| RFC 9791 | MNA Use Cases | July 2025 |
| Saad, et al. | Informational | [Page] |
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.¶
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 athttps://www.rfc-editor.org/info/rfc9791.¶
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 (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 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.¶
This document describes use cases that introduce functions that requirespecial processing by forwarding hardware. The current state of the art requiresallocating a new Special-Purpose Label (SPL)[RFC3032] 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)[RFC9613] approach was proposed to extend the MPLS architecture.MNA is expected to enable functions that may require carrying additionalancillary data within the MPLS packets, as well as a means to indicate that theancillary data is present and a specific action needs to be performed on thepacket.¶
This document lists various use cases that could benefit extensivelyfrom the MNA framework[RFC9789].Supporting a solution of the general MNA framework providesa common foundation for future network actions that can be exercisedin the MPLS data plane.¶
The following terminology is used in the document:¶
MPLS Fast Reroute[RFC4090][RFC5286][RFC7490][SR-TI-LFA] is a usefuland widely deployed tool for minimizing packet loss in the case of a link ornode failure.¶
Several cases exist where, once a Fast Reroute (FRR) has taken place in an MPLS network anda packet is rerouted away from the failure, a second FRR impactsthe 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 loopbetween the Label Switching Routers (LSRs) that activated FRR until the packet's TTLexpires. 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.¶
MNA can be used to carry information essential for collecting operational informationand measuring various performance metrics that reflect the experience of the packet marked by MNA.Optionally, the operational state and telemetry information collected onthe LSR may be transported using MNA techniques.¶
In situ Operations, Administration, and Maintenance (IOAM),defined in[RFC9197] and[RFC9326], might be used to collectoperational and telemetry information while a packet traverses a particular pathin a network domain.¶
IOAM can run in two modes: Ingress to Egress (I2E) and Hop by Hop (HbH). In I2Emode, only the encapsulating and decapsulating nodes will process IOAM datafields. In HbH mode, the encapsulating and decapsulating nodesand intermediate IOAM-capable nodes process IOAM data fields. The IOAM data fields,defined in[RFC9197], can be used to derive the operational state of the networkexperienced by the packet with the IOAM Header that traversed the path through the IOAM domain.¶
Several IOAM Option-Types have been defined:¶
With all IOAM Option-Types except for Direct Export (DEX), the collectedinformation is transported in the trigger IOAM packet.In the IOAM DEX Option-Type[RFC9326], the operational state and telemetry information arecollected according to a specified profile and exported in a manner andformat defined by a local policy. In IOAM DEX, the user data packet is onlyused to trigger the IOAM data to be directly exported or locally aggregatedwithout being carried in the IOAM trigger packets.¶
The Alternate Marking Method (AMM), defined in[RFC9341] and[RFC9342]), is an exampleof a hybrid performance measurement method[RFC7799] that can be used in the MPLS networkto measure packet loss and packet delay performance metrics.[RFC8957] definesthe 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.¶
An RFC 9543 Network Slice Service[RFC9543]provides connectivity coupled with network resource commitments and is expressed in terms of one or moreconnectivity constructs.Section 5 of [NS-IP-MPLS] defines a Network Resource Partition (NRP) Policyas 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 amarking in their network-layer header to identify this association, which is referred to as an NRP Selector. The NRP Selector mapsa packet to the associated network resources and provides thecorresponding forwarding treatment onto the packet.¶
A router that requires the forwarding of a packet that belongs to an NRPmay have to decide on the forwarding action to take based on selectednext hop(s) and decide on the forwarding treatment (e.g., scheduling and drop policy) toenforce based on the associated per-hop behavior.¶
In this case, routers that forward traffic over a physical link shared by multipleNRPs 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 packetsand carry data essential for these actions. For example, MNA can carry the NRP Selector[NS-IP-MPLS] in MPLS packets.¶
[RFC8595] describes how Service Function Chaining can be realized inan MPLS network by emulating the Network Service Header (NSH)[RFC8300] using only MPLS label stack entries.¶
The approach in[RFC8595] introduces some limitations, which are discussed in[SFP-VERIF]. However, the approach can benefitfrom the MNA framework introduced in[RFC9789].¶
MNA can be used to extend NSH emulation using MPLSlabels[RFC8595] to support the functionality of NSH Context Headers,whether fixed or variable length. For example, MNA could support Flow ID[RFC9263] that may be used for load-balancing amongService Function Forwarders and/or the Service Functionswithin the same Service Function Path.¶
In Segment Routing (SR), an ingress node steers a packet through an ordered list of instructionscalled "segments". Each of these instructions represents afunction to be called at a specific location in the network. Afunction is locally defined on the node where it is executed and mayrange from simply moving forward in the segment list to any complexuser-defined behavior.¶
Network Programming combines SR functions to achieve anetworking 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 functionalequivalent of FUNC::ARG in Segment Routing over IPv6 as described in[RFC8986].¶
Several services can be transported over MPLS networks today.These include providing Layer 3 (L3) connectivity (e.g., for unicast andmulticast L3 services) and Layer 2 (L2) connectivity (e.g., for unicastPWs, multicast E-Tree, and broadcast Ethernet LAN (E-LAN) L2 services). Inthose 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)[RFC4385][RFC5085] 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 isset to 0b0000 to identify the presence of the PW CW.¶
In addition to providing connectivity to user traffic, MPLS may also transport OAMdata (e.g., over MPLS Generic Associated Channels (G-AChs)[RFC5586]). In this case, the first nibble ofthe data that immediately follows the MPLS BoS is set to 0b0001. Itindicates the presence of a control channel associated with a PW, LSP, or section.¶
Bit Index Explicit Replication (BIER)[RFC8296] traffic can also be encapsulatedover MPLS. In this case, BIER has defined 0b0101 as the value for the first nibbleof the data that immediately appears after the BoS for anyBIER-encapsulated packet over MPLS.¶
For PWs, the G-ACh[RFC7212] uses the first four bits of the PW control wordto provide the initial discrimination between data packets andpackets belonging to the associated channel, as described in[RFC4385].¶
MPLS can be used as the data plane for Deterministic Networking (DetNet)[RFC8655].The DetNet sub-layers, forwarding, and serviceare realized using the MPLS label stack, the DetNet control word[RFC8964],and the DetNet Associated Channel Header[RFC9546].¶
MNA-based solutions for the use cases described in this document and proposedin the future are expected to allow for coexistence and backward compatibility with all existing MPLS services.¶
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 helpensure 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 anetwork slice traffic flow as it traverses the transport network.¶
This document has no IANA actions.¶
Section 7 of [RFC9789] 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 documentand will be addressed in future solution documents. It is strongly recommendedthat these solution documents undergo review by a security expert early in their development,ideally during the Working Group Last Call phase.¶
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 (GDFs), defined in[GDF], provide a new mechanism tosupport functions analogous to those supported through the IPv6 ExtensionHeaders mechanism. For example, GDF can support fragmentation/reassemblyfunctionality in the MPLS network by using the Generic Fragmentation Header.MNA can support GDF by placing a GDF header in an MPLS packet within thepost-stack data block[RFC9789]. Multiple GDF headers, organized as a list of headers, can alsobe present in the same MPLS packet.¶
The routers in a network can perform two distinct functions on incomingpackets: forwarding (where the packet should be sent) and scheduling(when the packet should be sent). IEEE-802.1 Time-Sensitive Networking (TSN) andDetNet provide several mechanisms for scheduling under theassumption that routers are time-synchronized. The most effective mechanismsfor delay minimization involve per-flow resource allocation.¶
Segment Routing (SR) is a forwarding paradigm that allows encoding forwardinginstructions in the packet in a stack data structure rather than beingprogrammed into the routers. The SR instructions are contained within a packetin the form of a First-In, First-Out stack, dictating the forwarding decisions ofsuccessive routers. Segment routing may be used to choose a path sufficientlyshort to be capable of providing bounded end-to-end latency but doesnot 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 thedelivery of such packets to their final destination within agiven time budget. One approach to address this use case in SR over MPLS (SR-MPLS) isdescribed in[SRTSN].¶
One efficient data structure for inserting local deadlines intothe headers is a "stack", similar to that used in SR tocarry forwarding instructions. The number of deadline values in thestack equals the number of routers the packet needs to traverse inthe network, and each deadline value corresponds to a specificrouter. The Top of Stack (ToS) corresponds to the first router'sdeadline, while the MPLS BoS refers to the last. Alllocal deadlines in the stack are later than or equal to the current time(upon which all routers agree), and times closer to the ToS arealways earlier than or equal to times closer to the MPLS BoS.¶
The ingress router inserts the deadline stack into the packet headers; no otherrouter 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 theassociated deadline carried in the packet (either in the MPLS stack as in-stack data orafter BoS as post-stack data).¶
The authors gratefully acknowledge the input of the members of the MPLS Open Design Team. Also, the authors sincerely thankLoa Andersson,Xiao Min,Jie Dong, andYaron Sheffer for their thoughtful suggestions and help in improving the document.¶