Summary

  • RFC 3985 defined a pseudowire as emulation of a service’s essential attributes with only the minimum necessary functionality, not reconstruction of a physical wire.
  • It separated forwarding, payload-semantic transformation, PW encapsulation and PSN transport, so a native-looking customer interface did not prove complete fidelity, delivery or inherited circuit security.

The ambiguity begins with the noun. A wire sounds physical, exclusive and continuous. A pseudowire is an agreement about observable properties. Unless the agreement lists those properties, two parties can call the same service faithful while measuring different things.

RFC 3985, published in March 2005, supplied the PWE3 architecture. Its status, errata and Datatracker record preserve an Informational document later updated in terminology. Its defining sentence was deliberately modest: emulate essential attributes, with the minimum necessary functionality, to the required degree of faithfulness.

That language inherited the requirements in RFC 3916. The status page, errata and Datatracker history describe a customer perspective in which the PW appears as an unshared link, while acknowledging deficiencies that can prevent some applications from working. Appearance and equivalence were never the same claim.

RFC 3985 divided responsibility. The PW encapsulates service-specific packets, cells or bit streams, carries them through an IP path or MPLS tunnel, manages identifiers and boundary needs such as timing, order and alarms. Switching belongs to a Forwarder. Translation or any operation requiring knowledge of payload meaning belongs to Native Service Processing. The functional definitions of FWRD and NSP sit outside PWE3.

This boundary matters because semantic intervention creates authority. A forwarder chooses which PW receives a payload. An NSP may modify, inject or translate payloads. The homogeneous PW should not silently become a universal service interpreter. If an intermediate representation is introduced, RFC 3985 says it effectively becomes a new framing type requiring documentation and assured interoperability.

The alternative was the principle of minimum intervention: transport payload as received, with as few modifications as possible. RFC 3985 connects that principle to RFC 1958, whose status, errata and Datatracker record preserve general Internet architectural guidance. Minimal handling decouples payload evolution from PW evolution and reduces subtle misrepresentation. It does not forbid necessary adaptation; it makes adaptation visible and accountable.

The reference model also draws three different spans. The emulated service runs from customer edge to customer edge. The pseudowire runs between provider edges. The PSN tunnel lies inside that path and can carry multiple PWs. An attachment circuit only joins a CE to a PE. A tunnel-up signal therefore says nothing by itself about the remote attachment, the Forwarder decision, the NSP transformation or the CE’s received service.

Even layers can be intentionally empty. If a payload needs no extra information, the encapsulation layer may add none. If the PSN already meets the assumed requirement, convergence may add nothing. This is disciplined omission, not missing implementation. Complexity has to earn its place through service need.

The customer-facing illusion nevertheless encounters packet-network physics. RFC 3985 lists loss, delay, delay variation and reordering; a transient can produce sustained impairment. Some applications tolerate that. PBX interconnection may demand much more care. In some cases traffic engineering is necessary, and in others the requested guarantees may be impossible.

Later specifications filled particular boxes without erasing the boundary. RFC 4385, with status, errata and Datatracker, standardized an MPLS PW control word for items such as flags, length and sequencing. A valid control word proves selected metadata was carried, not that every essential service attribute survived.

RFC 8077, its status, errata and Datatracker history later specified LDP setup and maintenance. A successfully signaled PW is a setup receipt. It is not a delivery receipt from one CE application to the other.

Congestion remained another independent condition. RFC 7893, together with its status, errata and Datatracker record, notes that PWs do not inherently reserve or control bandwidth. The word “circuit” at the edge does not allocate resources inside a shared PSN.

Security is the sharpest break with the physical metaphor. RFC 3985 says PWE3 itself provides no integrity, confidentiality or delivery protection for native data. Protocols that assumed a local or circuit environment may become more vulnerable across an emulated Ethernet. IPsec can protect traffic between PEs, but the document explicitly says it cannot provide equivalent services to the native service.

For procurement, the right acceptance sheet is therefore a matrix: which payload, order, timing, status, alarms, MTU, validation, congestion behavior and security properties are essential; which transformations are enabled; which deficiencies are accepted. “PW up” belongs on the sheet, but it cannot replace the sheet.

RFC 3985’s historical achievement was restraint. It did not disguise a packet network as a metaphysical wire. It defined a small, layered contract that could make unlike infrastructures interoperate without claiming that every original property survived. The apparent circuit at the customer edge was a useful interface. Evidence still had to cross every boundary beneath it.

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