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draft-ietf-mpls-mldp-recurs-fec
Network Working Group IJsbrand Wijnands
Internet Draft Eric C. Rosen
Intended Status: Proposed Standard Cisco Systems, Inc.
Expires: October 16, 2010
Maria Napierala
AT&T
Nicolai Leymann
Deutsche Telekom
April 16, 2010
Using mLDP through a Backbone where there is no Route to the Root
draft-wijnands-mpls-mldp-recurs-fec-01.txt
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Abstract
The control protocol used for constructing Point-to-Multipoint and
Multipoint-to-Multipoint Label Switched Paths ("MP LSPs") contains a
field that identifies the address of a "root node". Intermediate
nodes are expected to be able to look up that address in their
routing tables. However, if the route to the root node is a BGP
route, and the intermediate nodes are part of a BGP-free core, this
is not possible. This document specifies procedures which enable a
MP LSP to be constructed through a BGP-free core. In these
procedures, the root node address is temporarily replaced by an
address which is known to the intermediate nodes.
Table of Contents
1 Introduction .......................................... 3
2 The Recursive Opaque Value Type ....................... 5
2.1 Encoding .............................................. 5
2.2 Procedures ............................................ 5
3 The VPN-Recursive MP FEC Element ...................... 6
3.1 Encoding .............................................. 6
3.2 Procedures ............................................ 7
3.2.1 Unsegmented Inter-AS P-tunnels ........................ 7
3.2.2 Limited Carrier's Carrier Function .................... 9
4 IANA Considerations ................................... 10
5 Security Considerations ............................... 11
6 Acknowledgments ....................................... 11
7 Authors' Addresses .................................... 11
8 Normative References .................................. 12
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1. Introduction
[MLDP] defines several LDP FEC element encodings: P2MP, MP2MP
Upstream, and MP2MP Downstream.
The encoding for these three FEC elements is shown in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Address Family | Address Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Root Node Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Length | . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
~ ~
| Opaque Value |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MLDP FEC Element Encoding
Figure 1
Note that a P2MP or MP2MP label switched path ("MP LSP") is
identified by the combination of a "root node" and a variable length
"opaque value". The root node also plays a special role in the MLDP
procedures - MLDP messages that are "about" a particular MP LSP are
forwarded to the LDP adjacency that is the next hop on the route to
the root node.
Sometimes it is desirable for a MP LSP to pass through a part of the
network in which there is no route to the root node. For instance,
consider the topology of Figure 2:
CE1----PE1---P1---- ...----P2 ----PE2----CE2----R
Figure 2
where CE1 and CE2 are "customer edge routers", PE1 and PE2 are
"provider edge routers", but the provider's core is "BGP-free". That
is, PE1 has a BGP-learned route for R, in which PE2 is the BGP next
hop. However, the provider's interior routers (such as P1 and P2) do
not have any BGP-learned routes, and in particular do not have any
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routes to R.
In such an environment, data packets from CE1 address to R would get
encapsulated by PE1, tunneled to PE2, decapsulated by PE2, and
forwarded to CE2.
Suppose now that CE1 is trying to set up a MP LSP whose root is R,
and the intention is that the provider's network will participate in
the construction of the LSP. Then the MLDP messages identifying the
LSP must be passed from CE1 to PE1, from PE1 to P1, ..., from P2 to
PE2, from PE2 to CE2, and from CE2 to R.
To begin the process, CE1 creates a MP FEC element with the address
of R as the root node address, and passes that FEC element via MLDP
to PE1. However, PE1 cannot use this same FEC element to identify
the LSP in the LDP messages it sends to P1, because P1 does not have
a route to R.
However, PE1 does know that PE2 is the "BGP next hop" on the path to
R. What is needed is a method whereby:
- PE1 can tell P1 to set up an LSP as if the root node were PE2,
and
- PE2 can determine that the LSP in question is really rooted at R,
not at PE2 itself,
- PE2 can determine the original FEC element that CE1 passed to
PE1, so that PE2 can pass it on to CE2.
This document defines the procedures that allow CE1 to create an LSP
rooted at R. These procedures require PE1 to modify the MP FEC
element before sending an MLDP message to P1. The modified FEC
element has PE2 as the root, and the original FEC element as the
opaque value. This requires a new type of opaque value. Since the
opaque value contains a FEC element, we call this a "Recursive Opaque
Value". When PE2 sends an mLDP message to CE2, it replaces the FEC
element with the opaque value, thus undoing the recursion. Details
are in section 2.
Section 3 defines a "VPN Recursive Opaque Value". Whereas the
"Recursive Opaque Value" carries the original FEC, the "VPN Recursive
Opaque Value" carries the original FEC plus a Route Distinguisher
(RD). This has several possible uses in an L3VPN context. Details
are in section 3.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC2119].
2. The Recursive Opaque Value Type
2.1. Encoding
We define a new Opaque Value Type, the Recursive Opaque Value Type.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ ~
| P2MP or MP2MP FEC Element |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Recursive Opaque Value Type
Figure 3
The "opaque value" itself is a P2MP or MP2MP FEC element, encoded
exactly as specified in [MLDP], with a type field, a length field,
and value field of is own. The length field of the Recursive Opaque
Value Type thus includes the type and length fields of the FEC
element that is the value field.
2.2. Procedures
In the topology of Figure 2, let us suppose that CE1 sends PE1 an MP
FEC element whose root node is R, and whose opaque value is Q. We
will refer to this FEC element as "CE1-FEC". We may think of CE1-FEC
as an ordered pair, as follows:
CE1-FEC = <root=R, opaque_value=Q>.
PE1 determines that the root node R matches a BGP route, with a BGP
next hop of PE2. PE1 also knows by its configuration that the
interior routers on the path to PE2 are "BGP-free", and thus have no
route to R.
PE1 therefore MUST create a new MP FEC element, whose root node
address is the address of PE2, and whose opaque value is a Recursive
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(type 6) Opaque Value whose value field contains CE1-FEC. We refer
to this FEC element as PE2-FEC. PE1 then MUST send this FEC element
to P1.
PE2-FEC = <root=PE2, opaque_value=CE1-FEC>, or
PE2-FEC = <root=PE2, opaque_value=<root=R,
opaque_value=Q>>
As far as the interior routers are concerned, they are being
requested to build a MP LSP whose root node is PE2. They MUST NOT
interpret the opaque value at all.
When PE2-FEC arrives at PE2, PE2 notes that it is the identified root
node, and that the opaque value is a Recursive (type 6) opaque value.
Therefore it MUST replace PE2-FEC with the contents of the type 6
opaque value (i.e., with CE1-FEC) before doing any further
processing. This will result in CE1-FEC being sent on to CE2, and
presumably further from CE2 to R. Note that CE1-FEC will contain the
LSP root node specified by CE1; the presumption is that PE2 has a
route to this root node.
3. The VPN-Recursive MP FEC Element
3.1. Encoding
We define a new Opaque Value Type, the VPN-Recursive Opaque Value
Type.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 7 | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Route Distinguisher (8 octets) +-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ ~
| P2MP or MP2MP FEC Element |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
VPN-Recursive Opaque Value Type
Figure 3
The "opaque value" consists of an eight-octet Route Distinguisher
(RD), followed by a P2MP or MP2MP FEC element, encoded exactly as
specified in [MLDP], with a type field, a length field, and value
field of is own. The length field of the Recursive Opaque Value Type
thus includes the 8 octets of RD plus the type and length fields of
the FEC element that is the value field.
3.2. Procedures
3.2.1. Unsegmented Inter-AS P-tunnels
Consider the Inter-AS VPN scenario depicted in Figure 4.
PE1 --- P1 ---- ASBR1 ... ASBR2 ---- P2 ---- PE2
Figure 4
Suppose this is an "option B" VPN interconnect ([VPN] section 10).
This means that the Autonomous System Border Router (ASBR) in the
first Autonomous System (i.e., ASBR1) does not have a route to PE
routers in other ASes (such as PE2). Suppose also that the MVPN
policy is to instantiate PMSIs [MVPN] using mLDP, and that
"unsegmented inter-AS P-tunnels" [MVPN] are being used.
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In this scenario, PE1 may need to join a P2MP or MP2MP LSP whose root
is PE2. P1 has no route to PE2, and all PE1 knows about the route to
PE2 is that ASBR1 is the BGP next hop. Since P1 has no root to PE2,
PE1 needs to originate an mLDP message with a FEC element that
identifies ASBR1 as the root. This FEC element must contain enough
information to enable ASBR1 to find the next hop towards PE2 even
though ASBR1 does not have a route to PE2.
Although ASBR1 does not have a route to PE2, it does have a BGP
Intra-AS I-PMSI A-D route [MVPN] whose NLRI contains PE2's IP address
together with a particular RD. PE1 also has this Inter-AS I-PMSI A-D
route. The LSP needs to be set up along the path established by the
Intra-AS I-PMSI A-D routes. Therefore one must use a Recursive FEC
element that contains the RD as well as the as well as the address of
PE2. The "VPN-Recursive FEC Element" defined herein is used for this
purpose.
This enables us to provide the same functionality, for mLDP P-tunnels
that is provided for PIM P-tunnels in section 8.1.3.2 of [MVPN]
though the use of the MVPN Join Attribute.
At PE1 in Figure 4, the LSP to be created is associated with a
particular VRF. PE1 looks up in that VRF the Intra-AS I-PMSI A-D
route originated by PE2. It finds that the BGP next hop of that
route is ASBR1. So it creates a P2MP or MP2MP FEC element whose root
is ASBR1, and whose opaque value is a VPN-Recursive FEC element. The
VPN-Recursive FEC element itself consists of a root, an RD, and an
opaque value, set as follows:
- The root is PE2
- The RD is the RD from the NLRI of the Intra-AS A-D route
originated by PE2.
- The opaque value is chosen (by some method outside the scope of
this document) so as to be unique in the context of PE2. (E.g.,
it may have been specified in a PMSI tunnel attribute originated
by PE2.) We will refer to this opaque value as "Q".
The resulting FEC element can be informally represented as
<root=ASBR1, opaque_value=<root=PE2, RD, opaque_value=Q>>.
PE1 can now begin setting up the LSP by using this FEC element in an
LDP label mapping message sent towards ASBR1.
When ASBR1 receives, over a non-VRF interface, an mLDP label mapping
message containing this FEC element, it sees that it is the root, and
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that the opaque value is a VPN-Recursive (type 7) FEC element. It
parses the VPN-Recursive FEC element and extracts the root value,
PE2.
If ASBR1 has a route to PE2, it continues setting up the LSP by using
the following FEC element:
<root=PE2, opaque_value=Q>
However, if ASBR1 does not have a route to PE2, it looks for an
Intra-AS I-PMSI A-D route whose NLRI contains PE2's address along
with the specified RD value. Say the BGP next hop of that route is
ASBR2. Then ASBR1 continues setting up the LSP by using the
following FEC element:
<root=ASBR2, opaque_value=<root=PE2, RD, opaque_value=Q>>.
Note that in this case, the root has changed from ASBR1 to ASBR2, but
the opaque value is the unchanged VPN-Recursive FEC element.
3.2.2. Limited Carrier's Carrier Function
Another possible use of the VPN recursive FEC is to provide a limited
version of "Carrier's Carrier Service". Referring again to the
topology of Figure 2, suppose that PE1/PE2 are offering "Carrier's
Carrier VPN Service" [VPN] to CE1/CE2. CE1 sends PE1 an MP FEC
element whose root node is R, and whose opaque value is Q. We will
refer to this FEC element as "CE1-FEC". However, PE1's route to R
will be in a VRF ("Virtual Routing and Forwarding Table"). Therefore
the FEC-element created by PE1 must contain some identifier that PE2
can use to find the proper VRF in which to look up the address of R.
When PE1 looks up the address of R in a VRF, it will find a route in
the VPN-IP address family. The next hop will be PE2, but there will
also be a Route Distinguisher (RD) as part of that NLRI of the
matching route. In this case, the new FEC element created by PE1
MUST have the address of PE2 as the root node address, and MUST have
a VPN-Recursive (type 7) opaque value. The value field of the type 7
opaque value MUST consist of the 8-octet RD followed by CE1-FEC.
As far as the interior routers are concerned, they are being
requested to build a MP LSP whose root node is PE2. They MUST NOT
interpret the opaque value at all.
When an mLDP label mapping message containing PE2-FEC arrives at PE2
over a VRF interface, PE2 notes that it is the identified root node,
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and that the opaque value is a VPN-recursive (type 7) opaque value.
Therefore it MUST replace PE2-FEC with the contents of the VPN-
recursive opaque value (i.e., with CE1-FEC) before doing any further
processing. It uses the VRF to lookup up the path to R. This will
result in CE1-FEC being sent on to CE2, and presumably further from
CE2 to R.
In this scenario, the RD in the VPN-Recursive Opaque Value also
ensures uniqueness of the FEC Element within the inner carrier's
network.
This way of providing Carrier's Carrier service has limited
applicability, as it only works under the following conditions:
- Both the inner carrier and the outer carrier are using
unsegmented mLDP P-tunnels
- The inner carrier is not aggregating the P-tunnels of the outer
carrier, but is content to carry each such P-tunnel in a single
P-tunnel of its own.
The carrier's carrier scenario can be distinguished from the inter-AS
scenario by the fact that in the former, the mLDP messages are being
exchanged on VRF interfaces.
4. IANA Considerations
[MLDP] defines a registry for "The LDP MP Opaque Value Element Type".
This document requires the assignment of two new code points in this
registry:
- Type 6.
An opaque value of this type is itself a TLV that encodes an mLDP
FEC type, as defined in [MLDP].
- Type 7
An opaque value of this type consists of an eight-octet Route
Distinguisher as defined in [VPN], followed by a TLV that encodes
an mLDP FEC type, as defined in [MLDP].
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5. Security Considerations
TBD
6. Acknowledgments
The authors wish to thank Toerless Eckert for his contribution to
this work.
7. Authors' Addresses
IJsbrand Wijnands
Cisco Systems, Inc.
De kleetlaan 6a Diegem 1831
Belgium
E-mail: ice@cisco.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA, 01719
E-mail: erosen@cisco.com
Maria Napierala
AT&T Labs
200 Laurel Avenue, Middletown, NJ 07748
E-mail: mnapierala@att.com
Nicolai Leymann
Deutsche Telekom
Winterfeldtstrasse 21
Berlin 10781
Germany
E-mail: n.leymann@telekom.de
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8. Normative References
[MLDP] "Label Distribution Protocol Extensions for Point-to-
Multipoint and Multipoint-to-Multipoint Label Switched Paths", Minei,
Kompella, Wijnands, Thomas, draft-ietf-mpls-ldp-p2mp-08.txt, October
2009
[MVPN] "Multicast in MPLS/BGP IP VPNs", Rosen, Aggarwal, et. al.,
draft-ietf-l3vpn-2547bis-mcast-10.txt, January 2009
[RFC2119] "Key words for use in RFCs to Indicate Requirement
Levels.", Bradner, March 1997
[VPN] "BGP/MPLS IP Virtual Private Networks (VPNs)", Rosen, Rekhter,
RFC 4364, February 2006
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