unsigned char *packetBuf; 用来存储解码分片包
unsigned int packetLen; 记录已经存储的长度

每一个RTP包头长度RTP_HEADER_LEN为12，
RTP头后面就是H264数据，
对于RTP里的FU-A分片的H264数据需要重新组包后才能喂给解码器

定义:
unsigned char *data UDP接收数据
unsigned int len 接收UDP数据长度
PACKET_LEN = 1024 * 300
默认RTP的P=0（如果为1则在rtp包末尾减1的非有效荷载即可）

H264数据:

unsigned char *payload = data + RTP_HEADER_LEN;

fu_indicator：FU-A分片包定义头

unsigned char fu_indicator = *payload; 

fu_header: FU-A分片包定义头

unsigned char fu_header = *(payload + 1);

startBit == 1表示分片的起始包

unsigned char startBit = fu_header >> 7;

endBit == 1表示分片的结束包

unsigned char endBit = (fu_header & 0x40) >> 6;

payload_len H264的长度

unsigned int payload_len = len - RTP_HEADER_LEN;

futype: 判断当前接受到包的类型

uint8_t futype = fu_indicator & 0x1f;
:28即表示接收到的FU-A包 拼包只拼此类型的包其他为完整的h264包
:7 sps
:8 pps

nal_fua 用于在拼包时重写H264头 即fu_indicator的前3位和fu_header的后5位

uint8_t nal_fua = (fu_indicator & 0xe0) | (fu_header & 0x1f);

定义每个包的分割头，用于在组包时添加到每个包的包头

 uint8_t header[4] = {0x00, 0x00, 0x00, 0x01};

组包过程

• PACKET_LEN实例化一块内存的长度,用来存储分片数据
• block 即将数据传给解码器解码

if (futype == 28) {
        if (endBit == 1) {
            memcpy(packetBuf+packetLen, payload + 2, payload_len - 2);
            packetLen += (payload_len - 2);
            block(packetBuf, packetLen);
            packetLen = 0;
            memset(packetBuf, 0, PACKET_LEN);
        } else if (startBit == 1) {
            memcpy(packetBuf, header, sizeof(header));
            *(packetBuf + sizeof(header)) = nal_fua;
            memcpy(packetBuf + sizeof(header) + 1, payload + 2, payload_len-2);
            packetLen = payload_len - 2 + 4 + 1 ;
        } else {
            if (packetLen > 0) {
                memcpy(packetBuf + packetLen, payload + 2, payload_len - 2);
                packetLen += (payload_len - 2);
            }
        }
    }

单包

• 比如futype==7 || futype==8
• 不需组包，直接拼接header
• PACKET_LEN实例化一块内存的长度,用来存储分片数据
• block 即将数据传给解码器解码

unsigned int pl = payload_len + sizeof(header);
memcpy(packetBuf, header, sizeof(header));
memcpy(packetBuf + sizeof(header), payload, payload_len);
block(packetBuf, pl);
memset(packetBuf, 0, PACLET_LEN);

解RTP FU-A结束