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RFC 7032 - LDP Downstream-on-Demand in Seamless MPLS


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Internet Engineering Task Force (IETF)                  T. Beckhaus, Ed.
Request for Comments: 7032                           Deutsche Telekom AG
Category: Standards Track                                    B. Decraene
ISSN: 2070-1721                                                   Orange
                                                         K. Tiruveedhula
                                                        Juniper Networks
                                                 M. Konstantynowicz, Ed.
                                                              L. Martini
                                                     Cisco Systems, Inc.
                                                            October 2013

               LDP Downstream-on-Demand in Seamless MPLS

Abstract

   Seamless MPLS design enables a single IP/MPLS network to scale over
   core, metro, and access parts of a large packet network
   infrastructure using standardized IP/MPLS protocols.  One of the key
   goals of Seamless MPLS is to meet requirements specific to access
   networks including high number of devices, device position in network
   topology, and compute and memory constraints that limit the amount of
   state access devices can hold.  This can be achieved with LDP
   Downstream-on-Demand (DoD) label advertisement.  This document
   describes LDP DoD use cases and lists required LDP DoD procedures in
   the context of Seamless MPLS design.

   In addition, a new optional TLV type in the LDP Label Request message
   is defined for fast-up convergence.

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/rfc7032.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   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.

Table of Contents

   1. Introduction ....................................................4
   2. Reference Topologies ............................................6
      2.1. Access Topologies with Static Routing ......................6
      2.2. Access Topologies with Access IGP .........................10
   3. LDP DoD Use Cases ..............................................11
      3.1. Initial Network Setup .....................................12
           3.1.1. AN with Static Routing .............................12
           3.1.2. AN with Access IGP .................................13
      3.2. Service Provisioning and Activation .......................14
      3.3. Service Changes and Decommissioning .......................16
      3.4. Service Failure ...........................................17
      3.5. Network Transport Failure .................................17
           3.5.1. General Notes ......................................17
           3.5.2. AN Failure .........................................18
           3.5.3. AN/AGN Link Failure ................................19
           3.5.4. AGN Failure ........................................20
           3.5.5. AGN Network-Side Reachability Failure ..............20
   4. LDP DoD Procedures .............................................20
      4.1. LDP Label Distribution Control and Retention Modes ........21
      4.2. LDP DoD Session Negotiation ...............................23
      4.3. Label Request Procedures ..................................23
           4.3.1. Access LSR/ABR Label Request .......................23
           4.3.2. Label Request Retry ................................24
      4.4. Label Withdraw ............................................25
      4.5. Label Release .............................................26
      4.6. Local-Repair ..............................................27
   5. LDP Extension for LDP DoD Fast-Up Convergence ..................27
   6. IANA Considerations ............................................29
      6.1. LDP TLV Type ..............................................29
   7. Security Considerations ........................................29
      7.1. LDP DoD Native Security Properties ........................30
      7.2. Data-Plane Security .......................................31
      7.3. Control-Plane Security ....................................31
   8. Acknowledgements ...............................................32
   9. References .....................................................33
      9.1. Normative References ......................................33
      9.2. Informative References ....................................33

1.  Introduction

   Seamless MPLS design [SEAMLESS-MPLS] enables a single IP/MPLS network
   to scale over core, metro, and access parts of a large packet network
   infrastructure using standardized IP/MPLS protocols.  One of the key
   goals of Seamless MPLS is to meet requirements specific to access
   including high number of devices, device position in network
   topology, and compute and memory constraints that limit the amount of
   state access devices can hold.

   In general, MPLS Label Switching Routers (LSRs) implement either LDP
   or RSVP for MPLS label distribution.

   The focus of this document is on LDP, as Seamless MPLS design does
   not include a requirement for general-purpose explicit traffic
   engineering and bandwidth reservation.  This document concentrates on
   the unicast connectivity only.  Multicast connectivity is a subject
   for further study.

   In Seamless MPLS design [SEAMLESS-MPLS], IP/MPLS protocol
   optimization is possible due to relatively simple access network
   topologies.  Examples of such topologies involving access nodes (ANs)
   and aggregation nodes (AGNs) include:

   a.  A single AN homed to a single AGN.

   b.  A single AN dual-homed to two AGNs.

   c.  Multiple ANs daisy-chained via a hub-AN to a single AGN.

   d.  Multiple ANs daisy-chained via a hub-AN to two AGNs.

   e.  Two ANs dual-homed to two AGNs.

   f.  Multiple ANs chained in a ring and dual-homed to two AGNs.

   The amount of IP Routing Information Base (RIB) and Forwarding
   Information Base (FIB) state on ANs can be easily controlled in the
   listed access topologies by using simple IP routing configuration
   with either static routes or dedicated access IGP.  Note that in all
   of the above topologies, AGNs act as the access area border routers
   (access ABRs) connecting the access topology to the rest of the
   network.  Hence, in many cases, it is sufficient for ANs to have a
   default route pointing towards AGNs in order to achieve complete
   network connectivity from ANs to the network.

   However, the amount of MPLS forwarding state requires additional
   consideration.  In general, MPLS routers implement LDP Downstream
   Unsolicited (LDP DU) label advertisements [RFC5036] and advertise
   MPLS labels for all valid routes in their RIB tables.  This is seen
   as an inadequate approach for ANs, which require a small subset of
   the total routes (and associated labels) based on the required
   connectivity for the provisioned services.  Although filters can be
   applied to those LDP DU label advertisements, it is not seen as a
   suitable tool to facilitate any-to-any AN-driven connectivity between
   access and the rest of the MPLS network.

   This document describes an AN-driven "subscription model" for label
   distribution in the access network.  The approach relies on the
   standard LDP DoD label advertisements as specified in [RFC5036].  LDP
   DoD enables on-demand label distribution ensuring that only required
   labels are requested, provided, and installed.  Procedures described
   in this document are equally applicable to LDP IPv4 and IPv6 address
   families.  For simplicity, the document provides examples based on
   the LDP IPv4 address family.

   The following sections describe a set of reference access topologies
   considered for LDP DoD usage and their associated IP routing
   configurations, followed by LDP DoD use cases and LDP DoD procedures
   in the context of Seamless MPLS design.

   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 [RFC2119].

2.  Reference Topologies

   LDP DoD use cases are described in the context of a generic reference
   end-to-end network topology based on Seamless MPLS design
   [SEAMLESS-MPLS] as shown in Figure 1.

                 +-------+  +-------+  +------+  +------+
              ---+ AGN11 +--+ AGN21 +--+ ABR1 +--+ LSR1 +--> to LSR/AGN
   +--------+/   +-------+  +-------+  +------+  +------+
   | Access |             \/                   \/
   | Network|             /\                   /\
   +--------+    +-------+  +-------+  +------+  +------+
             \---+ AGN12 +--+ AGN22 +--+ ABR2 +--+ LSR2 +--> to LSR/AGN
                 +-------+  +-------+  +------+  +------+

      static routes
      or access IGP        IGP area             IGP area
     <----Access----><--Aggregation Domain--><----Core----->
     <------------------------- MPLS ---------------------->

       Figure 1: Seamless MPLS End-to-End Reference Network Topology

   The access network is either single- or dual-homed to AGN1x, with
   either a single parallel link or multiple parallel links to AGN1x.

   Seamless MPLS access network topologies can range from a single- or
   dual-homed access node to a chain or ring of access nodes, and it can
   use either static routing or access IGP (IS-IS or OSPF).  The
   following sections describe reference access topologies in more
   detail.

2.1.  Access Topologies with Static Routing

   In most cases, access nodes connect to the rest of the network using
   very simple topologies.  Here, static routing is sufficient to
   provide the required IP connectivity.  The following topologies are
   considered for use with static routing and LDP DoD:

   a.  [I1] topology - a single AN homed to a single AGN.

   b.  [I] topology - multiple ANs daisy-chained to a single AGN.

   c.  [V] topology - a single AN dual-homed to two AGNs.

   d.  [U2] topology - two ANs dual-homed to two AGNs.

   e.  [Y] topology - multiple ANs daisy-chained to two AGNs.

   The reference static routing and LDP configuration for [V] access
   topology is shown in Figure 2.  The same static routing and LDP
   configuration also applies to the [I1] topology.

          +----+                        +-------+
          |AN1 +------------------------+ AGN11 +-------
          |    +-------\    /-----------+       +-\    /
          +----+        \  /            +-------+  \  /
                         \/                         \/
                         /\                         /\
          +----+        /  \            +-------+  /  \
          |AN2 +-------/    \-----------+ AGN12 +-/    \
          |    +------------------------+       +-------
          +----+                        +-------+

          --(u)->                        <-(d)--

             <----- static routing -------> <------ IGP ------>
                                            <---- LDP DU ----->
             <--------- LDP DoD ----------> <-- labeled BGP -->

      (u) static routes: 0/0 default, (optional) /32 routes
      (d) static routes: AN loopbacks

             Figure 2: [V] Access Topology with Static Routes

   In line with the Seamless MPLS design, static routes configured on
   AGN1x and pointing towards the access network are redistributed in
   either IGP or BGP labeled IP routes [RFC3107].

   The reference static routing and LDP configuration for [U2] access
   topology is shown in Figure 3.

             +----+                        +-------+
       (d1)  |AN1 +------------------------+ AGN11 +-------
        |    |    +                        +       +-\    /
        v    +-+--+                        +-------+  \  /
               |                                       \/
               |                                       /\
        ^    +-+--+                        +-------+  /  \
        |    |AN2 +                        + AGN12 +-/    \
       (d2)  |    +------------------------+       +-------
             +----+                        +-------+

             --(u)->                        <-(d)--

                <----- static routing -------> <------ IGP ------>
                                               <---- LDP DU ----->
                <--------- LDP DoD ----------> <-- labeled BGP -->

    (u)  static route 0/0 default, (optional) /32 routes
    (d)  static route for AN loopbacks
    (d1) static route for AN2 loopback and 0/0 default with
         lower preference
    (d2) static route for AN1 loopback and 0/0 default with
         lower preference

             Figure 3: [U2] Access Topology with Static Routes

   The reference static routing and LDP configuration for [Y] access
   topology is shown in Figure 4.  The same static routing and LDP
   configuration also applies to the [I] topology.

                                       +-------+
                                       |       |---/
                                  /----+ AGN11 |
    +----+   +----+   +----+     /     |       |---\
    |    |   |    |   |    +----/      +-------+
    |ANn +...|AN2 +---+AN1 |
    |    |   |    |   |    +----\      +-------+
    +----+   +----+   +----+     \     |       |---/
                                  \----+ AGN12 |
           <-(d2)--  <-(d1)--          |       |---\
    --(u)-> --(u)->   --(u)->          +-------+
                                       <-(d)--

        <------- static routing --------> <------ IGP ------>
                                          <---- LDP DU ----->
        <----------- LDP DoD -----------> <-- labeled BGP -->

     (u)  static routes: 0/0 default, (optional) /32 routes
     (d)  static routes: AN loopbacks [1..n]
     (d1) static routes: AN loopbacks [2..n]
     (d2) static routes: AN loopbacks [3..n]

             Figure 4: [Y] Access Topology with Static Routes

   Note that in all of the above topologies, parallel Equal-Cost
   Multipath (ECMP) (or Layer 2 Link Aggregation Group (L2 LAG)) links
   can be used between the nodes.

   ANs support Inter-area LDP [RFC5283] in order to use the IP default
   route to match the LDP Forwarding Equivalence Class (FEC) advertised
   by AGN1x and other ANs.

2.2.  Access Topologies with Access IGP

   A dedicated access IGP instance is used in the access network to
   perform the internal routing between AGN1x and connected AN devices.
   Examples of such an IGP could be IS-IS, OSPFv2 and v3, or RIPv2 and
   RIPng.  This access IGP instance is distinct from the IGP of the
   aggregation domain.

   The following topologies are considered for use with access IGP
   routing and LDP DoD:

   a.  [U] topology - multiple ANs chained in an open ring and dual-
       homed to two AGNs.

   b.  [Y] topology - multiple ANs daisy-chained via a hub-AN to two
       AGNs.

   The reference access IGP and LDP configuration for [U] access
   topology is shown in Figure 5.
                                       +-------+
        +-----+   +-----+   +----+     |       +---/
        | AN3 |---| AN2 |---|AN1 +-----+ AGN11 |
        +-----+   +-----+   +----+     |       +---\
           .                           +-------+
           .
           .                           +-------+
        +-----+   +-----+   +----+     |       +---/
        |ANn-2|---|ANn-1|---|ANn +-----+ AGN12 |
        +-----+   +-----+   +----+     |       +---\
                                       +-------+

        <---------- access IGP ------------> <------ IGP ------>
                                             <---- LDP DU ----->
        <------------ LDP DoD -------------> <-- labeled BGP -->

               Figure 5: [U] Access Topology with Access IGP

   The reference access IGP and LDP configuration for [Y] access
   topology is shown in Figure 6.
                                           +-------+
                                           |       |---/
                                      /----+ AGN11 |2
        +----+   +----+   +----+     /     |       |---\
        |    |   |    |   |    +----/      +-------+
        |ANn +...|AN2 +---+AN1 |
        |    |   |    |   |    +----\      +-------+
        +----+   +----+   +----+     \     |       |---/
                                      \----+ AGN12 |
                                           |       |---\
                                           +-------+

        <---------- access IGP ------------> <------ IGP ------>
                                             <---- LDP DU ----->
        <------------ LDP DoD -------------> <-- labeled BGP -->

               Figure 6: [Y] Access Topology with Access IGP

   Note that in all of the above topologies, parallel ECMP (or L2 LAG)
   links can be used between the nodes.

   In both of the above topologies, ANs (ANn ... AN1) and AGN1x share
   the access IGP and advertise their IPv4 and IPv6 loopbacks and link
   addresses.  AGN1x advertises a default route into the access IGP.

   ANs support Inter-area LDP [RFC5283] in order to use the IP default
   route for matching the LDP FECs advertised by AGN1x or other ANs.

3.  LDP DoD Use Cases

   LDP DoD use cases described in this document are based on the
   Seamless MPLS scenarios listed in Seamless MPLS design
   [SEAMLESS-MPLS].  This section illustrates these use cases focusing
   on services provisioned on the access nodes and clarifies expected
   LDP DoD operation on the AN and AGN1x devices.  Two representative
   service types are used to illustrate the service use cases: MPLS
   Pseudowire Edge-to-Edge (PWE3) [RFC4447] and BGP/MPLS IP VPN
   [RFC4364].

   Described LDP DoD operations apply equally to all reference access
   topologies described in Section 2.  Operations that are specific to
   certain access topologies are called out explicitly.

   References to upstream and downstream nodes are made in line with the
   definition of upstream and downstream LSRs [RFC3031].

3.1.  Initial Network Setup

   An access node is commissioned without any services provisioned on
   it.  The AN can request labels for loopback addresses of any AN, AGN,
   or other nodes within the Seamless MPLS network for operational and
   management purposes.  It is assumed that AGN1x has the required
   IP/MPLS configuration for network-side connectivity in line with
   Seamless MPLS design [SEAMLESS-MPLS].

   LDP sessions are configured between adjacent ANs and AGN1x using
   their respective loopback addresses.

3.1.1.  AN with Static Routing

   If access static routing is used, ANs are provisioned with the
   following static IP routing entries (topology references from
   Section 2 are listed in square brackets):

   a.  [I1, V, U2] - Static default route 0/0 pointing to links
       connected to AGN1x.  Requires support for Inter-area LDP
       [RFC5283].

   b.  [U2] - Static /32 routes pointing to the other AN.  Lower
       preference static default route 0/0 pointing to links connected
       to the other AN.  Requires support for Inter-area LDP [RFC5283].

   c.  [I, Y] - Static default route 0/0 pointing to links leading
       towards AGN1x.  Requires support for Inter-area LDP [RFC5283].

   d.  [I, Y] - Static /32 routes to all ANs in the daisy-chain pointing
       to links towards those ANs.

   e.  [I1, V, U2] - Optional - Static /32 routes for specific nodes
       within the Seamless MPLS network, pointing to links connected to
       AGN1x.

   f.  [I, Y] - Optional - Static /32 routes for specific nodes within
       the Seamless MPLS network, pointing to links leading towards
       AGN1x.

   The upstream AN/AGN1x requests labels over an LDP DoD session(s) from
   downstream AN/AGN1x for configured static routes if those static
   routes are configured with an LDP DoD request policy and if they are
   pointing to a next hop selected by routing.  It is expected that all
   configured /32 static routes to be used for LDP DoD are configured
   with such a policy on an AN/AGN1x.

   The downstream AN/AGN1x responds to the Label Request from the
   upstream AN/AGN1x with a label mapping if the requested route is
   present in its RIB and there is a valid label binding from its
   downstream neighbor or if it is the egress node.  In such a case, the
   downstream AN/AGN1x installs the advertised label as an incoming
   label in its label information base (LIB) and its label forwarding
   information base (LFIB).  The upstream AN/AGN1x also installs the
   received label as an outgoing label in its LIB and LFIB.  If the
   downstream AN/AGN1x does have the route present in its RIB, but does
   not have a valid label binding from its downstream neighbor, it
   forwards the request to its downstream neighbor.

   In order to facilitate ECMP and IP Fast Reroute (IPFRR) Loop-Free
   Alternate (LFA) local-repair [RFC5286], the upstream AN/AGN1x also
   sends LDP DoD Label Requests to alternate next hops per its RIB, and
   installs received labels as alternate entries in its LIB and LFIB.

   The AGN1x on the network side can use BGP labeled IP routes [RFC3107]
   in line with the Seamless MPLS design [SEAMLESS-MPLS].  In such a
   case, AGN1x will redistribute its static routes pointing to local ANs
   into BGP labeled IP routes to facilitate network-to-access traffic
   flows.  Likewise, to facilitate access-to-network traffic flows,
   AGN1x will respond to access-originated LDP DoD Label Requests with
   label mappings based on its BGP labeled IP routes reachability for
   requested FECs.

3.1.2.  AN with Access IGP

   If access IGP is used, an AN(s) advertises its loopbacks over the
   access IGP with configured metrics.  The AGN1x advertises a default
   route over the access IGP.

   Routers request labels over LDP DoD session(s) according to their
   needs for MPLS connectivity (via Label Switching Paths (LSPs)).  In
   particular, if AGNs, as per Seamless MPLS design [SEAMLESS-MPLS],
   redistribute routes from the IGP into BGP labeled IP routes
   [RFC3107], they request labels over LDP DoD session(s) for those
   routes.

   Identical to the static route case, the downstream AN/AGN1x responds
   to the Label Request from the upstream AN/AGN1x with a label mapping
   (if the requested route is present in its RIB and there is a valid
   label binding from its downstream neighbor), and installs the
   advertised label as an incoming label in its LIB and LFIB.  The
   upstream AN/AGN1x also installs the received label as an outgoing
   label in its LIB and LFIB.

   Identical to the static route case, in order to facilitate ECMP and
   IPFRR LFA local-repair, the upstream AN/AGN1x also sends LDP DoD
   Label Requests to alternate next hops per its RIB, and it installs
   received labels as alternate entries in its LIB and LFIB.

   The AGN1x on the network side can use labeled BGP [RFC3107] in line
   with Seamless MPLS design [SEAMLESS-MPLS].  In such a case, AGN1x
   will redistribute routes received over the access IGP (and pointing
   to local ANs), into BGP labeled IP routes to facilitate network-to-
   access traffic flows.  Likewise, to facilitate access-to-network
   traffic flows, the AGN1x will respond to access-originated LDP DoD
   Label Requests with label mappings based on its BGP labeled IP routes
   reachability for requested FECs.

3.2.  Service Provisioning and Activation

   Following the initial setup phase described in Section 3.1, a
   specific access node, referred to as AN*, is provisioned with a
   network service.  AN* relies on LDP DoD to request the required MPLS
   LSP(s) label(s) from the downstream AN/AGN1x node(s).  Note that LDP
   DoD operations are service agnostic; that is, they are the same
   independently of the services provisioned on the AN*.

   For illustration purposes, two service types are described: MPLS PWE3
   [RFC4447] service and BGP/MPLS IPVPN [RFC4364].

   MPLS PWE3 service: For description simplicity, it is assumed that a
   single segment pseudowire is signaled using targeted LDP (tLDP)
   FEC128 (0x80), and it is provisioned with the pseudowire ID and the
   loopback IPv4 address of the destination node.  The following IP/MPLS
   operations need to be completed on the AN* to successfully establish
   such PWE3 service:

   a.  LSP labels for destination /32 FEC (outgoing label) and the local
       /32 loopback (incoming label) need to be signaled using LDP DoD.

   b.  A tLDP session over an associated TCP/IP connection needs to be
       established to the PWE3 destination Provider Edge (PE).  This is
       triggered either by an explicit tLDP session configuration on the
       AN* or automatically at the time of provisioning the PWE3
       instance.

   c.  Local and remote PWE3 labels for specific FEC128 PW ID need to be
       signaled using tLDP and PWE3 signaling procedures [RFC4447].

   d.  Upon successful completion of the above operations, AN* programs
       its RIB/LIB and LFIB tables and activates the MPLS PWE3 service.

   Note: Only minimum operations applicable to service connectivity have
   been listed.  Other non-IP/non-MPLS connectivity operations that are
   required for successful service provisioning and activation are out
   of scope in this document.

   BGP/MPLS IPVPN service: For description simplicity, it is assumed
   that the AN* is provisioned with a unicast IPv4 IPVPN service (VPNv4
   for short) [RFC4364].  The following IP/MPLS operations need to be
   completed on the AN* to successfully establish VPNv4 service:

   a.  BGP peering sessions with associated TCP/IP connections need to
       be established with the remote destination VPNv4 PEs or Route
       Reflectors.

   b.  Based on configured BGP policies, VPNv4 BGP Network Layer
       Reachability Information (NLRI) needs to be exchanged between AN*
       and its BGP peers.

   c.  Based on configured BGP policies, VPNv4 routes need to be
       installed in the AN* VPN Routing and Forwarding (VRF) RIB and
       FIB, with corresponding BGP next hops.

   d.  LSP labels for destination BGP next-hop /32 FEC (outgoing label)
       and the local /32 loopback (incoming label) need to be signaled
       using LDP DoD.

   e.  Upon successful completion of above operations, AN* programs its
       RIB/LIB and LFIB tables, and activates the BGP/MPLS IPVPN
       service.

   Note: Only minimum operations applicable to service connectivity have
   been listed.  Other non-IP/-MPLS connectivity operations that are
   required for successful service provisioning are out of scope in this
   document.

   To establish an LSP for destination /32 FEC for any of the above
   services, AN* looks up its local routing table for a matching route
   and selects the best next hop(s) and associated outgoing link(s).

   If a label for this /32 FEC is not already installed based on the
   configured static route with LDP DoD request policy or access IGP RIB
   entry, AN* sends an LDP DoD label mapping request.  A downstream
   AN/AGN1x LSR(s) checks its RIB for presence of the requested /32 and
   associated valid outgoing label binding, and if both are present,
   replies with its label for this FEC and installs this label as
   incoming in its LIB and LFIB.  Upon receiving the label mapping, the
   AN* accepts this label based on the exact route match of the
   advertised FEC and route entry in its RIB or based on the longest

   match in line with Inter-area LDP [RFC5283].  If the AN* accepts the
   label, it installs it as an outgoing label in its LIB and LFIB.

   In access topologies [V] and [Y], if AN* is dual-homed to two AGN1x
   and routing entries for these AGN1x's are configured as equal-cost
   paths, AN* sends LDP DoD Label Requests to both AGN1x devices and
   installs all received labels in its LIB and LFIB.

   In order for AN* to implement IPFRR LFA local-repair, AN* also sends
   LDP DoD Label Requests to alternate next hops per its RIB, and
   installs received labels as alternate entries in its LIB and LFIB.

   When forwarding PWE3 or VPNv4 packets, AN* chooses the LSP label
   based on the locally configured static /32 or default route or
   default route signaled via access IGP.  If a route is reachable via
   multiple interfaces to AGN1x nodes and the route has multiple equal-
   cost paths, AN* implements ECMP functionality.  This involves AN*
   using a hash-based load-balancing mechanism and sending the PWE3 or
   VPNv4 packets in a flow-aware manner with appropriate LSP labels via
   all equal-cost links.

   The ECMP mechanism is applicable in an equal manner to parallel links
   between two network elements and multiple paths towards the
   destination.  The traffic demand is distributed over the available
   paths.

   The AGN1x on the network side can use labeled BGP [RFC3107] in line
   with Seamless MPLS design [SEAMLESS-MPLS].  In such a case, the AGN1x
   will redistribute its static routes (or routes received from the
   access IGP) pointing to local ANs into BGP labeled IP routes to
   facilitate network-to-access traffic flows.  Likewise, to facilitate
   access-to-network traffic flows, the AGN1x will respond to access-
   originated LDP DoD Label Requests with label mappings based on its
   BGP labeled IP routes reachability for requested FECs.

3.3.  Service Changes and Decommissioning

   Whenever the AN* service gets decommissioned or changed and
   connectivity to a specific destination is no longer required, the
   associated MPLS LSP label resources are to be released on AN*.

   MPLS PWE3 service: If the PWE3 service gets decommissioned and it is
   the last PWE3 to a specific destination node, the tLDP session is no
   longer needed and is to be terminated (automatically or by
   configuration).  The MPLS LSP(s) to that destination is no longer
   needed either.

   BGP/MPLS IPVPN service: Deletion of a specific VPNv4 (VRF) instance
   via local or remote reconfiguration can result in a specific BGP next
   hop(s) no longer being needed.  The MPLS LSP(s) to that destination
   is no longer needed either.

   In all of the above cases, the following operations related to LDP
   DoD apply:

   o  If the /32 FEC label for the aforementioned destination node was
      originally requested based on either tLDP session configuration
      and default route or required BGP next hop and default route, AN*
      deletes the label from its LIB and LFIB, and releases it from the
      downstream AN/AGN1x by using LDP DoD procedures.

   o  If the /32 FEC label was originally requested based on the static
      /32 route configuration with LDP DoD request policy, the label is
      retained by AN*.

3.4.  Service Failure

   A service instance can stop being operational due to a local or
   remote service failure event.

   In general, unless the service failure event modifies required MPLS
   connectivity, there is no impact on the LDP DoD operation.

   If the service failure event does modify the required MPLS
   connectivity, LDP DoD operations apply as described in Sections 3.2
   and 3.3.

3.5.  Network Transport Failure

   A number of different network events can impact services on AN*.  The
   following sections describe network event types that impact LDP DoD
   operation on AN and AGN1x nodes.

3.5.1.  General Notes

   If service on any of the ANs is affected by any network failure and
   there is no network redundancy, the service goes into a failure
   state.  Upon recovery from network failure, the service is to be
   re-established automatically.

   The following additional LDP-related functions need to be supported
   to comply with Seamless MPLS [SEAMLESS-MPLS] fast service restoration
   requirements:

   a.  Local-repair: AN and AGN1x support local-repair for adjacent link
       or node failure for access-to-network, network-to-access, and
       access-to-access traffic flows.  Local-repair is to be
       implemented by using either IPFRR LDP LFA, simple ECMP, or
       primary/backup switchover upon failure detection.

   b.  LDP session protection: LDP sessions are configured with LDP
       session protection to avoid delay upon the recovery from link
       failure.  LDP session protection ensures that FEC label binding
       is maintained in the control plane as long as the LDP session
       stays up.

   c.  IGP-LDP synchronization: If access IGP is used, LDP sessions
       between ANs, and between ANs and AGN1x, are configured with IGP-
       LDP synchronization to avoid unnecessary traffic loss in case the
       access IGP converged before LDP and there is no LDP label binding
       to the best downstream next hop.

3.5.2.  AN Failure

   If the AN fails, adjacent AN/AGN1x nodes remove all routes pointing
   to the failed node from their RIB tables (including /32 loopback
   belonging to the failed AN and any other routes reachable via the
   failed AN).  In turn, this triggers the removal of associated
   outgoing /32 FEC labels from their LIB and LFIB tables.

   If access IGP is used, the AN failure will be propagated via IGP link
   updates across the access topology.

   If a specific /32 FEC(s) is no longer reachable from those
   ANs/AGN1x's, they also send LDP Label Withdraw messages to their
   upstream LSRs to notify them about the failure, and remove the
   associated incoming label(s) from their LIB and LFIB tables.
   Upstream LSRs, upon receiving a Label Withdraw, remove the signaled
   labels from their LIB/LFIB tables, and propagate LDP Label Withdraws
   across their upstream LDP DoD sessions.

   In the [U] topology, there may be an alternative path to routes
   previously reachable via the failed AN.  In this case, adjacent
   AN/AGN1x pairs invoke local-repair (IPFRR LFA, ECMP) and switch over
   to an alternate next hop to reach those routes.

   AGN1x is notified about the AN failure via access IGP (if used)
   and/or cascaded LDP DoD Label Withdraw(s).  AGN1x implements all
   relevant global-repair IP/MPLS procedures to propagate the AN failure
   towards the core network.  This involves removing associated routes
   (in the access IGP case) and labels from its LIB and LFIB tables, and

   propagating the failure on the network side using labeled BGP and/or
   core IGP/LDP DU procedures.

   Upon the AN coming back up, adjacent AN/AGN1x nodes automatically add
   routes pointing to recovered links based on the configured static
   routes or access IGP adjacency and link state updates.  This is then
   followed by LDP DoD label signaling and subsequent binding and
   installation of labels in LIB and LFIB tables.

3.5.3.  AN/AGN Link Failure

   Depending on the access topology and the failed link location,
   different cases apply to the network operation after AN link failure
   (topology references from Section 2 in square brackets):

   a.  [all] - link failed, but at least one ECMP parallel link remains.
       Nodes on both sides of the failed link stop using the failed link
       immediately (local-repair) and keep using the remaining ECMP
       parallel links.

   b.  [I1, I, Y] - link failed, and there are no ECMP or alternative
       links and paths.  Nodes on both sides of the failed link remove
       routes pointing to the failed link immediately from the RIB,
       remove associated labels from their LIB and LFIB tables, and send
       LDP Label Withdraw(s) to their upstream LSRs.

   c.  [U2, U, V, Y] - link failed, but at least one ECMP or alternate
       path remains.  The AN/AGN1x node stops using the failed link and
       immediately switches over (local-repair) to the remaining ECMP
       path or alternate path.  The AN/AGN1x removes affected next hops
       and labels.  If there is an AGN1x terminating the failed link, it
       immediately removes routes pointing to the failed link from the
       RIB, removes any associated labels from the LIB and LFIB tables,
       and propagates the failure on the network side using labeled BGP
       and/or core IGP procedures.

   If access IGP is used, AN/AGN1x link failure will be propagated via
   IGP link updates across the access topology.

   LDP DoD will also propagate the link failure by sending Label
   Withdraws to upstream AN/AGN1x nodes, and Label Release messages to
   downstream AN/AGN1x nodes.

3.5.4.  AGN Failure

   If an AGN1x fails adjacent access then, depending on the access
   topology, the following cases apply to the network operation
   (topology references from Section 2 are shown in square brackets):

   a.  [I1, I] - ANs are isolated from the network - An AN adjacent to
       the failure immediately removes routes pointing to the failed
       AGN1x from the RIB, removes associated labels from the LIB and
       LFIB tables, and sends LDP Label Withdraw message(s) to its
       upstream neighbors.  If access IGP is used, an IGP link update is
       sent.

   b.  [U2, U, V, Y] - at least one ECMP or alternate path remains.  AN
       adjacent to failed AGN1x stops using the failed link and
       immediately switches over (local-repair) to the remaining ECMP
       path or alternate path by following LDP [RFC5036] procedures.
       (Appendix A.1.7 "Detect Change in FEC Next Hop")

   Network-side procedures for handling AGN1x failure have been
   described in Seamless MPLS [SEAMLESS-MPLS].

3.5.5.  AGN Network-Side Reachability Failure

   If AGN1x loses network reachability to a specific destination or set
   of network-side destinations, AGN1x sends LDP Label Withdraw messages
   to its upstream ANs, withdrawing labels for all affected /32 FECs.
   Upon receiving those messages, ANs remove those labels from their LIB
   and LFIB tables, and use alternative LSPs instead (if available) as
   part of global-repair.

   If access IGP is used, and AGN1x gets completely isolated from the
   core network, it stops advertising the default route 0/0 into the
   access IGP.

4.  LDP DoD Procedures

   All LDP Downstream-on-Demand implementations follow the Label
   Distribution Protocol as specified in [RFC5036].  This section does
   not update [RFC5036] procedures, but illustrates LDP DoD operations
   in the context of use cases identified in Section 3 in this document,
   for information only.

   In the MPLS architecture [RFC3031], network traffic flows from the
   upstream LSR to the downstream LSR.  The use cases in this document
   rely on the downstream assignment of labels, where labels are
   assigned by the downstream LSR and signaled to the upstream LSR as
   shown in Figure 7.

                    +----------+      +------------+
                    | upstream |      | downstream |
              ------+   LSR    +------+    LSR     +----
          traffic   |          |      |            |  address
          source    +----------+      +------------+  (/32 for IPv4)
                                                      traffic
                   label distribution for IPv4 FEC    destination
                     <-------------------------

                            traffic flow
                     ------------------------->

                 Figure 7: LDP Label Assignment Direction

4.1.  LDP Label Distribution Control and Retention Modes

   The LDP specification [RFC5036] defines two modes for label
   distribution control, following the definitions in the MPLS
   architecture [RFC3031]:

   o  Independent mode: An LSR recognizes a particular FEC and makes a
      decision to bind a label to the FEC independently from
      distributing that label binding to its label distribution peers.
      A new FEC is recognized whenever a new route becomes valid on the
      LSR.

   o  Ordered mode: An LSR needs to bind a label to a particular FEC if
      it knows how to forward packets for that FEC (i.e., it has a route
      corresponding to that FEC) and if it has already received at least
      one Label Request message from an upstream LSR.

   Using independent label distribution control with LDP DoD and access
   static routing would prevent the access LSRs from propagating label
   binding failure along the access topology, making it impossible for
   an upstream LSR to be notified about the downstream failure and for
   an application using the LSP to switch over to an alternate path,
   even if such a path exists.

   The LDP specification [RFC5036] defines two modes for label
   retention, following the definitions in the MPLS architecture
   [RFC3031]:

   o  Conservative label retention mode: If operating in DoD mode, an
      LSR will request label mappings only from the next-hop LSR
      according to routing.  The main advantage of the conservative
      label retention mode is that only the labels that are required for
      the forwarding of data are allocated and maintained.  This is
      particularly important in LSRs where the label space is inherently

      limited, such as in an ATM switch.  A disadvantage of the
      conservative label retention mode is that if routing changes the
      next hop for a given destination, a new label must be obtained
      from the new next hop before labeled packets can be forwarded.

   o  Liberal label retention mode: When operating in DoD mode with
      liberal label retention mode, an LSR might choose to request label
      mappings for all known prefixes from all peer LSRs.  The main
      advantage of the liberal label retention mode is that reaction to
      routing changes can be quick because labels already exist.  The
      main disadvantage of the liberal label retention mode is that
      unneeded label mappings are distributed and maintained.

   Note that the conservative label retention mode would prevent LSRs
   from requesting and maintaining label mappings for any backup routes
   that are not used for forwarding.  In turn, this would prevent the
   access LSRs (AN and AGN1x nodes) from implementing any local
   protection schemes that rely on using alternate next hops in case of
   the primary next-hop failure.  Such schemes include IPFRR LFA if
   access IGP is used, or a primary and backup static route
   configuration.  Using LDP DoD in combination with liberal label
   retention mode allows the LSR to request labels for the specific FEC
   from primary next-hop LSR(s) and the alternate next-hop LSR(s) for
   this FEC.

   Note that even though LDP DoD operates in a liberal label retention
   mode, if used with access IGP and if no LFA exists, the LDP DoD will
   introduce additional delay in traffic restoration as the labels for
   the new next hop will be requested only after the access IGP
   convergence.

   Adhering to the overall design goals of Seamless MPLS
   [SEAMLESS-MPLS], specifically achieving a large network scale without
   compromising fast service restoration, all access LSRs (AN and AGN1x
   nodes) use LDP DoD advertisement mode with:

   o  Ordered label distribution control: enables propagation of label
      binding failure within the access topology.

   o  Liberal label retention mode: enables pre-programming of alternate
      next hops with associated FEC labels.

   In Seamless MPLS [SEAMLESS-MPLS], an AGN1x acts as an access ABR
   connecting access and metro domains.  To enable failure propagation
   between those domains, the access ABR implements ordered label
   distribution control when redistributing routes/FECs between the

   access side (using LDP DoD and static or access IGP) and the network
   side (using labeled BGP [RFC3107] or core IGP with LDP Downstream
   Unsolicited label advertisements).

4.2.  LDP DoD Session Negotiation

   An access LSR/ABR proposes the DoD label advertisement by setting the
   "A" value to 1 in the Common Session Parameters TLV of the
   Initialization message.  The rules for negotiating the label
   advertisement mode are specified in the LDP specification [RFC5036].

   To establish a DoD session between the two access LSR/ABRs, both
   propose the DoD label advertisement mode in the Initialization
   message.  If the access LSR only supports LDP DoD and the access ABR
   proposes the Downstream Unsolicited mode, the access LSR sends a
   Notification message with status "Session Rejected/Parameters
   Advertisement Mode" and then closes the LDP session as specified in
   the LDP specification [RFC5036].

   If an access LSR is acting in an active role, it re-attempts the LDP
   session immediately.  If the access LSR receives the same Downstream
   Unsolicited mode again, it follows the exponential backoff algorithm
   as defined in the LDP specification [RFC5036] with a delay of 15
   seconds and subsequent delays growing to a maximum delay of 2
   minutes.

   In case a PWE3 service is required between the adjacent access
   LSR/ABR, and LDP DoD has been negotiated for IPv4 and IPv6 FECs, the
   same LDP session is used for PWE3 FECs.  Even if the LDP DoD label
   advertisement has been negotiated for IPv4 and IPv6 LDP FECs as
   described earlier, the LDP session uses a Downstream Unsolicited
   label advertisement for PWE3 FECs as specified in PWE3 LDP [RFC4447].

4.3.  Label Request Procedures

4.3.1.  Access LSR/ABR Label Request

   The upstream access LSR/ABR will request label bindings from an
   adjacent downstream access LSR/ABR based on the following trigger
   events:

   a.  An access LSR/ABR is configured with /32 static route with LDP
       DoD Label Request policy in line with the initial network setup
       use case described in Section 3.1.

   b.  An access LSR/ABR is configured with a service in line with
       service use cases described in Sections 3.2 and 3.3.

   c.  Configuration with access static routes: An access LSR/ABR link
       to an adjacent node comes up, and an LDP DoD session is
       established.  In this case, the access LSR sends Label Request
       messages for all /32 static routes configured with an LDP DoD
       policy and all /32 routes related to provisioned services that
       are covered by the default route.

   d.  Configuration with access IGP: An access LSR/ABR link to an
       adjacent node comes up, and an LDP DoD session is established.
       In this case, the access LSR sends Label Request messages for all
       /32 routes learned over the access IGP and all /32 routes related
       to provisioned services that are covered by access IGP routes.

   e.  In all above cases, requests are sent to any next-hop LSRs and
       alternate LSRs.

   The downstream access LSR/ABR will respond with a Label Mapping
   message with a non-null label if any of the below conditions are met:

   a.  Downstream access LSR/ABR: The requested FEC is an IGP or static
       route, and there is an LDP label already learned from the next-
       next-hop downstream LSR (by LDP DoD or LDP DU).  If there is no
       label for the requested FEC and there is an LDP DoD session to
       the next-next-hop downstream LSR, the downstream LSR sends a
       Label Request message for the same FEC to the next-next-hop
       downstream LSR.  In such a case, the downstream LSR will respond
       back to the requesting upstream access LSR only after getting a
       label from the next-next-hop downstream LSR peer.

   b.  Downstream access ABR only: The requested FEC is a BGP labeled IP
       routes [RFC3107], and this BGP route is the best selected for
       this FEC.

   The downstream access LSR/ABR can respond with a label mapping with
   an explicit-null or implicit-null label if it is acting as an egress
   for the requested FEC, or it can respond with a "No Route"
   notification if no route exists.

4.3.2.  Label Request Retry

   Following the LDP specification [RFC5036], if an access LSR/ABR
   receives a "No Route" notification in response to its Label Request
   message, it retries using an exponential backoff algorithm similar to
   the backoff algorithm mentioned in the LDP session negotiation
   described in Section 4.2.

   If there is no response to the Label Request message sent, the LDP
   specification [RFC5036] (Section A.1.1) states that the LSR does not
   send another request for the same label to the peer and mandates that
   a duplicate Label Request be considered a protocol error and be
   dropped by the receiving LSR by sending a Notification message.

   Thus, if there is no response from the downstream peer, the access
   LSR/ABR does not send a duplicate Label Request message.

   If the static route corresponding to the FEC gets deleted or if the
   DoD request policy is modified to reject the FEC before receiving the
   Label Mapping message, then the access LSR/ABR sends a Label Abort
   message to the downstream LSR.

   To address the case of slower convergence resulting from described
   LDP behavior in line with the LDP specification [RFC5036], a new LDP
   TLV extension is proposed and described in Section 5.

4.4.  Label Withdraw

   If an MPLS label on the downstream access LSR/ABR is no longer valid,
   the downstream access LSR/ABR withdraws this FEC/label binding from
   the upstream access LSR/ABR with the Label Withdraw message [RFC5036]
   with a specified label TLV or with an empty label TLV.

   The downstream access LSR/ABR withdraws a label for a specific FEC in
   the following cases:

   a.  If an LDP DoD ingress label is associated with an outgoing label
       assigned by a labeled BGP route and this route is withdrawn.

   b.  If an LDP DoD ingress label is associated with an outgoing label
       assigned by LDP (DoD or DU), and the IGP route is withdrawn from
       the RIB or the downstream LDP session is lost.

   c.  If an LDP DoD ingress label is associated with an outgoing label
       assigned by LDP (DoD or DU) and the outgoing label is withdrawn
       by the downstream LSR.

   d.  If an LDP DoD ingress label is associated with an outgoing label
       assigned by LDP (DoD or DU), the next hop in the route has
       changed, and

       *  there is no LDP session to the new next hop.  To minimize the
          probability of this, the access LSR/ABR implements LDP-IGP
          synchronization procedures as specified in [RFC5443].

       *  there is an LDP session but no label from a downstream LSR.
          See note below.

   e.  If an access LSR/ABR is configured with a policy to reject
       exporting label mappings to an upstream LSR.

   The upstream access LSR/ABR responds to the Label Withdraw message
   with the Label Release message [RFC5036].

   After sending the Label Release message to the downstream access
   LSR/ABR, the upstream access LSR/ABR resends the Label Request
   message, assuming the upstream access LSR/ABR still requires the
   label.

   The downstream access LSR/ABR withdraws a label if the local route
   configuration (e.g., /32 loopback) is deleted.

   Note: For any events inducing next-hop change, a downstream access
   LSR/ABR attempts to converge the LSP locally before withdrawing the
   label from an upstream access LSR/ABR.  For example, if the next hop
   changes for a particular FEC and if the new next hop allocates labels
   by the LDP DoD session, then the downstream access LSR/ABR sends a
   Label Request on the new next-hop session.  If the downstream access
   LSR/ABR doesn't get a label mapping for some duration, then and only
   then does the downstream access LSR/ABR withdraw the upstream label.

4.5.  Label Release

   If an access LSR/ABR no longer needs a label for a FEC, it sends a
   Label Release message [RFC5036] to the downstream access LSR/ABR with
   or without the label TLV.

   If an upstream access LSR/ABR receives an unsolicited label mapping
   on a DoD session, it releases the label by sending a Label Release
   message.

   The access LSR/ABR sends a Label Release message to the downstream
   LSR in the following cases:

   a.  If it receives a Label Withdraw from the downstream access
       LSR/ABR.

   b.  If the /32 static route with LDP DoD Label Request policy is
       deleted.

   c.  If the service gets decommissioned and there is no corresponding
       /32 static route with LDP DoD Label Request policy configured.

   d.  If the next hop in the route has changed and the label does not
       point to the best or alternate next hop.

   e.  If it receives a Label Withdraw from a downstream DoD session.

4.6.  Local-Repair

   To support local-repair with ECMP and IPFRR LFA, the access LSR/ABR
   requests labels on both the best next-hop and the alternate next-hop
   LDP DoD sessions, as specified in the Label Request procedures in
   Section 4.3.  If remote LFA is enabled, the access LSR/ABR needs a
   label from its alternate next hop toward the PQ node and needs a
   label from the remote PQ node toward its FEC/destination [RLFA].  If
   the access LSR/ABR doesn't already know those labels, it requests
   them.

   This will enable the access LSR/ABR to pre-program the alternate
   forwarding path with the alternate label(s) and invoke the IPFRR LFA
   switchover procedure if the primary next-hop link fails.

5.  LDP Extension for LDP DoD Fast-Up Convergence

   In some conditions, the exponential backoff algorithm usage described
   in Section 4.3.2 can result in a wait time that is longer than
   desired to get a successful LDP label-to-route mapping.  An example
   is when a specific route is unavailable on the downstream LSR when
   the label mapping request from the upstream is received, but later
   comes back.  In such a case, using the exponential backoff algorithm
   can result in a max delay wait time before the upstream LSR sends
   another LDP Label Request.

   This section describes an extension to the LDP DoD procedure to
   address fast-up convergence, and as such is to be treated as a
   normative reference.  The downstream and upstream LSRs SHOULD
   implement this extension if fast-up convergence is desired.

   The extension consists of the upstream LSR indicating to the
   downstream LSR that the Label Request SHOULD be queued on the
   downstream LSR until the requested route is available.

   To implement this behavior, a new Optional Parameter is defined for
   use in the Label Request message:

                  Optional Parameter      Length     Value
                  Queue Request TLV         0      see below

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |1|0|  Queue Request (0x0971)   |         Length (0x00)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     U-bit = 1
       Unknown TLV bit.  Upon receipt of an unknown TLV, due to the
       U-bit being set (=1), the unknown TLV MUST be silently ignored
       and the rest of the message processed as if the unknown TLV
       did not exist.  In case the requested route is not available,
       the downstream LSR MUST ignore this unknown TLV and send a
       "No Route" notification back.  This ensures backward
       compatibility.

     F-bit = 0
       Forward unknown TLV bit.  This bit applies only when the U-bit is
       set and the LDP message containing the unknown TLV is to be
       forwarded.  Due to the F-bit being clear (=0), the unknown TLV is
       not forwarded with the message.

     Type = 0x0971
       Queue Request TLV (allocated by IANA).

     Length = 0x00
       Specifies the length of the Value field in octets.

   The specified operation is as follows.

   To benefit from the fast-up convergence improvement, the upstream LSR
   sends a Label Request message with a Queue Request TLV.

   If the downstream LSR supports the Queue Request TLV, it verifies if
   a route is available; if so, it replies with a label mapping as per
   existing LDP procedures.  If the route is not available, the
   downstream LSR queues the request and replies as soon as the route
   becomes available.  In the meantime, it does not send a "No Route"
   notification back.  When sending a Label Request with the Queue
   Request TLV, the upstream LSR does not retry the Label Request
   message if it does not receive a reply from its downstream peer.

   If the upstream LSR wants to abort an outstanding Label Request while
   the Label Request is queued in the downstream LSR, the upstream LSR
   sends a Label Abort Request message, making the downstream LSR remove
   the original request from the queue and send back a Label Request
   Aborted notification [RFC5036].

   If the downstream LSR does not support the Queue Request TLV, and the
   requested route is not available, it ignores this unknown TLV and
   sends a "No Route" notification back, in line with [RFC5036].  In
   this case, the upstream LSR invokes the exponential backoff algorithm
   described in Section 4.3.2, following the LDP specification
   [RFC5036].

   This procedure ensures backward compatibility.

6.  IANA Considerations

6.1.  LDP TLV Type

   This document uses a new Optional Parameter, Queue Request TLV, in
   the Label Request message defined in Section 5.  IANA already
   maintains a registry of LDP parameters called the "TLV Type Name
   Space" registry, as defined by RFC 5036.  The following assignment
   has been made:

                          TLV type  Description
                          0x0971    Queue Request TLV

7.  Security Considerations

   MPLS LDP DoD deployment in the access network is subject to the same
   security threats as any MPLS LDP deployment.  It is recommended that
   baseline security measures be considered, as described in "Security
   Framework for MPLS and GMPLS Networks" [RFC5920] and the LDP
   specification [RFC5036] including ensuring authenticity and integrity
   of LDP messages, as well as protection against spoofing and denial-
   of-service attacks.

   Some deployments require increased measures of network security if a
   subset of access nodes are placed in locations with lower levels of
   physical security, e.g., street cabinets (common practice for Very
   high bit-rate Digital Subscriber Line (VDSL) access).  In such cases,
   it is the responsibility of the system designer to take into account
   the physical security measures (environmental design, mechanical or
   electronic access control, intrusion detection) as well as monitoring
   and auditing measures (configuration and Operating System changes,
   reloads, route advertisements).

   But even with all this in mind, the designer still needs to consider
   network security risks and adequate measures arising from the lower
   level of physical security of those locations.

7.1.  LDP DoD Native Security Properties

   MPLS LDP DoD operation is request driven, and unsolicited label
   mappings are not accepted by upstream LSRs by design.  This
   inherently limits the potential of an unauthorized third party
   injecting unsolicited label mappings on the wire.

   This native security property enables an ABR LSR to act as a gateway
   to the MPLS network and to control the requests coming from any
   access LSR and prevent cases when the access LSR attempts to get
   access to an unauthorized FEC or remote LSR after being compromised.

   In the event that an access LSR gets compromised and manages to
   advertise a FEC belonging to another LSR (e.g., in order to 'steal'
   third-party data flows, or breach the privacy of a VPN), such an
   access LSR would also have to influence the routing decision for
   affected FECs on the ABR LSR to attract the flows.  The following
   measures need to be considered on an ABR LSR to prevent such an event
   from occurring:

   a.  Access with static routes: An access LSR cannot influence ABR LSR
       routing decisions due to the static nature of routing
       configuration, a native property of the design.

   b.  Access with IGP - access FEC "stealing": If the compromised
       access LSR is a leaf in the access topology (leaf node in
       topologies I1, I, V, Y described earlier), this will not have any
       adverse effect, due to the leaf IGP metrics being configured on
       the ABR LSR.  If the compromised access LSR is a transit LSR in
       the access topology (transit node in topologies I, Y, U), it is
       only possible for this access LSR to attract traffic destined to
       the nodes upstream from it.  Such a 'man-in-the-middle attack'
       can quickly be detected by upstream access LSRs not receiving
       traffic and by the LDP TCP session being lost.

   c.  Access with IGP - network FEC "stealing": The compromised access
       LSR can use IGP to advertise a "stolen" FEC prefix belonging to
       the network side.  This case can be prevented by giving a better
       administrative preference to the BGP labeled IP routes versus
       access IGP routes.

   In summary, the native properties of MPLS in access design with LDP
   DoD prevent a number of security attacks and make their detection
   quick and straightforward.

   The following two sections describe other security considerations
   applicable to general MPLS deployments in the access network.

7.2.  Data-Plane Security

   Data-plane security risks applicable to the access MPLS network
   include:

   a.  Labeled packets from a specific access LSR that are sent to an
       unauthorized destination.

   b.  Unlabeled packets that are sent by an access LSR to remote
       network nodes.

   The following mechanisms apply to MPLS access design with LDP DoD
   that address listed data-plane security risks:

   1.  addressing (a): Access and ABR LSRs do not accept labeled packets
       over a particular data link, unless from the access or ABR LSR
       perspective this data link is known to attach to a trusted system
       based on control-plane security as described in Section 7.3 and
       the top label has been distributed to the upstream neighbor by
       the receiving access or ABR LSR.

   2.  addressing (a) - The ABR LSR restricts network reachability for
       access devices to a subset of remote network LSRs, based on
       control-plane security as described in Section 7.3, FEC filters,
       and routing policy.

   3.  addressing (a): Control-plane authentication as described in
       Section 7.3 is used.

   4.  addressing (b): The ABR LSR restricts IP network reachability to
       and from the access LSR.

7.3.  Control-Plane Security

   Similar to Inter-AS MPLS/VPN deployments [RFC4364], control-plane
   security is a prerequisite for data-plane security.

   To ensure control-plane security access, LDP DoD sessions are
   established only with LDP peers that are considered trusted from the
   local LSR perspective, meaning they are reachable over a data link
   that is known to attach to a trusted system based on employed
   authentication mechanism(s) on the local LSR.

   The security of LDP sessions is analyzed in the LDP specification
   [RFC5036] and in [RFC6952] ("Analysis of BGP, LDP, PCEP, and MSDP
   Issues According to the Keying and Authentication for Routing
   Protocols (KARP) Design Guide").  Both documents state that LDP is
   subject to two different types of attacks: spoofing and denial-of-
   service attacks.

   The threat of spoofed LDP Hello messages can be reduced by following
   guidelines listed in the LDP specification [RFC5036]: accepting Basic
   Hellos only on interfaces connected to trusted LSRs, ignoring Basic
   Hellos that are not addressed to all routers in this subnet multicast
   group, and using access lists.  LDP Hello messages can also be
   secured using an optional Cryptographic Authentication TLV as
   specified in "LDP Hello Cryptographic Authentication" [CRYPTO-AUTH]
   that further reduces the threat of spoofing during the LDP discovery
   phase.

   Spoofing during the LDP session communication phase can be prevented
   by using the TCP Authentication Option (TCP-AO) [RFC5925], which uses
   a stronger hashing algorithm, e.g., SHA1 as compared to the
   traditionally used MD5 authentication.  TCP-AO is recommended as
   being more secure as compared to the TCP/IP MD5 authentication option
   [RFC5925].

   The threat of a denial-of-service attack targeting a well-known UDP
   port for LDP discovery or a TCP port for LDP session establishment
   can be reduced by following the guidelines listed in [RFC5036] and in
   [RFC6952].

   Access IGP (if used) and any routing protocols used in the access
   network for signaling service routes also need to be secured
   following best practices in routing protocol security.  Refer to the
   KARP IS-IS security analysis document [KARP-ISIS] and to [RFC6863]
   ("Analysis of OSPF Security According to the Keying and
   Authentication for Routing Protocols (KARP) Design Guide") for
   further analysis of security properties of IS-IS and OSPF IGP routing
   protocols.

8.  Acknowledgements

   The authors would like to thank Nischal Sheth, Nitin Bahadur, Nicolai
   Leymann, George Swallow, Geraldine Calvignac, Ina Minei, Eric Gray,
   and Lizhong Jin for their suggestions and review.  Additional thanks
   go to Adrian Farrel for thorough pre-publication review, and to
   Stephen Kent for review and guidance specifically for the security
   section.

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.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

   [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
              Heron, "Pseudowire Setup and Maintenance Using the Label
              Distribution Protocol (LDP)", RFC 4447, April 2006.

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

   [RFC5283]  Decraene, B., Le Roux, JL., and I. Minei, "LDP Extension
              for Inter-Area Label Switched Paths (LSPs)", RFC 5283,
              July 2008.

9.2.  Informative References

   [CRYPTO-AUTH]
              Zheng, L., Chen, M., and M. Bhatia, "LDP Hello
              Cryptographic Authentication", Work in Progress, August
              2013.

   [KARP-ISIS]
              Chunduri, U., Tian, A., and W. Lu, "KARP IS-IS security
              analysis", Work in Progress, March 2013.

   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, May 2001.

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

   [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
              Synchronization", RFC 5443, March 2009.

   [RFC5920]  Fang, L., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, July 2010.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, June 2010.

   [RFC6863]  Hartman, S. and D. Zhang, "Analysis of OSPF Security
              According to the Keying and Authentication for Routing
              Protocols (KARP) Design Guide", RFC 6863, March 2013.

   [RFC6952]  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP, and MSDP Issues According to the Keying
              and Authentication for Routing Protocols (KARP) Design
              Guide", RFC 6952, May 2013.

   [RLFA]     Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote LFA FRR", Work in Progress, May 2013.

   [SEAMLESS-MPLS]
              Leymann, N., Ed., Decraene, B., Filsfils, C.,
              Konstantynowicz, M., Ed., and D. Steinberg, "Seamless MPLS
              Architecture", Work in Progress, July 2013.

Authors' Addresses

   Thomas Beckhaus (editor)
   Deutsche Telekom AG
   Heinrich-Hertz-Strasse 3-7
   Darmstadt  64307
   Germany

   Phone: +49 6151 58 12825
   EMail: thomas.beckhaus@telekom.de

   Bruno Decraene
   Orange
   38-40 rue du General Leclerc
   Issy Moulineaux cedex 9  92794
   France

   EMail: bruno.decraene@orange.com

   Kishore Tiruveedhula
   Juniper Networks
   10 Technology Park Drive
   Westford, Massachusetts  01886
   USA

   Phone: 1-(978)-589-8861
   EMail: kishoret@juniper.net

   Maciek Konstantynowicz (editor)
   Cisco Systems, Inc.
   10 New Square Park, Bedfont Lakes
   London
   United Kingdom

   EMail: maciek@cisco.com

   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO  80112
   USA

   EMail: lmartini@cisco.com

 

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