packetization-mode=1 in SDP

利用wireshark进行抓包时，总是发现一连串RTP包的Payload中以0x7C开头，如图:

**RTP payload start with 0x7c**

到底是为什么呢?

Actually, you was correct for h264 stream,
if the NAL value (first byte) is 0x7C, it means that the I-Frame is fragmented.
No other frames (P and B) can be fragmented, so if there is packetization-mode=1 in SDP, then it means that the I-Frames are fragmented, and therefore if you read 0x7C as first byte, then it is I-Frame. Read more here: http://www.rfc-editor.org/rfc/rfc3984.txt.

如果sdp信息的packetization-mode=1, 则读到0x7C时，表明该payload是I帧的一部分(I帧被拆分成多个部分来进行传输)。
如果是通过rtsp的方式传输流，packetization-mode参数可以在DESCRIBE的响应报文中看到。

DESCRIBE

packetization-mode in SDP

References:

http://stackoverflow.com/questions/1957427/detect-mpeg4-h264-i-frame-idr-in-rtp-stream