Network Working Group                                          A. Azimov
Internet-Draft                                              E. Bogomazov
Intended status: Standards Track                             Qrator Labs
Expires: September 10, 2020                                      R. Bush
                                      Internet Initiative Japan & Arrcus
                                                                K. Patel
                                                            Arrcus, Inc.
                                                               K. Sriram
                                                                 US NIST
                                                           March 9, 2020

     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 peer to another peer or to a transit provider,
   passing a route learned from one transit provider to another transit
   provider or to a peer.  Today, approaches to leak prevention rely on
   marking routes by operator configuration, with no check that the
   configuration corresponds to that of the BGP neighbor, or enforcement
   that the two BGP 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 BGP speakers to enforce appropriate configuration on both
   sides.  Propagated routes are then marked with an 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 September 10, 2020.

Copyright Notice

   Copyright (c) 2020 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
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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Peering Relationships . . . . . . . . . . . . . . . . . . . .   3
   3.  BGP Role  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  BGP Role Capability . . . . . . . . . . . . . . . . . . . . .   4
   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 . . . . . . . . . . . . . . . . . . . . .   7
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   8
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     12.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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1.  Introduction

   A BGP route leak occurs when a route is learned from a transit
   provider or peer and then announced to another provider or peer.  See
   [RFC7908].  These are usually the result of misconfigured or absent
   BGP route filtering or lack of coordination between two BGP 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 often vulnerable to misconfiguration and even
   attack [Streibelt].  There is also the vulnerability that the
   community signal may be stripped, accidentally or maliciously.

   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 BGP Path Attribute is specified that SHOULD be automatically
   configured using BGP roles.  This attribute prevents networks from
   creating leaks, and detects leaks created by third parties.

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 BGP peering and
   the corresponding rules for route propagation:

   Provider:  MAY send to a customer all available prefixes.

   Customer:  MAY send to a provider their own prefixes 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 RS Client all available prefixes.

   Route Server Client (RS-client):  MAY send to an RS its own prefixes
      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.

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   Peer:  MAY send to a peer its own prefixes 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 new configuration option that SHOULD be configured on
   each BGP session.  It reflects the real-world agreement between two
   BGP speakers about their relationship.

   Allowed Role values for eBGP sessions 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.

4.  BGP Role Capability

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

   o  Type - <TBD1>;

   o  Length - 1 (octet);

   o  Value - integer corresponding to speaker's BGP Role.

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                      | 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
   BGP 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 with its own BGP Role (if it is set).  The
   allowed pairings are (first a sender's Role, second the receiver's

                      | Sender Role | Receiver Role |
                      | Provider    | Customer      |
                      | Customer    | Provider      |
                      | RS          | RS-Client     |
                      | RS-Client   | RS            |
                      | Peer        | Peer          |

                    Table 2: Allowed Role Capabilities

   If the observed Role pair is not in the above table, then the
   receiving speaker MUST send a Role Mismatch Notification (code 2,
   subcode <TBD2>) and reset the BGP session.

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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 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 byte
   long, transitive BGP Path attribute with the Type Code <TBD3>.  The
   purpose of this attribute is to guarantee that once route is sent to
   customer, peer or RS-client it will go only to customers.  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 neighbor's ASN, 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 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).

   2.  If route is sent to a customer or peer, or an RS-Client 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.

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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
   overcome 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 mis-configuration.

   As opposed to communities, BGP attributes may not be generally
   modified or filtered 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.

   As the BGP Role reflects the peering relationship between neighbors,
   it might have other uses beyond the route leaks 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
   this document.  Note that such uses would require local policy

   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:

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   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 in Table 1.
   Future assignments may be made by a standard action procedure

   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 mis-configuration.

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

11.  Acknowledgments

   The authors wish to thank Douglas Montgomery, Brian Dickson, Andrei
   Robachevsky, and Daniel Ginsburg for their contributions to a variant
   of this work.

12.  References

12.1.  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,

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   [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, <>.

12.2.  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-01 (work in progress), July

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [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, <>.

   [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,

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

Authors' Addresses

   Alexander Azimov
   Qrator Labs


   Eugene Bogomazov
   Qrator Labs


   Randy Bush
   Internet Initiative Japan & Arrcus


   Keyur Patel
   Arrcus, Inc.


   Kotikalapudi Sriram


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