MPLS Working Group J. Rajamanickam, Ed. Internet-Draft R. Gandhi, Ed. Intended status: Standards Track Cisco Systems, Inc. Expires: 17 July 2026 R. Zigler Broadcom J. Dong Huawei Technologies 13 January 2026 Post-Stack MPLS Network Action (MNA) Solution draft-ietf-mpls-mna-ps-hdr-05 Abstract This document defines the Post-Stack MPLS Network Action (MNA) solution for carrying Network Actions and Ancillary Data after the MPLS label stack, based on the In-Stack MNA solution defined in "MPLS Network Action (MNA) Sub-Stack Solution." 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 solution document addresses the Post-Stack network action and Post-Stack data-specific requirements found in RFC 9613. 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 17 July 2026. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Rajamanickam, et al. Expires 17 July 2026 [Page 1] Internet-Draft Post-Stack MNA Solution January 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 . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Post-Stack MPLS Header Presence Bit Carried in In-Stack MNA Sub-Stack . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2. Post-Stack MPLS Header Encoding . . . . . . . . . . . . . 6 3.2.1. Post-Stack MPLS Header Type . . . . . . . . . . . . . 6 3.2.2. Post-Stack Network Action Encoding . . . . . . . . . 7 3.2.3. Example MPLS Packet with Post-Stack MPLS Header for MNA . . . . . . . . . . . . . . . . . . . . . . . . . 7 4. In-Stack Network Action Special Opcodes . . . . . . . . . . . 9 4.1. In-Stack Network Action Opcode for PSMH Start Offset for MNA . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. In-Stack Network Action Opcode for Offset of End of Post-Stack MPLS Header for MNA . . . . . . . . . . . . . 9 5. Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.1. Processing Rules for P Bit . . . . . . . . . . . . . . . 10 5.2. Network Action Processing Order . . . . . . . . . . . . . 10 5.3. Node Capability Signaling . . . . . . . . . . . . . . . . 10 6. Node Responsibilities . . . . . . . . . . . . . . . . . . . . 11 6.1. Encapsulating Node Responsibilities . . . . . . . . . . . 11 6.2. Transit Node Responsibilities . . . . . . . . . . . . . . 11 6.3. Penultimate Node Responsibilities . . . . . . . . . . . . 12 6.4. Decapsulating Node Responsibilities . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8.1. First Nibble for Post-Stack MPLS Header . . . . . . . . . 12 8.2. Post-Stack MPLS Header Types Registry . . . . . . . . . . 12 8.3. Network Action Opcodes . . . . . . . . . . . . . . . . . 13 9. Appendix A: Examples . . . . . . . . . . . . . . . . . . . . 14 9.1. Examples of Post-Stack MPLS Header Encoding . . . . . . . 14 9.1.1. NAS that only Indicates Post-Stack MPLS Header . . . 14 9.1.2. NAS that Indicates Post-Stack MPLS Header Start Offset . . . . . . . . . . . . . . . . . . . . . . . 15 9.1.3. Post-Stack Network Actions with Two Opcodes . . . . . 16 Rajamanickam, et al. Expires 17 July 2026 [Page 2] Internet-Draft Post-Stack MNA Solution January 2026 9.1.4. Post-Stack Network Action with two Different Scopes . . . . . . . . . . . . . . . . . . . . . . . 17 9.2. In-Stack and Post-Stack Network Actions . . . . . . . . . 18 9.2.1. NAS with In-Stack and Post-Stack NAs . . . . . . . . 18 9.2.2. NASes with Different In-Stack and Post-Stack Scopes . . . . . . . . . . . . . . . . . . . . . . . 19 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 10.1. Normative References . . . . . . . . . . . . . . . . . . 20 10.2. Informative References . . . . . . . . . . . . . . . . . 21 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21 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. Forthcoming applications 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. AD can be used to carry additional information, such as data for In Situ OAM (IOAM) 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 MNA. This document defines the Post-Stack MPLS Network Action (MNA) solution for carrying Network Actions and Ancillary Data after the MPLS label stack. The solution is based on the In-Stack MNA solution defined in [I-D.ietf-mpls-mna-hdr]. The solution addresses the Post- Stack network action and Post-Stack data-specific requirements found 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. Rajamanickam, et al. Expires 17 July 2026 [Page 3] Internet-Draft Post-Stack MNA Solution January 2026 2.2. Abbreviations The terminology defined in [RFC9789] and [RFC9613] are used in this document. +==============+===================+=========================+ | Abbreviation | Meaning | Reference | +==============+===================+=========================+ | AD | Ancillary Data | [RFC9613] | +--------------+-------------------+-------------------------+ | bSPL | Base Special | [RFC9017] | | | Purpose Label | | +--------------+-------------------+-------------------------+ | BOS | Bottom Of Stack | [RFC3032] | +--------------+-------------------+-------------------------+ | 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 | [RFC3032] | +--------------+-------------------+-------------------------+ | 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 | | Rajamanickam, et al. Expires 17 July 2026 [Page 4] Internet-Draft Post-Stack MNA Solution January 2026 | | Bit | | +--------------+-------------------+-------------------------+ | PSD | Post-Stack Data | [RFC9613] and [RFC9789] | +--------------+-------------------+-------------------------+ | PSMH | Post-Stack MPLS | This document | | | Header | | +--------------+-------------------+-------------------------+ | TC | Traffic Class | [RFC5462] | +--------------+-------------------+-------------------------+ | TTL | Time To Live | [RFC3032] | +--------------+-------------------+-------------------------+ Table 1: Abbreviations 3. Overview The Post-Stack MNA solution contains two main parts: * Post-Stack MPLS Header Presence Bit Carried in In-Stack MNA Sub- Stack * Post-Stack MPLS Header Encoding that includes Post-Stack MPLS Header Type and Post-Stack Network Actions 3.1. Post-Stack MPLS Header 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 Post-Stack MPLS Header 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: Post-Stack MPLS Header Presence Bit Carried in In-Stack MNA Sub-Stack The following flags are carried in In-Stack NAS as defined in [I-D.ietf-mpls-mna-hdr] and shown in Figure 1. These flags are also applicable to Post-Stack MNA: * IHS (2 Bit): Indicates the combined scope of the In-Stack and the Post-Stack Network Actions. In-Stack NAS for each scope with P bit set has its corresponding Post-Stack MPLS Header. Rajamanickam, et al. Expires 17 July 2026 [Page 5] Internet-Draft Post-Stack MNA Solution January 2026 * U (1 Bit): Indicates the combined Unknown Action Handling of the In-Stack and the Post-Stack Network Actions. 3.2. Post-Stack MPLS Header Encoding The Post-Stack MPLS Header (PSMH) for MNA is encoded after the Bottom of the Stack (BOS), 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 for MNA carries one or more Post-Stack Network Actions and their Ancillary Data. The PSMH for MNA consists of two main parts: * Post-Stack MPLS Header Type * Post-Stack Network Action Encoding 3.2.1. Post-Stack MPLS Header Type The Post-Stack MPLS Header type is the top-header for all the Post- Stack Network Actions that are encoded in the PSMH for each scope 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 | TYPE = MNA-POST-STACK-HDR = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Post-Stack MPLS Header Type * PFN (4 bits): The Post-Stack First Nibble (PFN) [RFC9790] identifies the start of the PSMH. A new value (value TBA1) is to be assigned by IANA for the Post-Stack MPLS Header. This value is used for all Post-Stack MPLS Header types, including the type for MNA defined in this document. * Reserve (4 bits): Reserved bits. * PSMH-LEN (8 bits): The PSMH total length in 4-octet units that includes Post-Stack Network Actions. The length excludes the 4-byte PSMH type header. Rajamanickam, et al. Expires 17 July 2026 [Page 6] Internet-Draft Post-Stack MNA Solution January 2026 * TYPE (16 bits): Type is set to 1 to indicate Post-Stack MPLS Header Type for MNA. See Section 8.2. Note that other types of Post-Stack headers (besides MNA) can be defined in the future and is 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 8.3. * 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 associated with the Post-Stack Network Action. * CONTINUED POST-STACK DATA: Further Post-Stack Data associated with the Post-Stack Network Action, as indicated by the value of PS- NAL. The padding rules for Post-Stack Data that does not fill a multiple of 32-bit units are described in the document that defines the NA indicated by the MNA-PS-OP value. 3.2.3. Example MPLS Packet with Post-Stack MPLS Header for MNA Rajamanickam, et al. Expires 17 July 2026 [Page 7] Internet-Draft Post-Stack MNA Solution January 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 | TBA1 |Reserve| PSMH-LEN | TYPE = MNA-POST-STACK-HDR = 1 |PSMHT S +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - M | MNA-PS-OP |R|R| PS-NAL | POST-STACK DATA | | H +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+PSNA | ~ CONTINUED POST-STACK DATA ~ | - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Example MPLS Packet with Post-Stack MPLS Header for MNA An example of an MPLS packet encoding that carries an MPLS header and a Post-Stack MPLS header for the MNA Post-Stack Header type is shown in Figure 4. The encoding contains the following fields. * MPLS HDR: MPLS Header containing an MPLS label stack and MNA Sub- Stacks (NAS). * NAS: Network Action Sub-Stack. It has the P bit set to 1 to indicate the presence of a Post-Stack MPLS header for the MNA Post-Stack Header type 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 Post-Stack MPLS header type (PSMHT) and Post-Stack network actions (PSNA). * PSMHT: Post-Stack MPLS Header Type. This includes the type of Post-Stack MPLS header, version, and Post-Stack MPLS Header length. In this example, the type is set to the MNA Post-Stack Header. * PSNA: Post-Stack Network Actions. Applicable only when the type is set to the MNA Post-Stack Header. An MPLS packet may have more than one Post-Stack Network Action, not shown in Figure 4. Rajamanickam, et al. Expires 17 July 2026 [Page 8] Internet-Draft Post-Stack MNA Solution January 2026 4. In-Stack Network Action Special Opcodes 4.1. In-Stack Network Action Opcode for PSMH Start Offset for MNA Opcode: TBA2 Purpose: This opcode carries the start offset of the PSMH for MNA 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 MPLS BOS in 4-octet units. Scope: This opcode can be used with any scope. This opcode allows the existing Post-Stack Headers (PSHs) [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 MPLS BOS. A data value of 1 indicates that the PSMH starts at 4-octets after the BOS. 4.2. In-Stack Network Action Opcode for Offset of End of Post-Stack MPLS Header for MNA Opcode: TBA3 Purpose: This opcode carries the offset of the end of the Post-Stack MPLS Header for MNA 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 MPLS BOS in 4-octet units. A data value of 5 indicates that the Post-Stack MPLS Header ends at 20-octets after the BOS. Scope: This opcode can be used with any scope. The offset of the end of the PSMH for MNA and the offset of the start of the PSMH for MNA 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 Post-Stack MPLS Header checks). Rajamanickam, et al. Expires 17 July 2026 [Page 9] Internet-Draft Post-Stack MNA Solution January 2026 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 corresponding Post-Stack MPLS Header 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 TBA2. 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 P bit MUST be set to 1 when the network action with opcode TBA3 is added to the In-Stack Network Action Sub-Stack. The node that recognizes the network action with Opcode TBA3 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 Post-Stack MPLS Header as defined in this document. Conversely, the node that does not support the P bit skips processing the Post-Stack MPLS Header 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 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 ingress node that is adding a Post-Stack MPLS Header MUST ensure that the egress node is capable of Post-Stack MNA processing and can remove the PSMH from the packet. * Each participating node MUST signal the Post-Stack network actions that it supports to the encapsulating node. Rajamanickam, et al. Expires 17 July 2026 [Page 10] Internet-Draft Post-Stack MNA Solution January 2026 * Each participating node MUST signal its "Readable Label Depth including Post-Stack MPLS Header" 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 Post-Stack MPLS Header. The above capability signaling will be added in the appropriate protocols. Signaling details are outside the scope of this document. 6. Node Responsibilities This section defines the specific responsibilities for nodes along an MPLS path for processing a Post-Stack MPLS Header. 6.1. Encapsulating Node Responsibilities The encapsulating node MAY add a Post-Stack MPLS Header to the packet in accordance with its policies, the placement restrictions, and the limitations. The encapsulating node MUST NOT add a Post-Stack MPLS Header to the packet if the decapsulating node does not support Post-Stack MPLS Header. If the encapsulating node is also a transit node, then it MUST also respect transit node responsibilities. The encapsulating node MUST add the Post-Stack MPLS Header in the same order as the corresponding In-Stack NAS in the MPLS header. 6.2. Transit Node Responsibilities A transit node MAY modify the Ancillary Data in the Post-Stack MPLS Header. 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. Rajamanickam, et al. Expires 17 July 2026 [Page 11] Internet-Draft Post-Stack MNA Solution January 2026 6.3. Penultimate Node Responsibilities In addition to the transit node responsibilities above, the penultimate node MUST NOT remove an HBH or 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. 6.4. Decapsulating Node Responsibilities The decapsulating node MUST remove the Post-Stack MPLS Headers 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. IANA Considerations 8.1. First Nibble for Post-Stack MPLS Header This document requests that IANA allocate a value (TBA1) for the Post-Stack MPLS Header from the "Post-Stack First Nibble" registry created by [RFC9790] to indicate the start of the PSMH after the BOS. +==========+=======+========================+===============+ | Protocol | Value | Description | Reference | +==========+=======+========================+===============+ | PSMH | TBA1 | Post-Stack MPLS Header | This document | +----------+-------+------------------------+---------------+ Table 2: Post-Stack First Nibble Registry 8.2. 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 procedure for this registry is "IETF Review", "Experimental Use" and "Private Use". The fields are "Type" (integer), "Description" (string), and "Reference" (string). Rajamanickam, et al. Expires 17 July 2026 [Page 12] Internet-Draft Post-Stack MNA Solution January 2026 The assignments for this registry are: +=============+==============================+===============+ | Type | Description | Reference | +=============+==============================+===============+ | 0 | Reserved, not to be assigned | 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 The initial assignment for this registry is: +======+=====================================+===============+ | Type | Description | Reference | +======+=====================================+===============+ | 1 | Post-Stack MPLS Header Type For MNA | This document | +------+-------------------------------------+---------------+ Table 4: Post-Stack MPLS Header Types 8.3. 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 17 July 2026 [Page 13] Internet-Draft Post-Stack MNA Solution January 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 | | | +--------+-------------+----------------+-------------------------+ | TBA2 | Offset of | In-Stack Only | This document | | | Start of | | | | | Post-Stack | | | | | MPLS Header | | | +--------+-------------+----------------+-------------------------+ | TBA3 | Offset of | In-Stack Only | This document | | | End of | | | | | Post-Stack | | | | | MPLS Header | | | +--------+-------------+----------------+-------------------------+ | 127 | Opcode | In-Stack and | [I-D.ietf-mpls-mna-hdr] | | | Range | Post-Stack | | | | Extension | | | | | Beyond 127 | | | +--------+-------------+----------------+-------------------------+ Table 5: Network Action Opcodes 9. Appendix A: Examples 9.1. Examples of Post-Stack MPLS Header Encoding 9.1.1. NAS that only Indicates Post-Stack MPLS Header Rajamanickam, et al. Expires 17 July 2026 [Page 14] Internet-Draft Post-Stack MNA Solution January 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|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ [0| ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |1| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBA1 |Reserve| PSMH-LEN=1 | TYPE = MNA-POST-STACK-HDR = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP |R|R| PS-NAL=0 | POST-STACK DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: NAS that only indicates Post-Stack MPLS Header In some cases, the NAS may encode only the presence of Post-Stack MPLS Header. The Post-Stack MPLS Header starts immediately after the BOS. 9.1.2. NAS that Indicates Post-Stack MPLS Header Start Offset Rajamanickam, et al. Expires 17 July 2026 [Page 15] Internet-Draft Post-Stack MNA Solution January 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=TBA2 | Post-Stack Offset = 2 |1|IHS|0| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ [0| ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |1| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBA1 |Reserve| PSMH-LEN=1 | TYPE = MNA-POST-STACK-HDR = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP |R|R| PS-NAL=0 | POST-STACK DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: NAS Indicates Post-Stack MPLS Header Start Offset The NAS may encode the start offset of the Post-Stack MPLS Header with a non-zero value, for example, when it is after another header such as G-ACh or Control Word header. In this example, the PSMH starts at an offset of 8 bytes after the BoS. 9.1.3. Post-Stack Network Actions with Two Opcodes Rajamanickam, et al. Expires 17 July 2026 [Page 16] Internet-Draft Post-Stack MNA Solution January 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| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBA1 |Reserve| PSMH-LEN=3 | TYPE = MNA-POST-STACK-HDR = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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 NA with Two Opcodes This is an example where the Post-Stack MPLS Header encodes two different Post-Stack Network Actions. Details: PSMH-LEN=3: This is the total length of Post-Stack MPLS Header. MNA-PS-OP=2: Post-Stack NA Opcode 2. PS-NAL=0: Post-Stack Network Action does not contain any additional data. MNA-PS-OP=3: Post-Stack NA Opcode 3. PS-NAL=1: Post-Stack Network Action contains 1 additional 4-octet Ancillary Data. 9.1.4. Post-Stack Network Action with two Different Scopes Rajamanickam, et al. Expires 17 July 2026 [Page 17] Internet-Draft Post-Stack MNA Solution January 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=TBA2 | Post-Stack Offset = 2 |1|I2E|1| NASL=0|U|NAL=0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBA1 |Reserve| PSMH-LEN=1 | TYPE = MNA-POST-STACK-HDR = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=2|R|R| PS-NAL=0 | POST-STACK DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBA1 |Reserve| PSMH-LEN=2 | TYPE = MNA-POST-STACK-HDR = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP=3|R|R| PS-NAL=1 | POST-STACK DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | POST-STACK DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Optional Payload + Padding | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Post-Stack NA with two Different Scopes This is an example of Post-Stack MPLS Header encoding, that encodes two different different scoped Post-Stack Network Actions. The first scope is Hop-By-Hop and the second scope is Ingress-To-Egress. Details: The offset of the Hop-By-Hop scoped Post-Stack Network Action is 0. Opcode TBA2 carries the offset of the Ingress-To-Egress scoped Post-Stack Network Action. The data is 2, i.e., the Post-Stack MPLS Header starts 8 bytes after the MPLS Bottom of Stack. 9.2. In-Stack and Post-Stack Network Actions 9.2.1. NAS with In-Stack and Post-Stack NAs Rajamanickam, et al. Expires 17 July 2026 [Page 18] Internet-Draft Post-Stack MNA Solution January 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| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBA1 |Reserve| PSMH-LEN=1 | TYPE = MNA-POST-STACK-HDR = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP |R|R| PS-NAL=0 | POST-STACK DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: NAS with In-Stack and Post-Stack NAs In some cases, the NAS may encode In-Stack NAs and indicate the presence of a Post-Stack MPLS Header. The IHS field indicates the scope of both the In-Stack and Post-Stack NAs. 9.2.2. 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| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBA1 |Reserve| PSMH-LEN=1 | TYPE = MNA-POST-STACK-HDR = 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MNA-PS-OP |R|R| PS-NAL=0 | POST-STACK DATA | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ ~ Payload ~ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Rajamanickam, et al. Expires 17 July 2026 [Page 19] Internet-Draft Post-Stack MNA Solution January 2026 Figure 10: NASes with Different In-Stack and Post-Stack Scopes In some cases, the label stack may need to carry In-Stack NAs with Hop-By-Hop scope and Post-Stack NAs 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 NA with the Hop-By-Hop scope and the second NAS will encode the presence of I2E-scoped Post-Stack NAs. 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-18, 13 January 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, . Rajamanickam, et al. Expires 17 July 2026 [Page 20] Internet-Draft Post-Stack MNA Solution January 2026 [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, . [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, . [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 [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, . 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. Rajamanickam, et al. Expires 17 July 2026 [Page 21] Internet-Draft Post-Stack MNA Solution January 2026 Contributors The following people have substantially contributed to this document: Jisu Bhattacharya Cisco Systems, Inc. Email: jisu@cisco.com 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 Jie Dong Huawei Technologies Beijing China Email: jie.dong@huawei.com Rajamanickam, et al. Expires 17 July 2026 [Page 22]