MPLS Transport Profile Data Plane Architecture
draft-fbb-mpls-tp-data-plane-00

Abstract

The Multiprotocol Label Switching (MPLS) Transport Profile (MPLS-TP) is the set of MPLS protocol functions applicable to the construction and operation of packet-switched transport networks. This document specifies the subset of these functions that comprises the MPLS-TP data plane: the architectural layer concerned with the encapsulation and forwarding of packets within an MPLS-TP network.

This document is a product of a joint Internet Engineering Task Force (IETF) / International Telecommunication Union Telecommunication Standardization Sector (ITU-T) effort to include an MPLS Transport Profile within the IETF MPLS and PWE3 architectures to support the capabilities and functionalities of a packet transport network.

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

Status of this Memo

This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.

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This Internet-Draft will expire on August 5, 2010.

Copyright Notice

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Table of Contents

1.  Introduction
    1.1.  Scope
    1.2.  Terminology
2.  MPLS-TP Packet Encapsulation and Forwarding
3.  MPLS-TP Transport Entities
    3.1.  Label Switched Paths
        3.1.1.  LSP Packet Encapsulation and Forwarding
        3.1.2.  LSP Payloads
        3.1.3.  LSP Types
    3.2.  Sections
    3.3.  Pseudowires
4.  MPLS-TP Generic Associated Channel
5.  Media-Specific Considerations
    5.1.  Ethernet
        5.1.1.  Destination MAC Address Determination
    5.2.  Other Media
6.  Security Considerations
7.  IANA Considerations
8.  References
    8.1.  Normative References
    8.2.  Informative References
§  Authors' Addresses

1.  Introduction

The MPLS Transport Profile (MPLS-TP) [I‑D.ietf‑mpls‑tp‑framework] (Bocci, M., Bryant, S., Frost, D., Levrau, L., and L. Berger, “A Framework for MPLS in Transport Networks,” April 2010.) is the set of protocol functions that meet the requirements [RFC5654] (Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N., and S. Ueno, “Requirements of an MPLS Transport Profile,” September 2009.) for the application of MPLS to the construction and operation of packet-switched transport networks. Packet transport networks are defined and described in [I‑D.ietf‑mpls‑tp‑framework] (Bocci, M., Bryant, S., Frost, D., Levrau, L., and L. Berger, “A Framework for MPLS in Transport Networks,” April 2010.).

This document specifies the subset of protocol functions that comprises the MPLS-TP data plane: the architectural layer concerned with the encapsulation and forwarding of packets within an MPLS-TP network.

This document is a product of a joint Internet Engineering Task Force (IETF) / International Telecommunication Union Telecommunication Standardization Sector (ITU-T) effort to include an MPLS Transport Profile within the IETF MPLS and PWE3 architectures to support the capabilities and functionalities of a packet transport network.

1.1.  Scope

This document has the following purposes:

• To identify the data-plane functions within the MPLS Transport Profile
• To indicate which of these data-plane functions an MPLS-TP implementation is required to support

Note that the MPLS-TP functions discussed in this document are considered OPTIONAL unless stated otherwise.

1.2.  Terminology

Additional definitions and terminology can be found in [I‑D.ietf‑mpls‑tp‑framework] (Bocci, M., Bryant, S., Frost, D., Levrau, L., and L. Berger, “A Framework for MPLS in Transport Networks,” April 2010.) and [RFC5654] (Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N., and S. Ueno, “Requirements of an MPLS Transport Profile,” September 2009.).

2.  MPLS-TP Packet Encapsulation and Forwarding

This document defines the encapsulation and forwarding functions applicable to packets traversing an MPLS-TP Label Switched Path (LSP), Pseudowire (PW), or Section (see Section 3 (MPLS-TP Transport Entities) for the definitions of these transport entities). Encapsulation and forwarding functions for packets outside an MPLS-TP LSP, PW, or Section, and mechanisms for delivering packets to or from MPLS-TP LSPs, PWs, and Sections, are outside the scope of this document.

3.  MPLS-TP Transport Entities

The MPLS Transport Profile includes the following data-plane transport entities:

• Label Switched Paths (LSPs)
• Sections
• Pseudowires (PWs)

3.1.  Label Switched Paths

MPLS-TP LSPs are ordinary MPLS LSPs as defined in [RFC3031] (Rosen, E., Viswanathan, A., and R. Callon, “Multiprotocol Label Switching Architecture,” January 2001.) except as specifically noted otherwise in this document.

3.1.1.  LSP Packet Encapsulation and Forwarding

Encapsulation and forwarding of packets traversing MPLS-TP LSPs MUST follow standard MPLS packet encapsulation and forwarding as defined in [RFC3031] (Rosen, E., Viswanathan, A., and R. Callon, “Multiprotocol Label Switching Architecture,” January 2001.) and [RFC3032] (Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, “MPLS Label Stack Encoding,” January 2001.), except as explicitly stated otherwise in this document.

Data-plane support for Internet Protocol (IP) packet encapsulation, addressing, and forwarding is OPTIONAL.

Data-plane Quality of Service capabilities are included in the MPLS-TP in the form of the MPLS Differentiated Services (DiffServ) architecture [RFC3270] (Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen, “Multi-Protocol Label Switching (MPLS) Support of Differentiated Services,” May 2002.). Both E-LSP and L-LSP MPLS DiffServ modes are included. The Traffic Class field (formerly the EXP field) of an MPLS label follows the definition of [RFC5462] (Andersson, L. and R. Asati, “Multiprotocol Label Switching (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic Class" Field,” February 2009.) and [RFC3270] (Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen, “Multi-Protocol Label Switching (MPLS) Support of Differentiated Services,” May 2002.) and MUST be processed according to the rules specified in those documents.

The Pipe and Short Pipe DiffServ tunneling and TTL processing models described in [RFC3270] (Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P., Krishnan, R., Cheval, P., and J. Heinanen, “Multi-Protocol Label Switching (MPLS) Support of Differentiated Services,” May 2002.) and [RFC3443] (Agarwal, P. and B. Akyol, “Time To Live (TTL) Processing in Multi-Protocol Label Switching (MPLS) Networks,” January 2003.) are included in the MPLS-TP. The Uniform model is outside the scope of the MPLS-TP.

Per-platform, per-interface or other context-specific label space [RFC5331] (Aggarwal, R., Rekhter, Y., and E. Rosen, “MPLS Upstream Label Assignment and Context-Specific Label Space,” August 2008.) MAY be used for MPLS-TP LSPs. Note that the requirements of a particular LSP type may dictate which label spaces it can use.

Per-packet Equal-Cost Multi-Path (ECMP) load-balancing is outside the scope of the MPLS-TP.

Penultimate Hop Popping (PHP) MUST be disabled by default on MPLS-TP LSPs.

Fragmentation procedures as specified in [RFC3032] (Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, “MPLS Label Stack Encoding,” January 2001.) are outside the scope of the MPLS-TP.

3.1.2.  LSP Payloads

The MPLS-TP includes support for the following LSP payload types:

• Network-layer protocol packets
• Pseudowire packets

The rules for processing LSP payloads that are network-layer protocol packets SHALL be as specified in [RFC3032] (Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, “MPLS Label Stack Encoding,” January 2001.) except as specifically noted otherwise in this document.

The rules for processing LSP payloads that are pseudowire packets SHALL be as specified in [RFC3985] (Bryant, S. and P. Pate, “Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture,” March 2005.) and the attendant standards defined by the IETF Pseudowire Emulation Edge-to-Edge (PWE3) Working Group.

Note that the payload of an MPLS-TP LSP may be a packet type that itself contains one or more MPLS labels. This is true, for instance, when the payload is a pseudowire or another MPLS-TP LSP. From the data-plane perspective, however, an MPLS-TP packet is an MPLS packet as specified in [RFC3032] (Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D., Li, T., and A. Conta, “MPLS Label Stack Encoding,” January 2001.), and so in particular has precisely one label stack, and one label in the stack with its S (Bottom of Stack) bit set to 1.

3.1.3.  LSP Types

The MPLS-TP includes the following LSP types:

• Point-to-point unidirectional
• Point-to-point associated bidirectional
• Point-to-point co-routed bidirectional
• Point-to-multipoint unidirectional

Point-to-point unidirectional LSPs are supported by the basic MPLS architecture [RFC3031] (Rosen, E., Viswanathan, A., and R. Callon, “Multiprotocol Label Switching Architecture,” January 2001.) and are REQUIRED to function in the same manner in the MPLS-TP data plane except as explicitly stated otherwise in this document.

A point-to-point associated bidirectional LSP between LSRs A and B consists of two unidirectional point-to-point LSPs, one from A to B and the other from B to A, which are regarded as a pair providing a single logical bidirectional transport path. The nodes A and B are REQUIRED to be aware of this pairing relationship, but other nodes need not be.

A point-to-point co-routed bidirectional LSP is a point-to-point associated bidirectional LSP with the additional constraint that its two unidirectional component LSPs follow the same path in the network. This means that if one of the component LSPs follows the path through the nodes N0, ..., Nk, originating on N0 and terminating on Nk, then the path of the other component LSP is Nk, ..., N0, and that at each node an ingress interface of one component LSP is an egress interface of the other. In addition, each node along the path is REQUIRED to be aware of the pairing relationship between the component LSPs.

A point-to-multipoint unidirectional LSP functions in the same manner in the data plane, with respect to basic label processing and packet-switching operations, as a point-to-point unidirectional LSP, with one difference: an LSR may have more than one (egress interface, outgoing label) pair associated with the LSP, and any packet it transmits on the LSP is transmitted out all associated egress interfaces. Point-to-multipoint LSPs are described in [RFC4875] (Aggarwal, R., Papadimitriou, D., and S. Yasukawa, “Extensions to Resource Reservation Protocol - Traffic Engineering (RSVP-TE) for Point-to-Multipoint TE Label Switched Paths (LSPs),” May 2007.) and [RFC5332] (Eckert, T., Rosen, E., Aggarwal, R., and Y. Rekhter, “MPLS Multicast Encapsulations,” August 2008.).

3.2.  Sections

Two MPLS-TP LSRs are considered to be topologically adjacent at a particular layer n >= 0 of the MPLS-TP LSP hierarchy if there exists a link between them at the next lowest network layer. Such a link, if it exists, will be either an MPLS-TP LSP (if n > 0) or a data-link provided by the underlying server layer network (if n = 0), and is referred to as an MPLS-TP Section at layer n of the MPLS-TP LSP hierarchy. Thus, the links traversed by a layer n+1 MPLS-TP LSP are layer n MPLS-TP sections.

Note that the MPLS label stack associated with an MPLS-TP section at layer n consists of n labels, in the absence of stack optimisation mechanisms such as PHP. Note also that in order for two LSRs to exchange MPLS-TP control packets over a section, an additional label, the Generic Associated Channel Label (GAL) (see Section 4 (MPLS-TP Generic Associated Channel)) must appear at the bottom of the label stack.

3.3.  Pseudowires

The data-plane architectures for Single-Segment Pseudowires [RFC3985] (Bryant, S. and P. Pate, “Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture,” March 2005.), Multisegment Pseudowires [RFC5659] (Bocci, M. and S. Bryant, “An Architecture for Multi-Segment Pseudowire Emulation Edge-to-Edge,” October 2009.) and Point-to-Multipoint Pseudowires [I‑D.ietf‑pwe3‑p2mp‑pw‑requirements] (Heron, G., Wang, L., Aggarwal, R., Vigoureux, M., Bocci, M., Jin, L., JOUNAY, F., Niger, P., Kamite, Y., DeLord, S., and L. Martini, “Requirements for Point-to-Multipoint Pseudowire,” January 2010.), and the associated data-plane pseudowire protocol functions, as defined by the IETF Pseudowire Emulation Edge-to-Edge (PWE3) Working Group, are included in the MPLS-TP.

This document specifies no modifications or extensions to pseudowire data-plane architectures or protocols.

4.  MPLS-TP Generic Associated Channel

The MPLS Generic Associated Channel (G-ACh) mechanism is specified in [RFC5586] (Bocci, M., Vigoureux, M., and S. Bryant, “MPLS Generic Associated Channel,” June 2009.) and included in the MPLS-TP. The G-ACh provides an auxiliary logical data channel associated with MPLS-TP Sections, LSPs, and PWs in the data plane. The primary purpose of the G-ACh in the context of MPLS-TP is to support control, management, and OAM traffic associated with MPLS-TP transport entities. The G-ACh MUST NOT be used to transport client layer network traffic in MPLS-TP networks.

5.  Media-Specific Considerations

This section discusses considerations for support of the MPLS-TP data plane by specific underlying server layer media.

5.1.  Ethernet

5.1.1.  Destination MAC Address Determination

When two MPLS-TP nodes are connected by a point-to-point Ethernet link, the question arises as to what destination Ethernet Media Access Control (MAC) address should be specified in Ethernet frames transmitted to the peer node over the link. The problem of determining this address does not arise in IP/MPLS networks because of the presence of the Ethernet Address Resolution Protocol (ARP) [RFC0826] (Plummer, D., “Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware,” November 1982.) or IP version 6 Neighbor Discovery protocol [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.), which allow the unicast MAC address of the peer device to be learned dynamically.

If existing mechanisms are available in an MPLS-TP network to determine the destination unicast MAC addresses of peer nodes - for example if the network also happens to be an IP/MPLS network - such mechanisms SHOULD be used. The remainder of this section discusses the available options when this is not the case.

One possibility is for each node to be statically configured with the MAC address of its peer. Static MAC address configuration MAY be used in an MPLS-TP network, but can present an administrative burden and lead to operational problems.

Another possibility is to use the Ethernet broadcast address, but this may lead to excessive frame distribution and processing at the Ethernet layer. Broadcast traffic may also be treated specially by some devices and this may not be desirable for MPLS-TP data frames.

The preferred approach is therefore to use as the destination MAC address an Ethernet multicast address reserved for MPLS-TP. The address allocated for this purpose by the Internet Assigned Numbers Authority (IANA) is 01-00-5E-XX-XX-XX. An MPLS-TP implementation MUST process Ethernet frames received with this destination MAC address by default.

[Editor's note: Decide what to say about multipoint Ethernet and switched links. Decide if there is a need for protocol mechanisms to support learning of the Ethernet unicast MAC addresses of MPLS-TP peers that are not also IP peers.]

5.2.  Other Media

[Editor's note: Decide whether any considerations for media other than Ethernet need to be mentioned.]

6.  Security Considerations

TBD

7.  IANA Considerations

A future version of this document will detail IANA considerations for the allocation of a standard Ethernet multicast MAC address for use in MPLS-TP networks.

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses