CMSC 417 Midterm #2 (Fall 1997)- Solutions
Values of a packet sequence numbers that are not permitted since it is impossible to distinguish a new packet from and old one. (3 points)
Transport service access point. A end-point for communication at the transport layer. This is generally specified with a host name and port. (3 points)
The problem that two generals canít agree when to attack over a channel that looses messages. This is analogous to the problem of having two parties agree to shutdown a transport level connection. (3 points)
Classless Inter-Domain Routing: A replacement for the class A,B,C networks used on the Internet. It is designed to reduce routing table size and internal fragmentation of the IP address space. (3 points)
A way to combine multiple samples that assigns a greater weight to recent values than older ones. Avg =a Sample + (1- a ) Avg (3 points)
The problem is that sleep is a blocking call, and it would be difficult to wakeup a sleeping process when the ack arrives. It is a specific instance of the problem that is experienced when trying to have a thread wait on the OR of two or more conditions.
The initial time stamp is used in the connect request and connect ack packets. Once the initial sequence number has been agreed upo,. it canít be re-used until the maximum lifetime of the packet on the network has been exceeded to prevent a stale packet form being confused with a new one. However, once the connection is established, any sequence number can be used for each packet as long as the sequence number is not repeated within the maximum lifetime of a packet. (10 points)
If the tick rate is too fast, it will role over before the maximum packet lifetime and could cause an old connection request to be confused with a current one. If it is too slow, it will limit the rate at which connections can be made (10 points)
To efficiently use the network, a protocol must be able to have a window size at least as large as the end-to-end delay bandwidth product. The TCP option to shift the bits of the window size to permit more outstanding bytes is one way to meet this need. (9 points)
Since more data can be transferred in the same roundtrip time, making requests and responses larger can reduce the apparent latency of a message. For example, a directory protocol could request the contents of the entire directory rather than one file at a time. (9 points)
Leaky bucket permits a network device to accept a large burst of data and then slowly transmit it onto the network. A token bucket permits the burst to go directly onto the network (if there are tokens available). Either approach will ensure that the average traffic rate will be maintained. (108 points)
Per connection traffic shaping is useful when there are several different classes of traffic using the same link and hosts. By performing shaping on a per connection basis, applications with different (even conflicting) resource requirements can be accommodated. (5 points)
Per host traffic shaping is useful when two connections represent traffic that must travel over the same resource constrained link. By shaping the traffic as a unit, we can ensure that the capacity of the single link is not exceeded. (5 points)
Selective-repeat is good for communication channels with high probability of packet corruption or loss since it conserves bandwidth by only re-transmitting the damaged packet. Go-back-n is useful for more reliable links since it permits the receiver side to have only one buffer since all packets will arrive in order even after a loss. Go-back-n has less efficient utilization of link bandwidth since packets that were received correctly may still need to be re-transmitted. With go-back-n, packets will arrive in-order so the receive window is reduced to one.