EECS325 Homework Assignment 2 solution


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1. (1pt) Under what circumstances can the HTTP response
have empty body?
2. (1pt) Suppose Alice, with a Web-based e-mail account
(such as Hotmail or Gmail), sends a message to Bob, who
accesses his mail from his mail server using POP3. Discuss
how the message gets from Alice’s host to Bob’s host. List
the series of application-layer protocols that are used to
move the message between the two hosts.
3. (2pt) List and explain the two different approaches for DNS
4. (2pt) Is it possible for an organization’s Web server and
mail server to have exactly the same alias for a host name
(e.g., What would be the types for the RRs that
contain the hostnames of the web and the mail servers?
5. (3pt) Suppose TCP operates over a 1 Gbps link.
a. Assume TCP could utilize the full bandwidth
continuously and the average packet size is 60 bytes,
how long does it take for the TCP sequence numbers
to wrap around completely?
b. Suppose an added 32-bit timestamp field increments
1000 times during the wraparound time you found
above. How long would it take for the timestamp to
wrap around?
6. (3pt) Suppose you have the following 8-bit bytes:
01010011 01100110 01110100.
a. What is the 1s complement of the sum of these 8-bit
bytes? (Although TCP and UDP use 16-bit words in
computing the checksum, you only need to consider
8-bit based checksum for this problem)
b. Why use 1s complement of the sum instead of the
sum itself as the checksum?
7. (6pt – including 2 extra pts) Consider two network
entities, A and B, which are connected by a perfect
bidirectional channel (i.e., any message sent will be
received correctly; the channel will not corrupt, lose, or reorder packets). A and B are to deliver data messages to
each other in an alternating manner: First, A must deliver a
message to B, then B must deliver a message to A, then A
must deliver a message to B and so on. If an entity is in a
state where it should not attempt to deliver a message to
the other side, and there is an event like
rdt_send(data) call from above that attempts to pass
data down for transmission to the other side, this call from
above can simply be ignored with a call to
rdt_unable_to_send(data), which informs the higher
layer that it is currently not able to send data. [Note: This
simplifying assumption is made so you don’t have to worry
about buffering data.]
Draw a FSM specification for this protocol (one FSM for A,
and one FSM for B). Note that you do not have to worry
about a reliability mechanism here; the main point of this
question is to create a FSM specification that reflects the
synchronized behavior of the two entities. You should use
the following events and actions that have the same
meaning as protocol rdt1.0 in Figure 3.9:
rdt_send(data), packet = make_pkt(data),
udt_send(packet), rdt_rcv(packet),
extract(packet,data), deliver_data(data).
Make sure your protocol reflects the strict alternation of
sending between A and B. Also, make sure to indicate the
initial states for A and B in your FSM descriptions.