Some Thoughts on current network routing

During the May Day holiday, I drove to Zhejiang with my friends and found the drawback of the "destination-based shortest path algorithm". This algorithm may have caused Regular congestion of the expressway in a certain period of time!
Starting from Jiading District, g1501 was smooth, but once it was quickly transferred to g60 Shanghai Kun, it immediately became congested. In fact, as early as g1501, there was a bulletin board saying that Shanghai Kun had a rapid construction, however, we still moved to the Shanghai-Kunming fast, leaving S19/g15 as a discarded decoration... do you know why? Very easy, thanks to the recent Shanghai-kun fast journey! People believe in navigation too much, and few people have nothing to do with map research. So many people are on the map. Of course, this does not include me. A lot of navigation is based on the Dijkstra algorithm to calculate the shortest path. In addition to the distance weight, construction, congestion, and other factors will also be used as the weight, but this is not important, the most important thing is that the results of the Dijkstra algorithm will aggregate all the traffic that reaches or passes through the same point to a route. This is the root of the problem. Such an algorithm is inherently cohesive, this result puts a great deal of pressure on a route to reach a key node, which leads to abnormal congestion on certain expressways during holidays, however, there are many open routes that seem to have been forgotten. Not only is it a highway network, but it is also the same for a fast road network in a city. For several times, I did not go through the yan'an road tunnel to Pudong, but not from the North-South elevated to Central, saving time, the yan'an Road Tunnel is almost the shortest path from yan'an road to Lujiazui, while the North-South elevated road is also the shortest path from Huaihai Road to North Central Road.
By comparing the current three large networks, it is advantageous to understand the network technology. They are the expressway network, railway network, and IP network.
For a railway network, a train is guided in different directions based on how the railway tracks are forked. The turnout workers work like a routing protocol, but the railway network routes are planned in advance, that is, it is a combination of statistical multiplexing and time-sharing multiplexing. The railway network is characterized by the fact that "data packets" (locomotive or EMU) are not autonomous and rely entirely on the routes configured in advance. However, routes are dynamically adjusted. how to adjust the routes is accurate for a specific time, for example, the switch is switched to the left at 08:10 every day and reset at every day. The combination of time-sharing and statistical multiplexing is because there is no instantaneous burst traffic on the railway network, and all traffic is controllable. Even during the Spring Festival, a crowded train and an empty car are, in the railway view, it is a car.
Unlike the railway network, the expressway network is completely statistically reused. However, the "data packets" on the expressway network are self-guided and self-routed, that is to say, where to go is not controlled, No matter what third party, all the drivers themselves, just because the "Shortest Path" has been deeply rooted in the hearts of the people, so that all the drivers will go to the location where they will be blocked, in fact, it is possible to get to the destination more quickly by taking the time cost into consideration. As taxes and fees increase, drivers tend to avoid congestion unless they use a navigation that works based entirely on the shortest path algorithm. The route of the expressway network is actually a policy route, and the policy is completely determined by the driver (except during the control period ). It is such a combination of the shortest distance routing and Policy Routing that the expressway can often cope with traffic bursts. The only thing to note is that there should be no small accidents such as friction, real-time traffic information makes the expressway network look like a CSMA/CD/CA protocol.
An IP network is like a combination of a railway network and a highway network. How to route data packets from a route table completely dependent on a router is completely independent. An IP network is more like a railway network, but it is not as time-sharing as a railway network, in terms of multiplexing, similar to the highway network, it is completely statistically reused. There is no turnout behind the topology change mechanism, and the dynamic routing protocol will handle the topology change on its own. However, because data packets are not autonomous, this change in routing tends to direct traffic to another route, this is the feature of IP routing. congestion on the IP network is characterized by packet loss retransmission, and the network is more secure than a memory card ,... in the case of a large amount of uncertain traffic bursts, congestion is more serious. Therefore, the problem of the shortest path algorithm in high traffic is more serious than that in the expressway network. Fortunately, you can rely on Policy Routing and Static Routing to slightly change this situation.
Although it seems that all of these networks work well now, the routing of IP networks also needs to be changed based on the experience of the expressway network.
In the era when accessibility is a problem, there are not many paths, so there is not much traffic (of course, the number of paths and traffic are mutually reinforcing. On the way to Zhejiang, A friend said that it would take one day for him to remit a trip to Puxi from South China when he was a child ...), the overhead ratio of each path is relatively simple with a single unified measurement. Therefore, various measurement weighting operations are combined into a single measurement. In the era where accessibility is no longer a problem, path development is very large, and various traffic increases. Therefore, separate calculation of each measurement is more scientific than a single measurement, and each measurement has a minimum overhead, these measurements include path length, time overhead, packet loss rate (accident rate), congestion index, and monitored degree... our goal is to reach our destination. Faster is just one way. When time is not urgent, we can select a more comfortable path, that is, the optimal path in a measurement may be the sub-optimal path in a measurement. As mentioned above, when congestion on the shortest path becomes a cost, we should consider whether to use the shortest path but the X path to smooth the congestion cost. Blindly moving towards the shortest path is a paranoid behavior.
The preceding multi-metric separation idea has a premise that the number of lines and the topology should be ensured. Generally, no matter what network it is, a loop is usually required to guide the transit traffic, the central radiation line is used to distribute traffic in the core area. At the same time, it is necessary to traverse/traverse the line to quickly guide the traffic between one end and another end at the peripheral side of the core area. If this mode is used, china's IP network planning is indeed unreasonable. Many regions do not directly or intercommunicate with each other between core nodes. Therefore, the communication between the two nodes has to bypass the fourth or even nth side, which leads to loop congestion, chinanet does not have a line, except for its location, and there is very little radiation. This is very similar to Beijing's urban roads. Shanghai's Urban Expressway is more reasonable, and its radiation lines are generally not express roads, however, if the subway is added, the situation will be much better.
In addition to the algorithm of Selecting optimal routes for different measurements separately, the current routing protocol assumes that sufficient feedback mechanisms can alleviate many problems, for example, when a router finds its own transit traffic congestion, it actively sends an IGP link status update message to prevent additional traffic from flowing in again. Although this can be mitigated, there are still two problems, the first is to introduce traffic to another link, and the second is to cause excessive convergence. So we don't just need to declare link update when there is congestion. To address the second problem, we should introduce a "suggested update" in IGP, which I think is better, this is just to introduce a loop problem that will cut off the router introduced by those "do not accept suggestions. The reason why the expressway does not have a loop is because the driver's route is determined by himself, but there are still some new users or blind circles, or even several circles...
Comparing the IP network cabling topology of China and the United States, we will find many unreasonable places. From Beijing to Danqiao data center to New York, the number of outbound hops is much higher than the number of inbound hops, in turn, from New York to Beijing, the number of outbound hops is smaller than the number of inbound hops. For example, if you are in Hefei, Anhui province, you may have to go around the backbone network to Beijing during the inbound operation. If you think that New York is a bit inappropriate than Hefei, in this case, I used a line trial between Shanghai and valpalesau, Chile. The same number of hops in China is much larger than that in other countries, and the operator switched to as is quite powerful, most carriers need to carry a considerable amount of transit traffic, which indicates that the domestic IP network is still in the stage of a fairly 0-based National Highway, far from reaching the expressway stage, however, compared with the huge toll stations on the expressway, a data packet may not only go through a carrier network... in addition to the large ring, the local radiation interconnection is almost none, and there is no cross-direction or longitudinal line. This can only be blocked and inspected on the large ring line according to the shortest path solution. In addition, there are no other solutions, choose another line for separate measurement. Is this more convenient for auditing and more convenient for Wall efficiency? I don't know... of course, due to the loss of light speed in the media and the limitations of the media, the latency of the Cross-Pacific optical fiber cable is still greater than that of the middle, but this is not the reason for the disconnection, you know that you have traveled 100 kilometers in an hour on Shen Hai, and the effect is different from that when you opened 100 meters in Beijing's four-byte Ring Road for 10 minutes.
When I got home from work, I never took a path. Instead, I tried different routes cyclically in a few days. Later, my company moved, and I spent several days taking different routes to work, so I was late, early arrival... finally, I found five routes and used one of them separately under different circumstances, which can fix the commuting time at a certain time, So it rarely appears that "Today is too congested, so it's late. "," I didn't get on the bus today, so it's late. Of course, I usually like to see maps and road conditions when I am fine. This can also help me establish a policy route...
The first visible and useful large-scale network in the world is the ancient Rome road network. In those chaotic times, I thought it was the road network that maintained the external hegemony and internal peace of the Roman Empire, the decline of the Empire took 300 years. When the network was demolished, the Roman Empire actually did not exist... in the age of cold weapons, it took 300 years to tear down a network, showing how well the high availability of the network was designed. When the man-man broke through the Rhine Donau line of defense, the network topology was dynamically changed, and the Empire's defense was also changed from line-based defense to in-depth defense, forming a new line-based defense in the internal circle, when the Neman broke the inner circle again, the empire was split into two parts, and the network topology was dynamically updated, but each part was still smooth... is it worth learning from? In fact, the current Backbone Network Self-healing Ring is very similar to the road network in ancient Rome...