MPLS Network Actions Framework

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Table of Contents

1. Introduction

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

The document provides the foundation for the development of a common set network actions and information elements supporting additional operational models and capabilities of MPLS networks. Some of these actions are defined in existing MPLS specifications, while others require extensions to existing specifications to meet the requirements found in [I-D.ietf-mpls-mna-requirements].¶

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 on a failure, and whether or not a packet has metadata relevant to the forwarding decisions along the path.¶

This document generalizes the concept of "forwarding actions" into "network actions" to include any action that an MPLS router is requested to take on the packet. That includes any forwarding action, but may include other operations (such as security functions, OAM procedures, etc.) that are not directly related to forwarding of the packet.¶

MNA technologies may use the TC and TTL fields in an MPLS LSE for an alternative purpose.¶

2. Structure

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

A network action sub-stack contains:¶

• Network Action Sub-Stack Indicator: The first LSE in the NAS contains a label with special semantics, called the MNA label, that is used to indicate the start of a network action sub-stack.¶

• Network Action Indicators: 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: A set of zero or more LSEs that carry ancillary data for the present network actions. 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.¶

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.¶

A solution must specify the order that network actions are to be applied to the packet.¶

3. Encoding

Several possibilities to carry NAIs have been discussed in MNA drafts and in the MPLS Open DT [MPLSODT]. In this section, we enumerate the possibilities and some considerations for the various alternatives.¶

All types of network actions are represented in the MPLS label stack by a set of LSEs termed a 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.¶

[I-D.ietf-mpls-mna-requirements] requires that a solution not add unnecessary LSEs to the sub-stack (Section 3.1, requirement 7). Accordingly, solutions should also make efficient use of the bits within the sub-stack, as inefficient use of the bits will result in the addition of unnecessary LSEs.¶

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

   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|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Label:  Label value, 20 bits
                x:      Bit available for solution definition
                S:      Bottom of Stack, 1 bit

4. Semantics

5. Definition of a Network Action

Network actions should be defined in a document and 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. The action may apply to each transit node (HBH), only the egress node that pops the final label off of the label stack, or specific nodes along the label switched path.¶

• State: The document should specify if the network action can modify state in the 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 signalled appropriately. 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 what management considerations apply to the solutions they are describing. Solutions 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.¶

Some proposals to add encryption to the MPLS forwarding plane have been suggested [I-D.ietf-mpls-opportunistic-encrypt], but no mechanisms have been agreed upon at the time of publication of this document. Additionally, MPLS does not provide any cryptographic integrity protection on the MPLS headers. However, both such mechanisms would significantly complicate the operation of any MNA solution and would likely be significantly deleterious to forwarding performance. This is particularly the case where an MNA solution used an in-stack approach that needed to employ on-path modification of the MNA data.¶

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

A cornerstone of MPLS security is that packets entering an MPLS network from an untrusted third party are immediately encapsulated in an MPLS label stack specified by the MPLS network operator and thus are unable to present any labels to the network forwarding system.¶

It is thus difficult for an attacker to pass a raw MPLS-encoded packet into a network, and operators have considerable experience in excluding such packets at the network 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.¶

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 vulnerabilites 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 this has been benign, and indeed useful for the purposes of ECMP. However where the first network's traffic has MNA information this may be exposed to MNA capable forwarders causing unpredictable behaviour 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.¶

There are a number of mitigations that 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 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 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 can result in the thirty 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 code point from the "IGP MSD-Types" registry in the "Interior Gateway Protocol (IGP) Parameters" namespace for "Readable Label Depth", referencing this document.¶

9. Acknowledgements

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

The authors would like to thank Adrian Farrel for his contributions and to John Drake and Jie Dong for their comments.¶

Authors' Addresses