MPLS Transport Profile (MPLS-TP) Next-Hop Ethernet Addressing
Extracted elements (26)
A reserved Ethernet multicast address (01-00-5E-90-00-00) is recommended as a non-negotiated destination MAC for point-to-point links because it avoids the administrative complexity of static configuration while limiting distribution to only nodes that recognize the MPLS-TP multicast address.
Because IP functionality is optional in MPLS-TP networks, ARP and IPv6 Neighbor Discovery may not be available for MAC address learning. The GAP-based mechanism provides unicast MAC discovery specifically for MPLS-TP nodes without IP support, avoiding the operational burden of static MAC configuration.
Static MAC configuration was rejected as the default approach because it presents an administrative burden and causes operational problems on hardware replacement, especially across provider boundaries. Broadcast (FF-FF-FF-FF-FF-FF) was also rejected due to excessive frame distribution and potential special handling by devices.
GAP messages for Ethernet Interface Parameters are multicast to 01-00-5e-80-00-0d (as defined in Section 7 of RFC 7212), while the MPLS-TP point-to-point data address is 01-00-5E-90-00-00; implementors must distinguish between these two addresses.
Since MPLS-TP packets are MPLS packets, existing procedures in RFC 3032 and RFC 5332 for encapsulation of MPLS packets over Ethernet apply directly; this document adds only the MAC address determination layer on top.
After the Source MAC Address TLV lifetime has expired (due to peer ceasing transmission, removing the TLV, or disconnecting), the expired MAC address MUST NOT be used, and the node MUST return to selecting MAC addresses as though no advertisements were received.
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 the destination MAC address 01-00-5E-90-00-00 (the IANA-allocated MPLS-TP multicast address for point-to-point links).
Broadcast or multicast addressing used as a placeholder for an unknown unicast MAC address MUST NOT be used on links not known to be point-to-point. The implementation MUST provide a means for the operator to declare that a link is point-to-point if it supports these addressing modes.
If a minimum MFS is configured for a link and the peer-advertised MFS is lower, the operator MUST be notified of the mismatch; the operator may then choose to shut down the link or verify via OAM.
If existing mechanisms such as ARP, IPv6 Neighbor Discovery, or LLDP are available to determine destination unicast MAC addresses, these methods SHOULD be used. If none are available and both peers implement the GAP procedures in Section 4, the GAP mechanism SHOULD be used.
It is RECOMMENDED that a node that has been reconfigured, rebooted, or reconnected send a GAP Ethernet Interface Parameters message and issue a GAP Request for its peers' Ethernet Interface Parameters at the earliest opportunity, to expedite link initialization.
It is RECOMMENDED that implementations support the GAP-based MAC discovery procedures in Section 4 so that unicast addressing can be used rather than multicast/broadcast placeholders.
The Ethernet address and parameters advertised for an interface SHOULD be persistent across restarts. On system restart, any data retained from prior GAP peer advertisements MUST be discarded to prevent operation on stale data.
Values sent and received by this protocol MUST be accessible for operator inspection, and where local configuration is updated by received information, it MUST be clear why the configured value changed. If received values change, the new values MUST be used and the change made visible.
When a link 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.
When a multipoint Ethernet link serves as a section for a point-to-multipoint MPLS-TP LSP and multicast Ethernet addressing is used, the addressing and encapsulation procedures of RFC 5332 SHALL be used.
When the MAC address in a received Source MAC Address TLV changes, the new MAC address MUST be used (per Section 5.2 of RFC 7212).
Where a link changes type (e.g., 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 configured for that circumstance.
A new GAP application 'Ethernet Interface Parameters' (Application ID 0x0001) is defined to advertise Ethernet-specific parameters of the sending interface over a G-ACh link. It contains Source MAC Address and Maximum Frame Size TLV objects.
IANA allocated Application ID 0x0001 ('Ethernet Interface Parameters') in the 'G-ACh Advertisement Protocol Application Registry' to identify the new GAP application defined by this document.
IANA allocated Ethernet multicast address 01-00-5E-90-00-00 from the 'IANA Multicast 48-bit MAC Addresses' block in the 'Ethernet Numbers' registry for use by MPLS-TP LSRs over point-to-point links.
IANA created a new registry 'G-ACh Advertisement Protocol: Ethernet Interface Parameters' within the 'Generic Associated Channel (G-ACh) Parameters' registry, with Type IDs 0 (Source MAC Address) and 1 (Maximum Frame Size), Type ID range 0-255, and allocation policy of IETF Review or IESG Approval.
An attacker could disrupt communications by injecting false Source MAC Address or MFS values in GAP advertisements. This is mitigated by the cryptographic authentication mechanisms described in RFC 7212. Physical security of links is recommended to prevent disclosure of TLV contents.
Using broadcast or multicast Ethernet destination MAC addresses on links not confirmed to be point-to-point can cause MPLS-TP data frames to be distributed to unintended devices. Operators must ensure MPLS-TP nodes correctly identify link type and restrict multicast frame delivery on multipoint links.
Maximum Frame Size (MFS) TLV (type=1, length=4): carries the maximum Ethernet frame size in octets as a 32-bit unsigned integer in network byte order, following a 1-byte Type, 1-byte Reserved, and 2-byte Length field.
Source MAC Address TLV (type=0, length=8): carries the sender's EUI-64 unicast MAC address (48-bit MAC mapped to EUI-64 per IEEE), encoded as 8 bytes following a 1-byte Type, 1-byte Reserved, and 2-byte Length field.