MPLS Network Action (MNA) Post-Stack Header Specification
draft-ietf-mpls-mna-ps-hdr-17
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
|
|
|---|---|---|---|
| Authors | Jaganbabu Rajamanickam , Rakesh Gandhi , Royi Zigler , Jie Dong , Jisu Bhattacharya | ||
| Last updated | 2026-08-05 | ||
| Replaces | draft-jags-mpls-ps-mna-hdr | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
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RTGDIR IETF Last Call Review due 2026-06-09
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
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| Document shepherd | Adrian Farrel | ||
| Shepherd write-up | Show Last changed 2026-04-14 | ||
| IESG | IESG state | IESG Evaluation | |
| Consensus boilerplate | Yes | ||
| Telechat date |
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Has a DISCUSS. Has enough positions to pass once DISCUSS positions are resolved. |
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| Responsible AD | Jim Guichard | ||
| Send notices to | adrian@olddog.co.uk | ||
| IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-mpls-mna-ps-hdr-17
MPLS Working Group J. Rajamanickam, Ed.
Internet-Draft R. Gandhi, Ed.
Updates: 9994, 9789 (if approved) Cisco Systems, Inc.
Intended status: Standards Track R. Zigler
Expires: 6 February 2027 Broadcom
J. Dong
Huawei Technologies
J. Bhattacharya
Cisco Systems, Inc.
5 August 2026
MPLS Network Action (MNA) Post-Stack Header Specification
draft-ietf-mpls-mna-ps-hdr-17
Abstract
This document specifies the MPLS Network Action (MNA) Post-Stack
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.
This document updates RFC 9789: the definition of Readable Label
Depth (RLD).
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."
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This Internet-Draft will expire on 6 February 2027.
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Processing Rules for P Bit . . . . . . . . . . . . . . . 13
5.1.1. Pushing Labels . . . . . . . . . . . . . . . . . . . 13
5.2. Network Action Processing Order . . . . . . . . . . . . . 14
5.3. Node Capability Signaling . . . . . . . . . . . . . . . . 14
5.4. Post-Stack MNA and BIER Header . . . . . . . . . . . . . 14
6. Processing the Network Action Sub-Stack and Post-Stack
Header . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Encapsulating Node Responsibilities . . . . . . . . . . . 15
6.2. Transit Node Responsibilities . . . . . . . . . . . . . . 15
6.3. Penultimate Node Responsibilities . . . . . . . . . . . . 16
6.4. Egress Node Responsibilities . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Operational Considerations . . . . . . . . . . . . . . . . . 16
8.1. Manageability Considerations . . . . . . . . . . . . . . 17
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9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
9.1. First Nibble for Post-Stack MPLS Header . . . . . . . . . 17
9.2. Post-Stack MPLS Header Types Registry . . . . . . . . . . 18
9.3. Network Action Opcodes . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 22
A.1. Examples of PSMH Encoding . . . . . . . . . . . . . . . . 22
A.1.1. NAS that only indicates PSMH . . . . . . . . . . . . 22
A.1.2. NAS that Indicates PSMH Start Offset . . . . . . . . 23
A.1.3. Post-Stack Network Actions with Two Opcodes . . . . . 23
A.1.4. Post-Stack Network Actions with Two Different
Scopes . . . . . . . . . . . . . . . . . . . . . . . 24
A.2. Examples of In-Stack and Post-Stack Network Actions . . . 26
A.2.1. NAS with In-Stack and Post-Stack Network Actions . . 26
A.2.2. NASes with Different In-Stack and Post-Stack
Scopes . . . . . . . . . . . . . . . . . . . . . . . 27
A.2.3. NAS with a BIER Header . . . . . . . . . . . . . . . 28
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 29
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
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) MPLS Network Action (MNA).
The requirements for Post-Stack Network Actions and Post-Stack Data
(PSD) are described in [RFC9613].
This document specifies the MNA Post-Stack 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].
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This document updates [RFC9994]: the "Network Action Opcodes"
registry fields, the Post-Stack MNA applicability of the opcodes, MNA
scope, and Unknown action handling.
This document updates RFC 9789: the definition of Readable Label
Depth (RLD).
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] |
+--------------+---------------------------------+---------------+
| MTU | Maximum Transmission Unit | [RFC3032] |
+--------------+---------------------------------+---------------+
| NAI | Network Action Indicator | [RFC9613] |
+--------------+---------------------------------+---------------+
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| NAL | Network Action Length | [RFC9994] |
+--------------+---------------------------------+---------------+
| NAS | Network Action Sub-Stack | [RFC9789] |
+--------------+---------------------------------+---------------+
| NASL | Network Action Sub-Stack Length | [RFC9994] |
+--------------+---------------------------------+---------------+
| 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] |
+--------------+---------------------------------+---------------+
| PSH | Post-Stack Header | [RFC9790] |
+--------------+---------------------------------+---------------+
| 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 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 word in size, this definition is updated
by this document to read:
"the number of 4-octet words, starting from the top of the stack,
that a router can read in an incoming MPLS packet with no
performance impact".
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3. Overview
Two main parts are specified to support Post-Stack MNA:
* The Post-Stack MPLS Header (PSMH) Presence Bit Carried in an In-
Stack Network Action Sub-Stack (NAS).
* 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 Post-
Stack 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
The bit at position 20 in LSE Format B, specified as the R (Reserved)
bit in Section 4.2 of [RFC9994], in a 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 Network Action
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.
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* 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. For the NAI opcodes (TBA1 and TBA2) defined in this
document, the U bit for the opcode also indicates the unknown
action handling for the network actions in the associated PSMH.
In the absence of these opcodes, the U bit in the LSE Format B in
the NAS also indicates the unknown action handling for the network
actions in the associated PSMH.
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. Valid
range for PSMH-Len is 1 to 255.
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* 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.
As described in Section 1 of [RFC9790], the correct interpretation of
the PFN in a Post-Stack Header (PSH) depends on the context
established by the preceding Label Stack Entry (LSE) or group of
LSEs. For the PFN in a PSMH, that context is provided by the P bit
in the associated NAS.
3.2.1.1. Post-Stack MPLS Base Header for MNA
A Post-Stack MPLS Base Header shown in Figure 2 is employed for a NAS
with the P bit set to carry Post-Stack 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.
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.
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.
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.
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* 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.
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.
3.2.2.1. Updates to In-Stack Network Action Opcodes
This document updates [RFC9994]: the "Network Action Opcodes"
registry fields and the Post-Stack 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".
* The opcode 1 for Flag-Based Network Action Indicators without AD,
defined in [RFC9994], is not applicable as Post-Stack.
* The opcode 2 for No operation opcode, defined in [RFC9994], is
also applicable as Post-Stack.
* The opcode 127 for opcode range extension beyond 127, defined in
[RFC9994], is also applicable as Post-Stack.
3.2.3. Example Encoding of an MPLS Packet with PSMH
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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 in the NAS is 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.
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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 that points to
the start of the PSMH Base Header from the BoS in 4-octet units. A
data value of 1 indicates that the start of the PSMH Base Header is 4
octets after the BoS. Valid range in both 13-bit data in Format B
LSE and 20-bit data in Format C LSE is 0 to 2046, and uses the
11-bits from the right-hand side. The unused bits in the data MUST
be set to 0 and ignored on receipt.
Scope: This opcode can be used with any scope.
This opcode allows existing PSHs [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 that does not start immediately after the BoS (that is, one or
more PSHs are present between the BoS and the PSMH) MUST be indicated
by opcode TBA1 in the associated NAS.
If the P bit is set in the first NAS and the PSMH Start Offset opcode
is absent, the PSMH start offset defaults to 0 (that is, the PSMH is
encoded immediately after the BoS). In every NAS beyond the first
NAS with the P bit set, this opcode MUST be present and MUST carry
the offset of the associated PSMH. If this opcode is absent in a NAS
beyond the first NAS with the P bit set, the packet is recognized as
malformed and MUST be dropped regardless of the setting of the U bit.
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 an offset value of 0 to influence
the processing order in a NAS. Aside from this, the opcode TBA1 does
not have any other implication for other opcodes.
The opcode has the offset value set to 0 when the PSMH starts
immediately after the BoS.
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The offset MUST be within the valid range and MUST NOT be greater
than the size of the MPLS packet minus the size of the label stack,
where all sizes are in 4-octet units. Otherwise, the packet is
recognized as malformed and MUST be dropped regardless of the setting
of the U bit.
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 that points to
the last 4-octet entry in the PSMH from the BoS in 4-octet units. A
data value of 2 indicates that the last 4-octet entry of the PSMH is
8 octets after the BoS. Valid range in both 13-bit data in Format B
LSE and 20-bit data in Format C LSE is 1 to 2047, and uses the
11-bits from the right-hand side. The unused bits in the data MUST
be set to 0 and ignored on receipt.
Scope: This opcode can be used with any scope.
The start and end offsets of the PSMH allow the NAS parser
implementation to know the size and depth of the PSMH without having
to parse the PSMH (e.g., for checking against RLD).
The opcode TBA2 is optional. When the opcode TBA2 is added to a NAS
along with the opcode TBA1, it MUST be added immediately after the
opcode TBA1 and with the same U bit value in both NAIs. Otherwise,
the opcode TBA2 can be placed anywhere in a NAS. The opcode TBA2
does not affect the processing order of the network actions in the
NAS and the associated PSMH. Aside from this, the opcode TBA2 does
not have any other implication for other opcodes.
The end offset minus the start offset MUST equal the PSMH-Len of the
associated PSMH. Otherwise, the packet is recognized as malformed
and MUST be dropped regardless of the setting of the U bit.
When the opcode TBA1 is present in the NAS, the end offset value in
the opcode TBA2 MUST be greater than the start offset value in the
opcode TBA1 but MUST NOT be greater by more than 255 (maximum size of
PSMH-Len). Otherwise, the packet is recognized as malformed and MUST
be dropped regardless of the setting of the U bit.
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The offset MUST be within the valid range and MUST NOT be greater
than the size of the MPLS packet minus the size of the label stack,
where all sizes are in 4-octet units. Otherwise, the packet is
recognized as malformed and MUST be dropped regardless of the setting
of the U bit.
5. Procedure
5.1. Processing Rules for P Bit
The P bit MUST be set to 1 in a NAS when the associated PSMH is added
to the packet.
The P bit MUST be set to 1 when the network action with opcode TBA1
is added to a NAS. 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.
The P bit MUST be set to 1 when the network action with opcode TBA2
is added to a NAS. 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.
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.
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 received NAS MUST also be
copied to the new NAS. In this case, more than one HbH NAS with the
P bit set shares the PSMH; such NASes are identified by having the
same offsets in the duplicate copies of the NASes.
However, merging of two NASes with the P bit set is out of scope for
this document.
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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 NAS 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 Post-Stack MNA capabilities of the
transit and egress nodes, as described in Section 2.3 of [RFC9789].
Information about the Post-Stack MNA capabilities of the nodes may be
configured, collected through management protocols, or distributed by
control protocols (such as advertising by routing protocols). The
node responsible for selecting a path through the MPLS network learns
about the Post-Stack MNA capabilities of nodes using mechanisms that
are out of scope for this document.
The node responsible for selecting a path through the MPLS network
needs to know the path maximum transmission unit (MTU) of nodes in
the path [RFC3032]. Information about the path MTU of the nodes may
be configured, collected through management protocols, or distributed
by control protocols (such as MTU signaling for Label Distribution
Protocol [RFC3988] and Path MTU Discovery for IPv6 [RFC8201]). The
mechanisms to learn the path MTU of nodes in the path are out of
scope for this document.
The encapsulating node needs to learn about the RLD of the nodes in
the path as described in Section 2.3.1 of [RFC9789], so that Post-
Stack MNA can be read by nodes in the path. Information about the
RLD of the nodes may be configured, collected through management
protocols, or distributed by control protocols (such as advertising
by routing protocols). The mechanisms to learn the RLD of nodes in
the path 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).
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[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. Any BIER-related examples in
this document are informational and do not define normative behavior
for BFR processing.
6. Processing the Network Action Sub-Stack and Post-Stack Header
The processing of the NAS 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 NOT add a PSMH to the packet if the
transit node that removes the NAS does not support the P bit.
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 with the exception that the duplicate
copies of NASes with the P bit set share the PSMH.
6.2. Transit Node Responsibilities
A transit node processing a network action MAY modify the AD in the
PSMH. Such modification MUST NOT change the length of the PSMH.
A transit node that supports the P bit and removes a NAS with the P
bit set MUST also remove the associated PSMH. If the PSMH is shared
with a duplicate copy of the NAS with the P bit set that remains in
the packet, the PSMH MUST be retained.
A transit node unable to read the complete PSMH due to the RLD limit
simply skips the processing of the PSMH.
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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. If a PSMH is shared by the
duplicate copies of the NASes with the P bit set, the PSMH MUST be
removed only after all duplicate copies of the NASes are removed.
7. Security Considerations
The security considerations in [RFC3032], [RFC9789], and [RFC9994]
also apply to this document.
The MPLS domain border nodes MUST ensure that the MPLS packets with
Post-Stack MNA from any domain with a different administrative
control can be filtered.
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 Post-Stack MNA application
documents are encouraged to address them.
The MPLS OAM packets, for example using G-ACh (with PFN 0x1)
[RFC5586], may be transmitted with a PSMH that carries Post-Stack MNA
for network diagnostics. However, the specification for this is
outside the scope of this document.
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An operator may consider limiting the number and size of PSMHs as
well as the 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 Post-Stack 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 Post-Stack MNA packet drops or frequent encounters of
malformed PSMH, to alert operators to potential issues.
Comprehensive logging of Post-Stack 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
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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. This 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:
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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 current IANA table with RFC-ietf-mpls-mna-nrp-selector-07 refers
to [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] |
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| |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,
<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, March 1997,
<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, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.
[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, <https://www.rfc-editor.org/info/rfc5462>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[RFC9017] Andersson, L., Kompella, K., and A. Farrel, "Special-
Purpose Label Terminology", RFC 9017,
DOI 10.17487/RFC9017, April 2021,
<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, July 2025,
<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, July 2025,
<https://www.rfc-editor.org/info/rfc9790>.
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,
<https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
mna-nrp-selector-07>.
[RFC3988] Black, B. and K. Kompella, "Maximum Transmission Unit
Signalling Extensions for the Label Distribution
Protocol", RFC 3988, DOI 10.17487/RFC3988, February 2005,
<https://www.rfc-editor.org/info/rfc3988>.
[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, <https://www.rfc-editor.org/info/rfc4385>.
[RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586,
DOI 10.17487/RFC5586, June 2009,
<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, June 2011,
<https://www.rfc-editor.org/info/rfc6291>.
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[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
[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, <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, August 2024,
<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, July 2025,
<https://www.rfc-editor.org/info/rfc9791>.
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
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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=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
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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
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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 | S1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| MNA-PS-OP=L |R|R| PS-NAL=0 | Post-Stack Data | E1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| 0x0 |Reserve| PSMH-Len=2 | Type = MNA Post-Stack Header | S2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| MNA-PS-OP=M |R|R| PS-NAL=1 | Post-Stack Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| Post-Stack Data | E2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+
| |
| 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.
For the first PSMH, the start offset is S1 = 0, the end offset is E1
= 1, PSMH-Len is 1 which equals E1 minus S1.
For the second PSMH, the start offset is S2 = 2 and the end offset is
E2 = 4, PSMH-Len is 2 which equals E2 minus S2.
Details:
The start 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.
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PS-NAL=0: Post-Stack Network Action does not contain any
additional data.
Opcode TBA1 carries the start offset of the I2E-scoped Post-Stack
Network Action. The data is 2, i.e., there are 8 octets between
the BoS and the PSMH start.
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:
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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.2.2. 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.
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PS-NAL=0: Post-Stack Network Action does not contain any
additional data.
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| 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.
This example is informational and does not define normative behavior
for BFR processing. The specification on how to process this packet
is out of scope for this document.
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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, Ted Lemon for
Intdir, and Tony Przygienda for the RtgDir review, which helped
improve this document. The authors would like to thank Ketan
Talaulikar, Eric Vyncke, Mahesh Jethanandani, Gunter Van de Velde,
and Deb Cooley 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
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Jisu Bhattacharya
Cisco Systems, Inc.
U.S.A.
Email: jisu@cisco.com
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