| Network Working Group | O. Friel |
| Internet-Draft | E. Lear |
| Intended status: Standards Track | M. Pritikin |
| Expires: September 3, 2018 | Cisco |
| M. Richardson | |
| Sandelman Software Works | |
| March 02, 2018 |
BRSKI over IEEE 802.11
draft-friel-brski-over-802dot11-00
This document outlines the challenges associated with implementing Bootstrapping Remote Secure Key Infrastructures over IEEE 802.11 and IEEE 802.1x networks. Multiple options are presented for discovering and authenticating to the correct IEEE 802.11 SSID. This initial draft is a discussion document and no final recommendations are made on the recommended approaches to take.
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This Internet-Draft will expire on September 3, 2018.
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Bootstrapping Remote Secure Key Infrastructures (BRSKI) [I-D.ietf-anima-bootstrapping-keyinfra] describes how a device can bootstrap against a local network using an Initial Device Identity X.509 [IEEE802.1AR] IDevID certificate that is pre-installed by the vendor on the device in order to obtain an [IEEE802.1AR] LDevID. The BRSKI flow assumes the device can obtain an IP address, and thus assumes the device has already connected to the local network. Further, the draft states that BRSKI use of IDevIDs:
The draft does not describe any mechanisms for how an [IEEE802.11] enabled device would discover and select a suitable [IEEE802.11] SSID when multiple SSIDs are available. A typical deployment scenario could involve a device begin deployed in a location were twenty or more SSIDs are being broadcast, for example, in a multi-tenanted building or campus where multiple independent organizations operate [IEEE802.11] networks.
In order to reduce the administrative overhead of installing new devices, it is desirable that the device will automatically discover and connect to the correct SSID without the installer having to manually provision any network information or credentials on the device. It is also desirable that the device does not discover, connect to, and automatically enroll with the wrong network as this could result in a device that is owned by one organization connecting to the network of a different organization in a multi-tenanted building or campus.
Additionally, as noted above, the BRSKI draft does not describe how BRSKI could potentially align with [IEEE802.1X] authentication mechanisms.
This document outlines multiple different potential mechanisms that would enable a bootstrapping device to choose between different available 802.11 SSIDs in order to execute the BRSKI flow. This document also outlines several options for how 802.11 networks enforcing 802.1X authentication could enable the BRSKI flow, and describes the required device behaviour.
802.11u: an amendment to the IEEE 802.11-2007 standard to add features that improve interworking with external networks.
ANQP: Access Network Query Protocol
AP: IEEE 802.11 Access Point
CA: Certificate Authority
EAP: Extensible Authentication Protocol
EST: Enrollment over Secure Transport
HotSpot 2.0 / HS2.0: Wi-Fi Alliance standard that enables cell phones to roam seamlessly between cellular and IEEE 802.11 networks.
IDevID: Initial Device Identifier
LDevID: Locally Significant Device Identifier
SSID: 802.11 Service Set Identifier
STA: IEEE 802.11 station
WLC: Wireless LAN Controller
This section outlines multiple different mechanisms that could potentially be leveraged that would enable a bootstrapping device to choose between multiple different available 802.11 SSIDs. As noted previously, this draft does not make any final recommendations.
As will be seen in the following sections, there are several discovery options where the deivce can choose an incorrect SSID and attempt to join the wrong network. For example, the device is being deployed by one organization in a multi-tenant campus, and chooses to connect to the SSID of a different organization. The device is dependent upon the incorrect network rejecting its BRSKI enrollment attempt. It is possible that the device could end up enrolled with the wrong network.
A standardized naming convention for SSIDs offering BRSKI services is defined such as:
Where:
Device manufacturers would bake the well-known prefix string and character delimiter into device firmware. Network operators configuring SSIDs which offer BRSKI services would have to ensure that the SSID of those networks begins with this prefix. On bootstrap, the device would scan all available SSIDs and look for ones with this given prefix.
If multiple SSIDs are available with this prefix, then the device could simply round robin through these SSIDs and attempt to start the BRSKI flow on each one in turn until it succeeds.
This mechanism suffers from the limitations outlined in Section 2.1 - it does nothing to prevent a device enrolling against the same network.
Another issue with defining a specific naming convention for the SSID is that this may require network operators to have to deploy a new SSID. In general, network operators attempt to keep the number of unique SSIDs deployed to a minimum as each deployed SSID eats up a percentage of available air time and network capacity. A good discussion of SSID overhead and an SSID overhead [calculator] is available.
[IEEE802.11aq] is currently being worked by the IEEE, but is not yet finalized, and is not yet supported by any vendors in shipping product. 802.11aq defines new elements that can be included in 802.11 Beacon, Probe Request and Probe Response frames, and defines new elements for ANQP frames.
The extensions allow an AP to broadcast support for backend services, where allowed services are those registered in the [IANA] Service Name and Transport Protocol Port Number Registry. The services can be advertised in 802.11 elements that include either:
Bloom filters simply serve to reduce the size of Beacon and Probe Response frames when a large number of services are advertised. If a bloom filter is used by the AP, and a device discovers a potential service match in the bloom filter, then the device can query the AP for the full list of service name hashes using newly defined ANQP elements.
If BRSKI were to leverage 802.11aq, then the 802.11aq specification would need to be pushed and supported, and a BRSKI service would need to be defined in [IANA].
This mechanism suffers from the limitations outlined in Section 2.1 - it does nothing to prevent a device enrolling against the same network.
[IEEE802.11u] defines mechanisms for interworking. An introduction to 802.11u is given in the appendices. The 802.11u NAI Realm IE could be a possible mechanism for advertising BRSKI capability. BRSKI could possibly piggy back on top of NAI Realm and simply advertise an NAI Realm of “_bootstrapks”. Wireless LAN Controllers (WLC) appear to allow this kind of configuration today.
Note that today some WLCs tie 802.11u configuration to HS2.0 configuration i.e. you cannot enable advertising 802.11u bits without also advertising HS2.0 in Beacons - but thats a WLC implementation gap not a standards gap.
The key conceptual difference with this NAI Realm proposal is that BRSKI uses this special realm name more as a logical service advertisement rather than as a backhaul internet provider advertisement. Leveraging the NAI Realm to advertise a service is conceptually very similar to what 802.11aq is attempting to achieve.
Leveraging NAI Realm would not require any 802.11 specification changes, and could be defined by this IETF draft. Note that the author’s are not aware of any currently defined IETF or IANA namespaces that define NAI Realms. Device manufacturers would bake the well-known NAI Realm string into device firmware. Network operators configuring SSIDs which offer BRSKI services would have to ensure that the SSID offered an NAI Realm with this specific name. On bootstrap, the device would scan all available SSIDs and use ANQP to query for NAI Realms matching the BRSKI service name.
Additionally (or alternatively…) as NAI Realm includes advertising the EAP mechanism required, if a new EAP-BRSKI were to be defined, then this could be advertised. Devices could then scan for an NAI Realm that enforced EAP-BRSKI, and ignore the realm name.
This mechanism suffers from the limitations outlined in Section 2.1 - it does nothing to prevent a device enrolling against the same network.
Additionally, as the IEEE is attempting to standardize logical service advertisement via 802.11aq, 802.11aq would seem to be the more appropriate option than overloading NAI Realm. However, it is worth noting that configuring of NAI Realms is supported today by WLCs.
It is possible that an SSID may be configured to provide unrestricted and unauthenticated internet access. This could be advertised in the Interworking Information IE by including:
If such a network were discovered, a device could attempt to use the BRSKI well-known vendor cloud Registrar. Possibly this could be a default fall back mechanism that a device could use when determining which SSID to use.
Of the various elements currently defined by 802.11u for potentially advertising BRSKI, NAI Realm is the only element that is a possible option, as outlined above. The Roaming Consortium and 3GPP Cellular Network elements are not suitable at all. Another option that has been suggested in the IETF mailers is defining an extension to 802.11u specifically for advertising BRSKI service capability.
802.11aq appears to be the proposed mechanism for generically advertising any service capability, provided that service is registered with [IANA]. It is probably a better approach to encourage adoption of 802.11aq and register a service name for BRSKI with [IANA] rather than attempt to define a completely new BRSKI-specific 802.11u extension.
This is probably a bad idea for reasons to be documented…
The [DPP] specification defines how an entity that is already trusted by a network can assist an untrusted entity in enrolling with the network. The description below assumes the 802.11 network is in infrastructure mode. DPP introduces multiple key roles including:
h- Responder: A logical entity that responds to the Initiator of the DPP Authentication Protocol. The Responder may be the Configurator or the Enrollee.
In order to support a plug and play model for installation of devices, where the device is simply powered up for the first time and automatically discovers the network without the need for a helper or supervising application, for example an application running on a smart cell phone or tablet that performs the role of Configurator, then this implies that the AP must perform the role of the Configurator and the device or STA performs the role of Enrollee. Note that the AP may simply proxy DPP messages through to a backend WLC, but from the perspective of the device, the AP is the Configurator.
The DPP specification also mandates that the Initiator must know in advance and validate the bootstrapping public key of the Responder. For BRSKI purposes, the DPP bootstrapping public key will be the [IEEE802.1AR] IDevID of the device. As the boostrapping device cannot know in advance the bootstrapping public key of a specific operators network, this implies that the Configurator must take on the role of the Initiator. Therefore, the AP must take on the roles of both the Configurator and the Initiator.
More details to be added…
When the bootstrapping device determines which SSID to connect to, there are multiple potential options available for how the device authenticates with the network while bootstrapping. Several options are outlined in this section. This list is not exhaustive.
At a high level, authentication can generally be split into two phases using two different credentials:
The authentication options outlined in this document include:
These mechanisms are described in more detail in the following sections.
[IEEE802.11i] allows an SSID to advertise multiple authentication mechanisms. A very brief introduction to 802.11i is given in the appendices. For example, a single SSID could advertise both PSK and 802.1X authentication mechanisms. When a network operator needs to enforce two different authentication mechanisms, one for pre-BRSKI devices and one for post-BRSKI devices, the operator has two options:
Devices should be flexible enough to handle both potential scenarios. When discovering a pre-BRSKI SSID, the device should also discover the authentication mechanism enforced by the SSID that is advertising BRSKI support. If the device supports the authentication mechanism being advertised, then the device can connect to the SSID in order to initiate the BRSKI flow. For example, the device may support 802.1X as a pre-BRSKI authentication mechanism, but may not support PSK as a pre-BRSKI authentication mechanism.
Once the device has completed the BRKSI flow and has obtained an LDevID, a mechanism is needed to tell the device which SSID to use for post-BRSKI network access. This may be a different SSID to the pre-BRSKI SSID. The mechanism by which the post-BRSKI SSID is advertised to the device is out-of-scope of this version of this document.
If a device has to perform two different authentications, one for pre-BRSKI and one for post-BRSKI, network policy will typically assign the device to different VLANs for these different stages, and assign the device different IP addresses depending on which network segment the device is assigned to. This will generally be true even if a single SSID is used for both pre-BRSKI and post-BRSKI connections. Therefore, the bootstrapping device must be able to completely reset its network connection and network software stack, and obtain a new IP address between pre-BRSKI and post-BRSKI connections.
The device connects to an unauthenticated network pre-BRSKI. The device connects to a network enforcing 802.1X EAP-TLS post-BRSKI. The device uses its LDevID as the post-BRSKI 802.1X credential.
To be completed..
The device connects to a network enforcing PSK pre-BRSKI. The mechanism by which the PSK is provisioned on the device for pre-BRSKI authentication is out-of-scope of this version of this document. The device connects to a network enforcing 802.1X EAP-TLS post-BRSKI. The device uses the LDevID obtained via BRSKI as the post-BRSKI 802.1X credential.
When the device connects to the post-BRSKI network that is enforcing 802.1X EAP-TLS, the device uses its LDevID as its credential. The device should verify the certificate presented by the server during that EAP-TLS exchange against the trusted CA list it obtained during BRSKI.
If the 802.1X network enforces a tunneled EAP method, for example [RFC7170], where the device must present an additional credential such as a password, the mechanism by which that additional credential is provisioned on the device for post-BRSKI authentication is out-of-scope of this version of this document. NAI Realm may be used to advertise the EAP methods being enforced by an SSID. It is to be determined if guidelines should be provided on use of NAI Realm for advertising EAP method in order to streamline BRSKI.
The device connects to a network enforcing 802.1X EAP-TLS pre-BRSKI. The device uses its IDevID as the pre-BRSKI 802.1X credential. The device connects to a network enforcing 802.1X EAP-TLS post-BRSKI. The device uses its LDevID as the post-BRSKI 802.1X credential.
When the device connects to a pre-BRSKI network that is enforcing 802.1X EAP-TLS, the device uses its IDevID as its credential. The deivce should not attempt to verify the certificate presented by the server during that EAP-TLS exchange, as it has not yet discovered the local domain trusted CA list.
When the device connects to the post-BRSKI network that is enforcing 802.1X EAP-TLS, the device uses its LDevID as its credential. The deivce should verify the certificate presented by the server during that EAP-TLS exchange against the trusted CA list it obtained during BRSKI.
Again, if the post-BRSKI network enforces a tunneled EAP method, the mechanism by which that second credential is provisioned on the device is out-of-scope of this version of this document.
A new EAP method, let’s call it EAP-BRSKI, is defined that encapsulates the full BRSKI flow. At a high level, this enables the device to obtain an LDevID during the Layer 2 authentication stage. This has multiple advantages including:
The device discovers and connects to a network enforcing 802.1X EAP-BRSKI. A high level EAP-BRSKI flow would look something like:
The EAP server continues with TLS tunnel establishment:
At this stage, the EAP server has some policy decisions to make:
Once the EAP flow has successfully compelted, then:
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802.11i-2004 is an IEEE standard from 2004 that improves connection security. 802.11i-2004 is incorporated into 802.11-2014. 802.11i defines the Robust Security Network IE which includes information on:
The RSN IEs are included in Beacon and Probe Response frames. STAs can use this frame to determine the authentication mechanisms offered by a particular AP e.g. PSK or 802.1X.
802.11u-2011 is an IEEE standard from 2011 that adds features that improve interworking with external networks. 802.11u-2011 is incorporated into 802.11-2016.
STAs and APs advertise support for 802.11u by setting the Interworking bit in the Extended Capabilities IE, and by including the Interworking IE in Beacon, Probe Request and Probe Response frames.
The Interworking IE includes information on:
802.11u introduced Access Network Query Protocol (ANQP) which enables STAs to query APs for information not present in Beacons/Probe Responses.
ANQP defines these key IEs for enabling the STA to determine which network to connect to:
STAs can use some or all of the above IEs to make a suitable decision on which SSID to pick.
HotSpot 2.0 is an example of a specification built on top of 802.11u and defines 10 additional ANQP elements using the standard vendor extensions mechanisms defined in 802.11. It also defines a HS2.0 Indication element that is included in Beacons and Probe Responses so that STAs can immediately tell if an SSID supports HS2.0.