1. What is routing Routing is the act of passing information from the source through the network to the destination. On the way, at least one intermediate node is encountered. Routing is often compared to bridging, and to the careless person, they seem to accomplish the same thing. The main difference is that bridging occurs at the second layer (link layer) of the OSI reference protocol, while routing occurs at the third layer (network layer). This difference makes the two use different information in the process of transmitting information, so as to complete their tasks in different ways. The topic of routing has long appeared in the computer world, but commercial success was not achieved until the mid-1980s. The main reason for this time delay was that the network in the 1970s was very simple, and later large networks were more common. Second, the composition of routing Routing involves two basic actions: determining the best path and transmitting information through the network. In the process of routing, the latter is also called (data) exchange. The exchange is relatively simple, and the path selection is complicated. 1. Path selection Metric is a metric used by routing algorithms to determine the best path to a destination, such as path length. To help route selection, the routing algorithm initializes and maintains a routing table containing path information. The path information varies according to the routing algorithm used. The routing algorithm populates the routing table based on a lot of information. The destination / next hop address pair tells the router that the best way to reach this destination is to send the packet to the router representing the "next hop". When the router receives a packet, it checks its destination address and attempts to compare this address with "Jump" connection. The routing table may also include other information. The routing table compares the metrics to determine the best path. These metrics vary according to the routing algorithm used. The common metrics are described below. Routers communicate with each other and maintain their routing tables by exchanging routing information. The routing update information usually contains all or part of the routing table. By analyzing the routing update information from other routers, the router can build a detailed network topology. Another example of information sent between routers is link state broadcast information, which informs the link state of other router senders. The link information is used to build a complete topology map so that routers can determine the best path. 2. Exchange The exchange algorithm is relatively simple and is the same for most routing protocols. In most cases, a host decides to send data to another host. After obtaining the router's address through some method, the source host sends For a physical (MAC) address packet, the protocol address is directed to the destination host. After checking the destination protocol address of the data packet, the router determines whether it knows how to forward the packet. If the router does not know how to forward the packet, it usually discards it. If the router knows how to forward, it changes the destination physical address to the physical address of the next hop and sends it to it. The next hop may be the final destination host. If not, it is usually another router, which will perform the same steps. When a packet flows through the network, its physical address is changing, but its protocol address remains unchanged. The above describes the exchange between the source system and the destination system, and ISO defines the layered terms used to describe this process. In this terminology, a network device without the ability to forward packets is called an end system (ES--end system), and one with this capability is called an intermediary system (IS--intermediate system). IS is further divided into intra-domain IS (intradomain IS) that can communicate in the routing domain and interdomain IS (interdomain IS) that can both communicate in the routing domain. The routing domain is usually considered to be a part of the network under unified management, and it complies with a specific set of management rules, also known as an autonomous system (utonomous system). In some protocols, routing domains can be divided into routing intervals, but intra-domain routing protocols can still be used to exchange data within and between intervals. Three, routing algorithm Routing algorithms can be distinguished based on multiple characteristics. First, the specific goals of the algorithm designer affect the operation of the routing protocol; second, there are multiple routing algorithms, each of which affects the network and router resources differently; and finally, the routing algorithm uses multiple metrics to affect Calculation of good path. The following sections analyze the characteristics of these routing algorithms. 1. Design goals Routing algorithms usually have one or more of the following design goals: Simple optimization, low consumption, robust, stable and rapid aggregation flexibility Optimization refers to the routing algorithm's ability to select the best path, calculated according to the metric value and weight. For example, there is a routing algorithm that may use hop count and delay, but the weight of the delay may be larger. Of course, the routing protocol must strictly define the algorithm for calculating the metric. The routing algorithm can also be designed as simple as possible. In other words, routing protocols must efficiently provide their functions and minimize software and application overhead. Efficiency is especially important when the software that implements the routing algorithm must run on a computer with limited physical resources. The routing algorithm must be robust, that is, it must still be able to handle normally in the presence of abnormal or unforeseen events, such as hardware failures, high loads, and incorrect implementation. Because routers are located at the connection points of the network, they can cause major problems when they fail. The best routing algorithms are usually those that have passed the test of time and proved to be stable under various network conditions. In addition, routing algorithms must be able to aggregate quickly. Aggregation is the process by which all routers agree on the best path. When a certain network event makes the path broken or unavailable, the router distributes the routing update information through the network, prompting the recalculation of the best path, and finally making all routers agree. Routing algorithms with very slow aggregation may generate routing loops or network interruptions. Routing algorithms should also be flexible, that is, they should adapt quickly and accurately to various network environments. For example, assuming that a certain network segment is broken, when knowing the problem, many routing algorithms will quickly select the next best path for the path that usually uses this network segment. Routing algorithms can be designed to adapt to network bandwidth, router queue size, and network delay. 2. Algorithm type The differences between routing algorithms include: Static and dynamic single-path and multi-path flat and layered host intelligence and router intelligence intra-domain and inter-domain link state and distance vector (1) Static and dynamic The static routing algorithm is difficult to be regarded as an algorithm, but it is just a table mapping established by the network management before starting routing. These mappings themselves do not change unless the network administrator changes them. Algorithms that use static routing are easier to design and work well in networks where network communication is predictable and simple. Because static routing systems cannot respond to changes in the network, they are generally considered unsuitable for today's large, volatile networks. The main routing algorithms in the 1990s were all dynamic routing algorithms, adapting to changes in the network environment by analyzing the received routing update information. If the information indicates that the network has changed, the routing software recalculates the route and sends out new routing update information. This information penetrates the network, prompting the router to recalculate and make corresponding changes to the routing table. Dynamic routing algorithms can be supplemented with static routing where appropriate. For example, the last alternative route (router of last resort), as the route of all non-routable packets, ensures that all data has at least a way to deal with it. (2) Single path and multipath Some complex routing protocols support multiple paths to the same destination. Unlike single-path algorithms, these multi-path algorithms allow data to be multiplexed on multiple lines. The advantages of multipath algorithms are obvious: they can provide better throughput and reliability. (3) Flat and layered Some routing protocols operate in a flat space, others have routing levels. In a flat routing system, each router is equal to all other routers; in a hierarchical routing system, some routers form a routing backbone, and data flows from non-backbone routers to the backbone router, and then transmits on the backbone until they are Reach the destination area, where they reach the destination from the last backbone router through one or more non-backbone routers. Routing systems are usually designed with groups of logical nodes, called domains, autonomous systems, or intervals. In a layered system, some routers can communicate with routers in other domains, while others can only communicate with routers in the domain. In a very large network, there may be other levels, and the highest-level routers constitute the routing backbone. The main advantage of hierarchical routing is that it mimics the structure of most companies, so it can support its communication well. Most network communication takes place in the group (domain). Because routers in the domain only need to know about other routers in the domain, their routing algorithms can be simplified, and the traffic for routing updates can be reduced accordingly according to the routing algorithm used. (4) Host intelligence and router intelligence Some routing algorithms assume that the source node determines the entire path, which is usually called source routing. In the source routing system, the router only acts as a storage and forwarding device and unconsciously sends the packet to the next hop. Other routing algorithms assume that the host knows nothing about the path. In these algorithms, the router decides the path through the network based on its own calculations. In the former system, the host has the intelligence to decide the route, while the latter has the ability for the router. The compromise between host intelligence and router intelligence is actually the balance between optimal routing and extra overhead. Host intelligent systems can usually choose a better path because they explore all possible paths before sending data, and then choose the best path based on the definition of "optimization" by a particular system. However, determining the behavior of all paths usually requires a lot of exploration traffic and a long time. (5) Intra-domain and inter-domain Some routing algorithms only work within the domain, while others work both within and between domains. The essence of these two algorithms is different. The reason it follows is that the optimized intra-domain routing algorithm does not have to be an optimized inter-domain routing algorithm. (6) Link state and distance vector Link state algorithms (also called short path first algorithms) distribute routing information to each node of the network, but each router only sends the routing table that describes its own link state. In the distance vector algorithm (also called Bellman-Ford algorithm), each router sends all or part of the routing table, but only to its neighbors. In other words, the link state algorithm sends less update information everywhere, while the distance vector algorithm only sends more update information to neighboring routers. Because link state algorithms aggregate faster, they have less tendency to generate routing loops than distance algorithms. On the other hand, link state algorithms require more CPU and memory resources, so link state algorithms are more expensive to implement and support. Although there are differences, these two algorithm types work well in most environments. 3. Routing metrics The routing table contains information used by the switching software to select the best path. But how is the routing table created? What is the nature of the information they contain? How does the routing algorithm use this information to decide which path is better? Routing algorithms use many different metrics to determine the best path. Complex routing algorithms can select routes based on multiple metrics and combine them into a composite metric. Commonly used metrics are as follows: Path length reliability delay bandwidth load communication cost Path length is the most commonly used routing metric. Some routing protocols allow network managers to manually assign value to each network link. In this case, the length of the route is the sum of the costs of the individual links. Other routing protocols define the number of hops, that is, the number of network products, such as routers, that a packet must pass on from the source to the destination. Reliability, in routing algorithms, refers to the dependence of network links (usually described in terms of bit error rate). Some network links may fail more than others. After a network fails, some network links may be easier or faster to repair than others . Any reliability factor can be calculated when assigning a reliability rate, usually the network manager assigns a metric value to the network link. Routing delay refers to the time it takes for a packet to travel from the source to the destination through the network. Many factors affect the delay, including the bandwidth of the intermediate network link, the port queue of each router that passes, the degree of congestion of all intermediate network links, and the physical distance. Because delay is a mixture of many important variables, it is a more commonly used and effective metric. Bandwidth refers to the available circulating capacity of the link. When all other conditions are equal, a 10 Mbps Ethernet link is preferable to a 64 kbps dedicated line. Although bandwidth is the maximum throughput that a link can achieve, routing through links with larger bandwidth is not necessarily better than routing through slower links. For example, if a fast link is busy, the packet may take longer to reach its destination. Load refers to network resources, such as how busy the router is. The load can be calculated in many ways, including CPU usage and the number of packets processed per second. Continuous monitoring of these parameters is itself very resource intensive. Communication cost is another important metric. Especially, some companies may have more operational costs than performance. Even if the line delay may be longer, they would rather send data over their own line than using expensive public lines. 4. Network Protocol The routed protocol (Routed Protocol) is transmitted by the routing protocol (RouTIng Protocol). These network protocols perform various functions required for communication between user applications of the source and destination devices, and these functions may vary greatly in different protocols. The network protocol takes place in the upper four layers of the OSI reference model: the transport layer, the session layer, the presentation layer, and the application layer. The terms routed protocol (routable protocol) and rouTIng protocol (routing protocol) are often confused. The routed protocol is routed in the network, such as IP, DECnet, AppleTalk, Novell NetWare, OSI, Banyan VINES, and Xerox Network System (XNS). The routing protocol is a protocol that implements routing algorithms. Simply put, it guides network protocols. Routing protocols such as IGRP, EIGRP, OSPF, EGP, BGP, IS-IS and RIP.
A rice cooker or rice steamer is an automated kitchen appliance
designed to boil or steam rice. It consists of a heat source, a cooking bowl,
and a thermostat. The thermostat measures the temperature of the cooking bowl
and controls the heat. Complex rice cookers may have many more sensors and
other components, and may be multipurpose. Cooking rice has traditionally
required constant attention to ensure the rice was cooked properly, and not
burnt. Electric rice cookers automate the process by mechanically or
electronically controlling heat and timing, thus freeing up a heating element
on the cooking range that had to be otherwise occupied for rice cooking.
Although the rice cooker does not necessarily speed up the cooking process,
with an electric rice cooker the cook's involvement in cooking rice is reduced
to simply measuring the rice, preparing the rice properly and using the correct
amount of water. Once the rice cooker is set to cook, the rice will be cooked
with no further attention.
Features:
For modern home
rice cookers, the smallest single-person model cooks 1 rice cup (180 ml),
whereas large models can cook 10 cups. Commercial models can cook 20 or more
cups. As a possible source of confusion, model specifications and names may
list either cooked or uncooked capacity. Rice roughly doubles in size during
cooking; therefore, a 10 cup (uncooked) rice cooker can produce up to 20 cups
of cooked rice. The prices vary greatly, depending on the capacity, features,
materials used, and the country of origin.
The majority of
modern electric rice cookers are equipped with a stay-warm or keep-warm feature,
which keeps the rice at an optimal temperature for serving without over-cooking
it. Some gas cookers also have electric stay-warm mechanism. However, the
usefulness of this feature degrades over time, a microwave may be more energy
efficient or better suited to reheat rice that will sit longer than four hours.
Some rice
cookers use induction heating, with one or more induction heaters directly
warming the pot. This can improve energy efficiency.
Most modern
rice cookers use aluminium for the inner cooking bowl. There are some models
that use stainless steel instead of aluminium. Various other materials, such as
copper, pure carbon, ceramic, and diamond powder coating, may be used for
higher heat conductivity or better taste.
The
pressure-cooking models can raise the water's boiling point higher, e.g., from
100 °C at 1.0 atm up to about 110 °C at 1.4 atm, which speeds cooking. The
pressure-cooking models can also be used in high altitude areas, where the
boiling temperature is below 100 Celsius. Pressure cookers are also suitable
for cooking brown rice (which contains oils and bran fiber that cook
differently from pure white rice starch). Some pressure rice cookers have a
varying pressure control mechanism (named the "dual-pressure" method)
that creates repeated pressure/release cycles during the cooking.
There also
exist mechanisms to collect and return the boiled over liquid to the inner rice
bowl.
Many cookers now have microprocessor-controlled cooking cycles, which are often used to adjust for rice and
cooking type.
Applications
Rice cookers are typically used for the preparation of plain or
lightly seasoned rice. Each rice cooker model may be optimized to cook a
certain type of rice best. For example, most Japanese rice cookers are
optimized for cooking Japanese rice and may not be the best for other types of
rice[citation needed], although cooking time can be lengthened simply by more
water.
The typical method of cooking long grain rice
is boil-and-strain and/or steaming method. The absorption method used in
Japanese rice cookers will produce slightly different texture and taste,
usually stickier rice.
Brown rice generally needs longer cooking
times than white rice, unless it is broken or flourblasted (which perforates
the bran).
Different varieties of rice need different
cooking times, depending on their grain size, grain shape, and grain
composition. There are three main types of Asian rice: Oryza sativa subsp.
indica, i.e., Indian rice (long grain rice, e.g., basmati rice and Thai jasmine
rice), O. sativa subsp. javanica, i.e., Java rice (large grain rice) and O.
sativa subsp. japonica, i.e., Japanese rice (medium grain rice, e.g., Calrose
rice, short grain rice, e.g., most Japanese rice and risotto rice).
African rice, Oryza glaberrima, is an entirely
separate species, but can be cooked in the same way. Zizania is not even in the
same genus, although it is often called a rice (or "water oats"); it,
too, can also be cooked in a rice cooker.
A
rice cooker can be used to cook many boiled or steamed granular foods, such as
pot barley, bulgar wheat, and dal. Provided the ingredients have similar
cooking times, a rice cooker can cook mixtures such as khichdi. Some rice
cookers can be used as automated couscoussiers, cooking couscous and a stew
simultaneously.
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