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Versions: (draft-bryant-pals-ethernet-cw) 00 01 02 03 04 05 06 07 RFC 8469

PALS Working Group                                             S. Bryant
Internet-Draft                                                  A. Malis
Updates: 4448 (if approved)                                       Huawei
Intended status: Standards Track                             I. Bagdonas
Expires: January 3, 2019                                         Equinix
                                                           July 02, 2018


                Use of Ethernet Control Word RECOMMENDED
                     draft-ietf-pals-ethernet-cw-07

Abstract

   The pseudowire (PW) encapsulation of Ethernet, as defined in RFC
   4448, specifies that the use of the control word (CW) is optional.
   In the absence of the CW an Ethernet pseudowire packet can be
   misidentified as an IP packet by a label switching router (LSR).
   This in turn may lead to the selection of the wrong equal-cost-multi-
   path (ECMP) path for the packet, leading in turn to the misordering
   of packets.  This problem has become more serious due to the
   deployment of equipment with Ethernet MAC addresses that start with
   0x4 or 0x6.  The use of the Ethernet PW CW addresses this problem.
   This document recommends the use of the Ethernet pseudowire control
   word in all but exceptional circumstances.

   This document updates RFC 4448.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on January 3, 2019.








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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Specification of Requirements . . . . . . . . . . . . . . . .   3
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Recommendation  . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Equal Cost Multi-path (ECMP)  . . . . . . . . . . . . . . . .   5
   6.  Mitigations . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Operational Considerations  . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The pseudowire(PW) encapsulation of Ethernet, as defined in
   [RFC4448], specifies that the use of the control word (CW) is
   optional.  It is common for label switching routers (LSRs) to search
   past the end of the label stack to determine whether the payload is
   an IP packet, and if the payload is an IP packet, to select the next
   hop based on the so called "five-tuple" (IP source address, IP
   destination address, protocol/next-header, transport layer source
   port and transport layer destination port).  In the absence of a PW
   CW an Ethernet pseudowire packet can be misidentified as an IP packet
   by a label switching router (LSR) selecting the equal-cost-multi-path
   (ECMP) path based on the five-tuple.  This in turn may lead to the
   selection of the wrong ECMP path for the packet, leading in turn to
   the misordering of packets.  Further discussion of this topic is
   published in [RFC4928].



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   Flow misordering can also happen in a single path scenario when
   traffic classification and differential forwarding treatment
   mechanisms are in use.  These errors occur when a forwarder
   incorrectly assumes that the packet is IP and applies forwarding
   policy based on fields in the PW payload.

   IPv4 and IPv6 packets respectively start with the values 0x4 and 0x6.
   Misidentification can arise if an Ethernet PW packet without a CW is
   carrying an Ethernet packet with a destination address that starts
   either of these values.

   This problem has recently become more serious for a number of
   reasons.  Firstly, due to the deployment of equipment with Ethernet
   MAC addresses that start with 0x4 or 0x6 assigned by the IEEE
   Registration Authority Committee (RAC).  Secondly, concerns over
   privacy have led to the use of MAC address randomization which
   assigns local MAC addresses randomly for privacy.  Random assignment
   results in addresses starting with one of these two values one time
   in eight.

   The use of the Ethernet PW CW addresses this problem.

   This document recommends the use of the Ethernet pseudowire control
   word in all but exceptional circumstances.

2.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Background

   Ethernet pseudowire encapsulation is specified in [RFC4448].  In
   particular the reader is drawn to section 4.6, part of which is
   quoted below for the convenience of the reader:













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       "The control word defined in this section is based on the Generic
       PW MPLS Control Word as defined in [RFC4385].  It provides the
       ability to sequence individual frames on the PW, avoidance of
       equal-cost multiple-path load-balancing (ECMP) [RFC2992], and
       Operations and Management (OAM) mechanisms including VCCV
       [RFC5085].

       "[RFC4385] states, "If a PW is sensitive to packet misordering
       and is being carried over an MPLS PSN that uses the contents
       of the MPLS payload to select the ECMP path, it MUST employ a
       mechanism which prevents packet misordering." This is necessary
       because ECMP implementations may examine the first nibble after
       the MPLS label stack to determine whether the labeled packet
       is IP or not.  Thus, if the source MAC address of an Ethernet
       frame carried over the PW without a control word present begins
       with 0x4 or 0x6, it could be mistaken for an IPv4 or IPv6
       packet.  This could, depending on the configuration and
       topology of the MPLS network, lead to a situation where all
       packets for a given PW do not follow the same path.  This may
       increase out-of-order frames on a given PW, or cause OAM packets
       to follow a different path than actual traffic (see
       Section 4.4.3, "Frame Ordering").

       "The features that the control word provides may not be needed
       for a given Ethernet PW.  For example, ECMP may not be present
       or active on a given MPLS network, strict frame sequencing may
       not be required, etc.  If this is the case, the control word
       provides little value and is therefore optional.  Early Ethernet
       PW implementations have been deployed that do not include a
       control word or the ability to process one if present.  To
       aid in backwards compatibility, future implementations MUST
       be able to send and receive frames without the control word
       present."

   At the time when pseudowires were first deployed, some equipment of
   commercial significance was unable to process the Ethernet Control
   Word.  In addition, at that time it was considered that no Ethernet
   MAC address had been issued by the IEEE Registration Authority
   Committee (RAC) that starts with 0x4 or 0x6, and thus it was thought
   to be safe to deploy Ethernet PWs without the CW.

   Since that time the RAC has issued Ethernet MAC addresses start with
   0x4 or 0x6 and thus the assumption that in practical networks there
   would be no confusion between an Ethernet PW packet without the CW
   and an IP packet is no longer correct.

   Possibly through the use of unauthorized Ethernet MAC addresses, this
   assumption has been unsafe for a while, leading some equipment



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   vendors to implement more complex, proprietary, methods to
   discriminate between Ethernet PW packets and IP packets.  Such
   mechanisms rely on the heuristics of examining the transit packets in
   trying to find out the exact payload type of the packet and cannot be
   reliable due to the random nature of the payload carried within such
   packets.

   A posting on the NANOG email list highlighted this problem:

   https://mailman.nanog.org/pipermail/nanog/2016-December/089395.html

   RFC EDITOR Please delete this paragraph.
   Kramdown does not include references when they are only found in
   literal text so I include them here: [RFC4385] [RFC2992] [RFC5085] as
   a fixup.

4.  Recommendation

   The ambiguity between an MPLS payload that is an Ethernet PW and one
   that is an IP packet is resolved when the Ethernet PW control word is
   used.  This document updates [RFC4448] to state that where both the
   ingress PE and the egress PE support the Ethernet pseudowire control
   word, then the CW MUST be used.

   Where the application of ECMP to an Ethernet PW traffic is required,
   and where both the ingress and the egress PEs support [RFC6790]
   (Entropy Label Indicator/Entropy Label (ELI/EL)) or both the ingress
   and the egress PEs support [RFC6391] (FAT PW), then either method may
   be used.  The use of both methods on the same PW is not normally
   necessary and should be avoided unless circumstances require it.  In
   the case of multi-segment PWs, if ELI/EL is used then it SHOULD be
   used on every segment of the PW.  The method by which usage of ELI/EL
   on every segment is guaranteed is out of scope of this document.

5.  Equal Cost Multi-path (ECMP)

   Where the volume of traffic on an Ethernet PW is such that ECMP is
   required then one of two methods may be used:

   o Flow-Aware Transport (FAT) of Pseudowires over an MPLS Packet
   Switched Network specified in [RFC6391], or

   o LSP entropy labels specified in [RFC6790]

   RFC6391 works by increasing the entropy of the bottom of stack label.
   It requires that both the ingress and egress provider edge (PE)s
   support this feature.  It also requires that sufficient LSRs on the
   LSP between the ingress and egress PE be able to select an



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   ECMP path on an MPLS packet with the resultant stack depth.

   RFC6790 works by including an entropy value in the LSP part of the
   label stack.  This requires that the Ingress and Egress PEs support
   the insertion and removal of the EL and the entropy label indicator,
   and that sufficient LSRs on the LSP are able to preform ECMP based on
   the EL.

   In both cases there are considerations in getting Operations,
   Administration, and Maintenance (OAM) packets to follow the same path
   as a data packet.  This is described in detail section 7 of
   [RFC6391], and section 6 of RFC6790.  However in both cases the
   situation is improved compared to the ECMP behavior in the case where
   the Ethernet PW CW was not used, since there is currently no known
   method of getting a PW OAM packet to follow the same path as a PW
   data packet subjected to ECMP based on the five tuple of the IP
   payload.

   The PW label is pushed before the LSP label.  As the EL/ELI labels
   are part of the LSP layer rather than part of the PW layer, they are
   pushed after the PW label has been pushed.

6.  Mitigations

   Where it is not possible to use the Ethernet PW CW, the effects of
   ECMP can be disabled by carrying the PW over a traffic engineered
   path that does not subject the payload to load balancing (for example
   [RFC3209]).  However such paths may be subjected to link bundle load
   balancing and of course the single LSP has to carry the full PW load.

7.  Operational Considerations

   In some cases, the inclusion of a CW in the PW is determined by
   equipment configuration.  Furthermore, it is possible that the
   default configuration in such cases is to disable use of the CW.
   Care needs to be taken to ensure that software that implements this
   recommendation does not depend on existing configuration settings
   that prevents the use of control word.  It is recommended that
   platform software emits a rate limited message indicating that CW can
   be used but is disabled due to existing configuration.

   Instead of including a payload type in the packet, MPLS relies on the
   control plane to signal the payload type that follows the bottom of
   the label stack.  Some LSRs attempt to deduce the packet type by MPLS
   payload inspection, in some cases looking past the PW CW.  If the
   payload appears to be IP or IP carried in an Ethernet header they
   perform an ECMP calculation based on what they assume to be the five
   tuple fields.  However deduction of the payload type in this way is



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   not an exact science, and where a packet that is not IP is mistaken
   for an IP packet the result can be packets delivered out of order.
   Misordering of this type can be difficult for an operator to
   diagnose.  Operators should be aware when enabling capability that
   allows information gleaned from packet inspection past the PW CW to
   be used in any ECMP calculation, that this may cause Ethernet frames
   to be delivered out of order despite the presence of the CW.

8.  Security Considerations

   This document expresses a preference for one existing and widely
   deployed Ethernet PW encapsulation over another.  These methods have
   identical security considerations, which are discussed in [RFC4448].
   This document introduces no additional security issues.

9.  IANA Considerations

   This document makes no IANA requests.

10.  Acknowledgments

   The authors thank Job Snijders for drawing attention to this problem.
   The authors also thank Pat Thaler for clarifying the matter of local
   MAC address assignment.  We thank Sasha Vainshtein for his valuable
   review comments.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
              February 2006, <https://www.rfc-editor.org/info/rfc4385>.

   [RFC4448]  Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
              "Encapsulation Methods for Transport of Ethernet over MPLS
              Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
              <https://www.rfc-editor.org/info/rfc4448>.







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   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128,
              RFC 4928, DOI 10.17487/RFC4928, June 2007,
              <https://www.rfc-editor.org/info/rfc4928>.

   [RFC6391]  Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V.,
              Regan, J., and S. Amante, "Flow-Aware Transport of
              Pseudowires over an MPLS Packet Switched Network",
              RFC 6391, DOI 10.17487/RFC6391, November 2011,
              <https://www.rfc-editor.org/info/rfc6391>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
              <https://www.rfc-editor.org/info/rfc2992>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC5085]  Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
              Circuit Connectivity Verification (VCCV): A Control
              Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
              December 2007, <https://www.rfc-editor.org/info/rfc5085>.

Authors' Addresses

   Stewart Bryant
   Huawei

   Email: stewart.bryant@gmail.com


   Andrew G Malis
   Huawei

   Email: agmalis@gmail.com



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   Ignas Bagdonas
   Equinix

   Email: ibagdona.ietf@gmail.com>















































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