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Versions: 00                                                            
Network Working Group                                            H. Wang
Internet-Draft                                                  Y. Huang
Intended status: Standards Track                                 J. Dong
Expires: 22 April 2022                                            Huawei
                                                         19 October 2021


                Advertising S-BFD Discriminators in BGP
                 draft-wang-bess-sbfd-discriminator-00

Abstract

   This document defines the method of transmitting S-BFD Discriminators
   through BGP attributes.  This method helps services create S-BFD
   sessions more easily.

Requirements Language

   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].

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 22 April 2022.

Copyright Notice

   Copyright (c) 2021 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 (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



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   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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Motivations . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  EVPN Layer 3 Service Over SRv6 BE Use Case  . . . . . . .   3
     3.2.  EVPN Layer 3 Service Over SPv6 Policy Use Case  . . . . .   4
   4.  Procedure . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  BGP Encoding  . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Process . . . . . . . . . . . . . . . . . . . . . . . . .   6
       4.2.1.  Egress Process  . . . . . . . . . . . . . . . . . . .   6
       4.2.2.  Transit Process . . . . . . . . . . . . . . . . . . .   6
       4.2.3.  Ingress Process . . . . . . . . . . . . . . . . . . .   7
   5.  Error handling  . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   [RFC7880] defines Seamless Bidirectional Forwarding Detection (S-BFD)
   mechanism.  S-BFD is a simplified mechanism for using BFD with a
   large proportion of negotiation aspects eliminated, thus providing
   benefits such as quick provisioning, as well as improved control and
   flexibility for network nodes initiating path monitoring.  Currently,
   S-BFD can be used in service deployment to simplify the deployment.

2.  Motivations

   An important thing for S-BFD is to check the reachability of
   services, so that service interruption can be quickly detected when
   there is a failure on the service path and services can be switched
   to a backup path quickly.

   [RFC7880] defines Seamless Bidirectional Forwarding Detection (S-BFD)
   mechanism.  Generally, the administrator needs to manually deploy
   S-BFD discriminators on the device to create S-BFD sessions.





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   For the deployment of S-BFD in IPv4 network, the reflector can use
   the LSR-ID address as the discriminator.  This reduces the number of
   discriminators deployed on the transmit end.  This mode cannot be
   used for IPv6 because the discriminator has only four bytes.

   [RFC7883] [RFC7884] defines IS-IS and OSPF to flood BFD
   discriminators.  However, this mode is based on nodes and cannot
   traverse an IGP area.  In addition, without the knowledge of services
   to be detected, a large number of unnecessary S-BFD sessions may be
   created.

   It is suggested to use BGP to distribute BFD discriminator
   information.  BGP can transmit routes across domains, and service
   routes can driven the establishment of end-to-end S-BFD sessions.

3.  Scenarios


3.1.  EVPN Layer 3 Service Over SRv6 BE Use Case

            +-----------------------+ +-------------------+
            |                       | |                   |
            | +-----+      +-----+  | | +-----+           |
            | , PE1 |------|ASBR1|------|ASBR3\           |
            |/+-----+      +-----+  | | +-----+\          |
            /                       | |         \         |
           /|                       | |          \        |
    +-----/ |                       | |           '-----+ | +-----+
    | CE1 | |                       | |           | PE3 |---| CE2 |
    +------ |                       | |           /-----+ | +-----+
           `,                       | |          /        |
            |.+-----+      +-----+  | | +-----+ /         |
            | ' PE2 |------|ASBR2|------|ASBR4|-          |
            | +-----+      +-----+  | | +-----+           |
            |                       | |                   |
            |        AS65001        | |      AS65002      |
            +-----------------------+ +-------------------+
   Figure 1: EVPN Layer 3 Service Over SRv6 BE

   Figure 1 shows a SRv6 BE-based seamless scenario, PE1 and PE2 are
   dual-homed to CE1, and PE3 is dual-homed to CE2.  PE1, PE2, and PE3
   cross BGP ASes.

   CE1 accesses PE1 and PE2 through Layer 3 and advertises its private
   network routes to PE1.  PE1 encapsulates the routes into Type 5
   routes in the EVPN format and sends them to PE3.  After receiving
   Type 5 routes advertised by PE1 and PE2, PE3 generates primary and
   backup entries for the routes to speed up service switchover.



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   To speed up fault detection, we may configure an S-BFD session on PE3
   to detect PE1 and PE2.  In traditional mode, a discriminator needs to
   be assigned to PE1 and PE2, and two S-BFD sessions needs to be
   configured on PE3 to detect the VPN SID's reachability of PE1 and
   PE2.  In this scenario, the ingress PE forward services based on the
   reachability of the VPN SID.  To reduce the number of S-BFD sessions,
   we may detect SRv6 locator routes.

   There are large number of such PEs exist on the network.  Each PE is
   configured with several S-BFD sessions to detect PE1 and PE2, which
   increases the deployment complexity.

3.2.  EVPN Layer 3 Service Over SPv6 Policy Use Case

              +-----+      +-----+
              , PE1 |------| P1  |
             /+-----+      +-----+\
            /                      \
           /                        \
    +-----/         AS65001          '-----+   +-----+
    | CE1 |                          | PE3 |---| CE2 |
    +------                          /-----+   +-----+
           `,                       /
             .+-----+      +-----+ /
              ' PE2 |------| P2P |-
              +-----+      +-----+
   Figure 2: EVPN Layer 3 Service Over SRv6 Policy

   Figure 2 shows a SRv6 Policy scenario, CE1 is dual-homed to PE1 and
   PE2, and PE3 is dual-homed to PE1 and PE2.

   CE1 accesses PE1 and PE2 through Layer 3 and advertises its private
   network routes to PE1.  PE1 encapsulates the routes into Type 5
   routes in the EVPN format and sends them to PE3.

   After receiving Type 5 routes advertised by PE1 and PE2, PE3
   generates primary and backup entries for the routes to speed up
   service switchover.

   Configure S-BFD sessions on PE3 to detect PE1 and PE2 can speed up
   the fault detection.  In traditional mode, a discriminator needs to
   be assigned to PE1 and PE2, and S-BFD sessions is configured on PE3
   to detect the SRv6 Policy's endpoint of PE1 and PE2.

   There are large number of such PEs exist on the network, each PE must
   be configured with S-BFD sessions to detect PE1 and PE2, which
   increases the deployment complexity.




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4.  Procedure


4.1.  BGP Encoding

   [RFC9026] defines the "BFD Discriminators" (38) attribute, which is
   an optional transitive BGP attribute that conveys the Discriminators
   and other optional attributes used to establish BFD sessions.

   The attribute defined at [RFC9026] is used to transmit P2MP BFD
   session creation information through the BFD Discriminator attribute
   in MVPN scenarios.  For non-multicast services, such as L3VPN
   services, L2VPN services, and native IP services, BFD discriminators
   are also required to create an S-BFD session.

   The format of the BFD Discriminator attribute is as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+
     |    BFD Mode   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       BFD Discriminator                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                         Optional TLVs                         ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     Figure 3: Format of the BFD Discriminator Attribute

   o BFD Mode:

   The BFD Mode field is 1 octet long.  [RFC9026] defines only the P2MP
   BFD session for MVPN.  This document defines two new types of SBFD
   session types based on the preceding scenarios.

   SBFD for SRv6 Locator Session Mode, which dedicated to detecting the
   locator.  The temporary type is 176, and is to be allocated by IANA.

   SBFD for Common Session Mode, which is for general SBFD session.  The
   temporary type is 177, and is to be allocated by IANA.  This mode is
   not only for SRv6, but also can be used for other scenarios.

   o BFD Discriminators:

   The field length is 4 octets.  Used to describe the discriminator for
   S-BFD session.

   o Optional TLVs:




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   Variable-length fields are optional.  Indicates the additional
   information required for creating a S-BFD session.  The format is as
   follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |     Length    |           Value             ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 4: Format of the Optional TLV

   In this document, S-BFD for SRv6 Locator Session and S-BFD for Common
   Session must carry an IP addresses except discriminators, which reuse
   the Source IP Address TLV defined in [RFC9026].

   If the mode is set to SBFD for SRv6 Locator Session, the SRv6 Locator
   address used for the service is carried.

   If the mode is set to SBFD for Common Session, the next-hop address
   used for the service is carried.

   For details about the error handling, see section "Error Handling".

4.2.  Process

   In BGP families, such as L3VPN or EVPN, routes can carry the BGP
   attribute as required so that S-BFD sessions can be established based
   on the attribute.  The following uses S-BFD for SRv6 Locator Session
   as an example.  If mode is set to SBFD for Common Session, the
   processing method is similar.

4.2.1.  Egress Process

   As shown in figure 1, the S-BFD discriminator is configured on PE1.
   After obtaining the information, BGP encapsulates the attribute into
   the EVPN route and sets the BFD Mode to SBFD for Locator Session,
   when advertising the EVPN route.  The Discriminator value is local
   discriminator value.  The optional TLV carries the local PE's locator
   address used by the VPN.

4.2.2.  Transit Process

   Here is the seamless scenario, the ASBR does not re-allocate the
   VPNSID.  Therefore, the ASBR does not need to modify the VPNSID, and
   not to change the BFD discriminator attribute.






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4.2.3.  Ingress Process

   After receiving the EVPN Type 5 routes from PE1 and PE2, PE3 imports
   the routes to the VRF of PE3 based on the route targets.  Routes
   triggers establish the S-BFD sessions based on <discriminator,
   locator ip> information to detect SRv6 BE connectivity.

   In addition, routes with the same prefix from PE1 and PE2 form
   primary and backup paths.  When the primary path or the egress node
   is in fault, S-BFD detects that fault and forms switch to backup path
   quickly.

   To avoid the waste of redundant resources, assume that the ASBR re-
   assigns the SID in Option B and the ASBR does not recognize the
   attribute.  In this case, the SID and locator carried in the route
   received by PE3 do not match the Source IP carried in the Optional
   TLV in the BFD attribute.  Therefore, PE3 does not need to establish
   an S-BFD session to remote PE, which can avoid resource waste.

5.  Error handling

   Error handling complies with [RFC7606].  In this document, the BFD
   discriminator information is used only to establish an S-BFD session.
   Therefore, if the BFD discriminator information is invalid, the BFD
   attirbute will be discard and not transmit to other devices.

   For BFD discriminator attribute, the following case will be
   processed:

   o The BFD Discriminator value in receiving BFD Discriminator
   attribute is 0, the attribute is invalid.

   For BFD mode type is S-BFD for SRv6 Locator Session, the following
   case will be processed:

   o The BFD discriminator attribute doesn't contain optional TLV with
   type set to 1, the attribute is invalid.

   o The optional TLV type is 1 but the length is not 16, the attribute
   is invalid.

   o The optional TLV type is 1 but the value is all 0, the attribute is
   invalid.








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   o If multiple Source IP Optional TLVs are carried, the first source
   IP address should be used as the destination to establish an S-BFD
   session.  For EVPN type 2 MAC-IP routes may use the first and the
   second IP address because it may carry two SRv6 SIDs with different
   locators.  Other source IP addresses should be ignored.

   o If a non-Source IP Optional TLV is carried, the Optional TLV will
   be ignored.

   For BFD mode type is S-BFD for Common Session, the following case
   will be processed:

   o The BFD discriminator attribute doesn't contain optional TLV with
   type set to 1, the attribute is invalid.

   o The optional TLV type is 1 but the length is not 4 or 16, the
   attribute is invalid.

   o The optional TLV type is 1 but the value is all 0, the attribute is
   invalid.

   o If multiple Source IP Optional TLVs are carried, only the first
   source IP address should be used as the destination to establish an
   S-BFD session.  Other source IP addresses should be ignored.

   o If a non-Source IP Optional TLV is carried, the Optional TLV will
   be ignored.

6.  IANA Considerations

   This document defines two new BFD modes in the BFD Discriminator
   attribute.  The following values are recommended to be assigned by
   IANA:

                   Value  Description
                   ----  -------------------------
                   176   S-BFD for SRv6 Locator Session
                   177   S-BFD for Common Session

7.  Security Considerations

   The new S-BFD Discriminators sub-TLV does not introduce any new
   security risks for BGP.

   When creating an S-BFD session, the initiator verifies the S-BFD
   session based on routing information.  This reduces the number of
   invalid S-BFD sessions and avoid attribute attack.




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8.  Acknowledgements


9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

9.2.  References

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <https://www.rfc-editor.org/info/rfc7606>.

   [RFC7880]  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
              Pallagatti, "Seamless Bidirectional Forwarding Detection
              (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
              <https://www.rfc-editor.org/info/rfc7880>.

   [RFC7883]  Ginsberg, L., Akiya, N., and M. Chen, "Advertising
              Seamless Bidirectional Forwarding Detection (S-BFD)
              Discriminators in IS-IS", RFC 7883, DOI 10.17487/RFC7883,
              July 2016, <https://www.rfc-editor.org/info/rfc7883>.

   [RFC7884]  Pignataro, C., Bhatia, M., Aldrin, S., and T. Ranganath,
              "OSPF Extensions to Advertise Seamless Bidirectional
              Forwarding Detection (S-BFD) Target Discriminators",
              RFC 7884, DOI 10.17487/RFC7884, July 2016,
              <https://www.rfc-editor.org/info/rfc7884>.

   [RFC9026]  Morin, T., Ed., Kebler, R., Ed., and G. Mirsky, Ed.,
              "Multicast VPN Fast Upstream Failover", RFC 9026,
              DOI 10.17487/RFC9026, April 2021,
              <https://www.rfc-editor.org/info/rfc9026>.

Authors' Addresses










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   Haibo Wang
   Huawei
   No. 156 Beiqing Road
   Beijing
   100095
   P.R. China

   Email: rainsword.wang@huawei.com


   Yang Huang
   Huawei
   No. 156 Beiqing Road
   Beijing
   100095
   P.R. China

   Email: yang.huang@huawei.com


   Jie Dong
   Huawei
   No. 156 Beiqing Road
   Beijing
   100095
   P.R. China

   Email: jie.dong@huawei.com























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