The grief over a lost pet

The sorrow over a lost pet This pain might be hard to the point that the individual may feel more steamed at a pet than a human cherished one; there are numerous sentiments, and stages that are engaged with losing a pet. Five phases that are remembered for losing a pet are refusal, bartering, outrage, melancholy, and goals. Pam Brown once stated, If there is a paradise, its specific our creatures are to be there. Their lives become so interlaced with our own, it would take in excess of a chief heavenly messenger to detangle them (Brown, n.d.). I got up to a crisp spring breeze blasting through my window, the hints of recently conceived winged animals peeping, and the smell of blossoming blossoms. However, for reasons unknown something inside me asked and argued for me not get up. Something yelled inside me, yelled inside this little twelve-year-old young lady that this world was loaded with outrage, detest, and oblivious automatons moving around like individuals, individuals that were not living yet simply existing. Against my bodies, wishes and my terrible considerations I hurled myself up and started to begin my morning. I recall it was about 9:00 AM and I had a chiropractor arrangement at about early afternoon. I meandered around the house as though I had never been there, feeling lost, in a new body yet with no explanation behind this inclination just propelled myself on. Accepting the inclination would leave in the long run, I discovered the kitchen in my night wear. Just to be welcomed by the main thing on the planet that filled the void inside me, my pooch Shiloh. A blaze of memory came to me and I lived in that memory remaining in my kitchen recollect the memory of the battles it took me to persuade my mother that getting a canine was the most ideal treatment and disclosing to her that it would recuperate everything. I started to laugh to myself, I didn't know whether I was snickering in light of the fact that she trusted me and let me get him or on the grounds that I had hung a line of BS to her with expectations of a yes answer and here it worked out that my BS was correct. I got Shiloh from our neighborhood pet store. Each Saturday this pet store would have one feline and one pooch up for selection. When I had persuaded my mother this was the response to everything, we prepared and went to search for my clinician in a mutts body. The drive to the pet store was depleting I could barely contain myself. I watched the lines out and about stream by the vehicle; I felt that perhaps in the event that I focused on something, that it may place me in a mesmerizing and before I knew it that, we would be there. That didn't work, really it made me very sick and when I looked into, we had traveled possibly about a mile such a great amount for that splendid thought. I started to envision what my canine would resemble dark, white, or spotted. Perhaps with long hair and those tempting little dog eyes each canine proprietor knows. I concocted a huge number of names, just worrying myself more, what neckline I would pick, would I get a kid or a young lady hound. The most exaggerated inquiry in my mind was if my line of BS was going to work out, would it fill that sentiment of contempt, dejection, and uselessness. At that point I wound up asking would the person like me? That to me was one of the most clear explanation I required assistance, I was stressed if a canine was going to like me. We maneuvered into the parking area of the pet store, the hints of the tires moving over the asphalt and the dramatic end got up me from my trance. There was that last inquiry ringing in my ear, would the person like me? I understood we were there and felt this pressure in my chest, hands, and me in general. This is the thing that I had been sitting tight for and out of nowhere I felt frightened imagine a scenario where I picked an inappropriate pooch consider the possibility that that believing that everybody discusses, the sentiment of realizing its the correct decision isn't there. I assembled my musings and pushed my stomach from my throat back to where it had a place and left what I trusted would be the remainder of those emotions in the vehicle. As my hand got a handle on the handle and opened the entryway the sound of the one canine resonated in my ear. At that point nothing else made a difference, I was for once in my life numb to those horrendous emotions. The apprehension, dread, and tension more likely than not locked on to another person that was in short proximity. There was a line before the two confines out of nowhere an idea came into my head imagine a scenario in which somebody before me receives the pooch first. I immediately attempted to snatch the idea and discard it I did this so distinctively that I thought maybe I had acted it out in the center of the store, fortunately, I was not excessively insane. That is the point at which I heard the young man before me state EW, mama, I dont need this pooch. A liberating sensation flew tossed me. As the individuals before me cleared out I saw him, sitting in the pen alone totally mindful of his sentiment of being undesirable not feeling sufficient to return home with a young man. His dull earthy colored eyes spacey practically like tears, his shading practically like the grainy sand, and little spots practically like God had peppered him just on his feet before he sent him out the door. My consideration concentrated on why on the planet this young man didn't need him. That is the point at which I saw his back right leg was limp. The lady educated us that he didn't use this leg he was brought into the world with it yet had more need of adoration and devotion then I had ever felt. I understood he was much the same as me however simply didn't have the ability to state it. I envisioned him shouting out as I had done on different occasions to my dad I am here! Love me, need me, and allow me to give you how magnificent I am. I required him, I needed him, and there was definitely in my psyche that I needed to be that individual to give him what I so ached for. In that occurrence, I took a gander at my mother and stated, He is the main thing I need in li fe at this moment, and he is mine. We brought Shiloh home, the entire ride home I glared into his pecan eyes and saw that that coating was no more. In my eyes, I had given him what I yearned for and for that second, I felt true serenity, an inclination that everything would have been alright. That day he turned into my beginning and end, my reality. I marked on to a quiet agreement that day, a settlement of kinship, reliability and love that would be unrivaled by anybody. A canine that adores genuinely, without judging and needn't bother with anything clarified or asked he just knows. I adored him and he cherished me, my inquiry was addressed he loved me. There was nothing on the planet that would remove him from me, we were powerful together, or so I thought. A year had passed by and it was the greatest year of my life, he was great. He astonished me his leg never annoyed him maybe his hardest battle in life had skimmed away. He ran on three legs and jumped around in the yard as though he were a gazelle wandering the fields of Africa. I contemplated internally that an individual who has never possessed a canine has missed a superb piece of life. I returned to my faculties remaining in my kitchen the morning of my chiropractic arrangement asking why this day is so not quite the same as whatever other day, Why I considered the entirety of this so inside and out. I gazed at those equivalent pecan eyes expressing gratitude toward God for letting me own such a valiant, faithful canine. I went into the restroom just to see I despite everything was not dressed, my hair was rumpled, pointing every which way, much like roadways on a guide. I concluded that since it was just 9:00 in the first part of the day that I had the opportunity to take Shiloh outside and play for a little while, realizing that he would not pass judgment on me on what I looked like I remained in my night wear. Opening the entryway I felt the fresh breeze coast over my face, my exposed feet on the sun washed patio, seeing the incredible warmth under my toes. I extended the extent that I could reach, gazing at the sun as though I were getting a handle on it in my grasp, Shiloh did likewise. As I opened my eyes, I understood the bovines over the road were eagerly concentrating on us as though they needed to participate in a gazing match. We sat in the front for a spell, simply tuning in to the stirring leaves, the popping of pine trees branches as though they were all extending as one appreciating very similar things we were. I stayed there, appreciating how straightforward life could be the point at which you had what you required right close by. The smell of newly baled roughage filled the air and the sound of the child calves over the road calling to one another to play. A day like some other day, the scents and hints of a typical day, however something was all the while waiting, something that was obscure, which I believe, is the reason I was so delicate to this in disguise feeling. I strolled through the hosed dew grass, came to down, got the felt secured toy, and started to hurl it around for Shiloh. My mother had woken up and come outside to search for me. She remained on the yard and revealed to me that I ought to most likely begin to prepare for my arrangement seeing with regards to how I was still in my night robe and honestly a wreck. My mother turned and went inside to prepare. Much to my dismay that the inclination I had been having throughout the morning would before long show itself in evident structure. It was as though the following seven minutes were stuck in a time travel. As I pivoted for Shiloh I saw he had went over the way to the animal dwellingplace. Stressed and mad I did the main thing I thought of and last thing that he would intentionally hear, I called his name, SHILOH come here! He at that point did what he specialized in, tuned in to me. Shiloh came stumbling into the asphalt of the street; I heard his toenails cutting on the asphalt. An unexpected liberating sensation came over me, trailed by complete and express injury. I could smell the diesel, hear th e sound of the thundering motor, I ran for my life and let out a ghastliness filled yell that partook in me with it. I saw I was past the point of no return, I saw my beginning and end, my reality take the effect that I hustled so difficult to take for him. I tumbled to my knees, feeling the virus mud mush around my knees. I didn't feel anything, not a heartbeat. I didn't hear anything, no winged creatures, no wind, the trees that were simply extending in the breeze had halted as though they knew the seriousness of the circumstance. A piece of me kicked the bucket that day that I have never gotten back. My mother surged out

King Lear: Family Relationships, Human Nature and Its Failings Essay

â€Å"I love your highness as indicated by my bond; no more nor less† (I. I. 94-95). Great morning instructors and HSC understudies. Lord Lear, an ageless story of family connections, human instinct and its failings. Be that as it may, what makes this play â€Å"timeless†? The way that it contains all inclusive subjects of adoration, envy and family connections makes it pertinent to present day times despite the fact that it was composed for a 16thcentury crowd. Two pundits that have remarked on the topical worries of family connections and human instinct are Maggie Tomlinson in â€Å"A savage world† and Jim Young in â€Å"Still through the hawthorn blows the cold wind†, the two of which I’ll be talking about, today in detail. The idea of family connections is a predominant topic that can be seen on numerous levels, for example, the crumbling, reestablishment and the idea of familial bonds. There are numerous family connections in the plot of King Lear, with the two significant ones identifying with the sub plot of Gloucester and the primary plot of Lear. In both these connections, double-crossing is the main consideration that adds to the weakening of the family relationship. In Gloucester’s case, through the basic demonstration of mortifying Edmund, where Gloucester says in his essence â€Å"There was acceptable game at his creation and the whoreson must be acknowledged† (1. 1. 21-24), he made a fracture in the relationship. Maggie Tomlinson raises a fairly noteworthy moment that she remarks on the idea of the relationship and the trust that is manhandled. She states â€Å"The proof is basically not the kind of thing any one not to mention a dad would accept in† This shows the trust that is worked in these sorts of connections and its capacity to be misused. Family connections are additionally observed between the girls and King Lear. Shakespeare keenly examines the idea of connections through Lear’s test to see who cherishes him the most. Goneril and Regan are depicted as manipulative individuals with the endowment of words and dignified habits, however it very well may be noticed that Cordelia likewise cherishes her dad yet can't communicate it where she says â€Å"I am certain my love’s more heavy than my tongue† (I. I. 76-78). With the nonattendance of a maternal figure, one could address the amount Lear love’s, not to mention, thinks about his little girl. The way that he ousts Cordelia, when she can't communicate her adoration, shows the little information he has of her shortcomings and qualities or the condition of his brain. Subsequent to giving up his capacity, Lear requests love from his little girls Goneril and Regan, yet doesn't get, so he starts to argue. Jim Young remarks on this breaking down relationship, where Lear’s point of view is that his girls owe him love in light of the material blessings he has given them †Thy half of the realm thou hast not overlook, where in I thee endowed† ( II. iv. 177-181). This stresses the commitments of the constrained relationship rather than its characteristic event. Another perspective that is profoundly examined in King Lear is human instinct and its failings. To characterize human instinct it is the characteristics of mankind that are thought to be shared by every individual, making it an immortal subject. To be human is to blame and to gain from one’s errors. Allurement is a center perspective that causes these shortcomings and is a piece of human instinct. All through the play, enticement can be seen particularly through that of Lear. Its human instinct to feel love however one of Lear’s defects is his pride, he needs to be lauded, hear the amount he is cherished. In any case, this imperfection in his inclination of allurement causes his defeat and the loss of his rational soundness. In his disarray, he turns out to be allegorically visually impaired. It is just during the tempest that he gets his own test, where things may change or stop. It is in this tempest that he returns to nature as a basic being, the place the main thing that recognized him from a creature, was stripped , that is the capacity to think and reason. Here, he is deprived of all garments, and subsequently respect introducing the fizzling of ones nature. Youthful proceeds to state that Lear just becomes rational as a result of everyone around him particularly the Fool. The Fool holds on with Lear and offers his in sufferings yet is explicit around one point: â€Å"Never give your capacity to anyone†. It is human instinct to need force and regard, and when Lear parts with it, as observed through the losing of his knights, he himself turns into an imbecile. Finally, Shakespeare additionally explores human nature’s association with reclamation in Edmond. Edmond looks for recovery before he bites the dust, where passing is the redemptive equity. Realizing that he was not to live, he attempted to change his shrewd nature by informing others to proceed to spare Cordelia from his deadly courier, yet as Maggie Tomlinson stated, Shakespeare cunningly consolidates the endeavors of an individual to change their inclination. Here, Edmond comes up short and is liable for Cordelia’s passing. Tomlinson raises the inquiry if whether this shows we can endeavor to change, yet it is our human instinct to be preservationist and not stay into a new area, and thus Edmond attempts to do great by sparing Cordelia yet just comes up short. Ruler Lear will keep on staying an ageless story, and illuminate crowds about family connections and human instinct, for quite a long time to come. One could possibly think about whether those in Shakespeare’s time valued the play, the sum it is refreshing at this point. Much obliged to you.

Short Story Advice from the Masters of the Craft

Short Story Advice from the Masters of the Craft Writing a short story is one of the most difficult and complex endeavors a writer can undertake. The process of ensuring change within characters over the span of only a few pages is more difficult than it seems to the untrained eye, and telling a story within such limited space takes much patience to get it right.So what do the masters of short-story writing have to say about the genre and the process? Take a look at the quotes below and consider the suggestions for your own approaches to the genre.A short story is a love affair; a novel is a marriage. A short story is a photograph; a novel is a film.Lorrie MooreThinking of a short story in much the same way as a journalistic photographer thinks of a photograph will put you on the right track in the creation process. When setting up a shot, a photographer will make every attempt to include details that give depth to the subject of the photograph. A photograph of a young girl standing alone with a flower might not have much depth, bu t widen the frame to a photograph of that same girl holding a flower in front of the gravesite of her father and suddenly the entire picture takes on a much deeper meaning. Widen the frame even more, and the viewer only sees a graveyard with a figure standing alone.If the photographer focuses the frame too narrowly, the meaning is absent because of the lack of visual information; if the writer focuses too broadly, the meaning is lost in a sea of other distracting visual elements. In the same sense, when writing a short story, you have to include visual, sensory elements of setting that give greater depth to your characters. Add too many, however, and the theme is lost. In such, finding the perfect frame for the attempt is your greatest challenge.A short story must have a single mood and every sentence must build towards it.Edgar Allan PoePoe is undoubtedly one of the most prolific and influential authors of the genre. He is often referenced when discussing the importance of building tension and creating a mood within a piece. In this quote, he cautions against writing about anything that isnt a necessary progression toward the storys denouement. Simply put, if the main character has a cousin in England but that cousin has nothing to do with the story, dont mention him.For the same reason you should know the end before you even start writing, you should also know the target mood you want to accomplish before beginning the first sentence. In the same way that every plot point in your short story must be moving toward the conclusion, every sentence you write should attempt to convey the targeted mood. When you use this formula and ensure its application, the likelihood of getting off track or having too broad a scope will be decreased significantly.With a novel, which takes perhaps years to write, the author is not the same man he was at the end of the book as he was at the beginning. It is not only that his characters have developedâ€"he has developed with them, and this nearly always gives a sense of roughness to the work: a novel can seldom have the sense of perfection which you find in Chekhovs story, The Lady with the Dog.Graham GreeneWhen writing a short story, you should aim for perfection in every word. If a novelist does this, her book would likely never be ready for publicationâ€"it would simply take too long to finish. This is where the uniqueness of a short story sets it apart from any other genre (except poetry). Every word, every description, every element of setting, every movement must have purposeâ€"and that purpose is to guide the story and its characters toward the resolution.I believe that the short story is as different a form from the novel as poetry is, and the best stories seem to me to be perhaps closer in spirit to poetry than to novels.Tobias WolffWhile were on the topic of poetry, we cant leave out this wonderful quote from Tobias Wolff that demonstrates the difference between sitting down to work on a novel vers us sitting down to write a short story. When a poet writes, the process is often a period of agonizing over every single word. This process involves analyzing the word, considering its connotations, and searching for any other word that might fit better to convey the exact emotion the author wishes to convey.The process of writing a short story should be very similar to the poets process. The author needs to agonize over word choice, setting, clothing… anything that is included in the story. The sound of the language is as important in a short story as it is in poetry. Every word should be selected carefully to convey the right mood and the right emotion, and every action must have a purpose.Im a failed poet. Maybe every novelist wants to write poetry first, finds he cant and then tries the short story which is the most demanding form after poetry. And failing at that, only then does he take up novel writing.William FaulknerOften, a writer will take on a short story thinking that it will be simpler than writing a novel. As far as time commitment, this may be the case, although as William Faulkner points out, the short story is one of the most demanding forms of literature to write. It requires much of the same level of research as a novel, but must be condensed like poetry to tell only what is most relevant, most crucial and most poignant about a characters interaction with time, place and situation.

Rene Descartes Contribution To Psychology Free Essay Example, 2000 words

Descartes enlisted himself in Army at the age of 22; it is unclear as to what his exact duties were in Army. It is also, however, believed that he was serving in the so-called engineering wing of the army. It was also here that he developed his passion for mathematics and applied to engineer as this unit was responsible for the development of different structures and machines to protect the soldiers during the battle. It is also, however, further argued that Descartes may also have been involved in the education of the soldiers and therefore must have been involved in the development of education programs for the young noblemen who were part of the army. It was during this period that Descartes developed a close relationship with Isaac Beeckman which kind of intensified the interest of Descartes in the subject of science. It is also argued that the relationship between the two was more than just a relationship of the teacher and student and Beeckman served as the mentor to provide h im the necessary intellectual stimulus to consider the science and philosophy as the important fields of knowledge to peruse. We will write a custom essay sample on Rene Descartes' Contribution To Psychology or any topic specifically for you Only $17.96 $11.86/pageorder now Descartes left the army in 1619 and it was after this period that he started to consider writing on the subject of philosophy and other sciences on a very serious note. His early works include Discourse on Method which outlines his own style and orientation towards the field of philosophy. Over the period of time, he seems to work on different drafts of the various writings which he either abandoned writing or delayed the publication of them. It was also during this era that Galileo was condemned by the Church and fearing to meet the same fate, he decided not to publish his work which subsequently published with the name The World. In The World, he actually combined his earlier work on the optics and meteorology and further extended the work to discuss extensively on the field of Physics. It was also during this period that his status as a serious philosopher started to emerge and his work found its way to the elite class of Paris.

Diabetes The Fifth Leading Cause Death - 1769 Words

In the United States in 2010, â€Å"diabetes was the seventh leading cause of death† and out of all these, â€Å"a total of 234,051 death certificates [listed] diabetes as an underlying or contributing cause of death† (American Diabetes Association, 2014). In 2012, â€Å"29.1 million people or 9.3% of U.S. population have diabetes† (Center for Disease Contro, 2014). Out of 29.1 million, 21.0 million people are diagnosed and 8.1 million people are undiagnosed. About 25.9% are ages 65 and older and approximately 0.25% is under the age of 20. With the growing number of fast food restaurants opening on almost every corner, these numbers are, sadly, not very surprising. People these days have schedules that are so hectic that they almost always don’t have time to care about their well-being and have such unhealthy lifestyles. With that, serious health complications arise and unfortunately, diabetes mellitus is one of the most prevalent. Diabetes mellitus is a disease characterized by the body’s inability to metabolize glucose. Glucose is the body’s main source of fuel for energy. Too much or too little of it can cause some serious complications in the body. Normal glucose level in the blood should be between 70-120mg/dl. An increased level of more than 250 mg/dl is called hyperglycemia. Signs and symptoms â€Å"include the three ‘polys’: polyuria (excessive urination), polydipsia (excessive thirst), and polyphagia (excessive hunger)† (Rosdahl, 2012). Other signs and symptoms may includeShow MoreRelatedObesity Is Prevalent Between Children And Adolescents In1177 Words   |  5 PagesObesity is the second leading cause of death after smoking in the US. It also causes cancer, and it is associated with unhealthy eating and less exercise or physical activity. 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Mgt 520 Final Exam Study Free Essays

MGMT520 Final Exam Study Guide Finals open on Saturday April 20  at 12:01 a. m. MT (Saturday morning) Finals close on Thursday April 25 at 11:59 p. We will write a custom essay sample on Mgt 520 Final Exam Study or any similar topic only for you Order Now m. MT (Thursday night) PLEASE DON’T WAIT TILL THE LAST MINUTE – THE SYSTEM IS BUSY AND MAY SLOW DOWN AND ANYTHING CAN HAPPEN. YOU MAY WANT TO PRINT THIS GUIDE. 1. The final exam is â€Å"open book, open notes. † The maximum time you can spend in the exam is 3 hours, 30 minutes. If you have not clicked the Submit For Grade button by then, you will be automatically exited from the exam. In the final exam environment, the Windows clipboard is disabled, and so you will not be able to copy exam questions or answers to or from other applications. There are three pages to your final, and each page begins with a story. The stories are quite interesting and will make the questions flow easily. The questions that follow are then taken from each story. There is a little overlap. Remember questions are scrambled, so while they vary, all TCO’s will be tested. 2. You should click the Save Answers button in the exam frequently. This helps prevent connection timeouts that might occur with certain Internet Service Providers, and also minimizes lost answers in the event of connection problems. If your internet connection does break, when you reconnect you will normally be able to get back into your final exam without any trouble. Remember, though, that the exam timer continues to run while students are disconnected, so students should try to re-login as quickly as possible. The Help Desk cannot grant any student additional time on the exam. . See Syllabus â€Å"Due Dates for Assignments Exams† for due date information. 4. Reminders: * You will only be able to enter your online Final Exam one time * Click the â€Å"Save Answers† button often * If you lose your Internet connection during your Final Exam, logon again and try to access your Final Exam. If you are unable to enter the Final Exam, contact first the help desk and then your instructor. * You will always be able to see the time remaining in the Final Exam at the top right of the page . Assessments with Multiple Pages: * Make sure you click the â€Å"Save Answers† button before advancing to the next page (we also suggest clicking on save answers while you are working) * Complete all of the pages before submitting your Final Exam for instructor review; check your work and be sure to answer all the parts of questions. * Do NOT use your browser’s ‘Back’ and ‘Forward’ buttons during the Final Exam * Please use the provided links for navigation 6. Submitting Your Final Exam: When you are finished with the Final Exam, click on the â€Å"Submit for Grade† button * Please note: Once you click the â€Å"Submit for Grade† button, you will NOT be able to edit or change any of your answers 7. Exam Questions * The final exam covers all course TCOs and Weeks 1-7. * The exam has two short answer questions worth 15 points each (TCO I and D. ) (Plan about 10 minutes each). * The e xam has 7 essay questions worth 30 points each (TCO A, B, C, E, F, G, and H) (Plan about 23-25 minutes each. ) This gives you about a 30 minute buffer. The exam has a total of 240 points. * The final exam contains 3 pages, which can be completed in any order. You may go back and forth between the pages. * On the short answer questions, just answer the question asked, with any brief detail to explain why you answered that way. If a list is requested, provide it. * On the essay questions your answers should be succinct, fully address each part of the question, and demonstrate your knowledge and understanding in a concise but complete answer. You can use bullets where appropriate (i. e. listing elements, defenses, or steps. Ensure you analyze and give reasons for answers as partial credit is given even if the answer is wrong. * Remember always use proper citation when quoting other sources! Place any quoted or borrowed material (even a short phrase) in quotation marks with the source ( URL, author/date/page #) immediately following the end of the passage. Even cite paraphrased information. Quoted or paraphrased material should not dominate a student’s work; use it sparingly to support your own thoughts, ideas, and examples. Failure to properly cite material can jeopardize a passing grade on the exam. Your work may be submitted to turnitin. com, an online plagiarism checking service. * If you reference your text, it’s OK to just say Jennings, p__. 8. Some of the key study areas are as follows: (while these are key areas remember that the exam is comprehensive for all the assigned course content and this study guide may not be all inclusive. * TCO A: Given an organizational requirement to conform business practices to both the law and best ethical practices, apply appropriate ethical theories to shape a business decision. Schools of thought * Ethical models – you will need to apply them to a factual situation much as you did in your midterm * TCO B: Given instances of federal regulation of business and commercial practices, determine the constitutional and regulatory bases for such regulation, and formulate a strategy by which an  impacted business can influence or contest regulating outcomes. * APA * process of regulation promulgation * Legal challenges to regulati ons (recall our Week 2 assignment, especially #5) TCO C: Given an example of corporate liability arising from the sale of defective and dangerous products, develop a business strategy that includes ethical considerations to minimize liability for claims of product liability and breach of warranty. * Strict Liability 402A – know the elements, relate them to the facts! * Negligence – don’t forget to cite the elements and relate them to the facts! * Warranties – again, expressed and implied, and relate them to the facts. * Defenses to all of these claims – don’t forget to use our terms: contributory negligence, assumption of risk, comparative negligence; relate them to the facts! TCO D: Given a business requirement to form a contract for the sale of goods and services to a customer, define the elements of a contract, and determine whether a duly formed contract is enforceable under the common law or Uniform Commercial Code. * Contract formation – remember the elements! * Contract performance * Defenses to contract performance * TCO E: Given specified circumstances of an employment relationship, determine the circumstances under which an employer is liable to an employee for employment discrimination or wrongful discharge. Creation of the agency relationship, including respondeat superior, negligent hiring, etc. * Responsibilities of the agent and principal * Theories of discrimination under Title VII – disparate treatment, impact; don’t forget sexual harassment, which is also covered, as well as age under the ADEA * Defenses to a Title VII charge * Enforcement of Title VII * TCO F: Given specified circumstances of business ownership of real and intellectual property, evaluate the rights of business to the protection of its property and the obligations arising out of the use of the property. Theories to protect business intellectual property (patents, copyright, etc. ) * Know the difference between app ropriation (a privacy tort) and misappropriation (trade secrets). * Enforcement of business property rights (e. g. , product disparagement, trademark infringement, etc. ) * Defenses to these claims * TCO G: Given examples of anticompetitive or unfair trade practices, apply applicable antitrust or other consumer protection laws, and determine appropriate business strategies to prevent trade practices liabilities. Statutory protections for consumers * Bankruptcy * Restraints of trade * TCO H: Given a conflict between corporate stakeholders over a business decision, evaluate the legal and ethical responsibilities of corporate directors, officers, and controlling shareholders. * Duties and obligations of directors of a corporation * Insider Trading – be sure you know the elements * TCO I: Given specified circumstances of a business decision to expand to international markets, determine what international legal requirements or regulatory controls apply. Principles of international law * Resolution of international disputes * Jurisdiction in a private action between citizens or companies of different countries – understand sovereign immunity and how it applies and who may use it as a defense. * Jennings’s Article â€Å"Why an International Code of Ethics Would be Good† 9. Areas that were discussed in the threads will be prime targets. 10. Assignments will also be prime targets for revisiting. Finally, if you have any questions for me, please post them to our Q;A, or email me. Good luck on the exam! How to cite Mgt 520 Final Exam Study, Papers

Visual Rhetoric free essay sample

In this piece of visual rhetoric, there is a very strong message conveyed. Depicted in the medium are lips, very disturbing lips. The creator of this piece uses image to connect to the viewer visually, expressing a very serious tone. Image is a very powerful tool; it makes the author more credible and the audience more apt to agree and believe. The creator of this image intends to affect its viewers in a powerful way: through surprise and contrast, and it is well executed. The viewer of the image should look at it and ponder its meaning. No words accompany this picture, permitting the viewer to have free interpretation. However, the core message is clear. The core message is about smoking. The design of this anti-smoking advertisement is very intriguing, being a cropped picture of a woman’s bright red lips. They are not normal lips; they are lips with a black hole through them, as if burned by a cigarette. We will write a custom essay sample on Visual Rhetoric or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page The image is composed so that the viewer’s eyes are drawn right to the hole, a stain in perfection. The bright, cherry lipstick contrasted with the revolting, charcoal hole—placed right where a cigarette would sit—appalls the viewer. This placement was very effective in conveying a message about smoking. In the background of the picture there is a pore less, porcelain-like skin surrounding the lips. This, coupled with the red lips, only makes the woman appear even more perfect, despite the gaping, metaphorical hole in her lips. Light is used to brighten the picture, contrasting with the hole as well. Light is often utilized to symbolize hope and knowledge, ideas that smoking is not associated with. There are subtle shadows sitting in the corners of the woman’s mouth and below her lower lip, making the picture even more realistic. The viewer’s eyes move from the focal point (the burn hole) to the red lips, and then to the perfect skin, finally ending on the shadows in the teeth. The creator of this image was well versed in making an affective, persuasive design. The visual text in this advertisement assumes that its viewers are smokers so that they can persuade the smoker to quit. Another viewer could be a non-smoker, allowing the anti-smoking message to act as a preventative. All of the viewers might not concur with the idea that smoking ruins what might be close to perfection. For smokers in denial, this might be a difficult concept to grasp. The author was simply trying to persuade them to make the best decision, even if it meant doing so with a contorted image. This instills a negative opinion about smoking in the viewer. This visual rhetoric makes a lasting impression that, through thought, can be interpreted in different ways. It will invoke ideas about life and happiness in its viewers. The purpose of this visual rhetoric piece is to persuade, inform, and almost warn its viewers. Through the shocking hole in the lips, there is a significant amount of persuasion, for it does not appeal to many people. They are warned that smoking will stain their lives permanently. The argument, simply stated, is that smoking is bad. The image will evoke the value of a good life that does not include the depicted vice of smoking. Women often have desires to be perfect, while men may desire to be with a perfect woman, and glistening red lips symbolize this. This appeal of perfection is interrupted by the cigarette hole, crushing the desires of the viewers, and creating an instinctive affliction towards smoking.

Twice Gone, Twice Returned Essays - English-language Films, Films

Twice Gone, Twice Returned An Analytical Essay on The Horse Whisperer ?Twice gone to hell and twice returned? (449) is the phrase Nicholas Evans uses to describe Grace's emotional journey in the final chapter of The Horse Whisperer. In fact, Grace's development is an important theme throughout the novel. Pivotal to Grace's development were the two times she went ?to hell? after which a critical change can be seen in the girls outlook on life and her relationship with her mother. By breaking up the novel into three smaller sections it is easier to see this progression. Prior to Grace's accident she is indifferent towards her relationship with her mother. After Grace's accident she begins to confront this relationship and to some extent rebuilds it. Later, this relationship is nearly destroyed, but when Grace once again revisits the horror of death, she finally is able to break through and reaches a peace with her mother. Not much is known of Grace's relationship with her mother near the beginning of the novel, however, a brief background related by Annie does give some insight into the relationship. Annie recalls a conversation between her and Grace in which they are discussing a mother daughter photo shoot: Why don't we ever do this?' Grace said, not looking up. Annie answered, rather too tartly, that she thought it was immoral, like product placement. And Grace had nodded thoughtfully, still not looking at her. ?Uh-huh,' she said, matter-of-fact, flipping on to something else. ? I guess people think you're younger if you make out you haven't got kids.' This comment and the fact that it had been uttered without a trace of malice had given Annie such a shock that for several weeks she thought of little else then her relationship with Grace, or as she now saw it, her lack of one? (39). It's stated twice that Grace is not looking at her mother while speaking to her. Clearly Grace is either intentionally not looking at her mother, ignoring the lack of communication between them, or doesn't care enough to even bother. Either way, this demonstrates the lack of active communication between the two. Grace's snide remark to Annie's seemingly legit excuse also shows a lack of trust in Grace towards Annie. Lastly, Annie herself admits to observing the lack of relationship between her daughter and herself. Grace's accident forever changes the relationship between her and her mother. When Grace is in her coma, she is eventually faced with a choice: ? In the distance she could see a circle of light and somehow she knew she had the choice of going towards it or turning and going in the other direction where there was also light, but of a dimmer, less welcoming kind. ? (77). The brighter light represents death and consequently freedom from earthly problems. The dimmer light represents life and the problems Grace will have to face when she wakes up from the coma. At this point Grace chooses to head towards the dimmer light. She realizes that it is less inviting, but she chooses it regardless. Compared to how Grace handled choices in earlier examples, this choice is more mature. It symbolizes Grace's willingness to finally confront her problems. More importantly however than Grace's willingness to head to that tunnel, is who Grace knows is at the end of it. ?Then she heard voices. They were coming from the place where the light was dimmer. She couldn't see who it was but she knew one of the voices was her mother's.? (77) Grace can't see her mother, but she chooses to head towards her voice anyway. With no way of actually knowing what awaits her, Grace has chosen to trust her mother's voice and symbolically her mother. Thus begins a long, difficult healing process between the two, which may not be apparent at first as Grace fights to hold on to her former state of mind. Indeed at first it seems as though Grace may be reverting to her old state of mind. She tries constantly to hurt Annie as is shown during the scene outside of the Little Big Horn memorial. Still something in Grace makes her realize what she's doing to her mother isn't

Compound Conjugations of Avoir

Compound Conjugations of Avoir The verb avoir (to have) is one of the main irregular verbs in French. Like the other irregular verbs, the conjugation of avoir  doesnt follow the same patterns as other verbs, so getting a handle the proper use of this verb requires a fair amount of memorization. There are two kinds of conjugations in French: simple and compound. Here are the compound tenses (and conjugations) of the verb avoir, which are generally used to describe something in the past tense. Pass compos Pluperfect Past subjunctive j ai eu avais eu aie eu tu as eu avais eu aies eu il a eu avait eu ait eu nous avons eu avions eu ayons eu vous avez eu aviez eu ayez eu ils ont eu avaient eu aient eu Future perfect Conditional perfect Pluperfect subjunctive j aurai eu aurais eu eusse eu tu auras eu aurais eu eusses eu il aura eu aurait eu et eu nous aurons eu aurions eu eussions eu vous aurez eu auriez eu eussiez eu ils auront eu auraient eu eussent eu Past anterior Conditional perfect, 2nd form j eus eu eusse eu tu eus eu eusses eu il eut eu et eu nous emes eu eussions eu vous etes eu eussiez eu ils eurent eu eussent eu Past imperative Past infinitive Perfect participle tu aie eu avoir eu ayant eu nous ayons eu vous ayez eu

Vulnerability Tools Essay Example | Topics and Well Written Essays - 2500 words

Vulnerability Tools - Essay Example nformation systems. Nessus is a comprehensive and open source security scanner. Plug-in architecture allows users to customize it as per their systems and networks. The security scanner frequently updates itself and provides full reporting, host scanning, and real-time vulnerability searches. Security audit features of Nessus are (Messmer, 2005): Credentialed and un-credentialed port scanning Network based vulnerability scanning Credentialed based patch audits for Windows and most Unix platforms Credentialed configuration auditing of most Windows, Unix platforms Robust and comprehensive credentialed security testing of 3rd party applications such as iTunes, JAVA, Skype and Firefox Custom and embedded web application vulnerability testing SQL database configuration auditing Cisco Router configuration auditing Software enumeration on Unix and Windows Testing anti-virus installs for out-of date signatures and configuration errors Another popular and open source tool for vulnerability an alysis is Wireshark. This tool, which was previously named as Ethereal, also provides functionality for packet sniffing. A relatively easy GUI along with various filtering and sorting options makes this tool perfect for non-savvy IT staff within organizations (Scalisi, 2010). Comparing Nessus and Wireshark Wireshark is considered to be at top of the list for network protocol analyzers. Wireshark not only provides vulnerability analysis, as its functionality can be resembled with â€Å"tcpdump.† It emphasizes protocols and represents data streams on the GUI. The major advantage that this tool has is the compatibility of operating systems, as it supports OS X, Windows, UNIX and Linux. Moreover, it also extensively supports Voice over IP that is a significant option for the organization, as international and corporate organizations use VoIP for communication purposes to save cost and at the same time deliver quality. Nessus, on the other hand, is used in more than 75,000 organiz ations around the globe and it is considered to be one of the world’s most popular vulnerability scanner (Ferguson, n.d.). However, the third version, i.e. version 3, has now been converted to a proprietary license as the scanning engine is still free and updates are also available after a week on a release. Relating with the Scenarios When Nessus is incorporated in a large enterprise, most probably, a government organization such as Department of Defense (DOD) networks, it will initiate a port scan and target the defined host or a network. After opening the port, it examines all the services that are running on the system or network and tests all the detected services against vulnerabilities defined in the Nessus vulnerability database (Kim, n.d.). As this tool can develop a testing platform for network resilience, the report generation is very comprehensive that is ideal for large enterprises. As it is an easy remote based vulnerability analysis tool, it can be best suited for large enterprises that are geographically dispersed in more than one continent

Remembered Event (Male) Essay Example | Topics and Well Written Essays - 1000 words

Remembered Event (Male) - Essay Example The fact is that I realized that I fell in love with that girl. Definitely, it was love on the spot. I wanted to help her but couldn’t even pronounce a word, for there was lump in my throat. The girl saw my efforts and came closer. With the first sentence of her and probably due to her marvelous smile the stupor chaining me was broken. Suddenly I found my ability to speak and after that very first moment of our conversation or even before it I caught myself on the thought that that girl, her charming name was Emma, was my destiny, my soul-mate and my love for the whole life. Probably, you may think that I am too sentimental for a male. But to my mind man’s obduration works only in the context of a single status or a one when he just doesn’t know what is love or simply doesn’t experience those feelings that I learnt when met Emma. Thus, after that day I laid siege to Emma by the means of every possible way. Surely, my addresses were romantic ones, since romanticism had opened within me to the extent that days and nights I spent inventing plan for our dates. After some time of my tremulous attention Emma said â€Å"yes† to my proposal of relations. And we started dating. There was no doubt that it was the beginning of the happiest time of my life. At least, I thought in such a way, for I was flying high above the sky. Every day I was planning something new and interesting to make happy my beloved Emma. Time passed apart from her was seemed everlasting. Seconds and minutes spent together were the greatest happiness for us. I felt Emma was my blessing of destiny. We even had our places in parks, cinemas, cafes and just in the streets. We enjoyed each other every moment of our dates without thinking of any serious questions and issues, which were waiting for us in the nearest future. Approximately after seven months of our romantic relations Emma told me that there was no future for us, as her parents didn’t see me as a good life

Implementation of New Computer Network

Implementation of New Computer Network Here we are going to implement an new computer network for this company that 25 employees have been working in. Suppose you want to build a computer network, one that has potential to grow to global proportions to support applications as diverse as teleconferencing, video-on-demand, electronic commerce, distributed computing, and digital libraries. What available technologies would serve as the underlying building blocks, and what kind of software architecture would you design t integrate these building blocks into an effective communication service? Suppose you want to build a computer network, one that has the potential togrow to global proportions and to support applications as diverse as teleconferencing, video-on-demand, electronic commerce, distributed computing, and digital libraries. What available technologies would serve as the underlying building blocks, and what kind of software architecture would you design to integrate these building blocks into an effective communication service? Answering this question is the overriding goal of — to describe the available building materials and then to show how they can be used to construct a network from the ground up. Before we can understand how to design a computer network, we should first agree on exactly what a computer network is. At one time, the term network meant the set of serial lines used to attach dumb terminals to mainframe computers. To some, the term implies the voice telephone network. To others, the only interesting network is the cable network used to disseminate video signals. The main thing these networks have in common is that they are specialized to handle one particular kind of data (keystrokes, voice, or video) and they typically connect to special-purpose devices (terminals, hand receivers, and television sets). What distinguishes a computer network from these other types of networks? Probably the most important characteristic of a computer network is its generality. Computer networks are built primarily from general-purpose programmable hardware, and they are not optimized for a particular application like making phone calls or delivering television signals. Instead, they are able to carry many different types of data, and they support a wide, and ever-growing, range of applications. This chapter looks at some typical applications of computer networks and discusses the requirements that a network designer who wishes to support such applications must be aware of. Once we understand the requirements, how do we proceed? Fortunately, we will not be building the first network. Others, most notably the community of researchers responsible for the Internet, have gone before us. We will use the wealth of experience generated from the Internet to guide our design. This experience is embodied in a network architecture that identifies the available hardware and software components and shows how they can be arranged to form a complete network system. To start us on the road toward understanding how to build a network, this chapter does four things. First, it explores the requirements that different applications and different communities of people (such as network users and network operators) place on the network. Second, it introduces the idea of a network architecture, which lays the foundation for the rest of the book. Third, it introduces some of the key elements in the implementation of computer networks. Finally, it identifies the key metrics that are used to evaluate the performance of computer networks. 1.1 APPLICATIONS Most people know the Internet through its applications: the World Wide Web, email, streaming audio and video, chat rooms, and music (file) sharing. The Web, for example, presents an intuitively simple interface. Users view pages full of textual and graphical objects, click on objects that they want to learn more about, and a corresponding new page appears. Most people are also aware that just under the covers, each selectable object on a page is bound to an identifier for the next page to be viewed. This identifier, called a Uniform Resource Locator (URL), is used to provide a way of identifying all the possible pages that can be viewed from your web browser. For example, http://www.cs.princeton.edu/~llp/index.html is the URL for a page providing information about one of this books authors: the string http indicates that the HyperText Transfer Protocol (HTTP) should be used to download the page, www.cs.princeton.edu is the name of the machine that serves the page, and /~llp/index.html uniquely identifies Larrys home page at this site. What most Web users are not aware of, however, is that by clicking on just one such URL, as many as 17 messages may be exchanged over the Internet, and this assumes the page itself is small enough to fit in a single message. This number includes up to six messages to translate the server name (www.cs.princeton.edu) into its Internet address (128.112.136.35), three messages to set up a Transmission Control Protocol (TCP) connection between your browser and this server, four messages for your browser to send the HTTP get request and the server to respond with the requested page (and for each side to acknowledge receipt of that message), and four messages to tear down the TCP connection. Of course, this does not include the millions of messages exchanged by Internet nodes throughout the day, just to let each other know that they exist and are ready to serve web pages, translate names to addresses, and forward messages toward their ultim ate destination. Another widespread application of the Internet is the delivery of streaming audio and video. While an entire video file could first be fetched from a remote machine and then played on the local machine, similar to the process of downloading and displaying a web page, this would entail waiting for the last second of the video file to be delivered before starting to look at it. Streaming video implies that the sender and the receiver are, respectively, the source and the sink for the video stream. That is, the source generates a video stream (perhaps using a video capture card), sends it across the Internet in messages, and the sink displays the stream as it arrives. There are a variety of different classes of video applications. One class of video application is video-on-demand, which reads a pre-existing movie from disk and transmits it over the network. Another kind of application is videoconferencing, which is in some ways the more challenging (and, for networking people, interesting) case because it has very tight timing constraints. Just as when using the telephone, the interactions among the participants must be timely. When a person at one end gestures, then that action must be displayed at the other end as quickly as possible. Too much delay makes the system unusable. Contrast this with video-on-demand where, if it takes several seconds from the time the user starts the video until the first image is displayed, the service is still deemed satisfactory. Also, interactive video usually implies that video is flowing in both directions, while a video-on-demand application is most likely sending video in only one direction. One pioneering example of a videoconferencing tool, developed in the early and mid-1990s, is vic. shows the control panel for a vic session. vic is actually one of a suite of conferencing tools designed at Lawrence Berkeley Laboratory and UC Berkeley. The others include a whiteboard application (wb) that allows users to send sketches and slides to each other, a visual audio tool called vat, and a session directory (sdr) that is used to create and advertise videoconferences. All these tools run on Unix—hence their lowercase names—and are freely available on the Internet. Many similar tools are available for other operating systems. It is interesting to note that while video over the Internet is still considered to be in its relative infancy at the time of this writing (2006), that the tools to support video over IP have existed for well over a decade. Although they are just two examples, downloading pages from the Web and participating in a videoconference demonstrate the diversity of applications that can be built on top of the Internet, and hint at the complexity of the Internets design. Starting from the beginning, and addressing one problem at time, the rest of this book explains how to build a network that supports such a wide range of applications. Chapter 9 concludes the book by revisiting these two specific applications, as well as several others that have become popular on todays Internet. 1.2 REQUIREMENTS We have just established an ambitious goal for ourselves: to understand how to build a computer network from the ground up. Our approach to accomplishing this goal will be to start from first principles, and then ask the kinds of questions we would naturally ask if building an actual network. At each step, we will use todays protocols to illustrate various design choices available to us, but we will not accept these existing artifacts as gospel. Instead, we will be asking (and answering) the question of why networks are designed the way they are. While it is tempting to settle for just understanding the way its done today, it is important to recognize the underlying concepts because networks are constantly changing as the technology evolves and new applications are invented. It is our experience that once you understand the fundamental ideas, any new protocol that you are confronted with will be relatively easy to digest. The first step is to identify the set of constraints and requirements that influence network design. Before getting started, however, it is important to understand that the expectations you have of a network depend on your perspective: An application programmer would list the services that his application needs, for example, a guarantee that each message the application sends will be delivered without error within a certain amount of time. A network designer would list the properties of a cost-effective design, for example, that network resources are efficiently utilized and fairly allocated to different users. A network provider would list the characteristics of a system that is easy to administer and manage, for example, in which faults can be easily isolated and whereitiseasytoaccountfor usage. This section attempts to distill these different perspectives into a high-level introduction to the major considerations that drive network design, and in doing so, identifies the challenges addressed throughout the rest of this book. 1.2.1 Connectivity Starting with the obvious, a network must provide connectivity among a set of computers. Sometimes it is enough to build a limited network that connects only a few select machines. In fact, for reasons of privacy and security, many private (corporate) networks have the explicit goal of limiting the set of machines that are connected. In contrast, other networks (of which the Internet is the prime example) are designed to grow in a way that allows them the potential to connect all the computers in the world. A system that is designed to support growth to an arbitrarily large size is said to scale. Using the Internet as a model, this book addresses the challenge of scalability. Links, Nodes, and Clouds Network connectivity occurs at many different levels. At the lowest level, a network can consist of two or more computers directly connected by some physical medium, such as a coaxial cable or an optical fiber. We call such a physical medium a link,and we often refer to the computers it connects as nodes. (Sometimes a node is a more specialized piece of hardware rather than a computer, but we overlook that distinction for the purposes of this discussion.) As illustrated in, physical links are sometimes limited to a pair of nodes (such a link is said to be point-to-point), while in other cases, more than two nodes may share a single physical link (such a link is said to be multiple-access). Whether a given link supports point-to-point or multiple-access connectivity depends on how the node is attached to the link. It is also the case that multiple-access links are often limited in size, in terms of both the geographical distance they can cover and the number of nodes they can connect. If computer networks were limited to situations in which all nodes are directly connected to each other over a common physical medium, then networks would either be very limited in the number of computers they could connect, or the number of wires coming out of the back of each node would quickly become both unmanageable and very expensive. Fortunately, connectivity between two nodes does not necessarily imply a direct physical connection between them—indirect connectivity may be achieved among a set of cooperating nodes. Consider the following two examples of how a collection of computers can be indirectly connected. shows a set of nodes, each of which is attached to one or more point- to-point links. Those nodes that are attached to at least two links run software that forwards data received on one link out on another. If organized in a systematic way, these forwarding nodes form a switched network. There are numerous types of switched networks, of which the two most common are circuit-switched and packet-switched. The former is most notably employed by the telephone system, while the latter is used for the overwhelming majority of computer networks and will be the focus of this book. The important feature of packet-switched networks is that the nodes in such a network send discrete blocks of data to each other. Think of these blocks of data as corresponding to some piece of application data such as a file, a piece of email, or an image. We call each block of data either a packet or a message, and for now we use these terms interchangeably; we discuss the reason they are not always the same in Section 1.2.2. Packet-switched networks typically use a strategy called store-and-forward. As the name suggests, each node in a store-and-forward network first receives a complete packet over some link, stores the packet in its internal memory, and then forwards the complete packet to the next node. In contrast, a circuit-switched network first establishes a dedicated circuit across a sequence of links and then allows the source node to send a stream of bits across this circuit to a destination node. The major reason for using packet switching rather than circuit switching in a computer network is efficiency, discussed in the next subsection. The cloud in distinguishes between the nodes on the inside that implement the network (they are commonly called switches, and their primary function is to store and forward packets) and the nodes on the outside of the cloud that use the network (they are commonly called hosts, and they support users and run application programs). Also note that the cloud in is one of the most important icons of computer networking. In general, we use a cloud to denote any type of network, whether it is a single point-to-point link, a multiple-access link, or a switched network. Thus, whenever you see a cloud used in a figure, you can think of it as a placeholder for any of the networking technologies covered in this book. A second way in which a set of computers can be indirectly connected is shown in . In this situation, a set of independent networks (clouds) are interconnected to form an internetwork, or internet for short. We adopt the Internets convention of referring to a generic internetwork of networks as a lowercase i internet, and the currently operational TCP/IP Internet as the capital I Internet. A node that is connected to two or more networks is commonly called a router or gateway, and it plays much the same role as a switch—it forwards messages from one network to another. Note that an internet can itself be viewed as another kind of network, which means that an internet can be built from an interconnection of internets. Thus, we can recursively build arbitrarily large networks by interconnecting clouds to form larger clouds. Just because a set of hosts are directly or indirectly connected to each other does not mean that we have succeeded in providing host-to-host connectivity. The final requirement is that each node must be able to state which of the other nodes on the network it wants to communicate with. This is done by assigning an address to each node. An address is a byte string that identifies a node; that is, the network can use a nodes address to distinguish it from the other nodes connected to the network. When a source node wants the network to deliver a message to a certain destination node, it specifies the address of the destination node. If the sending and receiving nodes are not directly connected, then the switches and routers of the network use this address to decide how to forward the message toward the destination. The process of determining systematically how to forward messages toward the destination node based on its address is called routing. This brief introduction to addressing and routing has presumed that the source node wants to send a message to a single destination node (unicast). While this is the most common scenario, it is also possible that the source node might want to broadcast a message to all the nodes on the network. Or a source node might want to send a message to some subset of the other nodes, but not all of them, a situation called multicast. Thus, in addition to node-specific addresses, another requirement of a network is that it supports multicast and broadcast addresses. The main idea to take away from this discussion is that we can define a network recursively as consisting of two or more nodes connected by a physical link, or as two or more networks connected by a node. In other words, a network can be constructed from a nesting of networks, where at the bottom level, the network is implemented by some physical medium. One of the key challenges in providing network connectivity is to define an address for each node that is reachable on the network (including support for broadcast and multicast connectivity), and to be able to use this address to route messages toward the appropriate destination node(s). 1.2.2 Cost-Effective Resource Sharing As stated above, this book focuses on packet-switched networks. This section explains the key requirement of computer networks—efficiency—that leads us to packet switching as the strategy of choice. Given a collection of nodes indirectly connected by a nesting of networks, it is possible for any pair of hosts to send messages to each other across a sequence of links and nodes. Of course, we want to do more than support just one pair of communicating hosts—we want to provide all pairs of hosts with the ability to exchange messages. The question, then, is how do all the hosts that want to communicate share the network, especially if they want to use it at the same time? And, as if that problem isnt hard enough, how do several hosts share the same link when they all want to use it at the same time? To understand how hosts share a network, we need to introduce a fundamental concept, multiplexing, which means that a system resource is shared among multiple users. At an intuitive level, multiplexing can be explained by analogy to a timesharing computer system, where a single physical CPU is shared (multiplexed) among multiple jobs, each of which believes it has its own private processor. Similarly, data being sent by multiple users can be multiplexed over the physical links that make up a network. To see how this might work, consider the simple network illustrated in , where the three hosts on the left side of the network (senders S1S3) are sending data to the three hosts on the right (receivers R1R3) by sharing a switched network that contains only one physical link. (For simplicity, assume that host S1 is sending data to host R1, and so on.) In this situation, three flows of data—corresponding to the three pairs of hosts—are multiplexed onto a single physical link by switch 1 and then demultiplexed back into separate flows by switch 2. Note that we are being intentionally vague about exactly what a flow of data corresponds to. For the purposes of this discussion, assume that each host on the left has a large supply of data that it wants to send to its counterpart on the right. There are several different methods for multiplexing multiple flows onto one physical link. One common method is synchronous time-division multiplexing (STDM). The idea of STDM is to divide time into equal-sized quanta and, in a round-robin fashion, give each flow a chance to send its data over the physical link. In other words, during time quantum 1, data from S1 to R1 is transmitted; during time quantum 2, data from S2 to R2 is transmitted; in quantum 3, S3 sends data to R3. At this point, the first flow (S1 to R1) gets to go again, and the process repeats. Another method is frequency-division multiplexing (FDM). The idea of FDM is to transmit each flow over the physical link at a different frequency, much the same way that the signals for different TV stations are transmitted at a different frequency on a physical cable TV link. Although simple to understand, both STDM and FDM are limited in two ways. First, if one of the flows (host pairs) does not have any data to send, its share of the physical link—that is, its time quantum or its frequency—remains idle, even if one of the other flows has data to transmit. For example, S3 had to wait its turn behind S1 and S2 in the previous paragraph, even if S1 and S2 had nothing to send. For computer communication, the amount of time that a link is idle can be very large—for example, consider the amount of time you spend reading a web page (leaving the link idle) compared to the time you spend fetching the page. Second, both STDM and FDM are limited to situations in which the maximum number of flows is fixed and known ahead of time. It is not practical to resize the quantum or to add additional quanta in the case of STDM or to add new frequencies in the case of FDM. The form of multiplexing that we make most use of in this book is called statistical multiplexing. Although the name is not all that helpful for understanding the concept, statistical multiplexing is really quite simple, with two key ideas. First, it is like STDM in that the physical link is shared over time—first data from one flow is transmitted over the physical link, then data from another flow is transmitted, and so on. Unlike STDM, however, data is transmitted from each flow on demand rather than during a predetermined time slot. Thus, if only one flow has data to send, it gets to transmit that data without waiting for its quantum to come around and thus without having to watch the quanta assigned to the other flows go by unused. It is this avoidance of idle time that gives packet switching its efficiency. As defined so far, however, statistical multiplexing has no mechanism to ensure that all the flows eventually get their turn to transmit over the physical link. That is, once a flow begins sending data, we need some way to limit the transmission, so that the other flows can have a turn. To account for this need, statistical multiplexing defines an upper bound on the size of the block of data that each flow is permitted to transmit at a given time. This limited-size block of data is typically referred to as a packet, to distinguish it from the arbitrarily large message that an application program might want to transmit. Because a packet-switched network limits the maximum size of packets, a host may not be able to send a complete message in one packet. The source may need to fragment the message into several packets, with the receiver reassembling the packets back into the original message. In other words, each flow sends a sequence of packets over the physical link, with a decision made on a packet-by-packet basis as to which flows packet to send next. Notice that if only one flow has data to send, then it can send a sequence of packets back-to-back. However, should more than one of the flows have data to send, then their packets are interleaved on the link. depicts a switch multiplexing packets from multiple sources onto a single shared link. The decision as to which packet to send next on a shared link can be made in a number of different ways. For example, in a network consisting of switches interconnected by links such as the one in the decision would be made by the switch that transmits packets onto the shared link. (As we will see later, not all packet-switched networks actually involve switches, and they may use other mechanisms to determine whose packet goes onto the link next.) Each switch in a packet-switched network makes this decision independently, on a packet-by-packet basis. One of the issues that faces a network designer is how to make this decision in a fair manner. For example, a switch could be designed to service packets on a first-in-first-out (FIFO) basis. Another approach would be to transmit the packets from each of the different flows that are currently sending data through the switch in a round-robin manner. This might be done to ensure that certain flows receive a particular share of the links b andwidth, or that they never have their packets delayed in the switch for more than a certain length of time. A network that attempts to allocate bandwidth to particular flows is sometimes said to support quality of service (QoS), a topic that we return to in Chapter 6. Also, notice in that since the switch has to multiplex three incoming packet streams onto one outgoing link, it is possible that the switch will receive packets faster than the shared link can accommodate. In this case, the switch is forced to buffer these packets in its memory. Should a switch receive packets faster than it can send them for an extended period of time, then the switch will eventually run out of buffer space, and some packets will have to be dropped. When a switch is operating in this state, it is said to be congested. The bottom line is that statistical multiplexing defines a cost-effective way for multiple users (e.g., host-to-host flows of data) to share network resources (links and nodes) in a fine-grained manner. It defines the packet as the granularity with which the links of the network are allocated to different flows, with each switch able to schedule the use of the physical links it is connected to on a per-packet basis. Fairly allocating link capacity to different flows and dealing with congestion when it occurs are the key challenges of statistical multiplexing. 1.2.3 Support for Common Services While the previous section outlined the challenges involved in providing costeffective connectivity among a group of hosts, it is overly simplistic to view a computer network as simply delivering packets among a collection of computers. It is more accurate to think of a network as providing the means for a set of application processes that are distributed over those computers to communicate. In other words, the next requirement of a computer network is that the application programs running on the hosts connected to the network must be able to communicate in a meaningful way. When two application programs need to communicate with each other, there are a lot of complicated things that need to happen beyond simply sending a message from one host to another. One option would be for application designers to build all that complicated functionality into each application program. However, since many applications need common services, it is much more logical to implement those common services once and then to let the application designer build the application using those services. The challenge for a network designer is to identify the right set of common services. The goal is to hide the complexity of the network from the application without overly constraining the application designer. Intuitively, we view the network as providing logical channels over which application-level processes can communicate with each other; each channel provides the set of services required by that application. In other words, just as we use a cloud to abstractly represent connectivity among a set of computers, we now think of a channel as connecting one process to another. shows a pair of application-level processes communicating over a logical channel that is, in turn, implemented on top of a cloud that connects a set of hosts. We can think of the channel as being like a pipe connecting two applications, so that a sending application can put data in one end and expect that data to be delivered by the network to the application at the other end of the pipe. Thechallengeistorecognize what functionality the channels should provide to application programs. For example, does the application require a guarantee that messages sent over the channel are delivered, or is it acceptable if some messages fail to arrive? Is it necessary that messages arrive at the recipient process in the same order in which they are sent, or does the recipient not care about the order in which messages arrive? Does the network need to ensure that no third parties are able to eavesdrop on the channel, or is privacy not a concern? In general, a network provides a variety of different types of channels, with each application selecting the type that best meets its needs. The rest of this section illustrates the thinking involved in defining useful channels. Identifying Common Communication Patterns Designing abstract channels involves first understanding the communication needs of a representative collection of applications, then extracting their common communication requirements, and finally incorporating the functionality that meets these requirements in the network. One of the earliest applications supported on any networ Implementation of New Computer Network Implementation of New Computer Network Here we are going to implement an new computer network for this company that 25 employees have been working in. Suppose you want to build a computer network, one that has potential to grow to global proportions to support applications as diverse as teleconferencing, video-on-demand, electronic commerce, distributed computing, and digital libraries. What available technologies would serve as the underlying building blocks, and what kind of software architecture would you design t integrate these building blocks into an effective communication service? Suppose you want to build a computer network, one that has the potential togrow to global proportions and to support applications as diverse as teleconferencing, video-on-demand, electronic commerce, distributed computing, and digital libraries. What available technologies would serve as the underlying building blocks, and what kind of software architecture would you design to integrate these building blocks into an effective communication service? Answering this question is the overriding goal of — to describe the available building materials and then to show how they can be used to construct a network from the ground up. Before we can understand how to design a computer network, we should first agree on exactly what a computer network is. At one time, the term network meant the set of serial lines used to attach dumb terminals to mainframe computers. To some, the term implies the voice telephone network. To others, the only interesting network is the cable network used to disseminate video signals. The main thing these networks have in common is that they are specialized to handle one particular kind of data (keystrokes, voice, or video) and they typically connect to special-purpose devices (terminals, hand receivers, and television sets). What distinguishes a computer network from these other types of networks? Probably the most important characteristic of a computer network is its generality. Computer networks are built primarily from general-purpose programmable hardware, and they are not optimized for a particular application like making phone calls or delivering television signals. Instead, they are able to carry many different types of data, and they support a wide, and ever-growing, range of applications. This chapter looks at some typical applications of computer networks and discusses the requirements that a network designer who wishes to support such applications must be aware of. Once we understand the requirements, how do we proceed? Fortunately, we will not be building the first network. Others, most notably the community of researchers responsible for the Internet, have gone before us. We will use the wealth of experience generated from the Internet to guide our design. This experience is embodied in a network architecture that identifies the available hardware and software components and shows how they can be arranged to form a complete network system. To start us on the road toward understanding how to build a network, this chapter does four things. First, it explores the requirements that different applications and different communities of people (such as network users and network operators) place on the network. Second, it introduces the idea of a network architecture, which lays the foundation for the rest of the book. Third, it introduces some of the key elements in the implementation of computer networks. Finally, it identifies the key metrics that are used to evaluate the performance of computer networks. 1.1 APPLICATIONS Most people know the Internet through its applications: the World Wide Web, email, streaming audio and video, chat rooms, and music (file) sharing. The Web, for example, presents an intuitively simple interface. Users view pages full of textual and graphical objects, click on objects that they want to learn more about, and a corresponding new page appears. Most people are also aware that just under the covers, each selectable object on a page is bound to an identifier for the next page to be viewed. This identifier, called a Uniform Resource Locator (URL), is used to provide a way of identifying all the possible pages that can be viewed from your web browser. For example, http://www.cs.princeton.edu/~llp/index.html is the URL for a page providing information about one of this books authors: the string http indicates that the HyperText Transfer Protocol (HTTP) should be used to download the page, www.cs.princeton.edu is the name of the machine that serves the page, and /~llp/index.html uniquely identifies Larrys home page at this site. What most Web users are not aware of, however, is that by clicking on just one such URL, as many as 17 messages may be exchanged over the Internet, and this assumes the page itself is small enough to fit in a single message. This number includes up to six messages to translate the server name (www.cs.princeton.edu) into its Internet address (128.112.136.35), three messages to set up a Transmission Control Protocol (TCP) connection between your browser and this server, four messages for your browser to send the HTTP get request and the server to respond with the requested page (and for each side to acknowledge receipt of that message), and four messages to tear down the TCP connection. Of course, this does not include the millions of messages exchanged by Internet nodes throughout the day, just to let each other know that they exist and are ready to serve web pages, translate names to addresses, and forward messages toward their ultim ate destination. Another widespread application of the Internet is the delivery of streaming audio and video. While an entire video file could first be fetched from a remote machine and then played on the local machine, similar to the process of downloading and displaying a web page, this would entail waiting for the last second of the video file to be delivered before starting to look at it. Streaming video implies that the sender and the receiver are, respectively, the source and the sink for the video stream. That is, the source generates a video stream (perhaps using a video capture card), sends it across the Internet in messages, and the sink displays the stream as it arrives. There are a variety of different classes of video applications. One class of video application is video-on-demand, which reads a pre-existing movie from disk and transmits it over the network. Another kind of application is videoconferencing, which is in some ways the more challenging (and, for networking people, interesting) case because it has very tight timing constraints. Just as when using the telephone, the interactions among the participants must be timely. When a person at one end gestures, then that action must be displayed at the other end as quickly as possible. Too much delay makes the system unusable. Contrast this with video-on-demand where, if it takes several seconds from the time the user starts the video until the first image is displayed, the service is still deemed satisfactory. Also, interactive video usually implies that video is flowing in both directions, while a video-on-demand application is most likely sending video in only one direction. One pioneering example of a videoconferencing tool, developed in the early and mid-1990s, is vic. shows the control panel for a vic session. vic is actually one of a suite of conferencing tools designed at Lawrence Berkeley Laboratory and UC Berkeley. The others include a whiteboard application (wb) that allows users to send sketches and slides to each other, a visual audio tool called vat, and a session directory (sdr) that is used to create and advertise videoconferences. All these tools run on Unix—hence their lowercase names—and are freely available on the Internet. Many similar tools are available for other operating systems. It is interesting to note that while video over the Internet is still considered to be in its relative infancy at the time of this writing (2006), that the tools to support video over IP have existed for well over a decade. Although they are just two examples, downloading pages from the Web and participating in a videoconference demonstrate the diversity of applications that can be built on top of the Internet, and hint at the complexity of the Internets design. Starting from the beginning, and addressing one problem at time, the rest of this book explains how to build a network that supports such a wide range of applications. Chapter 9 concludes the book by revisiting these two specific applications, as well as several others that have become popular on todays Internet. 1.2 REQUIREMENTS We have just established an ambitious goal for ourselves: to understand how to build a computer network from the ground up. Our approach to accomplishing this goal will be to start from first principles, and then ask the kinds of questions we would naturally ask if building an actual network. At each step, we will use todays protocols to illustrate various design choices available to us, but we will not accept these existing artifacts as gospel. Instead, we will be asking (and answering) the question of why networks are designed the way they are. While it is tempting to settle for just understanding the way its done today, it is important to recognize the underlying concepts because networks are constantly changing as the technology evolves and new applications are invented. It is our experience that once you understand the fundamental ideas, any new protocol that you are confronted with will be relatively easy to digest. The first step is to identify the set of constraints and requirements that influence network design. Before getting started, however, it is important to understand that the expectations you have of a network depend on your perspective: An application programmer would list the services that his application needs, for example, a guarantee that each message the application sends will be delivered without error within a certain amount of time. A network designer would list the properties of a cost-effective design, for example, that network resources are efficiently utilized and fairly allocated to different users. A network provider would list the characteristics of a system that is easy to administer and manage, for example, in which faults can be easily isolated and whereitiseasytoaccountfor usage. This section attempts to distill these different perspectives into a high-level introduction to the major considerations that drive network design, and in doing so, identifies the challenges addressed throughout the rest of this book. 1.2.1 Connectivity Starting with the obvious, a network must provide connectivity among a set of computers. Sometimes it is enough to build a limited network that connects only a few select machines. In fact, for reasons of privacy and security, many private (corporate) networks have the explicit goal of limiting the set of machines that are connected. In contrast, other networks (of which the Internet is the prime example) are designed to grow in a way that allows them the potential to connect all the computers in the world. A system that is designed to support growth to an arbitrarily large size is said to scale. Using the Internet as a model, this book addresses the challenge of scalability. Links, Nodes, and Clouds Network connectivity occurs at many different levels. At the lowest level, a network can consist of two or more computers directly connected by some physical medium, such as a coaxial cable or an optical fiber. We call such a physical medium a link,and we often refer to the computers it connects as nodes. (Sometimes a node is a more specialized piece of hardware rather than a computer, but we overlook that distinction for the purposes of this discussion.) As illustrated in, physical links are sometimes limited to a pair of nodes (such a link is said to be point-to-point), while in other cases, more than two nodes may share a single physical link (such a link is said to be multiple-access). Whether a given link supports point-to-point or multiple-access connectivity depends on how the node is attached to the link. It is also the case that multiple-access links are often limited in size, in terms of both the geographical distance they can cover and the number of nodes they can connect. If computer networks were limited to situations in which all nodes are directly connected to each other over a common physical medium, then networks would either be very limited in the number of computers they could connect, or the number of wires coming out of the back of each node would quickly become both unmanageable and very expensive. Fortunately, connectivity between two nodes does not necessarily imply a direct physical connection between them—indirect connectivity may be achieved among a set of cooperating nodes. Consider the following two examples of how a collection of computers can be indirectly connected. shows a set of nodes, each of which is attached to one or more point- to-point links. Those nodes that are attached to at least two links run software that forwards data received on one link out on another. If organized in a systematic way, these forwarding nodes form a switched network. There are numerous types of switched networks, of which the two most common are circuit-switched and packet-switched. The former is most notably employed by the telephone system, while the latter is used for the overwhelming majority of computer networks and will be the focus of this book. The important feature of packet-switched networks is that the nodes in such a network send discrete blocks of data to each other. Think of these blocks of data as corresponding to some piece of application data such as a file, a piece of email, or an image. We call each block of data either a packet or a message, and for now we use these terms interchangeably; we discuss the reason they are not always the same in Section 1.2.2. Packet-switched networks typically use a strategy called store-and-forward. As the name suggests, each node in a store-and-forward network first receives a complete packet over some link, stores the packet in its internal memory, and then forwards the complete packet to the next node. In contrast, a circuit-switched network first establishes a dedicated circuit across a sequence of links and then allows the source node to send a stream of bits across this circuit to a destination node. The major reason for using packet switching rather than circuit switching in a computer network is efficiency, discussed in the next subsection. The cloud in distinguishes between the nodes on the inside that implement the network (they are commonly called switches, and their primary function is to store and forward packets) and the nodes on the outside of the cloud that use the network (they are commonly called hosts, and they support users and run application programs). Also note that the cloud in is one of the most important icons of computer networking. In general, we use a cloud to denote any type of network, whether it is a single point-to-point link, a multiple-access link, or a switched network. Thus, whenever you see a cloud used in a figure, you can think of it as a placeholder for any of the networking technologies covered in this book. A second way in which a set of computers can be indirectly connected is shown in . In this situation, a set of independent networks (clouds) are interconnected to form an internetwork, or internet for short. We adopt the Internets convention of referring to a generic internetwork of networks as a lowercase i internet, and the currently operational TCP/IP Internet as the capital I Internet. A node that is connected to two or more networks is commonly called a router or gateway, and it plays much the same role as a switch—it forwards messages from one network to another. Note that an internet can itself be viewed as another kind of network, which means that an internet can be built from an interconnection of internets. Thus, we can recursively build arbitrarily large networks by interconnecting clouds to form larger clouds. Just because a set of hosts are directly or indirectly connected to each other does not mean that we have succeeded in providing host-to-host connectivity. The final requirement is that each node must be able to state which of the other nodes on the network it wants to communicate with. This is done by assigning an address to each node. An address is a byte string that identifies a node; that is, the network can use a nodes address to distinguish it from the other nodes connected to the network. When a source node wants the network to deliver a message to a certain destination node, it specifies the address of the destination node. If the sending and receiving nodes are not directly connected, then the switches and routers of the network use this address to decide how to forward the message toward the destination. The process of determining systematically how to forward messages toward the destination node based on its address is called routing. This brief introduction to addressing and routing has presumed that the source node wants to send a message to a single destination node (unicast). While this is the most common scenario, it is also possible that the source node might want to broadcast a message to all the nodes on the network. Or a source node might want to send a message to some subset of the other nodes, but not all of them, a situation called multicast. Thus, in addition to node-specific addresses, another requirement of a network is that it supports multicast and broadcast addresses. The main idea to take away from this discussion is that we can define a network recursively as consisting of two or more nodes connected by a physical link, or as two or more networks connected by a node. In other words, a network can be constructed from a nesting of networks, where at the bottom level, the network is implemented by some physical medium. One of the key challenges in providing network connectivity is to define an address for each node that is reachable on the network (including support for broadcast and multicast connectivity), and to be able to use this address to route messages toward the appropriate destination node(s). 1.2.2 Cost-Effective Resource Sharing As stated above, this book focuses on packet-switched networks. This section explains the key requirement of computer networks—efficiency—that leads us to packet switching as the strategy of choice. Given a collection of nodes indirectly connected by a nesting of networks, it is possible for any pair of hosts to send messages to each other across a sequence of links and nodes. Of course, we want to do more than support just one pair of communicating hosts—we want to provide all pairs of hosts with the ability to exchange messages. The question, then, is how do all the hosts that want to communicate share the network, especially if they want to use it at the same time? And, as if that problem isnt hard enough, how do several hosts share the same link when they all want to use it at the same time? To understand how hosts share a network, we need to introduce a fundamental concept, multiplexing, which means that a system resource is shared among multiple users. At an intuitive level, multiplexing can be explained by analogy to a timesharing computer system, where a single physical CPU is shared (multiplexed) among multiple jobs, each of which believes it has its own private processor. Similarly, data being sent by multiple users can be multiplexed over the physical links that make up a network. To see how this might work, consider the simple network illustrated in , where the three hosts on the left side of the network (senders S1S3) are sending data to the three hosts on the right (receivers R1R3) by sharing a switched network that contains only one physical link. (For simplicity, assume that host S1 is sending data to host R1, and so on.) In this situation, three flows of data—corresponding to the three pairs of hosts—are multiplexed onto a single physical link by switch 1 and then demultiplexed back into separate flows by switch 2. Note that we are being intentionally vague about exactly what a flow of data corresponds to. For the purposes of this discussion, assume that each host on the left has a large supply of data that it wants to send to its counterpart on the right. There are several different methods for multiplexing multiple flows onto one physical link. One common method is synchronous time-division multiplexing (STDM). The idea of STDM is to divide time into equal-sized quanta and, in a round-robin fashion, give each flow a chance to send its data over the physical link. In other words, during time quantum 1, data from S1 to R1 is transmitted; during time quantum 2, data from S2 to R2 is transmitted; in quantum 3, S3 sends data to R3. At this point, the first flow (S1 to R1) gets to go again, and the process repeats. Another method is frequency-division multiplexing (FDM). The idea of FDM is to transmit each flow over the physical link at a different frequency, much the same way that the signals for different TV stations are transmitted at a different frequency on a physical cable TV link. Although simple to understand, both STDM and FDM are limited in two ways. First, if one of the flows (host pairs) does not have any data to send, its share of the physical link—that is, its time quantum or its frequency—remains idle, even if one of the other flows has data to transmit. For example, S3 had to wait its turn behind S1 and S2 in the previous paragraph, even if S1 and S2 had nothing to send. For computer communication, the amount of time that a link is idle can be very large—for example, consider the amount of time you spend reading a web page (leaving the link idle) compared to the time you spend fetching the page. Second, both STDM and FDM are limited to situations in which the maximum number of flows is fixed and known ahead of time. It is not practical to resize the quantum or to add additional quanta in the case of STDM or to add new frequencies in the case of FDM. The form of multiplexing that we make most use of in this book is called statistical multiplexing. Although the name is not all that helpful for understanding the concept, statistical multiplexing is really quite simple, with two key ideas. First, it is like STDM in that the physical link is shared over time—first data from one flow is transmitted over the physical link, then data from another flow is transmitted, and so on. Unlike STDM, however, data is transmitted from each flow on demand rather than during a predetermined time slot. Thus, if only one flow has data to send, it gets to transmit that data without waiting for its quantum to come around and thus without having to watch the quanta assigned to the other flows go by unused. It is this avoidance of idle time that gives packet switching its efficiency. As defined so far, however, statistical multiplexing has no mechanism to ensure that all the flows eventually get their turn to transmit over the physical link. That is, once a flow begins sending data, we need some way to limit the transmission, so that the other flows can have a turn. To account for this need, statistical multiplexing defines an upper bound on the size of the block of data that each flow is permitted to transmit at a given time. This limited-size block of data is typically referred to as a packet, to distinguish it from the arbitrarily large message that an application program might want to transmit. Because a packet-switched network limits the maximum size of packets, a host may not be able to send a complete message in one packet. The source may need to fragment the message into several packets, with the receiver reassembling the packets back into the original message. In other words, each flow sends a sequence of packets over the physical link, with a decision made on a packet-by-packet basis as to which flows packet to send next. Notice that if only one flow has data to send, then it can send a sequence of packets back-to-back. However, should more than one of the flows have data to send, then their packets are interleaved on the link. depicts a switch multiplexing packets from multiple sources onto a single shared link. The decision as to which packet to send next on a shared link can be made in a number of different ways. For example, in a network consisting of switches interconnected by links such as the one in the decision would be made by the switch that transmits packets onto the shared link. (As we will see later, not all packet-switched networks actually involve switches, and they may use other mechanisms to determine whose packet goes onto the link next.) Each switch in a packet-switched network makes this decision independently, on a packet-by-packet basis. One of the issues that faces a network designer is how to make this decision in a fair manner. For example, a switch could be designed to service packets on a first-in-first-out (FIFO) basis. Another approach would be to transmit the packets from each of the different flows that are currently sending data through the switch in a round-robin manner. This might be done to ensure that certain flows receive a particular share of the links b andwidth, or that they never have their packets delayed in the switch for more than a certain length of time. A network that attempts to allocate bandwidth to particular flows is sometimes said to support quality of service (QoS), a topic that we return to in Chapter 6. Also, notice in that since the switch has to multiplex three incoming packet streams onto one outgoing link, it is possible that the switch will receive packets faster than the shared link can accommodate. In this case, the switch is forced to buffer these packets in its memory. Should a switch receive packets faster than it can send them for an extended period of time, then the switch will eventually run out of buffer space, and some packets will have to be dropped. When a switch is operating in this state, it is said to be congested. The bottom line is that statistical multiplexing defines a cost-effective way for multiple users (e.g., host-to-host flows of data) to share network resources (links and nodes) in a fine-grained manner. It defines the packet as the granularity with which the links of the network are allocated to different flows, with each switch able to schedule the use of the physical links it is connected to on a per-packet basis. Fairly allocating link capacity to different flows and dealing with congestion when it occurs are the key challenges of statistical multiplexing. 1.2.3 Support for Common Services While the previous section outlined the challenges involved in providing costeffective connectivity among a group of hosts, it is overly simplistic to view a computer network as simply delivering packets among a collection of computers. It is more accurate to think of a network as providing the means for a set of application processes that are distributed over those computers to communicate. In other words, the next requirement of a computer network is that the application programs running on the hosts connected to the network must be able to communicate in a meaningful way. When two application programs need to communicate with each other, there are a lot of complicated things that need to happen beyond simply sending a message from one host to another. One option would be for application designers to build all that complicated functionality into each application program. However, since many applications need common services, it is much more logical to implement those common services once and then to let the application designer build the application using those services. The challenge for a network designer is to identify the right set of common services. The goal is to hide the complexity of the network from the application without overly constraining the application designer. Intuitively, we view the network as providing logical channels over which application-level processes can communicate with each other; each channel provides the set of services required by that application. In other words, just as we use a cloud to abstractly represent connectivity among a set of computers, we now think of a channel as connecting one process to another. shows a pair of application-level processes communicating over a logical channel that is, in turn, implemented on top of a cloud that connects a set of hosts. We can think of the channel as being like a pipe connecting two applications, so that a sending application can put data in one end and expect that data to be delivered by the network to the application at the other end of the pipe. Thechallengeistorecognize what functionality the channels should provide to application programs. For example, does the application require a guarantee that messages sent over the channel are delivered, or is it acceptable if some messages fail to arrive? Is it necessary that messages arrive at the recipient process in the same order in which they are sent, or does the recipient not care about the order in which messages arrive? Does the network need to ensure that no third parties are able to eavesdrop on the channel, or is privacy not a concern? In general, a network provides a variety of different types of channels, with each application selecting the type that best meets its needs. The rest of this section illustrates the thinking involved in defining useful channels. Identifying Common Communication Patterns Designing abstract channels involves first understanding the communication needs of a representative collection of applications, then extracting their common communication requirements, and finally incorporating the functionality that meets these requirements in the network. One of the earliest applications supported on any networ