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Network Working Group                                            R. Bush
Internet-Draft                                              Arrcus & IIJ
Intended status: Informational                                  K. Patel
Expires: April 23, 2020                                           Arrcus
                                                        October 21, 2019

                  BGP Topology Discovery Requirements


   For wide scale routing protocols to build their topology and
   reachability databases they need to discover the encapsulation data
   on a link, link IP layer 3 attributes, attributes for IP layer 3 and
   above protocols on that link, and link liveness.  We refer to this as
   neighbor discovery.  BGP-LS and its enhancements provide an API to
   present much of these data to BGP protocols, but do not directly
   collect these data.  This document explores the needs and criteria
   for the data needed.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on April 23, 2020.

Copyright Notice

   Copyright (c) 2019 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
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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Architectural Considerations  . . . . . . . . . . . . . . . .   2
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   In a massive scale datacenter or similar environment BGP([RFC4271])
   and BGP-like protocols, e.g.  BGP-SPF (see [I-D.ietf-lsvr-bgp-spf]),
   provide massive scale-out without centralization using a tried and
   tested scalable distributed control plane transport, offering a
   scalable routing solution.  But BGP4, BGP-SPF, and similar protocols
   need layer 3 topology discovery; meaning IP encapsulations on a link,
   layer 3 IP addressing data on a link, attributes for IP layer 3 and
   above protocols on that link, and assured link liveness from the
   network to build and maintain the routing topology.

   BGP-LS [RFC7752] and its extensions provide an API which BGP4 and
   BGP-SPF can use to get the and distribute topology data.  But BGP-LS
   itself does not gather the data, it merely presents it.  So the IP
   topology data must be gathered.

   What topology data do BGP-like protocols actually need?  What level
   of freshness is needed?  What are the requirements for scale,
   extensibility, security, etc?

2.  Architectural Considerations

   Massive Data Centers (MDCs) have on the order of 10,000 racks, often
   with two Top Of Rack (TOR) devices per rack, on the order of 40
   hardware servers per rack, each with order 100 virtual services or
   machines, each with their own layer 3 IP address.  Given service
   mobility, any initial IP address aggregation fragments over time.  To
   provide this level of scaling reliably, stably, and security imposes
   architectural constraints on any discovery protocol.

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   o  Deployable - If it is not easily deployable, it is a pointless
      exercise.  To be deployable, it must be easy to provision (e.g.
      zero touch provisioning, Open Config, YANG, et alia), easily
      reconfigured, easily measured, and easily monitored.

   o  Simple - If it isn't simple, it will not scale.  Simplicity
      requires restraint in design.  'Union Protocols' which are the sum
      of everyone's desires are complex disasters waiting to happen.
      Often they do not wait.  Prefer 'Intersection Protocols' which
      include only those things which everyone absolutely needs.

   o  Securable - Security properties should be analyzed.  Again,
      simplicity is key; complex protocols increase in complexity over
      time, and security vulnerabilities increase significantly with
      complexity.  As [RFC5218] 2.2.3 says "The more successful a
      protocol becomes, the more attractive a target it will be."

   o  Extensible - As [RFC5218] Section 2.2.1 said, successful protocols
      are extensible beyond the original expectation.  MDC and similar
      needs are expanding and we are still learning about the space.
      Simplicity and extensibility should go a long way to adaptability;
      complex protocols are hard to extend, especially when they are
      poorly understood.

   o  Implementable - It must be reasonably easy to implement and
      deploy.  Some implications are:

      *  Packet formats should be easy to generate and easily parsable.
         Type/length/Value (TLV) formats are preferred.

      *  The protocols should be free to use and deploy; i.e. not be
         constrained by Intellectual Property Right (IPR) claims.

      *  Again, simpler protocols are simpler to implement, deploy,
         measure, monitor, etc.

      *  Performance Problems arise if the protocol was not designed to

   o  Low Impact - The MDCs chose BGP for, among other reasons, it is
      quiet and only transmits changes, not repeated flooding of the
      same information.  This allows great scale.  The discovery
      protocol should be similar in this regard, not flood or chatter
      the same information repeatedly.  It should support fast and quiet
      session restoration in case of link failure and restoration when
      there has been no actual change in end point attributes.

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   o  Compatible - It must be compatible with the various routing
      technologies used in MDCs.  The new discovery protocol will
      discover the IP layer 3 encapsulations, learn layer 3 addressing
      data from the network, confirm link liveness, in order to allow
      upper layer routing protocol(s), e..g.  BGP4, BGP-SPF, and BGP-LS,
      to build maintain and distribute the topology.

3.  Requirements

   The target for the discovery protocol(s) is a massive datacenter
   scale deployment using BGP or similar routing, e.g.  BGP4 or
   [I-D.ietf-lsvr-bgp-spf]; but should be generally usable by other
   routing protocols, e.g.  EVPN, and in other similar environments.

   The IETF is very good at finding corner cases which expand needs and
   complicate protocols.  This effort should resist this tendency.

   It would be easiest for the BGP-like protocols to consume the data if
   they are presented via the BGP-LS [RFC7752] API as used in
   [I-D.ietf-lsvr-bgp-spf] Section 4.

   BGP-like protocols will need at least the following information about
   the topology:

   Node Identity:  Each node in the topology must have an identity/
      identifier which must be unique in the topology.

      A node must have one or more links to other nodes or it is, ab
      definito, not in the topology.

      A node has IP layer 3 attributes such as encapsulations and IP

   Link Identity:  A link is between two nodes.  Each end of a link is a
      node/device interface.

      Each link in the topology must be uniquely identified and the
      identities of the nodes on the link must be identified.  This
      includes LAGged links.

      As MAC addresses are not unique in actual deployment, they may not
      be assumed to uniquely identify a link.  Multiple VLANs between a
      port pair on two devices are a simple example of this.  Link
      Aggregation Groups (LAGs), Multi-Chassis LAGs etc.  must be

      A link might be on a tunnel interface; though the tunnel type may
      be restricted..

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   Link Liveness:  Because adjacencies and topology changes must be
      quickly detected, The stability of each link should be able to be
      monitored and reported.  As this can be noisy, it must be able to
      be tuned by the operator, and expensive operations should be

   Encapsulations:  The encapsulation(s) (IPv4, IPv6, ...) on each link
      must be known.  One or more of the common AFI/SAFIs must be
      supported on each link, IPv4, IPv6, MPLS, etc.

      It is assumed that the set of encapsulations is the same across
      the entire topology.

   Addresses:  The available addresses on the node interfaces for each
      encapsulation must be known.  More than one address for an
      encapsulation type must be supported.

      As BGP-like protocols will be peering between the nodes, there may
      be a preferred encapsulation and address on an link, or a loopback
      interface may be used.

   Upper Layer Protocol Parameters  To facilitate peering of upper layer
      protocols across a link, e.g.  BGP, the protocol should support
      signaling of the parameters for these protocols, e.g. peer AS
      number, peering address(es), etc.

   Mobility:  Fast detection of [micro-]service mobility must be

   EVPNs:  EVPN end-point discovery must be supported.

4.  Security Considerations

   While this document has no security considerations per se, it does
   make a plea for securability in protocol design.

   Mis-wires, malicious devices being plugged into ports, and monkey in
   the middle attacks should be considered.

   There should at least be assurance that the end-point a device opened
   a session with six months ago is the same one sending PDUs today.

5.  IANA Considerations

   This document has no IANA considerations.

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6.  Acknowledgments

   The authors thank Victor Kuarsingh and Gunter Van De Velde for

7.  References

7.1.  Normative References

              Patel, K., Lindem, A., Zandi, S., and W. Henderickx,
              "Shortest Path Routing Extensions for BGP Protocol",
              draft-ietf-lsvr-bgp-spf-06 (work in progress), September

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,

7.2.  Informative References

   [RFC5218]  Thaler, D. and B. Aboba, "What Makes for a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,

Authors' Addresses

   Randy Bush
   Arrcus & IIJ
   5147 Crystal Springs
   Bainbridge Island, WA  98110
   United States of America

   Email: randy@psg.com

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   Keyur Patel
   2077 Gateway Place, Suite 400
   San Jose, CA  95119
   United States of America

   Email: keyur@arrcus.com

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