Computer Networking: A Top-Down Approach Ch 3 & 4

Compute the Internet checksum value for these two 16-bit words: 11110101 11010011 and 10110011 01000100

01010110 11100111

What is meant by transport-layer demultiplexing?

Receiving a transport-layer segment from the network layer, extracting the payload (data) and delivering the data to the correct socket.

Consider the rdt 2.0 sender and receiver shown below, with FSM transitions at the sender labeled S1, S2, S3; and receiver transitions labeled R1 and R2 Which of the following sequences of transitions could possibly occur as a result of an initial rdt_send() call at the sender, and possible later message corruption and subsequent error recovery

S1, R2, S3 S1, R1, S2 S1, R1, S2, R2, S3

Which of the fields below are in a UDP segment header?

Source port number Destination port number Internet checksum Length (of UDP header plus payload)

What is meant by transport-layer multiplexing?

Taking data from one socket (one of possibly many sockets), encapsulating a data chunk with header information--thereby creating a transport layer segment--and eventually passing this segment to the network layer

Over what set of bytes is the checksum field in the UDP header computed over?

The entire UDP segment, expect the checksum field itself, and the IP sender and receive address fields.

Where is transport-layer functionality primarily implemented?

Transport layer functions are implemented primarily at the hosts at the "edge" of the network.

True or False: It is possible for two TCP segments with source port 80 to be sent by the sending host to different clients.

True

True or False: It is possible for two UDP segments to be sent from the same socket with source port 5723 at a server to two different clients

True

True or False: The transport layer provides for host-to-host delivery service?

True

True or False: When multiple UDP clients send UDP segments to the same destination port number at a receiving host, those segments (from different senders) will always be directed to the same socket at the receiving host.

True

Why is the UDP header length field needed?

Because the payload section can be of variable length, and this lets UDP know where the segment ends.

The transport layer sits on top of the network layer, and provides its services using the services provided to it by the network layer. Thus it's important that we know what is meant by the network layer's "best effort" delivery service. True or False: The network layer's best-effort delivery service means that IP makes its "best effort" to deliver segments between communicating hosts, but it makes no guarantees. In particular, it does not guarantee segment delivery, it does not guarantee orderly delivery of segments, and it does not guarantee the integrity of the data in the segments.

Correct/True

Which of the following statements are true about a checksum?

A checksum is computed at a sender by considering each byte within a packet as a number, and then adding these numbers (each number representing a bytes) together to compute a sum (which is known as a checksum) The sender-computed checksum value is often included in a checksum field within a packet header The receiver of a packet with a checksum field will add up the received bytes, just as the sender did, and compare this locally-computed checksum with the checksum value in the packet header. If these values are different then the receiver knows that one of the bits in the received packet has been changed during transmission from sender to receiver.

True or False: When computing the Internet checksum for two numbers, a single flipped bit in each of the two numbers will always result in a changed checksum

False

True or False: When multiple TCP clients send TCP segments to the same destination port number at a receiving host, those segments (from different senders) will always be directed to the same socket at the receiving host

False

Consider the purposes/goals/use of different reliable data transfer protocol mechanisms. For the given purpose/goal/use match it to the RDT mechanism that is used to implement the given purpose/goal/use

NAK: Lets the sender know that a packet was NOT received correctly at the receiver Checksum: Used by sender or receiver to detect bits flipped during a packet's transmission Sequence numbers: Allows for duplicate detection at receiver ACK: Lets the sender know that a packet was received correctly at the receiver Retransmission: Allows the receiver to eventually receive a packet that was corrupted or lost in an earlier transmission

Check all of the services below that are provided by the TCP protocol.

Reliable data delivery A byte stream abstraction, that does not preserve boundaries between message data sent in different socket send calls at the sender A flow-control service that ensures that a sender will not send at such a high rate so as to overflow receiving host buffers A congestion control service to ensure that multiple senders to not overload network links. In-order data delivery

True or False: On the sending side, the UDP sender will take each application-layer chunk of data written into a UDP socket and send it in a distinct UDP datagram. And then on the receiving side, UDP will deliver a segment's payload into the appropriate socket, preserving the application-defined message boundary.

True

True or False: When computing the Internet checksum for two numbers, a single flipped bit (i.e., in just one of the two numbers) will always result in a changed checksum

True

Compute the internet checksum value for these two 16-bit words: 01000001 11000100 and 00100000 00101011

10011110 00010000

Check all of the services below that are provided by the UDP protocol

A message abstraction, that preserves boundaries between message data sent in different socket send calls at the sender