Network Working Group                                          A. Azimov
Internet-Draft                                              E. Bogomazov
Intended status: Standards Track                             Qrator Labs
Expires: April 25, 2019                                          R. Bush
                                               Internet Initiative Japan
                                                                K. Patel
                                                            Arrcus, Inc.
                                                             J. Snijders
                                                        October 22, 2018

   Verification of AS_PATH Using the Resource Certificate Public Key
      Infrastructure and Autonomous System Provider Authorization


   This document defines the semantics of an Autonomous System Provider
   Authorization object in the Resource Public Key Infrastructure to
   verify the AS_PATH attribute of routes advertised in the Border
   Gateway Protocol.

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
   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 April 25, 2019.

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Copyright Notice

   Copyright (c) 2018 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Anomaly Propagation . . . . . . . . . . . . . . . . . . . . .   3
   3.  Autonomous System Provider Authorization  . . . . . . . . . .   4
   4.  Customer-Provider Verification Procedure  . . . . . . . . . .   4
   5.  AS_PATH Verification  . . . . . . . . . . . . . . . . . . . .   5
   6.  Disavowal of Provider Authorizaion  . . . . . . . . . . . . .   6
   7.  Siblings (Complex Relations)  . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Border Gateway Protocol (BGP) was designed with no mechanisms to
   validate BGP attributes.  Two consequences are BGP Hijacks and BGP
   Route Leaks [RFC7908].  BGP extensions are able to partially solve
   these problems.  For example, ROA-based Origin Validation [RFC6483]
   can be used to detect and filter accidental mis-originations, and
   [I-D.ymbk-idr-bgp-eotr-policy] can be used to detect accidental route
   leaks.  While these upgrades to BGP are quite useful, they still rely
   on transitive BGP attributes, i.e. AS_PATH, that can be manipulated
   by attackers.

   BGPSec [RFC8205] was designed to solve the problem of AS_PATH
   validation.  Unfortunately, strict cryptographic validation brought
   unaffordable computational overhead for BGP routers.  BGPSec also
   proved to be vulnerable to downgrade attacks that can nullify all the
   work of AS_PATH signing.  As a result, to abuse the AS_PATH or any

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   other signed transit attribute, an attacker merely needs to downgrade
   to 'old' BGP-4.

   An alternative approach was introduced with soBGP
   [I-D.white-sobgp-architecture].  Instead of strong cryptographic
   AS_PATH validation, it was suggested to create an AS_PATH security
   function based on a shared database of ASN adjacencies.  While such
   an approach has reasonable computational cost, the two side
   adjacencies don't provide a way to automate anomaly detection without
   high adoption rate - an attacker can easily up a one-way adjacency.
   SO-BGP suggested sharing data about adjacencies using additional BGP
   messages, which is recursively complex thus significantly increasing
   adoption complexity.  In addition, the general goal to verify all
   AS_PATHs was not achievable given the indirect adjacencies at
   internet exchange points.

   Instead of the general goal of checking AS_PATH correctness, this
   document focuses on solving real-world operational problems -
   automatic detection of malicious hijacks and route leaks.  To achieve
   this goal a new AS_PATH verification procedure is defined which is
   able to automatically detect invalid (malformed) AS_PATHs in
   announcements that are received from customers and peers.  This
   procedure uses a shared signed database of customer-to-provider
   relationships that is built using a new RPKI object - Autonomous
   System Provider Authorization (ASPA).  This technique provides
   benefits for the participants even in a state of early adoption.

2.  Anomaly Propagation

   Both route leaks and hijacks have similar effects on ISP operations -
   they redirect traffic, resulting in increased latency, packet loss,
   or possible MiTM attacks.  But the level of risk depends
   significantly on the propagation of these BGP anomalies.  For
   example, a hijack that is propagated only to customers may
   concentrate traffic in a particular ISP's customer cone; while if the
   anomaly is propagated through peers, upstreams, or reaches Tier-1
   networks, thus distributing globally, traffic may be redirected at
   the level of entire countries and/or global providers.

   The ability to constrain propagation of BGP anomalies to upstreams
   and peers, without requiring support from the source of the anomaly
   (which is critical if source has malicious intent), should
   significantly improve the security of inter-domain routing and solve
   the majority of problems.

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3.  Autonomous System Provider Authorization

   As described in [RFC6480], the RPKI is based on a hierarchy of
   resource certificates that are aligned to the Internet Number
   Resource allocation structure.  Resource certificates are X.509
   certificates that conform to the PKIX profile [RFC5280], and to the
   extensions for IP addresses and AS identifiers [RFC3779].  A resource
   certificate is a binding by an issuer of IP address blocks and
   Autonomous System (AS) numbers to the subject of a certificate,
   identified by the unique association of the subject's private key
   with the public key contained in the resource certificate.  The RPKI
   is structured so that each current resource certificate matches a
   current resource allocation or assignment.

   ASPAs are digitally signed objects that bind a selected AFI Provider
   AS number to a Customer AS number (in terms of BGP announcements not
   business), and are signed by the holder of the Customer AS.  An ASPA
   attests that a Customer AS holder (CAS) has authorized a particular
   Provider AS (PAS) to propagate the Customer's IPv4/IPv6 announcements
   onward, e.g. to the Provider's upstream providers or peers.  The ASPA
   record profile is described in [I-D.azimov-sidrops-aspa-profile].

4.  Customer-Provider Verification Procedure

   This section describes an abstract procedure that checks that pair of
   ASNs (AS1, AS2) is included in the set of signed ASPAs.  The
   semantics of its usa are defined in next section.  The procedure
   takes (AS1, AS2, ROUTE_AFI) as input parameters and returns three
   types of results: "valid", "invalid" and "unknown".

   A relying party (RP) must have access to a local cache of the
   complete set of cryptographically valid ASPAs when performing
   customer-provider verification procedure.

   1.  Retrieve all cryptographically valid ASPAs in a selected AFI with
       a customer value of AS1.  This selection forms the set of
       "candidate ASPAs."

   2.  If the set of candidate ASPAs is empty, then the procedure exits
       with an outcome of "unknown."

   3.  If there is at least one candidate ASPA where the provider field
       is AS2, then the procedure exits with an outcome of "valid."

   4.  Otherwise, the procedure exits with an outcome of "invalid."

   Since an AS1 may have different set providers in different AFI, it
   should also have different set of corresponding ASPAs.  In this case,

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   the output of this procedure with input (AS1, AS2, ROUTE_AFI) may
   have different output for different ROUTE_AFI values.

5.  AS_PATH Verification

   The AS_PATH attribute identifies the autonomous systems through which
   an UPDATE message has passed.  AS_PATH may contain two types of
   components: ordered AS_SEQes and unordered AS_SETs, as defined in

   The value of each AS_SEQ component can be described as set of pairs
   {(AS(I), prepend(I)), (AS(I-1), prepend(I-1))...}.  In this case, the
   sequence {AS(I), AS(I-1),...} represents different ASNs, that packet
   should pass towards the destination.  When a route is received from a
   customer or a literal peer, each pair (AS(I-1), AS(I)) MUST belong to
   customer-provider or sibling relationship.  If there are other types
   of relationships, it means that the route was leaked or the AS_PATH
   attribute was malformed.  The goal of the above procedure is to check
   the correctness of this statement.

   For 32-bit AS number compatible BGP speakers, if a route from
   ROUTE_AFI address family is received from a customer or peer, its
   AS_PATH MUST be verified as follows:

   1.  If the closest AS in the AS_PATH is not the receiver's neighbor
       ASN then procedure halts with the outcome "invalid";

   2.  If in one of AS_SEQ segments there is a pair (AS(I-1), AS(I)),
       and customer-provider verification procedure (Section 4) with
       parameters (AS(I-1), AS(I), ROUTE_AFI) returns "invalid" then the
       procedure also halts with the outcome "invalid";

   3.  If the AS_PATH has at least one AS_SET segment then procedure
       halts with the outcome "unverifiable";

   4.  Otherwise, the procedure halts with an outcome of "valid".

   For BGP speakers that are not 32-bit AS compatible, the above
   procedure is slightly different.  In point 2 if at least one AS(I-1),
   AS(I) is equal to AS_TRANS(23456), the corresponding pair must be
   passed without check using the customer-provider verification

   If the output of the AS_PATH verification procedure is "invalid" the
   LOCAL_PREF SHOULD be set to 0 or the route MAY be dropped.  If an
   "invalid" route has no alternative route(s) and it is propagated to
   other ASes despite the above, it MUST be marked with the
   GRACEFUL_SHUTDOWN community to avoid possible stable oscillations,

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   when an unchecked route received from a provider becomes preferred
   over an invalid route received from a customer.  This also allows
   customers to detect malformed routes received from upstream

   If the output of the AS_PATH verification procedure is 'unverifiable'
   it means that AS_PATH can't be fully verified.  Such routes should be
   treated with caution and SHOULD be processed the same way as
   "invalid" routes.  This policy goes with full correspondence to

   The above AS_PATH verification procedure is able to check routes
   received from customers and peers.  The ASPA mechanism combined with
   BGP Roles [I-D.ietf-idr-bgp-open-policy] and ROA-based Origin
   Validation [RFC6483] provide a fully automated solution to detect and
   filter hijacks and route leaks, including malicious ones.

6.  Disavowal of Provider Authorizaion

   An ASPA is a positive attestation that an AS holder has authorized
   its provider to redistribute received routes to the provider's
   providers and peers.  This does not preclude the provider AS from
   redistribution to its other customers.  By creating an ASPA where the
   provider AS is 0, the customer indicates that no provider should
   further announce its routes.  Specifically, AS 0 is reserved to
   identify provider-free networks, Internet exchange meshes, etc.

   An ASPA with a provider AS of 0 is a statement by the customer AS
   that the its routes should not be received by any relying party AS
   from any of its customers or peers.

   By convention, an ASPA with a provider AS of 0 should be the only
   ASPA issued by a given AS holder; although this is not a strict
   requirement.  A provider 0 ASPA may coexist with ASPAs that have
   different provider AS values; though in such cases, the presence or
   absence of the provider AS 0 ASPA does not alter the AS_PATH
   verification procedure.

7.  Siblings (Complex Relations)

   There are peering relationships which can not be described as
   strictly simple peer-peer or customer-provider; e.g. when both
   parties are intentionally sending prefixes received from each other
   to their peers and/or upstreams.

   In this case, two symmetric ASPAs records {(AS1, AS2), (AS2, AS1)}
   must be created by AS1 and AS2 respectively.

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8.  Security Considerations

   ASPA issuers should be aware of the verification implication in
   issuing an ASPA - an ASPA implicitly invalidates all routes passed to
   upstream providers other than the provider ASs listed in the
   collection of ASPAs.  It is the Customer AS's duty to maintain a
   correct set of ASPAs.

   While the ASPA provides a check of an AS_PATH for routes received
   from customers and peers, it doesn't provide full support for routes
   that are received from upstream providers.  So, this mechanism
   guarantees detection of both malicious and accidental route leaks and
   provides partial support for detection of malicious hijacks: upstream
   transit ISPs will still be able to send hijacked prefixes with
   malformed AS_PATHs to their customers.

9.  Acknowledgments

   The authors wish to thank authors of [RFC6483] since its text was
   used as an example while writing this document.

10.  References

10.1.  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,

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

10.2.  Informative References

              Azimov, A., Uskov, E., Bush, R., Patel, K., Snijders, J.,
              and R. Housley, "A Profile for Autonomous System Provider
              Authorization", draft-azimov-sidrops-aspa-profile-00 (work
              in progress), June 2018.

              Azimov, A., Bogomazov, E., Bush, R., Patel, K., and K.
              Sriram, "Route Leak Prevention using Roles in Update and
              Open messages", draft-ietf-idr-bgp-open-policy-02 (work in
              progress), January 2018.

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              Kumari, W. and K. Sriram, "Deprecation of AS_SET and
              AS_CONFED_SET in BGP", draft-kumari-deprecate-as-set-
              confed-set-12 (work in progress), July 2018.

              White, R., "Architecture and Deployment Considerations for
              Secure Origin BGP (soBGP)", draft-white-sobgp-
              architecture-02 (work in progress), June 2006.

              Azimov, A., Bogomazov, E., Bush, R., and K. Patel, "Route
              Leak Detection and Filtering using Roles in Update and
              Open messages", draft-ymbk-idr-bgp-eotr-policy-02 (work in
              progress), March 2018.

   [RFC3779]  Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
              Addresses and AS Identifiers", RFC 3779,
              DOI 10.17487/RFC3779, June 2004,

   [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,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <>.

   [RFC6483]  Huston, G. and G. Michaelson, "Validation of Route
              Origination Using the Resource Certificate Public Key
              Infrastructure (PKI) and Route Origin Authorizations
              (ROAs)", RFC 6483, DOI 10.17487/RFC6483, February 2012,

   [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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   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <>.

Authors' Addresses

   Alexander Azimov
   Qrator Labs


   Eugene Bogomazov
   Qrator Labs


   Randy Bush
   Internet Initiative Japan


   Keyur Patel
   Arrcus, Inc.


   Job Snijders
   NTT Communications
   Theodorus Majofskistraat 100
   Amsterdam  1065 SZ
   The Netherlands


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