Internet Draft Andrew G. Malis Document: draft-ietf-pwe3-fcs-retention-04.txt Tellabs Expires: March 2006 David Allan Nortel Networks Nick Del Regno MCI September 2005 PWE3 Frame Check Sequence Retention IPR Statement By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Status of this Memo 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/1id-abstracts.html 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. Intellectual Property Statement...............................2 2. Specification of Requirements.................................2 3. Overview......................................................2 4. Signaling FCS Retention With MPLS-based Pseudowires...........4 5. Signaling FCS Retention With L2TPv3-based Pseudowires.........5 6. Security Considerations.......................................5 Malis et al Expires March 2006 [Page 1]
PWE3 FCS Retention September 2005 7. Applicability Statement.......................................6 8. IANA Considerations...........................................6 9. Acknowledgement...............................................7 10. Normative References.........................................7 11. Full Copyright Statement.....................................8 12. Authors' Addresses...........................................8 1. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. 2. 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]. 3. Overview The specifications for Ethernet, Frame Relay, HDLC, and PPP pseudowire encapsulation [1] [2] [3] [9] [10] [11] 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.] Malis et al Expires March 2006 [Page 2]
PWE3 FCS Retention September 2005 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. Not only can a PW-aware device internally corrupt an encapsulated payload, but ANY LSR or router 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. 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 is 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]. This capability may be used only with Ethernet pseudowires that use "raw mode" [1], Frame Relay pseudowires that use "port mode" [2] [3], and HDLC and PPP pseudowires [3]. Note that this mechanism is not intended to carry errored frames through the pseudowire; as usual, the FCS MUST be examined at the Malis et al Expires March 2006 [Page 3]
PWE3 FCS Retention September 2005 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. 4. Signaling FCS Retention With MPLS-based Pseudowires When using the signaling procedures in [4], there is a Pseudowire Interface Parameter Sub-TLV type used to signal the desire to retain the FCS when advertising a VC label [5]: Parameter Length Description 0x0A 4 FCS Retention Indicator The presence of this parameter 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 is not present in the VC FEC element. The parameter includes a 16-bit FCS length field, which indicates the length of the original FCS being retained. For Ethernet pseudowires, this length will always be set to 4. For HDLC, PPP, and Frame Relay pseudowires, this length will be set to either 2 or 4. Since the FCS length on these interfaces is a local setting, retaining the FCS only makes sense if the FCS length is identical on both ends of the pseudowire. Including the FCS length in this parameter allows the PEs to ensure that the FCS is only retained when it makes sense. Since unknown parameters 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 with an identical setting for the FCS length has been included in the advertisements for both directions on a pseudowire. If the ingress PE recognizes the FCS Retention Indicator parameter, but does not wish to retain the FCS with the indicated length, 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. 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. Malis et al Expires March 2006 [Page 4]
PWE3 FCS Retention September 2005 5. Signaling FCS Retention With L2TPv3-based Pseudowires When using the signaling procedures in [7], the FCS Retention AVP, Attribute Type L2TP-TBA-1, is used. The Attribute Value field for this AVP has the following format: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | FCS Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The FCS Length is a 2-octet unsigned integer. The presence of this AVP in an ICRQ or ICRP message indicates that an LCCE (PE) requests its peer to retain FCS for the L2TP session being established. If the receiving LCCE recognizes the AVP and complies with the FCS retention request, it MUST include an FCS Retention AVP as an acknowledgement in a corresponding ICRP or ICCN message. FCS Retention is always bidirectional, thus FCS is only retained if both LCCEs send an FCS Retention AVP during session establishment. The Attribute Value is a 16-bit FCS length field, which indicates the length of the original FCS being retained. For Ethernet pseudowires, this length will always be set to 4. For HDLC, PPP, and Frame Relay pseudowires, this length will be set to either 2 or 4. Since the FCS length on these interfaces is a local setting, retaining the FCS only makes sense if the FCS length is identical on both ends of the pseudowire. Including the FCS length in this AVP allows the PEs to ensure that the FCS is only retained when it makes sense. The Length of this AVP is 8. The M bit for this AVP MUST be set to 0 (zero). This AVP MAY be hidden (the H bit MAY be 1 or 0). 6. 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 Malis et al Expires March 2006 [Page 5]
PWE3 FCS Retention September 2005 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. 7. 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 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. 8. IANA Considerations This document does not specify any new registries for IANA to maintain. Note that [5] allocates the FCS Retention Indicator interface parameter, so no further IANA action is required. Malis et al Expires March 2006 [Page 6]
PWE3 FCS Retention September 2005 This specification does require IANA to assign one value within the L2TP "Control Message Attribute Value Pairs" section as per [8]. The new AVP is encoded as L2TP-TBA-1 in this document, and should be referred to in the IANA L2TP parameters registry as "FCS Retention." 9. Acknowledgement The authors would like to thank Mark Townsley for the text in section 5. 10. Normative References [1] Martini, L. et al, "Encapsulation Methods for Transport of Ethernet Frames Over IP and MPLS Networks", draft-ietf- pwe3-ethernet-encap-10.txt, June 2005, work in progress [2] Martini, L. et al, "Frame Relay Encapsulation over Pseudo-Wires", draft-ietf-pwe3-frame-relay-05.txt, April 2005, 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-05.txt, May 2005, work in progress [4] Martini, L. et al, "Pseudowire Setup and Maintenance using the Label Distribution Protocol", draft-ietf-pwe3-control-protocol- 17.txt, June 2005, work in progress [5] Martini, L. et al, "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3)", draft-ietf-pwe3-iana-allocation- 11.txt, June 2005, work in progress [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [7] Lau, J., et al, "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005. [8] Townsley, W., "Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers Authority (IANA) Considerations Update", RFC 3438, December 2002. [9] Aggarwal, R. et al, "Transport of Ethernet Frames over L2TPv3", draft-ietf-l2tpext-pwe3-ethernet-03.txt, April 2005, work in progress Malis et al Expires March 2006 [Page 7]
PWE3 FCS Retention September 2005 [10]Townsley, W. et al, "Frame-Relay over L2TPv3", draft-ietf- l2tpext-pwe3-fr-06.txt, June 2005, work in progress [11]Pignataro, C. et al, "HDLC Frames over L2TPv3", draft-ietf- l2tpext-pwe3-hdlc-06.txt, June 2005, work in progress 11. Full Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 12. Authors' Addresses Andrew G. Malis Tellabs 90 Rio Robles Dr. San Jose, CA 95134 Email: Andy.Malis@tellabs.com David Allan 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 March 2006 [Page 8]
