A verdade e a mentira sobre a teoria computador

Question 1 Answer the following using True/False. You do not need to explain your answers1. The UDP transport protocol provides reliable transfer over unreliable channels. FALSE2. IP fragments are re-assembled at the destination. TRUE3. The IPv6 address space is 1 billion times larger than the IPv4 space. FALSE4. The End-to-End argument argues for putting extra functionality in the network FALSE5. A bridge works at the Data Link layer of the protocol stack TRUE6. No collisions can occur when the CSMA/CD is used. FALSE7. Virtual Circuit Switching requires a circuit establishment phase before any data can be sent. TRUE8. The Internet checksum detects all two-bit errors. FALSE9. An Ethernet bridge requires setting up its forwarding table before it can forward any packets. FALSE10. In RIP each node broadcasts its routing table to all nodes in the network. FALSE

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Page 3 Question 2: Assume you wish to transfer an n-byte file along a path composed of the source, destination, six point-to-point links, and five switches. Suppose each link has a propagation delay of 2ms, bandwidth of 4Mbps, and that the switches support both circuit and packet switching. Thus you can either break the file up into 1-KB packets, or set up a circuit through the switches and send the file as one contiguous bit stream. Suppose that packets have 24 bytes of packet header information and 1000 bytes of payload, that store-and-forward packet processing at each switch incurs a 1-ms delay after the packet has been completely received, that packets may be sent continuously without waiting for acknowledgements, and that circuit setup requires a 1-KB message to make one round-trip on the path incurring a 1-ms delay at each switch after the message has been completely received. Assume switches introduce no delay to data traversing a circuit. You may also assume that file size is a multiple of 1000 bytes. (a)For what file size n bytes is the total number of bytes sent across the network less for circuits than for packets?Solution (10 points) The number of bytes sent in the VC case is B c = 2(p+h)+n Note that the first packet has to complete a round trip before the circuit can be established. The number of bytes sent in the packet switching case is: B p = n/1000*(p+h) Where p is the number of data in the packet and h is the size of the packet header. Wewant: B c < B p Solving for n we find: n > (p+h)*2000/(p+h-1000) (1) Replacing p with 1000 and h with 24 we get: n > 85,333,33 Therefore n = 86,000. (b)For what file size n bytes is the total latency incurred before the entire file arrives at the destination less for circuits than for packets? The time required to complete the data transfer in the packet switching case is: T p = T f + (c-1)*T x

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Page 4 Where T f is the time needed for the first packet to completely arrive at the destination, T x is the packet transmission time (p+h/b, b is the link bw) and c is the number of packets (n/1000). The intuition is that after the first packet is delivered at the destination one packet arrives for every transmission time. T f can be expressed as: T f = (s+1)* T x + (s+1)* T g + s* T s Where T g is the propagation delay, s is the number of switches, and T s is the switch processing time. The time T c for the VC case is: T c = 2* T f + 6* T g + c* T ’x Where T ’x is the packet transmission time for the VC case (p/b). The reason is that in the VC case, 2* T f seconds are required to establish the circuit, then 6*T g are required for the first data byte to arrive at the destination and finally for c*T ’x all the data to arrive. Wewant: T p > T c (2) By solving (2) for n, we find that n = 903,000 (c) How sensitive are those results to the number of switches along the path? To the bandwidth of the links? To the ratio of packet size to packet header size? Solution: (5 points) From (1) we can see that in (a) only the payload to header size ratio affects the number of bits sent. On the other hand, number of switches and link bandwidth are irrelevant (5 points) In similar way we can see from (2) that for (b) the link bandwidth, number of switches, and payload to header size ratio are all important. When the number of switches increases n increases. When the link bandwidth increases n also increases. Finally, when the size p decreases n decreases. Note: For 449 the final grade is computed by (a+b+c)*2/3

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Page 5 Question 3: For the network below, give global distance-vector tables for all nodes in the network when (a) Each node knows only the distance to its immediate neighbors. (b) Each node has reported the information it had in the preceding step to its immediate neighbors (c) Step (b) happens a second time