MPLS Network Action (MNA) Sub-Stack Solution

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Table of Contents

1. Introduction

[RFC3032] defines the encoding of the MPLS label stack, the basic structure used to define a forwarding path. Forthcoming applications require MPLS packets to perform special network actions and carry optional Ancillary Data (AD) that can affect the packet forwarding decision or trigger Operations, Administration, and Maintenance (OAM) logging, for example. Ancillary Data can be used to carry additional information, such as a network slice identifier or an entropy value for load-balancing. Several MNA applications are described in [I-D.ietf-mpls-mna-usecases]. User-defined network actions allow new, local actions to be defined.¶

Network actions can be encoded with or without Ancillary Data (AD), either in the label stack or after the label stack. This solution document specifies In-stack network action and In-stack data (ISD) specific requirements found in [I-D.ietf-mpls-mna-requirements].¶

This document defines the syntax and semantics of network actions and ancillary data encoded in an MPLS Label Stack. In-stack actions and ancillary data are contained in a Network Action Sub-Stack (NAS), which is recognized by a new base Special Purpose Label (bSPL). This document follows the framework specified in [I-D.ietf-mpls-mna-fwk].¶

2. Conventions Used in This Document

3. Overview

The MPLS Network Action Sub-Stack (NAS) is a set of Label Stack Entries (LSEs) that appear as part of an MPLS Label Stack and serve to encode information about the network actions that should be invoked for the packet. Multiple NASes may appear in a label stack.¶

This document describes how network actions and their optional ancillary data are encoded as part of an NAS as a stack of LSEs. Mechanisms that allow sharing of ancillary data AD between multiple network actions encoded in the same NAS can be described in other documents and do not rely on any explicit provision in the encodings described in this document.¶

5. The MNA Sub-Stack

The MNA Sub-Stack MUST begin with a Format A LSE (Section 4.1). The label field of the LSE contains the MNA bSPL (value TBA) to indicate the presence of the MNA Sub-Stack.¶

The TC and TTL fields of the Format A LSE retain their semantics as defined in [RFC3032] and [RFC5462]. The TTL and TC fields in the Format A LSE are copied from the forwarding label at the top of the label stack. The penultimate node on the path may copy the TTL and TC fields from the preceding LSE to the next LSE on the label stack, overwriting the TTL and TC fields of the next LSE, as specified in Section 3.5 of [RFC3443]. If the node performing this copy is not aware of MNA, this could overwrite the values in the first LSE of the MNA sub-stack.¶

The second LSE in a NAS MUST be a Format B LSE (Section 4.2). This LSE contains an initial opcode plus additional fields that describe the NAS.¶

The Format B LSE (Section 4.2) could optionally carry additional data in Format D (Section 4.4) LSEs, up to the length encoded in the LSE's NAL value.¶

An NAS MAY contain more Format C (Section 4.3) and Format D (Section 4.4) LSEs, up to the length encoded in the NASL value. All Format D LSEs MUST follow a Format C or B LSE and be included in that LSE's NAL value.¶

6. Special Opcodes

Below are the special opcodes used to build a basic In-stack MNA solution. In future, additional special opcodes can be defined and their code-points can be assigned from the "Network Action Opcodes" IANA registry.¶

7. NAS placement in the Label Stack

The node adding an NAS to the label stack will need to place a copy of the NAS where it can be read by the relevant nodes. Each downstream node along the path will have a Readable Label Depth (RLD) [I-D.ietf-mpls-mna-fwk]. If the NAS is to be processed by a downstream MNA-capable node, then the entire NAS MUST be placed so that it is within RLD by the time the packet reaches the downstream MNA-capable node and the NAS MUST NOT appear at the top of the stack at any MNA incapable node on the path.¶

If the label stack is deep, several copies of the NAS may need to be encoded in the label stack.¶

For a NAS with HBH scope, every node will process the top copy of the NAS.¶

For a NAS with Select scope, it is processed by the node that brings it to the top of stack and then the NAS is removed from the stack. The select-scoped NAS needs to be inserted after the forwarding label and needs to be inserted before the next forwarding label. It could be inserted before or after a HBH NAS.¶

For I2E scope, only one copy of the NAS needs to be added at the bottom of the stack.¶

Transit, non-penultimate nodes that pop a forwarding label and expose a copy of a NAS MUST remove it.¶

A node performing Penultimate Hop Popping (PHP) that pops the forwarding label with only the NAS(es) remaining on the stack MUST NOT remove the NAS(es). Instead, it forwards the packet with the NAS(es) at the top of stack to the next node.¶

The node that receives the NAS at the top of the label stack MUST remove it.¶

8. Node Capability Signaling

Encapsulating Node is the node that pushes an NAS on to the Label stack.¶

The encapsulating node MUST make sure that the NAS can be processed by the transit and egress nodes.¶

• The path computation system needs to know the MSD and RLD that can be imposed at the ingress node of a given SR path [RFC8664]. This ensures that the label stack depth of a computed path does not exceed the maximum number of labels (i.e., MSD) the node is capable of imposing and the maximum number of labels that can be read by the MNA-processing nodes in the path. The MSD needs to include the MNA Sub-Stacks to be added.¶
• The node responsible for selecting a path through the MPLS network needs to know and consider the MNA-capabilities and RLD of the transit nodes, and the MNA-capabilities of the end point. it supports.¶
• Information about the capabilities of the nodes may be configured, collected through management protocols, or distributed by control protocols (such as advertising by routing protocols).¶
• The mechanisms by which the capabilities of the nodes are known by the node responsible for selecting a path through the MPLS network are out of scope for this document.¶

9. Processing the Network Action Sub-Stack

This section defines the specific responsibilities for nodes along an LSP.¶

10. Network Action Indicator Opcode Definition

The following information MUST be defined for new Network Action Indicator opcode request in the document that specifies the Network Action.¶

A request for a new NAI MUST include the following information:¶

• Format: The definition of the new Network Action MUST specify the LSE Formats. The opcode can define Network Action in Format B or C or both B and C. Both Format B and C LSEs MAY optionally carry Format D LSE(s).¶
• Scope: The request MUST specify at-least one scope (I2E, HBH, Select) for the Network Action. The request MAY specify more than one scope.¶
• Ancillary Data: A request MUST specify the quantity, syntax, and semantics of any associated Ancillary Data. The Ancillary Data MAY be variable length, but the length MUST be computable based on the data present in the NAS.¶
• Processing: The request MUST specify the detailed procedure for processing the network action.¶
• Interactions: The definition of the new Network Action MUST specify its interaction with other currently defined Network Action if there is any.¶

An assignment for an NAI MAY make requests from any combination of the "Network Action Opcodes" or "Network Action Flags Without Ancillary Data" assignments. This decision should optimize for eventual encoding efficiency. If the NAI does not require any ancillary data, then a flag is preferred as only one bit is used in the encoding.¶

11. Backward Compatibility

This section discusses interactions between MNA-capable and legacy, non-MNA-capable nodes.¶

An MNA-encapsulating node MUST ensure that the MPLS Network Action Sub-Stack indicator is not at the top of the MPLS Label Stack when the packet arrives at a non-MNA-capable node. If such a packet did arrive at a non-MNA-capable node, it will most likely be dropped.¶

Legacy nodes may scan the label stack, potentially looking for a label field containing a bSPL. To ensure that the LSE formats described herein do not appear to contain a bSPL value, the opcode value of 0 has been reserved. By ensuring that there is a non-zero value in the high order 7 bits, we are assured that the high order 20 bits cannot be misinterpreted as containing a bSPL value (0-15).¶

The TC and TTL fields of the Format A LSE are not re-purposed for encoding, as the penultimate node on the MPLS packet path may propagate TTL from the transport (or forwarding) label to the next label on the label stack, overwriting the TTL on the next label. If the penultimate node is a legacy node, it might perform this action, potentially corrupting other values stored in the TC and TTL fields. To protect against this, we retain the TC and TTL fields in the Format A LSE.¶

12. Implementation Status

[Note to the RFC Editor - remove this section before publication, as well as remove the reference to [RFC7942]]¶

This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶

13. Security Considerations

The security considerations in [RFC3032] also apply to this document.¶

In addition, MNA-creates a new dimension in security concerns:¶

• The actions of an encapsulating node can affect any or all of the nodes along the path. In the most common and benign situations, such as a syntactically incorrect packet could result in packet loss or corruption.¶
• The semantics of a network action are unbounded and may be insecure. A network action could be defined that made arbitrary changes to the memory of the forwarding router, which could then be used by the encapsulating node to compromise every MNA-capable router in the network. The IETF needs to ensure that only secure network actions are defined.¶
• The MNA architecture supports locally-defined network actions. For such actions, there will be limited oversight to ensure that the semantics do not create security issues. Implementors and network operators will need to ensure that locally-defined network actions do not compromise the security of the network.¶
• The MNA architecture supports modifying the AD on the intermediate nodes, so the critical network functions should either not rely on the data or should be aware of the risks and use other means to verify the security of the whole network.¶
• The "private Use" opcodes in "Network Action Opcodes" Section 14.4 and "Network Action Flags Without Ancillary Data" Section 14.3 Registry are subject to the considerations described in [RFC8126].¶

14. IANA Considerations

15. Examples

 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=MNA bSPL               | TC  |0|    TTL        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Opcode=1   |         Flags           |R|IHS|S|U|NASL=0 |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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=MNA bSPL                   | TC  |0|    TTL        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Opcode=2  |        Data=0           |R|IHS|S|U|NASL=2 |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Opcode=1  |        Flag-Based NAIs        |0|U| NAIs  |NAL=1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Additional Flag-Based NAIs                |S|Flag-Based-NAIs|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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=bSPL (TBA)             | TC  |0|    TTL        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Opcode=8  |      Ancillary Data     |R|IHS|S|U|NASL=0 |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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=MNA bSPL               | TC  |0|    TTL        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Opcode=10  |    Ancillary Data       |R|IHS|0|U|NASL=1 |NAL=1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|            Ancillary Data                 |S|Ancillary Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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=MNA bSPL               | TC  |0|    TTL        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Opcode=2   |        Data=0           |R|IHS|0|U|NASL=2 |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Opcode=9   |        Ancillary Data         |0|U|  AD   |NAL=1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|            Ancillary Data                 |S|Ancillary Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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=MNA bSPL              | TC  |S|    TTL        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=8    |      Ancillary Data     |R|IHS|S|U|NASL=2 |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=7    |      Ancillary Data7          |S|U|  AD7  |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=1    |      Flag-Based NAIs          |S|U|  NAI  |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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=MNA bSPL           | TC  |S|    TTL        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=8    |      Ancillary Data     |R|IHS|S|U|NASL=3 |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=1    |        0x01                   |S|U|  NAI  |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=7    |      Ancillary Data7          |S|U|  AD7  |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opcode=1    |        0x02                   |S|U|  NAI  |NAL=0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Acknowledgments

The authors of this document would like to thank the MPLS Working Group Open Design Team for the discussions and comments on this document. The authors would also like to thank Amanda Baber for reviewing the IANA Considerations and providing many useful suggestions. The authors would like to thank Loa Andersson, Stewart Bryant, Greg Mirsky, Joel M. Halpern and Adrian Farrel for reviewing this document and providing many useful suggestions. The authors would like to thank Fabian Ihle and Michael Menth, both from University of Tuebingen, for implementing the solution defined in this document in P4 pipeline.¶

Contributors

The following people have substantially contributed to this document:¶

Jisu Bhattacharya
Cisco Systems, Inc.
Email: [email protected]


Bruno Decraene
Orange
Email: [email protected]


Weiqiang Cheng
China Mobile
Email: [email protected]


Xiao Min
ZTE Corp.
Email: [email protected]


Luay Jalil
Verizon
Email: [email protected]


Jie Dong
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing  100095
China
Email: [email protected]


Tianran Zhou
Huawei Technologies
China
Email: [email protected]


Bin Wen
Comcast
Email: [email protected]


Sami Boutros
Ciena
Email: [email protected]


Tony Li
Juniper Networks
United States
Email: [email protected]


John Drake
Juniper Networks
United States
Email: [email protected]

Authors' Addresses