Internet Engineering Task Force (IETF) N. Kumar, Ed.

Internet Engineering Task Force (IETF) N. Kumar, Ed.
Request for Comments: 8287 C. Pignataro, Ed.
Category: Standards Track Cisco
ISSN: 2070-1721 G. Swallow
Southend Technical Center
N. Akiya
Big Switch Networks
S. Kini
Individual
M. Chen
Huawei
December 2017

Label Switched Path (LSP) Ping/Traceroute for Segment Routing (SR)
IGP-Prefix and IGP-Adjacency Segment Identifiers (SIDs)
with MPLS Data Planes

Abstract

A Segment Routing (SR) architecture leverages source routing and
tunneling paradigms and can be directly applied to the use of a
Multiprotocol Label Switching (MPLS) data plane. A node steers a
packet through a controlled set of instructions called "segments" by
prepending the packet with an SR header.

The segment assignment and forwarding semantic nature of SR raises
additional considerations for connectivity verification and fault
isolation for a Label Switched Path (LSP) within an SR architecture.
This document illustrates the problem and defines extensions to
perform LSP Ping and Traceroute for Segment Routing IGP-Prefix and
IGP-Adjacency Segment Identifiers (SIDs) with an MPLS data plane.

Status of This Memo

This is an Internet Standards Track document.

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

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8287.

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Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Coexistence of SR-Capable and Non-SR-Capable Node
Scenarios . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Challenges with Existing Mechanisms . . . . . . . . . . . . . 5
4.1. Path Validation in Segment Routing Networks . . . . . . . 5
5. Segment ID Sub-TLV . . . . . . . . . . . . . . . . . . . . . 7
5.1. IPv4 IGP-Prefix Segment ID . . . . . . . . . . . . . . . 7
5.2. IPv6 IGP-Prefix Segment ID . . . . . . . . . . . . . . . 8
5.3. IGP-Adjacency Segment ID . . . . . . . . . . . . . . . . 9
6. Extension to Downstream Detailed Mapping TLV . . . . . . . . 11
7. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. FECs in Target FEC Stack TLV . . . . . . . . . . . . . . 11
7.2. FEC Stack Change Sub-TLV . . . . . . . . . . . . . . . . 12
7.3. Segment ID POP Operation . . . . . . . . . . . . . . . . 13
7.4. Segment ID Check . . . . . . . . . . . . . . . . . . . . 13
7.5. TTL Consideration for Traceroute . . . . . . . . . . . . 19
8. Backward Compatibility with Non-SR Devices . . . . . . . . . 19
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9.1. New Target FEC Stack Sub-TLVs . . . . . . . . . . . . . . 20
9.2. Protocol in the Segment ID Sub-TLV . . . . . . . . . . . 20
9.3. Adjacency Type in the IGP-Adjacency Segment ID . . . . . 20
9.4. Protocol in the Label Stack Sub-TLV of the Downstream
Detailed Mapping TLV . . . . . . . . . . . . . . . . . . 21
9.5. Return Code . . . . . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 21
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . 22
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 24
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25

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1. Introduction

"Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures"
[RFC8029] defines a simple and efficient mechanism to detect data-
plane failures in Label Switched Paths (LSPs) by specifying
information to be carried in an MPLS "echo request" and "echo reply"
for the purposes of fault detection and isolation. Mechanisms for
reliably sending the echo reply are defined. The functionality
defined in [RFC8029] is modeled after the Ping/Traceroute paradigm
(ICMP echo request [RFC792]) and is typically referred to as "LSP
Ping" and "LSP Traceroute". [RFC8029] supports hierarchical and
stitching LSPs.

[SR] introduces and describes an SR architecture that leverages the
source routing and tunneling paradigms. A node steers a packet
through a controlled set of instructions called "segments" by
prepending the packet with an SR header. A detailed definition of
the SR architecture is available in [SR].

As described in [SR] and [SR-MPLS], the SR architecture can be
directly applied to an MPLS data plane, the SID will be 20 bits, and
the SR header is the label stack. Consequently, the mechanics of
data-plane validation of [RFC8029] can be directly applied to SR
MPLS.

Unlike LDP or RSVP, which are the other well-known MPLS control plane
protocols, the basis of Segment ID assignment in SR architecture is
not always on a hop-by-hop basis. Depending on the type of Segment
ID, the assignment can be unique to the node or within a domain.

This nature of SR raises additional considerations for validation of
fault detection and isolation in an SR network. This document
illustrates the problem and describes a mechanism to perform LSP Ping
and Traceroute for Segment Routing IGP-Prefix and IGP-Adjacency SIDs
within an MPLS data plane.

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1.1. Coexistence of SR-Capable and Non-SR-Capable Node Scenarios

[INTEROP] describes how SR operates in a network where SR-capable and
non-SR-capable nodes coexist. In such a network, one or more
SR-based LSPs and non-SR-based LSPs are stitched together to achieve
an end-to-end LSP. This is similar to a network where LDP and RSVP
nodes coexist and the mechanism defined in Section 4.5.2 of [RFC8029]
is applicable for LSP Ping and Trace.

Section 8 of this document explains one of the potential gaps that is
specific to SR-Capable and non-SR-capable node scenarios and explains
how the existing mechanism defined in [RFC8029] handles it.

2. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Terminology

This document uses the terminology defined in [SR] and [RFC8029];
readers are expected to be familiar with those terms.

4. Challenges with Existing Mechanisms

The following example describes the challenges with using the current
MPLS Operations, Administration, and Maintenance (OAM) mechanisms on
an SR network.

4.1. Path Validation in Segment Routing Networks

[RFC8029] defines the MPLS OAM mechanisms that help with fault
detection and isolation for an MPLS data-plane path by the use of
various Target Forwarding Equivalence Class (FEC) Stack sub-TLVs that
are carried in MPLS echo request packets and used by the responder
for FEC validation. While it is obvious that new sub-TLVs need to be
assigned for SR, the unique nature of the SR architecture raises the
need for additional operational considerations for path validation.
This section discusses the challenges.

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L1
+--------+
| L2 |
R3-------R6
/ \
/ \
R1----R2 R7----R8
\ /
\ /
R4-------R5

Figure 1: Segment Routing Network

The Node Segment IDs for R1, R2, R3, R4, R5, R6, R7, and R8 are 5001,
5002, 5003, 5004, 5005, 5006, 5007, and 5008, respectively.

9136 --> Adjacency Segment ID from R3 to R6 over link L1.
9236 --> Adjacency Segment ID from R3 to R6 over link L2.
9124 --> Adjacency segment ID from R2 to R4.
9123 --> Adjacency Segment ID from R2 to R3.

The forwarding semantic of the Adjacency Segment ID is to pop the
Segment ID and send the packet to a specific neighbor over a specific
link. A malfunctioning node may forward packets using the Adjacency
Segment ID to an incorrect neighbor or over an incorrect link. The
exposed Segment ID (of an incorrectly forwarded Adjacency Segment ID)
might still allow such a packet to reach the intended destination,
even though the intended strict traversal was broken.

In the topology above, assume that R1 sends traffic with a segment
stack as {9124, 5008} so that the path taken will be
R1-R2-R4-R5-R7-R8. If the Adjacency Segment ID 9124 is misprogrammed
in R2 to send the packet to R1 or R3, the packet may still be
delivered to R8 (if the nodes are configured with the same SR Global
Block (SRGB)) [SR] but not via the expected path.

MPLS traceroute may help with detecting such a deviation in the
above-mentioned scenario. However, in a different example, it may
not be helpful, for example, if R3 forwards a packet with Adjacency
Segment ID 9236 via link L1 (due to misprogramming) when it was
expected to be forwarded over link L2.

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5. Segment ID Sub-TLV

The format of the following Segment ID sub-TLVs follows the
philosophy of the Target FEC Stack TLV carrying FECs corresponding to
each label in the label stack. When operated with the procedures
defined in [RFC8029], this allows LSP Ping/Traceroute operations to
function when the Target FEC Stack TLV contains more FECs than
received label stacks at the responder nodes.

Three new sub-TLVs are defined for the Target FEC Stack TLV (Type 1),
the Reverse-Path Target FEC Stack TLV (Type 16), and the Reply Path
TLV (Type 21).

Sub-Type Sub-TLV Name
-------- ---------------
34 IPv4 IGP-Prefix Segment ID
35 IPv6 IGP-Prefix Segment ID
36 IGP-Adjacency Segment ID

See Section 9.2 for the registry for the Protocol field specified
within these sub-TLVs.

5.1. IPv4 IGP-Prefix Segment ID

The IPv4 IGP-Prefix Segment ID is defined in [SR]. The format is as
specified 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Prefix Length | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IPv4 Prefix

This field carries the IPv4 Prefix to which the Segment ID is
assigned. In case of an Anycast Segment ID, this field will carry
the IPv4 Anycast address. If the prefix is shorter than 32 bits,
trailing bits SHOULD be set to zero.

Prefix Length

The Prefix Length field is one octet. It gives the length of the
prefix in bits (values can be 1-32).

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Protocol

This field is set to 1, if the responder MUST perform FEC
validation using OSPF as the IGP protocol. Set to 2, if the
responder MUST perform Egress FEC validation using the
Intermediate System to Intermediate System (IS-IS) as the IGP
protocol. Set to 0, if the responder can use any IGP protocol for
Egress FEC validation.

Reserved

The Reserved field MUST be set to 0 when sent and MUST be ignored
on receipt.

5.2. IPv6 IGP-Prefix Segment ID

The IPv6 IGP-Prefix Segment ID is defined in [SR]. The format is as
specified 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IPv6 Prefix |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Prefix Length | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IPv6 Prefix

This field carries the IPv6 prefix to which the Segment ID is
assigned. In case of an Anycast Segment ID, this field will carry
the IPv4 Anycast address. If the prefix is shorter than 128 bits,
trailing bits SHOULD be set to zero.

Prefix Length

The Prefix Length field is one octet, it gives the length of the
prefix in bits (values can be 1-128).

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Protocol

Set to 1 if the responder MUST perform FEC validation using OSPF
as the IGP protocol. Set to 2 if the responder MUST perform
Egress FEC validation using IS-IS as the IGP protocol. Set to 0
if the responder can use any IGP protocol for Egress FEC
validation.

Reserved

MUST be set to 0 on send and MUST be ignored on receipt.

5.3. IGP-Adjacency Segment ID

This sub-TLV is applicable for any IGP-Adjacency defined in [SR].
The format is as specified 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Adj. Type | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| Local Interface ID (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| Remote Interface ID (4 or 16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| Advertising Node Identifier (4 or 6 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
| Receiving Node Identifier (4 or 6 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Adj. Type (Adjacency Type)

Set to 1 when the Adjacency Segment is a Parallel Adjacency as
defined in [SR]. Set to 4 when the Adjacency Segment is IPv4
based and is not a Parallel Adjacency. Set to 6 when the
Adjacency Segment is IPv6 based and is not a Parallel Adjacency.
Set to 0 when the Adjacency Segment is over an unnumbered
interface.

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Protocol

Set to 1 if the responder MUST perform FEC validation using OSPF
as the IGP protocol. Set to 2 if the responder MUST perform
Egress FEC validation using IS-IS as the IGP protocol. Set to 0
if the responder can use any IGP protocol for Egress FEC
validation.

Reserved

MUST be set to 0 on send and MUST be ignored on receipt.

Local Interface ID

An identifier that is assigned by the local Label Switching Router
(LSR) for a link to which the Adjacency Segment ID is bound. This
field is set to a local link address (IPv4 or IPv6). For IPv4,
this field is 4 octets; for IPv6, this field is 16 octets. If
unnumbered, this field is 4 octets and includes a 32-bit link
identifier as defined in [RFC4203] and [RFC5307]. If the
Adjacency Segment ID represents Parallel Adjacencies [SR], this
field is 4 octets and MUST be set to 4 octets of zeroes.

Remote Interface ID

An identifier that is assigned by the remote LSR for a link on
which the Adjacency Segment ID is bound. This field is set to the
remote (downstream neighbor) link address (IPv4 or IPv6). For
IPv4, this field is 4 octets; for IPv6, this field is 16 octets.
If unnumbered, this field is 4 octets and includes a 32-bit link
identifier as defined in [RFC4203] and [RFC5307]. If the
Adjacency Segment ID represents Parallel Adjacencies [SR], this
field is 4 octets and MUST be set to 4 octets of zeroes.

Advertising Node Identifier

This specifies the Advertising Node Identifier. When the Protocol
field is set to 1, then this field is 4 octets and carries the
32-bit OSPF Router ID. If the Protocol field is set to 2, then
this field is 6 octets and carries the 48-bit IS-IS System ID. If
the Protocol field is set to 0, then this field is 4 octets and
MUST be set to zero.

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Receiving Node Identifier

This specifies the downstream node identifier. When the Protocol
field is set to 1, then this field is 4 octets and carries the
32-bit OSPF Router ID. If the Protocol field is set to 2, then
this field is 6 octets and carries the 48-bit IS-IS System ID. If
the Protocol field is set to 0, then this field is 4 octets and
MUST be set to zero.

6. Extension to Downstream Detailed Mapping TLV

In an echo reply, the Downstream Detailed Mapping TLV [RFC8029] is
used to report for each interface over which a FEC could be
forwarded. For a FEC, there are multiple protocols that may be used
to distribute label mapping. The Protocol field of the Downstream
Detailed Mapping TLV is used to return the protocol that is used to
distribute the label carried in the Downstream Label field. The
following protocols are defined in [RFC8029]:

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

With SR, OSPF or IS-IS can be used for label distribution. This
document adds two new protocols as follows:

Protocol # Signaling Protocol
---------- ------------------
5 OSPF
6 IS-IS

See Section 9.4.

7. Procedures

This section describes aspects of LSP Ping and Traceroute operations
that require further considerations beyond [RFC8029].

7.1. FECs in Target FEC Stack TLV

When LSP echo request packets are generated by an initiator, FECs
carried in the Target FEC Stack TLV may need to differ to support an
SR architecture. The following defines the Target FEC Stack TLV
construction mechanics by an initiator for SR scenarios.

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Ping

The initiator MUST include FEC(s) corresponding to the
destination segment.

The initiator MAY include FECs corresponding to some or all of
the segments imposed in the label stack by the initiator to
communicate the segments traversed.

Traceroute

The initiator MUST initially include FECs corresponding to all
segments imposed in the label stack.

When a received echo reply contains the FEC Stack Change TLV
with one or more of the original segments being popped, the
initiator MAY remove a corresponding FEC(s) from the Target FEC
Stack TLV in the next (TTL+1) traceroute request, as defined in
Section 4.6 of [RFC8029].

When a received echo reply does not contain the FEC Stack
Change TLV, the initiator MUST NOT attempt to remove any FECs
from the Target FEC Stack TLV in the next (TTL+1) traceroute
request.

As defined in [SR-OSPF] and [SR-IS-IS], the Prefix SID can be
advertised as an absolute value, an index, or as a range. In any of
these cases, the initiator MUST derive the Prefix mapped to the
Prefix SID and use it in the IGP-Prefix Segment ID defined in
Sections 5.1 and 5.2. How the responder uses the details in the
SR-FEC sub-TLV to perform the validation is a local implementation
matter.

7.2. FEC Stack Change Sub-TLV

[RFC8029] defines a FEC Stack Change sub-TLV that a router must
include when the FEC stack changes.

The network node that advertised the Node Segment ID is responsible
for generating a FEC Stack Change sub-TLV with the Post Office
Protocol (POP) operation type for the Node Segment ID, regardless of
whether or not Penultimate Hop Popping (PHP) is enabled.

The network node that is immediately downstream of the node that
advertised the Adjacency Segment ID is responsible for generating the
FEC Stack Change sub-TLV for POP operation for the Adjacency Segment
ID.

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7.3. Segment ID POP Operation

The forwarding semantic of the Node Segment ID with the PHP flag is
equivalent to usage of Implicit Null in MPLS protocols. The
Adjacency Segment ID is also similar in a sense that it can be
thought of as a locally allocated segment that has PHP enabled when
destined for the next-hop IGP Adjacency Node. Procedures described
in Section 4.4 of [RFC8029] rely on the Stack-D and Stack-R
explicitly having the Implicit Null value. Implementations SHOULD
use the Implicit Null for the Node Segment ID PHP and Adjacency
Segment ID PHP cases.

7.4. Segment ID Check

This section modifies the procedure defined in Section 4.4.1 of
[RFC8029]. Step 4 defined in Section 4.4.1 of [RFC8029] is modified
as below:

4. If the label mapping for FEC is Implicit Null, set the
FEC-status to 2 and proceed to step 4a. Otherwise,
if the label mapping for FEC is Label-L, proceed to step 4a.
Otherwise, set the FEC-return-code to 10 ("Mapping for this
FEC is not the given label at stack-depth"), set the
FEC-status to 1, and return.

4a. Segment Routing IGP-Prefix and IGP-Adjacency SID Validation:

If the Label-stack-depth is 0 and the Target FEC Stack sub-TLV
at FEC-stack-depth is 34 (IPv4 IGP-Prefix Segment ID), {

Set the Best-return-code to 10, "Mapping for this FEC is not
the given label at stack-depth <RSC>" if any below
conditions fail:

/* The responder LSR is to check if it is the egress of the
IPv4 IGP-Prefix Segment ID described in the Target FEC Stack
sub-TLV, and if the FEC was advertised with the PHP bit
set.*/

- Validate that the Node Segment ID is advertised for the
IPv4 Prefix by IGP Protocol {

o When the Protocol field in the received IPv4 IGP-
Prefix Segment ID sub-TLV is 0, use any locally
enabled IGP protocol.

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o When the Protocol field in the received IPv4 IGP-
Prefix Segment ID sub-TLV is 1, use OSPF as the IGP
protocol.

o When the Protocol field in the received IPv4 IGP-
Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP
protocol.

o When the Protocol field in the received IPv4 IGP-
Prefix Segment ID sub-TLV is an unrecognized value, it
MUST be treated as a Protocol value of 0.

}

- Validate that the Node Segment ID is advertised with the
No-PHP flag. {

o When the Protocol is OSPF, the NP-Flag defined in
Section 5 of [SR-OSPF] MUST be set to 0.

o When the Protocol is IS-IS, the P-Flag defined in
Section 6.1 of [SR-IS-IS] MUST be set to 0.

}

If it can be determined that no protocol associated with the
Interface-I would have advertised the FEC-Type at FEC-stack-
depth, set the Best-return-code to 12, "Protocol not
associated with interface at FEC-stack-depth" and return.

Set FEC-Status to 1 and return.

}

Else, if the Label-stack-depth is greater than 0 and the Target
FEC Stack sub-TLV at FEC-stack-depth is 34 (IPv4 IGP-Prefix
Segment ID), {

Set the Best-return-code to 10 if any below conditions fail:

- Validate that the Node Segment ID is advertised for the
IPv4 Prefix by the IGP protocol {

o When the Protocol field in the received IPv4 IGP-
Prefix Segment ID sub-TLV is 0, use any locally
enabled IGP protocol.

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o When the Protocol field in the received IPv4 IGP-
Prefix Segment ID sub-TLV is 1, use OSPF as the IGP
protocol.

o When the Protocol field in the received IPv4 IGP-
Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP
protocol.

o When the Protocol field in the received IPv4 IGP-
Prefix Segment ID sub-TLV is an unrecognized value, it
MUST be treated as a Protocol value of 0.

}

If it can be determined that no protocol associated with
Interface-I would have advertised the FEC-Type at FEC-stack-
depth, set the Best-return-code to 12, "Protocol not
associated with interface at FEC stack-depth" and return.

Set FEC-Status to 1 and return.

}

Else, if the Label-stack-depth is 0 and the Target FEC sub-TLV
at FEC-stack-depth is 35 (IPv6 IGP-Prefix Segment ID), {

Set the Best-return-code to 10 if any of the below
conditions fail:

/* The LSR needs to check if it is being a tail-end for the
LSP and have the prefix advertised with the PHP bit set*/

- Validate that the Node Segment ID is advertised for the
IPv6 Prefix by the IGP protocol {

o When the Protocol field in the received IPv6 IGP-
Prefix Segment ID sub-TLV is 0, use any locally
enabled IGP protocol.

o When the Protocol field in the received IPv6 IGP-
Prefix Segment ID sub-TLV is 1, use OSPF as the IGP
protocol.

o When the Protocol field in the received IPv6 IGP-
Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP
protocol.

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o When the Protocol field in the received IPv6 IGP-
Prefix Segment ID sub-TLV is an unrecognized value, it
MUST be treated as a Protocol value of 0.

}

- Validate that the Node Segment ID is advertised with the
No-PHP flag. {

o When the Protocol is OSPF, the NP-flag defined in
Section 5 of [SR-OSPFV3] MUST be set to 0.

o When the Protocol is IS-IS, the P-Flag defined in
Section 6.1 of [SR-IS-IS] MUST be set to 0.

}

If it can be determined that no protocol associated with
Interface-I would have advertised the FEC-Type at FEC-stack-
depth, set the Best-return-code to 12, "Protocol not
associated with interface at FEC stack-depth" and return.

Set the FEC-Status to 1 and return.

}

Else, if the Label-stack-depth is greater than 0 and the Target
FEC sub-TLV at FEC-stack-depth is 35 (IPv6 IGP-Prefix Segment
ID), {

Set the Best-return-code to 10 if any below conditions fail:

- Validate that the Node Segment ID is advertised for the
IPv4 Prefix by the IGP protocol {

o When the Protocol field in the received IPv6 IGP-
Prefix Segment ID sub-TLV is 0, use any locally
enabled IGP protocol.

o When the Protocol field in the received IPv6 IGP-
Prefix Segment ID sub-TLV is 1, use OSPF as the IGP
protocol.

o When the Protocol field in the received IPv6 IGP-
Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP
protocol.

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RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

o When the Protocol field in the received IPv6 IGP-
Prefix Segment ID sub-TLV is an unrecognized value, it
MUST be treated as a Protocol value of 0.

}

If it can be determined that no protocol associated with
Interface-I would have advertised the FEC-Type at FEC-stack-
depth, set the Best-return-code to 12, "Protocol not
associated with interface at FEC stack-depth" and return.

Set the FEC-Status to 1 and return.

}

Else, if the Target FEC sub-TLV at FEC-stack-depth is 36
(IGP-Adjacency Segment ID), {

Set the Best-return-code to 35 (Section 9.5) if any below
conditions fail:

When the Adj. Type is 1 (Parallel Adjacency):

o Validate that the Receiving Node Identifier is the
local IGP identifier.

o Validate that the IGP-Adjacency Segment ID is
advertised by the Advertising Node Identifier of the
Protocol in the local IGP database {

* When the Protocol field in the received IGP-
Adjacency Segment ID sub-TLV is 0, use any locally
enabled IGP protocol.

* When the Protocol field in the received IGP-
Adjacency Segment ID sub-TLV is 1, use OSPF as the
IGP protocol.

* When the Protocol field in the received IGP-
Adjacency Segment ID sub-TLV is 2, use IS-IS as the
IGP protocol.

* When the Protocol field in the received IGP-
Adjacency Segment ID sub-TLV is an unrecognized
value, it MUST be treated as a Protocol value of 0.

}

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When the Adj. Type is 4 or 6 (IGP Adjacency or LAN
Adjacency):

o Validate that the Remote Interface ID matches the
local identifier of the interface (Interface-I) on
which the packet was received.

o Validate that the Receiving Node Identifier is the
local IGP identifier.

o Validate that the IGP-Adjacency Segment ID is
advertised by the Advertising Node Identifier of
Protocol in the local IGP database {

* When the Protocol field in the received IGP-
Adjacency Segment ID sub-TLV is 0, use any locally
enabled IGP protocol.

* When the Protocol field in the received IGP-
Adjacency Segment ID sub-TLV is 1, use OSPF as the
IGP protocol.

* When the Protocol field in the received IGP-
Adjacency Segment ID sub-TLV is 2, use IS-IS as the
IGP protocol.

* When the Protocol field in the received IGP-
Adjacency Segment ID sub-TLV is an unrecognized
value, it MUST be treated as a Protocol value of 0.

}

Set the FEC-Status to 1 and return.

}

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RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

7.5. TTL Consideration for Traceroute

The LSP Traceroute operation can properly traverse every hop of the
SR network for the Uniform Model as described in [RFC3443]. If one
or more LSRs employ a Short Pipe Model, as described in [RFC3443],
then the LSP Traceroute may not be able to properly traverse every
hop of the SR network due to the absence of TTL copy operation when
the outer label is popped. The Short Pipe is one of the most
commonly used models. The following TTL manipulation technique MAY
be used when the Short Pipe Model is used.

When tracing an LSP according to the procedures in [RFC8029], the TTL
is incremented by one in order to trace the path sequentially along
the LSP. However, when a source-routed LSP has to be traced, there
are as many TTLs as there are labels in the stack. The LSR that
initiates the traceroute SHOULD start by setting the TTL to 1 for the
tunnel in the LSP's label stack it wants to start the tracing from,
the TTL of all outer labels in the stack to the max value, and the
TTL of all the inner labels in the stack to zero. Thus, a typical
start to the traceroute would have a TTL of 1 for the outermost label
and all the inner labels would have a TTL of 0. If the FEC Stack TLV
is included, it should contain only those for the inner-stacked
tunnels. The Return Code/Subcode and FEC Stack Change TLV should be
used to diagnose the tunnel as described in [RFC8029]. When the
tracing of a tunnel in the stack is complete, then the next tunnel in
the stack should be traced. The end of a tunnel can be detected from
the Return Code when it indicates that the responding LSR is an
egress for the stack at depth 1. Thus, the traceroute procedures in
[RFC8029] can be recursively applied to traceroute a source-routed
LSP.

8. Backward Compatibility with Non-SR Devices

[INTEROP] describes how SR operates in a network where SR-capable and
non-SR-capable nodes coexist. In such networks, there may not be any
FEC mapping in the responder when the initiator is SR-capable, while
the responder is not (or vice-versa). But this is not different from
RSVP and LDP interoperation scenarios. When LSP Ping is triggered,
the responder will set the FEC-return-code to Return 4, "Replying
router has no mapping for the FEC at stack-depth".

Similarly, when an SR-capable node assigns Adj-SID for a non-SR-
capable node, the LSP traceroute may fail as the non-SR-capable node
is not aware of the "IGP Adjacency Segment ID" sub-TLV and may not
reply with the FEC Stack Change sub-TLVs. This may result in any
further downstream nodes replying back with a Return Code of 4,
"Replying router has no mapping for the FEC at stack-depth".

Kumar, et al. Standards Track [Page 19]

RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

9. IANA Considerations

9.1. New Target FEC Stack Sub-TLVs

IANA has assigned three new sub-TLVs from the "sub-TLVs for TLV Types
1, 16, and 21" subregistry of the "Multi-Protocol Label Switching
(MPLS) Label Switched Paths (LSPs) Ping Parameters" registry [IANA].

Sub-Type Sub-TLV Name Reference
-------- ----------------- ------------
34 IPv4 IGP-Prefix Segment ID Section 5.1
35 IPv6 IGP-Prefix Segment ID Section 5.2
36 IGP-Adjacency Segment ID Section 5.3

9.2. Protocol in the Segment ID Sub-TLV

IANA has created a new "Protocol in the Segment ID sub-TLV" (see
Section 5) registry under the "Multi-Protocol Label Switching (MPLS)
Label Switched Paths (LSPs) Ping Parameters" registry. Code points
in the range of 0-250 will be assigned by Standards Action [RFC8126].
The range of 251-254 is reserved for experimental use and will not be
assigned. The value of 255 is marked "Reserved". The initial
entries into the registry are:

Value Meaning Reference
---------- ---------------- ------------
0 Any IGP protocol This document
1 OSPF This document
2 IS-IS This document

9.3. Adjacency Type in the IGP-Adjacency Segment ID

IANA has created a new "Adjacency Type in the IGP-Adjacency Segment
ID" registry (see Section 5.3) under the "Multi-Protocol Label
Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry. Code points in the range of 0-250 will be assigned by
Standards Action. The range of 251-254 is reserved for experimental
use and will not be assigned. The value of 255 is marked "Reserved".
The initial entries into the registry are:

Value Meaning
---------- ----------------
0 Unnumbered Interface Adjacency
1 Parallel Adjacency
4 IPv4, Non-parallel Adjacency
6 IPv6, Non-parallel Adjacency

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9.4. Protocol in the Label Stack Sub-TLV of the Downstream Detailed
Mapping TLV

IANA has created a new "Protocol in the Label Stack sub-TLV of the
Downstream Detailed Mapping TLV" registry under the "Multi-Protocol
Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry. Code points in the range of 0-250 will be assigned by
Standards Action. The range of 251-254 is reserved for experimental
use and will not be assigned. The value of 255 is marked "Reserved".
The initial entries into the registry are:

Value Meaning Reference
---------- ---------------- ------------
0 Unknown Section 3.4.1.2 of RFC 8029
1 Static Section 3.4.1.2 of RFC 8029
2 BGP Section 3.4.1.2 of RFC 8029
3 LDP Section 3.4.1.2 of RFC 8029
4 RSVP-TE Section 3.4.1.2 of RFC 8029
5 OSPF Section 6 of this document
6 IS-IS Section 6 of this document
7-250 Unassigned
251-254 Reserved for
Experimental Use This document
255 Reserved This document

9.5. Return Code

IANA has assigned a new Return Code from the "Multi-Protocol Label
Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" in the
0-191 (Standards Action) range from the "Return Codes" subregistry.

Value Meaning Reference
---------- ----------------- ------------
35 Mapping for this FEC is not associated Section 7.4 of
with the incoming interface this document

10. Security Considerations

This document defines additional MPLS LSP Ping sub-TLVs and follows
the mechanisms defined in [RFC8029]. All the security considerations
defined in [RFC8029] will be applicable for this document and, in
addition, they do not impose any additional security challenges to be
considered.

Kumar, et al. Standards Track [Page 21]

RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

11. References

11.1. Normative References

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

[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,
<https://www.rfc-editor.org/info/rfc3443>.

[RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
<https://www.rfc-editor.org/info/rfc4203>.

[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
<https://www.rfc-editor.org/info/rfc5307>.

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

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

11.2. Informative References

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

[INTEROP] Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., and
S. Litkowski, "Segment Routing interworking with LDP",
Work in Progress, draft-ietf-spring-segment-routing-ldp-
interop-09, September 2017.

Kumar, et al. Standards Track [Page 22]

RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

[RFC792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.

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

[SR] Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", Work in Progress, draft-ietf-spring-
segment-routing-14, December 2017.

[SR-IS-IS] Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura,
"IS-IS Extensions for Segment Routing", Work in Progress,
draft-ietf-isis-segment-routing-extensions-15, December
2017.

[SR-MPLS] Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing with MPLS
data plane", Work in Progress, draft-ietf-spring-segment-
routing-mpls-11, October 2017.

[SR-OSPF] Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", Work in Progress,
draft-ietf-ospf-segment-routing-extensions-24, December
2017.

[SR-OSPFV3]
Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
Extensions for Segment Routing", Work in Progress,
draft-ietf-ospf-ospfv3-segment-routing-extensions-10,
September 2017.

Kumar, et al. Standards Track [Page 23]

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Acknowledgements

The authors would like to thank Stefano Previdi, Les Ginsberg, Balaji
Rajagopalan, Harish Sitaraman, Curtis Villamizar, Pranjal Dutta,
Lizhong Jin, Tom Petch, Victor Ji, Mustapha Aissaoui, Tony
Przygienda, Alexander Vainshtein, and Deborah Brungard for their
review and comments.

The authors would like to thank Loa Andersson for his comments and
recommendation to merge documents.

Contributors

The following are key contributors to this document:

Hannes Gredler, RtBrick, Inc.
Tarek Saad, Cisco Systems, Inc.
Siva Sivabalan, Cisco Systems, Inc.
Balaji Rajagopalan, Juniper Networks
Faisal Iqbal, Cisco Systems, Inc.

Kumar, et al. Standards Track [Page 24]

RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

Authors' Addresses

Nagendra Kumar (editor)
Cisco Systems, Inc.
7200-12 Kit Creek Road
Research Triangle Park, NC 27709-4987
United States of America

Email: [email protected]

Carlos Pignataro (editor)
Cisco Systems, Inc.
7200-11 Kit Creek Road
Research Triangle Park, NC 27709-4987
United States of America

Email: [email protected]

George Swallow
Southend Technical Center

Email: [email protected]

Nobo Akiya
Big Switch Networks

Email: [email protected]

Sriganesh Kini
Individual

Email: [email protected]

Mach(Guoyi) Chen
Huawei

Email: [email protected]

Kumar, et al. Standards Track [Page 25]