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计算机网络代写 | COMS3200 – Assignment 1

这个作业是完成本地网络、网络交换机等相关的问题

COMS3200 – Assignment 1

Part A (30 marks)
Answer each of the following questions in the associated quiz on Blackboard, following the
specified instructions. All answers will be automatically marked.
The figure below illustrates a simple network map that included two (2) edges: Local Network
(grey area) and Global Network (white area).
In Local Network, the three (3) circuit switches: SC1, SC2 and SC3 are interconnected by
two (2) links: LC1 and LC2. Each of these circuit links has three (3) slots. The hosts are each
directly connected to one of the switches. In this part, we only need to know the
communication scenario between Host A and Host B. The network has reserved one slot on
each of the two links. In the figure below, the dedicated end-to-end connection uses the second
slot in both links LC1 and LC2 (see the red coloured line).
We assume that device C is the combination of modem and router in the Local Network, are
connected together and connect to SC3 through LC3. The data transmission from device C
in Local Network through L1 is considered the data from client C in the Global Network
view.
2
In Global Network, packet-switches are used in the entirety of the network. We have a
scenario where a client requests a web page from a remote server on a remote island via a
slow satellite link above the earth in a geostationary orbit. There is one (1) client C, one (1)
server S, and four (4) DNS servers: D1 is a Local DNS Server, D2 is a Root DNS Server, D3
is a Top-level Domain (TLD) DNS Server, and D4 is Authoritative DNS Server. We don’t
need to know where the Link L7 connect to in this assignment.
Scenario 1 – Local Network (8 marks)
Host A sends a file of 2,880,000 bits to Host B at the time t0 = 0 mili-second (msec). Link
LC1 and link LC2 uses time-division multiplexing (TDM), all of them have one (1) frame,
and it is one (1) second that is divided by three (3) slots. The total bit rate is 9 Mbps (from
SC1 to SC3). It needs 500 msecs to establish an end-to-end circuit before Host A can begin
to transmit the file and the second (2nd) slot of the circuits in LC1 and LC2 is allocated for
the transmission between Host A and Host B. At the time t0 + 500 msecs, the first (1st) slot
of the circuits in LC1 and LC2 start to transmit. At the time t1, Host B has received all bits
of the file from Host A. After 200 msecs from t1, Host B sends a file of 960,000 bits to Host
A. Host A has received all bits of the file from Host B at time t2.
There are some assumptions for the Local Network:
1. There is no propagation delays in this networking.
2. There is no transmission delays between A and SC1, as well as B and SC3.
3. The network starts with no data are transmissing at the time t0 = 0.
4. The slot of the circuits is allocated for the transmission can be used in both direction.
5. All switches have no processing delays (this is unrealistic, just to simplify calculations).
Task: Answer the questions below in mili-seconds (msecs) rounded to three decimal places.
Question 1: What is the value of t1? (4 marks)
Question 2: What is the value of t2? (4 marks)
Scenario 2 – Global Network (17 marks)
A program on one of the hosts in the Local Network starts the process of retrieving a webpage
from the Server S. The following events happen sequentially:
1. At the time t0 = 0 mili-second (msec), Client C sends a DNS request to D1. D1 receives
the DNS request from Client C successfully at the time t1.
2. Right after t1 without any processing delays, D1 sends a response with Server S’s IP
address to Client C. Client C receives the DNS response from D1 successfully at the
time t2.
3. Right after t2 without any processing delays, C sends a TCP request (SYN) to open a
connection to Server S. Server S receives the TCP SYN request from Client C
successfully at the time t3.
4. Right after t3 without any processing delays, Server S acknowledges (SYN/ACK) the
TCP request and opens the connections with Client C. Client C receives the TCP
(SYN/ACK) acknowledgement from Server S successfully at the time t4.
5. Right after t4 without any processing delays, Client C sends a HTTP GET request to
Server S (which includes the ACK back to S for its SYN). Server S receives the HTTP
GET packet from Client C successfully at the time t5.
6. Right after t5 without any processing delays, Server S sends the web page to Client C,
which requires 7 packets, which includes the HTTP response plus the HTML file.
Client C receive the first (1st) packet of the HTTP response from Server S successfully
at the time t6 and the seventh (or last) packet of the HTTP response at the time t7.
7. After receiving each data packet without any processing delays, Client C sends an
ACK message back to Server S. Note that S does not need to wait for an ACK before
sending the next data packet. After receiving the last data packet acknowledgement
without any processing delays, S sends a TCP FIN packet to close the connection.
Part B (20 marks)
Answer each of the following questions in the associated quiz on Blackboard, following the
specified instructions. All answers will be automatically marked.
In this part, you need to know how to use Wireshark in analysing a packet capture file, names
a1.pcap. This packet capture shows a client downloading a javascript resource file from a
server. Some of the relevant IETF RFCs may help you understand more of the following
questions. In particular:
1. RFC 793 for Transmission Control Protocol
2. RFC 791 for Internet Protocol
3. RFC 2018 for TCP Selective Acknowledgment Options
4. RFC 7323 for TCP Extensions for High Performance
Wireshark displays TCP sequence numbers as a value relative to the first sequence number.
You will need to disable this to answer any questions that ask for a raw sequence number. It
is highly recommended that you turn off TCP Reassembly to understand the order of packet
transmission.


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