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5.1 TCP Format

The unit of transfer between two TCP layers is

frequently called a segment. Segments are ex-

changed to establish connections, to transfer user

data, to send acknowledgements, to advertise

window size and to release connections. The for-

mat of a TCP segment is illustrated in Figure 9.

The TCP header starts with the Source portand

the Destination port, similar to the UDP header

format. The Sequence numberidentifies the

position in the sender’s byte stream of the data

in the segment. The Acknowledgement number

identifies the position of the highest byte that

the source has received. Note that the sequence

number refers to the flow in the same direction

as the segment, while the acknowledgement

number refers to the stream flowing in the oppo-

site direction as the segment. The Offsetfield

contains the integer specifying the offset of the

data portion in the segment. This is needed as

the length of the Options field varies. After the

Offset field, 4 bits are reserved for future use.

The Codefield gives the contents of the segment

by setting 6 flags (urgent pointer is valid – URG,

acknowledgement field is valid – ACK, a push is

requested – PSH, reset of the connection – RST,

synchronise sequence numbers – SYN, sender

reached end of its stream – FIN).

The TCP layer at the receiver uses the Window

field to advertise how much data it is willing to

accept. Having the Urgent pointerfield allows

TCP to specify that some data is urgent. The

value shows where the urgent data is located.

The Checksumis calculated in a similar manner

as for UDP by appending a pseudo header.

Note that acknowledgements, windows, etc.

refer to volume/byte and not to number of seg-

ments. The TCP acknowledgement scheme is

called cumulate as it reports how much of the

stream has been well received. A motivation for

this is that it is simple to implement. In addition,

a lost acknowledgement does not necessarily

lead to a retransmission. On the other hand, the

sender would not receive information on which

information that is successfully received and

may prepare for retransmitting all segments

starting with the one being lost.

5.2 TCP Connection Handling

In order to establish a TCP connection three

messages are commonly exchanged as illustrated

in Figure 10. In the two first messages the flag

called SYN in the Code field is set. In the two

last messages the ACK flag as part of the Code

field is set.

To close a TCP connection, the FIN flag (part

of the Code field) is set. The receiver acknowl-

edges the segment and the session is over.

A TCP connection is unidirectional although

acknowledgements are transferred in the oppo-

site direction.

The units of user data seen at the sender side and

the receiver side may differ. For instance, at the

sender side a TCP entity may receive two blocks

of 10 kbyte, while at the receiver side, the TCP

entity delivers 20 blocks of 1 kbyte. Further-

more, the TCP entity may collect user data

before sending it, unless a Push flagis used or

Urgent datais indicated. Then the information

is sent without awaiting further user data.

To accommodate interactive users, TCP pro-

vides a push operation that can be used to force

delivery of bytes (the PSH flag as part of the

Code field is set in the segment). An example of

this is when TCP is used to transfer keystrokes.

Similar to UDP, TCP has also some port num-

bers that are reserved, although most ports are

available for dynamic binding. Port numbers

below 256 are called well-known portsand are

reserved for “standard” services, like FTP and

Telnet. Having the sender and receiver aligned

a pair of sockets sets up a TCP service. Each of

the sockets has a socket address consisting of the

IP address (32 bit or 128 bit) and a port number

(16 bit). A TCP connection can then be identi-

fied by the socket addresses at both ends,

(receiver_socket number, sender_socket num-

ber). A socket can be used for a number of paral-

lel flows. All connections are full duplex and

point-to-point.

Figure 9 TCP segment format

Figure 10 Messages
exchanged for establishing
a TCP connection

Offset Reserved

Source port Destination port

Sequence number

Acknowledgement number

Window

Urgent pointer

Options

Data

Padding

04 8 16 24 31

Code

Checksum

SYN-flag, seq = x

SYN-flag, ACK-flag, seq = y

, ack = x + 1

ACK-flag, ack = y + 1

source destination

seq = sequence number

ack = acknowledgement number