wdiff rfc9789.alt-original rfc9789.txt

MPLS Working Group

Internet Engineering Task Force (IETF) L. Andersson
Internet-Draft
Request for Comments: 9789 Huawei Technologies
Intended status:
Category: Informational S. Bryant
Expires: 30 June 2025
ISSN: 2070-1721 University of Surrey 5GIC
M. Bocci
Nokia
T. Li
Juniper Networks
27 December 2024
May 2025

MPLS Network Actions Action (MNA) Framework
draft-ietf-mpls-mna-fwk-15

Abstract

This document describes an architectural framework for the MPLS Network Actions
Action (MNA) technologies. MNA technologies are used to indicate
actions that impact the forwarding or other processing (such as
monitoring) of the packet along the Label Switched Path (LSP) of the
packet and to transfer any additional data needed for these actions.

The

This document provides the foundation for the development of a common
set of network actions and information elements supporting additional
operational models and capabilities of MPLS networks.

Status of This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents not an Internet Standards Track specification; it is
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This document is a product of the Internet Engineering Task Force
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This Internet-Draft will expire on 30 June 2025.
https://www.rfc-editor.org/info/rfc9789.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirement Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1. Normative Definitions . . . . . . . . . . . . . . . . 4
1.2.2.
1.3. Abbreviations . . . . . . . . . . . . . . . . . . . . 4
2. Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Scopes . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2. Partial Processing . . . . . . . . . . . . . . . . . . . 8
2.3. Signaling . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.1. Readable Label Depth . . . . . . . . . . . . . . . . 9
2.4. State . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. The MNA Label . . . . . . . . . . . . . . . . . . . . . . 11
3.1.1. Existing Base SPL . . . . . . . . . . . . . . . . . . 11
3.1.2. New Base SPL . . . . . . . . . . . . . . . . . . . . 11
3.1.3. New Extended SPL . . . . . . . . . . . . . . . . . . 11
3.1.4. User-Defined Label . . . . . . . . . . . . . . . . . 12
3.2. TC and TTL . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.1. TC and TTL retained . . . . . . . . . . . . . . . . . 12 Retained
3.2.2. TC and TTL Repurposed . . . . . . . . . . . . . . . . 12
3.3. Length of the NAS . . . . . . . . . . . . . . . . . . . . 13
3.3.1. Last/Continuation Bits . . . . . . . . . . . . . . . 13
3.3.2. Length Field . . . . . . . . . . . . . . . . . . . . 13
3.4. Encoding of Scopes . . . . . . . . . . . . . . . . . . . 14
3.5. Encoding a Network Action . . . . . . . . . . . . . . . . 14
3.5.1. Bit Catalogs . . . . . . . . . . . . . . . . . . . . 14
3.5.2. Operation Codes . . . . . . . . . . . . . . . . . . . 15
3.6. Encoding of Post-Stack Data . . . . . . . . . . . . . . . 15
3.6.1. First Nibble Considerations . . . . . . . . . . . . . 15
4. Semantics . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5. Definition of a Network Action . . . . . . . . . . . . . . . 16
6. Management Considerations . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1.
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
10.2.
9.2. Informative References . . . . . . . . . . . . . . . . . 20
Acknowledgements
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22

1. Introduction

This document describes an architectural framework for the MPLS Network Actions
Action (MNA) technologies. MNA technologies are used to indicate
actions for Label Switched Paths (LSPs) and/or MPLS packets and to
transfer data needed for these actions.

The

This document provides the foundation for the development of a common
set of network actions and information elements supporting additional
operational models and capabilities of MPLS networks. MNA solutions
derived from this framework are intended to address the requirements
found in [RFC9613]. In addition, MNA may support actions that
overlap existing MPLS functionality. This may be beneficial for
numerous reasons, such as making it more efficient to combine
existing functionality and new functions in the same MPLS packet.

MPLS forwarding actions are instructions to MPLS routers to apply
additional actions when forwarding a packet. These might include
load-balancing a packet given its entropy, whether or not to perform
fast-reroute
Fast Reroute on a failure, and whether or not a packet has metadata
relevant to the forwarding actions along the path.

This document generalizes the concept of MPLS "forwarding actions"
into to
"network actions" to that include any action that an MPLS router is
requested to take on the packet. That includes Network actions include any MPLS
forwarding
action, actions but may also include other operations (such as
security functions,
OAM Operations, Administration, and Maintenance (OAM)
procedures, etc.) that are not directly related to forwarding of the
packet. MPLS network actions are always triggered by an MNA packet
but may have implications for subsequent traffic, including non-MNA
packets, as discussed in Section 2.4.

MNA technologies may redefine the semantics of the Label, Traffic
Class (TC), and Time to Live (TTL) fields in an MPLS Label Stack
Entry (LSE) within a Network Action Sub-Stack (NAS).

1.1. Requirement 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. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.

Although this is an Informational document, these conventions are
applied to achieve clarity in the requirements that are presented.

1.2. Terminology

1.2.1. Normative Definitions

This document adopts the definitions of the following terms and
abbreviations from [RFC9613] as normative: "Network Action", "Network
Action Indication Indicator (NAI)", "Ancillary Data (AD)", and "Scope".

In addition, this document also defines the following terms:

Network Action Sub-Stack (NAS): A set of related, contiguous LSEs in
the MPLS label stack for carrying information related to network
actions. The Label, TC, and TTL values in the LSEs in the NAS may
be redefined, but the meaning of the S bit is unchanged.

Network Action Sub-Stack Indicator (NSI): The first LSE in the NAS
contains a special label that indicates the start of the NAS.

1.2.2.

1.3. Abbreviations

+==============+=====================+=====================+
| Abbreviation | Meaning | Reference |
+==============+=====================+=====================+
| AD | Ancillary Data | [RFC9613] |
+--------------+---------------------+---------------------+
| BIER | Bit Index Explicit | [RFC8279] |
| | Replication | |
+--------------+---------------------+---------------------+
| BoS | Bottom of Stack | [RFC6790] |
+--------------+---------------------+---------------------+
| bSPL | Base Special Special- | [RFC9017] |
| | Purpose Label | |
+--------------+---------------------+---------------------+
| ECMP | Equal Cost Equal-Cost | [RFC9522] |
| | Multipath | |
+--------------+---------------------+---------------------+
| EL | Entropy Label | [RFC6790] |
+--------------+---------------------+---------------------+
| ERLD | Entropy Readable | [RFC8662] |
| | Label Depth | |
+--------------+---------------------+---------------------+
| eSPL | Extended Special Special- | [RFC9017] |
| | Purpose Label | |
+--------------+---------------------+---------------------+
| HBH HbH | Hop by hop Hop | In the MNA context, |
| | | this document. |
+--------------+---------------------+---------------------+
| I2E | Ingress to Egress | In the MNA context, |
| | | this document. |
+--------------+---------------------+---------------------+
| IGP | Interior Gateway | |
| | Protocol | |
+--------------+---------------------+---------------------+
| ISD | In-stack data In-Stack Data | [RFC9613] |
+--------------+---------------------+---------------------+
| LSE | Label Stack Entry | [RFC3032] |
+--------------+---------------------+---------------------+
| MNA | MPLS Network Action | [RFC9613] |
| | Actions | |
+--------------+---------------------+---------------------+
| MSD | Maximum SID Depth | [RFC8491] |
+--------------+---------------------+---------------------+
| NAI | Network Action | [RFC9613] |
| | Indicator | |
+--------------+---------------------+---------------------+
| NAS | Network Action Sub- | This document |
| | Stack | |
+--------------+---------------------+---------------------+
| NSI | Network Action Sub- | This document |
| | Stack Indicator | |
+--------------+---------------------+---------------------+
| PSD | Post-stack data Post-Stack Data | [RFC9613] and |
| | | Section 3.6 |
+--------------+---------------------+---------------------+
| RLD | Readable Label | This document |
| | Depth | |
+--------------+---------------------+---------------------+
| SID | Segment Identifier | [RFC8402] |
+--------------+---------------------+---------------------+
| SPL | Special Purpose Special-Purpose | [RFC9017] |
| | Label | |
+--------------+---------------------+---------------------+

Table 1: Abbreviations

2. Structure

An MNA solution specifies one or more network actions to apply to an
MPLS packet. These network actions and their ancillary data may be
carried in sub-stacks within the MPLS label stack and/or post-stack
data. A solution must specify where in the label stack the network
actions action sub-stacks occur,
occur in the label stack, if and how frequently they should be
replicated within the label stack, and how the network action sub-
stack and post-stack data are encoded.

It seems highly likely that some ancillary data will be needed at
many points along an LSP. Replication of ancillary data throughout
the label stack would be highly inefficient, as would a full rewrite
of the label stack at each hop, so hop; thus, MNA allows encoding of network
actions and ancillary data deeper in the label stack, requiring
implementations to look past the first LSE. Processing of the label
stack past the top of stack top-of-stack LSE was first introduced with the Entropy
Label (EL) [RFC6790].

A network action sub-stack contains:

* Network Action Sub-Stack Indicator (NSI): The first LSE in the NAS
contains a special purpose special-purpose label, called the MNA label, which is
used to indicate the start of a network action sub-stack.

* Network Action Indicators (NAI): (NAIs): Optionally, a set of indicators
that describes the set of network actions. If the set of
indicators is not in the sub-stack, a solution could encode them
in post-stack data. A network action is said to be present if
there is an indicator in the packet that invokes the action.

* In-Stack Data (ISD): A set of zero or more LSEs that carry
ancillary data for the network actions that are present. Network
action indicators are not considered ancillary data.

Each network action present in the network action sub-stack may have
zero or more LSEs of in-stack data. The ordering of the in-stack
data LSEs corresponds to the ordering of the network action
indicators. The encoding of the in-stack data, if any, for a network
action must be specified in the document that defines the network
action. In-stack data may be referenced by multiple network actions.

As an example, in-stack data might look like the following label
stack with an embedded NAS:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Action Sub-Stack |0| |
~ ~
| Network Action Sub-Stack continued |0| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |0| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |1| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload |

Figure 1: A label stack Label Stack with an embedded Embedded Network Action Sub-Stack

Certain network actions may also specify that data is carried after
the label stack. This is called post-stack data. The encoding of
the post-stack data, if any, for a network action must be specified
in the document that defines the network action. If multiple network
actions are present and have post-stack data, the ordering of their
post-stack data corresponds to the ordering of the network action
indicators.

As an example, post-stack data might appear as a label stack followed
by post-stack data, followed by the payload:

Figure 2: A label stack followed Label Stack Followed by post-stack data Post-Stack Data

A solution must specify the order for network actions to be applied
to the packet for the actions to have consistent semantics. Since
there are many possible orderings, especially with bit catalogs
(Section 3.5.1), the solution must provide an unambiguous
specification. The precise semantics of an action are dependent on
the contents of the packet, including any ancillary data, and the
state of the router.

This document assumes that the MPLS WG will select not more than one
solution for the encoding of ISD and not more than one solution for
the encoding of PSD.

2.1. Scopes

A network action may need to be processed by every node along the
path,
path or some subset of the nodes along its path. Some of the scopes
that an action may have are:

* Hop-by-hop (HBH): Hop by Hop (HbH): Every node along the path will perform the
action.

* Ingress-to-Egress Ingress to Egress (I2E): Only the last node on the path will
perform the action.

* Select: Only specific nodes along the path will perform the
action.

If a solution supports the select scope, it must describe how it
specifies the set of nodes to perform the actions.

This framework does not place any constraints on the scope of, or the
ancillary data for, a network action. Any network action may appear
in any scope or combination of scopes, may have no ancillary data,
and may require in-stack data, data and/or post-stack data. Some
combinations may be sub-optimal, suboptimal, but this framework does not restrict
the combinations in an MNA solution. A specific MNA solution may
define such constraints.

2.2. Partial Processing

As described in [RFC3031], legacy devices that do not recognize the
MNA label will discard the packet if the top label is the MNA label.

Devices that do recognize the MNA label might not implement all of
the network actions that are present. A solution must specify how
unrecognized network actions that are present should be handled.

One alternative is that an implementation should stop processing
network actions when it encounters an unrecognized network action.
Subsequent present network actions would not be applied. The result
is dependent on the solution's order of operations.

Another alternative is that an implementation should drop any packet
that contains any unrecognized present network actions.

A third alternative is that an implementation should perform all
recognized present network actions, actions but ignore all unrecognized
present network actions.

Other alternatives may also be possible. The solution should specify
the alternative adopted.

In some solutions, an indication may be provided in the packet or in
the action as to how the forwarder should proceed if it does not
recognize the action. Where an action needs to be processed at every
hop, it is recommended that care be taken not to construct an LSP
that traverses nodes that do not support that action. It is
recognised that
recognized that, in some circumstances circumstances, it may not be possible to
construct an LSP that avoids such nodes, such as when a network is
re-converging
reconverging following a failure or when IPFRR IP Fast Reroute (IPFRR)
[RFC5714] is taking place.

2.3. Signaling

A node that wishes to make use of MNA and apply network actions to a
packet must understand the nodes that the packet will transit,
whether or not the nodes support MNA, and the network actions that
are to be invoked. These capabilities are presumed to be signaled by
protocols that are out-of-scope out of scope for this document and are presumed to
have per-network action per-network-action granularity. If a solution requires
alternate signaling, it must specify that explicitly.

2.3.1. Readable Label Depth

Readable Label Depth (RLD) is defined 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. [RFC8662] introduced Entropy
Readable Label Depth (ERLD). Readable Label Depth is the same
concept, but it is generalized and not specifically associated with
the Entropy Label (EL) or MNA.

ERLD is not redundant with RLD because ERLD specifically specifies a value of zero
if a system does not support the Entropy Label. Since a system could
reasonably support MNA or other MPLS functions and needs to advertise
an RLD value but not support the Entropy Label, another advertised
value is required.

A node that pushes an NAS onto the label stack is responsible for
ensuring that all nodes that are expected to process the NAS will
have the entire NAS within their RLD. A node SHOULD use signaling
(e.g., [RFC9088], the signaling described in [RFC9088] and [RFC9089]) to
determine this. An exception might be, for example, when the node
has out-of-band knowledge that all nodes along the path do not have
RLD limitations and thus could avoid the unnecessary overhead of
using signaling.

Per [RFC8662], a node that does not support EL will advertise a value
of zero for its ERLD, so advertising ERLD alone does not suffice in
all cases. A node MAY advertise both ERLD and RLD RLD, and it SHOULD do
so if its ERLD and RLD values are different. Again, if a node has out-
of-band
out-of-band knowledge that all nodes do not have RLD limitations,
then signaling can be avoided. If a node's ERLD and RLD values are
the same, it MAY only advertise ERLD for efficiency reasons. If a
node supports MNA but does not support EL, then it SHOULD advertise
RLD unless it has out-of-band knowledge that no nodes in the domain
have RLD restrictions.

RLD is advertised by an IGP MSD-Type value of (TBA) 3 and MAY be advertised
as a Node Maximum Segment Identifier (SID) Depth (MSD), MSD, Link MSD, or both.

An MNA node MUST use the RLD determined by selecting the first
advertised non-zero value from:

* The RLD advertised for the link. link

* The RLD advertised for the node. node

* The non-zero ERLD for the node. node

A node's RLD is a function of its hardware capabilities and is not
expected to depend on the specifics of the MNA solution.

2.4. State

A network action can affect the state stored in the network. This
implies that a packet may affect how subsequent packets are handled.
In particular, one packet may affect subsequent packets in the same
LSP.

3. Encoding

Several possible ways to encode NAIs have been proposed. This
section summarizes the proposals and some considerations for the
various alternatives.

When network actions are carried in the MPLS label stack, then
regardless of their type, they are represented by a set of LSEs
termed a network action sub-stack Network Action Sub-Stack (NAS). An NAS consists of a
special label, optionally followed by LSEs that specify which network
actions are to be performed on the packet and the in-stack ancillary
data for each indicated network action. Different network actions
may be placed together in one NAS or may be carried in different sub-
stacks.

[RFC9613] requires that a solution not add unnecessary LSEs to the
sub-stack (Section 3.1, (see requirement 9). 9 in Section 3.1 of [RFC9613]).
Accordingly, solutions should also make efficient use of the bits
within the sub-stack (except the S-bit), as inefficient use of the
bits could result in the addition of unnecessary LSEs.

3.1. The MNA Label

The first LSE in a network action sub-stack contains a special label
that indicates a network action sub-stack. A solution has several
choices for this special label.

3.1.1. Existing Base SPL

A solution may reuse an existing Base SPL (bSPL). If it elects to do
so, it must explain how the usage is backward compatible, including
in the case where there is ISD.

If an existing inactive bSPL is selected that will not be backward
compatible, then it must first be retired per [RFC7274] and then
reallocated.

3.1.2. New Base SPL

A solution may select a new bSPL.

3.1.3. New Extended SPL

A solution may select a new Extended SPL (eSPL). If it elects to do
so, it must address the requirement for the minimal number of LSEs.

3.1.4. User-Defined Label

A solution may allow the network operator to define the label that
indicates the network action sub-stack. This creates management
overhead for the network operator to coordinate the use of this label
across all nodes on the path using management or signaling protocols.
The user-defined label could be network-wide or LSP-specific. If a
solution elects to use a user-defined label, the solution should
justify this overhead.

3.2. TC and TTL

In the first LSE of the network action sub-stack, only the 20 bits of
the Label Value value and the Bottom of Stack bit are used by the NSI; the
TC field (3 bits) and the TTL (8 bits) are not used. This could
leave 11 bits that could be used for MNA purposes.

3.2.1. TC and TTL retained Retained

If the solution elects to retain the TC and TTL fields, then the
first LSE of the network action sub-stack would appear as described
in [RFC3032]:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: A Label Stack Entry

Label: Label value, 20 bits
TC: Traffic Class, 3 bits
S: Bottom of Stack, 1 bit
TTL: Time To Live

Figure 3: A Label Stack Entry

Further LSEs would be needed to encode NAIs. If a solution elects to
retain these the TC and TTL fields, it must address the requirement for the
minimal number of LSEs.

3.2.2. TC and TTL Repurposed

If the solution elects to reuse the TC and TTL fields, then the first
LSE of the network action sub-stack would appear as: as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label |x x x|S|x x x x x x x x|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4

Label: Label value, 20 bits
x: Bit available for use in solution definition
S: Bottom of Stack, 1 bit

The solution may use more LSEs to contain NAIs. If a solution elects
to use more LSEs LSEs, it must address the requirement for the minimal
number of LSEs.

3.3. Length of the NAS

A solution must have a mechanism (such as an indication of the length
of the NAS) to enable an implementation to find the end of the NAS.
This must be easily processed even by implementations that do not
understand the full contents of the NAS. Two options are described
below,
below; other solutions may be possible.

3.3.1. Last/Continuation Bits

A solution may use a bit per LSE to indicate whether or not the NAS
continues into the next LSE or not. LSE. The bit may indicate continuation by
being set or by being clear. The overhead of this approach is one
bit per LSE and has the advantage that it can effectively encode an
arbitrarily sized NAS. This approach is efficient if the NAS is
small.

3.3.2. Length Field

A solution may opt to have a fixed size length fixed-size Length field at a fixed
location within the NAS. The fixed size of the length Length field may not
be large enough to support all possible NAS contents. This approach
may be more efficient if the NAS is longer long, but not longer than can be
described by the length Length field.

Advice from one

One hardware designer recommends a length Length field as this minimizes
branching in the logic.

3.4. Encoding of Scopes

A solution may choose to explicitly encode the scope of each action
contained in a network action sub-stack. For example, a NAS might
contain Action A (HBH), (HbH), Action B (HBH), (HbH), and Action C (HBH). (HbH). A
solution may alternately choose to have the scope encoded implicitly,
based on the actions present in the network action sub-stack. For
example, a NAS might contain HBH scope actions: the following actions with HbH scope: A,
B, and C. This choice may have performance implications as an
implementation might have to parse the network actions that are
present in a network action sub-
stack sub-stack only to discover that there are
no actions for it to perform.

For example, suppose that an NAS is embedded in a label stack at a
depth of 6 six LSEs and that the NAS contains 3 three actions, each with
Select scope. These actions are not applicable at the current node
and should be ignored. If the scope is encoded explicitly with each
action, then an implementation must parse each action. However, if
the scope is encoded as part of the NAS, then an implementation need only
needs to parse the start of the NAS and need not parse individual actions.

Solutions need to consider the order of scoped NAIs and their
associated AD within individual sub-stacks and the order of per-scope
sub-stacks
sub-stacks, so that network actions and the AD can be most readily found
and need not be processed by nodes that are not required to handle those
actions.

3.5. Encoding a Network Action

Two options for encoding NAIs are described below, below; other solutions
may be possible. Any solution should allow the encoding of an
arbitrary number of NAIs.

3.5.1. Bit Catalogs

A solution may opt to encode the set of network actions as a list of
bits, sometimes known as a catalog. The solution must provide a
mechanism to determine how many LSEs are devoted to the catalog when
the NAIs are carried in-stack. A set bit in the catalog would
indicate that the corresponding network action is present.

Catalogs are efficient if the number of present network actions is
relatively high and if the size of the necessary catalog is small.
For example, if the first 16 actions are all present, a catalog can
encode this in 16 bits. However, if the number of possible actions
is large, then a catalog can become inefficient. Selecting only one
action that is the 256th action would require a catalog of 256 bits,
which would require more than one LSE when the NAIs are carried in-
stack.

A solution may include a bit remapping bit-remapping mechanism so that a given
domain may optimize for its commonly used actions.

3.5.2. Operation Codes

A solution may opt to encode the set of present network actions as a
list of operation codes (opcodes). Each opcode is a fixed number of
bits. The size of the opcode bounds the number of network actions
that the solution can support.

Opcodes are efficient if there are only one or two active network
actions. For example, if an opcode is 8 bits, then two active
network actions could be encoded in 16 bits. However, if 16 actions
are required, then opcodes would consume 128 bits. Opcodes are
efficient at encoding a large number of possible actions. If only
the 256th action is to be selected, that still requires 8 bits.

3.6. Encoding of Post-Stack Data

A solution may carry some NAI and AD as PSD. For ease of parsing,
all AD should be co-located with its NAI.

If there are multiple instances of post-stack data, they should occur
in the same order as their relevant network action sub-stacks and
then in the same order as their relevant network actions occur within
the network action sub-stacks.

3.6.1. First Nibble Considerations

The first nibble after the label stack has been used to convey
information in certain cases [RFC4385]. A consolidated view of the
uses of the first nibble uses is provided in [I-D.ietf-mpls-1stnibble]. [RFC9790].

For example, in [RFC4928] [RFC4928], this nibble is investigated to find out if
it has the value "4" or "6". If it is does not, it is assumed that the
packet payload is not IPv4 or IPv6, and Equal Cost Equal-Cost Multipath (ECMP)
is not performed.

It should be noted that this is an inexact method. For example, an
Ethernet Pseudowire pseudowire without a control word might have "4" or "6" in
the first nibble and thus will be ECMP'ed.

Nevertheless, the method is implemented and deployed, deployed; it is used
today and will be for the foreseeable future.

The use of the first nibble for Bit Index Explicit Replication (BIER)
is specified in [RFC8296]. BIER sets the first nibble to 5. The
same is true for a BIER payload as for any use of the first nibble:
it is not possible to conclude that the payload is BIER even if the
first nibble is set to 5 because an Ethernet pseudowire without a
control word might begin with a 5. However, the BIER approach meets
the design goal of [RFC8296] to determine that the payload is IPv4,
IPv6 or with the header of a pseudowire packet with a control word,
rather than being a payload belonging to a BIER or some other type of
packet.

[RFC4385] allocates 0b0000 for the pseudowire control word and 0b0001
as the control word for the pseudowire Associated Channel Header
(ACH).

A PSD solution should specify the contents of the first nibble, the
actions to be taken for the value, and the interaction with post-
stack data used concurrently by other MPLS applications.

4. Semantics

For MNA to be consistent across implementations and predictable in
operational environments, its semantics need to be entirely
predictable. An MNA solution MUST specify a deterministic order for
processing each of the Network Actions network actions in a packet. Each network
action must specify how it interacts with all other previously
defined network actions. Private network actions are network actions
that are not publicly documented. Private network actions MUST be
included in the ordering of network actions, but the interactions of
private actions with other actions are outside of the scope of this
document.

5. Definition of a Network Action

Network actions should be defined in a document that must contain:

Name: The name of the network action.

Network Action Indicator: The bit position or opcode that indicates
that the network action is active.

Scope: The document should specify which nodes should perform the
network action as described in Section 2.1.

State: The document should specify if the network action can modify
state in the network, and network and, if so, the state that may be modified
and its side effects.

Required/Optional: The document should specify whether a node is
required to perform the network action.

In-Stack Data: The number of LSEs of in-stack data, if any, and its
encoding. If this is of a variable length, then the solution must
specify how an implementation can determine this length without
implementing the network action.

Post-Stack Data: The encoding of post-stack data, if any. If this
is of a variable length, then the solution must specify how an
implementation can determine this length without implementing the
network action.

A solution should create an IANA registry for network actions.

6. Management Considerations

Network operators will need to be cognizant of which network actions
are supported by which nodes and will need to ensure that this is
signaled. Some solutions may require network-wide configuration to
synchronize the use of the labels that indicate the start of an NAS.
Solution documents must make clear clearly state what management considerations
apply to the solutions they are describing. Solutions Solution documents must
describe mechanisms for performing network diagnostics in the
presence of MNAs.

7. Security Considerations

An analysis of the security of MPLS systems is provided in [RFC5920],
which also notes that the MPLS forwarding plane has no built-in
security mechanisms.

Central to the security of MPLS networks is operational security of
the network; network, something that operators of MPLS networks are well
versed in. The deployment of link-level security (e.g., Media Access
Control Security (MACsec) [MACsec]) prevents link traffic observation
covertly acquiring the label stack for an attack. This is
particularly important in the case of a network deploying MNA,
because the MNA information may be sensitive.
Thus Thus, the
confidentiality and authentication achieved through the use of link-level link-
level security is particularly advantageous.

Some additional proposals to add encryption to the MPLS forwarding
plane have been suggested [I-D.ietf-mpls-opportunistic-encrypt], [MPLS-OPP-SEC], but no mechanisms have been
agreed upon at the time of publication of this document. [I-D.ietf-mpls-opportunistic-encrypt]
[MPLS-OPP-SEC] offers hop-by-
hop hop-by-hop security that encrypts the label
stack and is functionally equivalent to that provided by MACsec
[MACsec]. Alternatively, it also offers end-to-end encryption of the
MPLS payload with no cryptographic integrity protection of the MPLS
label stack.

Particular care would be is needed when introducing any end-to-end security
mechanism to allow an in-stack MNA solution that needed needs to employ on-path on-
path modification of the MNA data, data or where post-stack MNA data needed needs
to be examined on-path.

A cornerstone of MPLS security is to protect the network from
processing MPLS labels that originated outside the network.

Operators have considerable experience in excluding MPLS-encoded
packets at the network boundaries boundaries, for example, by excluding all MPLS
packets and all packets that are revealed to be carrying an MPLS
packet as the payload of IP tunnels. Where such packets are accepted
into an MPLS network from an untrusted third party, non-MPLS packets
are immediately encapsulated in an MPLS label stack specified by the
MPLS network operator operator, and MPLS packets have additional label stack
entries imported as specified by the MPLS network operator. Thus, it
is difficult for an attacker to pass an MPLS-encoded packet into a
network or to present any instructions to the network forwarding
system.

Within a single well-managed domain, an adjacent domain may be
considered to be trusted provided that it is sufficiently shielded
from third-party traffic ingress and third-party traffic observation.
In such a situation, no new security vulnerabilities are introduced
by MNA.

In some inter-domain applications (including carrier's carrier) where
a first network's MPLS traffic is encapsulated directly over a second
MPLS network by simply pushing additional MPLS LSEs, the contents of
the first network's payload and label stack may be visible to the
forwarders in the second network. Historically Historically, this has been benign, benign
and indeed useful for ECMP. However, if the first network's traffic
has MNA information information, this may be exposed to MNA-capable forwarders
causing
and cause unpredictable behavior or modification of the customer MPLS
label stack or MPLS payload. This is an increased vulnerability
introduced by MNA that SHOULD be addressed in any MNA solution.

Several mitigations are available to an operator:

a. Reject all incoming packets containing MNA information that do
not come from a trusted network. Note that it may be acceptable
to accept and process MNA information from a trusted network.

b. Fully encapsulate the inbound packet in a new additional MPLS
label stack such that the forwarder finds a Bottom of Stack (BoS)
bit imposed by the carrier network and only finds MNA information
added by the carrier network.

A mitigation that we reject as unsafe is having the ingress LSR Label
Switching Router (LSR) push sufficient additional labels such that
any MNA information received in packets entering the network from a
third-party network is made inaccessible due to it being below the
RLD. This is unsafe in the presence of an overly conservative RLD
value which and can result in the third-party MNA information becoming
visible to and acted on by an MNA forwarder in the carrier network.

8. IANA Considerations

This document requests that

IANA allocate a has allocated the following code point from in the "IGP MSD-Types"
registry [MSD] in within the "Interior Gateway Protocol (IGP)
Parameters" namespace for "Readable registry group:

10. Depth | MPLS | RFC 9789 |
+-------+----------------------+------------+-----------+

Table 2: New IGP MSD-Type

9. References

10.1.

9.1. Normative References

[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>.

[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031,
DOI 10.17487/RFC3031, January 2001,
<https://www.rfc-editor.org/info/rfc3031>.

[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>.

[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>.

[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
<https://www.rfc-editor.org/info/rfc5920>.

[RFC7274] Kompella, K., Andersson, L., and A. Farrel, "Allocating
and Retiring Special-Purpose MPLS Labels", RFC 7274,
DOI 10.17487/RFC7274, June 2014,
<https://www.rfc-editor.org/info/rfc7274>.

[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>.

[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>.

[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>.

10.2.

9.2. Informative References

[I-D.ietf-mpls-opportunistic-encrypt]

[MPLS-OPP-SEC]
Farrel, A. and S. Farrell, "Opportunistic Security in MPLS
Networks", Work in Progress, Internet-Draft, draft-ietf-
mpls-opportunistic-encrypt-03, 28 March 2017,
<https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
opportunistic-encrypt-03>.

[I-D.ietf-mpls-1stnibble]
Kompella, K., Bryant, S., Bocci, M., Mirsky, G.,
Andersson, L., and J. Dong, "IANA Registry and Processing
Recommendations for the First Nibble Following a Label
Stack", Work in Progress, Internet-Draft, draft-ietf-mpls-
1stnibble-13, 5 December 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
1stnibble-13>.

[RFC4928] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
Cost Multipath Treatment in MPLS Networks", BCP 128,
RFC 4928, DOI 10.17487/RFC4928, June 2007,
<https://www.rfc-editor.org/info/rfc4928>.

[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010,
<https://www.rfc-editor.org/info/rfc5714>.

[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.

[RFC8279] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
Explicit Replication (BIER)", RFC 8279,
DOI 10.17487/RFC8279, November 2017,
<https://www.rfc-editor.org/info/rfc8279>.

[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>.

[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.

[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.

[RFC8662] Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
Shakir, R., and J. Tantsura, "Entropy Label for Source
Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
DOI 10.17487/RFC8662, December 2019,
<https://www.rfc-editor.org/info/rfc8662>.

[RFC9088] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label Depth Using IS-IS", RFC 9088,
DOI 10.17487/RFC9088, August 2021,
<https://www.rfc-editor.org/info/rfc9088>.

[RFC9089] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label Depth Using OSPF", RFC 9089,
DOI 10.17487/RFC9089, August 2021,
<https://www.rfc-editor.org/info/rfc9089>.

[RFC9522] Farrel, A., Ed., "Overview and Principles of Internet
Traffic Engineering", RFC 9522, DOI 10.17487/RFC9522,
January 2024, <https://www.rfc-editor.org/info/rfc9522>.

[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, May 2025,
<https://www.rfc-editor.org/info/rfc9790>.

[MACsec] IEEE Computer Society, IEEE, "IEEE 802.1AE Media Standard for Local and metropolitan area
networks-Media Access Control (MAC) Security", August 2006. IEEE Std
802.1AE-2018, DOI 10.1109/ieeestd.2018.8585421, 26
December 2018,
<https://ieeexplore.ieee.org/document/8585421>.

[MSD] IANA, "IGP MSD-Types", December 2024,
<https://www.iana.org/assignments/igp-parameters/igp-
parameters.xhtml#igp-msd-types>.

9.
<https://www.iana.org/assignments/igp-parameters/>.

Acknowledgements

This document is the result of work started in MPLS Open Design Team,
with participation by the MPLS, PALS, and DETNET working groups. Working Groups.

The authors would like to thank Adrian Farrel for his contributions
and contributions.
The authors would also like to thank John Drake, Toerless Eckert, and
Jie Dong for their comments.

Authors' Addresses

Loa Andersson
Huawei Technologies
Email: loa@pi.nu

Stewart Bryant
University of Surrey 5GIC
Email: sb@stewartbryant.com

Matthew Bocci
Nokia
Email: matthew.bocci@nokia.com

Tony Li
Juniper Networks
Email: tony.li@tony.li