





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.



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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.






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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.







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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.











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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.





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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:






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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



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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



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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.



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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.













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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



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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].




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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.








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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>.








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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>.

























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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.




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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.

























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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















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