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Secure Inter-Domain Routing                                    K. Sriram
Internet-Draft                                             D. Montgomery
Intended status: Informational                                   US NIST
Expires: October 19, 2017                                 April 17, 2017


  Design Discussion and Comparison of Protection Mechanisms for Replay
              Attack and Withdrawal Suppression in BGPsec
          draft-sriram-replay-protection-design-discussion-08

Abstract

   In the context of BGPsec, a withdrawal suppression occurs when an
   adversary AS suppresses a prefix withdrawal with the intension of
   continuing to attract traffic for that prefix based on a previous
   (signed and valid) BGPsec announcement that was earlier propagated.
   Subsequently if the adversary AS had a BGPsec session reset with a
   neighboring BGPsec speaker and when the session is restored, the AS
   replays said previous BGPsec announcement (even though it was
   withdrawn), then such a replay action is called a replay attack.  The
   BGPsec protocol should incorporate a method for protection from
   Replay Attack and Withdrawal Suppression (RAWS), at least to control
   the window of exposure.  This informational document provides design
   discussion and comparison of multiple alternative RAWS protection
   mechanisms weighing their pros and cons.  This is meant to be a
   companion document to the standards track draft-ietf-sidrops-bgpsec-
   rollover that will specify a method to be used with BGPsec for RAWS
   protection.

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/.

   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 October 19, 2017.






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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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Description and Scenarios of Replay Attacks and Withdrawal
       Suppression . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Classification of Solutions . . . . . . . . . . . . . . . . .   4
   4.  Expiration Time Method  . . . . . . . . . . . . . . . . . . .   5
   5.  Key Rollover Method . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Periodic Key Rollover Method  . . . . . . . . . . . . . .   7
     5.2.  Event-driven Key Rollover Method  . . . . . . . . . . . .   9
       5.2.1.  EKR-A: EKR where Update Expiry is Enforced by CRL . .  10
       5.2.2.  EKR-B: EKR where Update Expiry is Enforced by
               NotAfter Time . . . . . . . . . . . . . . . . . . . .  11
       5.2.3.  EKR with Separate Key for Each Incoming-Outgoing
               Peering-Pair  . . . . . . . . . . . . . . . . . . . .  12
   6.  Summary of Pros and Cons  . . . . . . . . . . . . . . . . . .  13
   7.  Summary and Conclusions . . . . . . . . . . . . . . . . . . .  15
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. Informative References  . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   In BGP or BGPsec, prefix or route withdrawals happen, and a
   withdrawal can be explicit (i.e. route simply withdrawn) or implicit
   (i.e. a new route announcement replaces the previous).  In the
   context of BGPsec, a withdrawal suppression occurs when an adversary
   AS suppresses a prefix withdrawal with the intension of continuing to
   attract traffic for that prefix based on a previous (signed and
   valid) BGPsec announcement that was earlier propagated.  Subsequently
   if the adversary AS has a BGPsec session reset with a neighboring



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   BGPsec speaker and when the session is restored, the AS replays said
   previous BGPsec announcement (even though it was withdrawn), then
   such a replay action is called a replay attack.  The BGPsec protocol
   [I-D.ietf-sidr-bgpsec-protocol] requires a method for protection from
   Replay Attack and Withdrawal Suppression (RAWS), at least to control
   the window of exposure (see Sections 4.3, 4.4 of [RFC7353]).

   In this informational document, we provide design discussion and
   comparison of various RAWS protection mechanisms that may be used in
   conjunction with the BGPsec protocol.  This is meant to be a
   companion document to the standards track document
   [I-D.ietf-sidrops-bgpsec-rollover] that will specify a method to be
   used with BGPsec for RAWS protection.  Here we consider four
   alternative mechanisms - one based on the explicit Expiration Time
   approach and three variants based on the Key Rollover approach.  We
   provide a detailed comparison among these mechanisms, weighing their
   pros and cons.  This document is meant to help inform the decision
   process leading to an exact description for the mechanism to be
   finalized and formally specified in
   [I-D.ietf-sidrops-bgpsec-rollover].

2.  Description and Scenarios of Replay Attacks and Withdrawal
    Suppression

   The following are examples of various forms of replay attack and
   withdrawal suppression (RAWS):

   Example 1: AS1 has AS2 and AS3 as eBGPsec peers.  At time x, AS1 had
   announced a prefix (P) to AS2 and AS3.  At a later time (x+d), AS1
   sends a Withdraw for prefix P to AS2.  AS2 suppresses the Withdraw
   (does not send to its peers any explicit or implicit Withdraw).  AS2
   continues to attract some of the data for prefix P by pretending to
   still have a valid (signed) route for P.  In effect, AS2 can conduct
   a Denial of Service (DOS) attack on a server located at prefix P.
   (See slide #15 in [RAWS-discussion] for an illustration.)

   Example 2: AS1 has AS2 and AS3 as eBGPsec peers.  AS2 and AS3 are
   also eBGPsec peers.  At time x, AS1 announced a prefix P to AS2 and
   AS3.  AS3 also propagates to AS2 its route (via AS1) for prefix P.
   At a later time (x+d), AS1 discontinues its peering with AS2.  AS2
   should propagate an alternate longer path via AS3 for prefix P and
   thus implicitly withdraw the route via AS1.  However, AS2 suppresses
   it.  AS2 can thus make some traffic destined for prefix P to flow via
   itself.  This enables AS2 to eavesdrop on the data but not cause a
   DOS attack.  AS2 may also choose to DoS attack hosts in prefix P.
   (See slide #16 in [RAWS-discussion] for an illustration.)





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   Example 3: AS1 has AS2 and AS3 as eBGPsec peers.  AS2 and AS3 are
   also eBGPsec peers.  At time x, AS1 announced a prefix P to AS2
   without prepending (Update: AS1{pCount=1} P) but announced the same
   prefix to AS3 with prepending (Update: AS1{pCount=2} P).  Thus AS1
   had preferred its ingress data traffic for prefix P to come in via
   AS2.  At a later time (x+d), AS1 switches ingress data path
   preference to AS3 over AS2 - announcing prefix P to AS3 without
   prepending (Update: AS1{pCount=1} P) and to AS2 with prepending
   (Update: AS1{pCount=2} P).  AS2 suppresses the new prepended path
   announcement (does not send to its peers any new update about P).
   Thus AS2 continues to attract more of AS1's ingress data traffic and
   generates more revenue for itself at the expense of AS1.  (See slide
   #17 in [RAWS-discussion] for an illustration.)

   As illustrated above, the mechanisms and motivations for RAWS may
   differ.

   In the context of the examples mentioned above, a requirement for
   RAWS protection can be stated as follows.  An update that AS1 sends
   to AS2 at time x should expire at time x+w.  This capability would
   allow other ASes to detect actions by AS2 to suppress the Withdraw or
   replay the update from AS1 for prefix P after time x+w.  This limits
   the RAWS vulnerability window.  (Note: If no peering or policy change
   affecting prefix P occurs during the vulnerability window, then a
   typical solution would include a method for extending the validity
   period of the route(s) beyond x+w.)  We will later discuss what a
   reasonable window size, w, should be.

   The obvious downside of any mechanism that support this capability is
   that it will require AS1 to send a new update before time x+w, and
   this update will need to propagate via all the paths that the
   original update traversed.  Thus more update traffic will result than
   if the RAWS protection mechanism were not employed, and this traffic
   will require cryptographic processing by all of the routers along the
   paths.  Thus the creation of a mechanism to counter RAWS attacks
   potentially introduces a new opportunity for DoS attacks against
   eBGPsec routers.

3.  Classification of Solutions

   Mechanisms for RAWS protection can be classified into two broad
   categories as follows:

   o  Expiration Time (ET) Method: This method uses an explicit
      Expiration Time field in the BGPsec update.  (Note: Explicit
      Expire Time field was included in an earlier version of the BGPsec
      protocol specification [draft-ietf-sidr-bgpsec-protocol-01].)




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   o  Key Rollover (KR) Method: In this method, the update expiration is
      enforced by a key rollover.  Router transitions to a new
      certificate with a new pair of keys, and the previous router
      certificate either expires or is revoked.

   The Key Rollover method can be further characterized into the
   following sub categories:

   o  Periodic Key Rollover (PKR): Key rollovers happen at periodic
      intervals.

   o  Event-driven Key Rollover (EKR): Key rollovers happen only when
      peering or policy change events occur.

      *  EKR-A: EKR where expiry of previous update is enforced by CRL.

      *  EKR-B: EKR where expiry of previous update is controlled by
         NotAfter time (router certificate is not revoked at the time
         when the event happens).

   In Section 4, Section 5, and Section 6 we describe the various
   methods listed above, and discuss their pros and cons.

4.  Expiration Time Method

   The details of the Expiration Time (ET) method are as follow:

   o  Explicit Expiration Time is used for origin's signature.

   o  Expiration Time field is required in the BGPsec update.

   o  Periodic re-origination (beaconing) of prefixes is performed by
      origin ASes.  The value in the ET field in the update is extended
      at beaconing time, and thereby the update is refreshed.  Every
      prefix in the Internet is re-originated and propagates through the
      Internet once every 'beacon' interval.

   o  These beacons are distributed actions by prefix owners and are
      intended to be jittered in time to reduce burstiness.  The beacon
      interval can be different at each originating AS.

   o  Beacon interval granularity: TBD but preferably in fairly granular
      units (days).  It is important to limit the ability of each AS to
      specify a short beacon interval, to prevent an AS from using this
      mechanism to cause BGPsec to thrash.

   Discussion of Pros and Cons:




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   Pro: This method is easy on transit routers.  In the event of peering
   or policy change, BGPsec with the ET method behaves the same way as
   BGP-4 in terms of which prefix routes are propagated.  That is, the
   router re-evaluates best paths factoring in peering or policy
   changes, and propagates only those prefix routes that have a change
   in best path.  In other words, there is no necessity for a transit
   BGPsec router to re-propagate and refresh prefixes on all peering
   links.  This is because prefix updates are refreshed anyway once
   every beacon interval by all prefix originators.  There is low
   steady-state traffic associated with beaconing (see Figure on slide
   #8 in [RAWS-discussion]), but there are no huge bursts or spikes in
   workload due to peering or policy change events at transit routers.

   Con: Equipment vendor can potentially facilitate unnecessary frequent
   beaconing if ISP urges and pays (dollar attack!).  This possibility
   is mitigated by having a well thought-out granularity for ET, for
   example, setting the unit for advertising ET to one day (rather than
   one minute).

   Con: A change in on-the-wire BGPsec protocol would be needed in case
   the unit of the ET field (granularity) needs to be changed.

5.  Key Rollover Method

   Key Rollover (KR) method has three variations as outlined in
   Section 3.  Those will be discussed later in this section.  The
   following features are common to all variants of the KR method:

   o  In the KR method, it is best if the BGPsec router has two pairs of
      certificates as follows: A pair of origination certificates
      (current and next) for signing prefixes being originated by the AS
      of the router, and a pair of transit certificates (current and
      next) for signing transit prefixes.

   o  Note: If a BGPsec router only originates prefixes (i.e. has no
      transit prefixes), then it needs to maintain only a pair of
      origination certificates and need not maintain the extra pair of
      transit certificates.  (This would be the case for the vast
      majority of ASes, since most are stubs.)

   o  The three KR methods differ in how the rollover of certificates
      (or keys) is done:

      *  Certificate rollovers are Periodic vs. Event-driven.

      *  In the Event-driven method, the expiry of old update is (A)
         Enforced by CRL vs. (B) Controlled by NotAfter time.




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      *  In (A), certificate's NotAfter field is set to a very large
         value and CRL is issued to revoke the certificate when
         necessary.  In (B), NotAfter field set to a permissible
         vulnerability window time, and CRL to revoke certificate is not
         required.

   Discussion of Pros and Cons (common to all Key Rollover methods):

   Pro: The KR method functions by manipulating the RPKI objects
   (certificates, keys, NotAfter field in certificate, etc.) to refresh
   updates or to cause expiry of previously propagated updates.  Unlike
   the ET method, it does not rely on any explicit field in the update.
   Hence, an advantage of the KR method over the ET method is that in
   case any parameters need to change or if the method itself is
   modified, then there is no impact on the BGPsec protocol on the wire.

   Con: The KR method increases the number of objects in the RPKI
   repository system, by requiring at least two certificates for every
   transit AS.  It also introduces additional churn in the global RPKI
   as these certificates expire (or are revoked) and are replaced.

   Con: There is also added update churn.  The amount of update churn
   varies depending on the type of KR method used (see Section 5.1 and
   Section 5.2).

   We will now describe and discuss in detail the variants of the KR
   method.

5.1.  Periodic Key Rollover Method

   The details of the Periodic Key Rollover (PKR) method are as follow.

   o  Router's origination certificate's NotAfter time is used
      effectively as expiration time for origin's signature.

   o  Each origination router re-originates (i.e. beacons) before
      NotAfter time of the current origination certificate.  Beaconing
      is periodic re-origination of prefixes by origin ASes.

   o  At beaconing time, the next origination certificate becomes the
      new current certificate, and the new update is signed with the
      private key of this new current certificate and re-originated.

   o  A new 'next' origination certificate is created and propagated at
      or before beaconing time.  This can also be done with a good lead
      time.  In practice, multiple 'next' certificates for each router
      could be propagated and kept in the in the RPKI repositories.




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      They must have contiguous or slightly overlapping validity
      periods.

   o  Every prefix in the Internet is re-originated and propagates
      through the Internet once every 'beacon' interval.

   o  The re-originations or beacons are distributed actions by prefix
      owners and jittered in time by design to reduce burstiness.  The
      beacon interval can be different at different originating ASes.

   o  Beacon (or re-origination) interval granularity: TBD but
      preferably in fairly granular units (days).

   o  Transit certificates can have large NotAfter time (e.g., whatever
      duration is required normally for key maintenance).

   o  When a peering or policy change event occurs at a transit router,
      the router does not perform any reactive key rollover.  The router
      re-evaluates best paths factoring in peering or policy changes,
      and propagates only those prefix routes that have a change in best
      path (similar to BGP-4).  There is no necessity for the BGPsec
      router to re-propagate and refresh prefixes on all peering links.
      This is because prefix updates are refreshed anyway once every re-
      origination (i.e. beaconing) interval by all prefix originators.

   Discussion of Pros and Cons:

   Several of the same pros/cons of the Expiration Time method also
   apply here for the PKR method.

   Pro: The main pro for the PKR method is the same as that for the
   Expiration Time (ET) method.  That is, being easy on transit routers
   as discussed in Section 4.  Just as in the ET method, there is low
   steady-state traffic associated with periodic re-originations (i.e.
   beaconing) (see Figure on slide #8 in [RAWS-discussion]), but there
   are no huge bursts or spikes in workload due to peering or policy
   change events at transit routers.  (See comparisons with the EKR
   methods in Section 5.2.)

   Pro: The common pro discussed previously for all KR methods, namely,
   not requiring change of protocol on the wire when a parameter change
   occurs (e.g., change of beacon interval units) is naturally
   applicable here.

   Con: Churn in the RPKI is of concern.  Every BGPsec router renews and
   propagates its 'next' origination certificate once in every beacon
   (i.e. re-origination) interval.




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5.2.  Event-driven Key Rollover Method

   The common details of the Event-driven Key Rollover (EKR) methods are
   as follow.

   o  Key rollover is reactive to events (not periodic).

   o  If a peering or policy change event involves only prefixes being
      originated at the AS of the router, then the router rolls only the
      origination key.

   o  If a peering change event involves transit prefixes at the AS of
      the router, then the router rolls its transit key as well as the
      origination key.  Both keys are rolled because any peering
      relationship change also requires refresh of prefixes originated
      by the router.

   o  If a key rollover takes place, then a corresponding (origination
      or transit) new 'next' certificate is propagated in RPKI.

   Discussion of Pros and Cons:

   Pro: As long as no triggering events occur, there is no added update
   churn in BGPsec.

   Con: Whenever the transit key is rolled, there is a storm of BGPsec
   updates at routers in transit ASes.  For example, consider BGPsec
   capable transit AS5 that is connected to four BGPsec non-stub
   customers (AS1, AS2, AS3, AS4).  Assume each AS has a single BGPsec
   router in it.  AS1 through AS4 each receives almost full table
   (approximately 600K signed prefix updates) from AS5.  Assume also
   that AS1 and its customers together originate 100 prefixes in total;
   likewise for AS2, AS3 and AS4.  Now consider that an event occurs
   whereby the peering between AS1 and AS5 is discontinued.  As a result
   of this event, in the EKR method, the AS5 router signs and re-
   propagates approximately 3x600K = 1.8 Million signed prefix updates
   to AS2, AS3 and AS4 combined.  In addition, it also sends 4x100 = 400
   Withdraws, which are negligible.  In comparison, in the PKR method,
   reacting to the same event, the BGPsec router at AS5 sends only 4x100
   = 400 Withdraws and signs/re-propagates ZERO prefix updates.  (An
   illustration can be found in slide #9 in [RAWS-discussion].  Also,
   additional peering change scenarios and quantitative comparisons can
   be found in slides #10 and #11 in [RAWS-discussion].)

   It remains to be seen through measurement and modeling how the impact
   of such large bursts of workload in the EKR method at the time of
   event occurrence can be managed in route processors, e.g., by
   jittering and throttling the workload.



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5.2.1.  EKR-A: EKR where Update Expiry is Enforced by CRL

   EKR-A builds on the common principles as described for EKR above in
   Section 5.2.  The additional details of EKR-A operation are as
   follow:

   o  NotAfter time of origination and transit certificates is set to a
      large value (e.g., one year or whatever period needed for normal
      key maintenance).

   o  Whenever key rollover (for origination or transit) occurs, then a
      CRL is propagated for the certificate that was used until that
      time.  So the old update expires (due to invalid state) only when
      the CRL propagates and reaches each relying router.

   o  This method relies on end-to-end CRL propagation through the RPKI
      system to enforce expiry of a previous update whenever the need
      arises.

   o  The CRL either propagates all the way to the relying router, or
      the RPKI cache server of the router receives the CRL and then
      sends a withdrawal of the {AS, SKI, Pub Key} tuple to the router.
      Either way, the CRL must in effect propagate all the way to the
      relying router.

   o  Thus the attack vulnerability window with the EKR-A method is
      governed by the end-to-end CRL propagation time.

   Discussion of Pros and Cons:

   The following pro and con for the EKR-A method are in addition to the
   common pros and cons listed above for the KR and EKR methods
   (Section 5 and Section 5.2).

   Pro: EKR-A has much less RPKI churn than PKR or EKR-B (see
   Section 5.2.2).

   Con: Router needs to receive a CRL or a withdraw of {AS, SKI, Pub
   Key} tuple in order to know an update has expired.  Hence, the RAWS
   vulnerability window is determined by the CRL propagation time which
   can vary widely from one relying router to another router that may be
   in different regions.  It is anticipated that this would be no worse
   than 24 hours, but needs to be confirmed by measurements in an
   operational or emulated RPKI systems [rpki-delay].







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5.2.2.  EKR-B: EKR where Update Expiry is Enforced by NotAfter Time

   EKR-B builds on the common principles as described for EKR above in
   Section 5.2.  The additional details of EKR-B operation are as
   follow:

   o  NotAfter time of current origination and transit certificates is
      set to a value determined by the desired vulnerability window
      (~day).

   o  Update expiry is controlled by NotAfter time (router certificate
      is not revoked at the time when the event happens).

   o  If no triggering event occurs to cause origination key rollover
      within a pre-set time (NotAfter), then new origination (current
      and next) certificates are issued only to extend the NotAfter time
      but the corresponding key pairs and SKIs remain unchanged.

   o  Do likewise (i.e. similar to what the above bullet says) for the
      transit (current and next) certificates and keys.

   o  A previous update automatically becomes invalid at the earliest
      NotAfter time of the certificates used in the signatures unless
      each of those certificates' NotAfter time has been extended.

   o  Changes in certificates to extend their NotAfter time need not
      propagate end-to-end (all the way to the relying routers); they
      may propagate only up to the RPKI cache server of the relying
      router.  RPKI cache server would send a withdraw for an {AS, SKI,
      Pub Key} tuple to a relying router if the NotAfter time of the
      certificate has passed.

   o  Changes in certificates to advance NotAfter time can be scheduled
      and propagated (in RPKI) reasonably well in advance.

   Discussion of Pros and Cons:

   The following pro and con for EKR-B are in addition to the common
   pros and cons listed above for the KR and EKR methods (Section 5 and
   Section 5.2).

   Pro: Update expiration is automatic in case the NotAfter time of any
   of the certificates used to validate the update has not been
   extended.  So the RAWS vulnerability window is predictable and not
   influenced by the RPKI end-to-end propagation time.

   Pro: Routers do not get any RPKI updates from the RPKI cache server
   when a certificate changes but the corresponding key pair and SKI



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   remain unchanged.  Routers do not receive NotAfter time from their
   RPKI cache server.  There is no need for it.  Instead, the RPKI cache
   server keeps track of NotAfter time, and provides to routers only
   valid {AS, SKI, Pub Key} tuples.  This saves some RPKI state
   maintenance workload at the routers.

   Con: EKR-B has much more RPKI churn than EKR-A because both
   origination and transit certificates need to be reissued periodically
   to extend their validity time (even in the absence of any peering or
   policy change events).

5.2.3.  EKR with Separate Key for Each Incoming-Outgoing Peering-Pair

   This is a place holder section where we mention another variant of
   the EKR method.  This idea has not been considered or vetted by the
   SIDR WG yet.  So we only mention it here briefly.

   As noted earlier, the EKR methods considered so far generate a huge
   spike in workload whenever the transit key rollover takes place.  One
   way to reduce that workload is to have a separate signing key for
   each incoming-outgoing peering pair.  For example, consider a BGPsec
   router in AS4 that has peers in AS1, AS2, and AS3.  The router will
   hold six signing keys, one each corresponding to (AS1, AS2), (AS2,
   AS1), (AS1, AS3), (AS3, AS1), (AS2, AS3), and (AS3, AS2) peering-
   pairs.  Note that the directionality of peering is included here and
   is necessary.  The key corresponding to (AS-i, AS-j) would only be
   used to sign updates received from AS-i and being forwarded to AS-j.
   In the general case, when the BGPsec router has n peers, the number
   of transit keys will be n(n-1).  Since there would be a Current and a
   Next key (for rollover), the number of transit keys held in the
   router for signing will be actually 2n(n-1).  When a peering or
   policy change occurs, the router would rollover only those specific
   keys that correspond to the peering-pairs over which the prefix
   updates are affected.  In the above example, suppose a policy change
   between AS4 and AS1 causes AS4 to prepend prefixes sent to AS1
   (pCount changed from 1 to 2).  Then AS4 would do key rollover only
   for (AS2, AS1) and (AS3, AS1) peering-pairs, and not for any of the
   others.  This would substantially reduce the quantity of prefix
   updates that are signed and re-propagated.  In general, when peering
   or policy changes occur, this method will reduce the number of prefix
   updates to be re-propagated to exactly the same as that with normal
   BGP.  That means that this method would also be on par with the ET
   and PKR methods in terms of update churn when a peering or policy
   change takes place.  The downside of this method is that the router
   needs to maintain 2n(n-1) key pairs if it has n BGPsec peers.






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   Detailed discussion and comparison of this method with other methods
   can be provided in a later version of this document if the idea picks
   up interest in the WG.

6.  Summary of Pros and Cons

   Table 1 below summarizes the pros and cons for the various RAWS
   protection methods.  This summary follows from the discussion above
   in Section 4 and Section 5.

   +----------+---------------------------+----------------------------+
   | Method   | Pros                      | Cons                       |
   +----------+---------------------------+----------------------------+
   | Expirati | 1. The background load    | 1. Prefix owner can abuse  |
   | on Time  | due to beaconing is low   | by beaconing too           |
   | (ET)     | and not bursty.           | frequently.                |
   |          | ---                       | ---                        |
   |          | 2. Transit AS does NOT    | 2. Any change to the units |
   |          | have a huge spike in      | (granularity) of ET field  |
   |          | workload even when a      | entails a change to on-    |
   |          | peering or policy change  | the-wire BGPsec protocol.  |
   |          | happens at that AS.       |                            |
   |          | Beaconing facilitates     |                            |
   |          | this.                     |                            |
   |          | ---                       | ---                        |
   |          | 3. Does not add to RPKI   |                            |
   |          | churn.                    |                            |
   | -------- | ------------------------- | -------------------------- |
   | Periodic | 1. The background load    | 1. Prefix owner can abuse  |
   | Key      | due to beaconing is low   | by beaconing (i.e. re-     |
   | Rollover | and not bursty.           | originating) too           |
   | (PKR)    |                           | frequently.                |
   |          | ---                       | ---                        |
   |          | 2. Transit AS does NOT    | 2. Adds to RPKI churn. A   |
   |          | have a huge spike in      | pair of certificates       |
   |          | workload even when a      | (current and next) for     |
   |          | peering change happens at | each origination router    |
   |          | that AS. Beaconing (i.e.  | are rolled once every      |
   |          | periodic re-origination)  | beacon (i.e. re-           |
   |          | facilitates this.         | origination) interval.     |
   |          |                           | Significantly more RPKI    |
   |          |                           | churn than that with EKR-A |
   |          |                           | or EKR-B methods.          |
   |          | ---                       | ---                        |
   |          | 3. If the periodic re-    |                            |
   |          | origination (i.e.         |                            |
   |          | beaconing) interval units |                            |
   |          | change, BGPsec protocol   |                            |



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   |          | on the wire remains       |                            |
   |          | unaffected.               |                            |
   |          | ---                       | ---                        |
   |          | 4. Changes in the method  |                            |
   |          | (while still based on Key |                            |
   |          | Rollover) can be          |                            |
   |          | accommodated without      |                            |
   |          | requiring any change to   |                            |
   |          | on-the-wire BGPsec        |                            |
   |          | protocol.                 |                            |
   | -------- | ------------------------- | -------------------------- |
   | Event    | 1. No update churn for    | 1. Whenever the transit    |
   | driven   | long periods when no      | key is rolled (in response |
   | Key      | peering or policy changes | to a peering or policy     |
   | Rollover | occur.                    | change event), there is a  |
   | Type A   |                           | storm of BGPsec updates,   |
   | (EKR-A)  |                           | especially at routers in   |
   |          |                           | large transit ASes.        |
   |          | ---                       | ---                        |
   |          | 2. The added churn in     | 2. The RAWS vulnerability  |
   |          | RPKI is much lower than   | window is dependent on     |
   |          | that in the EKR-B method. | end-to-end CRL             |
   |          |                           | propagation. It may vary   |
   |          |                           | significantly from one     |
   |          |                           | relying router to another  |
   |          |                           | that may be in different   |
   |          |                           | regions.                   |
   |          | ---                       | ---                        |
   |          | 3. Same as Pro #4 for the |                            |
   |          | PKR method.               |                            |
   | -------- | ------------------------- | -------------------------- |
   | Event    | 1. Same as Pro #1 for the | 1. Same as Con #1 for the  |
   | driven   | EKR-A method.             | EKR-A method.              |
   | Key      |                           |                            |
   | Rollover |                           |                            |
   | Type B   |                           |                            |
   | (EKR-B)  |                           |                            |
   |          | ---                       | ---                        |
   |          | 2. The RAWS vulnerability | 2. The added churn in RPKI |
   |          | window is enforced by     | is much higher than that   |
   |          | NotAfter time in          | in the EKR-A method.       |
   |          | certificates and is       |                            |
   |          | therefore predictable.    |                            |
   |          | ---                       | ---                        |
   |          | 3. Same as Pro #4 for the |                            |
   |          | PKR method.               |                            |
   +----------+---------------------------+----------------------------+




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               Table 1: Table with Summary of Pros and Cons

7.  Summary and Conclusions

   We have attempted to provide insights into the operation of multiple
   alternative methods for RAWS protection.  It is hoped that the SIDR
   WG will utilize the analysis presented here as input for deciding on
   the choice of a mechanism for protection from RAWS.  Once that
   decision is made, the chosen mechanism would be included in the
   standards track document [I-D.ietf-sidrops-bgpsec-rollover].

   Some important considerations for the decision making can be possibly
   listed as follow:

   1.  The Expiration Time (ET) method is best (on par with the PKR
       method) in terms of preventing huge update workloads during
       peering and policy change events at transit routers with several
       peers.  It has no added RPKI churn.  But the ET method has the
       disadvantage of requiring on-the-wire protocol change if some
       parameters (e.g., the units of beacon interval) change.

   2.  The Periodic Key Rollover (PKR) method operates the same way as
       the ET method for preventing huge update workloads during peering
       and policy change events at transit routers with several peers.
       It does not have the disadvantage of requiring on-the-wire
       protocol change if some parameters (e.g., the units of beaconing/
       re-origination periodicity) change.  But it has the downside of
       added RPKI churn.

   3.  The Event-driven Key Roll (EKR-A and EKR-B) methods have
       significantly less RPKI churn than the PKR method.  They also
       have no BGPsec update churn during long quiet periods when no
       peering or policy change events occur.  But they suffer the
       drawback of creating huge update workloads during peering and
       policy change events at transit routers with several peers.  Can
       this workload be jittered or flow controlled to spread it over
       time without convergence delay concerns?  May be - needs further
       study.

   4.  The EKR-A method relies on end-to-end CRL propagation through the
       RPKI system to enforce expiry of a previous update when needed.
       By contrast, in the EKR-B method the update expiry is controlled
       by NotAfter time of the certificates used in update signatures.
       In EKR-B method, previous update automatically becomes invalid at
       the earliest NotAfter time of the certificates used in the
       signatures unless each of those certificates' NotAfter time has
       been extended.  Also, in EKR-B method, changes in certificates to
       extend their NotAfter time need not propagate end-to-end (all the



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       way to the relying routers); they may propagate only up to the
       RPKI cache server of the relying router (see Section 5.2.2).  The
       changes in certificates to advance NotAfter time can be scheduled
       and propagated (in RPKI) reasonably well in advance.

   5.  Besides being out-of-band relative to the BGPsec protocol on the
       wire, the other good thing about the Key Rollover method is that
       once the basics of the mechanism are implemented, there may be
       flexibility to implement PKR, EKR-A or EKR-B on top of it.  It
       may also be possible to switch from one method to another (within
       this class) if necessary based on operational experience; this
       transition would not require any change to on-the-wire BGPsec
       protocol.

8.  Acknowledgements

   The authors would like to thank Steve Kent for extensive review and
   many useful suggestions on an earlier version of this document.
   Thanks are also due to Roque Gagliano and Brian Weis for helpful
   discussions.  Further, we are thankful to Oliver Borchert and Okhee
   Kim for comments and suggestions.

9.  IANA Considerations

   This memo includes no request to IANA.

10.  Security Considerations

   This memo requires no security considerations of its own since it is
   targeted to be an informational RFC in support of
   [I-D.ietf-sidrops-bgpsec-rollover] and
   [I-D.ietf-sidr-bgpsec-protocol]  . The reader is therefore directed
   to the security considerations provided in those documents.

11.  Informative References

   [I-D.ietf-sidr-bgpsec-protocol]
              Lepinski, M. and K. Sriram, "BGPsec Protocol
              Specification", draft-ietf-sidr-bgpsec-protocol-22 (work
              in progress), January 2017.

   [I-D.ietf-sidrops-bgpsec-rollover]
              Weis, B., Gagliano, R., and K. Patel, "BGPsec Router
              Certificate Rollover", draft-ietf-sidrops-bgpsec-
              rollover-00 (work in progress), March 2017.






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   [RAWS-discussion]
              Sriram, K. and D. Montgomery, "Discussion of Key Rollover
              Mechanisms for Replay-Attack Protection", Presented
              at IETF-85 SIDR WG Meeting, November 2012,
              <http://www.ietf.org/proceedings/85/slides/
              slides-85-sidr-4.pdf>.

   [RFC7353]  Bellovin, S., Bush, R., and D. Ward, "Security
              Requirements for BGP Path Validation", RFC 7353,
              DOI 10.17487/RFC7353, August 2014,
              <http://www.rfc-editor.org/info/rfc7353>.

   [rpki-delay]
              Kent, S. and K. Sriram, "RPKI rsync Download Delay
              Modeling", Presented at IETF-86 SIDR WG Meeting, March
              2013, <http://www.ietf.org/proceedings/86/slides/
              slides-86-sidr-1.pdf>.

Authors' Addresses

   Kotikalapudi Sriram
   US NIST

   Email: ksriram@nist.gov


   Doug Montgomery
   US NIST

   Email: dougm@nist.gov





















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