Internet Engineering Task Force (IETF) N. Akiya

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
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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.

Akiya, et al. Standards Track [Page 13]

RFC 8611 LSP Ping for LAG June 2019

5.2. Individual End-to-End Path Verification

When the Remote Interface Index Sub-TLVs are available from an LSR
with TTL=n, then the validation of LAG member link traversal can be
performed by the downstream LSR of TTL=n+1. The initiator LSR
follows the procedures described in Section 4.3.

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

6. LSR Capability TLV

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

The format of the LSR Capability TLV is as below:

LSR Capability TLV Type is 4. Length is 4. The LSR Capability TLV
has the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSR Capability Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: LSR Capability TLV

Where:

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

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

Akiya, et al. Standards Track [Page 14]

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The LSR Capability Flags field is 4 octets in length; this
document defines the following flags:

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

This document defines two flags. The unallocated flags MUST be
set to zero when sending and ignored on receipt. Both the U and
the D flag MUST be cleared in the MPLS echo request message when
sending and ignored on receipt. Zero, one, or both of the flags
(U and D) MAY be set in the MPLS echo reply message.

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

U Upstream LAG Info Accommodation

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

D Downstream LAG Info Accommodation

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

7. LAG Description Indicator Flag: G

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

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

Akiya, et al. Standards Track [Page 15]

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The G flag is defined as below:

The Bit Number is 3.

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

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

G LAG Description Indicator

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

8. Local Interface Index Sub-TLV

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

The format of the Local Interface Index Sub-TLV is below:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Interface Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: Local Interface Index Sub-TLV

Where:

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

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

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

Akiya, et al. Standards Track [Page 16]

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9. Remote Interface Index Sub-TLV

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

The format of the Remote Interface Index Sub-TLV is below:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Interface Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6: Remote Interface Index Sub-TLV

Where:

o The Type field is 2 octets in length, and the value is 5.

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

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

10. Detailed Interface and Label Stack TLV

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

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

Akiya, et al. Standards Track [Page 17]

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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Type | Reserved (Must Be Zero) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. List of Sub-TLVs .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: Detailed Interface and Label Stack TLV

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

Address Type

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

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

IP Address and Interface

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

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

Akiya, et al. Standards Track [Page 18]

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

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

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

10.1. Sub-TLVs

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

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

10.1.1. Incoming Label Stack Sub-TLV

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

Incoming Label Stack Sub-TLV Type is 1. Length is variable, and its
format is as below:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 8: Incoming Label Stack Sub-TLV

Akiya, et al. Standards Track [Page 19]

RFC 8611 LSP Ping for LAG June 2019

10.1.2. Incoming Interface Index Sub-TLV

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

Incoming Interface Index Sub-TLV Type is 2. Length is 8, and its
format is as below:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface Index Flags | Reserved (Must Be Zero) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Incoming Interface Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 9: Incoming Interface Index Sub-TLV

Interface Index Flags

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

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

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

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

M LAG Member Link Indicator

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

Incoming Interface Index

An Index assigned by the LSR to this interface. It's normally an
unsigned integer and in network byte order.

Akiya, et al. Standards Track [Page 20]

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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.

Akiya, et al. Standards Track [Page 21]

RFC 8611 LSP Ping for LAG June 2019

13. IANA Considerations

13.1. LSR Capability TLV

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

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

13.1.1. LSR Capability Flags

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

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

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

13.2. Local Interface Index Sub-TLV

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

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

13.2.1. Interface Index Flags

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

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

Akiya, et al. Standards Track [Page 22]

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

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

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

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

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

13.3. Remote Interface Index Sub-TLV

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

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

13.4. Detailed Interface and Label Stack TLV

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

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

13.4.1. Sub-TLVs for TLV Type 6

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

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

Akiya, et al. Standards Track [Page 23]

RFC 8611 LSP Ping for LAG June 2019

This registry conforms with RFC 8029.

The registration procedures for this sub-TLV registry are:

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

The initial allocations for this registry are:

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

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

Akiya, et al. Standards Track [Page 24]

RFC 8611 LSP Ping for LAG June 2019

13.4.2. Interface and Label Stack Address Types

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

13.5. DS Flags

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

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

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

14. References

14.1. Normative References

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

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

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

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

Akiya, et al. Standards Track [Page 25]

RFC 8611 LSP Ping for LAG June 2019

14.2. Informative References

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

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

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

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

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

Akiya, et al. Standards Track [Page 26]

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Appendix A. LAG with Intermediate L2 Switch Issues

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

A.1. Equal Numbers of LAG Members

R1 ==== S1 ==== R2

The issue with this LAG provisioning model is that packets traversing
a LAG member from Router 1 (R1) to intermediate L2 switch (S1) can
get load-balanced by S1 towards Router 2 (R2). Therefore, MPLS echo
request messages traversing a specific LAG member from R1 to S1 can
actually reach R2 via any of the LAG members, and the sender of the
MPLS echo request messages has no knowledge of this nor any way to
control this traversal. In the worst case, MPLS echo request
messages with specific entropies will exercise every LAG member link
from R1 to S1 and can all reach R2 via the same LAG member link.
Thus, it is impossible for the MPLS echo request sender to verify
that packets intended to traverse a specific LAG member link from R1
to S1 did actually traverse that LAG member link and to
deterministically exercise "receive" processing of every LAG member
link on R2. (Note: As far as we can tell, there's not a better
option than "try a bunch of entropy labels and see what responses you
can get back", and that's the same remedy in all the described
topologies.)

A.2. Deviating Numbers of LAG Members

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

There are deviating numbers of LAG members on the two sides of the L2
switch. The issue with this LAG provisioning model is the same as
with the previous model: the sender of MPLS echo request messages has
no knowledge of the L2 load-balancing algorithm nor entropy values to
control the traversal.

A.3. LAG Only on Right

R1 ---- S1 ==== R2

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

Akiya, et al. Standards Track [Page 27]

RFC 8611 LSP Ping for LAG June 2019

A.4. LAG Only on Left

R1 ==== S1 ---- R2

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

Acknowledgements

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

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

Akiya, et al. Standards Track [Page 28]

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Authors' Addresses

Nobo Akiya
Big Switch Networks

Email: [email protected]

George Swallow
Southend Technical Center

Email: [email protected]

Stephane Litkowski
Orange

Email: [email protected]

Bruno Decraene
Orange

Email: [email protected]

John E. Drake
Juniper Networks

Email: [email protected]

Mach(Guoyi) Chen
Huawei

Email: [email protected]

Akiya, et al. Standards Track [Page 29]