Assignment 2: CMPT 371 

Assignment 2: CMPT 371 

1) Consider a host running a IPv4 resolver and a local DNS server. Do not consider IPv6 AAAA
records. Assume the domain names given in A, B and C are hosts (not DNS servers). Assume
the following domains are delegated by the DNS server for the nodes above them.
ca. com. gov. postulates.ca. seas.com. mammal.gov. integers.postulates.ca.
beaches.seas.com.
Assume all other domains are not delegated.
Both the host and the server have recently been rebooted. The server has made three queries
since it was rebooted. The queries were for the A records for each of the following hosts. The
hosts (not running DNS servers) are listed in the order the queries are made.
i. numbers.integers.postulates.ca
ii. landscapes.beaches.seas.com.
iii. lion.cat.mammal.gov.
 Answers should all be in the form of DNS records for example
 NS record for each DNS server for domain fresh.fruit.gov.
 A record for each DNS server for domain favorit.color.ca.
 AAAA record for fish.shark.seafood.com.
a) [4 points] What DNS records would you expect to find in the cache of the local DNS
server after the queries to resolve the address for numbers.integers.postulates.ca.?
b) [6 points] Consider each of the following hosts. For each host answer the following two
questions.
 Which DNS server would be authoritative for each of the following hosts?
 Why is that DNS server you chose authoritative for the host?
I. Tiger.cat.mammal.gov.
II. Colorful.sunsets.beaches.seas.com.
III. Arithmetic.integers.postulates.ca.
c) [6 points] After the three queries above for hosts i, ii, and iii, an additional query for
host wolf.mammal.gov. is made.
I. [2 points] What is the first DNS server queried? Why?
II. [2 points] What DNS record is the query requesting, in the query to the DNS
server in 1?
III. [2 points] What is returned by DNS query in II?
d) [4 points] For each of the following two domains you will state which DNS server would
be contacted first and the number of iterative queries needed to obtain an answer to
the query.
I. After the query for wolf.mammal.gov. the next query was for
sandy.beaches.seas.com.
II. After the query for sandy.beaches.seas.com the next query was for a.b.c.d.edu
2) [15 points] Consider bit torrent as an example of a peer to peer application.
The host labeled X is has just joined the illustrated bit torrent. As indicated below the host it
is the newest host in the torrent. Answer the following questions. Each answer should be no
longer than three sentences.
a) How does host X obtain an initial list of potential peers?
b) What is the purpose of the tracker?
c) How does a potential peer from X’s list become a neighboring peer?
d) What does it mean to unchoke a peer?
e) What does it mean for a peer to be optimistically unchoked?

X
NEWEST HOST
IN TORRENT
tracker:
3) [50 points] You will write two Python (compatible with v3.5) socket programs to implement
a slightly modified version of the protocol (rdt3.0) from your text (that we will/have discussed
in class. These two processes will exchange simple packets that are used to implement the
protocol. Please note that the form of the packets has been changed from the example in
your textbook. This means that the arguments of the makepkt function in the figures must
be changed to a list of the variables in the fields of the packets described below. The
arguments of makepkt should be in the order Field1, Field2, Field3, Field4
Your programs must run on the Linux machines in the CSIL labs. If your programs do not run
in the Linux environment in CSIL you will receive a 0 for this problem. Beware socket
programs are often not easily portable between operating systems.
The contents of the simple packets you send between your sender (client) and receiver
(server) sockets will include 4 values.
 Field 1: Packet contents: a 32 bit integer (data).
The integer cannot be assumed to follow any pattern
Packet contents for an ACK or NACK packets is 0
 Field 2: Sequence Number for Packet: a Boolean
Value is True if sequence number is 1
Value is False if sequence number is 0
 Field 3: Is this an ACK? Boolean
True if the packet is an ACK, False if the packet is not an ACK
 Field 4: Sequence Number for ACK: a Boolean
Value is True if sequence number is 1
Value is False if sequence number is 0
In your sender program you will
 Use one instance of a pseudo random number generator random( ) to produce uniformly
distributed random floating point numbers in the range [0.0, 5.0). These pseudo random
numbers will be used to simulate random arrival time of packets. When a packet is sent,
the send is followed immediately by a call to this random number generator. The pseudo
random number returned will be interpreted as the delay in seconds before the next
packet will be generated. This delay occurs after the process enters either “wait for call 0 from above” or “wait for call 1 from above”. It is the time between entering one of these states and building the next packet (waiting for enough data for a packet).
 Use a second instance of a pseudo random number generator random() to generate
random number that will be used to determine if an ACK and NACK that has just arrived
has been corrupted. This instance of the pseudo random number generator random
should generate uniformly distributed pseudo random numbers between [0.0 and 1.0).
If the number generated is less that the input value of the probability that an ACK packet
has been be corrupted, then the ACK packet that has just arrived will be considered to be
corrupted.
 Read the values of the following quantities used in your program at the start of your
program. You will not read any quantities not in this list into your program. You will read
the specified quantities in the order they are specified below.
o The seed for the random number generator used for timing
o The number of packets to send
o The seed for the random number generator used for determining if ACKs or NACKshave been corrupted.
o The probability that an ACK has been corrupted. Between [0 and 1)
o The round trip travel time (to be used for timer for determining if a packet has not received an ACK)
 Print three messages immediately before a packet is sent.
o You must print the messages exactly as given (character by character), with the
sole exception of items in bold which will contain actual data values.
o First message: one of the four following messages should be printed
 A packet with sequence number 0 is about to be sent
 A packet with sequence number 1 is about to be sent
 A packet with sequence number 0 is about to be resent
 A packet with sequence number 1 is about to be resent
o Second message should be printed
 Packet to send contains: data = 123 seq = 0 isack=False ack = 0o Third message: one of the two following messages should be printed.
 Starting timer for ACK0
 Starting timer for ACK1
 Print the following messages immediately after an uncorrupted ACK packet is received.
o An ACK will be received. Print the message for the packet received followed by the
third message.
o You must print the messages exactly as given below, with the sole exception of
items in bold which will contain actual data values.
o First message: one of the two following messages should be printed.
 An ACK0 packet has just been received
 An ACK1 packet has just been received
o Second message should be printed  Packet received contains: data = 0 seq = 1 isack= False ack = 0
o Third message: one of the two following messages should be printed.
 Stopping timer for ACK0
 Stopping timer for ACK1
 Print the following message immediately after a corrupted packet is received.
o You must print the message exactly as given below.
 A Corrupted ACK packet has just been received
 Print the following messages if a timer expires.
There are two reasons the timer may expire.
For a lost packet/ack the best way to handle this timeout is using the timeout associated with the socket recv().
For a corrupted ack or an incorrect sequence number something will be received before the timeout. You will need to check what is received, if it is a corrupted ack or incorrect ack you will need to monitor the time until the timer expires then send your next packet. You must print the message exactly as given below.
o One of the two following messages should be printed.
 ACK0 timer expired
 ACK1 timer expired

 Print the following messages immediately before the sender moves to another state or
returns to the same state.
o Only the appropriate one of the following messages should be printed for each
transition.
o These messages must be printed exactly as given below.
 The sender is moving to state WAIT FOR CALL 0 FROM ABOVE
 The sender is moving to state WAIT FOR CALL 1 FROM ABOVE
 The sender is moving back to state WAIT FOR CALL 0 FROM ABOVE
 The sender is moving back to state WAIT FOR CALL 1 FROM ABOVE
 The sender is moving to state WAIT FOR ACK0
 The sender is moving to state WAIT FOR ACK1
 The sender is moving back to state WAIT FOR ACK0
 The sender is moving back to state WAIT FOR ACK1
 In your receiver program you will
 Use an instance of a pseudo random number generator random() to generate uniformly
distributed pseudo random numbers between [0.0 and 1.0). These pseudo random
numbers will be used to determine if a packet that has just arrived has been corrupted.
If the pseudo random number generated is less that the input value of the probability that
the packet has been be corrupted, then the packet that has just arrived will be considered
to be corrupted.
 Read the values of the following quantities used in your program at the start of your
program. You will not read any quantities not in this list into your program. You will read
the specified quantities in the order they are specified below.
o The seed for the random number generator used for determining if packets have
been corrupted.
o The probability that a packet has been corrupted
 Print the following messages immediately before an ACK is sent.
o One of the first two messages should be sent followed by the third message.
o You must print the messages exactly as given, with the sole exception of items in
bold which will contain actual data values.
 An ACK0 is about to be sent
 An ACK1 is about to be sent  Packet to send contains: data = 0 seq = 0 isack = True ack = 0 Print the following messages immediately after an uncorrupted packet is received.
o One of the first four messages should be sent followed by the fifth message,
including the actual contents of the packet just received
o You must print the messages exactly as given, with the sole exception of items in
bold which will contain actual data values.
 A packet with sequence number 0 has been received
 A packet with sequence number 1 has been received
 A duplicate packet with sequence number 0 has been received
 A duplicate packet with sequence number 1 has been received
 Packet received contains: data 333 seq = 0 isack = True ack = 1 Print the following message immediately after a corrupted packet is received.
You must print the message exactly as given.
 A Corrupted packet has just been received
 Print the following messages Immediately before the sender moves to another state or
returns to the same state
o the appropriate one of the following messages should be printed
 The receiver is moving back to state WAIT FOR 0 FROM BELOW
 The receiver is moving back to state WAIT FOR 1 FROM BELOW
 The receiver is moving to state WAIT FOR 0 FROM BELOW
 The receiver is moving to state WAIT FOR 1 FROM BELOW
4) Consider a link with the following properties:
 75-meter link length
 Bit rate of 5Mibps (1Mib = 220 bits) in each direction.
 Propagation velocity through the link is 2.5*108
 m/s.
 Size of one HTTP response (containing one HTTP object) is 80Kibits bits
 (1Kibit = 210 `bits) including headers.
 Packets containing HTTP requests, SYNs, ACKs, and FINs are 1024 bits long
 including headers.
Assume that a request is made for a particular webpage. The initial response is a single
object. While processing that single object 13 additional objects are requested. Consider
how long it would take to obtain all 14 of these objects in each of the following
scenarios. Include the time used by both the 3 way handshake to establish the TCP
connection and the 3 way handshake to close the TCP connection.
a) [5 points] Assume that each object is requested using a non-persistent HTTP
connection. Assume that only one non-persistent connections can be in use at any
given time. Each non-persistent connection has a rate of 5Mibps. The first FIN sent
is a separate packet (no piggybacked on an object or request).
b) [5 points] Assume a single persistent HTTP connection is used for all 14 objects. No
pipelining is used.