IPSecME Working Group                                             Y. Nir
Internet-Draft                                               Check Point
Intended status: Standards Track                              V. Smyslov
Expires: September 2, 2016                                    ELVIS-PLUS
                                                           March 1, 2016

      Protecting Internet Key Exchange Protocol version 2 (IKEv2)
       Implementations from Distributed Denial of Service Attacks


   This document recommends implementation and configuration best
   practices for Internet Key Exchange Protocol version 2 (IKEv2)
   Responders, to allow them to resist Denial of Service and Distributed
   Denial of Service attacks.  Additionally, the document introduces a
   new mechanism called "Client Puzzles" that help accomplish this task.

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   This Internet-Draft will expire on September 2, 2016.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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
     1.1.  The Stateless Cookie  . . . . . . . . . . . . . . . . . .   3
     1.2.  Conventions Used in This Document . . . . . . . . . . . .   4
   2.  The Vulnerability . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Puzzles . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Retention Periods for Half-Open SAs . . . . . . . . . . . . .   8
   5.  Rate Limiting . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Plan for Defending a Responder  . . . . . . . . . . . . . . .  10
     6.1.  Session Resumption  . . . . . . . . . . . . . . . . . . .  12
   7.  Using Puzzles in the Protocol . . . . . . . . . . . . . . . .  12
     7.1.  Puzzles in IKE_SA_INIT Exchange . . . . . . . . . . . . .  12
       7.1.1.  Presenting Puzzle . . . . . . . . . . . . . . . . . .  13
       7.1.2.  Solving Puzzle and Returning the Solution . . . . . .  15
       7.1.3.  Computing Puzzle  . . . . . . . . . . . . . . . . . .  16
       7.1.4.  Analyzing Repeated Request  . . . . . . . . . . . . .  16
       7.1.5.  Making Decision whether to Serve the Request  . . . .  18
     7.2.  Puzzles in IKE_AUTH Exchange  . . . . . . . . . . . . . .  19
       7.2.1.  Presenting Puzzle . . . . . . . . . . . . . . . . . .  19
       7.2.2.  Solving Puzzle and Returning the Solution . . . . . .  20
       7.2.3.  Computing Puzzle  . . . . . . . . . . . . . . . . . .  21
       7.2.4.  Receiving Puzzle Solution . . . . . . . . . . . . . .  21
   8.  DoS Protection after IKE SA is created  . . . . . . . . . . .  22
   9.  Payload Formats . . . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  PUZZLE Notification . . . . . . . . . . . . . . . . . . .  23
     9.2.  Puzzle Solution Payload . . . . . . . . . . . . . . . . .  24
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  24
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  25
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     14.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   Denial of Service (DoS) attacks have always been considered a serious
   threat.  These attacks are usually difficult to defend against since
   the amount of resources the victim has is always bounded (regardless
   of how high it is) and because some resources are required for
   distinguishing a legitimate session from an attack.

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   The Internet Key Exchange protocol (IKE) described in [RFC7296]
   includes defense against DoS attacks.  In particular, there is a
   cookie mechanism that allows the IKE Responder to effectively defend
   itself against DoS attacks from spoofed IP-addresses.  However, bot-
   nets have become widespread, allowing attackers to perform
   Distributed Denial of Service (DDoS) attacks, which are more
   difficult to defend against.  This document presents recommendations
   to help the Responder thwart (D)DoS attacks.  It also introduces a
   new mechanism -- "puzzles" -- that can help accomplish this task.

   The IKE_SA_INIT Exchange described in Section 1.2 of [RFC7296]
   involves the Initiator sending a single message.  The Responder
   replies with a single message and also allocates memory for a
   structure called a half-open IKE Security Association (SA).  This
   half-open SA is later authenticated in the IKE_AUTH Exchange.  If
   that IKE_AUTH request never comes, the half-open SA is kept for an
   unspecified amount of time.  Depending on the algorithms used and
   implementation, such a half-open SA will use from around 100 bytes to
   several thousands bytes of memory.

   This creates an easy attack vector against an IKE Responder.
   Generating the IKE_SA_INIT request is cheap, and sending multiple
   such requests can either cause the Responder to allocate too much
   resources and fail, or else if resource allocation is somehow
   throttled, legitimate Initiators would also be prevented from setting
   up IKE SAs.

   An obvious defense, which is described in Section 5, is limiting the
   number of half-open SAs opened by a single peer.  However, since all
   that is required is a single packet, an attacker can use multiple
   spoofed source IP addresses.

1.1.  The Stateless Cookie

   Section 2.6 of [RFC7296] offers a mechanism to mitigate this DoS
   attack: the stateless cookie.  When the server is under load, the
   Responder responds to the IKE_SA_INIT request with a calculated
   "stateless cookie" - a value that can be re-calculated based on
   values in the IKE_SA_INIT request without storing Responder-side
   state.  The Initiator is expected to repeat the IKE_SA_INIT request,
   this time including the stateless cookie.

   Attackers that have multiple source IP addresses with return
   routability, such as in the case of bot-nets, can fill up a half-open
   SA table anyway.  The cookie mechanism limits the amount of allocated
   state to the size of the bot-net, multiplied by the number of half-
   open SAs allowed per peer address, multiplied by the amount of state

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   allocated for each half-open SA.  With typical values this can easily
   reach hundreds of megabytes.

   The mechanism described in Section 3 adds a proof of work for the
   Initiator by calculating a pre-image for a partial hash value.  This
   sets an upper bound, determined by the attacker's CPU, to the number
   of negotiations it can initiate in a unit of time.

1.2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  The Vulnerability

   If we break down what a Responder has to do during an initial
   exchange, there are three stages:

   1.  When the IKE_SA_INIT request arrives, the Responder:

       *  Generates or re-uses a Diffie-Hellman (D-H) private part.

       *  Generates a Responder Security Parameter Index (SPI).

       *  Stores the private part and peer public part in a half-open SA

   2.  When the IKE_AUTH request arrives, the Responder:

       *  Derives the keys from the half-open SA.

       *  Decrypts the request.

   3.  If the IKE_AUTH request decrypts properly:

       *  Validates the certificate chain (if present) in the IKE_AUTH

   Yes, there's a stage 4 where the Responder actually creates Child
   SAs, but when talking about (D)DoS, we never get to this stage.

   Stage #1 is pretty light on CPU power, but requires some storage, and
   it's very light for the Initiator as well.  Stage #2 includes
   private-key operations, so it's much heavier CPU-wise.  Stage #3
   includes public key operations, typically more than one.

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   To attack such a Responder, an attacker can attempt to either exhaust
   memory or to exhaust CPU.  Without any protection, the most efficient
   attack is to send multiple IKE_SA_INIT requests and exhaust memory.
   This should be easy because those requests are cheap.

   There are obvious ways for the Responder to protect itself even
   without changes to the protocol.  It can reduce the time that an
   entry remains in the half-open SA database, and it can limit the
   amount of concurrent half-open SAs from a particular address or
   prefix.  The attacker can overcome this by using spoofed source

   The stateless cookie mechanism from Section 2.6 of [RFC7296] prevents
   an attack with spoofed source addresses.  This doesn't completely
   solve the issue, but it makes the limiting of half-open SAs by
   address or prefix work.  Puzzles do the same thing only more of it.
   They make it harder for an attacker to reach the goal of getting a
   half-open SA.  They don't have to be so hard that an attacker can't
   afford to solve a single puzzle; it's enough that they increase the
   cost of a half-open SAs for the attacker so that it can create only a

   Reducing the amount of time an abandoned half-open SA is kept attacks
   the issue from the other side.  It reduces the value the attacker
   gets from managing to create a half-open SA.  For example, if a half-
   open SA is kept for 1 minute and the capacity is 60,000 half-open
   SAs, an attacker would need to create 1,000 half-open SAs per second.
   Reduce the retention time to 3 seconds, and the attacker needs to
   create 20,000 half-open SAs per second.  By introducing a puzzle,
   each half-open SA becomes more expensive for an attacker, making it
   more likely to thwart an exhaustion attack against Responder memory.

   At this point, filling up the half-open SA database is no longer the
   most efficient DoS attack.  The attacker has two ways to do better:

   1.  Go back to spoofed addresses and try to overwhelm the CPU that
       deals with generating cookies, or

   2.  Take the attack to the next level by also sending an IKE_AUTH

   It seems that the first thing cannot be dealt with at the IKE level.
   It's probably better left to Intrusion Prevention System (IPS)

   On the other hand, sending an IKE_AUTH request is surprisingly cheap.
   It requires a proper IKE header with the correct IKE SPIs, and it
   requires a single Encrypted payload.  The content of the payload

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   might as well be junk.  The Responder has to perform the relatively
   expensive key derivation, only to find that the MAC on the Encrypted
   payload on the IKE_AUTH request does not check.  Depending on the
   Responder implementation, this can be repeated with the same half-
   open SA (if the Responder does not delete the half-open SA following
   an unsuccessful decryption - see discussion in Section 4).

   Here too, the number of half-open SAs that the attacker can achieve
   is crucial, because each one allows the attacker to waste some CPU
   time.  So making it hard to make many half-open SAs is important.

   A strategy against DDoS has to rely on at least 4 components:

   1.  Hardening the half-open SA database by reducing retention time.

   2.  Hardening the half-open SA database by rate-limiting single IPs/

   3.  Guidance on what to do when an IKE_AUTH request fails to decrypt.

   4.  Increasing cost of half-open SA up to what is tolerable for
       legitimate clients.

   Puzzles have their place as part of #4.

3.  Puzzles

   The puzzle introduced here extends the cookie mechanism from
   [RFC7296].  It is loosely based on the proof-of-work technique used
   in Bitcoins [bitcoins].

   A puzzle is sent to the Initiator in two cases:

   o  The Responder is so overloaded that no half-open SAs may be
      created without solving a puzzle, or

   o  The Responder is not too loaded, but the rate-limiting method
      described in Section 5 prevents half-open SAs from being created
      with this particular peer address or prefix without first solving
      a puzzle.

   When the Responder decides to send the challenge notification in
   response to a IKE_SA_INIT request, the notification includes three

   1.  Cookie - this is calculated the same as in [RFC7296], i.e. the
       process of generating the cookie is not specified.

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   2.  Algorithm, this is the identifier of a Pseudo-Random Function
       (PRF) algorithm, one of those proposed by the Initiator in the SA

   3.  Zero Bit Count (ZBC).  This is a number between 8 and 255 (or a
       special value - 0, see Section that represents the
       length of the zero-bit run at the end of the output of the PRF
       function calculated over the cookie that the Initiator is to
       send.  The values 1-8 are explicitly excluded, because they
       create a puzzle that is too easy to solve for it to make any
       difference in mitigating DDoS attacks.  Since the mechanism is
       supposed to be stateless for the Responder, either the same ZBC
       is used for all Initiators, or the ZBC is somehow encoded in the
       cookie.  If it is global then it means that this value is the
       same for all the Initiators who are receiving puzzles at any
       given point of time.  The Responder, however, may change this
       value over time depending on its load.

   Upon receiving this challenge, the Initiator attempts to calculate
   the PRF using different keys.  When enough keys are found such that
   the resulting PRF output calculated using each of them has a
   sufficient number of trailing zero bits, that result is sent to the

   The reason for using several keys in the results rather than just one
   key is to reduce the variance in the time it takes the initiator to
   solve the puzzle.  We have chosen the number of keys to be four (4)
   as a compromise between the conflicting goals of reducing variance
   and reducing the work the Responder needs to perform to verify the
   puzzle solution.

   When receiving a request with a solved puzzle, the Responder verifies
   two things:

   o  That the cookie part is indeed valid.

   o  That the PRFs of the transmitted cookie calculated with the
      transmitted keys has a sufficient number of trailing zero bits.

   Example 1: Suppose the calculated cookie is
   739ae7492d8a810cf5e8dc0f9626c9dda773c5a3 (20 octets), the algorithm
   is PRF-HMAC-SHA256, and the required number of zero bits is 18.
   After successively trying a bunch of keys, the Initiator finds the
   following four 3-octet keys that work:

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         |  Key   | Last 32 Hex PRF Digits           | # 0-bits |
         | 061840 | e4f957b859d7fb1343b7b94a816c0000 |    18    |
         | 073324 | 0d4233d6278c96e3369227a075800000 |    23    |
         | 0c8a2a | 952a35d39d5ba06709da43af40700000 |    20    |
         | 0d94c8 | 5a0452b21571e401a3d00803679c0000 |    18    |

                Table 1: Three solutions for 18-bit puzzle

   Example 2: Same cookie, but modify the required number of zero bits
   to 22.  The first 4-octet keys that work to satisfy that requirement
   are 005d9e57, 010d8959, 0110778d, and 01187e37.  Finding these
   requires 18,382,392 invocations of the PRF.

               | # 0-bits | Time to Find 4 keys (seconds) |
               |    8     |                        0.0025 |
               |    10    |                        0.0078 |
               |    12    |                        0.0530 |
               |    14    |                        0.2521 |
               |    16    |                        0.8504 |
               |    17    |                        1.5938 |
               |    18    |                        3.3842 |
               |    19    |                        3.8592 |
               |    20    |                       10.8876 |

   Table 2: The time needed to solve a puzzle of various difficulty for
           the cookie = 739ae7492d8a810cf5e8dc0f9626c9dda773c5a3

   The figures above were obtained on a 2.4 GHz single core i5.  Run
   times can be halved or quartered with multi-core code, but would be
   longer on mobile phone processors, even if those are multi-core as
   well.  With these figures 18 bits is believed to be a reasonable
   choice for puzzle level difficulty for all Initiators, and 20 bits is
   acceptable for specific hosts/prefixes.

4.  Retention Periods for Half-Open SAs

   As a UDP-based protocol, IKEv2 has to deal with packet loss through
   retransmissions.  Section 2.4 of [RFC7296] recommends "that messages
   be retransmitted at least a dozen times over a period of at least
   several minutes before giving up".  Retransmission policies in
   practice wait at least one or two seconds before retransmitting for
   the first time.

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   Because of this, setting the timeout on a half-open SA too low will
   cause it to expire whenever even one IKE_AUTH request packet is lost.
   When not under attack, the half-open SA timeout SHOULD be set high
   enough that the Initiator will have enough time to send multiple
   retransmissions, minimizing the chance of transient network
   congestion causing IKE failure.

   When the system is under attack, as measured by the amount of half-
   open SAs, it makes sense to reduce this lifetime.  The Responder
   should still allow enough time for the round-trip, enough time for
   the Initiator to derive the D-H shared value, and enough time to
   derive the IKE SA keys and the create the IKE_AUTH request.  Two
   seconds is probably as low a value as can realistically be used.

   It could make sense to assign a shorter value to half-open SAs
   originating from IP addresses or prefixes that are considered suspect
   because of multiple concurrent half-open SAs.

5.  Rate Limiting

   Even with DDoS, the attacker has only a limited amount of nodes
   participating in the attack.  By limiting the amount of half-open SAs
   that are allowed to exist concurrently with each such node, the total
   amount of half-open SAs is capped, as is the total amount of key
   derivations that the Responder is forced to complete.

   In IPv4 it makes sense to limit the number of half-open SAs based on
   IP address.  Most IPv4 nodes are either directly attached to the
   Internet using a routable address or are hidden behind a NAT device
   with a single IPv4 external address.  IPv6 networks are currently a
   rarity, so we can only speculate on what their wide deployment will
   be like, but the current thinking is that ISP customers will be
   assigned whole subnets, so we don't expect the kind of NAT deployment
   that is common in IPv4.  For this reason, it makes sense to use a
   64-bit prefix as the basis for rate limiting in IPv6.

   The number of half-open SAs is easy to measure, but it is also
   worthwhile to measure the number of failed IKE_AUTH exchanges.  If
   possible, both factors should be taken into account when deciding
   which IP address or prefix is considered suspicious.

   There are two ways to rate-limit a peer address or prefix:

   1.  Hard Limit - where the number of half-open SAs is capped, and any
       further IKE_SA_INIT requests are rejected.

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   2.  Soft Limit - where if a set number of half-open SAs exist for a
       particular address or prefix, any IKE_SA_INIT request will
       require solving a puzzle.

   The advantage of the hard limit method is that it provides a hard cap
   on the amount of half-open SAs that the attacker is able to create.
   The downside is that it allows the attacker to block IKE initiation
   from small parts of the Internet.  For example, if an network service
   provider or some establishment offers Internet connectivity to its
   customers or employees through an IPv4 NAT device, a single malicious
   customer can create enough half-open SAs to fill the quota for the
   NAT device external IP address.  Legitimate Initiators on the same
   network will not be able to initiate IKE.

   The advantage of a soft limit is that legitimate clients can always
   connect.  The disadvantage is that an adversary with sufficient CPU
   resources can still effectively DoS the Responder.

   Regardless of the type of rate-limiting used, there is a huge
   advantage in blocking the DoS attack using rate-limiting for
   legitimate clients that are away from the attacking nodes.  In such
   cases, adverse impacts caused by the attack or by the measures used
   to counteract the attack can be avoided.

6.  Plan for Defending a Responder

   This section outlines a plan for defending a Responder from a DDoS
   attack based on the techniques described earlier.  The numbers given
   here are not normative, and their purpose is to illustrate the
   configurable parameters needed for defeating the DDoS attack.

   Implementations may be deployed in different environments, so it is
   RECOMMENDED that the parameters be settable.  As an example, most
   commercial products are required to undergo benchmarking where the
   IKE SA establishment rate is measured.  Benchmarking is
   indistinguishable from a DoS attack and the defenses described in
   this document may defeat the benchmark by causing exchanges to fail
   or take a long time to complete.  Parameters should be tunable to
   allow for benchmarking (if only by turning DDoS protection off).

   Since all countermeasures may cause delays and work on the
   Initiators, they SHOULD NOT be deployed unless an attack is likely to
   be in progress.  To minimize the burden imposed on Initiators, the
   Responder should monitor incoming IKE requests, searching for two

   1.  A general DDoS attack.  Such an attack is indicated by a high
       number of concurrent half-open SAs, a high rate of failed

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       IKE_AUTH exchanges, or a combination of both.  For example,
       consider a Responder that has 10,000 distinct peers of which at
       peak 7,500 concurrently have VPN tunnels.  At the start of peak
       time, 600 peers might establish tunnels at any given minute, and
       tunnel establishment (both IKE_SA_INIT and IKE_AUTH) takes
       anywhere from 0.5 to 2 seconds.  For this Responder, we expect
       there to be less than 20 concurrent half-open SAs, so having 100
       concurrent half-open SAs can be interpreted as an indication of
       an attack.  Similarly, IKE_AUTH request decryption failures
       should never happen.  Supposing the the tunnels are established
       using EAP (see Section 2.16 of [RFC7296]), users enter the wrong
       password about 20% of the time.  So we'd expect 125 wrong
       password failures a minute.  If we get IKE_AUTH decryption
       failures from multiple sources more than once per second, or EAP
       failure more than 300 times per minute, that can also be an
       indication of a DDoS attack.

   2.  An attack from a particular IP address or prefix.  Such an attack
       is indicated by an inordinate amount of half-open SAs from that
       IP address or prefix, or an inordinate amount of IKE_AUTH
       failures.  A DDoS attack may be viewed as multiple such attacks.
       If they are mitigated well enough, there will not be a need enact
       countermeasures on all Initiators.  Typical measures might be 5
       concurrent half-open SAs, 1 decrypt failure, or 10 EAP failures
       within a minute.

   Note that using counter-measures against an attack from a particular
   IP address may be enough to avoid the overload on the half-open SA
   database and in this case the number of failed IKE_AUTH exchanges
   never exceeds the threshold of attack detection.  This is a good
   thing as it prevents Initiators that are not close to the attackers
   from being affected.

   When there is no general DDoS attack, it is suggested that no cookie
   or puzzles be used.  At this point the only defensive measure is the
   monitoring of the number of half-open SAs, and setting a soft limit
   per peer IP or prefix.  The soft limit can be set to 3-5, and the
   puzzle difficulty should be set to such a level (number of zero-bits)
   that all legitimate clients can handle it without degraded user

   As soon as any kind of attack is detected, either a lot of
   initiations from multiple sources or a lot of initiations from a few
   sources, it is best to begin by requiring stateless cookies from all
   Initiators.  This will force the attacker to use real source
   addresses, and help avoid the need to impose a greater burden in the
   form of cookies on the general population of Initiators.  This makes
   the per-node or per-prefix soft limit more effective.

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   When cookies are activated for all requests and the attacker is still
   managing to consume too many resources, the Responder MAY increase
   the difficulty of puzzles imposed on IKE_SA_INIT requests coming from
   suspicious nodes/prefixes.  It should still be doable by all
   legitimate peers, but it can degrade experience, for example by
   taking up to 10 seconds to solve the puzzle.

   If the load on the Responder is still too great, and there are many
   nodes causing multiple half-open SAs or IKE_AUTH failures, the
   Responder MAY impose hard limits on those nodes.

   If it turns out that the attack is very widespread and the hard caps
   are not solving the issue, a puzzle MAY be imposed on all Initiators.
   Note that this is the last step, and the Responder should avoid this
   if possible.

6.1.  Session Resumption

   When the Responder is under attack, it MAY choose to prefer
   previously authenticated peers who present a Session Resumption
   ticket (see [RFC5723] for details).  The Responder MAY require such
   Initiators to pass a return routability check by including the COOKIE
   notification in the IKE_SESSION_RESUME response message, as allowed
   by Section 4.3.2. of [RFC5723].  Note that the Responder SHOULD cache
   tickets for a short time to reject reused tickets (Section 4.3.1),
   and therefore there should be no issue of half-open SAs resulting
   from replayed IKE_SESSION_RESUME messages.

7.  Using Puzzles in the Protocol

   This section describes how the puzzle mechanism is used in IKEv2.  It
   is organized as follows.  The Section 7.1 describes using puzzles in
   the IKE_SA_INIT exchange and the Section 7.2 describes using puzzles
   in the IKE_AUTH exchange.  Both sections are divided into subsections
   describing how puzzles should be presented, solved and processed by
   the Initiator and the Responder.

7.1.  Puzzles in IKE_SA_INIT Exchange

   IKE Initiator indicates the desire to create a new IKE SA by sending
   IKE_SA_INIT request message.  The message may optionally contain a
   COOKIE notification if this is a repeated request performed after the
   Responder's demand to return a cookie.

   HDR, [N(COOKIE),] SA, KE, Ni, [V+][N+]   -->

   According to the plan, described in Section 6, the IKE Responder
   should monitor incoming requests to detect whether it is under

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   attack.  If the Responder learns that (D)DoS attack is likely to be
   in progress, then its actions depend on the volume of the attack.  If
   the volume is moderate, then the Responder requests the Initiator to
   return a cookie.  If the volume is so high, that puzzles need to be
   used for defense, then the Responder requests the Initiator to solve
   a puzzle.

   The Responder MAY choose to process some fraction of IKE_SA_INIT
   requests without presenting a puzzle while being under attack to
   allow legacy clients, that don't support puzzles, to have a chance to
   be served.  The decision whether to process any particular request
   must be probabilistic, with the probability depending on the
   Responder's load (i.e. on the volume of attack).  The requests that
   don't contain the COOKIE notification MUST NOT participate in this
   lottery.  In other words, the Responder must first perform return
   routability check before allowing any legacy client to be served if
   it is under attack.  See Section 7.1.4 for details.

7.1.1.  Presenting Puzzle

   If the Responder makes a decision to use puzzles, then it MUST
   include two notifications in its response message - the COOKIE
   notification and the PUZZLE notification.  The format of the PUZZLE
   notification is described in Section 9.1.

                             <--   HDR, N(COOKIE), N(PUZZLE), [V+][N+]

   The presence of these notifications in an IKE_SA_INIT response
   message indicates to the Initiator that it should solve the puzzle to
   get better chances to be served.  Selecting Puzzle Difficulty Level

   The PUZZLE notification contains the difficulty level of the puzzle -
   the minimum number of trailing zero bits that the result of PRF must
   contain.  In diverse environments it is next to impossible for the
   Responder to set any specific difficulty level that will result in
   roughly the same amount of work for all Initiators, because
   computation power of different Initiators may vary by the order of
   magnitude, or even more.  The Responder may set difficulty level to
   0, meaning that the Initiator is requested to spend as much power to
   solve puzzle, as it can afford.  In this case no specific value of
   ZBC is required from the Initiator, however the larger the ZBC that
   Initiator is able to get, the better the chances it will have to be
   served by the Responder.  In diverse environments it is RECOMMENDED
   that the Initiator sets difficulty level to 0, unless the attack
   volume is very high.

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   If the Responder sets non-zero difficulty level, then the level
   should be determined by analyzing the volume of the attack.  The
   Responder MAY set different difficulty levels to different requests
   depending on the IP address the request has come from.  Selecting Puzzle Algorithm

   The PUZZLE notification also contains identifier of the algorithm,
   that must be used by Initiator to compute puzzle.

   Cryptographic algorithm agility is considered an important feature
   for modern protocols ([RFC7696]).  This feature ensures that protocol
   doesn't rely on a single build-in set of cryptographic algorithms,
   but has a means to replace one set with another and negotiate new set
   with the peer.  IKEv2 fully supports cryptographic algorithm agility
   for its core operations.

   To support this feature in case of puzzles, the algorithm that is
   used to compute puzzle needs to be negotiated during IKE_SA_INIT
   exchange.  The negotiation is performed as follows.  The initial
   request message sent by Initiator contains SA payload with the list
   of transforms the Initiator supports and is willing to use in the IKE
   SA being established.  The Responder parses received SA payload and
   finds mutually supported set of transforms of type PRF.  It selects
   most preferred transform from this set and includes it into the
   PUZZLE notification.  There is no requirement that the PRF selected
   for puzzles be the same, as the PRF that is negotiated later for the
   use in core IKE SA crypto operations.  If there are no mutually
   supported PRFs, then negotiation will fail anyway and there is no
   reason to return a puzzle.  In this case the Responder returns
   NO_PROPOSAL_CHOSEN notification.  Note that PRF is a mandatory
   transform type for IKE SA (see Sections 3.3.2 and 3.3.3 of [RFC7296])
   and at least one transform of this type must always be present in SA
   payload in IKE_SA_INIT request message.  Generating Cookie

   If Responder supports puzzles then cookie should be computed in such
   a manner, that the Responder is able to learn some important
   information from the sole cookie, when it is later returned back by
   Initiator.  In particular - the Responder should be able to learn the
   following information:

   o  Whether the puzzle was given to the Initiator or only the cookie
      was requested.

   o  The difficulty level of the puzzle given to the Initiator.

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   o  The number of consecutive puzzles given to the Initiator.

   o  The amount of time the Initiator spent to solve the puzzles.  This
      can be calculated if the cookie is timestamped.

   This information helps the Responder to make a decision whether to
   serve this request or demand more work from the Initiator.

   One possible approach to get this information is to encode it in the
   cookie.  The format of such encoding is a local matter of Responder,
   as the cookie would remain an opaque blob to the Initiator.  If this
   information is encoded in the cookie, then the Responder MUST make it
   integrity protected, so that any intended or accidental alteration of
   this information in returned cookie is detectable.  So, the cookie
   would be generated as:

   Cookie = <VersionIDofSecret> | <AdditionalInfo> |
                     Hash(Ni | IPi | SPIi | <AdditionalInfo> | <secret>)

   Alternatively, the Responder may continue to generate cookie as
   suggested in Section 2.6 of [RFC7296], but associate the additional
   information, that would be stored locally, with the particular
   version of the secret.  In this case the Responder should have
   different secrets for every combination of difficulty level and
   number of consecutive puzzles, and should change the secrets
   periodically, keeping a few previous versions, to be able to
   calculate how long ago the cookie was generated.

   The Responder may also combine these approaches.  This document
   doesn't mandate how the Responder learns this information from the

7.1.2.  Solving Puzzle and Returning the Solution

   If the Initiator receives a puzzle but it doesn't support puzzles,
   then it will ignore the PUZZLE notification as an unrecognized status
   notification (in accordance to Section 3.10.1 of [RFC7296]).  The
   Initiator also MAY ignore the PUZZLE notification if it is not
   willing to spend resources to solve the puzzle of the requested
   difficulty, even if it supports puzzles.  In both cases the Initiator
   acts as described in Section 2.6 of [RFC7296] - it restarts the
   request and includes the received COOKIE notification into it.  The
   Responder should be able to distinguish the situation when it just
   requested a cookie from the situation when the puzzle was given to
   the Initiator, but the Initiator for some reason ignored it.

   If the received message contains a PUZZLE notification and doesn't
   contain a COOKIE notification, then this message is malformed because

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   it requests to solve the puzzle, but doesn't provide enough
   information to do it.  In this case the Initiator SHOULD resend
   IKE_SA_INIT request.  If this situation repeats several times, then
   it means that something is wrong and the IKE SA cannot be

   If the Initiator supports puzzles and is ready to deal with them,
   then it tries to solve the given puzzle.  After the puzzle is solved
   the Initiator restarts the request and returns the puzzle solution in
   a new payload called a Puzzle Solution payload (denoted as PS, see
   Section 9.2) along with the received COOKIE notification back to the

   HDR, N(COOKIE), [PS,] SA, KE, Ni, [V+][N+]   -->

7.1.3.  Computing Puzzle

   General principals of constructing puzzles in IKEv2 are described in
   Section 3.  They can be summarized as follows: given unpredictable
   string S and pseudo-random function PRF find N different keys Ki
   (where i=[1..N]) for that PRF so that the result of PRF(Ki,S) has at
   least the specified number of trailing zero bits.  This specification
   requires that the solution to the puzzle contains 4 different keys
   (i.e.  N=4).

   In the IKE_SA_INIT exchange it is the cookie that plays the role of
   unpredictable string S.  In other words, in IKE_SA_INIT the task for
   the IKE Initiator is to find the four different, equal-sized keys Ki
   for the agreed upon PRF such that each result of PRF(Ki,cookie) where
   i = [1..4] has a sufficient number of trailing zero bits.  Only the
   content of the COOKIE notification is used in puzzle calculation,
   i.e. the header of the Notification payload is not included.

   Note, that puzzles in the IKE_AUTH exchange are computed differently
   than in the IKE_SA_INIT_EXCHANGE.  See Section 7.2.3 for details.

7.1.4.  Analyzing Repeated Request

   The received request must at least contain a COOKIE notification.
   Otherwise it is an initial request and it must be processed according
   to Section 7.1.  First, the cookie MUST be checked for validity.  If
   the cookie is invalid, then the request is treated as initial and is
   processed according to Section 7.1.  It is RECOMMENDED that a new
   cookie is requested in this case.

   If the cookie is valid then some important information is learned
   from it or from local state based on identifier of the cookie's
   secret (see Section for details).  This information helps the

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   Responder to sort out incoming requests, giving more priority to
   those of them, which were created by spending more of the Initiator's

   First, the Responder determines if it requested only a cookie, or
   presented a puzzle to the Initiator.  If no puzzle was given, then it
   means that at the time the Responder requested a cookie it didn't
   detect the (D)DoS attack or the attack volume was low.  In this case
   the received request message must not contain the PS payload, and
   this payload MUST be ignored if for any reason the message contains
   it.  Since no puzzle was given, the Responder marks the request with
   the lowest priority since the Initiator spent little resources
   creating it.

   If the Responder learns from the cookie that the puzzle was given to
   the Initiator, then it looks for the PS payload to determine whether
   its request to solve the puzzle was honored or not.  If the incoming
   message doesn't contain a PS payload, then it means that the
   Initiator either doesn't support puzzles or doesn't want to deal with
   them.  In either case the request is marked with the lowest priority
   since the Initiator spent little resources creating it.

   If a PS payload is found in the message, then the Responder MUST
   verify the puzzle solution that it contains.  The solution is
   interpreted as four different keys.  The result of using each of them
   in the PRF (as described in Section 7.1.3) must contain at least the
   requested number of trailing zero bits.  The Responder MUST check all
   the four returned keys.

   If any checked result contains fewer bits than were requested, it
   means that the Initiator spent less resources than expected by the
   Responder.  This request is marked with the lowest priority.

   If the Initiator provided the solution to the puzzle satisfying the
   requested difficulty level, or if the Responder didn't indicate any
   particular difficulty level (by setting ZBC to zero) and the
   Initiator was free to select any difficulty level it can afford, then
   the priority of the request is calculated based on the following

   o  The Responder must take the smallest number of trailing zero bits
      among the checked results and count it as the number of zero bits
      the Initiator got.

   o  The higher number of zero bits the Initiator got, the higher
      priority its request should receive.

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   o  The more consecutive puzzles the Initiator solved, the higher
      priority it should receive.

   o  The more time the Initiator spent solving the puzzles, the higher
      priority it should receive.

   After the priority of the request is determined the final decision
   whether to serve it or not is made.

7.1.5.  Making Decision whether to Serve the Request

   The Responder decides what to do with the request based on its
   priority and Responder's current load.  There are three possible

   o  Accept request.

   o  Reject request.

   o  Demand more work from Initiator by giving it a new puzzle.

   The Responder SHOULD accept an incoming request if its priority is
   high - it means that the Initiator spent quite a lot of resources.
   The Responder MAY also accept some of low-priority requests where the
   Initiators don't support puzzles.  The percentage of accepted legacy
   requests depends on the Responder's current load.

   If the Initiator solved the puzzle, but didn't spend much resources
   for it (the selected puzzle difficulty level appeared to be low and
   the Initiator solved it quickly), then the Responder SHOULD give it
   another puzzle.  The more puzzles the Initiator solves the higher its
   chances are to be served.

   The details of how the Responder makes a decision for any particular
   request, are implementation dependent.  The Responder can collect all
   the incoming requests for some short period of time, sort them out
   based on their priority, calculate the number of available memory
   slots for half-open IKE SAs and then serve that number of requests
   from the head of the sorted list.  The rest of requests can be either
   discarded or responded to with new puzzles.

   Alternatively, the Responder may decide whether to accept every
   incoming request with some kind of lottery, taking into account its
   priority and the available resources.

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7.2.  Puzzles in IKE_AUTH Exchange

   Once the IKE_SA_INIT exchange is completed, the Responder has created
   a state and is waiting for the first message of the IKE_AUTH exchange
   from the Initiator.  At this point the Initiator has already passed
   return routability check and has proved that it has performed some
   work to complete IKE_SA_INIT exchange.  However, the Initiator is not
   yet authenticated and this fact allows malicious Initiator to perform
   an attack, described in Section 2.  Unlike DoS attack in IKE_SA_INIT
   exchange, which is targeted on the Responder's memory resources, the
   goal of this attack is to exhaust a Responder's CPU power.  The
   attack is performed by sending the first IKE_AUTH message containing
   garbage.  This costs nothing to the Initiator, but the Responder has
   to do relatively costly operations of computing the D-H shared secret
   and deriving SK_* keys to be able to verify authenticity of the
   message.  If the Responder doesn't keep the computed keys after an
   unsuccessful verification of the IKE_AUTH message, then the attack
   can be repeated several times on the same IKE SA.

   The Responder can use puzzles to make this attack more costly for the
   Initiator.  The idea is that the Responder includes a puzzle in the
   IKE_SA_INIT response message and the Initiator includes a puzzle
   solution in the first IKE_AUTH request message outside the Encrypted
   payload, so that the Responder is able to verify puzzle solution
   before computing D-H shared secret.  The difficulty level of the
   puzzle should be selected so that the Initiator would spend
   substantially more time to solve the puzzle than the Responder to
   compute the shared secret.

   The Responder should constantly monitor the amount of the half-open
   IKE SA states that receive IKE_AUTH messages that cannot be decrypted
   due to integrity check failures.  If the percentage of such states is
   high and it takes an essential fraction of Responder's computing
   power to calculate keys for them, then the Responder may assume that
   it is under attack and SHOULD use puzzles to make it harder for

7.2.1.  Presenting Puzzle

   The Responder requests the Initiator to solve a puzzle by including
   the PUZZLE notification in the IKE_SA_INIT response message.  The
   Responder MUST NOT use puzzles in the IKE_AUTH exchange unless the
   puzzle has been previously presented and solved in the preceding
   IKE_SA_INIT exchange.

                             <--   HDR, SA, KE, Nr, N(PUZZLE), [V+][N+]

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   The difficulty level of the puzzle in IKE_AUTH exchange should be
   chosen so that the Initiator would spend more time to solve the
   puzzle than the Responder to compute the D-H shared secret and the
   keys, needed to decrypt and verify the IKE_AUTH request message.  On
   the other hand, the difficulty level should not be too high,
   otherwise the legitimate clients would experience an additional delay
   while establishing IKE SA.

   Note, that since puzzles in the IKE_AUTH exchange are only allowed to
   be used if they were used in the preceding IKE_SA_INIT exchange, the
   Responder would be able to estimate the computational power of the
   Initiator and to select the difficulty level accordingly.  Unlike
   puzzles in IKE_SA_INIT, the requested difficulty level for IKE_AUTH
   puzzles MUST NOT be zero.  In other words, the Responder must always
   set specific difficulty level and must not let the Initiator to
   choose it on its own.  Selecting Puzzle Algorithm

   The algorithm for the puzzle is selected as described in
   Section  There is no requirement, that the algorithm for the
   puzzle in the IKE_SA INIT exchange be the same, as the algorithm for
   the puzzle in IKE_AUTH exchange, however it is expected that in most
   cases they will be the same.

7.2.2.  Solving Puzzle and Returning the Solution

   If the IKE_SA_INIT response message contains the PUZZLE notification
   and the Initiator supports puzzles, it MUST solve the puzzle.  Note,
   that puzzle construction in the IKE_AUTH exchange differs from the
   puzzle construction in the IKE_SA_INIT exchange and is described in
   Section 7.2.3.  Once the puzzle is solved the Initiator sends the
   IKE_AUTH request message, containing the Puzzle Solution payload.

               [IDr,] AUTH, SA, TSi, TSr}   -->

   The Puzzle Solution payload MUST be placed outside the Encrypted
   payload, so that the Responder would be able to verify the puzzle
   before calculating the D-H shared secret and the SK_* keys.

   If IKE Fragmentation [RFC7383] is used in IKE_AUTH exchange, then the
   PS payload MUST be present only in the first IKE Fragment message, in
   accordance with the Section 2.5.3 of [RFC7383].  Note, that
   calculation of the puzzle in the IKE_AUTH exchange doesn't depend on
   the content of the IKE_AUTH message (see Section 7.2.3).  Thus the

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   Initiator has to solve the puzzle only once and the solution is valid
   for both unfragmented and fragmented IKE messages.

7.2.3.  Computing Puzzle

   The puzzles in the IKE_AUTH exchange are computed differently than in
   the IKE_SA_INIT exchange (see Section 7.1.3).  The general principle
   is the same; the difference is in the construction of the string S.
   Unlike the IKE_SA_INIT exchange, where S is the cookie, in the
   IKE_AUTH exchange S is a concatenation of Nr and SPIr.  In other
   words, the task for IKE Initiator is to find the four different keys
   Ki for the agreed upon PRF such that each result of PRF(Ki,Nr | SPIr)
   where i=[1..4] has a sufficient number of trailing zero bits.  Nr is
   a nonce used by the Responder in IKE_SA_INIT exchange, stripped of
   any headers.  SPIr is IKE Responder's SPI from the IKE header of the
   SA being established.

7.2.4.  Receiving Puzzle Solution

   If the Responder requested the Initiator to solve a puzzle in the
   IKE_AUTH exchange, then it MUST silently discard all the IKE_AUTH
   request messages without the Puzzle Solution payload.

   Once the message containing a solution to the puzzle is received, the
   Responder MUST verify the solution before performing computationlly
   intensive operations i.e. computing the D-H shared secret and the
   SK_* keys.  The Responder MUST verify all the four returned keys.

   The Responder MUST silently discard the received message if any
   checked verification result is not correct (contains insufficient
   number of trailing zero bits).  If the Responder successfully
   verifies the puzzle and calculates the SK_* key, but the message
   authenticity check fails, then it SHOULD save the calculated keys in
   the IKE SA state while waiting for the retransmissions from the
   Initiator.  In this case the Responder may skip verification of the
   puzzle solution and ignore the Puzzle Solution payload in the
   retransmitted messages.

   If the Initiator uses IKE Fragmentation, then it is possible, that
   due to packet loss and/or reordering the Responder could receive non-
   first IKE Fragment messages before receiving the first one,
   containing the PS payload.  In this case the Responder MAY choose to
   keep the received fragments until the first fragment containing the
   solution to the puzzle is received.  However, in this case the
   Responder SHOULD NOT try to verify authenticity of the kept fragments
   until the first fragment with the PS payload is received and the
   solution to the puzzle is verified.  After successful verification of

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   the puzzle the Responder could calculate the SK_* key and verify
   authenticity of the collected fragments.

8.  DoS Protection after IKE SA is created

   Once IKE SA is created there is usually not much traffic over it.  In
   most cases this traffic consists of exchanges aimed to create
   additional Child SAs, rekey, or delete them and check the liveness of
   the peer.  With a typical setup and typical Child SA lifetimes, there
   are typically no more than a few such exchanges, often less.  Some of
   these exchanges require relatively little resources (like liveness
   check), while others may be resource consuming (like creating or
   rekeying Child SA with D-H exchange).

   Since any endpoint can initiate a new exchange, there is a
   possibility that a peer would initiate too many exchanges that could
   exhaust host resources.  For example, the peer can perform endless
   continuous Child SA rekeying or create overwhelming number of Child
   SAs with the same Traffic Selectors etc.  Such behavior may be caused
   by buggy implementation, misconfiguration or be intentional.  The
   latter becomes more of a real threat if the peer uses NULL
   Authentication, described in [RFC7619].  In this case the peer
   remains anonymous, allowing it to escape any responsibility for its

   The following recommendations for defense against possible DoS
   attacks after IKE SA is established are mostly intended for
   implementations that allow unauthenticated IKE sessions; however,
   they may also be useful in other cases.

   o  If the IKEv2 window size is greater than one, then the peer could
      initiate multiple simultaneous exchanges that could increase host
      resource consumption.  Since currently there is no way in IKEv2 to
      decrease window size once it was increased (see Section 2.3 of
      [RFC7296]), the window size cannot be dynamically adjusted
      depending on the load.  For that reason, it is NOT RECOMMENDED to
      ever increase the IKEv2 window size above its default value of one
      if the peer uses NULL Authentication.

   o  If the peer initiates requests to rekey IKE SA or Child SA too
      often, implementations can respond to some of these requests with
      the TEMPORARY_FAILURE notification, indicating that the request
      should be retried after some period of time.

   o  If the peer creates too many Child SA with the same or overlapping
      Traffic Selectors, implementations can respond with the
      NO_ADDITIONAL_SAS notification.

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   o  If the peer initiates too many exchanges of any kind,
      implementations can introduce an artificial delay before
      responding to request messages.  This delay would decrease the
      rate the implementation need to process requests from any
      particular peer, making it possible to process requests from the
      others.  The delay should not be too long to avoid causing the IKE
      SA to be deleted on the other end due to timeout.  It is believed
      that a few seconds is enough.  Note, that if the Responder
      receives retransmissions of the request message during the delay
      period, the retransmitted messages should be silently discarded.

   o  If these counter-measures are inefficient, implementations can
      delete the IKE SA with an offending peer by sending Delete

9.  Payload Formats

9.1.  PUZZLE Notification

   The PUZZLE notification is used by the IKE Responder to inform the
   Initiator about the necessity to solve the puzzle.  It contains the
   difficulty level of the puzzle and the PRF the Initiator should use.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   |Protocol ID(=0)| SPI Size (=0) |      Notify Message Type      |
   |              PRF              |  Difficulty   |

   o  Protocol ID (1 octet) -- MUST be 0.

   o  SPI Size (1 octet) - MUST be 0, meaning no Security Parameter
      Index (SPI) is present.

   o  Notify Message Type (2 octets) -- MUST be <TBA by IANA>, the value
      assigned for the PUZZLE notification.

   o  PRF (2 octets) -- Transform ID of the PRF algorithm that must be
      used to solve the puzzle.  Readers should refer to the section
      "Transform Type 2 - Pseudo-Random Function Transform IDs" in
      [IKEV2-IANA] for the list of possible values.

   o  Difficulty (1 octet) -- Difficulty Level of the puzzle.  Specifies
      minimum number of trailing zero bits (ZBC), that each of the

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      results of PRF must contain.  Value 0 means that the Responder
      doesn't request any specific difficulty level and the Initiator is
      free to select appropriate difficulty level on its own (see
      Section for details).

   This notification contains no data.

9.2.  Puzzle Solution Payload

   The solution to the puzzle is returned back to the Responder in a
   dedicated payload, called the Puzzle Solution payload and denoted as
   PS in this document.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   |                                                               |
   ~                     Puzzle Solution Data                      ~
   |                                                               |

   o  Puzzle Solution Data (variable length) -- Contains the solution to
      the puzzle - four different keys for the selected PRF.  This field
      MUST NOT be empty.  All the keys MUST have the same size,
      therefore the size of this field is always a mutiple of 4 bytes.
      If the selected PRF accepts only fixed-size keys, then the size of
      each key MUST be of that fixed size.  If the PRF agreed upon
      accepts keys of any size, then then the size of each key MUST be
      between 1 octet and the preferred key length of the PRF
      (inclusive).  It is expected that in most cases the keys will be 4
      (or even less) octets in length, however it depends on puzzle
      difficulty and on the Initiator's strategy to find solutions, and
      thus the size is not mandated by this specification.  The
      Responder determines the size of each key by dividing the size of
      the Puzzle Solution Data by 4 (the number of keys).  Note that the
      size of Puzzle Solution Data is the size of Payload (as indicated
      in Payload Length field) minus 4 - the size of Payload Header.

   The payload type for the Puzzle Solution payload is <TBA by IANA>.

10.  Operational Considerations

   The difficulty level should be set by balancing the requirement to
   minimize the latency for legitimate Initiators and making things
   difficult for attackers.  A good rule of thumb is for taking about 1
   second to solve the puzzle.  A typical Initiator or bot-net member in

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   2014 can perform slightly less than a million hashes per second per
   core, so setting the difficulty level to n=20 is a good compromise.
   It should be noted that mobile Initiators, especially phones are
   considerably weaker than that.  Implementations should allow
   administrators to set the difficulty level, and/or be able to set the
   difficulty level dynamically in response to load.

   Initiators should set a maximum difficulty level beyond which they
   won't try to solve the puzzle and log or display a failure message to
   the administrator or user.

11.  Security Considerations

   When selecting parameters for the puzzles, in particular the puzzle
   difficulty, care must be taken.  If the puzzles appeared too easy for
   majority of the attackers, then the puzzles mechanism wouldn't be
   able to prevent DoS attack and would only impose an additional burden
   on the legitimate Initiators.  On the other hand, if the puzzles
   appeared to be too hard for majority of the Initiators then many
   legitimate users would experience unacceptable delay in IKE SA setup
   (or unacceptable power consumption on mobile devices), that might
   cause them to cancel connection attempt.  In this case the resources
   of the Responder are preserved, however the DoS attack can be
   considered successful.  Thus a sensible balance should be kept by the
   Responder while choosing the puzzle difficulty - to defend itself and
   to not over-defend itself.  It is RECOMMENDED that the puzzle
   difficulty be chosen so, that the Responder's load remain close to
   the maximum it can tolerate.  It is also RECOMMENDED to dynamically
   adjust the puzzle difficulty in accordance to the current Responder's

   Solving puzzles requires a lot of CPU power, that would increase
   power consumption.  This would influence battery-powered Initiators,
   e.g. mobile phones or some IoT devices.  If puzzles are hard then the
   required additional power consumption may appear to be unacceptable
   for some Initiators.  The Responder SHOULD take this possibility into
   considerations while choosing the puzzles difficulty and while
   selecting which percentage of Initiators are allowed to reject
   solving puzzles.  See Section 7.1.4 for details.

   If the Initiator uses NULL Authentication [RFC7619] then its identity
   is never verified, that may be used by attackers to perform DoS
   attack after IKE SA is established.  Responders that allow
   unauthenticated Initiators to connect must be prepared deal with
   various kinds of DoS attacks even after IKE SA is created.  See
   Section 8 for details.

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12.  IANA Considerations

   This document defines a new payload in the "IKEv2 Payload Types"

     <TBA>       Puzzle Solution                   PS

   This document also defines a new Notify Message Type in the "IKEv2
   Notify Message Types - Status Types" registry:

     <TBA>       PUZZLE

13.  Acknowledgements

   The authors thank Tero Kivinen, Yaron Sheffer and Scott Fluhrer for
   their contribution into design of the protocol.  In particular, Tero
   Kivinen suggested the kind of puzzle where the task is to find a
   solution with requested number of zero trailing bits.  Yaron Sheffer
   and Scott Fluhrer suggested a way to make puzzle difficulty less
   erratic by solving several weaker puzles.  The authors also thank
   David Waltermire for his carefull review of the draft and all others
   who commented the document.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <http://www.rfc-editor.org/info/rfc7296>.

   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2
              (IKEv2) Message Fragmentation", RFC 7383,
              DOI 10.17487/RFC7383, November 2014,

              "Internet Key Exchange Version 2 (IKEv2) Parameters",

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14.2.  Informative References

              Nakamoto, S., "Bitcoin: A Peer-to-Peer Electronic Cash
              System", October 2008, <https://bitcoin.org/bitcoin.pdf>.

   [RFC5723]  Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
              Protocol Version 2 (IKEv2) Session Resumption", RFC 5723,
              DOI 10.17487/RFC5723, January 2010,

   [RFC7619]  Smyslov, V. and P. Wouters, "The NULL Authentication
              Method in the Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015,

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,

Authors' Addresses

   Yoav Nir
   Check Point Software Technologies Ltd.
   5 Hasolelim st.
   Tel Aviv  6789735

   EMail: ynir.ietf@gmail.com

   Valery Smyslov
   PO Box 81
   Moscow (Zelenograd)  124460
   Russian Federation

   Phone: +7 495 276 0211
   EMail: svan@elvis.ru

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