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RFC 8611

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

Pages: 29
Proposed Standard
Updates:  8029
Updated by:  9041
Part 1 of 3 – Pages 1 to 11
None   None   Next

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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
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.
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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
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.
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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-
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.
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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



(page 11 continued on part 2)

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