How does concept of "redirection" works in ICMP?

Question

I am trying to understand this figure, what it is trying to say?

What I already know?

Routing table is updated dynamically. However, for efficiency, hosts do not take part in the routing update process because there are terrible number of hosts. (Can you explain this with figure? What does it mean by host don't take part in routing update process?)

Hosts thus use static routing. Host usually know only 1 IP address of router, the default router. Thus host may send datagram to wrong router.

Solution is that router send redirection message back to host.

But how is this working in this figure?

Say at first,host A sends datagram to R1. Turns out it was the wrong router. So it sends redirection message to A. Now, A learns that it send message to wrong router.

Now what? A only knows about R1, it doesn't have information about R2, how will it send data to R2? I am really confused about this.

If R1 sends the message to R2, what is the point of R1 sending redirection message to A? I mean what difference would it make here if there was no redirection message?

Please clarify the doubts.

Answer 1

Now what? A only knows about R1, it doesn't have information about R2, how will it send data to R2?

R1 sends R2's IP address in the Redirect Message to A. That is the whole point of the Redirect Message: to tell the host where to redirect the datagram to.

This is all pretty well explained in RFC 792 Internet Control Message Protocol:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Type      |     Code      |          Checksum             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Gateway Internet Address                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Internet Header + 64 bits of Original Data Datagram      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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Description

The gateway sends a redirect message to a host in the following situation. A gateway, G1, receives an internet datagram from a host on a network to which the gateway is attached. The gateway, G1, checks its routing table and obtains the address of the next gateway, G2, on the route to the datagram's internet destination network, X. If G2 and the host identified by the internet source address of the datagram are on the same network, a redirect message is sent to the host. The redirect message advises the host to send its traffic for network X directly to gateway G2 as this is a shorter path to the destination. The gateway forwards the original datagram's data to its internet destination.