Internet-Draft Post-Stack MNA Specification July 2026
Rajamanickam, et al. Expires 19 January 2027 [Page]
Workgroup:
MPLS Working Group
Internet-Draft:
draft-ietf-mpls-mna-ps-hdr-10
Published:
Intended Status:
Standards Track
Expires:
Authors:
J. Rajamanickam, Ed.
Cisco Systems, Inc.
R. Gandhi, Ed.
Cisco Systems, Inc.
R. Zigler
Broadcom
J. Dong
Huawei Technologies
J. Bhattacharya
Cisco Systems, Inc.

Post-Stack MPLS Network Action (MNA) Header Specification

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.

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 19 January 2027.

▲

Table of Contents

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 purposes, for example, 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 Post-Stack network actions and Post-Stack Data (PSD) are described in [RFC9613].

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. 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].

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.

Table 1: Abbreviations
Abbreviation Meaning Reference
AD Ancillary Data [RFC9613]
bSPL Base Special-Purpose Label [RFC9017]
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]
OAM Operations, Administration, and Maintenance [RFC6291]
P bit Post-Stack MPLS Header Presence Bit This document
PS Post-Stack This document
PSD Post-Stack Data [RFC9613] and [RFC9789]
PSMH Post-Stack MPLS Header This document
TC Traffic Class [RFC5462]
TTL Time to Live [RFC3032]

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 Network Action Sub-Stack

Bit 20 in LSE Format B carried in the In-Stack Network Action Sub-Stack (NAS) described in [RFC9994] 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 [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.

The following additional fields are carried in an NAS as defined in [RFC9994] and shown in Figure 1. These fields are also applicable to Post-Stack MNA as defined below:

  • IHS (2 bits): 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.

The Opcode and the associated ancillary data are encoded as defined in [RFC9994].

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 (PSNAs) 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.
  • 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 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 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 Actions Encoding
  • MNA-PS-OP (7 bits): Post-Stack Network Action Opcode. 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.
  • 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.

3.2.3. Encoding of an MPLS Packet with 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
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
M |           MNA Label (value 4)         | 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|               |
- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -
| |  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: 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 with the following fields.

  • MPLS HDR: MPLS Header containing an MPLS label stack and NAS.
  • NAS: Network Action Sub-Stack. MNA Label set to 4 [RFC9994]. The P bit set to 1 to indicate the presence of the corresponding PSMH in the packet. An MPLS packet may carry more than one NAS, not shown in Figure 4.

The example encoding in Figure 4 contains a PSMH with the following fields.

  • PSMH: Post-Stack MPLS Header. This includes the Post-Stack MPLS Base Header and the Post-Stack Network Actions encoding.
  • B: The Post-Stack MPLS Base Header for the "MNA Post-Stack Header" type.
  • PSNA: Post-Stack Network Actions encoding. Applicable for the "MNA Post-Stack Header" type. An MPLS packet may have more than one Post-Stack Network Actions; this is not shown in Figure 4.

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.

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 [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 Readable Label Depth with PSMH checks).

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 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.

A node that supports the P bit processes the Post-Stack Network Actions in the PSMH as defined in this document. Conversely, a 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

An encapsulating node that adds 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.
  • 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 [RFC9994] is extended in this document to include the length of the PSMH.

The above capability signaling will be added in appropriate protocols in the future. Signaling details are outside the scope of 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 the multicast packets to all its next hops by the Bit-Forwarding Routers (BFR).

[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] is 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 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. 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 [RFC9994] 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 [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.

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 network diagnostics and post-mortem analysis.

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.

Table 2: Post-Stack First Nibble Registry
Protocol Value Description Reference
MPLS 0x0 Post-Stack MPLS Header This document

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:

Table 3: Post-Stack MPLS Header Types Registry
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

The initial assignments for this registry are:

Table 4: Post-Stack MPLS Header Types
Type Description Reference
0 Reserved This document
1 MNA Post-Stack Header Type This document

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:

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 as In-Stack only, Post-Stack only, or In-Stack and Post-Stack.

The resulting entries in the registry are as follows:

Table 5: Network Action Opcodes
Opcode Description In-Stack Only, Post-Stack Only, In-Stack and Post-Stack Reference
0 Reserved Not Applicable [RFC9994]
1 Flag-Based Network Action Indicators without AD In-Stack Only [RFC9994]
2 No-Operation Opcode In-Stack and Post-Stack [RFC9994]
TBA1 Post-Stack MPLS Header Start Offset In-Stack Only This document
TBA2 Post-Stack MPLS Header End Offset In-Stack Only This document
127 Opcode Range Extension Beyond 127 In-Stack and Post-Stack [RFC9994]

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, , <https://www.rfc-editor.org/info/rfc9994>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[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, , <https://www.rfc-editor.org/info/rfc3032>.
[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, , <https://www.rfc-editor.org/info/rfc4385>.
[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, , <https://www.rfc-editor.org/info/rfc5462>.
[RFC5586]
Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed., "MPLS Generic Associated Channel", RFC 5586, DOI 10.17487/RFC5586, , <https://www.rfc-editor.org/info/rfc5586>.
[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, , <https://www.rfc-editor.org/info/rfc6291>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC9017]
Andersson, L., Kompella, K., and A. Farrel, "Special-Purpose Label Terminology", RFC 9017, DOI 10.17487/RFC9017, , <https://www.rfc-editor.org/info/rfc9017>.
[RFC9789]
Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS Network Actions (MNAs) Framework", RFC 9789, DOI 10.17487/RFC9789, , <https://www.rfc-editor.org/info/rfc9789>.
[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, , <https://www.rfc-editor.org/info/rfc9790>.

10.2. Informative References

[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, , <https://www.rfc-editor.org/info/rfc8296>.
[RFC9613]
Bocci, M., Ed., Bryant, S., and J. Drake, "Requirements for Solutions that Support MPLS Network Actions (MNAs)", RFC 9613, DOI 10.17487/RFC9613, , <https://www.rfc-editor.org/info/rfc9613>.
[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, , <https://www.rfc-editor.org/info/rfc9791>.

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 (value 4)         | 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.

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 (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  |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 

 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=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 

 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=NoOp|    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=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 

 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  |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 (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=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                             ~
~                                                               ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.

A.2.3. NAS with a BIER Header 

 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|    Next 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  |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 which 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 set to the number of LSEs in the BIER header (3+N in Figure 11) minus one.

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 and Tony Przygienda for the RtgDir review, which 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
Rakesh Gandhi (editor)
Cisco Systems, Inc.
Canada
Royi Zigler
Broadcom
Jie Dong
Huawei Technologies
Beijing
China
Jisu Bhattacharya
Cisco Systems, Inc.
U.S.A.