Description
Part 1: TCP
• The TCP is a connection‐oriented protocol meaning that it
establishes an end‐to‐end connection before any data is sent.
• Typically, TCP connections go through three phases: connection
establishment, data transfer and termination. These phases in TCP
segments are identified by flags.
• TCP uses an end‐to‐end flow control protocol to avoid having the
sender send data too fast for the TCP receiver to receive and
process it reliably.
• It applies a sliding window flow control protocol. In each TCP
segment, the receiver specifies in the receive window field the
amount of additionally received data (in bytes) that it is willing to
buffer for the connection. The sending host can send only up to that
amount of data before it must wait for an acknowledgment and
window update from the receiving host.
• TCP applies congestion control mechanisms which control the rate
of data entering the network, keeping the data flow below a rate
that would trigger collapse.
• For each connection, TCP maintains a congestion window, limiting
the total number of unacknowledged packets that may be in transit
end‐to‐end.
• This is somewhat analogous to TCP’s sliding window used for flow
control.
• TCP uses a mechanism called slow start to increase the congestion
window after a connection is initialized or after a timeout.
• It starts with a window of two times the maximum segment size
(MSS). Although the initial rate is low, the rate of increase is very
rapid; for every packet acknowledged, the congestion window
increases by 1 MSS so that the congestion window effectively
doubles for every round‐trip time (RTT).
o Use Start up your web browser and clear the browser’s cache
memory.
o Open up the Wireshark and start packet capture.
o Go to the https://www.info308a.ece.mcgill.ca/Lab1Ex2.html.
o Stop Wireshark packet capture.
o Find the IP address of the website i.e. : ***.***.***.***
o In the filter filed, type “ip.addr == ***.***.***.*** && tcp” to see only
the TCP packets. Press Apply
1. How many TCP datagrams are exchanged between your computer and the
server to establish the TCP connection? Why each of these segments is
needed to setup the TCP connection?
2. Which end point started the TCP Connection‐Establishment phase?
3. What flags are set in each of these TCP datagrams?
4. What is the initial value of the sequence number on the client’s side?
5. What is the initial value of the sequence number on the server’s side?
6. What is the value of the Acknowledgement field in the SYN ACK datagram?
How did the server determine that value?
7. For the TCP SYN datagram, determine the following
8. the source port number
9. the destination port number
10.the size of the window
11.the header length
12.For the TCP SYN ACK datagram, determine the following
13.the source port number
14.the destination port number
15.the size of the window
16.the header length
17.What is the usage of the window field in the TCP segments?
18.Consider the TCP segment containing the HTTP GET as the first segment in
the TCP connection. For the first three TCP segments, answer the following
questions:
19.When was each segment sent?
20.At what time was the ACK for each segment received?
Part 2: HTTP
The Overview:
• We will investigate the basic HTTP GET request/response
interaction by retrieving a simple HTML file which is very short,
and has no embedded objects.
o Start up your web browser.
o Open up the Wireshark. In the filter field, type “http”
(without the quotation marks), so that only captured
HTTP messages will be displayed later in the packet‐listing
window.
o Note: After you have changed the expression in the filter
input box, do not forget to press the Apply button (or the
Enter/Return key twice), to apply this filter string to the
displayed trace file.
o Start Wireshark packet capture.
o Go back to your web browser and enter:
o https://www.info308a.ece.mcgill.ca/Lab1Ex1.html
o Stop Wireshark packet capture.
30 What HTTP request method is used to retrieve the HTML file?
31 What is the URI of the requested file?
32 What HTTP version is your browser running? What are the other versions of
HTTP?
33 What languages does your browser accept for response?
34 What is the IP address of your computer?
35 What is the server’s IP address?
36 What is the relationship between source and destination IP addresses of the
first GET and the source and destination IP addresses of the first response?
37 What is the status code of the first response message? What does this code
indicate? What code is returned if the requested file cannot be found on the
server?
38 When was the last time that the received HTML file was modified at the
server?
39 What is the size of the content that is returned to your browser?
• In the previous exercise, we retrieved a short HTML file. Here, we
will see what happens when we download a long HTML file.
o Start up your web browser.
o Open up the Wireshark.
o Start Wireshark packet capture.
o Enter the following URL into your browser
o https://www.info308a.ece.mcgill.ca/Lab1Ex2.html
o Stop Wireshark packet capture, and type “http” in the filter
field.
40 How many HTTP GET request messages are sent by your web browser?
41 By inspecting the entire trace, determine the number of packets that contain
HTTP header. Explain your answer.
42 How many TCP segments are transmitted to your computer? Why multiple
segments are required to retrieve this single HTML file?
43 Determine the length of these TCP segments. Do they have the same size?
Explain your answer.
44 Which message and what field in that message indicate that the server was
able to process the request successfully?
45 What is the status code of the first response message?
46 What is the value of the content size of the first response message?
47 What is the etag (identity tag) of the first response message?
48 What is the application of etag in conditional HTTP request? Which line in
the second response contains the etag value of the first response?
49 Which HTTP GET contains the “IF‐MODIFIED‐SINCE” line? What is the usage
of this field?
50 What is the status code of the second response message? What does this
code mean?
51 What is the content length of the second response? Explain.
52 How many HTTP GET requests are sent by your web browser?
53 What is the content type of each response message?
54 Did your browser download the two images serially or in parallel? Explain.
What are the pros and cons of each approach?
55 Has the HTTP used persistent or non‐persistent connection? Explain your
answer.
56 What is the requested URL in the frame#101? What HTTP field contains the
username and password information? What are the submitted values for the
username and the password?
57 What HTTP request method is used in the frame#172? What HTTP field
contains the username and password information? Explain the difference
between this request method and the GET method.
58 What is the status code of the frame#174? What is the description of this
code?