Pseudowire Emulation Edge-to-Edge (PWE3) Frame Check Sequence Retention
Extracted elements (19)
FCS retention is presented as one alternative for enhancing end-to-end pseudowire integrity, alongside end-to-end IPsec between PEs or P-router internal integrity mechanisms. The document notes it may be wasteful to use an integrity mechanism stronger than the FCS generated by the source CE.
Standard pseudowire encapsulations remove the original FCS at ingress and regenerate it at egress, leaving the payload vulnerable to undetected corruption during traversal of the encapsulating network. Any LSR or router in the path can corrupt the encapsulated payload, and because FCS is recalculated at egress, corruption passes transparently through all Layer 2 switches. This document defines FCS retention to close that gap.
When pseudowires connect 802.1 bridges, this mechanism allows pseudowires to comply with the 802.1 requirement that all media interconnecting bridges have at least 32-bit FCS protection, enabling use in bridge interconnect scenarios.
FCS length must be identical on both ends of the pseudowire for retention to make sense. Since FCS length on HDLC, PPP, and Frame Relay interfaces is a local setting, including the FCS length in the parameter or AVP allows PEs to ensure FCS is only retained when the lengths match.
This signaling is backward compatible and interoperable with systems that do not implement FCS retention. Unknown parameters in MPLS signaling are silently ignored per RFC 4447, and the M bit being zero in L2TPv3 ensures non-implementing peers can ignore the AVP.
FCS is retained ONLY if the FCS Retention Indicator with an identical FCS length setting has been included in advertisements for both directions on a pseudowire. Unknown parameters are silently ignored, providing backward compatibility.
FCS retention is OPTIONAL. The default behavior is that the FCS is removed at the ingress PE and regenerated at the egress PE. This capability applies only to Ethernet pseudowires in raw mode, Frame Relay pseudowires in port mode, and HDLC and PPP pseudowires.
If a receiving LCCE recognizes the FCS Retention AVP and complies with the FCS retention request, it MUST include an FCS Retention AVP as acknowledgement in the corresponding ICRP or ICCN message. FCS retention is always bidirectional.
If PWE3 signaling is not in use for a pseudowire, then whether the FCS is to be retained MUST be identically provisioned in both PEs at the pseudowire endpoints. If there is no provisioning support, the default is to remove the FCS.
In MPLS-based pseudowires, the sender MUST NOT retain the FCS if the FCS Retention Indicator parameter is not present in the VC FEC element.
The egress PE MAY also examine the FCS; if this is done, errored frames MUST be discarded. The egress PE MAY also generate an alarm or count errored frames.
The FCS MUST be examined at the ingress PE, and errored frames MUST be discarded. This mechanism is not intended to carry errored frames through the pseudowire.
The M bit for the FCS Retention AVP MUST be set to 0 (zero), meaning the AVP is not mandatory and unknown receivers may ignore it.
For L2TPv3-based pseudowires, the FCS Retention AVP has Attribute Type 92. Its presence in an ICRQ or ICRP message indicates that the LCCE requests its peer retain FCS for the L2TP session being established.
The FCS Retention Indicator is a Pseudowire Interface Parameter Sub-TLV with type 0x0A and length 4, used in MPLS-based pseudowire signaling (LDP) to request that the ingress PE retain the FCS for the advertised VC label.
IANA assigned L2TP Control Message Attribute Value Pair type 92 (FCS Retention) within the L2TP parameters registry. The FCS Retention Indicator interface parameter (0x0A) was previously allocated by RFC 4446; no new MPLS registry action is required.
FCS retention enhances data integrity of transparent Ethernet, Frame Relay, and HDLC pseudowires by including the original CE-generated FCS in the encapsulation. When the payload passes FCS checking at the destination CE, it confirms the payload was not altered during network transit. The cost is two to four additional octets per payload packet.
The FCS Retention AVP (Attribute Type 92) has a 2-octet unsigned integer FCS Length field and a total AVP Length of 8. The M bit MUST be set to 0; the H bit MAY be 0 or 1.
The FCS Retention Indicator Sub-TLV includes a 16-bit FCS length field indicating the length of the original FCS being retained. For Ethernet pseudowires this is always 4; for HDLC, PPP, and Frame Relay pseudowires it is either 2 or 4.