Distance vector routing (5 p total)
- Assuming that weight of each link is 1, what would be enough for
"infinity" in this network, if no path goes through same link
or network node twice? (1 p)
Answer:
The length of the longest path in the network is 5 (without loops). The
smallest possible number for "infinity" is longest path + 1, so in this
network the length for infinity is 6.
Note: Some of you have answered 6, "because maximum number of _links_
in this network is 5". The answer is correct, but the explanation isn't,
even if you happen to get the correct answer in this network (assuming
the weights equal to 1). For example,
the weight of each link in the longest path could be 10 - then the total
length of the longest path would be 50 and infinity would be 51. Maximum
number of links would still be 5.
Grading
Correct answer (with or without explanation): 1 point.
- Show what happens when link B-C gets down. (2 p)
Answer:
- B tells A that connection to C is broken. However, A has
connection
to C, so B updates it's route to C to go through A.
C tells A that connection to B is broken. A tells C that there is a
connection via A. C updates its route to CAB.
C tells D that connection to B is broken. D updates its route
to B to DEAB.
- C now tells D that the new route to B is CAB. Because route DCAB is
shorter than DEAB, D updates its route to DCAB. Because no other
connections were using link BC, all the routes are now updated.
Grading:
- The answer is correct and explained clearly: 2 points
- Clear explanation, but small mistakes: 1 point
- Correct answer, but explained unclearly: 1 point
- The link between E and F comes up. Show the propagation of news
for routes to F. The weight of the new link is 1.(2 p)
Answer:
- E notes that it has new link to F. Because it's shorter than EDF, E
updates its route to EF.
F updates its route to FE.
- E tells A that it has new link to F. A uptdates its route to AEF.
E tells D that it has new route to F. It's as long as route DF, so D
can either update the route to DEF or leave the route DF, depending of the
algorithm used.
F tells D that it has new route to E. It's longer than route DE, so D
makes no changes.
- A tells B that it has new route to F. The new route BAEF is as long
as the old route BCDF, so B can either change the route or make no
changes.
A tells C that it has new route to F. C has a shorter route so it
makes no changes.
If D has changed its route to F, it tells C that it has a new route.
C can change route or make no changes.
Grading:
- The answer is correct and explained clearly: 2 points
- Clear explanation, but small mistakes: 1 point
- Correct answer, but explained unclearly: 1 point
2. Internetworking (10 points)
Write a short essay about internetworking. The essay should contain
and explain the following terms:
- internetworking
- virtual circuit
- tunneling
- fragmentation
- firewall
- gateway
- bridge
Answer:
Note! These are just explanations of the terms. This is not enough to
get points for this exercise, as the purpose was to write an
essay. See the grading instructions for this exercise.
- internetworking
Internetworking means connecting several different networks possibly of
different types to each other.
- virtual circuit
The virtual circuit is a connection built between the source subnet or
node and
destination subnet or node over a packet or cell switched network
in a way transparent to the source
and destination (both source and destination think they have physical
connection, while they don't).
- tunneling
Tunneling means transport of protocol PDU:s in the payload of a PDU in the
same layer.
- fragmentation
Fragmentation means breaking the incoming packets larger than maximum
packet size in the network to smaller fragments, which are then sent
in separate packets.
- firewall
A device between two parts of networks, forcing a security policy for
traffic between the separated parts of the network.
- gateway
Gateway is a network device that provides a connection between the
different, incompatible networks, providing the necessary translations.
There are several kinds of gateways such as tansport gateways,
application gateways, etc.
- bridge
Bridge is a store-and-forward device in data link layer that connects two
LANs with the same protocol.
Grading:
- First of all, you should note that the purpose was to write an
essay. Just explaining terms is not an essay, and things
listed in row is not an essay either. So if no answer was given at
all or the answer was not an essay: 0 points
- You can also give 0 points if the essay is so unclear (for example,
written in very bad English) that you can't understand anything in it
after you have read it 3 times. If you're not sure with that (for
example,
you have bad English skills yourself), you can ask us for advise via e-mail.
- The best answer is a "good" essay having clear structure, good
language and it is telling the things it's supposed to. If the
essay is "good" and all terms are explained correctly: 10 points.
- For each term explained wrong or not explained at all: -1 point
- If there are problems with the structure of the essay or with it's
language, you can reduce from 0 to 3 points depending of how serious the
problems are. Some small typos are not a
problem, but for example always starting a sentence with small letter
is. So if there are problems with how the essay is written: -1, -2 or
-3 points
- If you have reasons to suspect that the essay is plagiated,
please contact us by e-mail immediately. Attach the answer file to
your email and tell us, why do you think it's plagiated. The essay
could be plagiated, if:
- Writer's English skills change dramatically in the middle of the
essay or are dramatically different to other parts of the
exercise. For example, writer that usually has problems with even
the simpliest words suddenly starts to write complicated sentences
in exellent English.
- The essay seems to be a part of a larger text. It may for
example contain references to things that are not present in an
essay (like "as discussed earlier").
- Different parts of the essay seem to have no logical
connection. It may be because the writer have problems to think
logically, or parts of the essay have been plagiated from different
sources.
- You have already seen the same text or part of it somewhere.
3. ARP & RARP (4 points total)
- What is RARP and when is it needed? (1 p)
Answer:
Reverse Address Resolution Protocol (RARP) is used when a host knowing its Ethernet address doesn't know
its IP address. This could for example be a diskless workstation,
which doesn't know its IP address after booting. The host sends a
request to RARP server, and the server replies with host's IP address.
Grading:
- Correct answer: 1 point
- Incorrect or no answer: 0 points
- Give at least one example of how ARP can be a security risk. (1
p)
Answer:
- No authentication: it's possible that a host (A) replies to ARP
request sent to someone else (B), so all the data intended to B goes to
A.
- Moreover, if A has a connection to B, it can send changed data to
B (man-in-the-middle).
- It's possible to make a rush if a host sends a lot of packets to
non-existing IPs: a router would ask all the network nodes about
non-existing computers (broadcast storm or DoS-attack).
Grading:
- Correct example (one of those listed or something else): 1 point
- Incorrect answer or no answer: 0 points
Please note that the name of a security problem is not important, as
long as the explanation is correct.
- A host gets two answers to an ARP request. Is there a problem?
Why? (1 p)
Answer:
Yes, there is a problem - a host's network adapter should have only
one MAC
address. It may be that there are problems with network configuration
or that there is an attack going on.
Grading:
- Correct answer with explanation: 1 point
- Incorrect or no answer: 0 points
- What if a host gets two answers to a RARP request? (1
p)
Answer:
If a host gets two (different) answers to a RARP request, it means
that the host has two different IP addresses. In theory, if "host"
means a computer, there's no problem - a computer can have two
different IP addresses. However, usually in this case "host" means a
network card, which normally has only one Ethernet address and is
usually configured to have only one IP address, too. So in this case
there probably is a configuration problem in the network.
Grading:
- Here, the correct answer may be both "yes" and "no" depending of
writer's point of view, so you should grade this basing of the
explanation. If the answer is explained correctly: 1 point
- Incorrect or unexplained answer or no answer: 0 points
Please note: The question was a bit misleading - "two answers" should
of course have been "two different answers". However, the answer
remain the same here: there's no problem as long as the answers are
same, and there may be a problem if the answers are different.
Sending the review
Send your grades as an answer (attachment of the submission message) again
following
the returning
instructions. The file you send should contain
- #T<number of each exercise> <points for this exercise>
- #<comments for each exercise>
For example, your answer file could look like this:
#T1 8
#Cool! Everything's correct!
#T2 0
#You were supposed to write an essay, not just explanations of the
terms!
#T3 3
#No answer to 3 c)
The second part should be returned to same series as the first part
(series 3). You can write feedback for this homework round in
the body of your mail as explained in returning instructions, but you
won't get extra points for it.
The deadline for returning is 23.10.2001
Return your answer as explained in the
general instructions for returning your answer.
Remember to check that you get a positive
acknowledgment for your answer.
This page is maintained by the
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The page has last been updated 12.10.2001.
URL:
http://www.tcm.hut.fi/Studies/T-110.300/2001/Homeworks/model3.html
