"TCP/IP detailed Volume One" Reading notes-----Ping

1, Ping is used to test whether the host can reach the command, in fact, is essentially the echo type of ICMP message. Also, ping can be used to calculate RTT (round-trip time), which is the round-trip delay between two hosts.

2, with the increase of network security awareness, router access control and firewall and other network security measures, so that the accessibility of the host not only depends on the status of the network layer, but also with the type of protocol and port number and other factors related, so now the failure of ping command can not represent the destination host is unreachable.

3, IP record route: Many versions of the ping command after adding the-r option, the ability to let the datagram pass through each router in the IP datagram header of the optional part (IP datagram head maximum 60 bytes, Immediately following the 20-byte fixed header is the optional part) to fill in its own IP address. When the data is reported to the destination host, these IP addresses are then filled into the ICMP echo reply message, and the routers on the return path do the same. Finally, the source host can parse the entire interaction process with the destination host from the return message of the ping command to pass through those routers.

Of course, we can see that there is a problem behind such a mechanism because the optional field in the header of the IP data has a maximum of 40 bytes. In these 40 bytes, minus the three identity bytes associated with the record route, only 37 bytes can be used, which means that only 9 IP v4 addresses can be stored. This is not enough in the current Internet environment.

4, IP timestamp option: This is similar to the IP record route, but instead changes the original IP address to the time elapsed through the router. And it has four bits of the overflow field, in addition to the three identity bytes that are the same as the record route, to identify whether the record's timestamp overflow, and the other four bits of the flags field to identify the different timestamp types. One of the types that is worth mentioning is that the IP address of the router is also recorded while recording the timestamp. In fact, if we think about it, we can find that the timestamp of passing through the router is insignificant, but if the IP address is also recorded, then the optional field in the header of the IP data can only record four records.

"TCP/IP detailed Volume One" Reading notes-----Ping