Internet-Draft IKEv2 support for Child SA PFS policy no March 2024
Wouters Expires 4 September 2024 [Page]
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Standards Track
P. Wouters

IKEv2 support for Child SA PFS policy notification


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 for the Perfect Forward Secrecy (PFS) and Key Exchange (KE) method setting of the initial Child SA.

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This Internet-Draft will expire on 4 September 2024.

Table of Contents

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 eying 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 do PFS for Child SAs, or to only use the same KE methods for both the IKE SA and all Child SAs. But it is currently not possible to know this when the initial Child SA is established. The peers find out about this problem only at the next 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 Initial Exchanges

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      !
~ Key Eexchange methods list (KE list)                          ~

The Key Exchange method list (KE list) contains KE values from the IANA "Transform Type 4 - Key Exchange Method Transform IDs" registry. Each entry is two octets. If the KE list payload is a not a multiple of two, the entire payload MUST be ignored.

3. Usage of the CHILD_PFS_INFO Notify

The CHILD_PFS_INFO Notify payload is only sent during the IKE_AUTH Exchange that also contains the Key Exchange Method (KE) transform type. This is typically the last (or only) IKE_AUTH Exchange. When creating additional Child SA's using the CREATE_CHILD_SA Exchange, the Exchange already contains an optional KE payload and list of Key Exchange Method Transform IDs. When rekeying such existing additional Child SA, a proper KE value (NONE or otherwise) is already learned from the initial establishment of the Child SA during the initial CREATE_CHILD_SA Exchange and no CHILD_PFS_INFO is required.

If PFS is completely disallowed, the KE list contains only the value 0 (NONE). If PFS is optional but allowed, the KE list contains the value 0 (NONE) as well as the other accepted values, eg 19 for "256-bit random ECP group".

Unknown KE list entries MUST be ignored.

A peer MUST return NO_PROPOSAL_CHOSEN if the CHILD_PFS_NOTIFY values received does not include an acceptable value that can be used for the KE payload and Transform Type 4 during a Child SA rekey. This ensures that incomptabile peers will immediately fail the initial negotiation.

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.

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

The Libreswan Project
An initial IKE implementation using the Private Use value 40961 for the Notify payload
Level of maturity:
Implements the draft's example reasons
Implementation experience:
Libreswan Development:

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

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

8.2. Informative References

Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, DOI 10.17487/RFC6982, , <>.
Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, , <>.

Author's Address

Paul Wouters