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RFC 8611

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

Pages: 29
Group: MPLS
Proposed STD
Updates:  8029
Part 1 of 3 – Pages 1 to 11
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Top   ToC   RFC8611 - Page 1
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.
Top   ToC   RFC8611 - Page 2
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
Top   ToC   RFC8611 - Page 3
     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.
Top   ToC   RFC8611 - Page 4
   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
Top   ToC   RFC8611 - Page 5
   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;
Top   ToC   RFC8611 - Page 6
   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.
Top   ToC   RFC8611 - Page 7
   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-
Top   ToC   RFC8611 - Page 8
   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.
Top   ToC   RFC8611 - Page 9
   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.
Top   ToC   RFC8611 - Page 10
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].
Top   ToC   RFC8611 - Page 11
   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



(page 11 continued on part 2)

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