]> University of Tuebingen

Tuebingen Germany fabian.ihle@uni-tuebingen.de

University of Tuebingen

Tuebingen Germany michael.menth@uni-tuebingen.de

Routing Multiprotocol Label Switching egress protection resilience The MPLS Egress Protection Framework specifies a fast reroute framework for protecting IP/MPLS services. To that end, bypass tunnels have to be signaled to the Point of Local Repair (PLR). Further, the PLR must maintain the bypass forwarding state on a per-transport-tunnel basis. This document presents the concept of Stateless MNA-based Egress Protection (SMEP). SMEP protects egress routers by providing alternative MPLS egress labels in-stack. This mechanism does not require a bypass forwarding state in PLRs. An example for the application of SMEP is given using a MPLS network action for stack management. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Multiprotocol Label Switching Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction The MPLS egress protection framework in establishes bypass tunnels for egress routers on an egress failure, i.e., on a node or a link failure. This is referred to as egress protection. The protection mechanism relies on a Point of Local Repair (PLR) to perform local failure detection and local repair. Typically, this PLR is the penultimate router. When an egress failure occurs, packets are rerouted to an alternative egress router. The PLR node maintains the bypass forwarding state, which is a mapping of specific labels to bypass tunnels. The bypass tunnels are signaled using existing mechanisms, i.e., via an IGP, or topology-driven label distribution protocols such as LDP. This document defines the concept of Stateless MNA-based Egress Protection (SMEP). SMEP provides an alternative to the rerouting mechanism defined for the PLR, allowing the PLR to be stateless.

Terminology 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 when, and only when, they appear in all capitals, as shown here.

Abbreviations This document makes use of the terms defined in and in . Further abbreviations used in this document: Abbreviations.

Abbreviation	Meaning	Reference
BML	Bypass MPLS Label	This document
SMEP	Stateless MNA-based Egress Protection	This document

Concept of SMEP With SMEP, the egress bypass tunnel is indicated by one or multiple alternative MPLS forwarding labels which are located at the bottom of stack. We call those labels Bypass MPLS Labels (BMLs). On an egress failure, SMEP uses the BMLs in the stack to protect the egress tunnel. The PLR node is required to install the MPLS forwarding entries for the bypass tunnels using the BMLs. However, it does not need to maintain a table that maps transport tunnels to backup paths. Likewise, the PLR is not involved in the signaling of such information. Instead, this information is supplied in the MPLS stack from the ingress node to the PLR. Signaling is only needed between ingress, egress, and the protector, but not with the PLR anymore. Details of the signaling are not contained in this draft. The general concepts and mechanisms described in still apply.

Stack Management with POP-N-LSE Operation Network Action for SMEP The MPLS Network Action (MNA) framework encodes network actions and their data for processing by MPLS nodes. defines the encoding of such network actions and their data in so-called Network Action Substacks (NAS) in the MPLS stack. introduces a network action for stack management. It features a POP-N-LSE operation that pops a number POP-N of LSEs below the NAS. The POP-N-LSE operation facilitates the application of SMEP. To that end, the POP-N-LSE operation is added directly above the BMLs where POP-N is the number of labels in the bypass tunnel. The processing at the PLR is as follows:

1. If the PLR does not detect an egress failure
   • The PLR executes the POP-N-LSE action and pops all BMLs.
   • The packet is forwarded as usual to the egress node.
2. If the PLR detects an egress failure
   • The POP-N-LSE action is ignored and is popped along with the top-of-stack label.
   • The BML is now the top-of-stack label. The packet is forwarded to the protector based on the BML.

Example A simple example topology using MNA-based egress protection with an SR bypass tunnel is shown in . The example network contains an LSP R1-R2-R3 and a bypass tunnel R2-R3'-R3''. The MPLS stack using SMEP pushed by the ingress LER for the example topology is shown in . The label stack contains the forwarding labels L1, L2, L3, L3', and L3'', and the POP-N-LSE operation destined for the PLR. Since there are two BMLs to reach the protector, POP-N = 2. Labels L1 and L2 are used to forward to the penultimate router. Label L3 is used to route to the egress node, and labels L3' and L3'' are used to route to the protector. If the egress link or router R3 fails, the PLR can use the bypass tunnel of router R3' and R3''.

Example using the POP-N-LSE operation for SMEP.

MPLS stack pushed by the ingress LER.

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''. R2 pops the top-of-stack label L3 and forwards the packet as usual to the egress. If the LSR R2 detects an egress failure, it becomes the PLR. The POP-N-LSE action is ignored and the NAS is popped along with the top-of-stack label. This stack is shown in . This time, the BMLs L3' and L3'' are at the top-of-stack. The packet is forwarded according to those labels to the alternative egress node R3''.

MPLS stack at the PLR on egress failure.

Security Considerations The security issues discussed in and in apply to this document.

IANA Considerations This document makes no request of IANA.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document specifies a fast reroute framework for protecting IP/MPLS services and MPLS transport tunnels against egress node and egress link failures. For each type of egress failure, it defines the roles of Point of Local Repair (PLR), protector, and backup egress router and the procedures of establishing a bypass tunnel from a PLR to a protector. It describes the behaviors of these routers in handling an egress failure, including local repair on the PLR and context-based forwarding on the protector. The framework can be used to develop egress protection mechanisms to reduce traffic loss before global repair reacts to an egress failure and control-plane protocols converge on the topology changes due to the egress failure. Cisco Systems, Inc. Cisco Systems, Inc. Broadcom Futurewei Technologies Juniper Networks This document defines the MPLS Network Actions (MNA) sub-stack solution for carrying Network Actions and Ancillary Data in the label stack. MNA 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 solution document specifies In-stack network action and In-stack data specific requirements found in "Requirements for MPLS Network Actions". This document follows the architectural framework for the MNA technologies specified in "MPLS Network Actions (MNA) Framework". University of Tuebingen University of Tuebingen The MPLS Network Action (MNA) framework provides a general mechanism for the encoding and processing of network actions and their data. This document introduces a network action to the MPLS Network Action (MNA) framework for stack management operations including MOVE-N-LSE and POP-N-LSE. The MOVE-N-LSE operation elevates a specified number of LSEs within the stack and the POP-N-LSE operation removes a specified number of LSEs. The stack management operations can be used to facilitate various applications. As an example, a mechanism called HBH preservation which reduces the readable label depth (RLD) requirement in nodes is presented.