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‫ – מבוא לרשתות תקשורת‬3 '‫עבודה מס‬
18.12.07 : ‫תאריך הגשה‬
‫הגשה בזוגות לתא הבודק‬
1) Suppose within your web browser you click on a link to obtain a web page. Suppose
that the IP address for the associated URL is not cached in your local host, so that a DNS
look-up is necessary to obtain the IP address. Suppose that n DNS servers are visited
before your host receives the IP address from DNS. The successive visits incur an RTT of
RTT1, …, RTTn. Further suppose that the web page associated with the link contains
exactly one object, a small amount of HTML text. Let RTT0 denote the RTT between the
local host and the server containing the object. Assuming zero transmission time of the
object, how much time elapses from when the client clicks on the link until the client
receives the object?
2) Referring problem 1, suppose the HTML file indexes three very small objects on the
same server. Neglecting transmission times, how much time elapses with (1) nonpersistent HTML with no parallel TCP connections (2) non-persistent HTML with no
parallel TCP connections (3) persistent HTML with pipeline?
3) Suppose you are downloading a file using some P2P file-sharing system. The
bottleneck in the internet is your residential access link, which is a 128 kbps full-duplex
line. While you are downloading all of a sudden ten other users start uploading the file
from your computer. Will the simultaneous uploads slow down your downloads?
4) Why do HTTP, FTP, SMTP, POP3, run on top of TCP rather than UDP?
5) What is the overlay network in P2P file-sharing system? Does it include routers? What
are the edges in the overlay network?
6) What are the differences between POP3 and IMAP?
7) The UDP server described in the class requires only one socket, whereas the TCP
server needs two. Why? If the TCP server were to support n simultaneous connections,
each from a different client host, how many sockets would the TCP server need?
8) Consider the GBN and SR protocols. Suppose the sequence number space is of size k
and sender window size w. Show that for avoiding cases of ambiguity of received ACKs
at the sender, k ≥ 2w must hold.
9) We have seen that a sequence number (SN) space of {0,1} is sufficient for the stopand-wait ARQ protocol. That is, the protocol works properly even if the SN “wraps
around”: 0 1 0 1 ....
a. However, use an example to show that this SN space is inadequate for a sliding
window ARQ protocol with sender window size (SWS) = 2 frames. Assume that
receiver window size (RWS) = 1 frame. Note that RWS is the maximum number
of out-of-order frames that the receiver is willing to accept. Backup your example
with a diagram depicting the messages passing between the sender and receiver.
b. Assume n = 16 (SN = 0, …, 15) and the receiver’s NFE (next frame expected)
is equal to 12. How would a receiver determine whether a received frame is old
(to be discarded) or new (to be accepted)? Which of the frames with sequences 011 are old and which are new, if NFE=12
c. What should be the receiver’s RWS if it is required to buffer all out-of-ordered
frames (n=16)? RWS is the maximum number of out-of-order frames that the
receiver is willing to accept.
10) Suppose that a TCP connection is established between hosts A and B.
a. Can A send 10 successive segments, each 1024 bytes long, to B without any
ACKs received? If so state all protocol conditions required, if not explain why!
b. Is it possible that ACKs from B to A are lost and nevertheless A will not
retransmit any of the 10 segments? Explain!
c. Depict the mechanism in TCP enabling retransmitting a segment before the
RTO timer expires.
d. Suppose TCP updates the RTO according to the following algorithm:
Also assume that every odd segment transmitted by A to B is lost (segments form
B to A are not lost). Also assume that the RTT is not known but constant and the
TCP Sliding Window from A to B is constant and equals 1.
1. How many times will a segment from A to B be transmitted?
2. Suppose A calculates the SampleRTT from the transmission of the
second segment. What is the segment transmission rate from A to B given
that RTT equals 10 seconds?
3. Suppose the RTO is calculated by RTO = 2 × EstimatedRTT , what is
the rate now?