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Secure Inter-Domain Routing                                    K. Sriram
Internet-Draft                                             D. Montgomery
Intended status: Informational                                   US NIST
Expires: April 13, 2015                                 October 10, 2014


Design Discussion and Comparison of Replay-Attack Protection Mechanisms
                               for BGPSEC
          draft-sriram-replay-protection-design-discussion-04

Abstract

   The BGPSEC protocol requires a method for protection from replay
   attacks, at least to control the window of exposure.  In the context
   of BGPSEC, a replay attack occurs when an adversary suppresses a
   prefix withdrawal (implicit or explicit) or replays a previously
   received BGPSEC announcement for a prefix that has since been
   withdrawn.  This informational document provides design discussion
   and comparison of multiple alternative replay-attack protection
   mechanisms weighing their pros and cons.  It is meant to be a
   companion document to the standards track I-D.-ietf-sidr-bgpsec-
   rollover that will specify a method to be used with BGPSEC for
   replay-attack 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
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   This Internet-Draft will expire on April 13, 2015.

Copyright Notice

   Copyright (c) 2014 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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   (http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definition of Replay Attack . . . . . . . . . . . . . . . . .   3
   3.  Classification of Solutions . . . . . . . . . . . . . . . . .   4
   4.  Expire Time Method  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Key Rollover Method . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Periodic Key Rollover Method  . . . . . . . . . . . . . .   7
     5.2.  Event-driven Key Rollover Method  . . . . . . . . . . . .   8
       5.2.1.  EKR-A: EKR where Update Expiry is Enforced by CRL . .   9
       5.2.2.  EKR-B: EKR where Update Expiry is Enforced by
               NotValidAfter Time  . . . . . . . . . . . . . . . . .  10
       5.2.3.  EKR with Separate Key for Each Incoming-Outgoing
               Peering-Pair  . . . . . . . . . . . . . . . . . . . .  11
   6.  Summary of Pros and Cons  . . . . . . . . . . . . . . . . . .  12
   7.  Summary and Conclusions . . . . . . . . . . . . . . . . . . .  14
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   The BGPSEC protocol [I-D.ietf-sidr-bgpsec-protocol] requires a method
   for protection from replay attacks, at least to control the window of
   exposure [I-D.ietf-sidr-bgpsec-reqs].  In the context of BGPSEC, a
   replay attack occurs when an adversary suppresses a prefix withdrawal
   or replays a previously received BGPSEC announcement for a prefix
   that has since been withdrawn.

   In this informational document, we provide design discussion and
   comparison of various replay-attack protection mechanisms that may be
   used in conjunction with the BGPSEC protocol.  It is meant to be a
   companion document to the standards track document
   [I-D.ietf-sidr-bgpsec-rollover] that will specify a method to be used
   with BGPSEC for replay-attack protection.  Here we consider four
   alternative mechanisms - one based on the explicit Expire Time



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   approach and three different variants based on the Key Rollover
   approach.  We provide a detailed comparison between 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-sidr-bgpsec-rollover].

2.  Definition of Replay Attack

   In the context of BGPSEC, a replay attack occurs when an adversary
   suppresses a prefix withdrawal (implicit or explicit).  A replay
   attack occurs also when the adversary replays a previously received
   BGPSEC announcement for a prefix that has since been withdrawn.  In
   the rest of this document, we will refer to either of these two
   situations as repay attack.  The following are examples of replay
   attacks:

   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 towards itself by
   pretending to still have a signed and valid route for P.  In effect,
   AS2 can conduct a DOS attack on a server located at AS1 at prefix P.
   (See slide #15 in [replay-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 had 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 send an implicit Withdraw.  However, AS2 suppresses it.  AS2 can
   thus make a significant part of traffic destined for prefix P to flow
   via itself and eavesdrop on the data but not cause a DOS attack.
   (See slide #16 in [replay-discussion] for an illustration.)

   Example 3: AS1 has AS2 and AS3 as eBGPSEC peers.  AS2 and AS3 are
   also eBGPSEC peers.  At time x, AS1 had 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 - announces prefix P without prepending (Update:
   AS1{pCount=1} P) to AS3 and with prepending (Update: AS1{pCount=2} P)
   to AS2.  AS2 suppresses the new prepended path announcement (does not
   send to its peers any new update about P).  Thus AS2 carries more of
   AS1's ingress data traffic and generates more revenue for itself at




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   the expense of AS1.  (See slide #17 in [replay-discussion] for an
   illustration.)

   Thus the scenarios and motivations for replay attacks may differ as
   illustrated by the examples above.

   A requirement for replay-attack protection can be stated as follows.
   The update that AS1 sent to AS2 at time x should expire at time x+w.
   That means, AS2 can suppress the Withdraw or possibly replay the
   update from AS1 for prefix P until at most x+w.  This limits the
   replay 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.)

3.  Classification of Solutions

   Mechanisms for replay-attack protection can be classified into two
   broad categories as follows:

   o  Expire Time (ET) Method: This method uses an explicit expire time
      field in the BGPSEC update.

   o  Key Rollover (KR) Method: In this method, the update expiry is
      enforced by a key rollover.  Router rolls over to a new signing
      cert with a new pair of keys, and the previous router cert 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
         NotValidAfter time.

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







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4.  Expire Time Method

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

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

   o  Expire 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
      jittered in time by design to reduce burstiness.  The beacon
      interval can be different at different originating ASes.

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

   Discussion of Pros and Cons:

   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 the 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
   [replay-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, if the unit of ET is 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.








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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
      certs as follows: A pair of origination certs (current and next)
      for signing prefixes being originated by the AS of the router, and
      a pair of transit certs (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 certs and need not maintain the extra pair of transit
      certs.

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

      *  Cert 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 NotValidAfter time.

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

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

   Pro: The KR method functions by manipulating the RPKI objects (certs,
   keys, NotValidAfter field in cert, 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 introduces additional churn in the global RPKI
   system.

   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).





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   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 cert's NotValidAfter time is used as the
      implicit expire time for origin's signature.

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

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

   o  A new 'next' cert is created and propagated at beaconing time.
      This can also be done with a good lead time.  In practice,
      multiple 'next' certs can be kept in the pipeline.  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 certs can have very large NotValidAfter time (say ~years).

   o  When a peering or policy change event occurs at a transit router,
      the router (i.e.  BGPSEC router with PKR) does not perform any 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:





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

   Pro: The main pro for the PKR method is the same as that for the
   Expire 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 [replay-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 pro discussed above for the KR method regarding parameter
   changes (e.g., beacon interval units) not requiring change of
   protocol on the wire is naturally applicable here.

   Con: Churn in the RPKI is of concern.  Every BGPSEC router rolls two
   origination certs (current and next) once in every beacon (i.e., re-
   origination) interval.

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 the transit key as well as the
      origination key.

   o  If a key rollover takes place, then a corresponding (origination
      or transit) new 'next' cert 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 (400K



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   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
   3x400K = 1.2 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, following the same
   event, the router at AS5 sends only 4x100 = 400 Withdraws and signs/
   re-propagates ZERO prefix updates.  (An illustration can be found in
   slide #9 in [replay-discussion].  Also, additional peering change
   scenarios and quantitative comparisons can be found in slides #10 and
   #11 in [replay-discussion].)

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

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  NotValidAfter time of origination and transit certs is set to a
      large value (~year).

   o  Whenever key rollover (for origination or transit) occurs, then
      CRL is propagated for the old cert.  So the old update expires
      (due to invalid state) only when the CRL propagates and reaches
      the 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 cert 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:




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   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
   replay-attack 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].

5.2.2.  EKR-B: EKR where Update Expiry is Enforced by NotValidAfter 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  NotValidAfter time of current origination and transit certs is set
      to a value determined by the desired vulnerability window (~day).

   o  Update expiry is controlled by NotValidAfter time and CRL is not
      sent for the old cert when key rollover happens.

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

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

   o  Likewise for the transit (current and next) certs and keys.

   o  Changes in certs to extend their NotValidAfter 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 NotValidAfter time of
      the cert has passed.





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   o  The changes in certs to advance NotValidAfter time can be
      scheduled and propagated in RPKI 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 expiry is automatic in case the NotValidAfter time of any
   of the certs used to sign the update has not been extended.  So the
   replay-attack 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 cert changes but the key pair and SKI remain unchanged.  Routers
   do not receive NotValidAfter time from their RPKI cache server.
   There is no need for it.  Instead, the RPKI cache server keeps track
   of NotValidAfter 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 certs need to be reissued periodically to
   extend their validity time (in the absence of any 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 whetted 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 at a
   router.  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.  They 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



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   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.

   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 replay-
   attack protection methods.  This summary follows from the discussion
   above in Section 4 and Section 5.

   +----------+---------------------------+----------------------------+
   | Method   | Pros                      | Cons                       |
   +----------+---------------------------+----------------------------+
   | Expire   | 1. The background load    | 1. Prefix owner can abuse  |
   | 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 certs (current and |



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   |          | workload even when a      | next) for each origination |
   |          | peering change happens at | router are rolled once     |
   |          | that AS. Beaconing (i.e.  | every beacon (i.e. re-     |
   |          | periodic re-origination)  | origination) interval.     |
   |          | facilitates this.         | 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   |                            |
   |          | 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 replay-attack       |
   |          | RPKI is much lower than   | vulnerability window is    |
   |          | that in the EKR-B method. | dependent on 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   |                           |                            |



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   | (EKR-B)  |                           |                            |
   |          | ---                       | ---                        |
   |          | 2. The replay-attack      | 2. The added churn in RPKI |
   |          | vulnerability window is   | is much higher than that   |
   |          | enforced by NotValidAfter | in the EKR-A method.       |
   |          | time in certs and is      |                            |
   |          | therefore predictable.    |                            |
   |          | ---                       | ---                        |
   |          | 3. Same as Pro #4 for the |                            |
   |          | PKR method.               |                            |
   +----------+---------------------------+----------------------------+

               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 replay-attack protection.  It is hoped that
   the SIDR WG will take the insights and trade-offs presented here as
   input for deciding on the choice of a mechanism for protection from
   replay attacks.  Once that decision is made, the chosen mechanism
   would be included in the standards track document
   [I-D.ietf-sidr-bgpsec-rollover].

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

   1.  The Expire 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



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       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 NotValidAfter time of the certs used in update signatures.  In
       EKR-B, previous update automatically becomes invalid at the
       earliest NotValidAfter time of the certs used in the signatures
       unless each of those certs' NotValidAfter time has been extended.
       In the latter method, changes in certs to extend their
       NotValidAfter 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 (see Section 5.2.2).  The
       changes in certs to advance NotValidAfter time can be scheduled
       and propagated in RPKI 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 Roque Gagliano, Brian Weis and Steve
   Kent for helpful discussions.  Further, we are thankful to fellow
   NIST BGP team members 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-sidr-bgpsec-rollover] and [I-D.ietf-sidr-bgpsec-protocol].
   The reader is therefore directed to the security considerations
   provided in those documents.







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11.  References

11.1.  Normative References

   [I-D.ietf-sidr-bgpsec-protocol]
              Lepinski, M., "BGPSEC Protocol Specification", draft-ietf-
              sidr-bgpsec-protocol-09 (work in progress), July 2014.

11.2.  Informative References

   [I-D.ietf-sidr-bgpsec-reqs]
              Bellovin, S., Bush, R., and D. Ward, "Security
              Requirements for BGP Path Validation", draft-ietf-sidr-
              bgpsec-reqs-12 (work in progress), July 2014.

   [I-D.ietf-sidr-bgpsec-rollover]
              Gagliano, R., Patel, K., and B. Weis, "BGPSEC router key
              rollover as an alternative to beaconing", draft-ietf-sidr-
              bgpsec-rollover-02 (work in progress), April 2013.

   [replay-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>.

   [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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