Packet Sent And Received Through NAT Router - An Essay On Networking.

Your company has recently been expanded and currently 40 users of your company need simultaneous access to the internet. However company has only one public IP address. The network diagram of the company is presented below. Consider an example situations when three computers in switch-1 are communicating with remote server at the same time using one public IP. Assume PC-1 under switch-1 is using TCP port (1020) for this example. The public IP is assigned to the border router’s (NAT router) serial interface. Explain (for PC-1) how packet is sent to and received from the destination remote server by PC-1 through the steps: 1. source and destination addressesin outgoing packetsfrom PC-1; 2. how router keeps track of incoming packet from remote server and forwards to correct sources with private IP inside the company host by using router’s entries. 

1) How IP packet is created from ICMP packet at PC-A, i.e., describe the encapsulation of the ICMP payload into an IP packet (indicating values of relevant IP header fields).
2) How IP at PC-A determines whether PC-B is a local or remote destination including any calculations performed.
3) Once the location is determined in (2), a destination MAC address is required to prepare a frame. If PC-B is in remote, which device’s MAC address will be used for encapsulation of IP packet into a frame; mention the IP of that
devices? Explain why this device.
4) Once the frame is prepared and given to the physical layer of PC-A, list and briefly describe the remaining steps to deliver the frame to next device’s IP protocol, including decapsulation, routing decisions, and encapsulation, until the ICMP payload can be processed at PC-B. 

How packet is sent to and received from the destination remote server by PC-1

Solution

Activities taking place at PC-1

1. The message will start at the top of the protocol stack at PC-1 and it finds its way downwards.
2. The packet will go through application and proceed to TCP layer. TCP layer will assign the packet a port number. The assignment is key because a lot of programs are using TCP stack while sending messages, it is important to assign a port number to enable us know which program’s port should be listening to the specified port. For example, an outgoing packet is assigned a port number of 1020
3. After the packet goes through the TCP layer, it proceeds to IP layer where it is assigned the destination address, for instance a packet leaving PC-1 destine to remote server, will have destination address of the remote server.  Given that our packet has a destination address and a port number, the packet is ready to be sent over internet.  The physical layer turns our packets into electronic signals and transmits them over the WAN link.
4. On the opposite end of the WAN link has a direct connection to internet. Internet Service Provider’s router examine destination address by determining where to send it.
5. Finally, the packet arrives at remote server. The packets are received at the bottom of the remote server’s TCP stack and walks through upwards.
6. While the packet goes up, all the routing information that was added PC-1’s stack is striped off.
7. Once the packet arrives at the top of TCP stack, it has assembled in the original message, this is explained by Kocharians and Vinson (Kocharians & Vinson, 2014).

How router keeps track of incoming and outgoing traffic

Once a router receives a packet from the switch, it will strip off Layer 2 header information (ie MAC address) present on packet and it will search the destination IP address in from the packet. It will then find the route for the destination prefix, in case it matches, it will attach its exit inter MAC address to appear as source address and connected computing device as MAC address as destination MAC address. The packet will be forwarded to the exit interface. In case no route will be found within the routing table, that packet will be dropped.

Routing table for our example will look as below

1. PC-1 pings 200.10.4.5.59
2. Internet Protocol works with ARP to decide which network the ping is targeted by searching the IP address and issue mask of PC-A. Packet is sent to the router to be routed to the correct remote node network.
3. Since PC-A has send packet to the router, it is required to know MAC address of router’s interface that connects to its network.
4. Router will identify that IP address, it has to reply. It sends back to PC-A a reply. It costs the ARP some time to dispatch the information and query the device to respond. At times, time to live happens to disappear implying that ping has expired.
5. The router will respond with its MAC address of the interface that connects to the network segment. PC-A will have the whole thing        it requires so as to transmit a packet exiting native router. Network layer hands over to the datalink layer downwards. A packet is created with ICMP request. The packet will include destination and source Internet Protocol address together with ping ECHO request.
6. The datalink layer of PC-A will create a frame. The frame contents will be destination and source MAC addresses and type that will be specifying network layer protocol. CRC will be attached at the end of the frame to make sure the remote receiving node will drop out to if the frame is tarnished.
7. PC-1’s datalink layer hands over to Physical layer. Physical layer encodes the frame to 0s and 1s to a digital signal. Digital signal leaves out of local physical layer.
8. The router’s receives the frame. It does CRC check and finalizes it with a CRC value PC-A included to that frame to ensure the frame is not corrupted.
9. Here and now destination MAC address of received frame has been confirmed. Router will disseminate the packet to IP protocol. PC-A is kept in router’s memory buffer.
10. Internet Protocol searches the packet destination Internet Protocol address to decide if the packet belongs to the router.
11. Router requires to build a frame and it dispatches it to destination i.e. PC-B.
12. PC-B will respond with MAC address of its NIC together with ARP reply. The router has all it requires to reply.
13. PC-B receives the frame. It performs CRC check. Internet Protocol checks destination of Internet Protocol address. In case it ties with PC-B’s, the protocol column of the packet will decide on the course of the packet.
14. PC-B will generate a new ping echo reply packet. The echo will have destination Internet Protocol of PC-A and its IP address as the source address. The packet protocol begins all again the journey in the opposite direction.
15. RIP and EIGRP, Ospf
16. RIPv1 and IGRP
17. Explanation: rip and eigrp, ospf, uses classless addresses during the configuration of the network address. Classless routing send subnet mask with their updates. Hence, allowing VLSM masks. In our network we have links with class A and B network but however, the mask is /24. In case a classful routing protocol is used these networks will take their default subnet masks i.e. for 172.16.0.0 will automatically pick /16 whereas 10.0.0.0 will pick /8 subnet mask. This implies that wrong information will be routed. When using classful routing protocol, subnet mask should always remain consistent in the entire network.
18. Network addresses will be 10.10.10.0 for the WAN interfaces and 172.16.10.0 fa0/0 interface on the router on the left and 172.16.20.0 on the router on the right.
19. Routing tables across the network require to converge in minimum time to avoid traffic loss, in our above mentioned protocols, OSPF converges faster than any other routing protocol. This is attributed to the small sub-divisions whose OSPF is built on.  
20. exchange of data and acknowledgement

Stop and wait together with Automatic repeat request is a control technique incorporated to stop and wait flow control protocol. In case an error is noticed by recipient, it drops and sends a NAK requesting the sender to resend. If the frame never arrives at the recipient, the sender has a timer. Every time the frame is dispatched, clock timer is set, in case there is no acknowledgement or negative acknowledgement is received by sender during timeout span, the frame is dispatched again by sender.

Of course timeout may introduce a problem. For instance, sender does a retransmission but the recipient really received previous transmission. This implies that the recipient has same copies. In avoidance of this, acknowledgements and frames are categorized as 1 or 0 for ACK1 for frame 0 and ACK0 for frame 1.