MPLS                                                            D. Frost
Internet-Draft                                                 S. Bryant
Intended status: Standards Track                           Cisco Systems
Expires: October 10, 2013                                       M. Bocci
                                                          April 08, 2013

                  MPLS-TP Next-Hop Ethernet Addressing


   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
   presents considerations for link-layer addressing of Ethernet frames
   carrying MPLS-TP packets.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 10, 2013.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   The MPLS Transport Profile (MPLS-TP) [RFC5921] 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.  The MPLS-TP data plane consists of those MPLS-TP functions
   concerned with the encapsulation and forwarding of MPLS-TP packets
   and is described in [RFC5960].

   This document presents considerations for link-layer addressing of
   Ethernet frames carrying MPLS-TP packets.  Since MPLS-TP packets are
   MPLS packets, existing procedures ([RFC3032], [RFC5332]) for the
   encapsulation of MPLS packets over Ethernet apply.  Because IP
   functionality is only optional in an MPLS-TP network, IP-based
   protocols for Media Access Control (MAC) address learning, such as
   the Address Resolution Protocol (ARP) [RFC0826] and IP version 6
   Neighbor Discovery [RFC4861], may not be available.  This document
   specifies the options for the determination and selection of the
   next-hop Ethernet MAC address when MPLS-TP is used between nodes that
   do not have an IP dataplane.

1.1.  Terminology

   Term    Definition
   ------- ---------------------------
   ARP     Address Resolution Protocol
   G-ACh   Generic Associated Channel
   LSP     Label Switched Path
   LSR     Label Switching Router
   MAC     Media Access Control
   MPLS-TP MPLS Transport Profile

   Additional definitions and terminology can be found in [RFC5960] and

1.2.  Requirements Language

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

2.  Point-to-Point Link Addressing

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   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, or if
   Link Layer Discovery Protocol (LLDP) [LLDP] is in use, or if it
   implements the procedures in Section 4 of this document -- such
   mechanisms SHOULD be used.  The remainder of this section discusses
   the available options when this is not the case.

   Each node MAY be statically configured with the MAC address of its
   peer.  Note however that static MAC address configuration 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 approach which may be considered is to use the Ethernet
   broadcast address FF-FF-FF-FF-FF-FF as the destination MAC address in
   frames carrying MPLS-TP packets over a link that is known to be
   point-to-point.  This may, however, 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.

   In view of the above considerations, the approach which SHOULD be
   used, is therefore to configure both nodes to use the method
   described in this document which uses, as a destination MAC address,
   an Ethernet multicast address reserved for 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-90-00-00.  An
   MPLS-TP implementation MUST process Ethernet frames received over a
   point-to-point link with this destination MAC address by default.

   The use of broadcast or multicast addressing for the purpose
   described in this section, i.e.  as a placeholder for the unknown
   unicast MAC address of the destination, is applicable only when the
   attached Ethernet link is known to be point-to-point.  If a link is
   not known to be point-to-point, these forms of broadcast or multicast

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   addressing MUST NOT be used.  Thus the implementation MUST provide a
   means for the operator to declare that a link is point-to-point if it
   supports these addressing modes.  Moreover, the operator is cautioned
   that it is not always clear whether a given link is, or will remain,
   strictly point-to-point, particularly when the link is supplied by an
   external provider; point-to-point declarations must therefore be used
   with care.  Because of these caveats it is RECOMMENDED that
   implementations support the procedures in Section 4 so that unicast
   addressing can be used.

3.  Multipoint Link Addressing

   When a multipoint Ethernet link serves as a section [RFC5960] 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 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.  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.

4.  MAC Address Discovery via the G-ACh Advertisement Protocol

   The G-ACh Advertisement Protocol (GAP) [I-D.ietf-mpls-gach-adv]
   provides a simple means of informing listeners on a link of the
   sender's capabilities and configuration.  When used for this purpose
   on an Ethernet link, GAP messages are multicast to the address
   01-00-5e-80-00-0d (see [I-D.ietf-mpls-gach-adv] Section 7).  If these
   messages contain the unicast MAC address of the sender, then
   listeners can learn this address and use it in the future when
   transmitting frames containing MPLS-TP packets.  Since the GAP does
   not rely on IP, this provides a means of unicast MAC discovery for
   MPLS-TP nodes without IP support.

   This document defines a new GAP application "Ethernet Interface
   Parameters" (TBD1), to support the advertisement of Ethernet-specific
   parameters associated with the sending interface.  The following
   Type-Length-Value (TLV) objects are defined for this application; the
   TLV format is as defined in [I-D.ietf-mpls-gach-adv]:

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      Source MAC Address (type = 0, length = 8): The Value of this
      object is an EUI-64 [EUI-64] unicast MAC address assigned to one
      of the interfaces of the sender that is connected to this data
      link.  The IEEE-defined mapping from 48-bit MAC addresses to
      EUI-64 form is used.

      Maximum Frame Size (MFS) (type = 1, length = 4): The Value of this
      object is a 32-bit unsigned integer encoded in network byte order
      that specifies the maximum frame size octets of an an Ethernet
      Frame that can be sent over the sending interface.  Where MAC
      address learning occurs by some other means, this TLV group MAY be
      used to advertise only the MFS.  If multiple advertisements are
      made for the same parameter, use of these advertisements is

       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
      |     Type=0    |    Reserved   |           Length=8            |
      |                MAC Address in EUI-64 Format                   |
      |                                                               |

                       Figure 1: Source MAC Address Object Format

      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
     |      Type=1    |    Reserved   |          Length=4            |
     |                     Maximum Frame Size (MFS)                  |

                           Figure 2: MFS Object Format

   Per [I-D.ietf-mpls-gach-adv], MAC Address Discovery information needs
   to be periodically retransmitted and is to be retained by a receiver
   based on the period of time indicated by the associated Lifetime
   field.  To expedite the initialization of a link it is RECOMMENDED
   that a node that has been reconfigured, rebooted or is aware that it
   have been disconnected from its peer send a GAP Ethernet Interface
   Parameter message, and that it issues a GAP request message for the
   Ethernet parameters at the earliest opportunity.

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   When the MAC address in the received Source MAC Address TLV changes
   the new MAC address MUST be used (see Section 5.2 of

   If a minimum MFS is configured for a link and the MFS advertised by
   the peer is lower than that minimum, the operator MUST be notified of
   the MFS mismatch.  Under these circumstances the operator may choose
   to configure the LSR to shut the link, thereby triggering a fault,
   and hence causing the end-to-end path to be repaired.  Alternatively
   the operator may choose to configure the LSR to leave the link up so
   that an OAM message can be used to verify the actual MFS.

   In the event a GAP message is not received within the previously
   received associated Lifetime, the receiving node MUST assume that it
   is now connected to a node that does not support these advertisements
   and must behave as configured for this eventuality.

5.  Manageability Considerations

   The values sent and received by this protocol MUST be made accessible
   for inspection by network operators, and where local configuration is
   updated by the received information, it MUST be clear why the
   configured value has been changed.  The advertised information SHOULD
   be persistent across restarts.  Received advertisements MUST be
   discarded across restarts.  If the received values change, the new
   values MUST be used and the change made visible to the network

6.  Security Considerations

   The use of broadcast or multicast Ethernet destination MAC addresses
   for frames carrying MPLS-TP data packets can potentially result in
   such frames being distributed to devices other than the intended
   destination node or nodes when the Ethernet link is not point-to-
   point.  The operator SHOULD take care to ensure that MPLS-TP nodes
   are aware of the Ethernet link type (point-to-point or multipoint).
   In the case of multipoint links, the operator SHOULD either ensure
   that no devices are attached to the link that are not authorized to
   receive the frames, or take steps to mitigate the possibility of
   excessive frame distribution, for example by configuring the Ethernet
   switch to appropriately restrict the delivery of multicast frames to
   authorized ports.

   An attacker could disrupt communications by modifying the Source MAC
   Address or the MFS values, however this is mitigated by the use of
   cryptographic authentication as described in [I-D.ietf-mpls-gach-adv]
   which also describes other considerations applicable to the GAP
   protocol.  Visibility into the contents of either of the TLVs could

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   provide information that is useful for an attacker.  This is best
   addressed by physical security of the links.

7.  IANA Considerations

7.1.  Ethernet Multicast Address Allocation

   IANA has allocated an Ethernet multicast address from the "IANA
   Multicast 48-bit MAC Addresses" address block in the "Ethernet
   Numbers" registry for use by MPLS-TP LSRs over point-to-point links
   as described in Section 2.  The allocated address is
   01-00-5E-90-00-00.  IANA is requested to update the reference to
   point to the RFC number assigned to this document.

7.2.  G-ACh Advertisement Protocol Allocation

   IANA is requested to allocate a new Application ID in the "G-ACh
   Advertisement Protocol Applications" registry
   [I-D.ietf-mpls-gach-adv] (currently located in the "Pseudowire Name
   Spaces (PWE3)"), as follows:

   Application ID            Description                 Reference
   ------------------------- --------------------------- ---------------
   TBD1 to be assigned by    Ethernet Interface          (this draft)
   IANA                      Parameters

7.3.  Creation of Ethernet Interface Parameters Registry

   IANA is requested to create a new registry, "G-ACh Advertisement
   Protocol: Ethernet Interface Parameters" within the "Pseudowire Name
   Spaces (PWE3)" with fields and initial allocations as follows:

   Type Name          Type ID Reference
   ------------------ ------- ------------
   Source MAC Address 0       (this draft)
   Maximum Frame Size 1       (this draft)

   The range of the Type ID field is 0 - 255.

   The allocation policy for this registry is IETF Review.

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8.  Acknowledgements

   We thank Adrian Farrel for his valuable review comments on this

9.  References

9.1.  Normative References

   [EUI-64]   , "[EUI64] IEEE, "Guidelines for 64-bit Global Identifier
              (EUI-64) Registration Authority", http://
    , March
              1997.", .

              Frost, D., Bryant, S., and M. Bocci, "MPLS Generic
              Associated Channel (G-ACh) Advertisement Protocol", draft-
              ietf-mpls-gach-adv-06 (work in progress), February 2013.

   [LLDP]     , "IEEE, "Station and Media Access Control Connectivity
              Discovery (802.1AB)", September 2009.", .

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

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

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

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

   [RFC5960]  Frost, D., Bryant, S., and M. Bocci, "MPLS Transport
              Profile Data Plane Architecture", RFC 5960, August 2010.

9.2.  Informative References

   [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.

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

   [RFC5921]  Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
              Berger, "A Framework for MPLS in Transport Networks", RFC
              5921, July 2010.

Authors' Addresses

   Dan Frost
   Cisco Systems


   Stewart Bryant
   Cisco Systems


   Matthew Bocci


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