Pseudowire Emulation Edge-to-Edge (PWE3) Asynchronous Transfer Mode (ATM) Transparent Cell Transport Service
Extracted elements (13)
Discarding idle/unassigned cells at the ingress PE allows the service to benefit from statistical multiplexing bandwidth savings even though it otherwise emulates a transparent ATM link.
The transparent port service uses 'N-to-one' VCC cell transport mode so that ATM CEs view the service as a direct TDM leased line connection, enabling migration of ATM services to a PSN without reprovisioning CE devices.
Congestion control considerations for this service are identical to those in section 15 of RFC 4717; implementers should anticipate a future RFC describing additional ATM-over-pseudowire congestion control techniques.
This service is a natural application of the 'N-to-one' VCC cell transport mode defined in RFC 4717; the encapsulation, interface parameters, and pseudowire type assignment are fully inherited from that specification.
ATM ILMI signaling from CEs MUST be transported transparently and PEs must not act on it, but PEs must act on physical interface failure by withdrawing PW labels or using pseudowire status signaling.
ATM OAM cells MUST be transported transparently; PEs do not act on them. On PSN or pseudowire failure, PEs bring down ATM interfaces to CEs (e.g., generating LOS) rather than generating OAM cells.
Cell concatenation MAY be used for transparent cell-relay transport to save PSN bandwidth, but if used, it MUST be agreed upon between the ingress and egress PEs.
The egress PE MUST NOT change the VPI, VCI, PTI, or CLP bits when sending cells on the egress ATM port.
The transparent cell transport service MUST be used with pseudowires of type 0x0003, 'ATM transparent cell transport', as defined in RFC 4446.
When the pseudowire is set up using the PWE3 control protocol, the ingress PE MUST NOT exceed the 'Maximum Number of concatenated ATM cells' value received in the Label Mapping message, and MUST NOT use cell concatenation if this parameter is omitted by the egress PE.
The ingress PE discards idle/unassigned cells received from the ingress ATM port and maps all other received cells to a single pseudowire, emulating an ATM transmission convergence layer connection between two ports.
The 'Maximum Number of concatenated ATM cells' Pseudowire Interface Parameter Sub-TLV (Interface Parameter ID = 0x02) governs the cell concatenation limit and is exchanged in the LDP Label Mapping message for the pseudowire.
This document introduces no new security considerations beyond those in RFC 4717 and RFC 4447. The security of the transported ATM service is only as good as the security of the PSN, which may be less rigorous than a native ATM service.