New Understanding of TCP/IP protocol (4)

New Understanding of TCP/IP (4) some basic terms in TCP/IP, such as MSL, TTL, and RTT. MSL (max segment lifetime) indicates the maximum message survival time, that is, the maximum time for a packet to exist on the network. packets exceeding this time will be discarded, in actual application, this value is 30 s, 1 min, 2 min, etc. TTL (time to live) indicates the maximum number of routers that a packet passes through. This value is set by the active host. When a packet passes through a vro, its count is reduced by 1. When the value is 0, this packet is discarded and ICMP notifies the source host. The effect of MSL is greater than TTL. RTT (round-trip time) indicates the round-trip time between the client and the server. TCP contains an algorithm for dynamically estimating the RTT. The initiator of TCP connection closure receives a close request from the peer end, and sends a close confirmation before entering time_wait. The wait time is 2MSL, mainly the time occupied by the port, if SO_ADDRREUSE is set, close_wait is the status of the disabled side. Based on some of the above concepts, when a network exception occurs, there will be a semi-open connection and semi-closed connection, and the emergence of rst commands. For example, after the server is restarted, the client that has previously established a connection sends data without knowing it. This is a semi-closed connection, and the server sends the rst command. Semi-open connections do not receive timely responses during the connection process. The biggest difference between semi-open connections and semi-closed connections is that semi-closed connections cannot be used and semi-open connections can be used. The client sends syn to the server, but the server port is open. At this time, the server also sends rst. If the rst command is run on the server, you can use netstat to check the connection status to confirm the cause of the problem.