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INTERNET-DRAFT Charlie Kaufman, Editor
draft-ietf-ipsec-ikev2-17.txt
Obsoletes: 2407, 2408, 2409 September 23, 2004
Expires: March 2005
Internet Key Exchange (IKEv2) Protocol
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026. Internet-Drafts are working documents of
the Internet Engineering Task Force (IETF), its areas, and its
working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This document is a submission by the IPSEC Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted
to the ipsec@lists.tislabs.com mailing list.
Distribution of this memo is unlimited.
This Internet-Draft expires in March 2005.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes version 2 of the Internet Key Exchange (IKE)
protocol. IKE is a component of IPsec used for performing mutual
authentication and establishing and maintaining security associations
(SAs).
This version of the IKE specification combines the contents of what
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were previously separate documents, including ISAKMP (RFC 2408), IKE
(RFC 2409), the Internet DOI (RFC 2407), NAT Traversal, Legacy
authentication, and remote address acquisition.
Version 2 of IKE does not interoperate with version 1, but it has
enough of the header format in common that both versions can
unambiguously run over the same UDP port.
Table of Contents
1 Introduction...............................................3
1.1 Usage Scenarios..........................................5
1.2 The Initial Exchanges....................................7
1.3 The CREATE_CHILD_SA Exchange.............................9
1.4 The INFORMATIONAL Exchange..............................10
1.5 Informational Messages outside of an IKE_SA.............12
2 IKE Protocol Details and Variations.......................12
2.1 Use of Retransmission Timers............................13
2.2 Use of Sequence Numbers for Message ID..................13
2.3 Window Size for overlapping requests....................14
2.4 State Synchronization and Connection Timeouts...........15
2.5 Version Numbers and Forward Compatibility...............16
2.6 Cookies.................................................18
2.7 Cryptographic Algorithm Negotiation.....................20
2.8 Rekeying................................................21
2.9 Traffic Selector Negotiation............................23
2.10 Nonces.................................................25
2.11 Address and Port Agility...............................26
2.12 Reuse of Diffie-Hellman Exponentials...................26
2.13 Generating Keying Material.............................27
2.14 Generating Keying Material for the IKE_SA..............28
2.15 Authentication of the IKE_SA...........................29
2.16 Extensible Authentication Protocol Methods.............30
2.17 Generating Keying Material for CHILD_SAs...............32
2.18 Rekeying IKE_SAs using a CREATE_CHILD_SA exchange......33
2.19 Requesting an internal address on a remote network.....33
2.20 Requesting a Peer's Version............................35
2.21 Error Handling.........................................35
2.22 IPComp.................................................36
2.23 NAT Traversal..........................................37
2.24 ECN (Explicit Congestion Notification).................40
3 Header and Payload Formats................................40
3.1 The IKE Header..........................................40
3.2 Generic Payload Header..................................43
3.3 Security Association Payload............................44
3.4 Key Exchange Payload....................................54
3.5 Identification Payloads.................................55
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3.6 Certificate Payload.....................................57
3.7 Certificate Request Payload.............................60
3.8 Authentication Payload..................................62
3.9 Nonce Payload...........................................62
3.10 Notify Payload.........................................63
3.11 Delete Payload.........................................71
3.12 Vendor ID Payload......................................72
3.13 Traffic Selector Payload...............................73
3.14 Encrypted Payload......................................76
3.15 Configuration Payload..................................77
3.16 Extensible Authentication Protocol (EAP) Payload.......82
4 Conformance Requirements..................................84
5 Security Considerations...................................86
6 IANA Considerations.......................................89
7 Acknowledgements..........................................89
8 References................................................90
8.1 Normative References....................................90
8.2 Informative References..................................91
Appendix A: Summary of Changes from IKEv1...................94
Appendix B: Diffie-Hellman Groups...........................96
Change History (To be removed from RFC).....................97
Editor's Address...........................................108
Full Copyright Statement...................................108
Intellectual Property Statement............................108
Requirements Terminology
Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
"MAY" that appear in this document are to be interpreted as described
in [Bra97].
The term "Expert Review" is to be interpreted as defined in
[RFC2434].
1 Introduction
IP Security (IPsec) provides confidentiality, data integrity, access
control, and data source authentication to IP datagrams. These
services are provided by maintaining shared state between the source
and the sink of an IP datagram. This state defines, among other
things, the specific services provided to the datagram, which
cryptographic algorithms will be used to provide the services, and
the keys used as input to the cryptographic algorithms.
Establishing this shared state in a manual fashion does not scale
well. Therefore a protocol to establish this state dynamically is
needed. This memo describes such a protocol-- the Internet Key
Exchange (IKE). This is version 2 of IKE. Version 1 of IKE was
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defined in RFCs 2407, 2408, and 2409. This single document is
intended to replace all three of those RFCs.
Definitions of the primitive terms in this document (such as Security
Association or SA) can be found in [RFC2401bis].
IKE performs mutual authentication between two parties and
establishes an IKE security association (SA) that includes shared
secret information that can be used to efficiently establish SAs for
ESP [RFC2406] and/or AH [RFC2402] and a set of cryptographic
algorithms to be used by the SAs to protect the traffic that they
carry. In this document, the term "suite" or "cryptographic suite"
refers to a complete set of algorithms used to protect an SA. An
initiator proposes one or more suites by listing supported algorithms
that can be combined into suites in a mix and match fashion. IKE can
also negotiate use of IPComp [IPCOMP] in connection with an ESP
and/or AH SA. We call the IKE SA an "IKE_SA". The SAs for ESP and/or
AH that get set up through that IKE_SA we call "CHILD_SA"s.
All IKE communications consist of pairs of messages: a request and a
response. The pair is called an "exchange". We call the first
messages establishing an IKE_SA IKE_SA_INIT and IKE_AUTH exchanges
and subsequent IKE exchanges CREATE_CHILD_SA or INFORMATIONAL
exchanges. In the common case, there is a single IKE_SA_INIT exchange
and a single IKE_AUTH exchange (a total of four messages) to
establish the IKE_SA and the first CHILD_SA. In exceptional cases,
there may be more than one of each of these exchanges. In all cases,
all IKE_SA_INIT exchanges MUST complete before any other exchange
type, then all IKE_AUTH exchanges MUST complete, and following that
any number of CREATE_CHILD_SA and INFORMATIONAL exchanges may occur
in any order. In some scenarios, only a single CHILD_SA is needed
between the IPsec endpoints and therefore there would be no
additional exchanges. Subsequent exchanges MAY be used to establish
additional CHILD_SAs between the same authenticated pair of endpoints
and to perform housekeeping functions.
IKE message flow always consists of a request followed by a response.
It is the responsibility of the requester to ensure reliability. If
the response is not received within a timeout interval, the requester
needs to retransmit the request (or abandon the connection).
The first request/response of an IKE session (IKE_SA_INIT) negotiates
security parameters for the IKE_SA, sends nonces, and sends Diffie-
Hellman values.
The second request/response (IKE_AUTH) transmits identities, proves
knowledge of the secrets corresponding to the two identities, and
sets up an SA for the first (and often only) AH and/or ESP CHILD_SA.
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The types of subsequent exchanges are CREATE_CHILD_SA (which creates
a CHILD_SA), and INFORMATIONAL (which deletes an SA, reports error
conditions, or does other housekeeping). Every request requires a
response. An INFORMATIONAL request with no payloads (other than the
empty Encrypted payload required by the syntax) is commonly used as a
check for liveness. These subsequent exchanges cannot be used until
the initial exchanges have completed.
In the description that follows, we assume that no errors occur.
Modifications to the flow should errors occur are described in
section 2.21.
1.1 Usage Scenarios
IKE is expected to be used to negotiate ESP and/or AH SAs in a number
of different scenarios, each with its own special requirements.
1.1.1 Security Gateway to Security Gateway Tunnel
+-+-+-+-+-+ +-+-+-+-+-+
! ! IPsec ! !
Protected !Tunnel ! Tunnel !Tunnel ! Protected
Subnet <-->!Endpoint !<---------->!Endpoint !<--> Subnet
! ! ! !
+-+-+-+-+-+ +-+-+-+-+-+
Figure 1: Security Gateway to Security Gateway Tunnel
In this scenario, neither endpoint of the IP connection implements
IPsec, but network nodes between them protect traffic for part of the
way. Protection is transparent to the endpoints, and depends on
ordinary routing to send packets through the tunnel endpoints for
processing. Each endpoint would announce the set of addresses
"behind" it, and packets would be sent in Tunnel Mode where the inner
IP header would contain the IP addresses of the actual endpoints.
1.1.2 Endpoint to Endpoint Transport
+-+-+-+-+-+ +-+-+-+-+-+
! ! IPsec Transport ! !
!Protected! or Tunnel Mode SA !Protected!
!Endpoint !<---------------------------------------->!Endpoint !
! ! ! !
+-+-+-+-+-+ +-+-+-+-+-+
Figure 2: Endpoint to Endpoint
In this scenario, both endpoints of the IP connection implement
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IPsec, as required of hosts in [RFC2401bis]. Transport mode will
commonly be used with no inner IP header. If there is an inner IP
header, the inner addresses will be the same as the outer addresses.
A single pair of addresses will be negotiated for packets to be
protected by this SA. These endpoints MAY implement application layer
access controls based on the IPsec authenticated identities of the
participants. This scenario enables the end-to-end security that has
been a guiding principle for the Internet since [RFC1958], [RFC2775],
and a method of limiting the inherent problems with complexity in
networks noted by [RFC3439]. While this scenario may not be fully
applicable to the IPv4 Internet, it has been deployed successfully in
specific scenarios within intranets using IKEv1. It should be more
broadly enabled during the transition to IPv6 and with the adoption
of IKEv2.
It is possible in this scenario that one or both of the protected
endpoints will be behind a network address translation (NAT) node, in
which case the tunneled packets will have to be UDP encapsulated so
that port numbers in the UDP headers can be used to identify
individual endpoints "behind" the NAT (see section 2.23).
1.1.3 Endpoint to Security Gateway Transport
+-+-+-+-+-+ +-+-+-+-+-+
! ! IPsec ! ! Protected
!Protected! Tunnel !Tunnel ! Subnet
!Endpoint !<------------------------>!Endpoint !<--- and/or
! ! ! ! Internet
+-+-+-+-+-+ +-+-+-+-+-+
Figure 3: Endpoint to Security Gateway Tunnel
In this scenario, a protected endpoint (typically a portable roaming
computer) connects back to its corporate network through an IPsec
protected tunnel. It might use this tunnel only to access information
on the corporate network or it might tunnel all of its traffic back
through the corporate network in order to take advantage of
protection provided by a corporate firewall against Internet based
attacks. In either case, the protected endpoint will want an IP
address associated with the security gateway so that packets returned
to it will go to the security gateway and be tunneled back. This IP
address may be static or may be dynamically allocated by the security
gateway. In support of the latter case, IKEv2 includes a mechanism
for the initiator to request an IP address owned by the security
gateway for use for the duration of its SA.
In this scenario, packets will use tunnel mode. On each packet from
the protected endpoint, the outer IP header will contain the source
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IP address associated with its current location (i.e., the address
that will get traffic routed to the endpoint directly) while the
inner IP header will contain the source IP address assigned by the
security gateway (i.e., the address that will get traffic routed to
the security gateway for forwarding to the endpoint). The outer
destination address will always be that of the security gateway,
while the inner destination address will be the ultimate destination
for the packet.
In this scenario, it is possible that the protected endpoint will be
behind a NAT. In that case, the IP address as seen by the security
gateway will not be the same as the IP address sent by the protected
endpoint, and packets will have to be UDP encapsulated in order to be
routed properly.
1.1.4 Other Scenarios
Other scenarios are possible, as are nested combinations of the
above. One notable example combines aspects of 1.1.1 and 1.1.3. A
subnet may make all external accesses through a remote security
gateway using an IPsec tunnel, where the addresses on the subnet are
routed to the security gateway by the rest of the Internet. An
example would be someone's home network being virtually on the
Internet with static IP addresses even though connectivity is
provided by an ISP that assigns a single dynamically assigned IP
address to the user's security gateway (where the static IP addresses
and an IPsec relay is provided by a third party located elsewhere).
1.2 The Initial Exchanges
Communication using IKE always begins with IKE_SA_INIT and IKE_AUTH
exchanges (known in IKEv1 as Phase 1). These initial exchanges
normally consist of four messages, though in some scenarios that
number can grow. All communications using IKE consist of
request/response pairs. We'll describe the base exchange first,
followed by variations. The first pair of messages (IKE_SA_INIT)
negotiate cryptographic algorithms, exchange nonces, and do a Diffie-
Hellman exchange.
The second pair of messages (IKE_AUTH) authenticate the previous
messages, exchange identities and certificates, and establish the
first CHILD_SA. Parts of these messages are encrypted and integrity
protected with keys established through the IKE_SA_INIT exchange, so
the identities are hidden from eavesdroppers and all fields in all
the messages are authenticated.
In the following description, the payloads contained in the message
are indicated by names such as SA. The details of the contents of
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each payload are described later. Payloads which may optionally
appear will be shown in brackets, such as [CERTREQ], would indicate
that optionally a certificate request payload can be included.
The initial exchanges are as follows:
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni -->
HDR contains the SPIs, version numbers, and flags of various sorts.
The SAi1 payload states the cryptographic algorithms the initiator
supports for the IKE_SA. The KE payload sends the initiator's
Diffie-Hellman value. Ni is the initiator's nonce.
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
The responder chooses a cryptographic suite from the initiator's
offered choices and expresses that choice in the SAr1 payload,
completes the Diffie-Hellman exchange with the KEr payload, and sends
its nonce in the Nr payload.
At this point in the negotiation each party can generate SKEYSEED,
from which all keys are derived for that IKE_SA. All but the headers
of all the messages that follow are encrypted and integrity
protected. The keys used for the encryption and integrity protection
are derived from SKEYSEED and are known as SK_e (encryption) and SK_a
(authentication, a.k.a. integrity protection). A separate SK_e and
SK_a is computed for each direction. In addition to the keys SK_e
and SK_a derived from the DH value for protection of the IKE_SA,
another quantity SK_d is derived and used for derivation of further
keying material for CHILD_SAs. The notation SK { ... } indicates
that these payloads are encrypted and integrity protected using that
direction's SK_e and SK_a.
HDR, SK {IDi, [CERT,] [CERTREQ,] [IDr,]
AUTH, SAi2, TSi, TSr} -->
The initiator asserts its identity with the IDi payload, proves
knowledge of the secret corresponding to IDi and integrity protects
the contents of the first message using the AUTH payload (see section
2.15). It might also send its certificate(s) in CERT payload(s) and
a list of its trust anchors in CERTREQ payload(s). If any CERT
payloads are included, the first certificate provided MUST contain
the public key used to verify the AUTH field. The optional payload
IDr enables the initiator to specify which of the responder's
identities it wants to talk to. This is useful when the machine on
which the responder is running is hosting multiple identities at the
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same IP address. The initiator begins negotiation of a CHILD_SA
using the SAi2 payload. The final fields (starting with SAi2) are
described in the description of the CREATE_CHILD_SA exchange.
<-- HDR, SK {IDr, [CERT,] AUTH,
SAr2, TSi, TSr}
The responder asserts its identity with the IDr payload, optionally
sends one or more certificates (again with the certificate containing
the public key used to verify AUTH listed first), authenticates its
identity and protects the integrity of the second message with the
AUTH payload, and completes negotiation of a CHILD_SA with the
additional fields described below in the CREATE_CHILD_SA exchange.
The recipients of messages 3 and 4 MUST verify that all signatures
and MACs are computed correctly and that the names in the ID payloads
correspond to the keys used to generate the AUTH payload.
1.3 The CREATE_CHILD_SA Exchange
This exchange consists of a single request/response pair, and was
referred to as a phase 2 exchange in IKEv1. It MAY be initiated by
either end of the IKE_SA after the initial exchanges are completed.
All messages following the initial exchange are cryptographically
protected using the cryptographic algorithms and keys negotiated in
the first two messages of the IKE exchange. These subsequent
messages use the syntax of the Encrypted Payload described in section
3.14. All subsequent messages included an Encrypted Payload, even if
they are referred to in the text as "empty".
Either endpoint may initiate a CREATE_CHILD_SA exchange, so in this
section the term initiator refers to the endpoint initiating this
exchange.
A CHILD_SA is created by sending a CREATE_CHILD_SA request. The
CREATE_CHILD_SA request MAY optionally contain a KE payload for an
additional Diffie-Hellman exchange to enable stronger guarantees of
forward secrecy for the CHILD_SA. The keying material for the
CHILD_SA is a function of SK_d established during the establishment
of the IKE_SA, the nonces exchanged during the CREATE_CHILD_SA
exchange, and the Diffie-Hellman value (if KE payloads are included
in the CREATE_CHILD_SA exchange).
In the CHILD_SA created as part of the initial exchange, a second KE
payload and nonce MUST NOT be sent. The nonces from the initial
exchange are used in computing the keys for the CHILD_SA.
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The CREATE_CHILD_SA request contains:
Initiator Responder
----------- -----------
HDR, SK {[N], SA, Ni, [KEi],
[TSi, TSr]} -->
The initiator sends SA offer(s) in the SA payload, a nonce in the Ni
payload, optionally a Diffie-Hellman value in the KEi payload, and
the proposed traffic selectors in the TSi and TSr payloads. If this
CREATE_CHILD_SA exchange is rekeying an existing SA other than the
IKE_SA, the leading N payload of type REKEY_SA MUST identify the SA
being rekeyed. If this CREATE_CHILD_SA exchange is not rekeying an
existing SA, the N payload MUST be omitted. If the SA offers include
different Diffie-Hellman groups, KEi MUST be an element of the group
the initiator expects the responder to accept. If it guesses wrong,
the CREATE_CHILD_SA exchange will fail and it will have to retry with
a different KEi.
The message following the header is encrypted and the message
including the header is integrity protected using the cryptographic
algorithms negotiated for the IKE_SA.
The CREATE_CHILD_SA response contains:
<-- HDR, SK {SA, Nr, [KEr],
[TSi, TSr]}
The responder replies (using the same Message ID to respond) with the
accepted offer in an SA payload, and a Diffie-Hellman value in the
KEr payload if KEi was included in the request and the selected
cryptographic suite includes that group. If the responder chooses a
cryptographic suite with a different group, it MUST reject the
request. The initiator SHOULD repeat the request, but now with a KEi
payload from the group the responder selected.
The traffic selectors for traffic to be sent on that SA are specified
in the TS payloads, which may be a subset of what the initiator of
the CHILD_SA proposed. Traffic selectors are omitted if this
CREATE_CHILD_SA request is being used to change the key of the
IKE_SA.
1.4 The INFORMATIONAL Exchange
At various points during the operation of an IKE_SA, peers may desire
to convey control messages to each other regarding errors or
notifications of certain events. To accomplish this IKE defines an
INFORMATIONAL exchange. INFORMATIONAL exchanges MUST ONLY occur
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after the initial exchanges and are cryptographically protected with
the negotiated keys.
Control messages that pertain to an IKE_SA MUST be sent under that
IKE_SA. Control messages that pertain to CHILD_SAs MUST be sent under
the protection of the IKE_SA which generated them (or its successor
if the IKE_SA was replaced for the purpose of rekeying).
Messages in an INFORMATIONAL Exchange contain zero or more
Notification, Delete, and Configuration payloads. The Recipient of an
INFORMATIONAL Exchange request MUST send some response (else the
Sender will assume the message was lost in the network and will
retransmit it). That response MAY be a message with no payloads. The
request message in an INFORMATIONAL Exchange MAY also contain no
payloads. This is the expected way an endpoint can ask the other
endpoint to verify that it is alive.
ESP and AH SAs always exist in pairs, with one SA in each direction.
When an SA is closed, both members of the pair MUST be closed. When
SAs are nested, as when data (and IP headers if in tunnel mode) are
encapsulated first with IPComp, then with ESP, and finally with AH
between the same pair of endpoints, all of the SAs MUST be deleted
together. Each endpoint MUST close its incoming SAs and allow the
other endpoint to close the other SA in each pair. To delete an SA,
an INFORMATIONAL Exchange with one or more delete payloads is sent
listing the SPIs (as they would be expected in the headers of inbound
packets) of the SAs to be deleted. The recipient MUST close the
designated SAs. Normally, the reply in the INFORMATIONAL Exchange
will contain delete payloads for the paired SAs going in the other
direction. There is one exception. If by chance both ends of a set
of SAs independently decide to close them, each may send a delete
payload and the two requests may cross in the network. If a node
receives a delete request for SAs for which it has already issued a
delete request, it MUST delete the outgoing SAs while processing the
request and the incoming SAs while processing the response. In that
case, the responses MUST NOT include delete payloads for the deleted
SAs, since that would result in duplicate deletion and could in
theory delete the wrong SA.
A node SHOULD regard half closed connections as anomalous and audit
their existence should they persist. Note that this specification
nowhere specifies time periods, so it is up to individual endpoints
to decide how long to wait. A node MAY refuse to accept incoming data
on half closed connections but MUST NOT unilaterally close them and
reuse the SPIs. If connection state becomes sufficiently messed up, a
node MAY close the IKE_SA which will implicitly close all SAs
negotiated under it. It can then rebuild the SAs it needs on a clean
base under a new IKE_SA.
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The INFORMATIONAL Exchange is defined as:
Initiator Responder
----------- -----------
HDR, SK {[N,] [D,] [CP,] ...} -->
<-- HDR, SK {[N,] [D,] [CP], ...}
The processing of an INFORMATIONAL Exchange is determined by its
component payloads.
1.5 Informational Messages outside of an IKE_SA
If an encrypted IKE packet arrives on port 500 or 4500 with an
unrecognized SPI, it could be because the receiving node has recently
crashed and lost state or because of some other system malfunction or
attack. If the receiving node has an active IKE_SA to the IP address
from whence the packet came, it MAY send a notification of the
wayward packet over that IKE_SA in an informational exchange. If it
does not have such an IKE_SA, it MAY send an Informational message
without cryptographic protection to the source IP address. Such a
message is not part of an informational exchange, and the receiving
node MUST NOT respond to it. Doing so could cause a message loop.
2 IKE Protocol Details and Variations
IKE normally listens and sends on UDP port 500, though IKE messages
may also be received on UDP port 4500 with a slightly different
format (see section 2.23). Since UDP is a datagram (unreliable)
protocol, IKE includes in its definition recovery from transmission
errors, including packet loss, packet replay, and packet forgery. IKE
is designed to function so long as (1) at least one of a series of
retransmitted packets reaches its destination before timing out; and
(2) the channel is not so full of forged and replayed packets so as
to exhaust the network or CPU capacities of either endpoint. Even in
the absence of those minimum performance requirements, IKE is
designed to fail cleanly (as though the network were broken).
While IKEv2 messages are intended to be short, they contain
structures with no hard upper bound on size (in particular, X.509
certificates), and IKEv2 itself does not have a mechanism for
fragmenting large messages. IP defines a mechanism for fragmentation
of oversize UDP messages, but implementations vary in the maximum
message size supported. Further, use of IP fragmentation opens an
implementation to denial of service attacks [KPS03]. Finally, some
NAT and/or firewall implementations may block IP fragments.
All IKEv2 implementations MUST be able to send, receive, and process
IKE messages that are up to 1280 bytes long, and they SHOULD be able
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to send, receive, and process messages that are up to 3000 bytes
long. IKEv2 implementations SHOULD be aware of the maximum UDP
message size supported and MAY shorten messages by leaving out some
certificates or cryptographic suite proposals if that will keep
messages below the maximum. Use of the "Hash and URL" formats rather
then including certificates in exchanges where possible can avoid
most problems. Implementations and configuration should keep in mind,
however, that if the URL lookups are only possible after the IPsec SA
is established, recursion issues could prevent this technique from
working.
2.1 Use of Retransmission Timers
All messages in IKE exist in pairs: a request and a response. The
setup of an IKE_SA normally consists of two request/response pairs.
Once the IKE_SA is set up, either end of the security association may
initiate requests at any time, and there can be many requests and
responses "in flight" at any given moment. But each message is
labeled as either a request or a response and for each
request/response pair one end of the security association is the
initiator and the other is the responder.
For every pair of IKE messages, the initiator is responsible for
retransmission in the event of a timeout. The responder MUST never
retransmit a response unless it receives a retransmission of the
request. In that event, the responder MUST ignore the retransmitted
request except insofar as it triggers a retransmission of the
response. The initiator MUST remember each request until it receives
the corresponding response. The responder MUST remember each response
until it receives a request whose sequence number is larger than the
sequence number in the response plus its window size (see section
2.3).
IKE is a reliable protocol, in the sense that the initiator MUST
retransmit a request until either it receives a corresponding reply
OR it deems the IKE security association to have failed and it
discards all state associated with the IKE_SA and any CHILD_SAs
negotiated using that IKE_SA.
2.2 Use of Sequence Numbers for Message ID
Every IKE message contains a Message ID as part of its fixed header.
This Message ID is used to match up requests and responses, and to
identify retransmissions of messages.
The Message ID is a 32 bit quantity, which is zero for the first IKE
request in each direction. The IKE_SA initial setup messages will
always be numbered 0 and 1. Each endpoint in the IKE Security
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Association maintains two "current" Message IDs: the next one to be
used for a request it initiates and the next one it expects to see in
a request from the other end. These counters increment as requests
are generated and received. Responses always contain the same message
ID as the corresponding request. That means that after the initial
exchange, each integer n may appear as the message ID in four
distinct messages: The nth request from the original IKE initiator,
the corresponding response, the nth request from the original IKE
responder, and the corresponding response. If the two ends make very
different numbers of requests, the Message IDs in the two directions
can be very different. There is no ambiguity in the messages,
however, because the (I)nitiator and (R)esponse bits in the message
header specify which of the four messages a particular one is.
Note that Message IDs are cryptographically protected and provide
protection against message replays. In the unlikely event that
Message IDs grow too large to fit in 32 bits, the IKE_SA MUST be
closed. Rekeying an IKE_SA resets the sequence numbers.
2.3 Window Size for overlapping requests
In order to maximize IKE throughput, an IKE endpoint MAY issue
multiple requests before getting a response to any of them if the
other endpoint has indicated its ability to handle such requests. For
simplicity, an IKE implementation MAY choose to process requests
strictly in order and/or wait for a response to one request before
issuing another. Certain rules must be followed to assure
interoperability between implementations using different strategies.
After an IKE_SA is set up, either end can initiate one or more
requests. These requests may pass one another over the network. An
IKE endpoint MUST be prepared to accept and process a request while
it has a request outstanding in order to avoid a deadlock in this
situation. An IKE endpoint SHOULD be prepared to accept and process
multiple requests while it has a request outstanding.
An IKE endpoint MUST wait for a response to each of its messages
before sending a subsequent message unless it has received a
SET_WINDOW_SIZE Notify message from its peer informing it that the
peer is prepared to maintain state for multiple outstanding messages
in order to allow greater throughput.
An IKE endpoint MUST NOT exceed the peer's stated window size for
transmitted IKE requests. In other words, if the responder stated its
window size is N, then when the initiator needs to make a request X,
it MUST wait until it has received responses to all requests up
through request X-N. An IKE endpoint MUST keep a copy of (or be able
to regenerate exactly) each request it has sent until it receives the
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corresponding response. An IKE endpoint MUST keep a copy of (or be
able to regenerate exactly) the number of previous responses equal to
its declared window size in case its response was lost and the
initiator requests its retransmission by retransmitting the request.
An IKE endpoint supporting a window size greater than one SHOULD be
capable of processing incoming requests out of order to maximize
performance in the event of network failures or packet reordering.
2.4 State Synchronization and Connection Timeouts
An IKE endpoint is allowed to forget all of its state associated with
an IKE_SA and the collection of corresponding CHILD_SAs at any time.
This is the anticipated behavior in the event of an endpoint crash
and restart. It is important when an endpoint either fails or
reinitializes its state that the other endpoint detect those
conditions and not continue to waste network bandwidth by sending
packets over discarded SAs and having them fall into a black hole.
Since IKE is designed to operate in spite of Denial of Service (DoS)
attacks from the network, an endpoint MUST NOT conclude that the
other endpoint has failed based on any routing information (e.g.,
ICMP messages) or IKE messages that arrive without cryptographic
protection (e.g., Notify messages complaining about unknown SPIs). An
endpoint MUST conclude that the other endpoint has failed only when
repeated attempts to contact it have gone unanswered for a timeout
period or when a cryptographically protected INITIAL_CONTACT
notification is received on a different IKE_SA to the same
authenticated identity. An endpoint SHOULD suspect that the other
endpoint has failed based on routing information and initiate a
request to see whether the other endpoint is alive. To check whether
the other side is alive, IKE specifies an empty INFORMATIONAL message
that (like all IKE requests) requires an acknowledgment (note that
within the context of an IKE_SA, an "empty" message consists of an
IKE header followed by an Encrypted payload that contains no
payloads). If a cryptographically protected message has been received
from the other side recently, unprotected notifications MAY be
ignored. Implementations MUST limit the rate at which they take
actions based on unprotected messages.
Numbers of retries and lengths of timeouts are not covered in this
specification because they do not affect interoperability. It is
suggested that messages be retransmitted at least a dozen times over
a period of at least several minutes before giving up on an SA, but
different environments may require different rules. To be a good
network citizen, retranmission times MUST increase exponentially to
avoid flooding the network and making an existing congestion
situation worse. If there has only been outgoing traffic on all of
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the SAs associated with an IKE_SA, it is essential to confirm
liveness of the other endpoint to avoid black holes. If no
cryptographically protected messages have been received on an IKE_SA
or any of its CHILD_SAs recently, the system needs to perform a
liveness check in order to prevent sending messages to a dead peer.
Receipt of a fresh cryptographically protected message on an IKE_SA
or any of its CHILD_SAs assures liveness of the IKE_SA and all of its
CHILD_SAs. Note that this places requirements on the failure modes of
an IKE endpoint. An implementation MUST NOT continue sending on any
SA if some failure prevents it from receiving on all of the
associated SAs. If CHILD_SAs can fail independently from one another
without the associated IKE_SA being able to send a delete message,
then they MUST be negotiated by separate IKE_SAs.
There is a Denial of Service attack on the initiator of an IKE_SA
that can be avoided if the initiator takes the proper care. Since the
first two messages of an SA setup are not cryptographically
protected, an attacker could respond to the initiator's message
before the genuine responder and poison the connection setup attempt.
To prevent this, the initiator MAY be willing to accept multiple
responses to its first message, treat each as potentially legitimate,
respond to it, and then discard all the invalid half open connections
when it receives a valid cryptographically protected response to any
one of its requests. Once a cryptographically valid response is
received, all subsequent responses should be ignored whether or not
they are cryptographically valid.
Note that with these rules, there is no reason to negotiate and agree
upon an SA lifetime. If IKE presumes the partner is dead, based on
repeated lack of acknowledgment to an IKE message, then the IKE SA
and all CHILD_SAs set up through that IKE_SA are deleted.
An IKE endpoint may at any time delete inactive CHILD_SAs to recover
resources used to hold their state. If an IKE endpoint chooses to
delete CHILD_SAs, it MUST send Delete payloads to the other end
notifying it of the deletion. It MAY similarly time out the IKE_SA.
Closing the IKE_SA implicitly closes all associated CHILD_SAs. In
this case, an IKE endpoint SHOULD send a Delete payload indicating
that it has closed the IKE_SA.
2.5 Version Numbers and Forward Compatibility
This document describes version 2.0 of IKE, meaning the major version
number is 2 and the minor version number is zero. It is likely that
some implementations will want to support both version 1.0 and
version 2.0, and in the future, other versions.
The major version number should only be incremented if the packet
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formats or required actions have changed so dramatically that an
older version node would not be able to interoperate with a newer
version node if it simply ignored the fields it did not understand
and took the actions specified in the older specification. The minor
version number indicates new capabilities, and MUST be ignored by a
node with a smaller minor version number, but used for informational
purposes by the node with the larger minor version number. For
example, it might indicate the ability to process a newly defined
notification message. The node with the larger minor version number
would simply note that its correspondent would not be able to
understand that message and therefore would not send it.
If an endpoint receives a message with a higher major version number,
it MUST drop the message and SHOULD send an unauthenticated
notification message containing the highest version number it
supports. If an endpoint supports major version n, and major version
m, it MUST support all versions between n and m. If it receives a
message with a major version that it supports, it MUST respond with
that version number. In order to prevent two nodes from being tricked
into corresponding with a lower major version number than the maximum
that they both support, IKE has a flag that indicates that the node
is capable of speaking a higher major version number.
Thus the major version number in the IKE header indicates the version
number of the message, not the highest version number that the
transmitter supports. If the initiator is capable of speaking
versions n, n+1, and n+2, and the responder is capable of speaking
versions n and n+1, then they will negotiate speaking n+1, where the
initiator will set the flag indicating its ability to speak a higher
version. If they mistakenly (perhaps through an active attacker
sending error messages) negotiate to version n, then both will notice
that the other side can support a higher version number, and they
MUST break the connection and reconnect using version n+1.
Note that IKEv1 does not follow these rules, because there is no way
in v1 of noting that you are capable of speaking a higher version
number. So an active attacker can trick two v2-capable nodes into
speaking v1. When a v2-capable node negotiates down to v1, it SHOULD
note that fact in its logs.
Also for forward compatibility, all fields marked RESERVED MUST be
set to zero by a version 2.0 implementation and their content MUST be
ignored by a version 2.0 implementation ("Be conservative in what you
send and liberal in what you receive"). In this way, future versions
of the protocol can use those fields in a way that is guaranteed to
be ignored by implementations that do not understand them.
Similarly, payload types that are not defined are reserved for future
use and implementations of version 2.0 MUST skip over those payloads
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and ignore their contents.
IKEv2 adds a "critical" flag to each payload header for further
flexibility for forward compatibility. If the critical flag is set
and the payload type is unrecognized, the message MUST be rejected
and the response to the IKE request containing that payload MUST
include a Notify payload UNSUPPORTED_CRITICAL_PAYLOAD, indicating an
unsupported critical payload was included. If the critical flag is
not set and the payload type is unsupported, that payload MUST be
ignored.
While new payload types may be added in the future and may appear
interleaved with the fields defined in this specification,
implementations MUST send the payloads defined in this specification
in the order shown in the figures in section 2 and implementations
SHOULD reject as invalid a message with those payloads in any other
order.
2.6 Cookies
The term "cookies" originates with Karn and Simpson [RFC2522] in
Photuris, an early proposal for key management with IPsec, and it has
persisted. The ISAKMP fixed message header includes two eight octet
fields titled "cookies", and that syntax is used by both IKEv1 and
IKEv2 though in IKEv2 they are referred to as the IKE SPI and there
is a new separate field in a Notify payload holding the cookie. The
initial two eight octet fields in the header are used as a connection
identifier at the beginning of IKE packets. Each endpoint chooses one
of the two SPIs and SHOULD choose them so as to be unique identifiers
of an IKE_SA. An SPI value of zero is special and indicates that the
remote SPI value is not yet known by the sender.
Unlike ESP and AH where only the recipient's SPI appears in the
header of a message, in IKE the sender's SPI is also sent in every
message. Since the SPI chosen by the original initiator of the IKE_SA
is always sent first, an endpoint with multiple IKE_SAs open that
wants to find the appropriate IKE_SA using the SPI it assigned must
look at the I(nitiator) Flag bit in the header to determine whether
it assigned the first or the second eight octets.
In the first message of an initial IKE exchange, the initiator will
not know the responder's SPI value and will therefore set that field
to zero.
An expected attack against IKE is state and CPU exhaustion, where the
target is flooded with session initiation requests from forged IP
addresses. This attack can be made less effective if an
implementation of a responder uses minimal CPU and commits no state
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to an SA until it knows the initiator can receive packets at the
address from which it claims to be sending them. To accomplish this,
a responder SHOULD - when it detects a large number of half-open
IKE_SAs - reject initial IKE messages unless they contain a Notify
payload of type COOKIE. It SHOULD instead send an unprotected IKE
message as a response and include COOKIE Notify payload with the
cookie data to be returned. Initiators who receive such responses
MUST retry the IKE_SA_INIT with a Notify payload of type COOKIE
containing the responder supplied cookie data as the first payload
and all other payloads unchanged. The initial exchange will then be
as follows:
Initiator Responder
----------- -----------
HDR(A,0), SAi1, KEi, Ni -->
<-- HDR(A,0), N(COOKIE)
HDR(A,0), N(COOKIE), SAi1, KEi, Ni -->
<-- HDR(A,B), SAr1, KEr, Nr, [CERTREQ]
HDR(A,B), SK {IDi, [CERT,] [CERTREQ,] [IDr,]
AUTH, SAi2, TSi, TSr} -->
<-- HDR(A,B), SK {IDr, [CERT,] AUTH,
SAr2, TSi, TSr}
The first two messages do not affect any initiator or responder state
except for communicating the cookie. In particular, the message
sequence numbers in the first four messages will all be zero and the
message sequence numbers in the last two messages will be one. 'A' is
the SPI assigned by the initiator, while 'B' is the SPI assigned by
the responder.
An IKE implementation SHOULD implement its responder cookie
generation in such a way as to not require any saved state to
recognize its valid cookie when the second IKE_SA_INIT message
arrives. The exact algorithms and syntax they use to generate
cookies does not affect interoperability and hence is not specified
here. The following is an example of how an endpoint could use
cookies to implement limited DOS protection.
A good way to do this is to set the responder cookie to be:
Cookie = <VersionIDofSecret> | Hash(Ni | IPi | SPIi | <secret>)
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where <secret> is a randomly generated secret known only to the
responder and periodically changed and | indicates concatenation.
<VersionIDofSecret> should be changed whenever <secret> is
regenerated. The cookie can be recomputed when the IKE_SA_INIT
arrives the second time and compared to the cookie in the received
message. If it matches, the responder knows that SPIr was generated
since the last change to <secret> and that IPi must be the same as
the source address it saw the first time. Incorporating SPIi into the
calculation assures that if multiple IKE_SAs are being set up in
parallel they will all get different cookies (assuming the initiator
chooses unique SPIi's). Incorporating Ni into the hash assures that
an attacker who sees only message 2 can't successfully forge a
message 3.
If a new value for <secret> is chosen while there are connections in
the process of being initialized, an IKE_SA_INIT might be returned
with other than the current <VersionIDofSecret>. The responder in
that case MAY reject the message by sending another response with a
new cookie or it MAY keep the old value of <secret> around for a
short time and accept cookies computed from either one. The
responder SHOULD NOT accept cookies indefinitely after <secret> is
changed, since that would defeat part of the denial of service
protection. The responder SHOULD change the value of <secret>
frequently, especially if under attack.
2.7 Cryptographic Algorithm Negotiation
The payload type known as "SA" indicates a proposal for a set of
choices of IPsec protocols (IKE, ESP, and/or AH) for the SA as well
as cryptographic algorithms associated with each protocol.
An SA payload consists of one or more proposals. Each proposal
includes one or more protocols (usually one). Each protocol contains
one or more transforms - each specifying a cryptographic algorithm.
Each transform contains zero or more attributes (attributes are only
needed if the transform identifier does not completely specify the
cryptographic algorithm).
This hierarchical structure was designed to efficiently encode
proposals for cryptographic suites when the number of supported
suites is large because multiple values are acceptable for multiple
transforms. The responder MUST choose a single suite, which MAY be
any subset of the SA proposal following the rules below:
Each proposal contains one or more protocols. If a proposal is
accepted, the SA response MUST contain the same protocols in the
same order as the proposal. The responder MUST accept a single
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proposal or reject them all and return an error. (Example: if a
single proposal contains ESP and AH and that proposal is accepted,
both ESP and AH MUST be accepted. If ESP and AH are included in
separate proposals, the responder MUST accept only one of them).
Each IPsec protocol proposal contains one or more transforms. Each
transform contains a transform type. The accepted cryptographic
suite MUST contain exactly one transform of each type included in
the proposal. For example: if an ESP proposal includes transforms
ENCR_3DES, ENCR_AES w/keysize 128, ENCR_AES w/keysize 256,
AUTH_HMAC_MD5, and AUTH_HMAC_SHA, the accepted suite MUST contain
one of the ENCR_ transforms and one of the AUTH_ transforms. Thus
six combinations are acceptable.
Since the initiator sends its Diffie-Hellman value in the
IKE_SA_INIT, it must guess the Diffie-Hellman group that the
responder will select from its list of supported groups. If the
initiator guesses wrong, the responder will respond with a Notify
payload of type INVALID_KE_PAYLOAD indicating the selected group. In
this case, the initiator MUST retry the IKE_SA_INIT with the
corrected Diffie-Hellman group. The initiator MUST again propose its
full set of acceptable cryptographic suites because the rejection
message was unauthenticated and otherwise an active attacker could
trick the endpoints into negotiating a weaker suite than a stronger
one that they both prefer.
2.8 Rekeying
IKE, ESP, and AH security associations use secret keys which SHOULD
only be used for a limited amount of time and to protect a limited
amount of data. This limits the lifetime of the entire security
association. When the lifetime of a security association expires the
security association MUST NOT be used. If there is demand, new
security associations MAY be established. Reestablishment of
security associations to take the place of ones which expire is
referred to as "rekeying".
To allow for minimal IPsec implementations, the ability to rekey SAs
without restarting the entire IKE_SA is optional. An implementation
MAY refuse all CREATE_CHILD_SA requests within an IKE_SA. If an SA
has expired or is about to expire and rekeying attempts using the
mechanisms described here fail, an implementation MUST close the
IKE_SA and any associated CHILD_SAs and then MAY start new ones.
Implementations SHOULD support in place rekeying of SAs, since doing
so offers better performance and is likely to reduce the number of
packets lost during the transition.
To rekey a CHILD_SA within an existing IKE_SA, create a new,
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equivalent SA (see section 2.17 below), and when the new one is
established, delete the old one. To rekey an IKE_SA, establish a new
equivalent IKE_SA (see section 2.18 below) with the peer to whom the
old IKE_SA is shared using a CREATE_CHILD_SA within the existing
IKE_SA. An IKE_SA so created inherits all of the original IKE_SA's
CHILD_SAs. Use the new IKE_SA for all control messages needed to
maintain the CHILD_SAs created by the old IKE_SA, and delete the old
IKE_SA. The Delete payload to delete itself MUST be the last request
sent over an IKE_SA.
SAs SHOULD be rekeyed proactively, i.e., the new SA should be
established before the old one expires and becomes unusable. Enough
time should elapse between the time the new SA is established and the
old one becomes unusable so that traffic can be switched over to the
new SA.
A difference between IKEv1 and IKEv2 is that in IKEv1 SA lifetimes
were negotiated. In IKEv2, each end of the SA is responsible for
enforcing its own lifetime policy on the SA and rekeying the SA when
necessary. If the two ends have different lifetime policies, the end
with the shorter lifetime will end up always being the one to request
the rekeying. If an SA bundle has been inactive for a long time and
if an endpoint would not initiate the SA in the absence of traffic,
the endpoint MAY choose to close the SA instead of rekeying it when
its lifetime expires. It SHOULD do so if there has been no traffic
since the last time the SA was rekeyed.
If the two ends have the same lifetime policies, it is possible that
both will initiate a rekeying at the same time (which will result in
redundant SAs). To reduce the probability of this happening, the
timing of rekeying requests SHOULD be jittered (delayed by a random
amount of time after the need for rekeying is noticed).
This form of rekeying may temporarily result in multiple similar SAs
between the same pairs of nodes. When there are two SAs eligible to
receive packets, a node MUST accept incoming packets through either
SA. If redundant SAs are created though such a collision, the SA
created with the lowest of the four nonces used in the two exchanges
SHOULD be closed by the endpoint that created it.
Note that IKEv2 deliberately allows parallel SAs with the same
traffic selectors between common endpoints. One of the purposes of
this is to support traffic QoS differences among the SAs (see section
4.1 of [RFC2983]). Hence unlike IKEv1, the combination of the
endpoints and the traffic selectors may not uniquely identify an SA
between those endpoints, so the IKEv1 rekeying heuristic of deleting
SAs on the basis of duplicate traffic selectors SHOULD NOT be used.
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The node that initiated the surviving rekeyed SA SHOULD delete the
replaced SA after the new one is established.
There are timing windows - particularly in the presence of lost
packets - where endpoints may not agree on the state of an SA. The
responder to a CREATE_CHILD_SA MUST be prepared to accept messages on
an SA before sending its response to the creation request, so there
is no ambiguity for the initiator. The initiator MAY begin sending on
an SA as soon as it processes the response. The initiator, however,
cannot receive on a newly created SA until it receives and processes
the response to its CREATE_CHILD_SA request. How, then, is the
responder to know when it is OK to send on the newly created SA?
From a technical correctness and interoperability perspective, the
responder MAY begin sending on an SA as soon as it sends its response
to the CREATE_CHILD_SA request. In some situations, however, this
could result in packets unnecessarily being dropped, so an
implementation MAY want to defer such sending.
The responder can be assured that the initiator is prepared to
receive messages on an SA if either (1) it has received a
cryptographically valid message on the new SA, or (2) the new SA
rekeys an existing SA and it receives an IKE request to close the
replaced SA. When rekeying an SA, the responder SHOULD continue to
send requests on the old SA until it one of those events occurs. When
establishing a new SA, the responder MAY defer sending messages on a
new SA until either it receives one or a timeout has occurred. If an
initiator receives a message on an SA for which it has not received a
response to its CREATE_CHILD_SA request, it SHOULD interpret that as
a likely packet loss and retransmit the CREATE_CHILD_SA request. An
initiator MAY send a dummy message on a newly created SA if it has no
messages queued in order to assure the responder that the initiator
is ready to receive messages.
2.9 Traffic Selector Negotiation
When an IP packet is received by an RFC2401 compliant IPsec subsystem
and matches a "protect" selector in its SPD, the subsystem MUST
protect that packet with IPsec. When no SA exists yet it is the task
of IKE to create it. Maintenance of a system's SPD is outside the
scope of IKE (see [PFKEY] for an example protocol), though some
implementations might update their SPD in connection with the running
of IKE (for an example scenario, see section 1.1.3).
Traffic Selector (TS) payloads allow endpoints to communicate some of
the information from their SPD to their peers. TS payloads specify
the selection criteria for packets that will be forwarded over the
newly set up SA. This can serve as a consistency check in some
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scenarios to assure that the SPDs are consistent. In others, it
guides the dynamic update of the SPD.
Two TS payloads appear in each of the messages in the exchange that
creates a CHILD_SA pair. Each TS payload contains one or more Traffic
Selectors. Each Traffic Selector consists of an address range (IPv4
or IPv6), a port range, and an IP protocol ID. In support of the
scenario described in section 1.1.3, an initiator may request that
the responder assign an IP address and tell the initiator what it is.
IKEv2 allows the responder to choose a subset of the traffic proposed
by the initiator. This could happen when the configuration of the
two endpoints are being updated but only one end has received the new
information. Since the two endpoints may be configured by different
people, the incompatibility may persist for an extended period even
in the absence of errors. It also allows for intentionally different
configurations, as when one end is configured to tunnel all addresses
and depends on the other end to have the up to date list.
The first of the two TS payloads is known as TSi (Traffic Selector-
initiator). The second is known as TSr (Traffic Selector-responder).
TSi specifies the source address of traffic forwarded from (or the
destination address of traffic forwarded to) the initiator of the
CHILD_SA pair. TSr specifies the destination address of the traffic
forwarded from (or the source address of the traffic forwarded to)
the responder of the CHILD_SA pair. For example, if the original
initiator request the creation of a CHILD_SA pair, and wishes to
tunnel all traffic from subnet 192.0.1.* on the initiator's side to
subnet 192.0.2.* on the responder's side, the initiator would include
a single traffic selector in each TS payload. TSi would specify the
address range (192.0.1.0 - 192.0.1.255) and TSr would specify the
address range (192.0.2.0 - 192.0.2.255). Assuming that proposal was
acceptable to the responder, it would send identical TS payloads
back. [Note: the IP address range 192.0.1.* has been reserved for use
in examples in RFCs and similar documents. This document needed two
such ranges, and so also used 192.0.2.*. This should not be confused
with any actual address].
The responder is allowed to narrow the choices by selecting a subset
of the traffic, for instance by eliminating or narrowing the range of
one or more members of the set of traffic selectors, provided the set
does not become the NULL set.
It is possible for the responder's policy to contain multiple smaller
ranges, all encompassed by the initiator's traffic selector, and with
the responder's policy being that each of those ranges should be sent
over a different SA. Continuing the example above, the responder
might have a policy of being willing to tunnel those addresses to and
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from the initiator, but might require that each address pair be on a
separately negotiated CHILD_SA. If the initiator generated its
request in response to an incoming packet from 192.0.1.43 to
192.0.2.123, there would be no way for the responder to determine
which pair of addresses should be included in this tunnel, and it
would have to make a guess or reject the request with a status of
SINGLE_PAIR_REQUIRED.
To enable the responder to choose the appropriate range in this case,
if the initiator has requested the SA due to a data packet, the
initiator SHOULD include as the first traffic selector in each of TSi
and TSr a very specific traffic selector including the addresses in
the packet triggering the request. In the example, the initiator
would include in TSi two traffic selectors: the first containing the
address range (192.0.1.43 - 192.0.1.43) and the source port and IP
protocol from the packet and the second containing (192.0.1.0 -
192.0.1.255) with all ports and IP protocols. The initiator would
similarly include two traffic selectors in TSr.
If the responder's policy does not allow it to accept the entire set
of traffic selectors in the initiator's request, but does allow him
to accept the first selector of TSi and TSr, then the responder MUST
narrow the traffic selectors to a subset that includes the
initiator's first choices. In this example, the responder might
respond with TSi being (192.0.1.43 - 192.0.1.43) with all ports and
IP protocols.
If the initiator creates the CHILD_SA pair not in response to an
arriving packet, but rather - say - upon startup, then there may be
no specific addresses the initiator prefers for the initial tunnel
over any other. In that case, the first values in TSi and TSr MAY be
ranges rather than specific values, and the responder chooses a
subset of the initiator's TSi and TSr that are acceptable. If more
than one subset is acceptable but their union is not, the responder
MUST accept some subset and MAY include a Notify payload of type
ADDITIONAL_TS_POSSIBLE to indicate that the initiator might want to
try again. This case will only occur when the initiator and responder
are configured differently from one another. If the initiator and
responder agree on the granularity of tunnels, the initiator will
never request a tunnel wider than the responder will accept. Such
misconfigurations SHOULD be recorded in error logs.
2.10 Nonces
The IKE_SA_INIT messages each contain a nonce. These nonces are used
as inputs to cryptographic functions. The CREATE_CHILD_SA request
and the CREATE_CHILD_SA response also contain nonces. These nonces
are used to add freshness to the key derivation technique used to
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obtain keys for CHILD_SA, and to ensure creation of strong
pseudorandom bits from the Diffie-Hellman key. Nonces used in IKEv2
MUST be randomly chosen, MUST be at least 128 bits in size, and MUST
be at least half the key size of the negotiated prf. ("prf" refers to
"pseudo-random function", one of the cryptographic algorithms
negotiated in the IKE exchange). If the same random number source is
used for both keys and nonces, care must be taken to ensure that the
latter use does not compromise the former.
2.11 Address and Port Agility
IKE runs over UDP ports 500 and 4500, and implicitly sets up ESP and
AH associations for the same IP addresses it runs over. The IP
addresses and ports in the outer header are, however, not themselves
cryptographically protected, and IKE is designed to work even through
Network Address Translation (NAT) boxes. An implementation MUST
accept incoming requests even if the source port is not 500 or 4500,
and MUST respond to the address and port from which the request was
received. It MUST specify the address and port at which the request
was received as the source address and port in the response. IKE
functions identically over IPv4 or IPv6.
2.12 Reuse of Diffie-Hellman Exponentials
IKE generates keying material using an ephemeral Diffie-Hellman
exchange in order to gain the property of "perfect forward secrecy".
This means that once a connection is closed and its corresponding
keys are forgotten, even someone who has recorded all of the data
from the connection and gets access to all of the long-term keys of
the two endpoints cannot reconstruct the keys used to protect the
conversation without doing a brute force search of the session key
space.
Achieving perfect forward secrecy requires that when a connection is
closed, each endpoint MUST forget not only the keys used by the
connection but any information that could be used to recompute those
keys. In particular, it MUST forget the secrets used in the Diffie-
Hellman calculation and any state that may persist in the state of a
pseudo-random number generator that could be used to recompute the
Diffie-Hellman secrets.
Since the computing of Diffie-Hellman exponentials is computationally
expensive, an endpoint may find it advantageous to reuse those
exponentials for multiple connection setups. There are several
reasonable strategies for doing this. An endpoint could choose a new
exponential only periodically though this could result in less-than-
perfect forward secrecy if some connection lasts for less than the
lifetime of the exponential. Or it could keep track of which
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exponential was used for each connection and delete the information
associated with the exponential only when some corresponding
connection was closed. This would allow the exponential to be reused
without losing perfect forward secrecy at the cost of maintaining
more state.
Decisions as to whether and when to reuse Diffie-Hellman exponentials
is a private decision in the sense that it will not affect
interoperability. An implementation that reuses exponentials MAY
choose to remember the exponential used by the other endpoint on past
exchanges and if one is reused to avoid the second half of the
calculation.
2.13 Generating Keying Material
In the context of the IKE_SA, four cryptographic algorithms are
negotiated: an encryption algorithm, an integrity protection
algorithm, a Diffie-Hellman group, and a pseudo-random function
(prf). The pseudo-random function is used for the construction of
keying material for all of the cryptographic algorithms used in both
the IKE_SA and the CHILD_SAs.
We assume that each encryption algorithm and integrity protection
algorithm uses a fixed size key, and that any randomly chosen value
of that fixed size can serve as an appropriate key. For algorithms
that accept a variable length key, a fixed key size MUST be specified
as part of the cryptographic transform negotiated. For algorithms
for which not all values are valid keys (such as DES or 3DES with key
parity), they algorithm by which keys are derived from arbitrary
values MUST be specified by the cryptographic transform. For
integrity protection functions based on HMAC, the fixed key size is
the size of the output of the underlying hash function. When the prf
function takes a variable length key, variable length data, and
produces a fixed length output (e.g., when using HMAC), the formulas
in this document apply. When the key for the prf function has fixed
length, the data provided as a key is truncated or padded with zeros
as necessary unless exceptional processing is explained following the
formula.
Keying material will always be derived as the output of the
negotiated prf algorithm. Since the amount of keying material needed
may be greater than the size of the output of the prf algorithm, we
will use the prf iteratively. We will use the terminology prf+ to
describe the function that outputs a pseudo-random stream based on
the inputs to a prf as follows: (where | indicates concatenation)
prf+ (K,S) = T1 | T2 | T3 | T4 | ...
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where:
T1 = prf (K, S | 0x01)
T2 = prf (K, T1 | S | 0x02)
T3 = prf (K, T2 | S | 0x03)
T4 = prf (K, T3 | S | 0x04)
continuing as needed to compute all required keys. The keys are taken
from the output string without regard to boundaries (e.g., if the
required keys are a 256 bit AES key and a 160 bit HMAC key, and the
prf function generates 160 bits, the AES key will come from T1 and
the beginning of T2, while the HMAC key will come from the rest of T2
and the beginning of T3).
The constant concatenated to the end of each string feeding the prf
is a single octet. prf+ in this document is not defined beyond 255
times the size of the prf output.
2.14 Generating Keying Material for the IKE_SA
The shared keys are computed as follows. A quantity called SKEYSEED
is calculated from the nonces exchanged during the IKE_SA_INIT
exchange and the Diffie-Hellman shared secret established during that
exchange. SKEYSEED is used to calculate seven other secrets: SK_d
used for deriving new keys for the CHILD_SAs established with this
IKE_SA; SK_ai and SK_ar used as a key to the integrity protection
algorithm for authenticating the component messages of subsequent
exchanges; SK_ei and SK_er used for encrypting (and of course
decrypting) all subsequent exchanges; and SK_pi and SK_pr which are
used when generating an AUTH payload.
SKEYSEED and its derivatives are computed as follows:
SKEYSEED = prf(Ni | Nr, g^ir)
{SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr }
= prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr )
(indicating that the quantities SK_d, SK_ai, SK_ar, SK_ei, SK_er,
SK_pi, and SK_pr are taken in order from the generated bits of the
prf+). g^ir is the shared secret from the ephemeral Diffie-Hellman
exchange. g^ir is represented as a string of octets in big endian
order padded with zeros if necessary to make it the length of the
modulus. Ni and Nr are the nonces, stripped of any headers. If the
negotiated prf takes a fixed length key and the lengths of Ni and Nr
do not add up to that length, half the bits must come from Ni and
half from Nr, taking the first bits of each.
The two directions of traffic flow use different keys. The keys used
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to protect messages from the original initiator are SK_ai and SK_ei.
The keys used to protect messages in the other direction are SK_ar
and SK_er. Each algorithm takes a fixed number of bits of keying
material, which is specified as part of the algorithm. For integrity
algorithms based on a keyed hash, the key size is always equal to the
length of the output of the underlying hash function.
2.15 Authentication of the IKE_SA
When not using extensible authentication (see section 2.16), the
peers are authenticated by having each sign (or MAC using a shared
secret as the key) a block of data. For the responder, the octets to
be signed start with the first octet of the first SPI in the header
of the second message and end with the last octet of the last payload
in the second message. Appended to this (for purposes of computing
the signature) are the initiator's nonce Ni (just the value, not the
payload containing it), and the value prf(SK_pr,IDr') where IDr' is
the responder's ID payload excluding the fixed header. Note that
neither the nonce Ni nor the value prf(SK_pr,IDr') are transmitted.
Similarly, the initiator signs the first message, starting with the
first octet of the first SPI in the header and ending with the last
octet of the last payload. Appended to this (for purposes of
computing the signature) are the responder's nonce Nr, and the value
prf(SK_pi,IDi'). In the above calculation, IDi' and IDr' are the
entire ID payloads excluding the fixed header. It is critical to the
security of the exchange that each side sign the other side's nonce.
Note that all of the payloads are included under the signature,
including any payload types not defined in this document. If the
first message of the exchange is sent twice (the second time with a
responder cookie and/or a different Diffie-Hellman group), it is the
second version of the message that is signed.
Optionally, messages 3 and 4 MAY include a certificate, or
certificate chain providing evidence that the key used to compute a
digital signature belongs to the name in the ID payload. The
signature or MAC will be computed using algorithms dictated by the
type of key used by the signer, and specified by the Auth Method
field in the Authentication payload. There is no requirement that
the initiator and responder sign with the same cryptographic
algorithms. The choice of cryptographic algorithms depends on the
type of key each has. In particular, the initiator may be using a
shared key while the responder may have a public signature key and
certificate. It will commonly be the case (but it is not required)
that if a shared secret is used for authentication that the same key
is used in both directions. Note that it is a common but typically
insecure practice to have a shared key derived solely from a user
chosen password without incorporating another source of randomness.
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This is typically insecure because user chosen passwords are unlikely
to have sufficient unpredictability to resist dictionary attacks and
these attacks are not prevented in this authentication method.
(Applications using password-based authentication for bootstrapping
and IKE_SA should use the authentication method in section 2.16,
which is designed to prevent off-line dictionary attacks). The pre-
shared key SHOULD contain as much unpredictability as the strongest
key being negotiated. In the case of a pre-shared key, the AUTH
value is computed as:
AUTH = prf(prf(Shared Secret,"Key Pad for IKEv2"), <msg octets>)
where the string "Key Pad for IKEv2" is 17 ASCII characters without
null termination. The shared secret can be variable length. The pad
string is added so that if the shared secret is derived from a
password, the IKE implementation need not store the password in
cleartext, but rather can store the value prf(Shared Secret,"Key Pad
for IKEv2"), which could not be used as a password equivalent for
protocols other than IKEv2. As noted above, deriving the shared
secret from a password is not secure. This construction is used
because it is anticipated that people will do it anyway. The
management interface by which the Shared Secret is provided MUST
accept ASCII strings of at least 64 octets and MUST NOT add a null
terminator before using them as shared secrets. It MUST also accept a
HEX encoding of the Shared Secret. The management interface MAY
accept other encodings if the algorithm for translating the encoding
to a binary string is specified. If the negotiated prf takes a fixed
size key, the shared secret MUST be of that fixed size.
2.16 Extensible Authentication Protocol Methods
In addition to authentication using public key signatures and shared
secrets, IKE supports authentication using methods defined in RFC
3748 [EAP]. Typically, these methods are asymmetric (designed for a
user authenticating to a server), and they may not be mutual. For
this reason, these protocols are typically used to authenticate the
initiator to the responder and MUST be used in conjunction with a
public key signature based authentication of the responder to the
initiator. These methods are often associated with mechanisms
referred to as "Legacy Authentication" mechanisms.
While this memo references [EAP] with the intent that new methods can
be added in the future without updating this specification, some
simpler variations are documented here and in section 3.16. [EAP]
defines an authentication protocol requiring a variable number of
messages. Extensible Authentication is implemented in IKE as
additional IKE_AUTH exchanges that MUST be completed in order to
initialize the IKE_SA.
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An initiator indicates a desire to use extensible authentication by
leaving out the AUTH payload from message 3. By including an IDi
payload but not an AUTH payload, the initiator has declared an
identity but has not proven it. If the responder is willing to use an
extensible authentication method, it will place an EAP payload in
message 4 and defer sending SAr2, TSi, and TSr until initiator
authentication is complete in a subsequent IKE_AUTH exchange. In the
case of a minimal extensible authentication, the initial SA
establishment will appear as follows:
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
HDR, SK {IDi, [CERTREQ,] [IDr,]
SAi2, TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
EAP }
HDR, SK {EAP} -->
<-- HDR, SK {EAP (success)}
HDR, SK {AUTH} -->
<-- HDR, SK {AUTH, SAr2, TSi, TSr }
For EAP methods that create a shared key as a side effect of
authentication, that shared key MUST be used by both the initiator
and responder to generate AUTH payloads in messages 5 and 6 using the
syntax for shared secrets specified in section 2.15. The shared key
from EAP is the field from the EAP specification named MSK. The
shared key generated during an IKE exchange MUST NOT be used for any
other purpose.
EAP methods that do not establish a shared key SHOULD NOT be used, as
they are subject to a number of man-in-the-middle attacks [EAPMITM]
if these EAP methods are used in other protocols that do not use a
server-authenticated tunnel. Please see the Security Considerations
section for more details. If EAP methods that do not generate a
shared key are used, the AUTH payloads in messages 7 and 8 MUST be
generated using SK_pi and SK_pr respectively.
The initiator of an IKE_SA using EAP SHOULD be capable of extending
the initial protocol exchange to at least ten IKE_AUTH exchanges in
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the event the responder sends notification messages and/or retries
the authentication prompt. Once the protocol exchange defined by the
chosen EAP authentication method has successfully terminated, the
responder MUST send an EAP payload containing the Success message.
Similarly, if the authentication method has failed, the responder
MUST send an EAP payload containing the Failure message. The
responder MAY at any time terminate the IKE exchange by sending an
EAP payload containing the Failure message.
Following such an extended exchange, the EAP AUTH payloads MUST be
included in the two messages following the one containing the EAP
Success message.
2.17 Generating Keying Material for CHILD_SAs
CHILD_SAs are created either by being piggybacked on the IKE_AUTH
exchange, or in a CREATE_CHILD_SA exchange. Keying material for them
is generated as follows:
KEYMAT = prf+(SK_d, Ni | Nr)
Where Ni and Nr are the Nonces from the IKE_SA_INIT exchange if this
request is the first CHILD_SA created or the fresh Ni and Nr from the
CREATE_CHILD_SA exchange if this is a subsequent creation.
For CREATE_CHILD_SA exchanges including an optional Diffie-Hellman
exchange, the keying material is defined as:
KEYMAT = prf+(SK_d, g^ir (new) | Ni | Nr )
where g^ir (new) is the shared secret from the ephemeral Diffie-
Hellman exchange of this CREATE_CHILD_SA exchange (represented as an
octet string in big endian order padded with zeros in the high order
bits if necessary to make it the length of the modulus).
A single CHILD_SA negotiation may result in multiple security
associations. ESP and AH SAs exist in pairs (one in each direction),
and four SAs could be created in a single CHILD_SA negotiation if a
combination of ESP and AH is being negotiated.
Keying material MUST be taken from the expanded KEYMAT in the
following order:
All keys for SAs carrying data from the initiator to the responder
are taken before SAs going in the reverse direction.
If multiple IPsec protocols are negotiated, keying material is
taken in the order in which the protocol headers will appear in
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the encapsulated packet.
If a single protocol has both encryption and authentication keys,
the encryption key is taken from the first octets of KEYMAT and
the authentication key is taken from the next octets.
Each cryptographic algorithm takes a fixed number of bits of keying
material specified as part of the algorithm.
2.18 Rekeying IKE_SAs using a CREATE_CHILD_SA exchange
The CREATE_CHILD_SA exchange can be used to rekey an existing IKE_SA
(see section 2.8). New initiator and responder SPIs are supplied in
the SPI fields. The TS payloads are omitted when rekeying an IKE_SA.
SKEYSEED for the new IKE_SA is computed using SK_d from the existing
IKE_SA as follows:
SKEYSEED = prf(SK_d (old), [g^ir (new)] | Ni | Nr)
where g^ir (new) is the shared secret from the ephemeral Diffie-
Hellman exchange of this CREATE_CHILD_SA exchange (represented as an
octet string in big endian order padded with zeros if necessary to
make it the length of the modulus) and Ni and Nr are the two nonces
stripped of any headers.
The new IKE_SA MUST reset its message counters to 0.
SK_d, SK_ai, SK_ar, and SK_ei, and SK_er are computed from SKEYSEED
as specified in section 2.14.
2.19 Requesting an internal address on a remote network
Most commonly occurring in the endpoint to security gateway scenario,
an endpoint may need an IP address in the network protected by the
security gateway, and may need to have that address dynamically
assigned. A request for such a temporary address can be included in
any request to create a CHILD_SA (including the implicit request in
message 3) by including a CP payload.
This function provides address allocation to an IRAC (IPsec Remote
Access Client) trying to tunnel into a network protected by an IRAS
(IPsec Remote Access Server). Since the IKE_AUTH exchange creates an
IKE_SA and a CHILD_SA, the IRAC MUST request the IRAS controlled
address (and optionally other information concerning the protected
network) in the IKE_AUTH exchange. The IRAS may procure an address
for the IRAC from any number of sources such as a DHCP/BOOTP server
or its own address pool.
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Initiator Responder
----------------------------- ---------------------------
HDR, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, CP(CFG_REQUEST),
SAi2, TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
CP(CFG_REPLY), SAr2,
TSi, TSr}
In all cases, the CP payload MUST be inserted before the SA payload.
In variations of the protocol where there are multiple IKE_AUTH
exchanges, the CP payloads MUST be inserted in the messages
containing the SA payloads.
CP(CFG_REQUEST) MUST contain at least an INTERNAL_ADDRESS attribute
(either IPv4 or IPv6) but MAY contain any number of additional
attributes the initiator wants returned in the response.
For example, message from initiator to responder:
CP(CFG_REQUEST)=
INTERNAL_ADDRESS(0.0.0.0)
INTERNAL_NETMASK(0.0.0.0)
INTERNAL_DNS(0.0.0.0)
TSi = (0, 0-65536,0.0.0.0-255.255.255.255)
TSr = (0, 0-65536,0.0.0.0-255.255.255.255)
NOTE: Traffic Selectors contain (protocol, port range, address range)
Message from responder to initiator:
CP(CFG_REPLY)=
INTERNAL_ADDRESS(192.0.2.202)
INTERNAL_NETMASK(255.255.255.0)
INTERNAL_SUBNET(192.0.2.0/255.255.255.0)
TSi = (0, 0-65536,192.0.2.202-192.0.2.202)
TSr = (0, 0-65536,192.0.2.0-192.0.2.255)
All returned values will be implementation dependent. As can be seen
in the above example, the IRAS MAY also send other attributes that
were not included in CP(CFG_REQUEST) and MAY ignore the non-
mandatory attributes that it does not support.
The responder MUST NOT send a CFG_REPLY without having first received
a CP(CFG_REQUEST) from the initiator, because we do not want the IRAS
to perform an unnecessary configuration lookup if the IRAC cannot
process the REPLY. In the case where the IRAS's configuration
requires that CP be used for a given identity IDi, but IRAC has
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failed to send a CP(CFG_REQUEST), IRAS MUST fail the request, and
terminate the IKE exchange with a FAILED_CP_REQUIRED error.
2.20 Requesting the Peer's Version
An IKE peer wishing to inquire about the other peer's IKE software
version information MAY use the method below. This is an example of
a configuration request within an INFORMATIONAL Exchange, after the
IKE_SA and first CHILD_SA have been created.
An IKE implementation MAY decline to give out version information
prior to authentication or even after authentication to prevent
trolling in case some implementation is known to have some security
weakness. In that case, it MUST either return an empty string or no
CP payload if CP is not supported.
Initiator Responder
----------------------------- --------------------------
HDR, SK{CP(CFG_REQUEST)} -->
<-- HDR, SK{CP(CFG_REPLY)}
CP(CFG_REQUEST)=
APPLICATION_VERSION("")
CP(CFG_REPLY)
APPLICATION_VERSION("foobar v1.3beta, (c) Foo Bar Inc.")
2.21 Error Handling
There are many kinds of errors that can occur during IKE processing.
If a request is received that is badly formatted or unacceptable for
reasons of policy (e.g., no matching cryptographic algorithms), the
response MUST contain a Notify payload indicating the error. If an
error occurs outside the context of an IKE request (e.g., the node is
getting ESP messages on a nonexistent SPI), the node SHOULD initiate
an INFORMATIONAL Exchange with a Notify payload describing the
problem.
Errors that occur before a cryptographically protected IKE_SA is
established must be handled very carefully. There is a trade-off
between wanting to be helpful in diagnosing a problem and responding
to it and wanting to avoid being a dupe in a denial of service attack
based on forged messages.
If a node receives a message on UDP port 500 or 4500 outside the
context of an IKE_SA known to it (and not a request to start one), it
may be the result of a recent crash of the node. If the message is
marked as a response, the node MAY audit the suspicious event but
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MUST NOT respond. If the message is marked as a request, the node MAY
audit the suspicious event and MAY send a response. If a response is
sent, the response MUST be sent to the IP address and port from
whence it came with the same IKE SPIs and the Message ID copied. The
response MUST NOT be cryptographically protected and MUST contain a
Notify payload indicating INVALID_IKE_SPI.
A node receiving such an unprotected Notify payload MUST NOT respond
and MUST NOT change the state of any existing SAs. The message might
be a forgery or might be a response the genuine correspondent was
tricked into sending. A node SHOULD treat such a message (and also a
network message like ICMP destination unreachable) as a hint that
there might be problems with SAs to that IP address and SHOULD
initiate a liveness test for any such IKE_SA. An implementation
SHOULD limit the frequency of such tests to avoid being tricked into
participating in a denial of service attack.
A node receiving a suspicious message from an IP address with which
it has an IKE_SA MAY send an IKE Notify payload in an IKE
INFORMATIONAL exchange over that SA. The recipient MUST NOT change
the state of any SA's as a result but SHOULD audit the event to aid
in diagnosing malfunctions. A node MUST limit the rate at which it
will send messages in response to unprotected messages.
2.22 IPComp
Use of IP compression [IPCOMP] can be negotiated as part of the setup
of a CHILD_SA. While IP compression involves an extra header in each
packet and a CPI (compression parameter index), the virtual
"compression association" has no life outside the ESP or AH SA that
contains it. Compression associations disappear when the
corresponding ESP or AH SA goes away, and is not explicitly mentioned
in any DELETE payload.
Negotiation of IP compression is separate from the negotiation of
cryptographic parameters associated with a CHILD_SA. A node
requesting a CHILD_SA MAY advertise its support for one or more
compression algorithms though one or more Notify payloads of type
IPCOMP_SUPPORTED. The response MAY indicate acceptance of a single
compression algorithm with a Notify payload of type IPCOMP_SUPPORTED.
These payloads MUST NOT occur messages that do not contain SA
payloads.
While there has been discussion of allowing multiple compression
algorithms to be accepted and to have different compression
algorithms available for the two directions of a CHILD_SA,
implementations of this specification MUST NOT accept an IPComp
algorithm that was not proposed, MUST NOT accept more than one, and
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MUST NOT compress using an algorithm other than one proposed and
accepted in the setup of the CHILD_SA.
A side effect of separating the negotiation of IPComp from
cryptographic parameters is that it is not possible to propose
multiple cryptographic suites and propose IP compression with some of
them but not others.
2.23 NAT Traversal
NAT (Network Address Translation) gateways are a controversial
subject. This section briefly describes what they are and how they
are likely to act on IKE traffic. Many people believe that NATs are
evil and that we should not design our protocols so as to make them
work better. IKEv2 does specify some unintuitive processing rules in
order that NATs are more likely to work.
NATs exist primarily because of the shortage of IPv4 addresses,
though there are other rationales. IP nodes that are "behind" a NAT
have IP addresses that are not globally unique, but rather are
assigned from some space that is unique within the network behind the
NAT but which are likely to be reused by nodes behind other NATs.
Generally, nodes behind NATs can communicate with other nodes behind
the same NAT and with nodes with globally unique addresses, but not
with nodes behind other NATs. There are exceptions to that rule.
When those nodes make connections to nodes on the real Internet, the
NAT gateway "translates" the IP source address to an address that
will be routed back to the gateway. Messages to the gateway from the
Internet have their destination addresses "translated" to the
internal address that will route the packet to the correct endnode.
NATs are designed to be "transparent" to endnodes. Neither software
on the node behind the NAT nor the node on the Internet require
modification to communicate through the NAT. Achieving this
transparency is more difficult with some protocols than with others.
Protocols that include IP addresses of the endpoints within the
payloads of the packet will fail unless the NAT gateway understands
the protocol and modifies the internal references as well as those in
the headers. Such knowledge is inherently unreliable, is a network
layer violation, and often results in subtle problems.
Opening an IPsec connection through a NAT introduces special
problems. If the connection runs in transport mode, changing the IP
addresses on packets will cause the checksums to fail and the NAT
cannot correct the checksums because they are cryptographically
protected. Even in tunnel mode, there are routing problems because
transparently translating the addresses of AH and ESP packets
requires special logic in the NAT and that logic is heuristic and
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unreliable in nature. For that reason, IKEv2 can negotiate UDP
encapsulation of IKE and ESP packets. This encoding is slightly less
efficient but is easier for NATs to process. In addition, firewalls
may be configured to pass IPsec traffic over UDP but not ESP/AH or
vice versa.
It is a common practice of NATs to translate TCP and UDP port numbers
as well as addresses and use the port numbers of inbound packets to
decide which internal node should get a given packet. For this
reason, even though IKE packets MUST be sent from and to UDP port
500, they MUST be accepted coming from any port and responses MUST be
sent to the port from whence they came. This is because the ports may
be modified as the packets pass through NATs. Similarly, IP addresses
of the IKE endpoints are generally not included in the IKE payloads
because the payloads are cryptographically protected and could not be
transparently modified by NATs.
Port 4500 is reserved for UDP encapsulated ESP and IKE. When working
through a NAT, it is generally better to pass IKE packets over port
4500 because some older NATs handle IKE traffic on port 500 cleverly
in an attempt to transparently establish IPsec connections between
endpoints that don't handle NAT traversal themselves. Such NATs may
interfere with the straightforward NAT traversal envisioned by this
document, so an IPsec endpoint that discovers a NAT between it and
its correspondent MUST send all subsequent traffic to and from port
4500, which NATs should not treat specially (as they might with port
500).
The specific requirements for supporting NAT traversal are listed
below. Support for NAT traversal is optional. In this section only,
requirements listed as MUST only apply to implementations supporting
NAT traversal.
IKE MUST listen on port 4500 as well as port 500. IKE MUST respond
to the IP address and port from which packets arrived.
Both IKE initiator and responder MUST include in their IKE_SA_INIT
packets Notify payloads of type NAT_DETECTION_SOURCE_IP and
NAT_DETECTION_DESTINATION_IP. Those payloads can be used to detect
if there is NAT between the hosts, and which end is behind the
NAT. The location of the payloads in the IKE_SA_INIT packets are
just after the Ni and Nr payloads (before the optional CERTREQ
payload).
If none of the NAT_DETECTION_SOURCE_IP payload(s) received matches
the hash of the source IP and port found from the IP header of the
packet containing the payload, it means that the other end is
behind NAT (i.e., someone along the route changed the source
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address of the original packet to match the address of the NAT
box). In this case this end should allow dynamic update of the
other ends IP address, as described later.
If the NAT_DETECTION_DESTINATION_IP payload received does not
match the hash of the destination IP and port found from the IP
header of the packet containing the payload, it means that this
end is behind a NAT. In this case, this end SHOULD start sending
keepalive packets as explained in [Hutt04].
The IKE initiator MUST check these payloads if present and if they
do not match the addresses in the outer packet MUST tunnel all
future IKE and ESP packets associated with this IKE_SA over UDP
port 4500.
To tunnel IKE packets over UDP port 4500, the IKE header has four
octets of zero prepended and the result immediately follows the
UDP header. To tunnel ESP packets over UDP port 4500, the ESP
header immediately follows the UDP header. Since the first four
bytes of the ESP header contain the SPI, and the SPI cannot
validly be zero, it is always possible to distinguish ESP and IKE
messages.
The original source and destination IP address required for the
transport mode TCP and UDP packet checksum fixup (see [Hutt04])
are obtained from the Traffic Selectors associated with the
exchange. In the case of NAT traversal, the Traffic Selectors MUST
contain exactly one IP address which is then used as the original
IP address.
There are cases where a NAT box decides to remove mappings that
are still alive (for example, the keepalive interval is too long,
or the NAT box is rebooted). To recover in these cases, hosts that
are not behind a NAT SHOULD send all packets (including
retransmission packets) to the IP address and port from the last
valid authenticated packet from the other end (i.e., dynamically
update the address). A host behind a NAT SHOULD NOT do this
because it opens a DoS attack possibility. Any authenticated IKE
packet or any authenticated UDP encapsulated ESP packet can be
used to detect that the IP address or the port has changed.
Note that similar but probably not identical actions will likely
be needed to make IKE work with Mobile IP, but such processing is
not addressed by this document.
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2.24 ECN (Explicit Congestion Notification)
When IPsec tunnels behave as originally specified in [RFC2401], ECN
usage is not appropriate for the outer IP headers because tunnel
decapsulation processing discards ECN congestion indications to the
detriment of the network. ECN support for IPsec tunnels for
IKEv1-based IPsec requires multiple operating modes and negotiation
(see RFC3168</