Dynamic Link Exchange Protocol (DLEP) Link Identifier Extension
Extracted elements (22)
Routers must not make any assumptions about the meaning of Link Identifiers or how they are generated; the method for generating Link Identifiers is a modem implementation matter and out of scope. This keeps the identifier opaque and avoids coupling the router to modem-internal addressing schemes.
This extension was designed to support radio systems that operate at Layer 3 rather than Layer 2 bridge mode, point-to-multipoint tunnel systems (e.g., SD-WAN), and modems providing wide-area connectivity (e.g., LTE, 802.11) where the concept of a Layer 2 reachable remote router does not apply. The core DLEP assumption that every destination has a MAC address on the local Layer 2 domain fails in these cases.
This extension is built on top of core DLEP (RFC 8175) and requires mutual negotiation during session initialization. A modem that requires the extension to describe destinations but is paired with a router that does not advertise support must fall back to using Session Messages and Attached Subnet Data Items rather than terminating the session, preserving basic interoperability.
A Link Identifier is 4 octets by default. If a modem wishes to use a different length, it MUST announce this using the Link Identifier Length Data Item in the Session Initialization Response Message.
All DLEP Destination Up Messages containing a Link Identifier Data Item MUST contain Layer 3 information — one or more IPv4 or IPv6 Address Data Items or Attached Subnet Data Items — since the MAC address is that of the Gateway Node, not the final destination.
During the lifetime of a DLEP session, the length of Link Identifiers MUST remain constant; the Length field of the Link Identifier Data Item MUST NOT differ between destinations.
If a modem receives a Destination Announce Message from a router containing an unknown Link Identifier, the modem MUST terminate the session by issuing a Session Termination Message with status code 131 ('Invalid Destination') and transition to the Session Termination state.
If a modem requires this extension but the router does not advertise support, the modem SHOULD NOT immediately terminate the DLEP session; it SHOULD instead use DLEP Session Messages and Attached Subnet Data Items to provide general information.
If a router receives a Destination Up Message specifying a Link Identifier that has already been used, the router MUST respond with a Destination Up Response Message containing a Status Data Item with status code 130 ('Invalid Data') and transition to the Session Termination state.
Implementations supporting this extension SHOULD announce at least one valid IPv4 or IPv6 address of the Gateway Node to avoid requiring the router to use an additional IP address resolution protocol before populating its Routing Information Base.
Link Identifier Data Items referring to a new link MUST first appear in a DLEP Destination Up Message from the modem to the router. A router MUST NOT send a Destination Announce Message with a Link Identifier referring to a link not previously announced by the modem.
The IP addresses in IPv4 or IPv6 Address Data Items MUST be the addresses in use on the public side of any Network Address Translation.
To use this extension, both DLEP peers MUST announce support by including value 3 ('Link Identifiers') in the Extension Data Item during session initialization. If a modem requires the extension but the router does not advertise support, the modem MUST NOT include a Link Identifier Data Item in any DLEP Message.
When a Link Identifier Data Item is used, the MAC Address Data Item MUST contain the MAC address of the Gateway Node, not the final destination.
When routing packets to a Layer 3 DLEP Destination via the modem network, the MAC address of the Gateway Node MUST be used as the Layer 2 destination of the packet.
Within a single DLEP session, all Link Identifiers MUST be unique per MAC address, making a Layer 3 DLEP Destination uniquely identified by the pair {MAC Address, Link Identifier}. Link Identifiers MUST NOT be reused within the lifetime of a single DLEP peer-to-peer session.
A Layer 3 DLEP Destination is a DLEP Destination not directly addressable within the local Layer 2 domain but reachable via a node addressable within it. The Gateway Node is the last MAC-addressable device in the local Layer 2 domain on the path toward the Layer 3 DLEP Destination.
IANA assigned extension type code 3 ('Link Identifiers') to the 'Extension Type Values' registry within the 'Dynamic Link Exchange Protocol (DLEP) Parameters' registry, under the 'Specification Required' policy.
IANA assigned two new Data Item Type codes: 26 ('Link Identifier Length') and 27 ('Link Identifier') to the 'Data Item Type Values' registry within the 'Dynamic Link Exchange Protocol (DLEP) Parameters' registry, under the 'Specification Required' policy.
This extension adds no additional security mechanisms or features beyond those of core DLEP (RFC 8175). The security considerations of RFC 8175 apply in full to this extension, and no features introduced here require extra security considerations.
The Link Identifier Data Item (Type 27) carries a variable-length Link Identifier field — default 4 octets, but may differ if announced via the Link Identifier Length Data Item. It may be used wherever a MAC Address Data Item is defined as usable in core DLEP.
The Link Identifier Length Data Item (Type 26) has a fixed 2-octet payload containing the Link Identifier Length field that specifies the length in octets of Link Identifiers used by the modem for the session. It is sent only in the Session Initialization Response Message.