Solutions 2

ECEN 619 - HW#2: Please answer the following questions:
Due on Feb 19, 2013, in class. (No late homework accepted)
1) Give 3 examples of network attacks.
Spam, denial of service attacks, viruses.
2) What is HTTP protocol and which layer does HTTP sit at?
HTTP is Hypertext Transfer Protocol
HTTP assumes messages can be exchanged directly between HTTP client and
HTTP server
In fact, HTTP client and server are processes running in two different
machines across the internet.
HTTP uses the reliable stream transfer service provided by TCP
HTTP builds on top of TCP
HTTP protocol sits at application layer.
3) What is DNS protocol and which layer does DNS sit at?
DNS is Domain Name System protocol
DNS protocol allows queries of different types
(1) Name-to-address or Address-to-name
(2) Mail exchange
DNS usually involves short messages and so uses service provided by UDP
DNS builds in top of UDP
DNS protocol sits at application layer.
4) What is UDP protocol and which layer does UDP sit at?
UDP is User Datagram Protocol
UDP provides best-effort datagram service between two processes in two
computers across the internet
Port numbers distinguish various processes in the same machine
UDP is connectionless
Datagram is sent immediately when data is ready
Quick, simple, but not reliable
TCP builds on top of IP
UDP protocol sits at transport layer.
5) Describe the differences between the connection-oriented transport protocol and
connectionless transport protocol.

(1) The connection-oriented transport protocol specifies that the connection need to
be established before data can be exchanged, which cause the data can arrive in
order. It guarantees delivery successfully by sending ACK. An example for the
connection-oriented is TCP.
(2) The connectionless transport protocol states different from the connectionoriented
protocol. The delivery is not reliable. An example sor the connectionless
transport protocol is UDP.
6) Describe the differences between the packet-switching networks and circuitswitching
networks.
(1) In circuit switching networks, the data transmission path is dedicated to an entire
data block, which implies that there no other data transmission can share the same
channel. The channel can only be used by others when the current data has been
finished transmitted.
(2) In packet switching networks, the data is partitioned into a number of data units
(packets) with their own sequence number. These data packets are sent through
different channels in different times, but they will be reassembled in the original
order at receiver.
7) Describe the TCP Tahoe and TCP Reno algorithm, respectively.
TCP-tahoe (Jacobson, 1988)
- Time-out based protocol –use timeout to detect packet loss and
congestions
- Protocol:
i. After every acknowledgement of a new packet:
If W

ii.
iii.
when the duplicate acknowledgement exceeds a threshold, retransmit
“next expected” packet:
set W t =W/2, then set W= W t (i.e. halve the window);
resume congestion avoidance using new window once retransmission
is acknowledged.
Upon timer expiry, the algorithm goes into slow start as before:
Set W t =W/2;
Set W=1
8) What are the major differences between TCP Tahoe and TCP Reno?
TCP Reno introduces fast retransmissions and fast recovery. TCP Reno uses triple
duplicate ACKs to the same sequence number. The missing segment is retransmitted
immediately
9) Describe the principle of “Self-clocking” of TCP protocol.
Packets after the first burst are sent only in response to an ACK, the sender’s packet
spacing will exactly match the packet time on the slowest link in the path. Since the
packet transmission is dominated by ACK, self-clocking is achieved
Sender sends packets back-to-back to receiver
The vertical line is bandwidth
The horizontal line is time
Each of shaded box is a packet
Bandwidth*Time = Bits, and so the area of each box is the packet size.
The number of bits doesn’t change as a packet goes through the network so a
packet squeezed into the smaller long-haul bandwidth must spread out in time.
The time P b represents the minimum packet spacing on the slowest link in the
path (the bottleneck)
As the packets leave the bottleneck for the destination net, nothing changes
the inter packet interval so on the receiver’s net packet spacing P r = P b .
If the receiver processing time is the same for all packets, the spacing
between ACKs on the receiver’s net A r = P r = P b
10) What are the major factors to determine the "timeout" clock value?
The Round Trip Time (RTT)
11) Why is it difficult to design the "timeout" clock value for TCP in Internet?
The timeout value needs to be longer than RTT, which varies randomly. If the
timeout value set too short, then there will have unnecessary retransmissions. If the
timeout value is set too long, then TCP alright waste time to wait for the lost packet.
12) Is there the optimal value for "timeout" clock for TCP in Internet?
Non.

13) During which phase does TCP loose throughput most significantly?
Slow start phase.
14) Why does TCP Reno outperform TCP Tahoe?
Because when three duplicate ACKs are received, TCP Reno will have a congestion
window and set the slow start equal to the congestion window. Basing on this
technique, TCP Reno can do fast retransmission.
TCP Reno can retransmit the data faster than TCP Tahoe when the three duplicate
ACKs have been received. Because when the above situation occurs, TCP Reno will
skip the slow start phase and enter the congestion avoidance phase, unlike TCP
Tahoe that has to start from slow start phase with window size equal to 1.
15) Describe the "triple-duplicate ACK" control mechanism in TCP-Reno.
If triple-duplicate-ACK with same sequence number has been received. TCP starts
the congestion avoidance, which is set the congestion window size to half instead of
one with window size in slow start phase. After triple-duplicate-ACK has been
received, TCP Reno will start fast retransmission and fast recovery.
16) When will the "fast retransmissions and fast recovery" control action be kicked in?
When triple-duplicate-ACK with same sequence number has been received, fast
retransmission and fast recovery starts.
17) Can TCP Reno solely rely on the "triple-duplicate ACK" control mechanism for
flow control and why?
No, because timeout must be used when there is severe congestion or connection is
broken.
18) Can TCP flow control totally eliminate packet losses and why?
No, because TCP creates packet losses.
19) Is the "Slow Start" really SLOW for TCP in Internet?
No, because the slow start increases the congestion window exponentially, which is
fast increase.
20) What are the control laws for "Congestion Avoidance Phase" and "Slow Start Phase"
for TCP?
Congestion avoidance phase: linearly increasing congestion window (CWND)
Slow-start phase: exponentially increasing congestion window size (CWND).