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<pre>Internet Engineering Task Force (IETF) IJ. Wijnands, Ed.
Request for Comments: 7715 K. Raza
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 A. Atlas
Juniper Networks, Inc.
J. Tantsura
Ericsson
Q. Zhao
Huawei Technology
January 2016
<span class="h1">Multipoint LDP (mLDP) Node Protection</span>
Abstract
This document describes procedures to support node protection for
Point-to-Multipoint and Multipoint-to-Multipoint Label Switched Paths
(P2MP and MP2MP LSPs) that have been built by the Multipoint Label
Distribution Protocol (mLDP). In order to protect a node N, the
Point of Local Repair (PLR) Label Switching Router (LSR) of N must
learn the Merge Point (MPT) LSR(s) of node N such that traffic can be
redirected to them in case node N fails. Redirecting the traffic
around the failed node N depends on existing Point-to-Point (P2P)
Label Switched Paths (LSPs). The pre-established LSPs originate from
the PLR LSR and terminate on the MPT LSRs while bypassing LSR N.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc7715">http://www.rfc-editor.org/info/rfc7715</a>.
<span class="grey">Wijnands, et al. Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc7715">RFC 7715</a> mLDP Node Protection January 2016</span>
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-1.1">1.1</a>. Conventions Used in This Document . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-1.2">1.2</a>. Terminology . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-2">2</a>. PLR Determination . . . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-2.1">2.1</a>. Transit Node Procedure . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-2.2">2.2</a>. MP2MP Root Node Procedure . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-2.3">2.3</a>. PLR Information Encoding . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-3">3</a>. Using the tLDP Session . . . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-4">4</a>. Link or Node Failure . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-4.1">4.1</a>. Reconvergence after Node or Link Failure . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-4.1.1">4.1.1</a>. Node Failure . . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-4.1.2">4.1.2</a>. Link Failure . . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-4.1.3">4.1.3</a>. Switching to New Primary Path . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-5">5</a>. mLDP Capabilities for Node Protection . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-5.1">5.1</a>. PLR Capability . . . . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-5.2">5.2</a>. MPT Capability . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-5.3">5.3</a>. The Protected LSR . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-5.4">5.4</a>. The Node Protection Capability . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-6">6</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
<a href="#section-7">7</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
<a href="#section-8">8</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
<a href="#section-8.1">8.1</a>. Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
<a href="#section-8.2">8.2</a>. Informative References . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
<span class="grey">Wijnands, et al. Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc7715">RFC 7715</a> mLDP Node Protection January 2016</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
This document describes procedures to support node protection for
Point-to-Multipoint and Multipoint-to-Multipoint Label Switched Paths
(P2MP and MP2MP LSPs) that have been built by the Multipoint Label
Distribution Protocol (mLDP) [<a href="./rfc6388" title=""Label Distribution Protocol Extensions for Point- to-Multipoint and Multipoint-to-Multipoint Label Switched Paths"">RFC6388</a>]. In order to protect a node
N, the Point of Local Repair (PLR) LSR of N must learn the Merge
Point (MPT) LSR(s) of node N such that traffic can be redirected to
them in case node N fails. Redirecting the traffic around the failed
node N depends on existing P2P LSPs. The pre-established LSPs
originate from the PLR LSR and terminate on the MPT LSRs while
bypassing LSR N. The procedures to set up these P2P LSPs are outside
the scope of this document, but one can imagine using techniques
based on the Resource Reservation Protocol for Traffic Engineering
(RSVP-TE) [<a href="./rfc5420" title=""Encoding of Attributes for MPLS LSP Establishment Using Resource Reservation Protocol Traffic Engineering (RSVP-TE)"">RFC5420</a>] or Label Distribution Protocol (LDP) Loop-Free
Alternate (LFA) [<a href="./rfc5286" title=""Basic Specification for IP Fast Reroute: Loop-Free Alternates"">RFC5286</a>] to accomplish this.
The solution described in this document notifies the PLR(s) of the
MPT LSR(s) via signaling using a Targeted LDP (tLDP) session
[<a href="./rfc7060" title=""Using LDP Multipoint Extensions on Targeted LDP Sessions"">RFC7060</a>]. By having a tLDP session with the PLR, no additional
procedures need to be defined in order to support Make-Before-Break
(MBB), Graceful Restart (GR), and Typed Wildcard Forwarding
Equivalence Class (FEC). All this is achieved at the expense of
having additional tLDP sessions between each MPT and PLR LSR.
In order to allow a node to be protected against failure, the LSRs
providing the PLR and the MPT functionality as well as the protected
node MUST support the functionality described in this document. LDP
capability negotiation [<a href="./rfc5561" title=""LDP Capabilities"">RFC5561</a>] is used to signal the availability
of the functionality between the participating nodes; these nodes
MUST support capability negotiation.
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Conventions Used in This Document</span>
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 <a href="./rfc2119">RFC 2119</a> [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
The term "node" is used to refer to an LSR; "node" and "LSR" are used
interchangeably in this document. The terms "PLR" and "MPT" are used
as shorthand to refer to "PLR LSR" and "MPT LSR", respectively.
<span class="grey">Wijnands, et al. Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc7715">RFC 7715</a> mLDP Node Protection January 2016</span>
<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>. Terminology</span>
mLDP: Multipoint LDP
PLR: Point of Local Repair
The LSR that redirects the traffic to one or more Merge Point
LSRs.
MPT: Merge Point
The LSR that merges the backup LSP with the primary LSP. Note,
there can be multiple MPT LSRs for a single MP-LSP node
protection.
tLDP: Targeted LDP
MP LSP: Multi-Point LSP
Either a P2MP or MP2MP LSP.
root node:
The root of either a P2MP or MP2MP LSP as defined in [<a href="./rfc6388" title=""Label Distribution Protocol Extensions for Point- to-Multipoint and Multipoint-to-Multipoint Label Switched Paths"">RFC6388</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. PLR Determination</span>
In order for an MPT to establish a tLDP session with a PLR, it first
has to learn the PLR for a particular MP LSP. It is the
responsibility of the protected node N to advertise the address of
the PLR to the MPT. The PLR address for an MP LSP on node N is the
address of the upstream LDP peer, but only when node N is NOT the
root node of the MP2MP LSP. If the upstream LDP peer is unable to
function as PLR, the procedures in this document do not apply and are
out of the scope. If node N is the root node, the procedures are
slightly different as described in <a href="#section-2.2">Section 2.2</a>. The procedures that
follow assume that all the participating nodes (N, PLRs, MPTs) are
enabled (e.g., by a user configuration) to support and implement the
PLR determination feature.
The procedures as documented in this RFC require the protected node
to be directly connected to the PLR and MPT nodes. This is because
mLDP depends on unicast routing to determine the upstream LSR and
unicast routing (by default) only has information about the next hop
and not beyond that. Support for non-directly connected PLR and MPT
nodes is outside the scope of this document.
<span class="grey">Wijnands, et al. Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc7715">RFC 7715</a> mLDP Node Protection January 2016</span>
<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>. Transit Node Procedure</span>
Below are the procedures for when the protected node is a transit
node along the path to the root.
root
^
|
(LSR1)
. | .
. | .
. (N) .
. / \ .
. / \.
(LSR2) (LSR3)
| |
N: The node being protected.
...: Backup LSPs from LSR1 to LSR2 and LSR3.
Figure 1
Node N uses the root address of the MP LSP to determine the upstream
LSR for a given MP LSP following the procedures as documented in
<a href="./rfc6388#section-2.4.1.1">Section 2.4.1.1 of [RFC6388]</a>. The upstream LSR in Figure 1 is LSR1
because it is the first hop along the shortest path to reach the root
address. After determining the upstream LSR, node N (which has the
node protection feature enabled) MUST advertise the address of LSR1
as the PLR address to the downstream members of the MP LSP (i.e.,
LSR2 and LSR3) if the given downstream member has announced support
for node protection (see <a href="#section-5">Section 5</a> regarding capability negotiation).
For the format and encoding of PLR address information, see <a href="#section-2.3">Section</a>
<a href="#section-2.3">2.3</a>.
Note, in order for the protected traffic to reach nodes LSR2 and
LSR3, LSR1 MUST have two unidirectional LSPs to LSR2 and LSR3,
bypassing node N. The procedures for setting up these LSPs are
outside the scope of this document.
<span class="grey">Wijnands, et al. Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc7715">RFC 7715</a> mLDP Node Protection January 2016</span>
<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>. MP2MP Root Node Procedure</span>
Below are the procedures for when the protected node is the root of
an MP2MP LSP. Consider figure 2 below.
|
(LSR1)
. | .
. | .
. (N) . root
. / \ .
. / \.
(LSR2)....(LSR3)
| |
N: The MP2MP root node being protected.
...: Backup LSPs between LSR1, LSR2, and LSR3.
Figure 2
Assume that LSR1, LSR2, and LSR3 are all members of an MP2MP LSP for
which N is the root node. Since N is the root of the MP2MP LSP,
there is no upstream LSR and no 'single' PLR LSR for protecting node
N. In order to protect node N, all the directly connected members of
the MP2MP must participate in protecting node N by acting both as PLR
and MPT LSR. An LSR will act as MPT for traffic coming from the
other LSR(s) and it will act as PLR for traffic it is sending to the
other LSR(s). Since node N knows the members of the MP2MP LSP, it
will advertise the member list to its directly connected members,
excluding the member it is sending to. For example, node N will
advertise list {LSR3,LSR1} to LSR2 excluding LSR2 from it. Instead
of advertising a single PLR when node N is not the root, a list of
PLRs is advertised using the procedures documented in <a href="#section-2.3">Section 2.3</a>.
It should be noted that the MP2MP root node protection mechanism
doesn't replace the Root Node Redundancy (RNR) procedures as
described in <a href="./rfc6388#section-7">Section 7 of [RFC6388]</a>. The node protection procedures
in this document will help in restoring traffic for the existing
MP2MP LSPs after node failure, but a new root node has to be elected
eventually in order to allow new MP2MP LSPs to be created.
Note, in order for the protected traffic to be exchanged between
nodes LSR1, LSR2, and LSR3, bidirectional LSPs have to exist between
the LSRs, bypassing node N. The procedures for setting up these LSPs
are outside the scope of this document.
<span class="grey">Wijnands, et al. Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
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<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>. PLR Information Encoding</span>
The upstream LSR address is conveyed via an LDP Notification message
with an MP Status TLV, where the MP Status TLV contains a new "PLR
Status Value Element" that specifies the address of the PLR.
The new "PLR Status Value Element" is encoded as described below.
PLR Status Element:
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 = 2 | Length | Addr Family |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Fam cont | Num PLR entry | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
| PLR entry (1 or more) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where
Type: PLR Status Value Element (Type 2).
Length: The Length field is an unsigned integer that encodes the
length of the Status Value following the Length field. The
encoded Length varies based on the Addr Family and the number of
PLR entries.
Addr Family: Two-octet quantity containing a value from IANA's
"Address Family Numbers" registry [<a href="#ref-AFI" title=""Address Family Numbers"">AFI</a>] that encodes the address
family for the PLR address encoded in the PLR entry.
Num PLR entry: Element as an unsigned integer followed by the
number of "PLR entry" fields in the format specified below.
<span class="grey">Wijnands, et al. Standards Track [Page 7]</span></pre>
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The format of a "PLR Entry" is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A| Reserved | PLR address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ PLR address (cont) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where
A bit: 0 = Withdraw, 1 = Add.
Reserved: 15 bits; MUST be zero on transmit and ignored on
receipt.
PLR address: PLR address encoded according to the Address Family
field encoded in the PLR Status Value Element. Note that the
length of the PLR address field is specific to the Address Family
that is encoded.
The size of a "PLR Entry" is the 2 octets ("A bit + Reserved") + PLR
address length. The length of the PLR address is dependent on the
Address Family as encoded in the PLR Status Value Element. The size
of a "PLR entry" is 6 octets and 18 octets, respectively, for an IPv4
PLR address and an IPv6 PLR address.
If the PLR address on N changes for a given MP LSP, N needs to
trigger a new PLR Status to update the MPT(s). Node N can advertise
or withdraw a given PLR from its PLR set by setting the A bit to 1 or
0 respectively in the corresponding PLR entry. Removing a PLR
address is likely due to a link failure; see the procedures as
documented in <a href="#section-4.1">Section 4.1</a>. To remove all PLR addresses belonging to
the encoded Address Family, an LSR N MUST encode a PLR Status Value
Element with no PLR entry and the "Num PLR entry" field MUST be set
to zero.
Both the PLR Status and an MP FEC TLV [<a href="./rfc5036" title=""LDP Specification"">RFC5036</a>] MUST be included in
the LDP Notification message so that a receiver is able to associate
the PLR Status with the MP LSP.
<span class="grey">Wijnands, et al. Standards Track [Page 8]</span></pre>
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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Using the tLDP Session</span>
The receipt of a PLR MP Status (with PLR addresses) for an MP LSP on
a receiving LSR makes it an MPT for node protection. If not already
established, the MPT LSR MUST establish a tLDP session with all of
the learned PLR addresses using the procedures as documented in
[<a href="./rfc7060" title=""Using LDP Multipoint Extensions on Targeted LDP Sessions"">RFC7060</a>].
Using Figure 1 as the reference topology, let us assume that both
LSR2 and LSR3 are MPTs and have established a tLDP session with the
PLR being LSR1. Assume that both LSR2 and LSR3 have a FEC <R,X> with
an upstream LSR N and label Ln assigned to FEC towards N. The MPTs
will create a secondary upstream LSR for the FEC <R,X> (using the
received PLR address) and assign label Lpx to it. The MPTs will do
that for each PLR address that was learned for the MP LSP. In this
example, the MPTs will have a FEC <R,X> with two local labels
associated with it. Label Ln that was assigned to N using the normal
mLDP procedures, and Label Lpx that was assigned to PLR (LSR1) for
the purpose of node protection. Note, when the protected node is an
MP2MP root node, there will be an upstream LSR for each PLR address
that was advertised along with a unique Label Lpx.
The receipt of a FEC Label Mapping alone over the tLDP session from
MPT on a PLR conveys the label information but does not convey the
node being protected. The information about a protected node is
known to the MPT LSR and needs to be communicated to the PLR as well.
For this reason, the FEC Label Mapping (FEC <R,X> : Lpx) sent by the
MPT over the tLDP session to the PLR MUST include a Status TLV with
an MP Status and a new LDP MP Status Value Element called the
"Protected Node Status Value Element". This new value element is
used to specify the address of the node being protected. The
"Protected Node Status Value Element" has the following format:
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 = 3 | Length | Addr Family |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Fam cont | Node address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type : Protected Node Status Value Element (Type 3).
Length: The Length field is an unsigned integer that encodes the
length of the Status Value following the Length field. The
encoded Length varies based on the Address Family and is 6 octets
for Address Family + IPv4 address and 18 octets for Address Family
+ IPv6 address.
<span class="grey">Wijnands, et al. Standards Track [Page 9]</span></pre>
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Addr Family: Two-octet quantity containing a value from IANA's
"Address Family Numbers" registry [<a href="#ref-AFI" title=""Address Family Numbers"">AFI</a>] that encodes the address
family for the node address.
Node address: Protected node address encoded according to the
Address Family field.
When a PLR receives a Label Mapping for FEC <R,X> that includes a
Protected Node Status, it will only use that label binding once the
Node advertised in the Status value becomes unreachable. If the LSP
is an MP2MP LSP, the PLR would have assigned a Label Mapping for the
upstream MP2MP FEC Element to the MPT (<a href="./rfc6388#section-3">[RFC6388], Section 3</a>) for FEC
<R,X>. This label binding on the MPT MUST only be used once node N
becomes unreachable.
The procedures to determine if a node is unreachable is a local
decision and not spelled out in this document. Typically, link
failure or Bidirectional Forwarding Detection (BFD) can be used to
determine and detect node unreachability.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Link or Node Failure</span>
Consider the following topology:
root
^
|
. (LSR1)
. / | .
. (M) | .
. \ | .
. (N) .
. / \ .
. / \.
(LSR2) (LSR3)
| |
N: The node being protected.
M: The backup node to protect link LSR1 - N.
...: Backup LSPs from LSR1 to LSR2 and LSR3.
Figure 3
Assume that LSR1 is the PLR for protected node N and that LSR2 and
LSR3 are MPTs for node N. When LSR1 discovers that node N is
unreachable, it cannot immediately determine whether it is the link
from LSR1 to N or the actual node N that has failed. In Figure 3,
<span class="grey">Wijnands, et al. Standards Track [Page 10]</span></pre>
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the link between LSR1 and N is also protected using Fast Reroute
(FRR) [<a href="./rfc4090" title=""Fast Reroute Extensions to RSVP-TE for LSP Tunnels"">RFC4090</a>] link protection via node M. LSR1 MAY simultaneously
invoke both link protection via node M to N using redirection of the
traffic and node protection directly to LSR1 and LSR2. If only the
link failed, LSR2 and LSR3 will receive the packets twice due to the
two protection mechanisms. To prevent duplicate packets being
forwarded to the receivers on the tree, LSR2 and LSR3 need to
determine from which upstream node they should accept the packets.
This can be either from the primary upstream LSR N or from the
secondary upstream LSR1, but never both at the same time. The
selection between the primary upstream LSR or (one or more) secondary
upstream LSRs (on LSR2 and LSR3) is based on the reachability of the
protected node N. As long as N is reachable from an MPT, the MPT
should accept and forward the MPLS packets from N. Once N becomes
unreachable, the LSPs from secondary upstream PLR LSRs (LSR1 in our
example) are activated. Note that detecting if N is unreachable is a
local decision and not spelled out in this document.
Typically, link failure or BFD can be used to determine and detect
node unreachability.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Reconvergence after Node or Link Failure</span>
Consider the following topology:
root
^
_ |
/. (LSR1)
/. /. | .\
/. (M). | .\
(P). \. | .\
\. ( N ) .(Q)
\. / \ ./
\. / \ ./
(LSR2) (LSR3)
| |
N: The node being protected.
M: The backup node to protect link 'LSR1 - N'.
P and Q: The nodes on the new primary path after
failure of node N.
...: P2P backup LSPs.
Figure 4
<span class="grey">Wijnands, et al. Standards Track [Page 11]</span></pre>
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Assume that LSR1 has detected that node N is unreachable and invoked
both the link protection and node protection procedures as described
in this example. LSR1 is acting as PLR and sending traffic over both
the backup P2P LSP to node N (via M) and the P2P LSPs directly to
LSR2 and LSR3, acting as MPT LSRs. The sequence of events is
dependent on whether the link from LSR1 to N has failed or node N
itself has failed. The nodes downstream from the protected node (and
participating in node protection) MUST have the capability to
determine that the protected node has become unreachable. Otherwise,
the procedures below cannot be applied.
<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>. Node Failure</span>
If node N failed, both LSR2 and LSR3 will have changed the primary
upstream LSR to the secondary upstream LSR (LSR1) due to node N being
unreachable. With that, the label bindings previously assigned to
LSR1 will be activated on the MPTs (LSR2 and LSR3) and the label
binding to N will be disabled. Traffic is now switched over to the
label bindings that were installed for node protection.
<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>. Link Failure</span>
If the link 'LSR1 - N' has failed, both LSR2 and LSR3 will not change
the primary upstream LSR because node N is still reachable. LSR2 and
LSR3 will receive traffic over two different bindings, the primary
label binding assigned to node N (due to link protection via node M)
as well as over the binding assigned to LSR1 for the node protection.
Since the secondary upstream LSRs have not been activated, the
traffic received due to node protection will be dropped. Node N will
reconverge and update LSR2 and LSR3 (<a href="#section-2.3">Section 2.3</a>) with the
information that the PLR address (LSR1) is no longer applicable and
must be removed. In response, LSR2 and LSR3 MUST send a Label
Withdraw to LSR1 to withdraw the label binding. This will stop the
traffic being forwarded over the backup P2P LSPs for node protection.
LSR1 will respond back with a Label Release as soon as the binding
has been removed.
<span class="h4"><a class="selflink" id="section-4.1.3" href="#section-4.1.3">4.1.3</a>. Switching to New Primary Path</span>
The network will eventually reconverge and a new best path to the
root will be found by LSR2 and LSR3. LSR2 will find that P is its
new primary upstream LSR to reach the root and LSR3 will find Q.
Note that although the current active upstream LSR can either be node
N or LSR1 (depending on link or node failure), it does not matter for
the following procedures. Both LSR2 and LSR3 SHOULD use the Make-
Before-Break (MBB) procedures as described in <a href="./rfc6388#section-8">Section 8 of [RFC6388]</a>
to switch to the new primary upstream node. As soon as the new
primary upstream LSRs P and Q are activated, a Label Withdraw message
<span class="grey">Wijnands, et al. Standards Track [Page 12]</span></pre>
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MUST be sent to the old upstream LSR. Note that an upstream LSR
switchover from a tLDP neighbor to a directly connected LDP neighbor
is no different compared to switching between two directly connected
neighbors. After the Label Withdraw message has been received by
LSR1 or node N, forwarding will stop and a Label Release will be
sent.
When it is determined that after reconvergence there is no more
interest in the tLDP session between the MPT and the PLR, the tLDP
session MAY be taken down. It is possible that having no more
interest in the tLDP session is temporarily due to link flapping. In
order to avoid the tLDP session from flapping, it is RECOMMENDED to
apply a delay before tearing down the session. Determining the delay
is a local implementation matter. If the operator is not concerned
with the tLDP session flapping and/or other procedures are in place
to avoid this altogether, there is no need to apply the delay.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. mLDP Capabilities for Node Protection</span>
In order to describe the capabilities of the participating LSRs, this
document is organizing it per role in the network, i.e., Point of
Local Repair (PLR), Merge Point (MPT), and protected node (as
depicted in Figure 1).
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. PLR Capability</span>
A PLR node should handle the following conditions:
1. Accept an incoming tLDP session from the MPT LSR.
2. Support the receipt of a "Protected Node Status Value Element"
status in an MP Status TLV over tLDP session.
3. Upon node failure detection, capable of switching traffic towards
one or more MPT(s) over a P2P LSP (bypassing N) using the labels
previously advertised for MP LSPs over the tLDP session.
An LSR capable of performing these actions will advertise itself as
PLR capable in the Node Protection Capability (see <a href="#section-5.4">Section 5.4</a>).
This is a unidirectional capability announced from PLR to the
protected LSR.
<span class="grey">Wijnands, et al. Standards Track [Page 13]</span></pre>
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<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. MPT Capability</span>
An MPT node should handle the following conditions;
1. Support the receipt of "PLR Status Value Element" in an MP Status
TLV from a protected node N.
2. Support to transmit "Protected Node Status Value Element" in an MP
Status TLV to a PLR.
An LSR capable of performing these actions will advertise itself as
MPT capable in the Node Protection Capability (see <a href="#section-5.4">Section 5.4</a>).
This is a unidirectional capability from MPT to the protected LSR.
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. The Protected LSR</span>
A protected node should handle the following conditions:
1. Determine the PLR and MPT capability for directly connected
upstream and downstream LSRs for a given MP FEC.
2. Support transmitting of "PLR Status Value Element" in an MP Status
TLV to one or more downstream MPT LSRs.
The protected LSR does not advertise any capability for mLDP Node
Protection because it does not need to receive any of the defined MP
Status values as described above. However, the protected node does
play an important role in the signaling and setup of the node
protection. For a given FEC, the protected node can only send PLR
information to a downstream LSR if the PLR has signaled the PLR
capability and the downstream LSR has signaled the MPT capability.
When the downstream LSR (acting as MPT) receives the PLR Status, it
can implicitly infer that the advertised LSR(s) are PLR capable. The
MPT LSR can now proceed with setting up a tLDP session with the
PLR(s) and MP LSP node protection signaling.
<span class="grey">Wijnands, et al. Standards Track [Page 14]</span></pre>
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<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>. The Node Protection Capability</span>
We define a single capability "MP Node Protection Capability" to
announce the PLR and MPT capability.
The format of the capability parameter TLV is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Type = 0x0972 | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved |P|M| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where
U/F bits: MUST be set to 1 and 0, respectively (as per [<a href="./rfc5561" title=""LDP Capabilities"">RFC5561</a>]).
Type: MP Node Protection Capability (Type = 0x0972).
Length: Unsigned integer; MUST be set to 2.
S bit: Set to 1 to announce and 0 to withdraw the capability (as
per [<a href="./rfc5561" title=""LDP Capabilities"">RFC5561</a>]).
P bit: Set to 1 to indicate the PLR is capable of MP LSP node
protection.
M bit: Set to 1 to indicate the MPT is capable of MP LSP node
protection.
Reserved: MUST be zero on transmit and ignored on receipt.
The above capability can be sent in an LDP Initialization message to
announce capability at the session establishment time, or it can be
sent in an LDP Capability message to dynamically update (announce or
withdraw) its capability towards its peer using procedures specified
in [<a href="./rfc5561" title=""LDP Capabilities"">RFC5561</a>].
An LSR that supports the PLR functionality LSR MAY send this
capability to its downstream MP peers with P bit set; whereas, an LSR
that supports the MPT functionality MAY send this capability to its
upstream peer with M bit set. Moreover, an LSR that supports both
the PLR and MPT functionality MAY sent this capability to its peers
with both P and M bit set.
<span class="grey">Wijnands, et al. Standards Track [Page 15]</span></pre>
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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Security Considerations</span>
The procedures in this document add two new TLVs to existing LDP
messages. Those TLVs can be protected by the mechanisms that are
used to protect LDP messages as described in [<a href="./rfc6388" title=""Label Distribution Protocol Extensions for Point- to-Multipoint and Multipoint-to-Multipoint Label Switched Paths"">RFC6388</a>] and [<a href="./rfc5920" title=""Security Framework for MPLS and GMPLS Networks"">RFC5920</a>].
If it were possible to attack the mechanisms described in this
document, an LSR (a PLR or a MPT) could be induced to support a large
number of tLDP sessions and set up an even larger number of LSPs.
The security mechanisms described in [<a href="./rfc6388" title=""Label Distribution Protocol Extensions for Point- to-Multipoint and Multipoint-to-Multipoint Label Switched Paths"">RFC6388</a>] and [<a href="./rfc5920" title=""Security Framework for MPLS and GMPLS Networks"">RFC5920</a>] are
believed to be adequate, but an implementation could provide
additional protection by counting such protection sessions and LSPs
and producing a log message to the operator if a threshold is
crossed.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. IANA Considerations</span>
IANA has allocated the following two new code points from the "LDP MP
Status Value Element type" registry within the "Label Distribution
Protocol (LDP) Parameters" registry.
Value | Name | Reference
------+----------------------------------------+-----------
2 | PLR Status Value Element | this doc
------+----------------------------------------+-----------
3 | Protected Node Status Value Element | this doc
IANA has assigned the following new code point for a new Capability
Parameter TLV. The code point has been assigned from the IETF
Consensus range of the "TLV Type Name Space" registry within the
"Label Distribution Protocol (LDP) Parameters" registry.
Value | Description | Reference | Notes/Reg Date
------+-------------------------------+-----------+---------------
0x0972| MP Node Protection Capability | this doc |
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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. References</span>
<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
DOI 10.17487/RFC2119, March 1997,
<<a href="http://www.rfc-editor.org/info/rfc2119">http://www.rfc-editor.org/info/rfc2119</a>>.
[<a id="ref-RFC5036">RFC5036</a>] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", <a href="./rfc5036">RFC 5036</a>, DOI 10.17487/RFC5036,
October 2007, <<a href="http://www.rfc-editor.org/info/rfc5036">http://www.rfc-editor.org/info/rfc5036</a>>.
[<a id="ref-RFC6388">RFC6388</a>] Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
Thomas, "Label Distribution Protocol Extensions for Point-
to-Multipoint and Multipoint-to-Multipoint Label Switched
Paths", <a href="./rfc6388">RFC 6388</a>, DOI 10.17487/RFC6388, November 2011,
<<a href="http://www.rfc-editor.org/info/rfc6388">http://www.rfc-editor.org/info/rfc6388</a>>.
[<a id="ref-RFC5561">RFC5561</a>] Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
Le Roux, "LDP Capabilities", <a href="./rfc5561">RFC 5561</a>,
DOI 10.17487/RFC5561, July 2009,
<<a href="http://www.rfc-editor.org/info/rfc5561">http://www.rfc-editor.org/info/rfc5561</a>>.
[<a id="ref-RFC7060">RFC7060</a>] Napierala, M., Rosen, E., and IJ. Wijnands, "Using LDP
Multipoint Extensions on Targeted LDP Sessions", <a href="./rfc7060">RFC 7060</a>,
DOI 10.17487/RFC7060, November 2013, <<a href="http://www.rfc-editor.org/info/rfc7060">http://www.rfc-</a>
<a href="http://www.rfc-editor.org/info/rfc7060">editor.org/info/rfc7060</a>>.
[<a id="ref-AFI">AFI</a>] IANA, "Address Family Numbers",
<<a href="http://www.iana.org/assignments/address-family-numbers">http://www.iana.org/assignments/address-family-numbers</a>>.
<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>. Informative References</span>
[<a id="ref-RFC4090">RFC4090</a>] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels", <a href="./rfc4090">RFC 4090</a>,
DOI 10.17487/RFC4090, May 2005,
<<a href="http://www.rfc-editor.org/info/rfc4090">http://www.rfc-editor.org/info/rfc4090</a>>.
[<a id="ref-RFC5286">RFC5286</a>] Atlas, A., Ed., and A. Zinin, Ed., "Basic Specification
for IP Fast Reroute: Loop-Free Alternates", <a href="./rfc5286">RFC 5286</a>,
DOI 10.17487/RFC5286, September 2008,
<<a href="http://www.rfc-editor.org/info/rfc5286">http://www.rfc-editor.org/info/rfc5286</a>>.
<span class="grey">Wijnands, et al. Standards Track [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc7715">RFC 7715</a> mLDP Node Protection January 2016</span>
[<a id="ref-RFC5420">RFC5420</a>] Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and A.
Ayyangarps, "Encoding of Attributes for MPLS LSP
Establishment Using Resource Reservation Protocol Traffic
Engineering (RSVP-TE)", <a href="./rfc5420">RFC 5420</a>, DOI 10.17487/RFC5420,
February 2009, <<a href="http://www.rfc-editor.org/info/rfc5420">http://www.rfc-editor.org/info/rfc5420</a>>.
[<a id="ref-RFC5920">RFC5920</a>] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", <a href="./rfc5920">RFC 5920</a>, DOI 10.17487/RFC5920, July 2010,
<<a href="http://www.rfc-editor.org/info/rfc5920">http://www.rfc-editor.org/info/rfc5920</a>>.
Acknowledgments
The authors thank Nagendra Kumar, Duan Hong, Martin Vigoureux, Kenji
Fujihira, Loa Andersson, and Ben Campbell for their comments on this
document. Also, many thanks to Elwyn Davies and Adrian Farrel for
the detailed review and contribution to this document.
Contributors
The following individual contributed to this document:
Eric Rosen
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
United States
Email: erosen@juniper.net
<span class="grey">Wijnands, et al. Standards Track [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc7715">RFC 7715</a> mLDP Node Protection January 2016</span>
Authors' Addresses
IJsbrand Wijnands (editor)
Cisco Systems, Inc.
De kleetlaan 6a
Diegem 1831
Belgium
Email: ice@cisco.com
Kamran Raza
Cisco Systems, Inc.
2000 Innovation Drive
Ottawa, Ontario K2K-3E8
Canada
Email: skraza@cisco.com
Alia Atlas
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
United States
Email: akatlas@juniper.net
Jeff Tantsura
Ericsson
300 Holger Way
San Jose, CA 95134
United States
Email: jeff.tantsura@ericsson.com
Quintin Zhao
Huawei Technology
125 Nagog Technology Park
Acton, MA 01719
United States
Email: quintin.zhao@huawei.com
Wijnands, et al. Standards Track [Page 19]
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