Internet Draft Andrew G. Malis Document: draft-ietf-pwe3-fcs-retention-01.txt Tellabs Expires: November 2004 David Allan Nortel Networks Nick Del Regno MCI May 2004 PWE3 Frame Check Sequence Retention Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document defines a mechanism for preserving frame FCS through Ethernet, Frame Relay, and HDLC pseudowires. Table of Contents 1. Specification of Requirements.................................2 2. Overview......................................................2 3. FCS Retention With MPLS-based Pseudowires.....................3 4. FCS Retention With L2TPv3-based Pseudowires...................4 5. Security Considerations.......................................4 6. IANA Considerations...........................................4 7. References....................................................5 8. Authors' Addresses............................................5 Malis et al Expires November 2004 [Page 1] PWE3 FCS Retention May 2004 1. Specification of Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [6]. 2. Overview The specifications for Ethernet, Frame Relay, HDLC, and PPP pseudowire encapsulation [1] [2] [3] include a mode of use where frames are transparently delivered across the pseudowire without any header or other alterations by the pseudowire ingress or egress Provider Edge (PE). [Note that this mode is inherent for HDLC and PPP Pseudowires.] However, these specifications all specify that the original Frame Check Sequence (FCS) be removed at ingress and regenerated at egress, which means that the frames may be subject to unintentional alteration during their traversal of the pseudowire from the ingress to the egress PE. Thus, the pseudowire cannot be absolutely guaranteed to be "transparent" in nature. To be more precise, pseudowires, as currently defined, leave the payload vulnerable to undetected errors caused by the encapsulating network. As defined for MPLS, not only can a PW-aware device internally corrupt an encapsulated payload, but ANY MPLS LSR in the path can corrupt the encapsulated payload. In the event of such corruption, there is no way to detect the corruption through the path of the pseudowire. Further, because the FCS is calculated upon network egress, any corruption will pass transparently through ALL Layer 2 switches(Ethernet and Frame Relay) through which the packets travel. Only at the endpoint, assuming the corrupted packet even reaches the correct endpoint, can the packet be discarded, and depending on the contents of the packet, the corruption may not ever be detected. Not only does the encapsulation technique leave the payload unprotected, it also subverts the error checking mechanisms already in place in SP and customer networks by calculating FCS on questionable data. In a perfect network comprising perfect equipment, this is not an issue. However, as there is no such thing, it is an issue. SPs should have the option of saving overhead by yielding the ability to detect faults. Equally, SPs should have the option to sacrifice the overhead of carrying the original FCS end-to-end to ensure the ability to detect faults in the encapsulating network. Malis et al Expires November 2004 [Page 2] PWE3 FCS Retention May 2004 This document defines such a mechanism to allow the ingress PE to retain the original frame FCS on ingress to the network, and relieves the egress PE of the task of regenerating the FCS. This in an OPTIONAL mechanism for pseudowire implementations. For interoperability with systems that do not implement this document, the default behavior is that the FCS is removed at the ingress PE and regenerated at the egress PE, as specified in [1], [2], and [3]. Note that this mechanism is not intended to carry errored frames through the pseudowire; as usual, the FCS MUST be examined at the ingress PE and errored frames MUST be discarded. The FCS MAY also be examined by the egress PE; if this is done, errored frames MUST be discarded. The egress PE MAY also wish to generate an alarm or count the number of errored frames. 3. Signaling FCS Retention With MPLS-based Pseudowires When using the signaling procedures in [4], there is a Virtual Circuit FEC element parameter ID used to signal the desire to retain the FCS when advertising a VC label: Parameter ID Length Description 0x?? 2 FCS Retention Indicator Note: The next currently available parameter number is 0x0A [5]. This parameter number will be added to [5] if the timing permits while it is still a draft, otherwise it will be assigned by IANA. This note is to be removed once the assignment has been made. The presence of this parameter ID in the VC FEC element indicates that the egress PE requests the ingress PE to retain the FCS for the VC label being advertised. It does not obligate the ingress PE to retain the FCS; it is simply an indication that the ingress PE MAY retain the FCS. The sender MUST NOT retain the FCS if this parameter ID is not present in the VC FEC element. Since unknown parameter IDs are silently ignored [4], backwards compatibility with systems that do not implement this document is provided by requiring that the FCS is retained ONLY if the FCS Retention Indicator has been included in the advertisements for both directions on a pseudowire. If the ingress PE recognizes the FCS Retention Indicator parameter ID, but does not wish to retain the FCS, it need only issue its own label mapping message for the opposite direction without including the FCS Retention Indicator. This will prevent FCS retention in either direction. Malis et al Expires November 2004 [Page 3] PWE3 FCS Retention May 2004 If PWE3 signaling [4] is not in use for a pseudowire, then whether or not the FCS is to be retained MUST be identically provisioned in both PEs at the pseudowire endpoints. If there is no provisioning support for this option, the default behavior is to remove the FCS. 4. Signaling FCS Retention With L2TPv3-based Pseudowires This section will be added in a subsequent revision. 5. Security Considerations This mechanism enhances the data integrity of transparent Ethernet, Frame Relay, and HDLC pseudowires, because the original FCS, as generated by the Customer Edge (CE), is included in the encapsulation. When the encapsulated payload passes FCS checking at the destination CE, it is clear that the payload was not altered during its transmission through the network (or at least to the accuracy of the original FCS; but that is demonstratably better than no FCS at all). Of course, nothing comes for free; this requires the additional overhead of carrying the original FCS (in general, either two or four octets per payload packet). This signaling is backwards compatible and interoperable with systems that do not implement this document. 6. Applicability Statement In general, this document is intended to further extend the applicability of the services defined by [1], [2], and [3] to make them more suitable for use in deployments where data integrity is an issue (or at least, is as much an issue as in the original services that defined the FCS usage in the first place). There are some situations where this extension is not necessary, such as where the inner payloads have their own error-checking capabilities (such as TCP). But for inner payloads that do rely on the error- detecting capabilities of the link layer (such as SNA), this additional protection can be invaluable. When pseudowires are being used to connect 802.1 bridges, this document allows pseudowires to comply with the requirement that all media interconnecting 802.1 bridges have (at least) 32-bit FCS protection. Note that this document is one possible alternative for a service provider to enhance the end-to-end data integrity of pseudowires. Other mechanisms may include the use of end-to-end IPSec between Malis et al Expires November 2004 [Page 4] PWE3 FCS Retention May 2004 the PEs, or internal mechanisms in the P routers to assure the integrity of packets as they are switched between ingress and egress interfaces. Service providers may wish to compare the relative strengths of each approach when planning their pseudowire deployments; however, an argument can be made that it may be wasteful for a SP to use an end-to-end integrity mechanism that is STRONGER than the FCS generated by the source CE and checked by the destination CE. 7. IANA Considerations This document requires IANA to allocate a PWE3 Virtual Circuit FEC element parameter ID [5]. 8. References [1] Martini, L. et al, "Encapsulation Methods for Transport of Ethernet Frames Over IP and MPLS Networks", draft-ietf-pwe3- ethernet-encap-06.txt, April 2004, work in progress [2] Martini, L. et al, "Frame Relay Encapsulation over Pseudo- Wires", draft-ietf-pwe3-frame-relay-02.txt, February 2004, work in progress [3] Martini, L. et al, "Encapsulation Methods for Transport of PPP/HDLC Frames Over IP and MPLS Networks", draft-ietf-pwe3- hdlc-ppp-encap-mpls-03.txt, April 2004, work in progress [4] Martini, L. et al, "Transport of Layer 2 Frames Over MPLS", draft-ietf-pwe3-control-protocol-06.txt, March 2004, work in progress [5] Martini, L. et al, "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3)", draft-ietf-pwe3-iana-allocation-04.txt, April 20034, work in progress [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. 9. Authors' Addresses Andrew G. Malis Tellabs 90 Rio Robles Dr. San Jose, CA 95134 Email: Andy.Malis@tellabs.com David Allan Malis et al Expires November 2004 [Page 5] PWE3 FCS Retention May 2004 Nortel Networks 3500 Carling Ave. Ottawa, Ontario, CANADA Email: dallan@nortelnetworks.com Nick Del Regno MCI 400 International Parkway Richardson, TX 75081 Email: nick.delregno@mci.com Malis et al Expires November 2004 [Page 6]