MPLS can be used to provide a network layer to support packet transport services. In some operational environments, it is desirable that the operation and maintenance of such an MPLS based packet transport network follow operational models typical in traditional optical transport networks (for example, SONET/SDH), while providing additional OAM, survivability and other maintenance functions targeted at that environment.
MPLS-TP defines a profile of MPLS targeted at transport applications. This profile defines the specific MPLS characteristics and extensions required to meet transport requirements, while retaining compliance to the standard IETF MPLS architecture and label switching paradigm. The basic requirements are architecture for MPLS-TP are described by the IETF in RFC 5654, RFC 5921, and RFC 5960, to meet two objectives:
To enable MPLS to be deployed in a transport network and operated in a similar manner to existing transport technologies.
To enable MPLS to support packet transport services with a similar degree of predictability to that found in existing transport networks.
To meet these objectives, MPLS-TP has a number of high level characteristics:
It does not modify the MPLS forwarding architecture, which is based on existing pseudowire and LSP constructs. Point-to-point LSPs may be unidirectional or bidirectional. Bidirectional LSPs must be congruent (that is, co-routed and follow the same path in each direction). The system supports bidirectional co-routed MPLS-TP LSPs.
There is no LSP merging.
OAM, protection, and forwarding of data packets can operate without IP forwarding support. When static provisioning is used, there is no dependency on dynamic routing or signaling.
LSP and pseudowire monitoring is only achieved through the use of OAM and does not rely on control plane or routing functions to determine the health of a path. For example, LDP hello failures do not trigger protection.
MPLS-TP can operate in the absence of an IP control plane and IP forwarding of OAM traffic. MPLS-TP is only supported on static LSPs and PWs.
The system supports MPLS-TP on LSPs and PWs with static labels. MPLS-TP is not supported on dynamically signaled LSPs and PWs. MPLS-TP is supported for Epipe, Apipe, and Cpipe VLLs, and Epipe spoke SDP termination on IES, VPRN and VPLS. Static PWs may use SDPs that use either static MPLS-TP LSPs or RSVP-TE LSPs.
The following MPLS-TP OAM and protection mechanisms, defined by the IETF, are supported:
MPLS-TP Generic Associated Channel for LSPs and PWs (RFC 5586)
MPLS-TP Identifiers (RFC 6370)
Proactive CC, CV, and RDI using BFD for LSPs (RFC 6428)
On-Demand CV for LSPs and PWs using LSP Ping and LSP Trace (RFC 6426)
1-for-1 Linear protection for LSPs (RFC 6378)
Static PW Status Signaling (RFC 6478)
The system can play the role of an LER and an LSR for static MPLS-TP LSPs, and a PE/T-PE and an S-PE for static MPLS-TP PWs. It can also act as a S-PE for MPLS-TP segments between an MPLS network that strictly follows the transport profile, and an MPLS network that supports both MPLS-TP and dynamic IP/MPLS.