Encapsulation For MPLS Performance Measurement with Alternate-Marking Method

Abstract

This document defines the encapsulation for MPLS performance measurement with the Alternate-Marking method, which performs flow-based packet loss, delay, and jitter measurements on the MPLS traffic.¶

1. Introduction

[RFC9341] describes a performance measurement method, which can be used to measure packet loss, delay, and jitter on data traffic. Since this method is based on marking consecutive batches of packets, it is referred to as the Alternate-Marking Method. [RFC8372] outlines key considerations for developing a solution for MPLS flow identification, intended for use in performance monitoring of MPLS flows.¶

This document defines the encapsulation for MPLS performance measurement with the Alternate-Marking method, which performs flow-based packet loss, delay, and jitter measurements on the MPLS traffic. The encapsulation defined in this document supports performance monitoring at the intermediate nodes and MPLS flow identification at both transport and service layers.¶

Note that in parallel to the work of this document, there is ongoing work on MPLS Network Actions (MNA) [RFC9613]. The MPLS performance measurement with the Alternate-Marking method can also be achieved by MNA encapsulation. In addition, MNA will provide a broader use case applicability. That means the MNA encapsulation is expected to provide a more advanced solution, when published as an RFC and it is agreed that this document will be made Historic at that time.¶

2. Conventions Used in This Document

3. Flow-based PM Encapsulation in MPLS

This document defines the Flow-based MPLS performance measurement encapsulation with alternate marking method, as shown in figure 1.¶

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Extension Label (15)         |  TC |S|      TTL      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Flow-ID Label Indicator (TBA1)    |  TC |S|      TTL      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             Flow-ID Label             |L|D|T|S|      TTL      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Flow-ID Label Indicator (FLI) is an Extended Special Purpose Label (eSPL), which is combined with the Extension Label (XL, value 15) to form a Composite Special Purpose Label (cSPL), as defined in [RFC9017]. The FLI is defined in this document as value TBA1.¶

The Traffic Class (TC) and Time To Live (TTL) fields of the XL and FLI MUST use the same values of the label immediately preceding the XL. The Bottom of the Stack (BoS) bit [RFC3032] for the XL and FLI MUST be zero. If any XL or FLI processed by a node has the BoS bit set, the node MUST discard the packet and MAY log an error.¶

The Flow-ID Label (FL) is used as an MPLS flow identification [RFC8372]. Its value MUST be unique within the administrative domain. The Flow-ID Label values MAY be allocated by an external NMS or controller based on the measurement object instances (such as LSP or PW). There is a one-to-one mapping between a Flow-ID and a flow. The specific method on how to allocate the Flow-ID Label values is described in Section 5.¶

The FL, preceded by a cSPL, can be placed either at the bottom or in the middle, but not at the top, of the MPLS label stack, and it MAY appear multiple times within a label stack. Section 3.1 of this document provides several examples to illustrate the application of FL in a label stack. The TTL for the FL MUST be zero to ensure that it is not used inadvertently for forwarding. The BoS bit for the FL depends on whether the FL is placed at the bottom of the MPLS label stack, i.e., the BoS bit for the FL is set only when the FL is placed at the bottom of the MPLS label stack.¶

Besides the flow identification, a color-marking field is also necessary for the Alternate-Marking method. To achieve the purpose of coloring the MPLS traffic, and to distinguish between hop-by-hop measurement and edge-to-edge measurement, the TC for the FL is defined as follows:¶

• L(oss) bit is used for coloring the MPLS packets for loss measurement. Setting the bit means color 1 and unsetting the bit means color 0.¶

• D(elay) bit is used for coloring the MPLS packets for delay/jitter measurement. Setting the bit means color for delay measurement.¶

• T(ype) bit is used to indicate the measurement type. When the T bit is set to 1, that means edge-to-edge performance measurement. When the T bit is set to 0, that means hop-by-hop performance measurement.¶

Considering the FL is not used as a forwarding label, the repurposing of the TC for the FL is feasible and viable.¶

+----------------------+
|          LSP         |
|         Label        |
+----------------------+ <--+
|       Extension      |    |
|         Label        |    |
+----------------------+    |--- cSPL
|     Flow-ID Label    |    |
|       Indicator      |    |
+----------------------+ <--+
|        Flow-ID       |
|         Label        |
+----------------------+
|      Application     |
|         Label        |
+----------------------+ <= Bottom of stack
|                      |
|        Payload       |
|                      |
+----------------------+

+----------------------+
|          LSP         |
|         Label        |
+----------------------+
|      Application     |
|         Label        |
+----------------------+ <--+
|       Extension      |    |
|         Label        |    |
+----------------------+    |--- cSPL
|     Flow-ID Label    |    |
|       Indicator      |    |
+----------------------+ <--+
|        Flow-ID       |
|         Label        |
+----------------------+ <= Bottom of stack
|                      |
|        Payload       |
|                      |
+----------------------+

+----------------------+
|          LSP         |
|         Label        |
+----------------------+ <--+
|       Extension      |    |
|         Label        |    |
+----------------------+    |--- cSPL
|     Flow-ID Label    |    |
|       Indicator      |    |
+----------------------+ <--+
|        Flow-ID       |
|         Label        |
+----------------------+
|      Application     |
|         Label        |
+----------------------+ <--+
|       Extension      |    |
|         Label        |    |
+----------------------+    |--- cSPL
|     Flow-ID Label    |    |
|       Indicator      |    |
+----------------------+ <--+
|        Flow-ID       |
|         Label        |
+----------------------+ <= Bottom of stack
|                      |
|        Payload       |
|                      |
+----------------------+

4. Procedures of Encapsulation, Look-up and Decapsulation

The procedures for Flow-ID label encapsulation, look-up and decapsulation are summarized as follows:¶

• The MPLS ingress node [RFC3031] inserts the XL, FLI and FL into the MPLS label stack. At the same time, the ingress node sets the Flow-ID Label value, the two color-marking bits and the T bit, as defined in Section 3.¶

• If the edge-to-edge measurement is applied, i.e., the T bit is set to 1, then only the MPLS ingress/egress node [RFC3031] is the processing node, otherwise all the MPLS nodes along the LSP are the processing nodes. The processing node looks up the FL with the help of the XL and FLI, and exports the collected data, such as the Flow-ID, block counters and timestamps, to an external NMS/controller, referring to the Alternate-Marking method. Section 6 of [I-D.ietf-ippm-alt-mark-deployment] describes protocols for collected data export, and the details on how to export the collected data are outside the scope of this document. Note that while looking up the Flow-ID label, the transit node needs to perform some deep labels inspection beyond the label (at the top of the label stack) used to make forwarding decisions.¶

• The processing node MUST pop the XL, FLI and FL from the MPLS label stack when it needs to pop the preceding forwarding label. The egress node MUST pop the whole MPLS label stack, and this document doesn't introduce any new process to the decapsulated packet.¶

5. Procedures of Flow-ID allocation

There are at least two ways of allocating Flow-ID. One way is to allocate Flow-ID by a manual trigger from the network operator, and the other way is to allocate Flow-ID by an automatic trigger from the ingress node. Details are as follows:¶

• In the case of a manual trigger, the network operator would manually input the characteristics (e.g. IP five tuples and IP DSCP) of the measured flow, then the NMS/controller would generate one or two Flow-IDs based on the input from the network operator, and provision the ingress node with the characteristics of the measured flow and the corresponding allocated Flow-ID(s).¶

• In the case of an automatic trigger, the ingress node would identify the flow entering the measured path, export the characteristics of the identified flow to the NMS/controller by IPFIX [RFC7011], then the NMS/controller would generate one or two Flow-IDs based on the characteristics exported from the ingress node, and provision the ingress node with the characteristics of the identified flow and the corresponding allocated Flow-ID(s).¶

The policy pre-configured at the NMS/controller decides whether one Flow-ID or two Flow-IDs would be generated. If the performance measurement on the MPLS service is enabled, then one Flow-ID applied to the MPLS service would be generated. If the performance measurement on the MPLS transport is enabled, then one Flow-ID applied to the MPLS transport would be generated. If both of them are enabled, then two Flow-IDs are respectively applied to the MPLS service and the MPLS transport would be generated. In this case, a transit node needs to look up both of the two Flow-IDs by default. However, this behaviour can be changed through configuration, such as by setting it to look up only the Flow-ID applied to the MPLS transport.¶

Whether using the two methods mentioned above or other methods to allocate Flow-ID, the NMS/controller MUST ensure that every generated Flow-ID is unique within the administrative domain and MUST NOT have any value in the reserved label space (0-15) [RFC3032]. Specifically, the statement of "Flow-ID is unique" means that the values of Flow-ID are distinct and non-redundant for any flow at any given time within an administrative domain, such that no two flows share the same Flow-ID. This uniqueness ensures that each flow can be individually identified, tracked, and differentiated from others for accurate performance monitoring and management.¶

6. FLC and FRLD Considerations

Analogous to the Entropy Label Capability (ELC) defined in Section 5 of [RFC6790] and the Entropy Readable Label Depth (ERLD) defined in Section 4 of [RFC8662], the Flow-ID Label Capability (FLC) and the Flow-ID Readable Label Depth (FRLD) are defined in this document. Both FLC and FRLD have similar semantics with the ELC and ERLD to a router, except that the Flow-ID is used in its flow identification function while the Entropy is used in its load-balancing function.¶

The ingress node MUST insert each FL at an appropriate depth, which ensures the node to which the FL is exposed has the FLC. The ingress node SHOULD insert each FL within an appropriate FRLD, which is the minimum FRLD of all the on-path nodes that need to read and use the FL in question. How the ingress node knows the FLC and FRLD of all the on-path nodes is outside the scope of this document.¶

When the SR paths are used for transport, the label stack grows as the number of on-path segments increases. If the number of on-path segments is high, that may become a challenge for the FL to be placed within an appropriate FRLD. To overcome this potential challenge, an implementation MAY allow the ingress node to place FL between SID labels. This means that multiple identical FLs at different depths MAY be interleaved with SID labels. When this occurs, sophisticated network planning may be needed, which is beyond the scope of this document.¶

7. Equal-Cost Multipath Considerations

Analogous to what's described in Section 5 of [RFC8957], under conditions of Equal-Cost Multipath (ECMP), the introduction of the FL may lead to the same problem as caused by the Synonymous Flow Label (SFL) [RFC8957]. The two solutions proposed for SFL would also apply here. Specifically, adding FL to an existing flow may cause that flow to take a different path. If the operator expects to resolve this problem, they can choose to apply entropy labels [RFC6790] or add FL to all flows.¶

8. Security Considerations

As specified in Section 7.1 of [RFC9341], "for security reasons, the Alternate-Marking Method MUST only be applied to controlled domains". That requirement applies when the MPLS performance measurement with Alternate-Marking Method is taken into account, which means the MPLS encapsulation and related procedures defined in this document MUST only be applied to controlled domains, otherwise the potential attacks discussed in Section 10 of [RFC9341] may be applied to the deployed MPLS networks.¶

As specified in Section 3, the value of a Flow-ID label MUST be unique within the administrative domain. In other words, the administrative domain is the scope of a Flow-ID label. The method for achieving multi-domain performance measurement with the same Flow-ID label is outside the scope of this document. The Flow-ID label MUST NOT be signaled and distributed outside the administrative domain. Improper configuration that allows the Flow-ID label to be passed from one administrative domain to another would result in Flow-ID conflicts.¶

To prevent packets carrying Flow-ID labels from leaking from one domain to another, domain boundary nodes MUST deploy policies (e.g., ACL) to filter out these packets. Specifically, at the sending edge, the domain boundary node MUST filter out the packets that carry the Flow-ID Label Indicator and are sent to other domains. At the receiving edge, the domain boundary node MUST drop the packets that carry the Flow-ID Label Indicator and are from other domains. Note that packet leakage is neither breaching privacy nor can be a source of DoS.¶

9. Implementation Status

[Note to the RFC Editor - remove this section before publication, as well as remove the reference to [RFC7942].¶

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶

According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶

10. IANA Considerations

From the "Extended Special-Purpose MPLS Label Values" registry in the "Special-Purpose Multiprotocol Label Switching (MPLS) Label Values" namespace, a new value for the Flow-ID Label Indicator is requested from IANA as follows:¶

11. Acknowledgements

The authors would like to acknowledge Loa Andersson, Tarek Saad, Stewart Bryant, Rakesh Gandhi, Greg Mirsky, Aihua Liu, Shuangping Zhan, Ming Ke, Wei He, Ximing Dong, Darren Dukes, Tony Li, James Guichard, Daniele Ceccarelli, Eric Vyncke, John Scudder, Gunter van de Velde, Roman Danyliw, Warren Kumari, Murray Kucherawy, Deb Cooley, Zaheduzzaman Sarker, and Deboraha Brungard for their careful review and very helpful comments.¶

They also wish to acknowledge Italo Busi and Chandrasekar Ramachandran for their insightful MPLS-RT review and constructive comments.¶

Additionally, the authors would like to thank Dhruv Dhody for the English grammar review.¶

12. Contributors

Minxue Wang
China Mobile
Email: wangminxue@chinamobile.com¶

Wen Ye
China Mobile
Email: yewen@chinamobile.com¶

13. References