1. Briefly define the following terms:(24%)
(a) TCP Socket:
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A process send messages into, and receives messages from, the network
through a software interface called a socket. Let’s consider an analogy to help
us understand processes and sockets. A process is analogous to a house ad its
socket is analogous to its door. When a process wants to send a message to
another process on another host, it shoves the message out its door (socketed).
This sending process assumes that there is a transportation infrastructure on
the other side of its door that will transport the message to the door of the
destination process. Once the message arrives at the destination host, the
message passes through the receiving process’s door (socket), and the
receiving process then acts on the message.
(b) Congestion window
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The congestion window, dented cwnd, imposes a constraint on the rate at
which a TCP sender can send traffic into the network. Specifically, the amount
of unacknowledged data at a sender may not exceed the minimum of cwnd
and rwnd, that is:
LastByteSent – LastByteAcked <= min {cwnd, rwnd}
(c) ICMP
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ICMP is used by hosts and routers to communicate network-layer information
to each other. The most typical use of ICMP is for error reporting. For example,
when running a Telnet, FTP, or HTTP session, you may have encountered an
error message such as “Destination network unreachable.” This message had
its origins in ICMP. At some point, an IP router was unable to find a path to the
host specified in your Telnet, FTP, or HTTP application. That router created and
sent a type-3 ICMP message to your host indicating the error.
(d) Routing vs. forwarding
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Forwarding. When a packet arrives at a router’s input link, the router must
move the packet to the appropriate output link. For example, a packet
arriving from Host H1 to Router R1 must be forwarded to the next router on a
path to H2. In Section 4.3, we’ll look inside a router and examine how a
packet is actually forwarded from an input link at a router to an output link.
Routing. The network layer must determine the route or path taken by
packets as they flow from a sender to a receiver. The algorithms that calculate
these paths are referred to as routing algorithms. A routing algorithm would
determine, for example, the path along which packets flow from H1 to H2
(e) Exponential backoff
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When transmitting a given frame, after experiencing the nth collision in a row
for this frame, the adapter chooses a value for K at random from
{0,1,2,…,2m-1} where m=min(n,10).
(f) CSMA/CD
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CSMA/CD does the following:
1. An adapter may begin to transmit at any time; that is, there is no notion of
time slots.
2. An adapter never transmits a frame when it senses that some other adapter
is transmitting; that is, it uses carrier sensing.
3. A transmitting adapter aborts its transmission as soon as it detects that
another adapter is also transmitting; that is, it uses collision detection.
4. Before attempting a retransmission, an adapter waits a random time that is
typically small compared with the time to transmit a frame.
2. TCP congestion control algorithm has three major components. Briefly describe
the functionality of each component.(9%)
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(1) Slow start:
When a TCP connection begins, the value of cwnd is typically initialized to a small
value of 1 MSS, resulting in an initial sending rate of roughly MSS/RTT. For
example, if MSS=500bytes and RTT=200msec, the resulting initial sending rate is
only about 20 kbps, since the available bandwidth to the TCP sender may be
much larger than MSS/RTT, the TCP sender would like to find the amount of
available bandwidth quickly. Thus, in the slow-start state, the value cwnd begins
at 1 MSS and increase by 1 MSS every time a transmitted segment is first
acknowledged.
(2) Congestion avoidance:
On entry to the congestion-avoidance state, the value of cwnd is approximately
half its value when congestion was last encountered—congestion could be just
around the corner! Thus, rather than doubling the value of cwnd every RTT, TCP
adopts a more conservative approach and increases the value of cwnd by just a
single MSS every RTT.
(3) fast recovery:
In fast recovery, the value of cwnd is increased by 1 MSS for every duplicate ACK
received for the missing segment that caused TCP to enter the fast-recovery state.
Eventually, when an ACK arrives for the missing segment, TCP enters the
congestion-avoidance state after deflating cwnd. If a timeout event occurs, fast
recovery transitions to the slow-start state after performing the same actions as
in slow start and congestion avoidance: The value of cwnd is set to 1 MSS, and
the value of ssthresh is set to half the value of cwnd when the loss event
occurred.