代写program编程、代做Python编程设计

日期：2024-04-26 09:49

Project 3: TCP Implementation

Introduction

TCP serves many purposes: it provides reliable, in-order delivery of bytes, it makes

sure the sender does not send too fast and overwhelm the receiver (flow control), and

it makes sure the sender does not send too fast and overwhelm the network

(congestion control). It also aims to be fair: when multiple senders share the same

link, they should receive roughly the same proportion of bandwidth. In this project,

you will demonstrate your understanding of the TCP basics by implementing the TCP

protocol.

Be prepared: this is a single-person project, and your skills will be exercised. So start

early and feel more than welcome to ask questions. However, please note that the TAs

are not allowed to debug your code for you during their office hours. They cannot

touch your keyboard, and they will only spend a maximum of 10 minutes reading

your code. This helps them to assist more students effectively. Therefore, it is

recommended that you visit their office hours with specific questions instead of vague

statements like "my code doesn’t work." For debugging, logging or tools like

Wireshark1

can be helpful. There are many ways to do this; be creative.

TCP Recap

TCP is a transport layer protocol that enables different devices to communicate. As a

reminder the basic setup is the following.

You have an initiator (client) and a listener (server). The listener is waiting to receive

a connection from the initiator. After a connection is received, they perform a TCP

handshake to initiate the connection. Afterward, they can read and write to each other.

From the application layer, reads and writes to the socket are buffered before being

sent over the network. This means that multiple reads or writes might be combined

into a single packet or the opposite, that a single read or write to a socket might be

split into many packets.

To establish reliable data transfer, TCP must manage many different variables and

data structures. Here are some examples of the details you’ll need to track.

1 https://www.wireshark.org/

Let’s say we have sockets A and B, and that socket A wants to start sending data to

socket B. A will store the data in a buffer that it will pull from for sending packets.

Socket A will then create packets using data from the buffer and send as many as it is

allowed to send based on the congestion control algorithm used. As Socket B receives

packets, it stores the data transmitted in a buffer. This buffer helps with maintaining

the principle of in-order data transmission. B sends ACKs as responses to A to notify

A that various bytes have been received up to a certain point. B is also tracking the

next byte requested by the application reading from the socket, and when it receives

this byte, it will forward as much data in order that it can to the application buffer. For

example, if B is looking for byte number 400, it will not write any other bytes into the

application’s buffer until it receives byte number 400. After receiving byte number

400, it will write in as many other bytes as it can. (If B had bytes 400-1000 then all

would be written to the application buffer at the time of receiving the packet with byte

400. As packets are ACK’d, socket A will release memory used for storing data as

they no longer need to hold onto it. Finally, either side can initiate closing the

connection where the close handshake begins.

As both sides (initiator and listener) can both send and receive, you’ll be tracking a lot

of data and information. It’s important to write down everything each side knows

while writing your implementation and to utilize interfaces to keep your code module

and re-usable.

Project Specification

You are implementing the interfaces in tcpSock.py, such as case_socket and

case_close. Your code will be tested by us creating other Python files that will

utilize your interface to perform communications. The starter code has an example of

how we might perform the tests, we have a client.py and server.py which utilize the

sockets to send information back and forth. You can add additional helper functions to

tcpSock or change the implementation of the 4 core functions (socket, close,

read, and write). However, you cannot change the function signature of the 4 core

functions. Further, we will be utilizing grading.py to help us test your code. We may

change any of the values for the variables present in the file to make sure you aren’t

hard-coding anything. Namely, we will be varying the packet length and the initial

window variables.

Starter Code

The following files have been provided for you to use:

• packet.py: this file describes the basic packet format and header. You are not

allowed to modify this file.

• grading.py: these are variables that we will use to test your implementation;

please do not make any changes here, as we will replace them when running

tests.

• server.py: this is the starter code for the server side of your transport

protocol.

• client.py: this is the starter code for the client side of your transport

protocol.

• tcpSock.py: this contains the main socket functions required of your TCP

socket, including reading, writing, opening, and closing.

• backend.py: this file contains the code used to emulate the buffering and

sending of packets. This is where you should spend most of your time.

All the communication between your server and the client will use UDP as the

underlying protocol. All packets will begin with the common header described in

packet.py as follows:

− Identifier [4 bytes]

− Source Port [2 bytes]

− Destination Port [2 bytes]

− Sequence Number [4 bytes]

− Acknowledgement Number [4 bytes]

− Header Length [2 bytes]

− Packet Length [2 bytes]

− Flags [1 byte]

− Advertised Window [2 bytes]

All multi-byte integer fields must be transmitted in network byte order.

socket.ntoh and socket.hton and other related functions will be very

important for you to call. All integers must be unsigned, and the identifier should

be set to 3425. You are not allowed to change any of the fields in the header.

Additionally, packet length cannot exceed 1400 in order to prevent packets from

being broken into parts.

You can verify that your headers are sent correctly using wireshark or tcpdump.

You can view packet data sent including the full Ethernet frames. When viewing

your packet, you should see something similar to the below image; in this case, the

payload starts at 0x0035. The identifier - 3425 - shows up in hex as 0x0d61.

Implementation Tasks

1. TCP Handshakes - Implement TCP start and end handshakes before data

transmission starts and ends [1]. This should happen in the constructor and

destructor for case tcp socket.

2. Flow Control - You will notice that data transfer is very slow. That is because

the starter code is using the Stop-and-Wait algorithm, transmitting one packet

at a time. You can do much better by using a window of outstanding packets to

send on the network. Extend the implementation to: 1) Change the sequence

numbers and ACK numbers to represent the number of bytes sent and received

(rather than segments) 2) Implement TCP’s sliding window algorithm to send a

window of packets and utilize the advertised window to limit the amount of

data sent by the sender [2].

3. RTT Estimation - You will notice that loss recovery is very slow! One reason

for this is the starter code uses a fixed retransmission timeout (RTO) of 3

seconds. Implement an adaptive RTO by estimating the RTT [3].

4. Duplicate ACK Retransmission - another reason loss recovery is slow is that

the starter code relies on timeouts to detect packet loss. One way to recover

more quickly is to retransmit whenever you see triple duplicate ACKs.

Implement retransmission upon receipt of three duplicate ACKs.

For 325 students, you only need to implement Tasks 1 and 2; you will get extra

credits for implementing all 4 tasks. For 425 students, you will need to implement

all of them.

References

[1] TCP connection establishment and termination:

https://book.systemsapproach.org/e2e/tcp.html#connection-establishment-and-termination

[2] TCP sliding window:

https://book.systemsapproach.org/direct/reliable.html#sliding-window

https://book.systemsapproach.org/e2e/tcp.html#sliding-window-revisited

[3] Adaptive retransmission:

https://book.systemsapproach.org/e2e/tcp.html#adaptive-retransmission

Useful Links

Programming in Python

The 8th edition of the textbook (and what we’ve discussed in class) uses sockets in Python. A

Python socket tutorial is http://docs.python.org/howto/sockets.html

It may also help by reading the system implementation of epoll at

https://codebrowser.dev/glibc/glibc/sysdeps/unix/sysv/linux/sys/epoll.h.html. You

will better understand the epoll events.

REMINDERS

o Submit your code to Canvas in a zip file

o If your code does not run, you will not get credits.

o Document your code (by inserting comments in your code)

o DUE: 11:59 pm, Friday, April 26th

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