Encapsulation Methods for Transport of Asynchronous Transfer Mode (ATM) over MPLS Networks
Extracted elements (28)
A control word is used to prevent ECMP misclassification: without it, an ATM cell whose VPI begins with 0x4 or 0x6 could be mistaken for an IPv4/IPv6 packet by ECMP implementations that inspect the first nibble after the MPLS label stack, causing misordering or OAM path divergence.
The AAL5 PDU transparent mode carries the full CPCS-PDU including PAD, trailer, and CRC without processing, explicitly to support scrambled AAL5 PDUs (ATM security agents) that the PE cannot reassemble; the unprocessed CRC also allows integrity checking at the egress.
The AAL5 SDU mode strips the PAD and CPCS-PDU trailer at ingress to avoid the bandwidth waste of even small TCP packets requiring two ATM cells over AAL5 on native ATM; the trailer is regenerated at egress.
The encapsulations defined in RFC 4717 are identical to those in ITU-T Y.1411 (cell mode) and Y.1412 (frame mode), enabling interoperability between IETF and ITU-T implementations.
The sequence number value 0 in the control word indicates that the sequence number check algorithm is not used; a PE that negotiated not to use sequence number processing and receives a non-zero sequence number SHOULD send a PW status message indicating a receive fault and disable the PW.
ATM PWs that consume more than their fair share of PSN bandwidth in the presence of TCP-friendly services SHOULD be halted; PEs SHOULD monitor for congestion using explicit congestion notification, VCCV, or packet loss measurement.
For ILMI support, when a PW label mapping is withdrawn or a PW status notification is received, the egress PE MUST notify its ATM client of the failure by deleting the VC using ILMI.
Future implementations MUST be able to send and receive frames without a control word present, to aid backwards compatibility with early ATM PW deployments that did not include a control word.
If an encapsulated layer 2 PDU exceeds the tunnel MTU, the ingress router MUST drop it; if the payload exceeds the destination layer 2 interface MTU, the egress router MUST drop it.
If the PE detects a PW failure (label withdraw, targeted LDP session failure, or PW status TLV notification), a proper AIS F5 OAM cell MUST be generated for all affected ATM PVCs.
In AAL5 CPCS-SDU mode, the egress PE MUST regenerate the PAD and AAL5 CPCS-PDU trailer before transmitting the AAL5 frame on the egress ATM port.
In One-to-one mode, the control word is REQUIRED; for VCC the VCI field is absent, while for VPC the VCI field MUST be present with each cell.
In SDU/PDU OAM emulation mode, if an F5 end-to-end loopback OAM cell with loopback indication value of 1 is received and the PE has a label mapping for the ATM VC, the PE MUST decrement the loopback indication and loop back the cell; otherwise it MUST discard the cell.
The ATM N-to-one cell mode is the only REQUIRED encapsulation mode; all other modes (One-to-one cell, AAL5 SDU, AAL5 PDU) are OPTIONAL.
The egress PE MUST be aware of whether the ingress PE will send a control word over a specific PW, achieved by configuration or by signaling as defined in RFC 4447.
The ingress and egress PE MUST agree on a maximum number of cells in a single pseudowire PDU, via pseudowire-specific signaling or static configuration; the ingress PE MUST NOT exceed this limit.
The ingress LSR PE1 MUST set the S bit of the PW label to 1 to denote the VC label is at the bottom of the MPLS label stack.
When multiple VCCs or VPCs are transported in one N-to-one pseudowire, VPI/VCI values MUST be unique within the PW.
The ATM-specific interface parameter sub-TLV 0x02 specifies the Maximum Number of concatenated ATM cells (2 octets) that the egress PE can process as a single PDU; this parameter is REQUIRED for PW types 3, 9, 0x0a, 0xc, and 0xd.
The security of transported ATM services is only as good as the security of the PSN; this may be less rigorous than native ATM. The document specifies only encapsulations, not the PSN transport protocols, which carry their own security considerations (see RFC 4447, RFC 3985).
Defect handling defines four failure locations: (a) CE-to-PE ATM connection, (b) ATM side of the PW, (c) PSN side of the PE, (d) in the PSN. For failures at (a)/(b), the ingress PE generates F4/F5 AIS toward the PSN; for failures at (c)/(d), the egress PE generates F4/F5 AIS toward the CE.
AAL5 CPCS-SDU encapsulation prepends a 32-bit control word with fields Res(4), T, E, C, U, Res(2), Length(6), and optional 16-bit Sequence Number; the PAD and CPCS-PDU trailer are stripped at ingress and regenerated at egress.
AAL5 transparent PDU encapsulation uses the Generic Control Word with an ATM-specific byte containing M, V, Res, U, E, C bits, followed by the full AAL5 CPCS-PDU (n×48 bytes) including PAD and trailer; control word presence is MANDATORY.
ATM N-to-one cell encapsulation carries each ATM cell as a 4-byte header (VPI, VCI, PTI, CLP — without HEC) plus 48-byte payload (52 bytes per cell); multiple cells may be concatenated and the cell count is inferred from the frame length.
ATM One-to-one VCC cell encapsulation uses the Generic Control Word followed by an 8-bit ATM-specific byte (M, V, Res, PTI[3], C) and 48-byte cell payload; when multiple cells are concatenated the ATM-specific byte MUST be repeated per cell (49 bytes per cell).
ATM One-to-one VPC cell encapsulation adds a 16-bit VCI field after the ATM-specific byte, resulting in 51 bytes per encapsulated 53-byte ATM cell when multiple cells are concatenated.
The Generic Control Word (used for One-to-one cell mode and AAL5 PDU mode) is 32 bits: 4-bit zero indicator, 4-bit reserved, 16-bit optional sequence number, and 8-bit ATM-specific flags.
The Preferred Control Word (used for N-to-one cell mode and AAL5 SDU mode) is 32 bits: 4-bit zero indicator, 4-bit flags, 6-bit length field (set to packet length if < 64 bytes, else 0; MUST be 0 for N-to-one), and 16-bit sequence number.