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RFC 7916

Proposed STD
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Operational Management of Loop-Free Alternates

 


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Internet Engineering Task Force (IETF)                 S. Litkowski, Ed.
Request for Comments: 7916                                   B. Decraene
Category: Standards Track                                         Orange
ISSN: 2070-1721                                              C. Filsfils
                                                                 K. Raza
                                                           Cisco Systems
                                                            M. Horneffer
                                                        Deutsche Telekom
                                                               P. Sarkar
                                                  Individual Contributor
                                                               July 2016


             Operational Management of Loop-Free Alternates

Abstract

   Loop-Free Alternates (LFAs), as defined in RFC 5286, constitute an IP
   Fast Reroute (IP FRR) mechanism enabling traffic protection for IP
   traffic (and, by extension, MPLS LDP traffic).  Following early
   deployment experiences, this document provides operational feedback
   on LFAs, highlights some limitations, and proposes a set of
   refinements to address those limitations.  It also proposes required
   management specifications.

   This proposal is also applicable to remote-LFA solutions.

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 7841.

   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/rfc7916.

[Page 2] 
Copyright Notice

   Copyright (c) 2016 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.

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Table of Contents

   1. Introduction ....................................................4
      1.1. Requirements Language ......................................4
   2. Definitions .....................................................4
   3. Operational Issues with Default LFA Tiebreakers .................5
      3.1. Case 1: PE Router Protecting against Failures
           within Core Network ........................................5
      3.2. Case 2: PE Router Chosen to Protect against Core
           Failures while P Router LFA Exists .........................7
      3.3. Case 3: Suboptimal P Router Alternate Choice ...............8
      3.4. Case 4: No-Transit LFA Computing Node ......................9
   4. Need for Coverage Monitoring ....................................9
   5. Need for LFA Activation Granularity ............................10
   6. Configuration Requirements .....................................11
      6.1. LFA Enabling/Disabling Scope ..............................11
      6.2. Policy-Based LFA Selection ................................12
           6.2.1. Connected versus Remote Alternates .................12
           6.2.2. Mandatory Criteria .................................13
           6.2.3. Additional Criteria ................................14
           6.2.4. Evaluation of Criteria .............................14
           6.2.5. Retrieving Alternate Path Attributes ...............18
           6.2.6. ECMP LFAs ..........................................23
   7. Operational Aspects ............................................24
      7.1. No-Transit Condition on LFA Computing Node ................24
      7.2. Manual Triggering of FRR ..................................25
      7.3. Required Local Information ................................26
      7.4. Coverage Monitoring .......................................26
      7.5. LFAs and Network Planning .................................27
   8. Security Considerations ........................................28
   9. References .....................................................28
      9.1. Normative References ......................................28
      9.2. Informative References ....................................30
   Contributors ......................................................31
   Authors' Addresses ................................................31

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1.  Introduction

   Following the first deployments of Loop-Free Alternates (LFAs), this
   document provides feedback to the community about the management
   of LFAs.

   o  Section 3 provides real use cases illustrating some limitations
      and suboptimal behavior.

   o  Section 4 provides requirements for LFA simulations.

   o  Section 5 proposes requirements for activation granularity and
      policy-based selection of the alternate.

   o  Section 6 expresses requirements for the operational management of
      LFAs and, in particular, a policy framework to manage alternates.

   o  Section 7 details some operational considerations of LFAs, such as
      IS-IS overload bit management and troubleshooting information.

1.1.  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].

2.  Definitions

   o  Per-prefix LFA computation: Evaluation for the best alternate is
      done for each destination prefix, as opposed to the "per-next-hop"
      simplification technique proposed in Section 3.8 of [RFC5286].

   o  PE router: Provider Edge router.  These routers connect customers
      to each other.

   o  P router: Provider router.  These routers are core routers without
      customer connections.  They provide transit between PE routers,
      and they form the core network.

   o  Core network: subset of the network composed of P routers and
      links between them.

   o  Core link: network link part of the core network, i.e., a link
      between P routers.

   o  Link-protecting LFA: alternate providing protection against link
      failure.

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   o  Node-protecting LFA: alternate providing protection against node
      failure.

   o  Connected alternate: alternate adjacent (at the IGP level) to the
      Point of Local Repair (PLR) (i.e., an IGP neighbor).

   o  Remote alternate: alternate that does not share an IGP adjacency
      with the PLR.

3.  Operational Issues with Default LFA Tiebreakers

   [RFC5286] introduces the notion of tiebreakers when selecting the LFA
   among multiple candidate alternate next hops.  When multiple LFAs
   exist, [RFC5286] has favored the selection of the LFA that provides
   the best coverage against the failure cases.  While this is indeed a
   goal, it is one among multiple goals, and in some deployments this
   leads to the selection of a suboptimal LFA.  The following sections
   detail real use cases related to such limitations.

   Note that the use case for LFA computation per destination
   (per-prefix LFA) is assumed throughout this analysis.  We also assume
   in the network figures that all IP prefixes are advertised with
   zero cost.

3.1.  Case 1: PE Router Protecting against Failures within Core Network

         P1 --------- P2 ---------- P3 --------- P4
         |      1           100           1       |
         |                                        |
         | 100                                    | 100
         |                                        |
         |      1           100           1       |  1     5k
         P5 --------- P6 ---------- P7 --------- P8 --- P9 -- PE1
         | |         | |            |             |
       5k| |5k     5k| |5k          | 5k          | 5k
         | |         | |            |             |
         | +-- PE4 --+ |            +---- PE2 ----+
         |             |                   |
         +---- PE5 ----+                   | 5k
                                           |
                                          PE3

         Px routers are P routers using n * 10 Gbps links.
         PEs are connected using links with lower bandwidth.

                                 Figure 1

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   In Figure 1, let us consider the traffic flowing from PE1 to PE4.
   The nominal path is P9-P8-P7-P6-PE4.  Let us now consider the failure
   of link P7-P8.  As the P4 primary path to PE4 is P8-P7-P6-PE4, P4 is
   not an LFA for P8 (because P4 will loop traffic back to P8), and the
   only available LFA is PE2.

   When the core link P8-P7 fails, P8 switches all traffic destined to
   PE4/PE5 towards the node PE2.  Hence, a PE node and PE links are used
   to protect against the failure of a core link.  Typically, PE links
   have less capacity than core links, and congestion may occur on PE2
   links.  Note that although PE2 is not directly affected by the
   failure, its links become congested, and its traffic will suffer from
   the congestion.

   In summary, in the case of P8-P7 link failure, the impact on customer
   traffic is:

   o  From PE2's point of view:

      *  without LFA: no impact.

      *  with LFA: traffic is partially dropped (but possibly
         prioritized by a QoS mechanism).  It must be highlighted that
         in such a situation, traffic not affected by the failure may be
         affected by the congestion.

   o  From P8's point of view:

      *  without LFA: traffic is totally dropped until convergence
         occurs.

      *  with LFA: traffic is partially dropped (but possibly
         prioritized by a QoS mechanism).

   Besides the congestion aspects of using a PE router as an alternate
   to protect against a core failure, a service provider may consider
   this to be a bad routing design and would want to prevent it.

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3.2.  Case 2: PE Router Chosen to Protect against Core Failures while
      P Router LFA Exists

          P1 --------- P2 ------------ P3 ------- P4
          |      1           100       |     1    |
          |                            |          |
          | 100                        | 30       | 30
          |                            |          |
          |     1         50       50  |    10    |   1    5k
          P5 --------- P6 --- P10 ---- P7 ------- P8 --- P9 -- PE1
          | |         | |        \                |
        5k| |5k     5k| |5k       \ 5k            | 5k
          | |         | |          \              |
          | +-- PE4 --+ |           +---- PE2 ----+
          |             |                  |
          +---- PE5 ----+                  | 5k
                                           |
                                          PE3

             Px routers are P routers meshed with n * 10 Gbps links.
             PEs are meshed using links with lower bandwidth.

                                 Figure 2

   In Figure 2, let us consider the traffic coming from PE1 to PE4.  The
   nominal path is P9-P8-P7-P10-P6-PE4.  Let us now consider the failure
   of the link P7-P8.  For P8, P4 is a link-protecting LFA and PE2 is a
   node-protecting LFA.  PE2 is chosen as the best LFA, due to the
   better type of protection that it provides.  Just as in case 1, this
   may lead to congestion on PE2 links upon LFA activation.

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3.3.  Case 3: Suboptimal P Router Alternate Choice

                             +--- PE3 ---+
                            /             \
                      1000 /               \ 1000
                          /                 \
                  +----- P1 ---------------- P2 ----+
                  |      |        500        |      |
                  | 10   |                   |      | 10
                  |      |                   |      |
                  R5     | 10                | 10   R7
                  |      |                   |      |
                  | 10   |                   |      | 10
                  |      |        500        |      |
                  +---- P3 ----------------- P4 ----+
                          \                 /
                      1000 \               / 1000
                            \             /
                             +--- PE1 ---+

                   Px routers are P routers.
                   P1-P2 and P3-P4 links are 1 Gbps links.
                   All other inter-Px links are 10 Gbps links.

                                 Figure 3

   In Figure 3, let us consider the failure of link P1-P3.  For
   destination PE3, P3 has two possible alternates:

   o  P4, which is node-protecting

   o  R5, which is link-protecting

   P4 is chosen as the best LFA, due to the better type of protection
   that it provides.  However, for bandwidth capacity reasons, it
   may not be desirable to use P4.  A service provider may prefer to use
   high-bandwidth links as the preferred LFA.  In this example,
   preferring the shortest path over the type of protection may achieve
   the expected behavior, but in cases where metrics do not reflect the
   bandwidth, this technique would not work and some other criteria
   would need to be involved when selecting the best LFA.

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3.4.  Case 4: No-Transit LFA Computing Node

                               P1       P2
                               |   \  /   |
                            50 | 50 \/ 50 | 50
                               |    /\    |
                               PE1-+  +-- PE2
                                \        /
                              45 \      / 45
                                  -PE3-
                         (No-transit condition set)

                                 Figure 4

   The IS-IS and OSPF protocols define some way to prevent a router from
   being used for transit.

   The IS-IS overload bit is defined in [ISO10589], and the OSPF R-bit
   is defined in [RFC5340].  Also, the OSPF stub router is defined in
   [RFC6987] as a method to prevent transit on a node by advertising
   MaxLinkMetric on all non-stub links.

   In Figure 4, PE3 has its no-transit condition set (permanently, for
   design reasons) and wants to protect traffic using an LFA for
   destination PE2.

   On PE3, the loop-free condition is not satisfied: 100 !< 45 + 45.
   PE1 is thus not considered as an LFA.  However, thanks to the
   no-transit condition on PE3, we know that PE1 will not loop the
   traffic back to PE3.  So, PE1 is an LFA to reach PE2.

   In the case of a no-transit condition set on a node, LFA behavior
   must be clarified.

4.  Need for Coverage Monitoring

   As per [RFC6571], LFA coverage depends strongly on the network
   topology that is in use.  Even if the remote-LFA mechanism [RFC7490]
   significantly extends the coverage of the basic LFA specification,
   there are still some cases where protection would not be available.
   As network topologies are constantly evolving (network extension,
   additional capacity, latency optimization, etc.), the protection
   coverage may change.  Fast Reroute (FRR) functionality may be
   critical for some services supported by the network; a service
   provider must always know what type of protection coverage is
   currently available on the network.  Moreover, predicting protection
   coverage in the event of network topology changes is mandatory.

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   Today, network simulation tools associated with "what if" scenarios
   are often used by service providers for the overall network design
   (capacity, path optimization, etc.).  Sections 7.3, 7.4, and 7.5 of
   this document propose the addition of LFA information into such tools
   and within routers, so that a service provider may be able to:

   o  evaluate protection coverage after a topology change.

   o  adjust the topology change to cover the primary need (e.g.,
      latency optimization, bandwidth increase) as well as LFA
      protection.

   o  constantly monitor the LFA coverage in the live network and
      receive alerts.

   Documentation of LFA selection algorithms by implementers (default
   and tuning options) is important in order to make it possible for
   third-party modules to model these policy-based LFA selection
   algorithms.

5.  Need for LFA Activation Granularity

   As in all FRR mechanisms, an LFA installs backup paths in the
   Forwarding Information Base (FIB).  Depending on the hardware used by
   a service provider, FIB resources may be critical.  Activating LFAs
   by default on all available components (IGP topologies, interfaces,
   address families, etc.) may lead to a waste of FIB resources, as
   generally only a few destinations in a network should be protected
   (e.g., loopback addresses supporting MPLS services) compared to the
   number of destinations in the Routing Information Base (RIB).

   Moreover, a service provider may implement multiple different FRR
   mechanisms in its networks for different applications (e.g.,
   Maximally Redundant Trees (MRTs), TE FRR).  In this scenario, an
   implementation MAY allow the computation of alternates for a specific
   destination even if the destination is already protected by another
   mechanism.  This will provide redundancy and permit the operator to
   select the best option for FRR, using a policy language.

   Section 6 provides some implementation guidelines.

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6.  Configuration Requirements

   Controlling the selection of the best alternate and the granularity
   of LFA activation is a requirement for service providers.  This
   section defines configuration requirements for LFAs.

6.1.  LFA Enabling/Disabling Scope

   The granularity of LFA activation SHOULD be controlled (as alternate
   next hops consume memory in the forwarding plane).

   An implementation of an LFA SHOULD allow its activation, with the
   following granularities:

   o  Per routing context: Virtual Routing and Forwarding (VRF),
      virtual/logical router, global routing table, etc.

   o  Per interface.

   o  Per protocol instance, topology, area.

   o  Per prefix: Prefix protection SHOULD have a higher priority
      compared to interface protection.  This means that if a specific
      prefix must be protected due to a configuration request, an LFA
      MUST be computed and installed for that prefix even if the primary
      outgoing interface is not configured for protection.

   An implementation of an LFA MAY allow its activation, with the
   following criteria:

   o  Per address family: IPv4 unicast, IPv6 unicast.

   o  Per MPLS control plane: For MPLS control planes that inherit
      routing decisions from the IGP routing protocol, the MPLS
      data plane may be protected by an LFA.  The implementation may
      allow an operator to control this inheritance of protection from
      the IP prefix to the MPLS label bound to this prefix.  The
      inheritance of protection will concern IP-to-MPLS, MPLS-to-MPLS,
      and MPLS-to-IP entries.  As an example, LDP and Segment Routing
      extensions [SEG-RTG-ARCH] for IS-IS and OSPF are control-plane
      eligible for this inheritance of protection.

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6.2.  Policy-Based LFA Selection

   When multiple alternates exist, the LFA selection algorithm is based
   on tiebreakers.  Current tiebreakers do not provide sufficient
   control regarding how the best alternate is chosen.  This document
   proposes an enhanced tiebreaker allowing service providers to manage
   all specific cases:

   1.  An LFA implementation SHOULD support policy-based decisions for
       determining the best LFA.

   2.  Policy-based decisions SHOULD be based on multiple criteria, with
       each criterion having a level of preference.

   3.  If the defined policy does not allow the determination of a
       unique best LFA, an implementation SHOULD pick only one based on
       its own decision.  For load-balancing purposes, an implementation
       SHOULD also support the election of multiple LFAs.

   4.  The policy SHOULD be applicable to a protected interface or a
       specific set of destinations.  In the case of applicability to
       the protected interface, all destinations primarily routed on
       that interface SHOULD use the policy for that interface.

   5.  The choice of whether or not to dynamically re-evaluate policy
       (in the event of a policy change) is left to the implementation.
       If a dynamic approach is chosen, the implementation SHOULD
       recompute the best LFAs and reinstall them in the FIB without
       service disruption.  If a non-dynamic approach is chosen, the
       policy would be taken into account upon the next IGP event.  In
       this case, the implementation SHOULD support a command to
       manually force the recomputation/reinstallation of LFAs.

6.2.1.  Connected versus Remote Alternates

   In addition to connected LFAs, tunnels (e.g., IP, LDP, RSVP-TE,
   Segment Routing) to distant routers may be used to complement LFA
   coverage (tunnel tail used as virtual neighbor).  When a router has
   multiple alternate candidates for a specific destination, it may have
   connected alternates and remote alternates (reachable via a tunnel).
   Connected alternates may not always provide an optimal routing path,
   and it may be preferable to select a remote alternate over a
   connected alternate.  Some uses of tunnels to extend LFA [RFC5286]
   coverage are described in [RFC7490] and [TI-LFA].  [RFC7490] and
   [TI-LFA] present some use cases for LDP tunnels and Segment Routing
   tunnels, respectively.  This document considers any type of tunneling
   techniques to reach remote alternates (IP, Generic Routing

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   Encapsulation (GRE), LDP, RSVP-TE, the Layer 2 Tunneling Protocol
   (L2TP), Segment Routing, etc.) and does not restrict the remote
   alternates to the uses presented in these other documents.

   In Figure 1, there is no P router alternate for P8 to reach PE4 or
   PE5, so P8 is using PE2 as an alternate; this may generate congestion
   when FRR is activated.  Instead, we could have a remote alternate for
   P8 to protect traffic to PE4 and PE5.  For example, a tunnel from P8
   to P3 (following the shortest path) can be set up, and P8 would be
   able to use P3 as a remote alternate to protect traffic to PE4 and
   PE5.  In this scenario, traffic will not use a PE link during FRR
   activation.

   When selecting the best alternate, the selection algorithm MUST
   consider all available alternates (connected or tunnel).  For
   example, with remote LFAs, computation of PQ sets [RFC7490] SHOULD be
   performed before the selection of the best alternate.

6.2.2.  Mandatory Criteria

   An LFA implementation MUST support the following criteria:

   o  Non-candidate link: A link marked as "non-candidate" will never be
      used as an LFA.

   o  A primary next hop being protected by another primary next hop of
      the same prefix (ECMP case).

   o  Type of protection provided by the alternate: link protection or
      node protection.  In the case of preference for node protection,
      an implementation SHOULD support fallback to link protection if
      node protection is not available.

   o  Shortest path: lowest IGP metric used to reach the destination.

   o  Shared Risk Link Groups (SRLGs) (as defined in Section 3 of
      [RFC5286]; see also Section 6.2.4.1 for more details).

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6.2.3.  Additional Criteria

   An LFA implementation SHOULD support the following criteria:

   o  A downstream alternate: Preference for a downstream path over a
      non-downstream path SHOULD be configurable.

   o  Link coloring with "include", "exclude", and preference-based
      systems (see Section 6.2.4.2).

   o  Link bandwidth (see Section 6.2.4.3).

   o  Alternate preference / node coloring (see Section 6.2.4.4).

6.2.4.  Evaluation of Criteria

6.2.4.1.  SRLGs

   Section 3 of [RFC5286] proposes the reuse of GMPLS IGP extensions to
   encode SRLGs [RFC5307] [RFC4203].  Section 3 of [RFC5286] also
   describes the algorithm to compute SRLG protection.

   When SRLG protection is computed, an implementation SHOULD allow the
   following:

   o  Exclusion of alternates in violation of SRLGs.

   o  Maintenance of a preference system between alternates based on
      SRLG violations.  How the preference system is implemented is out
      of scope for this document, but here are two examples:

      *  Preference based on the number of violations.  In this case,
         more violations = less preferred.

      *  Preference based on violation cost.  In this case, each SRLG
         violation has an associated cost.  The lower violation costs
         are preferred.

   When applying SRLG criteria, the SRLG violation check SHOULD be
   performed on sources to alternates as well as alternates to
   destination paths, based on the SRLG set of the primary path.  In the
   case of remote LFAs, PQ-to-destination path attributes would be
   retrieved from the Shortest Path Tree (SPT) rooted at the PQ.

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6.2.4.2.  Link Coloring

   Link coloring is a powerful system to control the choice of
   alternates.  Link colors are markers that will allow the encoding of
   properties of a particular link.  Protecting interfaces are tagged
   with colors.  Protected interfaces are configured to include some
   colors with a preference level and exclude others.

   Link color information SHOULD be signaled in the IGP, and
   administrative-group IGP extensions [RFC5305] [RFC3630] that are
   already standardized, implemented, and widely used SHOULD be used for
   encoding and signaling link colors.

                                    PE2
                                    |  +---- P4
                                    | /
                           PE1 ---- P1 --------- P2
                                    |     10 Gbps
                             1 Gbps |
                                    |
                                    P3

                                 Figure 5

   In the example in Figure 5, the P1 router is connected to three P
   routers and two PEs.  P1 is configured to protect the P1-P4 link.  We
   assume that, given the topology, all neighbors are candidate LFAs.
   We would like to enforce a policy in the network where only a core
   router may protect against the failure of a core link and where
   high-capacity links are preferred.

   In this example, we can use the proposed link coloring by:

   o  Marking the PE links with the color RED.

   o  Marking the 10 Gbps core link with the color BLUE.

   o  Marking the 1 Gbps core link with the color YELLOW.

   o  Configuring the protected interface P1->P4 as follows:

      *  Include BLUE, preference 200.

      *  Include YELLOW, preference 100.

      *  Exclude RED.

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   Using this, PE links will never be used to protect against P1-P4 link
   failure, and the 10 Gbps link will be preferred.

   The main advantage of this solution is that it can easily be
   duplicated on other interfaces and other nodes without change.  A
   service provider has only to define the color system (associate a
   color with a level of significance), as it is done already for TE
   affinities or BGP communities.

   An implementation of link coloring:

   o  SHOULD support multiple "include" and "exclude" colors on a single
      protected interface.

   o  SHOULD provide a level of preference between included colors.

   o  SHOULD support the configuration of multiple colors on a single
      protecting interface.

6.2.4.3.  Bandwidth

   As mentioned in previous sections, not taking into account the
   bandwidth of an alternate could lead to congestion during FRR
   activation.  We propose that the bandwidth criteria be based on the
   link speed information, for the following reasons:

   o  If a router S has a set of X destinations primarily forwarded to
      N, using per-prefix LFAs may lead to having a subset of X
      protected by a neighbor N1, another subset by N2, another subset
      by Nx, etc.

   o  S is not aware of traffic flows to each destination, so in the
      case of FRR activation, S is not able to evaluate how much traffic
      will be sent to N1, N2, Nx, etc.

   Based on this, it is not useful to gather available bandwidth on
   alternate paths, as the router does not know how much bandwidth it
   requires for protection.  The proposed link speed approach provides a
   good approximation at low cost, as information is easily available.

   The bandwidth criteria of the policy framework SHOULD work in at
   least the following two ways:

   o  Prune: Exclude an LFA if the link speed to reach it is lower than
      the link speed of the primary next-hop interface.

   o  Prefer: Prefer an LFA based on its bandwidth to reach it compared
      to the link speed of the primary next-hop interface.

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6.2.4.4.  Alternate Preference / Node Coloring

   Rather than tagging interfaces on each node (using link colors) to
   identify the types of alternate nodes (as an example), it would be
   helpful if routers could be identified in the IGP.  This would allow
   grouped processing on multiple nodes.  As an implementation needs to
   exclude some specific alternates (see Section 6.2.3), an
   implementation SHOULD be able to:

   o  give preference to a specific alternate.

   o  give preference to a group of alternates.

   o  exclude a specific alternate.

   o  exclude a group of alternates.

   A specific alternate may be identified by its interface, IP address,
   or router ID, and a group of alternates may be identified by a marker
   (tag) advertised in IGP.  The IGP encoding and signaling for marking
   groups of alternates SHOULD be done according to [RFC7917] and
   [RFC7777].  Using a tag/marker is referred to as "node coloring", as
   compared to the link coloring option presented in Section 6.2.4.2.

   Consider the following network:

                                  PE3
                                  |
                                  |
                                  PE2
                                  |   +---- P4
                                  |  /
                         PE1 ---- P1 -------- P2
                                  |    10 Gbps
                           1 Gbps |
                                  |
                                  P3

                                 Figure 6

   In the example above, each node is configured with a specific tag
   flooded through the IGP.

   o  PE1,PE3: 200 (non-candidate).

   o  PE2: 100 (edge/core).

   o  P1,P2,P3: 50 (core).

Top      ToC       Page 18 
   A simple policy could be configured on P1 to choose the best
   alternate for P1->P4 based on the function or role of the router,
   as follows:

   o  criterion 1 -> alternate preference: exclude tags 100 and 200.

   o  criterion 2 -> bandwidth.

6.2.5.  Retrieving Alternate Path Attributes

6.2.5.1.  Alternate Path

   The alternate path is composed of two distinct parts: PLR to
   alternate and alternate to destination.

                             N1 -- R1 ---- R2
                            /50     \       \
                           /         R3 --- R4
                          /                   \
                          S -------- E ------- D
                          \\                  //
                           \\                //
                            N2 ---- PQ ---- R5

                                 Figure 7

   In Figure 7, we consider a primary path from S to D, with S using E
   as the primary next hop.  All metrics are 1, except that {S,N1} = 50.
   Two alternate paths are available:

   o  {S,N1,R1,R2|R3,R4,D}, where N1 is a connected alternate.  This
      consists of two sub-paths:

      *  {S,N1}: path from the PLR to the alternate.

      *  {N1,R1,R2|R3,R4,D}: path from the alternate to the destination.

   o  {S,N2,PQ,R5,D}, where the PQ is a remote alternate.  Again, the
      path consists of two sub-paths:

      *  {S,N2,PQ}: path from the PLR to the alternate.

      *  {PQ,R5,D}: path from the alternate to the destination.

   As displayed in Figure 7, some parts of the alternate path may fan
   out to multiple paths due to ECMP.

Top      ToC       Page 19 
6.2.5.2.  Alternate Path Attributes

   Some criteria listed in the previous sections require the retrieval
   of some characteristics of the alternate path (SRLG, bandwidth,
   color, tag, etc.).  We call these characteristics "path attributes".
   A path attribute can record a list of node properties (e.g., node
   tag) or link properties (e.g., link color).

   This document defines two types of path attributes:

   o  Cumulative attribute: When a path attribute is cumulative, the
      implementation SHOULD record the value of the attribute on each
      element (link and node) along the alternate path.  SRLG, link
      color, and node color are cumulative attributes.

   o  Unitary attribute: When a path attribute is unitary, the
      implementation SHOULD record the value of the attribute only on
      the first element along the alternate path (first node, or first
      link).  Bandwidth is a unitary attribute.

                             N1 -- R1 ---- R2
                            /               \
                           / 50              R4
                          /                   \
                          S -------- E ------- D

                                 Figure 8

   In Figure 8, N1 is a connected alternate to reach D from S.  We
   consider that all links have a RED color except {R1,R2}, which is
   BLUE.  We consider all links to be 10 Gbps except {N1,R1}, which is
   2.5 Gbps.  The bandwidth attribute collected for the alternate path
   will be 10 Gbps.  As the attribute is unitary, only the link speed of
   the first link {S,N1} is recorded.  The link color attribute
   collected for the alternate path will be {RED,RED,BLUE,RED,RED}.  As
   the attribute is cumulative, the value of the attribute on each link
   along the path is recorded.

6.2.5.3.  Connected Alternate

   For an alternate path using a connected alternate:

   o  Attributes from the PLR to the alternate are retrieved from the
      interface connected to the alternate.  If the alternate is
      connected through multiple interfaces, the evaluation of
      attributes SHOULD be done once per interface (each interface is
      considered as a separate alternate) and once per ECMP group of
      interfaces (Layer 3 bundle).

Top      ToC       Page 20 
   o  Path attributes from the alternate to the destination are
      retrieved from the SPT rooted at the alternate.  As the alternate
      is a connected alternate, the SPT has already been computed to
      find the alternate, so there is no need for additional
      computation.

                             N1 -- R1 ---- R2
                          50//50             \
                           //                 \
                        i1//i2                 \
                         S -------- E -------- D

                                 Figure 9

   In Figure 9, we consider a primary path from S to D, with S using E
   as the primary next hop.  All metrics are considered as 1 except
   {S,N1} links, which are using a metric of 50.  We consider the
   following SRLGs on links:

   o  {S,N1} using i1: SRLG1,SRLG10.

   o  {S,N1} using i2: SRLG2,SRLG20.

   o  {N1,R1}: SRLG3.

   o  {R1,R2}: SRLG4.

   o  {R2,D}: SRLG5.

   o  {S,E}: SRLG10.

   o  {E,D}: SRLG6.

   S is connected to the alternate using two interfaces: i1 and i2.

   If i1 and i2 are not part of an ECMP group, the evaluation of
   attributes is done once per interface, and each interface is
   considered as a separate alternate path.  Two alternate paths will be
   available with the associated SRLG attributes:

   o  Alternate path #1: {S,N1 using if1,R1,R2,D}:
      SRLG1,SRLG10,SRLG3,SRLG4,SRLG5.

   o  Alternate path #2: {S,N1 using if2,R1,R2,D}:
      SRLG2,SRLG20,SRLG3,SRLG4,SRLG5.

   Alternate path #1 is sharing risks with the primary path and may be
   pruned, or its preference may be revoked, per user-defined policy.

Top      ToC       Page 21 
   If i1 and i2 are part of an ECMP group, the evaluation of attributes
   is done once per ECMP group, and the implementation considers a
   single alternate path {S,N1 using if1|if2,R1,R2,D} with the following
   SRLG attributes: SRLG1,SRLG10,SRLG2,SRLG20,SRLG3,SRLG4,SRLG5.  The
   alternate path is sharing risks with the primary path and may be
   pruned, or its preference may be revoked, per user-defined policy.

6.2.5.4.  Remote Alternate

   For alternate path using a remote alternate (tunnel):

   o  Attributes on the path from the PLR to the alternate are retrieved
      using the PLR's primary SPT (when using a PQ node from the
      P-space) or the immediate neighbor's SPT (when using a PQ from the
      extended P-space).  These are then combined with the attributes of
      the link(s) to reach the immediate neighbor.  In both cases, no
      additional SPT is required.

   o  Attributes from the remote alternate to the destination path may
      be retrieved from the SPT rooted at the remote alternate.  An
      additional forward SPT is required for each remote alternate
      (PQ node), as indicated in Section 2.3.2 of [REMOTE-LFA-NODE].  In
      some remote-alternate scenarios, like [TI-LFA], alternate-to-
      destination path attributes may be obtained using a different
      technique.

   The number of remote alternates may be very high.  In the case of
   remote LFAs, simulations of real-world network topologies have shown
   that as many as hundreds of PQs are possible.  The computational
   overhead of collecting all path attributes of all such PQs to
   destination paths could grow beyond reasonable levels.

   To handle this situation, implementations need to limit the number of
   remote alternates to be evaluated to a finite number before
   collecting alternate path attributes and running the policy
   evaluation.  Section 2.3.3 of [REMOTE-LFA-NODE] provides a way to
   reduce the number of PQs to be evaluated.

   Some other remote alternate techniques using static or dynamic
   tunnels may not require this pruning.

Top      ToC       Page 22 
                  Link            Remote              Remote
                  alternate       alternate           alternate
                 -------------  ------------------   -------------
   Alternates    |  LFA      |  |   rLFA (PQs)   |   |  Static/  |
                 |           |  |                |   |  Dynamic  |
   sources       |           |  |                |   |  tunnels  |
                 -------------  ------------------   -------------
                      |                   |                  |
                      |                   |                  |
                      |        --------------------------    |
                      |        |  Prune some alternates |    |
                      |        | (sorting strategy)     |    |
                      |        --------------------------    |
                      |                   |                  |
                      |                   |                  |
                  ------------------------------------------------
                  |          Collect alternate attributes        |
                  ------------------------------------------------
                                          |
                                          |
                               -------------------------
                               |    Evaluate policy    |
                               -------------------------
                                          |
                                          |
                                   Best alternates

                                 Figure 10

6.2.5.5.  Collecting Attributes in the Case of Multiple Paths

   As described in Section 6.2.5, there may be some situations where an
   alternate path or part of an alternate path fans out to multiple
   paths (e.g., ECMP).  When collecting path attributes in such a case,
   an implementation SHOULD consider the union of attributes of each
   sub-path.

   In Figure 7 (in Section 6.2.5.1), S has two alternate paths to
   reach D.  Each alternate path fans out to multiple paths due to ECMP.
   Consider the following link color attributes: all links are RED
   except {R1,R3}, which is BLUE.  The user wants to use an alternate
   path with only RED links.  The first alternate path
   {S,N1,R1,R2|R3,R4,D} does not fit the constraint, as {R1,R3} is BLUE.
   The second alternate path {S,N2,PQ,R5,D} fits the constraint and will
   be preferred, as it uses only RED links.

Top      ToC       Page 23 
6.2.6.  ECMP LFAs

                                     10
                                PE2 - PE3
                                 |     |
                              50 |  5  | 50
                                 P1----P2
                                 \\    //
                              50  \\  // 50
                                   PE1

                 Links between P1 and PE1 are L1 and L2.
                 Links between P2 and PE1 are L3 and L4.

                                 Figure 11

   In Figure 11, the primary path from PE1 to PE2 is through P1, using
   ECMP on two parallel links -- L1 and L2.  In the case of standard
   ECMP behavior, if L1 is failing, the post-convergence next hop would
   become L2 and ECMP would no longer be in use.  If an LFA is
   activated, as stated in Section 3.4 of [RFC5286], "alternate
   next-hops may themselves also be primary next-hops, but need not be"
   and "alternate next-hops should maximize the coverage of the failure
   cases."  In this scenario, there is no alternate providing node
   protection, so PE1 will prefer L2 as the alternate to protect L1;
   this makes sense compared to post-convergence behavior.

   Consider a different scenario, again referring to Figure 11, where L1
   and L2 are configured as a Layer 3 bundle using a local feature and
   L3/L4 comprise a second Layer 3 bundle.  Layer 3 bundles are
   configured as if a link in the bundle is failing; the traffic must be
   rerouted out of the bundle.  Layer 3 bundles are generally introduced
   to increase bandwidth between nodes.  In a nominal situation, ECMP is
   still available from PE1 to PE2, but if L1 is failing, the
   post-convergence next hop would become the ECMP on L3 and L4.  In
   this case, LFA behavior SHOULD be adapted in order to reflect the
   bandwidth requirement.

Top      ToC       Page 24 
   We would expect the following FIB entry on PE1:

                   On PE1: PE2 +--> ECMP -> L1
                                |     |
                                |     +----> L2
                                |
                                +--> LFA (ECMP) -> L3
                                      |
                                      +----------> L4

                                 Figure 12

   If L1 or L2 is failing, traffic must be switched on the LFA ECMP
   bundle rather than using the other primary next hop.

   As mentioned in Section 3.4 of [RFC5286], protecting a link within an
   ECMP by another primary next hop is not a MUST.  Moreover, as already
   discussed in this document, maximizing coverage against the failure
   cases may not be the right approach, and a policy-based choice of an
   alternate may be preferred.

   An implementation SHOULD allow setting a preference to protect a
   primary next hop with another primary next hop.  An implementation
   SHOULD also allow setting a preference to protect a primary next hop
   with a NON-primary next hop.  An implementation SHOULD allow the use
   of an ECMP bundle as an LFA.

7.  Operational Aspects

7.1.  No-Transit Condition on LFA Computing Node

   In Section 3.5 of [RFC5286], the setting of the no-transit condition
   (through the IS-IS overload bit or the OSPF R-bit) in an LFA
   computation is only taken into account for the case where a neighbor
   has the no-transit condition set.

   In addition to Inequality 1 (Loop-Free Criterion)
   (Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D))
   [RFC5286], the IS-IS overload bit or the OSPF R-bit of the LFA
   calculating neighbor (S) SHOULD be taken into account.  Indeed, if it
   has the IS-IS overload bit set or the OSPF R-bit clear, no neighbor
   will loop traffic back to itself.

   An OSPF router acting as a stub router [RFC6987] SHOULD behave as if
   the R-bit was clear regarding the LFA computation.

Top      ToC       Page 25 
7.2.  Manual Triggering of FRR

   Service providers often perform manual link shutdown (using a
   router's command-line interface (CLI)) to perform network
   changes/tests.  A manual link shutdown may be done at multiple
   levels: physical interface, logical interface, IGP interface,
   Bidirectional Forwarding Detection (BFD) session, etc.  In
   particular, testing or troubleshooting FRR requires that manual
   shutdown be performed on the remote end of the link, as a local
   shutdown would not generally trigger FRR.

   To permit such a situation, an implementation SHOULD support
   triggering/activating LFA FRR for a given link when a manual shutdown
   is done on a component that currently supports FRR activation.

   An implementation MAY also support FRR activation for a specific
   interface or a specific prefix on a primary next-hop interface and
   revert without any action on any running component of the node (links
   or protocols).  In this use case, the FRR activation time needs to be
   controlled by a timer in case the operator forgot to revert the
   traffic to the primary path.  When the timer expires, the traffic is
   automatically reverted to the primary path.  This will simplify the
   testing of the FRR path; traffic can then be reverted back to the
   primary path without causing a global network convergence.

   For example:

   o  If an implementation supports FRR activation upon a BFD
      session-down event, that implementation SHOULD support FRR
      activation when a manual shutdown is done on the BFD session.  But
      if an implementation does not support FRR activation upon a BFD
      session-down event, there is no need for that implementation to
      support FRR activation upon manual shutdown of a BFD session.

   o  If an implementation supports FRR activation upon a physical
      link-down event (e.g., Rx laser "off" detection, error threshold
      raised), that implementation SHOULD support FRR activation when a
      manual shutdown of a physical interface is done.  But if an
      implementation does not support FRR activation upon a physical
      link-down event, there is no need for that implementation to
      support FRR activation upon manual shutdown of a physical link.

   o  A CLI command may allow switching from the primary path to the FRR
      path to test the FRR path for a specific interface or prefix.
      There is no impact on the control plane; only the data plane of
      the local node may be changed.  A similar command may allow
      switching traffic back from the FRR path to the primary path.

Top      ToC       Page 26 
7.3.  Required Local Information

   The introduction of LFAs in a network requires some enhancements to
   standard routing information provided by implementations.  Moreover,
   due to "non-100%" coverage, coverage information is also required.

   Hence, an implementation:

   o  MUST be able to display, for every prefix, the primary next hop as
      well as the alternate next-hop information.

   o  MUST provide coverage information per LFA activation domain (area,
      level, topology, instance, virtual router, address family, etc.).

   o  MUST provide the number of protected prefixes as well as
      non-protected prefixes globally.

   o  SHOULD provide the number of protected prefixes as well as
      non-protected prefixes per link.

   o  MAY provide the number of protected prefixes as well as
      non-protected prefixes per priority if the implementation supports
      prefix-priority insertion in the RIB/FIB.

   o  SHOULD provide a reason for choosing an alternate (policy and
      criteria) and for excluding an alternate.

   o  SHOULD provide the list of non-protected prefixes and the reason
      why they are not protected (e.g., no protection required, no
      alternate available).

7.4.  Coverage Monitoring

   It is pretty easy to evaluate the coverage of a network in a nominal
   situation, but topology changes may change the level of coverage.  In
   some situations, the network may no longer be able to provide the
   required level of protection.  Hence, it becomes very important for
   service providers to receive alerts regarding changes in coverage.

   An implementation SHOULD:

   o  provide an alert system if total coverage (for a node) is below a
      defined threshold or when coverage returns to normal.

   o  provide an alert system if coverage for a specific link is below a
      defined threshold or when coverage returns to normal.

Top      ToC       Page 27 
   An implementation MAY:

   o  trigger an alert if a specific destination is not protected
      anymore or when protection comes back up for this destination.

   Although the procedures for providing alerts are beyond the scope of
   this document, we recommend that implementations consider standard
   and well-used mechanisms like syslog or SNMP traps.

7.5.  LFAs and Network Planning

   The operator may choose to run simulations in order to ensure a
   certain type of full coverage for the whole network or a given subset
   of the network.  This is particularly likely if he operates the
   network in the sense of the third backbone profile described in
   Section 4 of [RFC6571]; that is, he seeks to design and engineer the
   network topology in such a way that a certain level of coverage is
   always achieved.  Obviously, a complete and exact simulation of the
   IP FRR coverage can only be achieved if the behavior is deterministic
   and the algorithm used is available to the simulation tool.  Thus, an
   implementation SHOULD:

   o  Behave deterministically in its LFA selection process.  That is,
      in the same topology and with the same policy configuration, the
      implementation MUST always choose the same alternate for a given
      prefix.

   o  Document its behavior.  The implementation SHOULD provide enough
      documentation regarding its behavior to allow an implementer of a
      simulation tool to foresee the exact choice of the LFA
      implementation for every prefix in a given topology.  This SHOULD
      take into account all possible policy configuration options.  One
      possible way to document this behavior is to disclose the
      algorithm used to choose alternates.

Top      ToC       Page 28 
8.  Security Considerations

   The policy mechanism introduced in this document allows the tuning of
   the selection of the alternate.  This is not seen as a security
   threat, because:

   o  all candidates are already eligible as per [RFC5286] and
      considered usable.

   o  the policy is based on information from the router's own
      configuration and from the IGP, both of which are considered
      trusted.

   Hence, this document does not introduce any new security
   considerations as compared to [RFC5286].

   As noted above, the policy mechanism introduced in this document
   allows the tuning of the selection of the best alternate but does not
   change the list of alternates that are eligible.  As described in
   Section 7 of [RFC5286], this best alternate "can be used anyway when
   a different topological change occurs, and hence this can't be viewed
   as a new security threat."

9.  References

9.1.  Normative References

   [ISO10589] International Organization for Standardization,
              "Intermediate System to Intermediate System intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode network service (ISO 8473)",
              ISO Standard 10589, 2002.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <http://www.rfc-editor.org/info/rfc3630>.

   [RFC4203]  Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
              <http://www.rfc-editor.org/info/rfc4203>.

Top      ToC       Page 29 
   [RFC5286]  Atlas, A., Ed., and A. Zinin, Ed., "Basic Specification
              for IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <http://www.rfc-editor.org/info/rfc5286>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305,
              October 2008, <http://www.rfc-editor.org/info/rfc5305>.

   [RFC5307]  Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
              <http://www.rfc-editor.org/info/rfc5307>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <http://www.rfc-editor.org/info/rfc5340>.

   [RFC6571]  Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
              B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
              <http://www.rfc-editor.org/info/rfc6571>.

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              DOI 10.17487/RFC6987, September 2013,
              <http://www.rfc-editor.org/info/rfc6987>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <http://www.rfc-editor.org/info/rfc7490>.

   [RFC7777]  Hegde, S., Shakir, R., Smirnov, A., Li, Z., and B.
              Decraene, "Advertising Node Administrative Tags in OSPF",
              RFC 7777, DOI 10.17487/RFC7777, March 2016,
              <http://www.rfc-editor.org/info/rfc7777>.

   [RFC7917]  Sarkar, P., Ed., Gredler, H., Hegde, S., Litkowski, S.,
              and B. Decraene, "Advertising Node Administrative Tags in
              IS-IS", RFC 7917, DOI 10.17487/RFC7917, July 2016,
              <http://www.rfc-editor.org/info/rfc7917>.

Top      ToC       Page 30 
9.2.  Informative References

   [REMOTE-LFA-NODE]
              Sarkar, P., Ed., Hegde, S., Bowers, C., Gredler, H., and
              S. Litkowski, "Remote-LFA Node Protection and
              Manageability", Work in Progress,
              draft-ietf-rtgwg-rlfa-node-protection-05, December 2015.

   [SEG-RTG-ARCH]
              Filsfils, C., Ed., Previdi, S., Ed., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", Work in Progress,
              draft-ietf-spring-segment-routing-09, July 2016.

   [TI-LFA]   Francois, P., Filsfils, C., Bashandy, A., Decraene, B.,
              and S. Litkowski, "Topology Independent Fast Reroute using
              Segment Routing", Work in Progress,
              draft-francois-segment-routing-ti-lfa-00, November 2013.

Top      ToC       Page 31 
Contributors

   Significant contributions were made by Pierre Francois, Hannes
   Gredler, Chris Bowers, Jeff Tantsura, Uma Chunduri, Acee Lindem, and
   Mustapha Aissaoui, whom the authors would like to acknowledge.

Authors' Addresses

   Stephane Litkowski (editor)
   Orange

   Email: stephane.litkowski@orange.com


   Bruno Decraene
   Orange

   Email: bruno.decraene@orange.com


   Clarence Filsfils
   Cisco Systems

   Email: cfilsfil@cisco.com


   Kamran Raza
   Cisco Systems

   Email: skraza@cisco.com


   Martin Horneffer
   Deutsche Telekom

   Email: Martin.Horneffer@telekom.de


   Pushpasis Sarkar
   Individual Contributor

   Email: pushpasis.ietf@gmail.com