CNT5106C Computer Networks, Summer 2010
Instructor: Prof. Ahmed Helmy
On the Internet Architecture, Elementary Queuing Theory and Application Layer
I. Internet and layered protocol architecture:
(5 points) In the layered protocol architecture the transport layer functionality
includes congestion control and error recovery (e.g., retransmission). One suggested that
this functionality should be done strictly at the end points (i.e., at the hosts) without aid
from the network. Do you agree? Why? Elaborate showing the design trade-offs.
In general, error recovery (e.g., re-transmission) is specific to application needs.
Some applications require 100% packet recovery, even with delays and jitters (such as
TCP-based applications, http, ftp and telnet traffic). Other applications may be tolerant to
loss but less tolerant to delays and jitter, such as voice applications. Re-transmissions and
packet recovery add to the jitters and the delays and hence may not be desirable for real-
time or voice applications. Hence it is not a good idea, in general, to include error
recovery at the network layer (that is not aware of application needs) and it is better to
implement such functionality at the transport layer end-to-end.
In cases of lossy channels in the network (such as X.25 in the early networking
days, or wireless links) it may be desirable to reduce the bit error rates on those links by
including error recovery at the end points of those links. [In general, most links nowadays
have very low BER, and for wireless links the MAC (such as IEEE 802.11) layer
provides Ack’ed delivery].
For congestion control, a similar argument may be given. That is, congestion
reaction may be application specific and is better implemented end-to-end. Congestion
, on the other hand, may provide useful information to the end points to react
appropriately. Since losses in the network may be due to congestion or other factors, a
signal from the network to the end point may help distinguish congestion errors from
other errors. Only congestion errors should trigger ‘back off’ or rate cut at the end points.
So, network assistance in congestion notification may help in some scenarios. [extra: In
other scenarios network assistance may prevent synchronization effects of congestion
control, e.g., RED, or may prevent/isolate misbehavior, e.g., WFQ.].
(5 points) What advantage does a circuit-switched network have over a packet-
switched network? How can it establish such advantage?
A circuit-switched network can guarantee a certain amount of end-to-end bandwidth for
the duration of a call. Most packet-switched networks today (including the Internet)
cannot make any end-to-end guarantees for bandwidth.
Circuit-switched networks use admission control, and reserve a circuit (in TDM it is done