Question

1. which of the following statements is not true for local DNS servers?

Option:

1. A local DNS server acts like a proxy.
2. IPS often provide default name servers.
3. A local DNS server provides authorization hostname to IP address mappings.
4. A local DNS servers has a local cache to speed up.

2. Which of the following is unlikely to be one of the transport layers services?

Option:

1. Segmenting
2. Reliable data transfer
3. congestion control
4. host-to-host communication

3. What is the data unit exchanged in the transport layer?

Option:

1. Datagrams
2. Messages
3. Segments
4. Frames

4. Which of the following fields cannot be found in the TCP header?

Option:

1. Sequence number
2. Source address
3. Source port address
4. Checksum

5. What flag is used for connection setup in the TCP header?

Option:

1. URG
2. ACK
3. SYN
4. PSH

6. which of the following statements would not be true for Go-back-N?

Option:

1. The receiver discards out-of-order packets
2. it is a pipelined protocol.
3. if a timeout occurs, the sender resends all packets that have not yet been acknowledged.
4. the receiver sends an ACK in case of receiving a packet with sequence number in the previous window.

Answer 1

1. (i) There is nothing in common in DNS and Proxy servers. These both are different terms and defined separately. The default nameservers are provided at the time of domain registration along with an IP address. We also can change these later. A DNS server maps an IP address to its hostname, in local DNS server, the hostname is the name of the domain so it maps the hostname to IP address. A DNS server keeps tracking on the records made by a user on the server so it maintains a local cache for faster processing or we can say for a better speedup.

2. (iv) Transport layer provides process-to-process communication. Segmentation is done at the transport layer as data is sent into packets instead of data stream. The transport layer is responsible for Reliable data transfer and it also controls errors and congestion while sending a packet.

3. (iii) The data is sent in segments from one to another system via the transport layer. Datagrams and segments are data units in the transport layer. So I'm not pretty sure about it whether datagram or segments.

4. (iii) TCP header contains a sequence number, source, and destination addresses, acknowledge, header length, flag bits, window size, checksum, urgent pointer, and options. So Source port address can not be found in TCP header.

5. (iii) In TCP header 6 flag bits exist there as SYN(used for connection establishment or in three-way handshaking), ACK(used to acknowledge packets that are received by the host), FIN(to terminate the connection), RST(reset the whole communication), PSH(sending packets of maximum segment size).

6. (ii) Do-back-N is not a pipelined protocol, it is N to 1 protocol. In Go-back-N corrupted frames/packets are not accepted by the receiver and silently discarded by the receiver. In case of no acknowledgment from the receiver, the sender retransmits all the packets that are not being acknowledged. The receiver also sends the ACK in case of receiving a packet with different sequence numbers and windows.