<P>A TCP Header contains a lot of information, but the most important of it is the Source and Destination IP and Port numbers and yes also the sequence number.</P>

<P>**************</P>
<P>HACKING TRUTH: Learn more about Ports, IP's, Sockets in the Net Tools Manual</P>
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<P>The source which is your computer(x) now knows what the IP Addresses and Port Numbers of the Destination and Source computers are. It now calculates the Checksum value by adding up all the octets of the datagram and puts the final checksum value to the TCP Header. The different octets and not the datagrams are then numbered. An octet would be a smaller broken down form of the entire data. TCP then puts all this information into the TCP header of each datagram. A TCP Header of a datagram would finally look like -:</P>

<P>&nbsp;</P>
<pre>   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<P>    |          Source Port          |       Destination Port        |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |                        Sequence Number                        |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |                    Acknowledgment Number                      |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |  Data |           |U|A|P|R|S|F|                               |</P>
<P>    | Offset| Reserved  |R|C|S|S|Y|I|            Window             |</P>
<P>    |       |           |G|K|H|T|N|N|                               |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |           Checksum            |         Urgent Pointer        |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |   The Actual Data form the next 500 octets                    |</P>
<P>    |                                                               |</P>
</pre>
<P>&nbsp;</P>
<P>There are certain new fields in the TCP header which you may not know off. Let's see what these new fields signify. The Windows field specifies the octets of new data which is ready to be processed. You see not all computers connected to the Internet run at the same speed and to ensure that a faster system does not send datagrams to a slow system at a rate which is faster than it can handle, we use the Window field. As the computer receives data , the space in the Window field gets decreased indicating that the receiver has received the data. When it reaches zero the sender stops sending further packets. Once the receiver finishes processing the received data, it increases the Window field, which in turn indicates that the receiver has processed the earlier sent data and is ready to receive more chunks of data.</P>

<P>The Urgent Field tells the remote computer to stop processing the last octet and instead receive the new octet. This is normally not commonly used.</P>

<P>The TCP protocol is a reliable protocol, which means that we have a guarantee that the data will arrive at the destination properly and without any errors.  It ensures that the data being received by the receiving end is arranged in the same correct order in which it was sent.</P>

<P>The TCP Protocol relies on a virtual circuit between the client and the host. The circuit is opened via a 3 part process known as the three part handshake. It supports full duplex transportation of data which means that it provides a path for two way data transfer. Hence using the TCP protocol, a computer can send and receive datagrams at the same time.</P>

<P>Read RFC 793 for further in depth details about the TCP protocol.</P>

<P>The User Datagram Protocol or the UDP Protocol</P>

<P>The User Data protocol or the UDP is yet another protocol which is a member of the Transport Layer. TCP is the standard protocol used by all systems for communications. TCP is used to break down the data to be transported into smaller datagrams, before they (the datagrams) are sent across a network. Thus we can say that TCP is used where more than a single or multiple datagrams are involved. </P>

<P>Sometimes, the data to be transported is able to fit into a single datagram. We do not need to break the data into smaller datagrams as the size of the data is pretty small. The perfect example of such data is the DNS system. To send out the query for a particular domain name, a single datagram is more than enough. Also the IP that is returned by the Domain Name  Server does not require more than one datagram for transportation. So in such cases instead of making use of the complex TCP protocol, applications fall back to the UDP protocol.</P>

<P>The UDP protocol works almost the way TCP works. But the only differences being that TCP breaks the data to be transferred into smaller chunks, does sequencing by inserting  a sequence number in the header and no error control. Thus we can conclude by saying that the UDP protocol is an unreliable protocol with no way to confirm that the data has reached the destination.</P>

<P>The UDP protocol does insert a USP header to the single datagram it is transporting. The UDP header contains the Source and Destination IP Addresses and Port Numbers and also the Checksum value. The UDP header is comparatively smaller than the TCP Header.</P>

<P>It is used by those applications where small chunks of data are involved. It offers services to the User's Network Applications like NFS(Network File Sharing) and SNMP.</P>

<P>Read RFC 768 for further in depth details about the UDP protocol.</P>
<P> </P>
<P>THE NETWORK LAYER</P>

<P>The IP Protocol</P>

<P>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. </P>

<P>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-:</P>
<pre>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |Version|  IHL  |Type of Service|          Total Length         |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |         Identification        |Flags|      Fragment Offset    |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |  Time to Live |    Protocol   |         Header Checksum       |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |                       Source Address                          |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |                    Destination Address                        |</P>
<P>    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+</P>
<P>    |  TCP header info followed by the actual data being transferred|</P>
<P>    |                                                               |</P>
</pre>
<P>&nbsp;</P>
<P>The Source and destination IP addresses and needed so that厀ell it is obvious isn't it? The Protocol number is added so that the IP protocol knows to which Transport Protocol the datagram has to be passed.</P>
<P>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. </P>

<P>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?</P>

<P>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.</P>

<P>The Internet Control Message Protocol or the ICMP</P>

<P>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</P>

<P>Read RFC 792 for further in depth details about the ICMP protocol.</P>

<P>The Link Layer</P>

<P>Almost all networks use Ethernet. Each machine in a network has it's own IP address and it's Ether Address. The Ether Address of a computer is different than it's IP address. An Ether Address is a 42 bit address while the IP address is only a 32 bit address. A Network must know which computer to deliver the datagram to. Right? For this the Ether Header is used. </P>

<P>The Ether Header is a 14 octet header that contains the Source and Destination Ethernet address, and a type code. Ether too calculates it's own Checksum value. The Type code relates to the protocol families to be used within the Network. The Ether Layer passes the datagram to the protocol specified by this field after inserting the Ether Header. There is simply no connection between the Ethernet Address and the IP address of a machine. Each machine needs to have a Ethernet to IP address translation table on its hard disk.</P>