A Childless Initiation of the Internet Key Exchange Version 2 (IKEv2) Security Association (SA)
An IKEv2 extension that sets up and authenticates a security association without also creating a Child SA.
Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A Childless Initiation of the Internet Key Exchange Version 2 (IKEv2) Security Association (SA)", RFC 6023, DOI 10.17487/RFC6023, October 2010, <https://www.rfc-editor.org/info/rfc6023>.
@misc{rfc6023,
series = {Request for Comments},
number = 6023,
howpublished = {RFC 6023},
publisher = {RFC Editor},
doi = {10.17487/RFC6023},
url = {https://www.rfc-editor.org/info/rfc6023},
author = {Y. Nir and H. Tschofenig and H. Deng and R. Singh},
title = {{A Childless Initiation of the Internet Key Exchange Version 2 (IKEv2) Security Association (SA)}},
pagetotal = 7,
year = 2010,
month = oct,
} RFC text
Contents
Independent Submission Y. Nir
Request for Comments: 6023 Check Point
Category: Experimental H. Tschofenig
ISSN: 2070-1721 NSN
H. Deng
China Mobile
R. Singh
Cisco
October 2010
A Childless Initiation of
the Internet Key Exchange Version 2 (IKEv2) Security Association (SA)
Abstract
This document describes an extension to the Internet Key Exchange
version 2 (IKEv2) protocol that allows an IKEv2 Security Association
(SA) to be created and authenticated without generating a Child SA.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6023.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
1. Introduction
IKEv2, as specified in [RFC5996], requires that the IKE_AUTH exchange
try to create a Child SA along with the IKEv2 SA. This requirement
is sometimes inconvenient or superfluous, as some implementations
need to use IKEv2 for authentication only, while others would like to
set up the IKEv2 SA before there is any actual traffic to protect.
The extension described in this document allows the creation of an
IKEv2 SA without also attempting to create a Child SA. The terms
IKEv2, IKEv2 SA, and Child SA and the various IKEv2 exchanges are
defined in [RFC5996]
An IKEv2 SA without any Child SA is not a fruitless endeavor. Even
without Child SAs, an IKEv2 SA allows:
o Checking the liveness status of the peer via liveness checks.
o Quickly setting up Child SAs without public key operations and
without user interaction.
o Authentication of the peer.
o Detection of NAT boxes between two hosts on the Internet.
1.1. Conventions Used in This Document
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].
2. Usage Scenarios
Several scenarios motivated this proposal:
o Interactive remote access VPN: the user tells the client to
"connect", which may involve interactive authentication. There is
still no traffic, but some may come later. Since there is no
traffic, it is impossible for the gateway to know what selectors
to use (how to narrow down the client's proposal).
o Location-aware security, as in [SecureBeacon]. The user is
roaming between trusted and untrusted networks. While in an
untrusted network, all traffic should be encrypted, but on the
trusted network, only the IKEv2 SA needs to be maintained.
o An IKEv2 SA may be needed between peers even when there is not
IPsec traffic. Such IKEv2 peers use liveness checks, and report
to the administrator the status of the "VPN links".
o IKEv2 may be used on some physically secure links, where
authentication is necessary but traffic protection is not. An
example of this is the Passive Optical Network (PON) links as
described in [3GPP.33.820].
o Childless IKEv2 can be used for [RFC5106] where we use IKEv2 as a
method for user authentication.
o A node receiving IPsec traffic with an unrecognized Security
Parameter Index (SPI) should send an INVALID_SPI notification. If
this traffic comes from a peer, which it recognizes based on its
IP address, then this node may set up an IKEv2 SA so as to be able
to send the notification in a protected INFORMATIONAL exchange.
o A future extension may have IKEv2 SAs used for generating keying
material for applications, without ever requiring Child SAs. This
is similar to what [RFC5705] is doing in Transport Layer Security
(TLS).
In some of these cases, it may be possible to create a dummy Child SA
and then remove it, but this creates undesirable side effects and
race conditions. Moreover, the IKEv2 peer might see the deletion of
the Child SA as a reason to delete the IKEv2 SA.
3. Protocol Outline
The decision of whether or not to support an IKE_AUTH exchange
without the piggy-backed Child SA negotiation is ultimately up to the
responder. A supporting responder MUST include the Notify payload,
described in Section 4, within the IKE_SA_INIT response.
A supporting initiator MAY send the modified IKE_AUTH request,
described in Section 5, if the notification was included in the
IKE_SA_INIT response. The initiator MUST NOT send the modified
IKE_AUTH request if the notification was not present.
A supporting responder that has advertised support by including the
notification in the IKE_SA_INIT response MUST process a modified
IKE_AUTH request, and MUST reply with a modified IKE_AUTH response.
Such a responder MUST NOT reply with a modified IKE_AUTH response if
the initiator did not send a modified IKE_AUTH request.
A supporting responder that has been configured not to support this
extension to the protocol MUST behave as the same as if it didn't
support this extension. It MUST NOT advertise the capability with a
notification, and it SHOULD reply with an INVALID_SYNTAX Notify
payload if the client sends an IKE_AUTH request that is modified as
described in Section 5.
4. CHILDLESS_IKEV2_SUPPORTED Notification
The Notify payload is as described in [RFC5996]
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 ! Childless Notify Message Type !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Protocol ID (1 octet) MUST be 1, as this message is related to an
IKEv2 SA.
o SPI Size (1 octet) MUST be zero, in conformance with section 3.10
of [RFC5996].
o Childless Notify Message Type (2 octets) - MUST be 16418, the
value assigned for CHILDLESS_IKEV2_SUPPORTED.
5. Modified IKE_AUTH Exchange
For brevity, only the Extensible Authentication Protocol (EAP)
version of an AUTH exchange will be presented here. The non-EAP
version is very similar. The figures below are based on Appendix C.3
of [RFC5996].
first request --> IDi,
[N(INITIAL_CONTACT)],
[[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
[IDr],
[CP(CFG_REQUEST)],
[V+][N+]
first response <-- IDr, [CERT+], AUTH,
EAP,
[V+][N+]
/ --> EAP
repeat 1..N times |
\ <-- EAP
last request --> AUTH
last response <-- AUTH,
[CP(CFG_REPLY)],
[V+][N+]
Note what is missing:
o The optional notifications: IPCOMP_SUPPORTED, USE_TRANSPORT_MODE,
ESP_TFC_PADDING_NOT_SUPPORTED, and NON_FIRST_FRAGMENTS_ALSO.
o The SA payload.
o The traffic selector payloads.
o Any notification, extension payload or VendorID that has to do
with Child SA negotiation.
6. Security Considerations
This protocol variation inherits all the security properties of
regular IKEv2 as described in [RFC5996].
The new notification carried in the initial exchange advertises the
capability, and cannot be forged or added by an adversary without
being detected, because the response to the initial exchange is
authenticated with the AUTH payload of the IKE_AUTH exchange.
Furthermore, both peers have to be configured to use this variation
of the exchange in order for the responder to accept a childless
proposal from the initiator.
7. IANA Considerations
IANA has assigned a notify message type from the "IKEv2 Notify
Message Types" registry with the name "CHILDLESS_IKEV2_SUPPORTED" and
the value "16418".
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
8.2. Informative References
[3GPP.33.820] 3GPP, "Security of H(e)NB", 3GPP TR 33.820 8.0.0,
March 2009.
[RFC5106] Tschofenig, H., Kroeselberg, D., Pashalidis, A.,
Ohba, Y., and F. Bersani, "The Extensible
Authentication Protocol-Internet Key Exchange
Protocol version 2 (EAP-IKEv2) Method", RFC 5106,
February 2008.
[RFC5705] Rescorla, E., "Keying Material Exporters for
Transport Layer Security (TLS)", RFC 5705,
March 2010.
[SecureBeacon] Sheffer, Y. and Y. Nir, "Secure Beacon: Securely
Detecting a Trusted Network", Work in Progress,
June 2009.
Authors' Addresses
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 67897
Israel
EMail: ynir@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
Hui Deng
China Mobile
53A,Xibianmennei Ave.
Xuanwu District
Beijing 100053
China
EMail: denghui02@gmail.com
Rajeshwar Singh Jenwar
Cisco Systems, Inc.
O'Shaugnessy Road
Bangalore, Karnataka 560025
India
Phone: +91 80 4103 3563
EMail: rsj@cisco.com
AI ELI5
On this page
§1Why a SA Without a Child SA
IKEv2, as specified in RFC 5996, requires that the IKE_AUTH exchange try to create a Child SA along with the IKEv2 SA. Some implementations only need IKEv2 for authentication, and others want to set up the IKEv2 SA before there is any traffic to protect.
The RFC argues that an IKEv2 SA without a Child SA is still useful. It allows liveness checks of the peer, quick setup of Child SAs later without public key operations or user interaction, authentication of the peer, and detection of NAT boxes between two hosts.
It is like showing your ID at the door first and ordering food later. Showing your ID is useful even if you have not ordered anything yet.
§2Usage Scenarios
The RFC lists several scenarios that motivated the extension:
- Interactive remote access VPN, where the user says "connect" but there is no traffic yet, so the gateway cannot know which selectors to use.
- Location-aware security, where all traffic is encrypted on an untrusted network but only the IKEv2 SA needs to be kept on a trusted one.
- IKEv2 SAs kept only for liveness checks and for reporting the status of VPN links.
- Physically secure links, such as Passive Optical Network links, where authentication is needed but traffic protection is not.
- Use of IKEv2 for user authentication in RFC 5106.
- Sending an INVALID_SPI notification in a protected INFORMATIONAL exchange.
- A possible future extension that uses IKEv2 SAs to generate keying material for applications.
In some of these cases a dummy Child SA could be created and then removed, but the RFC says this causes undesirable side effects and race conditions. The peer might also treat the deletion of the Child SA as a reason to delete the IKEv2 SA.
Sometimes you only need to know your friend is really who they say, or that they are still there. You do not need to start a game yet.
§3How Support Is Negotiated
The responder decides whether to allow an IKE_AUTH exchange without a piggy-backed Child SA negotiation. A supporting responder MUST include the notification in its IKE_SA_INIT response.
An initiator MAY send the modified IKE_AUTH request only if it saw that notification, and MUST NOT send it otherwise. A responder that advertised support MUST process the modified request and reply with a modified response, and MUST NOT send a modified response if the initiator did not send a modified request.
A responder configured not to support the extension must act as if it did not implement it. It MUST NOT advertise the capability, and it SHOULD reply with an INVALID_SYNTAX Notify payload if a client sends a modified request anyway.
The host has to put up a sign saying "ID check only is fine here." Guests may only use that shortcut if they saw the sign.
§4The Notification and the Modified Exchange
The CHILDLESS_IKEV2_SUPPORTED notification uses the ordinary Notify payload from RFC 5996. Its Protocol ID MUST be 1 because the message relates to an IKEv2 SA, its SPI Size MUST be zero, and its 2-octet message type MUST be 16418.
The modified IKE_AUTH exchange is shown in its EAP form; the non-EAP form is very similar. Compared with a normal exchange, the following are missing:
- The optional notifications IPCOMP_SUPPORTED, USE_TRANSPORT_MODE, ESP_TFC_PADDING_NOT_SUPPORTED and NON_FIRST_FRAGMENTS_ALSO.
- The SA payload.
- The traffic selector payloads.
- Any notification, extension payload or VendorID that has to do with Child SA negotiation.
The special form is the normal one with the parts about setting up the chat torn out. Only the ID-checking parts stay.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
NEXT-PAYLOAD | Next Payload | 1 byte | Type of the next payload. | |
CRITICAL | C | 1 bits | Critical bit. | |
RESERVED | RESERVED | 7 bits | Reserved. | |
PAYLOAD-LENGTH | Payload Length | 2 bytes | Length of the payload. | |
PROTOCOL-ID | Protocol ID | 1 byte | 1 | MUST be 1 (IKEv2 SA). |
SPI-SIZE | SPI Size | 1 byte | 0 | MUST be zero. |
MESSAGE-TYPE | Childless Notify Message Type | 2 bytes | 16418 | MUST be 16418. |
§5Security and IANA
The variation inherits all the security properties of regular IKEv2. The new notification cannot be forged or added by an adversary without detection, because the response to the initial exchange is authenticated by the AUTH payload of the IKE_AUTH exchange. Both peers also have to be configured to use the variation before the responder accepts a childless proposal.
IANA assigned the notify message type name CHILDLESS_IKEV2_SUPPORTED with the value 16418 in the "IKEv2 Notify Message Types" registry.
The sign cannot be swapped by a sneaky person without getting caught, because the later ID check covers it.