TCP is a set of protocols that are used in conjunction with Internet protocol
(IP) to regulate and route the transmission of messages from one computer
to another over the Internet. The job
of TCP is to receive information from the layer above, divide the information
into manageable chunks called packets, add the TCP header to each packet and
pass it on to the IP layer for routing to the destination.
TCP is a reliable protocol in that it checks to make sure that packets
do indeed arrive at the destination and that they are not corrupted either
maliciously or due to network errors. Also
there is no guarantee that the packets arrive at the destination in the proper
order (this is despite of the fact that all packets have the same IP address)
so one other thing that TCP does is to number the packets so that the receiver
can reassemble the information.
For
example if you have requested a web page from a server on the Internet, TCP at
the sender divides the HTML document into packets, numbers the packets.... and
passes it to IP layer.
Also
there is obviously receive functionalities at the receiving end.
TCP at the destination receives the packets from the IP layer and uses
the sequence numbers to reassemble the information. It also does some error checking and
transmits confirmations of receipts to the sender.
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What happens if a packet is lost and does not
arrive at the sender? |
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TCP is
referred to as a connection-oriented protocol.
This is because a connection is established through a handshake process
and this connection is maintained and kept open for the duration of the message
exchange. We said one of the things TCP does
is to break up the data into manageable packets. Well TCP at each end communicates the maximum
size of packet it can handle and the smaller is decided upon. Having divided the data into packets, TCP
adds a header to each packet. This
header is information that TCP at the receiving end needs to do its job. The most important information in the header
are the sequence number for the packet (packets do not generally arrive at
destination in the right order) and port number. TCP at the receiving end uses the sequence
number to reassemble the packets into original data and check for missing
packets. Port numbers are used to keep
track of conversations. Keep in mind
that you could have a number of applications open at each end, a number of
which may require communication. Under
these circumstances one machine's TCP would need to handle a number of
conversations. To do this TCP at each
end allocates a port number to each conversation. One other important thing that is placed in
the header is "checksum". This
is a number that is calculated for each packet.
TCP at the receiver computes its own and compares with checksum in the
header. If they don't agree, packet is
declared bad. At this point message that
is passed on to the IP layer looks as follows;
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Where do the TCP fit into the OSI? |
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User Datagram Protocol is also a transmission layer
protocol. What are the similarities
and differences of UDP with TCP? |
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