代做ICMP、代写Python、代做Python语言程序、代写Network

日期：2018-11-25 09:18

Course Work 1 – Network Application Development

Coursework Weight: 60%

In this coursework, you will develop a number of small networking-based applications. These

are designed to increase your competency in developing socket-based applications, as well as

increasing your familiarity with a number of key technologies and measures. These are in

widespread use, and commonly deployed to evaluate networks and to provide services over

them.

Coursework 1 is split into a number of smaller tasks: ICMP Ping Client, Traceroute Client, Web

Server and Web Proxy. Importantly, the tasks build upon each other; the work you do in Task

1.1 will be fundamental to Task 1.2, and similarly, the work completed in Task 2.1 will greatly

assist you in Task 2.2. Marks will be awarded for meeting certain criteria within each task.

These are outlined in more detail within each task description. You are encouraged to

progress as far as possible with each task. Do note however, that Task 1 and Task 2 are

independent; attempting both of them is advised, even if you do not fully complete each.

Submission and Assessment

The submission for all work completed in this practical is due by the end of Week 12. Please

submit 4 distinct Python scripts, named according to each task. Even though you can reuse

code from earlier tasks in the later tasks, it will simplify the marking procedure if you submit

each solution independently.

During the marking session (scheduled for Week 13), you will be expected to demonstrate

the functionality of each of these scripts. You will mainly be assessed on functionality, but

expect to be able to walk-through and explain your code. As we will also be providing you

with a few small snippets of code (to use in your own solution), you will not be expected to

explain these in great detail. However, a general understanding of how these functions work

will be beneficial to your overall learning and comprehension. There will also be a small

proportion of marks available for a consistent code style and useful commenting. Resilient

code, using try and except statements to catch errors is also preferred, and will be rewarded

accordingly.

Task 1.1: ICMP Ping

The first task is to recreate the ping client discussed in Lecture 3: Delay, Loss & Throughput.

Remember that ping is a tool used to measure delay and loss in computer networks. It does

this by sending messages to another host. Once a message has reached that host, it is sent

back to the sender. By measuring the amount of time taken to receive that response, we can

determine the delay in the network. Similarly, by tracking the responses returned from our

messages, we can determine if any have been lost in the network.

ping traditionally uses Internet Control Message Protocol (ICMP) messages to achieve this

behaviour. More details can be found in RFC777. For this task, we will be sending echo request

messages (with an ICMP type code of 8). These requests are useful to us because on reaching the

client, the client will respond with an echo reply message (with an ICMP type code of 0). By

timing the period of time elapsed between sending the request and receiving the reply, we

can accurately determine the network delay between the two hosts.

Remember, you are recreating ping without the use of external libraries; they are explicitly

prohibited!

Implementation Tips

There are a number of aspects to consider when writing your implementation. Carefully think

about the logic required; use a whiteboard if need be. A ping client sends one ICMP echo

request message at a time and waits until it receives a response. Measuring the time between

sending the message and receiving it will give us the network delay incurred in transit.

Repeating this process provides us with a number of delay measurements over time, showing

any deviation that may occur.

To assist you in your implementation, we have provided skeleton code for this task. This can

be found on the SCC. 203 Moodle page. It contains suggested functions, as well as an overview

of functionality to be implemented by each. These are given as comments and are to be

treated as guidance only. Note that you may have to change the parameters passed to each

function as you advance with the task. The following Python libraries will also be useful to

your implementation:

https://docs.python.org/2/library/socket.html

https://docs.python.org/2/library/struct.html

https://docs.python.org/2/library/time.html

https://docs.python.org/2/library/select.html

https://docs.python.org/2/library/binascii.html

Note that to run a privileged socket, such as socket.SOCK_RAW, your script must be run with

elevated privileges (such as sudo). Note that an alternative solution using unprivileged

sockets, such as socket.SOCK_DGRAM, is also acceptable.

We have provided you with a checksum function (included in the skeleton code) which can

be freely used in your solutions without penalty. It is important that when passing a packet

to this function, the checksum field must contain a dummy value of 0. Once the checksum has

been calculated, it can be immediately inserted in the packet to send.

The ICMP header contains both an identifier and a sequence number. These can be used by

your application to match an echo request with its corresponding echo reply. It is also worth

noting that the data included in an echo request packet will be included in its entirety within

the corresponding echo reply. Use these features to your advantage.

Debugging and Testing

Any host, whether this be a PC, laptop, phone or server, should respond to a message

generated by your application. In reality, this is not always the case, as both networks and

hosts can choose to disregard these packets, and may do so for a number of reasons (including

security). For the purposes of this task, using any well-known server is acceptable. As an

example, the following popular sites will respond to an echo request: lancaster.ac.uk,

baidu.com, or youkou. These will all return with relatively low delays. To rigorously test your

application, using servers located further afield will usually return larger delays. For example,

the US Department of Education (www.ed.gov) can be queried.

To confirm that the server you have chosen to test with does in fact respond to ICMP echo

request messages, feel free to use the existing built-in ping tool to verify reachability.

Once you are sending packets, you can use the Wireshark tool to inspect these. Wireshark

will also let you investigate the packets that you receive back. In both cases, it provides a

useful method to ensure that these contain the expected information. This is a very useful

tool for debugging, especially if you are getting unexpected errors; this will show exactly what

is being sent from your script. Wireshark is installed in the virtual machine and can be started

from the graphical interface. Once started, you can capture packets on the eth0 interface (as

highlighted in Figure 1). It may also be useful to filter packets to icmp only, using the filter bar

found towards the top of the interface (also highlighted in Figure 1).

Practical 1 Network Application Development Lent Term, 2018

SCC. 203 Computer Networks 4

Figure 1: Wireshark Interface

Marking Criteria

You will be awarded the majority of marks for a functioning replica of the ping tool. That

is, you can successfully send and receive ICMP echo messages, timing the delay between.

This is then reported in the terminal window. Your application should continue to perform

these measurements until stopped.

If you are unsure about the accuracy of the delay measured by your own tool, use the builtin

ping tool to confirm your results. We’re not expecting the results to be perfectly

identical (delay changes all the time), but showing that they are close is expected.

Additional marks will be awarded for the following aspects:

Taking an IP or host name as an argument

Once stopped, show minimum, average and maximum delay across all measurements

Configurable measurement count, set using an optional argument

Configurable timeout, set using an optional argument

Measuring and reporting packet loss, including unreachable destinations

Handling different ICMP error codes, such as Destination Host Unreachable and

Destination Network Unreachable

Practical 1 Network Application Development Lent Term, 2018

SCC. 203 Computer Networks 5

Please note that the features mentioned above are considered supplementary; you do not

have to complete them all, and you can still receive a satisfactory mark without completing

any of them. They are intentionally challenging and designed to stretch you.

Task 1.2: Traceroute

The second aspect of this task is to recreate the traceroute tool, again in Python. As

discussed in Lecture 3: Delay, Loss & Throughput, this is used to measure latency between the

host and each hop along the route to a destination. This too uses an ICMP echo request

message, but with an important modification: the Time To Live (TTL) value is initially set to

1. This ensures that we get a response from the first hop; the network device closest to the

host we are running the script on. When the message arrives at this device, the TTL

counter is decremented. When it reaches 0 (in this case at the first hop), the message is

returned to the client with an ICMP type of 11. This indicates that TTL has been exceeded.

As with the previous task, by measuring the time taken to receive this response, delay can

be calculated at each hop in the network. This process can be repeated, increasing the TTL

each time, until we receive an echo reply back (with an ICMP type of 0). This tells us that we

have reached the destination, so we can stop the script.

Implementation Tips

As with the previous task, make sure you think carefully about the logic here. Remember

you can build upon your Task 1.1 implementation, although you should submit two separate

scripts; one for each subtask.

As before, the checksum function included in the skeleton code can be used without

penalty.

The same Python documentation as noted in Task 1.1 will be useful for this task too. Of

particular note is the socket.setsockopt(level, optname, value) function,

which can be used to set the TTL of a socket (and thus the packets leaving it):

https://docs.python.org/2/library/socket.html#socket.socket.setsockopt

Debugging and Testing

As with the previous task, every host on the path to your chosen destination should

respond to your echo request message. In reality, these messages are often filtered, including

within the lab network. As a result, it is especially difficult to test this tool with a remote

host. Instead, it is suggested that you test with a closer endpoint that is reachable:

lancaster.ac.uk. Although the number of hops is small (~5), it can still be used to

demonstrate the working of your application. If you run your script whilst attached to a

different network, such as that at home, your results likely differ. You will also be able to

reach external hosts more easily.

The traceroute utility can be used to confirm the results generated by your own

application. This is installed on the virtual machine if you wish to use it. Be aware that by

default, this tool actually sends messages over UDP instead of ICMP; this is done to avoid

Practical 1 Network Application Development Lent Term, 2018

SCC. 203 Computer Networks 6

the blocking discussed earlier. To force traceroute to send packets using ICMP, the -I

flag can be used. See the Linux man page for more details:

https://linux.die.net/man/8/traceroute

As with Task 1.1, Wireshark can be used to inspect the packets leaving your application.

Comparing these to those created using the traceroute utility will provide you with a

meaningful comparison.

Marking Criteria

The majority of marks will be awarded for ensuring that your implementation behaves in a

way similar to the traceroute utility. This includes providing delay measurements for

each of the nodes between your machine and the chosen remote host. You are expected to

increase the TTL of each message, until you reach this final destination.

Additional marks will be awarded for the following aspects:

Measuring and reporting packet loss, including unreachable destinations

Repeated measurements for each node

Configurable timeout, set using an optional argument

Configurable protocol (UDP or ICMP), set using an optional argument

Resolve the IP addresses found in the responses to their respective hostnames

As before, please note that the features mentioned above are considered supplementary;

you do not have to complete them all, and you can still receive a satisfactory mark without

completing any of them. They are intentionally challenging and designed to stretch you.

Task 2.1: Web Server

For the second task of this practical, you will be building a simple HTTP web server. Web

Servers are a fundamental part of the Internet; they serve the web pages and content that

we are all familiar with. You will be learning more about web servers and the operation of

the HTTP protocol in Lecture 6: Web & HTTP. Fundamentally, a web server receives a

HTTP GET request for an object (usually a file), located on the web server. Once it receives

this request, the web server will respond by returning this object back to the requester.

As with the previous task, we will be using network sockets to build our application and to

interact with the network. The Web Server differs from the ICMP Ping application in that it

will bind to an explicit socket, identified by a port number. This allows the Web Server to

listen constantly for incoming requests, responding to each in turn. HTTP traffic is usually

bound for port 80, with port 8080 a frequently used alternative. For the purposes of this

application, we suggest you bind to a high numbered port above 1024; these are

unprivileged sockets, which reduces the likelihood of conflict with existing running services

on the virtual machine. For interest, application developers can register port numbers with

the Internet Assigned Numbers Authority (IANA), reserving them for their application’s

use:

Practical 1 Network Application Development Lent Term, 2018

SCC. 203 Computer Networks 7

https://www.iana.org/assignments/service-names-port-numbers/service-names-portnumbers.xhtml

The application you build should respond to HTTP GET requests, and should be built to

HTTP/1.1 specification, as defined in RFC2616. These requests will contain a Request-URI,

which is used to define the path to the object requested. For example, a request with a URI

of 127.0.0.1:8000/index.html, will serve a file name index.html found in the

same directory as the Python script itself. The URI is broken down as follows:

127.0.0.1: Hostname of web server

8000: Port number that web server has bound to

index.html: File to be served

On successfully finding and loading the file, it will be sent back to client with the appropriate

header. This will contain the Status-Code 200, meaning that the file has been found OK, and

that it will be delivered to the client as expected. Your implementation needs only serve

files from the same directory in which the Python script is executed.

Implementation Tips

As before, we have provided skeleton code that can be used to aid you in this task. This can

be found on the course’s Moodle page. It contains suggested functions, as well as an

overview of functionality to be implemented by each. These are given as comments and are

to be treated as guidance only. Note that you may have to change the parameters passed

to each function as you advance with the task. An example HTML file (index.html) is

also provided in the same location. The following Python library and its documentation may

also serve as a pointer to helpful functions (the latter will be of particular interest):

https://docs.python.org/2/library/socket.html

https://docs.python.org/2/library/socket.html#socket.socket.accept

As a baseline, your implementation needs only to be single-threaded. This allows a

maximum of one request to be handled at a time.

Debugging and Testing

To test your web server application, you must generate a valid request. There are a number

of tools to achieve this. For example, the wget utility can be used to generate a request

(presuming your web server is running on port 8000):

wget 127.0.0.1:8000/index.html

An equally valid method is to use a web browser, such as the Chromium Web Browser

installed on the virtual machine. Simply point the browser to the same URL:

127.0.0.1:8000/index.html

If you are unsure about what a HTTP request should look like, Wireshark can again be used

to inspect packets. This includes both the HTTP request and response. This will help you

debugging the form and structure of your requests, identifying any issues that may be

Practical 1 Network Application Development Lent Term, 2018

SCC. 203 Computer Networks 8

present. If you are still using Wireshark from the previous task, make sure to remove the

icmp filter! http can be used instead. It will also be necessary to capture packets on the

loopback interface (lo), rather than the external interface (eth0).

If you wish to observe how a Web Server should behave (and examine the packets

generated by such), Python provides a handy way of starting a very simple HTTP server

implementation:

python -m SimpleHTTPServer

Requests to this server can be made using the methods described previously.

Marking Criteria

For this task, you will be awarded marks for a functioning Web Server, capable of handling

requests for content. You should be able to demonstrate that, given a request, the Web

Server will return the correct file, as well as producing a well-formed response header with

protocol version and response code set correctly.

Additional marks will be awarded for the following aspects:

Binding the Web Server to a configurable port, defined as an optional argument

When a requested file is not available on the server, return a response with the

status code Not Found (404)

Create a multithreaded server implementation, capable of handling multiple

concurrent connections

Write your own HTTP client to query the web server (this should be submitted as

an additional standalone Python file)

Please note that the features mentioned above are considered supplementary; you do not

have to complete them all, and you can still receive a satisfactory mark without completing

any of them. They are intentionally challenging and designed to stretch you.

Task 2.2: Web Proxy

Building on the Web Server described in Task 2.1, this task is concerned with building a

Web Proxy. This operates in much the same way as a web server, with one significant

difference: once configured to use the Proxy Cache application, a client will make all

requests for content via this proxy. Normally, when we make a request (without a Web

Proxy), the requests travels from the host machine to the destination. The Web Server

then processes the request and sends back a response message to the requesting client.

However, when we use a Web Proxy, we place this additional application between the

client and the web server. Now, both the request message sent by the client, and the

response message delivered by the web server, pass through the Web Proxy. In other

words, the client requests the objects via the Web Proxy. The Web Proxy will forward the

client’s request to the web server. The web server will then generate a response message

and deliver it to the proxy server, which in turn sends it to the client. The message flow is

as below:

Practical 1 Network Application Development Lent Term, 2018

SCC. 203 Computer Networks 9

As with the Web Server, your Web Proxy application is only expected to handle HTTP/1.1

GET requests. Similarly, the Web Proxy will also bind to a specific port (this can be the

same as the Web Server), and continue to listen on this port until stopped.

Debugging and Testing

As with Task 2.1, there are a number of ways to test your Web Proxy. For example, to

generate requests using wget, we can use the following:

wget lancaster.ac.uk -e use_proxy=yes -e http_proxy=127.0.0.1:8000

This assumes that the Web Proxy is running on the local machine and bound to port 8000.

In this case, the URL requested from the proxy is lancaster.ac.uk.

A web browser can also be used to the same effect. Many web browsers support the use of

a web proxy through configuration. For example, the Chromium browser included in the

virtual machine image can be started in the following way to utilise the Web Proxy:

chromium-browser --proxy-server=”127.0.0.1:8000”

A caveat when testing your Web Proxy: some websites have enabled HTTP Strict Transport

Security (HSTS) (RFC6797). This forces clients (including both wget and a web browser) to

use HTTPS rather than HTTP. HTTPS is a secure version of HTTP, but we will consider this

out of scope for this practical. To check if a website has this feature enabled, use the

following tool, and ensure that the website you are using for testing purposes is not listed:

https://hstspreload.org/

As with the other tasks, Wireshark can be used to capture and investigate packets sent to

and from your proxy. As the proxy will be receiving local requests from the web browser,

as well as making external requests to fetch content, it is necessary to capture packets on

both the external (eth0) and loopback (lo) interfaces.

Marking Criteria

For this task, the majority of marks will be awarded for demonstrating a working Web

Proxy. You are expected to show the functionality of such through the use of either wget

or a properly configured web browser. Note that you are not expected to demonstrate the

Web Proxy using a website with HSTS enabled (see above).

Additional marks will be awarded for the following aspects:

Client Web

Proxy

Web

Server

Request Request

Response Response

Practical 1 Network Application Development Lent Term, 2018

SCC. 203 Computer Networks 10

Binding the Web Proxy to a configurable port, defined as an optional argument

Support for other HTTP request types (PUT, DELETE, etc.)

Object caching: A typical Web Proxy will cache the web pages each time the client

makes a particular request for the first time. The basic functionality of caching works

as follows. When the proxy gets a request, it checks if the requested object is

cached, and if yes, it returns the object from the cache, without contacting the

server. If the object is not cached, the proxy retrieves the object from the server,

returns it to the client and caches a copy for future requests. In practice, the proxy

server must verify that the cached responses are still valid and that they are the

correct responses to the client's requests. You can read more about caching and

how it is handled in HTTP in RFC2068. Add the simple caching functionality

described above. You do not need to implement any replacement or validation

policies. Your implementation, however, will need to be able to write responses to

the disk (i.e., the cache) and fetch them from the disk when you get a cache hit. For

this you need to implement some internal data structure in the proxy to keep track

of which objects are cached and where they are on the disk. You can keep this data

structure in main memory; there is no need to make it persist.

As before, please note that the features mentioned above are considered supplementary;

you do not have to complete them all, and you can still receive a satisfactory mark without

completing any of them. They are intentionally challenging and designed to stretch you.