关于socket.io的EIO

从firebug查到请求的充列是 /socket.io/?EIO=3&transport=polling&t=1429843011171-0

/socket.io/?EIO=3&transport=polling&t=1429843012696-1&sid=b9HirfcuubHtEDvRAAAA

/socket.io/?EIO=3&transport=websocket&sid=b9HirfcuubHtEDvRAAAA

/socket.io/?EIO=3&transport=polling&t=1429843013752-2&sid=b9HirfcuubHtEDvRAAAA 后面的请求就用websocket开始通信了

我猜应该优先用websocket,再采用flash,再采用polling 为什么是一上来是发两次polling 再发webscoket请求 ws以后再发一次polling,然后用websocket通信 是不这个是polling是用来生成sid的?