    Internet Engineering Task Force (IETF)                          N. Akiya
    Request for Comments: 8611                           Big Switch Networks
    Updates: 8029                                                 G. Swallow
    Category: Standards Track                                           SETC
    ISSN: 2070-1721                                             S. Litkowski
                                                                 B. Decraene
                                                                      Orange
                                                                    J. Drake
                                                            Juniper Networks
                                                                     M. Chen
                                                                      Huawei
                                                                   June 2019

        Label Switched Path (LSP) Ping and Traceroute Multipath Support
                  for Link Aggregation Group (LAG) Interfaces

    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 the determination of
       the capabilities supported by a Label Switching Router (LSR).

       This document updates RFC 8029.

    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
       https://www.rfc-editor.org/info/rfc8611.

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    Copyright Notice

       Copyright (c) 2019 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
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
         1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
         1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
         1.3.  Requirements Language . . . . . . . . . . . . . . . . . .   4
       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 . . . . . . . . . . . . . . . .   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 . . . . . . . . .  14
       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
           10.1.2.  Incoming Interface Index Sub-TLV . . . . . . . . . .  20
       11. Rate-Limiting on Echo Request/Reply Messages  . . . . . . . .  21
       12. Security Considerations . . . . . . . . . . . . . . . . . . .  21
       13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
         13.1.  LSR Capability TLV . . . . . . . . . . . . . . . . . . .  22
           13.1.1.  LSR Capability Flags . . . . . . . . . . . . . . . .  22

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         13.2.  Local Interface Index Sub-TLV  . . . . . . . . . . . . .  22
           13.2.1.  Interface Index Flags  . . . . . . . . . . . . . . .  22
         13.3.  Remote Interface Index Sub-TLV . . . . . . . . . . . . .  23
         13.4.  Detailed Interface and Label Stack TLV . . . . . . . . .  23
           13.4.1.  Sub-TLVs for TLV Type 6  . . . . . . . . . . . . . .  23
           13.4.2.  Interface and Label Stack Address Types  . . . . . .  25
         13.5.  DS Flags . . . . . . . . . . . . . . . . . . . . . . . .  25
       14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
         14.1.  Normative References . . . . . . . . . . . . . . . . . .  25
         14.2.  Informative References . . . . . . . . . . . . . . . . .  26
       Appendix A.  LAG with Intermediate L2 Switch Issues . . . . . . .  27
         A.1.  Equal Numbers of LAG Members  . . . . . . . . . . . . . .  27
         A.2.  Deviating Numbers of LAG Members  . . . . . . . . . . . .  27
         A.3.  LAG Only on Right . . . . . . . . . . . . . . . . . . . .  27
         A.4.  LAG Only on Left  . . . . . . . . . . . . . . . . . . . .  28
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  28
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

    1.  Introduction

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

       o  Label switching over some member links of the LAG is successful,
          but fails 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 that is label switching successfully.

       With the above scenario, 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.

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       The work toward a solution to this problem was motivated by issues
       encountered in live networks.

    1.2.  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 - The LSR that sends the MPLS echo request message.

       o  Responder LSR - The LSR that receives the MPLS echo request
          message and sends the MPLS echo reply message.

    1.3.  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
       BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
       capitals, as shown here.

    2.  Overview of Solution

       This document defines a new 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 the
       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 new LSR Capability TLV to indicate that separate load-
       balancing information for each L2 next hop over LAG is desired in the
       MPLS echo reply.  The responder LSR places the same 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-
       balancing information (i.e., multipath information and interface

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       index).  This mechanism is applicable to all types of LSPs that can
       traverse LAG interfaces.  Many LAGs are built from peer-to-peer
       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 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 provide an example of 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-balancing information for the 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 information;

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

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       o  in Section 6, the LSR Capability TLV;

       o  in Section 7, the LAG Description Indicator flag;

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

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

       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
       mechanisms 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 a subset of features X, Y, and Z (i.e., not all of
          them) implemented.

       o  The responder LSR sent the MPLS echo reply message with contents C
          because it does not have features X, Y, or 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 the 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", according to the rules defined in Section 3
       of [RFC8029].  Then, the initiator LSR can send another MPLS echo
       request without including the LSR Capability TLV.

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

    3.1.  Initiator LSR Procedures

       If an initiator LSR does not know what capabilities a responder LSR
       can support, it can send an MPLS echo 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 receives 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 the outgoing LAG member links
             separately; otherwise, clear the Downstream LAG Info
             Accommodation flag.

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

    4.  Mechanism to Discover L2 ECMP

    4.1.  Initiator LSR Procedures

       Through 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.

       Once the initiator LSR knows that a responder can support this
       mechanism, then it sends an MPLS echo request carrying a DDMAP TLV
       with the LAG Description Indicator flag (G) set to the responder LSR.
       The LAG Description Indicator flag (G) indicates that separate load-

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       balancing information for each L2 next hop 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 receives an MPLS echo request message with the
       LAG Description Indicator flag set in the DDMAP TLV, 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 the outgoing
       interfaces include LAG interfaces:

       o  For each downstream interface that is a LAG interface:

          *  The responder LSR MUST include a DDMAP TLV when sending the
             MPLS echo reply.  There is a single DDMAP TLV for the LAG
             interface, with member links described using sub-TLVs.

          *  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.  It describes
                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.  It describes
                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.

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

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       Based on the procedures described above, every LAG member link will
       have a Local Interface Index Sub-TLV and a Multipath Data Sub-TLV
       entry 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, except as follows.  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.  The blocks of Local Interface Index, Remote
       Interface Index (optional), and Multipath Data Sub-TLVs for each
       member link MUST appear adjacent to each other and be in order of
       increasing local interface index.

       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 (DS Flags with G set)  ~
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Local Interface Index Sub-TLV of LAG member link #1           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Remote Interface Index Sub-TLV of LAG member link #1          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Multipath Data Sub-TLV LAG member link #1                     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Local Interface Index Sub-TLV of LAG member link #2           |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Remote Interface Index Sub-TLV of LAG member link #2          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | 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 the 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 in Section 4.2 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 that is expected to traverse the
          specific LAG member link described by the 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 according to the
       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].

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

          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 (DS Flags with 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 construct a DDMAP
       for a downstream LAG.  The Remote Interface Index Sub-TLV 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
       the initiator LSR traverses all available LAG member links and takes
       the results through additional 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

       When it receives an echo request with the LAG Description Indicator
       flag set, a responder LSR that understands that flag and is capable
       of describing the incoming LAG member link SHOULD use the following
       procedures, regardless of whether or not the 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 that received the MPLS echo request.

       These procedures allow the initiator LSR to utilize the Incoming
       Interface Index Sub-TLV in the Detailed Interface and the 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.

       Depending on the LAG traffic division algorithm, the messages may or
       may not traverse different member links.  The 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 entropies examined.  In other words,
       the messages with a set of entropies that load-balances to outgoing
       LAG member link X at TTL=n should all reach the next hop on the same
       incoming LAG member link Y at TTL=n+1.

    Akiya, et al.                Standards Track                   [Page 12]

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       With additional logic, the initiator LSR can perform the following
       checks in a scenario where it (a) knows that there is a LAG that has
       two LAG members, between TTL=n and TTL=n+1, and (b) 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 LAG member
             link 1.

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

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

       o  Error case:

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

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

          *  One or both MPLS echo request messages cannot reach the
             immediate downstream LSR on whichever link.

          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 procedures defined above 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 an L2 switch
       between the LSR at TTL=n and the LSR at TTL=n+1, there is no
       guarantee that every incoming interface at TTL=n+1 can be traversed,
       even when traversing every outgoing LAG member link at TTL=n.  Issues
       resulting from LAG with an L2 switch in between are further described
       in Appendix A.  LAG provisioning models in operator networks should
       be considered when analyzing the output of LSP Traceroute that is
       exercising L2 ECMPs.

    Akiya, et al.                Standards Track                   [Page 13]

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    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
       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 TLV that 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.

       The format of the LSR Capability TLV is as below:

       LSR Capability TLV Type is 4.  Length is 4.  The LSR Capability TLV
       has the 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 field is 2 octets in length, and the value is 4.

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

    Akiya, et al.                Standards Track                   [Page 14]

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          The LSR Capability Flags field is 4 octets in length; this
          document defines the 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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                 Reserved (Must Be Zero)                   |U|D|
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          This document defines two flags.  The unallocated 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.  Zero, one, or both of the flags
          (U and D) 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 new 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.

    Akiya, et al.                Standards Track                   [Page 15]

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       The G flag is defined as below:

          The Bit Number is 3.

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

       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 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, and the value is 4.

       o  The Length field is 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.
          It's normally an unsigned integer and in network byte order.

    Akiya, et al.                Standards Track                   [Page 16]

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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 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 5.

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

       o  The Remote Interface Index field is 4 octets in length; it is an
          interface index assigned by a downstream LSR to an ingress
          interface.  It's normally an unsigned integer and in network byte
          order.

    10.  Detailed Interface and Label Stack TLV

       The Detailed Interface and Label Stack TLV 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 6.  Length is K + Sub-
       TLV Length (sum of Sub-TLVs).  K is the sum of all fields of this TLV
       prior to the list of Sub-TLVs, but the length of K depends on the
       Address Type.  Details of this information is described below.  The
       Detailed Interface and Label Stack TLV has the following format:

    Akiya, et al.                Standards Track                   [Page 17]

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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  |             Reserved (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.  The other fields of the 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

    Akiya, et al.                Standards Track                   [Page 18]

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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],
             which is described in Section 3.4.2 of [RFC7439].  A solution
             should be considered and 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

    Akiya, et al.                Standards Track                   [Page 19]

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    10.1.2.  Incoming Interface Index Sub-TLV

       The Incoming Interface Index Sub-TLV 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 that
       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      |       Reserved (Must Be Zero) |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                   Incoming Interface Index                    |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 9: Incoming Interface Index Sub-TLV

       Interface Index Flags

          The 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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |   Reserved (Must Be Zero)   |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, the 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.  It's normally an
          unsigned integer and in network byte order.

    Akiya, et al.                Standards Track                   [Page 20]

    RFC 8611                    LSP Ping for LAG                   June 2019

    11.  Rate-Limiting on Echo Request/Reply Messages

       An LSP may be over several LAGs.  Each LAG may have many member
       links.  To exercise all the links, many echo request/reply messages
       will be sent in a short period.  It's possible that those messages
       may traverse a common path as a burst.  Under some circumstances,
       this might cause congestion at the common path.  To avoid potential
       congestion, it is RECOMMENDED that implementations randomly delay the
       echo request and reply messages at the initiator LSRs and responder
       LSRs.  Rate-limiting of ping traffic is further specified in
       Section 5 of [RFC8029] and Section 4.1 of [RFC6425], which apply to
       this document as well.

    12.  Security Considerations

       This document extends the LSP Traceroute mechanism [RFC8029] to
       discover and exercise L2 ECMP paths to determine problematic member
       link(s) of a LAG.  These on-demand diagnostic mechanisms are used by
       an operator within an MPLS control domain.

       [RFC8029] reviews the possible attacks and approaches to mitigate
       possible threats when using these mechanisms.

       To prevent leakage of vital information to untrusted users, a
       responder LSR MUST only accept MPLS echo request messages from
       designated trusted sources via filtering the source IP address field
       of received MPLS echo request messages.  As noted in [RFC8029],
       spoofing attacks only have a small window of opportunity.  If an
       intermediate node hijacks these messages (i.e., causes non-delivery),
       the use of these mechanisms will determine the data plane is not
       working as it should.  Hijacking of a responder node such that it
       provides a legitimate reply would involve compromising the node
       itself and the MPLS control domain.  [RFC5920] provides additional
       MPLS network-wide operation recommendations to avoid attacks.  Please
       note that source IP address filtering provides only a weak form of
       access control and is not, in general, a reliable security mechanism.
       Nonetheless, it is required here in the absence of any more robust
       mechanisms that might be used.

    Akiya, et al.                Standards Track                   [Page 21]

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    13.  IANA Considerations

    13.1.  LSR Capability TLV

       IANA has assigned value 4 (from the range 0-16383) for the LSR
       Capability TLV from the "TLVs" registry under the "Multiprotocol
       Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
       registry [IANA-MPLS-LSP-PING].

         Type    TLV Name                                    Reference
         -----   --------                                    ---------
           4     LSR Capability                              RFC 8611

    13.1.1.  LSR Capability Flags

       IANA has created a new "LSR Capability Flags" registry.  The initial
       contents are as follows:

         Value   Meaning                                     Reference
         -----   -------                                     ---------
           31    D: Downstream LAG Info Accommodation        RFC 8611
           30    U: Upstream LAG Info Accommodation          RFC 8611
         0-29    Unassigned

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

    13.2.  Local Interface Index Sub-TLV

       IANA has assigned value 4 (from the range 0-16383) for the Local
       Interface Index Sub-TLV from the "Sub-TLVs for TLV Type 20"
       subregistry of the "TLVs" registry in the "Multiprotocol Label
       Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
       registry [IANA-MPLS-LSP-PING].

         Sub-Type   Sub-TLV Name                             Reference
         --------   ------------                             ---------
            4       Local Interface Index                    RFC 8611

    13.2.1.  Interface Index Flags

       IANA has created a new "Interface Index Flags" registry.  The initial
       contents are as follows:

        Bit Number Name                                      Reference
        ---------- --------------------------------          ---------
             15    M: LAG Member Link Indicator              RFC 8611
           0-14    Unassigned

    Akiya, et al.                Standards Track                   [Page 22]

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

       Note that this registry is used by the Interface Index Flags field of
       the 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.

    13.3.  Remote Interface Index Sub-TLV

       IANA has assigned value 5 (from the range 0-16383) for the Remote
       Interface Index Sub-TLV from the "Sub-TLVs for TLV Type 20"
       subregistry of the "TLVs" registry in the "Multiprotocol Label
       Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
       registry [IANA-MPLS-LSP-PING].

         Sub-Type   Sub-TLV Name                             Reference
         --------   ------------                             ---------
           5        Remote Interface Index                   RFC 8611

    13.4.  Detailed Interface and Label Stack TLV

       IANA has assigned value 6 (from the range 0-16383) for the Detailed
       Interface and Label Stack TLV from the "TLVs" registry in the
       "Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs)
       Ping Parameters" registry [IANA-MPLS-LSP-PING].

         Type    TLV Name                                    Reference
         -----   --------                                    ---------
           6     Detailed Interface and Label Stack          RFC 8611

    13.4.1.  Sub-TLVs for TLV Type 6

       RFC 8029 changed the registration procedures for TLV and sub-TLV
       registries for LSP Ping.

       IANA has created a new "Sub-TLVs for TLV Type 6" subregistry under
       the "TLVs" registry of the "Multiprotocol Label Switching (MPLS)
       Label Switched Paths (LSPs) Ping Parameters" registry
       [IANA-MPLS-LSP-PING].

    Akiya, et al.                Standards Track                   [Page 23]

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       This registry conforms with RFC 8029.

       The registration procedures for this sub-TLV registry are:

       Range        Registration Procedure   Note
       -----        ----------------------   -----
       0-16383      Standards Action         This range is for mandatory
                                             TLVs or for optional TLVs that
                                             require an error message if
                                             not recognized.
       16384-31743  RFC Required             This range is for mandatory
                                             TLVs or for optional TLVs that
                                             require an error message if
                                             not recognized.
       31744-32767  Private Use              Not to be assigned
       32768-49161  Standards Action         This range is for optional TLVs
                                             that can be silently dropped if
                                             not recognized.
       49162-64511  RFC Required             This range is for optional TLVs
                                             that can be silently dropped if
                                             not recognized.
       64512-65535  Private Use              Not to be assigned

       The initial allocations for this registry are:

       Sub-Type     Sub-TLV Name             Reference Comment
       --------     ------------             --------- -------
       0            Reserved                 RFC 8611
       1            Incoming Label Stack     RFC 8611
       2            Incoming Interface Index RFC 8611
       3-31743      Unassigned
       31744-32767                           RFC 8611  Reserved for
                                                       Private Use
       32768-64511  Unassigned
       64512-65535                           RFC 8611  Reserved for
                                                       Private Use

       Note: IETF does not prescribe how the Private Use sub-TLVs are
       handled; however, if a packet containing a sub-TLV from a Private Use
       ranges is received by an LSR that does not recognize the sub-TLV, an
       error message MAY be returned if the sub-TLV is from the range
       31744-32767, and the packet SHOULD be silently dropped if it is from
       the range 64511-65535.

    Akiya, et al.                Standards Track                   [Page 24]

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    13.4.2.  Interface and Label Stack Address Types

       The Detailed Interface and Label Stack TLV shares the Interface and
       Label Stack Address Types with the Interface and Label Stack TLV.  To
       reflect this, IANA has updated the name of the registry from
       "Interface and Label Stack Address Types" to "Interface and Label
       Stack and Detailed Interface and Label Stack Address Types".

    13.5.  DS Flags

       IANA has assigned a new bit number from the "DS Flags" subregistry of
       the "Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs)
       Ping Parameters" registry [IANA-MPLS-LSP-PING].

       Note: the "DS Flags" subregistry was created by [RFC8029].

        Bit number Name                                        Reference
        ---------- ----------------------------------------    ---------
             3     G: LAG Description Indicator                RFC 8611

    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>.

       [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>.

       [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>.

    Akiya, et al.                Standards Track                   [Page 25]

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    14.2.  Informative References

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

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

       [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
                  Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
                  <https://www.rfc-editor.org/info/rfc5920>.

       [RFC6425]  Saxena, S., Ed., Swallow, G., Ali, Z., Farrel, A.,
                  Yasukawa, S., and T. Nadeau, "Detecting Data-Plane
                  Failures in Point-to-Multipoint MPLS - Extensions to LSP
                  Ping", RFC 6425, DOI 10.17487/RFC6425, November 2011,
                  <https://www.rfc-editor.org/info/rfc6425>.

       [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>.

    Akiya, et al.                Standards Track                   [Page 26]

    RFC 8611                    LSP Ping for LAG                   June 2019

    Appendix A.  LAG with Intermediate L2 Switch Issues

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

    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 the
       MPLS echo request messages has no knowledge of this nor any way to
       control this traversal.  In the worst case, MPLS echo request
       messages with specific entropies will exercise every LAG member link
       from R1 to S1 and can all reach R2 via the same LAG member link.
       Thus, it is impossible for the MPLS echo request sender to verify
       that packets intended to traverse a specific LAG member link from R1
       to S1 did actually traverse that LAG member link and to
       deterministically exercise "receive" processing of every LAG member
       link on R2.  (Note: As far as we can tell, there's not a better
       option than "try a bunch of entropy labels and see what responses you
       can get back", and that's the same remedy in all the described
       topologies.)

    A.2.  Deviating Numbers of LAG Members

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

       There are deviating numbers of LAG members on the two sides of the L2
       switch.  The issue with this LAG provisioning model is the same as
       with the previous model: the sender of MPLS echo request messages has
       no knowledge of the L2 load-balancing 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 an MPLS echo request sender to deterministically exercise both
       LAG member links from S1 to R2.  And without such, "receive"
       processing of R2 on each LAG member cannot be verified.

    Akiya, et al.                Standards Track                   [Page 27]

    RFC 8611                    LSP Ping for LAG                   June 2019

    A.4.  LAG Only on Left

       R1 ==== S1 ---- R2

       The 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
       the 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.

    Acknowledgements

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

       The authors also would like to extend sincere thanks to the MPLS RT
       review members who took the 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 mechanisms.  The
       suggestion by Yimin Shen to create two separate validation procedures
       had a big impact on the contents of this document.

    Akiya, et al.                Standards Track                   [Page 28]

    RFC 8611                    LSP Ping for LAG                   June 2019

    Authors' Addresses

       Nobo Akiya
       Big Switch Networks

       Email: nobo.akiya.dev@gmail.com

       George Swallow
       Southend Technical Center

       Email: swallow.ietf@gmail.com

       Stephane Litkowski
       Orange

       Email: stephane.litkowski@orange.com

       Bruno Decraene
       Orange

       Email: bruno.decraene@orange.com

       John E. Drake
       Juniper Networks

       Email: jdrake@juniper.net

       Mach(Guoyi) Chen
       Huawei

       Email: mach.chen@huawei.com

    Akiya, et al.                Standards Track                   [Page 29]