Route Leak Prevention and Detection using Roles in UPDATE and OPEN Messages
draft-ietf-idr-bgp-open-policy-16

Network Working Group                                          A. Azimov
Internet-Draft                                      Qrator Labs & Yandex
Intended status: Standards Track                            E. Bogomazov
Expires: February 11, 2022                                   Qrator Labs
                                                                 R. Bush
                                Internet Initiative Japan & Arrcus, Inc.
                                                                K. Patel
                                                                  Arrcus
                                                               K. Sriram
                                                                USA NIST
                                                         August 10, 2021

   Route Leak Prevention and Detection using Roles in UPDATE and OPEN
                                Messages
                   draft-ietf-idr-bgp-open-policy-16

Abstract

   Route leaks are the propagation of BGP prefixes that violate
   assumptions of BGP topology relationships, e.g., passing a route
   learned from one lateral peer to another lateral peer or a transit
   provider and 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 the BGP OPEN message to
   establish an agreement of the relationship on each eBGP session
   between autonomous systems in order to enforce appropriate
   configuration on both sides.  Propagated routes are then marked
   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Peering Relationships . . . . . . . . . . . . . . . . . . . .   4
   3.  BGP Role  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  BGP Role Capability . . . . . . . . . . . . . . . . . . .   5
     3.2.  Role Correctness  . . . . . . . . . . . . . . . . . . . .   5
   4.  BGP Only to Customer (OTC) Attribute  . . . . . . . . . . . .   6
   5.  Additional Considerations . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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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 [RFC7908].  These are usually the result of
   misconfigured or absent BGP route filtering or lack of coordination
   between autonomous systems (ASes).

   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 the
   BGP OPEN message to establish an agreement of the relationship on
   each eBGP session between autonomous systems in order to enforce
   appropriate configuration on both sides.  Propagated routes are then
   marked according to the agreed relationship, allowing both prevention
   and detection of route leaks.

   This document provides configuration automation using BGP Roles,
   which are negotiated using a BGP Role Capability in the OPEN message
   [RFC5492].  An eBGP speaker may require the use of this capability
   and confirmation of BGP Role with a neighbor for the BGP OPEN to
   succeed.

   An optional, transitive BGP Path Attribute, called Only to Customer
   (OTC), is specified in Section 4.  It prevents ASes from creating
   leaks, and detects leaks created by the ASes in the middle of an AS
   path.  The main focus/applicability is the Internet (IPv4 and IPv6
   unicast route advertisements).

1.1.  Terminology

   In the rest of this document, the term "Peer" is used to refer to a
   "lateral peer" for simplicity.  Also, the terms Provider and Customer
   are used to refer to a transit provider and a transit customer,
   respectively.  Further, the terms RS and RS-Client are used to refer
   to a Route Server and its client, respectively.

   The terms "local AS" and "remote AS" are used to refer to the two
   ends of an eBGP session.  The "local AS" is the AS where the protocol
   action being described is to be performed, and "remote AS" is the AS
   at the other end of the eBGP session in consideration.

   The use of the term "route is ineligible" in this document has the
   same meaning as in [RFC4271], i.e., "route is ineligible to be
   installed in Loc-RIB and will be excluded from the next phase of
   route selection."

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2.  Peering Relationships

   The terms defined and used in this document (see below) do not
   necessarily represent 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 BGP Roles for eBGP peering and the
   corresponding rules for route propagation:

   Provider:  MAY propagate any available route to a Customer.

   Customer:  MAY propagate any route learned from a Customer, or
      locally originated, to a Provider.  All other routes MUST NOT be
      propagated.

   Route Server (RS):  MAY propagate any available route to a Route
      Server Client (RS-Client).

   RS-Client:  MAY propagate any route learned from a Customer, or
      locally originated, to an RS.  All other routes MUST NOT be
      propagated.

   Peer:  MAY propagate any route learned from a Customer, or locally
      originated, to a Peer.  All other routes MUST NOT be propagated.

   A 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.  Automatic
   enforcement of these rules should significantly reduce route leaks
   that may otherwise occur due to manual configuration mistakes.

3.  BGP Role

   The BGP Role characterizes the relationship between the eBGP speakers
   forming a session.  BGP Role is configured on a per-session basis.
   An eBGP speaker SHOULD configure the BGP Role locally based on the
   local AS's knowledge of its Role.  The only exception is when the
   eBGP connection is complex (see Section 5).  BGP Roles are mutually
   confirmed using the BGP Role Capability (described in Section 3.1) on
   each eBGP session between autonomous systems (ASes).  One of the
   Roles described below SHOULD be configured at the local AS for each
   eBGP session with a neighbor (remote AS) (see definitions in
   Section 1.1).

   Allowed Roles for eBGP sessions are:

   o  Provider - the local AS is a transit Provider of the remote AS;

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   o  Customer - the local AS is a transit Customer of the remote AS;

   o  RS - the local AS is a Route Server (usually at an Internet
      exchange point) and the remote AS is its RS-Client;

   o  RS-Client - the local AS is a client of an RS and the RS is the
      remote AS;

   o  Peer - the local and remote ASes are Peers (i.e., have a lateral
      peering relationship).

3.1.  BGP Role Capability

   The BGP Role Capability is defined as follows:

   o  Code - 9

   o  Length - 1 (octet)

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

                 +-------+------------------------------+
                 | Value | Role name (for the local AS) |
                 +-------+------------------------------+
                 |   0   | Provider                     |
                 |   1   | RS                           |
                 |   2   | RS-Client                    |
                 |   3   | Customer                     |
                 |   4   | Peer (Lateral Peer)          |
                 | 5-255 | Unassigned                   |
                 +-------+------------------------------+

                    Table 1: Predefined BGP Role Values

   If BGP Role is locally configured, the eBGP speaker MUST advertise
   BGP Role Capability in the BGP OPEN message.  An eBGP speaker MUST
   NOT advertise multiple versions of the BGP Role Capability.

3.2.  Role Correctness

   Section 3.1 described how BGP Role encodes the relationship on each
   eBGP session between autonomous systems (ASes).

   The mere receipt of BGP Role Capability does not automatically
   guarantee the Role agreement between two eBGP neighbors.  If the BGP
   Role Capability is advertised, and one is also received from the
   peer, the roles MUST correspond to the relationships in Table 2.  If

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   the roles do not correspond, the BGP speaker MUST reject the
   connection using the Role Mismatch Notification (code 2, subcode 8).

                    +---------------+----------------+
                    | Local AS Role | Remote AS Role |
                    +---------------+----------------+
                    | Provider      | Customer       |
                    | Customer      | Provider       |
                    | RS            | RS-Client      |
                    | RS-Client     | RS             |
                    | Peer          | Peer           |
                    +---------------+----------------+

                Table 2: Allowed Pairs of Role Capabilities

   If the BGP Role Capability is sent, but one is not received, then the
   connection MAY be rejected using the Role Mismatch Notification (code
   2, subcode 8); this mode of operation is called the "strict mode".
   For backward compatibility, if the BGP speaker does not receive the
   capability from its peer, it SHOULD ignore the absence of BGP Role
   Capability and proceed with session establishment; this SHOULD be the
   default non-strict mode of operation.  In this case, the locally
   configured BGP Role is used for the procedures described in
   Section 4.

   If an eBGP speaker receives multiple but identical BGP Role
   Capabilities with the same value in each, then the speaker MUST
   consider it to be a single BGP Role Capability and proceed [RFC5492].
   If multiple BGP Role Capabilities are received and not all of them
   have the same value, then the BGP speaker MUST reject the connection
   using the Role Mismatch Notification (code 2, subcode 8).

   The BGP Role value for the local AS is used in the route leak
   prevention and detection procedures described in Section 4.

4.  BGP Only to Customer (OTC) Attribute

   The Only to Customer (OTC) Attribute is an optional transitive path
   attribute with Attribute Type Code 35 and a length of 4 octets.  The
   purpose of this attribute is to guarantee that once a route is sent
   to a Customer, Peer, or RS-Client, it will subsequently go only to
   Customers.  The attribute value is an AS number determined by the
   policy described below.

   The following ingress policy applies to the processing of the OTC
   Attribute:

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   1.  If a route with the OTC Attribute is received from a Customer or
       RS-Client, then it is a route leak and MUST be considered
       ineligible (see Section 1.1).

   2.  If a route with the OTC Attribute is received from a Peer and at
       least one of the OTC Attributes has a value that is not equal to
       the remote (i.e., Peer's) AS number, then it is a route leak and
       MUST be considered ineligible.

   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 a value
       equal to the AS number of the remote AS.

   The following egress policy applies to the processing of the OTC
   Attribute:

   1.  If a route is to be advertised to a Customer, Peer, or RS-Client
       (when the sender is an RS), and the OTC Attribute is not present,
       then an OTC Attribute MUST be added with a value equal to the AS
       number of the local AS.

   2.  If a route already contains the OTC Attribute, it MUST NOT be
       propagated to Providers, Peers, or RS(s).

   The described policies provide both leak prevention for the local AS
   and leak detection and mitigation multiple hops away.  In the case of
   prevention at the local AS, the presence of an OTC Attribute
   indicates to the egress router that the route was learned from a
   Peer, Provider, or RS, and it can be advertised only to the
   customers.  The same OTC Attribute which is set locally also provides
   a way to detect route leaks by an AS multiple hops away if a route is
   received from a Customer, Peer, or RS-Client.

   The OTC Attribute may be set by the egress policy of remote AS or by
   the ingress policy of local AS.  In both scenarios, the OTC value
   will be the same.  This makes the scheme more robust and benefits
   early adopters.

   If an eBGP speaker receives an UPDATE with an OTC Attribute with a
   length different from 4 octets, then the UPDATE SHALL be considered
   malformed.  If malformed, the UPDATE message SHALL be handled using
   the approach of "treat-as-withdraw" [RFC7606].

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

   Correct implementation of the procedures specified in this document
   is not expected to result in the presence of multiple OTC Attributes
   in an UPDATE.  However, if an eBGP speaker receives multiple OTC

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   Attributes with a route, then the only difference in the processing
   is in Step 2 of the ingress policy.

   The described ingress and egress policies are applicable only for
   unicast IPv4 and IPv6 address families and MUST not affect other
   address families by default.  The operator MUST NOT have the ability
   to modify the policies defined in this section.

5.  Additional Considerations

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

   No Roles SHOULD be configured on a 'complex' eBGP session (assuming
   it is not segregated) and in that case, the OTC Attribute processing
   MUST be done relying on configuration on a per-prefix basis.  Also,
   in this case, the per-prefix peering configuration MUST follow the
   same definitions of peering relations as described in Section 2.
   However, in this case, there are no built-in measures to check
   correctness of the per-prefix peering configuration.

   The incorrect setting of BGP Roles and/or OTC Attributes may affect
   prefix propagation.  Further, this document does not specify any
   special handling of incorrect AS numbers in the OTC Attribute.  Such
   errors should not happen with proper configuration.

6.  IANA Considerations

   IANA has registered a new BGP Capability described in Section 3.1 in
   the "Capability Codes" registry's "IETF Review" range [RFC5492].  The
   description for the new capability is "BGP Role".  IANA has assigned
   the value 9 [to be removed upon publication:
   https://www.iana.org/assignments/capability-codes/capability-
   codes.xhtml].  This document is the reference for the new capability.

   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" in the Capability Codes registry.  Assignments consist of a
   Value and a corresponding Role name.  Initially, this registry is to
   be populated with the data contained in Table 1 found in Section 3.1.
   Future assignments may be made by the "IETF Review" policy as defined
   in [RFC8126].  The registry is as shown in Table 3.

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        +-------+--------------------------------+---------------+
        | Value | Role name (for the local AS)   |   Reference   |
        +-------+--------------------------------+---------------+
        |   0   | Provider                       | This document |
        |   1   | RS                             | This document |
        |   2   | RS-Client                      | This document |
        |   3   | Customer                       | This document |
        |   4   | Peer (i.e., Lateral Peer)      | This document |
        | 5-255 | To be assigned by IETF Review  |
        +-------+--------------------------------+---------------+

                    Table 3: IANA Registry for BGP Role

   IANA has registered a new OPEN Message Error subcode named the "Role
   Mismatch" (see Section 3.2) in the OPEN Message Error subcodes
   registry.  IANA has assigned the value 8 [to be removed upon
   publication: https://www.iana.org/assignments/bgp-parameters/bgp-
   parameters.xhtml#bgp-parameters-6].  This document is the reference
   for the new subcode.

   IANA has also registered a new path attribute named "Only to Customer
   (OTC)" (see Section 4) in the "BGP Path Attributes" registry.  IANA
   has assigned code value 35 [To be removed upon publication:
   http://www.iana.org/assignments/bgp-parameters/bgp-
   parameters.xhtml#bgp-parameters-2].  This document is the reference
   for the new attribute.

7.  Security Considerations

   The security considerations of BGP (as specified in [RFC4271] and
   [RFC4272]) apply.

   This document proposes a mechanism using BGP Role for the prevention
   and detection of route leaks that are the result of BGP policy
   misconfiguration.  A misconfiguration of the BGP Role may affect
   prefix propagation.  For example, if a downstream (i.e., towards a
   Customer) peering link were misconfigured with a Provider or Peer
   role, this will limit the number of prefixes that can be advertised
   in this direction.  On the other hand if an upstream provider were
   misconfigured (by a local AS) with the Customer role, this may result
   in propagating routes that are received from other Providers or
   Peers.  But the BGP Role negotiation and the resulting confirmation
   of Roles make such misconfigurations unlikely.

   Setting the strict mode of operation for BGP Role negotiation as the
   default may result in a situation where the eBGP session will not
   come up after a software update.  Such an implementation of this
   document is strongly discouraged.

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   Removing the OTC Attribute or changing its value can limit the
   opportunity of route leak detection.  Such activity can be done on
   purpose as part of a Man in the Middle (MITM) attack.  For example,
   an AS can remove the OTC Attribute on a received route and then leak
   the route to its transit provider.  Such malicious activity cannot be
   prevented without cryptographically signing the BGP UPDATE [RFC8205]
   or out of band detection [I-D.ietf-sidrops-aspa-verification], but
   such schemes are beyond the scope of this document.

   Adding an OTC Attribute when the route is advertised from Customer to
   Provider will limit the propagation of the route.  Such a route may
   be considered as ineligible by the immediate Provider or its Peers or
   upper layer Providers.  This kind of OTC Attribute addition is
   unlikely to happen on the Provider side because it will limit the
   traffic volume towards its Customer.  On the Customer side, adding an
   OTC Attribute for traffic engineering purposes is also discouraged
   because it will limit route propagation in an unpredictable way.

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>.

   [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>.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <https://www.rfc-editor.org/info/rfc5492>.

   [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>.

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   [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, <https://www.rfc-editor.org/info/rfc7908>.

   [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,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  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,
              <https://ieeexplore.ieee.org/document/974523>.

   [I-D.ietf-sidrops-aspa-verification]
              Azimov, A., Bogomazov, E., Bush, R., Patel, K., and J.
              Snijders, "Verification of AS_PATH Using the Resource
              Certificate Public Key Infrastructure and Autonomous
              System Provider Authorization", draft-ietf-sidrops-aspa-
              verification-07 (work in progress), February 2021.

   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <https://www.rfc-editor.org/info/rfc8205>.

Acknowledgements

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

Contributors

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   Brian Dickson
   Independent
   Email: brian.peter.dickson@gmail.com

   Doug Montgomery
   USA National Institute of Standards and Technology
   Email: dougm@nist.gov

Authors' Addresses

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

   Email: a.e.azimov@gmail.com

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

   Email: eb@qrator.net

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

   Email: randy@psg.com

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

   Email: keyur@arrcus.com

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

   Email: ksriram@nist.gov

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