This assignment is due no later than 6:20 p.m. on Tuesday, 29 October.
This homework assignment has six questions; answer all of them. Answers
should no longer than half a page, or around 100 to 150 words. Please site
references you use to answer your questions, including the textbooks you’re using.
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Imagine a network in which a connection starts off providing a datagram
service, but then changes to a virtual-circuit service at some point during the
connection’s operation. Describe two advantages a network might
achieve by providing connection-service changing behavior. Describe two costs
a network might have to incur providing connection-service changing behavior.
Advantages to the network include:
- Virtual-circuit traffic is easier to switch than is datagram traffic.
- Setting up virtual circuits allows the network to even out hot spots by
directing traffic away from overloaded routers or links within the network.
Disadvantages to the network include (pick two):
- Maintaining routers capable of both datagram and virtual-circuit
transmission increases the complexity of designing and running the network.
- Defining and keeping track of datagram transmission to created suitable
virtual circuits requires time and space overhead unexpected in datagram
service.
- Virtual circuits without admission control is so hard to implement it's
probably impossible.
- Virtual circuits don't react (or at all) to changes in network
characteristics (such as router or link failures) as well as datagram routing
does.
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A colleague of yours is building a network and comes to you for advise in
designing the network addresses. The only constraint your colleague has for
network addresses is that all bits identify hosts; no bits in the address can
be considered as identifying a network. Describe to your colleague two
advantages and two disadvantages of such an addressing scheme.
Note that this question asks you to analyze your colleague's scheme; it does
not ask you to design or implement it. You may make whatever assumptions about
design or implementation you feel convenient, as long as your assumptions are
consistent (for example, assuming the addresses are fixed length in one part of
your answer means the fixed-length assumption holds for all parts of your
answer. An answer shouldn't assume fixed-length addresses in one part and
variable-length addresses in another part).
Your colleague is essentially building a LAN. Advantages include (pick two):
- Simplified switching table maintenance (bridging vs routing).
- A host-only address space can be smaller (less overhead) than a
network-host address space.
- LAN-style switching (bridging) is easier to provide than is
network-style switching (routing).
Disadvantages include:
- Scalability problems because every host is directly accessible to
every other host (ahierarchical address space).
- LAN-style switching (bridging) is uses network resources less
efficiently than does network-style switching (routing).
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Continuing with question 1, briefly describe (no more than 100 or so words) how
a network might implement a connection that changes from providing datagram
service to providing virtual-circuit service.
In (slightly) decreasing order of ridiculousness:
- Answer 1: The network nails-up virtual circuits between all pairs of
hosts in the network so that all traffic through the network goes over
virtual circuits. Initially host-to-host transmissions receive
datagram-level service over the virtual circuit. As the entry router notes a
flow developing between specific hosts, the traffic between those hosts is
promoted to better quality service over the circuit.
- Answer 2: The network nails-up virtual circuits between all pairs of
hosts in the network and also supports datagram transmission separate from
the virtual circuits. Initially hosts use datagram transmission. As the
entry router notes a flow developing between specific hosts, the traffic
between those hosts is promoted from datagram to virtual-circuit
transmission.
- Answer 3: Hosts initially communicate with datagram transmission. When
an entry router observes a consistent traffic flow between pairs of hosts, it
builds a virtual circuit consistent with the flow and begins sending
appropriate PDUs over the circuit, leaving the other PDUs to datagram
transmission.
There are many other answers possible.
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A fully connected network N with n nodes uses a link-state routing
routing algorithm at each router. A colleague of yours believes that once
N is “turned-on” (which is assumed to be an instantaneous, global
operation) any router in N will have to receive around n2 routing
advertisements from other routers before it has a complete and accurate routing
table. Your colleague is wrong; explain why. Assume N’s topology
is stable.
Note this question asks you to explain why your colleague is wrong. You
can’t explain why your colleague is wrong by only giving the right
answer. Giving the right answer only shows that your colleague is wrong, it
doesn’t explain why your colleague is wrong. This question asks you to
explain why your colleague is wrong.
The total routing network transmits O(n2) advertisements for each round of
the link-state algorithm (assuming efficient network broadcast, and assuming
link state operates in rounds, which it doesn't). But each router only needs
to receive a single advertisement from every other router to be able to build
the network model used to compute routes.
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Continuing with question 1, suppose the layers above the connection-changing
network layer make service-changing connections the only form of end-to-end
connections available at the application layer. Describe two benefits a
network-applications developer might obtain by using service-changing
connections in an application. Describe two problems the developer might
encounter by using a service-changing connection in an application.
Note that in question 1 your answer involves the network’s point of
view, while in this question you answer involves a network-application
developer’s point of view (that is, the answers for one question are
not appropriate answers for the other question).
Advantages to the network-application developer include:
- Reduced application workload to correct unreliable transmission results
once the connection switches from datagram to virtual circuits
(assuming the application implements reliability).
- No admission or traffic characterization required for (eventual)
virtual-circuit service.
Disadvantages to the network-application developer include (pick two):
- Exploiting virtual-circuit transmission requires extra effort, doesn't
replace the effort needed to deal with datagram transmission.
- Until the switch over, datagram transmission performance may suffer in
competition with virtual-circuit traffic.
- Virtual-circuit transmission is more sensitive to network failure than
is datagram transmission.
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A router R in a network has n > 0 interfaces. Describe how the
router can determine the number of different networks to which it is attached.
The router can look in its routing table for a list of connected networks. A
(destination) network address is a field in each routing-table entry (Kurose &
Ross, p. 287).
However, some networks are accessible via the default route and are not listed
explicitly in the routing table (K&R, p. 325). To catch these cases, the
router should also look at the source and destination address PDU fields.
However, if a router never receives a PDU for a network accessible via the
default route, the router never learns about the network. To find all attached
networks, routers would have to coordinate using a special-purpose protocol
similar to a routing protocol, but one that spans default routes. Such a
special-purpose protocol is unscalable and is not generally practical.