Methods for Detection and Mitigation of BGP Route Leaks
draft-ietf-idr-route-leak-detection-mitigation-03

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IDR and SIDR                                                   K. Sriram
Internet-Draft                                             D. Montgomery
Intended status: Standards Track                                 US NIST
Expires: November 24, 2016                                    B. Dickson

                                                                K. Patel
                                                                   Cisco
                                                          A. Robachevsky
                                                        Internet Society
                                                            May 23, 2016

        Methods for Detection and Mitigation of BGP Route Leaks
           draft-ietf-idr-route-leak-detection-mitigation-03

Abstract

   [I-D.ietf-grow-route-leak-problem-definition] provides a definition
   of the route leak problem, and also enumerates several types of route
   leaks.  This document first examines which of those route-leak types
   are detected and mitigated by the existing origin validation (OV)
   [RFC 6811].  It is recognized that OV offers a limited detection and
   mitigation capability against route leaks.  This document specifies
   enhancements that significantly extend the route-leak prevention,
   detection, and mitigation capabilities of BGP.  One solution
   component involves carrying a per-hop route-leak protection (RLP)
   field in BGP updates.  The RLP field is proposed be carried in a new
   optional transitive attribute, called BGP RLP attribute.  The
   solution is meant to be initially implemented as an enhancement of
   BGP without requiring BGPsec [I-D.ietf-sidr-bgpsec-protocol].
   However, when BGPsec is deployed in the future, the solution can be
   incorporated in BGPsec, enabling cryptographic protection for the RLP
   field.  That would be one way of implementing the proposed solution
   in a secure way.  The document also includes a stopgap method for
   detection and mitigation of route leaks for an intermediate phase
   when OV is deployed but BGP protocol on the wire is unchanged.

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 http://datatracker.ietf.org/drafts/current/.

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   This Internet-Draft will expire on November 24, 2016.

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   Copyright (c) 2016 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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Related Prior Work  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Mechanisms for Detection and Mitigation of Route Leaks  . . .   4
     3.1.  Route-Leak Protection (RLP) Field Encoding by Sending
           Router  . . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.1.  BGP RLP Attribute . . . . . . . . . . . . . . . . . .   8
       3.1.2.  Carrying RLP Flag Values in the BGPsec Flags  . . . .   9
     3.2.  Intra-AS Messaging for Route Leak Prevention  . . . . . .   9
     3.3.  Recommended Actions at a Receiving Router for Detection
           of Route Leaks  . . . . . . . . . . . . . . . . . . . . .  10
     3.4.  Possible Actions at a Receiving Router for Mitigation . .  11
   4.  Stopgap Solution when Only Origin Validation is Deployed  . .  11
   5.  Design Rationale and Discussion . . . . . . . . . . . . . . .  12
     5.1.  Is route-leak solution without cryptographic protection a
           serious attack vector?  . . . . . . . . . . . . . . . . .  12
     5.2.  Combining results of route-leak detection, OV and BGPsec
           validation for path selection decision  . . . . . . . . .  13
     5.3.  Are there cases when valley-free violations can be
           considered legitimate?  . . . . . . . . . . . . . . . . .  14
     5.4.  Comparison with other methods, routing security BCP . . .  15
     5.5.  Per-Hop RLP Flags or Single RLP Flag per Update?  . . . .  15
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17

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   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   [I-D.ietf-grow-route-leak-problem-definition] provides a definition
   of the route leak problem, and also enumerates several types of route
   leaks.  This document first examines which of those route-leak types
   are detected and mitigated by the existing Origin Validation (OV)
   [RFC6811] method.  OV and BGPsec path validation
   [I-D.ietf-sidr-bgpsec-protocol] together offer mechanisms to protect
   against re-originations and hijacks of IP prefixes as well as man-in-
   the-middle (MITM) AS path modifications.  Route leaks (see
   [I-D.ietf-grow-route-leak-problem-definition] and references cited at
   the back) are another type of vulnerability in the global BGP routing
   system against which OV offers only partial protection.  BGPsec (i.e.
   path validation) provides cryptographic protection for some aspects
   of BGP update messages, but in its current form BGPsec doesn't offer
   any protection against route leaks.

   For the types of route leaks enumerated in
   [I-D.ietf-grow-route-leak-problem-definition], where the OV method
   does't offer a solution, this document specifies enhancements that
   significantly extend the route-leak prevention, detection, and
   mitigation capabilities of BGP.  One solution component involves
   carrying a per-hop route-leak protection (RLP) field in BGP updates.
   The RLP field is proposed be carried in a new optional transitive
   attribute, called BGP RLP attribute.  The solution is meant to be
   initially implemented as an enhancement of BGP without requiring
   BGPsec.  However, when BGPsec is deployed in the future, the solution
   can be incorporated in BGPsec, enabling cryptographic protection for
   the RLP field.  That would be one way of implementing the proposed
   solution in a secure way.  It is not claimed that the solution
   detects all possible types of route leaks but it detects several
   types, especially considering some significant route-leak occurrences
   that have been observed in recent years.  The document also includes
   a stopgap method (in Section 4) for detection and mitigation of route
   leaks for an intermediate phase when OV is deployed but BGP protocol
   on the wire is unchanged.

2.  Related Prior Work

   The basic idea and mechanism embodied in the proposed solution is
   based on setting an attribute in BGP route announcement to manage the
   transmission/receipt of the announcement based on the type of
   neighbor (e.g. customer, transit provider, etc.).  Documented prior

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   work related to said basic idea and mechanism dates back to at least
   the 1980's.  Some examples of prior work are: (1) Information flow
   rules described in [proceedings-sixth-ietf] (see pp. 195-196); (2)
   Link Type described in [RFC1105-obsolete] (see pp. 4-5); (3)
   Hierarchical Recording described in
   [draft-kunzinger-idrp-ISO10747-01] (see Section 6.3.1.12).  The
   problem of route leaks and possible solution mechanisms based on
   encoding peering-link type information, e.g.  P2C (i.e.  Transit-
   Provider to Customer), C2P (i.e.  Customer to Transit-Provider), p2p
   (i.e. peer to peer) etc., in BGPsec updates and protecting the same
   under BGPsec path signatures have been discussed in IETF SIDR WG at
   least since 2011. [draft-dickson-sidr-route-leak-solns] attempted to
   describe these mechanisms in a BGPsec context.  The draft expired in
   2012. [draft-dickson-sidr-route-leak-solns] defined neighbor
   relationships on a per link basis, but in the current document the
   relationship is encoded per prefix, as routes for prefixes with
   different business models are often sent over the same link.  Also
   [draft-dickson-sidr-route-leak-solns] proposed a second signature
   block for the link type encoding, separate from the path signature
   block in BGPsec.  By contrast, in the current document when BGPsec-
   based solution is considered, cryptographic protection is provided
   for Route-Leak Protection (RLP) encoding using the same signature
   block as that for path signatures (see Section 3.1).

3.  Mechanisms for Detection and Mitigation of Route Leaks

   Referring to the enumeration of route leaks discussed in
   [I-D.ietf-grow-route-leak-problem-definition], Table 1 summarizes the
   route-leak detection capability offered by OV and BGPsec for
   different types of route leaks.  (Note: Prefix filtering is not
   considered here in this table.  Please see Section 4.)

   A detailed explanation of the contents of Table 1 is as follows.  It
   is readily observed that route leaks of Types 1, 2, 3, and 4 are not
   detected by OV or BGPsec in its current form.  Clearly, Type 5 route
   leak involves re-origination or hijacking, and hence can be detected
   by OV.  In the case of Type 5 route leak, there would be no existing
   ROAs to validate a re-originated prefix or more specific, but instead
   a covering ROA would normally exist with the legitimate AS, and hence
   the update will be considered Invalid by OV.

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   +---------------------------------+---------------------------------+
   | Type of Route Leak              | Current State of Detection      |
   |                                 | Coverage                        |
   +---------------------------------+---------------------------------+
   | Type 1: Hairpin Turn with Full  | Neither OV nor BGPsec (in its   |
   | Prefix                          | current form) detects Type 1.   |
   | ------------------------------- | ------------------------------- |
   | Type 2: Lateral ISP-ISP-ISP     | Neither OV nor BGPsec (in its   |
   | Leak                            | current form) detects Type 2.   |
   | ------------------------------- | ------------------------------- |
   | Type 3: Leak of Transit-        | Neither OV nor BGPsec (in its   |
   | Provider Prefixes to Peer       | current form) detects Type 3.   |
   | ------------------------------- | ------------------------------- |
   | Type 4: Leak of Peer Prefixes   | Neither OV nor BGPsec (in its   |
   | to Transit Provider             | current form) detects Type 4.   |
   | ------------------------------- | ------------------------------- |
   | Type 5: Prefix Re-Origination   | OV detects Type 5.              |
   | with Data Path to Legitimate    |                                 |
   | Origin                          |                                 |
   | ------------------------------- | ------------------------------- |
   | Type 6: Accidental Leak of      | For internal prefixes never     |
   | Internal Prefixes and More      | meant to be routed on the       |
   | Specifics                       | Internet, OV helps detect their |
   |                                 | leak; they might either have no |
   |                                 | covering ROA or have an AS0-ROA |
   |                                 | to always filter them. In the   |
   |                                 | case of accidental leak of more |
   |                                 | specifics, OV may offer some    |
   |                                 | detection due to ROA maxLength. |
   +---------------------------------+---------------------------------+

     Table 1: Examination of Route-Leak Detection Capability of Origin
               Validation and Current BGPsec Path Validation

   In the case of Type 6 leaks involving internal prefixes that are not
   meant to be routed in the Internet, they are likely to be detected by
   OV.  That is because such prefixes might either have no covering ROA
   or have an AS0-ROA to always filter them.  In the case of Type 6
   leaks that are due to accidental leak of more specifics, they may be
   detected due to violation of ROA maxLength.  BGPsec (i.e. path
   validation) in its current form does not detect Type 6.  However,
   route leaks of Type 6 are least problematic due to the following
   reasons.  In the case of leak of more specifics, the offending AS is
   itself the legitimate destination of the leaked more-specific
   prefixes.  Hence, in most cases of this type, the data traffic is
   neither misrouted not denied service.  Also, leaked announcements of
   Type 6 are short-lived and typically withdrawn quickly following the
   announcements.  Further, the MaxPrefix limit may kick-in in some

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   receiving routers and that helps limit the propagation of sometimes
   large number of leaked routes of Type 6.

   Realistically, BGPsec may take a much longer time being deployed than
   OV.  Hence solution proposals for route leaks should consider both
   scenarios: (A) OV only (without BGPsec) and (B) OV plus BGPsec.
   Assuming an initial scenario A, and based on the above discussion and
   Table 1, it is evident that the solution method should focus
   primarily on route leaks of Types 1, 2, 3, and 4.  Section 3.1 and
   Section 3.3 describe a simple addition to BGP that facilitates
   detection of route leaks of Types 1, 2, 3, and 4.  The simple
   addition involves a per-hop Route-Leak Protection (RLP) field.  The
   RLP fields are carried in an optional transitive attribute in BGP,
   called BGP RLP attribute.  When BGPsec is deployed, the RLP field
   will be accommodated in the existing Flags field (see
   [I-D.ietf-sidr-bgpsec-protocol]) which is cryptographically protected
   under path signatures.  Section 3.2 describes intra-AS messaging and
   common practice for route leak prevention in major ISPs' networks.

3.1.  Route-Leak Protection (RLP) Field Encoding by Sending Router

   The key principle is that, in the event of a route leak, a receiving
   router in a transit-provider AS (e.g. referring to Figure 1, ISP2
   (AS2) router) should be able to detect from the update message that
   its customer AS (e.g.  AS3 in Figure 1) SHOULD NOT have forwarded the
   update (towards the transit-provider AS).  This means that at least
   one of the ASes in the AS path of the update has indicated that it
   sent the update to its customer or lateral (i.e. non-transit) peer,
   but forbade any subsequent 'Up' forwarding (i.e. from a customer AS
   to its transit-provider AS).  For this purpose, a Route-Leak
   Protection (RLP) field to be set by a sending router is proposed to
   be used for each AS hop.

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                                      /\              /\
                                       \ route-leak(P)/
                                        \ propagated /
                                         \          /
              +------------+    peer    +------------+
        ______| ISP1 (AS1) |----------->|  ISP2 (AS2)|---------->
       /       ------------+  prefix(P) +------------+ route-leak(P)
      | prefix |          \   update      /\        \  propagated
       \  (P)  /           \              /          \
        -------   prefix(P) \            /            \
                     update  \          /              \
                              \        /route-leak(P)  \/
                              \/      /
                           +---------------+
                           | customer(AS3) |
                           +---------------+

        Figure 1: Illustration of the basic notion of a route leak.

   For the purpose of route-leak detection and mitigation proposed in
   this document, the RLP field value SHOULD be set to one of two values
   as follows:

   o  0: This is the default value (i.e. "nothing specified"),

   o  1: This is the 'Do not Propagate Up or Lateral' indication; sender
      indicating that the route SHOULD NOT be forwarded 'Up' towards a
      transit-provider AS or to a lateral (i.e. non-transit) peer AS.

   The RLP indications SHOULD be set on a per prefix and per neighbor AS
   basis.  This is because updates for prefixes with different business
   models are often sent over the same link between ASes.

   There are two different scenarios when a sending AS SHOULD set value
   1 in the RLP field: (a) when sending the update to a customer AS, and
   (b) when sending the update to a lateral peer (i.e. non-transit) AS.
   In essence, in both scenarios, the intent of RLP = 1 is that the
   neighbor AS and any receiving AS along the subsequent AS path SHOULD
   NOT forward the update 'Up' towards its (receiving AS's) transit-
   provider AS or laterally towards its peer (i.e. non-transit) AS.
   When sending an update 'Up' to a transit-provider AS, the RLP
   encoding SHOULD be set to the default value of 0.  When a sending AS
   sets the RLP encoding to 0, it is indicating to the receiving AS that
   the update can be propagated in any direction (i.e. towards transit-
   provider, customer, or lateral peer).  This two-state specification
   in the RLP field works for detection and mitigation of route leaks of

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   Types 1, 2, 3, and 4 which are the focus here (see Section 3.3 and
   Section 3.4).

   An AS SHOULD NOT rewrite/reset the values set by any preceding ASes
   in their respective RLP fields.

   The proposed RLP encoding SHOULD be carried in BGP-4 [RFC4271]
   updates in a new BGP optional transitive attribute (see
   Section 3.1.1).  And it SHOULD be carried in BGPsec in the Flags
   field (see Section 3.1.2).

3.1.1.  BGP RLP Attribute

   The BGP RLP attribute is a new BGP optional transitive attribute.
   The attribute type code for the RLP attribute is to be assigned by
   IANA.  The value field of the RLP attribute is defined as a set of
   one or more RLP TLVs (Type, Length, Value) as described below:

   +-----------------------+ -\
   | ASN: N                |   |
   +-----------------------+    >  (Most recently added)
   | RLP: N                |   |
   +-----------------------+ -/
    ...........
   +-----------------------+ -\
   | ASN: 1                |   |
   +-----------------------+    >  (Least recently added)
   | RLP: 1                |   |
   +-----------------------+ -/

                         Figure 2: RLP TLV format.

   RLP TLV has these two components:

   ASN: Four octets encoding the public registered AS number of a BGP
   speaker.

   RLP Flag: One octet encoding the RLP Flag bits.  Usage of these flag
   bits was described above and will be further discussed in subsequent
   sections.

   If all ASes in the AS_PATH of a route are upgraded to participate in
   RLP, then the ASNs in the RLP TLV in Figure 2 will correspond one-to-
   one with sequence of ASes in the AS_PATH (excluding prepends).  If
   some ASes do not participate, then one or more {ASN, RLP} tuples may
   be missing in the RLP TLV relative to the AS_PATH.

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3.1.2.  Carrying RLP Flag Values in the BGPsec Flags

   In BGPsec enabled routers, the RLP encoding SHOULD be accommodated in
   the existing Flags field in BGPsec updates.  The Flags field is part
   of the Secure_Path Segment in BGPsec updates
   [I-D.ietf-sidr-bgpsec-protocol].  It is one octet long, and one Flags
   field is available for each AS hop, and currently only the first bit
   is used in BGPsec.  So there are 7 bits that are currently unused in
   the Flags field.  Two (or more if needed) of these bits can be
   designated for the RLP field.  Since the BGPsec protocol
   specification requires a sending AS to include the Flags field in the
   data that are signed over, the RLP field for each hop (assuming it
   would be part of the Flags field) will be protected under the sending
   AS's signature.

3.2.  Intra-AS Messaging for Route Leak Prevention

   The following procedure (or similar) for intra-AS messaging (i.e.
   between ingress and egress routers) for prevention of route leaks is
   a fairly common practice used by large ISPs.  (Note: This information
   was gathered through private discussions with operators of large ISP
   networks.)

   Routes are tagged on ingress to an AS with communities for origin,
   including the type of eBGP peer it was learned from (customer,
   transit-provider or peer), geographic location, etc.  The community
   attributes are carried across the AS with the routes.  Routes that
   the AS originates directly are tagged with similar origin communities
   when they are redistributed into BGP from static, IGP, etc.  These
   communities are used along with additional logic in route policies to
   determine which routes are to be announced to which eBGP peers and
   which are to be dropped.  Route policy is applied to eBGP sessions
   based on what set of routes they should receive (transit, full
   routes, internal-only, default-only, etc.).  In this process, the
   ISP's AS also ensures that routes learned from a transit-provider or
   a lateral peer (i.e. non-transit) at an ingress router are not leaked
   at an egress router to another transit-provider or peer.

   Addionally, in many cases, ISP network operators' outbound policies
   require explicit matches for expected communities before passing
   routes.  This helps ensure that that if an update has made it into
   the routing table (i.e.  RIB) but has missed its ingress community
   tagging (due to a missing/misapplied ingress policy), it will not be
   inadvertently leaked.

   The above procedure (or a simplified version of it) is also
   applicable when an AS consists of a single eBGP router.  It is
   recommended that all AS operators SHOULD implement the procedure

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   described above (or similar that is appropriate for their network) to
   prevent route leaks that they have direct control over.

   In the above described common practice, the IPS's ingress and egress
   routers primarily rely on pre-configured knowledge of the peer type
   for each of its eBGP peers.  However, as an additional measure of
   route-leak alertness, the ingress router SHOULD examine the RLP value
   set by the eBGP peer from which the route is received.  If said eBGP
   peer is a customer (for the route), then the RLP value is expected to
   be set to 0 (see Section 3.1).  And if said eBGP peer is a transit-
   provider or lateral peer, then the RLP field is expected to be set to
   1.  If the observed RLP value differs from the expectation, then the
   event SHOULD be logged, and said eBGP peer SHOULD be notified.

3.3.  Recommended Actions at a Receiving Router for Detection of Route
      Leaks

   An example set of receiver actions that work to detect and mitigate
   route leaks of Types 1, 2, 3, and 4 are provided here.  This example
   algorithm serves as a proof of concept.  However, other receiver
   algorithms or procedures can be designed (based on the same sender
   specification as in Section 3.1) and may perform with greater
   efficacy, and are by no means excluded.

   A recommended receiver algorithm for detecting a route leak is as
   follows:

   A receiving router SHOULD mark an update as a 'Route Leak' if ALL of
   the following conditions hold true:

   1.  The update is received from a customer or lateral peer AS.

   2.  The update has the RLP field set to 1 (i.e.  'Do not Propagate Up
       or Lateral') indication for one or more hops (excluding the most
       recent) in the AS path.

   The reason for stating "excluding the most recent" in the above
   algorithm is as follows.  An ISP should look at RLP values set by
   ASes preceding the immediate sending AS in order to ascertain a leak.
   The receiving router already knows that the most recent hop in the
   update is from its customer or lateral-peer AS to itself, and it does
   not need to rely on the RLP field value set by said AS for detection
   of route leaks.  (Note: The utility of RLP value set by the peer
   sending the update was discussed above in Section 3.2.)

   If the RLP encoding is secured by BGPsec (see Section 3.1) and hence
   protected against tampering by intermediate ASes, then there would be

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   added certainty in the route-leak detection algorithm described above
   (see discussions in Section 5.1 and Section 5.2).

3.4.  Possible Actions at a Receiving Router for Mitigation

   After applying the above detection algorithm, a receiving router may
   use any policy-based algorithm of its own choosing to mitigate any
   detected route leaks.  An example receiver algorithm for mitigating a
   route leak is as follows:

   o  If an update from a customer or lateral peer AS is marked as a
      'Route Leak', then the receiving router SHOULD prefer an alternate
      unmarked route if available.

   o  If no alternate unmarked route is available, then the route marked
      as a 'Route Leak' MAY be accepted.

   A basic principle here is that if an AS receives and marks a customer
   route as 'Route Leak', then the AS should override the "prefer
   customer route" policy, and instead prefer an alternate 'clean' route
   learned from another customer, a lateral peer, or a transit provider.
   This can be implemented by adjusting the local preference for the
   routes in consideration.

4.  Stopgap Solution when Only Origin Validation is Deployed

   A stopgap method is described here for detection and mitigation of
   route leaks for the intermediate phase when OV is deployed but BGP
   protocol on the wire is unchanged.  The stopgap solution can be in
   the form of construction of a prefix filter list from ROAs.  A
   suggested procedure for constructing such a list comprises of the
   following steps:

   o  ISP makes a list of all the ASes (Cust_AS_List) that are in its
      customer cone (ISP's own AS is also included in the list).  (Some
      of the ASes in Cust_AS_List may be multi-homed to another ISP and
      that is OK.)

   o  ISP downloads from the RPKI repositories a complete list
      (Cust_ROA_List) of valid ROAs that contain any of the ASes in
      Cust_AS_List.

   o  ISP creates a list of all the prefixes (Cust_Prfx_List) that are
      contained in any of the ROAs in Cust_ROA_List.

   o  Cust_Prfx_List is the allowed list of prefixes that is permitted
      by the ISP's AS, and will be forwarded by the ISP to upstream
      ISPs, customers, and peers.

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   o  A route for a prefix that is not in Cust_Prfx_List but announced
      by one of ISP's customers is 'marked' as a potential route leak.
      Further, the ISP's router SHOULD prefer an alternate route that is
      Valid (i.e. valid according to origin validation) and 'clean'
      (i.e. not marked) over the 'marked' route.  The alternate route
      may be from a peer, transit provider, or different customer.

   Special considerations with regard to the above procedure may be
   needed for DDoS mitigation service providers.  They typically
   originate or announce a DDoS victim's prefix to their own ISP on a
   short notice during a DDoS emergency.  Some provisions would need to
   be made for such cases, and they can be determined with the help of
   inputs from DDoS mitigation service providers.

   For developing a list of all the ASes (Cust_AS_List) that are in the
   customer cone of an ISP, the AS path based Outbound Route Filter
   (ORF) technique [draft-ietf-idr-aspath-orf] can be helpful (see
   discussion in Section 5.4).

5.  Design Rationale and Discussion

   This section provides design justifications for the methodology
   specified in Section 3, and also answers some questions that are
   anticipated or have been raised in the IETF IDR and SIDR working
   group meetings.

5.1.  Is route-leak solution without cryptographic protection a serious
      attack vector?

   It has been asked if a route-leak solution without BGPsec, i.e. when
   RLP bits are not protected, can turn into a serious new attack
   vector.  The answer seems to be: not really!  Even the NLRI and
   AS_PATH in BGP updates are attack vectors, and RPKI/OV/BGPsec seek to
   fix that.  Consider the following.  Say, if 99% of route leaks are
   accidental and 1% are malicious, and if route-leak solution without
   BGPsec eliminates the 99%, then perhaps it is worth it (step in the
   right direction).  When BGPsec comes into deployment, the route-leak
   protection (RLP) bits can be mapped into BGPsec (using the Flags
   field) and then necessary security will be in place as well (within
   each BGPsec island as and when they emerge).

   Further, let us consider the worst-case damage that can be caused by
   maliciously manipulating the RLP bits in an implementation without
   cryptographic protection (i.e. sans BGPsec).  Manipulation of the RLP
   bits can result in one of two types of attacks: (a) Upgrade attack
   and (b) Downgrade attack.  Descriptions and discussions about these
   attacks follow.  In what follows, P2C stands for transit provider to
   customer (Down); C2P stands for customer to transit provider (Up),

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   and p2p stands for peer to peer (lateral or non-transit
   relationship).

   (a) Upgrade attack: An AS that wants to intentionally leak a route
   would alter the RLP encodings for the preceding hops from 1 (i.e.
   'Do not Propagate Up or Lateral') to 0 (default) wherever applicable.
   This poses no problem for a route that keeps propagating in the
   'Down' (P2C) direction.  However, for a route that propagates 'Up'
   (C2P) or 'Lateral' (p2p), the worst that can happen is that a route
   leak goes undetected.  That is, a receiving router would not be able
   to detect the leak for the route in question by the RLP mechanism
   described here.  However, the receiving router may still detect and
   mitigate it in some cases by applying other means such as prefix
   filters [RFC7454].  If some malicious leaks go undetected (when RLP
   is deployed without BGPsec) that is possibly a small price to pay for
   the ability to detect the bulk of route leaks that are accidental.

   (b) Downgrade attack: RLP encoding is set to 1 (i.e.  'Do not
   Propagate Up or Lateral') when it should be set to 0 (default).  This
   would result in a route being mis-detected and marked as a route
   leak.  By default RLP encoding is set to 0, and that helps reduce
   errors of this kind (i.e. accidental downgrade incidents).  Every AS
   or ISP wants reachability for prefixes it originates and for its
   customer prefixes.  So an AS or ISP is not likely to change an RLP
   value 0 to 1 intentionally.  If a route leak is detected (due to
   intentional or accidental downgrade) by a receiving router, it would
   prefer an alternate 'clean' route from a transit provider or peer
   over a 'marked' route from a customer.  It may end up with a
   suboptimal path.  In order to have reachability, the receiving router
   would accept a 'marked' route if there is no alternative that is
   'clean'.  So RLP downgrade attacks (intentional or accidental) would
   be quite rare, and the consequences do not appear to be grave.

5.2.  Combining results of route-leak detection, OV and BGPsec
      validation for path selection decision

   Combining the results of route-leak detection, OV, and BGPsec
   validation for path selection decision is up to local policy in a
   receiving router.  As an example, a router may always give precedence
   to outcomes of OV and BGPsec validation over that of route-leak
   detection.  That is, if an update fails OV or BGPsec validation, then
   the update is not considered a candidate for path selection.
   Instead, an alternate update is chosen that passed OV and BGPsec
   validation and additionally was not marked as route leak.

   If only OV is deployed (and not BGPsec), then there are six possible
   combinations between OV and route-leak detection outcomes.  Because
   there are three possible outcomes for OV (NotFound, Valid, and

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   Invalid) and two possible outcomes for route-leak detection (marked
   as leak and not marked).  If OV and BGPsec are both deployed, then
   there are twelve possible combinations between OV, BGPsec validation,
   and route-leak detection outcomes.  As stated earlier, since BGPsec
   protects the RLP encoding, there would be added certainty in route-
   leak detection outcome if an update is BGPsec valid (see
   Section 5.1).

5.3.  Are there cases when valley-free violations can be considered
      legitimate?

   There are studies in the literature [Anwar] [Giotsas] [Wijchers]
   observing and analyzing the behavior of routes announced in BGP
   updates using data gathered from the Internet.  In particular, the
   studies have focused on how often there appear to be valley-free
   (e.g.  Gao-Rexford [Gao] model) violations, and if they can be
   explained [Anwar].  One important consideration for explanation of
   violations is per-prefix routing policies, i.e. routes for prefixes
   with different business models are often sent over the same link.
   One encouraging result reported in [Anwar] is that when per-prefix
   routing policies are taken into consideration in the data analysis,
   more than 80% of the observed routing decisions fit the valley-free
   model (see Section 4.3 and SPA-1 data in Figure 2).  [Anwar] also
   observes, "it is well known that this model [the basic Gao-Rexford
   model and some variations of it] fails to capture many aspects of the
   interdomain routing system.  These aspects include AS relationships
   that vary based on the geographic region or destination prefix, and
   traffic engineering via hot-potato routing or load balancing."  So
   there may be potential for explaining the remaining (20% or less)
   violations of valley-free as well.

   One major design factor in the methodology described in this document
   is that the Route-Leak Protection (RLP) encoding is per prefix.  So
   the proposed solution is consistent with ISPs' per-prefix routing
   policies.  Large global and other major ISPs will be the likely early
   adopters, and they are expected to have expertise in configuring
   policies (including per prefix policies, if applicable), and make
   proper use of the RLP indications on a per prefix basis.  When said
   large ISPs participate in this solution deployment, it is envisioned
   that they would form a ring of protection against route leaks, and
   co-operatively avoid many of the common types of route leaks that are
   observed.  Route leaks may still happen occasionally within the
   customer cones (if some customer ASes are not participating or not
   diligently implementing RLP), but said leaks would be much less
   likely to propagate from one large participating ISP to another.

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5.4.  Comparison with other methods, routing security BCP

   It is reasonable to ask if techniques considered in BCPs such
   as[RFC7454] (BGP Operations and Security) and [NIST-800-54] may be
   adequate to address route leaks.  The prefix filtering
   recommendations in the BCPs may be complementary but not adequate.
   The difficulty is in ISPs' ability to construct prefix filters that
   represent their customer cones (CC) accurately, especially when there
   are many levels in the hierarchy within the CC.  In the RLP-encoding
   based solution described here, AS operators signal for each route
   propagated, if it SHOULD NOT be subsequently propagated to a transit
   provider or peer.

   AS path based Outbound Route Filter (ORF) described in
   [draft-ietf-idr-aspath-orf] is also an interesting complementary
   technique.  It can be used as an automated collaborative messaging
   system (implemented in BGP) for ISPs to try to develop a complete
   view of the ASes and AS paths in their CCs.  Once an ISP has that
   view, then AS path filters can be possibly used to detect route
   leaks.  One limitation of this technique is that it cannot duly take
   into account the fact that routes for prefixes with different
   business models are often sent over the same link between ASes.
   Also, the success of AS path based ORF depends on whether ASes at all
   levels of the hierarchy in a CC participate and provide accurate
   information (in the ORF messages) about the AS paths they expect to
   have in their BGP updates.

5.5.  Per-Hop RLP Flags or Single RLP Flag per Update?

   The route-leak detection and mitigation mechanism described in this
   document is based on setting RLP flags on a per-hop basis.  There is
   another possible mechanism based on a single RLP flag per update.

   Method A - Per-Hop RLP Flags: The sender on each hop in the AS path
   sets RLP = 1 if sending the update to a cutomer or lateral peer
   (regardless of what the previous ASes in the path set their bits to).
   No AS (if operating correctly) would rewrite/reset the RLP flags set
   by any preceding AS (see Section 3.1).

   Method B - Single RLP Flag per Update: As it propagates, the update
   always has only one RLP flag.  Once an AS (in the update path)
   determines that it is sending an update towards a customer or lateral
   peer AS, it sets the RLP flag.  Once the flag is set, it would be
   required that subsequent ASes in the path should always leave the
   flag set.

   Method B is functionally deficient when compared to Method A for
   detection of route leaks.  This becomes quite evident from the

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   illustration in Figure 3.  With Method B in use, it is evident that
   when AS3 receives an update, {p [AS2, AS1] RLP =1] from AS2, there is
   no way for it (AS3) to distinguish and tell if AS2 is leaking a route
   learned from a transit-provider or lateral peer (AS1), or
   legitimately forwarding a route learned from a customer (AS1).
   Method A does not suffer from this inadequacy because if Method A
   were used, then AS3 in Figure 3 will have the benefit of seeing the
   RLP field values set by AS1 and AS2 individually.  Thus in Method A,
   a legitimate customer route forwarded from AS2 to AS3 will be
   distinguishable from a leaked transit-provider or lateral-peer route.

        +-----+         +-----+  peer to peer (p2p) relation +-----+
     p--| AS1 |-------->| AS2 |----------------------------->| AS3 |
        +-----+         +-----+            Update -->        +-----+
                                Method B: {p [AS2 AS1] RLP=1}
                        (independent of AS1's relationship with AS2)
           ** Method B fails to differentiate leak versus legitimate

              Method A: {p [AS2 AS1] {RLP2=1 RLP1=1}} -- route leak
               (if AS1 is transit-provider or lateral peer of AS2)
              Method A: {p [AS2 AS1] {RLP2=1 RLP1=0}} -- legitimate
                   (if AS1 is a customer of AS2)
       ** Method A successfully differentiates leak versus legitimate

        Figure 3: Illustration of the basic notion of a route leak.

   In general, Method A is more robust than Method B in the presence of
   faulty operation (including route leaks) or lack of upgrade (to
   perform RLP) on part of an AS in the AS path.  [Sriram] further
   illustrates this (see slides 5-9) under multiple partial deployment /
   faulty implementation scenarios.

   Further, it is feasible to provide cryptographic protection for the
   RLP encoding in the case Method A with the help of the BGPsec
   protocol (see Section 3.1.2).  Method B is not amenable to be mapped
   into BGPsec.

6.  Security Considerations

   The proposed Route-Leak Protection (RLP) field requires cryptographic
   protection in order to prevent malicious route leaks.  Since it is
   proposed that the RLP field be included in the Flags field in the
   Secure_Path Segment in BGPsec updates, the cryptographic security
   mechanisms in BGPsec are expected to also apply to the RLP field.
   The reader is therefore directed to the security considerations
   provided in [I-D.ietf-sidr-bgpsec-protocol].

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7.  IANA Considerations

   A request will be made to IANA for assignment of an attribute type
   code for the proposed new BGP RLP attribute.

8.  Acknowledgements

   The authors wish to thank Danny McPherson and Eric Osterweil for
   discussions related to this work.  Also, thanks are due to Jared
   Mauch, Jeff Haas, Warren Kumari, Amogh Dhamdhere, Jakob Heitz, Geoff
   Huston, Randy Bush, Alexander Azimov, Ruediger Volk, Sue Hares, Wes
   George, Chris Morrow, and Sandy Murphy for comments, suggestions, and
   critique.  The authors are also thankful to Padma Krishnaswamy,
   Oliver Borchert, and Okhee Kim for their comments and review.

9.  References

9.1.  Normative References

   [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,
              <http://www.rfc-editor.org/info/rfc4271>.

9.2.  Informative References

   [Anwar]    Anwar, R., Niaz, H., Choffnes, D., Cunha, I., Gill, P.,
              and N. Katz-Bassett, "Investigating Interdomain Routing
              Policies in the Wild",  ACM Internet Measurement
              Conference (IMC), October 2015,
              <http://www.cs.usc.edu/assets/007/94928.pdf>.

   [Cowie2010]
              Cowie, J., "China's 18 Minute Mystery",  Dyn
              Research/Renesys Blog, November 2010,
              <http://research.dyn.com/2010/11/
              chinas-18-minute-mystery/>.

   [Cowie2013]
              Cowie, J., "The New Threat: Targeted Internet Traffic
              Misdirection",  Dyn Research/Renesys Blog, November 2013,
              <http://research.dyn.com/2013/11/
              mitm-internet-hijacking/>.

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   [draft-dickson-sidr-route-leak-solns]
              Dickson, B., "Route Leaks -- Proposed Solutions",  IETF
              Internet Draft (expired), March 2012,
              <https://tools.ietf.org/html/draft-dickson-sidr-route-
              leak-solns-01>.

   [draft-ietf-idr-aspath-orf]
              Patel, K. and S. Hares, "AS path Based Outbound Route
              Filter for BGP-4",  IETF Internet Draft (expired), August
              2007, <https://tools.ietf.org/html/draft-ietf-idr-aspath-
              orf-09>.

   [draft-kunzinger-idrp-ISO10747-01]
              Kunzinger, C., "Inter-Domain Routing Protocol (IDRP)",
               IETF Internet Draft (expired), November 1994,
              <https://tools.ietf.org/pdf/draft-kunzinger-idrp-
              ISO10747-01.pdf>.

   [Gao]      Gao, L. and J. Rexford, "Stable Internet routing without
              global coordination",  IEEE/ACM Transactions on
              Networking, December 2001,
              <http://www.cs.princeton.edu/~jrex/papers/
              sigmetrics00.long.pdf>.

   [Gill]     Gill, P., Schapira, M., and S. Goldberg, "A Survey of
              Interdomain Routing Policies",  ACM SIGCOMM Computer
              Communication Review, January 2014,
              <https://www.cs.bu.edu/~goldbe/papers/survey.pdf>.

   [Giotsas]  Giotsas, V. and S. Zhou, "Valley-free violation in
              Internet routing - Analysis based on BGP Community data",
               IEEE ICC 2012, June 2012.

   [Hiran]    Hiran, R., Carlsson, N., and P. Gill, "Characterizing
              Large-scale Routing Anomalies: A Case Study of the China
              Telecom Incident",  PAM 2013, March 2013,
              <http://www3.cs.stonybrook.edu/~phillipa/papers/
              CTelecom.html>.

   [Huston2012]
              Huston, G., "Leaking Routes", March 2012,
              <http://labs.apnic.net/blabs/?p=139/>.

   [Huston2014]
              Huston, G., "What's so special about 512?", September
              2014, <http://labs.apnic.net/blabs/?p=520/>.

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   [I-D.ietf-grow-route-leak-problem-definition]
              Sriram, K., Montgomery, D., McPherson, D., Osterweil, E.,
              and B. Dickson, "Problem Definition and Classification of
              BGP Route Leaks", draft-ietf-grow-route-leak-problem-
              definition-06 (work in progress), May 2016.

   [I-D.ietf-sidr-bgpsec-protocol]
              Lepinski, M. and K. Sriram, "BGPsec Protocol
              Specification", draft-ietf-sidr-bgpsec-protocol-15 (work
              in progress), March 2016.

   [Kapela-Pilosov]
              Pilosov, A. and T. Kapela, "Stealing the Internet: An
              Internet-Scale Man in the Middle Attack", DEFCON-16 Las
              Vegas, NV, USA, August 2008,
              <https://www.defcon.org/images/defcon-16/dc16-
              presentations/defcon-16-pilosov-kapela.pdf>.

   [Kephart]  Kephart, N., "Route Leak Causes Amazon and AWS Outage",
               ThousandEyes Blog, June 2015,
              <https://blog.thousandeyes.com/route-leak-causes-amazon-
              and-aws-outage>.

   [Khare]    Khare, V., Ju, Q., and B. Zhang, "Concurrent Prefix
              Hijacks: Occurrence and Impacts",  IMC 2012, Boston, MA,
              November 2012, <http://www.cs.arizona.edu/~bzhang/
              paper/12-imc-hijack.pdf>.

   [Labovitz]
              Labovitz, C., "Additional Discussion of the April China
              BGP Hijack Incident",  Arbor Networks IT Security Blog,
              November 2010,
              <http://www.arbornetworks.com/asert/2010/11/additional-
              discussion-of-the-april-china-bgp-hijack-incident/>.

   [LRL]      Khare, V., Ju, Q., and B. Zhang, "Large Route Leaks",
               Project web page, 2012,
              <http://nrl.cs.arizona.edu/projects/
              lsrl-events-from-2003-to-2009/>.

   [Luckie]   Luckie, M., Huffaker, B., Dhamdhere, A., Giotsas, V., and
              kc. claffy, "AS Relationships, Customer Cones, and
              Validation",  IMC 2013, October 2013,
              <http://www.caida.org/~amogh/papers/asrank-IMC13.pdf>.

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   [Madory]   Madory, D., "Why Far-Flung Parts of the Internet Broke
              Today",  Dyn Research/Renesys Blog, September 2014,
              <http://research.dyn.com/2014/09/
              why-the-internet-broke-today/>.

   [Mauch]    Mauch, J., "BGP Routing Leak Detection System",  Project
              web page, 2014,
              <http://puck.nether.net/bgp/leakinfo.cgi/>.

   [Mauch-nanog]
              Mauch, J., "Detecting Routing Leaks by Counting",
              NANOG-41 Albuquerque, NM, USA, October 2007,
              <https://www.nanog.org/meetings/nanog41/presentations/
              mauch-lightning.pdf>.

   [NIST-800-54]
              Kuhn, D., Sriram, K., and D. Montgomery, "Border Gateway
              Protocol Security",  NIST Special Publication 800-54, July
              2007, <http://csrc.nist.gov/publications/nistpubs/800-54/
              SP800-54.pdf>.

   [Paseka]   Paseka, T., "Why Google Went Offline Today and a Bit about
              How the Internet Works",  CloudFare Blog, November 2012,
              <http://blog.cloudflare.com/
              why-google-went-offline-today-and-a-bit-about/>.

   [proceedings-sixth-ietf]
              Gross, P., "Proceedings of the April 22-24, 1987 Internet
              Engineering Task Force", April 1987,
              <https://www.ietf.org/proceedings/06.pdf>.

   [RFC1105-obsolete]
              Lougheed, K. and Y. Rekhter, "A Border Gateway Protocol
              (BGP)",  IETF RFC (obsolete), June 1989,
              <https://tools.ietf.org/html/rfc1105>.

   [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
              Austein, "BGP Prefix Origin Validation", RFC 6811,
              DOI 10.17487/RFC6811, January 2013,
              <http://www.rfc-editor.org/info/rfc6811>.

   [RFC7454]  Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations
              and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454,
              February 2015, <http://www.rfc-editor.org/info/rfc7454>.

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   [Sriram]   Sriram, K., Montgomery, D., Dickson, B., Patel, K., and A.
              Robachevsky , "Methods for Detection and Mitigation of BGP
              Route Leaks",  IETF-95 IDR WG Meeting), April 2016,
              <https://www.ietf.org/proceedings/95/slides/slides-95-idr-
              13.pdf>.

   [Toonk]    Toonk, A., "What Caused Today's Internet Hiccup", August
              2014, <http://www.bgpmon.net/
              what-caused-todays-internet-hiccup/>.

   [Toonk2015-A]
              Toonk, A., "What caused the Google service interruption",
              March 2015, <http://www.bgpmon.net/
              what-caused-the-google-service-interruption/>.

   [Toonk2015-B]
              Toonk, A., "Massive route leak causes Internet slowdown",
              June 2015, <http://www.bgpmon.net/
              massive-route-leak-cause-internet-slowdown/>.

   [Wijchers]
              Wijchers, B. and B. Overeinder, "Quantitative Analysis of
              BGP Route Leaks",  RIPE-69, November 2014,
              <https://ripe69.ripe.net/presentations/157-RIPE-69-
              Routing-WG.pdf>.

   [Zmijewski]
              Zmijewski, E., "Indonesia Hijacks the World",  Dyn
              Research/Renesys Blog, April 2014,
              <http://research.dyn.com/2014/04/
              indonesia-hijacks-world/>.

Authors' Addresses

   Kotikalapudi Sriram
   US NIST

   Email: ksriram@nist.gov

   Doug Montgomery
   US NIST

   Email: dougm@nist.gov

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   Brian Dickson

   Email: brian.peter.dickson@gmail.com

   Keyur Patel
   Cisco

   Email: keyupate@cisco.com

   Andrei Robachevsky
   Internet Society

   Email: robachevsky@isoc.org

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