Encapsulation Methods for Transport of Frame Relay over Multiprotocol Label Switching (MPLS) Networks
Extracted elements (26)
Frame relay BECN and FECN bits are transparent to the MPLS network and cannot reflect MPLS network congestion status. This is an inherent limitation of the emulation model, noted in the applicability statement.
Frame relay port mode PW was not given a separate encapsulation definition because it is identical to HDLC PW encapsulation; it does not process frame relay status messages or alarms (Q.922/Q.933), simplifying the design by reuse.
The legacy 'Martini mode' Control Word (PW type 0x0001) was defined for backward compatibility with existing implementations, differing from the new standard (PW type 0x0019) only in the reversal of the B (BECN) and F (FECN) bits.
Frame relay LMI is terminated locally at the PE connected to the frame relay attachment circuit and is not transported across the pseudowire. SVC, SPVC, and PVC link integrity check support are outside the scope of this document.
The legacy bit ordering (B and F reversed) is used for PW type 0x0001 'Frame Relay DLCI (Martini Mode)', while the new bit ordering is used for PW type 0x0019 'Frame Relay DLCI'. The IANA Pseudowire Type registry (RFC4446) governs these allocations.
A PE MUST NOT clear (set to 0) the DE bit if it was received with a value of 1. The DE bit MAY be set by ingress frame policing if not already set, but clearing it is prohibited.
A PE MUST NOT clear the FECN (F) or BECN (B) bits if received as 1. These bits MAY be set to reflect detected congestion on the PE, but are never cleared from 1 to 0.
Frame relay PWs that consume more than their fair share in the presence of services that reduce transmission rate SHOULD be halted. Whenever possible, frame relay PWs should run over traffic-engineered PSNs providing bandwidth allocation and admission control.
If PE2 supports received sequence number processing, the procedures in RFC4385 Section 4.2 MUST be used. For sequence number sending, if PE1 supports packet sequencing, RFC4385 Section 4.1 MUST be followed.
If the PW packet length (payload plus control word) is less than 64 octets, the Length field MUST be set to the packet's length; otherwise the Length field MUST be set to zero. The egress PE uses this field to remove padding.
QoS information SHOULD be represented in the EXP field of the PW MPLS label. If more than one MPLS label is imposed by the ingress LSR, the EXP field of labels higher in the stack SHOULD also carry the same value.
The C/R bit and DE, FECN, and BECN bits MUST be copied from the PW Control Word into the regenerated frame relay header during decapsulation. Padding MUST be removed before forwarding.
The Control Word is REQUIRED for frame relay one-to-one mode. Bits 0 to 3 of the Control Word MUST be set to 0 to indicate PW data.
The ingress LSR (PE1) MUST set the S bit of the PW label to 1, denoting that the PW label is at the bottom of the MPLS label stack.
The interface parameter information, when applicable, MUST be used to validate that the PEs and ingress/egress ports have the necessary capabilities to interoperate. The Frame Relay Header Length Sub-TLV (0x08) is OPTIONAL and indicates the FR header length (2, 3, or 4 octets); default is 2.
The maximum length of the payload field MUST be agreed upon by the two PEs, achievable by using the MTU interface parameter when the PW is established via RFC4447. It is RECOMMENDED to support a frame size of at least 1600 bytes.
The PE MUST provide frame relay PVC status signaling to the frame relay network. If a service-affecting condition is detected for a DLCI, the PE MUST communicate the PW status to the remote PE. The egress PE SHOULD generate corresponding errors and alarms on the egress frame relay PVC.
When a PE detects significant congestion while receiving PW PDUs, the BECN bits of frame relay frames transmitted on the same PW SHOULD be set to notify the remote PE. Additionally, FECN bits SHOULD be set in FR frames sent out the attachment circuit.
The Frame Relay Specific Interface Parameter Sub-TLV type 0x08 (Frame Relay Header Length) is an optional 16-bit value indicating the FR header length in octets, with valid values of 2, 3, or 4 and a default of 2 if absent.
The general frame relay pseudowire packet consists of an MPLS Transport header, a Pseudowire (PW) Header for multiplexing PWs within an MPLS tunnel, a Control Word carrying protocol control information, and a Frame Relay Service Payload.
The sequence number field in the Control Word is a 16-bit unsigned circular space. The value 0 indicates the sequence number check algorithm is not used. Processing of the sequence number field is OPTIONAL.
Two mapping modes are defined: one-to-one mapping (one frame relay VC per pseudowire) and many-to-one mapping (port mode, where multiple frame relay VCs on a port map to one PW). Port mode encapsulation is identical to HDLC PW encapsulation (RFC4618).
PWE3 provides no means of protecting the contents or delivery of PW packets on behalf of the native service. Security must be leveraged from the MPLS Tunnel Layer. A more detailed discussion is given in RFC3985, RFC4447, and RFC3916.
For one-to-one mapping mode, the PW packet format over MPLS PSN consists of: MPLS Tunnel label(s) (n*4 octets), PW label (4 octets), Control Word (4 octets), and Payload (frame relay frame information field, n octets).
The Control Word for one-to-one mapping mode is 4 octets: bits 0-3 MUST be 0 (PW data indicator), bit 4 is FECN (F), bit 5 is BECN (B), bit 6 is DE (D), bit 7 is C/R (C), bits 8-9 are FRG (fragmentation per RFC4623), bits 10-15 are Length, and bits 16-31 are the Sequence Number.
The Martini Legacy Mode Control Word differs from the standard control word only in that the B (BECN) and F (FECN) bits are reversed: bit 4 is BECN and bit 5 is FECN. This format is used for PW type 0x0001 'Frame Relay DLCI (Martini Mode)'.