Network Working Group                                          A. Azimov
Internet-Draft                                      Qrator Labs & Yandex
Intended status: Standards Track                            E. Bogomazov
Expires: July 20, 2021                                       Qrator Labs
                                                                 R. Bush
                                Internet Initiative Japan & Arrcus, Inc.
                                                                K. Patel
                                                               K. Sriram
                                                                USA NIST
                                                        January 16, 2021

     Route Leak Prevention using Roles in Update and Open messages


   Route leaks are the propagation of BGP prefixes which violate
   assumptions of BGP topology relationships; e.g. passing a route
   learned from one lateral peer to another lateral peer or a transit
   provider, passing a route learned from one transit provider to
   another transit provider or a lateral peer.  Existing approaches to
   leak prevention rely on marking routes by operator configuration,
   with no check that the configuration corresponds to that of the eBGP
   neighbor, or enforcement that the two eBGP speakers agree on the
   relationship.  This document enhances BGP OPEN to establish agreement
   of the (peer, customer, provider, Route Server, Route Server client)
   relationship of two neighboring eBGP speakers to enforce appropriate
   configuration on both sides.  Propagated routes are then marked with
   an Only to Customer (OTC) attribute according to the agreed
   relationship, allowing both prevention and detection of route leaks.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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 July 20, 2021.

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
   ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Peering Relationships . . . . . . . . . . . . . . . . . . . .   3
   3.  BGP Role  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  BGP Role Capability . . . . . . . . . . . . . . . . . . . . .   5
   5.  Role correctness  . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Strict mode . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  BGP Only to Customer (OTC) Attribute  . . . . . . . . . . . .   6
   7.  Enforcement . . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Additional Considerations . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     11.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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1.  Introduction

   A BGP route leak occurs when a route is learned from a transit
   provider or lateral peer and then announced to another provider or
   lateral peer.  See [RFC7908].  These are usually the result of
   misconfigured or absent BGP route filtering or lack of coordination
   between two eBGP speakers.

   The mechanism proposed in
   [I-D.ietf-grow-route-leak-detection-mitigation] uses large-
   communities to perform detection and mitigation of route leaks.
   While signaling using communities is easy to implement and deploy
   quickly, it normally relies on operator-maintained policy
   configuration, which is vulnerable to misconfiguration [Streibelt].
   The community signal can also be stripped at the ISP boundaries.

   This document provides configuration automation using 'BGP roles',
   which are negotiated using a new BGP Capability Code in OPEN message
   (see Section 4 in [RFC5492]).  Either or both BGP speakers MAY be
   configured to require that this capability be agreed for the BGP OPEN
   to succeed.

   A new optional, transitive BGP Path Attribute Only to Customer (OTC)
   is specified that SHOULD be automatically configured using BGP roles.
   This attribute prevents networks from creating leaks, and detects
   leaks created by third parties.

   In the rest of this document, we use the term "peer" to refer to
   "lateral peer" for simplicity.

2.  Peering Relationships

   Despite the use of terms such as "customer", "peer", etc. in this
   document, these are not necessarily business relationships based on
   payment agreements.  These terms are used to represent restrictions
   on BGP route propagation, sometimes known as the Gao-Rexford model
   [Gao].  The following is a list of various roles in eBGP peering and
   the corresponding rules for route propagation:

   Provider:  MAY send to a customer all available prefixes.

   Customer:  MAY send to a provider prefixes which the sender
      originates and prefixes learned from any of their customers.  A
      customer MUST NOT send to a provider prefixes learned from its
      peers, from other providers, or from Route Servers.

   Route Server (RS):  MAY send to an Route Server client (RS-client)
      all available prefixes.

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   RS-client:  MAY send to an RS prefixes which the sender originates
      and prefixes learned from its customers.  An RS-client MUST NOT
      send to an RS prefixes learned from its peers or providers, or
      from another RS.

   Peer:  MAY send to a peer prefixes which the sender originates and
      prefixes learned from its customers.  A peer MUST NOT send to a
      peer prefixes learned from other peers, from its providers, or
      from RS(s).

   Of course, any BGP speaker may apply policy to reduce what is
   announced, and a recipient may apply policy to reduce the set of
   routes they accept.  Violation of the above rules may result in route
   leaks and MUST NOT be allowed.  Automatic enforcement of these rules
   should significantly reduce route leaks that may otherwise occur due
   to manual configuration mistakes.  While enforcing the above rules
   will address most BGP peering scenarios, their configuration is not
   part of BGP itself; therefore, configuration of ingress and egress
   prefix filters is still strongly advised.

3.  BGP Role

   BGP Role is a new configuration option that is configured on a per-
   session basis.  BGP Roles reflect the agreement between two BGP
   speakers about their relationship.  One of the Roles described below
   SHOULD be configured on each eBGP session between ISPs that carry
   IPv4 and(or) IPv6 unicast prefixes.

   Allowed Role values for eBGP sessions between ISPs are:

   o  Provider - sender is a transit provider to neighbor;

   o  Customer - sender is a transit customer of neighbor;

   o  RS - sender is a Route Server, usually at an Internet exchange
      point (IX);

   o  RS-client - sender is client of an RS;

   o  Peer - sender and neighbor are peers.

   Since BGP Role reflects the relationship between two BGP speakers, it
   could also be used for other purposes besides route leak mitigation.

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4.  BGP Role Capability

   The TLV (type, length, value) of the BGP Role capability are:

   o  Type - <TBD1>;

   o  Length - 1 (byte);

   o  Value - integer corresponding to speaker's BGP Role (see Table 1).

                      | Value | Role name           |
                      |   0   | Sender is Provider  |
                      |   1   | Sender is RS        |
                      |   2   | Sender is RS-client |
                      |   3   | Sender is Customer  |
                      |   4   | Sender is Peer      |

                    Table 1: Predefined BGP Role Values

5.  Role correctness

   Section 3 described how BGP Role encodes the relationship between two
   eBGP speakers.  But the mere presence of BGP Role doesn't
   automatically guarantee role agreement between two BGP peers.

   To enforce correctness, the BGP Role check is applied with a set of
   constraints on how speakers' BGP Roles MUST correspond.  Of course,
   each speaker MUST announce and accept the BGP Role capability in the
   BGP OPEN message exchange.

   If a speaker receives a BGP Role capability, it MUST check the value
   of the received capability (i.e., the sender's role) with its own BGP
   Role.  The allowed pairings are as follows:

                    | Sender's Role | Receiver's Role |
                    | Provider      | Customer        |
                    | Customer      | Provider        |
                    | RS            | RS-client       |
                    | RS-client     | RS              |
                    | Peer          | Peer            |

                Table 2: Allowed Pairs of Role Capabilities

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   If the role of the receiving speaker for the eBGP session in
   consideration is included in Table 1 and the observed Role pair is
   not in the above table, then the receiving speaker MUST reject the
   eBGP connection, send a Role Mismatch Notification (code 2, subcode
   <TBD2>), and also send a Connection Rejected Notification [RFC4486]
   (Notification with error code 6, subcode 5).

5.1.  Strict mode

   A new BGP configuration option "strict mode" is defined with values
   of true or false.  If set to true, then the speaker MUST refuse to
   establish a BGP session with a neighbor which does not announce the
   BGP Role capability in the OPEN message.  If a speaker rejects a
   connection, it MUST send a send a Role Mismatch Notification (code 2,
   subcode <TBD2>), and also send a Connection Rejected Notification
   [RFC4486] (Notification with error code 6, subcode 5).  By default,
   strict mode SHOULD be set to false for backward compatibility with
   BGP speakers that do not yet support this mechanism.

6.  BGP Only to Customer (OTC) Attribute

   Newly defined here, the Only to Customer (OTC) is an optional, 4
   bytes long, transitive BGP Path attribute with the Type Code <TBD3>.
   The purpose of this attribute is to guarantee that once a route is
   sent to customer, peer, or RS-client, it will subsequently go only to
   customers.  The value of OTC is an AS number determined by policy as
   described below.  The semantics and usage of the OTC attribute are
   made clear by the ingress and egress policies described below.

   The following ingress policy applies to the OTC attribute:

   1.  If a route with OTC attribute is received from a Customer or RS-
       client, then it is a route leak and MUST be rejected.

   2.  If a route with OTC attribute is received from a Peer and its
       value is not equal to the sending neighbor's Autonomous System
       (AS) number, then it is a route leak and MUST be rejected.

   3.  If a route is received from a Provider, Peer, or RS and the OTC
       attribute is not present, then it MUST be added with value equal
       to the sending neighbor's AS number.

   The egress policy MUST be:

   1.  A route with the OTC attribute set MUST NOT be sent to Providers,
       Peers, or RS(s).

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   2.  If route is sent to a Customer or Peer, or an RS-client (when the
       sender is an RS) and the OTC attribute is not present, then it
       MUST be added with value equal to AS number of the sender.

   Once the OTC attribute has been set, it MUST be preserved unchanged.

7.  Enforcement

   Having the relationship unequivocally agreed between the two peers in
   BGP OPEN is critical; BGP implementations MUST enforce the
   relationship/role establishment rules (see Section 5) in order to
   ameliorate operator policy configuration errors (if any).

   Similarly, the application of that relationship on prefix propagation
   using OTC MUST be enforced by the BGP implementations, and not
   exposed to user misconfiguration.

   As opposed to communities, BGP attributes may not be generally
   modified or stripped by the operator; BGP router implementations
   enforce such treatment.  This is the desired property for the OTC
   marking.  Hence, this document specifies OTC as an attribute.

8.  Additional Considerations

   There are peering relationships that are 'complex', i.e., both
   parties are intentionally sending prefixes received from each other
   to their non-transit peers and/or transit providers.  If multiple BGP
   peerings can segregate the 'complex' parts of the relationship, the
   complex peering roles can be segregated into different normal BGP
   sessions, and BGP Roles MUST be used on each of the resulting normal
   (non-complex) BGP sessions.

   No Roles SHOULD be configured on a 'complex' BGP session (assuming it
   is not segregated) and in that case, OTC MUST be set by configuration
   on a per-prefix basis.  However, there are no built-in measures to
   check correctness of OTC use if BGP Role is not configured.

   The incorrect setting of BGP Roles and/or OTC attributes may affect
   prefix propagation.  Further, this document doesn't specify any
   special handling of incorrect/private ASNs in OTC attribute; such
   errors should not happen with proper configuration.

   As the BGP Role reflects the peering relationship between neighbors,
   it might have other uses beyond the route leak solution discussed so
   far.  For example, BGP Role might affect route priority, or be used
   to distinguish borders of a network if a network consists of multiple
   ASs.  Though such uses may be worthwhile, they are not the goal of

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   this document.  Note that such uses would require local policy

   The use of BGP Roles are specified for unicast IPv4 and IPv6 address
   families.  While BGP roles can be configured on other address
   families its applicability for these cases is out of scope of this

   As BGP role configuration results in automatic creation of inbound/
   outbound filters, existence of roles should be treated as existence
   of Import and Export policy [RFC8212].

9.  IANA Considerations

   This document defines a new Capability Codes option [to be removed
   upon publication:
   capability-codes.xhtml ] [RFC5492], named "BGP Role" with an assigned
   value <TBD1>.  The length of this capability is 1.

   The BGP Role capability includes a Value field, for which IANA is
   requested to create and maintain a new sub-registry called "BGP Role
   Value".  Assignments consist of Value and corresponding Role name.
   Initially this registry is to be populated with the data contained in
   Table 1 found in Section 4.  Future assignments may be made by a
   Standard Action procedure [RFC8126].  The allocation policy for new
   entries up to and including value 127 is "Expert Review" [RFC8126].
   The allocation policy for values 128 through 251 is "First Come First
   Served".  The values from 252 through 255 are for "Experimental Use".

   This document defines a new subcode, "Role Mismatch" with an assigned
   value <TBD2> in the OPEN Message Error subcodes registry [to be
   removed upon publication:
   parameters/bgp-parameters.xhtml#bgp-parameters-6] [RFC4271].

   This document defines a new optional, transitive BGP Path Attributes
   option, named "Only to Customer (OTC)" with an assigned value <TBD3>
   [To be removed upon publication:
   parameters/bgp-parameters.xhtml#bgp-parameters-2] [RFC4271].  The
   length of this attribute is four bytes.

10.  Security Considerations

   This document proposes a mechanism for prevention of route leaks that
   are the result of BGP policy misconfiguration.

   A misconfiguration in OTC setup may affect prefix propagation.  But
   the automation that is provided by BGP roles should make such
   misconfiguration unlikely.

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11.  References

11.1.  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,

   [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,

   [RFC4486]  Chen, E. and V. Gillet, "Subcodes for BGP Cease
              Notification Message", RFC 4486, DOI 10.17487/RFC4486,
              April 2006, <>.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

11.2.  Informative References

   [Gao]      Gao, L. and J. Rexford, "Stable Internet routing without
              global coordination",  IEEE/ACM Transactions on
              Networking, Volume 9, Issue 6, pp 689-692, DOI
              10.1109/90.974523, December 2001,

              Sriram, K. and A. Azimov, "Methods for Detection and
              Mitigation of BGP Route Leaks", draft-ietf-grow-route-
              leak-detection-mitigation-04 (work in progress), October

   [RFC7908]  Sriram, K., Montgomery, D., McPherson, D., Osterweil, E.,
              and B. Dickson, "Problem Definition and Classification of
              BGP Route Leaks", RFC 7908, DOI 10.17487/RFC7908, June
              2016, <>.

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   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [RFC8212]  Mauch, J., Snijders, J., and G. Hankins, "Default External
              BGP (EBGP) Route Propagation Behavior without Policies",
              RFC 8212, DOI 10.17487/RFC8212, July 2017,

              Streibelt, F., Lichtblau, F., Beverly, R., Feldmann, A.,
              Cristel, C., Smaragdakis, G., and R. Bush, "BGP
              Communities: Even more Worms in the Routing Can",


   The authors wish to thank Andrei Robachevsky, Daniel Ginsburg, Jeff
   Haas, Ruediger Volk, Pavel Lunin, Gyan Mishra, Ignas Bagdonas, Sue
   Hares, and John Scudder for comments, suggestions, and critique.


   Brian Dickson

   Doug Montgomery
   USA National Institute of Standards and Technology

Authors' Addresses

   Alexander Azimov
   Qrator Labs & Yandex
   Ulitsa Lva Tolstogo 16
   Moscow  119021
   Russian Federation


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   Eugene Bogomazov
   Qrator Labs
   1-y Magistralnyy tupik 5A
   Moscow  123290
   Russian Federation


   Randy Bush
   Internet Initiative Japan & Arrcus, Inc.
   5147 Crystal Springs
   Bainbridge Island, Washington  98110
   United States of America


   Keyur Patel
   2077 Gateway Place, Suite #400
   San Jose, CA  95119


   Kotikalapudi Sriram
   USA National Institute of Standards and Technology
   100 Bureau Drive
   Gaithersburg, MD  20899
   United States of America


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