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Cisco Certified Network Associate : 640-802 Exam

Exam Number/Code: 640-802
Exam Name: Cisco Certified Network Associate

“Cisco Certified Network Associate”, also known as 640-802 exam, is a Cisco certification.
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With the complete collection of questions and answers Q&as with Expert Explanations, Pass4sure has assembled to take you through 254 Q&As to your 640-802 Exam preparation. In the 640-802 exam resources, you will cover every field and category in CCNA helping to ready you for your successful Cisco Certification.
QUESTION 36:
Which fields are included in the TCP header? (Choose three).
A. Source Port
B. Acknowledgement Number
C. Request Number
D. Destination Address
E. Window
F. Data
Answer: A, B, E
Explanation:
TCP header:
Source Port: 16 bits.
Destination Port: 16 bits.
Sequence Number: 32 bits.
The sequence number of the first data byte in this segment. If the SYN bit is set, the
sequence number is the initial sequence number and the first data byte is initial sequence
number + 1.
Acknowledgment Number: 32 bits.
If the ACK bit is set, this field contains the value of the next sequence number the sender
of the segment is expecting to receive. Once a connection is established this is always
sent.
Data Offset: 4 bits.
The number of 32-bit words in the TCP header. This indicates where the data begins. The
length of the TCP header is always a multiple of 32 bits.
Reserved: 3 bits.
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Must be cleared to zero.
ECN, Explicit Congestion Notification: 3 bits.
Added in RFC 3168.
Control Bits: 6 bits.
Window: 16 bits, unsigned.
The number of data bytes beginning with the one indicated in the acknowledgment field
which the sender of this segment is willing to accept.
Checksum: 16 bits.
This is computed as the 16-bit one’s complement of the one’s complement sum of a
pseudo header of information from the IP header, the TCP header, and the data, padded
as needed with zero bytes at the end to make a multiple of two bytes.
Urgent Pointer: 16 bits, unsigned.
If the URG bit is set, this field points to the sequence number of the last byte in a
sequence of urgent data.
Options: 0 to 44 bytes.
Options occupy space at the end of the TCP header. All options are included in the
checksum. An option may begin on any byte boundary. The TCP header must be padded
with zeros to make the header length a multiple of 32 bits.
Data: Variable length.
QUESTION 37:
Acknowledgements, sequencing, and flow control are functions that are handled by
which layer of the OSI model?
A. Layer 5
B. Layer 4
C. Layer 7
D. Layer 6
E. Layer 3
F. Layer 2
G. Layer 1
Answer: B
QUESTION 38:
A receiving host has failed to receive all of the segments that it should acknowledge.
What can the host do to improve the reliability of this communication session?
A. Start a new session using UDP
B. Obtain a new IP address from the DHCP server
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C. Use a different source port for the session
D. Decrease the sequence number
E. Decrease the window size
Answer: E
Explanation:
A TCP window the amount of outstanding (unacknowledged by the recipient) data a
sender can send on a particular connection before it gets an acknowledgment back from
the receiver that it has gotten some of it.
For example if a pair of hosts are talking over a TCP connection that has a TCP window
size of 64 KB (kilobytes), the sender can only send 64 KB of data and then it must stop
and wait for an acknowledgment from the receiver that some or all of the data has been
received. If the receiver acknowledges that all the data has been received then the sender
is free to send another 64 KB.
One way to improve the reliability of the TCP connection is to reduce the window size
that the receiver needs to receive before sending an acknowledgement. However, this will
reduce throughput as more segments and acknowledgements will need to be sent in order
to transfer the same amount of data.
QUESTION 39:
You have set up an Internet based FTP server, where people can upload and
download files. In terms of the OSI model, what is the highest layer used during the
FTP sessions.
A. Application
B. Presentation
C. Session
D. Transport
E. Internet
F. Data Link
G. Physical
Answer: A