Request for Comments: 4720
Category: Standards Track
N. Del Regno
Pseudowire Emulation Edge-to-Edge (PWE3)
Frame Check Sequence Retention
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The IETF Trust (2006).
This document defines a mechanism for preserving Frame Check Sequence (FCS) through Ethernet, Frame Relay, High-Level Data Link Control (HDLC), and PPP pseudowires.
Table of Contents
1. Overview ........................................................1 2. Specification of Requirements ...................................3 3. Signaling FCS Retention with MPLS-Based Pseudowires .............3 4. Signaling FCS Retention with L2TPv3-Based Pseudowires ...........4 5. Security Considerations .........................................5 6. Applicability Statement .........................................5 7. IANA Considerations .............................................6 8. Acknowledgement .................................................6 9. Normative References ............................................6
The specifications for Ethernet, Frame Relay, HDLC, and PPP pseudowire encapsulation       include a mode of use whereby 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 absolutely be guaranteed to be "transparent" in nature.
To be more precise, pseudowires, as currently defined, leave the payload vulnerable to unintended modification occurring while transiting 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 that 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 it 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 , , and .
This capability may be used only with Ethernet pseudowires that use "raw mode" , Frame Relay pseudowires that use "port mode"  , and HDLC and PPP pseudowires .
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.
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 .
3. Signaling FCS Retention with MPLS-Based Pseudowires
When using the signaling procedures in , there is a Pseudowire Interface Parameter Sub-TLV type used to signal the desire to retain the FCS when advertising a VC label :
Parameter Length Description 0x0A 4 FCS Retention Indicator
The presence of this parameter indicates that the egress PE requests that the ingress PE 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 , backward compatibility with systems that do not implement this document is provided by requiring that the FCS be 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  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 for this option, the default behavior is to remove the FCS.
4. Signaling FCS Retention with L2TPv3-Based Pseudowires
This section uses the following terms as defined in :
Incoming-Call-Request (ICRQ) Incoming-Call-Reply (ICRP) Incoming-Call-Connected (ICCN) Attribute Value Pair (AVP) L2TP Control Connection Endpoint (LCCE)
When using the signaling procedures in , the FCS Retention AVP, Attribute Type 92, 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 that its peer 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 doing so 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).
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 demonstrably 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 backward 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 , , and  to make them more suitable for use in deployments where data integrity is an issue (or at least is as much of 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 ensure 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 an 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 does not specify any new registries for IANA to maintain.
Note that  allocates the FCS Retention Indicator interface parameter; therefore, no further IANA action is required.
IANA assigned one value within the L2TP "Control Message Attribute Value Pairs" section as per . The new AVP is 92 and is referred to in the IANA L2TP parameters registry as "FCS Retention".
The authors would like to thank Mark Townsley for the text in Section 4.
9. Normative References
 Martini, L., Rosen, E., El-Aawar, N., and G. Heron, "Encapsulation Methods for Transport of Ethernet over MPLS Networks", RFC 4448, April 2006.  Martini, L., Ed., Kawa, C., Ed., and A. Malis, Ed., "Encapsulation Methods for Transport of Frame Relay over Multiprotocol Label Switching (MPLS) Networks", RFC 4619, September 2006.  Martini, L., Rosen, E., Heron, G., and A. Malis, "Encapsulation Methods for Transport of PPP/High-Level Data Link Control (HDLC) over MPLS Networks", RFC 4618, September 2006.  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron, "Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)", RFC 4447, April 2006.  Martini, L., "IANA Allocations for Pseudowire Edge to Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.  Townsley, W., "Layer Two Tunneling Protocol (L2TP) Internet Assigned Numbers Authority (IANA) Considerations Update", BCP 68, RFC 3438, December 2002.  Aggarwal, R., Townsley, M., and M. Dos Santos, "Transport of Ethernet Frames over Layer 2 Tunneling Protocol Version 3 (L2TPv3)", RFC 4719, November 2006.  Townsley, M., Wilkie, G., Booth, S., Bryant, S., and J. Lau, "Frame Relay over Layer 2 Tunneling Protocol Version 3 (L2TPv3)", RFC 4591, August 2006.  Pignataro, C. and M. Townsley, "High-Level Data Link Control (HDLC) Frames over Layer 2 Tunneling Protocol, Version 3 (L2TPv3)", RFC 4349, February 2006.
Andrew G. Malis
90 Rio Robles Dr.
San Jose, CA 95134
3500 Carling Ave.
Ottawa, Ontario, CANADA
Nick Del Regno
400 International Parkway
Richardson, TX 75081
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