RFC 9357: Label Switched Path (LSP) Object Flag Extension for Stateful PCE

Abstract

RFC 8231 describes a set of extensions to the Path Computation Element Communication Protocol (PCEP) to enable stateful control of MPLS-TE and GMPLS Label Switched Paths (LSPs) via PCEP. One of the extensions is the LSP object, which includes a Flag field with a length of 12 bits. However, all bits of the Flag field have already been assigned.¶

This document defines a new LSP-EXTENDED-FLAG TLV for the LSP object for an extended Flag field.¶

1. Introduction

[RFC5440] describes the Path Computation Element Communication Protocol (PCEP), which is used between a PCE and a Path Computation Client (PCC) (or other PCE) to enable computation of Multi-protocol Label Switching for Traffic Engineering (MPLS-TE) Label Switched Paths (LSPs).¶

PCEP Extensions for the Stateful PCE Model [RFC8231] describes a set of extensions to PCEP to enable active control of MPLS-TE and Generalized MPLS (GMPLS) tunnels. One of the extensions is the LSP object, which contains a Flag field; bits in the Flag field are used to indicate delegation, synchronization, removal, etc.¶

As defined in [RFC8231], the length of the Flag field is 12 bits, and all of the bits have already been defined as shown in Table 1. This document extends the Flag field of the LSP object for other use by defining a new LSP-EXTENDED-FLAG TLV for an extended Flag field in the LSP object (see Section 3.1).¶

2. Conventions Used in this Document

3. PCEP Extension

The LSP object is defined in Section 7.3 of [RFC8231]. This document defines a new LSP-EXTENDED-FLAG TLV for an extended Flag field in the LSP object.¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Type=64             |           Length              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
//                 LSP Extended Flags                          //
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4. Advice for Specification of New Flags

Following the model provided in Section 3.1 of [RFC8786], we provide the following advice for new specifications that define additional flags. Each such specification is expected to describe the interaction between these new flags and any existing flags. In particular, new specifications are expected to explain how to handle the cases when both new and preexisting flags are set. They are also expected to discuss any security implications of the additional flags (if any) and their interactions with existing flags.¶

5. Backward Compatibility

The LSP-EXTENDED-FLAG TLV defined in this document does not introduce any backward compatibility issues. An implementation that does not understand or support the LSP-EXTENDED-FLAG TLV MUST ignore the TLV, as per [RFC5440]. Future documents that define bits in the LSP-EXTENDED-FLAG TLV are expected to also define the error handling required for cases in which the LSP-EXTENDED-FLAG TLV is missing when it MUST be present.¶

Further, any additional bits in the LSP-EXTENDED-FLAG TLV that are not understood by an implementation MUST be ignored. It is expected that future documents that define bits in the LSP-EXTENDED-FLAG TLV will take that into consideration.¶

7. Management Considerations

Implementations receiving set LSP Extended Flags that they do not recognize MAY log this. That could be helpful for diagnosing backward compatibility issues with future features that utilize those flags.¶

8. Security Considerations

[RFC8231] sets out security considerations for PCEP when used for communication with a stateful PCE. This document does not change those considerations. For LSP object processing, see [RFC8231].¶

The flags for the LSP object and their associated security considerations are specified in [RFC8231], [RFC8281], [RFC8623], and [BIND-LABEL-SID].¶

This document provides for the future addition of flags in the LSP object. Any future document that specifies new flags must also discuss any associated security implications. No additional security issues are raised in this document beyond those that exist in the referenced documents. Note that [RFC8231] recommends that the stateful PCEP extension be authenticated and encrypted using Transport Layer Security (TLS) [RFC8253] [PCEPS-TLS1.3], as per the recommendations and best current practices in [RFC9325]. Assuming that the recommendation is followed, then the flags will be protected by TLS.¶

9. References

Appendix A. Working Group Discussion

The working group discussed the idea of a fixed length (with 32 bits) for the LSP-EXTENDED-FLAG TLV. Though 32 bits would be sufficient for quite a while, the use of variable length with a multiple of 32 bits allows for future extensibility where we would never run out of flags and there would not be a need to define yet another TLV in the future. Further, note that [RFC5088] and [RFC5089] use the same approach for the PCE-CAP-FLAGS sub-TLV and are found to be useful.¶

Contributors

The following people have substantially contributed to this document:¶