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Versions: (draft-akiya-mpls-lsp-ping-lag-multipath) 00 01 02 03 04 05

Internet Engineering Task Force                                 N. Akiya
Internet-Draft                                       Big Switch Networks
Updates: 8029 (if approved)                                   G. Swallow
Intended status: Standards Track                           Cisco Systems
Expires: April 26, 2019                                     S. Litkowski
                                                             B. Decraene
                                                                  Orange
                                                                J. Drake
                                                        Juniper Networks
                                                                 M. Chen
                                                                  Huawei
                                                        October 23, 2018


       Label Switched Path (LSP) Ping/Trace Multipath Support for
                Link Aggregation Group (LAG) Interfaces
               draft-ietf-mpls-lsp-ping-lag-multipath-05

Abstract

   This document defines extensions to the MPLS Label Switched Path
   (LSP) Ping and Traceroute mechanisms as specified in RFC 8029.  The
   extensions allow the MPLS LSP Ping and Traceroute mechanisms to
   discover and exercise specific paths of Layer 2 (L2) Equal-Cost
   Multipath (ECMP) over Link Aggregation Group (LAG) interfaces.
   Additionally, a mechanism is defined to enable determination of the
   capabilities of an LSR supported.

   This document updates RFC8029.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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 https://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
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 26, 2019.

Copyright Notice

   Copyright (c) 2018 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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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
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview of Solution  . . . . . . . . . . . . . . . . . . . .   4
   3.  LSR Capability Discovery  . . . . . . . . . . . . . . . . . .   6
     3.1.  Initiator LSR Procedures  . . . . . . . . . . . . . . . .   7
     3.2.  Responder LSR Procedures  . . . . . . . . . . . . . . . .   7
   4.  Mechanism to Discover L2 ECMP Multipath . . . . . . . . . . .   7
     4.1.  Initiator LSR Procedures  . . . . . . . . . . . . . . . .   7
     4.2.  Responder LSR Procedures  . . . . . . . . . . . . . . . .   8
     4.3.  Additional Initiator LSR Procedures . . . . . . . . . . .  10
   5.  Mechanism to Validate L2 ECMP Traversal . . . . . . . . . . .  11
     5.1.  Incoming LAG Member Links Verification  . . . . . . . . .  11
       5.1.1.  Initiator LSR Procedures  . . . . . . . . . . . . . .  11
       5.1.2.  Responder LSR Procedures  . . . . . . . . . . . . . .  12
       5.1.3.  Additional Initiator LSR Procedures . . . . . . . . .  12
     5.2.  Individual End-to-End Path Verification . . . . . . . . .  13
   6.  LSR Capability TLV  . . . . . . . . . . . . . . . . . . . . .  14
   7.  LAG Description Indicator Flag: G . . . . . . . . . . . . . .  15
   8.  Local Interface Index Sub-TLV . . . . . . . . . . . . . . . .  16
   9.  Remote Interface Index Sub-TLV  . . . . . . . . . . . . . . .  17
   10. Detailed Interface and Label Stack TLV  . . . . . . . . . . .  17
     10.1.  Sub-TLVs . . . . . . . . . . . . . . . . . . . . . . . .  19
       10.1.1.  Incoming Label Stack Sub-TLV . . . . . . . . . . . .  19



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       10.1.2.  Incoming Interface Index Sub-TLV . . . . . . . . . .  20
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     12.1.  LSR Capability TLV . . . . . . . . . . . . . . . . . . .  21
       12.1.1.  LSR Capability Flags . . . . . . . . . . . . . . . .  21
     12.2.  Local Interface Index Sub-TLV  . . . . . . . . . . . . .  22
       12.2.1.  Interface Index Flags  . . . . . . . . . . . . . . .  22
     12.3.  Remote Interface Index Sub-TLV . . . . . . . . . . . . .  22
     12.4.  Detailed Interface and Label Stack TLV . . . . . . . . .  23
       12.4.1.  Sub-TLVs for TLV Type TBD4 . . . . . . . . . . . . .  23
     12.5.  DS Flags . . . . . . . . . . . . . . . . . . . . . . . .  23
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     14.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Appendix A.  LAG with intermediate L2 Switch Issues . . . . . . .  25
     A.1.  Equal Numbers of LAG Members  . . . . . . . . . . . . . .  25
     A.2.  Deviating Numbers of LAG Members  . . . . . . . . . . . .  25
     A.3.  LAG Only on Right . . . . . . . . . . . . . . . . . . . .  26
     A.4.  LAG Only on Left  . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

1.1.  Terminology

   The following acronyms/terms are used in this document:

   o  MPLS - Multiprotocol Label Switching.

   o  LSP - Label Switched Path.

   o  LSR - Label Switching Router.

   o  ECMP - Equal-Cost Multipath.

   o  LAG - Link Aggregation Group.

   o  Initiator LSR - LSR which sends MPLS echo request.

   o  Responder LSR - LSR which receives MPLS echo request and sends
      MPLS echo reply.

1.2.  Background

   The MPLS Label Switched Path (LSP) Ping and Traceroute mechanisms
   [RFC8029] are powerful tools designed to diagnose all available Layer
   3 (L3) paths of LSPs, including diagnostic coverage of L3 Equal-Cost



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   Multipath (ECMP).  In many MPLS networks, Link Aggregation Group
   (LAG) as defined in [IEEE802.1AX], which provides Layer 2 (L2) ECMP,
   is often used for various reasons.  MPLS LSP Ping and Traceroute
   tools were not designed to discover and exercise specific paths of L2
   ECMP.  This raises a limitation for the following scenario when an
   LSP traverses over a LAG:

   o  Label switching over some member links of the LAG is successful,
      but will be failed over other member links of the LAG.

   o  MPLS echo request for the LSP over the LAG is load balanced on one
      of the member links which is label switching successfully.

   With the above scenarios, MPLS LSP Ping and Traceroute will not be
   able to detect the label switching failure of the problematic member
   link(s) of the LAG.  In other words, lack of L2 ECMP diagnostic
   coverage can produce an outcome where MPLS LSP Ping and Traceroute
   can be blind to label switching failures over a problematic LAG
   interface.  It is, thus, desirable to extend the MPLS LSP Ping and
   Traceroute to have deterministic diagnostic coverage of LAG
   interfaces.

   The need for a solution of this problem was motivated by issues
   encountered in live networks.

2.  Overview of Solution

   This document defines an optional TLV to discover the capabilities of
   a responder LSR and extensions for use with the MPLS LSP Ping and
   Traceroute mechanisms to describe Multipath Information for
   individual LAG member links, thus allowing MPLS LSP Ping and
   Traceroute to discover and exercise specific paths of L2 ECMP over
   LAG interfaces.  The reader is expected to be familiar with mechanics
   of Downstream Mapping described in Section 3.3 of [RFC8029] and
   Downstream Detailed Mapping TLV (DDMAP) described in Section 3.4 of
   [RFC8029].

   The solution consists of the MPLS echo request containing a DDMAP TLV
   and the optional LSR capability TLV to indicate that separate load
   balancing information for each L2 nexthop over LAG is desired in the
   MPLS echo reply.  The Responder LSR places the same optional LSR
   capability TLV in the MPLS echo reply to provide acknowledgement back
   to the initiator LSR.  It also adds, for each downstream LAG member,
   load balance information (i.e., multipath information and interface
   index).  This mechanism is applicable to all types of LSPs which can
   traverse over LAG interfaces.  Many LAGs are built from p2p links,
   with router X and router X+1 having direct connectivity and the same
   number of LAG members.  It is possible to build LAGs asymmetrically



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   by using Ethernet switches between two routers.  Appendix A lists
   some use cases for which the mechanisms defined in this document may
   not be applicable.  Note that the mechanisms described in this
   document do not impose any changes to scenarios where an LSP is
   pinned down to a particular LAG member (i.e.  the LAG is not treated
   as one logical interface by the LSP).

   The following figure and description provides an example using an LDP
   network.

     <----- LDP Network ----->

             +-------+
             |       |
     A-------B=======C-------E
             |               |
             +-------D-------+

     ---- Non-LAG
     ==== LAG comprising of two member links

         Figure 1: Example LDP Network


   When node A is initiating LSP Traceroute to node E, node B will
   return to node A load balance information for following entries.

   1.  Downstream C over Non-LAG (upper path).

   2.  First Downstream C over LAG (middle path).

   3.  Second Downstream C over LAG (middle path).

   4.  Downstream D over Non-LAG (lower path).

   This document defines:

   o  In Section 3, a mechanism to discover capabilities of responder
      LSRs;

   o  In Section 4, a mechanism to discover L2 ECMP multipath
      information;

   o  In Section 5, a mechanism to validate L2 ECMP traversal;

   o  In Section 6, the LSR Capability TLV;

   o  In Section 7, the LAG Description Indicator flag;



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   o  In Section 8, the Local Interface Index Sub-TLV;

   o  In Section 9, the Remote Interface Index Sub-TLV;

   o  In Section 10, the Detailed Interface and Label Stack TLV;

3.  LSR Capability Discovery

   The MPLS Ping operates by an initiator LSR sending an MPLS echo
   request message and receiving back a corresponding MPLS echo reply
   message from a responder LSR.  The MPLS Traceroute operates in a
   similar way except the initiator LSR potentially sends multiple MPLS
   echo request messages with incrementing TTL values.

   There have been many extensions to the MPLS Ping and Traceroute
   mechanism over the years.  Thus it is often useful, and sometimes
   necessary, for the initiator LSR to deterministically disambiguate
   the differences between:

   o  The responder LSR sent the MPLS echo reply message with contents C
      because it has feature X, Y and Z implemented.

   o  The responder LSR sent the MPLS echo reply message with contents C
      because it has subset of features X, Y and Z implemented but not
      all.

   o  The responder LSR sent the MPLS echo reply message with contents C
      because it does not have features X, Y and Z implemented.

   To allow the initiator LSR to disambiguate the above differences,
   this document defines the LSR Capability TLV (described in
   Section 6).  When the initiator LSR wishes to discover the
   capabilities of the responder LSR, the initiator LSR includes the LSR
   Capability TLV in the MPLS echo request message.  When the responder
   LSR receives an MPLS echo request message with the LSR Capability TLV
   included, if it knows the LSR Capability TLV, then it MUST include
   the LSR Capability TLV in the MPLS echo reply message with the LSR
   Capability TLV describing features and extensions supported by the
   local LSR.  Otherwise, an MPLS echo reply MUST be sent back to the
   initiator LSR with the return code set to "One or more of the TLVs
   was not understood".  Then the initiator LSR can send another MPLS
   echo request without including the LSR Capability TLV.

   It is RECOMMENDED that implementations supporting the LAG Multipath
   extensions defined in this document include the LSR Capability TLV in
   MPLS echo request messages.





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3.1.  Initiator LSR Procedures

   If an initiator LSR does not know what capabilities a responder LSR
   can support, it can send an MPLS each request message and carry the
   LSR Capability TLV to the responder to discover the capabilities that
   the responder LSR can support.

3.2.  Responder LSR Procedures

   When a responder LSR received an MPLS echo request message that
   carries the LSR Capability TLV, the following procedures are used:

   If the responder knows how to process the LSR Capability TLV, the
   following procedures are used:

   o  The responder LSR MUST include the LSR Capability TLV in the MPLS
      echo reply message.

   o  If the responder LSR understands the "LAG Description Indicator
      flag":

      *  Set the "Downstream LAG Info Accommodation flag" if the
         responder LSR is capable of describing outgoing LAG member
         links separately; otherwise, clear the "Downstream LAG Info
         Accommodation flag".

      *  Set the "Upstream LAG Info Accommodation flag" if responder LSR
         is capable of describing incoming LAG member links separately;
         otherwise, clear the "Upstream LAG Info Accommodation flag".

   o  If the responder LSR does not understand the "LAG Description
      Indicator flag":

      *  Clear both the "Downstream LAG Info Accommodation flag" and the
         "Upstream LAG Info Accommodation flag".

   If the responder does not know the LSR Capability TLV, an MPLS echo
   reply with the return code set to "One or more of the TLVs was not
   understood" MUST be sent back to the initiator LSR.

4.  Mechanism to Discover L2 ECMP Multipath

4.1.  Initiator LSR Procedures

   Through the "LSR Capability Discovery" as defined in Section 3, the
   initiator LSR can understand whether the responder LSR can describe
   incoming/outgoing LAG member links separately in the DDMAP TLV.




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   Once the initiator LSR knows the capabilities that a responder
   supports, then it sends an MPLS echo request carrying a DDMAP with
   the "LAG Description Indicator flag" (G) set to the responder LSR.
   The "LAG Description Indicator flag" (G) indicates that separate load
   balancing information for each L2 nexthop over a LAG is desired in
   the MPLS echo reply.  The new "LAG Description Indicator flag" is
   described in Section 7.

4.2.  Responder LSR Procedures

   When a responder LSR received an MPLS echo request message with the
   "LAG Description Indicator flag" set, if the responder LSR
   understands the "LAG Description Indicator flag" and is capable of
   describing outgoing LAG member links separately, the following
   procedures are used, regardless of whether or not outgoing interfaces
   include LAG interfaces:

   o  For each downstream that is a LAG interface:

      *  The responder LSR MUST include a DDMAP TLV when sending the
         MPLS echo reply.

      *  The responder LSR MUST set the "LAG Description Indicator flag"
         in the DS Flags field of the DDMAP TLV.

      *  In the DDMAP TLV, the Local Interface Index Sub-TLV, Remote
         Interface Index Sub-TLV and Multipath Data Sub-TLV are used to
         describe each LAG member link.  All other fields of the DDMAP
         TLV are used to describe the LAG interface.

      *  For each LAG member link of the LAG interface:

         +  The responder LSR MUST add a Local Interface Index Sub-TLV
            (described in Section 8) with the "LAG Member Link Indicator
            flag" set in the Interface Index Flags field, describing the
            interface index of this outgoing LAG member link (the local
            interface index is assigned by the local LSR).

         +  The responder LSR MAY add a Remote Interface Index Sub-TLV
            (described in Section 9) with the "LAG Member Link Indicator
            flag" set in the Interface Index Flags field, describing the
            interface index of the incoming LAG member link on the
            downstream LSR (this interface index is assigned by the
            downstream LSR).  How the local LSR obtains the interface
            index of the LAG member link on the downstream LSR is
            outside the scope of this document.





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         +  The responder LSR MUST add an Multipath Data Sub-TLV for
            this LAG member link, if the received DDMAP TLV requested
            multipath information.

   Based on the procedures described above, every LAG member link will
   have a Local Interface Index Sub-TLV and a Multipath Data Sub-TLV
   entries in the DDMAP TLV.  The order of the Sub-TLVs in the DDMAP TLV
   for a LAG member link MUST be Local Interface Index Sub-TLV
   immediately followed by Multipath Data Sub-TLV.  A LAG member link
   may also have a corresponding Remote Interface Index Sub-TLV.  When a
   Local Interface Index Sub-TLV, a Remote Interface Index-Sub-TLV and a
   Multipath Data Sub-TLV are placed in the DDMAP TLV to describe a LAG
   member link, they MUST be placed in the order of Local Interface
   Index Sub-TLV, Remote Interface Index-Sub-TLV and Multipath Data Sub-
   TLV.

   A responder LSR possessing a LAG interface with two member links
   would send the following DDMAP for this LAG interface:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~   DDMAP fields describing LAG interface with DS Flags G set   ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |[MANDATORY] Local Interface Index Sub-TLV of LAG member link #1|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |[OPTIONAL] Remote Interface Index Sub-TLV of LAG member link #1|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |[MANDATORY] Multipath Data Sub-TLV LAG member link #1          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |[MANDATORY] Local Interface Index Sub-TLV of LAG member link #2|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |[OPTIONAL] Remote Interface Index Sub-TLV of LAG member link #2|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |[MANDATORY] Multipath Data Sub-TLV LAG member link #2          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Label Stack Sub-TLV                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 2: Example of DDMAP in MPLS Echo Reply


   When none of the received multipath information maps to a particular
   LAG member link, then the responder LSR MUST still place the Local
   Interface Index Sub-TLV and the Multipath Data Sub-TLV for that LAG
   member link in the DDMAP TLV, the value of Multipath Length field of
   the Multipath Data Sub-TLV is set to zero.




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4.3.  Additional Initiator LSR Procedures

   The procedures above allow an initiator LSR to:

   o  Identify whether or not the responder LSR can describe outgoing
      LAG member links separately, by looking at the LSR Capability TLV.

   o  Utilize the value of the "LAG Description Indicator flag" in DS
      Flags to identify whether each received DDMAP TLV describes a LAG
      interface or a non-LAG interface.

   o  Obtain multipath information which is expected to traverse the
      specific LAG member link described by corresponding interface
      index.

   When an initiator LSR receives a DDMAP containing LAG member
   information from a downstream LSR with TTL=n, then the subsequent
   DDMAP sent by the initiator LSR to the downstream LSR with TTL=n+1
   through a particular LAG member link MUST be updated with following
   procedures:

   o  The Local Interface Index Sub-TLVs MUST be removed in the sending
      DDMAP.

   o  If the Remote Interface Index Sub-TLVs were present and the
      initiator LSR is traversing over a specific LAG member link, then
      the Remote Interface Index Sub-TLV corresponding to the LAG member
      link being traversed SHOULD be included in the sending DDMAP.  All
      other Remote Interface Index Sub-TLVs MUST be removed from the
      sending DDMAP.

   o  The Multipath Data Sub-TLVs MUST be updated to include just one
      Multipath Data Sub-TLV.  The initiator LSR MAY just keep the
      Multipath Data Sub-TLV corresponding to the LAG member link being
      traversed, or combine the Multipath Data Sub-TLVs for all LAG
      member links into a single Multipath Data Sub-TLV when diagnosing
      further downstream LSRs.

   o  All other fields of the DDMAP are to comply with procedures
      described in [RFC8029].

   Figure 3 is an example that shows how to use the DDMAP TLV to notify
   which member link (link #1 in the example) will be chosen to send the
   MPLS echo request message to the next downstream LSR:







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      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~   DDMAP fields describing LAG interface with DS Flags G set   ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |[OPTIONAL] Remote Interface Index Sub-TLV of LAG member link #1|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Multipath Data Sub-TLV LAG member link #1         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Label Stack Sub-TLV                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 3: Example of DDMAP in MPLS Echo Request

5.  Mechanism to Validate L2 ECMP Traversal

   Section 4 defines the responder LSR procedures to constructs a DDMAP
   for a downstream LAG.  The Remote Interface Index Sub-TLVs that
   describes the incoming LAG member links of the downstream LSR is
   optional, because this information from the downstream LSR is often
   not available on the responder LSR.  In such case, the traversal of
   LAG member links can be validated with procedures described in
   Section 5.1.  If LSRs can provide the Remote Interface Index Sub-
   TLVs, then the validation procedures described in Section 5.2 can be
   used.

5.1.  Incoming LAG Member Links Verification

   Without downstream LSRs returning remote Interface Index Sub-TLVs in
   the DDMAP, validation of the LAG member link traversal requires that
   initiator LSR traverses all available LAG member links and taking the
   results through a logic.  This section provides the mechanism for the
   initiator LSR to obtain additional information from the downstream
   LSRs and describes the additional logic in the initiator LSR to
   validate the L2 ECMP traversal.

5.1.1.  Initiator LSR Procedures

   An MPLS echo request carrying a DDMAP TLV with the "Interface and
   Label Stack Object Request flag" and "LAG Description Indicator flag"
   set is sent to indicate the request for Detailed Interface and Label
   Stack TLV with additional LAG member link information (i.e.
   interface index) in the MPLS echo reply.








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5.1.2.  Responder LSR Procedures

   A responder LSR that understands the "LAG Description Indicator flag"
   and is capable of describing incoming LAG member link MUST use the
   following procedures, regardless of whether or not incoming interface
   was a LAG interface:

   o  When the "I" flag ( "Interface and Label Stack Object Request
      flag") of the DDMAP TLV in the received MPLS echo request is set:

      *  The responder LSR MUST add the Detailed Interface and Label
         Stack TLV (described in Section 10) in the MPLS echo reply.

      *  If the incoming interface is a LAG, the responder LSR MUST add
         the Incoming Interface Index Sub-TLV (described in
         Section 10.1.2) in the Detailed Interface and Label Stack TLV.
         The "LAG Member Link Indicator flag" MUST be set in the
         Interface Index Flags field, and the Interface Index field set
         to the LAG member link which received the MPLS echo request.

   These procedures allow initiator LSR to:

   o  Utilize the Incoming Interface Index Sub-TLV in the Detailed
      Interface and Label Stack TLV to derive, if the incoming interface
      is a LAG, the identity of the incoming LAG member.

5.1.3.  Additional Initiator LSR Procedures

   Along with procedures described in Section 4, the procedures
   described in this section will allow an initiator LSR to know:

   o  The expected load balance information of every LAG member link, at
      LSR with TTL=n.

   o  With specific entropy, the expected interface index of the
      outgoing LAG member link at TTL=n.

   o  With specific entropy, the interface index of the incoming LAG
      member link at TTL=n+1.

   Expectation is that there's a relationship between the interface
   index of the outgoing LAG member link at TTL=n and the interface
   index of the incoming LAG member link at TTL=n+1 for all discovered
   entropies.  In other words, set of entropies that load balances to
   outgoing LAG member link X at TTL=n should all reach the nexthop on
   same incoming LAG member link Y at TTL=n+1.





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   With additional logics, the initiator LSR can perform the following
   checks in a scenario where the initiator LSR knows that there is a
   LAG, with two LAG members, between TTL=n and TTL=n+1, and has the
   multipath information to traverse the two LAG member links.

   The initiator LSR sends two MPLS echo request messages to traverse
   the two LAG member links at TTL=n+1:

   o  Success case:

      *  One MPLS echo request message reaches TTL=n+1 on an LAG member
         link 1.

      *  The other MPLS echo request message reaches TTL=n+1 on an LAG
         member link 2.

      The two MPLS echo request messages sent by the initiator LSR reach
      at the immediate downstream LSR from two different LAG member
      links.

   o  Error case:

      *  One MPLS echo request message reaches TTL=n+1 on an LAG member
         link 1.

      *  The other MPLS echo request message also reaches TTL=n+1 on an
         LAG member link 1.

      One or two MPLS echo request messages sent by the initiator LSR
      cannot reach the immediate downstream LSR, or the two MPLS echo
      request messages reach at the immediate downstream LSR from the
      same LAG member link.

   Note that the above defined procedures will provide a deterministic
   result for LAG interfaces that are back-to-back connected between
   LSRs (i.e. no L2 switch in between).  If there is a L2 switch between
   the LSR at TTL=n and the LSR at TTL=n+1, there is no guarantee that
   traversal of every LAG member link at TTL=n will result in reaching
   from different interface at TTL=n+1.  Issues resulting from LAG with
   L2 switch in between are further described in Appendix A.  LAG
   provisioning models in operated network should be considered when
   analyzing the output of LSP Traceroute exercising L2 ECMPs.

5.2.  Individual End-to-End Path Verification

   When the Remote Interface Index Sub-TLVs are available from an LSR
   with TTL=n, then the validation of LAG member link traversal can be




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   performed by the downstream LSR of TTL=n+1.  The initiator LSR
   follows the procedures described in Section 4.3.

   The DDMAP validation procedures for the downstream responder LSR are
   then updated to include the comparison of the incoming LAG member
   link to the interface index described in the Remote Interface Index
   Sub-TLV in the DDMAP TLV.  Failure of this comparison results in the
   return code being set to "Downstream Mapping Mismatch (5)".

6.  LSR Capability TLV

   This document defines a new optional TLV which is referred to as the
   "LSR Capability TLV.  It MAY be included in the MPLS echo request
   message and the MPLS echo reply message.  An MPLS echo request
   message and an MPLS echo reply message MUST NOT include more than one
   LSR Capability TLV.  The presence of an LSR Capability TLV in an MPLS
   echo request message is a request that a responder LSR includes an
   LSR Capability TLV in the MPLS echo reply message, with the LSR
   Capability TLV describing features and extensions that the responder
   LSR supports.

   When a responder LSR received an MPLS echo request message that
   carries an LSR Capability TLV, if the responder LSR knows how to
   process the LSR Capability TLV, an LSR Capability TLV MUST be
   included in the MPLS echo reply message.  Otherwise, if the responder
   does not know the LSR Capability TLV, an MPLS echo reply with the
   return code set to "One or more of the TLVs was not understood" MUST
   be sent back to the initiator LSR.

   The format of the LSR Capability TLV is as below:

   LSR Capability TLV Type is TBD1.  Length is 4.  The value field of
   the LSR Capability TLV has following format:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      LSR Capability Flags                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 4: LSR Capability TLV

   Where:

      The Type is 2 octets in length and the value is TBD1.




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      The Length filed is 2 octets in length, and the value is 4.

      The LSR Capability Flags is 4 octets in length, this document
      defines following flags:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Must Be Zero (Reserved)                  |U|D|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      This document defines two flags.  The remaining flags MUST be set
      to zero when sending and ignored on receipt.  Both the U and the D
      flag MUST be cleared in the MPLS echo request message when
      sending, and ignored on receipt.  Neither, either or both the U
      and the D flag MAY be set in the MPLS echo reply message.

      Flag  Name and Meaning
      ----  ----------------

         U  Upstream LAG Info Accommodation

            An LSR sets this flag when the LSR is capable of
            describing a LAG member link in the Incoming Interface
            Index Sub-TLV in the Detailed Interface and
            Label Stack TLV.

         D  Downstream LAG Info Accommodation

            An LSR sets this flag when the LSR is capable of
            describing LAG member links in the Local Interface
            Index Sub-TLV and the Multipath Data Sub-TLV in the
            Downstream Detailed Mapping TLV.

7.  LAG Description Indicator Flag: G

   This document defines a flag, the "G" flag (LAG Description
   Indicator), in the DS Flags field of the DDMAP TLV.

   The "G" flag in the MPLS echo request message indicates the request
   for detailed LAG information from the responder LSR.  In the MPLS
   echo reply message, the "G" flag MUST be set if the DDMAP TLV
   describes a LAG interface.  It MUST be cleared otherwise.

   The "G" flag is defined as below:

      The Bit Number is TBD5.




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       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      | MBZ |G|MBZ|I|N|
      +-+-+-+-+-+-+-+-+

      RFC-Editor-Note: Please update above figure to place the G flag in
      the bit number TBD5.

  Flag  Name and Meaning
  ----  ----------------

     G  LAG Description Indicator

        When this flag is set in the MPLS echo request, the responder LSR
        is requested to respond with detailed LAG information. When this
        flag is set in the MPLS echo reply, the corresponding DDMAP TLV
        describes a LAG interface.

8.  Local Interface Index Sub-TLV

   The Local Interface Index Sub-TLV is an optional TLV, it describes
   the interface index assigned by the local LSR to an egress interface.
   One or more Local Interface Index sub-TLVs MAY appear in a DDMAP TLV.

   The format of the Local Interface Index Sub-TLV is 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Local Interface Index                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 5: Local Interface Index Sub-TLV

   Where:

   o  The "Type" field is 2 octets in length, the value is TBD2.

   o  The "Length" filed 2 octets in length, and the value is 4.

   o  The "Local Interface Index" field is 4 octets in length, it is an
      interface index assigned by a local LSR to an egress interface.







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9.  Remote Interface Index Sub-TLV

   The Remote Interface Index Sub-TLV is an optional TLV, it describes
   the interface index assigned by a downstream LSR to an ingress
   interface.  One or more Remote Interface Index sub-TLVs MAY appear in
   a DDMAP TLV.

   The format of the Remote Interface Index Sub-TLV is as 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Remote Interface Index                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 6: Remote Interface Index Sub-TLV

   Where:

   o  The "Type" field is 2 octets in length, and the value is TBD3.

   o  The "Length" field is 2 octets in length, and the value is 4.

   o  The "Remote Interface Index" is 4 octets in length, it is an
      interface index assigned by a downstream LSR to an ingress
      interface.

10.  Detailed Interface and Label Stack TLV

   The "Detailed Interface and Label Stack" object is a TLV that MAY be
   included in an MPLS echo reply message to report the interface on
   which the MPLS echo request message was received and the label stack
   that was on the packet when it was received.  A responder LSR MUST
   NOT insert more than one instance of this TLV into the MPLS echo
   reply message.  This TLV allows the initiator LSR to obtain the exact
   interface and label stack information as it appears at the responder
   LSR.

   Detailed Interface and Label Stack TLV Type is TBD4.  Length is K +
   Sub-TLV Length (sum of Sub-TLVs).  K is the sum of all fields of this
   TLV prior to Sub-TLVs, but the length of K depends on the Address
   Type.  Details of this information is described below.  The Value
   field has following format:






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      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Address Type  |             Must Be Zero                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   IP Address (4 or 16 octets)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Interface (4 or 16 octets)                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                      List of Sub-TLVs                         .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 7: Detailed Interface and Label Stack TLV

   The Detailed Interface and Label Stack TLV format is derived from the
   Interface and Label Stack TLV format (from [RFC8029]).  Two changes
   are introduced.  The first is that the label stack is converted into
   a sub-TLV.  The second is that a new sub-TLV is added to describe an
   interface index.  These fields of Detailed Interface and Label Stack
   TLV have the same use and meaning as in [RFC8029].  A summary of
   these fields is as below:

      Address Type

         The Address Type indicates if the interface is numbered or
         unnumbered.  It also determines the length of the IP Address
         and Interface fields.  The resulting total length of the
         initial part of the TLV is listed as "K Octets".  The Address
         Type is set to one of the following values:

            Type #        Address Type           K Octets
            ------        ------------           --------
                 1        IPv4 Numbered                16
                 2        IPv4 Unnumbered              16
                 3        IPv6 Numbered                40
                 4        IPv6 Unnumbered              28

      IP Address and Interface

         IPv4 addresses and interface indices are encoded in 4 octets;
         IPv6 addresses are encoded in 16 octets.

         If the interface upon which the echo request message was
         received is numbered, then the Address Type MUST be set to IPv4



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         Numbered or IPv6 Numbered, the IP Address MUST be set to either
         the LSR's Router ID or the interface address, and the Interface
         MUST be set to the interface address.

         If the interface is unnumbered, the Address Type MUST be either
         IPv4 Unnumbered or IPv6 Unnumbered, the IP Address MUST be the
         LSR's Router ID, and the Interface MUST be set to the index
         assigned to the interface.

         Note: Usage of IPv6 Unnumbered has the same issue as [RFC8029],
         described in Section 3.4.2 of [RFC7439].  A solution should be
         considered an applied to both [RFC8029] and this document.

10.1.  Sub-TLVs

   This section defines the sub-TLVs that MAY be included as part of the
   Detailed Interface and Label Stack TLV.  Two sub-TLVs are defined:

           Sub-Type    Sub-TLV Name
           ---------   ------------
             1         Incoming Label stack
             2         Incoming Interface Index

10.1.1.  Incoming Label Stack Sub-TLV

   The Incoming Label Stack sub-TLV contains the label stack as received
   by an LSR.  If any TTL values have been changed by this LSR, they
   SHOULD be restored.

   Incoming Label Stack Sub-TLV Type is 1.  Length is variable, and its
   format is as 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Label                 | TC  |S|      TTL      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                                                               .
     .                                                               .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Label                 | TC  |S|      TTL      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 8: Incoming Label Stack Sub-TLV




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10.1.2.  Incoming Interface Index Sub-TLV

   The Incoming Interface Index object is a Sub-TLV that MAY be included
   in a Detailed Interface and Label Stack TLV.  The Incoming Interface
   Index Sub-TLV describes the index assigned by a local LSR to the
   interface which received the MPLS echo request message.

   Incoming Interface Index Sub-TLV Type is 2.  Length is 8, and its
   format is as 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Type              |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Interface Index Flags      |         Must Be Zero          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Incoming Interface Index                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 9: Incoming Interface Index Sub-TLV

   Interface Index Flags

      Interface Index Flags field is a bit vector with following format.

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Must Be Zero (Reserved)   |M|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      One flag is defined: M.  The remaining flags MUST be set to zero
      when sending and ignored on receipt.

     Flag  Name and Meaning
     ----  ----------------

        M  LAG Member Link Indicator

           When this flag is set, interface index described in
           this sub-TLV is a member of a LAG.


   Incoming Interface Index

      An Index assigned by the LSR to this interface.




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11.  Security Considerations

   This document extends LSP Traceroute mechanism to discover and
   exercise L2 ECMP paths.  As a result of supporting the code points
   and procedures described in this document, additional processing are
   required by initiator LSRs and responder LSRs, especially to compute
   and handle the additional multipath information.  Due to additional
   processing, it is critical that proper security measures described in
   [RFC8029] are followed.

   The LSP Traceroute allows an initiator LSR to discover the paths of
   tested LSPs, providing detailed knowledge of the MPLS network.
   Exposing such information to a malicious user is considered
   dangerous.  To prevent leakage of vital information to untrusted
   users, a responder LSR MUST only accept MPLS echo request messages
   from trusted sources via filtering source IP address field of
   received MPLS echo request messages.[RFC8029] provides additional
   recommendations to avoid attacks and recommendations to follow if an
   operator desires to prevent tracing.

12.  IANA Considerations

12.1.  LSR Capability TLV

   The IANA is requested to assign new value TBD1 for LSR Capability TLV
   from the "Multiprotocol Label Switching Architecture (MPLS) Label
   Switched Paths (LSPs) Ping Parameters - TLVs" registry.

     Value   Meaning                                      Reference
     -----   -------                                      ---------
     TBD1    LSR Capability TLV                           this document

12.1.1.  LSR Capability Flags

   The IANA is requested to create and maintain a registry entitled "LSR
   Capability Flags" with following registration procedures:

    Registry Name: LAG Interface Info Flags

    Bit number Name                                        Reference
    ---------- ----------------------------------------    ---------
            31 D: Downstream LAG Info Accommodation        this document
            30 U: Upstream LAG Info Accommodation          this document
          0-29 Unassigned

   Assignments of LSR Capability Flags are via Standards Action
   [RFC8126].




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12.2.  Local Interface Index Sub-TLV

   The IANA is requested to assign new value TBD2 (from the range
   4-31743) for the Local Interface Index Sub-TLV from the
   "Multiprotocol Label Switching Architecture (MPLS) Label Switched
   Paths (LSPs) Ping Parameters - TLVs" registry, "Sub-TLVs for TLV
   Types 20" sub-registry.

     Value   Meaning                                      Reference
     -----   -------                                      ---------
     TBD2    Local Interface Index Sub-TLV                this document

12.2.1.  Interface Index Flags

   The IANA is requested to create and maintain a registry entitled
   "Interface Index Flags" with following registration procedures:

    Registry Name: Interface Index Flags

    Bit number Name                                        Reference
    ---------- ----------------------------------------    ---------
            15 M: LAG Member Link Indicator                this document
          0-14 Unassigned

   Assignments of Interface Index Flags are via Standards Action
   [RFC8126].

   Note that this registry is used by the Interface Index Flags field of
   following Sub-TLVs:

   o  The Local Interface Index Sub-TLV which may be present in the
      "Downstream Detailed Mapping" TLV.

   o  The Remote Interface Index Sub-TLV which may be present in the
      "Downstream Detailed Mapping" TLV.

   o  The Incoming Interface Index Sub-TLV which may be present in the
      "Detailed Interface and Label Stack" TLV.

12.3.  Remote Interface Index Sub-TLV

   The IANA is requested to assign new value TBD3 (from the range
   32768-49161) for the Remote Interface Index Sub-TLV from the
   "Multiprotocol Label Switching Architecture (MPLS) Label Switched
   Paths (LSPs) Ping Parameters - TLVs" registry, "Sub-TLVs for TLV
   Types 20" sub-registry.





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     Value   Meaning                                      Reference
     -----   -------                                      ---------
     TBD3    Remote Interface Index Sub-TLV               this document

12.4.  Detailed Interface and Label Stack TLV

   The IANA is requested to assign new value TBD4 for Detailed Interface
   and Label Stack TLV from the "Multiprotocol Label Switching
   Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters -
   TLVs" registry ([IANA-MPLS-LSP-PING]).

     Value   Meaning                                      Reference
     -----   -------                                      ---------
     TBD4    Detailed Interface and Label Stack TLV       this document

12.4.1.  Sub-TLVs for TLV Type TBD4

   The IANA is requested to create and maintain a sub-registry entitled
   "Sub-TLVs for TLV Type TBD4" under "Multiprotocol Label Switching
   Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters -
   TLVs" registry.

   Initial values for this sub-registry, "Sub-TLVs for TLV Types TBD4",
   are described below.

     Sub-Type     Name                                    Reference
     -----------  --------------------------------------  ---------
     1            Incoming Label Stack                    this document
     2            Incoming Interface Index                this document
     3-16383      Unassigned (mandatory TLVs)
     16384-31743  Experimental
     32768-49161  Unassigned (optional TLVs)
     49162-64511  Experimental

   Assignments of Sub-Types in the mandatory and optional spaces are via
   Standards Action [RFC8126].  Assignments of Sub-Types in the
   experimental space is via Specification Required [RFC8126].

12.5.  DS Flags

   The IANA is requested to assign a new bit number from the "DS flags"
   sub-registry from the "Multi-Protocol Label Switching (MPLS) Label
   Switched Paths (LSPs) Ping Parameters - TLVs" registry
   ([IANA-MPLS-LSP-PING]).

   Note: the "DS flags" sub-registry is created by [RFC8029].





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    Bit number Name                                        Reference
    ---------- ----------------------------------------    ---------
          TBD5 G: LAG Description Indicator                this document

13.  Acknowledgements

   The authors would like to thank Nagendra Kumar, Sam Aldrin, for
   providing useful comments and suggestions.  The authors would like to
   thank Loa Andersson for performing a detailed review and providing
   number of comments.

   The authors also would like to extend sincere thanks to the MPLS RT
   review members who took time to review and provide comments.  The
   members are Eric Osborne, Mach Chen and Yimin Shen.  The suggestion
   by Mach Chen to generalize and create the LSR Capability TLV was
   tremendously helpful for this document and likely for future
   documents extending the MPLS LSP Ping and Traceroute mechanism.  The
   suggestion by Yimin Shen to create two separate validation procedures
   had a big impact to the contents of this document.

14.  References

14.1.  Normative References

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

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

14.2.  Informative References

   [IANA-MPLS-LSP-PING]
              IANA, "Multi-Protocol Label Switching (MPLS) Label
              Switched Paths (LSPs) Ping Parameters",
              <http://www.iana.org/assignments/mpls-lsp-ping-parameters/
              mpls-lsp-ping-parameters.xhtml>.





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   [IEEE802.1AX]
              IEEE Std. 802.1AX, "IEEE Standard for Local and
              metropolitan area networks - Link Aggregation", November
              2008.

   [RFC7439]  George, W., Ed. and C. Pignataro, Ed., "Gap Analysis for
              Operating IPv6-Only MPLS Networks", RFC 7439,
              DOI 10.17487/RFC7439, January 2015,
              <https://www.rfc-editor.org/info/rfc7439>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Appendix A.  LAG with intermediate L2 Switch Issues

   Several flavors of "LAG with L2 switch" provisioning models and the
   corresponding MPLS data plane ECMP traversal validation issues are
   described in this section .

A.1.  Equal Numbers of LAG Members

   R1 ==== S1 ==== R2

   The issue with this LAG provisioning model is that packets traversing
   a LAG member from Router 1 (R1) to intermediate L2 switch (S1) can
   get load balanced by S1 towards Router 2 (R2).  Therefore, MPLS echo
   request messages traversing a specific LAG member from R1 to S1 can
   actually reach R2 via any of the LAG members, and the sender of MPLS
   echo request messages has no knowledge of this nor no way to control
   this traversal.  In the worst case, MPLS echo request messages with
   specific entropies to exercise every LAG members from R1 to S1 can
   all reach R2 via same LAG member.  Thus it is impossible for MPLS
   echo request sender to verify that packets intended to traverse
   specific LAG member from R1 to S1 did actually traverse that LAG
   member, and to deterministically exercise "receive" processing of
   every LAG member on R2.

A.2.  Deviating Numbers of LAG Members

              ____
   R1 ==== S1 ==== R2

   There are deviating number of LAG members on the two sides of the L2
   switch.  The issue with this LAG provisioning model is the same as
   previous model, sender of MPLS echo request messages have no




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   knowledge of L2 load balance algorithm nor entropy values to control
   the traversal.

A.3.  LAG Only on Right

   R1 ---- S1 ==== R2

   The issue with this LAG provisioning model is that there is no way
   for MPLS echo request sender to deterministically exercise both LAG
   members from S1 to R2.  And without such, "receive" processing of R2
   on each LAG member cannot be verified.

A.4.  LAG Only on Left

   R1 ==== S1 ---- R2

   MPLS echo request sender has knowledge of how to traverse both LAG
   members from R1 to S1.  However, both types of packets will terminate
   on the non-LAG interface at R2.  It becomes impossible for MPLS echo
   request sender to know that MPLS echo request messages intended to
   traverse a specific LAG member from R1 to S1 did indeed traverse that
   LAG member.

Authors' Addresses

   Nobo Akiya
   Big Switch Networks

   Email: nobo.akiya.dev@gmail.com


   George Swallow
   Cisco Systems

   Email: swallow@cisco.com


   Stephane Litkowski
   Orange

   Email: stephane.litkowski@orange.com


   Bruno Decraene
   Orange

   Email: bruno.decraene@orange.com




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   John E. Drake
   Juniper Networks

   Email: jdrake@juniper.net


   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com









































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