RFC 7473: Controlling State Advertisements of Non-negotiated LDP Applications

Text version

Internet Engineering Task Force (IETF)
Request for Comments: 7473
Category: Standards Track
ISSN: 2070-1721

K. Raza
S. Boutros
Cisco Systems, Inc.
March 2015

Controlling State Advertisements of Non-negotiated LDP Applications

Abstract

There is no capability negotiation done for Label Distribution Protocol (LDP) applications that set up Label Switched Paths (LSPs) for IP prefixes or that signal point-to-point (P2P) Pseudowires (PWs) for Layer 2 Virtual Private Networks (L2VPNs). When an LDP session comes up, an LDP speaker may unnecessarily advertise its local state for such LDP applications even when the peer session is established for some other applications like Multipoint LDP (mLDP) or the Inter- Chassis Communication Protocol (ICCP). This document defines a solution by which an LDP speaker announces to its peer its disinterest in such non-negotiated applications, thus disabling the unnecessary advertisement of corresponding application state, which would have otherwise been advertised over the established LDP session.

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 Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7473.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

   1. Introduction ....................................................3
   2. Conventions Used in This Document ...............................4
   3. Non-negotiated LDP Applications .................................4
      3.1. Uninteresting State ........................................5
           3.1.1. Prefix-LSPs .........................................5
           3.1.2. P2P-PWs .............................................5
   4. Controlling State Advertisement .................................5
      4.1. State Advertisement Control Capability .....................6
      4.2. Capabilities Procedures ....................................8
           4.2.1. State Control Capability in an
                  Initialization Message ..............................9
           4.2.2. State Control Capability in a Capability Message ....9
   5. Applicability Statement .........................................9
   6. Operational Examples ...........................................11
      6.1. Disabling Prefix-LSPs and P2P-PWs on an ICCP Session ......11
      6.2. Disabling Prefix-LSPs on a L2VPN/PW tLDP Session ..........11
      6.3. Disabling Prefix-LSPs Dynamically on an
           Established LDP Session ...................................12
      6.4. Disabling Prefix-LSPs on an mLDP-only Session .............12
      6.5. Disabling IPv4 or IPv6 Prefix-LSPs on a Dual-Stack LSR ....12
   7. Security Considerations ........................................13
   8. IANA Considerations ............................................13
   9. References .....................................................14
      9.1. Normative References ......................................14
      9.2. Informative References ....................................14
   Acknowledgments ...................................................15
   Authors' Addresses ................................................15

1. Introduction

The LDP Capabilities specification [RFC5561] introduced a mechanism to negotiate LDP capabilities for a given feature between peer Label Switching Routers (LSRs). The capability mechanism ensures that no unnecessary state is exchanged between peer LSRs unless the corresponding feature capability is successfully negotiated between the peers.

Newly defined LDP features and applications, such as Typed Wildcard Forwarding Equivalence Class (FEC) [RFC5918], Inter-Chassis Communication Protocol [RFC7275], mLDP [RFC6388], and L2VPN Point-to- multipoint (P2MP) PW [RFC7338] make use of LDP capabilities framework for their feature negotiation. However, the earlier LDP applications allowed LDP speakers to exchange application state without any capability negotiation. This, in turn, results in the unnecessary advertisement of state when a given application is not enabled on one of the LDP speakers. These earlier LDP applications include (i) application to establish LSPs for IP unicast prefixes and (ii) application to signal when L2VPN P2P PW [RFC4447] [RFC4762]. For example, when bringing up and using an LDP peer session with a remote Provider Edge (PE) LSR for purely ICCP-signaling reasons, an LDP speaker may unnecessarily advertise labels for IP (unicast) prefixes to this ICCP-related LDP peer.

Another example of unnecessary state advertisement can be cited when LDP is to be deployed in an IP dual-stack environment. For instance, an LSR that is locally enabled to set up LSPs for both IPv4 and IPv6 prefixes may advertise (address and label) bindings for both IPv4 and IPv6 address families towards an LDP peer that is interested in IPv4 bindings only. In this case, the advertisement of IPv6 bindings to the peer is unnecessary, as well as wasteful, from the point of view of LSR memory/CPU and network resource consumption.

To avoid this unnecessary state advertisement and exchange, currently an operator is typically required to configure and define filtering policies on the LSR, which introduces unnecessary operational overhead and complexity for such deployments.

This document defines a solution based on LDP Capabilities [RFC5561] by which an LDP speaker may announce to its peer(s) its disinterest (or non-support) for state to set up IP Prefix LSPs and/or to signal L2VPN P2P PW at the time of session establishment. This capability helps in avoiding unnecessary state advertisement for such feature applications. This document also states the mechanics to dynamically disable or enable the state advertisement for such applications during the session lifetime. The "uninteresting" state of an application depends on the type of application and is described later in Section 3.1.

2. Conventions Used in This Document

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].

The term "IP" in this document refers to both IPv4 and IPv6 unicast address families.

3. Non-negotiated LDP Applications

For the applications that existed prior to the definition of the LDP Capabilities framework [RFC5561], an LDP speaker typically advertises, without waiting for any capabilities exchange and negotiation, its corresponding application state to its peers after the session establishment. These early LDP applications include:

o IPv4/IPv6 Prefix LSPs Setup
o L2VPN P2P FEC 128 and FEC 129 PWs Signaling

The rest of This document uses the following shorthand terms for these earlier LDP applications:

"Prefix-LSPs": Refers to an application that sets up LDP LSPs corresponding to IP routes/prefixes by advertising label bindings for Prefix FEC (as defined in RFC 5036).

"P2P-PWs": Refers to an application that signals FEC 128 and/or FEC 129 L2VPN P2P PWs using LDP (as defined in RFC 4447).

To disable unnecessary state exchange for such LDP applications over an established LDP session, a new capability is being introduced in this document. This new capability controls the advertisement of application state and enables an LDP speaker to notify its peer its disinterest in the state of one or more of these "Non-negotiated" LDP applications at the time of session establishment. Upon receipt of such a capability, the receiving LDP speaker, if supporting the capability, disables the advertisement of the state related to the application towards the sender of the capability. This new capability can also be sent later in a Capability message either to disable a previously enabled application's state advertisement or to enable a previously disabled application's state advertisement.

3.1. Uninteresting State

A uninteresting state of a non-negotiated LDP application:

is the application state that is of no interest to an LSR and need not be advertised to the LSR;

need not be advertised in any of the LDP protocol messages;

is dependent on application type and specified accordingly.

3.1.1. Prefix-LSPs

For the Prefix-LSP application type, the uninteresting state refers to any state related to IP Prefix FEC (such as FEC label bindings, LDP Status). This document, however, does not classify IP address bindings (advertised via ADDRESS message) as a uninteresting state and allows the advertisement of IP address bindings. The reason for this allowance is that an LSR typically uses peer IP address(es) to map an IP routing next hop to an LDP peer in order to implement its control plane procedures. For example, mLDP [RFC6388] uses a peer's IP address(es) to determine its upstream LSR to reach the Root node as well as to select the forwarding interface towards its downstream LSR. Hence, in an mLDP-only network, while it is desirable to disable advertisement of label bindings for IP (unicast) prefixes, disabling advertisement of IP address bindings will break mLDP functionality. Similarly, other LDP applications may also depend on learnt peer IP addresses; hence, this document does not put IP address binding into a uninteresting state category to facilitate such LDP applications.

3.1.2. P2P-PWs

For the P2P-PW application type, the uninteresting state refers to any state related to P2P PW FEC 128 / FEC 129 (such as FEC label bindings, Media Access Control (MAC) address withdrawal, and LDP PW Status). In this document, the term "state" will mean to refer to the "uninteresting state" for an application, as defined in this section.

4. Controlling State Advertisement

To control advertisement of uninteresting state related to non- negotiated LDP applications defined in Section 3, a new capability TLV is defined as follows.

4.1. State Advertisement Control Capability

The "State Advertisement Control Capability" is a new Capability Parameter TLV defined in accordance with Section 3 of LDP Capabilities specification [RFC5561]. The format of this new 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|  SAC Capability (0x050D)  |           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|  Reserved   |                                               |
   +-+-+-+-+-+-+-+-+
   |                                                               |
   ~            State Advertisement Control Element(s)             ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Format of a "State Advertisement Control Capability" TLV

The value of the U-bit for the TLV MUST be set to 1 so that a receiver MUST silently ignore this TLV if unknown to it, and continue processing the rest of the message. Whereas, The value of F-bit MUST be set to 0. Once advertised, this capability cannot be withdrawn; thus, the S-bit MUST be set to 1 in an Initialization and Capability message.

The capability data associated with this State Advertisement Control (SAC) Capability TLV is one or more State Advertisement Control Elements, where each element indicates enabling/disabling of advertisement of uninteresting state for a given application. The format of a SAC Element is defined as follows:

                         0 1 2 3 4 5 6 7
                        +-+-+-+-+-+-+-+-+
                        |D| App |Unused |
                        +-+-+-+-+-+-+-+-+

Figure 2: Format of "State Advertisement Control Element"

Where:
D-bit: Controls the advertisement of the state specified in the "App" field:

1: Disable state advertisement
0: Enable state advertisement

When sent in an Initialization message, the D-bit MUST be set to 1.

App: Defines the legacy application type whose state advertisement is

to be controlled. The value of this field is defined as follows:

1: IPv4 Prefix-LSPs (LSPs for IPv4 prefixes)
2: IPv6 Prefix-LSPs (LSPs for IPv6 prefixes)
3: FEC 128 P2P-PW (L2VPN PWid FEC signaling)
4: FEC 129 P2P-PW (L2VPN Generalized PWid FEC signaling)

Any other value in this field MUST be treated as an error.

Unused:

Must Be Zero (MBZ) on transmit and ignored on receipt.

The "Length" field of the SAC Capability TLV (in octets) is computed as follows:

Length (in octets) = 1 + number of SAC elements

For example, if there are two SAC elements present, then the "Length" field is set to 3 octets. A receiver of this capability TLV can deduce the number of elements present in the TLV by using the "Length" field.

This document uses the term "element" to refer to a SAC Element.

As described earlier, the SAC Capability TLV MAY be included by an LDP speaker in an Initialization message to signal to its peer LSR that state exchange for one or more applications needs to be disabled on the given peer session. This TLV can also be sent later in a Capability message to selectively enable or disable these applications. If there is more than one element present in a SAC Capability TLV, the elements MUST belong to distinct app types and the app type MUST NOT appear more than once. If a receiver receives such a malformed TLV, it SHOULD discard this TLV and continue processing the rest of the message. If an LSR receives a message with a SAC capability TLV containing an element with the "App" field set to a value other than defined above, the receiver MUST ignore and discard the element and continue processing the rest of the TLV.

To control more than one application state, a sender LSR can either send a single capability TLV in a message with multiple elements present or send separate messages with a capability TLV specifying one or more elements. A receiving LSR, however, MUST treat each incoming capability TLV with an element corresponding to a given application type as an update to its existing policy for the given type.

To understand capability updates from an example, let us consider two LSRs, S (LDP speaker) and P (LDP peer), both of which support all the non-negotiated applications listed earlier. By default, these LSRs will advertise state for these applications, as configured, to their peer as soon as an LDP session is established. Now assume that P receives from S a SAC capability in an Initialization message with "IPv6 Prefix-LSPs" and "FEC 129 P2P-PW" applications disabled. This updates P's outbound policy towards S to advertise state related to only IPv4 Prefix-LSPs and FEC 128 P2P-PW applications. Later, P receives another capability update from S via a Capability message with "IPv6 Prefix-LSPs" enabled and "FEC 128 P2P-PWs" disabled. This results in P's outbound policy towards S to advertise both IPv4 and IPv6 Prefix-LSPs application state and disable both FEC 128 and FEC 129 P2P-PWs signaling. Finally, P receives another update from S via a Capability message that specifies to disable all four non- negotiated applications states, resulting in P outbound policy towards S to block/disable state for all these applications and only advertise state for any other application, as applicable.

4.2. Capabilities Procedures

The SAC capability conveys the desire of an LSR to disable the receipt of unwanted/unnecessary state from its LDP peer. This capability is unilateral and unidirectional in nature, and a receiving LSR is not required to send a similar capability TLV in an Initialization or Capability message towards the sender of this capability. This unilateral behavior conforms to the procedures defined in the Section 6 of LDP Capabilities [RFC5561].

After this capability is successfully negotiated (i.e., sent by an LSR and received/understood by its peer), then the receiving LSR MUST NOT advertise any state related to the disabled applications towards the capability-sending LSR until and unless these application states are explicitly enabled again via a capability update. Upon receipt of a capability update to disable an enabled application state during the lifetime of a session, the receiving LSR MUST also withdraw from the peer any previously advertised state corresponding to the disabled application.

If a receiving LDP speaker does not understand the SAC capability TLV, then it MUST respond to the sender with an "Unsupported TLV" notification as described in "LDP Capabilities" [RFC5561]. If a receiving LDP speaker does not understand or does not support an application specified in an application control element, it SHOULD silently ignore/skip such an element and continue processing rest of the TLV.

4.2.1. State Control Capability in an Initialization Message

The LDP Capabilities framework [RFC5561] dictates that the S-bit of the capability parameter in an Initialization message MUST be set to 1 and SHOULD be ignored on receipt.

An LDP speaker determines (e.g., via some local configuration or default policy) if it needs to disable Prefix-LSPs and/or P2P-PW applications with a peer LSR. If there is a need to disable, then the SAC TLV needs to be included in the Initialization message with respective SAC elements included with their D-bit set to 1.

An LDP speaker that supports the SAC capability MUST interpret the capability TLV in a received Initialization message such that it disables the advertisement of the application state towards the capability sending LSR for Prefix-LSPs and/or P2P-PW applications if their SAC element's D-bit is set to 1.

4.2.2. State Control Capability in a Capability Message

If the LDP peer supports "Dynamic Announcement Capability" [RFC5561], then an LDP speaker may send a SAC capability in a Capability message towards the peer. Once advertised, these capabilities cannot be withdrawn; hence, the S-bit of the TLV MUST be set to 1 when sent in a Capability message.

An LDP speaker may decide to send this TLV towards an LDP peer if one or more of its Prefix-LSPs and/or P2P-PW applications get disabled, or if a previously disabled application gets enabled again. In this case, the LDP speaker constructs the TLV with appropriate SAC elements and sends the corresponding capability TLV in a Capability message.

Upon receipt of this TLV in a Capability message, the receiving LDP speaker reacts in the same manner as it reacts upon the receipt of this TLV in an Initialization message. Additionally, the peer withdraws/advertises the application state to/from the capability- sending LDP speaker according to the capability update.

5. Applicability Statement

The procedures defined in this document may result in a disabling announcement of label bindings for IP Prefixes and/or P2P PW FECs and, hence, should be used with caution and discretion. This document recommends that this new SAC capability and its procedures SHOULD be enabled on an LSR only via a configuration knob. This knob could either be a global LDP knob or be implemented per LDP neighbor. Hence, it is recommended that an operator SHOULD enable this capability and its associated procedures on an LSR towards a neighbor only if it is known that such bindings advertisement and exchange with the neighbor is unnecessary and wasteful.

The following table summarizes a non-exhaustive list of typical LDP session types on which this new SAC capability and its procedures are expected to be applied to disable advertisement of uninteresting state:

   +===============================+=================================+
   | Session Type(s)               | Uninteresting State             |
   +===============================+=================================+
   | P2P-PW FEC 128-only           | IP Prefix LSPs + P2P-PW FEC 129 |
   |-------------------------------|---------------------------------|
   | P2P-PW only (FEC 128/129)     | IP Prefix LSPs                  |
   |-------------------------------|---------------------------------|
   | IPv4-only on a Dual-Stack LSR | IPv6 Prefix LSPs + P2P-PW       |
   |-------------------------------|---------------------------------|
   | IPv6-only on a Dual-Stack LSR | IPv4 Prefix LSPs + P2P-PW       |
   |-------------------------------|---------------------------------|
   | mLDP-only                     | IP Prefix LSPs + P2P-PW         |
   |-------------------------------|---------------------------------|
   | ICCP-only                     | IP Prefix LSPs + P2P-PW         |
   +-------------------------------+---------------------------------+

It is to be noted that if an application state needs changing after session initialization (e.g., to enable a previously disabled application or to disable a previously enabled application), the procedures defined in this document expect LSR peers to support the LDP "Dynamic Announcement" Capability to announce the change in SAC capability via an LDP Capability message. However, if any of the peering LSRs do not support this capability, the alternate option is to force reset the LDP session to advertise the new SAC capability accordingly during the following session initialization.

The following are some additional important points that an operator needs to consider regarding the applicability of this new capability and associated procedures defined in this document:

An operator SHOULD disable Prefix-LSP state on any Targeted LDP (tLDP) session that is established for ICCP-only and/or PW-only purposes.

An operator MUST NOT disable Prefix-LSP state on any tLDP session that is established for reasons related to remote Loop-Free Alternate (LFA) Fast Re-Route (FRR) [RLFA].

In a remote network that is LFA FRR [RLFA] enabled, it is RECOMMENDED not to disable Prefix-LSP state on a tLDP session even if the current session type is PW-only and/or ICCP-only. This is recommended because any remote/tLDP neighbor could potentially be picked as a remote LFA PQ node.

This capability SHOULD be enabled for Prefix-LSPs in the scenarios when it is desirable to disable (or enable) advertisement of "all" the prefix label bindings. For scenarios in which a "subset" of bindings need to be filtered, the existing filtering procedures pertaining to label binding announcement should be used.

Using label advertisement filtering policies in conjunction with the procedures defined in this document for Prefix-LSPs is allowed. In such cases, the label bindings will be announced as per the label filtering policy for the given neighbor when Prefix- LSP application is enabled.

6. Operational Examples

6.1. Disabling Prefix-LSPs and P2P-PWs on an ICCP Session

Consider two PE routers, LSR1 and LSR2, that understand/support SAC capability TLV and have an established LDP session to exchange ICCP state related to dual-homed devices connected to these LSRs. Let us assume that both LSRs are provisioned not to exchange any state for Prefix-LSPs (IPv4/IPv6) and P2P-PWs (FEC 128/129) application.

To indicate their disinterest in these applications, the LSRs will include a SAC capability TLV (with four SAC elements corresponding to these four applications with D-bit set to 1 for each one) in the Initialization message. Upon receipt of this TLV in Initialization message, the receiving LSR will disable the advertisement of IPv4/IPv6 label bindings, as well as P2P PW FEC 128/129 signaling, towards its peer after session establishment.

6.2. Disabling Prefix-LSPs on a L2VPN/PW tLDP Session

Consider LSR1 and LSR2 have an established tLDP session for P2P-PW applications to exchange label bindings for FEC 128/129. Given that there is no need to exchange IP label bindings amongst the PE LSRs over a PW tLDP session in most typical deployments, let us assume that LSRs are provisioned to disable IPv4/IPv6 Prefix-LSPs application state on the given PW session.

To indicate their disinterest in Prefix-LSP applications over a PW tLDP session, the LSRs will follow/apply the same procedures as described in previous section. As a result, only P2P-PW-related state will be exchanged between these LSRs over this tLDP session.

6.3. Disabling Prefix-LSPs Dynamically on an Established LDP Session

Assume that LSRs from previous sections were initially provisioned to exchange both Prefix-LSP and P2P-PW state over the session between them and also support the "Dynamic Announcement" Capability of [RFC5561]. Now, assume that LSR1 is dynamically provisioned to disable (IPv4/IPv6) Prefix-LSPs over a tLDP session with LSR2. In this case, LSR1 will send a SAC capability TLV in a Capability message towards LSR2 with application control elements defined for IPv4 and IPv6 Prefix-LSPs with the D-bit set to 1. Upon receipt of this TLV, LSR2 will disable Prefix-LSPs application state(s) towards LSR1 and withdraw all previously advertised application state from LSR1. To withdraw label bindings from its peer, LSR2 MAY use a single Prefix FEC Typed Wildcard Label Withdraw message [RFC5918] if the peer supports the Typed Wildcard FEC capability.

This dynamic disability of Prefix-LSPs application does not impact L2VPN P2P-PW application on the given session, and both LSRs should continue to exchange state related to PW Signaling applications.

6.4. Disabling Prefix-LSPs on an mLDP-only Session

Assume that LSR1 and LSR2 have formed an LDP session to exchange mLDP state only. In typical deployments, LSR1 and LSR2 also exchange bindings for IP (unicast) prefixes upon mLDP session, which is unnecessary and wasteful for an mLDP-only LSR.

Using the procedures defined earlier, an LSR can indicate its disinterest in Prefix-LSP application state to its peer upon session establishment time or dynamically later via an LDP capabilities update.

In reference to Section 3.1, the peer disables the advertisement of any state related to IP Prefix FECs, but it still advertises IP address bindings that are required for the correct operation of mLDP.

6.5. Disabling IPv4 or IPv6 Prefix-LSPs on a Dual-Stack LSR

In IP dual-stack scenarios, LSR2 may advertise unnecessary state (e.g., IPv6 prefix label bindings) towards peer LSR1 corresponding to IPv6 Prefix-LSP applications once a session is established mainly for exchanging state for IPv4. The similar scenario also applies when advertising IPv4 Prefix-LSP state on a session meant for IPv6. The SAC capability and its procedures defined in this document can help to avoid such unnecessary state advertisement.

Consider an IP dual-stack environment where LSR2 is enabled for Prefix-LSPs application for both IPv4 and IPv6, but LSR1 is enabled for (or interested in) only IPv4 Prefix-LSPs. To avoid receiving unwanted state advertisement for IPv6 Prefix-LSP applications from LSR2, LSR1 can send a SAC capability with an element for IPv6 Prefix- LSPs with the D-bit set to 1 in the Initialization message towards LSR2 at the time of session establishment. Upon receipt of this capability, LSR2 will disable all IPv6 label binding advertisements towards LSR1. If IPv6 Prefix-LSP applications are later enabled on LSR1, LSR1 can update the capability by sending a SAC capability in a Capability message towards LSR2 to enable this application dynamically.

7. Security Considerations

The proposal introduced in this document does not introduce any new security considerations beyond those that already apply to the base LDP specification [RFC5036] and to MPLS and GMPLS [RFC5920].

8. IANA Considerations

This document defines a new LDP capability parameter TLV. IANA has assigned the following value from "TLV Type Name Space" in the "Label Distribution Protocol (LDP) Parameters" registry as the new code point for the new LDP capability TLV code point.

   +--------+---------------------+-----------+-----------------------+
   | Value  | Description         | Reference |Notes/Registration Date|
   +--------+---------------------+-----------+-----------------------+
   | 0x050D | State Advertisement | RFC 7473  |                       |
   |        | Control Capability  |           |                       |
   +--------+---------------------+-----------+-----------------------+

9. References

9.1 Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.
   
   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, October 2007,
              <http://www.rfc-editor.org/info/rfc5036>.
   
   [RFC5561]  Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
              Le Roux, "LDP Capabilities", RFC 5561, July 2009,
              <http://www.rfc-editor.org/info/rfc5561>.

9.2. Informative References

   [RFC4447]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
              G. Heron, "Pseudowire Setup and Maintenance Using the
              Label Distribution Protocol (LDP)", RFC 4447, April 2006,
              <http://www.rfc-editor.org/info/rfc4447>.
   
   [RFC4762]  Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
              LAN Service (VPLS) Using Label Distribution Protocol (LDP)
              Signaling", RFC 4762, January 2007,
              <http://www.rfc-editor.org/info/rfc4762>.
   
   [RFC5918]  Asati, R., Minei, I., and B. Thomas, "Label Distribution
              Protocol (LDP) 'Typed Wildcard' Forward Equivalence Class
              (FEC)", RFC 5918, August 2010,
              <http://www.rfc-editor.org/info/rfc5918>.
   
   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, July 2010,
              <http://www.rfc-editor.org/info/rfc5920>.
   
   [RFC6388]  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", RFC 6388, November 2011,
              <http://www.rfc-editor.org/info/rfc6388>.
   
   [RFC7275]  Martini, L., Salam, S., Sajassi, A., Bocci, M.,
              Matsushima, S., and T. Nadeau, "Inter-Chassis
              Communication Protocol for Layer 2 Virtual Private Network
              (L2VPN) Provider Edge (PE) Redundancy", RFC 7275, June
              2014, <http://www.rfc-editor.org/info/rfc7275>.
   
   [RFC7338]  Jounay, F., Ed., Kamite, Y., Ed., Heron, G., and M. Bocci,
              "Requirements and Framework for Point-to-Multipoint
              Pseudowires over MPLS Packet Switched Networks", RFC 7338,
              September 2014, <http://www.rfc-editor.org/info/rfc7338>.
   
   [RLFA]     Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Re-Route
              (FRR)", draft-ietf-rtgwg-remote-lfa-11, Work in Progress,
              January 2015.

Acknowledgments

The authors would like to thank Eric Rosen and Alexander Vainshtein for their review and valuable comments. We also acknowledge Karthik Subramanian and IJsbrand Wijnands for bringing up mLDP use case.

Authors' Addresses

   Kamran Raza
   Cisco Systems, Inc.
   2000 Innovation Drive
   Ottawa, ON K2K-3E8
   Canada
   EMail: skraza@cisco.com

Sami Boutros
Cisco Systems, Inc.
3750 Cisco Way
San Jose, CA 95134
United States
EMail: sboutros@cisco.com