IKEv2 Optional SA&TS Payloads in Child Exchange

The Internet Key Exchange protocol version 2 (IKEv2) specified in is used in the IP Security (IPSec) architecture for the purposes of Security Association (SA) parameters negotiation and authenticated key exchange. The protocol uses UDP as the transport for its messages, which size varies from less than one hundred bytes to several kilobytes. According to , the secret keys used by IKE/IPSec SAs should be used only for a limited amount of time and to protect a limited amount of data. When the lifetime of an SA expires or is about to expire, the peers can rekey the SA to reestablish a new SA to take the place of the old one. For security gateways/ePDG in 4G networks and cRAN/Cloud in 5G networks, they will support more than 100,000 IKE/IPSEC tunnels. So on an average, for every second there may be hundreds or thousands of IKE SAs and Child SAs that are rekeying. This takes huge amount of bandwidth, packet fragmentation and more processing resources. For these devices, these problems can be solved by introducing the solution described in this document. This is also useful in Internet of Things (IoT) devices which utilizing lower power consumption technology. For these devices, reducing the length of IKE/Child SA rekeying messages can save the bandwidth consumption. At the same time, it can also save the computing processes by less payload are included. Most devices don’t prefer to change cryptographic suites frequently. By taking this advantage the SA and TS payloads can be made optional at the time of rekeying IKE SAs and Child SAs. In such situation, only a new SPI value is needed to create the new IKE SA and Child SA. So a new Notify payload which contains the needed SPI value can be sent instead of the SA and TS payloads. In case of rekeying IKE SAs, the SA payloads can be optimized if there is no change of cryptographic suites. In case of rekeying Child SAs, the SA and TS payloads can be optimized if there is no change of cryptographic suites and ACL configuration.

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 when, and only when, they appear in all capitals, as shown here.

This section provides protocol details and contains the normative parts. Before using this new optimization, the IPSec implementation who supports it has to know that the peer also supports it. Without the support on both sides, the optimized rekeying messages sent by one peer may be unrecognizable for the other peer. To prevent this failure from happening, the first step is to negotiate the support of this optimization between the two peers. There are two specific rekeying SAs cases: rekeying IKE SAs and rekeying Child SAs. After the negotiation, the initiator can optimize the rekeying message payloads in both cases. In other words, once the negotiation of support for optimizing payloads at rekeying IKE SAs and Child SAs is complete, both IKE SAs and Child SAs rekeying are supported by the two sides. The responder can react based on the received rekeying message.

The initiator indicates its support for optimizing payloads at rekeying IKE SAs and Child SAs by including a Notify payload of type MINIMAL_REKEY_SUPPORTED in the IKE_AUTH request message. By observing the MINIMAL_REKEY_SUPPORTED notification in the received message, the responder can deduce the initiator’s support of this extension. If the responder also supports this extension, it includes the MINIMAL_REKEY_SUPPORTED notification in the corresponding response message. After receiving the response message, the initiator can also know the support of this extension of the responder side. The IKE_AUTH message exchange in this case is shown below:

<-- HDR, SK {IDr, [CERT,] AUTH, SAr2, TSi, TSr, N(MINIMAL_REKEY_SUPPORTED)} ]]> If the responder doesn’t support this extension, it MUST ignore the MINIMAL_REKEY_SUPPORTED notification sent by the initiator and MUST NOT respond error to the initiator. With no MINIMAL_REKEY_SUPPORTED notification in the response message, the initiator can deduce that the responder doesn’t support this extension. In this case, the IKE SAs and Child SAs rekeyings happen as the usual way without the optimizations defined in this document. The IKE_AUTH message exchange in this case is shown below:

<-- HDR, SK {IDr, [CERT,] AUTH, SAr2, TSi, TSr} ]]>

The payload optimization at rekeying IKE SAs MUST NOT be used unless both peers have indicated their support of this extension by using the negotiation method described in . At the time of rekeying an IKE SA, when the initiator determines there is no change on its cryptographic suites since this IKE SA was created or last rekeyed, it MUST send the REKEY_OPTIMIZED notification payload instead of the SA payloads in the rekeying request message. In this REKEY_OPTIMIZED notification, it contains the initiator’s new Security Parameter Index (SPI) used for creating the new IKE SA. After receiving the initiator’s rekeying request message with the REKEY_OPTIMIZED notification and no SA payloads, the responder knows that the initiator wants to optimize the rekeying payload. Then when it determines that there is also no change in its cryptographic suites, the responder MUST send the rekeying respond message to the initiator with the REKEY_OPTIMIZED notification payload instead of the SA payloads. In this REKEY_OPTIMIZED notification, it contains the responder’s new SPI used for creating the new IKE SA. According to the initiator’s new SPI and the responder’s new SPI, the initiator and the responder can rekey the IKE SA on both sides. The CREATE_CHILD_SA message exchange in this case is shown below:

<-- HDR, SK {N(REKEY_OPTIMIZED), Nr, KEr} ]]> The initiator sends a REKEY_OPTIMIZED notification payload, a Nonce payload and a Diffie-Hellman value in the KEi payload. A new initiator SPI is supplied in the SPI field of the REKEY_OPTIMIZED notification payload. These messages also follow the original Perfect Forwarding Secrecy (PFS) with the signature and encryption algorithms used as last message. The responder replies (using the same Message ID to respond) with a REKEY_OPTIMIZED notification payload, a Nonce payload and a Diffie-Hellman value in the KEr payload. A new responder SPI is supplied in the SPI field of the REKEY_OPTIMIZED notification payload. This REKEY_OPTIMIZED notification MUST be included in a CREATE_CHILD_SA exchange message when there is no SA payloads included. When the REKEY_OPTIMIZED notification payload is included, the SA payload MUST NOT be included.

The payload optimization at rekeying Child SAs MUST NOT be used unless both peers have indicated their support of this extension by using the negotiation method described in . At the time of rekeying a Child SA, when the initiator determines there is no change in its cryptographic suites and ACL configuration since this Child SA was created or last rekeyed, it MUST send the REKEY_OPTIMIZED notification payload instead of the SA and TS payloads in the rekeying request message. In this REKEY_OPTIMIZED notification, it contains the initiator’s new Security Parameter Index (SPI) used for creating the new Child SA. After receiving the initiator’s rekeying request message with the REKEY_OPTIMIZED notification and no SA and TS payloads, the responder knows that the initiator wants to optimize the rekeying payload. Then when it determines that there is also no change in its cryptographic suites and ACL configuration, the responder MUST send the rekeying respond message to the initiator with the REKEY_OPTIMIZED notification payload instead of the SA and TS payloads. In this REKEY_OPTIMIZED notification, it contains the responder’s new SPI used for creating the new Child SA. According to the old SPIs included in the REKEY_SA payloads and the new SPIs included in the REKEY_OPTIMIZED payloads, the initiator and the responder can rekey the Child SA on both sides. The CREATE_CHILD_SA message exchange in this case is shown below:

<-- HDR, SK {N(REKEY_OPTIMIZED), Nr, [KEr,]} ]]> This REKEY_OPTIMIZED notification MUST be included in a CREATE_CHILD_SA exchange message when there is no SA and TS payloads included at the time of rekeying Child SAs. When the REKEY_OPTIMIZED notification payload is included, the SA and TS payloads MUST NOT be included.

The MINIMAL_REKEY_SUPPORTED notification is used by the initiator and responder to inform their ability of optimizing payloads at the time of rekeying IKE SAs and Child SAs to the peers. It is formatted as follows:

Protocol ID (1 octet) - MUST be 0. SPI Size (1 octet) - MUST be 0, meaning no SPI is present. Notify Message Type (2 octets) - MUST be <Need to get value from IANA>, the value assigned for the MINIMAL_REKEY_SUPPORTED notification. This notification contains no data.

The REKEY_OPTIMIZED notification is used to replace the SA payloads at the time of rekeying IKE SAs when there is no change of cryptographic suites in initiator and responder, and to replace the SA payloads and TS payloads at the time of rekeying Child SAs when there is no change of cryptographic suites and ACL configuration in initiator and responder. It is formatted as follows:

Protocol ID (1 octet) - MUST be 1. SPI Size (1 octet) - MUST be 8 when used at the time of rekeying IKE SAs and be 4 when used at the time of rekeying Child SAs. Notify Message Type (2 octets) - MUST be <Need to get value from IANA>, the value assigned for the REKEY_OPTIMIZED notification. SPI (4 octets or 8 octets) - Security Parameter Index. The initiator sends initiator SPI. The responder sends responder SPI.

This document defines two new Notify Message Types in the “IKEv2 Notify Message Types - Status Types” registry. IANA is requested to assign codepoints in this registry.

When using the payload optimization defined in this document, the rekeying of IKE SAs and Child SAs are using the same cryptographic suites. If changes to the configurations are wanted, such as supporting a new cryptographic algorithm, the rekeying won’t apply these changes. The initiator or responder should start a new IKE SA or Child SA to apply the new changes.

Special thanks go to Paul Wouters, Valery Smyslov, and Antony Antony.