Research on the use and convergence of RIP Routing Protocol in dial-up networks

In the RIP routing protocol, slow convergence is a special feature. What slow convergence? Here we will introduce it in detail. In addition, we will explain how to use the RIP routing protocol for dial-up networks. I hope the summary in this article will be helpful for your learning.

Slow convergence

Split horizon update can be used to solve the slow convergence problem. when the range separation technology is used, the router records the interfaces that receive the routes. When the router advertises a route, it does not send the route back through that interface. in this example, vror2 R2 does not advertise the route from 1 to 2 to R1. Therefore, once the connection between R1 and network 1 fails, it will no longer advertise the route. after several rounds of Route Selection updates, all machines will know that network 1 is inaccessible. however, the split range update technology cannot solve all topology problems.

Another way to consider slow convergence is to use the concept of information flow. if the router advertises a short circuit route to a network, all the receiving routers will immediately install the route. when a router stops notifying a route, the protocol should work according to the timeout mechanism before determining that the route is not reachable. when a timeout occurs, the router looks for an alternative route and starts to spread this information. unfortunately, the router does not know whether the alternative route depends on the route that just disappears. therefore, we should not spread the negative information quickly. there is a warning sentence or a sentence:

Good news spread fast, and bad news spread slowly.

Another technique that solves the slow convergence problem uses the hold down method. the suppression method forces the router involved in the protocol to ignore any route information about the network within a fixed period of time after receiving the information about the network's inaccessibility. the typical suppression duration is 60 seconds. the idea of this technology is to wait for enough time to make sure that all machines receive bad messages and will not mistakenly accept outdated messages. it should be noted that all the machines involved in the RIP routing protocol must follow the suppression policy, otherwise the routing loop will still occur. the disadvantage of the suppression technology is that if a routing loop is found, the routing loop will be maintained during the suppression period. more seriously, all the incorrect routes are retained during the suppression period, even if there is an alternative route.

The last technique to solve the slow convergence problem is the toxic reversal of poison reverse ). when a connection disappears, the router retains the route for several update cycles. However, when a route is broadcast, the cost of the route is set to be infinite. to improve the efficiency of the toxicity reversal method, It should be combined with triggered updates. the update trigger technology reduces the time for errors due to messages of good news.

Unfortunately, although trigger update technology, toxicity reversal technology, inhibition technology, and split-range technology can solve some problems, they bring about some new problems. for example, when many routers share a public network structure and adopt the update trigger technology, a broadcast can change the router routing table and lead to a new broadcast. if the second round of broadcast changes the route table, it will cause more broadcasts. this produces a broadcast avalanche.

The use of broadcast technology may lead to a routing loop) and the use of suppression technology to prevent slow convergence, which can make the RIP routing protocol work very low on the wide area network. broadcasting consumes a lot of valuable bandwidth. even if no broadcast avalanche occurs, periodic broadcast of all machines means that network traffic increases as the number of routers increases. the possible routing loop may be fatal when the line capacity is limited. when the packets in circles make the capacity of the line saturated, it is very difficult or even impossible for the router to exchange some routing packets to break this loop. similarly, in the wide area network, the suppression period may be too long, so that the timer used by the high-level protocol times out and thus the connection is interrupted. despite these well-known problems, many organizations still use RIP as IGP on the wide area network.

In view of the characteristics of the dial-up network routing, it is impossible for us to regularly broadcast the routing on the dial-up network for 30 seconds, just like on the LAN and the leased line network, therefore, we cannot process the time-out of a route entry on the dial-up network like a route entry on the LAN.

Based on this consideration, in order to ensure the timeliness and feasibility of the route, we stipulate that the route modification packet on the dial-up network should be sent in the following situations:

1. When receiving a specific request for Route modification;

2. When the routing database is changed by the new route message received from another interface;

3. When you know that a sink is reachable;

4. When an interface goes DOWN from circuit to circuit UP

There is such a problem that the packets of the RIP routing protocol are sent through UDP port 520, while UDP is an unreliable transmission method, this clearly requires consideration of the possibility of packet transmission failure. to this end, we adopt a re-transmission and confirmation mechanism for message transmission during transmission. when a request message is sent or modified, if the sender fails to receive the confirmation message sent by the sender within five seconds, the sender resends the request message or confirmation message. if the confirmation message is not received 5 seconds after the resend, the resend continues until the sender receives the confirmation message. if 10 times after re-transmission is considered to have received the confirmation message from the sink, the transmission fails and the Metric value of the corresponding route is set to 16. when receiving a message, when receiving a request or modification message from the source, you must confirm the received message and send the corresponding confirmation message to the source.

If a route modification packet is received from the router of the next station on the dial-up network, the route entry in the modified packet will always be considered reachable until a route message fails to be sent.

The difference between the Dialing Network and the LAN and the leased line network for timeout processing is also reflected in the routing database. in a routing database, there are two types of routes: "temporary routing" and "permanent routing ". "Temporary routing" is applicable to lan routing and Wan leased line routing. this route is broadcast every 30 seconds, and timeout processing is performed after the route refresh packet is not received within a certain period of time. "permanent routing" refers to a route that includes a dial-up network. No timeout processing is performed if no refresh message is received, however, if a message fails to be sent to a sink, the route is changed to "temporary route ".