wdiff rfc8029v1.txt rfc8029.txt

Internet Engineering Task Force (IETF) K. Kompella
Request for Comments: 8029 Juniper Networks, Inc.
Obsoletes: 4379, 6424, 6829, 7537 G. Swallow
Category: Standards Track
Updates: 1122 C. Pignataro, Ed.
ISSN: 2070-1721
Category: Standards Track N. Kumar
ISSN: 2070-1721 Cisco
S. Aldrin
Google
M. Chen
Huawei
January
March 2017

Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures

Abstract

This document describes a simple and efficient mechanism that can be
used to detect
data-plane failures in Multiprotocol Label Switching (MPLS) Label
Switched Paths (LSPs). There are two parts to this
document: information carried in It defines a probe message called an MPLS "echo "MPLS
echo request" and "echo a response message called an "MPLS echo reply" for
returning the purposes result of fault detection and isolation, and
mechanisms for reliably sending the probe. The MPLS echo reply. request is intended
to contain sufficient information to check correct operation of the
data plane and to verify the data plane against the control plane,
thereby localizing faults.

This document obsoletes RFCs 4379, 6424, 6829, and 7537. 7537, and updates
RFC 1122.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8029.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Structure of This Document . . . . . . . . . . . . . . . 5
1.3. Contributors . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Scope of This Specification . . . . . . . . . . . . . . . 5
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Use of Address Range 127/8 . . . . . . . . . . . . . . . 8
2.2. Router Alert Option . . . . . . . . . . . . . . . . . . . 9
3. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Return Codes . . . . . . . . . . . . . . . . . . . . . . 14
3.2. Target FEC Stack . . . . . . . . . . . . . . . . . . . . 16
3.2.1. LDP IPv4 Prefix . . . . . . . . . . . . . . . . . . . 17
3.2.2. LDP IPv6 Prefix . . . . . . . . . . . . . . . . . . . 18
3.2.3. RSVP IPv4 LSP . . . . . . . . . . . . . . . . . . . . 18
3.2.4. RSVP IPv6 LSP . . . . . . . . . . . . . . . . . . . . 18
3.2.5. VPN IPv4 Prefix . . . . . . . . . . . . . . . . . . . 19
3.2.6. VPN IPv6 Prefix . . . . . . . . . . . . . . . . . . . 20
3.2.7. L2 VPN Endpoint . . . . . . . . . . . . . . . . . . . 20
3.2.8. FEC 128 Pseudowire - IPv4 (Deprecated) . . . . . . . 21
3.2.9. FEC 128 Pseudowire - IPv4 (Current) . . . . . . . . . 21
3.2.10. FEC 129 Pseudowire - IPv4 . . . . . . . . . . . . . . 22
3.2.11. FEC 128 Pseudowire - IPv6 . . . . . . . . . . . . . . 23
3.2.12. FEC 129 Pseudowire - IPv6 . . . . . . . . . . . . . . 24
3.2.13. BGP Labeled IPv4 Prefix . . . . . . . . . . . . . . . 25
3.2.14. BGP Labeled IPv6 Prefix . . . . . . . . . . . . . . . 25
3.2.15. Generic IPv4 Prefix . . . . . . . . . . . . . . . . . 25
3.2.16. Generic IPv6 Prefix . . . . . . . . . . . . . . . . . 26
3.2.17. Nil FEC . . . . . . . . . . . . . . . . . . . . . . . 26
3.3. Downstream Mapping (Deprecated) . . . . . . . . . . . . . 27
3.4. Downstream Detailed Mapping TLV . . . . . . . . . . . . . 27
3.4.1. Sub-TLVs . . . . . . . . . . . . . . . . . . . . . . 30
3.4.2. Downstream Router and Interface . . . . . . . . . . . 37
3.5. Pad TLV . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.6. Vendor Enterprise Number . . . . . . . . . . . . . . . . 38
3.7. Interface and Label Stack . . . . . . . . . . . . . . . . 38
3.8. Errored TLVs . . . . . . . . . . . . . . . . . . . . . . 40
3.9. Reply TOS Octet TLV . . . . . . . . . . . . . . . . . . . 40
4. Theory of Operation . . . . . . . . . . . . . . . . . . . . . 41
4.1. Dealing with Equal-Cost Multipath (ECMP) . . . . . . . . 41
4.2. Testing LSPs That Are Used to Carry MPLS Payloads . . . . 42
4.3. Sending an MPLS Echo Request . . . . . . . . . . . . . . 42
4.4. Receiving an MPLS Echo Request . . . . . . . . . . . . . 43
4.4.1. FEC Validation . . . . . . . . . . . . . . . . . . . 49
4.5. Sending an MPLS Echo Reply . . . . . . . . . . . . . . . 50
4.5.1. Addition of a New Tunnel . . . . . . . . . . . . . . 51
4.5.2. Transition between Tunnels . . . . . . . . . . . . . 52
4.6. Receiving an MPLS Echo Reply . . . . . . . . . . . . . . 53
4.7. Issue with VPN IPv4 and IPv6 Prefixes . . . . . . . . . . 55
4.8. Non-compliant Routers . . . . . . . . . . . . . . . . . . 56
5. Security Considerations . . . . . . . . . . . . . . . . . . . 56
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 58
6.1. TCP and UDP Port Number . . . . . . . . . . . . . . . . . 58
6.2. MPLS LSP Ping Parameters . . . . . . . . . . . . . . . . 58
6.2.1. Message Types, Reply Modes, Return Codes . . . . . . 58
6.2.2. TLVs . . . . . . . . . . . . . . . . . . . . . . . . 59
6.2.3. Global Flags . . . . . . . . . . . . . . . . . . . . 60
6.2.4. Downstream Detailed Mapping Address Type . . . . . . 61
6.2.5. DS Flags . . . . . . . . . . . . . . . . . . . . . . 61
6.2.6. Multipath Types . . . . . . . . . . . . . . . 62
6.2.7. Pad Type . . . . . . . . . . . . . . . . . . . . . . 62
6.2.8. Interface and Label Stack Address Type . . . . . . . 63
6.3. IPv4 Special-Purpose Address Registry . . . . . . . . . . 63
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 64 63
7.1. Normative References . . . . . . . . . . . . . . . . . . 64
7.2. Informative References . . . . . . . . . . . . . . . . . 65
Appendix A. Deprecated TLVs and Sub-TLVs (Non-normative) . . . . 67 68
A.1. Target FEC Stack . . . . . . . . . . . . . . . . . . . . 68
A.1.1. FEC 128 Pseudowire - IPv4 (Deprecated) . . . . . . . 68
A.2. Downstream Mapping (Deprecated) . . . . . . . . . . . . . 68
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 72
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 73

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 It defines
a probe message called an MPLS "echo "MPLS echo request" and "echo reply", and mechanisms a response message
called an "MPLS echo reply" for
transporting returning the echo reply. result of the probe.
The first part aims at providing enough MPLS echo request is intended to contain sufficient information
to check correct operation of the data plane, as well as a mechanism
to verify the data plane against the control plane, thereby
localizing faults. The second part suggests two methods of
providing 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] [RFC1122], and this
specification updates that RFC. The motivation for this change and
the details of this exceptional use are discussed in Section 2.1
below.

This document obsoletes RFC 4379 [RFC4379], RFC 6424 [RFC6424], RFC
6829 [RFC6829], and RFC 7537 [RFC7537].

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" (Section 4) 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 MPLS 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 This Specification

The primary goal of this document is to provide a clean and updated
LSP Ping ping specification.

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

o Update all references and citations.

* Obsoleted RFCs 2434, 2030, and 3036 are respectively replaced
with RFCs 5226, 5905, and 5036.

* Additionally, three some informative references were published as
RFCs: RFCs 4447, 4761, 5085, and 5085. 5885.

o Incorporate all outstanding RFC errata.

* See [Err108], [Err742], [Err1418], [Err1714], [Err1786],
[Err2978], [Err3399].

o Replace EXP with Traffic Class (TC), based on the update from RFC
5462.

o Incorporate the updates from RFC 6829, by adding the pseudowire
(PW) Forwarding Equivalence Classes (FECs) advertised over IPv6
and obsoleting RFC 6829.

o Incorporate the updates from RFC 7506, by adding the IPv6 Router
Alert Option (RAO) for MPLS Operations, Administration, and
Maintenance (OAM).

o Incorporate newly defined bits on the Global Flags field from RFCs
6425 and 6426.

o Update the IPv4 addresses used in examples to utilize the
documentation prefix. Add examples with IPv6 addresses.

o Incorporate the updates from RFC 6424, by deprecating the
Downstream Mapping TLV (DSMAP) and adding the Downstream Detailed
Mapping TLV (DDMAP); updating two new Return Codes; adding the
motivations of tunneled or stitched LSPs; updating the procedures,
IANA considerations, and security considerations; and obsoleting
RFC 6424.

o Incorporate the updates from RFC 7537, by updating the IANA
Considerations section and obsoleting RFC 7537.

o Finally, obsolete RFC 4379.

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
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 to
confirm 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.

An LSP traceroute may cross a tunneled or stitched LSP en route to
the destination. While performing end-to-end LSP validation in such
scenarios, the FEC information included in the packet by the
Initiator may be different from the one assigned by the transit node
in a different segment of a stitched LSP or tunnel. Let us consider
a simple case.

A B C D E
o -------- o -------- o --------- o --------- o
\_____/ | \______/ \______/ | \______/
LDP | RSVP RSVP | LDP
| |
\____________________/
LDP

When an LSP traceroute is initiated from Router A to Router E, the
FEC information included in the packet will be LDP while Router C
along the path is a pure RSVP node and does not run LDP.
Consequently, node C will be unable to perform FEC validation. The
MPLS echo request should contain sufficient information to allow any
transit node within a stitched or tunneled LSP to perform FEC
validations to detect any misrouted echo requests.

One way these tools can be used is to periodically ping a 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 Multipath (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 IPv4 link-local
addresses. Use of the private address space was deemed ineffective
since the leading MPLS-based service is an IPv4 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. Older
deployed routers may not (correctly) implement IPv4 link-local
addresses and 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 an
IPv4-mapped IPv6 address for IPv6 was chosen for a number of reasons.

RFC 1122 allocates the 127/8 as an the "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 range provides an easy
means of identifying possible LSP packets.

2.2. Router Alert Option

This document requires the use of the RAO set in an 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
the 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/reply 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 |R|T|V|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

At the time of writing, three flags are defined: the R, T, and V
bits; 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 T (Respond Only If TTL Expired) flag MUST be set only in the echo
request packet by the sender. If the T flag is set to 1 in an
incoming echo request, and the TTL of the incoming MPLS label is more
than 1, then the receiving node MUST drop the incoming echo request
and MUST NOT send any echo reply to the sender. This flag MUST NOT
be set in the echo reply packet. If this flag is set in an echo
reply packet, then it MUST be ignored. The T flag is defined in
Section 3.4 of [RFC6425].

The R (Validate Reverse Path) flag is defined in [RFC6426]. When
this flag is set in the echo request, the Responder SHOULD return
reverse-path FEC information, as described in Section 3.4.2 of
[RFC6426].

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]. [RFC5085][RFC5885]. 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 code point is application specific and thus beyond the scope of
this document.

Return Codes and Subcodes are described in Section 3.1.

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 64-bit NTP timestamp 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 64-bit NTP timestamp
format in which 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 of how TLV and sub-TLV lengths are computed, and how
sub-TLVs are padded to be 4-octet aligned, are 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 (Deprecated)
3 Pad
4 Unassigned
5 Vendor Enterprise Number
6 Unassigned
7 Interface and Label Stack
8 Unassigned
9 Errored TLVs
10 Reply TOS Byte
20 Downstream Detailed Mapping

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.

In Sections 3.2 through 3.9 and their various subsections, only the
Value field of the TLV is included.

3.1. Return Codes

The Return Code is set to zero by the sender of an Echo Request. echo request. The
receiver of said Echo Request echo request can set it to one of the values listed
below in the corresponding Echo Reply echo reply that it generates. 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
14 See DDMAP TLV for meaning of Return Code and Return
Subcode (See Note 2)
15 Label switched with FEC change

Note 1

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

Note 2

The Return Code is per "Downstream Detailed Mapping TLV"
(Section 3.4). This Return Code MUST be used only in the message
header and MUST be set only in the MPLS echo reply message. If
the Return Code is set in the MPLS echo request message, then it
MUST be ignored. When this Return Code is set, each Downstream
Detailed Mapping TLV MUST have an appropriate Return Code and
Return Subcode. This Return Code MUST be used when there are
multiple downstreams for a given node (such as Point-to-Multipoint
(P2MP) or ECMP), and the node needs to return a Return Code/Return
Subcode for each downstream. This Return Code MAY be used even
when there is only one downstream for a given node.

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 Unassigned
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 have been defined and 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 192.0.2.1 (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 a FEC Stack TLV with one
FEC in it, namely, of type LDP IPv4 prefix, with prefix 192.0.2.1/32,
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 3.2.5) of 203.0.113.0/24 in VPN foo. Say further that LSR Y
with loopback address 192.0.2.1 announced prefix 203.0.113.0/24 with
Route Distinguisher (RD) RD-foo-Y (which may in general be different
from the RD that LSR X uses in its own advertisements for VPN foo),
label 23456, and BGP next hop 192.0.2.1 [RFC4271]. Finally, suppose
that LSR X receives a label binding of 1001 for 192.0.2.1 via LDP. X
has two choices in sending an MPLS echo request: X can send an MPLS
echo request with a FEC Stack TLV with a single FEC of type VPN IPv4
prefix with a prefix of 203.0.113.0/24 and a Route Distinguisher an RD of RD-foo-Y.
Alternatively, X can send a FEC Stack TLV with two FECs, the first of
type LDP IPv4 with a prefix of 192.0.2.1/32 and the second of type of
IP VPN with a prefix 203.0.113.0/24 with an RD of 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.)

If, for example, an LSR Y has an LDP mapping for the IPv6 address
2001:db8::1 (say, label 2001), then to verify that label 2001 does
reach an egress LSR that announced this prefix via LDP, LSR Y can
send an MPLS echo request with a FEC Stack TLV with one LDP IPv6
prefix FEC, with prefix 2001:db8::1/128, and with a label of 2001.

If an end-to-end path comprises of one or more tunneled or stitched
LSPs, each transit node that is the originating point of a new tunnel
or segment SHOULD reply back notifying the FEC stack change along
with the new FEC details, for example, if LSR X has an LDP mapping
for IPv4 prefix 192.0.2.10 on LSR Z (say, label 3001). Say further
that LSR A and LSR B are transit nodes along the path, which also
have an RSVP tunnel over which LDP is enabled. While replying back,
A SHOULD notify that the FEC changes from LDP to <RSVP, LDP>. If the
new tunnel is a transparent pipe, i.e., the data-plane trace will not
expire in the middle of the tunnel, then the transit node SHOULD NOT
reply back notifying the FEC stack change or the new FEC details. If
the transit node wishes to hide the nature of the tunnel from the
ingress of the echo request, then the transit node MAY notify the FEC
stack change and include Nil FEC as the new FEC.

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 Value fields are taken from RFC
3209 [RFC3209], Sections 4.6.1.1 and 4.6.2.1.

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 Endpoint 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 Value fields are taken from RFC
3209 [RFC3209], Sections 4.6.1.2 and 4.6.2.2.

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 Endpoint 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 Value field consists of the RD 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 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 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 Value field consists of the RD 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 RD 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 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 VPLS Edge Identifier (VE ID) are defined in [RFC4761]. This
document uses the simpler term L2 VPN endpoint when referring to a
VPLS BGP NLRI. The RD 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 Pseudowire (PW) Type in
Section 3.2.9.

When an L2 VPN endpoint is encoded in a label stack, the following
format is used. The value Value field consists of an RD (8 octets), the
sender's (of the ping) 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.1.1 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 fields to the local FEC information, the match
MUST be exact.

When a FEC 128 is encoded in a label stack, the following format is
used. The value Value field consists of the Sender's Provider Edge (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:

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 a 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 Value (continued) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AII Type | TAII Length | TAII Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ TAII Value (continued) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TAII (cont.) | 0-3 octets of zero padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.11. 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 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.

3.2.12. 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 a 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.

3.2.13. 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 Value field consists of 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.14. 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.

3.2.15. 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, the
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.

3.2.16. 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.

3.2.17. 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.

3.3. Downstream Mapping (Deprecated)

See Appendix A.2 for more details.

3.4. Downstream Detailed Mapping TLV

The Downstream Detailed Mapping object is a TLV that MAY be included
in an MPLS echo request message. Only one Downstream Detailed
Mapping object may appear in an echo request. The presence of a
Downstream Detailed Mapping object is a request that Downstream
Detailed Mapping objects be included in the MPLS echo reply. If the
replying router is the destination (Label Edge Router) of the FEC,
then a Downstream Detailed Mapping TLV SHOULD NOT be included in the
MPLS echo reply. Otherwise, the replying router SHOULD include a
Downstream Detailed Mapping object for each interface over which this
FEC could be forwarded. For a more precise definition of the notion
of "downstream", see Section 3.4.2, "Downstream Router and
Interface".

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | Address Type | DS Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Address (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Interface Address (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Return Code | Return Subcode| Sub-TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. List of Sub-TLVs .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Downstream Detailed Mapping TLV format is derived from the
deprecated Downstream Mapping TLV format (see Appendix A.2.) The key
change is that variable length and optional fields have been
converted into sub-TLVs.

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 Address Type is set
to one of the following values:

Type # Address Type
------ ------------
1 IPv4 Numbered
2 IPv4 Unnumbered
3 IPv6 Numbered
4 IPv6 Unnumbered

DS Flags

The DS Flags field is a bit vector of various flags 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 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 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
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 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
Address field, this indicates that it MUST bypass interface
verification but continue with label validation.

If the originator of an Echo Request echo request packet wishes to obtain
Downstream Detailed 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 Address to 224.0.0.2; for IPv6, the address MUST be
set to FF02::2. In both cases, the interface index MUST be set to
0. If an LSR receives an Echo Request echo request packet with the all-routers
multicast address, then this indicates that it MUST bypass both
interface and label stack validation but return Downstream Mapping
TLVs using the information provided.

Return Code

The Return Code is set to zero by the sender of an Echo Request. echo request.
The receiver of said Echo Request echo request can set it in the corresponding
Echo Reply
echo reply that it generates to one of the values specified in
Section 3.1 other than 14.

If the receiver sets a non-zero value of the Return Code field in
the Downstream Detailed Mapping TLV, then the receiver MUST also
set the Return Code field in the echo reply header to "See DDMAP
TLV for Return Code and Return Subcode" (Section 3.1). An
exception to this is if the receiver is a bud node [RFC4461] and
is replying as both an egress and a transit node with a Return
Code of 3 ("Replying router is an egress for the FEC at stack-
depth <RSC>") in the echo reply header.

If the Return Code of the echo reply message is not set to either
"See DDMAP TLV for Return Code and Return Subcode" (Section 3.1)
or "Replying router is an egress for the FEC at stack-depth
<RSC>", then the Return Code specified in the Downstream Detailed
Mapping TLV MUST be ignored.

Return Subcode

The Return Subcode is set to zero by the sender. The receiver can
set this field to an appropriate value as specified in
Section 3.1: The Return Subcode is filled in with the stack-depth
for those codes that specify the stack-depth. For all other
codes, the Return Subcode MUST be set to zero.

If the Return Code of the echo reply message is not set to either
"See DDMAP TLV for Return Code and Return Subcode" (Section 3.1)
or "Replying router is an egress for the FEC at stack-depth
<RSC>", then the Return Subcode specified in the Downstream
Detailed Mapping TLV MUST be ignored.

Sub-TLV Length

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

3.4.1. Sub-TLVs

This section defines the sub-TLVs that MAY be included as part of the
Downstream Detailed Mapping TLV.

Sub-Type Value Field
--------- ------------
1 Multipath data
2 Label stack
3 FEC stack change

3.4.1.1. Multipath Data Sub-TLV
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 | Multipath Length |Reserved (MBZ) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| (Multipath Information) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The multipath data sub-TLV includes Multipath Information.

Multipath Type

The type of the encoding for the Multipath Information.

The following Multipath Types are defined in this document:

Key Type Multipath Information
--- ---------------- ---------------------
0 no multipath Empty (Multipath Length = 0)
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 will be forwarded out this one
interface.

Types 2, 4, 8, and 9 specify that the supplied Multipath
Information will serve to exercise this path.

Multipath Length

The length in octets of the Multipath Information.

MBZ

MUST be set to zero when sending; MUST be ignored on receipt.

Multipath Information

Encoded multipath data (e.g., encoded address or label values),
according to the Multipath Type. See Section 3.4.1.1.1 for
encoding details.

3.4.1.1.1. Multipath Information Encoding

The Multipath Information encodes labels or addresses that will
exercise this path. The Multipath Information depends on the
Multipath Type. The contents of the field are shown in the table
above. IPv4 addresses are drawn from the range 127/8; IPv6 addresses
are drawn from the range 0:0:0:0:0:FFFF:7F00:0/104. Labels are
treated as numbers, i.e., they are right justified in the field. For
Type 4, ranges indicated by address pairs MUST NOT overlap and MUST
be in ascending sequence.

Type 8 allows a more dense encoding of IP addresses. The IP prefix
is formatted as a base IP address with the non-prefix low-order bits
set to zero. The maximum prefix length is 27. Following the prefix
is a mask of length 2^(32 - prefix length) bits for IPv4 and
2^(128 - prefix length) bits for IPv6. Each bit set to 1 represents
a valid address. The address is the base IPv4 address plus the
position of the 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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 is
formatted as a base label value with the non-prefix low-order bits
set to zero. The maximum prefix (including leading zeros due to
encoding) length is 27. Following the prefix is a mask of length
2^(32 - prefix length) bits. Each bit set to one represents a valid
label. The label is the base label plus the position of the bit in
the mask where the bits are numbered left to right beginning with
zero. Label values of all the odd numbers between 1152 and 1279
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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 IP
addresses MUST be picked from the set provided. If none of these
labels or addresses map to a particular downstream interface, then
for that interface, the type MUST be set to 0. If the received
Multipath Information is null (i.e., Multipath Length = 0, or for
Types 8 and 9, a mask of all zeros), the type MUST be set to 0.

For example, suppose LSR X at hop 10 has two downstream LSRs, Y and
Z, for the FEC in question. The received X could return Multipath
Type 4, 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 to LSR Y. Y, which has three
downstream LSRs, U, V, and W, computes that 127.1.1.1->127.1.1.127
would go to U and 127.1.1.128-> 127.1.1.255 would go to V. Y would
then respond with 3 Downstream Detailed Mapping TLVs: to U, with
Multipath Type 4 (127.1.1.1->127.1.1.127); to V, with Multipath Type
4 (127.1.1.127->127.1.1.255); and to W, with Multipath Type 0.

Note that computing Multipath Information may impose a significant
processing burden on the receiver. A receiver MAY thus choose to
process a subset of the received prefixes. The sender, on receiving
a reply to a Downstream Detailed Mapping with partial information,
SHOULD assume that the prefixes missing in the reply were skipped by
the receiver and MAY re-request information about them in a new echo
request.

The encoding of Multipath Information in scenarios where a few LSRs
apply Entropy-label-based load-balancing while other LSRs are non-EL
(IP-based) load balanced will be defined in a different document.

The encoding of Multipath Information in scenarios where LSRs have
Layer 2 ECMP over Link Aggregation Group (LAG) interfaces will be
defined in a different document.

3.4.1.2. Label Stack Sub-TLV

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Downstream Label | Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Label stack Stack sub-TLV contains 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. The number of label/protocol pairs present in the sub-TLV
is determined based on the sub-TLV data length. When the Downstream
Detailed Mapping TLV is sent in the echo reply, this sub-TLV MUST be
included.

Downstream Label

A Downstream downstream label is 24 bits, in 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 TC field [RFC5462] is bits 20-22, and S is
bit 23. The replying router SHOULD fill in the TC field and S
bit; the LSR receiving the echo reply MAY choose to ignore these.

Protocol

This specifies the label distribution protocol for the Downstream
label. Protocol values are taken from the following table:

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

3.4.1.3. FEC Stack Change Sub-TLV

A router MUST include the FEC stack change sub-TLV when the
downstream node in the echo reply has a different FEC Stack than the
FEC Stack received in the echo request. One or more FEC stack change
sub-TLVs MAY be present in the Downstream Detailed Mapping TLV. The
format is as below.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Operation Type | Address Type | FEC-tlv length| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Peer Address (0, 4, or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. FEC TLV .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Operation Type

The operation type specifies the action associated with the FEC
stack change. The following operation types are defined:

Type # Operation
------ ---------
1 Push
2 Pop

Address Type

The Address Type indicates the remote peer's address type. The
Address Type is set to one of the following values. The length of
the peer address is determined based on the address type. The
address type MAY be different from the address type included in
the Downstream Detailed Mapping TLV. This can happen when the LSP
goes over a tunnel of a different address family. The address
type MAY be set to Unspecified if the peer address is either
unavailable or the transit router does not wish to provide it for
security or administrative reasons.

Type # Address Type Address length
------ ------------ --------------
0 Unspecified 0
1 IPv4 4
2 IPv6 16

FEC TLV Length

Length in octets of the FEC TLV.

Reserved

This field is reserved for future use and MUST be set to zero.

Remote Peer Address

The remote peer address specifies the remote peer that is the next
hop for the FEC being currently traced. If the operation type is
PUSH, the remote peer address is the address of the peer from
which the FEC being pushed was learned. If the operation type is
POP,
pop, the remote peer address MAY be set to Unspecified.

For upstream-assigned labels [RFC5331], an operation type of POP pop
will have a remote peer address (the upstream node that assigned
the label), and this SHOULD be included in the FEC stack change
sub-TLV. The remote peer address MAY be set to Unspecified if the
address needs to be hidden.

FEC TLV

The FEC TLV is present only when the FEC-tlv length field is non-
zero. The FEC TLV specifies the FEC associated with the FEC stack
change operation. This TLV MAY be included when the operation
type is POP. pop. It MUST be included when the operation type is PUSH.
The FEC TLV contains exactly one FEC from the list of FECs
specified in Section 3.2. A Nil FEC MAY be associated with a PUSH
operation if the responding router wishes to hide the details of
the FEC being pushed.

FEC stack change sub-TLV operation rules are as follows:

a. A FEC stack change sub-TLV containing a PUSH operation MUST NOT
be followed by a FEC stack change sub-TLV containing a POP pop
operation.

b. One or more POP pop operations MAY be followed by one or more PUSH
operations.

c. One FEC stack change sub-TLV MUST be included per FEC stack
change. For example, if 2 labels are going to be pushed, then
one FEC stack change sub-TLV MUST be included for each FEC.

d. A FEC splice operation (an operation where one FEC ends and
another FEC starts, MUST be performed by including a POP pop type FEC
stack change sub-TLV followed by a PUSH type FEC stack change
sub-TLV.

e. A Downstream Detailed Mapping TLV containing only one FEC stack
change sub-TLV with Pop pop operation is equivalent to IS_EGRESS
(Return Code 3, Section 3.1) for the outermost FEC in the FEC
stack. The ingress router performing the MPLS LSP traceroute MUST
treat such a case as an IS_EGRESS for the outermost FEC.

3.4.2. Downstream Router and Interface

The 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 L and TTL=1 at LSR X, X
must be able to compute which LSRs could receive the packet if it was
originated with TTL=n+1, over which interface the request would
arrive and what label stack those LSRs would see. (It is outside the
scope of this document to specify how this computation is done.) The
set of these LSRs/interfaces consists 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 is the LSR originating the echo request is a special
case. X needs to figure out what LSRs would receive the MPLS echo
request for a given FEC Stack that X originates with TTL=1.

The set of downstream routers at X may be alternative paths (see the
discussion below on ECMP) or simultaneous paths (e.g., for MPLS
multicast). In the former case, the Multipath Information is used as
a hint to the sender as to how it may influence the choice of these
alternatives.

3.5. Pad TLV

The value part of the Pad TLV contains a variable number (>= 1) of
octets. The first octet takes values from the following table; all
the other octets (if any) are ignored. The receiver SHOULD verify
that the TLV is received in its entirety, but otherwise ignores the
contents of this TLV, apart from the first octet.

Value Meaning
----- -------
0 Reserved
1 Drop Pad TLV from reply
2 Copy Pad TLV to reply
3-250 Unassigned
251-254 Reserved for Experimental Use
255 Reserved

The Pad TLV can be added to an Echo Request echo request to create a message of a
specific length in cases where messages of various sizes are needed
for troubleshooting. The first octet allows for controlling the
inclusion of this additional padding in the respective Echo Reply. echo reply.

3.6. Vendor Enterprise Number

"Private Enterprise Numbers" [IANA-ENT] are maintained by IANA. The
Length of this TLV is always 4; the value is the Structure of
Management Information (SMI) Private Enterprise Code, in network
octet order, of the vendor with a Vendor Private extension to any of
the fields in the fixed part of the message, in which case this TLV
MUST be present. If none of the fields in the 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 used, the
Vendor Enterprise Number TLV MUST be included in the message.

3.7. Interface and Label Stack

The Interface and Label Stack TLV MAY be 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 also determines the length of the IP Address and
Interface fields. The resulting total for the initial part of the
TLV is listed in the table below as "K Octets". The Address Type
is set to one of the following values:

Type # Address Type K Octets
------ ------------ --------
0 Reserved 4
1 IPv4 Numbered 12
2 IPv4 Unnumbered 12
3 IPv6 Numbered 36
4 IPv6 Unnumbered 24
5-250 Unassigned
251-254 Reserved for Experimental Use
255 Reserved

IP Address and Interface

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

If the interface upon which the echo request message was received
is numbered, then the Address Type MUST be set to IPv4 or IPv6,
the IP Address MUST be set to either the LSR's Router ID or the
interface address, and the Interface MUST be set to the interface
address.

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

Label Stack

The label stack of the received echo request message. If any TTL
values have been changed by this router, they SHOULD be 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 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
. .
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.9. Reply TOS Octet 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 Type of Service (TOS)
octet set to the value specified in the TLV. This TLV has a length
of 4 with the following value 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reply-TOS Byte| Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4. Theory of Operation

An MPLS echo request is used to test a particular LSP. The LSP to be
tested is identified by the "FEC Stack"; for example, if the LSP was
set up via LDP, and a label is mapped to an egress IP address of
198.51.100.1, the FEC Stack contains a single element, namely, an LDP
IPv4 prefix sub-TLV with value 198.51.100.1/32. If the LSP being
tested is an RSVP LSP, the FEC Stack consists of a single element
that captures the RSVP Session and Sender Template that uniquely
identifies the LSP.

FEC Stacks can be more complex. For example, one may wish to test a
VPN IPv4 prefix of 203.0.113.0/24 that is tunneled over an LDP LSP
with egress 192.0.2.1. The FEC Stack would then contain two
sub-TLVs, the bottom being a VPN IPv4 prefix, and the top being an
LDP IPv4 prefix. If the underlying (LDP) tunnel were not known, or
was considered irrelevant, the FEC Stack could be a single element
with just the VPN IPv4 sub-TLV.

When an MPLS echo request is received, the receiver is expected to
verify that the control plane and data plane are both healthy (for
the FEC Stack being pinged), and that the two planes are in sync.
The procedures for this are in Section 4.4.

4.1. Dealing with Equal-Cost Multipath (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 from to get to the
desired endpoint. Finally, LSPs may have backup paths, detour paths,
and other alternative paths to take should the primary LSP go down.

To deal with

Regarding the last two first: points stated above: it is assumed that the
LSR sourcing MPLS echo requests can force the echo request into any
desired LSP, so choosing among multiple LSPs at the ingress is not an
issue. The problem of probing the various flavors of backup paths
that will typically not be used for forwarding data unless the
primary LSP is down will not be addressed here.

Since 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 the algorithms that a given LSR uses to distribute packets
over alternative paths may be proprietary.

To achieve some degree of coverage of alternate paths, there is a
certain latitude in choosing the destination IP address and source
UDP port for an MPLS echo request. This is clearly not sufficient;
in the case of traceroute, more latitude is offered by means of the
Multipath Information of the Downstream Detailed Mapping TLV. This
is used as follows. An ingress LSR periodically sends an MPLS LSP
traceroute message to determine whether there are multipaths for a
given LSP. If so, each hop will provide some information as to how
each of its downstream paths can be exercised. The ingress can then
send MPLS echo requests that exercise these paths. If several
transit LSRs have ECMP, the 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 to carry MPLS payloads (such as LSPs used
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 of
the LSP ping may forward the MPLS echo request successfully over an
interface not configured to carry MPLS payloads because of the use of
penultimate hop popping. Since the receiving router has no means to
ascertain whether the IP packet was sent unlabeled or implicitly
labeled, the addition of labels shimmed above the MPLS echo request
(using the Nil FEC) will prevent a router from forwarding such a
packet out to unlabeled interfaces.

4.3. Sending an MPLS Echo Request

An MPLS echo request is a UDP packet. The IP header is set as
follows: the source IP address is a routable address of the sender;
the destination IP address is a (randomly chosen) IPv4 address from
the range 127/8 or an IPv6 address from the range
0:0:0:0:0:FFFF:7F00:0/104. The IP TTL is set to 1. The source UDP
port is chosen by the sender; the destination UDP port is set to 3503
(assigned by IANA for MPLS echo requests). The Router Alert IP
option
Option of value 0x0 [RFC2113] for IPv4 or value 69 [RFC7506] for IPv6
MUST be set in the IP header.

An MPLS echo request is sent with a label stack corresponding to the
FEC Stack being tested. Note that further labels could be applied
if, for example, the normal route to the topmost FEC in the stack is
via a Traffic Engineered Tunnel [RFC3209]. If all of the FECs in the
stack correspond to Implicit Null labels, the MPLS 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 the innermost label to 1, to prevent the ping
request going farther than it should. Examples of where this SHOULD
be done include pinging a VPN IPv4 or IPv6 prefix, an L2 VPN
endpoint, or a pseudowire. Preventing the ping request from going
too far can also be accomplished by inserting a Router Alert label
above this label; however, this may lead to the 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]. [RFC5085][RFC5885].

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

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

The TimeStamp Sent is set to the time of day in NTP format that the
echo request is sent. The TimeStamp Received is set to zero.

An MPLS echo request MUST have a FEC Stack TLV. Also, the Reply Mode
must be set to the desired Reply Mode; the Return Code and Subcode
are set to zero. In the "traceroute" mode, the echo request SHOULD
include a Downstream Detailed Mapping TLV.

4.4. Receiving an MPLS Echo Request

Sending an MPLS echo request to the control plane is triggered by one
of the following packet processing exceptions: Router Alert option,
IP TTL expiration, MPLS TTL expiration, MPLS Router Alert label, or
the destination address in the 127/8 address range. The control
plane further identifies it by UDP destination port 3503.

For reporting purposes, the bottom of the stack is considered to be a
stack-depth of 1. This is to establish an absolute reference for the
case where the actual stack may have more labels than there are FECs
in the Target FEC Stack.

Furthermore, in all the error codes Return Codes listed in this document, a stack-
depth
stack-depth of 0 means "no value specified". This allows
compatibility with existing implementations that do 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. If the packet is not well-
formed, LSR X SHOULD send an MPLS Echo Reply echo reply with the Return Code
set to "Malformed echo request received" and the Subcode set to
zero. If there are any TLVs not marked as "Ignore" (i.e., if the
TLV type is less than 32768, see Section 3) that LSR X does not
understand, LSR X SHOULD send an MPLS "TLV not understood" (as
appropriate), and set the Subcode 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 field's 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 echo request was
received.

Stack-R: the label stack on the packet as it was received.

Stack-D: the label stack carried in the "Label Stack stack
sub-TLV" 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 the label being verified.
Initialized to the number of labels in the
received label stack S.

FEC-stack-depth: depth of the FEC in the Target FEC Stack that
should be used to verify the current actual
label. Requires no initialization.

Best-return-code: contains the Return Code 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 to Best-return-code, but for the Echo
Reply echo
reply Subcode.

FEC-status: result value returned by 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, 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 and the Target FEC Stack description in the body of the echo
request, and reports any errors. */

/* The algorithm iterates as follows. */

3. Label Validation:

If Label-stack-depth is 0 {

/* The LSR needs to report that it is a tail end for the LSP */

Set FEC-stack-depth to 1, set Label-L to 3 (Implicit Null).
Set Best-return-code to 3 ("Replying router is an egress for
the FEC at stack-depth"), set Best-rtn-subcode to the value of
FEC-stack-depth (1), and go to step 5 (Egress Processing).

}

/* This step assumes there is always an entry for well-known label
values */

Set Label-L to the value extracted from Stack-R at depth
Label-stack-depth. Look up Label-L in the Incoming Label Map
(ILM) to determine if the label has been allocated and an
operation is associated with it.

If there is no entry for Label-L {

/* Indicates a temporary or permanent label synchronization
problem, and the 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 associated label operation from the corresponding
Next Hop Label Forwarding Entry (NHLFE), and proceed to step 4
(Label Operation Check).

}

4. Label Operation Check

If the label operation is "Pop and Continue Processing" {

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

Iterate to the next label by decrementing Label-stack-depth,
and loop back to step 3 (Label Validation).

}

If the label operation is "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 a Downstream Detailed Mapping TLV is present in the received
echo request {

If the IP address in the TLV 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 and
Stack-R.

}

Else {

Verify that the IP address, interface address, and label
stack in the Downstream Detailed Mapping TLV match
Interface-I and Stack-R. If there is a mismatch, set
Best-return-code to 5, "Downstream Mapping Mismatch". An
Interface and Label Stack TLV SHOULD be included in 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 the NHLFE entry.

/* Note: this Return Code is set even if Label-stack-depth
is one */

If the output interface is not MPLS enabled {

Set Best-return-code to Return Code 9, "Label switched
but no MPLS forwarding at stack-depth" and set
Best-rtn-subcode to Label-stack-depth and go to step 7
(Send Reply Packet).

}

If a Downstream Detailed Mapping TLV is present {

A Downstream Detailed Mapping TLV SHOULD be included in
the echo reply (see Section 3.4) filled in with
information about the current ECMP path.

}

If no Downstream Detailed Mapping TLV is present, or 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 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 Detailed
Mapping TLV. This is done by walking through Stack-D (the
Downstream labels) from the bottom, decrementing the number of
labels for each non-Implicit Null label, while incrementing
FEC-stack-depth for each label. If the 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 is
considered to be entry 1.
*/

Set FEC-stack-depth to 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 number of FECs in the FEC stack is greater than or equal
to FEC-stack-depth {
Perform the FEC Checking procedure (see Section 4.4.1).

If FEC-status is 2, set Best-return-code to 10 ("Mapping for
this FEC is not the given label at stack-depth").

If 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 Reply Packet).
}

5. Egress Processing:

/* These steps are performed by the LSR that identified itself as
the tail-end LSR for an LSP. */

If the received echo request contains no Downstream Detailed
Mapping TLV, or the 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 in
the Downstream Detailed Mapping TLV match Interface-I and Stack-R.
If not, set Best-return-code to 5, "Downstream Mapping Mismatch".
A Received Interface and Label Stack TLV SHOULD be created for the
echo response packet. Go to step 7 (Send Reply Packet).

6. Egress FEC Validation:

/* This is a loop for all entries in the Target FEC Stack starting
with FEC-stack-depth. */
Perform FEC checking by following the algorithm described in
Section 4.4.1 for Label-L and the FEC at FEC-stack-depth.

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

If FEC-status (the result of the check) is 1,
go to step 7 (Send Reply Packet).

/* Iterate to the next FEC entry */

++FEC-stack-depth.
If FEC-stack-depth > the number of FECs in the FEC-stack,
go to step 7 (Send Reply Packet).

If FEC-status is 0 {

++Label-stack-depth.
If Label-stack-depth > the 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.
Loop back 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 the above process. The procedures for sending the echo
reply are found in Section 4.5.

4.4.1. FEC Validation

/* This section describes validation of a FEC entry within the Target
FEC Stack and accepts a FEC, Label-L, and Interface-I.

If the outermost FEC of the Target FEC stack is the Nil FEC, then the
node MUST skip the Target FEC validation completely. This is to
support FEC hiding, in which the outer hidden FEC can be the Nil FEC.
Else, 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 FEC-return-code to 10 ("Mapping for this FEC is 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 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 no mapping for the FEC at stack-
depth". Set FEC-status to 1. Return.

4. If the label mapping for FEC is Implicit Null, set FEC-status to
2 and proceed to step 5. Otherwise, if the label mapping for FEC
is Label-L, proceed to step 5. Otherwise, set FEC-return-code to
10 ("Mapping for this FEC is not the given 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 the FEC. If it can be determined that no protocol
associated with Interface-I would have advertised a 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 a UDP packet. It MUST ONLY be sent in response
to an MPLS echo request. The source IP address is a routable address
of the replier; the source port is the well-known UDP port for LSP
ping. The destination IP address and UDP port are copied from the
source IP address and UDP port of the echo request. The IP TTL is
set to 255. If the Reply Mode in the echo request is "Reply via an
IPv4 UDP packet with Router Alert", then the IP header MUST contain
the Router Alert IP option 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 case be the Router Alert label (1) (see
[RFC3032]).

The format of the echo reply is 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 set to the time of
day that the echo request is 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 and Subcode, as determined
in the previous section.

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

If the replying router is the destination of the FEC, then Downstream
Detailed Mapping TLVs SHOULD NOT be included in the echo reply.

If the echo request contains a Downstream Detailed Mapping TLV, and
the replying router is not the destination of the FEC, the replier
SHOULD compute its downstream routers and corresponding labels for
the incoming label and add Downstream Detailed Mapping TLVs for each
one to the echo reply it sends back. A replying node should follow
the procedures defined in Section 4.5.1 if there is a FEC stack
change due to tunneled LSP. If the FEC stack change is due to
stitched LSP, it should follow the procedures defined in
Section 4.5.2.

If the Downstream Detailed Mapping TLV contains Multipath Information
requiring more processing than the receiving router is willing to
perform, the responding router MAY choose to respond with only a
subset of multipaths contained in the echo request Downstream
Detailed Mapping. (Note: The originator of the echo request MAY send
another echo request with the Multipath Information that was not
included in the reply.)

Except in the case of Reply Mode 4, "Reply via application-level
control channel", echo replies are always sent in the context of the
IP/MPLS network.

4.5.1. Addition of a New Tunnel

A transit node knows when the FEC being traced is going to enter a
tunnel at that node. Thus, it knows about the new outer FEC. All
transit nodes that are the origination point of a new tunnel SHOULD
add the FEC stack change sub-TLV (Section 3.4.1.3) to the Downstream
Detailed Mapping TLV in the echo reply. The transit node SHOULD add
one FEC stack change sub-TLV of operation type PUSH, per new tunnel
being originated at the transit node.

A transit node that sends a Downstream FEC stack change sub-TLV in
the echo reply SHOULD fill the address of the remote peer, which is
the peer of the current LSP being traced. If the transit node does
not know the address of the remote peer, it MUST set the address type
to Unspecified.

The Label stack Stack sub-TLV MUST contain one additional label per FEC
being PUSHed. The label MUST be encoded as defined in
Section 3.4.1.2. The label value MUST be the value used to switch
the data traffic. If the tunnel is a transparent pipe to the node,
i.e., the data-plane trace will not expire in the middle of the new
tunnel, then a FEC stack change sub-TLV SHOULD NOT be added, and the
Label stack Stack sub-TLV SHOULD NOT contain a label corresponding to the
hidden tunnel.

If the transit node wishes to hide the nature of the tunnel from the
ingress of the echo request, then it MAY not want to send details
about the new tunnel FEC to the ingress. In such a case, the transit
node SHOULD use the Nil FEC. The echo reply would then contain a FEC
stack change sub-TLV with operation type PUSH and a Nil FEC. The
value of the label in the Nil FEC MUST be set to zero. The remote
peer address type MUST be set to Unspecified. The transit node
SHOULD add one FEC stack change sub-TLV of operation type PUSH, per
new tunnel being originated at the transit node. The Label stack Stack
sub-TLV MUST contain one additional label per FEC being PUSHed. The
label value MUST be the value used to switch the data traffic.

4.5.2. Transition between Tunnels

A transit node stitching two LSPs SHOULD include two FEC stack change
sub-TLVs. One with a POP pop operation for the old FEC (ingress) and one
with the PUSH operation for the new FEC (egress). The replying node
SHOULD set the Return Code to "Label switched with FEC change" to
indicate change in the FEC being traced.

If the replying node wishes to perform FEC hiding, it SHOULD respond
back with two FEC stack change sub-TLVs, one POP pop followed by one
PUSH. The POP pop operation MAY either exclude the FEC TLV (by setting
the FEC TLV length to 0) or set the FEC TLV to contain the LDP FEC.
The PUSH operation SHOULD have the FEC TLV containing the Nil FEC.
The Return Code SHOULD be set to "Label switched with FEC change".

If the replying node wishes to perform FEC hiding, it MAY choose to
not send any FEC stack change sub-TLVs in the echo reply if the
number of labels does not change for the downstream node and the FEC
type also does not change (Nil FEC). In such case, the replying node
MUST NOT set the Return Code to "Label switched with FEC change".

4.6. Receiving an MPLS Echo Reply

An LSR X should only receive an MPLS echo reply in response to an
MPLS echo request that it sent. Thus, on receipt of an MPLS echo
reply, X should parse the packet to ensure that it is well-formed,
then attempt to match up the echo reply with an echo request that it
had previously sent, using the destination UDP port and the Sender's
Handle. If no match is found, then X jettisons the echo reply;
otherwise, it checks the Sequence Number to see if it matches.

If the echo reply contains Downstream Detailed Mappings, and X wishes
to traceroute further, it SHOULD copy the Downstream Detailed
Mapping(s) into its next echo request(s) (with TTL incremented by
one).

If one or more FEC stack change sub-TLVs are received in the MPLS
echo reply, the ingress node SHOULD process them and perform some
validation.

The FEC stack changes are associated with a downstream neighbor and
along a particular path of the LSP. Consequently, the ingress will
need to maintain a FEC stack per path being traced (in case of
multipath). All changes to the FEC stack resulting from the
processing of a FEC stack change sub-TLV(s) should be applied only
for the path along a given downstream neighbor. The following
algorithm should be followed for processing FEC stack change
sub-TLVs.

push_seen = FALSE
fec_stack_depth = current-depth-of-fec-stack-being-traced
saved_fec_stack = current_fec_stack

while (sub-tlv = get_next_sub_tlv(downstream_detailed_map_tlv))

if (sub-tlv == NULL) break

if (sub-tlv.type == FEC-Stack-Change) {

if (sub-tlv.operation == POP) {
if (push_seen) {
Drop the echo reply
current_fec_stack = saved_fec_stack
return
}

if (fec_stack_depth == 0) {
Drop the echo reply
current_fec_stack = saved_fec_stack
return
}

Pop FEC from FEC stack being traced
fec_stack_depth--;
}

if (sub-tlv.operation == PUSH) {
push_seen = 1
Push FEC on FEC stack being traced
fec_stack_depth++;
}
}
}

if (fec_stack_depth == 0) {
Drop the echo reply
current_fec_stack = saved_fec_stack
return
}

The next MPLS echo request along the same path should use the
modified FEC stack obtained after processing the FEC stack change
sub-TLVs. A non-Nil FEC guarantees that the next echo request along
the same path will have the Downstream Detailed Mapping TLV validated
for IP address, interface address, and label stack mismatches.

If the top of the FEC stack is a Nil FEC and the MPLS echo reply does
not contain any FEC stack change sub-TLVs, then it does not
necessarily mean that the LSP has not started traversing a different
tunnel. It could be that the LSP associated with the Nil FEC
terminated at a transit node, and at the same time, a new LSP started
at the same transit node. The Nil FEC would now be associated with
the new LSP (and the ingress has no way of knowing this). Thus, it
is not possible to build an accurate hierarchical LSP topology if a
traceroute contains Nil FECs.

A reply from a downstream node with Return Code 3, may not
necessarily be for the FEC being traced. It could be for one of the
new FECs that was added. On receipt of an IS_EGRESS reply, the LSP
ingress should check if the depth of Target FEC sent to the node that
just responded was the same as the depth of the FEC that was being
traced. If it was not, then it should pop an entry from the Target
FEC stack and resend the request with the same TTL (as previously
sent). The process of popping a FEC is to be repeated until either
the LSP ingress receives a non-IS_EGRESS reply or until all the
additional FECs added to the FEC stack have already been popped.
Using an IS_EGRESS reply, an ingress can build a map of the
hierarchical LSP structure traversed by a given FEC.

When the MPLS echo reply Return Code is "Label switched with FEC
change", the ingress node SHOULD manipulate the FEC stack as per the
FEC stack change sub-TLVs contained in the Downstream Detailed
Mapping TLV. A transit node can use this Return Code for stitched
LSPs and for hierarchical LSPs. In case of ECMP or P2MP, there could
be multiple paths and Downstream Detailed Mapping TLVs with different
Return Codes (see Section 3.1, Note 2). The ingress node should
build the topology based on the Return Code per ECMP path/P2MP
branch.

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 label stack of depth greater than 1, with the innermost
label having a TTL 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 can shrink to a single label
before the ping hits the egress PE; this will result 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 the LSR that receives
such a ping to realize that the ping terminated prematurely and to
send back error code Return 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 reach the egress PE.

4.8. Non-compliant Routers

If the egress for the FEC Stack being pinged does not support MPLS LSP
ping, then no reply will be sent, resulting in possible "false
negatives". If When in "traceroute" mode, if a transit LSR does not
support LSP ping, so 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 with TTL=n+1, n+2, ..., n+k to probe LSRs
further down the path. In such a case, the echo request for TTL > n
SHOULD be sent with the Downstream Detailed Mapping TLV "Downstream
IP Address" field set to the ALLROUTERs multicast address until a
reply is received with a Downstream Detailed Mapping TLV. The label stack
Stack TLV MAY be omitted from the Downstream Detailed Mapping TLV.
Furthermore, the "Validate FEC Stack" flag SHOULD NOT be set until an
echo reply packet with a Downstream Detailed Mapping TLV is received.

5. Security Considerations

Overall, the security needs for LSP ping are similar to those of ICMP
ping.

There are at least three approaches to attacking LSRs using the
mechanisms defined here. One is a Denial-of-Service (DoS) attack, by
sending MPLS echo requests/replies to LSRs and 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 requests and replies. The third is an
unauthorized source using an LSP ping to obtain information about the
network.

To avoid potential DoS attacks, it is RECOMMENDED that
implementations regulate the LSP ping traffic going to the control
plane. A rate limiter SHOULD be applied to the well-known UDP port
defined in Section 6.1.

Unsophisticated replay and spoofing attacks involving faking or
replaying MPLS echo reply messages are unlikely to be effective.
These replies would have to match the Sender's Handle and Sequence
Number of an outstanding MPLS echo request message. A non-matching
replay would be discarded as the 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 TimeStamp
Sent by requiring an exact match on this field.

To protect against unauthorized sources using MPLS echo request
messages to 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 not clear how to prevent hijacking (non-delivery) 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) to secure the data carried in
MPLS echo requests 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.

The value part of the Pad TLV contains a variable number of octets.
With the exception of the first octet, these contents, if any, are
ignored on receipt, and can therefore serve as a clandestine channel.

When MPLS LSP Ping ping is used within an administrative domain, a
deployment can increase security by using border filtering of
incoming LSP Ping ping packets as well as outgoing LSP Ping ping packets.

Although this document makes special use of 127/8 addresses, 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 to 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 RFC 1812. In particular,
the default behavior is to treat packets destined to a 127/8 address
as "martians".

If a network operator wants to prevent tracing inside a tunnel, one
can use the Pipe Model [RFC3443], i.e., hide the outer MPLS tunnel by
not propagating the MPLS TTL into the outer tunnel (at the start of
the outer tunnel). By doing this, MPLS LSP traceroute packets will not
expire in the outer tunnel, and the outer tunnel will not get traced.

If one doesn't wish to expose the details of the new outer LSP, then
the Nil FEC can be used to hide those details. Using the Nil FEC
ensures that the trace progresses without false negatives and all
transit nodes (of the new outer tunnel) perform some minimal
validations on the received MPLS echo requests.

6. IANA Considerations

6.1. TCP and UDP Port Number

The TCP and UDP port number 3503 has been allocated by IANA for LSP
echo requests and replies.

6.2. MPLS LSP Ping Parameters

IANA maintains the "Multiprotocol Label Switching (MPLS) Label
Switched Paths (LSPs) Ping Parameters" registry at
[IANA-MPLS-LSP-PING].

The following subsections detail the name spaces managed by IANA.
For some 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", and "Vendor Private Use".

Values from "Specification Required" ranges MUST be registered with
IANA. The request MUST be made via an Experimental RFC that describes the format
and procedures for using the code point; the actual assignment is
made during the 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
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.

6.2.1. Message Types, Reply Modes, Return Codes

IANA has created and will maintain registries for Message Types,
Reply Modes, 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 in the range 252-255 are for Vendor Private Use and
MUST NOT be allocated.

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

Message Types defined in this document are the following:

Value Meaning
----- -------
1 MPLS Echo Request
2 MPLS Echo Reply

Reply Modes defined 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 with Router Alert
4 Reply via application-level control channel

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

IANA has updated the reference for each these values to this
document.

6.2.2. TLVs

IANA has created and maintains a registry for the Type field of top-
level TLVs as well as for any associated sub-TLVs. Note that the
meaning 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 the
ranges 0-16383 and 32768-49161 are made via Standards Action as
defined in [RFC5226]; assignments in the ranges 16384-31743 and
49162-64511 are made via "Specification Required"; values in the
ranges 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
---- -------- -----------
1 Target FEC Stack
1 LDP IPv4 prefix
2 LDP IPv6 prefix
3 RSVP IPv4 LSP
4 RSVP IPv6 LSP
5 Unassigned
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 (Deprecated)
3 Pad
4 Unassigned
5 Vendor Enterprise Number
6 Unassigned
7 Interface and Label Stack
8 Unassigned
9 Errored TLVs
Any value The TLV not understood
10 Reply TOS Byte
20 Downstream Detailed Mapping

IANA has updated the reference for each of these values to this
document.

6.2.3. Global Flags

IANA has created a "Global Flags" subregistry of the "Multiprotocol
Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry.

This registry tracks the assignment of 16 flags in the Global Flags
field of the MPLS LSP ping echo request message. The flags are
numbered from 0 (most significant bit, transmitted first) to 15.

New entries are assigned by Standards Action.

Initial entries in the registry are as follows:

6.2.4. Downstream Detailed Mapping Address Type

This document extends RFC 4379 by defining a new address type for use
with the Downstream Mapping and Downstream Detailed Mapping TLVs.
IANA has established a registry to assign address types for use with
the Downstream Mapping and Downstream Detailed Mapping TLVs, which
initially allocates the following assignments:

Type # Address Type K Octets Reference
------ ------------ -------- ---------
1 IPv4 Numbered 16 RFC 8029 [RFC8029]
2 IPv4 Unnumbered 16 RFC 8029 [RFC8029]
3 IPv6 Numbered 40 RFC 8029 [RFC8029]
4 IPv6 Unnumbered 28 RFC 8029 [RFC8029]
5 Non IP 12 [RFC6426]

Downstream Detailed Mapping Address Type Registry

Because the field in this case is an 8-bit field, the allocation
policy for this registry is "Standards Action".

6.2.5. DS Flags

This document defines the Downstream Mapping (DSMAP) TLV and the
Downstream Detailed Mapping (DDMAP) TLV, which have Type 2 and Type
20, respectively, assigned from the "TLVs" subregistry of the
"Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs)
Ping Parameters" registry.

DSMAP has been deprecated by DDMAP, but both TLVs share a field: DS
Flags.

IANA has created and now maintains a registry entitled "DS Flags".

The registration policy for this registry is Standards Action
[RFC5226].

IANA has made the following assignments:

Bit Number Name Reference
---------- ------------------------------------------- ---------
7 N: Treat as a Non-IP Packet RFC 8029 [RFC8029]
6 I: Interface and Label Stack Object Request RFC 8029 [RFC8029]
5 E: ELI/EL push indicator [RFC8012]
4 L: Label-based load balance indicator [RFC8012]
3-0 Unassigned

6.2.6. Multipath Types

IANA has created and now maintains a registry entitled "Multipath
Types".

The registration policies policy [RFC5226] for this registry are as follows:

0-250 Standards Action
251-254 Experimental Use
255 is Standards Action
Action.

IANA has made the following assignments:

Value Meaning Reference
---------- ---------------------------------------- ---------
0 no multipath RFC 8029 [RFC8029]
1 Unassigned
2 IP address RFC 8029 [RFC8029]
3 Unassigned
4 IP address range RFC 8029 [RFC8029]
5-7 Unassigned
8 Bit-masked IP address set RFC 8029 [RFC8029]
9 Bit-masked label set RFC 8029 [RFC8029]
10 IP and label set [RFC8012]
11-250 Unassigned
251-254 Reserved for Experimental Use RFC 8029 [RFC8029]
255 Reserved RFC 8029 [RFC8029]

6.2.7. Pad Type

IANA has created and now maintains a registry entitled "Pad Types".

The registration policies policy [RFC5226] for this registry are:

0-250 Standards Action
251-254 Experimental Use
255 is Standards Action
Action.

IANA has made the following initial assignments:

Registry Name: Pad Types

Value Meaning Reference
---------- ---------------------------------------- ---------
0 Reserved RFC 8029 [RFC8029]
1 Drop Pad TLV from reply RFC 8029 [RFC8029]
2 Copy Pad TLV to reply RFC 8029 [RFC8029]
3-250 Unassigned
251-254 Experimental Use RFC 8029 [RFC8029]
255 Reserved RFC 8029 [RFC8029]

6.2.8. Interface and Label Stack Address Type

IANA has created and now maintains a registry entitled "Interface and
Label Stack Address Types".

The registration policies policy [RFC5226] for this registry are:

0-250 Standards Action
251-254 Experimental Use
255 is Standards Action
Action.

IANA has made the following initial assignments:

Registry Name: Interface and Label Stack Address Types

Value Meaning Reference
---------- ---------------------------------------- ---------
0 Reserved RFC 8029 [RFC8029]
1 IPv4 Numbered RFC 8029 [RFC8029]
2 IPv4 Unnumbered RFC 8029 [RFC8029]
3 IPv6 Numbered RFC 8029 [RFC8029]
4 IPv6 Unnumbered RFC 8029 [RFC8029]
5-250 Unassigned
251-254 Experimental Use RFC 8029 [RFC8029]
255 Reserved RFC 8029 [RFC8029]

6.3. IPv4 Special-Purpose Address Registry

IANA has updated the reference in Note 1 of the "IANA IPv4 Special-
Purpose Address Registry" [IANA-SPECIAL-IPv4] to point to this
document.

7. References

7.1. 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 in RFCs 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>.

[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and 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 G. Swallow, "Detecting Multi-Protocol
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., and 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>.

7.2. Informative References

[Err108] RFC Errata, Erratum ID 108, RFC 4379.

[Err742] RFC Errata, Erratum ID 742, RFC 4379.

[Err1418] RFC Errata, Erratum ID 1418, RFC 4379.

[Err1714] RFC Errata, Erratum ID 1714, RFC 4379.

[Err1786] RFC Errata, Erratum ID 1786, RFC 4379.

[Err2978] RFC Errata, Erratum ID 2978, RFC 4379.

[Err3399] RFC Errata, Erratum ID 3399, RFC 4379.

[Err742] RFC Errata, Erratum ID 742, RFC 4379.

[IANA-ENT]
IANA, "PRIVATE ENTERPRISE NUMBERS",
<http://www.iana.org/assignments/enterprise-numbers>.

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

[IANA-SPECIAL-IPv4]
IANA, "IANA IPv4 Special-Purpose Address Registry",
<http://www.iana.org/assignments/
iana-ipv4-special-registry>.

[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 Label Information 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>.

[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks",
RFC 3443, DOI 10.17487/RFC3443, January 2003,
<http://www.rfc-editor.org/info/rfc3443>.

[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>.

[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., and
G. Heron, "Pseudowire Setup and Maintenance Using the
Label Distribution Protocol (LDP)", RFC 4447,
DOI 10.17487/RFC4447, April 2006,
<http://www.rfc-editor.org/info/rfc4447>.

[RFC4461] Yasukawa, S., Ed., "Signaling Requirements for Point-to-
Multipoint Traffic-Engineered MPLS Label Switched Paths
(LSPs)", RFC 4461, DOI 10.17487/RFC4461, April 2006,
<http://www.rfc-editor.org/info/rfc4461>.

[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>.

[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, DOI 10.17487/RFC5331, August 2008,
<http://www.rfc-editor.org/info/rfc5331>.

[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
2009, <http://www.rfc-editor.org/info/rfc5462>.

[RFC5885] Nadeau, T., Ed. and C. Pignataro, Ed., "Bidirectional
Forwarding Detection (BFD) for the Pseudowire Virtual
Circuit Connectivity Verification (VCCV)", RFC 5885,
DOI 10.17487/RFC5885, June 2010,
<http://www.rfc-editor.org/info/rfc5885>.

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

[RFC6426] Gray, E., Bahadur, N., Boutros, S., and R. Aggarwal, "MPLS
On-Demand Connectivity Verification and Route Tracing",
RFC 6426, DOI 10.17487/RFC6426, November 2011,
<http://www.rfc-editor.org/info/rfc6426>.

[RFC6829] Chen, M., Pan, P., Pignataro, C., and R. Asati, "Label
Switched Path (LSP) Ping for Pseudowire Forwarding
Equivalence Classes (FECs) Advertised over IPv6",
RFC 6829, DOI 10.17487/RFC6829, January 2013,
<http://www.rfc-editor.org/info/rfc6829>.

[RFC7537] Decraene, B., Akiya, N., Pignataro, C., Andersson, L., and
S. Aldrin, "IANA Registries for LSP Ping Code Points",
RFC 7537, DOI 10.17487/RFC7537, May 2015,
<http://www.rfc-editor.org/info/rfc7537>.

[RFC8012] Akiya, N., Swallow, G., Pignataro, C., Malis, A., and S.
Aldrin, "Label Switched Path (LSP) and Pseudowire (PW)
Ping/Trace over MPLS Networks Using Entropy Labels (ELs)",
RFC 8012, DOI 10.17487/RFC8012, November 2016,
<http://www.rfc-editor.org/info/rfc8012>.

Appendix A. Deprecated TLVs and Sub-TLVs (Non-normative)

This appendix describes deprecated elements, which are non-normative
for an implementation. They are included in this document for
historical and informational purposes.

A.1. Target FEC Stack

A.1.1. FEC 128 Pseudowire - IPv4 (Deprecated)

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 a FEC 128 is encoded in a label stack, the following format is
used. The value Value field consists of the Remote PE IPv4 Address (the
destination address of the targeted LDP session), the PW ID, and the
encapsulation type 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote PE IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type | Must Be Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This FEC is deprecated and is retained only for backward
compatibility. Implementations of LSP ping SHOULD accept and process
this TLV, but SHOULD send LSP ping echo requests with the new TLV
(see Section 3.2.9), 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 (Deprecated)

The 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 echo
reply. If the replying router is the destination of the FEC, then a
Downstream Mapping TLV SHOULD NOT 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 a more precise definition of the notion of "downstream", see
Section 3.4.2, "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. downstream labels. Values for 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
5 Non IP 12

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

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 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 Address to 224.0.0.2; for IPv6, the address MUST be
set to FF02::2. In both cases, the interface index MUST be set to
0. If an LSR receives an Echo Request echo request packet with the all-routers
multicast address, then this indicates that it MUST bypass both
interface and label stack validation, but return Downstream
Mapping TLVs using the information provided.

Multipath Type

The following Multipath Types are defined:

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

Types 2, 4, 8, and 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 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 Section 3.4.1.1.1 for 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 downstream label is 24 bits, in 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 TC bits are bits 20-22, and bit 23 is the S
bit. The replying router SHOULD fill in the TC and S bits; the
LSR receiving the echo reply MAY choose to ignore these bits.

Protocol

The protocol is taken from the following table:

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

Acknowledgements

The original acknowledgements from RFC 4379 state the following:

This document is 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 the Multipath Information sub-field of the
Downstream Mapping TLV was adapted from text suggested by Curtis
Villamizar.

We would like to thank Loa Andersson for motivating the advancement
of this specification.

We also would like to thank Alexander Vainshtein, Yimin Shen, Curtis
Villamizar, David Allan, Vincent Roca, Mirja Kuhlewind, and Elwyn
Davies for their review and useful comments.

Authors' Addresses

Kireeti Kompella
Juniper Networks, Inc.

Email: kireeti.kompella@gmail.com

George Swallow
Cisco Systems, Inc.

Email: swallow.ietf@gmail.com

Carlos Pignataro (editor)
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)

Mach(Guoyi) Chen
Huawei

Email: mach.chen@huawei.com