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<TITLE>Hacking Truths: What They Don't Teach in Manuals!!! Ankit Fadia nkit@bol.net.in</TITLE>
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<FONT SIZE=2><P>_________________________________________________________________________</P>
<P>TCP\IP: A Mammoth Description By Ankit Fadia </FONT><A HREF="mailto:ankit@bol.net.in"><FONT SIZE=2>ankit@bol.net.in</FONT></A></P>
<FONT SIZE=2><P>_________________________________________________________________________</P>
<P>TCP\IP or Transmission Control Protocol \ Internet Protocol is a stack or collection of various protocols. A protocol is basically the commands or instructions using which two computers within a local network or the Internet can exchange data or information and resources. </P>
<P>Transmission Control Protocol \ Internet Protocol or the TCP\IP was developed around the time of the ARPAnet. It is also known as the Protocol Suite. It consists of various protocols but as the TCP</P>
<P>(Transmission Control Protocol) and the IP (Internet Protocol) are the most, well known of the suite of protocols, the entire family or suite is called the TCP\IP suite.</P>
<P>The TCP\ IP Suite is a stacked suite with various layers stacked on each other, each layer looking after one aspect of the data transfer. Data is transferred from one layer to the other. The Entire TCP\ IP suite can be broken down into the below layers-:</P>
<P>Layer Name Protocol </P>
<P>Link Layer (Hardware, Ethernet) ARP, RARP, PPP, Ether</P>
<P>Network Layer(The Invisible Layer) IP, ICMP</P>
<P>Transport Layer UDP, TCP</P>
<P>Application Layer(The Visible Layer) The Actual running Applications like-: FTP client, Browser</P>
<P>Physical Layer (Not part of TCP \IP) Physical Data Cables, Telephone wires</P>
<P>Data travels from the Link Layer down to the Physical Layer at the source and at the destination it travels from the Physical Layer to the Link Layer. We will later discuss what each layer and each protocol does.</P>
<P>The TCP\IP suite not only helps to transfer data but also has to correct various problems that might occur during the data transfer. There are basically two types of most common errors that might occur during the process of data transfer. They are-:</P>
<P>Data Corruption -: In this kind of error, the data reaches the destination after getting corrupted.</P>
<P>Data Loss -: In this kind of error, the entire collection of packets which constitute the data to be transferred does not reach the destination.</P>
<P>TCP\IP expects such errors to take place and has certain features which prevent, such error which might occur.</P>
<P>Checksums-: A checksum is a value (Normally, a 16 Bit Value) that is formed by summing up the Binary Data in the used program for a given data block. The program being used is responsible for the calculation of the Checksum value. The data being sent by the program sends this calculated checksum value, along with the data packets to the destination. When the program running at the destination receives the data packets, it re-calculates the Checksum value. If the Checksum value calculated by the Destination program matches with the Checksum Value attached to the Data Packets by the Source Program match, then the data transfer is said to be valid and error free. Checksum is calculated by adding up all the octets in a datagram.</P>
<P> </P>
<P>Packet Sequencing-: All data being transferred on the net is broken down into packets at the source and joined together at the destination. The data is broken down into packets in a particular sequence at the source. This means that, for example, the first byte has the first sequence number and the second byte the second sequence number and so on. These packets are free to travel independently on the net, so sometimes, when the data packets reach the destination they arrive, out of sequence, which means that the packet which had the first sequence number attached to it does not reach the destination first. Sequencing defines the order in which the hosts receive the data packets or messages. The application or the layer running at the destination automatically builds up the data from the sequence number in each packet.</P>
<P>The source system breaks the data to be transferred into smaller packets and assigns each packet a unique sequence number. When the destination gets the packets, it's starts rearranging the packets by reading the sequence numbers of each packet to make the data received usable.</P>
<P>For example, say you want to transfer a 18000 octet file. Not all networks can handle the entire 18000 octet packets at a time. So the huge file is broken down into smaller say 300 octet packets. Each packet has been assigned a unique sequence number. Now when the packets reach the destination the packets are put back together to get the usable data. Now during the transportation process, as the packets can move independently on the net, it is possible that the packet 5 will arrive at the destination before packet 4 arrives. In such a situation, the sequence numbers are used by the destination to rearrange the data packets in such a way that even if Data packet 5 arrived earlier, Packet 4 will always precede Packet 5.</P>
<P>A data can easily be corrupted while it is being transferred from the source to the destination. Now if a error control service is running then if it detects data corruption, then it asks the source to re-send the packets of data. Thus only non corrupted data reaches the destination. An error control service detects and controls the same two types of errors-:</P>
<P>1.) Data Loss</P>
<P>2.) Data Corruption</P>
<P>The Checksum values are used to detect if the data has been modified or corrupted during the transfer from source to destination or any corruption in the communication channel which may have caused data loss.</P>
<P>Data Corruption is detected by the Checksum Values and by performing Cyclic Redundancy Checks</P>
<P>(CRC 's). CRC 's too like the Checksums are integer values but require intensely advanced calculation and hence are rarely used.</P>
<P>There is yet another way of detecting data corruption-: Handshaking. </P>
<P>This feature ensures demands that both the source and destination must transmit and receive acknowledgement messages, that confirm transfer of uncorrupted data. Such acknowledgement messages are known as ACK messages.</P>
<P>Let's take an example of a typical scenario of data transfer between two systems.</P>
<P>Source Sends MSG1 to Destination. It will not send MSG2 to Destination unless and until it gets the MSG ACK and destination will not send more requests for data or the next request message (MSG2) unless it gets the ACK from Source confirming that the MSG1 ACK was received by it. If the source does not get a ACK message from the destination, then something which is called a timed-out occurs and the source will re send the data to destination.</P>
<P>So this means that if A sends a data packet to B and B checksums the data packet and finds the data corrupted, then it can simply delete for a time out to take place. Once the time out takes place, A will re send the data packet to B. But this kind of system of deleting corrupt data is not used as it is inefficient and time consuming. </P>
<P>Instead of deleting the corrupt data and waiting for a time out to take place, the destination (B) sends a not acknowledged or NACK message to source(A). When A gets the NACK message, instead of waiting for a time out to take place, it straightaway resends the data packet.</P>
<P>An ACK message of 1000 would mean that all data up to 1000 octets has been received till now.</P>
<P>TCP/ IP is a layered suite of protocols. All layers are equally important and with the absence of even a single layer, data transfer would not have been possible. Each TCP/ IP layer contributes to the entire process of data transfer. An excellent example, is when you send an email. For sending mail there is a separate protocol, the SMTP protocol which belongs to the Application layer. The SMTP Application protocol like all other application layer protocols assumes that there is a reliable connection existing between the two computers. For the SMTP application protocol to do what it is designed for, i.e. to send mail, it requires the existence of all other Layers as well. The Physical Layer i.e. cables and wires is required to transport the data physically. The Transmission Control Protocol or the TCP protocol which belongs to the Transport Layer is needed to keep track of the number of packets sent and for error correction. It is this protocol that makes sure that the data reaches the other end. The TCP protocol is called by the Application Protocol to ensure error free communication between the source and destination. For the TCP layer to do its work properly i.e. to ensure that the data packets reach the destination, it requires the existence of the Internet Protocol or IP. The IP protocol contains the Checksum and Source and Destination IP address.</P>
<P>You may wonder why do we need different protocols like TCP and IP and why not bundle them into the same Application protocol.? The TCP protocol contains commands or functions which are needed by various application protocols like FTP, SMTP and also HTTP. The TCP protocol also calls on the IP protocol, which in turn contains commands or functions which some application protocols require while others don抰. So rather than bundling the entire TCP and IP protocol set into specific application protocols, it is better to have different protocols which are called whenever required.</P>
<P>The Link Layer which is the Hardware or Ethernet layer is also needed for transportation of the data packets. The PPP or the Point to Point Protocol belongs to this layer. Before we go on let's get accustomed with certain TCP\IP terms. Most people get confused between datagrams and packets and think that they are one and the same thing . You see, a datagram is a unit of data which is used by various protocols and a packet is a physical object or thing which moves on a physical medium like a wire. There is a remarkable difference between a Packet and a Datagram, but it is beyond the scope of this book. To make things easier I will use only the term datagram (Actually this is the official term.)while discussing various protocols.</P>
<P>Two different main protocols are involved in transporting packets from source to destination.</P>
<P>1.) The Transmission Control Protocol or the TCP Protocol</P>
<P>2.) The Internet Protocol or the IP protocol.</P>
<P>Besides these two main protocols, the Physical Layer and the Ethernet Layer are also indispensable to data </P>
<P>transfer. </P>
<P>THE TRANSPORT LAYER</P>
<P>The TCP protocol</P>
<P>The Transmission Control Protocol is responsible for breaking up the data into smaller datagrams and putting the datagrams back to form usable data at the destination. It also resends the lost datagrams to destination where the received datagrams are reassembled in the right order. The TCP protocol does the bulk of work but without the IP protocol, it cannot transfer data.</P>
<P>Let's take an example to make things more clearer. Let's say your Internet Protocol Address or IP address is xxx.xxx.xxx.xxx or simply x and the destination's IP is yyy.yyy.yyy.yyy or simply y. Now As soon as the three-way connection is established between x and y, x knows the destination IP address and also the Port to which it is connected to. Both x and y are in different networks which can handle different sized packets. So in order to send datagrams which are in receivable size, x must know what is the maximum datagram size which y can handle. This too is determined by both x and y during connection time.</P>
<P>So once x knows the maximum size of the datagram which y can handle, it breaks down the data into smaller chunks or datagrams. Each datagram has it's own TCP header which too is put by TCP.</P>
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