TCP Features : Basic Data Transfer

Basic data transfer is the capability of TCP to transfer a continuous stream of octets in each direction. The octets are sent among application processes running on remote systems that use TCP. The application processes then group a set of bytes that need to be sent/received into a message segment. Message segments can be of arbitrary length. Ultimately, the messages have to be sent in IP datagrams that are limited by the MTU size of a network interface. However, at the TCP level, there is no real restriction on message size because the details of accommodating the message segments in IP datagrams
is the task of the IP Layer. For reasons of efficiency in managing messages, TCP connections typically negotiate a maximum segment size.

Messages sent by TCP have an octet stream orientation (see Figure below). TCP keeps track of each octet that is sent/received. The TCP has no inherent notion of a block of data, unlike other transport protocols, which typically keep track of the Transport Protocol Data Unit (TPDU) number and not the octet number. TCP can be used to provide multiple virtual-circuit connections between two TCP hosts.

Application processes that use TCP send data in whatever size is convenient for sending. For example, an application can send data that is as little as one octet or as big as several kilo-octets. TCP numbers each octet that it sends. The octets are delivered to the application processes at the receiving end in the order in which they are sent. This process is called sequencing of octets.

An application can send data to TCP a few octets at a time. TCP buffers this data and sends these octets either as a single message or as several smaller message segments. All that TCP guarantees is that data arrives at the receiver in the order in which it was sent. For example, if an application sends 1,024 octets of data over a period of ten seconds, the data can be sent across the network in a single TCP packet of 1,024 octets, or in four TCP packets of 256 octets, or in any combination of octets.

Because TCP sends data as a stream of octets, there is no real end-of-message marker in the data stream. To ensure that all the data submitted to the TCP module has been transmitted, a push function is required to be implemented by TCP. The push causes the TCP promptly to send any data that it has received from an application up to that point. The actual data that is sent by TCP is treated as an unstructured stream of octets. TCP does not contain any facility to superimpose an application-dependent structure on the data. For example, you cannot tell TCP to treat the data as a set of records in a database and to send one record at a time. Any such structuring must be handled by the application processes that communicate by using TCP.