BGPsec Considerations for Autonomous System (AS) Migration
RFC 8206

Internet Engineering Task Force (IETF)                         W. George
Request for Comments: 8206                                       Neustar
Updates: 8205                                                  S. Murphy
Category: Standards Track                                  PARSONS, Inc.
ISSN: 2070-1721                                           September 2017

       BGPsec Considerations for Autonomous System (AS) Migration

Abstract

   This document discusses considerations and methods for supporting and
   securing a common method for Autonomous System (AS) migration within
   the BGPsec protocol.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8206.

Copyright Notice

   Copyright (c) 2017 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
     1.2.  Documentation Note  . . . . . . . . . . . . . . . . . . .   3
   2.  General Scenario  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  RPKI Considerations . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Origin Validation . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Path Validation . . . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Outbound Announcements (PE-->CE)  . . . . . . . . . .   5
       3.2.2.  Inbound Announcements (CE-->PE) . . . . . . . . . . .   6
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Outbound (PE-->CE)  . . . . . . . . . . . . . . . . . . .   8
     5.2.  Inbound (CE-->PE) . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Other Considerations  . . . . . . . . . . . . . . . . . .   9
     5.4.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   A method of managing a BGP Autonomous System Number (ASN) migration
   is described in RFC 7705 [RFC7705].  Since it concerns the handling
   of AS_PATH attributes, it is necessary to ensure that the process and
   features are properly supported in BGPsec [RFC8205] because BGPsec is
   explicitly designed to protect against changes in the BGP AS_PATH,
   whether by choice, by misconfiguration, or by malicious intent.  It
   is critical that the BGPsec protocol framework be able to support
   this operationally necessary tool without creating an unacceptable
   security risk or exploit in the process.

1.1.  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.

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1.2.  Documentation Note

   This document uses ASNs from the range reserved for documentation as
   described in RFC 5398 [RFC5398].  In the examples used here, they are
   intended to represent Globally Unique ASNs, not ASNs reserved for
   private use as documented in Section 10 of RFC 1930 [RFC1930].

2.  General Scenario

   This document assumes that the reader has read and understood the ASN
   migration method discussed in RFC 7705 [RFC7705] including its
   examples (see Section 2 of the referenced document), as they will be
   heavily referenced here.  The use case being discussed in RFC 7705
   [RFC7705] is as follows: For whatever the reason, a provider is in
   the process of merging two or more ASes, where eventually one
   subsumes the other(s).  BGP AS confederations [RFC5065] are not
   enabled between the ASes, but a mechanism is being used to modify
   BGP's default behavior and allow the migrating Provider Edge (PE)
   router to masquerade as the old ASN for the Provider-Edge-to-
   Customer-Edge (PE-CE) eBGP (external BGP) session, or to manipulate
   the AS_PATH, or both.  While BGPsec [RFC8205] does have a method to
   handle standard confederation implementations, it is not applicable
   in this exact case.  This migration requires a slightly different
   solution in BGPsec than for a standard confederation because unlike
   in a confederation, eBGP peers may not be peering with the "correct"
   external ASN, and the forward-signed updates are for a public ASN,
   rather than a private one; so, there is no expectation that the BGP
   speaker would strip the affected signatures before propagating the
   route to its eBGP neighbors.

   In the examples in Section 5.4, AS64510 is being subsumed by AS64500,
   and both ASNs represent a Service Provider (SP) network (see Figures
   1 and 2 in RFC 7705 [RFC7705]).  AS64496 and 64499 represent
   end-customer networks.  References to PE, CE, and P routers mirror
   the diagrams and references in RFC 7705.

3.  RPKI Considerations

   The methods and implementation discussed in RFC 7705 [RFC7705] are
   widely used during network integrations resulting from mergers and
   acquisitions, as well as network redesigns; therefore, it is
   necessary to support this capability on any BGPsec-enabled routers/
   ASNs.  What follows is a discussion of the potential issues to be
   considered regarding how ASN migration and BGPsec [RFC8205]
   validation might interact.

   One of the primary considerations for this document and migration is
   that service providers (SPs) rarely stop after one

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   merger/acquisition/divestiture; they end up accumulating several
   legacy ASNs over time.  Since SPs are using migration methods that
   are transparent to customers and therefore do not require
   coordination with customers, they do not have as much control over
   the length of the transition period as they might with something
   completely under their administrative control (e.g., a key roll).
   Because they are not forcing a simultaneous migration (i.e., both
   ends switch to the new ASN at an agreed-upon time), there is no
   incentive for a given customer to complete the move from the old ASN
   to the new one.  This leaves many SPs with multiple legacy ASNs that
   don't go away very quickly, if at all.  As solutions were being
   proposed for Resource Public Key Infrastructure (RPKI)
   implementations to solve this transition case, the WG carefully
   considered operational complexity and hardware scaling issues
   associated with maintaining multiple legacy ASN keys on routers
   throughout the combined network.  While SPs who choose to remain in
   this transition phase indefinitely invite added risks because of the
   operational complexity and scaling considerations associated with
   maintaining multiple legacy ASN keys on routers throughout the
   combined network, saying "don't do this" is of limited utility as a
   solution.  As a result, this solution attempts to minimize the
   additional complexity during the transition period, on the assumption
   that it will likely be protracted.  Note that while this document
   primarily discusses service provider considerations, it is not solely
   applicable to SPs, as enterprises often migrate between ASNs using
   the same functionality.  What follows is a discussion of origin and
   path validation functions and how they interact with ASN migrations.

3.1.  Origin Validation

   Route Origin Validation as defined by RFC 6480 [RFC6480] does not
   require modification to enable AS migration, as the existing protocol
   and procedure allow for a solution.  In the scenario discussed in RFC
   7705 [RFC7705], AS64510 is being replaced by AS64500.  If there are
   any existing routes originated by AS64510 on the router being moved
   into the new ASN, new Route Origination Authorizations (ROAs) for the
   routes with the new ASN should be generated, and they should be
   treated as new routes to be added to AS64500.  However, we also need
   to consider the situation where one or more other PEs are still in
   AS64510 and are originating one or more routes that may be distinct
   from any that the router under migration is originating.  PE1 (which
   is now a part of AS64500 and instructed to use "Replace Old AS" as
   defined in [RFC7705] to remove AS64510 from the path) needs to be
   able to properly handle routes originated from AS64510.  If the route
   now shows up as originating from AS64500, any downstream peers'
   validation check will fail unless a ROA is *also* available for
   AS64500 as the origin ASN.  In addition to generating a ROA for 65400
   for any prefixes originated by the router being moved, it may be

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   necessary to generate ROAs for 65400 for prefixes that are
   originating on routers still in 65410, since the AS replacement
   function will change the origin AS in some cases.  This means that
   there will be multiple ROAs showing different ASes authorized to
   originate the same prefixes until all routers originating prefixes
   from AS64510 are migrated to AS64500.  Multiple ROAs of this type are
   permissible per Section 3.2 of RFC 6480 [RFC6480] so managing origin
   validation during a migration like this is merely applying the
   defined case where a set of prefixes are originated from more than
   one ASN.  Therefore, for each ROA that authorizes the old ASN (e.g.,
   AS64510) to originate a prefix, a new ROA MUST also be created that
   authorizes the replacing ASN (e.g., AS64500) to originate the same
   prefix.

3.2.  Path Validation

   BGPsec path validation requires that each router in the AS path
   cryptographically sign its update to assert that "every Autonomous
   System (AS) on the path of ASes listed in the UPDATE message has
   explicitly authorized the advertisement of the route to the
   subsequent AS in the path" (see Section 1 of RFC 8205 [RFC8205]).
   Since the referenced AS-migration technique explicitly modifies the
   AS_PATH between two eBGP peers who are not coordinating with one
   another (are not in the same administrative domain), no level of
   trust can be assumed; therefore, it may be difficult to identify
   legitimate manipulation of the AS_PATH for migration activities when
   compared to manipulation due to misconfiguration or malicious intent.

3.2.1.  Outbound Announcements (PE-->CE)

   When PE1 is moved from AS64510 to AS64500, it will be provisioned
   with the appropriate keys for AS64500 to allow it to forward-sign
   routes using AS64500.  However, there is no guidance in the BGPsec
   protocol specification [RFC8205] on whether or not the forward-signed
   ASN value is required to match the configured remote AS to validate
   properly.  That is, if CE1's BGP session is configured as "remote AS
   64510", the presence of "local AS 64510" on PE1 will ensure that
   there is no ASN mismatch on the BGP session itself, but if CE1
   receives updates from its remote neighbor (PE1) forward-signed from
   AS64500, there is no guidance as to whether the BGPsec validator on
   CE1 still considers those valid by default.  Section 6.3 of RFC 4271
   [RFC4271] mentions this match between the ASN of the peer and the
   AS_PATH data, but it is listed as an optional validation, rather than
   a requirement.  We cannot assume that this mismatch will be allowed
   by vendor implementations, so using it as a means to solve this
   migration case is likely to be problematic.

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3.2.2.  Inbound Announcements (CE-->PE)

   Inbound is more complicated, because the CE doesn't know that PE1 has
   changed ASNs, so it is forward-signing all of its routes with
   AS64510, not AS64500.  The BGPsec speaker cannot manipulate previous
   signatures and therefore cannot manipulate the previous AS path
   without causing a mismatch that will invalidate the route.  If the
   updates are simply left intact, the ISP would still need to publish
   and maintain valid and active public keys for AS 64510 if it is to
   appear in the BGPsec_PATH signature so that receivers can validate
   that the BGPsec_PATH signature arrived intact/whole.  However, if the
   updates are left intact, this will cause the AS path length to be
   increased, which is unacceptable as discussed in RFC 7705 [RFC7705].

4.  Requirements

   In order to be deployable, any solution to the described problem
   needs to consider the following requirements, listed in no particular
   order.  BGPsec:

   o  MUST support AS migration for both inbound and outbound route
      announcements (see Sections 3.2.1 and 3.2.2), without reducing
      BGPsec's protections for route path.

   o  MUST NOT require any reconfiguration on the remote eBGP neighbor
      (CE).

   o  SHOULD NOT require global (i.e., network-wide) configuration
      changes to support migration.  The goal is to limit required
      configuration changes to the devices (PEs) being migrated.

   o  MUST NOT lengthen the AS path during migration.

   o  MUST operate within existing trust boundaries, e.g., can't expect
      remote side to accept pCount=0 (see Section 4.2 of RFC 8205
      [RFC8205]) from untrusted/non-confederation neighbor.

5.  Solution

   As noted in Section 4.2 of RFC 8205 [RFC8205], BGPsec already has a
   solution for hiding ASNs where increasing the AS path length is
   undesirable.  So a simple solution would be to retain the keys for
   AS64510 on PE1 and forward-sign towards CE1 with AS64510 and
   pCount=0.  However, this would mean passing a pCount=0 between two
   ASNs that are in different administrative and trust domains such that
   it could represent a significant attack vector to manipulate BGPsec-
   signed paths.  The expectation for legitimate instances of pCount=0
   (to make a route server that is not part of the transit path

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   invisible) is that there is some sort of existing trust relationship
   between the operators of the route server and the downstream peers
   such that the peers could be explicitly configured by policy to
   accept pCount=0 announcements only on the sessions where they are
   expected.  For the same reason that things like "Local AS" [RFC7705]
   are used for ASN migration without end-customer coordination, it is
   unrealistic to assume any sort of coordination between the SP and the
   administrators of CE1 to ensure that they will by policy accept
   pCount=0 signatures during the transition period; therefore, this is
   not a workable solution.

   A better solution presents itself when considering how to handle
   routes coming from the CE toward the PE, where the routes are
   forward-signed to AS64510, but will eventually need to show AS64500
   in the outbound route announcement.  Because both AS64500 and AS64510
   are in the same administrative domain, a signature from AS64510
   forward-signed to AS64500 with pCount=0 would be acceptable as it
   would be within the appropriate trust boundary so that each BGP
   speaker could be explicitly configured to accept pCount=0 where
   appropriate between the two ASNs.  At the very simplest, this could
   potentially be used at the eBGP boundary between the two ASNs during
   migration.  Since the AS_PATH manipulation described above usually
   happens at the PE router on a per-session basis and does not happen
   network-wide simultaneously, it is not generally appropriate to apply
   this AS-hiding technique across all routes exchanged between the two
   ASNs, as it may result in routing loops and other undesirable
   behavior.  Therefore, the most appropriate place to implement this is
   on the local PE that still has eBGP sessions with peers expecting to
   peer with AS64510 (using the transition mechanisms detailed in RFC
   7705 [RFC7705]).  Since that PE has been moved to AS64500, it is not
   possible for it to forward-sign AS64510 with pCount=0 without some
   minor changes to the BGPsec behavior to address this use case.

   AS migration is using AS_PATH and remote AS manipulation to act as if
   a PE under migration exists simultaneously in both ASNs even though
   it is only configured with one global ASN.  This document describes
   applying a similar technique to the BGPsec signatures generated for
   routing updates processed through this migration machinery.  Each
   routing update that is received from or destined to an eBGP neighbor
   that is still using the old ASN (64510) will be signed twice, once
   with the ASN to be hidden and once with the ASN that will remain
   visible.  In essence, we are treating the update as if the PE had an
   internal BGP hop and the update was passed across an eBGP session
   between AS64500 and AS64510, configured to use and accept pCount=0,
   while eliminating the processing and storage overhead of creating an
   actual eBGP session between the two ASNs within the PE router.  This
   will result in a properly secured AS path in the affected route
   updates, because the PE router will be provisioned with valid keys

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   for both AS64500 and AS64510.  An important distinction here is that
   while AS migration under standard BGP4 is manipulating the AS_PATH
   attribute, BGPsec uses an attribute called the "Secure_Path" (see
   Section 3.1 of RFC 8205 [RFC8205]) and BGPsec-capable neighbors do
   not exchange AS_PATH information in their route announcements.
   However, a BGPsec neighbor peering with a non-BGPsec-capable neighbor
   will use the information found in the Secure_Path to reconstruct a
   standard AS_PATH for updates sent to that neighbor.  Unlike in the
   Secure_Path where the ASN to be hidden is still present but ignored
   when considering the AS path (due to pCount=0), when reconstructing
   an AS_PATH for a non-BGPsec neighbor, the pCount=0 ASNs will not
   appear in the AS_PATH at all (see Section 4.4 of RFC 8205 [RFC8205]).
   This document is not changing existing AS_PATH reconstruction
   behavior, merely highlighting it for clarity.

   The procedure to support AS migration in BGPsec is slightly different
   depending on whether the PE under migration is receiving the routes
   from one of its eBGP peers ("inbound" as in Section 3.2.2) or
   destined toward the eBGP peers ("outbound" as in Section 3.2.1).

5.1.  Outbound (PE-->CE)

   When a PE router receives an update destined for an eBGP neighbor
   that is locally configured with AS-migration mechanisms as discussed
   in RFC 7705 [RFC7705], it MUST generate a valid BGPsec signature as
   defined in RFC 8205 [RFC8205] for _both_ configured ASNs.  It MUST
   generate a signature from the new (global) ASN forward-signing to the
   old (local) ASN with pCount=0, and then it MUST generate a forward
   signature from the old (local) ASN to the target eBGP ASN with
   pCount=1 as normal.

5.2.  Inbound (CE-->PE)

   When a PE router receives an update from an eBGP neighbor that is
   locally configured with AS-migration mechanisms (i.e., the opposite
   direction of the previous route flow), it MUST generate a signature
   from the old (local) ASN forward-signing to the new (global) ASN with
   pCount=0.  It is not necessary to generate the second signature from
   the new (global) ASN because the Autonomous System Border Router
   (ASBR) will generate that when it forward-signs towards its eBGP
   peers as defined in normal BGPsec operation.  Note that a signature
   is not normally added when a routing update is sent across an iBGP
   (internal BGP) session.  The requirement to sign updates in iBGP
   represents a change to the normal behavior for this specific
   AS-migration scenario only.

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5.3.  Other Considerations

   In the inbound case discussed in Section 5.2, the PE is adding BGPsec
   attributes to routes received from or destined to an iBGP neighbor
   and using pCount=0 to mask them.  While this is not prohibited by
   BGPsec [RFC8205], BGPsec-capable routers that receive updates from
   BGPsec-enabled iBGP neighbors MUST accept updates with new (properly
   formed) BGPsec attributes, including the presence of pCount=0 on a
   previous signature, or they will interfere with this method.  In a
   similar fashion, any BGPsec-capable route-reflectors in the path of
   these updates MUST reflect them transparently to their BGPsec-capable
   clients.

   In order to secure this set of signatures, the PE router MUST be
   provisioned with valid keys for _both_ configured ASNs (old and new),
   and the key for the old ASN MUST be kept valid until all eBGP
   sessions are migrated to the new ASN.  Downstream neighbors will see
   this as a valid BGPsec path, as they will simply trust that their
   upstream neighbor accepted pCount=0 because it was explicitly
   configured to do so based on a trust relationship and business
   relationship between the upstream and its neighbor (the old and new
   ASNs).

   Additionally, Section 4 of RFC 7705 [RFC7705] discusses methods in
   which AS migrations can be completed for iBGP peers such that a
   session between two routers will be treated as iBGP even if the
   neighbor ASN is not the same ASN on each peer's global configuration.
   As far as BGPsec is concerned, this requires the same procedure as
   when the routers migrating are applying AS-migration mechanisms to
   eBGP peers, but the router functioning as the "ASBR" between old and
   new ASN is different.  In eBGP, the router being migrated has direct
   eBGP sessions to the old ASN and signs from old ASN to new with
   pCount=0 before passing the update along to additional routers in its
   global (new) ASN.  In iBGP, the router being migrated is receiving
   updates (that may have originated either from eBGP neighbors or other
   iBGP neighbors) from its downstream neighbors in the old ASN and MUST
   sign those updates from old ASN to new with pCount=0 before sending
   them on to other peers.

5.4.  Example

   The following example will illustrate the method being used above.
   As with previous examples, PE1 is the router being migrated, AS64510
   is the old ASN, which is being subsumed by AS64500, the ASN to be
   permanently retained.  64505 is another external peer, used to
   demonstrate what the announcements will look like to a third-party
   peer that is not part of the migration.  Some additional notation is
   used to delineate the details of each signature as follows:

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   The origin BGPsec Signature Segment takes the form:
   sig(Target ASN, (pCount,...,Origin ASN), NLRI) key.

   Intermediate BGPsec Signature Segments take the form:
   sig(Target ASN,...,(pCount,...,Signer ASN),...,NLRI) key.

   (pCount,...,ASN) refers to the new Secure_Path Segment added to the
   BGPsec_PATH attribute by the ASN (Origin ASN or Signer ASN).

   "Equivalent AS_PATH" refers to what the AS_PATH would look like if it
   was reconstructed to be sent to a non-BGPsec peer, while the
   Securedpath shows the AS path as represented between BGPsec peers.

   Note: The representation of Signature Segment generation is being
   simplified here somewhat for the sake of brevity; the actual details
   of the signing process are as described in Sections 4.1 and 4.2 of
   [RFC8205].  For example, what is covered by the signature also
   includes Flags, Algorithm Suite Identifier, NLRI length, etc.  Also,
   the key is not carried in the update; instead, the Subject Key
   Identifier (SKI) is carried.

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   Before Merger

                                       64505
                                       |
             ISP B                     ISP A
   CE-1 <--- PE-1 <------------------- PE-2 <--- CE-2
   64496     Old_ASN: 64510   Old_ASN: 64500     64499

   CE-2 to PE-2:  sig(64500, (pCount=1,...,64499), N)K_64499-CE2
                  Equivalent AS_PATH=(64499)
                  Securedpath=(64499)
                  length=sum(pCount)=1

   PE-2 to 64505: sig(64505,...,(pCount=1,...,64500),...,N)K_64500-PE2
                  sig(64500, (pCount=1,...,64499), N)K_64499-CE2
                  Equivalent AS_PATH=(64500,64499)
                  Securedpath=(64500,64499)
                  length=sum(pCount)=2

   PE-2 to PE-1:  sig(64510,...,(pCount=1,...,64500),...,N)K_64500-PE2
                  sig(64500, (pCount=1,...,64499), N)K_64499-CE2
                  Equivalent AS_PATH=(64500,64499)
                  Securedpath=(64500,64499)
                  length=sum(pCount)=2

   PE-1 to CE-1:  sig(64496,...,(pCount=1,...,64510),...,N)K_64510-PE1
                  sig(64510,...,(pCount=1,...,64500),...,N)K_64500-PE2
                  sig(64500, (pCount=1,...,64499), N)K_64499-CE2
                  Equivalent AS_PATH= (64510,64500,64499)
                  Securedpath=(64510,64500,64499)
                  length=sum(pCount)=3

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   Migrating, route flow outbound PE-1 to CE-1

                                     64505
                                     |
           ISP A'                    ISP A'
 CE-1 <--- PE-1 <------------------- PE-2 <--- CE-2
 64496     Old_ASN: 64510   Old_ASN: 64500     64499
           New_ASN: 64500   New_ASN: 64500

 CE-2 to PE-2:  sig(64500, (pCount=1,...,64499), N)K_64499-CE2
                Equivalent AS_PATH=(64499)
                Securedpath=(64499)
                length=sum(pCount)=1

 PE-2 to 64505: sig(64505,...,(pCount=1,...,64500),...,N)K_64500-PE2
                sig(64500, (pCount=1,...,64499), N)K_64499-CE2
                Equivalent AS_PATH=(64500,64499)
                Securedpath=(64500,64499)
                length=sum(pCount)=2

 PE-2 to PE-1:  sig(64500, (pCount=1,...,64499), N)K_64499-CE2
                Equivalent AS_PATH=(64499)
                Securedpath=(64499)
                length=sum(pCount)=1
 #PE-2 sends to PE-1 (in iBGP) the exact same update
 #as it received from AS64499.

 PE-1 to CE-1:  sig(64496,...,(pCount=1,...,64510),...,N)K_64510-PE1
                sig(64510,...,(pCount=0,...,64500),...,N)K_64500-PE2 (*)
                sig(64500, (pCount=1,...,64499), N)K_64499-CE2
                Equivalent AS_PATH=(64510,64499)
                Securedpath=(64510, 64500 (pCount=0),64499)
                length=sum(pCount)=2 (length is NOT 3)
 #PE-1 adds the Secure_Path Segment in (*) acting as AS64500
 #PE-1 accepts (*) with pCount=0 acting as AS64510,
 #as it would if it received (*) from an eBGP peer

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   Migrating, route flow inbound CE-1 to PE-1

                                    64505
                                    |
          ISP A'                    ISP A'
CE-1 ---> PE-1 -------------------> PE-2 ---> CE-2
64496     Old_ASN: 64510   Old_ASN: 64500     64499
          New_ASN: 64500   New_ASN: 64500

CE-1 to PE-1:  sig(64510, (pCount=1,...,64496), N)K_64496-CE1
               Equivalent AS_PATH=(64496)
               Securedpath=(64496)
               length=sum(pCount)=1

PE-1 to PE-2:  sig(64500,...,(pCount=0,...,64510),...,N)K_64510-PE1 (**)
               sig(64510, (pCount=1,...,64496), N)K_64496-CE1
               Equivalent AS_PATH=(64496)
               Securedpath=(64510 (pCount=0),64496)
               length=sum(pCount)=1 (length is NOT 2)
#PE-1 adds the Secure_Path Segment in (**) acting as AS64510
#PE-1 accepts (**) with pCount=0 acting as AS64500,
#as it would if it received (**) from an eBGP peer
#PE-1, as AS64500, sends the update including (**) to PE-2 (in iBGP)

PE-2 to 64505: sig(64505,...,(pCount=1,...,64500),...,N)K_64500-PE2
               sig(64500,...,(pCount=0,...,64510),...,N)K_64510-PE1
               sig(64510, (pCount=1,...,64496), N)K_64496-CE1
               Equivalent AS_PATH=(64500,64496)
               Securedpath=(64500,64510 (pCount=0), 64496)
               length=sum(pCount)=2 (length is NOT 3)

PE-2 to CE-2:  sig(64499,...,(pCount=1,...,64500),...,N)K_64500-PE2
               sig(64500,...,(pCount=0,...,64510),...,N)K_64510-PE1
               sig(64510, (pCount=1,...,64496), N)K_64496-CE1
               Equivalent AS_PATH=(64500,64496)
               Securedpath=(64500, 64510 (pCount=0), 64496)
               length=sum(pCount)=2 (length is NOT 3)

6.  IANA Considerations

   This document does not require any IANA actions.

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7.  Security Considerations

   RFC 7705 [RFC7705] discusses a process by which one ASN is migrated
   into and subsumed by another.  Because this process involves
   manipulating the AS_Path in a BGP route to make it deviate from the
   actual path that it took through the network, this migration process
   is attempting to do exactly what BGPsec is working to prevent.
   BGPsec MUST be able to manage this legitimate use of AS_Path
   manipulation without generating a vulnerability in the RPKI route
   security infrastructure, and this document was written to define the
   method by which the protocol can meet this need.

   The solution discussed above is considered to be reasonably secure
   from exploitation by a malicious actor because it requires both
   signatures to be secured as if they were forward-signed between two
   eBGP neighbors.  This requires any router using this solution to be
   provisioned with valid keys for both the migrated and subsumed ASN so
   that it can generate valid signatures for each of the two ASNs it is
   adding to the path.  If the AS's keys are compromised, or zero-length
   keys are permitted, this does potentially enable an AS_PATH
   shortening attack, but these are existing security risks for BGPsec.

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>.

   [RFC7705]  George, W. and S. Amante, "Autonomous System Migration
              Mechanisms and Their Effects on the BGP AS_PATH
              Attribute", RFC 7705, DOI 10.17487/RFC7705, November 2015,
              <https://www.rfc-editor.org/info/rfc7705>.

   [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>.

   [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/rfc8105>.

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8.2.  Informative References

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, DOI 10.17487/RFC1930, March 1996,
              <https://www.rfc-editor.org/info/rfc1930>.

   [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>.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065,
              DOI 10.17487/RFC5065, August 2007,
              <https://www.rfc-editor.org/info/rfc5065>.

   [RFC5398]  Huston, G., "Autonomous System (AS) Number Reservation for
              Documentation Use", RFC 5398, DOI 10.17487/RFC5398,
              December 2008, <https://www.rfc-editor.org/info/rfc5398>.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <https://www.rfc-editor.org/info/rfc6480>.

Acknowledgements

   Thanks to Kotikalapudi Sriram, Shane Amante, Warren Kumari, Terry
   Manderson, Keyur Patel, Alia Atlas, and Alvaro Retana for their
   review comments.

   The authors particularly wish to acknowledge Kotikalapudi Sriram,
   Oliver Borchert, and Michael Baer for their review and suggestions
   for the examples in Section 5.4, which made an important contribution
   to the quality of the text.

   Additionally, the solution presented in this document is an amalgam
   of several Secure Inter-Domain Routing (SIDR) interim meeting
   discussions plus a discussion at IETF 85, collected and articulated
   thanks to Sandy Murphy.

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Authors' Addresses

   Wesley George
   Neustar
   45980 Center Oak Plaza
   Sterling, VA  20166
   United States of America

   Email: wesgeorge@puck.nether.net

   Sandy Murphy
   PARSONS, Inc.
   7110 Samuel Morse Drive
   Columbia, MD  21046
   United States of America

   Phone: +1 443-430-8000
   Email: sandy@tislabs.com

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