Networks

Which of the following statements are true about a checksum?

-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 two values are different then the receiverknows that one of the bits in the received packet has been changed during transmission from sender to receiver. -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

Which of the following characteristics below are associated with a UDP socket?

-socket(AF_INET, SOCK_DGRAM) creates this type of socket -data from different clients can be received on the same socket -provides unreliable transfer of a groups of bytes ("a datagram"), from client to server -the application must explicitly specify the IP destination address and port number for each group of bytes written into a socket

Application Layer

Protocols that are part of a distributed network application.

The shared link's utilization

Rbottleneck / (R / 4)

Which characteristics are associated with the technique of circuit switching?

Reserves resources needed for a call from source to destination. Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) are two approaches for implementing this technique. This technique was the basis for the telephone call switching during the 20th century and into the beginning of this current century.

Transport layer data unit

Segment

what is meant by the term "encapsulation"?

Taking data from the layer above, adding header fields appropriate for this layer, and then placing the data in the payload field of the "packet" for that layer

Bottleneck link

The bottleneck link is the link with the smallest capacity between RS, RC, and R/4

Check all of the phrases below that state a true property of a local DNS server.

The local DNS server can decrease the name-to-IP-address resolution time experienced by a querying local host over the case when a DNS is resolved via querying into the DNS hierarchy. The local DNS server record for a remote host is sometimes different from that of the authoritative server for that host

What are some reasons for discarding received-but- out-of-sequence packets at the receiver in GBN?

The sender will resend that packet in any case. The implementation at the receiver is simpler

Transport Layer

Transfer of data between one process and another process (typically on different hosts).

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

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

digital signature

Used to detect tampering/changing of message contents, and to identify the originator of a message

Cable Access Network

Wired. Up to 10's to 100's of Mbps downstream per user.

Ethernet speed

Wired. Up to 100's Gbps per link.

what transport services are provided to the application by UDP?

best effort service

Network layer data unit

datagram

manifest

file containing the location and encoding rate of files corresponding to video segments in a video.

Link layer data unit

frame

enter deep

A CDN approach that stores content in access networks, close to clients.

access control

Limiting use of resources or capabilities to given users

Application layer data unit

Message

Lets the sender know that a packet was NOT received correctly at the receiver.

NAK

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

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

Which of the following characteristics below are associated with a TCP socket?

-when contacted, the server will create a new server-side socket to communicate with that client -a server can perform an accept() on this type of socket -provides reliable, in-order byte-stream transfer (a "pipe"), from client to server -socket(AF_INET, SOCK_STREAM) creates this type of socket

In DASH (Dynamic, Adaptive Streaming over HTTP), a server divides a video file into chunks that ...

... are stored, each encoded at multiple rates (video quality). The client plays the video chunk-by-chunk, with each chunk requested at encoding rate that fits the available bandwidth at the time

Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms. What is the maximum channel utilization of a stop-and-wait protocol for this channel?

0.1

Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms. What is the channel utilization of a pipelined protocol with an arbitrarily high level of pipelining for this channel?

1.0

Suppose a Web server has five ongoing connections that use TCP receiver port 80, and assume there are no other TCP connections (open or being opened or closed) at that server. How many TCP sockets are in use at this server?

6

Choose one the following two definitions that makes the correct distinction between routing versus forwarding. A) Forwarding is the local action of moving arriving packets from router's input link to appropriate router output link, while routing is the global action of determining the source-destination paths taken by packets. B) Routing is the local action of moving arriving packets from router's input link to appropriate router output link, while forwarding is the global action of determining the source-destination paths taken by packets.

A

Which of the following descriptions below correspond to a "nuts-and-bolts" view of the Internet?

A "network of networks". A collection of hardware and software components executing protocols that define the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. A collection of billions of computing devices, and packet switches interconnected by links.

DASH

An approach that allows a client to adapt the encoding rate of retrieved video to network congestion conditions.

Why is the UPD header length field needed?

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

In SR, why does the receiver have to acknowledge packets with sequence numbers that are less than (and to the left of) those in its window, which starts at rcv_base

Because the sender may not have received an ACK for that packet yet

Which of the following physical layer technologies has the highest transmission rate and lowest bit error rate in practice? A) 802.11 WiFi Channel B) 4G/5G cellular C) Fiber optic cable D) Satellite channel E) Twisted pair (e.g., CAT5, CAT6) F) Coaxial cable

C) Fiber optic cable

Used by sender or receiver to detect bits flipped during a packet's transmission

Checksum

Which characteristics are associated with the technique of packet switching?

Congestion loss and variable end-end delays are possible with this technique. Resources are used on demand, not reserved in advance. This technique is used in the Internet. Data may be queued before being transmitted due to other user's data that's also queueing for transmission.

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. True or False.

Correct! The network layer's best effort service doesn't really provide much service at all, does it?

Network Layer

Delivery of datagrams from a source host to a destination host (typically)

What are some reasons for not discarding received-but- out-of-sequence packets at the receiver in GBN?

Even though that packet will be retransmitted, its next retransmission could be corrupted, so don't discard a perfectly well-received packet, silly!

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

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

What is the role of an authoritative name server in the DNS?

It provides the definitive answer to the query with respect to a name in the authoritative name server's domain

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

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

We learned that in HTTP web browser caching, HTTP local web server caching, and in local DNS caching, that a user benefits (e.g., shorter delays over the case of no caching) from finding a local/nearby copy of a requested item. In which of the following forms of caching does a user benefit from its not only from its own recent requests (and cached replies) but also from recent requests made from other users?

Local DNS server caching HTTP local web caching

what transport services are provided to the application by TCP?

Loss-free data transfer. The service will reliably transfer all data to the receiver, recovering from packets dropped in the network due to router buffer overflow. Congestion control. The service will control senders so that the senders do not collectively send more data than links in the network can handle. Flow Control. The provided service will ensure that the sender does not send so fast as to overflow receiver buffers.

encryption

Provides confidentiality by encoding contents

authentication

Proving you are who you say you are

The client's utilization

Rbottleneck / RC

The server's utilization

Rbottleneck / RS

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.

firewall

Specialized "middleboxes" filtering or blocking traffic, inspecting packet contents inspections

What approach is taken by a CDN to stream content to hundreds of thousands of simultaneous users?

Store/serve multiple copies of videos at multiple geographically distributed sites

What is meant by transport-layer multiplexing?

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

When we say that the Internet is a "network of networks," we mean?

The Internet is made up of access networks at the edge, tier-1 networks at the core, and interconnected regional and content provider networks as well. The Internet is made up of a lot of different networks that are interconnected to each other.

How does the networked application running on a server know the client IP address and the port number to reply to in response to a received datagram?

The application code at the server determines client IP address and port # from the initial segment sent by client, and must explicitly specify these values when sending into a socket back to that client

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

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

Maximum achievable end-to-end throughput

The maximum achievable end-end throughput is the capacity of the link with the minimum capacity

Suppose that the local DNS server caches all information coming in from all root, TLD, and authoritative DNS servers for 20 time units. (Thus, for example, when a root server returns the name and address of a TLD server for .com, the cache remembers that this is the TLD server to use to resolve a .com name). Assume also that the local cache is initially empty, that iterative DNS queries are always used, that DNS requests are just for name-to-IP-address translation, that 1 time unit is needed for each server-to-server or host-to-server (one way) request or response, and that there is only one authoritative name server (each) for any .edu or .com domain. Consider the following DNS requests, made by the local host at the given times: t=0, the local host requests that the name gaia.cs.umass.edu be resolved to an IP address. t=1, the local host requests that the name icann.org be resolved to an IP address. t=5, the local host requests that the name cs.umd.edu be resolved to an IP address. (Hint: be careful!) t=10, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. t=12, the local host requests that the name cs.mit.edu be resolved to an IP address. t=30, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. (Hint: be careful!) Which of the requests require 8 time units to be resolved?

The request at t=1. The request at t=30. The request at t=0.

In the SR receiver window (see diagram below, taken from PPT slides and video), why haven't the red packets been delivered yet?

There is a packet with a lower sequence number than any of the red packets that has yet to be received, so in-order delivery of data in the red packets up to the application layer is not possible

Which characteristics are associated with a client-server approach to structuring network applications

There is a server with a well known server IP address. There is a server that is always on. HTTP uses this application structure

What happens when a socket connect() procedure is called/invoked?

This procedure creates a new socket at the client, and connects that socket to the specified server

Manifest file. What is the purpose of a manifest file in a streaming multimedia setting?

To let a client know where it can retrieve different video segments, encoded at different rates

Physical Layer

Transfer of a bit into and out of a transmission media.

Link Layer

Transfer of data between neighboring network devices.

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: 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

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

Digital Subscriber Line (DSL)

Wired. Up to 10's of Mbps downstream per user.

WiFi

Wireless. 10's to 100's of Mbps per device.

4G Cellular Data

Wireless. Up to 10's Mbps per device.

Which of the following statements about pipelining are true?

With a pipelined sender, there may be transmitted packets "in flight" - propagating through the channel - packets that the sender has sent but that the receiver has not yet received. A pipelined sender can have transmitted multiple packets for which the sender has yet to receive an ACK from the receiver

Physical layer data unit

bit

Allows the receiver to eventually receive a packet that was corrupted or lost in an earlier transmission.

retransmission

Allows for duplicate detection at receiver.

sequence numbers

Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms. How many packets can the sender transmit before it starts receiving acknowledgments back?

10

Which of the following descriptions below correspond to a "services" view of the Internet?

A place I go for information, entertainment, and to communicate with people. A platform for building network applications.

Which human scenarios involve a protocol?

A student raising her/his hand to ask a really insightful question, followed by the teaching acknowledging the student, listening carefully to the question, and responding with a clear, insightful answer. And then thanking the student for the question, since teachers love to get questions. One person asking, and getting, the time to/from another person. Two people introducing themselves to each other

chunk

A unit of video, each of which may be encoded at multiple different rates, stored in different files

What is meant by a cumulative acknowledgment, ACK(n)?

A cumulative ACK(n) acks all packets with a sequence number up to and including n as being received

Lets the sender know that a packet was received correctly at the receiver.

ACK

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

Which characteristics are associated with a P2P approach to structuring network applications

A process requests service from those it contacts and will provide service to processes that contact it. There is not a server that is always on.