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Control Word Reserved bit for use in E-Tree
draft-delord-pwe3-cw-bit-etree-06

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Simon DeLord , Raymond Key , Wim Henderickx , Lucy Yong , Lizhong Jin
Last updated 2011-10-16 (Latest revision 2011-04-15)
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draft-delord-pwe3-cw-bit-etree-06
Network Working Group                       Simon Delord, Alcatel-Lucent
Internet Draft                                       Raymond Key, Huawei
Category: Standard Track          Frederic Jounay, Orange France Telecom
Expires: April 2012                       Wim Henderickx, Alcatel-Lucent
                                                       Lucy Yong, Huawei
                                                        Lizhong Jin, ZTE

                                                        October 16, 2011

            Control Word Reserved bit for use in E-Tree 
                 draft-delord-pwe3-cw-bit-etree-06 

Status of this Memo 
 
   This Internet-Draft is submitted to IETF in full conformance with the 
   provisions of BCP 78 and BCP 79. 
 
   Internet-Drafts are working documents of the Internet Engineering 
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   This Internet-Draft will expire on April 16, 2012. 

 
Abstract 
    
   The extension described in this document allows a pair of PEs 
   connected via an Ethernet Pseudowire (PW) to signal via the use of
   the Control Word (CW) whether the Ethernet frame encapsulated is 
   coming from a Root AC or a Leaf AC. This allows a PE receiving this
   frame to decide whether it can be forwarded to a Leaf AC or not. 
   Such forward or drop decision is an additional filtering action 
   after the MAC-based forwarding decision. This applies to both P2P PW
   and P2MP PW.    

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Table of Contents 
    
   1. Introduction....................................................3 
   2. Terminology.....................................................3
   3. Scope and Applicability.........................................4 
   3.1. Scope & Problem...............................................4
   3.2. Proposed Approach.............................................5
   3.3. Applicability.................................................5 
   4. Control Word Extensions.........................................6 
   4.1. Ethernet Control Word Reserved Bit............................6 
   5. Procedures......................................................7 
   5.1. Detailed PE PW Setup Procedures...............................7
   5.2. Detailed PE Forwarding Procedures.............................7
   5.2.1. Forwarding PE...............................................7
   5.2.2. Receiving PE................................................7
   6. Security Considerations.........................................8 
   7. IANA Considerations.............................................8 
   8. Acknowledgments.................................................8 
   9. References......................................................8 
   9.1. Normative References..........................................8 
   9.2. Informative References........................................8 
   Authors' Addresses.................................................9
   Intellectual Property and Copyright Statements.....................9 

Conventions used in this document 
    
   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]. 

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1. Introduction 
    
   This document proposes to use a specific bit within the Control Word
   (CW) present on top of an Ethernet PW for a PE to indicate to a 
   remote PE connected via a Pseudowire (PW) whether the Ethernet frame
   carried comes from a Root AC or a Leaf AC in an E-Tree construct 
   using VPLS. This applies to both P2P PW and P2MP PW.
   
   [RFC4385] describes the preferred design of a Pseudowire Emulation
   Edge-to-Edge (PWE3) Control Word to be used over an MPLS packet 
   switched network. 
 
   [RFC4447] specifies extensions to Label Distribution Protocol (LDP) 
   for PEs to setup PWs. It also describes the procedures for PEs to
   signal to each other whether the CW is to be used or not.   
 
   [RFC4448] specifies the encapsulation of Ethernet/802.3 PDUs within
   a PW. 

   [Draft VPLS ETree Req] provides the functional requirements for MEF 
   E-Tree support in VPLS.
    
2. Terminology

   E-Tree 
   
   An Ethernet VPN in which each Root AC can communicate with every
   other AC, whereas Leaf ACs can only communicate with Root ACs. Each 
   AC on an E-Tree construct is designated as either a Root AC or a 
   Leaf AC. There can be multiple Root ACs and Leaf ACs per E-Tree 
   construct.

   Root AC 

   An ingress frame at a Root AC can be delivered to one or more of
   any of the other ACs in the E-Tree. Please note that this AC is
   bidirectional.

   Leaf AC 
   
   Ingress frame at a Leaf AC can only be delivered to one or more Root
   ACs in the E-Tree. Ingress frame at a Leaf AC MUST NOT be delivered
   to any Leaf ACs in the E-Tree. Please note that this AC is 
   bidirectional.

 
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3. Scope and Applicability 
    
3.1. Scope & Problem
    
   One important application for carriers is the deployment of E-Tree
   services over an MPLS backbone using VPLS.
 
                    +---------+          +---------+
                    |   PE1   |          |   PE2   |
                    |         |          |         |
   +---+            |  +---+  |          |  +---+  |            +---+
   |CE1+-----AC1----+--+   |  |          |  |   +--+----AC3-----+CE3|
   +---+  (Root AC) |  | V |  |          |  | V |  | (Root AC)  +---+
                    |  |   |  | Ethernet |  |   |  |
                    |  | S +--+----PW----+--+ S |  |
   +---+            |  |   |  |          |  |   |  |            +---+
   |CE2+-----AC2----+--+ I |  |          |  | I +--+----AC4-----+CE4|
   +---+  (Leaf AC) |  |   |  |          |  |   |  | (Leaf AC)  +---+ 
                    |  +---+  |          |  +---+  |
                    |         |          |         |
                    +---------+          +---------+
                     <---------- E-Tree ----------> 
                                  
                  Figure 1. E-Tree over MPLS using VPLS 

   Figure 1 describes a scenario where PE1 and PE2 need to establish an
   E-Tree construct between different Ethernet endpoints. Each PE has 2
   ACs connected to a VSI. These VSIs are then linked together via an
   Ethernet PW. 

   AC1 is a Root AC on PE1 and AC2 is a Leaf AC on PE1.

   AC3 is a Root AC on PE2 and AC4 is a Leaf AC on PE2.
  
   With an E-Tree construct:
      - AC1 can receive from and transmit to AC2, AC3 and AC4
      - AC3 can receive from and transmit to AC1, AC2 and AC4
      - AC2 and AC4 can receive from and transmit to AC1 and AC3
      - AC2 and AC4 cannot receive from or transmit to each other
 
   When an Ethernet Frame is received on PE1 via AC1, the frame can be 
   transmitted to AC2 and via the Ethernet PW connecting PE1 and PE2 to
   AC3 and AC4.

   However when an Ethernet Frame is received on PE1 via AC2, the frame
   can be transmitted to AC3 via the Ethernet PW but not to AC4.

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   When PE2 receives an Ethernet frame from PE1 via the Ethernet PW,
   there is no indication whether it is from a Leaf AC or a Root AC, PE2
   is therefore unable to decide whether it can be forwarded to AC4
   (Leaf AC) as per the E-Tree construct.

3.2. Proposed Approach

   A simple fix to this problem is to carry one additional bit of
   information (0 or 1) for each Ethernet payload frame on the Ethernet
   PW.

   This per-payload signaling approach indicates whether the payload
   frame is from a Leaf AC or a Root AC on the ingress PE.

   Based on this per-payload extra-information, the egress PE can decide
   whether the payload frame can be forwarded to a Leaf AC or not.

   [RFC4385] defines in section 3 the Preferred PW MPLS Control Word.

    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 0| Flags |FRG|  Length   | Sequence Number               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Flags (bits 4 to 7): 
         These bits MAY be used by for per-payload signaling. Their
         semantics MUST be defined in the PW specification.

   [RFC4448] defines in section 4.6 the Control Word for Ethernet PW.

    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 0|   Reserved            |       Sequence Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Flags (bits 4 to 7 of the Control Word) are "reserved for future
   use" bits in the current Ethernet PW standard that can be used for
   such per-payload signaling.

3.3. Applicability 

   The proposed approach in this document will allow the egress PE to
   make a decision for each Ethernet frame arriving on the PW from the
   ingress PE whether this frame can be forwarded towards a Leaf AC.
   

    

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   Obviously, each PE is also responsible for locally identifying which
   ACs are Root ACs or Leaf ACs and for not forwarding Ethernet frames 
   between Leaf ACs. The most common procedure for doing so is to apply
   a split horizon rule between Leaf ACs. However this is out of scope
   of this document.

4. Control Word Extensions 
    
4.1. Ethernet Control Word Reserved Bit 
        
   [RFC4448] defines in section 4.6 the CW format for an Ethernet PW.

   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), and Operations and Management
   (OAM) mechanisms including VCCV.

   The Control Word is defined as follows:

    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 0|L| Reserved            |       Sequence Number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the above diagram, the first 4 bits MUST be set to 0 to indicate
   PW data.  

   Leaf bit (L). 
   
   The L-bit is used to flag the presence of an Ethernet frame from
   a Leaf AC as follows:

       L = 1 The Ethernet Frame comes from a Leaf AC
       L = 0 The Ethernet Frame comes from a Root AC

   The rest of the first 16 bits are reserved for future use. They MUST 
   be set to 0 when transmitting, and MUST be ignored upon receipt.    

   The next 16 bits provide a sequence number that can be used to
   guarantee ordered frame delivery.  The processing of the sequence
   number field is OPTIONAL.

   The sequence number space is a 16-bit, unsigned circular space.  The
   sequence number value 0 is used to indicate that the sequence number
   check algorithm is not used.  The sequence number processing
   algorithm is found in [RFC4385].

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5. Procedures 
       
   [RFC4447] defines the procedures by which PEs maintain and exchange   
   PW information via LDP.  
   
   This document does not change any of the procedures defined in 
   [RFC4447]. The rules for negotiating the use of the CW are defined 
   in section 6.  

5.1. Detailed PE PW Setup Procedures 
    
   In order for a PEs to signal if an Ethernet frame travelling on an 
   Ethernet PW comes from a Root AC or a Leaf AC, it MUST use the 
   extension defined in this document.

   The procedure MUST follow the standard procedures described in 
   [RFC4447].
   
   The Label Mapping messages that are sent in order to set up these 
   PWs MUST have c=1.  When a Label Mapping message for a PW of one 
   of these types is received and c=0, a Label Release message MUST
   be sent, with an "Illegal C-bit" status code.  In this case, the 
   PW will not be enabled. 

5.2. Detailed PE Forwarding Procedures
   
5.2.1. Forwarding PE
 
   The L-bit MUST only be used when the first 4 bits of the CW are 
   equal to 0000.

   The L-bit MUST be set to 0 when the first 4 bits of the CW are equal
   to 0001.

   If the Ethernet frame that has to be sent across an Ethernet PW (or
   set of Ethernet PWs) comes from a Root AC, the L bit MUST be set to
   0.

   If the Ethernet frame that has to be sent across an Ethernet PW (or 
   set of Ethernet PWs) comes from a Leaf AC, the L bit MUST be set to
   1.

5.2.2 Receiving PE
   
   Upon reception of an Ethernet Frame where the L bit in the CW is set
   to 0, this frame can be forwarded to any AC belonging to this VPN 
   instance.

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   Upon reception of an Ethernet Frame where the L bit in the CW is set
   to 1, this frame can be forwarded to any Root AC belonging to this 
   VPN instance but it MUST NOT be forwarded to any Leaf AC. This 
   forward or drop decision is an additional filtering action after the
   MAC-based forwarding decision.

6. Security Considerations 
    
   This will be added in later version of this document.

7. IANA Considerations 
    
   There are no specific IANA considerations in this document.
    
8. Acknowledgments 
    
   The authors would like to thank Yuji Kamite for his valuable
   comments. 

9. References    

9.1. Normative References 
    
   [RFC2119]    Bradner, S., Key words for use in RFCs to Indicate 
                Requirement Levels, BCP 14, RFC 2119, March 1997. 
   
   [RFC4385]    Bryant,S., Swallow, G., and Al, Pseudowire Emulation 
                Edge-to-Edge (PWE3) Control Word for Use over an MPLS
                PSN, February 2006.

   [RFC4447]    Martini, L., and al, Pseudowire Setup and Maintenance
                Using the Label Distribution Protocol (LDP), April 2006 
    
   [RFC4448]    Martini, L., and al, Encapsulation Methods for 
                Transport of Ethernet over MPLS Networks, April 2006

9.2. Informative References 
   
   [Draft VPLS ETree Req]
                Key, et al., Requirements for MEF E-Tree Support in
                VPLS, draft-ietf-l2vpn-etree-reqt-00 (work in progress),
                October 2011

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Authors' Addresses 

   Simon Delord
   Alcatel-Lucent
   Email: simon.delord@gmail.com

   Raymond Key
   Huawei
   Email: raymond.key@ieee.org

   Frederic Jounay     
   Orange France Telecom     
   2, avenue Pierre-Marzin     
   22307 Lannion Cedex, France    
   Email: frederic.jounay@orange.com

   Wim Henderickx
   Alcatel-Lucent
   Copernicuslaan 50
   2018 Antwerp, Belgium
   Email: wim.henderickx@alcatel-lucent.com

   Lucy Yong
   Huawei USA
   1700 Alma Dr. Suite 500
   Plano, TX  75075, USA
   Email: lucy.yong@huawei.com

   Lizhong Jin
   ZTE Corporation
   889, Bibo Road
   Shanghai, 201203, China
   Email: lizhong.jin@zte.com.cn

Copyright Notice 
    
   Copyright (c) 2011 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.      
       

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