< draft-ietf-ipsecme-ddos-protection-06.txt   draft-ietf-ipsecme-ddos-protection-07.txt >
IPSecME Working Group Y. Nir IPSecME Working Group Y. Nir
Internet-Draft Check Point Internet-Draft Check Point
Intended status: Standards Track V. Smyslov Intended status: Standards Track V. Smyslov
Expires: October 17, 2016 ELVIS-PLUS Expires: January 2, 2017 ELVIS-PLUS
April 15, 2016 July 1, 2016
Protecting Internet Key Exchange Protocol version 2 (IKEv2) Protecting Internet Key Exchange Protocol version 2 (IKEv2)
Implementations from Distributed Denial of Service Attacks Implementations from Distributed Denial of Service Attacks
draft-ietf-ipsecme-ddos-protection-06 draft-ietf-ipsecme-ddos-protection-07
Abstract Abstract
This document recommends implementation and configuration best This document recommends implementation and configuration best
practices for Internet Key Exchange Protocol version 2 (IKEv2) practices for Internet Key Exchange Protocol version 2 (IKEv2)
Responders, to allow them to resist Denial of Service and Distributed Responders, to allow them to resist Denial of Service and Distributed
Denial of Service attacks. Additionally, the document introduces a Denial of Service attacks. Additionally, the document introduces a
new mechanism called "Client Puzzles" that help accomplish this task. new mechanism called "Client Puzzles" that help accomplish this task.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 17, 2016. This Internet-Draft will expire on January 2, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. The Vulnerability . . . . . . . . . . . . . . . . . . . . . . 3 3. The Vulnerability . . . . . . . . . . . . . . . . . . . . . . 3
4. Defense Measures while IKE SA is being created . . . . . . . 6 4. Defense Measures while the IKE SA is being created . . . . . 6
4.1. Retention Periods for Half-Open SAs . . . . . . . . . . . 6 4.1. Retention Periods for Half-Open SAs . . . . . . . . . . . 6
4.2. Rate Limiting . . . . . . . . . . . . . . . . . . . . . . 6 4.2. Rate Limiting . . . . . . . . . . . . . . . . . . . . . . 6
4.3. The Stateless Cookie . . . . . . . . . . . . . . . . . . 7 4.3. The Stateless Cookie . . . . . . . . . . . . . . . . . . 7
4.4. Puzzles . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.4. Puzzles . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.5. Session Resumption . . . . . . . . . . . . . . . . . . . 10 4.5. Session Resumption . . . . . . . . . . . . . . . . . . . 10
4.6. Keeping computed Shared Keys . . . . . . . . . . . . . . 10 4.6. Keeping computed Shared Keys . . . . . . . . . . . . . . 11
4.7. Preventing Attacks using "Hash and URL" Certificate 4.7. Preventing "Hash and URL" Certificate Encoding Attacks . 11
Encoding . . . . . . . . . . . . . . . . . . . . . . . . 11 4.8. IKE Fragmentation . . . . . . . . . . . . . . . . . . . . 12
4.8. IKE Fragmentation . . . . . . . . . . . . . . . . . . . . 11 5. Defense Measures after an IKE SA is created . . . . . . . . . 12
5. Defense Measures after IKE SA is created . . . . . . . . . . 11
6. Plan for Defending a Responder . . . . . . . . . . . . . . . 13 6. Plan for Defending a Responder . . . . . . . . . . . . . . . 13
7. Using Puzzles in the Protocol . . . . . . . . . . . . . . . . 15 7. Using Puzzles in the Protocol . . . . . . . . . . . . . . . . 15
7.1. Puzzles in IKE_SA_INIT Exchange . . . . . . . . . . . . . 15 7.1. Puzzles in IKE_SA_INIT Exchange . . . . . . . . . . . . . 15
7.1.1. Presenting Puzzle . . . . . . . . . . . . . . . . . . 16 7.1.1. Presenting a Puzzle . . . . . . . . . . . . . . . . . 16
7.1.2. Solving Puzzle and Returning the Solution . . . . . . 18 7.1.2. Solving a Puzzle and Returning the Solution . . . . . 18
7.1.3. Computing Puzzle . . . . . . . . . . . . . . . . . . 19 7.1.3. Computing a Puzzle . . . . . . . . . . . . . . . . . 19
7.1.4. Analyzing Repeated Request . . . . . . . . . . . . . 19 7.1.4. Analyzing Repeated Request . . . . . . . . . . . . . 19
7.1.5. Making Decision whether to Serve the Request . . . . 20 7.1.5. Deciding if to Serve the Request . . . . . . . . . . 21
7.2. Puzzles in IKE_AUTH Exchange . . . . . . . . . . . . . . 21 7.2. Puzzles in an IKE_AUTH Exchange . . . . . . . . . . . . . 22
7.2.1. Presenting Puzzle . . . . . . . . . . . . . . . . . . 22 7.2.1. Presenting Puzzle . . . . . . . . . . . . . . . . . . 22
7.2.2. Solving Puzzle and Returning the Solution . . . . . . 23 7.2.2. Solving Puzzle and Returning the Solution . . . . . . 23
7.2.3. Computing Puzzle . . . . . . . . . . . . . . . . . . 23 7.2.3. Computing the Puzzle . . . . . . . . . . . . . . . . 24
7.2.4. Receiving Puzzle Solution . . . . . . . . . . . . . . 24 7.2.4. Receiving the Puzzle Solution . . . . . . . . . . . . 24
8. Payload Formats . . . . . . . . . . . . . . . . . . . . . . . 24 8. Payload Formats . . . . . . . . . . . . . . . . . . . . . . . 25
8.1. PUZZLE Notification . . . . . . . . . . . . . . . . . . . 24 8.1. PUZZLE Notification . . . . . . . . . . . . . . . . . . . 25
8.2. Puzzle Solution Payload . . . . . . . . . . . . . . . . . 25 8.2. Puzzle Solution Payload . . . . . . . . . . . . . . . . . 25
9. Operational Considerations . . . . . . . . . . . . . . . . . 26 9. Operational Considerations . . . . . . . . . . . . . . . . . 26
10. Security Considerations . . . . . . . . . . . . . . . . . . . 26 10. Security Considerations . . . . . . . . . . . . . . . . . . . 27
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 28 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
13.1. Normative References . . . . . . . . . . . . . . . . . . 28 13.1. Normative References . . . . . . . . . . . . . . . . . . 28
13.2. Informative References . . . . . . . . . . . . . . . . . 28 13.2. Informative References . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction 1. Introduction
Denial of Service (DoS) attacks have always been considered a serious Denial of Service (DoS) attacks have always been considered a serious
threat. These attacks are usually difficult to defend against since threat. These attacks are usually difficult to defend against since
the amount of resources the victim has is always bounded (regardless the amount of resources the victim has is always bounded (regardless
of how high it is) and because some resources are required for of how high it is) and because some resources are required for
distinguishing a legitimate session from an attack. distinguishing a legitimate session from an attack.
The Internet Key Exchange protocol version 2 (IKEv2) described in The Internet Key Exchange protocol version 2 (IKEv2) described in
[RFC7296] includes defense against DoS attacks. In particular, there [RFC7296] includes defense against DoS attacks. In particular, there
is a cookie mechanism that allows the IKE Responder to effectively is a cookie mechanism that allows the IKE Responder to defend itself
defend itself against DoS attacks from spoofed IP-addresses. against DoS attacks from spoofed IP-addresses. However, bot-nets
However, bot-nets have become widespread, allowing attackers to have become widespread, allowing attackers to perform Distributed
perform Distributed Denial of Service (DDoS) attacks, which are more Denial of Service (DDoS) attacks, which are more difficult to defend
difficult to defend against. This document presents recommendations against. This document presents recommendations to help the
to help the Responder thwart (D)DoS attacks. It also introduces a Responder counter (D)DoS attacks. It also introduces a new mechanism
new mechanism -- "puzzles" -- that can help accomplish this task. -- "puzzles" -- that can help accomplish this task.
2. Conventions Used in This Document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. The Vulnerability 3. The Vulnerability
The IKE_SA_INIT Exchange described in Section 1.2 of [RFC7296] The IKE_SA_INIT Exchange described in Section 1.2 of [RFC7296]
involves the Initiator sending a single message. The Responder involves the Initiator sending a single message. The Responder
replies with a single message and also allocates memory for a replies with a single message and also allocates memory for a
structure called a half-open IKE Security Association (SA). This structure called a half-open IKE Security Association (SA). This
half-open SA is later authenticated in the IKE_AUTH Exchange. If 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 that IKE_AUTH request never comes, the half-open SA is kept for an
unspecified amount of time. Depending on the algorithms used and unspecified amount of time. Depending on the algorithms used and
implementation, such a half-open SA will use from around 100 bytes to implementation, such a half-open SA will use from around 100 bytes to
several thousands bytes of memory. several thousands bytes of memory.
This creates an easy attack vector against an IKE Responder. This creates an easy attack vector against an IKE Responder.
Generating the IKE_SA_INIT request is cheap, and sending multiple Generating the IKE_SA_INIT request is cheap. Sending large amounts
such requests can either cause the Responder to allocate too much of IKE_SA_INIT requests can cause a Responder to use up all its
resources and fail, or else if resource allocation is somehow resources. If the Responder tries to defend against this by
throttled, legitimate Initiators would also be prevented from setting throttling new requests, this will also prevent legitimate Initiators
up IKE SAs. from setting up IKE SAs.
An obvious defense, which is described in Section 4.2, is limiting An obvious defense, which is described in Section 4.2, is limiting
the number of half-open SAs opened by a single peer. However, since 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 all that is required is a single packet, an attacker can use multiple
spoofed source IP addresses. spoofed source IP addresses.
If we break down what a Responder has to do during an initial If we break down what a Responder has to do during an initial
exchange, there are three stages: exchange, there are three stages:
1. When the IKE_SA_INIT request arrives, the Responder: 1. When the IKE_SA_INIT request arrives, the Responder:
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* Derives the keys from the half-open SA. * Derives the keys from the half-open SA.
* Decrypts the request. * Decrypts the request.
3. If the IKE_AUTH request decrypts properly: 3. If the IKE_AUTH request decrypts properly:
* Validates the certificate chain (if present) in the IKE_AUTH * Validates the certificate chain (if present) in the IKE_AUTH
request. request.
Yes, there's a stage 4 where the Responder actually creates Child The fourth stage where the Responder creates the Child SA is not
SAs, but when talking about (D)DoS, we never get to this stage. reached by attackers who cannot pass the authentication step.
Stage #1 is pretty light on CPU power, but requires some storage, and 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 it's very light for the Initiator as well. Stage #2 includes
private-key operations, so it's much heavier CPU-wise. Stage #3 private-key operations, so it is much heavier CPU-wise. Stage #3 may
includes public key operations, typically more than one. include public key operations if certificates are involved. These
operations are often more computationly expensive than those
performed at stage #2.
To attack such a Responder, an attacker can attempt to either exhaust To attack such a Responder, an attacker can attempt either to exhaust
memory or to exhaust CPU. Without any protection, the most efficient memory or to exhaust CPU. Without any protection, the most efficient
attack is to send multiple IKE_SA_INIT requests and exhaust memory. attack is to send multiple IKE_SA_INIT requests and exhaust memory.
This should be easy because those requests are cheap. This is easy because IKE_SA_INIT requests are cheap.
There are obvious ways for the Responder to protect itself even There are obvious ways for the Responder to protect itself without
without changes to the protocol. It can reduce the time that an changes to the protocol. It can reduce the time that an entry
entry remains in the half-open SA database, and it can limit the remains in the half-open SA database, and it can limit the amount of
amount of concurrent half-open SAs from a particular address or concurrent half-open SAs from a particular address or prefix. The
prefix. The attacker can overcome this by using spoofed source attacker can overcome this by using spoofed source addresses.
addresses.
The stateless cookie mechanism from Section 2.6 of [RFC7296] prevents The stateless cookie mechanism from Section 2.6 of [RFC7296] prevents
an attack with spoofed source addresses. This doesn't completely an attack with spoofed source addresses. This doesn't completely
solve the issue, but it makes the limiting of half-open SAs by solve the issue, but it makes the limiting of half-open SAs by
address or prefix work. Puzzles, introduced in Section 4.4, do the address or prefix work. Puzzles, introduced in Section 4.4,
same thing only more of it. They make it harder for an attacker to accomplish the same thing only more of it. They make it harder for
reach the goal of getting a half-open SA. They don't have to be so an attacker to reach the goal of getting a half-open SA. Puzzles do
hard that an attacker can't afford to solve a single puzzle; it's not have to be so hard that an attacker cannot afford to solve a
enough that they increase the cost of a half-open SAs for the single puzzle; it is enough that puzzles increase the cost of
attacker so that it can create only a few. creating a half-open SAs, so the attacker is limited in the amount
they can create.
Reducing the amount of time an abandoned half-open SA is kept attacks Reducing the lifetime of an abandoned half-open SA also reduces the
the issue from the other side. It reduces the value the attacker impact of such attacks. For example, if a half-open SA is kept for 1
gets from managing to create a half-open SA. For example, if a half- minute and the capacity is 60 thousand half-open SAs, an attacker
open SA is kept for 1 minute and the capacity is 60,000 half-open would need to create one thousand half-open SAs per second. If the
SAs, an attacker would need to create 1,000 half-open SAs per second. retention time is reduced to 3 seconds, the attacker would need to
Reduce the retention time to 3 seconds, and the attacker needs to create 20 thousand half-open SAs per second to get the same result.
create 20,000 half-open SAs per second. By introducing a puzzle, By introducing a puzzle, each half-open SA becomes more expensive for
each half-open SA becomes more expensive for an attacker, making it an attacker, making it more likely to prevent an exhaustion attack
more likely to thwart an exhaustion attack against Responder memory. against Responder memory.
At this point, filling up the half-open SA database is no longer the 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: most efficient DoS attack. The attacker has two alternative attacks
to do better:
1. Go back to spoofed addresses and try to overwhelm the CPU that 1. Go back to spoofed addresses and try to overwhelm the CPU that
deals with generating cookies, or deals with generating cookies, or
2. Take the attack to the next level by also sending an IKE_AUTH 2. Take the attack to the next level by also sending an IKE_AUTH
request. request.
It seems that the first thing cannot be dealt with at the IKE level. If an attacker is so powerfull that it is able to overwhelm the
It's probably better left to Intrusion Prevention System (IPS) Responder's CPU that deals with generating cookies, then the attack
technology. cannot be dealt with at the IKE level and must be handled by means of
the Intrusion Prevention System (IPS) technology.
On the other hand, sending an IKE_AUTH request is surprisingly cheap. On the other hand, the second alternative of sending an IKE_AUTH
It requires a proper IKE header with the correct IKE SPIs, and it request is very cheap. It requires generating a proper IKE header
requires a single Encrypted payload. The content of the payload with the correct IKE SPIs and a single Encrypted payload. The
might as well be junk. The Responder has to perform the relatively content of the payload is irrelevant and might be junk. The
expensive key derivation, only to find that the MAC on the Encrypted Responder has to perform the relatively expensive key derivation,
payload on the IKE_AUTH request does not check. Depending on the only to find that the MAC on the Encrypted payload on the IKE_AUTH
Responder implementation, this can be repeated with the same half- request fails the integrity check. If a Responder does not hold on
open SA. Puzzles can make attacks of such sort more costly for an to the calculated SKEYSEED and SK_* keys (which it should in case a
attacker. See Section 7.2 for details. valid IKE_AUTH comes in later) this attack might be repeated on the
same half-open SA. Puzzles make attacks of such sort more costly for
an attacker. See Section 7.2 for details.
Here too, the number of half-open SAs that the attacker can achieve 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 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. 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: A strategy against DDoS has to rely on at least 4 components:
1. Hardening the half-open SA database by reducing retention time. 1. Hardening the half-open SA database by reducing retention time.
2. Hardening the half-open SA database by rate-limiting single IPs/ 2. Hardening the half-open SA database by rate-limiting single IPs/
prefixes. prefixes.
3. Guidance on what to do when an IKE_AUTH request fails to decrypt. 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 4. Increasing the cost of half-open SAs up to what is tolerable for
legitimate clients. legitimate clients.
Puzzles have their place as part of #4. Puzzles are used as a solution for strategy #4.
4. Defense Measures while IKE SA is being created 4. Defense Measures while the IKE SA is being created
4.1. Retention Periods for Half-Open SAs 4.1. Retention Periods for Half-Open SAs
As a UDP-based protocol, IKEv2 has to deal with packet loss through As a UDP-based protocol, IKEv2 has to deal with packet loss through
retransmissions. Section 2.4 of [RFC7296] recommends "that messages retransmissions. Section 2.4 of [RFC7296] recommends "that messages
be retransmitted at least a dozen times over a period of at least be retransmitted at least a dozen times over a period of at least
several minutes before giving up". Retransmission policies in several minutes before giving up". Many retransmission policies in
practice wait at least one or two seconds before retransmitting for practice wait one or two seconds before retransmitting for the first
the first time. time.
Because of this, setting the timeout on a half-open SA too low will 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. 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 When not under attack, the half-open SA timeout SHOULD be set high
enough that the Initiator will have enough time to send multiple enough that the Initiator will have enough time to send multiple
retransmissions, minimizing the chance of transient network retransmissions, minimizing the chance of transient network
congestion causing IKE failure. congestion causing an IKE failure.
When the system is under attack, as measured by the amount of half- When the system is under attack, as measured by the amount of half-
open SAs, it makes sense to reduce this lifetime. The Responder open SAs, it makes sense to reduce this lifetime. The Responder
should still allow enough time for the round-trip, enough time for 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 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 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. 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 It could make sense to assign a shorter value to half-open SAs
originating from IP addresses or prefixes that are considered suspect originating from IP addresses or prefixes that are considered suspect
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Even with DDoS, the attacker has only a limited amount of nodes Even with DDoS, the attacker has only a limited amount of nodes
participating in the attack. By limiting the amount of half-open SAs participating in the attack. By limiting the amount of half-open SAs
that are allowed to exist concurrently with each such node, the total 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 amount of half-open SAs is capped, as is the total amount of key
derivations that the Responder is forced to complete. derivations that the Responder is forced to complete.
In IPv4 it makes sense to limit the number of half-open SAs based on 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 IP address. Most IPv4 nodes are either directly attached to the
Internet using a routable address or are hidden behind a NAT device Internet using a routable address or are hidden behind a NAT device
with a single IPv4 external address. For IPv6, ISPs assign between a with a single IPv4 external address. For IPv6, ISPs assign between a
/48 and a /64, so it makes sense to use a 64-bit prefix as the basis /48 and a /64, so it does not make sense for rate-limiting to work on
for rate limiting in IPv6. single IPv6 IPs. Instead, ratelimits should be done based on either
the /48 or /64 of the misbehaving IPv6 address observed.
The number of half-open SAs is easy to measure, but it is also 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 worthwhile to measure the number of failed IKE_AUTH exchanges. If
possible, both factors should be taken into account when deciding possible, both factors should be taken into account when deciding
which IP address or prefix is considered suspicious. which IP address or prefix is considered suspicious.
There are two ways to rate-limit a peer address or prefix: 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 1. Hard Limit - where the number of half-open SAs is capped, and any
further IKE_SA_INIT requests are rejected. further IKE_SA_INIT requests are rejected.
2. Soft Limit - where if a set number of half-open SAs exist for a 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 particular address or prefix, any IKE_SA_INIT request will be
require solving a puzzle. required to solve a puzzle.
The advantage of the hard limit method is that it provides a hard cap 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. 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 The disadvantage is that it allows the attacker to block IKE
from small parts of the Internet. For example, if an network service initiation from small parts of the Internet. For example, if an
provider or some establishment offers Internet connectivity to its network service provider or some establishment offers Internet
customers or employees through an IPv4 NAT device, a single malicious connectivity to its customers or employees through an IPv4 NAT
customer can create enough half-open SAs to fill the quota for the device, a single malicious customer can create enough half-open SAs
NAT device external IP address. Legitimate Initiators on the same to fill the quota for the NAT device external IP address. Legitimate
network will not be able to initiate IKE. Initiators on the same network will not be able to initiate IKE.
The advantage of a soft limit is that legitimate clients can always The advantage of a soft limit is that legitimate clients can always
connect. The disadvantage is that an adversary with sufficient CPU connect. The disadvantage is that an adversary with sufficient CPU
resources can still effectively DoS the Responder. resources can still effectively DoS the Responder.
Regardless of the type of rate-limiting used, there is a huge Regardless of the type of rate-limiting used, legitimate initiators
advantage in blocking the DoS attack using rate-limiting for that are not on the same network segments as the attackers will not
legitimate clients that are away from the attacking nodes. In such be affected. This is very important as it reduces the adverse impact
cases, adverse impacts caused by the attack or by the measures used caused by the measures used to counteract the attack, and allows most
to counteract the attack can be avoided. initiators to keep working even if they do not support puzzles.
4.3. The Stateless Cookie 4.3. The Stateless Cookie
Section 2.6 of [RFC7296] offers a mechanism to mitigate DoS attack: Section 2.6 of [RFC7296] offers a mechanism to mitigate DoS attacks:
the stateless cookie. When the server is under load, the Responder the stateless cookie. When the server is under load, the Responder
responds to the IKE_SA_INIT request with a calculated "stateless responds to the IKE_SA_INIT request with a calculated "stateless
cookie" - a value that can be re-calculated based on values in the cookie" - a value that can be re-calculated based on values in the
IKE_SA_INIT request without storing Responder-side state. The IKE_SA_INIT request without storing Responder-side state. The
Initiator is expected to repeat the IKE_SA_INIT request, this time Initiator is expected to repeat the IKE_SA_INIT request, this time
including the stateless cookie. This mechanism prevents DoS attacks including the stateless cookie. This mechanism prevents DoS attacks
from spoofed IP addresses, since an attacker needs to have a routable from spoofed IP addresses, since an attacker needs to have a routable
IP address to return the cookie. IP address to return the cookie.
Attackers that have multiple source IP addresses with return Attackers that have multiple source IP addresses with return
routability, such as in the case of bot-nets, can fill up a half-open 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 SA table anyway. The cookie mechanism limits the amount of allocated
state to the number of attackers, multiplied by the number of half- state to the number of attackers, multiplied by the number of half-
open SAs allowed per peer address, multiplied by the amount of state open SAs allowed per peer address, multiplied by the amount of state
allocated for each half-open SA. With typical values this can easily allocated for each half-open SA. With typical values this can easily
reach hundreds of megabytes. reach hundreds of megabytes.
4.4. Puzzles 4.4. Puzzles
The puzzle introduced here extends the cookie mechanism from The puzzle introduced here extends the cookie mechanism of [RFC7296].
[RFC7296]. It is loosely based on the proof-of-work technique used It is loosely based on the proof-of-work technique used in Bitcoins
in Bitcoins [bitcoins]. This sets an upper bound, determined by the [bitcoins]. Puzzles set an upper bound, determined by the attacker's
attacker's CPU, to the number of negotiations it can initiate in a CPU, to the number of negotiations the attacker can initiate in a
unit of time. unit of time.
A puzzle is sent to the Initiator in two cases: A puzzle is sent to the Initiator in two cases:
o The Responder is so overloaded that no half-open SAs may be o The Responder is so overloaded that no half-open SAs may be
created without solving a puzzle, or created without solving a puzzle, or
o The Responder is not too loaded, but the rate-limiting method o The Responder is not too loaded, but the rate-limiting method
described in Section 4.2 prevents half-open SAs from being created described in Section 4.2 prevents half-open SAs from being created
with this particular peer address or prefix without first solving with this particular peer address or prefix without first solving
a puzzle. a puzzle.
When the Responder decides to send the challenge notification in When the Responder decides to send the challenge to solve a puzzle in
response to a IKE_SA_INIT request, the notification includes three response to a IKE_SA_INIT request, the message includes at least
fields: three components:
1. Cookie - this is calculated the same as in [RFC7296], i.e. the 1. Cookie - this is calculated the same as in [RFC7296], i.e. the
process of generating the cookie is not specified. process of generating the cookie is not specified.
2. Algorithm, this is the identifier of a Pseudo-Random Function 2. Algorithm, this is the identifier of a Pseudo-Random Function
(PRF) algorithm, one of those proposed by the Initiator in the SA (PRF) algorithm, one of those proposed by the Initiator in the SA
payload. payload.
3. Zero Bit Count (ZBC). This is a number between 8 and 255 (or a 3. Zero Bit Count (ZBC). This is a number between 8 and 255 (or a
special value - 0, see Section 7.1.1.1) that represents the special value - 0, see Section 7.1.1.1) that represents the
length of the zero-bit run at the end of the output of the PRF 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 function calculated over the cookie that the Initiator is to
send. The values 1-8 are explicitly excluded, because they send. The values 1-8 are explicitly excluded, because they
create a puzzle that is too easy to solve for it to make any create a puzzle that is too easy to solve. Since the mechanism
difference in mitigating DDoS attacks. Since the mechanism is is supposed to be stateless for the Responder, either the same
supposed to be stateless for the Responder, either the same ZBC ZBC is used for all Initiators, or the ZBC is somehow encoded in
is used for all Initiators, or the ZBC is somehow encoded in the the cookie. If it is global then it means that this value is 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 same for all the Initiators who are receiving puzzles at any
given point of time. The Responder, however, may change this given point of time. The Responder, however, may change this
value over time depending on its load. value over time depending on its load.
Upon receiving this challenge, the Initiator attempts to calculate Upon receiving this challenge, the Initiator attempts to calculate
the PRF using different keys. When enough keys are found such that the PRF output using different keys. When enough keys are found such
the resulting PRF output calculated using each of them has a that the resulting PRF output calculated using each of them has a
sufficient number of trailing zero bits, that result is sent to the sufficient number of trailing zero bits, that result is sent to the
Responder. Responder.
The reason for using several keys in the results rather than just one The reason for using several keys in the results, rather than just
key is to reduce the variance in the time it takes the initiator to one key, is to reduce the variance in the time it takes the initiator
solve the puzzle. We have chosen the number of keys to be four (4) to solve the puzzle. We have chosen the number of keys to be four
as a compromise between the conflicting goals of reducing variance (4) as a compromise between the conflicting goals of reducing
and reducing the work the Responder needs to perform to verify the variance and reducing the work the Responder needs to perform to
puzzle solution. verify the puzzle solution.
When receiving a request with a solved puzzle, the Responder verifies When receiving a request with a solved puzzle, the Responder verifies
two things: two things:
o That the cookie part is indeed valid. o That the cookie is indeed valid.
o That the PRFs of the transmitted cookie calculated with the o That the results of PRF of the transmitted cookie calculated with
transmitted keys has a sufficient number of trailing zero bits. the transmitted keys has a sufficient number of trailing zero
bits.
Example 1: Suppose the calculated cookie is Example 1: Suppose the calculated cookie is
739ae7492d8a810cf5e8dc0f9626c9dda773c5a3 (20 octets), the algorithm 739ae7492d8a810cf5e8dc0f9626c9dda773c5a3 (20 octets), the algorithm
is PRF-HMAC-SHA256, and the required number of zero bits is 18. is PRF-HMAC-SHA256, and the required number of zero bits is 18.
After successively trying a bunch of keys, the Initiator finds the After successively trying a bunch of keys, the Initiator finds the
following four 3-octet keys that work: following four 3-octet keys that work:
+--------+----------------------------------+----------+ +--------+----------------------------------+----------+
| Key | Last 32 Hex PRF Digits | # 0-bits | | Key | Last 32 Hex PRF Digits | # 0-bits |
+--------+----------------------------------+----------+ +--------+----------------------------------+----------+
skipping to change at page 10, line 34 skipping to change at page 10, line 39
longer on mobile phone processors, even if those are multi-core as longer on mobile phone processors, even if those are multi-core as
well. With these figures 18 bits is believed to be a reasonable well. With these figures 18 bits is believed to be a reasonable
choice for puzzle level difficulty for all Initiators, and 20 bits is choice for puzzle level difficulty for all Initiators, and 20 bits is
acceptable for specific hosts/prefixes. acceptable for specific hosts/prefixes.
Using puzzles mechanism in the IKE_SA_INIT exchange is described in Using puzzles mechanism in the IKE_SA_INIT exchange is described in
Section 7.1. Section 7.1.
4.5. Session Resumption 4.5. Session Resumption
When the Responder is under attack, it MAY choose to prefer When the Responder is under attack, it SHOULD prefer previously
previously authenticated peers who present a Session Resumption authenticated peers who present a Session Resumption ticket
ticket (see [RFC5723] for details). The Responder MAY require such [RFC5723]. However, the Responder SHOULD NOT serve resumed
Initiators to pass a return routability check by including the COOKIE Initiators exclusively because dropping all IKE_SA_INIT requests
notification in the IKE_SESSION_RESUME response message, as allowed would lock out legitimate Initiators that have no resumption ticket.
by Section 4.3.2. of [RFC5723]. Note that the Responder SHOULD cache When under attack the Responder SHOULD require Initiators presenting
tickets for a short time to reject reused tickets (Section 4.3.1), Session Resumption Tickets to pass a return routability check by
and therefore there should be no issue of half-open SAs resulting including the COOKIE notification in the IKE_SESSION_RESUME response
from replayed IKE_SESSION_RESUME messages. message, as described in 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.
Several kinds of DoS attacks are possible on servers supported IKE Several kinds of DoS attacks are possible on servers supported IKE
Session Resumption. See Section 9.3 of [RFC5723] for details. Session Resumption. See Section 9.3 of [RFC5723] for details.
4.6. Keeping computed Shared Keys 4.6. Keeping computed Shared Keys
Once the IKE_SA_INIT exchange is finished, the Responder is waiting Once the IKE_SA_INIT exchange is finished, the Responder is waiting
for the first message of the IKE_AUTH exchange from the Initiator. for the first message of the IKE_AUTH exchange from the Initiator.
At this point the Initiator is not yet authenticated and this fact At this point the Initiator is not yet authenticated, and this fact
allows a malicious peer to perform an attack, described in Section 3 allows an attacker to perform an attack, described in Section 3. The
- it can just send a garbage in the IKE_AUTH message thus forcing the attacker can just send garbage in the IKE_AUTH message forcing the
Responder to perform CPU costly operations to compute SK_* keys. Responder to perform costly CPU operations to compute SK_* keys.
If the received IKE_AUTH message failed to decrypt correctly (or If the received IKE_AUTH message failed to decrypt correctly (or
failed to pass ICV check), then the Responder SHOULD still keep the failed to pass ICV check), then the Responder SHOULD still keep the
computed SK_* keys, so that if it happened to be an attack, then the computed SK_* keys, so that if it happened to be an attack, then an
malicious Initiator cannot get advantage of repeating the attack attacker cannot get advantage of repeating the attack multiple times
multiple times on a single IKE SA. The responder can also use on a single IKE SA. The responder can also use puzzles in the
puzzles in the IKE_AUTH exchange as decribed in Section 7.2. IKE_AUTH exchange as decribed in Section 7.2.
4.7. Preventing Attacks using "Hash and URL" Certificate Encoding 4.7. Preventing "Hash and URL" Certificate Encoding Attacks
In IKEv2 each side may use "Hash and URL" Certificate Encoding to In IKEv2 each side may use the "Hash and URL" Certificate Encoding to
instruct the peer to retrieve certificates from the specified instruct the peer to retrieve certificates from the specified
location (see Section 3.6 of [RFC7296] for details). Malicious location (see Section 3.6 of [RFC7296] for details). Malicious
initiators can use this feature to mount a DoS attack on responder by initiators can use this feature to mount a DoS attack on the
providing an URL pointing to a large file possibly containing responder by providing an URL pointing to a large file possibly
garbage. While downloading the file the responder consumes CPU, containing garbage. While downloading the file the responder
memory and network bandwidth. consumes CPU, memory and network bandwidth.
To prevent this kind of attacks the responder should not blindly To prevent this kind of attack, the responder should not blindly
download the whole file. Instead it SHOULD first read the initial download the whole file. Instead, it SHOULD first read the initial
few bytes, decode the length of the ASN.1 structure from these bytes, few bytes, decode the length of the ASN.1 structure from these bytes,
and then download no more than the decoded number of bytes. Note, and then download no more than the decoded number of bytes. Note,
that it is always possible to determine the length of ASN.1 that it is always possible to determine the length of ASN.1
structures used in IKEv2 by analyzing the first few bytes, if they structures used in IKEv2, if they are DER-encoded, by analyzing the
are DER-encoded. However, since the content of the file being first few bytes. However, since the content of the file being
downloaded can be under attacker's control, implementations should downloaded can be under the attacker's control, implementations
not blindly trust the decoded length and SHOULD check whether it should not blindly trust the decoded length and SHOULD check whether
makes sense before continue downloading. Implementations SHOULD also it makes sense before continuing to download the file.
apply a configurable hard limit to the number of pulled bytes and Implementations SHOULD also apply a configurable hard limit to the
SHOULD provide an ability for an administrator to either completely number of pulled bytes and SHOULD provide an ability for an
disable this feature or to limit its use to a configurable list of administrator to either completely disable this feature or to limit
trusted URLs. its use to a configurable list of trusted URLs.
4.8. IKE Fragmentation 4.8. IKE Fragmentation
IKE Fragmentation described in [RFC7383] allows IKE peers to avoid IP IKE Fragmentation described in [RFC7383] allows IKE peers to avoid IP
fragmentation of large IKE messages. Attackers can mount several fragmentation of large IKE messages. Attackers can mount several
kinds of DoS attacks using IKE Fragmentation. See Section 5 of kinds of DoS attacks using IKE Fragmentation. See Section 5 of
[RFC7383] for details. [RFC7383] for details on how to mitigate these attacks.
5. Defense Measures after IKE SA is created 5. Defense Measures after an IKE SA is created
Once IKE SA is created there is usually not much traffic over it. In Once an IKE SA is created there usually are only a limited amount of
most cases this traffic consists of exchanges aimed to create IKE messages exchanged. This IKE traffic consists of exchanges aimed
additional Child SAs, rekey, or delete them and check the liveness of to create additional Child SAs, IKE rekeys, IKE deletions and IKE
the peer. With a typical setup and typical Child SA lifetimes, there liveness tests. Some of these exchanges require relatively little
are typically no more than a few such exchanges, often less. Some of resources (like liveness check), while others may be resource
these exchanges require relatively little resources (like liveness consuming (like creating or rekeying Child SA with D-H exchange).
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 Since any endpoint can initiate a new exchange, there is a
possibility that a peer would initiate too many exchanges that could possibility that a peer would initiate too many exchanges that could
exhaust host resources. For example, the peer can perform endless exhaust host resources. For example, the peer can perform endless
continuous Child SA rekeying or create overwhelming number of Child continuous Child SA rekeying or create an overwhelming number of
SAs with the same Traffic Selectors etc. Such behavior may be caused Child SAs with the same Traffic Selectors etc. Such behavior can be
by buggy implementation, misconfiguration or be intentional. The caused by broken implementations, misconfiguration, or as an
latter becomes more of a real threat if the peer uses NULL intentional attack. The latter becomes more of a real threat if the
Authentication, described in [RFC7619]. In this case the peer peer uses NULL Authentication, as described in [RFC7619]. In this
remains anonymous, allowing it to escape any responsibility for its case the peer remains anonymous, allowing it to escape any
actions. See Section 3 of [RFC7619] for details. responsibility for its behaviour. See Section 3 of [RFC7619] for
details on how to mitigate attacks when using NULL Authentication.
The following recommendations for defense against possible DoS The following recommendations apply especially for NULL Authenticated
attacks after IKE SA is established are mostly intended for IKE sessions, but also apply to authenticated IKE sessions, with the
implementations that allow unauthenticated IKE sessions; however, difference that in the latter case, the identified peer can be locked
they may also be useful in other cases. out.
o If the IKEv2 window size is greater than one, then the peer could o If the IKEv2 window size is greater than one, peers are able to
initiate multiple simultaneous exchanges that could increase host initiate multiple simultaneous exchanges that increase host
resource consumption. Since currently there is no way in IKEv2 to resource consumption. Since there is no way in IKEv2 to decrease
decrease window size once it was increased (see Section 2.3 of window size once it has been increased (see Section 2.3 of
[RFC7296]), the window size cannot be dynamically adjusted [RFC7296]), the window size cannot be dynamically adjusted
depending on the load. For that reason, it is NOT RECOMMENDED to depending on the load. It is NOT RECOMMENDED to allow an IKEv2
ever increase the IKEv2 window size above its default value of one window size greater than one when NULL Authentication has been
if the peer uses NULL Authentication. used.
o If the peer initiates requests to rekey IKE SA or Child SA too o If a peer initiates an abusive amount of CREATE_CHILD_SA exchanges
often, implementations can respond to some of these requests with to rekey IKE SAs or Child SAs, the Responder SHOULD reply with
the TEMPORARY_FAILURE notification, indicating that the request TEMPORARY_FAILURE notifications indicating the peer must slow down
should be retried after some period of time. their requests.
o If the peer creates too many Child SA with the same or overlapping o If a peer creates many Child SA with the same or overlapping
Traffic Selectors, implementations can respond with the Traffic Selectors, implementations MAY respond with the
NO_ADDITIONAL_SAS notification. NO_ADDITIONAL_SAS notification.
o If the peer initiates too many exchanges of any kind, o If a peer initiates many exchanges of any kind, the Responder MAY
implementations can introduce an artificial delay before introduce an artificial delay before responding to each request
responding to each request message. This delay would decrease the message. This delay would decrease the rate the Responder needs
rate the implementation need to process requests from any to process requests from any particular peer, and frees up
particular peer, making it possible to process requests from the resources on the Responder that can be used for answering
others. Note, that if the Responder receives retransmissions of legitimate clients. If the Responder receives retransmissions of
the request message during the delay period, the retransmitted the request message during the delay period, the retransmitted
messages should be silently discarded. The delay should not be messages MUST be silently discarded. The delay must be short
too long to avoid causing the IKE SA to be deleted on the other enough to avoid legitimate peers deleting the IKE SA due to a
end due to timeout. It is believed that a few seconds is enough. timeout. It is believed that a few seconds is enough. Note
Note however, that even a few seconds may be too long in those however, that even a few seconds may be too long when settings
settings, that rely on immediate response to the request message, rely on an immediate response to the request message, e.g. for the
e.g. for the purposes of quick detection of a dead peer. purposes of quick detection of a dead peer.
o If these counter-measures are inefficient, implementations can o If these counter-measures are inefficient, implementations MAY
delete the IKE SA with an offending peer by sending Delete delete the IKE SA with an offending peer by sending Delete
Payload. Payload.
In IKEv2 client can request various configuration attributes from In IKE, a client can request various configuration attributes from
server. Most often those attributes include internal IP addresses. server. Most often these attributes include internal IP addresses.
Malicious clients can try to exhaust server's IP address pool by Malicious clients can try to exhaust a server's IP address pool by
continuously requesting a large number of internal addresses. Server continuously requesting a large number of internal addresses. Server
implementations SHOULD limit the number of IP addresses allocated to implementations SHOULD limit the number of IP addresses allocated to
any particular client. Note, that it is not possible with clients any particular client. Note, this is not possible with clients using
using NULL Authentication, since their identity cannot be verified. NULL Authentication, since their identity cannot be verified.
6. Plan for Defending a Responder 6. Plan for Defending a Responder
This section outlines a plan for defending a Responder from a DDoS This section outlines a plan for defending a Responder from a DDoS
attack based on the techniques described earlier. The numbers given attack based on the techniques described earlier. The numbers given
here are not normative, and their purpose is to illustrate the here are not normative, and their purpose is to illustrate the
configurable parameters needed for defeating the DDoS attack. configurable parameters needed for surviving DDoS attacks.
Implementations may be deployed in different environments, so it is Implementations are deployed in different environments, so it is
RECOMMENDED that the parameters be settable. As an example, most RECOMMENDED that the parameters be settable. For example, most
commercial products are required to undergo benchmarking where the commercial products are required to undergo benchmarking where the
IKE SA establishment rate is measured. Benchmarking is IKE SA establishment rate is measured. Benchmarking is
indistinguishable from a DoS attack and the defenses described in indistinguishable from a DoS attack and the defenses described in
this document may defeat the benchmark by causing exchanges to fail this document may defeat the benchmark by causing exchanges to fail
or take a long time to complete. Parameters should be tunable to or take a long time to complete. Parameters SHOULD be tunable to
allow for benchmarking (if only by turning DDoS protection off). allow for benchmarking (if only by turning DDoS protection off).
Since all countermeasures may cause delays and work on the Since all countermeasures may cause delays and additional work for
Initiators, they SHOULD NOT be deployed unless an attack is likely to the Initiators, they SHOULD NOT be deployed unless an attack is
be in progress. To minimize the burden imposed on Initiators, the likely to be in progress. To minimize the burden imposed on
Responder should monitor incoming IKE requests, searching for two Initiators, the Responder should monitor incoming IKE requests, for
things: two scenarios:
1. A general DDoS attack. Such an attack is indicated by a high 1. A general DDoS attack. Such an attack is indicated by a high
number of concurrent half-open SAs, a high rate of failed number of concurrent half-open SAs, a high rate of failed
IKE_AUTH exchanges, or a combination of both. For example, IKE_AUTH exchanges, or a combination of both. For example,
consider a Responder that has 10,000 distinct peers of which at consider a Responder that has 10,000 distinct peers of which at
peak 7,500 concurrently have VPN tunnels. At the start of peak peak 7,500 concurrently have VPN tunnels. At the start of peak
time, 600 peers might establish tunnels at any given minute, and time, 600 peers might establish tunnels within any given minute,
tunnel establishment (both IKE_SA_INIT and IKE_AUTH) takes and tunnel establishment (both IKE_SA_INIT and IKE_AUTH) takes
anywhere from 0.5 to 2 seconds. For this Responder, we expect 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 there to be less than 20 concurrent half-open SAs, so having 100
concurrent half-open SAs can be interpreted as an indication of concurrent half-open SAs can be interpreted as an indication of
an attack. Similarly, IKE_AUTH request decryption failures an attack. Similarly, IKE_AUTH request decryption failures
should never happen. Supposing the the tunnels are established should never happen. Supposing that the tunnels are established
using EAP (see Section 2.16 of [RFC7296]), users enter the wrong using EAP (see Section 2.16 of [RFC7296]), users may be expected
password about 20% of the time. So we'd expect 125 wrong to enter a wrong password about 20% of the time. So we'd expect
password failures a minute. If we get IKE_AUTH decryption 125 wrong password failures a minute. If we get IKE_AUTH
failures from multiple sources more than once per second, or EAP decryption failures from multiple sources more than once per
failure more than 300 times per minute, that can also be an second, or EAP failures more than 300 times per minute, this can
indication of a DDoS attack. also be an indication of a DDoS attack.
2. An attack from a particular IP address or prefix. Such an 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 is indicated by an inordinate amount of half-open SAs from a
IP address or prefix, or an inordinate amount of IKE_AUTH specific IP address or prefix, or an inordinate amount of
failures. A DDoS attack may be viewed as multiple such attacks. IKE_AUTH failures. A DDoS attack may be viewed as multiple such
If they are mitigated well enough, there will not be a need enact attacks. If these are mitigated successfully, there will not be
countermeasures on all Initiators. Typical measures might be 5 a need to enact countermeasures on all Initiators. For example,
concurrent half-open SAs, 1 decrypt failure, or 10 EAP failures measures might be 5 concurrent half-open SAs, 1 decrypt failure,
within a minute. or 10 EAP failures within a minute.
Note that using counter-measures against an attack from a particular 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 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 database. In this case the number of failed IKE_AUTH exchanges will
never exceeds the threshold of attack detection. This is a good never exceed the threshold of attack detection.
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 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 or puzzles be used. At this point the only defensive measure is to
monitoring of the number of half-open SAs, and setting a soft limit monitor the number of half-open SAs, and setting a soft limit per
per peer IP or prefix. The soft limit can be set to 3-5, and the peer IP or prefix. The soft limit can be set to 3-5. If the puzzles
puzzle difficulty should be set to such a level (number of zero-bits) are used, the puzzle difficulty should be set to such a level (number
that all legitimate clients can handle it without degraded user of zero-bits) that all legitimate clients can handle it without
experience. degraded user experience.
As soon as any kind of attack is detected, either a lot of 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 initiations from multiple sources or a lot of initiations from a few
sources, it is best to begin by requiring stateless cookies from all sources, it is best to begin by requiring stateless cookies from all
Initiators. This will force the attacker to use real source Initiators. This will This will mitigate attacks based on IP address
addresses, and help avoid the need to impose a greater burden in the spoofing, and help avoid the need to impose a greater burden in the
form of cookies on the general population of Initiators. This makes form of puzzles on the general population of Initiators. This makes
the per-node or per-prefix soft limit more effective. the per-node or per-prefix soft limit more effective.
When cookies are activated for all requests and the attacker is still When cookies are activated for all requests and the attacker is still
managing to consume too many resources, the Responder MAY increase managing to consume too many resources, the Responder MAY start to
the difficulty of puzzles imposed on IKE_SA_INIT requests coming from use puzzles for these requests or increase the difficulty of puzzles
suspicious nodes/prefixes. It should still be doable by all imposed on IKE_SA_INIT requests coming from suspicious nodes/
legitimate peers, but it can degrade experience, for example by prefixes. This should still be doable by all legitimate peers, but
taking up to 10 seconds to solve the puzzle. the use of puzzles at a higher difficulty may degrade the user
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 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 nodes causing multiple half-open SAs or IKE_AUTH failures, the
Responder MAY impose hard limits on those nodes. Responder MAY impose hard limits on those nodes.
If it turns out that the attack is very widespread and the hard caps 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. 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 Note that this is the last step, and the Responder should avoid this
if possible. if possible.
skipping to change at page 15, line 26 skipping to change at page 15, line 39
This section describes how the puzzle mechanism is used in IKEv2. It This section describes how the puzzle mechanism is used in IKEv2. It
is organized as follows. The Section 7.1 describes using puzzles in 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 the IKE_SA_INIT exchange and the Section 7.2 describes using puzzles
in the IKE_AUTH exchange. Both sections are divided into subsections in the IKE_AUTH exchange. Both sections are divided into subsections
describing how puzzles should be presented, solved and processed by describing how puzzles should be presented, solved and processed by
the Initiator and the Responder. the Initiator and the Responder.
7.1. Puzzles in IKE_SA_INIT Exchange 7.1. Puzzles in IKE_SA_INIT Exchange
IKE Initiator indicates the desire to create a new IKE SA by sending IKE Initiator indicates the desire to create a new IKE SA by sending
IKE_SA_INIT request message. The message may optionally contain a an IKE_SA_INIT request message. The message may optionally contain a
COOKIE notification if this is a repeated request performed after the COOKIE notification if this is a repeated request performed after the
Responder's demand to return a cookie. Responder's demand to return a cookie.
HDR, [N(COOKIE),] SA, KE, Ni, [V+][N+] --> HDR, [N(COOKIE),] SA, KE, Ni, [V+][N+] -->
According to the plan, described in Section 6, the IKE Responder According to the plan, described in Section 6, the IKE Responder
should monitor incoming requests to detect whether it is under should monitor incoming requests to detect whether it is under
attack. If the Responder learns that (D)DoS attack is likely to be attack. If the Responder learns that a (D)DoS attack is likely to be
in progress, then its actions depend on the volume of the attack. If in progress, then its actions depend on the volume of the attack. If
the volume is moderate, then the Responder requests the Initiator to 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 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 used for defense, then the Responder requests the Initiator to solve
a puzzle. a puzzle.
The Responder MAY choose to process some fraction of IKE_SA_INIT The Responder MAY choose to process some fraction of IKE_SA_INIT
requests without presenting a puzzle while being under attack to requests without presenting a puzzle while being under attack to
allow legacy clients, that don't support puzzles, to have a chance to allow legacy clients, that don't support puzzles, to have a chance to
be served. The decision whether to process any particular request be served. The decision whether to process any particular request
must be probabilistic, with the probability depending on the must be probabilistic, with the probability depending on the
Responder's load (i.e. on the volume of attack). The requests that Responder's load (i.e. on the volume of attack). The requests that
don't contain the COOKIE notification MUST NOT participate in this don't contain the COOKIE notification MUST NOT participate in this
lottery. In other words, the Responder must first perform return lottery. In other words, the Responder must first perform a return
routability check before allowing any legacy client to be served if routability check before allowing any legacy client to be served if
it is under attack. See Section 7.1.4 for details. it is under attack. See Section 7.1.4 for details.
7.1.1. Presenting Puzzle 7.1.1. Presenting a Puzzle
If the Responder makes a decision to use puzzles, then it MUST If the Responder makes a decision to use puzzles, then it includes
include two notifications in its response message - the COOKIE two notifications in its response message - the COOKIE notification
notification and the PUZZLE notification. The format of the PUZZLE and the PUZZLE notification. Note that the PUZZLE notification MUST
notification is described in Section 8.1. always be accompanied with the COOKIE notification, since the content
of the COOKIE notification is used as an input data when solving
puzzle. The format of the PUZZLE notification is described in
Section 8.1.
<-- HDR, N(COOKIE), N(PUZZLE), [V+][N+] <-- HDR, N(COOKIE), N(PUZZLE), [V+][N+]
The presence of these notifications in an IKE_SA_INIT response The presence of these notifications in an IKE_SA_INIT response
message indicates to the Initiator that it should solve the puzzle to message indicates to the Initiator that it should solve the puzzle to
get better chances to be served. have a better chance to be served.
7.1.1.1. Selecting Puzzle Difficulty Level 7.1.1.1. Selecting the Puzzle Difficulty Level
The PUZZLE notification contains the difficulty level of the puzzle - The PUZZLE notification contains the difficulty level of the puzzle -
the minimum number of trailing zero bits that the result of PRF must the minimum number of trailing zero bits that the result of PRF must
contain. In diverse environments it is next to impossible for the contain. In diverse environments it is next to impossible for the
Responder to set any specific difficulty level that will result in Responder to set any specific difficulty level that will result in
roughly the same amount of work for all Initiators, because roughly the same amount of work for all Initiators, because
computation power of different Initiators may vary by the order of computation power of different Initiators may vary by an order of
magnitude, or even more. The Responder may set difficulty level to magnitude, or even more. The Responder may set the difficulty level
0, meaning that the Initiator is requested to spend as much power to to 0, meaning that the Initiator is requested to spend as much power
solve puzzle, as it can afford. In this case no specific value of to solve a puzzle as it can afford. In this case no specific value
ZBC is required from the Initiator, however the larger the ZBC that of ZBC is required from the Initiator, however the larger the ZBC
Initiator is able to get, the better the chances it will have to be that Initiator is able to get, the better the chance is that it will
served by the Responder. In diverse environments it is RECOMMENDED be served by the Responder. In diverse environments it is
that the Initiator sets difficulty level to 0, unless the attack RECOMMENDED that the Initiator set the difficulty level to 0, unless
volume is very high. the attack volume is very high.
If the Responder sets non-zero difficulty level, then the level If the Responder sets a non-zero difficulty level, then the level
should be determined by analyzing the volume of the attack. The should be determined by analyzing the volume of the attack. The
Responder MAY set different difficulty levels to different requests Responder MAY set different difficulty levels to different requests
depending on the IP address the request has come from. depending on the IP address the request has come from.
7.1.1.2. Selecting Puzzle Algorithm 7.1.1.2. Selecting the Puzzle Algorithm
The PUZZLE notification also contains identifier of the algorithm, The PUZZLE notification also contains identifier of the algorithm,
that must be used by Initiator to compute puzzle. that must be used by Initiator to compute puzzle.
Cryptographic algorithm agility is considered an important feature Cryptographic algorithm agility is considered an important feature
for modern protocols ([RFC7696]). This feature ensures that protocol for modern protocols ([RFC7696]). Algorithm agility ensures that a
doesn't rely on a single build-in set of cryptographic algorithms, protocol doesn't rely on a single built-in set of cryptographic
but has a means to replace one set with another and negotiate new set algorithms, but has a means to replace one set with another, and
with the peer. IKEv2 fully supports cryptographic algorithm agility negotiate new algorithms with the peer. IKEv2 fully supports
for its core operations. cryptographic algorithm agility for its core operations.
To support this feature in case of puzzles, the algorithm that is To support crypto agility in case of puzzles, the algorithm that is
used to compute puzzle needs to be negotiated during IKE_SA_INIT used to compute a puzzle needs to be negotiated during the
exchange. The negotiation is performed as follows. The initial IKE_SA_INIT exchange. The negotiation is performed as follows. The
request message sent by Initiator contains SA payload with the list initial request message sent by the Initiator contains an SA payload
of transforms the Initiator supports and is willing to use in the IKE with the list of transforms the Initiator supports and is willing to
SA being established. The Responder parses received SA payload and use in the IKE SA being established. The Responder parses the
finds mutually supported set of transforms of type PRF. It selects received SA payload and finds a mutually supported PRFs. The
most preferred transform from this set and includes it into the Responder selects the preferred PRF from the list of mutually
PUZZLE notification. There is no requirement that the PRF selected supported ones and includes it into the PUZZLE notification. There
for puzzles be the same, as the PRF that is negotiated later for the is no requirement that the PRF selected for puzzles be the same as
use in core IKE SA crypto operations. If there are no mutually the PRF that is negotiated later for use in core IKE SA crypto
supported PRFs, then negotiation will fail anyway and there is no operations. If there are no mutually supported PRFs, then IKE SA
reason to return a puzzle. In this case the Responder returns negotiation will fail anyway and there is no reason to return a
NO_PROPOSAL_CHOSEN notification. Note that PRF is a mandatory puzzle. In this case the Responder returns a NO_PROPOSAL_CHOSEN
transform type for IKE SA (see Sections 3.3.2 and 3.3.3 of [RFC7296]) notification. Note that PRF is a mandatory transform type for IKE SA
and at least one transform of this type must always be present in SA (see Sections 3.3.2 and 3.3.3 of [RFC7296]) and at least one
payload in IKE_SA_INIT request message. transform of this type must always be present in the SA payload in an
IKE_SA_INIT request message.
7.1.1.3. Generating Cookie 7.1.1.3. Generating a Cookie
If Responder supports puzzles then cookie should be computed in such If the Responder supports puzzles then a cookie should be computed in
a manner, that the Responder is able to learn some important such a manner that the Responder is able to learn some important
information from the sole cookie, when it is later returned back by information from the sole cookie, when it is later returned back by
Initiator. In particular - the Responder should be able to learn the Initiator. In particular - the Responder should be able to learn the
following information: following information:
o Whether the puzzle was given to the Initiator or only the cookie o Whether the puzzle was given to the Initiator or only the cookie
was requested. was requested.
o The difficulty level of the puzzle given to the Initiator. o The difficulty level of the puzzle given to the Initiator.
o The number of consecutive puzzles given to the Initiator. o The number of consecutive puzzles given to the Initiator.
o The amount of time the Initiator spent to solve the puzzles. This o The amount of time the Initiator spent to solve the puzzles. This
can be calculated if the cookie is timestamped. can be calculated if the cookie is timestamped.
This information helps the Responder to make a decision whether to This information helps the Responder to make a decision whether to
serve this request or demand more work from the Initiator. serve this request or demand more work from the Initiator.
One possible approach to get this information is to encode it in the 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, cookie. The format of such encoding is an implementation detail of
as the cookie would remain an opaque blob to the Initiator. If this Responder, as the cookie would remain an opaque blob to the
information is encoded in the cookie, then the Responder MUST make it Initiator. If this information is encoded in the cookie, then the
integrity protected, so that any intended or accidental alteration of Responder MUST make it integrity protected, so that any intended or
this information in returned cookie is detectable. So, the cookie accidental alteration of this information in the returned cookie is
would be generated as: detectable. So, the cookie would be generated as:
Cookie = <VersionIDofSecret> | <AdditionalInfo> | Cookie = <VersionIDofSecret> | <AdditionalInfo> |
Hash(Ni | IPi | SPIi | <AdditionalInfo> | <secret>) Hash(Ni | IPi | SPIi | <AdditionalInfo> | <secret>)
Alternatively, the Responder may continue to generate cookie as Note, that according to the Section 2.6 of [RFC7296], the size of the
cookie cannot exceed 64 bytes.
Alternatively, the Responder may continue to generate a cookie as
suggested in Section 2.6 of [RFC7296], but associate the additional suggested in Section 2.6 of [RFC7296], but associate the additional
information, that would be stored locally, with the particular information, using local storage identified with the particular
version of the secret. In this case the Responder should have version of the secret. In this case the Responder should have
different secrets for every combination of difficulty level and different secrets for every combination of difficulty level and
number of consecutive puzzles, and should change the secrets number of consecutive puzzles, and should change the secrets
periodically, keeping a few previous versions, to be able to periodically, keeping a few previous versions, to be able to
calculate how long ago the cookie was generated. calculate how long ago a cookie was generated.
The Responder may also combine these approaches. This document The Responder may also combine these approaches. This document
doesn't mandate how the Responder learns this information from the doesn't mandate how the Responder learns this information from a
cookie. cookie.
7.1.2. Solving Puzzle and Returning the Solution 7.1.2. Solving a Puzzle and Returning the Solution
If the Initiator receives a puzzle but it doesn't support puzzles, If the Initiator receives a puzzle but it doesn't support puzzles,
then it will ignore the PUZZLE notification as an unrecognized status then it will ignore the PUZZLE notification as an unrecognized status
notification (in accordance to Section 3.10.1 of [RFC7296]). The notification (in accordance to Section 3.10.1 of [RFC7296]). The
Initiator also MAY ignore the PUZZLE notification if it is not Initiator MAY ignore the PUZZLE notification if it is not willing to
willing to spend resources to solve the puzzle of the requested spend resources to solve the puzzle of the requested difficulty, even
difficulty, even if it supports puzzles. In both cases the Initiator if it supports puzzles. In both cases the Initiator acts as
acts as described in Section 2.6 of [RFC7296] - it restarts the described in Section 2.6 of [RFC7296] - it restarts the request and
request and includes the received COOKIE notification into it. The includes the received COOKIE notification into it. The Responder
Responder should be able to distinguish the situation when it just should be able to distinguish the situation when it just requested a
requested a cookie from the situation when the puzzle was given to cookie from the situation where the puzzle was given to the
the Initiator, but the Initiator for some reason ignored it. Initiator, but the Initiator for some reason ignored it.
If the received message contains a PUZZLE notification and doesn't If the received message contains a PUZZLE notification and doesn't
contain a COOKIE notification, then this message is malformed because contain a COOKIE notification, then this message is malformed because
it requests to solve the puzzle, but doesn't provide enough it requests to solve the puzzle, but doesn't provide enough
information to do it. In this case the Initiator MUST ignore the information to allow the puzzle to be solved. In this case the
received message and continue to wait until either the valid one is Initiator MUST ignore the received message and continue to wait until
received or the retransmission timer fires. If it fails to receive either a valid PUZZLE notification is received or the retransmission
the valid message after several retransmissions of IKE_SA_INIT timer fires. If it fails to receive a valid message after several
request, then it means that something is wrong and the IKE SA cannot retransmissions of IKE_SA_INIT requests, then it means that something
be established. is wrong and the IKE SA cannot be established.
If the Initiator supports puzzles and is ready to deal with them, If the Initiator supports puzzles and is ready to solve them, then it
then it tries to solve the given puzzle. After the puzzle is solved tries to solve the given puzzle. After the puzzle is solved the
the Initiator restarts the request and returns the puzzle solution in Initiator restarts the request and returns back to the Responder the
a new payload called a Puzzle Solution payload (denoted as PS, see puzzle solution in a new payload called a Puzzle Solution payload
Section 8.2) along with the received COOKIE notification back to the (denoted as PS, see Section 8.2) along with the received COOKIE
Responder. notification.
HDR, N(COOKIE), [PS,] SA, KE, Ni, [V+][N+] --> HDR, N(COOKIE), [PS,] SA, KE, Ni, [V+][N+] -->
7.1.3. Computing Puzzle 7.1.3. Computing a Puzzle
General principals of constructing puzzles in IKEv2 are described in General principals of constructing puzzles in IKEv2 are described in
Section 4.4. They can be summarized as follows: given unpredictable Section 4.4. They can be summarized as follows: given unpredictable
string S and pseudo-random function PRF find N different keys Ki 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 (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 least the specified number of trailing zero bits. This specification
requires that the solution to the puzzle contains 4 different keys requires that the solution to the puzzle contain 4 different keys
(i.e. N=4). (i.e. N=4).
In the IKE_SA_INIT exchange it is the cookie that plays the role of 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 unpredictable string S. In other words, in the IKE_SA_INIT the task
the IKE Initiator is to find the four different, equal-sized keys Ki for the IKE Initiator is to find the four different, equal-sized keys
for the agreed upon PRF such that each result of PRF(Ki,cookie) where Ki for the agreed upon PRF such that each result of PRF(Ki,cookie)
i = [1..4] has a sufficient number of trailing zero bits. Only the where i = [1..4] has a sufficient number of trailing zero bits. Only
content of the COOKIE notification is used in puzzle calculation, the content of the COOKIE notification is used in puzzle calculation,
i.e. the header of the Notification payload is not included. i.e. the header of the Notification payload is not included.
Note, that puzzles in the IKE_AUTH exchange are computed differently 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. than in the IKE_SA_INIT_EXCHANGE. See Section 7.2.3 for details.
7.1.4. Analyzing Repeated Request 7.1.4. Analyzing Repeated Request
The received request must at least contain a COOKIE notification. The received request must at least contain a COOKIE notification.
Otherwise it is an initial request and it must be processed according 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 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 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 processed according to Section 7.1. It is RECOMMENDED that a new
cookie is requested in this case. cookie is requested in this case.
If the cookie is valid then some important information is learned If the cookie is valid, then some important information is learned
from it or from local state based on identifier of the cookie's from it, or from local state based on identifier of the cookie's
secret (see Section 7.1.1.3 for details). This information helps the secret (see Section 7.1.1.3 for details). This information helps the
Responder to sort out incoming requests, giving more priority to Responder to sort out incoming requests, giving more priority to
those of them, which were created by spending more of the Initiator's those which were created by spending more of the Initiator's
resources. resources.
First, the Responder determines if it requested only a cookie, or First, the Responder determines if it requested only a cookie, or
presented a puzzle to the Initiator. If no puzzle was given, then it presented a puzzle to the Initiator. If no puzzle was given, this
means that at the time the Responder requested a cookie it didn't 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 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 the received request message must not contain the PS payload, and
this payload MUST be ignored if for any reason the message contains this payload MUST be ignored if the message contains a PS payload for
it. Since no puzzle was given, the Responder marks the request with any reason. Since no puzzle was given, the Responder marks the
the lowest priority since the Initiator spent little resources request with the lowest priority since the Initiator spent little
creating it. resources creating it.
If the Responder learns from the cookie that the puzzle was given to 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 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 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 message doesn't contain a PS payload, this means that the Initiator
Initiator either doesn't support puzzles or doesn't want to deal with either doesn't support puzzles or doesn't want to deal with them. In
them. In either case the request is marked with the lowest priority either case the request is marked with the lowest priority since the
since the Initiator spent little resources creating it. Initiator spent little resources creating it.
If a PS payload is found in the message, then the Responder MUST If a PS payload is found in the message, then the Responder MUST
verify the puzzle solution that it contains. The solution is verify the puzzle solution that it contains. The solution is
interpreted as four different keys. The result of using each of them 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 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 requested number of trailing zero bits. The Responder MUST check all
the four returned keys. of the four returned keys.
If any checked result contains fewer bits than were requested, it If any checked result contains fewer bits than were requested, this
means that the Initiator spent less resources than expected by the means that the Initiator spent less resources than expected by the
Responder. This request is marked with the lowest priority. Responder. This request is marked with the lowest priority.
If the Initiator provided the solution to the puzzle satisfying the If the Initiator provided the solution to the puzzle satisfying the
requested difficulty level, or if the Responder didn't indicate any requested difficulty level, or if the Responder didn't indicate any
particular difficulty level (by setting ZBC to zero) and the particular difficulty level (by setting ZBC to zero) and the
Initiator was free to select any difficulty level it can afford, then Initiator was free to select any difficulty level it can afford, then
the priority of the request is calculated based on the following the priority of the request is calculated based on the following
considerations: considerations:
o The Responder must take the smallest number of trailing zero bits 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 among the checked results and count it as the number of zero bits
the Initiator got. the Initiator solved for.
o The higher number of zero bits the Initiator got, the higher o The higher number of zero bits the Initiator provides, the higher
priority its request should receive. priority its request should receive.
o The more consecutive puzzles the Initiator solved, the higher o The more consecutive puzzles the Initiator solved, the higher
priority it should receive. priority it should receive.
o The more time the Initiator spent solving the puzzles, the higher o The more time the Initiator spent solving the puzzles, the higher
priority it should receive. priority it should receive.
After the priority of the request is determined the final decision After the priority of the request is determined the final decision
whether to serve it or not is made. whether to serve it or not is made.
7.1.5. Making Decision whether to Serve the Request 7.1.5. Deciding if to Serve the Request
The Responder decides what to do with the request based on its The Responder decides what to do with the request based on the
priority and Responder's current load. There are three possible request's priority and the Responder's current load. There are three
actions: possible actions:
o Accept request. o Accept request.
o Reject request. o Reject request.
o Demand more work from Initiator by giving it a new puzzle. o Demand more work from the Initiator by giving it a new puzzle.
The Responder SHOULD accept an incoming request if its priority is The Responder SHOULD accept an incoming request if its priority is
high - it means that the Initiator spent quite a lot of resources. high - this means that the Initiator spent quite a lot of resources.
The Responder MAY also accept some of low-priority requests where the The Responder MAY also accept some low-priority requests where the
Initiators don't support puzzles. The percentage of accepted legacy Initiators don't support puzzles. The percentage of accepted legacy
requests depends on the Responder's current load. requests depends on the Responder's current load.
If the Initiator solved the puzzle, but didn't spend much resources If the Initiator solved the puzzle, but didn't spend much resources
for it (the selected puzzle difficulty level appeared to be low and for it (the selected puzzle difficulty level appeared to be low and
the Initiator solved it quickly), then the Responder SHOULD give it the Initiator solved it quickly), then the Responder SHOULD give it
another puzzle. The more puzzles the Initiator solves the higher its another puzzle. The more puzzles the Initiator solves the higher its
chances are to be served. chances are to be served.
The details of how the Responder makes a decision for any particular The details of how the Responder makes a decision for any particular
request, are implementation dependent. The Responder can collect all request are implementation dependent. The Responder can collect all
the incoming requests for some short period of time, sort them out of the incoming requests for some short period of time, sort them out
based on their priority, calculate the number of available memory based on their priority, calculate the number of available memory
slots for half-open IKE SAs and then serve that number of requests 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 from the head of the sorted list. The remainder of requests can be
discarded or responded to with new puzzles. either discarded or responded to with new puzzle requests.
Alternatively, the Responder may decide whether to accept every Alternatively, the Responder may decide whether to accept every
incoming request with some kind of lottery, taking into account its incoming request with some kind of lottery, taking into account its
priority and the available resources. priority and the available resources.
7.2. Puzzles in IKE_AUTH Exchange 7.2. Puzzles in an IKE_AUTH Exchange
Once the IKE_SA_INIT exchange is completed, the Responder has created 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 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 from the Initiator. At this point the Initiator has already passed
return routability check and has proved that it has performed some the return routability check and has proved that it has performed
work to complete IKE_SA_INIT exchange. However, the Initiator is not some work to complete IKE_SA_INIT exchange. However, the Initiator
yet authenticated and this fact allows malicious Initiator to perform is not yet authenticated and this allows a malicious Initiator to
an attack, described in Section 3. Unlike DoS attack in IKE_SA_INIT perform an attack, described in Section 3. Unlike a DoS attack in
exchange, which is targeted on the Responder's memory resources, the the IKE_SA_INIT exchange, which is targeted on the Responder's memory
goal of this attack is to exhaust a Responder's CPU power. The resources, the goal of this attack is to exhaust a Responder's CPU
attack is performed by sending the first IKE_AUTH message containing power. The attack is performed by sending the first IKE_AUTH message
garbage. This costs nothing to the Initiator, but the Responder has containing garbage. This costs nothing to the Initiator, but the
to do relatively costly operations of computing the D-H shared secret Responder has to perform relatively costly operations when computing
and deriving SK_* keys to be able to verify authenticity of the the D-H shared secret and deriving SK_* keys to be able to verify
message. If the Responder doesn't keep the computed keys after an authenticity of the message. If the Responder doesn't keep the
unsuccessful verification of the IKE_AUTH message, then the attack computed keys after an unsuccessful verification of the IKE_AUTH
can be repeated several times on the same IKE SA. 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 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 Initiator. The idea is that the Responder includes a puzzle in the
IKE_SA_INIT response message and the Initiator includes a puzzle IKE_SA_INIT response message and the Initiator includes a puzzle
solution in the first IKE_AUTH request message outside the Encrypted solution in the first IKE_AUTH request message outside the Encrypted
payload, so that the Responder is able to verify puzzle solution payload, so that the Responder is able to verify puzzle solution
before computing D-H shared secret. The difficulty level of the before computing the D-H shared secret. The difficulty level of the
puzzle should be selected so that the Initiator would spend puzzle should be selected so that the Initiator would spend
substantially more time to solve the puzzle than the Responder to substantially more time to solve the puzzle than the Responder to
compute the shared secret. compute the shared secret.
The Responder should constantly monitor the amount of the half-open The Responder should constantly monitor the amount of the half-open
IKE SA states that receive IKE_AUTH messages that cannot be decrypted IKE SA states that receive IKE_AUTH messages that cannot be decrypted
due to integrity check failures. If the percentage of such states is due to integrity check failures. If the percentage of such states is
high and it takes an essential fraction of Responder's computing high and it takes an essential fraction of Responder's computing
power to calculate keys for them, then the Responder may assume that 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 it is under attack and SHOULD use puzzles to make it harder for
attackers. attackers.
7.2.1. Presenting Puzzle 7.2.1. Presenting Puzzle
The Responder requests the Initiator to solve a puzzle by including The Responder requests the Initiator to solve a puzzle by including
the PUZZLE notification in the IKE_SA_INIT response message. The the PUZZLE notification in the IKE_SA_INIT response message. The
Responder MUST NOT use puzzles in the IKE_AUTH exchange unless the Responder MUST NOT use puzzles in the IKE_AUTH exchange unless a
puzzle has been previously presented and solved in the preceding puzzle has been previously presented and solved in the preceding
IKE_SA_INIT exchange. IKE_SA_INIT exchange.
<-- HDR, SA, KE, Nr, N(PUZZLE), [V+][N+] <-- HDR, SA, KE, Nr, N(PUZZLE), [V+][N+]
7.2.1.1. Selecting Puzzle Difficulty Level 7.2.1.1. Selecting Puzzle Difficulty Level
The difficulty level of the puzzle in IKE_AUTH exchange should be The difficulty level of the puzzle in the IKE_AUTH exchange should be
chosen so that the Initiator would spend more time to solve the 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 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 keys needed to decrypt and verify the IKE_AUTH request message. On
the other hand, the difficulty level should not be too high, the other hand, the difficulty level should not be too high,
otherwise the legitimate clients would experience an additional delay otherwise legitimate clients will experience an additional delay
while establishing IKE SA. while establishing the IKE SA.
Note, that since puzzles in the IKE_AUTH exchange are only allowed to 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 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 Responder would be able to estimate the computational power of the
Initiator and to select the difficulty level accordingly. Unlike Initiator and select the difficulty level accordingly. Unlike
puzzles in IKE_SA_INIT, the requested difficulty level for IKE_AUTH puzzles in the IKE_SA_INIT, the requested difficulty level for
puzzles MUST NOT be zero. In other words, the Responder must always IKE_AUTH puzzles MUST NOT be zero. In other words, the Responder
set specific difficulty level and must not let the Initiator to must always set a specific difficulty level and must not let the
choose it on its own. Initiator to choose it on its own.
7.2.1.2. Selecting Puzzle Algorithm 7.2.1.2. Selecting the Puzzle Algorithm
The algorithm for the puzzle is selected as described in The algorithm for the puzzle is selected as described in
Section 7.1.1.2. There is no requirement, that the algorithm for the Section 7.1.1.2. There is no requirement that the algorithm for the
puzzle in the IKE_SA INIT exchange be the same, as the algorithm for 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 the puzzle in IKE_AUTH exchange; however, it is expected that in most
cases they will be the same. cases they will be the same.
7.2.2. Solving Puzzle and Returning the Solution 7.2.2. Solving Puzzle and Returning the Solution
If the IKE_SA_INIT response message contains the PUZZLE notification If the IKE_SA_INIT response message contains the PUZZLE notification
and the Initiator supports puzzles, it MUST solve the puzzle. Note, and the Initiator supports puzzles, it MUST solve the puzzle. Note,
that puzzle construction in the IKE_AUTH exchange differs from the that puzzle construction in the IKE_AUTH exchange differs from the
puzzle construction in the IKE_SA_INIT exchange and is described in 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 Section 7.2.3. Once the puzzle is solved the Initiator sends the
IKE_AUTH request message, containing the Puzzle Solution payload. IKE_AUTH request message, containing the Puzzle Solution payload.
HDR, PS, SK {IDi, [CERT,] [CERTREQ,] HDR, PS, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SA, TSi, TSr} --> [IDr,] AUTH, SA, TSi, TSr} -->
The Puzzle Solution payload MUST be placed outside the Encrypted The Puzzle Solution (PS) payload MUST be placed outside the Encrypted
payload, so that the Responder would be able to verify the puzzle payload, so that the Responder is able to verify the puzzle before
before calculating the D-H shared secret and the SK_* keys. calculating the D-H shared secret and the SK_* keys.
If IKE Fragmentation [RFC7383] is used in IKE_AUTH exchange, then the 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 PS payload MUST be present only in the first IKE Fragment message, in
accordance with the Section 2.5.3 of [RFC7383]. Note, that accordance with the Section 2.5.3 of [RFC7383]. Note, that
calculation of the puzzle in the IKE_AUTH exchange doesn't depend on 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 the content of the IKE_AUTH message (see Section 7.2.3). Thus the
Initiator has to solve the puzzle only once and the solution is valid Initiator has to solve the puzzle only once and the solution is valid
for both unfragmented and fragmented IKE messages. for both unfragmented and fragmented IKE messages.
7.2.3. Computing Puzzle 7.2.3. Computing the Puzzle
The puzzles in the IKE_AUTH exchange are computed differently than in A puzzle in the IKE_AUTH exchange is computed differently than in the
the IKE_SA_INIT exchange (see Section 7.1.3). The general principle IKE_SA_INIT exchange (see Section 7.1.3). The general principle is
is the same; the difference is in the construction of the string S. 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 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 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 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) 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 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 a nonce used by the Responder in the IKE_SA_INIT exchange, stripped
any headers. SPIr is IKE Responder's SPI from the IKE header of the of any headers. SPIr is the IKE Responder's SPI from the IKE header
SA being established. of the SA being established.
7.2.4. Receiving Puzzle Solution 7.2.4. Receiving the Puzzle Solution
If the Responder requested the Initiator to solve a puzzle in the If the Responder requested the Initiator to solve a puzzle in the
IKE_AUTH exchange, then it MUST silently discard all the IKE_AUTH IKE_AUTH exchange, then it MUST silently discard all the IKE_AUTH
request messages without the Puzzle Solution payload. request messages without the Puzzle Solution payload.
Once the message containing a solution to the puzzle is received, the Once the message containing a solution to the puzzle is received, the
Responder MUST verify the solution before performing computationlly Responder MUST verify the solution before performing computationlly
intensive operations i.e. computing the D-H shared secret and the intensive operations i.e. computing the D-H shared secret and the
SK_* keys. The Responder MUST verify all the four returned keys. SK_* keys. The Responder MUST verify all four of the returned keys.
The Responder MUST silently discard the received message if any The Responder MUST silently discard the received message if any
checked verification result is not correct (contains insufficient checked verification result is not correct (contains insufficient
number of trailing zero bits). If the Responder successfully number of trailing zero bits). If the Responder successfully
verifies the puzzle and calculates the SK_* key, but the message verifies the puzzle and calculates the SK_* key, but the message
authenticity check fails, then it SHOULD save the calculated keys in authenticity check fails, then it SHOULD save the calculated keys in
the IKE SA state while waiting for the retransmissions from the the IKE SA state while waiting for the retransmissions from the
Initiator. In this case the Responder may skip verification of the Initiator. In this case the Responder may skip verification of the
puzzle solution and ignore the Puzzle Solution payload in the puzzle solution and ignore the Puzzle Solution payload in the
retransmitted messages. retransmitted messages.
If the Initiator uses IKE Fragmentation, then it is possible, that If the Initiator uses IKE Fragmentation, then it is possible, that
due to packet loss and/or reordering the Responder could receive non- due to packet loss and/or reordering the Responder could receive non-
first IKE Fragment messages before receiving the first one, first IKE Fragment messages before receiving the first one containing
containing the PS payload. In this case the Responder MAY choose to the PS payload. In this case the Responder MAY choose to keep the
keep the received fragments until the first fragment containing the received fragments until the first fragment containing the solution
solution to the puzzle is received. However, in this case the to the puzzle is received. In this case the Responder SHOULD NOT try
Responder SHOULD NOT try to verify authenticity of the kept fragments to verify authenticity of the kept fragments until the first fragment
until the first fragment with the PS payload is received and the with the PS payload is received and the solution to the puzzle is
solution to the puzzle is verified. After successful verification of verified. After successful verification of the puzzle, the Responder
the puzzle the Responder could calculate the SK_* key and verify can then calculate the SK_* key and verify authenticity of the
authenticity of the collected fragments. collected fragments.
8. Payload Formats 8. Payload Formats
8.1. PUZZLE Notification 8.1. PUZZLE Notification
The PUZZLE notification is used by the IKE Responder to inform the The PUZZLE notification is used by the IKE Responder to inform the
Initiator about the necessity to solve the puzzle. It contains the Initiator about the need to solve the puzzle. It contains the
difficulty level of the puzzle and the PRF the Initiator should use. difficulty level of the puzzle and the PRF the Initiator should use.
1 2 3 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 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 | | Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Protocol ID(=0)| SPI Size (=0) | Notify Message Type | |Protocol ID(=0)| SPI Size (=0) | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PRF | Difficulty | | PRF | Difficulty |
skipping to change at page 25, line 29 skipping to change at page 25, line 39
o Notify Message Type (2 octets) -- MUST be <TBA by IANA>, the value o Notify Message Type (2 octets) -- MUST be <TBA by IANA>, the value
assigned for the PUZZLE notification. assigned for the PUZZLE notification.
o PRF (2 octets) -- Transform ID of the PRF algorithm that must be o PRF (2 octets) -- Transform ID of the PRF algorithm that must be
used to solve the puzzle. Readers should refer to the section used to solve the puzzle. Readers should refer to the section
"Transform Type 2 - Pseudo-Random Function Transform IDs" in "Transform Type 2 - Pseudo-Random Function Transform IDs" in
[IKEV2-IANA] for the list of possible values. [IKEV2-IANA] for the list of possible values.
o Difficulty (1 octet) -- Difficulty Level of the puzzle. Specifies o Difficulty (1 octet) -- Difficulty Level of the puzzle. Specifies
minimum number of trailing zero bits (ZBC), that each of the the minimum number of trailing zero bits (ZBC), that each of the
results of PRF must contain. Value 0 means that the Responder results of PRF must contain. Value 0 means that the Responder
doesn't request any specific difficulty level and the Initiator is doesn't request any specific difficulty level and the Initiator is
free to select appropriate difficulty level on its own (see free to select an appropriate difficulty level on its own (see
Section 7.1.1.1 for details). Section 7.1.1.1 for details).
This notification contains no data. This notification contains no data.
8.2. Puzzle Solution Payload 8.2. Puzzle Solution Payload
The solution to the puzzle is returned back to the Responder in a The solution to the puzzle is returned back to the Responder in a
dedicated payload, called the Puzzle Solution payload and denoted as dedicated payload, called the Puzzle Solution payload and denoted as
PS in this document. PS in this document.
skipping to change at page 26, line 4 skipping to change at page 26, line 14
1 2 3 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 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 | | Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
~ Puzzle Solution Data ~ ~ Puzzle Solution Data ~
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Puzzle Solution Data (variable length) -- Contains the solution to o Puzzle Solution Data (variable length) -- Contains the solution to
the puzzle - four different keys for the selected PRF. This field the puzzle - four different keys for the selected PRF. This field
MUST NOT be empty. All the keys MUST have the same size, MUST NOT be empty. All of the keys MUST have the same size,
therefore the size of this field is always a mutiple of 4 bytes. 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 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 each key MUST be of that fixed size. If the agreed upon PRF
accepts keys of any size, then then the size of each key MUST be 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 between 1 octet and the preferred key length of the PRF
(inclusive). It is expected that in most cases the keys will be 4 (inclusive). It is expected that in most cases the keys will be 4
(or even less) octets in length, however it depends on puzzle (or even less) octets in length, however it depends on puzzle
difficulty and on the Initiator's strategy to find solutions, and difficulty and on the Initiator's strategy to find solutions, and
thus the size is not mandated by this specification. The thus the size is not mandated by this specification. The
Responder determines the size of each key by dividing the size of 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 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 size of Puzzle Solution Data is the size of Payload (as indicated
in Payload Length field) minus 4 - the size of Payload Header. in Payload Length field) minus 4 - the size of Payload Header.
The payload type for the Puzzle Solution payload is <TBA by IANA>. The payload type for the Puzzle Solution payload is <TBA by IANA>.
9. Operational Considerations 9. Operational Considerations
The difficulty level should be set by balancing the requirement to The puzzle difficulty level should be set by balancing the
minimize the latency for legitimate Initiators and making things requirement to minimize the latency for legitimate Initiators with
difficult for attackers. A good rule of thumb is for taking about 1 making things difficult for attackers. A good rule of thumb is for
second to solve the puzzle. A typical Initiator or bot-net member in taking about 1 second to solve the puzzle. A typical Initiator or
2014 can perform slightly less than a million hashes per second per bot-net member in 2014 can perform slightly less than a million
core, so setting the difficulty level to n=20 is a good compromise. hashes per second per core, so setting the difficulty level to n=20
It should be noted that mobile Initiators, especially phones are is a good compromise. It should be noted that mobile Initiators,
considerably weaker than that. Implementations should allow especially phones are considerably weaker than that. Implementations
administrators to set the difficulty level, and/or be able to set the should allow administrators to set the difficulty level, and/or be
difficulty level dynamically in response to load. able to set the difficulty level dynamically in response to load.
Initiators should set a maximum difficulty level beyond which they 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 won't try to solve the puzzle and log or display a failure message to
the administrator or user. the administrator or user.
10. Security Considerations 10. Security Considerations
When selecting parameters for the puzzles, in particular the puzzle Care must be taken when selecting parameters for the puzzles, in
difficulty, care must be taken. If the puzzles appeared too easy for particular the puzzle difficulty. If the puzzles are too easy for
majority of the attackers, then the puzzles mechanism wouldn't be the majority of attacker, then the puzzle mechanism wouldn't be able
able to prevent DoS attack and would only impose an additional burden to prevent (D)DoS attacks and would only impose an additional burden
on the legitimate Initiators. On the other hand, if the puzzles on legitimate Initiators. On the other hand, if the puzzles are too
appeared to be too hard for majority of the Initiators then many hard for the majority of Initiators, then many legitimate users would
legitimate users would experience unacceptable delay in IKE SA setup experience unacceptable delays in IKE SA setup (and unacceptable
(or unacceptable power consumption on mobile devices), that might power consumption on mobile devices), that might cause them to cancel
cause them to cancel connection attempt. In this case the resources the connection attempt. In this case the resources of the Responder
of the Responder are preserved, however the DoS attack can be are preserved, however the DoS attack can be considered successful.
considered successful. Thus a sensible balance should be kept by the Thus a sensible balance should be kept by the Responder while
Responder while choosing the puzzle difficulty - to defend itself and choosing the puzzle difficulty - to defend itself and to not over-
to not over-defend itself. It is RECOMMENDED that the puzzle defend itself. It is RECOMMENDED that the puzzle difficulty be
difficulty be chosen so, that the Responder's load remain close to chosen so, that the Responder's load remains close to the maximum it
the maximum it can tolerate. It is also RECOMMENDED to dynamically can tolerate. It is also RECOMMENDED to dynamically adjust the
adjust the puzzle difficulty in accordance to the current Responder's puzzle difficulty in accordance to the current Responder's load.
load.
Solving puzzles requires a lot of CPU power, that would increase Solving puzzles requires a lot of CPU power that increases power
power consumption. This would influence battery-powered Initiators, consumption. This additional power consumption can negatively affect
e.g. mobile phones or some IoT devices. If puzzles are hard then the battery-powered Initiators, e.g. mobile phones or some IoT devices.
required additional power consumption may appear to be unacceptable If puzzles are too hard, then the required additional power
for some Initiators. The Responder SHOULD take this possibility into consumption may appear to be unacceptable for some Initiators. The
considerations while choosing the puzzles difficulty and while Responder SHOULD take this possibility into consideration while
selecting which percentage of Initiators are allowed to reject choosing the puzzle difficulty, and while selecting which percentage
solving puzzles. See Section 7.1.4 for details. 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 If the Initiator uses NULL Authentication [RFC7619] then its identity
is never verified, that may be used by attackers to perform DoS is never verified. This condition may be used by attackers to
attack after IKE SA is established. Responders that allow perform a DoS attack after the IKE SA is established. Responders
unauthenticated Initiators to connect must be prepared to deal with that allow unauthenticated Initiators to connect must be prepared to
various kinds of DoS attacks even after IKE SA is created. See deal with various kinds of DoS attacks even after the IKE SA is
Section 5 for details. created. See Section 5 for details.
To prevent amplification attacks implementations must strictly follow To prevent amplification attacks implementations must strictly follow
the retransmission rules described in Section 2.1 of [RFC7296]. the retransmission rules described in Section 2.1 of [RFC7296].
11. IANA Considerations 11. IANA Considerations
This document defines a new payload in the "IKEv2 Payload Types" This document defines a new payload in the "IKEv2 Payload Types"
registry: registry:
<TBA> Puzzle Solution PS <TBA> Puzzle Solution PS
This document also defines a new Notify Message Type in the "IKEv2 This document also defines a new Notify Message Type in the "IKEv2
Notify Message Types - Status Types" registry: Notify Message Types - Status Types" registry:
<TBA> PUZZLE <TBA> PUZZLE
12. Acknowledgements 12. Acknowledgements
The authors thank Tero Kivinen, Yaron Sheffer and Scott Fluhrer for The authors thank Tero Kivinen, Yaron Sheffer, and Scott Fluhrer for
their contribution into design of the protocol. In particular, Tero their contributions to the design of the protocol. In particular,
Kivinen suggested the kind of puzzle where the task is to find a Tero Kivinen suggested the kind of puzzle where the task is to find a
solution with requested number of zero trailing bits. Yaron Sheffer solution with a requested number of zero trailing bits. Yaron
and Scott Fluhrer suggested a way to make puzzle difficulty less Sheffer and Scott Fluhrer suggested a way to make puzzle difficulty
erratic by solving several weaker puzles. The authors also thank less erratic by solving several weaker puzles. The authors also
David Waltermire for his carefull review of the document, Graham thank David Waltermire and Paul Wouters for their careful reviews of
Bartlett for pointing out to the possibility of "Hash & URL" related the document, Graham Bartlett for pointing out to the possibility of
attack, and all others who commented the document. the "Hash & URL" related attack, and all others who commented the
document.
13. References 13. References
13.1. Normative References 13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
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