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Versions: (draft-tschofenig-ipsecme-ikev2-resumption)
00 01 02 03 04 05 06 07 08 09 RFC 5723
Network Working Group Y. Sheffer
Internet-Draft Check Point
Intended status: Standards Track H. Tschofenig
Expires: September 10, 2009 Nokia Siemens Networks
L. Dondeti
V. Narayanan
QUALCOMM, Inc.
March 9, 2009
IKEv2 Session Resumption
draft-ietf-ipsecme-ikev2-resumption-02.txt
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Abstract
The Internet Key Exchange version 2 (IKEv2) protocol has a certain
computational and communication overhead with respect to the number
of round-trips required and the cryptographic operations involved.
In remote access situations, the Extensible Authentication Protocol
(EAP) is used for authentication, which adds several more round trips
and consequently latency.
To re-establish security associations (SAs) upon a failure recovery
condition is time consuming especially when an IPsec peer (such as a
VPN gateway) needs to re-establish a large number of SAs with various
end points. A high number of concurrent sessions might cause
additional problems for an IPsec peer during SA re-establishment.
In order to avoid the need to re-run the key exchange protocol from
scratch it would be useful to provide an efficient way to resume an
IKE/IPsec session. This document proposes an extension to IKEv2 that
allows a client to re-establish an IKE SA with a gateway in a highly
efficient manner, utilizing a previously established IKE SA.
A client can reconnect to a gateway from which it was disconnected.
The proposed approach requires passing opaque data from the IKEv2
responder to the IKEv2 initiator, which is later made available to
the IKEv2 responder for re-authentication. We use the term ticket to
refer to the opaque data that is created by the IKEv2 responder.
This document does not specify the format of the ticket but
recommendations are provided.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Usage Scenario . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Requesting a Ticket . . . . . . . . . . . . . . . . . . . 7
4.2. Receiving a Ticket . . . . . . . . . . . . . . . . . . . . 8
4.3. Presenting a Ticket . . . . . . . . . . . . . . . . . . . 8
4.3.1. Protection of the IKE_SESSION_RESUME Exchange . . . . 10
4.3.2. Presenting a Ticket: The DoS Case . . . . . . . . . . 10
4.3.3. Requesting a Ticket during Resumption . . . . . . . . 11
4.4. IKE Notifications . . . . . . . . . . . . . . . . . . . . 11
4.5. TICKET_OPAQUE Notify Payload . . . . . . . . . . . . . . . 11
4.6. TICKET_OPAQUE' Notify Payload . . . . . . . . . . . . . . 12
4.7. Processing Guidelines for IKE SA Establishment . . . . . . 12
5. Ticket Recommendations . . . . . . . . . . . . . . . . . . . . 13
5.1. Ticket Content . . . . . . . . . . . . . . . . . . . . . . 13
5.2. Ticket Identity and Lifecycle . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7.1. Stolen Tickets . . . . . . . . . . . . . . . . . . . . . . 15
7.2. Forged Tickets . . . . . . . . . . . . . . . . . . . . . . 15
7.3. Denial of Service Attacks . . . . . . . . . . . . . . . . 15
7.4. Key Management for Tickets By Value . . . . . . . . . . . 16
7.5. Ticket Lifetime . . . . . . . . . . . . . . . . . . . . . 16
7.6. Ticket by Value Format . . . . . . . . . . . . . . . . . . 16
7.7. Identity Privacy, Anonymity, and Unlinkability . . . . . . 16
7.8. Replay Protection in the IKE_SESSION_RESUME Exchange . . . 17
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Ticket Format . . . . . . . . . . . . . . . . . . . . 18
A.1. Recommended Ticket by Value Format . . . . . . . . . . . . 19
A.2. Recommended Ticket by Reference Format . . . . . . . . . . 19
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 20
B.1. -02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
B.2. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
B.3. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
B.4. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B.5. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B.6. -04 . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B.7. -03 . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B.8. -02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B.9. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
B.10. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The Internet Key Exchange version 2 (IKEv2) protocol has a certain
computational and communication overhead with respect to the number
of round-trips required and the cryptographic operations involved.
In particular the Extensible Authentication Protocol (EAP) is used
for authentication in remote access cases, which increases latency.
To re-establish security associations (SA) upon a failure recovery
condition is time-consuming, especially when an IPsec peer, such as a
VPN gateway, needs to re-establish a large number of SAs with various
end points. A high number of concurrent sessions might cause
additional problems for an IPsec responder.
In many failure cases it would be useful to provide an efficient way
to resume an interrupted IKE/IPsec session. This document proposes
an extension to IKEv2 that allows a client to re-establish an IKE SA
with a gateway in a highly efficient manner, utilizing a previously
established IKE SA.
A client can reconnect to a gateway from which it was disconnected.
One way to ensure that the IKEv2 responder is able to recreate the
state information is by maintaining IKEv2 state (or a reference into
a state store) in a "ticket", an opaque data structure. This ticket
is created by the server and forwarded to the client. The IKEv2
protocol is extended to allow a client to request and present a
ticket. This document does not mandate the format of the ticket
structure but a recommendation is provided. In Appendix A a ticket
by value and a ticket by reference format is proposed.
This approach is similar to the one taken by TLS session resumption
[RFC5077] with the required adaptations for IKEv2, e.g., to
accommodate the two-phase protocol structure. We have borrowed
heavily from that specification.
The proposed solution should additionally meet the following goals:
o Using only symmetric cryptography to minimize CPU consumption.
o Allowing a gateway to push state to clients.
o Providing cryptographic agility.
o Having no negative impact on IKEv2 security features.
The following are non-goals of this solution:
o Providing load balancing among gateways.
o Specifying how a client detects the need for a failover.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document uses terminology defined in [RFC4301], [RFC4306], and
[RFC4555]. In addition, this document uses the following terms:
Ticket: An IKEv2 ticket is a data structure that contains all the
necessary information that allows an IKEv2 responder to re-
establish an IKEv2 security association.
In this document we use the term ticket and thereby refer to an
opaque data structure that may either contain IKEv2 state as
described above or a reference pointing to such state.
3. Usage Scenario
This specification envisions two usage scenarios for efficient IKEv2
and IPsec SA session re-establishment.
The first is similar to the use case specified in Section 1.1.3 of
the IKEv2 specification [RFC4306], where the IPsec tunnel mode is
used to establish a secure channel between a remote access client and
a gateway; the traffic flow may be between the client and entities
beyond the gateway.
The second use case focuses on the usage of transport (or tunnel)
mode to secure the communicate between two end points (e.g., two
servers). The two endpoints have a client-server relationship with
respect to a protocol that runs using the protections afforded by the
IPsec SA.
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(a)
+-+-+-+-+-+ +-+-+-+-+-+
! ! IKEv2/IKEv2-EAP ! ! Protected
! Remote !<------------------------>! ! Subnet
! Access ! ! Access !<--- and/or
! Client !<------------------------>! Gateway ! Internet
! ! IPsec tunnel ! !
+-+-+-+-+-+ +-+-+-+-+-+
(b)
+-+-+-+-+-+ +-+-+-+-+-+
! ! IKE_SESSION_RESUME ! !
! Remote !<------------------------>! !
! Access ! ! Access !
! Client !<------------------------>! Gateway !
! ! IPsec tunnel ! !
+-+-+-+-+-+ +-+-+-+-+-+
Figure 1: Resuming a Session with a Remote Access Gateway
In this scenario, an end host (an entity with a host implementation
of IPsec [RFC4301] ) establishes a tunnel mode IPsec SA with a
gateway in a remote network using IKEv2. The end host in this
scenario is sometimes referred to as a remote access client. At a
later stage when a client needs to re-establish the IKEv2 session it
may choose to establish IPsec SAs using a full IKEv2 exchange or the
IKE_SESSION_RESUME exchange (shown in Figure 1).
In this scenario, the client needs to get an IP address from the
remote network so that traffic can be encapsulated by the remote
access gateway before reaching the client. In the initial exchange,
the gateway may acquire IP addresses from the address pool of a local
DHCP server. The session resumption exchange may need to support the
assignment of a new IP address.
The protocol defined in this document supports the re-allocation of
an IP address to the client, if this capability is provided by the
network. This capability is implicit in the use of the IKE
configuration mechanism, which allows the client to present its
existing IP address and receive the same address back, if allowed by
the gateway.
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4. Protocol Details
This section provides protocol details and contains the normative
parts. This document defines two protocol exchanges, namely
requesting a ticket, see Section 4.1, and presenting a ticket, see
Section 4.3.
4.1. Requesting a Ticket
A client MAY request a ticket in the following exchanges:
o In an IKE_AUTH exchange, as shown in the example message exchange
in Figure 2 below.
o In a CREATE_CHILD_SA exchange, when an IKE SA is rekeyed.
o In an Informational exchange, if the gateway previously replied
with an N(TICKET_ACK) instead of providing a ticket.
o In an Informational exchange, when the ticket lifetime is about to
expire.
o In an IKE_SESSION_RESUME exchange, see Section 4.3.3.
Normally, a client requests a ticket in the third message of an IKEv2
exchange (the first of IKE_AUTH). Figure 2 shows the message
exchange for this typical case.
Initiator Responder
----------- -----------
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
HDR, SK {IDi, [CERT,] [CERTREQ,] [IDr,]
AUTH, SAi2, TSi, TSr, N(TICKET_REQUEST)} -->
Figure 2: Example Message Exchange for Requesting a Ticket
The notification payloads are described in Section 4.4. The above is
an example, and IKEv2 allows a number of variants on these messages.
A complete description of IKEv2 can be found in [RFC4718].
When an IKEv2 responder receives a request for a ticket using the
N(TICKET_REQUEST) payload it MUST perform one of the following
operations if it supports the extension defined in this document:
o it creates a ticket and returns it with the N(TICKET_OPAQUE)
payload in a subsequent message towards the IKEv2 initiator. This
is shown in Figure 3.
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o it returns an N(TICKET_NACK) payload, if it refuses to grant a
ticket for some reason.
o it returns an N(TICKET_ACK), if it cannot grant a ticket
immediately, e.g., due to packet size limitations. In this case
the client MAY request a ticket later using an Informational
exchange, at any time during the lifetime of the IKE SA.
4.2. Receiving a Ticket
The IKEv2 initiator receives the ticket and may accept it provided
the IKEv2 exchange was successful, as described in Section 4.1. The
ticket may be used later with an IKEv2 responder that supports this
extension. Figure 3 shows how the initiator receives the ticket.
Initiator Responder
----------- -----------
<-- HDR, SK {IDr, [CERT,] AUTH, SAr2, TSi,
TSr, N(TICKET_OPAQUE) }
Figure 3: Receiving a Ticket
4.3. Presenting a Ticket
A client MAY initiate a regular (non-ticket-based) IKEv2 exchange
even if it is in possession of a valid ticket. Note that the client
can only judge validity in the sense of the ticket lifetime. A
client MUST NOT present a ticket when it knows that the ticket's
lifetime has expired.
It is up to the client's local policy to decide when the
communication with the IKEv2 responder is seen as interrupted and a
new exchange needs to be initiated and the session resumption
procedure to be initiated.
Tickets are intended for one-time use, i.e., a client MUST NOT reuse
a ticket. A reused ticket SHOULD be rejected by a gateway.
This document specifies a new IKEv2 exchange type called
IKE_SESSION_RESUME whose value is TBA by IANA. This exchange is
somewhat similar to the IKE_AUTH exchange, and results in the
creation of a Child SA. The client SHOULD NOT use this exchange type
unless it knows that the gateway supports it.
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Initiator Responder
----------- -----------
HDR, Ni, N(TICKET_OPAQUE'), [N+,]
SK { [IDr,] SAi2, TSi, TSr [, CP(CFG_REQUEST)]} -->
Note: The initiator presents the TICKET_OPAQUE' payload to the
responder as the lifetime field attached to the ticket is not
relevant to the responder as it is included in the protected form
inside the ticket.
The exchange type in HDR is set to 'IKE_SESSION_RESUME'. The
initiator sets the SPIi value in the HDR to a new random value and
the SPIr value is set to 0.
See Section 4.3.1 for details on computing the protected (SK)
payload.
When the IKEv2 responder receives a ticket using the
N(TICKET_OPAQUE') payload it MUST perform one of the following steps
if it supports the extension defined in this document:
o If it is willing to accept the ticket, it responds as shown in
Figure 4.
o It responds with an unprotected N(TICKET_NACK) notification, if it
rejects the ticket for any reason. In that case, the initiator
should re-initiate a regular IKE exchange. One such case is when
the responder receives a ticket for an IKE SA that has previously
been terminated on the responder itself, which may indicate
inconsistent state between the IKEv2 initiator and the responder.
However, a responder is not required to maintain the state for
terminated sessions.
o When the responder receives a ticket for an IKE SA that is still
active and if the responder accepts it, then the old SAs SHOULD be
silently deleted without sending a DELETE informational exchange.
Consequently, all the IPsec child SAs are deleted as well once the
old IKE SA is deleted.
Initiator Responder
----------- -----------
<-- HDR, SK {Nr, SAr2, [TSi, TSr],
[CP(CFG_REPLY)]}
Figure 4: IKEv2 Responder accepts the ticket
Again, the exchange type in HDR is set to 'IKE_SESSION_RESUME'. The
initiator sets the SPIi value in the HDR to a new random value and
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the SPIr value is set to 0.
The SK payload is protected using the cryptographic parameters
derived from the ticket, see Section 4.3.1 below.
At this point a new IKE SA is created by both parties, see
Section 4.7. This is followed by normal derivation of a child SA,
per Section 2.17 of [RFC4306].
4.3.1. Protection of the IKE_SESSION_RESUME Exchange
The two messages of this exchange are protected by a "subset" IKE SA.
The key material is derived from the ticket, as follows:
{SK_d2 | SK_ai | SK_ar | SK_ei | SK_er} = prf+(SK_d_old, Ni)
where SK_d_old is the SK_d value of the original IKE SA, as retrieved
from the ticket. Ni guarantees freshness of the key material. SK_d2
is used later to derive the new IKE SA, see Section 4.7.
See [RFC4306] for the notation. "prf" is determined from the SA value
in the ticket.
4.3.2. Presenting a Ticket: The DoS Case
When receiving the first message of the IKE_SESSION_RESUME exchange,
the gateway may decide that it is under a denial-of-service attack.
In such a case, the gateway SHOULD defer the establishment of session
state until it has verified the identity of the client. We use a
variation of the IKEv2 Cookie mechanism, whereby the cookie is
protected.
In the two messages that follow, the gateway responds that it is
unwilling to resume the session until the client is verified, and the
client resubmits its first message, this time with the cookie:
Initiator Responder
----------- -----------
<-- HDR, SK{N(COOKIE)}
HDR, Ni, N(TICKET_OPAQUE), [N+,]
SK {N(COOKIE), [IDr,] SAi2, TSi, TSr [, CP(CFG_REQUEST)]} -->
Assuming the cookie is correct, the gateway now replies normally.
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This now becomes a 4-message exchange. The entire exchange is
protected as defined in Section 4.3.1.
See Section 2.6 and Section 3.10.1 of [RFC4306] for more guidance
regarding the usage and syntax of the cookie. Note that the cookie
is completely independent of the IKEv2 ticket.
4.3.3. Requesting a Ticket during Resumption
When resuming a session, a client will typically request a new ticket
immediately, so it is able to resume the session again in the case of
a second failure. Therefore, the N(TICKET_REQUEST) and
N(TICKET_OPAQUE) notifications may be piggybacked as protected
payloads to the IKE_SESSION_RESUME exchange.
The returned ticket (if any) will correspond to the IKE SA created
per the rules described in Section 4.7.
4.4. IKE Notifications
This document defines a number of notifications. The notification
numbers are TBA by IANA.
+-------------------+--------+-----------------+
| Notification Name | Number | Data |
+-------------------+--------+-----------------+
| TICKET_OPAQUE | TBA1 | See Section 4.5 |
| TICKET_REQUEST | TBA2 | None |
| TICKET_ACK | TBA3 | None |
| TICKET_NACK | TBA4 | None |
| TICKET_OPAQUE' | TBA5 | See Section 4.6 |
+-------------------+--------+-----------------+
4.5. TICKET_OPAQUE Notify Payload
The data for the TICKET_OPAQUE Notify payload consists of the Notify
message header, a lifetime field and the ticket itself. The four
octet lifetime field contains the number of seconds until the ticket
expires (encoded as an unsigned integer).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload !C! Reserved ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Protocol ID ! SPI Size = 0 ! Notify Message Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Lifetime !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Ticket ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: TICKET_OPAQUE Notify Payload
4.6. TICKET_OPAQUE' Notify Payload
The data for the TICKET_OPAQUE' Notify payload consists of the Notify
message header, and the ticket itself. Unlike the TICKET_OPAQUE
payload no lifetime value is included in the TICKET_OPAQUE' Notify
payload.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload !C! Reserved ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Protocol ID ! SPI Size = 0 ! Notify Message Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Ticket ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: TICKET_OPAQUE' Notify Payload
4.7. Processing Guidelines for IKE SA Establishment
When a ticket is presented, the gateway needs to obtain the ticket
per value. In case a ticket by reference was provided by the client
the gateway needs to resolve the reference in order to obtain the
ticket by value. In case the client has already provided the ticket
per value it can parses the ticket. In either case, the gateway
needs to process the ticket by value in order to restore the state of
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the old IKE SA, and the client retrieves this state from its local
store. Both peers now create state for the new IKE SA as follows:
o The SA value (transforms etc.) is taken directly from the ticket.
o The sequence numbers are reset to 0.
o The IDi value is obtained from the ticket.
o The IDr value is obtained from the new exchange. The gateway MAY
make policy decisions based on the IDr value encoded in the
ticket.
o The SPI values are created anew, similarly to a regular IKE
exchange. SPI values from the ticket MUST NOT be reused. This
restriction is to avoid problems caused by collisions with other
SPI values used already by the initiator/responder.
The cryptographic material is refreshed based on the ticket and the
nonce values, Ni, and Nr, from the current exchange. A new SKEYSEED
value is derived as follows:
SKEYSEED = prf(SK_d2, Ni | Nr)
where SK_d2 was computed earlier (Section 4.3.1).
The keys are derived as follows, unchanged from IKEv2:
{SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr} =
prf+(SKEYSEED, Ni | Nr | SPIi | SPIr)
where SPIi, SPIr are the SPI values created in the new IKE exchange.
See [RFC4306] for the notation. "prf" is determined from the SA value
in the ticket.
5. Ticket Recommendations
5.1. Ticket Content
When passing a ticket by value to the client, the ticket content MUST
be integrity protected and encrypted. A ticket by reference does not
need to be encrypted, as it does not contain any sensitive material,
such as keying material. However, access to the storage where that
sensitive material is stored MUST be protected so that only
unauthorized access is not allowed. We note that such a ticket is
analogous to the concept of 'stub', as defined in
[I-D.xu-ike-sa-sync], or the concept of a Session ID from TLS.
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When the state is passed by value, the ticket MUST encode at least
the following state from an IKE SA.
o IDi, IDr.
o SPIi, SPIr.
o SAr (the accepted proposal).
o SK_d.
The ticket by value MUST include a key identity field, so that keys
for encryption and authentication can be changed, and when necessary,
algorithms can be replaced.
In addition, the ticket by value and the ticket by reference MUST
contain a protected ticket expiration value that is readable for the
client.
5.2. Ticket Identity and Lifecycle
Each ticket is associated with a single IKE SA. In particular, when
an IKE SA is deleted, the client MUST delete its stored ticket. A
ticket is therefore associated with the tuple (IDi, IDr).
The lifetime of the ticket carried in the N(TICKET_OPAQUE)
notification SHOULD be the minimum of the IKE SA lifetime (per the
gateway's local policy) and its re-authentication time, according to
[RFC4478]. Even if neither of these are enforced by the gateway, a
finite lifetime MUST be specified for the ticket.
The gateway SHOULD set the expiration date for the ticket to a larger
value than the lifetime of the IKE SA. The key that is used to
protect the ticket MUST have a lifetime that is significantly longer
than the lifetime of an IKE SA.
6. IANA Considerations
This document requires a number of IKEv2 notification status types in
Section 4.4, to be registered by IANA. The corresponding registry
was established by IANA.
The document defines a new IKEv2 exchange in Section 4.3. The
corresponding registry was established by IANA.
7. Security Considerations
This section addresses security issues related to the usage of a
ticket.
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7.1. Stolen Tickets
An man-in-the-middle may try to eavesdrop on an exchange to obtain a
ticket by value and use it to establish a session with the IKEv2
responder. This can happen in different ways: by eavesdropping on
the initial communication and copying the ticket when it is granted
and before it is used, or by listening in on a client's use of the
ticket to resume a session. However, since the ticket's contents is
encrypted and the attacker does not know the corresponding secret
key, a stolen ticket cannot be used by an attacker to succesfully
resume a session. An IKEv2 responder MUST use strong encryption and
integrity protection of the ticket to prevent an attacker from
obtaining the ticket's contents, e.g., by using a brute force attack.
Since a ticket by reference does not need to be encrypted. When an
adversary is able to eavesdrop on an exchange, as described in the
previous paragraph, then the ticket by reference may be obtained.
The adversary MUST NOT be able to resolve the ticket via the
reference, i.e., access control MUST be enforced to ensure disclosure
only to authorized entities.
7.2. Forged Tickets
A malicious user could forge or alter a ticket by value in order to
resume a session, to extend its lifetime, to impersonate as another
user, or to gain additional privileges. This attack is not possible
if the content of the ticket by value is protected using a strong
integrity protection algorithm.
In case of a ticket by reference an adversary may attempt to
construct a faked ticket by reference to point to state information
stored by the IKEv2 responder. This attack will fail because the
adversary is not in possession of the keying material associated with
the IKEv2 SA.
7.3. Denial of Service Attacks
An adversary could generate and send a large number of tickets by
value to a gateway for verification. To minimize the possibility of
such denial of service, ticket verification should be lightweight
(e.g., using efficient symmetric key cryptographic algorithms).
When an adversary chooses to send a large number of tickets by value
then this may lead to an amplification attack as the IKEv2 is forced
to resolve the reference to a ticket in order to determine that the
adversary is not in possession of the keying material corresponding
to the stored state or that the reference is void. To minimize this
attack the protocol to resolve the reference should be as lightweight
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as possible. and should not generate a large number of messages.
7.4. Key Management for Tickets By Value
A full description of the management of the keys used to protect the
ticket by value is beyond the scope of this document. A list of
RECOMMENDED practices is given below.
o The keys should be generated securely following the randomness
recommendations in [RFC4086].
o The keys and cryptographic protection algorithms should be at
least 128 bits in strength.
o The keys should not be used for any other purpose than generating
and verifying tickets.
o The keys should be changed regularly.
o The keys should be changed if the ticket format or cryptographic
protection algorithms change.
7.5. Ticket Lifetime
An IKEv2 responder controls the validity period of the state
information by attaching a lifetime to a ticket. The chosen lifetime
is based on the operational and security requirements of the
environment in which this IKEv2 extension is deployed. The responder
provides information about the ticket lifetime to the IKEv2
initiator, allowing it to manage its tickets.
7.6. Ticket by Value Format
Great care must be taken when defining a ticket format such that the
requirements outlined in Section 5.1 are met. In particular, if
confidential information, such as a secret key, is transferred to the
client it MUST be done using channel security to prevent attackers
from obtaining or modifying the ticket. Also, the ticket by value
MUST have its integrity and confidentiality protected with strong
cryptographic techniques to prevent a breach in the security of the
system.
7.7. Identity Privacy, Anonymity, and Unlinkability
Since opaque state information is passed around between the IKEv2
initiator and the IKEv2 responder it is important that leakage of
information, such as the identities of an IKEv2 initiator and a
responder, MUST be avoided (e.g., with the help of encryption. Thus,
it prevents the disclosure of potentially sensitive information.
When an IKEv2 initiator presents a ticket as part of the
IKE_SESSION_RESUME exchange, confidentiality is not provided for the
exchange. There is thereby the possibility for an on-path adversary
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to observe multiple exchange handshakes where the same state
information is used and therefore to conclude that they belong to the
same communication end points.
This document therefore envisions that the ticket is presented to the
IKEv2 responder only once; multiple usage of the ticket is not
provided.
7.8. Replay Protection in the IKE_SESSION_RESUME Exchange
A major design goal of this protocol extension has been the two-
message exchange for session resumption. There is a tradeoff between
this abbreviated exchange and replay protection. It is RECOMMENDED
that an IKEv2 responder should cache tickets, and reject replayed
ones. However, some gateways may not do that in order to reduce
state size. An adversary may attempt to replay a ticket. To
mitigate these risks a client may be required by the gateway to show
that it knows the ticket's secret, before any state is committed on
the gateway side. Note that this is a stronger guarantee than the
regular IKE cookie mechanism, which only shows IP return routability
of the client. This is enabled by including the cookie in the
protected portion of the message.
For performance reasons, the cookie mechanism is optional, and
invoked by the gateway only when it suspects that it is the subject
of a denial-of-service attack.
In any case, a ticket replayed by an adversary only causes partial
IKE state to be created on the gateway. The IKE exchange cannot be
completed and an IKE SA cannot be created unless the client knows the
ticket's secret values.
8. Acknowledgements
We would like to thank Paul Hoffman, Pasi Eronen, Florian Tegeler,
Stephen Kent, Sean Shen, Xiaoming Fu, Stjepan Gros, Dan Harkins, Russ
Housely, Yoav Nir and Tero Kivinen for their comments. We would to
particularly thank Florian Tegeler and Stjepan Gros for their help
with their implementation efforts and Florian Tegeler for his formal
verification using the CASPER tool set.
We would furthermore like to thank the authors of
[I-D.xu-ike-sa-sync] (Yan Xu, Peny Yang, Yuanchen Ma, Hui Deng and Ke
Xu) for their input on the stub concept.
We would like to thank Hui Deng, Tero Kivinen, Peny Yang, Ahmad
Muhanna and Stephen Kent for their feedback regarding the ticket by
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reference concept.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
9.2. Informative References
[I-D.rescorla-stateless-tokens]
Rescorla, E., "How to Implement Secure (Mostly) Stateless
Tokens", draft-rescorla-stateless-tokens-01 (work in
progress), March 2007.
[I-D.xu-ike-sa-sync]
Xu, Y., Yang, P., Ma, Y., Deng, H., and H. Deng, "IKEv2 SA
Synchronization for session resumption",
draft-xu-ike-sa-sync-01 (work in progress), October 2008.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4478] Nir, Y., "Repeated Authentication in Internet Key Exchange
(IKEv2) Protocol", RFC 4478, April 2006.
[RFC4555] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, June 2006.
[RFC4718] Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
Implementation Guidelines", RFC 4718, October 2006.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
Appendix A. Ticket Format
This document does not specify a mandatory-to-implement or a
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mandatory-to-use ticket format. The format described in the sub-
sections are RECOMMENDED.
A.1. Recommended Ticket by Value Format
struct {
[authenticated] struct {
octet format_version; // 1 for this version of the protocol
octet reserved[3]; // sent as 0, ignored by receiver.
octet key_id[8]; // arbitrary byte string
opaque IV[0..255]; // actual length (possibly 0) depends
// on the encryption algorithm
[encrypted] struct {
opaque IDi, IDr; // the full payloads
octet SPIi[8], SPIr[8];
opaque SA; // the full SAr payload
octet SK_d[0..255]; // actual length depends on SA value
int32 expiration; // an absolute time value, seconds
// since Jan. 1, 1970
} ikev2_state;
} protected_part;
opaque MAC[0..255]; // the length (possibly 0) depends
// on the integrity algorithm
} ticket;
Note that the key defined by "key_id" determines the encryption and
authentication algorithms used for this ticket. Those algorithms are
unrelated to the transforms defined by the SA payload.
The reader is referred to [I-D.rescorla-stateless-tokens] that
recommends a similar (but not identical) ticket format, and discusses
related security considerations in depth.
A.2. Recommended Ticket by Reference Format
For implementations that prefer to pass a reference to IKE state in
the ticket, rather than the state itself, we RECOMMEND the following
format:
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struct {
[authenticated] struct {
octet format_version; // 1 for this version of the protocol
octet reserved[3]; // sent as 0, ignored by receiver.
octet key_id[8]; // arbitrary byte string
struct {
opaque state_ref; // reference to IKE state
int32 expiration; // an absolute time value, seconds
// since Jan. 1, 1970
} ikev2_state_ref;
} protected_part;
opaque MAC[0..255]; // the length depends
// on the integrity algorithm
} ticket;
Appendix B. Change Log
B.1. -02
Added a new TICKET_OPAQUE' payload that does not have a lifetime
field included.
Removed the lifetime usage from the IKE_SESSION_RESUME exchange
(utilizing the TICKET_OPAQUE') when presenting the ticket to the
gateway.
Removed IDi payloads from the IKE_SESSION_RESUME exchange.
Clarified that IPsec child SAs would be deleted once the old IKE SA
gets deleted as well.
Clarified the behavior of SPI and sequence number usage.
B.2. -01
Addressed issue#75, see
http://tools.ietf.org/wg/ipsecme/trac/ticket/75. This included
changes throughout the document to ensure that the ticket may contain
a reference or a value.
B.3. -00
Resubmitted document as a WG item.
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B.4. -01
Added reference to [I-D.xu-ike-sa-sync]
Included recommended ticket format into the appendix
Various editorial improvements within the document
B.5. -00
Issued a -00 version for the IPSECME working group. Reflected
discussions at IETF#72 regarding the strict focus on session
resumption. Consequently, text about failover was removed.
B.6. -04
Editorial fixes; references cleaned up; updated author's contact
address
B.7. -03
Removed counter mechanism. Added an optional anti-DoS mechanism,
based on IKEv2 cookies (removed previous discussion of cookies).
Clarified that gateways may support reallocation of same IP address,
if provided by network. Proposed a solution outline to the problem
of key exchange for the keys that protect tickets. Added fields to
the ticket to enable interoperability. Removed incorrect MOBIKE
notification.
B.8. -02
Clarifications on generation of SPI values, on the ticket's lifetime
and on the integrity protection of the anti-replay counter.
Eliminated redundant SPIs from the notification payloads.
B.9. -01
Editorial review. Removed 24-hour limitation on ticket lifetime,
lifetime is up to local policy.
B.10. -00
Initial version. This draft is a selective merge of
draft-sheffer-ike-session-resumption-00 and
draft-dondeti-ipsec-failover-sol-00.
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Authors' Addresses
Yaron Sheffer
Check Point Software Technologies Ltd.
5 Hasolelim St.
Tel Aviv 67897
Israel
Email: yaronf@checkpoint.com
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Lakshminath Dondeti
QUALCOMM, Inc.
5775 Morehouse Dr
San Diego, CA
USA
Phone: +1 858-845-1267
Email: ldondeti@qualcomm.com
Vidya Narayanan
QUALCOMM, Inc.
5775 Morehouse Dr
San Diego, CA
USA
Phone: +1 858-845-2483
Email: vidyan@qualcomm.com
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