tcp sockets

NETWORK

ARCHITECTURE 1

Project 1

By Chimmili Hari Kalyani (16212951) and Tanya

Srivastava (16211490)

Network Architecture

Project 1

By Chimmili Hari Kalyani and Tanya Srivastava

P a g e 1 | 12

1. Reviewed KR, P&D code and “UNIX Network Programming” by Stevens.

2. Obtained the source code for the client and server (posted below): simplex-

talk.c and simplex-talk-server.c. Make sure you can successfully compile

these programs using gcc. (Important: The code uses 5432 as the TCP port,

it is strongly suggested you choose a random port greater than 10000.)

Source code files are: SimpleClient.c, SimpleServer.c

Downloaded gcc (Cygwin) compiler and installed on my PC.

Chose TCP port# 55005

Compiled source code (both client and server)

3. Experiments with TCP sockets


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a. Opened two terminals. Started one server in one terminal and one

client in another terminal (as shown in the below figure).

Sent message from the client to the server successfully (is shown in

the figure below).

b. While the first client is running, started nine other clients in the

background with their input redirected from SampleText.txt file.


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c. Answer the following questions:

i. What happens to these ten clients?

Answer: The first 5 clients are blocked (excluding the first running

client) and those messages are not displayed on the server. The

client number 7, 8, 9 and 10 connections are refused.

ii. Do their connect()s fail, or time out, or succeed?

Answer: Connections are refused for the client number 7, 8, 9 and

10.

iii. Do any other calls block?

Answer: Yes, other than the first running client, the next five

client calls are blocked.


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iv. Wait a bit, then make the first client exit. What happens?

Answer: After exiting the first client, there are 5 messages

displayed on the server terminal. Only the first 5 clients are able to

communicate to the server successfully.

d. Changed the server constant MAX PENDING to 1 and recompiled the

server source code.

e. Opened two terminals. Started one server in one terminal and one

client in another terminal. Able to send messages from the client to

the server. Left both the server and the client in running mode.


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f. While the first client is running, started nine other clients in the

background with their input redirected from SampleText.txt file (as

shown in below).

g. Answer the following questions:

v. What happens to these ten clients?

Answer: The first client is blocked (excluding the first running client)

and the sampletext.txt message is not displayed on the server. The

other client (from 3 to 10) connections are refused.

vi. Do their connect()s fail, or time out, or succeed?

Answer: Connections are refused for the client number 3 to 10.


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vii. Do any other calls block?

Answer: Yes, other than the first running client, the second client call

is blocked.

viii. Wait a bit, then make the first client exit. What happens?

Answer: After exiting the first client, the second client text message

displayed on the server terminal. Only the second client is able to

communicate to the server successfully.

h. NOTE: As we do not have a second PC – to perform this step.


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We have used Hercules Utility Tool (running on my PC) in order to

mimic the second host.

Fig. Hercules utility tool

i. In Wireshark, under the Capture Options, entered tcp port 55005

as a capture filter and started as shown below.


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j. Started Client on my PC (host) and typed several messages. And

typed one very long message without hitting enter, I quitted the Client

and stopped capture on Wireshark.

Because I could not run server and client on separate hosts ( I ran both

on the same host) – unfortunately, I could not capture any packets

using WireShark, with “tcp port 55005” as a filter.


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k. Answer the following

i) How many packets were transmitted?

Answer: One packet

ii) What is the correspondence between messages and packets?

Answer: The long messages will be split into multiple packets of size

equal to ~1500 bytes, using TCP protocol.

l. Modified the client and server so that each time the client sends a

line to the server, the server sends the line back to the client (as shown

in figure below).


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The client (and server) are now make alternating calls to recv() and send().

4. Experiments with UDP sockets

a) Modified the simplex-talk client and server source code to use UDP as the

transport protocol, rather than TCP. In both client and server, changed

SOCK STREAM to SOCK DGRAM.

b) In the client, removed the call to connect(). Used the sendto() syscall in the

place of the send() syscall.

c) In the server, removed the calls to listen() and accept(), and used a single

loop that calls recvfrom() with the sockets.

d) What happens when 2 or more clients connect to the same UDP server?

In UDP (stateless protocol), we have observed there is no issue when more

than 2 clients connected and no client is ever in the "connected" state.


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Multiple clients are able to send packets to the same server, and their packets

arrived at the server (in random sequence).

e) How does this differ from TCP behavior?

We have observed that in TCP the messages sent from clients one after

the other (in order) and the messages received on the server is in the

same sequence (first send client message is received first).

In UDP, packets are sent individually. One packet per one read call.

In case of TCP, data is received as a “stream,” with nothing

distinguishing where one packet ends and another begins. There may

be multiple packets per read call.

In TCP, the long message sent from the multiple clients received

without lost or corrupted and all the messages are able to receive by the

server.

In case of UDP, the one very long message is not received correctly at

the server end (message received partially). The server did not receive

the long messages are sent from some of the clients, is shown in the

figure below.


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tcp sockets

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