for the receiver of tcp, when a packet arrives in sequence and all previous

已经接收的包都已确认，则说明该包是完整的，可以直接交给上层应用程序处理。TCP协议是面向连接的可靠传输协议，在数据传输过程中，数据被分割成一个个包进行传输，并通过序列号进行排序和重组。当TCP接收端收到一个包时，会首先确认该包是否按照正确的顺序到达，如果该包的序列号与之前接收到的包的序列号连续，且之前的所有包都已经被确认，则可以判断该包是按正确的顺序到达的。

当一个包按序到达时，接收端会认为之前所有的包也已到达，并已经按序被正确地接收和确认。此时，接收端可以将该包交给上层应用程序进行处理。因为TCP是可靠的协议，所以接收端可以确保上层应用程序接收到的是完整、正确的数据。

在实现过程中，接收端会维护一个接收缓冲区，用于存储已经接收到的包。当一个新的包到达时，接收端会检查该包的序列号是否与已经接收到的包的序列号连续，如果连续则将该包存储到接收缓冲区，并更新已确认的序列号。当上层应用程序需要获取数据时，接收端会从接收缓冲区中读取数据并传递给应用程序进行处理。

总之，当TCP接收端接收到一个按序到达的包时，并且之前的所有包都已经按序被正确接收和确认时，接收端可以直接将该包交给上层应用程序进行处理，确保应用程序接收到的数据是完整且正确的。

相关问题

What is the purpose of TCP fast retransmission

The purpose of TCP fast retransmission is to improve the reliability and performance of data transmission over a network. When a TCP sender detects the loss of a packet, it will typically wait for a certain period of time before retransmitting the packet. With fast retransmission, however, the sender can quickly detect that a packet has been lost by receiving duplicate acknowledgments from the receiver. When the sender receives three duplicate acknowledgments for the same packet, it assumes that the packet has been lost and immediately retransmits it. This helps to reduce the overall delay and improve the throughput of the network by avoiding unnecessary timeouts and retransmissions.