# uft.2 -- UDP file transfer

Part 1
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Stress-test your uft application in three ways:

1) Generate arbitrary control and data packets of the right size (but
   with fully or partly random contents) and observe how robust your
   application behaves.  Address the observed robustness problems in
   your implementation.

2) Examine your protocol design and your implementation with respect
   to two senders sending different file contents of a file with the
   same name to the same receiver at the same time.  What happens?

3) Temporarily change your client to direct a server to perform a file
   transmission at high bitrate to some third -- unsuspecting --
   target.  Is this possible with your design?  Discuss how misusing
   your server in such denial-of-service attacks could be prevented or
   at least made more difficult.




Part 2
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Extend your uft from assignment 1 to support adaptive congestion
control using some additive increase/multiplicative decrease (AIMD)
compatible scheme.  You may use TFRC, ack clocking or may invent
another scheme that adapts to changes in the available data rate and
behaves _somewhat_ fair in sharing network resources.

We will provide a tool "udppipe" that runs in user mode and allows for
traffic shaping according to some predefined bit rate and will thus
drop excess packets.  It will also delay incoming packets by the delay
(specified in milliseconds).  udppipe has the following command line
parameters:

$ udppipe -l [lhost:]lport -c [chost:]cport -b bitrate -d delay

                   lhost:lport         chost:cport
  +------------+        V+---------+        V+------------+
  | uft sender |-------->| udppipe |-------->|uft receiver|
  +------------+         +---------+         +------------+

The uft sender needs to send to the udppipe and the udppipe to the
uft receiver.  Packets in the reverse direction are just addressed
to packet origin as obtained from recvfrom().

Introducing congestion control obviously requires that you modify the
semantics of the -b option previously used to specify the transmission
bitrate.  Make a decision about potential new semantics and discuss
your choice in the written part.  *Any* revised semantics are allowed,
provided that it still influences transmission rate.  Also describe
potential drawbacks of your choice.

Your modified uft should work with a dynamic change from 10 kbit/s to
1 Mbit/s.

You should document your protocol design including header syntax and
interactions and discuss why you have chosen this particular design.