Network P. Wouters
Internet-Draft Aiven
Intended status: Standards Track 3 March 2025
Expires: 4 September 2025
IKEv2 support for Child SA PFS policy notification
draft-pwouters-ipsecme-child-pfs-info-01
Abstract
This document defines the CHILD_PFS_INFO Notify Message Status Type
Payload for the Internet Key Exchange Protocol Version 2 (IKEv2) to
support exchanging the policy settings for the Perfect Forward
Secrecy (PFS) and which Key Exchange (KE) method(s) setting of the
initial Child SA that are expected to be used during Child SA rekey.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Payload Format . . . . . . . . . . . . . . . . . . . . . 3
2. CHILD_PFS_INFO Notify Status Message Payload . . . . . . . . 3
3. Usage of the CHILD_PFS_INFO Notify . . . . . . . . . . . . . 4
4. Operational Considerations . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 6
6.1. Libreswan . . . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The IKEv2 [RFC7296] protocol uses the Keying Exchange (KE) payload,
formerly known as the Diffie-Hellman Group Transform payload to
create an ephemeral IKE connection. During an IKE rekey, a new KE
payload is used to create a new ephemeral IKE connection, resulting
in Perfect Forward Secrecy (PFS).
A Child SA optionally uses its own PFS settings by including its own
KE payload and list of acceptable Keying Exchange methods. During
Child SA rekeys, KE payloads of acceptable Keying Exchange methods
are exchanged to create PFS.
The Initial Exchanges establish both an IKE SA and a Child SA using
the Keying Exchange method negotiated for the IKE SA. Thus, after
the Initial Exchange, the peers are not aware of each others PFS
requirements for the existing Child SA. It is common practise to
either not perform PFS for Child SAs, or to only perform the same KE
methods for both the IKE SA and all Child SAs. The situation is even
more complex when Post-Quantum Key Exchange methods are used htat
might contain multiple KE payloads, which might not all be desired
for rekeying the Initial Child SA. It is currently not possible to
know the desired PFS configuration for rekey of the initial Child SA.
The peers find out about this problem only at the first Child SA
rekey, which is typically 1 to 8 hours later.
This document introduces the CHILD_PFS_INFO Notify payload to
exchange this information during the estaliblisment of the initial
Child SA.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Payload Format
All multi-octet fields representing integers are laid out in big
endian order (also known as "most significant byte first", or
"network byte order").
2. CHILD_PFS_INFO Notify Status Message Payload
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 ! Notify Message Type !
+---------------+---------------+-------------------------------+
! Transform Type| REQUIRED? | Child Key Exchange Method |
+---------------+-----------------------------------------------+
~ : : ~
+-------------------------------+-------------------------------+
* The Critical Flag (C) MUST NOT be set.
* Protocol ID (1 octet) - MUST be 0. MUST be ignored if not 0.
* SPI Size (1 octet) - MUST be 0. MUST be ignored if not 0.
* Notify Status Message Type (2 octets) - set to [TBD1]
* list of one or more Child Key Exchange Methods
Each allowed or mandatory Child Key Exchange Method, and its
Transform Type for which it is valid. These differ based on the Key
Exchange it is used in.
Each entry is four octets. If the KE list payload is a not a
multiple of four, the entire payload MUST be ignored.
The REQUIRED fields contains 0 for ALLOWED and 1 for MANDATORY.
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3. Usage of the CHILD_PFS_INFO Notify
The CHILD_PFS_INFO Notify payload is sent during the (last) IKE_AUTH
exchange.
Any peer MAY send the CHILD_PFS_INFO Notify payload to inform the
peer of its acceptable PFS settings. If a peer receives no
CHILD_PFS_INFO during the IKE_AUTH exchange, it MUST continue without
any error condition. This might result in a NO_PROPOSAL_CHOSEN error
during rekey time later when the initial Child SA fails to rekey.
When creating additional Child SA's using the CREATE_CHILD_SA
Exchange, the Exchange already negotiations all the required KEs and
the results can be remembered to apply to future rekey events for
this Child SA and CHILD_PFS_INFO MUST NOT be used.
If PFS is completely disallowed for the initial Child SA, the KE list
contains only the Transform Type with value 4, the REQUIRED set to 1
(MANDATORY) and the Child Key Exchange Method set to the value 0
(NONE).
If PFS is optional for the initial Child SA but allowed, the KE list
contains at least one entry for Transform Type with value 4, with one
value (e.g. 19 for "256-bit random ECP group") with REQUIRED set to
OPTIONAL (0).
If PFS is mandatory for the initial Child SA, the KE list contains at
least one entry for Transform Type with value 4, with one value (e.g.
19 for "256-bit random ECP group") with REQUIRED set to MANDATORY
(1).
To support PFS requiring additional Child Key Exchange Methods,
additional allowed Child Key Exchange Methods for Additional Key
Exchange Transform Types are specified that can be set to MANDATORY
or OPTIONAL. Every Transform Type ID with Key Exchange Method entry
in the list of Child Key Exchange Methods MUST have been used during
the initial IKE SA / Child SA establishment and MUST NOT contain the
value NONE (0).
Note that the Additional Key Exchange method order MUST remain the
same, but the specific Transform Type number in the range 6-12 might
be different if an Additional Key Exchange method was used
specifically for the IKE SA but not desired for the initial Child SA
rekey.
If the Child Key Exchange Method list contains any values (known or
unkown) that were not used during the initial IKE SA / Child SA
establishment, or any values which it is unwilling to use for PFS, it
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MUST fail the Child SA. This means an Initial Responder MUST return
NO_PROPOSAL_CHOSEN (and maintain the IKE SA). An Initial Initiator
MUST immediately send a DELETE notify for the Child SA (not the IKE
SA). This behaviour ensures that incomptabile peers will immediately
fail the initial Child SA and won't only later on during rekey fail
the Child SA.
4. Operational Considerations
This document is a result of Operational Considers that have shown
peers can run into broken IPsec connections at rekey time. These are
not obvious to the administrators as these usually do not sit around
for a few hours to wait and see if the rekey process worked
successfully. The CHILD_PFS_INFO results in immediate negotiation
failure that can be repaired before taking the IPsec connection into
production.
During rekey, the cryptographic strength of a rekeyed Child SA SHOULD
remain at least as strong as the Child SA being rekeyed. In practise
this means the negotiated algorithms remain the same. But some
deployments use stronger settings for the IKE SA compared to its
Child SAs, which means technically the initial Child SA uses a
stronger KE method than for rekeys. The CHILD_PFS_INFO payload
exposes such settings to the peers during the Initial Exchanges, and
peers can at that time accept or reject the child proposal. Once the
Initial Child SA containing CHILD_PFS_INFO is accepted, rekey
proposals are guaranteed to be acceptable to both parties. For
example, an IKE SA could be using KE method 15 (3072-bit MODP) and
specify in the CHILD_PFS_INFO that it accepts KE method 14 (2048-bit
MODP) for this Child SA rekey.
Deployments with a large number of Child SAs often use no PFS for
their Child SAs. It is computationally much cheaper to establish the
large number of Child SAs and then immediately rekey the IKE SA.
This method can also be used if the peer's Child SA KE methods are
unacceptable. If both peers accept the KE method of 0 (NONE), it can
decide to rekey the Child SA without PFS and immediately rekey the
IKE SA using its accepted KE method.
5. Security Considerations
This document introduces no new security considerations, as it only
causes an increased awareness of peer capabilities with respect to KE
methods.
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6. Implementation Status
[Note to RFC Editor: Please remove this section and the reference to
[RFC6982] before publication.]
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC7942], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
Authors are requested to add a note to the RFC Editor at the top of
this section, advising the Editor to remove the entire section before
publication, as well as the reference to [RFC7942].
6.1. Libreswan
Organization: The Libreswan Project
Name: https://libreswan.org/
Description: An initial IKE implementation using the Private Use
value 40961 for the Notify payload
Level of maturity: Beta
Coverage: Implements the draft's example reasons
Licensing: GPLv2
Implementation experience: TBD
Contact: Libreswan Development: swan-dev@libreswan.org
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7. IANA Considerations
This document defines one new IKEv2 Notify Message Type payload for
the IANA "IKEv2 Notify Message Types - Status Types" registry.
Value Notify Type Messages - Status Types Reference
----- ------------------------------ ---------------
[TBD1] CHILD_PFS_INFO [this document]
Figure 1
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
DOI 10.17487/RFC6982, July 2013,
<https://www.rfc-editor.org/info/rfc6982>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
Author's Address
Paul Wouters
Aiven
Email: paul.wouters@aiven.io
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