Final Exam of Computer Networks (ICE 1230)
2008.5. 27.
Answer in English and Total 15 points
1. Why is it said that packet switching employs statistical
multiplexing? Contrast statistical multiplexing with the
multiplexing that takes place in TDM. (1 point)
Answer: In a packet switched network, the packets from different sources
flowing on a link do not follow any fixed, pre-defined pattern. In TDM
circuit switching, each host gets the same slot in a revolving TDM frame.
2. In BitTorrent, suppose Alice provides chunks to Bob
throughout a 30-second interval. Will Bob necessarily
return the favor and provide chunks to Alice in the same
interval? Why or why not? (1 point)
Answer: It is not necessary that Bob will also provide chunks to
Alice. Alice has to be in the top 4 neighbors of Bob for Bob to
send out chunks to her; this might not occur even if Alice
provides chunks to Bob throughout a 30 second interval.
3. What is a way using Last-Modified: header line in HTTP? (1
point)
Answer: The header line is used by Conditional GET to check
whether the object asked has been modified.
4. We saw that there is no network-layer protocol that can be
used to identify the hosts participating in a multicast group.
Given this, how can multicast applications learn the
identities of the hosts that are participating in a multicast
group? (1 point)
Answer: The protocol must be built at the application layer. For
example, an application may
periodically multicast its identity to all other group members in
an application-layer message.
5 In these days, the Internet needs to include the wireless
communications network in addition to the wired networks.
What is a way to extend the congestion control of TCP in
this case? Why? (2 point)
Answer: Packet losses occur from network congestion and
wireless communication. Since the wired Internet does not
consider the packet loss due to errors in wireless
communication, the congestion control of TCP should be
revised. A way is …
6. Describe the overall steps of Link State Routing algorithm.
And show the technical issues (at least two) and their
solutions of its implementation as Open Shortest Path First.
(2 points)
1.
discover its neighbors and learn their network address.
2. measure the delay or cost to each of its neighbors.
3. construct a packet telling all it has just learned.
4. send this packet to all other routers.
5. compute the shortest path to every other router.
For example, OSPF has an architecture consisting of areas where the link
state packets are flooding inside. Sequence number is with age number in
order to solve …
7. Suppose two nodes start to transmit at the same time a
packet of length L over a broadcast channel of rate R.
Denote the propagation delay between the two nodes as
dprop. Will there be a collision dprop < L/R ? Why or why not?
(1 points)
There will be a collision in the sense that while a node is transmitting it will start
to receive a packet from the other node.
8. What are the purposes of the HLR and VLR in GSM
networks? What elements of mobile IP are similar to the
HLR and VLR? (1 point)
The home network in GSM maintains a database called the home location
register (HLR), which contains the permanent cell phone number and
subscriber profile information about each of its subscribers. The HLR also
contains information about the current locations of these subscribers. The
visited network maintains a database known as the visitor location register
(VLR) that contains an entry for each mobile user that is currently in the
portion of the network served by the VLR. VLR entries thus come and go as
mobile users enter and leave the network.
The edge router in home network in mobile IP is similar to the HLR in
GSM and the edge router in foreign network is similar to the VLR in
GSM.
9. What are some of the difficulties associated with the Intserv
model and per-flow reservation of resources? (1 point)
Scalability: Per-flow resource reservation implies the need for a router to
process resource reservations and maintain per-flow state for each flow
passing through the router. Flexibly service: The Intserv framework
provides for a small number of prespecified service classes.
10. What are the differences between message confidentiality
and message integrity? Can you have one without the
other? Justify your answer. (2 points)
Confidentiality is the property that the original plaintext message can not be determined
by an attacker who intercepts the ciphertext-encryption of the original plaintext message.
Message integrity is the property that the receiver can detect whether the message sent
(whether encrypted or not) was altered in transit. The two are thus different concepts,
and one can have one without the other. An encrypted message that is altered in transmit
may still be confidential (the attacker can not determine the original plaintext) but will
not have message integrity if the error is undetected. Similarly, a message that is altered
in transit (and detected) could have been sent in plaintext and thus would not be
confidential.
11. What is the purpose of a nonce in an end-point
authentication protocol? (1 point)
A nonce is used to ensure that the person being authenticated is “live.” Nonces thus are
used to combat playback attacks.
12. What is the role of ASN.1 in the ISO/OSI reference model’s
presentation layer? (1 point)
The role of the presentation layer is to allow the sending and receiving of data in a
machine-independent format (i.e., without regard to the particular storage and
architectural conventions of the sender and receiver).