MPLS                                                       D. Frost, Ed.
Internet-Draft                                            S. Bryant, Ed.
Intended status: Standards Track                           Cisco Systems
Expires: August 28, September 13, 2010                                M. Bocci, Ed.
                                                       February 24,
                                                          March 12, 2010

             MPLS Transport Profile Data Plane Architecture


   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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

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

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  MPLS-TP Packet Encapsulation and Forwarding  . . . . . . . . .  5
   3.  MPLS-TP Transport Entities . . . . . . . . . . . . . . . . . .  5
     3.1.  Label Switched Paths . . . . . . . . . . . . . . . . . . .  5  6
       3.1.1.  LSP Packet Encapsulation and Forwarding  . . . . . . .  5  6
       3.1.2.  LSP Payloads . . . . . . . . . . . . . . . . . . . . .  6
       3.1.3.  LSP Types  . . . . . . . . . . . . . . . . . . . . . .  6  7
     3.2.  Sections . . . . . . . . . . . . . . . . . . . . . . . . .  7  8
     3.3.  Pseudowires  . . . . . . . . . . . . . . . . . . . . . . .  8  9
   4.  MPLS-TP Generic Associated Channel . . . . . . . . . . . . . .  8  9
   5.  Media-Specific  Server Layer Considerations  . . . . . . . . . . . . . . . .  8 . 10
     5.1.  Ethernet Media . . . . . . . . . . . . . . . . . . . . . . 10
       5.1.1.  Point-to-Point Links . . .  8
       5.1.1.  Destination MAC Address Determination . . . . . . . .  8
     5.2.  Other Media . . . . . . 10
       5.1.2.  Multipoint Links . . . . . . . . . . . . . . . . .  9 . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 13

1.  Introduction

   The MPLS Transport Profile (MPLS-TP) [I-D.ietf-mpls-tp-framework] is
   the set of protocol functions that meet the requirements [RFC5654]
   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].

   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

   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:

   o  To identify the data-plane functions within the MPLS Transport

   o  To indicate which of these data-plane functions an MPLS-TP
      implementation is required to support support.

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

1.2.  Terminology

   Term    Definition
   ------- ------------------------------------------
   G-ACh   Generic Associated Channel
   GAL     G-ACh Label
   LER     Label Edge Router
   LSP     Label Switched Path
   LSR     Label Switching Router
   MAC     Media Access Control
   MPLS-TP MPLS Transport Profile
   OAM     Operations, Administration and Maintenance
   PW      Pseudowire
   QoS     Quality of Service
   TTL     Time To Live

   Additional definitions and terminology can be found in

   [I-D.ietf-mpls-tp-framework] and [RFC5654].

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 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.

   MPLS-TP packet encapsulation and forwarding operates according to the
   MPLS data-plane architecture described in [RFC3031] and [RFC3032],
   and the data-plane architectures for Single-Segment Pseudowires
   [RFC3985], Multi-Segment Pseudowires [RFC5659], and Point-to-
   Multipoint Pseudowires [I-D.ietf-pwe3-p2mp-pw-requirements], except
   as noted otherwise in this document.

   MPLS-TP forwarding is based on the label that identifies an LSP or
   PW.  The label value specifies the processing operation to be
   performed by the next hop at that level of encapsulation.  A swap of
   this label is an atomic operation in which the contents of the packet
   after the swapped label are opaque to the forwarder.  The only event
   that interrupts a swap operation is Time To Live (TTL) expiry.

   Further processing to determine the context of a packet occurs when a
   swap operation is interrupted in this manner, when a pop operation
   exposes a specific reserved label, or when the packet is received
   with the Generic Associated Channel Label (GAL) (see Section 4) at
   the top of the stack.  Otherwise the packet is forwarded according to
   the procedures in [RFC3032].

3.  MPLS-TP Transport Entities

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

   o  Label Switched Paths (LSPs)

   o  Sections

   o  Pseudowires (PWs)

3.1.  Label Switched Paths

   MPLS-TP LSPs are ordinary MPLS LSPs as defined in [RFC3031] 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] and [RFC3032], 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].  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] and [RFC3270] 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] and [RFC3443] 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] MAY be used for MPLS-TP LSPs.  Downstream [RFC3031] or
   upstream [RFC5331] label allocation schemes MAY be used for MPLS-TP
   LSPs.  Note that the requirements of a particular LSP type may
   dictate which label spaces or allocation schemes 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

   Fragmentation procedures as specified in [RFC3032] are outside the
   scope of the MPLS-TP.

3.1.2.  LSP Payloads

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

   o  Network-layer protocol packets

   o  Pseudowire packets

   The rules for processing LSP payloads that are network-layer protocol
   packets SHALL be as specified in [RFC3032] except as specifically
   noted otherwise in this document. [RFC3032].

   The rules for processing LSP payloads that are pseudowire packets
   SHALL be as specified in [RFC3985] and the attendant standards
   defined by the IETF Pseudowire Emulation Edge-to-Edge (PWE3) Working

   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], 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:

   o  Point-to-point unidirectional

   o  Point-to-point associated bidirectional

   o  Point-to-point co-routed bidirectional

   o  Point-to-multipoint unidirectional

   Point-to-point unidirectional LSPs are supported by the basic MPLS
   architecture [RFC3031] 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] and

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.  Such an LSP is referred to as a client of
   the section layer, and the section layer as the server layer with
   respect to its clients.

   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 G-ACh Label (GAL) (see Section 4) must appear at the bottom of
   the label stack.

3.3.  Pseudowires

   The data-plane architectures for Single-Segment Pseudowires
   [RFC3985], Multisegment Pseudowires [RFC5659]

   An MPLS-TP section may provide one or more of the following types of
   service to its client layer:

   o  Point-to-point bidirectional

   o  Point-to-point unidirectional

   o  Point-to-multipoint unidirectional

   The manner in which a section provides such a service is outside the
   scope of the MPLS-TP.

   Note that an LSP of any of the types listed in Section 3.1.3 may
   serve as a section for a client-layer transport entity as long as it
   supports the type of service the client requires.

3.3.  Pseudowires

   The data-plane architectures for Single-Segment Pseudowires
   [RFC3985], Multi-Segment Pseudowires [RFC5659], and Point-to-Multipoint Point-to-
   Multipoint Pseudowires [I-D.ietf-pwe3-p2mp-pw-requirements], 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] 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

   For pseudowires, the G-ACh uses the first four bits of the PW control
   word to provide the initial discrimination between data packets and
   packets belonging to the associated channel, as described in
   [RFC4385].  When this first nibble of a packet, immediately following
   the label at the bottom of stack, has a value of '1', then this
   packet belongs to a G-ACh.  The first 32 bits following the bottom of
   stack label then have a defined format called an Associated Channel
   Header (ACH), which further defines the content of the packet.  The
   ACH is therefore both a demultiplexer for G-ACh traffic on the PW,
   and a discriminator for the type of G-ACh traffic.

   When the the control message is carried over a section or an LSP,
   rather than over a PW, it is necessary to provide an indication in
   the packet that the payload is something other than a client data
   packet.  This is achieved by including a reserved label with a value
   of 13 in the label stack.  This reserved label is referred to as the
   G-ACh Label (GAL), and is defined in [RFC5586].  When a GAL is found,
   it indicates that the payload begins with an ACH.  The GAL is thus a
   demultiplexer for G-ACh traffic on the section or the LSP, and the
   ACH is a discriminator for the type of traffic carried on the G-ACh.
   Note however that MPLS-TP forwarding follows the normal MPLS model,
   and that a GAL is invisible to an LSR unless it is the top label in
   the label stack.  The only other circumstance under which the label
   stack may be inspected for a GAL is when the TTL has expired.  Any
   MPLS-TP component that intentionally performs this inspection must
   assume that it is asynchronous with respect to the forwarding of
   other packets.  All operations on the label stack are in accordance
   with [RFC3031] and [RFC3032].

5.  Media-Specific  Server Layer Considerations

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

   In general, the MPLS-TP network has no awareness of the internals of
   the server layer of which it is a client, requiring only that the
   server layer be capable of delivering the type of service required by
   the MPLS-TP transport entities that make use of it.  Note that what
   appears to be a single server layer link to the MPLS-TP network may
   be a complicated construct underneath, such as an LSP or a collection
   of underlying links operating as a bundle.  Special care may be
   needed in network design and operation when such constructs are used
   as a server layer for MPLS-TP.

5.1.  Ethernet Media

5.1.1.  Destination MAC Address Determination  Point-to-Point Links

   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] or IP version 6 Neighbor Discovery protocol [RFC4861],
   which allow the unicast MAC address of the peer device to be learned

   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.  For example, replacement of an
   Ethernet interface to resolve a hardware fault when this approach is
   used requires that the peer node be manually reconfigured with the
   new MAC address.  This is especially problematic if the peer is
   operated by another provider.

   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. MPLS-TP for use
   over point-to-point links.  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 over
   a point-to-point link with this destination MAC address by default.

   [Editor's note: Decide what to say about multipoint

   Note that this approach is applicable only when the attached Ethernet and
   switched links.  Decide if there
   link is known to be point-to-point.  If a need for protocol mechanisms link is not known to
   support be
   point-to-point, the reserved MAC address noted above MUST NOT be

   A further alternative is to adapt or introduce a protocol mechanism
   for 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 peers.  This topic is for media other
   than further study.

5.1.2.  Multipoint Links

   When a multipoint Ethernet need link serves as a section for a point-to-
   multipoint MPLS-TP LSP, and multicast destination MAC addressing at
   the Ethernet layer is used for the LSP, the addressing and
   encapsulation procedures specified in [RFC5332] SHALL be used.

   When a multipoint Ethernet link - that is, a link which is not known
   to be mentioned.] point-to-point - serves as a section for a point-to-point
   MPLS-TP LSP, unicast destination MAC addresses must be used for
   Ethernet frames carrying packets of the LSP.  Note that according to
   the discussion in the previous section, this implies the use of
   either static MAC address configuration or a protocol that enables
   peer MAC address discovery.

6.  Security Considerations


   This document serves primarily to specify which aspects of existing
   MPLS data-plane functionality apply to MPLS-TP.  As such it
   introduces no new security considerations in itself, but the security
   considerations documented in the specifications to which it refers
   apply as well to MPLS-TP.

7.  IANA Considerations

   A future version of this document will detail

   The authors request that IANA considerations for allocate an Ethernet Multicast Address
   from the allocation of a standard Ethernet multicast MAC address Multicast Addresses table in the ethernet-numbers
   registry for use
   in by MPLS-TP networks. LSRs over point-to-point links as
   described in Section 5.1.1.  The entry should specify an address of
   the form 01-00-5E-XX-XX-XX, a Type Field of 8847/8848, and a usage
   "MPLS-TP point-to-point (this draft)".

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC5654]  Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
              and S. Ueno, "Requirements of an MPLS Transport Profile",
              RFC 5654, September 2009.

   [RFC5586]  Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
              Associated Channel", RFC 5586, June 2009.

   [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", RFC 3270, May 2002.

   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks",
              RFC 3443, January 2003.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, February 2009.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, August 2008.

   [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)", RFC 4875, May 2007.

   [RFC5332]  Eckert, T., Rosen, E., Aggarwal, R., and Y. Rekhter, "MPLS
              Multicast Encapsulations", RFC 5332, August 2008.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, February 2006.

8.2.  Informative References

              Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
              Berger, "A Framework for MPLS in Transport Networks",
              draft-ietf-mpls-tp-framework-10 (work in progress),
              February 2010.

              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", draft-ietf-pwe3-p2mp-pw-requirements-02 (work
              in progress), January 2010.

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC5659]  Bocci, M. and S. Bryant, "An Architecture for Multi-
              Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
              October 2009.

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", STD 37,
              RFC 826, November 1982.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

Authors' Addresses

   Dan Frost (editor)
   Cisco Systems


   Stewart Bryant (editor)
   Cisco Systems


   Matthew Bocci (editor)