| Internet-Draft | MIAD Usecases | March 2022 |
| Saad, et al. | Expires 8 September 2022 | [Page] |
This document presents a number of use cases that have a common need for encoding MPLS function indicators and ancillary data inside MPLS packets. There has been significant recent interest in extending the MPLS data plane to carry such ancillary data to address a number of use cases described in this document.¶
The use cases described are not an exhaustive set, but rather the ones that are actively discussed by members of the IETF MPLS, PALS and DETNET working groups participating in the MPLS Open Design Team.¶
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This document describes important cases that require carrying additional ancillary data within the MPLS packets, as well as the means to indicate ancillary data is present.¶
These use cases have been identified by the MPLS working group design team working on defining MPLS function Indicators and Ancillary Data (MIAD) for the MPLS data plane. The MPLS ancillary data can be classified as:¶
The use cases described in this document will be used to assist in identifying requirements and issues to be considered for future resolution by the working group.¶
The following terminology is used in the document:¶
MPLS Fast Reroute (FRR) [RFC4090], [RFC5286] and [RFC7490] 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 FRR has taken place in an MPLS network and resulted in rerouting a packet away from the failure, a second FRR that impacts the same packet to rerouting is not helpful, and may even be disruptive.¶
For example, in such a case, the packet may continue to loop until its TTL expires. This can lead to link congestion and further packet loss. Thus, the attempt to prevent a packet from being dropped may instead affect many other packets. A proposal to address this is presented in [I-D.kompella-mpls-nffrr].¶
In-situ Operations, Administration, and Maintenance (IOAM) may record operational and telemetry information within the packet while the packet traverses a particular path in a network domain.¶
The term "in-situ" refers to the fact that the IOAM data fields are added to the data packets rather than being sent within the probe packets specifically dedicated to OAM or Performance Measurement (PM).¶
IOAM can run in two modes End-to-End (E2E) and Hop-by-Hop (HbH). In E2E mode, only the encapsulating and decapsulating nodes will process IOAM data fields. In HbH mode, the encapsulating and decapsulating nodes as well as intermediate IOAM-capable nodes process IOAM data fields.¶
The IOAM data fields are defined in [I-D.ietf-ippm-ioam-data], and can be used for various use-cases of OAM and PM.¶
[I-D.gandhi-mpls-ioam-sr] defines how IOAM data fields are transported using the MPLS data plane encapsulations, including Segment Routing (SR) with MPLS data plane (SR-MPLS).¶
The IOAM data may be added after the bottom of the MPLS label stack. The IOAM data fields can be of fixed or incremental size as defined in [I-D.ietf-ippm-ioam-data]. [I-D.gandhi-mpls-ioam] describes applicability of IOAM to MPLS dataplane. The encapsulating MPLS node needs to know if the decapsulating MPLS node can process the IOAM data before adding it in the packet. In HbH IOAM mode, nodes that are capable of processing IOAM will intercept and process the IOAM data. The presence of IOAM data will be transparent to nodes that does not support or do not participate in the IOAM process.¶
[I-D.ietf-teas-ietf-network-slices] specifies the definition of an IETF Network Slice. It further discusses the general framework for requesting and operating IETF Network Slices, their characteristics, and the necessary system components and interfaces.¶
Multiple network slices can be realized on top of a single shared network.¶
In order to overcome scale challenges, IETF Network Slices may be aggregated into groups according to similar characteristics. The slice aggregate [I-D.bestbar-teas-ns-packet] is a construct that comprises of the traffic flows of one or more IETF Network Slices of similar characteristics.¶
A router that requires forwarding of a packet that belongs to a slice aggregate may have to decide on the forwarding action to take based on selected next-hop(s), and the forwarding treatment (e.g., scheduling and drop policy) to enforce based on the associated per-hop behavior.¶
The routers in the network that forward traffic over links that are shared by multiple slice aggregates need to identify the slice aggregate packets in order to enforce the associated forwarding action and treatment.¶
An IETF network slice MAY support the following key features:¶
A Global Identifier as a Flow-Aggregate Selector (FAS) can be encoded in the MPLS packet as defined in [I-D.kompella-mpls-mspl4fa], [I-D.li-mpls-enhanced-vpn-vtn-id], and [I-D.decraene-mpls-slid-encoded-entropy-label-id]. The FAS is used to associate the packets with a flow aggregate that utilizes resources defined by Network Resource Partition (NRP) as described in [I-D.bestbar-teas-ns-packet].¶
The FAS can be encoded within an MPLS label carried in the packet's MPLS label stack. All packets that belong to the same flow aggregate MAY carry the same FAS in the MPLS label stack.¶
LSRs use the MPLS forwarding label to determine the forwarding next-hop(s), and can use the FAS present in the MPLS packet to infer the specific forwarding treatment that needs to be applied on the packet.¶
[RFC3031] states in Section 2.1 that: 'Some routers analyze a packet's network layer header not merely to choose the packet's next hop, but also to determine a packet's "precedence" or "class of service"'.¶
It is possible by assigning a unique MPLS forwarding label to each flow aggregate (FEC) to distinguish the packets forwarded to the same destination. from other flow aggregates. In this case, LSRs can use the top forwarding label to infer both the forwarding action and the forwarding treatment to be invoked on the packets. A similar approach is described in [I-D.ietf-spring-resource-aware-segments] and [I-D.bestbar-teas-ns-packet].¶
The routers in a network can perform two distinct functions on incoming packets, namely forwarding (where the packet should be sent) and scheduling (when the packet should be sent). Time Sensitive Networking (TSN) and Deterministic Networking 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 a low end-to-end latency but does not influence the queueing of individual packets in each router along that path.¶
TSN is required for networks transporting such time sensitive traffic, whose packets are required to be delivered to their final destination by a given time.¶
One efficient data structure for inserting local deadlines into the headers is a "stack", similar to that used in Segment Routing 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 Bottom-of-Stack (BoS) refers to the last's. All local deadlines in the stack are later or equal to the current time (upon which all routers agree), and times closer to the ToS are always earlier or equal to times closer to the BoS.¶
The ingress router inserts the deadline stack into the packet headers; no other router needs to be aware of the requirements of the time sensitive flows. Hence admitting a new flow only requires updating the information base of the ingress router.¶
MPLS LSRs that expose the Top of Stack (ToS) label can also inspect the associated "deadline" carried in the packet (either in MPLS stack or after BoS).¶
A number of different time formats commonly used in networking applications and can be used to encode the local deadlines.¶
For the forwarding sub-entry we could adopt like SR-MPLS standard 32-bit MPLS labels (which contain a 20-bit label and BoS bit), and thus SR-TSN stack entries could be 64-bits in size comprising a 32-bit MPLS label and the aforementioned nonstandard 32-bit timestamp.¶
Alternatively, an SR-TSN stack entry could be 96 bits in length comprising a 32-bit MPLS label and either of the standardized 64-bit timestamps.¶
[RFC8595] describes how Service Function Chaining (SFC) can be realized in an MPLS network by emulating the NSH by using only MPLS label stack elements.¶
A reference to the NSH SFC use case is defined in [RFC8596].¶
The approach in [RFC8595] introduces some limitations that are discussed in [I-D.lm-mpls-sfc-path-verification]. This approach, however, can benefit from the solution framework introduced with MIAD.¶
For example, it may be possible for the Network Service Header (NSH) to be embedded in an Extended Header (EH) to support the Path ID and any metadata that needs to be carried and and exchanged between Service Function Forwarders (SFFs).¶
In SR, an ingress node steers a packet through an ordered list of instructions, called "segments". Each one 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 Segment Routing (SR) functions to achieve a networking objective that goes beyond mere packet routing.¶
It may be desirable to encode a pointer to function and its arguments within an MPLS packet transport header. For example, in MPLS we can encode the FUNC::ARGs within the label stack or after the Bottom of Stack to support the equivalent of FUNC::ARG in SRv6 as described in [RFC8986].¶
Application-aware Networking (APN) as described in [I-D.li-apn-problem-statement-usecases] allows application-aware information (i.e., APN attributes) including APN identification (ID) and/or APN parameters (e.g. network performance requirements) to be encapsulated at network edge devices and carried in packets traversing an APN domain.¶
The APN data is carried in packets to facilitate service provisioning, perform fine-granularity traffic steering and network resource adjustment. To support APN in MPLS networks, mechanisms are needed to carry such APN data in MPLS encapsulated packets.¶
Two or more of the aforementioned use cases MAY co-exist in the same packet. Some examples of such use cases are described below.¶
IOAM may provide key functions with network slicing to help ensure that critical network slice SLOs are being met by the network provider.¶
In such a case, IOAM is able to collect key performance measurement parameters of network slice traffic flows as it traverses the transport network.¶
This may require, in addition to carrying a specific network slice selector (e.g., GISS), the MPLS network slice packets may have to also carry IOAM ancillary data.¶
Note that the IOAM ancillary data may have to be modified, and updated on some/all LSRs traversed by the network slice MPLS packets.¶
IOAM operation may also be desirable on MPLS packets that carry time-sensitive related data. Similarly, this may require the presence of multiple ancilary data (whether In-stack or Post-stack ancillary data) to be present in the same MPLS packet.¶
This document has no IANA actions.¶
This document introduces no new security considerations.¶
The authors gratefully acknowledge the input of the members of the MPLS Open Design Team.¶
The following individuals contributed to this document:¶
Loa Andersson Bronze Dragon Consulting Email: loa@pi.nu¶