Computer Networks (CITS3230) - Tutorial 5
(for the week commencing 26th May 2008)
- Draw a diagram showing the packet structure of a single Ethernet
packet carrying an email message using TCP/IP.
Highlight all fields that
identify forms of source/destination connectivity.
- What are some of the motivations behind the reserved port
scheme used in IPv4?
What problems would arise if such a scheme did not exist?
Can you suggest other mechanisms to resolve these problems?
- The client-server model supported by the Berkeley sockets
network API suggests that each server machine needs have tens,
possibly hundreds,
of server processes sitting idle just waiting for connections -
rather wasteful of operating system resources.
Develop some pseudo-code in Java or C
to demonstrate how the Internet supervisor daemon,
inetd,
addresses this potential problem.
- A and B are two people at two ends of a very slow telephone
connection. They are both completely cut off from the world except for
their connection with each other. In particular, they have no idea
what time it is in the real world (they are doing their Networks' project).
There is a delay AB in the transmission of what is being said by A to
B, and a delay BA in the transmission of what is being said by B to A.
The delay AB does not equal the delay BA. The delays, for the purpose
of this problem, are on the order of minutes in each direction.
Both A and B have watches. Before they begin to talk, the watches are
not set to the correct time, nor are they set to the same time.
Can you devise a mechanism such that A and B can, via their
telephone connection only, accurately synchronize their watches (to
within a few seconds)? The watches do not have to reflect time in the
real world, but must match each other. Both A and B will know the
mechanism before the call begins, and each will know whether they are A or B.
Chris McDonald.
May 2008.
From The Whole Internet Catalog & User's Guide
(1st ed., O'Reilly and Associates, 1992, p33),
under the section Export Laws we see:
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[...] However the list of restricted items has a lot of surprises, and does
cover a lot of things that you can learn as a student in any university.
Networking code and encryption code might be restricted, based upon their
capabilities. Many times, one little item is of concern, but by the time
the regulations are written, they cover a much wider area. For example,
during the Persian Gulf War, it was a lot harder to knock out Iraq's
command and control network than anticipated. It turned out they were
using commercial IP routers which were very good at finding alternative
routes quickly. Suddenly, exporting any router [from the US]
that could find alternate routes was restricted.
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