Cisco Systems, Inc.

tsaad.net@gmail.com

Independent

kiran.ietf@gmail.com

Futurewei Technologies

haoyu.song@futurewei.com

Ericsson

gregimirsky@gmail.com

RTG mpls Special Purpose Label MPLS data plane This document presents use cases that have a common feature that may be addressed by encoding network action indicators and associated ancillary data within MPLS packets. There is community interest in extending the MPLS data plane to carry such indicators and ancillary data to address these use cases. The use cases described in this document are not an exhaustive set but rather the ones that have been actively discussed by members of the IETF MPLS, PALS, and DetNet Working Groups from the beginning of work on MPLS Network Action (MNA) until the publication of this document.

Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. 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). Not all documents approved by the IESG are candidates for any level of Internet Standard; see 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 .

Copyright Notice Copyright (c) 2025 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 () 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.

Table of Contents

• .  Introduction
  • .  Terminology
  • .  Abbreviations
• .  Use Cases
  • .  No Further Fast Reroute
  • .  Applicability of Hybrid Measurement Methods
    • .  In Situ OAM
    • .  Alternate Marking Method
  • .  Network Slicing
  • .  NSH-Based Service Function Chaining
  • .  Network Programming
• .  Coexistence with the Existing MPLS Services Using Post-Stack Headers
• .  Coexistence of the MNA Use Cases
• .  IANA Considerations
• .  Security Considerations
• .  References
  • .  Normative References
  • .  Informative References
• .  Use Cases for Continued Discussion
  • .  Generic Delivery Functions
  • .  Delay Budgets for Time-Bound Applications
  • .  Stack-Based Methods for Latency Control
• Acknowledgements
• Contributors
• Authors' Addresses

Introduction This document describes use cases that introduce functions that require special processing by forwarding hardware. The current state of the art requires allocating a new Special-Purpose Label (SPL) or Extended Special-Purpose Label (eSPL). SPLs are a very limited resource, while eSPL requires an extra label stack entry per network action, which is expensive. Therefore, an MPLS Network Action (MNA) approach was proposed to extend the MPLS architecture. MNA is expected to enable functions that may require carrying additional ancillary data within the MPLS packets, as well as a means to indicate that the ancillary data is present and a specific action needs to be performed on the packet. This document lists various use cases that could benefit extensively from the MNA framework . Supporting a solution of the general MNA framework provides a common foundation for future network actions that can be exercised in the MPLS data plane.

Terminology The following terminology is used in the document:

RFC 9543 Network Slice:

Interpreted as defined in . This document uses "network slice" interchangeably as a shorter version of the term "RFC 9543 Network Slice".

MPLS Ancillary Data (also referred to in this document as "ancillary data"):

Data that can be classified as:

• residing within the MPLS label stack (referred to as "in-stack data"), and
• residing after the Bottom of Stack (BoS) (referred to as "post-stack data").

Abbreviations

AMM:

Alternative Marking Method

BoS:

Bottom of Stack

DEX:

Direct Export

eSPL:

extended Special-Purpose Label

FRR:

Fast Reroute

G-ACh:

Generic Associated Channel

HbH:

Hop by Hop

I2E:

Ingress to Egress

IOAM:

In situ Operations, Administration, and Maintenance

LSP:

Label Switched Path

LSR:

Label Switching Router

MNA:

MPLS Network Action

NRP:

Network Resource Partition

NSH:

Network Service Header

PW:

Pseudowire

SPL:

Special-Purpose Label

ToS:

Top of Stack

Use Cases

No Further Fast Reroute MPLS Fast Reroute is a useful and widely deployed tool for minimizing packet loss in the case of a link or node failure. Several cases exist where, once a Fast Reroute (FRR) has taken place in an MPLS network and a packet is rerouted away from the failure, a second FRR impacts the same packet on another node and may result in traffic disruption. In such a case, the packet impacted by multiple FRR events may continue to loop between the Label Switching Routers (LSRs) that activated FRR until the packet's TTL expires. That can lead to link congestion and further packet loss. To avoid that situation, packets that FRR has redirected will be marked using MNA to preclude further FRR processing.

Applicability of Hybrid Measurement Methods MNA can be used to carry information essential for collecting operational information and measuring various performance metrics that reflect the experience of the packet marked by MNA. Optionally, the operational state and telemetry information collected on the LSR may be transported using MNA techniques.

In Situ OAM In situ Operations, Administration, and Maintenance (IOAM), defined in and , might be used to collect operational and telemetry information while a packet traverses a particular path in a network domain. IOAM can run in two modes: Ingress to Egress (I2E) and Hop by Hop (HbH). In I2E mode, only the encapsulating and decapsulating nodes will process IOAM data fields. In HbH mode, the encapsulating and decapsulating nodes and intermediate IOAM-capable nodes process IOAM data fields. The IOAM data fields, defined in , can be used to derive the operational state of the network experienced by the packet with the IOAM Header that traversed the path through the IOAM domain. Several IOAM Option-Types have been defined:

• Pre-allocated Trace
• Incremental Trace
• Edge-to-Edge
• Proof-of-Transit
• Direct Export (DEX)

With all IOAM Option-Types except for Direct Export (DEX), the collected information is transported in the trigger IOAM packet. In the IOAM DEX Option-Type , the operational state and telemetry information are collected according to a specified profile and exported in a manner and format defined by a local policy. In IOAM DEX, the user data packet is only used to trigger the IOAM data to be directly exported or locally aggregated without being carried in the IOAM trigger packets.

Alternate Marking Method The Alternate Marking Method (AMM), defined in and ), is an example of a hybrid performance measurement method that can be used in the MPLS network to measure packet loss and packet delay performance metrics. defines the Synonymous Flow Label framework to realize AMM in the MPLS network. The MNA is an alternative mechanism that can be used to support AMM in the MPLS network.

Network Slicing An RFC 9543 Network Slice Service provides connectivity coupled with network resource commitments and is expressed in terms of one or more connectivity constructs. defines a Network Resource Partition (NRP) Policy as a policy construct that enables the instantiation of mechanisms to support one or more network slice services. The packets associated with an NRP may carry a marking in their network-layer header to identify this association, which is referred to as an NRP Selector. The NRP Selector maps a packet to the associated network resources and provides the corresponding forwarding treatment onto the packet. A router that requires the forwarding of a packet that belongs to an NRP may have to decide on the forwarding action to take based on selected next hop(s) and decide on the forwarding treatment (e.g., scheduling and drop policy) to enforce based on the associated per-hop behavior. In this case, routers that forward traffic over a physical link shared by multiple NRPs need to identify the NRP to which the packet belongs to enforce their respective forwarding actions and treatments. MNA technologies can signal actions for MPLS packets and carry data essential for these actions. For example, MNA can carry the NRP Selector in MPLS packets.

NSH-Based Service Function Chaining describes how Service Function Chaining can be realized in an MPLS network by emulating the Network Service Header (NSH) using only MPLS label stack entries. The approach in introduces some limitations, which are discussed in . However, the approach can benefit from the MNA framework introduced in . MNA can be used to extend NSH emulation using MPLS labels to support the functionality of NSH Context Headers, whether fixed or variable length. For example, MNA could support Flow ID that may be used for load-balancing among Service Function Forwarders and/or the Service Functions within the same Service Function Path.

Network Programming In Segment Routing (SR), an ingress node steers a packet through an ordered list of instructions called "segments". Each of these instructions represents a function to be called at a specific location in the network. A function is locally defined on the node where it is executed and may range from simply moving forward in the segment list to any complex user-defined behavior. Network Programming combines SR functions to achieve a networking objective beyond mere packet routing. Encoding a pointer to a function and its arguments within an MPLS packet transport header may be desirable. MNA can be used to encode the FUNC::ARGs to support the functional equivalent of FUNC::ARG in Segment Routing over IPv6 as described in .

Coexistence with the Existing MPLS Services Using Post-Stack Headers Several services can be transported over MPLS networks today. These include providing Layer 3 (L3) connectivity (e.g., for unicast and multicast L3 services) and Layer 2 (L2) connectivity (e.g., for unicast PWs, multicast E-Tree, and broadcast Ethernet LAN (E-LAN) L2 services). In those cases, the user service traffic is encapsulated as the payload in MPLS packets. For L2 service traffic, it is possible to use a Control Word (CW) immediately after the MPLS header to disambiguate the type of MPLS payload, prevent possible packet misordering, and allow for fragmentation. In this case, the first nibble of the data that immediately follows the MPLS BoS is set to 0b0000 to identify the presence of the PW CW. In addition to providing connectivity to user traffic, MPLS may also transport OAM data (e.g., over MPLS Generic Associated Channels (G-AChs) ). In this case, the first nibble of the data that immediately follows the MPLS BoS is set to 0b0001. It indicates the presence of a control channel associated with a PW, LSP, or section. Bit Index Explicit Replication (BIER) traffic can also be encapsulated over MPLS. In this case, BIER has defined 0b0101 as the value for the first nibble of the data that immediately appears after the BoS for any BIER-encapsulated packet over MPLS. For PWs, the G-ACh uses the first four bits of the PW control word to provide the initial discrimination between data packets and packets belonging to the associated channel, as described in . MPLS can be used as the data plane for Deterministic Networking (DetNet) . The DetNet sub-layers, forwarding, and service are realized using the MPLS label stack, the DetNet control word , and the DetNet Associated Channel Header . MNA-based solutions for the use cases described in this document and proposed in the future are expected to allow for coexistence and backward compatibility with all existing MPLS services.

Coexistence of the MNA Use Cases Two or more of the discussed cases may coexist in the same packet. That may require the presence of multiple ancillary data (whether in-stack or post-stack ancillary data) to be present in the same MPLS packet. For example, IOAM may provide essential functions along with network slicing to help ensure that critical network slice Service Level Objectives (SLOs) are being met by the network provider. In this case, IOAM can collect key performance measurement parameters of a network slice traffic flow as it traverses the transport network.

IANA Considerations This document has no IANA actions.

Security Considerations outlines security considerations for documents that do not specify protocols. The authors have verified that these considerations are fully applicable to this document. In-depth security analysis for each specific use case is beyond the scope of this document and will be addressed in future solution documents. It is strongly recommended that these solution documents undergo review by a security expert early in their development, ideally during the Working Group Last Call phase.

References Normative References Huawei Technologies University of Surrey 5GIC Nokia Juniper Networks Informative References Juniper Networks Juniper Networks Juniper Networks ZTE Some functionalities (e.g., fragmentation/reassembly and Encapsulating Security Payload) provided by IPv6 can be viewed as delivery functions independent of IPv6 or even IP entirely. This document proposes to provide those functionalities at different layers (e.g., MPLS, BIER or even Ethernet) independent of IP. Work in Progress Cisco Systems Inc. Juniper Networks Huawei Technologies Ericsson ZTE Corporation Realizing network slices may require the Service Provider to have the ability to partition a physical network into multiple logical networks of varying sizes, structures, and functions so that each slice can be dedicated to specific services or customers. Multiple network slices can be realized on the same network while ensuring slice elasticity in terms of network resource allocation. This document describes a scalable solution to realize network slicing in IP/MPLS networks by supporting multiple services on top of a single physical network by relying on compliant domains and nodes to provide forwarding treatment (scheduling, drop policy, resource usage) on to packets that carry identifiers that indicate the slicing service that is to be applied to the packets. Work in Progress This document specifies the encoding to be used by an LSR in order to transmit labeled packets on Point-to-Point Protocol (PPP) data links, on LAN data links, and possibly on other data links as well. This document also specifies rules and procedures for processing the various fields of the label stack encoding. [STANDARDS-TRACK] This document defines RSVP-TE extensions to establish backup label-switched path (LSP) tunnels for local repair of LSP tunnels. These mechanisms enable the re-direction of traffic onto backup LSP tunnels in 10s of milliseconds, in the event of a failure. Two methods are defined here. The one-to-one backup method creates detour LSPs for each protected LSP at each potential point of local repair. The facility backup method creates a bypass tunnel to protect a potential failure point; by taking advantage of MPLS label stacking, this bypass tunnel can protect a set of LSPs that have similar backup constraints. Both methods can be used to protect links and nodes during network failure. The described behavior and extensions to RSVP allow nodes to implement either method or both and to interoperate in a mixed network. [STANDARDS-TRACK] This document describes the preferred design of a Pseudowire Emulation Edge-to-Edge (PWE3) Control Word to be used over an MPLS packet switched network, and the Pseudowire Associated Channel Header. The design of these fields is chosen so that an MPLS Label Switching Router performing MPLS payload inspection will not confuse a PWE3 payload with an IP payload. [STANDARDS-TRACK] This document describes Virtual Circuit Connectivity Verification (VCCV), which provides a control channel that is associated with a pseudowire (PW), as well as the corresponding operations and management functions (such as connectivity verification) to be used over that control channel. VCCV applies to all supported access circuit and transport types currently defined for PWs. [STANDARDS-TRACK] This document describes the use of loop-free alternates to provide local protection for unicast traffic in pure IP and MPLS/LDP networks in the event of a single failure, whether link, node, or shared risk link group (SRLG). The goal of this technology is to reduce the packet loss that happens while routers converge after a topology change due to a failure. Rapid failure repair is achieved through use of precalculated backup next-hops that are loop-free and safe to use until the distributed network convergence process completes. This simple approach does not require any support from other routers. The extent to which this goal can be met by this specification is dependent on the topology of the network. [STANDARDS-TRACK] This document generalizes the applicability of the pseudowire (PW) Associated Channel Header (ACH), enabling the realization of a control channel associated to MPLS Label Switched Paths (LSPs) and MPLS Sections in addition to MPLS pseudowires. In order to identify the presence of this Associated Channel Header in the label stack, this document also assigns one of the reserved MPLS label values to the Generic Associated Channel Label (GAL), to be used as a label based exception mechanism. The MPLS Generic Associated Channel (G-ACh) provides an auxiliary logical data channel associated with a Label Switched Path (LSP), a pseudowire, or a section (link) over which a variety of protocols may flow. These protocols are commonly used to provide Operations, Administration, and Maintenance (OAM) mechanisms associated with the primary data channel. This document specifies simple procedures by which an endpoint of an LSP, pseudowire, or section may inform the other endpoints of its capabilities and configuration parameters, or other application-specific information. This information may then be used by the receiver to validate or adjust its local configuration, and by the network operator for diagnostic purposes. This document describes an extension to the basic IP fast reroute mechanism, described in RFC 5286, that provides additional backup connectivity for point-to-point link failures when none can be provided by the basic mechanisms. This memo provides clear definitions for Active and Passive performance assessment. The construction of Metrics and Methods can be described as either "Active" or "Passive". Some methods may use a subset of both Active and Passive attributes, and we refer to these as "Hybrid Methods". This memo also describes multiple dimensions to help evaluate new methods as they emerge. Bit Index Explicit Replication (BIER) is an architecture that provides optimal multicast forwarding through a "multicast domain", without requiring intermediate routers to maintain any per-flow state or to engage in an explicit tree-building protocol. When a multicast data packet enters the domain, the ingress router determines the set of egress routers to which the packet needs to be sent. The ingress router then encapsulates the packet in a BIER header. The BIER header contains a bit string in which each bit represents exactly one egress router in the domain; to forward the packet to a given set of egress routers, the bits corresponding to those routers are set in the BIER header. The details of the encapsulation depend on the type of network used to realize the multicast domain. This document specifies a BIER encapsulation that can be used in an MPLS network or, with slight differences, in a non-MPLS network. This document describes a Network Service Header (NSH) imposed on packets or frames to realize Service Function Paths (SFPs). The NSH also provides a mechanism for metadata exchange along the instantiated service paths. The NSH is the Service Function Chaining (SFC) encapsulation required to support the SFC architecture (defined in RFC 7665). This document describes how Service Function Chaining (SFC) can be achieved in an MPLS network by means of a logical representation of the Network Service Header (NSH) in an MPLS label stack. That is, the NSH is not used, but the fields of the NSH are mapped to fields in the MPLS label stack. This approach does not deprecate or replace the NSH, but it acknowledges that there may be a need for an interim deployment of SFC functionality in brownfield networks. This document provides the overall architecture for Deterministic Networking (DetNet), which provides a capability to carry specified unicast or multicast data flows for real-time applications with extremely low data loss rates and bounded latency within a network domain. Techniques used include 1) reserving data-plane resources for individual (or aggregated) DetNet flows in some or all of the intermediate nodes along the path of the flow, 2) providing explicit routes for DetNet flows that do not immediately change with the network topology, and 3) distributing data from DetNet flow packets over time and/or space to ensure delivery of each packet's data in spite of the loss of a path. DetNet operates at the IP layer and delivers service over lower-layer technologies such as MPLS and Time- Sensitive Networking (TSN) as defined by IEEE 802.1. RFC 8372 ("MPLS Flow Identification Considerations") describes the requirement for introducing flow identities within the MPLS architecture. This document describes a method of accomplishing this by using a technique called "Synonymous Flow Labels" in which labels that mimic the behavior of other labels provide the identification service. These identifiers can be used to trigger per-flow operations on the packet at the receiving label switching router. This document specifies the Deterministic Networking (DetNet) data plane when operating over an MPLS Packet Switched Network. It leverages existing pseudowire (PW) encapsulations and MPLS Traffic Engineering (MPLS-TE) encapsulations and mechanisms. This document builds on the DetNet architecture and data plane framework. The Segment Routing over IPv6 (SRv6) Network Programming framework enables a network operator or an application to specify a packet processing program by encoding a sequence of instructions in the IPv6 packet header. Each instruction is implemented on one or several nodes in the network and identified by an SRv6 Segment Identifier in the packet. This document defines the SRv6 Network Programming concept and specifies the base set of SRv6 behaviors that enables the creation of interoperable overlays with underlay optimization. In situ Operations, Administration, and Maintenance (IOAM) collects operational and telemetry information in the packet while the packet traverses a path between two points in the network. This document discusses the data fields and associated data types for IOAM. IOAM-Data-Fields can be encapsulated into a variety of protocols, such as Network Service Header (NSH), Segment Routing, Generic Network Virtualization Encapsulation (Geneve), or IPv6. IOAM can be used to complement OAM mechanisms based on, e.g., ICMP or other types of probe packets. Service Function Chaining (SFC) uses the Network Service Header (NSH) (RFC 8300) to steer and provide context metadata (MD) with each packet. Such metadata can be of various types, including MD Type 2, consisting of Variable-Length Context Headers. This document specifies several such Context Headers that can be used within a Service Function Path (SFP). In situ Operations, Administration, and Maintenance (IOAM) is used for recording and collecting operational and telemetry information. Specifically, IOAM allows telemetry data to be pushed into data packets while they traverse the network. This document introduces a new IOAM option type (denoted IOAM-Option-Type) called the "IOAM Direct Export (DEX) Option-Type". This Option-Type is 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 this document. This document describes the Alternate-Marking technique to perform packet loss, delay, and jitter measurements on live traffic. This technology can be applied in various situations and for different protocols. According to the classification defined in RFC 7799, it could be considered Passive or Hybrid depending on the application. This document obsoletes RFC 8321. This document generalizes and expands the Alternate-Marking methodology to measure any kind of unicast flow whose packets can follow several different paths in the network; this can result in a multipoint-to-multipoint network. The network clustering approach is presented and, for this reason, the technique described here is called "Clustered Alternate Marking". This document obsoletes RFC 8889. This document describes network slicing in the context of networks built from IETF technologies. It defines the term "IETF Network Slice" to describe this type of network slice and establishes the general principles of network slicing in the IETF context. The document discusses the general framework for requesting and operating IETF Network Slices, the characteristics of an IETF Network Slice, the necessary system components and interfaces, and the mapping of abstract requests to more specific technologies. The document also discusses related considerations with monitoring and security. This document also provides definitions of related terms to enable consistent usage in other IETF documents that describe or use aspects of IETF Network Slices. This document defines format and usage principles of the Deterministic Networking (DetNet) service Associated Channel over a DetNet network with the MPLS data plane. The DetNet service Associated Channel can be used to carry test packets of active Operations, Administration, and Maintenance (OAM) protocols that are used to detect DetNet failures and measure performance metrics. This document specifies requirements for the development of MPLS Network Actions (MNAs) that affect the forwarding or other processing of MPLS packets. These requirements are informed by a number of proposals for additions to the MPLS information in the labeled packet to allow such actions to be performed, either by a transit or terminating Label Switching Router (i.e., the Label Edge Router - LER). ZTE Ericsson Work in Progress Individual Cisco Systems Cisco Systems INSA Lyon Orange Bell Canada This document presents Topology Independent Loop-free Alternate Fast Reroute (TI-LFA), aimed at providing protection of node and adjacency segments within the Segment Routing (SR) framework. This Fast Reroute (FRR) behavior builds on proven IP Fast Reroute concepts being LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding (DLFA). It extends these concepts to provide guaranteed coverage in any two-connected networks using a link-state IGP. An important aspect of TI-LFA is the FRR path selection approach establishing protection over the expected post-convergence paths from the point of local repair, reducing the operational need to control the tie-breaks among various FRR options. Work in Progress RAD Work in Progress

Use Cases for Continued Discussion Several use cases for which MNA can provide a viable solution have been discussed. The discussion of these aspirational cases is ongoing at the time of publication of the document.

Generic Delivery Functions Generic Delivery Functions (GDFs), defined in , provide a new mechanism to support functions analogous to those supported through the IPv6 Extension Headers mechanism. For example, GDF can support fragmentation/reassembly functionality in the MPLS network by using the Generic Fragmentation Header. MNA can support GDF by placing a GDF header in an MPLS packet within the post-stack data block . Multiple GDF headers, organized as a list of headers, can also be present in the same MPLS packet.

Delay Budgets for Time-Bound Applications The routers in a network can perform two distinct functions on incoming packets: forwarding (where the packet should be sent) and scheduling (when the packet should be sent). IEEE-802.1 Time-Sensitive Networking (TSN) and DetNet provide several mechanisms for scheduling under the assumption that routers are time-synchronized. The most effective mechanisms for delay minimization involve per-flow resource allocation. Segment Routing (SR) is a forwarding paradigm that allows encoding forwarding instructions in the packet in a stack data structure rather than being programmed into the routers. The SR instructions are contained within a packet in the form of a First-In, First-Out stack, dictating the forwarding decisions of successive routers. Segment routing may be used to choose a path sufficiently short to be capable of providing bounded end-to-end latency but does not influence the queueing of individual packets in each router along that path. When carried over the MPLS data plane, a solution is required to enable the delivery of such packets to their final destination within a given time budget. One approach to address this use case in SR over MPLS (SR-MPLS) is described in .

Stack-Based Methods for Latency Control One efficient data structure for inserting local deadlines into the headers is a "stack", similar to that used in SR to carry forwarding instructions. The number of deadline values in the stack equals the number of routers the packet needs to traverse in the network, and each deadline value corresponds to a specific router. The Top of Stack (ToS) corresponds to the first router's deadline, while the MPLS BoS refers to the last. All local deadlines in the stack are later than or equal to the current time (upon which all routers agree), and times closer to the ToS are always earlier than or equal to times closer to the MPLS BoS. The ingress router inserts the deadline stack into the packet headers; no other router needs to know the requirements of the time-bound flows. Hence, admitting a new flow only requires updating the ingress router's information base. MPLS LSRs that expose the ToS label can also inspect the associated deadline carried in the packet (either in the MPLS stack as in-stack data or after BoS as post-stack data).

Acknowledgements The authors gratefully acknowledge the input of the members of the MPLS Open Design Team. Also, the authors sincerely thank , , , and for their thoughtful suggestions and help in improving the document.

Contributors Bronze Dragon Consulting

loa@pi.nu

Authors' Addresses Cisco Systems, Inc.

tsaad.net@gmail.com

Independent

kiran.ietf@gmail.com

Futurewei Technologies

haoyu.song@futurewei.com

Ericsson

gregimirsky@gmail.com