| Internet-Draft | MPLS Inspection MSD | September 2023 |
| Liu | Expires 14 March 2024 | [Page] |
This document defines a new type of MSD, Base MPLS Inspection MSD to reflect the Readable Label Depth(RLD), and the mechanism to signal this MSD using IGP and BGP-LS.¶
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[I-D.ietf-mpls-mna-fwk] specifies an architectural framework for the MPLS Network Actions (MNA) technologies. MNA technologies are used to indicate actions for Label Switched Paths (LSPs) and/or MPLS packets and to transfer data needed for these actions.¶
[I-D.ietf-mpls-mna-hdr] defines the MPLS Network Action sub-stack(NAS) solution for carrying Network Actions and Ancillary Data in the label stack. The node adding an NAS to the label stack will need to place a copy of the NAS where it can be read by the relevant nodes. In order to put the NAS at the appropriate place into the MPLS label stack, the need for signaling RLD by every participanting node is proposed in [I-D.ietf-mpls-mna-hdr].¶
On the other hand, even if the MNA framework is not followed, as long as there're scenarios where at least part of the transit nodes are required to inspect beyond the top of stack, the requirement to obtain the maximum inspection depth of the nodes along the LSP exists.¶
Maximum SID Depth (MSD)[RFC8491] is originally introduced for SR-MPLS to express the number of SIDs supported by a node or a link on a node. In a non-SR MPLS network, MSD defines the maximum label depth.¶
This document defines a new type of MSD, Base MPLS Inspection MSD, and the mechanism to signal this MSD using IGP and BGP-LS.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
MNA: MPLS Network Actions¶
NAS: Network Action sub-stack¶
EL: Entropy Label¶
ERLD: Entropy Readable Label Depth¶
RLD: Readable Label Depth¶
The Base MPLS Inspection MSD is defined as the maximum number of labels a router can read in an MPLS packet received on its incoming interface(s) (starting from the top of the stack).¶
The Base MPLS Inspection MSD MAY be used by ingress LSRs to determine the position of the NAS, and whether it's necessary to insert multiple NAS at different positions in the label stack. When the label stack are determined by a centralized controller, the MSD of each intermediate LSR SHOULD be sent to the controller.¶
With Base MPLS Inspection MSD, application/network action-specified MSD analogous to ERLD-MSD[RFC9088] [RFC9089] MAY not needed. For example, a node can signal certain network action capability and the Base MPLS Inspection MSD to indicate that it can process this network action within the MSD.¶
Editor's note: The reason why ERLD-MSD is not reused to reflect the RLD is that the definition of ERLD is strongly related the router's ability to process entropy label. As specified in [RFC8662], the ERLD means that the router will perform load-balancing using the EL if the EL is placed within the first ERLD labels, and a router capable of reading N labels but not using an EL located within those N labels MUST consider its ERLD to be 0. Considering that implementations in strict accordance with the definition of ERLD may exist, defining a new MSD instead of reusing/updating ERLD is preferred in this document.¶
A new MSD-Type , called Base MPLS Inspection MSD, is defined. The MSD-Type code is to be assigned by IANA. The MSD-Value field is set to the maximum number of labels a router can read in the range between 0 to 255. The scope of the advertisement depends on the application. This MSD can be advertised on the per-node or per-link basis as in [RFC8491] and [RFC8476].¶
The absence of Base MPLS Inspection MSD advertisements indicates only that the advertising node does not support advertisement of this capability.¶
The IGP extensions defined in this document can be advertised via BGP-LS (distribution of Link-State and TE information using BGP) [RFC7752] using existing BGP-LS TLVs.¶
The Base MPLS Inspection MSD is advertised using the Node MSD TLV as well as the Link MSD TLV as defined in [RFC8814].¶
This document specifies the ability to advertise additional node capabilities using IS-IS, OSPF and BGP-LS. As such, the security considerations as described in [RFC5340], [RFC7684], [RFC7752], [RFC7770], [RFC7794], [RFC7981], [RFC8476], [RFC8491], [RFC8662], [RFC8814], [RFC9085] are applicable to this document.¶
Incorrectly setting of the Base MPLS Inspection MSD value may lead to poor or no execution of the network action.¶
This document requests the following allocation from IANA:¶
Type TBA in the IGP MSD-Types registry is requested to be assigned for the Base MPLS Inspection MSD.¶
The author would like to thank Greg Mirsky, Les Ginsberg, Tony Li and Gyan Mishra for their helpful review and comments.¶