CSCI 4211: Introduction To Computer Networks Fall 2017 Due

CSCI4211: Introduction to Computer Networks Fall 2017 Due Thursday

Consider the following switched network (Figure 1), where we have one Ethernet switch S, connecting three hosts: host Z on port 1, host X on port 2, and host H on port 3, as well as an IP router R (default router for the hosts) on port 0. These hosts lie on an IP network with the network prefix 128.101.1.0/24.

The IP address for the interface of router R that is connected to switch S is 128.101.1.254. The IP addresses for hosts Z, X, and H are shown in the figure. Additionally, the current switch (forwarding) table at switch S and the ARP caches at host X and router R are provided. Answer the following questions based on this network setup.

Paper For Above instruction

(a) Host X sending an IP datagram to host Z

When host X intends to send an IP datagram to host Z, it first needs to determine the MAC address associated with host Z’s IP address (128.101.1.10). Host X performs an ARP (Address Resolution Protocol) request broadcast within the local network, asking, “Who has IP 128.101.1.10? Tell me your MAC address.” Since host Z recognizes its IP in the request, it responds with an ARP reply containing its MAC address, say MAC_Z.

Switch S, which maintains a forwarding table, handles the ARP request by flooding it to all ports unless it already has an entry for the destination MAC address. When host Z replies, switch S updates its forwarding table, associating host Z’s MAC address with the port connected to host Z (port 1), ensuring subsequent packets to host Z are sent directly without flooding.

Router R will also receive the ARP request if it is within the broadcast domain. Since router R’s interface IP (128.101.1.254) falls within the local subnet, R’s ARP cache is checked. If R’s cache does not have the MAC address of host Z, R processes the ARP request but typically does not respond unless its IP matches the queried IP. When the ARP reply from host Z is received, router R updates its ARP cache accordingly.

Once host X learns host Z’s MAC address, it constructs an Ethernet frame encapsulating the IP datagram, with source MAC as host X’s MAC and destination MAC as host Z’s MAC. Host X sends this frame to switch S, which forwards it to host Z based on its MAC address table. If any routing or intermediate devices needed to process the packet, they would act according to their forwarding tables, but since both hosts are on the same local network, the datagram is delivered directly after MAC address resolution.

(b) Host X sending an IP datagram to host H

Replicating the above process, host X initiates an ARP request to find host H’s MAC address if it is not already cached. The ARP request is broadcast on the local network. Switch S flooding is handled similarly, updating its forwarding table when host H responds with its MAC address. The ARP reply from host H updates switch S’s table accordingly.

Once host X knows host H’s MAC address, it creates an Ethernet frame with source MAC as host X’s MAC and destination MAC as host H’s MAC. The frame is forwarded via switch S directly to host H. Since host H is on the same subnet, no routing is needed in this case.

The key difference compared to part (a) is the destination IP and MAC addresses corresponding to host H. The process illustrates how ARP resolution and switch forwarding work collaboratively to deliver IP datagrams within a local network.

(c) Host X sending an IP datagram to an external server W with IP 72.14.204.104

Initially, host X checks its ARP cache for server W’s MAC address. Given that server W’s MAC is not present, host X issues an ARP request broadcast seeking the MAC address corresponding to IP 72.14.204.104. The switch flood propagates this request, and since server W resides outside the local network (not in 128.101.1.0/24), the ARP response does not arrive directly from server W.

Host X recognizes that the destination IP address is outside its local subnet. Therefore, it sends the IP datagram to its default gateway, router R, with the Ethernet frame’s destination MAC as R’s MAC address (128.101.1.254). Within the IP datagram, the source IP is host X’s IP, and the destination IP is 72.14.204.104, indicating the packet is destined for the outside server.

The Ethernet frame encapsulating the IP datagram has source MAC as host X’s MAC and destination MAC as router R’s MAC. Router R, upon receiving the frame, checks its routing table, determines the next hop towards the IP network of server W, encapsulates the datagram in a new Ethernet frame with its interface MAC as source and the MAC of the next-hop router or server W as destination. This forwarding continues until the packet reaches server W.

This process demonstrates how host X utilizes ARP for local MAC address resolution, and how default routers facilitate delivery of remote IP datagrams through hierarchical addressing and forwarding.

Additional context from Figures and descriptions

The scenario provides insight into how ARP, MAC addressing, and switching work together within local networks and how IP forwarding determines the routing of datagrams destined outside the local subnet. The switch’s forwarding table evolution, ARP cache management at hosts and routers, and IP routing mechanisms collectively enable efficient data transfer across local and wide-area networks.

References

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