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Versions: (draft-singh-bess-bgp-vpls-control-flags) 00 01 02 03 04 05

BESS Working Group                                              R. Singh
INTERNET-DRAFT                                               K. Kompella
Intended Status: Proposed Standard                      Juniper Networks
Updates: RFC 4761 (once approved)                        S. Palislamovic
                                                          Alcatel-Lucent
Expires: January 3, 2019                                    July 2, 2018


           Updated processing of control flags for BGP VPLS
               draft-ietf-bess-bgp-vpls-control-flags-05


Abstract

   This document updates the meaning of the "control flags" fields
   inside the "layer2 info extended community" used for BGP-VPLS NLRI as
   defined in [RFC4761].


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
   Task Force (IETF), its areas, and its working groups.  Note that
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Copyright and License Notice

   Copyright (c) 2013 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



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   (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
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Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  3
   2  Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3  Updated meaning of control flags in the layer2 info extended
      community . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1 Control word (C-bit) . . . . . . . . . . . . . . . . . . . .  4
     3.2 Sequence flag (S-bit)  . . . . . . . . . . . . . . . . . . .  4
   4  Using p2mp LSP as transport for BGP VPLS  . . . . . . . . . . .  5
   5  Treatment of C and S bits in multi-homing scenarios . . . . . .  5
     5.1 Control word (C-bit) . . . . . . . . . . . . . . . . . . . .  5
     5.2 Sequence flag (S-bit)  . . . . . . . . . . . . . . . . . . .  6
   6  Illustrative diagram  . . . . . . . . . . . . . . . . . . . . .  6
   7  Security Considerations . . . . . . . . . . . . . . . . . . . .  7
   8  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  7
   9  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     9.1  Normative References  . . . . . . . . . . . . . . . . . . .  7
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  8






















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1  Introduction

   [RFC4761] describes the concepts and signaling for using BGP (Border
   Gateway Protocol) to setup a VPLS (virtual private LAN service). It
   specifies the BGP VPLS NLRI (network layer reachability information)
   by which a PE may require other PEs in the same VPLS to include (or
   not) control-word and sequencing information in VPLS frames sent to
   this PE.

   The use of control word (CW) helps prevent mis-ordering of IPv4 or
   IPv6 PW traffic over ECMP (equal cost multi-path) paths or LAG (link
   aggregation group) bundles. [RFC4385] describes the format for
   control-word that may be used over point-2-point PWs (pseudowires)
   and over a VPLS. It along with [RFC3985] also describes sequencing of
   frames.

   However, [RFC4761] does not specify the behavior of PEs in a mixed
   environment where some PEs support control-word/sequencing and others
   do not.

1.1  Terminology

   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 RFC 2119 [RFC2119].

2  Problem

   [RFC4761] specifies the VPLS BGP NLRI by which a given PE advertises
   the behavior expected from the multiple PEs participating in the same
   VPLS. The NLRI indicates the VPLS label that the various PE routers,
   which are referred to in the NLRI, should use when forwarding VPLS
   traffic to this PE. Additionally, by using the "control flags" this
   PE specifies whether the other PEs (in the same VPLS) should use
   control-word or sequenced-delivery for frames forwarded to this PE.
   These are respectively indicated by the C and the S bits in the
   "control flags" as specified in section 3.2.4 in [RFC4761].

   [RFC4761] requires that if the advertising PE sets the C and S bits,
   when forwarding VPLS traffic to the PE, the receiving PE MUST insert
   control word (CW) and by including sequence numbers respectively.

   However, in a BGP VPLS deployment there would often be cases where a
   PE receiving the VPLS BGP NLRI may not have the ability to insert a
   CW or include sequencing information inside PW frames. Thus, the
   behavior of processing CW and sequencing needs to be further
   specified.




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   This document updates the meaning of the control flags in layer2
   extended community in the BGP VPLS NLRI. It also specifies the
   forwarding behavior for a mixed-mode environment where not every PE
   in a VPLS has the ability or the configuration to honor the control
   flags received from the PE advertising the BGP NLRI.

3  Updated meaning of control flags in the layer2 info extended
   community

   Current specification does not allow for the CW setting to be
   negotiated. Rather, if a PE sets the C-bit, it expects to receive
   VPLS frames with a control word, and will send frames the same way.
   If the PEs at both ends of a pseudowire do not agree on the setting
   of the C-bit, the PW does not come up.  The expected behavior is
   similar for the S-bit.

   This memo updates the meaning of the C-bit and the S-bit in the
   control flags.

3.1 Control word (C-bit)

   If a PE sets the C-bit in its NLRI, it means that the PE has ability
   to send and receive frames with a control word.  If the PEs at both
   ends of a PW set the C-bit, control words MUST be used in both
   directions of the PW.  If both PEs send a C-bit of 0, control words
   MUST not be used on the PW.  These two cases behave as before.

   However, if the PEs don't agree on the setting of the C-bit, control
   words MUST not be used on that PW but the PW MUST NOT be prevented
   from coming up due to this mismatch. So, the PW MUST still come up.
   This behavior is new; the old behavior was that the PW doesn't come
   up.

3.2 Sequence flag (S-bit)

   Current BGP VPLS specification do not allow for S-bit setting to be
   negotiated either.  If the PE sets the S-bit, it expects to receive
   VPLS frames with sequence numbers, and will send the frames with
   sequence numbers as well.  This memo further specifies the existing
   behavior. If the PEs on the both ends of the PW set the S-bit, then
   both PEs MUST include the PW sequence numbers.  If the PEs at both
   ends of the PW do not agree on the setting of the S-bit, the PW
   SHOULD NOT come up at all.








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4  Using p2mp LSP as transport for BGP VPLS

   BGP VPLS can be used over point-2-point LSPs acting as transport
   between the VPLS PEs. Alternately, BGP VPLS may also be used over
   p2mp (point to multipoint) LSPs (label switched path) with the source
   of the p2mp LSP rooted at the PE advertising the VPLS BGP NLRI.

   In a network that uses p2mp LSPs as transport for BGP VPLS, in a
   given VPLS there may be some PEs that support control-word while
   others do not. Similarly, for sequencing of frames.

   In such a setup, a source PE that supports control-word should setup
   2 different p2mp LSPs such that:
     - one p2mp LSP will carry CW-marked frames to those PEs that
       advertised C-bit as 1, and
     - the other p2mp LSP will carry frames without CW to those PEs
       that advertised C-bit as 0.

     Using 2 different p2mp LSPs to deliver frames with and without CW
     to different PEs ensures that this PE honors the C-bit advertised
     by the other PEs.

     However, the set of leaves on the 2 p2mp LSPs (rooted at the given
     PE) MUST NOT contain any PEs that advertised a value for S-bit
     different from what this PE itself is advertising. PEs that
     advertised their S-bit value differently (from what this PE
     advertised) will not be on either of the p2mp LSPs. It is ensured
     that this PE is sending VPLS frames only to those PEs that agree
     with this PE on the setting of S-bit.

5  Treatment of C and S bits in multi-homing scenarios

5.1 Control word (C-bit)

     In multi-homed environment, different PEs may effectively represent
     the same service destination end point.  It could be assumed that
     the end-to-end PW establishment process should follow the same
     rules when it comes to control word requirement, meaning setting
     the C-bit would be enforced equally toward both primary and backup
     designated forwarder together.

     However, in the multi-homing case each PW SHOULD be evaluated
     independently. Assuming the below specified network topology, there
     could be the case where PW between PE2 and PE1 could have control
     word signaled via extended community and would be used in the VPLS
     frame, while PE2 to PE4 PW would not insert the control word in the
     VPLS frame due to C-bit mismatch.  The rest of PEs multi-homing
     behavior should simply follow the rules specified in draft-ietf-



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     bess-vpls-multihoming-00.


5.2 Sequence flag (S-bit)

     In multi-homed environment, different PEs may effectively represent
     the same service destination end point. In this case, the rules for
     end-to-end PW establishment SHOULD follow the same rules when it
     comes to sequence bit requirements.  Consider the case below with
     CE5 being multi-homed to PE4 and PE1.  The PW behavior is similar
     to the C-word scenario so that the insertion of S-bit evaluation
     SHOULD be independent per PW.  However, because S-bit mismatch
     between two end-point PEs yields in no PW establishment, in the
     case where PE4 doesn't support S-bit, only one PW would be
     established, between PE1 and PE2.  Thus, even though CE5 is
     physically multi-homed, due to PE4's lack of support for S-bit, and
     no PW between PE1 and PE4, CE5 would not be multi-homed any more.

6  Illustrative diagram

                                                          -----
                                                         /  A1 \
           ----                                     ____CE1     |
          /    \          --------       --------  /    |       |
         |  A2 CE2-      /        \     /        PE1     \     /
          \    /   \    /          \___/          | \     -----
           ----     ---PE2                        |  \
                       |                          |   \   -----
                       | Service Provider Network |    \ /     \
                       |                          |     CE5  A5
                       |            ___           |   /  \     /
                        \          /   \         PE4_/    -----
                        PE3       /     \       /
                |------/  \-------       -------
         ----  /   |    ----
        /    \/    \   /    \               CE = Customer Edge Device
       |  A3 CE3    --CE4 A4 |              PE = Provider Edge Router
        \    /         \    /
         ----           ----                A<n> = Customer site n

                           Figure 1: Example of a VPLS


   In the above topology, let there be a VPLS configured with the PEs as
   displayed. Let PE1 be the PE under consideration that is CW enabled.
   Let PE2 and PE3 also be CW enabled. Let PE4 not be CW enabled. PE1
   will advertise a VPLS BGP NLRI, containing the C/S bits marked as 1.
   PE2 and PE3 on learning of NLRI from PE1, shall include the control



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   word in VPLS frames being forwarded to PE1. However, PE4 which does
   not have the ability to include control-word.

   As per [RFC4761], PE1 would have an expectation that all other PEs
   forward traffic to it by including CW. That expectation cannot be met
   by PE4 in this example. Thus, as per [RFC4761] the PW between PE1 and
   PE4 does not come up.

   However, this document addresses how to support the mixed-CW
   environment as above. PE1 will bring up the PW with PE4 despite the
   CW mismatch. Additionally, it will setup its data-plane such that it
   will strip the control-word only for those VPLS frames that are
   received from PEs that are themselves indicating their desire to
   receive CW marked frames. So, PE1 will setup its data plane to strip-
   off the CW only for VPLs frames received from PEs PE2 and PE3. PE1
   will setup its data plane to not strip CW from frames received from
   PE4.

7  Security Considerations

   This document updates the behavior specified in [RFC4761]. The
   security considerations listed in [RFC4761] apply. However, there are
   no new security considerations due to the text of this document.

8  IANA Considerations

   This document does not make any requests from IANA.

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.

   [RFC4761]   Kompella, K., Y. Rekhter, Virtual Private LAN Service
               (VPLS) Using BGP for Auto-Discovery and Signaling,
               RFC 4761, January 2007.

   [RFC4385]   Bryant, S., Swallow G., Martini L., D. McPherson,
               Pseudowire Emulation Edge-to-Edge (PWE3) Control Word,
               RFC 4385, February 2006.

   [RFC3985]   Bryant, S., P. Pate, Pseudo Wire Emulation
               Edge-to-Edge (PWE3) Architecture, RFC3985, March 2005.





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Authors' Addresses

      Ravi Singh
      Juniper Networks
      1194 N. Mathilda Ave.
      Sunnyvale, CA  94089
      US
      EMail: ravis@juniper.net


      Kireeti Kompella
      Juniper Networks
      1194 N. Mathilda Ave.
      Sunnyvale, CA  94089
      US
      EMail: kireeti@juniper.net


      Senad Palislamovic
      Alcatel-Lucent
      EMail: senad.palislamovic@alcatel-lucent.com






























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