Layer Two Tunneling Protocol (L2TP) over Frame Relay
Extracted elements (15)
L2TP was designed to be independent of the media it runs over, insulating L2TP operation from media-specific details. The base specification uses UDP/IP, and this document extends it to native Frame Relay.
QoS mechanisms for L2TP over Frame Relay can be roughly provided with proprietary mechanisms localized within the LAC or LNS; QoS considerations are explicitly declared out of scope for this document.
FRF.12 (Frame Relay Fragmentation Implementation Agreement) may be used to handle MTU issues; if fragmentation is not supported, a minimum MTU of 1526 must be maintained by both endpoints.
All protocols over a Frame Relay circuit MUST encapsulate their packets within a Q.922 frame, and frames must contain information necessary to identify the protocol carried within the Frame Relay PDU.
For PVCs, the endpoints of the L2TP tunnel MUST be identified by DLCI assigned to the PVC at configuration time; this DLCI MAY be obtained as tunnel endpoints as defined in RFC 2868.
If fragmentation is not supported, the two Frame Relay endpoints MUST support an MTU size of at least 1526, calculated as PPP MRU + PPP header + max L2TP header + Frame Relay header.
L2TP MUST be able to share a Frame Relay virtual circuit (VC) with other protocols carried over the same VC.
The endpoints of the L2TP tunnel MUST be identified by X.121/E.164 addresses in the case of Frame Relay SVCs; these addresses MAY be obtained as tunnel endpoints as defined in RFC 2868.
The frame format for L2TP MUST be SNAP encapsulation as defined in RFC 1490 and FRF3.1, using NLPID followed by OUI and PID.
The RECOMMENDED default Frame Relay MTU is 1564, based on a PPP default MRU of 1500, to avoid packet discards on the Frame Relay interface.
There SHALL be no framing issues between PPP and Frame Relay; PPP frames from remote users are stripped of CRC, link framing, and transparency bytes before L2TP encapsulation and forwarding.
There SHALL NOT be any change in Frame Relay SVC signaling due to L2TP; L2TP tunnel creation triggers SVC connection setup but must not alter SVC signaling behavior.
NLPID value 0x80 is used for L2TP over Frame Relay, indicating the presence of a SNAP header. The OUI 0x00-00-5E identifies IANA and PID 0x0007 identifies the L2TP protocol.
There is no standard specification for Frame Relay security at the time of publication; the Frame Relay Forum was working on a Frame Relay Privacy Agreement, and basic security issues are deferred to the base L2TP specification (RFC 2661).
L2TP frames encapsulated in Frame Relay use: Q.922 Address (2 octets), Control 0x03 + pad 0x00, NLPID 0x80, OUI 0x00-00-5E (3 octets), PID 0x0007 (2 octets), L2TP packet payload, and FCS.