Computer Network Exam Questions and Answers

Question 1a: Connection-oriented vs. connectionless network architecture

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1. a) A computer network can be designed following a connection Oriented architecture or a connectionless network architecture. Suppose that in the network layer, routers are subjected to "stressful" conditions that might cause them to fail fairly often. Explain the actions that would need to be taken on such router failure at a high level, and which architecture better suits such a network environment

b) Traditional IP network routing is time consuming. When managing an IP network, for every packet at each routing node, it incurs four delays – queuing delay, processing delay, transmission delay and propagation delay. Of the four delays, explain which two delays are hard to reduce and provide a mechanism that can possibly reduce these two delays.

2. End-host A communicates with end-host B in a 100Mbps network. Assume the shortest path between A and B has three Multi-Protocol Label Switching (MPLS) routers and the minimum delay to establish a path is 0.15ms. Each packet has 512 bytes payload and 64 bytes header. Assume there is no propagation delay from A to B. The minimum delay to label the shortest path from A to B is 0.2ms. The delay to add a 32 bits label to a packet and check a packet label is 0.01ms. Calculate the minimum delay for transferring the 512KB file from A to B using the MPLS routers.

3. When designing a computer network in support of one-many communication scenarios, IP multicasting is normally applied which can be implemented with unicast emulation or network-layer multicast scheme. Consider a single sender and 64 receivers. Suppose the sender is connected to the receivers through a binary tree of routers. Each time a packet (or copy of a packet) is sent over a single link, it incurs a unit of "cost".

a) Calculate the cost of sending a multicast packet in the case of unicast emulation and network-layer multicast for this topology respectively.

b) Assess the complexities that IP multicast routing add to IP unicast routing and the role that Reverse Path Forwarding (RPF) plays in IP multicast routing.

4. Suppose that two Ethernet networks are interconnected through ATM network using interconnecting devices as shown in Figure Q4-1. The interconnecting device ID uses AAL5 in order to encapsulate Ethernet frames. The format of Ethernet frame is shown in and the format of Adaptation Layer 5 packet (PDU, protocol data unit) is given Assume that a TCP application running on the host H1 sends two characters CR (carriage return) and LF (line feed) to a TCP application running on the host.

Calculate the number of ATM cells needed to transfer the TCP segment carrying these two characters between H1 and H2. Assume that the TCP header size is 20 bytes and the IP header size is 20 bytes. Determine the length of the padding in the Ethernet frame and the padding length in the AAL5 PDU.