* Specific real space associated with historical events
In the memoir"The Dust of Someone's Feet", the space that the author mentioned a lot was the space of " the slope of Hang Ken six-way intersection " [25. 385]. That was the space of historical events and at the same time the space for the author's reminiscences: " In the years before 60, the slope of Hang Ken six-way intersection was very quiet at night. The dracontomelon and milk trees were not like a few years later when an American bomb destroyed two houses and several ancient trees " [25. 385, 386]. A brief reminiscence but enough to see the contrast in the same space between two different periods of time. After the Dien Bien Phu victory in 1954, our country again confronted the American empire. " The slope of Hang Ken six-way intersection " - a place that was no longer peaceful and " deserted " as before. An American bomb " destroyed two houses and several ancient trees ". The American empire destroyed many houses and many villages like that. To Hoai did not include statistics, but through his memories, he brought readers back to the space of history, the space of the war against American imperialism.
Going back in time, following the author's memories, readers are brought back to the space of the years after the August Revolution. During the years of fighting against French colonialism, the writers also participated in that historical event: " In the summer of 1949, Nguyen Tuan and I followed the 54th Battalion of the Capital Regiment to launch a campaign to destroy a series of strongholds along the Thao River bank, blocking the Northwest, the Dai Buc, Dai Phac, Khe Pia, Ngoi Mac, Ma Yen Son posts up to Pho Rang sub-region post " [25. 394]. The space was spread out very wide, To Hoai called it " the Thao River bank corridor stronghold ". Going to the field with the soldiers, witnessing the tactics in each campaign, the writers could fully see the difficulties and hardships of the war.
Referring to the historical events of the country, To Hoai wanted to show readers the difficult periods of the nation, the years when the country was in turmoil.
Maybe you are interested!
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Qos Assurance Methods for Multimedia Communications
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low. The EF PHB requires a sufficiently large number of output ports to provide low delay, low loss, and low jitter.
EF PHBs can be implemented if the output port's bandwidth is sufficiently large, combined with small buffer sizes and other network resources dedicated to EF packets, to allow the router's service rate for EF packets on an output port to exceed the arrival rate λ of packets at that port.
This means that packets with PHB EF are considered with a pre-allocated amount of output bandwidth and a priority that ensures minimum loss, minimum delay and minimum jitter before being put into operation.
PHB EF is suitable for channel simulation, leased line simulation, and real-time services such as voice, video without compromising on high loss, delay and jitter values.
Figure 2.10 Example of EF installation
Figure 2.10 shows an example of an EF PHB implementation. This is a simple priority queue scheduling technique. At the edges of the DS domain, EF packet traffic is prioritized according to the values agreed upon by the SLA. The EF queue in the figure needs to output packets at a rate higher than the packet arrival rate λ. To provide an EF PHB over an end-to-end DS domain, bandwidth at the output ports of the core routers needs to be allocated in advance to ensure the requirement μ > λ. This can be done by a pre-configured provisioning process. In the figure, EF packets are placed in the priority queue (the upper queue). With such a length, the queue can operate with μ > λ.
Since EF was primarily used for real-time services such as voice and video, and since real-time services use UDP instead of TCP, RED is generally
not suitable for EF queues because applications using UDP will not respond to random packet drop and RED will strip unnecessary packets.
2.2.4.2 Assured Forwarding (AF) PHB
PHB AF is defined by RFC 2597. The purpose of PHB AF is to deliver packets reliably and therefore delay and jitter are considered less important than packet loss. PHB AF is suitable for non-real-time services such as applications using TCP. PHB AF first defines four classes: AF1, AF2, AF3, AF4. For each of these AF classes, packets are then classified into three subclasses with three distinct priority levels.
Table 2.8 shows the four AF classes and 12 AF subclasses and the DSCP values for the 12 AF subclasses defined by RFC 2597. RFC 2597 also allows for more than three separate priority levels to be added for internal use. However, these separate priority levels will only have internal significance.
PHB Class
PHB Subclass
Package type
DSCP
AF4
AF41
Short
100010
AF42
Medium
100100
AF43
High
100110
AF3
AF31
Short
011010
AF32
Medium
011100
AF33
High
011110
AF2
AF21
Short
010010
AF22
Medium
010100
AF23
High
010110
AF1
AF11
Short
001010
AF12
Medium
001100
AF13
High
001110
Table 2.8 AF DSCPs
The AF PHB ensures that packets are forwarded with a high probability of delivery to the destination within the bounds of the rate agreed upon in an SLA. If AF traffic at an ingress port exceeds the pre-priority rate, which is considered non-compliant or “out of profile”, the excess packets will not be delivered to the destination with the same probability as the packets belonging to the defined traffic or “in profile” packets. When there is network congestion, the out of profile packets are dropped before the in profile packets are dropped.
When service levels are defined using AF classes, different quantity and quality between AF classes can be realized by allocating different amounts of bandwidth and buffer space to the four AF classes. Unlike
EF, most AF traffic is non-real-time traffic using TCP, and the RED queue management strategy is an AQM (Adaptive Queue Management) strategy suitable for use in AF PHBs. The four AF PHB layers can be implemented as four separate queues. The output port bandwidth is divided into four AF queues. For each AF queue, packets are marked with three “colors” corresponding to three separate priority levels.
In addition to the 32 DSCP 1 groups defined in Table 2.8, 21 DSCPs have been standardized as follows: one for PHB EF, 12 for PHB AF, and 8 for CSCP. There are 11 DSCP 1 groups still available for other standards.
2.2.5.Example of Differentiated Services
We will look at an example of the Differentiated Service model and mechanism of operation. The architecture of Differentiated Service consists of two basic sets of functions:
Edge functions: include packet classification and traffic conditioning. At the inbound edge of the network, incoming packets are marked. In particular, the DS field in the packet header is set to a certain value. For example, in Figure 2.12, packets sent from H1 to H3 are marked at R1, while packets from H2 to H4 are marked at R2. The labels on the received packets identify the service class to which they belong. Different traffic classes receive different services in the core network. The RFC definition uses the term behavior aggregate rather than the term traffic class. After being marked, a packet can be forwarded immediately into the network, delayed for a period of time before being forwarded, or dropped. We will see that there are many factors that affect how a packet is marked, and whether it is forwarded immediately, delayed, or dropped.
Figure 2.12 DiffServ Example
Core functionality: When a DS-marked packet arrives at a Diffservcapable router, the packet is forwarded to the next router based on
Per-hop behavior is associated with packet classes. Per-hop behavior affects router buffers and the bandwidth shared between competing classes. An important principle of the Differentiated Service architecture is that a router's per-hop behavior is based only on the packet's marking or the class to which it belongs. Therefore, if packets sent from H1 to H3 as shown in the figure receive the same marking as packets from H2 to H4, then the network routers treat the packets exactly the same, regardless of whether the packet originated from H1 or H2. For example, R3 does not distinguish between packets from h1 and H2 when forwarding packets to R4. Therefore, the Differentiated Service architecture avoids the need to maintain router state about separate source-destination pairs, which is important for network scalability.
Chapter Conclusion
Chapter 2 has presented and clarified two main models of deploying and installing quality of service in IP networks. While the traditional best-effort model has many disadvantages, later models such as IntServ and DiffServ have partly solved the problems that best-effort could not solve. IntServ follows the direction of ensuring quality of service for each separate flow, it is built similar to the circuit switching model with the use of the RSVP resource reservation protocol. IntSer is suitable for services that require fixed bandwidth that is not shared such as VoIP services, multicast TV services. However, IntSer has disadvantages such as using a lot of network resources, low scalability and lack of flexibility. DiffServ was born with the idea of solving the disadvantages of the IntServ model.
DiffServ follows the direction of ensuring quality based on the principle of hop-by-hop behavior based on the priority of marked packets. The policy for different types of traffic is decided by the administrator and can be changed according to reality, so it is very flexible. DiffServ makes better use of network resources, avoiding idle bandwidth and processing capacity on routers. In addition, the DifServ model can be deployed on many independent domains, so the ability to expand the network becomes easy.
Chapter 3: METHODS TO ENSURE QoS FOR MULTIMEDIA COMMUNICATIONS
In packet-switched networks, different packet flows often have to share the transmission medium all the way to the destination station. To ensure the fair and efficient allocation of bandwidth to flows, appropriate serving mechanisms are required at network nodes, especially at gateways or routers, where many different data flows often pass through. The scheduler is responsible for serving packets of the selected flow and deciding which packet will be served next. Here, a flow is understood as a set of packets belonging to the same priority class, or originating from the same source, or having the same source and destination addresses, etc.
In normal state when there is no congestion, packets will be sent as soon as they are delivered. In case of congestion, if QoS assurance methods are not applied, prolonged congestion can cause packet drops, affecting service quality. In some cases, congestion is prolonged and widespread in the network, which can easily lead to the network being "frozen", or many packets being dropped, seriously affecting service quality.
Therefore, in this chapter, in sections 3.2 and 3.3, we introduce some typical network traffic load monitoring techniques to predict and prevent congestion before it occurs through the measure of dropping (removing) packets early when there are signs of impending congestion.
3.1. DropTail method
DropTail is a simple, traditional queue management method based on FIFO mechanism. All incoming packets are placed in the queue, when the queue is full, the later packets are dropped.
Due to its simplicity and ease of implementation, DropTail has been used for many years on Internet router systems. However, this algorithm has the following disadvantages:
− Cannot avoid the phenomenon of “Lock out”: Occurs when 1 or several traffic streams monopolize the queue, making packets of other connections unable to pass through the router. This phenomenon greatly affects reliable transmission protocols such as TCP. According to the anti-congestion algorithm, when locked out, the TCP connection stream will reduce the window size and reduce the packet transmission speed exponentially.
− Can cause Global Synchronization: This is the result of a severe “Lock out” phenomenon. Some neighboring routers have their queues monopolized by a number of connections, causing a series of other TCP connections to be unable to pass through and simultaneously reducing the transmission speed. After those monopolized connections are temporarily suspended,
Once the queue is cleared, it takes a considerable amount of time for TCP connections to return to their original speed.
− Full Queue phenomenon: Data transmitted on the Internet often has an explosion, packets arriving at the router are often in clusters rather than in turn. Therefore, the operating mechanism of DropTail makes the queue easily full for a long period of time, leading to the average delay time of large packets. To avoid this phenomenon, with DropTail, the only way is to increase the router's buffer, this method is very expensive and ineffective.
− No QoS guarantee: With the DropTail mechanism, there is no way to prioritize important packets to be transmitted through the router earlier when all are in the queue. Meanwhile, with multimedia communication, ensuring connection and stable speed is extremely important and the DropTail algorithm cannot satisfy.
The problem of choosing the buffer size of the routers in the network is to “absorb” short bursts of traffic without causing too much queuing delay. This is necessary in bursty data transmission. The queue size determines the size of the packet bursts (traffic spikes) that we want to be able to transmit without being dropped at the routers.
In IP-based application networks, packet dropping is an important mechanism for indirectly reporting congestion to end stations. A solution that prevents router queues from filling up while reducing the packet drop rate is called dynamic queue management.
3.2. Random elimination method – RED
3.2.1 Overview
RED (Random Early Detection of congestion; Random Early Drop) is one of the first AQM algorithms proposed in 1993 by Sally Floyd and Van Jacobson, two scientists at the Lawrence Berkeley Laboratory of the University of California, USA. Due to its outstanding advantages compared to previous queue management algorithms, RED has been widely installed and deployed on the Internet.
The most fundamental point of their work is that the most effective place to detect congestion and react to it is at the gateway or router.
Source entities (senders) can also do this by estimating end-to-end delay, throughput variability, or the rate of packet retransmissions due to drop. However, the sender and receiver view of a particular connection cannot tell which gateways on the network are congested, and cannot distinguish between propagation delay and queuing delay. Only the gateway has a true view of the state of the queue, the link share of the connections passing through it at any given time, and the quality of service requirements of the
traffic flows. The RED gateway monitors the average queue length, which detects early signs of impending congestion (average queue length exceeding a predetermined threshold) and reacts appropriately in one of two ways:
− Drop incoming packets with a certain probability, to indirectly inform the source of congestion, the source needs to reduce the transmission rate to keep the queue from filling up, maintaining the ability to absorb incoming traffic spikes.
− Mark “congestion” with a certain probability in the ECN field in the header of TCP packets to notify the source (the receiving entity will copy this bit into the acknowledgement packet).
Figure 3. 1 RED algorithm
The main goal of RED is to avoid congestion by keeping the average queue size within a sufficiently small and stable region, which also means keeping the queuing delay sufficiently small and stable. Achieving this goal also helps: avoid global synchronization, not resist bursty traffic flows (i.e. flows with low average throughput but high volatility), and maintain an upper bound on the average queue size even in the absence of cooperation from transport layer protocols.
To achieve the above goals, RED gateways must do the following:
− The first is to detect congestion early and react appropriately to keep the average queue size small enough to keep the network operating in the low latency, high throughput region, while still allowing the queue size to fluctuate within a certain range to absorb short-term fluctuations. As discussed above, the gateway is the most appropriate place to detect congestion and is also the most appropriate place to decide which specific connection to report congestion to.
− The second thing is to notify the source of congestion. This is done by marking and notifying the source to reduce traffic. Normally the RED gateway will randomly drop packets. However, if congestion
If congestion is detected before the queue is full, it should be combined with packet marking to signal congestion. The RED gateway has two options: drop or mark; where marking is done by marking the ECN field of the packet with a certain probability, to signal the source to reduce the traffic entering the network.
− An important goal that RED gateways need to achieve is to avoid global synchronization and not to resist traffic flows that have a sudden characteristic. Global synchronization occurs when all connections simultaneously reduce their transmission window size, leading to a severe drop in throughput at the same time. On the other hand, Drop Tail or Random Drop strategies are very sensitive to sudden flows; that is, the gateway queue will often overflow when packets from these flows arrive. To avoid these two phenomena, gateways can use special algorithms to detect congestion and decide which connections will be notified of congestion at the gateway. The RED gateway randomly selects incoming packets to mark; with this method, the probability of marking a packet from a particular connection is proportional to the connection's shared bandwidth at the gateway.
− Another goal is to control the average queue size even without cooperation from the source entities. This can be done by dropping packets when the average size exceeds an upper threshold (instead of marking it). This approach is necessary in cases where most connections have transmission times that are less than the round-trip time, or where the source entities are not able to reduce traffic in response to marking or dropping packets (such as UDP flows).
3.2.2 Algorithm
This section describes the algorithm for RED gateways. RED gateways calculate the average queue size using a low-pass filter. This average queue size is compared with two thresholds: minth and maxth. When the average queue size is less than the lower threshold, no incoming packets are marked or dropped; when the average queue size is greater than the upper threshold, all incoming packets are dropped. When the average queue size is between minth and maxth, each incoming packet is marked or dropped with a probability pa, where pa is a function of the average queue size avg; the probability of marking or dropping a packet for a particular connection is proportional to the bandwidth share of that connection at the gateway. The general algorithm for a RED gateway is described as follows: [5]
For each packet arrival
Caculate the average queue size avg If minth ≤ avg < maxth
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Space/Time Art – A Picture of the World Through the Eyes of Women -
The Element of Fantasy in the Art of Building Space and Time -
Art Space in Kim Lan's Short Stories -
Nguyen Khai's artistic style in novels after 1980 through three novels: Human Time, A Tiny Human World, God Laughs - 15
The country was submerged in the smoke and fire of war to appreciate the independence and peace of today.

In his memoirs, To Hoai once mentioned the space of Thuong Yen forest. Thuong Yen forest " in a Red Dao ethnic village in the middle of a dense forest on the banks of the Lo River on Binh Ca wharf " [25. 414] appeared with harsh images of a revolutionary base. " Never before had the agency been huddled in such a dangerous place. Nguyen Tuan feared this place the most. Coming to Thuong Yen, in the humid forest, Nguyen Tuan was truly tormented by malaria. " [25. 414]. Those who held the pen and "taste the honey and lie on thorns" with the soldiers, they understood more deeply the hardships of the resistance war against the invaders. When they were able to go directly to the " dangerous " places, the soldiers on the literary and artistic front had more valuable materials to write about, to reconstruct the fierce years of war.
Recreating the days of bombs and bullets, To Hoai not only describes the space of Thuong Yen forest, but the writer also takes the reader to a very special space where " The shops and meat coupons only sell fat - which is rarely available. People poured into the streets, all day long, lining up, lining up to buy bunches of jute. Bombs were falling overhead, and on the ground, there were long lines of people buying and selling like this, who knows what will happen. " [25. 596]. The war " bombs were falling overhead ", and " on the ground, there were long lines of people buying and selling ". At this time, life and death were as fragile as a thread. What will be the fate of those people lining up to buy bunches of jute? To Hoai did not describe further, but from the fact that people poured into the streets, we can also imagine the fierceness of the situation. From this realistic space, the writer evokes for us the life of the Vietnamese people during the difficult and harsh period, in the suffocating atmosphere of war. To Hoai always describes events from an objective point of view. The author does not analyze or comment but lets the reader feel for themselves.
The reality of life through events. The space of this event makes the reader feel anxious because death seems to be present right above everyone's head. Gloom and danger cover the space filled with bombs and gunpowder.
The author has distilled and placed meaningful historical and social events in a very open space. Although the events are not arranged in chronological order but according to the author's recollection, they have contributed significantly to reflecting the realistic picture of life in a turbulent period.
* Specific real space associated with private events
In To Hoai's memoirs, in addition to the historical event space, there is also the private event space, which is the family space, of individuals whose changes affect the character's life and psychology.
We have known many portraits of writers through To Hoai's humanistic view of life. Once again, readers have the opportunity to meet them again through a realistic space associated with personal events. When the resistance war broke out, the families of writers and artists had to evacuate to safe places:
“ During the resistance war, Nguyen Hong and Kim Lan evacuated their families to Cau Den hamlet on Nha Nam. Painters Tran Van Can, Ta Thuc Binh, and the whole family of Uncle Ngo Tat To also gathered on the low hill that was still deserted at that time. Areas in Xuan Ang (Phu Tho) or Quan Tin (Thanh Hoa) also gathered villages of resistance artists. It is said that painter Tran Van Can was given a field by the commune, and he was very good at plowing .” [25. 422]. The event of evacuation during the resistance war is no longer strange to us – the people of the invaded country. Evacuating to avoid the sweep of the enemy. Families of artists and writers all had to temporarily leave their familiar living places. They went to deserted areas, which gradually became villages of resistance artists and writers.
War. Even the painter Tran Van Can, who was only good at using a brush to draw, is now " very good at plowing ".
After the peace, many writers moved to Hanoi. And Nguyen Hong's return to Hanoi was a change in his family life as well as in his own life. " Returning to Hanoi, Nguyen Hong and his wife and children rented the second floor of an old Miriben townhouse next to the eye hospital near Hom market... Only the children running in and out made it so confusing. She applied to work as an employee at a state-owned book store, selling at the corner counter of the current general store, at the entrance to Hang Bai street " [25. 422]. The space became narrower than the days " under Nghia Dung beach ". Everything was difficult at that time. And then there were many events that affected Nguyen Hong and the life of the writer's family. Incidents happened. After the extraordinary conference to discuss and confirm the structure of the Writers' Association, Nguyen Hong prepared to return to Nha Nam - where the writer had left. Let's look at the space of Nguyen Hong's room: " The room was still as messy with mosquito nets as usual. But if you notice, it was different. Everything was packed and tied up like I had seen before, early in the morning, things in the houses were ready to be thrown out to the fence to avoid the planes. I nodded, but I really didn’t understand how… Then one day, I heard Kim Lan say that Nguyen Hong had moved to Nha Nam. I don’t know if his wife and children were carrying things like the day they happily returned to Hanoi or if they hired a three-wheeled cart to push them. ” [25. 492]. Life in Hanoi could not hold Nguyen Hong. A man full of self-respect and “ determination ” [25. 492] made Nguyen Hong “ definitely not stay anymore ” [25. 493]. “ So he left everything, went back to Nha Nam, Cau Den hamlet, Doi Chay hamlet, and lived on the hill like the years of evacuation… On the hill with sparse bamboo, there were still a few houses of villagers, a primary school, thatched roof, dilapidated walls, and desolate. He still lived in the same house as when he fled from the West. The house has mud walls, the fence has cactus, the well has a puddle of water in the ravine at the foot of the hill. Far away, ahead, begins
The undulating low forests on Yen The Thuong. ” [25. 493]. It was the historical and social events that affected the family events, the psychology and actions of the character. When he was insulted and hurt, he did not hesitate to express his attitude: “ - Yes, Nha Nam. Enough, enough. I will not play with you anymore. I will return to Nha Nam .”
In his memoirs, especially in "Cat bui chan ai" (The Dust of One's Feet) in specific real-life spaces, especially the space of events associated with individuals, we see that To Hoai never forgot his late friend Nguyen Tuan. The writer with the " arrogant " style has appeared many times, each time he appears, the reader is surprised. This is the image of the writer Nguyen Tuan when he was young: " The train from Phnom Penh arrived in Poipet just before dusk. It was the fall of 1930 (…). At the end of the row of seats, there were two young people (…). The train rumbled all night to Vong Cac. They could not sleep. They were still nervous because of their luck and praised themselves for being wise and skillful in hiding the secret police. So they safely left Cambodia. By mid-afternoon, the train entered the city of Vong Cac. The two were bewildered in the middle of nowhere. Around them, in the big station, there were crowds of Siamese, Chinese, and Vietnamese people, chattering and chattering. Four Siamese policemen in white uniforms and gold-rimmed hats and a group of soldiers carrying rifles gathered around. One of them spoke Vietnamese and asked:
- Are you Tuan and Thiep?
True to his given name. The police already knew, just asking to perform a spell. Confused, not knowing how to answer, the rope tied them both up and jerked their elbows. The gun butt pushed them back, pushing them to walk through the crowded streets. ” [25. 500, 505]. In his reminiscences, To Hoai built an image of the young man Nguyen Tuan “ with a young face, not even twenty years old ” [25. 502], “ Disgusted with the suffocating, deadlocked life of the people in a colony, Nguyen Tuan always dreamed of faraway lands. ” [17. 599].
That is why he tried every way to go abroad, and his illegal wandering trip of the colonial regime led to his arrest in Thailand and finally being sent back to Thanh Hoa prison in 1930. To Hoai chose a very special event to reveal the personality of writer Nguyen Tuan. That personality created an "I" with its own color from the ego of "nomadism". To Hoai respected the truth. He recorded authentic events when: " In Thanh Hoa, when he was no longer under house arrest, Nguyen Tuan went to Hanoi, wrote to make a living and created every character as my character. " [25. 519]. Escaping from house arrest, not "wandering" like before, but " In the book, it is similar to real life, that person's name is Nguyen or Bach, after drinking and drinking to his heart's content, he left the place of entertainment, but did not return home, at that time, Uncle Nguyen's family had moved from Thanh Hoa to live in Nga Tu So on Moc Thuong Dinh, his hometown. ” [25. 519]. We see the contradiction in Nguyen Tuan through this very real detail. When returning to his homeland, Nguyen fell into a vicious circle and then became bored. This is clearly shown in his works before the August Revolution. Violent, sad, but also sometimes heartbreaking and above all we still see a personality, an honest soul that is not necessarily closed to life.
In the memoir Cat bui chan ai , the letters Nguyen Tuan sent to To Hoai during his trip to several provinces in the Northwest left readers with an unforgettable impression of the natural scenery of the mountainous region, especially the space associated with Nguyen Tuan's personal events in a very special landscape. " The previous letter was written from Simacai on a sunny market day. This letter was written after three days of walking 70km in a row, in the rain, the day before there was a big flood blocking the stream, so we had to go around to Meo village to avoid the flood. The day before it rained, the next day it rained, the day after it rained again. (…). I was still feverish, after several days of walking in the rain I still had a fever. I brought all the ointment with me, had to ask the local people to give me more ointment to withdraw."
like that. Tired, hard, but the images collected about postal transportation are not too poor. The Lao Cai line is basically what I have grasped. ” [25. 573]. Although Nguyen Tuan was always tormented by the disease “ his bones were already cold because of rheumatism this season ” [25. 572], he was still enthusiastic about going and exploring. Everything the writer “collected” became the material for his later works.
In the Hoa Lo prison, the interview between “us” and an American prisoner named John, from Ohio, took place very specifically. To Hoai described very carefully Nguyen Tuan’s attitude and emotional state: “ Nguyen Tuan sneered (…) ”, “ Nguyen Tuan spoke seriously (…) ”, “ Nguyen Tuan smiled faintly (…) ”, “ Nguyen Tuan shook his pipe. That was his way of regaining his composure. So he was annoyed. The heavy popping sounds of the ashtray in the quiet night made the wrinkles on Nguyen Tuan’s forehead stretch out. Nguyen Tuan said, leisurely (…) ”, “ Nguyen Tuan stood up, angrily pointing his pipe at John’s face (…) ”, “ Nguyen Tuan shook his head gently: “Killing someone and saying that people must love each other! I don’t understand!” ”, “ Nguyen Tuan slowly stood up, raised his hand: “Remind me: when you come back, remember to study geography again. There are no two United States, no two Vietnams. Remember that” ” [25. 630, 631, 632]. With a natural but also very “mischievous” narrative tone, To Hoai showed readers a Nguyen Tuan with full nuances of mood and emotion in a very special space. To Hoai was honest when he commented: “ Nguyen Tuan is quiet, very political and has a self-righteous look. (…). Nguyen Tuan is logical and tough. ” [25. 632]. It is not by chance that To Hoai placed his character in such a space. In this space, Nguyen Tuan clearly revealed the attitude and viewpoint of a person on the other side of the battle line with the invaders. With descriptive language that is “ as natural as the breath of life ” [40. 137] along with a very special event, To Hoai built
A complete portrait of Nguyen Tuan, not only a normal person but also a person “ also in politics ” [25. 632]. The person who was once passionate about those trips, at times he refused to go to Paris, perhaps because “ the two hollow legs of that strong man made it difficult to travel, erasing all the feelings of the distant world. ” [25. 661]. Perhaps the departure of that talented, “arrogant, proud” man had been foretold. Before going to Cat Ba, “ I came to show off to Nguyen Tuan a small but interesting trip. ” [25. 662]. And the expressions of Nguyen Tuan: “ The yellow stick of Sapa, the sound of the stick hitting the wooden surface was sparse and disjointed. I could guess the difficult, heavy footsteps. Nguyen Tuan walked in, his face was pale and purple, not red from wine. ” [25. 662] showed the tiredness of old age and travel. No one can avoid the laws of life: " In the morning, someone came to tell Dao Vu and me:
- Last night, I heard on the radio that Mr. Nguyen Tuan had passed away.
I thought it was still the same day I sat alone at his house, but Nguyen Tuan had been lying still since the morning I came here. Nguyen Tuan! Nguyen Tuan oh! Oh! Oh! ” [25. 663, 664]. Revealing the sorrowful mood from this specific event, the reader can feel the author's profound sadness at the eternal departure of Nguyen Tuan.
An event that cannot be ignored when writing about the poet Nguyen Binh is the loss of his child. To Hoai attached this event to a harsh space so that we can understand the pain of a father who lost his child: “ Hien (Nguyen Binh's daughter) was plump and chubby, but soon she became skinny, scabby, and had terrible pimples. Every day, her father carried her on one shoulder, like a cat carrying its baby. Wherever she went, swarms of green-gray flies chased after her. One night, her father got drunk and carried Hien out into the street. At Ba Trieu intersection - oh, how could so many confidences, tears, and smiles of the writer of the story

![Qos Assurance Methods for Multimedia Communications
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low. The EF PHB requires a sufficiently large number of output ports to provide low delay, low loss, and low jitter.
EF PHBs can be implemented if the output ports bandwidth is sufficiently large, combined with small buffer sizes and other network resources dedicated to EF packets, to allow the routers service rate for EF packets on an output port to exceed the arrival rate λ of packets at that port.
This means that packets with PHB EF are considered with a pre-allocated amount of output bandwidth and a priority that ensures minimum loss, minimum delay and minimum jitter before being put into operation.
PHB EF is suitable for channel simulation, leased line simulation, and real-time services such as voice, video without compromising on high loss, delay and jitter values.
Figure 2.10 Example of EF installation
Figure 2.10 shows an example of an EF PHB implementation. This is a simple priority queue scheduling technique. At the edges of the DS domain, EF packet traffic is prioritized according to the values agreed upon by the SLA. The EF queue in the figure needs to output packets at a rate higher than the packet arrival rate λ. To provide an EF PHB over an end-to-end DS domain, bandwidth at the output ports of the core routers needs to be allocated in advance to ensure the requirement μ > λ. This can be done by a pre-configured provisioning process. In the figure, EF packets are placed in the priority queue (the upper queue). With such a length, the queue can operate with μ > λ.
Since EF was primarily used for real-time services such as voice and video, and since real-time services use UDP instead of TCP, RED is generally
not suitable for EF queues because applications using UDP will not respond to random packet drop and RED will strip unnecessary packets.
2.2.4.2 Assured Forwarding (AF) PHB
PHB AF is defined by RFC 2597. The purpose of PHB AF is to deliver packets reliably and therefore delay and jitter are considered less important than packet loss. PHB AF is suitable for non-real-time services such as applications using TCP. PHB AF first defines four classes: AF1, AF2, AF3, AF4. For each of these AF classes, packets are then classified into three subclasses with three distinct priority levels.
Table 2.8 shows the four AF classes and 12 AF subclasses and the DSCP values for the 12 AF subclasses defined by RFC 2597. RFC 2597 also allows for more than three separate priority levels to be added for internal use. However, these separate priority levels will only have internal significance.
PHB Class
PHB Subclass
Package type
DSCP
AF4
AF41
Short
100010
AF42
Medium
100100
AF43
High
100110
AF3
AF31
Short
011010
AF32
Medium
011100
AF33
High
011110
AF2
AF21
Short
010010
AF22
Medium
010100
AF23
High
010110
AF1
AF11
Short
001010
AF12
Medium
001100
AF13
High
001110
Table 2.8 AF DSCPs
The AF PHB ensures that packets are forwarded with a high probability of delivery to the destination within the bounds of the rate agreed upon in an SLA. If AF traffic at an ingress port exceeds the pre-priority rate, which is considered non-compliant or “out of profile”, the excess packets will not be delivered to the destination with the same probability as the packets belonging to the defined traffic or “in profile” packets. When there is network congestion, the out of profile packets are dropped before the in profile packets are dropped.
When service levels are defined using AF classes, different quantity and quality between AF classes can be realized by allocating different amounts of bandwidth and buffer space to the four AF classes. Unlike
EF, most AF traffic is non-real-time traffic using TCP, and the RED queue management strategy is an AQM (Adaptive Queue Management) strategy suitable for use in AF PHBs. The four AF PHB layers can be implemented as four separate queues. The output port bandwidth is divided into four AF queues. For each AF queue, packets are marked with three “colors” corresponding to three separate priority levels.
In addition to the 32 DSCP 1 groups defined in Table 2.8, 21 DSCPs have been standardized as follows: one for PHB EF, 12 for PHB AF, and 8 for CSCP. There are 11 DSCP 1 groups still available for other standards.
2.2.5.Example of Differentiated Services
We will look at an example of the Differentiated Service model and mechanism of operation. The architecture of Differentiated Service consists of two basic sets of functions:
Edge functions: include packet classification and traffic conditioning. At the inbound edge of the network, incoming packets are marked. In particular, the DS field in the packet header is set to a certain value. For example, in Figure 2.12, packets sent from H1 to H3 are marked at R1, while packets from H2 to H4 are marked at R2. The labels on the received packets identify the service class to which they belong. Different traffic classes receive different services in the core network. The RFC definition uses the term behavior aggregate rather than the term traffic class. After being marked, a packet can be forwarded immediately into the network, delayed for a period of time before being forwarded, or dropped. We will see that there are many factors that affect how a packet is marked, and whether it is forwarded immediately, delayed, or dropped.
Figure 2.12 DiffServ Example
Core functionality: When a DS-marked packet arrives at a Diffservcapable router, the packet is forwarded to the next router based on
Per-hop behavior is associated with packet classes. Per-hop behavior affects router buffers and the bandwidth shared between competing classes. An important principle of the Differentiated Service architecture is that a routers per-hop behavior is based only on the packets marking or the class to which it belongs. Therefore, if packets sent from H1 to H3 as shown in the figure receive the same marking as packets from H2 to H4, then the network routers treat the packets exactly the same, regardless of whether the packet originated from H1 or H2. For example, R3 does not distinguish between packets from h1 and H2 when forwarding packets to R4. Therefore, the Differentiated Service architecture avoids the need to maintain router state about separate source-destination pairs, which is important for network scalability.
Chapter Conclusion
Chapter 2 has presented and clarified two main models of deploying and installing quality of service in IP networks. While the traditional best-effort model has many disadvantages, later models such as IntServ and DiffServ have partly solved the problems that best-effort could not solve. IntServ follows the direction of ensuring quality of service for each separate flow, it is built similar to the circuit switching model with the use of the RSVP resource reservation protocol. IntSer is suitable for services that require fixed bandwidth that is not shared such as VoIP services, multicast TV services. However, IntSer has disadvantages such as using a lot of network resources, low scalability and lack of flexibility. DiffServ was born with the idea of solving the disadvantages of the IntServ model.
DiffServ follows the direction of ensuring quality based on the principle of hop-by-hop behavior based on the priority of marked packets. The policy for different types of traffic is decided by the administrator and can be changed according to reality, so it is very flexible. DiffServ makes better use of network resources, avoiding idle bandwidth and processing capacity on routers. In addition, the DifServ model can be deployed on many independent domains, so the ability to expand the network becomes easy.
Chapter 3: METHODS TO ENSURE QoS FOR MULTIMEDIA COMMUNICATIONS
In packet-switched networks, different packet flows often have to share the transmission medium all the way to the destination station. To ensure the fair and efficient allocation of bandwidth to flows, appropriate serving mechanisms are required at network nodes, especially at gateways or routers, where many different data flows often pass through. The scheduler is responsible for serving packets of the selected flow and deciding which packet will be served next. Here, a flow is understood as a set of packets belonging to the same priority class, or originating from the same source, or having the same source and destination addresses, etc.
In normal state when there is no congestion, packets will be sent as soon as they are delivered. In case of congestion, if QoS assurance methods are not applied, prolonged congestion can cause packet drops, affecting service quality. In some cases, congestion is prolonged and widespread in the network, which can easily lead to the network being frozen, or many packets being dropped, seriously affecting service quality.
Therefore, in this chapter, in sections 3.2 and 3.3, we introduce some typical network traffic load monitoring techniques to predict and prevent congestion before it occurs through the measure of dropping (removing) packets early when there are signs of impending congestion.
3.1. DropTail method
DropTail is a simple, traditional queue management method based on FIFO mechanism. All incoming packets are placed in the queue, when the queue is full, the later packets are dropped.
Due to its simplicity and ease of implementation, DropTail has been used for many years on Internet router systems. However, this algorithm has the following disadvantages:
− Cannot avoid the phenomenon of “Lock out”: Occurs when 1 or several traffic streams monopolize the queue, making packets of other connections unable to pass through the router. This phenomenon greatly affects reliable transmission protocols such as TCP. According to the anti-congestion algorithm, when locked out, the TCP connection stream will reduce the window size and reduce the packet transmission speed exponentially.
− Can cause Global Synchronization: This is the result of a severe “Lock out” phenomenon. Some neighboring routers have their queues monopolized by a number of connections, causing a series of other TCP connections to be unable to pass through and simultaneously reducing the transmission speed. After those monopolized connections are temporarily suspended,
Once the queue is cleared, it takes a considerable amount of time for TCP connections to return to their original speed.
− Full Queue phenomenon: Data transmitted on the Internet often has an explosion, packets arriving at the router are often in clusters rather than in turn. Therefore, the operating mechanism of DropTail makes the queue easily full for a long period of time, leading to the average delay time of large packets. To avoid this phenomenon, with DropTail, the only way is to increase the routers buffer, this method is very expensive and ineffective.
− No QoS guarantee: With the DropTail mechanism, there is no way to prioritize important packets to be transmitted through the router earlier when all are in the queue. Meanwhile, with multimedia communication, ensuring connection and stable speed is extremely important and the DropTail algorithm cannot satisfy.
The problem of choosing the buffer size of the routers in the network is to “absorb” short bursts of traffic without causing too much queuing delay. This is necessary in bursty data transmission. The queue size determines the size of the packet bursts (traffic spikes) that we want to be able to transmit without being dropped at the routers.
In IP-based application networks, packet dropping is an important mechanism for indirectly reporting congestion to end stations. A solution that prevents router queues from filling up while reducing the packet drop rate is called dynamic queue management.
3.2. Random elimination method – RED
3.2.1 Overview
RED (Random Early Detection of congestion; Random Early Drop) is one of the first AQM algorithms proposed in 1993 by Sally Floyd and Van Jacobson, two scientists at the Lawrence Berkeley Laboratory of the University of California, USA. Due to its outstanding advantages compared to previous queue management algorithms, RED has been widely installed and deployed on the Internet.
The most fundamental point of their work is that the most effective place to detect congestion and react to it is at the gateway or router.
Source entities (senders) can also do this by estimating end-to-end delay, throughput variability, or the rate of packet retransmissions due to drop. However, the sender and receiver view of a particular connection cannot tell which gateways on the network are congested, and cannot distinguish between propagation delay and queuing delay. Only the gateway has a true view of the state of the queue, the link share of the connections passing through it at any given time, and the quality of service requirements of the
traffic flows. The RED gateway monitors the average queue length, which detects early signs of impending congestion (average queue length exceeding a predetermined threshold) and reacts appropriately in one of two ways:
− Drop incoming packets with a certain probability, to indirectly inform the source of congestion, the source needs to reduce the transmission rate to keep the queue from filling up, maintaining the ability to absorb incoming traffic spikes.
− Mark “congestion” with a certain probability in the ECN field in the header of TCP packets to notify the source (the receiving entity will copy this bit into the acknowledgement packet).
Figure 3. 1 RED algorithm
The main goal of RED is to avoid congestion by keeping the average queue size within a sufficiently small and stable region, which also means keeping the queuing delay sufficiently small and stable. Achieving this goal also helps: avoid global synchronization, not resist bursty traffic flows (i.e. flows with low average throughput but high volatility), and maintain an upper bound on the average queue size even in the absence of cooperation from transport layer protocols.
To achieve the above goals, RED gateways must do the following:
− The first is to detect congestion early and react appropriately to keep the average queue size small enough to keep the network operating in the low latency, high throughput region, while still allowing the queue size to fluctuate within a certain range to absorb short-term fluctuations. As discussed above, the gateway is the most appropriate place to detect congestion and is also the most appropriate place to decide which specific connection to report congestion to.
− The second thing is to notify the source of congestion. This is done by marking and notifying the source to reduce traffic. Normally the RED gateway will randomly drop packets. However, if congestion
If congestion is detected before the queue is full, it should be combined with packet marking to signal congestion. The RED gateway has two options: drop or mark; where marking is done by marking the ECN field of the packet with a certain probability, to signal the source to reduce the traffic entering the network.
− An important goal that RED gateways need to achieve is to avoid global synchronization and not to resist traffic flows that have a sudden characteristic. Global synchronization occurs when all connections simultaneously reduce their transmission window size, leading to a severe drop in throughput at the same time. On the other hand, Drop Tail or Random Drop strategies are very sensitive to sudden flows; that is, the gateway queue will often overflow when packets from these flows arrive. To avoid these two phenomena, gateways can use special algorithms to detect congestion and decide which connections will be notified of congestion at the gateway. The RED gateway randomly selects incoming packets to mark; with this method, the probability of marking a packet from a particular connection is proportional to the connections shared bandwidth at the gateway.
− Another goal is to control the average queue size even without cooperation from the source entities. This can be done by dropping packets when the average size exceeds an upper threshold (instead of marking it). This approach is necessary in cases where most connections have transmission times that are less than the round-trip time, or where the source entities are not able to reduce traffic in response to marking or dropping packets (such as UDP flows).
3.2.2 Algorithm
This section describes the algorithm for RED gateways. RED gateways calculate the average queue size using a low-pass filter. This average queue size is compared with two thresholds: minth and maxth. When the average queue size is less than the lower threshold, no incoming packets are marked or dropped; when the average queue size is greater than the upper threshold, all incoming packets are dropped. When the average queue size is between minth and maxth, each incoming packet is marked or dropped with a probability pa, where pa is a function of the average queue size avg; the probability of marking or dropping a packet for a particular connection is proportional to the bandwidth share of that connection at the gateway. The general algorithm for a RED gateway is described as follows: [5]
For each packet arrival
Caculate the average queue size avg If minth ≤ avg < maxth
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