Connecting MPLS-SPRING Islands over IP Networks
draft-xu-spring-islands-connection-over-ip-05

Abstract

MPLS-SPRING is an MPLS-based source routing paradigm in which a sender of a packet is allowed to partially or completely specify the route the packet takes through the network by imposing stacked MPLS labels to the packet. To facilitate the incremental deployment of this new technology, this document describes a mechanism which allows the outermost LSP be replaced by an IP-based tunnel.

Status of This Memo

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Terminology
• 3. Packet Forwarding Procedures
• 4. Acknowledgements
• 5. IANA Considerations
• 6. Security Considerations
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Authors' Addresses

1. Introduction

MPLS-SPRING [I-D.ietf-spring-segment-routing-mpls] is a MPLS-based source routing paradigm in which a sender of a packet is allowed to partially or completely specify the route the packet takes through the network by imposing stacked MPLS labels to the packet. To facilitate the incremental deployment of this new technology, this document describes a mechanism which allows the outermost LSP to be replaced by an IP-based tunnel (e.g., MPLS-in-IP/GRE tunnel [RFC4023], MPLS-in-UDP tunnel [RFC7510] or MPLS-in-L2TPv3 tunnel [RFC4817] and etc) when the nexthop along the LSP is not MPLS-SPRING-enabled. The tunnel destination address would be the address of the egress of the outmost LSP (e.g., the egress of the active segment).

This mechanism is much useful in the MPLS-SPRING-based Service Function Chainning (SFC) case [I-D.xu-sfc-using-mpls-spring] where only a few specific routers (e.g., Service Function Forwarders (SFF) and classifiers) are required to be MPLS-SPRING-capable while the remaining routers are just required to support IP forwarding capability. In addition, this mechanism is also useful in some specific Traffic Engineering scenarios where only a few routers (e.g., the entry and exit nodes of each plane in the dual-plane network ) are specified as segments of explicit paths. In this way, only a few routers are required to support the MPLS-SPRING capability while all the other routers just need to support IP forwarding capability, which would significantly reduce the deployment cost of this new technology. Furthermore, since there is no need to run any other label distribution protocol (e.g., LDP), the network provisioning is greatly simplified, which is one of the major claimed benefits of the MPLS-SPRING technology.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

2. Terminology

This memo makes use of the terms defined in [RFC3031], [I-D.ietf-spring-segment-routing-mpls] and [I-D.xu-sfc-using-mpls-spring] .

3. Packet Forwarding Procedures

Assume an MPLS-SPRING-enabled router X prepares to forward an MPLS packet to the next segment (i.e., the node segment of MPLS-SPRING-enabled router Y) which is identified by the top label of the MPLS packet. If the next-hop router of the best path to Y is a non-MPLS router, X couldn't map the packet's top label into an Next Hop Label Forwarding Entry (NHLFE) , even though the top label itself is a valid incoming label. If the label is not a Penultimate Hop Popping (PHP) label (i.e., the NP-flag [I-D.ietf-isis-segment-routing-extensions] associated with the corresponding prefix SID of that top label is set), X SHOULD swap the top label to the corresponding label significant to Y and then encapsulate the MPLS packet into an IP-based tunnel. The tunnel destination address is the IP address of Y (e.g., the /32 or /128 prefix FEC associated with that top label) and the tunnel source address is the IP address of X. If the top label is a PHP label and not at the bottom of the label stack, X SHOULD pop that top label before performing the above encapsulation. The IP encapsulated packet would be forwarded according to the IP forwarding table. Upon receipt of that IP encapsulated packet, Y would decapsulate it and then process the decapsulated MPLS packet accordingly.

As for which tunnel encapsulation type should be used by X, it can be manually specified on X or learnt from Y's advertisement of its tunnel encapsulation capability. How to advertise the tunnel encapsulation capability using IS-IS or OSPF are specified in [I-D.xu-isis-encapsulation-cap] and [I-D.ietf-ospf-encapsulation-cap] respectively.

4. Acknowledgements

Thanks Joel Halpern, Bruno Decraene and Loa Andersson for their insightful comments on this draft.

5. IANA Considerations

No action is required for IANA.

6. Security Considerations

TBD.

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses