详解RTP协议之H264封包和解包实战 / 开普饭

0.引言

为了更好理解本篇文章，可以先阅读前面几篇文章，文章列表如下:

详解RTP协议之H264封包细节(1)

详细解析RTSP框架和数据包分析(1)

建议:阅读本文前，一定要阅读前面的文章，只有理解了原理，才能够正真读懂代码。

1.RTP实战源码框架

在win上主要是支持qt，使用2015编译器。linux上支持cmake编译。主要是支持跨平台支持。

使用函数int get_annexb_nalu (nalu_t *nalu, FILE *bits)，一开始是读取本地h264文件，解析出不带startcode的nalu，接着是函数static void rtp_h264_pack_get_info(void* pack, uint16_t* seq, uint32_t* timestamp)，经过函数就会得到RTP包，处理RTP包就有2个流程，第一个是通过网络发送出去，然后播放。另外一个流程是通过RTP_unpack去解码，生成nalu，再加上h264的start code，就可以存储在本地文件，然后再播放。框架如下图所示:

main.c是用户自定义发送和写文件，测试函数。

rtp-packet.c是RTP Packet序列化和反序列化功能。

rtp-payload.c主要是RTP封包和拆包的接口层。

rtp-h264-packet.c主要是H264封包RTP的实现层。

rtp-h264-unpacket.c主要是H264拆包RTP的实现层。

这里以视频数据为例子，整个源码的数据对象的关系是，在封包流程中，nalu指向rtp结构，再指向rtp序列化，最终才是sendto的数据。在解封装的流程中，recv的数据指向rtp反序列化，再到rtp结构，最终解析出nalu，加上startcode，存在本地文件，保存为h264，可以播放。

2.RTP实战测试效果

(1)播放命令

ffplay h264.sdp -protocol_whitelist 'file,http,https,rtp,udp,tcp,tls'

界面如下:

(2)注意，需要在如下的目录下，准备好输入h264文件，即把h264文件放到这个debug目录，因为代码中需要打开输入的h264文件。

(3)h264.sdp是代码生成，整个日志打印过程，其界面如下:

3.源码详解

3.1 初始化参数和回调函数注册

(1)rtp header头部定义。按照前面文章描述的头部结构来定义。封包流程中，注册函数，源码如下:

/** * @brief rtp_payload_encode_input 这里是通用的接口 * @param encoder * @param data data具体是什么媒体类型的数据，接口不关注，具体由ctx->encoder->input去处理 * @param bytes * @param timestamp * @return */int rtp_payload_encode_input(void* encoder, const void* data, int bytes, uint32_t timestamp){ struct rtp_payload_delegate_t* ctx; ctx = (struct rtp_payload_delegate_t*)encoder; return ctx->encoder->input(ctx->packer, data, bytes, timestamp);}

RTP Header固定长度为12个字节。rtp packet(包括header payload)的数据结构定义如下:

void* rtp_payload_decode_create(int payload, const char* name, struct rtp_payload_t *handler, void* cbparam){    struct rtp_payload_delegate_t* ctx;    ctx = calloc(1, sizeof(*ctx));    if (ctx)    {        if (rtp_payload_find(payload, name, ctx) < 0            || NULL == (ctx->packer = ctx->decoder->create(handler, cbparam)))        {            free(ctx);            return NULL;        }    }    return ctx;}

(2)网络发送到本地，这里使用的UDP发送。

int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){ int hdrlen; uint32_t i; uint8_t* ptr; if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4)) { assert(0); // RTP version field must equal 2 (p66) return -1; } // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展 hdrlen = RTP_FIXED_HEADER pkt->rtp.cc * 4 (pkt->rtp.x ? 4 : 0); if (bytes < hdrlen pkt->extlen) return -1; ptr = (uint8_t *)data; nbo_write_rtp_header(ptr, &pkt->rtp); ptr = RTP_FIXED_HEADER; // pkt contributing source for (i = 0; i < pkt->rtp.cc; i , ptr = 4) { nbo_w32(ptr, pkt->csrc[i]); // csrc列表封装到头部 } // pkt header extension if (1 == pkt->rtp.x) { // 5.3.1 RTP Header Extension assert(0 == (pkt->extlen % 4)); nbo_w16(ptr, pkt->reserved); nbo_w16(ptr 2, pkt->extlen / 4); memcpy(ptr 4, pkt->extension, pkt->extlen); // extension封装到头部 ptr = pkt->extlen 4; } return hdrlen pkt->extlen;}

3.2 详解封装RTP包流程源码

(1)函数rtp_payload_encode_create(int payload, const char* name, uint16_t seq, uint32_t ssrc, struct rtp_payload_t *handler, void* cbparam)接收用户参数，并设置回调函数。源码如下:

// 把可读RTP packet封装成要发送出去的数据 序列化int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){    int hdrlen;    uint32_t i;    uint8_t* ptr;    if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4))    {        assert(0); // RTP version field must equal 2 (p66)        return -1;    }    // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展    hdrlen = RTP_FIXED_HEADER   pkt->rtp.cc * 4   (pkt->rtp.x ? 4 : 0);    if (bytes < hdrlen   pkt->extlen)        return -1;    ptr = (uint8_t *)data;    nbo_write_rtp_header(ptr, &pkt->rtp);    ptr  = RTP_FIXED_HEADER;    // pkt contributing source    for (i = 0; i < pkt->rtp.cc; i  , ptr  = 4)    {        nbo_w32(ptr, pkt->csrc[i]);     // csrc列表封装到头部    }    // pkt header extension    if (1 == pkt->rtp.x)    {        // 5.3.1 RTP Header Extension        assert(0 == (pkt->extlen % 4));        nbo_w16(ptr, pkt->reserved);        nbo_w16(ptr   2, pkt->extlen / 4);        memcpy(ptr   4, pkt->extension, pkt->extlen);   // extension封装到头部        ptr  = pkt->extlen   4;    }    return hdrlen   pkt->extlen;}

(2)通过如下代码可以看出，ctx->encoder->create指向的函数是 rtp_h264_pack_create，也就是正真的实现体。源码如下:

struct rtp_payload_encode_t *rtp_h264_encode(){ static struct rtp_payload_encode_t packer = { rtp_h264_pack_create, rtp_h264_pack_destroy, rtp_h264_pack_get_info, rtp_h264_pack_input, }; return &packer;}

(3)函数rtp_h264_pack_create，用来存储外部设置的参数。源码如下:

// 把可读RTP packet封装成要发送出去的数据 序列化int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){    int hdrlen;    uint32_t i;    uint8_t* ptr;    if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4))    {        assert(0); // RTP version field must equal 2 (p66)        return -1;    }    // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展    hdrlen = RTP_FIXED_HEADER   pkt->rtp.cc * 4   (pkt->rtp.x ? 4 : 0);    if (bytes < hdrlen   pkt->extlen)        return -1;    ptr = (uint8_t *)data;    nbo_write_rtp_header(ptr, &pkt->rtp);    ptr  = RTP_FIXED_HEADER;    // pkt contributing source    for (i = 0; i < pkt->rtp.cc; i  , ptr  = 4)    {        nbo_w32(ptr, pkt->csrc[i]);     // csrc列表封装到头部    }    // pkt header extension    if (1 == pkt->rtp.x)    {        // 5.3.1 RTP Header Extension        assert(0 == (pkt->extlen % 4));        nbo_w16(ptr, pkt->reserved);        nbo_w16(ptr   2, pkt->extlen / 4);        memcpy(ptr   4, pkt->extension, pkt->extlen);   // extension封装到头部        ptr  = pkt->extlen   4;    }    return hdrlen   pkt->extlen;}

(4)封包流程中，也是使用相同的回调函数，源码如下:

// 把可读RTP packet封装成要发送出去的数据 序列化int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){ int hdrlen; uint32_t i; uint8_t* ptr; if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4)) { assert(0); // RTP version field must equal 2 (p66) return -1; } // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展 hdrlen = RTP_FIXED_HEADER pkt->rtp.cc * 4 (pkt->rtp.x ? 4 : 0); if (bytes < hdrlen pkt->extlen) return -1; ptr = (uint8_t *)data; nbo_write_rtp_header(ptr, &pkt->rtp); ptr = RTP_FIXED_HEADER; // pkt contributing source for (i = 0; i < pkt->rtp.cc; i , ptr = 4) { nbo_w32(ptr, pkt->csrc[i]); // csrc列表封装到头部 } // pkt header extension if (1 == pkt->rtp.x) { // 5.3.1 RTP Header Extension assert(0 == (pkt->extlen % 4)); nbo_w16(ptr, pkt->reserved); nbo_w16(ptr 2, pkt->extlen / 4); memcpy(ptr 4, pkt->extension, pkt->extlen); // extension封装到头部 ptr = pkt->extlen 4; } return hdrlen pkt->extlen;}

(5)通过网络发送到本地。源码如下:

// 把可读RTP packet封装成要发送出去的数据 序列化int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){    int hdrlen;    uint32_t i;    uint8_t* ptr;    if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4))    {        assert(0); // RTP version field must equal 2 (p66)        return -1;    }    // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展    hdrlen = RTP_FIXED_HEADER   pkt->rtp.cc * 4   (pkt->rtp.x ? 4 : 0);    if (bytes < hdrlen   pkt->extlen)        return -1;    ptr = (uint8_t *)data;    nbo_write_rtp_header(ptr, &pkt->rtp);    ptr  = RTP_FIXED_HEADER;    // pkt contributing source    for (i = 0; i < pkt->rtp.cc; i  , ptr  = 4)    {        nbo_w32(ptr, pkt->csrc[i]);     // csrc列表封装到头部    }    // pkt header extension    if (1 == pkt->rtp.x)    {        // 5.3.1 RTP Header Extension        assert(0 == (pkt->extlen % 4));        nbo_w16(ptr, pkt->reserved);        nbo_w16(ptr   2, pkt->extlen / 4);        memcpy(ptr   4, pkt->extension, pkt->extlen);   // extension封装到头部        ptr  = pkt->extlen   4;    }    return hdrlen   pkt->extlen;}

(6)获取nalu的sps、pps，并封装成RTP包。源码如下:

int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){ int hdrlen; uint32_t i; uint8_t* ptr; if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4)) { assert(0); // RTP version field must equal 2 (p66) return -1; } // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展 hdrlen = RTP_FIXED_HEADER pkt->rtp.cc * 4 (pkt->rtp.x ? 4 : 0); if (bytes < hdrlen pkt->extlen) return -1; ptr = (uint8_t *)data; nbo_write_rtp_header(ptr, &pkt->rtp); ptr = RTP_FIXED_HEADER; // pkt contributing source for (i = 0; i < pkt->rtp.cc; i , ptr = 4) { nbo_w32(ptr, pkt->csrc[i]); // csrc列表封装到头部 } // pkt header extension if (1 == pkt->rtp.x) { // 5.3.1 RTP Header Extension assert(0 == (pkt->extlen % 4)); nbo_w16(ptr, pkt->reserved); nbo_w16(ptr 2, pkt->extlen / 4); memcpy(ptr 4, pkt->extension, pkt->extlen); // extension封装到头部 ptr = pkt->extlen 4; } return hdrlen pkt->extlen;}

(7)rtp-payload.c的函数都是接口层，正真实现体都在rtp-h264-pack.c和rtp-h264-unpack.c里。

int rtp_packet_serialize(const struct rtp_packet_t *pkt, void* data, int bytes){    int hdrlen;    hdrlen = rtp_packet_serialize_header(pkt, data, bytes);    if (hdrlen < RTP_FIXED_HEADER || hdrlen   pkt->payloadlen > bytes)        return -1;    memcpy(((uint8_t*)data)   hdrlen, pkt->payload, pkt->payloadlen);    return hdrlen   pkt->payloadlen;}

(8)接口ctx->encoder->input(ctx->packer, data, bytes, timestamp)执行的实现体rtp_h264_pack_input(void* pack, const void* h264, int bytes, uint32_t timestamp)，通过如下源码，可以看出。

// 把可读RTP packet封装成要发送出去的数据 序列化int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){ int hdrlen; uint32_t i; uint8_t* ptr; if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4)) { assert(0); // RTP version field must equal 2 (p66) return -1; } // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展 hdrlen = RTP_FIXED_HEADER pkt->rtp.cc * 4 (pkt->rtp.x ? 4 : 0); if (bytes < hdrlen pkt->extlen) return -1; ptr = (uint8_t *)data; nbo_write_rtp_header(ptr, &pkt->rtp); ptr = RTP_FIXED_HEADER; // pkt contributing source for (i = 0; i < pkt->rtp.cc; i , ptr = 4) { nbo_w32(ptr, pkt->csrc[i]); // csrc列表封装到头部 } // pkt header extension if (1 == pkt->rtp.x) { // 5.3.1 RTP Header Extension assert(0 == (pkt->extlen % 4)); nbo_w16(ptr, pkt->reserved); nbo_w16(ptr 2, pkt->extlen / 4); memcpy(ptr 4, pkt->extension, pkt->extlen); // extension封装到头部 ptr = pkt->extlen 4; } return hdrlen pkt->extlen;}

(9)从这里就可以实现打包，源码如下:

int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){    int hdrlen;    uint32_t i;    uint8_t* ptr;    if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4))    {        assert(0); // RTP version field must equal 2 (p66)        return -1;    }    // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展    hdrlen = RTP_FIXED_HEADER   pkt->rtp.cc * 4   (pkt->rtp.x ? 4 : 0);    if (bytes < hdrlen   pkt->extlen)        return -1;    ptr = (uint8_t *)data;    nbo_write_rtp_header(ptr, &pkt->rtp);    ptr  = RTP_FIXED_HEADER;    // pkt contributing source    for (i = 0; i < pkt->rtp.cc; i  , ptr  = 4)    {        nbo_w32(ptr, pkt->csrc[i]);     // csrc列表封装到头部    }    // pkt header extension    if (1 == pkt->rtp.x)    {        // 5.3.1 RTP Header Extension        assert(0 == (pkt->extlen % 4));        nbo_w16(ptr, pkt->reserved);        nbo_w16(ptr   2, pkt->extlen / 4);        memcpy(ptr   4, pkt->extension, pkt->extlen);   // extension封装到头部        ptr  = pkt->extlen   4;    }    return hdrlen   pkt->extlen;}

(10)函数rtp_h264_pack_nalu(packer, p1, (int)nalu_size)意思是，正如前面文章所讲，是直接打包，就是没有切片，一个nalu打包为一个RTP包。源码如下:

// 把可读RTP packet封装成要发送出去的数据 序列化int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){ int hdrlen; uint32_t i; uint8_t* ptr; if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4)) { assert(0); // RTP version field must equal 2 (p66) return -1; } // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展 hdrlen = RTP_FIXED_HEADER pkt->rtp.cc * 4 (pkt->rtp.x ? 4 : 0); if (bytes < hdrlen pkt->extlen) return -1; ptr = (uint8_t *)data; nbo_write_rtp_header(ptr, &pkt->rtp); ptr = RTP_FIXED_HEADER; // pkt contributing source for (i = 0; i < pkt->rtp.cc; i , ptr = 4) { nbo_w32(ptr, pkt->csrc[i]); // csrc列表封装到头部 } // pkt header extension if (1 == pkt->rtp.x) { // 5.3.1 RTP Header Extension assert(0 == (pkt->extlen % 4)); nbo_w16(ptr, pkt->reserved); nbo_w16(ptr 2, pkt->extlen / 4); memcpy(ptr 4, pkt->extension, pkt->extlen); // extension封装到头部 ptr = pkt->extlen 4; } return hdrlen pkt->extlen;}

(11)函数调用rtp_packet_serialize(&packer->pkt, rtp, n)，就是一个打包的过程，就是严格按照RTP的SPEC来实现。源码如下:

// 拿到一帧RTP序列化后的数据static int rtp_encode_packet(void* param, const void *packet, int bytes, uint32_t timestamp, int flags){    struct rtp_h264_test_t* ctx = (struct rtp_h264_test_t*)param;//拿到用户传递进来的参数    int ret = 0;    ret = sendto(                ctx->fd,                (void*)packet,                bytes,                0,                (struct sockaddr*)&ctx->addr,                ctx->addr_size);    uint8_t *nalu = (uint8_t *)packet;    printf('rtp send packet -> nalu_type:%d,0x%02x,0x%02x, bytes:%d, timestamp:%u\n',           nalu[12]&0x1f,  nalu[12],  nalu[13], bytes, timestamp);    ret = rtp_payload_decode_input(ctx->decoder_h264, packet, bytes);       // 重新又解封装    return 0;}

// 把可读RTP packet封装成要发送出去的数据 序列化int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){ int hdrlen; uint32_t i; uint8_t* ptr; if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4)) { assert(0); // RTP version field must equal 2 (p66) return -1; } // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展 hdrlen = RTP_FIXED_HEADER pkt->rtp.cc * 4 (pkt->rtp.x ? 4 : 0); if (bytes < hdrlen pkt->extlen) return -1; ptr = (uint8_t *)data; nbo_write_rtp_header(ptr, &pkt->rtp); ptr = RTP_FIXED_HEADER; // pkt contributing source for (i = 0; i < pkt->rtp.cc; i , ptr = 4) { nbo_w32(ptr, pkt->csrc[i]); // csrc列表封装到头部 } // pkt header extension if (1 == pkt->rtp.x) { // 5.3.1 RTP Header Extension assert(0 == (pkt->extlen % 4)); nbo_w16(ptr, pkt->reserved); nbo_w16(ptr 2, pkt->extlen / 4); memcpy(ptr 4, pkt->extension, pkt->extlen); // extension封装到头部 ptr = pkt->extlen 4; } return hdrlen pkt->extlen;}

(12)把可读RTP packet封装成要发送出去的数据，与上面的函数是反向关系。这里把头部和数据分开处理。从packet读取一个个字节，然后封包去处理。

int rtp_packet_serialize_header(const struct rtp_packet_t *pkt, void* data, int bytes){    int hdrlen;    uint32_t i;    uint8_t* ptr;    if (RTP_VERSION != pkt->rtp.v || 0 != (pkt->extlen % 4))    {        assert(0); // RTP version field must equal 2 (p66)        return -1;    }    // RFC3550 5.1 RTP Fixed Header Fields(p12) 是否要扩展    hdrlen = RTP_FIXED_HEADER   pkt->rtp.cc * 4   (pkt->rtp.x ? 4 : 0);    if (bytes < hdrlen   pkt->extlen)        return -1;    ptr = (uint8_t *)data;    nbo_write_rtp_header(ptr, &pkt->rtp);    ptr  = RTP_FIXED_HEADER;    // pkt contributing source    for (i = 0; i < pkt->rtp.cc; i  , ptr  = 4)    {        nbo_w32(ptr, pkt->csrc[i]);     // csrc列表封装到头部    }    // pkt header extension    if (1 == pkt->rtp.x)    {        // 5.3.1 RTP Header Extension        assert(0 == (pkt->extlen % 4));        nbo_w16(ptr, pkt->reserved);        nbo_w16(ptr   2, pkt->extlen / 4);        memcpy(ptr   4, pkt->extension, pkt->extlen);   // extension封装到头部        ptr  = pkt->extlen   4;    }    return hdrlen   pkt->extlen;}

(13)RTP打包H264函数，把带startcode的nalu也传递进来。如果nalu_size RTP_FIXED_HEADER <= (size_t)packer->size，这就是前面所说的打包的一种方式，正好吻合起来，如下:

static int rtp_h264_pack_fu_a(struct rtp_encode_h264_t *packer, const uint8_t* nalu, int bytes){ int r, n; unsigned char *rtp; // RFC6184 5.3. NAL Unit Header Usage: Table 2 (p15) // RFC6184 5.8. Fragmentation Units (FUs) (p29) uint8_t fu_indicator = (*nalu & 0xE0) | 28; // FU-A 固定的逻辑 uint8_t fu_header = *nalu & 0x1F; // 设置为s置位1 r = 0; nalu = 1; // skip NAL Unit Type byte bytes -= 1; assert(bytes > 0); // FU-A start for (fu_header |= FU_START; 0 == r && bytes > 0; packer->pkt.rtp.seq) { if (bytes RTP_FIXED_HEADER <= packer->size - N_FU_HEADER) { assert(0 == (fu_header & FU_START)); fu_header = FU_END | (fu_header & 0x1F); // FU-A end 整个nalu结束了 packer->pkt.payloadlen = bytes; } else { packer->pkt.payloadlen = packer->size - RTP_FIXED_HEADER - N_FU_HEADER; } packer->pkt.payload = nalu; n = RTP_FIXED_HEADER N_FU_HEADER packer->pkt.payloadlen; rtp = (uint8_t*)packer->handler.alloc(packer->cbparam, n); if (!rtp) return -ENOMEM; packer->pkt.rtp.m = (FU_END & fu_header) ? 1 : 0; // set marker flag n = rtp_packet_serialize_header(&packer->pkt, rtp, n); if (n != RTP_FIXED_HEADER) { assert(0); return -1; } /*fu_indicator fu_header*/ rtp[n 0] = fu_indicator; rtp[n 1] = fu_header; memcpy(rtp n N_FU_HEADER, packer->pkt.payload, packer->pkt.payloadlen); // packer->cbparam用户的参数 r = packer->handler.packet(packer->cbparam, rtp, n N_FU_HEADER packer->pkt.payloadlen, packer->pkt.rtp.timestamp, 0); packer->handler.free(packer->cbparam, rtp); bytes -= packer->pkt.payloadlen; nalu = packer->pkt.payloadlen; fu_header &= 0x1F; // clear flags } return r;}

(14)把一个比较大的nalu，分多次进行打包，使用FU的方式。代码的思想，如同前面的文章所描述。代码如下:

static int rtp_h264_pack_fu_a(struct rtp_encode_h264_t *packer, const uint8_t* nalu, int bytes){    int r, n;    unsigned char *rtp;    // RFC6184 5.3. NAL Unit Header Usage: Table 2 (p15)    // RFC6184 5.8. Fragmentation Units (FUs) (p29)    uint8_t fu_indicator = (*nalu & 0xE0) | 28; // FU-A 固定的逻辑    uint8_t fu_header = *nalu & 0x1F;           // 设置为s置位1    r = 0;    nalu  = 1; // skip NAL Unit Type byte    bytes -= 1;    assert(bytes > 0);    // FU-A start    for (fu_header |= FU_START; 0 == r && bytes > 0;   packer->pkt.rtp.seq)    {        if (bytes   RTP_FIXED_HEADER <= packer->size - N_FU_HEADER)        {            assert(0 == (fu_header & FU_START));            fu_header = FU_END | (fu_header & 0x1F); // FU-A end  整个nalu结束了            packer->pkt.payloadlen = bytes;        }        else        {            packer->pkt.payloadlen = packer->size - RTP_FIXED_HEADER - N_FU_HEADER;        }        packer->pkt.payload = nalu;        n = RTP_FIXED_HEADER   N_FU_HEADER   packer->pkt.payloadlen;        rtp = (uint8_t*)packer->handler.alloc(packer->cbparam, n);        if (!rtp) return -ENOMEM;        packer->pkt.rtp.m = (FU_END & fu_header) ? 1 : 0; // set marker flag        n = rtp_packet_serialize_header(&packer->pkt, rtp, n);        if (n != RTP_FIXED_HEADER)        {            assert(0);            return -1;        }        /*fu_indicator   fu_header*/        rtp[n   0] = fu_indicator;        rtp[n   1] = fu_header;        memcpy(rtp   n   N_FU_HEADER, packer->pkt.payload, packer->pkt.payloadlen);        // packer->cbparam用户的参数        r = packer->handler.packet(packer->cbparam, rtp, n   N_FU_HEADER   packer->pkt.payloadlen, packer->pkt.rtp.timestamp, 0);        packer->handler.free(packer->cbparam, rtp);        bytes -= packer->pkt.payloadlen;        nalu  = packer->pkt.payloadlen;        fu_header &= 0x1F; // clear flags    }    return r;}

(15)函数rtp_h264_pack_fu_a(struct rtp_encode_h264_t *packer, const uint8_t* nalu, int bytes)表示一个大的nalu分多次打包的过程，这个原理前面的文章，已经讲过了。源码如下:

static int rtp_h264_pack_fu_a(struct rtp_encode_h264_t *packer, const uint8_t* nalu, int bytes){ int r, n; unsigned char *rtp; // RFC6184 5.3. NAL Unit Header Usage: Table 2 (p15) // RFC6184 5.8. Fragmentation Units (FUs) (p29) uint8_t fu_indicator = (*nalu & 0xE0) | 28; // FU-A 固定的逻辑 uint8_t fu_header = *nalu & 0x1F; // 设置为s置位1 r = 0; nalu = 1; // skip NAL Unit Type byte bytes -= 1; assert(bytes > 0); // FU-A start for (fu_header |= FU_START; 0 == r && bytes > 0; packer->pkt.rtp.seq) { if (bytes RTP_FIXED_HEADER <= packer->size - N_FU_HEADER) { assert(0 == (fu_header & FU_START)); fu_header = FU_END | (fu_header & 0x1F); // FU-A end 整个nalu结束了 packer->pkt.payloadlen = bytes; } else { packer->pkt.payloadlen = packer->size - RTP_FIXED_HEADER - N_FU_HEADER; } packer->pkt.payload = nalu; n = RTP_FIXED_HEADER N_FU_HEADER packer->pkt.payloadlen; rtp = (uint8_t*)packer->handler.alloc(packer->cbparam, n); if (!rtp) return -ENOMEM; //视频拆包时,nalu的结束标志 packer->pkt.rtp.m = (FU_END & fu_header) ? 1 : 0; // set marker flag n = rtp_packet_serialize_header(&packer->pkt, rtp, n); if (n != RTP_FIXED_HEADER) { assert(0); return -1; } /*fu_indicator fu_header*/ rtp[n 0] = fu_indicator; rtp[n 1] = fu_header; memcpy(rtp n N_FU_HEADER, packer->pkt.payload, packer->pkt.payloadlen); // packer->cbparam用户的参数，回调用户定义的函数，前面已介绍过 r = packer->handler.packet(packer->cbparam, rtp, n N_FU_HEADER packer->pkt.payloadlen, packer->pkt.rtp.timestamp, 0); packer->handler.free(packer->cbparam, rtp); bytes -= packer->pkt.payloadlen; nalu = packer->pkt.payloadlen; fu_header &= 0x1F; // clear flags } return r;}

(16)前面的源码已经分析过，函数rtp_encode_packet(void* param, const void *packet, int bytes, uint32_t timestamp, int flags)是用户定义的函数，主要是用作发送数据和自定义封装函数(把这个两个函数写在一起不是很合适)。函数packer->handler.packet(packer->cbparam, rtp, n, packer->pkt.rtp.timestamp, 0)，这里会回调用户定义的rtp_encode_packet(void* param, const void *packet, int bytes, uint32_t timestamp, int flags)，就通过网络去发送数据到指定的IP上，我这里就是到本机。用户的参数和定义的函数就是通过 struct rtp_payload_t handler_rtp_decode_h264和struct rtp_h264_test_t ctx来完成。再回顾下这个源码，如下:

// 拿到一帧RTP序列化后的数据static int rtp_encode_packet(void* param, const void *packet, int bytes, uint32_t timestamp, int flags){    struct rtp_h264_test_t* ctx = (struct rtp_h264_test_t*)param;//拿到用户传递进来的参数    int ret = 0;    ret = sendto(                ctx->fd,                (void*)packet,                bytes,                0,                (struct sockaddr*)&ctx->addr,                ctx->addr_size);    uint8_t *nalu = (uint8_t *)packet;    printf('rtp send packet -> nalu_type:%d,0x%02x,0x%02x, bytes:%d, timestamp:%u\n',           nalu[12]&0x1f,  nalu[12],  nalu[13], bytes, timestamp);    ret = rtp_payload_decode_input(ctx->decoder_h264, packet, bytes);       // 重新又解封装    return 0;}

3.3 解封RTP包流程源码

(1)函数rtp_payload_decode_create(int payload, const char* name, struct rtp_payload_t *handler, void* cbparam)接收外部参数。源码如下:

void* rtp_payload_decode_create(int payload, const char* name, struct rtp_payload_t *handler, void* cbparam){ struct rtp_payload_delegate_t* ctx; ctx = calloc(1, sizeof(*ctx)); if (ctx) { if (rtp_payload_find(payload, name, ctx) < 0 || NULL == (ctx->packer = ctx->decoder->create(handler, cbparam))) { free(ctx); return NULL; } } return ctx;}

(2)通过如下代码可以看出，ctx->decoder->create指向的函数是rtp_h264_unpack_create(struct rtp_payload_t *handler, void* param)，也就是正真的实现体。源码如下:

struct rtp_payload_decode_t *rtp_h264_decode(){    static struct rtp_payload_decode_t unpacker = {        rtp_h264_unpack_create,        rtp_h264_unpack_destroy,        rtp_h264_unpack_input,    };    return &unpacker;}

static void* rtp_h264_unpack_create(struct rtp_payload_t *handler, void* param){ struct rtp_decode_h264_t *unpacker; unpacker = (struct rtp_decode_h264_t *)calloc(1, sizeof(*unpacker)); if(!unpacker) return NULL; memcpy(&unpacker->handler, handler, sizeof(unpacker->handler)); unpacker->cbparam = param; unpacker->flags = -1; return unpacker;}

(3)函数rtp_payload_decode_input(ctx->decoder_h264, packet, bytes)就是处在接口层，正真的实现是由ctx->decoder->input指向的函数rtp_h264_unpack_input(void* p, const void* packet, int bytes)(该函数才是正真的实现体)，根据packer不同的类型去实现解封装，源码如下:

//解封装static int rtp_h264_unpack_input(void* p, const void* packet, int bytes){    int r;    uint8_t nalt;    struct rtp_packet_t pkt;    struct rtp_decode_h264_t *unpacker;    unpacker = (struct rtp_decode_h264_t *)p;    // 反序列化    if(!unpacker || 0 != rtp_packet_deserialize(&pkt, packet, bytes) || pkt.payloadlen < 1)        return -EINVAL;    if (-1 == unpacker->flags)    {        unpacker->flags = 0;        unpacker->seq = (uint16_t)(pkt.rtp.seq - 1); // disable packet lost    }    if ((uint16_t)pkt.rtp.seq != (uint16_t)(unpacker->seq   1))    {        unpacker->flags = RTP_PAYLOAD_FLAG_PACKET_LOST;        unpacker->size = 0; // discard previous packets    }    unpacker->seq = (uint16_t)pkt.rtp.seq;    //就是payload的第一个字节    nalt = ((unsigned char *)pkt.payload)[0];    switch(nalt & 0x1F)    {    case 0: // reserved    case 31: // reserved        // 这里最好是报错 然后返回错误值        assert(0);        return 0; // packet discard    //对应不同的类型,就做不同的解释    case 24: // STAP-A        return rtp_h264_unpack_stap(unpacker, (const uint8_t*)pkt.payload, pkt.payloadlen, pkt.rtp.timestamp, 0);    case 25: // STAP-B        return rtp_h264_unpack_stap(unpacker, (const uint8_t*)pkt.payload, pkt.payloadlen, pkt.rtp.timestamp, 1);    case 26: // MTAP16        return rtp_h264_unpack_mtap(unpacker, (const uint8_t*)pkt.payload, pkt.payloadlen, pkt.rtp.timestamp, 2);    case 27: // MTAP24        return rtp_h264_unpack_mtap(unpacker, (const uint8_t*)pkt.payload, pkt.payloadlen, pkt.rtp.timestamp, 3);    case 28: // FU-A        return rtp_h264_unpack_fu(unpacker, (const uint8_t*)pkt.payload, pkt.payloadlen, pkt.rtp.timestamp, 0);    case 29: // FU-B        return rtp_h264_unpack_fu(unpacker, (const uint8_t*)pkt.payload, pkt.payloadlen, pkt.rtp.timestamp, 1);    default: // 1-23 NAL unit        r = unpacker->handler.packet(unpacker->cbparam, (const uint8_t*)pkt.payload, pkt.payloadlen, pkt.rtp.timestamp, unpacker->flags);        unpacker->flags = 0;        unpacker->size = 0;        return 0 == r ? 1 : r; // packet handled    }}

(4)如果是1对1的RTP包，直接就反序列化即可。如收到一个UDP包，有1000字节，就可以解析出这个udp包的rtp信息。通过收到的数据，解析出来可读的RTP Packet。

// RFC3550 RTP: A Transport Protocol for Real-Time Applications// 5.1 RTP Fixed Header Fields (p12)/* 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 | = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = | contributing source (CSRC) identifiers || .... | - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */// 通过收到的数据，解析出来可读的RTP packet 反序列化int rtp_packet_deserialize(struct rtp_packet_t *pkt, const void* data, int bytes){ uint32_t i, v; int hdrlen; const uint8_t *ptr; if (bytes < RTP_FIXED_HEADER) // RFC3550 5.1 RTP Fixed Header Fields(p12) return -1; ptr = (const unsigned char *)data; memset(pkt, 0, sizeof(struct rtp_packet_t)); // pkt header v = nbo_r32(ptr); pkt->rtp.v = RTP_V(v); pkt->rtp.p = RTP_P(v); pkt->rtp.x = RTP_X(v); pkt->rtp.cc = RTP_CC(v); pkt->rtp.m = RTP_M(v); pkt->rtp.pt = RTP_PT(v); pkt->rtp.seq = RTP_SEQ(v); pkt->rtp.timestamp = nbo_r32(ptr 4); pkt->rtp.ssrc = nbo_r32(ptr 8); assert(RTP_VERSION == pkt->rtp.v); // 调试的时候用 hdrlen = RTP_FIXED_HEADER pkt->rtp.cc * 4; // 解析带csrc时的总长度 if (RTP_VERSION != pkt->rtp.v || bytes < hdrlen (pkt->rtp.x ? 4 : 0) (pkt->rtp.p ? 1 : 0)) return -1; // 报错 // pkt contributing source for (i = 0; i < pkt->rtp.cc; i ) { pkt->csrc[i] = nbo_r32(ptr 12 i * 4); } assert(bytes >= hdrlen); pkt->payload = (uint8_t*)ptr hdrlen; // 跳过头部 拿到payload pkt->payloadlen = bytes - hdrlen; // payload长度 // pkt header extension if (1 == pkt->rtp.x) { const uint8_t *rtpext = ptr hdrlen; assert(pkt->payloadlen >= 4); pkt->extension = rtpext 4; pkt->reserved = nbo_r16(rtpext); pkt->extlen = nbo_r16(rtpext 2) * 4; if (pkt->extlen 4 > pkt->payloadlen) { assert(0); return -1; } else { //有扩展 pkt->payload = rtpext pkt->extlen 4; pkt->payloadlen -= pkt->extlen 4; } } // padding if (1 == pkt->rtp.p) { uint8_t padding = ptr[bytes - 1]; if (pkt->payloadlen < padding) { assert(0); return -1; } else { pkt->payloadlen -= padding; } } return 0;}

(5)将视频RTP包解封装为nalu，其原理就是封包的逆向过程，通过不同类型的数据包采用不同的打拆包方式，1对多的RTP拆包，源码如下:

// 5.8. Fragmentation Units (FUs) (p29)/* 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 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - |  FU indicator |   FU header   |              DON              | - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -||                                                               ||                          FU payload                           ||                                                               ||                                - - - - - - - - - - - - - - - - |                               :   ...OPTIONAL RTP padding     | - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */static int rtp_h264_unpack_fu(struct rtp_decode_h264_t *unpacker, const uint8_t* ptr, int bytes, uint32_t timestamp, int fu_b){    int r, n;    uint8_t fuheader;    //uint16_t don;    r = 0;    n = fu_b ? 4 : 2;   // payload前面有几个字节是供fu_a或fu_b使用， fu_a只有2个字节; fu_b是4个字节    if (bytes < n || unpacker->size   bytes - n > RTP_PAYLOAD_MAX_SIZE)    {        assert(0);        return -EINVAL; // error    }    if (unpacker->size   bytes - n   1 /*NALU*/ > unpacker->capacity)    {        void* p = NULL;        int size = unpacker->size   bytes   1;        size  = size / 4 > 128000 ? size / 4 : 128000;        p = realloc(unpacker->ptr, size);        if (!p)        {            // set packet lost flag            unpacker->flags = RTP_PAYLOAD_FLAG_PACKET_LOST;            unpacker->size = 0;            return -ENOMEM; // error        }        unpacker->ptr = (uint8_t*)p;        unpacker->capacity = size;    }    fuheader = ptr[1];    //don = nbo_r16(ptr   2);    if (FU_START(fuheader))//头    {#if 0        if (unpacker->size > 0)        {            unpacker->flags |= RTP_PAYLOAD_FLAG_PACKET_CORRUPT;            unpacker->handler.packet(unpacker->cbparam, unpacker->ptr, unpacker->size, unpacker->timestamp, unpacker->flags);            unpacker->flags = 0;            unpacker->size = 0; // reset        }#endif        unpacker->size = 1; // NAL unit type byte        unpacker->ptr[0] = (ptr[0]/*indicator*/ & 0xE0) | (fuheader & 0x1F);        assert(H264_NAL(unpacker->ptr[0]) > 0 && H264_NAL(unpacker->ptr[0]) < 24);    }    else    {        if (0 == unpacker->size)//中间包        {            unpacker->flags = RTP_PAYLOAD_FLAG_PACKET_LOST;            return 0; // packet discard        }        assert(unpacker->size > 0);    }    unpacker->timestamp = timestamp;    if (bytes > n)    {        assert(unpacker->capacity >= unpacker->size   bytes - n);        memmove(unpacker->ptr   unpacker->size, ptr   n, bytes - n);        unpacker->size  = bytes - n;    }    if(FU_END(fuheader))//中间过程    {        if(unpacker->size > 0)      // 多次传入数据后等到FU_END的时候难道一个完整的nalu            r = unpacker->handler.packet(unpacker->cbparam, unpacker->ptr, unpacker->size, timestamp, unpacker->flags);        unpacker->flags = 0;        unpacker->size = 0; // reset    }    return 0 == r ? 1 : r; // packet handled}

(6)函数unpacker->handler.packet(unpacker->cbparam, (const uint8_t*)pkt.payload, pkt.payloadlen, pkt.rtp.timestamp, unpacker->flags)，也会回调用户自定义的函数去写入文件，注意：写文件前，需要加上startcode。源码如下:

static int rtp_decode_packet(void* param, const void *packet, int bytes, uint32_t timestamp, int flags){ static const uint8_t start_code[4] = { 0, 0, 0, 1 }; struct rtp_h264_test_t* ctx = (struct rtp_h264_test_t*)param; static uint8_t buffer[2 * 1024 * 1024]; assert(bytes 4 < sizeof(buffer)); assert(0 == flags); size_t size = 0; if (0 == strcmp('H264', ctx->encoding) || 0 == strcmp('H265', ctx->encoding)) { memcpy(buffer, start_code, sizeof(start_code));//添加startcode size = sizeof(start_code); } else if (0 == strcasecmp('mpeg4-generic', ctx->encoding)) { int len = bytes 7; uint8_t profile = 2; uint8_t sampling_frequency_index = 4; uint8_t channel_configuration = 2; buffer[0] = 0xFF; /* 12-syncword */ buffer[1] = 0xF0 /* 12-syncword */ | (0 << 3)/*1-ID*/ | (0x00 << 2) /*2-layer*/ | 0x01 /*1-protection_absent*/; buffer[2] = ((profile - 1) << 6) | ((sampling_frequency_index & 0x0F) << 2) | ((channel_configuration >> 2) & 0x01); buffer[3] = ((channel_configuration & 0x03) << 6) | ((len >> 11) & 0x03); /*0-original_copy*/ /*0-home*/ /*0-copyright_identification_bit*/ /*0-copyright_identification_start*/ buffer[4] = (uint8_t)(len >> 3); buffer[5] = ((len & 0x07) << 5) | 0x1F; buffer[6] = 0xFC | ((len / 1024) & 0x03); size = 7; } memcpy(buffer size, packet, bytes); size = bytes; printf('nalu get -> bytes:%d, timestamp:%u\n', size, timestamp); // TODO: // check media file fwrite(buffer, 1, size, ctx->out_file);}

4.总结

详解RTP协议之H264封包和解包实战

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