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Versions: (draft-beckhaus-ldp-dod) 00 01 02 03 05 06 07 08 09 RFC 7032

Network Working Group                                   T. Beckhaus, Ed.
Internet-Draft                                       Deutsche Telekom AG
Intended status: Standards Track                             B. Decraene
Expires: January 14, 2014                                         Orange
                                                         K. Tiruveedhula
                                                        Juniper Networks
                                                 M. Konstantynowicz, Ed.
                                                              L. Martini
                                                     Cisco Systems, Inc.
                                                           July 13, 2013


               LDP Downstream-on-Demand in Seamless MPLS
                       draft-ietf-mpls-ldp-dod-09

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, including
   high number of devices, their position in network topology and their
   compute and memory constraints that limit the amount of state access
   devices can hold.This can be achieved with LDP Downstream-on-Demand
   (LDP 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.

Requirements Language

   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 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.





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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on January 14, 2014.

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
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   (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
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Reference Topologies  . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Access Topologies with Static Routing . . . . . . . . . .   5
     2.2.  Access Topologies with Access IGP . . . . . . . . . . . .   7
   3.  LDP DoD Use Cases . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Initial Network Setup . . . . . . . . . . . . . . . . . .   9
       3.1.1.  AN with Static Routing  . . . . . . . . . . . . . . .   9
       3.1.2.  AN with Access IGP  . . . . . . . . . . . . . . . . .  11
     3.2.  Service Provisioning and Activation . . . . . . . . . . .  11
     3.3.  Service Changes and Decommissioning . . . . . . . . . . .  14
     3.4.  Service Failure . . . . . . . . . . . . . . . . . . . . .  14
     3.5.  Network Transport Failure . . . . . . . . . . . . . . . .  15
       3.5.1.  General Notes . . . . . . . . . . . . . . . . . . . .  15
       3.5.2.  AN Node Failure . . . . . . . . . . . . . . . . . . .  15
       3.5.3.  AN/AGN Link Failure . . . . . . . . . . . . . . . . .  16
       3.5.4.  AGN Node Failure  . . . . . . . . . . . . . . . . . .  17
       3.5.5.  AGN Network-side Reachability Failure . . . . . . . .  18
   4.  LDP DoD Procedures  . . . . . . . . . . . . . . . . . . . . .  18
     4.1.  LDP Label Distribution Control and Retention Modes  . . .  19
     4.2.  LDP DoD Session Negotiation . . . . . . . . . . . . . . .  20
     4.3.  Label Request Procedures  . . . . . . . . . . . . . . . .  21
       4.3.1.  Access LSR/ABR Label Request  . . . . . . . . . . . .  21
       4.3.2.  Label Request Retry . . . . . . . . . . . . . . . . .  22
     4.4.  Label Withdraw  . . . . . . . . . . . . . . . . . . . . .  23



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     4.5.  Label Release . . . . . . . . . . . . . . . . . . . . . .  24
     4.6.  Local Repair  . . . . . . . . . . . . . . . . . . . . . .  24
   5.  LDP Extension for LDP DoD Fast-Up Convergence . . . . . . . .  24
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     6.1.  LDP TLV TYPE  . . . . . . . . . . . . . . . . . . . . . .  26
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
     7.1.  LDP DoD Native Security Properties  . . . . . . . . . . .  27
     7.2.  Data Plane Security . . . . . . . . . . . . . . . . . . .  28
     7.3.  Control Plane Security  . . . . . . . . . . . . . . . . .  29
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  30
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   Seamless MPLS design [I-D.ietf-mpls-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, their position
   in network topology and their compute and memory constraints that
   limit the amount of state access devices can hold.

   In general MPLS Label Switching Routers 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.  Document concentrates on the
   unicast connectivity only.  Multicast connectivity is subject for
   further study.

   In Seamless MPLS design [I-D.ietf-mpls-seamless-mpls], IP/MPLS
   protocol optimization is possible due to a relatively simple access
   network topologies.  Examples of such topologies involving access
   nodes (AN) and aggregation nodes (AGN) 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.



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

   The amount of IP RIB and 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 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.

   The amount of MPLS forwarding state however requires additional
   consideration.  In general MPLS routers implement LDP Downstream
   Unsolicited (LDP DU) label advertisement [RFC5036] and advertise MPLS
   labels for all valid routes in their RIB.  This is seen as an
   inadequate approach for ANs, which requires a small subset of the
   total routes (and associated labels) based on the required
   connectivity for the provisioned services.  And although filters can
   be applied to those LDP DU labels 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 access node driven "subscription model"
   for label distribution in the access.  The approach relies on the
   standard LDP Downstream-on-Demand (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 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.

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
   [I-D.ietf-mpls-seamless-mpls] shown in Figure 1

                  +-------+  +-------+  +------+  +------+
               ---+ AGN11 +--+ AGN21 +--+ ABR1 +--+ LSR1 +--> to LSR/AGN
    +--------+/   +-------+  +-------+  +------+  +------+
    | Access |             \/                   \/
    | Network|             /\                   /\
    +--------+    +-------+  +-------+  +------+  +------+



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              \---+ 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 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 use
   either static routing or access IGP ( ISIS 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 [I1] topology.

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



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               +----+                        +-------+

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

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

           (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 unicast (BGP-LU) [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 -----------> <-- BGP LU -->

        (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.






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   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 [I] topology.

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

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

           (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 ECMP (or 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 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.
   Example of such IGP could be ISIS, OSPFv2&v3, RIPv2&RIPng.  This
   access IGP instance is distinct from the IGP of the aggegation
   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.




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   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 -------------> <-- BGP LU -->

              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 -------------> <-- BGP LU -->

              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.




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   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 advertise 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
   [I-D.ietf-mpls-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 PWE3 [RFC4447] and BGP/MPLS IPVPN [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 LSR [RFC3031].

   LDP DoD procedures follow the LDP specification [RFC5036], and are
   equally applicable to LDP IPv4 and IPv6 address families.  For
   simplicity examples are provided for LDP IPv4 address family only.

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 Seamless MPLS network for operational and
   management purposes.  It is assumed that AGN1x has required IP/MPLS
   configuration for network-side connectivity in line with Seamless
   MPLS design [I-D.ietf-mpls-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):






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   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 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.

   Upstream AN/AGN1x requests labels over LDP DoD session(s) from
   downstream AN/AGN1x for configured static routes if those static
   routes are configured with 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 policy on AN/AGN1x.

   Downstream AN/AGN1x responds to the Label Request from the upstream
   AN/AGN1x with a Label Mapping if requested route is present in its
   RIB, and there is a valid label binding from its downstream or it is
   the egress node.  In such case downstream AN/AGN1x installs the
   advertised label as an incoming label in its label table (LIB) and
   its forwarding table (LFIB).  Upstream AN/AGN1x also installs the
   received label as an outgoing label in their 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, it forwards the
   request to its downstream.

   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 install received labels as alternate entries in its LIB
   and LFIB.

   AGN1x node on the network side can use BGP labeled unicast [RFC3107]
   in line with the Seamless MPLS design [I-D.ietf-mpls-seamless-mpls].
   In such a case AGN1x will be redistributing its static routes



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   pointing to local ANs into BGP labeled unicast to facilitate network-
   to-access traffic flows.  Likewise, to facilitate access-to-network
   traffic flows, AGN1x will be responding to access-originated LDP DoD
   label requests with label mappings based on its BGP labeled unicast
   reachability for requested FECs.

3.1.2.  AN with Access IGP

   If access IGP is used, AN(s) advertise their loopbacks over the
   access IGP with configured metrics.  AGN1x advertise a default route
   over the access IGP.

   Routers request labels over LDP DoD session(s) according to their
   needs for MPLS connectivity (LSPs).  In particular if AGNs, as per
   Seamless MPLS design [I-D.ietf-mpls-seamless-mpls], redistribute
   routes from the IGP into BGP labeled unicast [RFC3107], they request
   labels over LDP DoD session(s) for those routes.

   Identically to the static route case, 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), and installs the advertised label as an
   incoming label in its LIB and LFIB.  Upstream AN/AGN1x also installs
   the received label as an outgoing label in their LIB and LFIB.

   Identically to the static route case, in order to facilitate ECMP and
   IPFRR LFA local-repair, 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.

   AGN1x node on the network side can use BGP labeled unicast [RFC3107]
   in line with Seamless MPLS design [I-D.ietf-mpls-seamless-mpls].  In
   such case AGN1x will be redistributing routes received over the
   access IGP (and pointing to local ANs), into BGP labeled unicast to
   facilitate network-to-access traffic flows.  Likewise, to facilitate
   access-to-network traffic flows AGN1x will be responding to access
   originated LDP DoD label requests with label mappings based on its
   BGP labeled unicast 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 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*.




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   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 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.  Targeted LDP session over an associated TCP/IP connection needs
       to be established to the PWE3 destination PE.  This is triggered
       by either an explicit targeted LDP 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 targeted LDP 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/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 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 NLRIs need to be
       exchanged between AN* and its BGP peers.

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





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   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,
   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.  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 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 are configured as equal cost
   paths, AN* sends LDP DoD label requests to both AGN1x devices and
   install 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
   install 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 Equal Cost Multi-Path (ECMP) functionality.
   This involves AN* using 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.





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   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.

   AGN1x node on the network side can use BGP labeled unicast [RFC3107]
   in line with Seamless MPLS design [I-D.ietf-mpls-seamless-mpls].  In
   such case AGN1x will be redistributing its static routes (or routes
   received from the access IGP) pointing to local ANs into BGP labeled
   unicast to facilitate network-to-access traffic flows.  Likewise, to
   facilitate access-to-network traffic flows AGN1x will be responding
   to access originated LDP DoD label requests with label mappings based
   on its BGP labeled unicast reachability for requested FECs.

3.3.  Service Changes and Decommissioning

   Whenever AN* service gets decommissioned or changed and connectivity
   to specific destination is not 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 targeted LDP
   session is not 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,
   local or remote re-configuration can result in specific BGP next-
   hop(s) being no longer needed.  The MPLS LSP(s) to that destination
   is no longer needed either.

   In all of the above cases the following LDP DoD related operations
   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 release it from
      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.



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   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 Section 3.2
   and Section 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.  When the network failure is recovered from, the service is to
   be re-established automatically.

   The following additional LDP-related functions need to be supported
   to comply with Seamless MPLS [I-D.ietf-mpls-seamless-mpls] fast
   service restoration requirements as follows:

   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 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 downstream best next-hop.

3.5.2.  AN Node Failure

   AN node fails and all links to adjacent nodes go down.

   Adjacent AN/AGN1x nodes remove all routes pointing to the failed
   link(s) from their RIB tables (including /32 loopback belonging to



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   the failed AN and any other routes reachable via the failed AN).
   This in turn triggers the removal of associated outgoing /32 FEC
   labels from their LIB and LFIB tables.

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

   If a specific /32 FEC(s) is not reachable anymore from those AN/
   AGN1x, they also send LDP Label Withdraw to their upstream LSRs to
   notify about the failure, and remove the associated incoming label(s)
   from their LIB and LFIB tables.  Upstream LSRs upon receiving Label
   Withdraw remove the signaled labels from their LIB/LFIB tables, and
   propagate LDP Label Withdraw across their upstream LDP DoD sessions.

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

   AGN1x gets notified about the AN failure via either 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 access IGP case) and labels from its LIB and LFIB tables,
   and propagating the failure on the network side using BGP-LU and/or
   core IGP/LDP-DU procedures.

   Upon 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.








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   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 tabels, 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 - AN/AGN1x node stops using the failed link and
       immediately switchover (local-repair) to the remaining ECMP path
       or alternate path.  AN/AGN1x removes affected next-hops and
       labels from its tables and invoke LDP Label Withdraw as per point
       (a) above.  If there is an AGN1x node terminating the failed
       link, it removes routes pointing to the failed link immediately
       from the RIB, remove associated labels from their LIB and LFIB
       tabels, and propagate the failure on the network side using BGP-
       LU 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
   downstream AN/AGN1x nodes.

3.5.4.  AGN Node Failure

   AGN1x fails and all links to adjacent access nodes go down.

   Depending on the access topology, following cases apply to the
   network operation after AGN1x node failure (topology references from
   Section 2 in square brackets):

   a.  [I1, I] - ANs are isolated from the network - AN adjacent to the
       failure removes routes pointing to the failed AGN1x node
       immediately from the RIB, removes associated labels from their
       LIB and LFIB tabels, and sends LDP label withdraw(s) to their
       upstream LSRs.  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 switchover (local-repair) to the remaining ECMP path
       or alternate path.  AN removes affected routes and labels from
       its tables and invoke LDP Label Withdraw as per point (a) above.

   Network side procedures for handling AGN1x node failure have been
   described in Seamless MPLS [I-D.ietf-mpls-seamless-mpls].




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3.5.5.  AGN Network-side Reachability Failure

   AGN1x loses network reachability to a specific destination or set of
   network-side destinations.

   In such event 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.  In turn ANs also send Label Withdraw messages for
   affected /32 FECs to their upstream ANs.

   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

   Label Distribution Protocol is specified in [RFC5036], and all LDP
   Downstream-on-Demand implementations follow [RFC5036] specification.
   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
   upstream to 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









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4.1.  LDP Label Distribution Control and Retention Modes

   LDP protocol specification [RFC5036] defines two modes for label
   distribution control, following the definitions in 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
   upstream LSR to be notified about the downstream failure and for an
   application using the LSP to switchover to an alternate path, even if
   such a path exists.

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

   o  Conservative mode - If operating in Downstream on Demand mode, an
      LSR will request label mappings only from the next hop LSR
      according to routing.  The main advantage of the conservative 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 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 mode - When operating in Downstream on Demand mode with
      Liberal Label retention, 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 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



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   that are not used for forwarding.  This in turn 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 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 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 get requested only after the access IGP
   convergence.

   Adhering to the overall design goals of Seamless MPLS
   [I-D.ietf-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 - enables pre-programming of alternate
      next-hops with associated FEC labels.

   In Seamless MPLS [I-D.ietf-mpls-seamless-mpls] AGN1x node acts as an
   access ABR connecting access and metro domains.  To enable failure
   propagation between those domains, access ABR implements ordered
   label distribution control when redistributing routes/FEC between the
   access-side (using LDP DoD and static or access IGP) and the network-
   side ( using BGP labeled unicast [RFC3107] or core IGP with LDP
   Downstream Unsolicited label advertisement.

4.2.  LDP DoD Session Negotiation

   Access LSR/ABR propose the Downstream-on-Demand label advertisement
   by setting "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 LDP protocol specification
   [RFC5036].

   To establish a Downstream-on-Demand session between the two access
   LSR/ABRs, both propose the Downstream-on-Demand label advertisement
   mode in the Initialization message.  If the access LSR only supports
   LDP DoD and the access ABR proposes Downstream Unsolicited mode, the
   access LSR sends a Notification message with status "Session Rejected



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   /Parameters Advertisement Mode" and then close the LDP session as
   specified in LDP protocol 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 protocol specification [RFC5036] with 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 LDP DoD label
   advertisement has been negotiated for IPv4 and IPv6 LDP FECs as
   described earlier, LDP session uses 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

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

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

   b.  Access LSR/ABR is configured with a service in line with service
       use cases described in Section 3.2 and Section 3.3.

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

   d.  Configuration with access IGP - Access LSR/ABR link to adjacent
       node comes up and LDP DoD session is established.  In this case
       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 next-hop LSR(s) and
       alternate LSR(s).





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

   a.  Downstream access LSR/ABR - requested FEC is an IGP or static
       route and there is an LDP label already learnt 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, downstream LSR sends a Label
       Request message for the same FEC to the next-next-hop downstream
       LSR.  In such case 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 - requested FEC is a BGP labelled
       unicast route [RFC3107] and this BGP route is the best selected
       for this FEC.

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

4.3.2.  Label Request Retry

   Following LDP specification 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 algoritm mentioned in the LDP session
   negotiation described in Section 4.2.

   If there is no response to the sent Label Request message, the LDP
   specification [RFC5036] (section A.1.1, page# 100) states that the
   LSR does not send another request for the same label to the peer and
   mandates that a duplicate Label Request is considered a protocol
   error and is 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 again.

   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 LDP specification [RFC5036], a new LDP TLV
   extension is proposed and described in Section 5.



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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.

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

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

   b.  If 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 downstream LDP session is lost.

   c.  If 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 LDP DoD ingress label is associated with an outgoing label
       assigned by LDP (DoD or DU), route next-hop changed and

       *  there is no LDP session to the new next-hop.  To minimize
          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 downstream LSR.  See
          note below.

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

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

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

   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, downstream access LSR/
   ABR is to attempt to converge the LSP locally before withdrawing the
   label from an upstream access LSR/ABR.  For example if the next-hop



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   changes for a particular FEC and if the new next-hop allocates labels
   by LDP DoD session, then the downstream access LSR/ABR sends a Label
   Request on the new next-hop session.  If downstream access LSR/ABR
   doesn't get Label Mapping for some duration, then and only then
   downstream access LSR/ABR withdraws the upstream label.

4.5.  Label Release

   If an access LSR/ABR does not need any longer 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 upstream access LSR/ABR receives an unsolicited Label Mapping on
   DoD session, they release the label by sending Label Release message.

   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 route next-hop 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, 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, 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.  If access LSR/ABR
   doesn't already know those labels, it requests them.

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

5.  LDP Extension for LDP DoD Fast-Up Convergence




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   In some conditions, the exponential backoff algorithm usage described
   in Section 4.3.2 can result in a longer than desired wait time 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 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 the improvement in 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 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 requested route is not available, the downstream
       LSR MUST ignore this unknown TLV and send a "no route"
       notification back. 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 F-bit being clear (=0), the unknown TLV is not
       forwarded with the containing message.

    Type
       Queue Request Type value to be allocated by IANA.



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


   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
   route is available and if so it replies with 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 to
   remove the original request from the queue and send back a
   notification Label Request Aborted [RFC5036].

   If the downstream LSR does not support the Queue Request TLV, and
   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 standard LDP specification LDP
   specification [RFC5036].

   This described procedure ensures backward compatitibility.

6.  IANA Considerations

6.1.  LDP TLV TYPE

   This document uses a new a new Optional Parameter Queue Request TLV
   in the Label Request message defined in Section 5.  IANA already
   maintains a registry of name LDP "TLV TYPE NAME SPACE" defined by
   RFC5036.  The following value is suggested for assignment:

                        TLV type  Description
                        0x0971    Queue Request TLV


7.  Security Considerations




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   MPLS LDP Downstream on Demand deployment in the access network is
   subject to similar security threats as any MPLS LDP deployment.  It
   is recommended that baseline security measures are 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 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, routes
   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 Downstream on Demand operation is request driven and
   unsolicited label mappings are not accepted by upstream LSR by
   design.  This inherently limits the potential of an unauthorized
   third party injecting unsolicited label mappings on the wire.

   This native security property enables 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 when 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 a privacy of a VPN), such Access
   LSR would also have to influence the routing decision for affected
   FEC on the ABR LSR to attract the flows.  Following measures need to
   be considered on ABR LSR to prevent such event from occurring:

   a.  Access with static routes - Access LSR can not influence ABR LSR
       routing decisions due to static nature of routing configuration,
       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



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       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 be quickly detected by upstream Access LSRs not receiving
       traffic and LDP TCP session being lost.

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

   Following two sections describe other security considerations
   applicable to general MPLS deployments in the access.

7.2.  Data Plane Security

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

   a.  Labelled packets from specific Access LSR are sent to an
       unauthorized destination.

   b.  Unlabelled packets are sent by Access LSR to remote network
       nodes.

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

   1.  addressing (a) - Access and ABR LSRs are not accepting 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 described in
       Section 7.3, and the top label has been distributed to the
       upstream neighbour by the receiving Access or ABR LSR.

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





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   3.  addressing (a) - use control plane authentication described in
       Section 7.3.

   4.  addressing (b) - ABR LSR restricts IP network reachability to and
       from the Access LSR.

7.3.  Control Plane Security

   Similarly to Inter-AS MPLS/VPN deployments [RFC4364], the control
   plane security is prerequisite to the 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 sesions is analyzed in LDP specification
   [RFC5036] and in Analysis of BGP, LDP, PCEP and MSDP Issues According
   to KARP Design Guide [I-D.ietf-karp-routing-tcp-analysis].  Both
   documents state that LDP is subject to two different types of attacks
   - spoofing and denial of service attacks.

   Threat of spoofed LDP Hello messages can be reduced by following
   guidelines listed in LDP specification [RFC5036]: accepting Basic
   Hellos only on interfaces connected to trusted LSRs, ignoring Basic
   Hellos that are not addressed to All Routers on this Subnet multicast
   group, using access lists.  LDP Hello messages can be also secured
   using an optional Cryptographic Authentication TLV specified in LDP
   Hello Cryptographic Authentication
   [I-D.ietf-mpls-ldp-hello-crypto-auth], what further reduces the
   threat of spoofing during LDP discovery phase.

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

   The threat of the Denial of Service targetting well-known UDP port
   for LDP discovery and TCP port for LDP session establishment can be
   reduced by following the guidelines listed in [RFC5036] and in
   [I-D.ietf-karp-routing-tcp-analysis].

   Access IGP (if used) and any routing protocols used in access network
   for signaling service routes needs also to be secured following
   routing protocol security best practices.  Refer to KARP IS-IS
   security analysis [I-D.ietf-karp-isis-analysis] and Analysis of OSPF



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   Security According to KARP Design Guide [RFC6863] 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, 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

   [I-D.ietf-karp-isis-analysis]
              Chunduri, U., Tian, A., and W. Lu, "KARP IS-IS security
              analysis", draft-ietf-karp-isis-analysis-00 (work in
              progress), March 2013.

   [I-D.ietf-karp-routing-tcp-analysis]
              Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP and MSDP Issues According to KARP Design
              Guide", draft-ietf-karp-routing-tcp-analysis-07 (work in
              progress), April 2013.



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   [I-D.ietf-mpls-ldp-hello-crypto-auth]
              Zheng, L., Chen, M., and M. Bhatia, "LDP Hello
              Cryptographic Authentication", draft-ietf-mpls-ldp-hello-
              crypto-auth-01 (work in progress), January 2013.

   [I-D.ietf-mpls-seamless-mpls]
              Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
              M., and D. Steinberg, "Seamless MPLS Architecture", draft-
              ietf-mpls-seamless-mpls-03 (work in progress), May 2013.

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

   [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.

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






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   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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