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Assignment #4 AIT 622 – Summer 2008
Due: Tuesday, July 1 1. Select a subnet mask for so that there will be at least 16,000 subnets with at least 700 host addresses available on each subnet. Proposed Mask 11111111 11111111 11111100 00000000 Subnets: 214 - 2 = 16,382 Hosts: 210 -2 = 1,022 2. Select a subnet mask for so that there are at least 500 subnets with at least 100 host addresses per subnet. Proposed Mask 11111111 11111111 11111111 10000000 Subnets: 29 - 2 = 510 Hosts: 27 -2 = 126 3. How many subnets are available if a class C address has six bits of subnetting? How many host addresses are available per subnet? Mask 11111111 11111111 11111111 11111100 Subnets: 26 -2 = 62 Hosts: 22 -2 = 2 4. Suppose a 28-bit mask is used to subnet How many subnets are available? Give the mask and list the subnet addresses in binary and dotted-quad notation. Class C address 11111111 11111111 11111111 11110000 Subnets: 24 -2 = 14 Hosts: 24 -2 = 14 5. Suppose a 20-bit mask is used to subnet How many subnets are available? Give the mask and list the subnet addresses in binary and dotted-quad notation. Class B address 11111111 11111111 11110000 00000000
Subnets: 24 -2 = 14 Hosts: 212 -2 = 4094 CHAPTER 12 6. How does TCP provide a reliable transfer service by using IP, which is unreliable? Transmission Control Protocol (TCP) provides a reliable byte-stream transfer service between two endpoints on an internet. TCP depends on IP to move packets around the network on its behalf. IP is inherently unreliable, so TCP protects against data loss, data corruption, packet reordering and data duplication by adding checksums and sequence numbers to transmitted data and, on the receiving side, sending back packets that acknowledge the receipt of data. 7. What “specializations” are made to the simplified sliding window protocol for TCP? What two important problems does the specialized mechanism used by TCP solve? TCP has a multi-stage flow-control mechanism which continuously adjusts the sender's data rate in an attempt to achieve maximum data throughput while avoiding congestion and subsequent packet losses in the network. It also attempts to make the best use of network resources by acking as much data as possible into a single IP packet, although this behaviour can be overridden by applications that demand immediate data transfer and don't care about the inefficiencies of small network packets. 8. Describe the connection abstraction used by TCP. With TCP a given port number dose not correspond o a single object. Instead, TCP has been built on the connection abstraction, in which the objects to be identified are virtual circuit connections, not individual ports. TCP uses the connection, not the protocol port, as its fundamental abstraction; connections are identified by a pair of endpoints. 9. Why does TCP support an option to specify a Maximum Segment Size (MSS) when it also has a window advertisement mechanism? TCP provides a means for a device to specify that the MSS it wants to use is either smaller or larger than the default value of 536. A device can inform the other of the MSS it wants to use during the connection establishment process. A device that chooses to do so includes in its SYN message the TCP option called, appropriately, Maximum Segment Size. The other device receives this option and records the MSS for the connection. Each device can specify the MSS it wants for the segments it receives independently. Devices may wish to use a larger MSS if they know for a fact that the MTUs of the networks the segments will pass over are larger than the IP minimum of 576. This is most commonly the case when large amounts of data are sent on a local network; the process of path MTU discovery is used to determine the appropriate MSS. A smaller MSS might be advisable if it were known that a particular optional feature was in place that would consistently increase the size of the IP header. Employing IPSec for security would be a good example. Pg: 202 in the book.
10. Why is the TCP acknowledgement scheme called cumulative? What are the advantages and disadvantages of cumulative acknowledgements? The TCP acknowledgement scheme is called cumulative because it reports how much of the stream has accumulated. Advantage: Lost acknowledgements do not necessarily fore retransmission. Disadvantage: The sender does not receive information about all successful transmissions, but only about a single position in the stream that has been received. 11. Why is an adaptive retransmission algorithm used for TCP? In general terms, how does it work? To accommodate the varying delays encountered in an internet environment, TCP uses an adaptive retransmission algorithm that monitors delays on each connection and adjusts its timeout parameter accordingly. TCP records the time at which each segment is sent and the time at which an acknowledgement arrives for the data in that segment. Fro the two times, TCP computes an elapsed time known as a sample round trip time or round trip sample. Whenever it obtains a new round trip sample, TCP adjusts its notion of the average round trip time for the connection. 12. What problem does Karn’s algorithm solve? Karn’s Algorithm avoids the problem of ambiguous acknowledgements by only adjusting the estimated round trip for unambiguous acknowledgements. 13. What is congestion? What two techniques does the TCP standard recommend to avoid congestion? Describe the techniques. Congestion is a condition of severe delay caused by and overload of datagrams at one or more switching points. To avoid congestion, the TCP standard now recommends using two techniques: slow-start and multiplicative decrease. Multiplicative decrease congestion avoidance: upon loss of a segment, reduce the congestion window by half (down to a minimum of at least one segment). For those segments that remain in the allowed window, backoff the retransmission timer exponentially. Slow-Start (Additive) Recovery: Whenever starting traffic on a new connection or increasing traffic after a period of congestion, start the congestion window at the size of a single segment and increase the congestion window by one segment each time an acknowledgement arrives.
14. What is silly window syndrome? Describe the two approaches taken to implement silly window syndrome avoidance on the receive side; which is best? In general terms, describe the Nagle algorithm that is used for silly window syndrome avoidance on the send side. Two approaches to avoid silly window syndrome are, 1 TCP acknowledges each segment that arrives, but does not advertise an increase in its window until the window reaches the limits specified by the silly window avoidance heuristic. In the second approach, TCP delays sending an acknowledgement when silly window avoidance specifies that the window is not sufficiently large to advertise. Delayed acknowledgments are most likely the best because delayed acknowledgements can decrease traffic and thereby increase throughput. Nagle algorithm: Send-side silly window avoidance: When a sending application generates additional data to be sent over a connection for which previous data has been transmitted but not acknowledged, place the new data in the output buffer as usual, but do not send additional segments until there is sufficient data to fill a maximum-sized segment. If still waiting to send when an acknowledgement arrives, send all data that has accumulated in the buffer. Apply the rule even when the users request a push operation.
Other things you should know (but don’t need to answer as part of the homework!) 1. Describe the properties of the TCP reliable stream delivery service
2. Describe the operation of the sliding window protocol.
3. Describe the differences and characteristics of the tail-drop policy and the random early discard policy.
4. Understand how TCP works as related to the TCP state machine on page 223.