MPLS Working Group J. Rajamanickam, Ed. Internet-Draft R. Gandhi, Ed. Updates: 9994 (if approved) Cisco Systems, Inc. Intended status: Standards Track R. Zigler Expires: 1 February 2027 Broadcom J. Dong Huawei Technologies J. Bhattacharya Cisco Systems, Inc. 31 July 2026 Post-Stack MPLS Network Action (MNA) Header Specification draft-ietf-mpls-mna-ps-hdr-13 Abstract This document specifies the Post-Stack MPLS Network Action (MNA) Header encoding and procedures for carrying Network Action encodings and Ancillary Data after the MPLS label stack, based on the MNA Sub- Stack including In-Stack Network Actions and Data specified in RFC 9994. 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. This document updates RFC 9994: the "Network Action Opcodes" registry fields, the Post-Stack MNA applicability of the opcodes, MNA scope, and Unknown action handling. 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 1 February 2027. Rajamanickam, et al. Expires 1 February 2027 [Page 1] Internet-Draft Post-Stack MNA Specification July 2026 Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. 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 . . . . . . . . . . . . . . 4 2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Readable Label Depth . . . . . . . . . . . . . . . . . . 5 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. PSMH Presence Bit Carried in In-Stack Network Action Sub-Stack . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2. PSMH Encoding . . . . . . . . . . . . . . . . . . . . . . 7 3.2.1. Post-Stack MPLS Base Header . . . . . . . . . . . . . 7 3.2.2. Post-Stack Network Actions Encoding . . . . . . . . . 8 3.2.3. Example Encoding of an MPLS Packet with PSMH . . . . 9 4. In-Stack Network Action Special Opcodes . . . . . . . . . . . 11 4.1. Opcode for PSMH Start Offset . . . . . . . . . . . . . . 11 4.2. Opcode for PSMH End Offset . . . . . . . . . . . . . . . 12 5. Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.1. Processing Rules for P Bit . . . . . . . . . . . . . . . 12 5.1.1. Pushing Labels . . . . . . . . . . . . . . . . . . . 13 5.2. Network Action Processing Order . . . . . . . . . . . . . 13 5.3. Node Capability Signaling . . . . . . . . . . . . . . . . 13 5.4. Post-Stack MNA and BIER Header . . . . . . . . . . . . . 14 6. Processing the Network Action Sub-Stack and Post-Stack Header . . . . . . . . . . . . . . . . . . . . . . . . . 14 6.1. Encapsulating Node Responsibilities . . . . . . . . . . . 14 6.2. Transit Node Responsibilities . . . . . . . . . . . . . . 14 6.3. Penultimate Node Responsibilities . . . . . . . . . . . . 15 6.4. Egress Node Responsibilities . . . . . . . . . . . . . . 15 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 8. Operational Considerations . . . . . . . . . . . . . . . . . 15 8.1. Manageability Considerations . . . . . . . . . . . . . . 16 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 9.1. First Nibble for Post-Stack MPLS Header . . . . . . . . . 16 Rajamanickam, et al. Expires 1 February 2027 [Page 2] Internet-Draft Post-Stack MNA Specification July 2026 9.2. Post-Stack MPLS Header Types Registry . . . . . . . . . . 17 9.3. Network Action Opcodes . . . . . . . . . . . . . . . . . 17 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 10.1. Normative References . . . . . . . . . . . . . . . . . . 19 10.2. Informative References . . . . . . . . . . . . . . . . . 20 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 21 A.1. Examples of PSMH Encoding . . . . . . . . . . . . . . . . 21 A.1.1. NAS that only indicates PSMH . . . . . . . . . . . . 21 A.1.2. NAS that Indicates PSMH Start Offset . . . . . . . . 21 A.1.3. Post-Stack Network Actions with Two Opcodes . . . . . 22 A.1.4. Post-Stack Network Actions with Two Different Scopes . . . . . . . . . . . . . . . . . . . . . . . 23 A.2. Examples of In-Stack and Post-Stack Network Actions . . . 25 A.2.1. NAS with In-Stack and Post-Stack Network Actions . . 25 A.2.2. NASes with Different In-Stack and Post-Stack Scopes . . . . . . . . . . . . . . . . . . . . . . . 26 A.2.3. NAS with a BIER Header . . . . . . . . . . . . . . . 26 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 27 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 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, as described in [RFC9791]. AD can be used to carry additional information for network slice purposes, as described in [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. The requirements for PS Network Actions and Post-Stack Data (PSD) are described in [RFC9613]. This document specifies the PS MPLS Network Action (MNA) header encoding and procedures for carrying Network Action encodings and AD after the MPLS label stack. The specification is based on the MNA Sub-Stack, including In-Stack Network Actions and Data, specified in [RFC9994]. This document follows the framework specified in [RFC9789]. Rajamanickam, et al. Expires 1 February 2027 [Page 3] Internet-Draft Post-Stack MNA Specification July 2026 This document updates [RFC9994]: the "Network Action Opcodes" registry fields, the PS MNA applicability of the opcodes, MNA scope, and Unknown action handling. 2. Conventions Used in This Document 2.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2.2. Abbreviations The terminology defined in [RFC9613] is used in this document. +==============+=================================+===============+ | Abbreviation | Meaning | Reference | +==============+=================================+===============+ | AD | Ancillary Data | [RFC9613] | +--------------+---------------------------------+---------------+ | bSPL | Base Special-Purpose Label | [RFC9017] | +--------------+---------------------------------+---------------+ | BIER | Bit Index Explicit Replication | [RFC8296] | +--------------+---------------------------------+---------------+ | BoS | Bottom of Stack | [RFC9789] | +--------------+---------------------------------+---------------+ | HbH | Hop-by-Hop | [RFC9789] | +--------------+---------------------------------+---------------+ | I2E | Ingress to Egress | [RFC9789] | +--------------+---------------------------------+---------------+ | IHS | I2E, HbH, or Select | [RFC9994] | +--------------+---------------------------------+---------------+ | ISD | In-Stack Data | [RFC9613] | +--------------+---------------------------------+---------------+ | LSE | Label Stack Entry | [RFC9789] | +--------------+---------------------------------+---------------+ | MNA | MPLS Network Action | [RFC9789] | +--------------+---------------------------------+---------------+ | NAI | Network Action Indicator | [RFC9613] | +--------------+---------------------------------+---------------+ | NAL | Network Action Length | [RFC9994] | +--------------+---------------------------------+---------------+ | NAS | Network Action Sub-Stack | [RFC9789] | +--------------+---------------------------------+---------------+ | NASL | Network Action Sub-Stack Length | [RFC9994] | Rajamanickam, et al. Expires 1 February 2027 [Page 4] Internet-Draft Post-Stack MNA Specification July 2026 +--------------+---------------------------------+---------------+ | OAM | Operations, Administration, and | [RFC6291] | | | Maintenance | | +--------------+---------------------------------+---------------+ | PFN | Post-Stack First Nibble | [RFC9790] | +--------------+---------------------------------+---------------+ | P bit | Post-Stack MPLS Header Presence | This document | | | Bit | | +--------------+---------------------------------+---------------+ | PS | Post-Stack | This document | +--------------+---------------------------------+---------------+ | PSD | Post-Stack Data | [RFC9613] and | | | | [RFC9789] | +--------------+---------------------------------+---------------+ | PSMH | Post-Stack MPLS Header | This document | +--------------+---------------------------------+---------------+ | PSNA | Post-Stack Network Action | This document | +--------------+---------------------------------+---------------+ | TC | Traffic Class | [RFC5462] | +--------------+---------------------------------+---------------+ | TTL | Time to Live | [RFC3032] | +--------------+---------------------------------+---------------+ | U bit | Unknown action handling bit | [RFC9994] | +--------------+---------------------------------+---------------+ Table 1: Abbreviations 2.3. Readable Label Depth The Readable Label Depth (RLD) is defined in Section 2.3.1 of [RFC9789] as: "the number of LSEs, starting from the top of the stack, that a router can read in an incoming MPLS packet with no performance impact", Since an LSE is one 4-octet unit word in size, this definition is updated by this document to read: "the number of 4-octet unit words, starting from the top of the stack, that a router can read in an incoming MPLS packet with no performance impact". 3. Overview Two main parts are specified to support PS MNA: Rajamanickam, et al. Expires 1 February 2027 [Page 5] Internet-Draft Post-Stack MNA Specification July 2026 * 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 AD. 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 PS MNA. 3.1. PSMH Presence Bit Carried in In-Stack Network Action Sub-Stack The bit at position 20 in LSE Format B, specified as the R (Reserved) bit in Section 4.2 of [RFC9994], in the In-Stack Network Action Sub- Stack (NAS) 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 the Format B LSE in a NAS as defined in [RFC9994] and shown in Figure 1. * S (1 bit): The BoS [RFC3032]. * NASL (4 bits): The Network Action Sub-Stack Length. * NAL (3 bits): The Network Action Length. * Opcode (7 bits): The Opcode encoded as defined in [RFC9994]. * Data (13 bits): The AD encoded as defined in [RFC9994]. This document updates [RFC9994] for the following fields carried in a Format B LSE in a NAS with P bit set: * IHS (2 bits): The scope of all the NAIs encoded in the NAS, as well as in the associated PSMH. * U (1 bit): Indicates the combined Unknown action handling of the NAIs encoded in the NAS as well as the NAIs in the associated PSMH. Rajamanickam, et al. Expires 1 February 2027 [Page 6] Internet-Draft Post-Stack MNA Specification July 2026 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., a non-zero start offset). Network action opcodes to specify the PSMH start and end offsets are 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 (PSNAs) and their AD 3.2.1. Post-Stack MPLS Base Header Post-Stack MPLS Base Header is defined as 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 | Post-Stack MPLS Header Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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. The length excludes the 4-octet Post-Stack MPLS Base Header. * Type (16 bits): Post-Stack MPLS Header Type. See Section 9.2. Note that other types of PSMHs can be defined in the future (besides the "MNA Post-Stack Header" defined in this document) and are outside the scope of this document. Rajamanickam, et al. Expires 1 February 2027 [Page 7] Internet-Draft Post-Stack MNA Specification July 2026 3.2.1.1. Post-Stack MPLS Base Header for MNA A Post-Stack MPLS Base Header shown in Figure 2 is employed for an NAS with the P bit set to carry PS MNA. The Post-Stack MPLS Header Type is set to 1 for "MNA Post-Stack Header" and the PSMH-Len includes the encoding of the Post-Stack Network Actions. If a node does not recognize the Post-Stack MPLS Header Type, the packet is handled based on the setting of the U bit in the NAS. PSMH-Len MUST be greater than 0, otherwise, the packet is recognized as malformed and MUST be dropped regardless of the setting of the U bit in the NAS. If a node encounters an unexpected PFN in a PSMH, the packet is recognized as malformed and MUST be dropped regardless of the setting of the U bit in the NAS. 3.2.2. Post-Stack Network Actions 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 AD 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. Value for MNA-PS-OP is assigned from the same registry shared by Post-Stack and In-Stack network action opcodes. See Section 9.3. * R (2 bits): Reserved bits. MUST be set to zero on transmission and ignored upon receipt. * 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): PSD associated with the Post-Stack Network Action. Rajamanickam, et al. Expires 1 February 2027 [Page 8] Internet-Draft Post-Stack MNA Specification July 2026 * 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 4-octet units are described in the document that defines the Network Action, as indicated by the MNA-PS-OP value. The sum of PS-NAL plus one (for the 4-octet network action word) for all network actions encoded in a PSMH MUST equal the PSMH-Len. If not, the packet is recognized as malformed and MUST be dropped regardless of the setting of the U bit in the NAS. 3.2.2.1. Updates to In-Stack Network Action Opcodes This document updates [RFC9994]: the "Network Action Opcodes" registry fields and the PS MNA applicability of the opcodes defined in [RFC9994] as follows. See Section 9.3. * A document defining a new network action opcode MUST specify the applicability as "In-Stack", "Post-Stack" or "In-Stack and Post- Stack". * In case of opcode 1 for Flag-Based NAIs Without AD, defined in [RFC9994], when carried in Post-Stack, the same bit position of the flags are mapped in ISD and PSD. * The packets with opcode 127, defined in [RFC9994], when carried in Post-Stack, MUST be dropped regardless of the setting of the U bit in the NAS. 3.2.3. Example Encoding of an MPLS Packet with PSMH Rajamanickam, et al. Expires 1 February 2027 [Page 9] Internet-Draft Post-Stack MNA Specification July 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 (value 4) | TC |0| TTL | N P +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A L | Opcode=2 | 0 |1|IHS|0| NASL=0|U|NAL=0| S S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - H ~ |0| ~ D +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ R | |1| | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - | | 0x0 |Reserve| PSMH-Len | Type = MNA Post-Stack Header | B P +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - S | MNA-PS-OP |R|R| PS-NAL | Post-Stack Data | P M +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ S H ~ Continued Post-Stack Data ~ N | ~ ~ A - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Example Encoding of an MPLS Packet with PSMH An example encoding of an MPLS packet shown in Figure 4 consists of the following two main parts: * MPLS HDR: MPLS header includes an MPLS label stack and NAS. * PSMH: Post-Stack MPLS Header includes the Post-Stack MPLS Base Header and the Post-Stack Network Actions encoding. The MPLS header includes the following field: * NAS: Network Action Sub-Stack. MNA Label set to 4 [RFC9994]. The P bit is set to 1 to indicate the presence of the associated PSMH in the packet. An MPLS packet may carry more than one NAS, not shown in Figure 4. The PSMH includes the following fields: * B: The Post-Stack MPLS Base Header for the "MNA Post-Stack Header" type. * PSNA: Post-Stack Network Actions encoding. Applicable to 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. Rajamanickam, et al. Expires 1 February 2027 [Page 10] Internet-Draft Post-Stack MNA Specification July 2026 4. In-Stack Network Action Special Opcodes 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 [RFC9994]). Data: The data value of the LSE contains the offset from the BoS in 4-octet units. A data value of 1 indicates that the PSMH starts at 4 octets after the BoS. Scope: This opcode can be used with any scope. This opcode allows existing Post-Stack Headers [RFC9790], such as the Pseudowire (PW) Control Word (0x0) [RFC4385] and Generic Associated Channel (G-ACh) (0x1) [RFC5586], as well as any other PSH defined in the future, to be placed after the BoS and before the PSMH. A PSMH in the packet that follows the PSHs, MUST be added with opcode TBA1 in the NAS that specifies the offset of the PSMH. If the P bit is set in a given NAS and the PSMH Start Offset opcode is absent, the PSMH is encoded immediately after the BoS. This is the case when no PSH is present. This opcode MUST be added in every NAS beyond the first NAS, when the P bit is set in the NAS, to carry the offset of the associated PSMH. This opcode can be placed in a NAS to process the Post-Stack Network Actions in a certain order with respect to the other Opcodes in the NAS. This opcode can be used with offset value 0 to influence the processing order in a NAS. The opcode has the offset value set to 0 when the PSMH starts immediately after the BoS. The offset value in the opcode TBA1 MUST NOT be geater than the size of the MPLS packet minus the size of the label stack. Otherwise, the packet is recognized as malformed and MUST be dropped regardless of the setting of the U bit in the NAS. Rajamanickam, et al. Expires 1 February 2027 [Page 11] Internet-Draft Post-Stack MNA Specification July 2026 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 [RFC9994]). 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 checking against RLD). When the opcode TBA2 is added in a NAS, it MUST be added immediately after the PSMH Start Offset opcode (value TBA1) when the opcode TBA1 is also present in the NAS. The offset value in the opcode TBA2 MUST be greater than the offset value in the opcode TBA1 but MUST NOT be greater by 256 (maximum size of PSMH-Len). Otherwise, the packet is recognized as malformed and MUST be dropped regardless of the setting of the U bit in the NAS. The offset value in the opcode TBA2 MUST NOT be geater than the size of the MPLS packet minus the size of the label stack. Otherwise, the packet is recognized as malformed and MUST be dropped regardless of the setting of the U bit in the NAS. This opcode is optional and can be placed anywhere in a NAS and does not affect the processing order of the network actions in the NAS and the associated PSMH. 5. Procedure 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 associated PSMH is added in the packet. 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. If the P bit is not set, the node that recognizes the network action with opcode TBA1 MUST process the packet based on the U bit. Rajamanickam, et al. Expires 1 February 2027 [Page 12] Internet-Draft Post-Stack MNA Specification July 2026 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. If the P bit is not set, the node that recognizes the network action with opcode TBA2 MUST process the packet based on the U bit. A node that supports the P bit processes the Post-Stack Network Actions in the PSMH as defined in this document, based on the setting of the U bit in the NAS. Conversely, a node that does not support the P bit simply skips processing the Post-Stack Network Actions in the PSMH, irrespective of the setting of the U bit in the NAS. 5.1.1. Pushing Labels When an MNA-capable node pushes labels as described in Section 7.1 of [RFC9994], and copies the network actions (including their AD) from the received topmost NAS with HbH scope in the new NAS with HbH scope, the P bit and the PSMH start and PSMH end offset opcodes and their ADs containing offset values from the NAS MUST also be copied. In this case, more than one NAS with P bit set employs the same PSMH. However, merging of two NASes with the P bit set is out of scope for this document. 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 and that order can be changed by using the PSMH Start Offset opcode as described in Section 4.1. 5.3. Node Capability Signaling The node responsible for selecting a path through the MPLS network needs to know and consider the PS MNA capabilities of the transit nodes as well as the PS MNA capabilities of the egress node as described in Section 2.3 of [RFC9789]. The encapsulating node also MUST learn about the RLD capabilities of the nodes in the path. Information about the PS MNA capabilities and the RLD of the nodes may be configured, collected through management protocols, or distributed by control protocols (such as advertising by routing protocols). Rajamanickam, et al. Expires 1 February 2027 [Page 13] Internet-Draft Post-Stack MNA Specification July 2026 The node responsible for selecting a path through the MPLS network learns about the PS MNA and RLD capabilities of nodes in the path using the mechanisms that are out of scope for this document. 5.4. Post-Stack MNA and BIER Header [RFC8296] specifies the Bit Index Explicit Replication (BIER) encapsulation for an MPLS network. The BIER header, defined in Section 2 of [RFC8296], is used to replicate and forward multicast packets to all their next hops by Bit-Forwarding Routers (BFRs). [RFC9994] does not specify an encoding format to carry a NAS along with the BIER header that is processed on a BFR. Since the PSMH uses the NAS defined in [RFC9994], the processing of a PSMH on a BFR node is 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 [RFC9994] also applies 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 egress node does not support PSMH. The encapsulating node MUST ensure that the packet with all PSMHs added does not exceed the path MTU, as specified in Section 8 of [RFC9994]. The encapsulating node MUST add the associated PSMH in the same order as the NAS in the MPLS header. Note that duplicate copies of NASes with the P bit set employs the same PSMH. 6.2. Transit Node Responsibilities A transit node within the scope of the MAY modify the AD in the associated PSMH. Such modification MUST NOT change the length of the PSMH. Rajamanickam, et al. Expires 1 February 2027 [Page 14] Internet-Draft Post-Stack MNA Specification July 2026 A transit node that removes a NAS with the P bit set MUST also remove the associated PSMH, whether or not it supports PS MNA processing. A transit node unable to read the complete PSMH due to the RLD limit, simply skips the procesing of the PSMH. A transit node unable to remove the associated PSMH because of either the RLD limit or because it does not implement the PSMH Start Offset opcode, MUST NOT remove the NAS with the P bit set. 6.3. Penultimate Node Responsibilities In addition to the transit node responsibilities above, the penultimate node MUST NOT remove a Hop-by-Hop (HbH) or Ingress to Egress (I2E) NAS [RFC9994] 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. 6.4. Egress Node Responsibilities The egress node MUST remove the associated PSMHs from the packet when it removes the NASes with the P bit set. 7. Security Considerations The security considerations in [RFC3032], [RFC9789], and [RFC9994] also apply to this document. System designers must be aware that information included in Post- Stack AD 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. Otherwise, sensitive data MUST NOT be transmitted using these mechanisms. 8. Operational Considerations The operational considerations in [RFC9994] 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. The MPLS OAM packets, for example using G-ACh (with PFN 0x1) [RFC5586], may be transmitted carrying MNA with PSMH for performing network diagnostics. However, the specification for this is outside the scope of this document. Rajamanickam, et al. Expires 1 February 2027 [Page 15] Internet-Draft Post-Stack MNA Specification July 2026 An operator may consider limiting the number and size of PSMHs as well as number of network actions enabled in the MPLS network. This can help minimize the size of the MPLS packets that routers need to process. 8.1. Manageability Considerations A PS MNA implementation MAY collect the following counters: * Packets with PSMH received * Packets with PSMH exceeding RLD * 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 network diagnostics and post-mortem analysis. 9. IANA Considerations 9.1. First Nibble for Post-Stack MPLS Header This document requests that IANA add the following entry to the "Post-Stack First Nibble" registry created by [RFC9790]. +==========+=======+========================+===============+ | Protocol | Value | Description | Reference | +==========+=======+========================+===============+ | MPLS | 0x0 | Post-Stack MPLS Header | This document | +----------+-------+------------------------+---------------+ Table 2: Post-Stack First Nibble Registry Rajamanickam, et al. Expires 1 February 2027 [Page 16] Internet-Draft Post-Stack MNA Specification July 2026 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" within the "MPLS Network Actions" registry group. The registration procedures for this registry are "IETF Review", "Experimental Use", and "Private Use". The fields are "Type" (integer), "Description" (string), and "Reference" (string). This registry is added under "MPLS Network Actions" because the container is defined by an MNA document. Note that adding this registry there makes discovery easier, but the Type space is not MNA- specific, the document itself anticipates non-MNA PSMH types whose codepoints would then be allocated from a registry added under MNA. The Registration Procedures for this registry are: +=============+=========================+ | Range | Registration Procedures | +=============+=========================+ | 1-65520 | IETF Review | +-------------+-------------------------+ | 65521-65524 | Experimental Use | +-------------+-------------------------+ | 65525-65535 | Private Use | +-------------+-------------------------+ Table 3: Registration Procedures for the Post-Stack MPLS Header Types Registry The initial assignments for this registry are: +======+=======================+===============+ | Type | Description | Reference | +======+=======================+===============+ | 0 | Reserved | This document | +------+-----------------------+---------------+ | 1 | MNA Post-Stack Header | This document | +------+-----------------------+---------------+ Table 4: Post-Stack MPLS Header Types 9.3. Network Action Opcodes IANA maintains the "Network Action Opcodes" registry that is created by [RFC9994]. IANA is requested to perform two actions for this registry: Rajamanickam, et al. Expires 1 February 2027 [Page 17] Internet-Draft Post-Stack MNA Specification July 2026 1. Assign two code points for network actions defined in this document. 2. Add a new column for Applicability with values to indicate whether network action opcodes may be used as In-Stack only, Post- Stack only, or In-Stack and Post-Stack. By default, the unassigned values are applicable for In-Stack and Post-Stack Network Actions. The IANA table with RFC-ietf-mpls-mna-nrp-selector-07 is to be updated to reflect [I-D.ietf-mpls-mna-nrp-selector]. The resulting entries in the registry are as follows: +=======+============+=============+================================+ |Opcode |Description |Applicability|Reference | +=======+============+=============+================================+ |0 |Reserved |Not |[RFC9994] | | | |Applicable | | +-------+------------+-------------+--------------------------------+ |1 |Flag-Based |In-Stack Only|[RFC9994] | | |Network | | | | |Action | | | | |Indicators | | | | |without AD | | | +-------+------------+-------------+--------------------------------+ |2 |No operation|In-Stack and |[RFC9994] | | |opcode |Post-Stack | | +-------+------------+-------------+--------------------------------+ |3 |13-bit NRP |In-Stack Only|[I-D.ietf-mpls-mna-nrp-selector]| | |Selector | | | +-------+------------+-------------+--------------------------------+ |4 |20-bit NRP |In-Stack Only|[I-D.ietf-mpls-mna-nrp-selector]| | |Selector | | | +-------+------------+-------------+--------------------------------+ |5 |20-bit |In-Stack Only|[I-D.ietf-mpls-mna-nrp-selector]| | |Entropy and | | | | |NRP Selector| | | +-------+------------+-------------+--------------------------------+ |6-110 |Unassigned |In-Stack and | | | | |Post-Stack | | +-------+------------+-------------+--------------------------------+ |111-114|Reserved for|In-Stack and |[RFC9994] | | |Experimental|Post-Stack | | | |Use | | | +-------+------------+-------------+--------------------------------+ |115-126|Reserved for|In-Stack and |[RFC9994] | Rajamanickam, et al. Expires 1 February 2027 [Page 18] Internet-Draft Post-Stack MNA Specification July 2026 | |Private Use |Post-Stack | | +-------+------------+-------------+--------------------------------+ |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 Range|In-Stack and |[RFC9994] | | |Extension |Post-Stack | | | |Beyond 127 | | | +-------+------------+-------------+--------------------------------+ Table 5: Network Action Opcodes 10. References 10.1. Normative References [RFC9994] Rajamanickam, J., Ed., Gandhi, R., Ed., Zigler, R., Song, H., and K. Kompella, "MPLS Network Action (MNA) Sub-Stack Specification Including In-Stack Network Actions and Data", RFC 9994, DOI 10.17487/RFC9994, 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, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Rajamanickam, et al. Expires 1 February 2027 [Page 19] Internet-Draft Post-Stack MNA Specification July 2026 [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, . [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 [I-D.ietf-mpls-mna-nrp-selector] Li, T., Beeram, V. P., Drake, J., Saad, T., and I. Meilik, "MPLS Network Actions for Network Resource Partition Selector", Work in Progress, Internet-Draft, draft-ietf- mpls-mna-nrp-selector-07, 11 June 2026, . [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, . [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, . [RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A., Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation for Bit Index Explicit Replication (BIER) in MPLS and Non- MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January 2018, . Rajamanickam, et al. Expires 1 February 2027 [Page 20] Internet-Draft Post-Stack MNA Specification July 2026 [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. 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 (value 4) | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode=2 | 0 |1|IHS|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ |0| ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |1| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=1 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=L |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. A.1.2. NAS that Indicates PSMH Start Offset Rajamanickam, et al. Expires 1 February 2027 [Page 21] Internet-Draft Post-Stack MNA Specification July 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 (value 4) | 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=L |R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: NAS that 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 1 February 2027 [Page 22] Internet-Draft Post-Stack MNA Specification July 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 (value 4) | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode=2 | 0 |1|IHS|1| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=3 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=L |R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=M |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 length of PSMH contents after the Post- Stack MPLS Base Header. MNA-PS-OP=L: Post-Stack Network Action Opcode L. A network action allocation is outside of this document. PS-NAL=0: Post-Stack Network Action does not contain any additional data. MNA-PS-OP=M: Post-Stack Network Action Opcode M. A network action allocation is outside of this document. PS-NAL=1: Post-Stack Network Action contains one additional 4-octet AD. A.1.4. Post-Stack Network Actions with Two Different Scopes Rajamanickam, et al. Expires 1 February 2027 [Page 23] Internet-Draft Post-Stack MNA Specification July 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 (value 4) | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode=2 | 0 |1|HbH|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label (value 4) | 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=L |R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=2 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=M |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 Post- Stack Network Actions with different scopes. The first scope is HbH, and the second scope is I2E. Details: The offset of the HbH-scoped Post-Stack Network Action is 0. PSMH-Len=1: This is the length of PSMH contents after the first Post-Stack MPLS Base Header. MNA-PS-OP=L: Post-Stack Network Action Opcode L. A network action allocation is outside of this document. PS-NAL=0: Post-Stack Network Action does not contain any additional data. 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. Rajamanickam, et al. Expires 1 February 2027 [Page 24] Internet-Draft Post-Stack MNA Specification July 2026 PSMH-Len=2: This is the length of PSMH contents after the second Post-Stack MPLS Base Header. MNA-PS-OP=M: Post-Stack Network Action Opcode M. A network action allocation is outside of this document. PS-NAL=1: Post-Stack Network Action contains one additional 4-octet AD. A.2. Examples of In-Stack and Post-Stack Network Actions A.2.1. NAS with In-Stack and Post-Stack Network Actions 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 (value 4) | 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=L |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. Details: PSMH-Len=1: This is the length of PSMH contents after the first Post-Stack MPLS Base Header. MNA-PS-OP=L: Post-Stack Network Action Opcode L. A network action allocation is outside of this document. Rajamanickam, et al. Expires 1 February 2027 [Page 25] Internet-Draft Post-Stack MNA Specification July 2026 PS-NAL=0: Post-Stack Network Action does not contain any additional data. 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 (value 4) | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode=1 | Flag-Based NAIs |0|HbH|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA Label (value 4) | TC |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Opcode=2 | 0 |1|I2E|1| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x0 |Reserve| PSMH-Len=1 | Type = MNA Post-Stack Header | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=L |R|R| PS-NAL=0 | Post-Stack Data | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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. The first NAS will encode the In-Stack Network Action with the HbH scope and the second NAS will encode the presence of an I2E- scoped Post-Stack Network Action. Details: PSMH-Len=1: This is the length of PSMH contents after the first Post-Stack MPLS Base Header. MNA-PS-OP=L: Post-Stack Network Action Opcode L. A network action allocation is outside of this document. PS-NAL=0: Post-Stack Network Action does not contain any additional data. A.2.3. NAS with a BIER Header Rajamanickam, et al. Expires 1 February 2027 [Page 26] Internet-Draft Post-Stack MNA Specification July 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 (value 4) | TC |0| TTL | N +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A | Opcode=TBA1 |Post-Stack Offset = 2+N |1|I2E|0| NASL=0|U|NAL=0| S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - | BIFT-id (20 bits) | TC |1| TTL | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ B | 0101b | Ver=0 | BSL | Entropy | I +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ E |OAM|Rsv| DSCP | Proto | BFIR-id | R +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ H | | D ~ BitString (N*4 octets) ~ R | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - | 0x0 |Reserve| PSMH-Len=1 | Type = MNA Post-Stack Header | P +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ S | MNA-PS-OP=L |R|R| PS-NAL=0 | Post-Stack Data | MH +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: NAS with a BIER Header In this example, as shown in Figure 11, a multicast packet with a BIER header, defined in Section 2 of [RFC8296], carries an MPLS encapsulation that is processed by non-BFR nodes. A NAS is carried before the BIER header in the MPLS label stack and the associated PSMH is carried below the BIER header. The PSMH start offset is the length of the BIER header that follows the BoS, in 4-octet units: two 4-octet words plus the BitString (N words), i.e., (2+N)*4 octets in Figure 11. The first four octets of the BIER header are the BoS LSE itself and are not counted for the offset. Acknowledgments The authors would like to thank the authors and contributors of RFC 9994, as this document borrows some text from an 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. The authors also thank Chongfeng Xie for the OpsDir review and Tony Rajamanickam, et al. Expires 1 February 2027 [Page 27] Internet-Draft Post-Stack MNA Specification July 2026 Przygienda for the RtgDir review, which helped improve this document. The authors would like to thank Ketan Talaulikar for the IESG review and for providing many useful comments to improve this document. Contributors The following people have substantially contributed to this document: John Drake Juniper Networks United States Email: je_drake@yahoo.com 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 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 1 February 2027 [Page 28]