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‎draft-ihle-mpls-mna-stack-management.md‎

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normative:
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informative:
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IhMe25-2:
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-: ihme25-2
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title: Reduced Hardware Requirements for MPLS Network Actions Through Stack Management
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author:
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-
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ins: F. Ihle
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name: Fabian Ihle
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org: University of Tuebingen
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-
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ins: M. Menth
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name: Michael Menth
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org: University of Tuebingen
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date: 2025-06-06
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ann: Accepted for publication in ACM/IRTF Applied Networking Research Workshop.
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--- abstract
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The MPLS Network Action (MNA) framework provides a general mechanism for the encoding and processing of network actions and their data.
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This document introduces a network action for stack management operations.
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Those operations include a MOVE-N-LSE and a POP-N-LSE operation.
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With the MOVE-N-LSE operation, a number of N LSEs from below the NAS are brought to the top-of-stack.
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This can be leveraged to enable for more efficient MPLS processing with MNA.
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With the MOVE-N-LSE operation, a number of N LSEs from below the NAS is brought to the top-of-stack.
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This can be leveraged to enable more efficient MPLS processing with MNA.
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An example for this is given in this document using a mechanism called HBH preservation.
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With the POP-N LSE operation, a number of N LSEs below the NAS is popped.
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With the POP-N-LSE operation, a number of N LSEs below the NAS is popped.
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This can be used to facilitate mechanisms such as Stateless MNA-based Egress Protection (SMEP).
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SMEP provides an alternative to the rerouting mechanism defined for the PLR in {{?RFC8679}} allowing the PLR to be stateless.
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In this context, the POP-N LSE operation eliminates the need to encode Bypass MPLS Labels (BML) in the ancillary data of the SMEP network action.
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However, the stack management operations are not tied to specific use cases and may be used in a more generalized way.
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SMEP provides an alternative to the rerouting mechanism defined for the PLR in {{?RFC8679}} allowing the PLR to be stateless by carrying Bypass MPLS Labels (BML) in the stack.
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However, the stack management operations are not tied to specific use cases and may also be applied for various other use cases.
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# Conventions and Definitions
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Further abbreviations used in this document:
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| Abbreviation | Meaning | Reference
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| ---------- | -------------------------------- | -------------------
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| BML | Bypass MPLS Label | {{?I-D.ihle-mpls-mna-stateless-egress-protection-00}}
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| SMEP | Stateless MNA-based Egress Protection | {{?I-D.ihle-mpls-mna-stateless-egress-protection-00}}
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| Abbreviation | Meaning | Reference |
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| ------------ | ------------------------------------- | ----------------------------------------------------- |
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| BML | Bypass MPLS Label | {{?I-D.ihle-mpls-mna-stateless-egress-protection-00}} |
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| SMEP | Stateless MNA-based Egress Protection | {{?I-D.ihle-mpls-mna-stateless-egress-protection-00}} |
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{: #table_abbrev title="Abbreviations."}
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# Stack Management Operations
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## Network Action Encoding
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The network action for stack management operations is encoded as follows:
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- Network Action Indication: The stack management network action is indicated by opcode TBA1.
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- Network action indication: The stack management network action is indicated by opcode TBA1.
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- Format: The stack management network action MUST be encoded using a Format B or a Format C LSE as defined in {{?I-D.ietf-mpls-mna-hdr}}, see {{fig-lseops-encoding-b}} and {{fig-lseops-encoding-c}}.
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- Scope: The stack management network action can be used in any scope, i.e., in the select, HBH, and I2E scope. The scope depends on the use case.
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- Ancillary Data: The stack management network actions requires four bits of in-stack ancillary data per operation (pop and move) to encode the parameter N.
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The parameter N corresponds to the number of LSEs to apply the operation to.
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For example, the POP-N-LSE operation with N=2 pops 2 LSEs that follow after the NAS.
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In Format B, the four least-signifact bits of the ancillary data field contain the number of LSEs for the MOVE-N-LSE operation.
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The next four bits contain the number of LSEs for the POP-N-LSE operation.
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All other ancillary data bits MUST be set to zero and are reserved for future use.
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For Format C, the same encoding of POP-N-LSEs and MOVE-N-LSEs applies to the first data field of the LSE.
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The second data field, i.e., the 4-bit wide ancillary data field, MUST be set to zero and is reserved for future use.
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- Ancillary data: The stack management network action requires four bits of in-stack ancillary data per operation (pop and move) to encode the number of labels, i.e., the numbers `MOVE-N` and `POP-N`.
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- In Format B, the four least-significant bits of the ancillary data field contain the value of `MOVE-N`.
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The next four bits contain the value of `POP-N`.
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All other ancillary data bits MUST be set to zero and are reserved for future use.
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- In Format C, the same encoding of POP-N-LSEs and MOVE-N-LSEs applies to the first data field of the LSE.
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The second data field, i.e., the 4-bit wide ancillary data field, MUST be set to zero and is reserved for future use.
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- Post-Stack Data: None.
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- Post-stack data: None.
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{{fig-lseops-encoding-b}} and {{fig-lseops-encoding-c}} illustrate the stack management operation encoding in Format B and C.
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~~~~
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{::include ./drawings/lseops-encoding-b.txt}
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## Processing
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A node that processes the stack management network action MUST perform the MOVE-N-LSE and POP-N-LSE operation according to the corresponding parameter N > 0.
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If N = 0, the corresponding operation is not performed.
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A node that processes the stack management network action MUST perform the MOVE-N-LSE and POP-N-LSE operation according to the corresponding parameter `N > 0`.
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If `N = 0`, the corresponding operation is not applied.
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- For `MOVE-N > 0`, `MOVE-N` LSEs below the NAS are brought to the top-of-stack.
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- For `POP-N > 0`, `POP-N` LSEs below the NAS are popped.
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The ingress LER MUST ensure that no invalid state, e.g., a MOVE-N-LSE operation with N > \#LSEs below the NAS, results from the processing of the stack management network action.
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The ingress LER MUST ensure that no invalid state, e.g., a MOVE-N-LSE operation with `MOVE-N > #LSEs below the NAS`, results from the processing of the stack management network action.
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If multiple NAS with the stack management network action are present in the stack and are processed by a single node, all values for `MOVE-N` or `POP-N` are summed up respectively.
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The operation, i.e., pop or move, is then applied based on the summed up value.
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# Use Cases and Examples
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This section illustrates one use case for each operation in an example.
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This section explains one use case for each operation and illustrates it in an example.
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## MOVE-N-LSE
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First, we describe the concept of HBH preservation.
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Then, we explain how the MOVE-N-LSE operation is used for this concept, and give an example.
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### HBH Preservation
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In the MNA framework, each node must be able to access the HBH-scoped Network Action Substack (NAS).
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The HBH-scoped NAS must be within the readable label depth (RLD) of each node.
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To achieve this, redundant copies of the NAS are placed at different positions in the MPLS stack {{?I-D.ietf-mpls-mna-hdr}}.
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This increases the overall stack size.
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Further, routers must scan through unrelated stack entries to find the HBH-scoped NAS.
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This adds parsing overhead and is difficult on hardware with limited RLD.
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The HBH preservation mechanism keeps the HBH-scoped NAS within reach of all nodes by placing the NAS always below the top-of-stack label.
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When a node pops the top label, it moves the next forwarding label below the NAS to the top.
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This prevents the NAS from being exposed and removed.
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Nodes can process it without scanning through unrelated labels.
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This avoids redundant copies and reduces RLD pressure {{IhMe25-2}}.
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### MOVE-N-LSE Operation for HBH Preservation
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The stack management network action with the MOVE-N-LSE operation is added to the HBH-scoped NAS with `MOVE-N = 1` to apply the HBH preservation mechanism.
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Each node popping the top-of-stack label and processing this action brings one LSE from below the NAS to the top.
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This prevents the HBH-scoped NAS from being exposed to the top.
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However, this mechanism poses a challenge when there are MNA-incapable nodes on the path.
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An MNA-incapable node pops the top-of-stack forwarding label and exposes the HBH-scoped NAS to the top, leading to packet drop at the next hop.
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To fix this, the MOVE-N-LSE operation can be leveraged to bring LSEs at the preceding node of an MNA-incapable node to the top using a select-scoped NAS.
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This way, forwarding labels for upcoming MNA-incapable nodes are brought to the top and the HBH-scoped NAS is never exposed.
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The MNA-capabilities are signaled to the ingress LER.
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Therefore, the ingress LER places a select-scoped network action for the preceding node of MNA-incapable nodes in the MPLS stack.
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No information about MNA capabilities of neighboring nodes is required in a transit node as all node capabilities are signaled to the ingress LER.
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## POP-N-LSE: Stateless MNA-based Egress Protection (SMEP)
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Stateless MNA-based Egress Protection (SMEP) is a proposed mechanism within the MNA framework {{?I-D.ihle-mpls-mna-stateless-egress-protection-00}}.
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SMEP encodes egress protection information such as Bypass MPLS Labels (BMLs) in the MPLS label stack as network actions.
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This approach eliminates the need for the Point of Local Repair (PLR) to maintain bypass forwarding states or engage in additional signaling for bypass tunnel mappings.
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Instead, all necessary information is delivered to the PLR via the MPLS stack enabling failover to alternative egress paths in case of egress node or link failures.
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### Example
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An example is illustrated in {{fig-hbh-preservation}}.
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~~~~
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{::include ./drawings/hbh-preservation.txt}
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~~~~
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{: #fig-hbh-preservation title="Example using the MOVE-N-LSE operation."}
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However, in the proposed SMEP network action encoding, the 20 bit BMLs are split across the AD fields of a network action.
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This requires the PLR to extract the BML from the network action first.
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With the POP-N-LSE operation, the BMLs are contained as forwarding LSEs in the MPLS stack without the need to encode the bypass tunnel information in the AD fields of a network action.
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An example is given in {{fig-smep}}.
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In {{fig-hbh-preservation}}, a MNA-capable node is followed by two MNA-incapable nodes.
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Therefore, the stack management network action with `MOVE-N = 2` for the MOVE-N-LSE operation is pushed by the ingress LER as a select-scoped NAS for the LSR R1.
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Further, to enable the HBH preservation mechanism, the stack management action with `MOVE-N = 1` for the MOVE-N-LSEs operation is added to the HBH-scoped NAS.
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R1 brings three LSEs to the top-of-stack, two from the MOVE-N-LSE operation in the select-scoped NAS and one from the HBH-scoped NAS.
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At the MNA-incapable LSRs R2 and R3, the forwarding label is popped without exposing the NAS to the top.
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Finally, the MNA-capable LSR R4 processes the HBH-scoped NAS and brings one LSE from below the NAS to the top.
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## POP-N-LSE
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First, we describe the concept of Stateless MNA-based Egress Protection (SMEP).
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Then, we explain how the POP-N-LSE operation is used for this concept, and give an example.
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### Stateless MNA-based Egress Protection (SMEP)
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The egress protection framework defined in {{?RFC8679}} is comprehensive.
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It provides a mechanism for rerouting traffic in the event of an egress failure, and explains how rerouted services and their associated context can be restored.
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Stateless MNA-based Egress Protection (SMEP) provides an alternative to the rerouting mechanism defined for the PLR, allowing the PLR to be stateless {{?I-D.ihle-mpls-mna-stateless-egress-protection-00}}.
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Thus, the PLR does not need to maintain a table that maps transport tunnels to backup paths.
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Likewise, the PLR is not involved in the signaling of such information.
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Instead, this information is supplied from the ingress to the PLR in the network action.
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Signaling is only needed between ingress, egress, and the protector, but not with the PLR anymore.
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However, in the proposed SMEP network action encoding in {{?I-D.ihle-mpls-mna-stateless-egress-protection-00}}, the 20 bit BMLs are split across the AD fields of a network action.
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This requires the PLR to extract the BML from the network action.
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### POP-N-LSE Operation for SMEP
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The POP-N-LSE operation simplifies the application of SMEP.
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The BMLs are contained as forwarding LSEs in the MPLS stack without the need to encode the bypass tunnel information in the AD fields of a network action.
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The processing at the PLR is as follows:
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1. If the PLR does not detect an egress failure
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- The PLR executes the POP-N-LSE action and pops all BMLs.
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- The packet is forwarded as usual to the egress node.
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2. If the PLR detects an egress failure
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- The POP-N-LSE action is ignored and is popped along with the top-of-stack label.
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- The BML is now the top-of-stack label. The packet is forwarded to the protector based on the BML.
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### Example
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An example of the POP-N-LSE Operation for SMEP is given in {{fig-smep}}.
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{::include ./drawings/smep.txt}
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The example network contains an LSP R1-R2-R3 and a bypass tunnel R2-R3'-R3''.
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The label stack contains the forwarding labels L1, L2, L3, L3', and L3''.
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If there is no egress failure, the LSR R2 executes the POP-N-LSE action and pops the BMLs L3' and L3''.
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If there is no egress failure, the LSR R2 executes the POP-N-LSE action with `POP-N = 2` and pops the BMLs L3' and L3''.
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The packet is forwarded as usual according to the top-of-stack label L3.
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If the LSR R2 detects an egress failure it becomes the PLR.
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The POP-N-LSE action is ignored and the NAS is popped along with the top-of-stack label.
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This time, the BMLs L3' and L3'' are at the top-of-stack.
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The packet is forwarded according to those labels to the alternative egress node R3''.
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This mechanism requires configuration or an indication to only execute the POP-N-LSE operation if there is no egress failure detected.
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## MOVE-N-LSE: HBH Preservation
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An efficient way to structure the MPLS stack with different NAS is to place to HBH-scoped NAS always below the top-of-stack forwarding label.
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This reduces the required allocation of resources to a minimum as no irrelevant LSEs are parsed to find the HBH NAS located deeper in the stack.
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Placing the HBH-scoped NAS close to the top and popping the top-of-stack forwarding label exposes the HBH-scoped NAS to the top which results in popping this NAS.
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To prevent this, an LSR MAY bring the forwarding label from below the NAS to the top-of-stack using the MOVE-N-LSE operation.
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# Implementation Status
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In the following example, the stack management network action with the MOVE-N-LSE operation is added to the HBH-scoped NAS if one is present in the stack.
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This brings one LSE from below the NAS to the top.
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The MOVE-N-LSEs operation is only applied if the top-of-stack forwarding label is popped.
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However, this mechanism poses a challenge when there are MNA-incapable nodes on the path.
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An MNA-incapable node pops the top-of-stack forwarding label and exposes the HBH-scoped NAS to the top, leading to packet drop at the next hop.
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To fix this, the MOVE-N-LSE operation can be leveraged to bring N LSEs at the preceding node of an MNA-incapable node to the top using a select-scoped NAS.
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This way, forwarding labels for the upcoming N MNA-incapable nodes are brought to the top and the HBH-scoped NAS is never exposed to the top.
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The MNA-capabilities are signaled to the ingress LER.
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Therefore, the ingress LER places a select-scoped network action for the preceding node of MNA-incapable nodes in the MPLS stack.
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No information about MNA capabilities of neighboring nodes is required in a transit node as all node capabilities are signalled to the ingress LER.
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\[Note to the RFC Editor - remove this section before publication, as well as remove the reference to {{?rfc7942}}\]
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If multiple NAS with the stack management network action and N > 0 for the MOVE-N-LSE operation are present in the stack, all values for N are summed up and that number of LSEs below the last processed NAS is brought to the top.
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An example is illustrated in {{fig-hbh-preservation}}.
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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}}.
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The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs.
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Please note that the listing of any individual implementation here does not imply endorsement by the IETF.
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Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors.
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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.
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~~~~
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{::include ./drawings/hbh-preservation.txt}
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~~~~
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{: #fig-hbh-preservation title="Example using the MOVE-N-LSE operation."}
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## University of Tuebingen Implementation
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The stack management network action defined in this document has been implemented using the MOVE-N-LSE operation for HBH preservation with a P4 pipeline {{IhMe25-2}}.
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The implementation code can be found at [https://github.com/uni-tue-kn/P4-MNA](https://github.com/uni-tue-kn/P4-MNA).
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In {{fig-hbh-preservation}}, a MNA-capable node is followed by two MNA-incapable nodes.
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Therefore, the stack management network action with N=2 for the MOVE-N-LSE operation is pushed by the ingress LER as a select-scoped NAS for the LSR R1.
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Further, to enable the HBH preservation mechanism, the stack management action with N=1 for the MOVE-N-LSEs operation is added to the HBH-scoped NAS.
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LSR R1 brings three LSEs to the top-of-stack, two from the MOVE-N-LSE operation in the select-scoped NAS and one from the HBH-scoped NAS.
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At the MNA-incapable LSRs R2 and R3, the forwarding label is popped without exposing the HBH-scoped NAS to the top.
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Finally, the MNA-capable LSR R4 processes the HBH-scoped NAS and brings one LSE from below the NAS to the top as indicated in the HBH-scoped NAS.
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# Security Considerations
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This document requests that IANA allocates a new codepoint with the name "Stack Management" in the "Network Action Opcodes Registry" introduced in {{?I-D.ietf-mpls-mna-hdr}}.
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| MNA Opcode | Description | Reference
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| ---------- | -------------------------------- | -------------------
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| TBA1 | Stack Managament| This document
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| MNA Opcode | Description | Reference |
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| ---------- | ---------------- | ------------- |
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| TBA1 | Stack Management | This document |
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{: #table_iana title="Stack Management Opcode IANA allocation."}
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