Network Working Group Hannes Gredler
Internet Draft Juniper Networks
Intended status: Standards Track
Expires: April 2014 Yakov Rekhter
Juniper Networks
October 1 2013
Supporting Source/Explicitly Routed Tunnels via Stacked LSPs
draft-gredler-spring-mpls-00.txt
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Abstract
This document describes how source/explicitly routed tunnels could be
realized using stacked Label Switched Paths (LSPs).
This document also describes how use of IS-IS/OSPF as a label
distribution protocol fits into the MPLS architecture.
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Table of Contents
1 Specification of Requirements ......................... 3
2 Terminology ........................................... 4
3 Introduction .......................................... 4
4 Constructing Explicitly Routed Tunnels by using Stacked LSPs 5
4.1 Examples of Constructing Explicitly Routed Tunnels by Stacked LSPs 7
4.1.1 Explicitly Routed Tunnel with Strict Hops ............. 8
4.1.2 Explicitly Routed Tunnel with Loose Hops .............. 10
5 IS-IS or OSPF as Label Distribution Protocol .......... 12
5.1 Example of IS-IS/OSPF as Label Distribution Protocols . 13
6 IANA Considerations ................................... 14
7 Security Considerations ............................... 14
8 Acknowledgements ...................................... 14
9 Normative References .................................. 14
10 Informative References ................................ 14
11 Authors' Addresses .................................... 15
1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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2. Terminology
We use the term "explicitly routed tunnels" as a synonym for such
terms as "source routed tunnels" and "source-initiated routed
tunnels".
Note that the term "source routed tunnel", or "source-initiated
routed tunnel" does not imply that intermediate nodes of such a
tunnel forward packets traversing the tunnel based upon source
addresses of these packets. In the context of "source routed tunnels"
and "source-initiated routed tunnels" the term "source" refers to the
tunnels' ingress.
3. Introduction
MPLS architecture [RFC3031] defines the concept of explicitly routed
tunnel as follows:
If a Tunneled Packet travels from Ru to Rd over a path other
than the Hop-by-hop path, we say that it is in an "Explicitly
Routed Tunnel"
where Ru and Rd are Label Switch Routers (LSRs).
To realize explicitly routed tunnels [RFC3031] proposes to use
explicitly routed LSPs:
An "Explicitly Routed LSP Tunnel" is a LSP Tunnel that is also an
Explicitly Routed LSP
Up until now there have been two possible protocols to instantiate/signal
such explicitly routed LSPs - RSVP-TE ([RFC3209]) and CR-LDP
([RFC3212]).
MPLS architecture ([RFC3031]) defines the notion of LSP hierarchy,
as LSP tunnels within LSPs. Use of MPLS label stack mechanism allows
LSP hierarchy to nest to any depth.
In this document we specify the procedures to realize explicitly
routed point-to-point tunnels by using LSP hierarchy, thus defining
yet another possible mechanism to realize such tunnels.
An essential part of MPLS is the notion of label distribution
protocol. On the subject of whether it should be one, or more then
one label distribution protocol, MPLS architecture ([RFC3031]) said
the following:
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THE ARCHITECTURE DOES NOT ASSUME THAT THERE IS ONLY A SINGLE
LABEL DISTRIBUTION PROTOCOL. In fact, a number of different
label distribution protocols are being standardized.
Up until now IETF standardized the following label distribution
protocols for unicast: LDP ([RFC5036]), CR-LDP ([RFC3212]), RSVP-TE
[RFC3209] and BGP ([RFC3107], [RFC4364], [RFC4761]).
Recently there have been proposals ([gredler-isis], [gredler-ospf],
[previdi-isis], [psenak-ospf]) to extend IS-IS [RFC1142] and OSPF
[RFC1583] to make them yet another label distribution protocols.
This document describes how use of IS-IS or OSPF as label
distribution protocols fits into the MPLS architecture. This document
also describes the benefits of using IS-IS/OSPF as label distribution
protocols for the purpose of constructing explicitly routed tunnels
with stacked LSPs.
4. Constructing Explicitly Routed Tunnels by using Stacked LSPs
Instead of explicitly routed LSPs, one can use LSP hierarchy (stack
of LSPs) to construct explicitly routed point-to-point tunnels as
follows.
Consider an explicitly routed point-to-point tunnel with an explicit
route <R(0), R(1), R(2), ... R(n)>, where R(0) is the ingress of the
tunnel and R(n) is the egress of the tunnel. Denote the LSPs needed
to realize such a tunnel via an LSP stack as <LSP(1), LSP(2), ...
LSP(n)>, where LSP(1) is the topmost and LSP(n) is the bottommost LSP
in the stack. These LSPs are constructed as follows:
+ All the LSPs in the stack are constructed with the same ingress -
R(0).
+ LSP(i) is constructed with R(i) as its egress (e.g., LSP(1) is
constructed with R(1) as its egress, LSP(2) with R(2) as its
egress, etc... LSP(n) with R(n) as its egress).
+ For every 0 < i < n, the first intermediate point of LSP(i+1).
is constructed to be the egress of LSP(i).
+ The first intermediate point of LSP(1) is constructed to be one
hop away from R(0). If R(1) is one hop away from R(0), then this
intermediate point is also the egress of LSP(1) (in which case
LSP(1) is a one-hop LSP). If R(1) is more than one hop away from
R(0), then this intermediate point is some router other than
R(1), and R(1) is still the egress of that LSP.
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+ The first intermediate point of any LSP in the stack, could be
either single or multi-hop away from the egress of that LSP.
When R(i) and R(i+1) are single hop away from each other, this
corresponds to a strict hop for the explicitly routed tunnel, in
which case the first intermediate point of LSP(i+1) is one hop away
from the egress of that LSP. When R(i) and R(i+1) are multi-hop away
from each other, this corresponds to a loose hop for the explicitly
routed tunnel, in which case the first intermediate point of LSP(i+1)
is multi-hop away from the egress of that LSP.
Following the above procedures, the LSPs in the stack satisfy the
following properties:
+ All LSPs in the stack have the same ingress.
+ The egress of a given LSP in the stack is the first intermediate
point of the next LSP in the stack.
+ The first intermediate point of the LSP at the top of the stack
is one hop away from the ingress.
+ The first intermediate point of any LSP in the stack could be
either single or multi-hop away from the egress of that LSP.
(Thus the egress of a given LSP in the stack could be either
single or multi-hop away from the egress of the next LSP in the
stack.)
Such stack of LSPs provides the functionality to forward a packet
through a sequence of egresses of the LSPs on the stack - the
sequence of these egresses represents the explicit route of the
explicitly routed point-to-point tunnel constructed by using these
stacked LSPs. The ingress of all these LSPs is the ingress of the
tunnel.
When the first intermediate point of a given LSP in the stack is
multi-hop away from the egress of that LSP, the existing label
distribution protocols (LDP, RSVP-TE, etc. ) can be used to establish
a multi-hop LSP fragment for this LSP. When IS-IS or OSPF, in
addition to being a routing protocol, is also used as a label
distribution protocol (see section "IS-IS or OSPF as Label
Distribution Protocol"), it can also be used to establish such multi-
hop LSP fragment.
To construct the label stack associated with the stack of LSPs the
ingress uses the following procedures:
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+ For (n > i > 0) R(0) obtains from R(i) label binding for LSP(i+1)
and places the label onto the stack, starting from the bottommost
label (the label that corresponds to LSP(n).
In section "IS-IS or OSPF as Label Distribution Protocol" we
describe how IS-IS or OSPF with appropriate extensions could be
used as a label distribution protocol to obtain such label
bindings.
If the first intermediate point of LSP(i+1) is either (a) a
single hop away from the egress of that LSP, or (b) multi-hop
away, and LDP is used as a label distribution protocol to
establish a multi-hop LSP fragment between the first intermediate
point and the egress of that LSP, then R(0) can use targeted LDP
session with R(i) to obtain such label bindings.
+ For LSP(1) if R(1) is one hop away from R(0), then no label is
needed, and the label stack construction terminates with the
topmost label that R(0) obtains from R(1) for LSP(2).
+ Otherwise, if R(1) is more than one hop away from R(0), then R(0)
places the label it binds to LSP(1) at the top of the stack.
Since the MPLS label stack mechanism allows stack of LSPs to nest to
any depth, use of LSP hierarchy for explicitly routed tunnels does
not place any protocol restrictions on the number of entries in the
explicit route of an explicitly routed tunnel. Note though that there
may be some other restrictions (e.g., due to MTU, or hardware) that
would place an upper bound on the depth of the label stack, and thus
on the number of entries in the explicit route. Also, the depth of
the label stack may have implications on ECMP, and specifically on
the use of the Entropy label (see [kini] for more).
4.1. Examples of Constructing Explicitly Routed Tunnels by Stacked LSPs
In this section we illustrate how to construct an explicitly routed
tunnel by using stacked LSPs. The first example illustrates this for
an explicitly routed tunnel with strict hops. The second example
illustrates this for an explicitly routed tunnel with loose hops.
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4.1.1. Explicitly Routed Tunnel with Strict Hops
Consider a network topology shown below:
R0-----R4
| /|
| / |
| / |
| / |
| / |
R1 R3
\ /
\ /
R2
Assume that R0 wants to construct an explicitly routed tunnel with
(R0, R1, R2, R3, R4) as strict hops. R0 constructs this tunnel using
the following stack of LSPs:
LSP1: (R0, R1) - top of the stack
LSP2: (R0, R1, R2)
LSP3: (R0, R2, R3)
LSP4: (R0, R3, R4) - bottom of the stack
Note that this stack of LSPs meets the requirements specified in
section "Constructing Explicitly Routed Tunnels by using Stacked
LSPs". Specifically,
+ All four LSPs in the stack have the same ingress - R0, which is
also the ingress of the explicitly routed tunnel.
+ The egress of LSP1, R1, is the first intermediate point of the
next LSP in the stack, LSP2. The egress of LSP2, R2, is the first
intermediate point of the next LSP in the stack, LSP3. Likewise,
the egress of LSP3, R3, is the first intermediate point of the
next (and the last) LSP in the stack, LSP4.
+ The LSP at the top of the stack, LSP1, has its first intermediate
point, R1, one hop away from its ingress, R0. Because of that,
this intermediate point is also the egress of that LSP, and that
LSP is a one-hop LSP.
+ In that particular example the first intermediate point of every
LSP in the stack is one hop away from the egress of that LSP.
That is, the first intermediate point of LSP2, R1, is one hop
away from the egress of that LSP, R2; the first intermediate
point of LSP3, R2, is one hop away from the egress of that LSP,
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R3; and the first intermediate point of LSP4, R3, is one hop away
from the egress of that LSP, R4. As a result, the egress of a
given LSP in the stack is one hop away from the egress of the
next LSP in the stack, which makes all the hops of the explicitly
routed tunnel strict.
The first intermediate point of each of these LSPs creates label
bindings for these LSPs as follows. R3 creates label binding for LSP4
by binding a particular label, L1, to the address of R4, creating a
Next Hop Label Forwarding Entry (NHLFE) whose next hop is the link
from R3 to R4, and setting the Incoming Label Map (ILM) so that L1
maps to that NHLFE. Likewise, R2 creates label binding for LSP3 by
binding a particular label, L2, to the address of R3, creating an
NHLFE whose next hop is the link from R2 to R3, and setting the ILM
so that L2 maps to that NHLFE. Finally, R1 creates label binding for
LSP2 by binding a particular label, L3, to the address of R2,
creating an NHLFE whose next hop is the link from R1 to R2, and
setting the ILM so that L3 maps to that NHLFE.
To get from the first hop of LSP4, R0, to the second hop of LSP4, R3,
the packet has to go through the LSP tunnel provided by LSP3. To get
from the first hop of LSP3, R0, to the second hop of LSP3, R2, a
packet has to go through the LSP tunnel provided by LSP2. To get from
the first hop of LSP2, R0, to the second hop of LSP2, R1, a packet
has to go through the LSP tunnel provided by LSP1.
In order to accomplish this R0 constructs the label stack for the
explicitly routed tunnel as follows:
+ Step 1: R0 obtains label binding L1 created by R3 for LSP4 (R0,
R3, R4), and starts building the label stack by pushing L1 onto
the label stack.
+ Step 2: R0 obtains label binding L2 created by R2 for LSP3 (R0,
R2, R3), and pushes L2 into the stack. At this point the stack
contains (L2, L1).
+ Step 3: R0 obtains label binding L3 created by R1 for LSP2 (R0,
R1, R2), and pushes L3 into the stack. At this point the stack
contains (L3, L2, L1).
+ Step 4: Since R0 and R1 are one hop away from each other the
label stack construction is completed (R0 does not need a label
for one-hop LSP1).
So far we did not say anything about how R0 obtains from R3 label
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binding for LSP4, from R2 label binding for LSP3, and from R1 label
binding for LSP2.
At least in principle, these label bindings could be obtain by such
already defined label distribution protocols as LDP (to be more
precise, targeted LDP if the two routers are more than one hop away
from each other). E.g., if one uses targeted LDP, then R0 would need
to maintain a targeted LDP session with R3 and another targeted LDP
session with R2 (R0 would maintain a "vanilla" LDP session with R1).
Using these LDP sessions R0 would obtain from R3 label binding for
LSP4, from R2 label binding for LSP3, and from R1 label binding for
LSP2.
In section "IS-IS or OSPF as Label Distribution Protocol" we describe
how IS-IS or OSPF with appropriate extensions could be used as a
label distribution protocol to obtain such label bindings.
4.1.2. Explicitly Routed Tunnel with Loose Hops
Consider a network topology shown below:
R0---R1
/ \
R2 R5---R6
\ /
R3---R4
Assume that R0 wants to construct an explicitly routed tunnel with
(R0, R4, R6) as loose hops. R0 constructs this tunnel using the
following stack of LSPs:
LSP1: (R0, R2, R3, R4) - top of the stack
LSP2: (R0, R4, R5, R6) - bottom of the stack
Note that this stack of LSPs meets the requirements specified in
section "Constructing Explicitly Routed Tunnels by using Stacked
LSPs". Specifically,
+ Both LSPs in the stack have the same ingress - R0, which is also
the ingress of the explicitly routed tunnel.
+ The egress of LSP1, R4, is the first intermediate point of the
next LSP in the stack, LSP2.
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+ The LSP at the top of the stack, LSP1, has its first intermediate
point, R2, one hop away from its ingress, R0. However, this
intermediate point is not the egress of that LSP, and therefore
this LSP is a multi-hop LSP.
+ In that particular example the first intermediate point of every
LSP in the stack is multi-hop away from the egress of that LSP
That is, the first intermediate point of LSP1, R2, is multi-hop
away from the egress of that LSP, R4; the first intermediate
point of LSP2, R4, is multi-hop away from the egress of that LSP,
R6. As a result, the egress of a given LSP in the stack is
multi-hop away from the egress of the next LSP in the stack,
which makes the hops of the explicitly routed tunnel loose.
In this example we assume that LDP is used as a label distribution
protocol for both LSP1 and LSP2. Since R0 and R4 are not IGP
neighbors, they are remote label distribution peers. Thus R0 and R4
use targeted LDP for label distribution. All other routers use
"vanilla" LDP procedures.
To get from the first hop of LSP2, R0, to its second hop, R4, the
packet has to go through the LSP tunnel provided by LSP1.
In order to accomplish this R0 constructs the label stack for the
explicitly routed tunnel as follows:
+ Step 1: R0 (using targeted LDP) obtains label binding L1 created
by R4 for LSP2 (R0, R4, R5, R6), and starts building the label
stack by pushing L1 onto the label stack.
+ Step 2: R0 (using "vanilla" LDP procedures) obtains label binding
L2 created by R2 for LSP1 (R0, R3, R4), and pushes L2 into the
stack. At this point the stack contains (L2, L1).
+ Step 3: Since R0 and R1 are one hop away from each other the
label stack construction is completed (R0 does not need a label
for one-hop LSP1).
A reader familiar with Remote LFA FRR [R-LFA] should be able to
notice that the example described in this section is nothing more
than an instance of Remote LFA FRR, where Remote LFA FRR provides
fast reroute to the traffic going from R0 to R6 in the presence of
the (R0, R2) link failure, with R0 being the Point of Local Repair
(PLR), R4 being the PQ-node, and R6 being the ultimate destination.
The explicitly routed tunnel (R0, R4, R6) consists of the PLR as the
head-node, the PQ-node as the next loose hop, and the ultimate
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destination as yet another loose hop.
5. IS-IS or OSPF as Label Distribution Protocol
When OSPF or IS-IS, in addition to being a routing protocol, is also
used as a label distribution protocol (as proposed in [gredler-isis],
[gredler-ospf], [previdi-isis], [psenak-ospf), the OSPF/IS-IS Link
State Advertisements originated by a router carry label bindings for
LSPs that either transit or originated by the router. Doing this
allows to extend such LSPs. The criteria for selecting among all
these LSPs a subset for which the router would originate label
binding advertisements in IS-IS/OSPF are purely local to the router.
The router could be either single or multi-hop away from the egresses
of the LSPs in the subset. Existing label distribution protocols
(LDP, RSVP-TE, etc.) can be used to establish multi-hop LSP fragments
if the router is multi-hop away from the egress of a particular LSP
in the subset. When IS-IS or OSPF, in addition to being a routing
protocol, is also used as a label distribution protocol, it can also
be used to establish such multi-hop LSP fragments.
MPLS architecture [RFC3031] defines the notion of local/remote label
distribution peers as follows:
When two LSRs are IGP neighbors, we will refer to them as "local
label distribution peers". When two LSRs may be label distribution
peers, but are not IGP neighbors, we will refer to them as "remote
label distribution peers."
Following OSPF/IS-IS procedures each router passes Link State
Advertisements originated by other routers unmodified. When these
advertisements carry label binding information, this information is
also passed unmodified. Therefore, the router that originates label
bindings advertisements in IS-IS/OSPF can be either single or multi-
hop away from the routers that receive and use these bindings. In
the former case the IGP neighbors of the router that originates the
advertisements will be the local label distribution peers of the
router. In the latter case other routers in the same IGP domain will
be the remote label distribution peers of the router.
Use of OSPF or IS-IS as a label distribution protocol provides
scalable support for remote label distribution peering in terms of
the number of label distribution peers a given router has to
maintain. This is because label distribution protocol messages (Link
State Advertisements) are exchanged only between IGP neighbors,
without requiring control plane peering between a router that
originates Link State Advertisements and each of its remote label
distribution peers.
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It is important to note that the existing MPLS control plane already
has mechanisms/protocols to support remote label distribution peering
(using BGP or targeted LDP [RFC5036]). Thus the practical relevance
of the ability to provide scalable support for remote label
distribution peering with IS-IS or OSPF as a label distribution
protocol depends on a particular use case.
If for a given subset of routers within an MPLS network each router
within the subset is assigned a distinct index, then one could
compress announcements of labels bound to the LSPs whose FECs are the
IP addresses of these routers by (a) advertising these indices in IS-
IS/OSPF, and (b) making each router advertise a label block in IS-
IS/OSPF as well. A router R1 that advertises a given label block
algorithmically binds a FEC associated with an IP address of some
other router R2 to the label from that block that is identified by
the index that R2 advertises in IGP. A router R1 that receives label
block originated by some other router R2 can determine the label
bound to a FEC associated with an IP address of some other router R3
by using the index advertised by R3 as an offset into the label block
advertised by R2. Note that to avoid wasting labels this scheme
requires a fairly dense assignment of indices. Also note that to
expand the number of labels that a router advertises using label
blocks, the router may advertise more than one label block.
Note though, that the benefits of scaling improvements in terms of
label distribution peering come at a cost, as every router in the
domain ends up keeping all the labels assigned/bounded by every other
router in the domain, whether it really needs to know them or not.
Whether this cost is of practical significance depends on the
encoding of label bindings (e.g., use of label blocks vs enumerating
each label binding).
5.1. Example of IS-IS/OSPF as Label Distribution Protocols
In this section we illustrate how IS-IS/OSPF with extensions, as
defined in [gredler-isis], [gredler-ospf], [previdi-isis], [psenak-
ospf] could be used as a label distribution protocol to support
explicitly routed tunnels realized by stacked LSPs. For the purpose
of this illustration we assume the scenario described in section
"Example of Constructing Explicitly Routed Tunnels by Stacked LSPs".
In that example one of the key issues is the ability of R0 to obtain
from R3 label binding for LSP4, from R2 label binding for LSP3, and
from R1 label binding for LSP2.
To obtains such label bindings, the Link State Advertisement
originated by R3 carries label L1 (this is the label that R3 binds to
LSP4). Using IS-IS/OSPF procedures this Link State Advertisement is
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propagated by R2 and R1 (as well as by R4) to R0. This is how R0
obtains from R3 label binding for LSP4. In a similar fashion, the
Link State Advertisement originated by R2 carries label L2 (which is
the label that R2 binds to LSP3). Using IS-IS/OSPF procedures this
Link State Advertisement is propagated by R1 (as well as by R3 and
R4) to R0. This is how R0 obtains from R2 label binding for LSP3.
Likewise, the Link State Advertisement originated by R1 carries label
L3 (which is the label that R1 binds to LSP2). Using IS-IS/OSPF
procedures this Link State Advertisement is delivered to R0. This is
how R0 obtains from R1 label binding for LSP2.
Note that while from R0's perspective both R2 and R3 are remote label
distribution peers, R0 does not maintain any control plane peering
(e.g., targeted LDP) with either R2 or R3.
6. IANA Considerations
This document introduces no new IANA Considerations.
7. Security Considerations
TBD
8. Acknowledgements
We would like to thank John Drake and John Scudder for their review
and comments.
9. Normative References
[RFC3031] Rosen, E., et. al., "Multiprotocol Label Switching
Architecture", RFC3031, January 2001
10. Informative References
[kini] Kini, S., et. al., "Entropy labels for source routed stacked
tunnels", draft-kini-mpls-entropy-label-src-stacked-tunnels, work in
progress
[gredler] Gredler, H., et. al., "Advertising MPLS labels in IGPs"
draft-gredler-rtgwg-igp-label-advertisement, work in progress
[gredler-isis] Gredler, H., et. al., "Advertising MPLS labels in IS-
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IS" draft-gredler-isis-label-advertisement, work in progress
[gredler-ospf] Gredler, H., et. al., "Advertising MPLS labels in
OSPF" draft-gredler-ospf-label-advertisement, work in progress
[previdi-isis] Previdi, S., et. al., "IS-IS Extensions for Segment
Routing" draft-previdi-isis-segment-routing-extensions, work in
progress
[psenak-ospf] Psenak, P., et. al., "OSPF Extensions for Segment
Routing" draft-psenak-ospf-segment-routing-extensions, work in
progress
[R-LFA] Bryant, S., et. al., "Remote LFA FRR", draft-ietf-rtgwg-
remote-lfa, work in progress
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119, March 1997
[RFC3107] Rekhter, Y., et. al., "Carrying Label Information in
BGP-4", RFC3107, May 2001
[RFC3209] Awduche, D., et. al., "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC3209, December 2001
[RFC3212] Jamoussi, B., et. al., "Constraint-Based LSP Setup using
LDP", RFC3212, January 2002
[RFC4364] Rosen E., et. al., "BGP/MPLS IP Virtual Private Networks
(VPNs)", RFC4364, February 2006
[RFC4761] Kompella, K., et. al., "Virtual Private LAN Service (VPLS)
Using BGP for Auto-Discovery and Signaling", RFC4761, January 2007
[RFC5036] L. Andersson, et. al., "LDP Specification", RFC5036,
October 2007
11. Authors' Addresses
Hannes Gredler
Juniper Networks
e-mail: hannes@juniper.net
Yakov Rekhter
Juniper Networks
e-mail: yakov@juniper.net
Gredler & Rekhter [Page 15]
Internet Draft draft-gredler-spring-mpls-00.txt October 2013
Gredler & Rekhter [Page 16]