Internet Engineering Task Force (IETF) D. Frost

Internet Engineering Task Force (IETF) D. Frost
Request for Comments: 7213 Blue Sun
Category: Standards Track S. Bryant
ISSN: 2070-1721 Cisco Systems
M. Bocci
Alcatel-Lucent
June 2014

MPLS Transport Profile (MPLS-TP) Next-Hop Ethernet Addressing

Abstract

The 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

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7213.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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RFC 7213 MPLS-TP Ethernet Addressing June 2014

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 optional in an MPLS-TP network, IP-based protocols
for Media Access Control (MAC) address learning, such as the Address
Resolution Protocol (ARP) [RFC826] and IPv6 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
data plane.

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
[RFC5654].

1.2. 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 [RFC2119].

2. Point-to-Point Link Addressing

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

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of the presence of the Ethernet Address Resolution Protocol (commonly
referred to as the Address Resolution Protocol and shortened to ARP)
[RFC826] or IPv6 Neighbor Discovery (ND) protocol [RFC4861], 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, or if
the Link Layer Discovery Protocol (LLDP) [LLDP] is in use, these
methods SHOULD be used. If ARP, ND, and LLDP are not available, and
both peers implement the procedures in Section 4 of this document,
then the GAP mechanism described in this memo SHOULD be used. The
remainder of this section discusses alternative options that might be
employed when none of the preceding mechanisms for learning MAC
addresses are available.

One common approach is for each node to be statically configured with
the MAC address of its peer. However, 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 that 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, this document recommends that,
if a non-negotiated address is to be used, both nodes are configured
to use 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 default
to passing to the MPLS sub-system any MPLS packets received from a
point-to-point Ethernet link with this destination MAC address.

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

Frost, et al. Standards Track [Page 3]

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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 therefore need to 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 that 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) [RFC7212] 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
Section 7 of [RFC7212]). 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" (0x0001) 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 [RFC7212]:

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.

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RFC 7213 MPLS-TP Ethernet Addressing June 2014

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 in octets of 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
undefined.

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 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

MFS Object Format

Per [RFC7212], 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 has been
disconnected from its peer send a GAP Ethernet Interface Parameters
message, and that it issue a GAP Request message for the Ethernet
Interface Parameters of its peers, at the earliest opportunity.

When the MAC address in the received Source MAC Address TLV changes,
the new MAC address MUST be used (see Section 5.2 of [RFC7212]).

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 down the link, thereby triggering a

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fault and 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.

A peer node could cease transmission of G-ACh advertisements, or
cease to include a Source MAC Address TLV in advertisements, or cease
to be connected, any of which will cause the TLV lifetime to expire.
After the Source MAC Address TLV lifetime has expired, this MAC
Address MUST NOT be used as the peer MAC address. The node MUST
return to selecting MAC addresses as though no advertisements were
received, using the method 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 received information, it MUST be clear why the configured
value has been changed. If the received values change, the new
values MUST be used and the change made visible to the network
operators.

The Ethernet address and associated parameters advertised for an
interface SHOULD be persistent across restarts. In the event of a
system restart, any data that has been retained as a consequence of
prior Ethernet Interface Parameters advertisements from GAP peers
MUST be discarded; this prevents incorrect operation on the basis of
stale data.

Where the link changes to a new type, i.e., from point-to-point to
point-to-multipoint, the network operator SHOULD be informed. If the
new link type is incompatible with the Ethernet addressing method in
use, the system MUST take the action that is configured under those
circumstances.

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).

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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 [RFC7212], which also
describes other considerations applicable to the GAP protocol.
Visibility into the contents of either of the TLVs could 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 has updated the reference to point to the
RFC number assigned to this document.

7.2. G-ACh Advertisement Protocol Allocation

IANA has allocated a new Application ID in the "G-ACh Advertisement
Protocol Application Registry", as follows:

Application ID Description Reference
-------------- ----------------------------- ---------
0x0001 Ethernet Interface Parameters This RFC

7.3. Creation of Ethernet Interface Parameters Registry

IANA has created a new registry, "G-ACh Advertisement Protocol:
Ethernet Interface Parameters" within the "Generic Associated Channel
(G-ACh) Parameters" registry with fields and initial allocations as
follows:

Type Name Type ID Reference
------------------ ------- ---------
Source MAC Address 0 This RFC
Maximum Frame Size 1 This RFC

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

The allocation policy for this registry is IETF Review or IESG
Approval.

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RFC 7213 MPLS-TP Ethernet Addressing June 2014

8. Acknowledgements

We thank Adrian Farrel for his valuable review comments on this
document.

9. References

9.1. Normative References

[EUI-64] IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
Registration Authority", March 1997,
<http://standards.ieee.org/regauth/oui/tutorials/
EUI64.html>.

[LLDP] IEEE, "Station and Media Access Control Connectivity
Discovery", IEEE 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.

[RFC7212] Frost, D., Bryant, S., and M. Bocci, "MPLS Generic
Associated Channel (G-ACh) Advertisement Protocol", RFC
7212, June 2014.

9.2. Informative References

[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.

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RFC 7213 MPLS-TP Ethernet Addressing June 2014

[RFC826] 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.

Authors' Addresses

Dan Frost
Blue Sun

EMail: [email protected]

Stewart Bryant
Cisco Systems

EMail: [email protected]

Matthew Bocci
Alcatel-Lucent

EMail: [email protected]

Frost, et al. Standards Track [Page 9]