Connection-Oriented Communication

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Connection-Oriented Communication

The Transmission Control Protocol (TCP) is a connection-based protocol that establishes a connection, or session, between two machines before any data is transferred. TCP exists within the Transport layer, between the Application layer and the IP layer, providing a reliable and guaranteed delivery mechanism to a destination machine. Connection-based protocols guarantee the delivery of packets by tracking the transmission and receipt of individual packets during communication. A session is able to track the progress of individual packets by monitoring when a packet is sent, in what order it was sent, and by notifying the sender when it is received so it can send more. Figure 2.17 illustrates how TCP sets up a connection-oriented session between two machines.

The first step in the communication process is to send a message indicating a desire to synchronize the systems. This is equivalent to dialing a phone number and waiting for someone to answer. The second step is for the machine to send an acknowledgment that it is listening and willing to accept data. This step is equivalent to a person answering the phone, and then waiting for the caller to say something. The third step is for the calling machine to send a message indicating that it understands the receiving machine’s willingness to listen and that data transmission will now begin.

After the TCP session has been created, the machines begin to communicate just as people do during a phone call. In the example of the telephone, if the caller uses a cellular phone and some of the transmission is lost, the user indicates she did not receive the message by saying “What did you say? I didn’t hear that.” This indicates to the sender that he needs to resend the data.

Figure 2.18 illustrates the format of a TCP header. The header includes all the parameters that are used to guarantee delivery of packets and to provide error-checking and control. Notice that the header specifies a source and destination port for the communication. This tells the machine where it is supposed to send the data, and from where the data came.

Included in the header are sections defining the sequence numbers and acknowledgment numbers that help verify the delivery of a datagram. A datagram or packet is simply the data that is being transferred to the destination machine. This data often has to be broken up into smaller pieces (datagrams) because the underlying network can only transmit so much data at one time. Other parameters include the SYN and FIN options for starting and ending communication sessions between two machines, the size of the window to be used in transferring data, a checksum for verifying the header information, and other options that can be specific implementations of TCP/IP. The last part of the frame is the actual data being transmitted. A full discussion of each of these parameters is beyond the scope of this book or the TCP/IP test.

More academic texts and RFCs on the Internet describe in fuller detail the specifications for each parameter. I recommend looking for sources that speak to you in your language. Some resources are engineering texts; some are much too simple. Look for a happy medium to begin with and work your way into the more complex.

During the initialization of a TCP session, often called the “three-way handshake,” both machines agree on the best method to track how much data is to be sent at any one time, acknowledgment numbers to be sent upon receipt of data, and when the connection is no longer necessary because all data has been transmitted and received. It is only after this session is created that data transmission begins. To provide reliable delivery, TCP places packets in sequenced order and requires acknowledgment that these packets reached their destination before it sends new data. TCP is typically used for transferring large amounts of data, or when the application requires acknowledgment that data has been received. Given all the additional overhead information that TCP needs to keep track of, the format of a TCP packet can be somewhat complex.

Try to visualize TCP as being similar to a phone call. Imagine Shey decides to call Kim on the phone. Shey picks up the phone and dials Kim’s phone number. This is equivalent to TCP sending out a synchronization request to another machine. Kim happens to have caller ID and can identify Shey before picking up the phone. Kim decides to speak to Shey and picks up the phone with a greeting, something like “Hi,” indicating her willingness to communicate. This is equivalent to a machine sending an acknowledgment that it has received a synchronization request and is willing to respond. Shey now says “Hi,” indicating that he has heard Kim and is ready to communicate, in the same way that a sending machine verifies that it has received the other machine’s willingness to communicate. Now Shey and Kim can talk about anything they want, secure in the knowledge that their messages are being received.

After the transfer of data is complete, the TCP session is broken down in a similar three-step fashion. In the case of Shey and Kim, Shey may indicate his need to get off the phone because he’s run out of things to say. Kim says, “Oh, no problem, goodnight.” Shey ends the three step sequence by saying “Goodnight.” Machines use the same type of process to break down a TCP session. The sending machine indicates that it has run out of data to send and wants to close the connection. The receiving machine indicates it has received all the data and that closing the connection is fine. The sending machine then simply closes the connection.

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