在哪个阶段连接到AP？802.11b 协议

网络工程 无线的 IEEE-802.11
2021-07-09 08:24:41

我看到几个设备试图连接到一个 AP（使用wireshark），几个设备发送和接收“探测响应”消息（802.11）， 但它们甚至不发送关联请求。这是为什么？只有在探测响应之后，连接才被认为是关闭的还是打开的？ 他们怎么知道不发送关联请求？这是一个没有发送关联请求的设备的响应包后记在此处输入图片说明 

Frame 442: 243 bytes on wire (1944 bits), 243 bytes captured (1944 bits) on interface 0
    Interface id: 0 (\\.\airpcap00)
    Encapsulation type: IEEE 802.11 plus radiotap radio header (23)
    Arrival Time: Mar 18, 2014 21:32:31.171606000 Jerusalem Standard Time
    [Time shift for this packet: 0.000000000 seconds]
    Epoch Time: 1395171151.171606000 seconds
    [Time delta from previous captured frame: 0.002142000 seconds]
    [Time delta from previous displayed frame: 0.096003000 seconds]
    [Time since reference or first frame: 13.290039000 seconds]
    Frame Number: 442
    Frame Length: 243 bytes (1944 bits)
    Capture Length: 243 bytes (1944 bits)
    [Frame is marked: False]
    [Frame is ignored: False]
    [Protocols in frame: radiotap:wlan_radio:wlan]
Radiotap Header v0, Length 20
    Header revision: 0
    Header pad: 0
    Header length: 20
    Present flags
        Present flags word: 0x000018ee
    Flags: 0x10
        .... ...0 = CFP: False
        .... ..0. = Preamble: Long
        .... .0.. = WEP: False
        .... 0... = Fragmentation: False
        ...1 .... = FCS at end: True
        ..0. .... = Data Pad: False
        .0.. .... = Bad FCS: False
        0... .... = Short GI: False
    Data Rate: 1.0 Mb/s
    Channel frequency: 2412 [BG 1]
    Channel flags: 0x00a0, Complementary Code Keying (CCK), 2 GHz spectrum
    Antenna signal: -70dBm
    Antenna noise: -100dBm
    Signal Quality: 76
    Antenna: 0
    dB antenna signal: 30dB
802.11 radio information
    PHY type: 802.11b (4)
    Short preamble: False
    Data rate: 1.0 Mb/s
    Channel: 1
    Frequency: 2412MHz
    Signal strength (dBm): -70dBm
    Noise level (dBm): -100dBm
    [Duration: 1976µs]
IEEE 802.11 Probe Response, Flags: ........C
    Type/Subtype: Probe Response (0x0005)
    Frame Control Field: 0x5000
        .... ..00 = Version: 0
        .... 00.. = Type: Management frame (0)
        0101 .... = Subtype: 5
        Flags: 0x00
    .000 0001 0011 1010 = Duration: 314 microseconds
    Receiver address: SamsungE_74:b9:f9 (d0:22:be:74:b9:f9)
    Destination address: SamsungE_74:b9:f9 (d0:22:be:74:b9:f9)
    Transmitter address: Sagemcom_fb:5d:9d (00:78:9e:fb:5d:9d)
    Source address: Sagemcom_fb:5d:9d (00:78:9e:fb:5d:9d)
    BSS Id: Sagemcom_fb:5d:9d (00:78:9e:fb:5d:9d)
    .... .... .... 0000 = Fragment number: 0
    0010 1000 0011 .... = Sequence number: 643
    Frame check sequence: 0xeb99cc98 [correct]
    [FCS Status: Good]
IEEE 802.11 wireless LAN
    Fixed parameters (12 bytes)
        Timestamp: 0x0000008df2ce0b90
        Beacon Interval: 0.102400 [Seconds]
        Capabilities Information: 0x0411
            .... .... .... ...1 = ESS capabilities: Transmitter is an AP
            .... .... .... ..0. = IBSS status: Transmitter belongs to a BSS
            .... ..0. .... 00.. = CFP participation capabilities: No point coordinator at AP (0x00)
            .... .... ...1 .... = Privacy: AP/STA can support WEP
            .... .... ..0. .... = Short Preamble: Not Allowed
            .... .... .0.. .... = PBCC: Not Allowed
            .... .... 0... .... = Channel Agility: Not in use
            .... ...0 .... .... = Spectrum Management: Not Implemented
            .... .1.. .... .... = Short Slot Time: In use
            .... 0... .... .... = Automatic Power Save Delivery: Not Implemented
            ...0 .... .... .... = Radio Measurement: Not Implemented
            ..0. .... .... .... = DSSS-OFDM: Not Allowed
            .0.. .... .... .... = Delayed Block Ack: Not Implemented
            0... .... .... .... = Immediate Block Ack: Not Implemented
    Tagged parameters (183 bytes)
        Tag: SSID parameter set: HOTBOX-9810
            Tag Number: SSID parameter set (0)
            Tag length: 11
            SSID: HOTBOX-9810
        Tag: Supported Rates 1(B), 2(B), 5.5(B), 11(B), 18, 24, 36, 54, [Mbit/sec]
            Tag Number: Supported Rates (1)
            Tag length: 8
            Supported Rates: 1(B) (0x82)
            Supported Rates: 2(B) (0x84)
            Supported Rates: 5.5(B) (0x8b)
            Supported Rates: 11(B) (0x96)
            Supported Rates: 18 (0x24)
            Supported Rates: 24 (0x30)
            Supported Rates: 36 (0x48)
            Supported Rates: 54 (0x6c)
        Tag: DS Parameter set: Current Channel: 1
            Tag Number: DS Parameter set (3)
            Tag length: 1
            Current Channel: 1
        Tag: ERP Information
            Tag Number: ERP Information (42)
            Tag length: 1
            ERP Information: 0x00
        Tag: ERP Information
            Tag Number: ERP Information (47)
            Tag length: 1
            ERP Information: 0x00
        Tag: RSN Information
            Tag Number: RSN Information (48)
            Tag length: 24
            RSN Version: 1
            Group Cipher Suite: 00:0f:ac (Ieee 802.11) TKIP
            Pairwise Cipher Suite Count: 2
            Pairwise Cipher Suite List 00:0f:ac (Ieee 802.11) AES (CCM) 00:0f:ac (Ieee 802.11) TKIP
            Auth Key Management (AKM) Suite Count: 1
            Auth Key Management (AKM) List 00:0f:ac (Ieee 802.11) PSK
            RSN Capabilities: 0x000c
        Tag: Extended Supported Rates 6, 9, 12, 48, [Mbit/sec]
            Tag Number: Extended Supported Rates (50)
            Tag length: 4
            Extended Supported Rates: 6 (0x0c)
            Extended Supported Rates: 9 (0x12)
            Extended Supported Rates: 12 (0x18)
            Extended Supported Rates: 48 (0x60)
        Tag: HT Capabilities (802.11n D1.10)
            Tag Number: HT Capabilities (802.11n D1.10) (45)
            Tag length: 26
            HT Capabilities Info: 0x18fc
            A-MPDU Parameters: 0x1b
            Rx Supported Modulation and Coding Scheme Set: MCS Set
            HT Extended Capabilities: 0x0000
            Transmit Beam Forming (TxBF) Capabilities: 0x00000000
            Antenna Selection (ASEL) Capabilities: 0x00
        Tag: HT Information (802.11n D1.10)
            Tag Number: HT Information (802.11n D1.10) (61)
            Tag length: 22
            Primary Channel: 1
            HT Information Subset (1 of 3): 0x08
            HT Information Subset (2 of 3): 0x0004
            HT Information Subset (3 of 3): 0x0000
            Rx Supported Modulation and Coding Scheme Set: Basic MCS Set
        Tag: Vendor Specific: Broadcom
            Tag Number: Vendor Specific (221)
            Tag length: 9
            OUI: 00:10:18 (Broadcom)
            Vendor Specific OUI Type: 2
            Vendor Specific Data: 0201f02c0000
        Tag: Vendor Specific: Microsoft Corp.: WPA Information Element
            Tag Number: Vendor Specific (221)
            Tag length: 28
            OUI: 00:50:f2 (Microsoft Corp.)
            Vendor Specific OUI Type: 1
            Type: WPA Information Element (0x01)
            WPA Version: 1
            Multicast Cipher Suite: 00:50:f2 (Microsoft Corp.) TKIP
            Unicast Cipher Suite Count: 2
            Unicast Cipher Suite List 00:50:f2 (Microsoft Corp.) AES (CCM) 00:50:f2 (Microsoft Corp.) TKIP
            Auth Key Management (AKM) Suite Count: 1
            Auth Key Management (AKM) List 00:50:f2 (Microsoft Corp.) PSK
        Tag: Vendor Specific: Microsoft Corp.: WMM/WME: Parameter Element
            Tag Number: Vendor Specific (221)
            Tag length: 24
            OUI: 00:50:f2 (Microsoft Corp.)
            Vendor Specific OUI Type: 2
            Type: WMM/WME (0x02)
            WME Subtype: Parameter Element (1)
            WME Version: 1
            WME QoS Info: 0x80
            Reserved: 00
            Ac Parameters ACI 0 (Best Effort), ACM no, AIFSN 3, ECWmin/max 4/10 (CWmin/max 15/1023), TXOP 0
            Ac Parameters ACI 1 (Background), ACM no, AIFSN 7, ECWmin/max 4/10 (CWmin/max 15/1023), TXOP 0
            Ac Parameters ACI 2 (Video), ACM no, AIFSN 2, ECWmin/max 3/4 (CWmin/max 7/15), TXOP 94
            Ac Parameters ACI 3 (Voice), ACM no, AIFSN 2, ECWmin/max 2/3 (CWmin/max 3/7), TXOP 47
2个回答

这些设备只是在寻找可用的网络，而不是试图与它们关联。这是寻找网络并确定 AP 功能是什么的设备的正常行为。

在您告诉它连接到该网络之前，设备不会发送关联请求。

但是甚至不发送关联请求。这是为什么？只有在探测响应之后，连接才被认为是关闭的还是打开的？他们怎么知道不发送关联请求？

他们只是在检查周围无线网络的可用性。这并不一定表明他们确实想要加入或关联到网络。

当无线客户端希望加入无线网络（或检查漫游到新网络的可能性）时，它们会发出探测请求。通常，这些本质上是通用的，以及设备如何发现它们周围的网络。可以把它想象成客户端设备大喊：“任何无线网络都能听到我的声音吗？”

探测请求也可以定向到特定网络。“网络ABCXYZ能听到我说话吗？” 这是“隐藏”SSID 所必需的，并且是许多移动设备尝试重新连接到存储网络的默认方式。

当接入点听到探测请求时，如果探测请求本质上是通用的，或者是针对配置的无线网络，它会以探测响应进行响应。此行为由 IEEE 802.11 网络标准定义。因此，您的 AP 将始终响应来自无线客户端的任何一般探测请求。

您可能自己也经历过这种情况而没有意识到。当您想将无线设备加入网络时，您可能会看到一个无线网络列表，并选择了您想要加入的网络。该列表主要是根据无线客户端在发出探测请求时从该区域的 AP 收到的探测响应生成的。

所有 AP 还会定期使用信标帧来通告自己，这是客户端可以在该区域找到无线网络的另一种方式。然而，这是一个较慢的发现过程，因此绝大多数客户端主要使用探测请求/响应来进行发现。

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