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Versions: 00 01 02 03 04 05 06 07 08                                    
LSR Working Group                                                A. Wang
Internet-Draft                                             China Telecom
Intended status: Standards Track                                   Z. Hu
Expires: June 11, 2020                               Huawei Technologies
                                                        December 9, 2019

                    Prefix Unreachable Announcement


   This document describes the mechanism that can be used to announce
   the unreachable prefixes for service fast convergence.

Status of This Memo

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

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   This Internet-Draft will expire on June 11, 2020.

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   Copyright (c) 2019 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   2
   3.  Scenario Description  . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Inter-Area Node Failure Scenario  . . . . . . . . . . . .   3
     3.2.  Inter-Area Links Failure Scenario . . . . . . . . . . . .   3
     3.3.  Intra-Area Node Failure Scenario  . . . . . . . . . . . .   4
   4.  Inter-area prefix unreachable solution  . . . . . . . . . . .   4
   5.  Intra-area prefix unreachable solution  . . . . . . . . . . .   5
   6.  Implementation Consideration  . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   6
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   OSPF and IS-IS have the summary route and default route mechanism on
   area border router or L1L2 border router, which can be used to
   increase the scalability of these IGP protocols.  Such summary
   mechanism can also reduce the SPF calculation time when the link
   oscillation occurs in another area.

   The summary route and the default route may cover the host route or
   link prefixes of intra area or inter area.  But in some situations,
   the router needs to know the exact reachability information about
   prefix in other area, especially when the prefix is unreachable but
   it is located within the summary range.

   With the introduction of SRv6, more and more services are migrated
   from the MPLS data plane to the IPv6 data plane.  The biggest
   difference between IPv6 and MPLS is that IPv6 has aggregation
   ability, so we need to reconsider how to know the prefix reachability
   in the case of aggregation.

   This document introduces the mechanism that can be used in such
   situation, to announce the unreachable prefixes which are located in
   the summary address range.

2.  Conventions used in this document

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

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3.  Scenario Description

   Figure1 illustrates the topology scenario when OSPF is running in
   multi-area.  R0-R4 are routers in backbone area, S1-S4,T1-T4 are
   internal routers in area 1 and area 2 respectively.  R1 and R3 are
   area border routers between area 0 and area 1.  R2 and R4 are area
   border routers between area 0 and area 2.  Ps2 is the host address of
   S2 and Pt2 is the host address T2.

    | +--+        +--+   ++-+   ++-+    +-++   + -+        +--+|
    | |S1+--------+S2+---+R1+---|R0+----+R2+---+T1+--------+T2||
    | +-++     Ps2+-++   ++-+   +--+    +-++   ++++    Pt2 +-++|
    |   |           |     |               |     ||           | |
    |   |           |     |               |     ||           | |
    | +-++        +-++   ++-+           +-++   ++++        +-++|
    | |S4+--------+S3+---+R3+-----------+R4+---+T3+--------+T4||
    | +--+        +--+   ++-+           +-++   ++-+        +--+|
    |                     |               |                    |
    |                     |               |                    |
    |         Area 1      |     Area 0    |      Area 2        |

   Figure1: OSPF Inter-Area Prefix Unreachable Announcement Scenario

3.1.  Inter-Area Node Failure Scenario

   If the area border router R2/R4 does the summary action, then one
   summary address that cover the prefixes of area 2 will be announced
   to area 0 and area 1, instead of the detail address.  When the node
   T2 is down, Pt2 become unreachable.  But there will be no change to
   the summary prefix.  Except the border router R2/R4, the other
   routers within area 0 and area 1 do not know the unreachable status
   of this prefix.  When these routers send traffic to prefix Pt2, the
   traffic will be dropped.

3.2.  Inter-Area Links Failure Scenario

   In other situation, if the link between T1/T2 and T1/T3 are broken,
   R2 will not be able to reach node T2.  But as R2 and R4 do the
   summary announcement, and the summary address covers the prefix of
   Pt2, other nodes in area 0 area 1 will still send traffic to T2 via
   the border router R2.  When R2 receives such traffic, it will drop
   the packet.

   In such situation, the border router R2 should notify other routers
   that it can't reach the prefix Pt2, and lets the other routers to
   select R4 as the bypass router to reach prefix Pt2.

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3.3.  Intra-Area Node Failure Scenario

   For intra area, there are some situations that the border routers,
   for example R1/R3 in Figure 1, announces the summary address that
   cover also the prefix addresses in area 1.  In this situation, when
   node S2 failures, node S1 should send traffic to the back up path
   that bypass the failure node.  But the back up path can't be
   triggered because node S1 still think it can reach the prefix Ps2
   because it has the summary route that announced by the border router

   From the above scenarios, we can conclude that in such situations,
   the mechanism for Prefix Unreachable Announcement (PUA) should be
   designed to alleviate the traffic loss.

4.  Inter-area prefix unreachable solution

   [RFC7794] and [I-D.ietf-lsr-ospf-prefix-originator] both define one
   sub-TLV "Prefix Source Router ID" to announce the originator router
   information of one prefix.  This TLV can be used to announce the
   prefix unreachable information when the link or node is down.

   According to the procedure described in section 5 of
   [I-D.ietf-lsr-ospf-prefix-originator], the ABR has the responsibility
   to add the prefix originator information when it receive the Router
   LSA from other routers in the same area.  When the ABR does the
   summary work and receives one updated LSA that omits the prefix
   belong to failed link which is within the range of summary address,
   the ABR should announce one new Summary LSA, which includes the
   information about this prefix, but with the prefix originator set to
   NULL(all 0 address).

   When one node in one area is down, the ABR has also the ability to
   detect the missing neighbor from the neighbor list.  It should then
   announce one new Summary LSA that includes the loopback addresses of
   this node, with the prefix originator set also to NULL(all 0

   For IS-IS, the above procedure is similar.  The level-1/2 router will
   accomplish the above work when it judges that one prefix within the
   summary address range is missing.

   These LSAs will be transported via the traditional flooding

   When the routers in other area receives such LSA, they will generate
   automatically one black-hole route, with the prefix as the
   destination, and the next hop be set to Null.

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5.  Intra-area prefix unreachable solution

   In the intra-area scenario, like S1 illustrated in Figure 1, it will
   learn two types of prefixes, one is summary route, another is host
   route.  When node S2 is down, S2 will withdraw the host route.  But
   S1 can still match the summary route via the longest mask matching.
   For this scenario, when node S2 is down, S1 needs to keep the S2 host
   route for a period of time but updates S2 host route to black hole
   route.  S1 will match the black hole route via the longest mask
   matching.  Such mechanism can be used to trigger a SRv6 VPN for PE
   switching, or SRv6 TE mid-point protection.

   The period for keeping the black hole route should be configured, to
   ensure the related protocols or services be converged.

6.  Implementation Consideration

   The above procedures will only be triggered under the following

   1.  The ABR or Level 1/2 router do the summary work.

   2.  The link prefix or loopback address of the node within the
   summary address range become unreachable.

   The Summary LSA that includes the unreachable prefix, with the prefix
   originator set to NULL value, will be announced across the ABR
   router, reach the routers in other areas.  It's behavior is still the
   same as that defined in OSPFv2 [RFC2328] or OSPFv3 [RFC5340]

   Considering the balances of reachable information and unreachable
   information announcement capabilities, the implementation of this
   mechanism should set one MAX_Address_Announcement (MAA) threshold
   value that can be configurable.  Then, the ABR should make the
   following decisions to announce the prefixes:

   1.  If the number of unreachable prefixes is less than MAA, the ABR
   should advertise the summary address and the PUA.

   2.  If the number of reachable address is less than MAA, the ABR
   should advertise the detail reachable address only.

   3.  If the number of reachable prefixes and unreachable prefixes
   exceed MAA, then advertise the summary address with MAX metric.

   When the receiver receives such LSA, it will do the following
   judgements and actions:

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   1.  If all the source that announces the summary route announces the
   prefix unreachable information, the receiver should add the black
   hole route to this prefix.

   2.  If not, the receiver should prefer the router that does not
   include the prefix unreachable information to reach this prefix.

   3.  The receiver router should keep the black hole routes for PUA as
   one configurable time(MAX_T_PUA) to allow the services that depends
   on them converged.  After the MAX_T_PUA time, such black hole routes
   can be deleted then.

7.  Security Considerations

   Security concerns for OSPF are addressed in [RFC5709]

   Advertisement of the additional information defined in this document
   may raise some compatible issues when the node does not recognize it
   or consider such information is illegal.  During deployment, the
   operator should make sure all the routers within its domain have
   support such features.

8.  IANA Considerations


9.  Acknowledgement

   Thanks Peter Psenak, Les Ginsberg and Acee Lindem for their
   suggestions and comments on this draft.

10.  Normative References

              Wang, A., Lindem, A., Dong, J., Psenak, P., and K.
              Talaulikar, "OSPF Prefix Originator Extension", draft-
              ietf-lsr-ospf-prefix-originator-05 (work in progress),
              November 2019.

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

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,

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   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,

   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, DOI 10.17487/RFC5709, October
              2009, <https://www.rfc-editor.org/info/rfc5709>.

   [RFC7794]  Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
              U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
              and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
              March 2016, <https://www.rfc-editor.org/info/rfc7794>.

Authors' Addresses

   Aijun Wang
   China Telecom
   Beiqijia Town, Changping District
   Beijing  102209

   Email: wangaj3@chinatelecom.cn

   Zhibo Hu
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing  100095

   Email: huzhibo@huawei.com

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