MPLS Working Group J. Rajamanickam, Ed. Internet-Draft R. Gandhi, Ed. Intended status: Standards Track Cisco Systems, Inc. Expires: 25 December 2026 R. Zigler Broadcom J. Dong Huawei Technologies J. Bhattacharya Cisco Systems, Inc. 23 June 2026 Post-Stack MPLS Network Action (MNA) Header Specification draft-ietf-mpls-mna-ps-hdr-09 Abstract This document defines the Post-Stack MPLS Network Action (MNA) Header Specification for carrying Network Action encodings and Ancillary Data after the MPLS label stack, based on the In-Stack MNA Specification defined in "MPLS Network Action (MNA) Sub-Stack Specification." MPLS Network Actions can be used to influence packet forwarding decisions, carry additional Operations, Administration, and Maintenance information in the MPLS packet, or perform user- defined operations. This document follows the framework specified in RFC 9789. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 25 December 2026. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Rajamanickam, et al. Expires 25 December 2026 [Page 1] Internet-Draft Post-Stack MNA Specification June 2026 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. PSMH Presence Bit Carried in In-Stack MNA Sub-Stack . . . 5 3.2. PSMH Encoding . . . . . . . . . . . . . . . . . . . . . . 6 3.2.1. Post-Stack MPLS Base Header . . . . . . . . . . . . . 6 3.2.2. Post-Stack Network Action Encoding . . . . . . . . . 7 3.2.3. Encoding of an MPLS Packet with PSMH . . . . . . . . 7 4. In-Stack Network Action Special Opcodes . . . . . . . . . . . 8 4.1. Opcode for PSMH Start Offset . . . . . . . . . . . . . . 9 4.2. Opcode for PSMH End Offset . . . . . . . . . . . . . . . 9 5. Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.1. Processing Rules for P Bit . . . . . . . . . . . . . . . 10 5.2. Network Action Processing Order . . . . . . . . . . . . . 10 5.3. Node Capability Signaling . . . . . . . . . . . . . . . . 10 6. Processing the Network Action Sub-Stack and Post-Stack Header . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.1. Encapsulating Node Responsibilities . . . . . . . . . . . 11 6.2. Transit Node Responsibilities . . . . . . . . . . . . . . 11 6.3. Penultimate Node Responsibilities . . . . . . . . . . . . 11 6.4. Decapsulating Node Responsibilities . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 8. Operational Considerations . . . . . . . . . . . . . . . . . 12 8.1. Manageability Considerations . . . . . . . . . . . . . . 12 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 9.1. First Nibble for Post-Stack MPLS Header . . . . . . . . . 13 9.2. Post-Stack MPLS Header Types Registry . . . . . . . . . . 13 9.3. Network Action Opcodes . . . . . . . . . . . . . . . . . 14 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 10.1. Normative References . . . . . . . . . . . . . . . . . . 15 10.2. Informative References . . . . . . . . . . . . . . . . . 17 Appendix A. Appendix A: Examples . . . . . . . . . . . . . . . . 17 A.1. Examples of PSMH Encoding . . . . . . . . . . . . . . . . 17 A.1.1. NAS that only Indicates PSMH . . . . . . . . . . . . 17 A.1.2. NAS that Indicates PSMH Start Offset . . . . . . . . 18 Rajamanickam, et al. Expires 25 December 2026 [Page 2] Internet-Draft Post-Stack MNA Specification June 2026 A.1.3. Post-Stack Network Actions with Two Opcodes . . . . . 18 A.1.4. Post-Stack Network Actions with two Different Scopes . . . . . . . . . . . . . . . . . . . . . . . 19 A.2. Examples of In-Stack and Post-Stack Network Actions . . . 20 A.2.1. NAS with In-Stack and Post-Stack Network Actions . . 20 A.2.2. NASes with Different In-Stack and Post-Stack Scopes . . . . . . . . . . . . . . . . . . . . . . . 21 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 1. Introduction [RFC3032] defines the encoding of the MPLS label stack, the basic structure used to define a forwarding path. There are applications that require MPLS packets to perform special network actions and carry optional Ancillary Data (AD) that can affect the packet forwarding decision or trigger Operations, Administration, and Maintenance (OAM) logging, for example as described in [RFC9791]. Ancillary Data can be used to carry additional information, for network slice purpose, as an example [RFC9791]. In some cases, more AD may be required than can be carried in the MPLS header, so these kinds of network actions and their AD are encoded after the Bottom of Stack (BOS). This network action with AD is called the Post-Stack (PS) MNA. This document defines the Post-Stack MPLS Network Action (MNA) header specification for carrying Network Action encodings and Ancillary Data after the MPLS label stack. The specification is based on the In-Stack MNA Specification defined in [I-D.ietf-mpls-mna-hdr]. The requirements for Post-Stack network actions and Post-Stack data (PSD) are described in [RFC9613]. This document follows the framework specified in [RFC9789]. 2. Conventions Used in This Document 2.1. Requirements Language 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. 2.2. Abbreviations The terminology defined in [RFC9789] and [RFC9613] is used in this document. Rajamanickam, et al. Expires 25 December 2026 [Page 3] Internet-Draft Post-Stack MNA Specification June 2026 +==============+===================+=========================+ | Abbreviation | Meaning | Reference | +==============+===================+=========================+ | AD | Ancillary Data | [RFC9613] | +--------------+-------------------+-------------------------+ | bSPL | Base Special | [RFC9017] | | | Purpose Label | | +--------------+-------------------+-------------------------+ | BOS | Bottom Of Stack | [RFC9789] | +--------------+-------------------+-------------------------+ | HBH | Hop-By-Hop Scope | [RFC9789] | +--------------+-------------------+-------------------------+ | I2E | Ingress-To-Egress | [RFC9789] | | | Scope | | +--------------+-------------------+-------------------------+ | IHS | I2E, HBH, or | [I-D.ietf-mpls-mna-hdr] | | | Select Scope | | +--------------+-------------------+-------------------------+ | ISD | In-Stack Data | [RFC9613] | +--------------+-------------------+-------------------------+ | LSE | Label Stack Entry | [RFC9789] | +--------------+-------------------+-------------------------+ | MNA | MPLS Network | [RFC9789] | | | Actions | | +--------------+-------------------+-------------------------+ | NAI | Network Action | [RFC9613] | | | Indicator | | +--------------+-------------------+-------------------------+ | NAL | Network Action | [I-D.ietf-mpls-mna-hdr] | | | Length | | +--------------+-------------------+-------------------------+ | NAS | Network Action | [RFC9789] | | | Sub-Stack | | +--------------+-------------------+-------------------------+ | NASL | Network Action | [I-D.ietf-mpls-mna-hdr] | | | Sub-Stack Length | | +--------------+-------------------+-------------------------+ | OAM | Operations, | [RFC6291] | | | Administration, | | | | and Maintenance | | +--------------+-------------------+-------------------------+ | P bit | Post-Stack MPLS | This document | | | Header Presence | | | | Bit | | +--------------+-------------------+-------------------------+ | PS | Post-Stack | This document | +--------------+-------------------+-------------------------+ | PSD | Post-Stack Data | [RFC9613] and [RFC9789] | Rajamanickam, et al. Expires 25 December 2026 [Page 4] Internet-Draft Post-Stack MNA Specification June 2026 +--------------+-------------------+-------------------------+ | PSMH | Post-Stack MPLS | This document | | | Header | | +--------------+-------------------+-------------------------+ | TC | Traffic Class | [RFC5462] | +--------------+-------------------+-------------------------+ | TTL | Time To Live | [RFC3032] | +--------------+-------------------+-------------------------+ Table 1: Abbreviations 3. Overview Two main parts are specified to support Post-Stack MNA: * The Post-Stack MPLS Header (PSMH) Presence Bit Carried in In-Stack MNA Sub-Stack. * The PSMH encoding that includes a Post-Stack MPLS Base Header and one or more Post-Stack Network Actions and their Ancillary Data. Note that the PSMH can be encoded for use cases other than the PS MNA. However, this document only describes the procedure related to the Post-Stack MNA. 3.1. PSMH Presence Bit Carried in In-Stack MNA Sub-Stack Bit 20 in LSE Format B carried in the In-Stack Network Action Sub- Stack (NAS) described in [I-D.ietf-mpls-mna-hdr] is defined as the P bit in this document to indicate the presence of the PSMH in the packet after the BOS. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode | 13-bit Data (Format B) |P|IHS|S| NASL |U| NAL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: PSMH Presence Bit Carried in In-Stack MNA Sub-Stack The following fields are carried in an NAS as defined in [I-D.ietf-mpls-mna-hdr] and shown in Figure 1. * S (1 Bit): The BoS. * NASL (4 Bits): The Network Action Sub-Stack Length. * NAL (3 Bits): The Network Action Length. Rajamanickam, et al. Expires 25 December 2026 [Page 5] Internet-Draft Post-Stack MNA Specification June 2026 The following fields are carried in an NAS as defined in [I-D.ietf-mpls-mna-hdr] and shown in Figure 1. These fields are also applicable to Post-Stack MNA as defined below: * IHS (2 Bit): The scope of all the NAIs encoded in the NAS as well as in the corresponding PSMH. * U (1 Bit): Indicates the combined Unknown Action Handling of the In-Stack and the Post-Stack Network Actions. 3.2. PSMH Encoding The PSMH is encoded after the LSE for which the BOS bit is set, either immediately after the BOS (i.e., start offset of 0) or after any other Post-Stack headers that follow the BOS (i.e., start offset of non-zero), as described in Section 4. The PSMH consists of two main parts: * Post-Stack MPLS Base Header * Encoding for one or more Post-Stack Network Actions and their Ancillary Data 3.2.1. Post-Stack MPLS Base Header For PS MNA, the Post-Stack MPLS Base Header is added for each scope. The Post-Stack MPLS Base Header added in a PSMH is shown in Figure 2. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PFN |Reserve| PSMH-Len | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Post-Stack MPLS Base Header * PFN (4 bits): The Post-Stack First Nibble (PFN) [RFC9790] identifies the start of the PSMH. The PFN is set to 0x0 for the PSMH. * Reserve (4 bits): Reserved bits. MUST be set to zero on transmission and ignored upon receipt. * PSMH-Len (8 bits): The PSMH total length in 4-octet units that includes Post-Stack Network Actions. The length excludes the 4 octet Post-Stack MPLS Base Header. Rajamanickam, et al. Expires 25 December 2026 [Page 6] Internet-Draft Post-Stack MNA Specification June 2026 * Type (16 bits): Type is set to 1 for "MNA Post-Stack Header" type. See Section 9.2. Note that other types of PSMHs (besides MNA) can be defined in the future and are outside the scope of this document. 3.2.2. Post-Stack Network Action Encoding The format shown in Figure 3 encodes a single Post-Stack Network Action. By repeating this format, multiple Post-Stack Network Actions and their corresponding Ancillary Data can be encoded. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP |R|R| PS-NAL | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Continued Post-Stack Data ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Post-Stack Network Action Encoding * MNA-PS-OP (7 bits): Post-Stack Network Action Opcode. See Section 9.3. * R (2 bits): Reserved bits. * PS-NAL (7 bits): Post-Stack Network Action Length in 4-octet units. This excludes the first 4 octets starting with MNA-PS-OP. * Post-Stack Data (16 bits): Post-Stack Data (PSD) associated with the Post-Stack Network Action. * Continued Post-Stack Data: Further PSD associated with the Post- Stack Network Action, as indicated by the value of the PS-NAL. The padding rules for PSD that do not fill a multiple of 32-bit units are described in the document that defines the Network Action, as indicated by the MNA-PS-OP value. 3.2.3. Encoding of an MPLS Packet with PSMH Rajamanickam, et al. Expires 25 December 2026 [Page 7] Internet-Draft Post-Stack MNA Specification June 2026 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 - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - M | MNA Label | TC |0| TTL | N P +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A L |Opcode=2=NoOp| 0 |1|IHS|0| NASL=0|U|NAL=0| S S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - H ~ |0| ~ D +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R | |1| | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - P | 0x0 |Reserve| PSMH-Len | Type = MNA Post-Stack Header |Base S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - M | MNA-PS-OP |R|R| PS-NAL | Post-Stack Data | | H +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+PSNA | ~ Continued Post-Stack Data ~ | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Encoding of an MPLS Packet with PSMH An example of an MPLS packet encoding that carries an MPLS Header and a PSMH is shown in Figure 4. The example encoding contains an MPLS HDR and a PSMH with the following fields. * MPLS HDR: MPLS Header containing an MPLS label stack and NAS. * NAS: Network Action Sub-Stack. It has the P bit set to 1 to indicate the presence of a PSMH in the packet. An MPLS packet may carry more than one NAS, not shown in Figure 4. * PSMH: Post-Stack MPLS Header. This includes the Base Header and PSNA. * Base: The Post-Stack MPLS Base Header for the "MNA Post-Stack Header" type. * PSNA: Post-Stack Network Actions. Applicable for the "MNA Post- Stack Header" type. An MPLS packet may have more than one Post- Stack Network Action; this is not shown in Figure 4. 4. In-Stack Network Action Special Opcodes Rajamanickam, et al. Expires 25 December 2026 [Page 8] Internet-Draft Post-Stack MNA Specification June 2026 4.1. Opcode for PSMH Start Offset Opcode: TBA1 Purpose: This opcode carries the start offset of the PSMH from the BOS. LSE Format: B or C (defined in [I-D.ietf-mpls-mna-hdr]). Data: The data value of the LSE contains the offset from the BOS in 4-octet units. Scope: This opcode can be used with any scope. This opcode allows the existing Post-Stack Headers [RFC9790], e.g., the Generic Control Word (0000b) [RFC4385] and G-ACh (0001b) [RFC5586], and any other PSH defined in the future to be placed immediately after the BOS. If in the given NAS, the P bit is set and the PSMH Start Offset opcode is absent, the PSMH is encoded immediately after the BOS. A data value of 1 indicates that the PSMH starts at 4 octets after the BOS. 4.2. Opcode for PSMH End Offset Opcode: TBA2 Purpose: This opcode carries the offset of the end of the PSMH from the BOS. LSE Format: B or C (defined in [I-D.ietf-mpls-mna-hdr]). Data: The data value of the LSE contains the offset from the BOS in 4-octet units. A data value of 5 indicates that the PSMH ends at 20 octets after the BOS. Scope: This opcode can be used with any scope. The offset of the end of the PSMH and the offset of the start of the PSMH allow the NAS parser implementation to know the size of the PSMH without having to parse the PSMH (e.g., for Readable Label Depth with PSMH checks). 5. Procedure Rajamanickam, et al. Expires 25 December 2026 [Page 9] Internet-Draft Post-Stack MNA Specification June 2026 5.1. Processing Rules for P Bit The P bit MUST be set to 1 in an In-Stack Network Action Sub-Stack when the corresponding PSMH is added in the packet. By default, the PSMH starts immediately after the BOS. The offset of the PSMH that does not start immediately after the BOS is indicated using the PSMH Start Offset Opcode TBA1. The P bit MUST be set to 1 when the network action with opcode TBA1 is added to the In-Stack Network Action Sub-Stack. The node that recognizes the network action with Opcode TBA1 MUST process the packet according to the U flag if the P bit is not set. The P bit MUST be set to 1 when the network action with opcode TBA2 is added to the In-Stack Network Action Sub-Stack. The node that recognizes the network action with Opcode TBA2 MUST process the packet according to the U flag if the P bit is not set. The node that supports the P bit, processes the Post-Stack Network Actions in the PSMH as defined in this document. Conversely, the node that does not support the P bit skips processing the PSMH altogether. 5.2. Network Action Processing Order The Post-Stack Network Actions are processed in the same order they are encoded after the BOS. By default, they are processed after the In-Stack Network Actions in the Network Action Sub-Stack. However, Post-Stack Network Action Opcodes for applications (such as In Situ OAM (IOAM) as described in [RFC9791]) can be added to the In-Stack Network Action Sub-Stack to process Post-Stack Network Actions in a certain order with respect to the Opcodes in the In-Stack Network Action Sub-Stack. 5.3. Node Capability Signaling The encapsulating node that is adding a PSMH MUST ensure that the decapsulating node is capable of Post-Stack MNA processing and can remove the PSMH from the packet. * Each participating transit and decapsulating node MUST signal the Post-Stack network actions that it supports to the encapsulating node. Rajamanickam, et al. Expires 25 December 2026 [Page 10] Internet-Draft Post-Stack MNA Specification June 2026 * Each participating transit and decapsulating node MUST signal its "Readable Label Depth including PSMH" that can be encoded by the encapsulating node. Note that the "Readable Label Depth" defined in [I-D.ietf-mpls-mna-hdr] is extended in this document to include the length of the PSMH. The above capability signaling will be added in the appropriate protocols in future. Signaling details are outside the scope of this document. 6. Processing the Network Action Sub-Stack and Post-Stack Header The processing of the Network Action Sub-Stack described in [I-D.ietf-mpls-mna-hdr] are also applicable to the procedures defined in this document. This section defines the specific responsibilities for nodes along an MPLS path for processing a PSMH. 6.1. Encapsulating Node Responsibilities The encapsulating node MAY add a PSMH to the packet. The location of the header MAY be in accordance with local policy. The location may also be subject to any placement restrictions inherent in the implementation. The encapsulating node MUST NOT add a PSMH to the packet if the decapsulating node does not support PSMH. The encapsulating node MUST add the PSMH in the same order as the corresponding NAS in the MPLS header. 6.2. Transit Node Responsibilities A transit node MAY modify the Ancillary Data in the PSMH. A transit node MUST respect the Unknown Action Handling flag encoded in the corresponding NAS when processing the PSMH. A transit node that removes an NAS with the Select scope, MUST also remove the associated PSMH. 6.3. Penultimate Node Responsibilities In addition to the transit node responsibilities above, the penultimate node MUST NOT remove an Hop-By-Hop (HBH) or Ingress-To- Egress (I2E) NAS [I-D.ietf-mpls-mna-hdr] and the associated PSMH when the NAS is exposed after removing the forwarding (transport) label. This allows the egress node to receive and process the NAS and the associated PSMH. Rajamanickam, et al. Expires 25 December 2026 [Page 11] Internet-Draft Post-Stack MNA Specification June 2026 6.4. Decapsulating Node Responsibilities The decapsulating node MUST remove the PSMHs from the packet when it removes the NASes. 7. Security Considerations The security considerations in [RFC3032], [RFC9789], and [I-D.ietf-mpls-mna-hdr] also apply to this document. System designers must be aware that information included in Post- Stack Ancillary Data may be transmitted "in the clear." Network actions that require the exchange of sensitive data, must be defined in such a way that the data is encrypted in transit. 8. Operational Considerations The operational considerations in [I-D.ietf-mpls-mna-hdr] also apply to this document. Performance and scale assessments are outside the scope of this document; the authors of any future PS MNA application documents are encouraged to address them. 8.1. Manageability Considerations A PS MNA implementation MAY collect the following counters: * NAS with P flag received * NAS with P flag processed * MNA per-post-stack-network-action counters * Packets with PSMH dropped due to unknown actions * Packets with PSMH skipped due to unknown actions * Packets with PSMH dropped due to malformed PSMH Additionally, tracking both successful invocations and failures for each specific post-stack network action is RECOMMENDED to provide granular visibility. Nodes MAY generate rate-limited notifications or alarms for significant operational events, such as sustained high rates of PS MNA packet drops or frequent encounters of malformed PSMH, to alert operators to potential issues. Comprehensive logging of PS MNA processing details and outcomes can aid in the network diagnostics and post-mortem analysis. Rajamanickam, et al. Expires 25 December 2026 [Page 12] Internet-Draft Post-Stack MNA Specification June 2026 9. IANA Considerations 9.1. First Nibble for Post-Stack MPLS Header This document requests that IANA update the "Post-Stack First Nibble" registry created by [RFC9790] to include the PFN value 0x0 for the Post-Stack MPLS Header. This should be added as a new entry in the registry. +==========+=======+========================+===============+ | Protocol | Value | Description | Reference | +==========+=======+========================+===============+ | MPLS | 0x0 | Post-Stack MPLS Header | This document | +----------+-------+------------------------+---------------+ Table 2: Post-Stack First Nibble Registry 9.2. Post-Stack MPLS Header Types Registry This document requests that IANA create a new registry with the name "Post-Stack MPLS Header Types" as follows. The registration procedures for this registry are "IETF Review", "Experimental Use", and "Private Use". The fields are "Type" (integer), "Description" (string), and "Reference" (string). Note: This is a new registry group in parallel with other MPLS registries (e.g., Post-Stack First Nibble); an example location can be: https://www.iana.org/assignments/post-stack-mpls-header-types/post- stack-mpls-header-types.xhtml The Registration Procedure(s) for this registry are: +=============+==================+===============+ | Type | Description | Reference | +=============+==================+===============+ | 0 | Reserved | This document | +-------------+------------------+---------------+ | 1-65520 | IETF Review | This document | +-------------+------------------+---------------+ | 65521-65524 | Experimental Use | This document | +-------------+------------------+---------------+ | 65525-65535 | Private Use | This document | +-------------+------------------+---------------+ Table 3: Post-Stack MPLS Header Types Registry Rajamanickam, et al. Expires 25 December 2026 [Page 13] Internet-Draft Post-Stack MNA Specification June 2026 The initial assignments for this registry are: +======+============================+===============+ | Type | Description | Reference | +======+============================+===============+ | 0 | Reserved | This document | +------+----------------------------+---------------+ | 1 | MNA Post-Stack Header Type | This document | +------+----------------------------+---------------+ Table 4: Post-Stack MPLS Header Types 9.3. Network Action Opcodes IANA maintains the "Network Action Opcodes" registry that will be created by [I-D.ietf-mpls-mna-hdr]. IANA is requested to perform two actions for this registry: 1. Assign two code points for network actions defined in this document. 2. Add a new column to indicate whether network action opcodes may be used In-Stack or Post-Stack. The resulting entries in the registry are as follows: Rajamanickam, et al. Expires 25 December 2026 [Page 14] Internet-Draft Post-Stack MNA Specification June 2026 +========+=============+================+=========================+ | Opcode | Description | In-Stack Only, | Reference | | | | Post-Stack | | | | | Only, In-Stack | | | | | and Post-Stack | | +========+=============+================+=========================+ | 0 | Reserved | Not Applicable | [I-D.ietf-mpls-mna-hdr] | +--------+-------------+----------------+-------------------------+ | 1 | Flag-Based | In-Stack Only | [I-D.ietf-mpls-mna-hdr] | | | Network | | | | | Action | | | | | Indicators | | | | | without AD | | | +--------+-------------+----------------+-------------------------+ | 2 | No | In-Stack and | [I-D.ietf-mpls-mna-hdr] | | | operation | Post-Stack | | | | Opcode | | | +--------+-------------+----------------+-------------------------+ | TBA1 | Post-Stack | In-Stack Only | This document | | | MPLS Header | | | | | Start | | | | | Offset | | | +--------+-------------+----------------+-------------------------+ | TBA2 | Post-Stack | In-Stack Only | This document | | | MPLS Header | | | | | End Offset | | | +--------+-------------+----------------+-------------------------+ | 127 | Opcode | In-Stack and | [I-D.ietf-mpls-mna-hdr] | | | Range | Post-Stack | | | | Extension | | | | | Beyond 127 | | | +--------+-------------+----------------+-------------------------+ Table 5: Network Action Opcodes 10. References 10.1. Normative References [I-D.ietf-mpls-mna-hdr] Rajamanickam, J., Gandhi, R., Zigler, R., Song, H., and K. Kompella, "MPLS Network Action (MNA) Sub-Stack Specification including In-Stack Network Actions and Data", Work in Progress, Internet-Draft, draft-ietf-mpls- mna-hdr-21, 24 February 2026, . Rajamanickam, et al. Expires 25 December 2026 [Page 15] Internet-Draft Post-Stack MNA Specification June 2026 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001, . [RFC4385] Bryant, S., Swallow, G., Martini, L., and D. McPherson, "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385, February 2006, . [RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic Class" Field", RFC 5462, DOI 10.17487/RFC5462, February 2009, . [RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., "MPLS Generic Associated Channel", RFC 5586, DOI 10.17487/RFC5586, June 2009, . [RFC6291] Andersson, L., van Helvoort, H., Bonica, R., Romascanu, D., and S. Mansfield, "Guidelines for the Use of the "OAM" Acronym in the IETF", BCP 161, RFC 6291, DOI 10.17487/RFC6291, June 2011, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9017] Andersson, L., Kompella, K., and A. Farrel, "Special- Purpose Label Terminology", RFC 9017, DOI 10.17487/RFC9017, April 2021, . [RFC9789] Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS Network Actions (MNAs) Framework", RFC 9789, DOI 10.17487/RFC9789, July 2025, . Rajamanickam, et al. Expires 25 December 2026 [Page 16] Internet-Draft Post-Stack MNA Specification June 2026 [RFC9790] Kompella, K., Bryant, S., Bocci, M., Mirsky, G., Ed., Andersson, L., and J. Dong, "IANA Registry and Processing Recommendations for the First Nibble Following a Label Stack", RFC 9790, DOI 10.17487/RFC9790, July 2025, . 10.2. Informative References [RFC9613] Bocci, M., Ed., Bryant, S., and J. Drake, "Requirements for Solutions that Support MPLS Network Actions (MNAs)", RFC 9613, DOI 10.17487/RFC9613, August 2024, . [RFC9791] Saad, T., Makhijani, K., Song, H., and G. Mirsky, "Use Cases for MPLS Network Action Indicators and Ancillary Data", RFC 9791, DOI 10.17487/RFC9791, July 2025, . Appendix A. Appendix A: Examples A.1. Examples of PSMH Encoding A.1.1. NAS that only Indicates PSMH 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Opcode=2=NoOp| 0 |1|IHS|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ [0| ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |1| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=1 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP |R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: NAS that only indicates PSMH In this example, as shown in Figure 5, the NAS may encode only the presence of PSMH. The PSMH starts immediately after the BOS. Rajamanickam, et al. Expires 25 December 2026 [Page 17] Internet-Draft Post-Stack MNA Specification June 2026 A.1.2. NAS that Indicates PSMH Start Offset 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode=TBA1 | Post-Stack Offset = 2 |1|IHS|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ [0| ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |1| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=1 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP |R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: NAS Indicates PSMH Start Offset The NAS may encode the start offset of the PSMH with a non-zero value, for example, when it is after another header such as G-ACh or Control Word header. In this example, as shown in Figure 6, the PSMH starts at an offset of 8 octets after the BoS. A.1.3. Post-Stack Network Actions with Two Opcodes Rajamanickam, et al. Expires 25 December 2026 [Page 18] Internet-Draft Post-Stack MNA Specification June 2026 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Opcode=2=NoOp| 0 |1|IHS|1| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=3 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=2|R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=3|R|R| PS-NAL=1 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Optional Payload + Padding | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Post-Stack Network Actions with Two Opcodes In this example, as shown in Figure 7, the PSMH encodes two different Post-Stack Network Actions. Details: PSMH-Len=3: This is the total length of PSMH. MNA-PS-OP=2: Post-Stack Network Action Opcode 2. PS-NAL=0: Post-Stack Network Action does not contain any additional data. MNA-PS-OP=3: Post-Stack Network Action Opcode 3. PS-NAL=1: Post-Stack Network Action contains 1 additional 4 octet Ancillary Data. A.1.4. Post-Stack Network Actions with two Different Scopes Rajamanickam, et al. Expires 25 December 2026 [Page 19] Internet-Draft Post-Stack MNA Specification June 2026 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Opcode=2=NoOp| 0 |1|HBH|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode=TBA1 | Post-Stack Offset = 2 |1|I2E|1| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=1 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=2|R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=2 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=3|R|R| PS-NAL=1 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Optional Payload + Padding | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Post-Stack Network Actions with two Different Scopes In this example, as shown in Figure 8, the PSMH encodes two differently scoped Post-Stack Network Actions. The first scope is HBH, and the second scope is I2E. Details: The offset of the HBH scoped Post-Stack Network Action is 0. Opcode TBA1 carries the offset of the I2E scoped Post-Stack Network Action. The data is 2, i.e., the PSMH starts 8 octets after the BOS. A.2. Examples of In-Stack and Post-Stack Network Actions A.2.1. NAS with In-Stack and Post-Stack Network Actions Rajamanickam, et al. Expires 25 December 2026 [Page 20] Internet-Draft Post-Stack MNA Specification June 2026 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode=1 | Flag-Based NAIs |1|IHS|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ [0| ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |1| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=1 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP |R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: NAS with In-Stack and Post-Stack Network Actions In this example as shown in Figure 9, the NAS may encode In-Stack Network Actions and indicate the presence of a PSMH. The IHS field indicates the scope of both the In-Stack and Post-Stack Network Actions. A.2.2. NASes with Different In-Stack and Post-Stack Scopes 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode=1 | Flag-Based NAIs |0|HBH|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Opcode=2=NoOp| 0 |1|I2E|1| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=1 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP |R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Rajamanickam, et al. Expires 25 December 2026 [Page 21] Internet-Draft Post-Stack MNA Specification June 2026 Figure 10: NASes with Different In-Stack and Post-Stack Scopes In this example as shown in Figure 10, the label stack may carry In- Stack Network Actions with HBH scope and Post-Stack Network Actions with I2E scope. In this case, there will be two NASes in the label stack. In this case, the first NAS will encode the In-Stack Network Action with the HBH scope and the second NAS will encode the presence of I2E-scoped Post-Stack Network Action. Acknowledgments The authors would like to thank the authors and contributors of the draft-ietf-mpls-mna-hdr as this document borrows some text from the earlier version of that document. The authors would like to thank Greg Mirsky, Loa Andersson, Haoyu Song, Adrian Farrel, Yao Liu, and Joel Halpern for reviewing this document and providing many useful comments. Thank you to Chongfeng Xie for the Opsdir review that helped improve this document. Contributors The following people have substantially contributed to this document: John Drake Juniper Networks United States Email: jdrake@juniper.net Authors' Addresses Jaganbabu Rajamanickam (editor) Cisco Systems, Inc. Canada Email: jrajaman@cisco.com Rakesh Gandhi (editor) Cisco Systems, Inc. Canada Email: rgandhi@cisco.com Royi Zigler Broadcom Email: royi.zigler@broadcom.com Rajamanickam, et al. Expires 25 December 2026 [Page 22] Internet-Draft Post-Stack MNA Specification June 2026 Jie Dong Huawei Technologies Beijing China Email: jie.dong@huawei.com Jisu Bhattacharya Cisco Systems, Inc. U.S.A. Email: jisu@cisco.com Rajamanickam, et al. Expires 25 December 2026 [Page 23]