Router is set as a default router for both HostA and HostB. • HostA and HostB\'s

Question

Router is set as a default router for both HostA and HostB.

• HostA and HostB's ARP tables are cleared, the router ARP tables are not cleared (i.e., removing the need of two transmissions to router – no packet dropping).

• Then HostB pings HostA

a. Below find a list of packets that may or may not be transmitted, on each indicated LAN, when the HostB pings HostA. For each frame/packet, indicate if it was or was not transmitted on the indicated LAN (True or False).

b. For those that you determine were transmitted (i.e., TRUE), arrange in order of transmission, i.e., indicate the sequence of packet transmission events as they play out on the two LANs.

c. Fill in the fields of the various frames/packets transmitted, e.g., Ethernet source and destination addresses, IP source and destination addresses, etc. for the frames/packets that traverse the two LANs. (Use the following notation for your answer: HostA_eth0, or Router_eth1, HostA_IP, HostB_IP, etc., for broadcast use the hexadecimal notation). Assume the Router interface on LAN1 is eth0 and IP address is IP1 and on LAN2 it is eth1 and IP2

[Transmissions on LAN1]

ARP Request: sent? True or False

eth_src =

eth_dst =

sender_ip =

target ip =

ARP Reply: sent? True or False

eth_src =

eth_dst =

sender_ip =

target ip =

ICMP Echo Request: sent? True or False

eth_src =

eth_dest =

ip_src =

ip_dest =

ICMP Echo Reply: sent? True or False

eth_src =

eth_dest =

ip_src =

ip_dest =

[Transmissions on LAN2]

ARP Request: sent? True or False

eth_src =

eth_dst =

sender_ip =

target ip =

ARP Reply: sent? True or False

eth_src =

eth_dst =

sender_ip =

target ip =

ICMP Echo Request: sent? True or False

eth_src =

eth_dest =

ip_src =

ip_dest =

ICMP Echo Reply: sent? True or False

eth_src =

eth_dest =

ip_src =

ip_dest =

Explanation / Answer

First, lets get a bit around about some key terms and understand the working of the OSI Layer, ARP and ICMP and then we would apply that working to the required example above and solve it.

Now in the OSI layer we have different layers with different application . The Layer 2 (Data Link layer) is the layer to handle the MAC Address for the transmission while the Layer 3 (Network Layer) handles the tracking of the IP address for any packet. Now for any machine it has two main address as the Physical Address (MAC address) and the Logical Address (IP address). The MAC adderss is the one which never changes while the IP address do change.

1. ARP : The ARP stands for Address Resolution Protocol. It is basically mapping protocol. It basically maps the IP address with the MAC address at the different layers of the OSI. Now the ARP is basically mapping of the IP address to an MAC address for an transmission.

Now how does this ARP things works: Suppose you have two remote host connected with each other through the an gateway. The two host are unknown to each other and the gateway. Now suppose the one host wants to transmit the data to the 2nd host. The 1st host knows the IP address of the 2nd host but does not where it is located. So first the ARP process would actually check if there is any log entry of the 2nd host and its reachability. If the information is present with the 1st host then the data is sent to the 2nd host with the ip address of the 2nd host and the MAC address.

But if suppose the 1st host does not have an entry for the 2nd host, in that case the 1st host will broadcast for the required ip to all its neighbours to check if they have the required IP. In our case the two host are connected through an gateway, so the broadcast moves to the router. Now the process of the broadcasting is an recurrsive process. Now the gateway after receiving the broadcast checks the frame for information and finds that the packet from 1st host has an different IP than its. So, the process of broadcasting again is triggerd but this time from the gateway.

As the gateway is connected to the Host 2, the broadcast from gateway would this time go to the host 2. The Host 2 would decouple the packet and check if the IP address of the required target matches. This time it matches and thus the Host 2 receives the message. The Host 2 on receiving the message also makes an entry in the ARP cache which is an tracking inventory of the connected hosts and how to reach them. Now as the packet is received it has the entry of the source host and the gateway address through which it can accessed. Also the Gateway intermediate also make suck entries for their knowledge.

After receiving the message the Host 2 sends the confirmation reply which again travels back to the Host 1 and the same way thus creating and making all aware of an bidirectional route between the hosts and the gateways.

2. IMCP Message: This is actually the PING request which is basically used for sending the simple message along the network to check the connection.

Now the questin you had asked for about:
Transmission on LAN1

ARP Request: sent? True or False - True

eth_src = broadcast mac (in our case we would have the router mac address)

eth_dst = host1 mac

sender_ip = host2 ip

target ip = host1 ip

ARP Reply: sent? True or False - True

eth_src = host1 mac

eth_dst = broadcast mac

sender_ip = host1 ip

target ip = host2 ip

ICMP Echo Request: sent? True or False - False

ICMP Echo Reply: sent? True or False-False

[Transmissions on LAN2]

ARP Request: sent? True or False - True

eth_src =host2 mac

eth_dst =

broadcase mac

sender_ip = host2 ip

target ip = host1 ip

ARP Reply: sent? True or False - true

eth_src = broadcast ip

eth_dst = host2 ip

sender_ip =host 1 ip

target ip = host 2 ip

ICMP Echo Request: sent? True or False - True

eth_src =host 2 mac

eth_dest =host 1 mac

ip_src =host 2 ip

ip_dest = host 1 ip

ICMP Echo Reply: sent? True or False - true

eth_src = host 1mac

eth_dest = host 2 mac

ip_src = host1 ip

ip_dest = host2 ip

As discussed first, the gateway in this case is the router, so the request from host2 would first go the router, as both the hosts have their ARP cleared and have no knowledge of the nodes. the router has the details of the nodes so when it receives the broadcast it actually send the packet to the desired node on the basis of the ARP cache available.

Also,the follow of the message would depending on the OS and other factors but basically in the case where the ARP is cleared, the flow would be

1. Host 2 -> ARP Request
2. Router -> Receives the ARP Request and routes the packet to the host1 as the router has the required ARP available
3.Host1 -> ARP reply
4. Host 2- > IMCP Request
5. Host 1 -> IMCP Reply
6. Host 1 -> ARP Request
7. Host 2 -> ARP Reply

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