Project Part #3 Solution

For part 3 of the project, your team will continue to improve the socket library you have developed in part 1. For this part, you will add windowing to the messages sent by your socket library. The client and server have been modified to use random packet sizes for sending and receiving.

 

As part of the project part 1, you will be given a number of files. You can also find them in the sakai site under "Resources" - "Project resources" - "Part 3" .

 

1.  client3.c : This is the original client source code. file. You may not alter the code for this file. It must run using your sock352.py file.

 

2.  server3.c : This is the original server file You may not alter the code for this file. It must run using your your sock352.py file.

 

3. sock352.py: This is a new library for part 3. You must fill in the methods defined in this file, as below.

 

 

 

Your library must implement the following methods as defined in the sock352.py file:

 

def init(UDPportTx,UDPportRx): def readKeyChain(filename): def   init  (self):

def bind(self,address): def connect(self,*args): def listen(self,backlog): def accept(self,*args): def close(self):

def send(self,buffer):

def recv(self,nbytes):

def readKeyChain(filename):

 

 

 

3. The 352 RDP v1 protocol:

 

Recall as in TCP, 352 RDP v1 maps the abstraction of a logical byte stream onto a model of an unreliable packet network.  352 RDP v1  thus closely follows TCP for the underlying packet protocol. A connection has 3 phases: Set-up, data transfer, and termination.  352 RDP v1 uses a much simpler timeout strategy than TCP for handling lost packets.

 

Packet structure:

 

The CS 352 RDP v1 packet as defined as:

 

< –-----------------------------32 Bits –-----------------------------
Version
Flags
Option
Protocol
Header Length
Packet Checksum
Source Port
Destination Port
 

 

Sequence Number
 

 

Acknowledgement Number
Receiver’s Window
Payload Length
 

 

 

The flags field is defined as:

 

< –-----------------------------8 Bits –-----------------------------
 
 
 
Has Option
RESET
ACK
FIN
SYN
 

Connection Set Up:

 

The client initiates a connection by sending a packet with the SYN bit set in the flags field, picking a random sequence number, and setting the sequence_no field to this number. If no connection is currently open, the server responds with both the SYN and ACK bits set, picks a random number for

it's sequence_no field and sets the ack_no field to the client's incoming  sequence_no+1. If there is an existing connection, the server responds with the  sequence_no+1, but the RST flag set.

 

Connection additions for part 3: Window sizes

 

 

Figure 1: Windowing

Figure 1 show how windowing works. The initial window size is advertised as W, in the is assignment W = 32KB. . Suppose the client sends an amount of data less than the window, of size X bytes. When the application layer calls recv() in sizes of less than X, the receiver must keep the data in a queue. For each recv() call, the receiver must generate and empty ACK with the new window size, which is the space left in the queue.

 

If the sender tries to exceed the window size, the send() call can not return until there is some space in the queue. The send() call should loop receiving packets until there is some space. The send() call can return less data was actually sent than was requested.

 

Data exchange:

352 RDP follows a simplified Go-Back-N protocol for data exchange, as described in section Kurose and Ross., Chapter 3.4.3, pages 218-223 and extended to TCP style byte streams as described in Chapter 3.5.2, pages 233-238.

 

When the client sends data, if it is larger than the maximum UDP packet size (64K bytes, minus the size of the sock352 header), it is first broken up into segments, that is, parts of the application byte- stream, of up to 64K. If the client makes a call smaller than 64K, then the data is sent in a single UDP packet of that size, with the payload_len field set appropriately.  Segments are acknowledged as the last segment received in-order (that is, go-back-N).  Data is delivered to the higher level application in- order based on the read() calls made. If insufficient data exists for a read() call, partial data can

be returned and the number of bytes set in the call's return value.

 

Not that just like TCP, the ACK field is set for each data packet.

 

Timeouts and retransmissions:

352 RDP v1 uses a single timer model of timeouts and re-transmission, similar  to TCP in that there should be a single timer  per connection, although each segment has a logical timeout. The timeout for a segment is 0.2 seconds. That is, if a packet has not been acknowledged after 0.2 seconds it should be re-transmitted, and the logical timeout would be set again set to 0.2 seconds in the future for that segment. The timeout used for a connection should be the timeout of the oldest segment.

 

There are two strategies for implementing timeouts. One approaches uses Unix signals and other uses a separate thread. These will be covered in class and recitation.

 

Connection termination:

Connection termination will follow a similar algorithm as TCP, although simpified.  In this model, each side closes it's send side separtately, see pages 255-256 of Kurose and Ross and pages 39-40 of

Stevens. In version 1, it is OK for the client to end the connection with a FIN bit set when it both gets the last ACK and close has been called. That is, close cannot terminate until the last ACK is received from the server. The sever can terminate the connection under the same confitions.

 

If the socket receives an FIN from the other side, and it's data buffer is empty, the socket can be closed after a timeout of 5 seconds.

3. Grading:

Functionality: 80%

15% - Part 1 working (i.e. no encryption, tested with client1.py and server1.py)

15% - Part 2 working (encryption, tested with client2.py and server2.py)

50% - Part 3 working (windowing with random packet sizes – client3.py and server3.py) Style: 20%

 

Style:

Style points are given by the instructor and TA after reading the code. Style is subjective, but will be graded on a scale from 1-5 where 1 is incomprehensible code and 5 means it is perfectly clear what the programmer intended.

 

4. What to hand in

 

You must hand in a single archived file, either zip, tar, gzipped tar, bzipped tar or WinRAR (.zip, .tar,

.tgz, .rar) that contains:  (1) README.TXT file with your team members, (2) the cllient1.py, client2.py and client3.py source code, (3) the server1.py, server2.py and server3.py source code, (4) a single sock352.py which works for all clients and servers and (5) any other files your library needs to work.

 

Your archive file must include a file called “README.TXT” that includes the names of the project partners for the project!