Lab 5: Best Effort Netimg


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Lab 5: Best Effort Netimg

This lab introduces you to UDP­based cient­server programming. It allows you to modify the receiver
window and maximum segment size of the client and the packet drop probability of the server and
observe what happens when there is no flow control and no error recovery at the transport layer. You will
re­build the simple client­server remote image viewer from Lab1 using UDP. You’re provided with a
skeleton code tar ball and a reference Linux binary executable of the client, refnetimg, and the server
refimgdb. You should be able to run your client against the provided server and the provided client
against your server. The provided Makefile builds netimg and imgdb. You can download the support code
from the Course Folder. As usual, you can search for the string “YOUR CODE HERE” in the code to
find places where your code must go. You may also want to consult the lecture notes on UDP socket.
Task 1: Server Side
Your first task is to write the server code. You can search for the string “Lab5 Task 1” in socks.cpp and
imgdb.cpp to find places where “Task 1” related code must be filled in. Fill in the couple lines of code to
create a UDP socket in function socks.cpp:socks_servinit(). So that you don’t have to re­type your
code for Lab 6, the imgdb.cpp released for Lab 5 already includes instructions/comments for Lab 6 tasks,
so be careful when you search for “Task 1” that you work only on “Lab5 Task 1” for this lab.
The server does not have any required command­line option. Upon start up, the server initializes a UDP
socket and obtain an ephemeral port number from the kernel which it prints out to the console. Then it
waits for a query packet from the client, loads the requested image from file from the same
folder/directory it is launched from, sends the dimensions of the image to the client, and transfers the
image to the client.
% imgdb [ ‐d <prob ]
The optional ‐d command line argument allows you to change the probability that the server drops a
segment instead of sending it to the client. The probability is of type float and I recommend that you play
with values ranging from 0.011 to 0.11. You can also try larger values and see how it affects the resulting
image. To disable dropping, set the value to ­1.0. The implemenation of this option is provided to you.
The definition of the query packet, iqry_t, is depicted in the following figure and can be found in
For this lab, the query packet must be of type NETIMG_SYNQRY, also defined in netimg.h. In addition to the
filename of the image the client is searching for, the query message carries the maximum segment size
(mss), the receiver’s window size (rwnd), and the forward error correction window size, the latter two are
used in Lab 6 and PA3. The function imgdb::recvqry() is similar to the one from Lab 3: it receives a
query packet, checks that the packet is of the correct version and type and returns one of the NETIMG error
codes defined in netimg.h. Unlike the one in Lab 3, this imgdb::recvqry calls the recvfrom() socket API
to receive a UDP query packet. Upon successful return from recvfrom(), the client’s address and port
number must be recorded in imgdb::client member variable. You are to write the call to recvfrom(). It
shouldn’t take more than 2­3 lines of code.
As in Lab 3, imgdb::handleqry() then calls imgdb::readimg() to load the requested image from file. If
the image is found, handleqry() stores the mss, rwnd, and fwnd information to the eponymous imgdb class
member variables and calls imgdb::marshall_imsg() to prepare an imsg_t packet with the dimensions of
the image. The definition of imsg_t is the same as for Lab 3 and is defined in netimg.h. Then handleqry()
calls imgdb::sendimg() to send first the imsg_t packet, followed by the image itself. While we used the
send() socket API to send the imsg_t packet in Lab 3, you are to write imgdb::sendpkt() to send the
packet using the sendto() socket API. The destination of the sendto() call should be the client whose
address and port number you stored earlier in imgdb::client in your call to recvfrom(). In this lab,
imgdb::sendpkt() is a simple wrapper function of not more than 1 or 2 lines of code. It becomes more
complicated in PA3.
Finally, the function imgdb::sendimg() sends the image contained in the provided image argument. The
local variable ip points to the start of the image. Unlike in previous assignments, we will be
encapsulating chunks of our image data in packets with header ihdr_t attached. The definition of ihdr_t
can be found in netimg.h.
As usual, the first field is the version field, which must be of value NETIMG_VERS. The type field follows
and must be of value NETIMG_DATA. Next comes the size field, which records the size of the attached data,
in bytes, not including the header. The last field is the sequence number corresponding to the first byte of
the attached data. We will use the byte offset from the start of the image in memory as the sequence
number of each byte. So the first byte of the image has sequence number 0. The k­th byte has sequence
number k­1. A packet carrying a chunk of an image starting at the k­th byte of the image has sequence
number k­1. Since the sequence number field in ihdr_t is an int, the largest image we can transfer is
Prior to sending the image, you must ensure that the sender buffer can hold at least one packet of size
imgdb::mss, as specified by the client in its iqry_t message. The size of the sender buffer is a socket
option that you can query and set using the getsockopt() and setsockopt() socket APIs respectively.
This takes about 5 lines of code.
To send the image, you are to divide up the image into chunks of datasize, where datasize has been
computed for you as mss ‐ sizefo(ihdr_t) ‐ NETIMG_UDPIP. Obviously, the last chunk of the image may
be smaller than datasize. To marshall together a packet consisting of an ihdr_t header followed by a
chunk of data, you are NOT ALLOWED to make any copy of the image data. Instead, you are
REQUIRED to use the sendmsg() socket API. See the comments in imgdb::sendimg() for further
instructions. This task should take about 20 lines of code. Since we cannot tell from observing the
behavior of your code whether you have used sendmsg() and whether you have made copies of the image
data, this task will be graded by inspecting your code. If you don’t know how to use sendmsg() or how to
implement this task without copying, please consult the teaching staff. We will be weighing this task and
the corresponding use of recvmsg() in the client side together as being worth 1/3 of the grade points for
this lab. Please remember that if you cannot implement a required task, you must inform the teaching
staff in your writeup for PA3 and that if you turn in a different implementation, it will be considered
That’s all for Task 1. It should take about 30 lines of code in total, of which only the call to sendmsg()
should be unfamiliar to you.
Task 2: Client Side
Your second task is to write the client code. The client takes two required command line arguments,
exactly the same as in previous assignments:
% netimg ‐s <server:<port ‐q <image.tga [ ‐w <rcvwin ‐m <mss ]
where the ‐s option specifies the server’s name and port number and the ‐q option specifies the image file
name. The client also behaves the same as in previous assignments, except that instead of using TCP, it
uses UDP to transfer file. The optional ‐m command line argument allows you to set the client’s
maximum segment size, in bytes. The maximum segment size includes all headers, including the UDP/IP
headers of 28 bytes. The optional ‐w command line argument allows you to set the client’s receiver
window size, in terms of number of maximum­size segments. The implementation of both of these
optional arguments is provided to you.
You can search for the string “Lab5 Task 2” in socks.cpp and netimg.cpp to find places where “Task 2”
related code must be filled in. As with task 1, so that you don’t have to re­type your code for Lab 6, the
netimg.cpp released for Lab 5 already includes instructions/comments for Lab 6 tasks, so be careful when
you search for “Task 2” that you work only on “Lab5 Task 2” for this lab.
The function socks.cpp:socks_clntinit(sname, port, rcvbuf) creates a UDP socket and connects to the
server whose name is provided in sname, at the specified port. Since this a UDP socket, the call to
connect() in this function simply stores the server’s address and port number so that subsequent calls to
send and receive packets do not need to provide the server address. You are only asked to copy over the 2
line socket creation code from your socks_servinit() function here. Also in this function write another
3­6 lines of code to set the socket receive buffer to be of size at least rcvbuf bytes. Different systems set
different maximum limit on the receive buffer. It is set at 1.5 MB on CAEN’s eecs489 hosts. So want to
retrieve the actual receive buffer size allocated to better understand the observed behavior of your code.
The function netimage_sendqry() has been provided to you. In addition to the filename of the image the
client is searching for, the query message also carries the receiver’s window size (rwnd) and maximum
segment size (mss). The function netimg_recvimsg() is not changed from previous assignments and is
also provided to you.
If the image is found, the client initializes the graphics library and put the socket in non­blocking mode
before going into an infinite loop, receiving image data from the network and displaying it. In main()
write 1 line of code to set the socket non­blocking. We’ll be relying on the socket being non­blocking in
subsequent assignments related to this lab.
Finally, in netimg_recvimg() receives the image data one packet at a time. Recall that each data packet
consists of an ihdr_t header, followed by a chunk of data. You are NOT allowed to make any copy of the
image data. Instead, you are REQUIRED to use the recvmsg() socket API. See the comments in
imgdb_recvimg() for further instructions. This task should take about 20 lines of code similar and inverse
to the ones you wrote for imgdb::sendimg(). Again, this task will be graded by inspecting your code.
Please note in your writeup for PA3 if you are not able implement this task using recvmsg().
That’s it for Task 2. You will write a total of about 30 lines of code, most of which are very similar to the
ones you wrote for Task 1.
Testing Your Code
In addition to the skeletal code and Makefile, we’ve also provided a reference implementation of both
server and client that run on the CAEN eecs489 hosts. You should test your code with various sizes of
receiver window and/or maximum segment. When the receiver buffer is too small, most of the arriving
UDP packets will be dropped and the displayed image will only be partially completed. You should also
play with the server’s drop probability and observe how higher drop probability causes more gaps/streaks
in the displayed image.
Your home network firewall may block UDP packets, preventing you from running your client and/or
server to connect to the server/client running on the CAEN eecs489 hosts. You could use “ssh ‐Y” to
connect to the CAEN eecs489 hosts and have X­window events forwarded to your local host. This works
on Linux and Mac OS X (with XQuartz installed). On Windows, you can use MobaXterm. I would
recommend against using VNC as students have reported OpenGL incompatibilities with VNC in the
past. In both cases, the resulting image display becomes unbearably slow (worse with VNC). If your
home network firewall blocks UDP, my suggestion is to test your client and server locally on localhost
and after everything is working, then you test them on the CAEN eecs489 hosts over MWireless on
campus or over UMVPN where the local network firewall doesn’t block UDP. Similarly for testing
interoperability against the reference implementation.
Submission Instructions
Do NOT use any libraries or compiler options not already used in the provided Makefile, to ensure that
we will be able to build your code for grading. If we can’t compile your code, you will be heavily
Test your compilation on CAEN eecs489 hosts! Your submission must compile and run without errors
on CAEN eecs489 hosts using the provided Makefile, unmodified. Both your client and server must
interoperate with the reference server and client respectively.
Your “Lab5 files” comprises your socks.cpp, netimg.cpp, and imgdb.cpp files.
To turn in your Lab5:
1. Submit the SHA1’s of your Lab5 files on the CTools Assignments page. (If the URL doesn’t work
for you, just click the “Assignments” item on the left menu of the CTools page for EECS 489.)
Once you’ve submitted your SHA1’s, don’t make any more changes to the files, or your SHA1’s
will become invalid.
2. Upload your Lab5 files by pointing your web browser to Course folder and navigate to your lab5
folder under your uniqname. Or you can scp the files to your lab5 folder on IFS:
This path is accessible from any machine you’ve logged into using your ITCS (
password. Please report any problems to ITCS.
3. Keep your own backup copy! Don’t make any more changes to the files once you’ve submitted
your final SHA1’s.
The timestamp on your SHA1 submission on CTools’ Assignments page will be your time of submission.
If this is past the deadline, your submission will be considered late. You are allowed multiple
“submissions” without late­policy implications as long as you respect the deadline.
Do NOT turn in an archival (.zip or .tgz) file, instead please turn in your solution files individually.
Turn in ONLY the files you have modified. Do not turn in support code we provided that you haven’t
modified. Do not turn in any binary files (object files, executables, or images) with your assignment.
Do remove all printf()’s or cout’s and cerr’s and any other logging statements you’ve added for
debugging purposes. You should debug using a debugger, not with printf()’s. If we can’t understand the
output of your code, you will get zero point.

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