The Stop and Wait ARQ solves the main three problems but may cause big performance issues as the sender always waits for acknowledgement even if it has the next packet ready to send. Consider a situation where you have a high bandwidth connection and propagation delay is also high (you are connected to some server in some other country through a high-speed connection). To solve this problem, we can send more than one packet at a time with a larger sequence number. We will be discussing these protocols in the next articles.
I am implementing a client/server echo program using the Stop-and-Wait protocol. Part of the implementation is to randomly introduce drops from the server. I am actually randomly dropping ACKs as well as FRAMEs. A side effect of this is the introduction of a corner case:
C Program For Stop And Wait Protocol
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In a batch script, a START command without /wait will run the program and just continue, so a script containing nothing but a START command will close the CMD console and leave the new program running.
The above will start the calculator and wait before continuing. However if you replace calc.exe with Winword.exe, to run Word instead, then the /wait will stop working, this is because Winword.exe is a stub which launches the main Word application and then exits.
Districts face several delicate policy issues and must decide which responsibilities the school bears and which responsibilities fall to parents and other caregivers. In addition, those responsibilities must be further clarified to reflect policies when students are traveling between home and their school bus stops and while waiting for the bus. Most school transportation professionals agree that it is the parents' responsibility to supervise students at these times. However, many also recognize that this may be an unrealistic expectation due to work schedules, disabilities, or other circumstances. In some cases, accommodations may have to be made for these situations. Regardless of how these situations will be handled, clearly stating and communicating expectations about parents' responsibilities is vital.
After following existing policy, the next step to consider is school bus stop location options. It is impractical to discuss school bus stops without discussing bus routes. Clearly they are closely related and the characteristics of one have implications for the other. For example, if a route involves travel along a busy road, and a stop is designated along that segment of the route, students who wait at that stop will have to contend with traffic on that road. Information here will be presented with the school bus stop as the central point of focus but with the recognition that there is a need to balance the desired characteristics of a bus stop with the realities of what the school bus route will allow.
Street-side characteristics include the conditions on the road where the school bus stops to load and unload students. To provide the safest environment for students to walk between home and the school bus stop and wait at the stop:
In addition to the on-street characteristics, characteristics about the off-street location of the school bus stop are also critical to ensuring student safety during transport to school. This section addresses the school bus stop itself. For the safest areas for students to wait for, and load onto or off of the bus:
Before understanding the stop and Wait protocol, we first know about the error control mechanism. The error control mechanism is used so that the received data should be exactly same whatever sender has sent the data. The error control mechanism is divided into two categories, i.e., Stop and Wait ARQ and sliding window. The sliding window is further divided into two categories, i.e., Go Back N, and Selective Repeat. Based on the usage, the people select the error control mechanism whether it is stop and wait or sliding window.
Here stop and wait means, whatever the data that sender wants to send, he sends the data to the receiver. After sending the data, he stops and waits until he receives the acknowledgment from the receiver. The stop and wait protocol is a flow control protocol where flow control is one of the services of the data link layer.
Therefore, the idea of stop and wait protocol in the receiver's side is also very simple, i.e., consume the packet, and once the packet is consumed, the acknowledgment is sent. This is known as a flow control mechanism.
The above figure shows the working of the stop and wait protocol. If there is a sender and receiver, then sender sends the packet and that packet is known as a data packet. The sender will not send the second packet without receiving the acknowledgment of the first packet. The receiver sends the acknowledgment for the data packet that it has received. Once the acknowledgment is received, the sender sends the next packet. This process continues until all the packet are not sent. The main advantage of this protocol is its simplicity but it has some disadvantages also. For example, if there are 1000 data packets to be sent, then all the 1000 packets cannot be sent at a time as in Stop and Wait protocol, one packet is sent at a time.
Suppose the sender sends the data and it has also been received by the receiver. On receiving the packet, the receiver sends the acknowledgment. In this case, the acknowledgment is lost in a network, so there is no chance for the sender to receive the acknowledgment. There is also no chance for the sender to send the next packet as in stop and wait protocol, the next packet cannot be sent until the acknowledgment of the previous packet is received.
The WS is equal to N. If we say the protocol is GB-4, then WS = 4. In order to implement pipelining, the window size should always be greater than one. If window size is one, the protocol reduces to stop-and -wait protocol.
A channel has a bit rate of 4 Kbps and one way propagation delay of 20 msec. The channel uses stop and wait protocol. The transmission time of the acknowledgement frame is negligible. To get a channel efficiency of at least 50%, the minimum frame size should be-
A sender uses the stop and wait ARQ protocol for reliable transmission of frames. Frames are of size 1000 bytes and the transmission rate at the sender is 80 Kbps. Size of an acknowledgement is 100 bytes and the transmission rate at the receiver is 8 Kbps. The one way propagation delay is 100 msec.
Using stop and wait protocol, sender wants to transmit 10 data packets to the receiver. Out of these 10 data packets, every 4th data packet is lost. How many packets sender will have to send in total?
In programs that never end (such as operating systems), infinite busy waiting can be implemented by using unconditional jumps as shown by this NASM syntax: jmp $. The CPU will unconditionally jump to its own position forever. A busy wait like this can be replaced with:
In low-level programming, busy-waits may actually be desirable. It may not be desirable or practical to implement interrupt-driven processing for every hardware device, particularly those that are seldom accessed. Sometimes it is necessary to write some sort of control data to hardware and then fetch device status resulting from the write operation, status that may not become valid until a number of machine cycles have elapsed following the write. The programmer could call an operating system delay function, but doing so may consume more time than would be expended in spinning for a few clock cycles waiting for the device to return its status.
Monit is a utility for managing and monitoring processes, programs, files, directories and filesystems on a Unix system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations. E.g. Monit can start a process if it does not run, restart a process if it does not respond and stop a process if it uses too much resources. You can use Monit to monitor files, directories and filesystems for changes, such as timestamps changes, checksum changes or size changes.
Monit is controlled via an easy to configure control file based on a free-format, token-oriented syntax. Monit logs to syslog or to its own log file and notifies you about error conditions via customisable alert messages. Monit can perform various TCP/IP network checks, protocol checks and can utilise SSL for such checks. Monit provides a HTTP(S) interface and you may use a browser to access the Monit program.
You can use Monit to monitor daemon processes or similar programs running on localhost. Monit is particularly useful for monitoring daemon processes, such as those started at system boot time. For instance sendmail, sshd, apache and mysql. In contrast to many other monitoring systems, Monit can act if an error situation should occur, e.g.; if sendmail is not running, monit can start sendmail again automatically or if apache is using too many resources (e.g. if a DoS attack is in progress) Monit can stop or restart apache and send you an alert message. Monit can also monitor process characteristics, such as how much memory or cpu cycles a process is using.
The event list can also be negated to send alerts for all events except those which are listed, by prepending the list with the word not. For example, to receive all alerts except notification about Monit program start and stop:
In the case of a process check, Monit will wait up to 30 seconds for the start/stop action to finish before giving up and report an error. You can override this timeout using the TIMEOUT option or globally using the set limits.
Program checks are asynchronous. Meaning that Monit will not wait for the program to exit, but instead, Monit will start the program in the background and immediately continue checking the next service entry in monitrc. At the next cycle, Monit will check if the program has finished and if so, collect the program's exit status. If the status indicate a failure, Monit will raise an alert message containing the program's error (stderr) output, if any. If the program has not exited after the first cycle, Monit will wait another cycle and so on. If the program is still running after 5 minutes, Monit will kill it and generate a program timeout event. It is possible to override the default timeout (see the syntax below). 2ff7e9595c
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