• 针对SDP里的m= 做的逻辑代码
• 根据SDP反映到sender和receivers ，并关联到内部的cricket::ChannelInterface 。
• 文档说： 管理着 RtpSenders, RtpReceivers, and BaseChannel 与m=
• 例如：当SDP的 m= 变化时，RtpTransceiver 可以控制 senders和 receivers 与 channel 关联发生变化，比如add stop

RtpTransceiver 父类是一个interface，且共有继承了rtc::RefCountInterface

• api\rtp_transceiver_interface.h
  

同时支持planB 和 unified plan 的SDP

• // With Plan B SDP, an RtpTransceiver can have any number of senders and
  // receivers which map to a=ssrc lines in the m= section.
  // With Unified Plan SDP, an RtpTransceiver will have exactly one sender and one
  // receiver which are encapsulated by the m= sectio