05 (work in progress), June 2008.
[MLDP-IN-BAND-SIGNALLING] "In-band signaling for Point-to-Multipoint
and Multipoint-to-Multipoint Label Switched Paths", I. Wijnands et
al., draft-wijnands-mpls-mldp-in-band-signaling (work in progress)
[MVPN-BGP] "BGP Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", R. Aggarwal et al., draft-ietf-l3vpn-2547bis-mcast-bgp (work
in progress)
[RFC1997] R. Chandra, P. Traina, T. Li, "BGP Communities Attribute",
RFC1997, August 1996.
[RFC2119] "Key words for use in RFCs to Indicate Requirement
Levels.", Bradner, RFC2119, March 1997.
8. Non-normative References
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
Specification (Revised)", RFC 4601, August 2006.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
9. Authors' Addresses
Yakov Rekhter
Juniper Networks, Inc.
e-mail: yakov@juniper.net
Rahul Aggarwal
e-mail: raggarwa_1@yahoo.com
Nicolai Leymann
Deutsche Telekom
Winterfeldtstrasse 21
Berlin 10781
Germany
e-mail: nicolai.leymann@t-systems.com
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