Transport Layer Protocol:

Transmission Control Protocol

- Introduction to TCP- Operation of TCP

- Characteristics of TCP- TCP three-way handshake process

- Application of TCP- Relationship between TCP and IP

- Standard TCP/IP services- Port numbers and socket address

Introduction: Transport Layer Protocol

• The Transport layer is where sessions are established and data packets are exchanged between hosts. Two core protocols are found at this layer are, TCP, UDP

• A transport layer provides end-to-end or host-to-host communication services.

• The transport layer provides services such as connection-oriented data stream support, reliability, flow control, and multiplexing.

Introduction: TCP (Transmission Control Protocol)• Second transport layer protocol, TCP.

• Connection oriented and reliable protocol.

• Creates virtual connection between two TCPs to send data.

• TCP uses flow control and error control mechanisms at the transport level.

• Heavy weight protocol, ensure that packet is delivered or not, if not TCP resends that packet and packet must be delivered sequentially.

TCP Operations

• OPEN to open a connection

• CLOSE to close a connection

• SEND to send data to an open connection

• RECEIVE to receive data from an open connection

• STATUS to find information about a connection

TCP Message Format

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9

0 1 2 3

0 1

Source Port Destination Port

Sequence Number

Acknowledgement Number

DataOffset

ReservedURG

ACK

PSH

RST

SYN

FIN

Window

Checksum Urgent Pointer

Options Padding

data

Source port: 16 bits The source port numberDestination port: 16 bits The destination port number

Sequence Number (SEQ):

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

Acknowledgement Number (ACQ):

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 where 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: Acknowledgement  PSH: Push function  RST: Reset the connection  SYN: Synchronize sequence numbers  FIN: No more data from senderWindow: 16 bits The number of data octets beginning with the one indicated in the acknowledgement field

which the sender of this segment is willing to accept.

Checksum: 16 bits Checksum field is calculated to verify the data correctness.

Characteristics of TCP

• Basic Data Transfer: Continuous transfer of octets in each direction

• Reliability: Must have to recover from data that is damaged, lost, duplicated or delivered or not.

• Flow Control: How much to send and how much to receive. (Acknowledgement)

• Multiplexing: Uses different ports and addresses to do simultaneously communication. (Many Processes)

• Connection: Reliability and Flow Control mechanism requires TCPs initialize or connection status.

• Security: The users of TCP may indicate the security and precedence of their communication

TCP three-way handshake process

• It is usually a process that takes place when a computer is about to communicate with a foreign device to establish rules for communication.

• To establish a connection, each device must send a SYN and receive an ACK for it from the other device. Thus, conceptually, we need to have four control messages pass between the devices. However, it's inefficient to send a SYN and an ACK in separate messages when one could communicate both simultaneously. Thus, in the normal sequence of events in connection establishment, one of the SYNs and one of the ACKs is sent together by setting both of the relevant bits (a message sometimes called a SYN+ACK). This makes a total of three messages, and for this reason the connection procedure is called a three-way handshake.

• E.g. : SYN: The active open is performed by the client sending a SYN to the server. The client sets the segment's sequence number to a random value A.

• SYN-ACK: In response, the server replies with a SYN-ACK. The acknowledgment number is set to one more than the received sequence number i.e. A+1, and the sequence number that the server chooses for the packet is another random number, B.

• 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.

Host A sends a TCP SYNchronize packet to Host B

Host B receives A's SYN

Host B sends a SYNchronize-ACKnowledgement

Host A receives B's SYN-ACK

Host A sends ACKnowledge

Host B receives ACK.

TCP socket connection is ESTABLISHED

Applications of TCP

• Used to send large number of files.

• Used for traffic that you need all the data for. i.e. HTML, pictures, etc. 

• Reliable for use.

• In TCP the data will not be corrupted.

• TCP has a mechanism to control the errors.

• It is a connection oriented protocol,  It establishes a path, or a virtual connection all the way through switches routers proxies etc. and then starts any communication.

• Used for security purposes.

E.g.: TCP is mailing a letter with a return receipt at the post office, except that the post master will organize the letters in-order-of mailing and only deliver them in-order.

Relationship between TCP and IP

• The Internet Protocol Suite (commonly known as TCP/IP) is the set of communications protocols used for the Internet and other similar networks.

• It is named from two of the most important protocols in it: • the Transmission Control Protocol (TCP) and• the Internet Protocol (IP), which were the first two networking

protocols defined in this standard.

Transmission Control Protocol (TCP)

• Transmission Control Protocol (TCP) is the core protocol of the Internet Protocol Suite.

• TCP provides the delivery of a stream of bytes from a program from one computer to another computer.

• TCP provides reliable, ordered, and error-checked delivery of a stream of octets between applications running on hosts communicating over an IP network.

• TCP is the connection-oriented protocol.

Internet Protocol (IP)

• IP has the task of delivering packets from the source host to the destination host solely based on the IP addresses in the packet headers.

• Internet Protocol is connectionless protocol.

Summary

• Both TCP and IP are the connection protocols from Internet Protocol Suite or TCP/IP model.

• TCP is a core operating on a relatively high level; IP operates at a lower level.

• TCP is the protocol from Transport Layer whereas IP is the primary protocol in the Internet Layer.

• TCP provides communication services at an intermediate level between an application program and the IP; IP encapsulates all data, and is connectionless.

Standard TCP/IP services

• Telnet

• FTP/Anonymous FTP

• TFTP

Source: http://docs.oracle.com/

Telnet:

- Telnet is a user command and an underlying TCP/IP protocol for accessing remote computers.

- The Telnet protocol enables terminals and terminal-oriented processes to communicate on a network running TCP/IP.

- Telnet provides a user interface through which two hosts can communicate on a character-by-character or line-by-line basis.

- 23 is the port number.

Anonymous / File Transfer Protocol (FTP)

- The File Transfer Protocol (FTP) is a standard network protocol used to transfer computer files from one host to another host over a TCP-based network, such as the Internet.

- Anonymous FTP is a common way to get access to a server in order to view or download files that are publicly available.

- 20/21 is the port number.

Trivial File Transfer Protocol (TFTP)

• Trivial File Transfer Protocol (TFTP) is a simple, lock-step, File Transfer Protocol which allows a client to get from or put a file onto a remote host.

• Trivial File Transfer Protocol (TFTP) is an Internet software utility for transferring files that is simpler to use than the File Transfer Protocol (FTP) but less capable.

• It is used where user authentication and directory visibility are not required.

• 69 is the port number.

Port Numbers

• A port number is a way to identify a specific process to which an Internet or other network message is to be forwarded when it arrives at a server.

• For the Transmission Control Protocol, a port number is a 16-bit integer that is put in the header appended to a message unit.

Socket Address

• A socket address is the combination of an IP address and a port number, much like one end of a telephone connection is the combination of a phone number and a particular extension.

• Based on this address, internet sockets deliver incoming data packets to the appropriate application process or thread.

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