The Tcp Service Model Information Technology Essay

The Tcp Service Model Information Technology Essay The transport service is implemented by a transport protocol used between two transport entities. The transport protocol have to deal with error control, sequencing, and flow control. UDP is a simple protocol and it has some niche uses, such as client-server interactions and multimedia, but for the most Internet applications, reliable, sequenced delivery is needed.UDP cannot provide this, so another protocol is required. It is called TCP and is the main workhouse of the internet. THE TCP SERVICE MODEL: TCP service is obtained by both the sender and receiver creating end points, called sockets. A socket may be used for multiple connections at the same time. All TCP connections are full duplex and point to point. Full duplex means that the traffic can go in both directions at the same time. Point-to-point connection contains exactly two end points. Another feature of the TCP service is URGENT DATA. When the urgent data are received at the destination. The receiving application is interrupted so it can stop whatever it was doing and read the data stream to find the urgent data. The start of the urgent data is not marked while the end is marked so the application knows when it is over. This scheme basically provides a crude signaling. THE TCP PROTOCOL: A key feature of TCP, and one which dominates the protocol design, is that every byte on a TCP connection has its own 32-bit sequencer. When the internet began, the lines between routers were mostly 56-kbps leased lines, so a host blasting away at full speed took over 1 week to cycle through the sequence numbers. Separate 32-bit sequence numbers are used for acknowledgements and for the window mechanism. The sending and receiving TCP entities exchange data in the form of segments. What should a segment have? 1.Each segment including the TCP header, must fit in the 65,515-bytes IP payload. 2.Each network has a maximum transfer unit, or MTU, and each segment must fit in the MTU. THE TCP SEGMENT HEADER: The following is the dissection of TCP header field by field. The Source port and Destination port fields identify the local end points of the connection. The source port number is of 16 bits and indentifies the sending host TSAP(client port number). The destination port number is of 16 bits and is used to identify the receiver host TSAP(server port number). The sequence number is of 32 bits size. Since TCP supports Byte-stream, in which each byte is numbered, big space is allocated for numbering i.e.(2 power 32=4096 million).With the help of these numbers only ,one can be able to differentiate the old delayed duplicate with fresh ones. The acknowledgement number is of 32 bits size. It specifies the next byte expected. The length of TCP header is of 4 bits size and indicates the number of rows. Next comes a 6-bit field that is UNUSED. The fact that this field has survived intact for over a quarter of a century is testimony to how well thought out TCP is. Lesser protocols would have needed it to fix bugs in the original design. Six 1-bit flags: 1. URG is set to 1 if the urgent flag pointer is in use. 2. The ACK bit is set to 1 to indicate that the acknowledgement number is valid. If ACK is 0, the segment does not contain an acknowledgement so the ACKNOWLEDGEMENT NUMBER is ignored. 3. The PSH bit indicates pushed data. 4. The RST bit is used to reset a connection that has become confused due to a host crash or some other reason. It is also used to reject an invalid segment or refuse an attempt to open a connection. In general, if you get a segment with the RST bit on, you have problem on your hands. 5. The SYN bit is Synchronization flag. It is used to establish connections. The connection request has SYN=1 and ACK=0 to indicate that the piggyback acknowledgement field is not in use. The connection reply does bear an acknowledgement, so it has SYN=1 and ACK=1. 6. The FIN bit is finish flag. It is used to release a connection. It specifies that the sender has no more data to transmit. CHECKSUM: A checksum is also provided for extra reliability. It checks the header, the data and the conceptual pseudoheader. When performing this computation, the TCP Checksum field is set to zero and the data field is padded out with an additional zero byte if its length is an odd number. PSEUDOHEADER: The pseudoheader contains the 32-bit IP addresses of the source and destination machines, the protocol number for TCP (6), and the byte count for the TCP segment. Including the pseudoheader in the TCP checksum computation helps detect misbelieved packets. TCP CONNECTION MANAGEMENT MODELING: The steps required to establish and release connections can be represented in a finite state machine with the 11 states listed below. In each state, certain events are legal .when a legal event happens, some action may be taken. If some other event happens, an error is reported. State Description CLOSED No connection is active or pending LISTEN The server is waiting for an incoming call SYN, RCVD A connection request has arrived: wait for ACK SYN SENT The application has started to open a connection ESTABLISH The normal data transfer state FIN WAIT 1 The application has said it is finished FIN WAIT 2 The other side has agreed to release TIMED WAIT Wait for all packets to die off CLOSING Both sides have tried to close simultaneously CLOSING WAIT The other side has initiated a release LAST ACK Wait for all packets to die off Each connection starts in the CLOSED state. When it performs either a passive open {LISTEN}, or an active open {CONNECT}, it leaves the state. If the other side does the opposite one, a connection is established and the state becomes ESTABLISHED. Connection release can be initiated by either side. The state returns to CLOSED, after it is completed. Description of the figure: The heavy solid line is the normal path for a client. The heavy dashed line is the normal path for a server. The light lines are unusual events. Each transition is labeled by the event causing it and the action resulting from it, separated by a slash. The event can either be a user à ¢Ãƒ ¢Ã¢â‚¬Å¡Ã‚ ¬initiated system call {CONNECT, LISTEN, SEND or CLOSE}, a segment arrival [SYN, FIN, ACK or RST}, or in one case, a timeout of twice the maximum packet lifetime. The action is the sending of a control segment {SYN, FIN or RST} or nothing, indicated by -.Comments is shown in parentheses. Figure: TCP connection management finite state machine. TCP TRANSMISSION POLICY: Windows Management in TCP is not directly tied to acknowledgements as it is in most data link protocols. If the sender transmits a 2048-byte segment that is correctly received, the receiver will acknowledge the segment. However, since it now has only 2048 bytes of buffer space (until the application removes some data from the buffer), it will advertise a window of 2048 starting at the next byte expected. The window management in TCP is shown in the following figure. Now the sender transmits another 2048 bytes, which are acknowledged, but the advertised window is 0. The sender must stop until the application process on the receiving host has removed some data from the buffer, at which time TCP can advertise a larger window. Senders are not required to transmit data as soon as they come in from the application. When the first 2 KB of data came in, TCP, knowing that it had a 4 KB window available, would have been completely correct in just buffering the data until another 2KB came in, to be able to transmit a segment with a 4KB payload. This freedom can be exploited to improve performance. On the following grounds the sender com still send segments upon receiving the win=0(window size) or (buffer available). When the window=0;the sender may not normally send segments, with two exceptions i)URGENT DATA MAY BE SENT To allow the user to kill the process running on the remote machine. ii)The sender may send a 1byte segment to make the receiver re-announce the next byte expected and window size. SILLY WINDOW SYNDROME: To transmit 1byte of message TCP overhead 20 bytes, IP are required.In other words just to transmit 1 byte, extra 40 Bytes are required. Whenever there is a 1 byte room available at the receiver buffer, the window update segment is sent. Since the receiver is requested for 1 byte the sender sends 1 byte which leads to the receiver buffer to be full over again. Asking for 1 Byte and sending 1 byte appears to be silly, hence the name silly window syndrome. To avoid Silly window syndrome, Nagle suggested NAGLES APPROACH: When the data comes to the sender 1 byte at a time, just send the first Byte and buffer, all the rest until the outstanding Byte is acknowledged. Then send all the buffered characters in one 1 TCP segment and start buffering again until they are all acknowledged. Nagles approach cannot be implemented for all applications. In particular, when as X-windows application is being run over the internet, mouse movements have to be sent to the remote computer. Gathering them and sending them in bursts makes the mouse cursor more erratically, which makes users dissatisfied. CLARKS ALGORITHM: This approach is to prevent the receiver from sending a window update for 1byte instead, it is forced to wait until it has a decent amount of space available and then advertise that. Nagles algorithm is with respect to sender and Clarkà ¢Ãƒ ¢Ã¢â‚¬Å¡Ã‚ ¬Ãƒ ¢Ã¢â‚¬Å¾Ã‚ ¢s algorithm with respect to solve Silly window syndrome. The overall goal is for the sender not to send small segments and the receiver not to ask for them. TCP CONGESTION CONTROL: When the load offered to any network is more than it can handle, congestion builds up. The Internet is no exception. Although network layer also tries to manage congestion, most of the heavy lifting is done by TCP because the real solution to congestion is to slow down the data rate. In theory, congestion can be dealt with by employing a principle borrowed from physics: the law of conservation of packets. The idea is to refrain from injecting a new packet into the network until an old one leaves.TCP attempts to achieve this goal by dynamically manipulating the window size. The first step in managing congestion is detecting it. A timeout caused by a lost packet could have been caused by either (1) noise on a transmission line or (2) packet discard at a congested router. Nowadays, packet loss due to transmission errors is relatively rare because most long-haul trunks are fiber. All the Internet TCP algorithms assume that timeouts are caused by congestion and monitor timeouts for signs of trouble the way miners watch their canaries. TCP TIMER MANAGEMENT: TCP uses multiple timers to do its work. The most important of these is the retransmission timer. When a segment is sent, a retransmission timer is started. If the segment is acknowledged before the timer expires, the timer is stopped. If, on the other hand, the timer goes off before the acknowledgement comes in, and the segment is retransmitted. A second timer is the persistence timer. It is designed to prevent the following deadlock. The receiver sends an acknowledgement with a window size of 0, telling the sender to wait.Later, the receiver updates the window, but the packet with the update is lost. Now both the sender and the receiver are waiting for each other to do something. When the persistence timer goes off, the sender transmits a probe to the receiver. The response to the probe gives the window size. If it is still zero, the persistence timer is set again and the cycle repeats. If it is nonzero, data can now be sent. A third timer that some implementations use is the keep alive timer. When a connection has been idle for a long time, the keep alive timer may go off to cause one side to check whether the other side is still here. If it fails to respond, the connection is terminated. This feature is controversial because it adds overhead and may terminate an otherwise healthy connection due to transient network partition. The last timer used on each TCP connection is the one used in the TIMED WAIT state while closing. It runs for twice the maximum packet lifetime to make sure that when a connection is closed; all packets created by it have died off.

Blood Pressure Essay -- Health, Diseases, Walking

Distance Other studies have attempted to measure the effect of distance on blood pressure response in order to find if there is any correlation between distance and a significant blood pressure reduction. One of these studies (see Moreau et al, 2001) involved 24 sedentary post-menopausal American women (15 in the intervention group ) with high blood pressure. The results found that, after a 2-year walking programme, that there was a statistically significant drop of 13 mmHg in the systolic blood pressure of those participants who were taking anti-hypertensive drugs and of 11 mmHg in those not receiving medication, whereas there was no difference found in their diastolic blood pressure and in the blood pressure of the control group. The participants in the intervention group were instructed to walk 3 kms per day in addition to their normal daily physical activity. After the first year the reduction in blood pressure was significant: 6 mm Hg in the systolic blood pressure of the participants wh o were taking anti-hypertensive medication and 7mmHg in the non-medication group. In this study , the magnitude of the reduction of systolic blood pressure as a result of a walking programme is similar to that which has been found in earlier studies in response to traditional exercise (see Seals et al,1997, and Hagberg et al, 2000). Frequency Several studies have examined the impact of the frequency of walking per week on blood pressure in order to determine what frequency is most effective in terms of blood pressure reduction response. A study by Gettman et al, conducted in 1976, which compared the effect of fast walking between one, three and five days per week on blood pressure response, found that the difference in frequency was no... ...ffect on systolic blood pressure only. Lee et al (2010) found that moderate to high intensity waking was more likely to decrease blood pressure than low intensity walking. This result supports the findings of the study conducted by Quinn in the year 2000. Nemoto et al (2007) compared the effect of continuous walking at moderate intensity (50% of peak aerobic capacity) and interval walking at high intensity (70% of peak aerobic capacity) on reducing blood pressure. They found that blood pressure reduction was greater in the group who walked at high intensity: 9 mmHg for systolic blood pressure and 5 mmHg for diastolic. The results of a study conducted in India by Gosh (2006) to measure the effect of walking intensity (2 Km/30min) for 20 weeks, showed that there was a significant reduction in systolic/diastolic blood pressure by approximately 10 mmHg/4 mmHg.

Everybody’s Guilty Essay

In â€Å" Everybody’s Guilty – The Ecological Dilemma, â€Å" author and professor of Human Ecology at University of California, Santa Barbara, Garrett Hardin, explains the current issue with invisible reverberations. Hardin calls attention to the readers about how innocent actions by individuals can inflict on the environment. â€Å" We all acquiesce in the system of arrangements and practices that has created our ecological crisis† (Hardin, 40). In order to approve of our actions, individuals tend to hide from reality behind symbols and/or words. Incorporating rhetoric into our everyday lives does this. Garrett Hardin begins his argument by explaining the naming process that may have unintended consequences and then finishes with potentially solutions to this ecological crisis. For example, using the word â€Å"healthy† attracts an audience that is ultimately looking to eat correctly. Without looking at the ingredients, people believe the false advertisement and buy the food that says â€Å"healthy† on the box rather than what is actually good for them. By doing this, consumers are using an unethical form of persuasion while not being aware of the consequences or changes this action may have on people. Hardin wants the readers to view our world not as a society, but â€Å"as an ecological system† (Hardin, 40). Instead of acting as a whole, people need to understand that as individuals, everyone needs to play their part to improve the world we live on. Hardin suggests, â€Å"we [as individuals] can never do merely one thing† (Hardin, 41). The choices that are made everyday can have an impact on something a lot larger than what was intended. In the article, â€Å"Everybody’s Guilty†, Hardin uses the example of damming the Nile at Aswan. By building this dam, workers not only secured a water source for electricity and irrigation, which was their only intention, but they also caused deep distress to Egypt. This proves Hardin’s argument. Although the thought of a dependable water source is efficient, many times the consequences are overseen. Egypt now is suffering from periodic floods, which is slowly eliminating the nutrients within the water. To bring this example into an easier perspective, lets look at what average people do everyday such as pumping gas. Individuals unconsciously pump their gas with the only intention to be able to get from one place to another. Everyone knows the amount of consequences that may arise from this certain action, but no one really pays attention. The amount of pollution that is created from pumping gas is at alarming rates and is slowly taking a toll on the atmosphere. Transportation is the largest single source of air pollution in the United States. This includes air pollution emitted during vehicle operation, refueling, and manufacturing. These simple actions cause over a third of the contaminated chemicals in the air, and all people are worried about is if they will make it to work on time. With both the amount of cars on the road, and the escalating population rates, Hardin is trying to persuade readers that people need to take action and by persuading the audience, Hardin is engaging in a form of rhetoric. â€Å"Pollution will not be controlled unless population is controlled† (Hardin, 44). â€Å"Even what we call ‘success’ may prove to be a bitter failure. Increasing the size of the population is generally held to be a good thing, but an even larger proportion of the world’s people is becoming convinced that the world is already overpopulated† (Hardin, 42). Increasing the population not only limits resources, it also increases the amount of misunderstanding to people â€Å"never do[ing] one thing†, which can potentially lead to a larger ecological crisis. There are many problems when it comes to population. Technology is expensive and with population growing, the price of natural resources is increasing as well. There are solutions to this issue though, but cannot be implemented right away. Hardin looks â€Å"toward voluntarism and persuasion to help create a climate of opinion that can some day support stronger measures† (Hardin, 45). By â€Å"doing the right thing†, Hardin rhetorically persuades the readers to engage in thoughtful actions that decreases the population. The first step to this solution is to create a 100% effective birth control. Society knows that contraception is not completely effective, but because of this, Hardin suggests we create a system for acceptability towards abortions if necessary. If birth control fails, abortions should be included as a â€Å"back up plan† with the cost of being preferably free. The problem with this proposal is that abortions are frowned upon in other countries. To avoid the abortion issue, young girls need to be taught to become independent and goal oriented instead of becoming a teenage mother. Now-a-days girls are so interested in creating their own little families and having children that they tend to forget the hardships of having child. We need to educate all of society so as to reduce this social pressure, and consequently reduce the number of semi-reluctant mothers, who are probably not the best ones to raise children anyway† (Hardin, 46). Having a child when the woman is a child herself is not healthy, but is a prime example to Hardin’s argument. By creating children for the wrong reasons relates to creating an invisible reverberations. At the time it sounds great to have a child, but the aftermath consequences do not balance out. As a society, individuals need to create a culture of acceptability to smaller families. The world is slowly becoming more accustomed to this new culture, but still needs improvement. Garrett Hardin wants to leave his readers with a prolonged thought to help the world. He enriches his readers with evidence that support his idea of having an ecological crisis. Hardin explains that everyone is at fault when it comes to harming the world, but there are ways to solve this conflict and that is population control. Although the solution may not be easy, Hardin wants people to understand the consequences of their actions, because until then, the ecological crisis is going to continue to increase.

Pregnancies And Development Of A Fetus - 1574 Words

Starting from the beginning every single human being on the planet Earth started out as a sperm and egg. During conception, the sperm fertilizes the egg and you develop into what scientists refer to as a zygote. Through research scientist have been able to determine when implantation is futile, the first two weeks of a zygotes life is uncertain and as many as 30% zygotes don’t survive. Every week of a pregnancy is important to the growth and development of a baby. Have you ever thought about when a baby develops fingernails, hair or even their teeth? A baby develops in three trimesters: the most development happens within the first and the second trimester. All the while, the baby is only viable when it reaches the third trimester. Through the three trimesters each characteristic develops within a certain time period, through this rigorous process the child develops unique characteristics such as their fingerprints, hair, and teeth. There are many things that affect the develo pment of a fetus throughout pregnancy such as prescription and nonprescription drugs, illegal drugs, tobacco, alcohol, radiation, environmental pollution, infectious diseases, maternal nutrition, and maternal emotional stress. Before taking prescription and nonprescription drugs, a mother should consult a physician because even if she thinks a drug is safe to use it can cause development issues and be really harmful to the baby. Illegal drugs are highly- addictive and can make the baby addicted to theShow MoreRelatedThe Effects Of Pregnancy On Children s Development Of The Fetus1761 Words   |  8 Pagesthe dangerous decision to drink, smoke, or use drugs throughout their pregnancy is increasing daily. When an expecting mother uses a substance while pregnant, she poses a serious threat to both her health and her child’s. 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