Tcp Tutorial-11

The IP Protocol


Both the TCP and the UDP protocols, after inserting the headers to the datagram(s) given to them pass them to the Internet Protocol or the IP Protocol. The main job of the IP protocol is to find a way of transporting the datagrams to the destination receiver. It does not do any kind of error checking.

The IP protocol too adds it's own IP Header to each datagram. The IP header contains the source and destination IP addresses, the protocol number and yet another checksum. The IP header of a particular datagram looks like-:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL |Type of Service| Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TCP header info followed by the actual data being transferred| | |

number is added so that the IP protocol knows to which Transport Protocol the datagram has to be passed.

You see various Transport Protocols are used like for example TCP or UDP. So this protocol number is inserted to tell IP the protocol to which the datagram has to be passed.

It too inserts it's own Checksum value which is different from the Checksum Value inserted by the Transport Protocols. This Checksum has to be inserted as without it the Internet Protocol will not be able to verify if the Header has been damaged in the transfer process and hence the datagram might reach a wrong destination. The Time to Live field specifies a value which is decreased each time the datagram passes through a network. Remember Tracert?

The Internet Protocol Header contains other fields as well, but they are quite advanced and cannot be included in a manual which gives an introduction to the TCP\IP protocol. To learn more about the IP protocol read RFC 791.

The Internet Control Message Protocol or the ICMP

The ICMP protocol allows hosts to transfer information on errors that might have occurred during the data transfer between two hosts. It is basically used to display error messages about errors that might occur during the data transfer. The ICMP is a very simple protocol without any headers. It is most commonly used to diagnose Network Problems. The famous utility PING is a part of the ICMP protocol. ICMP requests do not require the user or application to mention any port number as all ICMP requests are answered by the Network Software itself. The ICMP protocol too handles only a single datagram. That's why we say in PING only a single datagram is sent to the remote computer. This protocol can remote many network problems like Host Down, Congested Network etc