Csci4211 Introduction To Computer Networks Fall 2017 Due Thu

Csci4211 Introduction To Computer Networksfall 2017due Thursday Nov

Csci4211 Introduction To Computer Networksfall 2017due Thursday Nov

Analyze a network scenario involving MAC addressing, ARP, switching, IP forwarding, virtual circuits, distance vector routing, and ICMP Redirect messages. Address specific questions about how host X communicates with hosts Z, H, and a remote server W, including ARP request handling, switch table updates, and router forwarding. Also, examine the setup of virtual circuits between various hosts and routers, explain the difference between virtual circuit and circuit switching, and analyze routing issues like the count-to-infinity and poisoned reverse problems. Additionally, analyze scenarios involving IP forwarding, default routers, and ICMP Redirect messages, especially focusing on how routers and hosts handle forwarding decisions, ARP resolution, and potential routing inefficiencies.

Paper For Above instruction

In modern computer networks, understanding the interplay between hardware addressing mechanisms, such as MAC addresses and protocols like ARP, and network-layer forwarding strategies is essential. This paper explores a typical switched network scenario with integrated routing and switching components, focusing on the processes involved when hosts communicate within and outside their local network. We analyze the ARP process, MAC address learning by switches, and how IP packets are forwarded based on ARP cache entries and switch tables. Furthermore, we delve into the distinctions between virtual circuit switching and traditional circuit switching, highlighting their operational differences and use cases.

ARP and Switching in a Local Network

When host X intends to send an IP datagram to host Z, it first needs to resolve the MAC address corresponding to Z's IP address, 128.101.1.x. Assuming host X has the IP address of Z via DNS, but not the MAC address, it broadcasts an ARP request onto the network, asking "Who has IP 128.101.1.x? Tell me." Switch S, which manages local forwarding, receives the broadcast and floods it to all ports, except the port on which it was received. When host Z recognizes its IP in the ARP request, it responds unicast with its MAC address. This ARP response is sent directly back to host X, updating X's ARP cache with Z's MAC address.

ARP Request Handling by Switch and Router

The switch, real-time, learns the MAC address associated with the source IP from the frame's source MAC address and updates its forwarding table accordingly. When ARP requests are broadcast, switches do not process them beyond flooding, as ARP frames are broadcast at the data link layer. If host Z responds, the ARP reply is unicast, and the switch updates its MAC table with the new mapping of port to MAC address. Regarding the router R, the ARP request for Z's IP may reach R if the MAC entry for Z is not yet in its ARP cache or if traffic needs to be routed; R, if it receives the broadcast, may do nothing unless it needs the MAC for forwarding.

Delivering IP Datagrams Within the Local Network

Once host X learns Z's MAC address, it encapsulates the IP datagram within an Ethernet frame with source MAC X and destination MAC Z. When host X sends the frame, switch S forwards it based on its MAC table, delivering it directly to host Z's port. Rerouting does not occur at the network layer for local delivery—the switch handles this. If R is involved—for example, if the initial packet is destined for a different network or the MAC address is unknown—the packet is forwarded via R based on its routing table. In such cases, R examines the IP header, determines that the destination is within its subnet, and either delivers it locally or forwards it based on its own ARP cache and forwarding logic.

Communication with a Remote Server Outside the Local Network

If host X needs to contact server W with IP 72.14.204.104 outside the local network, it first consults its ARP cache. Since W's MAC address is not known, X sends an ARP request broadcasting "Who has 72.14.204.104?" Once the ARP reply arrives with W's MAC address, X updates its ARP cache. Host X recognizes that W is outside the local network, so it forwards the IP datagram to its default gateway router R. The Ethernet frame encapsulating the IP packet has source MAC of X and destination MAC of R's interface. At the network layer, the IP header contains source IP of X and destination IP of W. R then routes the packet towards W via the Internet, encapsulating the IP packet within subsequent Ethernet frames at each hop, updating MAC addresses accordingly.

Differences Between Virtual Circuit and Circuit Switching

Virtual circuit switching establishes a logical connection between hosts over a shared physical network, with dedicated virtual paths, whereas circuit switching assigns a dedicated physical path for the duration of the communication. Virtual circuits are more flexible and efficient for packet-switched networks as they reuse the same path dynamically across sessions, while circuit switching is characteristic of traditional telephony where a fixed physical circuit is established and maintained for the entire session.

Virtual Circuit Translation Tables

In the network depicted, establishing virtual circuits involves assigning VCI numbers sequentially and propagating translation tables at each router. For example, when Host A connects to host K, each router along the path updates its translation table by associating the local VCI with the remote VCI. As subsequent connections are established (B to J, B to D, D to F, E to L), routers maintain a table mapping incoming VCI to outgoing VCI, ensuring correct packet forwarding within the virtual circuit. These tables are constructed dynamically, ensuring efficient label switching and data flow.

Distance Vector Routing and its Challenges

The count-to-infinity problem occurs in distance vector routing when routers update their routing tables based on outdated or incorrect information, causing routing loops and incrementally increasing hop counts. Poisoned reverse is a technique used to mitigate this issue by advertising a route as unreachable (infinite metric) to the neighbor that provided the bad route information. However, this method can be circumvented or may not fully prevent looping, especially in complex networks with multiple pathways, thus requiring additional techniques like split horizon or hold-down timers.

IP Forwarding and ICMP Redirects

In the scenario where host A sends a packet to host D across LANs, router R1 receives the packet, examines its forwarding table, and forwards it either directly or via R2, depending on network topology. If R1 determines a better route to D through R2, it might send an ICMP Redirect message to host A, informing it to send future packets directly to R2's MAC address. Host A, which lacks R2’s MAC, resolves it via ARP. When host A sends subsequent packets, it consults its routing table, which may be updated upon receiving redirect messages, optimizing the routing path. R2, upon receiving packets from R1, forwards them toward D, encapsulated in Ethernet frames with appropriate MAC and IP addresses, ensuring proper delivery from source to destination across multiple hops.

Routing Inefficiencies and Network Dynamics

Routing entries can become suboptimal due to outdated topology information, slow convergence, or failure to receive updated routing advertisements. For example, R1 initially routing traffic via R2 instead of R3 may be caused by delayed updates, incomplete information, or network failures. When R2 detects that R3 offers a better route, R1 can learn this through ICMP Redirects or routing protocol updates, leading to potential route optimization over time. Dynamic routing protocols like OSPF or EIGRP facilitate such updates, but network conditions and protocol configurations influence the effectiveness of route selection, which can impact overall network performance and latency.

Conclusion

Effective network communication relies on intricate interactions involving ARP, MAC address learning, switching, IP forwarding, and routing protocols. Understanding how each component adapts, learns, and updates allows network administrators to troubleshoot and optimize network performance. Techniques like ICMP Redirects and poisoned reverse serve specific purposes but require careful configuration and awareness to prevent routing issues such as looping or suboptimal routing choices. Continuous advancements in routing algorithms and switching technology contribute to more efficient and resilient networks, underpinning the modern Internet infrastructure.

References

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