Explanation Of IP Packet Creation And Delivery Using NAT Router In An Essay.

Overall Explanation

Overall Explanations

Explained clearly outgoing packet from PC-1 to router, NAT router’s activities (address change in outgoing packet, router update, address change at incoming packet form destination, how router identify inside source)

Explained well but there is error in one of the aforementioned points

Average explanation, but there are errors two points as mentioned above

Mostly incorrect and have errors in three points as mentioned above

You have tried to answer but there are many errors, one point is mostly correct and rest of the points are incorrect (as mentioned above)

You have failed to explain the problem and/or did not attempt. Router entries

Correctly written router entry for PC-1 with all components in the entry (private, public and ports)

Address the following points in your explanation:

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.

Identification of network addresses (remote or local) :

o You have clearly explained the process and correctly shown calculation of network addresses (with result in decimal format).

o Your explanation is correct for one of the two.

o You have failed to computation or did not attempt.

IP Address of device :

o You have identified the device name and IP of that device and explained this device should be the destination for frame.

o Your explanation shows that you have done mistake in either the device name or IP of that device, and your explanation is partly incorrect.

o Your explanation shows that you have done mistake in both the device name and IP of that device, and your explanation is mostly incorrect.

o You have failed to explain the problem and/or did not attempt.

ICMP packet delivery :

o You explained correctly the steps of frame delivery to next device’s IP protocol, decapsulation, routing decisions, and encapsulation and rest of the steps of delivery of ICMP packet to PC-B (decapsulation steps, indication of higher layer protocol at IP).

o Your explanation is mostly correct but one steps has incorrect explanation/missing one step mentioned in the above.

o You explained well however you have errors in any two steps mentioned in the above.

o Your explanation is average and have errors in any three steps mentioned in the above.

o Your explanation is largely incorrect and have four incorrect steps mentioned in the above.

o Your explanation is mostly incorrect, have some correct aspect in one of the above steps.

o You have failed to explain and/or did not attempt.

NAT is a property in a router which allow clients to access services through public IP address in a network. This eliminates the need of having a separate domain name for a network

If a packet is sent from PC-1 (192.168.10.7/24) connected to the switch through port 1020, the packet will be routed to the default gateway (192.168.10.1/24) of the router. A router with a NAT loopback feature detects the incoming packet as 209.165.200.226/27 and is the interface of its WAN network. It determines the destination of the packet depending on the port forwarding rule. Data is then directed to the host address remote server that receives the packets. If the DNAT rule is not applicable the router will drop the packet. An ICMP destination unreachable is replied. If the rule is still there then the router rewrites the source IP address. The client (192.168.10.7/24) sends the packets as incoming from (192.168.10.1/24) then the server receives the packet as incoming from 209.165.200.226/27. When the request is replied by the server the process is repeated. This process is identical for external senders. Thus this is a two way communication.

Source and destination addresses

Source address:   192.168.10.7/24

Destination address:       209.165.200.226/27

First PC-A (200.10.5.70/28) initiates the session by sending a broadcast ARP packet. The address resolution packet is used in discovering the layer two address that is linked to layer three addresses in the router. The broadcast then instructs the router to send the packet to all ports that are connected to it. Computer PC-B (200.10.4.59/27) within layer three then responds with a unicast ARP reply. The results are captured so that they can be looked up later. This makes the ARP not to be sent frequently. This creates an ICMP request that is crafted. The source PC-A (200.10.5.70/28) address will be originating computers’ ether address. The destination addresses in the router and PC-B (200.10.4.59/27) will then be revealed from ARP. The packet is then sent to the router through a cable Fa0/0 (200.10.5.70/28). The router then examines the destination address of PC-B (200.10.4.59/27) and looks for it from the CAM table, this specifies the physical port in the ether address. The packet is then transmitted through the wire to the correct physical ports. When PC-B (200.10.4.59/27) computer receives the packet, it first examines the address and makes sure that it is similar as that one in the physical interface. If the address is similar it then checks if PC-B (200.10.4.59/27) matches with the address in layer 3 of the router as assigned in the physical interface. If the packet matches, it is handled by upper layer protocols (Pigeon, 2014) and if there is no match the packet is then dropped. For the ICMP reply, things work in the same way, except that the responding computer doesn't need to send an ARP request. The source computer receives reply from the connected computer if the pinging process is successful.

When PC-A (200.10.5.70/28) sends packet with the local network, PC-B (200.10.4.59/27) listens to the incoming signal MAC address. If there is no answer within the network then the packet is directed to the default gateway and configured in IP stack of the sending host. If the destination address has not found any MAC address then the network is remote. If an address exits then the network is local.

Router Entries

The source machine MAC address is used in encapsulation. This is because at the source machine, the domain name is converted into IP number. The target IP number is loaded in the packet from the routing table. Then the MAC address of the next router is obtained through the Arp request. The frame is then moved to the MAC address. This process continues until the last router gets data. The router then identifies the address of the destination host. The last router of the network is the one that knows the MAC address of the target machine.

Once the last router finds the MAC address of the destination computer, it determines the shortest distance that it will use to send data to the destination host. The router then uses the public IP address to transmit data to the host computer.

Routing protocols

• Routing information protocol  (RIP)
• Open Shortest Path First (OSPF)

RIP - this dynamic routing protocol adds policies to the system and selects the best route in the network for packets to flow in. The routing protocol employs use of hop-count metric. This prevents routing loops during packet transmission. It also implements route poisoning. This prevents packets from being transmitted in incorrect paths (Thorenoor, 2010). The tables are also updated after thirty seconds. This improves the working speed of the router. It is most perfect because it can easily be configured

OSPF - This tests status of the links of its neighbors before it sends the packets. It stabilizes the networked before it is used. It uses the IP addresses directly, does not rely on the TCP. It also calculates the shortest route to be used in the network basing on the cost of the route, not living behind bandwidth. It is a more reliable routing protocol that can easily be implemented (Black, 2013).

BGP - it uses the TCP protocol while the network requires usage of IP protocols. It is mostly used in autonomous system, this makes it unfavorable to be used in the network. The routing protocol also does not allow automatic configuration of the routers thus they are configured manually

RIP is set to use two IP addresses with 32 bit while OSPF uses routes that are based on the telnet.  OSPF uses multicasting while RIP uses commands in the transfer of network signals. BGP uses TCP to transfer the signals while OSPF and RIP uses IP address for communication between the routers. This makes OSPF and RIP suitable for the network

Question 4:

As in the above figure, selective acknowledgement will use ACK values in TCP to show packet loss. The packets that are lost during transmission are shown by suck TCP heads. Typically, duplicate acknowledgements means one or more packets have been lost during the communication process. They always show the symptoms of packet loss. Mostly when three duplicate acknowledgements are received by the receiver, it retransmit the missing packet before the timer expires (Campbell, 2014). This can be called faster transmission. Several duplicate acknowledgment are used for single packet.

References

Black, U.D., 2013. IP routing protocols: RIP, OSPF, BGP, PNNI, and Cisco routing protocols. Prentice Hall Professional.

Campbell, M.J., Finn, D.J., Tucker, G.K., Vahey, M.D. and Vedder, R.W., Hughes Aircraft Co, 2014. Data-flow multiprocessor architecture with three dimensional multistage interconnection network for efficient signal and data processing. U.S. Patent 5,021,947.

Pigeon, N., Sagem Communications SAS, 2014. Method and device for allocating MAC addresses in a carrier-current communication network. U.S. Patent 8,842,697.

Thorenoor, S.G., 2010, April. Dynamic routing protocol implementation decision between EIGRP, OSPF and RIP based on technical background using OPNET modeler. In Computer and Network Technology (ICCNT), 2010 Second International Conference on (pp. 191-195). IEEE.