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Internet Engineering Task Force (IETF)                           K. RazaRequest for Comments: 7473                                    S. BoutrosCategory: Standards Track                            Cisco Systems, Inc.ISSN: 2070-1721                                               March 2015Controlling State Advertisements of Non-negotiated LDP ApplicationsAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7473.Raza & Boutros               Standards Track                    [Page 1]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 2015Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Table of Contents1. Introduction ....................................................32. Conventions Used in This Document ...............................43. Non-negotiated LDP Applications .................................43.1. Uninteresting State ........................................53.1.1. Prefix-LSPs .........................................53.1.2. P2P-PWs .............................................54. Controlling State Advertisement .................................54.1. State Advertisement Control Capability .....................64.2. Capabilities Procedures ....................................8           4.2.1. State Control Capability in an                  Initialization Message ..............................94.2.2. State Control Capability in a Capability Message ....95. Applicability Statement .........................................96. Operational Examples ...........................................116.1. Disabling Prefix-LSPs and P2P-PWs on an ICCP Session ......116.2. Disabling Prefix-LSPs on a L2VPN/PW tLDP Session ..........11      6.3. Disabling Prefix-LSPs Dynamically on an           Established LDP Session ...................................126.4. Disabling Prefix-LSPs on an mLDP-only Session .............126.5. Disabling IPv4 or IPv6 Prefix-LSPs on a Dual-Stack LSR ....127. Security Considerations ........................................138. IANA Considerations ............................................139. References .....................................................149.1. Normative References ......................................149.2. Informative References ....................................14   Acknowledgments ...................................................15   Authors' Addresses ................................................15Raza & Boutros               Standards Track                    [Page 2]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 20151.  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 dynamicallyRaza & Boutros               Standards Track                    [Page 3]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 2015   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   inSection 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 inRFC 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:   o  "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 inRFC 5036).   o  "P2P-PWs": Refers to an application that signals FEC 128 and/or      FEC 129 L2VPN P2P PWs using LDP (as defined inRFC 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.Raza & Boutros               Standards Track                    [Page 4]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 20153.1.  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 inSection 3, a new capability   TLV is defined as follows.Raza & Boutros               Standards Track                    [Page 5]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 20154.1.  State Advertisement Control Capability   The "State Advertisement Control Capability" is a new Capability   Parameter TLV defined in accordance withSection 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.Raza & Boutros               Standards Track                    [Page 6]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 2015   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.Raza & Boutros               Standards Track                    [Page 7]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 2015   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 theSection 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.Raza & Boutros               Standards Track                    [Page 8]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 20154.2.1.  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 thisRaza & Boutros               Standards Track                    [Page 9]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 2015   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].Raza & Boutros               Standards Track                   [Page 10]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 2015   -  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 Examples6.1.  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.Raza & Boutros               Standards Track                   [Page 11]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 2015   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 toSection 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 whenRaza & Boutros               Standards Track                   [Page 12]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 2015   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  |           |                       |   +--------+---------------------+-----------+-----------------------+Raza & Boutros               Standards Track                   [Page 13]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 20159.  References9.1  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>.Raza & Boutros               Standards Track                   [Page 14]

RFC 7473        State Adv. Control of Non-negotiated Apps     March 2015   [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.comRaza & Boutros               Standards Track                   [Page 15]