CMSC 417 Computer Networks Fall 2007
Programming Assignment 3 — Simple Transport Protocol
Assigned: Oct. 17 Due: Oct. 28, 11:59PM.
1 Introduction
You will design a simple transport protocol that provides reliable datagram service. Your protocol
will be responsible for ensuring data is delivered in order, without duplicates or errors. Since the
local area networks in the university are far too reliable, we will provide a header file (emulation.h)
and an object file (emulation.o) for emulating an unreliable network. To used them, you have to
include the header file and then link with the object file. There is a macro defined in emulation.h
to replace all call of sendto with emulate_sendto which can be configured to drop, damage, or
duplicate packets on demand. If the packet is not dropped, then it is sent to (using UDP) the
destination node as if sendto is invoked. Note that the emulate_sendto may corrupt a packet
and then send it, or may send multiple copies of the same packet. While debugging, You can select
the behavior of the function, and use it to test parts of your code. However, your selection will be
ignored in grading, and the function will randomly drop, duplicate and forward packets.
For the assignment, you will transfer a named file reliably from between two nodes (a sender
and a receiver). You do NOT have to implement connection open/close etc. You may assume that
the receiver is run first and will wait indefinitely, and the sender can just send a named file to the
receiver.
2 Packet Format
To make use of the emulation and specify the behavior of the emulate_sendto function, the (UDP)
data part of each packet will contain the following header:
123
01234567890123456789012345678901
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|version|L|D|C|U| Frequency | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Your own header & Data |
| ....... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version: Always set to 0011
Disposition: The packet disposition consists of four bits (three used) and tells the function how to handle
this packet. The bits are:
–L: Loss bit
–D: Duplication bit
–C: Corruption bit
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