MPLS-TP Requirements
draft-ietf-mpls-tp-requirements-04

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Abstract

This document specifies the requirements of an MPLS Transport Profile (MPLS-TP). This document is a product of a joint International Telecommunications Union (ITU)-IETF effort to include an MPLS Transport Profile within the IETF MPLS architecture to support the capabilities and functionalities of a packet transport network as defined by International Telecommunications Union - Telecommunications Standardization Sector (ITU-T).

This work is based on two sources of requirements; MPLS architecture as defined by IETF, and packet transport networks as defined by ITU-T.

The requirements expressed in this document are for the behavior of the protocol mechanisms and procedures that constitute building blocks out of which the MPLS transport profile is constructed. The requirements are not implementation requirements.

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.

Table of Contents

1.  Introduction
    1.1.  Terminology
    1.2.  Transport network overview
    1.3.  Layer network overview
2.  MPLS-TP Requirements
    2.1.  General requirements
    2.2.  Layering requirements
    2.3.  Data plane requirements
    2.4.  Control plane requirements
    2.5.  Network Management (NM) requirements
    2.6.  Operation, Administration and Maintenance (OAM) requirements
    2.7.  Network performance management (PM) requirements
    2.8.  Recovery & Survivability requirements
        2.8.1.  Data plane behavior requirements
        2.8.2.  Triggers for protection, restoration, and reversion
        2.8.3.  Management plane operation of protection and restoration
        2.8.4.  Control plane and in-band OAM operation of recovery
        2.8.5.  Topology-specific recovery mechanisms
    2.9.  QoS requirements
    2.10.  Security requirements
3.  IANA Considerations
4.  Security Considerations
5.  Acknowledgements
6.  References
    6.1.  Normative References
    6.2.  Informative References
§  Authors' Addresses

1.  Introduction

For many years, transport networks (e.g. Synchronous Optical Networking (SONET)/Synchronous Digital hierarchy (SDH)) have provided carriers with a high benchmark for reliability and operational simplicity. With the accelerating growth and penetration of:

• Packet-based services such as Ethernet, Voice over IP (VoIP), Layer 2 (L2)/Layer 3 (L3) Virtual Private Networks (VPNs), IP Television (IPTV), Radio Access Network (RAN) backhauling, etc.
• Applications with various bandwidth and QoS requirements.

Carriers are in need of technologies capable of efficiently supporting packet-based services and applications on their transport networks. The need to increase their revenue while remaining competitive forces operators to look for the lowest network Total Cost of Ownership (TCO). Investment in equipment and facilities (Capital Expenditure (CAPEX)) and Operational Expenditure (OPEX) should be minimized.

Carriers are considering migrating or evolving to packet transport networks in order to reduce their costs and to improve their ability to support services with guaranteed Service Level Agreements (SLAs). For carriers it is important that migrating from their existing transport networks to packet transport networks should not involve dramatic changes in network operation, should not necessitate extensive retraining, and should not require major changes to existing work practices. The aim is to preserve the look-and-feel to which carriers have become accustomed in deploying their transport networks, while providing common, multi-layer operations, resiliency, control and management for packet, circuit and lambda transport networks.

Transport carriers require control and deterministic usage of network resources. They need end-to-end control to engineer network paths and to efficiently utilize network resources. They require capabilities to support static (Operations Support System (OSS) based) or dynamic (control plane) provisioning of deterministic, protected and secured services and their associated resources.

Carriers will still need to cope with legacy networks (which are composed of many layers and technologies), thus the packet transport network should interwork as appropriate with other packet and transport networks (both horizontally and vertically). Vertical interworking is also known as client/server or network interworking. Horizontal interworking is also known as peer-partition or service interworking. For more details on each type of interworking and some of the issues that may arise (especially with horizontal interworking) see Y.1401 (International Telecommunications Union, “Principles of interworking,” February 2008.) [ITU.Y1401.2008].

MPLS is a maturing packet technology and it is already playing an important role in transport networks and services. However, not all of MPLS's capabilities and mechanisms are needed and/or consistent with transport network operations. There is therefore the need to define an MPLS Transport Profile (MPLS-TP) in order to support the capabilities and functionalities needed for packet transport network services and operations through combining the packet experience of MPLS with the operational experience of existing transport networks.

MPLS-TP will enable the migration of transport networks to a packet-based network that will efficiently scale to support packet services in a simple and cost effective way. MPLS-TP needs to combine the necessary existing capabilities of MPLS with additional minimal mechanisms in order that it can be used in a transport role.

This document specifies the requirements of an MPLS Transport Profile (MPLS-TP). The requirements are for the the behavior of the protocol mechanisms and procedures that constitute building blocks out of which the MPLS transport profile is constructed. That is, the requirements indicate what features are to be available in the MPLS toolkit for use by MPLS-TP. The requirements in this document do not describe what functions an MPLS-TP implementation supports. The purpose of this document is to identify the toolkit and any new protocol work that is required.

Although this document is not a protocol specification, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are used as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) and are to be interpreted as instructions to the protocol designers producing solutions that satisfy the requirements set out in this document.

This document is a product of a joint ITU-IETF effort to include an MPLS Transport Profile within the IETF MPLS architecture to support the capabilities and functionalities of a packet transport network as defined by ITU-T.

This work is based on two sources of requirements, MPLS architecture as defined by IETF and packet transport networks as defined by ITU-T. The requirements of MPLS-TP are provided below. The relevant functions of MPLS are included in MPLS-TP, except where explicitly excluded.

Although both static and dynamic configuration of MPLS-TP transport paths (including Operations, Administration and Maintenance (OAM) and protection capabilities) is required by this document, it MUST be possible for operators to be able to completely operate (including OAM and protection capabilities) an MPLS-TP network in the absence of any control plane protocols for dynamic configuration.

1.1.  Terminology

Note: Mapping between the terms in this section and ITU-T terminology will be described in a subsequent document.

Note: The definition of segment in a GMPLS/ASON context (i.e. as defined in RFC4397 (Bryskin, I. and A. Farrel, “A Lexicography for the Interpretation of Generalized Multiprotocol Label Switching (GMPLS) Terminology within the Context of the ITU-T's Automatically Switched Optical Network (ASON) Architecture,” February 2006.) [RFC4397]) encompasses both segment and concatenated segment as defined in this document.

Associated bidirectional path: A path that supports traffic flow in both directions but which is constructed from a pair of unidirectional paths (one for each direction) which are associated with one another at the path's ingress/egress points. The forward and backward directions may or may not follow the same route (links and nodes) across the network.

Bidirectional path: A path where the forward and backward directions follow the same route (links and nodes) across the network.

Concatenated Segment: A serial-compound link connection as defined in G.805 (International Telecommunications Union, “Generic functional architecture of transport networks,” March 2000.) [ITU.G805.2000]. A concatenated segment is a contiguous part of an LSP or multi-segment PW that comprises a set of segments and their interconnecting nodes in sequence.

Co-routed bidirectional path: A bidirectional path where the forward and backward directions follow the same route (links and nodes) across its layer network.

Domain: A domain represents a collection of entities (for example network elements) that are grouped for a particular purpose, examples of which are administrative and/or managerial responsibilities, trust relationships, addressing schemes, infrastructure capabilities, aggregation, survivability techniques, distributions of control functionality, etc. Examples of such domains include IGP areas and Autonomous Systems.

Layer network: Layer network is defined in G.805 (International Telecommunications Union, “Generic functional architecture of transport networks,” March 2000.) [ITU.G805.2000]. A layer network provides for the transfer of client information and independent operation of the client OAM. A Layer Network may be described in a service context as follows: one layer network may provide a (transport) service to higher client layer network and may, in turn, be a client to a lower layer network. A layer network is a logical construction somewhat independent of arrangement or composition of physical network elements. A particular physical network element may topologically belong to more than one layer network, depending on the actions it takes on the encapsulation(s) associated with the logical layers (e.g. the label stack), and thus could be modeled as multiple logical elements. A layer network may consist of zero or more sublayers. For additional explanation of how layer networks relate to the OSI concept of layering see Appendix I of Y.2611 (International Telecommunications Union, “High-level architecture of future packet-based networks,” December 2006.) [ITU.Y2611.2006].

Link: A physical or logical connection between a pair of LSRs that are adjacent at the (sub)layer network under consideration. A link may carry zero, one or more LSPs or PWs. A packet entering a link will emerge with the same label stack entry values.

Logical Ring: An MPLS-TP logical ring is constructed from a set of LSRs and logical data links (such as MPLS-TP LSP tunnels or MSPL-TP pseudowires) and physical data links that form a ring topology.

Path: See Transport path.

Physical Ring: An MPLS-TP physical ring is constructed from a set of LSRs and physical data links that form a ring topology.

Ring Topology: In an MPLS-TP ring topology each LSR is connected to exactly two other LSRs, each via a single point-to-point bidirectional MPLS-TP capable data link. A ring may also be constructed from only two LSRs where there are also exactly two links. Rings may be connected to other LSRs to form a larger network. Traffic originating or terminating outside the ring may be carried over the ring. Client network nodes (such as CEs) may be connected directly to an LSR in the ring.

Section: A section is a server layer (which may be MPLS-TP or a different technology) which provides for encapsulation and OAM of a MPLS-TP transport path client layer. A section layer may provide for aggregation of multiple MPLS-TP clients. Note that G.805 (International Telecommunications Union, “Generic functional architecture of transport networks,” March 2000.) [ITU.G805.2000] defines the section layer as one of the two layer networks in a transmission media layer network. The other layer network is the physical media layer network.

Segment: A link connection as defined in G.805 (International Telecommunications Union, “Generic functional architecture of transport networks,” March 2000.) [ITU.G805.2000]. A segment is the part of an LSP that traverses a single link or the part of a PW that traverses a single link (i.e. that connects a pair of adjacent {S|T}-PEs).

Sublayer: Sublayer is defined in G.805 (International Telecommunications Union, “Generic functional architecture of transport networks,” March 2000.) [ITU.G805.2000]. The distinction between a layer network and a sublayer is that a sublayer is not directly accessible to clients outside of its encapsulating layer network and offers no direct transport service for a higher layer (client) network.

Tandem Connection: A tandem connection is an arbitrary part of a transport path that can be monitored (via OAM) independently from the end-to-end monitoring (OAM). It may be a monitored segment, a monitored concatenated segment or any other monitored ordered sequence of contiguous hops and/or segments (and their interconnecting nodes) of a transport path.

Transport path: A network connection as defined in G.805 (International Telecommunications Union, “Generic functional architecture of transport networks,” March 2000.) [ITU.G805.2000]. In an MPLS-TP environment a transport path corresponds to an LSP or a PW.

Transport path layer: A layer network which provides point-to-point or point-to-multipoint transport paths which are used to carry a higher (client) layer network or aggregates of higher (client) layer networks, for example the transport service layer. It provides for independent OAM (of the client OAM) in the transport of the clients.

Transport service layer: A layer network in which transport paths are used to carry a customer’s (individual or bundled) service (may be point-to-point, point-to-multipoint or multipoint-to-multipoint services).

Transmission media layer: A layer network which provides sections (two-port point-to-point connections) to carry the aggregate of network transport path or network service layers on various physical media.

Unidirectional path: A path that supports traffic flow in only one direction.

1.2.  Transport network overview

The connection (or transport path) service is the basic service provided by a transport network. The purpose of a transport network is to carry its clients (i.e. the stream of client PDUs or client bits) between endpoints in the network (typically over several intermediate nodes). These endpoints may be service switching points or service terminating points. The connection services offered to customers are aggregated into large transport paths with long-holding times and independent OAM (of the client OAM), which contribute to enabling the efficient and reliable operation of the transport network. These transport paths are modified infrequently.

Aggregation and hierarchy are beneficial for achieving scalability and security since:

1. They reduce the number of provisioning and forwarding states in the network core.
2. They reduce load and the cost of implementing service assurance and fault management.
3. Clients are encapsulated and layer associated OAM overhead is added. This allows complete isolation of customer traffic and its management from carrier operations.

An important attribute of a transport network is that it is able to function regardless of which clients are using its connection service or over which transmission media it is running. The client, transport network and server layers are from a functional and operations point of view independent layer networks. Another key characteristic of transport networks is the capability to maintain the integrity of the client across the transport network. A transport network must provide the means to commit quality of service objectives to clients. This is achieved by providing a mechanism for client network service demarcation for the network path together with an associated network resiliency mechanism. A transport network must also provide a method of service monitoring in order to verify the delivery of an agreed quality of service. This is enabled by means of carrier-grade OAM tools.

Clients are first encapsulated. These encapsulated client signals may then be aggregated into a connection for transport through the network in order to optimize network management. Server layer OAM is used to monitor the transport integrity of the client layer or client aggregate. At any hop, the aggregated signals may be further aggregated in lower layer transport network paths for transport across intermediate shared links. The encapsulated client signals are extracted at the edges of aggregation domains, and are either delivered to the client or forwarded to another domain. In the core of the network, only the server layer aggregated signals are monitored; individual client signals are monitored at the network boundary in the client layer network. Although the connectivity of the client of the transport path layer may be point-to-point, point-to-multipoint or multipoint-to-multipoint, the transport path layer itself only provides point-to-point or point-to-multipoint transport paths which are used to carry the client.

Quality-of-service mechanisms are required in the packet transport network to ensure the prioritization of critical services, to guarantee BW and to control jitter and delay.

1.3.  Layer network overview

A layer network provides its clients with a transport service and the operation of the layer network is independent of whatever client happens to use the layer network. Information that passes between any client to the layer network is common to all clients and is the minimum needed to be consistent with the definition of the transport service offered. The client layer network can be connectionless, connection oriented packet switched, or circuit switched. The transport service transfers a payload (individual packet payload for connectionless networks, a sequence of packets payloads in the case of connection oriented packet switched networks, and a deterministic schedule of payloads in the case of circuit switched networks) such that the client can populate the payload without affecting any operation within the serving layer network.

The operations within a layer network that are independent of the clients include the control of forwarding, the control of resource reservation, the control of traffic demerging, and the OAM of the transport service. All of these operations are internal to a layer network. By definition, a layer network does not rely on any client information to perform these operations and therefore all information required to perform these operations is independent of whatever client is using the layer network.

A layer network will have common features in order to support the control of forwarding, resource reservation, and OAM. For example, a layer network will have a common addressing scheme for the end points of the transport service and a common set of transport descriptors for the transport service. However, a client may use a different addressing scheme or different traffic descriptors (consistent with performance inheritance).

It is sometimes useful to independently monitor a smaller domain within a layer network (or the transport services as the traverse this smaller domain) but the control of forwarding or the control of resource reservation involved retain their common elements. These smaller monitored domains are sublayers.

It is sometimes useful to independently control forwarding within smaller domain within a layer network but the control of resource reservation and OAM retain their common elements. These smaller domains are partitions of the layer network.

2.  MPLS-TP Requirements

2.1.  General requirements

1

The MPLS-TP data plane MUST be a subset of the MPLS data plane as defined by the IETF. When MPLS offers multiple options in this respect, MPLS-TP SHOULD select the minimum sub-set (necessary and sufficient subset) applicable to a transport network application.

2

Any new functionality that is defined to fulfil the requirements for MPLS-TP MUST be agreed within the IETF through the IETF consensus process and MUST re-use (as far as practically possible) existing MPLS standards.

3

Mechanisms and capabilities MUST be able to interoperate, without a gateway function, with existing IETF MPLS [RFC3031] (Rosen, E., Viswanathan, A., and R. Callon, “Multiprotocol Label Switching Architecture,” January 2001.) and IETF PWE3 [RFC3985] (Bryant, S. and P. Pate, “Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture,” March 2005.) control and data planes where appropriate.

4

MPLS-TP and its interfaces, both internal and external, MUST be sufficiently well-defined that interworking equipment supplied by multiple vendors will be possible both within a single network, and between networks.

5

MPLS-TP MUST be a connection-oriented packet switching model with traffic engineering capabilities that allow deterministic control of the use of network resources.

6

MPLS-TP MUST support traffic engineered point to point (P2P) and point to multipoint (P2MP) transport paths.

7

MPLS-TP MUST support the logical separation of the control and management planes from the data plane.

8

MPLS-TP MUST allow the physical separation of the control and management planes from the data plane.

9

MPLS-TP MUST support static provisioning of transport paths via an OSS, i.e. via the management plane.

10

Mechanisms in an MPLS-TP network that satisfy functional requirements that are common to general transport networks (i.e., independent of technology) SHOULD be operable in a way that is similar to the way the equivalent mechanisms are operated in other transport networks.

11

Static provisioning MUST NOT depend on the presence of any element of a control plane.

12

MPLS-TP MUST support the capability for network operation (including OAM and recovery) via the management plane (without the use of any control plane protocols).

13

A solution MUST be provided to support dynamic provisioning of MPLS-TP transport paths via a control plane.

14

The MPLS-TP data plane MUST be capable of forwarding data and taking recovery actions independently of the control or management plane used to operate the MPLS-TP layer network. That is, the MPLS-TP data plane MUST continue to operate normally if the management plane or control plane that configured the transport paths fails.

15

MPLS-TP MUST support mechanisms to avoid or minimize traffic impact (e.g. packet delay, reordering and loss) during network reconfiguration.

16

MPLS-TP MUST support transport paths through multiple homogeneous domains.

17

MPLS-TP MUST NOT dictate the deployment of any particular network topology either physical or logical, however:

A.

It MUST be possible to deploy MPLS-TP in rings.

B.

It MUST be possible to deploy MPLS-TP in arbitrarily interconnected rings with one or two points of interconnection.

C.

MPLS-TP MUST support rings of at least 16 nodes in order to support the upgrade of existing TDM rings to MPLS-TP. MPLS-TP SHOULD support rings with more than 16 nodes.

18

MPLS-TP MUST be able to scale at least as well as existing transport technologies with growing and increasingly complex network topologies as well as with increasing bandwidth demands, number of customers, and number of services.

19

MPLS-TP SHOULD support mechanisms to safeguard against the provisioning of transport paths which contain forwarding loops.

2.2.  Layering requirements

20

A generic and extensible solution MUST be provided to support the transport of one or more client layer networks (e.g. MPLS-TP, Ethernet, ATM, FR, etc.) over an MPLS-TP layer network.

21

A solution MUST be provided to support the transport of MPLS-TP transport paths over one or more server layer networks (such as MPLS-TP, Ethernet, SONET/SDH, OTN, etc.). Requirements for bandwidth management within a server layer network are outside the scope of this document.

22

In an environment where an MPLS-TP layer network is supporting a client network, and the MPLS-TP layer network is supported by a server layer network then operation of the MPLS-TP layer network MUST be possible without any dependencies on the server or client network.

23

It MUST be possible to operate the layers of a multi-layer network that includes an MPLS-TP layer autonomously.

The above are not only technology requirements, but also operational. Different administrative groups may be responsible for the same layer network or different layer networks.

24

It MUST be possible to hide MPLS-TP layer network addressing and other information (e.g. topology) from client layers.

2.3.  Data plane requirements

25

The identification of each transport path within its aggregate MUST be supported.

26

A label in a particular link MUST uniquely identify the transport path within that link.

27

A transport path's source MUST be identifiable at its destination within its layer network.

28

MPLS-TP MUST be capable of using P2MP server (sub-)layer capabilities when supporting P2MP MPLS-TP transport paths (for example context-specific labels [RFC5331] (Aggarwal, R., Rekhter, Y., and E. Rosen, “MPLS Upstream Label Assignment and Context-Specific Label Space,” August 2008.)).

29

It MUST be possible to operate and configure the MPLS-TP data (transport) plane without any IP forwarding capability in the MPLS-TP data plane.

30

MPLS-TP MUST support unidirectional, bidirectional and co-routed bidirectional point-to-point transport paths.

31

The forward and backward directions of a co-routed bidirectional transport path MUST follow the same links and nodes within its (sub-)layer network.

32

The intermediate nodes at each (sub-)layer MUST be aware about the pairing relationship of the forward and the backward directions belonging to the same bidirectional transport path.

33

MPLS-TP MAY support transport paths with asymmetric bandwidth requirements, i.e. the amount of reserved bandwidth differs between the forward and backward directions.

34

MPLS-TP MUST support unidirectional point-to-multipoint transport paths.

35

MPLS-TP MUST be extensible in order to accommodate new types of client networks and services.

36

MPLS-TP SHOULD support mechanisms to enable the reserved bandwidth associated with a transport path to be increased without impacting the existing traffic on that transport path

37

MPLS-TP SHOULD support mechanisms to enable the reserved bandwidth of a transport path to be decreased without impacting the existing traffic on that transport path, provided that the level of existing traffic is smaller than the reserved bandwidth following the decrease.

38

MPLS-TP MUST support mechanisms which ensure the integrity of the transported customer's service traffic as required by its associated SLA. Loss of integrity may be defined as packet corruption, re-ordering or loss during normal network conditions.

39

MPLS-TP MUST support mechanisms to detect when loss of integrity of the transported customer's service traffic has occurred.

40

MPLS-TP MUST support an unambiguous and reliable means of distinguishing users' (client) packets from MPLS-TP control packets (e.g. control plane, management plane, OAM and protection switching packets).

2.4.  Control plane requirements

This section defines the requirements that apply to MPLS-TP when a control plane is deployed.

The ITU-T has defined an architecture for Automatically Switched Optical and Transport Networks (ASON/ASTN) in G.8080 (International Telecommunications Union, “Architecture for the automatically switched optical network (ASON),” June 2006.) [ITU.G8080.2006]. The control plane for MPLS-TP MUST fit within the ASON/ASTN architecture.

An interpretation of the ASON/ASTN control plane requirements in the context of GMPLS can be found in [RFC4139] (Papadimitriou, D., Drake, J., Ash, J., Farrel, A., and L. Ong, “Requirements for Generalized MPLS (GMPLS) Signaling Usage and Extensions for Automatically Switched Optical Network (ASON),” July 2005.) and [RFC4258] (Brungard, D., “Requirements for Generalized Multi-Protocol Label Switching (GMPLS) Routing for the Automatically Switched Optical Network (ASON),” November 2005.).

Additionally:

41

The MPLS–TP control pane SHOULD support control plane topology and data plane topology independence.

42

The MPLS-TP control plane MUST be able to be operated independent of any particular client or server layer control plane.

43

The MPLS-TP control plane MUST support establishing all the connectivity patterns defined for the MPLS-TP data plane (e.g., unidirectional and bidirectional P2P, unidirectional P2MP, etc.) including configuration of protection functions and any associated maintenance functions.

44

The MPLS-TP control pane MUST support the configuration and modification of OAM maintenance points as well as the activation/deactivation of OAM when the transport path or transport service is established or modified.

45

An MPLS-TP control plane MUST support operation of the recovery functions described in Section 2.8.

46

An MPLS-TP control plane MUST scale gracefully to support a large number of transport paths, nodes and links.

2.5.  Network Management (NM) requirements

For requirements related to NM functionality (Management Plane in ITU-T terminology) for MPLS-TP, see the MPLS-TP NM requirements document [I‑D.gray‑mpls‑tp‑nm‑req] (Lam, H., Mansfield, S., and E. Gray, “MPLS TP Network Management Requirements,” January 2009.).

2.6.  Operation, Administration and Maintenance (OAM) requirements

For requirements related to OAM functionality for MPLS-TP, see the MPLS-TP OAM requirements document [I‑D.ietf‑mpls‑tp‑oam‑requirements] (Vigoureux, M., Ward, D., and M. Betts, “Requirements for OAM in MPLS Transport Networks,” November 2008.).

2.7.  Network performance management (PM) requirements

For requirements related to PM functionality for MPLS-TP, see the MPLS-TP OAM requirements document [I‑D.ietf‑mpls‑tp‑oam‑requirements] (Vigoureux, M., Ward, D., and M. Betts, “Requirements for OAM in MPLS Transport Networks,” November 2008.).

2.8.  Recovery & Survivability requirements

Network survivability plays a critical role in the delivery of reliable services. Network availability is a significant contributor to revenue and profit. Service guarantees in the form of SLAs require a resilient network that rapidly detects facility or node failures and restores network operation in accordance with the terms of the SLA.

The requirements in this section use the recovery terminology defined in RFC 4427 [RFC4427] (Mannie, E. and D. Papadimitriou, “Recovery (Protection and Restoration) Terminology for Generalized Multi-Protocol Label Switching (GMPLS),” March 2006.).

47

MPLS-TP MUST provide protection and restoration mechanisms.

A.

Recovery techniques used for P2P and P2MP SHOULD be identical to simplify implementation and operation. However, this MUST NOT override any other requirement.

48

MPLS-TP recovery mechanisms MUST be applicable at various levels throughout the network including support for link, path segment and end-to-end path, and pseudowire segment, and end-to-end pseudowire recovery.

49

MPLS-TP recovery paths MUST meet the SLA protection objectives of the service.

A.

MPLS-TP MUST provide mechanisms to guarantee 50ms recovery times from the moment of fault detection in networks with spans less than 1200 km.

B.

For protection it MUST be possible to require protection of 100% of the traffic on the protected path.

C.

Recovery objectives SHOULD be configurable per transport path, and SHOULD include bandwidth and QoS.

50

The recovery mechanisms MUST all be applicable to any topology.

51

The recovery mechanisms MUST operate in synergy with (including coordination of timing) the recovery mechanisms present in any underlying server transport network (for example, Ethernet, SDH, OTN, WDM) to avoid race conditions between the layers.

52

MPLS-TP protection mechanisms MUST support priority logic to negotiate and accommodate coexisting requests (i.e., multiple requests) for protection switching (e.g., administrative requests and requests due to link/node failures).

53

MPLS-TP recovery and reversion mechanisms MUST prevent frequent operation of recovery in the event of an intermittent defect.

2.8.1.  Data plane behavior requirements

General protection and survivability requirements are expressed in terms of the behavior in the data plane.

2.8.1.1.  Protection

54

MPLS-TP MUST support 1+1 protection.

A.

MPLS-TP 1+1 support MUST include bidirectional protection switching for P2P connectivity, and this SHOULD be the default behavior for 1+1 protection.

B.

Unidirectional 1+1 protection for P2MP connectivity MUST be supported.

C.

Unidirectional 1+1 protection for P2P connectivity is not required.

55

MPLS-TP MUST support 1:n protection (including 1:1 protection).

A.

MPLS-TP 1:n support MUST include bidirectional protection switching for P2P connectivity, and this SHOULD be the default behavior for 1:n protection.

B.

Unidirectional 1:n protection for P2MP connectivity MUST be supported.

C.

Unidirectional 1:n protection for P2P connectivity is not required.

D.

The action of protection switching MUST NOT cause user data to loop. Backtracking is allowed.

Note: Support for extra traffic (as defined in G.870 (International Telecommunications Union, “Terms and definitions for optical transport networks (OTN),” March 2008.) [ITU.G870.2008]) is not required in MPLS-TP.

2.8.1.2.  Restoration

56

The restoration LSP MUST be able to share resources with the LSP being replaced (sometimes known as soft rerouting).

57

Restoration priority MUST be supported so that an implementation can determine the order in which transport paths should be restored (to minimize service restoration time as well as to gain access to available spare capacity on the best paths).

58

Preemption priority MUST be supported to allow restoration to displace other transport paths in the event of resource constraint.

2.8.1.3.  Sharing of protection resources

59

MPLS-TP SHOULD support 1:n (including 1:1) shared mesh restoration.

60

MPLS-TP MUST support the sharing of protection bandwidth by allowing best effort traffic.

61

MPLS-TP MUST support the definition of shared protection groups to allow the coordination of protection actions resulting from triggers caused by events at different locations in the network.

62

MPLS-TP MUST support sharing of protection resources such that protection paths that are known not to be required concurrently can share the same resources.

2.8.1.4.  Reversion

63

MPLS-TP protection mechanisms MUST support revertive and non- revertive behavior. Reversion MUST be the default behavior.

64

MPLS-TP restoration mechanisms MAY support revertive and non- revertive behavior.

2.8.2.  Triggers for protection, restoration, and reversion

Recovery actions may be triggered from different places as follows:

65

MPLS-TP MUST support physical layer fault indication triggers.

66

MPLS-TP MUST support OAM-based triggers.

67

MPLS-TP MUST support management plane triggers (e.g., forced switch, etc.).

68

There MUST be a mechanism to allow administrative recovery actions to be distinguished from recovery actions initiated by other triggers.

69

Where a control plane is present, MPLS-TP SHOULD support control plane triggers.

2.8.3.  Management plane operation of protection and restoration

All functions described here are for control by the operator.

70

It MUST be possible to configure of protection paths and protection-to-working path relationships (sometimes known as protection groups).

71

There MUST be support for pre-calculation of recovery paths.

72

There MUST be support for pre-provisioning of recovery paths.

73

The external controls as defined in [RFC4427] (Mannie, E. and D. Papadimitriou, “Recovery (Protection and Restoration) Terminology for Generalized Multi-Protocol Label Switching (GMPLS),” March 2006.) MUST be supported.

74

There MUST be support for the configuration of timers used for recovery operation.

75

Restoration resources MAY be pre-planned and selected a priori, or computed after failure occurrence.

76

When preemption is supported for recovery purposes, it MUST be possible for the operator to configure it.

77

The management plane MUST provide indications of protection events and triggers.

78

The management plane MUST allow the current protection status of all transport paths to be determined.

2.8.4.  Control plane and in-band OAM operation of recovery

79

The MPLS-TP control plane (which is not mandatory in an MPLS-TP implementation) MUST support:

A.

establishment and maintenance of all recovery entities and functions

B.

signaling of administrative control

C.

protection state coordination (PSC)

80

In-band OAM MAY be used for:

A.

signaling of administrative control

B.

protection state coordination

2.8.5.  Topology-specific recovery mechanisms

81

MPLS-TP MAY support recovery mechanisms that are optimized for specific network topologies. These mechanisms MUST be interoperable with the mechanisms defined for arbitrary topology (mesh) networks to enable protection of end-to-end transport paths.

Note that topology-specific recovery mechanisms are subject to the development of requirements using the normal IETF process.

2.8.5.1.  Ring protection

Several service providers have expressed a high level of interest in operating MPLS-TP in ring topologies and require a high level of survivability function in these topologies. The requirements listed below have been collected from these service providers and from the ITU-T.

The main objective in considering a specific topology (such as a ring) is to determine whether it is possible to optimize any mechanisms such that the performance of those mechanisms within the topology is significantly better than the performance of the generic mechanisms in the same topology. The benefits of such optimizations are traded against the costs of developing, implementing, deploying, and operating the additional optimized mechanisms noting that the generic mechanisms MUST continue to be supported.

Within the context of recovery in MPLS-TP networks, the optimization criteria considered in ring topologies are as follows:

a.

Minimize the number of OAM MEs that are needed to trigger the recovery operation – less than are required by other recovery mechanisms.

b.

Minimize the number of elements of recovery in the ring – less than are required by other recovery mechanisms.

c.

Minimize the number of labels required for the protection paths across the ring – less than are required by other recovery mechanisms.

d.

Minimize the amount of management plane transactions during a maintenance operation (e.g., ring upgrade) – less than are required by other recovery mechanisms.

It may be observed that the requirements in this section are fully compatible with the generic requirements expressed above, and that no requirements that are specific to ring topologies have been identified.

82

MPLS-TP MUST include recovery mechanisms that operate in any single ring supported in MPLS-TP, and continue to operate within the single rings even when the rings are interconnected.

83

When a network is constructed from interconnected rings, MPLS-TP MUST support recovery mechanisms that protect user data that traverses more than one ring. This includes the possibility of failure of the ring-interconnect nodes and links.

84

MPLS-TP recovery in a ring MUST protect unidirectional and bidirectional P2P transport paths.

85

MPLS-TP recovery in a ring MUST protect unidirectional P2MP transport paths.

86

MPLS-TP 1+1 and 1:1 protection in a ring MUST support switching time within 50 ms from the moment of fault detection in a network with a 16 nodes ring with less than 1200 km of fiber.

87

The protection switching time in a ring MUST be independent of the number of LSPs crossing the ring.

88

Recovery actions in a ring MUST be data plane functions triggered by different elements of control. The triggers are configured by management or control planes and are subject to configurable policy.

89

The configuration and operation of recovery mechanisms in a ring MUST scale well with:

A.

the number of transport paths (must be better than linear scaling)

B.

the number of nodes on the ring (must be at least as good as linear scaling)

C.

the number of ring interconnects (must be at least as good as linear scaling)

90

MPLS-TP recovery in ring topologies MAY support multiple failures without reconfiguring the protection actions.

91

Recovery techniques used in a ring MUST NOT prevent the ring from being connected to a general MPLS-TP network in any arbitrary way, and MUST NOT prevent the operation of recovery techniques in the rest of the network.

92

MPLS-TP Recovery mechanisms applicable to a ring MUST be equally applicable in physical and logical rings.

93

Recovery techniques in a ring SHOULD be identical to those in general networks to simplify implementation. However, this MUST NOT override any other requirement.

94

Recovery techniques in logical and physical rings SHOULD be identical to simplify implementation and operation. However, this MUST NOT override any other requirement.

95

The default recovery scheme in a ring MUST be bidirectional recovery in order to simplify the recovery operation.

96

The recovery mechanism in a ring MUST support revertive switching, which MUST be the default behaviour. This allows optimization of the use of the ring resources, and restores the preferred quality conditions for normal traffic (e.g., delay) when the recovery mechanism is no longer needed.

97

The recovery mechanisms in a ring MUST support ways to allow administrative protection switching, to be distinguished from protection switching initiated by other triggers.

98

It MUST be possible to lockout (disable) protection mechanisms on selected links (spans) in a ring (depending on operator’s need). This may require lockout mechanisms to be applied to intermediate nodes within a transport path.

99

MPLS-TP recovery mechanisms in a ring MUST include a mechanism to allow an implementation to handle coexisting requests (i.e., multiple requests – not necessarily arriving simultaneously) for protection switching based on priority.

100

MPLS-TP recovery and reversion mechanisms in a ring MUST offer a way to prevent frequent operation of recovery in the event of an intermittent defect.

101

MPLS-TP MUST support the sharing of protection bandwidth in a ring by allowing best effort traffic.

102

MPLS-TP MUST support sharing of ring protection resources such that protection paths that are known not to be required concurrently can share the same resources.

103

MUST support the coordination of triggers caused by events at different locations in a ring. Note that this is the ring equivalent of the definition of shared protection groups.

2.9.  QoS requirements

Carriers require advanced traffic management capabilities to enforce and guarantee the QoS parameters of customers’ SLAs.

Quality of service mechanisms are REQUIRED in an MPLS-TP network to ensure:

104

Support for differentiated services and different traffic types with traffic class separation associated with different traffic.

105

Prioritization of critical services.

106

Enabling the provisioning and the guarantee of Service Level Specifications (SLS), with support for hard and relative end-to-end bandwidth guaranteed.

107

Support of services, which are sensitive to jitter and delay.

108

Guarantee of fair access, within a particular class, to shared resources.

109

Guaranteed resources for in-band control and management plane traffic regardless of the amount of data plane traffic.

110

Carriers are provided with the capability to efficiently support service demands over the MPLS-TP network. This MUST include support for a flexible bandwidth allocation scheme.

2.10.  Security requirements

For a description of the security threats relevant in the context of MPLS and GMPLS and the defensive techniques to combat those threats see the Security Framework for MPLS & GMPLS Networks [I‑D.draft‑ietf‑mpls‑mpls‑and‑gmpls‑security‑framework] (Fang, L. and M. Behringer, “Security Framework for MPLS and GMPLS Networks,” November 2008.).

3.  IANA Considerations

This document makes no request of IANA.

Note to RFC Editor: this section may be removed on publication as an RFC.

4.  Security Considerations

For a description of the security threats relevant in the context of MPLS and GMPLS and the defensive techniques to combat those threats see the Security Framework for MPLS & GMPLS Networks [I‑D.draft‑ietf‑mpls‑mpls‑and‑gmpls‑security‑framework] (Fang, L. and M. Behringer, “Security Framework for MPLS and GMPLS Networks,” November 2008.).

5.  Acknowledgements

The authors would like to thank all members of the teams (the Joint Working Team, the MPLS Interoperability Design Team in the IETF, and the T-MPLS Ad Hoc Group in the ITU-T) involved in the definition and specification of MPLS Transport Profile.

The authors would also like to thank Loa Andersson, Lou Berger, Italo Busi, John Drake, Adrian Farrel, Eric Gray, Neil Harrison, Huub van Helvoort, Wataru Imajuku, Julien Meuric, Tom Nadeau, Hiroshi Ohta, George Swallow, Tomonori Takeda and Maarten Vissers for their comments and enhancements to the text.

An ad hoc discussion group consisting of Stewart Bryant, Italo Busi, Andrea Digiglio, Li Fang, Adrian Farrel, Jia He, Huub van Helvoort, Feng Huang, Harald Kullman, Han Li, Hao Long and Nurit Sprecher provided valuable input to the requirements for deployment and survivability in ring topologies.

6.  References

6.1. Normative References

6.2. Informative References

Authors' Addresses