[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]

Versions: 00

Interdomain Routing                                             R. White
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                                D. Sharp
Expires: April 3, 2020                                  Cumulus Networks
                                                                 D. Dutt
                                                     Stardust Consulting
                                                                B. Sadhu
                                                                  VMWare
                                                             J. Tantsura
                                                            Apstra, Inc.
                                                         October 1, 2019


                  Link Local Next Hop Handling for BGP
                       draft-white-linklocalnh-00

Abstract

   BGP, described in [RFC4271], was originally designed to provide
   reachability between domains and between the edges of a domain.  As
   such, BGP assumes the next hop towards any reachable destination may
   not reside on the advertising speaker, but rather may either be
   through a router connected to the same subnet as the speaker, or
   through a router only reachable by traversing multiple hops through
   the network.  Because of this, BGP does not recognize the use of IPv6
   link local addresses, as described in [RFC4291], as a valid next hop
   for forwarding purposes.

   However, BGP speakers are now often deployed on point-to-point links
   in networks where multihop reachability of any kind is not assumed or
   desired (all next hops are assumed to be the speaker reachable
   through a directly connected point-to-point link).  This is common,
   for instance, in data center fabrics.  In these situations, a global
   IPv6 address is not required for the advertisement of reachability
   information; in fact, providing global IPv6 addresses in these kinds
   of networks can be detrimental to Zero Touch Provisioning (ZTP).

   This draft standardizes the operation of BGP over a point-to-point
   link using link local IPv6 addressing only.

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




White, et al.             Expires April 3, 2020                 [Page 1]


Internet-Draft    Link Local Next Hop Handling for BGP      October 2019


   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 April 3, 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
   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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Changes to BGP Next Hop Attribute to Support Link Local on
       Point-to-Point  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Receiver Processing of IPv6 Link Local Forwarding Addresses .   4
   4.  Error handling  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   BGP, described in [RFC4271], was originally designed to provide
   reachability between domains and between the edges of a domain.  As
   such, BGP assumes the next hop towards any reachable destination may
   not reside on the advertising speaker, but rather may either be
   through a router connected to the same subnet as the speaker, or
   through a router only reachable by traversing multiple hops through



White, et al.             Expires April 3, 2020                 [Page 2]


Internet-Draft    Link Local Next Hop Handling for BGP      October 2019


   the network.  Because of this, BGP does not recognize the use of IPv6
   link local addresses, as described in [RFC4271], as a valid next hop
   for forwarding purposes.

   However, BGP speakers are now often deployed on point-to-point links
   in networks where multihop reachability of any kind is not assumed or
   desired (all next hops are assumed to be the speaker reachable
   through a directly connected point-to-point link).  This is common,
   for instance, in data center fabrics.  In these situations, a global
   IPv6 address is not required for the advertisement of reachability
   information; in fact, providing global IPv6 addresses in these kinds
   of networks can be detrimental to Zero Touch Provisioning (ZTP).

2.  Changes to BGP Next Hop Attribute to Support Link Local on Point-to-
    Point

   [RFC2545], section 2, notes link local IPv6 addresses are not
   generally suitable for use in the Next Hop field of the
   MP_REACH_NLRI.  In order to support the many uses of link local
   addresses, however, [RFC2545] constructs the Next Hop field in IPv6
   route advertisements by setting the length of the field to 32, and
   including both a link local and global IPv6 address in the resulting
   enlarged field.  In this way, the receiving BGP speaker can use the
   global IPv6 address to build local forwarding information, and the
   link local address for ICMPv6 redirects, etc.  [RFC2545] does not,
   however, provide an explanation for situations where there is only a
   link local IPv6 address in the Next Hop field of the MP_REACH_NLRI.
   The result is each implementation that supports link local peering
   along with forwarding to a link local address has implemented the
   construction of the Next Hop field in the MP_REACH_NLRI when there is
   only a link local address available in slightly different ways.

   If an implementation intends to send a single link local forwarding
   address in the Next Hop field of the MP_REACH_NLRI, it MUST set the
   length of the Next Hop field to 16 and include only the IPv6 link
   local address in the Next Hop field.

   If an implementation intends to send both a link local and global
   IPv6 forwarding address in the Next Hop field of the MP_REACH_NLRI,
   it MUST set the length of the Next Hop field to 32 and include both
   the IPv6 link local and global IPv6 forwarding addresses in the Next
   Hop field.  If both link local and global IPv6 forwarding addresses
   are carried in the Next Hop Field, the speaker SHOULD provide a local
   configuration option to determine which address is preferred for
   forwarding.






White, et al.             Expires April 3, 2020                 [Page 3]


Internet-Draft    Link Local Next Hop Handling for BGP      October 2019


3.  Receiver Processing of IPv6 Link Local Forwarding Addresses

   On receiving an MP_REACH_NLRI with a Next Hop length of 16,
   implementations SHOULD form the forwarding information using the IPv6
   next hop contained in the Next Hop field, regardless of whether it is
   a link local or globally reachable IPv6 address.

   Implementations MAY check the validity of any IPv6 link local address
   used to calculate forwarding information by insuring the address is
   in the local neighbor table for the interface on which the BGP update
   was received (or through which the BGP speaker from which the update
   was received is reachable).  There MUST be a configuration option to
   enable/disable this check.

   Note: It is possible that checking the IPv6 neighbor table for the
   existence or validity of a link local next hop may make instances
   where a link is being overwhelmed through some form of Deinal of
   Service (DoS) attack worse than they would otherwise be.  If the IPv6
   neighbor cache is overrun in a way that causes the link local address
   being used for BGP peering to be removed from the table, which is
   possible through an on-link DoS attack, any fresh BGP update will
   cause the entire peering session to fail if the implementation is
   checking the validity of link local next hops as described above.
   Operators should carefully assess the use of validation against the
   local IPv6 neighbor table to determine if it is appropriate for any
   particular peering session.

4.  Error handling

   A BGP speaker receiving an MP_REACH_NLRI with the length of the Next
   Hop Field set to 32, where the update contains anything other than a
   link local IPv6 address and a global IPv6 address, SHOULD consider
   this a malformed UPDATE message, and proceed as described in the
   following paragraphs.  In order to support backwards compatibility
   with existing implementations, an implementation MAY ignore a second
   link local IPv6 address or 0::0/0 included with an IPv6 link local
   address when the length of the Next Hop Field is set to 32; in this
   case, the implementation SHOULD report the existence of this
   additional information so the operator can correct the sending BGP
   implementation.

   If the Next Hop field is malformed, the implementation MUST handle
   the mailformed UPDATE message using the approach of "treat-as-
   withdraw", as described in section 7.3 of [RFC7606].  It MAY send a
   NOTIFICATION message as described in section 4 of [RFC4271], using
   the UPDATE error message code (8 - Invalid NEXT_HOP Attribute)
   indicating there is an invalid NEXT_HOP field




White, et al.             Expires April 3, 2020                 [Page 4]


Internet-Draft    Link Local Next Hop Handling for BGP      October 2019


   If the Next Hop field is properly formed, but the link local next hop
   is not reachable (as determined by an examination of the IPv6
   neighbor table), the implementation MAY handle the mailformed UPDATE
   message using the approach of "treat-as-withdraw", as described in
   section 7.3 of [RFC7606] (see note above on checking the local
   neighbor table for the correctness of the next hop).  The
   implementation MAY send a NOTIFICATION message as described in
   section 4 of [RFC4271] using the UPDATE error message code (TBA),
   indicating a link local address was included in the MP_REACH_NLRI,
   but the link local address included cannot be reached.  As this could
   indicate a security breach of some type (see the security
   considerations section below), the operator SHOULD have a local
   configuration option to terminate the peering session until manual
   intervention is initiated.

5.  Acknowledgements

   The authors would like to thank Don Slice, Jeff Haas, and John
   Scudder for their contributions to this draft.

6.  IANA Considerations

   This memo requests IANA assign a number from the "Error Subcodes"
   registry defined in the IANA Considerations section in [RFC4271].
   This allocation will be for a new UPDATE error subcode, code (TBA),
   with a value of "Unreachable Link Local Address."

7.  Security Considerations

   The mechanism described in this draft can be used as a component of
   ZTP for building BGP adjacencies across point-to-point links.  This
   method, then, can be used by an attacker to form a peering session
   with a BGP speaker, ultimately advertising incorrect routing
   information into a routing domain in order to misdirect traffic or
   cause a denial of service.  By using link local IPv6 addresses, the
   attacker would be able to forego the use of a valid IPv6 address
   within the domain, making such an attack easier.

   Operators SHOULD carefully consider security when deploying link
   local addresses for BGP peering.  Operators SHOULD filter traffic on
   links where BGP peering is not intended to occur to prevent speakers
   from accepting BGP session requests, as well as other mechanisms
   described in [RFC7454].

   Operators MAY also use some form of cryptographic validation on links
   within the network to prevent unauthorized devices from forming BGP
   peering sessions.  Authentication, such as the TCP authentication
   described in [RFC5925], may provide some relief, if it is present and



White, et al.             Expires April 3, 2020                 [Page 5]


Internet-Draft    Link Local Next Hop Handling for BGP      October 2019


   correctly configured.  However, the distribution and management of
   keys in an environment where global addresses are not present on BGP
   speakers may be challenging.

   Operators also MAY instruct a BGP peer which has received an UPDATE
   with an unreachable NEXT_HOP to disable the peering session over
   which the invalid NEXT_HOP was received pending manual intervention.

8.  References

8.1.  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>.

   [RFC2545]  Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
              Extensions for IPv6 Inter-Domain Routing", RFC 2545,
              DOI 10.17487/RFC2545, March 1999,
              <https://www.rfc-editor.org/info/rfc2545>.

   [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,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC7454]  Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
              and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
              February 2015, <https://www.rfc-editor.org/info/rfc7454>.

   [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>.








White, et al.             Expires April 3, 2020                 [Page 6]


Internet-Draft    Link Local Next Hop Handling for BGP      October 2019


8.2.  Informative References

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              DOI 10.17487/RFC2629, June 1999,
              <https://www.rfc-editor.org/info/rfc2629>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

Authors' Addresses

   Russ White
   Juniper Networks

   Email: russ@riw.us


   Donald Sharp
   Cumulus Networks

   Email: sharpd@cumulusnetworks.com


   Dinesh Dutt
   Stardust Consulting

   Email: ddutt@hobbesdutt.com


   Biswajit Sadhu
   VMWare

   Email: biswajit.sadhu@gmail.com


   Jeff Tantsura
   Apstra, Inc.

   Email: jefftant.ietf@gmail.com










White, et al.             Expires April 3, 2020                 [Page 7]

Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/