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<>UDP协议
* 源端口号,目的端口号:发送端和接收端的端口号
* 长度:UDP用户数据报的长度,最小值是8字节(只有首部)
* 校验和:检测UDP用户数据报在传输中是否出错,出错就丢弃
* 源端口号,目的端口号:发送端和接收方的窗口号
* 序列号:本报文段的数据第一个字节的序号
* 确认号:期望收到的对方下一个报文段的第一个数据字节的序号
* 首部长度:TCP报文段的数据起始处距离TCP报文段的起始处有多长,以4字节为计算单位
* 保留:保留为今后使用,目前置为0
* 紧急URG:此位置1,表明紧急指针字段有效,告诉系统此报文段有紧急数据,应尽快传送
* 确认ACK:=1时确认号字段有效,在连接建立后所有传达的报文段都必须把 ACK 置1
* 推送PSH:当两个应用进程进行交互式的通信时,有时在一端的应用进程希望在键入一个命令后立即就能够收到对方的响应。在这种情况下,TCP
就可以使用推送(push)操作,这时,发送方TCP 把 PSH 置 1,并立即创建一个报文段发送出去,接收方收到 PSH = 1
的报文段,就尽快地(即“推送”向前)交付给接收应用进程,而不再等到整个缓存都填满后再向上交付
* 复位RST:用于复位相应的TCP连接
* 同步SYN:仅在三次握手建立TCP连接时有效。
SYN=1,ACK=0时,表明这是一个连接请求报文段,若对方同意建立连接,则应在相应的报文段中使用SYN=1和ACK=1
* 终止FIN:用来释放一个连接,当FIN=1时,表明此报文段发送方的数据已经发送完毕,并要求释放连接
* 窗口:指发送本报文段的一方的接受窗口(不是自己的发送窗口)
* 校验和:校验和字段检验的范围包括首部和数据两部分,在计算校验和时需要加上 12 字节的伪头部
* 紧急指针:仅在 URG = 1
时才有意义,它指出本报文段中的紧急数据的字节数(紧急数据结束后就是普通数据),即指出了紧急数据的末尾在报文中的位置,注意:即使窗口为零时也可发送紧急数据
* 选项:长度可变,最多40字节