rtsp剥离负载，发送在连接建立以后，OPTION \DESCRIPT\SETUP\PLAY等方法已相继发送，udp(tcp) 数据传输通道、媒体信息已相应建立，ffmpeg通过注册回调函数read_packet开始读取发来的负载信息，并组装成AVPacket格式返回。
AVInputFormat ff_rtsp_demuxer = {
.name = "rtsp",
.long_name = NULL_IF_CONFIG_SMALL("RTSP input"),
.priv_data_size = sizeof(RTSPState),
.read_probe = rtsp_probe,
.read_header = rtsp_read_header,
.read_packet = rtsp_read_packet,
.read_close = rtsp_read_close,
.read_seek = rtsp_read_seek,
.flags = AVFMT_NOFILE,
.read_play = rtsp_read_play,
.read_pause = rtsp_read_pause,
.priv_class = &rtsp_demuxer_class,
};
对应回调函数rtsp_read_packet，主要做了两件事1、通过套接字读取远端数据 2、解析数据剥离负载
一、读取远端数据
static int rtsp_read_packet(AVFormatContext *s, AVPacket *pkt)
{
RTSPState *rt = s->priv_data;
int ret;
RTSPMessageHeader reply1, *reply = &reply1;
char cmd[1024];
retry:
......................
ret = ff_rtsp_fetch_packet(s, pkt);
................
}此过程发生再ff_rtsp_fetch_packet中，
int ff_rtsp_fetch_packet(AVFormatContext *s, AVPacket *pkt)
{
RTSPState *rt = s->priv_data;
.....
switch(rt->lower_transport) {
default:
#if CONFIG_RTSP_DEMUXER
case RTSP_LOWER_TRANSPORT_TCP:
len = ff_rtsp_tcp_read_packet(s, &rtsp_st, rt->recvbuf, RECVBUF_SIZE);
break;
#endif
case RTSP_LOWER_TRANSPORT_UDP:
case RTSP_LOWER_TRANSPORT_UDP_MULTICAST:
len = udp_read_packet(s, &rtsp_st, rt->recvbuf, RECVBUF_SIZE, wait_end);
if (len > 0 && rtsp_st->transport_priv && rt->transport == RTSP_TRANSPORT_RTP)
ff_rtp_check_and_send_back_rr(rtsp_st->transport_priv, rtsp_st->rtp_handle, NULL, len);
break;
....
}
if (len < 0)
return len;
if (len == 0)
return AVERROR_EOF;
if (rt->transport == RTSP_TRANSPORT_RDT) {
ret = ff_rdt_parse_packet(rtsp_st->transport_priv, pkt, &rt->recvbuf, len);
} else if (rt->transport == RTSP_TRANSPORT_RTP) {
ret = ff_rtp_parse_packet(rtsp_st->transport_priv, pkt, &rt->recvbuf, len);
.....
}如果是使用UDP协议，那么通过读取数据报，得到远端发送的rtp数据。
二、解析音视频负载
在（一）得到远端数据后，ffmpeg马上判断传输类型，并调用ff_rdt_parse_packet/ff_rtp_parse_packet解析。这里我们使用的是rtp数据，所以只看ff_rtp_parse_packet。下面是该函数的核心代码
static int rtp_parse_one_packet(RTPDemuxContext *s, AVPacket *pkt,
uint8_t **bufptr, int len)
{
.....
if ((buf[0] & 0xc0) != (RTP_VERSION << 6))
return -1;
if (RTP_PT_IS_RTCP(buf[1])) {
return rtcp_parse_packet(s, buf, len);
}
....
if ((s->seq == 0 && !s->queue) || s->queue_size <= 1) {
/* First packet, or no reordering */
return rtp_parse_packet_internal(s, pkt, buf, len);
} else {
uint16_t seq = AV_RB16(buf + 2);
int16_t diff = seq - s->seq;
if (diff < 0) {
/* Packet older than the previously emitted one, drop */
av_log(s->ic, AV_LOG_WARNING,
"RTP: dropping old packet received too late\n");
return -1;
} else if (diff <= 1) {
/* Correct packet */
rv = rtp_parse_packet_internal(s, pkt, buf, len);
return rv;
} else {
/* Still missing some packet, enqueue this one. */
rv = enqueue_packet(s, buf, len);
if (rv < 0)
return rv;
*bufptr = NULL;
/* Return the first enqueued packet if the queue is full,
* even if we're missing something */
if (s->queue_len >= s->queue_size) {
av_log(s->ic, AV_LOG_WARNING, "jitter buffer full\n");
return rtp_parse_queued_packet(s, pkt);
}
return -1;
}
}
}这部分开头做了一层过滤，通过PT数值过滤掉RTCP数据，随后判断了rtp-seq值，正常来说seq应该是连续增的，否则做相应的异常处理，得到正常的rtp数据后，便开始真正分析rtp包了（rtp_parse_packet_internal）
static int rtp_parse_packet_internal(RTPDemuxContext *s, AVPacket *pkt,
const uint8_t *buf, int len)
{
unsigned int ssrc;
int payload_type, seq, flags = 0;
int ext, csrc;
AVStream *st;
uint32_t timestamp;
int rv = 0;
csrc = buf[0] & 0x0f;
ext = buf[0] & 0x10;
payload_type = buf[1] & 0x7f;
if (buf[1] & 0x80)
flags |= RTP_FLAG_MARKER;
seq = AV_RB16(buf + 2);
timestamp = AV_RB32(buf + 4);
ssrc = AV_RB32(buf + 8);
/* store the ssrc in the RTPDemuxContext */
s->ssrc = ssrc;
/* NOTE: we can handle only one payload type */
if (s->payload_type != payload_type)
return -1;
st = s->st;
// only do something with this if all the rtp checks pass...
if (!rtp_valid_packet_in_sequence(&s->statistics, seq)) {
av_log(s->ic, AV_LOG_ERROR,
"RTP: PT=%02x: bad cseq %04x expected=%04x\n",
payload_type, seq, ((s->seq + 1) & 0xffff));
return -1;
}
if (buf[0] & 0x20) {
int padding = buf[len - 1];
if (len >= 12 + padding)
len -= padding;
}
s->seq = seq;
len -= 12;
buf += 12;
len -= 4 * csrc;
buf += 4 * csrc;
if (len < 0)
return AVERROR_INVALIDDATA;
/* RFC 3550 Section 5.3.1 RTP Header Extension handling */
if (ext) {
if (len < 4)
return -1;
/* calculate the header extension length (stored as number
* of 32-bit words) */
ext = (AV_RB16(buf + 2) + 1) << 2;
if (len < ext)
return -1;
// skip past RTP header extension
len -= ext;
buf += ext;
}
if (s->handler && s->handler->parse_packet) {
rv = s->handler->parse_packet(s->ic, s->dynamic_protocol_context,
s->st, pkt, ×tamp, buf, len, seq,
flags);
} else if (st) {
if ((rv = av_new_packet(pkt, len)) < 0)
return rv;
memcpy(pkt->data, buf, len);
pkt->stream_index = st->index;
} else {
return AVERROR(EINVAL);
}
// now perform timestamp things....
finalize_packet(s, pkt, timestamp);
return rv;
}
解析函数跳过了前面的head，直接指到rtp音视频数据所在的地方，并调用s->handler->parse_packet，将数据剥离到AVPacket中。现在疑问的是，parse_packet是在哪定义的呢。
经过一番寻找，不难发现，定义该结构的地方如下，再通过GDB断点，打印堆栈，发现就是在解析DESCRIPT负载数据的时候。这一部分定义了rtp所有可能的负载类型，并按ffmpeg特有的风格模块化，各自提供解析回调函数供外界调用。
void ff_register_dynamic_payload_handler(RTPDynamicProtocolHandler *handler)
{
handler->next = rtp_first_dynamic_payload_handler;
rtp_first_dynamic_payload_handler = handler;
}
void ff_register_rtp_dynamic_payload_handlers(void)
{
ff_register_dynamic_payload_handler(&ff_ac3_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_amr_nb_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_amr_wb_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_dv_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_g726_16_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_g726_24_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_g726_32_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_g726_40_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_g726le_16_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_g726le_24_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_g726le_32_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_g726le_40_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_h261_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_h263_1998_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_h263_2000_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_h263_rfc2190_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_h264_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_hevc_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_ilbc_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_jpeg_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_mp4a_latm_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_mp4v_es_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_mpeg_audio_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_mpeg_audio_robust_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_mpeg_video_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_mpeg4_generic_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_mpegts_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_ms_rtp_asf_pfa_handler);
ff_register_dynamic_payload_handler(&ff_ms_rtp_asf_pfv_handler);
ff_register_dynamic_payload_handler(&ff_qcelp_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_qdm2_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_qt_rtp_aud_handler);
ff_register_dynamic_payload_handler(&ff_qt_rtp_vid_handler);
ff_register_dynamic_payload_handler(&ff_quicktime_rtp_aud_handler);
ff_register_dynamic_payload_handler(&ff_quicktime_rtp_vid_handler);
ff_register_dynamic_payload_handler(&ff_svq3_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_theora_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_vc2hq_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_vorbis_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_vp8_dynamic_handler);
ff_register_dynamic_payload_handler(&ff_vp9_dynamic_handler);
ff_register_dynamic_payload_handler(&gsm_dynamic_handler);
ff_register_dynamic_payload_handler(&opus_dynamic_handler);
ff_register_dynamic_payload_handler(&realmedia_mp3_dynamic_handler);
ff_register_dynamic_payload_handler(&speex_dynamic_handler);
ff_register_dynamic_payload_handler(&t140_dynamic_handler);
}
RTPDynamicProtocolHandler *ff_rtp_handler_find_by_name(const char *name,
enum AVMediaType codec_type)
{
RTPDynamicProtocolHandler *handler;
for (handler = rtp_first_dynamic_payload_handler;
handler; handler = handler->next)
if (handler->enc_name &&
!av_strcasecmp(name, handler->enc_name) &&
codec_type == handler->codec_type)
return handler;
return NULL;
}