Simulation of TCP Layer

Transcription

1 39 Simulation of TCP Layer Preeti Grover, M.Tech, Computer Science, Uttrakhand Technical University, Dehradun ABSTRACT The Transmission Control Protocol (TCP) represents the most deployed transport protocol used in the Internet so far. The Transmission Control Protocol (TCP) is intended for use as a highly reliable host to host protocol between hosts in packet switched computer communication networks, and in interconnected systems of such networks. It acts as an intermediary between the application layer and network layer. In this paper I simulate different features of TCP layer. With the help of Nagle s Theorem various feature of TCP can be performed. Keywords Simulation, Nagle s Theorem, Functionalities of TCP, Functions used. 1. INTRODUCTION The Simulation of TCP Layer is a process to show various functionality of the Transport Layer of the TCP/IP Reference model. The Transmission Control Protocol is intended to use as a highly reliable host to host protocol between hosts in packet switched computer communication network, and in interconnected systems of such network security. The following figure illustrates the position of the Tranport Layer. Application Layer Transport Layer Network Layer Data Link Layer and Physical Layer Fig.1 The Transport Layer is responsible for end to end Successful delievery of data on the network. It also ensures the order of data is maintained or not. There are two types of protocols used in the Transport layer, TCP and UDP. The TCP is also known as Transmission Control Protocol and the UDP is known as User Datagram Protocol. TCP is a connection oriented, reliable data transfer service, while UDP provides a connectionless and unreliable data transfer service. Both TCP and UDP are widely in use. In our project we will be talking about the functionalities of the TCP protocol. Functionalities of TCP layer are as follows: Connection Establishment between hosts. Segmentation of data received by Application Layer. Adding TCP Header to the data. Error Control using checksum. Flow Control. Connection Termination. Connection Establishment: The "three-way handshake" is the procedure used to establish a connection. This procedure normally is initiated by one TCP and responded to by another TCP. The procedure also works if two TCP simultaneously initiate the procedure. When simultaneous attempt occurs, each TCP receives a "SYN" segment which carries no acknowledgment after it has sent a "SYN". TCP Segmentation: Transmission Control Protocol accepts data from a data stream, segments it into chunks, and adds a TCP header creating a TCP segment. The TCP segment is then encapsulated into an Internet Protocol (IP) datagram. A TCP segment is "the packet of information that TCP uses to exchange data with its peers. TCP Header: A TCP segment consists of a segment header and a data section. The TCP header contains 10 mandatory fields, and an optional extension field. The data section follows the header. Its contents are the payload data carried for the application. The length of the data section is not specified in the TCP segment header. It can be calculated by subtracting the combined length of the TCP header and the encapsulating IP header from the total IP datagram length. Error Control: Error control is one of the very prominent feature of the TCP. In our simulation of TCP layer, we have used the logic of Checksum for error control. Each TCP header contains the information of checksum. Flow Control: TCP uses an end to end flow control protocol to avoid having the sender send data too fast for the TCP receiver to receive and process it reliably. TCP senders and receivers typically employ flow control logic to specifically avoid repeatedly sending small segments. I implemented Nagle's algorithm to control the flow of segments.

2 40 Connection Termination: The connection termination phase uses, at most, a four way handshake, with each side of the connection terminating independently. When an endpoint wishes to stop its half of the connection, it transmits a FIN packet, which the other end acknowledges with an ACK. TCP is a connection oriented, end to end reliable protocol designed to fit into a layered hierarchy of protocols which support multi network applications. The TCP provides for reliable inter process communication between pairs of processes in host computers attached to distinct but interconnected computer communication networks. Very few assumptions are made as to the reliability of the communication protocols below the TCP layer. the destination TCP. The receiving TCP places the data from a segment into the receiving user's buffer and notifies the receiving user. The TCP include control information in the segments which they use to ensure reliable ordered data transmission. A TCP segment consists of a segment header and a data section. The TCP header contains 10 mandatory fields, and an optional extension field. 2. IMPLEMENTATIONS DETAILS CONNECTION ESTABLISHMENT: To establish a connection,tcp uses a threeway handshake. Before a client attempts to connect with a server, the server must first bind to a port to open it up for connections: this is called a passive open. Once the passive open is established, a client may initiate an active open. To establish a connection, the threeway (or 3step) handshake occurs: 1. SYN: The active open is performed by the client sending a SYN to the server. It sets the segment's sequence number to a random value A. 2. SYNACK: In response, the server replies with a SYNACK. The acknowledgment number is set to one more than the received sequence number (A + 1), and the sequence number that the server chooses for the packet is another random number, B. 3. ACK: Finally, the client sends an ACK back to the server. The sequence number is set to the received acknowledgement value i.e. A + 1, and the acknowledgement number is set to one more than the received sequence number i.e. B + 1. At this point, both the client and server have received an acknowledgment of the connection. The threeway handshake reduces the possibility of false connections. The simplest threeway handshake: TCP SEGMENT: Processes transmit data by calling on the TCP and passing buffers of data as arguments. The TCP packages the data from these buffers into segments and calls on the Internet module to transmit each segment to Fig 2. TCP HEADER: TCP segments are sent as Internet datagrams. The Internet Protocol header carries several information fields, including the source and destination host addresses. A TCP header follows the Internet header, supplying information specific to the TCP protocol. TCP Header contains the informations related to sequence number of the data segment, acknowledgement number, window size, checksum, urgent pointer, header length and several flags. The informations of the TCP header are very important as they allow the user in implementing the errorcontrol and several algorithms for congestion and flow control. The transport layer adds the header at the sending end and removes it at the receiving end. These informations have been used in the simulation and used them for the error free and ordered delivery of data.. Fig 3.

3 41 Source Port: 16 bits The source port number. Destination Port: 16 bits The destination port number. Sequence Number: 32 bits The sequence number of the first data octet in this segment (except when SYN is present). If SYN is present the sequence number is the initial sequence number (ISN) and the first data octet is ISN+1. Acknowledgment Number: 32 bits If the ACK control bit is set this field contains the value of the next sequence number the sender of the segment is expecting to receive. Once a connection is established this is always sent. Data Offset: 4 bits The number of 32 bit words in the TCP Header. This indicates when the data begins. The TCP header (even one including options) is an integral number of 32 bits long. Reserved: 6 bits Reserved for future use. Must be zero. Control Bits: 6 bits (from left to right) URG: Urgent Pointer field significant. ACK: Acknowledgment field significant. PSH: Push Function. RST: Reset the connection. SYN: Synchronize sequence numbers. FIN: No more data from sender. Window: 16 bits The number of data octets beginning with the one indicated in the acknowledgment field which the sender of this segment is willing to accept. Checksum: 16 bits The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header and text. If a segment contains an odd number of header and text octets to be check summed, the last octet is padded on the right with zeros to form a 16 bit word for checksum purposes. The pad is not transmitted as part of the segment. While computing the checksum, the checksum field itself is replaced with zeros. The checksum also covers a 96 bit pseudo header conceptually prefixed to the TCP header. This pseudo header contains the Source Address, the Destination Address, the Protocol, and TCP length. This gives the TCP protection against misrouted segments. This information is carried in the Internet Protocol and is transferred across the TCP/Network interface in the arguments or results of calls by the TCP on the IP. The TCP Length is the TCP header length plus the data length in octets (this is not an explicitly transmitted quantity, but is computed), and it does not count the 12 octets of the pseudo header. Urgent Pointer: 16 bits This field communicates the current value of the urgent pointer as a positive offset from the sequence number in this segment. The urgent pointer points to the sequence number of the octet following the urgent data. This field is only be interpreted in segments with the URG control bit set. Options: variable Options may occupy space at the end of the TCP header and are a multiple of 8 bits in length. All options are included in the checksum. An option may begin on any octet boundary. Padding: The TCP header padding is used to ensure that the TCP header ends and data begins on a 32 bit boundary. The padding is composed of zeros. TRANSMISSION OF DATA There are some key features of TCP that helps to transfer the data from one host to another that makes the data more reliable and avoid the data loss. Ordered Data Transfer: TCP uses a sequence number to identify each byte of data. The sequence number identifies the order of the bytes sent from one computer to another so that the data can be reconstructed in order, regardless of any fragmentation, disordering, or packet loss that may occur during transmission. Retransmission Of Lost Packets: TCP primarily uses a ACK scheme, where the receiver sends an ACK signifying that the receiver has received all data preceding the ACK sequence number. The sender sets the sequence number field to the sequence number of the first payload byte in the segment's data field, and the receiver sends an ACK specifying the sequence number of the next byte they expect to receive. If sender not received the ACK from receiver for a particular data then sender retransmit that particular data so it helps to avoid the loss of data. Error Control using Checksum: Error control is one of the very prominent features of the TCP. In simulation of TCP layer, the logic of Checksum for error control has been used..each TCP header contains the information of checksum. This checksum is calculated before sending the data on the network towards the receiver. The data is divided into 16 bit fragments, the one's

4 42 complement is calculated, binary sum done and finally the one's complement of the binary sum is called the checksum. This checksum has been added to the data at the end. At the receiver end the Checksum function is used to evaluate the data on the basis of the checksum received. Once the data is found correct, one message saying that the data is found correct is printed. This mechanism is followed at either end. Flow Control: TCP uses an end to end flow control protocol to avoid having the sender send data too fast for the TCP receiver to receive and process it reliably. Having a mechanism for flow control is essential in an environment where machines of diverse network speeds communicate. I used Flow Control mechanism to avoid the congestion during the transmission of data and improves the efficiency of TCP layer to transmit the data. In this research work, I implemented the Nagle's Algorithm to control the flow of data transmission. The Nagle algorithm (by John Nagle) is a method for congestion control, so the sender won't flood the receiver with data. When the sender sends a packet to the receiver, then the sender will wait for an ACK from the receiver before sending the following packets. Buffer Implementation: When the data segments are received at the receiver's end, the header is removed from it. The left data is put into the buffer. This process is continues till even one data segment is left to be sent. Once all data segments are received at the receiver's end, it resembles all the data segments and the final data is delivered. This process is continued at each end. CONNECTION TERMINATION The connection termination phase uses, at most, a fourway handshake, with each side of the connection terminating independently. FIN ACK FIN ACK The FIN segment can hold last chunk of data to be sent on the network from the sender side. This also indicates that the sender of FIN segment has closed connection from its side, and will not send any further data, but it can still accept data from the other side. For final termination of connection both side have to send the FIN segment and acknowledge the termination of connection. If the termination is acknowledged from both ends then the connection is said to be terminated finally. 3. DETAILS TO THE PROGRAMMER The Transport Layer is responsible for end to end successful delivery of data on the network. It also ensures the order of data is maintained or not. There are two types of protocols used in the Transport layer, TCP and UDP. The TCP is also known as Transmission Control Protocol and the UDP is known as User Datagram Protocol. TCP is a connection oriented, reliable data transfer service, while UDP provides a connectionless and unreliable data transfer service. Both TCP and UDP are widely in use. In this paper we will be talking about the functionalities of the TCP protocol. Functionalities of TCP layer are as follows: Connection Establishment: The "three-way handshake" is the procedure used to establish a connection. This procedure normally is initiated by one TCP and responded to by another TCP. The procedure also works if two TCP simultaneously initiate the procedure. When simultaneous attempt occurs, each TCP receives a "SYN" segment which carries no acknowledgment after it has sent a "SYN". TCP Segmentation: Transmission Control Protocol accepts data from a data stream, segments it into chunks, and adds a TCP header creating a TCP segment. The TCP segment is then encapsulated into an Internet Protocol (IP) datagram. A TCP segment is "the packet of information that TCP uses to exchange data with its peers. TCP Header: A TCP segment consists of a segment header and a data section. The TCP header contains 10 mandatory fields, and an optional extension field. The data section follows the header. Its contents are the payload data carried for the application. The length of the data section is not specified in the TCP segment header. It can be calculated by subtracting the combined length of the TCP header and the encapsulating IP header from the total IP datagram length. Error Control: Error control is one of the very prominent feature of the TCP. In our simulation of TCP layer, we have used the logic of Checksum for error-control. Each TCP header contains the information of checksum. Flow Control: TCP uses an end to end flow control protocol to avoid having the sender send data too fast for the TCP receiver to receive and process it reliably. TCP senders and receivers typically employ flow control logic to specifically avoid repeatedly sending small segments. I

5 43 implemented Nagle's algorithm to control the flow of segments. Connection Termination: The connection termination phase uses, at most, a four way handshake, with each side of the connection terminating independently. When an endpoint wishes to stop its half of the connection, it transmits a FIN packet, which the other end acknowledges with an ACK DIGRAMATICAL REPRESENTATION This function converts a given binary string into its character string equivalent. static void data_recover( ) This function stored the recieved data to the final destination. static void checksum_fetch( ) This function fetches checksum of any recieving packets. static void complete(int x) This function takes a binary number as an input and add the required number of zero's before it. static void delay( ) This function makes some delay between the segments. static void checksum( ) This function calculates the checksum for packet and stored it in segment. 3.3 USAGE On running this simulation, the following options that will appear on the main screen are as follow: 1) Enter n for normal mode. 2) Enter exit for exit. If you press the 'n' for normal mode then it will come to the next option i.e. 1) Enter the packet number that you want to corrupt. After entering the packet number, the next option shows on the main screen i.e. Fig FUNCTIONS IMPLEMENTED Fig FUNCTIONS IMPLEMENTED static void dec bin(int x) This function takes a decimal number as input and converts it into its binary equivalent. static void bin_dec( ) This function converts a given binary number into its decimal equivalent. static void binstr_numstr( ) 1) Enter the file name whose data you want to transmit. After giving the file name, the connection will be established between the client and server through socket programming then the data transmission will take place. This layer will include the end to end message transfer capabilities independent of the underlying network, along with error control, segmentation, flow control, congestion control, and application addressing (port numbers). Step 1: Run the both side (Client & Server) program. When user run both side program successfully, then Server side waiting for client port. Step 2: After running the client program successfully user have to enter n to perform simulation in normal mode or exit to close the terminal.

6 44 Step 3: If user enter n to run the software in normal mode, then it will come to the next option i.e. enter the data packet number that you want to corrupt. Step 4: After performing step 3, user need to enter a valid input data file (.txt file) which needs to be located in same location where programming file exist. Step 5: Now after step 4 both side start the connecting to each other and user have to wait for connection establishment message. Step 6: Now after establishing connection the simulation at TCP layer starts and user needs to wait until the last data segment sent and then FIN request will send automatically between the client and receiver to terminate the connection. Step 7: Finally when simulation process is completed and all the data received by other side. Step 8: After the complete data received if the user wants to send another file then again user can enter 'n' for normal mode or enter 'exit' for exit. 3.4 ASSUMPTIONS The port number should not lie between because these are already reserved ports. Data for transmission will be textual data. INPUT: 1) Receiver s IP address & port address. 2) Text Message. OUTPUT: Text message after transmission. 4. GAP ANALYSIS The real world applications of today are mostly full duplex connections. But in this simulation I have made a half duplex connection. i.e. in real world application the client or the server are able to send data at any point of time, even simultaneously. But in our simulation once the server is sending some data, the client will be in receiving mode, it is supposed to receive data only, it will not be able to send any data. At the same time if the client is sending some data, the server has to be in the receiving mode, it will not be able to send any data at that point of time. proper point of time, it is dropped. The sender is requested to retransmit the segment again. In this simulation we have not been able to show the timestamping and retransmission of a segment due to it. While we have shown if any data segment is not received as per the order, it is again retransmitted from the sender side. We can also implement the timer control so that the particular segment can be send in the particular time. Like we can set a particular time, if we set a timing to send a segment in 3 sec then that segment should be send in that 3 secs only, if it doesn't happen then that segment will dropped and retransmitted. 5. CONCLUSION All the features that are needed to be in the simulation of TCP layer are included. In this connection is established between the client and server through the mechanism of threeway handshaking. While transmission of data,it provided the ordered data transfer and retransmit the lost data. It also avoid error by the mechanism of checksum so that the delivered data should be error free. In this simulation Nagle's theorem is implemented for the reliable transmission of the data that the next segment will not send until it received the ACK so it will help to avoid the congestion. After transmitting the data successfully, it terminates the connection using the fourway handshake mechanism. 6. REFERENCES [1] Cerf, V., and R. Kahn, "A Protocol for Packet Network Intercommunication", IEEE Transactions on Communications, Vol. COM22, No. 5, pp , May [2] Andrew S. Tanenbaum ( ). Computer Networks (Fourth Ed.). Prentice Hall. [3] Dalal, Y. and C. Sunshine, "Connection Management in Transport Protocols", Computer Networks, Vol. 2, No. 6, pp , December [4] RFC 793, RFC 896 [5]. Figures from TCP/IP Guide(google.com) In the real world applications the packets are often timestamped so that due to congestion in network, if some TCP segments don t reach at the receiver side at