RFC 9791: Use Cases for MPLS Network Action Indicators and Ancillary Data

1.1. Terminology

The following terminology is used in the document:¶

RFC 9543 Network Slice:

Interpreted as defined in [RFC9543]. 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").¶

1.2. 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¶

2.1. No Further Fast Reroute

MPLS Fast Reroute [RFC4090] [RFC5286] [RFC7490] [SR-TI-LFA] 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.¶

2.2. 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.¶

2.3. Network Slicing

An RFC 9543 Network Slice Service [RFC9543] provides connectivity coupled with network resource commitments and is expressed in terms of one or more connectivity constructs. Section 5 of [NS-IP-MPLS] 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 [NS-IP-MPLS] in MPLS packets.¶

2.4. NSH-Based Service Function Chaining

[RFC8595] describes how Service Function Chaining can be realized in an 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 benefit from the MNA framework introduced in [RFC9789].¶

MNA can be used to extend NSH emulation using MPLS labels [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 among Service Function Forwarders and/or the Service Functions within the same Service Function Path.¶

2.5. 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 [RFC8986].¶

A.1. Generic Delivery Functions

Generic Delivery Functions (GDFs), defined in [GDF], 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 [RFC9789]. Multiple GDF headers, organized as a list of headers, can also be present in the same MPLS packet.¶

A.2. 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 [SRTSN].¶

A.3. 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).¶