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BIER                                                            Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                               N. Warnke
Expires: January 7, 2020                                Deutsche Telekom
                                                             I. Wijnands
                                                           Cisco Systems
                                                              D. Awduche
                                                            July 6, 2019

           Tethering A BIER Router To A BIER-incapable Router


   This document specifies optional procedures to optimize the handling
   of Bit Index Explicit Replication (BIER) incapable routers, by
   tethering a BIER router to a BIER incapable router.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 7, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Terminologies . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Additional Considerations . . . . . . . . . . . . . . . . . .   4
   4.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Advertising from Helped Node  . . . . . . . . . . . . . .   6
     4.2.  Advertising from Helper Node  . . . . . . . . . . . . . .   7
     4.3.  Procedures for BGP Signaling  . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Terminologies

   Familiarity with BIER architecture, protocols and procedures is
   assumed.  Some terminologies are listed below for convenience.

   [To be added].

2.  Introduction

   Consider the following scenario where router X does not support BIER.

                              ------ BFR2 ------- BFER2
      BFER1 ---  BFR1 ---- X ------- BFR3 ------- BFER3
                              ------ BFRn ------- BFERn

   For BFR1 to forward BIER traffic towards BFR2...BFRn, it needs to
   tunnel individual copies through X.  This degrades to "ingress"
   replication to those BFRs.  If X's connections to BFRs are long

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   distance or bandwidth limited, and n is large, it becomes very

   A solution to the inefficient tunneling from BFRs is to tether a BFRx
   to X:

                              ------ BFR2 ------- BFER2
      BFER1 ---  BFR1 ---- X ------- BFR3 ------- BFER3
                          / \  .........
                         /   \
                      BFRx    ------ BFRn ------- BFERn

   Instead of BFR1 tunneling to BFR2, ..., BFRn directly, BFR1 will get
   BIER packets to BFRx, who will then tunnel to BFR2, ..., BFRn.  There
   could be fat and local pipes between the tethered BFRx and X, so
   ingress replication from BFRx is acceptable.

   For BFR1 to tunnel BIER packets to BFRx, the BFR1-BFRx tunnel need to
   be announced in IGP as a forwarding adjacency so that BFRx will
   appear on the SPF tree.  This need to happen in a BIER specific
   topology so that unicast traffic would not be tunneled to BFRx.
   Obviously this is operationally cumbersome.

   Section 6.9 of BIER architecture specification [RFC8279] describes a
   method that tunnels BIER packets through incapable routers without
   the need to announce tunnels.  However that does not work here,
   because BFRx will not appear on the SPF tree of BFR1.

   There is a simple solution to the problem though.  Even though X does
   not support BIER forwarding, it could advertises BIER information as
   if it supported BIER so BFRs will send BIER packets to it.  The BIER
   packets have a BIER label in front of the BIER header and X will use
   the BIER label to label switch to BFRx, who will in turn do BIER
   forwarding to other BFRs but via tunneling as described in section
   6.9 of BIER architecture spec.

   Even though X advertises as if it supported BIER, BFRx needs to know
   that X does not really support BIER so it will tunnel to other BFRs
   through X.  The knowledge is through static provisioning or through
   additional signaling.  In the latter case, X could advertise that
   BFRx is its helper node, so that other BFRs could optionally use the
   Section 6.9 method to tunnel to BFRx, instead of sending native BIER
   packets to X and rely on X label switching to BFRx.  This also allows
   it to work in the non-MPLS case.

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   Alternatively, instead of for X to advertise that it supports BIER
   but relies on helper BFRx, BFRx could advertise that it is X's helper
   and other BFRs will use BFRx (instead of X's children on the SPF
   tree) to replace X during its post-SPF processing as described in
   section 6.9 of BIER architecture spec.  That way, X does not need any
   special knowledge, provisioning or procedure.

   The two options both have pros and cons - the first option only needs
   X and BFRx to support the new procedure while the second option does
   not require anything to be done to the BIER incapable X.

   BFRx could also be connected to other routers in the network so that
   it could send BIER packets through other routers as well, not
   necessarily tunneling through X.  To prevent routing loops, smallest
   metric, which is 1, must be announced for links between X and BFRx in
   both directions.

3.  Additional Considerations

   While the example shows a local connection between BFRx and X, it
   does not have to be like that.  As long as packets can arrive at BFRx
   without requiring X to do BIER forwarding, it should work.  For
   example, X could label switch incoming BIER packets through a multi-
   hop tunnel to BFRx, or other BFRs could tunnel BIER packets to BFRx
   based on X's advertisement that BFRx is its helper.  However, BFRx
   must make sure that if X appears in its SPF paths to some BFERs, then
   it must tunnel BIER packets for those BFERs directly to X's BFR
   children on BFRx's SPF tree.

   Additionally, the helper BRFx can be a transit helper, i.e., it has
   other connections (instead of being a stub helper that is only
   connected to X), as long as BFRx won't send BIER packets tunneled to
   it back towards the tunnel ingress:

                                 ------ BFR2 ------- BFER2
         BFER1 ---  BFR1 ---- X ------- BFR3 ------- BFER3
                            BFRx ------ BFR4 ------- BFER4
                                 ------ BFR5 ------- BFER5

   In the following example, there is a connection between BFR1 and
   BFRx.  If the link metrics are all 1 on the three sides of
   BFR1-X-BFRx triangle, loop won't happen but if the BFRx-X metric is 3
   while other two sides of the triangle has metric 1 then BFRx will

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   send BIER packets tunneled to it from BFR1 back to BFR1, causing a

                              ------ BFR2 ------- BFER2
      BFER1 ---  BFR1 ---- X ------- BFR3 ------- BFER3
                   \      / \  .........
                    \    /   \
                      BFRx    ------ BFRn ------- BFERn

   This can easily be prevented if BFR1 does an SPF calculation with the
   helper BFRx as the root.  For any BFERn reached via X from BFR1, if
   BFRx's SPF path to BFERn includes BFR1 then BFR1 must not use the
   helper.  Instead, BFR1 must directly tunnel packets for BFERn to X's
   BFR (grand-)child on BFR1's SPF path to BFERn, per section 6.9 of

   Notice that this SPF calculation on BFR1 with BFRx as the root is no
   different from the SPF done for a neighbor as part of LFA
   calculation.  In fact, BFR1 tunneling packets to X's helper is no
   different from sending packets to a LFA backup.

   Also notice that, instead of a dedicated helper BFRx, any one or
   multiple ones of BFR2..N can also be the helper (as long as the
   connection between that BFR and X has enough bandwidth for
   replication to multiple helpers through X).  To allow multiple
   helpers to help the same non-BFR, the "I am X's helper" advertisement
   carries a priority.  BFR1 will choose the helper advertising the
   highest priority among those satisfying the loop-free condition
   described above.  When there are multiple helpers advertising the
   same priority and satisfying the loop-free condition, any one or
   multiple ones could be used solely at the discretion of BFR1.
   However, if multiple ones are used, it means that multiple copies may
   be tunneled through X.

   The following situation where a helper BFRxy helps two different non-
   BFRs X and Y also works.  It's just a special situation of a transit

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                              ----- BFR2 ------- BFER2
                          X ------- BFR3 ------- BFER3
                        / | \
                      /    \  ----- BFR4 ------- BFER4
                    /       \
          BFIR1 -- BFR1      BFRxy ------------- BFERxy
                    \       /
                      \    /  ----- BFR5 ------- BFER5
                        \ | /
                          Y ------- BFR6 ------- BFER6
                              ----- BFRn ------- BFERn

4.  Specification

   The procedures in this document apply when a BFRx is tethered to a
   BIER incapable router X as X's helper for BIER forwarding.

   BFRx MUST not send BIER packets natively to X even if X advertises
   BIER information.  BFRx knows that X does not really support BIER
   either from provisioning or from the BIER Helper Node sub-sub-TLV
   advertised by X.

   Procedures for BGP signaling is described in Section 4.3.

   Either of the following two methods may be used for ISIS [RFC8401]
   and OSPF [RFC8444].  The sub-sub-TLVs for both methods have the same
   format: the value is BIER prefix of the helper/helped node followed
   by a one-octet priority field, and one-octet reserved field.  The
   length is 6 for IPv4 and 18 for IPv6 respectively.

4.1.  Advertising from Helped Node

   For non-MPLS encapsulation, X MUST advertise a BIER Helper Node sub-
   sub-TLV that specifies the BIER prefix of the helper BFRx.  Other
   BFRs MUST use the Section 6.9 procedure modified as following: X is
   treated as BIER incapable (because of the BIER Helper Node sub-sub-
   TLV), and is replaced with the BFRx (instead of X's children on the
   SPF tree) during the post-SPF processing.

   This requires other BFRs to recognize the BIER Helper Node sub-sub-
   TLV.  The same procedure MAY be used For MPLS encapsulation, though
   with the following alternative for MPLS encapsulation, tethering is
   transparent to other BFRs (except the helper node BFRx) - they do not
   need to be aware that X does not support BIER at all.

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   For MPLS encapsulation, X MAY advertises BIER information as if it
   supported BIER forwarding, including the MPLS Encapsulation sub-sub-
   TLV with a label range.  X MUST set up its forwarding state such that
   incoming packets with a BIER label in its advertised label range are
   label switched to BFRx, either over a direct link or through a
   tunnel.  The incoming label is swapped to a BIER label advertised by
   BFRx for the <sub-domain, bsl, set> that the incoming label
   corresponds to.

   Notice that both methods can be used for MPLS encapsulation at the
   same time.  In that case another BFR may send BIER packets to X
   natively, or tunnel to BFRx directly.

4.2.  Advertising from Helper Node

   With this method, the helper node (BFRx) MUST advertise a BIER Helped
   Node sub-sub-TLV that specifies the BIER incapable node (X) that this
   node helps.  When other BFRs follow the post-SPF processing
   procedures as specified in section 6.9 of the BIER architecture spec
   [RFC8279], they replace the helped node on the SPF tree with the
   helper node (instead of the children of the helped node).

4.3.  Procedures for BGP Signaling

   Suppose that the BIER domain uses BGP signaling
   [I-D.ietf-bier-idr-extensions] instead of IGP.  BFR1..N advertises
   BIER prefixes that are reachable through them, with BIER Path
   Attributes (BPA) attached.  There are three situations regarding X's

   (1)  X does not participate in BGP peering at all

   (2)  X re-advertises the BIER prefixes but does not do next-hop-self

   (3)  X re-advertises the BIER prefixes and does next-hop-self

   With (1) and (2), the BFR1..N will tunnel BIER packets directly to
   each other.  It works but not efficiently as explained earlier.  With
   (3), BIER forwarding will not work, because BFR1..N would try to send
   BIER packets to X though X does not advertise any BIER information.
   If Tunnel Encapsulation Attribute (TEA) [I-D.ietf-idr-tunnel-encaps]
   is used as specified in [I-D.zzhang-bier-multicast-as-a-service] with
   (3), then it becomes similar to (2) - works but still not

   To make tethering work well with BGP signaling, the following can be

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   o  Configure a BGP session between X and its helper BFRx.  X re-
      advertises BIER prefixes (with BPA) to BFRx without changing the
      tunnel destination address in the TEA.

   o  BFRx advertises its own BIER prefix with BPA to X, and sets the
      tunnel destination address in the TEA to itself.  X then re-
      advertises BFRx's BIER prefix to BFR1..N, without changing the
      tunnel destination address in the TEA.

   o  For BIER prefixes (with BIER Path Attribute) that X re-advertises
      to other BFRs, the tunnel destination in the TEA is changed to the
      helper BFRx.

   With the above, BFR1..N will tunnel BIER packets to BFRx (following
   the tunnel destination address in the TEA), who will then tunnel
   packets to other BFRs (again following the tunnel destination address
   in the TEA).  Notice that what X does is not specific to BIER at all.

5.  Security Considerations

   This specification does not introduce additional security concerns
   beyond those already discussed in BIER architecture and OSPF/ISIS/BGP
   extensions for BIER signaling.

6.  IANA Considerations

   This document requests two new sub-sub-TLV type values from the "Sub-
   sub-TLVs for BIER Info Sub-TLV" registry in the "IS-IS TLV
   Codepoints" registry:

        Type    Name
        ----    ----
        TBD1    BIER Helper Node
        TBD2    BIER Helped Node

   This document also requests two new sub-TLV type values from the
   OSPFv2 Extended Prefix TLV Sub-TLV registry:

        Type    Name
        ----    ----
        TBD3    BIER Helper Node
        TBD4    BIER Helped Node

7.  Acknowledgements

   The author wants to thank Eric Rosen and Antonie Przygienda for their
   review, comments and suggestions.

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8.  Normative References

              Xu, X., Chen, M., Patel, K., Wijnands, I., and T.
              Przygienda, "BGP Extensions for BIER", draft-ietf-bier-
              idr-extensions-06 (work in progress), January 2019.

              Patel, K., Velde, G., Ramachandra, S., and E. Rosen, "The
              BGP Tunnel Encapsulation Attribute", draft-ietf-idr-
              tunnel-encaps-12 (work in progress), May 2019.

              Zhang, Z., Rosen, E., and L. Geng, "Multicast/BIER As A
              Service", draft-zzhang-bier-multicast-as-a-service-00
              (work in progress), October 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,

   [RFC8401]  Ginsberg, L., Ed., Przygienda, T., Aldrin, S., and Z.
              Zhang, "Bit Index Explicit Replication (BIER) Support via
              IS-IS", RFC 8401, DOI 10.17487/RFC8401, June 2018,

   [RFC8444]  Psenak, P., Ed., Kumar, N., Wijnands, IJ., Dolganow, A.,
              Przygienda, T., Zhang, J., and S. Aldrin, "OSPFv2
              Extensions for Bit Index Explicit Replication (BIER)",
              RFC 8444, DOI 10.17487/RFC8444, November 2018,

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks

   EMail: zzhang@juniper.net

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   Nils Warnke
   Deutsche Telekom

   EMail: Nils.Warnke@telekom.de

   IJsbrand Wijnands
   Cisco Systems

   EMail: ice@cisco.com

   Daniel Awduche

   EMail: daniel.awduche@verizon.com

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