wdiff draft-smack-mpls-rfc4379bis-08.txt draft-smack-mpls-rfc4379bis-09.txt

Network Working Group K. Kompella
Internet-Draft Juniper Networks, Inc. Networks
Obsoletes: 4379, 6829 (if approved) C. Pignataro
Intended status: Standards Track N. Kumar
Expires: May 22, June 20, 2016 Cisco
S. Aldrin
Google
M. Chen
Huawei
November 19,
December 18, 2015

Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures
draft-smack-mpls-rfc4379bis-08
draft-smack-mpls-rfc4379bis-09

Abstract

This document describes a simple and efficient mechanism that can be
used to detect data plane failures in Multi-Protocol Label Switching
(MPLS) Label Switched Paths (LSPs). There are two parts to this
document: information carried in an MPLS "echo request" and "echo
reply" for the purposes of fault detection and isolation, and
mechanisms for reliably sending the echo reply.

This document obsoletes RFC 4379.

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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material or to cite them other than as "work in progress."

This Internet-Draft will expire on May 22, June 20, 2016.

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Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Structure of This Document . . . . . . . . . . . . . . . 4
1.3. Contributors . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Scope of RFC4379bis work . . . . . . . . . . . . . . . . 5
1.5. ToDo . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Use of Address Range 127/8 . . . . . . . . . . . . . . . 6
2.2. Router Alert Option . . . . . . . . . . . . . . . . . . . 8
3. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Return Codes . . . . . . . . . . . . . . . . . . . . . . 13
3.2. Target FEC Stack . . . . . . . . . . . . . . . . . . . . 13
3.2.1. LDP IPv4 Prefix . . . . . . . . . . . . . . . . . . . 15
3.2.2. LDP IPv6 Prefix . . . . . . . . . . . . . . . . . . . 15
3.2.3. RSVP IPv4 LSP . . . . . . . . . . . . . . . . . . . . 15
3.2.4. RSVP IPv6 LSP . . . . . . . . . . . . . . . . . . . . 16
3.2.5. VPN IPv4 Prefix . . . . . . . . . . . . . . . . . . . 16
3.2.6. VPN IPv6 Prefix . . . . . . . . . . . . . . . . . . . 17
3.2.7. L2 VPN Endpoint . . . . . . . . . . . . . . . . . . . 18
3.2.8. FEC 128 Pseudowire - IPv4 (Deprecated) . . . . . . . 18
3.2.9. FEC 128 Pseudowire - IPv4 (Current) . . . . . . . . . 19 18
3.2.10. FEC 129 Pseudowire - IPv4 . . . . . . . . . . . . . . 20 19
3.2.11. BGP Labeled IPv4 Prefix . . . . . . . . . . . . . . . 21 20
3.2.12. BGP Labeled IPv6 Prefix . . . . . . . . . . . . . . . 21 20
3.2.13. Generic IPv4 Prefix . . . . . . . . . . . . . . . . . 22 21
3.2.14. Generic IPv6 Prefix . . . . . . . . . . . . . . . . . 22 21
3.2.15. Nil FEC . . . . . . . . . . . . . . . . . . . . . . . 23 22
3.2.16. FEC 128 Pseudowire - IPv6 . . . . . . . . . . . . . . 23 22
3.2.17. FEC 129 Pseudowire - IPv6 . . . . . . . . . . . . . . 24 23
3.3. Downstream Mapping (Deprecated) . . . . . . . . . . . . . 24
3.4. Downstream Detailed Mapping . . . . . . . . . 25
3.3.1. . . . . . . 24
3.4.1. Multipath Information Encoding . . . . . . . . . . . 28
3.3.2. 24
3.4.2. Downstream Router and Interface . . . . . . . . . . . 30
3.4. 26
3.5. Pad TLV . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5. 27
3.6. Vendor Enterprise Number . . . . . . . . . . . . . . . . 31
3.6. 27
3.7. Interface and Label Stack . . . . . . . . . . . . . . . . 32
3.7. 27
3.8. Errored TLVs . . . . . . . . . . . . . . . . . . . . . . 33
3.8. 29
3.9. Reply TOS Byte TLV . . . . . . . . . . . . . . . . . . . 33 29
4. Theory of Operation . . . . . . . . . . . . . . . . . . . . . 34 29
4.1. Dealing with Equal-Cost Multi-Path (ECMP) . . . . . . . . 34 30
4.2. Testing LSPs That Are Used to Carry MPLS Payloads . . . . 35 31
4.3. Sending an MPLS Echo Request . . . . . . . . . . . . . . 35 31
4.4. Receiving an MPLS Echo Request . . . . . . . . . . . . . 36 32
4.4.1. FEC Validation . . . . . . . . . . . . . . . . . . . 42 38
4.5. Sending an MPLS Echo Reply . . . . . . . . . . . . . . . 43 39
4.6. Receiving an MPLS Echo Reply . . . . . . . . . . . . . . 44 40
4.7. Issue with VPN IPv4 and IPv6 Prefixes . . . . . . . . . . 44 40
4.8. Non-compliant Routers . . . . . . . . . . . . . . . . . . 45 41
5. Security Considerations . . . . . . . . . . . . . . . . . . . 45 41
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46 42
6.1. Message Types, Reply Modes, Return Codes . . . . . . . . 47 43
6.2. TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . 47 43
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 48 44
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 49 45
8.1. Normative References . . . . . . . . . . . . . . . . . . 49 45
8.2. Informative References . . . . . . . . . . . . . . . . . 50 46
Appendix A. Deprecated TLVs . . . . . . . . . . . . . . . . . . 47
A.1. FEC 128 Pseudowire . . . . . . . . . . . . . . . . . . . 47
A.2. Downstream Mapping(DSMAP) . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 51

1. Introduction

This document describes a simple and efficient mechanism that can be
used to detect data plane failures in MPLS Label Switched Paths
(LSPs). There are two parts to this document: information carried in
an MPLS "echo request" and "echo reply", and mechanisms for
transporting the echo reply. The first part aims at providing enough
information to check correct operation of the data plane, as well as
a mechanism to verify the data plane against the control plane, and
thereby localize faults. The second part suggests two methods of
reliable reply channels for the echo request message for more robust
fault isolation.

An important consideration in this design is that MPLS echo requests
follow the same data path that normal MPLS packets would traverse.
MPLS echo requests are meant primarily to validate the data plane,
and secondarily to verify the data plane against the control plane.
Mechanisms to check the control plane are valuable, but are not
covered in this document.

This document makes special use of the address range 127/8. This is
an exception to the behavior defined in RFC 1122 [RFC1122] and
updates that RFC. The motivation for this change and the details of
this exceptional use are discussed in section 2.1 below.

1.1. Conventions

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].

The term "Must Be Zero" (MBZ) is used in object descriptions for
reserved fields. These fields MUST be set to zero when sent and
ignored on receipt.

Terminology pertaining to L2 and L3 Virtual Private Networks (VPNs)
is defined in [RFC4026].

Since this document refers to the MPLS Time to Live (TTL) far more
frequently than the IP TTL, the authors have chosen the convention of
using the unqualified "TTL" to mean "MPLS TTL" and using "IP TTL" for
the TTL value in the IP header.

1.2. Structure of This Document

The body of this memo contains four main parts: motivation, MPLS echo
request/reply packet format, LSP ping operation, and a reliable
return path. It is suggested that first-time readers skip the actual
packet formats and read the Theory of Operation first; the document
is structured the way it is to avoid forward references.

1.3. Contributors

A mechanism used to detect data plane failures in Multi-Protocol
Label Switching (MPLS) Label Switched Paths (LSPs) was originally
published as RFC 4379 in February 2006. It was produced by the MPLS
Working Group of the IETF and was jointly authored by Kireeti
Kompella and George Swallow.

The following made vital contributions to all aspects of the original
RFC 4379, and much of the material came out of debate and discussion
among this group.

Ronald P. Bonica, Juniper Networks, Inc.
Dave Cooper, Global Crossing
Ping Pan, Hammerhead Systems
Nischal Sheth, Juniper Networks, Inc.
Sanjay Wadhwa, Juniper Networks, Inc.

1.4. Scope of RFC4379bis work

The goal of this document is to take LSP Ping to an Internet
Standard.

[RFC4379] defines the basic mechanism for MPLS LSP validation that
can be used for fault detection and isolation. The scope of this
document also is to address various updates to MPLS LSP Ping,
including:

1. Updates to all references and citations. Obsoleted RFCs 2434,
2030, and 3036 are respectively replaced with RFCs 5226, 5905,
and 5036. Additionally, these three documents published as RFCs:
RFCs 4447, 5085, and 4761.
2. Incorporate all outstanding Errata. These include Erratum with
IDs: 108, 1418, 1714, 1786, 3399, 742, and 2978.
3. Replace EXP with Traffic Class (TC), based on the update from RFC
5462.
4. Incorporate the updates from RFC 6829, adding the PW FECs
advertised over IPv6.
5. Incorporate the updates from RFC 7506, adding IPv6 Router Alert
Option for MPLS OAM.

1.5. ToDo

This section should be empty, and removed, prior to publication.
ToDos:

1. Evaluation of which of the RFCs that updated RFC 4379 need to be
incorporated into this 4379bis document. Specifically, these
RFCs updated RFC 4379: 6424, 6425, 6426 and 7537. RFCs that
updated RFC 4379 and are incorporated into this 4379bis, will be
Obsoleted by 4379bis.
2. Review IANA Allocations

2. Motivation

When an LSP fails to deliver user traffic, the failure cannot always
be detected by the MPLS control plane. There is a need to provide a
tool that would enable users to detect such traffic "black holes" or
misrouting within a reasonable period of time, and a mechanism to
isolate faults.

In this document, we describe a mechanism that accomplishes these
goals. This mechanism is modeled after the ping/traceroute paradigm:
ping (ICMP echo request [RFC0792]) is used for connectivity checks,
and traceroute is used for hop-by-hop fault localization as well as
path tracing. This document specifies a "ping" mode and a
"traceroute" mode for testing MPLS LSPs.

The basic idea is to verify that packets that belong to a particular
Forwarding Equivalence Class (FEC) actually end their MPLS path on a
Label Switching Router (LSR) that is an egress for that FEC. This
document proposes that this test be carried out by sending a packet
(called an "MPLS echo request") along the same data path as other
packets belonging to this FEC. An MPLS echo request also carries
information about the FEC whose MPLS path is being verified. This
echo request is forwarded just like any other packet belonging to
that FEC. In "ping" mode (basic connectivity check), the packet
should reach the end of the path, at which point it is sent to the
control plane of the egress LSR, which then verifies whether it is
indeed an egress for the FEC. In "traceroute" mode (fault
isolation), the packet is sent to the control plane of each transit
LSR, which performs various checks that it is indeed a transit LSR
for this path; this LSR also returns further information that helps
check the control plane against the data plane, i.e., that forwarding
matches what the routing protocols determined as the path.

One way these tools can be used is to periodically ping an FEC to
ensure connectivity. If the ping fails, one can then initiate a
traceroute to determine where the fault lies. One can also
periodically traceroute FECs to verify that forwarding matches the
control plane; however, this places a greater burden on transit LSRs
and thus should be used with caution.

2.1. Use of Address Range 127/8

As described above, LSP ping is intended as a diagnostic tool. It is
intended to enable providers of an MPLS-based service to isolate
network faults. In particular, LSP ping needs to diagnose situations
where the control and data planes are out of sync. It performs this
by routing an MPLS echo request packet based solely on its label
stack. That is, the IP destination address is never used in a
forwarding decision. In fact, the sender of an MPLS echo request
packet may not know, a priori, the address of the router at the end
of the LSP.

Providers of MPLS-based services also need the ability to trace all
of the possible paths that an LSP may take. Since most MPLS services
are based on IP unicast forwarding, these paths are subject to equal-
cost multi-path (ECMP) load sharing.

This leads to the following requirements:

1. Although the LSP in question may be broken in unknown ways, the
likelihood of a diagnostic packet being delivered to a user of an
MPLS service MUST be held to an absolute minimum.

2. If an LSP is broken in such a way that it prematurely terminates,
the diagnostic packet MUST NOT be IP forwarded.

3. A means of varying the diagnostic packets such that they exercise
all ECMP paths is thus REQUIRED.

Clearly, using general unicast addresses satisfies neither of the
first two requirements. A number of other options for addresses were
considered, including a portion of the private address space (as
determined by the network operator) and the newly designated IPv4
link local addresses. Use of the private address space was deemed
ineffective since the leading MPLS-based service is an IPv4 Virtual
Private Network (VPN). VPNs often use private addresses.

The IPv4 link local addresses are more attractive in that the scope
over which they can be forwarded is limited. However, if one were to
use an address from this range, it would still be possible for the
first recipient of a diagnostic packet that "escaped" from a broken
LSP to have that address assigned to the interface on which it
arrived and thus could mistakenly receive such a packet.
Furthermore, the IPv4 link local address range has only recently been
allocated. Many deployed routers would forward a packet with an
address from that range toward the default route.

The 127/8 range for IPv4 and that same range embedded in as
IPv4-mapped IPv6 addresses for IPv6 was chosen for a number of
reasons.

RFC 1122 allocates the 127/8 as "Internal host loopback address" and
states: "Addresses of this form MUST NOT appear outside a host."
Thus, the default behavior of hosts is to discard such packets. This
helps to ensure that if a diagnostic packet is misdirected to a host,
it will be silently discarded.

RFC 1812 [RFC1812] states:

A router SHOULD NOT forward, except over a loopback interface, any
packet that has a destination address on network 127. A router
MAY have a switch that allows the network manager to disable these
checks. If such a switch is provided, it MUST default to
performing the checks.

This helps to ensure that diagnostic packets are never IP forwarded.

The 127/8 address range provides 16M addresses allowing wide
flexibility in varying addresses to exercise ECMP paths. Finally, as
an implementation optimization, the 127/8 provides an easy means of
identifying possible LSP packets.

2.2. Router Alert Option

This document requires the use of the Router Alert Option (RAO) set
in IP header in order to have the transit node process the MPLS OAM
payload.

[RFC2113] defines a generic Option Value 0x0 for IPv4 RAO that alerts
transit router to examine the IPv4 packet. [RFC7506] defines MPLS
OAM Option Value 69 for IPv6 RAO to alert transit routers to examine
the IPv6 packet more closely for MPLS OAM purposes.

The use of the Router Alert IP Option in this document is as follows:

In case of an IPv4 header, the generic IPv4 RAO value 0x0
[RFC2113] SHOULD be used. In case of an IPv6 header, the IPv6 RAO
value (69) for MPLS OAM [RFC7506] MUST be used.

3. Packet Format

An MPLS echo request is a (possibly labeled) IPv4 or IPv6 UDP packet;
the contents of the UDP packet have 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version Number | Global Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Reply mode | Return Code | Return Subcode|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's Handle |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Sent (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Sent (seconds fraction) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Received (seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TimeStamp Received (seconds fraction) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLVs ... |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Version Number is currently 1. (Note: the version number is to
be incremented whenever a change is made that affects the ability of
an implementation to correctly parse or process an MPLS echo request/
reply. These changes include any syntactic or semantic changes made
to any of the fixed fields, or to any Type-Length-Value (TLV) or sub-
TLV assignment or format that is defined at a certain version number.
The version number may not need to be changed if an optional TLV or
sub-TLV is added.)

The Global Flags field is a bit vector with the following format:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MBZ |V|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

One flag is defined for now, the V bit; the rest MUST be set to zero
when sending and ignored on receipt.

The V (Validate FEC Stack) flag is set to 1 if the sender wants the
receiver to perform FEC Stack validation; if V is 0, the choice is
left to the receiver.

The Message Type is one of the following:

Value Meaning
----- -------
1 MPLS echo request
2 MPLS echo reply

The Reply Mode can take one of the following values:

Value Meaning
----- -------
1 Do not reply
2 Reply via an IPv4/IPv6 UDP packet
3 Reply via an IPv4/IPv6 UDP packet with Router Alert
4 Reply via application level control channel

An MPLS echo request with 1 (Do not reply) in the Reply Mode field
may be used for one-way connectivity tests; the receiving router may
log gaps in the Sequence Numbers and/or maintain delay/jitter
statistics. An MPLS echo request would normally have 2 (Reply via an
IPv4/IPv6 UDP packet) in the Reply Mode field. If the normal IP
return path is deemed unreliable, one may use 3 (Reply via an IPv4/
IPv6 UDP packet with Router Alert). Note that this requires that all
intermediate routers understand and know how to forward MPLS echo
replies. The echo reply uses the same IP version number as the
received echo request, i.e., an IPv4 encapsulated echo reply is sent
in response to an IPv4 encapsulated echo request.

Some applications support an IP control channel. One such example is
the associated control channel defined in Virtual Circuit
Connectivity Verification (VCCV) [RFC5085]. Any application that
supports an IP control channel between its control entities may set
the Reply Mode to 4 (Reply via application level control channel) to
ensure that replies use that same channel. Further definition of
this codepoint is application specific and thus beyond the scope of
this document.

Return Codes and Subcodes are described in the next section.

The Sender's Handle is filled in by the sender, and returned
unchanged by the receiver in the echo reply (if any). There are no
semantics associated with this handle, although a sender may find
this useful for matching up requests with replies.

The Sequence Number is assigned by the sender of the MPLS echo
request and can be (for example) used to detect missed replies.

The TimeStamp Sent is the time-of-day (according to the sender's
clock) in NTP format [RFC5905] when the MPLS echo request is sent.
The TimeStamp Received in an echo reply is the time-of-day (according
to the receiver's clock) in NTP format that the corresponding echo
request was received.

TLVs (Type-Length-Value tuples) have 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Types are defined below; Length is the length of the Value field in
octets. The Value field depends on the Type; it is zero padded to
align to a 4-octet boundary. TLVs may be nested within other TLVs,
in which case the nested TLVs are called sub-TLVs. Sub-TLVs have
independent types and MUST also be 4-octet aligned.

Two examples follow. The Label Distribution Protocol (LDP) IPv4 FEC of how TLV and sub-TLV has the following format: length are computed, and of how
sub-TLVs are padded to be 4-octet aligned as follows:

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 = 1 (LDP IPv4 FEC) | Length = 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Length for this TLV is 5. A Target FEC Stack TLV that contains
an LDP IPv4 FEC sub-TLV and a VPN IPv4 prefix sub-TLV has the
following format:

A description of the Types and Values of the top-level TLVs for LSP
ping are given below:

Type # Value Field
------ -----------
1 Target FEC Stack
2 Downstream Mapping
3 Pad
4 Not Assigned
5 Vendor Enterprise Number
6 Not Assigned
7 Interface and Label Stack
8 Not Assigned
9 Errored TLVs
10 Reply TOS Byte

Types less than 32768 (i.e., with the high-order bit equal to 0) are
mandatory TLVs that MUST either be supported by an implementation or
result in the return code of 2 ("One or more of the TLVs was not
understood") being sent in the echo response.

Types greater than or equal to 32768 (i.e., with the high-order bit
equal to 1) are optional TLVs that SHOULD be ignored if the
implementation does not understand or support them.

3.1. Return Codes

The Return Code is set to zero by the sender. The receiver can set
it to one of the values listed below. The notation <RSC> refers to
the Return Subcode. This field is filled in with the stack-depth for
those codes that specify that. For all other codes, the Return
Subcode MUST be set to zero.

Value Meaning
----- -------
0 No return code
1 Malformed echo request received
2 One or more of the TLVs was not understood
3 Replying router is an egress for the FEC at stack-
depth <RSC>
4 Replying router has no mapping for the FEC at stack-
depth <RSC>
5 Downstream Mapping Mismatch (See Note 1)
6 Upstream Interface Index Unknown (See Note 1)
7 Reserved
8 Label switched at stack-depth <RSC>
9 Label switched but no MPLS forwarding at stack-depth
<RSC>
10 Mapping for this FEC is not the given label at stack-
depth <RSC>
11 No label entry at stack-depth <RSC>
12 Protocol not associated with interface at FEC stack-
depth <RSC>
13 Premature termination of ping due to label stack
shrinking to a single label

Note 1

The Return Subcode contains the point in the label stack where
processing was terminated. If the RSC is 0, no labels were
processed. Otherwise the packet would have been label switched at
depth RSC.

3.2. Target FEC Stack

A Target FEC Stack is a list of sub-TLVs. The number of elements is
determined by looking at the sub-TLV length fields.

Sub-Type Length Value Field
-------- ------ -----------
1 5 LDP IPv4 prefix
2 17 LDP IPv6 prefix
3 20 RSVP IPv4 LSP
4 56 RSVP IPv6 LSP
5 Not Assigned
6 13 VPN IPv4 prefix
7 25 VPN IPv6 prefix
8 14 L2 VPN endpoint
9 10 "FEC 128" Pseudowire - IPv4 (deprecated)
10 14 "FEC 128" Pseudowire - IPv4
11 16+ "FEC 129" Pseudowire - IPv4
12 5 BGP labeled IPv4 prefix
13 17 BGP labeled IPv6 prefix
14 5 Generic IPv4 prefix
15 17 Generic IPv6 prefix
16 4 Nil FEC
24 38 "FEC 128" Pseudowire - IPv6
25 40+ "FEC 129" Pseudowire - IPv6

Other FEC Types will be defined as needed.

Note that this TLV defines a stack of FECs, the first FEC element
corresponding to the top of the label stack, etc.

An MPLS echo request MUST have a Target FEC Stack that describes the
FEC Stack being tested. For example, if an LSR X has an LDP mapping
[RFC5036] for 192.168.1.1 (say, label 1001), then to verify that
label 1001 does indeed reach an egress LSR that announced this prefix
via LDP, X can send an MPLS echo request with an FEC Stack TLV with
one FEC in it, namely, of type LDP IPv4 prefix, with prefix
192.168.1.1/32, and send the echo request with a label of 1001.

Say LSR X wanted to verify that a label stack of <1001, 23456> is the
right label stack to use to reach a VPN IPv4 prefix [see section
Section 3.2.5] of 10/8 in VPN foo. Say further that LSR Y with
loopback address 192.168.1.1 announced prefix 10/8 with Route
Distinguisher RD-foo-Y (which may in general be different from the
Route Distinguisher that LSR X uses in its own advertisements for VPN
foo), label 23456 and BGP next hop 192.168.1.1 [RFC4271]. Finally,
suppose that LSR X receives a label binding of 1001 for 192.168.1.1
via LDP. X has two choices in sending an MPLS echo request: X can
send an MPLS echo request with an FEC Stack TLV with a single FEC of
type VPN IPv4 prefix with a prefix of 10/8 and a Route Distinguisher
of RD-foo-Y. Alternatively, X can send an FEC Stack TLV with two
FECs, the first of type LDP IPv4 with a prefix of 192.168.1.1/32 and
the second of type of IP VPN with a prefix 10/8 with Route
Distinguisher of RD-foo-
Y. RD-foo-Y. In either case, the MPLS echo request
would have a label stack of <1001, 23456>. (Note: in this example,
1001 is the "outer" label and 23456 is the "inner" label.)

3.2.1. LDP IPv4 Prefix

The IPv4 Prefix FEC is defined in [RFC5036]. When an LDP IPv4 prefix
is encoded in a label stack, the following format is used. The value
consists of 4 octets of an IPv4 prefix followed by 1 octet of prefix
length in bits; the format is given below. The IPv4 prefix is in
network byte order; if the prefix is shorter than 32 bits, trailing
bits SHOULD be set to zero. See [RFC5036] for an example of a
Mapping for an IPv4 FEC.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.2. LDP IPv6 Prefix

The IPv6 Prefix FEC is defined in [RFC5036]. When an LDP IPv6 prefix
is encoded in a label stack, the following format is used. The value
consists of 16 octets of an IPv6 prefix followed by 1 octet of prefix
length in bits; the format is given below. The IPv6 prefix is in
network byte order; if the prefix is shorter than 128 bits, the
trailing bits SHOULD be set to zero. See [RFC5036] for an example of
a Mapping for an IPv6 FEC.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 prefix |
| (16 octets) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.3. RSVP IPv4 LSP

The value has the format below. The value fields are taken from RFC
3209, sections 4.6.1.1 and 4.6.2.1. See [RFC3209].

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 tunnel end point address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 tunnel sender address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | LSP ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.4. RSVP IPv6 LSP

The value has the format below. The value fields are taken from RFC
3209, sections 4.6.1.2 and 4.6.2.2. See [RFC3209].

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 tunnel end point address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Tunnel ID |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 tunnel sender address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Must Be Zero | LSP ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.5. VPN IPv4 Prefix

VPN-IPv4 Network Layer Routing Information (NLRI) is defined in
[RFC4365]. This document uses the term VPN IPv4 prefix for a VPN-
IPv4 NLRI that has been advertised with an MPLS label in BGP. See
[RFC3107].

When a VPN IPv4 prefix is encoded in a label stack, the following
format is used. The value field consists of the Route Distinguisher
advertised with the VPN IPv4 prefix, the IPv4 prefix (with trailing 0
bits to make 32 bits in all), and a prefix length, as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Route Distinguisher (RD) is an 8-octet identifier; it does not
contain any inherent information. The purpose of the RD is solely to
allow one to create distinct routes to a common IPv4 address prefix.
The encoding of the RD is not important here. When matching this
field to the local FEC information, it is treated as an opaque value.

3.2.6. VPN IPv6 Prefix

VPN-IPv6 Network Layer Routing Information (NLRI) is defined in
[RFC4365]. This document uses the term VPN IPv6 prefix for a VPN-
IPv6 NLRI that has been advertised with an MPLS label in BGP. See
[RFC3107].

When a VPN IPv6 prefix is encoded in a label stack, the following
format is used. The value field consists of the Route Distinguisher
advertised with the VPN IPv6 prefix, the IPv6 prefix (with trailing 0
bits to make 128 bits in all), and a prefix length, as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 prefix |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Route Distinguisher is identical to the VPN IPv4 Prefix RD,
except that it functions here to allow the creation of distinct
routes to IPv6 prefixes. See section Section 3.2.5. When matching this
field to local FEC information, it is treated as an opaque value.

3.2.7. L2 VPN Endpoint

VPLS stands for Virtual Private LAN Service. The terms VPLS BGP NLRI
and VE ID (VPLS Edge Identifier) are defined in [RFC4761]. This
document uses the simpler term L2 VPN endpoint when referring to a
VPLS BGP NLRI. The Route Distinguisher is an 8-octet identifier used
to distinguish information about various L2 VPNs advertised by a
node. The VE ID is a 2-octet identifier used to identify a
particular node that serves as the service attachment point within a
VPLS. The structure of these two identifiers is unimportant here;
when matching these fields to local FEC information, they are treated
as opaque values. The encapsulation type is identical to the PW Type
in section 3.2.8 below.

When an L2 VPN endpoint is encoded in a label stack, the following
format is used. The value field consists of a Route Distinguisher (8
octets), the sender (of the ping)'s VE ID (2 octets), the receiver's
VE ID (2 octets), and an encapsulation type (2 octets), formatted as
follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Distinguisher |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's VE ID | Receiver's VE ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encapsulation Type | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.8. FEC 128 Pseudowire - IPv4 (Deprecated)

See Appendix A.1 for details

3.2.9. FEC 128 Pseudowire - IPv4 (Current)

FEC 128 (0x80) is defined in [RFC4447], as are the terms PW ID
(Pseudowire ID) and PW Type (Pseudowire Type). A PW ID is a non-zero
32-bit connection ID. The PW Type is a 15-bit number indicating the
encapsulation type. It is carried right justified in the field below
termed encapsulation type with the high-order bit set to zero.

Both of these fields are treated in this protocol as opaque values.
When matching these field to the local FEC information, the match
MUST be exact.

When an FEC 128 is encoded in a label stack, the following format is
used. The value field consists of the sender's PE IPv4 address (the
source address of the targeted LDP session), the remote PE IPv4
address (the destination address of the targeted LDP session), the PW
ID, and the encapsulation type as follows:

This

3.2.10. FEC is deprecated and is retained only for backward
compatibility. Implementations of LSP ping SHOULD accept 129 Pseudowire - IPv4

FEC 129 (0x81) and process
this TLV, but SHOULD send LSP ping echo requests with the new TLV
(see next section), unless explicitly configured to use the old TLV.

An LSR receiving this TLV SHOULD use the source IP address of the LSP
echo request to infer the sender's PE address.

3.2.9. FEC 128 Pseudowire - IPv4 (Current)

FEC 128 (0x80) is defined in [RFC4447], as are the terms PW ID
(Pseudowire ID) and PW Type, Attachment Group Identifier
(AGI), Attachment Group Identifier Type (Pseudowire Type). A PW ID is a non-zero
32-bit connection ID. (AGI Type), Attachment
Individual Identifier Type (AII Type), Source Attachment Individual
Identifier (SAII), and Target Attachment Individual Identifier (TAII)
are defined in [RFC4447]. The PW Type is a 15-bit number indicating
the encapsulation type. It is carried right justified in the field
below
termed encapsulation type PW Type with the high-order bit set to zero.

Both of these All the other
fields are treated in this protocol as opaque values.
When matching values and copied directly from the FEC
129 format. All of these field to values together uniquely define the local FEC information,
within the match
MUST be exact. scope of the LDP session identified by the source and
remote PE IPv4 addresses.

When an FEC 128 129 is encoded in a label stack, the following format is
used. The value field consists Length of this TLV is 16 + AGI length + SAII length + TAII
length. Padding is used to make the sender's PE IPv4 address (the
source address total length a multiple of 4;
the targeted LDP session), the remote PE IPv4
address (the destination address length of the targeted LDP session), the PW
ID, and padding is not included in the encapsulation type as follows: Length field.

3.2.10. FEC 129 Pseudowire - IPv4

FEC 129 (0x81) and the terms PW Type, Attachment Group Identifier
(AGI), Attachment Group Identifier Type (AGI Type), Attachment
Individual Identifier Type (AII Type), Source Attachment Individual
Identifier (SAII), and Target Attachment Individual Identifier (TAII)
are defined in [RFC4447]. The PW Type is a 15-bit number indicating
the encapsulation type. It is carried right justified in the field
below PW Type with the high-order bit set to zero. All the other
fields are treated as opaque values and copied directly from the FEC
129 format. All of these values together uniquely define the FEC
within the scope of the LDP session identified by the source and
remote PE IPv4 addresses.

When an FEC 129 is encoded in a label stack, the following format is
used. The Length of this TLV is 16 + AGI length + SAII length + TAII
length. Padding is used to make the total length a multiple of 4;
the length of the padding is not included in the Length field.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender's PE IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote PE IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type | AGI Type | AGI Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ AGI Value ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | SAII Length | SAII Value SAII Length | SAII Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SAII Value (continued) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | TAII Length | TAII Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TAII Value (continued) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TAII (cont.) | 0-3 octets of zero padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.11. BGP Labeled IPv4 Prefix

BGP labeled IPv4 prefixes are defined in [RFC3107]. When a BGP
labeled IPv4 prefix is encoded in a label stack, the following format
is used. The value field consists the IPv4 prefix (with trailing 0
bits to make 32 bits in all), and the prefix length, as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.12. BGP Labeled IPv6 Prefix

BGP labeled IPv6 prefixes are defined in [RFC3107]. When a BGP
labeled IPv6 prefix is encoded in a label stack, the following format
is used. The value consists of 16 octets of an IPv6 prefix followed
by 1 octet of prefix length in bits; the format is given below. The
IPv6 prefix is in network byte order; if the prefix is shorter than
128 bits, the trailing bits SHOULD be set to zero.

3.2.13. Generic IPv4 Prefix

The value consists of 4 octets of an IPv4 prefix followed by 1 octet
of prefix length in bits; the format is given below. The IPv4 prefix
is in network byte order; if the prefix is shorter than 32 bits,
trailing bits SHOULD be set to zero. This FEC is used if the
protocol advertising the label is unknown or may change during the
course of the LSP. An example is an inter-AS LSP that may be
signaled by LDP in one Autonomous System (AS), by RSVP-TE [RFC3209]
in another AS, and by BGP between the ASes, such as is common for
inter-AS VPNs.

3.2.14. Generic IPv6 Prefix

The value consists of 16 octets of an IPv6 prefix followed by 1 octet
of prefix length in bits; the format is given below. The IPv6 prefix
is in network byte order; if the prefix is shorter than 128 bits, the
trailing bits SHOULD be set to zero.

3.2.15. Nil FEC

At times, labels from the reserved range, e.g., Router Alert and
Explicit-null, may be added to the label stack for various diagnostic
purposes such as influencing load-balancing. These labels may have
no explicit FEC associated with them. The Nil FEC Stack is defined
to allow a Target FEC Stack sub-TLV to be added to the Target FEC
Stack to account for such labels so that proper validation can still
be performed.

The Length is 4. Labels are 20-bit values treated as numbers.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Label is the actual label value inserted in the label stack; the MBZ
fields MUST be zero when sent and ignored on receipt.

3.2.16. FEC 128 Pseudowire - IPv6

The FEC 128 Pseudowire IPv6 sub-TLV has a structure consistent with
the FEC 128 Pseudowire IPv4 sub-TLV as described in Section 3.2.9.
The value field consists of the Sender's PE IPv6 address (the source
address of the targeted LDP session), the remote PE IPv6 address (the
destination address of the targeted LDP session), the PW ID, and the
encapsulation type as follows:

Sender's PE IPv6 Address: The source IP address of the target IPv6
LDP session. 16 octets.

Remote PE IPv6 Address: The destination IP address of the target IPv6
LDP session. 16 octets.

PW ID: Same as FEC 128 Pseudowire IPv4 in Section 3.2.9.

PW Type: Same as FEC 128 Pseudowire IPv4 in Section 3.2.9.

3.2.17. FEC 129 Pseudowire - IPv6

The FEC 129 Pseudowire IPv6 sub-TLV has a structure consistent with
the FEC 129 Pseudowire IPv4 sub-TLV as described in Section 3.2.10.
When an FEC 129 is encoded in a label stack, the following format is
used. The length of this TLV is 40 + AGI (Attachment Group
Identifier) length + SAII (Source Attachment Individual Identifier)
length + TAII (Target Attachment Individual Identifier) length.
Padding is used to make the total length a multiple of 4; the length
of the padding is not included in the Length field.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Sender's PE IPv6 Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Remote PE IPv6 Address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type | AGI Type | AGI Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ AGI Value ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | SAII Length | SAII Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SAII Value (continued) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | TAII Length | TAII Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TAII Value (continued) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TAII (cont.) | 0-3 octets of zero padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sender's PE IPv6 Address: The source IP address of the target IPv6
LDP session. 16 octets.

Remote PE IPv6 Address: The destination IP address of the target IPv6
LDP session. 16 octets.

The other fields are the same as FEC 129 Pseudowire IPv4 in
Section 3.2.10.

3.3. Downstream Mapping

The (Deprecated)

See Appendix A.2 for more details.

3.4. Downstream Detailed Mapping object

The format of this TLV is a TLV that MAY be included in an
echo request message. Only one Downstream Mapping object may appear defined in an echo request. The presence section 3.3 of a Downstream Mapping object is a
request [RFC6424]

3.4.1. Multipath Information Encoding

The Multipath Information encodes labels or addresses that Downstream Mapping objects be included will
exercise this path. The Multipath Information depends on the
Multipath Type. The contents of the field are shown in the echo
reply. If table
above. IPv4 addresses are drawn from the replying router is range 127/8; IPv6 addresses
are drawn from the destination of range 0:0:0:0:0:FFFF:7F00/104. Labels are treated
as numbers, i.e., they are right justified in the FEC, then a
Downstream Mapping TLV SHOULD field. For Type 4,
ranges indicated by Address pairs MUST NOT overlap and MUST be included in the echo reply.
Otherwise the replying router SHOULD include a Downstream Mapping
object for each interface over which this FEC could be forwarded.
For
ascending sequence.

Type 8 allows a more precise definition of the notion dense encoding of "downstream", see
section 3.3.2, "Downstream Router and Interface". IP addresses. The Length is K + M + 4*N octets, where M IP prefix
is formatted as a base IP address with the Multipath Length,
and N non-prefix low-order bits
set to zero. The maximum prefix length is 27. Following the number prefix
is a mask of Downstream Labels. Values length 2^(32-prefix length) bits for K are found in
the description of Address Type below. IPv4 and
2^(128-prefix length) bits for IPv6. Each bit set to 1 represents a
valid address. The Value field address is the base IPv4 address plus the
position of a
Downstream Mapping has the following format: bit in the mask where the bits are numbered left to
right beginning with zero. For example, the IPv4 addresses
127.2.1.0, 127.2.1.5-127.2.1.15, and 127.2.1.20-127.2.1.29 would be
encoded as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | Address Type | DS Flags |
|0 1 1 1 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream IP Address (4 or 16 octets) |
|1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Interface Address (4 or 16 octets) |

Those same addresses embedded in IPv6 would be encoded as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multipath Type| Depth Limit | Multipath Length |
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. (Multipath Information) .
. .
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
|0 1 1 1 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
|1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Maximum Transmission Unit (MTU)

Type 9 allows a more dense encoding of labels. The MTU label prefix is the size in octets of the largest MPLS frame (including
formatted as a base label stack) that fits on value with the interface non-prefix low-order bits
set to the Downstream LSR.

Address Type zero. The Address Type indicates if the interface maximum prefix (including leading zeros due to
encoding) length is numbered or
unnumbered. It also determines 27. Following the length prefix is a mask of the Downstream IP
Address and Downstream Interface fields. length
2^(32-prefix length) bits. Each bit set to one represents a valid
label. The resulting total for label is the initial part base label plus the position of the TLV is listed bit in
the table below as "K
Octets". The Address Type is set mask where the bits are numbered left to one right beginning with
zero. Label values of all the following values:

Type # Address Type K Octets
------ ------------ -------- odd numbers between 1152 and 1279
would be encoded as follows:

0 1 2 3
0 1 IPv4 Numbered 16 2 IPv4 Unnumbered 16 3 IPv6 Numbered 40 4 IPv6 Unnumbered 28

DS Flags

The DS Flags field is a bit vector with the following format: 5 6 7 8 9 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Rsvd(MBZ) |I|N|
+-+-+-+-+-+-+-+-+

Two flags are defined currently, I 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If the received Multipath Information is non-null, the labels and N. The remaining flags IP
addresses MUST be picked from the set to zero when sending and ignored on receipt.

Flag Name and Meaning
---- ----------------
I Interface and Label Stack Object Request

When this flag is set, it indicates that the replying
router SHOULD include an Interface and Label Stack
Object in the echo reply message.

N Treat as a Non-IP Packet

Echo request messages will be used to diagnose non-IP
flows. However, provided. If none of these messages are carried in IP
packets. For a router that alters its ECMP algorithm
based on the FEC
labels or deep packet examination, this flag
requests that the router treat this as it would if the
determination of an IP payload had failed.

Downstream IP Address and Downstream Interface Address

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

If the interface map to the a particular downstream LSR is numbered, interface, then
for that interface, the
Address Type type MUST be set to IPv4 0. If the received
Multipath Information is null (i.e., Multipath Length = 0, or IPv6, for
Types 8 and 9, a mask of all zeros), the Downstream IP
Address type MUST be set to either the 0.

For example, suppose LSR X at hop 10 has two downstream LSR's Router ID or LSRs, Y and
Z, for the interface address FEC in question. The received X could return Multipath
Type 4, with low/high IP addresses of the 127.1.1.1->127.1.1.255 for
downstream LSR, LSR Y and the Downstream
Interface Address MUST be set to the 127.2.1.1->127.2.1.255 for downstream LSR's interface
address.

If the interface LSR Z.
The head end reflects this information to the downstream LSR is unnumbered, the Address
Type MUST be IPv4 Unnumbered or IPv6 Unnumbered, the Downstream IP
Address MUST be the Y. Y, which has three
downstream LSR's Router ID, LSRs, U, V, and the Downstream
Interface Address MUST be set W, computes that 127.1.1.1->127.1.1.127
would go to the index assigned by the
upstream LSR U and 127.1.1.128-> 127.1.1.255 would go to the interface.

If an LSR does not know the IP address of its neighbor, V. Y would
then it
MUST set the Address Type to either IPv4 Unnumbered or IPv6
Unnumbered. For IPv4, it must set the Downstream IP Address respond with 3 "Downstream Detailed Mapping" TLVs: to
127.0.0.1; for IPv6 the address is set U, with
Multipath Type 4 (127.1.1.1->127.1.1.127); to 0::1. In both cases,
the interface index MUST be set V, with Multipath Type
4 (127.1.1.127->127.1.1.255); and to 0. If an LSR receives an Echo
Request packet W, with either of these addresses in the Downstream IP
Address field, this indicates Multipath Type 0.

Note that it MUST bypass interface
verification but continue with label validation.

If computing Multipath Information may impose a significant
processing burden on the originator receiver. A receiver MAY thus choose to
process a subset of an Echo Request packet wishes the received prefixes. The sender, on receiving
a reply to obtain a Downstream Detailed Mapping information but does not know the expected
label stack, then it with partial information,
SHOULD set the Address Type to either IPv4
Unnumbered or IPv6 Unnumbered. For IPv4, it MUST set assume that the
Downstream IP Address to 224.0.0.2; for IPv6 prefixes missing in the address MUST be
set to FF02::2. In both cases, reply were skipped by
the interface index MUST be set to
0. If an LSR receives an Echo Request packet with the all-routers
multicast address, then this indicates that it MUST bypass both
interface receiver, and label stack validation, but return Downstream
Mapping TLVs using the MAY re-request information provided.

Multipath Type about them in a new echo
request.

The following encoding of Multipath Types information in scenarios where few LSRs
apply Entropy label based load balancing while other LSRs are defined:

Key Type Multipath Information
--- ---------------- ---------------------
0 no non-EL
(IP based) load balancing will be defined in a different document.

The encoding of multipath Empty (Multipath Length = 0) information in scenarios where LSR have
Layer 2 IP address IP addresses
4 IP address range low/high address pairs
8 Bit-masked IP IP address prefix and bit mask
address set
9 Bit-masked label set Label prefix and bit mask

Type 0 indicates that all packets ECMP over Link Aggregation Group (LAG) interfaces will be forwarded out this one
interface.

Types 2, 4, 8,
defined in different document.

3.4.2. Downstream Router and 9 specify that the supplied Multipath
Information will serve to exercise this path.

Depth Limit Interface

The Depth Limit is applicable only to notion of "downstream router" and "downstream interface" should
be explained. Consider an LSR X. If a packet that was originated
with TTL n>1 arrived with outermost label stack L and is the
maximum number of labels considered in the hash; this SHOULD TTL=1 at LSR X, X
must be
set able to zero compute which LSRs could receive the packet if unspecified or unlimited.

Multipath Length

The length in octets of the Multipath Information.

Multipath Information

Address or label values encoded according to the Multipath Type.
See the next section below for encoding details.

Downstream Label(s)

The set of labels in it was
originated with TTL=n+1, over which interface the request would
arrive and what label stack as it those LSRs would have appeared if
this router were forwarding see. (It is outside the packet through
scope of this interface.
Any Implicit Null labels are explicitly included. Labels are
treated as numbers, i.e., they are right justified in the field.

A Downstream Label document to specify how this computation is 24 bits, in the same format as an MPLS label
minus the TTL field, i.e., done.) The
set of these LSRs/interfaces consists of the MSBit downstream routers/
interfaces (and their corresponding labels) for X with respect to L.
Each pair of downstream router and interface requires a separate
Downstream Detailed Mapping to be added to the label reply.

The case where X is bit 0, the
LSBit is bit 19, LSR originating the Traffic Class (TC) bits are bits 20-22, and
bit 23 echo request is a special
case. X needs to figure out what LSRs would receive the S bit. MPLS echo
request for a given FEC Stack that X originates with TTL=1.

The replying router SHOULD fill in set of downstream routers at X may be alternative paths (see the TC
and S bits;
discussion below on ECMP) or simultaneous paths (e.g., for MPLS
multicast). In the LSR receiving former case, the echo reply MAY choose Multipath Information is used as
a hint to ignore the sender as to how it may influence the choice of these bits. Protocol
alternatives.

3.5. Pad TLV

The Protocol is taken from value part of the following table:

Protocol # Signaling Protocol
---------- ------------------
0 Unknown
1 Static
2 BGP
3 LDP
4 RSVP-TE

3.3.1. Multipath Information Encoding

The Multipath Information encodes labels or addresses that will
exercise this path. Pad TLV contains a variable number (>= 1) of
octets. The Multipath Information depends on first octet takes values from the
Multipath Type. The contents of following table; all
the field other octets (if any) are shown ignored. The receiver SHOULD verify
that the TLV is received in its entirety, but otherwise ignores the table
above. IPv4 addresses are drawn
contents of this TLV, apart from the range 127/8; IPv6 addresses
are drawn first octet.

Value Meaning
----- -------
1 Drop Pad TLV from the range 0:0:0:0:0:FFFF:7F00/104. Labels are treated
as numbers, i.e., they reply
2 Copy Pad TLV to reply
3-255 Reserved for future use

3.6. Vendor Enterprise Number

SMI Private Enterprise Numbers are right justified in the field. For Type 4,
ranges indicated maintained by Address pairs MUST NOT overlap and MUST be in
ascending sequence.

Type 8 allows a more dense encoding of IP addresses. IANA. The IP prefix Length is formatted as a base IP address with
always 4; the non-prefix low-order bits
set to zero. The maximum prefix length value is 27. Following the prefix
is a mask SMI Private Enterprise code, in network
octet order, of length 2^(32-prefix length) bits for IPv4 and
2^(128-prefix length) bits for IPv6. Each bit set to 1 represents the vendor with a
valid address. The address is Vendor Private extension to any of
the base IPv4 address plus fields in the
position fixed part of the bit message, in which case this TLV
MUST be present. If none of the mask where fields in the bits fixed part of the
message have Vendor Private extensions, inclusion of this TLV is
OPTIONAL. Vendor Private ranges for Message Types, Reply Modes, and
Return Codes have been defined. When any of these are numbered left to
right beginning with zero. For example, used, the IPv4 addresses
127.2.1.0, 127.2.1.5-127.2.1.15,
Vendor Enterprise Number TLV MUST be included in the message.

3.7. Interface and 127.2.1.20-127.2.1.29 would Label Stack

The Interface and Label Stack TLV MAY be
encoded included in a reply message
to report the interface on which the request message was received and
the label stack that was on the packet when it was received. Only
one such object may appear. The purpose of the object is to allow
the upstream router to obtain the exact interface and label stack
information as follows: it appears at the replying LSR.

The Length is K + 4*N octets; N is the number of labels in the label
stack. Values for K are found in the description of Address Type
below. The Value field of this 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 1 1 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0|
| Address Type | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0|
| IP Address (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Those same addresses embedded
| Interface (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. Label Stack .
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 IPv6 would be encoded the table below as follows:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 "K Octets". The Address Type
is set to one of the following values:

Type # Address Type K Octets
------ ------------ --------
1 IPv4 Numbered 12
2 IPv4 Unnumbered 12
3 IPv6 Numbered 36
4 5 6 7 8 9 0 1 2 3 IPv6 Unnumbered 24

IP Address and Interface

IPv4 addresses and interface indices are encoded in 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 1 1 1 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 9 allows a more dense encoding of labels. The label prefix octets; IPv6
addresses are encoded in 16 octets.

If the interface upon which the echo request message was received
is
formatted as a base label value with numbered, then the non-prefix low-order bits Address Type MUST be set to zero. The maximum prefix (including leading zeros due IPv4 or IPv6,
the IP Address MUST be set to
encoding) length is 27. Following either the prefix is a mask of length
2^(32-prefix length) bits. Each bit LSR's Router ID or the
interface address, and the Interface MUST be set to one represents a valid
label. The label the interface
address.

If the interface is unnumbered, the base label plus Address Type MUST be either
IPv4 Unnumbered or IPv6 Unnumbered, the position of IP Address MUST be the bit in
LSR's Router ID, and the mask where Interface MUST be set to the bits are numbered left index
assigned to right beginning with
zero. the interface.

Label values Stack

The label stack of all the odd numbers between 1152 and 1279
would received echo request message. If any TTL
values have been changed by this router, they SHOULD be encoded as follows:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 restored.

3.8. Errored TLVs

The following TLV is a TLV that MAY be included in an echo reply to
inform the sender of an echo request of mandatory TLVs either not
supported by an implementation or parsed and found to be in error.

The Value field contains the TLVs that were not understood, encoded
as sub-TLVs.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 2 3 4 5 6 7 8 9 0 1 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
| Type = 9 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
| Value |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.9. Reply TOS Byte TLV

This TLV MAY be used by the originator of the echo request to request
that an echo reply be sent with the IP header TOS byte set to the
value specified in the TLV. This TLV has a length of 4 with the
following value field.

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 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1|
| Reply-TOS Byte| Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If the received Multipath Information is non-null, the labels and IP
addresses MUST be picked from the set provided. If none

4. Theory of these
labels or addresses map Operation

An MPLS echo request is used to test a particular downstream interface, then LSP. The LSP to be
tested is identified by the "FEC Stack"; for that interface, example, if the type MUST be LSP was
set up via LDP, and is to 0. an egress IP address of 10.1.1.1, the FEC
Stack contains a single element, namely, an LDP IPv4 prefix sub-TLV
with value 10.1.1.1/32. If the received
Multipath Information LSP being tested is null (i.e., Multipath Length = 0, or for
Types 8 and 9, a mask an RSVP LSP, the
FEC Stack consists of all zeros), a single element that captures the type MUST RSVP Session
and Sender Template that uniquely identifies the LSP.

FEC Stacks can be set to 0. more complex. For example, suppose LSR X at hop 10 has one may wish to test a
VPN IPv4 prefix of 10.1/8 that is tunneled over an LDP LSP with
egress 10.10.1.1. The FEC Stack would then contain two downstream LSRs, Y sub-TLVs, the
bottom being a VPN IPv4 prefix, and
Z, for the FEC in question. The received X top being an LDP IPv4 prefix.
If the underlying (LDP) tunnel were not known, or was considered
irrelevant, the FEC Stack could return Multipath
Type 4, be a single element with low/high IP addresses of 127.1.1.1->127.1.1.255 for
downstream LSR Y and 127.2.1.1->127.2.1.255 for downstream LSR Z.
The head end reflects this information just the VPN
IPv4 sub-TLV.

When an MPLS echo request is received, the receiver is expected to LSR Y. Y, which has three
downstream LSRs, U, V,
verify that the control plane and data plane are both healthy (for
the FEC Stack being pinged) and W, computes that 127.1.1.1->127.1.1.127
would go to U the two planes are in sync. The
procedures for this are in section 4.4 below.

4.1. Dealing with Equal-Cost Multi-Path (ECMP)

LSPs need not be simple point-to-point tunnels. Frequently, a single
LSP may originate at several ingresses, and 127.1.1.128-> 127.1.1.255 would go to V. Y would
then respond terminate at several
egresses; this is very common with 3 Downstream Mappings: LDP LSPs. LSPs for a given FEC
may also have multiple "next hops" at transit LSRs. At an ingress,
there may also be several different LSPs to U, with Multipath Type 4
(127.1.1.1->127.1.1.127); choose from to V, with Multipath Type 4
(127.1.1.127->127.1.1.255); get to the
desired endpoint. Finally, LSPs may have backup paths, detour paths,
and other alternative paths to W, take should the primary LSP go down.

To deal with Multipath Type 0.

Note the last two first: it is assumed that computing Multipath Information may impose a significant
processing burden on the receiver. A receiver MAY thus choose to
process a subset of LSR sourcing
MPLS echo requests can force the received prefixes. echo request into any desired LSP,
so choosing among multiple LSPs at the ingress is not an issue. The sender, on receiving
a reply to a Downstream Mapping with partial information, SHOULD
assume that
problem of probing the prefixes missing in various flavors of backup paths that will
typically not be used for forwarding data unless the reply were skipped by primary LSP is
down will not be addressed here.

Since the
receiver, actual LSP and MAY re-request information about them in path that a new echo
request.

3.3.2. Downstream Router and Interface

The notion of "downstream router" and "downstream interface" should given packet may take may not be explained. Consider an LSR X. If
known a packet priori, it is useful if MPLS echo requests can exercise all
possible paths. This, although desirable, may not be practical,
because the algorithms that was originated
with TTL n>1 arrived with outermost label L and TTL=1 at a given LSR X, X
must be able uses to compute which LSRs could receive the packet if it was
originated with TTL=n+1, distribute packets
over which interface alternative paths may be proprietary.

To achieve some degree of coverage of alternate paths, there is a
certain latitude in choosing the request would
arrive destination IP address and what label stack those LSRs would see. (It source
UDP port for an MPLS echo request. This is outside clearly not sufficient;
in the
scope case of this document to specify how this computation traceroute, more latitude is done.) The
set offered by means of these LSRs/interfaces consists the
Multipath Information of the downstream routers/
interfaces (and their corresponding labels) for X with respect to L.
Each pair of downstream router and interface requires a separate Downstream Detailed Mapping to be added to the reply.

The case where X TLV. This
is the used as follows. An ingress LSR originating the echo request is a special
case. X needs to figure out what LSRs would receive the periodically sends an MPLS echo
request
traceroute message to determine whether there are multipaths for a
given FEC Stack that X originates with TTL=1.

The set LSP. If so, each hop will provide some information how each of
its downstream routers at X may be alternative paths (see the
discussion below on ECMP) or simultaneous paths (e.g., for can be exercised. The ingress can then send
MPLS
multicast). In the former case, echo requests that exercise these paths. If several transit
LSRs have ECMP, the Multipath Information is ingress may attempt to compose these to exercise
all possible paths. However, full coverage may not be possible.

4.2. Testing LSPs That Are Used to Carry MPLS Payloads

To detect certain LSP breakages, it may be necessary to encapsulate
an MPLS echo request packet with at least one additional label when
testing LSPs that are used as
a hint to the sender carry MPLS payloads (such as LSPs used
to how it carry L2VPN and L3VPN traffic. For example, when testing LDP or
RSVP-TE LSPs, just sending an MPLS echo request packet may influence not detect
instances where the choice of these
alternatives.

3.4. Pad TLV

The value part router immediately upstream of the Pad TLV contains a variable number (>= 1) destination of
octets. The first octet takes values from
the following table; all LSP ping may forward the other octets (if any) are ignored. The receiver SHOULD verify
that MPLS echo request successfully over an
interface not configured to carry MPLS payloads because of the TLV is received in its entirety, but otherwise ignores use of
penultimate hop popping. Since the
contents receiving router has no means to
differentiate whether the IP packet was sent unlabeled or implicitly
labeled, the addition of this TLV, apart from labels shimmed above the first octet.

Value Meaning
----- -------
1 Drop Pad TLV MPLS echo request
(using the Nil FEC) will prevent a router from reply
2 Copy Pad TLV to reply
3-255 Reserved for future use

3.5. Vendor Enterprise Number

SMI Private Enterprise Numbers are maintained by IANA. forwarding such a
packet out unlabeled interfaces.

4.3. Sending an MPLS Echo Request

An MPLS echo request is a UDP packet. The Length IP header is
always 4; set as
follows: the value source IP address is the SMI Private Enterprise code, in network
octet order, of the vendor with a Vendor Private extension to any routable address of the fields in sender;
the fixed part of destination IP address is a (randomly chosen) IPv4 address from
the message, in which case this TLV
MUST be present. If none of range 127/8 or IPv6 address from the fields in range
0:0:0:0:0:FFFF:7F00/104. The IP TTL is set to 1. The source UDP
port is chosen by the fixed part of sender; the
message have Vendor Private extensions, inclusion of this TLV destination UDP port is
OPTIONAL. Vendor Private ranges set to 3503
(assigned by IANA for Message Types, Reply Modes, and
Return Codes have been defined. When any MPLS echo requests). The Router Alert IP
option of these are used, the
Vendor Enterprise Number TLV value 0x0 [RFC2113] for IPv4 or value 69 [RFC7506] for IPv6
MUST be included set in IP header.

An MPLS echo request is sent with a label stack corresponding to the message.

3.6. Interface and Label Stack

The Interface and Label
FEC Stack TLV MAY being tested. Note that further labels could be included in a reply message
to report applied
if, for example, the interface on which normal route to the request message was received and topmost FEC in the label stack that was on the packet when it was received. Only
one such object may appear. The purpose is
via a Traffic Engineered Tunnel [RFC3209]. If all of the object is to allow FECs in the upstream router
stack correspond to obtain Implicit Null labels, the exact interface and label stack
information as it appears at MPLS echo request is
considered unlabeled even if further labels will be applied in
sending the replying LSR.

The Length packet.

If the echo request is K + 4*N octets; N labeled, one MAY (depending on what is being
pinged) set the number TTL of labels in the innermost label
stack. Values for K are found in to 1, to prevent the description of Address Type
below. The Value field ping
request going farther than it should. Examples of where this SHOULD
be done include pinging a Downstream Mapping has VPN IPv4 or IPv6 prefix, an L2 VPN endpoint
or a pseudowire. Preventing 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Type | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IP Address (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Interface (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. Label Stack .
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Address Type

The Address Type indicates if the interface is numbered or
unnumbered. It ping request from going too far can
also determines be accomplished by inserting a Router Alert label above this
label; however, this may lead to the length of undesired side effect that MPLS
echo requests take a different data path than actual data. For more
information on how these mechanisms can be used for pseudowire
connectivity verification, see [RFC5085].

In "ping" mode (end-to-end connectivity check), the IP Address TTL in the
outermost label is set to 255. In "traceroute" mode (fault isolation
mode), the TTL is set successively to 1, 2, and
Interface fields. so on.

The resulting total for sender chooses a Sender's Handle and a Sequence Number. When
sending subsequent MPLS echo requests, the initial part sender SHOULD increment
the Sequence Number by 1. However, a sender MAY choose to send a
group of echo requests with the
TLV same Sequence Number to improve the
chance of arrival of at least one packet with that Sequence Number.

The TimeStamp Sent is listed set to the time-of-day in NTP format that the table below as "K Octets".
echo request is sent. The Address Type TimeStamp Received is set to one of the following values:

Type # Address Type K Octets
------ ------------ --------
1 IPv4 Numbered 12
2 IPv4 Unnumbered 12
3 IPv6 Numbered 36
4 IPv6 Unnumbered 24

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 zero.

An MPLS echo request message was received
is numbered, then the Address Type MUST be set to IPv4 or IPv6,
the IP Address MUST be set to either the LSR's Router ID or the
interface address, and have an FEC Stack TLV. Also, the Interface MUST Reply
Mode 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 desired reply mode; the
LSR's Router ID, Return Code and the Interface MUST be
Subcode are set to zero. In the index
assigned to the interface.

Label Stack

The label stack of "traceroute" mode, the received echo request message. If any TTL
values have been changed by this router, they
SHOULD be restored.

3.7. Errored TLVs

The following TLV is include a TLV that MAY be included in Downstream Detailed Mapping TLV.

4.4. Receiving an echo reply to
inform the sender of MPLS Echo Request

Sending an MPLS echo request of mandatory TLVs either not
supported to the control plane is triggered by an implementation one
of the following packet processing exceptions: Router Alert option,
IP TTL expiration, MPLS TTL expiration, MPLS Router Alert label, or parsed and found to be
the destination address in error.

The Value field contains the TLVs that were not understood, encoded
as sub-TLVs.

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 = 9 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.8. Reply TOS Byte TLV

This TLV MAY be used 127/8 address range. The control
plane further identifies it by UDP destination port 3503.

For reporting purposes the originator bottom of the echo request stack is considered to request
that an echo reply be sent with the IP header TOS byte set stack-
depth of 1. This is to establish an absolute reference for the
value specified case
where the actual stack may have more labels than there are FECs in
the TLV. This TLV has Target FEC Stack.

Furthermore, in all the error codes listed in this document, a length stack-
depth of 4 0 means "no value specified". This allows compatibility
with existing implementations that do not use the
following value Return Subcode
field.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reply-TOS Byte| Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4. Theory of Operation

An LSR X that receives an MPLS echo request then processes it as
follows.

1. General packet sanity is used to test a particular LSP. The LSP to be
tested is identified by the "FEC Stack"; for example, if verified. If the LSP was
set up via LDP, and packet is to an egress IP address of 10.1.1.1, the FEC
Stack contains a single element, namely, not well-
formed, LSR X SHOULD send an LDP IPv4 prefix sub-TLV MPLS Echo Reply with value 10.1.1.1/32. If the LSP being tested is an RSVP LSP, the
FEC Stack consists of a single element that captures the RSVP Session Return Code
set to "Malformed echo request received" and Sender Template that uniquely identifies the LSP.

FEC Stacks can be more complex. For example, one may wish Subcode to test a
VPN IPv4 prefix of 10.1/8 zero.
If there are any TLVs not marked as "Ignore" that is tunneled over LSR X does not
understand, LSR X SHOULD send an LDP LSP with
egress 10.10.1.1. The FEC Stack would then contain two sub-TLVs, the
bottom being a VPN IPv4 prefix, MPLS "TLV not understood" (as
appropriate), and the top being Subcode set to zero. In the latter case,
the misunderstood TLVs (only) are included as sub-TLVs in an LDP IPv4 prefix.
If
Errored TLVs TLV in the underlying (LDP) tunnel were reply. The header fields Sender's
Handle, Sequence Number, and Timestamp Sent are not known, or was considered
irrelevant, examined, but
are included in the FEC Stack could be a single element with just MPLS echo reply message.

The algorithm uses the following variables and identifiers:

Interface-I: the interface on which the VPN
IPv4 sub-TLV.

When an MPLS echo request is received, was
received.

Stack-R: the receiver is expected to
verify that label stack on the control plane and data plane are both healthy (for packet as it was received.

Stack-D: the FEC Stack being pinged) and that the two planes are label stack carried in sync. The
procedures for this are the "Label Stack sub-
TLV" in section 4.4 below.

4.1. Dealing with Equal-Cost Multi-Path (ECMP)

LSPs need not be simple point-to-point tunnels. Frequently, a single
LSP may originate at several ingresses, and terminate at several
egresses; this is very common with LDP LSPs. LSPs for a given FEC
may also have multiple "next hops" at transit LSRs. At an ingress,
there may also be several different LSPs to choose Downstream Detailed Mapping TLV (not
always present)

Label-L: the label from to get to the
desired endpoint. Finally, LSPs may have backup paths, detour paths,
and other alternative paths actual stack currently being
examined. Requires no initialization.

Label-stack-depth: the depth of label being verified. Initialized
to take should the primary LSP go down.

To deal with number of labels in the last two first: it is assumed received label
stack S.

FEC-stack-depth: depth of the FEC in the Target FEC Stack that
should be used to verify the LSR sourcing
MPLS echo requests can force current actual
label. Requires no initialization.

Best-return-code: contains the return code for the echo request into any desired LSP,
so choosing among multiple LSPs at reply
packet as currently best known. As the ingress is not an issue. The
problem of probing algorithm
progresses, this code may change depending on the various flavors
results of backup paths further checks that will
typically not be used it performs.

Best-rtn-subcode: similar to Best-return-code, but for forwarding data unless the primary LSP is
down will not be addressed here.

Since Echo
Reply Subcode.

FEC-status: result value returned by the actual LSP and path that a given packet may take may not be
known a priori, it is useful if MPLS echo requests can exercise all
possible paths. This, although desirable, may not be practical,
because FEC Checking
algorithm described in section 4.4.1.

/* Save receive context information */

2. If the algorithms that a given echo request is good, LSR uses to distribute packets X stores the interface over alternative paths may be proprietary.

To achieve some degree of coverage of alternate paths, there is a
certain latitude in choosing
which the destination IP address and source
UDP port for an MPLS echo request. This is clearly not sufficient; was received in Interface-I, and the case label stack
with which it came in Stack-R.

/* The rest of traceroute, more latitude is offered by means the algorithm iterates over the labels in Stack-R,
verifies validity of label values, reports associated label switching
operations (for traceroute), verifies correspondence between the
Multipath Information
Stack-R and the Target FEC Stack description in the body of the Downstream Mapping TLV. This is used echo
request, and reports any errors. */
/* The algorithm iterates as follows. An ingress */

3. Label Validation:

If Label-stack-depth is 0 {

/* The LSR periodically sends an MPLS traceroute
message needs to determine whether there are multipaths for a given LSP.
If so, each hop will provide some information how each of report its
downstream paths can be exercised. The ingress can then send MPLS
echo requests that exercise these paths. If several transit LSRs
have ECMP, being a tail-end for the ingress may attempt to compose these LSP */

Set FEC-stack-depth to exercise all
possible paths. However, full coverage may not be possible.

4.2. Testing LSPs That Are Used 1, set Label-L to Carry MPLS Payloads

To detect certain LSP breakages, it may be necessary 3 (Implicit Null).
Set Best-return-code to encapsulate 3 ("Replying router is an MPLS echo request packet with egress for
the FEC at least one additional label when
testing LSPs that are used to carry MPLS payloads (such as LSPs used stack depth"), set Best-rtn-subcode to carry L2VPN and L3VPN traffic. For example, when testing LDP or
RSVP-TE LSPs, just sending an MPLS echo request packet may not detect
instances where the router immediately upstream of the destination value of
the LSP ping may forward the MPLS echo request successfully over
FEC-stack-depth (1) and go to step 5 (Egress Processing).

}

/* This step assumes there is always an
interface not configured entry for well-known label
values */

Set Label-L to carry MPLS payloads because of the use of
penultimate hop popping. Since value extracted from Stack-R at depth Label-
stack-depth. Look up Label-L in the receiving router has no means Incoming Label Map (ILM) to
differentiate whether the IP packet was sent unlabeled or implicitly
labeled, the addition of labels shimmed above the MPLS echo request
(using
determine if the Nil FEC) will prevent a router from forwarding such a
packet out unlabeled interfaces.

4.3. Sending label has been allocated and an MPLS Echo Request

An MPLS echo request is a UDP packet. The IP header operation is set as
follows: the source IP address
associated with it.

If there is no entry for L {

/* Indicates a routable address of the sender; temporary or permanent label synchronization
problem the destination IP address is a (randomly chosen) IPv4 address from LSR needs to report an error */

Set Best-return-code to 11 ("No label entry at stack-depth")
and Best-rtn-subcode to Label-stack-depth. Go to step 7 (Send
Reply Packet).

}

Else {

Retrieve the range 127/8 or IPv6 address associated label operation from the range
0:0:0:0:0:FFFF:7F00/104. The IP TTL is set corresponding
NHLFE and proceed to 1. The source UDP
port is chosen by the sender; step 4 (Label Operation check).

}

4. Label Operation Check

If the destination UDP port label operation is set to 3503
(assigned by IANA for MPLS echo requests). The "Pop and Continue Processing" {

/* Includes Explicit Null and Router Alert IP
option of value 0x0 [RFC2113] for IPv4 or value 69 [RFC7506] for IPv6
MUST be set in IP header.

An MPLS echo request is sent with a label stack corresponding cases */
Iterate to the
FEC Stack being tested. Note that further labels could be applied
if, for example, the normal route next label by decrementing Label-stack-depth and
loop back to step 3 (Label Validation).

}

If the topmost FEC in the stack label operation is
via a Traffic Engineered Tunnel [RFC3209]. "Swap or Pop and Switch based on Popped
Label" {

Set Best-return-code to 8 ("Label switched at stack-depth") and
Best-rtn-subcode to Label-stack-depth to report transit
switching.

If all of the FECs a Downstream Detailed Mapping TLV is present in the
stack correspond to Implicit Null labels, the MPLS received
echo request is
considered unlabeled even if further labels will be applied in
sending the packet. {

If the echo request is labeled, one MAY (depending on what is being
pinged) set the TTL of IP address in the innermost label to 1, TLV is 127.0.0.1 or 0::1 {

Set Best-return-code to prevent the ping
request going farther than it should. Examples of where this 6 ("Upstream Interface Index
Unknown"). An Interface and Label Stack TLV SHOULD be done include pinging a VPN IPv4 or IPv6 prefix, an L2 VPN endpoint
or a pseudowire. Preventing
included in the ping request from going too far can
also be accomplished by inserting a Router Alert reply and filled with Interface-I and
Stack-R.

}

Else {

Verify that the IP address, interface address, and label above this
label; however, this may lead to
stack in the undesired side effect that MPLS
echo requests take Downstream Detailed Mapping TLV match
Interface-I and Stack-R. If there is a different data path than actual data. For more
information on how these mechanisms can mismatch, set
Best-return-code to 5, "Downstream Mapping Mismatch". An
Interface and Label Stack TLV SHOULD be used for pseudowire
connectivity verification, see [RFC5085].

In "ping" mode (end-to-end connectivity check), included in the TTL
reply and filled in based on Interface-I and Stack-R. Go
to step 7 (Send Reply Packet).

}

For each available downstream ECMP path {

Retrieve output interface from the
outermost label NHLFE entry.

/* Note: this return code is set to 255. In "traceroute" mode (fault isolation
mode), even if Label-stack-depth
is one */

If the TTL output interface is set successively not MPLS enabled {
Set Best-return-code to 1, 2, Return Code 9, "Label switched
but no MPLS forwarding at stack-depth" and so on.

The sender chooses a Sender's Handle set Best-rtn-
subcode to Label-stack-depth and goto Send_Reply_Packet.

}

If a Sequence Number. When
sending subsequent MPLS echo requests, the sender Downstream Detailed Mapping TLV is present {

A Downstream Detailed Mapping TLV SHOULD increment be included in
the Sequence Number by 1. However, a sender MAY choose to send a
group of echo requests reply (see Section 3.4) filled in with
information about the same Sequence Number to improve current ECMP path.

}

If no Downstream Detailed Mapping TLV is present, or the
chance of arrival of at least one packet with that Sequence Number.

The TimeStamp Sent
Downstream IP Address is set to the time-of-day in NTP format that ALLROUTERS multicast
address, go to step 7 (Send Reply Packet).

If the
echo request is sent. The TimeStamp Received "Validate FEC Stack" flag is not set to zero.

An MPLS echo request MUST have an FEC Stack TLV. Also, and the Reply
Mode must be set LSR is not
configured to the desired reply mode; the Return Code and
Subcode are set perform FEC checking by default, go to zero. In step 7
(Send Reply Packet).

/* Validate the "traceroute" mode, Target FEC Stack in the received echo request
SHOULD include a request.

First determine FEC-stack-depth from the Downstream Detailed
Mapping TLV.

4.4. Receiving an MPLS Echo Request

Sending an MPLS echo request to the control plane This is triggered done by one
of walking through Stack-D (the
Downstream labels) from the following packet processing exceptions: Router Alert option,
IP TTL expiration, MPLS TTL expiration, MPLS Router Alert label, or bottom, decrementing the destination address in number of
labels for each non-Implicit Null label, while incrementing
FEC-stack-depth for each label. If the 127/8 address range. The control
plane further identifies it by UDP destination port 3503.

For reporting purposes Downstream Detailed
Mapping TLV contains one or more Implicit Null labels, FEC-
stack-depth may be greater than Label-stack-depth. To be
consistent with the above stack-depths, the bottom of stack is
considered to be stack-
depth of entry 1. This is
*/

Set FEC-stack-depth to establish an absolute reference for the case
where 0. Set i to Label-stack-depth.

While (i > 0 ) do {

++FEC-stack-depth.
if Stack-D[FEC-stack-depth] != 3 (Implicit Null)
--i.
}

If the actual stack may have more labels than there are number of FECs in the Target FEC Stack.

Furthermore, in all stack is greater than or equal
to FEC-stack-depth {
Perform the error codes listed in FEC Checking procedure (see subsection 4.4.1
below).

If FEC-status is 2, set Best-return-code to 10 ("Mapping for
this document, a stack-
depth of 0 means "no value specified". This allows compatibility
with existing implementations that do FEC is not use the Return Subcode
field.

An LSR X that receives an MPLS echo request then processes it as
follows.

1. General packet sanity is verified. given label at stack-depth").

If the packet return code is not well-
formed, LSR X SHOULD send an MPLS Echo Reply with the Return Code 1, set Best-return-code to "Malformed echo request received" FEC-return-
code and the Subcode Best-rtn-subcode to zero.
If there FEC-stack-depth.
}

Go to step 7 (Send Reply Packet).
}

5. Egress Processing:

/* These steps are any TLVs not marked as "Ignore" that performed by the LSR X does not
understand, that identified itself as
the tail-end LSR X SHOULD send for an MPLS "TLV not understood" (as
appropriate), and the Subcode set to zero. In the latter case,
the misunderstood TLVs (only) are included as sub-TLVs in an
Errored TLVs TLV in the reply. The header fields Sender's
Handle, Sequence Number, and Timestamp Sent are not examined, but
are included in the MPLS echo reply message.

The algorithm uses the following variables and identifiers:

Interface-I: the interface on which the MPLS LSP. */

If received echo request was
received.

Stack-R: the label stack on contains no Downstream Detailed Mapping
TLV, or the packet as it was received.

Stack-D: Downstream IP Address is set to 127.0.0.1 or 0::1 go
to step 6 (Egress FEC Validation).

Verify that the IP address, interface address, and label stack carried in
the Downstream Detailed Mapping TLV (not always present)

Label-L: the label from the actual stack currently being
examined. Requires no initialization.

Label-stack-depth: the depth of label being verified. Initialized match Interface-I and Stack-R.
If not, set Best-return-code to the number of labels in the received label
stack S.

FEC-stack-depth: depth of the FEC in the Target FEC 5, "Downstream Mapping Mis-match".
A Received Interface and Label Stack that
should TLV SHOULD be used to verify the current actual
label. Requires no initialization.

Best-return-code: contains the return code created for the
echo reply
packet as currently best known. As the algorithm
progresses, this code may change depending on the
results of further checks that it performs.

Best-rtn-subcode: similar response packet. Go to Best-return-code, but step 7 (Send Reply Packet).

6. Egress FEC Validation:

/* This is a loop for all entries in the Echo
Reply Subcode.

FEC-status: result value returned Target FEC Stack starting
with FEC-stack-depth. */

Perform FEC checking by following the FEC Checking algorithm described in section 4.4.1.

/* Save receive context information */

2. If the echo request is good, LSR X stores the interface over
which the echo was received in Interface-I,
subsection 4.4.1 for Label-L and the label stack
with which it came in Stack-R.

/* The rest of the algorithm iterates over the labels in Stack-R,
verifies validity of label values, reports associated label switching
operations (for traceroute), verifies correspondence between the
Stack-R FEC at FEC-stack-depth.

Set Best-return-code to FEC-code and Best-rtn-subcode to the Target FEC Stack description value
in the body FEC-stack-depth.

If FEC-status (the result of the echo
request, and reports any errors. */

/* The algorithm iterates as follows. */

3. Label Validation:

If Label-stack-depth check) is 0 {

/* The LSR needs to report its being a tail-end for the LSP */

Set FEC-stack-depth to 1, set Label-L
go to 3 (Implicit Null).
Set Best-return-code step 7 (Send Reply Packet).

/* Iterate to 3 ("Replying router is an egress for the next FEC at stack depth"), set Best-rtn-subcode to entry */

++FEC-stack-depth.
If FEC-stack-depth > the value number of
FEC-stack-depth (1) and FECs in the FEC-stack,
go to step 5 (Egress Processing).

}

/* This step assumes there 7 (Send Reply Packet).

If FEC-status is always an entry for well-known label
values */

Set Label-L to 0 {

++Label-stack-depth.
If Label-stack-depth > the value number of labels in Stack-R,
Go to step 7 (Send Reply Packet).

Label-L = extracted label from Stack-R at depth Label-
stack-depth. Look up Label-L in the Incoming Label Map (ILM)
Label-stack-depth.
Loop back to
determine if the label has been allocated and step 6 (Egress FEC Validation).
}

7. Send Reply Packet:

Send an operation is
associated MPLS echo reply with it.

If there is no entry a Return Code of Best-return-code,
and a Return Subcode of Best-rtn-subcode. Include any TLVs
created during the above process. The procedures for L { sending the
echo reply are found in subsection 4.5.

4.4.1. FEC Validation

/* Indicates a temporary or permanent label synchronization
problem This subsection describes validation of an FEC entry within the LSR needs to report
Target FEC Stack and accepts an error FEC, Label-L, and Interface-I. The
algorithm performs the following steps. */

1. Two return values, FEC-status and FEC-return-code, are
initialized to 0.

2. If the FEC is the Nil FEC {

If Label-L is either Explicit_Null or Router_Alert, return.

Else {

Set Best-return-code FEC-return-code to 11 ("No 10 ("Mapping for this FEC is not the
given label entry at stack-depth")
and Best-rtn-subcode to Label-stack-depth. Go stack-depth").
Set FEC-status to step 7 (Send
Reply Packet). 1
Return.
}
Else {

Retrieve

}

3. Check the associated FEC label operation from mapping that describes how traffic received
on the corresponding
NHLFE and proceed LSP is further switched or which application it is
associated with. If no mapping exists, set FEC-return-code to step 4 (Label Operation check).

}
Return 4, "Replying router has no mapping for the FEC at stack-
depth". Set FEC-status to 1. Return.

4. Label Operation Check If the label operation mapping for FEC is "Pop and Continue Processing" {

/* Includes Explicit Null and Router Alert label cases */

Iterate Implicit Null, set FEC-status to the next label by decrementing Label-stack-depth
2 and
loop back proceed to step 3 (Label Validation).

}

If 5. Otherwise, if the label operation mapping for FEC
is "Swap or Pop and Switch based on Popped
Label" {

Set Best-return-code Label-L, proceed to 8 ("Label switched step 5. Otherwise, set FEC-return-code to
10 ("Mapping for this FEC is not the given label at stack-depth") and
Best-rtn-subcode stack-
depth"), set FEC-status to Label-stack-depth 1, and return.

5. This is a protocol check. Check what protocol would be used to report transit
switching.
advertise FEC. If a Downstream Mapping TLV it can be determined that no protocol
associated with Interface-I would have advertised an FEC of that
FEC-Type, set FEC-return-code to 12 ("Protocol not associated
with interface at FEC stack-depth"). Set FEC-status to 1.

6. Return.

4.5. Sending an MPLS Echo Reply

An MPLS echo reply is present a UDP packet. It MUST ONLY be sent in the received response
to an MPLS echo
request {

If the request. The source IP address in is a routable address
of the TLV replier; the source port is 127.0.0.1 or 0::1 {

Set Best-return-code to 6 ("Upstream Interface Index
Unknown"). An Interface and Label Stack TLV SHOULD be
included in the reply and filled with Interface-I well-known UDP port for LSP
ping. The destination IP address and
Stack-R.

}

Else {

Verify that UDP port are copied from the
source IP address, interface address, address and label
stack in UDP port of the Downstream Mapping TLV match Interface-I and
Stack-R. If there echo request. The IP TTL is a mismatch,
set Best-return-code to
5, "Downstream Mapping Mismatch". An Interface and Label
Stack TLV SHOULD be included in 255. If the reply and filled in
based on Interface-I and Stack-R. Go to step 7 (Send Reply Packet).

}

For each available downstream ECMP path {

Retrieve output interface from Mode in the NHLFE entry.

/* Note: echo request is "Reply via an
IPv4 UDP packet with Router Alert", then the IP header MUST contain
the Router Alert IP option of value 0x0 [RFC2113] for IPv4 or 69
[RFC7506] for IPv6. If the reply is sent over an LSP, the topmost
label MUST in this return code case be the Router Alert label (1) (see
[RFC3032]).

The format of the echo reply is set even if Label-stack-depth the same as the echo request. The
Sender's Handle, the Sequence Number, and TimeStamp Sent are copied
from the echo request; the TimeStamp Received is one */

If set to the output interface time-of-
day that the echo request is not MPLS enabled {

Set Best-return-code received (note that this information is
most useful if the time-of-day clocks on the requester and the
replier are synchronized). The FEC Stack TLV from the echo request
MAY be copied to the reply.

The replier MUST fill in the Return Code 9, "Label switched
but no MPLS forwarding at stack-depth" and set Best-rtn-
subcode to Label-stack-depth and goto Send_Reply_Packet.

} Subcode, as determined
in the previous subsection.

If the echo request contains a Downstream Mapping TLV Pad TLV, the replier MUST interpret
the first octet for instructions regarding how to reply.

If the replying router is present {

A the destination of the FEC, then Downstream
Detailed Mapping TLV TLVs SHOULD NOT be included in the echo
reply (see section 3.3) filled in with information about
the current ECMP path.

}

} reply.

If no the echo request contains a Downstream Detailed Mapping TLV is present, or TLV, and
the Downstream IP
Address is set to the ALLROUTERS multicast address, go to step
7 (Send Reply Packet).

If the "Validate FEC Stack" flag is not set and the LSR replying router is not
configured to perform FEC checking by default, go to step 7
(Send Reply Packet).

/* Validate the Target FEC Stack in the received echo request.

First determine FEC-stack-depth from the Downstream Mapping
TLV. This is done by walking through Stack-D (the Downstream
labels) from destination of the bottom, decrementing FEC, the number of replier
SHOULD compute its downstream routers and corresponding labels for
each non-Implicit Null
the incoming label, while incrementing FEC-stack-
depth and add Downstream Detailed Mapping TLVs for each label.
one to the echo reply it sends back.

If the Downstream Detailed Mapping TLV contains
one or Multipath Information
requiring more Implicit Null labels, FEC-stack-depth may be
greater processing than Label-stack-depth. To be consistent with the
above stack-depths, the bottom receiving router is considered to be entry 1.
*/

Set FEC-stack-depth to 0. Set i willing to Label-stack-depth.

While (i > 0 ) do {

++FEC-stack-depth.
if Stack-D[FEC-stack-depth] != 3 (Implicit Null)
--i.
}

If
perform, the number responding router MAY choose to respond with only a
subset of FECs multipaths contained in the FEC stack is greater than or equal
to FEC-stack-depth {
Perform echo request Downstream
Detailed Mapping. (Note: The originator of the FEC Checking procedure (see subsection 4.4.1
below).

If FEC-status is 2, set Best-return-code to 10 ("Mapping for
this FEC is echo request MAY send
another echo request with the Multipath Information that was not
included in the given label at stack-depth").

If reply.)

Except in the return code is 1, set Best-return-code to FEC-return-
code and Best-rtn-subcode to FEC-stack-depth.
}

Go to step 7 (Send case of Reply Packet).
}

5. Egress Processing:

/* These steps Mode 4, "Reply via application level
control channel", echo replies are performed by always sent in the LSR that identified itself as context of the tail-end
IP/MPLS network.

4.6. Receiving an MPLS Echo Reply

An LSR for X should only receive an LSP. */

If received MPLS echo reply in response to an
MPLS echo request contains no Downstream Mapping TLV, or that it sent. Thus, on receipt of an MPLS echo
reply, X should parse the Downstream IP Address is set packet to 127.0.0.1 or 0::1 go ensure that it is well-formed,
then attempt to step 6
(Egress FEC Validation).

Verify match up the echo reply with an echo request that it
had previously sent, using the IP address, interface address, destination UDP port and label stack in the Downstream Mapping TLV match Interface-I and Stack-R. Sender's
Handle. If not,
set Best-return-code no match is found, then X jettisons the echo reply;
otherwise, it checks the Sequence Number to 5, "Downstream Mapping Mis-match". A
Received Interface see if it matches.

If the echo reply contains Downstream Detailed Mappings, and Label Stack TLV X wishes
to traceroute further, it SHOULD be created for copy the Downstream Detailed
Mapping(s) into its next echo response packet. Go to step 7 (Send Reply Packet).

6. Egress FEC Validation:

/* This request(s) (with TTL incremented by
one).

4.7. Issue with VPN IPv4 and IPv6 Prefixes

Typically, an LSP ping for a VPN IPv4 prefix or VPN IPv6 prefix is
sent with a loop for all entries in the Target FEC Stack starting label stack of depth greater than 1, with FEC-stack-depth. */

Perform FEC checking by following the algorithm described in
subsection 4.4.1 for Label-L and innermost
label having a TTL of 1. This is to terminate the FEC ping at FEC-stack-depth.

Set Best-return-code to FEC-code and Best-rtn-subcode the egress
PE, before it gets sent to the value
in FEC-stack-depth.

If FEC-status (the result of customer device. However, under
certain circumstances, the check) is 1,
go to step 7 (Send Reply Packet).

/* Iterate label stack can shrink to a single label
before the next FEC entry */

++FEC-stack-depth.
If FEC-stack-depth > ping hits the number of FECs egress PE; this will result in the FEC-stack,
go to step 7 (Send Reply Packet).

If FEC-status ping
terminating prematurely. One such scenario is 0 {

++Label-stack-depth.
If Label-stack-depth > a multi-AS Carrier's
Carrier VPN.

To get around this problem, one approach is for the number of labels in Stack-R,
Go LSR that receives
such a ping to step 7 (Send Reply Packet).

Label-L = extracted label from Stack-R at depth
Label-stack-depth.
Loop realize that the ping terminated prematurely, and send
back error code 13. In that case, the initiating LSR can retry the
ping after incrementing the TTL on the VPN label. In this fashion,
the ingress LSR will sequentially try TTL values until it finds one
that allows the VPN ping to step 6 (Egress FEC Validation).
}

7. Send Reply Packet:

Send an MPLS echo reply with a Return Code of Best-return-code,
and a Return Subcode of Best-rtn-subcode. Include any TLVs
created during reach the above process. The procedures egress PE.

4.8. Non-compliant Routers

If the egress for sending the
echo reply are found in subsection 4.5.

4.4.1. FEC Validation

/* This subsection describes validation of an FEC entry within the
Target FEC Stack and accepts an FEC, Label-L, and Interface-I. The
algorithm performs the following steps. */

1. Two return values, FEC-status and FEC-return-code, are
initialized to 0.

2. If the FEC is the Nil FEC { being pinged does not support MPLS
ping, then no reply will be sent, resulting in possible "false
negatives". If Label-L is either Explicit_Null or Router_Alert, return.

Else {

Set FEC-return-code to 10 ("Mapping for this FEC is in "traceroute" mode, a transit LSR does not the
given label at stack-depth").
Set FEC-status to 1
Return.
}

}

3. Check the FEC label mapping that describes how traffic received
on the support
LSP is further switched or which application it is
associated with. If no mapping exists, set FEC-return-code to
Return 4, "Replying router has ping, then no mapping reply will be forthcoming from that LSR for some
TTL, say, n. The LSR originating the FEC at stack-
depth". Set FEC-status to 1. Return.

4. If echo request SHOULD try sending
the label mapping for FEC is Implicit Null, set FEC-status to
2 and proceed echo request with TTL=n+1, n+2, ..., n+k to step 5. Otherwise, if probe LSRs further
down the label mapping path. In such a case, the echo request for FEC
is Label-L, proceed to step 5. Otherwise, TTL > n SHOULD
be sent with Downstream Detailed Mapping TLV "Downstream IP Address"
field set FEC-return-code to
10 ("Mapping for this FEC is not the given ALLROUTERs multicast address until a reply is
received with a Downstream Detailed Mapping TLV. The label at stack-
depth"), set FEC-status to 1, and return.

5. This is a protocol check. Check what protocol would be used to
advertise FEC. If it can stack TLV
MAY be determined that no protocol
associated with Interface-I would have advertised an omitted from the Downstream Detailed Mapping TLV.
Furthermore, the "Validate FEC of that
FEC-Type, Stack" flag SHOULD NOT be set FEC-return-code to 12 ("Protocol not associated
with interface at FEC stack-depth"). Set FEC-status to 1.

6. Return.

4.5. Sending until an MPLS Echo Reply

An MPLS
echo reply is a UDP packet. It MUST ONLY be sent in response
to an MPLS echo request. The source IP address is packet with a routable address
of the replier; the source port Downstream Detailed Mapping TLV is received.

5. Security Considerations

Overall, the well-known UDP port security needs for LSP ping are similar to those of ICMP
ping. The destination IP address and UDP port

There are copied from at least three approaches to attacking LSRs using the
source IP address
mechanisms defined here. One is a Denial-of-Service attack, by
sending MPLS echo requests/replies to LSRs and UDP port thereby increasing
their workload. The second is obfuscating the state of the MPLS data
plane liveness by spoofing, hijacking, replaying, or otherwise
tampering with MPLS echo request. requests and replies. The IP TTL third is
set an
unauthorized source using an LSP ping to 255. If the Reply Mode in obtain information about the echo request
network.

To avoid potential Denial-of-Service attacks, it is "Reply via an
IPv4 UDP packet with Router Alert", then RECOMMENDED that
implementations regulate the IP header MUST contain LSP ping traffic going to the Router Alert IP option of value 0x0 [RFC2113] for IPv4 or 69
[RFC7506] for IPv6. If control
plane. A rate limiter SHOULD be applied to the well-known UDP port
defined below.

Unsophisticated replay and spoofing attacks involving faking or
replaying MPLS echo reply is sent over an LSP, the topmost
label MUST in this case messages are unlikely to be effective.
These replies would have to match the Router Alert label (1) (see
[RFC3032]).

The format Sender's Handle and Sequence
Number of the an outstanding MPLS echo reply is the same request message. A non-matching
replay would be discarded as the echo request. The
Sender's Handle, sequence has moved on, thus a spoof
has only a small window of opportunity. However, to provide a
stronger defense, an implementation MAY also validate the Sequence Number, and TimeStamp
Sent are copied
from the by requiring an exact match on this field.

To protect against unauthorized sources using MPLS echo request; the TimeStamp Received is set request
messages to the time-of-
day obtain network information, it is RECOMMENDED that
implementations provide a means of checking the source addresses of
MPLS echo request messages against an access list before accepting
the message.

It is received (note that this information is
most useful not clear how to prevent hijacking (non-delivery) of echo
requests or replies; however, if the time-of-day clocks on the requester and the
replier these messages are synchronized). The FEC Stack TLV from indeed hijacked,
LSP ping will report that the echo request
MAY be copied data plane is not working as it should.

It does not seem vital (at this point) to secure the reply.

The replier MUST fill data carried in the Return Code
MPLS echo requests and Subcode, as determined
in replies, although knowledge of the previous subsection.

If state of
the echo request contains MPLS data plane may be considered confidential by some.
Implementations SHOULD, however, provide a Pad TLV, the replier MUST interpret
the first octet for instructions regarding how to reply.

If the replying router is the destination means of filtering the FEC, then Downstream
Mapping TLVs SHOULD NOT
addresses to which echo reply messages may be included sent.

Although this document makes special use of 127/8 address, these are
used only in conjunction with the echo reply.

If the echo request contains UDP port 3503. Furthermore, these
packets are only processed by routers. All other hosts MUST treat
all packets with a Downstream Mapping TLV, and destination address in the
replying range 127/8 in
accordance to RFC 1122. Any packet received by a router is not with a
destination address in the range 127/8 without a destination UDP port
of the FEC, the replier SHOULD
compute its downstream routers and corresponding labels for the
incoming label, and add Downstream Mapping TLVs for each one 3503 MUST be treated in accordance to RFC 1812. In particular,
the
echo reply it sends back.

If the Downstream Mapping TLV contains Multipath Information
requiring more processing than the receiving router default behavior is willing to
perform, the responding router MAY choose treat packets destined to respond with only a
subset of multipaths contained in the 127/8 address
as "martians".

6. IANA Considerations

The TCP and UDP port number 3503 has been allocated by IANA for LSP
echo request Downstream
Mapping. (Note: requests and replies.

The originator of following sections detail the echo request MAY send another
echo request with the Multipath Information that was not included in new name spaces to be managed by
IANA. For each of these name spaces, the reply.)

Except in space is divided into
assignment ranges; the case of Reply Mode 4, "Reply via application level
control channel", echo replies following terms are always sent used in describing the context of the
IP/MPLS network.

4.6. Receiving an MPLS Echo Reply

An LSR X should only receive an MPLS echo reply
procedures by which IANA allocates values: "Standards Action" (as
defined in response to an
MPLS echo [RFC5226]), "Specification Required", and "Vendor Private
Use".

Values from "Specification Required" ranges MUST be registered with
IANA. The request that it sent. Thus, on receipt of MUST be made via an MPLS echo
reply, X should parse the packet to ensure Experimental RFC that it is well-formed,
then attempt to match up
describes the echo reply with an echo request that it
had previously sent, format and procedures for using the destination UDP port and code point; the Sender's
Handle. If no match
actual assignment is found, then X jettisons made during the echo reply;
otherwise, IANA actions for the RFC.

Values from "Vendor Private" ranges MUST NOT be registered with IANA;
however, the message MUST contain an enterprise code as registered
with the IANA SMI Private Network Management Private Enterprise
Numbers. For each name space that has a Vendor Private range, it checks
must be specified where exactly the Sequence SMI Private Enterprise Number to
resides; see if it matches.

If below for examples. In this way, several enterprises
(vendors) can use the echo reply contains Downstream Mappings, and X wishes to
traceroute further, it SHOULD copy the Downstream Mapping(s) into its
next echo request(s) (with TTL incremented by one).

4.7. Issue with VPN IPv4 same code point without fear of collision.

6.1. Message Types, Reply Modes, Return Codes

The IANA has created and IPv6 Prefixes

Typically, an LSP ping will maintain registries for a VPN IPv4 prefix or VPN IPv6 prefix is
sent with a label stack of depth greater than 1, with the innermost
label having a TTL Message Types,
Reply Modes, and Return Codes. Each of 1. This is to terminate the ping at the egress
PE, before it gets sent to the customer device. However, under
certain circumstances, the label stack these can shrink to a single label
before the ping hits take values in the egress PE; this will result
range 0-255. Assignments in the ping
terminating prematurely. One such scenario is a multi-AS Carrier's
Carrier VPN.

To get around this problem, one approach is for range 0-191 are via Standards
Action; assignments in the LSR that receives
such a ping to realize that range 192-251 are made via "Specification
Required"; values in the ping terminated prematurely, range 252-255 are for Vendor Private Use,
and send
back error code 13. In that case, the initiating LSR can retry the
ping after incrementing MUST NOT be allocated.

If any of these fields fall in the TTL on Vendor Private range, a top-level
Vendor Enterprise Number TLV MUST be present in the VPN label. In message.

Message Types defined in this fashion, document are the ingress LSR will sequentially try TTL values until it finds one
that allows the VPN ping to reach the egress PE.

4.8. Non-compliant Routers

If the egress for the FEC Stack being pinged does not support following:

Value Meaning
----- -------
1 MPLS
ping, then no echo request
2 MPLS echo reply will be sent, resulting in possible "false
negatives". If

Reply Modes defined in "traceroute" mode, a transit LSR does this document are the following:

Value Meaning
----- -------
1 Do not support
LSP ping, then no reply will be forthcoming from that LSR for some
TTL, say, n. The LSR originating the echo request SHOULD try sending
the echo request
2 Reply via an IPv4/IPv6 UDP packet
3 Reply via an IPv4/IPv6 UDP packet with TTL=n+1, n+2, ..., n+k to probe LSRs further
down the path. In such Router Alert
4 Reply via application level control channel

Return Codes defined in this document are listed in section 3.1.

6.2. TLVs

The IANA has created and will maintain a case, the echo request registry for TTL > n SHOULD
be sent with Downstream Mapping TLV "Downstream IP Address" the Type field set
to
of top-level TLVs as well as for any associated sub-TLVs. Note the ALLROUTERs multicast address until
meaning of a reply sub-TLV is received with a
Downstream Mapping TLV. The label stack MAY be omitted from scoped by the
Downstream Mapping TLV. Furthermore, the "Validate FEC Stack" flag
SHOULD NOT be set until an echo reply packet with a Downstream
Mapping TLV is received.

5. Security Considerations

Overall, the security needs The number spaces for LSP ping are similar to those the
sub-TLVs of ICMP
ping.

There various TLVs are at least three approaches to attacking LSRs using the
mechanisms defined here. One is a Denial-of-Service attack, by
sending MPLS echo requests/replies to LSRs and thereby increasing
their workload. independent.

The second is obfuscating the state of the MPLS data
plane liveness by spoofing, hijacking, replaying, or otherwise
tampering with MPLS echo requests valid range for TLVs and replies. The third is an
unauthorized source using an LSP ping to obtain information about the
network.

To avoid potential Denial-of-Service attacks, it sub-TLVs is RECOMMENDED that
implementations regulate the LSP ping traffic going to the control
plane. A rate limiter SHOULD be applied to 0-65535. Assignments in the well-known UDP port
defined below.

Unsophisticated replay
range 0-16383 and spoofing attacks involving faking or
replaying MPLS echo reply messages 32768-49161 are unlikely to be effective.
These replies would have to match made via Standards Action as
defined in [RFC5226]; assignments in the Sender's Handle range 16384-31743 and Sequence
Number of an outstanding MPLS echo request message. A non-matching
replay would be discarded
49162-64511 are made via "Specification Required" as defined above;
values in the sequence has moved on, thus range 31744-32767 and 64512-65535 are for Vendor
Private Use, and MUST NOT be allocated.

If a spoof TLV or sub-TLV has only a small window Type that falls in the range for Vendor
Private Use, the Length MUST be at least 4, and the first four octets
MUST be that vendor's SMI Private Enterprise Number, in network octet
order. The rest of opportunity. However, the Value field is private to provide a
stronger defense, an implementation MAY also validate the TimeStamp
Sent by requiring an exact match on vendor.

TLVs and sub-TLVs defined in this field.

To protect against unauthorized sources using MPLS echo request
messages to obtain network information, it document are the following:

Type Sub-Type Value Field
---- -------- -----------
1 Target FEC Stack
1 LDP IPv4 prefix
2 LDP IPv6 prefix
3 RSVP IPv4 LSP
4 RSVP IPv6 LSP
5 Not Assigned
6 VPN IPv4 prefix
7 VPN IPv6 prefix
8 L2 VPN endpoint
9 "FEC 128" Pseudowire - IPv4 (Deprecated)
10 "FEC 128" Pseudowire - IPv4
11 "FEC 129" Pseudowire - IPv4
12 BGP labeled IPv4 prefix
13 BGP labeled IPv6 prefix
14 Generic IPv4 prefix
15 Generic IPv6 prefix
16 Nil FEC
24 "FEC 128" Pseudowire - IPv6
25 "FEC 129" Pseudowire - IPv6
2 Downstream Mapping
3 Pad
4 Not Assigned
5 Vendor Enterprise Number
6 Not Assigned
7 Interface and Label Stack
8 Not Assigned
9 Errored TLVs
Any value The TLV not understood
10 Reply TOS Byte

7. Acknowledgements

The original acknowledgements from RFC 4379 state the following:

This document is RECOMMENDED that
implementations provide a means the outcome of many discussions among many
people, including Manoj Leelanivas, Paul Traina, Yakov Rekhter,
Der-Hwa Gan, Brook Bailey, Eric Rosen, Ina Minei, Shivani
Aggarwal, and Vanson Lim.

The description of checking the source addresses Multipath Information sub-field of
MPLS echo request messages against an access list before accepting the message.

It is not clear how
Downstream Mapping TLV was adapted from text suggested by Curtis
Villamizar.

We would like to prevent hijacking (non-delivery) thank Loa Andersson for motivating the advancement
of echo
requests or replies; however, if these messages are indeed hijacked,
LSP ping will report that the data plane is not working as it should.

It does not seem vital (at this point) bis specification.

We also would like to secure the data carried in
MPLS echo requests thank Alexander Vainshtein, Yimin Shen, Curtis
Villamizar, David Allan for their review and replies, although knowledge of the state of
the MPLS data plane may be considered confidential by some.
Implementations SHOULD, however, provide a means of filtering the
addresses to which echo reply messages may be sent.

Although this document makes special comments.

8. References

8.1. Normative References

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>.

[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<http://www.rfc-editor.org/info/rfc1812>.

[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113,
DOI 10.17487/RFC2113, February 1997,
<http://www.rfc-editor.org/info/rfc2113>.

[RFC2119] Bradner, S., "Key words for use of 127/8 address, these are
used only in conjunction with the UDP port 3503. Furthermore, these
packets are only processed by routers. All other hosts MUST treat
all packets with a destination address in the range 127/8 in
accordance RFCs to Indicate
Requirement Levels", BCP 14, RFC 1122. Any packet received by a router with a
destination address in the range 127/8 without a destination UDP port
of 3503 MUST be treated in accordance to 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 1812. In particular,
the default behavior is to treat packets destined to a 127/8 address
as "martians".

6. IANA Considerations

The TCP 3032, DOI 10.17487/RFC3032, January 2001,
<http://www.rfc-editor.org/info/rfc3032>.

[RFC4026] Andersson, L. and UDP port number 3503 has been allocated by IANA for LSP
echo requests T. Madsen, "Provider Provisioned Virtual
Private Network (VPN) Terminology", RFC 4026,
DOI 10.17487/RFC4026, March 2005,
<http://www.rfc-editor.org/info/rfc4026>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and replies.

The following sections detail the new name spaces to be managed by
IANA. For each of these name spaces, the space is divided into
assignment ranges; the following terms are used in describing the
procedures by which IANA allocates values: "Standards Action" (as
defined in [RFC5226]), "Specification Required", S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.

[RFC4379] Kompella, K. and "Vendor Private
Use".

Values from "Specification Required" ranges MUST be registered with
IANA. The request MUST be made via an Experimental G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC that
describes the format 4379,
DOI 10.17487/RFC4379, February 2006,
<http://www.rfc-editor.org/info/rfc4379>.

[RFC5226] Narten, T. and procedures for using the code point; the
actual assignment is made during the IANA actions H. Alvestrand, "Guidelines for the RFC.

Values from "Vendor Private" ranges MUST NOT be registered with IANA;
however, the message MUST contain Writing an enterprise code as registered
with the
IANA SMI Private Network Management Private Enterprise
Numbers. For each name space that has a Vendor Private range, it
must be specified where exactly the SMI Private Enterprise Number
resides; see below for examples. In this way, several enterprises
(vendors) can use the same code point without fear of collision.

6.1. Message Types, Reply Modes, Return Codes

The IANA has created Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.

[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and will maintain registries W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.

[RFC6424] Bahadur, N., Kompella, K., and G. Swallow, "Mechanism for
Performing Label Switched Path Ping (LSP Ping) over MPLS
Tunnels", RFC 6424, DOI 10.17487/RFC6424, November 2011,
<http://www.rfc-editor.org/info/rfc6424>.

[RFC7506] Raza, K., Akiya, N., and C. Pignataro, "IPv6 Router Alert
Option for MPLS Operations, Administration, and
Maintenance (OAM)", RFC 7506, DOI 10.17487/RFC7506, April
2015, <http://www.rfc-editor.org/info/rfc7506>.

8.2. Informative References

[RFC0792] Postel, J., "Internet Control Message Types,
Reply Modes, Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<http://www.rfc-editor.org/info/rfc792>.

[RFC3107] Rekhter, Y. and Return Codes. Each of these can take values in the
range 0-255. Assignments in the range 0-191 are via Standards
Action; assignments in the range 192-251 are made via "Specification
Required"; values E. Rosen, "Carrying Label Information in the range 252-255 are
BGP-4", RFC 3107, DOI 10.17487/RFC3107, May 2001,
<http://www.rfc-editor.org/info/rfc3107>.

[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for Vendor LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<http://www.rfc-editor.org/info/rfc3209>.

[RFC4365] Rosen, E., "Applicability Statement for BGP/MPLS IP
Virtual Private Use, Networks (VPNs)", RFC 4365,
DOI 10.17487/RFC4365, February 2006,
<http://www.rfc-editor.org/info/rfc4365>.

[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and MUST NOT be allocated.

If any of these fields fall in
G. Heron, "Pseudowire Setup and Maintenance Using the Vendor Private range, a top-level
Vendor Enterprise Number TLV MUST be present in the message.

Message Types
Label Distribution Protocol (LDP)", RFC 4447,
DOI 10.17487/RFC4447, April 2006,
<http://www.rfc-editor.org/info/rfc4447>.

[RFC4761] Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
<http://www.rfc-editor.org/info/rfc4761>.

[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <http://www.rfc-editor.org/info/rfc5036>.

[RFC5085] Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
Circuit Connectivity Verification (VCCV): A Control
Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
December 2007, <http://www.rfc-editor.org/info/rfc5085>.

Appendix A. Deprecated TLVs

A.1. FEC 128 Pseudowire

FEC 128 (0x80) is defined in this document [RFC4447], as are the following:

Value Meaning
----- -------
1 MPLS echo request
2 MPLS echo reply

Reply Modes defined terms PW ID
(Pseudowire ID) and PW Type (Pseudowire Type). A PW ID is a non-zero
32-bit connection ID. The PW Type is a 15-bit number indicating the
encapsulation type. It is carried right justified in this document are the following:

Value Meaning
----- -------
1 Do not reply
2 Reply via an IPv4/IPv6 UDP packet
3 Reply via an IPv4/IPv6 UDP packet field below
termed encapsulation type with Router Alert
4 Reply via application level control channel

Return Codes defined the high-order bit set to zero. Both
of these fields are treated in this document are listed protocol as opaque values.

When an FEC 128 is encoded in section 3.1.

6.2. TLVs

The IANA has created and will maintain a registry for label stack, the Type following format is
used. The value field consists of top-level TLVs as well as for any associated sub-TLVs. Note the
meaning remote PE IPv4 address (the
destination address of a sub-TLV is scoped by the TLV. The number spaces for the
sub-TLVs of various TLVs are independent.

The valid range for TLVs and sub-TLVs is 0-65535. Assignments in targeted LDP session), the
range 0-16383 PW ID, and 32768-49161 are made via Standards Action as
defined in [RFC5226]; assignments in the range 16384-31743 and
49162-64511 are made via "Specification Required"
encapsulation type as defined above;
values in the range 31744-32767 and 64512-65535 are for Vendor
Private Use, and MUST NOT be allocated.

If a TLV or sub-TLV has a Type that falls in the range for Vendor
Private Use, the Length MUST be at least 4, and the first four octets
MUST be that vendor's SMI Private Enterprise Number, in network octet
order. The rest of the Value field is private to the vendor.

TLVs and sub-TLVs defined in this document are the following:

Type Sub-Type Value Field
---- -------- ----------- follows:

0 1 Target FEC Stack 2 3
0 1 LDP IPv4 prefix 2 LDP IPv6 prefix 3 RSVP IPv4 LSP 4 RSVP IPv6 LSP 5 Not Assigned 6 VPN IPv4 prefix 7 VPN IPv6 prefix 8 L2 VPN endpoint 9 "FEC 128" Pseudowire - IPv4 (Deprecated)
10 "FEC 128" Pseudowire - IPv4
11 "FEC 129" Pseudowire - IPv4
12 BGP labeled IPv4 prefix
13 BGP labeled IPv6 prefix
14 Generic IPv4 prefix
15 Generic IPv6 prefix
16 Nil FEC
24 "FEC 128" Pseudowire - IPv6
25 "FEC 129" Pseudowire - IPv6 0 1 2 Downstream Mapping 3 Pad 4 Not Assigned 5 Vendor Enterprise Number 6 Not Assigned 7 Interface and Label Stack 8 Not Assigned 9 Errored TLVs
Any value The TLV not understood
10 Reply TOS Byte

7. Acknowledgements

The original acknowledgements from RFC 4379 state the following: 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote PE IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This document FEC is the outcome deprecated and is retained only for backward
compatibility. Implementations of many discussions among many
people, including Manoj Leelanivas, Paul Traina, Yakov Rekhter,
Der-Hwa Gan, Brook Bailey, Eric Rosen, Ina Minei, Shivani
Aggarwal, LSP ping SHOULD accept and Vanson Lim. process
this TLV, but SHOULD send LSP ping echo requests with the new TLV
(see next section), unless explicitly configured to use the old TLV.

An LSR receiving this TLV SHOULD use the source IP address of the LSP
echo request to infer the sender's PE address.

A.2. Downstream Mapping(DSMAP)

The description Downstream Mapping object is a TLV that MAY be included in an
echo request message. Only one Downstream Mapping object may appear
in an echo request. The presence of a Downstream Mapping object is a
request that Downstream Mapping objects be included in the Multipath Information sub-field echo
reply. If the replying router is the destination of the FEC, then a
Downstream Mapping TLV was adapted from text suggested by Curtis
Villamizar.

We would like to thank Loa Andersson for motivating SHOULD NOT be included in the advancement
of this bis specification. We also would like to thank Alexander
Vainshtein echo reply.
Otherwise the replying router SHOULD include a Downstream Mapping
object for his review and comments.

8. References

8.1. Normative References

[RFC1122] Braden, R., Ed., "Requirements each interface over which this FEC could be forwarded.
For a more precise definition of the notion of "downstream", see
section 3.3.2, "Downstream Router and Interface".

The Length is K + M + 4*N octets, where M is the Multipath Length,
and N is the number of Downstream Labels. Values for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>.

[RFC1812] Baker, F., Ed., "Requirements K are found in
the description of Address Type below. The Value field of a
Downstream Mapping 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | Address Type | DS Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream IP Address (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Interface Address (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multipath Type| Depth Limit | Multipath Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. (Multipath Information) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Maximum Transmission Unit (MTU)

The MTU is the size in octets of the largest MPLS frame (including
label stack) that fits on the interface to the Downstream LSR.

Address Type
The Address Type indicates if the interface is numbered or
unnumbered. It also determines the length of the Downstream IP
Address and Downstream 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

DS Flags

The DS Flags field is a bit vector with the following format:

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Rsvd(MBZ) |I|N|
+-+-+-+-+-+-+-+-+

Two flags are defined currently, I and N. The remaining flags MUST
be set to zero when sending and ignored on receipt.

Flag Name and Meaning
---- ----------------
I Interface and Label Stack Object Request

When this flag is set, it indicates that the replying
router SHOULD include an Interface and Label Stack
Object in the echo reply message.

N Treat as a Non-IP Packet

Echo request messages will be used to diagnose non-IP
flows. However, these messages are carried in IP
packets. For a router that alters its ECMP algorithm
based on the FEC or deep packet examination, this flag
requests that the router treat this as it would if the
determination of an IP payload had failed.

Downstream IP Address and Downstream Interface Address

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

If the interface to the downstream LSR is numbered, then the
Address Type MUST be set to IPv4 or IPv6, the Downstream IP
Address MUST be set to either the downstream LSR's Router ID or
the interface address of the downstream LSR, and the Downstream
Interface Address MUST be set to the downstream LSR's interface
address.

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

If an LSR does not know the IP address of its neighbor, then it
MUST set the Address Type to either IPv4 Unnumbered or IPv6
Unnumbered. For IPv4, it must set the Downstream IP Address to
127.0.0.1; for IPv6 the address is set to 0::1. In both cases,
the interface index MUST be set to 0. If an LSR receives an Echo
Request packet with either of these addresses in the Downstream IP
Address field, this indicates that it MUST bypass interface
verification but continue with label validation.

If the originator of an Echo Request packet wishes to obtain
Downstream Mapping information but does not know the expected
label stack, then it SHOULD set the Address Type to either IPv4
Unnumbered or IPv6 Unnumbered. For IPv4, it MUST set the
Downstream IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<http://www.rfc-editor.org/info/rfc1812>.

[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113,
DOI 10.17487/RFC2113, February 1997,
<http://www.rfc-editor.org/info/rfc2113>.

[RFC2119] Bradner, S., "Key words Address to 224.0.0.2; for use in RFCs IPv6 the address MUST be
set to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<http://www.rfc-editor.org/info/rfc3032>.

[RFC4026] Andersson, L. and T. Madsen, "Provider Provisioned Virtual
Private Network (VPN) Terminology", RFC 4026,
DOI 10.17487/RFC4026, March 2005,
<http://www.rfc-editor.org/info/rfc4026>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., FF02::2. In both cases, the interface index MUST be set to
0. If an LSR receives an Echo Request packet with the all-routers
multicast address, then this indicates that it MUST bypass both
interface and S. Hares, Ed., "A
Border Gateway Protocol label stack validation, but return Downstream
Mapping TLVs using the information provided.

Multipath Type

The following Multipath Types are defined:

Key Type Multipath Information
--- ---------------- ---------------------
0 no multipath Empty (Multipath Length = 0)
2 IP address IP addresses
4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.

[RFC4379] Kompella, K. IP address range low/high address pairs
8 Bit-masked IP IP address prefix and G. Swallow, "Detecting Multi-Protocol bit mask
address set
9 Bit-masked label set Label Switched (MPLS) Data Plane Failures", RFC 4379,
DOI 10.17487/RFC4379, February 2006,
<http://www.rfc-editor.org/info/rfc4379>.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.

[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., prefix and W. Kasch,
"Network Time Protocol Version 4: Protocol bit mask

Type 0 indicates that all packets will be forwarded out this one
interface.

Types 2, 4, 8, and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.

[RFC7506] Raza, K., Akiya, N., 9 specify that the supplied Multipath
Information will serve to exercise this path.

Depth Limit

The Depth Limit is applicable only to a label stack and C. Pignataro, "IPv6 Router Alert
Option is the
maximum number of labels considered in the hash; this SHOULD be
set to zero if unspecified or unlimited.

Multipath Length

The length in octets of the Multipath Information.

Multipath Information

Address or label values encoded according to the Multipath Type.
See the next section below for MPLS Operations, Administration, and
Maintenance (OAM)", RFC 7506, DOI 10.17487/RFC7506, April
2015, <http://www.rfc-editor.org/info/rfc7506>.

8.2. Informative References

[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<http://www.rfc-editor.org/info/rfc792>.

[RFC3107] Rekhter, Y. and E. Rosen, "Carrying encoding details.

Downstream Label(s)

The set of labels in the label stack as it would have appeared if
this router were forwarding the packet through this interface.
Any Implicit Null labels are explicitly included. Labels are
treated as numbers, i.e., they are right justified in the field.

A Downstream Label Information is 24 bits, in
BGP-4", RFC 3107, DOI 10.17487/RFC3107, May 2001,
<http://www.rfc-editor.org/info/rfc3107>.

[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<http://www.rfc-editor.org/info/rfc3209>.

[RFC4365] Rosen, E., "Applicability Statement for BGP/MPLS IP
Virtual Private Networks (VPNs)", RFC 4365,
DOI 10.17487/RFC4365, February 2006,
<http://www.rfc-editor.org/info/rfc4365>.

[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., the same format as an MPLS label
minus the TTL field, i.e., the MSBit of the label is bit 0, the
LSBit is bit 19, the Traffic Class (TC) bits are bits 20-22, and
G. Heron, "Pseudowire Setup
bit 23 is the S bit. The replying router SHOULD fill in the TC
and Maintenance Using S bits; the
Label Distribution LSR receiving the echo reply MAY choose to ignore
these bits. Protocol (LDP)", RFC 4447,
DOI 10.17487/RFC4447, April 2006,
<http://www.rfc-editor.org/info/rfc4447>.

[RFC4761] Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using

The Protocol is taken from the following table:

Protocol # Signaling Protocol
---------- ------------------
0 Unknown
1 Static
2 BGP for Auto-Discovery and
Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
<http://www.rfc-editor.org/info/rfc4761>.

[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <http://www.rfc-editor.org/info/rfc5036>.

Authors' Addresses

Kireeti Kompella
Juniper Networks, Inc.

Email: kireeti.kompella@gmail.com
Carlos Pignataro
Cisco Systems, Inc.

Email: cpignata@cisco.com

Nagendra Kumar
Cisco Systems, Inc.

Email: naikumar@cisco.com

Sam Aldrin
Google

Email: aldrin.ietf@gmail.com

Mach(Guoyi) Chen
Huawei

Email: mach.chen@huawei.com