A Childless Initiation of the IKE SA

IKEv2, as specified in requires that in an IKE_AUTH exchange, a child SA is created along with the IKE SA. This requirement is sometimes inconvenient, as some implementations need to use IKE for authentication only, while other implementations would like to set up the IKE SA before there is any actual traffic to protect. An IKE SA without any child SA is not a fruitless endeavor. Even without Child SAs, an IKE SA allows: Checking the liveness status of the peer via liveness checks. Quickly setting up child SAs without public key operations, and/or without user interaction. Authentication of the peer.

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 .

Several scenarios motivated this proposal: 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). Location aware security, as in . 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 IKE SA needs to be maintained. A future extension may have IKE SAs used for generating keying material for applications, without ever requiring child SAs. This is similar to what is doing in 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 IKE peer might see the deletion of the Child SA as a reason to delete the IKE SA.

The decision of whether or not to support an IKE_AUTH exchage without the piggy-backed child SA negotiation is ultimately up to the reponsder. A supporting resonder MUST include the VID payload, described in , within the IKE_INIT response. A supporting initiator MAY send the modified IKE_AUTH request, described in , if the VID payload was included in the IKE_INIT response. The initiator MUST NOT send the modified IKE_AUTH request if the VID was not present. A supporting responder that advertised the VID payload in the IKE_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.

The VID payload is as described in with the data as follows:

This value was obtained by hashing the string "Will do IKE_AUTH without child SA payloads"

For brevity, only the EAP version of an AUTH exchange will be presented here. The non-EAP version is very similar. The figures below are based on appendix A.3 of .

IDi, [N(INITIAL_CONTACT)], [[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+], [IDr], [CP(CFG_REQUEST)], [V+] first response <-- IDr, [CERT+], AUTH, EAP, [V+] / --> EAP repeat 1..N times | \ <-- EAP last request --> AUTH last response <-- AUTH, [CP(CFG_REPLY)], [N(ADDITIONAL_TS_POSSIBLE)], [V+] ]]> Note what is 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.

TBA

There are no IANA considerations for this document.