Basic understanding of network operation

Question

Assume two stations, A and B, are connected by two intermediate routers.

A----R1-----R2----B

Now, when A sends a packet to B, then the OSI model will come into the picture. Like, A's TCP layer will add A's port address (let's say x), and then the Network layer will set the Source IP address as A and Destination IP address as B. I understand till this point, but I have some confusion regarding the next process. I am listing the it below:

1. What will be the destination MAC address in the DLL header when A is transmitting? Will it be R1's address or B's address?
2. If A has sent a packet to R1, will R1 change network header here, or will it only read the destination IP address and forward to R2 as is without any modification?
3. At R1, the only sublayers of OSI that will be accessed are Physical layer, DLL, and Network layer only? Is this correct understanding?
4. Will R1 change the destination MAC address in the DLL, and set it to R2's address, or will it be B's MAC address in DLL throughout the communication?

I don't feel the need to separate this into four questions. Please let me know if this is not allowed here.

Answer 1

HostA will look at the destination network address and compare it to its own network. It will realize that the destination network is a different network, so it will create a frame to its configured gateway, which should be R1.

R1 will receive the frame from HostA, strip off the frame to get to the packet, inspect the packet's destination address, look up the destination address in its routing table to discover that it should send the packet out its interface toward R2. It will build a new frame for the interface to R2 and forward the new frame out that interface.

R2 will do the same thing that R1 does.

Also, understand that not every layer-2 protocol uses MAC addresses. There is a possibility that the link between the routers uses something like HDLC, PPP, frame relay, or another WAN protocol that doesn't use MAC addresses. The routers strip off the frames, so if there are MAC addresses in the frames, they are lost. The routers will build new frames for the new interfaces out which they will forward frames. The routers are not changing the MAC address on the packets, but they end up with new frames with new MAC addresses, or no MAC addresses at all, depending on the layer-2 protocols of the new interfaces.

The routers may change the packet header. For example, with IPv4, the TTL is decremented, and that requires that the Header Checksum be recalculated and replaced, too. IPv6 will decrement the Hop Count, but it has no checksum.

Answer 2

When A is sending packet to B

Source MAC address is = A device mac address

Destination mac address is =.

If destination is on same networks then destination mac address is destination mac address.

If destination on different networks then destination mac address is source device gateway mac address.