Supporting In Situ Operations, Administration, and Maintenance Using MPLS Network Actions

Supporting In Situ Operations, Administration, and Maintenance Using MPLS Network Actions Cisco Systems, Inc.

Canada rgandhi@cisco.com

Individual

gregimirsky@gmail.com

Futurewei Technologies

United States of America haoyu.song@futurewei.com

Comcast

United States of America Bin_Wen@cable.comcast.com

Comcast

United States of America Voitek_Kozak@comcast.com

Routing MPLS Working Group Internet-Draft IOAM MPLS Network Action In situ Operations, Administration, and Maintenance (IOAM), defined in RFC 9197, collects operational and telemetry information in the packet using IOAM-Data-Fields while the packet traverses a path between two points in the network. Several IOAM Option-Types are available, for example, Pre-allocated Trace, Proof of Transit (POT), Edge-to-Edge (E2E), and Incremental Trace, that can be used to collect information for calculating various performance metrics. RFC 9326 defines the IOAM Direct Export (IOAM-DEX) Option-Type, which is used as a trigger for IOAM data to be directly exported or locally aggregated without being pushed into in-flight data packets. MPLS Network Actions (MNA) techniques indicate actions to be performed on any combination of Label Switched Paths, MPLS packets, and the node itself, and to transport data needed for these actions. This document employs the MNA mechanisms to collect and transport the operational state and telemetry information using IOAM-Data-Fields as well as IOAM-DEX.

Introduction In situ Operations, Administration, and Maintenance (IOAM) collects operational and telemetry information in the packet using IOAM-Data-Fields while the packet traverses a path between two points in the network. Several IOAM Option-Types are available, for example, Pre-allocated Trace, Proof of Transit (POT), Edge-to-Edge (E2E), and Incremental Trace, that can be used to collect information for calculating various performance metrics. Such mechanisms transport the collected information from an IOAM encapsulating node to an IOAM decapsulating node (both typically located at the edge of the IOAM domain within the data path). IOAM Direct Export (IOAM-DEX) is an IOAM Option-Type used as a trigger for IOAM data to be directly exported or locally aggregated without being pushed into in-flight data packets. The exporting method and format are outside the scope of . MPLS Network Actions (MNA) techniques indicate actions to be performed on any combination of Label Switched Paths, MPLS packets, and the node itself, and allow for the transport of data needed for these actions. defines mechanisms for carrying a Network Action Sub-Stack (NAS) as part of the MPLS label stack, i.e., the In-Stack MNA solution. defines mechanisms for carrying MNA and Ancillary Data (AD) below the MPLS label stack, i.e., as the Post-Stack MNA solution. describes various use cases that can be realized using the MNA solution, including the IOAM and IOAM-DEX. This document employs the MNA mechanisms to collect and transport the operational state and telemetry information using IOAM Option-Types, including Pre-allocated Trace, POT, and E2E, as well as IOAM-DEX. The mechanism for exporting the collected information for the IOAM-DEX Option-Type is outside the scope of this document. Also, support for the Incremental Trace IOAM Option-Type is outside the scope of this document.

Conventions Used in This Document

Acronyms The terminology defined in , , and is used in this document. Acronyms

Abbreviation	Meaning	Reference
AD	Ancillary Data
BoS	Bottom of Stack
HbH	Hop-by-Hop
I2E	Ingress to Egress
E2E	Edge to Edge
IHS	I2E, HbH, or Select
IOAM	In situ Operations, Administration, and Maintenance
IOAM-DEX	IOAM Direct Export
IOAM-DEX-MNA-ISD	IOAM Direct Export as MPLS Network Action ISD	This document
ISD	In-Stack Data
LSE	Label Stack Entry
MNA	MPLS Network Action
MPLS	Multiprotocol Label Switching
NAI	Network Action Indicator
NAL	Network Action Length
NAS	Network Action Sub-Stack
NASL	Network Action Sub-Stack Length
OAM	Operations, Administration, and Maintenance
P bit	Post-Stack Data Presence bit
POT	Proof of Transit
RLD	Readable Label Depth
PFN	Post-Stack First Nibble
PSD	Post-Stack Data
PSMH	Post-Stack MPLS Header
S bit	Bottom of Stack bit
TC	Traffic Class
TTL	Time to Live
U bit	Unknown Network Action Handling bit

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

Applicability of IOAM and IOAM-DEX in an MPLS Network Pre-allocated Trace, POT, and E2E IOAM Option-Types use user packets to collect and transport the operational state and telemetry information. This document defines the Post-Stack MNA solution supporting Pre-allocated Trace, POT, and E2E IOAM Option-Types (). However, for some use cases, e.g., mobile backhaul, in which network resources are closely controlled, collecting and transporting telemetry information within a user packet may increase the complexity of network operations. In such environments, IOAM nodes can collect and export the operational state and telemetry information using IOAM-DEX as defined in . IOAM-DEX collects the on-path telemetry information defined as IOAM data in . In this document, IOAM-DEX is realized as In-Stack Data in and as Post-Stack Data in . The procedure defined in this document for carrying IOAM Option-Types using the MNA solution can be applied to user traffic packets and active measurement test packets. uses the mechanisms defined in this document to transport IOAM Option-Types using the MNA solution, with Simple Two-Way Active Measurement Protocol (STAMP) test packets for Hop-by-Hop (HbH) and E2E measurements. also describes active measurement methods where the mechanisms defined in this document can carry IOAM Option-Types for on-path telemetry in MPLS networks using MNA.

Realization of IOAM and IOAM-DEX as MPLS Network Actions

Realization of IOAM and IOAM-DEX as Post-Stack MNA The presence of a corresponding Post-Stack MPLS Header (PSMH) is indicated using the P bit in the NAS as defined in . An example of a NAS and the corresponding PSMH carrying IOAM and IOAM-DEX Option-Types is shown in .

An Example of In-Stack Network Action Sub-Stack for Post-Stack MPLS Header Carrying IOAM and IOAM-DEX The MNA Label (value 4) is defined in . S (1-bit): The BoS . P (1-bit): The P bit is set as specified in . The NAL and NASL fields are set as specified in . Opcode (7-bit): The opcode is set as specified in . The Data (13-bit, next to the Opcode field): The data is set as specified in . The U bit is set as specified in for In-Stack Network Actions and updated in for Post-Stack Network Actions. The IHS scope field is set as defined in and updated in to indicate the I2E, HbH, or Select processing scope for the Post-Stack Network Action and the associated Post-Stack Data when the P bit is set. If both edge and intermediate nodes need to process the IOAM Option-Types, then the IHS scope MUST be set to "HbH, value 0x1". If only edge nodes need to process the IOAM Option-Types, then the IHS scope MUST be set to "I2E, value 0x0". The I2E scope allows skipping IOAM processing on the intermediate nodes, i.e., it avoids the need to parse all IOAM Option-Types to detect the one that requires HbH processing.

MPLS Network Action Post-Stack MPLS Header for IOAM and IOAM-DEX An example encoding of PSMH carrying IOAM and IOAM-DEX in Post-Stack Data (PSD) is shown in .

An Example of Post-Stack MPLS Header Carrying IOAM and IOAM-DEX The Post-Stack MPLS Base Header is added and contains the following fields as defined in .

PFN (4-bit):: The Post-Stack First Nibble (PFN) (value 0x0) as defined in .
PSMH-Len (8-bit):: The PSMH total length in units of 4 octets as defined in . The value excludes the first 4-octet unit for Post-Stack MPLS Base Header. When carrying N IOAM and IOAM-DEX network actions, PSMH-Len equals (1 + PS-NAL) * N.
Type (16-bit):: Type for MNA Post-Stack Header (value 1) as defined in .

The PSMH is added after the Post-Stack MPLS Base Header and contains the Post-Stack Network Action Opcode for IOAM and IOAM-DEX, the length in 4-octet units, and the IOAM Option-Type with IOAM-Data-Fields in the Post-Stack ancillary data as shown in . The IOAM-Data-Fields MUST follow the definitions corresponding to their IOAM Option-Types (e.g., see Section 4 of and the IOAM-DEX Option-Type in Section 3.2 of ). An IOAM Option-Type is added to the PSMH and contains the following fields:

MNA-PS-OP (7-bit):: Set to Opcode TBA1 (Network Action Opcode for IOAM and IOAM-DEX in PSD) for the IOAM Option-Type defined in , and the IOAM-DEX Option-Type defined in .
R (3 x 1-bit):: Three reserved bits appear in the PSMH row: two bits following MNA-PS-OP and one bit preceding IOAM-Opt-Type. All reserved bits MUST be set to 0 on transmission and MUST be ignored on receipt.
IOAM-Opt-Type (7-bit):: The field to carry the IOAM Option-Type, as defined in the "IOAM Option-Type Registry" in and .
PS-NAL (7-bit):: Unsigned integer representing the length of the IOAM-Data-Fields in 4-octet units for the MNA-PS-OP. The value excludes the first 4-octet unit.
IOAM Option-Type and Data Space:: IOAM-Data-Fields as specified by the IOAM-Opt-Type field, beginning from the Namespace-ID field. IOAM-Data-Fields are defined according to the IOAM Option-Type (e.g., see Section 4 of and Section 3.2 of ).
Block-Number (8-bit):: The Block-Number is used for the alternate marking method to aggregate the IOAM-Data-Fields collected in the data plane and to correlate IOAM-Data-Fields exported from different nodes along the packet path, e.g., to compute measurement metrics for each block of a data flow. When the Flow ID field is present in the IOAM-Data-Fields in the PSMH, the Block-Number is scoped per data flow within the IOAM Namespace identified by the Namespace-ID field. When the Flow ID field is absent, the Block-Number is scoped per IOAM Namespace (identified by the Namespace-ID field) and applies to all flows sharing that Namespace. The Block-Number MUST be initialized to 0 at the start of operation and wraps from 255 back to 0 on overflow. It is incremented sequentially at every measurement interval configured at the encapsulating node. When the alternate marking method is not used, the Block-Number SHOULD be set to 0.

Multiple IOAM and IOAM-DEX Option-Types in PSMH An example of multiple Post-Stack Network Actions with the same scope carrying different IOAM Option-Types is shown in .

Example of Multiple Post-Stack Network Actions with IOAM and IOAM-DEX with the Same Scope

Realization of IOAM-DEX as In-Stack MNA This document adopts the IOAM-DEX Option-Type format defined in to support IOAM-DEX in an MPLS network using MNA In-Stack Data (ISD) . To support the direct export of the operational state and telemetry information, the IOAM-DEX-MNA-ISD Binary Large OBject (blob) is placed as part of the ISD block in an MPLS label stack according to the MNA encoding procedures defined in . By setting the IHS field, the IOAM-DEX-MNA-ISD can be configured to operate in HbH, I2E, or Select modes to collect the operational state and telemetry information using an MNA Opcode ().

An Example of IOAM-DEX Encapsulation as an MNA Opcode Here, the enclosed elements are defined as follows:

The MNA Label (value 4) is defined in .
S (1-bit): The BoS .
P (1-bit): The P bit is set as specified in .
The IHS, U bit, and NAL fields are set as specified in .
NASL (4-bit): The number of LSEs that compose the IOAM-DEX-MNA-ISD blob.
Opcode (7-bit): Set to TBA2 for the Network Action Opcode for IOAM-DEX in ISD.
Data (13-bit): Data field is not used and MUST be set to 0.
IOAM-DEX-MNA-ISD: IOAM-DEX in MPLS Network Action ISD encoding.

IOAM-DEX MAY be carried simultaneously as both In-Stack Data using Opcode TBA2 and Post-Stack Data using Opcode TBA1 in the same MPLS packet. Each Network Action operates independently: TBA2 triggers direct export at nodes processing the In-Stack NAS, while TBA1 triggers direct export at nodes processing the Post-Stack NAS. The Opcodes TBA1 and TBA2 MUST NOT be carried in the same NAS; otherwise the packet MUST be treated as malformed and MUST be dropped.

IOAM-DEX Option-Type Encoding as MNA ISD The IOAM-DEX-MNA-ISD blob in a NAS uses the LSE Format D, as defined in Section 4.4 of , that maps to the IOAM-DEX Option-Type format . In addition to the requirement to preserve the S bit, the most significant bit in LSE Format D is always set to 1, avoiding a possible mix-up of the LSE with one of the Base Special Purpose Labels . The format of the IOAM-DEX-MNA-ISD blob in an NAS is shown in .

IOAM-DEX Option-Type Format for MPLS Network Action ISD Where fields are defined as follows:

Namespace-ID (16-bit): Identifier of the IOAM Namespace, as defined in Section 3.2 of .
Reserved (6-bit): The Reserved field MUST be set to 0 on transmit and ignored on receipt.
Flags (8-bit): The flags field comprises eight one-bit subfields. The subfields in the Flags field are allocated by IANA in the IOAM DEX Flags registry, as defined in Section 4.2 of .
IOAM-Trace-Type-MNA (22-bit): The IOAM-Trace-Type-MNA corresponds to the IOAM-Trace-Type field defined in Section 3.2 of . The interpretation of bit positions, from bit 0 through bit 21, is as specified in . Note that bits as specified in are right-shifted one bit, so for example, bit 0 in (leftmost bit for hop_Lim and node_id in short format) is mapped to bit 1 (second leftmost bit) in the MNA encoding. The registry that contains the assigned code points is found in IANA's IOAM Trace-Type registry .
O (1-bit): This is a one-bit flag identical to the interpretation of bit 22 of the IOAM-Trace-Type field as defined in Section 3.2 of , marked as "variable-length Opaque State Snapshot" per ; assigned code points are found in IANA's IOAM Trace-Type registry .
R (1-bit): This is a one-bit flag identical to the interpretation of bit 23 of the IOAM-Trace-Type field as defined in Section 3.2 of , marked as "reserved" per ; assigned code points are found in IANA's IOAM Trace-Type registry . Similarly to , the R bit is reserved to allow for future extensions of the IOAM-Trace-Type-MNA bit field, i.e., it indicates the presence of the extended IOAM-Trace-Type-MNA field in the next LSE in Format D. The format of that extended LSE is outside the scope of this document and reserved for future specification.
The concatenation of the 22-bit IOAM-Trace-Type-MNA field with O and R flags, explained above, is identical to the 24-bit IOAM-Trace-Type field in Section 3.2 of , and in the interpretation of those bits as defined in and assigned code points found in IANA's IOAM Trace-Type registry .
Ext-Flags (6-bit): This field comprises six one-bit subfields. The allocation of the subfields in the Ext-Flags field is according to Section 4.3 of . Four flags have been assigned by IANA: two as defined in Section 4.3 of indicating the presence of the optional Flow ID MNA and Sequence Number MNA fields, and two as defined in Section 4.1 of indicating the presence of the optional Multicast Branch ID field. The length of the Ext-Flags field in the IOAM-DEX Option-Type in MNA is shorter by two one-bit fields compared to the length of the Extension Flags field defined in Section 3.2 of . In the 6-bit Ext-Flags field of the IOAM-DEX-MNA-ISD header, four flags have been mapped to those IANA assigned flags in the IOAM-DEX Option-Type, and two flags (corresponding to unassigned bits in the RFC 9326 Extension-Flags field) remain unassigned. displays the detailed format of the Ext-Flags field.
In the IOAM-DEX Option-Type defined in , the IOAM-Trace-Type and Reserved fields together form the second 32-bit word, aligning the optional fields to 4-octet boundaries. In the case of the IOAM-DEX in MNA, for optional fields such as Flow ID MNA and Sequence Number MNA, such alignment is achieved without adding such Reserved field as padding.
Flow ID MNA: An optional 4-octet field containing a 30-bit Flow ID (after removing leading 1 and S bits). The semantics of the Flow ID MNA field are as those of the Flow ID field defined in Section 3.2 of , with the following differences. The Flow ID MNA field carries a 30-bit value, compared to the 32-bit Flow ID field in ; therefore, the Flow ID space differs accordingly, and the 30-bit value will wrap around faster than the 32-bit value. The most significant bit MUST be set to 1. The bit 23 MUST be set according to the definition of the S bit in .
Sequence Number MNA: An optional 4-octet field containing a 30-bit sequence number (after removing leading 1 and S bits). The semantics of the Sequence Number MNA field are as those of the Sequence Number field defined in Section 3.2 of , with the following differences. The Sequence Number MNA field carries a 30-bit value, compared to the 32-bit Sequence Number field in ; therefore, the sequence number space differs accordingly, and the 30-bit value will wrap around faster than the 32-bit value. The most significant bit MUST be set to 1. The bit 23 MUST be set according to the definition of the S bit in . In MPLS networks, when it is known that the label stack information is used for load-balancing flows, the 19-bit-long part of the Sequence Number MNA, starting from the bit 1 position of the LSE, MUST remain immutable for a particular packet flow that the value of the Flow ID MNA field identifies. In MPLS networks, when it is known that other load-balancing techniques are used, all bits of the Sequence Number MNA field can be varied. Mechanisms to learn the load-balancing techniques used by the nodes in MPLS networks are outside the scope of this document.

Ext-Flags Field Format Where fields are defined as follows:

F (1-bit): One-bit flag. When the F flag is set to 1, it indicates the presence of the Flow ID field in the IOAM-DEX-MNA-ISD header.
Q (1-bit): One-bit flag. When the Q flag is set to 1, it indicates the presence of the Sequence Number field in the IOAM-DEX-MNA-ISD header. The designation as Q flag is used to avoid conflicting with the existing S bit for Bottom-of-Stack (BoS) in the MPLS header. This flag corresponds to the Sequence Number flag (labeled "S" in ) in the IOAM DEX Extension-Flags registry.
N (1-bit): One-bit flag. When the N flag is set to 1, it indicates the presence of the Multicast Branching Node ID in the Multicast Branch ID field, as defined in Section 4.1 of .
I (1-bit): One-bit flag. When the I flag is set to 1, it indicates the presence of the Multicast Branching Interface Index in the Multicast Branch ID field, as defined in Section 4.1 of . The N and I flags MUST be both set or both cleared; otherwise the header is considered malformed and the packet MUST be dropped.
A (1-bit): Unassigned one-bit flags. They MUST be set to zero on transmission and MUST be ignored upon receipt.

Considerations for IOAM and IOAM-DEX in MPLS Networks

Ingress-to-Egress Scope IOAM and IOAM-DEX Network Actions The Network Actions with I2E scope carry IOAM Option-Types that require processing on the encapsulating and egress nodes only. The IOAM Option-Type carried can be the IOAM E2E Option-Type (value 3) defined in or the IOAM-DEX Option-Type (value 4) defined in . The Network Actions with I2E scope MUST NOT carry any IOAM Option-Type that requires IOAM processing on the intermediate nodes, as it will not be processed by them when the IHS scope is set to "I2E, value 0x0". The IOAM and IOAM-DEX Network Action procedure with I2E scope is summarized as follows:

The encapsulating node inserts a NAS with the IHS scope set to "I2E, value 0x0", as well as one or more Network Actions for IOAM and IOAM-DEX in the MPLS packet.
The egress node processes the IOAM-Data-Fields using the procedures defined in and . An example of IOAM processing is to export the IOAM-Data-Fields for streaming telemetry.
The egress node MUST remove the Network Actions and IOAM-Data-Fields from the received packet. The decapsulated packet is forwarded downstream similar to the regular data packets or terminated locally.

Hop-by-Hop Scope IOAM and IOAM-DEX Network Actions The Network Actions with HbH scope carry IOAM Option-Types that require processing at the intermediate and/or encapsulating and egress nodes. The IOAM Option-Type carried can be Pre-allocated Trace (value 0), POT (value 2), or E2E (value 3) defined in , or IOAM-DEX (value 4) defined in . Note that the MPLS Network Action defined in this document is not supported for the IOAM Incremental Trace Option-Type (value 1), which requires HbH processing. The IOAM and IOAM-DEX Network Action procedure with HbH scope is summarized as follows:

The encapsulating node inserts a NAS with the IHS scope set to "HbH, value 0x1", as well as one or more Network Actions for IOAM and IOAM-DEX in the MPLS packet.
The intermediate node that is enabled for the IOAM function processes the IOAM-Data-Fields as defined in and when the node recognizes the HbH scope in the NAS.
The intermediate node forwards the data packet downstream.
The processing on the egress node is the same as in the I2E case.

Both HbH and I2E scope Network Actions for IOAM and IOAM-DEX may be carried in the Post-Stack MNA in an MPLS packet. In this case, the PSMH carrying HbH-scoped Network Actions MUST be added after the BoS and before the PSMH carrying I2E-scoped Network Actions. This minimizes the Readable Label Depth (RLD) required for intermediate node processing.

Node Capability An MNA-capable egress node that supports the network action for IOAM and IOAM-DEX may not support the specific IOAM Option-Type or IOAM-Data-Fields carried in the packet. In that case, the egress node MUST handle the Network Action according to the U bit as defined in and updated in . The MNA-capable intermediate node that does not support the Network Action for IOAM and IOAM-DEX, or the IOAM Option-Type or IOAM-Data-Fields carried in the packet, MUST handle the Network Action according to the U bit as defined in and updated in . The intermediate node that does not support the MNA skips processing the IOAM and IOAM-DEX functions. The encapsulating node needs to know if the intermediate and egress nodes can support the IOAM and IOAM-DEX network actions. Information about the IOAM and IOAM-DEX capabilities of the nodes may be configured, collected through management protocols, or distributed by control protocols (such as advertising by routing protocols). The signaling extension for this capability exchange is outside the scope of this document.

Nested MPLS Encapsulation When a packet is received with an MPLS Network Action for IOAM-DEX in ISD or IOAM and IOAM-DEX in PSD, and the nested MPLS encapsulating node needs to add a different Network Action for IOAM-DEX in ISD, the node MUST add a new NAS with the network action for IOAM-DEX ISD as part of the new MPLS encapsulation. When a packet is received with an MPLS header, the nested MPLS encapsulating node MUST NOT add new Network Action for IOAM and IOAM-DEX in PSD. This is because inserting a PSMH in an MPLS-encapsulated packet received from the network is not permitted by .

Readable Label Depth Consideration The encapsulating node needs to ensure that the IOAM Option-Types and their IOAM-Data-Fields and IOAM-DEX data in a NAS and the associated PSMH are added within the RLD of the downstream MNA-capable nodes so that they can process the IOAM-Data-Fields and IOAM-DEX data.

Operational Considerations Operational considerations discussed in and , management considerations discussed in , and management and deployment considerations discussed in apply to this document. The performance considerations discussed in Section 5 of are also applicable here. Additional operational considerations are described in for deploying IOAM and IOAM-DEX in MPLS networks. Further operational considerations also include policies controlling the processing of the collected operational state and telemetry information, and their transport. An implementation MAY collect the following counters:

Packets with IOAM network actions received
Packets with IOAM-DEX network actions received
Packets with IOAM network actions dropped due to unknown IOAM function
Packets with IOAM-DEX network actions dropped due to unknown IOAM-DEX function
Packets with IOAM dropped due to malformed network action
Packets with IOAM-DEX dropped due to malformed network action

Security Considerations Security considerations discussed in , , , , , , and apply to this document. The usage of MPLS Network Actions defined in this document for IOAM and IOAM-DEX is intended for deployment in a single network administrative domain. As such, it assumes that the operator enabling the IOAM and IOAM-DEX operations has previously verified the integrity of the path. However, operators need to properly secure the IOAM and IOAM-DEX in the domain to avoid malicious configuration and use, which could include injecting malicious IOAM and IOAM-DEX packets into the domain.

IANA Considerations

Network Action Opcodes IANA is requested to assign code points from its Network Action Opcodes registry (created in and updated in ) as specified in . Network Action Opcodes

Opcode	Description	In-Stack Only, Post-Stack Only, In-Stack and Post-Stack	Reference
TBA1	Network Action for IOAM and IOAM-DEX in PSD	Post-Stack Only	This document
TBA2	Network Action for IOAM-DEX in ISD	In-Stack Only	This document

References Normative References Informative References IOAM Trace-Type IANA

Appendix A: Examples

Example A1: In-Stack and Post-Stack Network Action Processing Order The PSMH Start Offset Network Action can be added to interleave Post-Stack Network Actions with In-Stack Network Actions. The following example shows how to process the Post-Stack Network Action before some of the In-Stack Network Actions.

Example of Post-Stack and In-Stack Network Action Processing Order In this example, shown in , opcode L is processed first, followed by opcode 1 for Flag-Based NAIs, then opcode TBA3 (PSMH Start Offset defined in ), and the opcode TBA1 in the corresponding PSMH, and finally opcode M. The network action allocation for Opcode L and Opcode M is outside the scope of this document.

Acknowledgments The authors would like to thank Adrian Farrel and Xueyan Song for reviewing this document and providing review comments. The authors would also like to thank Patrick Khordoc, Sagar Soni, Shwetha Bhandari, Vengada Prasad Govindan, Tarek Saad, Stewart Bryant, Xiao Min, Jaganbabu Rajamanickam, and Cheng Li for reviewing the early version of this document. The authors would also like to thank Mach Chen, Andrew Malis, Matthew Bocci, and Nick Delregno for the MPLS-RT expert review of the early version of this document. The authors also thank Matthew Bocci for the early Rtgdir review and Giuseppe Fioccola for the PerfMetrDir review, which helped improve this document.

Contributors The following people have substantially contributed to this document: