Yarmouk University – Faculty of Information Technology and CS Computer Science

Department

Transport Layer

23.2

23-2 USER DATAGRAM PROTOCOL (UDP)

The User Datagram Protocol (UDP) is called a

connectionless, unreliable transport protocol. It does

not add anything to the services of IP except to provide

process-to-process communication instead of host-to-

host communication.

23.3

Table 23.1 Well-known ports used with UDP

23.4

Figure 23.9 User datagram format

23.5

UDP length = IP length – IP header’s length

Note

23.6

23-3 TCP

TCP is a connection-oriented protocol; it creates a

virtual connection between two TCPs to send data. In

addition, TCP uses flow and error control mechanisms

at the transport level.

23.7

Table 23.2 Well-known ports used by TCP

23.8

The bytes of data being transferred in each connection are numbered by TCP.The numbering starts with a randomly generated number.

USING sequence number and the acknowledgment number FIELDS. These twofields refer to the byte number and not the segment number.

Note

23.9

Figure 23.16 TCP segment format

Field Description

Source port address. This is a 16-bit

Destination port address

This is a 16-bit field

Sequence number This 32-bit field defines the number assigned to the first byte of data contained in this 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 defines x + I 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 can be between 5 (5 x 4 =20) and 15 (15 x 4 =60).

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

Control This field defines 6 different control bits or flags

Window size This field defines the size of the window, in bytes, that the otherparty must maintain. Note that the length of this field is 16 bits, which means thatthe maximum size of the window is 65,535 bytes. This value is normally referredto as the receiving window (rwnd) and is determined by the receiver.

Checksum This 16-bit field

Urgent pointer •This l6-bit field, which is valid only if the urgent flag is set, isused when the segment contains urgent data. It defines the number that must beadded to the sequence number to obtain the number of the last urgent byte in thedata section of the segment

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

23.11

Figure 23.17 Control field

These bits enable flow control, connection establishment and termination, connection abortion, and the mode of data transfer in TCP.

23.12

Table 23.3 Description of flags in the control field

Connection establishment using three-way handshaking

Random

With the connection now established, the client will send data whose first byte will be given sequence number 4,568. The server's first byte of data will be numbered 12,999.

Connection establishment using three-way handshaking

23.15

Figure 23.18 Connection establishment using three-way handshaking

23.16

• A SYN segment cannot carry data, but it consumes one sequence number.• A SYN + ACK segment cannot carry data, but does consume one sequence number.• An ACK segment, if carrying no data, consumes no sequence number.

Note

simultaneous Connection establishment using three-way handshaking

It will not require a 3rd ACK segments. It will be handled in 2 way handshake

23.18

Figure 23.19 Data transfer

23.19

Connection termination using three-way handshaking

23.20

• The FIN segment consumes one sequence number if it not carry data.• The FIN + ACK segment consumes one sequence number if it does not carry data.

Note

Simultaneous Connection termination

Connection termination using Half-close Four-way handshaking

Connection termination using Half-close Four-way handshaking

23.24

Connection termination using Half-close Four-way handshaking

One End can stop sending datawhile receiving data.And normally issued by theCLIENT

The termination can be only forthe outbound connection (i.e.Sorting) for the client(requester) and the server canterminate the inboundconnection and maintain theoutbound

Request for thetermination but the 3rd

way is not initiated yet.Until the server finishesthe processing and senda FIN back to the client.

In the half closedconnection, the clientcloses outbound, thuscannot send out dataonly ACK. And servercloses inboundconnection, thus he stillcan send data ifneeded.

23.25

• ACK segments do not consume sequence numbers and are not acknowledged.• Data may arrive out of order and be temporarily stored by the receiving TCP, but

TCP guarantees that no out-of-order segment is delivered to the process.

Note

23.26

Figure 23.24 Normal operation

23.27

Figure 23.25 Lost segment

23.28

The receiver TCP delivers only ordered data to the process.

Note