المحاضره الثامنه

1. Connection oriented
2. Point to point
3. Complete reliability - delivered as sent
4. Full duplex communication
5. Stream interface
6. Reliable connection startup
7. Graceful connection shutdown
? when a Application at site A wants to send to and receive data
from another application at site B, the following three phases
occur:
1. The two TCPs establish a virtual connection between them.
2. Data are exchanged in both directions.
3. The connection is terminated.

? Note that this is a virtual connection, not a physical connection.
The TCP segment is encapsulated in an IP datagram and can be
sent out of order, or lost, or corrupted, and then resent. Each may
be routed over a different path to reach the destination. There is
no physical connection. TCP creates a stream-oriented
environment in which it accepts the responsibility of delivering the
bytes in order to the other site.
? TCP has no knowledge of the underlying
Internet structure

TCP offers full-duplex service, where data can flow
in both directions at the same time. Each TCP
endpoint then has its own sending and receiving
buffer, and segments move in both directions.
? IP is unreliable
? Application programmers typically require
reliability (although not always)
? The Transmission Control Protocol (TCP)
establishes a reliable end-to-end
communication service on top of IP
? It uses an acknowledgment mechanism to
check the safe and sound arrival of data
? Unreliable delivery by the communication
system (IP)
? Unreliable end hosts (e.g., crashes and
reboots
? TCP handles these through a variety of
techniques
• Sender sets a
timer
• Receiver sends a
acknowledgement
• Timeout results in
retransmission
• Sensible timer values vary greatly on an
Internet
• TCP monitors the delay on a connection
and adapts the timer
– notes time taken to receive
acknowledgements
– computes weighted average and variance
over many transmissions and uses these to
set the timer
• TCP uses a window mechanism
• Each end of the connection allocates a buffer
and notifies the other end of its size
• Receiver sends available window size in each
acknowledgement (window advertisement)
• Receiver sends window advertisement when
the application consumes some data
• Zero window advertisement tells the sender
to stop transmitting until further notice
? Uses special synchronization (SYN) and finish
(FIN) messages to open and close connections
? Also confirms that all data has been received at
both sides
? TCP monitors congestion through message
loss
? first lost message, TCP backs right off and sends
just one small message
? if this is not lost, doubles data size and sends two
? continues exponential growth until half receiver’s
window size is reached then slows down rate of
increase
? TCP messages are called segments
? Source port address. This is a 16-bit field that defines the
port number of the application program in the host that is
sending the segment.

? Destination port address. This is a 16-bit field that defines
the port number of the application program in the host that is
receiving the segment.

? Sequence number. This 32-bit field defines the number
assigned to the first byte of data contained in this segment.
TCP is a stream transport protocol. To ensure connectivity,
each byte to be transmitted is numbered. The sequence
number tells the destination which byte in this sequence is the
first byte in the segment.

? Acknowledgment number. This 32-bit field defines the
byte number that the receiver of the segment is expecting to
receive from the other party. If the receiver of the segment has
successfully received byte number x from the other party, it
returns x + 1 as the acknowledgment number.
? Header length. This 4-bit field indicates the number of 4-
byte words in the TCP header. The length of the header can be
between 20 and 60 bytes. Therefore, the value of this field is
always between 5 (5× 4 = 20) and 15 (15 × 4 = 60).

? Reserved. This is a 6-bit field reserved for future use.

? Control. This field defines 6 different control bits or flags as
shown in Figure. One or more of these bits can be set at a time.
These bits enable flow control, connection establishment and
termination, connection abortion, and the mode of data
transfer in TCP. A brief description of each bit is shown in the
figure.
? Window size. This field defines the window size of the sending TCP in
bytes. Note that the length of this field is 16 bits, which means that the
maximum size of the window is 65,535 bytes. This value is normally referred
to as the receiving window (rwnd) and is determined by the receiver. The
sender must obey the dictation of the receiver in this case.

? Checksum. This 16-bit field contains the checksum. The calculation of the
checksum for TCP.

? Urgent pointer. This 16-bit field, which is valid only if the urgent flag is
set, is
used when the segment contains urgent data. It defines a value that must be
added
to the sequence number to obtain the number of the last urgent byte in the
data section of the segment

? Options. There can be up to 40 bytes of optional information in the TCP
header.