wdiff draft-akiya-mpls-lsp-ping-lag-multipath-04.txt draft-akiya-mpls-lsp-ping-lag-multipath-05.txt

Internet Engineering Task Force N. Akiya
Internet-Draft G. Swallow
Updates: 4379,6424 (if approved) Cisco Systems
Intended status: Standards Track S. Litkowski
Expires: June 2, 24, 2015 B. Decraene
Orange
J. Drake
Juniper Networks
November 29,
December 21, 2014

Label Switched Path (LSP) Ping/Trace Multipath Support for
Link Aggregation Group (LAG) Interfaces
draft-akiya-mpls-lsp-ping-lag-multipath-04
draft-akiya-mpls-lsp-ping-lag-multipath-05

Abstract

This document defines an extension to the MPLS Label Switched Path
(LSP) Ping and Traceroute as specified in RFC 4379. The extension
allows the MPLS LSP Ping and Traceroute to discover and exercise
specific paths of Layer 2 (L2) Equal-Cost Multipath (ECMP) over Link
Aggregation Group (LAG) interfaces.

This document updates RFC4379 and RFC6424.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on June 2, 24, 2015.

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Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. LSR Capability Discovery . . . . . . . . . . . . . . . . . . 6
4. Mechanism to Discover L2 ECMP Multipath . . . . . . . . . . . 5
3.1. 7
4.1. Initiator LSR Procedures . . . . . . . . . . . . . . . . 5
3.2. 7
4.2. Responder LSR Procedures . . . . . . . . . . . . . . . . 5
3.3. 7
4.3. Additional Initiator LSR Procedures . . . . . . . . . . . 7
4. 9
5. Mechanism to Validate L2 ECMP Traversal . . . . . . . . . . . 8
4.1. 10
5.1. Incoming LAG Member Links Verification . . . . . . . . . 11
5.1.1. Initiator LSR Procedures . . . . . . . . . . . . . . . . 8
4.2. 11
5.1.2. Responder LSR Procedures . . . . . . . . . . . . . . . . 9
4.3. 11
5.1.3. Additional Initiator LSR Procedures . . . . . . . . . 12
5.2. Individual End-to-End Path Verification . . . . 9
5. LAG Interface Info TLV . . . . . 13
6. LSR Capability TLV . . . . . . . . . . . . . . 11
6. DDMAP TLV DS Flags: G . . . . . . . 14
7. LAG Description Indicator Flag: G . . . . . . . . . . . . . . 12
7. 15
8. Local Interface Index Sub-TLV . . . . . . . . . . . . . . . . 16
9. Remote Interface Index Sub-TLV . . . . . . 12
8. . . . . . . . . . 17
10. Detailed Interface and Label Stack TLV . . . . . . . . . . . 13
8.1. 18
10.1. Sub-TLVs . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1.1. 20
10.1.1. Incoming Label Stack Sub-TLV . . . . . . . . . . . . 15
8.1.2. 20
10.1.2. Incoming Interface Index Sub-TLV . . . . . . . . . . 16
9. 20
11. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. 21
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
10.1. LAG Interface Info 22
12.1. LSR Capability TLV . . . . . . . . . . . . . . . . . 17
10.1.1. LAG Interface Info . . 22
12.1.1. LSR Capability Flags . . . . . . . . . . . . . . 18
10.2. . . 22
12.2. Local Interface Index Sub-TLV . . . . . . . . . . . . . . . . 18
10.2.1. 22
12.2.1. Interface Index Flags . . . . . . . . . . . . . . . 18
10.3. 23
12.3. Remote Interface Index Sub-TLV . . . . . . . . . . . . . 23
12.4. Detailed Interface and Label Stack TLV . . . . . . . . . 19
10.3.1. 23
12.4.1. Sub-TLVs for TLV Type TBD3 TBD4 . . . . . . . . . . . . . 19
10.4. 24
12.5. DS Flags . . . . . . . . . . . . . . . . . . . . . . . . 19
11. 24
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
12. 24
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. 25
14.1. Normative References . . . . . . . . . . . . . . . . . . 20
12.2. 25
14.2. Informative References . . . . . . . . . . . . . . . . . 20 25
Appendix A. LAG with L2 Switch Issues . . . . . . . . . . . . . 21 26
A.1. Equal Numbers of LAG Members . . . . . . . . . . . . . . 21 26
A.2. Deviating Numbers of LAG Members . . . . . . . . . . . . 21 26
A.3. LAG Only on Right . . . . . . . . . . . . . . . . . . . . 21 26
A.4. LAG Only on Left . . . . . . . . . . . . . . . . . . . . 22 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 27

1. Introduction

1.1. Terminology

The following acronyms/terminologies 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 - LSR which sends MPLS echo request.

o Responder LSR - LSR which receives MPLS echo request and sends
MPLS echo reply.

1.2. Background

The MPLS Label Switched Path (LSP) Ping and Traceroute as specified
in [RFC4379] are powerful tools designed to diagnose all available
layer 3 (L3) paths of LSPs, i.e. i.e., provides diagnostic coverage of L3
Equal-Cost Multipath (ECMP). In many MPLS networks, Link Aggregation
Group (LAG) as defined in [IEEE802.1AX], which provide Layer 2 (L2)
ECMP, are often used for various reasons. MPLS LSP Ping and
Traceroute tools were not designed to discover and exercise specific
paths of L2 ECMP. Result The result raises a limitation for following
scenario when LSP X traverses over LAG Y:

o Label switching of LSP X over one or more member links of LAG Y is
succeeding.
have succeeded.

o Label switching of LSP X over one or more member links of LAG Y is
failing.
have failed.

o MPLS echo request for LSP X over LAG Y is load balanced over a
member link which is label switching successfully.

With the above scenario, MPLS LSP Ping and Traceroute will not be
able to detect the MPLS label switching failure of problematic member
link(s) of the LAG. In other words, lack of L2 ECMP discovery and exercise
capability diagnostic
coverage can produce an outcome where MPLS LSP Ping and Traceroute
can be blind to label switching failures over problematic LAG interface that are
impacting MPLS traffic.
interface. It is, thus, desirable to extend the MPLS LSP Ping and
Traceroute to have deterministic diagnostic coverage of LAG
interfaces.

Creation of this document was motivated by issues encountered in live
networks.

2. Overview

This document defines an extension to the MPLS LSP Ping and
Traceroute to describe Multipath Information for LAG member links
separately, thus allowing MPLS LSP Ping and Traceroute to discover
and exercise specific paths of L2 ECMP over LAG interfaces. Reader
is expected to be familiar with mechanics of the MPLS LSP Ping and
Traceroute described in Section 3.3 of [RFC4379] and Downstream
Detailed Mapping TLV (DDMAP) described in Section 3.3 of [RFC6424].

MPLS echo request carries a DDMAP and an optional TLV to indicate
that separate load balancing information for each L2 nexthop over LAG
is desired in MPLS echo reply. Responder LSR places the same
optional TLV in the MPLS echo reply to provide acknowledgement back
to the initiator. It also adds, for each downstream LAG member, a
load balance information (i.e. multipath information and interface
index). The following figure and the texts provides an example using
an LDP network. However the problem and the mechanism is applicable
to all types of LSPs which can traverse over LAG interfaces.

<----- 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 balance information for 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 discover capabilities of responder LSRs;

o In Section 4, a mechanism to discover L2 ECMP multipath
information;

o In Section 4, 5, a mechanism to validate L2 ECMP traversal in some
LAG provisioning models;

o In Section 5, 6, the LAG Interface Info LSR Capability TLV;

o In Section 6, 7, the LAG Description Indicator flag;

o In Section 7, 8, the Local Interface Index Sub-TLV;

o In Section 8, 9, the Remote Interface Index Sub-TLV;

o In Section 10, the Detailed Interface and Label Stack TLV;

o In Appendix A, issues with LAG having an L2 Switch.

Note that the mechanism described in this document does 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).

3. Mechanism

Also note that many LAGs are built from p2p links, and thus router X
and router X+1 have the same number of LAG members. It is possible
to build LAGs asymmetrically by using Ethernet switches in the
middle. Appendix A lists some cases which this document does not
address; if an operator deploys LAGs in a manner similar to what's
shown in Appendix A, the mechanisms in this document may not suit
them.

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 has been many extensions to the MPLS Ping and Traceroute
mechanism over the years. Thus it is often useful, and sometimes
necessary, for the initiator LSR to deterministically disambiguate
the difference 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 subset of features X, Y and Z implemented but not
all.

o The responder LSR sent the MPLS echo reply message with contents C
because it does not have features X, Y and 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 reply message with the LSR Capability TLV
included, then the responder LSR MUST include the LSR Capability TLV
in the MPLS echo reply message with the LSR Capability TLV describing
features and extensions supported by the local LSR.

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

4. Mechanism to Discover L2 ECMP Multipath

3.1.

4.1. Initiator LSR Procedures

The MPLS echo request carries a DDMAP and with the LAG Interface Info TLV
(described "LAG Description
Indicator flag" (G) set in Section 5) the DS Flags to indicate that separate
load balancing information for each L2 nexthop over LAG is desired in MPLS echo
reply.

3.2. Responder LAG is desired in
MPLS echo reply. The new "LAG Description Indicator flag" is
described in Section 7.

4.2. Responder LSR Procedures

This section describes the handling of the new TLVs by nodes which
understand the "LAG Description Indicator flag". There are two cases
- nodes which understand the "LAG Description Indicator flag" but
which for some reason cannot describe LAG members separately, and
nodes which both understand the "LAG Description Indicator flag" and
are able to describe LAG members separately. Note that Section 6,
Section 8 and Section 9 describe the new TLVs referenced by this
section , and looking over the definition of the new TLVs first may
make it easier to read this section.

A responder LSR Procedures

Responder LSRs that understand the LAG Interface Info TLV "LAG Description Indicator flag"
but are
unable to describe is not capable of describing outgoing LAG member links separately are to use
uses the following procedures:

o The responder LSR MUST add the LAG Interface Info TLV in If the received MPLS echo reply. This will allow request message had the initiator LSR to understand that Capability
TLV, the responder LSR understood MUST include the LAG Interface Info TLV. LSR Capability TLV in the
MPLS echo reply message.

o The responder LSR MUST clear the Downstream "Downstream LAG Info
Accommodation
flag flag" in the LAG Interface Info LSR Capability Flags field of the LAG Interface
Info LSR
Capability TLV. This will allow the initiator LSR to understand
that the responder LSR understood the LAG Interface Info TLV but cannot describe outgoing LAG member links
separately in the DDMAP.

The

A responder LSRs LSR that understands the LAG Interface Info TLV "LAG Description Indicator flag"
and
are able to describe is capable of describing outgoing LAG member links separately are to use
uses the follow procedures, regardless of whether or not outgoing
interfaces include LAG interfaces:

o The If the received MPLS echo request message had the LSR Capability
TLV, the responder LSR MUST add include the LAG Interface Info LSR Capability TLV in the
MPLS echo reply. reply message.

o The responder LSR MUST set the Downstream "Downstream LAG Info Accommodation
flag
flag" in the LAG Interface Info LSR Capability Flags field of the LAG Interface
Info LSR Capability TLV.

o For each downstream that is a LAG interface:

* The responder LSR MUST add DDMAP in the MPLS echo reply.

* The responder LSR MUST set the LAG "LAG Description Indicator flag flag"
in the DS Flags field (described in Section 6) of the DDMAP.

* In the DDMAP, Local Interface Index Sub-TLV, Remote Interface
Index Sub-TLV and Multipath Data Sub-
TLV Sub-TLV are to describe each
LAG member link. All other fields of the DDMAP are to describe
the LAG interface.

* For each LAG member link of this LAG interface:

+ The responder LSR MUST add an a Local Interface Index Sub-TLV
(described in Section 8) with the "LAG Member Link Indicator
flag" set in the Interface Index Flags field, describing 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 7) 9) with the "LAG Member Link Indicator
flag" set in the Interface Index Flags field, describing 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 Indicator
flag set in member link on the Interface Index Flags field, describing downstream LSR is
outside the scope of this
LAG member link. document.

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

Based on the procedures described above, every LAG member link will
have the a Local Interface Index Sub-TLV and the a Multipath Data Sub-TLV
entries in the DDMAP. When both The order of the Sub-TLVs in the DDMAP for a
LAG member link MUST be Local Interface Index Sub-TLV immediately
followed by Multipath Data Sub-TLV. 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 the a
Multipath Data Sub-TLV are placed in the DDMAP to describe a LAG
member link, Interface Index Sub-TLV they MUST be added first with placed in the order of Local Interface
Index Sub-TLV, Remote Interface Index-Sub-TLV and Multipath Data Sub-TLV immediately following.

For example, a Sub-
TLV.

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 with DS Flags G set | ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|[MANDATORY] Local Interface Index Sub-TLV of LAG member link #1 | #1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|[OPTIONAL] Remote Interface Index Sub-TLV of LAG member link #1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|[MANDATORY] Multipath Data Sub-TLV LAG member link #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|[MANDATORY] Local Interface Index Sub-TLV of LAG member link #2 | #2|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|[OPTIONAL] Remote Interface Index Sub-TLV of LAG member link #2|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|[MANDATORY] Multipath Data Sub-TLV LAG member link #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label Stack Sub-TLV |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Example of DDMAP in MPLS Echo Reply

3.3.

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, with the Multipath Length field of the
Multipath Data Sub-TLV being zero.

4.3. Additional Initiator LSR Procedures

Above

The procedures above allow an initiator LSR to:

o Require the responder LSR to always add the LAG Interface Info TLV
in the MPLS echo reply. This allows the initiator LSR to identify Identify whether or not the responder LSR understands the LAG Interface
Info TLV and can describe outgoing
LAG member links separately. separately, by looking at the LSR Capability TLV.

o Utilize the value of the LAG "LAG Description Indicator flag flag" in DS
Flags to identify whether each received DDMAP describes a LAG
interface or a non-LAG interface.

o Obtain multipath information which is expected to traverse the
specific LAG member link described by 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 with following
procedures:

o The Local Interface Index Sub-TLVs MUST NOT 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 SHOULD MUST be updated to include just the one
Multipath Data Sub-TLV. The initiator MAY keep just the Multipath
Data Sub-TLV corresponding to the LAG member link being traversed. The
initiator LSR MAY traversed,
or combine the Multipath Data Sub-TLVs for all LAG member links
into a single Multipath Data Sub-TLV, but there MUST
be only one Multipath Data Sub-TLV in the sending DDMAP. when diagnosing further
downstream LSRs.

o All other fields of the DDMAP are to comply with procedures
described in [RFC6424].

Using the DDMAP example described in the Figure 2, the DDMAP being
sent by the initiator LSR through LAG member link #1 to the next
downstream LSR should be:

Figure 3: Example of DDMAP in MPLS Echo Request

5. Mechanism to Validate L2 ECMP Traversal

This document does

Section 4 defines the responder LSR procedures to constructs a DDMAP
for a downstream LAG, and also defines that inclusion of the Remote
Interface Index Sub-TLVs describing the incoming LAG member links of
the downstream LSR is optional. The reason why it is optional for
the responder LSR to include the Remote Interface Index Sub-TLVs is
that this information from the downstream LSR is often not update available
on the FEC validation responder LSR. In such case, the traversal of LAG member
links can be validated with procedures nor described in Section 5.1. If
LSRs can provide the Remote Interface Index Sub-TLVs in DDMAP
objects, then the validation procedures, specified procedures described in [RFC4379] 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
initiator LSR traverses all available LAG member links and [RFC6424]
respectively. Rather this document taking the
results through a logic. This section provides the mechanism for the
initiator LSR to obtain additional information from the downstream
LSRs when incoming and/or outgoing interfaces are LAGs. With this
additional information, it is and describes the responsibility of additional logic in the initiator LSR to
validate the L2 ECMP traversal.

4.1.

5.1.1. Initiator LSR Procedures

The MPLS echo request is sent with a DDMAP with DS Flags I set the "Interface and
Label Stack Object Request flag" and "LAG Description Indicator flag"
set in the optional LAG Interface Info TLV DS Flags 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.

4.2.

5.1.2. Responder LSR Procedures

Responder LSRs

A responder LSR that understands the LAG Interface Info TLV "LAG Description Indicator flag"
but unable
to describe is not capable of describing incoming LAG member link are is to use
following procedures:

o The responder LSR MUST add the LAG Interface Info TLV in If the received MPLS
echo reply. This will allow echo request message had the initiator LSR to understand that Capability
TLV, the responder LSR understood MUST include the LAG Interface Info TLV. LSR Capability TLV in the
MPLS echo reply message.

o The responder LSR MUST clear The Upstream the "Upstream LAG Info Accommodation
flag
flag" in the LAG Interface Info LSR Capability Flags field of the LAG Interface
Info LSR Capability TLV.
This will allow the initiator LSR to understand that the responder
LSR understood the LAG Interface Info TLV but cannot describe incoming LAG member link.

The

A responder LSRs LSR that understands the LAG Interface Info TLV "LAG Description Indicator flag"
and
able to describe is capable of describing incoming LAG member link MUST use the
following procedures, regardless of whether or not incoming interface
was a LAG interface:

o Add If the LAG Interface Info received MPLS echo request message had the LSR Capability
TLV, the responder LSR MUST include the LSR Capability TLV in the
MPLS echo reply to provide
acknowledgement back to the initiator. message.

o The Upstream LAG Info
Accommodation flag responder LSR MUST be set in the "Upstream LAG Interface Info Accommodation
flag" in the LSR Capability Flags
field. field of the LSR Capability TLV.

o When the received DDMAP had "Interface and Label Stack Object
Request flag" set in the DS Flags I set, field, the responder LSR MUST
add the Detailed Interface and Label Stack TLV (described in
Section 8) 10) in the MPLS echo reply.

o When the received DDMAP had "Interface and Label Stack Object
Request flag" set in the DS Flags I set field and the incoming interface
was a LAG, the responder LSR MUST add the Incoming Interface Index
Sub-TLV (described in Section 8.1.2). 10.1.2) in the Detailed Interface
and Label Stack TLV. The LAG "LAG Member Link Indicator flag flag" MUST be
set in the Interface Index Flags field, and the Interface Index
field set to the LAG member link which received the MPLS echo
request.

These procedures allow initiator LSR to:

o Identify whether or not the responder LSR understands the LAG
Interface Info TLV and can describe the
incoming LAG member links
(the responder link, by looking at the LSR is mandated to always add Capability TLV.

o Utilize the LAG Incoming Interface
Info TLV Index Sub-TLV in the MPLS echo reply).

4.3. Detailed
Interface and Label Stack TLV to identify, if the incoming
interface was a LAG, the identity of the incoming LAG member.

5.1.3. Additional Initiator LSR Procedures

Along with procedures described in Section 3, described 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.

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 discovered
entropies. In other words, set of entropies that load balances to
outgoing LAG member link X at TTL=n should all reach the nexthop on
same incoming LAG member link Y at TTL=n+1.

With additional logics added in logics, the initiator LSR, LSR can perform following
checks
can be performed: in a scenario where the initiator knows that there is a LAG,
with two LAG members, between TTL=n and TTL=n+1, and has the
multipath information to traverse the two LAG members.

The initiator LSR sends two MPLS echo request messages to traverse
the two LAG members at TTL=1:

o Success case:

* Traversing One MPLS echo request message reaches TTL=n+1 on an LAG member=1 at TTL=n results in member
1.

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

The two MPLS echo request messages sent by the incoming interface at TTL=n+1.

* Traversing initiator LSR reach
two different LAG member=2 members at TTL=n results in LAG member=2' as the incoming interface at TTL=n+1. immediate downstream LSR.

o Error case:

* Traversing One MPLS echo request message reaches TTL=n+1 on an LAG member=1 at TTL=n results in member
1.

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

One or two MPLS echo request messages sent by the incoming interface at TTL=n+1.

* Traversing initiator LSR
does not reach the immediate downstream LSR, or the two MPLS echo
request messages reach a same LAG member=2 member at TTL=n results in LAG member=1' as the incoming interface at TTL=n+1. immediate
downstream LSR.

Note that defined procedures will provide a deterministic result for
LAG interfaces that are back-to-back connected between routers (i.e.
no L2 switch in between). If there is a L2 switch between LSR at
TTL=n and LSR at TTL=n+1, there is no guarantee that traversal of
every LAG member link at TTL=n will result in reaching different
interface index at TTL=n+1. Issues resulting from LAG with L2 switch
in between are further described in Appendix A. LAG provisioning
models in operated network should be considered when analyzing operated network should be considered when analyzing the
output of LSP Traceroute exercising L2 ECMPs.

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 by the downstream responder LSR are
then updated to include the comparison of the incoming LAG member
link (which MPLS echo request was received on) to the interface index
described in the Remote Interface Index Sub-TLV in the DDMAP.

Failure of this comparison results in the return code being set to
"Downstream Mapping Mismatch (5)".

A responder LSR that is not able to perform the above additional
DDMAP validation procedures is considered to lack the upstream LAG
capability. Thus, if the received MPLS echo request contained the
LSR Capability TLV, then the responder LSR MUST include the LSR
Capability TLV in the
output of LSP Traceroute exercising L2 ECMPs.

5. MPLS echo reply and the LSR Capability TLV MUST
have the "Upstream LAG Interface Info Accomodation flag" cleared.

6. LSR Capability TLV

The LAG Interface Info LSR Capability object is a new TLV that 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 LAG Interface Info LSR Capability object. Presence of LAG Interface
Info an LSR Capability
object in an MPLS echo request message is a request that a responder
LSR describes upstream and
downstream LAG interfaces according to procedures defined includes an LSR Capability object in this
document. If the MPLS echo reply message,
with the LSR Capability object describing features and extensions
supported. When the received MPLS echo request message contains an
LSR Capability object, an responder LSR is able to accommodate this request,
then MUST include the LAG Interface Info LSR
Capability object MUST be included in the MPLS echo reply message.

LAG Interface Info

LSR Capability TLV Type is TBD1. Length is 4. The Value value field of LAG Interface
the LSR Capability TLV has following format:

Figure 4: LAG Interface Info LSR Capability TLV

LAG Interface Info

LSR Capability Flags

LAG Interface Info

The LSR Capability Flags field is a bit vector with following
format.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero (Reserved) |U|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Two flags are defined: U and D. The remaining flags MUST be set
to zero when sending and ignored on receipt. Both U and D flags
MUST be cleared in MPLS echo request message when sending, and
ignored on receipt. Either Neither, either or both U and D flags MAY be
set in MPLS echo reply message.

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

U Upstream LAG Info Accommodation

When

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

D Downstream LAG Info Accommodation

When

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

6. DDMAP TLV DS Flags:

7. LAG Description Indicator Flag: G

One flag, G, is added in DS Flags field of the DDMAP TLV. The G flag
of the DS Flags field has no meaning in the MPLS echo request
message. The G flag MUST therefore be cleared when sending, and
ignored on the receipt of the DS Flags field in the MPLS echo request message. 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.

DS Flags

DS Flags G is added, in Bit Number TBD4, TBD5, in DS Flags bit vector.

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

RFC-Editor-Note: Please update above figure to place the flag G in
the bit number TBD4. TBD5.

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

G LAG Description Indicator

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

8. Local Interface Index Sub-TLV

The Local Interface Index object is a Sub-TLV that MAY be included in
a DDMAP TLV. Zero or more Local Interface Index object MAY appear in
a DDMAP TLV. The Local Interface Index Sub-TLV describes the index
assigned by the local LSR to the egress interface.

The Local Interface Index Sub-TLV Type is TBD2. Length is 8, and the
Value field has following format:

Figure 5: Local Interface Index Sub-TLV

Interface Index Flags

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero (Reserved) |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, interface index described in
this sub-TLV is set, DDMAP describes a LAG member of a LAG.

Local Interface Index

An Index assigned by the LSR to this interface.

9. Remote Interface Index Sub-TLV

The Remote Interface Index object is a Sub-TLV that MAY be included
in a DDMAP TLV. Zero or more Remote Interface Index object MAY
appear in a DDMAP TLV. The Remote Interface Index Sub-TLV describes
the index assigned by
local the downstream LSR to the egress ingress interface.

The Remote Interface Index Sub-TLV Type is TBD2. TBD3. Length is 8, and
the Value field has following format:

Figure 5: 6: Remote Interface Index Sub-TLV

Interface Index Flags

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero (Reserved) |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, interface index described in
this sub-TLV is a member of a LAG.

Remote Interface Index

An Index assigned by the downstream LSR to this the ingress interface.

10. Detailed Interface and Label Stack TLV

The "Detailed Interface and Label Stack" object is a TLV that MAY be
included in a 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. 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 TBD3. TBD4. Length is K +
Sub-TLV Length (sum of Sub-TLVs). K is the sum of all fields of this
TLV prior to Sub-TLVs, but the length of K depends on the Address
Type. Details of this information is described below. The Value
field has 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Type | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | Sub-TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. List of Sub-TLVs .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6: 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 [RFC4379]). Two changes
are introduced. First is that label stack, which is of variable
length, is converted into a sub-TLV. Second is that a new sub-TLV is
added to describe an interface index. The fields of Detailed
Interface and Label Stack TLV have the same use and meaning as in
[RFC4379]. A summary of the fields taken from the Interface and
Label Stack TLV 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 for the initial part
of the TLV is listed in the table below 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
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 [RFC4379],
described in Section 3.4.2 of [I-D.ietf-mpls-ipv6-only-gap]. A
solution should be considered an applied to both [RFC4379] and
this document.

Sub-TLV Length
Total length in octets of the sub-TLVs associated with this
TLV.

8.1.

10.1. Sub-TLVs

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

Sub-Type Value Field
--------- ------------
1 Incoming Label stack
2 Incoming Interface Index

8.1.1.

10.1.1. Incoming Label Stack Sub-TLV

The Incoming Label Stack sub-TLV contains the label stack as received
by the 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 the
Value field has following format:

Figure 7: 8: Incoming Label Stack Sub-TLV

8.1.2.

10.1.2. Incoming Interface Index Sub-TLV

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

Incoming Interface Index Sub-TLV Type is 2. Length is 8, and the
Value field has the same format as the Local Interface Index Sub-TLV
described in Section 7, 8, and has following format:

Figure 8: 9: Incoming Interface Index Sub-TLV

Interface Index Flags

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero (Reserved) |M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

One flag is defined: M. The remaining flags MUST be set to zero
when sent 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.

11. Security Considerations

This document extends LSP Traceroute mechanism to discover and
exercise L2 ECMP paths. As a result of supporting the code points
and procedures described in this document, additional processing are
required by initiator LSRs and responder LSRs, especially to compute
and handle increasing number of multipath information. Due to
additional processing, it is critical that proper security measures
described in [RFC4379] and [RFC6424] are followed.

The LSP Traceroute allows an initiator LSR to discover the paths of
tested LSPs, providing deep knowledge of the MPLS network. Exposing
such information to a malicious user is considered dangerous. To
prevent leakage of vital information to untrusted users, a responder
LSR MUST only accept MPLS echo request messages from trusted sources
via filtering source IP address field of received MPLS echo request
messages.

10.

12. IANA Considerations

10.1. LAG Interface Info

12.1. LSR Capability TLV

The IANA is requested to assign new value TBD1 for LAG Interface Info LSR Capability TLV
from the "Multiprotocol Label Switching Architecture (MPLS) Label
Switched Paths (LSPs) Ping Parameters - TLVs" registry.

Value Meaning Reference
----- ------- ---------
TBD1 LAG Interface Info LSR Capability TLV this document

10.1.1. LAG Interface Info

12.1.1. LSR Capability Flags

The IANA is requested to create and maintain a registry entitled "LAG
Interface Info "LSR
Capability Flags" with following registration procedures:

Registry Name: LAG Interface Info Flags

Bit number Name Reference
---------- ---------------------------------------- ---------
15
31 D: Downstream LAG Info Accommodation this document
14
30 U: Upstream LAG Info Accommodation this document
0-13
0-29 Unassigned

Assignments of LAG Interface Info LSR Capability Flags are via Standards Action
[RFC5226].

10.2.

12.2. Local Interface Index Sub-TLV

The IANA is requested to assign new value TBD2 (from the range
4-31743) for the Local Interface Index Sub-TLV from the
"Multiprotocol Label Switching Architecture (MPLS) Label Switched
Paths (LSPs) Ping Parameters - TLVs" registry, "Sub-
TLVs "Sub-TLVs for TLV
Types 20" sub-registry.

Value Meaning Reference
----- ------- ---------
TBD2 Local Interface Index Sub-TLV this document

10.2.1.

12.2.1. Interface Index Flags

The IANA is requested to create and maintain a registry entitled
"Interface Index Flags" with following registration procedures:

Registry Name: Interface Index Flags

Bit number Name Reference
---------- ---------------------------------------- ---------
15 M: LAG Member Link Indicator this document
0-14 Unassigned

Assignments of Interface Index Flags are via Standards Action
[RFC5226].

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 and 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.

10.3.

12.3. Remote Interface Index Sub-TLV

The IANA is requested to assign new value TBD3 (from the range
32768-49161) for the Remote Interface Index Sub-TLV from the
"Multiprotocol Label Switching Architecture (MPLS) Label Switched
Paths (LSPs) Ping Parameters - TLVs" registry, "Sub-TLVs for TLV
Types 20" sub-registry.

Value Meaning Reference
----- ------- ---------
TBD3 Remote Interface Index Sub-TLV this document

12.4. Detailed Interface and Label Stack TLV

The IANA is requested to assign new value TBD3 TBD4 for Detailed Interface
and Label Stack TLV from the "Multiprotocol Label Switching
Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters -
TLVs" registry ([IANA-MPLS-LSP-PING]).

Value Meaning Reference
----- ------- ---------
TBD3
TBD4 Detailed Interface and Label Stack TLV this document

10.3.1.

12.4.1. Sub-TLVs for TLV Type TBD3 TBD4

The IANA is requested to create and maintain a sub-registry entitled
"Sub-TLVs for TLV Type TBD3" TBD4" under "Multiprotocol Label Switching
Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters -
TLVs" registry.

Initial values for this sub-registry, "Sub-TLVs for TLV Types TBD3", TBD4",
are described below.

Sub-Type Name Reference
--------- ----------------------------------------
----------- -------------------------------------- ---------
1 Incoming Label Stack this document
2 Incoming Interface Index this document
4-65535
3-16383 Unassigned (mandatory TLVs)
16384-31743 Experimental
32768-49161 Unassigned (optional TLVs)
49162-64511 Experimental

Assignments of Sub-Types in the mandatory and optional spaces are are
via Standards Action [RFC5226].

10.4. Assignments of Sub-Types in the
experimental space is via Specification Required [RFC5226].

12.5. DS Flags

The IANA is requested to assign a new bit number from the "DS flags"
sub-registry from the "Multi-Protocol Label Switching (MPLS) Label
Switched Paths (LSPs) Ping Parameters - TLVs" registry
([IANA-MPLS-LSP-PING]).

Note: the "DS flags" sub-registry is created by
[I-D.ietf-mpls-lsp-ping-registry].

Bit number Name Reference
---------- ---------------------------------------- ---------
TBD4
TBD5 G: LAG Description Indicator this document

11.

13. Acknowledgements

Authors

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

12.

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

14. References

12.1.

14.1. Normative References

[I-D.ietf-mpls-lsp-ping-registry]
Decraene, B., Akiya, N., Pignataro, C., Andersson, L., and
S. Aldrin, "IANA registries for LSP ping Code Points",
draft-ietf-mpls-lsp-ping-registry-00 (work in progress),
November 2014.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.

[RFC6424] Bahadur, N., Kompella, K., and G. Swallow, "Mechanism for
Performing Label Switched Path Ping (LSP Ping) over MPLS
Tunnels", RFC 6424, November 2011.

12.2.

14.2. Informative References

[I-D.ietf-mpls-ipv6-only-gap]
George, W. and C. Pignataro, "Gap Analysis for Operating
IPv6-only MPLS Networks", draft-ietf-mpls-ipv6-only-gap-04
(work in progress), November 2014.

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

[IEEE802.1AX]
IEEE Std. 802.1AX, "IEEE Standard for Local and
metropolitan area networks - Link Aggregation", November
2008.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.

Appendix A. LAG with L2 Switch Issues

Several flavors of "LAG with L2 switch" provisioning models are
described in this section, with MPLS data plane ECMP traversal
validation issues with each.

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 R1 to S1 can get load balanced by S1 towards R2.
Therefore, MPLS echo request messages traversing specific LAG member
from R1 to S1 can actually reach R2 via any LAG members, and sender
of MPLS echo request messages have no knowledge of this nor no way to
control this traversal. In the worst case, MPLS echo request
messages with specific entropies to exercise every LAG members from
R1 to S1 can all reach R2 via same LAG member. Thus it is impossible
for MPLS echo request sender to verify that packets intended to
traverse specific LAG member from R1 to S1 did actually traverse that
LAG member, and to deterministically exercise "receive" processing of
every LAG member on R2.

A.2. Deviating Numbers of LAG Members

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

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

A.4. LAG Only on Left

R1 ==== S1 ---- R2

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 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.

Authors' Addresses

Nobo Akiya
Cisco Systems

Email: nobo@cisco.com

George Swallow
Cisco Systems

Email: swallow@cisco.com

Stephane Litkowski
Orange

Email: stephane.litkowski@orange.com

Bruno Decraene
Orange

Email: bruno.decraene@orange.com

John E. Drake
Juniper Networks

Email: jdrake@juniper.net