Question

When it comes to packets the value in the “8-bit Hop Limit” field of the packet header decrements by one each time it is forwarded by a network node. The IPv6 packet is discarded if the value in this field reaches 0. The Hop Limit field can accommodate a maximum value of 255, which also makes this the maximum possible number of hops. What were to happen if there were no hop limits put on a packet? What does your research show you is the average amount of hops a packet usually takes across the internet to its destination? Why does a series of routers in one network not surge the hop count past 255 on the way to its destination?

Answer 1

Since you already know how the Hop limit works, and that if the hop limit field is less than or equal to 0, it is discarded.

Now, coming directly to the question.

Q1) What were to happen if there were no hop limits put on a packet?

There is a fair chance that due to congestion in a network, the data packet may take on different routes to reach to the destination. Now, if the network is really congested, and there is no hop limit, the packets may keep on travelling in the network indefinitely (or an infinite loop). Now, suppose another packet is sent and that too keeps on looping in the network because of the congestion. Then another one and so on. Do you see the chain effect? As these packets keep on looping in the network, the congestion doesn't seem to go away, but keeps on increasing. Thus, as a way of congestion control, there is a hop limit so that in case a packet ends up in a loop, instead of allowing it to add on to the network traffic, it's better to drop it.

Q2) What does your research show you is the average amount of hops a packet usually takes across the internet to its destination?

You can do a practical exercise if you're on a windows machine by opening you command prompt and typing "tracert www.somewebsite.com" //in place of some website, try a govt website of a country which is exactly half way across the earth from your country

Here, you would notice the average hop is less than 50. When the IP standards were being designed, the designers thought (using their own analysis method) that 255 would be sufficient number of upper bound hops for packet to reach across the globe, and with the above command, you can see it to be true. For instance, a tracert between USA and India takes 8 hops.

Q3) Why does a series of routers in one network not surge the hop count past 255 on the way to its destination?

Well, technically, the series of routers between a source and the destination may go way above 255 (in case of congestion or bad routing algorithm). But as the Internet is growing, more long-distance links are being built (or already have been built), making it possible for a packet to get from one country to another in less than a few hops, if not, then the infrastructure is not right. Though this assumption was controversial, it was accepted. The routing algorithms are able to find short paths between destination and source.