EVPN VPWS layer 2 Attributes Extended community for Control-Word behavior
draft-gs-bess-evpn-l2-attribute-cw-00

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BESS Workgroup                                    Gangadhara Reddy Chavva
                                                                 Ericsson
                                                         Satishkumar Rodd
Internet-Draft                                                   Ericsson
Intended status: Standards Track                        
Expires: June 24, 2019                                  December 24, 2018

             EVPN VPWS layer 2 Attributes Extended community for 
                         Control-Word behavior 
                  draft-gs-bess-evpn-l2-attribute-cw-00

Abstract

   This document aims to define a negotiation mechanism for L2
   capabilities in an EVPN scenario spefic to EVPN-VPWS control word 
   behavior.

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   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Specification of Requirements . . . . . . . . . . . . . . . .   2
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   4.  EVPN Layer 2 Attributes Extended Community  . . . . . . . . .   3
   5.  Usage of Control Flags  . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   EVPN [RFC7432] is ambiguous about the VPWS session  
   if the control-word(C-flag) in L2 attributes between PE devices are 
   different.

   The objective of this document is to explain the mechanism to  
   negaotiate C-flag between PE devices which helps establish EVPN-VPWS 
   session where PE C-flags don't align initially.
   
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 [RFC2119].

3.  Terminology

   EVPN: BGP MPLS-Based Ethernet VPN defined in [RFC7432]

   EVPN ELAN: In order to distinguish EVPN VPWS, EVPN ELAN speficies
   EVPN defined in [RFC7432]

   CW: Control Word defined in [RFC4448]

   PE: Provider Edge

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4.  EVPN Layer 2 Attributes Extended Community

   EVPN Layer 2 Attributes Extended Community is defined in EVPN VPWS
   [RFC8214]. A mechanism to achieve interoperability between devices
   with different CW capabilities is defined in this document. 
   EVPN Layer 2 Attributes Extended Community is advertised
   along with Ethernet Auto-discovery(E-AD). The definition of 
   EVPN Layer 2 Attributes Extended Community is same with [RFC8214]. 
   it is listed as below for convenience.

               +-------------------------------------------+
               |  Type (0x06) / Sub-type (0x04) (2 octets) |
               +-------------------------------------------+
               |  Control Flags  (2 octets)                |
               +-------------------------------------------+
               |  L2 MTU (2 octets)                        |
               +-------------------------------------------+
               |  Reserved (2 octets)                      |
               +-------------------------------------------+

   Figure 1: EVPN Layer 2 Attributes Extended Community

   The definition of Control Flags is as below:

           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
          +-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+
          |   MBZ                  |C|P|B|  (MBZ = MUST Be Zero)
          +-+-+-+-+-+-+-+-+-+-+--+-+-+-+-+

   Figure 2: Control Flags

   The P bit and B bit operations are defined in [RFC8214]. 

   C bit represents the control word capability of the PE. It indicates
   if set to C=0 that, PE is not capable of processing control-word(CW)
   in the data traffic. C bit SHOULD be same among PEs belonging to same
   Ethernet Segment(ESI).
   
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   Other bits in Control Flags are reserved for future investigation and
   MUST be zero.
   
   If local PE does not support EVPN Layer 2 Attributes Extended
   Community, this community MUST be ignored.

5.  Usage of Control Flags

   The description below is based on the network topology showed in
   Figure 3:

                        +--------+       +--------+
                        |   PE1  |       |   PE2  |
                        |  (CW)  |-------|  (NCW) |
                        +--------+       +--------+
 
   Figure 3: Network Topology for Control Word Interoperability

   PE1 is control-word capable where as PE2 is not capable of control-word.
   PE2 capability for CW is either Disabled or Not supported.
   
   With the setup mentioned in Figure 3, the current EVPN-VPWS RFC[RFC8214]
   does not explicitly mention the state of the VPWS session between two 
   PEs. It also states PE2 MUST put control-word for any data to be sent
   to PE1 over that link. But here the problem is PE2 may not be capable of
   prepending control-word to traffic. Hence PE1 will receive the packets
   without control-word even when its expecting with CW.
     
   This document proposes a solution for the above problem. It clearly
   defines the behavior for a PE in this scenario.
   
   The solution require but does not mandate an administrative command 
   enforce-cw. The behavior of this is exaplined in following statements. 
   
   enforce-cw enabled:
   -------------------
   1) When PE1 receives E-AD(C-bit=0) from PE2, PE1 MUST NOT fall back 
   to C-bit=0, but the VPWS session state will be brought DOWN.
   2) When PE2 receives E-AD(C-bit=1) from PE1, PE2 will establish session
   in DOWN state.
   
   enforce-cw disabled:
   ------------------
   1) When PE1 receives E-AD(C-bit=0) from PE2, PE1 MUST fall back 
   to C-bit=0, and initiate new E-AD message towards PE2 with C-bit=0.
   And mark VPWS session UP.
   2) When PE2 receives E-AD(C-bit=1) from PE1, PE2 will establish session
   in DOWN state. 
   
   This document does not suppress any E-AD messages in any scenario. Every PE
   MUST be sending their E-AD message at-least once per VPWS-session.
   

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   C-bit Handling Procedures Diagram where enforce-cw is disabled.

                   ------------------
               Y   | Received       |       N
            -------| E-AD message   |--------------
            |      ------------------             |
        --------------                            |
        |            |                            |
     -------      -------                         |
     | C=0 |      | C=1 |                         |
     -------      -------                         |
        |            |                            |
        |    ----------------                     |
        |    | Control Word |     N               |
        |    |    Capable?  |-----------          |
        |    ----------------          |          |
        |          Y |                 |          |
        |            |                 |          |
        |   ----------------           |          |
        |   | Control Word |  N        |          |
        |   |  enabled?    |----       |          |
        |   ----------------   |       |          |
        |          Y |         |       |          |
        |            |         |       |   ----------------
        |            |         |       |   | Control Word |
        |            |         |       |   |  enabled?    |
        |            |         |       |   ----------------
        |            |         |       |     N |     Y |
        |            |         |       |       |       |
      Send         Send      Send    Send    Send    Send
       C=0          C=1       C=0     C=0     C=0     C=1
                               |       |       |       |
                            ----------------------------------
                            | If receive the same as sent,   |
                            | VPWS setup is complete. If not:|
                            ----------------------------------
                               |       |       |       |
                              ------------------- -----------
                              |     Receive     | | Receive |
                              |       C=1       | |   C=0   |
                              ------------------- -----------
                                       |               |
                               Make session DOWN       |
                               Wait for the next      Send
                               message                C=0
                                                
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   C-bit Handling Procedures Diagram where enforce-cw is enabled.
 
                   ------------------
               Y   | Received       |       N
            -------| E-AD message   |-------------------
            |      ------------------                  |
        ------------------------------                 |          
        |                            |                 |          
     -------                      -------              |          
     | C=0 |                      | C=1 |              |          
     -------                      -------              |          
        |                                        |                 |          
        |                            |        ----------------    
                |                                |        | Control Word |  N 
        |                            |        |  enabled?    |--- 
        |                            |        ----------------  | 
        |                            |               Y |        | 
        |                            |                 |        |
   ----------------             ----------------       |        |
   | Control Word |  N          | Control Word |  N    |        |
   |  enabled?    |-------      |  enabled?    |----   |        |
   ----------------      |      ----------------   |   |        |
          Y |            |             Y |         |   |        |
            |            |               |         |   |        |
            |            |               |         |   |        |
            |            |               |         |   |        |
            |            |               |         |   |        |
            |            |               |         |   |        |
            |            |               |         |   |        |
          Send         Send            Send      Send Send         Send 
           C=1          C=0             C=1       C=0  C=1      C=0
                    |            |               |         |  
         -------------- -------------  ------------- --------------        
     |  Make VPWS | | Make VPWS |  | Make VPWS | |      Make VPWS |
     |Session DOWN| | Session UP|  | Session UP| |Session DOWN|
     -------------- -------------  ------------- --------------
   

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6.  Security Considerations

   There are no new security considerations due to the text of this
   document.

7.  IANA Considerations

   This document does not make any requests from IANA.

8.  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>.
                          
   [RFC4447]  Martini, L., El-Aawar, N., Heron, G., Rosen, E., Tappan,
              D., and T. Smith, "Pseudowire Setup and Maintenance
              using the Label Distribution Protocol (LDP)", RFC 4447,
              April 2006.
                          <https://www.rfc-editor.org/info/rfc4447>
                          
   [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>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC8214]  Boutros, S., Sajassi, A., Salam, S., Drake, J., and J.
              Rabadan, "Virtual Private Wire Service Support in Ethernet
              VPN", RFC 8214, DOI 10.17487/RFC8214, August 2017,
              <https://www.rfc-editor.org/info/rfc8214>.

   [RFC8469]  Bryant, S., Malis, A., and I. Bagdonas, "Recommendation to
              Use the Ethernet Control Word", RFC 8469,
              DOI 10.17487/RFC8469, November 2018,
              <https://www.rfc-editor.org/info/rfc8469>.

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Author's Address

   Gangadhara Reddy Chavva
   Ericsson.
   EMail: meetgangadhara@gmail.com
   
   Satishkumar Rodd
   Ericsson.
   EMail: rodd.satish@gmail.com
   

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