Networking Working Group                                       D. Beller
Internet-Draft                                            Alcatel-Lucent
Intended Status: Standards Track                               A. Farrel
Created: August 27, 2009                              Old Dog Consulting
Expires: February 27, 2010

   An Inband Data Communication Network For the MPLS Transport Profile


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   The Generic Associated Channel (G-ACh) has been defined as a
   generalization of the pseudowire (PW) associated control channel to
   enable the realization of a control/communication channel associated
   with Multiprotocol Label Switching (MPLS) Label Switched Paths
   (LSPs), MPLS PWs, MPLS LSP segments, and MPLS sections between
   adjacent MPLS-capable devices.

   The MPLS Transport Profile (MPLS-TP) is a profile of the MPLS
   architecture that identifies elements of the MPLS toolkit that may be
   combined to build a carrier grade packet transport network based on
   MPLS packet switching technology.

   This document describes how the G-ACh may be used to provide the
   infrastructure that forms part of the Management Communication
   Network (MCN) and a Signaling Communication Network (SCN).
   Collectively, the MCN and SCN may be referred to as the Data
   Communication Network (DCN). This document explains how MCN and SCN
   messages are encapsulated, carried on the G-ACh, and demultiplexed

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   for delivery to the management or signaling/routing control plane
   components on an MPLS-TP node.

   It should be noted that the use of the G-ACh to provide connectivity
   for the DCN is intended for use only where the MPLS-TP network is not
   capable of encapsulating or delivering native DCN messages.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [RFC2119].

1. Introduction

   The associated channel header (ACH) is specified in [RFC4385]. It is
   a packet header format for use on pseudowires (PWs) in order to
   identify packets used for OAM and similar functions.

   The use of the ACH is generalized in [RFC5586] and can be applied on
   any Multiprotocol Label Switching (MPLS) Label Switching Path (LSP).
   This is referred to as the Generic Associated Channel (G-ACh) and is
   intended to create a control/management communication channel
   associated with the LSP that can be used to carry packets used for
   OAM and similar functions (e.g., control/management plane messages).

   The purpose of a packet carried on the G-ACh is indicated by the
   value carried by the Channel Type field of the ACH and a registry of
   values is maintained by IANA ([RFC4446] and [RFC4385]). The
   combination of the ACH and the ACH TLVs that may be appended to the
   ACH is referred in this document as the G-ACh header.

   The MPLS transport profile (MPLS-TP) is described in [MPLS-TP] and in
   [TP-REQ]. MPLS-TP is the application of MPLS to construct a packet
   transport network. It constitutes a profile of MPLS that enables
   operational models typical in transport networks, which includes
   additional OAM, survivability and other maintenance functions not
   previously supported by MPLS.

   Label Switching Routers (LSRs) in MPLS networks may be operated using
   management protocols or control plane protocols. Messaging in these
   protocols is normally achieved using IP packets exchanged over IP-
   capable interfaces. However, some nodes in MPLS-TP networks may be
   constructed without support for direct IP encapsulation on their
   line-side interfaces, and without access to an out-of-fiber data
   communication network. In order that such nodes can communicate using
   management plane or control plane protocols, channels must be
   provided, and the only available mechanism is to use an MPLS label.

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   The G-ACh provides a suitable mechanism for this purpose, and this
   document defines processes and procedures to allow the G-ACh to be
   used to build a management communication network (MCN) and a
   signaling communication network (SCN) together known as the data
   communication network (DCN) [G.7712].

1.1. Requirements

   The requirements presented in this section are based on those
   communicated to the IETF by the ITU-T.

   1. A packet encapsulation mechanism must be provided to support the
      transport of MCN and SCN packets over the G-ACh.

   2. The G-ACh carrying the MCN and SCN packets shall support the
      following application scenarios:

      a. The G-ACh interconnects two adjacent MPLS-TP nodes (used when
         the server layer does not provide a Management Communication
         Channel (MCC) or a Signalling Communication Channel (SCC)).

      b. The G-ACh is carried by an MPLS-TP tunnel that traverses
         another operator's domain (carrier's carrier scenario)

   3. The G-ACh shall provide two independent channels: an MCC to build
      the MCN and an SCC to build the SCN. The G-ACh packet header shall
      indicate whether the packet is an MCC or an SCC packet in order to
      forward it to the management or control plane application for

   4. The channel separation mechanism shall allow the use of separate
      rate limiters and traffic shaping functions for each channel (MCC
      and SCC) ensuring that the flows do not exceed their assigned
      traffic profiles. The rate limiters and traffic shapers are
      outside the scope of the MCC and SCC definitions.

   5. The G-ACh that carries the MCC and SCC shall be capable of
      carrying different OSI layer 3 (network layer) PDUs. These shall
      include IPv4, IPv6, and OSI PDUs. The G-ACh header of the MCC/SCC
      packet shall indicate which layer 3 PDU is contained in the
      payload field of the packet such that the packet can be delivered
      to the related layer 3 process within the management and control
      plane application, respectively, for further processing.

   6. The G-ACh is not required to provide specific security mechanisms.
      However, the management or control plane protocols that operate
      over the MCC or SCC are required to provide adequate security
      mechanisms in order not to be susceptible to security attacks.

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2. Procedures

   Figure 1 depicts the format of an MCC/SCC packet that is sent on the
   G-ACh. The Channel Type field indicates the function of the ACH
   message so, to send an MCC/SCC packet on the G-ACh, the MCC/SCC
   message is prepended with an ACH with the Channel Type set to
   indicate that the message is an MCC or SCC message. The ACH MUST
   include the ACH Protocol ID TLV [ACH-TLV] to identify the protocol
   type of the MCC or SCC message, and MAY include further ACH TLVs.

    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
   |0 0 0 1|Version|   Reserved    |         Channel Type          |
   |                         ACH TLV Header                        |
   ~                      zero or more ACH TLVs                    ~
   ~                                                               ~
   |                      ACH Protocol ID TLV                      |
   |                         MCC/SCC Message                       |
   ~                                                               ~

                 Figure 1: G-ACh MCC/SCC Packet

   o The Channel Type field determines whether the message is an MCC or
     an SCC message. See Section 5 for the codepoint assignments.

   o No other ACH TLV (except the ACH protocol ID TLV - see below) has
     been identified for use on a DCN message. If a message is received
     carrying an ACH TLV that is not understood in the context of a DCN
     message, the ACH TLV SHOULD be silently ignored and processing of
     the message SHOULD continue.

   o The ACH Protocol ID TLV identifies the PDU type of the MCC/SCC
     message. The ACH MUST be present in a DCN message and MUST be
     placed last in the sequence of ACH TLVs so that it comes
     immediately before the MCC/SCC message. Note that this means that
     the PID field of the TLV is immediately adjacent to the message
     itself [ACH-TLV].

     The ACH Protocol ID TLV is defined in [ACH-TLV] and uses the PPP
     protocol identifiers to distinguish different protocols [RFC1661],

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   When the G-ACh sender receives an MCC message that is to be sent over
   the MCC, the sender creates the G-ACh header, provides an ACH
   Protocol ID TLV indicating the MCC layer 3 PDU type, sets the Channel
   Type field to MCC, and prepends the MCC message with the G-ACh
   header. The same procedure is applied when a control plane message is
   to be sent over the SCC. In this case, the sender sets the Channel
   Type field to SCC.

   If the G-ACh is associated with an MPLS section, the GAL is added to
   the message as defined in [RFC5586]. The TTL field MUST be set to 1,
   and the S-bit of the GAL MUST be set to 1.

   If the G-ACh is associated with an LSP, the GAL is added to the
   packet and the LSP label is pushed on top of the GAL as defined in
   [RFC5586]. The TTL field of the GAL MUST be set to 1, and the S-bit
   of the GAL MUST be set to 1.

   The DCN channel MUST NOT be used to transport user traffic and SHALL
   only be used to carry management or control plane messages.
   Procedures that ensure this (such as deep packet inspection) are
   outside the scope of this specification.

   When a receiver has received a packet on the G-ACh with the ACH
   Channel Type set to MCC or SCC, it SHALL look at the PID field
   carried in the ACH Protocol ID TLV. If the TLV is absent, the message
   SHALL be silently discarded, although a local system MAY increment a
   counter that records discarded or errored packets, and MAY log an
   event. If the PID value is known by the receiver it delivers the
   the MCC/SCC message to the appropriate processing entity. If the PID
   value is unknown, the receiver SHALL silently discard the received
   packet, MAY increment a counter that records discarded or errored
   messages, and MAY log an event.

   It must be noted that according to [RFC5586] a receiver MUST NOT
   forward a GAL packet based on the GAL label as is normally the case
   for MPLS packets. If the GAL appears at the bottom of the label
   stack, it MUST be processed as described in the previous paragraph.

   Note that there is no requirement for MPLS-TP devices to support IP
   or OSI forwarding in the fast (forwarding) path. Thus, if a message
   is received on the MCC or SCC and is not targeted to an address of
   the receiving MPLS-TP node the packet might not be forwarded in the
   fast path. A node MAY apply layer 3 forwarding procedures in the slow
   path and MAY discard or reject the message using the layer 3 protocol
   if it is unable to forward it. Thus, protocols making use of the DCN
   should make no assumptions about the forwarding capabilities unless
   they are determined a priori or through the use of a routing
   protocol. Furthermore it is important that user data (i.e., data

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   traffic) is not routed through the DCN as this would potentially
   cause the traffic to be lost or delayed, and might significantly
   congest the DCN.

2.1. Pseudowire Setup

   Provider Edge nodes may wish to set up PWs using a signaling protocol
   that uses remote adjacencies (such as LDP [RFC5036]). In the absence
   of an IP-based control plane network, these PEs MUST first set up an
   LSP tunnel across the MPLS-TP network. This tunnel can be used both
   to carry the PW once it has been set up and to provide a G-ACh based
   DCN for control plane communications between the PEs.

3. Applicability

   The DCN is intended to provide connectivity between management
   stations and network nodes, and between pairs of network nodes, for
   the purpose of exchanging management plane and control plane

   Appendix A of [NM-REQ] describes how Control Channels (CCh) that are
   the links in an MPLS-TP DCN can be out-of-fiber and out-of-band, in-
   fiber and out-of-band, or in-band with respect to the user data
   carried by the MPLS-TP network. The Appendix also explains how the
   DCN can be constructed from a mix of different types of links and
   how routing and forwarding can be used within the DCN to facilitate
   multi-hop delivery of management and control plane messages.

   The G-ACh used as described in this document allows the creation of
   a "data channel associated CCh" (type 6 in Appendix A of [NM-REQ])
   and an "in-band CCh" (type 7 in Appendix A of [NM-REQ]). In the
   former case, the G-ACh is associated with an MPLS-TP section. In the
   latter case, the G-ACh is associated with an MPLS-TP LSP or PW and
   may span one or more hops in the MPLS-TP network.

   There is no need to create a CCh for every LSP between a pair of
   Indeed, where the nodes are physically adjacent, the G-ACh associated
   with the MPLS-TP section would normally be used. Where nodes are
   virtually adjacent (that is, connected by LSP tunnels), one or two of
   the LSPs might be selected to provide the CCh and a back-up CCh.

4. Security Considerations

   The G-ACh provides a virtual link between MPLS-TP nodes and might be
   used to induce many forms of security attack. Protocols that operate
   over the MCN or SCN are REQUIRED to include adequate security
   mechanisms and implementations MUST allow operators to configure the
   use of those mechanisms.

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5. IANA Considerations

   Channel Types for the Generic Associated Channel are allocated from
   the IANA PW Associated Channel Type registry defined in [RFC4446] and
   updated by [RFC5586].

   IANA is requested to allocate two further Channel Types as follows:
     xx  Management Communication Channel (MCC)
     yy  Signaling Communication Channel (SCC)

6. Normative References

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

   [RFC4385]  Bryant, S., et al., "Pseudowire Emulation Edge-to-Edge
              (PWE3) Control Word for Use over an MPLS PSN", RFC 4385,
              February 2006.

   [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", RFC 4446, April 2006 .

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

   [ACH-TLV]  Bryant, S., et al., "Definition of ACH TLV Structure",
              draft-bryant-mpls-tp-ach-tlv, work in progress.

7. Informative References

   [MPLS-TP]  Bryant, S., Bocci, M., Lasserre, M., "A Framework for MPLS
              in Transport Networks", draft-ietf-mpls-tp-framework, work
              in progress.

   [TP-REQ]   B. Niven-Jenkins, Ed., D. Brungard, Ed., M. Betts, Ed.,
              N. Sprecher, S. Ueno, "MPLS-TP Requirements",
              draft-ietf-mpls-tp-requirements, work in progress.

   [NM-REQ]   Lam, H.-K., Mansfield, S., and Gray, E., "MPLS TP Network
              Management Requirements", draft-ietf-mpls-tp-nm-req, work
              in progress.

   [G.7712]   ITU-T Recommendation G.7712, "Architecture and
              specification of data communication network", June 2008.

   [RFC1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

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   [RFC3818]  Schryver, V., "IANA Considerations for the Point-to-Point
              Protocol (PPP)", BCP 88, RFC 3818, June 2004.

   [RFC5036]  Andersson, L., Minei, I., and Thomas, B., "LDP
              Specification", RFC 5036, October 2007.

8. Acknowledgements

   The editors wish to thank Pietro Grandi, Martin Vigoureux, Kam Lam,
   Ben Niven-Jenkins, and Francesco Fondelli for their contribution to
   this document, and the MEAD team for thorough review.

   Study Group 15 of the ITU-T provided the basis for the requirements
   text in Section 1.1.

9. Authors' Addresses

   Dieter Beller
   Alcatel-Lucent Germany

   Adrian Farrel
   Old Dog Consulting

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