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rfc:rfc7769

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
                                                               Comcast
                                                         February 2016
Media Access Control (MAC) Address Withdrawal over Static Pseudowire

Abstract

 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)
 environment.

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
 http://www.rfc-editor.org/info/rfc7769.

Sivabalan, et al. Standards Track [Page 1] 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

Sivabalan, et al. Standards Track [Page 2] 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
 PW.
 Thus, MAC withdraw signaling for static PW reuses the following
 concepts:
  1. in-band signaling mechanisms used by static PW status signaling

and

  1. 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
 unusable.

Sivabalan, et al. Standards Track [Page 3] 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",
 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].

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

Sivabalan, et al. Standards Track [Page 4] 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
 field.
 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.

Sivabalan, et al. Standards Track [Page 5] 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:
  1. the Sequence Number field is of size 32 bits.
  1. 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

Sivabalan, et al. Standards Track [Page 6] 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
 timeout.
 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
 retransmission.
 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.

Sivabalan, et al. Standards Track [Page 7] 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,
            <http://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, <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,
            <http://www.rfc-editor.org/info/rfc4447>.
 [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,
            <http://www.rfc-editor.org/info/rfc4762>.
 [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,
            <http://www.rfc-editor.org/info/rfc5085>.

Sivabalan, et al. Standards Track [Page 8] 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,
            <http://www.rfc-editor.org/info/rfc5226>.
 [RFC6073]  Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
            Aissaoui, "Segmented Pseudowire", RFC 6073,
            DOI 10.17487/RFC6073, January 2011,
            <http://www.rfc-editor.org/info/rfc6073>.
 [RFC6478]  Martini, L., Swallow, G., Heron, G., and M. Bocci,
            "Pseudowire Status for Static Pseudowires", RFC 6478,
            DOI 10.17487/RFC6478, May 2012,
            <http://www.rfc-editor.org/info/rfc6478>.
 [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,
            <http://www.rfc-editor.org/info/rfc7361>.

Sivabalan, et al. Standards Track [Page 9] RFC 7769 MAC Address Withdrawal over Static PW February 2016

Authors' Addresses

 Siva Sivabalan
 Cisco Systems, Inc.
 2000 Innovation Drive
 Kanata, Ontario  K2K 3E8
 Canada
 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
 Comcast
 1800 Bishops Gate Blvd
 Mount Laurel, NJ  08075
 United States
 Email: mannan_venkatesan@cable.comcast.com

Sivabalan, et al. Standards Track [Page 10]

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