BESS Working Group                                              R. Singh
INTERNET-DRAFT                                               K. Kompella
Intended Status: Proposed Standard                      Juniper Networks
Updates: 4761 (if approved)                              S. Palislamovic
Expires: February 18, September 6, 2019                                 March 5, 2019                               August 17, 2018

           Updated processing of control flags Control Flags for BGP VPLS


   This document updates the meaning of the "control flags" fields
   inside Control Flags field in the "layer2 info extended community"
   Layer2 Info Extended Community used for BGP-VPLS NLRI as defined in
   RFC4761. If approved, this This document updates RFC4761.

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Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  3
   2  Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3  Updated meaning of control flags Control Flags in the layer2 info extended
      community Layer2 Info Extended
      Community . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1 Control word (C-bit) . . . . . . . . . . . . . . . . . . . .  4
     3.2 Sequence flag (S-bit)  . . . . . . . . . . . . . . . . . . .  4
   4  Using p2mp LSP Point-to-MultiPoint (P2MP) LSPs 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
     9.2 Informative References . . . . . . . . . . . . . . . . . . .  7  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  8

1  Introduction

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

   The use of control word the Control Word (CW) helps prevent mis-ordering of IPv4
   or IPv6 PW Psuedo-Wire (PW) traffic over ECMP (equal cost multi-path) Equal Cost Multi-Path (ECMP)
   paths or LAG (link
   aggregation group) Link Aggregation Group (LAG) bundles. [RFC4385] describes
   the format for
   control-word CW that may be used over point-2-point Point-to-Point PWs (pseudowires) and over a
   VPLS. It along Along with [RFC3985] [RFC3985], the document also describes sequencing of 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 Control Word/sequencing and others
   do not.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
   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 from 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" Control Flags this PE
   specifies whether the other PEs (in the same VPLS) should use
   control-word 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" 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 MUST, respectively, insert control word (CW) and by including
   include sequence numbers respectively. 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
   CW or include sequencing information inside PW frames. Thus, the
   behavior of processing CW and sequencing needs to be further

   This document updates the meaning of the control flags 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 Control Flags in the layer2 info extended

   Current Layer2 Info Extended

   The current specification does not allow for the CW setting to be
   negotiated. Rather, 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 both the two 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 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 in either direction on that PW but the PW MUST
   NOT be prevented from coming up due to this mismatch. So, the PW MUST
   still come up. up but not use control word in either direction. This
   behavior is new; changed from the old behavior was that described in [RFC4761] where
   the PW doesn't does not come up.

3.2 Sequence flag (S-bit)

   Current BGP VPLS specification do not allow for S-bit setting to be
   negotiated either.  If  In typical implementations, if the PE sets the S-bit, S-
   bit, it expects to receive VPLS frames with sequence seqence numbers, and will
   send the outgoing frames with sequence numbers as well.  This memo
   further specifies the existing expected 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. up.

4  Using p2mp LSP 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 (point to multipoint) LSPs (label switched path)
   P2MP Label Switched Path (LSPs) with the source of the p2mp P2MP LSP
   rooted at the PE advertising the VPLS BGP NLRI.

   In a network that uses p2mp P2MP LSPs as transport for BGP VPLS, in a
   given VPLS there VPLS,there may be
   some PEs that support control-word CW while others do may not. Similarly, for the
   sequencing of VPLS frames.

   In such a setup, a source PE that supports control-word CW should setup
   2 two
   different p2mp P2MP LSPs such that:
          - one p2mp One P2MP LSP will carry transport CW-marked frames to those PEs
            that advertised the C-bit as 1, and 1.
          - the The other p2mp P2MP LSP will carry transport frames without CW to those
            PEs that advertised C-bit as 0.

     Using 2 two different p2mp P2MP LSPs to deliver frames with and without
     the CW to different PEs ensures that this a P2MP root PE honors the C-bit C-
     bit advertised by the other P2MP PEs.

     However, the set of leaves on the 2 p2mp two P2MP LSPs (rooted at the
     given PE) MUST NOT contain any PEs that advertised a value for the
     S-bit different from what this the root PE itself is advertising. PEs
     that advertised their S-bit value differently (from what this the P2MP
     root PE advertised) will not be on either of the p2mp P2MP LSPs. It is ensured This
     ensures that this the P2MP root PE is sending VPLS frames only to those
     PEs that agree
     with this PE 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  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. 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. forwarders.

     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 CW
     signaled via extended community and would be used in the VPLS
     frame, while PE2 to PE4 PW would not insert the control word CW 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-
     bess-vpls-multihoming-00. [VPLS-MULTIHOMING].

5.2 Sequence flag (S-bit)

     In multi-homed environment, different PEs may effectively represent
     the same service destination end point. end-point. In this case, the rules for
     end-to-end PW establishment SHOULD follow the same rules behavior as
     listed in section 3.2 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 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 yields results 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. multi-

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

   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] [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 CW only for those VPLS frames that are received from
   PEs that are themselves indicating have indicated their desire to receive CW marked frames. So,
   PE1 will setup its data plane to strip-
   off strip the CW only for VPLs frames
   received from PEs PE2 and PE3. PE1 will setup its data plane data-plane to not strip
   the 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 behavior changes in this

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.

   [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
      1194 N. Mathilda Ave.
      1133 Innovation Way
      Sunnyvale, CA  94089

      Kireeti Kompella
      Juniper Networks
      1194 N. Mathilda Ave.
      1133 Innovation Way
      Sunnyvale, CA  94089

      Senad Palislamovic