GAS简单笔记

参考
【 https://en.wikipedia.org/wiki/Generic_Advertisement_Service 】
【 https://www.cisco.com/c/dam/global/zh_cn/products/products_netsol/wireless/pdf/cco_network_group_10_v7.pdf 】

对于WLAN的配置，包括无线控制器的配置，单个和多个SSID的配置将都会涉及必要的Passpoint信息。这些额外的Passpoint信息会在信标或探针的响应信息中进行添加，这样拥有Passpoint功能的手机客户端设备可以通过检测和查询无线接入点以获取进一步的信息。在查询过程中，一种被称为ANQP(Access Network Query Protocol – 接入网络查询协议)的标准被遵循。该协议通过标准的2路或4路握手过程从无线接入点和ANQP服务器中获得足够的信息，以确定手机客户端设备可以验证和关联的最佳无线接入点。这个过程被称为GAS（Generic Advertisement Service – 通用通告服务）协议，该标准在IEEE802.11u中定义。

Generic Advertisement Service (GAS): An IEEE 802.11u service that provides over-the-air transportation for frames of higher-layer advertisements between Wi-Fi stations (802.11 Stations) or between a server in an external network and a station. GAS may be used prior stations are authenticated, or associated to a wireless Access Point (AP) in a Basic Service Set (BSS). GAS supports higher-layer protocols that employ a query/response mechanism.

In a BSS infrastructure, the purpose of Generic Advertisement Service (GAS) is to enable a station to identify the availability of information related to network services. While the specification of network services information is out of scope of IEEE 802.11, there is a need for stations to query for information on network services provided by SSPNs or other external networks beyond an AP (Access Point) before they associate to the wireless LAN. GAS defines a generic container to advertise network services information over an IEEE 802.11 network. Public Action frames are used to transport this information.

There are a number of reasons why providing information pre-association is beneficial.
It supports more informed decision making about an IEEE 802.11 infrastructure with which to associate. This is generally more efficient than requiring a non-AP STA to associate with an AP before discovering the information and then deciding whether or not to stay associated.
It is possible to query multiple networks in parallel.
The station can discover information about APs that are not part of the same administrative group as the AP with which it is associated, supporting the selection of an AP belonging to a different IEEE 802.11 infrastructure that has an appropriate SSP roaming agreement in place.