RFC 7769

Internet Engineering Task Force (IETF)                      S. Sivabalan
Request for Comments: 7769                                    S. Boutros
Category: Standards Track                            Cisco Systems, Inc.
ISSN: 2070-1721                                                  H. Shah
                                                             Ciena Corp.
                                                               S. Aldrin
                                                             Google Inc.
                                                           M. Venkatesan
                                                           February 2016

  Media Access Control (MAC) Address Withdrawal over Static Pseudowire


   This document specifies a mechanism to signal Media Access Control
   (MAC) address withdrawal notification using a pseudowire (PW)
   Associated Channel (ACH).  Such notification is useful when
   statically provisioned PWs are deployed in a Virtual Private LAN
   Service (VPLS) or Hierarchical Virtual Private LAN Service (H-VPLS)

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 7769          MAC Address Withdrawal over Static PW    February 2016

Copyright Notice

   Copyright (c) 2016 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  MAC Withdraw OAM Message  . . . . . . . . . . . . . . . . . .   4
   4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Operation of Sender . . . . . . . . . . . . . . . . . . .   6
     4.2.  Operation of Receiver . . . . . . . . . . . . . . . . . .   7
   5.  Security Consideration  . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  MPLS G-Ach Type . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Sequence Number TLV . . . . . . . . . . . . . . . . . . .   8
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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RFC 7769          MAC Address Withdrawal over Static PW    February 2016

1.  Introduction

   An LDP-based MAC address withdrawal mechanism is specified in
   [RFC4762] to remove dynamically learned MAC addresses when the source
   of those addresses can no longer forward traffic.  This is
   accomplished by sending an LDP Address Withdraw Message with a MAC
   List TLV containing the MAC addresses to be removed from all other
   Provider Edge nodes over the LDP sessions.  [RFC7361] describes an
   optimized MAC withdrawal mechanism that can be used to remove only
   the set of MAC addresses that need to be relearned in H-VPLS
   networks.  [RFC7361] also describes optimized MAC withdrawal
   operations in PBB-VPLS networks.

   A PW can be signaled via the LDP or can be statically provisioned.
   In the case of a static PW, an LDP-based MAC withdrawal mechanism
   cannot be used.  This is analogous to the problem and solution
   described in [RFC6478] where a PW OAM (Operations, Administration,
   and Maintenance) message has been introduced to carry the PW status
   TLV using the in-band PW Associated Channel.  In this document, we
   use a PW OAM message to withdraw MAC address(es) learned via a static

   Thus, MAC withdraw signaling for static PW reuses the following

      -  in-band signaling mechanisms used by static PW status signaling

      -  MAC withdrawal mechanisms described by [RFC4762] and [RFC7361].

   MAC withdraw signaling is a best effort scheme.  It is an attempt to
   optimize network convergence by reducing blackholes caused by PW
   failover for protected PWs.  The protocol defined in this document
   addresses possible loss of the MAC withdraw signal due to network
   congestion, but does not guarantee delivery, as is the case for the
   LDP-based MAC withdraw signaling.  In the event that MAC withdraw
   signaling does not reach the intended target, the fallback to MAC
   re-learning due to bi-directional traffic or as a last resort aging
   out of MAC addresses in the absence of frames from the sources, will
   resume the traffic via new PW path.  Such fallbacks would cause
   temporary blackouts but does not render a network permanently

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RFC 7769          MAC Address Withdrawal over Static PW    February 2016

2.  Terminology

   The following terminology is used in this document:

   ACK:  Acknowledgement for MAC withdraw message

   LDP:  Label Distribution Protocol

   MAC:  Media Access Control

   MPLS:  Multiprotocol Label Switching

   PW:  Pseudowire

   PW OAM:  PW Operations, Administration, and Maintenance

   TLV:  Type, Length, and Value

   VPLS:  Virtual Private LAN Services

   In addition, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "OPTIONAL" in this document are to be interpreted as described in

3.  MAC Withdraw OAM Message

   LDP provides reliable packet transport for control plackets for
   dynamic PWs.  This can be contrasted with static PWs that rely on
   retransmission and acknowledgments (ACKs) for reliable OAM packet
   delivery as described in [RFC6478].  The proposed solution for MAC
   withdrawal over a static PW also relies on retransmissions and ACKs.
   However, an ACK is mandatory.  A given MAC withdrawal notification is
   sent as a PW OAM message, and the sender retransmits the message a
   configured number of times in the absence of an ACK response for the
   sequence-numbered message.  The receiver removes the MAC address(es)
   for a given sequence-number MAC withdraw signaling message and sends
   the ACK response.  The receipt of the same or lower sequence-number
   message is responded to with an ACK but does not cause removal of MAC
   addresses.  A new TLV to carry the sequence number has been defined.

   The format of the MAC address withdraw OAM message is shown in Figure
   1.  The MAC withdraw PW OAM message follows the same guidelines used
   in [RFC6478], whereby the first 4 bytes of the OAM message header are
   followed by a message-specific field and a set of TLVs relevant for
   the message.  Since the MAC withdrawal PW OAM message is not
   refreshed forever, a MAC address withdraw OAM message MUST contain a
   "Sequence Number TLV"; otherwise, the entire message is dropped.  It

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RFC 7769          MAC Address Withdrawal over Static PW    February 2016

   MAY contain the MAC Flush Parameter TLV defined in [RFC7361] when
   static PWs are deployed in H-VPLS and PBB-VPLS scenarios.  The first
   2 bits of the sequence-number TLV are reserved and MUST be set to 0
   on transmit and ignored on receipt.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |0 0 0 1|Version|   Reserved    |  MAC Withdraw OAM Msg (0x28)  |
      |            Reserved           |  TLV Length   |A|R| Flags     |
      |Res|   Sequence No. TLV (0x1)  |  Sequence Number TLV Length   |
      |                        Sequence Number                        |
      |                                                               |
      |                         MAC List TLV                          |
      ~                MAC Flush Parameter TLV (optional)             ~
      |                                                               |
      |                                                               |

            Figure 1: MAC Address Withdraw PW OAM Packet Format

   In this section, the MAC List TLV and MAC Flush Parameter TLV are
   collectively referred to as "MAC TLV(s)".  The definition and
   processing rules of the MAC List TLV are described by [RFC4762], and
   the corresponding rules of the MAC Flush Parameter TLV are governed
   by [RFC7361].

   "TLV Length" is the total length of all TLVs in the message, and
   "Sequence Number TLV Length" is the length of the Sequence Number

   A single bit (called "A-bit") is set by a receiver to acknowledge
   receipt and processing of a MAC Address Withdraw OAM Message.  In the
   acknowledge message, with the A-bit set, the MAC TLVs are excluded.

   A single bit (called "R-bit") is set to indicate if the sender is
   requesting reset of the sequence numbers.  The sender sets this bit
   when the pseudowire is restarted and has no local record of previous
   send and expected receive sequence numbers.

   The Sequence Number TLV MUST be the first TLV in the message.

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RFC 7769          MAC Address Withdrawal over Static PW    February 2016

   The lack of a reliable transport protocol for the in-band OAM
   necessitates a presence of sequencing and acknowledgement scheme so
   that the receiver can recognize newer message from retransmitted
   older messages.  [RFC4385] describes the details of sequence-number
   handling, which includes overflow detection for a Sequence Number
   field size of 16 bits.  This document leverages the same scheme with
   the two exemptions:

      -  the Sequence Number field is of size 32 bits.

      -  overflow detection is simplified such that a sequence number
         that exceeds 2,147,483,647 (0x7FFFFFFF) is considered an
         overflow and reset to 1.

4.  Operation

   This section describes how the initial MAC Withdraw OAM Messages are
   sent and retransmitted, as well as how the messages are processed and
   retransmitted messages are identified.

4.1.  Operation of Sender

   Each PW is associated with a counter to keep track of the sequence
   number of the transmitted MAC withdrawal messages.  Whenever a node
   sends a new set of MAC TLVs, it increments the transmitted sequence-
   number counter and includes the new sequence number in the message.
   The transmit sequence number is initialized to 1 at the onset, after
   the wrap and after the sequence number reset request receipt.  Hence
   the transmit sequence number is set to 2 in the first MAC withdraw
   message sent after the sequence number is initialized to 1.

   The sender expects an ACK from the receiver within a time interval we
   call "Retransmit Time", which can be either a default (1 second) or a
   configured value.  If the ACK does not arrive within the Retransmit
   Time, the sender retransmits the message with the same sequence
   number as the original message.  The retransmission MUST cease when
   an ACK is received.  In order to avoid continuous retransmissions in
   the absence of acknowledgements, a method of suppressing
   retransmissions MUST be implemented.  A simple and well-used approach
   is to cease retransmission after a small number of transmissions.  In
   the absence of an ACK response, a one second retransmission with two
   retries is RECOMMENDED.  However, both the interval and the number of
   retries are a local matter that present no interworking issues; thus,
   the operator MAY configure different values.  Alternatively, an
   increasing backoff delay with a larger number of retries MAY be
   implemented to improve scaling issues.  Whilst there are no
   interworking issues with any of these methods, the implementer must
   be mindful to not introduce network congestion and must take into

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RFC 7769          MAC Address Withdrawal over Static PW    February 2016

   account the decaying value of the delayed MAC withdraw signaling
   against possible relearning due to bidirectional traffic or MAC

   During the period of retransmission, if a need to send a new MAC
   withdraw message with updated sequence number arises, then
   retransmission of the older unacknowledged withdraw message MUST be
   suspended and retransmit time for the new sequence number MUST be
   initiated.  In essence, a sender engages in retransmission logic only
   for the most recently sent withdraw message for a given PW.

   In the event that a pseudowire is deleted and re-added or the router
   is restarted with configuration, the local node may lose information
   about the previously sent sequence number.  This becomes problematic
   for the remote peer as it will continue to ignore the received MAC
   withdraw messages with lower sequence numbers.  In such cases, it is
   desirable to reset the sequence numbers at both ends of the
   pseudowire.  The reset R-bit is set in the first MAC withdraw to
   notify the remote peer to reset the send and receive sequence
   numbers.  The R-bit must be cleared in subsequent MAC withdraw
   messages after the acknowledgement is received.

4.2.  Operation of Receiver

   Each PW is associated with a register to keep track of the expected
   sequence number of the MAC withdrawal message and is initialized to
   1.  Whenever a MAC withdrawal message is received, and if the
   sequence number on the message is greater than the value in the
   register, the MAC addresses contained in the MAC TLVs are removed,
   and the register is updated with the received sequence number.  The
   receiver sends an ACK whose sequence number is the same as that in
   the received message.

   If the sequence number in the received message is smaller than or
   equal to the value in the register, the MAC TLVs are not processed.
   However, an ACK with the received sequence number MUST be sent as a
   response.  The receiver processes the ACK message as an
   acknowledgement for all the MAC withdraw messages sent up to the
   sequence number present in the ACK message and terminates

   The handling of the sequence number is described in Section 3.

   A MAC withdraw message with the R-bit set MUST be processed by
   resetting the send and receive sequence number first.  The rest of
   MAC withdraw message processing is performed as described above.  The
   acknowledgement is sent with the R-bit cleared.

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RFC 7769          MAC Address Withdrawal over Static PW    February 2016

5.  Security Consideration

   The security measures described in [RFC4447], [RFC5085], and
   [RFC6073] are adequate for the proposed mechanism.

6.  IANA Considerations

6.1.  MPLS G-Ach Type

   IANA has assigned a new channel type (0x0028) from the "MPLS
   Generalized Associated Channel (G-ACh) Types (including Pseudowire
   Associated Channel Types)" registry.  The description of the new
   channel type is "MAC Withdraw OAM Message".

6.2.  Sequence Number TLV

   IANA has assigned a new TLV Type (0x0001) from the existing LDP "TLV
   Type Name Space" registry.  The description for the new TLV Type is
   "Sequence Number TLV".

7.  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,

   [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, <http://www.rfc-editor.org/info/rfc4385>.

   [RFC4447]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
              G. Heron, "Pseudowire Setup and Maintenance Using the
              Label Distribution Protocol (LDP)", RFC 4447,
              DOI 10.17487/RFC4447, April 2006,

   [RFC4762]  Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
              LAN Service (VPLS) Using Label Distribution Protocol (LDP)
              Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,

   [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,

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RFC 7769          MAC Address Withdrawal over Static PW    February 2016

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [RFC6073]  Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
              Aissaoui, "Segmented Pseudowire", RFC 6073,
              DOI 10.17487/RFC6073, January 2011,

   [RFC6478]  Martini, L., Swallow, G., Heron, G., and M. Bocci,
              "Pseudowire Status for Static Pseudowires", RFC 6478,
              DOI 10.17487/RFC6478, May 2012,

   [RFC7361]  Dutta, P., Balus, F., Stokes, O., Calvignac, G., and D.
              Fedyk, "LDP Extensions for Optimized MAC Address
              Withdrawal in a Hierarchical Virtual Private LAN Service
              (H-VPLS)", RFC 7361, DOI 10.17487/RFC7361, September 2014,

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RFC 7769          MAC Address Withdrawal over Static PW    February 2016

Authors' Addresses

   Siva Sivabalan
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8

   Email: msiva@cisco.com

   Sami Boutros
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134
   United States

   Email: sboutros@cisco.com

   Himanshu Shah
   Ciena Corp.
   3939 North First Street
   San Jose, CA  95134
   United States

   Email: hshah@ciena.com

   Sam Aldrin
   Google Inc.

   Email: aldrin.ietf@gmail.com

   Mannan Venkatesan
   1800 Bishops Gate Blvd
   Mount Laurel, NJ  08075
   United States

   Email: mannan_venkatesan@cable.comcast.com

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