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Versions: (draft-singh-bess-bgp-vpls-control-flags) 00 01 02 03 04 05 06 07 08

BESS Working Group                                              R. Singh
INTERNET-DRAFT                                               K. Kompella
Intended Status: Proposed Standard                      Juniper Networks
Updates: 4761 (if approved)                              S. Palislamovic
                                                                   Nokia
Expires: October 20, 2019                                 April 18, 2019


           Updated processing of Control Flags for BGP VPLS
               draft-ietf-bess-bgp-vpls-control-flags-08


Abstract

   This document updates the meaning of the Control Flags field in the
   Layer2 Info Extended Community used for BGP-VPLS NLRI as defined in
   RFC4761. This document updates 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
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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Copyright and License Notice

   Copyright (c) 2018 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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   described in the Simplified BSD License.



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 Point-to-MultiPoint (P2MP) LSPs as transport for BGP
      VPLS  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5  Illustrative diagram  . . . . . . . . . . . . . . . . . . . . .  6
   6  Treatment of C and S bits in multi-homing scenarios . . . . . .  7
     6.1 Control word (C-bit) . . . . . . . . . . . . . . . . . . . .  7
     6.2 Sequence flag (S-bit)  . . . . . . . . . . . . . . . . . . .  7
   7  Security Considerations . . . . . . . . . . . . . . . . . . . .  8
   8  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  8
   9  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     9.1  Normative References  . . . . . . . . . . . . . . . . . . .  8
     9.2 Informative References . . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9




















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1  Introduction

   "Virtual Private LAN Service (VPLS) Using BGP for Auto-Discovery and
   Signaling" ([RFC4761]) describes the concepts and signaling for using
   Border Gateway Protocol (BGP) to setup a VPLS. It specifies the BGP
   VPLS Network Layer Reachability Information (NLRI) by which a
   provider-edge router (PE) may require other PEs in the same VPLS to
   include (or not) the control-word and sequencing information in VPLS
   frames sent to this PE.

   The use of the Control Word (CW) helps prevent mis-ordering of IPv4
   or IPv6 Pseudo-Wire (PW) traffic over Equal Cost Multi-Path (ECMP)
   paths or Link Aggregation Group (LAG) bundles. [RFC4385] describes
   the format for CW that may be used over Point-to-Point PWs and over a
   VPLS. Along with [RFC3985], the document also describes sequence
   number usage for VPLS 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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2  Problem

   [RFC4761] specifies the VPLS BGP NLRI by which a given PE advertises
   the behavior expected by 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,
   the receiving PE MUST, respectively, insert control word (CW) and
   include sequence numbers when forwarding VPLS traffic to the
   advertising PE.

   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



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   CW or include sequencing information inside PW frames. Thus, the
   behavior of processing CW and sequencing needs to be further
   specified.

   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

   The current specification does not allow for the CW setting to be
   negotiated. In a typical implementation, 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 the two ends of a PW do not agree
   on the setting of the C-bit, the PW does not come up.  The 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 at both ends of the PW do not agree on the
   setting of the C-bit, control words MUST NOT be used in either
   direction on that PW but the PW MUST NOT be prevented from coming up
   due to this mismatch. So, the PW will still come up but not use
   control word in either direction. This behavior is changed from the
   behavior described in [RFC4761] where the PW does not come up.

3.2 Sequence flag (S-bit)

   If a PE sets the S-bit in its NLRI, it means that the PE has ability
   to set sequence numbers as listed in section 4.1 in [RFC4385] and
   process sequence numbers as listed in section 4.2 in [RFC4385]. If
   the PEs at both ends of a PW set the S-bit, non-zero sequence numbers
   MUST be used in both directions of the PW.  If both PEs send a S-bit
   of 0, sequence numbers MUST NOT be used on the PW.  These two cases
   behave as before.




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   Current BGP VPLS specification does not allow for S-bit setting to be
   negotiated either.  In a typical implementation, if the PE sets the
   S-bit in the advertised NLRI, it expects to receive VPLS frames with
   non-zero sequence numbers, and will send outgoing frames over the PW
   with non-zero sequence numbers.

   This memo further specifies the expected behavior when the PEs at the
   ends of the PW advertise differing S-bit values. If the PEs at both
   ends of the PW do not agree on the setting of the S-bit, then the PW
   SHOULD NOT come up. This is to avoid running into out-of-sequence
   ordering scenarios when the multiple PEs that are enabling multi-
   homing for a site have differing S-bit advertisements as listed in
   section 4.2 in [RFC4385]. However, if a deployment is known to not
   utilize multi-homing, a user-configurable way to override this
   recommendation MAY BE provided by an implementation whereby the PW is
   allowed to come up. In that case the PE advertising S-bit as 0 should
   set sequence numbers in the frames as zero and the PW receiving the
   frames should not have an expectation to receive non-zero sequence
   numbers.


4  Using Point-to-MultiPoint (P2MP) LSPs 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 Label Switched Path (LSPs) 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 a VPLS, there may
   be some PEs that support CW while others may not. Similarly, for the
   sequencing of VPLS frames.

   In such a setup, a source PE that supports CW should setup two
   different P2MP LSPs such that:
          - One P2MP LSP will transport CW-marked frames to those PEs
            that advertised the C-bit as 1.
          - The other P2MP LSP will transport frames without CW to those
            PEs that advertised C-bit as 0.

     Using two different P2MP LSPs to deliver frames with and without
     the CW to different PEs ensures that a P2MP root PE honors the C-
     bit advertised by the other P2MP PEs.

     However, the set of leaves on the two P2MP LSPs (rooted at the
     given PE) MUST NOT contain any PEs that advertised a value for the
     S-bit different from what the root PE itself is advertising. PEs
     that advertised their S-bit value differently (from what the P2MP
     root PE advertised) will not be on either of the P2MP LSPs. This



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     ensures that the P2MP root PE is sending VPLS frames only to those
     PEs that agree on the setting of S-bit.

     The ingress router for the P2MP LSP should send separate NLRIs for
     the cases of using control-word and for not using control-word.

5  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
   and sequencing enabled. Let PE2 and PE3 also be CW enabled and
   sequencing enabled. Let PE4 not be CW enabled or have the ability to
   include sequence numbers. PE1 will advertise a VPLS BGP NLRI,
   containing the C/S bits marked as 1. PE2 and PE3 on learning of NLRI
   from PE1, will include the CW and non-zero sequence numbers in the
   VPLS frames being forwarded to PE1 as listed in section 4 in
   [RFC4385]. However, PE4 which does not have the ability to include CW
   or include non-zero sequence numbers, will not.

   As per [RFC4761], PE1 would have an expectation that all other PEs
   forward CW-containing frames which have non-zero sequence numbers.
   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 and



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   mixed sequencing-ability of PEs described above. PE1 will not bring
   up the PW with PE4 due to the S-bit mismatch, unless overridden by
   local configuration on PE1 and PE4 as specified in section 3.2. If
   PE4 instead was to advertise a C-bit of 0 and an S-bit of 1, then
   despite the CW mismatch the PW between PE1 and PE4 would come up.
   Additionally PE1 would setup its data-plane such that it will strip
   the CW only for those VPLS frames that are received from PEs that
   have indicated their desire to receive CW marked frames. So, PE1 will
   setup its data plane to strip the CW only for VPLs frames received
   from PE2 and PE3 and it will expect to process PW frames containing
   non-zero sequence numbers as listed in section 4.2 in [RFC4385]. PE1
   will setup its data-plane to not strip the CW from frames received
   from PE4 and it it would expect PE4 to send frames with non-zero
   sequence numbers. All frames sent by PE4 to PE1 over the PW would
   have a non-zero sequence number.

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

6.1 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
   forwarders.

   However, in the multi-homing case each PW SHOULD be evaluated
   independently. Assuming the network topology specified in section 5,
   there could be the case where PW between PE2 and PE1 could have CW
   signaled via extended community and would be used in the VPLS frame,
   while PE2 to PE4 PW would not insert the CW in the VPLS frame due to
   C-bit mismatch.  The rest of PEs multi-homing behavior should simply
   follow the rules specified in [VPLS-MULTIHOMING].


6.2 Sequence flag (S-bit)

   In a 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 behavior as listed
   in section 3.2 when it comes to sequence bit requirements.  Consider
   the case described in section 5 with CE5 being multi-homed to PE4 and
   PE1.  The PW behavior is similar to the CW scenario so that the
   insertion of S-bit evaluation SHOULD be independent per PW.  However,
   because S-bit mismatch between two end-point PEs results in no PW
   establishment, in the case where PE4 doesn't support S-bit. So, only
   one PW would be established, between PE1 and PE2.  Thus, even though



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   CE5 is physically multi-homed, due to PE4's lack of support for
   sending frames with non-zero sequence numbers there would be no PW
   between PE2 and PE4. CE5 would effectively not be multi-homed.


7  Security Considerations

   This document updates the behavior specified in [RFC4761]. The
   security considerations listed in [RFC4761] apply. This document
   essentially addresses BGP-VPLS behavior for PEs when the C-bit and/or
   S-bit value advertised by a given PE are different from what another
   PE in the VPLS is advertising. Any bit-flipping media errors leading
   to causing this mismatch of C/S bits between PEs do not adversely
   affect the availability of the PWs. Rather they cause control-words
   to not be used or cause the NRLI-advertising PE to not expect non-
   zero sequenced frames, for the C-bit and the S-bit respectively being
   mismatched across PEs. This is no worse than the previous behavior
   where any bit-flipping media errors leading to mismatch of C/S bit
   between PEs would cause the PW to not come up.

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.

   [RFC8174]   Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
               2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
               May 2017.

9.2 Informative References

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



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   [VPLS-MULTIHOMING] Kothari, B., et al, BGP based Multi-homing in
               Virtual Private LAN Service,
               draft-ietf-bess-vpls-multihoming-02, September 2018.



Authors' Addresses

      Ravi Singh
      Juniper Networks
      1133 Innovation Way
      Sunnyvale, CA  94089
      US
      EMail: ravis@juniper.net


      Kireeti Kompella
      Juniper Networks
      1133 Innovation Way
      Sunnyvale, CA  94089
      US
      EMail: kireeti@juniper.net


      Senad Palislamovic
      Nokia
      600 Mountain Avenue
      Murray Hill, NJ 07974-0636
      US

      EMail: Senad.palislamovic@nokia.com




















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