課程名稱︰計算機網路
課程性質︰資訊系大三必修
課程教師︰周承復
開課學院：電資學院
開課系所︰資訊系
考試日期（年月日）︰102/10
考試時限（分鐘）：120
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
Question1:"Quickies"(15%)
Answer each of the following question briefly, i.e., in at most a few sentence
a)(5%)Please explain what is head-of-the-line blocking in a router.
b)(5%)TCP's current EstimatedRTT is 100ms. Upon receipt of next sample,
the EstimatedRTT is 5ms. Is this possible?If so,when.If not, why not?
c)(5%)What is meant by network-assited congestion control?
Give an example of a congestion control protocol that take this approach.
Does TCP take this approach? Explain your answer.
Question2: A reliable data transfer protocol(25%)
Suppose we have two network entities, A and B. B has a suply of data messages
that will be sent to A according to the following conventions.
When A gets a request from the layer above to get the next data(D) message
from B, A must send a request(R) message to B on the A-to-B channel. Only when
B receives an R message can it send a data(D) message back to A on the B-to-A
channel. A should deliver exactly one copy of each D message to the layer
above. R messages can be lost (but not corrupt) in the A-to-B channel; D
message, once sent, are always delivered correctly. The delay along both
channel is unknown and variable. Assume no reordering occurs.
Design (give an FSM description of) a protocol that incorporate the
appropriate mechanisms to compensate for the loss-prone A-to-B channel and
implements message passing to the layer above at entity A, as discussed above.
Use only those mechanisms that are absolutely necessary.
Use a pencil or a rough sheet before finalizing your answer.
Qusetion3:Routing Algorithms(30%)
Consider the network in the figure below.
The numbers on links between the nodes represent the costs corresponding
to these links.Assume that nodes initially know only the costs to their
neighbors.
D--(5)--E--(3)--A
| | |
(2) (9) |
| | |
C--(1)--B---(7)--
a)(10%)Using the distance-vector algorithm, show the distance tables at node E
Assume that the algorithm works in a synchronous manner, where all nodes
simultaneously receive distance vectors from their neighbors, compute their
new distance vectors, and inform their neighbors if their distance vectors
have changed.
b)(5%)Create a routing loop between the nodes B and C by changing the cost of
the link between the nodes C and D. What is the minimum change in link cost
that creates the routing loop? What is this problem alternatively called?
c)(5%)How does the distance-vector-like protocol solve this problem?
If the distance-vector-like protocol was used for routing in the above network
, what is the finite number that would play the rule of (無限大)?
d)(5%)If the link-state-like protocol was used in the above network, how would
it handle the routing loop? How do nodes learn the link costs in the link-
state-like protocol?
e)(5%)Is there any other method to solve this problem?
Question4:Transport Layer(20%)
Consider the following plot of TCP window size as a function of time for two
TCP connection A and B. In this problem we will suppose that both TCP senders
are sending large files. We also assume that the packet loss events are
independent in connection A and B.
a)(5%)Considering the above value of congestion window(CongWin) for these
connections, can we identify the type of TCP connections(Reno or Tahoe)that
have been used by connection A and B? Justify your answers.
b)(5%)What are the values of the Threshold parameter between the 1st and the
14th transmission rounds for each connection?
c)(5%)At the 12th transmission round for connection A, is segment loss
detected by a triple duplicate ACK or by timeout? Justfy your answer.
d)(5%)Assume that the segment size is 1460 bytes and that a total of 87600
bytes have been successfully transmitted over connection A before the 13th
transmission round. At which transmission round the cumulative amount of
successful transmitted data is equal to 163520 bytes? Again we assume that
there is neither timeout nor dupACK after the 13th transmission round.
(附圖為折線圖)
Question5:(20%)
Compare GBN, SR, and TCP(no delayed ack).Assume that the timeout values for
all 3 protocols are sufficiently long such that 5 consecutive data segment
and their corresponding ACKs can be received (if not lost in the channel) by
the receiveing host(Host B) and the 2nd segment(sent from A)is lost.In the end
, all 5 data segment have been correctly received by host B.
a)(15%)How many segments has lost A sent in total and how many acks has host B
sent in total? What are their sequence numbers?Answer this question for all 3
protocols.
b)(5%)If the timeout values for all three protocols are much longer than 5 RTT
,then which protocol successfully delivers all 5 data segments in shortest
time interval?
Question6:(10%)
Consider the network shown below. Suppose AS3 and AS2 are running OSPF for
their intra-AS routing protocol. Suppose AS1 and AS4 are running RIP for their
intra-AS routing protocol. Suppose eBGP and iBGP are used for the inter-AS
routing protocol. Initially suppose there is no physical link between AS2 &AS4
(a) Router 3a learns about prefix X from which routing protocol?
(b) Router 3c learns about X from which routing protocol?
(c) Router 1d learns about X from which routing protocol?
(d) Router 1c learns about X form which routing protocol?
(圖參考作業)
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