Miracast 协议详解
一、Miracast 概述
Miracast 基于 Wi‑Fi Display (WFD) 标准，用于将移动设备屏幕无线投射到显示设备。主要组件：
- Wi‑Fi Direct (P2P)：建立直连
- RTSP：会话控制
- RTP/RTCP：音视频传输
- UIBC：用户输入回传
二、Miracast 交互流程
阶段1：设备发现与 P2P 连接
Source (手机) Sink (接收端) | | |--- Probe Request (WFD IE) ---->| |<-- Probe Response (WFD IE) ----| | | |--- P2P Device Discovery ------>| |<-- P2P Device Discovery ------| | | |--- P2P GO Negotiation -------->| |<-- P2P GO Negotiation ---------| | | |--- P2P Provisioning ---------->| |<-- P2P Provisioning -----------| | | |--- DHCP (获取IP) ------------->| |<-- DHCP (分配IP) --------------|阶段2：RTSP 会话建立
RTSP 用于能力协商和会话控制。
2.1 M1 - RTSP Options
Source -> Sink: OPTIONS * RTSP/1.0 CSeq: 1 Require: org.wfa.wfd1.0 User-Agent: WFD/1.0 Sink -> Source: RTSP/1.0 200 OK CSeq: 1 Public: org.wfa.wfd1.0, GET_PARAMETER, SET_PARAMETER2.2 M2 - Get Parameter (能力查询)
Source -> Sink: GET_PARAMETER rtsp://192.168.49.1/wfd1.0 RTSP/1.0 CSeq: 2 Content-Type: text/parameters Content-Length: <length> wfd_video_formats wfd_audio_codecs wfd_client_rtp_ports wfd_uibc_capability wfd_standby_resume_capabilitySink 响应包含支持的能力：
Sink -> Source: RTSP/1.0 200 OK CSeq: 2 Content-Type: text/parameters Content-Length: <length> wfd_video_formats: 00 00 01 01 00000001 00 00000000 00000000 00 0000 0000 00 none none wfd_audio_codecs: AAC 00000001 00 wfd_client_rtp_ports: RTP/AVP/UDP;unicast 19000 0 mode=play wfd_uibc_capability: input_category_list=HIDC;hidc_cap_list=none2.3 M3 - Set Parameter (能力协商)
Source 选择参数并发送：
Source -> Sink: SET_PARAMETER rtsp://192.168.49.1/wfd1.0 RTSP/1.0 CSeq: 3 Content-Type: text/parameters Content-Length: <length> wfd_video_formats: 00 00 01 01 00000001 00 00000000 00000000 00 0000 0000 00 none none wfd_audio_codecs: AAC 00000001 00 wfd_client_rtp_ports: RTP/AVP/UDP;unicast 19000 0 mode=play wfd_uibc_setting: enable2.4 M4 - Setup (建立传输通道)
Source -> Sink: SETUP rtsp://192.168.49.1/wfd1.0 RTSP/1.0 CSeq: 4 Transport: RTP/AVP/UDP;unicast;client_port=19000-19001 Require: org.wfa.wfd1.0 Sink -> Source: RTSP/1.0 200 OK CSeq: 4 Transport: RTP/AVP/UDP;unicast;client_port=19000-19001;server_port=19002-19003 Session: 12345678;timeout=602.5 M5 - Play (开始播放)
Source -> Sink: PLAY rtsp://192.168.49.1/wfd1.0 RTSP/1.0 CSeq: 5 Session: 12345678 Sink -> Source: RTSP/1.0 200 OK CSeq: 5 Session: 12345678阶段3：RTP/RTCP 音视频传输
RTP 包结构
RTP Header (12 bytes): 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P|X| CC |M| PT | sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | synchronization source (SSRC) identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ RTP Payload: - Video: H.264 NAL units - Audio: AAC framesRTCP 包结构
RTCP SR (Sender Report): 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V=2|P| RC | PT=SR=200 | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of sender | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NTP timestamp, most significant word | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NTP timestamp, least significant word | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sender's packet count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sender's octet count | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+阶段4：UIBC (用户输入回传)
UIBC 允许 Sink 将触摸、键盘等输入回传给 Source。
Source -> Sink (M16): SET_PARAMETER rtsp://192.168.49.1/wfd1.0 RTSP/1.0 CSeq: 16 Content-Type: text/parameters Content-Length: <length> wfd_uibc_setting: enable wfd_uibc_capability: input_category_list=HIDC;hidc_cap_list=mouse keyboard三、WiFi 数据包解析
1. Probe Request/Response (WFD IE)
Probe Request Frame: [802.11 Header] [SSID: "DIRECT-xx"] [Supported Rates] [WFD IE] ← Miracast关键信息 - Device Type (Source/Sink) - Session Management Control Port (7236) - Maximum Throughput - WFD Information ElementsWFD IE 结构：
WFD IE (Information Element): Element ID: 221 (Vendor Specific) Length: Variable OUI: 50-6F-9A (Wi-Fi Alliance) OUI Type: 0x0A (WFD) Sub-elements: - Device Type (0x00) - Session Management Control Port (0x01) - Maximum Throughput (0x02) - WFD Extended Capability (0x03) - Local IP Address (0x04) - Associated BSSID (0x05) - WFD Session Information (0x06) - WFD Alternative MAC Address (0x07)2. P2P Action Frame
P2P Action Frame: [802.11 Header] [Category: Public Action (0x04)] [Action: P2P (0x7F)] [P2P Attributes] - P2P Capability - P2P Device Info - P2P Group Owner Intent - P2P Configuration Timeout - P2P Listen Channel - P2P Operating Channel3. RTSP 数据包（TCP）
Ethernet Header (14 bytes) IP Header (20 bytes) TCP Header (20 bytes) RTSP Message: OPTIONS * RTSP/1.0\r\n CSeq: 1\r\n Require: org.wfa.wfd1.0\r\n User-Agent: WFD/1.0\r\n \r\n4. RTP 数据包（UDP）
Ethernet Header (14 bytes) IP Header (20 bytes) UDP Header (8 bytes) RTP Header (12 bytes) RTP Payload (H.264 NAL or AAC)四、关键参数说明
视频格式参数 (wfd_video_formats)
格式: 00 00 01 01 00000001 00 00000000 00000000 00 0000 0000 00 none none 解析: - 00 00: Native (1920x1080p60) - 01: Preferred Display Mode - 01: H264 Profile (High) - 00000001: H264 Level (4.0) - 00: CEA Mask (支持的分辨率位图) - 00000000: VESA Mask - 00000000: HH Mask - 00: Latency - 0000: Minimum Slice Size - 0000: Slice Encapsulation - 00: Frame Rate Control - none: 3D Video Formats - none: 3D Video Formats音频编解码参数 (wfd_audio_codecs)
格式: AAC 00000001 00 解析: - AAC: 音频编解码器 - 00000001: 支持的AAC模式位图 - 00: LPCM模式（不支持）五、完整交互时序图
Source Sink | | |-- Wi-Fi Probe (WFD IE) ------>| |<-- Wi-Fi Probe Response ------| | | |-- P2P Device Discovery ------->| |<-- P2P Device Discovery ------| | | |-- P2P GO Negotiation -------->| |<-- P2P GO Negotiation --------| | | |-- P2P Provisioning ----------->| |<-- P2P Provisioning ----------| | | |-- DHCP Request --------------->| |<-- DHCP Response (IP分配) -----| | | |-- RTSP OPTIONS -------------->| |<-- RTSP 200 OK --------------| | | |-- RTSP GET_PARAMETER -------->| |<-- RTSP 200 OK (能力列表) -----| | | |-- RTSP SET_PARAMETER -------->| |<-- RTSP 200 OK --------------| | | |-- RTSP SETUP ---------------->| |<-- RTSP 200 OK (端口信息) -----| | | |-- RTSP PLAY ----------------->| |<-- RTSP 200 OK --------------| | | |-- RTP Video Stream ---------->| |-- RTP Audio Stream ---------->| |<-- RTCP SR/RR ----------------| | | |-- RTSP TEARDOWN ------------->| |<-- RTSP 200 OK --------------|
