Making Route Servers Aware of Data Link Failures at IXPs
draft-ietf-idr-rs-bfd-00

Network Working Group                                            R. Bush
Internet-Draft                                 Internet Initiative Japan
Intended status: Standards Track                                 J. Haas
Expires: January 2, 2016                                      J. Scudder
                                                  Juniper Networks, Inc.
                                                               A. Nipper
                                                            T. King, Ed.
                                                  DE-CIX Management GmbH
                                                            July 1, 2015

        Making Route Servers Aware of Data Link Failures at IXPs
                        draft-ietf-idr-rs-bfd-00

Abstract

   When route servers are used, the data plane is not congruent with the
   control plane.  Therefore, the peers on the Internet exchange can
   lose data connectivity without the control plane being aware of it,
   and packets are dropped on the floor.  This document proposes the use
   of BFD between the two peering routers to detect a data plane
   failure, and then uses BGP next hop cost to signal the state of the
   data link to the route server(s).

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
   be interpreted as described in [RFC2119] only when they appear in all
   upper case.  They may also appear in lower or mixed case as English
   words, without normative meaning.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 2, 2016.

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Copyright Notice

   Copyright (c) 2015 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
   (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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   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.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Mutual Discovery of Route Server Client Routers . . . . .   3
     2.2.  Tracking Connectivity . . . . . . . . . . . . . . . . . .   4
   3.  Advertising Client Router Connectivity to the Route Server  .   4
   4.  Utilizing Next Hop Unreachability Information at Client
       Routers . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Recommendations for Using BFD . . . . . . . . . . . . . . . .   5
   6.  Bootstrapping . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Other Considerations  . . . . . . . . . . . . . . . . . . . .   6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   In configurations (typically Internet exchanges) where EBGP routing
   information is exchanged between client routers through the agency of
   a route server [I-D.ietf-idr-ix-bgp-route-server], but traffic is
   exchanged directly, operational issues can arise when partial data
   plane connectivity exists among the route server client routers.
   This is because, as the data plane is not congruent with the control
   plane, the client routers on the Internet exchange can lose data
   connectivity without the control plane - the route server - being
   aware of it, and packets are dropped on the floor.

   To remedy this, two basic problems need to be solved:

   1.  Client routers must have a means of verifying connectivity
   amongst themselves, and

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   2.  Client routers must have a means of communicating the knowledge
   so gained back to the route server.

   The first can be solved by application of Bidirectional Forwarding
   Detection [RFC5880].  The second can be solved by use of BGP NH-SAFI
   [I-D.ietf-idr-bgp-nh-cost].  There is a subsidiary problem that must
   also be solved.  Since one of the key value propositions offered by a
   route server is that client routers need not be configured to peer
   with each other:

   3.  Client routers must have a means (other than configuration) to
   know of one another's existence.

   This can also be solved by an application of BGP NH-SAFI.

   Throughout this document, we generally assume that the route server
   being discussed is able to represent different RIBs towards different
   clients, as discussed in section 2.3.2.1.
   [I-D.ietf-idr-ix-bgp-route-server].  These procedures (other than the
   use of BFD to track next hop reachability) have limited value if this
   is not the case.

2.  Operation

   Below, we detail procedures where a route server tells its client
   routers about other client routers (by sending it their next hops
   using NH-SAFI), the client router verifies connectivity to those
   other client routers (using BFD) and communicates its findings back
   to the route server (again using NH-SAFI).  The route server uses the
   received NH-SAFI routes as input to the route selection process it
   performs on behalf of the client.

2.1.  Mutual Discovery of Route Server Client Routers

   Strictly speaking, what is needed is not for a route server client
   router to know of other (control-plane) client routers, but rather to
   know (so that it can validate) all the next hops the route server
   might choose to send the client router, i.e. to know of potential
   forwarding plane relationships.

   In effect, this requirement amounts to knowing the BGP next hops the
   route server is aware of in its Adj-RIBs-In.  Fortunately,
   [I-D.ietf-idr-bgp-nh-cost] defines a construct that contains exactly
   this data, the "Next-Hop Information Base", or NHIB, as well as
   procedures for a BGP speaker to communicate its NHIB to its peer.
   Thus, the problem can be solved by the route server advertising its
   NHIB to its client router, following those procedures.

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   We observe that (as per NH-SAFI) the cost advertised in the route
   server's Adj-NHIB-Out need not reflect a "real" IGP cost, the only
   requirement being that the advertised costs are commensurate.  A
   route server MAY choose to advertise any fixed cost other than all-
   ones (which is a reserved value in NH-SAFI).  This specification does
   not suggest semantics be imputed to the NH-SAFI advertised by the
   route server and received by the client, other than "this next hop is
   present in the control plane, you might like to track it".  The route
   server is not allowed to advertise a next hop as NH_UNREACHABLE.

   A route server client SHOULD use BFD (or other means beyond the scope
   of this document) to track forwarding plane connectivity [RFC5880] to
   each next hop depicted in the received NH-SAFI.

2.2.  Tracking Connectivity

   For each next hop in the Adj-NHIB-In received from the route server,
   the client router SHOULD use some means to confirm that data plane
   connectivity does exist to that next hop.

   For each next hop in the Adj-NHIB-In received from the route server,
   the client router SHOULD setup a BFD session to it if one is not
   already available and track the reachability of this next hop.

   For each next hop being tracked, a corresponding NH-SAFI route should
   be placed in the client router's own Adj-NHIB-Out to be advertised to
   the route server.  Any next hop for which connectivity has failed
   should have its cost advertised as NH_UNREACHABLE.  (This may also be
   done as a result of policy even if connectivity exists.)  Any other
   next hop should have some feasible cost advertised.  The values
   advertised may be all equal, or may be set according to policy or
   other implementation-specific means.

   If the test of connectivity between one client router and another
   client router has failed the client router that detected this failure
   should perform connectivity test for a configurable amount of time
   (preferable 24 hours) on a regular basis (e.g. every 5 minutes).  If
   during this time no connectivity can be restored no more testing is
   performed and this client router is advertised as NH_UNREACHABLE
   until manually changed or the client router is rebooted.

3.  Advertising Client Router Connectivity to the Route Server

   As discussed above, a client router will advertise its Adj-NHIB-Out
   to the route server.  The route server should use this information as
   input to its own decision process when computing the Adj-RIB-Out for
   this peer.  This peer-dependent Adj-RIB-Out is then advertised to
   this peer.  In particular, the route server MUST exclude any routes

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   whose next hops the client has declared to be NH_UNREACHABLE.  The
   route server MAY also consider the advertised cost to be the "IGP
   cost" section 9.1 [RFC4271] when doing this computation.

4.  Utilizing Next Hop Unreachability Information at Client Routers

   A client router detecting an unreachable next hop signals this
   information to the route server as described above.  Also, it treats
   the routes as unresolvable as per section 9.1.2.1 [RFC4271] and
   proceeds with route selection as normal.

   Changes in nexthop reachability via these mechanisms should receive
   some amount of consideration toward avoiding unnecessary route
   flapping.  Similar mechanisms exist in IGP implementations and should
   be applied to this scenario.

5.  Recommendations for Using BFD

   The RECOMMENDED way a client router can confirm the data plane
   connectivity to its next hops is available, is the use of BFD in
   asynchronous mode.  Echo mode MAY be used if both client routers
   running a BFD session support this.  The use of authentication in BFD
   is OPTIONAL as there is a certain level of trust between the
   operators of the client routers at a particular IXP.  If trust cannot
   be assumed, it is recommended to use pair-wise keys (how this can be
   achieved is outside the scope of this document).  The ttl/hop limit
   values as described in section 5 [RFC5881] MUST be obeyed in order to
   secure BFD sessions from packets coming from outside the IXP.

   There is interdependence between the functionality described in this
   document and BFD from an administrative point of view.  To streamline
   behaviour of different implementations the following is RECOMMENDED:

   o  If BFD is administratively shut down by the administrator of a
      client router then the functionality described in this document
      MUST also be administratively shut down.
   o  If the administrator enables the functionality described in this
      document on a client router then BFD MUST be automatically
      enabled.

   The following values of the BFD configuration of client routers (see
   section 6.8.1 [RFC5880]) are RECOMMENDED in order to allow a fast
   detection of lost data plane connectivity:

   o  DesiredMinTxInterval: 1,000,000 (microseconds)
   o  RequiredMinRxInterval: 1,000,000 (microseconds)
   o  DetectMult: 3

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   The configuration values above are a trade-off between fast detection
   of data plane connectivity and the load client routers must handle
   keeping up the BFD communication.  Selecting smaller
   DesiredMinTxInterval and RequiredMinRxInterval values generates lots
   of BFD packets, especially at larger IXPs with many hundreds of
   client routers.

   The configuration values above are selected in order to handle brief
   interrupts on the data plane.  Otherwise, if a BFD session detects a
   brief data plane interrupt to a particular client router, it will
   cause to signal the route server that it should remove routes from
   this client router and tell it shortly afterwards to add the routes
   again.  This is disruptive and computational expensive on the route
   server.

   The configuration values above are also partially impacted by BGP
   advertisement time in reaction to events from BFD.  If the
   configuration values are selected so that BFD detects data plane
   interrupts a lot faster than the BGP advertisement time, a data plane
   connectivity flapping could be detected by BFD but the route server
   is not informed about them because BGP is not able to transport this
   information fast enough.

   As discussed, finding good configuration values is hard so a client
   router administrator MAY select better suited values depending on the
   special needs of the particular deployment.

6.  Bootstrapping

   If the route server starts it does not know anything about
   connectivity states between client routers.  So, the route server
   assumes optimistically that all client routers are able to reach each
   other unless told otherwise.

7.  Other Considerations

   For purposes of routing stability, implementations may wish to apply
   hysteresis ("holddown") to next hops that have transitioned from
   reachable to unreachable and back.

8.  Normative References

   [I-D.ietf-idr-bgp-nh-cost]
              Varlashkin, I., Raszuk, R., Patel, K., Bhardwaj, M., and
              S. Bayraktar, "Carrying next-hop cost information in BGP",
              draft-ietf-idr-bgp-nh-cost-02 (work in progress), May
              2015.

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   [I-D.ietf-idr-ix-bgp-route-server]
              Jasinska, E., Hilliard, N., Raszuk, R., and N. Bakker,
              "Internet Exchange BGP Route Server", draft-ietf-idr-ix-
              bgp-route-server-07 (work in progress), June 2015.

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

   [RFC2439]  Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
              Flap Damping", RFC 2439, November 1998.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June
              2010.

Authors' Addresses

   Randy Bush
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, Washington  98110
   US

   Email: randy@psg.com

   Jeffrey Haas
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: jhaas@juniper.net

   John G. Scudder
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: jgs@juniper.net

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   Arnold Nipper
   DE-CIX Management GmbH
   Lichtstrasse 43i
   Cologne  50825
   Germany

   Email: arnold.nipper@de-cix.net

   Thomas King (editor)
   DE-CIX Management GmbH
   Lichtstrasse 43i
   Cologne  50825
   Germany

   Email: thomas.king@de-cix.net

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